Boosting the NIR-I luminescence of lanthanide nanoparticles excited in NIR-II by plasmonic arrays

The exploitation of lanthanide-induced NIR luminescence holds significant potential for advancing biosensing and imaging technologies, because of its deeper imaging penetration depth and lower biosensing noise than visible light. However, the low quantum efficiency of light generation, along with the high cost and poor sensitivity of light detection for NIR-II photons, have largely constrained the application of NIR II luminescence for bio-imaging and sensing. To overcome these challenges, we have developed a new strategy for NIR bioimaging and sensing, by using Tm3+ doped NaYF4 upconversion nanoparticles (UCNPs) which emit NIR-I upconversion luminescence (UCL) at 808 nm under NIR-II excitation at 1208 nm. The 1208 excitation enables deeper imaging penetration due to negligible autofluorescence, which also reduces biosensing noise, resulting in an enhanced signal-tonoise ratio (S/N) from a minimal noise background. The NIR-I emission at 808 nm can enable deep penetration depth within the bio-tissue, meanwhile the highly sensitive detection of NIR I photons is much more achievable than that of NIR II photons. The intrinsically weak NIR UCL signals were further amplified by up to 210-folds using a pioneering plasmonic approach, achieved by coupling UCNPs with a periodic array of silver hole-caps nanoarrays (Ag-HCNA). Three-dimensional finite-difference timedomain (3D-FDTD) simulations and lifetime analyses revealed substantial electric field enhancements under excitation of 1208 nm and accelerated radiative decay at 808 nm, leading to both excitation and emission enhancement. The potential use of Ag-HCNAs conjugated with UCNPs in amplifying UCL signals for an achievable fluorescence immunoassay nanoplatform was also verified. This was achieved by immobilizing streptavidin-functionalized UCNPs (SA-UNCPs) onto biotinylated Bovine Serum Albumin (b-BSA)-pre-grafted substrates, resulting in up to 113-fold increase in UCL intensity. This innovative plasmonic-enhanced UCL platform offers significant advantages, such as the costeffective detection of NIR 808 nm emission by silicon-based detectors, improved signal-to-noise ratio due to increased light penetration depth, and reduced autofluorescence through NIR-II excitation.