We have studied the mechanisms of water-based quenching of the upconversion photoluminescence of upconverting nanophosphors (UCNPs) via luminescence decay measurements for a better understanding of the non-radiative deactivation pathways responsible for the relatively low upconversion luminescence efficiency in aqueous solutions. This included both upconversion luminescence measurements and the direct excitation of emissive energy states of Er(3+) and Yb(3+) dopants in NaYF4:Yb(3+),Er(3+) UCNPs by measuring the decays at 550 and 655 nm upon 380 nm excitation and at 980 nm upon 930 nm excitation, respectively. The luminescence intensities and decays were measured from both bare and silanized NaYF4:Yb(3+),Er(3+) and NaYF4:Yb(3+),Tm(3+) UCNPs in H2O and D2O. The measurements revealed up to 99.9% quenching of the upconversion photoluminescence intensity of both Er(3+) and Tm(3+) doped bare nanophosphors by water. Instead of the multiphonon relaxation of excited energy levels of the activators, the main mechanism of quenching was found to be the multiphonon deactivation of the Yb(3+) sensitizer ion caused by OH-vibrations on the surface of the nanophosphor. Due to the nonlinear nature of upconversion, the quenching of Yb(3+) has a higher order effect on the upconversion emission intensity with the efficient Yb-Yb energy migration in the ∼35 nm nanocrystals making the whole nanophosphor volume susceptible to surface quenching effects. The study underlines the need of efficient surface passivation for the use of UCNPs as labels in bioanalytical applications performed in aqueous solutions.

Photophysical studies of nonlinear lanthanide-doped photon upconverting nanoparticles (UCNPs) increasingly used in biophotonics and photovoltaics require absolute measurements of the excitation power density (P)-dependent upconversion luminescence (UCL) and luminescence quantum yields (Φ) for quantifying the material performance, UCL deactivation pathways, and possible enhancement factors. We present here the P-dependence of the UCL spectra, Φ, and slope factors of the different emission bands of representative 25 nm-sized oleate-capped β-NaYF:17% Yb, 3% Er UCNPs dispersed in toluene and as powder as well as Φ of 3 μm-sized upconversion particles (UCμP), all measured with a newly designed integrating sphere setup, enabling controlled variation of P over four orders of magnitude. This includes quantifying the influence of the beam shape on the measured Φ and comparison of experimental Φ with simulations utilizing the balancing power density model of the Andersson-Engels group and the simulated Φ of UCμP from the Berry group, underpinned by closely matching decay kinetics of our UC material. We obtained a maximum Φ of 10.5% for UCμP and a Φ of 0.6% and 2.1% for solid and dispersed UCNPs, respectively. Our results suggest an overestimation of the contribution of the purple and an underestimation of that of the red emission of β-NaYF:Yb,Er: microparticles by the simulations of the Berry group. Moreover, our measurements can be used as a guideline to the absolute determination of UCL and Φ.

Many studies have shown that the oral mucosa and salivary glands are sensitive to estrogen action. However, the expression of estrogen receptors (ERs) within these tissues is an area of controversy. ERs exist as two subtypes (ER and ER ), and we hypothesized that the incongruity between ER expression and estrogen sensitivity may result from differential expression of ER subtypes in oral tissues. To test this hypothesis, we analyzed oral mucosal and salivary gland samples for ER and ER protein expression by immunohistochemistry from a cross-section of patients attending hospital for surgical problems of the head and neck. ER was not detected in oral buccal and gingival epithelium or in salivary glands. In contrast, ER was widely expressed at high levels in all oral tissues studied. Within these tissues, ER was observed primarily in keratinocytes and salivary gland acinar and ductal cells. Our results demonstrating the expression of only the ER subtype within oral tissues may explain the contradictory results from previous studies investigating ER expression in these tissues. Importantly, these results suggest that estrogens may act via ER in oral tissues and explain the effect of hormonal changes on the oral mucosa as well as on saliva secretion and composition.

The disintegration of hexagonal NaYF4:Yb3+,Er3+ upconverting nanoparticles (UCNP) was studied by incubating various nanoparticle concentrations in aqueous suspensions over time while monitoring the upconversion emission intensity and measuring the dissolved particle-constituting ion concentrations. The results revealed that the ions dissolved into water resulting apparently in anisotropic structural disintegration of the UCNPs as observed with transmission electron microscopy. The UCNP disintegration caused partial loss of active ions Yb3+ and Er3+ from the host matrix and therefore decrease in the upconversion luminescence intensity. The decrease, however, was strongly dependent on the UCNP concentration, and dramatic drop in the intensity was observed especially at diluted nanoparticle suspensions, where the nanoparticles disintegrated almost completely until the solubility equilibrium was achieved. At the concentrated suspensions the equilibrium was achieved already with minimal disintegration, and the change in the luminescence intensity was negligible. Further, due to the high impact of fluoride ions on the solubility equilibrium the disintegration of the UCNPs could be prevented by adding fluoride to the suspension. The reported disintegration of NaYF4:Yb3+,Er3+ nanoparticles in diluted aqueous suspensions should be taken into consideration when the UCNPs are used at low concentrations in analytical applications and in guiding the design of improved shell-stabilized UCNPs.