Surface plasmon-coupled emission (SPCE) has emerged as a potential sensing platform owing to its >50% fluorescence signal collection efficiency. Further advancements toward boosting the coupling efficiency can be achieved by relevant spacer and cavity engineering. Several composites of metal nanoparticles (NPs) have been used along with different templates such as low dimensional carbon substrates (1D, 2D, and 3D), ceramics, proteins, and DNA, to name a few. However, they fundamentally suffer from intrinsic parasitic losses in metals and require nonzero nanogaps between them and the metal thin film for hot-spot generation. Here, we report the first-time application and significance of high refractive index (HRI) dielectric, rare earth, biocompatible Nd2O3 NPs as salient spacers to achieve template-free and metal NP-free, 118-fold emission enhancements in SPCE platform using a simple optical setup. The primary focus is on the effect of volume and size of nanoenvironment on the coupling of Nd2O3 nanorods with silver (Ag) thin film. In addition to this, we report a new cavity format as pseudo-MDHD (metal–dielectric–high refractive index dielectric) framework analogous to MDM (metal–dielectric–metal). This study also elaborates on the importance of Mie resonances and resonant light scattering in analyzing the emission enhancements obtained using spacer, cavity, and extended cavity interfaces. This work also demonstrates the first-time utility of cost-effective smartphone based SPCE studies for monitoring tannic acid (TA), a hazardous chemical in environmental water, at picomolar limit of detection (LOD) using HRI dielectric Nd2O3 nanorods.

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.

Although luminescence spectroscopy has been a promising sensing technology with widespread applications in point-of-care diagnostics and chem-bio detection, it fundamentally suffers from low signal collection efficiency, considerable background noise, poor photostability, and intrinsic omnidirectional emission properties. In this regard, surface plasmon-coupled emission, a versatile plasmon-enhanced detection platform with >50% signal collection efficiency, high directionality, and polarization has previously been explored to amplify the limit of detection of desired analytes. However, high Ohmic loss in metal-dependent plasmonic platforms has remained an inevitable challenge. Here, we develop a hybrid nanocavity interface on a template-free and loss-less photonic crystal-coupled emission (PCCE) platform by the quintessential integration of high refractive index dielectric Nd2O3 “Huygens sources” and sharp-edged silver nanoprisms (NPrs). While efficient forward light scattering characteristics of Nd2O3 nanorods (NRs) present 460-fold emission enhancements in PCCE, the tunable localized plasmon resonances of NPrs display high electromagnetic field confinement at sharp nanotips and protrusions, boosting the enhancements 947-fold. The judicious use of silver NPr (AgNPr) metal-Nd2O3 dielectric hybrid resonances in conjugation with surface-trapped Bloch surface waves of the one-dimensional photonic crystal (1DPhC) displayed unprecedented >1300-fold enhancements. The experimental results are validated by excellent correlations with numerical calculations. The multifold hotspots generated by zero and nonzero nanogaps between the coassembly of NPrs, NRs, and 1DPhCs are used for (i) determination of hyper and hypothyroidism levels through monitoring the concentration of iodide (I–) ions and (ii) single-molecule detection (zeptomolar) of the stress hormone, cortisol, through the synthesized cortisol-rhodamine B conjugate obtained using a simple esterification reaction.

Coupling of photons with molecular emitters in different nanocavities have resulted in transformative plasmonic applications. The rapidly expanding field of surface plasmoncoupled emission (SPCE) has synergistically employed subwavelength optical properties of localized surface plasmon resonance (LSPR) supported by nanoparticles (NPs) and propagating surface plasmon polaritons assisted by metal thin films for diagnostic and point-of-care analysis. Gold nanoparticles (AuNPs) significantly quench the molecular emission from fluorescent molecules (at close distances <5 nm). More often, complex strategies are employed for providing a spacer layer around the AuNPs to avoid direct contact with fluorescent molecules, thereby preventing quenching. In this study we demonstrate a rapid and facile strategy with the use of Au-decorated SiO 2 NPs (AuSil), a metal (Au)-dielectric (SiO 2 ) hybrid material for dequenching the otherwise quenched fluorescence emission from radiating dipoles and to realize 88-fold enhancement using the SPCE platform. Different loading of AuNPs were studied to tailor fluorescence emission enhancements in spacer, cavity, and extended (ext.) cavity nanointerfaces. We also present femtomolar detection of spermidine using this nanohybrid in a highly desirable ext. cavity interface. This interface serves as an efficient coupling configuration with dual benefits of spacer and cavity architectures that has been widely explored hitherto. The multifold hot-spots rendered by the AuSil nanohybrids assist in augmented electromagnetic (EM)-field intensity that can be captured using a smartphone-based SPCE platform presenting excellent reliability and reproducibility in spermidine detection.

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.

A variety of materials such as low dimensional carbon substrates (1D, 2D, and 3D), nanoprisms, nanocubes, proteins, ceramics, and DNA to name a few, have been explored in surface plasmon-coupled emission (SPCE) platform. While these offer new physicochemical insights, investigations have been limited to silver as primary plasmonic material. Although, gold nanoparticles (AuNPs) exhibit robust performance, its intrinsic property to quench the emission from radiating dipoles (at distances < 5 nm) has impeded its utility. Despite the use of metal-dielectric resonances (with Au decorated SiO2 NPs) and sharp nanotips (from Au nanostars) for dequenching the emission, the enhancements obtained has been less than 200-fold in SPCE platform. To address these long-standing challenges, we demonstrate the utility of gold soret colloids (AuSCs) and photonic crystal-coupled emission (PCCE) platform. The soret nano-assemblies synthesized using adiabatic cooling technique presented integrated hotspots when taken with high refractive index Nd2O3 ‘Huygens sources’. The collective and coherent coupling between localized Mie and delocalized Bragg plasmons (of sorets), dielectric plasmons (of Nd2O3), highly confined and intense Bloch surface waves (of PCCE platform) aided in realization of dequenched, as well as amplified > 1500-fold enhancements at the photoplasmonic nanocavity interface, presenting new opportunities for multidisciplinary applications.

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.

No abstract