Gold nanostars can display tunable optical properties in the visible and near IR, which lead to strong electromagnetic field enhancement at their tips. We report generalized application of gold nanostars for ultrasensitive identification of molecules, based on both localized surface plasmon resonance (LSPR) and surface enhanced Raman scattering (SERS). We address the requirements of plasmonic sensors, related to sufficiently large areas where nanoparticles are uniformly immobilized with high density, as well as mechanical flexibility, which offers additional advantages for real-world applications. Gold nanostar monolayers were thus immobilized on transparent, flexible polydimethylsiloxane substrates, and their refractive index sensitivity and SERS performance were studied. The application of such substrates for LSPR based molecular sensing is demonstrated via detection of a model analyte, mercaptoundecanoic acid. We further demonstrate SERS-based pesticide detection on fruit skin, by simply covering the fruit surface with the flexible plasmonic substrate, at the area where the target molecule is to be detected. The transparency of the substrate allows SERS detection through backside excitation, thereby facilitating practical implementation.

Solution processed polydimethylsiloxane/gold nanostar flexible substrates for plasmonic sensing

Seven members of the homologous series of the liquid crystal dimers, the bis(ω-(cholesteryloxycarbonyl)alkyl)disulfides, which contain a sulfur−sulfur link in the flexible spacer have been synthesised and their liquid crystal properties characterised. The dimers are referred to using the acronym Chol-n-SS-n-Chol in which n denotes the number of carbon atoms linking the cholesteryl-based groups and the sulfur atoms, and was varied between 3, 5, 6, 8, 10, 11 and 12. All seven homologues exhibit a chiral nematic phase and for the longest three members a smectic A phase was also observed. An odd−even effect is apparent in both the transition temperatures and the values of the entropy change associated with the chiral nematic−isotropic transition, ΔSN*I /R, in which dimers with even values of n show the higher values. This is interpreted in terms of the average molecular shapes in which the C−S−S−C dihedral angle is around 90°. The values of ΔSN*I/R shown by these dimers are very small for liquid crystal dimer...

Cholesteryl-based liquid crystal dimers containing a sulfur–sulfur link in the flexible spacer

The first series of liquid crystal dimers to contain an isoflavone-based moiety has been prepared, the α-(4'-methoxy-3-phenyl-4H-1-benzopyran-4-one-7-oxy)-ω-(4-methoxyazobenzene-4'-oxy)alkanes. All eight members of this homologous series exhibit an enantiotropic nematic phase. The nematic–isotropic transition temperatures and associated entropy changes exhibit a dramatic dependence on the length and parity of the flexible spacer. This characteristic behaviour for a homologous series of liquid crystal dimers has been interpreted in terms of how the flexible alkyl spacer controls the average molecular shape. A comparison of the transitional properties of this series with those of the symmetric parent dimers, the α,ω-bis(4-methoxyazobenzene-4'-oxy)alkanes, reveals that replacing a methoxyazobenzene unit by the isoflavone-based group reduces the melting point but increases the nematic–isotropic transition temperature. Three other non-symmetric dimers are reported: 1-(4'-methoxy-3-phenyl-4H-1-benzopyran-4-one-...

Non-symmetric liquid crystal dimers containing an isoflavone moiety

Six new non-symmetric dimers belonging to the family of compounds the (R,S)-(benzylidene-4ʹ-substituted-aniline)-2ʺ-methylbutyl-4ʹʹʹ-(4ʹʹʹʹ-phenyloxy) benzoate-oxy) hexanoates have been prepared and their thermal behaviour characterised. The terminal substituents on the 4-benzylideneaniline fragment are H, CH3, C2H5, Cl, Br and I. All six dimers exhibit an enantiotropic nematic phase, and the efficiency of the terminal substituent in enhancing the nematic–isotropic transition temperature is largely consistent with that established for conventional low molar mass liquid crystals. This trend is discussed in terms of the size of the terminal substituent and its effect on the shape of the molecule. The halogen-substituted and ethyl-substituted dimers also exhibit smectic behaviour. For the halogen-substituted dimers, the driving force for smectic phase formation is considered to be, at least in part, the specific anisotropic interaction between the unlike mesogenic units. In contrast, for the ethyl-substitute...

Non-symmetric dimers: effects of varying the mesogenic linking unit and terminal substituent

This paper describes a set of design parameters for manipulating the branch length, branch direction, and overall size of gold nanostars (AuNS) synthesized by a seedless process. We controlled the structure of AuNS by varying four different synthetic factors: mechanical agitation, type of Good’s buffer, concentration ratio of precursors, and pH of growth solution. We found that stirring the precursors increased the yield of branched particles. Additionally, the AuNS morphology could be controlled by changing the buffer type and concentration ratio of precursors, which tuned the localized plasmon resonances from the visible to the near-infrared. We determined that performing the synthesis at a pH less than the pKa of the buffer was critical for the stability of the AuNS; when the pH was greater than the pKa, the nanoparticles aggregated in solution. We expanded the library of Good’s buffers able to form AuNS and optimized the mechanical and chemical synthesis conditions, which improved the structural homog...

Manipulating the Anisotropic Structure of Gold Nanostars using Good’s Buffers

A series of six non-symmetric liquid crystal dimers α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-[pentyl-4-(4′-phenyl)benzoateoxy]hexane have been synthesized and characterized. These dimers vary from one another in terms of the terminal substituent (H, CH3, C2H5, F, Cl and Br) with different polarizabilities and molecular sizes. Investigation on the thermal behaviour shows that the dimer with a more polarizable terminal substituent of Br atom exhibits a smectic A phase as well as an enantiotropic nematic phase. Influence of the molecular size of the terminal substituent in stabilizing the liquid crystalline properties is also studied.

Synthesis, thermal and optical behaviour of non-symmetric liquid crystal dimers α-(4-benzylidene-substituted-aniline-4'-oxy)-ω-[pentyl-4-(4'-phenyl)benzoateoxy]hexane

We describe a simple, one-pot, silver-assisted synthesis of branched gold nanoparticles (GNPs) (nanostars) using Ag NO3 and the mild reducing agent, ascorbic acid, in the presence of a lyotropic liquid crystal (LLC) template formed by Triton X-100 in water either in the hexagonal phase or the micellar phase above the critical micellar concentration. The LLC template is found to assist in the formation of well-defined nanostars with long, twinned thorns and provides the necessary colloidal stability that prevents the final nanostars from irreversible aggregation. In addition, the length of the thorns on the nanostars can be tailored by controlling the Au/Ag ratio in the growth solution. We discuss the contributions of the competing mechanisms that result in an optimal concentration of AgNO3 for the synthesis of nanostars with considerably longer thorns, which is only possible in the LLC template that slows down the growth of the thorns on the initially formed GNP seeds.

Huey-Charn Lee

Papers

Cholesteryl-based liquid crystal dimers containing a sulfur–sulfur link in the flexible spacer

Synthesis, thermal and optical behaviour of non-symmetric liquid crystal dimers α-(4-benzylidene-substituted-aniline-4'-oxy)-ω-[pentyl-4-(4'-phenyl)benzoateoxy]hexane

A versatile, one-pot synthesis of gold nanostars with long, well-defined thorns using a lyotropic liquid crystal template