Adsorption of O1-[6-(methylselanyl)hexanoyl]glycerol (SeOG) on the gold surface was investigated by cyclic voltammetry, phase-sensitive AC voltammetry, electrochemical impedance spectroscopy and piezoelectric microgravimetry. SeOG adsorption results in a stable and compact surface layer with the coverage degree close to unity for an adsorption time of 30 to 80 min and 4.6 mM SeOG acetonitrile solution. Such a layer displays minute defects (pinholes) with the radius of ca. 1-3 μm, separated by 6-50 μm intervals (depending on the adsorption time). The adsorbed compound undergoes anodic desorption in the gold oxide region and also undergoes a cathodic process leading to the removal of the surface layer. Both these processes are similar to those demonstrated by short-chain alkanethiols and have been interpreted as a indication for the conversion of the selena to selenol function as a result of a dissociative adsorption process. Apparently, the main component of the surface layer is O1-(6-selanylhexanoyl)glycerol that results by the cleavage of the C6-Se bond in SeOG. The two free hydroxy groups in SeOG allow to use it as a bridge for binding other compounds to the gold surface. This possibility was illustrated by building up surface layers of a carotenoid derivative (O1-(8'-apo-β-apo-caroten-8'-oyl)-O2-[6-(methylselanyl)hexanoyl]glycerol, II) or carotenoid- and phosphocholine-derivatized SeOG (O1-(8'-apo-β-caroten-8'-oyl)-O2-[6-(methylselanyl)hexanoyl]-O3-{[2-(trimethylammonio)ethoxy]phosphoryl}glycerol, III). The compound III generates a less densely packed layer due to the constraints induced by the phosphocholine substituent. Each of these compounds undergoes anodic reactions that are typical of carotenoids in the adsorbed state. However, the polar and hydrophilic phosphocholine residue in III shifts the anodic peak to a less pozitive potential. SeOG allows therefore to tune the molecular environment of a surface attached compound by means of a suitable co-substituent.
We developed a biosensor based on supported lipid films (sBLM) with incorporated calixarenes for detection of cytochrome c (cyt c) using electrochemical impedance spectroscopy (EIS). Calix[6]arene (CALIX) was incorporated into soybean phosphatidylcholine (SBPC) liposomes in different molar ratios (SBPC:CALIX = 10 : 1, 30 : 1 and 100 : 1). The lipid layer containing calixarenes has been formed on the surface of a gold electrode previously covered with an octadecanethiol monolayer. Both the sBLM capacitance and the electron transfer resistance showed noticeable modification at a cyt c concentration as low as 10 nM. However, a much higher concentration of free lysine (> 30 mM) was necessary to induce a similar effect. Nonspecific interaction of cyt c with sBLM without CALIX was negligible.
Despite the large body of ancient Ethiopian works of art in the form of murals, icons, and illuminated manuscripts, the physicochemical examinations carried out on them are few. This study is an in situ investigation of the wall paintings in the early 12th century Yemrehanna Krestos Church, Ethiopia. Fast, onsite, nondestructive analysis of the painting materials was carried out using a portable X-ray energy fluorescence dispersive spectrometer. It is believed to be the first onsite technical examination of Ethiopian mural paintings. This work resulted in information about the painting materials and the existence of different painting programs. The analysis revealed that the main pigments were red and yellow ochre, minium, cinnabar, orpiment, gypsum, lead, and white and carbon black; those typically employed in medieval times with no indication of later conservation-restoration intervention. Correlation between concentration of elements and multivariate statistical analysis was used to identify the most probable compounds and to classify sets of pigments used in two painting programs. The portable X-ray energy dispersive fluorescence spectrometer analyzer is found valuable to guide the in situ analysis and assess potential sites for microsampling for further investigations with complementary analytical techniques. However, in this expedition, we were unable to collect sufficient microsamples to warrant adequate complementary analyses. Characterization and documentation of the materials of the church murals support art historical studies and eventually conservation intervention plans.
The system Co(II)-phenylthiourea (PTU)-borax buffer was investigated by cathodic stripping voltammetry (CSV) at a hanging mercury drop electrode. The results of the voltammetric measurements showed that the presence of both PTU and Co(II) gives rise to a new irreversible peak at about -1.5 V. Based upon our previous results obtained in the study of other sulfur compounds and the sulfide ion itself, the peak was ascribed to the catalytic hydrogen evolution superimposed on the reduction of the coordinated Co(II) ion. The catalyst itself is a Co(II) complex with the sulfide ion produced by the decomposition of the analyte during the deposition step. The influence of PTU and cobalt concentration, accumulation conditions and stripping parameters was investigated and complementary data on thiourea are included. The results showed that the measurement of the catalytic hydrogen evolution peak current can be used as a basis for a simple, accurate and rapid method for the determination of PTU within the concentration range 10-100 nM. The catalytic method is relatively free of interferences and could be a suitable alternative for cases in which the stripping peak due to mercury ion reduction in the accumulated mercury compound is disturbed by some interference.
The differential-pulse cathodic stripping voltammetry of cysteine (CysH), cystine (Cys) and N-acetylcysteine (Ac-CysH) was studied at a hanging mercury drop electrode at pH 7 in the presence of nickel ion.
3-(N-Morpholino)propanesulfonic acid (MOPS) and phosphate-acetate buffer were used as supporting electrolytes. In the presence of CysH, after accumulation at potentials ranging between 0.0 and -0.4 V, the catalytic reduction of nickel ion gives a peak at -0.6 V versus the Ag-AgC1 reference electrode. An additional effect of nickel ion is the suppression of the cathodic stripping peak due to mercury cysteinate reduction, thus permitting the simultaneous determination of another thiol with no catalytic activity (e.g., Ac-CysH). Consequently, CysH (or Cys) and Ac-C ysH can be determined simultaneously or independently in the same sample. Cys is reduced at potentials preceding the catalytic peak, which is actually due to the CysH thus produced. Some differences between the behaviour of Cys and CysH are due to different accumulation mechanisms. The catalytic stripping voltammetry of CysH or Cys exhibits good sensitivity (detection limit about 1 nmol dm-3 for 3 min accumulation). The stripping voltammetric method described appears to be the first involving adsorptive accumulation of a metal complex in which the organic ligand is determined catalytically. Further, this particular method affords some selectivity in the determination of sulfur compounds, which are normally determined by cathodic stripping voltammetry of their anodically accumulated mercury complexes.
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