Sensors and Actuators B 138 (2009) 351–361

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Sensors and Actuators B: Chemical

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SWCNT nano-composite optical sensors for VOC and gas trace detection

M. Consalesa,∗, A. Crescitelli a, M. Penzab, P. Aversab, P. Delli Veneri c, M. Giordanod, A. Cusanoa

a Optoelectronic Division – Engineering Department, University of Sannio, Benevento, Italyb Department of Physical Technologies and New Materials, ENEA, Brindisi, Italyc Department of Solar Technologies, ENEA, Via Vecchio Macello, 80055 Portici, Italyd Institute of Composite and Biomedical Materials, CNR, Portici, Italy

a r t i c l e i n f o

Article history:Received 1 December 2008Received in revised form 29 January 2009Accepted 16 February 2009Available online 4 March 2009

Keywords:Optical fiber sensorsCarbon nanotube nano-compositesRoom temperature chemical detection

a b s t r a c t

The feasibility of exploiting novel chemo-sensitive coatings based on cadmium arachidate (CdA)/single-walled carbon nanotube (SWCNT) composites for the development of high performance opticalchemo-sensors has been experimentally investigated and proved. SWCNT nano-composite overlays havebeen transferred on the distal end of standard optical fibers by the Langmuir–Blodgett deposition tech-nique in order to realize extrinsic low-finesse Fabry-Perot interferometers. A structural and morphologicalcharacterization of the fabricated optical probes has been carried out by scanning electron microscopyobservations and Raman spectroscopy analysis. Reflectance measurements have been carried out to testthe sensing performance of the proposed devices to volatile organic compound (VOC) and gas exposure.Also, VOC trace detection has been evaluated in different environments, such as air and water. Obtainedresults have also been compared in the same operating conditions to those obtained in the case of stan-dard SWCNT overlays deposited by using the same deposition technique and optical transducers in orderto outline the effect of the nano-composite matrix both on the material integration effectiveness andsensing performance. The results shown here demonstrate that the use of nano-composite coatings notonly promotes a better adhesion of the sensitive overlay to the fiber surface, thus improving the robust-

ness of the chemo-optical sensors, but also significantly enhances their sensitivities from three to seventimes with respect to the SWCNT overlay directly deposited on the fiber surface. Moreover, resolutions inthe range 30–250 part per billion (ppb) and 0.6–1.5 part per million (ppm) have been estimated in case

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. Introduction

In the last decades, strong efforts have been devoted to theesign and realization of always more accurate and less expen-ive sensing systems with high capabilities of detecting chemicalraces in the field of environmental monitoring, chemical engineer-ng, medical applications, safety and food control. A sensor system

ith enhanced performance must operate with highly sensitiveaterials and highly efficient transducers, along with appropriate

lectronic interfaces. Also, simplicity, rapidity, robustness, as wells size and reliability are essential qualities that have become ofundamental importance when monitoring the environment.

A current trend for enhancing the detection properties of

hemical sensors has become in recent years the development ofovel nanostructured materials of different chemical composition,roduced as nano-particles, nano-wires, or nanotubes, as demon-trated by the large number of literatures published every year in

∗ Corresponding author. Tel.: +39 0824305810; fax: +39 0824305840.E-mail address: consales@unisannio.it (M. Consales).

925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2009.02.041

, respectively.© 2009 Elsevier B.V. All rights reserved.

this field [1,2]. These materials have their unique structures whichare dominated by a wire-like structure whose diameter varies overa broad range from several nanometers to a micrometer. In partic-ular, carbon-based nanostructures exhibit unique properties andmorphological flexibility, which renders them inherently multi-functional and compatible with organic and inorganic systems.Among them, SWCNTs have been considered ever since their dis-covery as one of the most promising nanomaterials due to theirhollow nanostructure and high specific surface area which pro-vides attractive characteristics for chemical sensing [3,4]. In fact,the unique morphology of SWCNTs confers them the amazing capa-bility to reversibly adsorb molecules of numerous environmentalpollutants undergoing a modulation of their electrical, geometri-cal and optical properties, such as resistivity, dielectric constant,thickness [5–7], so that carbon nanotubes (CNTs)-based chemicalsensors offer the possibility of excellent sensitivity, low operating

temperature, rapid response time and sensitivities to various kindsof chemicals.

On this line of argument, various configurations employing CNTsas chemo-sensitive overlay have been impressively studied to fab-ricate innovative gas and vapor phase chemical sensors [8–13]. In

3 d Actuators B 138 (2009) 351–361

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52 M. Consales et al. / Sensors an

articular, the peculiarity of SWCNTs to change their optical prop-rties due to the adsorption of environmental pollutant moleculesas for the first time demonstrated in 2004 [7]. In that case,ano-scale thin films of SWCNT bundles were transferred on theptical fiber termination by means of the Langmuir–Blodgett (LB)echnique and used as sensitive coatings for the development ofolatile organic compounds (VOCs) optical chemo-sensors. Initially,linker–buffer material of CdA pre-deposited on the sensor surface

o promote the SWCNT adhesion was used. However, successivemprovements in the LB deposition procedure allowed the SWCNTverlays to be directly deposited on the fiber tip [14], resulting introng enhancements of the sensing performance of un-bufferedonfigurations with respect to the buffered cases, both in terms ofensitivity and response times [15]. In particular, detection limitsf less than one ppm, high repeatability and response times of fewinutes have been obtained so far by means of the integration of

WCNTs with the optical fiber technology [15]. In addition, recentlyhe sensing properties of carbon nanotubes were also exploitedor the detection of chemical pollutants in aqueous environmentst room temperature [16], demonstrating the feasibility of suchanostructured materials to be successfully employed as sensitiveoatings for a wide range of environmental monitoring applica-ions.

However, the adhesion of CNTs to the fiber end-face still rep-esents a challenge that must be addressed before the sensors cane exploited in practical applications of environment monitoring.n the other hand, as mentioned, although the use of the bufferedonfiguration address this issue, it severely limits the sensing per-ormance of the final devices.

In last years, the embedding of controlled quantity of CNTnside host-matrices of a foreign material for the synthesis of

nano-composite with advanced sensing features is becomingn interesting approach that has attracted much attention in thecientific community. Indeed, this approach seems to have theotentiality to improve sensing performance, such as sensitivitynd selectivity, extending the detection capacities to an ever-ncreasing number of chemicals [12,17–21].

In light of these considerations, in this contribution, our atten-ion has been focused on the use of novel sensitive coatingsomposed of SWCNTs embedded in CdA matrixes. The use of CdAatrix ensures a better adhesion of the carbon tubes to the fiber

ptic surface, thus improving the sensor robustness and reliabil-ty that are factors of crucial importance especially for chemicaletection in aqueous environments. It also leads to enhancederformance in terms of sensitivities, mainly due to the higherefractive index of the nano-composite overlays with respect tohose based on standard SWCNTs.

Although CdA/SWCNT nano-composites have already been suc-essfully exploited in case of surface acoustic waves (SAW)-basedas and VOC sensors [22,23], in this manuscript, the sensing perfor-ance of optical chemo-sensors coated by such nano-scale coatings

ave been for the first time assessed against several chemicalsither in air or water environments, at room temperature. In partic-lar, to the best of our knowledge, no experimental data have beeneported on the capability of this CdA/SWCNT composites inte-rated with optical fiber technology to detect trace chemicals bothn gas and in liquid phase, leading to significant improvements inhe performance of optical fiber sensors based on standard SWCNTayers.

. Principle of operation

This section is focused on the sensing configuration adopted forhe realization of chemo-optical nanosensors, i.e. the reflectometriconfiguration, and on its principle of operation. As schematicallyepresented in Fig. 1, it is essentially based on a low finesse and

Fig. 1. Schematic view of the Fabry-Perot-based configuration.

extrinsic Fabry-Perot (FP) interferometer created by the depositionof a thin sensitive nanocoatings on the terminal face of properlycut and prepared silica optical fibers (SOF). The thin film acts as anoptical cavity where the first mirror is given by the fiber-sensitivelayer interface whereas the second one is provided by the sensitivelayer-external medium interface.

This configuration has been massively exploited in fiber optic-based sensing in the past due to its high sensitivity, especially forthe detection and measurements of various physical, chemical andbiomedical parameters [24,25]. This characteristic, combined withthe possibility to integrate a number of sensitive materials withthe optical fibers by means of very simple, low cost and versatiledeposition techniques makes it one of the most attractive and usefuloptoelectronic configuration especially suitable for environmentalmonitoring applications.

The principle of operation relies on the fact that the presence of agiven analyte within the test environment induces the adsorption ofits molecules within the sensitive overlay. This interaction, in turn,changes the complex refractive index (nfilm) and thickness (dfilm) ofthe overlay, leading to a modulation of the intensity of light reflectedback into the fiber. As a matter of fact the fiber–film reflectance canbe expressed as [26]:

R =∣∣∣ r12 + r23 · e−i·kfilm

1 · r12 · r23 · e−i·kfilm

∣∣∣2(1)

with:

r12 = nf − ∼nfilm

nf + ∼nfilm

; r23 =∼nfilm − next∼nfilm + next

kfilm =2� ·

(2 · ∼

nfilm · dfilm

)�

= 4� · n · dfilm

�− i

4� · k · dfilm

�

= ˇfilm − i˛ · dfilm

(2)

where nfilm = n − i · k, ˛ = 4�k/� is the overlay absorption coeffi-cient of the sensitive fiber coating, nf and next are the optical fiberand external medium refractive index respectively, and � is the opti-cal wavelength. Thus, the reflectance changes due to the chemicalinteraction between the sensing overlay and the target analyte canbe expressed as follows:

�R =(

�R

�n

)· �n +

(�R

�˛

)· �˛ +

(�R

�dfilm

)· �dfilm

= Sn · �n + S˛ · �˛ + Sd · �dfilm (3)

where Sn, S˛ and Sd represent the reflectance sensitivities againstthe variations of the effective refractive index, the absorption coeffi-cient and the overlay thickness, respectively. They strongly dependupon the geometrical and electro-optical properties of the sensitivenanocoatings and upon the environmental condition (for exam-

ple vapor or liquid phase) and, for this reason, have to be properlyconsidered case by case.

In particular, several effects could be involved to promotea reflectance change as a consequence of the analyte moleculeadsorption within the sensitive overlay: first of all, swelling of

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M. Consales et al. / Sensors an

he SWCNT nano-composite overlay that leads to a consequentncrease of the film thickness; also, refractive index variations arexpected due to the film density variation as expressed by theorentz–Lorentz law [27]. In addition, according to the plasma opticffect [28–30], a change either in the real part of the refractive indexr in the absorption coefficient could be possible as a consequencef the free carrier concentration change induced by charge trans-er mechanisms during the analyte sorption. Finally, modificationsf film reflectance could also be possible due to optical absorptionodifications induced by the chemical interaction with the target

nalyte.In addition, it is noteworthy that, when very low chemical con-

entrations are considered (as in this work), it can be safely assumedhat the analyte molecule adsorption occurs at constant overlayhickness (�dfilm = 0 in Eq. (3)).

. Sensor fabrication

The realization of thin films based on SWCNTs with a control-able thickness is an important basis for the future development ofheir scientific understanding and technological applications. Herehe Langmuir–Blodgett (LB) technique has been chosen as way toransfer nanometer-scale layers of CdA/SWCNT composite upon theistal end of standard single-mode fibers. It enables the precise con-rol of the layer thickness, its homogeneous deposition over largereas and the possibility to make multilayer structures with varyingayer composition [31]. An additional advantage of the LB techniques that almost any kind of solid substrate can be used for the depo-

ition. However, these advantages have to be traded with the lowpeed of the deposition procedure as well as the limited number ofaterials suitable for this technique.The CdA has been chosen as host–matrix material to incor-

orate the SWCNTs in the nano-composite due to its peculiar

ig. 2. SEM photograms, of CdA/SWCNT nano-composite multilayers composed of ten mnd (c) 75 wt.%.

ators B 138 (2009) 351–361 353

amphiphilic molecular structure suitable for LB deposition process[7,11].

With regard to the deposition process, two separate solutionsof arachidic acid in chloroform and SWCNTs in chloroform havebeen mixed in order to prepare a final solution of chloroform witharachidic acid and SWCNTs. The concentrations and the volumesof the initial solutions were chosen to obtain the desired weightpercentages of the filler-component (SWCNTs) with respect tothe matrix-component (CdA). Different concentrations of arachidicacid and SWCNTs in the final solution could also be exploited forthe preparation of composites with different weight percentagesenabling the possibility to additionally tailor the sensing perfor-mance [22]. The mixed solution was then accurately dispersedand stirred in an ultrasonic bath for 1 h. Successively, 160 �l ofthe mixed solution were spread onto a sub-phase constituted byacetate buffer with CdCl2 10−4 M. The pH and the temperature ofthe sub phase were kept constant at 6.0 and 20 ◦C, respectively. Themonolayer of the nano-composite was compressed with a barrierrate of 15 mm/min up to a surface pressure of 27 mN/m. The singlecomposite monolayer was deposited upon the fiber surface with adipping rate of 14 mm/min. The optical fibers used for the depo-sition have been previously accurately polished from the acrylicprotection and cleaved with a precision cleaver. Then, they havebeen washed in chloroform and dried with gaseous nitrogen to beready for the SWCNT composite deposition. Repeated dipping of thefiber substrates through the condensed Langmuir layer have beenperformed, resulting in the deposition of multilayered CdA/SWCNTfilms one monolayer at time. The thickness values obtainable by

using the aforementioned processing stage ranged from few tens ofnanometers to hundreds of nanometers [22], depending upon thenumber of SWCNTs-based monolayers. In addition, a CdA/SWCNTs-based overlay can be seen as a mixture of two media, graphite-likeand CdA (with the former being the inclusion and the latter the host

onolayers, realized by using a SWCNT filler content percentage of (a and b) 27 wt.%

3 d Actuators B 138 (2009) 351–361

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54 M. Consales et al. / Sensors an

aterial) and its optical behavior can be estimated by means of anffective medium approximation [32,33]. This means that the effec-ive index of such a layer is significantly influenced by the volumeraction of the inclusion that strongly depends upon the number ofeposited monolayers as well as the nanotube distribution on theber end-face. Theoretical values for the effective refractive indexf CdA/SWCNTs-based overlay can range from 1.6 to 1.75 [32] forolume fractions of graphite-like material inclusion ranging from.3 to 0.7. If a volume fraction of 0.5 is considered, the film refractive

ndex can be estimated to be approx. 1.7. This value for the volumeraction is only approximative and has been evaluated on the basisf the SEM observations of the CdA/SWCNT films. Of course, in ordero provide a more precise value for the nano-composite refractivendex, a more accurate measurement of the SWCNT volume fractionhould be addressed.

. Structural and morphological characterization

The rational design of a chemical sensor and of its perfor-ance is something which is possible only if the sensitive material

roperties and the way they are affected by different depositionarameters or ambient conditions are well known and understood.o fulfil this aim an extensive characterization of the fabricateddA/SWCNTs-based chemo-optical sensors has been carried out

n order to investigate the structural and morphological featuresf the so produced sensitive nanocoatings. Fig. 2 shows the typ-cal scanning electron microscope (SEM) images of CdA/SWCNTano-composite overlay composed of ten monolayers and fabri-ated by using a percentage of SWCNT filler content of approx. 27nd 75 wt.%. They reveal both the tendency of carbon nanotubes toggregate, forming bundles, as well as their good coverage of theubstrate. Moreover, qualitative scotch-tests revealed a fair adhe-ion of the nano-composite to the substrate. In addition, during thexperimental testing, we noticed for the CdA/SWCNTs-based sen-ors higher durability and robustness with respect to the standardWCNTs-based ones.

A detailed morphological and structural characterization of thearbon nanotubes-based composites is reported elsewhere [22].n particular, from X-ray diffraction measurements performed onB films composed of 20 monolayers of CdA/SWCNT compositeswith SWCNTs-filler weight percentages ranging from 0 to 75 wt.%),eposited on glass substrates, nano-composite multilayer peri-ds in the range 5.51–5.56 nm have been estimated, which is ingreement with the total length of the CdA molecule reported initerature [34].

The obtained values indicate that the embedded SWCNTs doot influence the periodicity of the CdA molecule and that thetructural order of the CdA host-matrix is maintained also in theano-composite, even at high content of SWCNTs-filler. Here, weeport Raman spectroscopy analyses conducted to characterize theabricated LB films of CdA/SWCNTs (already deposited upon theptical fiber tip) and compare them with the standard SWCNTsne. To this aim, a Raman microscope functioning in backscatteringonfiguration employing a HeNe laser (633 nm) and 50× and 100×bjective lenses was used.

The results are shown in Fig. 3, where typical Raman spectraf fiber optic probes coated with SWCNT and carbon nanotubes-ased composite layers have been reported (the spectrum in figure

s referred to a sample with a filler content of 25 wt.%).The characteristic multi-peak feature “G-band” at about

580 cm−1, corresponding to carbon atoms vibration tangentiallyith respect to the nanotube walls [35], together with the less

emarkable disorder-induced “D-band” peak typically in the range300–1400 cm−1, representing the degree of defects or danglingonds [35], can be easily revealed. In particular, the observation of

Fig. 3. Typical Raman spectra of SWCNT and CdA/SWCNT LB layers directlydeposited on the optical fiber tip.

the two most intense G peaks (labelled G+ and G−) confirms thesingle-walled nature of the carbon tubes while their predominantsemiconducting behavior can be derived by the Lorentzian line-shape of the G− feature which, on the contrary, is broadened formetallic SWCNTs [35].

In addition the large ratio of G to D peaks observed here givesus an indication of an ordered structure of the deposited SWCNToverlay.

No significant differences between the two recorded Ramanspectra can be observed, thus revealing that no structural degra-dation of the carbon nanotubes occurred as a consequence of theirinclusion within the CdA matrix.

It is also worth noting that, since the Raman spectra have beenperformed on SWCNTs-based thin film already deposited on theoptical fiber end-face, the results shown here also confirm theirsuccessful integration with the optical fiber technology.

5. Interrogation system

An important issue to address when dealing with sensors isthe design and development of a proper demodulation unit ableto provide a continuous interrogation of single or multiple sensorprobes by minimizing size, complexity and increasing the cost-effectiveness. So far, a variety of schemes have been proposedfor the interrogation of a fiber optic sensor based on the FP cav-ity, the most used ones relying on spectrum-modulating approachand single wavelength reflectometry [36]. Here the attention hasbeen focused on this last technique, which is simple to implementand requires just few widespread commercial and low-cost opto-electronic components while preserving excellent performance.In addition it enables the fabrication of cost-effective, reliable,robust and portable equipments, which are factors of crucial impor-tance for in situ and long-term monitoring applications and for thedesired technology transfer to the market.

The interrogation system, reported in Fig. 4 and described morein detail in [15], basically involves a superluminescent light emit-ting diode (with central wavelength � = 1310 nm and a bandwidthof approx. 40 nm), an optical isolator, a 2 × 2 coupler and two pho-todetectors. It provides an output signal I that is proportional to the

fiber–film interface reflectance R and that is insensitive to eventualfluctuations of the optical power levels along the whole measure-ment chain. In the followings, the relative change of the sensoroutput �I/I0 has been considered (where I0 is the output signal

M. Consales et al. / Sensors and Actuators B 138 (2009) 351–361 355

of the

ititl

F(

Fig. 4. Schematic representation

n the reference or initial condition), which, in turn, corresponds to

he relative reflectance change occurring at the fiber-sensitive layernterface (�R/R0). Synchronous detection is typically implementedo enhance the system performance, by amplitude modulating theight source at 500 Hz and retrieving the photodetector voltages by

ig. 5. Relative reflectance changes of the opto-chemical sensor coated by 10 monolayera) toluene and (b) ethanol vapors, at room temperature; the inset in (a) reports the nume

exploited interrogation system.

using a dual channel lock-in amplifier. The minimum �R/R0 that

can be detected by means of the proposed interrogation system,calculated by considering the maximum scattering on the sensorresponse in a steady-state level for a time interval of at least 10 min,is typically in the range 1–6 × 10−4. In addition, a Time Division

s of CdA/SWCNT nano-composite (25 wt.%), exposed to different concentrations ofrical fitting of the sensor response for the first toluene pulse.

3 d Actuators B 138 (2009) 351–361

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ture, preliminary experiments have been conducted with the aimof measuring the electrical response of a SWCNTs-based compos-ite layer upon NO2 exposure. To this aim, a direct measure of thed.c. electrical conductance of the composite films deposited onto

56 M. Consales et al. / Sensors an

ultiplexing (TDM) approach is typically exploited to perform theuasi-simultaneous interrogation of up to eight optical probes byeans of a multi-channel fiber optic switch.

. Sensing performance

The sensing performance of the fabricated probes have beennvestigated both in air and water environments at room tempera-ure and have been compared with those obtained with the sameransducers coated by standard SWCNT overlays.

.1. Room temperature detection of hydrocarbon vapors andaseous NO2

Here the attention is focused on trace detection of four VOCstoluene, xylene, ethanol and isopropanol) and one gas (NO2) bysing a sample coated by 10 monolayers of nanotubes-based com-osite with a SWCNT filler weight percentage of approximately5 wt.%. The results obtained from the experimental measurementsre shown in Fig. 5. In particular, here the �R/R0 occurred as aonsequence of the exposure to toluene and ethanol vapors haveeen reported. The analyte adsorption within the SWCNTs-basedano-composite overlay was able to induce a significant increase ofhe fiber–film reflectance as consequence of the complex refractivendex changes in the FP sensing cavity.

The experimental data clearly reveal the capability to detectery low concentrations of the tested pollutants at ppm levels, asell as its quite good attitude to recover the initial baseline sig-al upon the complete analyte molecule desorption. This feature

s of great importance for chemical sensing applications since itnables the sensor to be easily and quickly reused after a giveneasurement, avoiding ad hoc cleaning procedures, that are costly

nd time-consuming. In particular, the sensor exhibited a completend fast reversibility of the response in case of toluene exposure,owever, this is not the case of ethanol exposure for which the ref-rence signal corresponding to the condition of uncontaminatedmbient is not fully recovered. This behavior was also observed onsopropanol exposure and could be attributed to the higher polarityf this kind of chemical species (alcohols).

Different xylene and ethanol sensitivities have been measuredor the CdA/SWCNTs-based fiber probe (approx. 3.2 × 10−3 ppm−1

nd approx. 5 × 10−4 ppm−1, respectively). In addition, the differentesponse dynamic can be attributed mainly to the different diffusiv-ties of toluene and ethanol molecules within the nano-compositeverlay.

Fig. 6 reports the response of the same CdA/SWCNTs-based sen-or to five NO2 pulses with concentrations in the range 1–10 ppm.

Also in this case the sensor turned out to be capable of detect-ng very small traces of the gas under test and, similarly to whatbserved for alcohol detection, a baseline shift occurred upon mul-iple exposures of NO2. However, in this case the drift is thoughto be mainly caused by the fact that the time extent between twouccessive NO2 exposures is not sufficiently high to let the sensor toompletely recover its initial baseline. This is demonstrated by theact that after the last exposure the baseline value is similar to thene recorded before the first exposure. Combined with this effect,slight drift of the baseline intensity is also present, that could bettributed to a strongest interaction between NO2 and CdA/SWCNTverlay. Also a chemical modification of the overlay could be possi-le. In both cases, a modification of the nano-composite refractive

ndex is expected. The same can be said for the case of ethanoldsorption.

It is also worth noting that for NO2 detection the fiber opticransducer exhibited a �R/R0 of opposite sign with respect to thatbserved in case of vapor detection. This interesting behavior has

Fig. 6. Relative reflectance changes of the opto-chemical sensor coated by 10 mono-layers of CdA/SWCNT nano-composite (25 wt.%), exposed to different concentrationsof gaseous NO2, at room temperature.

also been observed for the fiber optic sensors in the standard con-figuration (i.e. fiber optic coated by pure SWCNTs), and could beascribed to the electrical nature of the analyte under investigation,electron donor (vapors) or acceptor (NO2). In particular, it couldbe due to the influence that the charge transfer induced by theanalyte molecule adsorption within the sensitive material has onthe optical properties of SWCNTs-based overlay itself. In light ofthis consideration, the plasma optic effect [28–30] is also beingconsidered, that allows one to relate the modulation of the opti-cal properties of sensitive overlays (refractive index and absorptioncoefficient) to the changes in its free carrier concentration. In orderto better clarify this aspect, further measurements are currentlyin progress involving more analytes with varying charge transferproperties as well as transducers with different principles of opera-tion (mainly resistive, capacitive and mass-sensitive) coated by thesame SWCNTs-based materials. However, to support our conjec-

Fig. 7. Electrical resistance versus time of a CdA/SWCNT thin film upon exposure toNO2 gas, at room temperature. The substrate used is Cr/Au patterned alumina with200 �m pitch interdigitated transducers.

M. Consales et al. / Sensors and Actu

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ig. 8. Comparison between the sensor characteristic curves obtained for the fiveested chemicals.

ough alumina (5.0 mm length × 5.0 mm width × 0.5 mm thickness)ubstrates with 200 �m pitch interdigitated Cr/Au (20/200-nmhick) pattern by means of two-pole probe method with an elec-rometer (Keithley 617) has been carried out. Fig. 7 reports theypical time response of the electrical resistance of a LB layer ofdA/SWCNT composite upon exposure of NO2, at room tempera-ure. As expected, it decreases when the composite film is exposedo the oxidizing NO2 gas. In particular, electron charge trans-er occurs from SWCNTs-based composite to NO2 because of thelectron-accepting power of the NO2 molecules. Thus, the NO2 gasepletes electrons from the SWCNTs-based composite, increaseshe concentration of electrical holes in the p-type SWCNTs-basedomposite, hence causing the electrical resistance to decrease. Evenf a partial desorption and an unreached saturation level can bebserved, similar to the case of optical sensors, however, a clearlectrical response modulated by the gas adsorption is demon-trated, revealing that a charge transfer effectively occurs betweenhe realized LB SWCNT overlays and the analyte under investigation.

An almost linear behavior in the sensor calibration curvesreported in Fig. 8) has been found towards most of the tested chem-cals in the investigated ranges, as well as higher sensitivities (seeig. 9a) in case of exposure to aromatic hydrocarbons (3.2 × 10−3

nd 1.3 × 10−3 ppm−1, respectively, for xylene and toluene) thano alcohols (5 × 10−4 and 4 × 10−4 ppm−1, respectively, for iso-ropanol and ethanol).

In addition the sensitivity towards NO2 was of approx.1.3 × 10−3 ppm−1. By considering the minimum detectable �R/R0chievable with the exploited interrogation system, resolutionscalculated as �R/R0min/C, where C is the analyte concentration)n the range 30–80 and 200–250 ppb have been estimated, respec-ively, for hydrocarbon and alcohol detection, while the minimumoncentration of NO2 that can be detected turned out to be circa0 ppb.

It is worth noting that the response to toluene exposures did noteach the equilibrium after 60 min and that the considered valuesor �R/R0 are not exactly the ones at the equilibrium. For this reasonn error analysis aimed to estimate the exact values of the sensoresponse at the equilibrium has been carried out. To this aim, theransient responses of the SWCNTs-based sensor have been fitted

see the inset in Fig. 5a) by means of the “pulse cumulative withower term” function (a built-in function of the software “Tableurve 2D”, Systat Software Inc.) which provided r2 values higherhan 0.997. By using these adjusted values for �R/R0, a difference inhe sensor sensitivities �S = S∞ − S60 (where S∞ and S60 are the sen-

ators B 138 (2009) 351–361 357

sitivities obtained considering the values at equilibrium and after60 min, respectively) of approx. 2 × 10−5 ppm−1 was appreciated.This value for �S leads to an error in the resolution estimation ofapprox. 1 ppb. The same can also be said for NO2 and xylene expo-sures, for which resolution errors of approx. 10 and 2 ppb have beenestimated.

Moreover, from the analysis of the mean response time (calcu-lated as the average of the times needed for the output signal topass from 10% to 90% of the total signal shift occurring upon ana-lyte exposures), it turned out that the CdA/SWCNT composite-basedsensor provides a faster response (see Fig. 9b) in case of alco-hol exposure (8 and 9 min, respectively) than in the hydrocarbonone (33 and 31 min, respectively). In particular, the ratio betweenthe mean response time in case of toluene and ethanol detectionis approx. 3.4, while the one obtained with standard SWCNTs isapprox. 15. This means that the presence of the CdA matrix notonly slows the sensor response down but also leads to significantdifferences in the adsorption dynamics of the two analytes. In addi-tion, since the diffusivity depends upon the exploited SWCNT fillercontent, it is expected that variations in the SWCNT weight per-centage within the CdA matrix could be able promote differencesin the sensor response times. In order to investigate this aspect,further experimental measurements will be performed, involvingnano-composite-based sensors with different weight percentagesof CNTs.

The opto-chemical sensors based on CdA/SWCNTs exhibitedquite high response times; this could be mainly due to the factthat the molecules of the target analyte adsorb not only at theside-wall of the carbon nanotubes, but also in the interstitial sitesbetween the tubes [37]. In particular, in case of film consistingof bundles of SWCNTs, the latter contribution could significantlyslow down the sensor response times [3]. However, it is worthnoting that these times are comparable with those obtained bymeans of many sensors based on different transducing principles(conductometric, resonator, mass-sensitive sensors, etc.) but inte-grating the same sensitive materials [3,4,11,38]. Also, it can beattributed to the presence of the CdA matrix and thus to the dif-fusion rates of analyte molecules inside the CNT composite. It isalso important noting that the content of SWCNTs within it canalso influence the diffusion times and the sensor sensitivity (asalready reported for the electrical case with CdA/SWCNTs-basedSAW sensors [22,23]). These aspects, however, are still under inves-tigation. The bar plots reported in Fig. 9 can also be used to comparethe sensing performance of the nano-composite-based probe withthe results obtained with that coated by ten monolayers of SWC-NTs directly deposited on the fiber end. It can be clearly observedthat the use of the novel CdA/SWCNT nano-composite coatings notonly improves the robustness of the chemo-optical sensor, but alsosignificantly enhances its sensitivity.

As a matter of fact, sensor sensitivities from three to seven timeshigher have been observed for the investigated chemicals withrespect to the counterpart optoelectronic sensor directly coated bySWCNTs.

This can be ascribed to the higher refractive index of this newfiber nanocoatings with respect to those based on standard SWCNTones.

However, much higher response times have been obtained inmost of the cases, probably due, as already said, to the presenceof the CdA matrix and, thus to slower analyte molecule diffusiontimes.

6.2. Chemical trace detection in aqueous environments

Once their excellent chemical adsorption capabilities in air atroom temperature is verified, the sensing characteristics of theCdA/SWCNT nano-composite have also been investigated for the

358 M. Consales et al. / Sensors and Actuators B 138 (2009) 351–361

Fig. 9. Comparison between sensitivity (a) and mean response times (b) of the CdA/SWCNTs and SWCNTs-based fiber optic sensing configurations obtained for the five testedchemicals.

Fig. 10. Schematic view of the experimental set-up exploited for the chemical trace detection in water.

M. Consales et al. / Sensors and Actuators B 138 (2009) 351–361 359

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constant. Nevertheless, the results reported so far prove that theproposed composite-based nanosensors exhibited enhanced per-formance with respect to the standard SWCNTs-based ones (eitherin terms of robustness or sensitivity) [16], thus revealing their

ig. 11. Time responses of the opto-chemical sensor coated by 20 monolayersf CdA/SWCNT nano-composite (75 wt.%), exposed to different concentrations ofoluene in water at room temperature.

romatic hydrocarbon detection in aqueous environment. In thisase, a fiber optic chemo-sensor coated by 20 monolayers ofdA/SWCNT nano-composite was exploited for the detection of

ow concentrations of toluene and xylene in water. To this aim aroper experimental set-up was designed and realized. In partic-lar, as schematically reported in Fig. 10, the probe under testingas inserted in a Pyrex beaker containing pure water. The presencef toluene and xylene within the test ambient has been promotedy their injection inside the beaker.

The injected volumes have been selected, each time, in ordero obtain the desired analyte concentrations. The polluted wateras been continuously stirred to ensure the maximum dispersionf the analyte. In addition, after each analyte exposure, the capabil-ties of the reflectometric nanosensor to recover the initial steadytate level have been investigated by restoring the initial condi-ion of uncontaminated water. Pure water, in fact, was continuouslynjected in the test chamber, while the contaminated water, previ-usly present in it, contemporarily stilled out.

Fig. 11 reports the transient responses of a fiber optic probeoated by 20 monolayers of nano-composite (75 wt.%) as a con-equence of several toluene injections with concentrations rangingrom 20 to 100 ppm (�l/l). Different to that in air case, here a signifi-ant reflectance decrease occurred on analyte adsorption within theensing nanolayer. This could be ascribed to the different SWCNTs-ller content used in this case and, as consequence, to a differentefractive index of the sensitive overlay as well as to the dependencef the reflectance upon the surrounding refractive index (in accor-ance with Eqs. (1) and (2)) combined with the different adsorptionharacteristics occurring in the two environments. In addition, aood repeatability has been observed in the sensor response whenxposed to two successive 100 ppm xylene injections.

Similar results were also obtained in case of xylene detectioneasurements carried out considering the same concentration

ange. The calibration curves (reporting the sensor output ver-us the analyte concentration) are shown in Fig. 12. They alsoemonstrate that for the detection in aqueous ambient a lin-ar dependence exists between the fiber–film reflectance changend the concentration of the two organic analytes. In addition,he typical higher affinity of carbon nanotubes-based sensorsowards xylene (the sensor sensitivity is 1.0 × 10−3 ppm−1) than

−4 −1

oluene (4 × 10 ppm ) was confirmed [15]. The sensor resolutionbtained in this case are, respectively, of circa 0.6 and 1.5 ppm.

In addition, to compare the performance of the proposed sen-or with those obtained with the one coated by a SWCNT layer,he characteristic curves of the standard SWCNTs-based transduc-

Fig. 12. Comparison between the sensor characteristic curves obtained in corre-spondence of toluene and xylene injections in water environment.

ers against toluene in water have also been reported in Fig. 12. Theresults clearly reveal that a significant enhancement in the sensorsensitivity can be obtained by the use of SWCNTs-based compos-ite (4 × 10−4 ppm−1) with respect to the standard carbon nanotubecounterpart (1.2 × 10−4 ppm−1) [16]. The CdA/SWCNT compositecoating exhibited behavior similar to those of the standard SWCNTones not only concerning their sensitivity features but also for theircharacteristic times. As a matter of the fact, an analysis of the sen-sor characteristic times revealed a mean response time againsttoluene (approximately 11 min) smaller than the one obtained incase of xylene testing (approximately 14 min), as summarized inFig. 13, that is the typical trend observed for the SWCNTs-basedtransducers [15]. Different from that in air case, if it is comparedwith the characteristic times observed for the sensor coated by thestandard SWCNT overlays against toluene (approximately 20 min)[16], the composite-based transducers exhibited a significantlyhigher rapidity of the response on analyte injections within the testbeaker.

However, further measurements are currently in progress inorder to investigate the influence of the experimental condition onthe dynamic features of the sensors under investigation, involvinga fully automatic set-up which allows one to tailor and keep thetime needed for injecting and stilling out the contaminated water

Fig. 13. Mean response times obtained exposing the chemo-optical nanosensorscoated by the CdA/SWCNT nano-composite and the standard SWCNT layer to tolueneand xylene in water, at room temperatures.

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trong potentiality and possible exploitation for practical on-fieldnvironmental monitoring applications.

. Conclusion

In conclusion, in this contribution the authors demonstratedor the first time the possibility to exploit CdA/SWCNTs nano-omposites as highly sensitive materials to be integrated withptical fiber technology for the development of advanced fiber optichemical nano sensors. With respect to the standard SWCNTs-basedptoelectronic sensors, this combination improves the adhesion ofhe carbon nanotubes-based sensitive overlay to the fiber optic sen-or surface and, at the same time, leads to enhanced performancen terms of sensors sensitivity. This could enable the fabrication of

ore robust and reliable optical chemo-sensors characterized bydvanced sensing features both for in air and for in water chemicaletection. Further works will be focused to a deeper characteriza-ion of the fabrication process in terms of repeatability and to thenvestigation of dependence of the sensing performance upon theanotube content within the composite.

Also, another issue to address in the near future is related tohe poor selectivity of the proposed SWCNTs-based chemo-sensors.his is a very common aspect in chemical sensing applicationss demonstrated by the strong effort which is currently devotedy the researchers on how to improve the sensor discriminationbilities among different analytes. Two possible approaches are cur-ently under investigations. The first one is a direct approach whichelies on the sensitive layer functionalization in order to have aigher affinity of the material towards specific chemical species39]. The second one is based on the use of a hybrid system com-osed of multiple transducers coated by the same material (or, inhe most general case, by different materials) in the form of anrray. Here, sensors with a poor selective response, when consid-red collectively, provide unique patterns typical for each analyte.he generated response patterns are interpreted by pattern recog-ition algorithm for the selective detection [11,40].

cknowledgment

The authors wish to thank Emanuele Serra (ENEA CR. Casaccia)or the SEM observations.

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Biographies

Marco Consales received the master degree cum laude in telecommunications engi-neering on January 2004 from University of Naples Federico II, Italy and the PhDdegree in “Information Engineering” on June 2007 from University of Sannio, in Ben-evento, Italy, with tutors Prof. Antonello Cutolo and Prof. Andrea Cusano. At present

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M. Consales et al. / Sensors an

ime he is research associate at the same university. His research activity is focusedn the area of optoelectronics for sensing applications. His main interest is in the fieldf fiber optic sensors based on nanostructured sensitive materials for chemical andnvironmental applications. He is co-author of a national patent and of numerousublications on international journals and conference proceedings. He is a revieweror the Institute of Physics (IOP) and IEEE.

lessio Crescitelli received the master degree in telecommunications engineering inay 2005 from University of Naples Federico II, Italy. He worked with the University

f Fisciano, Salerno, to improve the food and flower conservation. At present time hes a PhD student at University of Sannio, Benevento, Italy. His interest in fiber opticensors is focused on chemical sensing based on reflectometric measurements.

ichele Penza graduated in physics from the University of Bari (Italy) in 1990.e was a fellow of the INFM in 1991. Since 1992, he has worked at the Sciencend Technology Park, Brindisi (Italy) first as fellow and then as researcher. Hisain scientific activities and interests are preparation and characterization of thin

lms for acoustic, electrical and optical sensing devices, SAW, QCM and TFBARas/vapor sensors, microacoustic sensors, sensor arrays for chemical detection,unctional characterization of sensing transducers and arrays, processing of sensoranostructured materials, design and fabrication of sensing devices, pattern recogni-ion, environmental monitoring applications. He has co-authored peer-review about20 scientific papers published in leading refereed journals in the sensors field,ome invited speakers and about 30 congress communications, disseminated inational/international conferences, 2 filed Italian patents on gas sensors. He spent a-month period for scientific training at the Russian Academy of Sciences (Moscow)

n 1993, and at other research organizations (CNR, Universities). He is IEEE mem-er, member of Italian Physical Society, member of Italian Association on Sensorsnd Microsystems. Actually, he is a researcher of ENEA, Italian National Agencyor New Technologies, Energy and Environment, at Department of Physical Tech-ologies and New Materials working in the field of chemical sensors and sensoranomaterials.

atrizia Aversa obtained chemical diploma in 1982. Since 1990, she was fellowt CNRSM and then, starting from 1992 she has worked as technician in thetaff at the Science and Technology Park, Brindisi (Italy). Her main activity is theangmuir–Blodgett deposition of thin films for gas sensors applications, chemicalnalysis. She spent a 1-year period for technical and scientific training at Chemistry

epartment, University of Pisa (Italy) and attended to numerous technical work-

hops and schools. Since 2002 she is in the technical staff of ENEA, Italian Nationalgency for New Technologies, Energy and the Environment.

aola Delli Veneri received the degree in physics in 1994 at the University “FedericoI” of Naples. Since 1997 she has been researcher at ENEA-Portici Research Center. Her

ators B 138 (2009) 351–361 361

main professional interest is in thin film silicon solar cells technology. In particular,she has a long experience in growth and characterization of thin film materials anddevices.

Michele Giordano received the degree in chemical engineering from Universityof Naples “Federico II” 1992. He completed his PhD in materials engineering in1996 at the same university. He joined the National Research Council (CNR) in1996. Associate researcher at the Institute of Composite and Biomedical Materialsof CNR since 2005. Research activities are within the area of materials engi-neering and science. In particular, the main research focuses are nano-compositeand composite materials (mainly polymer based) including multiscale design andprocessing of multifunctional composite materials, structural health managementsystems and thin films engineering for sensing and optoelectronic applications. Heis author of more than 80 peer reviewed papers and more than 150 conferencecommunications.

Andrea Cusano received his master degree cum laude in electronic engineeringon November 27, 1998 from University of Naples “Federico II”, Italy and his PhD in“Information Engineering” from the same university, with tutor professor AntonelloCutolo. His PhD thesis was focused on the development of fiber optic optoelectronicsensor for smart materials applications. On December 2002, he was nominated per-manent researcher at the Engineering Department of University of Sannio, startinghis activity on December 30, 2002, in the Optoelectronic Division. From 1999 hisactivity is focused in the field of optoelectronic devices for sensing and telecom-munication applications. He is co-author of over 90 articles published on severalhigh level international journals, and of over 130 communications to internationalconferences. He is also co-author of 4 chapters in international books. He servedas co-Editor for the publication of two Special Issues focused on optical fiber sen-sors: the first will be published by IEEE Sensors Journal and the second one willbe published in Current Analytical Chemistry (a Bentham publications). He is co-author of 4 national patents regarding the development of innovative fiber opticsensors. He is also co-founder of the spin-off society “OptoSmart S.r.l”, whose corebusiness is the developing of fiber optic devices for the structural and environ-mental monitoring. In 2006, he won a national competition for the position ofassociate professor and actually he is associate professor at the University of San-nio. He acts also as referee of several high level international journals, such as

Frequency Control, Sensors and Actuators A and B, Optics Express, Optics Communica-tions, Sensors. He is also associate Editor of Sensors & Transducers Journal, Journalof Sensors, Open Optics Journal and The Open Environmental & Biological MonitoringJournal.