Electron transport through thin organic films in metal--insulator--metal junctions based on self-assembled monolayers.

This paper describes an experimentally simple system for measuring rates of electron transport across organic thin films having a range of molecular structures. The system uses a metal--insulator--metal junction based on self-assembled monolayers (SAMs); it is particularly easy to assemble. The junction consists of a SAM supported on a silver film (Ag-SAM(1)) in contact with a second SAM supported on the surface of a drop of mercury (Hg-SAM(2))--that is, a Ag-SAM(1)SAM(2)-Hg junction. SAM(1) and SAM(2) can be derived from the same or different thiols. The current that flowed across junctions with SAMs of aliphatic thiols or aromatic thiols on Ag and a SAM of hexadecane thiol on Hg depended both on the molecular structure and on the thickness of the SAM on Ag: the current density at a bias of 0.5 V ranged from 2 x 10(-10) A/cm(2) for HS(CH(2))(15)CH(3) on Ag to 1 x 10(-6) A/cm(2) for HS(CH(2))(7)CH(3) on Ag, and from 3 x 10(-6) A/cm(2) for HS(Ph)(3)H (Ph = 1,4-C(6)H(4)) on Ag to 7 x 10(-4) A/cm(2) for HSPhH on Ag. The current density increased roughly linearly with the area of contact between SAM(1) and SAM(2), and it was not different between Ag films that were 100 or 200 nm thick. The current--voltage curves were symmetrical around V = 0. The current density decreased with increasing distance between the electrodes according to the relation I = I(0)e(-beta d(Ag,Hg)), where d(Ag,Hg) is the distance between the electrodes, and beta is the structure-dependent attenuation factor for the molecules making up SAM(1). At an applied potential of 0.5 V, beta was 0.87 +/- 0.1 A(-1) for alkanethiols, 0.61 +/- 0.1 A(-1) for oligophenylene thiols, and 0.67 +/- 0.1 A(-1) for benzylic derivatives of oligophenylene thiols. The values of beta did not depend significantly on applied potential over the range of 0.1 to 1 V. These junctions provide a test bed with which to screen the intrinsic electrical properties of SAMs made up of molecules with different structures; information obtained using these junctions will be useful in correlating molecular structure and rates of electron transport.     

Kinetics of the reaction between the antioxidant Trolox and the free radical DPPH in semi-aqueous solution

Reaction of the free-radical diphenylpicrylhydrazyl (DPPH(.)) with Trolox (TrOH) was investigated in buffered hydroalcoholic media by using a stopped-flow system. DPPH was reduced to the hydrazine analogue DPPH-H with a measured stoichiometry of about 2. DPPH-H was characterized by an acid-base equilibrium (pKa = 8.6). Time-resolved absorption spectra recorded with an excess of either TrOH or DPPH indicated that no significant amount of the TrO radical was accumulated. The TrO radical formed in a first step further reacted quickly with DPPH(.). For 1 : 1 ethanol-buffer mixtures at pH 7.4, the bimolecular rate constants of the first and second steps were 1.1 x 10(4) M(-1) s(-1) and 2 x 10(6) M(-1) s(-1), respectively. A significant increase of the measured rate constant was observed for ethanol-buffer solutions as compared to ethanol. The rate was also increased at higher pH. A deuterium isotopic effect of 2.9 was measured. These data are discussed with regards to mechanisms involving either electron or proton exchange as rate determining steps in the reaction of DPPH with Trolox. The importance of solvent acidity control in investigation of antioxidant properties is outlined.     

Determination of physiological thiols by electrochemical detection with piazselenole and its application in rat breast cancer cells 4T-1

Glutathione (GSH), the most abundant cellular thiol, has been shown to play an important role in maintaining cellular redox equilibrium that is pivotal for cell growth and function. In the present paper a novel electrochemical probe of piazselenole containing a Se-N bond was well developed for the determination of GSH. The cyclic voltammogram of piazselenole scanned at 100 mV/s displayed an irreversible reduction peak at -0.106 V (vs Ag/AgCl electrode) and a significant peak current decrease could be further provoked with the addition of GSH into piazselenole solution. On the basis of the peak current decrease of piazselenole recorded by differential pulse voltammetry with the increase of GSH concentration, a working curve was constructed for GSH determination in the range of 5.0 x 10(-10) approximately 2.2 x 10(-8) M with the linear regression equation as DeltaiP (10(-6)A) = 0.0952 + 0.4287 x CGSH (10(-8) M) and the detection limit (3sigma) as 83 pM. The proposed method was satisfactorily applied to the extracts of rat breast cancer cells 4T-1 for intracellular thiols detection.     

Photodegradation kinetics of some thiols and thiones in aqueous suspension of zinc oxide

Photodegradation of several thiols and thiones including 2-pyrimidinethiol, thiazolidine-2-thione, imidazolidine-2-thione, 1,2,4- triazole 3-thiol and 2,5-dimercapto- 1,3,4-thiadiazole have been studied in aerated aqueous solution using zinc oxide under irradiation with 400W high pressure mercury lamp. Rate constants span the range from 2.44 x 10(-2) min x (-1) to 1.76 x 10(-1) min x (-1). The slower degradation rate of 2-imidazoline 2-thione could be related to oxidation potential of the corresponding ring. The Langmuir-Hinshelwood (L-H) rate constant k(r) and adsorption constant, (KA) for all five thiols and thiones are reported.     

Dosage des thiols par polarographie a impulsions

The determination of thiols has been studied by pulse polarography. A single peak is obtained if the concentration is less than 10^-4 M. The electrochemical reaction involves oxidation of mercury in presence of the thiol with adsorption of the mercury thiolate on the electrode. The method is more sensitive with a negative pulse; this corresponds to reduction of the mercury thiolate formed during the drop life. The sensitivity is about 10^-7 M, i.e. 3 p.p.b. calculated as sulfur. Peak potential values are given for H₂S, nine aliphatic thiols and benzenethiol. Peak height is proportional to concentration up to about 5 · 10^-5 M. The phenomena are more complicated for concentrations higher than 10^-4 M so that the method is not then analytically useful.     

Fluorescence and interactions with thiol compounds of Nile Red-adsorbed gold nanoparticles

We have investigated the interactions between a variety of thiols and Nile Red-adsorbed gold nanoparticles (NRAuNPs). After adding thiols to solutions of NRAuNPs, the solutions fluoresce strongly as a result of the displacement of a Nile Red-derived product from the surface of the AuNPs. We propose a mechanism for the formation of this NR product on the surface of AuNPs by conducting mass spectrometry, fluorescence, and capillary electrophoresis measurements. By recording the fluorescence changes of the NRAuNP solutions after addition of the thiols, we investigated the interactions between the thiols and NRAuNPs. Using the Langmuir isotherm model, we found that the displacement rate constants for thiols having one carboxyl residue, such as 3-mercaptopropionic acid, fall within the range 0.55-1.19 x 10(-2) s(-1). Thiols containing hydroxyl groups [e.g., 2-mercaptoethanol (2-ME)] or amino groups [e.g., N-(2-mercaptopropionyl)glycine (MPG)], or that have flat structures on the AuNP surface, such as mercaptosuccinic acid, exhibit double-exponential kinetics with first rate constants of 0.51-2.83 x 10(-2) s(-1) and second rate constants of 6.0-23.4 x 10(-4) s(-1). Our results reveal that steric effects and the charge density of the thiols both play important roles in determining the interactions with NRAuNPs. The interactions (displacement and/or induced aggregation) are also dependent on the size of NRAuNPs.     

Liquid-membrane electrode sensitive to ammonia and aliphatic amines based on 1-[(2-oxyphenyl)iminomethly]-2-oxybenzene-O,O',N-ammin-nickel(II) complex

In this study, an ammonia-gas-sensing liquid-state membrane electrode based on 1-[(2-oxyphenyl)iminomethyl]-2-oxybenzene-O,O',N-ammin-nickel(II) (Ni(II)-Schiff base) as an ionophore was developed. This membrane electrode gave a very good performance and exhibited linear response with near-Nernstian slope of 50.7+/-0.5 mV per decade, within a concentration range of 1.0x10(-1)-1.0x10(-3) M. The electrode performance depends on the pH of the test solution and the optimum pH range was 10.0-11.5. The effect of concentration of internal solutions in the electrode was also investigated. The selectivity properties of the electrode towards aliphatic amines at a pH value of 10.2 were investigated with the mixed-solution method. Furthermore, the response time and lifetime of the electrode prepared were determined.     

Radical scavenging activity against 1,1-diphenyl-2-picrylhydrazyl of ascorbic acid 2-glucoside (AA-2G) and 6-acyl-AA-2G

The radical scavenging activity of the stable derivatives, which are O-substituted at the C-2 position of ascorbic acid (AA), against 1,1-diphenyl-2-picrylhydrazyl (DPPH) was evaluated in buffer under different pH conditions, and compared with those of AA and alpha-tocopherol. AA was shown to have 50% radical scavenging ability (EC50) at a concentration of 2.2 x 10(-5) M against 0.1 mM DPPH in 60% ethanol. Ascorbyl 6-palmitate, a lipophilic AA derivative which has a free endiol group and is therefore unstable, also showed potent radical scavenging activity with EC50 of 2.9 x 10(-5) M. A typical lipophilic antioxidant, alpha-tocopherol gave a similar EC50 value as that of AA. In contrast, ascorbyl 2,6-dipalmitate, AA 2-phosphate and AA 2-sulfate exhibited negligible scavenging activity. On the other hand, 2-O-alpha-D-glucopyranosyl-L-ascorbic acid (AA-2G) and a series of 6-O-acyl-2-O-alpha-D-glucopyranosyl-L-ascorbic acids (6-Acyl-AA-2G) themselves exhibited the radical scavenging activity of EC50: 6.1 x 10(-5) M and 4.4 x 10(-5)-5.9 x 10(-5) M, respectively, although their activities were lower than that of AA. Among 6-Acyl-AA-2G derivatives, the EC50 values tended to decrease with increasing length of their acyl carbon group. Increasing pH of the buffer resulted in decrease in the scavenging activity of all compounds tested as expected. We speculate that the difference in the radical scavenging activity of derivatives O-substituted at the C-2 position of AA may be ascribed to the linkage type of the substituent group to the endiol-lactone resonance system and the degree of dissociation of the C-3 proton.     

Spectrophotometric determination of biologically active thiols with eosin, silver(I) and adenine.

A simple and highly sensitive spectrophotometric method for the determination of biologically active thiols based on the fading of eosin-silver(I)-adenine ternary complex was established. In the determination of 6-mercaptopurine (MP), Beer's law was obeyed in the range 0.02-0.30 microg ml(-1), with an effective molar absorptivity at 562 nm and the relative standard deviation being 3.5 x 10(5) dm3 mol(-1) cm(-1) and 0.72% (n = 5). Analytical data for various biologically active thiols were determined with the proposed method. This method is about 5-10 times more sensitive than the conventional spectrophotometric methods. A compound having a disulfide bond (-S-S-), such as cystine, could also be determined by the conversion of disulfides to free thiols with the sulfite ion. The procedure was successfully applied to assays of various biologically active thiols in actual medicines.     

Copper-catalyzed direct thiolation of azoles with aliphatic thiols

Cu(II)-catalyzed direct thiolation of azoles with thiols is described via intermolecular C-S bond formation/C-H functionalization under oxidative conditions. Both aryl thiols and aliphatic thiols are used as coupling partners, and furnished the thiolation products in moderate to good yields. The reaction is compatible with a wide range of heterocycles including oxazole, thiazole, imidazole and oxadiazole.     

Quantitative determination of SH groups using 19F NMR spectroscopy and disulfide of 2,3,5,6-tetrafluoro-4-mercaptobenzoic acid

A new method of measurement of thiol concentration by 19F NMR spectroscopy is developed. The method is based on the detection of products of the exchange reaction of thiols with a newly synthesized fluorinated disulfide, 2,3,5,6-tetrafluoro-4-mercaptobenzoic acid (BSSB). A significant broadening of the 19F NMR signal of BSSB in the presence of thiols was observed and attributed to the exchange reaction between the parent disulfide and 2,3,5,6-tetrafluoro-4-mercaptobenzoic acid. The rate constant for this reaction was found to be equal to (63 +/- 11) x 10(3) M(-1) s(-1) at pH 7.0. The method was applied for the measurement of concentration of glutathione and albumin in rat blood.     

Automated flow-injection pseudotitration of strong and weak acids, ascorbic acid and calcium, and catalytic pseudotitrations of aminopolycarboxylic acids by use of a microcomputer-controlled analyser

A simple, inexpensive, fully automated spectrophotometric system for flow-injection pseudotitrations is used to perform acid-base, redox, complexometric and catalytic "titrations". Peak widths (in time units) in the range 10-100 sec can be measured with a precision of better than 0.3% in most cases. Strong and weak acids in the range 5.0 x 10(-4)-1.0 x 10(-2)M are measured by using sodium hydroxide-Bromothymol Blue "titrant". Ascorbic acid (1 x 10(-4)-1 x 10(-2)M) is "titrated" with 2,6-dichlorophenolindophenol, and calcium (5.0 x 10(-4)-5.0 x 10(-2)M) with EDTA, with calmagite as indicator in the presence of magnesium. Aminopolycarboxylic acids (5 x 10(-6)-1 x 10(-2)M) are measured by use of catalytic indication based on the manganese-catalysed periodate-diethylaniline reaction. The ascorbic acid method has been applied to analysis of pharmaceutical preparations, and the calcium method to water analysis.     

Ion-selective electrodes for potentiometric determination of ranitidine hydrochloride, applying batch and flow injection analysis techniques.

New ranitidine hydrochloride (RaCl)-selective electrodes of the conventional polymer membrane type are described. They are based on incorporation of ranitidine-tetraphenylborate (Ra-TPB) ion-pair or ranitidine-phosphotungstate (RaPT) ion-associate in a poly(vinyl chloride) (PVC) membrane plasticized with dioctylphthalate (DOP) or dibutylphthalate (DBP). The electrodes are fully characterized in terms of the membrane composition, solution temperature, and pH. The sensors showed fast and stable responses. Nernstian response was found over the concentration range of 2.0 x 10(-5) M to 1.0 x 10(-2) M and 1.0 x 10(-5) M to 1.0 x 10(-2) M in the case of Ra-TPB electrode and over the range of 1.03 x 10(-5) M to 1.00 x 10(-2) M and 1.0 x 10(-5) M to 1.0 x 10(-2) M in the case of Ra-PT electrode for batch and FIA systems, respectively. The electrodes exhibit good selectivity for RaCl with respect to a large number of common ions, sugars, amino acids, and components other than ranitidine hydrochloride of the investigated mixed drugs. The electrodes have been applied to the potentiometric determination of RaCl in pure solutions and in pharmaceutical preparations under batch and flow injection conditions with a lower detection limit of 1.26 x 10(-5) M and 5.62 x 10(-6) M at 25 +/- 1 degrees C. An average recovery of 100.91% and 100.42% with a relative standard deviation of 0.72% and 0.53% has been achieved.     

Determination of selenium(IV) and tellurium(IV) by differential pulse polarography

Differential pulse polarographic methods for the determination of selenium(IV) and tellurium(IV) in nitric acid medium are described. The peak current is maximal when 0.25M nitric acid medium is used, the DPP peaks for Se(IV) and Te(IV) being at -0.54 and -0.8 V vs. Ag/AgCl respectively. The peak current is a linear function of selenium concentration over three ranges, 5.1 x 10(-6)-1.3 x 10(-5), 1.27 x 10(-5)-1.27 x 10(-4) and 1.27 x 10(-4)-7.60 x 10(-4)M Se(IV), with different slopes. The plot for Te(IV) is linear over the range 0.78 x 10(-6)-9.40 x 10(-5)M.     

A new spectrofluorometric method for the determination of ascorbic acid based on its activating effect on a hemoglobin-catalyzed reaction.

A spectrofluorometric method for the determination of ascorbic acid (AA) based on its activation on the hemoglobin-catalyzed reaction was proposed. The fluorescence intensity of the product was measured under the optimal experimental conditions, i.e. 4.0 x 10(-6) M H2O2, 6.0 x 10(-5) M p-cresol, 1.2 M NH3-NH4Cl (pH 10.4) and 2.0 x 10(-7) M hemoglobin. The order of additions of the reagents was also studied. The activation of AA was found to be associated with a high ammonia concentration. The linear range of the method was 9.0 x 10(-10)-3.6 x 10(-8) M of AA. The detection limit was calculated to be 3.0 x 10(-10) M. The relative standard deviation of this method is 1.6% at 7.0 x 10(-9) M for 11 determinations.     

Spectrofluorimetric determination of thiols by use of N[P-(2-benzoxazolyl)phenyl]maleimide

The weak fluorescence of N-[P-(2-benzoxazolyl)phenyl]maleimide (BOPM) can be greatly enhanced by thiol-containing compounds. A sensitive and simple spectrofluorimetric method based on the use of BOPM has been developed for the determination of thiols such as cysteine (Cys) and reduced glutathione (GSH). Calibration plots were linear in the concentration range from 0 to 1.6 x 10(-7) mol L(-1) for Cys and 0 to 1.7 x 10(-7) mol L(-1) for GSH. The detection limits (3a) were 2.36 x 10(-10) mol L(-1) for Cys and 1.49 x 10(-10) mol L(-1) for GSH. Many other amino acids (present at 100-fold greater concentrations) did not interfere with the determination. The proposed method has been used for the determination of Cys in protein hydrolysate and cystine electrolyte or GSH in serum, with recoveries of 95.4-103.7%.     

Differential pulse polarographic determination of ofloxacin in pharmaceuticals and biological fluids

A sensitive method is described for the determination of ofloxacin in its pure form, dosage forms and biological fluids. The proposed method depends upon the polarographic activity of ofloxacin in Britton Robinson buffers, whereby a well-defined cathodic wave is produced over the pH range 4.1-10.3. The wave was characterized as being irreversible, diffusion-controlled with limited adsorption properties. The current-concentration relationship was found to be rectilinear over the range 5x10(-5)-5x10(-4) M and 1x10(-5)-5x10(-4) M using the DC(t) and DPP modes respectively, with a minimum detectability (S/N=3) of 3x10(-7) M. The proposed method was successfully applied to the determination of ofloxacin in tablets and biological fluids. The results obtained were found to be in agreement with those obtained by a reference method.     

Determination of cationic surfactants in water samples by their enhanced resonance light scattering with azoviolet

A simple assay of cationic surfactants in water samples was developed based on the measurements of enhanced resonance light scattering (RLS). At pH 6.09 and ionic strength 0.03 M, the interactions of azoviolet (AV) with cationic surfactants, including zephiramine (Zeph) and cetyl trimethyl ammonium bromide (CTMAB), result in enhanced RLS signals characterized by the peaks of 470.0, 485.0 and 495.0 nm. The enhanced RLS intensity is proportional to the concentration of cationic surfactant of Zeph in the range of 0.2~6.0x10(-6) M, and to that of CTMAB in the range of 0.4~4.8x10(-6 )M. The limit of determination (3 sigma) is 2.1x10(-8) M and 3.8x10(-8) M for the two surfactants, respectively. Determinations of cationic surfactants in synthetic and tap water samples were successfully made with a recovery of 90.5~108.6%.     

Determination of copper with neocuproine by thermal-lens spectrometry

Conditions for the spectrophotometric determination of copper with 2,9-dimethyl-l,10-phenan-throline (neocuproine) in the presence of ascorbic acid in a water-ethanol solution (9 : 1) at pH 4.5–5.0 have been found. The detection limit is 3 x 10^-6 M. The concentration range is from 4.4 x 10^-6 to 3 x 10^-4 M. Conditions for the determination of copper(I) with neocuproine by thermal lens spectrometry have been proposed. The detection limit is 4 x 10^-7 M. The concentration range is from 7 x 10^-7 to 6 x 10^-5 M. Iron(II) at concentrations as high asn x 10^-4 M does not interfere with the determination of copper. Changes in the conditions for the photometric reaction associated with passing from spectrophotometric measurements to thermal lensing are discussed.     

Uranyl-selective PVC membrane electrodes based on some recently synthesized benzo-substituted macrocyclic diamides

Four different recently synthesized macrocyclic diamides were studied to characterize their abilities as uranyl ion carriers in PVC membrane electrodes. The electrodes based on macrocycle 1,18-diaza-3,4;15,16-dibenzo-5,8,11,14,21,24-hexaoxacyclohexaeicosane-2,17-dione resulted in a Nernstian response for UO(2)(2+) ion over wide concentration ranges. The linear concentration range for the polymeric membrane electrode (PME) is 3.0x10(-6)-8.2x10(-3) M with a detection limit of 2.2x10(-6) and that for the coated graphite electrode (CGE) is 5.0x10(-7)-1.5x10(-3) M with a detection limit of 3.5x10(-7) M. The electrodes manifest advantages of low resistance, very fast response and, most importantly, good selectivities relative to a wide variety of other cations.