Using SICM Charge Mapping to Investigate Acid Erosion

Objectives: To evaluate Scanning Ion Conductance Microscopy (SICM) as a quantitative technique for assessing acid erosion on enamel surfaces in vitro.
Methods: Bovine enamel specimens (4 x 4 mm) were polished using lapping diamond with a final grade of 50 nm to expose the enamel microstructure. SICM (150 nm tip size, 50 mM KCl electrolyte, pH 7.2) was used to build an accurate topographical map simultaneously with a bulk normalised assessment of the surface charge achieved through current time curves generated during a carefully controlled scan hopping protocol. The electrolyte was removed, and the tip retracted from the surface to a safe distance. The surface was treated with citric acid (1 % w/w, pH 3.6) for 30 s. The sample was rinsed, the electrolyte replaced and a second SICM map, with the original tip, obtained. COMSOL Multiphysics was used to model the experiment and through iterative modelling provided a means to calculate estimates of the real surface charge values from the experimentally obtained normalised current. The scan hopping protocol generates a grid with 1600 pixels. At each pixel a unitless normalised current value is obtained, the model allows for a conversion to surface charge in mCm^-2.
Results: The mean normalised current and resulting surface charge pre- and post-citric acid exposure is shown in the table below.


The mean surface charge for the post-citric acid treated enamel surface was significantly lower than the pre-treated surface (p<0.0001, Students t-test).
Conclusions: Citric acid exposure modifies the measured surface charge of enamel leaving the surface overall significantly more negatively charged. This research demonstrates that SICM is a potential quantitative tool for assessing enamel surface charge changes following acid erosion.