The determination of nitrite by a graphene quantum dot fluorescence quenching method without sample pretreatment

A method for quantitative analysis of nitrite was achieved based on fluorescence quenching of graphene quantum dots. To obtain reliable results, the effects of pH, temperature and reaction time on this fluorescence quenching system were studied. Under optimized conditions, decrease in fluorescence intensity of graphene quantum dots (F₀/F) showed a good linear relationship with nitrite concentration between 0.007692–0.38406 mmol/L and 0.03623–0.13043 μmol/L; the limits of detection were 9.8 μmol/L and 5.4 nmol/L, respectively. Variable temperature experiments, UV absorption spectra and thermodynamic calculations were used to determine the quenching mechanism, and indicated that it was an exothermic, spontaneous dynamic quenching process. This method was used to analyse urine samples, and showed that it could be applied to analyse biological samples.     

Enhanced fluorescence of tetrasulfonated zinc phthalocyanine by graphene quantum dots and its application in molecular sensing/imaging

When excited at 435 nm, tetra‐sulfonate zinc phthalocyanine (ZnPcS₄) emitted dual fluorescence at 495 and 702 nm. The abnormal fluorescence at 495 nm was experimentally studied and analyzed in detail for the first time. The abnormal fluorescence at 495 nm was deduced to originate from triplet–triplet (T–T) energy transfer of excited phthalocyanine (³*ZnPcS₄). Furthermore, graphene quantum dots (GQDs) enhanced the 495 nm fluorescence quantum yield (Q) of ZnPcS₄. The fluorescence properties of ZnPcS₄–GQDs conjugate were retained in a cellular environment. Based on the fluorescence of ZnPcS₄–GQDs conjugate, we designed and prepared an Apt29/thrombin/Apt15 sandwich thrombin sensor with high specificity and affinity. This cost‐saving, simple operational sensing strategy can be extended to use in sensing/imaging of other biomolecules.     

Assay of ceftazidime and cefepime based on fluorescence quenching of carbon quantum dots

A novel and sensitive method for the determination of ceftazidime and cefepime in an active pharmaceutical ingredient (API) has been developed based on the fluorescence quenching of poly(ethylene glycol) (PEG)2000‐capped carbon quantum dots (CQDs) prepared using a chemical oxidation method. The quenching of fluorescence intensity is proportional to the concentration of ceftazidime and cefepime over the range of 0.33–3.30 and 0.24–2.40 µg/mL, respectively. The mode of interaction between PEG2000‐capped CQDs and ceftazidime/cefepime in aqueous solutions was investigated using a fluorescence, UV/Vis and Fourier transform infrared spectrometry (FTIR) at physiological pH. UV/Vis and FTIR spectra demonstrated that ground state compounds were formed through hydrophobic interaction the fluorescence quenching of CQDs caused by ceftazidime and cefepime. The quenching constants decreased with increases in temperature, which was consistent with static quenching. Copyright © 2015 John Wiley & Sons, Ltd.     

Efficient fluorescence energy transfer system between fluorescein isothiocyanate and CdTe quantum dots for the detection of silver ions

We report a fluorescence resonance energy transfer (FRET) system in which the fluorescent donor is fluorescein isothiocyanate (FITC) dye and the fluorescent acceptor is CdTe quantum dot (QDs). Based on FRET quenching theory, we designed a method to detect the concentration of silver ions (Ag⁺). The results revealed a good linear trend over Ag⁺ concentrations in the range 0.01–8.96 nmol/L, a range that was larger than with other methods; the quenching coefficient is 0.442. The FRET mechanism and physical mechanisms responsible for dynamic quenching are also discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

量子点荧光标记技术的研究热点及面临的挑战

半导体量子点作为新型荧光标记物,在生物医学领域具有重要应用．与传统的有机染料及荧光蛋白等荧光标记物相比,半导体量子点具有发光颜色可调、激发范围宽、发射光谱窄、化学及光稳定性好、表面化学丰富以及生物偶联技术成熟等诸多优势,为生命体系的靶向示踪,高灵敏、原位、实时、动态荧光成像,DNA及蛋白质检测,靶向药物,临床医学,生物芯片和传感器等研究提供了新的发展契机．基于作者在半导体量子点生物荧光成像和安全性评价研究的基础,综述了半导体量子点荧光标记物在生命科学与医学领域应用的研究热点,并对半导体量子点荧光标记技术走向实用面临的挑战进行了评述．     

A selective determination of copper ions in water samples based on the fluorescence quenching of thiol‐capped CdTe quantum dots

CdTe quantum dots (QDs) capped with different stabilizers, i.e. thioglycolic acid (TGA), 3‐mercaptopropionic acid (MPA) and glutathione (GSH) were investigated as fluorescent probes for the determination of Cu²⁺. The stabilizer was shown to play an important role in both the sensitivity and selectivity for the determination of Cu²⁺. TGA‐capped CdTe QDs showed the highest sensitivity, followed by the MPA and GSH‐capped CdTe QDs, respectively. The TGA‐ and MPA‐capped CdTe QDs were not selective for Cu²⁺ that was affected by Ag⁺. The GSH‐capped CdTe QDs were insensitive to Ag⁺ and were used to determine Cu²⁺ in water samples. Under optimal conditions, quenching of the fluorescence intensity (F₀/F) increased linearly with the concentration of Cu²⁺ over a range of 0.10–4.0 µg/mL and the detection limit was 0.06 µg/mL. The developed method was successfully applied to the determination of Cu²⁺ in water samples. Good recoveries of 93–104%, with a relative standard deviation of < 6% demonstrated that the developed simple method was accurate and reliable. The quenching mechanisms were also described. Copyright © 2015 John Wiley & Sons, Ltd.     

ZnS quantum dots‐based fluorescence spectroscopic technique for the detection of quercetin

Water‐soluble ZnS quantum dots (QDs) modified by mercaptoacetic acid (MPA) were used to determinate quercetin in aqueous solutions by a fluorescence spectroscopic technique. The results showed that the fluorescence of the modified ZnS QDs could be quenched by quercetin effectively in physiological buffer solution. The optimum fluorescence intensity was found to be at incubation time 10 min, pH 7.0 and temperature 25°C. Under the optimal conditions, the detection limit of quercetin was 5.71 × 10^‐7 mol/L. Moreover, the quenching mechanism was discussed to be a static quenching procedure, which was proved by the quenching rate constant K_q (1.14 × 10¹³ L/mol/s). Copyright © 2013 John Wiley & Sons, Ltd. Copyright © 2013 John Wiley & Sons, Ltd.     

A highly sensitive and selective detection of picric acid using fluorescent sulfur‐doped graphene quantum dots

The development of an analytical probe to monitor highly mutagenic picric acid (PA) carries enormous significance for the environment and for health. A novel, simple and rapid fluorescence analytical assay using sulfur‐doped graphene quantum dots (SGQDs) was designed for the highly sensitive and selective detection of PA. SGQDs were synthesized via simple pyrolysis of 3‐mercaptopropionic acid and citric acid and characterized using advanced analytical techniques. Fluorescence intensity (FI) of SGQDs was markedly quenched by addition of PA, attributed to the inner filter effect and dominating static quenching mechanism between the two, in addition to a significant colour change. The calibration curve of the proposed assay exhibited a favourable linearity between quenched FI and PA concentration over the 0.1–100 μΜ range with a lowest detection limit of 0.093 μΜ and a correlation coefficient of 0.9967. The analytical assay was investigated for detection of trace amounts of PA in pond and rain water samples and showed great potential for practical applications with both acceptable recovery (98.0–100.8%) and relative standard deviation (1.24–4.67%). Analytical performance of the assay in terms of its detection limit, linearity range, and recovery exhibited reasonable superiority over previously reported methods, thereby holding enormous promise as a simple, sensitive, and selective method for detection of PA.     

The effects of graphene quantum dots on the maturation of mouse oocytes and development of offspring

Recently, graphene nanomaterials have attracted tremendous attention and have been utilized in various fields because of their excellent mechanical, thermal, chemical, optical properties, and good biocompatibility, especially in biomedical aspects. However, there is a concern that the unique characteristics of nanomaterials may have undesirable effects. Therefore, in this study, we sought to systematically investigate the effects of graphene quantum dots (GQDs) on the maturation of mouse oocytes and development of the offspring via in vitro and in vivo studies. In vitro, we found that the first polar body extrusion rate in the high dosage exposure groups (1.0–1.5 mg/ml) ₂ decreased significantly and the failure of spindle migration and actin cap formation after GQDs exposure was observed. The underlying mechanisms might be associated with reactive oxygen species accumulation and DNA damage. Moreover, transmission electron microscope studies showed that GQDs may have been internalized into oocytes, tending to accumulate in the nucleus and severely affecting mitochondrial morphology, which included swollen and vacuolated mitochondria accompanied by cristae alteration with a lower amount of dense mitochondrial matrix. In vivo, when pregnant mice were exposed to GQDs at 8.5 days of gestation (GD, 8.5), we found that high dosage of GQD exposure (30 mg/kg) significantly affected mean fetal length; however, all the second generation of female mice grew up normal, attained sexual maturity, and gave birth to a healthy offspring after mating with a healthy male mouse. The results presented in this study are important for the future investigation of GQDs for the biomedical applications.     

Mercaptopropionic acid–capped CdTe quantum dots as fluorescence probe for the determination of salicylic acid in pharmaceutical products

Mercaptopropionic acid (MPA)–capped cadmium telluride (CdTe) quantum dot (QDs) fluorescent probes were synthesized in aqueous solution and used for the determination of salicylic acid. The interaction between the MPA–capped CdTe QDs and salicylic acid was studied using fluorescence spectroscopy and some parameters that could modify the fluorescence were investigated to optimize the measurements. Under optimum conditions, the quenched fluorescence intensity of MPA–capped CdTe QDs was linearly proportional to the concentration of salicylic acid in the range of 0.5–40 µg mL^–1 with a coefficient of determination of 0.998, and the limit of detection was 0.15 µg mL^–1. The method was successfully applied to the determination of salicylic acid in pharmaceutical products, and satisfactory results were obtained that were in agreement with both the high pressure liquid chromatography (HPLC) method and the claimed values. The recovery of the method was in the range 99 ± 3% to 105 ± 9%. The proposed method is simple, rapid, cost effective, highly sensitivity and eminently suitable for the quality control of pharmaceutical preparation. The possible mechanisms for the observed quenching reaction was also discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Sensitive determination of enoxacin in pharmaceutical formulations by its quench effect on the fluorescence of glutathione‐capped CdTe quantum dots

A sensitive and simple method for the determination of enoxacin (ENX) was developed based on the fluorescence quenching effect of ENX for glutathione (GSH)‐capped CdTe quantum dots (QDs). Under optimum conditions, a good linear relationship was obtained from 4.333 × 10^?9 mol?L^?1 to 1.4 × 10^?5 mol?L^?1 with a correlation coefficient (R) of 0.9987, and the detection limit (3σ/K) was 1.313 × 10^?9 mol?L^?1. The corresponding mechanism has been proposed on the basis of electron transfer supported by ultraviolet–visible (UV) light absorption, fluorescence spectroscopy, and the measurement of fluorescence lifetime. The method has been applied to the determination of ENX in pharmaceutical formulations (enoxacin gluconate injections and commercial tablets) with satisfactory results. The proposed method manifested several advantages such as high sensitivity, short analysis time, low cost and ease of operation. Copyright © 2015 John Wiley & Sons, Ltd.     

Study of fluorescence quenching of mercaptosuccinic acid‐capped CdS quantum dots in the presence of some heavy metal ions and its application to Hg(II) ion determination

In this study, CdS quantum dots (QDs) capped with mercaptosuccinic acid (MSA) were prepared in one step. The size, shape, component and spectral properties of MSA‐capped CdS QDs were characterized by transmission electron microscopy (TEM), photoluminescence (PL) and infrared (IR) spectrometry. The results showed that the prepared QDs with an average diameter of 6 nm have favorable fluorescence, which is greatly influenced by the pH of the environment. The interaction of some heavy metal ions including Ag⁺, Hg²⁺, Cu²⁺, Ni²⁺ and Co²⁺ with MSA‐capped CdS QDs was investigated in different buffering pH media. Based on the fluorescence quenching of the QDs in the presence of each of the metal ions, the feasibility of their determinations was examined according to the Stern–Volmer equation. The investigations showed that Hg(II) ions can be determined in the presence of many co‐existing metal ions at a buffering pH of 5. This method was satisfactorily applied to the measurement of Hg(II) ions in some environmental samples. Copyright © 2014 John Wiley & Sons, Ltd.     

Interaction of glucose‐derived carbon quantum dots with silver and gold nanoparticles and its application for the fluorescence detection of 6‐thioguanine

The interaction of glucose‐derived carbon quantum dots (CQDs) with silver (Ag) and gold (Au) nanoparticles (NPs) was explored by fluorescence spectroscopy. Both metal NPs cause an efficient quenching of CQD fluorescence, which is likely due to the energy transfer process between CQDs as donors and metal NPs as acceptors. The Stern–Volmer plots were evaluated and corresponding quenching constants were found to be 1.9 × 10¹⁰ and 2.2 × 10⁸ M^?1 for AgNPs and AuNPs, respectively. The analytical applicability of these systems was demonstrated for turn‐on fluorescence detection of the anti‐cancer drug, 6‐thioguanine. Because the CQD–AgNP system had much higher sensitivity than the CQD–AuNP system, we used it as a selective fluorescence probe in a turn‐on assay of 6‐thioguanine. Under optimum conditions, the calibration graph was linear from 0.03 to 1.0 μM with a detection limit of 0.01 μM. The developed method was applied to the analysis of human plasma samples with satisfactory results.     

Ultrasensitive chemiluminescence assay for cimetidine detection based on the synergistic improving effect of Au nanoclusters and graphene quantum dots

A novel and sensitive chemiluminescence (CL) procedure based on the synergetic catalytic effects of gold nanoclusters (Au NCs) and graphene quantum dots (GQDs) was developed for the reliable measurement of cimetidine (CM). The initial experiments showed that the KMnO₄‐based oxidation of alkaline rhodamine B (RhoB) generated a very weak CL emission, which was intensively enhanced in the simultaneous presence of Au NCs and GQDs. CL intermediates can be adsorbed and gathered on the surface of Au NCs, becoming more stable. GQDs participate in the energy transferring processes and facilitate them. These improving effects were simultaneously obtained by adding both Au NCs and GQDs into the RhoB‐KMnO₄ reaction. Consequently, the increasing effect of the Au NCs/GQDs mixture was more than that of pure Au NCs or GQDs, and a new nano‐assisted powerful CL system was achieved. Furthermore, a marked quenching in the emission of the introduced CL system was observed in the presence of CM, so the system was examined to design a sensitive sensor for CM. After optimization of influencing parameters, the linear lessening in CL emission intensity of KMnO₄‐RhoB‐Au NCs/GQDs was verified for CM concentrations in the range 0.8–200 ng ml^?1. The limit of detection (3S_b/m) was 0.3 ng ml^?1. Despite being a simple CL method, good sensitivity was obtained for CM detection with reliable results for CM determination in human urine samples.     

Preparation and characterization of selenium‐decorated graphene quantum dots with high afterglow for application in photodynamic therapy

Graphene quantum dots (GQDs) was synthesized using a simple, rapid and affordable method and decorated with selenium at different molar ratios for the first time to obtain an efficient sample for use in photodynamic therapy. Surface modification of GQDs was carried out using polyethylene glycol (PEG) for conjugation with protoporphyrin IX (PpIX). Synthesized GQDs (Se: 0.3%) at 180°C had an emission spectrum that fairly coincided with the absorption profile of PpIX. A relative decrease of about 62.48% in the emission intensity of anthracene was recorded under illumination with UVC light in the presence of GQDs (Se: 0.3%) and the reduction for clung GQDs (Se: 0.3%) and PpIX during 90 min was about 70.68%. Singlet oxygen (¹O₂) generation was examined using a chemical method that showed significant enhancement in decomposition rate constant in clung GQDs–PEG–PpIX compared with GQDs and PpIX alone. Afterglow over 600 s showed that GQDs (Se: 0.3%) could be effective for near skin and even deep tumours.     

Ultrasensitive detection of glibenclamide based on its enhancing effect on the fluorescence emission of CdTe quantum dots

Glibenclamide (GB), as a sulfonylurea‐based medication is commonly prescribed for the treatment of type 2 diabetes. Due to its increasing consumption, there is a need to develop a simple, fast, and reliable detection method to follow its concentration in pharmaceutical and biological samples. Herein, a novel fluorometric method is developed for the sensitive measurement of GB. The method is based on the enhancing effect of GB on the fluorescence emission of mercaptopropionic acid (MPA) capped cadmium telluride quantum dots (CdTe QDs). QDs were synthesized in aqueous solution and were characterized by fluorescence spectroscopy, transmission electron microscopy (TEM), X‐ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT‐IR). Fluorescence intensity of QDs was enhanced by adding GB in a very low concentration. The effect of operative factors such as pH, buffer, contact time and concentration of CdTe QDs were investigated and in the optimized condition, a linear increase was achieved for the emission intensity of QDs by increasing GB concentration in the range 49–345 ng mL^?1, with a detection limit of 17.84 ng mL^?1. The offered method has an acceptable precision (relative standard deviations were < 2.8%) and was satisfactorily applied for the determination of GB in pharmaceutical products and human urine samples.     

Species-dependent effects of seawater acidification on alkaline phosphatase activity in dinoflagellates

Increases of atmospheric CO₂ cause ocean acidification (OA) and global warming, the latter of which can stratify the water column and impede nutrient supply from deep water. Phosphorus (P) is an essential nutrient for phytoplankton to grow. While dissolved inorganic phosphorus (DIP) is the preferred form of P, phytoplankton have evolved alkaline phosphatase (AP) to utilize dissolved organic phosphorus (DOP) when DIP is deficient. Although the function of AP is known to require pH > 7, how OA affects AP activity and hence the capacity of phytoplankton to utilize DOP is poorly understood. Here, we examined the effects of pH conditions (5.5–11) on AP activity from six species of dinoflagellates, an important group of marine phytoplankton. We observed a general pattern that AP activity declined sharply at pH 5.5, peaked between pH 7 and 8, and dropped at pH > 8. However, our data revealed remarkable interspecific variations in optimal pH and niche breadth of pH. Among the species examined, Fugacium kawagutii and Prorocentrum cordatum had an optimal pH at 8, and Alexandrium pacificum, Amphidinium carterae, Effrenium voratum, and Karenia mikimotoi showed an optimal pH of 7. However, whereas A. pacificum and K. mikimotoi had the broadest pH niche for AP (7–10) and F. kawagutii the second (8–10), Am. carterae, E. voratum, and P. cordatum exhibited a narrow pH range. The response of Am. carterae AP to pH changes was verified using purified AP heterologously expressed in Escherichia coli. These results in concert suggest OA will likely differentially impact the capacity of different phytoplankton species to utilize DOP in the projected more acidified and nutrient-limited future ocean.     

Low activity of alkaline phosphatase in two eutrophic reservoirs

We studied recycling of phosphate by enzymatic hydrolysis in two temperate very eutrophic reservoirs. To assess the potential importance of phosphate regeneration by alkaline phosphatase, we determined the activity of this enzyme in lake water concomitantly with the determinations of the concentrations of phosphomonoesters, soluble reactive phosphate, total soluble phosphate and total phosphate. Contrary to our expectations for such productive waters where algal blooms are frequent, during the study period this process of phosphate regeneration was not significant, probably because the product of hydrolysis (contained in the soluble reactive phosphate fraction) was always abundant. We conclude that, in spite of what has been observed repeatedly in natural lakes with similar trophic characteristics, the readily available fraction of phosphate in these reservoirs is large and for that reason alkaline phosphatase production is low. Therefore hydrolysis by this enzyme is not significant for growth. What seems intriguing is the small amount of phosphomonoesters found in the water; with no phosphatase activity this phosphate fraction should always be high, unless hydrolysis takes place either during phosphomonoester release or later due to their instability.     

Sensitive detection of sodium cromoglycate with glutathione‐capped CdTe quantum dots as a novel fluorescence probe

A sensitive and simple analytical strategy for the detection of sodium cromoglycate (SCG) has been established based on a readily detectable fluorescence quenching effect of SCG for glutathione‐capped (GSH‐capped) CdTe quantum dots (QDs). The fluorescence of GSH‐capped CdTe QDs could be efficiently quenched by SCG through electron transfer from GSH‐capped CdTe QDs to SCG. Under optimum conditions, the response was linearly proportional to the concentration of SCG between 0.6419 and 100 µg/mL, with a correlation coefficient (R) of 0.9964; the detection limit (3δ/K) was 0.1926 µg/mL. The optimum conditions and the influence of coexisting foreign substances on the reaction were also investigated. The very effective and simple method reported here has been successfully applied to the determination of SCG in synthetic and real samples. It is believed that the established approach could have good prospects for application in the fields of clinical diseases diagnosis and treatment. Copyright © 2015 John Wiley & Sons, Ltd.     

The unfolding intermediate state of calf intestinal alkaline phosphatase during denaturation in guanidine solutions

The equilibrium unfolding of calf intestinal alkaline phosphatase in guanidinium chloride (GdmCl) solutions was studied by following the fluorescence and ultraviolet difference spectra. At low concentrations of GdmCl (< 1.6 M), the fluorescence intensity decreased with a slight red shift of the emission maximum from 332 nm to 344 nm. An unfolding intermediate state was observed at a broad concentration range of GdmCl as a denaturant (between 1.6 and 2.6 M). This intermediate was characterized by increased fluorescence emission intensity, ultraviolet difference absorption at 236 nm and 260 nm, as well as increased binding to the protein and red shift of the fluorescence probe 1-anilinonaphthalene-8-sulfonic acid.