Sensing and imaging pH inside living cells are of paramount importance for better penetrating cellular functions and disease diagnostics. Herein, we engineered an original pH sensor by a simple one-step self-assembly of poly(ethylene glycol) (PEG)ylated phospholipid (DSPE-PEG) and a phenol red small molecule on the surface of upconversion nanoparticles (UCNPs) to form a phospholipid monolayer for sensing and imaging the change of intracellular pH. The sensor showed excellent reversibility and rapid response to the pH variations. Furthermore, this pH sensing system could measure spatial and temporal pH changes during endocytosis and interrogate the pH fluctuations inside cells under external stimuli. Our experimental results revealed that the pH sensor was able to map spatial and temporal pH fluctuations inside living cells, showing its potential application in diagnostics and pH-related study of cell biology.

In this paper, we report the obtention of red‐emitting Ca3(PO4)2:Eu3+ phosphor by the solid‐state reaction method. X‐ray diffraction and photoluminescence (PL) measurements were used to investigate the crystal phases, luminescent properties, and optimal Eu3+ doping concentration. To further improve the PL intensity, Na+ acting as a charge compensator was incorporated into the phosphor. The effects of Na+ concentration on the PL properties were studied. It was found that the charge‐compensation phosphor showed greatly enhanced red emission beyond the phosphor without charge compensation. Ca2.3Na0.4(PO4)2:0.3Eu3+ was determined to be the best composition. The charge‐compensation mechanism was analyzed in light of the principles on defect formation and reaction, and the changes of the site centrosymmetry for the activator. Effects of Na+ concentrations on the intensity of red emission. The inset showed the R/O ratios along with Na+ concentrations.

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