# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 4 Au NPs in Opto-Biodetection and Beyond
The Au nanostructures based on PVP and with lower MW presented higher grafting density. Note that the grafting density is an important parameter in cases where the biofunctionality (e.g., sugar moiety) is at the end of the polymer chain, thus graft density is proportional to the number of biorecognition moieties. The authors even linked amino-glicosides to the carboxylic acid group at the chain terminal, and observed that the shorter PVP@Au nanostructures showed stronger binding of Con A.  
Figure 13: Schematic representation of Au NPs biosensor based on LSPR (**a**) and based on quenching fluorescence (**b**). In the area of diagnosis, Au NPs can also be used as surface enhanced Raman scattering (SERS) platforms. This effect also results from the interaction of the electromagnetic radiation with the Au NPs, resulting in the local enhancement of the electric field in specific sites (hot spots), such as metal nanojunctions. Thus, the polarization of molecules adsorbed at the surface of the NP are also affected resulting in an enhancement of the Raman scattering signal, allowing the identification of such molecular species even in very diluted conditions [140; 141; 142]. The importance of polymer/Au nanocomposites in the preparation of these diagnosis platforms also results from the colloidal stabilization and chemical functionalization conferred by the polymer shells, namely when they are intended to be used in complex environments such as real biological samples [141; 142; 143] RAFT-polymer/Au nanocomposites have been prepared aiming the preparation of nanoassemblies, in which the NPs become close to each other, thus creating \"hot spots\" that result in improved SERS signals [144]. Additionally, the polymer shell in the plasmonic platforms composed by polymer/Au nanocomposites, can also provide responsive behavior by a judicious choice of the monomers. For instance, polymers based on poly(ethylene glycol) (PEG) or poly(ethylene oxide) (PEO), poly(\(N\)-isopropyl acrylamide) (PNIPAM) and poly(\(N\)-vinylcaprolactam (PNVCL) provide thermal responsiveness and acrylic acid pH responsiveness, as will be discussed below. Moreover, polymers based on PEG/PEO can also afford biocompatibility and antifouling properties to the nanocomposites, which is relevant to limit non-specific interactions between surfaces and proteins. These \"smart\" nanomaterials can be used in drug/gene delivery applications and the role of RAFT polymerization is very important here, because it allows control the molecular weight and composition of the polymer shell, which is crucial to tailor and adjust the sensitivity of the polymer to an external stimulus. In many works reporting RAFT-polymer/Au nanocomposites, only the concept of shell response to an external stimulus (such as change of pH and/or temperature) is demonstrated but without real applications for these \"smart\" nanomaterials. In the RAFT polymerization of NiPAM, using a PEO-based xanthate-type RAFT agent, the authors have demonstrated the influence of the PNiPAM molecular weight (MW) on the low critical solution temperature (LCST). Then in the presence of this polymer, Au NPs were synthesized using sodium borohydride as reducing agent (in THF/ethanol) and the resulting nanocomposite proved to be stable in water, however increasing the temperature the transparent colloid becomes turbid [102]. [PERSON] et al. have shown the impact of the MW balance of two blocks (PNiPAM and butyl acrylate, BA) on the cloud point value of the block copolymer. Here, the preparation of polymer/Au nanocomposites was performed via both _grafting to_ and in situ strategies; in this case, reversible changes occur in LSPR by increasing the temperature, as a result of polymer shell conformation [96]. Moreover, these authors have demonstrated that by increasing the NaCl concentration, the cloud point of the block copolymer decreased significantly. The same behavior was observed later for PNiPAM@AuNPs also prepared via RAFT polymerization.