# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 3 RAFT Polymerization
### Application to Gold Nanocomposites
Moreover, depending on the grafting taking place via the R group, or the Z group there are significant mechanistic differences. [PERSON] has reported a rather systematic discussion regarding these two approaches, as well as the _grafting to_ and in situ strategies and concluded that the _grafting from_ is intrinsically more complex and difficult to control. Nevertheless, judicious choice of reaction conditions, including the use of sacrificial RAFT agents can yield good results [73]. Figure 12: Image of the final NPs solutions 5 min after reduction at 20 \({}^{\circ}\)C. The polymer concentration increases from left to right (0 to 0.15 wt %). Reprinted with permission from [96]. Copyright 2010 American Chemical Society. Surface-initiated RAFT polymerization has been more widely explored using silica NPs, and has been reviewed by [PERSON] et al. [105]. In the SIP, the CTA is covalently bonded to the silica surface and then the monomer is polymerized, in organic medium (e.g., DMF or THF) from the surface of the silica particle. Also using organic medium, back in 2003, [PERSON] et al. [106] reported the RAFT polymerization of PNiPAM from a RAFT agent that was covalently grafted onto the surface of Au NPs (\(d\) = 3.2 nm) containing 11-mercapto-1-undecanol, in DMF. [PERSON] et al. have used this approach to prepare nanocomposites using CdS and CdSe quantum dots via miniemulsion [107]. Surprisingly, in aqueous media there are no reports regarding this type of Au NPs surface modification. In the last years a similar strategy using a macroCTA or macroRAFT agent, instead of CTA has been explored. This approach, named RAFT assisted encapsulating emulsion polymerization (REEP), involves chain extension via RAFT emulsion copolymerization from the macroRAFT agent adsorbed onto NP surface. This approach was first reported in 2008 by [PERSON] et al. [108] for the encapsulation of pigment NPs. The authors used a macroRAFT agent based on acrylic acid and butyl acrylate to stabilize the pigment and then a mixture of hydrophobic monomers (butyl acrylate and methyl methacrylate) was added in a controlled way. The macroRAFT agent acts as a surfactant during the RAFT emulsion polymerization thus allowing emulsion polymerization without the use of additional surfactants. Similar methodologies have been applied to several other particulates, such as platelets [109; 110; 111], carbon nanotubes [112; 113], graphene oxide [114], cerium oxide (CeO\({}_{2}\)) NPs [115; 116; 117], and SiO\({}_{2}\) NPs [111]. Nevertheless, in some methodologies additional surfactant was used, for example in the encapsulation of quantum dots (QDs) [118; 119] and cerium oxide (CeO\({}_{2}\)) NPs [120]. More recently, this approach was reported to encapsulate Au NPs [97] without adding any additional surfactant, affording core@shell type structures. The authors adsorbed a macroRAFT agent based on poly(ethylene glycol)methyl ether acrylate onto Au NPs (diameter around 15 nm) and then the chain extension was promoted by adding a mixture of hydrophobic monomers (methyl methacrylate and \(n\)-butyl acrylate). Even though this approach is not yet well-explored to prepare polymer/Au nanocomposites, when the macroRAFT has a strong tendency to self-assemble in aqueous medium, this seems to be the most promising path to prepare core@shell type nanostructures in which the polymer shell can be designed, tailored and controlled [39]. However, this strategy is not as straightforward as it may seem. Indeed, the use of amphiphilic macroRAFT may in some cases be associated with self-assembly which prevents, or at least limits adsorption on the surface yet, very little is discussed in the literature regarding this aspect. Furthermore, adsorption of the macroRAFT onto the surface to obtain stable colloids (which is crucial for successful REEP) has proven rather complex as it depends on the chemical and structural characteristics of the nanoparticles surface as well as of the chemical and structural characteristics of the macroRAFT used. [PERSON] et al. and [PERSON] et al.