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

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.