Electrochemical tuning of the activity and structure of a copper-cobalt micro-nano film on a gold electrode, and its application to the determination of glucose and of Chemical Oxygen Demand

Micro-nano structured Cu-Co was in situ fabricated on the surface of a gold electrode via electrochemical reduction of CuCl2 and Co(NO3)2. It is shown that the shape of the particles can be controlled by variation of deposition current, deposition time, pH value and the ratio of Cu(II) and Co(II) ions. If prepared under current of −200 μA in 0.1 M, pH 4.0 acetate buffer solution, the film possesses high catalytic activity towards the electrochemical oxidation of glucose at a largely increased oxidation current compared to a non-modified surface. The electrochemical activity of this sensor can be easily tuned. Glucose is a standard compound for evaluating the chemical oxygen demand (COD), and we have therefore studied the application of the sensor to the determination of this parameter. Under optimized conditions, the sensor has linear response to glucose in the 1.92-768 mg L−1 concentration range, and the detection limit is 0.609 mg L−1 (at an S/N ratio of 3). A large number of surface water samples was studied, and the results obtained by this method were found to be linearly correlated to those obtained by the dichromate method (r = 0.995; n = 33). Graphical Abstract This study describes the facile synthesis of micro-nano Cu-Co by one-step electrodeposition of Cu(II) and Co(II) on gold electrode. The alloy composite exhibited excellent electrocatalytic activities, and was successfully applied on the COD determination of glucose and water samples. Electronic supplementary material The online version of this article (doi:10.1007/s00604-014-1353-z) contains supplementary material, which is available to authorized users.

molar ratio of them changed while the total metal ion concentration stays the same.
The molar ratio of CuCl 2 and Co(NO 3 ) 2 on the catalytic activity of intermetallic composite was studied based on the oxidation signal of glucose in 0.1 M NaOH solution (Fig. S2). Fig. S2A shows the LSV curves on different Cu-Co films for glucose with COD of 19.2 mg L −1 , and Fig. S2B demonstrates the influence of different molar ratio of Cu 2+ , Co 2+ while the total metal concentration was conserved as 30 Mm. In the absence of Cu 2+ , the response current for glucose solution was very low (curve a). While gradually improving the Cu 2+ concentration to 20 mM, the response current increased slowly. If further improving Cu 2+ concentration to 27 mM, the response current increased linearly. As the concentration of Cu 2+ beyond 27 mM, the response signal decreased swiftly. This is because that the synergistic effects to oxygen-oxygen band breaking between the copper and cobalt become weaker when the concentration of Cu 2+ is too high. Clearly, the electrocatalytic activity of Cu-Co film can be tuned by the different concentration ratio of Cu 2+ and Co 2+ . Thus, for obtaining high electrochemical activity, Cu-Co sensing film prepared under 27 mM Cu 2+ and 3 mM Co 2+ was employed.

Influence of deposition medium and deposition time
Various micro-nano Cu-Co coatings were prepared using 27 mM CuCl 2 and 3 mM Co(NO 3 ) 2 in different solution including 0.01 M HCl (pH 2); 0.1 M HAc (pH 2.87); 0.1 M acetate buffer solution with pH of 3.6, 4, 4.6, 5 and 5.6; and 0.1 M NaNO 3 (pH 7). After that, the electrochemical activity of resulting micro-nano Cu-Co was analyzed by LSV based on the oxidation peak current of 19.2 mg L -1 glucose with reduction current of −200 A, and reduction time of 100 s. As shown in Fig. S3, the response current of glucose on the surface of micro-nano Cu-Co in 0.1 M NaOH solution greatly increases with pH value from 2 to 4. However, the oxidation current of glucose starts to decrease when the pH value is > 4. The pH value of solution not only affects the mass transport of Cu 2+ and Co 2+ , but also influences their electron transfer rate on gold surface. Herein, to achieve high response activity, micro-nano Cu-Co sensing film was prepared in 0.1 M, pH 4.0 acetate buffer to obtain high electrochemical activity.
The influence of deposition time on the electrochemical activity of micro-nano Cu-Co was also studied using 27 mM CuCl 2 and 3 mM Co(NO 3 ) 2 in 0.1 M, pH 4.0 acetate buffer with reduction current of −200 A. The LSV curves of glucose on different micro-nano Cu-Co/gold electrodes are given in Fig. S4A, and the oxidation current as a function of deposition time is displayed in Fig. S4B. When the deposition time extends from 80 to 100 s, the activity of prepared micro-nano Cu-Co toward the electrochemical oxidation of 19.2 mg L -1 glucose in 0.1 M NaOH solution remarkably increases, accompanied by notable oxidation current enhancement. However, the oxidation current of glucose on the surface of micro-nano Cu-Co gradually decreases with further extending the deposition time from 100 to 120 s. This is because that the particle size of deposits obviously increases when the deposition time exceeds 100 s.
Consequently, the electrochemical activity of micro-nano Cu-Co to the oxidation of glucose also decreases. Thus, 100-s oxidation deposition was used to prepare micro-nano Cu-Co sensing film for getting the best sensitivity.