Green Synthesis of Red‐Emitting Carbon Nanodots as a Novel “Turn‐on” Nanothermometer in Living Cells

Temperature measurements in biology and medical diagnostics, along with sensitive temperature probing of living cells, is of great importance; however, it still faces significant challenges. Herein, a novel “turn‐on” carbon‐dot‐based fluorescent nanothermometry device for spatially resolved temperature measurements in living cells is presented. The carbon nanodots (CNDs) are prepared by a green microwave‐assisted method and exhibit red fluorescence (λ_em=615 nm) with high quantum yields (15 %). Then, an on–off fluorescent probe is prepared for detecting glutathione (GSH) based on aggregation‐induced fluorescence quenching. Interestingly, the quenched fluorescence could be recovered by increasing temperature and the CNDs–GSH mixture could behave as an off–on fluorescent probe for temperature. Thus, red‐emitting CNDs can be utilized for “turn‐on” fluorescent nanothermometry through the fluorescence quenching and recovery processes, respectively. We employ MC3T3‐E1 cells as an example model to demonstrate the red‐emitting CNDs can function as “non‐contact” tools for the accurate measurement of temperature and its gradient inside a living cell.     

Determination of trans-fatty acids in food samples based on the precolumn fluorescence derivatization by high performance liquid chromatography

Trans-fatty acids are unsaturated fatty acids that are considered to have health risks. 1,3,5,7-Tetramethyl-8-butyrethylenediamine-difluoroboradiaza-s-indacene is a highly sensitive fluorescent labeling reagent for carboxylic acids developed by our lab. In this study, using this precolumn fluorescent derivatization reagent, a rapid and accurate high-performance liquid chromatography with fluorescence detection method was developed for the determination of two trans-fatty acids in food samples. Under the optimized derivative conditions, two trans-fatty acids were tagged with the fluorescent labeling reagent in the presence of 1-ethyl-3-(3-dimethyl-aminopropyl) carbodiimide at 25°C for 30 min. Then, the baseline separation of trans- and cis-fatty acids and their saturated fatty acid with similar structures was achieved with less interference using a reversed-phased C₁₈ column with isocratic elution in 14 min. With fluorescence detection at λ_ex/λ_em = 490 /510 nm, the linear range of the TFAs was 1.0-200 nM with low detection limits in the range of 0.1–0.2 nM (signal-to-noise ratio = 3). In addition, the proposed approach was successfully applied for the detection of trans-fatty acids in food samples, and the recoveries using this method ranged from 96.02 to 109.22% with low relative standard deviations of 1.2–4.3% (n = 6).     

Ultrasmall green-emitting carbon nanodots with 80% photoluminescence quantum yield for lysosome imaging

Carbon-based fluorescent nanomaterials have gained much attention in recent years. In this work, green-photoluminescent carbon nanodots (CNDs; also termed carbon dots, CDs) with amine termination were synthesized via the hydrothermal treatment of amine-containing spermine and rose bengal (RB) molecules. The CNDs have an ultrasmall size of ∼2.2 nm and present bright photoluminescence with a high quantum yield of ∼80% which is possibly attributed to the loss of halogen atoms (Cl and I) during the hydrothermal reaction. Different from most CNDs which have multicolor fluorescence emission, the as-prepared CNDs possess excitation-independent emission property, which can avoid fluorescence overlap with other fluorescent dyes. Moreover, the weakly basic amine-terminated surface endows the CNDs with the acidotropic effect. As a result, the CNDs can accumulate in the acidic lysosomes after cellular internalization and can serve as a favorable agent for lysosome imaging. Besides, the CNDs have a negligible impact on the lysosomal morphology even after 48 h incubation and exhibit excellent biocompatibility in the used cell models.     

ssDNA-QDs/GO multicolor fluorescence system for synchronous screening of hepatitis virus DNA

Viral hepatitis is a common infectious disease caused by five viruses (hepatitis virus A, B, C, D, and E). Given the diversity of hepatitis virus, rapid screening and accurate typing of viral hepatitis are the prerequisites for hepatitis therapy. Here, a multicolor fluorescence system was constructed by combining with the multi-color fluorescence properties of CdSe/ZnS quantum dots (QDs, emission wavelengths: 525 nm, 585 nm and 632 nm) and the broad-spectrum fluorescence quenching performance of GO. Taking advantage of the specific recognition of ssDNA modified CdSe/ZnS QDs to target hepatitis virus DNA, the constructed system could effectively distinguish hepatitis A virus DNA (HAV-DNA), hepatitis B virus DNA (HBV-DNA), and hepatitis C virus DNA (HCV-DNA) in a homogeneous solution. Based on the different adsorption property of GO for ssDNA and dsDNA, the fluorescence Forster resonance energy transfer (FRET) process between ssDNA modified QDs and GO could be regulated. The fluorescence signal of the constructed system presented a sensitive response to HAV-DNA, HBV-DNA, and HCV-DNA content in the range of 1.0–192 nM, 8.0–192 nM, and 1.0–128 nM, respectively. The limit of detection for HAV-DNA, HBV-DNA, and HCV-DNA is 0.46 nM, 1.53 nM, and 0.58 nM. The constructed system can be used to screen hepatitis virus DNA in real samples, which provides an alternative strategy for rapid screening and diagnosis of viral hepatitis.     

Turning waste into wealth: facile and green synthesis of carbon nanodots from pollutants and applications to bioimaging

In an effort to turn waste into wealth, Reactive Red 2 (RR2), a common and refractory organic pollutant in industrial wastewater, has been employed for the first time as a precursor to synthesize carbon nanodots (CNDs) by a facile, green and low-cost route, without utilization of any strong acids or other oxidizers. The detailed characterizations have confirmed that the synthesized CNDs exhibit good water dispersibility, with a mean particle size of 2.43 nm and thickness of 1–3 layers. Importantly, the excellent fluorescence properties and much reduced biotoxicity of the CNDs confer its potential applications in further biological imaging, which has been successfully verified in both in vitro (cell culture) and in vivo (zebrafish) model systems. Thus, it is demonstrated that the synthesized CNDs exhibit nice biocompatibility and fluorescence properties for bioimaging. This work not only provides a novel economical and environmentally friendly approach in recycling a chemical pollutant, but also greatly promotes the potential application of CNDs in biological imaging.

The pollutant reactive red 2 was employed to synthesize fluorescent carbon nanodots allowing biological imaging in vitro and in vivo.     

Pulsed lasers versus continuous light sources in capillary electrophoresis and fluorescence detection studies: Photodegradation pathways and models

Pulsed lasers are widely used in capillary electrophoresis (CE) studies to provide laser induced fluorescence (LIF) detection. Unfortunately pulsed lasers do not give linear calibration curves over a wide range of concentrations. While this does not prevent their use in CE/LIF studies, the non-linear behavior must be understood. Using 7-hydroxycoumarin (7-HC) (10–5000 nM), Tamra (10–5000 nM) and tryptophan (1–200 μM) as dyes, we observe that continuous lasers and LEDs result in linear calibration curves, while pulsed lasers give polynomial ones. The effect is seen with both visible light (530 nm) and with UV light (355 nm, 266 nm). In this work we point out the formation of byproducts induced by pulsed laser upon irradiation of 7-HC. Their separation by CE using two Zeta LIF detectors clearly shows that this process is related to the first laser detection. All of these photodegradation products can be identified by an ESI-/MS investigation and correspond to at least two 7HC dimers. By using the photodegradation model proposed by Heywood and Farnsworth (2010) and by taking into account the 7-HC results and the fact that in our system we do not have a constant concentration of fluorophore, it is possible to propose a new photochemical model of fluorescence in LIF detection. The model, like the experiment, shows that it is difficult to obtain linear quantitation curves with pulsed lasers while UV-LEDs used in continuous mode have this advantage. They are a good alternative to UV pulsed lasers. An application involving the separation and linear quantification of oligosaccharides labeled with 2-aminobezoic acid is presented using HILIC and LED (365 nm) induced fluorescence.     

From Metal–Organic Framework to Intrinsically Fluorescent Carbon Nanodots

Highly photoluminescent carbon nanodots (CNDs) were synthesized for the first time from metal–organic framework (MOF, ZIF‐8) nanoparticles. Coupled with fluorescence and non‐toxic characteristics, these carbon nanodots could potentially be used in biosafe color patterning.     

Synthesis and characterization of new selective Zn fluorescent probes for functionalization: in vitro Cell imaging applications

Herein the synthesis and characterization of new, lipophilic highly Zn²⁺-selective fluorescent probes are reported. High affinity for zinc (K_d 1.1–8.0 nM) over other biologically relevant metals and mixtures of metals was observed. Excitation at 360 nm afforded an emission spectrum with maximum at 530 nm for the zinc bound complex. The linear relationship between fluorescence intensity and zinc concentration indicates that FZnA-probes can be used for quantification. The probes have been synthesized in 28–45% overall yield and the feasibility for further functionalization with biologically relevant side chains has been demonstrated. In vitro studies using PC12 cells and 10 μM of one of the novel probes (FZnA-Ada) visualized endogenous labile Zn²⁺ after 45 min incubation time.     

Composition dependent frequency upconversion of Er/Yb-codoped lead chloride tellurite glasses

The green and red upconversion luminescence of Er³⁺ in lead chloride tellurite glasses excited at 980 nm is investigated. Three intense emission bands centered at 530, 545, and 658 nm corresponding to the transitions ⁴S_3/2→⁴I_15/2, ²H_11/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2, respectively, were simultaneously observed at room temperature. With increasing PbCl₂ content, the intensity of green (530 nm) emissions increase slightly, while the green (545 nm) and red (658 nm) emissions increase significantly. The results indicate that PbCl₂ has more influence on the green (545 nm) and red (658 nm) emissions than the green (530 nm) emission. The dependence of upconversion intensities on excitation power and possible upconversion mechanisms are discussed and evaluated.     

Customizing the Electrochemical Properties of Carbon Nanodots by Using Quinones in Bottom‐Up Synthesis

We show how the redox potentials of carbon nanodots (CNDs) can be modulated by employing quinones as electroactive precursors during a microwave‐assisted synthesis. We prepared and characterized a redox library of CNDs, demonstrating that this approach can promote the use of carbon nanodots for ad hoc applications, including photocatalysis.     

Intramolecular Hydrogen Bonds Quench Photoluminescence and Enhance Photocatalytic Activity of Carbon Nanodots

Understanding the photoluminescence (PL) and photocatalytic properties of carbon nanodots (CNDs) induced by environmental factors such as pH through surface groups is significantly important to rationally tune the emission and photodriven catalysis of CNDs. Through adjusting the pH of an aqueous solution of CNDs, it was found that the PL of CNDs prepared by ultrasonic treatment of glucose is strongly quenched at pH 1 because of the formation of intramolecular hydrogen bonds among the oxygen‐containing surface groups. The position of the strongest PL peak and its corresponding excitation wavelength strongly depend on the surface groups. The origins of the blue and green emissions of CNDs are closely related to the carboxyl and hydroxyl groups, respectively. The deprotonated COO^? and CO^? groups weaken the PL peak of the CNDs and shift it to the red. CNDs alone exhibit photocatalytic activity towards degradation of Rhodamine B at different pH values under UV irradiation. The photocatalytic activity of the CNDs is the highest at pH 1 because of the strong intramolecular hydrogen bonds formed among the oxygen‐containing groups.     

A dual-responsive fluorescence method for the detection of clenbuterol based on BSA-protected gold nanoclusters

The illegal feeding of clenbuterol (CLB) to domestic animals and the potential harm of it to human health lead an urgent requirement for the efficient detection of CLB, especially in the edible meat. In this paper we reported a new fluorescence method for the detection of trace amount of CLB by using the BSA-protected gold nanoclusters (AuNCs@BSA). Under the excitation of either 280 or 500 nm the emission of AuNCs@BSA was quenched obviously by diazotized CLB, supplying a dual-responsive fluorescence method to detect CLB in aqueous solution. In addition, the linear response of the fluorescence intensity of AuNCs@BSA to diazotized CLB allowed the quantitative detection of CLB in a range of 4.0 nM–300 μM upon excitation at two wavelength, and the limit of detection for CLB was 3.0 nM upon 280 nm excitation and 1.6 nM upon 500 nm excitation, respectively. In addition, the dual-responsive mechanism of AuNCs@BSA to CLB was investigated in detail by using several CLB analogues and reference compounds. Particularly, the proposed method was successfully applied to detect CLB in pork mince and the results were validated well by HPLC, illustrating it could be used as a reliable, rapid, and cost-effective technique for the determination of CLB residues in real samples.     

A new fluorescence method for determination of ozone in ambient air

A new simple method for determination of ozone in ambient air is presented. The reaction employed is based on the known ozonolysis of indigo dye. The indigotrisulfonate molecule contains one carbon–carbon double bond (C═C), which reacts with ozone and generates isatinsulfonates and sulfoanthranilate. The quantitatively formed sulfoanthranilate presents fluorescence (λ_ex 245 nm, λ_em 400 nm). Ozone was collected using two cellulose filters coated with 40 μL of 1.0 × 10^{− 3} mol L^{− 1} of indigotrisulfonate. The analytical response was linear in the range 0–150 ppbv ozone, and a detection limit of 7 ppbv was achieved using a sampling time of 15 min and an optimum sampling air flow rate of 0.4 L min^{− 1}. There was no interference from sulfur dioxide, formaldehyde or nitrogen dioxide. The ozonolysis mechanism and the reaction products are discussed.     

A simple synthesis of nitrogen doped porous graphitic carbon: Electrochemical determination of paracetamol in presence of ascorbic acid and p-aminophenol

Graphite paste electrode modified with nitrogen doped porous carbon (NDPC) is used for the detections of paracetamol (PCM), ascorbic acid (AA) and p-aminophenol (PAP) at relatively low concentration. NDPC is synthesized by direct carbonization of Zn(OAc)₂ incorporated melamine-formaldehyde resin microsphere. The NDPC shows small pore diameters centered at 3.14 nm and 8.12 nm and has a pseudo graphitic structure with reasonable porous matrix. The lower limit of detections (S/N = 3) for PCM, AA, and PAP are found to be 30 nM, 720 nM and 10 nM respectively. Under optimized experimental condition, the linear ranges of determination for PCM and AA are 1–400 μM, 10–2700 μM respectively in mixture. Similarly for PCM and PAP mixture, the linear ranges of determination are found to be 1–90 μM. It is also used for the analysis of urine and pharmaceutical products with better sensitivity.     

Aptamer‐Based Turn‐On Detection of Thrombin in Biological Fluids Based on Efficient Phosphorescence Energy Transfer from Mn‐Doped ZnS Quantum Dots to Carbon Nanodots

This paper presents the first example of a sensitive, selective, and stable phosphorescent sensor based on phosphorescence energy transfer (PET) for thrombin that functions through thrombin–aptamer recognition events. In this work, an efficient PET donor–acceptor pair using Mn‐doped ZnS quantum dots labeled with thrombin‐binding aptamers (TBA QDs) as donors, and carbon nanodots (CNDs) as acceptors has been constructed. Due to the π–π stacking interaction between aptamer and CNDs, the energy donor and acceptor are taken into close proximity, leading to the phosphorescence quenching of donors, TBA QDs. A maximum phosphorescence quenching efficiency as high as 95.9 % is acquired. With the introduction of thrombin to the “off state” of the TBA‐QDs‐CNDs system, the phosphorescence is “turned on” due to the formation of quadruplex‐thrombin complexes, which releases the energy acceptor CNDs from the energy donors. Based on the restored phosphorescence, an aptamer‐based turn‐on thrombin biosensor has been demonstrated by using the phosphorescence as a signal transduction method. The sensor displays a linear range of 0–40 nM for thrombin, with a detection limit as low as 0.013 nM in pure buffers. The proposed aptasensor has also been used to monitor thrombin in complex biological fluids, including serum and plasma, with satisfactory recovery ranging from 96.8 to 104.3 %. This is the first time that Mn‐doped ZnS quantum dots and CNDs have been employed as a donor–acceptor pair to construct PET‐based biosensors, which combines both the photophysical merits of phosphorescence QDs and the superquenching ability of CNDs and thus affords excellent analytical performance. We believe this proposed method could pave the way to a new design of biosensors using PET systems.     

Aptasensor for ampicillin using gold nanoparticle based dual fluorescence–colorimetric methods

A gold nanoparticle based dual fluorescence–colorimetric method was developed as an aptasensor to detect ampicillin using its single-stranded DNA (ssDNA) aptamer, which was discovered by a magnetic bead-based SELEX technique. The selected aptamers, AMP4 (5′-CACGGCATGGTGGGCGTCGTG-3′), AMP17 (5′-GCGGGCGGTTGTATAGCGG-3′), and AMP18 (5′-TTAGTTGGGGTTCAGTTGG-3′), were confirmed to have high sensitivity and specificity to ampicillin (K _d, AMP7 = 9.4 nM, AMP17 = 13.4 nM, and AMP18 = 9.8 nM, respectively). The 5′-fluorescein amidite (FAM)-modified aptamer was used as a dual probe for observing fluorescence differences and color changes simultaneously. The lower limits of detection for this dual method were a 2 ng/mL by fluorescence and a 10 ng/mL by colorimetry for ampicillin in the milk as well as in distilled water. Because these detection limits were below the maximum residue limit of ampicillin, this aptasensor was sensitive enough to detect antibiotics in food products, such as milk and animal tissues. In addition, this dual aptasensor will be a more accurate method for antibiotics in food products as it concurrently uses two detection methods: fluorescence and colorimetry.     

One-pot synthesis of strongly fluorescent DNA-CuInS2 quantum dots for label-free and ultrasensitive detection of anthrax lethal factor DNA

Herein, high quality DNA-CuInS₂ QDs are facilely synthesized through a one-pot hydrothermal method with fluorescence quantum yield as high as 23.4%, and the strongly fluorescent DNA-CuInS₂ QDs have been utilized as a novel fluorescent biosensor for label-free and ultrasensitive detection of anthrax lethal factor DNA. L-Cysteine (L-Cys) and a specific-sequence DNA are used as co-ligands to stabilize the CuInS₂ QDs. The specific-sequence DNA consists of two domains: phosphorothiolates domain (sulfur-containing variants of the usual phosphodiester backbone) controls the nanocrystal passivation and serves as a ligand, and the functional domain (non-phosphorothioates) controls the biorecognition. The as-prepared DNA-CuInS₂ QDs have high stability, good water-solubility and low toxicity. Under the optimized conditions, a linear correlation was established between the fluorescence intensity ratio I/I₀ (I₀ is the original fluorescence intensity of DNA-CuInS₂ QDs, and I is the fluorescence intensity of DNA-CuInS₂ QDs/GO with the addition of various concentrations of anthrax lethal factor DNA) and the concentration of anthrax lethal factor DNA in the range of 0.029–0.733 nmol L⁻¹ with a detection limit of 0.013 nmol L⁻¹. The proposed method has been successfully applied to the determination of anthrax lethal factor DNA sequence in human serum samples with satisfactory results. Because of low toxicity and fine biocompatibility, DNA-CuInS₂ QDs also hold potential applications in bioimaging.     

A carbon–carbon hybrid – immobilizing carbon nanodots onto carbon nanotubes

The thrust of this work is to integrate small and uniformly sized carbon nanodots (CNDs) with single-walled carbon nanotubes (SWCNT) of different diameters as electron donors and electron acceptors, respectively, and to test their synergetic interactions in terms of optoelectronic devices. CNDs (denoted pCNDs, where p indicates pressure) were prepared by pressure-controlled microwave decomposition of citric acid and urea. pCNDs were immobilized on single-walled carbon nanotubes by wrapping the latter with poly(4-vinylbenzyl trimethylamine) (PVBTA), which features positively charged ammonium groups in the backbone. Negatively charged surface groups on the CNDs lead to attractive electrostatic interactions. Ground state interactions between the CNDs and SWCNTs were confirmed by a full-fledged photophysical investigation based on steady-state and time-resolved techniques. As a complement, charge injection into the SWCNTs upon photoexcitation was investigated by ultra-short time-resolved spectroscopy.     

Fluorescent detection of amidinium-carboxylate and amidinium formation using anthracene-based diamidine: an application for the analysis of dicarboxylic acid binding

An anthracene-based diamidine (1) ‘turn-on’ fluorescent probe for the detection of dicarboxylic acids has been designed and synthesized. The fluorescence spectra of the diamidine 1 with carboxylic acids that showed two different fluorescence bands, which corresponded to the amidinium-carboxylate (λ_em=430-440 nm), and amidinium (λ_em=460-470 nm as a broad band) formation, were confirmed by DOSY NMR and TD-DFT calculations. These different fluorescence bands showed the binding mode of carboxylic acids and the stability of formed complexes toward diamidine 1. The fluorescent detection of amidinium-carboxylate formation using diamidine 1 was applicable to the detection of α,ω-dicarboxylic acids (C₆-C₁₃) and succeeded in the detection of α,ω-dicarboxylic acid (adipic acid) in human urine.