Wavelength-Independent Pumping Mechanism and Spectroscopic Property of Upconversion Nanoparticles for Laser-Flexible Microscopic Bioimaging

Upconversion nanoparticles (UCNPs) doped with rare earth sensitizers and activators show unique optical properties and promising prospects for emerging applications. Typically, the excitation laser wavelength for UCNPs is selected in accordance with different application demands. However, altering the excitation wavelengths usually influences the upconversion luminescence (UCL) mechanism, necessitating the modification of the lanthanide doping strategy in nanoparticles. By far, a strategy with an excitation wavelength independent of the UCL mechanism is still an urgent need for UCNP applications. Here, we report a wavelength-independent pumping scheme with a flexible excitation wavelength range of 700–900 nm. Specifically, the flexible excitation of Nd3+ was applied to sensitize various activator systems such as Tm3+, Er3+, and Ho3+, all of which can be excited by multiple near-infrared (NIR) wavelengths, displaying consistent and stable spectral characteristics. Through microscopic imaging of dispersed nanoparticles and stained cells, we verified that multiple excitation wavelengths have the possibility of achieving high signal-to-noise ratio imaging. We believe that this approach will provide an in-depth understanding of Nd3+-sensitized upconversion systems and further advance the design for enhancing NIR-stimulated UCL, potentially spanning biomedical imaging, information technology, photocatalysis, and various other emerging fields.