Label-Free Aptasensors Based on Localized Surface Plasmon Resonance

4 LABEL-FREE APTASENSORS BASED ON LOCALIZED SURFACE PLASMON RESONANCE 

For centuries, metal nanoparticles have never ceased to attract scientists and artists from many diverse cultures. In this section we briefly introduce a phenomenon of metal nanoparticles that still inspires scientists localized surface plasmon resonance (LSPR) (Hutter and Fendler, 2004). Metal nanoparticles show nonlinear electronic transport (single-electron transport of Coulomb blockade) and nonlinear/ultrafast optical response due to the SPR. Conduction electrons (—) and ionic cores (-F) in a metal form a plasma state. When external electric fields (i.e., electromagnetic waves, electron beams etc.) are applied to a metal, electrons move so as to screen perturbed charge distribution, move beyond the neutral states, return to the neutral states, and so on. This collective motion of electrons is called a plasma oscillation. SPR is a collective excitation mode of the plasma localized near the surface. Electrons confined in a nanoparticle conform the LSPR mode. The resonance frequency of the surface plasmon is different 

LSPR absorption bands are characteristic of the type of nanomaterial, the diameter of nanoparticles, and their distribution (Freeman et al., 1995 Nath and Chilkoti, 2002). LSPR can detect an immediate change in the interfacial refractive index (RI) of the surrounding medium (Mock et al., 2003 Raschke et al., 2003 Nath and Chilkoti, 2004), which is greatly affected by the attachment of biomolecules at the colloid-solution interface (Himmelhaus and Takei, 2000 Mock et al., 2003 Raschke et al., 2003 Nath and Chilkoti, 2004 Endo et al., 

LSPR phenomena have been utilized previously to monitor biomolecular interactions by several groups (Haes et al., 2004 Dahlin et al., 2005 Endo et al., 2005, 2006 Sonnichsen et al., 2005 Liu et al., 2006 Shin et al., 2007). 

Incidence light I Incidence light Optical probe 

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