A method to measure pH inside mesoporous particles using protein-bound SNARF1 fluorescent probe

We use fluorescence spectroscopy to measure the pH sensed by proteins which are immobilized in mesoporous silica particles by covalently attaching the pH-probe SNARF1 to the proteins. In contrast to previous attempts where pH-probes were attached to the pore surface, the present approach allows the pH to be measured without altering the properties of the silica material. When the particles are suspended in aqueous solutions of various pH the emission spectra of the labeled proteins indicate an environment inside the pores which is closer to neutral compared to the bulk solution. In a 1-butanol/water mixture (92.5/7.5%) the emission spectra of the immobilized proteins report a pH-behavior typical of the aqueous suspensions, in contrast to the blue-shifted emission spectrum of the protein in the outside organic solvent. This observation shows that the immobilized proteins experience an aqueous pore environment even though the surrounding solution is poor in water. The spectra of SNARF1-labeled bovine serum albumin and feruloyl esterase generated similar results suggesting that the method can be applied to different types of proteins. Taken together the results show that spectroscopic probes carried by immobilized macromolecules can be used to characterize the environment inside mesoporous particles without perturbing the properties of the material.

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► Measurement of apparent pH inside mesoporous silica using fluorescent probes.
► SNARF1 labeled proteins immobilized in SBA-15 without surface modifications.
► The probe reports differences in pH environment inside and outside the pores.