Rapid and sensitive colorimetric detection of ascorbic acid in food based on the intrinsic oxidase‐like activity of MnO2 nanosheets

In this paper, we report a colorimetric sensor for the rapid, selective detection of ascorbic acid (AA) in aqueous solutions. Single‐layered MnO₂ nanosheets were established as an artificial oxidase; consequently colorless 3,3´,5,5´‐tetramethylbenzidine (TMB) was oxidized to a blue product (oxTMB), with increase in absorbance at 650 nm. The absorbance of the reaction system decreased after introduction AA, which reduced MnO₂ into Mn²⁺. Under optimum conditions, a detection limit of 62.81 nM for AA in aqueous solutions could be achieved. The linear response range for AA was 0.25–30 μM with a correlation coefficient of 0.996. Importantly, the MnO₂ nanosheet–TMB chromogenic reaction exhibited great selectivity as there was no interference from other metal ions, amino acids and small biological molecules. The proposed colorimetric sensing of AA could be applied for fruit, juice and pharmaceutical samples. Moreover, the proposed sensor showed satisfying performance, including low cost, easy preparation, rapid detection, and good biocompatibility.     

The determination of glutamine with flow‐injection chemiluminescence detection and mechanism study

The main purpose of this study was to develop an inexpensive, simple, rapid and sensitive chemiluminescence (CL) method for the determination of glutamine (Gln) using a flow‐injection (FI) system. Gln was found to strongly inhibit the CL signal of the luminol–H₂O₂–CuSO₄ system in Na₂B₄O₇ solution. A new FI‐CL method was developed for the determination of Gln. Parameters affecting the reproducibility and CL detection were optimized systematically. Under the optimized conditions, the corresponding linear regression equation was established over the range of 5.0 × 10^?7 to 2.5 × 10^?6 mol/L with the detection limit of 1.8 × 10^?8 mol/L. The relative standard deviation was found to be 1.8% for 11 replicate determinations of 1.5 × 10^?6 mol/L Gln. The proposed method has been satisfactorily applied for the determination of Gln in real samples (Marzulene‐s granules) with recoveries in the range of 98.7–108.6%. The minimum sampling rate was about 100 samples/h. The possible mechanism of this inhibitory CL was studied by fluorescence spectrophotometer and UV–vis spectrophotometer. Copyright © 2009 John Wiley & Sons, Ltd.     

Sulfur and nitrogen co‐doped graphene quantum dot‐assisted chemiluminescence for sensitive detection of tryptophan and mercury (II)

A simple one‐step thermal treatment to prepare strong fluorescent sulfur and nitrogen co‐doped graphene quantum dots (SN‐GQD) using citric acid and l ‐cysteine as precursors was developed. The ultra‐weak chemiluminescence (CL) from the reaction of hydrogen peroxide (H₂O₂) and periodate (IO₄^?) was significantly enhanced by SN‐GQD in acidic medium. The enhanced CL was induced by excited‐state SN‐GQD (SN‐GQD*), which was produced from the transfer energy of (O₂)₂* and ¹O₂ to SN‐GQD and recombination of oxidant‐injected holes and electrons in SN‐GQD. In the presence of tryptophan (Trp), the CL intensity of the SN‐GQD–H₂O₂–KIO₄ system was greatly diminished. This finding was used to design a novel method for determination of Trp in the linear range 0.6–20.0 μM, with a limit of detection (LOD) of 58.0 nM. Furthermore, Hg²⁺ was detectable in the range 0.1–9.0 μM with a LOD of 64.0 nM, based on its marked enhancement of the SN‐GQD–H₂O₂–KIO₄ CL system. The proposed method was successfully applied to detect Trp in milk and human plasma samples and Hg²⁺ in drinking water samples, with recoveries in the range 95.7–107.0%.     

A red fluorescent carbon dots with good water solubility for rapid detection of Al3+ in actual samples

We developed a facile strategy for the fabrication of red fluorescent carbon nanodots (R-CDs) and demonstrated their applications for Al³⁺ sensing. Red-emission carbon dots (CDs) were synthesized using a simple hydrothermal treatment with citric acid and urea as precursors, manifesting intriguing red-emission behaviour at 610 nm. With increasing Al³⁺ concentration, the fluorescence band at 610 nm decreased gradually. Monitoring the intrinsic fluorescence variation (I_610nm), as-prepared CDs were developed as an effective platform for fluorescent Al³⁺ sensing, with a linear range of 0.5–60.0 μM and a detection limit of 3.0 nM. More importantly, R-CDs have been applied successfully to the analysis of Al³⁺ in actual samples with satisfactory recoveries in the range 97.12–102.05%, which indicated that obtained CDs could be implemented as an effective tool for the identification and detection of Al³⁺ in actual samples.     

Terbium (III) coordination polymer–copper (II) compound as fluorescent probe for time‐resolved fluorescence ‘turn‐on’ detection of hydrogen sulfide

With recognition of the biological importance of hydrogen sulfide (H₂S), we present a simple and effective fluorescent probe for H₂S using a Tb³⁺ coordination polymer–Cu²⁺ compound (DPA/Tb/G–Cu²⁺). Dipicolinic acid (DPA) and guanosine (G) can coordinate with Tb³⁺ to form a macromolecular coordination polymer (DPA/Tb/G). DPA/Tb/G specifically binds to Cu²⁺ in the presence of coexisting cations, and obvious fluorescence quenching is observed. The quenched fluorescence can be exclusively recovered upon the addition of sulfide, which is measured in the mode of time‐resolved fluorescence. The fluorescence intensities of the DPA/Tb/G–Cu²⁺ compound enhance linearly with increasing sulfide concentrations from 1 to 30 μM. The detection limit for sulfide in aqueous solution is estimated to be 0.3 μM (at 3σ). The DPA/Tb/G–Cu²⁺ compound was successfully applied to sense H₂S in human serum samples and exhibited a satisfactory result. It displays some desirable properties, such as fast detection procedure, high selectivity and excellent sensitivity. This method is very promising to be utilized for practical detection of H₂S in biological and environmental samples.     

A ratiometric fluorescent probe for sensing hydrogen peroxide based on a hemicyanine–naphthol fluorophore

The synthesis, properties and applications of a water‐soluble boronate‐functioned hemicyanine–naphthol hybrid as a novel ratiometric fluorescent sensor for hydrogen peroxide are presented. The dye displayed remarkable a colour change from pale orange (λ_em = 590 nm) to pink (λ_em = 690 nm) in the presence of H₂O₂, which could be rationalized by the chemoselective H₂O₂‐mediated transformation of arylboronate to phenolate with high selectivity and a fast response (within 2 min). A good linear relationship (R² = 0.9951) was obtained with the H₂O₂ concentration ranging from 0 to 25 μM, with a limit of detection of 0.09 μM according to the signal‐to‐noise ratio (S/N = 3). The advantages of this fluorophore include easy modification, excellent aqueous solubility and superior photostability, and it has been applied to the detection of trace amounts of hydrogen peroxide in water samples. Copyright © 2015 John Wiley & Sons, Ltd.     

Green synthesis of fluorescent carbon dots using chloroplast dispersions as precursors and application for Fe3+ ion sensing

Water‐soluble carbon dots (CDs) were synthesized using a one‐step hydrothermal treatment of chloroplast dispersions extracted from fresh leaves as a green carbon source. The CD solution showed an emission peak centred at 445 nm when excited at 300 nm. The synthesized CDs were uniform and monodispersed with an average size of 5.6 nm. When adding ferric(III) ions (Fe³⁺) to the solution of the original CDs, the fluorescence intensity decreased significantly. Based on the linear relationship between fluorescence intensity and concentration of Fe³⁺ ions, an effective method for rapid, sensitive and selective Fe³⁺ sensing in aqueous solution could be established. Under optimum conditions, the extent of the fluorescence quenching of prepared CDs strongly depended on the Fe³⁺ ions over a wide concentration range 1.0–100.0 μM with a detection limit (3σ/k) of 0.3 μM. Furthermore, the quantitative determination of Fe³⁺ ions in environmental water samples was realized.     

Highly luminescent S,N co‐doped carbon quantum dots‐sensitized chemiluminescence on luminol–H2O2 system for the determination of ranitidine

S,N co‐doped carbon quantum dots (N,S‐CQDs) with super high quantum yield (79%) were prepared by the hydrothermal method and characterized by transmission electron microscopy, photoluminescence, UV–Vis spectroscopy and Fourier transformed infrared spectroscopy. N,S‐CQDs can enhance the chemiluminescence intensity of a luminol–H₂O₂ system. The possible mechanism of the luminol–H₂O₂–(N,S‐CQDs) was illustrated by using chemiluminescence, photoluminescence and ultraviolet analysis. Ranitidine can quench the chemiluminescence intensity of a luminol–H₂O₂–N,S‐CQDs system. So, a novel flow‐injection chemiluminescence method was designed to determine ranitidine within a linear range of 0.5–50 μg ml^?1 and a detection limit of 0.12 μg ml^?1. The method shows promising application prospects.     

Turn‐off–on chemiluminescence determination of cyanide

A flow injection chemiluminescence (FI–CL) method was developed for the determination of cyanide (CN⁻) based on the recovered CL signal by Cu²⁺ inhibiting a glutathione (GSH)‐capped CdTe quantum dot (QD) and hydrogen peroxide system. In an alkaline medium, strong CL signals were observed from the reaction of CdTe QDs and H₂O₂, and addition of Cu²⁺ could cause significant CL inhibition of the CdTe QDs–H₂O₂ system. In the presence of CN⁻, Cu²⁺ can be removed from the surface of CdTe QDs via the formation of particularly stable [Cu(CN)_n]^(n‐1)– species, and the CL signal of the CdTe QDs–H₂O₂ system was efficiently recovered. Thus, the CL signals of CdTe QDs–H₂O₂ system were turned off and turned on by the addition of Cu²⁺ and CN⁻, respectively. Further, the results showed that among the tested ions, only CN⁻ could recover the CL signal, which suggested that the CdTe QDs–H₂O₂–Cu²⁺ CL system had highly selectivity for CN⁻. Under optimum conditions, the CL intensity and the concentration of CN⁻ show a good linear relationship in the range 0.0–650.0 ng/mL (R² = 0.9996). The limit of detection for CN⁻ was 6.0 ng/mL (3σ). This method has been applied to detect CN⁻ in river water and industrial wastewater with satisfactory results. Copyright © 2014 John Wiley & Sons, Ltd.     

Terephthalic acid: A dosimeter for the detection of hydroxyl radicals in vitro

Hydroxylation reactions of aromatic compounds have been used to detect hydroxyl radicals produced by gamma irradiation and ultrasound. The present study investigated the suitability of terephthalic acid (THA) as a hydroxyl radical dosimeter for general use in biologically relevant reactions. Hydroxyl radicals were generated by: (1) irradiating, THA with a 254 nm ultraviolet; (2) irradiating with gamma rays from a cesium source; and (3) generating hydroxyl radicals with 1 mM H₂O₂ and 10 μM Cu⁺². In each of the three experiments, a fluorescent product was generated which exhibited identical fluorescent excitation and emission spectra. THA is non-fluorescent, eliminating the problem of a high initial background. Because THA has four ring hydrogens, only one mon-hydroxylated isomer was formed. The hydrogen peroxide reaction was dependent on the presence of a metal and cupric ions were effective in enhancing the reaction. With a Cu⁺² concentration of 10 μM, the reation was linear between 0–30 mM H₂O₂. Catalase abolished the reaction at a concentration of 100 μg/ml and the effects could still be observed at 10 ng/ml, consistent with the very high rate at which catalase destroys hydrogen peroxide. Tertbutyl- hydroperoxide did not generate any fluorescence in this system which makes THA a very specific detector of hydroxyl radicals.     

A dual-mode optical assay for iron(II) and gallic acid based on Fenton reaction

The hydroxyl radicals ( · OH) produced by the Fenton reaction of iron(II) and hydrogen peroxide (H₂O₂) can oxidize the colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to blue oxidized TMB (Ox-TMB), resulting in a decrease in the fluorescence intensity of the reaction system and an increase in ultraviolet absorption. Ox-TMB had a visible absorption peak at 625 nm and a fluorescence peak around 420 nm. When gallic acid (GA) was added to the system, Ox-TMB was reduced to TMB, which made the color of the system disappear and the fluorescence recover. The linear ranges for determination of iron(II) were 0.5–10 μM (fluorometric) and 0.5–20 μM (colorimetric), and the detection limits were 0.25 μM (fluorometric) and 0.28 μM (colorimetric). The linear ranges for determination of GA were 0–80 μM (fluorometric) and 0–60 μM (colorimetric), and the detection limits were 0.31 μM (fluorometric) and 0.8 μM (colorimetric). The results of anti-interference experiments shew that this dual-mode assay had very good selectivity for the determination of iron(II) and GA.     

Pyrophosphate as substrate for alkaline phosphatase activity: A convenient flow‐injection chemiluminescence assay

A sensitive and convenient flow‐injection chemiluminescence (FI‐CL) turn‐on assay for alkaline phosphatase (ALP) activity without any label and synthesis is developed. Cu²⁺ can catalyze the luminol–H₂O₂ CL reaction. Pyrophosphate (PPi) can chelate Cu²⁺ and therefore the Cu²⁺‐mediated luminol‐H₂O₂ CL reaction is inhibited. The addition of ALP can catalyze the hydrolysis of PPi into phosphate ions, Cu²⁺ is released and the chemiluminescence recovers. A detection limit of 1 mU/mL ALP is obtained.     

Oxidative stress induces alkaloid production in Uncaria tomentosa root and cell cultures in bioreactors

The effect of oxidative stress on indole alkaloids accumulation by cell suspensions and root cultures of Uncaria tomentosa in bioreactors was investigated. Hydrogen peroxide (H₂O₂, 200 μM) added to U. tomentosa cell suspension cultures in shaken flasks induced the production of monoterpenoid oxindole alkaloids (MOA) up to 40.0 μg/L. In a stirred tank bioreactor, MOA were enhanced by exogenous H₂O₂ (200 μM) from no detection up to 59.3 μg/L. Root cultures grew linearly in shaken flasks with a μ=0.045 days^?1 and maximum biomass of 12.08±1.24 g DW/L (at day 30). Roots accumulated 3α‐dihydrocadambine (DHC) 2354.3±244.8 μg/g DW (at day 40) and MOA 348.2±32.1 μg/g DW (at day 18). Exogenous addition of H₂O₂ had a differential effect on DHC and MOA production in shaken flasks. At 200 μM H₂O₂, MOA were enhanced by 56% and DHC by 30%; while addition of 800 and 1000 μM H₂O₂, reduced by 30–40% DHC accumulation without change in MOA. Root cultures in the airlift reactor produced extracellular H₂O₂ with a characteristic biphasic profile after changing aeration. Maximum MOA was 9.06 mg/L at day 60 while at this time roots reached ca. 1 mg/L of DHC. Intracellular H₂O₂ in root cultures growing in the bioreactor was 0.87 μmol/g DW compared to 0.26 μmol/g DW of shaken flasks cultures. These results were in agreement with a higher activity of the antioxidant enzymes superoxide dismutase and peroxidase by 6‐ and 2‐times, respectively. U. tomentosa roots growing in the airlift bioreactor were exposed to an oxidative stress and their antioxidant system was active allowing them to produce oxindole alkaloids.     

Pre-treatment with H<Subscript>2</Subscript>O<Subscript>2</Subscript> induces salt tolerance in Barley seedlings

Barley seedlings were pre-treated with 1 and 5 μM H₂O₂ for 2 d and then supplied with water or 150 mM NaCl for 4 and 7 d. Exogenous H₂O₂ alone had no effect on the proline, malondialdehyde (MDA) and H₂O₂ contents, decreased catalase (CAT) activity and had no effect on peroxidase (POX) activity. Three new superoxide dismutase (SOD) isoenzymes appeared in the leaves as a result of 1 μM H₂O₂ treatment. NaCl enhanced CAT and POX activity. SOD activity and isoenzyme patterns were changed due to H₂O₂ pre-treatment, NaCl stress and leaf ageing. In pre-treated seedlings the rate of ¹⁴CO₂ fixation was higher and MDA, H₂O₂ and proline contents were lower in comparison to the seedlings subjected directly to NaCl stress. Cl⁻ content in the leaves 4 and 7 d after NaCl supply increased considerably, but less in pre-treated plants. It was suggested that H₂O₂ metabolism is involved as a signal in the processes of barley salt tolerance.     

Oxygen consumption during the fenton-type reaction between hydrogen peroxide and a ferrous-chelate (Fe2+-DTPA)

The Fenton-type reaction between ferrous diethylenetriamine pentaacetic acid (Fe²⁺-DTPA, 50–200 μM) and H₂O₂ (20–1000 μM) in phosphate buffer at pH 7.0 results in consumption of dissolved oxygen. This observation differs from many prior reports that oxygen is liberated when more concentrated solutions of H₂O₂ are decomposed by iron salts. The rate and total quantity of oxygen consumed were dependent upon the concentrations of ferrous chelate, H₂O₂, and excess DTPA. Evidence is provided that both the ferrous-DTPA chelate and free DTPA can participate in the oxygen-consuming reactions. Oxygen was also consumed during the Fenton reaction between ferrous ions and H₂O₂ when DTPA and phosphate buffer were omitted. Under these conditions, oxygen evolution was observed at higher H₂O₂ concentrations (e.g., 400 μM). The consumption of oxygen during the Fenton-type reaction of an iron chelate at neutral pH may be relavant to events that take place in biologic systems.     

A highly sensitive ratiometric fluorescent probe based on fluorescein coumarin for detecting hydrazine in actual water and biological samples

Hydrazine (N₂H₄) is a highly toxic and harmful chemical reagent. Fluorescent probes are simple and efficient tools for sensitive monitoring of N₂H₄ enrichment in the environment, humans, animals, and plants. In this work, a ratiometric fluorescent probe (FP-1) containing coumarin was used for hydrazine detection. The proposed FP-1 probe had a linear detection range of 0–250 μM and a limit of detection (LOD) of 0.059 μM (1.89 ppb). A large red Stokes shift was observed in fluorescence and UV–vis absorption spectra due to the hydrolysis of ester bonds between FP-1 and hydrazine. The hydrazine detection mechanism of FP-1 was also investigated using density functional theory (DFT) calculations. Finally, FP-1 could sensitively and selectively monitor hydrazine in actual water samples and BEAS-2B cells. Therefore, it has great application potential in environmental monitoring and disease diagnosis.     

Determination of puerarin in biological samples and its application to a pharmacokinetic study by flow‐injection chemiluminescence

A simple and sensitive flow injection–chemiluminescence (FI–CL) method has been developed for the determination of puerarin, based on the fact that puerarin can greatly inhibit CL of the luminol–H₂O₂–haemoglobin system. The inhibition of CL intensity was linear to the logarithm of the concentration of puerarin in the range 0.08–10.0 μg/mL (r² = 0.9912). The limit of detection was 0.05 μg/mL (3σ) and the relative standard deviation (RSD) for 1.0 μg/mL (n = 11) of puerarin solution was 1.4%. Coupled with solid‐phase extraction (SPE) as the sample pretreatment, the determination of puerarin in biological samples and a preliminary pharmocokinetic study of puerarin in rats were performed. The recoveries for plasma and urine at three different concentrations were 89.2–110.0% and 91.4–104.8%, respectively. The pharmacokinetics of puerarin in plasma of rat coincides with the two‐compartment open model. The T_1/2α, T_1/2β, CL/F, V_Z/F, AUC_{(0 – t)}, MRT_{(0 – ∞)}, T_max and C_max were 0.77 ± 0.21 h, 7.55 ± 2.64 h, 2.43 ± 1.02 L/kg/h, 11.40 ± 3.45 L/kg, 56.67 ± 10.65 mg/h/L, 5.04 ± 2.78 h, 1.00 ± 0.35 h and 19.70 ± 4.67 μg/mL, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

Pyruvate prevents peroxide‐induced injury of in vitro preimplantation bovine embryos

The impact of oxidative stress on the in vitro development of bovine embryos in synthetic oviduct fluid medium (mSOF) was assessed by using H₂O₂ as a stress inducer. In a preliminary experiment, a chemiluminescent method was used to measure the antioxidative capacity of the mSOF culture medium. Pyruvate was the mSOF component displaying the highest H₂O₂ degrading ability. Essential and nonessential amino acids also significantly reduced the H₂O₂ concentration, whereas lactate and glutamine were ineffective. The effect on further development of a short exposure of zygotes, 9–16‐cell stage embryos and blastocysts to 0 M; 10⁻⁷ M ; 10⁻⁶ M, and 10⁻⁵ M H₂O₂ in pyruvate‐free mSOF was evaluated. Developmental rates of the H₂O₂‐treated zygotes to the 5–8‐cell or blastocyst stages and survival of H₂O₂‐treated blastocysts were reduced in a dose‐dependent manner whereas the 9–16‐cell embryos were unaffected by those treatments. Blastocysts treated with H₂O₂ also tended to have lower numbers of bisbenzimide‐stained nuclei and showed increased nuclear fragmentation. Including pyruvate in the mSOF culture medium during a 10⁻⁵ M H₂O₂ pulse highly reduced the H₂O₂ concentration as measured by chemiluminescence and improved zygote and blastocyst development, but failed to prevent blastocyst nuclei degradation. These experiments suggest that bovine embryos show developmental change in sensitivity to exogenous H₂O₂, the 9–16‐cell embryos being more resistant than zygotes and blastocysts and that H₂O₂ and its toxic effects can be attenuated by including pyruvate in the medium. Mol. Reprod. Dev. 52:149–157, 1999. © 1999 Wiley‐Liss, Inc.     

Cu2+-catalyzed oxidative degradation of thyroglobulin

Thyroglobulin (Tg) was subjected to metal-catalyzed oxidation, and the oxidative degradation was analyzed by SDS-polyacrylamide gel electrophoresis under reducing conditions. In contrast to no effect of hydrogen peroxide (H₂O₂) alone on the Tg degradation, the inclusion of Cu²⁺ (30 μM), in combination with 2 mM H₂O₂, caused a remarkable degradation of Tg, time- and concentration-dependent. The action of Cu²⁺ was not mimicked by Fe²⁺, suggesting that Tg may interact selectively with Cu²⁺. A similar degradation of Tg was also observed with Cu²⁺corbate system, and the concentration of Cu²⁺ (5–10 μM), in combination with ascorbate, required for the effective degradation was smaller than that of Cu²⁺ (10–30 μM) in combination with H₂O₂. In support of involvement of H₂O₂ in the Cu²⁺ corbate action, catalase expressed a complete protection. However, hydroxyl radical scavengers such as dimethylsulfoxide or mannitol failed to prevent the oxidation of Tg whereas phenolic compounds, which can interact with Cu²⁺, diminished the oxidative degradation, presumably consistent with the mechanism for Cu²⁺-catalyzed oxidation of protein. Moreover, the amount of carbonyl groups in Tg was increased as the concentration (3–100 μM) of Cu²⁺ was enhanced, while the formation of acid-soluble peptides was not remarkable in the presence of Cu²⁺ up to 200 μM. In further studies, Tg pretreated with heat or trichloroacetic acid seemed to be somewhat resistant to Cu²⁺-catalyzed oxidation, implying a possible involvement of protein conformation in the susceptibility to the oxidation. Based on these observations, it is proposed that Tg could be degraded non-enzymatically by Cu²⁺-catalyzed oxidation.     

Analysis of methylparaben in cosmetics based on a chemiluminescence H2O2−NaIO4−CNQDs system

In this article, a simple, effective chemiluminescence (CL) method for the detection of methylparaben (MP) in cosmetic samples was developed based on an IO₄^?–H₂O₂–carbon nitrogen quantum dots (CNQDs) system without a separation process. The results indicated that the redox reaction between periodate and hydrogen peroxide released hydroxide radicals and superoxide radical anions in the presence of bicarbonate. These two radicals were responsible for the formation of excited luminophor CNQD* with a maximum wavelength at 480 nm. Due to the competitive reaction with hydroxide radicals, CL intensity was markedly diminished in the presence of MP. The relative standard deviation in the intraday assay was below 5.5% (n = 9), and the detection limit was as low as 0.50 μmol/L. The proposed method allowed for the successful, selective determination of MP in cosmetics.