Label-free in situ pH monitoring in a single living cell using an optical nanoprobe

Intracellular pH plays critical roles in cell and tissue functions during processes such as metabolism, proliferation, apoptosis, ion transportation, endocytosis and muscle contraction. It is thus an important biomarker that can readily be used to monitor the physiological status of a cell. Thus, disrupted intracellular pH may serve as an early indicator of cell dysfunction and deterioration. Various methods have been developed to detect cellular pH, such as pH-sensitive labelling reagents with fluorescent or Raman signals. However, excessive cellular uptake of these reagents will not only disrupt cell viability but also compromise effective long-term monitoring. Here, we present a novel fibre-optic fluorescent nanoprobe with a high spatial resolution for label-free, subcellular pH sensing. The probe has a fast response time (~20 s) with minimum invasiveness and excellent pH resolution (0.02 pH units) within a biologically relevant pH environment ranging from 6.17 to 8.11. Its applicability was demonstrated on cultured A549 lung cancer cells, and its efficacy was further testified in two typical cytotoxic cases using carbonylcyanide 3-chlorophenyl hydrazine, titanium dioxide and nanoparticles. The probe can readily detect the pH variations among cells under toxin/nanoparticles administration, enabling direct monitoring of the early onset of physiological or pathological events with high spatiotemporal resolution. This platform has excellent promise as a minimum invasive diagnostic tool for pH-related cellular mechanism studies, such as inflammation, cytotoxicity, drug resistance, carcinogenesis and stem cell differentiation.