Second harmonic generation correlation spectroscopy for single molecule experiments

Area of Science:

• Nonlinear Optics
• Spectroscopy
• Nanotechnology

Background:

• Correlation spectroscopic techniques are vital for analyzing molecular dynamics.
• Existing methods like fluorescence correlation spectroscopy (FCS) have limitations in detection sensitivity.
• There is a need for more sensitive techniques for single-molecule detection in complex environments.

Purpose of the Study:

• To introduce and validate Second Harmonic Generation Correlation Spectroscopy (SHGCS) as a novel method for single-molecule detection.
• To extend the detection limits of current correlation spectroscopic techniques.
• To demonstrate the applicability of SHGCS in complex biological media.

Main Methods:

• Utilized Barium titanium oxide (BaTiO3) nanocrystals (NCs) as a model system.
• Employed a femtosecond laser on a scanning confocal microscope for Second Harmonic Generation (SHG) signal acquisition.
• Analyzed SHG signal fluctuations using autocorrelation to determine diffusion times and particle concentrations.
• Investigated SHGCS performance in both transparent and turbid media, including serum.

Main Results:

• Achieved high signal-to-noise ratio (SNR) SHG signals from BTO NCs.
• Determined an average diffusion time of 6.43 ± 0.68 ms for water-dispersed BTO NCs.
• Established a detection limit of 814 ± 41 fM for SHGCS, approximately 100-fold lower than FCS.
• Successfully demonstrated the dynamics of BTO NCs in serum with high SNR and selectivity.

Conclusions:

• SHGCS offers a significant advancement in detection limits for correlation spectroscopy.
• The technique provides high SNR and selectivity, suitable for ultralow concentration measurements.
• SHGCS shows strong potential for single-particle or single-molecule analysis in complex biological and chemical systems.
• The sub-picomolar detection limit positions SHGCS as a powerful tool for advanced nanoscale research.