The synthesis and modification of highly fluorescent carbon quantum dots for reversible detection of water‐soluble phosphonate‐1‐hydroxyethane‐1,1‐diphosphonic acid by fluorescence spectroscopy

Photoluminescent (PL) carbon quantum dots (CQDs) were prepared successfully using a facile and green procedure. They exhibited striking blue fluorescence and excellent optical properties, with a quantum yield as high as 61.44%. Due to the fluorescence quenching effect and the stronger complexing ability of the phosphoric acid group of 1‐hydroxyethane‐1,1‐diphosphonic acid (HEDP) to Fe³⁺ , CQDs doped with Fe³⁺ were adequately constructed as an efficient and sensitive fluorescent probe for HEDP‐specific sensing. The proposed fluorescent probe had a sensitive and rapid response in the range 5–70 μ M. Furthermore, quantitative molecular surface (QMS) analysis based on the Multiwfn program was applied to explore the complexation mode of HEDP and metal ions. The distribution of electrostatic potential (ESP), average local ionization energy (ALIE), the minimum value points and the position of the lone pair electrons on the surface of molecular van der Waals were further determined. More strikingly, this experiment achieved the quantitative detection of water‐soluble phosphonate‐HEDP, for the first time using fluorescence spectrometry. It has been proved to be an effective and intuitive sensing method for the detection of HEDP in real samples.     

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.     

Fluorescent carbon nanoparticles obtained from charcoal via green methods and their application for sensing Fe3+ in an aqueous medium

Two green methods (microwave and hydrothermal) were employed for the preparation of water dispersible fluorescent carbon nanoparticles (CNPs) from activated charcoal. Microwave and hydrothermally synthesized carbon nanoparticles, (MW‐CNPs) and (HT‐CNPs), respectively were characterized by microscopic and spectroscopic techniques. A detailed study of their fluorescence characteristics was made. MW‐CNPs and HT‐CNPs were tested for metal ion selectivity in aqueous medium. MW‐CNPs showed selectivity for Fe³⁺ among the tested metal ions and important studies such as for interference, linear range and limit of detection were carried out. The application of MW‐CNPs for detection of Fe³⁺ in water was demonstrated.     

A solvothermal method to synthesize fluorescent carbon nanoparticles and application to photocatalysis and electrocatalysis

A novel solvothermal approach to synthesize fluorescent carbon nanoparticles (CNPs) was developed using glucose and ammonium oxalate as the carbon source, and glycol as the solvent. The solution of as‐prepared CNPs emitted blue‐green fluorescence under ultraviolet (UV) light at 365 nm, and the carbon nanoparticle formation was investigated through XRD, TEM, DLS, FT‐IR, UV, PL, XPS. The particle was well dispersed with an average diameter of about 10–30 nm. In contrast with previous methods, in this synthesis process neither strong acid treatment nor further surface modification was necessary. The solution of as‐prepared CNPs were applied to photocatalytic degradation of mountain green in the present H₂O₂, and the decolorization rate was over 97% when the reaction time was more than 7 h under visible light. The as‐prepared CNPs were also applied to electrocatalysis and showed excellent electrocatalytic activity. Copyright © 2014 John Wiley & Sons, Ltd.     

A real‐time fluorescence assay for protease activity and inhibitor screening based on the aggregation‐caused quenching of a perylene probe

We have established a real‐time and label‐free fluorescence turn‐on strategy for protease activity detection and inhibitor screening via peptide‐induced aggregation‐caused quenching of a perylene probe. Because of electrostatic interactions and high hydrophilicity, poly‐l ‐glutamic acid sodium salt (PGA; a negatively charged peptide) could induce aggregation of a positively charged perylene probe (probe 1) and the monomer fluorescence of probe 1 was effectively quenched. After a protease was added, PGA was enzymatically hydrolyzed into small fragments and probe 1 disaggregated. The fluorescence recovery of probe 1 was found to be proportional to the concentration of protease in the range from 0 to 1 mU/ml. The detection limit was down to 0.1 mU/ml. In the presence of a protease inhibitor, protease activity was inhibited and fluorescence recovery reduced. Moreover, we demonstrated the potential application of our method in a complex mixture sample including 1% human serum. Our method is simple, fast and cost effective.     

Carbon nanoparticle for highly sensitive and selective fluorescent detection of mercury(II) ion in aqueous solution

In this article, carbon nanoparticles (CNPs) were used as a novel fluorescent sensing platform for highly sensitive and selective Hg(2+) detection. To the best of our knowledge, this is the first example of CNPs obtained from candle soot used in this type of sensor. The general concept used in this approach is based on that adsorption of the fluorescently labeled single-stranded DNA (ssDNA) probe by CNP via π-π stacking interactions between DNA bases and CNP leads to substantial dye fluorescence quenching; however, in the presence of Hg(2+), T-Hg(2+)-T induced hairpin structure does not adsorb on CNP and thus retains the dye fluorescence. A detection limit as low as 10nM was achieved. The present CNP-based biosensor for Hg(2+) detection exhibits remarkable specificity against other possible metal ions. Furthermore, superior selectivity performance was observed when Hg(2+) detection was carried out in the presence of a large amount of other interference ions. Finally, in order to evaluate its potential practical application, Hg(2+) detection was conducted with the use of lake water other than pure buffer and it is believed that it holds great promise for real sample analysis upon further development.     

Fluorescence modulation of 1,7‐bis(4‐hydroxy‐3‐methoxyphenyl)‐1,6‐heptadiene‐3,5‐dione by silver nanoparticles and its possible analytical application

The effects of silver nanoparticles on the photophysical properties of 1,7‐bis(4‐hydroxy‐3‐methoxyphenyl)‐1,6‐heptadiene‐3,5‐dione, popularly known as curcumin, have been investigated using optical absorption and fluorescence techniques. Although absorption spectroscopy suggests a ground‐state complex formation, fluorescence quenching data confirms a simultaneous static and dynamic quenching, inferring ground as well as excited‐state complex formation. The recovery of fluorescence quenching of the curcumin–silver nanoparticle complex in the presence of ascorbic acid or uric acid emphasizes a strong interaction between the silver nanoparticles and ascorbic acid/uric acid, suggesting that fluorescence recovery after the quenching of curcumin–silver nanoparticle complexes has potential for ascorbic acid or uric acid assay development. Copyright © 2011 John Wiley & Sons, Ltd.     

Real‐time nucleic acid sequence–based amplification (NASBA) using an adenine‐induced quenching probe and an intercalator dye

Aims: We found that an adenine base caused fluorescence quenching of a fluorescein (FL)‐labelled probe in DNA:RNA hybrid sequences, and applied this finding to a nucleic acid sequence–based amplification (NASBA) method. Methods and Results: The present NASBA method employed a probe containing an FL‐modified thymine at its 3′ end and ethidium bromide (EtBr) on the basis of a combination of adenine‐induced quenching and fluorescence resonance energy transfer (FRET) between the FL donor and EtBr acceptor. This NASBA was used to detect Shiga toxin (STX) stx‐specific mRNA in STX‐producing Escherichia coli, demonstrating rapid quantification of the target gene with high sensitivity. Conclusion: Although the inherent quenching effect of adenine was inferior to that of guanine, FRET between the FL and EtBr moieties enhanced the adenine‐induced quenching, allowing rapid and sensitive real‐time NASBA detection. Significance and Impact of the Study: This study gives a novel real‐time diagnostic system based on NASBA for a sensitive mRNA (or viral RNA) detection.     

Silver‐doped graphite carbon nitride nanosheets as fluorescent probe for the detection of curcumin

Green fluorescent silver (Ag)‐doped graphite carbon nitride (Ag‐g‐C₃N₄) nanosheets have been fabricated by an ultrasonic exfoliating method. The fluorescence of the Ag‐g‐C₃N₄ nanosheets is quenched by curcumin. The fluorescence intensity decreases with the increase in the concentration of curcumin, indicating that the Ag‐g‐C₃N₄ nanosheets can function as a non‐toxic and facile fluorescence probe to detect curcumin. The fluorescence intensity of Ag‐g‐C₃N₄ nanosheets shows a linear relationship to curcumin in the concentration range 0.01–2.00 μM with a low detection limit of 38 nM. The fluorescence quenching process between curcumin and Ag‐g‐C₃N₄ nanosheets mainly is based on static quenching. The fluorescent probe has been successfully applied to analyse curcumin in human urine and serum samples with satisfactory results.     

Selective fluorescence quenching of papain–Au nanoclusters by self‐polymerization of dopamine

In this paper, we synthesized a papain‐stabilized fluorescent Au nanocluster (NC) probe and studied its interaction with dopamine. As fluorescence of papain–Au NCs is quenched in the presence of dopamine under alkaline conditions, we were able to establish a simple, selective analytical method for dopamine determination. By studying the fluorescence lifetime and dynamic light scattering of the NCs before and after interaction with dopamine, we found that this fluorescence quenching mechanism was possibly due to dopamine self‐polymerization that produced polydopamine that cross‐linked papain–Au NCs. Based on this new phenomenon, we proposed a highly selective analytical method for dopamine detection. Other small organic molecules, such as amino acids, ascorbic acid and uric acid did not interfere with dopamine detection. Dopamine in the range 20–100 μM can be linearly detected by the fluorescence quenching ratio of gold nanoclusters. Dopamine detection could be visually realized by watching color changes of papain–Au NCs under UV light or daylight, as both fluorescence and absorption of the papain–Au NCs changed during dopamine polymerization.     

BSA‐conjugated zinc oxide nanoparticles as luminescent probes for the determination of histidine

Zinc oxide nanoparticles doped with bovine serum albumin were used to determine histidine in aqueous solutions using a fluorescence spectroscopic technique. The results showed that histidine effectively quenched the fluorescence of the modified ZnO nanoparticles, whereas other amino acids did not significantly affect the light emission, thereby allowing selective and sensitive histidine detection in amino acid mixtures. Under optimal conditions (pH 7.0, 25 °C, 10 min preincubation), the detection limit for histidine was ~ 9.87 × 10^–7 mol/L. The high value of the determined quenching rate constant K_q (3.30 × 10¹³ L/mol/s) was consistent with a static quenching mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Carbon Nanoparticles in High‐Performance Perovskite Solar Cells

In the past few years, organic–inorganic metal halide ABX₃ perovskites (A = Rb, Cs, methylammonium, formamidinium (FA); B = Pb, Sn; X = Cl, Br, I) have rapidly emerged as promising materials for photovoltaic applications. Tuning the film morphology by various deposition techniques and additives is crucial to achieve solar cells with high performance and long‐term stability. In this work, carbon nanoparticles (CNPs) containing functional groups are added to the perovskite precursor solution for fabrication of fluorine‐doped tin oxide/TiO₂/perovskite/spiro‐OMeTAD/gold devices. With the addition of CNPs, the perovskite films are thermally more stable, contain larger grains, and become more hydrophobic. NMR experiments provide strong evidence that the functional groups of the CNPs interact with FA cations already in the precursor solution. The fabricated solar cells show a power‐conversion efficiency of 18% and negligible hysteresis.     

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.     

Nitrogen‐doped carbon dots as a fluorescent probe for the highly sensitive detection of Ag+ and cell imaging

An easy hydrothermal synthesis strategy was applied to synthesize green‐yellow emitting nitrogen‐doped carbon dots (N‐CDs) using 1,2‐diaminobenzene as the carbon source, and dicyandiamide as the dopant. The nitrogen‐doped CDs resulted in improvement in the electronic characteristics and surface chemical activities. N‐CDs exhibited bright fluorescence emission and could response to Ag⁺ selectively and sensitively. Other ions produced nearly no interference. A N‐CDs based fluorescent probe was then applied to sensitively determine Ag⁺ with a detection limit of 5 × 10^?8 mol/L. The method was applied to the determination of Ag⁺ dissolved in water. Finally, negligibly cytotoxic, excellently biocompatibile, and highly fluorescent carbon dots were applied for HepG2 cell imaging and the quenched fluorescence by adding Ag⁺, which indicated its potential applications.     

Highly luminescent nitrogen‐doped carbon dots for simultaneous determination of chlortetracycline and sulfasalazine

Here, we have presented a green and facile strategy to fabricate nitrogen‐doped carbon dots (N‐CDs) and their applications for determination of chlortetracycline (CTC) and sulfasalazine (SSZ). The fluorescent N‐CDs, prepared by one‐step hydrothermal reaction of citric acid and l ‐arginine, manifested numerous excellent features containing strong blue fluorescence, good water‐solubility, narrow size distribution, and a high fluorescence quantum yield (QY) of 38.8%. Based on the fluorescence quenching effects, the as‐synthesized N‐CDs as a fluorescent nanosensor exhibited superior analytical performances for quantifying CTC and SSZ. The linear range for CTC was calculated to be from 0.85 to 20.38 μg ml^?1 with a low detection limit of 0.078 μg ml^?1. Meanwhile, the linear range for SSZ was estimated to be from 0.34 to 6.76 μg ml^?1 with a low detection limit of 0.032 μg ml^?1. Therefore, the N‐CDs hold admirable application potential for constructing a fluorescent sensor for pharmaceutical analysis.     

Boron and nitrogen co‐doped carbon dots as a sensitive fluorescent probe for the detection of curcumin

In this present study, a fluorescent probe was developed to detect curcumin, which is derived from the rhizomes of the turmeric. We used a simple and economical way to synthesize boron and nitrogen co‐doped carbon dots (BNCDs) by microwave heating. The maximum emission wavelength of the BNCDs was 450 nm at an excitation wavelength of 360 nm. The as‐prepared BNCDs were characterized by multiple analytical techniques such as transmission electron microscopy, X‐ray photoelectron spectroscopy, X‐ray diffraction, and infrared spectroscopy. The synthesized carbon nanoparticles had an average particle diameter of 4.23 nm. The BNCDs exhibited high sensitivity to the detection of curcumin at ambient conditions. The changes of BNCDs fluorescent intensity show a good linear relationship with the curcumin concentrations in the range 0.2–12.5 μM. This proposed method has been successfully applied to detect the curcumin in urine samples with the recoveries of 96.5–105.5%.     

Nanographite‐based fluorescent biosensor for detecting microRNA using duplex‐specific nuclease‐assisted recycling

The development of a nanographite (NG)‐based fluorescent biosensor for detecting microRNA (miRNA) is reported. Duplex‐specific nuclease (DSN)‐assisted signal amplification was key to its function. In the absence of a target, with the assistance of p‐stacking interactions, the NG adsorbed the double carboxyfluorescein (FAM)‐labelled probe (DFP) whose surface was perfectly complementary to miRNA, leading to quenching of FAM fluorescence. In the presence of a target, double‐stranded DNA/RNA hybrids were repelled by the NG and fluorescence was restored. Meanwhile, the considerable increase in signal strength and sensitivity suggests DSN‐mediated target recycling as an application. The detection limit of the proposed biosensor for miRNA was 10 pmol/L; there was a linear correlation when the miRNA concentration ranged from 50 pmol/L to 5 nmol/L. Additionally, the method could distinguish let‐7b from most let‐7 miRNA family members and was successfully used in a sample assay. This biosensor is a novel and highly sensitive tool for miRNA detection and has great potential for biochemical research, disease diagnosis, and therapy.     

‘Turn‐off’ fluorescence strategy for determination of hexavalent chromium ions based on copper nanoclusters

Cu nanoclusters (CuNCs) capped by tannic acid (TA) (CuNCs@TA) can be used as a highly sensitive fluorescent probe for Cr(VI) detection. Therefore, a fluorescence detection method for Cr(VI) can be established according to the fluorescence quenching of CuNCs@TA that is caused immediately after the addition of Cr(VI). The fluorescence quenching efficiency of CuNCs@TA was linearly correlated with Cr(VI) concentration within the range 0.03–60 μM, and the detection limit for Cr(VI) was 5 nM. This method was demonstrated to be suitable for detecting Cr(VI) in actual water samples. We found that sodium thiosulfate (ST) can redox with Cr(VI) and therefore restore the fluorescence of CuNCs@TA. The mechanism of CuNCs@TA fluorescence quenching and enhancement by Cr(VI) and ST was investigated in detail. The ‘turn‐on’ fluorescent sensor is of practical significance and has broad application prospects.     

Spectrofluorimetric determination of folic acid in tablets and urine samples using 1,10‐phenanthroline‐terbium probe

A new spectrofluorimetric method was reported for the determination of folic acid (FA), based on its quenching effect on the fluorescence intensity of Tb³⁺–1,10‐phenanthroline complex as a fluorescent probe. The quenched fluorescence intensity at an emission wavelength of 545 nm was proportional to the concentration of FA in Tris–HCl buffer solution of pH 6.2. The effects of pH, time, order of addition of reagents, temperature and concentrations of Tb³⁺, buffer and 1,10‐phenanthroline were investigated and optimized. The linear range for the determination of FA was 0.01–1.1 mg/L. The detection limit was 0.003 mg/L and the relative standard deviation for replicated determination of 1 mg/L of folic acid was 1.2%. This method was simple, practical and relatively free from interference effects. It was successfully applied to assess FA in pharmaceutical tablets and urine samples. Copyright © 2010 John Wiley & Sons, Ltd.     

Preparation of aminated core‐shell fluorescent nanoparticles and their application to the synchronous fluorescence determination of γ‐globulin

Amino‐modified silica nanoparticles (FSNPs) doped with fluorescein isothiocyanate (FITC) were synthesized by using an aqueous core of reverse‐micelle microemulsion as the nanoreactor in an easy one‐pot method. Due to the FITC conjugating with (3‐aminopropyl)triethoxysilane (APTS), the nanoparticles prevent the FITC from leaching from the silica matrix when immersed in aqueous solution. SEM, FTIR, fluorescence lifetime, a photobleaching experiment and synchronous fluorescence spectra were used to characterize the FSNPs. The synchronous fluorescence signal of FSNPs was enhanced when trace amounts of γ‐globulin (γ‐G) were added. Under the optimal experimental conditions, the enhanced fluorescence intensity (ΔF) was linear with the concentration of γ‐G (c) in the range 0.3–4.8 µg/mL, with a detection limit of 0.04 µg/mL. The proposed method is simple, sensitive for the determination of trace amounts of γ‐G and used to determine the content of γ‐G in synthetic samples with satisfactory results. Copyright © 2008 John Wiley & Sons, Ltd.