Temperature-Controlled Electrochemical Microsensors and Microsensor Arrays

Abstract

There is a growing demand for rapid, simple, and low-cost chemical sensors for next generation bio-manufacturing, medical diagnostics and environmental monitoring. Current reliable technologies like mass spectrometry and fluorescence-based assays suffer from low throughput and relatively high cost. A preferred alternative method would use electrochemical techniques, which can be adapted for automated screening of multiple chemical/biochemical targets. Unfortunately, due to the complexity of real-world samples, which can contain multiple interfering species spanning several orders of magnitude in concentration, the broad range of electrochemical sensing strategies found in the literature have produced only limited commercial technologies. Actively controlling an electrode’s surface temperature, which is simultaneously one of the most critical and most neglected parameters in determining sensor response, offers a route towards improved electrochemical detection for real-world samples. Previous work has demonstrated certain advantages of both directly and indirectly heated electrodes in terms of sensitivity and selectivity[1,2]. In this work we present the design concepts, fabrication methods, and temperature characterization approaches for microfabricated, temperature-controlled electrodes. The use of modern microfabrication techniques provides several advantages by facilitating the construction of sensors with highly reproducible heating/electrochemical characteristics, the production of high spatial-density sensor arrays, and the compatibility of sensors with complementary microfluidic/MEMS components for sensing in ultra-small sample volumes. The sequential deposition of patterned metallic structures and insulating layers (e.g. SiO2) was used to fabricate Au disk electrodes (r ≤ 75 μm) with underlying platinum resistive heating elements as shown in Figure 1A. The small vertical electrode-heater separation (≲ 500 nm) allows for rapid and efficient heating of the electrode surface. Infrared and fluorescent temperature imaging techniques are used to evaluate temperature gradients and thermal response times, which impact sensor performance. We demonstrate temperature-dependent electrochemical measurements performed on model systems (e.g. Fe(CN)6 3-/4- and Ru(NH3)6 3+/2+) in which coupled heat and mass transport impart signal enhancement as shown in the cyclic voltammograms in Figure 1B. We also discuss the impact of various temperature control routines, such as temperature steps, pulses and oscillations, on electrochemical signal content. References (1) Gründler, P. Fresenius. J. Anal. Chem. 2000, 367, 324–328. (2) Flechsig, G.-U.; Walter, A. Electroanalysis 2012, 24, 23–31. Figure 1