Fluorescence-Free Single-Molecule Microscopy via Electronic Resonance Stimulated Raman Scattering

Vibrational spectroscopy is a powerful tool for spectral imaging of biological samples, thanks to its narrow bandwidth (10 cm⁻¹) compared to fluorescence. Single-molecule vibrational spectroscopy has been demonstrated with near-field amplification as in surface-enhanced Raman spectroscopy or fluorescence detection as in stimulated Raman excited fluorescence and bond-selective fluorescence-detected infrared-excited spectro-microscopy. However, these methods involve complex sample preparation or produce high backgrounds, limiting their practicality. In response to these issues, we developed electronic resonance stimulated Raman scattering (ER-SRS) to achieve single-molecule sensitivity in far-field vibrational microscopy without relying on fluorescence detection. ER-SRS has encountered difficulties due to large electronic backgrounds. To overcome this, we employed Raman-amplified nonfluorescent molecular probe (RANMP) alongside our synchronously pumped, independently tunable double optical parametric oscillators for effective optimization of the signal-to-background ratio. The optimization of probe and light source allowed us to successfully detect ER-SRS signal from single particles in solution and from single molecules embedded in polymer matrix. ER-SRS combined with RANMP, offering single-molecule sensitivity without the aid of fluorescence detection, will open new avenues in biological and chemical fields, particularly in multiplexed imaging.