Chiral Plasmonic Self-assembly Of Gold Nanorods Mediated By Polymer

Chiral inorganic nanomaterials have attracted tremendous interest because of their excellent optical properties.They extend the chiral signals of the traditional molecular materials in the ultraviolet and deep ultraviolet regions to the visible and near-infrared regions,and produce chiroptical activity that are generally stronger than the molecules.Thus,they show the potentials for a wide variety of applications in enantiomer separation,chiral catalysis,chiral sensing,biomedicine,and optical metamaterials.Many noble metal nanoparticles have a localized surface plasmon resonance(LSPR)phenomenon and exhibit unique optical properties in the visible and near-infrared regions,thus,the chiroptical properties of the noble metal nanoparticles is studied most.It is easy to construct chiral plasmonic nanomaterials by gold nanorods(GNRs)due to their anisotropic characteristic.To date,preparation of chiral plasmonic nanostructures has achieved great progress by using natural biomolecules and their derivatives,for example,amino acids,peptides,fibrils,cellulose nanocrystals,and DNA as linkers or templates.Chiral polymers are promising synthetic materials,which are likely to replace biomolecules.Due to structural diversity and complex supramolecular interactions,these synthetic polymers can be designed with hierarchical architectures,distinct characteristics,and even properties not inherent in natural biological molecules.We realized chiral self-assembly of GNRs using polymers synthesized by reversible addition-fragmentation chain transfer(RAFT)polymerization,producing chiral plasmonic nanostructures with strong chiroptical response.We synthesized poly(methacrylate hydroxyethyl-3-indole propionate)(PIPEMA)and poly(2-hydroxyethyl methacrylate)(PHEMA)from their achiral monomers by RAFT polymerization without any chiral reagents involved.Studies have shown that the main chains of PIPEMA and PHEMA exhibit a preferred-handed helical conformation,which is attributed to their syndiotacticity and stabilized by the steric hindrance of the side groups.Further studies have shown that the driving force of the chiral assembly of GNRs is that the side groups of polymer partially entered into the hydrophobic environment of the CTAB micelles,the left-handed helical main chains that are exposed to the solution entangle with each other due to the hydrophobic interactions,resulting the left-handed side-by-side assembly.Consequently,the chirality of the polymer was transferred to the chiral assemblies of GNRs,producing strong chiroptical signals in their LSPR range.In addition,the chiral signals of the GNR assemblies can be regulated by the molecular weight of the polymer.On the basis of the above research,we further explored the effect of the chirality of the side group and polymers with different end-group on the chiroptical activity of GNR assemblies.We synthesized the enantiomers of PIPEMA derivatives,namely,dithioester-terminated poly(methacrylate hydroxyethyl L-tryptophanate)(P(L-Trp-EMA))and poly(methacrylate hydroxyethyl D-tryptophanate)(P(D-Trp-EMA))by using L-tryptophan or D-tryptophan instead of 3-indolepropionic acid.Studies have shown that GNR assemblies induced by P(L-Trp-EMA),P(D-Trp-EMA),and their racemic mixture exhibit the same chirality,illustrating the chirality of the side group has less influence on the chiral assembly of GNRs.Since the chirality of GNR assemblies is determined by the preferred-handedness of the polymer main chains,it is indicated that the chiral side group of the polymer has negligible influence on the preferred-handedness of the main chain.Additionally,by modifying the end-group of the polymer main chain,we synthesized hydroxy-terminated and thiol-terminated poly(methacrylate hydroxyethyl L-tryptophanate)(P(L-Trp-EMA)-OH,P(L-Trp-EMA)-SH),respectively.Further studies have shown that GNR assemblies induced by P(L-Trp-EMA)-OH can only produce left-handed signal under different pH values,similar to P(L-Trp-EMA),while P(L-Trp-EMA)-SH can induce the chiral assembly of GNRs with right-handed nanostructures and left-handed nanostructures under different pH values,respectively.And the chiral transition from right-handedness to left-handness can be observed in the circular dichroism spectra.It is difficult to effectively regulate the optical activity and structure of chiral nanomaterials by chiral small molecules.However,polymers have unique advantages in manipulating assembly of nanoparticles,due to structural diversity and rich supramolecular interactions.We realized the regulation of the chiral assembly of GNRs by achiral polymer,namely,thiol-terminated polystyrene.We synthesized chiral cetyltrimethylammonium bromide(“chiral CTAB”,2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide),which is composed of two CTAB molecules and contains two carbon atoms with the same chirality.Compared to CTAB molecules,“chiral CTAB” can strongly adsorb on GNRs due to their two quaternary ammonium salts.Thus,the chiral GNR coated with “chiral CTAB” can be prepared by replacing achiral ligand CTAB,which provides a chiral source for chiral assembly of GNRs.Studies have shown that the chiroptical activity and the morphology of the GNR assemblie can be regulated by adjusting the molecular weight of the polystyrene grafted to GNRs coated with “chiral CTAB” and the kind of the poor solvent in the system,resulting chiral nanorings,nanospheres,and side-by-side assembly structures,respectively.