Mesoporous Fe₂O₃–CdS Heterostructures for Real-Time Photoelectrochemical Dynamic Probing of Cu²⁺

A three-dimensional (3D) mesoporous Fe2O3−CdS nanopyramid heterostructure is developed for solar-driven, realtime, and selective photoelectrochemical sensing of Cu in the living cells. Fabrication of the mesoporous Fe2O3 nanopyramids is realized by an interfacial aligned growth and self-assembly process, based on the van der drift model and subsequent selective in situ growth of CdS nanocrystals. The as-prepared mesoporous Fe2O3− CdS heterostructures achieve significant enhancement (∼3-fold) in the photocurrent density compared to pristine mesoporous Fe2O3, which is attributed to the unique mesoporous heterostructures with multiple features including excellent flexibility, high surface area (∼87 m/g), and large pore size (∼20 nm), enabling the PEC performance enhancement by facilitating ion transport and providing more active electrochemical reaction sites. In addition, the introduction of Cu enables the activation of quenching the charge transfer efficiency, thus leading to sensitive photoelectrochemical recording of Cu level in buffer and cellular environments. Furthermore, real-time monitoring (∼0.5 nM) of Cu released from apoptotic HeLa cell is performed using the as-prepared 3D mesoporous Fe2O3−CdS sensor, suggesting the capability of studying the nanomaterial−cell interfaces and illuminating the role of Cu as trace element. F cellular functions and medical research have elucidated the function mechanism of many physiological trace elements, which have the potential to greatly improve disease diagnosis. Among all the trace elements, copper is an important metal that is essential for most forms of life and the third most abundant transition metal in humans, which serves as a structural and catalytic cofactor for many proteins and enzymes such as cytochrome c oxidase and copper−zinc superoxide dismutase. Understanding how copper contributes to healthy and disease states requires advanced methods for real-time dynamic probing copper in the living cells rather than in ionic or molecular models. Several methods have been demonstrated to probe Cu in biological environments. Nonetheless, the ultralow concentrations and complex forms of Cu in living cells pose a challenging barrier to monitor the real-time dynamic production and concentration of Cu in physiological conditions. Semiconductor-based photoelectrochemical (PEC) biomolecule detection is a recently developed sensing approach that represents several attractive features. First, light and electricity inducers effectively eliminate the signal excitation and detection from the background noise. Second, the semiconductor nanostructure PEC conversion enables a combination of low intensity light source (such as sunlight) and low electric field for generation of reactive charge carriers and photocurrent, which facilitate the targeted electrochemical reactions without other possible interfering side reactions. Third, compared to conventional electrochemical detections progressing on the electrode surfaces, the PEC method can realize the evolution of oxygen or other reactive oxidative species in the interiors of bulky biomolecules, offering charge relay and efficient signal transducing. Among the semiconductor materials for the PEC conversion, α-Fe2O3 has been demonstrated as one of the most attractive candidates for PEC probing, due to its suitable band gap size (1.9−2.2 eV), photoand electrochemical stability, nontoxicity, low cost, and earth abundancy. Despite these seemingly favorable characteristics, several factors have limited the α-Fe2O3 photoactivity performance, including a short hole diffusion length (2−4 Received: March 3, 2015 Accepted: June 12, 2015 Published: June 12, 2015 Article