Dihydroxybenzene Isomers Research Articles

Simultaneous and Selective Electrochemical Determination of Catechol and Hydroquinone on A Nickel Oxide (NiO) Reduced Graphene Oxide (rGO) Doped Multiwalled Carbon Nanotube (fMWCNT) Modified Platinum Electrode

A new electrochemical sensor is reported based on a modified platinum (Pt) electrode sequentially drop-casting doped with functionalized multi-walled carbon nanotubes (fMWCNTs) decorated by reduced graphene oxide (rGO) and nickel oxide (NiO) nanoparticles (NPs). The NiO/rGO/fMWCNTs nanohybrid was characterized by Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF). This sensor was employed for the simultaneous determination of catechol and hydroquinone. In the cyclic voltammetry (CV) results for a binary mixture of catechol and hydroquinone, the peak potentials of the two dihydroxybenzene isomers were separated by more than 105 mV, demonstrating the ability of the fabricated sensor to simultaneously determine the analytes. The NiO/rGO/fMWCNTs/Pt electrode exhibited a wide linear range of 10–300 μM with detection limits of 19.86 and 40.18 nM based upon a signal-to-noise ratio of 3 for catechol and hydroquinone, respectively. Furthermore, using differential pulse voltammetry (DPV), the sensor maintained well-defined peaks, demonstrating suitable selectivity. In addition, the NiO/rGO/fMWCNTs/Pt nanocomposite exhibits stability values of 92.3% and 91.6% for catechol and hydroquinone with excellent repeatability. The sensor analyzed water samples with spike recoveries from 90% to 106.66% for catechol and 88% to 98.66% for hydroquinone, respectively.

Theoretical study of the chemical interactions between carbon fiber ultramicroelectrodes and the dihydroxybenzene isomers for electrochemical sensor understanding

Herein we present a study using the density functional theory in periodic systems to describe the interaction of carbon fiber ultramicroelectrodes during the detection of the dihydroxybenzene isomers: catechol (CT), resorcinol (RS), and hydroquinone (HQ). We used graphene as the surface model in the conformations: carbon basal plane, zig-zag, and armchair edges. We modeled the surface models functionalized with oxygenated groups. We predicted chemisorption for the oxidized form of RS when the COOH group is present in the armchair edge. Also, the susceptibility of the RS to form dimers and trimers bonded to the electrode surface. Experimentally, the electrooxidation of RS showed inactivation of the ultramicroelectrode, which can be explained considering the chemisorptions mentioned above and the formation of no-electroactive polymers. In the case of the other two molecules (CT and HQ), the model describes favorable interaction energies with the electrode; none of them as strong as RS, thus, experimentally, we did not see affections to the electrochemical responses, and we could measure a well-defined steady-state current at different concentrations of these molecules. Furthermore, the characteristic half-wave potential (E1/2) of the isomers ensured their detection and quantification in single or mixed solutions in concentrations from 40 μM to 1 mM.

Simultaneous detection of hazardous skin whitening agents in Indian cosmetic products using a green chromatographic technique

The present work mainly highlights the simultaneous detection of four skin whitening agents i.e. hydroquinone (HQ), resorcinol (RS), catechol (CC) and 3,3′-dichlorobenzidine (DCB) in facial creams and body lotion. Among these, the first three are positional isomers of dihydroxybenzene so simultaneous separation is difficult with the conventional reverse-phase high-performance liquid chromatographic technique (RP-HPLC). The selected skin whitening agents were detected in facial cream and body lotion using micellar liquid chromatography coupled to a photodiode array detector (MLC-PDA). In the present study, optimization of the method was accomplished using response surface methodology (RSM) with central composite design (CCD). The second-order polynomial model for predicting the optimal chromatographic run time was evaluated by the analysis of variance (ANOVA) and 3D response surface plots for the interactions between three variables were constructed. Three experimental parameters which were chosen as independent variables were; surfactant concentration (SDS), the volume percentage of organic modifier (OM) and pH of the mobile phase. The second-order polynomial model for predicting the optimal chromatographic run time was evaluated by the analysis of variance (ANOVA) and 3D response surface plots for the interactions between three variables were constructed. Three experimental parameters which were chosen as independent variables were; surfactant concentration (SDS), percentage of organic modifier (OM) and pH of the mobile phase. The second-order polynomial model for predicting the optimal chromatographic run time was evaluated by the analysis of variance (ANOVA) and 3D response surface plots for the interactions between three variables were constructed. Three experimental parameters which were chosen as independent variables were; surfactant concentration (SDS), percentage of organic modifier (OM) and pH of the mobile phase. The optimized mobile phase was 0.15 M SDS and 7% 1-butanol, buffered at pH 7 with 0.01 M NaH2PO4. The chromatographic run time for simultaneous determination of selected analytes was 7.5 min. The correlation coefficient (r2) values were satisfactory between 0.998–0.999 over the linear concentration range. Limits of detection (LODs) and the limits of quantification (LOQs) for the four skin whitening agents were in the range of 0.05–0.07 μg/mg and 0.11–0.14 μg/mg, respectively. Trueness (98.4–102.7%) and precision (< 4.3%) were acceptable. The developed method was fast, cost-effective, and green which could easily analyze complex matrices (facial creams, body lotion) without any pretreatment other than filtration. The results indicated that the MLC-PDA method proved to be more suitable for the simultaneous separation of selected positional isomers.

Ruthenium Red-functionalized sol-gel and multi-walled carbon nanotubes for electrochemical simultaneous detection of three dihydroxybenzene isomers

In this proof-of-concept study, sol–gel was functionalized with Ruthenium Red (RR) using 3-glycidoxypropyl trimethoxysilane as the coupling agent and multi-walled carbon nanotubes (MWCNTs) and characterized with FT-IR, SEM and TEM as well as electrochemical techniques. The functionalized sol–gel (fSG) nanocomposite was used for the modification of a graphite paste electrode (GPE/fSGRR-MWCNTs) for the speciation of dihydroxybenzene compounds, hydroquinone (HQ), catechol (CC), and resorcinol (RS) which are known as hazardous water pollutants. The DPV measurements were performed in phosphate buffer solution at pH 5.0 from −0.2 to 1.0 V vs. Ag/AgCl. The nanocomposite-modified electrode displayed stable and continuous three well-separated oxidation peaks at 0.20, 0.30 and 0.72 V for HQ, CC, and RS, respectively. The limit-of-detection (LOD) was determined to be 0.049, 0.008 and 0.015 μM for HQ, CC, and RS, respectively. The modified GPE/fSGRR-MWCNTs showed good stability, reproducibility, sensitivity, and selectivity properties to determine all three compounds. The electrochemical performance of the GPE/fSGRR-MWCNT has been evaluated for simultaneous determination of dihydroxybenzene isomers in creek water, residential wastewater, and commercial hair dye, with satisfactory recoveries between 95 and 104%. This modified GPE/fSGRR-MWCNTs showed better stability, reproducibility, sensitivity, and selectivity properties to determine all three compounds when compared with the similar modified electrodes recently reported in the literature. Therefore, this novel RR-containing nanocomposite-modified sensor can be a promising pre-screening tool for the inexpensive and rapid detection of HQ, CC, and RS in on-field environmental studies.

Simultaneous Detection of Dihydroxybenzene Isomers Using Electrochemically Reduced Graphene Oxide-Carboxylated Carbon Nanotubes/Gold Nanoparticles Nanocomposite.

An electrochemical sensor based on electrochemically reduced graphene oxide (ErGO), carboxylated carbon nanotubes (cMWCNT), and gold nanoparticles (AuNPs) (GCE/ErGO-cMWCNT/AuNPs) was developed for the simultaneous detection of dihidroxybenzen isomers (DHB) hydroquinone (HQ), catechol (CC), and resorcinol (RS) using differential pulse voltammetry (DPV). The fabrication and optimization of the system were evaluated with Raman Spectroscopy, SEM, cyclic voltammetry, and DPV. Under optimized conditions, the GCE/ErGO-cMWCNT/AuNPs sensor exhibited a linear concentration range of 1.2–170 μM for HQ and CC, and 2.4–400 μM for RS with a detection limit of 0.39 μM, 0.54 μM, and 0.61 μM, respectively. When evaluated in tap water and skin-lightening cream, DHB multianalyte detection showed an average recovery rate of 107.11% and 102.56%, respectively. The performance was attributed to the synergistic effects of the 3D network formed by the strong π–π stacking interaction between ErGO and cMWCNT, combined with the active catalytic sites of AuNPs. Additionally, the cMWCNT provided improved electrocatalytic properties associated with the carboxyl groups that facilitate the adsorption of the DHB and the greater amount of active edge planes. The proposed GCE/ErGO-cMWCNT/AuNPs sensor showed a great potential for the simultaneous, precise, and easy-to-handle detection of DHB in complex samples with high sensitivity.

Simultaneous Electrochemical Determination of Dihydroxybenzene Isomers at 1-Butyl-3-Methylimidazolium Hexafluorophosphate Modified Pencil Graphite Electrode

A simplistic and inexpensive electrochemical platform has been fabricated for the simultaneous detection of Dihydroxybenzene Isomers (DHBIs); named- Hydroquinone (HQ), Catechol (CC) and Resorcinol (RS); in aqueous system. Cyclic Voltammetry (CV) and Differential Pulse Voltammetry (DPV) were adopted for the detection technique. 2B pencil (Faber Castell) collected from the resident stationary shop was used for fabricating Pencil Graphite Electrode (PGE) used as working electrode. PGE was characterized by SEM and EDX. It was modified electrochemically by ionic liquid, 1-Butyl-3-Methylimidazolium hexafluorophosphate (BIHP). Total analyses were performed in phosphate buffer solution (PBS) at pH 7.0. BIHP-PGE showed good selectivity and robust anti-interference for detection of HQ, CC and RS simultaneously in aqueous media. For HQ, CC and RS, the limit of detection is 9.09 μML -1 , 8.15 μM L -1 and 26.78 μM L -1 respectively and sensitivity is 525.21 μA/mM/cm 2 , 585.68 μA/mM/cm 2 and 178.00 μA/mM/cm 2 respectively at BIHP-PGE in simultaneous detection.

Coomassie brilliant blue G 250 modified carbon paste electrode sensor for the voltammetric detection of dihydroxybenzene isomers

In this present study, coomassie brilliant blue G-250 (CBBG) modified electrode was fabricated for the specific and simultaneous detection of three dihydroxybenzene isomers such as resorcinol (RS), catechol (CC) and hydroquinone (HQ). The fabrication of the modified electrode was carried out by electrochemical polymerization of CBBG on the surface of unmodified electrode. The surface structures of bare and fabricated electrode were studied by scanning electron microscope (SEM). The established electrode portrays the very fine interface with these isomers and displayed the sufficient sensitivity and selectivity. The specific parameters of pH solution, scan rate and varying the concentration of analytes were optimized at the modified electrode. The sensor process was originated to be adsorption-controlled activity and the low limit of detection (LOD) for RS and CC was attained at 0.24 and 0.21 µM respectively. In the simultaneous study, designed sensor clearly implies the three well separated anodic peaks for RS, HQ and CC nevertheless in unmodified electrode it failed. Also, the constructed electrode was applied for the real sample analysis in tap water and obtained results are agreeable and it consistent in-between 92.80–99.48%.

Hybridized 1D–2D MnMoO4–MXene nanocomposites as high-performing electrochemical sensing platform for the sensitive detection of dihydroxybenzene isomers in wastewater samples

Hydroquinone (HQ) and catechol (CC) are the two major dihydroxybenzene isomers, are considered one of the toxic pollutants in wastewater, which often coexisted and impede each other during sample identification. For practical analysis and simultaneous detection of HQ and CC in wastewater, we fabricate a hybrid electrochemical sensor with electrospun one-dimensional (1D) MnMoO4 nanofibers coupled with a few-layered exfoliated two-dimensional (2D) MXene. The facilitated abundant defective edges of 1D MnMoO4 and 2D MXene nanoarchitecture accelerated the effect of synergistic signal amplification and exhibited high electrocatalytic activity towards the oxidation of hydroquinone and catechol. MnMoO4–MXene-GCE showed oxidation potentials of 0.102 V and 0.203 V for hydroquinone and catechol, respectively. It revealed the distinguished and simultaneous detection range of 0.101 V with a strong anodic peak current. Noteworthily, the proposed 1D–2D hybridized MnMoO4–MXene–GCE sensor exhibited a wide linear response from 5 nM to 65 nM for hydroquinone and catechol. Moreover, it showed a low detection limit of 0.26 nM and 0.30 nM for HQ and CC with high stability, respectively. The feasible 1D–2D MnMoO4–MXene nanocomposite-based biosensor effectively detected hydroquinone and catechol in hazardous water pollutants using the differential pulse voltammetric technique with recovery values.

Simultaneous determination of dihydroxybenzene isomers in cosmetics by synthesis of nitrogen-doped nickel carbide spheres and construction of ultrasensitive electrochemical sensor

N-doped nickel carbide spheres (N-NiCSs) were synthesised for the first time by controlling the type of surfactant, surfactant-to-Ni molar ratio, reaction temperature, and reaction time. The morphology, composition, and electrochemical behaviour of the synthesised spheres revealed that the spheres presented a large specific surface area, abundant pores, and good conductivity, with excellent electrocatalytic performance. A glassy carbon electrode-modified with N-NiCSs was used for the simultaneous identification of hydroquinone (HQ), catechol (CC), and resorcinol (RS) utilising differential pulse voltammetry. The oxidation peaks of HQ, CC, and RS were observed at 9.8, 119, and 470 mV, respectively (vs. SCE). Under optimal conditions, the oxidation peak currents of HQ, CC, and RS were linear in the concentration ranges of 0.005–100 μM, 0.05–200 μM, and 5–500 μM, respectively. The detection limits of HQ, CC, and RS were 0.00152 μM, 0.015 μM, and 0.24 μM (S/N = 3), respectively. The sensitivities of HQ, CC, and RS were 4.635, 2.069, and 0.985 μA μM−1 cm−2 (S/N = 3), respectively. The fabricated sensor was successfully used to detect HQ, CC, and RS in hair dye, whitening cream, and local tap water samples. Moreover, the sensor presented a good repeatability, reproducibility, and stability during cosmetic testing and a relatively wide linear range, an ultralow detection limit, and an ultrahigh sensitivity.

Electrochemical activation of copper oxide decorated graphene oxide modified carbon paste electrode surface for the simultaneous determination of hazardous Di-hydroxybenzene isomers

The construction of a consistent electrochemical sensor is essential for the determination of hazardous pollutants in water and environments. Hydroquinone (HQ), catechol (CT), and resorcinol (RS) frequently coexist as priority pollutants in water and interfere with each other during identification, which poses a challenge for simultaneous and sensitive determination for those di-hydroxybenzene isomers. In this work, copper oxide (CuO) decorated graphene oxide (GO) nanocomposite (CuO/GO) was prepared and used to modify carbon paste electrode (CPE). The as-prepared electrode (CGCPE) was electrochemically activated in the presence of NaOH solution (Et-CGCPE) for the simultaneous determination of di-hydroxybenzene isomers (HQ, CT, and RS), and showed a well-defined oxidation peak with a significantly improved electrochemical response. We ascribe this high performance to the formation of electroactive functional groups on the electrode surface during the electrochemical activation and the synergistic effect between GO and CuO. Under the optimized conditions, the results show a long linear concentration range of HQ (0.2–360 µM), CT (0.3–360 µM), and RS (0.7–250 µM) with the detection limits of 1.0 μM, 1.1 μM, and 1.8 μM, respectively. The Et-CGCPE was effectively applied to detect three di-hydroxybenzene isomers in lake water with good repeatability and stability.

Electrochemical Sensing Platforms of Dihydroxybenzene: Part 2 - Nanomaterials Excluding Carbon Nanotubes and Graphene.

The high surface-to-volume ratio and desirable chemical, thermal, and catalytic properties of nanomaterials have made them promising electrode materials for sensing applications. As such, different nanomaterials and their nanocomposite-based individual and/or simultaneous detection of dihydroxybenzene (DHB) have been reported in recent years. Due to the low degradation rate and high toxicity of DHB isomers, the development of innovative and robust sensors for their simultaneous detection has received considerable attention. In this review, applications of different nanomaterials (with the exception of carbon nanotubes, graphene, and their derivatives) for individual and/or simultaneous detection of DHB are briefly discussed. The focal point is on the characteristic features of the modified electrodes that improve their electrocatalytic activities toward DHB. Real sample analysis and electrolyte media are also summarized. This review includes studies published from 2011 to 2020.

Fabrication an electrochemical sensor based on composite of Cu-TCPP nanosheets and PSS functionalized graphene for simultaneous and sensitive determination of dihydroxybenzene isomers

Abstract A novel electrochemical sensor based on the combination of two-dimensional metal–organic framework Cu-porphyrin (Cu-TCPP) and poly(styrene sulfonate) functionalized graphene (PSS-Gr) denoated as PSS-Gr@Cu-TCPP was constructed and applied to simultaneously determine dihydroxybenzene isomers. The nanocomposite was synthesized by a simple ultrasonic mixing method and characterized by transmission electron microscopy (TEM), UV–Vis absorption and electrochemical techniques. The electrochemical behaviors of dihydroxybenzene isomers including hydroquinone (HQ), catechol (CT) and resorcinol (RS) on PSS-Gr@Cu-TCPP modified electrode were investigated and their oxidation peak potentials were separated evidently with the oxidation peak currents enhanced significantly on the modified electrode, which were attributed to the remarkable electrocatalytic activity of the nanocomposite on the oxidation of three isomers. Under the optimized conditions, the proposed sensor showed a wide linear concentration range of 1–200 μM, 0.08–120 μM and 5–100 μM with the low detection limit of 1 μM, 0.08 μM, 5 μM for hydroquinone, catechol and resorcinol. In addition, the fabricated electrochemical sensor was successfully applied to detect three dihydroxybenzene isomers in real samples with satisfactory recovery. This work provided a simple and sensitive approach for simultaneous determination of dihydroxybenzene isomers.

“Turn-on” fluorometric probe for hydroquinone and catechol based on an in situ reaction between protamine sulfate and dihydroxybenzene isomers and the formation of fluorescent polymer nanoparticles

Hydroquinone (HQ) and catechol (CC) pose serious risks to human and ecosystem health, so it is important to develop sensitive methods for detecting HQ and CC. Here, we describe an environmentally friendly turn-on fluorescent method for detecting HQ and CC involving monitoring the intrinsic fluorescence of fluorescent polymer nanoparticles (FPNs) synthesized by crosslinking protamine sulfate with HQ or CC in an aqueous solution. The FPN fluorescence intensities at excitation/emission peaks of 333/430 and 320/397 nm were related to the HQ and CC concentrations, respectively. A linear relationship was found between the fluorescence intensity and HQ or CC concentration. The detection limits for HQ and CC were 0.21 and 0.33 μM, respectively. Importantly, no interferences from 10 common interfering substances were found, meaning that the method is very selective. The method was successfully used to detect HQ and CC in spiked water samples and gave satisfactory results.

Competitive Hydrogenation and Hydrodeoxygenation of Oxygen-Substituted Aromatics over Rh/Silica: Catechol, Resorcinol and Hydroquinone

The competitive hydrogenation and hydrodeoxygenation (HDO) of dihydroxybenzene isomers, catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene) and hydroquinone (1,4-dihydroxybenzene), was studied in the liquid phase over a Rh/silica catalyst at 323 K and 3 barg hydrogen pressure. Under competitive hydrogenation conditions an order of reactivity of ortho > para > meta was observed. Catechol initially inhibited resorcinol and hydroquinone hydrogenation but not HDO suggesting separate sites for hydrogenation and HDO. When resorcinol and hydroquinone were reacted competitively, HDO became the favoured reaction. The data suggested that cyclohexane and cyclohexanone were primary products. At low dihydroxybenzene (DHB) conversion the ratio of HDO products was dependent upon DHB isomer. When all three DHB isomers were reacted together, initially 86% of the HDO yield came from catechol with the rest from hydroquinone. When resorcinol finally reacted, HDO products were produced first. Reaction of DHB isomers in pairs using deuterium instead of hydrogen revealed changes in kinetic isotope effect (KIE). The presence of competing reactants had a dramatic effect on the energetics of hydrogenation and HDO reactions of individual components, reinforcing the view that hydrogenation and HDO are mechanistically separate. This effect on reaction energetics observed when more than one substrate was present, highlights the limitations of studying one single model compound as a route to understanding the processes required for the upgrading of a true bio-oil feed.

Carbon-stabilized porous silicon as novel voltammetric sensor platforms

The use of carbon-stabilized porous silicon (pSi) as a voltammetric sensing platform able to discriminate the detection of various electroactive species with similar redox potentials is demonstrated for the first time. For this purpose, the voltammetric responses to the presence of the three dihydroxybenzene isomers hydroquinone, catechol and resorcinol were studied using thermally hydrocarbonized pSi (THCpSi) and thermally carbonized pSi (TCpSi), as well as a glassy carbon electrode (GCE) as a benchmark. Both THCpSi and TCpSi outperformed the electrochemical performance of a GCE, as shown by the electrochemical characterization performed in the presence of the redox pair [Fe(CN)6]3-/4-. Cyclic voltammetry results demonstrated that both THCpSi and TCpSi exhibited fast electron transfer kinetics, with TCpSi showing a slightly improved electrochemical performance. The electrode catalytic enhancement shown by these carbon-stabilized nanostructures was harnessed to allow the simultaneous voltammetric determination of hydroquinone, catechol and resorcinol. The analytical performance of TCpSi to the detection of the three dihydroxybenzene isomers was studied using differential pulse voltammetry, revealing good reproducibility, high sensitivity, low limits of detection (all below 5 µmol L−1) and good recoveries (between 98 and 102%) in the analysis of spiked tap water samples.

CNT/Al2O3 core-shell nanostructures for the electrochemical detection of dihydroxybenzene isomers.

We report CNT/Al2O3 core-shell nanostructures for the electrochemical detection of dihydroxybenzene (DHB) isomers. Amorphous films of Al2O3 (1.2-15.4 nm in thickness) are uniformly deposited onto the inner and outer walls of CNTs by atomic layer deposition. The effect of the Al2O3 shell thickness on the electrochemical detection of dihydroxybenzene isomers was explored using cyclic and square-wave voltammetry. The best sensing properties are found at a shell thickness of approx. 2.4 nm (CNT/Al2O3(9) sensor), where the oxidation peak currents (sensor-signal) increased ca. 10 times as compared to a sensor fabricated with non-coated CNTs. All of the three DHB isomers (hydroquinone, catechol and resorcinol) are independently detected in the concentration ranges of 2-1000 μmol L-1, 0.5-700 μmol L-1 and 3.5-500 μmol L-1, respectively. The sensors show reliable repeatability, reproducibility, long-term stability, and applicability in the analysis of real samples. Based on these findings, a plausible mechanism is proposed highlighting the role of the Al2O3-shell.

High-performance electrochemical sensing platform based on sodium alginate-derived 3D hierarchically porous carbon for simultaneous determination of dihydroxybenzene isomers.

Three-dimensional hierarchically porous carbon (denoted as SA-900) with a microporous, mesoporous and macroporous structure was facilely fabricated via direct carbonization of sodium alginate. SA-900 was fully characterized by N2 adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction and Raman spectroscopy to confirm its structure. SA-900 was coated onto a glassy carbon electrode surface to construct an ultrasensitive electrochemical sensing platform (SA-900/GCE). Electrochemical behaviors of hydroquinone (HQ), catechol (CC) and resorcinol (RC) on the SA-900/GCE surface were investigated, and it was found that SA-900 possesses excellent electrocatalytic activity towards them. Experimental conditions including carbonization temperature, pH value, SA-900 concentration, accumulation potential and accumulation time were optimized for quantitative assay. Under optimized conditions, linear ranges for simultaneous determination of HQ, CC and RC are 0.05-1.50 μM, 0.05-1.50 μM and 0.50-15.00 μM, respectively. Detection limits for HQ, CC and RC are calculated to be 0.0183 μM, 0.0303 μM and 0.3193 μM (S/N = 3). The SA-900/GCE based electrochemical sensing platform is applied for determining HQ, CC and RC in lake water samples with satisfactory results.

Tailoring molecular permeability of vertically-ordered mesoporous silica-nanochannel films on graphene for selectively enhanced determination of dihydroxybenzene isomers in environmental water samples

Herein we demonstrate a simple and rapid electrochemical method for selectively enhanced determination of catechol (CC) or hydroquinone (HQ) isomers in environmental water samples by tailoring the molecular permeability of vertically-ordered mesoporous silica-nanochannel films on electrochemically reduced graphene oxide (VMSF/ErGO). Such VMSF/ErGO composite film was fabricated on the gold electrode (AuE) surface using electrochemically assisted self-assembly approach. The as-prepared electrodes with surfactant micelles (SM) template inside silica nanochannels, designed as SM/VMSF/ErGO/AuE, possess hydrophobic hydrocarbon cores and show preferential response to CC via hydrophobic effect. After removing SM from silica nanochannels, the obtained VMSF/ErGO/AuE displays more sensitive response to HQ, which is due to the hydrogen bond effect between the silanol groups of silica walls and HQ. Given the potential-resolved and high electrocatalytic ability of ErGO, and molecular permeability and anti-fouling ability of VMSF, these two present sensors could detect CC and HQ in lake water with a low limit of detection (18 nM for CC and 16 nM for HQ), and a high sensitivity (0.33 μA/μM for CC and 0.37 μA/μM for HQ), without complicated sample pretreatment. Moreover, the proposed sensors provide a convenient, rapid and economic way for direct analysis of environmental water samples, exhibiting excellent long-term stability.

Determination and Correlation of Solubilities of Benzoic Acid, Salicylic Acid, Resorcinol and Hydroquinone in Water and in 1-Octanol at Temperatures from 297.25 K to 334.45 K

The solubilities of benzoic acid, salicylic acid, resorcinol and hydroquinone in water and in 1-octanol were measured by the dynamic method which is also called the synthetic method from 297.25 K to 334.45 K. Using differential scanning calorimetry (DSC Q2000 and SDT Q600), the melting temperature and the enthalpy of fusion of these solutes were determined. The obtained results show that the solubility of benzoic acid in water is greater than that of salicylic acid, but in the case of the two isomers of dihydroxybenzene, the solubility of resorcinol is approximately 100 times that of hydroquinone. In 1-octanol, the decreasing order of the solubility of these compounds is as follows: resorcinol > benzoic acid > salicylic acid > hydroquinone. The experimental solubilities were correlated using two regression equations. The correlation coefficient is greater than 0.9937 for one of these two equations for the binary studied systems where the solvent is either water or 1-octanol. New experimental data are provided for the solubility of resorcinol in water and salicylic acid, resorcinol, hydroquinone in 1-octanol.

Electrochemical Discrimination of Dihydroxybenzene Isomer at Different Thiols Self-Assembled Monolayers on Gold and Simultaneous Determination of Catechol and Hydroquinone

The discrimination effect of dihydroxybenzene isomer including catechol (CAT), hydroquinone (HQ) and resorcinol (RE) on bare gold, and a series of thiols self-assembled monolayers (SAMs) including mercaptohexanol (MCH), 6-mercaptohexanoic acid (MHA), mercaptoethylamine (MEA), thiol-modified double-stranded DNA (ds-DNA), single-stranded DNA (ss-DNA) is first investigated by cyclic voltammetry (CV) and linear scan voltammetry (LSV). Results show that CAT, HQ and RE can not be electrochemically discriminated simultaneously by bare gold or thiols SAMs based on their oxidation or reduction peaks. Furthermore, RE has a big interference on the electrochemical response of CAT and HQ. At acidic solution (pH 4.4), in presence of RE, CAT and HQ can not be discriminated on bare gold, ds-DNA or ss-DNA SAMs. However, CAT and HQ can be discriminated well at MCH, MHA, and MEA SAMs on gold based on the difference of reduction peak potentials (ΔE pc) as 219, 212 and 165 mV respectively. MCH SAMs is chosen to study simultaneous determination of CAT and HQ due to bigger ΔE pc and current response. We fix the potential at 0.7 V for 60 s to enrich the oxidation products of CAT and HQ, then investigated the relationships of reduction peak current (i pc) with the concentrations of CAT and HQ. The i pc values are linear with the concentrations of CAT and HQ in the range of 7.0 × 10−6∼1.04 × 10−3 M. The limit of detection (LOD) is calculated to be 5.9 × 10−7 M for CAT and 8.3 × 10−7 M for HQ (S/N = 3). The MCH SAMs on gold is applied to the simultaneous determination of CAT and HQ in lake water sample and satisfactory result is obtained.