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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