J. Eur. Opt. Society-Rapid Publ. 22, 28( 2026) 285
coupling bandwidth and field redistribution, enhancing fluorophore excitation and emission over a broad spectrum [ 14 – 16 ]. Also, broadband responses can result from constructive interference of multiple multipolar moments in disordered assemblies, broadening the spectral response by suppressing narrow reflectivity features [ 17 ]. Here, we propose lithography-free metasurfaces as a scalable, costeffective alternative to traditional nanofabrication for fluorescence biosensing. By using random metal nanoparticle aggregates, we achieved a 423-fold fluorescence enhancement while avoiding the high costs of precision-engineered structures [ 18 ]. Large-scale plasmonic metasurfaces can be fabricated also via laser-induced periodic surface structures or chemical synthesis. Although these devices offer efficient manufacturing of ordered arrays, it prioritizes polarization control over broadband fluorescence enhancement [ 19, 20 ]. Similarly, high-performance chemical systems like gold nanotubes face large-area integration challenges [ 21, 22 ]. Random aggregates overcome these issues through superior substrate compatibility and simple technological processes.
For fluorescence-based sensors Crystal Violet( CV), Nile Red( NR), and Rose Bengal( RB) fluorophores have been found to be highly useful, all exhibiting low quantum yields – 0.7, 0.11, 0.019, respectively [ 23 – 25 ]. Studies on these fluorophores investigated their interactions with nanostructures to understand the mechanisms and limitations of enhancement platforms. CV, an achiral weak emitter, has found extensive use in MEF intensification studies, particularly for the detection of single molecules [ 21 ] or in structures with localized intense fields [ 26 ]. NR was utilised in the study of enhanced emission on bimetallic plasmonic platforms( silver nanoparticles and gold nanolayers) derived from sol-gel [ 27 ]. RB aggregates have been extensively studied in the context of interaction with metal nanoparticles – AgNPs and GNPs [ 28 – 30 ]. Most high-performance MEF platforms reported in the literature rely on nanoparticle colloids, spacer-engineered films, or lithographically defined arrays to control distance and resonance conditions. While effective, these methods present fabrication complexity, limited throughput, and constrained active area, which complicate optimization and translation to large-area coatings or device-scale substrates. In contrast, thermally annealed Ag thin films demonstrate that simple post- processing can tune plasmonic response via morphology changes( particle reshaping and spacing), and Al plasmonics offers UV-VIS response with strong sensitivity to oxide and morphology [ 18, 31 ].
Previously, we demonstrated that influence of plasmonic metasurfaces on fluorescence intensity enhancement for rhodamine R6G, a hight quantum yield fluorophore, on silicon or glass substrate [ 18 ]. This work presents a simple plasmonic metasurface approach for intensifying the fluorescence emission of low-quantum-yield dyes. This method uses thin-film-derived nanostructures that do not require a specific configuration of the meta-atom geometry or lithography. These large area platforms can also provide scalable, morphology-driven, plasmon-enhanced fluorescence. We explore thermally annealed Ag nanostructured films( 15 – 25 nm) which develop from isolated islands to interconnected networks, and the ultrathin Al nanostructured films
( 2 – 6 nm), which exhibit intrinsic nanoscale dielectric separation, owing to native oxide. Native oxide provides nanoscale separation that minimise fluorescence quenching while enabling coupling to localized plasmonic fields. We used Crystal Violet, Nile Red, and Rose Bengal as low-emissivity and environmentally sensitive fluorophores to link fluorescence enhancement to morphological changes, which were confirmed by SEM and particular excitation settings for each dye.
2 Material and methods
In this investigation, silicon was utilized as the substrate, while the metals under analysis were aluminium( Al) and SILVER( Ag). The metasurfaces were manufactured on a large area of 1.5 cm 2 by electron beam evaporation using a TEMESCAL FC-2000( Temescal, Livermore, CA, USA) for Al and Elettrorava system( Elettrorava S. r. l., Torino, Italy) for Ag. This method was used to deposit either discontinuous layers( extremely thin – e. g. 2 nm, 4nmand6nm – thick mass equivalent – for Al) or thin continuous layers( 15 nm, 20 nm, and 25 nm for Ag) subjected to specific annealing conditions to achieve percolation of nanoaggregates. Table 1 show the process parameters employed for the fabrication of the proposed metasurfaces.
Stock solutions of Crystal Violet( CV), Nile Red( NR), and Rose Bengal( RB) fluorophores( from Carl ROTH) were prepared in ethanol at a concentration of 4 mM. Thin films were fabricated via drop-casting 50 lL aliquots of each dye solution onto the nanostructured surface containing metallic meta-atoms. The morphological characterisation of the samples obtained was performed using scanning electron microscopy( SEM) with a Field Emission Scanning Electron Microscope( FEG-SEM) – Nova NanoSEM 630( FEI Company, USA). Also, the meta-atoms size distributions were determined from high-resolution SEM images by measuring approximately 200 individual nanoaggregates for each sample. The resulting size histograms were fitted using a Gaussian function to extract the mean particle size, standard deviation( SD), and full width at half maximum( FWHM).
Steady-state fluorescence emission and optical absorption measurements were performed using an FLS920 fluorescence spectrometer( Edinburgh Instruments Ltd., UK) featuring a 450 W xenon arc lamp as the excitation source. Excitation wavelengths were selected according to the absorption maxima of each fluorophore: 550 nm – CV, 540 nm – NR, and 500 nm – RB. The obtained emission spectra revealed bands centred at wavelengths characteristic of thin-film fluorescence: 636 nm – CV, 635 nm – NR, and 571 nm RB.
3 Results and discussions
SEM micrographs of the 2 nm, 4 nm, and 6 nm( equivalent mass) Al thin layers( Fig. 1) reveal that ultrathin