In Situ, Real-Time Imaging of pH Gradients and Solution Flows through Silica Garden Membranes during Tube Growth.

Silica gardens are self-organized structures formed by the reaction between high-alkaline silicate and acidic metal solutions. This reaction generates complex tubular structures driven by osmosis and buoyancy, which are decorated by the precipitation of a diaphragm-like membrane featuring textural and chemical gradients. It is understood that the initial gradient of pH values and its evolution over time and space are the key parameters controlling textural and chemical patterns and morphogenesis. We have developed a technique that enables simultaneous visualization of membrane formation, fluid and particle flow, and pH evolution at microscopic resolution in real-time. This is achieved by exploiting the unusual behavior of pH probes in the saturated metal solution, which we have investigated in detail and utilized to perform ratiometric pH mapping in situ and in real-time, either at low magnification with a color camera or, at high magnification, with a confocal microscope. Using the same probes and setup, confocal fluorescence microscopy allows for mapping pH in the microtubes with higher resolution through ratiometric analysis.