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Langmuir 1998, 14, 1748-1752
Quartz Crystal Microbalance Sensor Deposited with Langmuir-Blodgett Films of Functionalized Polythiophenes and Application to Heavy Metal Ions Analysis S. C. Ng,*,† X. C. Zhou,† Z. K. Chen,† P. Miao,† H. S. O. Chan,† S. F. Y. Li,‡ and P. Fu† Department of Chemistry, National University of Singapore, Singapore 119260, Republic of Singapore, and Department of Chemistry and Institute of Materials Research and Engineering, National University of Singapore, Singapore 119260, Republic of Singapore Received March 19, 1997. In Final Form: October 30, 1997X Two novel functionalized polythiophenes, poly[3-(6-hydroxyhexyl)thiophene] and poly(3-octanethio2,2′-bithiophene), were used as selective coating materials for quartz crystal microbalance (QCM) sensors. These conductive polymers on admixture with stearyl alcohol were deposited on QCM devices by the Langmuir-Blodgett (LB) technique. These LB films exhibited a selective adsorption for the first-row transitions ions in the Irving-Williams order due to the complexation of metal ions with sulfur atoms in the polymers. These LB film coated QCM sensors were found applicable in detecting heavy metal ions in aqueous solution, which are major pollutants in wastewater. The coated QCM sensors can selectively adsorb heavy metal ions (Hg2+) from solution over a wide range from 0.1 to 100 ppm concentration by complexation with sulfur atoms in the polymers. The bound mercury could be removed by ethylenediaminetetraacetic acid (EDTA) solution. The presence of ferrous, lead, cobalt, and chromium cations in the solution did not interfere with the detection of mercury ions.
1. Introduction The quartz crystal microbalance (QCM) is an extremely sensitive mass sensor with detection capabilities in the sub-nanogram range as demonstrated in a number of gas and vapor systems.1,2 Sauerbrey has shown that if rigid layer behavior and no slip at the resonator-fluid boundary are assumed, the changes in resonant frequency can be used as a direct measurement of mass changes on the surface of the QCM according to eq 1,3 where ∆f is the measured frequency shift, k the constant, and ∆m the mass change.
∆f ) -k ∆m
(1)
Liquid phase measurements involving the use of a piezoelectric crystal coated with a reactive or absorbent layer were first reported by Konash and Bastiaans4 as part of their attempts to develop a mass detector for liquid chromatography. Nomura et al.5-7 has also demonstrated the utility of coated crystals for the selective sorption of ionic species from solution. In particular, copper oleate coating was used for the determination of 3-40 µM lead,5 poly(vinylpyridine) coating was used for the measurement of 5-35 µM copper (II),6 and silicone oil was used for the * To whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Chemistry and Institute of Materials Research and Engineering. X Abstract published in Advance ACS Abstracts, December 15, 1997. (1) Ward, M. D.; Buttry, D. A. Science 1990, 249, 1000. (2) McCallum, J. J. Analyst 1989, 114, 1173. (3) Sauerbrey, G. Z. Phys. 1959, 155, 206. (4) Konash, P. L.; Bastiaans, G. J. Anal. Chem. 1980, 52, 1929. (5) Nomura, T.; Okahara, T.; Hasegawa, T. Anal. Chim. Acta 1986, 182, 261. (6) Nomura, T.; Ando, M. Anal. Chim. Acta 1985, 172, 353. (7) Nomura, T.; Sakai, M. Anal. Chim. Acta 1986, 183, 301.
determination of 5-100 µM iron(III).7 Recently, a 4-(3aminopropyl)morpholine modified poly(glycidyl methacrylate-vinylpyrrolidinone) copolymer coated QCM developed by Hunter and Price8 was shown to exhibit increased specificity toward Cu2+ and Ni2+ ions over other first-row transition metal ions, and concentrations at 0.1 ppm were readily detectable. The applicability of a PVCcrown ether polymer coated QCM as a detector for various metal ions in ion chromatography was also reported by Jane et al.9 The detection and assessment of the heavy metal ions in water supplies or effluent streams or in specific processes are important tasks in environmental protection and human health. Currently, although there are several methods commonly employed in metal ions analysis, e.g., atomic adsorption spectroscopy,10 electrochemical separation techniques,11 high-performance liquid chromatography,12 and capillary electrophoresis,13 none of these techniques is amenable to real-time analysis. While atomic adsorption and electrochemical separation methods can offer good analytical performance in terms of precision and accuracy, they are nevertheless expensive from the viewpoint of reagent consumption and instrumentation capital cost. Accordingly, there is considerable interest in developing simple, rapid, and economically viable methods that will afford a facile analyses of heavy metal ions. The QCM sensor-based analysis is an alternative method which can provide real-time analysis, low cost in (8) Hunter, T. C.; Price, G. J. Analyst 1995, 120, 161. (9) Jane, Y. S.; Shih, J. S. Analyst 1995, 120, 5. (10) Hwang, T. J.; Jiang, S. J. J. Anal. At. Spectrosc. 1996, 11 (5), 353. (11) Hissner, F.; Mattusch, J.; Werner, G. Fresnious J. Anal. Chem. 1996, 354 (5-6), 718. (12) Das-Ak Chakraborty, R.; Cervera, M. L.; Delaguardia, M. Mikrochim. Acta 1996, 122 (3-4), 209. (13) Wen, J.; Cassidy, R. M. Anal. Chem. 1996, 68 (6), 1047.
S0743-7463(97)00296-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/06/1998
LB Films on QCM Sensors
instrumentation, and amenability to automation without requirement of expensive reagents. This paper is a preliminary report describing the development of a selective chemical sensor based on QCMs deposited with Langmuir-Blodgett (LB) films of readily synthesized functional polythiophenes for the determination of metal ions in solution. Here, the major deficiency of the QCM devices, namely, the absence of selectivity, is alleviated by the application of selective sorbents based on the specially designed functional polymers deposited on the crystal. Although polythiophenes are well-known electroactive polymers which have established application aspects in the areas of microelectronics,14,15 electrode materials,16,17 optoelectronics,18 and biosensors,19-21 to our knowledge there has been little impetus into their applications as ion sensors. Key advantages in the utility of polythiophenes stem from the amenity of the thiophene nucleus in lending itself to facile synthetic attachment of a whole range of pendant moieties as of the straightforward polymerization procedures for the derived functional monomers.22,23 In addition, deposition of our series of functional polythiophenes as films on the QCM devices can be readily effected by the Langmuir-Blodgett technique, which provides the added advantage of forming evenly coated layers on the crystal substrate as compared to the conventional dipping and spreading approach or the spin casting method. The derived ion sensors are anticipated to have potentials for the determination of metal contaminants in water supplies or effluent streams or in specific processes. 2. Experimental Section 2.1. Quartz Crystal Microbalance System. The QCM sensors employed were commercially available 10 MHz AT-cut unbonded quartz crystals with gold electrodes (0.51 cm diameter) on both sides purchased from International Crystal Manufacturing Co., Oklahoma City, OK. The crystal was installed in a detection cell with only one electrode exposed to the detection solution. A thermostat was used to control the temperature at 25 ( 1 °C through a thermostatic water jacket. The frequency of the vibrating crystal was measured by a TF830 universal frequency counter (Thurlby-Thandar Ltd, England) connected to a personal computer. The frequency data were recorded every 30 s. Provided that the crystal was carefully thermostated and the system was protected from drafts, the frequency response was stable to within (5 Hz over periods of 4-6 h in a stirred solution. From eq 1, a frequency change of 1 Hz corresponds to a mass change of 0.904 ( 0.01 ng on the electrode. 2.2. LB Film-Deposited QCM. The coating materials poly(3-(ω-hydroxyhexyl)thiophene) (P3HHT) and poly(3-(octylthio)2,2′-bithiophene) (P3OTBT, Mn ) 9600, dispersity 1.37) were chemically synthesized.24,25 P3HHT mixed with stearyl alcohol (14) Borroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; MacKay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539. (15) Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Jpn. J. Appl. Phys. 1991, 30, L1938. (16) Kawai, T.; Kuwabara, T.; Wang, S.; Yoshino, K. Jpn. J. Appl. Phys. 1990, 29, 602. (17) Bobacka, J.; Ivaska, A.; Grzeszczuk, M. Synth. Met. 1991, 44, 9. (18) Kobayashi, T. Relaxation Polymerization; Kobayashi, T., Ed.; World Scientific: Singapore, 1993; pp 1-79. (19) Kim, S. R.; Choi, S. A.; Kim, J. D.; Kim, K. J.; Lee, C.; Rhee, S. B. Synth. Met. 1995, 71, 2027. (20) Chan, H. S. O.; Ng, S. C.; Seow, S. H. Synth. Met. 1994, 66, 177. (21) Chan, H. S. O.; Toh, C. S.; Gan, L. M. J. Mater. Chem. 1995, 5, 631. (22) Roncali, J. Chem. Rev. 1992, 92, 711. (23) Toshima, N.; Susuma, H. Prog. Polym. Sci. 1995, 20, 155. (24) Ng, S. C.; Fu, P.; Yu, W. L.; Chan, H. S. O.; Tan, K. L. Synth. Met. 1997, 87, 119. (25) Ng, S. C.; Chan, H. S. O.; Huang, H. H.; Seow, R. S. H. J. Chem. Res. 1996, 232.
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Figure 1. Chemical structures of sensing materials: poly(3(ω-hydroxyhexyl)thiophene) (P3HHT) and poly(3-(octylthio)2,2′-bithiophene) (P3OTBT). in the molar ratio of 3:1 and P3OTBT mixed with stearyl alcohol in the molar ratio of 1:3 were used as the sensing materials (their chemical structures are shown in Figure 1). They were dissolved in double-distilled chloroform at concentrations of 0.37 and 0.48 mg mL-1, respectively, and were spread on the surface of ultrapurified water contained in a NIMA 622 LB trough. The quartz crystals were treated with hexamethyl disilazane after cleaning with chloroform, ethanol and ultrapurified water in an ultrasonator. The monolayers were transferred onto the substrate of gold-coated quartz crystals at a constant surface pressure of 30 mN m-1 at 20 °C by the vertical dipping method. The dipping speed was 5.0 mm min-1 for both the upstroke and downstroke. In all cases transfer ratios close to 1.00 were observed, leading to the formation of Y-type multilayers for both sensing materials. 2.3. Selectivity and Sensitivity Testing. All testing was performed using distilled water and solutions of analytical reagent grade metal nitrates at 25 °C. All concentrations are quoted in terms of parts per million of metal ion. Sauerbrey’s equation was considered valid as all solutions were below 2 mmol dm-3 and the amounts absorbed were well within the linear region.26
3. Results and Discussion 3.1. LB Deposited Conductive Polymer Films on QCM. As the thiophene nucleus is not sufficiently amphiphilic, polythiophenes or polybithiophenes generally require the use of a surface active material in order to form a stable monolayer at the air-water interface. Accordingly, for P3OTBT and P3HHT the isotherms were recorded after admixing the respective polymers with a 1:3, 3:1 mole ratio of stearyl alcohol. Figure 2 shows that there are two distinct phase transitions in the π-A isotherm for the P3OTBT mixture, while for P3HHT the changes are more gradual from the two-dimensional “gas” to “solid” phase. The first phase transition for P3OTBT at about 40 Å2 should correspond to the change of the adjacent thiophene rings from the nonplanar to planar conformation, while the second phase transition is from the liquid to a more condensed state. P3OTBT, which has an octylthio group at the C-3 position per bithiophene repeating unit, can freely align on the air-water interface with free rotation about the bonds linking adjacent thiophene rings. In the gaseous phase, most of the molecules are lying on the water surface with a consequent large area occupied. With an increase of compression, the conformation for the two adjacent thiophene rings will begin to take on planarity with the hydrophobic side chain oriented outward from the water surface, which is the cause of the first phase transition at about 40 Å2. On the other hand, P3HHT, which has a hydrophilic ω-hydroxyhexyl group at the C-3 position of each thiophene repeat unit, will align itself on the water surface with the polymer main chain oriented on the air side. Accordingly, (26) Guilbault, G. G.; Jordan, J. CRC Crit. Rev. Anal. Chem. 1988, 19 (1), 1.
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Figure 2. Pressure vs area isotherms (A) for the 3:1 mixture of P3HHT and stearyl alcohol and (B) for the 1:3 mixture of P3OTBT and stearyl alcohol.
Figure 3. Relationship of the frequency shift to the number of monolayers deposited onto the quartz crystals: A, P3HHT/ stearyl alcohol (3/1 ) molar ratio); B, P3OTBT/stearyl alcohol (1/3 ) molar ratio).
the area change from the gaseous state to solid state will be relatively smaller. In an attempt to determine the quality of the LB multilayers on the QCM sensors, the frequency change (∆F) of the QCM was measured before and after the LB films deposition. As shown in Figure 3, a plot of frequency shift versus thickness of the LB film coatings for both polymers gave straight lines through the origin. This result together with the unity transfer ratios suggested that the monolayers have been successfully deposited on the QCM sensor in an orderly fashion. 3.2. Reaction of Conductive Polymer Films with Metal Ions. Figure 4 shows the adsorption of crystals coated with polymer 10 layers of P3HHT/stearyl alcohol (3:1) to metal ion type when they are exposed to 20 ppm solution of various metal ions at neutral pH at 25 °C. The adsorption takes about 2 min to reach equilibrium. Similar response behaviors were observed on the polymer P3OTBT/stearyl alcohol (1:3) coated QCMs. The QCM coated with a LB film of stearyl alcohol only gave a very slight frequency shift to all of the metal ions tested. The large frequency response to the analytes on the polymercoated electrodes when identical experiments were per-
Ng et al.
Figure 4. Binding ability of the crystal coated with 10 layers of P3HHT/stearyl alcohol (3/1) films to heavy metal ions in 20 ppm solution at neutral pH: (1) Ag+, (2) Cu2+, (3) Hg2+, (4) Ni2+, (5) Co2+, (6) Zn2+, (7) Fe2+, and (8) typical response of crystal coated with 10 layers of stearyl alcohol only to 20 ppm Cu2+.
formed verified the participation of polythiophenes in the observed behavior. The complexation of metal ions with sulfur atoms in the polymers is the main driving force for the adsorption. The order of adsorption selectivity for the first-row transition ions exhibited by the films was Cu2+ > Ni2+ > Co2+ > Zn2+ > Fe2+, which agrees well with the Irving-Williams order based on ionic potentials and size. The somewhat stronger adsorption of Ag+, Hg2+ to the sensing polymer layer is attributable to the strong affinity of these metal ions to the sulfur atoms. Although the extent of metal ions adsorption does not appear to vary extensively, the coated QCM will nevertheless give a much higher frequency response to the Hg2+, Ag+ considering that these ions have atomic weights two to three times larger than the first-row transition ions. In order to remove adsorbed metal ions rapidly from the film after each adsorption run, the crystal was immersed into a stirred 1 mM ethylenediaminetetraacetic acid solution for 5 min, the trapped metal ions were removed, and the frequency was allowed to return to the initial value prior to the next analysis. Figure 5 shows the effect of the number of the coating layers on performance of the sensors. The frequency change increases with the increase in the number of layers, which indicates that the metal ions may penetrate into LB films. The response slope of the P3OTBT/stearyl alcohol (1:3) coating is slightly larger than the slope obtained for the P3HHT/stearyl alcohol (3:1) film, which indicates the thickness of P3OTBT/stearyl alcohol (1:3) film has a larger effect on the sensor response than the P3HHT/stearyl alcohol (3:1) film. This is because of the P3OTBT/stearyl alcohol (1:3) film is more porous. Although the 40 multilayer film coated QCM gave a relatively higher response to the analytes, the LB film was found to slip off into the solution after 7-10 detection runs. In contrast, the 20 multilayer films coated QCMs were found to be stable and reusable even after 50 measurement runs. 3.3. Calibration Graph, Reproducibility, Lower Detection Limit, and Interference. The calibration graphs of frequency change against Hg2+ and Ag+ concentrations were linear up to the concentration of 100 ppm, as shown in Figure 6. Beyond this concentration, relatively small frequency shifts were observed because of the saturated adsorption of analytes on the LB layers.
LB Films on QCM Sensors
Figure 5. Dependence of frequency changes of 20 ppm Hg2+ (A) and Ag+ (B) on the thickness of LB films: (0) P3HHT/ stearyl alcohol (3/1); (4) P3OTBT/stearyl alcohol (1/3).
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Figure 7. QCM deposited with 20 layers of P3HHT/stearyl alcohol (3/1) for successive additions (indicated by arrows) of 0.1 ppm Hg2+ solutions.
tions of 20 ppm Hg2+ for the P3HHT/stearyl alcohol (3:1) and P3OTBT/stearyl alcohol (1:3) polymers, respectively. The lower limit for reliable detection (LRD) of the QCM can be investigated by the adsorption of metal ions which gives frequency change three times larger than the system noise, a frequency shift of 30 Hz. Figure 7 depicts the response of the QCM to successive additions of 0.1 ppm of Hg2+. On each addition, a plateau in frequency change is clearly depicted. The masses of Hg2+ ions adsorbed on three successive additions of Hg(NO3)2 were 45.12, 37.96, and 36.17 ng, respectively. Figure 7 also demonstrates the high stability of the frequency response and reveals that concentrations at the 0.05 ppm level could be readily detected by the film indicating the usefulness of this sensing system at very low concentration. The effect of a number of interfering anions and cations on the determination of 5 ppm mercury(II) was investigated. Changes in frequency of more than (6% (i.e., twice the standard deviation) were considered to result from interference. It was found that a 2-fold ppm amount of all cations (Na+, K+, Ca2+, Pb2+, Cr2+, Co2+, Ni2+) except silver and cuprous in the solution did not interfere. Anions such as carbonate, perchlorate, sulfate, phosphate, and chloride at 2-fold ppm amount also did not interfere, while bromide, iodide, chromate, or thiocyanate all interfered by forming stable precipitates or complexes with mercury. 4. Conclusion
Figure 6. Calibration graph of QCMs coated with 20 layers of polymers to Hg2+ (0) and Ag+ (4) in neutral pH solution (a) P3HHT/stearyl alcohol (3/1) and (b) P3OTBT/stearyl alcohol (1/3).
The regression equations and correlation coefficients of Hg2+ in the above concentration ranges are ∆f ) 12.18C + 28.54, R2 ) 0.987 (n ) 8) for P3HHT/stearyl alcohol (3:1) and ∆f ) 10.76C + 14.58, R2 ) 0.997 (n ) 8) for P3OTBT/stearyl alcohol (1:3) coatings, respectively, where C is the concentration of the ion. The standard deviations were 7.04 Hz (3.2%) and 3.02 (2.40%) for six determina-
The properties of the multilayers of two novel functionalized polythiophenes deposited by the LB technique were studied by QCM devices. The binding ability of the heavy metal ions on the two polymer films was investigated. This preliminary study also shows that a sensitive and reasonably selective sensor based on the quartz crystal microbalance coated with LB films of functional polythiophenes can be prepared. Concentrations at the low ppm levels of Ag+ and Hg2+ ions were readily detectable and measurements could be extended to as high as 100 ppm. Although further development is needed to increase the specificity and long-term stability of this type of sensor, the approach adopted and the results generated in this work point the way to a range of chemical sensors by the use of target-oriented functional polymer coating materials
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and novel transducers such as the quartz crystal microbalance and piezoelectric devices. Acknowledgment. We thank the National University of Singapore for financial support through the research grants RP930603 and RP960613. P.M. and P.F. are
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grateful to NUS for the award of a research scholarship. X.C.Z. and Z.K.C. gratefully acknowledge the financial support from the National Science and Technology Board of Singapore. LA970296V
