UV–visible and synchronous fluorescence spectroscopic investigations of the complexation of Al(III) with caffeic acid, in aqueous low acidic medium

Effect of Cu2+ and Al3+ on the interaction of chlorogenic acid and caffeic acid with serum albumin

Despite the phenolic acids' health benefits, their interactions with proteins are still unclear. In this study, the interactions of Bovine Serum Albumin (BSA) with chlorogenic acid (CHA), caffeic acid (CA), and their Al³⁺, Cu²⁺ complexes were studied by using circular dichroism (CD) spectroscopy, fluorescence spectroscopy, and UV/Vis spectroscopy. It was found that esterification of carboxyl group of CA with quinic acid increased the binding affinities for BSA. After chelating with Cu²⁺ and Al³⁺, both CHA and CA exhibited high binding affinities for BSA. CHA could form CHA-Cu₂ and CHA-Al₂ complex with Cu²⁺ and Al³⁺. The result of CD spectroscopy demonstrated that the binding of CHA and Al³⁺ with BSA contributed to the folding of BSA secondary structure. In addition, with the presence of CHA, binding with Al³⁺ could also induce changes in BSA conformation. The binding sites of both CHA and CA were closed to Trp213.

Synthesis of Hybrid Polyphenol/Hydroxyapatite Nanomaterials with Anti-Radical Properties

Plant-derived natural bioactive molecules are of great therapeutic potential but, so far, their application in nanomedicine has scarcely been studied. This work aimed at comparing two methodologies, i.e., adsorption and in situ incorporation, to prepare hybrid polyphenol/hydroxyapatite nanoparticles. Two flavonoids, baicalin and its aglycone derivative baicalein, and two phenolic acids derived from caffeic acid, rosmarinic and chlorogenic acids, were studied. Adsorption of these polyphenols on pre-formed hydroxyapatite nanoparticles did not modify particle size or shape and loading was less than 10% (w/w). In contrast, presence of polyphenols during the synthesis of nanoparticles significantly impacted and sometimes fully inhibited hydroxyapatite formation but recovered particles could exhibit higher loadings. For most hybrid particles, release profiles consisted of a 24 h burst effect followed by a slow release over 2 weeks. Antioxidant properties of the polyphenols were preserved after adsorption but not when incorporated in situ. These results provide fruitful clues for the valorization of natural bioactive molecules in nanomedicine.

The role of ion-lipid interactions and lipid packing in transient defects caused by phenolic compounds

The transient disruption of membranes for the passive permeation of ions or small molecules is a complex process relevant to understanding physiological processes and biotechnology applications. Phenolic compounds are widely studied for their antioxidant and antimicrobial properties, and some of these activities are based on the interactions of the phenolic compound with membranes. Ions are ubiquitous in cells and are known to alter the structure of phospholipid bilayers. Yet, ion-lipid interactions are usually ignored when studying the membrane-altering properties of phenolic compounds. This study aims to assess the role of Ca2+ ions on the membrane-disrupting activity of two phenolic acids and to highlight the role of local changes in lipid packing in forming transient defects or pores. Results from tethered bilayer lipid membrane electrical impedance spectroscopy experiments showed that Ca2+ significantly reduces membrane disruption by caffeic acid methyl ester and caffeic acid. As phenolic acids are known metal chelators, we used UV-vis and fluorescence spectroscopy to exclude the possibility that Ca2+ interferes with membrane disruption by binding to the phenolic compound and subsequently preventing membrane binding. Molecular dynamics simulations showed that Ca2+ but not caffeic acid methyl ester or caffeic acid increases lipid packing in POPC bilayers. The combined data confirm that Ca2+ reduces the membrane-disrupting activity of the phenolic compounds, and that Ca2+-induced changes to lipid packing govern this effect. We discuss our data in the context of ion-induced pores and transient defects and how lipid packing affects membrane disruption by small molecules.

Caffeic Acid/Eu(III) Complexes: Solution Equilibrium Studies, Structure Characterization and Biological Activity

Caffeic acid (CFA) is one of the various natural antioxidants and chemoprotective agents occurring in the human diet. In addition, its metal complexes play fundamental roles in biological systems. Nevertheless, research on the properties of CFA with lanthanide metals is very scarce, and little to no chemical or biological information is known about these particular systems. Most of their properties, including their biological activity and environmental impact, strictly depend on their structure, stability, and solution behaviour. In this work, a multi-analytical-technique approach was used to study these relationships for the Eu(III)/CFA complex. The synthesized metal complex was studied by FT-IR, FT-Raman, elemental, and thermal (TGA) analysis. In order to examine the chemical speciation of the Eu(III)/CFA system in an aqueous solution, several independent potentiometric and spectrophotometric UV-Vis titrations were performed at different M:L (metal:ligand) and pH ratios. The general molecular formula of the synthesized metal complex in the solid state was [Eu(CFA)₃(H₂O)₃]∙2H₂O (M:L ratio 1:3), while in aqueous solution the 1:1 species were observed at the optimum pH of 6 ≤ pH ≤ 10, ([Eu(CFA)] and [Eu(CFA)(OH)]⁻). These results were confirmed by ¹H-NMR experiments and electrospray-ionization mass spectrometry (ESI-MS). To evaluate the interaction of Eu(III)/CFA and CFA alone with cell membranes, electrophoretic mobility assays were used. Various antioxidant tests have shown that Eu(III)/CFA exhibits lower antioxidant activity than the free CFA ligand. In addition, the antimicrobial properties of Eu(III)/CFA and CFA against Escherichia coli, Bacillus subtilis and Candida albicans were investigated by evaluation of the minimum inhibitory concentration (MIC). Eu(III)/CFA shows higher antibacterial activity against bacteria compared to CFA, which can be explained by the highly probable increased lipophilicity of the Eu(III) complex.

Strategies used by nature to fix the red, purple and blue colours in plants: a physical chemistry approach

While identified by the respective flavylium cation, anthocyanins are much more than this molecule. The flavylium cation (generally appearing only at very acidic pH values) is one of the molecules of a complex sequence of pH dependent molecular species reversibly interconnected by different chemical reactions. These species include the red flavylium cation, purple quinoidal base and blue or bluish anionic quinoidal bases. At the common pH of the vacuoles of simpler anthocyanins, the red flavylium cation is present only at very acidic pH values and at moderately acidic pHs there is no significant colour of the purple quinoidal base. Moreover, the blue or bluish anionic quinoidal base appearing around neutral pH values is not stable. Intermolecular (copigmentation) and intramolecular (in acylated anthocyanins) interactions increase the colour hue and yield bathochromic shifts in the absorption bands, permitting to extend the pH domain of the flavylium cation and increase the mole fraction of the quinoidal bases. Metal complexation is another strategy. In particular, the Al³⁺ cation plays an essential role in the blue colour of hydrangea. The most sophisticated structures are however the metaloanthocyanins, such as the one that gives the blue colour of commelina communis, constituted of six anthocyanins, six flavanones and two metals. In this work we discuss how physical chemical tools are indispensable to account for the chemical behaviour of these complex systems. The experimental procedures and the equations needed to calculate all equilibrium constants of anthocyanins and the consequent pH dependent mole fraction distributions in the absence or presence of copigments are described in detail. Reverse pH jumps monitored by stopped flow have been shown to be an indispensable tool to calculate these parameters.

Phenolic compounds alter the ion permeability of phospholipid bilayers via specific lipid interactions

This study aims to understand the role of specific phenolic-lipid interactions in the membrane-altering properties of phenolic compounds. We combine tethered lipid bilayer (tBLM) electrical impedance spectroscopy (EIS) with all-atom molecular dynamics (MD) simulations to study the membrane interactions of six phenolic compounds: caffeic acid methyl ester, caffeic acid, 3,4 dihydroxybenzoic acid, chlorogenic acid, syringic acid and p-coumaric acid. tBLM/EIS experiments showed that caffeic acid methyl ester, caffeic acid and 3,4 dihydroxybenzoic acid significantly increase the permeability of phospholipid bilayers to Na⁺ ions. In contrast, chlorogenic acid, syringic acid and p-coumaric acid showed no effect. Experiments with lipids lacking the phosphate group show a significant decrease in the membrane-altering effects indicating that specific phenolic-lipid interactions are critical in altering ion permeability. MD simulations confirm that compounds that alter ion permeability form stable interactions with the phosphate oxygen. In contrast, inactive phenolic compounds are superficially bound to the membrane surface and primarily interact with interfacial water. Our combined results show that compounds with similar structures can have very different effects on ion permeability in membranes. These effects are governed by specific interactions at the water-lipid interface and show no correlation with lipophilicity. Furthermore, none of the compounds alter the overall structure of the phospholipid bilayer as determined by area per lipid and order parameters. Based on data from this study and previous findings, we propose that phenolic compounds can alter membrane ion permeability by causing local changes in lipid packing that subsequently reduce the energy barrier for ion-induced pores.

Novel nano-engineered environmental sensor based on polymelamine/graphitic-carbon nitride nanohybrid material for sensitive and simultaneous monitoring of toxic heavy metals

Heavy metal ions (HMIs) pollution is always a serious issue worldwide. Therefore, monitoring HMIs in environmental water is an important and challenging step to ensure environmental health and human safety. In this study, we spotlight an effortless, single-step in-situ electrochemical polymerization deposition technique to fabricate a novel, low-cost, efficient, nano-engineered poly(melamine)/graphitic-carbon nitride nanonetwork (PM/g-C₃N₄) modified screen-printed carbon electrode (SPE) for sensitive, selective, and simultaneous electrochemical monitoring of toxic HMIs in environmental waters. g-C₃N₄ nanomaterial was prepared using melamine as a precursor via pyrolysis technique. As-prepared g-C₃N₄ and melamine monomer were electrochemically in-situ polymerized/deposited over pre-anodized SPE (ASPE) using cyclic voltammetry technique. XRD, XPS, and SEM were engaged to characterize the developed electrode. The fabricated PM/g-C₃N₄/ASPE was applied as an environmental sensor to selective and simultaneous electrochemical detection of Pb²⁺ and Cd²⁺ ions using differential pulse voltammetry technique. The developed sensor displayed excellent selectivity and sensitivity towards Pb²⁺ and Cd²⁺ with limit of detections of 0.008 µM and 0.02 µM, respectively. The fabricated PM/g-C₃N₄/ASPE sensor exhibits superior stability, repeatability, good anti-interference, and applicability for recognition of Pb²⁺ and Cd²⁺ ions in real water samples. These results proved that developed environmental sensor is low-cost, efficient, practical platform for rapid, selective, simultaneous monitoring of HMIs in the environment.

Using modulated current for energy minimization in the electrochemical treatment of effluents containing organic pollutants

Anodic oxidation of recalcitrant organic compounds suffers from loss of efficiency as the concentration decreases, leading to high energy consumption. Here, we propose a modulated current (MC) technique to control and maintain the applied current as close as possible to its limiting value throughout the electrolysis, thus ensuring high mineralization current efficiency. The efficacy of this technique was first validated for caffeic acid (CA) electrooxidation using a boron-doped diamond (BDD) anode and was then confirmed for the degradation of a wastewater containing phenolic compounds from wet coffee processing. Combining MC and constant current (CC) operation for CA electrolysis resulted in a substantial reduction of the specific energy consumption from 256 to 52.4 kWh kg^-1 TOC, due to improvement of the mineralization current efficiency from 17.9 to 77.1%. The MC+CC technique was also successful in reducing the energy consumption for a real coffee processing wastewater mineralization, demonstrating its suitability as a simple and effective tool that can be used to reduce the energy costs in electrochemical treatment of effluents containing organic pollutants.

Effect of biochar addition on the removal of organic and nitrogen pollutants from leachate treated with a semi-aerobic aged refuse biofilter

The effect of biochar on the removal of organic and nitrogen contaminants from leachate in a semi-aerobic aged refuse biofilter (SAARB) was investigated. A preset amount of biochar was mixed with the aged refuse to explore the enhancement ability of pollutant removal by characterizing the leachate effluent and gas. The results showed that biochar contributed to the removal of organic and nitrogen pollutants from the leachate and that increasing the amount of biochar added led to higher colour number, chemical oxygen demand, ammonia nitrogen, and total nitrogen removal efficiencies. Furthermore, the addition of biochar significantly increased the removal of large molecule organic pollutants from the leachate. The improved removal of organics was due to the considerable number of surface functional groups and the large surface area of the biochar, which effectively absorbed and removed a significant amount of the organic matter from the leachate. Biochar elevated the dissolved oxygen concentration in the semi-aerobic system, which facilitated the completion of the nitrification reaction. It also promoted denitrification by acting as a supplementary carbon source. The nitrous oxide (N₂O) emissions decreased as the amount of biochar added increased. When the biochar proportion reached 3%, the N₂O emission was only 1.11% of the original total nitrogen and the di-nitrogen emission was 19.61%. The findings of this study can be used to improve the treatment of leachate using biochar combined with a SAARB.

Antioxidant Packaging Films Based on Ethylene Vinyl Alcohol Copolymer (EVOH) and Caffeic Acid

The main objective of this research activity was to design and realize active films with tunable food functional properties. In detail, caffeic acid (CA), a polyphenol with high antioxidant effect, was used as active ingredient in poly (vinyl alcohol-co-ethylene) (EVOH) films at 5 wt.% and 15 wt.% and successfully realized by means of the solvent casting process. Optical, morphological, thermal and mechanical studies were considered to define the effect of the presence of the CA component on the structural properties of the matrix. In addition, moisture content and antioxidant activity were evaluated, to have clear information on the CA effect in terms of functional characteristics of realized food packaging systems. Results from tensile tests showed increased values for strength and deformation at break in EVOH_CA based films. Results from colorimetric and transparency analysis underlined that the presence of caffeic acid in EVOH copolymer induces some alterations, whereas the addition of the active ingredient determined a positive radical scavenging activity of the formulations, confirming the possibility of practically using these polymeric systems in the food packaging sector.

Electrocatalytic determination of Cd2+ and Pb2+ using an l-cysteine tungstophosphate self-assembled monolayer on a polycrystalline gold electrode

Constructing a sensitive electrochemical sensor based on (Cys)PW for Cd²⁺ and Pb²⁺ detection at the nanomolar level with remarkable selectivity.

The effective determination of Cd(ii) and Pb(ii) simultaneously based on an aluminum silicon carbide-reduced graphene oxide nanocomposite electrode

A platform for the simultaneous determination of Cd(ii) and Pb(ii) in aqueous solution has been applied based on an aluminum silicon carbide-reduced graphene oxide nanocomposite (Al₄SiC₄-RGO) modified bismuth film glassy carbon electrode (GCE) using square wave anodic stripping voltammetry (SWASV) for the first time. The Al₄SiC₄-RGO nanocomposite electrode was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Compared with the Al₄SiC₄ modified GCE and bare GCE, the electrochemical performance of the Al₄SiC₄-RGO nanocomposite electrode is obviously enhanced resulting from the synergistic effects of Al₄SiC₄, RGO and bismuth film. The chemical and electrochemical parameters that exert an influence on the deposition and stripping of metal ions, such as supporting electrolytes, pH values, concentrations of Bi³⁺, deposition potentials and deposition times, were carefully studied. Under optimal conditions, a linear relationship exists between the currents and the concentrations of Cd(ii) and Pb(ii) in the range of 50 to 2700 μg L^-1. The limits of detection (S/N = 3) are estimated to be 1.30 μg L^-1 for Pb(ii) and 2.15 μg L^-1 for Cd(ii). Compared with the related work reported in the literature, the analytical performance in this work has a lower determination limit and a wider detection linear range. In addition, this electrode also exhibits good stability and reproducibility. These results imply that the Al₄SiC₄-RGO nanocomposite might be a promising candidate for practical applications in the electrochemical detection of metal ions.

The study of the influence of Mg(ii) and Ca(ii) ions on caffeic acid autoxidation in weakly alkaline aqueous solution using MCR-ALS analysis of spectrophotometric data

UV-Vis spectrophotometry with MCR-ALS analysis was applied to study the influence of Mg(ii) and Ca(ii) ions on caffeic acid autoxidation.

Sensitive determination of trace Cd(ii) and Pb(ii) in soil by an improved stripping voltammetry method using two different in situ plated bismuth-film electrodes based on a novel electrochemical measurement system

In this study, a simple but effective electrochemical method was developed to minimize the interference from real soil samples and increase the sensitivity of Pb(ii) and Cd(ii) detection by square-wave anodic stripping voltammetry (SWASV) using a novel electrochemical measurement system, which can be used for the on-site determination of trace Cd(ii) and Pb(ii) in real soil samples. The method involved performing SWASV following double deposition and stripping steps at two in situ plated bismuth-film electrodes with drastically different surface properties. Pb(ii) and Cd(ii) were first deposited on an in situ plated bismuth-film graphite carbon paste electrode (Bi/GCPE). When the first deposition was finished, the GCPE was moved to a micro-electrolytic cell to perform the first stripping step. The following measurements were performed with the other deposition and stripping steps using a highly sensitive in situ plated bismuth-film multiwalled carbon nanotube–Nafion composite modified glassy carbon electrode (Bi/MWCNT–Nafion/GCE) as the working electrode. Pb(ii), Cd(ii) and Bi(iii) stripped from the GCPE in the micro-electrolytic cell were partially deposited on the MWCNT–Nafion/GCE, and the stripping current signals were obtained from their oxidation during the second stripping step. Considering the small volume of the micro-electrolytic cell, the concentrations of Cd(ii) and Pb(ii) were drastically higher than those in the bulk solution, and therefore, the detection limits were reduced. Under the optimized conditions, the concentrations in the linear range spanned from 1.0 to 45.0 μg L⁻¹ for both Pb(ii) and Cd(ii), with a detection limit of 0.03 μg L⁻¹ for Pb(ii) and 0.02 μg L⁻¹ for Cd(ii) (S/N = 3). Finally, analyses of real samples were performed to detect trace levels of Pb(ii) and Cd(ii) in soil with satisfactory results.

A double-stripping voltammetry method was designed and developed to improve the sensitivity and anti-interference ability for detection of heavy metals.

Complexation behaviour of caffeic, ferulic and p-coumaric acids towards aluminium cations: a combined experimental and theoretical approach

The formation of complexes of hydroxycinnamic acids with Al(iii) ions is strongly pH-dependent and the complexation occurs via a carboxylic function.

Highly sensitive and selective detection of cadmium with a graphite carbon nitride nanosheets/Nafion electrode

A sensitive g-C₃N₄ Cd²⁺ sensor is fabricated in this paper. The linear range and detection limit for Cd²⁺ detection could be as low as 1 nM and (0.5 nM). This sensor also has excellent stability and selectivity for Cd²⁺ against interferences.

Synthesis, chemical characterization, computational studies and biological activity of new DNA methyltransferases (DNMTs) specific inhibitor. Epigenetic regulation as a new and potential approach to cancer therapy

This work deals with the synthesis, the chemical characterization of dibutyltin(IV) complex of caffeic acid (Bu2Sn(IV)HCAF, caf1) and its cytotoxic action on tumor cells. The coordination environment at the tin center was investigated by FTIR, (119)Sn{(1)H} cross polarization magic angle spinning, electrospray ionization mass spectroscopy in the solid state and UV-vis, fluorescence and (1)H, (13)C and (119)Sn NMR spectroscopy in solution phases. Density functional theory study confirmed the proposed structures in solution phase and indicated the most probably stable conformation. The effects on viability of breast cancer MDA-MB231, colorectal cancer HCT116, hepatocellular carcinoma HepG2 and Chang liver cells, an immortalized non-tumor hepatic cell line, have been investigated. The effect of a variation in structure of caf1 was found to lead to a change in the respective antiproliferative properties: caf1 induces loss of viability in HCT116, MDA-MB-231, and HepG2; the complex shows only moderate effects in non-tumor Chang liver cells. caf1 exerts lower cytotoxic activity than Bu2SnCl2, suggesting that the binding with H3CAF modulates the marked cytotoxic activity exerted by Bu2SnCl2; caf1 displays a considerably more pronounced antitumoural effect towards cell lines than caffeic acid. It is known that caffeic acid can modulate DNA (cytosine-5)-methyltransferases 1 (DNMT1) mediated DNA methylation. In this paper we demonstrate that caf1 treatment was able to induce a time-dependent reduction of global DNA methylated status. This effect was also confirmed by a concomitant reduction DNMT1 expression level. The effect induced by caf1 was more evident not only with respect to untreated cells but also compared to H3CAF treated cells.

Degradation of hydroxycinnamic acid mixtures in aqueous sucrose solutions by the Fenton process

The degradation efficiencies and behaviors of caffeic acid (CaA), p-coumaric acid (pCoA), and ferulic acid (FeA) in aqueous sucrose solutions containing the mixture of these hydroxycinnamic acids (HCAs) were studied by the Fenton oxidation process. Central composite design and multiresponse surface methodology were used to evaluate and optimize the interactive effects of process parameters. Four quadratic polynomial models were developed for the degradation of each individual acid in the mixture and the total HCAs degraded. Sucrose was the most influential parameter that significantly affected the total amount of HCA degraded. Under the conditions studied there was a <0.01% loss of sucrose in all reactions. The optimal values of the process parameters for a 200 mg/L HCA mixture in water (pH 4.73, 25.15 °C) and sucrose solution (13 mass %, pH 5.39, 35.98 °C) were 77% and 57%, respectively. Regression analysis showed goodness of fit between the experimental results and the predicted values. The degradation behavior of CaA differed from those of pCoA and FeA, where further CaA degradation is observed at increasing sucrose and decreasing solution pH. The differences (established using UV/vis and ATR-FTIR spectroscopy) were because, unlike the other acids, CaA formed a complex with Fe(III) or with Fe(III) hydrogen-bonded to sucrose and coprecipitated with lepidocrocite, an iron oxyhydroxide.

Synthesis of micro sphere-like bismuth nanoparticles by microwave assisted polyol method; designing a novel electrochemical nanosensor for ultra-trace measurement of Pb2+ ions

Bi nanostructures were synthesized in the presence of propylene glycol as the solvent and surfactant through microwave method.

pH influence on the complexation site of Al(III) with protocatechuic acid. A spectroscopic and theoretical approach

Electronic spectroscopy techniques with the aid of quantum chemical calculations, and notably the Time-Dependent Density Functional Theory, can be used to probe the structure of metal complexes in solution. Here, we report the characterization of Al(III)-protocatechuate in aqueous solution, at pH=6.5. The exploitation of the UV-vis spectra of the system by chemometric methods highlights the formation of a single complex of stoichiometry 1:1. From different structural hypothesis, the comparison of theoretical and experimental spectra shows that Al(III) forms a monodentate complex with the carboxylate function. This hypothesis is confirmed by the calculation of the complexation reaction pathways. Previous studies report the formation of a chelate involving the ortho-dihydroxyl group, at pH=3.5. These results illustrate the important dependence of the protonation state of the carboxylic function on the Al(III) fixation site on the studied ligand.

Development of a novel cyano group containing electrochemically deposited polymer film for ultrasensitive simultaneous detection of trace level cadmium and lead

Poly(diphenylamine-co-2-aminobenzonitrile) (P(DPA-co-2ABN)), a cyano group containing conducting polyaniline derivative, has been electrodeposited developed as the new material and utilized for the simultaneous electrochemical determination of trace levels of cadmium (Cd(2+)) and lead (Pb(2+)). P(DPA-co-2ABN) film preconcentrates effectively through cyano chelation and electrochemically strips the heavy metal ions with well separated potentials, which are beneficially utilized for ppb level simultaneous detection of Cd(2+) and Pb(2+). Differential pulse voltammetry studies revealed that Cd(2+) and Pb(2+) ions were simultaneously stripped with well-defined, separated and sharp peaks for Cd(2+) and Pb(2+). The influence of various operational parameters such as pulse amplitude, pulse time, scan rate, initial potential, end potential, accumulation potential and accumulation time on the electrochemical stripping of heavy metals were investigated in details. Under the optimal conditions, good linear correlations were obtained from 1.26 to 907.8 ppm for Cd(2+) and 0.26 to 58.73 ppm for Pb(2+), respectively. Low detection limits for Cd(2+) and Pb(2+), 0.255 ppm and 0.165 ppm, respectively, were observed. The practical utility of the new procedure was demonstrated in real samples.

Revisiting caffeate's capabilities as a complexation agent to silver cation in mining processes by means of the dual descriptor--a conceptual DFT approach

Caffeic acid (C(9)H(8)O(4)) and its conjugate base C(9)H(7)O(4) (-) (anionic form-known as caffeate) were analyzed computationally through the use of quantum chemistry to assess their intrinsic global and local reactivity using the tools of conceptual density functional theory. The anionic form was found to be better at coordinating the silver cation than caffeic acid thus suggesting the use of caffeate as a complexation agent. The complexation capability of caffeate was compared with that of some of the most common ligand agents used to coordinate silver cations. Local reactivity descriptors allowed identification of the preferred sites on caffeate for silver cation coordination thus generating a plausible silver complex. All silver complexes were analyzed thermodynamically considering interaction energies in both gas and aqueous phases; the complexation free energy in aqueous phase was also determined. These results suggest that more attention be paid to the caffeate anion and its derivatives because this work has shed new light on the behavior of this anion in the recovery of silver cations that could be exploited in silver mining processes in a environmentally friendly way.

Electrochemical aptasensor for detection of copper based on a reagentless signal-on architecture and amplification by gold nanoparticles

A highly sensitive and specific electrochemical aptasensor for Cu(2+) detection based on gold nanoparticles (AuNPs) is presented. In this work, AuNPs offered a big surface area to immobilize a large number of aptamers and excellent electrochemical signal transduction. Its high sensitivity, low detection limit, and wide detection range are the main advantages over our former copper aptasensor. The peak current increased proportionally to the Cu(2+) concentration over the range from 0.1 nM to 10 μM with a detection limit of 0.1 pM. The presence of other divalent metal ions did not affect the detection of Cu(2+), which indicates a high specificity of Cu(2+) detection could be detected. Rapidity, simplicity, and excellent selectivity make it suitable for practical use in determination of Cu(2+) from lake samples.

The study of a new method to determine copper ion by square-wave voltammetry-extraction iodometry at the liquid/liquid interfaces

A new method of indirect determination of Cu(2+) was developed based on square-wave voltammetry by the oxidation of iodide in organic solvent at the liquid/liquid (L/L) interface. The limit of detection for the determination of Cu(2+) in this method was found to be 5 x 10(-4) mol/L, and the concentration ranged up to 1 x 10(-2) mol/L gave a linear limiting current versus concentration response. For the same simulated wastewater, this method showed high accuracy compared with the result tested by sodium diethyldithiocarbamate extraction spectrophotometry. This approach could be applied to the indirect determination of the oxidative heavy metals in the industrial wastewater.

Reaction pathways involved in the mechanism of AlIII chelation with caffeic acid: catechol and carboxylic functions competition

Density functional theory calculations on the AlIII-caffeic acid system are carried out to investigate the fixing mechanism of this metal ion to the two competing complexing sites in the ligand. This theoretical study was performed to explain the complex formation of 1:1 stoichiometry observed in aqueous medium at low pH values. Both complexation with the catechol and carboxylic functions are envisaged. The reaction pathways for the formation of these two chelates are calculated at the B3LYP/6-31G** level of theory. The complexation on the more acidic group is relatively straightforward and shows the intermediate formation of a monodentate complex followed by a chelation process. The complexation reaction pathway with the catechol function is more sophisticated, and several pathways are explored. Once more, the formation of a monodentate complex is achieved and the most favorable pathway for chelation involves the successive steps: 1) coordination of AlIII on the oxygen atom of a hydroxyl group, 2) deprotonation of this hydroxyl group, 3) ring closure with the other oxygen atom, and 4) deprotonation of the second hydroxyl. From an energetic point of view, this second pathway is more favorable. Notably the energy barrier necessary to form the chelate is lower for the catechol function than that calculated for the carboxylic group. The results of this purely theoretical study are in complete agreement with spectroscopic investigations performed on this system.