Phenylboronic acids
(BAs) are important synthetic receptors
that
bind reversibly to cis-diols enabling their use in molecular sensing.
When conjugated to magnetic iron oxide nanoparticles, BAs have potential
for application in separations and enrichment. Realizing this will
require a new understanding of their inherent binding modes and measurement
of their binding capacity and their stability in/extractability from
complex environments. In this work, 3-aminophenylboronic acid was
functionalized to superparamagnetic iron oxide nanoparticles (MNPs,
core diameter 8.9 nm) to provide stable aqueous suspensions of functionalized
particles (BA-MNPs). The progress of sugar binding and its impact
on BA-MNP colloidal stability were monitored through the pH-dependence
of hydrodynamic size and zeta potential during incubation with a range
of saccharides. This provided the first direct observation of boronate
ionization pKain grafted BA, which in
the absence of sugar shifted to a slightly more basic pH than free
BA. On exposure to sugar solutions under MNP-limiting conditions,
pKamoved progressively to lower pH as
maximum capacity was gradually attained. The pKashift is shown to be greater for sugars with greater BA binding
affinity, and on-particle sugar exchange effects were inferred. Colloidal
dispersion of BA-MNPs after binding was shown for all sugars at all
pHs studied, which enabled facile magnetic extraction of glucose from
agarose and cultured extracellular matrix expanded in serum-free media.
Bound glucose, quantified following magnetophoretic capture, was found
to be proportional to the solution glucose content under glucose-limiting
conditions expected for the application. The implications for the
development of MNP-immobilized ligands for selective magnetic biomarker
capture and quantitation from the extracellular environment are discussed.