Selective Wettability–Driven Evaporation-Enhanced Redox Cycling for Robust and Ultrasensitive Detection of Viral Particles

Timely detection of viral infections, particularly in settings outside centralized laboratories, is essential for controlling outbreaks. Small electrochemical biosensors are appealing because they are fast, require microliter sample volumes, and can be integrated into portable devices. However, shrinking the sensor area often weakens the signal, making it difficult to detect low viral loads. We previously showed that combining redox cycling with controlled droplet evaporation can boost the signal. Yet, this method faces signal variability due to user-dependent droplet placement. Here, we introduce a selective-wettability, evaporation-enhanced redox cycling (SW-E2RC) device that passively centers and pins the droplet on the sensing area, improving both signal strength and reproducibility. The chip combines a wettable sensing zone surrounded by water-repellent micropillars that guide the droplet into place and stabilize it during evaporation, concentrating redox-active species over the electrodes. Using SARS-CoV-2 and avian influenza H5N1 as model pathogens, we show that SW-E2RC reduces the LOD to 9.2 × 10³ copies/mL, corresponding to a ∼10³-fold improvement in sensitivity compared with E2RC. This platform can be adapted to various capture probes and targets, enabling more sensitive and reliable point-of-need viral diagnostics.