Fundamental principles of photoelectrochemical sensors with focus on hexavalent chromium detection

Hexavalent chromium constitutes a major hazard to the environment and human health. Consequently, monitoring methods capable of widespread deployment are necessary. However, traditional laboratory analytical methods, despite their accuracy, prove unsuitable for mass screening due to high cost and the need for stationary equipment. Photoelectrochemical sensors provide a promising alternative as they allow portable, cost-effective devices with low detection limits and inherent selectivity to chromium oxidation states. This review analyzes the current state of photoelectrochemical detection of Cr(vi), from fundamental principles to practical applications. The physicochemical foundations of PEC sensor operation are examined in detail: semiconductor-light interactions, electrolyte interface formation, and charge carrier generation and separation mechanisms. Major materials science strategies for improving analytical characteristics are analyzed: creation of semiconductor heterostructures, use of quantum dots and plasmonic nanostructures, transport network engineering, and molecular surface modification. The review shows the development from simple single-phase semiconductor systems to complex multi-component architectures designed to overcome fundamental limitations. Progress in materials design has allowed notable improvements in sensor performance through rational engineering of optical, electronic, and surface properties. Analysis of Cr(vi) detection systems shows general design principles that can be applied to photoelectrochemical sensing of other environmental contaminants. We provide an overview for researchers in photocatalysis, electrochemistry, and materials chemistry working on next-generation photoactive analytical systems.