Perspectives and Fabrication Challenges for Plasmon Based SERS Substrates

Abstract

Surface Enhanced Raman Scattering (SERS) has emerged as a powerful surface- sensitive technique for detection of surface-adsorbed analytes down to ultra-low concentrations. Its use across various contrasting disciplines has far surpassed the traditional molecule detection techniques like mass spectroscopy, gas chromatography, optical and chemical sensors, etc. Plasmon-based sensors are extremely sensitive as these sensors exploit the enhancement and confinement of optical fields in the close vicinity of the interfaces between a metal and a dielectric, enabling the design of biochemical sensors as scales far below the diffraction-limit. Such localized optical enhancements within quantum-confined structures are the basis of photonics. Therefore, SERS has made its mark in classical disciplines like material science, biology, medicine, chemistry, and the like, as well as interdisciplinary fields like criminal sciences, art restoration, etc. One of the demanding factors necessary for a reliable and good SERS signal is the substrate over which the analyte molecule is detected. This review tries to give an account of fabrication of such surfaces and the challenges associated therein. Beginning with a short introduction on the fundamental concepts of interaction of light with metal nanostructures, a basic theory of SERS is discussed from classical electromagnetic concepts. The concept of plasma oscillation has been introduced that brings the notion of plasma frequency. Theoretical treatments are explained using Drude and Lorentz models. The motion of free as well as bound electrons inside metals and dielectrics are derived using generalized equations of motion. Finally, a frequency-dependent permittivity relation is obtained. Conditions leading to localized surface plasmon resonances have been discussed. Enhancement factors arising out of electromagnetic and chemical contributions have been elaborated. The adopted techniques used for fabrication of useful SERS substrates have then been discussed in detail. Initially, the colloid based method has been reviewed. This has been done with respect to their stability, shape, dispersity and efficiency of SERS enhancement factor (EF). This is followed by a detailed account of template-based planar solid substrates. This further includes synthesis of immobilized metal nanoparticles, nano-sphere lithographic techniques, wet chemical etched substrates, electron beam lithographic techniques and ion beam patterned substrates. Finally, an account of flexible substrates used for SERS detection is provided. The strengths and weaknesses of each technique has been elucidated in the review. Finally, an effort has been made to give an idea of the research gaps in this area and the challenges involved there-in thereby indicating more work to be done before this technique can be routinely used as a low-cost commercial product.