  Pyranine (HPTS) is a remarkably interesting pH sensitive dye that has been
used for plenty of applications. Its high quantum yield and extremely sensitive
ratiometric fluorescence against pH change makes it a very favorable for pH
sensing applications and development of pH nano/microsensors. However, its
strong negative charge and lack of easily modifiable functional groups makes it
difficult to be used with charged substrates such as silica. This study reports
a noncovalent HPTS immobilization methodology on silica microparticles that
considers the retention of pH sensitivity as well as long term stability of the
pH microsensors. The study emphasizes on importance of surface charge for
governing the sensitivity of the immobilized HPTS dye molecules on silica
microparticles. Importance of methodology of immobilization that preserves the
sensitivity as well as stability of the microsensors is also assessed.
