Selective enhancement of red emission from upconversion nanoparticles via surface plasmon-coupled emission

Abstract

Upconversion nanoparticles (UCNPs) can convert low-energy light (e.g. near-infrared, NIR) to high-energy light (e.g. visible). UCNPs such as NaYF₄:Yb,Er will generate strong green emission at 540 nm and weak red emission at 650 nm. Considering the “optical window” of cells and tissues (600–900 nm) owing to the lack of efficient endogenous absorbers, red emission (600–700 nm) enhancement is especially important for deep tissue imaging. Therefore, it is of great significance to selectively enhance red emission of UCNPs. Placing UCNPs in proximity to gold nanorods (AuNRs) can enhance the emission of UCNPs. In this paper, we fabricate heteronanostructures composed of UCNPs and AuNRs and precisely control the distance with polyelectrolyte multilayers as spacers to enhance the red emission of UCNPs. Fluorescence spectra of UCNPs enhanced by AuNRs with different thicknesses of polyelectrolyte are measured. The results show that red emission intensity exhibits a strong dependence on the interparticle distance and spectral properties of these nanoparticles. At a optimized distance of 8 nm, the maximal fluorescence enhancement of 10.6-fold for red emission is achieved by tuning the localized surface plasmon resonance (LSPR) wavelength of AuNRs to match with the red emission. The enhancement of the red emission is further confirmed by a decreased decay time induced by surface plasmon-coupled emission (SPCE). The enhancement also contributes to the increased penetration depth from 7.5 mm to 12.5 mm. These results reveal that our strategy possesses the ability of selectively enhancing red emission of UCNPs and provides theoretical guidance on preparing versatile systems with enhanced red emission by the SPCE effect.