Multifunctional tunable Cu ₂ O and CuInS ₂ quantum dots on TiO ₂ nanotubes for efficient chemical oxidation of cholesterol and ibuprofen

In this study, a CuInS₂/Cu₂O/TiO₂ nanotube (TNT) heterojunction-based hybrid material is reported for the selective detection of cholesterol and ibuprofen. Anodic TNTs were co-decorated with Cu₂O and CuInS₂ quantum dots (QDs) using a modified chemical bath deposition (CBD) method. QDs help trigger the chemical oxidation of cholesterol by cathodically generating hydroxyl radicals (˙OH). The small size of QDs can be used to tune the energy levels of electrode materials to the effective redox potential of redox species, resulting in highly improved sensing characteristics. Under optimal conditions, CuInS₂/Cu₂O/TNTs show the highest sensitivity (∼12 530 μA mM^-1 cm^-2, i.e. up to 11-fold increase compared to pristine TNTs) for cholesterol detection with a low detection limit (0.013 μM) and a fast response time (1.3 s). The proposed biosensor was successfully employed for the detection of cholesterol in real blood samples. In addition, fast (4 s) and reliable detection of ibuprofen (with a sensitivity of ∼1293 μA mM^-1 cm^-2) as a water contaminant was achieved using CuInS₂/Cu₂O/TNTs. The long-term stability and favourable reproducibility of CuInS₂/Cu₂O/TNTs illustrate a unique concept for the rational design of a stable and high-performance multi-purpose electrochemical sensor.