Porous Co-Cu Electrode Fabricated By Electroplating through Sacrificial Glass Fiber Paper Template
Authors: Wang Xiaochen , Church Jared , Lee Woo Hyoung , Cho Hyoung J. 
Published: February 2020

Due to the over-fertilization and runoff, an increased phosphate concentration in natural water systems often results in serious environmental problems such as eutrophication and harmful algae blooming. In order to protect drinking water resources, online phosphate sensors for real-time monitoring of the exact phosphate level in natural water body are highly demanded.

Compared with traditional method such as liquid chromatography-mass spectrometry (LC-MS) which involves complex facilities and process, various types of environmental microsensors have already shown great potential in terms of simplified test procedures, fast response and in-situ­ measurement capability. However, there exist technical challenges such as complex fabrication processes, instability, and oxygen interferences for practical implementations.

Among these works, a cobalt-coated needle type phosphate potentiometric sensor was reported before by one of authors[1]. A sensing mechanism was proposed for a theoretical explanation of pH effect, potential shift and oxygen interference of Co electrode, which also pointed out a direction for improving the sensor performance for real environment application[1, 2]. Another study reported that alloying a different metal element inert to phosphate could stabilize the sensor electrode by affecting redox reaction and reducing the over activity of Co[3]. However, since this sensor is based on potentiometry, oxygen interference still exists during measurements. Enlightened by this work and electrochemistry knowledge[4], we seek to employ an amperometric technique by shifting the electrode surface redox reaction to eliminate oxygen interference. We designed and fabricated a textured, amperometric Co-Cu microelectrode using pulsed-electroplating(PEP)[5] through sacrificial glass fiber paper template and characterized the sensor for in situ phosphate monitoring.

The sensor fabrication process starts from photolithography electrode patterning on Au/Ti/SiO2 coated silicon substrate. A glass fiber filter paper (0.5μm pore size) was attached to the patterned Au electrode surface as a template and fixed by two glass plates. One end of glass fiber paper was dipped in an electrolyte (CoSO4, CuSO4). While the whole paper was soaked, a pulsed potential (duty cycle=0.5) was applied. During the electroplating process, the electrolyte was supplied continuously to electrode surface by a capillary effect so that Co-Cu electrode surfaces might conform to the texture of the paper template. After the PEP process, the electrode was dipped in a buffered oxide etchant to remove the glass fiber filter paper leaving behind a porous Co-Cu layer on the electrode.

Potentiometric tests were conducted first to confirm the electrode response to phosphate. The Co-Cu electrode responded immediately (< 30s) when phosphate was present in buffered solution (25 mM KHP, pH 4.0) with a sensitivity of -34mV per decade. Then we explored the electrode’s electrochemical property by cyclic voltammetry (CV) test in oxygen saturated and deoxygenated 10-2 M phosphate solutions, respectively. According to the CV results, amperiometric response under the negatively applied potentials after -300 mV vs. Ag/AgCl would experience oxygen interference, while under potentials exceeding -300 mV no oxygen interference was observed. After evaluating various calibration curves, the amperometic signals under -250 mV validate a good linear response to different logarithm of phosphate concentrations with stable sensitivity and significant suppression of oxygen interference effects.

[1] W. H. Lee, Y. Seo, and P. L. Bishop, "Characteristics of a cobalt-based phosphate microelectrode for in situ monitoring of phosphate and its biological application," Sens Actuators B Chem., vol. 137, pp. 121-128, 2009.

[2] R. K. Meruva and M. E. Meyerhoff, "Mixed Potential Response Mechanism of Cobalt Electrodes toward Inorganic Phosphate," Anal. Chem., vol. 68, pp. 2022-2026, 1996.

[3] T. Kidosaki, S. Takase, and Y. Shimizu, "Electrodeposited Cobalt-Iron Alloy Thin-Film for Potentiometric Hydrogen Phosphate-Ion Sensor," Journal of Sensor Technology, vol. 2, pp. 95-101, 2012.

[4] A. J. Bard and L. R.Faulkner, ELECTROCHEMICAL METHODS: Fundamentals and Applications, 2 ed.: JOHN WILEY &SONS, INC., 2001.

[5] P. E. Bradley and D. Landolt, "Pulse-plating of copper - cobalt alloys," Electrochimica Acta, vol. 45, pp. 1077-1087, 1999.