Second harmonic generation correlation spectroscopy for single molecule experiments.

We demonstrate a single molecule detection approach to further extend the detection limit of correlation spectroscopic techniques through the Second Harmonic Generation Correlation Spectroscopy (SHGCS). SHG signals with high signal-to-noise ratio (SNR) were obtained from Barium titanium oxide, BaTiO(3) (BTO) nanocrystals (NCs) upon excitation by a femto-second laser fitted to the scanning confocal bench. The fluctuation of SHG signals from BTO NCs in transparent and turbid media was examined and their diffusion time and particle concentration were evaluated by autocorrelation. Proof-of-concept measurements indicate that water-dispersed BTO NCs at different concentrations yield an average diffusion time of 6.43 ± 0.68 ms and the detection limit of SHGCS was found to be at 814 ± 41 fM, approximately 100 folds below the detection limit of fluorescence correlation spectroscopy (FCS). The dynamics of BTO NCs was demonstrated in serum with high SNR and selectivity to show its potential applicability in biomedicine. High SNR and the sub-picomolar detection limit positions SHGCS as an excellent technique for ultralow single particle or single molecule experimentation in a complex medium.