Diamond-Like Carbon Thin Film Electrodes for Microfluidic Bioelectrochemical Sensing Platforms.

This work aims to utilize diamond-like carbon (DLC) thin films for bioreceptor immobilization and amperometric biosensing in a microfluidic platform. A specific RF-PECVD method was employed to prepare DLC thin film electrodes with desirable surface and bulk properties. The films possessed a relatively high sp2 fraction, a moderate electrical conductivity (7.75 × 10-3 S cm-1), and an optical band gap of 1.67 eV. X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy revealed a presence of oxygen-containing functional groups on the DLC surface. The DLC electrodes were integrated into polydimethylsiloxane (PDMS) microfluidic electrochemical cells with the channel volume of 2.24 µL. Glucose oxidase (GOx) was chosen as a model bioreceptor to validate the employment of DLC electrodes for bioelectrochemical sensing. In-channel immobilization of glucose oxidase (GOx) at the DLC surface was realized through carbodiimide covalent linkages. Enzyme bound DLC electrode was confirmed with the redox potential at around -79 mV vs NHE in 0.1 M phosphate buffer pH 7.4. Amperometric flow-injection glucose sensing at a potential of -0.45 V vs Ag in the absence of standard redox mediators showed the increase of current response upon increasing the glucose concentration. The sensing mechanism is based on the reduction process of H2O2 liberated from the enzymatic activity. The proposed model for the catalytic H2O2 reduction to H2O on DLC electrodes was attributed to the dissociation of C-O bonds at the DLC surface.

Microfluidic chip-based nanoelectrode array as miniaturized biochemical sensing platform for prostate-specific antigen detection.

A microfluidic biosensor chip with an embedded three-electrode configuration is developed for the study of the voltammetric response of a nanoelectrode array with controlled inter-electrode distance in a nanoliter-scale sample volume. The on-chip three-electrode cell consists of a 5 × 5 array of Au working nanoelectrodes with radii between 60 and 120 nm, a Cl(2)-plasma-treated Ag/AgCl reference electrode, and a Au counter electrode. The nanoelectrode array is fabricated by creating high-aspect-ratio pores through an alumina insulating layer using an I(2) gas-assisted focused-ion-beam (FIB) milling, ion beam sculpting, and electrodeposition of Au. The glass substrate with the electrode pattern is assembled with a polydimethylsiloxane (PDMS) microchannel slab giving a volume of 180 nL for each channel. Cyclic voltammetry calibration with a standard redox species exhibits a significant increase of current density by two orders of magnitude compared to that obtained from a microelectrode. On-chip functionalization of the nanoelectrodes with a prostate-specific antigen (PSA) biosensor complex and detection of PSA based on a competitive immunoassay method are performed. The detection limit is approximately 10 pg/mL (∼270 fM), which corresponds to roughly 30,000 copies of PSA in the microchannel test volume.