Exploring Microenvironment Acidity Inside the Solvent-Filled Pores of Mesoporous Silica Thin Films via Single Molecule Spectroscopy

Single-molecule (SM) spectroscopy was used to study the local acidity inside solution-filled silica mesopores. The dual emission, pH-sensitive dye C-SNARF-1 was used as the fluorescent probe. Mesoporous materials were prepared as thin films supported on glass substrates. A microfluidic device was used to seal the upper surface of the films and to allow for their exposure to flowing aqueous solutions of the dye under different pH and ionic strength conditions. Single-molecule data were collected by two-color wide-field fluorescence imaging in the pseudototal internal reflection fluorescence mode. Pairs of fluorescence images were acquired simultaneously in bands centered around 580 and 640 nm, each having a 40 nm bandwidth. The ratio of C-SNARF-1 emission in these two bands (I580/I640) is highly sensitive to pH in bulk solution. The SM data show that C-SNARF-1 remains sensitive to pH in mesoporous materials, but that the emission ratios obtained are often larger than in bulk solution. Results obtained as a...