fulerene 60 organic sensor

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Sensors and Actuators B 92 (2003) 243–254 Fullerene C60-cryptand coated surface acoustic wave quartz crystal sensor for organic vapors Hung-Bin Lin, Jeng-Shong Shih Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan Received 24 October 2002; received in revised form 2 January 2003; accepted 14 January 2003 Abstract A fullerene C60-cryptand 22 (C60-di-propylamine-cryptand 22) coated surface acoustic wave (SAW) quartz crystal gas detection system with 250 MHz oscillator and a computer interface for signal acquisition and data processing was prepared to detect various organic vapors. The C60-cryptand 22 coated SAW quartz crystal sensor exhibited sensitivity to both polar and nonpolar organic molecules. The C60-cryptand 22 was demonstrated as a better and more sensitive coating material than fullerene C60 or cryptand 22 for polar organic molecules. Comparison of C60-cryptand 22 coated SAW crystal and quartz crystal microbalance (QCM) sensors was also made and the SAW sensor exhibited much better response than the piezoelectric sensor for organic molecules with the same concentrations. The C60-cryptand 22 coated SAW crystal sensor could be repeatedly reused for the detection of most of organic vapors, e.g. alcohols, aldehydes, ketones, ethers, alkanes and alkenes. The selectivity of the C60-cryptand 22 coated SAW crystal sensor for various polar organic vapors seem to be in the order: alcohols > aldehydes > ketones > ethers. For nonpolar organic molecules, the C60-cryptand 22 coated SAW crystal sensor showed sensitivity for alkenes, e.g. hexene and benzene than for alkanes, 1-hexane and cyclohexane. Effects of molar mass, steric structures and isomers of organic molecules on the SAW frequency responses were also investigated. The SAW detection system showed the good detection limit of 0.1–3.0mg/ml for these organic molecules. The C60-cryptand 22 coated SAW crystal sensor was successfully applied as a quite sensitive gas chromatography (GC) detector for various organic compounds. The C60-cryptand 22 coated surface acoustic wave GC detector compared well with the commercial thermal conductivity detector (TCD) for various organic molecules, e.g. organic halides, alcohols, alkenes and ketones. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Surface acoustic wave (SAW) quartz crystal sensor; Organic gas sensor; Fullerene C60; C60-cryptand 22; GC detector 1. Introduction Piezoelectric crystals, e.g. quartz and LiNbO 3 , are well known to be sensitive to pressure on their surfaces [1]. In addition, the oscillating frequency of a piezoelectric crystal decreases on adsorption of a foreign substance onto its sur- face. The variation of oscillating frequency is proportional to the mass of foreign molecules deposited on the crystal surface and the basic frequency of the piezoelectric crys- tal. The theoretical detection limit of an oscillating quartz crystal has been reported to be as small as 10 12 g for for- eign molecules according to the Sauerbrey’s equation [2]: F =−2.3 × 10 6 × F 2 0 × M s /A, where F (Hz) is the frequency shift due to the coating of the foreign molecules on quartz crystal, F 0 (Hz) the vibrational frequency of un- coated quartz crystal, M s (g) the mass of deposited coating Corresponding author. Tel.: +886-2-29350749; fax: +886-2-29309077. E-mail address: [email protected] (J.-S. Shih). and A (cm 2 ) the area coated. Piezoelectric crystals with appropriate coating adsorbents can be used as sensitive and highly selective gas/liquid sensors [3–8] for organic, metal ions and biological species. However, most of these piezoelectric crystal sensors are the bulk piezoelectric crys- tal sensors in which the vibrational wave propagated in the quartz crystal, from one side to another side surface, and use the 10–30MHz AT-cut quartz crystals. According to Sauerbrey’s equation, the frequency response (F) can be increased with higher vibrational frequency crystals. However, higher frequency bulk piezoelectric bulk crystal (>100 MHz) is difficult to be prepared and not commer- cially available. Thus, the surface acoustic wave sensor with acoustic wave of 200–400 MHz in which the acoustic wave on the surface of the piezoelectric crystal, launched and received on the same surface of the crystal [9–13]. The SAW sensor has demonstrated to be superior to the traditional piezoelectric crystal bulk sensor always called as quartz crystal microbalance (QCM) and the detection limit of SAW sensor have been reported to be as small as 10 15 g 0925-4005/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-4005(03)00159-X

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Page 1: Fulerene 60 Organic Sensor

Sensors and Actuators B 92 (2003) 243–254

Fullerene C60-cryptand coated surface acoustic wavequartz crystal sensor for organic vapors

Hung-Bin Lin, Jeng-Shong Shih∗Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan

Received 24 October 2002; received in revised form 2 January 2003; accepted 14 January 2003

Abstract

A fullerene C60-cryptand 22 (C60-di-propylamine-cryptand 22) coated surface acoustic wave (SAW) quartz crystal gas detectionsystem with 250 MHz oscillator and a computer interface for signal acquisition and data processing was prepared to detect various organicvapors. The C60-cryptand 22 coated SAW quartz crystal sensor exhibited sensitivity to both polar and nonpolar organic molecules. TheC60-cryptand 22 was demonstrated as a better and more sensitive coating material than fullerene C60 or cryptand 22 for polar organicmolecules. Comparison of C60-cryptand 22 coated SAW crystal and quartz crystal microbalance (QCM) sensors was also made andthe SAW sensor exhibited much better response than the piezoelectric sensor for organic molecules with the same concentrations. TheC60-cryptand 22 coated SAW crystal sensor could be repeatedly reused for the detection of most of organic vapors, e.g. alcohols, aldehydes,ketones, ethers, alkanes and alkenes. The selectivity of the C60-cryptand 22 coated SAW crystal sensor for various polar organic vaporsseem to be in the order: alcohols> aldehydes> ketones> ethers. For nonpolar organic molecules, the C60-cryptand 22 coated SAWcrystal sensor showed sensitivity for alkenes, e.g. hexene and benzene than for alkanes, 1-hexane and cyclohexane. Effects of molar mass,steric structures and isomers of organic molecules on the SAW frequency responses were also investigated. The SAW detection systemshowed the good detection limit of 0.1–3.0 mg/ml for these organic molecules. The C60-cryptand 22 coated SAW crystal sensor wassuccessfully applied as a quite sensitive gas chromatography (GC) detector for various organic compounds. The C60-cryptand 22 coatedsurface acoustic wave GC detector compared well with the commercial thermal conductivity detector (TCD) for various organic molecules,e.g. organic halides, alcohols, alkenes and ketones.© 2003 Elsevier Science B.V. All rights reserved.

Keywords:Surface acoustic wave (SAW) quartz crystal sensor; Organic gas sensor; Fullerene C60; C60-cryptand 22; GC detector

1. Introduction

Piezoelectric crystals, e.g. quartz and LiNbO3, are wellknown to be sensitive to pressure on their surfaces[1]. Inaddition, the oscillating frequency of a piezoelectric crystaldecreases on adsorption of a foreign substance onto its sur-face. The variation of oscillating frequency is proportionalto the mass of foreign molecules deposited on the crystalsurface and the basic frequency of the piezoelectric crys-tal. The theoretical detection limit of an oscillating quartzcrystal has been reported to be as small as 10−12 g for for-eign molecules according to the Sauerbrey’s equation[2]:�F = −2.3 × 106 × F 2

0 × Ms/A, where�F (Hz) is thefrequency shift due to the coating of the foreign moleculeson quartz crystal,F0 (Hz) the vibrational frequency of un-coated quartz crystal,Ms (g) the mass of deposited coating

∗ Corresponding author. Tel.:+886-2-29350749;fax: +886-2-29309077.E-mail address:[email protected] (J.-S. Shih).

and A (cm2) the area coated. Piezoelectric crystals withappropriate coating adsorbents can be used as sensitiveand highly selective gas/liquid sensors[3–8] for organic,metal ions and biological species. However, most of thesepiezoelectric crystal sensors are the bulk piezoelectric crys-tal sensors in which the vibrational wave propagated inthe quartz crystal, from one side to another side surface,and use the 10–30 MHz AT-cut quartz crystals. Accordingto Sauerbrey’s equation, the frequency response (�F) canbe increased with higher vibrational frequency crystals.However, higher frequency bulk piezoelectric bulk crystal(>100 MHz) is difficult to be prepared and not commer-cially available. Thus, the surface acoustic wave sensorwith acoustic wave of 200–400 MHz in which the acousticwave on the surface of the piezoelectric crystal, launchedand received on the same surface of the crystal[9–13].The SAW sensor has demonstrated to be superior to thetraditional piezoelectric crystal bulk sensor always called asquartz crystal microbalance (QCM) and the detection limitof SAW sensor have been reported to be as small as 10−15 g

0925-4005/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0925-4005(03)00159-X

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[14–18]for analyte molecules. In this study, A C60-cryptand22 coated SAW crystal detection system was preparedand applied to detect various polar and nonpolar organicvapors.

Artificial macrocyclic polyethers, cryptands and crownethers, have demonstrated a remarkable complexing abilitynot only for metal ions[19,20], but also for polar organicspecies[21,22], e.g. amines, alcohols, aldehydes and ke-tones. Thus, in this study, cryptand 22 was used to adsorband detect the polar organic vapors. However, cryptand 22cannot adsorb nonpolar organic molecules, e.g. alkanes andalkenes. Fullerene C60 is a new allotropic form of carbonand its physical and chemical properties have recently re-ceived substantial attention[23–25]. A characteristic featureof fullerene is its affinity to various organic molecules, espe-cially nonpolar organic molecules. Therefore, in this study,fullerene C60-cryptand 22 was synthesized and applied asthe coating material on the SAW crystal to adsorb and detectvarious nonpolar and polar molecules.

2. Experimental

2.1. Preparation of C60-cryptand 22 coating material

The coating material fullerene C60-cryptand 22 (C60-di-propylamine-cryptand 22) was obtained from the reactionbetween 7,16-bis (3-aminopropyl)-1,4,10,13- tetraoxa-7,16-diaza cyclo-octadecane (di-propyl amine-cryptand 22) andfullerene C60. Di-propyl amine-cryptand 22 was synthe-sized as described by Chiou and Shih[7] and obtainedas liquid product by the reaction of cryptand 22 (0.26 g,1.00 mmol) with acetonitrile (5 mmol), followed by thereduction with borohydride and extraction with chloro-form. Fullerene-cryptand 22 was obtained as a precipitateby adding the liquid product di-propyl amine-cryptand 22and fullerene C60 (0.2 g, 0.276 mmol) to toluene (10 ml)and stirring at room temperature for 5 days. The solidproduct was cleaned with toluene and methanol severaltimes to remove unreacted fullerene C60 and di-propylamine-cryptand 22. The product was identified by FTIRwith an absorption peak at around 1148 cm−1 for the C–Nvibration.

2.2. SAW crystal coating

The SAW quartz crystal plates with a basic resonant fre-quency of 250 MHz and the size of 8.0 mm× 2.0 mm con-taining two-port IDTs (interdigital transducer) electrodes onone side of the SAW crystal plates that were obtained fromMicrosensor systems Co., USA. The SAW quartz crystalswere coated with C60-cryptand 22/PVC (10 mg/10 mg) in10 ml tetrahydrofuran (THF) solution by the spin coat-ing with a microsyringe. After evaporation of the sol-vent, C60-cryptand 22 coated SAW quartz crystals wereobtained.

Fig. 1. Fullerene C60-cryptand 22 coated 250 MHz surface acoustic (SAW)sensor.

2.3. Apparatus

A JASCO FTIR-5300 Fourier transform infrared (FTIR)was applied to identify synthesized C60-cryptand 22.

The experimental set-up of the SAW quartz crystal de-tection system included an oscillator with a C60-cryptand22 coated SAW quartz crystal obtained by Microsensor sys-tems Co., USA, a frequency counter (Lutron FC-2700 Tai-wan counter Co.), RS232 connected to a PC microcomputerand 500 ml glass working cell as shown inFigs. 1 and 2.A BASIC computer program was written for digital signalacquisition and data processing through RS232 to the PCmicrocomputer. The SAW-GC (gas chromatography) detec-tion system a GC-14A gas chromatograph with a thermalconductivity detector and a Porapak Q 80/100 GC columnfor separation of various organic molecules was also set upas shown inFig. 3.

Fig. 2. Experimental set-up of the C60-cryptand 22 coated SAW detectionsystem.

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Fig. 3. Schematic diagram of the GC-SAW detection system based onC60-cryptand 22.

3. Results and discussion

3.1. Frequency response of C60-cryptand 22 coated SAWcrystal sensor

The frequency responses of the C60-cryptand 22 coated250 MHz surface acoustic wave (SAW) crystal sensor for

Fig. 4. Frequency responses of the C60-cryptand 22 coated SAW sensor for methanol; (b) propyl aldehyde; (c) hexane and (d) propyl amine vapors.

various organic vapors, e.g. alcohols, ketones, aldehydes,amines, alkane and alkenes, were investigated. As shownin Fig. 4, the 250 MHz C60-cryptand 22 coated SAWcrystal sensor exhibited fairly good sensitivity for theseorganic vapors.Fig. 4 also reveals that the frequency of theC60-cryptand 22 coated SAW crystal sensor could be com-pletely reversed after introducing pure N2 for desorption forvarious organic molecules except amines. The irreversibleresponse for amines (Fig. 4d) may be attributed to probablechemical adsorption of amine molecules onto C60 of adsor-bent C60-cryptand 22 as reported in the literature[25] whilephysical adsorption is found in all other organic vapors. Thisimplies that the C60-cryptand 22 coated SAW crystal sensorcan be repeatedly reused for the detection of most organicvapors, e.g. alcohols, aldehydes, ketones, ethers, alkanes andalkenes.

3.2. Comparison between SAW and QCM sensors

It is well known that the piezoelectric crystals, e.g. quartzand LiNbO3, are applied as quite sensitive pressure sensorsin both quartz crystal microbalance (QCM) and surfaceacoustic wave (SAW) detection systems. The compari-son between SAW and QCM crystal sensors for ethanol

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Fig. 5. Comparison between frequency responses of SAW and piezoelectric (PZ) crystal sensors based on C60-cryptand 22 (0.4�g) coatings for ethanol.

vapor with C60-cryptand 22 coating onto piezoelectriccrystals was made. As shown inFig. 5, the C60-cryptand22 coated SAW crystal sensor exhibited a quite excellentsensitivity of approximately 750 Hz/(mg l−1) for ethanol;in contrast, not quite good frequency response was foundby using the C60-cryptand 22 coated QCM crystal sen-sor. This indicates that the C60-cryptand 22 coated SAWcrystal sensor is a much more sensitive tool for trace or-ganic vapor as compared with the traditional QCM sen-sors, although the noisy signal (ca.±10 Hz) of the SAWsensor is slightly greater than that of the QCM sensor(ca.±2 Hz).

Fig. 6. Effect of coating material on the 250 MHz SAW sensor for methanol (14 mg/l).

3.3. Coating material effect

The coating material effect on the frequency responseof the SAW sensor was investigated. Various coating ma-terials, e.g. C60, cryptand 22 and C60-cryptand 22 wereused as adsorbents onto the SAW crystal plate to adsorband detect methanol molecules. AsFig. 6 reveals, the SAWcrystal with C60-cryptand 22 coating for methanol ex-hibits a significantly better frequency response than C60and cryptand 22 coated SAW crystals. It is not surpris-ing for the least frequency shift of the C60 coated SAWcrystal, because fullerene (C60) basically is an aromatic

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Fig. 7. Effect of coating load of C60-cryptand 22 on the frequency response of the SAW sensor for ethanol (14 mg/l).

nonpolar molecule and cannot strongly adsorb polar or-ganic molecules such as methanol. The quite larger fre-quency shifts of the C60-cryptand 22 and cryptand 22coated SAW crystals may be due to hydrogen bondingformation between the macrocyclic polyether cryptand 22and alcohol molecules. However, the enhancement of thefrequency response by the attachment of C60 to cryptand22 for C60-cryptand 22 may be attributed to the induc-tion of the C60 molecule with 60� electrons to cryptand22 which results in the strong adsorption of methanolonto the C60-cryptand 22 molecule and large frequency

Fig. 8. Frequency responses for (A) propanol; (B) propyl aldehyde and (C) acetone with the C60-cryptand 22 (0.4�g) coated SAW sensor.

response of the C60-cryptand 22 coated SAW crystalsensor.

3.4. Coating load effect

The effect of the amount of C60-cryptand 22 coating onthe frequency response of the SAW crystal gas sensor wasalso investigated for ethanol, as shown inFig. 7. The SAWcrystal with a thicker C60-cryptand 22 coating exhibiteda larger frequency response for the same concentration(14 mg/l) of ethanol. However, the frequency response

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Fig. 9. Molecular mass effect of various alcohols on the frequency response of the C60-cryptand 22 coated SAW sensor: (A) butanol; (B) propanol; (C)ethanol and (D) methanol.

apparently tends to level off with larger amounts (>0.4�g)of C60-cryptand 22 coating. This behavior implies thatthere is a limit of the adsorption of gas molecules in a smallarea of adsorbent and an excessive coating is unnecessaryfor a SAW sensor.

3.5. Frequency responses for polar organic vapors

The selectivity of the C60-cryptand 22-coated SAWcrystal sensor for various polar organic vapors was also

Fig. 10. Steric effect of butanol isomers on frequency response of the C60-cryptand 22 coated SAW sensor: (A) 1-butanol; (B) 2-butanol and (C)tert-butanol.

investigated. Various polar-organic molecules contain-ing the propyl group, e.g. propanol, propyl aldehyde andacetone, were detected via the C60-cryptand 22-coatedSAW crystal sensor. The frequency responses of theC60-cryptand 22-coated SAW crystal sensor for these or-ganic vapors appear to be in the following order (Fig. 8):propanol� propyl aldehyde> acetone. This result may beattributed to the formation of the stronger hydrogen bonds,ROH· · · O(cryptand 22) or ROH· · · N (cryptand 22), be-tween propanol and cryptand 22 than hydrogen bond

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Fig. 11. Isomer effect on frequency response of the C60-cryptand 22 coated SAW sensor for (A)n-butanol and (B) diethyl ether.

ROH· · · HN (cryptand 22) between propyl aldehyde andcryptand 22, as well as the fact that there is no hydrogenbond formation between acetone and cryptand 22.

3.6. Effect of molar mass of gas molecules

Various organic alcohols, e.g. methanol, ethanol, propanoland butanol, were detected by the C60-cryptand 22-coatedSAW crystal sensor. As clearly shown inFig. 9, the fre-quency shifts of the C60-cryptand 22-coated SAW crystal

Fig. 12. Frequency responses of the C60-cryptand 22 coated SAW sensor for (A) hexene and (B) hexane.

sensor for these linear alcohols are, apparently, in the fol-lowing order: butanol> propanol> ethanol> methanol.This result seems to indicate that a larger molar mass al-cohol has a greater frequency response, which is consistentwith the findings in general reports for piezoelectric detec-tors [26]. It is reasonable to assume that, if the number ofadsorbed molecules on an adsorbent is limited and is thesame for various adsorbates, a greater molar mass of adsor-bate would certainly lead to a larger frequency shift than inthe case of a smaller molecule.

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3.7. Steric and isomer effects

The steric effect of organic isomers such as butanol iso-mers was also investigated. The C60-cryptand 22-coatedSAW crystal sensor for various butanol isomers was ob-served to be in the following order: 1-butanol> 2-butanol>tert-butanol (Fig. 10). This result may be attributed to thegreater steric hindrance oftert- and 2-butanols than thatof 1-butanol. This result seem to imply that a linear or-ganic molecule like butanol has the smallest steric hindranceand the strongest adsorption on C60-cryptand 22 adsorbent,resulting in the best frequency response. Furthermore, thelinear organic amine can be expected to be tightly, hori-zontally, adsorbed on the C60-cryptand 22 adsorbent sur-face, exhibiting a more effective pressure on the surfaceof the quartz crystal. This causes a greater frequency re-sponse than the partial vertical adsorption of the branchedisomers onto C60-cryptand 22 of the SAW crystal sen-sor. Isomer effect of butanol and ether with same molarmass on the frequency response of the C60-cryptand 22coated SAW crystal sensor was also studied. As shown inFig. 11, much better frequency response for 1-butanol thanthat of diethyl ether was found, which may be attributedto the formation the hydrogen bonding between 1-butanoland C60-cryptand 22; in contrast, no possible hydrogenbonding was found between diethyl ether and C60-cryptand22.

3.8. Frequency responses of non-polar organic vapors

Because fullerene (C60) on C60-cryptand 22 adsor-bent of the SAW sensor basically is an aromatic nonpolargroup and therefore can be expected to adsorb and detectnon-polar organic molecules such as 1-hexene and hexane.As shown inFig. 12, the C60-cryptand 22 SAW crystalsensor exhibited quite sensitive response to 1-hexene. Fur-thermore, much better frequency response for 1-hexenethan that of hexane was observed as shown inFig. 12.This result may be attributed to the�–� interaction be-tween 1-hexene and C60, in contrast, no�–� interac-tion between C60 and saturated hydrocarbons such ashexane.

3.9. Detection limit of the SAW sensor for organic vapors

The detection limit of the C60-cryptand 22 coated SAWcrystal sensor for various organic vapors was also studied.IUPAC [27] recommends that the detection limit of a detec-tor (CL) can be estimated as follows:

CL = ksBm

(1)

where sB is a standard deviation of the blank signal us-ing factor 20 measurements,m the analytical sensitivitywhich can be estimated as the slope of the working curveas shown inFig. 8, and k the numerical factor chosen in

Table 1Detection limits of various organic vapors with the C60-cryptand 22coated SAW sensor

Organic vapors Detectionlimit (mg/l)

Organicvapors

Detectionlimit (mg/l)

Methanol 0.80 Diethyl ether 3.60Ethanol 0.70 Acetone 2.60n-Propanol 0.48 Propionaldehyde 0.85n-Butanol 0.25 Hexane 2.30iso-Butanol 0.27 Hexene 0.80tert-Butanol 0.70 – –

accordance with the confidence level desired. As suggestedby Long and Winefordner[28], the use ofk = 3 allowsa confidence level of 99.86% for a normal distribution ofthe blank signal. For example, the detection limit (CL) formethanol can be estimated to be approximately 0.80 mg/lfrom the analytical sensitivity (m) of 73.2 Hz/(mg ml−1)and the standard deviations of the blank signal (sB) of19.5 Hz withk = 3. The detection limits of C60-cryptand22 SAW crystal sensor for various organic vapors are for-mulated in Table 1. The C60-cryptand 22 coated SAWcrystal sensor obviously exhibited quite good sensitivitywith the detection limits of approximately 0.2–3.0 mg/l forvarious polar organic vapors, e.g. alcohols, acetone, ether,aldehyde, and non-polar organic vapors, e.g. hexane andhexane.

3.10. Performance of GC-SAW detector

The C60-cryptand 22 coated SAW crystal sensor was alsoapplied as a gas chromatographic (GC) detector (denotedas GC-SAW detector) to detect polar and nonpolar organicmolecules. The C60-cryptand 22 coated SAW detector wasconnected on line with a gas chromatograph as shown inFig. 3. In this study, various organic molecules were sepa-rated using a Porpak packed GC column, and then were de-tected with the C60-cryptand 22 coated GC-SAW detector.As shown inFig. 13, water and various organic molecules,e.g. ethanol, acetone, chloroform, ethyl ether and toluene,are separated with the Porapak GC column, and subse-quently detected with the C60-cryptand 22 coated GC-SAWdetector and a thermal conductivity detector (TCD). It canbe found fromFig. 13B, that the SAW detector gives quitegood responses with good sensitivity and good line shapesfor these organic compounds. Compared with the thermalconductivity detector, the C60-cryptand 22 coated SAWdetector seems to compare well with the more expensivecommercial thermal conductivity detector for these organicmolecules. The GC-SAW detector also was found to givegood sensitivity and good line shapes for some non-polarorganic molecules, e.g.n-hexane, cyclohexane and benzene(Fig. 14).

The frequency response of the C60-cryptand 22 coatedGC-SAW detector as a function of the concentration

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Fig. 13. Comparison of responses of water and various organic molecules with thermal conductivity detector (TCD) (A) and surface acoustic wave (SAW)GC detectors (B): (1) water, (2) ethanol, (3) acetone, (4) chloroform, (5) ethyl ether, (6) toluene.

Fig. 14. Frequency responses of the C60-cryptand 22 coated SAW gas chromatographic detector forn-hexane, cyclohexane and benzene.

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Fig. 15. Effect of concentration of acetone on (a) gas chromatogram and (b) the response of C60-cryptand 22 coated SAW gas chromatographic detector.

of the analyte, e.g. acetone, was found (Fig. 15a). TheC60-cryptand 22 coated GC-SAW detector seems to exhibita linear response to the amount of the analyte (Fig. 15b).This result implies that the organic compounds can bequantitatively detected with the C60-cryptand 22 coatedGC-SAW detector.

3.11. Effects of flow rate and reproducibility of theGC-SAW detector

The flow rate effect of the carrier gas (N2) on the fre-quency response of the C60-cryptand 22 coated GC-SAWdetector for organic molecules C60-cryptand 22 coatedGC-SAW detector was also examined. The increase in flowrate of the carrier gas was found to cause a decrease in the

frequency response of the GC-SAW detector for acetoneas shown inFig. 16. It is reasonable to assume that theslower flow rate easily leads to relatively complete equilib-rium in the detector cell and maximum adsorption of theanalyte molecules by the adsorbent C60-cryptand 22 onSAW crystal, resulting in a better frequency response of theC60-cryptand 22 coated GC-SAW detector.

The reproducibility of the C60-cryptand 22 coatedGC-SAW detector was also investigated with a series of 20repetitive injections of acetone into the gas chromatographwith the SAW detector. The response of the C60-cryptand22 coated GC-SAW detector exhibited a good reproducibil-ity with a relative standard deviation (R.S.D.) of 4.2% (Fig.17). The C60-cryptand 22 coated GC-SAW detector alsodemonstrated fast response and rapid return to the baseline.

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Fig. 16. Effect of flow rate of GC carrier gas N2 on the C60-cryptand 22 coated GC-SAW detector for acetone.

Fig. 17. Reproducibility of the response of C60-cryptand 22 coated SAW gas chromatographic detector for acetone with 20 repetitive injections.

4. Conclusion

In conclusion, the fullerene C60-cryptand 22 coated SAWcrystal sensor can be employed as a sensitive gas sensorfor various organic vapors and exhibits much more sensitivetool for trace organic vapor as compared with the traditionalQCM sensors. The C60-cryptand 22 coated SAW crystalsensor also could be successfully applied as a quite sen-sitive gas chromatographic (GC-SAW) detector for variouspolar and nonpolar organic compounds. The C60-cryptand22 coated GC-SAW crystal detector seems to compare well

with the more expensive commercial conductivity detectorfor various organic molecules. The fullerene C60-cryptand22 coated GC-SAW crystal detector also exhibited fast re-sponse and high reproducibility.

Acknowledgements

The authors would like to express their gratitude to theNational Science Council of the Republic of China in Taiwanfor the financial support.

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Biographies

Jeng-Shong Shihgraduated from the National Taiwan Normal Universitywith a BS degree in Chemistry in 1967, the National Ching-Hwa Uni-versity with a MS degree in Inorganic Chemistry in 1970 and receivedhis PhD degree in Analytical Chemistry from Michigan State Univer-sity, USA, in 1978. He is currently a professor with the Department ofChemistry, National Taiwan Normal University. His research interests in-clude chemical sensors, catalysts, surfactants, fullerenes and macrocyclicpolyethers.

Hung-Bin Lin received his MS degree in Analytical Chemistry fromNational Taiwan Normal University in July 2002. His research interest isthe development of various SAW sensors.