Multifunctional Role of Poly-L-Lysine in Plasmonic Metal Nanostructures

Poly-L-lysine (PLL) layers deposited by sequential spin coating as a multifunctional platform for plasmonic metal nanostructures were investigated. Using an identical deposition strategy, a cross-material comparison was performed for Pt, Al, and Ag nanoparticles and Ag nanowires. Increasing PLL thickness red-shifts the localized surface plasmon resonance (LSPR) by modifying the dielectric environment, and depends on nanoparticle size distribution, plasma frequency, and background dielectric contribution. Deposition of a 25 nm PLL layer results in absorption shifts of 63, 57, 28, and 45 nm for Al, Pt, and Ag nanoparticles and Ag nanowires, respectively. Mie theory simulations confirmed the experiments and revealed that larger particles exhibit stronger long-wavelength shifts. PLL-coated Al nanostructures preserved their optical response and photoluminescence-enhancing functionality after immersion in deionized water for up to 340 min. As a dielectric spacer, PLL controls the interaction between ZnO and Al nanoparticles and increased the ultraviolet (UV) photoluminescence enhancement factor of ZnO at 377 nm up to 2.1-fold relative to pure ZnO. The developed Al/PLL platform produced pronounced quenching of intrinsic IgG fluorescence at 330 nm. These findings established PLL coatings as a versatile unified strategy for precise spectral tuning of plasmonic resonances, controlling emitter/metal interactions, and stabilizing Al nanostructures for UV photonic and bioanalytical sensing.