Ratiometrically pH-Insensitive Upconversion Nanoprobe: Toward Simultaneously Quantifying Organellar Calcium and Chloride and Understanding the Interaction of the Two Ions in Lysosome Function

Calcium and chloride levels are closely related to lysosome dysfunction. However, the simultaneous measurement of calcium (Ca2+) and chloride (Cl-) in acidic subcellular organelles, which is conducive to a deep understanding of lysosome-related biological events, remains a challenge. In this study, we developed a pH-insensitive, ratiometric NIR nanoprobe for the simultaneous detection of Ca2+ and Cl- in acidic lysosomes and determined the roles of the two ions in lysosome function. The upconversion nanoprobe with blue, green, and red emissions was modified with a Ca2+-sensitive dye (Rhod-5N) and Cl-responsive fluorophore (10,10′-bis[3-carboxypropyl]-9,9′-biacridinium dinitrate, BAC). As a result of a dual-luminescence resonance energy transfer between upconversion nanoparticles (UCNPs) and Rhod-5N/BAC, the blue and green upconversion luminescence (UCL) of UCNPs were quenched and the red UCL was used as the reference signal. The ratiometric upconversion nanoprobe possesses a specific ability for the concurrent recognition of Ca2+ and Cl- ions independent of the influence of the environmental pH. To locate the probe in the lysosome, dextran was further modified with upconversion nanoparticles. Then, the nanoprobe with a high spatial resolution was constructed for the simultaneous monitoring of Ca2+ and Cl- in acidic lysosomes. Moreover, it was found that the reduction of lysosomal Cl- affects the release of lysosomal Ca2+, which further blocks the activities of specific lysosomal enzymes. The ratiometric NIR nanoprobe has great potential for decoding and evaluating lysosomal diseases.