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.     

Construction of amperometric biosensor modified with conducting polymer/carbon dots for the analysis of catechol

Phenolic compounds used in food industries and pesticide industry, are environmentally toxic and pollute the rivers and ground water. For that reason, detection of phenolic compounds such as catechol by using simple, efficient and cost-effective devices have been becoming increasingly popular. In this study, a suitable and a novel matrix was composed using a novel conjugated polymer, namely poly[1-(5-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophen-2-yl)furan-2-yl)-5-(2-ethylhexyl)-3-(furan-2-yl)-4H thieno[3,4-c]pyrrole-4,6(5H)-dione] (PFTBDT) and carbon dots (CDs) to detect catechol. PFTBDT and CDs were synthesized and the optoelectronic properties of PFTBDT were investigated via electrochemical and spectroelectrochemical studies. Laccase enzyme was immobilized onto the constructed film matrix on the graphite electrode. The proposed biosensor was found to have a low detection limit (1.23 μM) and a high sensitivity (737.44 μA/mM.cm⁻²) with a linear range of 1.25–175 μM. Finally, the applicability of the proposed enzymatic biosensor was evaluated in a tap water sample and a satisfactory recovery (96–104%) was obtained for catechol determination.     

Nitrogen dioxide multiphase chemistry: uptake kinetics on aqueous solutions containing phenolic compounds

The uptake coefficients of NO2 on aqueous solutions containing guaiacol, syringol and catechol were determined over the pH range from 1 to 13 using the wetted wall flowtube technique. The measured uptake coefficients were used to determine the rate coefficients for the reaction of the physically dissolved NO2 with the neutral and deprotonated forms of phenolic compounds listed above. These organic compounds are ubiquitous not only in biomass burning plumes but also in soils, where they form part of the building blocks of humic acids. The NO2 uptake kinetics on solutions containing guaiacol, syringol or catechol were observed to be strongly pH dependent with uptake coefficients increasing from below 10(-7), under acidic conditions, to more than 10(-5) at pH values above 10. This behaviour illustrates the difference of reactivity between the neutral phenolic species and the phenoxide ions. The corresponding second order rate coefficients were typically observed to increase from 10(5) M(-1) s(-1) for the neutral compounds to a minimum of 10(8) M(-1) s(-1) for the phenoxide ions.     

Electrochemical Behaviour and Antioxidant Activity of Some Natural Polyphenols

A number of natural polyphenols (chlorogenic acid ( 9 ), cordigol ( 11 ), cordigone ( 12 ), danthrone ( 1 ), 1,5-dihydroxy-3-methoxyxanthone ( 2 ), eriosematin ( 7 ), flemichin D ( 8 ), frutinone A ( 6 ), mangiferin ( 4 ), quercetin ( 5 ), 1,3,6,7-tetrahydroxyxanthone ( 3 ) and verbascoside ( 10 )) were investigated for their redox properties using cyclic voltammetry. The antioxidant properties of these compounds were also examined in two models, namely lipid peroxidation in rat synaptosomes and AAPH-mediated oxidation of serum albumin. Compounds with a catechol group ( 9, 4, 5, 3 and 10 ) were oxidized below 0.4 V and inhibited lipid peroxidation with IC₅₀ values between 2 and 8 μM . Compounds having one or more isolated phenolic groups and showing an oxidation potential between 0.45 and 0.8 V ( 11, 12 and 8 ) inhibited lipid peroxidation with IC₅₀ between 7 and 9 μM , except 2 (0.45 V), danthrone (0.96 V) and eriosematin which showed no or modest antioxidant activity. Some of the investigated compounds also protected albumin from oxidation, but no structure-activity relationship was apparent, suggesting that other factors beside redox potential influence this activity.     

Laccase Biosensor Based on N‐Succinimidyl‐3‐Thiopropionate‐Functionalized Gold Electrodes

A laccase biosensor, in which the enzyme was immobilized on N‐succinimidyl‐3‐thiopropionate (NSTP)‐modified gold electrodes, is reported. Two different approaches for the preparation of N‐succinimidyl‐terminated monolayers were evaluated: a) activation of a preformed 3‐mercaptopropionic acid (MPA) SAM by reaction with 1‐(3‐dimethylaminopropyl)‐ 3‐ethylcarbodiimide (EDC) and N‐hydroxysulfosuccinimide (NHS); b) assembling of dithiobisuccinimidyl propionate (DTSP). NSTP‐modified electrodes were characterized by cyclic voltammetry and electrochemical impedance spectroscopy. Biosensors prepared by covalent binding of the enzyme and by cross‐linking with glutaraldehyde atop NSTP‐modified electrodes were compared in terms of sensitivity and operational range for caffeic acid. A much better analytical performance was found using the latter approach. Variables affecting the amperometric detection (enzyme loading, pH and applied potential) were optimized. The operational stability and characteristics of functioning of the laccase biosensor in terms of repeatability of the amperometric measurements, reproducibility with different biosensors and useful lifetime, were evaluated. The kinetic parameters of the enzyme reactions and the analytical characteristics of the corresponding calibration plots were calculated for eight phenolic compounds. Limits of detection of 0.07 μM, 0.05 μM and 0.09 μM were obtained for caffeic acid, catechol and 3,4‐dihydroxyphenylacetic acid (DOPAC), respectively. The practical usefulness of the developed biosensor was evaluated by estimating the “pool” of phenolic compounds in olive oil mill wastewaters (OMW).     

Highly efficient quenching of coreactant electrogenerated chemiluminescence by phenolic compounds

We describe the quenching effects of phenolic compounds on the electrogenerated chemiluminescence (ECL) of the Ru(bpy)3(2+) (bpy = 2,2'-bipyridine)/tri-n-propylamine (TPrA) system in aqueous solution. First, the emissions via different ECL routes were examined in the presence of 1,4-benzoquinone. It was found that the interception of the ECL intermediate radicals by the quencher molecules significantly influenced the light emission, especially when the direct coreactant oxidation played a predominant role in producing ECL. The most efficient quenching was observed for the low-oxidation-potential (LOP) ECL at a low concentration of TPrA (<5 mM). The Stern-Volmer constant (K(SV)) of the LOP ECL quenching could be as high as 1.3 x 10(6) M(-1), approximately 700 times larger than that of the photoluminescence quenching. Other phenolic compounds, such as phenol, hydroquinone, catechol, and dopamine, would be oxidized at the potential where the ECL was generated, and the benzoquinone-containing products exhibited ECL quenching effects similar to that of 1,4-benzoquinone. The highly efficient quenching of the LOP ECL by the phenolic compounds may provide a new approach for the determination of these pharmaceutically and environmentally important molecules.     

Potentiometric studies on the complexation of copper(II) by phenolic acids as discrete ligand models of humic substances

Studies on the complexation of copper(II) by phenolic acids, as ligand models of humic substances were done by potentiometry. The acids under study were: 3,4-dihydroxyhydrocinnamic acid or hydrocaffeic acid (1), 3,4-dihydroxyphenylacetic acid (2) and 3,4-dihydroxybenzoic acid or protocatechuic acid (3). Acidity constants of the ligands and the formation constants of metal-ligand complexes were evaluated by computer programs. The carboxylic group of the phenolic acids has different pK_a1 values, being the dissociation constants intrinsically related with the distance between the function and the aromatic nucleus. The results obtained allow concluding that acidity constants of the catechol moiety of the compounds are similar with pK_a2 and pK_a3 values between 9.47-9.41 and 11.55-11.70. The complexation properties of the three ligands towards copper(II) ion are quite similar, being the species found not different either in nature or stability. Although the model ligands have some structural differences no significant differences were found in their complexation properties towards copper(II). So, it can be postulated that complexation process is intrinsically related with the presence of a catechol group.     

Azobioisosteres of Curcumin with Pronounced Activity against Amyloid Aggregation,Intracellular Oxidative Stress,and Neuroinflammation

Many (poly-)phenolic natural products, for example, curcumin and taxifolin, have been studied for their activity against specific hallmarks of neurodegeneration, such as amyloid-β 42 (Aβ42) aggregation and neuroinflammation. Due to their drawbacks, arising from poor pharmacokinetics, rapid metabolism, and even instability in aqueous medium, the biological activity of azobenzene compounds carrying a pharmacophoric catechol group, which have been designed as bioisoteres of curcumin has been examined. Molecular simulations reveal the ability of these compounds to form a hydrophobic cluster with Aβ42, which adopts different folds, affecting the propensity to populate fibril-like conformations. Furthermore, the curcumin bioisosteres exceeded the parent compound in activity against Aβ42 aggregation inhibition, glutamate-induced intracellular oxidative stress in HT22 cells, and neuroinflammation in microglial BV-2 cells. The most active compound prevented apoptosis of HT22 cells at a concentration of 2.5 μm (83 % cell survival), whereas curcumin only showed very low protection at 10 μm (21 % cell survival).     

A novel palygorskite-modified carbon paste amperometric sensor for catechol determination

A palygorskite-modified carbon paste electrode (CPE) was constructed using graphite powder mixed with palygorskite particles. Compared with the unmodified CPE, the resulting palygorskite-modified CPE remarkably increases the peak currents of catechol, and greatly lowers the peak potential separation. Therefore, the palygorskite exhibits catalytic activity to catechol and significantly improves the determining sensitivity. The electrocatalytic activity of palygorskite is attributed to its high adsorption capability and the –OH groups on its surface, which plays an important role in the electron transfer between the modified CPE and the catechol in the solution. The sensor shows a linear response range between 5 and 100 μM catechol with a correlation coefficient of 0.998. The detection limit was calculated as 0.57 μM (s/n = 3).     

Immobilization of CoII-(N,N′-bis(salicylidene)-2-aminobenzylamine) on Poly(pyrrole-co-o-anisidine)/Chitosan Composite Films: Application to Electrocatalytic Oxidation of Catechol

In this study, poly (pyrrole-co-o-anisidine)/chitosan composite (Cs) films were prepared by cyclic voltammetry technique on platinum electrode using different pyrrole and o-anisidine mole ratios. Immobilization process was accomplished in Co^II-(N,N′-bis(salicylidene)-2-aminobenzylamine)(CoL) dissolved 0.15 M acetonitrile-LiClO₄ solution by cyclic voltammetry technique at 0.2–2.0 V potential range. Three electrode methods were applied in all electrochemical studies. After immobilization process, the characterizations of the electro catalytic surfaces (Cs−CoL−Pt) were carried out by cyclic voltammetry and SEM images. The SEM images clearly indicated that the [CoL] complex is immobilized onto composite films. The electrocatalytic activity of the modified electrodes on the catechol was investigated using buffer solutions of different pH values. The results of catalytic studies revealed that, pH=10 buffer solution was the optimal solution and 1 : 1 Cs−CoL−Pt electrode was the best electrode for catechol oxidation. In square wave voltammetry measurements using this electrode, two linear working ranges were determined. The linear response ranges for catechol determination were found as 3.0 μM–6.0 μM and 16 μM–80 μM for the first and the second linear working ranges, respectively, with 1.1 μM detection limit.     

A comparison of amperometric screen-printed, carbon electrodes and their application to the analysis of phenolic compounds present in beers

In this paper a comparison between three commercially-available, screen-printable graphite inks for the construction of phenolic biosensors is made. The enzyme tyrosinase was immobilised within a polymer matrix and the substrate catechol was used to characterise the bio-electroanalytical response of each electrode. Biosensors fabricated from Gwent graphite inks exhibited the greatest sensitivity (5740 mA mol cm(-2)) compared to Dupont and Acheson graphite-based inks. This difference in sensitivity was attributed to a combination of a larger electroactive surface area, and thus a greater number of immobilised enzyme molecules. However, the dynamic range was considerably smaller (0.025-14 muM) indicating that the enzyme molecules were easily accessible to the substrate catechol. The surface properties of the biosensors were characterised using ac impedance, which indicated that the presence of the polymer on the electrode surface not only increased the charge-transfer kinetics of the three biosensors, but also increased the surface roughness of biosensors fabricated from Gwent inks. On the basis of these results Gwent graphite-based inks were used for analysis of phenolic compounds in lager beers by flow-injection analysis. The biosensor displayed favourable response characteristics, but cannot differentiate between the various phenolic compounds present in the samples. Nevertheless, the biosensor maybe suitable for indicating the phenolic status of beer or brew samples compared to time-consuming traditional methods, e.g. colorimetric or chromatographic methods.     

Amperometric phenol biosensor based on sol-gel silicate/Nafion composite film

An amperometric biosensor based on tyrosinase immobilized in silicate/Nafion composite film has been developed for the determination of phenolic compounds. The Nafion polymer in the composite was used not only to overcome the brittleness of the pure sol-gel-derived silicate film but also to increase the long-term stability of the biosensor. Tyrosinase was immobilized by a thin film of silicate/Nafion composite on a glassy carbon electrode. Phenolic compounds were determined by the direct reduction of biocatalytically-liberated quinone species at −200 mV versus Ag/AgCl (3 M NaCl). The process parameters for the fabrication of the enzyme electrode and various experimental variables such as pH and operating potential were explored for optimum analytical performance of the enzyme electrode. The biosensor can reach 95% of steady-state current in about 15 s. The sensitivities of the biosensor for catechol and phenol were 200 and 46 mA/M, respectively. A detection limit of 0.35 mM catechol was obtained with a signal-to-noise ratio of 3. The enzyme electrode retained 74% of its initial activity after 2 weeks of storage in 50 mM phosphate buffer at pH 7.     

Amperometric detection of phenolic compounds by polypyrrole-based composite carbon paste electrodes

This contribution describes new composite carbon paste electrodes (CPEs) for the determination of phenolic compounds. The composite CPEs were prepared by in situ generation of polypyrrole (PPy) within a paste containing the enzyme polyphenol oxidase (PPO). The best paste composition (enzyme/pyrrole monomer/carbon particles/Nujol) was determined for a model enzyme, glucose oxidase (GOx) according to the enzymatic activity of the resulting electrodes and to the enzyme leakage from the paste during storage in phosphate buffer. The in situ electrogenerated PPy improves the enzyme immobilisation within the paste since practically no enzyme was lost in solution after 72 h of immersion. Moreover, the enzyme activity remains particularly stable under storage since the biocomposite structure conserves 80% of its activity after 1 month of storage. Following the optimisation of the paste composition, PPO-based carbon paste biosensors were prepared and presented excellent analytical properties toward catechol detection with a sensitivity of 4.7 A M(-1) cm(-2) and a response time lower than 20 s. The resulting biosensors were applied to the determination of polyphenolic compounds (e.g., epicatechin and ferulic acid).     

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.     

A catechol biosensor based on a gold nanoparticles encapsulated-dendrimer

Tyrosinase has been immobilized on a Au nanoparticles encapsulated-dendrimer bonded conducting polymer on a glassy carbon electrode for the estimation of catechol. The modified electrode was characterized by cyclic voltammetry and AFM techniques. The principle of catechol estimation was based on the reduction of biocatalytically liberated quinone species at +0.2 V versus Ag/AgCl (3 M KCl), with good stability, sensitivity, and featuring a low detection limit (about 0.002 μM) and wide linear range (0.005 μM-120 μM). The electrochemical redox peak of catechol on the GCE/PolyPATT/Den(AuNPs)/tyrosinase was also investigated. A response time of 7 s, reusability up to 5 cycles and a shelf life of more than 2 months under refrigerated conditions were reported. Various parameters influencing biosensor performance have been optimized including pH, temperature, and applied potential. The utility and application of this nanobiosensor was tested in a real water samples.     

Determination of Phenolic Compounds Based on Co‐Immobilization of Methylene Blue and HRP on Multi‐Wall Carbon Nanotubes

This work evaluated an amperometric biosensor based on multi‐wall carbon nanotubes (MWCNT), chemically modified with methylene blue (Met) and horseradish peroxidase (HRP), for detection of phenolic compounds. The dependences of the biosensor response due to the enzyme immobilization procedure, HRP amounts, pH and working potential were investigated. The amperometric response for catechol using the proposed biosensor showed a very wide linear response range (1 to 150 μmol L^?1), good sensitivity (50 nA cm^?2 μmol^?1 L), excellent operational stability (after 300 determinations the response remained at 97%) and very good storage stability (lifetime>3 months). Based on all these characteristics, it is possible to affirm that the material is promising for phenol detection due to its good electrochemical response and enzyme stabilization. The biosensor response for various phenolic compounds was investigated.     

Phenolic constituents of licorice. III. Structures of glicoricone and licofuranone, and inhibitory effects of licorice constituents on monoamine oxidase

Two new phenolic compounds, glicoricone (3) and licofuranone (4), were isolated from a species of licorice brought from the northwestern region of China, and their structures were assigned. Among the twelve licorice constituents examined for the inhibition of monoamine oxidase (MAO), six compounds, 3, 4, genistein (6), licopyranocoumarin (7), licocoumarone (14) and glycyrrhisoflavone (15), inhibited the enzyme with the IC50 (concentration required for 50% inhibition of the enzyme activity) values of 6.0 x 10(-5)-1.4 x 10(-4) M. Glycyrrhizin (1) also inhibited MAO with the IC50 value of 1.6 x 10(-4) M.     

酚化合物对胰岛素淀粉样纤维化的抑制作用

采用牛胰岛素作为模型多肽分子, 对几种结构相近的简单多酚的抗多肽淀粉样纤维化作用进行了研究. 结果表明, 邻苯二酚和对苯二酚对胰岛素纤维化具有抑制作用, 并通过形成醌中间体对多肽链进行修饰, 与对苯醌作用类似; 而苯酚和间二苯酚在相同条件下, 既不能修饰多肽也无抑制纤维化作用. 在无氧条件下, 邻苯二酚和对苯二酚对胰岛素纤维化的抑制作用明显降低, 说明酚化合物经氧化形成的醌中间体是其抗胰岛素纤维化的主要活性结构.     

Selective Determination of Catechol in the Presence of Hydroquinone at Bare Indium Tin Oxide Electrodes via Peak‐Potential Separation and Redox Cycling by Hydrazine

The electrochemical, selective determination of catechol (CT) in the presence of hydroquinone (HQ) is not readily achieved, because the formal potentials of two phenolic compounds are very close. Here, we have developed a simple electrochemical method for the selective determination of CT by using bare indium tin oxide electrodes and employing CT redox cycling by hydrazine. The cyclic voltammetry of CT and HQ was investigated in Tris buffer (pH 9.0), phosphate buffered saline buffer (pH 7.4), and acetate buffer (pH 4.5). Especially in Tris buffer, the anodic peak potential of CT is much lower than that of HQ, resulting in a large difference between two peak potentials (ca. 0.4 V). The difference allows the selective determination of CT in the presence of excess HQ. The anodic current of CT is amplified using CT redox cycling by hydrazine, which also helps to stabilize CT and HQ in Tris buffer for several hours. The detection limits of CT in Tris buffer containing 0.1 mM HQ are 1 μM and 10 μM in the presence and absence of hydrazine, respectively.     

Pyrolyzed Photoresist Carbon Electrodes for Microchip Electrophoresis with Dual‐Electrode Amperometric Detection

The fabrication and evaluation of pyrolyzed photoresist films (PPF) for microchip capillary electrophoresis (CE) with dual‐electrode electrochemical (EC) detection is described. The sensitivity, linearity, and reproducibility were evaluated using catecholamines and related compounds, including dopamine (DA), 5‐hydroxyindole‐3‐acetic acid (5‐HIAA), ascorbic acid (AA), and catechol. Initial studies with DA show the response of the PPF electrodes to be linear between 25 and 500 μM (r²=0.999) with a limit of detection (LOD) of 5 μM (S/N=3) and sensitivity of 5.8 pA/μM. Selectivity was further enhanced by employing dual‐electrode detection in the series configuration for detection of species exhibiting chemically reversible redox reactions.