Detection of Cysteine Using Graphene Quantum Dots/Titanium Dioxide Nanotube-Based Biosensor

The objective of this research was to analyze the diagnostic value of a graphene quantum dots (GQDs)-titanium dioxide (TiO 2 ) nanotube-based biosensor for L-cysteine (Cys). TiO 2 nanotubes were prepared using anodic oxidation, GQDs were synthesized using thermal decomposition of citric acid, and TiO 2 -GQDs composite materials were fabricated using electrophoresis. A biosensor based on TiO 2 -GQDs was constructed, and the stability and photocatalytic efficiency of the TiO 2 -GQDs material were analyzed using X-ray diffraction (XRD) and a dual-beam ultraviolet-visible spectrophotometer (UVS). Cys detection was performed using the TiO 2 -GQDs biosensor, and the sensitivity, pH, time response, and selectivity of TiO 2 -GQDs towards Cys were analyzed using a fluorescence spectrophotometer (FS). The results revealed that TiO 2 nanotubes with optimal diameter and length were obtained at an oxidation voltage of 80 V and oxidation time of 2 hours, with a greatly increased specific surface area (SSA) compared to those at 40 V and 50 V ( P <0.05). Under constant oxidation voltage, longer oxidation time resulted in longer nanotube length. The microstructure of GQDs was clear, and the diffraction peaks and photocatalytic efficiency of GQDs in TiO 2 -GQDs increased with increasing GQDs content. The fluorescence quenching intensity (FQI) of TiO 2 -GQDs solution increased with increasing Cys concentration. After the addition of Cys, the FQI of TiO 2 -GQDs solution was much higher than that of 19 other amino acids (AAs) ( P <0.05). However, difference in the F 0 − F value between Cys and interference AAs in TiO 2 -GQDs solution was not great ( P >0.05). In conclusion, TiO 2 -GQDs material exhibited good stability and photocatalytic efficiency. The biosensor based on TiO 2 -GQDs demonstrated high sensitivity and selectivity towards high-concentration Cys solutions, showing promising applications in biomedicine.