1 research outputs found
Speciation of l‑DOPA on Nanorutile as a Function of pH and Surface Coverage Using Surface-Enhanced Raman Spectroscopy (SERS)
The adsorption configuration of organic molecules on
mineral surfaces
is of great interest because it can provide fundamental information
for both engineered and natural systems. Here we have conducted surface-enhanced
Raman spectroscopy (SERS) measurements to probe the attachment configurations
of DOPA on nanorutile particles under different pH and surface coverage
conditions. The Raman signal enhancement arises when a charge transfer
(CT) complex forms between the nanoparticles and adsorbed DOPA. This
Raman signal is exclusively from the surface-bound complexes with
great sensitivity to the binding and orientation of the DOPA attached
to the TiO<sub>2</sub> surface. Our SERS spectra show peaks that progressively
change with pH and surface coverage, indicating changing surface speciation.
At low pH and surface coverage, DOPA adsorbs on the surface lying
down, with probably three points of attachment, whereas at higher
pH and surface coverage DOPA stands up on the surface as a species
involving two attachment points via the two phenolic oxygens. Our
results demonstrate experimentally the varying proportions of the
two surface species as a function of environmental conditions consistent
with published surface complexation modeling. This observation opens
up the possibility to manipulate organic molecule attachment in engineered
systems such as biodetection devices. Furthermore, it provides a perspective
on the possible role of mineral surfaces in the chemical evolution
of biomolecules on the early Earth. Adsorbed biomolecules on mineral
surface in certain configurations may have had an advantage for subsequent
condensation reactions, facilitating the formation of peptides