The Analyst

The purpose of this paper is to develop a sensitive and low-cost colorimetric method for detection of melamine using simple circuitry. The stable p-nitroaniline-modified silver nanoparticles (Ag NPs) were synthesized by a zero-length covalent coupling chemistry, which showed sensitivity and selectivity towards melamine. The melamine induces the aggregation of nanoparticles due to electron donor-acceptor interaction between melamine and p-nitroaniline at the Ag NP interface, resulting in a shift in the surface plasmon band and a consequent color change of the Ag NPs from yellow to blue. The color change was monitored using UV-vis spectrophotometry. More importantly, this optical method was successfully utilized to a practical infant formula sample. As low as 0.1 ppm melamine in infant formula could be distinguished upon a color change of solution in two minutes.

The roles of CdS nanocrystals (quantum dots) stabilized by oleic acid ligands in toluene were investigated in the 1,1'-oxalyldiimidazole chemiluminescence (ODI-CL) reaction generated under various environmental conditions. CdS excited by high-energy intermediates formed in ODI-CL reaction emitted dim CL. Also, CdS acted as a catalyst to enhance the yield of high-energy intermediates, capable of transferring energy to fluorescent molecules, in aqueous ODI-CL reactions, whereas it acted as a strong quencher, capable of inhibiting the CL emission of excited fluorescent molecules, in non-aqueous ODI-CL reactions. Based on the role of CdS in the aqueous ODI-CL reaction, the limit of detection (LOD = signal/noise = 3, 0.1 μM) determined to quantify glucose using aqueous ODI-CL reaction in the presence of 2.75 μM CdS was four times lower than that in the absence of CdS. The range of recovery determined in the aqueous ODI-CL reaction in the presence of CdS was 91.7-104%. We expect that the aqueous ODI-CL reaction in the presence of CdS can be applied as a highly sensitive sensor in various research fields such as bioanalytical chemistry, environmental engineering, homeland security, and toxicology.

A novel competitive 1,1'-oxalyldiimidazole (ODI) chemiluminescent enzyme immunoassay (CLEIA) was developed as a method for rapid and simple screening of melamine in milk. Fat existing in milk acts as an inhibitor in the competitive binding interaction of melamine and anti-melamine in the presence of melamine-conjugated horseradish peroxidase. Thus, the calibration curve and sensitivity of competitive ODI CLEIA for the quantification of melamine in fat free milk were wider and better than those in milk containing fat. However, a centrifuge is not a good method for removing the inhibitor because a portion of the melamine is also removed with the fat. The incubation time (20 min) for the competitive binding interaction of anti-melamine and melamine in 20% milk diluted with PBS buffer of pH 7.4 was longer than that (10 min) in 100% milk even though the sensitivity of the former was better than latter. The limit of detection (1.12 ppb) determined in rapid ODI CLEIA (dynamic range: 3.8-125 ppb) for the quantification of melamine in 20% milk not containing fat was lower than those (6.3 and 9.0 ppb) calculated in relatively time-consuming luminol CLEIA and enzyme-linked immunosorbent assay (ELISA). Also, we expect that ODI-CLEIA (dynamic range: 62.5-2000 ppb) capable of directly quantifying melamine in 100% milk without any pretreatment can be applied as a new and simple method for rapid screening of melamine in milk.

Solvent and pH effects on fast and ultrasensitive 1,1'-oxalyldi(4-methyl)imidazole chemiluminescence (OD4MI-CL) were studied. The influences of these two factors on the complex OD4MI-CL reaction are discussed within a conceptual prototype for developing aqueous and non-aqueous capillary electrophoresis (ACE and NACE) devices with OD4MI-CL detection. The reaction channel length and OD4MI yield from the reaction between bis(2,4,6-trichlorophenyl) oxalate (TCPO) and 4-methylimidazole in the channel will be influenced by pH, water volume fraction, and cosolvent properties of the solution. Optimum OD4MI-CL efficiency is observed at pH 6.5 when 1-propanol, which has a low dielectric constant (epsilon = 20.8), is used as the NACE solvent in the separation channel. Water (epsilon = 80.1), the solvent in the ACE separation channel, acts similarly to a high dielectric constant organic solvent in NACE because the disadvantages normally associated with TCPO-CL reactions in water disappear due to the faster OD4MI-CL reaction versus OD4MI decomposition in aqueous solution. Therefore, it is expected that the OD4MI-CL detection system can be used in both NACE and ACE devices without requiring detector modifications. We also conclude that OD4MI-CL detection in NACE and ACE devices will be much more sensitive than the TCPO-CL detection used in current NACE devices.

In the present work, for the first time, investigation of the initial fast step of reaction between 1,1,1-trichloro-3-methyl-3-phospholene (TCMP) and methanol was studied under a dry inert atmosphere by a newly constructed CSF apparatus by means of a further development in the configuration of the previous stopped-flow spectrophotometer (SFS). Hence, it was necessary to make many changes to the stopped-flow apparatus: replacement of the spectrophotometer amplifier with a conductance amplifier and the use of a conductivity cell to replace the optical one. The conductivity cell was made of polyethylene capillary tube (1 mm internal diameter; i.d.) and its inside dimensions were 1 mm i.d. × 3 mm long. Two tube electrodes which are made of stainless steel (0.8 mm i.d. × 14 mm long) were fixed at opposite ends of the observation cell and their outer surfaces were connected to the conductivity bridge amplifier by the two lead wires. The mixer was a 3-way Teflon valve and the distance between the mixer and the end of the observation cell was 30 mm. For each run, at least 24 µL of solution was required for a typical trace with a dead time of about 5 ms. Because of the extreme sensitivity of TCMP to moisture, the stopped-flow (CSF) apparatus was used inside a glove bag under a dry nitrogen atmosphere. Kinetic parameters for pseudo first-order reaction involving k(obs) = 30 s(-1) at 22 °C and activation energy E(a) = 13.55 KJ mol(-1) were successfully calculated for the initial fast step of the reaction between TCMP and methanol at 22 °C.

A new fluorescence reagent, 2-methylbenzo[b][1,10]phenanthrolin-7(12H)-one (mBPO), synthesized in our laboratory was used as the probe for protein and its interaction with Bovine Serum Albumin (BSA) was investigated in detail in this paper. It was found that BSA had the ability to quench the fluorescence of mBPO at 411 nm (λ(ex) = 286 nm), and the quenched intensity of fluorescence was proportional to the concentration of BSA. Based on this fact, mBPO has been used as a fluorescence probe for the detection of BSA. Under the optimal conditions, the calibration graph is linear up to 0.5 mg L(-1) for BSA and the limit of detection (LOD) was 0.06 mg L(-1). The regression equation is y = 1048.8x + 7.2093 with R(2) = 0.9913. The mechanism for the interaction of mBPO with BSA was also studied, while the binding constant and the number of binding sites were calculated. According to the thermodynamics parameter, the binding mode between mBPO and BSA was deduced. The results suggested the interaction between mBPO and BSA to be hydrophobic force in nature. It also proved that the fluorescence quenching reaction was affected by the tryptophan residue of BSA. For there are two tryptophan (Trp) residues, in site 134 and site 212 of BSA, and mBPO maybe has interaction with them respectively.

To improve the electrocatalytic activities of carbon nanotubes (CNT) towards the oxidation of nicotinamide adenine dinucleotide (NADH), we derive them with a redox mediator, 1,10-phenanthroline-5,6-dione (PD), by the noncovalent functionalization method. The redox carbon nanotubes (PD/CNT/GC) show excellent electrocatalytic activities towards the oxidation of NADH (catalytic reaction rate constant, k(h) = 7.26 × 10(3) M(-1) s(-1)), so the determination of NADH can be achieved with a high sensitivity of 8.77 μA mM(-1) under the potential of 0.0 V with minimal interference. We also develop an amperometric ethanol biosensor by integration of alcohol dehydrogenase (ADH) within the redox carbon nanotubes (PD/CNT/GC). The ethanol biosensor exhibits a wide linear range up to 7 mM with a lower detection limit of 0.30 mM as well as a high sensitivity of 10.85 nA mM(-1).

A method for the determination of iron(II) in water in the presence of iron(III) with 1,10-phenanthroline is described, in which the interfering effect of iron(III) ions is suppressed by masking with complexones. In the absence of a chelating agent for iron(III) the colour intensity of samples being analysed is unstable, owing to redox processes induced by the effect of 1,10-phenanthroline on the iron(II)-iron(III) system, causing reduction of iron(III) and formation of the corresponding coloured iron(II) chelate compound in a stoicheiometrically proportional concentration. The advantages of complexones, especially nitrilotriacetic acid, over other chelating agents are discussed.

A functional polymer for the catalysis of the chemiluminescent reaction and molecular recognition ability of 1,10-phenanthroline was prepared based on the molecular imprinting method. The structural and catalytic roles of transition metal ion interactions were applied in the material design. A ternary complex, 4-vinylpyridine-Cu(II)-1,10-phenanthroline (2:1:1), was synthesized and used as a functional monomer. The ligand 1,10-phenanthroline in the ternary complex was the template used to form the molecularly imprinted polymer. Another monomer, styrene, and the cross-linking reagent divinylbenzene were copolymerized with the ternary complex. The polymer containing the ternary complex is an efficient catalyst for the decomposition of hydrogen peroxide. During the hydrogen peroxide decomposition, superoxide radical ion is formed, which reacts with 1,10-phenanthroline and gives a chemiluminescent emission. The 1,10-phenanthroline was destroyed during the chemiluminescent reaction, leaving a cavity and copper binding sites for another 1,10-phenanthroline molecule. The prepared polymer particles were packed into a glass tube and developed as a molecular recognition chemiluminescent flow-through sensor for 1,10-phenanthroline. The sensitivity and selectivity of the sensor were tested.

A simple and accurate spectrophotometric method is proposed for the determination of tannins in tea and beer samples based on the reduction of iron(III) to iron(II) by tannins at 80 degrees C for 20 min. The iron(II) was then reacted with 1,10-phenanthroline at pH 4.4 to form a coloured complex. Background correction could be effected by precipitating the tannins in the sample solution twice with gelatin and kaolin. Absorbance measurements were made at 540 nm and the calibration graph was linear from 0 to 5.5 micrograms ml-1 of tannic acid with a slope of 0.213 A p.p.m.-1. The precision for the determination of tannins in a tea sample containing 9.45% of tannins was 1.8%. Most of the ingredients commonly found in tea and beer samples do not interfere with the determination. Several tea and beer samples were analysed for their tannin content using the proposed method.

Methods are described for the determination of the nitrate esters, ethylene glycol dinitrate, propylene glycol 1,2-dinitrate, ethylene glycol mononitrate, propylene glycol 1-mononitrate and propylene glycol 2-mononitrate, in blood. These esters can be removed efficiently from blood by extraction with diethyl ether, and determination of the dinitrates can be carried out colorimetrically, after alkaline hydrolysis, by a diazotisation and coupling reaction. As the mononitrates are not determined by this procedure, they do not interfere with the determination of the corresponding dinitrate. Gas chromatography, with the electron-capture detector, provides a sensitive method for detecting and measuring the mononitrates, with adequate separation from the parent dinitrate, and the latter can also be determined by this method as an alternative to the colorimetric procedure. For both methods the effect of possible interferences is investigated, and the efficiency of recovery from blood over the range 0 to 25 µg of ester is reported.

The derivatization of amphetamine and methamphetamine with 1,2-naphthoquinone-4-sulfonate (NQS) into solid-phase extraction cartridges (C18) is described. Optimum conditions were the use of carbonate-hydrogencarbonate buffer of pH 10, a 10 min reaction time at 25 degrees C and an NQS concentration of 9.6 x 10(-3) M. The accuracy and the precision of the method were tested. The results obtained with the proposed liquid-solid procedure were compared with those obtained with a traditional liquid-liquid extraction with hexane-ethyl acetate. The procedure was used to measure amphetamine in pharmaceutical and urine samples.

An atomic-absorption spectrometric method for the determination of trace amounts of copper in solution with 1,2-naphthoquinone thiosemicarbazone is described. This compound reacts with copper in a weakly acidic medium to produce a complex that can be extracted into isobutyl methyl ketone. The atomic absorption of the organic phase is measured at 324.8 nm. The sensitivity of the method is 0.6 ng ml-1 for 1% absorption in aqueous solution and the presence of several milligrams of 55 foreign ions is tolerated. The method has been applied successfully to the analysis of waters, foods and analytical reagents.

Selenium(IV) reacts with 1,2-diamino-4-nitrobenzene to form 5-nitropiazselenol, which is detected by means of a gas chromatograph equipped with an electron-capture detector. Trace amounts of selenium(VI) and total selenium in milk, milk products and albumin were determined by this method with a practical detection limit of 0.005 µg. The amount of selenium(VI) found in milk was about 60% of the total selenium content.

A sensitive extraction-spectrophotometric method for the determination of iron has been developed. Iron(II) forms a complex cation with 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (PDT) in aqueous solution and may be extracted into 1,2-dichloroethane (DCE) with tetrabromophenolphthalein ethyl ester (TBPE) as an ion pair at pH 5–7.5. The maximum absorbance of the extracted species occurs at 610 nm. The calibration graph is rectilinear over the range 0–0.25 p.p.m. of iron(II) and the apparent molar absorptivity of the ion-association complex is 1.9 × 105 l mol–1 cm–1. The extracted species has a molar ratio of Fe(II): PDT : TBPE = 1:3:2. The method has been applied to the determination of iron in various sera and also to river and well waters.

High-performance liquid chromatographic methods have been developed for the determination of ampicillin (ABPC), amoxicillin (AMPC) and ciclacillin (ACPC) in serum and urine. The methods involve acetylation of these aminopenicillins with acetic anhydride in aqueous solutions (pH 9.0) at ambient temperature for 3 min followed by reaction with 2 M 1,2,4-triazole and 10–3M mercury(II) chloride in solution (pH 9.0) at 60 °C for 10 min. The resulting products were separated on a C18 column following stabilisation in an eluent containing sodium thiosulphate. They were detected at 328 nm for ABPC and AMPC and 327 nm for ACPC.

Liquid and poly(vinyl chloride) membrane electrodes that are highly selective and sensitive to benzene-1,2,4-tricarboxylic acid (trimellitic acid) were developed. Both electrodes exhibit a linear response in the range 10–1–10–5M over the pH range 6–9 with a near-Nernstian slope of 27–28 mV per concentration decade at 25 °C. As little as 1 mg ml–1 of trimellitic acid can be measured directly with an average recovery of 98.7%(standard deviation 1.8%) and without interference from many organic acids and inorganic anions.

A high-performance liquid chromatographic method has been developed for the assays of carbenicillin (CBPC), ticarcillin (TIPC) and sulbenicillin (SBPC) in serum and urine. These penicillins were reacted with 2 M 1,2,4-triazole and 10–3M mercury(II) chloride in solution (pH 9.0) at 60 °C for 10 min. The resulting products were separated on a C18 column using a mobile phase containing tetrabutylammonium bromide and sodium thiosulphate, and were detected at between 327 and 331 nm. The method has been applied to the assays of these penicillins in serum and urine samples, which were ultra-filtered with a YMT membrane, or filtered with an acrylate copolymer membrane. The procedure permits the accurate determination of penicillin concentrations in serum down to 0.05 µg ml–1 for SBPC and 0.1 µg ml–1 for CBPC and TIPC, and is specific to intact penicillins without interference from corresponding penicilloic acids.

We report the first use of 3-amino-5-mercapto-1,2,4-triazole (AMT) to construct a surface-enhanced Raman scattering (SERS) based pH nano- and microsensor, utilizing silver nanoparticles. We optimize the procedure of homogenous attachment of colloidal silver to micrometer-sized silica beads via an aminosilane linker. Such micro-carriers are potential optically trappable SERS microprobes. It is demonstrated that the SERS spectrum of AMT is strongly dependent on the pH of the surroundings, as the transformation between two different adsorption modes, upright (A form) and lying flat (B form) orientation, is provoked by pH variation. The possibility of tuning the nanosensor working range by changing the concentration of AMT in the surrounding solution is demonstrated. A strong correlation between the pH response of the nanosensor and the AMT concentration in solution is found to be controlled by the interactions between the surface and solution molecules. In the absence of the AMT monomer, the performance of both the nano- and microsensor is shifted substantially to the strongly acidic pH range, from 1.5 to 2.5 and from 1.0 to 2.0, respectively, which is quite unique even for SERS-based sensors.

This paper describes the development and preliminary testing of a competitive surface-enhanced Raman scattering (SERS) immunoassay for calcitriol, the 1,25-dihydroxy metabolite (1,25-(OH)(2)-D(3)) of vitamin D(3). Deficiencies in 1,25-(OH)(2)-D have been linked to renal disease, while elevations are linked to hypercalcemia. Thus, there has been a sharp increase in the clinical demand for measurements of this metabolite. The work herein extends the many attributes of SERS-based sandwich immunoassays that have been exploited extensively in the detection of large biolytes (e.g., DNA, proteins, viruses, and microorganisms) into a competitive immunoassay for the low level determination of a small biolyte, 1,25-(OH)(2)-D(3) (M(w) = 416 g mol(-1)). The assay uses surface modified gold nanoparticles as SERS labels, and has a dynamic range of 10-200 pg mL(-1) and a limit of detection of 8.4 ± 1.8 pg mL(-1). These analytical performance metrics match those of tests for 1,25-(OH)(2)-D(3) that rely on radio- or enzyme-labels, while using a much smaller sample volume and eliminating the disposal of radioactive wastes. Moreover, the SERS-based data from pooled-patient sera show strong agreement with that from radioimmunoassays. The merits and potential utility of this new assay are briefly discussed.

2-Nitro-5,6-dimethylindane-1,3-dione is a new orally active anti-asthmatic drug and is administered in the form of the monohydrate sodium salt. It possesses three electro-reducible groups, namely the two carbonyl and the nitro group, the reduction of which are pH dependent. The best defined waves for analytical purposes are those produced by the 4e reduction of the nitro group in acidic media, which, when combined with the fact that the known breakdown products and impurities do not contain this electro-active group, endows a polarographic method for the assay of this drug in raw materials and formulations with enhanced selectivity.

The presence of various BHC isomers in the human organism has received relatively little attention, and studies were often restricted to only one BHC isomer. A previously described one-step extraction and clean-up procedure before gas-liquid chromatographic determination of organochlorine pesticide residues in human blood has now been applied to various organs and depot fat. The identification of the BHC isomers was performed by gas-liquid chromatography with several stationary liquid phases. 2,2-Dimethylpropane-1,3-diol succinate was found to be the most satisfactory stationary liquid phase for the present purpose, as it gave distinctly separated peaks and characteristic relative retention times.

The electrooxidation of dopamine (DA), uric acid (UA) and their mixture on a gold electrode modified by a self-assembled monolayer of 2-(3,4-dihydroxyphenyl)-1,3-dithialone has been studied by cyclic voltammetry (CV), chronoamperometry and differential pulse voltammetry (DPV). CV was used to investigate the redox properties of the modified electrode at various scan rates and the apparent charge transfer rate constant (k(s)), and transfer coefficient (α) were calculated. The mediated oxidation of DA at the modified electrode under the optimum condition (pH = 7.0) in CV occurs at a potential about 220 mV less positive than that of the unmodified gold electrode. The values of electron transfer coefficients (α), catalytic rate constant (k) and diffusion coefficient (D) were calculated for DA, using electrochemical methods. DPV exhibited a linear dynamic range over the concentration range of 0.2-250.0 μM and a detection limit (3σ) of 0.07 μM for DA. The modified electrode was used for simultaneous determination of DA and UA by DPV. The results showed that the electrode is highly efficient for the catalytic electrooxidation of DA and UA, leading to a remarkable peak resolution (~350 mV) for two compounds. The electrode was used for the determination of DA in an injection sample.