Mobile Phone-Based Picomolar Detection of Tannic Acid on Nd₂O₃ Nanorod–Metal Thin-Film Interfaces

Abstract

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.

Plasmon-Coupled Directional Emission from Soluplus-Mediated AgAu Nanoparticles for Attomolar Sensing Using a Smartphone

“…The emitted photons from the fluorophores on a metal thin film (coupled to prism) are funneled through defined angles based on satisfying the surface plasmon polaritons (SPP) dispersion eq 1. [3][4][5]11 This angle is the angle of minimum reflectivity in complementary surface plasmon resonance (SPR) experiments, conducted without fluorophores. 2,4,5 In simple terms, in SPCE, the SPP dispersion relation given by eq 1 needs to be satisfied so as to allow coupling of emitted photons (allowed modes) from fluorophores to the metal thin film.…”

“…2,4,5 In simple terms, in SPCE, the SPP dispersion relation given by eq 1 needs to be satisfied so as to allow coupling of emitted photons (allowed modes) from fluorophores to the metal thin film. [3][4][5]11…”

“…All the optical experiments were performed in line with our earlier works. 7,9,11 The SPCE substrate attached to the BK7 prism (using index matching fluid; refractive index, 1.47) is mounted on a 360°r otating stage before the experimentation. The reverse Kretschmann (RK) configuration was utilized for all the experiments, where the 532 nm continuous wave laser source (35 mW) excites the radiating dipoles (coated on Ag thin nanofilm) directly.…”

“…Freshly prepared 1 and 2 wt % PVA spin coated at 3000 rpm for 1 min results in 30 and 60 nm thick coatings as reported earlier. 11,35 In the spacer nanointerface, the SPCE substrate (50 nm Ag) was first spin coated with 60 nm PVA doped with NPs of interest. This was followed by spin coating PVA doped with RhB to obtain an overcoat of 30 nm nanolayer containing radiating dipoles.…”

“…[7][8][9][10][11]14 Generally, the enhancement factor of SPCE substrate is 5−10-fold as compared to traditional luminescence. [7][8][9][10][11]15 Lakowicz and coworkers have demonstrated the significance of silver (Ag) NPs spacer layer engineering on SPCE platform to further augment the enhancement to 60-fold. 15 With this as a preliminary step for combining PEF and SPCE, diverse nanoarchitectures have been explored to further augment the SPCE enhancements.…”