Fluorescence-sensing technology is highly regarded in environmental monitoring, molecular detection, and optoelectronic devices because of its high sensitivity, specificity, and rapid response. However, many fluorescence-sensing materials suffer from low luminescence efficiency and poor photostability, limiting their practical applications. To improve luminescence performance, enhancing the fluorescence intensity and photostability is essential. Enhancing the fluorescence intensity can increase the detection sensitivity for trace targets, and enhancing the photostability can maintain consistent fluorescence intensity under prolonged excitation. Core-shell-structured metal nanoparticles have gained attention because of their easy preparation, controllable thickness, and unique localized surface plasmon resonance (LSPR) effects. These properties make them suitable for applications such as fluorescence enhancement, infrared absorption enhancement, and bioimaging. Although metal core-shell nanostructures can enhance fluorescence by more than 10-fold through the LSPR effects, their impact on photostability remains unclear. This study aims to investigate the LSPR mechanism for modulating the intersystem crossing of fluorescent molecules, thereby establishing a theoretical inverse relationship between the service life of fluorescent materials and their upper-state lifetime. We systematically investigated the effect of metal nanoparticle core-shell structures on the comprehensive optical properties of fluorescent materials using Au@SiO₂ nanoparticles as a plasmonic substrate and by adjusting the silica shell thickness. The results demonstrate that LSPR simultaneously modulates the upper-state lifetime, fluorescence signal enhancement, and photobleaching suppression. Photobleaching suppression and fluorescence enhancement improve as the silica shell thickness increases, peaking at a shell thickness of 9 nm. By precisely tuning the Au@SiO? nanostructure parameters, we achieved a 10-fold fluorescence enhancement. Importantly, this study reveals the LSPR-induced photostability regulation mechanism and the synergistic relationship between fluorescence enhancement and photostability improvement.

Research methods were started with a theoretical analysis based on Jablonski energy levels to establish the mathematical relationship between fluorescence lifetime and photostability under the LSPR modulation. Subsequently, Lumerical FDTD Solutions software was used to determine the LSPR resonance peaks of Au@SiO? nanoparticles with various shell thicknesses. For experimental validation, Au@SiO? nanoparticles with silica shell thicknesses of 3?12 nm were synthesized and assembled into the LSPR substrates via electrostatic adsorption onto quartz glass slides. A HORIBA Fluoromax-4 fluorescence spectrometer was then used to measure the fluorescence signals and upper-state lifetimes of two fluorescent materials (SABF? and 0F-2NHBoc) on different substrates. Finally, the photobleaching behavior of the fluorescent materials on various substrates was evaluated under 405 nm excitation light.

This study demonstrates that the silica shell thickness of Au@SiO₂ nanoparticles can be accurately adjusted to modulate the optical properties of fluorescent materials, thereby enabling the simultaneous enhancement of fluorescence intensity and photobleaching suppression. The experimental results reveal that an optimal silica shell thickness of 9 nm can achieve 6.4-fold and 9.7-fold fluorescence enhancement for SABF₂ and 0F-2NHBoc, respectively, while substantially suppressing photobleaching (Figs.5 and 6). Thereafter, an inverse relationship between the upper-state lifetimes of the fluorescent materials and their photostability was established. With the increase of silica shell thickness, the upper-state lifetime decreases, confirming the theoretical prediction of an inverse correlation between these parameters (Fig.7). Furthermore, this study highlights the broad applicability of the LSPR modulation. The results reveal that resonance wavelength matching is sufficient for modulating the optical properties of the fluorescence, regardless of the size and nature of the fluorescent molecules (Fig.4). This indicates that the LSPR-based modulation technique can be extensively applied to various fluorescent materials, providing a versatile approach for enhancing the intensity and stability of fluorescence emissions.

This study presents a systematic investigation of comprehensive optical property modulation of fluorescent materials using Au@SiO₂ core-shell nanostructure-based LSPR substrates. In addition to fluorescence enhancement, the impact of LSPR on photostability was examined. Theoretically, this study reveals an inverse mathematical relationship between the service life of the fluorescent material (indicating photostability) and its LSPR-modulated upper-state lifetime. Experimentally, the silica shell in the core-shell structure prevents direct contact between the fluorescent materials and metal nanoparticles. By adjusting the shell thickness, the upper-state lifetime, photobleaching suppression, and fluorescence enhancement are simultaneously regulated. At the optimal shell thickness, the upper-state lifetime is minimized and the photobleaching inhibition and fluorescence enhancement are maximized, consistent with the theoretical analysis. Notably, the LSPR-based modulation depends on resonance wavelength matching, which is independent of fluorescent molecular size or properties, highlighting its broad applicability. This enables the development of new high-stability fluorescent sensors.