High-capacitance air-brushed electrodes for capacitive label-free bioassays

Non-invasive assays for protein biomarkers of cancer allow both its early diagnosis and continuous treatment monitoring. Yet, accurate point-of-care (POC) diagnostic devices for cancer diagnosis and monitoring, needed in point-of-care (POC) sites and places with limited resources, are scarce, not the least, due to their high current cost or bulky equipment necessary for analysis. Here, we show that the capacitive cellulase-linked electrochemical enzyme-linked aptamer-sorbent assay (e-ELASA) on magnetic beads (MBs) performed with airbrushed graphite (Gr) electrodes accurately and economically detects HER-2/neu, the protein biomarker of some aggressive forms of cancers and target of anticancer therapy. The disposable Gr electrodes were produced by airbrushing inexpensive graphite-powder/chitosan water inks onto polyester transparency films, producing high-capacitance electrodes, whose apparent specific capacitance ranged between 3.61 and 8.88 mF cm^-2 as a function of the number of sprayed layers and graphite content in inks. The five-layer electrodes produced from 1.7 g of graphite powder (per 5 mL)/0.55 % chitosan water inks outperformed manually polished spectroscopic Gr electrodes earlier used in this label-free capacitive e-ELASA, as a result of the higher capacitive changes of the former, providing the same 0.1 fM limit of detection of HER-2/neu, in both buffer and 10 % serum, yet with a three-fold higher sensitivity. The portable and low cost airbrushed electrodes/e-ELASA set-up can be used for quick and accurate regular POC monitoring of HER-2/neu, particularly, in low and middle income settings, and, in perspective, the high-capacitance airbrushed electrodes can be adapted for other type label-free capacitive bioassays.