Sensors and Actuators B 115 (2006) 473–480

An enzymeless electrochemical sensor for the selective determination of creatinine in human urine J.-C. Chen a , A.S. Kumar a , H.-H. Chung a , S.-H. Chien b,c , M.-C. Kuo b,c , J.-M. Zen a,∗ a

Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan b Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan c Department of Chemistry, National Taiwan University, Taichung 10664, Taiwan

Received 5 May 2005; received in revised form 8 October 2005; accepted 12 October 2005 Available online 29 November 2005

Abstract Determination of creatinine in various biological fluids is useful for evaluation of renal, muscular and thyroid dysfunctions. An enzymeless electrochemical approach for the selective and quantitative recognition of creatinine in human urine has been demonstrated by using a preanodized screen-printed carbon electrode (SPE* ). During a preconcentration step (at 1.8 V versus Ag/AgCl), the formation of a stable carbon–carbon bond between the electro-generated C O of SPE* and the active methylene group of creatinine was identified by XPS. By using the SPE* together with a medium exchange procedure, the creatinine was selectively detected in the window of 0.37–3.6 mM with a slope and regression coefficient of 16.7 ␮A/mM and 0.998, respectively, by square-wave voltammetry. Ten successive detection of 0.37 mM creatinine showed a relative standard deviation of 3.4%, indicating a detection limit (signal/noise = 3) of 8.6 ␮M. Real human urine samples were analyzed by this method and compared with the results obtained from the Jaff´e reaction procedure. © 2005 Elsevier B.V. All rights reserved. Keywords: Creatinine; Human urine; Screen-printed electrode; Preanodization

1. Introduction Creatinine, the end product of the metabolism of creatine in mammals, is a clinically important index of renal glomerular filtration rates [1–4]. Determination of this metabolite in various biological fluids is also useful for evaluation of renal, muscular and thyroid dysfunctions. Several immunoassays or enzymatic methods have been developed for selective creatinine recognition and detection [5–10]. In general, the creatinine biosensors with measurable electrochemical signals are based on sequence enzymatic reactions either by detecting NH4 + or H2 O2 with a major interference of the endogenous creatine. The performance of the biosensor often depends on the stability of the enzyme layer and thus a recalibration of the sensor is necessary. The thickness of enzyme layer can also affect the response time as the species transport within the membrane limits the dynamics of the whole process of analytical signal generation.



Corresponding author. Fax: +886 4 22862547. E-mail address: [email protected] (J.-M. Zen).

0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.10.015

Although highly selective and relatively fast creatinine determinations in biofluids are possible with high-performance liquid chromatography (HPLC) [6,11,12], it requires tedious off-line preparations, expensive equipment, and HPLC-skilled personal and may not be well suited for clinical routine analysis. Therefore the development of rapid, cheap and selective creatinine sensors is promising and pressing need in clinical diagnosis. We report here an enzymeless approach using a preanodized screen-printed carbon electrode (SPE* ) for the selective and quantitative detection of creatinine in human urine. The idea is adopted from the Jaff´e reaction (Scheme 1), in which the active methylene group (i.e., adjacent carbony group with ␣-hydrogen to act as a nucleophilic donor) reacts with alkaline sodium picrate to give a red–yellow complex for the creatinine determination in clinical laboratories [2,13]. The Jaff´e reaction, however, has serious interference from co-existing biological samples like ascorbic acid (AA) and uric acid (UA) [2]. In the present case, the active methylene group of creatinine is selectively involved in a carbonyl–condensation reaction with the introduction of carbonyl functionalities at the SPE* to form a strong bonding on the surface. A simple medium exchange

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Scheme 1. Jaff´e reaction.

procedure can then lead to the selective recognition of creatinine without any interference from co-existing urinary chemicals like AA, UA, urea, ammonia and creatine. To the best of our knowledge, this is the first report for the creatinine determination using an enzymeless electrochemical sensor. Real sample assays were demonstrated for the human urine samples without any off-line treatments. 2. Experimental 2.1. Chemicals and materials Creatinine (hydrochloride and anhydrous), creatine, UA and AA were all obtained from Sigma and used as received without further purification. All other chemicals used were of ACS certified reagent grade. A pH 6.7 phosphate buffer solution (PBS) was used in all studies. Unless otherwise stated, the creatinine hydrochloride (Ctn-HCl) sample was taken as a standard for all the experiments. Water was obtained from a Millipore purification system (18 M/cm). Solutions and buffers were prepared by employing standard laboratory procedures. 2.2. Instrumentation and electrodes Voltammetric measurements were carried out with a BAS 100W workstation and a BAS VC-2 electrochemical cell (West Lafayette, IN, USA). The three-electrode configuration consisted of a SPE (Zensor R&D, Taiwan) as a working electrode,

Fig. 1. SW voltammograms for (A) 4 mM creatinine hydrochloride (Ctn-HCl), (B) 1 mM uric acid and (C) 1 mM ascorbic acid on SPE and SPE* (at step-1 and step-2) in pH 6.7 PBS. (D) SWV response (at step-2) for the 4 mM creatine (Cr), urea (50 mM) and NH4 Cl (50 mM) on the SPE* . Other conditions were the same as in Scheme 2. SWV parameters: frequency = 15 Hz, amplitude = 25 mV and step = 5 mV.

Scheme 2. Conceptional representation for the selective recognition of creatinine (Ctn) in presence of uric acid (UA), ascorbic acid (AA), urea, ammonium chloride (NH4 + ) and creatine (Cr) at the SPE* . Step-1: preconcentration of SPE* at 1.8 V vs. Ag/AgCl for 50 s in pH 6.7 PBS. Step-2: medium exchange of the SPE* where the preconcentrated electrode is submerged in water under constant stirring for 5 s. All the analytes except creatinine were retained on the SPE* during step-2. The C O surface functional group was generated by preanodization of a bare SPE at 2.0 V vs. Ag/AgCl for 400 s in pH 6.7 PBS.

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an Ag/AgCl reference electrode, and a platinum wire auxiliary electrode. X-ray photoelectron spectroscopy (XPS) analysis (Omicron DAR 400, Germany) was performed by using an Al K␣ Xray radiation source (1486.6 eV) with 0.1 eV of resolution. The pressure inside the analyzer was maintained at about 10−10 Torr during the measurements. Prior to the experiments, the binding energy (BE) was standardized with Au4f3/2 (84.0 eV, full width at half maximum (fwhm) = 1.20 eV). The C1s peak at 284.6 eV is taken uniformly as an internal standard. The high-resolution spectra were performed under ambient conditions and averaged by a number of scans to increase the signal-to-noise ratio. Quantitative XPS analysis was carried out by using an Origin 6.0 graphic program to pick up the intensity maximum and BE values. XPS peak areas were calculated by using in-built software programs. Peak sensitive parameters were used to calculate the atomic ratio factors. 2.3. Procedure A bare SPE was first thoroughly washed with a copious amount of Millipore water, followed by the preanodization at

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2.0 V for 400 s in pH 6.7 PBS. Electroanalysis of creatinine consists of two discrete steps as illustrated in Scheme 2. In the first step, creatinine is preconcentrated at the SPE* under a deposition potential of 1.8 V versus Ag/AgCl for 50 s. Then, a medium exchange procedure (step-2) is applied by submerging the preconcentrated SPE* in pure water under stirring for 5 s to clean up the electrode. Note that the strong bonding of the creatinine on the surface (i.e., SPE* -Ctn) was not affected by this clean process. Finally, the amount of creatinine is measured by SWV in pH 6.7 PBS. The optimized SWV parameters were as follows: frequency = 15 Hz, amplitude = 25 mV and step = 5 mV. Human urine samples were collected from two laboratory personals and stored in a refrigerator before use. The urine samples were suitably diluted with a blank base electrolyte and the amount of creatinine was measured by a standard addition method. In the Jaff´e reaction approach, 10 mL of a filtered urine sample is diluted to 10 times by using 1 M NaOH and 2.0 mL of a saturated picric acid solution [2]. After completely shaking for 10 min, the deep reddish-yellow color formed in the test sample was quantitatively detected with a spectrophotometer at 520 nm.

Fig. 2. High-resolution XPS responses for (A) C1s , O1s and (B) N1s core energy levels. (C) Conceptional representation for the surface characteristics.

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3. Results and discussion 3.1. Response characteristics To verify the necessity of both the preactivation process (i.e., preanodization and preconcentration) and medium exchange procedure to the selective recognition of creatinine, experiments were performed with creatinine and other co-existing samples in urine (such as UA, AA, ammonium, urea and creatine). Note that the UA interference was reported to be remarkable on both SPE and SPE* under the step-1 condition [10]. Classical biosensors often use a permselective membrane and/or an internal oxidant (like PbO2 ) in the composition to eliminate the interference [8–10]. These analytes were all preconcentrated at 1.8 V versus Ag/AgCl for 50 s in either pH 6.7 PBS or a human urine sample (step-1), followed by a medium exchange process to pure water (step-2). As can been in Fig. 1A, no faradic response of creatinine was observed at a bare SPE, while the SPE* yielded a well-defined SWV response at ∼0.42 V versus Ag/AgCl in pH 6.7 PBS. The preactivation process (step1) obviously has a dramatic effect on the electron-transfer property of the creatinine redox system. A medium exchange process (step-2) to clean up the electrode, on the other hand, turns out to be typical to achieve good selectivity to creatinine. As can be seen in Fig. 1B and C, the weak-adsorbed UA and AA were all washed out during the cleaning process. In other words, only the strongly adsorbed creatinine can still retain the anodic stripping

activity at the SPE* after step-2. Ammonium, urea and creatine at concentrations close to the levels in human urine also did not show any characteristic peak current signals at the SPE* , as shown in Fig. 1D. Scheme 2 illustrates how the preactivation process and medium exchange procedure can be applied to the selective recognition of creatinine. Most important of all, this observation can indeed assure the possibility of a simple and selective procedure for the creatinine determination in human urine. 3.2. Mechanistic studies Activation of carbon surface either by chemical or electrochemical methods can have a dramatic effect on the electrontransfer property of redox systems. The origins of these effects have been attributed to several factors. It is well documented that the electrochemical anodization procedure leads to the formation of C O (major) and/or C–OH groups on glassy carbon electrode (GCE) surface [14–19]. The anodic oxidation of highly oriented pyrolytic graphite (HOPG) results in the destruction of the basal plane and the exposure of a large quantity of edge plane graphite at the electrode surface and thus faster electron transfer characteristics [20,21]. In our own results, we also demonstrated that a preanodized GCE can selectively trigger the complexation with Pb2+ through the [ C O· · ·Pb2+ ] formation at a preconcentrtion potential of −1.2 V versus Ag/AgCl in acidic solutions [22,23].

Fig. 3. (A) SWV responses for the SPE* (step-1) with Ctn-HCl or Ctn-anhydrous samples in pH 6.7 PBS. (B) Peak current responses against the added [Cl− ]. Other conditions were the same as in Fig. 1.

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In this work, X-ray photoelectron spectroscopy (XPS) was applied to study the surface characteristics of SPE on a few ˚ dimension. Fig. 2A displays typical high-resolution XPS A responses of the C1s and O1s core energy levels for SPE, SPE* and SPE* -creatinine (SPE* -Ctn) samples. As can be seen, the bare SPE surface contains a major fraction of native carbon peak ( C , 285.6 eV) with minor fractions of –C–OH (∼286 eV), C O, –C–O–C (287 eV) groups. The results are in agreement with the reported peaks on a classical carbon surface [15–19]. Upon preanodization of a SPE, the C1s peak (especially the native carbon) was found to decrease by ∼20%. On the other hand, a ∼25% increase in the O1s level was observed after the electrochemical anodization procedure. This particular observation confirms the introduction of quinone functionalities ( C O, –C–OH, –C( O)–OH, etc.) upon the SPE* surface [18,19]. Based on the peak-senstive parameters, the carbon to nitrogen atomic ratio (NO1s /NC1s ) was calculated as 0.07 and 0.11 for SPE and SPE* , respectively. The marked increase in the ratio after the preanodization procedure proves the activation of the SPE surface. Note that the NO1s /NC1s ratio was reported to lie in the window of 0.04–0.15 under various GCE polishing treatments [16]. Mechanical polishing with organic solvents was found to reduce the NO1s /NC1s value, whereas the cloth/alumina polishing leads to a ∼4 times increase in the ratio. Surface carbon density is another important factor to increase the NO1s /NC1s

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Fig. 4. SWV responses for the SPE* (step-2) with increasing concentration of Ctn-HCl in pH 6.7 PBS. SWV conditions were the same as in Fig. 1.

ratio and it can be improved to 0.51 upon anodization of the GCE surface [17]. Even though SPE has less surface carbon density over that of GCE, the advantages of cheap, mass-producible, flexible and disposable features make SPE more attractive for practical analysis. In the present case, the active methylene group (i.e. a carbonyl group with ␣-hydrogen to act as a nucleophilic

Scheme 3. Possible chemical/electrochemical reaction pathway of creatinine on the SPE* .

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Fig. 5. Selective recognition of creatinine in two human urine samples (#1 and #2) at the SPE* by the proposed method. Other conditions were the same as in Fig. 1.

donor) of creatinine was proposed to get involved in the carbonyl–condensation reaction with surface C O functional groups of SPE* to form a strong covalent complex (i.e., SPE* -Ctn) on the surface. Fig. 2A and B clearly indicate the formation of the SPE* -Ctn through the observation of typical C1s , O1s and N1s XPS peak responses. Because of the extra source of nitro and oxygen groups from the SPE* -Ctn deposited on the SPE* , the NO1s /NC1s ratio increases up to 0.16. Compared to the literature results [24,25], the N1s XPS levels represent –C–N–CH3 (399.2 eV), –C–NH–C (400.2 eV) and C NH (401.1 eV) groups. No such N1s responses were noticed on control experiments with both SPE and SPE* . Apart from this, traces from the internal hydrogen bonding of –H–O–H– (O1s , 535.5 eV) and –N–H–O– (N1s , 402.7 eV) were also identified on the surface. These boding features may also partially help to stabilize the creatinine on the SPE* . Our mechanistic studies also suggested that the presence of chloride anions is necessary and essential to generate the methylene functional groups ([creatinine]:[Cl− ] = 1:1) in a neutral medium (in step-1). Control electrochemical experiments in the absence of chloride ions (Ctn-anhydrous as a sample) in pH 6.7 PBS did not show any faradic response on the SPE* . The proposed mechanism was illustrated in Fig. 3A. Furthermore, the addition of Cl− ions to the Ctn-anhydrous solution ([Ctnanhydrous]:[Cl− ] = 1:1) resulted in the same behavior as that of Ctn-HCl (Fig. 3B). Presumably the Cl− ions can lead to the formation of HCl with the labile ␣-hydrogen and in turn helps to the addition reaction for the case of the Ctn-anhydrous sample. This is indeed another advantage of the proposed method, as the [Cl− ] in human urine is in the range of 98–107 mM [2]. It is thus not necessary to add extra Cl− ions to generate the proposed reaction. Scheme 3 sketches a possible mechanism for the electrochemical behavior of Ctn on the SPE* . In the presence of chloride ions, creatinine loses one proton from the active methyl site and further gets reacted with the surface functional groups on the SPE* during precocentration procedure at 1.8 V versus Ag/AgCl

by forming a –C–C– link. In addition, stripping analysis can then be performed to selectively determine creatinine. Of course, further detailed product analysis is necessary to confirm the oxidative product and to the mechanism. 3.3. Analysis of urine samples As shown in Fig. 4, the reproducibility of both step-1 and step2 with Ctn-HCl is highly accurate and quantitative. The anodic peak current signals were found to systematically increase with an increase in creatinine from 0.36 to 3.7 mM with a slope and regression coefficient value of 16.7 ␮A/mM and 0.998, respectively, at the SPE* . Ten successive detection of 0.37 mM creatinine under the optimized SWV conditions yielded a relative standard deviation of 3.42%. The detection limit (signal/noise = 3) was calculated as 8.6 ␮M. In healthy individuals, the elimination of creatinine in urine occurs at an approximately constant rate and its concentration is used as a correlation factor in the study of the excretion of several substances [26–28]. In general, the concentration of creatinine in the urine samples lies in the range of 3.5–15 mM [2]. Fig. 5 shows the SWV responses for the urinary creatinine assays by a standard addition method. The creatinine levels determined in two human urine samples were 4.79 and 2.86 mM with recoveries of 97 and 102%, respectively. The concentrations determined by the Jaff´e reaction procedure were, on average, 30% higher than those determined by the proposed method. This result verifies the interferences inherent to the Jaff´e technique and is consistent with earlier observations [9,29,30]. Most important of all, the study indicates the applicability of the method to practical analysis. 4. Conclusions We demonstrate in this study first time a selective and quantitative recognition of creatinine in human urine without any interference from the co-existing common biochemical compounds with preactivated screen-printed carbon electrodes

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together with a medium exchange procedure. The formation of carbon–carbon bonds between the electrogenerated C O and the active methylene group of creatinine was found to be a key for the observation. Since the approach is simple and easy to extend into miniature devices, the disposable nature of SPE can offer a cheap and enzymeless way to clinical systems. Acknowledgment The authors gratefully acknowledge financial support from Yung Shin Pharmaceutical Industrial Co. Ltd., Taiwan. References [1] S.-K. Jung, G.S. Wilson, Polymeric mercaptosilane-modified platinum electrodes for elimination of interferents in glucose biosensors, Anal. Chem. 68 (1996) 591–596. [2] M.L. Bishop, J.L. Duben-Engelkirk, E.P. Fody, Clinical Chemistry, Principles, Procedures, Correlations, 4th ed., Lippincott Williams and Wilkins, New York, 2000, p. 440 (Chapter 21). [3] S. Hallan, A. Asberg, M. Lindberg, H. Johnsen, Validation of the modification of diet in renal disease formula for estimating GFR with special emphasis on calibration of the serum creatinine assay, Am. J. Kidney Dis. 44 (2004) 84–93. [4] C.M. Gibson, D.S. Pinto, S.A. Murphy, D.A. Morrow, H.-P. Hobbach, S.D. Wiviott, R.P. Giugliano, C.P. Cannon, E.M. Antman, E. Braunwald, Association of creatinine and creatinine clearance on presentation in acute myocardial infarction with subsequent mortality, J. Am. Coll. Cardiol. 42 (2003) 1535–1543. [5] A.J. Killard, M.R. Smyth, Creatinine biosensors: principles and designs, Trends Biotech. 18 (2000) 433–437. [6] T. Smith-Palmer, Separation methods applicable to urinary creatine and creatinine, J. Chromatogr. B 781 (2002) 93–106. [7] A. Benkert, F. Scheller, W. Schossler, C. Hentschel, B. Micheel, O. Behrsing, G. Scharte, W. Stocklein, A. Warsinke, Development of a creatinine ELISA and an amperometric antibody-based creatinine sensor with a detection limit in the nanomolar range, Anal. Chem. 72 (2000) 916–921. [8] T. Osaka, S. Komada, A. Amano, Y. Fujino, H. Mori, Electrochemical molecular sieving of the polyion complex film for designing highly sensitive biosensor for creatinine, Sens. Actuators B 65 (2000) 58–63. [9] B. Tombach, J. Schneider, F. Matzkies, R.M. Schaefer, G.C. Chemnitius, Amperometric creatinine biosensor for hemodialysis patients, Clin. Chim. Acta 312 (2001) 129. [10] J.H. Shin, Y.S. Choi, H.J. Lee, S.H. Choi, J. Ha, I.J. Yoon, H. Nam, G.S. Cha, A planar amperometric creatinine biosensor employing an insoluble oxidizing agent for removing redox-active interferences, Anal. Chem. 73 (2001) 5965–5971. [11] A.K. Hewavitharana, H.L. Bruce, Simultaneous determination of creatinine and pseudouridine concentrations in bovine plasma by reversedphase liquid chromatography with photodiode array detection, J. Chromatogr. B 784 (2003) 275–281. [12] G. Werner, V. Schneider, J. Emmert, Simultaneous determination of creatine, uric acid and creatinine by high-performance liquid chromatography with direct serum injection and multi-wavelength detection, J. Chromatogr. 525 (1990) 265–275. [13] J. McMurray, Organic Chemistry, 3rd ed., Cole Publishing Company, Pacific Grove, CA, 1992, p. 844. [14] L. Kavan, Electrochemical carbon, Chem. Rev. 97 (1997) 3061–3082. [15] R.L. McCreery, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 17, Marcel Dekker, New York, 1991, p. 221. [16] P. Chen, R.L. McCreery, Control of electron transfer kinetics at glassy carbon electrodes by specific surface modification, Anal. Chem. 68 (1996) 3958–3965.

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Biographies Jyh-Cheng Chen is currently pursuing his doctoral studies under the guidance of Prof. Jyh-Myng Zen. His research interests include electrochemistry, chemical sensors and physical organic chemistry. Annamalai Senthil Kumar is a postdoctoral fellow in Prof. J.-M. Zen’s laboratory. He obtained his PhD degree in department of Physical chemistry, University of Madras, India in 2000. His research interest includes interdisciplinary areas of material science, physical and analytical chemistry in particular of designing and application of chemically modified electrodes, electro-catalysis, nanoparticles and chemical- and bio-sensors. Hsieh-Hsun Chung is a postdoctoral fellow in Prof. J.-M. Zen’s laboratory. He obtained his doctoral degree in electrochemistry from National Chung Hsing University in 2002. His research interests include chemically modified electrodes, electrocatalysis and biosensors.

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Shu-Hua Chien is a professor and research fellow in Institute of Chemistry, Academia Sinica. Her main interest is in the structure and reactivity of supported metal catalysts with particular to heterogeneous catalysis at the gas/solid interface. Ming-Chih Kuo is a postdoctoral fellow in Dr. S.-H. Chien’s laboratory. Jyh-Myng Zen obtained his BS degree in Chemistry from National Tsing Hua University, Taiwan, in 1980. He served in the military 1980–1982. He studied electrochemistry with Larry R. Faulkner at University of Illinois,

Urbana-Champaign and graduated with a PhD in 1988. For the next three years, he was a postdoctoral researcher with Allen J. Bard and John B. Goodenough, first in the Chemistry Department and later at the Center for Material Science and Engineering at the University of Texas, Austin. He is currently a professor in the Department of Chemistry, National Chung Hsing University (NCHU), Taiwan. His research interests include chemically modified electrodes, electrocatalytsis, photoelectrochemistry, physical electrochemistry and chemical sensors.

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hydrodynamic flow (Hf) of the carrier base electrolyte.1,2. The ... provide high sensitivity with good resolution, it is not optimally suited for routine ... electrode. The FIA system contains a Cole-Parmer. 35 ... An electrochemical cell coupled wit