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Electrochimica Acta
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Manuscript Number: RH15-551
Title: Ionic Liquid-Carbon Nanomaterial Hybrids for Electrochemical
Sensor Applications
Article Type: Review Article
Keywords: ionic liquids; nanomaterials; Electrochemical Sensor; carbon
nanotube; graphene.
Corresponding Author: Dr. Mohammed AbdulHakim AlSaadi, Ph.D
Corresponding Author's Institution: University of Malaya
First Author: Ali Abo-Hamad
Order of Authors: Ali Abo-Hamad; Mohammed AbdulHakim AlSaadi, Ph.D; Maan
Hayyan; Ibrahim Juneidi; Mohd Ali Hashim
Abstract: Ionic liquids (ILs) have shown an increasing concern in
nanotechnology during the last decade. Their unique physicochemical
properties were highly utilized in electrochemistry, commonly as a form
of IL-carbon nanomaterial (IL-CNM) hybrids. Synergistic combination of
both components resulted in better use in numerous electrochemical
applications such as energy storage devices and sensor electrodes. The
need of high surface area, excellent electrical conductivity, high
sensitivity and catalytic activity was the key behind their useful
applications. This review aims to provide an overview about the synthetic
routes for electrochemical sensor fabrication based on IL-CNM hybrids.
The differences in sensing performance between the electrode designs are
also discussed. ILs can affect the structure and surface chemistry of
CNMs including carbon nanotube, graphene and fullerene. IL-CNM-modified
solid electrode was the most common and effective design used in academic
researches. However, the inclusion of biological components and metallic
nanoparticles were highly affecting the electrode performance. The
electrochemical techniques used for detection have varied based on
several considerations related to electrode design and targeted analyte.
They also played an important role in determining the sensor sensitivity
and detection limit.
Ionic Liquid-Carbon Nanomaterial Hybrids for Electrochemical Sensor
Applications
Ali Abo-Hamada,b
, Mohammed AbdulHakim AlSaadia,c*
, Maan Hayyana,d
, Ibrahim
Juneidia,b
Mohd Ali Hashima,b
aUniversity of Malaya Centre for Ionic Liquids (UMCiL), University of Malaya, Kuala
Lumpur 50603, Malaysia bDepartment of Chemical Engineering, University of Malaya, Kuala Lumpur 50603,
Malaysia cNanotechnology & Catalysis Research Centre (NANOCAT), University of Malaya,
Kuala Lumpur 50603, Malaysia d
Department of Civil Engineering, University of Malaya, Kuala Lumpur 50603,
Malaysia
*E-mail: [email protected], Tel: +60-16-3630693, Fax: +60-3-7967-5311
This work is a part of a huge literature review (collected from over 1000 articles) about
the potential applications of ionic liquids in nanotechnology. The authors have decided to
split this part since they discovered its specificity and importance in electrochemical
science. This work showed a good example of the unique existence and collaboration
between two promising materials (ionic liquids and carbon nanomaterials) to serve the
field of electrochemical sensors and biosensors. The ways of how both materials are
utilized at a time were discussed and interpreted as three basic designs of electrodes. The
article studies each design individually by explaining the steps of sensor fabrication and
summarizing all related examples to describe the case. Finally, the pros and cons of the
different designs were discussed along with the effect of the used electrochemical
techniques.
Suggested Referees:
- Inas Muen AlNashef
- Mohammed Harun Chakrabarti
- Madalina M. Barsan
*Cover Letter (including Suggested Referees)
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1
Ionic Liquid-Carbon Nanomaterial Hybrids for Electrochemical Sensor Applications
Ali Abo-Hamada,b
, Mohammed AbdulHakim AlSaadia,c*
, Maan Hayyana,d
, Ibrahim Juneidia,b
Mohd Ali Hashima,b
aUniversity of Malaya Centre for Ionic Liquids (UMCiL), University of Malaya, Kuala
Lumpur 50603, Malaysia bDepartment of Chemical Engineering, University of Malaya, Kuala Lumpur 50603,
Malaysia cNanotechnology & Catalysis Research Centre (NANOCAT), University of Malaya, Kuala
Lumpur 50603, Malaysia d
Department of Civil Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia
*E-mail: [email protected], Tel: +60-16-3630693, Fax: +60-3-7967-5311
Abstract
Ionic liquids (ILs) have shown an increasing concern in nanotechnology during the last
decade. Their unique physicochemical properties were highly utilized in electrochemistry,
commonly as a form of IL-carbon nanomaterial (IL-CNM) hybrids. Synergistic combination
of both components resulted in better use in numerous electrochemical applications such as
energy storage devices and sensor electrodes. The need of high surface area, excellent
electrical conductivity, high sensitivity and catalytic activity was the key behind their useful
applications. This review aims to provide an overview about the synthetic routes for
electrochemical sensor fabrication based on IL-CNM hybrids. The differences in sensing
performance between the electrode designs are also discussed. ILs can affect the structure and
surface chemistry of CNMs including carbon nanotube, graphene and fullerene. IL-CNM-
modified solid electrode was the most common and effective design used in academic
researches. However, the inclusion of biological components and metallic nanoparticles were
highly affecting the electrode performance. The electrochemical techniques used for
detection have varied based on several considerations related to electrode design and targeted
analyte. They also played an important role in determining the sensor sensitivity and
detection limit.
*Manuscript (including Abstract)Click here to view linked References
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Keywords: ionic liquids; nanomaterials; Electrochemical Sensor; carbon nanotube;
graphene.
Table of Contents
Abstract ................................................................................................................................................... 1
List of IL abbreviations ........................................................................................................................... 3
1. Introduction ..................................................................................................................................... 4
1.1. Electrochemical aspects of ILs ............................................................................................... 5
1.2. CNMs in electrochemistry ...................................................................................................... 8
2. IL-CNM hybrids ............................................................................................................................. 9
3. Electrochemical sensors based on ILs and CNMs ........................................................................ 13
3.1. Typical forms of IL-CNM based electrochemical sensors ................................................... 13
3.2. Fabrication of IL-CNM based sensors .................................................................................. 15
3.2.1. Nanocomposite film modified solid electrode .............................................................. 16
3.2.2. Nanocomposite paste electrode ..................................................................................... 21
3.2.3. Surface-modified carbon-IL paste electrode ................................................................. 25
4. Perspectives on IL-CNM based sensors ........................................................................................ 29
4.1. Preferences for design selection ............................................................................................ 30
4.2. Other factors affecting the sensor performance .................................................................... 32
Conclusions ........................................................................................................................................... 35
References ............................................................................................................................................. 41
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List of IL abbreviations
Abbreviation Cation
EMIm 1-ethyl-3-methylimidazolium
BMIm 1-butyl-3-methylimidazolium
OMIm 1-octyl-3-methylimidazolium
BPy4 butyl-pyridinium
APMIm 1-(3-Aminopropyl)-3-methylimidazolium
iBMIm 1-isobutyl-3-methylimidazolium
DBIm 1,3-dibutylimidazolium
DPIm 1,3-dipropylimidazolium
BPIm 3-butyl-1-[3-(N-pyrrolyl)propyl]imidazolium
C6MIm 1-hexyl-3-methylimidazolium
BMPy 1-Butyl- 3-methylpyridinium
B4MPy 1-butyl-4-methylpyridinium
HPy 1-hexylpyridinium
P(C6)3C14 trihexyltetradecyl phosphonium
BPIm 3-butyl-1-[3-(N-pyrrolyl)propyl]imidazolium
VEIm 1-vinyl-3-ethyl imidazolium
BMP 1-butyl-1-methyl-pyrrolidinium
BIm 1-butylimidazole
HeMIm 1-(2’-hydroxylethyl)-3-methylimidazolium
SBMIm 1-(4-sulfonylbutyl)-3-methylimidazole
MP N-methyl-piperidinium
OPy octylpyridinium
CMMIm 1-carboxymethy-3-methylimidazolium
AMIm 1-(2-acryloyloxy-ethyl)-3-methyl-imidazol-1-ium
HPAA 3-hydroxypropanaminium acetate
Py1,4 1-butyl-1-methylpyrrolidinium
BPy N-butylpyridinium
Abbreviation Anion
TFSI bis(trifluoromethylsulfonil)imide
FSI bis(fluorosulfonyl)imide
TFA trifluoroacetate
PF6 hexafluorophosphate
BF4 tetrafluoroborate
TFS trifluoromethanesulfonate
Ala alanine
HS hydrogen sulfate
Cys 2-amino-3-mercaptopropionic acid (L-cysteine)
ES ethyl sulfate
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1. Introduction
The entire inclusion of positive and negative ions combined together in a liquid form is the
main characteristic of ILs. However, an arbitrary definition was frequently used to describe
the IL as a molten salt which has a melting point below 100 °C [1]. The boundary between
ILs and molten salts was justified by the rapid improvement of liquid salt application in many
fields below this temperature. As common with normal salts, IL can include various types of
cations and anions. It is a combination of large organic cation such as imidazolium,
pyridinium and phosphonium and a relatively small anion which could either be a single atom
like Cl- and Br
- or bigger complex like ethyl sulfate, tetrafluoroborate and
hexafluorophosphate. ILs have unique properties compared to conventional solvents like low
volatility, high electric conductivity, and low toxicity which make them considered as green
media [2]. Their high solvation ability along with the good thermal and ionic conductivity
attracted the interests to utilize them in chemical and electrochemical synthesis [3]. Other
applications in electrochemical devices, lubricants composite materials, polymers and
nanoparticles were deeply established [4]. In the field of electrochemical analysis, novel
electrochemical sensors and biosensors were introduced as a form of modified electrodes[5].
CNMs have increasingly attracted attentions due to their structural and physical properties.
They can be existed in different dimensionalities of sp2 bonded graphitic carbon [6]. The
most common types of CNMs found in various studies and especially in the field of
electrochemical sensor applications are represented in Figure 1.
Zero-dimensional form which seen as an irregular sheet of graphene being curled up into a
sphere by incorporating pentagons in its structure represents fullerene particles (C60). The
size of fullerene may range form 30-300 carbon atoms. Graphene has the structure of two-
dimensional block of sp2 carbon sheets. Conceptually, rolling up one or more graphene sheets
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results in single or few layered graphene tubes known as single wall carbon nanotube
(SWCNT) or multi wall carbon nanotube (MWCNT) [6-8].
CNMs offer distinctive features in the field of sensor applications. The ability of CNMs to be
modified in different ways has led to enhance their usability and improve the level of
exploitation of their electronic properties and sensitivity.
Various analytes were successfully detected using CNMs based sensors. While CNM based
sensors have beaten traditional sensing technologies in performance, seeking for better
sensitivity, reusability, reversibility and real applicability are still challenges [9-12].
This review is to highlight the recent advances in electrochemical sensors based on the
combination of the two functional materials; ILs and CNMs. It is also to discuss the ways for
general sensor fabrication as well as the resulted performance. The viability to introduce
biological components like enzymes or biomolecules required for selective biosensors
fabrication was also discussed.
1.1. Electrochemical aspects of ILs
The efficiency of ILs in electrochemical technologies can be experienced by their
effectiveness to produce electrochemical elements (e.g. electrolytes and/or electrodes) to be
used in advanced batteries, supercapacitors, actuators, fuel cells and dye sensitized solar cells
[13]. Interestingly, the possibilities to design ideal tailor-made electrolytes for such devices
are unlimited since the combinations of cations and anions are countless [14].
Electrochemical properties of ILs are basically defined through their electrochemical
windows, conductivities, and viscosities.
It is important for any electrochemical solvent to identify its electrochemical window (EW)
before it can be used in electrochemical applications. Lithium-ion batteries and
supercapacitors, for example, can only operate within specific voltage ranges which represent
the EWs of their electrolytes. The EW of an electrolyte is commonly calculated by
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subtracting the reduction potential (cathodic limit) from the oxidation potential (anodic
limit) [15]. In other words, EW is the voltage range between which the tested substance does
not get oxidized nor reduced (i.e. inert within this range) [16]. Figure 2 shows EWs for some
common ILs used as electrolyte in lithium-ion batteries [17].
Electrochemical stability of ILs at different electrode materials, such as Pt, W and glassy
carbon (GC) was the matter of various studies. However, making a comparison from the
available data of previous studies in order to judge the best performance seems to be
improper due to the different types of materials used as working and reference electrodes.
However, stability windows were almost found in a broad range from 2 to 6 V, typically
4.5 V. In addition, the specific capacity of electrodes based on high carbon surface and ILs
might reach 180 μF/g [18].
Poor susceptibility of the ILs cations and anions towards electrochemical oxidation and
reduction contributed to their wide electrochemical window. Thus, interesting
electrochemical applications might be established based on these solvents such as
electrochemical sensors and biosensors.
Ionic conductivity of solvents depends on the availability of charge carriers and their
mobility. Unpredictably, ILs which consists entirely of ions do not always possess as high
conductivities as in concentrated aqueous electrolytes due to the obstructed ion mobility. This
phenomena result from the high viscosity of IL, ion aggregations and/or the large sizes of
available ions [19]. Moreover, ILs based on pyridinium, piperidinium, pyrrolidinium, and
tetraalkylammonium cations were characterized by substantially lower conductivities ranged
from 0.1 and 5 mS.cm-1
[18]. Generally, the conductivity of ILs for a given anion decreased
by changing the cation type following the trend: imidazolium>pyrrolidinium>ammonium.
Planarity of the imidazolium ring was found vital in supporting the high conductivity,
whereas pyrrolidinium ring itself is not planar and was found related to the lower
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7
conductivity [20]. Typically, there is a strong correlation between IL conductivity and
viscosity governed by Walden’s rule [21]. An inverse relationship between both properties is
present. Krossing et al. [22, 23] reported the strong dependence of IL viscosity (and,
consequently, the electrical conductivity) on its molecular volume. However, in the case of
using ILs as electrolytes, the properties of the electrode/IL interface are highly important. The
capacitance and microstructure of the electrical double layers (EDLs) at the interface
determine the performance of the electrochemical system [24]. ILs are structured of various
types of ions or ionic pairs wherever they existed (i.e. either in the bulk or at the electrode/IL
interface). However, the behavior of such ions in both positions is ruled by the intermolecular
forces including dipole-dipole, van der Waals, hydrogen-bonding, and Coulombic forces
[25]. Various investigations were performed to describe the arrangements of cations and
anions in the ILs and a nano-structural organization was frequently reported [26, 27]. This
organization was found helpful to explain the high solvation capability and unique
physicochemical properties in ILs [28]. For instance the nanometer-scale structuring for 1-
alkyl-3-methylimidazolium family [CnMIm] with hexafluorophosphate [PF6] using molecular
simulation was visualized as presented in Figure 3 [29]. Using the color coding for the two
types of domains: polar (red) and nonpolar (green), it was observed that the increase in the
alkyl chain causes the nonpolar domains to become larger and more connected.
Considering this complexity in the ILs shape compared to conventional aqueous electrolytes,
new theories for the EDLs at the electrode/IL interface shall be proposed. Following the
electrochemical impedance technique, imidazolium-based ILs was found to have large
capacitance at the interface with carbon gel electrodes built from 3D macroporous carbon
[30]. The electrochemical capacitor showed a specific energy of 58 W.h.kg-1
which is
comparable to commercial batteries. On the other hand, microporous carbon (i.e. CNT) has
only presented higher capacitance in aqueous electrolytes. Another study has focused on the
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role of ion chemistry and structure in carbonaceous-electrodes/ILs capacitive response [31].
The capacitance of the EDL at the negatively-charged electrode was determined by the cation
polarizability which affects in turn the thickness and dielectric constant of the EDL. A
monolayer of cations up against the electrified carbon surface was suggested to form the
EDL. Sato et al. [32] reported that ILs formed from aliphatic quaternary ammonium cation
(i.e. N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium) and [BF4]- or [TFSI]
- anions
showed wide potential windows, high ionic conductivity (over 3×10-3
S.cm-1
), and high EDL
capacity at the interface with activated-carbon-based electrodes compared to conventional
organic solvents. It is worth mentioning that the topography of electrode surface was also
found to influence the capacitance behavior at the IL/electrode interface. Studying the
differential capacitance at different electrode potentials revealed serious qualitative changes
in the capacitance for the different electrode roughness, see Figure 4 [33]. This suggested
paying more attention on electrode surface characterization and controlling its roughness in
order to ascertain a good level of energy density.
1.2. CNMs in electrochemistry
Since the discovery of CNMs, numerous applications were reported about the use of CNMs
to produce elements for electrochemical devices such as solar cells [34, 35], capacitors [36,
37], fuel cells [38, 39] and sensors [40]. Among these varieties, electrochemical sensors have
arguably been the most encountered developments in the field of electrochemistry due to the
huge number of studies reported [41-44]. The advantages of CNMs, such as their
electrochemical inertness, relatively low cost, wide potential window, and electrocatalytic
activity for various redox reactions, have significantly promoted their use in electrochemistry
[45]. Electrochemical sensors based on CNTs in general depend on the high aspect ratio of
CNTs. This property was found highly valuable to build up a vertically aligned arrays of
CNTs, or nanoarrays, with a correspondingly improved sensitivity and low detection limits of
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9
targeted analytes [46]. The electrocatalytic activity of CNT modified solid electrodes were
frequently reported as higher than other graphitic-based and bare solid electrodes. Figure 5
provides an example about the sensitivity difference between conventional electrodes (i.e.
carbon paste, graphite rod and GC) and a CNT-modified GC electrode for the detection of
DNA hybridization [47]. This example shows clearly the important role played by CNT to
enhance the electrode catalytic activity.
Graphene is also a promising electrode material due to unique and excellent properties
promoting their use in electrochemical sensors. Due to the high specific surface area of ~
2600 m2.g
-1 [48], high–speed electron mobility at room temperature [49, 50] and the
outstanding heat conductivity [51], graphene-based materials represent a suitable
electrochemical platform for various chemical and biochemical detections [52]. Just as CNT
and graphene, C60 was introduced for electrode modification by Compton et al. [53]. C60
films were immobilized by casting method onto the surface of noble metal based electrodes
then coated by Nafion protecting films. This design helped to increase the electrode
performance by reducing the required amount of C60. Goyal et al. described a successful
method for the determination of nandrolone in human serum and urine samples using C60-
modified GC electrode [54]. The typical CV diagram observed indicated the enhancement in
electrode activity due to the role of C60 film which acted as electron-mediator, see Figure 6.
Later on, the electrochemical behavior of C60 modified electrode was frequently investigated
in various aqueous and non-aqueous solutions [55, 56]. The electrochemical analysis studies
performed afterwards suggested the use of C60 based electrode as a good analytical tool for
electrochemical detection [57, 58].
2. IL-CNM hybrids
The first combination between CNM and ILs was reported in 2003 when Fukushima and
coworkers [59] produced the “bucky gel” by agitating an imidazolium based IL with pristine
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CNT until a thick paste was obtained. The interest of such combinations was aroused due to
the significant changes in the CNT rheological properties, the high thermal stability of the
combination, and the ability to produce high conductive polymeric gel from polymerizable IL
and CNT. Since then, numerous studies were performed to investigate the potential
application of different types of CNM-IL hybrids including graphene [60], fullerene [61] and
CNT [62].
Figure 7 shows some examples about the preparation and proposed applications of IL-CNT
bucky gels in electro-mechanic devices.
However, CNT-IL hybrids have found the highest interest compared to other hybrid types. A
great concern was expressed for the different applications of IL-CNT in chemical, physical
and biological fields [59, 63-69]. The combination was considered as a good solution to
address the drawbacks of bare CNT by the functionalizing and dispersing effects of the ILs in
the hybrid. CNT-IL hybrids were effectively applied in polymeric matrices and found to
improve their thermal, mechanical and electrical properties [70-72]. The tensile strength was
found to increase by 300% in the presence of 1-allyl-3-methylimidazolium chloride with 3%
weight fraction of CNTs [73]. Sekitani et al. reported the use of CNT-IL to produce a
stretchable composite with high conductivity sufficient for the application in high-
performance electronic circuits. Similarly, Tung et al. fabricated a transparent polymer matrix
with excellent thermal and electrical conductivity for optoelectronic devices [71]. Moreover,
Liu et al. studied the dispersibility of CNTs in a polymerized IL and reported an increase in
the tensile strength, storage modulus and glass transition temperature [72]. Zhu et al.
produced an effective bucky gel based on gold nanoparticles, MWCNTs and [SBMIm][PF6]
to be used for glucose electrochemical sensing [74], see Figure 8. The composite was casted
on the surface of solid electrode and was found highly active to catalyze the oxidation
reaction of glucose non-enzymatically. Generally, plenty of other applications of IL-CNT
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hybrids were also reported especially in biosensing [75-77], actuating [78, 79] and other
electrochemical applications [80].
Graphene has also been intensively studied since the first observation and characterization
were reported in 2004 [5, 49]. Graphene-IL hybrid materials have high attractive properties
which contributed significantly to new future applications in electrocatalysis [81],
supercapacitor [82], and micro-electro-mechanical lubrication systems [83]. Mo et al.
designed an ion exchange on the graphene surface for wettability control by IL assembly
starting from graphene oxide and 1-alkyl-3-(3-triethoxysilylpropyl)imidazolium chloride
[84]. This study has provided an alternate way for quantity detection of surface ions by
surface force.
Liu et al. investigated the solubility of C60 in different types of ILs based on 1-n-butyl-3-
methylimidazolium, 1-n-octyl-3-methylimidazolium and 1-n-benzyl-3-methylimidazolium
cations, and hexafluorophosphate, tetrafluoroborate and bis((trifluoromethyl)sulfonyl)imide
anions [85]. The C60 dissolution reached at concentration up to 0.10 mg.mL-1
and the C60-IL
hybrids showed an excellent potential for the use in optical, electrochemical, and biological
applications. Later on, Zhilei et al. reported the successful application of C60-IL hybrids for
glucose sensors [86]. The areas of application were extended to include gene delivery [87]
and gas chromatography development [88] in which the hybrid was used for the preparation
of stationary phases. Campisciano et al. studied the immobilization of palladium
nanoparticles using a series of C60-IL hybrids to be applied as catalysts for C–C coupling
reactions [89].
I the field of electrochemistry, the individual performance of ILs or CNMs was found less
than that achieved by their combination. A synergistic effect was discovered and contributed
to more developed electrochemical attitude and higher level of usability [90]. Furthermore, it
is easier to tune the solubility of modified CNMs by simple anion exchange reactions.
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Another impact of synergistic effect is the higher catalytic activity found for IL-
functionalized CNMs towards chemical or electrochemical reactions. It can also enhance the
capability to receive more functional materials like proteins/enzymes required for selective
sensitivity [91-95].
Biocompatibility of IL-CNM hybrids was found in high importance to enhance the catalytic
activity of the proposed sensors towards several analytes. The presence of IL in the hybrid
provides the necessary platform to immobilize biomaterials onto the hybrid structure through
a physical or chemical attachment. Due to the high solvation property of ILs, they can
dissolve the natural biomaterials under mild conditions [96] and, thus, ease the interaction
with the carbonaceous body under different possible kinetics. Figure 9 describes the most
common types of biological components used frequently with IL-CNM hybrids.
Various studies have reported the role of the strong hydrogen-bond-accepting ability to
dissolve cellulose in polar and low-melting-point ILs [97, 98]. Successful dissolution of
cellulose was initially achieved using [AMIm][Cl] [99] and [BMIm][Cl] [100]. The
capability to form the required hydrogen bonds was attributed to the presence of chloride
anion in the ILs [101]. Enzyme dissolution for the purpose of preservation at maintained
efficiency is usually difficult to be reached using ordinary ILs. Proteins are always
susceptible to denaturation due to the strong hydrogen bonds formed with carbohydrates in
order to dissolve them [102]. Fujita et al. studied the effect of IL structure and chemistry on
IL-protein interactions. Using cytochrome c as a protein model, an enhanced solubility and
stability was obtained in the aqueous solutions of ILs based on dihydrogen phosphate anion.
Moreover, cytochrome c was found to be dissolved in dry [AMIm][Cl] and [BMIm][Cl]
[103]. Effectively, an electrochemical investigation about the interaction between IL and
cytochrome c has suggested the use of IL as a permeable protecting layer to support the
electrode redox activity of immobilized cytochrome c on the surface of modified gold
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13
electrode [104]. Generally, according to a recent review, the major factors affecting the
enzyme activity and stability in ILs are: IL polarity, hydrophobicity and water miscibility,
viscosity, and impurities [105]. In fact, not only the ILs are responsible for the excellent
biocompatibility of IL-CNMs hybrids, but also the CNMs which can offer a good
compatibility with different biomolecules. Smart et al. reviewed the biocompatibility of
CNTs and investigated the type of interactions between the nanotubes and antibodies and the
immune system, fibroblasts, neural cells, osteoblasts, ion channels or cellular membranes
[106]. Biocompatibility of graphene was also reviewed and found highly valuable for the
application in electrochemical sensor [107]. Furthermore, the growing interest in C60
biocompatibility for the purpose of drug and gene delivery was recently reviewed [108]. The
design and synthesis of bio-functionalized C60 systems were found to significantly affect the
ability to cross the cell membrane and deliver the active molecules.
According to a recent review, IL-CNM hybrids may also have other promising features which
has opened the door for new potential applications [5]. To date, with the huge efforts placed
for IL-CNM hybrids development, no commercial designs have been commercially exposed
due to some economic considerations.
3. Electrochemical sensors based on ILs and CNMs
The main requirements for best electrochemical sensing performance are catalytic activity
and preparation simplicity. CNM-IL based electrodes have highly met these requirements,
therefore a several designs were suggested through the proper selection of IL type and/or
CNM [109].
3.1. Typical forms of IL-CNM based electrochemical sensors
In the advanced stages of sensor production, by incorporating a biological or biological-
derived element with the IL-CNM hybrid, new functions can be added to the electrode which
would be rather named as biosensor [110, 111]. Idiomatically, a biosensor is an analytical
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device used for the detection and measurement of an analyte by combining a biological
component with a physicochemical detector [112]. The typical design of biosensors requires
the inclusion of biomolecules in the body of the electrode. However, the designation of
“biosensors” might also be seen in case of electrodes without a biological combination but for
biomolecules detecting purposes [113]. Immunosensor were also introduced to express the
pharmaceutical tasks of the sensors [114]. They are basically depending on the affinity
reactions between antigens and antibodies. When an enzyme is coupled to the recognition
layer (i.e. either antigen or antibody) the antigen-antibody interaction occurs and causes an
electrical signal which would be interpreted as electrochemical detection [115]. CNT based
immunosensors are well-established in the field because of their small sizes which enables
their use in intracellular studies (in vivo) without damaging the structural integrity of the
studied cells [116, 117].
In 2004, Zhao et al. [118] were the first to introduce the use of CNM and IL combinations to
build up a novel electrode and study its electrochemical properties. A mixture of MWCNT
and [BMIM][PF6] was prepared and was susceptible to further modification by the addition
of enzyme-coated gold nanoparticles. The electrode was perfectly used to catalyze the
reduction of O2 and H2O2 in aqueous solutions. Graphene-IL combination was introduced
later and used to prepare electrochemical electrodes which were highly outperforming other
conventional electrodes used for the same purposes. Figure 10 illustrates a typical procedure
commonly followed to prepare a modified electrode for sensing aims [119].
Fullerene played also an important role in this respect and was combined with ILs for sensors
preparation. In some cases, C60-IL hybrids can be applied as accompanied with other type of
CNMs or some metal nanoparticles [120].
Very recently, a combination of C60, MWCNT and IL was developed in the form of a
nanostructured uniform film on the surface of GC electrode [121]. The composite film
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
15
showed an improved sensitivity for the detection of catecholamines compared to bare GC,
MWCNT-modified GC, C60-modifed GC, and MWCNT/C60-modified GC electrodes. This
revealed the synergetic effect of C60, CNT and IL which resulted in higher conductivity,
better electron transfer rate and anti-fouling properties along with the enhanced catalytic
activity.
3.2. Fabrication of IL-CNM based sensors
There are several routes to electrode preparation can be followed to design electrochemical
sensors using CNMs and ILs. Basically, three main models can be eventually achieved based
on the final electrode structure and the main steps of fabrication as described in Figure 11.
The most common procedure is to modify the surface of a ready-made electrode like GC or
graphite with a film of CNM/IL-based nanocomposite by (a) casting method, (b)
electrodeposition, (c) layer-by-layer self-assembly or (d) adsorption between CNM, ILs, and
biomolecules [62, 122-125] (Figure 11 a). The second apparatus is to prepare CNM/IL
nanocomposite pastes by the direct mixing of CNM with the IL in the presence or absence of
other additives in liquid or solid form [126, 127] (Figure 11 b). It is also possible to modify
the surface of graphite/IL paste with a film of CNM coating [128] (Figure 11 c). The main
role of IL in this case is to form the carbon paste benefiting from its binding capability and
thus give the electrode the required flexibility. Details on each case are provided later with
recent examples illustrating each case.
Basically, Nanocomposites have become promising materials due to their multi-functionality
and excellent properties acquired by the unique combinations of certain materials [129]. The
nanocomposite was previously defined as “a multiphase material where at least one of the
constituent phases has one dimension less than 100 nm”[45]. This concept highly fits the
hybrids of IL-CNM formed from at least two components: one type of IL with one type of
CNM.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
16
IL-CNM-based nanocomposites could include other additional materials (e.g. biomolecules,
graphite, water, paraffin) and/or a mixture of different CNM types or ILs. In the field of
electrochemical sensors, the liquid materials which are involved in the nanocomposite and
are required to provide the homogeneous thick structure were usually called as binders. The
main function of the binder is to work as a pasting liquid for the carbon powder. Therefore, it
was believed that the binder must have the following typical parameters: (a) high viscosity
and low volatility, (b) electroinactivity and chemical inertness, (c) minimal solubility in
aqueous solutions, and (d) immiscibility with organic solvents [130]. However, besides the
binding effect of ILs reported in several IL-based nanocomposites, ILs have successfully
proven to improve the electrocatalytic activity and selectivity in various electrochemical
systems [131, 132].
It is worth mentioning that the first invention of carbon paste electrode (CPE) dates back to
1958 when Ralph N. Adams produced a thick mixture of carbon and bromoform (as binder)
to act as paste electrode for iodide oxidation [133]. The used amount of binder was 7 mL per
1g of graphite which is conventionally a high amount according to the recent CPE designs.
The typical ratios (graphite/binder) that have been frequently considered as suitable for CPE
fabrication are 70:30 or 60:40 (w/w) using paraffin oil, mineral oil or IL as binder [134].
Generally, the overall production of IL-CNM based electrodes passes through various critical
steps until the final design is reached. Figure 12 illustrates all potential steps reported to date
to produce sensors or biosensors.
3.2.1. Nanocomposite film modified solid electrode
The fabrication of this type of electrode involves the preparation of an IL-nanocomposite
paste. The paste might include some types of CNMs and metal nanoparticles (e.g. CNT,
graphene, C60, Au, Pd, Pt and some metal oxide nanoparticles). Furthermore, other biological
additives are occasionally added to the nanocomposite paste to provide a kind of catalytic
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
17
activity for the reduction or oxidation reactions of the targeted analyte. This paste is used then
to modify the surface of a pre-produced solid electrode (e.g. GC, indium tin oxide (ITO) and
gold electrodes) through different procedures. The most common ways are: casting a few
drops of the paste on the electrode surface, rubbing the electrode surface with the paste or
simply by immersing the electrode directly in a diluted form of the paste. Electrodeposition
might be used instead or after the previous treatments in order to create a uniform nano-
structure film on the surface. This process has been recently referred to as “decoration” and
involves decorating the coating layer with metal nanoparticles which are desired for certain
catalytic reactions. Biosensors of this type of electrodes are usually produced by
immobilizing the biological compounds on the prepared film through physical adsorption. It
is noteworthy that this fabrication is the most popular to design electrochemical sensor
electrodes. Some examples of this case are summarized in Table 1 while the rest can be
reviewed in the supplementary (Table S.1).
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
18
Table 1 Details about electrodes characteristics and performances extracted from recent examples which used the CNM-IL nanocomposite modified solid electrode
IL Nanomaterial Targeted analyte, reaction
type and field of application
Biological
Component
Substrate
Electrode
Other Components
If Existed
Sensing Performance Technique Ref
[P(C6)3C14][TFSI] graphene penicillin
(hydrolysis/reduction)
in milk
penicillinase &
hematein
GC LOD= 1 pM (0.04 ppt) DPV [135]
[BMIm][BF4] IL-functionalized
graphene &
nanoporous TiO2
integrated with Cd+2
carbohydrate antigen15-3
(CA15-3)
(bio-interaction)
primary and
secondary
CA15-3
antibody
GC LOD= 0.008 U.mL-1
SWV [136]
[APMIm][Cl] IL-functionalized
graphene & Au NPs
alpha-fetoprotein (AFP) in
serum samples
anti-AFP GC poly(diallyldimethyl
ammonium chloride
& Prussian blue
LOD= 4.6 pg.mL-1
SWV [137]
[BMIm][BF4] MWCNT Sudan I (oxidation) in food
products like ketchup and
chilli sauce
GC cetyltrimethyl
ammonium bromide
LOD= 8.0 nM
S= 7.3 A.M-1
SWV [138]
[BMIm][BF4] polyhydroxy C60 glucose (oxidation) glucose oxidase GC LOD= 1 µM LSV [139]
[BMIm][BF4] C60 & MWCNT hydrazine (HZ) and
hydroxylamine (HA)
(oxidation) in water samples
chitosan GC LODHZ= 17 ±2 nM
SHZ= 0.684 A.M-1
LODHA= 28 ±2 nM
SHA= 0.487 A.M-1
DPV [140]
[MP][TFSI] N-doped graphene
nanoribbons
4-nonyl-phenol (4-NP) in
water samples
GC poly(o-phenylene-
diamine-co-o-
toluidine) & 4-NP
as template
LOD= 8 nM
S= 3.4 A.M-1
LSV [141]
[BMIm][PF6] MWCNT Cd+2
in water samples GC 2-Nitrophenyl octyl
ether & cadmium
ionophore I & PVC
LOD= 2.3 nM Potentiometric
, EMF
[142]
[BMIm][PF6] graphene & Au NPs E. sakazakii (reduction) horseradish
peroxidase-anti-E.
sakazakii
SPE LOD= 119 cfu.mL-1
CV [143]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
19
[BMIm][PF6] graphene oxide &
MWCNT
dopamine (oxidation) GC LOD= 3.0 pM
S= 8.9 A.M-1
SWV [144]
[BMIm][PF6] MWCNT pyrimethanil (oxidation) in
fruit samples
GC LOD= 16 nM
S= 0.12 A.M-1
DPV [145]
[OPy][PF6] mesoporous carbon &
Pd NPs
glucose (oxidation) in
human serum samples
GC Nafion LOD= 0.2 mM
S= 0.6994 µA.mM-1
Amp [146]
[CMMIm][HS] carbon nanohorns 4-aminophenylarsonic acid
(oxidation)
GC LOD= 0.5 µM
S= 19.1 µA.mM-1
Amp [147]
[APMIm][Br] IL-functionalized
graphene nanoribbons
& PdAg alloy NPs
nifedipine (reduction) in
pharmaceutical sample
GC LOD= 4 nM DPV [148]
[BMIm][PF6] NH2-functionalized
MWCNT
faropenem (oxidation) in
urine samples
GC LOD= 31 nM
S= 0.035 A.M-1
DPV [149]
[BMIm][PF6] graphene oxide &
CNT
Amaranth in drinks GC LOD= 0.1 nM
S= 4.92 A.M-1
SWV [150]
[APMIm][Br] IL-functionalized
graphene
5-hydroxytryptamine (5-
HT) and dopamine (DA)
(oxidation) in human serum
samples
GC LOD 5-HT= 0.067 µM
S5-HT = 0.0772 A.M-1
LOD DA= 0.33 µM
SDA= 0.0223 A.M-1
DPV [151]
Polymerized
[VEIm][BF4]
IL-functionalized
MWCNT
simultaneous determination
of ascorbic acid (AA),
dopamine (DA) and uric
acid (UA) (oxidation) in
human urine samples
GC LODAA= 1.65 µM
SAA= 0.012 A.M-1
LODDA= 2.01 µM
SDA= 0.041 A.M-1
LODUA= 0.46 µM
SUA= 0.229 A.M-1
DPV [152]
[BMIm][PF6] graphene oxide carbaryl (oxidation) in fruit
samples
GC LOD= 0.02 µM
S= 1.1 A.M-1
SWV [153]
[AMIm][Cl] IL-functionalized
graphene & Au NPs
simultaneous determination
of Sunset yellow (SY) and
Tartrazine (TZ) (oxidation)
in drinks
GC LODSY= 0.52 nM
SSY= 26.843 A.M-1
LODTZ= 0.83 nM
STZ= 9.83 A.M-1
SWV [154]
[BMIm][PF6] MWCNT Luteolin (oxidation) in
Chrysanthemum
GC LOD= 0.5 nM
S= 64.703 A.M-1
DPV [155]
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20
[BMIm][PF6] MWCNT serum amyloid A (SAA)
(bio-interaction) in human
serum samples
chitosan &
anti-SAA
GC carboxy-polypyrrole LOD= 0.3 pg.mL-1
DPV [156]
[BMIm][PF6],
[BMIm][BF4],
[PNMIm][BF4]
graphene & 2,6-DAP-
imprinted core-shell
NPs (SiO2 NPs)
2,6-Diaminopyridine (2,6-
DAP) in hair-dyes
GC 2,6-DAP (as
template)
LOD= 0.0275 mg.kg-1
S= 0.0995 µA.( mg.kg-
1)
-1
CV [157]
[APMIm][Cl] graphene & Au
nanodendrites
Fe+3
(reduction) in coastal
waters
GC Nafion LOD= 35 nM
S= 0.39 A.M-1
SWV [158]
Polymerized
[VEIm][BF4]
graphene phenylethanolamine
(reduction) in pig urine
samples
GC LOD= 0.002 µM
S= 0.582 A.M-1
DPV [159]
polymerized
[VEIm][Br]
graphene oxide dopamine (oxidation) GC polypyrrole LOD= 73.3 nM
S= 2.499 A.M-1
DPV [160]
[APMIm][Br] IL-functionalized
graphene
carbaryl (C) and
monocrotophos (M) in
spiked tomato juice samples
acetylcholinest
erase & gelatin
GC glutaraldehyde LODC= 5.3 × 10-15
M
LODM= 4.6 × 10-14
M
DPV [161]
[BMIm][PF6] graphene bisphenol A (oxidation) in
soda and milk samples
GC LOD= 8.0 nM
S= 85.27 A.M-1
LSV [162]
[AMIm][Cl] IL-functionalized
graphene & Au NPs
Sudan I in red chili, tomato
sauce, apple juice and grape
juice samples
GC LOD= 0.050 nM
S= 10.99 A.M-1
SWV [163]
NPs: nanoparticles, GC: glassy carbon, ITO: indium tin oxide, SPE: screen-printed electrode, LOD: limit of detection, S: sensitivity
ASV(ad): Adsorptive stripping voltammetry, ASV(an): Anodic stripping voltammetry, LSV: linear sweep voltammetry, Amp: amperometry, CV: Cyclic voltammetry, DPV:
Differential pulse voltammetry, SWV: Square wave voltammetry, CFFTLSV: Coulometric Fast Fourier Transform Linear Sweep Voltammetry, EMF: electromotive forces
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
21
3.2.2. Nanocomposite paste electrode
No surface modification is required to fabricate this type of electrode. IL-CNM
nanocomposite paste electrode is prepared by mixing an IL with a sort of CNMs using
mechanical or ultrasonic agitation. Graphite powder and liquid paraffin are commonly used
in the composite to save the amount required from CNM and IL. Other additives can be
added to the mixture as desired until a thick gel-like paste is obtained. This is followed by
filling the paste inside a syringe and then a copper wire is used to provide the outer electric
connection. The contact between the electrode and environment (i.e. electrolyte) is made
through the outer surface of the uncovered paste. Various examples for this type of sensors
are provided in Table 2 to describe the electrode characteristics and fields of application
Table S.2 in the supplementary provides the rest of other available studies reviewed for this
work.
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22
Table 2 Details about electrodes characteristics and performances extracted from recent examples which used the IL-CNM nanocomposite paste electrode
IL Nanomaterial Targeted analyte, reaction type
and field of application
Biological
component
Other Components If
Existed
Sensing Performance Technique Ref
[BMIm][Br] MWCNTs epinephrine (EP) and
acetaminophen (AC) (oxidation)
in human urine, pharmaceutical,
and serum samples
graphite powder & liquid
paraffin
LODEP = 0.09 µM
LODAC= 0.5 µM
DPV [164]
[C6MIm][PF6] graphene mangiferin (oxidation) in in
aqueous solutions such as serum
and urine
graphite powder LOD= 20.0 nM
S= 0.138 A.M-1
SWV [165]
[BMIm][PF6] MWCNTs hydrogen peroxide (reduction) chitosan CuCl2 LOD= 1.0 µM Amp [166]
[BMIm][Br] Pt NPs supported
on the MWCNTs
surface
Sudan I (oxidation) in food
samples such as in chilli sauce,
chilli powder, tomato sauce and
strawberry sauce
graphite powder LOD= 3 nM SWV [167]
[OPy][PF6] SWCNTs nitrite (oxidation) in milk
samples
LOD= 0.1 µM
S= 0.0798A.M-1
Amp [168]
[BMIm][Cl] NiO/CNTs
nanocomposite
morphine (oxidation) in human
urine and pharmaceutical
samples
liquid paraffin &
graphite powder
LOD= 0.01 µM
S= 0.0521 A.M-1
SWV [169]
[DPIm][Br] ZnO/MWCNT
nanocomposite
bisphenol A and (BPA) Sudan I
(oxidation) in food samples
liquid paraffin &
graphite powder
LOD (BPA)= 9.0 nM
S (BPA)= 0.406 A.M-1
LOD (Sudan I)= 80 nM
S (Sudan I)= 0.229 A.M-1
SWV [170]
[C6MIm][PF6] graphene sulfite (oxidation) in water
samples
benzoylferrocene &
paraffin oil & graphite
powder
LOD= 20 nM
S= 0.077 A.M-1
SWV [171]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
23
[BMPy][TFSI] MWCNT & Pd
NPs
ascorbic acid (AA), dopamine
(DA), and uric acid (UA)
(oxidation) in human serum and
urine samples
paraffin oil & graphite
powder
LODAA= 200 nM
SAA= 0.114 A.M-1
LODDA= 30 nM
SDA= 0.148 A.M-1
LODUA= 150 nM
SUA= 0.115 A.M-1
DPV [172]
[OPy][Cl] MWCNT nitrite (reduction) in meat
products
hemoglobin LOD= 1.46 µM
S= 548 µA.mM-1
.cm-2
CV [173]
not mentioned graphene Levodopa (oxidation) in human
serum and urine samples
1-(6,7-dihydroxy-2,4-
dimethylbenzofuran-3-
yl) ethanone, graphite
powder
LOD= 5.0 ± 1 nM
S= 1.2 A.M-1
DPV [174]
[C6MIm][PF6] MWCNT & Au
nanostructured
ascorbic acid (AA), dopamine
(DA), and uric acid (UA)
(oxidation) in human serum and
urine samples
paraffin oil & graphite
powder
LODAA= 120 nM
SAA= 0.110 A.M-1
LODDA= 30 nM
SDA= 0.129 A.M-1
LODUA= 30 nM
SUA= 0.266 A.M-1
DPV [175]
[DPIm][Br] ZnO/MWCNT
nanocomposite
noradrenaline (oxidation) in
urine samples
liquid paraffin &
graphite powder
LOD= 20 nM
S= 2.9464 A.M-1
SWV [176]
[OPy][PF6] MWCNT Cd+2
(reduction) in water
samples
4-((1H-1,2,4-triazol-3-
ylimino)methyl)phenol
LOD= 0.08 µg.L-1
S= 1.822 µA.(µg.L-1
)-1
ASV(an) [177]
[EMIm][PF6] MWCNT & Au
NPs
human serum albumin (HSA) in
biological fluids
HSA
antibody
paraffin oil & graphite
powder & 1,6-
hexanedithiol
LOD= 15.4 ng.mL-1
EIS [178]
[B4MPy][PF6] MWCNT dsDNA (oxidation) LOD= 0.249 mg.L-1
(16 pM)
S= 0.53 nA.pM-1
DPV [179]
[HPy][PF6] N-doped graphene rutin (oxidation) in
pharmaceutical tablet samples
graphite powder LOD= 0.23 nM
S= 424.89 A.M-1
DPV [180]
[Py1,4][TFSI] graphene &
alumina NPs
Hg+2
in water samples graphite powder LOD= 1.95 nM Potentiometric
, EMF
[181]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
24
[C6MIm][PF6] graphene oxide bisphenol A (oxidation) in water
samples
graphite powder LOD= 55.0 nM
S= 0.103 A.M-1
SWV [182]
[BPy][PF6] acidic-
functionalized
MWCNT
uric acid (oxidation) in human
urine samples
poly(β-cyclodextrin) &
graphite powder
LOD= 0.3 µM
S= 0.3552 A.M-1
LSW [183]
[BMIm][Br] NiO/CNTs
nanocomposite
quercetin (oxidation) in food and
pharmaceutical samples
liquid paraffin &
graphite powder
LOD= 0.03 µM
S= 0.0222 A.M-1
SWV [184]
[OPy][PF6] graphene Tl+, Pb
2+ and Hg
2+ (oxidation)
in
water and soil samples
2,4-
Cl2C6H3C(O)CHPPh3
LOD (Tl+)= 0.357 nM
LOD (Pb2+
)= 0.450 nM
LOD (Hg2+
)= 0.386 nM
S (Tl+)= 400.34 µA.M
-1
S (Pb2+
)= 305.78 µA.M-1
S (Hg2+
)= 342.95 µA.M-1
SWV [185]
[C6MIm][PF6] ZnO/CNTs
nanocomposite
Carbidopa (oxidation) in
pharmaceutical serum, water and
urine
liquid paraffin &
graphite powder
LOD= 0.05 µM
S= 0.9856 A.M-1
SWV [186]
[Py1,4][TFSI] IL-modified
mesoporous
carbon
carbendazim (oxidation) in
sugarcane samples
liquid paraffin &
graphite powder
LOD= 0.500 µg.L-1
S= 0.1227 µA.( µg.L-1
) -1
DPV [187]
1st:
[BMIm][Cl]
2nd
:
[BMIm][Br]
graphene caffeic acid (oxidation) 1st LOD= 5 µM
1st S= 0.2406 A.M
-1
2nd
LOD= 1.8 µM
2nd
S= 0.1176 A.M-1
DPV [188]
[C6MIm][BF4] MWCNT & nano
silica
[C6MIm]+ cation (ion exchange)
in water samples
liquid paraffin &
graphite powder
LOD= 1×10-5
mol.kg-1
Potentiometric
, EMF
[189]
[OPy][PF6] graphene & Ag
NPs
thiourea (oxidation) in orange
juice and waste water samples
LOD= 0.7 µM
S= 0.01476 A.M-1
Amp [190]
ECL: Electrochemiluminescence
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
25
3.2.3. Surface-modified carbon-IL paste electrode
The first step of fabrication for this type of sensor is to prepare an IL-carbon past electrode
(CILE). This is usually carried out by mixing graphite powder with an IL until a thick black
gel is obtained. The gel produced acts as a conductive paste and is used then to fill an empty
syringe or glass tube which represents the electrode body. The electric connection of the
electrode is made through a copper wire ingrained inside the past, while some part of the
paste is left out the syringe/tube to provide the contact with the environment. In the next step,
CILE is modified by immersing the electrode body inside a CNM-based suspension. This
could also be performed after the electrochemical deposition of metal nanoparticles to
enhance the electrode catalytic activity. Most of CNM-modified CILEs were used for the
purpose of bio and immuno sensing using voltammetry technique. Table 3 provides some
examples about this type of sensor including its performance and field of application.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
26
Table 3 Details about electrodes characteristics and performances extracted from recent examples which used the CNM modified CILE electrode
IL Nanomaterial Targeted analyte, reaction
type and field of application
Biological
component
Other Components
If Existed
Sensing Performance Technique Ref
[BMIm][PF6] MWCNTs & Co(OH)2
NPs
simultaneous trace
determination of levodopa (L-
dopa) and serotonin (5-HT)
(oxidation) in human serum
Nafion DPV:
LOD (L-dopa)= 0.12 µM
S (L-dopa) = 1.01 A.M-1
LOD (5-HT)= 0.023 µM
S (5-HT) = 1.291 A.M-1
Amp:
LOD (L-dopa)= 0.47 µM
S (L-dopa) = 2.55 A.M-1
LOD (5-HT)= 0.36 µM
S (5-HT) = 3.34 A.M-1
DPV,
Amp
[191]
[HPy][PF6] graphene oxide rutin (oxidation) in
pharmaceutical tablet samples
acridine orange LOD= 8.33 nM
S= 9.53 A.M-1
DPV [192]
[BMIm][BF4] SWCNTs rutin (oxidation) in
pharmaceutical tablet samples
LOD= 0.70 nM
S= 0.132 mA.µM-1
DPV [193]
[BMIm][PF6] MWCNTs & Co(OH)2
NPs
levodopa (L-Dopa) and
melatonin (Mel) (oxidation)
in pharmaceutical and human
urine samples
DPV:
LOD (L-Dopa)= 0.075 µM
S (L-Dopa)= 1.95 A.M-1
LOD (Mel)= 0.004 µM
S (Mel)= 7.491 A.M-1
Amp:
LOD (L-Dopa)= 0.10 µM
S (L-Dopa)= 1.49 A.M-1
LOD (Mel)= 0.042 µM
S (Mel)= 3.45 A.M-1
DPV,
Amp
[128]
[HPy][PF6] V2O5 nanobelts &
MWCNTs
Y. enterocolitica gene
sequence (hybridization) in
pathogenic pork meat sample
ssDNA: 5 -
CCGGCAAAACG
TCTGCGTGA-3
& chitosan
LOD= 1.76 pM DPV [194]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
27
[HPy][PF6] graphene & Au NPs rutin (oxidation) in
pharmaceutical tablet samples
LOD= 0.255 nM
S= 1.83 A.M-1
DPV [195]
[EMIm][BF4] MWCNTs trichloroacetic acid (TCA),
hydrogen peroxide and
sodium nitrite (reduction)
myoglobin Nafion LOD (TCA)= 0.10 mM
S (TCA)= 5.82 A.M-1
LOD (H2O2)= 6.0 µM
S (H2O2)= 1.85 A.M-1
LOD (nitrite)= 0.10 mM
CV [196]
IL-SPE:
[OPy][PF6]
graphene 1st: NADH (oxidation)
2nd
: hydrogen peroxide
(oxidation and reduction)
3rd
: glucose (oxidation)
glucose oxidase glutaraldehyde LOD (NADH)= 2.0 µM
LOD (H2O2)= 0.05 µM (by
oxidation) and 0.08 µM (by
reduction)
S (H2O2)= 6.286 µA.mM-1
(by
oxidation) and 4.278 µA.mM-1
(by reduction)
LOD (glucose)= 1.0 µM
S (glucose)= 22.78 µA.mM-1
.
cm-2
Amp [197]
[BPy4][PF6] Au NPs, graphene carcinoembryonic antigen
(oxidation) in human serum
samples
poly (L-Arginine) LOD= 0.01 ng.mL-1
DPV [198]
[BMIm][Br] MWCNTs of trichloroacetic acid (TCA)
and nitrite (reduction)
hemoglobin &
chitosan
LOD (TCA)= 5.25 µA.mM-1
S (TCA)= 0.4 mM
LOD (nitrite)= 0.1 mM
S (nitrite)= 0.756 A.M-1
CV [199]
[BMIm][PF6] graphene & CdS NPs methimazole LOD= 0.55 nM DFFTC-
LSV
[200]
[HPy][PF6] graphene dopamine (oxidation) in
human urine samples
dsDNA LOD= 0.027 µM
S= 0.156 A.M-1
DPV [201]
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28
[HPy][PF6] N-doped graphene trichloroacetic acid (TCA)
and hydrogen peroxide
(reduction)
hemoglobin &
chitosan
LOD (TCA)= 0.13 µM
S (TCA)= 0.0061 A.M-1
LOD (H2O2)= 0.13 µM
S (H2O2)= 0.0324 A.M-1
CV [202]
[EMIm][BF4] graphene oxide of trichloroacetic acid (TCA),
hydrogen peroxide, and nitrite
(reduction)
hemoglobin Nafion LOD (TCA)= 3.12 µM
S (TCA)= 0.0023 A.M-1
LOD (H2O2)= 0.0137 µM
S (H2O2)= 0.041 A.M-1
LOD (nitrite)= 0.0104 µM
S (nitrite)= 53.82 A.M-1
CV [203]
[HPy][PF6] NiO/graphene
nanocomposite
trichloroacetic acid (TCA)
and hydrogen peroxide
(reduction)
myoglobin Nafion LOD (TCA)= 0.23 mM
S (TCA)= 0.02197 A.M-1
LOD (H2O2)= 0.71 µM
S (H2O2)= 0.076 A.M-1
CV [204]
[OPy][PF6] graphene Cd+2
and Pb+2
(reduction) in
rice samples
cellulose acetate &
graphite powder
LOD (Cd+2
)= 0.08 µg.L-1
LOD (Pb+2
)= 0.10 µg.L-1
S= 1.1847 µA.( µg.L-1
)-1
S= 0.7325 µA.( µg.L-1
)-1
SWASV [205]
[HPy][PF6] graphene oxalic acid (oxidation) in
spinach samples
LOD= 0.48 µM
S= 0.0825 A.M-1
DPV [206]
[HPy][PF6] graphene & Au NPs folic acid (oxidation) in
different drug tablets
LOD= 2.7 nM
S= 2.665 A.M-1
DPV [207]
[HPy][PF6] 3D-graphene trichloroacetic acid
(reduction) in water and drug
samples
hemoglobin & chitosan
LOD= 0.133 mM
S= 0.00548 A.M-1
CV [208]
DFFTC-LSV: differential fast Fourier transform continuous linear sweep voltammetry
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29
4. Perspectives on IL-CNM based sensors
Although the IL-CNM-based electrochemical sensors can be fabricated in three main designs,
there are some important considerations that should be known before choosing the proper
design. Each design has its advantages and drawbacks and is preferred for certain types of
applications, therefore, the preferences for design selection necessitate balancing those
positive and negative aspects. The main advantages of electrochemical sensors that have a
paste structure are (a) the ease of their fabrication, (b) the simplicity of electrode surface
cleaning, (c) the high flexibility and excellent ability to compositionally adjust the paste
combination, (d) the infinite number of possible paste combinations which lead to wide
applicability in numerous fields, and (e) that they perfectly provide an adequate tool to study
the electron transfer mechanism in the cathodic and anodic ranges [134]. However, all paste-
based electrodes in general face several obstacles which limit their applicability in practical
analysis. Due to the fouling of electrode surface after the frequent using, the paste electrode
begins to lose its reproducibility and thus the surface must be cleaned and renewed after each
run. The paste electrodes have their individual physicochemical and electrochemical
properties which may differ from other probes produced by different preparation. This
requires an adequate experience of the user to calibrate and optimize the probes individually
and, thus, it represents a serious limitation for industrial-scale production. Aging of paste
electrodes is also a main shortcoming leading the electrode to lose its structure and function
with time as a result of the limited stability of the composite [130]. On the other hand, the
commercially available solid electrodes offer a very good choice to avoid the shortage of
paste-based electrodes. They possess a higher resistance towards chemical attack during the
electrochemical operation. The sensitivity of GCEs, as an example, is much higher than
CPEs, but they are more vulnerable to fouling effect which requires an entire polishing of the
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30
surface. However, when the surface of a solid electrode is modified in somehow, renewing
the surface can be very time consuming and may cause serious erosion if repeated.
Nevertheless, enhancements in electrode performance were already achieved for the paste-
based electrodes by the interference of IL and CNMs. Various studies have reported the
improvement in the electrode electrocatalytic activity and reproducibility either by including
a sort of IL-CNM hybrids in the structure of the nanocomposite paste or by modifying the
surface of the CILE with a layer of CNM. Furthermore, modifications of solid electrodes
using IL-CNM hybrids have also helped to reduce the drawbacks of the bare solid electrodes
and improve their performance to certain limits. In general, the surface modification of
CILEs or solid electrodes proved to raise the resistance to fouling and to limit the access of
interfering agents. The sensitivity, selectivity and electrocatalytic activity have also been
improved because of the role played by the multifunctional materials represented by IL,
CNMs and other biological/nanostructural additives.
4.1. Preferences for design selection
There are plenty of examples available for the use of IL-CNM-modified solid electrodes
compared to fewer available for the rest types of electrodes. The overall comparison indicates
the more advanced performances of the modified solid electrodes compared to the
nanocomposite pastes and CNM-modified CILEs when using the same electrochemical
technique and the CNM type. Keihan et al. [209] reported the use of IL-MWCNT hybrid to
modify the surface of GC for the detection of hydrogen peroxide. The electrode was further
modified by Prussian blue and showed a good sensitivity towards the electrochemical
reduction of hydrogen peroxide. The amperometric technique was used in the study and
showed a very low detection limit at 0.49 µM. Similarly, Rahimi et al. [210] prepared another
modified GC electrode using IL-MWCNT- catalase hybrid film for the amperometric
detection of hydrogen peroxide. The detection limit achieved was even lower and reached 3.7
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31
nM. On the other hand, the amperometric detection of hydrogen peroxide using a
nanocomposite paste electrode has recorded a lower performance than the previous electrode
style. The nanocomposite was prepared from IL, MWCNT, chitosan and CuCl2 and was used
for the electrochemical reduction of hydrogen peroxide [166]. The detection limit achieved
did not exceed 1.0 µM reflecting that the modified solid electrodes surpass the
nanocomposite-paste electrodes in terms of electrochemical performance. The amperometric
detection of nitrite was also investigated using the two types of electrodes; modified solid
electrode and nanocomposite paste electrode. The electrochemical oxidation of nitrite on the
surface of modified ITO electrode resulted in a very high sensitivity and low detection limit
(1.0 nM) [211]. This result was not comparable with the result achieved by a nanocomposite
paste electrode which used the same amperometric technique but achieved a hundred-time
higher detection limit value (100 nM) [168]. In fact, it is worth mentioning that the modified
ITO electrode was including nanoparticles of AuPt alloy along with the IL-MWCNT hybrid,
whereas the nanocomposite paste included only the IL-SWCNT. Although the comparison
was made between two studies that worked under different conditions, it can provide another
bonus to the modified solid electrodes compared to the nanocomposite pastes. The
electrochemical oxidation of dopamine using DPV technique revealed that the nanocomposite
paste could only beat the modified solid electrode when it is supported by metal
nanoparticles. IL-MWCNT-Au nanocomposite paste electrode have reached a good detection
limit at 30 nM for the electrochemical sensing of dopamine [175]. This limit was exactly half
the limit reached by IL-MWCNT modified GC electrode when no additives were involved
[212]. However, the inclusion of C60 along with the MWCNT and IL hybrid film on the
surface of GC electrode was found to further improve the sensitivity towards dopamine
oxidation. The study used the same electrochemical technique and achieved a detection limit
at 15 nM [213]. This would support our claim that the modified solid electrodes would
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32
originally perform better than the nanocomposite paste electrodes. However, the
nanocomposite paste electrode can be developed technically to improve its performance and
the inclusion of nanostructured metallic catalysts would make them competitive with the
modified solid electrodes. CNM-modified CILEs seemed to have similar electrochemical
activity to nanocomposite paste electrodes. The electrochemical detection of dopamine on the
surface of graphene-modified CILE showed a very close result to that obtained for the
previously discussed nanocomposite paste electrode (LOD = 27 nM) [201]. The detection of
rutin by electrochemical oxidation following DPV technique also showed quite similar results
for a nanocomposite paste electrode and CNM-modified CILE. On one side, chemically
modified graphene (N-doped graphene) was used with the IL to fabricate the nanocomposite
electrode [180], while gold-nanoparticles-modified-graphene was used to modify the CILE
on the other side [195]. The first electrode design has shown a detection limit of 0.23 nM
compared to 0.26 nM for the second design. Moreover, rutin has also been detected using
SWCNT-modified CILE by following the same detection technique as in the previously
mentioned studies [193]. The detection limit obtained was 0.70 nM which seemed to be a bit
close to the previous results and indicates the correspondence in the performances between
the nanocomposite paste electrodes and CNM-modified CILEs. The examples studied earlier
were chosen from the literature considering that the affecting variables on the electrode
performance were as close to each other as possible.
4.2. Other factors affecting the sensor performance
Electrochemical sensors can be classified into biosensors, immunosensors or normal
electrochemical sensors according to their compositions and purposes. However, based on
measurement methods, sensors are usually classified into amperometric when measuring the
redox current, potentiometric when measuring the potential, and conductometric when the
change in the resistance is monitored. Three main factors were found considerably affecting
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33
the performance of sensors proposed for same analyte detection. By using the data extracted
from the examples provided in sections 3.1.1, 3.1.2, and 3.1.3, various comparisons can be
made to study the effects of (a) the electrochemical technique used, (b) the addition of
biomolecules and (c) the presence of nanostructured additives in the sensor.
The electrochemical activity might differ when different electrochemical techniques are used.
Among the various electrochemical techniques used for the detection of different analytes,
amperometric technique has proved to be an informative and effective way to characterize the
sensor and maximize its ability to detect at very low concentrations with a wide detection
range. It can provide a direct and continuous measurement of the accumulated amounts of
analytes within just one run. Amperometric technique can be a good choice when the effect
of the interfering agents is absent or negligible and it is expected to suite the industrial
application. However, voltammetry could also offer a convenient tool for the sensitive and
selective detection of several analytes. DPV was found to be very comparable to the
amperometry and contributed to the recent improvements in the electrochemical activities
especially for the detection of molecules (e.g. glucose, dopamine, NADH… etc). Babaei et
al. [191] reported the electrochemical detection of levodopa using a modified CILE.
[BMIm][PF6] was used as binder and the electrode was coated with a film of MWCNT,
Co(OH)2 nanoparticles and Nafion to reduce the effect of the interfering compounds. By
following two electrochemical techniques (i.e. amperometry and DPV), the results showed
very close sensitivities (2.55 A.M-1
by amperometry and 1.01 A.M-1
by DPV) but with
relativity lower detection limit by DPV compared to amperometry. The same conclusion can
be drawn out from another study which used a similar electrode (with no Nafion added) and
by following the same techniques [128]. The sensitivity and detection limit obtained
amperometrically were close to those obtained by DPV.
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34
Potentiometric technique, which depends on the measurement of electromotive forces, was
frequently used for the detection of metal cations in aqueous solutions. IL-CNM-
nanocomposite paste electrodes were recently introduced as ion-selective electrodes for the
potentiometric measurements [214]. The significance of this type is the ability to include the
selective agents in the electrode structure as required for all potentiometric electrodes. The
inclusion of IL as binder have overcome the difficulties of using non-conductive mineral oils
which consists of multi-components and may unpredictably influence detection and analysis.
Eventually, the potentiometric technique using IL-CNM-based electrodes has become more
common than the voltammetric techniques for the detection of metal ions and has resulted
almost in similar performance.
EIS is an effective tool to study the properties at the interface of electrode surface [215]. The
information obtained is usually presented in the form of impedance spectrum called “Nyquist
plot”. Typical Nyquist plot includes a semicircle part reflecting the electron transfer limited
process at higher frequencies followed by a linear part of the diffusion limited process at
lower frequency [178]. The diameter of the semicircle represents the electron-transfer
resistance Ret which determines the kinetics of the probe. Therefore, any blocking behavior
for the redox couple on the electrode surface can be interpreted into signal for
characterization purposes. If certain substances are adsorbed onto the surface, Ret value will
vary dependently [216]. Impedimetric biosensors and immunosensors were effectively
prepared in the form of IL-CNM-modified solid electrodes [217-219] and IL-CNM-
nanocomposite paste electrode [178]. The resulted performances were superior to other
electrodes that used different electrochemical techniques in terms of the detection limits
obtained (10-13
– 10-14
M).
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35
Conclusions
This review summarizes the recent advances on the fabrication of electrochemical sensor
using IL and CNMs. Various examples were reviewed and compared in terms of the material
used in the electrode composition, the field of application and the resulted performances. ILs
have been mainly used with three types of CNMs: CNT, graphene and fullerene to fabricate
electrochemical sensors. Due to the good electrochemical stability, conductivity and wide
electrochemical windows of ILs, they have become very promising materials for electrodes
fabrication and modification. According to the reported studies, three main electrode designs
were found for the purpose of electrochemical detection. Each design has its advantages and
drawbacks and is preferred for certain types of applications. IL-CNM based electrodes
showed better performances compared to the electrodes that involved only an IL or CNM,
owing to their synergistic effect. Along with the proper selection of electrode design, the
presence of other additives and the electrochemical technique used are all as important to
determine the sensing performance of the proposed electrode.
Acknowledgements
The authors would like to express their thanks to University of Malaya HIR-MOHE
(D000003-16001) and University of Malaya Centre for Ionic Liquids (UMCiL) for their
support to this research.
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36
Figure 1 Common types of CNMs, (a) C60, (b) graphene, (c) SWCNT and (d) MWCNT
Figure 2 Electrochemical windows of some ILs (solid bars) and the lithium metal Fermi level [17]; 1-
butyl-3-methylimidazolium (BMIM), N,N-propylmethylpyrrolidinium (P13)
bis(trifluoromethylsulfonyl)imide (TFSI)
a b c d
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37
Figure 3 Snapshots of simulation boxes containing 700 ions of [CnMIm][PF6] using red/green coloring
code for (b-f) and Corey, Pauling, Koltun coloring code (a), where (a,b) represents [C2MIm][PF6], (c)
[C4MIm][PF6], (d) [C6MIm][PF6], (e) [C8MIm][PF6] and (f) [C12MIm][PF6] (from ref [29])
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
38
Figure 4 Differential capacitance of EDL (at [EMIm][FSI]/graphite interface) as a function of electrode
potential for flat and rough electrode surfaces at 393K (from ref [33])
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39
Figure 5 CVs for guanine solution using carbon paste (A), graphite rod (B), bare GC (C) and CNT-
modified GC (D) electrodes (from ref [47])
Figure 6 CVs of bare GC (dashed line) and C60-modified GC electrodes (solid line) recorded for 1.0 μM
nandrolone in phosphate buffer solution at pH 7.2 (from ref [54])
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
40
a
b c Figure 7 Bucky gel preparation using SWCNT and [BMIm][BF4] using grinding method [220], (b)
schematic representation of a proposed configuration of polymer-supported bucky-gel actuator based on
the ion transfer mechanism [221] and (c) proposed organic transistor array using bucky gel paste as
conducting wires (yellow parts) [220].
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
41
Figure 8 (a) Schematic illustration of the nanocomposite: IL-CNT-Au nanoparticles, (b) and (c) SEM and
TEM images of the nanocomposite respectively (from ref [74])
a
b c
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
42
Figure 9 The biocompatibility of IL-CNM hybrids with the different biological components
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
43
Figure 10 Typical procedure of using graphene/IL hybrid to modify glassy carbon electrode for sensor
application, (GrO: graphene oxide, GrO-IL: ionic-liquid-functionalized graphene oxide) [119]
a
b c
Figure 11 The three main apparatus to fabricate IL&CNM-based electrochemical sensors, nanocomposite
film-modified electrode (a), nanocomposite paste (b) and nano-structure coated carbon-IL paste.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
44
Figure 12 Available procedures to fabricate IL-CNM based electrochemical sensors.
IL+
CNM
Nanocomposite
Glassy carbon Nobel metal
Carbon-IL
paste electrode Nanocomposite
paste electrode
Sensors
By simple agitating
Additives might be
added as required
Paste
electrode
Modified
electrode
IL-functionalized
CNM Nanocomposite
coating
Bucky gel
electrode
Further
modification
if required
Biosensor
With biological
content Without biological
content
Other additives: metal nanoparticles
biological components
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
45
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Table S. 1 CNM-IL nanocomposite modified solid electrode examples
IL Nanomaterial Targeted analyte, reaction
type and field of application
Biological
Component
Substrate
Electrode
Other Components If
Existed
Sensing Performance Technique Ref
[APMIm][Br] MWCNTs &
Au NPs
oxygen (reduction) by
electrocatalysis reaction
GC - CV [1]
[BMIm][BF4] MWCNTs β-nicotinamide adenine
dinucleotide (NADH)
(oxidation) by
electrocatalysis reaction
chitosan GC LOD= 0.06 µM
S= 0.0844 A.M-1
Amp [2]
[BMIm][BF4],
[BMIm][PF6]
SWCNTs oxygen and hydrogen
peroxide (reduction)
heme-containing
proteins/enzymes
(myoglobin,
cytochrome c,
and horseradish
peroxidase)
GC - CV [3]
[OMIm][PF6] MWCNTs &
Au NPs
guanine (G) and adenine
(A) (oxidation) in milk,
plasma and urine samples
GC LODG= 0.005 µM
SG= 35.47 A.M-1
LODA= 0.005 µM
SA= 36.62 A.M-1
CV [4]
[BMIm][PF6] MWCNTs tryptophane (oxidation) rutin GC LOD= 0.03 µM
S= 0.5596 A.M-1
CV, DPV [5]
[AMIm][Br] MWCNTs hydrogen peroxide
(reduction)
GC Prussian blue LOD= 0.49 µM
S= 185.90 µA.mM-1
.cm-2
Amp [6]
[BMIm][PF6] graphitic
mesoporous
carbon
hydroquinone (HQ) and
catechol (CC) (oxidation)
GC LODHQ = 0.05 µM
LODCC= 0.06 µM
SHQ= 1.54 A.M-1
SCC= 1.52 A.M-1
CV, DPV [7]
[BPIm][Br] SWCNTs bisphenol A (oxidation) in
plastic drinking bottle
GC LOD= 1.0 nM
S= 0.1818 A.M-1
CV, DPV [8]
Supplementary Tables (S1 + S2)
[BPy][PF6] MWCNTs hydrogen peroxide
(reduction)
microperoxidase-
11 (MP-11)
GC Nafion LOD= 3.8 nM
S= 0.1086 A.M-1
CV, DPV [9]
[OMIm][PF6] MWCNTs Dopamine (oxidation) in
human blood serum
GC LOD= 0.10 µM CV, DPV [10]
[BMP][TFSI] MWCNTs 1st: NADH (oxidation) 2
nd:
ethanol (oxidation)
2nd
: oxidized
form of NADH
(NAD+)
GC LODNADH= 0.02 µM
SNADH= 0.44 A.M-1
.cm-2
LODethanol= 0.5 µM
Sethanol= 7.7 mA.M-1
.cm-2
Amp [11]
[BMIm][PF6] SWCNTs methylparathion (MP) and
p-nitrophenol (PNP)
(reduction)
in lake water and fruit
sample
GC LODMP= 1.0 nM
SMP= 31.00 A.M-1
SPNP= 16.11 A.M-1
CV [12]
[BMIm][BF4] MWCNTs hydrogen peroxide
(reduction)
catalase GC LOD= 3.7 nM
S= 4.9 A.M-1
Amp [13]
[OMIm][PF6] PtRuNi ternary
alloy NPs on
MWCNTs
alcohol (oxidation) GC LOD= 0.05 mM
S= 0.021 mA.mM-1
.cm-2
Amp [14]
[EMIm][OAc] MWCNTs glucose (oxidation) cellulose &
glucose oxidase
GC S= 6.57 μA.mM-1
. cm−2
Amp [15]
[BMIm][PF6] MWCNTs guanine (G) and adenine
(A) (oxidation)
GC PbO2 LODG= 6.0 nM
SG= 1.52 A.M-1
. cm−2
LODA= 0.30 nM
SA= 0.84 A.M-1
. cm−2
CV, DPV [16]
[EMIm][BF4] CeO2 NPs &
MWCNTs
Pb2+
(reduction) in water
samples
athrombin-
binding aptamer
GC LOD= 5 nM CV, DPV [17]
Polymerized
[VEIm][Br]
MWCNTs 1st: oxygen (reduction),
hydrogen peroxide
(oxidation/reduction) by
electrocatalysis reaction
2nd
: glucose
(oxidation/reduction)
None
/
glucose oxidase
GC 2,2 -
azobisisobutyronitril
e as an initiator &
Nafion
S= 0.853 A.M-1
CV [18]
[OMIm][PF6] SWCNTs folic acid (oxidation) GC LOD= 1.0 nM
S= 3.167 A.M-1
CV, DPV [19]
[BMIm][PF6] MWCNTs Oxygen (reduction) laccase GC - CV [20]
[EMIm][ES] carbon
nanofiber
phenolic compounds
(catechol, p-Cresol, Phenol,
and m-Cresol) (reduction)
tyrosinase GC polyaniline LOD (catechol, p-Cresol,
Phenol)= 0.1 nM, LOD (m-
Cresol)= 0.5 nM, S
(catechol)= 296±4 A.M-1
.cm-
2,S (p-Cresol)= 262±3 A.M
-
1.cm
-2, S (Phenol) = 253±4
A.M-1
.cm-2
, S (m-Cresol)=
47±1 A.M-1
.cm-2
Amp [21]
[BMIm][PF6] SWCNTs &
nanosized
shuttle-shaped
CeO2
phosphoenolpyruvate
carboxylase
(PEPCase) gene
(hybridization)
DNA (ssDNA):
5 -CAG CAC
CTA GGC ATA
GGT TC3
GC LOD= 2.3×10-13
M EIS [22]
[BMP][TFSI] MWCNTs prostate specific antigen
(PSA) (cathodic reaction) in
prostate tissue samples
Thionine & anti-
PSA
GC LOD= 20 pg.mL-1
S= 0.95 mA.ngmL-1
DPV [23]
[APMIm][Cl] graphene & Au
NPs
Carcinoembryonic (CEA)
(anodic reaction) in serum
samples
Carcinoembryoni
c antibody
(anti-CEA):
(Isotype:IgG1)
GC LOD= 0.1 fg.mL-1
DPV [24]
[OMIm][PF6] &
[BMIm][PF6]
MWCNTs Biomolecules: glucose and
NADH
GC - CV [25]
[P(C6)3C14][TFSI] MWCNTs &
AuPt alloy NPs
nitrite (oxidation) ITO LOD= 1.0 nM Amp [26]
[BMIm][PF6] TiO2-graphene
nanocomposite
hydrogen peroxide
(reduction)
chitosan &
hemoglobin
GC LOD= 0.3µM
S= 0.0707 A.M-1
Amp [27]
[EMIm][PF6] MWCNTs &
AuPt NPs
Myeloperoxidase (bio-
interaction)
anti-
myeloperoxidase
ITO poly(o-phenylene
diamine)
LOD= 0.05 ng.mL-1
Amp [28]
[BMIm][PF6] MWCNTs Indomethacin (oxidation) in
biological and
pharmaceutical samples
carbon
ceramic
LOD= 0.26 μM
S= 0.0686 A.M-1
DPV [29]
[BMP][TFSI] MWCNTs human immunoglobulin E
(reduction) in human serum
samples
chitosan GC glutaraldehyde LOD= 37 pM
S= 0.2367 μA.nM-1
DPV [30]
[EMIm][BF4] MWCNTs nitrite (reduction) in water
samples
hemoglobin GC LOD= 0.81 μM
S= 0.04592 A.M-1
Amp [31]
[P(C6)3C14][TFSI] MWCNTs &
AuPt alloy NPs
cysteine (oxidation) GC S= 0.0438 A.M-1
Amp [32]
[BMIm][BF4] MWCNTs food dyes: Sunset Yellow
(SY) and tartrazine (Tz)
(oxidation) in food samples
carbon-
ceramic
LODSY= 0.1 μM
SSY= 0.386 A.M-1
LODTz= 1.1 μM
STz= 0.216 A.M-1
DPV [33]
[BMIm][PF6] C60 NADH (oxidation) by
electrocatalysis reaction
GC - CV [34]
[iBMIm][TFSA] C60 Paraquat in meconium
samples
chitosan &
Polyclonal
antibody
GC ferrocene & 1-ethyl-
3-(3-
dimethylaminopropy
l) carbodiimide & N-
hydroxysuccinimide
LOD= 9.0 pM EIS [35]
[DBIm][TFSA] C60 deoxynivalenol (DON) in
food samples
chitosan &
antibody to DON
GC epichlorohydrin & 3-
(aminopropyl)
triethoxysilane &
glutaraldehyde
LOD= 0.3pg.mL-1
EIS [36]
[DBIm][Br] C60 glucose (oxidation) in
human serum samples
chitosan &
glucose oxidase
GC ferrocene LOD= 3nM
S= 234.67 A.M-1
.cm-2
Amp [37]
[BMIm][PF6] graphene cholesterol (oxidation) cholesterol
oxidase &
catalase
GC LOD= 0.05 μM
S= 4.163 A.M-1
.cm-2
Amp [38]
[BMP][TFSI] Au NPs &
polyamidoamin
e dendrimer &
MWCNT
prostate-specific antigen
(PSA) in human serum
samples
chitosan & anti-
PSA
GC thionine & phtaloyl
chloride
LOD (by DPV)= 0.001
ng.mL-1
, S= 0.0975
µA.(ng.mL-1
)-1
,
LOD (by EIS)= 0.5 ng.mL-1
DPV,
EIS
[39]
[HeMIm][TFSI] AuPt alloy NPs
& graphene &
Au NPs
carbaryl (1-naphthyl
methylcarbamate)
(oxidation) in cabbage and
apple peel
chitosan GC carbaryl imprinted
poly(p-
aminothiophenol) (p-
ATP)
LOD= 8.0 nM
S= 4.0 A.M-1
.mm2
DPV [40]
[BMIm][PF6] bamboo-like
MWCNTs &
prussin blue
NPs
hydrogen peroxide
(reduction)
GC LOD= 0.02 µM
S= 1.44 A.M-1
SWV [41]
[BMP][TFSI] Graphene & Au
NPs
glucose (oxidation) GC LOD= 0.062 µM
S= 97.8 µA.mM-1
. cm-2
Amp [42]
[BMIm][BF4] Graphene &
ZrO2 NPs
Alfuzosin (oxidation) chitosan GC LOD= 0.5 nM CFFTLSV [43]
[SBMIm][PF6] MWCNTs &
Au NPs
glucose (oxidation) in body
fluid
GC LOD= 2.0 µM CV [44]
polyethylenimine-
functionalized IL
MWCNTs &
Au NPs
1st: hydrogen peroxide
(reduction)
2nd
: glucose (oxidation)
2nd
: glucose
oxidase
GC S (H2O2)= 15.6µA.mM-1
. cm-
2
Amp [45]
[BIm][Br]
immobilized onto
an epoxy group
on a poly(glycidyl
methacrylate)
MWCNTs phenolics in commercial red
wines
chitosan &
tyrosinase
ITO acetic acid - CV [46]
[OMIm][PF6] MWCNTs simultaneous determination
of serotonin (5-HT) and
dopamine (DA) (oxidation)
GC LOD5-HT= 8 nM
LODDA= 60 nM
DPV [47]
polyelectrolyte-
functionalized IL
Pt NPs &
graphene
simultaneously determine
ascorbic acid and dopamine
(oxidation) in urine samples
GC - CV,
DPV
[48]
[OMIm][PF6] SWCNTs & Au
NPs
chloramphenicol
(reduction) in milk samples
GC LOD= 5.0 nM
S= 10.46 A.M-1
CV [49]
[BMIm][BF4] SH-
functionalized
MWCNTs &
Au NPs
cholesterol (oxidation) in
serum samples
cholesterol
oxidase &
chitosan
ITO Nafion &
glutaraldehyde
S= 200 µA.M-1
Amp [50]
[OMIm][PF6] MWCNTs sudan compounds
(oxidation) in soft drinks
GC LOD= 0.001 ppm to 0.005
ppm
S (sudan II)= 0.0315 µA per
ppm
Amp [51]
[APMIm][Br] graphene trinitrotoluene (reduction)
in real water samples
GC LOD= 4 ppb ASV (ad) [52]
[BMIm][PF6] MWCNTs simultaneous determination
of Hydroquinone (HQ)
and catechol (CC)
(oxidation)
GC LODHQ= 0.67 nM
SHQ= 1.6782 A.M-1
LODCC= 0.60 nM
SCC= 0.9042 A.M-1
DPV [53]
[BMIm][PF6] graphene glucose (oxidation) glucose oxidase gold LOD= 0.376 mM
S= 0.64 µA.mM-1
Amp [54]
[P(C6)3C14][TFSI] MWCNTs Sudan I (oxidation) in hot
chilli powder and ketchup
samples
GC cationic genimi
surfactant
(i.e.C12H25N(CH3)2–
C4H8–
N(CH3)2C12H25Br2,
C12–C4–C12)
LOD= 0.03 µM
S= 4.28 A.M-1
LSV [55]
[EMIm][BF4] MWCNTs hydrogen peroxide
(reduction)
chitosan &
Cytochrome c
GC acetic acid & NaOH LOD= 0.80 µM Amp [56]
1st: [BMIm][BF4],
2nd
: [BMIm][PF6],
3rd
: [BMIm][TFSI]
MWCNTs organophosphate
(oxidation)
organophosphoru
s hydrolase
gold 1st: S= 2.40 A.M
-1
2nd
: S= 4.37 A.M-1
3rd
: S= 2.46 A.M-1
Amp [57]
[APMIm][Br] SWCNTs glucose (oxidation-
reduction) by
electrocatalysis reaction
glucose oxidase GC - CV [58]
[BMIm][PF6]
graphene with
Pd NPs
MWCNTs with
Pd NPs
ascorbic acid, dopamine
(DA), uric acid, and glucose
(oxidation)
GC Nafion LODDA= 0.07 µM
SDA= 3.28 A.M-1
Amp [59]
[APMIm][Br] IL-
functionalized
MWCNTs &
Ag NPs
hydrogen peroxide
(reduction) in human serum
samples
GC LOD= 3.9 nM DPV [60]
[BMIm][BF4] IL-
functionalized
graphene oxide
& Au NPs
Hg+2
(reduction-oxidation)
in tap water
GC LOD= 0.03 nM ASV(an) [61]
[BMIm][Cys] IL-
functionalized
graphene
catechol (CC) and
hydroquinone (HQ)
(oxidation) in tap and river
water samples
GC LODCC= 1.0 µM
LODHQ= 0.85 µM
DPV [62]
[APMIm][Br] IL-
functionalized
SWCNTs & Au
NPs
glucose (reduction) glucose oxidase GC LOD= 0.8 µM CV [63]
[EMIm][TFS]
[BMIm][BF4]
[EMIm][TCB]
[EMIm][TFSI]
MWCNTs Acetylthiocholine
(oxidation)
carbon
paste
66%
graphite,
34%
paraffin)
LOD= 0.05 - 0.08 mM
S= 36-45 µA.mM-1
.cm-2
Amp [64]
[P(C6)3C14][TFSI] PtM; (M = Ru,
Pd and Au) NPs
& MWCNTs
glucose (oxidation) in
serum and urine samples
GC LOD= 0.05 mM
S= 10.7 µA.mM-1
.cm-2
Amp [65]
[APMIm][Br] IL-
functionalized
graphene
1st: NADH
2nd
: ethanol
(oxidation)
2nd
: chitosan &
alcohol
dehydrogenase
GC S (NADH)= 37.43 µA.mM-
1.cm
-2
LOD (ethanol)= 5.0 µM, S
(ethanol)= 6.91 nA.µM-1
.cm-2
Amp [66]
[BMIm][BF4] functionalized
MWCNTs &
TiO2 NPs
isoproterenol (IP) and
serotonin (5-HT)
(oxidation) in human serum
GC 9-(1,3-dithiolan-2-
yl)-6,7-dihydroxy-
3,3-dimethyl-3,4-
dihydrodibenzo[b,d]f
uran-1(2H)-one
(benzofuran
derivative)
LODIP= 28 ± 2 nM
SIP= 0.021 A.M-1
LOD5-HT = 0.154 µM
S5-HT = 0.039 A.M-1
DPV [67]
[BMIm][BF4] MWCNTs &
C60
Catecholamines:
norepinephrine (NE),
isoprenaline (IP) and
dopamine (DA) (oxidation)
in serum and urine samples
chitosan GC acetic acid LODNE= 18 ± 2 nM
LODIP= 22 ± 2 nM
LODDA= 15 ± 2 nM
SNE= 0.650 A.M-1
SIP= 0.550 A.M-1
SDA= 0.777 A.M-1
DPV [68]
[BMIm][PF6] MWCNTs simultaneous determination
of norepinephrine (NE)
and serotonin (5-HT)
(oxidation) in water and
human blood serum
samples
7-(1,3-dithiolan-
2-yl)-9,10-
dihydroxy-
6Hbenzofuro[
3,2-c]chromen-
6-one
GC LODNE = 49 nM
SNE = 0.306 A.M-1
LOD5-HT = 2 nM
S5-HT = 0.073 A.M-1
DPV [69]
[EMIm][Ala] amino acid, IL-
functionalized
graphene
catechol in water samples tyrosinase &
chitosan
GC LOD= 8 nM
S= 12.6 A.M-1
.cm-2
Amp [70]
[BMIm][Cys] IL-
functionalized
graphene
dopamine (DA) and uric
acid (UA) (oxidation) in
urine samples
GC LODDA= 0.679 µM
SDA= 0.0455 A.M-1
LODUA= 0.323 µM
SUA= 0.0195 A.M-1
DPV [71]
[BMIm][BF4] Prussian blue-
modified
MWCNT
hydrogen peroxide
(reduction)
SPE Nafion LOD= 0.35 µM
S= 0.436 A.M-1
.cm-2
Amp [72]
[BMIm][BF4] IL-
functionalized
graphene
bovine hemoglobin in
bovine blood samples
bovine
hemoglobin
(template)
GC polypyrrole LOD= 3.09×10-11
g.L-1
DPV [73]
[VEIm][BF4] reduced
graphene oxide
& PtPd NPs
glucose (oxidation) in
human serum samples
GC Nafion LOD= 2 µM
S= 1.47 µA.mM-1
.cm-2
Amp [74]
[APMIm][Br] IL-
functionalized
graphene
sunset yellow (oxidation) in
soft drinks
sunset yellow
(template)
GC Monomers: 1-(α-Methyl
acrylate)-3-
allylimidazolium
bromide,
Methacrylic acid,
and 4-vinyl pyridine
Initiator: 2,2‘-
Azobis-
(isobutyronitrile)
Cross linker: ethyleneglycol
dimethacrylate
LOD= 4.0 nM
S= 5.0 A.M-1
.mm-2
DPV [75]
[BMIm][BF4] graphene & Au
NPs
Theophylline (TP) and
caffeine (CAF) (oxidation)
in tea, energy drinks and
pharmaceutics
chitosan GC LODTP= 1.32 nM
STP= 38.10 A.M-1
LODCAF= 4.42 nM
SCAF= 33.06 A.M-1
DPV [76]
[BMIm][PF6] graphene &
MWCNTs
hydroquinone (HQ) and
catechol (CT) (oxidation) in
rain water
GC LODHQ= 0.1 µM
SHQ= 0.1140 A.M-1
LODCT= 0.06 µM
SCT= 0.2233 A.M-1
DPV [77]
[APMIm][Br] IL-
functionalized
graphene
hydrogen peroxide
(reduction)
ITO polyaniline LOD= 0.06 µM
S= 0.280 A.M-1
Amp [78]
[APMIm][Cl] graphene & Au
NPs & Prussian
blue NPs or
CdFe(CN)6 NPs
carcinoembryonic antigen
(CEA) and alpha-
fetoprotein (AFP) (bio-
interaction) in serum
samples
chitosan &
capture anti-
CEA and capture
anti-AFP
GC LODCEA= 10 ng.L-1
LODAFP= 6 ng.L-1
DPV [79]
Table S. 2 IL-CNM nanocomposite paste electrode examples
IL Nanomaterial Targeted Analyte, Field
and Reaction Type
Biological
component
Other Components If
Existed
Sensing Performance Technique Ref
[BMIm][Br] MWCNTs carbidopa (oxidation) in
human urine and serum
graphite powder &
paraffin
LOD= 0.06 µM
S= 0.0283 A.M-1
SWV [80]
[BMIm][BF4] MWCNTs Hg+2
in aqueous solution
in dental amalgam and
water samples
graphite powder & 1-(2-
ethoxyphenyl)-3-(3-
nitrophenyl)triazene
(ENTZ) as an ionophore
LOD= 2.5 nM
S= A.M-1
Potentiometric
, EMF
[81]
[BMP][TFSI] MWCNTs & Si NPs Ce+3
in aqueous solutions graphite powder & New
Schiff base, (Z)-2-((1H-
1,2,4-triazol-3-
ylimino)methyl)phenol,
as an efficient ionophore
LOD= 6.45 nM Potentiometric
, EMF
[82]
[BMIm][PF6] MWCNTs dextromethorphan
(oxidation) in commercial
pharmaceutical products
Carbon microparticles
(graphite fine powder)
LOD= 8.81 µM
S= 0.0268 A.M-1
Amp [83]
HPAA iron-doped natrolite
zeolite NPs & MWCNTs
dopamine (DA) and uric
acid (UA) (oxidation) in
human blood serum and
urine samples
paraffin oil LODDA= 0.116 µM
LODUA= 0.133 µM
DPV [84]
[BMIm][BF4] silica-nanospheres
functionalized with isatin
thiosemicarbazone &
MWCNTs
Cu+2
in real samples (tea,
coffee and multivitamin
tablets)
paraffin oil LOD= 0.501 nM Potentiometric
, EMF
[85]
[BMIm][PF6] MWCNTs Cd2+
in waste water
samples
2,2-thio-bis [4-methyl
(2-amino phenoxy)
phenyl ether] as
ionophore & graphite
powder
LOD= 7.9 nM Potentiometric
, EMF
[86]
[OPy][PF6] MWCNTs 1st: ascorbic acid,
ferricycanide, NADH, and
hydrogen peroxide
2nd
: glucose (oxidation)
1st: None
2nd
: glucose
oxidase
S (glucose)= 2 µA.mM -1
2nd
: Amp [87]
[BMIm][PF6] MWCNTs methamphetamine
hydrochloride (MA·HCl)
(chemiluminescence)
tris(2,2′-bipyridyl)-
ruthenium Ru(bpy)3Cl2
LOD= 8.0 nM ECL [88]
[BMIm][BF4] MWCNTs Yb+3
N'-(1-oxoacenaphthylen-
2(1H)-ylidene) furan-2-
carbohydrazide &
graphite powder
LOD= 0.01 nM Potentiometric
, EMF
[89]
[BMIm][BF4] MWCNTs Ce+3
N-[(2-hydroxyphenyl)
methylidene]-2-
furohydrazide &
graphite powder
LOD= 0.36 µM Potentiometric
, EMF
[48]
[BMIm][BF4] MWCNTs Er+3
[5-(dimethylamino)
naphthalene-1-sulfonyl
4-phenylsemicarbazide]
& graphite powder
LOD= 0.50 nM Potentiometric
, EMF
[90]
[BMIm][PF6] 3D graphene trinitrotoluene (reduction) LOD= 0.5ppb
S= 1.65 μA.cm-2
per ppb
ASV(abs) [91]
[BMIm][PF6] mesoporous carbon glucose (oxidation) glucose
oxidase
S= 0.18 nA. μM-1
CV [92]
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