Enhancement of Raman signals of pyrene due to the enhanced electric fields on the surface of silver nanoparticles has been investigated by controlling the thickness of the silica shell. Dimeric nanostructures having well-defined gaps between two silver nanoparticles were prepared, and the gap size (d) was varied from 1.5 to 40 nm. The molecules trapped at the dimeric junctions showed higher Raman signal enhancements when the gap was less than 15 nm due to the presence of amplified electric field, in agreement with our theoretical studies. The experimental Raman enhancement factors at the hot spots follow a 1/d(n) dependence, with n = 1.5, in agreement with the recent theoretical studies by Schatz and co-workers. Experimental results presented here on the distance dependence of surface enhanced Raman spectroscopy (SERS) enhancement at the hot spots can provide insight on the design of newer plasmonic nanostructures with optimal nanogaps.

Strong coupling interactions between plasmon and exciton-based excitations have been proposed to be useful in the design of optoelectronic systems. However, the role of various optical parameters dictating the plasmon-exciton (plexciton) interactions is less understood. Herein, we propose an inequality for achieving strong coupling between plasmons and excitons through appropriate variation of their oscillator strengths and spectral widths. These aspects are found to be consistent with experiments on two sets of free-standing plexcitonic systems obtained by (i) linking fluorescein isothiocyanate on Ag nanoparticles of varying sizes through silane coupling and (ii) electrostatic binding of cyanine dyes on polystyrenesulfonate-coated Au nanorods of varying aspect ratios. Being covalently linked on Ag nanoparticles, fluorescein isothiocyanate remains in monomeric state, and its high oscillator strength and narrow spectral width enable us to approach the strong coupling limit. In contrast, in the presence of polystyrenesulfonate, monomeric forms of cyanine dyes exist in equilibrium with their aggregates: Coupling is not observed for monomers and H-aggregates whose optical parameters are unfavorable. The large aggregation number, narrow spectral width, and extremely high oscillator strength of J-aggregates of cyanines permit effective delocalization of excitons along the linear assembly of chromophores, which in turn leads to efficient coupling with the plasmons. Further, the results obtained from experiments and theoretical models are jointly employed to describe the plexcitonic states, estimate the coupling strengths, and rationalize the dispersion curves. The experimental results and the theoretical analysis presented here portray a way forward to the rational design of plexcitonic systems attaining the strong coupling limits.

Recent advances in understanding the theoretical and experimental properties of excitons and plasmons have led to several technological breakthroughs. Though emerging from different schools of research, the parallels they possess both in their isolated and assembled forms are indeed interesting. Employing the larger framework of the dipolar coupling model, these aspects are discussed based on the excitonic transitions in chromophores and plasmonic resonances in noble metal nanostructures. The emergence of novel optical properties in linear, parallel, and helical assemblies of chromophores and nanostructures with varying separation distances, orientations, and interaction strengths of interacting dipolar components is discussed. The very high dipolar strengths of plasmonic transitions compared to the excitonic transitions, arising due to the collective nature of the electronic excitations in nanostructures, leads to the emergence of hot spots in plasmonically coupled assemblies. Correlations on the distance dependence of electric field with Raman signal enhancements have paved the way to the development of capillary tube-based plasmonic platforms for the detection of analytes.

Effective treatment of malignant melanoma requires an appropriate combination of therapeutic intervention with long-term prognosis as it often survives by monotherapies. Herein, we report a novel melanoma-targeted theranostic nanoenvelope (MTTNe: ISQ@BSA-AuNC@AuNR@DAC@DR5) which has been constructed by assembling a bovine serum albumin (BSA) stabilized gold nanocluster on a gold nanorod (BSA-AuNC@AuNR), a three-in-one theranostic modality, i.e., photothermal therapy (PTT), photodynamic therapy (PDT), and chemotherapy, tethered with a surface-enhanced Raman scattering (SERS) detection technique. The resultant MTTNe was coloaded with the melanoma-specific FDA approved drug dacarbazine (DAC) and a newly synthesized near-infrared (NIR) absorbing squaraine molecule ISQ that served partly as a photosensitizer and multiplex Raman reporter. Finally, a nanoenvelope was anchored with anti-DR5 monoclonal antibodies as a targeting motif for highly expressed melanoma-specific death receptors in malignant cells. Significant phototherapies of MTTNe were initiated upon an 808 nm single laser trigger which showed a synergistic effect of photothermal hyperthermia as well as singlet oxygen (1O2) driven photodynamic effect in the presence of ISQ followed by on-demand thermoresponsive drug release in the intracellular milieu. Moreover, a multiplex SERS spectral pattern of ISQ (1345 cm–1) and DAC (1269 cm–1) has been utilized for monitoring precise drug release kinetics and target-specific recognition on melanoma cells by Raman imaging. Therapeutic performance of the nanoenvelope was evaluated by in vitro cytotoxicity studies in human melanoma cells (A375) and confirmed the apoptotic phenomenon by molecular-level monitoring of intracellular SERS fingerprints. Finally, to address the biocompatibility of MTTNe, in vivo subacute toxicity was conducted on BALB/c mice. Hence, the current studies mark a footstep of a facile strategy for the treatment of melanoma by synergistic multimodal photothermal/photodynamic/chemotherapy.

Quartets of Au nanorods were designed by combining the methodologies of lateral and longitudinal assemblies. A high electric field prevailing at the quartet junctions results in large enhancement in the Raman signals of molecules. FDTD simulations showed that the displacement of the lateral dimers in quartets expands the scope of hot spot distribution.