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Conventional upconversion thermometry strategy, based on the thermally coupled levels (TCL) of lanthanide ions, confronts a dilemma in simultaneously achieving high absolute/relative temperature (T) sensitivities (Sa and Sr) and good signal discriminability. Herein, a novel thermometry strategy by utilizing the synergistic effect of dual-wavelength lasers to manipulate electron thermal distribution is proposed to go beyond the limitation of the TCL-based strategy. In the case of NaGdF4: 20%Yb3+, 2%Er3+ nanoparticles upon 980 & 1530 nm dual-excitation, it is found that more efficient utilization of excited photons results in a higher electron concentration in ladder-like energy levels of Er3+, owing to the diverse pumping routes, which increases transition rate of the T-dependent phonon-assisted cross-relaxation process, and in turn establishes a thermally-sensitive electronic connection between Er3+: 2H11/2,4S3/2 and Er3+: 4F9/2. Remarkably, not only a highest record value of Sa (0.0365 K−1) for the Er3+ doped host materials in the physiological temperature range (303–343 K) is achieved, but also a high Sr (1.29% K-1) and an excellent signal discriminability (Δλ = 112 nm) are obtained. Combining a much intensified upconversion (UC) signal and a good size/shape homogeneity in nanometer scale, the investigated NaGdF4: 20%Yb3+, 2%Er3+ upon dual-wavelength excitation is potentially applicable in the intracellular thermal sensing and imaging. This work exploits an effective way to develop high-performance T-sensors, and the proposed thermometry strategy can be extended to surges of the other lanthanide ions doped systems pumped by multiple-wavelength lasers.

Selenium has been successfully doped into graphite phase carbon nitride (g-C 3 N 4 ) by a simple thermal condensation method through calcinating the mixture of dicyandiamide, cyanuric acid and selenium dioxide. The morphology and composition of thus-prepared selenium doped graphite phase carbon nitride (Se-g-C 3 N 4 ) nanosheets were characterized by transmission electron microscopy (TEM), high resolution TEM, X-ray diffraction (XRD), fourier transform infrared (FT-IR) spectra and X-ray photoelectron spectroscopy (XPS). Characterization results revealed that the resultant products have typical ultrathin lamellar structure with the uniform lateral diameter of 20 nm. The amount of Se dopant is ca. 2.1 at% based on the XPS result. For the first time, ultrathin Se-g-C 3 N 4 nanosheets have been demonstrated to possess an intrinsic peroxidase activity by following the Michaelis–Menten kinetics and even have higher affinity to peroxidase substrates TMB and H 2 O 2 in comparison to that of HRP. Se-g-C 3 N 4 nanosheets is conducive to mediating electron transfer and thus to enhancing the catalytic oxidation of the substrate TMB in the presence of H 2 O 2 to produce a blue color change in aqueous solution. More importantly, a sensitive and selective method for xanthine detection was developed using xanthine oxidase and the as-prepared Se-g-C 3 N 4 nanosheets. On the basis of the high catalytic activity of Se-g-C 3 N 4 nanosheets, a rapid, sensitive, and convenient approach was developed for colorimetric detection of xanthine with detection limit 1.6 × 10 −8 mol L −1 . Taking the advantages of this, the novel Se-g-C 3 N 4 nanosheets represent a promising candidate as an enzyme mimic and may find a wide range of new applications in diagnostics and biotechnology fields.