Abstract
There have been significant advancements in the field of surface-enhanced Raman spectroscopy (SERS). Despite being an ultra-sensitive analytical technique, challenges, such as how to get a proper match between the SERS substrate and light for better signal enhancement to obtain a stable, sensitive SERS substrate, prevent its widespread applications. Finite-difference time-domain (FDTD) method, a numerical tool for modeling computational electrodynamics, has recently been used to investigate SERS for understanding the underlying physics, and optimally design and fabricate SERS substrates for molecular analysis. In this review, we summarize the trend of using FDTD method in SERS studies by providing an introduction of fundamental principles, the studies of optical responses, electromagnetic (EM) field distribution, enhancement factor (EF) of SERS, the application in design and fabrication of SERS substrates, and SERS for biosensing and environmental analysis. Finally, the critical issues of using inherently approximate FDTD method and future improvement for solving EM problems and SERS applications are discussed.
Highlights
The basic concept of surface-enhanced Raman spectroscopy (SERS) is that the Raman scattering signal of molecules at or close to the surface could be enhanced to a factor of 1010–1011 using large local field enhancements at metallic surfaces under the right conditions [1], [2]
With the assumption that the structure is infinite in the z dimension and that the fields are independent of z, the Maxwell's equations are split into two independent groups of equations that can be solved in the x–y plane only, which results in the transverse electric (TE) and transverse magnetic (TM) equations
In terms of fabrication method, Sharma et al presented three representative examples of SERS substrates that include the substrates prepared by bottom-up methods, namely silica-coated clusters of gold NPs, and top-down fabricated immobilized NR assembly (INRA) substrates and tip-enhanced Raman spectroscopy (TERS) tips [20], while Fan et al classified three general types of substrates, including metallic nanoparticle (MNP) immobilized in planar solid supports and metallic nanostructures fabricated using nanolithographic methods and template techniques [19]
Summary
The basic concept of surface-enhanced Raman spectroscopy (SERS) is that the Raman scattering signal of molecules at or close to the surface could be enhanced to a factor of 1010–1011 using large local field enhancements at metallic surfaces under the right conditions [1], [2]. SERS has been employed and widely used as a powerful spectroscopic tool for sensitive and specific detection of chemical [3], biological [4], and medical analytes [5] It has progressed from studies of model systems on roughened electrodes to highly sophisticated studies, such as single-molecule spectroscopy due to the amplification of electromagnetic (EM) fields generated by the excitation of localized surface plasmons [6]. A more comprehensive comparison of these modeling techniques, with respect to the operation, speed, expenses, and accuracy, would be beneficial to readers, which, is out of the scope of this review Among all these methods, FDTD technique has been widely used in SERS, as it includes Maxwell's equations that focus on the EM mechanism [15], shows computational electrodynamics that is an important parameter related to SERS intensity [16], and helps search for better noble metal/semiconductor substrates [17]. Other issues regarding the method limitations, research challenges, and future trends are discussed
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