Silver Nanoparticles as Optical Sensors

Abstract

The use of silver nanoparticles (AgNPs) as analytical and bioanalytical sensors is receiving significant attention. This relevance arises from its unusual optical, electronic, and chemical properties (Schultz et al., 2000; Taton et al., 2000; Yguerabide & Yguerabide, 1998). The optical excitation of the surface plasmon resonance (SPR) caused by a collective excitation of the conduction band electrons of the nanoparticle, produces an absorption with large molar extinction coefficients and relevant scattering, usually when the particle size is larger than a few tens of nanometers (Haes et al., 2004). Colloid suspensions display brilliant colors as a result of intense light absorption and scattering, a fact first recognized by Faraday (Faraday & Philos, 1857), more than a century ago. SPR is associated with the coupled oscillation of free electrons on the conduction band accompanying enhanced local electromagnetic field, which is intensely sensitive to surrounding medium conditions. When a nanoparticle is exposed to an electromagnetic wave, the electrons in the particle oscillate at the same frequency as the incident wave. Spectral characteristics of silver nanoparticles are strongly dependent on their size, shape, interparticle spacing and environment (b-Rao et al., 2002). Therefore, the geometry of noble metal nanoparticles would provide important control over linear and nonlinear optical properties (Bruzzone et al., 2005; b-Jiang et al., 2005; Roll et al., 2003). Optical scattering has shown to be usefull in imaging methods to detect biosystems and has been applied to the diagnostics of cancer cells (Hirsch et al., 2003; El-Sayed et al., 2005). Other potentialities are related to single-nanoparticle use as chemical and biological sensors (Cognet et al., 2003; McFarland & Van Duyne, 2003); changes in the plasmonic resonance wavelength of maximum absorption or scattering are monitored as a function of changing the chemical and physical environment of the surface of the nanoparticles. The spectral response, SPR or scattering band, must be very sensitives to the changes in the refractive index of the surroundings. Many theoretical and experimental studies (Kelly et al., 2003; Gole & Murphy, 2004) have been made on metal nanoparticles with different geometries with the purpose to find the best nanoparticle configuration and enhance the sensitivity of the plasmon resonance response. 12