The perfect energy level overlap of 2 H 11/2 , 4 S 3/2 , and 4 F 9/2 in Er 3+ ions with those of 5 F 3 , 5 F 4 / 5 S 2 , and 5 F 5 in adjacently codoped Ho 3+ ions allows efficient interenergy transfer. Therefore, in addition to routine activators, Er 3+ or Ho 3+ can further act as sensitizers to transfer the upconverted energy to nearby Ho 3+ or Er 3+ , resulting in enhanced upconversion luminescence due to the emission overlap. Proper codoping of Er 3+ /Ho 3+ or Ho 3+ /Er 3+ obviously elevates the maximum doping concentration (thus producing additional upconverted photons) to a level higher than that causing luminescence quenching and significantly enhances upconversion emissions compared with those of singly Er 3+ or Ho 3+ -doped host materials. Indeed, the so-far strongest red upconversion emission under 1532 nm excitation was obtained in LiYF 4 :Er/Ho@LiYF 4 nanoparticles and Ho 3+ -sensitized Er 3+ upconversion emissions excited by 1150 nm laser was simultaneously discovered. With great enhancement compared with that of singly Ho 3+ doped counterparts, this work demonstrates the generality and rationality of our design strategy.

An ultraviolet-visible light (UV-Vis)-reversible but fluorescence-irreversible chemosensor was developed for the detection of copper. Coordination between the probe, 2-pyridylaldehyde fluorescein hydrazone (FHP), and Cu(2+) gave a reversible UV-Vis response, Storage of the probe-Cu complex resulted in hydrolytic cleavage of the N═C bond, which released the fluorophore (ring-opened fluorescein hydrazine) and gave irreversible fluorescence. Thus, FHP becomes a multifunctional chemosensor, and its reversibility can be controlled by the reaction time. Cu(2+) in living cells could be detected using FHP and general fluorescence methods.

A new thiol-containing colorimetric probe has been developed by using a chromene derivative, 7-nitro-2,3-dihydro-1H-cyclopenta[b]chromen-1-one (1). The molecule exhibited high selectivity and sensitivity for detecting thiol species as cysteine, homocysteine, and glutathione in aqueous solution through a rapid visual color change from colorless to yellow.

Novel two-photon fluorogenic probe could simultaneously monitor changes in the mitochondrial viscosity and H2O2 levels using two different channels.

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