A graphene-based electrochemical sensor for sensitive determination of caffeine

An electrochemical sensor based on the electrocatalytic activity of graphene (Gr) for sensitive detection of caffeine is presented. The electrochemical behaviors of caffeine on Nafion-Gr modified glassy carbon electrode (Nafion-Gr/GCE) were investigated by cyclic voltammetry and differential pulse voltammetry. The results showed that the Nafion-Gr/GCE exhibited excellent electrocatalytic activity to caffeine. Caffeine can be effectively accumulated at Nafion-Gr/GCE and produce a sensitive anodic peak. Such electrocatalytic behavior of Gr is attributed to its unique physical and chemical properties, e.g., subtle electronic characteristics and strong adsorptive capability. This electrochemical sensor shows an excellent performance for detecting caffeine with a detection limit of 1.2×10(-7) M (S/N=3), a reproducibility of 5.2% relative standard deviation, and a satisfied recovery from 98.6% to 102.0%. The sensor shows great promise for simple and sensitive determination of caffeine.     

Determination of thiol compounds by solid-phase extraction using multi-walled carbon nanotubes as adsorbent coupled with high-performance liquid chromatography-fluorescence detection

Microchimica Acta - We describe a method for solid-phase extraction of biogenic thiols using multi-walled carbon nanotubes as adsorbent, and their subsequent determination via HPLC and fluorescence...     

Aptamer-based electrochemical assay of 17β-estradiol using a glassy carbon electrode modified with copper sulfide nanosheets and gold nanoparticles,and applying enzyme-based signal amplification

Microchimica Acta - We have developed an electrochemical method for the determination of 17β-estradiol. A glassy carbon electrode was modified with a composite made from copper sulfide...     

Ultra-trace determination of S-nitrosothiols in blood samples by spectrofluorimetry with 8-(3',4'-diaminophenyl)-difluoroboradiaza-s-indacene

Increasing evidence suggests that S-nitrosothiols (RSNO) may represent naturally occurring nitric oxide (NO) surrogates and function as intermediates in NO metabolism. In this work, a simple, sensitive, and selective micromethod is developed and validated for quantification of RSNO. A fluorescent probe 8-(3',4'-diaminophenyl)-difluoroboradiaza-s-indacence (DABODIPY) is firstly used to label RSNO. The derivatization reaction is performed in aqueous medium at 30 degrees C for 15min in the presence of 6x10(-5)molL(-1)Hg2+ and the derivative is detected by fluorescence at lambda(ex)/lambda(em)=500/510nm. A linear function of concentration in the range of (2.0-600.0)x10(-8)molL(-1) is observed with a correlation coefficient of 0.9992 and detection limit of 1.2x10(-9)molL(-1) (S/N=3). This technique has been successfully applied to quantify RSNO in some human blood samples including healthy persons and patients suffering from cardiovascular diseases.     

高效液相色谱-串联质谱联用测定脂肪中乙酰孕激素残留

建立了脂肪中5种乙酰孕激素,包括醋酸甲羟孕酮(MPA)、醋酸氯地孕酮(CMA)、醋酸甲地孕酮(MEGA)、甲烯雌醇醋酸酯(MLA)及17α-羟基孕酮醋酸酯(HPA)的液相色谱-串联质谱(LC-MS/MS)检测方法。考察了脂肪样品溶剂提取、固相萃取净化对样品分析的影响,确定样品各前处理过程的条件。采用电喷雾(ESI)阳离子,在选择反应监测模式(SRM)下,本方法对5种乙酰孕激素的检出限为0.2～0.3μg/kg,定量限为0.5μg/kg。在空白猪脂肪样品中以0.5、1.0和5.0μg/kg3个水平添加时,5种乙酰孕激素的回收率范围在60.5%～84.1%,RSD为8.9%～16.8%。     

Sensitive voltammetric determination of tyrosine using multi-walled carbon nanotubes/4-aminobenzeresulfonic acid film-coated glassy carbon electrode

A chemically modified electrode is constructed based on the multi-walled carbon nanotubes (MWNTs)/4-aminobenzeresulfonic acid (4-ABSA) film-coated glassy carbon electrode. The electrocatalytic oxidation of tyrosine (Tyr) is investigated on the surface of the MWNTs/4-ABSA-modified electrode using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The prepared modified electrode shows voltammetric responses with high sensitivity and selectivity for Tyr in optimal conditions, which makes it very suitable for sub-micromolar detection of Tyr. A sensitive oxidation peak at +0.64 V is employed to determine Tyr. Good linear relationship between the oxidation peak current and the Tyr concentration in the range of 1 × 10⁻⁷ to 5 × 10⁻⁵ mol/L is obtained in phosphate buffer solution with pH 7.0. By use of modified electrode, the voltammetric detection limit for Tyr in DPV measurements is 8 × 10⁻⁸ mol/L (S/N = 3). Good sensitivity, selectivity and stability of the low-cost modified electrode make it very suitable for the determination of trace amounts of Tyr in pharmaceutical and clinical preparations.     

Ultra-trace level determination of nitrite in human saliva by spectrofluorimetry using 1,3,5,7-tetramethyl-8-(3,4-diaminophenyl)-difluoroboradiaza-s-indacene

A rapid, highly sensitive and selective fluorogenic method for the determination of traces of nitrite is described. It is based on the reaction of weakly fluorescent 1,3,5,7-tetramethyl-8-(3,4-diaminophenyl)-difluoroboradiaza-s-indacence (DAMBO) and nitrite in acidic aqueous solution to give 1,3,5,7-tetramethyl-8-(5-benzotriazolyl)-difluoroboradiaza-s-indacene (DAMBO-T), which is highly fluorescent. The optimum reaction conditions and other analytical parameters are investigated to enhance the sensitivity of the method. The fluorescence enhancement at 507 nm is linearly related to the concentration of nitrite in the range of 6.0 × 10⁻⁹–5.0 × 10⁻⁷ mol L⁻¹ with a correlation coefficient of R = 0.9995 (n = 10) and a detection limit of 1.0 × 10⁻¹⁰ mol L⁻¹. The R.S.D. is 1.12% (n = 10). The method is applied to the determination of nitrite in human saliva samples with the recoveries of 96. 24–105.30%. Correspondence: Ke-Jing Huang, Department of Chemistry, Wuhan University, Wuhan 430072, P.R. China     

Sensitive determination of nitric oxide in some rat tissues using polymer monolith microextraction coupled to high-performance liquid chromatography with fluorescence detection

A simple, sensitive, selective, and low-cost method is proposed for rapidly determining nitric oxide (NO) in some rat tissues. Polymer monolith microextraction (PMME) using a poly(methacrylic acid–ethylene glycol dimethacrylate) (MAA-EGDMA) monolithic column was combined with derivatization of NO using 1,3,5,7-tetramethyl-8-(3′,4′-diaminophenyl)-difluoroboradiaza-s-indacene (TMDABODIPY), and this was used to analyze the derivatives of NO by high-performance liquid chromatography (HPLC) with fluorescence detection at λ _ex/λ _em = 498/507 nm. The baseline separation of TMDABODIPY and its NO derivative is performed under simple conditions in which a C₁₈ column is used and eluted with 50 mmol L⁻¹ ethanolamine and methanol. The conditions for the extraction of NO derivatives were optimized. The limit of detection of NO was 2 × 10⁻¹² mol L⁻¹ (S/N = 3). The linearity range of the method was 9 × 10⁻¹¹−4.5 × 10⁻⁸ mol L⁻¹. The interday and intraday relative standard deviations were less than 5%. The proposed method was successfully applied to the determination of NO levels in some rat tissue samples including heart, kidney, and liver with recoveries varying from 87.1 to 95.2%.     

New high-performance liquid chromatographic method for sensitive determination of pheomelanin in biological materials by precolumn fluorescence derivatization with naphthalene-2,3-dicarboxaldehyde

Pheomelanin is an important type of melanin distributed in the skin, eye and hair in the mammal, which is of great social, clinical and cosmetic significance. In this study, a new HPLC method with fluorescence detection is described originally for the sensitive determination of pheomelanin in biological materials. The pheomelanin polymer is decomposed into two specific degradation products, 3-amino-4-hydroxyphenylalanine (3-AHP) and 4-amino-3- hydroxyphenylalanine (4-AHP) with hydriodic acid. Then the two AHP isomers are derivatized using a fluorescent probe naphthalene-2,3-dicarboxaldehyde in the presence of cyanide. The resulting highly stable 2-substituted 1-cyanobenz[f] isoindole derivatives were separated on a 5NH(2)-MS aminopropyl packed HPLC column with binary isocratic elution profile and detected fluorimetrically. The assay shows high sensitivity of 0.11nM (2.2fmol per injection, the lowest reported) at signal-to-noise ratio of 3 for each AHPs, good accuracy and precision (RSDs<3.1%), and linearity (range of 0.02-10microM, r>0.995). The results obtained by using fluorescence detection have been compared with other detection systems (electrochemical and UV). The sensitivity can increase from 100 to respect electrochemical detection and 30000 times respect UV detection. The method has been used for the quantitative determination of pheomelanin in various biological samples, including cell cultures from five types of melanoma cell lines of human and rat origin, hair samples of various colors, melanoma tissue and the urines from human melanoma patients and healthy subjects. This original application of HPLC-fluorescence detection represents a powerful tool for investigating pheomelanin synthesis in vitro and in vivo under physiological and pathophysiological conditions.