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Schematic for Decyl Methacrylate (hydrophobic) PEBBLE
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Another matrix commonly used is Decyl Methacrylate. This material is hydrophobic, which allows the use of hydrophobic dyes and ionophores. An indirect sensing scheme is often used with these PEBBLEs which involves an "optically silent" ionophore, which is very selective for its analyte, but not fluorescent, and a fluorescent pH indicator. The sensing is accomplished by an ion exchange, where the analyte (say a sodium ion, which has a +1 charge) comes in and binds to the ionophore. But, to keep charge neutrality, a hydrogen ion must leave, which changes the pH inside the PEBBLE (and thus changes the fluorescence of the pH dye.
To keep these PEBBLEs from sticking to each other, polyethylene glycol (PEG) is used as a steric stabilizer in the formulation.These SEM images show the dramatic difference between using PEG and not!
Decyl Methacrylate formulations With PEG (left) and Without PEG (right - large chunks ~ 50 microns in size).
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