Photonic crystal enhanced fluorescence using integrated high refractive index nanodiamond-plasmonic gold cryosoret nano-assemblies for ultrasensitive detection

In the domain of fluorescence spectroscopy, augmenting the emission intensity using nano-micro-engineering methods have presented promising means to enhance the performance of the associated biosensing modalities. However, the quenching induced by certain plasmonic nanoparticles (NPs) in the so called ‘zone of inactivity’ has remained a major challenge. Consequently, interfacing the fluorescent reporters such as molecular dyes and quantum dots with gold nanoparticles (AuNPs) results in dramatic fluorescence quenching that hampers the associated point-of-care diagnostic (POC) tools. In this work we overcome some of the long-standing challenges of conventional fluorescence-based detection techniques, by integrating high refractive index (HRI) dielectric fluorescent nanodiamonds (NDs) with metallo-dielectric nano-assemblies. The hybrid plasmonic gold and HRI dielectric ND assembly called AuND cryosoret (AuNDCS) is synthesized using adiabatic cooling (-196°C) driven nano-assembly method called cryosoret nano-engineering. Interfacing AuND cryosoret over the lossless photonic crystal (PC) substrate yielded more than 600-fold enhancement in the emission through interaction with the radiating guided mode resonance (GMR) of the PC. The experimental fluorescence measurements are corroborated with extensive simulations of a variety of nano-constructs studied, including monomers (0D), dimers (1D), trimers (2D), tetramers (3D) to visualize the magnitude and spatial distribution of the electric and magnetic field hotspots. The high fluorescence intensity is further considered for detecting the cancer-specific target biomarker, micro-RNA 375 with femtomolar-level detection limits using the hybrid metallo-dielectric AuND cryosoret as the molecular tag. The prism-free and objective-free detection instrument demonstrated here using compact and inexpensive system is suitable for on-site POC diagnostics.