All-Optical Electrochemiluminescence by Light Downconversion at BiVO₄ Photoanodes

Electrochemiluminescence (ECL) is a powerful analytical technique. However, the necessity of an external power supply limits its use for portable sensing devices. Herein, we report a new sensing scheme based on an all-optical ECL (AO-ECL), which addresses this issue by emitting light without external electrical devices but through light excitation (λ_exc). In AO-ECL, the photovoltage generated by the semiconductor simultaneously drives the anodic ECL reaction (producing photons at λ_AO-ECL) and a cathodic charge transfer. While this approach significantly reduces the complexity of ECL instrumentation, current materials for such systems often suffer from relatively complex fabrication methods and all existing AO-ECL systems are upconversion systems (e.g., λ_exc > λ_AO-ECL). Here, we report for the first time an AO-ECL downconversion process (e.g., λ_exc < λ_AO-ECL) based on a bismuth vanadate (BiVO₄) electrode with the model luminol-H₂O₂ system. Because BiVO₄ has a wide absorption below 500 nm, the ECL emission spectrum further red-shifts to 510 nm. The strong responsiveness toward H₂O₂ of the BiVO₄ AO-ECL system enables a significant shift on ECL onset potential from +0.3 V to -0.3 V (vs Ag/AgCl). A nearly 2.6-fold enhancement in ECL intensity was achieved at 0.4 V and the distinct ECL signal is observed even without applied bias. Moreover, the platform enabled cadmium ion (Cd²⁺) detection, with the AO-ECL intensity rising 2.2-fold. The simplicity of BiVO₄ electrode fabrication combined with its cost-effectiveness positions this downconversion AO-ECL system as a potential candidate for the development of portable bioanalytical sensors and wireless bioimaging applications.