Abstract
Single-cell research is essential for understanding cell heterogeneity, cell differentiation, and carcinogenesis, among other important cellular processes. New techniques for intracellular pH monitoring are urgently needed to gain new insights into single-cell responses to external stimuli. In this study, fiber-optic reflection-based pH micro (μ)-probes (tip diameter: 500-3000 nm) were designed and fabricated using a novel hexagonal 1-in-6 fiber configuration. An organic-modified silicate (OrMoSils) sol-gel doped with a pH-sensitive dye, 2',7'-bis(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF), were coated onto the probe sensing tip for pH detection. These probes enabled neutral pH monitoring and quasi-real-time data acquisition (response time: 20 ± 5 s). The fluorescence signals of the newly developed probes were found to correlate linearly with pH (R(2) = 0.9869 when coupling laser power was at 8.2 mW) within a biologically relevant pH range (6.18-7.80). The pH resolution was 0.038 pH unit. The miniaturized probes were validated in single human lung cancer A549 cells to demonstrate applicability in single-cell experiments. In summary, novel pH μ-probes with excellent resolution and response times within a biologically relevant pH range were developed, and they can be used for measuring pH changes in single cells.