Investigation of the Importance of the Electronic Enhancement Mechanism for Surface-Enhanced Raman Scattering (SERS)

Abstract

Different mechanisms contribute to the increase of the inelastically scattered signal observed in surface-enhanced Raman spectroscopy (SERS). The so-called electronic (chemical) enhancement mechanism has been studied using a Kretschmann configuration by switching the electronic contribution on and off. For this, the direct coupling of the analyte molecules to the metal surface was prevented with the help of different intermediate layers between molecules and metal consisting of self-assembled monolayers or a metallic oxide layer. It has been observed that for a separation from the metal, despite the only very small (a few nanometers) distance from the metal surface introduced by the interlayers, the ratio between the enhancements determined for in-resonance and preresonance conditions is drastically reduced compared to the situation where the molecules can chemisorb on the metal surface. It was also observed that while in the case of direct contact to the metal surface, the resonance/preresonance enhancement ratio was strongly mode-dependent, such dependence did not occur when the analyte molecule was kept away from direct access to the surface. This confirms the transition to a pure electromagnetic enhancement when the molecules are separated from the metal surface and allows for the characterization of the two mechanisms.

Detection and quantification of small concentrations of moxifloxacin using surface‐enhanced Raman spectroscopy in a Kretschmann configuration

“…Similar to the resonant coupling of the excitation light to the SPPs under a well‐defined resonance angle, also the signal light is appearing within in the so‐called “Kretschmann cone.” Its direction is only slightly shifted from the direction of the incoming beam and can be efficiently detected in backward direction within the opening angle of the microscope objective used. [ 81 ] This effect is, for example, also used for directional surface plasmon‐coupled fluorescence detection based on surface plasmon‐coupled emission (SPCE) [ 82 ] and also contributes there to intense signals.…”

“…The self-built experimental setup was already described earlier in detail. [80,81] The Raman spectrometer, in which the KC has been integrated, uses an argon ion laser (Coherent Innova 308 series) as excitation source. A microscope setup guides the laser light to the sample and collects the Raman light in 180 backscattering geometry.…”

“…Its direction is only slightly shifted from the direction of the incoming beam and can be efficiently detected in backward direction within the opening angle of the microscope objective used. [81] This effect is, for example, also used for directional surface plasmoncoupled fluorescence detection based on surface plasmon-coupled emission (SPCE) [82] and also contributes there to intense signals.…”

Hybrid Enhancement of Surface-Enhanced Raman Scattering Using Few-Layer MoS2 Decorated with Au Nanoparticles on Si Nanosquare Holes

“…Surface-enhanced Raman scattering (SERS) has become a powerful method for the detection of biomolecules, owing to its many excellent advantages, including improvement of the Raman scattering efficiency, high sensitivity at the single-molecule level, and high accuracy in biomedical detection [ 1 , 2 , 3 ]. In the last few years, various SERS substrates, such as metallic structures [ 4 ], compound semiconductors [ 5 , 6 ], and two-dimensional (2D) materials [ 7 , 8 ] have garnered an increasing interest. Among SERS substrates, the main mechanisms for SERS enhancement can be divided into electromagnetic (EM) and chemical (CM) mechanisms [ 9 , 10 ].…”

A photo-responsive p-Si/TiO2/Ag heterostructure with charge transfer for recyclable surface-enhanced Raman scattering substrates

“…The electromagnetic (EM) mechanism usually enhances the Raman signal over a large range for local surface plasmon resonance (LSPR), [6][7][8] while chemical enhancement, usually interpreted by the charge transfer (CT) mechanism, can result in enormous selective Raman enhancement via changing the electronic structure of a detected molecule adsorbed on the metal material's surface. [9][10][11] However, for a long time, the EM mechanism has been thought to be the principal strategy given the very strong SERS activity of noble metal colloidal particles (including silver (Ag), gold (Au) and copper (Cu)). [12][13][14][15] The EM enhancement mechanism, commonly known as the "hot spot" effect, mainly comes from the randomly distributed noble metal nano aggregates, with great signal fluctuation.…”