# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 3 RAFT Polymerization
### Application to Gold Nanocomposites  
In general, the surface modification of inorganic nanoparticles using RAFT polymerization has been explored following three main approaches that are schematized in Figure 8. In the _\"grafing to\"_ and in the \"in situ\" strategies a previously prepared polymer is used. In the former, the polymer and the NPs are mixed and the polymer chains adsorb or bind covalently onto the NPs surface, respectively; whilst in the second strategy, NPs are synthesized using an inorganic precursor in the presence of the polymer. Regarding the third strategy, \"grafing from\", the polymerization is carried out from the inorganic surface which is previously functionalized with the polymerization mediator (e.g., CTA) [1; 68]. In the context of the preparation of polymer/Au nanocomposites, RAFT polymerization presents an advantage related with the use of CTAs. CTAs are usually a thiocarbonylthio compounds, therefore they have high affinity to gold surfaces due to the presence of sulfur atoms. Hence, polymers based on di-and tri-thio CTA agents have been used in the surface modification of gold nanoparticles due to the possibility of forming a strong linkage between the polymer and the NP surface [1; 68]. Back in 2003 [PERSON] et al. [80] have reported the immobilization of polymers prepared by RAFT polymerization onto gold films. After the preparation of the polymers, poly(sodium 4-styrenesulfonate), poly((_ar_-vinylbenzyl)trimethylammoniumchloride), poly(_N_,_N_-dimethyl-acrylamide), and poly(3-[2-(_N_-methylacrylamide)-ethyldimethyl ammonium] propane sulfonate-_b_-_N_,_N_-dimethylacrylamide), the authors reduced the dithioester end group of the CTA to thiol using NaBH\({}_{4}\). Then the thiol containing polymers were immobilized onto gold surfaces due to the high affinity of thiol to gold, as depicted in Figure 9.  
Figure 8: Common approaches to prepare polymer/Au nanocomposites. However, in 2006 [PERSON] et al. [81] demonstrated that CTAs (dithioesters and trithiocarbonates) are also able to chemisorb onto gold surfaces, consequently their reduction into thiols is not mandatory. This demonstration was very helpful to increase the range of RAFT polymers that can be used for surface modification of gold, since some RAFT polymers are incompatible with NaBH\({}_{4}\). Regarding the three strategies schematized in Figure 8, all of them present advantages and disadvantages in the preparation of polymer/Au nanocomposites. The _grafting to_ also referred as _post-modification_ is the easiest and most straightforward methodology since each component is synthesized individually allowing control over the size and shape of the inorganic particle as well as the molecular weight, structure and composition of the polymer, and only afterwards are they mixed yielding the nanocomposite [1; 68]. However, in this strategy, limited grafting density can be an issue, especially for polymers with high MW [82]. As mentioned before, chemisorption of the previously-prepared polymer onto Au NPs can be promoted through the thiol group, by removal of the RAFT agent [83; 84; 85; 86; 87] or can be carried out maintaining the RAFT agent (di- or tri-thio group). For example, hydrophilic polymers prepared by RAFT polymerization have been used to coat Au NPs. For instance, [PERSON] et al. [88] 2010, reported the stabilization of previously prepared Au NPs (_d_ = 20 nm) using temperature and pH responsive polymers prepared via RAFT polymerization: poly(2-aminoethylmethacrylamide) (PAEA), poly(acrylic acid) (PAA), poly(\(N\),\(N\)-dimethylaminoethyl acrylate) (PDMAEA), poly(oligoethylene oxide) acrylate (P(OEG-A)), poly(oligoethylene oxide acrylate-_co_-diethylene oxide acrylate) (P(OEG-A-_co_-DEG-A)) and poly(\(N\)-isopropyl acrylamide) (PNiPAM). Only the cationic polymers required special care during mixing with Au NPs to avoid aggregation due to the interactions with the citrate anions at the Au NPs surface. [PERSON] et al.