Synchrotron-based high-resolution photoelectron spectroscopy was applied to study the modification of the alkanethiol (AT) self-assembled monolayers on gold and silver substrates by nitrogen-oxygen downstream microwave plasma. Because of the low density and energy of the ionizing particles, the long-lived nitrogen and oxygen radicals provided the major impact of plasma treatment. The treatment resulted in massive damage and disordering of the initially well-ordered and chemically homogeneous AT films. The most pronounced processes are the complete (AT/Au) or partial (AT/Ag) oxidation of the pristine thiolate species, partial desorption of hydrogen and carbon-containing fragments with subsequent cross-linking within the residual hydrocarbon layer, and partial oxidation of this layer, and appearance of the nitrogen-containing entities. The plasma-treatment-induced changes in the alkyl matrix and at the S-substrate interface are only partly correlated. The rate and extent of the oxidation processes at this interface are noticeably larger for C18/Au than for C18/Ag, which suggests a stronger S-metal bond in the latter system. The results demonstrate that a smallest oxygen contamination should be avoided if one wants to perform a soft modification of thin organic layers or definite molecular entities attached to these layers through the exposure to plasma.

IntroductionDry-eye syndrome (DES) is a general eye disease. Eye drops are the common ophthalmological medication. However, the ocular barrier makes it difficult to attain high drug bioavailability. Nanomedicine is a promising alternative treatment for ocular diseases and may increase drug content in the affected eye.MethodsTo explore this potential, we constructed nanoparticles (NPs) containing an anti-inflammatory agent for DES treatment. The NPs were made of gelatin–epigallocatechin gallate (EGCG) with surface decoration by hyaluronic acid (HA) and designated “GEH”. The particle size, surface charge, and morphology were evaluated. The in vitro biocompatibility and anti-inflammation effect of nanoparticles were assayed via culturing with human corneal epithelium cells (HCECs) and in vivo therapeutic effect was examined in a DES rabbit’s model.ResultsThe synthesized GEH NPs had a diameter of approximately 250 nm and were positively charged. A coculture experiment revealed that 20 µg/mL GEH was not cytotoxic to HCECs and that an EGCG concentration of 0.2 µg/mL downregulated the gene expression of IL1B and IL6 in inflamed HCECs. Large amounts of GEH NPs accumulated in the cytoplasm of HCECs and the ocular surfaces of rats and rabbits, indicating the advantage of GEH NPs for ocular delivery of medication. Twice-daily topical treatment with GEH NPs was performed in a rabbit model of DES. The ocular surface of GEH-treated rabbits displayed normal corneal architecture with no notable changes in inflammatory cytokine levels in the cornea lysate. The treatment improved associated clinical signs, such as tear secretion, and fluorescein staining recovered.ConclusionWe successfully produced GEH NPs with high affinity for HCECs and animal eyes. The treatment can be delivered as eye drops, which retain the drug on the ocular surface for a longer time. Ocular inflammation was effectively inhibited in DES rabbits. Therefore, GEH NPs are potentially valuable as a new therapeutic agent delivered in eye drops for treating DES.

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High-resolution photoelectron spectroscopy was applied to study the modification of alkanethiolate (AT) self-assembled monolayers (SAMs) on gold and silver substrates by nitrogen-oxygen downstream microwave plasma. The dominating plasma-induced processes are the oxidation of the alkyl matrix and the thiolate headgroups and the desorption of the alkylsulfonate species. The rates and extent of these processes and the mechanism of film modification depend on the substrate and the length of the alkyl chain. The films on Ag were found to be much more resistant to degradation by the reactive plasma, which is related to stronger thiolate-metal and sulfonate-metal bonds as compared to those on Au and to a partial polymerization of the alkyl matrix in AT/Ag at the initial stages of the plasma treatment. On a given substrate, the length of the aliphatic chain noticeably affects the rates and extent of the oxidation and desorption processes. The major effect stems from a two-step character of the thiolate-sulfonate transformation process, which requires both the penetration of the reactive oxygen species to the headgroupsubstrate interface and the oxidation event itself. Whereas for short-chain SAMs the oxidation step is rate-determining, the process becomes diffusion-limited for the long-chain films.

Synchrotron-based high-resolution photoelectron spectroscopy was applied to study the modification of self-assembled monolayers (SAMs) of 4‘-methyl-1,1‘-biphenyl-4-thiols (BPT) on (111) gold and silver substrates by a nitrogen−oxygen downstream microwave plasma and the attachment of acrylic acid to the plasma-modified SAMs. The plasma treatment resulted in massive damage and disordering of the BPT films, with the extent and character being noticeably different for BPT/Au and BPT/Ag. Whereas for BPT/Au a profound desorption of the entire BPT moieties and a complete defragmentation of the residual hydrocarbon part occurred, only partial desorption and oxidation took place for BPT/Ag, where even a part of intact BPT moieties survived the plasma treatment. The differences in the response of BPT/Au and BPT/Ag to the plasma treatment are related to the stronger thiolate−substrate bonds for the latter system. Taking into account that an analogous difference was also observed in alkanethiolate films, one can consider it as a general property of thiol-derived SAMs. The extent of acrylic acid attachment to the plasma-treated BPT/Ag was found to be essentially larger than that to the hydrocarbon residues in the case of BPT/Au.