# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 3 RAFT Polymerization
### Application to Gold Nanocomposites
coated Au NPs (_d_ = 8 nm) with three polymers prepared by MADIX/ RAFT polymerization which in aqueous solutions have different behaviors: a cationic polymer, poly[(3-acryl-amidopropyl) trimethylammonium chloride] (PAPTAC), a thermoresponsive polymer PNiPAM, and a pH-responsive polymer PAA [89], and also reported the used of poly(\(N\)-vinyl caprolactam) [55], which is also a thermoresponsive polymer. The optical properties of the resulting nanocomposites changed by varying the pH and/or temperature of the colloids depending on the polymer used. [PERSON] et al. [90] in 2010 prepared Au NPs with different sizes from 5 to 47 nm which then were coated, in aqueous solution, with poly(poly(ethylene glycol)methacrylate) (Mn = 16,800 g/mol) and demonstrated thermoresponsiveness. [PERSON] et al. in 2011 [91] prepared a library of Au NPs (12, 28 and 51 nm) coated by post-polymerization modification of poly(pentafluorophenyl methacrylate) synthesized via RAFT polymerization with different chain lengths. This post-polymerization modification allowed the preparation of nanocomposites with different surface chemical functionalities (e.g., charge, polarity) and used them as optical sensors to study several biologically relevant media. In 2013, [PERSON] et al. [92] coated Au NPs (_d_ = 14 nm) with PNiPAM containing single or multiple trithiocarbonate (TTC) groups. The authors demonstrated that the TTC groups play an important role binding to the gold core, thus providing stability to the nanocomposite. Interestingly, using polymers with multiple TTC groups, the distance between the Au cores was kept constant even when polymers with higher MW were used. [PERSON]'s group has also been working on the _grafting to_ approach to prepare other controlled nanostructures with Au [93; 94]. Figure 9: Mechanism describing the immobilization of a RAFT-prepared (co)polymer onto gold surface. Reprinted with permission from [80]. Copyright 2003 American Chemical Society. The surface of gold nanorods bearing polymers prepared via RAFT was reported by [PERSON] et al. in 2007 [95]. Gold nanorods (\(AR=10\)) were firstly synthesized and subsequently modified with hydrophilic polymers poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) or poly(acrylic acid) (PAA), in aqueous solution, and with the hydrophobic polymer (polystyrene--PS) in DMF. The authors demonstrated that the polymers were grafted to the Au nanorods surface by the di- or tri-thio group of the RAFT agent without the need to reduce these groups to the corresponding thiol. Figure 10 illustrates the synthetic procedure as well as the different coordination modes. [PERSON] et al. [96] 2010 described the stabilization of previously prepared Au NPs (\(d=8\) nm) using not only a hydrophilic polymer (poly(\(N\)-isopropyl-acrylamide), PNiPAM) but also an amphiphilic diblock copolymer prepared by RAFT polymerization--poly(\(n\)-butyl acrylate-\(b\)-\(N\)-isopropyl-acrylamide) (PBA-\(b\)-PNiPAM). Interestingly, shell@core type nanostructures were obtained when the PNiPAM was used to stabilize the NPs, but when the PBA\({}_{\text{2K}}\)-\(b\)-PNiPAM\({}_{\text{8K}}\) was used, free polymer globules were also obtained. [PERSON] et al. [97] have shown that using amphiphilic diblock polymer (P(PEGA\({}_{\text{40}}\))-\(b\)-(MMA-\(co\)-BA)\({}_{\text{140}}\)-TTC) it is not possible to prepare, via this approach, core@shell type structures in aqueous medium. This happens because this amphiphilic copolymer formed well-defined micelles in water and the Au NPs, previously prepared, are not able to migrate to the core of the polymer sphere. Actually, these two last examples demonstrate a disadvantage of the _post-modification_ strategy in the preparation of polymer/Au nanocomposites towards bioapplications. The amphiphilic block copolymers in aqueous solutions can form well-organized aggregates, such as micelles, depending on their concentration in solution which compromises the adsorption of the polymer onto the Au surface.