# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 3 RAFT Polymerization
### Application to Gold Nanocomposites
In fact, in what concerns the coating of Au NPs using the amphiphilic block copolymers, the methodology normally involves the use of organic solvents, such as THF or DMF, where organized aggregates are not formed [98; 99; 100]. Figure 10: Proposed mechanisms describing synthesis, reduction, and immobilization onto a gold surface of RAFT-prepared poly(acrylic acid) (PAA) and polystyrene (PS). Reprinted with permission from [95]. Copyright 2007 American Chemical Society. _In situ_ preparation of this type of nanocomposites seems to be a very simple approach since it occurs in a one-pot procedure, i.e., the polymer with the desired molecular weight, composition and structure is prepared and the metal nanoparticles are generated in situ, usually using sodium borohydride (NaBH\({}_{4}\)) to reduce the inorganic precursor. However, this reducing agent can also reduce the dithioester- or trithiocarbonate-end groups of polymer chains leading to a thiol-ended polymer. This method involves several challenges such as the control over the size and shape of the nanoparticles [1; 68]. Nevertheless, in 2002 [PERSON] et al. [101], demonstrated a novel route for the preparation of copolymer-stabilized gold nanoparticles via in situ preparation. The authors prepared four water soluble copolymers (anionic, cationic, neutral, and zwitterionic (betaine) species): poly(sodium 2-acrylamido-2-methyl propane sulfonate) (PAMPS), poly((_a_-vinylbenzyl)-trimethylammonium chloride) (PVBTAC), poly(_N_,_N_-dimethyl-acrylamide) (PDMAm), and poly(3-[2-_N_-methylacrylamide]-ethyl dimethyl ammoniaic propane sulfonate-_block-N_,_N_-dimethylacrylamide) (PMAEDAPS-_b_-PDMAm). The metal salt (HAuCl\({}_{4}\)) was mixed with the dithioester end-capped copolymer and after addition of the reducing agent (NaBH\({}_{4}\)) copolymer-stabilized gold nanoparticles were obtained, as schematized in Figure 11. This procedure was also performed using the metal salts AgNO\({}_{3}\), Na\({}_{2}\)PtCl\({}_{6}\cdot\)6H\({}_{2}\)O and Na\({}_{3}\)RhCl\({}_{6}\) resulting in the corresponding copolymer-stabilized nanoparticles. Whilst the authors proved that it is possible to synthesize stable metal nanoparticles in the presence of different RAFT polymers, studies focusing on the particle size control have not been performed. In 2008, [PERSON] et al. [102] prepared poly(ethylene oxide-_b_-_N_-isopropylacrylamide) by RAFT polymerization of NiPAM using PEO-based RAFT agent, varying the length of the PNiPAM block. The copolymer was mixed with HAuCl\({}_{4}\), in THF, and reduced using NaBH\({}_{4}\) dissolved in ethanol, yielding Au NPs with 5 to 30 nm in diameter regardless of the concentration and the length of PNiPAM block. [PERSON] et al. [103] in 2011 reported the synthesis in situ of Au NPs in DMF using poly(styrene)-_b_-poly(ethylene oxide) (PS-_b_-PEO) block copolymer and a trithiocarbonate group located between the two blocks. The resulting Au NPs presented a diameter between 5 and 10 nm with \(\lambda_{\text{LSPR}}\) at 530 nm. On the other hand, [PERSON] et al. [96] in 2010 reported the in situ synthesis of Au NPs, in water at pH = 8, using an amphiphilic diblock copolymer prepared by RAFT polymerization--poly(_n_-butyl acrylate-_b_-_N_-isopropyl-acrylamide) (PBA-_b_-PNIPAM). First, the authors observed that when a higher amount of polymer was used the reduction of the gold precursor was slower. Also, the color of the colloidal solution varied from dark brown to light orange depending on the polymer concentration, as shown in Figure 12. This is directly correlated with the size of the Au NPs generated: without polymer \(d\) = 6 \(\pm\) 3 nm; for lower concentration (2.5 \(\times\) 10\({}^{-3}\) wt %) \(d\) = 8 \(\pm\) 6 nm, and for higher concentration 1.5 wt % \(d\) = 1.6 \(\pm\) 0.4 nm. It is noteworthy that the critical aggregation concentration of this copolymer is 3 \(\times\) 10\({}^{-4}\) wt %. [PERSON] et al.