Remote plasmonic-enhanced Raman spectroscopy with the plasmon-molecule coupling in distance over 100 nm

. We propose remote plasmonic-enhanced Raman scattering (RPERS) spectroscopy for molecular sensing and imaging applications. RPERS requires no contact between analyte molecules and metallic nanostructures, which overcomes the limitations of surface-enhanced Raman scattering (SERS). We constructed RPERS substrates consisting of silver nanoislands and columnar silica structures, which demonstrated a 2×10 7 enhancement in Raman scattering for Rhodamine 6G molecules, even when the metal nanostructures and analyte molecules were over 100 nm apart. The RPERS substrate also exhibited improved reproducibility (<15% RSD), long-term stability (>1 month), and sensitivity (>10 times) compared to conventional SERS substrates. We also confirmed the feasibility of RPERS for biophotonic analysis, i.e., enhancing Raman histological imaging of oesophagus tissues with oesophageal adventitia of a Wistar rat attached atop the columnar silica structure layer. Our demonstration is a promising advancement in the field of enhanced spectroscopy using plasmon and offers a solution to the challenges faced by conventional SERS spectroscopy. It has the potential to pave the way for future developments in remote plasmonic-enhanced spectroscopy.


Introduction
Raman spectroscopy is a versatile technique used to analyze molecular species and structural changes based on the Raman spectrum derived from the molecular vibrations of a sample 1,2 .It can be applied to various states of matter, such as solids, liquids, and gases, and is widely used in molecular sensing, bioimaging, and other applications.
However, the primary limitation of Raman spectroscopy is its weak Raman scattered light intensity, which results in low molecular detection sensitivity and long measurement times.To overcome this limitation, surface-enhanced Raman scattering (SERS) spectroscopy has been developed 3 .SERS utilizes plasmons generated by light excitation of metal nanostructures, which can significantly enhance the Raman scattered light intensity in the vicinity of the metal nanostructures (<10 nm) by 10 2 to 10 7 orders of magnitude.This results in significant improvements in molecular detection sensitivity (<nM) and measurement times (~ms).
Despite its potential, SERS spectroscopy has limitations that hinder its practical application.One concern is the possibility of denaturation of both the metal nanostructures and the measured molecules due to contact between them.Another challenge is achieving precise quantitative measurements without careful control of molecular positioning with 1 nm accuracy, as SERS signal intensity is particularly strong in metal nanogaps (hotspots).
To address these limitations, in the present study, we proposed plasmon-mediated long-range enhancement of Raman scattering via dielectric nanostructures, namely, remote plasmonic-enhanced Raman scattering (RPERS) spectroscopy that overcomes the limitations of SERS spectroscopy.

Fundamental characteristics of RPERS
Figure 1 illustrates the structure of the RPERS plate, which was composed of Ag nanoislands (AgNIs) and columnar SiO2 structures (CSS) on a float slide glass plate 4 .The AgNIs measured between 50-150 nm in lateral dimension and less than 20 nm in height, while the CSS layer, which was around 100 nm thick, acted as a protective layer for the AgNIs.The AgNIs and CSS layers were produced using a sputtering process, enabling the creation of an RPERS plate with a large area.
The fundamental enhancement capability of RPERS spectroscopy was demonstrated using Rhodamine 6G (R6G) molecules, as shown in Fig. 2a.Remarkably, we achieved an optical enhancement of 2×10 7 for the RPERS plate, compared to the slide glass plate, even though the distance between the metal nanostructures and the analyte molecules was over 100 nm.The detection sensitivity of 1.8 pM achieved by RPERS spectroscopy was comparable to that of general SERS spectroscopy, indicating that RPERS was a highly sensitive detection method.Furthermore, compared to the AgNI plate without CSS, which was used for SERS measurements, the RPERS plate demonstrated better signal linearity and signal-to-noise ratio, as shown in Fig. 2b.This is thought to be due to the uniform enhancement provided by RPERS, whereas SERS signals fluctuated significantly depending on the molecule's position with respect to hotspots.To demonstrate that the observed RPERS enhancement occurred even when the analyte molecules were separated from the AgNIs by a CSS layer that was more than 100 nm thick, we performed several tests from various viewpoints.Firstly, we rinsed the R6G molecules that were bound atop the CSS surface with ethanol for a few seconds.This resulted in the extinguishing of the enhanced Raman signals.Secondly, we examined the dependence of the RPERS enhancement on the molecular species and found that it was not equivalent to SERS.Thirdly, we conducted an adhesive tape test where we dispersed 2-naphthalene thiol fine powders onto the adhesive side of a tape.We observed enhanced Raman signals of 2-naphthalene thiol only when the tape was attached to the CSS surface of the RPERS plate.Moreover, the signals appeared and disappeared reversibly with the tape on and off the surface.These observations suggested that the analyte molecules were bound atop the CSS surface, providing an explanation for the enhanced Raman scattering even when the metal nanostructure and the analyte molecule were more than 100 nm apart.

RPERS spectroscopy for bioimaging
Finally, we demonstrated the potential of RPERS spectroscopy for bioimaging applications, as illustrated in Fig. 3.A tissue section of the oesophagus, including the oesophageal adventitia, from a Wistar rat was attached to the RPERS plate.The Raman signals were significantly enhanced by the RPERS plate, enabling clear and highresolution Raman imaging of the tissue section.

Conclusion
This study presented RPERS spectroscopy, which allowed for significant enhancement of Raman spectroscopy without requiring contact between analyte molecules and metallic nanostructures.The RPERS plate offers several advantages over conventional SERS plates, including improved reproducibility, stability, and sensitivity.Our demonstration represented a promising advance in the field of enhanced Raman spectroscopy using plasmon and provided a solution to the challenges faced by conventional SERS spectroscopy.This technology has the potential to pave the way for future developments in remote plasmonic-enhanced spectroscopy.