Nanoscale Localized Surface Plasmon Resonance Biosensors

The interaction of light with metallic nanoparticles (NPs) has aroused significant interest in recent years due to demonstrated applications in nanoscale lithography [1–3], surface-enhanced spectroscopies [4–9], and chemical and biological sensing applications [10–14]. In each of these examples, light can be localized, manipulated, and amplified on the nanometer scale by exciting a collective electron oscillation in the metallic NPs, known as localized surface plasmon resonance (LSPR). By controlling the size, shape, material and local dielectric environment of the NPs, the resonance condition of the plasmon can be tuned throughout the visible and near-infra-red (IR) ranges [15–21]. It is the latter property – namely the local dielectric environment – which forms the basis of LSPR-based biosensing experiments [10, 11, 14, 22–28]. Before discussing LSPR and its applications in detail, the basic physics of plasmons will be briefly reviewed [20]. When a metal surface (either bulk or nanoscale) is irradiated with electromagnetic radiation (light) of the appropriate frequency, a coherent oscillation of the metal’s conduction electrons is induced orthogonal to the propagation direction of the light. This oscillation is a ‘‘plasmon’’, and it can be analyzed as fluctuations in the metal’s surface electron density or, in other words, as a longitudinal electron density wave. In the case of metallic NPs with sizes less than the wavelength of the incident radiation, this plasmon is localized on the surface of the NP (in contrast to bulk metal, where the plasmon can propagate along the surface plane and evanescently decay perpendicular to the plane). This principle is illustrated in Figure 9.1, where the electron cloud of a metallic NP oscillates in phase with the incident electromagnetic field. The wavelength at which this resonance occurs depends on a number of factors related to both the NP itself, as well as its local environment. Typically, gold and silver NPs are chosen for most LSPR applications, although other metals such as 159