The organelle-specific pH is crucial for cell homeostasis. Aberrant pH of lysosomes has been manifested in myriad diseases. To probe lysosome responses to cell stress, we herein report the detection of lysosomal pH changes with a dual colored probe (CM-ROX), featuring a coumarin domain with "always-on" blue fluorescence and a rhodamine-lactam domain activatable to lysosomal acidity to give red fluorescence. With sensitive ratiometric signals upon subtle pH changes, CM-ROX enables discernment of lysosomal pH changes in cells undergoing autophagy, cell death, and viral infection.

A visual and fluorogenic detection method for a nerve agent simulant was developed based on a Lossen rearrangement of rhodamine-hydroxamate, in the presence of diethyl chlorophosphate, under alkaline conditions.

A hetero-organelle partition and lysosome activable sensor was developed for fluorogenic detection of mitochondrial depolarization in autophagy and cell death.

Determination of nitrite levels is of substantial interest in many applications from monitoring drinking water quality to clinical diagnosis, etc. N-(Rhodamine B)-lactam-o-phenylenediamine (RB-PDA) was identified from a panel of rhodamine-lactams for fluorogenic and visual detection of nitrite in aqueous media via analyte triggered formation of highly fluorescent and deep colored species.NSFC [21072162]; Natural Science Foundation of Fujian Province of China [2011J06004]; Fundamental Research Funds for the Central Universities [2011121020]; NEFTBS [J1030415

A chromogenic and fluorogenic detection of aldehydes was achieved via analyte triggered opening of the deoxylactam of N-(rhodamine B)-deoxylactam-ethylenediamine (dRB-EDA). The utility of the sensor was demonstrated by fluorescent labeling of aldehyde-displaying sialoproteins on cell surfaces.

Selective and continuous tracking of dynamic organelles is crucial for modern biology. We herein report a ship-in-a-bottle strategy for tagging lysosomes by a strain-promoted azide-alkyne cycloaddition to couple a pH sensor (RC) with mannose-6-carboxylate (M6C) actively transported into lysosomes through cell sorting. In contrast to classical acidotropic sensors, which are prone to dissipate from lysosomes, M6C-RC formed in situ is stably trapped in lysosomes without resort to lysosomal acidity and exhibits "always-on" blue fluorescence to pinpoint lysosomes and red-to-blue fluorescence ratios indicative of the lysosomal pH value. These advantages enable tracking of stressed lysosomes, and necrosis to be differentiated from apoptosis on the basis of lysosomal pH changes. The cell-sorting-mediated bioorthogonal tagging strategy offers a new route to track stressed organelles with disrupted physiological organelle-probe affinity.

N-(Rhodamine B)-benzimidazole (RB-IM), featuring an intramolecular spiro-benzimidazole, was developed for selective sensing of protons via opening of the spiro-ring to give fluorescent and colored species. The utility of RB-IM was demonstrated by imaging of lysosomes in live cells and staining of the intestine of zebrafish.Scheme 1 pH sensing by RB-IM and rhodamine-lactams.

A fluorescence-quenched substrate of sialidase enables fluorescence-on live cell imaging of sialidases, revealing up-regulation of lysosome-associated sialidase in cell senescence.