Metal-enhanced fluorescence of molecular probes in plasmonic nanostructures offers highly sensitive chemical and biomedical analyses, but a comprehensive theory of the phenomenon is far from being complete. In this study, a systematic theoretical analysis is provided for overall luminescence enhancement/quenching for fluorophores near silver spherical nanoparticles. The approach accounts for local intensity enhancement, radiative and nonradiative rates modification, light polarization, molecule position, and its dipole moment orientation. Numerical modeling has been performed for fluorescein-based labels (e.g., Alexa Fluor 488) widely used in biomedical studies and development. The maximal enhancement exceeding 50 times is predicted for nanoparticle diameter 50 nm, the optimal excitation wavelength being 370 nm. For long-wave excitation, bigger particles are more efficient. The experiments with a fluorescein isothiocyanate conjugate of bovine serum albumin confirmed theoretical predictions. The results provide an extensive and promising estimate for simple and affordable silver-based nanostructures to be used in fluorescent plasmonic sensors.

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Based on analysis of light propagation in multiple cavity aperiodic structures, a filter with three narrow passbands with high transmission is designed. The design is implemented for use in express early-stage oral cancer screening by means of Raman spectroscopy. A combination of SiO 2 ∕Nb 2 O 5 is used in a vacuum deposition process to ensure high refractive index contrast and durability of materials. The approach enables the development of simple and affordable Raman testers for multiple medical, forensic, and environmental applications.

A simple chemical technique was devised for the fabrication of silver nanostructured substrates which can be used for plasmonic enhancement of labeled proteins fluorescence. For bovine serum albumin labeled with fluorescein isothiocyanate, the obtained enhancement factor ranges from three to seven, depending on metal-luminophore spacing and silver nanoparticle size. For excitation with linear polarized light, the enhancement factor increases noticeably for p-polarization and decreases for s-polarization. The experimental results were interpreted in terms of the theoretical model in which the enhancement factor depends on incident light polarization, luminophoremetal spacing and silver nanoparticle size. Proposed plasmonic substrates can be considered as an affordable replacement of standard ones in different types of fluorescent assays for the purpose of increasing sensitivity.

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