Plasmonic sensing has been an important multidisciplinary research field and has been extensively used in detection of trace molecules in chemistry and biology. The sensing techniques are typically based on surface-enhanced spectroscopies and surface plasmon resonances (SPRs). This review article deals with some recent advances in surface-enhanced Raman scattering (SERS) sensors and SPR sensors using either localized surface plasmon resonances (LSPRs) or propagating surface plasmon polaritons (SPPs). The advances discussed herein present some improvements in SERS and SPR sensing, as well as a new type of nanowire-based SPP sensor.

This chapter introduce some fundamentals of plasmonics. Surface plasmons (SPs) are collective oscillations of conduction electrons in a metal surface, which can be excited by an electromagnetic (EM) wave. The splendid optical characteristics of SPs include, but are not limited to, preferential absorption and scattering of light of particular wavelengths, EM field confinement beyond the diffraction limit and local EM field enhancement. The Drude model does not consider interband transitions that contribute to damping and decrease the lifetime of plasmons. For nanoparticles, the electron oscillation is confined in three dimensions, resulting in the so-called localized surface plasmons. For large particles, the retardation effect becomes prominent, which leads to a red shift of the plasmon resonance. The plasmon coupling between adjacent metal nanostructures can largely shift the localized surface plasmon resonances frequencies and generate hugely enhanced EM fields in the nanogaps between the nanostructures. Plasmon-generated hot electrons attract much attention, as they may play important roles in photochemical and photovoltaic processes.

When light shines on matter, part of the momentum of the incident photons is transferred to matter, resulting in a back-force acting on it, the so-called "optical force". This force is usually extremely weak of the order of pN or less. However, under certain circumstances, the optical force can be so strong that it is possible to optically trap and manipulate objects at the micro-/nano-scale. In metal nanostructures, the optical force can even be enhanced due to the excitation of surface plasmons, and the optical manipulation at the sub-wavelength scale becomes possible, leading to the derivation of an attractive research branch-surface-plasmon-induced optical forces. This paper will discuss the recent advances on the optical trapping and manipulation using surface-plasmon-induced optical forces.

Optical force has opened up the unique possibility of manipulating objects of micro-/nanoscale sizes by means of light. A typical application of optical forces is the so-called optical tweezer that was discovered by Ashkin in 1971. Plasmon-assisted optical forces can be discussed in two slightly different categories: with and without a focused laser beam that provides additional gradient force from the laser itself. In nanostructures that support surface plasmons, an enhanced electromagnetic (EM) near field is induced under resonant excitation, leading to a series of interesting applications in surface-enhanced spectroscopy and sensing. For propagating surface plasmons, the scattering force is also an interesting component to study—it pushes the trapped particle to move along the propagation direction. For near-field-coupled metal nanoparticles, the optical force is enhanced because of the enhanced EM field gradient. Due to the larger polarizability, metal nanoparticles experience much higher optical forces, including the gradient, scattering, and absorption forces.