Spontaneous versus Stimulated Surface-Enhanced Raman Scattering of Liquid Water.

Paulina Filipczak,Michał Nejbauer,Tomasz Kardaś,Marcin Kozanecki,Czesław Radzewicz,Marcin Pastorczak

doi:10.1021/acs.jpcc.0c06937

Abstract

We have observed for the first time the surface-enhanced (SE) signal of water in an aqueous dispersion of silver nanoparticles in spontaneous (SERS) and femtosecond stimulated Raman (SE-FSRS) processes with different wavelengths of the Raman pump (515, 715, and 755 nm). By estimating the fraction of water molecules that interact with the metal surface, we have calculated enhancement factors (EF): 4.8 × 106 for SERS and (3.6–3.7) × 106 for SE-FSRS. Furthermore, we have tested the role of simultaneous plasmon resonance and Raman resonance conditions for the aν1 + bν3 overtone mode of water (755 nm) in SE-FSRS signal amplification. When the wavelength of the Raman pump is within the plasmon resonance of the metal nanoparticles, the Raman resonance has a negligible effect on the EF. However, the Raman resonance with the aν1 + bν3 mode strongly enhances the signal of the fundamental OH stretching mode of water.

Highlights

Raman spectroscopy provides rich chemical and structural information on the studied samples and is, potentially, an extremely useful tool in chemical, biomedical, and material sciences
It was estimated that the Raman signal enhancement factor (EF) in surfaceenhanced Raman scattering (SERS) is on the order of 106.2 Another approach to Raman signal amplification relies on nonlinear optical processes (i.e., coherent anti-Stokes Raman scattering (CARS) or stimulated Raman scattering (SRS))
The pronounced baseline in the raw Raman spectra of AgNPs originates from, as predicted by theory, resonance surface plasmon emission.[18]. It is clear from the corrected spectra that the Raman signal of water in AgNPs blue is uniformly amplified with respect to the pure water

Summary

■ INTRODUCTION

Raman spectroscopy provides rich chemical and structural information on the studied samples and is, potentially, an extremely useful tool in chemical, biomedical, and material sciences. It has been shown that photon conversion in a stimulated Raman process can be as high as 10%.1 Both CARS and SRS require a strong pump and are usually performed with short laser pulses. We compare EFs. Article of the SE-FSRS signal at different Raman pump and Raman probe wavelengths with spontaneous surface-enhanced Raman scattering (SERS) of the same samples. Absorption spectra of AgNP dispersions were obtained with the use of a double-beam UV− vis−NIR spectrophotometer (Cary 5000, Varian) with superb photometric performance in the 200−800 nm range in relation to pure water as a reference sample. The Raman probe spectrum with and without the presence of the Raman beam (Thorlabs chopper blocks for every second Raman pump pulse) in the sample was recorded by the spectrometer (spectrograph Andor Shamrock SR 500i with CCD camera Andor Newton U971N)

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES