Laser Induced Porous 3D-Graphene/MnO2 Electrodes Embedded in a Paper-Based Microfluidic Device for Point-of-Care Testing of Neurotransmitters in Finger-Prick Blood
Authors: Fishlock Sam Jeffery , Bhattacharya Gourav , McLaughlin James 
Published: May 2020

Microfluidic electrochemical paper-based devices (ePADS) are a promising platform for point of care (PoC) diagnostics. In this work, we present a microfluidic device for home-based detection of the neurotransmitter dopamine, which is crucial for further understanding of nervous system disorders such as Parkinson’s disease and other chronic mental illnesses including schizophrenia. Our device features a 3-electrode ePAD, where the electrodes are aligned along a wax-defined microfluidic channel, in a capillary flow device which uses Whatman filter paper as the porous wicking substrate. The working electrode was embedded directly into the Whatman filter-paper by using the paper as a precursor for laser-induced-graphene (LIG). To obtain LIG, we treat the Whatman filter in commercial flame-retardant spray and then treat with a CO2 infrared laser, this treatment produces a highly conductive, 3D porous graphene network with high surface area and electrical conductivity, directly in the paper matrix. The working and counter LIG electrodes are patterned by laser alongside the wax-defined microfluidic channel as shown in figure 1a, and a third reference electrode was defined in inkjet-printed silver (Dimatix DMP-2831) and then treated in commercial bleach to obtain an Ag/AgCl pseudo-reference electrode. The charge-transfer kinetics of the LIG surface was obtained by cyclic voltammetry (figure 1b) of the LIG electrode in a 3-electrode setup (platinum counter electrode and Ag/AgCl reference electrode in 3.0 M KCl). The LIG electrode was cycled in 0.1 M aqueous solution of KCl consisting of 5mM ferro-ferricyanide redox couple. The Randles–Sevcik analysis was used to obtain an electroactive surface area of 1.97 mm2 (2.5 times higher than the geometrical surface area) and the analysis also showed a rapid electron transport kinetic.

The working electrode was modified from the pristine LIG by initially (i) chemical activation in 2 molar urea solution to introduce nitrogen-functional groups on the LIG and (ii) electrodeposition of MnO2 nanoparticles over the N-functionalised graphene to produce a sensing electrode with low background current and rapid electron transfer characteristics.

Crucially for microfluidic devices to be used as a real PoC device, the patient needs to be able to simply deposit the biomarker. For neurotransmitter analysis, blood/plasma from a finger-prick enables a plausible route for sampling. Here, we show a novel blood sampling design of the ePAD cassette (holder) which enables rapid and low volume blood sampling, which is shown in figure 1c. The holder includes a hydrophilic microfluidic channel which rapidly samples 5 μL blood from a finger-prick droplet. An air vent at the channel end enables the channel to be filled to exactly 5.0 μL but not over-filled. The blood is prevented from entering the ePAD by a removable adhesive tab, which also covers a reservoir filled with running buffer (RB), (TBS + 0.05% Tween + 0.5% BSA). When the channel is filled with blood, the removal of the adhesive tab enables release of the defined volume of blood and RB into (i) a Pall Cytosep 1668 blood filtration membrane and then (ii) into the wax-patterned lateral flow channel of the ePAD device for rapid electrochemical detection of dopamine, creatinine and uric acid by square wave anodic stripping voltammetry.

Figure 1