Gold nanoparticle (Au-NP) seeds were adsorbed onto the surface of a self-assembled monolayer (SAM)-modified electrode. With the treatment of this modified electrode by Au-NPs growth solution containing different concentrations of H2O2 or cholesterol along with cholesterol oxidase (ChOx), the Au-NP seeds on the electrode surface were enlarged in varying degrees. As a result, the peak currents in corresponding cyclic voltammograms were inversely proportional to the concentration of H2O2 or cholesterol. ChOx was also further modified onto the surface of Au/SAM/Au-NP electrode to prepare Au/SAM/Au-NP/ChOx electrode. Using the enzyme-modified electrode to detect cholesterol, which also utilized the enlargement of the NPs, an extraordinary low detection limit of 5 x 10(-9) M was achieved and two linear dependence ranges of 7.5 x 10(-8)-1 x 10(-6) and 1 x 10(-6)-5 x 10(-5) M were obtained. Consequently, new kinds of H2O2 and cholesterol biosensors could be fabricated.
Trimethylacetic acid (TMAA) adsorption evolution on the rutile TiO 2 (110) surface from submonolayer to saturation coverages was examined at the atomic level by scanning tunneling microscopy using the same area analysis approach. Upon TMAA deprotonation, no evidence of terminal OH group formation has been found. It has been suggested that uncommon geometry associated with detached hydrogen atom takes place instead, with proton bonding to pair bridging oxygen atoms. Such a configuration is likely to be stabilized by adjacent adsorbed TMA groups and, in turn, be a factor in the formation of TMA (2 × 1) reconstruction at saturation coverage. It also shown that the surface defects do not have a major impact on TMAA adsorption.
The reactivity of natural dissolved organic matter toward sulfide and has not been well studied with regard to electron transfer, product formation, and kinetics. We thus investigated the abiotic transformation of sulfide upon reaction with reduced and nonreduced Sigma-Aldrich humic acid (HA), at pH 6 under anoxic conditions. Sulfide reacted with nonreduced HA at conditional rate constants of 0.227-0.325 h(-1). The main transformation products were elemental S (S0) and thiosulfate (S2O3(2-)), yielding electron accepting capacities of 2.82-1.75 μmol e- (mg C)(-1). Native iron contents in the HA could account for only 6-9% of this electron transfer. About 22-37% of S reacted with the HA to form organic S (Sorg). Formation of Sorg was observed and no inorganic transformation products occurred for reduced HA. X-ray absorption near edge structure spectroscopy supported Sorg to be mainly zerovalent, such as thiols, organic di- and polysulfides, or heterocycles. In conclusion, our results demonstrate that HA can abiotically reoxidize sulfide in anoxic environments at rates competitive to sulfide oxidation by molecular oxygen or iron oxides.
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