Essence: A Shear-Enhanced, Flow-through, Nanoporous and Capacitive Electrode Technology with Non-Planar Interdigitated Micro-Electrodes
Authors: Cheng Yu Hsuan , Li Zhenglong , Yang Juliana , Moura Pedro , Kim Hansin , Basuray Sagnik 
Published: February 2020

The most promising biosensor platforms are electrochemical biosensors due to their simple, low manufacturing and usage costs, multiplexed detection capabilities, short response time, as well as ease of miniaturization for portable point-of-care diagnostics. In this talk, we investigate new electrochemical sensing that utilizes a shear-enhanced, flow-through, nanoporous and capacitive electrode technology (hereafter called “ESSENCE”). ESSENCE consists of three layers, a top and bottom nonplanar interdigitated microelectrode array (IDμE) and a middle layer of vertically grown Carbon Nanotubes (CNTs). The fabrication protocol of ESSENCE is unique as unlike most fabrication protocols; it is neither top-down nor bottom-up. We call the technique side-to-side, where the final layer is a sidewall of the device. The steps are: (1) First, fabricate the bottom IDμE on a suitable substrate like glass or silicon. (2) Deposition and the etching process are used to make the microfluidic channel above the bottom IDμE. (3) The catalyst to grow the CNT pillars in the microchannel is patterned on the exposed bottom portion of the channel. (4) A sacrificial layer to support the upper electrode and protect catalyst is introduced on the bottom IDμE. (5) Top electrodes and catalyst are patterned on the top of the sacrificial layer. (6) The CNT’s are grown using well-established techniques. (7) Finally; the chip wall is closed to finish a non-panned interdigitated microfluidic sensor. The functionalization of the CNT is carried out using an in-house on-chip protocol. We exploited the unique electrochemical impedance spectroscopy signal from ESSENCE. Interestingly the tightly packed, porous, flow-through electrode of ESSENCE leads to a substantive increase in diffusion, leading to the formation of the parasitic double layer capacitor at highly elevated frequencies (MHz). This allows for measurement of the charge transfer resistance, an electrochemical signature of the binding of the target biomolecule at high frequencies. This leads to a significant increase in the signal to noise ratio. Further, the nano-porous nature of ESSENCE tremendously boosts the shear force in the device increasing selectivity. Thus, the use of ESSENCE technology boosts both sensitivities (decrease the occurrence of false negatives) and selectivity (decrease the occurrence of false positives), mitigates biofouling and decreases artifacts in the electrochemical signal like the parasitic double layer capacitance (polarization error). Hence, ESSENCE has four significant benefits over the current generation of electrochemical biosensors. (1) The electrode nanoporosity facilitates the development of shear forces of the order of a hydrogen bond, which significantly increases selectivity by mitigating non-specific adsorption. (2) Our preliminary results indicate that the interdigitated, nanoporous microelectrode design, results in a high signal to noise ratio (SNR) which is realized by increases in both sensitivity and specificity. As the ionic flux is confined to nanodomains due to nanostructured electrodes (nanoconfinement effects), it enables the ESSENCE technology to tremendously increase sensitivity and boosts the signal so significantly that even the surface functionalization of the electrodes can be resolved and characterized. (3) The nanoporous electrode architecture increases convective transport of the analyte of interest to the sensing element, thus overcoming diffusion limitations and reducing assay times. Additionally, the convection-mediated transport enhancement negates signal artifacts such as the parasitic double layer capacitance, thus facilitating rapid, high resolutions characterization of the binding signal due to a significant reduction in noise. (4) Finally, given the ease of controlling shear force through flow rate, challenges related to sensitivity and, device selectivity can be decoupled from each other (and each parameter investigated individually) using the shear force as a tuning design parameter. This decoupling allows us to mitigate the problems of biofouling, false positives, and false negatives, among others. Here, electrode fabrication involves many different well-established micro-fabrication techniques. Preliminary tests show very high sensitivity and selectivity with DNA, and a modified ELISA assay for p53, a biomarker for cancer.

Acknowledgement: The authors are supported by the National Science Foundation (NSF) Career grant (#1751759): "ASSURED" electrochemical platform for multiplexed detection of Cancer Biomarker Panel using Shear-Enhanced Nanoporous-Capacitive Electrodes

Figure Caption: The left figure shows the schematic of the different layers of the ESSENCE electrochemical sensor. The right figure shows an assembled chip with connection wires and inlet/outlet ports for fluid entry.

Figure 1