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Lanthanide ions have remarkable photophysical characteristics due to their internal enegy levels. In particular, they have a large anti stokes efficiency through upconversion (UC). In this process, multiple low energy photons (typically NIR) are sequentially absorbed and such energy is then reemited from long lived states (μs-ms) as higher energy radiation (visible or UV). NIR excitation light not only allows for deeper light penetration and reduced photodamage effects, but also offers lower autofluorescence, reduced light scattering, and phototoxicity. Lanthanide-doped upconversion nanoparticles (UCNPs) are therefore a promising new generation agents for biosensing and bioimaging, but their photophysics is still under study.Rare earth trivalent ions embedded in an inorganic host (β-NaYF4: 20%Yb3+, 2%Er3+) were synthesized via thermal techniques [2]. Hexagonal phase UCNPs with sizes ranging from 15 to 800 nm were obtained and characterized by XRD and SEM techniques.The emission spectra of the samples were measured with a commercial spectrofluorometer modified to use a 980 nm pulsed excitation source coupled with a time resolved detection system. This new arrangement facilitated the simultaneous measurement of the stationary emission spectra as well as the spectrally resolved excited state lifetimes of the samples (See Figure 1). The advantage to observe both the stationary and dynamic behavior of the system in situ, offers the possibility to perform combined studies to measure the dependence of the intensities of the emission maxima and the excited state lifetimes on the excitation pump power in different dispersion media for different samples.In particular, the variation of the ratio of the intensities of the green (520-560 nm) to red (630-670 nm) emission bands together with their respective excited state lifetimes as a function of the incident power, provided valuable information regarding the non-linear relationship between absorbed and emitted light power (quantum yield). These results are essential to elucidate how the competition process between the linear decay rate and the upconverted rate at the intermediate states takes place to get a better insight about the dynamics of the intra f-f transitions of the lanthanides for different power regimes enabling goal-oriented UCNP design.[1] Chen, G., Qiu, H., Prasad, P. N., & Chen, X., Chem. Rev., 2014, 114(10), pp 5161.[2] X. Ye, J.E. Collins, Y. Kang, J. Chen, D.T.N. Chen, A. Yodh and Christopher B. PNAS, 2010, 107(52), 22430.