Delayed voltammetric with respect to amperometric electrochemical detection of concentration changes in microchannels

Abstract

ARTICLEThis journal is © The Royal Society of Chemistry 2013 J. Name., 2013, 00, 1-3 | 1 The time response of an electrode incorporated into a fluidic channel to variations in analyte concentration of the outer-sphere redox probe ferrocenemethanol was investigated, both for amperometry (AMP) and cyclic voltammetry (CV). The experimental data show that the temporal resolution of CV is not as good as for AMP, as CV cannot properly detect fast concentration transients. The delayed response of CV was previously reported, for neurotransmitters, and mostly attributed to adsorption of the analyte to the electrode surface. By using an outer-sphere redox couple, we show that mass transport also significantly delays the response of CV. The experimental delay time in CV was understood from mass transfer limitations due to the relaxation of the diffusion layer during repeated potential scanning. Furthermore, a robust protocol for the analysis of fast concentration transients was established, using the impulse and modulation transfer functions of the system. This method was found to be more precise than the mere analysis of the undifferentiated traces in the time domain. As a proof of concept, the effect of increased viscosity was investigated, showing that AMP was more sensitive than CV to these variations. Overall, this analysis underlines further the enhanced temporal sensitivity of AMP over CV, at the expense of decreased chemical resolution, potentially having implications for in situ electrochemical detection of biologically relevant molecules.

Interplay between the potential waveform and diffusion layer dynamics determines the time-response of voltammetric detection in microchannels

“…the bulk analyte concentration, the diffusion layer first has to be established, thus inducing a delayed response over AMP. This phenomenon is qualitatively and quantitatively similar to the delayed response observed in an RC circuit, the diffusion layer actually behaving as an RC component [25], that gives rise to a characteristic relaxation time τD in the signal [28,29]:…”

“…The effects of the discharge currents generated by the RC constant of the electrode surface were here not considered because of the small (~0.2 ms) RC constant previously calculated for this type of device [25]. The results for the normalized currents are shown in Fig.…”

“…To further demonstrate the delayed response of CV with respect to AMP, the experiment detailed previously [25] for FcMeOH was repeated for the outer-sphere probe RuHex. A 4 s plug of 1 mM RuHex was injected (v= 10 µl s -1 ) into a stream of PBS.…”

“…The Reynolds' number in the microchannels section of the chip is ~100 at the highest flow rate considered (10 µl s -1 ). In all the cases considered previously for the detection of plugs of FcMeOH in microchannels [25], the CV response was delayed over the AMP traces. As FcMeOH was found to show outer-sphere properties in our system, with a fast electron transfer, this delay was attributed to mass transport, more precisely to the relaxation of the diffusion layer in response to the observed concentration changes [28,29].…”

“…In contrast, cyclic voltammetry (CV), or other potentiodynamic techniques, are popular because of its improved chemical resolution, allowing for the simultaneous quantitation of different analytes. In a previous report [25], we have compared the response of AMP and CV to the injections of a plug of 1 mM ferrocenemethanol (FcMeOH) in a microfluidic system. This redox probe was used because of its fast electron transfer [11,26,27], guaranteeing that the observations will arise from mass transport rather than secondary reactions (adsorption, formation of chemical bonds…).…”