Ennoblement of stainless steel in the presence of glucose oxidase: nature and role of interfacial processes.

The ennoblement of the free corrosion potential (E(corr)) of AISI 316L stainless steel which did not occur in synthetic fresh water (SFW), was observed after introduction of glucose oxidase (Gox) and glucose, or of hydrogen peroxide (H(2)O(2)). The composition of the surface was monitored using AFM and XPS, a detailed XPS analysis being based on the discrimination between oxygen of organic and inorganic nature proposed in a previous study. In H(2)O(2) medium, the main changes regarding the inorganic phase were the increase of the oxygen concentration in the passive film, the increase of the molar concentration ratio of oxidized species Fe(ox)/Cr(ox) and the growth of nanoparticles, presumably made of ferric oxide/hydroxide. In Gox medium, no significant changes were observed in both oxygen concentration and Fe(ox)/Cr(ox) ratio, but the density of colloidal particles decreased, indicating a dissolution of Fe oxide/hydroxide under the influence of gluconate. In contrast with H(2)O(2), in SFW and Gox the amount of organic compounds increased due to the accumulation of polysaccharides and proteins. The influence of glucose oxidase on the ennoblement of stainless steel is not due to indirect effects of H(2)O(2) through the change of surface composition. The E(corr) ennoblement seems to be directly due to the presence of H(2)O(2) and to the electrochemical behavior of H(2)O(2) and related oxygen species. This consideration is important for understanding and controlling microbial influenced corrosion.

Evolution of the passive film and organic constituents at the surface of stainless steel immersed in fresh water.

The evolution of the surface of a conventional stainless steel (AISI 316L) immersed in aqueous medium simulating fresh water (pH approximately 8) was studied using XPS and AFM. A detailed analysis of XPS spectra allowed a distinction to be made between oxygen of organic and inorganic nature. During the first 48 h, the main changes concern the inorganic phase: the oxygen concentration in the passive layer increases, owing both to oxidation of metal elements, including conversion of Fe(II) to Fe(III), and to hydration; the molar ratio of oxidized species Fe(ox)/Cr(ox) decreases slightly; the formation of colloidal particles, presumably made of ferric hydroxide, is observed by AFM. After longer periods of immersion, the Fe(ox)/Cr(ox) is higher, while the coating of colloidal particles reaches a full surface coverage. The amount of organic compounds increases further and the XPS spectra reveal the accumulation of polysaccharides and proteins, which indicate that these organic compounds are of biological origin.

Surface free energies and elemental surface compositions of human enamel after application of commercially available mouthrinses and adsorption of salivary constituents.

The adsorption of active agents from six commercially available mouthrinses to ground and polished enamel, with and without adsorbed salivary constituents, was monitored by contact angle measurements and X-ray Photoelectron Spectroscopy (XPS). Human enamel samples were treated with mouthrinses containing chlorhexidine (Peridex), stannous fluoride/amine fluoride (Meridol), thymol/benzoic acid (Listerine), sanguinarine (Veadent), sodium fluoride (Prodent), or cetylpyridinium chloride (Merocet). XPS indicated a sizeable adsorption of both active and non-active components for all products. After treatment, all enamel surface free energies increased except for the stannous fluoride/amine fluoride containing mouthrinse. It is suggested that non-active components in the products cause an increase in surface free energy. Despite this thermodynamically unfavorable increase in surface free energy, all rinses have plaque reducing effects, indicating that this unfavorable surface characteristic is overruled by the antibacterial properties of the components. Replacement of non-active components by less adsorbing surfactants could increase the efficiency of the products tested.

A comparison between the elemental surface compositions and electrokinetic properties of oral streptococci with and without adsorbed salivary constituents.

In order to characterize the functional cell surface, isoelectric points and elemental surface concentration, the ratios of nitrogen, oxygen and phosphorus to carbon of saliva-coated strains were determined by pH-dependent zeta potential measurements and X-ray photoelectron spectroscopy and compared with those of uncoated strains. The measurements of potential were carried out on completely hydrated cells, whereas the spectroscopy was on freeze-dried micro-organisms. The small increase in the nitrogen:carbon surface concentration ratio of saliva-coated streptococci in comparison to uncoated strains varied from 0.001 (Streptococcus mitis BA) to 0.029 (Streptococcus sanguis CH3) and was concurrent with an increase of the isoelectric point, ranging from 0.0 to 0.9. Increases in the oxygen:carbon ratio ranged from 0.006 (Strep. mitis BA) to 0.041 (Streptococcus mutans NS), whereas the phosphorus:carbon surface concentration ratio was unchanged after saliva treatment. Despite the fact that isoelectric and compositional measurements were made in different states of surface hydration, a decrease in the nitrogen:carbon ratio accompanied by an increase in oxygen:carbon ratio, was related to a decrease of the isoelectric point of the saliva-coated strains, and so analogous with previous observations for uncoated strains. Although there were changes in the physico-chemical properties of the strains upon saliva coating, all more or less kept their own surface identity despite adsorption of salivary constituents, possibly indicating some capacity to protect their own physico-chemical identity.

Elemental surface concentration ratios and surface free energies of human enamel after application of chlorhexidine and adsorption of salivary constituents.

In vitro adsorption of chlorhexidine from a commercially available chlorhexidine-containing (0.12%) mouthrinse (Peridex) on both ground and polished and on saliva-coated enamel was studied by X-ray photoelectron spectroscopy and contact angle measurements. Furthermore, adsorption of salivary constituents was studied on chlorhexidine-treated enamel. Changes in the elemental surface composition obtained by X-ray photoelectron spectroscopy clearly demonstrated adsorption of chlorhexidine from the mouthrinse as indicated by increased N and C signals which were in all cases accompanied by a higher enamel surface free energy, estimated from the contact angle data. In addition, it was found that salivary constituents adsorb less to chlorhexidine-treated enamel than to ground and polished enamel and, moreover, that adsorbed salivary constituents were desorbed by chlorhexidine. In vivo, the effect of a 3-week use of chlorhexidine was compared with the use of a placebo and habitual oral hygiene of 10 volunteers by means of clinical contact angle measurements. Also in vivo adsorption of chlorhexidine yielded elevated surface free energies with respect to habitual oral hygiene or a placebo. Thus in vivo, the well-known favourable effects of the bactericidal properties of chlorhexidine completely overrule the unfavourable effects of high enamel surface free energies.