# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 2 Colloidal Gold Nanoparticles: Optical Properties and Related Chemical Practices
In the FRET effect, the fluorescence emission band of the donor (fluorophore) overlaps the absorption band of the acceptor (Au NPs) resulting in several vibronic transitions in the donor that have practically the same energy as the corresponding transitions in the acceptor. The critical distance between donor and acceptor (e.g., using organic dyes) is up  
Figure 3: Absorbance spectra, photographs and TEM micrographs of gold nanospheres with different sizes (**a**) and gold nanorods with different aspect ratios (**b**). Adapted with permission from [7]. Copyright 2007 American Chemical Society; and adapted from [8] Copyright 2005 with permission from Elsevier, respectively. to 10 nm, for occurring energy transfer in conventional FRET. However, energy transfer between fluorophores and metal NPs has been described for larger distances due to the so called NSET. For distances between 2-30 nm an efficient energy transfer occurs between the dye and the NP, while for longer distances (>50 nm) the energy transfer can suffer oscillations [14; 15; 16; 17; 18]. There are several well-established methods to synthesize Au NPs, most of them are based on the reduction of a gold (III) salt in the presence of a stabilizing agent, whose synthesis conditions are varied in order to produce colloids characterized by distinct particle size distributions and stable either in aqueous or organic medium. The increasing interest in using Au NPs for a number of applications led to the development of new methods in which reducing agents have been used as more environmentally friendly colloidal stabilizers [19; 20; 21]. Some of the most relevant methods used to prepare Au NPs will be presented as follows. The most popular method to synthesize Au NPs has been the citrate method, originally introduced by [PERSON] et al. [22], and which involves the formation of Au colloids by the reduction of boiling tetrachloroauric acid (HAuCl4) using sodium citrate (Na3[(COO)3C3H5O]), in aqueous medium. The citrate method allows control over the average particle diameter within a wide range (~10-50 nm) varying the concentration ratio between the gold salt and sodium citrate. The citrate method yields NPs stabilized by interparticle electrostatic repulsions due to the adsorption of citrate anions at the particles' surfaces, which are thus negatively charged [23; 24]. Monodisperse and quasi-spherical Au NPs with average diameters up to 300 nm, have been reported via a seeded growth method using the citrate coated Au NPs as seeds [25; 26]. Smaller Au NPs can be synthesized by the method reported by [PERSON] and [PERSON] in 1994 [27], also known as the two-phase synthesis. In this method, the [AuCl4]\({}^{-}\) precursor species are transferred from the aqueous solution to an organic solvent (e.g., toluene) via a phase-transfer reagent and then reduced with aqueous sodium borohydride, at room temperature, in the presence of a long chain thiol (e.g., dodecanethiol), thus yielding thiol capped Au NPs which are stable in organic medium. In this case, the average particle size is varied by varying the alkanethiol:HAuCl4 molar ratio from 0:1 (8 nm) to 2:1 (2 nm). Although the reaction mechanism is not well understood, an increase of the amount of alkanethiol results in a decrease of the NPs average size regardless the amount of reducing agent employed [28]. The methods of phase exchange are particularly relevant because aqueous stable Au NPs are normally required for bioapplications. Noteworthy, hydrophobic Au NPs can be transferred from organic to aqueous media using for example DMAP (4-(dimethylamino)pyridine), which acts as a stabilizer and transfer agent [29; 30]. Methods using borohydride as a reducing agent in one-phase synthesis, namely in aqueous medium, have also been reported. In these procedures, thiol compounds are very often used as colloidal stabilizers and the particle size can be controlled by varying the ratio between the thiol ligand, the Au(III) species and the reducing agent [31; 32].