Surface plasmon-coupled emission (SPCE) has emerged as an interdisciplinary, versatile sensing platform because of its highly directional, solid-state, and polarized emission. Here, we report the distinct properties rendered by silver Soret colloids (Ag-SCs) such as nanovoids and nanocavities to observe 104-fold enhancement in the emission intensity of omnidirectionally radiating emitter dipoles. Unlike earlier reports utilizing templated silver nanoparticles (AgNPs) in spacer or cavity architectures, here we employ template-free, linker-less Ag-SCs. The Purcell factor (maximum of 120.6) obtained using the finite-difference time-domain simulations for Soret nanocavities is in excellent agreement with the trend in emission enhancements obtained experimentally. The thermal gradient created by adiabatic cooling of AgNPs drives their thermodiffusion, resulting in monodisperse nanoparticle assemblies (Ag-SCs). In addition, we report an extended-cavity architecture with Ag-SCs, as a novel pseudo-metal–dielectric–metal (MDM) interface, for achieving 80-fold SPCE. This study also features the unique properties of Ag-SCs as interfacial nanomaterials on the SPCE platform to achieve femtomolar detection of glutathione (GSH). The quenching of fluorescence from the Alizarin Red S–boric acid (ARS–BA) complex upon the addition of Cu2+ ions and the dequenching upon the GSH addition studied with Ag-SCs as the spacer layer remarkably increased the sensitivity of the analyte. The uniform and intense electromagnetic-field confinement provided by these intricate architectures and hybrid interfaces, along with their ease of fabrication and versatility for a variety of analytes, is critical to achieving augmented SPCE. This is accomplished without compromising the reliability of detection, as demonstrated with the use of a cellphone camera, Commission Internationale de l’Eclairage color space, and luminosity plots for turn-on fluorescence. The emission images were acquired using an android-phone camera by aligning it with its angular emission, making it amenable for point-of-care diagnostics.

The quest for auxiliary plasmonic materials with lossless properties began in the past decade. In the current study, a unique plasmonic response is demonstrated from a stratified high refractive index (HRI)–graphene oxide (GO) and low refractive index (LRI)–polymethyl methacrylate (PMMA) multistack. Graphene oxide plasmon-coupled emission (GraPE) reveals the existence of strong surface states on the terminating layer of the photonic crystal (PC) framework. The chemical defects in GO thin film are conducive for unraveling plasmon hybridization within and across the multistack. We have achieved a unique assortment of metal-dielectric-metal (MDM) ensuing a zero-normal steering emission on account of solitons as well as directional GraPE. This has been theoretically established and experimentally demonstrated with a metal-free design. The angle-dependent reflectivity plots, electric field energy (EFI) profiles, and finite-difference time-domain (FDTD) analysis from the simulations strongly support plasmonic modes with giant Purcell factors (PFs). The architecture presented prospects for the replacement of metal-dependent MDM and surface plasmon-coupled emission (SPCE) technology with low cost, easy to fabricate, tunable soliton [graphene oxide plasmon-coupled soliton emission (GraSE)], and plasmon [GraPE] engineering for diverse biosensing applications. The superiority of the GraPE platform for achieving 1.95 pg mL–1 limit of detection of human IFN-γ is validated experimentally. A variety of nanoparticles encompassing metals, intermetallics, rare-earth, and low-dimensional carbon–plasmonic hybrids were used to comprehend PF and cavity hot-spot contribution resulting in 900-fold fluorescence emission enhancements on a lossless substrate, thereby opening the door to unique light–matter interactions for next-gen plasmonic and biomedical technologies.

The intrinsically lossy nature of plasmonic-based detection platforms necessitates the use of alternative nanophotonic platforms such as one-dimensional photonic crystals (1DPhCs) to exploit properties pertaining to photonic stop band (PSB), Bloch surface waves (BSWs), microcavity, and band-edge modes. We present a highly desirable confinement of internal optical modes (IOMs) and large surface electromagnetic (EM) field due to BSWs on a plasmon-free, metal template-free, photonic crystal-coupled emission (PCCE) platform ensuing 44-fold emission enhancements of the, otherwise, omnidirectionally emitting radiating dipoles. The effect of dielectric thickness in the PCCE platform has also been explored, and the optimized thicknesses for enhanced coupling of both BSWs and IOMs with the radiating dipoles have been obtained. Cavity engineering involving quantum emitters sandwiched in hot spots between 1DPhCs and Ag nanoparticles (AgNPs) has delivered ∼200-fold emission enhancements on account of the improved local density of states (LDOS) via exceptional EM field trapping by BSWs, IOMs, and localized surface plasmon resonance (LSPR) of plasmonic nanoparticles. Experimental results that are in strong agreement with the numerically calculated data validate this augmentation in enhancements due to the amplified coupling between the radiating dipoles and modes supported by 1DPhCs. Moreover, the tightly entrapped optical energy within the hot spots between AgNPs and 1DPhCs was adopted for sensing environmentally hazardous Al 3+ ions at a 0.21 parts per quadrillion (ppq) limit of detection in drinking water samples with reliable and reproducible results, opening new avenues for investigating distinctive photonic crystal nanoarchitectures as a robust, practical, and user-friendly technology for multiplexed diagnostic fluorescence assays.

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We demonstrate for the first time the tuning of qubit emission based on cavity engineering on plasmonic silver thin films. This tunable transition from weak to strong coupling regime in plasmon-coupled fluorescence platform was achieved with the use of palladium nanocomposites. In addition to our recently established correlation between Purcell factor and surface plasmon-coupled emission enhancements, we now show that the qubit-cavity environment experiences the Purcell effect, Casimir force, internal fano resonance, and Rabi splitting. Finite-difference time-domain simulations and time correlated single photon counting studies helped probe the molecular structure of the radiating dipole, rhodamine-6G, in palladium-based nanocavities. The sensitivity of the qubit-cavity mode helped attain a DNA detection limit of 1 aM (attomolar) and multianalyte sensing at picomolar concentration with the use of a smartphone camera and CIE color space. We believe that this low-cost technology will lay the groundwork for mobile phone-based next-gen plasmonic sensing devices.

We demonstrate an important approach to correlate Purcell factor (PF) and surface plasmon-coupled emission (SPCE) enhancements with the use of finite-difference time-domain (FDTD) simulations and time-correlated single-photon counting (TCSPC) studies of a radiating dipole in cermet nanocavities. We observed >50-fold fluorescence enhancement with high directionality and polarization of Rhodamine 6G (Rh6G) emission trapped in the nanocavity created between the titanium-based ceramic nanoparticle and metallic silver thin film. Compositional variation with hybrid nanoparticles, TiC0N1 (TiN), TiC0.5N0.5 (TiCN), and TiC1N0 (TiC), brought about enhanced PFs and tunable fluorescence enhancements that were used for mobile-phone-based detection of tryptophan with nanomolar sensitivity. We hope that this study opens the door to next-gen plasmonics with the ability to tune and enhance the hot-spot electromagnetic field intensity of alternative plasmonic materials, as hybrid synergy spacers in the SPCE platform.

We demonstrate for the first time the use of Fe-based nanoparticles on N-doped graphene as spacer and cavity materials and study their plasmonic effect on the spontaneous emission of a radiating dipole. Fe-C-MF was produced by pyrolizing FeOOH and melamine formaldehyde precursor on graphene, while Fe-C-PH was produced by pyrolizing the Fe-phenanthroline complex on graphene. The use of the Fe-C-MF composite consisting of Fe-rich crystalline phases supported on N-doped graphene presented a spacer material with 116-fold fluorescence enhancements. On the other hand, the Fe-C-PH/Ag based cavity resulted in an 82-fold enhancement in Surface Plasmon-Coupled Emission (SPCE), with high directionality and polarization of Rhodamine 6G (Rh6G) emission owing to Casimir and Purcell effects. The use of a mobile phone as a cost-effective fluorescence detection device in the present work opens up a flexible perspective for the study of different nanomaterials as tunable substrates in cavity mode and spacer applications.