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Piezoelectric crystal membrane chemical sensors based on fullerene C60
Sensors and Actuators B 76 (2001) 347±353
Piezoelectric crystal membrane chemical sensors based on fullerene C60 Jeng-Shong Shih*, Yun-Ching Chao, Mine-Fag Sung, Guann-Jou Gau, Chyow-San Chiou Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan
Abstract Various reusable and sensitive piezoelectric (PZ) quartz crystal membrane sensors with home-made computer interfaces for signal acquisition and data processing were developed to detect organic/inorganic vapors and organic/inorganic species in solutions, respectively. Fullerene C60 and synthesized fullerene derivatives, e.g. C60-cryptand22 and C60-dibenzo-16-crown-5 were applied as coating materials on quartz crystals of the PZ crystal sensors. The oscillating frequency of the quartz crystal decreased due to the adsorption of organic or inorganic species onto coating material molecules on the crystal surface. The C60-coated PZ crystal sensor was ®rstly prepared to study the adsorption and the interaction between fullerene C60 and organic molecules. Only amines, dithios, alkynes and 1,3-dienes seem to perform chemisorption onto fullerene while physical adsorption is found in all other cases like alcohols, aldehydes and carboxylic acids. The reactivities of fullerene C60 and other conjugated ole®ns to various organic molecules seem to be in the order b-carotene > C60 > pyrene. The fullerene C60-coated PZ crystal sensor also can be employed to sensitively detect some organic molecules, e.g. aldehydes, carboxylic acids, amines and ole®ns. A C60-cryptand22-coated PZ gas sensor for both polar/nonpolar organic vapors was also prepared. The cryptand22-coated PZ gas sensor can be employed as a GC detector for organic molecules. The C60-cryptand22-coated crystal GC-PZ detector compared well with the commercial thermal conductivity detector (TCD). Various liquid PZ crystal sensor based on C60cryptand22 and dibenzo-16-crown-5-C60 were also prepared as liquid chromatographic (LC) detectors for metal ions, anions and polar/ nonpolar organic molecules in solutions. The frequency responses of the C60-cryptand22-coated LC-PZ detectors for various organic molecules and metal ions were compared well with the responses of a commercial UV±VIS detector and a conductivity detector which are generally used in LC systems. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Piezoelectric crystal chemical sensors; Fullerene C60; Gas sensors; Organic/inorganic liquid sensors
1. Introduction Piezoelectric (PZ) crystals, e.g. quartz, BaTiO3 and PLZT(Pb, La, ZrOx), are well known to be quite sensitive to pressure and any mass change on their surfaces and can be applied as microbalances for trace quantitative analysis [1]. Any increase in mass or pressure on the surface of the crystal will result in a decrease in the vibrational frequency of an oscillating PZ crystal. The variation of vibrational frequency, DF related to the mass, M (in g) of coating materials or absorbated foreign substance onto the crystal surface with area, A (in cm2) and fundamental frequency, F0 of the PZ crystal can be evaluated from Sauerbrey's equation [2] M 6 DF 2:3 10 A The theoretical detection limit 4 for absorbed foreign substances on quartz crystals is reported to be as small as *
Corresponding author. Tel.: 886-2-29350749; fax: 886-2-29309077. E-mail address: [email protected] (J.-S. Shih).
10 12 g. Therefore, the PZ crystals with appropriate adsorbents can be applied as quite sensitive gas [3±8] and liquid [9,10] sensors for adsorbing particular molecules. Owing to their high mass sensitivity, the PZ crystals can also be employed as a sensitive sensor to study the adsorption and interaction between an adsorbent on the crystal surface and various adsorbed compounds. It seems that the PZ crystal can be employed as a sensitive probe with appropriate adsorbents for various organic or inorganic compounds. Fullerene C60 is a new allotropic form of carbon [11,12], the study of fullerene's physical and chemical properties has recently received a lot of attention [13±17]. A characteristic feature of fullerene is its af®nity to various organic molecules. Fullerene C60 with 60 p-electrons potentially can be expected to be applied as a good adsorbent to adsorb and detect nonpolar and some polar organic molecules. However, fullerene cannot adsorb metal ions, anions and most polar organic species. Macrocyclic polyethers, e.g. crown ethers [18±20] and cryptands [21], have demonstrated remarkable complexing abilities to metal ions [19±21], anions [22] and polar organic molecules [23,24]. However,
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J.-S. Shih et al. / Sensors and Actuators B 76 (2001) 347±353
most crown ethers and cryptands can be dissolved in water, thus, naked crown ethers and cryptands cannot used as adsorbents on PZ crystals to detect various organic and inorganic molecules in liquid solutions. Therefore, in the present work, fullerene and fullerene derivatives, e.g. C60-cryptand22 and C60-dibenzo-16-crown-5-coated quartz crystals were prepared and applied to adsorb and detect various polar/nonpolar organic molecules and metal ions.
2.4. Reagents
2. Experimental
The fullerene C60-coated PZ quartz membrane crystal liquid sensor was set up and employed to detect various polar organic molecules in solutions, e.g. hexylamine, hexanoic acid, 2-hexanone and hexanal. As shown in Fig. 1a, the frequency shifts of the C60-coated PZ crystal liquid sensor for these organic molecules are in the order
2.1. Preparation of fullerene derivatives Fullerene-cryptand22 was obtained as a precipitate by addition of fullerene C60 (1.0 g, 1.38 mmol) and cryptand22 (0.36 g, 1.38 mmol) in anhydrous toluene (250 ml) and stirring at room temperature for 72 h. C60-dibenzo-16crown-5-oxyacetic acid was synthesized by the reaction of sym-dibenzo-16-crown-5-oxyacetic acid (56 mmol) and fullerene (C60, 56 mmol) in toluene for 72 h [25,26]. These products were identi®ed by FTIR and 13 C NMR.
Fullerene (C60) of >99% purity was obtained from Strem Chemicals (USA). All other chemicals used were of analytical grade. 3. Results and discussion 3.1. Fullerene C60-coated PZ crystal sensors
2.2. Crystal coating The PZ crystals used were AT-cut spherical quartz crystals, of radius 4.0 mm and thickness 0.18 mm with a basic resonant frequency of 10 MHz and provided with a silverplated metal electrodes on both the sides that were obtained from Taiwan Crystal Co. Both sides of the crystals were coated with an adsorbent, e.g. either C60 or C60-cryptand22/PVC (10 mg/10 mg) in 10 ml tetrahydrofuran (THF) solution via the spin coating with a microsyringe. After evaporation of the solvent, fullerene-coated PZ crystals were obtained. 2.3. Apparatus The experimental set-ups of PZ quartz gas/liquid crystal detection systems included oscillators, working cell with quartz crystals and assembled home-made computer interfaces. The oscillating frequency (F) of the C60 or C60 derivative-coated PZ crystal was converted to an electric potential (V) signal using an F/V converter TC 9400 chip. The voltage signal was processed with a 12-bit ADC (analog to digital converter) connected to a programming peripheral interface (PPI 8255) and microcomputer (PC/AT) for signal acquisition and data processing with a Basic computer program. Both JASCO FTIR-5300 Fourier transform infra-red (FTIR) and JASCO MX400 Fourier transform NMR spectrometries were applied to identify the synthesized various fullerene derivatives. A Perkin-Elmer 2400 elemental analyzer was also employed for elemental analysis of the fullerene derivatives.
Fig. 1. Frequency responses of the C60-coated (a) LC-PZ detector for polar organic molecules and (b) GC-PZ detector for nonpolar organic molecules.
J.-S. Shih et al. / Sensors and Actuators B 76 (2001) 347±353
hexylamine > hexanoic acid > 2-hexanone > hexanal. This result seems to imply that the high polar organic molecules with easily exchangeable protons such as hexylamine and hexanoic acid are more easily adsorbed onto fullerene molecule than less polar and without exchangeable proton molecules such as 2-hexanone and hexanal. Like alkenes, fullerene with 30 p-bonds may undergo nucleophilic and electrophilic attacks. Therefore, both nucleophilic addition of hexylamine to fullerene and electrophilic attack of hexanoic acid to fullerene can be expected. Fullerene C60 with 60 p-electrons can be expected to adsorb nonpolar organic molecules with p±p interaction. As expected, C60 molecule can adsorb hexene and hexyne with frequency responses in the order hexyne > hexene > hexane (Fig. 1b). The adsorption of organic molecules onto fullerene molecules could be either physical adsorption or chemical adsorption. The desorption study was performed to determine whether the adsorption is chemical or physical. As shown in Fig. 2, the frequency of the fullerene-coated crystal for hexylamine can only be partially reversed after introducing pure water. The partial irreversibility for hexamine seems to mean that the adsorption may be partial chemisorption or strong physical adsorption on inner layers of C60. Poorer reversibility was also found for hexyne. Infrared (IR) spectrum can be employed to infer the structure of the fullerene products obtained after adsorbing the organic molecules. For the C60-hexyne incorporated species, the peaks at 3441 cm 1 (C60±H), 1623 cm 1 (C=C), 1413 cm 1 (=C±H), and no 2100 cm 1 (CBC of hexyne), 574 and 526 cm 1 (C60) are found. It seems to indicate that the adsorption of hexyne onto C60 molecule may be partial chemisorption. The chemisorption also can be found in the cases of amines, dithios and dienes onto C60 molecules. Comparison of reactivities of C60 and other conjugated ole®ns, e.g. b-carotene and pyrene, to various organic molecules was also made. The frequency shifts of these
Fig. 2. Irreversible response curve of hexylamine with 4 mg C60-coated liquid piezoelectric detector.
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Fig. 3. Response curves of (a) propanol and (b) hexene with 6 mg C60-, bcarotene- and pyrene-coated piezoelectric gas crystal detector.
conjugated ole®ns-coated PZ crystal gas sensors for various polar/nonpolar organic molecules, such as propanol and hexene, were found to be in the order (Fig. 3) b-carotene > C60 > pyrene. This result seems to imply that the reactivity of C60 to organic molecules is greater than other aromatic ole®ns, e.g. pyrene and less that of linear conjugated ole®ns, e.g. b-carotene. The fullerene C60-coated PZ crystal gas sensor was also set up and employed to study the interaction between C60 and some inorganic vapors, e.g. ozone, HCl and HNO3. As shown in Fig. 4a, frequency of the C60-coated PZ crystal gas sensor shows increase after the adsorption of ozone molecule. The decomposition of fullerene C60 into small pieces after oxidation by ozone was reported in [27] which can lead to the decreased mass onto the PZ crystal and results in the decrease in the frequency of the C60-coated PZ crystal gas sensor. The oxidation of C60 also can be con®rmed by the
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Fig. 5. Frequency responses of n-propanol, propanal, propanon, hexane, hexene and hexyne with C60-cryptand22 and C60-coated GC-PZ crystal detectors.
Fig. 4. Response curves of (a) ozone in the air and (b) propanol under air And ozone with C60-coated piezoelectric gas detector.
irreversible response (Fig. 4a) after introducing the clean air and infrared spectrum of C60 after adsorbing ozone molecule with epoxide (C±O) peaks at 1030 and 1118 cm 1. After reacting with ozone molecule, fullerene C60 exhibited greater adsorbing ability to organic species, e.g. propanol, than original C60 molecule (Fig. 4b). This result may be attributed to the increased polarity of fullerene molecule after reacting with ozone molecule. Suzuki et al. [28] also reported that the electrophilic property of fullerene C60 increased after the oxidation of C60 with ozone. The irreversible response and the change in IR peaks were also found in the study on the interaction between C60 and HCl or HNO3 by using the C60-coated PZ crystal gas sensor. The reactivity of HNO3 toward C60 molecule seems greater than HCl. 3.2. C60-macrocyclic polyether-coated PZ crystal sensors Although fullerene C60 can adsorb some organic molecules, C60 is not a quite good adsorbent for polar organic
molecules and cannot absorb metal ions, anions and biologic species. Therefore, in this study, some fullerene C60-macrocyclic polyether derivatives, e.g. C60-cryptand22 and C60dibenzo-16-crown-5 were synthesized and applied as adsorbents to adsorb and detect nonpolar/polar organic molecules, metal ions and anions. The fullerene C60-cryptand22-coated PZ crystal gas sensor was developed and applied as a gas chromatographic (GC) detector to detect various polar and nonpolar organic molecules. The C60-cryptand22-coated PZ crystal GC-PZ detector (Fig. 5) exhibited more sensitivity to polar organic molecules, e.g. propanol, propanal, propanone, than pure fullerene C60-coated PZ crystal detector. This result may be attributed to the complexing ability of cryptand22 of C60-cryptand22 to polar organic molecules [24]. Furthermore, by comparing the frequency response peaks with C60-cryptand22-coated and C60-coated GC-PZ detectors, the response of C60-cryptand22-coatedGC-PZ detector shows better peak shape and narrower peak width than that of the C60-coated GC-PZ detector. In addition, the C60-cryptand22-coatedGC-PZ detector also exhibited better sensitivity to nonpolar organic molecules (Fig. 5), e.g. hexane, hexene and hexyne, than the C60-coated GC-PZ detector. The enhancement effect of cryptand22 attached to fullerene C60 molecule for the adsorption of nonpolar organic molecule may be attributed to the destruction of the conjugated structure of fullerene C60 and to the increased reactivity of C60 molecule after attaching a foreign molecule such as cryptand22. The performance of the C60-cryptand22-coated GC-PZ detector was also compared with the response of the commercial gas chromatographic detector such as thermal conductivity detector (TCD). As shown in Fig. 6, the C60cryptand22-coated PZ crystal detector seems to compare well with the commercial TCD for these organic molecules
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Fig. 6. Comparison of gas chromatograms with (A) C60-cryptand22coated piezoelectric (PZ) detector and (B) thermal conductivity detector (TCD) for various alcohols: (1) water, (2) sec-propanol, (3) n-propanol, (4) sec-butanol and (5) n-butanol.
such as alcohols. The better peak shapes and narrower haftway peak width (o1/2) by using the C60-cryptand22-coated GC-PZ detector than that of the TCD detector obviously can be found. Furthermore, the home-made PZ-GC detector is less expensive than the TCD or another commercial GC detector. In addition, the C60-cryptand22-coated GC-PZ detector also showed good linear response to the concentration of these organic species, good detection limit of about 3.7 mg for n-butanol and good reproducibility with R.S.D. of about 3% for methanol. The C60-cryptand22-coated PZ crystal sensor also can be used as a liquid chromatographic (LC) detector not only for organic molecules but also for the metal ions and anions in solutions. The C60-cryptand22-coated LC-PZ detector has demonstrated as a quite sensitive detector for various organic species, e.g. alcohols, amines and carboxylic acids, in solutions which is shown in Fig. 7. The frequency responses of the C60-cryptand22-coated LC-PZ detector for organic molecules compared well with the responses of a commercial UV±VIS detector which is generally used in a LC. Fig. 7 indicates that all alcohols can be responded by the C60-cryptand22-coated LC-PZ detector, but some nonUV±VIS absorbable alcohols, e.g. propanol, butanol and hexanol, were not detected by the commercial UV±VIS detector for LC. Like other macrocyclic polyethers, cryptands have demonstrated a remarkable complexing ability for various metal ions [21]. Thus, the C60-cryptand22-coated LC-PZ
Fig. 7. Comparison of liquid chromatograms with (a) commercial UV± VIS detector and (b) C60-cryptand22-coated LC-PZ detector for various alcohols: (1) propanol, (2) butanol, (3) hexanol, (4) phenol, (5) 4-chlorophenol and (6) 4-methoxy-phenol.
detector can also be employed to detect the metal ions in aqueous solutions. The effect of the amount of C60-cryptand22 coating on the frequency response of the quartz crystal detector was investigated. The crystal with more C60-cryptand22 coating exhibited a larger frequency shift for the same concentration of the K ions as shown in Fig. 8a. The cryptand22 of C60-cryptand22 not only can form complexes with cations, but also can form stable complexes with anions [22] when the cryptand is protonated as cryptand-NH in acidic solution (pH < 6). It means that the C60-cryptand22-coated LC-PZ detector is a switch-type multifunctional detector which can be used as a cationic detector at pH 7 and as an anionic detector at pH 6. As expected, the C60-cryptand22-coated LC-PZ detector can be employed to detect various carboxylic anions, e.g. HCOO , CH3COOH , CH3CH2COO and CH3(CH2)2 COO , as shown in Fig. 8b. As expected, effect of pH on the frequency response of the C60-cryptand22coated LC-PZ detector for ions was also observed in this study. The performance of the C60-cryptand22-coated LCPZ detector for various ions also compared well with the response of the conductivity detector which is generally
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Acknowledgements The authors would like to express their appreciation to the National Science Council of the Republic of China in Taiwan for ®nancial support. References
Fig. 8. Frequency responses of C60-cryptand22-coated LC-PZ detector for K ions (10 3 M potassium hydrogen phosphate) at pH 7 and various carboxylate anions: (1) HCOO , (2) CH3COO , (3) CH3CH2COO , (4) CH3(CH2)2COO (potassium salts) at pH 3.
used to detect various ions in LC. C60-dibenzo-16-crown-5 was also synthesized [25,26] in our laboratory and applied as a quite sensitive adsorbent for polar/nonpolar organic molecules and metal ions. Similarly, the C60dibenzo-16-crown-5-coated PZ crystal GC-PZ detector showed more sensitivity to polar as well as nonpolar organic molecules than pure fullerene C60-coated PZ crystal detector. 4. Conclusion In conclusion, the fullerene C60 and C60-macrocyclic polyethers, e.g. C60-cryptand22-coated PZ quartz crystal sensors can be applied to detect various organic molecules, metal ions and anions with good sensitivity, good detection limit and reproducibility.
[1] G.G. Guilbault, S.S. Lu, A.W. Czanderna, Application of Piezoelectric Quartz Crystal Microbalances, Elsevier, New York, 1984. [2] G. Sauerbrey, The use of quartz oscillators for weighing thin layers and for microweighing, Z. Phys. 155 (1959) 206±222. [3] W.H. King Jr., Piezoelectric sorption detector, Anal. Chem. 36 (1964) 1735±1738. [4] J. Hlavay, G.G. Guilbault, Applications of the piezoelectric crystal detector in analytical chemistry, Anal. Chem. 49 (1977) 1890±1896. [5] Y. Tomita, G.G. Guilbault, Coating for a piezoelectric crystal sensitive to organophosphorus pesticides, Anal. Chem. 52 (1980) 1484±1489. [6] J. Janata, A. Bezegh, Chemical sensors, Anal. Chem. 60 (1988) 62R±74R. [7] W. Gopel, J. Hesse, J.N. Zemel, Sensors: A Comprehensive Survey, VCH, Weinheim, New York, 1989. [8] C.J. Lu, J.S. Shih, Detection of polar organic vapors with coated piezoelectric crystals based on crown ethers, Anal. Chem. Acta 306 (1995) 129±137. [9] J. Rickert, A. Brecht, W. Gopel, QCM operation in liquids: constant sensitivity during formation of extended protein multilayers by affinity, Anal. Chem. 69 (1997) 1441±1448. [10] Y.S. Jane, J.S. Shih, Application of crown ether-coated piezoelectric crystal detector for ion chromatography, Analyst 120 (1995) 517±522. [11] H.W. Kroto, J.R. Heath, S.C. O'Brien, R.F. Curl, R.E. Smalley, C60: Buckminsterfullerene, Nature 318 (1985) 162±163. [12] W. Kraetschmer, L.D. Lamb, K. Fostiropoules, D.R. Huffman, Solid: C60, a new form of carbon, Nature 347 (1990) 354±358. [13] W.A. Scrivens, P.V. Bedworth, J.M. Tour, Purification of gram quantities of C60, a new inexpensive and facile method, J. Am. Chem. Soc. 114 (1992) 7917±7919. [14] R.E. Haufler, R.E. Smalley, Efficient production of C60 (Buckminster fullerene), C60H36 and the solvated buckide ion, J. Phys. Chem. 94 (1990) 8634±8636. [15] J.M. Hawkins, A. Meyer, T.A. Lewis, S. Loren, F.J. Hollander, Crystal structure of osmylated C60: confirmation of the soccer ball framework, Science 252 (1991) 312±313. [16] J. Chai, R.E. Smalley, Fullerenes with metals inside, J. Phys. Chem. 95 (1991) 7564±7568. [17] Q. Xie, E. Perez-Cordero, L. Echegoyen, Electrochemical detector of C606 and C706 : enhanced stability of fullerides in solution, J. Am. Chem. Soc. 114 (1992) 3978±3980. [18] C.J. Pedersen, Cyclic polyethers and their complexes with metal salts, J. Am. Chem. Soc. 89 (1967) 7017±7036. [19] L.M. Kolthoff, Application of macrocyclic compounds in chemical analysis, Anal. Chem. 51 (1979) 1R±22R. [20] J.S. Shih, Application of macrocyclic polyethers, J. Chin. Chem. Soc. 39 (1992) 551±559. [21] J.M. Lehn, J.P. Sauvage, Cryptates: stability and selectivity of alkali and alkaline ether macrobicyclic complexes, J. Am. Chem. Soc. 97 (1975) 6700±6707. [22] S.S. Tsai, J.S. Shih, Application of the bifunctional switch-type cryptand ion chromatographic stationary phase for ion separation, Sep. Sci. Technol. 33 (1998) 1407±1421. [23] D.J. Cram, Application of exchanger with crown ethers as anchor group in analytical and preparative chemistry, J. Chromatogr. 167 (1978) 307±320.
J.-S. Shih et al. / Sensors and Actuators B 76 (2001) 347±353 [24] C.S. Chiou, J.S. Shih, Piezoelectric cryptand-coated piezoelectric quartz crystal liquid chromatographic detector for cations, anions and organic molecules, Anal. Chim. Acta 392 (1999) 125±133. [25] C.H. Hung, Preparation and application of crown ether-fullerene piezoelectric crystal liquid chromatographic detector, M.S. Thesis, National Taiwan Normal University, Taiwan, 1996. [26] C.H. Hung, J.S. Shih, Fullerene-crown ether-coated piezoelectric crystal liquid chromatographic detector for metal ions and polar/ nonpolar organic molecules, J. Chin. Chem. Soc. 47 (2000) 1095±1104. [27] R. Taylor, J.P. Parsons, A.G. Avent, S.P. Rannard, T.J. Dennis, J.P. Hare, H.W. Kroto, D.R.M. Walton, Degradation of C60 by light, Nature 351 (1991) 277. [28] T. Suzuki, Y. Maruyama, T. Akasaka, W. Ando, K. Kobayashi, S. Naggase, Redox properties of organofullerenes, J. Am. Chem. Soc. 116 (1994) 1359±1363.
Biographies Jeng-Shong Shih received his BSc degree in Chemistry from National Taiwan Normal University, in 1967, MSc degree in Inorganic Chemistry from National Ching-Hwa University, in 1970 and PhD degree in
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Analytical Chemistry from Michigan State University, USA, in 1978. He is currently a professor in the Department of Chemistry, National Taiwan Normal University. His research interests include chemical sensors, catalysts, fullerenes and macrocyclic polyethers. Yun-Ching Chao received her MSc degree in Analytical Chemistry from National Taiwan Normal University, in 1996. Her research interest is the interaction between Fullerene C60 and organic molecules and ozone. Mine-Fag Sung received his MSc degree in Analytical Chemistry from National Taiwan Normal University, in 1997. His research interest is the comparison of reactivities between Fullerene C60 and other conjugated olefins. Guann-Jou Gau received his MSc degree in Analytical Chemistry from National Taiwan Normal University, in 1997. His research interest is the development of C60-cryptand22-coated piezoelectric crystal gas sensors for organic vapors. Chyow-San Chiou received his PhD degree in Analytical Chemistry from National Taiwan Normal University, in 1998. His research interest is the development of C60-cryptand22-coated piezoelectric crystal liquid chromatographic detector for cations, anions and organic molecules.
Piezoelectric crystal membrane chemical sensors based on fullerene C60 Jeng-Shong Shih*, Yun-Ching Chao, Mine-Fag Sung, Guann-Jou Gau, Chyow-San Chiou Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan
Abstract Various reusable and sensitive piezoelectric (PZ) quartz crystal membrane sensors with home-made computer interfaces for signal acquisition and data processing were developed to detect organic/inorganic vapors and organic/inorganic species in solutions, respectively. Fullerene C60 and synthesized fullerene derivatives, e.g. C60-cryptand22 and C60-dibenzo-16-crown-5 were applied as coating materials on quartz crystals of the PZ crystal sensors. The oscillating frequency of the quartz crystal decreased due to the adsorption of organic or inorganic species onto coating material molecules on the crystal surface. The C60-coated PZ crystal sensor was ®rstly prepared to study the adsorption and the interaction between fullerene C60 and organic molecules. Only amines, dithios, alkynes and 1,3-dienes seem to perform chemisorption onto fullerene while physical adsorption is found in all other cases like alcohols, aldehydes and carboxylic acids. The reactivities of fullerene C60 and other conjugated ole®ns to various organic molecules seem to be in the order b-carotene > C60 > pyrene. The fullerene C60-coated PZ crystal sensor also can be employed to sensitively detect some organic molecules, e.g. aldehydes, carboxylic acids, amines and ole®ns. A C60-cryptand22-coated PZ gas sensor for both polar/nonpolar organic vapors was also prepared. The cryptand22-coated PZ gas sensor can be employed as a GC detector for organic molecules. The C60-cryptand22-coated crystal GC-PZ detector compared well with the commercial thermal conductivity detector (TCD). Various liquid PZ crystal sensor based on C60cryptand22 and dibenzo-16-crown-5-C60 were also prepared as liquid chromatographic (LC) detectors for metal ions, anions and polar/ nonpolar organic molecules in solutions. The frequency responses of the C60-cryptand22-coated LC-PZ detectors for various organic molecules and metal ions were compared well with the responses of a commercial UV±VIS detector and a conductivity detector which are generally used in LC systems. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Piezoelectric crystal chemical sensors; Fullerene C60; Gas sensors; Organic/inorganic liquid sensors
1. Introduction Piezoelectric (PZ) crystals, e.g. quartz, BaTiO3 and PLZT(Pb, La, ZrOx), are well known to be quite sensitive to pressure and any mass change on their surfaces and can be applied as microbalances for trace quantitative analysis [1]. Any increase in mass or pressure on the surface of the crystal will result in a decrease in the vibrational frequency of an oscillating PZ crystal. The variation of vibrational frequency, DF related to the mass, M (in g) of coating materials or absorbated foreign substance onto the crystal surface with area, A (in cm2) and fundamental frequency, F0 of the PZ crystal can be evaluated from Sauerbrey's equation [2] M 6 DF 2:3 10 A The theoretical detection limit 4 for absorbed foreign substances on quartz crystals is reported to be as small as *
Corresponding author. Tel.: 886-2-29350749; fax: 886-2-29309077. E-mail address: [email protected] (J.-S. Shih).
10 12 g. Therefore, the PZ crystals with appropriate adsorbents can be applied as quite sensitive gas [3±8] and liquid [9,10] sensors for adsorbing particular molecules. Owing to their high mass sensitivity, the PZ crystals can also be employed as a sensitive sensor to study the adsorption and interaction between an adsorbent on the crystal surface and various adsorbed compounds. It seems that the PZ crystal can be employed as a sensitive probe with appropriate adsorbents for various organic or inorganic compounds. Fullerene C60 is a new allotropic form of carbon [11,12], the study of fullerene's physical and chemical properties has recently received a lot of attention [13±17]. A characteristic feature of fullerene is its af®nity to various organic molecules. Fullerene C60 with 60 p-electrons potentially can be expected to be applied as a good adsorbent to adsorb and detect nonpolar and some polar organic molecules. However, fullerene cannot adsorb metal ions, anions and most polar organic species. Macrocyclic polyethers, e.g. crown ethers [18±20] and cryptands [21], have demonstrated remarkable complexing abilities to metal ions [19±21], anions [22] and polar organic molecules [23,24]. However,
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most crown ethers and cryptands can be dissolved in water, thus, naked crown ethers and cryptands cannot used as adsorbents on PZ crystals to detect various organic and inorganic molecules in liquid solutions. Therefore, in the present work, fullerene and fullerene derivatives, e.g. C60-cryptand22 and C60-dibenzo-16-crown-5-coated quartz crystals were prepared and applied to adsorb and detect various polar/nonpolar organic molecules and metal ions.
2.4. Reagents
2. Experimental
The fullerene C60-coated PZ quartz membrane crystal liquid sensor was set up and employed to detect various polar organic molecules in solutions, e.g. hexylamine, hexanoic acid, 2-hexanone and hexanal. As shown in Fig. 1a, the frequency shifts of the C60-coated PZ crystal liquid sensor for these organic molecules are in the order
2.1. Preparation of fullerene derivatives Fullerene-cryptand22 was obtained as a precipitate by addition of fullerene C60 (1.0 g, 1.38 mmol) and cryptand22 (0.36 g, 1.38 mmol) in anhydrous toluene (250 ml) and stirring at room temperature for 72 h. C60-dibenzo-16crown-5-oxyacetic acid was synthesized by the reaction of sym-dibenzo-16-crown-5-oxyacetic acid (56 mmol) and fullerene (C60, 56 mmol) in toluene for 72 h [25,26]. These products were identi®ed by FTIR and 13 C NMR.
Fullerene (C60) of >99% purity was obtained from Strem Chemicals (USA). All other chemicals used were of analytical grade. 3. Results and discussion 3.1. Fullerene C60-coated PZ crystal sensors
2.2. Crystal coating The PZ crystals used were AT-cut spherical quartz crystals, of radius 4.0 mm and thickness 0.18 mm with a basic resonant frequency of 10 MHz and provided with a silverplated metal electrodes on both the sides that were obtained from Taiwan Crystal Co. Both sides of the crystals were coated with an adsorbent, e.g. either C60 or C60-cryptand22/PVC (10 mg/10 mg) in 10 ml tetrahydrofuran (THF) solution via the spin coating with a microsyringe. After evaporation of the solvent, fullerene-coated PZ crystals were obtained. 2.3. Apparatus The experimental set-ups of PZ quartz gas/liquid crystal detection systems included oscillators, working cell with quartz crystals and assembled home-made computer interfaces. The oscillating frequency (F) of the C60 or C60 derivative-coated PZ crystal was converted to an electric potential (V) signal using an F/V converter TC 9400 chip. The voltage signal was processed with a 12-bit ADC (analog to digital converter) connected to a programming peripheral interface (PPI 8255) and microcomputer (PC/AT) for signal acquisition and data processing with a Basic computer program. Both JASCO FTIR-5300 Fourier transform infra-red (FTIR) and JASCO MX400 Fourier transform NMR spectrometries were applied to identify the synthesized various fullerene derivatives. A Perkin-Elmer 2400 elemental analyzer was also employed for elemental analysis of the fullerene derivatives.
Fig. 1. Frequency responses of the C60-coated (a) LC-PZ detector for polar organic molecules and (b) GC-PZ detector for nonpolar organic molecules.
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hexylamine > hexanoic acid > 2-hexanone > hexanal. This result seems to imply that the high polar organic molecules with easily exchangeable protons such as hexylamine and hexanoic acid are more easily adsorbed onto fullerene molecule than less polar and without exchangeable proton molecules such as 2-hexanone and hexanal. Like alkenes, fullerene with 30 p-bonds may undergo nucleophilic and electrophilic attacks. Therefore, both nucleophilic addition of hexylamine to fullerene and electrophilic attack of hexanoic acid to fullerene can be expected. Fullerene C60 with 60 p-electrons can be expected to adsorb nonpolar organic molecules with p±p interaction. As expected, C60 molecule can adsorb hexene and hexyne with frequency responses in the order hexyne > hexene > hexane (Fig. 1b). The adsorption of organic molecules onto fullerene molecules could be either physical adsorption or chemical adsorption. The desorption study was performed to determine whether the adsorption is chemical or physical. As shown in Fig. 2, the frequency of the fullerene-coated crystal for hexylamine can only be partially reversed after introducing pure water. The partial irreversibility for hexamine seems to mean that the adsorption may be partial chemisorption or strong physical adsorption on inner layers of C60. Poorer reversibility was also found for hexyne. Infrared (IR) spectrum can be employed to infer the structure of the fullerene products obtained after adsorbing the organic molecules. For the C60-hexyne incorporated species, the peaks at 3441 cm 1 (C60±H), 1623 cm 1 (C=C), 1413 cm 1 (=C±H), and no 2100 cm 1 (CBC of hexyne), 574 and 526 cm 1 (C60) are found. It seems to indicate that the adsorption of hexyne onto C60 molecule may be partial chemisorption. The chemisorption also can be found in the cases of amines, dithios and dienes onto C60 molecules. Comparison of reactivities of C60 and other conjugated ole®ns, e.g. b-carotene and pyrene, to various organic molecules was also made. The frequency shifts of these
Fig. 2. Irreversible response curve of hexylamine with 4 mg C60-coated liquid piezoelectric detector.
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Fig. 3. Response curves of (a) propanol and (b) hexene with 6 mg C60-, bcarotene- and pyrene-coated piezoelectric gas crystal detector.
conjugated ole®ns-coated PZ crystal gas sensors for various polar/nonpolar organic molecules, such as propanol and hexene, were found to be in the order (Fig. 3) b-carotene > C60 > pyrene. This result seems to imply that the reactivity of C60 to organic molecules is greater than other aromatic ole®ns, e.g. pyrene and less that of linear conjugated ole®ns, e.g. b-carotene. The fullerene C60-coated PZ crystal gas sensor was also set up and employed to study the interaction between C60 and some inorganic vapors, e.g. ozone, HCl and HNO3. As shown in Fig. 4a, frequency of the C60-coated PZ crystal gas sensor shows increase after the adsorption of ozone molecule. The decomposition of fullerene C60 into small pieces after oxidation by ozone was reported in [27] which can lead to the decreased mass onto the PZ crystal and results in the decrease in the frequency of the C60-coated PZ crystal gas sensor. The oxidation of C60 also can be con®rmed by the
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Fig. 5. Frequency responses of n-propanol, propanal, propanon, hexane, hexene and hexyne with C60-cryptand22 and C60-coated GC-PZ crystal detectors.
Fig. 4. Response curves of (a) ozone in the air and (b) propanol under air And ozone with C60-coated piezoelectric gas detector.
irreversible response (Fig. 4a) after introducing the clean air and infrared spectrum of C60 after adsorbing ozone molecule with epoxide (C±O) peaks at 1030 and 1118 cm 1. After reacting with ozone molecule, fullerene C60 exhibited greater adsorbing ability to organic species, e.g. propanol, than original C60 molecule (Fig. 4b). This result may be attributed to the increased polarity of fullerene molecule after reacting with ozone molecule. Suzuki et al. [28] also reported that the electrophilic property of fullerene C60 increased after the oxidation of C60 with ozone. The irreversible response and the change in IR peaks were also found in the study on the interaction between C60 and HCl or HNO3 by using the C60-coated PZ crystal gas sensor. The reactivity of HNO3 toward C60 molecule seems greater than HCl. 3.2. C60-macrocyclic polyether-coated PZ crystal sensors Although fullerene C60 can adsorb some organic molecules, C60 is not a quite good adsorbent for polar organic
molecules and cannot absorb metal ions, anions and biologic species. Therefore, in this study, some fullerene C60-macrocyclic polyether derivatives, e.g. C60-cryptand22 and C60dibenzo-16-crown-5 were synthesized and applied as adsorbents to adsorb and detect nonpolar/polar organic molecules, metal ions and anions. The fullerene C60-cryptand22-coated PZ crystal gas sensor was developed and applied as a gas chromatographic (GC) detector to detect various polar and nonpolar organic molecules. The C60-cryptand22-coated PZ crystal GC-PZ detector (Fig. 5) exhibited more sensitivity to polar organic molecules, e.g. propanol, propanal, propanone, than pure fullerene C60-coated PZ crystal detector. This result may be attributed to the complexing ability of cryptand22 of C60-cryptand22 to polar organic molecules [24]. Furthermore, by comparing the frequency response peaks with C60-cryptand22-coated and C60-coated GC-PZ detectors, the response of C60-cryptand22-coatedGC-PZ detector shows better peak shape and narrower peak width than that of the C60-coated GC-PZ detector. In addition, the C60-cryptand22-coatedGC-PZ detector also exhibited better sensitivity to nonpolar organic molecules (Fig. 5), e.g. hexane, hexene and hexyne, than the C60-coated GC-PZ detector. The enhancement effect of cryptand22 attached to fullerene C60 molecule for the adsorption of nonpolar organic molecule may be attributed to the destruction of the conjugated structure of fullerene C60 and to the increased reactivity of C60 molecule after attaching a foreign molecule such as cryptand22. The performance of the C60-cryptand22-coated GC-PZ detector was also compared with the response of the commercial gas chromatographic detector such as thermal conductivity detector (TCD). As shown in Fig. 6, the C60cryptand22-coated PZ crystal detector seems to compare well with the commercial TCD for these organic molecules
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Fig. 6. Comparison of gas chromatograms with (A) C60-cryptand22coated piezoelectric (PZ) detector and (B) thermal conductivity detector (TCD) for various alcohols: (1) water, (2) sec-propanol, (3) n-propanol, (4) sec-butanol and (5) n-butanol.
such as alcohols. The better peak shapes and narrower haftway peak width (o1/2) by using the C60-cryptand22-coated GC-PZ detector than that of the TCD detector obviously can be found. Furthermore, the home-made PZ-GC detector is less expensive than the TCD or another commercial GC detector. In addition, the C60-cryptand22-coated GC-PZ detector also showed good linear response to the concentration of these organic species, good detection limit of about 3.7 mg for n-butanol and good reproducibility with R.S.D. of about 3% for methanol. The C60-cryptand22-coated PZ crystal sensor also can be used as a liquid chromatographic (LC) detector not only for organic molecules but also for the metal ions and anions in solutions. The C60-cryptand22-coated LC-PZ detector has demonstrated as a quite sensitive detector for various organic species, e.g. alcohols, amines and carboxylic acids, in solutions which is shown in Fig. 7. The frequency responses of the C60-cryptand22-coated LC-PZ detector for organic molecules compared well with the responses of a commercial UV±VIS detector which is generally used in a LC. Fig. 7 indicates that all alcohols can be responded by the C60-cryptand22-coated LC-PZ detector, but some nonUV±VIS absorbable alcohols, e.g. propanol, butanol and hexanol, were not detected by the commercial UV±VIS detector for LC. Like other macrocyclic polyethers, cryptands have demonstrated a remarkable complexing ability for various metal ions [21]. Thus, the C60-cryptand22-coated LC-PZ
Fig. 7. Comparison of liquid chromatograms with (a) commercial UV± VIS detector and (b) C60-cryptand22-coated LC-PZ detector for various alcohols: (1) propanol, (2) butanol, (3) hexanol, (4) phenol, (5) 4-chlorophenol and (6) 4-methoxy-phenol.
detector can also be employed to detect the metal ions in aqueous solutions. The effect of the amount of C60-cryptand22 coating on the frequency response of the quartz crystal detector was investigated. The crystal with more C60-cryptand22 coating exhibited a larger frequency shift for the same concentration of the K ions as shown in Fig. 8a. The cryptand22 of C60-cryptand22 not only can form complexes with cations, but also can form stable complexes with anions [22] when the cryptand is protonated as cryptand-NH in acidic solution (pH < 6). It means that the C60-cryptand22-coated LC-PZ detector is a switch-type multifunctional detector which can be used as a cationic detector at pH 7 and as an anionic detector at pH 6. As expected, the C60-cryptand22-coated LC-PZ detector can be employed to detect various carboxylic anions, e.g. HCOO , CH3COOH , CH3CH2COO and CH3(CH2)2 COO , as shown in Fig. 8b. As expected, effect of pH on the frequency response of the C60-cryptand22coated LC-PZ detector for ions was also observed in this study. The performance of the C60-cryptand22-coated LCPZ detector for various ions also compared well with the response of the conductivity detector which is generally
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Acknowledgements The authors would like to express their appreciation to the National Science Council of the Republic of China in Taiwan for ®nancial support. References
Fig. 8. Frequency responses of C60-cryptand22-coated LC-PZ detector for K ions (10 3 M potassium hydrogen phosphate) at pH 7 and various carboxylate anions: (1) HCOO , (2) CH3COO , (3) CH3CH2COO , (4) CH3(CH2)2COO (potassium salts) at pH 3.
used to detect various ions in LC. C60-dibenzo-16-crown-5 was also synthesized [25,26] in our laboratory and applied as a quite sensitive adsorbent for polar/nonpolar organic molecules and metal ions. Similarly, the C60dibenzo-16-crown-5-coated PZ crystal GC-PZ detector showed more sensitivity to polar as well as nonpolar organic molecules than pure fullerene C60-coated PZ crystal detector. 4. Conclusion In conclusion, the fullerene C60 and C60-macrocyclic polyethers, e.g. C60-cryptand22-coated PZ quartz crystal sensors can be applied to detect various organic molecules, metal ions and anions with good sensitivity, good detection limit and reproducibility.
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Biographies Jeng-Shong Shih received his BSc degree in Chemistry from National Taiwan Normal University, in 1967, MSc degree in Inorganic Chemistry from National Ching-Hwa University, in 1970 and PhD degree in
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Analytical Chemistry from Michigan State University, USA, in 1978. He is currently a professor in the Department of Chemistry, National Taiwan Normal University. His research interests include chemical sensors, catalysts, fullerenes and macrocyclic polyethers. Yun-Ching Chao received her MSc degree in Analytical Chemistry from National Taiwan Normal University, in 1996. Her research interest is the interaction between Fullerene C60 and organic molecules and ozone. Mine-Fag Sung received his MSc degree in Analytical Chemistry from National Taiwan Normal University, in 1997. His research interest is the comparison of reactivities between Fullerene C60 and other conjugated olefins. Guann-Jou Gau received his MSc degree in Analytical Chemistry from National Taiwan Normal University, in 1997. His research interest is the development of C60-cryptand22-coated piezoelectric crystal gas sensors for organic vapors. Chyow-San Chiou received his PhD degree in Analytical Chemistry from National Taiwan Normal University, in 1998. His research interest is the development of C60-cryptand22-coated piezoelectric crystal liquid chromatographic detector for cations, anions and organic molecules.