A sensitive enzymatic method for paraoxon detection based on enzyme inhibition and fluorescence quenching

A sensitive and selective method for the paraoxon detection based on enzyme inhibition and fluorescence quenching was presented in this study. Under the catalytic effect of acetylcholinesterase (AChE), acetylthiocholine (ATCh) hydrolysis released thiocholine (TCh) which could react with N-(7-dimethylamino-4-methylcoumarin-3-yl) maleimide (DACM) to produce a blue fluorescence compound. Subsequently, AChE catalytic activity was inhibited with the addition of paraoxon, which caused TCh decreased, leading to a significant decrease of the blue fluorescent compound. Meanwhile, p-nitrophenol, the hydrolysis product of paraoxon, would lead to a quenching of the fluorescence. Therefore, fluorescence intensity of the system would decrease dramatically by a combined effect of enzyme inhibition and fluorescence quenching. Under optimal experimental conditions, an excellent linear relationship between the decrease of fluorescence intensity and paraoxon concentration over the range from 5.5 × 10⁻¹² to 1.8 × 10⁻¹⁰ mol L⁻¹ was obtained. Fluorescence background caused by nonenzymatic hydrolysis of ATCh or other matters was relatively low, the proposed approach offered adequate sensitivity for the detection of paraoxon at 3.5 × 10⁻¹² mol L⁻¹.     

A new resonance scattering spectral method for the determination of trace amounts of iodate with rhodamine 6G

Under the conditions of 0.04 mol L⁻¹ HCl-8.0 × 10⁻⁴ mol L⁻¹ KI, there is a fluorescence peak at 540 nm and a synchronous fluorescence peak at 540 nm for rhodamine 6G (RhG). When there is IO₃⁻, it reacts with exceed I⁻ to form I₃⁻. And I₃⁻ and RhG combine into ion association particles. The particles exhibit three resonance scattering peaks at 320, 400 and 595 nm. And there is fluorescence quenching at 540 nm. Iodine concentration is proportional to the intensity of the resonance scattering intensity at 400 nm in the range of 1.0-20 × 10⁻⁷ mol L⁻¹. And a new resonance scattering spectral (RSS) method has been described for the determination of IO₃⁻ in salt samples. The spectral results have been verified that the formation of (RhG-I₃)_n association particles and solid-liquid interfaces are the main factor that cause the fluorescence quenching and resonance scattering effects.     

Triethanolamine-capped CdSe quantum dots as fluorescent sensors for reciprocal recognition of mercury (II) and iodide in aqueous solution

Water-soluble luminescent CdSe quantum dots surface-modified with triethanolamine (TEA-CdSe-QDs) were prepared with high stability. The fluorescence of the TEA-CdSe-QDs was greatly quenched only when Hg²⁺ and I⁻ coexisted in the solution, whereas addition of either Hg²⁺ or I⁻ individually has no noticeable effect on the fluorescence emission. Such a unique quenching effect could be used for reciprocal recognition of mercury (II) ions and/or iodide anions in aqueous solution with rather high selectivity and sensitivity. The detection limits of Hg²⁺ or I⁻ ion were 1.9 × 10⁻⁷ mol L^-1 or 2.8 × 10⁻⁷ mol L⁻¹, respectively. The adequate experiments showed that iodine (I) anions could bridge between TEA-CdSe-QDs and Hg²⁺ to form a stable complex (QDs-I⁻-Hg²⁺) and the following effective electron transfer from the QDs to the Hg²⁺ could be responsible for the fluorescence quenching of QDs.     

Study on the interaction between oxolinic acid aggregates and protein and its analytical application

It was found that oxolinic acid (OA) at high concentration can self-assemble into nano- to micro- meter scale OA aggregates in Tris-HCl (pH 7.48) buffer solution. The nanoparticles of OA were adopted as fluorescence probes in the quantitative analysis of proteins. Under optimum conditions, the fluorescence quenching extent of nanometer scale OA aggregates was in proportion to the concentration of albumins in the range of 3.0 × 10⁻⁸ to 3.0 × 10⁻⁵ g mL⁻¹ for bovine serum albumin (BSA) and 8.0 × 10⁻⁸ to 8.0 × 10⁻⁶ g mL⁻¹ for human serum albumin (HSA). The detection limits (S/N = 3) were 3.4 × 10⁻⁹ g mL⁻¹ for BSA, and 2.6 × 10⁻⁸ g mL⁻¹ for HSA, respectively. Samples were satisfactorily determined. The interaction mechanism of the system was studied using fluorescence, UV-vis, resonance light scattering (RLS) and transmission electron microscope (TEM) technology, etc., indicating that the nonluminescent complex was formed between serum albumin molecular and OA, to disaggregate the self-association of OA, which resulted in the dominated static fluorescence quenching in the system.     

Facile synthesis of N-acetyl-L-cysteine capped ZnS quantum dots as an eco-friendly fluorescence sensor for Hg2+

This paper described an investigation of a novel eco-friendly fluorescence sensor for Hg²⁺ ions based on N-acetyl-l-cysteine (NAC)-capped ZnS quantum dots (QDs) in aqueous solution. By using safe and low-cost materials, ZnS QDs modified by NAC were easily synthesized in aqueous medium via a one-step method. The quantitative detection of Hg²⁺ ions was developed based on fluorescence quenching of ZnS QDs with high sensitivity and selectivity. Under optimal conditions, its response was linearly proportional to the concentration of Hg²⁺ ions in a range from 0 to 2.4 × 10⁻⁶ mol L⁻¹ with a detection limit of 5.0 × 10⁻⁹ mol L⁻¹. Most of common physiologically relevant cations and anions did not interfere with the detection of Hg²⁺. The proposed method was applied to the trace determination of Hg²⁺ ions in water samples. The possible quenching mechanism was also examined by fluorescence and UV-vis absorption spectra.     

Flow injection system with in-line Winkler's procedure using 16-way valve and spectrofluorimetric determination of dissolved oxygen in environmental waters

Flow injection spectrofluorimetry with in-line Winklers procedure was developed for the dissolved oxygen (DO) determination. 2-Thionaphthol reacted with iodine produced by Winkler’s method to form fluorescence inactive disulfide compound. To automate the process completely, a 5-channel flow system with a newly designed 16-way valve was assembled. The system consisted of a dispersion coil (DC), a precipitate formation coil (PFC), a precipitate dissolving coil (PDC), and extraction coil (EC). A calibration can be constructed by using a standard iodine solution for dissolved oxygen. The calibration graph was linear over the range 1.2×10⁻⁴∼6.0×10⁻⁴ mol l⁻¹ iodine (1.96∼9.80 mg O l⁻¹)). The relative standard deviation (n=6) was below 0.3% for the 4×10⁻⁴ mol l⁻¹ iodine (6.27 mg O l⁻¹) determination. The sample throughput was 12/h.     

Preparation, characterization and evaluation of water-soluble l-cysteine-capped-CdS nanoparticles as fluorescence probe for detection of Hg(II) in aqueous solution

Water-soluble l-cysteine-capped-CdS nanoparticles were prepared in aqueous solution at room temperature through a straightforward one-pot process by using safe and low-cost inorganic salts as precursors, and characterized by transmission electron microscopy, X-ray diffraction spectrometry, Fourier transform infrared spectrometry, spectrofluorometry and ultraviolet-visible spectrometry. The prepared l-cysteine-capped-CdS nanoparticles were evaluated as fluorescence probe for Hg(II) detection. The fluorescence quenching of the l-cysteine-capped-CdS nanoparticles depended on the concentration and pH of Hg(II) solution. Maximum fluorescence quenching was observed at pH 7.4 with the excitation and emission wavelengths of 360 nm and 495 nm, respectively. Quenching of its fluorescence due to Hg(II) at the 20 nmol l⁻¹level was unaffected by the presence of 5 × 10⁶-fold excesses of Na(I) and K(I), 5 × 10⁵-fold excesses of Mg(II), 5 × 10⁴-fold excesses of Ca(II), 500-fold excesses of Al(III), 91-fold excesses of Mn(II), 23.5-fold excesses of Pb(II), 25-fold excesses of Fe(III), 25-fold excesses of Ag(I), 8.5-fold excesses of Ni(II) and 5-fold excesses of Cu(II). Under optimal conditions, the quenched fluorescence intensity increased linearly with the concentration of Hg(II) ranging from 16 nmol l⁻¹ to 112 nmol l⁻¹. The limit of detection for Hg(II) was 2.4 nmol l⁻¹. The developed method was applied to the detection of trace Hg(II) in aqueous solutions.     

Mn(II)-sodium dodecyl sulphate complex mimic enzyme-catalyzed fluorescence quenching of Pyronine B by hydrogen peroxide

Mn(II)-sodium dodecyl sulphate complex (Mn(II)-SDS) is used to mimic the active group of peroxidase. The catalytic characteristic of this mimic enzyme catalyst in the oxidation reaction of fluorescence substrate, tetraethyldiaminoxanthyl chloride (Pyronine B (PB)), with hydrogen peroxide has been studied. The experimental results show that Mn(II)-SDS complex has similar catalytic activity that of peroxidase. The steady-state catalytic rate depends upon mimic enzyme and substrate concentrations, and the Michaelis-Menten parameters K_m, V_max and K_cat are 7.6×10⁻⁶ M, 7.9×10⁻⁷ M s⁻¹ and 7.9 s⁻¹, respectively. The catalytic activity of Mn(II)-SDS complex is compared with those of HRP and Hemin. Though the catalytic activity of Mn(II)-SDS complex is 15.9% of that of HRP, it can catalyze the oxidation reaction of PB with hydrogen peroxide lead to fluorescence quenching of PB. Under optimum conditions, linear relationship between fluorescence quenching F₀/F and concentration of H₂O₂ is in the range of (0.0-3.6) × 10⁻⁷ M. The detection limit is determined to be 3.0×10⁻⁹ M. By coupling this mimic catalytic reaction with the catalytic reaction of glucose oxidase (GOD), glucose can be detected. Linear relationship between F₀/F and concentration of glucose is in the range of (0.0-1.4) × 10⁻⁷ M. The detection limit is determined to be 4.2×10⁻⁹ M. This method is applied to the determination of glucose in human serum and the results are in good agreement with the phenol-4-aminoantipyrine (4-AAP).     

Selective determination of bisphenol A (BPA) in water by a reversible fluorescence sensor using pyrene/dimethyl β-cyclodextrin complex

A bifurcated optical fiber chemical sensor for continuous monitoring of bisphenol A (BPA) has been proposed based on the fluorescence quenching (λ_ex/λ_em = 286/390 nm) of pyrene/dimethyl β-cyclodextrin (HDM-β-CD) supramolecular complex immobilized in a plasticized poly(vinyl chloride) (PVC) membrane, in which pyrene served as a sensitive fluorescence indicator probe. The decrease of the fluorescence intensity of pyrene/HDM-β-CD complex upon the addition of BPA was attributed to the displacement of pyrene by BPA, which has been utilized as the basis of the fabrication of a BPA-sensitive fluorescence sensor. The response mechanism of the sensor was discussed in detail. The sensor exhibited a dynamic detection range from 7.90 × 10⁻⁸ to 1.66 × 10⁻⁵ mol L⁻¹ with a detection limit of 7.00 × 10⁻⁸ mol L⁻¹, and showed excellent reproducibility, reversibility, selectivity, and lifetime. The proposed sensor was successfully used for the determination of BPA in water samples and landfill leachate.     

Photoinduced interactions between colloidal TiO2 nanoparticles and calf thymus-DNA

The interaction of colloidal TiO₂ nanoparticles with calf thymus-DNA was studied by using absorption, FT-IR, steady state and time resolved fluorescence spectroscopic techniques. The apparent association constant has been deduced (K_app = 2.85 × 10³ M⁻¹) from the absorption spectral changes of the DNA-colloidal TiO₂ nanoparticles using the Benesi–Hildebrand equation. Addition of colloidal TiO₂ nanoparticles quenched the fluorescence of EtBr–DNA. The number of binding sites (n = 0.97) and the apparent binding constant (K = 6.68 × 10³ M⁻¹) were calculated from relevant fluorescence quenching data. The quenching, through a static mechanism, was confirmed by time resolved fluorescence spectroscopy.     

Protein determination using methylene blue in a synchronous fluorescence technique

A new method for detecting protein by synchronous fluorescence enhancement was developed, based on the combination of near infrared (NIR) fluorescence and the dedimerization phenomenon of methylene blue (MB). Under analytical conditions, there are linear relationships between the enhancing extent of synchronous fluorescence of MB-sodium dodecyl benzene sulfonate (SDBS)-protein at 667 nm and the concentration of protein in the range of 8.0 × 10⁻⁸-4.0 × 10⁻⁵ g mL⁻¹ for bovine serum albumin (BSA), 1.0 × 10⁻⁷-3.5 × 10⁻⁵ g mL⁻¹ for egg albumin (EA). The detection limits (S/N = 3) of BSA and EA are 8.9 ng mL⁻¹ and 10.0 ng mL⁻¹, respectively. The fluorescence enhancement mechanism is discussed in detail. Results from multiple techniques indicate that the fluorescence enhancement of the system originates from the hydrophobic microenvironment provided by BSA and SDBS, and the formation of an MB-SDBS-BSA complex, as well as the deaggregation of some MB dimer.     

A novel fluorescence probe for cilnidipine assay

A general route for preparation of dye-encapsulated polymer particles via an emulsion polymerization process has been described. 1-Naphthaleneboronic acid (NBA) was encapsulated, the resultant particles were used as a fluorescence probe for cilnidipine assay based on fluorescence quenching. The sensitivity of NBA-encapsulated probe to cilnidipine was largely improved in comparison with that of free NBA. The probe showed a linear response toward cilnidipine over the concentration range of 2.0 × 10⁻⁷ to 1.1 × 10⁻⁵ mol l⁻¹, with high sensitivity, fast response time, and good selectivity.     

An ultrasensitive method for the determination of melamine using cadmium telluride quantum dots as fluorescence probes

An ultrasensitive and simple method for the determination of melamine was developed based on the fluorescence quenching of thioglycolic acid (TGA) capped CdTe quantum dots (QDs) at pH 11.0. In strong alkaline aqueous solution, the selectivity of the method has been greatly improved due to most heavy metal ions show no interference as they are in the precipitation form or in their anion form. Furthermore, CdTe quantum dots have higher quantum yields at higher pH. The method has a wider concentration range and lower detection limit. The influence factors on the determination of melamine were investigated and the optimum conditions were determined. Under optimum conditions, the fluorescence intensity change of TGA coated CdTe quantum dots was linearly proportional to melamine over a concentration range from 1.0 × 10⁻¹¹ to 1.0 × 10⁻⁵ mol L⁻¹ with a correlation coefficient of 0.9943 and a detection limit of 5 × 10⁻¹² mol L⁻¹. The mechanism of fluorescence quenching of the QDs has been proposed based on the infrared spectroscopy information and electrophoresis experiments in presence of melamine under alkaline condition. The proposed method was employed to detect trace melamine in milk powder and pet feeds with satisfactory results.     

Determination of arsenic based on quenching of CdS quantum dots fluorescence using the gas-diffusion flow injection method

A sequential injection analysis system for determination of arsenic based on hydride generation and fluorescence quenching of mercaptoacetic acid capped cadmium sulfide quantum dots (CdS-MAA QDs) is described. The generated arsine diffused across the PTFE membrane in a gas-diffusion unit and subsequently interacted with CdS-MAA QDs. The parameters affecting the arsine generation and the fluorescence quenching of QDs were studied. Under the optimum conditions, it was observed that a increase in the concentration of As(III) corresponded well to a decrease in fluorescence intensity according to the Stern-Volmer relationship. The extent of quenching was dependent on the concentration of arsenic in the range of 0.08-3.20 mmol L⁻¹, with the detection limit of 0.07 mg L⁻¹. The precision (%RSD) from eight replicates of the determination of As(III) 1.0 mg L⁻¹ was found to be 1.4%. The proposed method was applied to the determination of arsenic in ground water samples with satisfactory recoveries.     

Synchronous fluorescence determination of DNA based on the interaction between methylene blue and DNA

The fluorescence intensity of methylene blue (MB) quenched by DNA in the pH range of 6.5-8.0 was studied with synchronous fluorescence technology. A novel method for detecting single-stranded and double-stranded DNA was developed. The decreased fluorescence intensity at 664 nm is in proportion to the concentration of DNA in the range of 0.28-11.0 μmol L⁻¹ for ctDNA, 0.14-8.25 μmol L⁻¹ for thermally denatured ctDNA and 0.28-8.25 μmol L⁻¹ for hsDNA. The detection limits (S/N = 3) are 0.11, 0.04 and 0.04 μmol L⁻¹, respectively. The method is rapid, selective, and the reagents are lower toxic. It has been used for the determination of DNA in synthetic samples with good satisfaction. In addition, the interaction modes between MB and ctDNA and the mechanism of the fluorescence quenching were also discussed in detail. The experimental results from absorption spectra and fluorescence polarization indicate that the possible interaction modes between MB and DNA are the electrostatic binding and the intercalation binding.     

Flow injection fluorescence determination of dopamine using a photo induced electron transfer (PET) boronic acid derivative

An automated flow injection analysis system was developed for the fluorometric determination of dopamine in pharmaceutical injections. The method is based on the quenching effect of dopamine on m-dansylaminophenyl boronic acid (DAPB) fluorescence due to the reverse photo induced electron transfer (PET) mechanism. Effects of pH and interfering species on the determination of dopamine were examined. Calibration for dopamine, based on quenching data, was linear in the concentration range of 1.0 × 10⁻⁵ to 1.0 × 10⁻⁴ M. Detection limit (3 s) of the method was found to be 3.7 × 10⁻⁶ M. Relative standard deviation of 1.2% (n = 10) was obtained with 1.0 × 10⁻⁵ M dopamine standard solution. The proposed method was applied successfully for the determination of dopamine in pharmaceutical injection sample. The sampling rate was determined as 24 samples per hour.     

Fluorometric method for the determination of hydrogen peroxide and glucose with Fe3O4 as catalyst

In this paper, we utilized the instinct peroxidase-like property of Fe₃O₄ magnetic nanoparticles (MNPs) to establish a new fluorometric method for determination of hydrogen peroxide and glucose. In the presence of Fe₃O₄ MNPs as peroxidase mimetic catalyst, H₂O₂ was decomposed into radical that could quench the fluorescence of CdTe QDs more efficiently and rapidly. Then the oxidization of glucose by glucose oxidase was coupled with the fluorescence quenching of CdTe QDs by H₂O₂ producer with Fe₃O₄ MNPs catalyst, which can be used to detect glucose. Under the optimal reaction conditions, a linear correlation was established between fluorescence intensity ratio I₀/I and concentration of H₂O₂ from 1.8 × 10⁻⁷ to 9 × 10⁻⁴ mol/L with a detection limit of 1.8 × 10⁻⁸ mol/L. And a linear correlation was established between fluorescence intensity ratio I₀/I and concentration of glucose from 1.6 × 10⁻⁶ to 1.6 × 10⁻⁴ mol/L with a detection limit of 1.0 × 10⁻⁶ mol/L. The proposed method was applied to the determination of glucose in human serum samples with satisfactory results.     

Conjugated carbazole dimer as fluorescence carrier for preparation of iodine-sensitive chemical sensor

The carbazole derivative, 9-ethyl-3-carbazylidene carbazole hydrazone (ECCH) with two conjugated carbazole rings have been applied as a fluorescence carrier for preparation of an iodine sensitive optical chemical sensor. The response of the sensor is based on quenching of the fluorescence of ECCH by iodine. The conjugated carbazole dimer based sensor shows a linear response toward iodine in the concentration range 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol L⁻¹, with a detection limit of 8.0 × 10⁻⁷ mol L⁻¹ at pH of 7.0. The effect of composition of the sensor membrane was studied, and the experimental conditions were optimized. Most commonly coexisting ions do not interfer with the iodine assay. The sensor shows sufficient repeatability, selectivity, operational lifetime of two months and a fast response of less then 50 s. The sensor has been used for determination of iodine in water samples.     

A fluorescent chemical sensor for Fe based on blocking of intramolecular proton transfer of a quinazolinone derivative

In this paper, 2-(2′-hydroxy-phenyl)-4(3H)-quinazolinone (HPQ), a typical compound that exhibits excited state intramolecular proton transfer (ESIPT) reaction and possesses good photophysical properties, is synthesized and used as fluoroionophore for Fe³⁺ sensitive optochemical sensor. The decrease of fluorescence intensity of HPQ membrane upon the addition of Fe³⁺ was attributed to the blocking of ESIPT reactions of HPQ and quenching its fluorescence. The effect of the composition of the sensing membrane was studied, and experimental conditions were optimized. The sensor shows a linear response toward Fe³⁺ in the concentration range of 7.1 × 10⁻⁷ M to 1.4 × 10⁻⁴ M with a limit of detection of 8.0 × 10⁻⁸ M, and a working pH range from 2.5 to 4.5. It shows excellent selectivity for Fe³⁺ over a large number of cations such as alkali, alkaline earth and transitional metal ions. The proposed sensor is applied to the determination of the content of iron ions in pharmaceutical preparations samples with satisfactory results.     

Laser induced fluorescence and photochemical derivatization for trace determination of camptothecin

Laser-excited fluorescence was used for the selective determination of camptothecin in samples containing anti-cancer camptothecin-analogs (irinotecan and topotecan). The selectivity of the method was based on the UV photochemical derivatization in basic solution which increased the analyte fluorescence (337/450 nm) and eliminated fluorescence from the two campthotecin-analogs. The influence of UV exposure time and sodium hydroxide concentration was studied using an experimental design. Limit of detection was 4 × 10⁻¹⁰ mol L⁻¹ with linear fluorescence response up to 1 × 10⁻⁶ mol L⁻¹. Average recoveries of camptothecin (added to the samples to simulate a contamination) were 92 ± 4 and 94 ± 6% (n = 3) respectively in irinotecan and topotecan based pharmaceuticals.