Hydrophobic Gold Nanoparticles with Intrinsic Chirality for the Efficient Fabrication of Chiral Plasmonic Nanocomposites

Abstract

The development of plasmonic nanomaterials with chiral geometry has drawn extensive attention owing to their practical implications in chiral catalysis, chiral metamaterials, or enantioselective biosensing and medicine. However, due to the lack of effective synthesis methods of hydrophobic nanoparticles (NPs) showing intrinsic, plasmonic chirality, their applications are currently limited to aqueous systems. In this work, we resolve the problem of achieving hydrophobic Au NPs with intrinsic chirality by efficient phase transfer of water-soluble NPs using low molecular weight, liquid crystal-like ligands. We confirmed that, after the phase transfer, Au NPs preserve strong, far-field circular dichroism (CD) signals, attesting their chiral geometry. The universality of the method is exemplified by using different types of NPs and ligands. We further highlight the potential of the proposed approach to realize chiral plasmonic, inorganic/organic nanocomposites with block copolymers, liquid crystals, and compounds forming physical gels. All soft matter composites sustain plasmonic CD signals with electron microscopies confirming well-dispersed nanoinclusions. The developed methodology allows us to expand the portfolio of plasmonic NPs with intrinsic structural chirality, thereby broadening the scope of their applications toward soft-matter based systems.

Keywords: block copolymers; chiral metamolecules; chirality transfer; circular dichroism; gels; liquid crystals; phase transfer; reconfigurable nanostructures; supramolecular chirality.

Figure 1

Outline of hydrophobic chiral plasmonic NPs fabrication. (a) Scheme of the phase transfer of l-AuHN NPs into the hydrophobic environment through CTAB to HT1 ligand-exchange reaction. Molecular structures of (b) CTAB and (c) HT1. Representative SEM, HAADF TEM, and elemental mapping images of (d) l-AuHN@CTAB NPs and (e) l-AuHN@HT1 NPs after phase transfer to dichloromethane (DCM). (f) Vis–NIR spectra of L-AuHN (blue) and d-AuHN (red) NPs dispersions in water and dichloromethane (dotted and solid lines, respectively). (g) CD spectra of l- and d-AuHN@CTAB NPs (blue and red dotted lines, respectively) in water. (h) CD spectra of l- and d-AuHN@HT1 NPs (blue and red solid lines, respectively) in DCM.

Figure 2

Stability of hydrophobic, chiral NPs. (a) Vis–NIR spectra of d- and l-AuHN@HT1 NPs (upper and lower spectra, respectively) dispersed in THF at various storage times: 0 h, 72 h without HT1 excess, and 7 days with HT1 excess. (b) CD spectra corresponding to those shown in a. (c) G-factor calculated at the peak of the main CD band shown in panel b, compared to that of AuHN@CTAB in H₂O. (d and e) SEM micrographs and 3D models of dropcasted l-AuHN@HT1 NPs in THF (0 h) and l-AuHN@HT1 NPs in THF (72 h); insets present zoom into NPs.

Figure 3

Universality of phase transfer of chiral nanoparticles via hydrophobic thiol ligands (HT). (a) Structures of HT used for the phase transfer process. (b and c) Vis–NIR spectra of l- and d-AuNRs dispersions in water (AuNR@CTAC in H₂O) and dichloromethane (AuNR@HT2–5 in DCM). (d and e) Circular dichroism spectra of dispersions of hydrophilic and hydrophobic l-AuNR and d-AuNR. (f and g) Representative SEM micrographs of l-AuNR before (f) and after (g) ligand exchange, samples were dropcasted from water and dichloromethane dispersions, respectively. (h) Absolute value of g-factor calculated at the maximum of the main CD band for l-AuNR (at the top) and d-AuNR (bottom) coated with different hydrophobic thiols dispersed in DCM, compared to hydrophilic l- and d-AuNR coated with CTAC dispersed in water.

Figure 4

Compatibility of hydrophobic, chiral NPs with various organic materials serving as matrixes hosting chiral Au NPs. (a) Scheme of heat annealed d-AuNR@HT2 in M1 composite, comprising d-AuNR@HT2 NPs (shown in yellow) and M1 matrix (shown in blue), which is a liquid crystal forming helical nanofilaments in thin films. (b) Circular dichroism spectra of d-AuNR@HT2 in M1 sample after a heating–cooling cycle (155–30 °C) and d-AuNR@HT2 NPs dispersed in dichloromethane. (c) SEM micrograph of a heat annealed d-AuNR@HT2 in M1 sample. (d) Scheme of d-AuHN@HT1 in M2 gel composite, comprising d-AuHN@HT1 NPs (shown in yellow) and M2 matrix (shown in blue) forming physical gel by assembling into helical nanofilaments. (e) CD spectrum of d-AuHN@HT1 in M2 composite after gelation process and d-AuHN@HT1 NPs dispersed in dichloromethane; insets show optical photographs of the prepared samples in reflected and transmitted light. (f) TEM micrograph of d-AuHN@HT1 in M2 xerogel. (g) Scheme of l-AuHN@HT1 in M3 composite material comprising l-AuHN@HT1 NPs (yellow) and diblock copolymer (styrene blocked with polyethylene glycol, M3, bluish color) forming phase-separated spherical and cylindrical structures. (h) CD spectra of l-AuHN@HT1 in M3 composite dropcasted onto a solid substrate compared to CD spectra of AuHN@HT1 dispersion in DCM. (i) SEM micrograph of l-AuHN@HT1 in M3 composite.