Selective and sensitive detection of intracellular O₂ ^•- using Au NPs/cytochrome c as SERS nanosensors

A novel surface-enhanced Raman scattering (SERS) nanosensor was developed by modifying oxidized cytochrome c (Cyt c) on gold nanoparticles (Au NPs) for the sensitive and selective determination of intracellular superoxide anion radical (O₂^•-). On the basis of the differences in the SERS spectra between the oxidized and reduced form of Cyt c, this nanosensor could be employed to investigate O₂^•- concentration by measuring the SERS spectra of the reduced Cyt c. Using this SERS nanosensor, a detection limit of 1.0 × 10^-8 M for O₂ ^•- could be attained. Additionally, the selectivity of the SERS nanosensor for O₂^•- was examined, showing that other reactive oxygen species (ROS) and biologically relevant species did not influence the detection of O₂^•-. More importantly, the nanosensor could be delivered to the living HeLa and normal human liver cells and permitted the concentration of O₂^•- to be monitored in real time and in a noninvasive manner, which indicates that this nanosensor will be suitable for the qualitative and quantitative analysis of O₂^•- in biosystems, thus leading to a greater understanding of oxidative-stress-related diseases at a cellular level.