Silver-selective Sensor Using an Electrode-separated Piezoelectric Quartz Crystal Modified with a Chitosan Derivative

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1 2002 The Japan Society for Analytical Chemistry 881 Silver-selective Sensor Using an Electrode-separated Piezoelectric Quartz Crystal Modified with a Chitosan Derivative Shanhu BAO and Toshiaki NOMURA Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1, Asahi, Matsumoto , Japan Silver(I) adsorbed selectively onto a quartz plate modified with N-(2-pyridylmethyl)chitosan in an ammonium chloride buffer solution containing EDTA, and the frequency of the quartz plate increased. It was supposed that the increasing frequency was caused by the desorption of adsorbed water on the chitosan derivative, which was induced from the reaction of silver(i) with the chitosan derivative. The concentration of the buffer, ph, temperature, conductivity and eluent affected the frequency shift resulting from the adsorption of silver(i). The frequency decreased at a conductivity lower than 2.2 ms/cm, and increased with increasing conductivity above this value. The frequency shifts caused by the adsorption of silver(i) were proportional to the concentration over the range nm of silver(i), and the correlation coefficient was The detection limit and the relative standard deviation at 50 nm for five times were 6 nm and 3.4%, respectively. The proposed method was simple while showing higher sensitivity and selectivity. (Received March 29, 2002; Accepted June 3, 2002) Introduction The determinations of metal ions in solutions using a piezoelectric quartz crystal (PQC) were performed with electrodeposition 1 until In this method, after the analyte in a solution was electrodeposited onto the electrode of the quartz crystal for several minutes, the different frequencies of the quartz crystal before and after the deposition were measured under a dried condition. It was an inevitable fact that the precision of the analysis was poor because of the complicated operations, such as the reproducibility of setting the crystal to the oscillator and depositing the metal ions on the electrode. After the quartz crystal immersed in a solution oscillated, 2,3 direct electrodepositing determinations of metal ions in solutions such as copper(ii), 4 silver(i), 5,6 lead(ii), 7 gold(iii) 8 or anions such as cyanide, 2,9 iodide, 10 and bromide 11 were reported. Although the quantitative analysis of the various ions using the quartz crystal became more sensitive and simple, it was still inconvenient to use the electrode of the quartz crystal as a working electrode because of exfoliation and the short life of the electrode. Another determination method of metal ions at the µm level has been developed by a precipitation reaction, or ion-pair complexation 15 to deposit directly onto the quartz crystal. These methods need a pretreatment to separate the analytes, or to choose a highly selective precipitant. Nomura et al. applied a functional film to the surface of the quartz crystal for a selective concentration and determination of the metal ion simultaneously. Silicon oil was coated onto a quartz crystal for iron(iii) and aluminum(iii), 16 and poly(vinylpyridine) for copper(ii). 17 Chitosan has been widely used for the concentration and separation of metal ions as an absorption reagent, and a quantitative analysis of metal ions has been developed by coating chitosan onto a quartz crystal as a functional film. 18 Especially, metal-complexed chitosan and a chitosan derivative are useful for a peculiar selective separation of some metal ions. The chitosan film, however, can be used only in a basic solution because of dissolution in acidic. To conquer this weak point, the chitosan was combined chemically onto a quartz crystal plate with silane and glutaraldehyde, and then the frequency behavior and the adsorption characteristics of metal ions in solution were examined for N-(2-pyridylmethyl) chitosan (PMC) 23 modified on the quartz crystal plate. The chitosan derivative (PMC) film selectively adsorbed silver(i) in an ammonium chloride buffer solution containing EDTA, and could be applied to the determination of a trace amount of silver(i). Experimental Reagents All of the chemicals were bought from Wako Pure Chemical Industries, Ltd. The reagents for the film, {chitosan1000, (3- aminopropyl)triethoxylsilane (APTES), glutaraldehyde, and 2- pyridinecarbaldehyde} were used without further purification. The standard storage solutions of metal ions were prepared with analytical reagent-grade chemicals, and diluted to test solutions when used. All solutions were prepared with deionized water. Apparatus An electrode-separated PQC 23 constructed with two platinum plate electrodes and a quartz crystal plate of AT cut, 9 MHz (Kyushu Dentsu) was used. It was connected to a transistor oscillator and an applied 6.0 V with a DC voltage power supply (Metronix, 523B). The frequency was read out with a frequency counter (Iwasaki Communication, SC7204) and then recorded with a personal computer (NEC, PC-9801). Measurements were run in a flow cell; the solution temperature was controlled with a thermostated bath (TAIYO, C-650). Modification of PQC with a PMC The quartz crystal plate was washed with a 2 M sodium

2 882 ANALYTICAL SCIENCES AUGUST 2002, VOL. 18 hydroxide solution for 30 min, acetone and deionized water, and then dried. It was immersed in 10% (3-aminopropyl)- triethoxylsilane 22 acetone solution for 2 h, dried at 100 C for 1 h, and washed again with acetone. After that, it was immersed in a 0.1 M phosphate buffer solution (ph 7.0) containing 5% glutaraldehyde for 2 h, washed with water, immersed in a 5% acetic acid solution containing 0.5% chitosan overnight, immersed in a 5% acetic acid solution containing 5% 2- pyridinecarbaldehyde 20 overnight, and then treated with a 3% NaBH 4 solution. The expected structure of the obtained chitosan derivative film is shown in Scheme 1. The modified quartz crystal plate was washed with water and acetone and dried before use. Procedure A blank solution of ammonium chloride buffer was pumped through a flow cell containing electrode-separated PQC modified with the chitosan derivative. When the frequency was constant (F 1), a metal ion sample solution containing a buffer was pumped in place of the blank solution. The frequency increased with the adsorption of the ion. After 3 min, the sample solution was changed to the blank solution again, and the maximum frequency after the addition of the sample solution was read (F 2). The frequency shift resulting from the adsorption of metal ion ( F) was the difference between F 1 and F 2. After the measurement, the blank solution was changed to a nitric acid solution of 20 mm for 5 min, followed by deionzed water to desorb the metal ion. Results and Discussion Scheme 1 Frequency shifts of PQC modified with PMC The quartz crystal plate having the electrodes (Normal PQC) was coated with chitosan, PMC, or polystyrene and the frequencies of these Normal PQCs in air and water were measured (Fig. 1). The frequencies of these Normal PQCs in air and of the Normal PQC coated with polystyrene in water were increased with increasing weight of the coatings according to the Sauerbrey equation, f = f 2 ( m/a); also, the frequency shifts of the Normal PQC coated with polystyrene in air agreed perfectly with those in water. The deviation from the Sauerbrey equation in polystyrene would come from the spreading area of the polystyrene benzene solution, not only on the electrode, but over the whole plate. On the other hand, Fig. 1 Frequency shifts of a quartz crystal plate having gold electrodes (Normal PQC) coated with polystyrene (, ), chitosan (, ) and chitosan derivative (, ) in air (,, ) and water (,, ). The solid line is the theoretical value calculated from the sauerbrey equation. Temperature, 25 C; humidity, 60%. although the frequencies of the Normal PQCs coated with chitosan or chitosan derivative increased with increasing time in dried air, polystyrene or the uncoated one did not, as shown in Fig. 2. The frequencies of the Normal PQC coated with chitosan or PMC in water greatly increased, as shown in Fig. 1. It was supposed that the increasing frequency shifts resulted from the adsorption of water onto the chitosan or PMC, the water desorbed when copper(ii) reacted with it to form a complex. The frequency shifts corresponding to the decreased mass on the PQC, which was the difference between the desorbed water and adsorbed copper(ii), were observed. Adsorption behavior of metal ions on PMC Metal ions, such as silver(i), nickel(ii), copper(ii), cobalt(ii), cadmium(ii) and zinc(ii), in 60 mm ammonium chloride buffer solution (ph 9.5) containing 15 mm potassium nitrate were flowed to an electrode-separated PQC modified with PMC. In silver(i), nickel(ii), copper(ii) and cobalt(ii) solutions, the frequencies abruptly increased, as shown in Fig. 3, while they increased slowly when the chitosan film was used. 18 The frequencies in silver(i), copper(ii) and cobalt(ii) solutions increased in about 30 s, and then remained constant in spite of the sample solutions being flowed. On the other hand, the frequency in the nickel(ii) solution gradually increased, and no frequency shift was observed in cadmium(ii) and zinc(ii) solutions. Although it was supposed that the nickel(ii) would form a stable complex with the chitosan derivative, the complexes of silver(i), cobalt(ii) and copper(ii) were not very stable, and came to equilibrium immediately with their aminocomplexes in solution. On the other hand, silver(i) would coordinate with a donor atom in a distant place of the chitosan derivative, and cause a disorder of the structure, and thus desorb much water, although nickel(ii), cobalt(ii), copper(ii), etc. would coordinate with the nearby donor atoms (N). Only silver(i) among these metal ions was eluted from the film with the blank solution, and the frequency returned to that of the PQC coated with PMC film. Even if the blank solution containing EDTA was flowed, nickel(ii), copper(ii) and cobalt(ii) could not be eluted; that is, elution for the nickel(ii), copper(ii) and cobalt(ii) with EDTA from the PMC film was difficult. Therefore, 20 mm of a nitric acid solution was used to elute the metal ions.

3 883 Fig. 2 Frequency shifts of the Normal PQC in Fig. 1 coated with 5.2 µg polystyrene (a), 5.3 µg chitosan (b), and 5.2 µg chitosan derivative (c) and uncoated (d) in dried air. Temperature, 25 C; flow rate, 4.0 ml/min. Fig. 4 Frequency of the PQC unmodified ( ) and modified ( ) with PMC in ammonium salt buffer solution (ph 9.5). Reaction time, 3 min; other conditions as in Fig. 2. Fig. 3 Adsorption behavior of metal ions on an electrode-separated PQC modified with a chitosan derivative (PMC) in a 60 mm ammonium salt buffer (ph 9.5) solution containing 15 mm potassium nitrate and 1 µm metal ions. Reaction time, 140 s; other conditions as in Fig. 2. Influence of the ph and concentration of the buffer solution The frequencies of electrode-separated PQC modified with PMC in an ammonium chloride buffer solution (ph 9.5) over the range of 5 (0.37 ms/cm) to 100 mm (6.4 ms/cm) are shown in Fig. 4. The electrode-separated PQC modified with PMC did not vibrate in a lower conductivity solution than 0.37 ms/cm, pure water, and acetate buffer solution. It was supposed that the increasing mass of the PMC with water adhered by the hydrogen bond between the PMC and water was too heavy to vibrate. Although the frequencies of the electrode-separated PQC without film were approximately constant at larger than 20 mm buffer, those with PMC gradually increased with increasing concentration over 30 mm (2.2 ms/cm). The conductivity of the solutions, therefore, should be controlled with a buffer solution. In the silver(i) solution, the frequency decreased when the concentration of an ammonium chloride buffer solution was lower than 30 mm. On the other hand, the frequencies gradually increased with increasing concentration of the buffer solution above 30 mm, in spite of the fact that the frequency shifts caused by the adsorption of silver(i) would decrease with increasing the concentration of the buffer solution because Fig. 5 Frequency shifts of the PQC modified with PMC in various ph of a 60 mm ammonium salt buffer solution containing 100 nm silver ion with (a) and without (b) a supporting electrolyte (15 mm potassium nitrate). Other conditions as in Fig. 4. complexation of silver(i) with PMC would be difficult for the higher concentrations of the ammonium ion. The frequency shifts decreased with increasing ph prepared with a 60 mm ammonium chloride buffer solution (Fig. 5b), while almost the same frequency shifts were obtained at a constant conductivity (8.4 ms/cm) prepared with the addition of different amounts of potassium nitrate to a 60 mm ammonium chloride buffer solution, as if the ph was different (Fig. 5a). It was found that the frequency shifts depended on the conductivity. To obtain a larger frequency shift, caused by the adsorption of silver(i), a higher concentration of the buffer solution than 50 mm should be prepared. Flow rate and temperature influence The flow rate of a silver(i) solution over the range of 2 7 ml/min did not affect the frequency shifts when the passage time of the sample solution was 3 min. It was suggested that because the adsorption rate of silver(i) to the PMC film was fast, the reaction established an equilibrium condition immediately. The beginning adsorption speed of silver(i) onto the PMC film differed for different concentrations of silver(i). The frequencies, however, finally became constant in flowing silver(i), and the frequency shifts were proportional to the concentrations. A flow rate of 4 ml/min was selected. The frequency shifts were increased with increasing

4 884 ANALYTICAL SCIENCES AUGUST 2002, VOL. 18 Table 1 Effect of foreign ions Frequency shift/hz Frequency shift/hz Metal ion/µm 60 µm EDTA Metal ion/µm 60 µm EDTA 50 nm Ag(I) µm Pb(II) µm Ni(II) µm Al(III) µm Cu(II) 8 40 µm Cr(III) µm Co(II) 1 50 µm Mn(II) 3 20 µm Hg(II) µm Mg(II) 4 20 µm Fe(III) µm Sr(II) 3 20 µm Fe(II) µm Ca(II) 5 30 µm Cd(II) µm Ba(II) 6 30 µm Zn(II) 8 Fig. 6 Effect of the EDTA concentration on the adsorption of 1 µm metal ions in a 60 mm ammonium salt buffer solution containing 15 mm potassium nitrate., silver(i);, nickel(ii);, copper(ii);, cobalt(ii); other conditions as in Fig. 4. Frequency shifts of the PQC modified with PMC in a 60 mm ammonium salt buffer solutions containing 15 mm potassium nitrate and metal ions. [L] + [AgL + ] = [L 0]. (3) A rearrangement yields temperature of the solution over the range C, and an exponential function was found between the frequency shifts ( F) and the temperatures (T). The activation energy of the adhesion of silver(i) on the PMC film was about 41 kj mol 1, computed from a plot between ln F and 1/T. Effect of foreign ions The optimal concentration of EDTA for masking metal ions was examined before investigating the influence of foreign ions. The effects of the EDTA concentration on the adsorption of silver(i), nickel(ii), copper(ii) and cobalt(ii), each of 1 µm, are shown in Fig. 6. Equivalent concentrations of EDTA to metal ions could mask the metal ions, except for nickel(ii), which required at least a 10-fold concentration of EDTA. The frequency shifts in the silver(i) solution including EDTA were constant even if the concentrations of EDTA were increased, concluding the PMC film in EDTA solution was especially selectivity to silver(i). It was supposed that the formation constant between PMC and silver(i) would be larger than that between EDTA and silver(i), and that the complexes between PMC and other metal ions would be less stable than that of the EDTA complexes. The proposed method, therefore, is selective for silver(i). The frequency shifts in metal-ion solutions containing 60 µm EDTA are shown in Table 1. The existence of foreign metal ions at a few µm concentration would be permitted to be determined at the 50 nm level of silver(i) provided that 60 µm EDTA was added. Calibration graph for silver(i) In an ammonium chloride buffer solution, the ligand substitution reaction of silver(i) to the PMC film [L] can be represented as Ag(NH 3) 2+ + L = AgL + + 2NH 3. (1) The formation constant (K) for the reaction is [AgL K = + ][NH K 3] 2 = AgL, (2) [Ag(NH 3) 2+ ][L] K Ag(NH3 ) 2 + where K AgL and K Ag(NH3 ) 2 + are the formation constants for AgL + and Ag(NH 3) 2+, respectively. If L 0 represents the total concentration of all forms of PMC, then 1 [NH 1 1 = 3] 2 +. (4) [AgL + ] K[L0] [Ag(NH 3) 2+ ] [L0] If one supposes that [NH F = k [AgL + ] and 3] 2 = k, K k [L 0] Eq. (4) yields, 1 F k 1 = +, (5) [Ag(NH 3) 2+ ] k [L 0] where k and k are constants. A calibration graph was constructed under the following conditions: a 60 mm ammonium chloride buffer solution (ph 9.5) containing 15 mm potassium nitrate, a flow rate of 4.0 ml/min, and a sample passage time of 3 min. The frequency changes ( F, Hz) resulting from silver(i) adsorption increased with increasing concentration of silver(i) ([Ag + ], nm) over the range nm (Fig. 7a). A plot of 1/[Ag(NH 3) 2+ ] against to 1/ F according to Eq. (5) was linear, 1/ F = 0.568/[Ag + ]+0.002, having a correlation coefficient of (Fig. 7b). The frequency shifts ( F 1, Hz) for silver(i) without other metal ions was proportional to the concentrations (C 1, nm) over the range nm and F 1 = 3.51C ; the correlation coefficient was Containing 60 µm EDTA and each 1 µm of nickel(ii), copper(ii), cobalt(ii), cadmium(ii) and zinc(ii), the frequency shifts ( F 2, Hz) also depended upon the concentrations (C 2, nm), according to the equation F 2 = 3.89C , the correlation coefficient was The detection limit and the relative standard deviation for 50 nm silver(i) of five measurements were 6 nm and 3.4%, respectively. Conclusions N-(2-Pyridylmethyl) chitosan (PMC) modified onto an electrode-separated PQC by the intervention of silane and glutaraldehyde was a stable membrane, which could be used in acidic and basic solutions. Silver(I) could be selectively determined from the coexisting metal ions in an ammonium chloride buffer solution containing EDTA. Also, the frequency

5 885 Fig. 7 Relationship between the silver(i) concentration and the frequency shift (a) and the Langmuir response (b). Other conditions as in Fig. 4. shifts resulting from the adsorption of silver(i) increased with the addition of potassium nitrate as a supporting electrolyte. Although the frequency shift came back to the original one with only the blank solution, larger frequency shifts were obtained using a 20 mm nitric acid solution as the eluent. References 2. T. Nomura and A. Minemura, Nippon Kagaku Kaishi, 1980, P. L. Konash and G. J. Bastiaans, Anal. Chem., 1980, 52, T. Nomura, T. Nagamune, K. Izutsu, and T. S. West, Bunseki Kagaku, 1981, 30, T. Nomura and T. Nagamune, Anal. Chim. Acta, 1983, 155, T. Nomura and M. Iijima, Anal. Chim. Acta, 1981, 13, H. J. Schmidt, U. Pittermann, H. Schneider, and K. G. Weil, Anal. Chim. Acta, 1993, 273, C. Sanchez-Pedreno, J. A. Ortuno, and D. Martinez, Anal. Chim. Acta, 1992, 263, R. L. Bunde and J. J. Rosentreter, Microchem. J., 1993, 47, T. Nomura and T. Mimatsu, Anal. Chim. Acta, 1982, 143, S.-Z. Yao, L.-H. Nie, and Z.-H. Mo, Anal. Chim. Acta, 1989, 217, T. Nomura, Bunseki Kagaku, 1987, 36, T. Nomura and S. Yumoto, Bunseki Kagaku, 1991, 40, T. Nomura, M. Kumagai, and A. Sato, Anal. Chim. Acta, 1997, 343, T. Nomura and A. Sato, Bunseki Kagaku, 1997, 46, T. Nomura and M. Ando, Anal. Chim. Acta, 1985, 172, T. Nomura, T. Okuhara, and T. Hasegawa, Anal. Chim. Acta, 1986, 182, S. H. Bao and T. Nomura, Bunseki Kagaku, 2002, 51, Y. Baba and H. Hirakawa, Chem. Lett., 1992, Y. Baba, Y. Kawano, and H. Hirakawa, Bull. Chem. Soc. Jpn., 1996, 69, K. Inoue, Y. Baba, and K. Yoshizuka, Bull. Chem. Soc. Jpn., 1993, 66, M. Minunni, P. Skladal, and M. Mascini, Anal. Lett., 1994, 27(8), T. Nomura and T. Yamada, U. S. Patent, 1993, K. Sakurai, T. Shibano, K. Kimura, and T. Takashi, Sen-i Gakkaishi, 1985, 41, T J. L. Jones and J. P. Mieure, Anal. Chem., 1969, 41, 484.

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