Plasmon-enhanced optical chirality through hotspot formation in surfactant-directed self-assembly of gold nanorods

Plasmonically enhanced optical dichroism has attracted substantial interest for its application in optical sensing, where the interplay between chirality emanating from both molecules and plasmon-supporting structures has been regarded as a critical ingredient. Here, we experimentally demonstrate that suitably self-assembled achiral plasmonic nanostructures produce a high degree of enhancement in the optical dichroism observed from chiral molecules placed in their vicinity. Specifically, we identify a near-field enhancement associated with plasmonic hotpots as the mechanism enabling our observation of visible-NIR circular dichroism emanating from small amounts of chiral molecules. Our structures consist of linear arrays of gold nanorods obtained by introducing chiral anionic surfactants, such as modified bile salts, which lead to selective destabilization of a cetyltrimethylammonium bromide coating layer on Au nanorods, thereby promoting a tip-to-tip oriented assembly. The proposed mechanism of plasmonically-enhanced circular dichroism is supported by deriving a simple, yet general theoretical formalism that confirms the observed results, revealing the role of optical hotspots at the gaps of linear tip-to-tip nanorod assemblies as the origin of enhancement in the dichroism from chiral molecules. Importantly, it is the refractive rather than the absorption-mediated chiral response of the molecules that produces dichroism in the visible-NIR plasmonic regime, far from their UV absorption resonances. The observed self-assembly mechanism suggests that chiral analytes not directly interacting with the nanorod surfaces, but just able to induce tip-to-tip aggregation, can be revealed by a CD signature in the plasmonic region, thereby supporting potential applications in ultrasensitive analysis.

Deoxycholic acid and L-Phenylalanine enrich their hydrogel properties when combined in a zwitterionic derivative

Hypothesis: Sodium Deoxycholate (NaDC) and Phenylalanine (Phe) are important biological hydrogelators. NaDC hydrogels form by lowering the pH or by increasing the ionic strength. Phe gels form from saturated solution by thermal induction and slow kinetics. The resulting gels hold great potential in medicine and biology as drug carriers and models for fundamental self-assembly in pathological conditions. Based on this background it was hypothesized that a Phe substituted NaDC could provide a molecule with expanded gelling ability, merging those of the precursors. Experiments: We coupled both building blocks in a zwitterionic derivative bearing a Phe residue at the C3 carbon of NaDC. The specific zwitterionic structure, the concurrent use of Ca2+ ions for the carboxyl group coordination and the pH control generate conditions for the formation of hydrogels. The hydrogels were analyzed by combining UV and circular dichroism spectroscopies, rheology, small angle X-ray scattering and atomic force microscopy. Findings: Hydrogel appearance occurs in conditions that are uncovered in the case of the pure Phe and NaDC: self-standing gels form instantaneously at room temperature, in the 10–12 pH range and down to concentration of 0.17 wt%. Both thixotropic and shake resistant gels can form depending on the derivative concentration. The gels show an uncommon thermal stability in the scanned range of 20–60 °C. The reported system concurrently enriches the hydrogelation properties of two relevant building blocks. We anticipate some potential applications of such gels in materials science where coordination of metal ions can be exploited for templating inorganic nanostructures. © 2019 Elsevier Inc

C-12 vs C-3 substituted bile salts : An example of the effects of substituent position and orientation on the self-assembly of steroid surfactant isomers

Biomolecule derivatives are transversally used in nanotechnology. Deciphering their aggregation behavior is a crucial issue for the rational design of functional materials. To this end, it is necessary to build libraries of selectively functionalized analogues and infer general rules. In this work we enrich the highly applicative oriented collection of steroid derivatives, by reporting a rare example of C-12 selectively modified bile salt. While nature often exploits such position to encode functions, it is unusual and not trivial to prepare similar analogues in the laboratory. The introduction of a tert-butyl phenyl residue at C-12 provided a molecule with a self-assembly that remarkably switched from rigid pole-like structures to twisted ribbons at a biologically relevant critical temperature (∼25 °C). The system was characterized by microscopy and spectroscopy techniques and compared with the C-3 functionalized analogue. The twisted ribbons generate samples with a gel texture and a viscoelastic response. The parallel analysis of the two systems suggested that the observed thermoresponsive self-assemblies occur at similar critical temperatures and are probably dictated by the nature of the substituent, but involve aggregates with different structures depending on position and orientation of the substituent. This study highlights the self-assembly properties of two appealing thermoresponsive systems. Moreover, it adds fundamental insights hereto missing in the investigations of the relation between self-assembly and structure of synthetic steroids, which are valuable for the rational design of steroidal amphiphiles