# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 3 RAFT Polymerization
### Application to Gold Nanocomposites
[97] have also shown that using amphiphilic diblock polymer (P(PEGA\({}_{40}\))-_b_-(MMA-_co_-BA)\({}_{140}\)-TTC) the synthesis in situ of Au NPs in aqueous medium is influenced by the pH, the amount the copolymer, and the [HAuCl\({}_{4}\)]:[copolymer] ratio. Furthermore, in this case the Au NPs are generated in the outermost part of the polymer sphere (i.e., hydrophilic part). Figure 11: **(left)** Preparation of (co)polymer-stabilized transition metal nanoparticles, (right) UV-Vis spectra for HAuCl\({}_{4}\) sols and polymer-stabilized Au NPs. P1—PAMPS, P2—PVBTAC, P3—PDMA, and P4—PMAEDAPS-_b_-PDMAm. Adapted with permission from [101]. Copyright 2002 American Chemical Society. In addition, in 2013 [PERSON] et al. [104] demonstrated that the molecular weight of the polymer also influences the size of the Au NPs as well as the _end_-group of the polymer. In this study, the authors prepared PNIPAM by RAFT polymerization with three different molecular weights and then the terminal xanthate group was reduced to a thiol or a hydrogen. An aqueous solution of HAuCl\({}_{4}\) was added to the polymer at pH = 8 and reduced with NaBH\({}_{4}\). Indeed, as others had reported before, increasing the concentration of the polymer leads to a reduction of the NPs diameter. Additionally, small differences in the size were also found when different molecular weights were used. But significant differences in the size of the NPs, and consequently in the \(\lambda_{\text{LSPR}}\), were observed as function of the type of end-group (xanthine group--_X_, thiol--_SH_, and hydrogen--_H_). For instance, using higher PNIPAM concentration (0.1 wt %) diameters obtained are: \(d_{\text{H-end}}\) = 6.8 \(\pm\) 3.0 nm, \(d_{\text{SH-end}}\) = 3.2 \(\pm\) 0.4 nm, and \(d_{\text{X-end}}\) = 1.3 \(\pm\) 0.8 nm; and the respective \(\lambda_{\text{LSPR}}\) are 545, 521 nm and no plasmon band was detected. These results clearly show that the presence of sulphur atoms has an important role during the generation of the gold nuclei and in the growth of the NPs. The authors noted that the xanthate group strongly binds to gold blocking the growth of the NPs. It is important to remember that the reducing agent (NaBH\({}_{4}\)) could also reduce the xanthate to thiol although the authors believe that this did not occur in their case. In brief, despite of the apparent simplicity of the in situ synthesis, several parameters influence the generation of the NPs in the presence of the polymers. The molecular weight, the concentration and the chemical nature of the polymer influence the size of the NPs. Moreover, the hydrophilicity, hydrophobicity or amphiphilic nature of polymers and their subsequent configuration/organization in the solvent (aqueous or organic medium) also plays a significant role. However, the parameter that has the major impact is the functional groups containing sulfur, which strongly binds to gold and can block the growth of the NP. In general, for polymers containing sulfur, increasing the polymer concentration (consequently higher amount of sulfur atoms) results in a reduction of the size of NPs. Therefore, for each polymer, studies of the experimental conditions must be done in order to obtain the desired NP size in the nanocomposite. _Grafting from_ approach also known as surface-initiated controlled radical polymerization (SIP) is an elegant method which allows the well-controlled synthesis of the polymer from the surface of previously prepared nanoparticles. In this case, the nanoparticle is previously prepared, thus the size and the shape are controlled, being a methodology very attractive for coating anisotropic nanoparticles (e.g., nanorods). This methodology, requires two steps: (i) anchoring of the CTA onto the NPs surface and, (ii) the polymerization from NPs surface [1; 68]. The former can be a challenge since the CTA grafting density could influence the second step, the polymerization, namely in what regards the chain length, and living characteristics of the polymer.