We report on a method for direct determination of the molar concentration of gold nanoparticles (GNP) in solution by using resonance light scattering correlation spectroscopy (RLSCS). RLSCS is based on the correlation analysis of the fluctuations of resonance light scattering due to Brownian motion of single nanoparticles in a highly-focused laser volume. Similar to single molecule fluorescence correlation spectroscopy, the number of particles in the detection volume is reciprocally related to the G(0) value in the RLSCS plot. A model is established for quantification of GNP concentration, and the effect of laser illumination intensity was studied. An excellent linear relationship exists between the concentration of GNP and the reciprocal G(0) value of the correlation plots at low laser illumination intensity. The method was applied to the determination of the molar concentrations of GNP in sizes of 15, 20, 30, and 40 nm to give detection limits of 300, 80, 10, and 40 pM, respectively. The results obtained by RLSCS were in good agreement with that of combined atomic emission spectroscopy and transmission electron microscopy (AES-TEM). A sensitive microscale method was reported for the determination of the activity of the enzyme caspase 3 by RLSCS by using GNP as labeling probes. The assay is based on the measurement of the change of the molar concentration of GNP when peptide-labeled GNP substrates were cleaved by caspase 3. The method is shown to enable the determination of caspase 3 (with a 0.1 nM detection limit) and to monitoring drug-induced cell apoptosis.
Graphical abstract Enzyme activity and cell apoptosis assays by single particle resonance light scattering correlation spectroscopy (RLSCS) are reported.
