3D-Printed Microfluidics System Coupled with Electrochemical pH Control for Enhanced Chlorine Detection
Authors: O'Sullivan Shane , Diaz Fernando , Seymour Ian , O'Riordan Alan 
Published: February 2024

In this work we present the design of a novel technique for amplifying chlorine detection via a reusable microfluidics platform for use with electrochemical sensors. This platform enables experiments to be performed in hydrodynamic conditions, utilising convection as a secondary mode of mass transport for the analyte. All experiments have also been corroborated using COMSOL Multiphysics simulations. The platform is constructed using 3-D printing, which has been used for macroscale electrode microfluidics previously [1]. However, the sensors utilised herein are of the ultramicro scale with complex geometry. This geometry allows bipotentiostatic electrochemistry to be utilised enabling localised electrochemical pH control. This effect, while studied in quiescent conditions, remains poorly understood in hydrodynamic conditions at this scale, though has been investigated at the macroscale.[2] By utilising these two phenomena in tandem, we demonstrate, both experimentally and through simulations, the feasibility of this coupling at the ultramicro scale. Initially we demonstrate operability using ferrocene monocarboxylic acid to electrochemically characterise our sensors, under various controlled flowrates. A highly linear relationship was observed between the cubic root of flowrate in uL/min and the observed current at the working electrode. This cubic root relationship follows the expected theory when compared to the Levich equation as demonstrated by Rees et al [3]. Next we demonstrated the impact of flowrate on electrochemical pH control. This was done using the location of the gold oxide reduction peak, which has been shown to be indicative of local pH by Seymour et al [4]. We observed a visible shift in the location of this peak, with a set potential bias applied, once a flowrate was applied. This potential shift when compared to the static counterpart suggests that electrochemical pH control is less efficient under flow conditions, as expected when compared to the COMSOL simulations. However, this loss of efficiency can be compensated for as demonstrated by our work. By simply adjusting the potential bias as required, the gold oxide reduction peak can be shifted back to its initial location, meaning the desired local pH has been achieved. This compensated potential was then used for pH controlled detection of hypochlorous acid. The previously mentioned cubic root relationship was also demonstrated for the detection of a set concentration of hypochlorous acid under varying flow conditions. We report a near 30-fold enhancement of signal for a set concentration of hypochlorous acid, in water samples, when high flow is applied. Crucially, this enhancement was obtained while electrochemically regulating the local pH. Furthermore, the high linearity of the current enhancing effect of increasing flowrate is demonstrated. In boosting the signal, lower limits of detection can be obtained before signals are lost to background noise.
Figure 1 Hypochlorous acid detection in static and high flow conditions. Inset shows the activity of static scans in greater detail
References:

I. Belmonte and R. J. White, Analytica Chimica Acta, 2022, 1192, 339377.

A. M. Zimer, M. Medina da Silva, E. G. Machado, H. Varela, L. H. Mascaro and E. C. Pereira, Analytica Chimica Acta, 2015, 897, 17-23.

N. V. Rees and R. G. Compton, Russian Journal of Electrochemistry, 2008, 44, 368-389.

Seymour, B. O'Sullivan, P. Lovera, J. F. Rohan and A. O'Riordan, Sensors and Actuators B-Chemical, 2020, 325, 128774.

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