C A R B O N 4 8 ( 2 0 1 0 ) 1 2 6 2 – 1 2 7 0

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p-Hexafluoroisopropanol phenyl covalently functionalizedsingle-walled carbon nanotubes for detection of nerve agents

Lingtao Kong a,b, Jin Wang a, Xucheng Fu a, Yu Zhong a, Fanli Meng a, Tao Luo a,Jinhuai Liu a,*

a Key Laboratory of Biomimetic Sensing and Advanced Robot Technology, Institute of Intelligent Machines,

Chinese Academy of Sciences, Hefei, Anhui 230031, Chinab School of Chemistry and Chemical Engineering, Anhui University, Hefei, Anhui 230039, China

A R T I C L E I N F O

Article history:

Received 13 October 2009

Accepted 29 November 2009

Available online 3 December 2009

0008-6223/$ - see front matter � 2009 Elsevidoi:10.1016/j.carbon.2009.11.051

* Corresponding author: Fax: +86 551 5592420E-mail address: jhliu@iim.ac.cn (J. Liu).

A B S T R A C T

Novel p-hexafluoroisopropanol phenyl (HFIPPH) covalently functionalized single-walled

carbon nanotubes (SWCNTs) have been prepared through in situ diazonium reaction

between SWCNTs and p-hexafluoroisopropanol aniline; moreover, the hybridized material

can be characterized by ultraviolet vision near infrared spectroscopy, Raman spectroscopy,

thermogravimetric analysis, X-ray photoelectron spectrometry, field-emission scanning

electron microscopy and high resolution transmission electron microscopy. The results

reveal that the one-dimensional electronic structures of the functionalized tubes could

be basically maintained without damaging their electronic properties. Considered that

strong hydrogen-bonding can be formed between hexafluoroisopropanol groups and

dimethyl methylphosphonate (DMMP) (simulant of nerve agent sarin), the SWCNT-HFIPPH

sensing devices have been fabricated and employed to detect DMMP. Excellent sensitivity

and selectivity of the hybridized SWCNT-HFIPPH devices suggest that it has great capability

of detecting explosives and chemical warfare agents.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Developing novel sensing devices with low cost and power is

very important because these devices are being widely ap-

plied in defense, homeland security, biological detection,

and environmental monitor [1–6]. As one kind of novel sens-

ing devices, chemical microsensors with low power consump-

tion are being considered as leading candidates due to

unobservable variations of resistance in response to binding

of analytes. It should be pointed out that the chemical micro-

sensors with high sensitivity and selectivity are usually

hybridized devices of nonselective transducer and vapor con-

centrator which are composed of chemosensing materials,

e.g., metal oxides, organic semiconductors, and carbon nano-

tubes etc. As far as sensitivies of some chemosensing materi-

er Ltd. All rights reserved

.

als are concerned, high sensitivity and fast response time of

single-walled carbon nanotubes (SWCNTs) have been exhib-

ited, which could be ascribable to their unique quasi-one-

dimensional electronic structures [7–15]; however, selectivity

of pristine SWCNTs restricts its practical application and

physical/chemical functionalization approaches of SWCNTs

are being investigated due to enhancement of processibility

and sensing performance of decorated SWCNTs [16–19]. Addi-

tionally, decoration of SWCNTs contributes to improving dis-

solution and dispersion of SWCNTs in various solvents [20],

which opens the door for cost-effective methods to fabricate

sensors by simple dispensing or printing techniques. On the

other hand, the unique properties of SWCNTs can be associ-

ated with some other materials (e.g., conducting polymers,

metals and metal oxides) through chemical functionalization

.

C A R B O N 4 8 ( 2 0 1 0 ) 1 2 6 2 – 1 2 7 0 1263

[18,21–23], which leads to creating hybridized sensing materi-

als with enhanced sensitivity, selectivity and faster response

time. Therefore, decorated SWCNTs have been widely em-

ployed for constructing field-effect transistors (FETs) to detect

some gases [24–27].

In comparison with some other sensing materials, hexa-

fluoroisopropanol (HFIP) substituents have been highly at-

tracted recently in that strong hydrogen-bonding of HFIP

group contribute to causing some remarkable changes in

the physical, chemical, and biological properties of these no-

vel materials [28–31], which can improve sensitivities and

selectivities for detecting explosives and chemical warfare

agents [32,33]. By aid of hydrogen-bonding interaction, the

obvious absorption of organophosphorus vapors on the com-

pound involving HFIP substituents was firstly observed by

Barlow et al. [34,35]. Subsequently, these organic compounds

containing HFIP groups can be widely used as sensitive sen-

sors for detecting explosives, chemical agents, and volatile or-

ganic compound, which can be applied in acoustic wave

devices, chemiresistors, chemicapacitors, microcantilevers

and fluorescence sensing methods [36]. It should be men-

tioned that maximization of hydrogen-bond acidity of hydro-

xyl groups and minimization of hydrogen-bond basicity of

hydroxylic oxygen atoms by electron-withdrawing effect of

fluorine atoms in HFIP can efficiently inhibit self-association

of HFIP substituents [36]. Therefore, HFIP substituents, which

serve as absorbent layer on these sensors and interact with

the strong hydrogen-bond basic compounds via hydrogen-

bonding, contribute to absorbing the vapors into the polymer

film on the surface of the device and increasing sensory re-

sponse. More recently, investigations on sensors based on flu-

oroalcohol hydrogen-bond acidic groups functionalized

nanomaterials have been active. In contrast with bare

SWCNTs, decoration of polymer-HFIP hybrids can greatly im-

prove sensitivities and selectivities of the sensors [32]. Hence,

it can be predicted that HFIP derivative can be widely applied

as powerful intermediate sensing material possessing excel-

lent sensitivity and selectivity for detecting chemical warfare

agents, sarin or its simulant dimethyl methylphosphonate

(DMMP). Moreover, the sensors based on SWCNTs functional-

ized with HFIP aniline (HFIPA) can efficiently improve their

Fig. 1 – Schematic view of the SWCNT-HFIPPH hybrid and its

specific interaction with DMMP molecules through

hydrogen-bond.

sensitivities and selectivities thanks to efficient accumulation

of chemical vapors through the strong hydrogen-bond acidity

of HFIP groups in HFIPA. Hence, it is very important for us to

fabricate HFIPA-functionalized SWCNTs for sensing devices.

In the present paper, we firstly prepared SWCNT-p-hexafluo-

roisopropanol phenyl (HFIPPH) hybrids via in situ diazonium

reaction of HFIPA and fabricated chemical sensing devices

by SWCNT-HFIPPH, which are capable of detecting ppb con-

centrations of DMMP through the hydrogen-bonding interac-

tion (shown in Fig. 1).

2. Experimental

2.1. Instruments

Fourier transform infrared (FT-IR) spectra were recorded on a

Nexus-870 spectrophotometer. Nuclear magnetic resonance

(NMR) spectra were acquired at 25 �C using a Bruker Avance

spectrometer. Mass spectra (MS) were recorded on a Micro-

mass GCT-MS spectrometer. Ultraviolet vision near infrared

(UV–Vis-NIR) absorption spectra were recorded by using a Sol-

idspec-3700 spectrophotometer. Field-emission scanning

electron microscopy (FE-SEM) images were taken by a Sirion

200 field-emission scanning electron microscopy. Transmis-

sion electron microscopy (TEM) image was obtained from a

JEOL JEM-2010 instrument operated at 100 kV. Thermogravi-

metric analysis (TGA) was performed using a Shimadzu

TGA-50H analyzer in argon surroundings. X-ray photoelec-

tron spectrometry (XPS) was carried out on an ESCALab MK

II using non-monochromatized Mg Ka X-ray beams as the

excitation source. Binding energies were calibrated relative

to the C 1s peak at 284.6 eV.

2.2. Materials and chemicals

The hexafluoroacetone trihydrate (99%) was obtained from

Aldrich. Aniline (97%) and all other chemicals (analytical re-

agent grade) were purchased from Shanghai Chemical Co.

Ltd. (China). SWCNTs used in this study were purchased from

Chengdu Organic Chemicals Co. Ltd. (China), with an average

diameter of 1.1 nm and an average length of 5 lm. The purity

was declared to be above 90%.

2.3. Purification and disentanglement of SWCNTs

500 mg SWCNT raw soot was first heated in an air at 350 �C for

3 h. The remaining soot was sonicated in 100 mL 37 wt.%

hydrochloric acid for 4 h and then refluxed for 5 h. The mix-

ture solution was filtrated with a 0.20 lm Teflon filter under

vacuum. The sediment was washed with copious amount of

water, and then it was vacuum-dried at 60 �C for one day. Dis-

entanglement of SWCNT bundles was performed by oleum.

The obtained SWCNTs were soaked in oleum (20% SO3) 72 h

by an immersion blender so that the intercalation of acid into

the tightly entangled network of SWCNTs will loosen the

ropes. After diluting with deionized water, the mixture solu-

tion was centrifuged and the supernatant acid was decanted

off. The precipitate was washed with deionized water three

times and then ultrasonically dispersed into 100 mL 0.2% ben-

1264 C A R B O N 4 8 ( 2 0 1 0 ) 1 2 6 2 – 1 2 7 0

zalkonium chloride solution. The solution was centrifuged at

4000 rpm for 20 min. The supernatant solution was filtrated

with a 0.20 lm Teflon filter under vacuum, washed exten-

sively with deionized water until pH of the filtrate reached 6

and dried under vacuum. Finally, the purified SWCNTs were

obtained by annealing in Ar at 900 �C for 30 min to remove

the oxygen-based functional groups generated from the

oleum treatment. Approximate 100 mg of product was

obtained.

2.4. Preparation of HFIPA

The aniline (0.093 g, 1.0 mmol) and p-toluenesulfonic acid

(0.01 g) were dissolved in toluene (10 mL) in the presence of

a small amount of molecular sieves under an atmosphere

of Ar. After heating to 100 �C, toluene solution (5 mL) of

hexafluoroacetone trihydrate (0.220 g, 1.0 mmol) was then

added dropwise over 0.5 h. The temperature was increased

to 110 �C and maintained between 110 and 120 �C via heating

for 2 days under magnetic stirring. Afterwards, the mixture

was cooled and filtered to yield 0.16 g crude product. Recrys-

tallization generate pure compound (0.11 g, 69%) with rose

pink. 1H NMR (400 MHz, (CD3)2SO, 25 �C, TMS, d): 7.291 ppm

(d, Benzene-2H, J = 8.44 Hz), 6.619 ppm (d, Benzene-2H,

J = 8.74 Hz), 5.421 ppm (s, NH2), 8.158 ppm (s, OH); 13C NMR

(100 MHz, (CD3)2SO, 25 �C, TMS, d): 113.66, 117.15, 119.40,

122.25, 125.12, 127.97, 150.51 ppm, 19F NMR (376 MHz,

(CD3)2SO, 25 �C, TMS, d): �74.31 ppm; FT-IR (KBr) m/cm�1:

3397, 3328, 3069, 3052, 3030, 1617, 1520, 1373, 1308, 1264,

1204, 1221, 1188, 1151, 1121, 968, 943, 829, 707, 638, 566,

539, 503; MS: Calcd for C9H7NOF6 (M+), 259.0432, found

259.0441.

2.5. Preparation of SWCNT-HFIPPH hybrids

The reaction sequence is depicted in Fig. 2. In a typical exper-

iment, 6 mg of the purified SWCNTs was sonicated for 30 min

in 10 mL of 1,2-dichlorobenzene (ODCB). A solution of 100 mg

HFIPA (0.4 mmol) in 3 mL of acetonitrile was added to the

suspension. After transferring to a septum-capped reaction

flask and bubbling with nitrogen for 10 min, 0.1 mL of iso-

amyl nitrite was quickly added. The suspension was stirred

in the dark at 70 �C for 24 h. After cooled to room tempera-

ture, the suspension was diluted with 30 mL of dimethyl-

formamide (DMF), filtered through a 0.2 lm Teflon filter,

and washed extensively with DMF until the filtrate became

colorless. Subsequently, it has been dried in vacuum at

80 �C for 24 h to give a black powder of the modified SWCNTs

(5 mg).

Fig. 2 – Preparation o

3. Results and discussion

3.1. Synthesis and characterization of SWCNT-HFIPPHhybrids

SWCNTs were functionalized with HFIPPH via in situ genera-

tion of the corresponding aryl diazonium of HFIPA according

to a previously described method [37–47], which avoids the

use of unstable aryl diazonium salts. In the synthesis of

SWCNT-HFIPPH, the amino group on HFIPA was oxidized to

a diazo group by isoamyl nitrite and released N2 to form an

aryl radical. This active radical reacts with the double bonds

on the walls of SWCNTs to generate the functionalization

[48]. Thus the HFIP group is covalently attached to the surface

of the SWCNT with a benzene ring between these two moie-

ties. Raman spectroscopy of SWCNTs has been well devel-

oped by both theoretical works and experiments [49,50]. The

Raman spectrum of unfunctionalized SWCNTs (Fig. 3A) dis-

plays two strong bands, viz., the so-called radial breathing

mode (ca. 209 cm�1) (RBM) and tangential mode (ca. 1590 cm�1) (G-band). The multiple peaks observed in the RBM could

be ascribable to distribution of diameters in the SWCNT sam-

ple. The weaker band centered at ca. 1290 cm�1 is attributed

to disorder mode (D-band) in the hexagonal framework of

the nanotube wall, which is related with resonance-enhanced

scattering of an electron via phonon emission by a defect that

breaks the basic symmetry of the graphene plane [51]; fur-

thermore, increase in the relative intensity of the D-band

compared to the G-band can be attributed to an increased rel-

ative number of sp3-hybridized carbons in the framework of

nanotube, which can be used to estimate the degree of func-

tionalization [42]. The Raman spectrum of SWCNT-HFIPPH

(Fig. 3B) is significantly different from that of unfunctional-

ized SWCNTs, which is reflected from alterations in the rela-

tive intensities of the three main modes, i.e., the relative

intensity of RBM is decreased in comparison with the G-band;

however, the relative intensity of the D-band is obviously in-

creased, implying that SWCNTs could be effectively decorated

by HFIPPH. In addition, as seen in Fig. 3A, three bands for the

RBM of as-purified SWCNTs can be observed at 165, 209, and

267 cm�1, respectively, the intensity of the band at 209 cm�1 is

the largest. Krupke et al. reported that all metallic SWCNTs

have RBM frequencies in the range between 218 and 280

cm�1, and semiconducting SWCNTs in the range between

175 and 213 cm�1 [52]. Hence, the present nanotubes mainly

belong to the semiconducting SWCNTs.

Except that the functional degree of SWCNTs can be re-

flected by Raman spectrum, the functional degree of SWCNTs

can be evaluated by UV–Vis-NIR absorption experiment. Van

f SWCNT-HFIPPH.

Fig. 4 – UV–Vis-NIR absorption spectra in DMF of purified

SWCNTs (dashed line) and SWCNT-HFIPPH (solid line).

Fig. 5 – TGA of SWCNT-HFIPPH in argon atmosphere (10 �C/

min).

Fig. 3 – Raman spectra (514 nm excitation) of SWCNTs, (A)

purified SWCNTs (Inset: expanded view of the RBM); (B)

SWCNT-HFIPPH; (C) SWCNT-HFIPPH after 750 �C TGA

treatment in an argon atmosphere at 10 �C/min.

C A R B O N 4 8 ( 2 0 1 0 ) 1 2 6 2 – 1 2 7 0 1265

Hove singularities of SWCNTs are usually lost after a high de-

gree of covalent functionaliztion, which are ascribable to dis-

ruption of the delocalized p-conjugation in the sp2-hybridized

nanotubes [39]. UV–Vis-NIR absorption spectra of purified

SWCNTs (0.02 mg mL�1) and SWCNT-HFIPPH (0.03 mg mL�1)

in DMF are shown in Fig. 4. The extinction coefficients of

the purified SWCNTs and SWCNT-HFIPPH are 384 L mol�1

cm�1 and 249 L mol�1 cm�1, respectively. From Fig. 4, we can

find that the specific Van Hove singularities in the present

SWCNT-HFIPPH hybrids are basically retained, indicating that

the p-conjugation electron structure of the SWCNT is not se-

verely damaged [53]. In addition, an interesting feature in the

UV–Vis-NIR absorption spectrum of our purified SWCNTs in

DMF (Fig. 4) can be observed, i.e., the lack of band at 550 nm

indicates that there are not many metallic SWCNTs in the

sample [54], which is consistent with the Raman results.

On the other hand, quantitative estimation for the func-

tionalization degree of carbon nanotubes can be performed

through TGA due to measurement of mass loss accompany

with functional moieties removed from the pristine SWCNTs

in an inert environment by sufficient heating. Before TGA

experiment, the sample was further washed several times

with DMF, ethanol, and finally diethyl ether to remove the ex-

cess of ungrafted species and other impurities, and dried at

80 �C under vacuum. As shown in Fig. 5, when SWCNT-

HFIPPH hybrids are heated to 750 �C in an argon atmosphere

(10 �C/min), mass loss in the TGA experiment can be observed

as ca. 21%, which suggests that ca. 1 in 75 carbon atoms could

be functionalized on the basis of all the weight loss only de-

pended on the functional components. Again, it can be con-

sidered that Raman spectrum (Fig. 3C) of SWCNT-HFIPPH

after TGA experiment is quite similar to pristine SWCNTs,

which indicates that only intact nanotubes can be left if the

functional moieties are removed.

In order to further analyze the degree of functional groups

on the wall of the SWCNT, XPS experiment of SWCNT-HFIPPH

Fig. 6 – XPS analysis of SWCNT-HFIPPH. The XPS pass energy was 20.00 eV with a 45� takeoff angle and a 500 lm beam size.

1266 C A R B O N 4 8 ( 2 0 1 0 ) 1 2 6 2 – 1 2 7 0

hybrids has been carried out. As shown from Fig. 6, atomic

percent of C, F, O, and N atoms can be shown as 90.52%,

6.49%, 2.62%, and 0.37%, respectively. According to percentage

of the C atoms relative to the F atoms, the atomic ratio of car-

bon to fluorine is 14:1, suggesting that 84 C atoms per HFIP

can be estimated on the basis of per HFIP group including 6

F atoms. Assuming that all of the carbon atoms only come

from the SWCNT and the functional group decorated on the

side wall of SWCNTs, 1 in 78 carbon atoms could be function-

alized by one HFIP group because six carbon atoms can be

Fig. 7 – FE-SEM images of bare SWCNTs and SWCNT-HFIPPH hyb

of nanotubes. (B) FE-SEM image of SWCNT-HFIPPH showing exf

ascribable to aryl six-membered ring, which are agreement

with the TGA analysis.

The bare and functionalized SWCNTs can be observed di-

rectly by FE-SEM. As shown in Fig. 7A, a number of SWCNTs

are easily entangled, which leads to formation of large-sized

nanotube bundles. In contrast, FE-SEM (Fig. 7B) displays that

entanglement of HFIPPH decorated SWCNTs, can be effec-

tively exfoliated and well dispersed, which may be related

to larger solubility of SWCNT-HFIPPH hybrids in DMF com-

pared to bare SWCNTs. Moreover, no precipitation or floccula-

rids. (A) FE-SEM image of bare SWCNTs showing the bundles

oliation of nanotubes.

C A R B O N 4 8 ( 2 0 1 0 ) 1 2 6 2 – 1 2 7 0 1267

tion from the saturated solution of the SWCNT-HFIPPH hy-

brids can be observed in a couple of months. Additionally,

high resolution (HR)-TEM image of SWCNT-HFIPPH (Fig. 8)

shows the presence of individual SWCNTs with specific rug-

ged surface, which has been attributed to the attachment of

HFIPPH functional groups on the tube wall.

3.2. Protocol of SWCNT-HFIPPH hybridized devices

The electrodes for the sensors array were microfabricated on

silicon substrate using standard lithographic patterning. SiO2

film with one micron thickness was initially deposited on a

(1 0 0) oriented silicon wafer to insulate the substrate using

chemical vapor deposition (CVD). After photo lithographically

defining the electrode area, a Cr adhesion layer and ca. 300-

nm thickness Au layer were e-beam evaporated. Finally, the

electrodes were defined using lift-off techniques. As seen

from FE-SEM images of sensors array in Fig. 9A, the gap dis-

tance between electrodes can be fixed at 5 lm.

Protocols on fabrication of the SWCNT-HFIPPH sensing de-

vices have been detailed discussed as follows. When as-pre-

pared SWCNT-HFIPPH hybrids have been dispersed

(0.1 lg mL�1) in DMF using ultrasonic force for two hours,

transparent solution of SWCNT-HFIPPH hybrids could be ob-

served. Subsequently, a 0.1 lL drop of the solution was ex-

tracted and deposited onto the electrode gap using a

microsyringe. As shown in Fig. 9B, formation of a network of

SWCNT-HFIPPH bridged each electrode gap can be observed

after evaporation of the solution. It is important to anneal

the sensor at 150 �C for 60 min under argon atmosphere so

as to decrease the contact resistance between SWCNTs and

the gold electrodes and remove some DMF residues. The den-

sity of the SWCNTacross the gap can be adjusted via two ways,

i.e., controlling concentration of the SWCNT-HFIPPH hybrids

in the DMF solution and the size of droplet. As shown in

Fig. 9C, the SWCNT-HFIPPH/FET device can be used for making

the transistor measurements and the resistance of the

SWCNT-HFIPPH devices ranges from 0.5 to 1.5 MX.

3.3. Sensing behavior of SWCNT-HFIPPH hybridizeddevices

The response of the sensors to different concentrations of

DMMP was studied in a homemade measuring system. Nitro-

Fig. 8 – HR-TEM image of SWCNT-HFIPPH hybrids deposited

form suspension ethanol.

gen was used as the carrier and diluting gas. DMMP vapors

were generated by bubbling nitrogen through a glass tube.

The desired DMMP concentrations were prepared by dilution

with nitrogen using mass flow controllers. The experiments

were performed at room temperature and a flow rate of

1000 cm3 min�1, and nitrogen was used as a balance gas.

The real-time response curves (Fig. 10) have been presented

for the SWCNT-HFIPPH hybridized devices to DMMP under

various concentrations. The investigation of sensory response

was performed with a resistance measurement at a constant

bias voltage (0.1 V) between the two electrodes. The response

of the device is defined as DR/R0 = (R � R0)/R0, where R0 is the

resistance before the exposure to DMMP vapors and R is the

maximum resistance during the exposure. As shown in

Fig. 10A and B, good reversibility and response of the

SWCNT-HFIPPH hybridized sensors to DMMP can be observed,

respectively. The relative response (DR/R0) at a low concentra-

tion of 50 ppb DMMP is ca. 0.6%; furthermore, the DR/R0

change of 0.11% can be observed upon exposure to 25 ppb of

DMMP, which indicates that the sensing can be qualified as

ppb-level sensitivity. Accompany with increasing the concen-

tration, the response of the sensors can also be sharply in-

creased; however, the response curve (Fig. 10B) tends to be

flat when the concentration is increased to 8 ppm.

As an important evaluation for sensors, the selectivity of

the SWCNT-HFIPPH hybridized device for DMMP has been

investigated in comparison with some materials, e.g., hexane,

toluene, xylene, benzene, methanol and ethanol. The concen-

trations of all the materials have been diluted to 1% of satu-

rated vapor conditions and resistance measurement could

be performed under identical conditions described previously.

As shown in Fig. 10C, the DR/R0 (15.3%) of SWCNT-HFIPPH hy-

brids is obviously stronger than that (less than 3%) of the

other materials, which displays an excellent selectivity of

the chemresistor for DMMP. In addition, control experiments

of bare SWCNTs for detecting DMMP are also performed in or-

der to investigate the effects from HFIPPH decoration for sen-

sitivity and selectivity of functionalized SWCNTs. In the case

of the SWCNT-HFIPPH sensor, the response to DMMP is ca.

28% larger compared to the bare SWCNTs, at 4 ppm concen-

tration of DMMP, which can be observed from Fig. 10D. Inter-

estingly, if the concentration of DMMP is decreased to

0.5 ppm, the response of SWCNT-HFIPPH hybrids is much

higher than that of bare SWCNTs by approximate thirteen

times. Therefore, it can be concluded that the sensitivity

and selectivity for detecting DMMP have been remarkably in-

creased if SWCNTs are functionalized by HFIPPH in compari-

son with the bare SWCNTs, which is attributed to the fact that

strong hydrogen-bonding interaction between HFIP groups

and DMMP can efficiently improve the sensitivity for detect-

ing DMMP.

3.4. Sensing mechanism of SWCNT-HFIPPH hybrids fordetecting DMMP

In order to investigate the response mechanism of the

SWCNT-HFIPPH sensors to DMMP, the transfer characteristics

(Isd vs. Vg) of the transistor were measured by applying

100 mV and sweeping the gate voltage between �20 and

+20 V in steps of 0.5 V. Fig. 11 illustrates the transfer curves

Fig. 9 – (A) FE-SEM image of sensor array. (B) FE-SEM image of network of SWCNT-HFIPPH bridged electrode gap. (C) The

configuration of the SWCNT/FET device.

Fig. 10 – (A) Real-time response curves of the sensor upon exposure to different concentrations of DMMP. (B) Resistance

changes (DR/R0) of the sensor upon exposure to varying concentrations of DMMP. (C) Resistance changes of the sensor upon

exposure to different common materials and DMMP diluted to 1% of saturated vapor conditions. (D) Resistance changes of the

SWCNT-HFIPPH and bare SWCNT devices were compared at different concentrations of DMMP. (All of the tests were

performed at room temperature, with a bias voltage fixed at 0.1 V.)

1268 C A R B O N 4 8 ( 2 0 1 0 ) 1 2 6 2 – 1 2 7 0

Fig. 11 – The source–drain current versus gate voltage of the

sensor (Vsd = 100 mV). (A) For bare SWCNTs in air. (B and C)

For SWCNT-HFIPPH, under air and saturate DMMP vapors,

respectively.

C A R B O N 4 8 ( 2 0 1 0 ) 1 2 6 2 – 1 2 7 0 1269

of the bare SWCNT and the SWCNT-HFIPPH/FET devices be-

fore and after exposure to saturated DMMP vapors. After the

surfaces of the SWCNT were functionalized with HFIPPH,

the threshold voltage of the SWCNT-HFIPPH/FET device is

shifted towards a negative gate voltage by ca. 3.4 V as com-

pared to the bare SWCNTs. Similarly, after exposure to satu-

rated DMMP vapors, the threshold voltage can be

continually shifted towards the negative direction by ca.

2.5 V. Compared to the bare SWCNTs, the conductance of

the SWCNT-HFIPPH network is obviously decreased, which

can be due to the fact that the SWCNT is p-type semiconduc-

tor, covalent functionalization of the nanotube surface par-

tially alters the electronic structure, and causes a decrease

of the carrier density in the SWCNT, which further leads to

threshold voltage shifted to the negative direction. In addi-

tion, after exposure to the saturated DMMP vapors, the shift

of threshold voltage can be attributed to a charge transfer

process associated with the analyte and introduction of scat-

tering sites. The shift to negative voltage indicates that the

adsorption of DMMP affects a negative charge on the nano-

tubes. For additional positive charge from the gate bias is re-

quired to counteract the negative charge resulting from the

adsorption of DMMP. The result of electron transfer is consis-

tent with the fact that DMMP is a strong electron donor. The

observed transfer of negative charge can be ascribable to an

interaction of the SWCNT-HFIPPH with the partially negative

charge on the terminal phosphonate oxygen of DMMP.

4. Conclusions

Novel SWCNT-HFIPPH hybrids were synthesized by in situ dia-

zonium reaction between SWCNTs and HFIPA for the first

time. HFIPPH groups bind to the SWCNT surfaces through

covalent interactions. The resulting nanohybrids were char-

acterized by UV–Vis-NIR, Raman, TGA, XPS, FE-SEM and HR-

TEM. The Raman and UV–Vis-NIR absorption spectra show

that the one-dimensional electronic structures of the func-

tionalized tubes are mostly retained without damaging their

electronic properties. Based on the sensing material, chemi-

cal sensor of SWCNTs hybrids with high sensitivity and selec-

tivity has been easily fabricated via drop-casting approach,

which has successfully detected DMMP at very low concentra-

tions. The increase of the sensitivity and selectivity are

mainly ascribable to efficient accumulation of chemical va-

pors through the strong hydrogen-bond acidity of HFIP

groups. The achievement of hybridized SWCNT devices with

high performances is expected to accelerate the progress to-

ward the realization of low cost, low power, and nanoscale

chemiresistive sensor systems.

Acknowledgements

This work is supported by the National Basic Research Pro-

gram of China (Grant No. 2007CB936603), the National High

Technology Research and Development Program of China

(Grant No. 2007AA022005), the National Natural Science Foun-

dation of China (Grant Nos. 60604022, 10635070 and 6080102)

and Innovation Project of Chinese Academy of Sciences

(KSCX2-YW-G-058).

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