Molecular Recognition of Aromatic Nitro Compounds at Cyclodextrin Dithiocarbamate-modified Electrodes

Silver electrodes were modified by the adsorption of α-, β-, and γ-cyclodextrin dithiocarbamates (2α–γ) and characterized by reductive desorption experiments. Their molecular recognition properties were studied by cyclic voltammetry using three families of positional isomers of aromatic nitro compounds. Electrodes modified with 2α and 2β were selective for meta and para isomers while 2γ showed little selectivity. These observations are explained in terms of cavity sizes and guest structure. Computational studies suggest that the main reason for the observed selectivity is the different position of the NO² group in ortho and para isomers with respect to the CD cavity.     

流动注射在线预富集-高效液相色谱法测定四种硝基类化合物

建立了以香烟过滤嘴纤维作吸附剂,在线固相萃取-高效液相色谱（SPE—HPLC）测定水中邻硝基苯甲酸、对硝基苯胺、邻硝基苯酚、3-氯硝基苯四种硝基类化合物的方法。邻硝基苯甲酸、对硝基苯胺、邻硝基苯酚、3-氯硝基苯分别在0．006～4．80、0．003～2．40、0．002～1．60、0．002～1．60mg／L范围内峰面积与浓度呈线性关系,相关系数分别为0．9994、0．9996、0．9997和0．9996;检出限（S／N=3）分别为1．0、0．8、0．6,0．6μg／L;富集倍数分别为28．2、176．6、172．1、153．3。该法用于河水中四种硝基类化合物的测定,回收率为85．41％～116．44％,相对标准偏差在1．1％～5．4％范围内。     

Carbon-Paste Electrodes with Incorporated Lactate Oxidase and Mediators

Abstract

Amperometric bipelectrodes based on carbon paste modified with L-lactate oxidase and mediators are described which are sensitive to L-lactaie. Several ferrocenes and phenoxazine derivatives as mediators are evaluated, with Meldola blue exhibiting the best results.

With ferrocene derivatives the bioelectrodes were operated at potential of 0.2-0.4 V (vs SCE). The apparent Michaelis constants for the calibration curves of the bioelectrodes are different depending on the chosen mediator and varied in the range from 0.9 to 3.6 mM L-lactate. The pH optimum in the case of the 1,1′-dimethylferrocene bioelectrode was 9.3 and the limiting sensitivity was determined to be 31 μA/mM cm^?2. Covered by a Nation membrane, the electrode sensitivity was in the range of 3.2-4.4 μA/mM cm^?2 at pH 7.0 and the calibration range was linear up to 2-2.5 mM L-lactate.

The bioelectrode based on Meldola blue mediated electron transfer at potentials between 0.05-0.25 V. The Km(app) was 17.0 and 18.2 mM at 0.1 and 0.05 V, respectively. The pH optimum of the bioelectrodes was 7.9 and limiting sensitivity - 164μA/mM cm^?2.

At normal physiological level of ascorbic acid (50 μM) in blood the response of the Meldoia blue based sensors was 5.3 % at 0.1 V, whereas that of the 1,1′-dimethylferrocene bioelectrode was 83 % at 0.25 V in comparison to the response observed with physiological levels of L-lactate (2 mM).     

Voltammetric Determination of Thiourea at Conductive Diamond Electrodes

The oxidation of thiourea (TU) at boron‐doped diamond (BDD) electrodes was investigated by the use of anodic voltammetry. The results indicated that the overall TU oxidation reaction is rather complicated and takes place via two steps: a slow electron‐transfer yielding the corresponding free radical, followed by a fast oxidation of this radical, prior to its dimerization. It was found that in acidic media the voltammetric response is suitable for analytical applications, and unlike glassy carbon (GC), BDD electrodes exhibit very low susceptibility to adsorption. The same conclusion was supported by the results of AC voltammetric measurements. Based upon the voltammetric peak for the first step of TU oxidation, a method is proposed for the determination of this compound in the micromolar concentration range. The analytical performance characteristics of the method are comparable to those reported for TU determination by the use of platinum electrodes or enzyme‐modified platinum electrodes.     

Spectrophotometric Determination of Ampicillin with Some Nitro Compounds

Abstract

A spectrophotometric procedure for the determination of ampicillin (Amp.) in pure solutions and in its pharmaceutical preparations has been developed. This new method offers advantages of simplicity, rapidity and stability in comparison with the official BP (1980) method. The proposed method is based on the formation of a colour condensation product through the reaction of ampicillin and nitrobenzene derivatives in an alkaline aqueous-acetone medium 40% (v/v). Beer's law is obeyed in the range 0.5–28 μg ml^?1. The colours were produced within 20 min. after heating at 60±5°C and stable for at least 6 h. The method is relatively accurate (recovery 100±2%) and precise (RSD 1.9%) and can be used successfully for the preparation of capsules, syrup and ampoules.     

Voltammetric Study of Nitro Radical Anion Generated from Some Nitrofuran Compounds of Pharmacological Significance

The electrochemical behavior of 2‐(5‐amino‐ 1,3,4‐oxadiazolyl)‐5‐nitrofuran (NF359) and its comparison with well‐known drugs such as nifurtimox (NFX) and nitrofurazone (NFZ) in protic, mixed and aprotic media by cyclic voltammetry, tast and differential pulse polarography was studied. All the compounds were electrochemically reducible in all media being the reduction of the nitrofuran group the main voltammetric signal. The one‐electron reduction couple due to the nitro radical anion formation was visualized in mixed (for NF359 and NFZ) and aprotic media (for all compounds). By applying a cyclic voltammetric methodology we have calculated the decay constants (k₂) of the corresponding nitro radical anions in mixed and aprotic media. In mixed medium data fit well with a disproportionation reaction of the nitro radical anion but in aprotic medium fit better with a dimerization reaction. Also, considering cyclic voltammetric measurements in aprotic media we have estimated the reduction potential of the RNO₂/RNO₂^.? couple in aqueous medium, pH 7 (E¹₇ values) finding very good correlation with E¹₇ values obtained by pulse radiolysis. Furthermore we have calculated the equilibrium constants from the electron transfer from nitro radical anion to oxygen (k_O2) finding that nitro radical anion from NF359 is thermodynamically favored to react with oxygen in respect to both NFZ and NFX.     

Voltammetric Determination of Azidothymidine Using Silver Solid Amalgam Electrodes

A new simple and direct electroanalytical method was developed for the determination of azidothymidine in commercial pharmaceutical preparations. It is based on differential pulse voltammetry at silver solid amalgam electrode with polished surface (p‐AgSAE) or surface modified by mercury meniscus (m‐AgSAE). The electroreduction of azidothymidine in basic media at these electrodes gives rise to one irreversible cathodic peak. Its potential in 0.05 mol L^?1 borate buffer, pH 9.3 at ca. ?1050 mV is comparable to that using hanging mercury drop electrode (HMDE). Achieved limits of quantitation are in the 10^?7 mol L^?1 concentration range for both amalgam electrodes. According to the procedure based on the standard addition technique, the recoveries of known amounts of azidothymidine contained in pharmaceutical preparations available in capsules were 101.4±1.8% (m‐AgSAE), 100.3±3.5% (p‐AgSAE) and 102.0±1.0% (HMDE) (n=10). There was no significant difference between the values gained by proposed voltammetric methods and the HPLC‐UV recommended by the United States Pharmacopoeia regarding the mean values and standard deviations.     

用碳糊修饰电极测定氨基酸的伏安法研究

用10％氧化铝的修饰碳糊电极研究了测定组氨酸的伏安法。 在0.05 mol/L 丁二酸－硼砂（pH=3.5)底液中在－0.6 V出现一还原峰, 结合2.5次微分技术测定组氨酸, 在3.20 ～130 μmol/L 浓度范围内有良好的线性关系, 相对标准偏差为3.4％, 检出限为0.4 μmol/L。 检测灵敏度比文献报道有较大的提高。 文中还讨论了电极过程。     

A New Procedure for Voltammetric Lead Determination Based on Coprecipitation and Centrifugation Preconcentration

An arrangement of a voltammetric cell specially designed to be compatible to both centrifugation and voltammetry was designed, allowing direct measurement without any need for dissolution or filtration steps. The procedure for the preconcentration via coprecipitation is described. The system was succesfully tested using trace amounts of Pb(II) ions in aqueous solution preconcentrated with Al(OH)₃ as the carrier precipitate and resulting peak currents were found much higher than those without applying centrifugation. The effects of the parameters on the resulting signal were examined and the detection limit was found as 2.2×10^?9 M.     

DNA Hydrolysis and Voltammetric Determination of Guanine and Adenine Using Different Electrodes

Abstract

This work reports the development of a biosensor method for the label‐free detection of specific DNA sequences. In the initial phase, square wave voltammetry (SWV) was used in a comparative investigation into the electrochemical oxidation of purines (guanine and adenine) and DNA fragments at various electrode surfaces: carbon paste (CPE), glassy carbon electrode (GCE), and gold (AuE). Relative to the carbon electrodes, an approximate 4.0‐fold, 6.0‐fold, and 3.25‐fold increase in the anodic response was observed when guanine, adenine, and hydrolyzed DNA, respectively, were measured on the AuE. It was shown that the guanine and adenine bases could be successfully determined by use of SWV for a deoxyribonucleic acid sample following acid hydrolysis. This label‐free detection of hydrolyzed DNA on gold electrodes has significant advantages over methods using existing carbon electrode materials because of its higher sensitivity and the potential applicability of microfabrication techniques for the production of the requisite gold electrodes.

In another phase of development, the times and conditions for DNA hydrolysis and purine release were investigated. It was shown that under optimal conditions, trace levels of the purine bases could be readily detected following 20 min of hydrolysis at room temperature. The proposed method can be used to estimate the guanine and adenine contents in DNA with in a linear range of 5–30 ng ml^?1.

Finally, when appropriate probe sequences were first adsorbed on the surface of the screen‐printed gold electrode (SPGE), this electrochemical biosensor could be used to specifically detect sequences from ss corona virus aviair following hybridization and hydrolysis reactions on the sensor surface. No enhancement of the voltammetric response was observed when the sensor was challenged with a non‐complementary DNA sequence.     

Preconcentration and Determination of Metal Species at Polypyrrole Modified Glassy Carbon Electrodes

Metal species can be extracted/preconcentrated onto electropolymerized polypyrrole film electrodes without deliberate incorporation of chemically active counterions. Silver species preconcentrated on polypyrrole film can be determined electrochemically. The effects of polypyrrole film coverage, electropolymerization conditions,solution composition and the presence of other species have been investigated. Experimental results showed that the anodic peak current for incorporated silver species was proportional to silver concentration in solution in the range from 2 to 150 μM. The detection limit was estimated to be about 0.2 \μM (20 ppb). Most of the metal ions studied, including Ni, Cd, Pb, Zn and Fe, did not show obvious interference on the determination. Cu and Hg were also extracted onto polypyrrole film electrodes. Polypyrrole film electrodes were reuseable. The mechanism involved in the preconcentration was also investigated.     

Voltammetric Determination of Cymoxanil and Famoxadone at Different Types of Carbon Electrodes

Differential pulse voltammetry (DPV) at a carbon fibre rod electrode (CFRE) and a capillary carbon paste electrode (CPE) have been used for the determination of pesticides cymoxanil and famoxadone, respectively. In the cathodic potential range, optimum conditions were found for the determination of cymoxanil by DPV at CFRE at pH 4 with limit of quantification (L_Q) of 5.9×10^?7 mol L^?1. In the anodic area, determination of famoxadone by DPV at CPE was performed at optimum pH 2 with L_Q=1.4×10^?7 mol L^?1. Practical applicability of the newly developed methods was verified on spiked samples of river water and soil.     

Simultaneous Voltammetric Determination of Molybdenum and Copper in Manganese Compounds

 Adsorptive voltammetry was employed for the determination of copper and molybdenum in manganese compounds. As working electrode the hanging mercury drop electrode (HMDE) was used. The method was applied in aqueous solutions of MnCl₂ and Mn(NO₃)₂, as well as in pre-treated manganese dioxide and manganese ores. The detection limits are 3 μg/g for copper and 5 μg/g for molybdenum in the sample. The RSDs at concentration level of 8 μg/g are 2.2 and 3.2% for copper and molybdenum, respectively.     

Utilization of Carbon Paste Electrodes for the Voltammetric Determination of Chlortoluron

Conventional (CPE) and miniaturized (m‐CPE) carbon paste electrodes consisting of a carbon paste filled capillary were used for differential pulse voltammetric determination of chlortoluron in samples of river water and soil, in the latter case after the extraction by methanol. Britton‐Robinson buffer pH 3 with low content of methanol was found to be optimal for the determination. The achieved determination limits were 2.8 µmol L^?1 and 0.34 µmol L^?1 in river water, and 3.1 and 4.3 µg g^?1 in soil, using CPE and m‐CPE, respectively.     

甲醛的铂微粒修饰玻碳电极伏安法测定

采用电化学沉积法制备了铂微粒修饰玻碳电极(Pt/GCE).通过循环伏安法研究了甲醛在该电极上的电化学行为,并优化了实验参数,在此基础上建立了一种伏安法直接测定甲醛的方法.在酸性溶液中,甲醛的氧化峰电流与其浓度在1.0×10-5～1.0×10-3mol/L的范围内呈良好的线性关系(r=0.999 7),检出限为5.0 μmol/L,回收率为96%～104%,相对标准偏差为4.0%～4.5%.     

Voltammetric Determination of Phosphate in Brazilian Biodiesel Using Two Different Electrodes

The presence of contaminants, such as phosphate, in biodiesel, has several drawbacks for instance: current engines perform poorly, fuel tanks deteriorate, catalytic conversion is damaged, and particles emission is increased. Therefore, biodiesel quality control is extremely important for biodiesel acceptance and commercialization worldwide. In this context, a bare glassy carbon electrode (GCE) and another chemically modified electrode with iron hexacyanoferrate (Prussian Blue – PB) were developed for determination of phosphate in biodiesel. The LODs of 6.44 and 1.19 mg kg^?1, and LOQs of 21.43 and 3.97 mg kg^?1 were obtained for the bare GCE and the PB‐modified GCE, respectively. The methodology was employed for analysis of Brazilian biodiesel samples, and it led to satisfactory results, demonstrating its potential application for biodiesel quality control. Additionally, recovery and interference tests were conducted, which revealed that the developed methods are suitable for analysis of phosphate in biodiesel samples.     

Preconcentration and Voltammetric Determination of the Antihypertensive Doxazosin on a C8 Modified Carbon Paste Electrode

 An electrochemical study of the doxazosin oxidative process at carbon paste electrodes using different voltammetric techniques has been carried out. The process is irreversible and controlled by adsorption, giving rise to an oxidation wave around 1.0 V in citric acid-citrate buffer (pH 3.0). A mechanism based on the oxidation of the amine group is postulated. Two methods based on adsorptive stripping (AdS) of doxazosin at the C₈-modified carbon paste electrode (C₈-MCPE), before its voltammetric determination, are studied, using differential pulse voltammetry (DPV) and square wave voltammetry (SWV) as redissolution techniques. By means of AdS-DPV and C₈-MCPE, doxazosin can be determined over the 1.0 × 10⁻⁹ to 3.0 × 10⁻⁸ mol L⁻¹ range with a variation coefficient of 2.2% (2.0 × 10⁻⁸ mol L⁻¹) and a limit of detection of 7.4 ×10⁻¹⁰ mol L⁻¹. If AdS-SWV is used, a linear range from 1.0 × 10⁻⁹ to 4.0 × 10⁻⁸ mol L⁻¹ is obtained, the variation coefficient being 2.8% (2.0 × 10⁻⁸ mol L⁻¹, and the limit of detection reached 7.7 × 10⁻¹⁰ mol L⁻¹. The AdS-DPV procedure was applied to the determination of doxazosin in urine and formulations. Received March 13, 1999. Revision December 23, 1999.     

Adsorptive Stripping Voltammetric Determination of Phenol at an Electrochemically Pretreated Carbon-Paste Electrode with Solid Paraffin as a Binder

An adsorptive stripping voltammetric method for determination of phenol at an electrochemically pretreated carbon-paste electrode has been developed. Solid paraffin was used as the binder of the carbon-paste electrode. The carbon-paste electrode was pretreated in the solution of 0.001 mol L⁻¹NaOH by holding it at +1.8 V (versus an Ag/AgCl electrode) for 5 min. On the pretreated electrode, the adsorption of phenol was greatly enhanced. Phenol was accumulated in NH₃–NH₄Cl (pH 9.25) medium at the potential of +0.1 V (versus Ag/AgCl electrode) for a certain time and then determined by second order differential anodic stripping voltammetry. An oxidative peak was observed at about +0.66 V. The relationship between second order peak current and phenol concentration was linear in the range of 2.5 × 10⁻⁷–5.0 × 10⁻⁶mol L⁻¹phenol, and the detection limit was 5.0 × 10⁻⁸mol L⁻¹. The method has been applied to the determination of phenol in tap water and waste water. The relative standard deviation (six determinations) was less than 3.5%.