Biosensors and Bioelectronics 24 (2009) 3008–3013

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A disposable screen-printed silver strip sensor for single drop analysis of halide in biological samples Mei-Hsin Chiu a , Wan-Ling Cheng a , Govindan Muthuraman a , Cheng-Teng Hsu b , Hsieh-Hsun Chung b , Jyh-Myng Zen a,∗ a b

Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan Zensor R&D, Taichung City, Taiwan

a r t i c l e

i n f o

Article history: Received 17 December 2008 Received in revised form 4 March 2009 Accepted 4 March 2009 Available online 17 March 2009 Keywords: Screen-printed silver strip Single drop analysis Halide ion detection Ag/Agx O reference electrode

a b s t r a c t A screen-printed silver strip with three-electrode configuration of Ag-working, Ag-counter and Ag/Agx O reference electrodes was developed for simultaneous determination of chloride, bromide and iodide in aqueous solutions. It was fabricated simply by screen-printing silver ink onto a polypropylene (PP) base. The in-situ prepared Ag/Agx O reference electrode can avoid the leaching interference in chloride detection while using a conventional Ag/AgCl reference electrode. A single drop of analyte (50 ␮l) is enough to determine iodide, bromide and chloride by measuring the well-separated oxidation peak currents of respective silver halides. The calibration graph was linear from 10 ␮M to 20 mM for iodide and bromide and 100 ␮M to 20 mM for chloride and the detection limit (S/N = 3) was 3.05 ␮M, 2.95 ␮M and 18.83 ␮M for iodide, bromide and chloride, respectively. The strip is designed to be disposable and as such manual polishing is not necessary. The proposed sensor is not only simple to manufacture and easy to operate but also fast and precise with little detection volume. It is successfully applied to the determination of halide ions in real samples. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Halide ions analyses are important due to their presence in different domains, such as biomedical, food, environmental, etc. (Pickard and Emery, 1982; Taylor and Schultz, 1996; Katsu et al., ˇ 1997; Masadome et al., 1999; Sucman and Bednáˇr, 2003). Coulometric titration using electrogenerated Ag+ has long been used in the determination of halide but is, however, lack of selectivity (Kissinger and Heineman, 1984; Sawyer et al., 1995). To improve the selectivity, a quite complicated method by coupling of constant current chronopotentiometry and square-wave voltammetry was developed to simultaneous determination of Cl− , Br− and I− in a mixture (Parham and Zargar, 2002). Underpotentially deposited Ag adlayers on Au(III) surfaces with an effected electrochemical change by halide adsorption was also reported to measure Cl− , Br− and I− (Michalitsch et al., 2000; Michalitsch and Laibinis, 2001; Choi and Laibinis, 2004). The surface passivation on platinum electrodes in I− analysis was improved by using laser irradiation to remove solid iodine from the electrode surface (Akkermans et al., 1999). An alternative to noble metal electrodes is edge plane pyrolytic graphite electrode, which is inexpensive compared to their metallic counterparts (Lowe et al., 2005).

∗ Corresponding author. Fax: +886 4 22854007. E-mail address: [email protected] (J.-M. Zen). 0956-5663/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2009.03.004

Since disposable sensors hold a clear advantage for on-site testing, we report here a screen-printed silver electrode strip (SP-AgES) sensor for determination of halide in aqueous solutions. The proposed sensor strip is fabricated simply by screen-printing silver ink onto a polypropylene (PP) base with Ag-working, Ag-counter and Ag/Agx O reference electrodes. Silver electrodes have been reported to have the best electrocatalytic activity with an appropriate choice of supporting electrolyte in halide analysis (Ball et al., 1960; Arai et al., 1996; Suturovic´ et al., 1997; Yeom et al., 1999; Hukelmann ˇ and Oster, 2002; Sucman and Bednáˇr, 2003; Araújo et al., 2004). Silver surface exhibits a remarkable cage effect to promote high local concentration of either true or latent halide radicals (Rondinini et al., 2000). The use of Ag ink pseudo-reference electrode (i.e., Ag+/0 ) is not suitable for the strip since at ambient conditions Ag can be easily converted to its oxides. The subsequent chlorination of Ag ink in the detection of chloride ion may also induce the formation Ag/AgCl and hence a potential shift. Liquid-free reference electrodes (Hettiarachchi and MacDonald, 1987; Sato and Ohno, 1996; Desmond et al., 1997; Ciobanu et al., 2002, 2004) also exhibit fluctuation in potentials and drift with prolonged time during a series of measurements (Ives and Janz, 1969; Sawyer and Roberts, 1974) and was improved by polyion-sensitive polymer membranes (Nagy et al., 1997; Lee et al., 1998). To simplify the fabrication process and component, we focus on the electrochemical generation and characterization of Agx O on screen-printed silver electrode (AgSPE). The fabrication is straight-

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Fig. 1. Schematic representation of the set-up for single drop analysis of halide in aqueous solutions at the SP-AgES.

forward and eliminates the need to use glass or plastic tubes. It can also avoid the possible contamination of sample solutions with the leaching of Cl− from conventional Ag/AgCl reference electrode. We are interested in miniaturization volume of analytes with fast detection time (Chen et al., 2005; Chang and Zen, 2006, 2007; Zen et al., 2007) and this work affords clear-cut sensor fabrication over conventional ion-selective electrode (ISE)-based sensors in which membranes are used for their selectivity (Laitinen and Lin, 1963; Edmond, 1988; Janata, 1989; Spichiger-Keller, 1998; ´ Muchandani and Sadik, 2000; Zielinska et al., 2002). The planar and fully screen-printed SP-AgES and its analytical utilities are demonstrated especially on the stability and reproducibility of the Ag/Agx O reference electrode. Selective and accurate voltammetric determination method of halide ions was then established using the SP-AgES and applicable to the determination of halide ions in real samples. 2. Experimental 2.1. Materials KI, KBr, KCl, KNO2 , Na2 SO4 , Na2 C2 O4 , Na2 S·9H2 O (Showa) and KH2 PO4 , K2 HPO4 , KOH, KSCN, KCN, H3 PO4 (Riedel-de Haën) were used as received. Aqueous solutions were prepared with Millipore de-ionized water throughout this investigation. A 0.1 M stock solution of Cl− , Br− and I− were prepared with 0.1 M, pH 6 PBS and stored under refrigeration. Carrier and sample solution were prepared by suitable dilution of the stock solution before experiments. 2.2. Apparatus and procedure All the experiments were carried out with a CHI 703 electrochemical workstation (CH Instruments, Austin, TX, USA). Fig. 1 illustrates the set-up and schematic representation of the proposed

SP-AgES used in this study. The strip consists of Ag-working electrode (a), Ag/Agx O reference electrode (b) and Ag-counter electrode (c). A semi-automatic screen-printer was used in printing the conducting silver ink (Acheron, Japan) on a flexible polypropylene film (50 mm × 70 mm) and cured at 100◦ C for 10 min (Zen et al., 2000, 2002). After drying, an insulating layer was finally printed over the SPE leaving the working area of 7.07 mm2 . All experiments were conducted using 50 ␮l of aqueous solution unless otherwise mentioned. The Ag/Agx O reference electrode of the SP-AgES was prepared by scanning 10 cycles in the potential range of 0.0 V to +1.5 V (vs. Ag/AgCl) in 0.1 M, pH 6 PBS. The color of the electrode surface changes from grey to black after the electrochemical treatment (as shown in Fig. 1). 3. Results and discussion 3.1. Evaluation of Ag/Agx O reference electrode Although silver wire (Ag+/0 ) has been used as a quasi-reference electrode in electrochemical detection of chloride by underpotentially deposited silver films on polycrystalline gold (Choi and Laibinis, 2004), it is known that Ag quasi-reference electrode exhibits fluctuation in potentials. This is confirmed by comparing the shift in peak potential of Br− in pH 6 PBS between AgSPE (Ag+/0 ) and Ag/Agx O reference electrodes. As shown in Fig. 2A, with 10 different Ag-working strips, compared to a much higher R.S.D. of 7.74% (0.0925 ± 0.0069 V) observed at Ag+/0 , the Ag/Agx O shows a R.S.D. of only 1.74% (−0.153 ± 0.0027 V) in reproducibility (see supporting information Fig. S-1A). When a separate single drop analysis was carried out every once a week with the same piece of SP-AgES after washing with water and keeping in air without any protection, there was only a small shift of the peak potential for Br− in 0.1 M PBS with R.S.D. = 1.71% (−0.148 ± 0.0025 V) after 14 weeks duration (see supporting information Fig. S-1B). The use of Ag/Agx O refer-

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Fig. 2. (A) Cyclic voltammograms of the AgSPE coupled with Ag/AgCl reference electrode in 0.1 M, pH 6 PBS (a) first scan, (b) after 10 scans, (c) spiked with 300 ␮M Cl− and (d) Ag/Agx O as reference electrode after 100th scan. Scan rate = 50 mV s−1 . (B) The value of measured potential is compared with standard potential.

ence electrode can therefore improve the poor stability of Ag+/0 quasi-reference electrode. The development of Ag/Agx O reference electrode can also avoid the possible contamination of sample solutions from the leaching Cl− solution of the conventional Ag/AgCl reference electrode. The performance of reference electrode was first evaluated in pH 6 PBS. Note that the extent of contamination from Ag/AgCl reference electrode varies from the type and condition used and the one illustrated here represents a relatively serious case. No current response corresponding to Cl− was observed in the first cycle (a); whereas, a current response started to occur after 10 segments of scanning (b). The current response was further confirmed as the leaching Cl− by spiking with 300 ␮M Cl− (c). In the case of Ag/Agx O reference electrode, no such a Cl− ion peak can be observed even after 100th scan. The difference in the oxidation peak potential observed between Ag+/0 and Ag/Agx O in detecting Br− also indirectly confirms the electrochemical treatment to produce a different Ag electrode surface. The cyclic voltammogram of a reversible redox couple of Fe(CN)6 3−/4− was found to maintain good reversibility at the proposed SP-AgES. Compared to E1/2 = +0.24 V and +0.083 V observed at Ag/AgCl and Ag+/0 , respectively, a negative shift of E1/2 to −0.11 V was observed at the Ag/Agx O. The value of measured potential is compared for easy understanding with standard potential as depicted in Fig. 2B. Overall, the applicability of the Ag/Agx O reference electrode is promising and is essential for Cl− determination. The proposed SP-AgES sensor provides a viable alternative to established commercial reference electrodes.

Fig. 3. (A) Cyclic voltammograms of halide ions (500 ␮M each) and (B) log(v) vs. log(i) plot for 100 ␮M Br− at the SP-AgES. Other conditions are the same as in Fig. 2.

3.2. Electrochemical behavior of the SP-AgES Well-separated oxidation potential of −0.057 V, −0.19 V and −0.43 V (vs. Ag/Agx O) is observed for Cl− , Br− and I− , respectively, by single drop analysis of halide ions in pH 6 PBS (Fig. 3A). It is proposed earlier that, as a halide ion reacts with Ag, a silver halide precipitate is formed on the surface and consequently produces an oxidation wave on the voltammogram. Oxidation of the silver halide precipitate on the electrode surface leads to the generation of a oxidation current in accordance with the equation of AgX + e− → Ag + X− (X = Cl− , Br− and I− ) (Arai et al., 1996). The well-separated peak potentials were found to correlate with the solubility product (Ksp ), which determines thermodynamically the interaction tendency between the surface silver atoms and the halide anions (Raton, 1983–1984). The difference in the interaction tendency to form silver compounds (e.g., AgCl, AgBr and AgI) within an Ag-anion bond would then result in the shift in the oxidation/reduction potential. It is worth to note that the Ksp order of silver compounds in water is F− > Cl− > Br− > I− , particularly the Ksp of AgF is at least 12 orders of magnitude (182 g/100 ml water at 15 ◦ C) larger than others (e.g., AgCl = 1.76 × 10−10 , AgBr = 5.32 × 10−13 and AgI = 8.49 × 10−17 ) in the same group. The smaller the Ksp , the stronger the interaction tendency between the silver surface atoms and the halide anions, so F− is the weakest among the four. This means that the surface precipitate is readily achieved for Cl− , Br− and I− ions, but not for F− ions under similar conditions.

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We first assume that the same mechanism, in which halides are anodically accumulated on Hg electrodes as HgX(s) and then stripped off by scanning cathodically, can also be applied to Ag (Ball et al., 1960). Yet, instead of adsorption-controlled, a diffusioncontrolled process was observed for both oxidation and reduction reactions at the SP-AgES. According to the log(ip ) vs. log(v) plot, slope value of ∼0.5 was observed in both oxidation and reduction processes (Fig. 3B). In addition, since the ipc /ipa value is not equal to unity and the peak potential difference (Ep ) is more than 130 mV for all three halides, it is expected that the oxidation and reduction peaks should not be from a reversible redox process (Bard and Faulkner, 2001). The current function (ip /v1/2 C) of the reduction process was also found to have about twice higher than that of oxidation process in all studied scan rates. We think that chemical complication may be involved in the reduction process. Since the oxidation process shows diffusion-controlled one electron reaction and is more precise in the quantification of halide, the oxidation process is therefore used for further analytical application.

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Fig. 4. Simultaneous determination of halide ions at the SP-AgES by linear scan voltammetry in 0.1 M, pH 6 PBS. Concentration range of iodide and bromide is from 20 ␮M to 2 mM and chloride is from 100 ␮M to 10 mM. Scan rate = 50 mV s−1 .

3.3. Halide ions determination Because the Ag/Agx O reference electrode was in-situ generated and made the way easy for single drop analysis, its inter electrode assay was evaluated in the presence of Cl− , Br− and I− of various concentration. For 10 different strips to measure the same solutions, the R.S.D. was 0.4%, 1.74% and 3.17% for 100 ␮M I− , 100 ␮M Br− and 500 ␮M Cl− , respectively (see supporting information Fig. S-2 A and B). The current responses of halide ions were highly stable with a R.S.D. of <5%. Overall, the use of SP-AgES in deter-

mination of halide ions is very promising. Since each halide ion possesses well-separated oxidation peak potential (Fig. 3A), simultaneous determination of Cl− , Br− and I− is also possible. Fig. 4 shows voltammograms of Cl− , Br− and I− mixed solution at both low and high concentrations. Highly resolved voltammetric peaks of Cl− , Br− and I− verify the applicability of the SP-AgES in simultaneous determination of halide in aqueous solutions. The slope of the obtained calibration plot was 0.029, 0.027 and 0.028 (␮A/␮M) (correlation coefficient = 0.998) for Cl− , Br− and I− , respectively. A

Table 1 Recoveries of halides in serum, urine, sweat, sea water and pond water by standard addition method. Sample

Original detected value (mM)

Spike (mM)

Seruma

1.05 ± 0.007

1 2 3 4

2.10 3.07 4.22 5.27

± ± ± ±

0.005 0.010 0.009 0.008

103.21 102.15 105.58 105.40

± ± ± ±

0.68 0.34 0.23 0.17

Urinea

0.39 ± 0.003

1 2 3 4

1.36 2.35 3.32 4.52

± ± ± ±

0.007 0.006 0.042 0.012

98.05 98.23 97.97 103.53

± ± ± ±

2.07 1.03 0.69 0.52

Sweata

0.52 ± 0.948

1 2 3 4

1.54 2.55 3.41 4.59

± ± ± ±

0.007 0.012 0.025 0.031

102.11 101.27 96.23 101.75

± ± ± ±

1.73 0.87 0.58 0.43

Sea waterb

0.93 ± 0.017

1 2 3 4

1.89 2.96 3.92 4.89

± ± ± ±

0.038 0.036 0.045 0.129

98.37 101.06 99.91 99.32

± ± ± ±

1.11 1.86 2.04 1.56

Pond waterc

0.21 ± 0.063

0.2 0.3 0.4 0.5

0.39 0.48 0.57 0.68

± ± ± ±

0.018 0.202 0.106 0.209

97.16 95.26 95.12 96.45

± ± ± ±

2.03 0.84 0.73 1.19

Pond waterd

ND

0.1 0.2 0.3 0.4

0.098 0.195 0.302 0.389

± ± ± ±

0.002 0.005 0.007 0.001

97.61 97.36 100.55 97.41

± ± ± ±

2.24 2.66 2.34 0.34

Pond watere

ND

0.1 0.2 0.3 0.4

0.098 0.200 0.293 0.401

± ± ± ±

0.003 0.009 0.001 0.013

97.53 99.79 97.78 100.22

± ± ± ±

2.54 4.70 0.09 3.34

ND: not detectable. a Dilution factor = 100. b Dilution factor = 500. c Chloride. d Bromide. e Iodide.

Detected value after spike (mM)

Recovery (%) (n = 3)

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marized in Table 1, indicate that the proposed sensor strip can be practically applicable to the real matrices. 4. Conclusion

Fig. 5. Typical linear scan voltammograms in single drop analysis of chloride in serum real sample by standard addition method.

wide linear range of 10 ␮M to 20 mM, 20 ␮M to 20 mM and 100 ␮M to 20 mM was observed for I− , Br− and Cl− , respectively. The calculated detection limit (S/N = 3) was 3.05 ␮M, 2.95 ␮M and 18.83 ␮M for I− , Br− and Cl− , respectively. The selectivity in detecting halides was checked for different inorganic ions (see supporting information Table S-1). For NO2 − , C2 O4 2− and SO4 2− , the current variation shows very low interference effect (<±5%) even for 100 times excess of interfering ions to 100 ␮M I− , 100 ␮M Br− and 500 ␮M Cl− . This is as expected due to a relatively larger solubility of NO2 − , C2 O4 2− and SO4 2− than those of the halides in water. As to likely interferents, such as CN− , SCN− and S2− that have poorly soluble Ag salts, the interference is noticeable even at equal concentration of interfering ions. The interference effect is predictable and especially for SCN− to Br− (+22.93%), CN− to Cl− (−7.67%) and S2− to I− (−28.3%). Yet, since the amount of CN− , SCN− and S2− is often low in real samples and the interference is negligible when the concentration of interfering ions is 10 times less than the analytes. This is the reason why the selectivity of SP-AgES is still acceptable in real sample analysis and thus will be discussed later. Real sample analysis of Cl− in energy drink is first presented in this study. As mentioned earlier, the advantage of the proposed sensor is its readiness for detection simply by putting a single drop (50 ␮l) of real sample solution on the SP-AgES surface. The detected value of 17.39 mM by standard addition method is fairly close to the labeled value of 16.70 mM. The selectivity is also acceptable as the amount of Cl− added (334 ␮M) correlated well the value measured (347.9 ± 5.5 ␮M) with a recovery of ∼104%. The proposed strip was then applied to the chloride test in serum, urine, sweat and sea water. The results obtained are summarized in Table 1. Serum, urine and sweat samples were taken from a healthy person and the concentration of chloride value measured was 104.8 ± 0.71 mM, 38.8 ± 0.27 mM and 52.0 ± 0.95 mM, respectively. Typical voltammetric results by single drop analysis of Cl− in a serum real sample are shown in Fig. 5. The samples were also subject to a measurement by ion-selective electrode at a local hospital and a comparable value of 104 mM and 37 mM for serum and urine, respectively, was obtained. Overall, the proposed strip has been shown to provide accurate determination of Cl− in “real world” samples. Determination of halide in sea water was also demonstrated and the concentration of Cl− was found to be 0.45 M. This is quite reasonable as the concentration of Cl− was generally in the range of 0.40–0.55 M in sea water (Millero, 2002). Trace amount of Br− and I− in sea water, unfortunately, can not be detected by the strip. Pond water was further applied for simultaneous detection of halides by standard addition method. Good recoveries, as sum-

We successfully demonstrate a screen-printed silver strip with three-electrode configuration in detection of chloride, bromide and iodide by single drop analysis. The results show the evidence that the fabricated method and design of electrochemical strip by printing the conducting silver ink on a flexible polypropylene film surface are simple and precise. The Ag/Agx O reference electrode possesses better stability than the Ag ink quasi-reference electrode (Ag+/0 ). Good analytical performance was obtained in the determination of halide ions at the SP-AgES by linear scan voltammetry. In particular, the present developed system enables us to detect halide in aqueous solutions with only 50 ␮l of testing volume in a very short detection time (<20 s for one concentration). This purposed method is also promising for real sample analyses. It is thus a useful platform for the development of a highly sensitive halide sensor. The simplicity of the sensor fabrication and its anticipated scalability offer advantages for miniaturizing the present system into a microsensor format (Chang and Zen, 2006, 2007). Acknowledgements The authors gratefully acknowledge financial support from the National Science Council of Taiwan. This work is supported in part by the Ministry of Education, Taiwan under the ATU plan. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.bios.2009.03.004. References Akkermans, R.P., Fulian, Q., Roberts, S.L., Suárez, M.F., Compton, R.G., 1999. J. Phys. Chem. B 103, 8319–8327. Arai, K., Kusu, F., Noguchi, N., Takamura, K., Osawa, H., 1996. Anal. Biochem. 240, 109–113. Araújo, A.N., Montenegroa, M.C.B.S.M., Kousalová, L., Sklenárová, H., Solich, P., Olmos, R.P., 2004. Anal. Chim. Acta 505, 161–166. Ball, R.G., Manning, D.L., Menis, O., 1960. Anal. Chem. 32, 621–623. Bard, A.J., Faulkner, L.R., 2001. Electrochemical Methods, Fundamentals and Applications, 2nd ed. Wiley, New York. Chang, J.-L., Zen, J.-M., 2006. Electrochem. Commun. 8, 571–576. Chang, J.-L., Zen, J.-M., 2007. Electrochem. Commun. 9, 2744–2750. Chen, J.-C., Chung, H.-H., Hsu, C.-T., Tsai, D.-M., Kumar, A.S., Zen, J.-M., 2005. Sens. Actuators B 110, 364–369. Choi, H.G., Laibinis, P.E., 2004. Anal. Chem. 76, 5911–5917. Ciobanu, M., Wilburn, J.P., Buss, N.L., Ditavong, P., Lowy, D.A., 2002. Electroanalysis 14, 989–997. Ciobanu, M., Wilburn, J.P., Lowy, D.A., 2004. Electroanalysis 16, 1351–1358. Desmond, D., Lane, B., Alderman, J., Glennon, J.D., Diamond, D., Arrigan, D.W.M., 1997. Sens. Actuators B 44, 389–396. Edmond, T.E. (Ed.), 1988. Chemical Sensors. Chapman and Hall, New York (Chapter 3). Hukelmann, M., Oster, O., 2002. Clin. Chim. Acta 319, 75–76. Hettiarachchi, S., MacDonald, D.D., 1987. J. Electrochem. Soc. 134, 1307–1308. Ives, D., Janz, G., 1969. Reference Electrodes. Academic Press, New York, pp. 110–111, 333–335. Janata, J., 1989. Principles of Chemical Sensors. Plenum, New York (Chapter 4). Katsu, T., Mori, Y., Matsuka, N., Gomita, Y., 1997. J. Pharm. Biomed. Anal. 15, 1829–1832. Kissinger, P.T., Heineman, W.R., 1984. Laboratory Techniques in Electroanalytical Chemistry. Marcel Dekker, New York. Laitinen, H.A., Lin, Z.F., 1963. Anal. Chem. 35, 1405–1411. Lee, H.J., Hong, U.S., Lee, D.K., Shin, J.H., Nam, H., Cha, G.S., 1998. Anal. Chem. 70, 3377–3383. Lowe, E.R., Banks, C.E., Compton, R.G., 2005. Electroanalysis 17, 1627–1634. Masadome, T., Asano, Y., Nakamura, T., 1999. Talanta 50, 595–600. Michalitsch, R., Laibinis, P.E., 2001. Angew. Chem. Int. Ed. 40, 941–944. Michalitsch, R., Palmer, B.J., Laibinis, P.E., 2000. Langmuir 16, 6533–6540.

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