A vapor response mechanism study of surface-modifiedsingle-walled carbon nanotubes coated chemiresistorsand quartz crystal microbalance sensor arrays

Hung-Ling Lu a, Chia-Jung Lu a,n, Wei-Cheng Tian b, Horn-Jiunn Sheen c

a Department of Chemistry, National Taiwan Normal University, Taipei, Taiwanb Department of Electrical Engineering, National Taiwan University, Taipei, Taiwanc Institute of Applied Mechanics, National Taiwan University, Taipei, Taiwan

a r t i c l e i n f o

Article history:Received 25 April 2014Received in revised form8 August 2014Accepted 9 August 2014Available online 19 August 2014

Keywords:Carbon nano-tubesGas sensorVolatile organic compoundsChemiresistorQuartz crystal microbalance

a b s t r a c t

This paper compares the selectivity and discusses the response mechanisms of various surface-modified,single-walled carbon nanotube (SWCNT)-coated sensor arrays for the detection of volatile organiccompounds (VOCs). Two types of sensor platforms, chemiresistor and quartz crystal microbalance(QCM), were used to probe the resistance changes and absorption masses during vapor sensing. Foursensing materials were used in this comparison study: pristine, acidified, esterified, and surfactant(sodium dodecyl sulfate, SDS)-coated SWCNTs. SWCNT-coated QCMs reached the response equilibriumfaster than the chemiresistors did, which revealed a delay diffusion behavior at the inter-tube junction.In addition, the calibration lines for QCMs were all linear, but the chemiresistors showed curvaturecalibration lines which indicated less effectiveness of swelling at high concentrations. While the sorptionof vapor molecules caused an increase in the resistance for most SWCNTs due to the swelling, theacidified SWCNTs showed no responses to nonpolar vapors and a negative response to hydrogen bondacceptors. This discovery provided insight into the inter-tube interlocks and conductivity modulation ofacidified SWCNTs via a hydrogen bond. The results in this study provide a stepping-stone for furtherunderstanding of the mechanisms behind the vapor selectivity of surface-modified SWCNT sensorarrays.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Volatile organic compounds (VOCs) represent a special cate-gory of hazardous substances that pose adverse effects to both theenvironment and human health [1]. Due to the diversity inchemical structures, the analyses of VOCs are usually performedusing field sampling methods (i.e., canisters or adsorbent tubes)followed by in-lab analysis using a gas chromatograph–massspectrometer (GC–MS) [2]. Although these analytical methodsprovide accurate assessments of VOC concentrations, they areoften too expensive for continuous use and/or real-time analyses.On the other hand, chemical sensor arrays with cross sensitivity toVOCs can provide sufficient selectivity and detection limits forapplications where immediate attention or continuous monitoringis called for [3].

Over the past few decades, several types of chemical sensorshave been developed for the purpose of volatile compound orscent detection [4]. These sensor applications include metal oxidesensors [4], acoustic wave sensors (i.e., QCM and SAW) [5,6],optical sensors [7–9], and chemiresistors [10–12]. Recent researcheffort has been focused on improving the performance of thesesensors by either employing nano structures to conventionalmaterials such as metal oxide [13], or by applying newly devel-oped nano materials such as metal nanoparticles [8–12] and nanocarbon materials [14–16].

Among these newly developed nano materials for chemicalsensing applications, carbon nanotubes have drawn much attentiondue to their highly adsorptive surfaces and their susceptible con-ductivity to environmental changes. Both single-walled carbonnanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs)have been demonstrated as highly sensitive materials for gas andvapor detection [14,17]. The earliest gas sensor employing SWCNTswas a field effect transistor device that measured the conductivitychanges of a single carbon nano-tube [18]. Several recent studieshave shown that measuring the film of randomly stacked CNTs on aninterdigit electrode can also achieve a highly sensitive chemiresistor

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Talanta

http://dx.doi.org/10.1016/j.talanta.2014.08.0270039-9140/& 2014 Elsevier B.V. All rights reserved.

n Correspondence to: No. 88, Sec. 4, Ting-Chou Rd.,Taipei City, ROC, 11677,Taiwan. Tel.: þ886 2 77346132; fax: þ886 2 29324249.

E-mail address: cjlu@ntnu.edu.tw (C.-J. Lu).

Talanta 131 (2015) 467–474

[17,19]. Aside from measuring the conductivity changes, CNTs havealso been tested on QCM, SAW and optical fiber sensor platforms[20–24]. The efficacy of using CNT-based sensors for VOCs in theenvironment, for detecting explosives, and for breath analysis hasbeen recently reviewed [25–27].

In addition to the application of pristine CNTs as a sensingmaterial, both covalent and non-covalent surface modifications ofCNTs have been attempted in order to create a diversifiedselectivity for CNT-based sensors [28]. Non-covalent modificationscan be achieved via either a polymer composite [24,28–31] orsurface adsorption with π–π stacking attractions [32]. The swellingmechanism that is particularly important for polymer-coated CNTsensors was explored by Feller et al. [33]. Covalent modificationrequires a few more steps in the chemical synthesis. Functionalgroups that provide specific chemical forces can be grown on thesurface of CNTs [34]. Several recent studies have demonstratedsurface modifications using porphyrin or other metal complexeson CNTs for highly selective detection [21,35–38].

In the present study, we investigated a series of surface-modified SWCNTs coated on both chemiresistors and QCMs. Bothcovalent and non-covalent modified SWCNTs were tested. Bysimultaneously observing both sorption mass and conductivitychanges, we were able to investigate the mechanism behindselectivity changes as well as the progression of surface modifica-tions. Different response behaviors for both sensor platforms wereused to interpret how surface-modified SWCNTs react to incomingvapors as well as the role of the interactions among nanotubes in afilm. The experiment design in this work enabled the observationof different aspects of surface-modified SWCNTs during vaporsensing.

2. Experimental

2.1. Materials

High purity (490%) single walled carbon nanotubes were pur-chased from Sigma-Aldrich and used as obtained. Chemicals forsurface modification synthesis such as nitric acid, thionyl chloride,octanol and sodium dodecyl sulfate (SDS) were purchased from J. T.Baker. Organic solvents for both the synthesis process and test vaporgeneration were obtained from either Fluka (Japan) or Alfa Aesar (UK)with a purity of 98% or higher.

2.2. Surface modification of SWCNTs

Four different SWCNTs were tested in this study: pristine,acidic surface, ether surface, and SDS-coated (Fig. 1a). The non-covalent modification of SDS surface adsorption was prepared bysuspending SWCNTs in a SDS/toluene solution. The molar ratio ofSDS to the carbon atoms of SWCNTs was 1:10. The mixturesolution was stirred and then ultrasonically resonated for30 min. The SWCNT in the SDS solution was then filtered, rinsedwith excess amount of toluene and dried in an oven for later use.The covalent modification of SWCNTs started with acidification toform a carboxylic acid group at the surface. SWCNTs weresuspended in a mixed-acid solution containing H2SO4:HNO3¼3:1.The suspended solution was stirred and ultrasonically resonatedand then refluxed for 3 h. The acidified SWCNTs (i.e., SWCNT-COOH) were filtered, washed with an excess amount of deionizedwater and then dried in an oven.

Esterification was carried out by mixing 0.2 g of dried SWCNT-COOH with excess thionyl chloride (SOCl2) in dimethylformamide(DMF) with refluxing under nitrogen at 70 1C for 24 h. TheSWCNT-COOHs were converted to SWCNT-COCls in this step. Afterallowed to cool to room temperature, the SWCNT-COCls were

rinsed with a large amount of dehydrated tetrahydrofuran (THF)and dried in a vacuum oven. Dried SWCNT-COCl was then mixedwith 1 mL pyridine and 80 mL octanol and then refluxed at 117 1Cfor 4 days. After reflux, the solution was cooled to room tempera-ture. The excess octanol was removed by a filtering of the solidSWCNT-COOC8H17 (i.e., labeled as SWCNT-ESTER for later discus-sions) and the final product was washed repeatedly with ethanoland THF. In terms of interacting with analytes, the polar attractionwas more important than the weak dispersion from the interac-tion with an aliphatic chain in this SWCNT-ESTER structure.

2.3. Sensor devices

10-MHz, AT-cut, gold-electrode QCM devices were purchasedfrom Taitein electronics, CO., Taiwan. The 10-MHz driving circuitrywas constructed in-house. A screen-printed interdigit electrode (IDE)was obtained from Ghitron Technology Co., Ltd (Taiwan). There were5 pairs of carbon electrodes with lengths of 4 mm and widths of0.25 mm that were spaced 0.15 mm apart. The SWCNTs weredissolved in THF and spray-coated on both QCM and IDE. Thespray-coat procedure was performed using a small airbrush withregulated-pressure nitrogen gas from a cylinder. The electrodes wereburied under the SWCNT film. The resistance of the chemiresistorwas measured through a bridge circuit. The thickness of the filmswas examined by cross-section SEM images (i.e., see Supportinginformation). The thickness of the SWCNTs on IDE was �17 μm. Thethickness of the SWCNTs on QCM was �33 μm. We intentionallyadded more mass to the QCM to enhance the sensitivity for thisdevice. Fig. 1b shows the photos of both sensors as one unit. The

SWCNT SWCNT-SDS

SWCNT-COOH SWCNT-ESTER

O

HO

O

HO

O

O

O

O

Fig. 1. (a) Chemical structures of surface modified SWCNTs (b) Photo of QCM andchemiresistor sensor platforms.

H.-L. Lu et al. / Talanta 131 (2015) 467–474468

frequency shift and voltage signal was recorded via a data acquisitioninterface card (DAQ-Pad 6015, National Instruments, USA). The soft-ware for continuous acquisition of frequency and voltage changeswas written in-house using LabVIEW.

2.4. Instruments and apparatus

The infrared spectra of SWCNTs during the synthetic processwere measured by FTIR (Spectrum RX-1, Perkin Elmer). A fieldemission scanning electron microscope (FE-SEM, S4800, Hitachi,Japan) was used to observe the image of the SWCNT film. Afrequency counter (HP-5313A, Agilent, USA) and a resistancemeter (model 6487, Keithley, USA) were used to monitor theoscillating frequency of QCM and resistance changes of chemir-esistor during the spray coating of SWCNTs.

2.5. Vapor testing system

Test vapor was generated by a dynamic dilution system con-sisting of three mass flow controllers, stainless steel or Teflonmanifolds, adsorbent traps, and a solvent evaporator. Clean back-ground air was generated by passing compressed air through thetandem traps of a molecular sieve, charcoal, and a highly efficientparticle filter (HEPA) to remove moisture, VOCs, and fine particles,respectively. The temperature was maintained at 2571 1C andrelative humidity was kept below the lowest readable level (o1%)for a humidity meter (DO9861T-R1, Delta Ohm, Padua, Italy). Theorganic vapor concentration was generated by passing the back-ground air through a bubbler to produce a saturated vaporconcentration, and was diluted with different ratios of backgroundair, and both were controlled using mass flow controllers (5850i,Brooks Instrument, PA). Two solenoid 3-way valves were con-nected to the front of the test cell and vent that allowed computer-controlled switching between the background air and the testvapor concentration. The generated vapor concentration wasconfirmed by GC-FID (HP5890, Agilent). The test cell that wasused to house both the sensor arrays had an internal volume of�250 mL. The volumetric flow rates were maintained at 5 L/minto ensure that the switching between test atmosphere and back-ground air could be achieved in a few seconds.

3. Results and discussion

3.1. Characterization of surface modified SWCNTs

Fig. 2 shows the FTIR spectrums of pristine SWCNT, SWCNT-COOH,SWCNT-ESTER, and SWCNT-SDS. The pristine SWCNTs showed twosmall absorbance bands located at 1570 cm�1 that represented C¼Cstretching. Another small absorbance at 3410 cm�1 indicated a smallfraction of oxidization on the surface of “as-purchased” SWCNT. Afterthe acidification to SWCNT-COOH, narrower and sharper bandsappeared at 1580–1750 cm�1 which indicated the absorbance of theC¼O and COO� functional groups. Another huge and broad absor-bance band at 3000–3600 cm�1 was the contribution of –OH on thecarboxylic acid group. When the SWCNT-COOH was further convertedto a SWCNT-ESTER, the broad absorbance band at 3000–3600 cm�1

disappeared. Instead, a group of sharp absorbance peaks was seen at2800–3000 cm�1, which indicated the replacement of –OH by thelong alkane chain (–CH2–, sp3) of an ester group (i.e., from –COOH to –

COOR). The FTIR spectrum of the SWCNT-SDS showed both the similarsignature of sp3 band form the long alkane chain of SDS and theabsorbance signals of pristine SWCNT. These FTIR spectrums con-firmed the changes in chemical structure as we progressively modifiedthe SWCNT.

Fig. 2b shows an example SEM image of a spray-coated SWCNT-ESTER film. The morphologies of all four SWCNT sensing filmswere very similar regardless of surface modification. The randomlystacked nanotubes created a naturally porous film. The poresamong these randomly stacked nanotubes were ranged fromsubmicron to several microns and were beneficial for rapid gasdiffusion into the film. The gas response times of such a poroussensing film were expected to be short. A different study demon-strated by Feller et al. using polymer-coated CNTs as sensing filmshowed a faster rate of response equilibrium while significantlyreducing the film thickness [33]. This showed how the diffusionthrough the polymer matrix contributed significantly to theresponse time. Unlike the film created by dense polymer coatingswhere gas diffusion time through sensing film contributessignificantly to sensor response time, the small difference in thethickness of SWCNT film was expected to have a minor effect onthe response time due to its porous structure.

3.2. Response time differences between QCMs and chemiresistors

Fig. 3 shows examples for the response signals of a chemiresistor(CR) and that of a QCM sensor. The chemiresistor circuitry wasconfigured in such a way that the increases in resistance caused theincreases in output voltage. The frequency shift of the QCM duringvapor adsorption was always negative. The response signals of theQCMwere reversed and overlapped with the chemiresistor signal in

Tran

smitt

ance

Wavenumber (cm1)4000 3500 3000 2500 2000 1500

Fig. 2. (a) Infrared spectra of SWCNTs with and without surface modifications(b) SEM image of spray-coated SWCNT film.

H.-L. Lu et al. / Talanta 131 (2015) 467–474 469

order to compare the times required for reaching equilibrium forboth sensors. Fig. 3a shows the response signals of SWCNT-SDS-coated QCM and CR to 3000 ppm butanol. Fig. 3b shows the signalsof SWCNT-ESTER-coated QCM and CR to 3500 ppm butyl acetate. Inboth cases, the responses of the QCM reached equilibrium fasterthan the chemiresistor did. Similar results were observed in allcases of pristine SWCNT, SWCNT-SDS and SWCNT-ESTER. This wasparticularly noteworthy because both sensors were coated with thesame material at a similar thickness. As shown in the SEM image(Fig. 2b), the porous nature of randomly stacked SWCNT films wasexpected to enable a fast equilibrium between the gas phase andthe surface adsorption. However, the increases in resistance requirevapor molecules to intercalate between the junctions of twonanotubes. A proposed response mechanism is illustrated inFig. 4. With the onset of vapor concentration, the vapor moleculesdiffused rapidly into the porous SWCNT films and were readilyadsorbed onto the non-overlapped surfaces of SWCNTs. This wasthe adsorption-mass-increase stage, and a frequency shift of theQCM occurred. The remaining kinetic energy of adsorbed vapormolecules allowed the molecules to move into the overlappedregions between the nanotubes. The overlapped regions providedvan der Waal attraction from both sides of the nanotube wall, whichwas better stabilized by the adsorbates. Thus, the adsorbed mole-cules had a greater tendency to stay within the overlapped regions.The swelling between carbon nanotubes needed to overcome theattractions between nanotube surfaces and a slight displacement ofthese nanotubes. Therefore, even though it was thermodynamicallyfavorable for adsorbed molecules to partition into the overlappedregions between SWCNTs, a certain amount of reaction time wasstill required to complete the displacement of SWCNTs during theswelling. The response behavior of SWCNT-COOHs differed fromthat of the other three sensing materials, which will be addressed inthe following sections.

3.3. Response signals of surface modified SWCNTs

Fig. 5 shows examples of the response signals for both a non-polar vapor (i.e., octane) and a highly polar vapor (i.e., butanol) forthe four SWCNTs coated on both CR and QCM. As both Fig. 5a and bshow, the coating of SDS on SWCNTs can significantly enhance theresponse for QCM sensors, which means an increase in vaporpartition. When SDS was coated on the SWCNTs, the static chargeof the –SO3

�Naþ terminal was attracted to the surface of theSWCNTs due to the highly polarizable conjugating electrons onthe surface of the SWCNTs. After we had rinsed the SWCNT-SDSwith toluene, the excessive SDS was eliminated leaving only thestrongly adsorbed SDS that bind to the surface via a static charge-induced dipole attraction. The long alkane chain of the SDS layerprovided an additional “stationary phase” for the partition ofhydrophobic compounds. Hydrophilic compounds are more likelyto be attracted by the –SO3

�Naþ group of SDS. When octane wasabsorbed by SWCNT-SDS, octane could partition into the over-lapped alkane chain “spacer” and caused swelling between thenanotubes. As a result, the increased absorption of octane onSWCNT-SDS QCM showed an equivalent increase in the responseson the SWCNT-SDS chemiresistor. In the case of butanol, theSWCNT-SDS chemiresistor did not show sensitivity enhancementthat matched the increase in absorption mass. This was because theoverlapped regions of SWCNT-SDSs were spaced by non-polaralkane chains. Butanol was greatly attracted to the –SO3

�Naþ sitenear the SWCNT surface. An increase in mass adsorption can beseen in the QCM responses (Fig. 4b). The effectiveness of increasingthe distances between SWCNT-SDS nanotubes by butanol adsorp-tion was reduced because it did not contribute to the swelling of thenon-polar end of the SDS shell. Also, the static charge of the –

SO3�Naþ site hindered the hydrogen bond formation of n-butanol

directly to the surface of the SWCNTs. The possibility of reducingconductivity by scattering points [39,40] created from n-butanoladsorption was reduced; therefore, the resistance change exertedby n-butanol adsorption was less sensitive.

0

0.004

0.008

0.012

0 50 100 150 200Time (s)

CR

sign

al (V

)

-280

-210

-140

-70

0

CR

QCM

QCM

CR

-0.117

-0.111

-0.105

-0.099

0 50 100 150 200Time (s)

CR

sign

al (V

)

-80

-60

-40

-20

0

CR

QCM

QCM

CR

QC

M

f (Hz)

∇

QC

M

f (Hz)

∇

Fig. 3. Examples for overlapped response signals of QCM and chemiresistor (CR)(a) SWCNT-SDS sensors and the response to 3000 ppm butanol. (b) SWCNT-ESTERsensors and the response to 3500 ppm butyl acetate. (note: 2nd y-axis is inreversed order).

QCM response

CRresponse

QCM response

: vapormolecules

Fig. 4. Scheme for the response mechanism of pristine SWCNT-coated QCMs andchemiresistors.

H.-L. Lu et al. / Talanta 131 (2015) 467–474470

SWCNT-COOHs showed a sensing behavior that was moredistinct than that of the other three materials. Judging from theresponse of the SWCNT-COOH-coated QCM, there was only arelatively small but detectable amount of octane adsorption onthe SWCNT-COOH. However, the SWCNT-COOH-coated chemire-sistor showed absolutely no response to octane even at very highconcentrations. On the other hand, SWCNT-COOHs showed ahigher adsorption mass for butanol than that of either pristineSWCNTs or SWCNT-ESTERs, but lower than that of SWCNT-SDSs.Surprisingly, a huge reverse response (i.e., resistance decrease)was seen with the SWCNT-COOH chemiresistor (Fig. 5d). It shouldbe noted that the scale of the second y-axis on the right of Fig. 5dfor SWCNT-COOH is 20 times larger than the scale of the firsty-axis for the other three chemiresistors.

Unlike the NH3 or NO2 that were used as target gases forSWCNT sensors in earlier studies [25,41], nonpolar VOCs, such asoctane, lack the ability to change the electron density of SWCNT.Therefore, the chemiresistor responses of octane on SWCNT,SWCNT-ESTER and SWCNT-SDS are mainly due to the swellingbetween the carbon nanotubes that increases the resistance ofelectron-hopping at the junctions of nanotubes. SWCNT-COOH hasvery strong hydrogen bonding terminals. The massive hydrogenbonds at the junction regions held SWCNT-COOH nanotubestightly together. The adsorbed octane on the surface was notcapable of breaking the hydrogen bond “interlocks” between theSWCNT-COOH nanotubes (Fig. 6). Therefore, SWCNT-COOH-coatedchemiresistors showed no response to octane.

Butanol is neither a strong electron donor nor an acceptor bycomparison with either NH3 or NO2. The increases in resistanceindicate that the swelling remains to be the dominant mechanismfor pristine SWCNT, SWCNT-SDS and SWCNT-ESTER chemiresis-tors. However, a huge decrease in resistance was observed whenbutanol was adsorbed onto SWCNT-COOH. Although the hydroxylgroup (–OH) of butanol can act as a hydrogen bond donor oracceptor, its acidity is much lower than that of the carboxyl group

(–COOH). Therefore, we believe that hydroxyl groups acceptprotons from carboxyl groups when hydrogen bonds are formed,which allows the conjugating electrons of carboxylic groups toincrease the electron density on carbon nanotubes and the con-ductivity of SWCNT-COOH film to increase. This reverse responseis also know as Negative Vapour Coefficient (NVC) and is some-times observed with Poly aniline (PANI) or other IntrinsicallyConducting Polymers (ICP) [42,43].

-300

-250

-200

-150

-100

-50

0

0 200 400 600 800 1000Time (s)

-0.002

0.002

0.006

0.01

0.014

0 200 400 600 800 1000Time (s)

Sign

al (V

)

-0.04

-0.02

0

0

0.02

0.04

200 400 600 800 1000

Sign

al (V

)

-0.8

-0.4

0

0.4

0.8

Time (s)

Signal (V)

-500

-400

-300

-200

-100

0

0 200 400 600 800 1000Time (s)

1

1

11,2

22

3

3

3

4

4

4

4

f (H

z)

f (H

z)

23

Fig. 5. Vapor responses of various SWCNT-coated QCMs and chemiresistors. Test concentrations: octane: 1916–9580 ppm; butanol: 981–4866 ppm. (a) QCM vs. octane(b) QCM vs. butanol (c) chemiresistor vs. octane (d) chemiresistor vs. butanol.

OHO

HOO

OHO

HOO

OHO

octane

XX

OHO

HOO

X

butanol

HOO

strong acid weak

acid

HOO

H

HOO

H

OHO

O

O

Fig. 6. Scheme for the response mechanisms of SWCNT-COOH-coatedchemiresistors.

H.-L. Lu et al. / Talanta 131 (2015) 467–474 471

3.4. Calibration lines of SWCNTs coated QCMs and chemiresistors

Fig. 7 shows the calibration lines of toluene for both QCM andchemiresistor arrays. The y-axis units for the QCM arrays were Δfv/Δfs, which stands for the vapor response frequency shift (Δfv)normalized to the frequency shift induced by the coating thickness(Δfs) of SWCNT film. The output signal of the chemiresistor wasvoltage, which was converted to the change in resistance (ΔR) usingOhm’s law. The y-axis units for the chemiresistor calibrations wereexpressed as ΔR/R0, which represented a change in the resistancenormalized to the film resistance (R0) in clean air. The units on the y-axis of Fig. 7a and b are changes in parts per million (ppm). Thecalibration lines of QCM for all SWCNTs (Fig. 7a) were linear over awide range of vapor concentrations. On the contrary, the calibrationlines of chemiresistors were all curved for pristine SWCNTs, SWCNT-ESTERs, and SWCNT-SDSs. There was no calibration data for toluenefrom the SWCNT-COOH-coated chemiresistor, because it did notrespond to toluene. Due to the high signal-to-noise ratios of chemir-esistors, the responses of this array can be measured at lowerconcentrations. The inset of Fig. 7b shows the calibration lines of achemiresistor array at a lower concentration range. The chemiresistorresponses appeared to be closer to linear at low concentrations andgradually leveled off at high concentrations.

The linear calibrations of the QCM sensors indicated that thepartition coefficients for surface adsorption over the tested con-centration range were constant. The surface adsorption mass was

always in a linear proportion to the concentration in the testatmosphere. However, increases in adsorbed molecules at highconcentrations did not seem to swell the nanotube films aseffective as they had at lower concentrations. As illustrated inFig. 4, vapor molecules were first adsorbed onto the non-overlapped surface of SWCNTs. Some kinetic energy remained onthe adsorbed molecules, which allowed them to move along theSWCNT surface, and some of them were stabilized at the junctionpoint of the nanotubes. In general, the overlapped regionsprovided better stabilization by offering the conjugating π-electrons for dispersion attractions from both sides of two adja-cent SWCNT walls. A partition equilibrium exists between thenon-overlapped surface and the overlapped regions. The concen-tration inside the overlapped region could be higher than that onthe non-overlapped surface due to stabilization. At low concentra-tion, adsorbed molecules were rapidly diffused into the over-lapped region and effectively increased the resistance at thejunction point. When vapor concentrations were high, these smalloverlapped regions approached saturation. When a second or thirdlayer of adsorption occurred inside the overlapped region, thestability from both SWCNT walls was reduced due to the increasedseparation between SWCNT walls. The partition into the over-lapped region became less favorable than it was at low concentra-tion. Therefore, the calibration lines of swelling-dependentSWCNT chemiresistors appeared to be nonlinear over a largeconcentration range. This adsorption/diffusion behavior waspreviously fitted to an LHC (i.e., Langmuir Henry Clustering)model. Details regarding the use of this model can be foundelsewhere [33,44].

In addition to the discrepancy in the linearity between the QCMand chemiresistor calibrations, the relative sensitivities of theseSWCNTs were also different on two platforms. The sensitivitytrend for toluene on the QCM array was SWCNT-SDS4SWCNT¼SWCNT-COOH4SWCNT-ESTER. The sensitivity order oftoluene on the chemiresistor array was SWCNT-SDS4SWCNT-ESTER4SWCNTcSWCNT-COOH (i.e., no response). As indicatedearlier, the responses of the QCM revealed the partition coeffi-cients between the air and the SWCNT film. The responses of thechemiresistors were affected by at least three factors: the partitionbetween the air and the SWCNT surfaces, the partition betweennon-overlapped and overlapped surfaces, and the effectiveness ofthe swelling. The effectiveness of the swelling is mainly deter-mined two factors: the free volume associated with the absorbedorganic molecules, and the attractions between the nanotubes.Both factors could be affected by the surface modification of theSWCNTs. For example, the strong hydrogen bonds between the

0

5000

10000

15000

20000

25000

30000

0 3000 6000 9000 12000Conc. (ppm)

Δƒ/ƒ

0 (pp

m)

SWCNTSWCNT-ESTERSWCNT-SDSSWCNT-COOH

0

4000

8000

12000

16000

20000

0 2000 4000 6000 8000 10000Conc. (ppm)

ΔR/R

0 (pp

m)

SWCNT-COOH: no response

Fig. 7. Examples of toluene calibration curves for SWCNT-coated (a) QCMs and(b) chemiresistors. The inset in (b) shows the blow-up of low concentrationresponses for chemiresistors.

-16000000

-12000000

-8000000

-4000000

0

0 2000 4000 6000Conc. (ppm)

ΔR/R

0 (p

pm)

chlorobenzene

2-butanone

butyl acetate

butanol

Fig. 8. Calibration curves of four polar compounds tested with a SWCNT-COOHcoated chemiresistor.

H.-L. Lu et al. / Talanta 131 (2015) 467–474472

carboxylic acid groups on adjacent SWCNT-COOHs inhibited theswelling completely and resulted in the lack of a response tononpolar compounds in chemiresistor (Fig. 6).

Fig. 8 shows the calibration curves of several polar organic vaporsthat were tested using a SWCNT-COOH chemiresistor. Similar totoluene or octane, chlorobenzene showed only a barely detectableresponse on a SWCNT-COOH chemiresistor. The remaining com-pounds in Fig. 8 were able to either accept or partially stabilize thepositive charge (i.e., proton) from carboxylic terminals. The responsesof these compounds were all “negative,”which equated to a reductionin film resistance. In addition, unlike the other three swelling-

dependent chemiresistors, the negative calibration curves of SWCNT-COOHwere all linear, which indicated clear differences in the responsemechanism. Salehi-Khojin et al. recently proved that modulation ofthe conductivity of carbon nanotubes itself plays an important role inhighly defected SWCNTs [45]. In the present study, the reactions ofcovalent modifications (i.e., acidification) created a highly defectiveSWCNT. All these polar organics in Fig. 8 were basic related tocarboxylic acid. With the mechanism that we proposed in Fig. 6, thesemolecules attracted protons via hydrogen bonds accepting andreleasing some of the electron density back to the carbon nanotubes,thus, the conductivity of the chemiresistor was increased. We have

CR-octane

CR-toluene

CR-butanol

×103

CR-butanone

×102

QCM-octane

QCM-toluene

QCM-butanol

QCM-butanone

QCM-TCE CR-TCE

Fig. 9. Response patterns of five VOCs obtained from SWCNT-coated QCM and chemiresistor arrays. Coatings (from left to right): SWCNT, SWCNT-ESTER, SWCNT-SDS,SWCNT-COOH.

H.-L. Lu et al. / Talanta 131 (2015) 467–474 473

also tested several nonpolar organic vapors such as xylene andtrichloroethylene, and none of these compounds showed measurableresponses on the SWCNT-COOH chemiresistor.

3.5. QCM and chemiresistor response patterns

Response patterns are usually plotted as either a bar chart or aradar chart that shows the relative sensitivity of a sensor array to atarget analyte. The most common approach is to normalize thesensitivity (i.e., calibration slope) with respect to the highestsensitivity in the sensor array. In the present study we plottedthe response pattern of a QCM array in a conventional manner forfive tested vapors, as shown in Fig. 9a–e. The extraction ofresponse patterns from chemiresistor calibrations is somewhatdifficult because the calibrations of swelling-dependent chemir-esistors are nonlinear over the tested concentration range. Weused the first portion of a calibration curve at low concentrationrange that could be approximated as a straight line, and then usedthe slope of this section to draw the response patterns (Fig. 9f–j).

The response patterns of the QCM arrays indicated that thesurface modifications with SDS coating significantly increased theamount of absorption mass for most compounds. The covalentmodification of a SWCNT into a COOH or ester group can alter thepreferences to different functional groups to some degree. Theresponse patterns of the chemiresistors were dramatically differ-ent from those of the QCM arrays. Despite the unique sensingbehavior of the SWCNT-COOH chemiresistor, the relative sensitiv-ities of the other three chemiresistors were also highly distin-guishable. The additional mechanisms that were involved in theresistive changes led to the differences between chemiresistor andQCM patterns. It is noteworthy that the same set of SWCNTs canexpress different response patterns on two different sensor plat-forms. The versatile properties of sensing materials and the choiceof proper sensing platforms are essential for constructing a hybridarray [46]. This discrepancy between QCMs and chemiresistors canbe used to create a hybrid array that can potentially improve therecognition rate for vapors.

In the present study, the covalent surface modification started withconverting a pristine SWCNT to a SWCNT-COOH. Dramatic changes inchemiresistor selectivity were observed: insensitive to nonpolarcompounds and an increase in conductivity versus polar compounds.When the carboxyl acid groups further reacted to esters, thesecharacteristics disappeared and the behavior of SWCNT-ESTER che-miresistors returned to swelling-dependent. This result indicated thatthe proton of a carboxyl acid plays an essential role in the uniqueresponse selectivity of a SWCNT-COOH chemiresistor.

4. Conclusion

In the present study, QCM and chemiresistor platforms wereused to investigate the selectivity mechanisms of one pristine andthree surface-modified SWCNTs. The results of the QCM provideda direct indication of the VOC partition selectivity of SWCNTsurfaces. The chemiresistors responded to either swelling orcharge modulation depending on the type of SWCNT. Surface-acidified SWCNT-COOHs showed a unique H-bond interlock thatprohibited the swelling mechanism. However, the resistance ofSWCNT-COOHs could be modulated via H-bond formation. Theresponse time, linearity and relative sensitivities were all differentbetween QCM and chemiresistor arrays coated with the same setof SWCNTs. The signal transduction mechanisms behind SWCNT-coated QCMs and chemiresistors were quite different, whichimplies that the diversified properties of surface-modifiedSWCNTs can be used to create a highly selective hybrid array.

Acknowledgements

The funding for this work was supported by the NationalScience Council of Taiwan under the project number NSC-100-2113-M-003-001-MY2.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.talanta.2014.08.027.

References

[1] L. Mølhave, Indoor Air 1 (1991) 357–376.[2] B. Tolnai, J. Hlavay, D. Möller, H.-J. Prümke, H. Becker, M. Dostler, Microchem. J.

67 (2000) 163–169.[3] J. Gutiérrez, M.C. Horrillo, Talanta 124 (2014) 95–105.[4] D.L. García-González, N. Tena, R. Aparicio-Ruiz, R. Aparicio, Talanta 120 (2014)

342–348.[5] C.L. Li, C.J. Lu, Talanta 79 (2009) 851–855.[6] S.J. Patrash, E.T. Zellers, Anal. Chem. 65 (1993) 2055–2066.[7] J.C. Echeverrı´a, P. Vicente, J. Estella, J.J. Garrido, Talanta 99 (2012) 433–440.[8] Y.Q. Chen, C.J. Lu, Sens. Actuators, B 135 (2009) 492–498.[9] K.J. Chen, C.J. Lu, Talanta 73 (2007) 358–365.[10] W.H. Steinecker, M.P. Rowe, E.T. Zellers, Anal. Chem. 79 (2007) 4977–4986.[11] C.Y. Yang, C.L. Li, C.J. Lu, Anal. Chim. Acta 565 (2006) 17–26.[12] R.S. Jian, R.X. Huang, C.J. Lu, Talanta 88 (2012) 160–167.[13] M.A. Andio, P.N. Browning, P.A. Morris, S.A. Akbar, Sens. Actuators, B 165

(2012) 13–18.[14] E.S. Snow, F.K. Perkins, J.A. Robinson, Chem. Soc. Rev. 35 (2006) 790–798.[15] S. Basu, P. Bhattacharyya, Sens. Actuators, B 173 (2012) 1–21.[16] T.T. Tung, M. Castro, T.Y. Kim, K.S. Suh, J.F. Feller, J. Mater. Chem. 2 (2012)

21754–21766.[17] I. Sayago, H. Santos, M.C. Horrillo, M. Aleixandre, M.J. Fernández, E. Terrado,

I. Tacchini, R. Aroz, W.K. Maser, A.M. Benito, M.T. Martínez, J. Gutiérrez,E. Muñoz, Talanta 77 (2008) 758–764.

[18] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Science287 (2000) 622–625.

[19] J. Li, Y. Lu, Q. Ye, M. Cinke, J. Han, M. Meyyappan, Nano Lett. 3 (2003) 929–933.[20] M. Consales, S. Campopiano, A. Cutolo, M. Penza, P. Aversa, G. Cassano,

M. Giordano, A. Cusano, Sens. Actuators, B 118 (2006) 232–242.[21] L. Lvova, M. Mastroianni, G. Pomarico, M. Santonico, G. Pennazza, C. Di Natale,

R. Paolesse, A. D’Amico, Sens. Actuators, B 170 (2012) 163–171.[22] C. Viespe, C. Grigoriu, Sens. Actuators, B 147 (2010) 43–47.[23] I. Sayago, M.J. Fernández, J.L. Fontecha, M.C. Horrillo, C. Vera, I. Obieta,

I. Bustero, Sens. Actuators, B 156 (2011) 1–5.[24] S. Manivannan, A.M. Saranya, B. Renganathan, D. Sastikumar, G. Gobi, Kyu

Chang Park, Sens. Actuators, B 171–172 (2012) 634–638.[25] D.R. Kauffman, A. Star, Angew. Chem. Int. Ed. 47 (2008) 6550–6570.[26] U. Tisch, H. Haick, J. Breath Res 8 (2014) 1–8.[27] C. Di Natale, R. Paolesse, E. Martinelli, R. Capuano, Anal. Chim. Acta 824 (2014)

1–17.[28] S. Liu, Q. Shen, Y. Cao, L. Gan, Z. Wang, M.L. Steigerwald, X. Guo, Coord. Chem.

Rev. 254 (2010) 1101–1116.[29] J. Lu, J.F. Feller, B. Kumar, M. Castro, Y.S. Kim, Y.T. Park, J.C. Grunlan, Sens.

Actuators, B 155 (2011) 28–36.[30] S. Badhulika, N.V. Myung, A. Mulchandani, Talanta 123 (2014) 109–114.[31] B. Kumar, M. Castro, J.F. Feller, Sens. Actuators, B 161 (2012) 621–628.[32] Y.L. Zhao, J.F. Stoddart, Acc. Chem. Res. 42 (2008) 1161–1171.[33] B. Kumar, M. Castro, J.F. Feller, Chem. Senses 3 (2013) 1–7.[34] K. Balasubramanian, M. Burghard, Small 1 (2005) 180–192.[35] F. Wang, T.M. Swager, J. Am. Chem. Soc. 133 (2011) 11181–11193.[36] M. Penza, R. Rossi, M. Alvisi, M.A. Signore, E. Serra, R. Paolesse, A. D’Amico,

C. Di Natale, Sens. Actuators, B 144 (2010) 387–394.[37] M.D. Shirsat, T. Sarkar, J. Kakoullis Jr., N.V. Myung, B. Konnanath, A. Spanias,

A. Mulchandani, J. Phys. Chem. C 116 (2012) 3845–3850.[38] B. Esser, J.M. Schnorr, T.M. Swager, Angew. Chem. Int. Ed. 51 (2012)

5752–5756.[39] A. Star, T.-R. Han, V. Joshi, J.P. Gabriel, G. Gruner, Adv. Mater. 16 (2004)

2049–2052.[40] A. Star, J.P. Gabriel, K. Brasley, G. Gruner, Nano Lett. 3 (2003) 459–463.[41] N. Peng, Q. Zhang, C.L. Chow, O.K. Tan, N. Marzari, Nano Lett. 9 (2009)

1626–1630.[42] J. Lu, B. Kumar, M. Castro, J.F. Feller, Sens. Actuators, B 140 (2009) 451–460.[43] A. Bouvree, J.F. Feller, M. Castro, Y. Grohens, M. Rinaudo, Sens. Actuators, B 138

(2009) 138–147.[44] B. Kumar, J.F. Feller, M. Castro, J. Lu, Talanta 81 (2010) 908–915.[45] A. Salehi-Khojin, F. Khalili-Araghi, M.A. Kuroda, K.Y. Lin, J.-P. Leburton, R.

I. Masel, ACS Nano 5 (2011) 153–158.[46] C.L. Li, Y.F. Chen, M.H. Liu, C.J. Lu, Sens. Actuators, B 169 (2012) 349–359.

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