# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 4 Au NPs in Opto-Biodetection and Beyond  
The leading interest of Au NPs in bioapplications, namely in biosensing applications, is mainly due to the existence of LSPR, which is very sensitive to variation of the surrounding environment, including the state of aggregation of the NPs and their behavior in the presence of fluorophores. More specifically, these features allow the use of gold NPs in systems in order to identify specific (bio)analytes that induce the aggregation of NPs or, induce changes in the fluorescence (e.g., quenching effect) of a fluorophore close to the surface of the Au NPs. For example, these systems can be used in the identification of a specific strand of DNA, antibody-antigen or proteins-ligand [123; 124; 3; 125; 126; 3]. Several biosensors envisage the identification of biomolecules, such DNA and proteins, that result from aggregation of Au NPs induced by specific molecular recognition, thus shifting the LSPR band, a phenomenon that in certain cases can be observed by visual inspection. Hence, colloidal Au NPs that are well dispersed in water present a characteristic color, but when aggregated induce color changes, typically from red to purple-blue, depending on the particle aggregation state [58; 128; 129; 130; 131]. Figure 12(a) shows a schematic representation of Au NPs as a biosensing platform based on the LSPR. Regarding biosensing based on fluorescence quenching, receptor/ligand binding and release events can be monitor through changes in fluorescence intensity or lifetime of the fluorophore, reflecting binding and unbinding states of analytes to sensors, since the fluorescence intensity is dependent on the distance between the Au NP (quencher) and the fluorophore [16; 131; 132; 133; 134; 135; 136] (Figure 13b). The development of optical biosensors based on RAFT-polymer/Au nanostructures have been less exploited. Examples include the report of a biorecognition event (bioreceptor/bioanalyte) for the system sugar/lectin (e.g., glucose or mannose/Concanavalin A). For instance, glucose containing monomers are polymerized in an aqueous solution via RAFT polymerization and after reducing the dithio group from the Z-terminal to a thiol termination, the thiolated glycopolymer is mixed with Au NPs via a _\"grafting to\"_ approach. The biorecognition event is observed by the shift of the LSPR band to longer wavelengths and decrease of absorbance, which is the result of aggregation due to the increase of the concentration of Concanavalin A [137]. In a similar work, instead of glucose, the authors have used mannose moieties which showed stronger biorecognition abilities by increasing the density of this bioreceptor in the polymer shell [86]. In another example, the authors have prepared via RAFT polymerization a polymer containing \(N\)-(2-hydroxypropyl)methacrylamide (HMPA) and \(N\)-(3-aminopropyl)methacrylamide (APMA). While the HMPA is known by its non-immunogenic nature, the APMA is used to provide amine groups where a trisaccharide was covalently attached. The UV-Vis spectrum of glycopolymer-SH mixed with Au NPs showed a red-shift in the LSPR and a decrease in absorbance, and macroscopic aggregation of these functional NPs was even observed in the presence of the specific lectin. Furthermore, transmission electron microcopy analysis has shown that these multivalent nanostructures have specific affinity to hemagglutinin, which are proteins present on the surface of the influenza virus; nanostructures with no affinity to hemagglutinin, and used as control systems, were not observed around the surface of the virus [138]. Interestingly, other glycopolymer-stabilized Au NPs prepared by a similar strategy as mentioned above, were explored as platforms for synthetic anticancer vaccines [139]. In addition, [PERSON] et al. [82] coated Au NPs with poly(oligoethylenediyclo methacrylates) (POEGMA) and poly(\(N\)-vinylpyrrolidone) (PVP) with different MW prepared via RAFT, and compared the grafting density, stability in buffer solutions, temperature sensitivity and the optical response to a lectin.