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Determination of glucose using a piezoelectric quartz crystal and the silver mirror reaction
Analytica Chimica Acta 407 (2000) 17–21
Determination of glucose using a piezoelectric quartz crystal and the silver mirror reaction Oi-Wah Lau ∗ , Bing Shao Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, PR China Received 24 December 1997; received in revised form 14 September 1999; accepted 17 September 1999
Abstract The ‘silver mirror’ reaction is utilized to detect glucose with a piezoelectric quartz crystal. Silver produced in this reaction deposits on the surface of the quartz crystal electrode and forms an elastic film. Sauerbrey’s equation can be applied without the need to consider the effect of viscoelasticity. The properties of the solution were found to have little effect on the response signal. The deposited silver can easily be removed with dilute nitric acid. The quartz crystal can be used indefinitelywithout losing its sensitivity. This method can detect glucose with high sensitivity and good linearity in the range of 1–80 g ml−1 . ©2000 Elsevier Science B.V. All rights reserved. Keywords: Glucose; Piezoelectric quartz crystal; Silver mirror reaction
1. Introduction The piezoelectric quartz crystal ‘microbalance’ (QCM) has been used as a sensing device for the determination of components in solution for some time [1–7]. One of the main fields of application of the QCM is as a (bio)chemical sensor. For such an application a sensitive film is usually coated on the electrode, which can interact selectively with a certain component in the solution. However, the re-usability of these sensors is still a problem that does not have a satisfactory solution. Nomura [8] utilized the formation of BaSO4 from Ba(NO3 )2 and the adsorption of BaSO4 on the piezoelectric crystal electrode to determine SO4 2− in wa∗ Corresponding author. Tel.: +852-2609-6344; fax: +852-26035315. E-mail address: [email protected] (O.-W. Lau).
ter. No selective film was needed. The selectivity was obtained by Ba2+ interacting selectively with SO4 2− . BaSO4 could be removed from the electrode with EDTA, thus enabling the crystal to be used repeatedly. The silver mirror reaction is well known for testing for aldehydes. Sigga and Segal [9] used Tollen’s reagent as the oxidant to determine carbonyl groups: R–CHO + 2Ag(NH3 )2 + + H2 O → R–COOH + 2Ag + 2NH4 + The unused silver ions are determined by potentiometric titration with potassium iodide. As a smooth thin silver film is formed in a silver mirror reaction, it was thought that by monitoring the deposited silver film with a QCM, glucose, which reacts with the silver ions as above, could be detected. The silver mirror reaction was chosen because of its importance for detecting glucose. The deposited silver can be removed
0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 7 8 3 - 7
18
O.-W. Lau, B. Shao / Analytica Chimica Acta 407 (2000) 17–21
followed by two drops of 5% NaOH solution and seven drops of 2 M ammonia. The solution was mixed well by sucking in and squeezing out the solution several times with a dropper. The frequency value, f1 , was recorded after 1 min., before 1.00 ml of freshly prepared glucose solution was added and mixed as above. The frequency value, f2 , was recorded after 10 min. The difference between the two frequency values, 1f = f1 −f2 , was taken as the response signal. Doubly distilled water was used throughout. All experiments were performed at room temperature (i.e. 21 ± 1◦ C).
Fig. 1. Schematic diagram of the well-type cell.
from the electrode with dilute nitric acid, so that the The piezoelectric crystal can be used repeatedly without losing its sensitivity. Moreover, the deposited silver film is an ideal case for the application of Sauerbrey’s equation [1], as it has been found that there is no viscoelasticity so that interference due to non-mass factors is negligible.
2. Experimental 2.1. Apparatus Piezoelectric quartz crystals were purchased from the Beijing 707 Plant. A gold electrode was used with an active area of 0.28 cm2 for each side. A well-type cell was laboratory-made with the quartz crystal stuck to one end of a glass tube with epoxy glue forming the bottom of the well (Fig. 1). The oscillating circuit was the same as that described by Bruckenstein and Shay [10]. Crystals with a fundamental frequency of 9 MHz were used. The frequency values were measured with a frequency counter (Model T213, Sung, Hong Kong) with a resolution of 0.1 Hz. Stable oscillation of the QCM in liquids can be obtained using this system.
3. Results and discussion In the classical silver mirror reaction, 5% aqueous silver nitrate solution was used to prepare the Tollen’s reagent. After the aldehyde or glucose was added to Tollen’s reagent for 10 min, a precipitate of silver would appear [11]. Fig. 2 is the response of the system to 0.05 mg of glucose using 1 ml of 0.1% AgNO3 solution as a function of time. Fig. 2 shows clearly that the quartz crystal electrode responded to glucose and the frequency decreased by about 6 kHz in the first hour. As the frequency changes over the course of the reaction, it should be recorded at a fixed time after the start of the reaction for all determinations. In subsequent experiments, a waiting time of 10 min after the reaction started was chosen as a compromise between sensitivity and speed of the experiment.
2.2. Procedure The electrode was treated with 1M HNO3 for 30 min and cleaned with water. An aliquot of 1.00 ml of 0.1% AgNO3 solution was added into the analytical cell,
Fig. 2. Response of the system to 0.05 mg ml−1 of glucose under conditions as described in Section 2.2.
O.-W. Lau, B. Shao / Analytica Chimica Acta 407 (2000) 17–21
Fig. 3. Effect of the concentration of silver nitrate on the responding signal, other conditions as described in Section 2.2.
The effect of the concentration of silver nitrate on the response signal was assessed and the results are shown schematically in Fig. 3. Considering the linear response range, response time, sensitivity, reproducibility and precision, the concentration of AgNO3 solution selected was 0.1%. The deposited silver can be removed with dilute nitric acid (Fig. 4). Most of the deposited silver can be removed in 5 min. However, if the quartz crystal was treated with HNO3 for too long, the sensitivity of the crystal would be lowered (see Fig. 5), so a time of 30 min was selected for further experiments. The reproducibility of the effect on the quartz crystal frequency of treatment with 1 M nitric acid for 30 min is shown in Table 1.
Fig. 4. Removing deposited Ag with 1 M HNO3 .
19
Fig. 5. Effect of the time of treatment with 1 M nitric acid on the response signal; other conditions as described in Section 2.2.
The addition of the solutions of sodium hydroxide and ammonia to the test solution is to form a fresh Ag(NH3 )2 + solution. This addition causes the frequency to fluctuate. In order to record a precise value of f1 , an optimum waiting time is needed both for the attainment of equilibrium of the solution and for the stabilization of the frequency values. However, it has been found that the sensitivity decreased with waiting time after 5 min (Fig. 6). Since the difference between the response after a waiting time of 1 min was not much different from that after 5 min, the waiting time of 1 min was selected.
Fig. 6. Effect of waiting time in the presence of Ag(NH3 )2 + on the response signal; other conditions as described in Section 2.2.
20
O.-W. Lau, B. Shao / Analytica Chimica Acta 407 (2000) 17–21
Table 1 Recovery of crystal frequency after treatment with 1 M nitric acid for 30 min Number
1
2
3
4
Frequency before silver deposition (kHz) Frequency after silver deposition (kHz) Frequency after treatment with HNO3 (kHz) Recovery (%)
20.110 19.813 20.082 90.6
20.082 19.676 20.072 97.5
20.072 19.487 20.068 99.3
20.068 19.568 20.083 103
Fig. 7. Effect of time for water storage on the response signal; other conditions as described in Section 2.2.
Fig. 8. Calibration for glucose utilizing the silver mirror reaction; conditions as described in Section 2.2.
O.-W. Lau, B. Shao / Analytica Chimica Acta 407 (2000) 17–21 Table 2 Effect of liquid parameters on the frequency before and after silver depositiona Eq. (1)
1fb = 2355 (ρη)1/2 + 3201
Eq. (2)
1fa = 2407 (ρη)1/2 + 3097
Eq. (3)
1fa −1fb = 52 (ρη)1/2 − 104
Correlation coefficient = 0.9973 (n = 6) Correlation coefficient = 0.9973 (n = 6)
1fb , the frequency value before the reaction ; 1fa , the frequency value after the reaction and ρ and η are the density and the viscosity of the solution respectively. a
Before the AgNO3 solution was added to the cell, water was added to clean the cell after treatment with dilute nitric acid. If water was stored in the cell, the storage time also had an effect on the response signal (Fig. 7). The longer the water storage time, the lower was the sensitivity, but the reason for this is not clear. The storage of water may change the surface of the crystal and the deposition of silver may be sensitive to the surface of the crystal. In any case, water was not stored in the cell. The piezoelectric crystal responded linearly to the glucose concentration in the range 1–80 g ml−1 . When a sensitive film consisting of macromolecules such as antibodies is attached to a QCM, it is often viscous so that it is not easy to get linear calibration. In contrast, no sensitive film is needed for the silver mirror reaction, and the deposited silver film is elastic so that a good linear relation can be obtained as shown in Fig. 8. The selectivity of the system for glucose has been discussed previously [9,11]. It has been proven in our previous studies that the properties of the liquid may cause errors in mass-sensing by a QCM [12]. The relations between frequency and liquid parameters for the proposed sensor have also been determined before and after silver deposition (Table 2). Eq. (3) is the difference between Eqs. (1) and (2). As the coefficient of (ρη)1/2 is small in Eq. (3), the properties of the liquid have little effect on the response signal, possibly because there was little change in the interfacial nature before and after silver deposition, and the properties of the liquid also changed little before and after silver deposition so that the error due to liquid parameters is negligible. Hence in this method, it is not necessary to make
21
efforts to keep the density, viscosity, conductivity, etc. of the solution constant.
4. Conclusions Before this study, it was not easy to employ the silver mirror reaction to detect glucose quantitatively. With the help of a QCM, glucose can be determined with good sensitivity utilizing this reaction. The silver mirror reaction is well suited for the application of a QCM: good linear calibration is easily obtained due to the formation of the smooth, elastic silver film, and the crystal can be used repeatedly without losing sensitivity because the deposited silver can be removed with dilute nitric acid. However, the sensitivity of the crystal is affected by the state of its surface, which is easily altered by many factors. The sensitivities may differ greatly between different crystals, and even for the same crystal after different treatments. More work needs to be done to improve the reproducibility of the proposed method, which will make the method more useful. References [1] G. Sauerbrey, Z. Phys. 55 (1959) 206. [2] P.L. Konash, G.J. Bastiaans, Anal. Chem. 52 (1980) 1929. [3] T. Nomura, M. Okuhara, Anal. Chim. Acta 142 (1982) 281. [4] J.J. McCallum, Analyst 114 (1989) 1173. [5] M. Thompson, A.L. Kipling, W.C. Duncan-Hewitt, L.V. ˆ c-Vlasak, Analyst 116 (1991) 881. Rajakoviæ, B.A. Cavi´ [6] J.W. Grate, S.J. Martin, R.M. White, Anal. Chem. 65 (1993) 940A and 987A. [7] M.D. Ward, in I. Rubinstein (Ed.), Physical Electrochemistry: Principles, Methods and Applications, Marcel Dekker; New York 1995. [8] T. Nomura, Bunseki Kagaku, 36(1987) 93, Japan, CA106:162266aG. [9] S. Sigga, E. Segal, Anal. Chem. 25 (1953) 640. [10] S. Bruckenstein, M. Shay, Electrochim. Acta 30 (1985) 1295. [11] N.D. Cheronis, T.S. Ma, Organic Functional Group Analysis by Micro and Semimicro Methods, Interscience Publishers, New York, 1964, p.154. [12] O.W. Lau, B. Shao, W.M. Zhang, Anal. Chim. Acta 312 (1995) 217.
Determination of glucose using a piezoelectric quartz crystal and the silver mirror reaction Oi-Wah Lau ∗ , Bing Shao Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, PR China Received 24 December 1997; received in revised form 14 September 1999; accepted 17 September 1999
Abstract The ‘silver mirror’ reaction is utilized to detect glucose with a piezoelectric quartz crystal. Silver produced in this reaction deposits on the surface of the quartz crystal electrode and forms an elastic film. Sauerbrey’s equation can be applied without the need to consider the effect of viscoelasticity. The properties of the solution were found to have little effect on the response signal. The deposited silver can easily be removed with dilute nitric acid. The quartz crystal can be used indefinitelywithout losing its sensitivity. This method can detect glucose with high sensitivity and good linearity in the range of 1–80 g ml−1 . ©2000 Elsevier Science B.V. All rights reserved. Keywords: Glucose; Piezoelectric quartz crystal; Silver mirror reaction
1. Introduction The piezoelectric quartz crystal ‘microbalance’ (QCM) has been used as a sensing device for the determination of components in solution for some time [1–7]. One of the main fields of application of the QCM is as a (bio)chemical sensor. For such an application a sensitive film is usually coated on the electrode, which can interact selectively with a certain component in the solution. However, the re-usability of these sensors is still a problem that does not have a satisfactory solution. Nomura [8] utilized the formation of BaSO4 from Ba(NO3 )2 and the adsorption of BaSO4 on the piezoelectric crystal electrode to determine SO4 2− in wa∗ Corresponding author. Tel.: +852-2609-6344; fax: +852-26035315. E-mail address: [email protected] (O.-W. Lau).
ter. No selective film was needed. The selectivity was obtained by Ba2+ interacting selectively with SO4 2− . BaSO4 could be removed from the electrode with EDTA, thus enabling the crystal to be used repeatedly. The silver mirror reaction is well known for testing for aldehydes. Sigga and Segal [9] used Tollen’s reagent as the oxidant to determine carbonyl groups: R–CHO + 2Ag(NH3 )2 + + H2 O → R–COOH + 2Ag + 2NH4 + The unused silver ions are determined by potentiometric titration with potassium iodide. As a smooth thin silver film is formed in a silver mirror reaction, it was thought that by monitoring the deposited silver film with a QCM, glucose, which reacts with the silver ions as above, could be detected. The silver mirror reaction was chosen because of its importance for detecting glucose. The deposited silver can be removed
0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 7 8 3 - 7
18
O.-W. Lau, B. Shao / Analytica Chimica Acta 407 (2000) 17–21
followed by two drops of 5% NaOH solution and seven drops of 2 M ammonia. The solution was mixed well by sucking in and squeezing out the solution several times with a dropper. The frequency value, f1 , was recorded after 1 min., before 1.00 ml of freshly prepared glucose solution was added and mixed as above. The frequency value, f2 , was recorded after 10 min. The difference between the two frequency values, 1f = f1 −f2 , was taken as the response signal. Doubly distilled water was used throughout. All experiments were performed at room temperature (i.e. 21 ± 1◦ C).
Fig. 1. Schematic diagram of the well-type cell.
from the electrode with dilute nitric acid, so that the The piezoelectric crystal can be used repeatedly without losing its sensitivity. Moreover, the deposited silver film is an ideal case for the application of Sauerbrey’s equation [1], as it has been found that there is no viscoelasticity so that interference due to non-mass factors is negligible.
2. Experimental 2.1. Apparatus Piezoelectric quartz crystals were purchased from the Beijing 707 Plant. A gold electrode was used with an active area of 0.28 cm2 for each side. A well-type cell was laboratory-made with the quartz crystal stuck to one end of a glass tube with epoxy glue forming the bottom of the well (Fig. 1). The oscillating circuit was the same as that described by Bruckenstein and Shay [10]. Crystals with a fundamental frequency of 9 MHz were used. The frequency values were measured with a frequency counter (Model T213, Sung, Hong Kong) with a resolution of 0.1 Hz. Stable oscillation of the QCM in liquids can be obtained using this system.
3. Results and discussion In the classical silver mirror reaction, 5% aqueous silver nitrate solution was used to prepare the Tollen’s reagent. After the aldehyde or glucose was added to Tollen’s reagent for 10 min, a precipitate of silver would appear [11]. Fig. 2 is the response of the system to 0.05 mg of glucose using 1 ml of 0.1% AgNO3 solution as a function of time. Fig. 2 shows clearly that the quartz crystal electrode responded to glucose and the frequency decreased by about 6 kHz in the first hour. As the frequency changes over the course of the reaction, it should be recorded at a fixed time after the start of the reaction for all determinations. In subsequent experiments, a waiting time of 10 min after the reaction started was chosen as a compromise between sensitivity and speed of the experiment.
2.2. Procedure The electrode was treated with 1M HNO3 for 30 min and cleaned with water. An aliquot of 1.00 ml of 0.1% AgNO3 solution was added into the analytical cell,
Fig. 2. Response of the system to 0.05 mg ml−1 of glucose under conditions as described in Section 2.2.
O.-W. Lau, B. Shao / Analytica Chimica Acta 407 (2000) 17–21
Fig. 3. Effect of the concentration of silver nitrate on the responding signal, other conditions as described in Section 2.2.
The effect of the concentration of silver nitrate on the response signal was assessed and the results are shown schematically in Fig. 3. Considering the linear response range, response time, sensitivity, reproducibility and precision, the concentration of AgNO3 solution selected was 0.1%. The deposited silver can be removed with dilute nitric acid (Fig. 4). Most of the deposited silver can be removed in 5 min. However, if the quartz crystal was treated with HNO3 for too long, the sensitivity of the crystal would be lowered (see Fig. 5), so a time of 30 min was selected for further experiments. The reproducibility of the effect on the quartz crystal frequency of treatment with 1 M nitric acid for 30 min is shown in Table 1.
Fig. 4. Removing deposited Ag with 1 M HNO3 .
19
Fig. 5. Effect of the time of treatment with 1 M nitric acid on the response signal; other conditions as described in Section 2.2.
The addition of the solutions of sodium hydroxide and ammonia to the test solution is to form a fresh Ag(NH3 )2 + solution. This addition causes the frequency to fluctuate. In order to record a precise value of f1 , an optimum waiting time is needed both for the attainment of equilibrium of the solution and for the stabilization of the frequency values. However, it has been found that the sensitivity decreased with waiting time after 5 min (Fig. 6). Since the difference between the response after a waiting time of 1 min was not much different from that after 5 min, the waiting time of 1 min was selected.
Fig. 6. Effect of waiting time in the presence of Ag(NH3 )2 + on the response signal; other conditions as described in Section 2.2.
20
O.-W. Lau, B. Shao / Analytica Chimica Acta 407 (2000) 17–21
Table 1 Recovery of crystal frequency after treatment with 1 M nitric acid for 30 min Number
1
2
3
4
Frequency before silver deposition (kHz) Frequency after silver deposition (kHz) Frequency after treatment with HNO3 (kHz) Recovery (%)
20.110 19.813 20.082 90.6
20.082 19.676 20.072 97.5
20.072 19.487 20.068 99.3
20.068 19.568 20.083 103
Fig. 7. Effect of time for water storage on the response signal; other conditions as described in Section 2.2.
Fig. 8. Calibration for glucose utilizing the silver mirror reaction; conditions as described in Section 2.2.
O.-W. Lau, B. Shao / Analytica Chimica Acta 407 (2000) 17–21 Table 2 Effect of liquid parameters on the frequency before and after silver depositiona Eq. (1)
1fb = 2355 (ρη)1/2 + 3201
Eq. (2)
1fa = 2407 (ρη)1/2 + 3097
Eq. (3)
1fa −1fb = 52 (ρη)1/2 − 104
Correlation coefficient = 0.9973 (n = 6) Correlation coefficient = 0.9973 (n = 6)
1fb , the frequency value before the reaction ; 1fa , the frequency value after the reaction and ρ and η are the density and the viscosity of the solution respectively. a
Before the AgNO3 solution was added to the cell, water was added to clean the cell after treatment with dilute nitric acid. If water was stored in the cell, the storage time also had an effect on the response signal (Fig. 7). The longer the water storage time, the lower was the sensitivity, but the reason for this is not clear. The storage of water may change the surface of the crystal and the deposition of silver may be sensitive to the surface of the crystal. In any case, water was not stored in the cell. The piezoelectric crystal responded linearly to the glucose concentration in the range 1–80 g ml−1 . When a sensitive film consisting of macromolecules such as antibodies is attached to a QCM, it is often viscous so that it is not easy to get linear calibration. In contrast, no sensitive film is needed for the silver mirror reaction, and the deposited silver film is elastic so that a good linear relation can be obtained as shown in Fig. 8. The selectivity of the system for glucose has been discussed previously [9,11]. It has been proven in our previous studies that the properties of the liquid may cause errors in mass-sensing by a QCM [12]. The relations between frequency and liquid parameters for the proposed sensor have also been determined before and after silver deposition (Table 2). Eq. (3) is the difference between Eqs. (1) and (2). As the coefficient of (ρη)1/2 is small in Eq. (3), the properties of the liquid have little effect on the response signal, possibly because there was little change in the interfacial nature before and after silver deposition, and the properties of the liquid also changed little before and after silver deposition so that the error due to liquid parameters is negligible. Hence in this method, it is not necessary to make
21
efforts to keep the density, viscosity, conductivity, etc. of the solution constant.
4. Conclusions Before this study, it was not easy to employ the silver mirror reaction to detect glucose quantitatively. With the help of a QCM, glucose can be determined with good sensitivity utilizing this reaction. The silver mirror reaction is well suited for the application of a QCM: good linear calibration is easily obtained due to the formation of the smooth, elastic silver film, and the crystal can be used repeatedly without losing sensitivity because the deposited silver can be removed with dilute nitric acid. However, the sensitivity of the crystal is affected by the state of its surface, which is easily altered by many factors. The sensitivities may differ greatly between different crystals, and even for the same crystal after different treatments. More work needs to be done to improve the reproducibility of the proposed method, which will make the method more useful. References [1] G. Sauerbrey, Z. Phys. 55 (1959) 206. [2] P.L. Konash, G.J. Bastiaans, Anal. Chem. 52 (1980) 1929. [3] T. Nomura, M. Okuhara, Anal. Chim. Acta 142 (1982) 281. [4] J.J. McCallum, Analyst 114 (1989) 1173. [5] M. Thompson, A.L. Kipling, W.C. Duncan-Hewitt, L.V. ˆ c-Vlasak, Analyst 116 (1991) 881. Rajakoviæ, B.A. Cavi´ [6] J.W. Grate, S.J. Martin, R.M. White, Anal. Chem. 65 (1993) 940A and 987A. [7] M.D. Ward, in I. Rubinstein (Ed.), Physical Electrochemistry: Principles, Methods and Applications, Marcel Dekker; New York 1995. [8] T. Nomura, Bunseki Kagaku, 36(1987) 93, Japan, CA106:162266aG. [9] S. Sigga, E. Segal, Anal. Chem. 25 (1953) 640. [10] S. Bruckenstein, M. Shay, Electrochim. Acta 30 (1985) 1295. [11] N.D. Cheronis, T.S. Ma, Organic Functional Group Analysis by Micro and Semimicro Methods, Interscience Publishers, New York, 1964, p.154. [12] O.W. Lau, B. Shao, W.M. Zhang, Anal. Chim. Acta 312 (1995) 217.