# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 2 Colloidal Gold Nanoparticles: Optical Properties and Related Chemical Practices
Also, sodium citrate can be used in this case only as stabilizer since it does not reduce Au(III) extensively at room temperature [33]. However, there are reports on the synthesis of aqueous stable Au NPs using borohydride as a reducing agent without addition of further stabilizers during the synthesis but with judicious control on the reacting mixture pH, in this case set at 8 [34; 35]. Indeed, depending on the pH of the reacting mixture, the hydrolysis of the initial precursor [AuCl4]\({}^{-}\) results in Au(III) hydroxo complexes whose distinct chemical reactivity impacts on the nucleation and growth stages of the Au NPs, which in turn influences the final morphology observed for the NPs [36; 37]. On the other hand, [PERSON] et al. [38] have used both citrate and borohydride anions as reducing agents and have reported a major influence for the chloride concentration on the particle size distribution of Au NPs. The authors argued that by increasing the chloride concentration, the aggregation of the primary particles is promoted due to the decrease of surface charge, thereby leading to bigger particles. Hence, by controlling the pH and the chloride concentration, NPs with diameters in the 19-47 nm range were obtained by using sodium citrate as reducing agent and, Au NPs with diameters ranging 3-12 nm were obtained by using sodium borohydride as the reducing agent. Other authors even exploredgreen synthesis methods to prepare nanoparticles, in this case using plant extracts as reducing and stabilizing agents [39]. The syntheses of Au NPs have not been limited to nearly spherical nanostructures and anisotropic NPs have been reported, such as rods, prisms, wires and stars. In fact, the exploitation of distinct optical properties of Au NPs of variable morphology has been a cornerstone on the development of chemical strategies to functional nanomaterials. In a number of synthesis methods, the oriented particle growth occurs from Au(0) seeds (\(d<10\) nm) previously introduced in a growth solution containing the Au(III) precursors, a soft reducing agent (e.g., ascorbic acid) and a shape-directing agent, such as an halide counter ion (Cl\({}^{-}\), Br\({}^{-}\) and I\({}^{-}\)) [40; 41]. For example, NPs with a rod shape (nanorods, NRs) have been prepared using the seed-mediated growth method introduced by [PERSON] et al. 2001 [42] which was later improved, in order to obtain better control over the particle aspect ratio (AR) and thus allowing the synthesis of NRs with higher AR [43; 44; 45]. The first step of this method concerns the preparation of Au\({}^{0}\) seeds (1.5-5 nm in diameter) using BH\({}_{4}\)\({}^{-}\) as reducing agent and then mixing the ensuing seeds with a growth solution. The growth solution contains the Au(III) salt, ascorbic acid, the surfactant CTAB (hexadecyltrimethylammonium bromide) and, in some cases, AgNO\({}_{3}\). The ascorbic acid is used as a mild reducing agent in order to promote the growth of the Au particles but avoiding extensive reduction that would cause further nucleation events that would result in a polydispersed colloid. The surfactant CTAB used as stabilizing agent is very important to control the AR, namely the length of the NR, because it is used as a soft template for shaping the nanoparticle into a rod but also because CTAB molecules adsorb preferentially onto the faces along the length of the NR ([110] or [100] faces), so the growth is promoted at the ends of the NR, [111] faces, and a slow growth of the NR width is observed. Additionally, the presence of chemisorbed Ag\({}^{+}\) ions helps the shape induction. According to [PERSON] et al., Ag\({}^{+}\) is adsorbed at the Au NPs surface in the form of AgBr (Br\({}^{-}\) ions come from the CTAB) restricting the growth in that specific facet ([110]). It is noteworthy that Ag NPs are not generated in this case [42; 43; 44; 46] and other parameters influence the growth of Au NRs using these methodologies [47]. Although controversial, the presence of bromide ions seems to be essential to control the synthesis of Au NRs.