A self-powered photovoltaic colorimetric detector for sensing metal ions at ultralow concentrations

The accumulation of toxic metal ions from industrial activities poses significant environmental and health risks, thus necessitating the development of portable, rapid, and highly sensitive detection systems. We report a self-powered photovoltaic colorimetric sensor that is capable of detecting Al³⁺, Fe³⁺, and Cu²⁺ ions at nanomolar concentrations. Traditional spectrometer-based platforms are bulky and unsuitable for onsite applications, whereas conventional colorimetric sensors often suffer from limited sensitivity and poor reproducibility. To address these limitations, we utilize a rhodamine derivative (R6GH) that undergoes a ring-opening reaction upon interaction with target metal ions, which results in a visible color change under green LED illumination. The portable sensor integrates a Schottky junction that is fabricated by depositing gallium-doped zinc oxide (GZO) onto an n-type silicon substrate via atomic layer deposition, which enables the efficient conversion of optical signals into electrical outputs. The device operates in dual detection mode. In voltage mode, the detection limits are 16 nM for Al³⁺, 22 nM for Fe³⁺, and 41 nM for Cu²⁺. In current mode, the respective detection limits are 26, 18, and 34 nM. Compared with conventional chemosensors, this system offers an improvement in sensitivity of up to two orders of magnitude. Additionally, the sensor demonstrates excellent signal reproducibility, with a relative standard deviation (RSD) of less than 1.14 % across 560 switching cycles. The combination of high sensitivity, rapid response (<30 s), and stable, self-powered operation makes this device a promising candidate for real-time metal ion monitoring for the future of bioelectronic devices in healthcare.