Apoferritin-based tunable nano-indicator for intracellular pH sensing: Regulating response performances and minimizing effects of system fluctuations

Nano-indicator for intracellular pH sensing, with tunable response performances, has been engineered by flexibly using different dye pairs and controlling the dye feed ratio in bio-derived apoferritin, and effects of system fluctuations can be minimized by using ratiometric measurement. [Display omitted] •The nano-indicator can be easily and controllably fabricated without any derivatization.•The nano-indicator have good biocompatibility and precisely controlled particle size.•Response performances of the nano-indicator can be flexibly regulated.•Ratiometric signal can minimize the effect of system fluctuations.•pH fluctuations in cells treated with different stimuli can be estimated by the nano-indicator. A new class of apoferritin-based tunable nano-indicators, utilizing apoferritin (APO) as a template and the dye pair as a signal element, have been developed for intracellular pH sensing. Similar to cocktails making, the nano-indicator with a diameter of ∼10 nm can be flexibly and easily constructed just by mixing commercially available pH-sensitive and pH-insensitive dye pairs (without any derivatization) with APO in solution, based on the pH-dependent depolymerization/self-assembly properties of the bio-derived APO. By using different dye pairs (fluorescein/rhodamine B, fluorescein/RuBpy) for ratiometric design and controlling the dye feed ratio, the pH response performance of the APO-based nano-indicator including response linear range and sensitivity can be flexibly regulated. Compared with free dyes, it has lower cytotoxicity. Moreover, ratiometric signal of the nano-indicators can minimize the effect of system fluctuations, involving ambient temperature, probe local concentration and its solution environment. Furtherly, the nano-indicators have been successfully applied for intracellular pH measurement in the range of 5–7 and the detection of pH fluctuations in cells after different stimuli treatment.