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Kimberly A Millard, Suzanne F Groemminger, Andrew Hoteling, Daniel Hook, Katarzyna Wygladacz; Surface Characterization of the Interaction Between Peroxide Neutralizing Disks and Platinum Modulating Compounds. Invest. Ophthalmol. Vis. Sci. 2014;55(13):864.
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One-step hydrogen peroxide (H2O2) contact lens disinfecting systems utilize a platinum (Pt) coated disk to initiate the neutralization of H2O2. This reaction results in rapid neutralization once the Pt disk comes into contact with H2O2. The addition of a platinum modulating compound (PMC) to a H2O2 system can effectively delay neutralization while still maintaining safe residual H2O2 levels. Two PMCs tested were thiourea and carbamide. PMCs may have an affinity for the Pt surface that can irreversibly or reversibly block the surface from being catalytically active. This study investigates the interaction of thiourea and carbamide with the Pt disk surface using TOF-SIMS and XPS.
An untreated Pt disk and a Pt disk soaked for 24 hours in a phosphate buffered saline solution (PBS) were used as controls. The test Pt disks were soaked for 24 hours in either 2.0% carbamide/PBS or 2.0% thiourea/PBS. A flat section of each Pt disk was cut and mounted for analysis. TOF-SIMS data was collected using Bi3 primary ion beam positive mode using static SIMS conditions to characterize the PMC. The XPS elemental composition data was used to quantitate the relative amounts of each PMC adsorbed to the Pt disk surface.
TOF-SIMS showed that the intact molecular ion at m/z 61.04 (carbamide) and m/z 77.02 (thiourea) was observed on the surface of the disks soaked in the respective solutions. TOF-SIMS imaging demonstrated the distribution of these species on the surface was fairly uniform. The control Pt disks did not show a response for the carbamide or thiourea molecular ions. XPS showed a 2-fold increase in the amount of thiourea vs. carbamide adsorbed to the Pt disk surface. After rinsing, carbamide response decreased while thiourea did not.
TOF-SIMS and XPS were used to evaluate the surface characteristics of Pt coated disks in the presence of carbamide or thiourea. TOF-SIMS analysis showed the presence of both compounds on the platinum surface. XPS demonstrated quantitative elemental analysis of the Pt surface suggesting thiourea has a stronger affinity for the Pt surface than carbamide. The relative affinities of thiourea and carbamide for the Pt surface correlate with the delay of neutralization. Thiourea prevents neutralization while carbamide reversibly blocks neutralization resulting in a delay that allows for a faster overall disinfection rate.
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