A composite electrode composed of electrodeposited, nickel-iron nanostructured clusters onto a glassy carbon (GC) disk electrode was used as a working electrode to detect methylene blue at concentrations below 10 μM. The Ni-Fe clusters were prepared by pulse electrodeposition and a lateral composition variation was observed reflective of a local pH change across the Ni-Fe feature. The applied potential for the detection of MB at a pH of 4 was determined through voltammetry and demonstrated using chronoamperometry and electrochemical impedance spectroscopy (EIS) where the adsorption of MB influenced both the capacitance, C, and ohmic resistance, Rs. A peak present in it1/2 vs t chronoamperometry plots decreased with lower MB bulk concentration, while in contrast, the RsC parameters determined from equivalent circuit models of EIS had the opposite behavior having a larger signal with lower MB concentration, and hence providing a way to increase the detection signal at lower MB concentration.
Methylene blue (MB) is a redox-active molecule, which is used for biochemical studies as a redox indicator,1-3 and as a comparable model for pyocyanin, a bacteria metabolite.4 Electrochemical detection of MB has been demonstrated through adsorption on gold electrodes,5 enhanced with a gold modified alkanethiol layer,5-7 and with carbon nanotube modified gold electrodes.8 In this study, a low-cost approach was used to develop a novel Ni-Fe/glassy carbon (GC) composite electrode to detect methylene blue. Ni-Fe microstructures were deposited from a sulfate-boric acid electrolyte at a pH of 3 by a pulse galvanostatic method. A three-electrode cell was used, containing a glassy carbon disk (GC) working electrode, a platinum counter electrode and a silver chloride reference electrode (Ag/AgCl). Deposition was carried out with and without electrode rotation. Field-emission scanning electron microscopy (FESEM), energy-dispersive x-ray spectroscopy (EDX) and x-ray fluorescence analyzer (XRF) were used to characterize the surface morphology and the composition of the Ni-Fe clusters. The redox and adsorption behavior of MB were characterized with cyclic voltammetry and chronoamperometry/coulometry in a phosphate buffer solution (PBS) containing variable μM amounts of MB. The electrodeposition results showed interesting morphology and composition that impacted the MB electrochemistry, Fig. 1. The average composition of Ni-Fe clusters with a 2 s deposition time and without rotation was Ni-rich, 84 wt% Ni, 16 wt% Fe, and the morphology of the microscale clusters was composed of nanoscale nuclei in a circular shape with a hole in its center, attributed to gas evolution from the side reaction. The average size of the clusters was roughly 50 µm with different hole diameters (Fig. 1a). The average composition of Ni-Fe clusters with a 6 s deposition time and 400 rpm rotation rate had a contrasting composition, Fe-rich, 38 wt% Ni, 62 wt% Fe, and the morphology demonstrated microscale clusters with cracks. The average size of the microscale clusters was between 10 and 20 µm (Fig. 1b). The cyclic votammograms of the MB electrolyte, under quiescent conditions, were quasi-reversible, showing a single oxidation peak and a reduction peak. The Ni-Fe clusters impacted methylene blue (MB) chronoamperometry and coulometry, enhancing the response particularly for low concentrations of MB. Deviations from linearity in a plot of it1/2 vs. tunder reduction conditions, at a constant applied potential, indicated that the response was not diffusion controlled at times below 100 s, where the Ni-Fe/GC composite electrode had a significantly larger current signal, but with diffusion behavior observed at long times (Fig. 1c), with little to no improvement in the current in this region using the Ni-Fe/GC composite electrode compared to GC. The results show that the Ni-Fe/GC composite electrode can enhanced the electrochemical detection of MB at low MB concentration. Acknowledgement The authors wish to thank H. Bilan at the Center of Advanced Materials Processing (CAMP) at Clarkson University for SEM help. References Y. Wu and R.Y. Lai, Analytical Chemistry, 86, 8888 (2014). W. Yang, R. Y. Lai, Langmuir, 27, 14669 (2011). L. E. Korshoj, A. J. Zaitouna, and R. Y. Lai, Analytical Chemistry 86, 8888 (2014). M. Kasozi, S. Gromer, H. Adler, K. Zocher, S. Rahlfs, S. Wittlin, K. Fritz-Wolf, R. H. Schirmer, and K. Becker, Redox Report, 16, 154 (2011). Sagara, H. Kawamura, and N. Nakashima, Langmuir, 12, 4253 (1996). Barou, M. Bouvet, O. Heintz, and R. Meunier-Prest, Electrochimica Acta, 75, 387-392 (2012). Zhao, B. Zeng, and D. Pang, Electroanalysis 15, 1060 (2003). García-González, A. Costa-García, and M.T. Fernández-Abedul, Sensors and Actuators B, 202, 129 (2014). Figure 1
A low cost alternative to the electrochemical detection of methylene blue (MB) is demonstrated using a novel Ni–Fe/glassy carbon (GC) composite electrode. Ni-Fe microstructures were electrodeposited from a sulfate-boric acid electrolyte at a pH of 3 by a pulse galvanostatic method, and the morphology and composition was identified by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Deposition with the electrode rotation rate and duty cycle as variables were examined and both influenced the composition as well as morphology. For example, a 2 s deposition time and a 63 s relaxation time was Ni-rich, 84 wt% Ni, 16 wt% Fe, and the morphology of the microstructures had a circular shape with a central hole, attributed to gas evolution from the side reaction. The composition distribution had more iron at the edge and more nickel close to the center which may reflect a local pH change occurring during deposition. A longer duty cycle resulted in micro-sized deposits that included nano-cracks. A low duty cycle accompanied with rotation of the electrode resulted in no deposit, due to corrosion during of the off-time. Detection of MB was greatly enhanced with the Ni-Fe/GC composite electrode compared to GC alone. The MB concentration varied from 0.5 to 10 µM in a phosphate buffer solution (PBS). Voltammetry and chronoamperometry were used under conditions where a Ni-Fe surface oxide may be initially present. The limits of detection were investigated. When adsorption of MB is too high, then the current density is not sensitive to its concentration. Also, the reduction of the surface oxide on Ni-Fe can limit detection of MB. Therefore, voltammetry with high sweep rate and the chronoamperometry at short times is most sensitive to low concentrations of MB on the microstructured Ni-Fe nano-cluster/GC electrode.
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