Spin probe EPR spectroscopy is currently the only method to quantitatively report
on the orientational ordering of graphene oxide membranes. This technique is based
on the analysis of EPR spectra of a membrane containing stable radicals sorbed on
oxidized graphene planes. The efficiency of the method depends on the spin probe structure;
therefore, it is important to find stable paramagnetic substances that are most sensitive
to the alignment of graphene oxide membranes. In the present work, three novel stable
nitroxide radicals containing aromatic fragments with two nitrogen atoms were tested
as spin probes to study graphene oxide membranes. The spin-Hamiltonian parameters
of the radicals in graphite oxide powder and orientational order parameters of the
probes inside graphene oxide membrane were determined. The sensitivity of one of these
radicals to membrane orientational ordering was found to be higher than for any of
spin probes used previously. A likely reason for this higher sensitivity is the presence
of heteroatoms which can facilitate interaction between paramagnetic molecules and
oxygen-containing groups on the inner surface of the membrane. The new high-sensitivity
spin probe may significantly increase the potential of EPR spectroscopy for studying
the internal structure of graphene oxide membranes.
