We registered surface enhanced Raman scattering (SERS) spectra of the human lactoferrin molecules adsorbed on a silvered porous silicon (por-Si) from 10⁻⁶–10⁻¹⁸ M solutions. It was found that the por-Si template causes a negative surface potential of silver particles and their chemical resistivity to oxidation. These properties provided to attract positively charged lactoferrin molecules and prevent their interaction with metallic particles upon 473 nm laser excitation. The SERS spectra of lactoferrin adsorbed from 10⁻⁶ M solution were rather weak but a decrease of the concentration to 10⁻¹⁰ M led to an enormous growth of the SERS signal. This effect took place as oligomers of lactoferrin were broken down to monomeric units while its concentration was reduced. Oligomers are too large for a uniform overlap with electromagnetic field from silver particles. They cannot provide an intensive SERS signal from the top part of the molecules in contrast to monomers that can be completely covered by the electromagnetic field. The SERS spectra of lactoferrin at the 10⁻¹⁴ and 10⁻¹⁶ M concentrations were less intensive and started to change due to increasing contribution from the laser burned molecules. To prevent overheating the analyte molecules on the silvered por-Si were protected with graphene, which allowed the detection of lactoferrin adsorbed from the 10⁻¹⁸ M solution.

Plasmonic surface lattice resonance (SLR) is a phenomenon in which individual localized surface plasmon resonances (LSPRs) excited in periodically-arranged plasmonic nanoparticles couple through the interaction with the propagating diffracted incident light. The SLR optical absorption peak is by at least one order of magnitude more intense than the LSPR one, making SLR superior for applications in which LSPR is commonly used. Recently, we have developed a route for the fabrication of spherical virus-like particles (VLPs) with plasmonic Au cores and protein coronas, where the LSPR in the cores amplifies vibrational Raman signals originating from protein-antibody interactions [ACS Synth. Biol. 12 (2023) 2320]. The particles are promising for biosensing, however, the intensity of the signals recorded in solution is not strong enough to detect antibodies at very low concentrations. Here, we show that by ordering the VLPs in periodic nanoarrays exhibiting SLR amplifies the signals by two orders of magnitude, revealing superior potential of VLP-SLR arrays in ultrasensitive biodetection.

We demonstrate a surface lattice resonance (SLR)-based plasmonic nanolaser that leverages bulk production of colloidal nanoparticles and assembly on templates with single particle resolution. SLRs emerge from the hybridization of the plasmonic and photonic modes when nanoparticles are arranged into periodic arrays and it can provide feedback for stimulated emission. It has been shown that perfect arrays are not a strict prerequisite for producing lasing. Here, we propose using high-quality colloids instead. Silver colloidal nanocubes feature excellent plasmonic properties due to their single-crystal nature and low facet roughness. We use capillarity-assisted nanoparticle assembly to produce substrates featuring SLR and comprising single nanocubes. Combined with a laser dye pyrromethene-597, the nanocube array lases at 574 nm with <1.2 nm linewidth, <100 μJ/cm² lasing threshold, and produces a beam with <1 mrad divergence, despite less-than-perfect arrangement. Such plasmonic nanolasers can be produced on a large-scale and integrated in point-of-care diagnostics, photonic integrated circuits, and optical communications applications.