Fluorescent probe based on N-doped carbon dots for the detection of intracellular pH and glutathione

Carbon dots (CDs) as fluorescent probes have been widely exploited to detect biomarkers, however, tedious surface modification of CDs is generally required to achieve a relatively good detection ability. Here, we synthesized N-doped carbon dots (N-CDs) from triethylenetetramine (TETA) and m-phenylenediamine (m-PD) using a one-step hydrothermal method. When the pH increases from 3 to 11, the fluorescence intensity of the N-CDs gradually decreases. Furthermore, it displays a linear response to the physiological pH range of 5–8. Au³⁺ is reduced by amino groups on the surface of N-CDs to generate gold nanoparticles (AuNPs), causing fluorescence quenching of the N-CDs. If glutathione (GSH) is then added, the fluorescence of the N-CDs is recovered. The fluorescence intensity of the N-CDs is linearly correlated with the GSH concentration in the range of 50–400 μM with a limit of detection (LOD) of 7.83 μM. The fluorescence probe was used to distinguish cancer cells from normal cells using pH and to evaluate intracellular GSH. This work expands the application of CDs in multicomponent detection and provides a facile fluorescent probe for the detection of intracellular pH and GSH.

N-doped carbon dots used as a fluorescence probe can distinguish cancer cells from normal cells by pH and evaluate intracellular GSH.     

Nitrogen-doped carbon quantum dots for fluorescence detection of Cu2+ and electrochemical monitoring of bisphenol A

In this work, water-soluble nitrogen-doped carbon quantum dots (N-CDs) were synthetized at low temperature via a simple hydrothermal strategy, using citric acid as the carbon source and polyethylenimine (PEI) as the nitrogen source. The as-prepared N-CDs with near spherical structure and sizes of 4.5–7.5 nm exhibited blue luminescence and a fluorescence quantum yield of 40.2%. Both X-ray photoelectron spectroscopy (XPS) and FTIR spectroscopy measurements demonstrated the presence of the primary and secondary amines on the surface of the N-CDs. The fluorescence of N-CDs could be effectively quenched by Cu²⁺ owing to the formation of a copper–amine complex between Cu²⁺ and the amino groups on the surface of the N-CDs. Since this behavior was quite pronounced the fluorescence quenching was used for Cu²⁺ detection with high sensitivity and good selectivity. The linear range spanned the concentration of Cu²⁺ from 0.2 to 10 μM with a detection limit of 2 nM. In addition, the N-CDs could effectively electrochemically catalyze the oxidation of bisphenol A (BPA), which provided a promising method for BPA detection. The calibration range of BPA was 0.01 to 0.21 μM with a detection limit of 1.3 nM.

Nitrogen-doped carbon dots were applied in the fluorescence detection of Cu²⁺ and electrochemical detection of BPA.     

Urea-doped carbon dots as fluorescent switches for the selective detection of iodide ions and their mechanistic study

A facile and green strategy for the fabrication of fluorescent urea-doped carbon dots (N-CDs) has been explored. Significantly, the fluorescent N-CDs could recognize iodide ions (I⁻) with high selectivity, and their photoluminescence could be efficiently quenched by the addition of I⁻. The sensitivity analysis for I⁻ indicated a linear relationship in the range from 12.5 to 587 μM with the detection limit as low as 0.47 μM. Furthermore, the I⁻ induced fluorescence (FL) quenching mechanism was investigated employing a combination of techniques, including UV-vis/fluorescence spectroscopy, Density Functional Theory (DFT) calculation, TEM and time-resolved fluorescence decay measurements. The DFT calculation results demonstrated that the amino- and amide groups of N-CDs play a significant role in iodide recognition through the formation of multiple N–H⋯I⁻, C–H⋯I⁻ and C( O)N–H⋯I⁻ interactions with I⁻. The TEM experiment confirmed the aggregation process when I⁻ was added to the N-CDs solution. Moreover, the radiative decay rate of N-CDs, which was first measured and reported the kinetic behaviors of the FL-quenching process, decreased from 3.30 × 10⁷ s⁻¹ to 1.95 × 10⁷ s⁻¹ after the coordination with I⁻ ions. The reduced lifetime demonstrated that the excited energy dissipation led to a dynamic quenching process. Therefore, such carbon materials can function as effective fluorescent switches for the selective detection of I⁻ ions.

Urea-doped carbon dots (N-CDs) have been successfully fabricated for monitoring iodide ions; the reduced lifetime of N-CDs demonstrated that the excited energy dissipation led to a dynamic fluorescence quenching process.     

Green fluorescent nanomaterials for rapid detection of chromium and iron ions: wool keratin-based carbon quantum dots

Heavy metal ions produced by industrial activity have been a serious environmental problem, and their detection is critical for treatment of the heavy metal pollution. Among the variable heavy metal detection methods, fluorescent indicator methods have attracted wide attention due to the advantage of the convenience and nondestructive detection process. Carbon quantum dots have great application in this respect. In this study, nitrogen and sulfur doped fluorescent carbon quantum dots (N,S-CDs) were prepared based on wool keratin using a hydrothermal method, of which the morphology, chemical composition and optical properties were characterized. The prepared N,S-CDs were spherical nanoparticles with a diameter of 2–6 nm, showing wide fluorescence excitation and emission wavelength range and considerable quantum yield, and have sensitive response to Cr⁶⁺ and Fe³⁺. This study reveals a novel cost-effective and convenient synthetic route of green carbon quantum dots through naturally sourced materials, demonstrates the application potential of the wool keratin-based N,S-CDs in rapid detection of heavy metal, and opens up a new path for functional utilization of waste wool keratin.

Synthesis of carbon quantum dots from wool keratin and their potential in detecting chromium and iron ions.     

Synthesis and bioimaging of a BODIPY-based fluorescence quenching probe for Fe3+

Iron is the main substance for maintaining life. Real-time determination of ferric ion (Fe³⁺) in living cells is of great significance for understanding the relationship of Fe³⁺ concentration changes with various physiological and pathological processes. Fluorescent probes are suitable for the detection of trace metal ions in cells due to their low toxicity and high sensitivity. In this work, a boron-dipyrromethene-based fluorescent probe (BODIPY-CL) for selective detection of Fe³⁺ was synthesized. The fluorescence emission of BODIPY-CL was determined at 516 nm. In a pH range of 1 to 10, the probe BODIPY-CL exhibits a quenching response to Fe³⁺. Meanwhile, BODIPY-CL showed a highly selective response to Fe³⁺ compared with 16 kinds of metal ions. The stoichiometry ratio of BODIPY-CL bound to Fe³⁺ was nearly 2 : 1. The fluorescence quenching response obtained by the sensor was linear with the Fe³⁺ concentration in the range of 0–400 μM, and the detection limit was 2.9 μM. BODIPY-CL was successfully applied to image Fe³⁺ in cells. This study provides a promising fluorescent imaging probe for further research on the physiological and pathological effects of Fe³⁺.

A quenched fluorescence probe sensitive to Fe³⁺ ions was synthesized. The probe was successfully used to detect Fe³⁺ in living organisms.     

Remarkably selective biocompatible turn-on fluorescent probe for detection of Fe3+ in human blood samples and cells

The robust nature of a biocompatible fluorescent probe is demonstrated, by its detection of Fe³⁺ even after repeated rounds of quenching (reversibility) by acetate in real human blood samples and cells in vitro. Significantly trace levels of Fe³⁺ ions up to 8.2 nM could be detected, remaining unaffected by the existence of various other metal ions. The obtained results are validated by AAS and ICP-OES methods. A portable test strip is also fabricated for quick on field detection of Fe³⁺. As iron is a ubiquitous metal in cells and plays a prominent role in biological processes, the use of this probe to image Fe³⁺ in cells is a substantial development towards biosensing. Cytotoxicity studies also proved the nontoxic nature of this probe.

The robust nature of a biocompatible fluorescent probe is demonstrated, by its detection of Fe³⁺ even after repeated rounds of quenching (reversibility) by acetate in real human blood samples and cells in vitro.     

Reed-derived fluorescent carbon dots as highly selective probes for detecting Fe3+ and excellent cell-imaging agents

A kind of highly selective and sensitive fluorescent probe for detecting Fe³⁺, carbon dots (CDs), was prepared with renewable reed naturally containing C, N, O, and S elements as a green and eco-friendly carbon source by a simple hydrothermal process. The fluorescence of CDs without purification and surface modification can be quenched by Fe³⁺ in a wide concentration range of 0 to 362 μmol L⁻¹ (concentration of Fe³⁺), with detection limits as low as 0.014 μmol L⁻¹ in 0–50 μmol L⁻¹. Characterizations, such as TEM, XPS, Raman and FTIR, confirmed that the static quenching mechanism involved the generation of non-luminescent complexes between Fe³⁺ and functional groups (carboxyl group, sulfur-oxyl group and hydroxyl group) on the surface of CDs and with the aggregation of CDs. More importantly, CDs had good biocompatibility and nontoxicity according to an MTT cell-viability assay, and cells labeled with CDs emitted blue, green and red color fluorescence. Thus, the static quenching mechanism was confirmed. So, this reed-derived natural CD solution can be utilized in detecting Fe³⁺, culture cells, and cell imaging.

A highly selective and sensitive fluorescent probe for detecting Fe³⁺, carbon dots (CDs), was prepared with renewable reed naturally containing C, N, O, and S elements as a green and eco-friendly carbon source by a simple hydrothermal process.     

Miscanthus grass-derived carbon dots to selectively detect Fe3+ ions

Novel fluorescent carbon dots (CDs) were synthesized using an economically feasible and green one-step heating process. Miscanthus, a perennial grass and an inexpensive sustainable biomass, was utilized as the starting material to prepare CDs and doped CDs (nitrogen, phosphorous and nitrogen-phosphorous dual doped). The abundance of oxygen-containing functional groups in Miscanthus-derived CDs (MCD) and doped MCD was confirmed via Fourier-transform infrared (FTIR) and energy dispersive X-ray spectroscopy (EDS). The average size of MCD, N-doped MCD, P-doped MCD and dual-doped MCDs was found to be 7.87 ± 0.27, 4.6 ± 0.21, 6.7 ± 0.38 and 5.3 ± 0.32 nm, respectively. The synthesized MCD and doped MCD exhibited a quantum yield (QY) of 4.71, 11.65, 2.33 and 9.63% for the MCD, N-doped MCD, P-doped MCD and dual-doped MCD, respectively. MCD and doped MCD exhibited excellent excitation-dependent photoluminescence properties, with strong blue fluorescence upon irradiation with UV-light (365 nm). N-doped MCD exhibited superb selectivity towards Fe³⁺ ions, with a detection limit of 20 nM and a detection range from 0.02 to 2000 μM. The normalized linear relationship between the intensity of fluorescence emission of the prepared N-doped MCD and the concentration of Fe³⁺ ions was utilized to selectively and sensitively detect Fe³⁺ ions.

Fluorescent carbon dots for the selective and sensitive detection of Fe³⁺ ions with a wide detection range and very low detection limit.     

Environmentally friendly synthesis of photoluminescent biochar dots from waste soy residues for rapid monitoring of potentially toxic elements

Single-step environmentally friendly synthesis of biochar dots (BCDs) was developed using hydrothermal treatment of waste biomass. Using soy residue as the carbon precursor, the resultant BCDs had strong and stable photoluminescence. Characterization by atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy indicates that the BCDs prepared were water soluble, spherical, oxygenous and nitrogen-doped carbon nanoparticles with 10–20 nm in diameter. The fluorescence quantum yield of the BCDs was 3.7%. The use of the BCDs as a very effective fluorescent probe for label-free, rapid, and selective detection of Hg²⁺ and Fe³⁺ ions was further demonstrated with good linear relationships at 0–50 μM and 10–50 μM, respectively. The minimum detection limits of Hg²⁺ and Fe³⁺ were 100 nM and 30 nM. Furthermore, the feasibility of using the BCDs for monitoring of Hg²⁺ and Fe³⁺ in open waters was also established.

Waste biomass was used as a carbon precursor to prepare photoluminescent biochar dots for economical and eco-friendly monitoring of Hg²⁺ and Fe³⁺ ions.     

A novel method for the preparation of solvent-free,microwave-assisted and nitrogen-doped carbon dots as fluorescent probes for chromium(vi) detection and bioimaging

Chromium(vi) [Cr(vi)] has been shown to be toxic to organisms due to its mutagenicity and carcinogenicity. Therefore, the exploitation of probes with low toxicity and high sensitivity for Cr(vi) detection is needed. In this study, a one-step, solvent-free, and microwave-assisted method was developed for the preparation of nitrogen-doped carbon dots (N-CDs). The reaction could be finished in just three minutes, and the yield of the dots could reach 58.5%; the as-prepared N-CDs exhibited excellent water solubility, emitted bright cyan fluorescence with a high quantum yield of 38.88%, and possessed excitation- and concentration-dependent characteristics. The N-CDs could be effectively applied to Cr(vi) detection with a linear range of 1–100 μM, and the detection limit could be as low as 0.12 μM. The quenching mechanism was responsible for the inner filter effect, and the quenched fluorescence could be recovered with a linear range of 5–100 μM by the addition of ascorbic acid. We showed that the fluorescent probes could even be employed for the detection of Cr(vi) in river water and for bio-imaging because of their nearly zero cytotoxicity; this showed the potential application of these probes in ion detection and cellular bioimaging. Herein, we have provided an effective strategy to rapidly obtain high-quality N-CDs using a solid-phase microwave method, and the as-prepared N-CDs exhibit various potential applications in environmental and biological fields.

Herein, N-doped carbon dots with excellent fluorescence characteristics were prepared by a solvent-free, microwave-assisted method and employed for the fluorometric detection of Cr(vi) and bioimaging.     

Integration detection of mercury(ii) and GSH with a fluorescent “on-off-on” switch sensor based on nitrogen,sulfur co-doped carbon dots

Using aurine and citric acid as precursors, we have synthesized stable blue-fluorescent nitrogen and sulfur co-doped carbon dots (NS-CDs), with a high quantum yield of up to 68.94% via a thermal lysis method. The fluorescent NS-CDs were employed as a sensitive sensor for the integration detection of Hg²⁺ and glutathione (GSH). This was attributed to Hg²⁺ effectively quenching the fluorescence of the NS-CDs by static quenching, and then GSH was able to recover the fluorescence owing to the stronger binding between Hg²⁺ and the sulfhydryl of GSH. Based on the “on-off-on” tactic, the detection limits of Hg²⁺ ions and GSH were 50 nM and 67 nM respectively. The fluorescence sensor was successfully applied to detect Hg²⁺ ions and GSH in actual samples (tap water and fetal bovine serum). Furthermore, we have proved that the sensor had good reversibility. Overall, our NS-CDs can serve as effective sensors for environmental and biological analysis in the future.

NS-CDs are employed as a sensitive sensor for the integration detection of Hg²⁺ and GSH. Hg²⁺ effectively quenching the fluorescence by static quenching. GSH is able to recover the fluorescence owing to the stronger binding between Hg²⁺ and GSH.     

Dansyl-modified carbon dots with dual-emission for pH sensing,Fe3+ ion detection and fluorescent ink

In this work, a multifunctional ratiometric fluorescence (FL) nanohybrid (CSCDs@DC) was synthesized from chitosan based carbon dots (CSCDs) and dansyl chloride (DC) at room temperature. The CSCDs@DC revealed strong FL intensity, great stability and excellent anti-photobleaching properties. Herein, CSCDs@DC was responsive to pH value in the range of 1.5–4.0 and exhibited color-switchable FL properties between acidic and alkaline environments. In addition, CSCDs@DC showed good selectivity and sensitivity towards Fe³⁺ ions. A good linear relationship for the Fe³⁺ ion detection was obtained in the range from 0 μM to 100 μM, with a detection limit of 1.23 μM. What''s more, CSCDs@DC can be used as a fluorescent ink. It expressed superior optical properties after 3 months of storage or continuous exposure to UV light for 24 h. This study suggested that CSCDs@DC had potential in the detection of pH and metal ions, as well as showing promising application in the anti-counterfeiting field.

This work provided a new strategy for developing a multifunctional fluorescence platform which had potential in the detection of pH and metal ions, as well as showing promising application in the anti-counterfeiting field.     

Fluorescence sensing of tyrosinase activity based on amine rich carbon dots through direct interaction in a homogeneous system: detection mechanism and application

As a vital, copper-containing oxidase, tyrosinase (TYR) is useful as a biomarker for the screening of skin diseases. In this paper, a convenient and sensitive homogeneous fluorescence detection platform for the assay of TYR activity without any modified steps is described. Inspired by the fact that carbon dots (CDs) with excellent properties can be obtained through some surface modification, amine rich carbon dots (N-CDs) using a nitrogen doping process were developed as the fluorescent probe for this assay. The effect and the response mechanism of the degree of nitrogen doping in relation to the response of different CDs to the sensing of TYR activity using dopamine (DA) as a substrate were investigated. The DA was oxidized to o-dopaquinone with the catalyzation of TYR and quenched the fluorescence of the N-CDs by direct interaction. By using a set concentration of DA and other optimized reaction conditions, the fluorescence intensity of the N-CDs was directly applied to monitor the TYR activity. This assay for TYR activity showed a broad linear range from 0.05 to 6.0 U mL⁻¹ with a detection limit of 0.039 U mL⁻¹. The satisfactory recovery of the sensor for TYR activity in diluted human serum illustrated a potential clinical application.

As a vital, copper-containing oxidase, tyrosinase (TYR) is useful as a biomarker for the screening of skin diseases.     

Green synthesis of surface-passivated carbon dots from the prickly pear cactus as a fluorescent probe for the dual detection of arsenic(iii) and hypochlorite ions from drinking water

Efforts were made to develop a simple new approach for the green synthesis of surface-passivated carbon dots from edible prickly pear cactus fruit as the carbon source by a one-pot hydrothermal route. Glutathione (GSH) was passivated on the surface of the CDs to form a sensor probe, which exhibited excellent optical properties and water solubility. The prepared sensor was successfully characterized by UV-visible spectrophotometry, fluorescence spectrophotometry, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The simple sensing platform developed by the GSH-CDs was highly sensitive and selective with a “turn-off” fluorescence response for the dual detection of As³⁺ and ClO⁻ ions in drinking water. This sensing system exhibited effective quenching in the presence of As³⁺ and ClO⁻ ions to display the formation of metal complexes and surface interaction with an oxygen functional group. The oxygen-rich GSH-CDs afforded a better selectivity for As³⁺/ClO⁻ ions over other competitive ions. The fluorescence quenching measurement quantified the concentration range as 2–12 nM and 10–90 μM with the lower detection limit of 2.3 nM and 0.016 μM for the detection of As³⁺ and ClO⁻ ions, respectively. Further, we explored the potential applications of this simple, reliable, and cost-effective sensor for the detection of As³⁺/ClO⁻ ions in environmental samples for practical analysis.

Efforts were made to develop a simple new approach for the green synthesis of surface-passivated carbon dots from edible prickly pear cactus fruit as the carbon source by a one-pot hydrothermal route.     

A review on I–III–VI ternary quantum dots for fluorescence detection of heavy metals ions in water: optical properties,synthesis and application

Heavy metal contamination remains a major threat to the environment. Evaluating the concentrations of heavy metals in water environments is a crucial step towards a viable treatment strategy. Non-cadmium photo-luminescent I–III–VI ternary QDs have attracted increasing attention due to their low toxicity and extraordinary optical properties, which have made them popular in biological applications. Recently, ternary I–III–VI-QDs have gained growing interest as fluorescent detectors of heavy metal ions in water. Here, we review the research progress of ternary I–III–VI QDs for the fluorescence detection of heavy metal ions in water. First, we summarize the optical properties and synthesis methodologies of ternary I–III–VI QDs. Then, we present various detection mechanisms involved in the fluorescence detection of heavy metal ions, which are mostly attributed to direct interaction between these unique QDs and the metal ions, seen in the form of fluorescence quenching and fluorescence enhancement. We also display the potential applications in environmental remediation such as water treatment and associated challenges of I–III–VI QDs in the fluorescence detection of Cu²⁺ and other metal ions.

Ternary I–III–VI quantum dots used in the fluorescence detection of heavy metals ions in water.     

Co2+ detection,cell imaging,and temperature sensing based on excitation-independent green-fluorescent N-doped carbon dots

Green-fluorescent N-doped carbon dots (N-CDs) have been successfully fabricated using hydrothermal treatment of tyrosine and urea. The N-CDs obtained showed excitation-independent emission, superior stability and strong photoluminescence with a quantum yield of ca. 9.8%. Based on these striking behaviors, the as-prepared N-CDs have been utilized in Co²⁺ detection and temperature sensing. Due to an inner filter effect, the N-CDs obtained were dramatically quenched by Co²⁺ with linear ranges of 0.1 μM–10 μM, 25 μM–275 μM and 300 μM–400 μM, and they had a detection limit of 0.15 μM. The use of the as-prepared N-CDs has been extended to visualize Co²⁺ fluctuations in living cells. Additionally, the N-CDs obtained have also been applied for use as a temperature sensor with a linear range of 25–80 °C.

Green-fluorescent N-doped carbon dots (N-CDs) have been successfully fabricated using hydrothermal treatment of tyrosine and urea.     

Facile synthesis of N,P-doped carbon dots from maize starch via a solvothermal approach for the highly sensitive detection of Fe3+

Nitrogen/phosphorus-doped carbon dots (N, P-CDs) with a quantum yield as high as 76.5% were synthesized by carbonizing maize starch via a facile ethanol solvothermal approach. Transmission electron microscopy (TEM) measurement shows that the as-prepared N, P-CDs displayed a quasi-spherical shape with a mean size of ca. 2.5 nm. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy disclosed the presence of –OH, –NH₂, –COOH, and –CO functional groups over the surface of N, P-CDs. On the basis of excellent fluorescent properties with strong blue fluorescence emission at 445 nm upon excitation at 340 nm, these N, P-CDs were adopted as a fluorescent probe towards the effective detection of Fe³⁺ ions in water. The limit of detection (LOD) was as low as 0.1 μmol L⁻¹ and showed a better linear relationship in the range of 0.1 ∼ 50 μmol L⁻¹. In conclusion, these synthesized N, P-CDs can be efficiently used as a promising candidate for the detection of Fe³⁺ ions in some practical samples.

Nitrogen/phosphorus-doped carbon dots (N, P-CDs) with a quantum yield as high as 76.5% were synthesized by carbonizing maize starch via a facile ethanol solvothermal approach and utilized for the detection of Fe³⁺.     

Environmentally exploitable biocide/fluorescent metal marker carbon quantum dots

Carbon quantum dots are currently investigated to act as safe/potent alternatives for metal-based nanostructures to play the role of probes for environmental applications owing to their low toxicity, low cost, chemical inertness, biocompatibility and outstanding optical properties. The synthesis of biocide/fluorescent metal marker carbon quantum dots with hydrophilic character was performed via a quite simple and green technique. The natural biopolymer that was used in this study for the synthesis of carbon quantum dots is fragmented under strong alkaline conditions. Afterwards, under hydrothermal conditions, re-polymerization, aromatization and subsequent oxidation, the carbonic nanostructures were grown and clustered. Dialysis of the so-produced carbonic nanostructures was carried out to obtain highly purified/mono-dispersed carbon quantum dots with a size distribution of 1.5–6.5 nm. The fluorescence intensity of the synthesized carbon quantum dots under hydrothermal conditions for 3 h was affected by dialysis, however, the fluorescence intensity was significantly increased ca. 20 times. The synthesized carbon quantum dots were exploited as fluorescent markers in the detection of Zn²⁺ and Hg²⁺. The prepared carbon quantum dots also exhibited excellent antimicrobial potency against Bacillus cereus, Escherichia coli and Candida albicans. The detected minimal inhibitory concentration for the dialyzed CQDs towards the tested pathogens was 350–450 μL mL⁻¹. The presented approach is a simple and green technique for the scaled-up synthesis of biocide/fluorescent marker carbon quantum dots instead of metal-based nanostructures for environmental applications, without using toxic chemicals or organic solvents.

Synthesis of biocide/fluorescent metal marker carbon quantum dots with hydrophilic character for the detection of Zn²⁺ and Hg²⁺.     

Fish-scale-derived carbon dots as efficient fluorescent nanoprobes for detection of ferric ions

Herein, highly fluorescent carbon dots (CDs) with the incorporation of N and O functionalities were prepared through a facile and cost-effective hydrothermal reaction using fish scales of the crucian carp as the precursor. The as-prepared CDs exhibit strong fluorescent emissions at 430 nm with a relative quantum yield of 6.9%, low cytotoxicity, and robust fluorescence stability against photobleaching and good ionic strength. More significantly, the fluorescence of these CDs can be effectively and selectively quenched by Fe³⁺ ions, which enables the application of CDs as fluorescent Fe³⁺ nanoprobes with a linear range of 1–78 μmol L⁻¹ and a detection limit of 0.54 μmol L⁻¹. The proposed fluorescent CD nanoprobes can also be used for the assay of spiked Fe³⁺ in real water samples and human serums with high recoveries and low standard deviations. Hence, CDs can be potentially applied as safe and reliable fluorescent nanoprobes for environmental and clinical Fe³⁺ analyses.

Herein, highly fluorescent carbon dots (CDs) with the incorporation of N and O functionalities were prepared through a facile and cost-effective hydrothermal reaction using fish scales of the crucian carp as the precursor.

Fe³⁺ ions are a versatile chemical that have been widely used in the purification of drinking water, catalyzing certain reactions,^1,2 and electronic industry. In addition, Fe³⁺ is also an essential metal ion in hemoglobin, myosin, and cytochrome, which can regulate normal oxygen uptake, cellular metabolism, and enzymatic reactions in physiological DNA and RNA syntheses in human bodies.^3,4 The deficiency of Fe³⁺ causes various diseases such as iron deficiency, anemia, aplastic anemia, complicating further into limb weakness and reduced immune functions,⁵ whereas excessive Fe³⁺ can result in dysfunctions (kidney, heart, and liver), Alzheimer''s disease, and even cancer.^6–8 Hence, the accurate analysis of Fe³⁺ is of significant clinical importance. On the other hand, the overutilization of Fe resources and the discharge of Fe³⁺-containing sewage into aquatic environments result in a considerable threat to the natural environment; therefore, the accurate and reliable determination of Fe³⁺ in aquatic media is also imperative.⁹ Conventional Fe³⁺ detection techniques mainly include flame atomic absorption spectrophotometry (FAAS),¹⁰ inductively coupled plasma mass spectrometry (ICP-MS),¹¹ voltammetry,¹² and spectrometry.¹³ Although worthwhile detection accuracy can be achieved, these techniques usually require tedious sample preparation procedures or sophisticated instruments, which limit the rapid and handy analysis of Fe³⁺ in practice. Hence, it is essential to develop cost-effective analysis means for the quantitative detection of Fe³⁺.Because of its high sensitivity, rapid response, and simplicity in sample preparation, the fluorescent probe technique is recognized as an alternative means to quantitatively determine Fe³⁺ ions through fluorescence quenching interactions.^14–16 Currently, the major fluorescent probe materials for Fe³⁺ detection include organic fluorescent dyes,^17,18 noble metal nanoclusters,^19,20 and heavy-metal chalcogenide semiconductor quantum dots.^21,22 Nevertheless, organic fluorescent dyes suffer from photobleaching, and semiconductor quantum dots suffer from potential elemental toxicity and complicated size/morphology control issues. Therefore, it is essential to investigate alternative fluorescent probes for the sensitive and rapid detection of Fe³⁺ with high photostability, low cytotoxicity, and easy availability that can be suitable for widespread applications.Carbon quantum dots (CDs) represent a newly emerged fluorescent nanomaterial with the doping or decoration of heteroelements containing functionalities within/onto the tiny carbonaceous motif (below 10 nm).²³ Besides the high fluorescence intensity, CDs also exhibit high solubility, robust chemical inertness, high resistance toward photobleaching, and good biocompatibility; hence, CDs can serve as efficient and safe fluorescent nanomaterials for bioimaging,²⁴ anti-faking,²⁵ biosensing,²⁶ and photocatalysis.²⁷ In particular, CDs can serve as electron donors in contact with guest acceptors through electrostatic or coordination interactions: the excitation state electrons of CDs can be nonradiatively transferred to the guest analytes, and therefore, the fluorescence of CDs is effectively quenched. Hence, CDs are widely investigated as credible sensing nanoprobes for the analyses of various guest ions/molecules with high sensitivities and reliabilities.In general, CDs can be prepared through top-down and bottom-up strategies. The former approach mainly includes the chemical or electrochemical cutting of graphitic motifs (graphite, carbon nanotubes, or graphene) into tiny nanodots with tunable bandgaps, but this approach commonly necessitates harsh synthesis conditions and modification of functionalities to tune the electron structure, structural energy traps, and further the fluorescent activities. In contrast, the latter approach can more easily yield CDs through the pyrolysis, solvothermal, or wet-chemical condensation of small molecular organics, polymers, and other easily available biomass low-cost raw materials, which cater toward the cost expectation for practical applications. Up to now, various natural biomass precursors, such as fruit juices,^28–30 vegetables,^31,32 grass,³³ milk,³⁴ beer,³⁵ and even waste³⁶ have been successfully employed to fabricate fluorescent CDs. Fish scale is a commonly discarded biomass waste containing rich proteins, which is considered to be a potential precursor for the cost-effective preparation of CDs in a safe manner. The further utilization of the afforded fluorescent CDs as fluorescent nanoprobes for Fe³⁺ detection deserves to be investigated.In this work, the easily available fish scales of the crucian carp was employed as a precursor for the hydrothermal synthesis of CDs. The afforded water-soluble CDs exhibit strong bluish fluorescence with a quantum yield (QY) of 6.9%, high resistances against photobleaching, high ion strength, and low cytotoxicity. The fluorescence of CDs can be effectively quenched by Fe³⁺ ions via static quenching, which facilitates its application as fluorescent Fe³⁺ nanoprobes with a wide linear detection range and low detection limit. Such CD-based fluorescent Fe³⁺ nanoprobes can be further used in the determination of real water samples and human serum with high reproducibility, exhibiting their potential as reliable fluorescent nanoprobes for environmental and clinical Fe³⁺ detection.     

Sulfhydryl functionalized carbon quantum dots as a turn-off fluorescent probe for sensitive detection of Hg2+

Mercury ion (Hg²⁺) is one of the most toxic heavy metal ions and lowering the detection limit of Hg²⁺ is always a challenge in analytical chemistry and environmental analysis. In this work, sulfhydryl functionalized carbon quantum dots (HS-CQDs) were synthesized through a one-pot hydrothermal method. The obtained HS-CQDs were able to detect mercury ions Hg²⁺ rapidly and sensitively through fluorescence quenching, which may be ascribed to the formation of nonfluorescent ground-state complexes and electron transfer reaction between HS-CQDs and Hg²⁺. A modification of the HS-CQD surface by –SH was confirmed using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The HS-CQDs sensing system obtained a good linear relationship over a Hg²⁺ concentration ranging from 0.45 μM to 2.1 μM with a detection limit of 12 nM. Delightfully, the sensor has been successfully used to detect Hg²⁺ in real samples with satisfactory results. This means that the sensor has the potential to be used for testing actual samples.

Schematic presentation of the synthesis of HS-CQDs and the application as a “turn-off” fluorescent probe for Hg²⁺ detection.