# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 2 Colloidal Gold Nanoparticles: Optical Properties and Related Chemical Practices  
Colloidal Au NPs have been used in several bioapplications mainly due to properties associated to the existence of a Localized Surface Plasmon Resonance (LSPR) [2; 3; 4; 5]. The LSPR is a collective oscillation of the conduction electrons confined to the metal particle dimensions (Figure 2). As such, it occurs for particles much smaller than the wavelength of the incident light that in this situation probes the whole electron cloud. The interaction with the electromagnetic radiation originates accumulation of surface charge at opposite regions of the particle, that result in a restoring force due to oscillating dipoles whose electric field is opposed to the incident field. There is a characteristic resonance frequency for the oscillating dipoles originated by the polarization of the conduction electrons in the metal NPs. For small Au NPs (e.g., 15 nm), the LSPR frequency is located in the visible spectrum, giving rise to strong absorption when irradiated with light at that frequency. Light absorption and scattering at this wavelength region, which is stronger as compared to molecular dyes and becomes more relevant for bigger particles, explains the red color of such colloidal Au NPs. Figure 1: Functionalities associated to polymer/Au nanocomposite particles in some bioapplications. Inspired in [1]. Figure 2: Schematic illustration of the free electron coherent oscillation at a given wavelength for the excitation radiation and corresponding optical spectra of Au nanospheres (**a**) and nanorods (**b**), evidencing the Localized Surface Plasmon Resonance (LSPR) bands. The LSPR of Au NPs is influenced by their size and shape, by the dielectric constant of the surrounding medium and the distance between neighboring nanoparticles [2; 6; 7; 8]. Although for Au nanospheres of bigger sizes and anisotropic particles such as Au nanorods (NRs), the explanation becomes more complex, it suffices to state here that the optical spectra show significative differences. Hence, by increasing the NPs diameter the LSPR shifts to higher wavelengths as shown in Figure 3a[7]. For Au NRs the differences in the optical spectra are even more pronounced, showing an absorption band (transverse mode) corresponding to the electron cloud oscillation along the short axis (diameter), and another band (longitudinal mode) corresponding to the electron cloud oscillation along the long axis (Figure 3b). For the latter, the wavelength for maximum absorption is particularly sensitive to the length of the NRs. Figure 3b shows that by increasing the aspect ratio (AR = length/width) of Au NRs, the longitudinal band shifts to higher wavelengths and can reach the near-infrared (NIR) region, which is of great interest for bioapplications [8; 9]. The relevance of the LSPR of Au NPs for bioapplications is clearly perceived by taking in consideration that under light excitation at such frequencies, the systems will tend to relax either by scattering or by heat dissipation. The first process can be explored in optical biolabeling or biosensing for which a relatively diminutive amount of Au is required. The second process can be explored in photothermal namely by using Au NRs that show the longitudinal LSPR in the NIR therapeutic window (650-1350 nm), because excitation light of maximum penetration depth in biological tissues can be used. Additionally, the optical properties observed for Au colloids can be explored by associating these nanostructures to other optical active compounds, such as molecular fluorophores, which potentiate the use of Au NPs in several biosensing applications. Hence, the presence of certain fluorophores may experience fluorescence quenching when they are in the vicinity of AuNPs due to Forster resonance energy transfer (FRET) as well as to Nanoparticle Surface Energy Transfer (NSET); the latter is observed at distances nearly twice as far as FRET [10; 11; 12; 13; 14].