In situ fluorimetric detection of micrometer-scale pH gradients at the solid/liquid interface

In this study, we describe the development of a sensing system for the real-time analysis of the pH and pH gradients at the solid/liquid interface. A pH-sensitive fluorescent platform has been developed by ‘clicking’ an alkyne-modified N-methylpiperazine naphthalimide onto an azide monolayer on glass. The surface immobilisation of the molecular probe was assessed by X-ray photoelectron spectroscopy, while sensitivity and reversibility were demonstrated by fluorescence microscopy and contact angle goniometry. The pH titration of the platform allowed for the determination of the pK_a value of the pH-sensitive probe at the solid/liquid interface. The platform was employed for the real-time interfacial analysis of micron-scale pH gradients at the solid/liquid interface induced by the electrolysis of water as exerted by an interdigitated microelectrode array. The calibration by the platform titration allowed the visualisation and quantification of the pH gradient with a dynamic range of about three pH units and a sub-second temporal resolution.