CFTR and cystic fibrosis: Using air–liquid interface(ALI) cultures to model the respiratory epithelium, we show that SERS-MSfacilitates the optical measurement of trans-epithelial pH gradientsbetween the airway surface liquid and the basolateral culture medium.SERS-MS also enabled the successful quantification of pH changes inthe airway surface liquid following stimulation of the Cystic Fibrosistransmembrane conductance regulator (CFTR, the apical ion channelthat is dysfunctional in Cystic Fibrosis airways).