A two-photon fluorescent probe records the intracellular pH through ‘OR’ logic operation via internal calibration. - Podder, Arup; Won, Miae; Kim, Soobin; Verwilst, Peter; Maiti, Mrinmoy; Yang, Zhigang; Qu, Junle; Bhuniya, Sankarprasad; Kim, Jong Seung (Sensors & Actuators B: Chemical, 2018)