Carbon dots: synthetic methods and applications as fluorescent probes for the detection of metal ions, inorganic anions and organic molecules

Fluorescent and chromogenic organic probes to detect group 10 metal ions: design strategies and sensing applications

Group 10 metals including Ni, Pd and Pt have been extensively applied in various essential aspects of human social life, material science, industrial manufactures, medicines and biology. The ionic forms of these metals are involved in several biologically important processes due to their strong binding capability towards different biomolecules. However, the mishandling or overuse of such metals has been linked to serious contamination of our ecological system, more specifically in soil and water bodies with acute consequences. Therefore, the detection of group 10 metal ions in biological as well as environmental samples is of huge significance from the human health point of view. Related to this, considerable efforts are underway to develop adequately efficient and facile methods to achieve their selective detection. Optical sensing of metal ions has gained increasing attention of researchers, particularly in the environmental and biological settings. Innovatively designed optical probes (fluorescent or colorimetric) are usually comprised of three basic components: an explicitly tailored receptor unit, a signalling unit and a clearly defined reporter unit. This review deals with the recent progress in the design and fabrication of fluorescent or colorimetric organic sensors for the detection of group 10 metal ions (Ni(II), Pd(II) and Pt(II)), with attention to the general aspects for design of such sensors.

A Ratiometric Probe Based on Carbon Dots and Calcein & Eu3+ for the Fluorescent Detection of Sodium Tripolyphosphate

Sodium tripolyphosphate, a food additive, is applied broadly in food industry. However, excessive accumulation of sodium tripolyphosphate can result in electrolyte abnormality of the body. Therefore, it is of great importance to investigate an effective method for the detection of sodium tripolyphosphate. In this work, nitrogen-doped carbon dots (NCDs) with constant fluorescence were fabricated using a domestic microwave oven. A ratiometric fluorescent probe was constructed in which NCDs were as internal standard, calcein & Eu³⁺ were as the detection signal. The fluorescence of calcein at 515 nm was quenched by Eu³⁺, whereas the emission peak of NCDs at 446 nm was almost unchanged. Additionally, the fluorescence of calcein was recovered because of the strong interaction of sodium tripolyphosphate and Eu³⁺. The linear range for sodium tripolyphosphate was 0.5-6 µmol/L with detection limit of 0.12 µmol/L. Furthermore, the ratiometric fluorescent probe was applied for sodium tripolyphosphate detection in real milk samples.

Mesoporous silica imprinted carbon dots for the selective fluorescent detection of triclosan

A molecularly imprinted fluorescence sensor built as a mesoporous structured silica imprinted layer on the surface of carbon dots (CDs@m-MIP) was employed for the selective detection of triclosan (TRI). The fluorescence of this CDs@m-MIP was affected sensitively and selectively by TRI via an electron transfer-induced fluorescence quenching mechanism with a detection limit of TRI at 1.08 nM (range 1.72-138 nM) under the optimum setup (e.g., pH, response time, and CDs@m-MIP dose). This approach was used successfully to detect TRI in real water samples (e.g., sewage, river, and tap water). The recoveries of TRI were satisfactory in spiked river and tap water (in 94.7-99.5 %). The outcome of this research is thus expected to help develop highly efficient fluorescent sensing systems towards diverse hazardous compounds including TRI.

Carbon nanodots combined with loop-mediated isothermal amplification (LAMP) for detection of African swine fever virus (ASFV)

The spread of African swine fever virus (ASFV) caused huge economic costs, so early detection is particularly important. Here, we established a fluorescence biosensor based on carbon nanodots (CNDs) and loop-mediated isothermal amplification (LAMP) to ultra-sensitively detect ASFV. LAMP with high efficiency produced a large amount of pyro phosphoric acid and caused pH change in a short time. CNDs with strong light stability had a large fluorescence response at the emission wavelength of 585.5 nm to small pH change by the excitation wavelength of 550 nm. The biosensor realized “turn-off–on” mode for ASFV detection with the detection limit as low as 15.21 copies μL⁻¹. In addition, the biosensor had high accuracy in the actual sample assay. Therefore, the biosensor achieved rapid, sensitive, low-cost, and simple detection for ASFV. Moreover, the biosensor broadened the detection pathway of LAMP as a tool with great development prospect.

Carbon dots with red emission for bioimaging of fungal cells and detecting Hg2+ and ziram in aqueous solution

It is of importance for bioimaging of fungal cells using biocompatible and low toxic carbon dots (CDs) as labels in plant protection field because a clearer understanding on the infection mechanism of fungi on plant can be achieved. Meanwhile, long wavelength, especially, red/near-infrared (NIR) emissive CDs are more biocompatible than short wavelength emissive ones. In this work, CDs with red emission were synthesized by solvothermal pyrolysis of citric acid, acrylamide dissolved in formamide. Fungal cells stained by the CDs with red emission were brightly illuminated when imaged on a fluorescent microscope with excitation by a green laser pulse, suggesting the CDs are of an excellent label for bioimaging of fungal cell in red color region. Moreover, the CDs show a selective response to Hg²⁺ in the NaAc-HAc buffer solution, while ziram can form a more stable complex with Hg²⁺, leading to a recovery of the quenched fluorescence of the CDs. Therefore, methods for the detections of Hg²⁺ and ziram based on the "off-on" fluorescence of the CDs were established with limits of detection as low as 0.19 μM and 0.55 μg/mL.

Synthesis of bovine serum albumin capped boron-doped carbon dots for sensitive and selective detection of Pb(II) ion

Carbon dots have tremendous potential to be used for biochemical sensing and environmental testing due to its superior optical properties and excellent biocompatibility. The surface of carbon dots can be easily functionalized. In the present study boron doped carbon dots have been synthesized using one pot approach by microwave treatment method. The surface of boron doped carbon dots is capped with bovine serum albumin. The maximum fluorescence emission observed at 444 nm when excited upon 345 nm of wavelength. In the normal light, it is light green in colour but when exposed in long wavelength UV light it exhibited blue fluorescence. The carbon dots have an irregular shape with a diameter below 5 nm. The applicability of synthesized carbon dots as the fluorescent sensor has been checked using different metal ions. It is observed that Pb(II) ion shows appreciable and selective quenching. Linear relationship is exist between the decrease in fluorescence intensity and the concentrations of Pb(II) ion in the range from 1 ppb to 10 ppb concentration. Limit of detection is found to be 0.08 ppb. This study will be helpful in the development of new fluorescent nano-biosensors.

One-step microwave synthesis of N,S co-doped carbon dots from 1,6-hexanediamine dihydrochloride for cell imaging and ion detection

As a new member of the fluorescent carbon nanomaterial family, carbon dots (CDs) not only have outstanding photoluminescence properties and small size characteristics, but also contain favourable low cytotoxicity and biocompatibility, which could be the best choice to detect of ions to replace quantum dots for ions detection. Here, the N,S co-doped carbon dots (N/S-CDs) was synthesized by one-step microwave using 1,6-hexanediamine dihydrochloride and dimethyl sulfoxide as precursors, and their morphology and structure were characterized by TEM, XRD, XPS and FTIR. The optimal emission wavelength of the CDs was 512 nm with green fluorescence, and was red-shifted gradually as the excitation wavelength aggrandized. The synthesized CDs owned a well quantum yield of 24 %. It was further applied for the detection of MnO₄^- and Cr₂O₇^2- with an excellent detection limit of 0.34 μM and 0.23μM, respectively. Cr₂O₇^2- did not influence the N/S-CDs PL response of MnO₄^- in the presence of excessive Pb²⁺. Moreover, the obtained N/S-CDs demonstrated preeminent biocompatibility and could be resoundingly applied for cellular imaging.

Intrinsic lysosomal targeting fluorescent carbon dots with ultrastability for long-term lysosome imaging

Intrinsic lysosomal targeting carbon dots were synthesized with ultrastability for long-term lysosome imaging of living cells and drug-induced apoptotic cells.

Facile Synthesis of Nitrogen-Doped Carbon Dots from Lignocellulosic Waste

The current research mainly focuses on transforming low-quality waste into value-added nanomaterials and investigating various ways of utilising them. The hydrothermal preparation of highly fluorescent N-doped carbon dots (N-CDs) was obtained from the carboxymethylcellulose (CMC) of oil palm empty fruit bunches and linear-structured polyethyleneimines (LPEI). Transmission electron microscopy (TEM) analysis showed that the obtained N-CDs had an average size of 3.4 nm. The N-CDs were monodispersed in aqueous solution and were strongly fluorescent under the irradiation of ultra-violet light. A detailed description of the morphology and shape was established using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). It was shown that LPEI were successfully tuned the fluorescence (PL) properties of CDs in both the intrinsic and surface electronic structures, and enhanced the quantum yield (QY) up to 44%. The obtained N-CDs exhibited remarkable PL stability, long lifetime and pH-dependence behaviour, with the excitation/emission maxima of 350/465.5 nm. Impressively, PL enhancement and blue-shifted emission could be seen with the dilution of the original N-CDs solution. The obtained N-CDs were further applied as fluorescent probe for the identification of Cu2+ in aqueous media. The mechanism could be attributed to the particularly high thermodynamic affinity of Cu2+ for the N-chelate groups over the surface of N-CDs and the fast metal-to-ligand binding kinetics. The linear relationship between the relative quenching rate and the concentration of Cu2+ were applied between 1-30 µM, with a detection limit of 0.93 µM. The fluorescent probe was successfully applied for the detection of Cu2+ in real water. Moreover, a solid-state film of N-CDs was prepared in the presence of poly (vinyl alcohol) (PVA) polymer and found to be stable even after 72-h of continuous irradiation to UV-lamp. In contrast to the aqueous N-CDs, the composite film showed only an excitation independent property, with enhanced PL QY of around 47%. Due to the strong and stable emission nature of N-CDs in both aqueous and solid conditions, the obtained N-CDs are ideal for reducing the overall preparation costs and applying them for various biological and environmental applications in the future.

Bimetallic AuCu nanoclusters-based florescent chemosensor for sensitive detection of Fe3+ in environmental and biological systems

Assays of ferric ion (Fe³⁺) with high sensitivity and selectivity have been required to evaluate its amount in environmental and biological systems. Herein, a novel fluorometric penicillamine-capped bimetallic gold-copper nanoclusters (PA-AuCu bi-MNCs) sensor was constructed for facile, environmentally friendly and quantitative detection of Fe³⁺ through inner filter effect (IFE) mechanism. One-step green synthetic approach was applied for the synthesis of AuCu bi-MNCs by using d-penicillamine (D-PA) as template and stabilizer. In the presence of Fe³⁺, the emission of the PA-AuCu bi-MNCs was hindered that caused selective quenching of the fluorescence intensity. The response to Fe³⁺ allows for two linear dynamic ranges of 5.0 × 10^-7 M-7.0 × 10^-6 M and 7.0 × 10^-6 M-1.0 × 10^-4 M with a detection limit of 0.1 μM, which is approximately 53 times lower than the maximum level (5.37 μM) of Fe³⁺ in drinking water that had been reported by the World Health Organization. The independency of the system from most of the interferences is the important feature of this work. Beside the appropriate selectivity of the proposed method, it shows a considerable operation in various environmental samples including rain water, three types of river water and also in human blood serum as a biological matrix.

Ratio fluorometric determination of ATP base on the reversion of fluorescence of calcein quenched by Eu(III) ion using carbon dots as reference

A kind of nitrogen doped carbon dots (NCDs) with excellent stable luminescence performance was prepared by pyrolysis using ethanolamine as precursor. By simply mixing solution of NCDs and calcein-Eu³⁺, a ratio fluorometric probe with carbon dots as "internal reference" and calcein-Eu³⁺ as recognition group was constructed for ATP detection. The fluorescence of the calcein can be selectively quenched by Eu³⁺, and can be restored when ATP was added because Eu³⁺ ions exhibit higher affinity to the oxygen-donor atoms originated from phosphates than that from carboxylate groups. Meanwhile, fluorescence of NCDs was not affected by Eu³⁺, calcein or ATP. By adding NCDs as "internal reference" in the above system, a new ratiometric strategy for detecting ATP was conducted. The dynamic linear range for ATP detection was 5.0 × 10^-8 mol L^-1~ 2.0 × 10^-6 mol L^-1, and the detection limit was 2.0 × 10^-8 mol L^-1.The method was successfully applied to detecting ATP in adenosine disodium triphosphate injection. Compared with calcein- Eu³⁺ probe without NCDs as reference, the ratio fluorometric probe effectively reduced interference and improved the accuracy and sensitivity.

Phenanthroline-Derivative Functionalized Carbon Dots for Highly Selective and Sensitive Detection of Cu2+ and S2- and Imaging inside Live Cells

Developing effective methods for the instant detection of Cu²⁺ and S²⁻ is highly desired in the biological and environmental fields. Herein, a novel fluorescent nanoprobe was elaborately designed and synthesized by grafting a phenanthroline derivative onto the surface of carbon dots (CDs). The obtained functionalized CDs (FCDs) exhibited blue fluorescence (FL) with excellent photostability and possessed a mean diameter around 4 nm. Cu²⁺ can be selectively captured by the phenanthroline group of FCDs to generate an absorptive complex in situ, leading to obvious quenching of the FCDs’ FL signal through an inner filter effect. Furthermore, the FL of the FCD–Cu²⁺ can be effectively recovered by S²⁻ anions due to the release of FCDs from the FCD–Cu²⁺ complex owing to the formation of stable CuS (K_sp = 1.27 × 10⁻³⁶) between S²⁻ and Cu²⁺. The detection limits of the FCDs were determined to be 40.1 nM and 88.9 nM for Cu²⁺ and S²⁻, respectively. Moreover, this nanoprobe can also be used for the imaging of intracellular Cu²⁺ and S²⁻, which shows strong application prospects in the field of biology.

Synthesis of hydrophilic and hydrophobic carbon quantum dots from waste of wine fermentation

Wine lees are one of the main residues formed in vast quantities during the fermentation of wine. While toxic when applied to plants and wetlands, it is a biodegradable material, and several alternatives have been proposed for its valorization as: dietary supplement in animal feed, source for various yeast extracts and bioconversion feedstock. The implementation of stricter environment protection regulations resulted in increasing costs for wineries as their treatment process constitutes an unavoidable and expensive step in wine production. We propose here an alternative method to reduce waste and add value to wine production by exploiting this rich carbon source and use it as a raw material for producing carbon quantum dots (CQDs). A complete synthetic pathway is discussed, comprising the carbonization of the starting material, the screening of the most suitable solvent for the extraction of CQDs from the carbonized mass and their hydrophobic or hydrophilic functionalization. CQDs synthesized with the reported procedure show a bright blue emission (λ_max = 433 ± 13 nm) when irradiated at 366 nm, which is strongly shifted when the wavelength is increased (e.g. emission at around 515 nm when excited at 460 nm). Yields and luminescent properties of CQDs, obtained with two different methods, namely microwave and ultrasound-based extraction, are discussed and compared. This study shows how easy a residue can be converted into an added-value material, thus not only reducing waste and saving costs for the wine-manufacturing industry but also providing a reliable, affordable and sustainable source for valuable materials.

A "Turn-On" thiol functionalized fluorescent carbon quantum dot based chemosensory system for arsenite detection

Carbon quantum dots (CQDs) have emerged out as promising fluorescent probes for hazardous heavy metals detection in recent past. In this study, water soluble CQDs were synthesized by facile microwave pyrolysis of citric acid & cysteamine, and functionalized with ditheritheritol to impart thiol functionalities at surface for selective detection of toxic arsenite in water. Microscopic analysis reveals that the synthesized CQDs are of uniform size (diameter ∼5nm) and confirmed to have surface SH groups by FT-IR. The functionalized probe is then demonstrated for arsenite detection in water by "Turn-On" read out mechanism, which reduces the possibility of false positive signals associated with "turn off' probes reported earlier. The blue luminescent functionalized CQDs exhibit increase in fluorescence intensity on arsenite addition in 5-100ppb wide detection range. The probe can be used for sensitive detection of arsenite in environmental water to a theoretical detection limit (3s) of 0.086ppb (R²=0.9547) with good reproducibility at 2.6% relative standard deviation. The presented reliable, sensitive, rapid fCQDs probe demonstrated to exhibit high selectivity towards arsenite and exemplified for real water samples as well. The analytical performance of the presented probe is comparable to existing organic & semiconductor based optical probes.