Multifunctional tunable Cu2O and CuInS2 quantum dots on TiO2 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.

Voltage-Switchable Biosensor with Gold Nanoparticles on TiO2 Nanotubes Decorated with CdS Quantum Dots for the Detection of Cholesterol and H2O2.

A thin layer of gold nanoparticles (Au NPs) sputtered on cadmium sulfide quantum dots (CdS QDs) decorated anodic titanium dioxide nanotubes (TNTs) (Au/CdS QDs/TNTs) was fabricated and explored for the nonenzymatic detection of cholesterol and hydrogen peroxide (H2O2). Morphological studies of the sensor revealed the formation of uniform nanotubes decorated with a homogeneously dispersed CdS QDs and Au NPs layer. The electrochemical measurements showed an enhanced electrocatalytic performance with a fast electron transfer (∼2 s) between the redox centers of each analyte and electrode surface. The hybrid nanostructure (Au/CdS QDs/TNTs) electrode exhibited about a 6-fold increase in sensitivity for both cholesterol (10,790 µA mM-1 cm-2) and H2O2 (78,833 µA mM-1 cm-2) in analyses compared to the pristine samples. The hybrid electrode utilized different operational potentials for both analytes, which may lead to a voltage-switchable dual-analyte biosensor with a higher selectivity. The biosensor also demonstrated a good reproducibility, thermal stability, and increased shelf life. In addition, the clinical significance of the biosensor was tested for cholesterol and H2O2 in real blood samples, which showed maximum relative standard deviations of 1.8 and 2.3%, respectively. These results indicate that a Au/CdS QDs/TNTs-based hybrid nanostructure is a promising choice for an enzyme-free biosensor due to its suitable band gap alignment and higher electrocatalytic activities.

TiO2 nanotube array-modified electrodes for L-cysteine biosensing: experimental and density-functional theory study.

We report a non-enzymatic facile method for the detection of L-cysteine (L-Cyst) using free-standing TiO2 nanotube (TNT) array-modified glassy carbon electrodes (GCEs). Self-organized, highly ordered, and vertically oriented TNT arrays were fabricated by anodization of titanium sheets in ethylene glycol-based electrolyte. Detailed electrochemical measurements were performed and it was found that modified GCE exhibited high current compared to the pristine counterpart. The high current of the modified electrode was attributed to the high surface area and enhanced electrocatalytic activities of the TNTs toward the L-Cyst oxidation. Under the optimum conditions, the modified electrode exhibited a high sensitivity of ∼1.68 µA mM-1 cm-2 with a low detection limit of ∼0.1 mM. The fabricated electrode was found to be sensitive to pH and electrolyte temperature. The real sample analysis of the proposed method showed a decent recovery toward L-Cyst addition in human blood serum. Furthermore, the density-funcational theory (DFT) analysis revealed that TNTs have greater affinity toward L-Cyst, having stronger binding distance after its adsorption. The higher negative E ads values suggested a stable and chemisorption nature. The density of states results show that the E gap of TNTs is significantly reduced after L-Cyst adsorption. The modified GCE showed excellent selectivity, enhanced stability, and fast response, which make TNTs a promising candidate for the enzyme-free detection of other biological analytes.