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Table 2. Fluorescence enhancement factors for CV, NR, RB on nanostructured Al films( 2, 4, 6 nm) and Ag films( 15, 20, 25 nm).
Fluorophore
Aluminium( mass equivalent thickness)
Enhancement factor( EF) on specific layer Silver
|
2 nm |
4 nm |
6 nm |
15 nm |
20 nm |
25 nm |
Crystal Violet |
1 |
2 |
3 |
8.6 |
10 |
19 |
Nile Red |
3 |
3.2 |
2.3 |
2.4 |
4 |
4.4 |
Rose Bengal |
1.2 |
3 |
7.7 |
2 |
4 |
6 |
In contrast, Ag films exhibit substantially stronger enhancement for all fluorophores, particularly at larger Ag thicknesses. CV shows the highest enhancement overall, reaching EF = 19 on the 25 nm Ag film, while RB and NR reach maxima of EF = 6 and EF = 4.4, respectively. These results indicate that Ag nanostructures support more efficient plasmon-mediated fluorescence enhancement than Al under the studied conditions, with the degree of enhancement strongly dependent on both the fluorophore and metal layer thickness. Furthermore, the highest EF are obtained when using the 25 nm Ag film. The high filling factor observed for this metasurface platform suggests the onset of particle percolation, which influences significantly the plasmonic coupling. Additionally, the spiderweb pattern that forms after annealing – consisting of a high number of sharp angles – significantly intensifies the localized electromagnetic field.
4 Conclusion
In this study we presented a simple plasmonic metasurface approach for enhancing fluorescence of low quantum yield dyes using thin film derived nanostructures. We performed thermal annealing on Ag films( 15 – 25 nm), and deposited mass equivalent ultrathin Al films( 2 – 6 nm) to develop large plasmonic metasurface platforms comprising of randomly configurate metallic nano-aggregates. The native oxide of studied metals provides the separation required to partially mitigate the extreme near-field quenching while enabling coupling to localized plasmonic fields from nanoscale features. We investigated the behaviour of the fluorescence intensity for Crystal Violet, Nile Red, and Rose Bengal in the presence of metal nanostructures. The EF are calculated relative to the respective emission maxima of each fluorophore. The values vary depending on the type of fluorophore – specifically, their quantum efficiency – rather than the layer thickness. For Al, promising results concerning fluorescence intensity enhancement were obtained using 6 nm mass equivalent layers, with the best EF for Rose Bengal. At low thicknesses( e. g., 6 nm), Al forms monodisperse, nearly periodic nanostructures with very low heights. This leads to a uniform distribution of localized field enhancements. Since the structures are uniformly spaced, a large portion of the fluorophores can interact with the plasmonic resonances, resulting in consistent enhancement.
For Ag the fluorescence measurements revealed substantial intensity increases, with EF dependent on Ag thickness and fluorophore, the best result was obtained for Crystal Violet for 25 nm with an EF of 19. Silver forms large aggregates, evolving from hemispherical shapes to percolated spiderweb at a thickness of 25 nm. The enhancement occurs via two mechanisms: i) height matching ensures most molecules are within the plasmonic interaction zone of the 25 nm structures, and ii) natural spacing, where the native oxide layer prevents quenching by maintaining distance between the dyes and metal surface. Although aluminium is suitable for thin layers, the taller percolated structures offered by annealed silver thicker films exhibit significant enhancement of fluorescence emission.
The proposed low-cost, lithography-free platforms offer a versatile approach for enhancing the broadband spectral emission of a variety of fluorophores without specifically tailoring the geometry for each chromophore, providing a scalable, morphology-driven, plasmon-enhanced fluorescence. Furthermore, the fabrication method used in this study allows for nanostructuring large wafers of various substrate – silicon, glass, or ceramics, for a multitude of hands-on applications requiring a rapid response( e. g. refractive index-based sensors, fluorescence sensors, photocatalysis, etc.).
Acknowledgments
The authors acknowledge Gabriel Craciun for electron beam deposition using Elettrorava system. Funding
This research was funded by the Core Program of IMT-Bucharest within the National Research Development and Innovation Plan 2022-2026, carried out with the support of Romanian Ministry of Research, Innovation and Digitization, project no. 2307.
Conflicts of interest The authors have nothing to disclose. Data availability statement This article has no associated data generated and / or analyzed. Author contribution statement
Conceptualization, R. T. and V. A.; Methodology, D. C.; Validation, V. A, C. K. and R. T; Formal Analysis, V. A. and R. T.;