Mobile phone-based surface plasmon-coupled emission from streptavidin magnetic nanoparticles and graphene oxide hybrid nanointerface

Abstract

Fluorescence spectroscopy and related applications are increasingly explored for different point-of-care diagnostics and biophysicochemical senor development. However, low signal collection efficiency (< 1% of isotropic emission), low spectral resolution and requirement of cumbersome equipment in conventional fluorescence based methodologies are significant challenges [1]. In this context, recently metal nanothin films are being progressively studied to achieve highly polarized, low background noise, high spectral resolution and sharply directional emission, using the surface plasmon-coupled emission (SPCE) platform [1]. Several quintessential concepts, processes and applications have been examined using this SPCE technology [2]. Graphene oxide (GO) spacer nanolayer has been recently utilized to enhance the performance of SPCE platform [3,4]. However, the utility and inter-plasmonic coupling efficiency of streptavidin magnetic nanoparticles (SMNPs) has not been reported hitherto, to the best of our knowledge.

In this study, the silver (Ag) metal thin film (50 nm) was spin coated with GO monolayer, followed by plasmonic SMNPs. These samples were characterized using high-resolution transmission electron microscope. Further, rhodamine B, fluorophore overcoat was adopted and examined in reverse Kretschmann optical configuration [1,2]. This fluorophore nanolayer (30 nm) act as radiating dipoles whose emission is coupled with Ag thin film via the active SMNPs + GO spacer material used. A 532 nm continuous wave laser source was used and the emission was collected using a 550 nm long wave pass (LWP) filter and polarizer. The radiating dipoles were excited from the sample side and the emission was collected from the distal part of the prism, using OceanOptics spectrophotometer and also user-friendly mobile phone [3,4].

The blank samples without any NPs gave 10-fold emission enhancement of conventional fluorescence. Further, GO samples yielded 32-fold SPCE enhancements in line with our earlier report [4]. Furthermore, AgNPs displayed 65-fold SPCE enhancements on account of localized surface plasmon resonance (LSPR) in line with earlier reports [3-5]. While these AgNPs were taken as spacer nanomaterial on SPCE platform with a pre-coated monolayer of GO, the enhancements increased to 200-fold. These results are attributed to a combined effect of meta-enhanced fluorescence (MEF) and SPCE. On account of MEF the NPs in the spacer layer increases the emission and excitation rate as a result of which the fluorescence lifetime decreases, radiative decay rate increases with a concomitant increase in quantum yield of the fluorophores. Additionally, as the entire phenomena is occurring on metallic thin film in RK configuration, the emission is funneled through the curved part of the prism on account of its dispersive nature at defined angle. As a result of this, the collection efficiency of the emitted photons are dramatically increased.

Furthermore, the spacer GO nanolayer and SMNPs individually presented 30-fold and 100-fold emission enhancements in SPCE platform. However, when both GO and SMNPs were integrated on same spacer nanointerface intriguing results were obtained. The synergistic plasmonic coupling between the surface plasmon polaritons (SPPs) of Ag nanofilm, localized plasmon magnetic resonances of SMNPs and π-plasmons of GO resulted in augmented > 500-fold emission enhancement. The hybrid interfacial properties of plasmonic SMNPs and GO along with propagating surface plasmons of Ag thin nanofilm are primary reasons for realization of high SPCE enhancements (synergy of MEF and SPCE). The SPCE was highly directional with a narrow angular characteristic accompanied with highly p-polarised attribute of out-coupled emission. The emission was captured using cost-effective user-friendly mobile phone (in order to replace high cost detectors such as Ocean Optics) and the images were processed using ColorGrab app to obtain the luminosity values. Biocompatible SMNPs are generally used in enzyme-linked immunosorbent assay (ELISA) and related experimentation in the field of biosensing by tagging them with appropriate fluorophore moieties. In this study, an excellent agreement was observed between the SPCE enhancements and luminosity values, thereby portraying immense utility of the proposed platform in sensor devices, tailored for resource limited-settings.