Auxiliary electrode oxidation for naked-eye electrochemical determinations in microfluidics: Towards on-the-spot applications

Abstract

In this paper, we describe a new microfluidic cell to perform naked-eye electrochemical detection (NEED) through the oxidation of Ag-based auxiliary electrodes (AEs) in the presence of a cathodic current on the working electrode (WE). The chips were fabricated in polydimethylsiloxane by a simple, cost-effective, and cleanroom-free prototyping that allows reuse of each AE up to 60 times. Stainless steel microwires coated with Ag film and in electric contact to each other acted as AEs, whereas single microwires coated with Au and Ag films operated as WE and quasi-reference electrode, respectively. All the electrodes were reversibly introduced into the devices. The high contrast among nonoxidized and oxidized (black Ag2O nanostructures of up to 400 nm) Ag ensures the naked-eye monitoring of the AE oxidation extent. The number of oxidized AEs was taken as the analytical signal for semi-quantitative assays of p-benzoquinone as proof-of-concept. In addition to the low cost, a key advantage within the point-of-use testing philosophy, further contributions from our work include: i) simultaneous semi-quantitative naked-eye tests for the same analyte from a single WE to speed up electroanalytical assays and ii) the use of the time needed to oxidize one AE as the signal, which led to quantitative assays with enhanced sensitivity for hydrogen peroxide. To attain a fully portable and user-friendly NEED method, the sensor was integrated into a handheld platform that consisted of a homemade potentiostat and gravity-assisted pumping, with which on-site tests can be performed by nontrained personnel anywhere. The portable potentiostat has the lowest cost (US$7.72) reported in the literature compared with setups to measure electric current.