Lanthanide-doped upconversion nanoparticles
have received growing
attention in the development of low-background, highly sensitive and
selective sensors. Here, we report a water probe based on ligand-free
NaYF4:Yb/Er nanoparticles, utilizing their intrinsically
nonlinear upconversion process. The water molecule sensing was realized
by monitoring the upconversion emission quenching, which is mainly
attributed to efficient energy transfer between upconversion nanoparticles
and water molecules as well as water-absorption-induced excitation
energy attenuation. The nonlinear upconversion process, together with
power function relationship between upconversion emission intensity
and excitation power density, offers a sensitive detection of water
content down to 0.008 vol % (80 ppm) in an organic solvent. As an
added benefit, we show that noncontact detection of water can be achieved
just by using water attenuation effect. Moreover, these upconversion
nanoparticle based recyclable probes should be particularly suitable
for real-time and long-term water monitoring, due to their superior
chemical and physical stability. These results could provide insights
into the design of upconversion nanoparticle based sensors.