Biosensors are an important aspect of diagnostic medicine since they enable biomarker detection at point-of-care or at-home settings, making diagnosis faster and less resource-intensive than laboratory methods. In the past, proposed sensors have required external reagents, complicating use and increasing cost. They have used materials that may be toxic or difficult to synthesize, making the sensor infeasible to manufacture at scale. Moreover, some diseases present heterogeneously, requiring detection of multiple biomarkers. Consequently, few commercial biosensors exist. To truly advance the field, reagentless, multiplexed biosensors that are simple to produce and use are needed. Photoelectrochemical transducers separate optical excitation and electrochemical measurements, producing low signal-to-noise ratio. Zinc oxide (ZnO) nanoparticles are popular as photoactive thin films because of low cost and high electron mobility, although its rapid electron-hole recombination reduces photosensitivity. Linear polyethyleneimine (L-PEI) is a non-toxic photosensitizer that can reduce the work function of ZnO and is water-soluble, enabling facile fabrication via solution processing. Thus, the objective of this research was to produce a reagentless, multiplexed biosensor using a ZnO/L-PEI photoelectrode, fabricated using a simple drop-casting technique. This nanocomposite demonstrated slight increases in photon-to-electron conversion efficiency and photocurrent at 365 nm over unmodified ZnO, as well as improved adhesion to the indium tin oxide substrate. This platform can detect multiple biomarkers via DNA hybridization, with either signal-off or signal-on transduction. The signal-off response is from steric hinderance caused by probe-target hybridization. The unique signal-on mechanism is based on toehold-strand displacement, using a dual-strand DNA probe labelled with a gold nanoparticle label. With the single-strand target present, the probe forms a hairpin structure, bringing the label to the photoactive surface and forming a heterojunction with the modified ZnO substrate. With UV excitement, this injects hot electrons into the conduction band of ZnO and increases photocurrent. With their target present, the signal-off mechanism decreased photocurrent by 60% and the signal-on mechanism increased photocurrent by 35% respectively. Given this proof-of-concept, the use of this platform should be explored for detection of disease DNA in biological samples.

Nanocomposite zinc oxide photoelectrode for facile multiplexed analysis of DNA

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