Solvothermal synthesis of phosphorus and nitrogen doped carbon quantum dots as a fluorescent probe for iron(III)

Novel carbon dots from phenylenediamine for simultaneous detection of peroxydisulfate and phosphate with a smart phone by dual-channel of fluorometry and colorimetry

Carbon dots (CDs), one type of zero-dimensional carbon nanomaterial, showed extensive application in food analysis. Herein, CDs as fluorometry and colorimetry probes were developed to determine peroxydisulfate (PDS) and phosphate ion (Pi) in food samples. CDs were developed with one-pot hydrothermal process from 5-amino salicylic acid and o/m-phenylenediamine named o/m-CDs. o-CDs and m-CDs showed bright green fluorescence with quantum yield at 5.73 % and 6.40 %, which was quenched by PDS and Pi. Fluorometry was based on fluorescence quenching with LOD at 1.6 nM (PDS) and 5.2 nM (Pi). The colorimetry was based on color change of CDs from colorless to brown and indigo blue with LOD at 2.4 (PDS) and 21.1 μM (Pi). Interestingly, for both channels there was no interfering of each other. For portable detection, a wechat mini program of smart phone was employed to calculate the color change. Furthermore, the systems were potential for application in food safety analysis.

Quantum dot-based non-volatile memory: a comprehensive outlook

With the rise of digital technology, the use of memory devices is swiftly expanding, and this trend is expected to continue in the forthcoming years. Accordingly, researchers are exploring materials that surpass the performance of those used in traditional memory devices, and notably, there is a considerable interest in quantum dots (QDs). This is primarily due to the fact that quantum dots possess exceptional optical and electric properties. As a result, they have become appealing materials to enhance the performance of non-volatile memory devices. In this review, we outlined the various approaches employed for the synthesis of quantum dots as well as different types of quantum dots used for prototyping different non-volatile memory technologies and their current perspective. Additionally, we compared various key parameters, such as the ON/OFF ratio, retention time, memory window, charge trapping capacity, and multiple voltage levels, of these QD-based memories together with future outlook.

High acid-base tolerance and long storage time lanthanum cerium co-doped carbon quantum dots for Fe3+ detection

In this paper, lanthanum and cerium co-doped carbon quantum dots (LaCe-CQDs) was firstly synthesized by one step hydrothermal method. The obtained LaCe-CQDs shown sable fluorescence properties with pH values from 3 to 9 and after 4 weeks of storage. The average particle size of LaCe-CQDs, with excitation and emission wavelengths of 350 nm and 446 nm, is 3.27 ± 0.12 nm. Selective analysis of various metal ions revealed the sensitivity of LaCe-CQDs towards Fe3+ ions. Within the 0-60 μM, the fluorescence intensity exhibits a strong linear correlation with the concentration of Fe3+. The limit of detection (LOD) was determined to be 0.753 μM. Additionally, the accuracy of LaCe-CQDs were demonstrated in natural water samples. Therefore, LaCe-CQDs are a promising sensor for Fe3+ detection.

Carboxymethylcellulose-based aggregation-induced emission antibacterial material for multifunctional applications

Polysaccharides are ubiquitous in nature, typically harmless, and highly compatible with various tissues in biomedical contexts. These properties make them attractive for use in multifunctional materials. In this study, the aggregation-induced emission (AIE) antibacterial material (PLOCMC) was successfully synthesized by carboxymethylcellulose (CMC) and ε-Poly-Lysine (ε-PL). PLOCMC exhibits not only the AIE property but also a room temperature phosphorescent (RTP) phenomenon. This dual emission behavior enhances its potential applications in chemical sensing and anti-counterfeiting. Notably, PLOCMC shows low cytotoxicity and exhibits antibacterial activity against typical Gram-positive and Gram-negative bacteria, making it a potent agent against a variety of bacterial strains. Additionally, PLOCMC demonstrates specific responsiveness to Fe3+ ions and nitrite, indicating its potential utility in food safety and monitoring applications.

Highly-fluorescent extracts from Pterocarpus wood for Fe3+ ion detection

At present, excessive Fe3+ in daily water has become a threat to human health. Among the conventional detection methods for Fe3+, fluorescent probes have been applied on a large scale due to their simplicity and efficiency. However, the currently available fluorescent probes are difficult to synthesize, costly and environmentally unfriendly, limiting their applications. In this work, a fluorescent extract of Pterocarpus wood was successfully obtained, and the structure of some coumarin-based molecules in this extract was determined by 2D-NMR. Subsequently, the intensity of this fluorescence was optimized using response surface methodology (RSM), resulting in a high-intensity fluorescent probe. The probe was sensitive to the concentrations of Fe3+ and MnO4-, and could efficiently detects Fe3+ in the range of 2.7 μM-8.0 μM, with LOD and LOQ reaching 1.06 μM and 3.20 μM, respectively. Moreover, based on the strong complexation property of EDTA on Fe3+, this work designed the "switch-on" fluorescent probes. The experiment shows that both static and dynamic quenching exist in this system. The mechanism of complexation and oxidation of fluorescent molecules by the quencher is interpreted in the quenching reaction. In addition, the fluorescent probe has a high yield and low cost, it also performs well in actual water sample tests. This method is expected to be developed as a new way on Fe3+ detection.

Alfalfa biomass as a green source for the synthesis of N,S-CDs via microwave treatment. Application as a nano sensor for nifuroxazide in formulations and gastric juice

BACKGROUND

Researchers have investigated different techniques for synthesis of carbon dots. These techniques include Arc discharge, laser ablation, oxidation, water/solvothermal, and chemical vapor deposition. However, these techniques suffer from some limitations like the utilization of gaseous charged particles, high current, high temperature, potent oxidizing agents, non-environmentally friendly carbon sources, and the generation of uneven particle size. Therefore, there was a significant demand for the adoption of a new technology that combines the environmentally friendly aspects of both bio-based carbon sourcing and synthesis technique.

RESULTS

Medicago sativa L (alfalfa)-derived N, S-CDs have been successfully synthesized via microwave irradiation. The N,S-CDs exhibit strong fluorescence (λex/em of 320/420 nm) with fluorescence quantum yield of 2.2 % and high-water solubility. The produced N,S-CDs were characterized using TEM, EDX, Zeta potential analysis, IR, UV-Visible, and fluorescence spectroscopy. The average diameter of the produced N, S-CDs was 4.01 ± 1.2 nm, and the Zeta potential was -24.5 ± 6.63 mv. The stability of the produced nano sensors was also confirmed over wide pH range, long time, and in presence of different ions. The synthesized N, S-CDs were employed to quantify the antibacterial drug, nifuroxazide (NFZ), by fluorescence quenching via inner filter effect mechanism. The method was linear with NFZ concentration ranging from 1.0 to 30.0 μM. LOD and LOQ were 0.16 and 0.49 μM, respectively. The method was applied to quantify NFZ in simulated gastric juice (SGJ) with % recovery 99.59 ± 1.4 in addition to pharmaceutical dosage forms with % recovery 98.75 ± 0.61 for Antinal Capsules® and 100.63 ± 1.54 for Antinal suspension®. The Method validation was performed in compliance with the criteria outlined by ICH.

SIGNIFICANCE AND NOVELTY

The suggested approach primarily centers on the first-time use of alfalfa, an ecologically sustainable source of dopped-CDs, and a cost-effective synthesis technique via microwave irradiation, which is characterized by low energy consumption, minimized reaction time, and the ability to control the size of the produced CDs. This is in line with the growing global recognition of the implementation of green analytical chemistry principles.

Carbon Quantum Dots: Properties, Preparation, and Applications

Carbon quantum dots are a novel form of carbon material. They offer numerous benefits including particle size adjustability, light resistance, ease of functionalization, low toxicity, excellent biocompatibility, and high-water solubility, as well as their easy accessibility of raw materials. Carbon quantum dots have been widely used in various fields. The preparation methods employed are predominantly top-down methods such as arc discharge, laser ablation, electrochemical and chemical oxidation, as well as bottom-up methods such as templates, microwave, and hydrothermal techniques. This article provides an overview of the properties, preparation methods, raw materials for preparation, and the heteroatom doping of carbon quantum dots, and it summarizes the applications in related fields, such as optoelectronics, bioimaging, drug delivery, cancer therapy, sensors, and environmental remediation. Finally, currently encountered issues of carbon quantum dots are presented. The latest research progress in synthesis and application, as well as the challenges outlined in this review, can help and encourage future research on carbon quantum dots.

Fe-doping green fluorescent carbon dots via co-electrolysis of chrysoidine G and potassium ferrocyanide for sensitive Cr(VI) detection

In this study, we aimed to synthesis of Fe-doping green fluorescent carbon dots (G-CDs) through the co-electrolysis of chrysoidine G and potassium ferrocyanide for Cr(VI) detection. The use of potassium ferrocyanide improves the quantum yield and sensing performance of G-CDs toward Cr(VI). The G-CDs have a maximum excitation wavelength of 308 nm and an emission wavelength of 510 nm. Comprehensive analyses including Raman, FT-IR, and XPS provided insights into the chemical structure and composition of the G-CDs. Under optimal conditions, G-CDs demonstrated concentration-dependent quenching upon interaction with Cr(VI). A linear relationship within the range of 0.25-100 µM was established with a calibration equation of ΔF/F0 = 0.005 + 0.015CCr(VI), yielding an R2 value of 0.996 and a limit of detection of 0.15 μM. The applicability of the G-CDs method was demonstrated by successful Cr(VI) detection in water samples with recovery rates ranging from 98.8 % to 100.1 % and relative standard deviation within 3.0 %. The fluorescence lifetime and Zeta potential measurements confirmed that the mechanism was via a static quenching process, while redox reaction, nanoparticle aggregation, and surface charge variation also played significant roles.

Dual Applications of Cobalt-Oxide-Grafted Carbon Quantum Dot Nanocomposite for Two Electrode Asymmetric Supercapacitors and Photocatalytic Behavior

Carbon materials, such as graphene, carbon nanotubes, and quantum-dot-doped metal oxides, are highly attractive for energy storage and environmental applications. This is due to their large surface area and efficient optical and electrochemical activity. In this particular study, a composite material of cobalt oxide and carbon quantum dots (Co3O4-CQD) was prepared using cobalt nitrate and ascorbic acid (carbon source) through a simple one-pot hydrothermal method. The properties of the composite material, including the functional groups, composition, surface area, and surface morphology, were evaluated by using various methods such as ultraviolet, Fourier transform infrared, X-ray diffraction, Raman, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller, scanning electron microscopy, and transmission electron microscopy analysis. The electrochemical performance of the Co3O4-CQD composite has been studied using a three-electrode system. The results show that at 1 A g-1, the composite delivers a higher capacitance of 1209 F g-1. The asymmetric supercapacitor (Co3O4-CQD//AC) provided 13.88 W h kg-1 energy and 684.65 W kg-1 power density with a 96% capacitance retention. The Co3O4-CQD composite also demonstrated excellent photocatalytic activity (90% in 60 min) for the degradation of methylene blue dye under UV irradiation, which is higher than that of pristine Co3O4 and CQD. This demonstrates that the Co3O4-CQD composite is a promising material for commercial energy storage and environmental applications.

Facile preparation, optical mechanism elaboration, and bio-imaging application of fluorescent cellulose nanocrystals with tunable emission wavelength

Interfacing cellulose nanocrystals (CNCs) with fluorescent materials provides more possibilities for constructing of sensory/imaging platforms in biomedical applications. In this work, by harnessing the efficient extraction accompanied modification of CNCs and adjustable optical properties of carbon dots (CDs), we report the constructions and emission wavelength tuning of fluorescent CNCs (F-CNCs) composed of CNC nano-scaffolds and CDs. The as-prepared CNCs are densely decorated with citric acid (CA), which plays the role of carbon source for the in-situ synthesis of CDs on CNCs. For the F-CNCs carrying blue, green, and red emissive CDs, ethylenediamine (EDA), urea, and thiourea are the N or N/S sources. Fingerprints of chemical groups, morphological characters, and redox activities are resolved to elaborate the optical mechanisms of CDs with varying emission colors. The emission wavelength is adjusted by either changing the particle size or introducing new emission centers. Both are primarily achieved via precursor engineering. The F-CNCs reveal quantum yields (QYs) >22 % and negligible fluorescence quenching (< 6 %) upon continuous excitation as long as 24 h. Benefited from their cell membrane penetration capability, the F-CNCs with different emission wavelengths were challenged for multiplexed cytoplasm imaging.

A red fluorescent carbon dots with good water solubility for rapid detection of Al3+ in actual samples

We developed a facile strategy for the fabrication of red fluorescent carbon nanodots (R-CDs) and demonstrated their applications for Al3+ sensing. Red-emission carbon dots (CDs) were synthesized using a simple hydrothermal treatment with citric acid and urea as precursors, manifesting intriguing red-emission behaviour at 610 nm. With increasing Al3+ concentration, the fluorescence band at 610 nm decreased gradually. Monitoring the intrinsic fluorescence variation (I610nm ), as-prepared CDs were developed as an effective platform for fluorescent Al3+ sensing, with a linear range of 0.5-60.0 μM and a detection limit of 3.0 nM. More importantly, R-CDs have been applied successfully to the analysis of Al3+ in actual samples with satisfactory recoveries in the range 97.12-102.05%, which indicated that obtained CDs could be implemented as an effective tool for the identification and detection of Al3+ in actual samples.

A PEDOT enhanced covalent organic framework (COF) fluorescent probe for in vivo detection and imaging of Fe3

Simple and accurate in vivo monitoring of Fe3+ is essential for gaining a better understanding of its role in physiological and pathological processes. A novel fluorescent probe was synthesized via in situ solid-state polymerization of 3,4-ethylenedioxythiophene (PEDOT) in the pore channels of a covalent organic framework (COF). The PEDOT@COF fluorescent probe exhibited an absolute quantum yield (QY) 3 times higher than COF. In the presence of Fe3+ the PEDOT@COF 475 nm fluorescence emission, 365 nm excitation, is quenched within 180 s. Fluorescence quenching is linear with Fe3+ in the concentration range of 0-960 μM, with a detection limit of 0.82 μM. The fluorescence quenching mechanism was attributed to inner filter effect (IEF), photoinduced electron transfer (PET) and static quenching (SQE) between PEDOT@COF and Fe3+. A paper strip-based detector was designed to facilitate practical applicability, and the PEDOT@COF probe successfully applied to fluorescence imaging of Fe3+ levels in vivo. This work details a tool of great promise for enabling detailed investigations into the role of Fe3+ in physiological and pathological diseases.

Nitrogen and sulfur co-doped carbon quantum dots fluorescence quenching assay for detection of mercury (II)

Mercury is a highly toxic and potentially bioaccumulative heavy metal ion that can cause severe health problems in humans even at very low concentrations. Thus, the development of a simple, rapid, and sensitive assay for the effective detection of mercury ions at trace levels is of great importance. Here, nitrogen and sulfur co-doped carbon quantum dots (N,S-CQD) were synthesized by a simple hydrothermal treatment of chitosan in the presence of thiourea and citric acid with a quantum yield (QY) up to 33.0 % and used as a selective fluorescent probe to detect mercury ions (Hg²⁺). The effect of pH, ionic strength, and time on the fluorescence intensity of N,S-CQD were investigated and optimized. The synthesized N,S-CQD showed ultrasensitive ability to detect Hg²⁺ ions in the water samples, also in the presence of 11 interfering metal ions, with a low detection limit (∼4 nM) over a wide linear range from ∼5-160 nM. The sensing performance of N,S-CQD probe in real sample applications was evaluated by the detection of Hg²⁺ in lake water samples, which confirmed its potential application in environmental analysis.

Nitrogen-doped carbon dots: Recent developments in its fluorescent sensor applications

Doped carbon dots have attracted great attention from researchers across disciplines because of their unique characteristics, such as their low toxicity, physiochemical stability, photostability, and outstanding biocompatibility. Nitrogen is one of the most commonly used elements for doping because of its sizeable atomic radius, strong electronegativity, abundance, and availability of electrons. This distinguishes them from other atoms and allows them to perform distinctive roles in various applications. Here, we have reviewed the most current breakthroughs in nitrogen-doped CDs (N-CDs) for fluorescent sensor applications in the last five years. The first section of the article addresses several synthetic and sustainable ways of making N-CDs. Next, we briefly reviewed the fluorescent features of N-CDs and their sensing mechanism. Furthermore, we have thoroughly reviewed their fluorescent sensor applications as sensors for cations, anions, small molecules, enzymes, antibiotics, pathogens, explosives, and pesticides. Finally, we have discussed the N-CDs' potential future as primary research and how that may be used. We hope that this study will contribute to a better understanding of the principles of N-CDs and the sensory applications that they can serve.

Review on Carbon Dot-Based Fluorescent Detection of Biothiols

Biothiols, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play a vital role in gene expression, maintaining redox homeostasis, reducing damages caused by free radicals/toxins, etc. Likewise, abnormal levels of biothiols can lead to severe diseases, such as Alzheimer's disease (AD), neurotoxicity, hair depigmentation, liver/skin damage, etc. To quantify the biothiols in a biological system, numerous low-toxic probes, such as fluorescent quantum dots, emissive organic probes, composited nanomaterials, etc., have been reported with real-time applications. Among these fluorescent probes, carbon-dots (CDs) have become attractive for biothiols quantification because of advantages of easy synthesis, nano-size, crystalline properties, low-toxicity, and real-time applicability. A CDs-based biothiols assay can be achieved by fluorescent "Turn-On" and "Turn-Off" responses via direct binding, metal complex-mediated detection, composite enhanced interaction, reaction-based reports, and so forth. To date, the availability of a review focused on fluorescent CDs-based biothiols detection with information on recent trends, mechanistic aspects, linear ranges, LODs, and real applications is lacking, which allows us to deliver this comprehensive review. This review delivers valuable information on reported carbon-dots-based biothiols assays, the underlying mechanism, their applications, probe/CDs selection, sensory requirement, merits, limitations, and future scopes.

Fluorescence Turns on-off-on Sensing of Ferric Ion and L-Ascorbic Acid by Carbon Quantum Dots

This study used a hydrothermal approach to create a sensitive and focused nanoprobe. Using an “on-off-on” sensing mechanism, the nanoprobe was employed to detect and quantify ferric ions and L-ascorbic acid. Synthesis of the carbon quantum dots was achieved with a single hydrothermal step at 180°C for 24 hours using hot pepper as the starting material. The prepared CQDs showed high fluorescence with a quantum yield of 30% when excited at 350 nm, exhibiting excitation-dependent fluorescence. The emission of the CQDs can be quenched by adding ferric ions, which can be attributed to complex formation leading to nonradiative photoinduced electron transfer (PET). Adding L-ascorbic acid, which can convert ferric ions into ferrous ions, break the complex, and restore the fluorescence of CQD. The linear range and LOD were (10–90) μM and 1 μM for ferric ions, respectively, and L-ascorbic acid’s linear range was (5–100) μM while LOD was 0.1 μM quantification of both substances was accomplished. In addition, orange fruit was used as an actual sample source for ascorbic acid analysis, yielding up to 99% recovery.

Carbon Nanodots as Electron Transport Materials in Organic Light Emitting Diodes and Solar Cells

Charge injection and transport interlayers play a crucial role in many classes of optoelectronics, including organic and perovskite ones. Here, we demonstrate the beneficial role of carbon nanodots, both pristine and nitrogen-functionalized, as electron transport materials in organic light emitting diodes (OLEDs) and organic solar cells (OSCs). Pristine (referred to as C-dots) and nitrogen-functionalized (referred to as NC-dots) carbon dots are systematically studied regarding their properties by using cyclic voltammetry, Fourier-transform infrared (FTIR) and UV-Vis absorption spectroscopy in order to reveal their energetic alignment and possible interaction with the organic semiconductor's emissive layer. Atomic force microscopy unravels the ultra-thin nature of the interlayers. They are next applied as interlayers between an Al metal cathode and a conventional green-yellow copolymer-in particular, (poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1',3}-thiadiazole)], F8BT)-used as an emissive layer in fluorescent OLEDs. Electrical measurements indicate that both the C-dot- and NC-dot-based OLED devices present significant improvements in their current and luminescent characteristics, mainly due to a decrease in electron injection barrier. Both C-dots and NC-dots are also used as cathode interfacial layers in OSCs with an inverted architecture. An increase of nearly 10% in power conversion efficiency (PCE) for the devices using the C-dots and NC-dots compared to the reference one is achieved. The application of low-cost solution-processed materials in OLEDs and OSCs may contribute to their wide implementation in large-area applications.

Photocatalytic materials modified with carbon quantum dots for the degradation of organic pollutants under visible light: A review

In recent years, carbon quantum dots (CQDs) have received widespread attention owing to their non-toxicity, sustainability, excellent photostability, and intrinsic photoluminescence properties. In particular, CQDs have attracted considerable interest for visible-light-driven photocatalysis because of their excellent electron transfer characteristics and high light capture efficiency. Many studies have reported CQDs/photocatalyst composite systems constructed to make full use of the solar spectrum, improving the ability of photocatalytic materials to degrade organic pollutants. Here, we review the recent research on CQDs-based photocatalysts, and their ability to remove environmental pollutants, with a special emphasis on degradation mechanisms. Several improvements in the catalytic response of CQDs to visible light are also included. In addition, we discuss the aspects that should be considered to construct composite materials based on CQD characteristics and the potential applications of CQD-based photocatalysts for efficient utilization of visible light.

Synthesis and application of novel N, Si-carbon dots for the ratiometric fluorescent monitoring of the antibiotic balofloxacin in tablets and serum

A ratiometric fluorescent probe with blue-emission fluorescence based on N, Si-doped carbon dots (N, Si-CDs) for the detection of balofloxacin (BLFX) was synthesized by simple one-pot hydrothermal carbonization using methotrexate and 3-aminopropyltriethoxysilane (APTES) as carbon materials. The obtained N, Si-CDs showed dual-emission band fluorescence characterization at 374 nm and 466 nm. Furthermore, the synthesized N, Si-CD probe exhibited evidence of ratiometric fluorescence emission characteristics (F 466/F 374) toward BLFX along with a decrease in fluorescence intensity at 374 nm and an increase in fluorescence intensity at 466 nm. Based on this probe, a highly sensitive and fast detection method for the analysis of BLFX has been established with a linear range of 1-60 μM and a low detection limit of 0.1874 μM, as well as a rapid response time of 5.0 s. The developed assay has also been successfully applied for the detection of BLFX in tablets and rat serum.

Photoswitchable carbon-dot liposomes mediate catalytic cascade reactions for amplified dynamic treatment of tumor cells

Most biochemical reactions that occur in living organisms are catalyzed by a series of enzymes and proceed in a tightly controlled manner. The development of artificial enzyme cascades that resemble multienzyme complexes in nature is of current interest due to their potential in various applications. In this study, a nanozyme based on photoswitchable carbon-dot liposomes (CDsomes) was developed for use in programmable catalytic cascade reactions. These CDsomes prepared from triolein are amphiphilic and self-assemble into liposome-like structures in an aqueous environment. CDsomes feature excitation-dependent photoluminescence and, notably, can undergo reversible switching between a fluorescent on-state and nonfluorescent off-state under different wavelengths of light irradiation. This switching ability enables the CDsomes to exert photocatalytic oxidase- and peroxidase-like activities in their on- (bright) and off- (dark) states, respectively, resulting in the conversion of oxygen molecules into hydrogen peroxide (H₂O₂), followed by the generation of active hydroxyl radicals (OH). The two steps of oxygen activation can be precisely controlled in a sequential manner by photoirradiation at different wavelengths. Catalytic reversibility also enables the CDsomes to produce sufficient reactive oxygen species (ROS) to effectively kill tumor cells. Our results reveal that CDsomes is a promising photo-cycling nanozyme for precise tumor phototherapy through regulated programmable cascade reactions.

Carbon dots-based nanomaterials for fluorescent sensing of toxic elements in environmental samples: Strategies for enhanced performance

Rapid industrialization and manufacturing expansion have caused heavy metal pollution, which is a critical environmental issue faced by global population. In addition, the disadvantages presented by conventional detection methods such as the requirement of sophisticated instruments and qualified personnel have led to the development of novel nanosensors. Recently, carbon dots (CDs) have been presented as a multifunctional nanomaterial alternative for the accurate detection of heavy metal ions in water systems. The capacity of CDs to detect contaminants in wastewater -including heavy metals- can be found in the literature; however, to the best of our knowledge, none of them discusses the most recent strategies to enhance their performance. Therefore, in this review, beyond presenting successful examples of the use of CDs for the detection of metal ions, we further discuss the strategies to enhance their photoluminescence properties and their performance for environmental monitoring. In this manner, strategies such as heteroatom-doping and surface passivation are reviewed in detail, as well as describing the mechanisms and the effect of precursors and synthesis methods. Finally, the current challenges are described in detail to propose some recommendations for further research.

Maize starch derived boron doped carbon spheres via facile solvothermal route as the photoluminescence sensor for determination of pH and Cr(VI)

In this work, a kind of boron doped carbon spheres (B-CSs) was successfully synthesized utilizing maize starch as carbon source and boric acid as dopant via facile solvothermal method. The chemical structure of the prepared B-CSs was systemically investigated by TEM, FT-IR, XRD, XPS and EDS. The synthesized B-CSs feature spherical structure with average size of ∼254 nm and exhibit strong photoluminescence (PL) with maximum emission at a wavelength of ∼453 nm under irradiation at 350 nm, leading to a quantum yield of 6.2%. Furthermore, the aqueous pH and Cr(VI) has a significantly various impact on the PL intensity of B-CSs, which can be flexibly utilized as the PL sensor for detection aqueous pH and Cr(VI) in aqueous. Particularly, the B-CSs have a desirable sensitivity and selectivity for detection of Cr(VI) with a low detection limit of ∼0.34μmol l^-1. Conclusively, our work provides a novel and dual-functional fluorescent sensor for detection of the pH and toxic metal ions in water environment.

Application of a luminous intensity variation fluorescent probe for the detection of ferric ions

A luminous intensity variation fluorescent probe (Probe 1) for the detection of ferric ion was developed. The quantitative range of Fe³⁺ content detected was 0 to 600 μM with the LOD at 0.76 μM. Further, after 20 minutes of Fe³⁺ addition, the intensity of luminescence of Probe 1 solution gradually decreased with increased Fe³⁺ concentration. In addition, the B and G values of these images showed a linear relationship with Fe³⁺ concentration (0-500 μM). Probe 1 was successfully used for the rapid determination of Fe³⁺ concentration in real samples. This study demonstrates that Probe 1 is an excellent tool for the rapid determination of Fe³⁺ content in real samples using a smart phone without professional equipment.

Chitosan and κ-carrageenan-derived nitrogen and sulfur co-doped carbon dots "on-off-on" fluorescent probe for sequential detection of Fe3+ and ascorbic acid

This study develops a high sensitive and selective "on-off-on" fluorescent probe for sequential detection of iron ion (Fe³⁺) and ascorbic acid (AA) based on nitrogen and sulfur co-doped carbon dots (N, S-CDs), which were synthesized by using chitosan and κ-carrageenan as raw materials through one-step hydrothermal protocol. The synthesized N,S-CDs possess particularly high quantum yield (QY = 59.31%), excellent stability and excitation dependent behavior, showing great potential for practical applications. Furthermore, N,S-CDs provided high selectivity and strong anti-interference to Fe³⁺ due to its fluorescence quenching performance, revealing a wide linear concentration range from 1 to 100 μM for the detection of Fe³⁺ ion with an extremely low limit of detection of 57 nM, and presented reliable and accurate results in actual sample detection of Fe³⁺. The overall fluorescence quenching mechanism of N,S-CDs with Fe³⁺ was due to the formation of N,S-CDs/Fe³⁺ initiated to the aggregation and electron transfer of N,S-CDs, resulting in the static quenching of fluorescence. More interestingly, AA could reduce Fe³⁺ to Fe²⁺ and efficaciously recover the quenched fluorescence of N,S-CDs/Fe³⁺. N,S-CDs/Fe³⁺ as "turn-on" fluorescent probe was further applied for detecting AA in a linear range of 0.5-90 μM with a detection limit of 38 nM.

Microbial inhibition and biosensing with multifunctional carbon dots: Progress and perspectives

Carbon dots (CDs) and their doped counterparts including nitrogen-doped CDs (N@CDs) have been synthesized by bottom-up or top-down approaches from different precursors. The attractiveness of such emerging 2D‑carbon-based nanosized materials is attributed to their excellent biocompatibility, preparation, aqueous dispersibility, and functionality. The antimicrobial, optical, and electrochemical properties of CDs have been advocated for two important biotechnological applications: bacterial eradication and sensing/biosensing. CDs as well as N@CDs act as antimicrobial agents as their surfaces encompass functional hydroxyl, carboxyl, and amino groups that generate free radicals. As a new class of photoluminescent nanomaterials, CDs can be employed in diversified analytics. CDs with surface carboxyl or amino groups form nanocomposites with nanomaterials or be conjugated with biorecognition molecules toward the development of sensors/biosensors. The deployment of conductive CDs in electrochemical sensing has also increased significantly because of their quantum size, excellent biocompatibility, enzyme-mimicking activity, and high surface area. The review also addresses the ongoing challenges and promises of CDs in pathogenesis and analytics. Perspectives on the future possibilities include the use of CDs in microbial viability assay, wound healing, antiviral therapy, and medical devices.

Simple and Sensitive Multi-components Detection Using Synthetic Nitrogen-doped Carbon Dots Based on Soluble Starch

Although carbon dots (CDs) as fluorescent sensors have been widely exploited, multi-component detection using CDs without tedious surface modification is always a challenging task. Here, two kinds of nitrogen-doped CDs (NCD-m and NCD-o) based on soluble starch (SS) as carbon source were prepared through one-pot hydrothermal process using m-phenylenediamine and o-phenylenediamine as nitrogenous dopant respectively. Through fluorescence "on-off" mechanism of CDs, NCD-m and NCD-o could be used as a fluorescence sensor for detection of Fe 3+ and Ag + with LOD of 0.25 and 0.51 μM, respectively. Additionally, NCD-m could be used for indirect detection of ascorbic acid (AA) with LOD of 5.02 μM. Moreover, fluorescence intensity of NCD-m also exhibited the sensitivity to pH change from 2 to 13. More importantly, Both NCD-m and NCD-o had potential application for analysis of complicated real samples such as tap water, Vitamin C tablets and orange juice. Ultimately, the small size of NCD-m could contribute to reinforcing intracellular endocytosis, which allowed them to be used for bacteria imaging. Obviously, these easily obtainable nitrogen-doped CDs were able to be used for multi-components detection. Strategy for synthesis of nitrogen-doped carbon dots (NCDs) and a schematic for fabrication of as-prepared NCDs for detection of Fe 3+, Ag + and ascorbic acid (AA).

Synthesis of Fluorescent Nitrogen and Phosphorous Co-doped Carbon Quantum Dots for Sensing of Iron, Cell Imaging and Antioxidant Activities

Carbon quantum dots (CQD) as the result of their exceptional physical and chemical properties show tremendous potential in various field of applications like cell imaging and doping of CQDs with elements like nitrogen and phosphorous increase its fluorescence property. Herein, we have synthesized fluorescent nitrogen and phosphorous codoped carbon quantum dots (NPCQDs) via a one-pot hydrothermal method. Sesame oil, L-Aspartic acid, and phosphoric acid were used as carbon, nitrogen, and phosphorous sources, respectively. UV-Vis spectrophotometer, fluorescence spectrometer, Fourier transform infrared spectrometer (FTIR), X-ray diffraction spectrometer (XRD), field emission scanning microscopy (FESEM), and transmission electron microscopy (TEM) were employed to characterize the synthesized fluorescent NPCQDs. The as-synthesized NPCQDs with a particle size of 4.7 nm possess excellent water solubility, high fluorescence with high quantum yield (46%), high ionic stability, and resistance to photobleaching. MTT assay indicated the biocompatibility of NPCQDs and it was used for multicolor live-cell imaging. Besides, the NPCQDs show an effective probe of iron ions (Fe3+) in an aqueous solution with a high degree of sensitivity and selectivity. The DPPH assay showed its good antioxidant activity.

Fabrication of fluorescent hybrid nanomaterials based on carbon dots and its applications for improving the selective detection of Fe (III) in different matrices and cellular imaging

Considering that detection on cations or ions still meets some challenges in achieving the effectivity and selectivity just by employing one platform, the ingenious fabrication of nanomaterials exhibits an increasing research interests for the preponderance in improving or integrating the performance of single platform. Herein, a fluorescent hybrid nanomaterials based on an organic dye 4-methylumbelliferone (4-MU) as modifier and D-arginine as carbon cores has been developed via a facile one-step hydrothermal synthesis, forming carbon dots (CDs)/4-MU hybrid nanomaterials (CDs-4-MU). This kind of nanomaterials can improve the sensitive and selective detection of single CDs towards Fe³⁺ ions in different matrices. The detection mechanism of CDs-4-MU towards Fe³⁺ can be attributed to an electron transfer process between CDs-4-MU and Fe³⁺, leading to the fluorescence quenching. The limit of detection (LOD) and corresponding linear range in tris-HCl buffer solution are 0.68 μM and 2.29-200 μM, respectively. Furthermore, this nanomaterial can also achieve a detection of Fe³⁺ ions in real samples such as tap water, culture medium and fetal bovine serum. In particular, CDs-4-MU exhibits a good biocompatibility and can be uptaken by MC3T3 cells, thus can be applied for Fe³⁺ ions detection in cellular level and cellular imaging. Therefore, this work provides a versatile strategy for the synthesis of CDs-based hybrid nanomaterials and opens a new pathway for improving the ion detection in real samples, which is of significance in practical applications.

Microwave-assisted conversion of palm kernel shell biomass waste to photoluminescent carbon dots

In the present work, palm kernel shell (PKS) biomass waste has been used as a low-cost and easily available precursor to prepare carbon dots (CDs) via microwave irradiation method. The impacts of the reacting medium: water and diethylene glycol (DEG), and irradiation period, as well as the presence of chitosan on the CDs properties, have been investigated. The synthesized CDs were characterized by several physical and optical analyses. The performance of the CDs in terms of bacteria cell imaging and copper (II) ions sensing and removal were also explored. All the CDs possessed a size of 6-7 nm in diameter and the presence of hydroxyl and alkene functional groups indicated the successful transformation of PKS into CDs with carbon core consisting of C = C elementary unit. The highest quantum yield (44.0%) obtained was from the CDs synthesised with DEG as the reacting medium at irradiation period of 1 min. It was postulated that the high boiling point of DEG resulted in a complete carbonisation of PKS into CDs. Subsequently, the absorbance intensity and photoluminescence intensity were also much higher compared to other precursor formulation. All the CDs fluoresced in the bacteria culture, and fluorescence quenching occurred in the presence of heavy metal ions. These showed the potential of CDs synthesised from PKS could be used for cellular imaging and detection as well as removal of heavy metal ions.

Phosphorus-Doped Carbon Quantum Dots as Fluorometric Probes for Iron Detection

Carbon quantum dots (CQDs), a novel fluorescent nanomaterial, have been extensively employed/explored in various applications, that is, biosensors, bioimaging, nanomedicine, therapeutics, photocatalysis, electrocatalysis, energy storage system, and so forth. In this study, we report the synthesis, characterization, and the application of phosphorus-doped CQDs (PCQDs), synthesized using trisodium citrate and phosphoric acid by the hydrothermal method. The effect of phosphorus doping on optical features and the formation of PCQDs have been explored elaborately by controlling the concentrations of precursors, reaction time, and the temperature. The fluorescent quantum yield for PCQDs was determined to be 16.1% at an excitation/emission wavelength of 310/440 nm. Also, the optical and structural properties of PCQDs were determined by using various spectroscopic and microscopic techniques. Static quenching of fluorescence was determined upon the addition of Fe3+ to PCQDs because of the formation of the fluorescent inactive complex (PCQDs-Fe3+). Hence, this chemistry leads to the development of a new fluorometric assay for the detection of Fe3+. The lower limit of Fe3+ detection is determined to be 9.5 nM (3σ/slope), with the linear fit from 20 nM to 3.0 μM (R 2 = 0.99). We have validated this new assay in the raw, ejected, and purified water samples of the RO plant by the standard addition method. These results suggest the possibility of developing a new commercial assay for Fe3+ detection in blood, urine, and various industrial waste and sewage water samples. Furthermore, recycling the pollutant water into the freshwater using filters that consist of PCQDs offers a great deal.

Dual-Emitting N/S-Doped Carbon Dots-Based Ratiometric Fluorescent and Light Scattering Sensor for High Precision Detection of Fe(III) Ions

Precise and rapid sensing of Fe(III) under concerned facile conditions is important in environmental monitoring. Herein, a facile and label-free ratiometric sensor is constructed for selective determination of Fe(III) ions by coupling second-order scattering (SOS) and fluorescence. We were synthesized fluorescent N, S-doped carbon dots (N/S-CDs) via facile one-step hydrothermal treatment with an intensive fluorescence and a weak SOS signal and high quantum yield (32%). The fluorescence of N/S-CDs was quenched whereas the intensity of SOS was relatively increased by Fe(III) ions due to aggregation-induced fluorescence quenching or enhancement. Based on this effect, a novel fluorescent ratiometric probe with the combined fluorescence and SOS is proposed for the sensing of Fe(III) ions, and with the detection limit of 83 nM and linear range of 0.1-10 μM and 10-40 μM, respectively.

One-step preparation of single-layered graphene quantum dots for the detection of Fe3

Single-layer graphene quantum dots are highly desirable while their facile and controllable preparations remain challenging. Herein, single-layered graphene quantum dots (sl-GQDs) were developed via a facile one-step hydrothermal synthesis, with citric acid and β-cyclodextrin (CD) as starting materials. The sl-GQDs decorated with CD molecules emit green fluorescence with a quantum yield of 5.34%, and exhibit a good response exclusively to ferric ions for their structural oxygenous groups. The linear range of the proposed sensor for ferric ions was found in a wide concentration range of 0-85 μM. The detection limit is about 0.26 μM. The sl-GQDs based sensing platform also demonstrates its feasibility in real water sample analysis with recoveries of 93.8%-101.5%.

High Quantum Yield Fluorescent Carbon Nanodots for detection of Fe (III) Ions and Electrochemical Study of Quenching Mechanism

Carbon nanodots (CNDs) offer potential applications in photocatalysis, optoelectronics, bio-imaging, and sensing due to their excellent photoluminescence (PL) properties, biocompatibility, aqueous solubility, and easy functionalization. Recent emphasis on CNDs in the selective detection of metal ions is due to the growing concern for human and environmental safety. In this work, two types of fluorescent carbon nanodots (CNDs) are synthesized economically from ethylene diamine (E-CNDs) or urea (U-CNDs) in a single step microwave process. The as-prepared CNDs exhibit excellent PL at an excitation wavelength of 350 nm with a quantum yield of 64% for E-CNDs and 8.4% for U-CNDs with reference to quinine sulfate. Both E-CNDs and U-CNDs demonstrate high selectivity towards Fe (III) ions among different metal ions, by fluorescence quenching in a dose dependent manner. The limit of detection of E-CNDs and U-CNDs is observed to be 18 nM and 30 nM, respectively, in the linear response range of 0-2000 μM with a short response time (seconds). The CNDs detect Fe (III) ions in tap water and serum sample with no spiking and the recovery was ~100% with the Fe (III) samples. Cellular internalization studies confirm the localization of the CNDs and the optical imaging sensing of Fe (III) ions inside living cells. A charge transfer fluorescence quenching mechanism, specifically between the CNDs and Fe (III), is proposed and examined using cyclic voltammetry. The overall characteristics of the E-CNDs provides a potential sensing platform in highly sensitive and selective detection of Fe (III) ions.

Biocompatible iron(II)-doped carbon dots as T1-weighted magnetic resonance contrast agents and fluorescence imaging probes

The one-pot synthesis of iron-doped carbon quantum dots (Fe-CQDs) for use as both magnetic resonance (MR) and fluorescent (dual-mode) imaging nanoprobes is described. Comprehensive characterizations of the material confirmed the successful doping of the CQDs with Fe(II) ions. The imaging probe has a longitudinal relaxivity of 3.92 mM^-1∙s^-1 and a low r₂/r₁ ratio of 1.27, both of which are critical for T₁-weighted contrast agents. The maximum emission of Fe-CQDs locates at 450 nm under 375 nm excitation, which also can be applied to fluorescence imaging. Biotoxicity assessment showed good biocompatibility of the Fe-CQDs. The in-vitro experiments with A549 cells indicated that the Fe-CQDs are viable candidates as dual-mode (MR/fluorescence) imaging nanoprobes. For in-vivo experiments, they exhibit high contrast efficiency, thereby improving the positive contrast in T₁-weighted MR images. In-vivo time-dependent MRI of major organs showed that the Fe-CQDs undergo fast glomerular filtration and can evade immuno-absorption due to their ultra-small size and excellent biocompatibility. Graphical abstract Schematic presentation of the synthesis of Fe-CQDs and applications to magnetic resonance and fluorescent dual-mode imaging.

Recent Advancements in Doped/Co-Doped Carbon Quantum Dots for Multi-Potential Applications

Carbon quantum dots (CQDs)/carbon nanodots are a new class of fluorescent carbon nanomaterials having an approximate size in the range of 2–10 nm. The majority of the reported review articles have discussed about the development of the CQDs (via simple and cost-effective synthesis methods) for use in bio-imaging and chemical-/biological-sensing applications. However, there is a severe lack of consolidated studies on the recently developed CQDs (especially doped/co-doped) that are utilized in different areas of application. Hence, in this review, we have extensively discussed about the recent development in doped and co-doped CQDs (using elements/heteroatoms—e.g., boron (B), fluorine (F), nitrogen (N), sulphur (S), and phosphorous (P)), along with their synthesis method, reaction conditions, and/or quantum yield (QY), and their emerging multi-potential applications including electrical/electronics (such as light emitting diode (LED) and solar cells), fluorescent ink for anti-counterfeiting, optical sensors (for detection of metal ions, drugs, and pesticides/fungicides), gene delivery, and temperature probing.

Soluble non-toxic carbon nano-rods for the selective sensing of iron(iii) and chromium(vi)

A simpler methodology has reported for the fabrication of non-toxic functionalized soluble carbon nano-rods for the sensing of Fe(iii) and Cr(vi) in aqueous media.

Photoluminescence enhancement via microwave irradiation of carbon quantum dots derived from solvothermal synthesis of l-arginine

Photoluminescence enhancement of carbon quantum dots was achieved via solvothermal synthesis followed by microwave irradiation.

Improvement of selectivity via the surface modification of carbon nanodots towards the quantitative detection of mercury ions

Highly fluorescent carbon nanodots are promising fluorophores for biochemical, pharmaceutical, and environmental analysis due to their facile preparation, biocompatibility, tunability, and low-cost precursors.