Optical detection of phenolic compounds based on the surface plasmon resonance band of Au nanoparticles

An indirect colorimetric method is presented for detection of trace amounts of hydroquinone (1), catechol (2) and pyrogallol (3). The reduction of AuCl4(-) to Gold nanoparticles (Au-NPs) by these phenolic compounds in the presence of cetyltrimethylammonium chloride (CTAC) produced very intense surface plasmon resonance peak of Au-NPs. The plasmon absorbance of Au-NPs allows the quantitative colorimetric detection of the phenolic compounds. The calibration curves derived from the changes in absorbance at lambda = 568 nm were linear with concentration of hydroquinone, catechol and pyrogallol in the range of 7.0 x 10(-7) to 1.0 x 10(-4)M, 6.0 x 10(-6) to 2.0 x 10(-4)M and 6.0 x 10(-7) to 1.0 x 10(-4)M, respectively. The detection limits were 5.3 x 10(-7), 2.5 x 10(-6) and 3.2 x 10(-7)M for the hydroquinone, catechol and pyrogallol, respectively. The method was applied satisfactorily to the determination of phenolic compounds in water samples and pharmaceutical formulations.     

Radiolytic transformations of chlorinated phenols and chlorinated phenoxyacetic acids

Hydroxyl radical reactions of selected chlorinated aromatic phenols (2,4-dichlorophenol, 2,4,6-trichlorophenol, and pentachlorophenol) and chlorinated phenoxyacetic acids [2,4-dichlorophenoxyacetic acid (2,4-D), 2,4-D methyl ester, 2-(2,4-dichlorophenoxy)propionic acid (2,4-DP)] were studied using the radiolysis techniques of pulse radiolysis and gamma radiolysis. Hydroxyl radical addition was the prominent reaction pathway for the chlorinated phenoxyacetic acids and also for the chlorinated phenols at pH values below the pK(a) of the compounds. A very prominent change in (*)OH reactivity was observed with the chlorinated phenoxide ions in high pH solutions. Two different reaction pathways were clearly present between the hydroxyl radical and the chlorinated phenoxide ions. One of the reaction pathways was suppressed when the concentration of chlorinated phenoxide ions was increased 10-fold. Amid a greater electron-withdrawing presence on the aromatic ring (higher chlorinated phenoxide ions), the hydroxyl radical reacted preferably by way of addition to the aromatic ring. Steady-state experiments utilizing gamma radiolysis also showed a substantial decrease in oxidation with an increase in pH of substrate.     

Reactions of synthetic phenolic antioxidants with electrogenerated titrants and their analytical applications

Synthetic phenolic compounds (pyrogallol, catechol, hydroquinone, and their derivatives bearing heterocyclic fragments) react with electrogenerated titrants, halogens, and ferricyanide(III) ions. Stoichiometric coefficients of reactions are found. It is shown that the use of ferricyanide(III) ions as a titrant and a one-electron oxidant for the determination of this class of antioxidants offers advantages in comparison with titrants-halogens. The found amounts of pyrogallol, pyrocatechol, and hydroquinone derivatives in model solutions with the RSD 1–5% are fractions of milligrams. It is found that, in the series of the studied synthetic phenolic compounds, pyrocatechol derivatives possess the maximum antioxidant capacity (AOC).     

A multi-scale assessment of Pb(II) sorption on dolomite

Macroscopic sorption studies indicated that Pb sorption capacity was independent of pH over the pH range 5-7, while sorption as a function of reaction time up to two weeks for systems with no bulk precipitate phases showed continuous Pb uptake on dolomite. This could be due to diffusion of Pb into the micropores of dolomite as well as an increase in surface sites caused by particle size reduction during suspension mixing. Normalized XANES spectra for systems undersaturated with respect to Pb carbonate precipitates resembled the spectrum of Pb4(OH)4(4+), suggesting that Pb is mainly coordinated to dolomite as an inner-sphere surface complex. On the other hand, the XANES spectrum for 10(-3) M Pb at 1 atm CO2(g) in a 2 M Mg(NO3)2 background electrolyte solution resembled that of cerussite, while a sample at 5 x 10(-4) M Pb in equilibrium with air and 2 M Mg(NO3)2 resembled that of hydrocerussite. EXAFS analyses of sorption samples in chloride solutions showed that there were only first-shell contributions under 1 atm CO2(g), while higher shell contributions from Ca/Mg were seen at 10(-3.42) atm CO2(g). On the other hand, EXAFS samples prepared in nitrate solutions showed noticeable differences in speciation under different reaction conditions-from outer-sphere surface complexes at low Pb concentrations and pH, to inner-sphere surface complexes at moderate Pb concentrations and neutral pH, to the formation of Pb carbonate precipitates at the highest Pb loadings.     

Thermal reactions of guaiacol and syringol as lignin model aromatic nuclei

Thermal reactions of guaiacol (2-methoxyphenol) and syringol (2,6-dimethoxyphenol) were compared in a closed ampoule reactor (N₂/400-600 °C/40-600 s) to obtain information on the thermal reactivities of lignin aromatic nuclei, guaiacyl and syringyl types. For both compounds, the O-CH₃ bond homolysis, which was observed at >400 °C, initiated their decomposition. This homolysis was followed by several temperature-dependent reactions; radical-induced rearrangement to convert the aromatic OCH₃ to aromatic CH₃ structures and condensation into high molecular weight (MW) products were the next steps (≈400 °C); then, coke formation became extensive (≈450 °C); effective gas formation required higher temperature such as >550-600 °C. The syringol- and guaiacol-characteristic GC/MS-detectable low MW products were explained with the above mentioned reactions. As for the difference between guaiacol and syringol, coke and gas (especially CH₄ and CO₂) formation was more extensive in syringol. This effective coking can be explained by the influence of the additional OCH₃ group in syringol, which doubles the opportunity for coke formation. This, in turn, reduces the yields of GC/MS-detectable low MW products. Demethoxylation to form guaiacol was also observed in syringol, even though the reactivity was not so high. These reactions are discussed at the molecular level.     

Titania sol–gel-derived tyrosinase-based amperometric biosensor for determination of phenolic compounds in water samples. Examination of interference effects

For detection of phenolic compounds in environmental water samples we propose an amperometric biosensor based on tyrosinase immobilized in titania sol-gel. The analytical characteristics toward catechol, p-cresol, phenol, p-chlorophenol, and p-methylcatechol were determined. The linear range for catechol determination was 2.2 x 10(-7)-1.3 x 10(-5) mol L(-1) with a limit of detection of 9 x 10(-8) mol L(-1) and sensitivity 2.0 x 10(3) mA mol(-1) L. The influence of sample matrix components on the electrode response was studied according to Plackett-Burman experimental design. The potential interferents Mg(2+), Ca(2+), HCO3(-), SO4(2-), and Cl(-), which are usually encountered in waters, were taken into account in the examination. Cu(2+) was also taken into account, because CuSO(4) is sometimes added to a water sample, as a preservative, before determination of phenolic compounds. It was found that among the ions tested only Mg(2+) and Ca(2+) did not directly affect the electrode response. The developed biosensor was used for determination of catechol in spring and surface water samples using the standard addition method.     

Tandem mass spectrometry of metal nitrate negative ions produced by electrospray ionization

M(NO(3))(x)(-) ions are generated by electrospray ionization (ESI) of metal solutions in nitric acid in negative ion mode. Collision-induced reactions of these ions are monitored in a tandem mass spectrometer (MS) of quadrupole-octopole-quadrupole (QoQ) geometry. For Group 1 and 2 elements, the M(NO(3))(x)(-) ions dissociate into NO(3)(-) and neutral metal nitrate molecules. These elements also form some M(x)(NO(3))x+1- clusters, especially Li(+). Metal nitrate ions from transition elements and Group 13 elements fragment into oxo products and also undergo internal electron transfer to leave the M atom in a lower oxidation state. To calibrate the collision energy, the dissociation energy of O-NO(2)(-) is found to be 5.55 eV, about 0.76 eV lower than a value derived from thermochemistry. The product ions from Fe(NO(3))(4)(-) ions have low formation thresholds of only 0.5 to 2 eV.     

Simplex optimization of the synergic extraction of a bis-diketo copper(II) complex

Simplex optimization is used to maximize the synergic extraction of copper(II) acetylacetonate into cyclohexane through formation of the isoquinoline adduct. Concentrations of isoquinoline and acetylacetone and pH were optimized for 1 × 10^-4 M copper(II) solutions. Distribution coefficients of 0.25 were obtained above pH 7.9 for acetylacetone and isoquinoline concentrations above 3.0 × 10^-4 M and 2.0 × 10^-4 M, respectively.     

Tropospheric multiphase chemistry of 2,5- and 2,6-dimethylphenols: determination of the mass accommodation coefficients and the Henry's law constants

The uptake of 2,5-dimethylphenol and 2,6-dimethylphenol on aqueous surfaces was measured between 279 and 293 K, using the wetted-wall flow tube technique coupled with UV absorption spectroscopic detection. For both compounds, the uptake coefficients gamma were found to be independent of the KOH scavenger concentration in the range of 0.01 to 1 M (pH > pK(a)) and of the liquid-gas contact times. In addition, the uptake coefficients and the derived mass accommodation coefficients alpha show a negative temperature dependence in the investigated temperature range. The mass accommodation coefficients decrease from 1.1 x 10(-3) to 1.1 x 10(-4), and from 5.4 x 10(-4) to 6.4 x 10(-5) for 2,5-dimethylphenol and 2,6-dimethylphenol, respectively. These results are used to discuss the incorporation of these species into the liquid using the nucleation theory. Henry's law constants (HLC) of both compounds were directly measured using a dynamic equilibrium system based on the water/air equilibrium at the interface within the length of a microporous tube. The measurements were conducted over the range 278-293 K in both deionized water and 35 g L(-1) solution of NaCl. At 293 K and in pure water, HLC were found to be equal to (in units of M atm(-1)): 2,5-dimethylphenol, HLC = (1270 +/- 240); 2,6-dimethylphenol, HLC = (250 +/- 80). All of the values for HLC in 35 g L(-1) salt solution were 5-55% lower than the corresponding values in deionized water, depending on the compound and the temperature. These data (mass accommodation coefficients and Henry's law constants) were then used to estimate the partitioning of these phenolic compounds between gaseous and aqueous phases and the corresponding atmospheric lifetimes under clear sky (tau(gas)) and cloudy conditions (tau(multiphase)) have then been derived. The calculated multiphase lifetimes (in units of hours) are lower than those in gas phase at a cumulus temperature of 283 K (in parentheses): 2,5-dimethylphenol, 2.2 (3.5); 2,6-dimethylphenol, 3.8 (4.2).     

Phosphate adsorption on synthetic goethite and akaganeite

Low crystalline iron hydroxides such as goethite (alpha-FeOOH) and akaganeite (beta-FeOOH) were synthesized, and the selective adsorption of phosphate ions from phosphate-enriched seawater was examined. The results of the distribution coefficients (K(d)) of oxoanions in mixed anion solutions at pH 8 follow the selectivity order Cl-, NO3-, SO4(2-) < CO3(2-), HPO4(2-) for goethite, and Cl-, CO3(2-) < NO3- < SO4(2) < HPO4(2-) for akaganeite. In seawater, both adsorbents show high selectivity for phosphate ions despite the presence of large amounts of major cations and anions in seawater. The adsorption isotherms fitted better with the Freundlich equation and the maximum uptake of phosphate from phosphate-enriched seawater was 10 mg P/g at an equilibrium phosphate concentration of 0.3 mg P/L on both adsorbents. The phosphate adsorption/desorption cycles show that akaganeite is an excellent adsorbent even after 10 cycles and its chemical stability is good.     

Mechanism of the NO2 conversion to NO2- in an alkaline solution.

The reaction of NO2 and NaOH aqueous solution at room temperature was studied for elucidating the behavior of gaseous NO2 in an alkaline solution. Experimental runs related to NO2 absorption have been carried out in various pH solutions. The nitrite and nitrate ions formed in these absorption solutions were quantitatively analyzed. In the case of pH 5-12, both of the nitrite and nitrate ions were formed simultaneously. On the other hand, only the nitrite ion was formed when the pH of the absorption solution was higher than 13. In this paper, a new reaction mechanism was proposed to explain the selective formation of nitrite ion in the 10 M alkaline solution. In order to confirm the new reaction mechanism, H2(18)O was used as part of the absorption solution for detecting oxygen gas production. The amounts of reaction products: (18)O(18)O, (18)O(16)O and (16)O(16)O, were quantitatively determined. It was confirmed that the new reaction proceeds mainly in the 10 M alkaline solution.     

Electrospray ionisation mass spectrometric study of degradation products of quercetin, quercetin-3-glucoside and quercetin-3-rhamnoglucoside, produced by in vitro fermentation with human faecal flora

Eight phenolic compounds, obtained by in vitro fermentation of quercetin, quercetin-3-glucoside and quercetin-3-rhamnoglucoside were analysed by electrospray ionisation mass spectrometry (ESI-MS). Low-energy collision-induced dissociation tandem mass spectrometry (CID-MS/MS) was performed on the [M - H]- precursor ions to obtain specific fragmentation. Typical fragmentation of the phenolic acids was loss of 44 (CO2) and 18 (H2O) u. Production of m/z 108 by loss of neutral radicals, e.g. HCO2, CH3 or HCO, was also favoured. Structures of the compounds, numbered 1-8, were suggested based on the fragmentation patterns.     

Ferrocenyl catechols: synthesis, oxidation chemistry and anti-proliferative effects on MDA-MB-231 breast cancer cells

The synthesis and anti-tumoral properties of a series of compounds possessing a ferrocenyl group tethered to a catechol via a conjugated system is presented. On MDA-MB-231 breast cancer cell lines, the catechol compounds display a similar or greater anti-proliferative potency (IC(50) values ranging from 0.48-1.21 μM) than their corresponding phenolic analogues (0.57-12.7 μM), with the highest activity found for species incorporating the [3]ferrocenophane motif. On the electrochemical timescale, phenolic compounds appear to oxidize to the quinone methide, while catechol moieties form the o-quinone by a similar mechanism. Chemical oxidation of selected compounds with Ag(2)O confirms this interpretation and demonstrates the probable involvement of such oxidative metabolites in the in vitro activity of these species.     

Computational study on the molecular conformations of phenolic compounds

The study of the conformation of eight phenolic compounds (i.e. phenol, guaiacol, syringol, pyrocatechol, o-, m-, p-cresol and vanillin) was carried out using B3LYP (Becke, 3-parameter, Lee-Yan-Parr) method and cc-pVTZ (correlation consistent polarized polarisation Valence triple zêta) basis set. Potential energy surfaces were plotted in order to identify the minimum energy structures and the transition states. For the first time, structures of vanillin and syringol were completely determined. Moreover, our work also confirms that the most stable conformations of m-cresol and p-cresol are obtained when the methyl group presents a hydrogen atom at the perpendicular of the benzene ring. The possibility of intramolecular hydrogen bonding was also investigated. Guaiacol, pyrocatechol, syringol and vanillin present a structure stabilized by the presence of a moderate hydrogen bond. Finally, potential energy barriers and rotational potential functions were calculated. The comparison of the rotational potential parameters for each molecule highlights the presence of similarities specific to each substituent and their neighbours.     

Even-electron ions: a systematic study of the neutral species lost in the dissociation of quasi-molecular ions

The collision-induced dissociations of the even-electron [M + H](+) and/or [M - H](-) ions of 121 model compounds (mainly small aromatic compounds with one to three functional groups) ionized by electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) have been studied using an ion trap instrument, and the results are compared with the literature data. While some functional groups (such as COOH, COOCH(3), SO(3)H in the negative ion mode, or NO(2) in both the positive and negative ion modes) generally promote the loss of neutrals that are characteristic as well as specific, other functional groups (such as COOH in the positive ion mode) give rise to the loss of neutrals that are characteristic, but not specific. Finally, functional groups such as OH and NH(2) in aromatic compounds do not lead to the loss of a neutral that reflects the presence of these substituents. In general, the dissociation of [M + H](+) and [M - H](-) ions generated from aliphatic compounds or compounds containing an aliphatic moiety obeys the even-electron rule (loss of a molecule), but deviations from this rule (loss of a radical) are sometimes observed for aromatic compounds, in particular for nitroaromatic compounds. Thermochemical data and ab initio calculations at the CBS-QB3 level of theory provide an explanation for these exceptions. When comparing the dissociation behaviour of the even-electron [M + H](+) and/or [M - H](-) ions (generated by ESI or APCI) with that of the corresponding odd-electron [M](+) ions (generated by electron ionization, EI), three cases may be distinguished: (1) the dissociation of the two ionic species differs completely; (2) the dissociation involves the loss of a common neutral, yielding product ions differing in mass by one Da, or (3) the dissociations lead to a common product ion.     

New insights into the acid-promoted reaction of caffeic acid and its esters with nitrite: decarboxylation drives chain nitrosation pathways toward novel oxime derivatives and oxidation/fragmentation products thereof

In 0.05 M acetate buffer, pH 4, containing 1% methanol, caffeic acid (1a) (2 x 10(-3) M) reacted smoothly with nitrite (NO(2)(-)) (4 x 10(-3) M) to afford as main products the novel 2-hydroxy- and 2-methoxyaldoximes 7a,b, the 2-oxoaldoxime 9a, 3,4-dihydroxybenzoic acid, 3,4-dihydroxybenzaldehyde, and the known furoxan 3c and benzoxazinone 4b in smaller amounts. At lower 1a concentration (e.g., 1 x 10(-4) M), 7a was the main product, whereas with 0.1 M 1a and 0.5 M NO(2)(-) 3c and 9a were prevailing. At pH 2, 7a was still the most abundant product, together with 3,4-dihydroxybenzaldehyde and some 9a, whereas at pH 1 9a and 3,4-dihydroxybenzaldehyde were formed in higher yields. No evidence for ring nitration products, including the previously reported 4,5-dihydroxy-2-nitrobenzaldehyde, was obtained. At 2 x 10(-3) M concentration and at pH 4, caffeic acid methyl ester (1b) reacted with NO(2)(-) chiefly via ring nitration and/or dimerization to give 5a, the novel nitrated neolignan derivative 10, and the parent 6. Chlorogenic acid (1c) afforded only the ring nitrated derivative 5b. A unifying mechanism for the reaction of 1a and its esters with NO(2)(-) is proposed involving reversible formation of nitroso intermediates via chain nitrosation at the 2-position of the (E)-3-(3,4-dihydroxyphenyl)propenoic system. In the case of 1a, decarboxylation would drive the nitroso intermediates toward the formation of oximes 7a,b and 3c, reflecting nucleophilic addition of water, methanol, and NO(2)(-), and their oxidation or breakdown products, viz. 9a, 3,4-dihydroxybenzaldehyde, 3,4-dihydroxybenzoic acid, and the benzoxazinone 4b. In the case of esters 1b,c, to which decarboxylation is precluded, ring nitration or dimerization become the favored routes, triggered by preliminary oxidation at the catechol moiety.     

Coated-wire silver ion-selective electrode based on silver complex of cyclam.

A coated-wire ion-selective electrode (ISE) based on cyclam (1,4,8,11-tetraazacyclotetradecane) as a neutral carrier in a polyvinyl chloride (PVC) matrix was fabricated for the determination of Ag(I) ions. The coated-wire ISE exhibited a linear Nernstian response over the range 1 x 10(-1) to 1 x 10(-7) M with a slope of 59 +/- 2 mV per decade change and a detection limit of 5 x 10(-8) M. The ISE shows a greater preference for Ag over other cations with good precision. The electrode was selective towards Ag(I) ions in the presence of 13 different metal ions tested. The selectivity coefficients (K(ij)) were determined for Na(I), K(I), Mg(II), Ca(II), Ba(II), Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Pb(II) and Hg(II). The selectivity coefficients of these cations are in the range of 10(-4) to 10(-2). This ISE was used for the determination of free silver and total silver in electroplating bath solutions, additives and brighteners.     

Liquid-membrane dicyanoargentate-sensitive electrodes based on quaternary ammonium salts

Several quaternary ammonium salts have been synthesized and comparisons made of their efficiency as exchange substrates in dicyanoargentate-sensitive electrodes. The electrode prepared from hexadecyltrioctylammonium dicyanoargentate shows the best performance characteristics and has been studied in more detail. Its Nernstian response range is 10(-1)-10(-4)M, the optimum pH range is 10.6-12.6, and the detection limit is 3 x 10(-5)M. It is suggested that the detection limit of the electrode is controlled by interference from cyanide ions, and the difference between detection limits obtained with the various electrodes is discussed in the light of this. Selectivity coefficients for various interfering ions have been determined and related to the extractability of the ions. The electrode has been applied to determination of silver in cyanide-containing plating solutions.     

Photoinduced electron transfer in naphthalimide-pyridine systems: effect of proton transfer on charge recombination efficiencies

We studied the effect of proton-coupled electron transfer on lifetimes of the charge-separated radicals produced upon light irradiation of the thiomethyl-naphthalimide donor SMe-NI-H in the presence of nitro-cyano-pyridine acceptor (NO(2)-CN-PYR). The dynamics of electron and proton transfer were studied using femtosecond pump-probe spectroscopy in the UV/vis range. We find that the photoinduced electron transfer between excited SMe-NI-H and NO(2)-CN-PYR occurs with a rate of 1.1 × 10(9) s(-1) to produce radical ions SMe-NI-H(?+) and NO(2)-CN-PYR(?-). These initially produced radical ions in a solvent cage do not undergo a proton transfer, possibly due to unfavorable geometry between N-H proton of the naphthalimide and aromatic N-atom of the pyridine. Some of the radical ions in the solvent cage recombine with a rate of 2.3 × 10(10) s(-1), while some escape the solvent cage and recombine at a lower rate (k = 4.27 × 10(8) s(-1)). The radical ions that escape the solvent cage undergo proton transfer to produce neutral radicals SMe-NI(?) and NO(2)-CN-PYR-H(?). Because neutral radicals are not attracted to each other by electrostatic interactions, their recombination is slower that the recombination of the radical ions formed in model compounds that can undergo only electron transfer (SMe-NI-Me and NO(2)-CN-PYR, k = 1.2 × 10(9) s(-1)). The results of our study demonstrate that proton-coupled electron transfer can be used as an efficient method to achieve long-lived charge separation in light-driven processes.