Electrochemical Methods to Study Photoluminescent Carbon Nanodots: Preparation, Photoluminescence Mechanism and Sensing

Bright Green-Emissive Carbon Nanodots for Sensing and Intracellular Imaging of Cu2+ and Glutathione

Green-emissive carbon nanodots (CDs) with high quantum yield are prepared. The abundant functional groups on the surfaces of CDs can selectively interact with Cu2+. The formed cupric amine complexes induce significant fluorescence quenching. The "on-off" switching can be further adjusted to the fluorescence "on" mode by the introduction of glutathione (GSH), which hinders the interactions between CDs and Cu2+. Based on the fantastic optical behavior of CDs, highly sensitive detection of Cu2+ and GSH can be achieved. Intracellular imaging of the two targets is also validated.

Synthesis, properties and potential applications of photoluminescent carbon nanoparticles: A review

Photoluminescent-carbon nanoparticles (PL-CNPs) are a new class of materials that received immense interest among researchers due to their distinct characteristics, including photoluminescence, high surface-to-volume ratio, low cost, ease of synthesis, high quantum yield, and biocompatibility. By exploiting these outstanding properties, many studies have been reported on its utility as sensors, photocatalysts, probes for bio-imaging, and optoelectronics applications. From clinical applications to point-of-care test devices, drug loading to tracking of drug delivery, and other research innovations demonstrated PL-CNPs as an emerging material that could substitute conventional approaches. However, some of the PL-CNPs have poor PL properties and selectivity due to the presence of impurities (e.g., molecular fluorophores) and unfavourable surface charges by the passivation molecules, which impede their applications in many fields. To address these issues, many researchers have been paying great attention to developing new PL-CNPs with different composite combinations to achieve high PL properties and selectivity. Herein, we thoroughly discussed the recent development of various synthetic strategies employed to prepare PL-CNPs, doping effects, photostability, biocompatibility, and applications in sensing, bioimaging, and drug delivery fields. Moreover, the review discussed the limitations, future direction, and perspectives of PL-CNPs in possible potential applications.

Ratiometric Fluorescent Detection of Ultrasound-Regulated ATP Release: An Ultrasound-Resistant Cu,N-Doped Carbon Nanosphere

Focused ultrasound, as a protocol of cancer therapy, might induce extracellular adenosine triphosphate (ATP) release, which could enhance cancer immunotherapy and be monitored as a therapeutic marker. To achieve an ATP-detecting probe resistant to ultrasound irradiation, we constructed a Cu/N-doped carbon nanosphere (CNS), which has two fluorescence (FL) emissions at 438 and 578 nm to detect ultrasound-regulated ATP release. The addition of ATP to Cu/N-doped CNS was conducted to recover the FL intensity at 438 nm, where ATP enhanced the FL intensity probably via intramolecular charge transfer (ICT) primarily and hydrogen-bond-induced emission (HBIE) secondarily. The ratiometric probe was sensitive to detect micro ATP (0.2-0.6 μM) with the limit of detection (LOD) of 0.068 μM. The detection of ultrasound-regulated ATP release by Cu,N-CNS/RhB showed that ATP release was enhanced by the long-pulsed ultrasound irradiation at 1.1 MHz (+37%, p < 0.01) and reduced by the short-pulsed ultrasound irradiation at 5 MHz (-78%, p < 0.001). Moreover, no significant difference in ATP release was detected between the control group and the dual-frequency ultrasound irradiation group (+4%). It is consistent with the results of ATP detection by the ATP-kit. Besides, all-ATP detection was developed to prove that the CNS had ultrasound-resistant properties, which means it could bear the irradiation of focused ultrasound in different patterns and detect all-ATP in real time. In the study, the ultrasound-resistant probe has the advantages of simple preparation, high specificity, low limit of detection, good biocompatibility, and cell imaging ability. It has great potential to act as a multifunctional ultrasound theranostic agent for simultaneous ultrasound therapy, ATP detection, and monitoring.

Selective and sensitive detection of tartrazine in beverages by sulfur quantum dots with high fluorescence quantum yield

In this work, sulfur quantum dots (TPA-SQDs) protected by terephthalic acid as a stabilizer were synthesized using a one-pot method. When excited at 310 nm, the synthesized TPA-SQDs solution emitted strong blue fluorescence at 428 nm, and the absolute quantum yield was as high as 85.99%. The proposed SQDs can be used as a fluorescent probe to specifically quench tartrazine (TZ), showing a good linear relationship (R² = 0.996) at TZ concentrations of 0.1-20 μM, with a detection limit of 39 nM. By analysing the fluorescence lifetime, UV-Vis absorption spectrum and zeta potential of the assay system, it can be speculated that the fluorescence quenching mechanism of TZ on TPA-SQDs is the inner filter effect (IFE). The proposed method was applied to the detection of TZ in vitamin water and orange juice, and the results were consistent with the determination results by high-performance liquid chromatography. The recoveries and relative standard deviations were 93.2-102.6% and 1.34-2.88%, respectively, which provided an alternative method for the determination of TZ in beverages or other food samples.

Photoluminescence Mechanism of Carbon Dots: Triggering Multiple Color Emissions through Controlling the Degree of Protonation

Carbon dots (CDs) have excellent optical properties, low toxicity and easy preparation, which have led to them being widely used in biomedicine, sensing and optical devices. However, although great progress has been made in the preparation of CDs, the detailed exploration of their photoluminescence (PL) mechanism is still under debate due to their complex structures and surface functionalities. Here, we proposed a single change in the pH of the synthesis condition, which had no effect on the CDs intrinsic core states and avoided the mutual influence of multiple PL origins. The m-phenylenediamine (m-PD) served as a carbon source, whose protonation degree determined the surface state of the resulting CDs and the accompanying fluorescence characteristics. The as-obtained CDs materials can be applied in the chemical sensor and anti-counterfeiting fields in a targeted manner. Therefore, our work not only contributes to the explanation of the CDs PL mechanism, but also obtains a series of CDs materials with controllable PL properties.

Assessment of pesticide induced inhibition of Apis mellifera (honeybee) acetylcholinesterase by means of N-doped carbon dots/BSA nanocomposite modified electrochemical biosensor

This work describes the development and optimization of an electrochemical method to evaluate pesticide induced inhibition of honey bee (Apis mellifera) acetylcholinesterase (AChE) by means of acetylcholinesterase biosensor. The inhibition assay was based on the detection of changes in electrochemical activity of the enzyme caused by pesticide. As transducer, nitrogen doped carbon dots BSA (N-CD/BSA) nanocomposite electrodeposited on pencil graphite electrode was used to covalently immobilize AChE. The as-synthesized nanocomposite and fabricated electrodes were characterized for the structural, functional and electrochemical properties. Nanocomposite promoted the electron transfer reaction to catalyze the electro-oxidation of thiocholine and a large current response was obtained by cyclic voltammetry at 0.77 V, indicating successful immobilization of AChE. The sensitivity of Diazinon, an OP insecticide, for honeybee AChE was tested under optimal conditions and a linear response ranging 10-250 nM was obtained with a detection limit of 8.9 nM, and sensitivity 9 uA/nM/cm2. The method showed a good operational reproducibility and selectivity of biosensor. Further, the molecular docking provided additional support to the experimental data suggesting irreversible nature and contact toxicity of the pesticide for honey bee AChE. The developed biosensor has proved useful for the diazinon detection in wheat samples with 99% recovery rate.

Optical properties and photoactivity of carbon nanodots synthesized from olive solid wastes at different carbonization temperatures

Carbon nanodots (CNDs) have many fascinating properties, such as optical properties (UV-Visible absorption and fluorescence emission), which make them good candidates in many applications, such as photocatalysts for the degradation of several organic pollutants. This study aims to synthesize CNDs from olive solid wastes at different carbonization temperatures from 300 to 900 °C and study the effect on the optical properties of the CNDs, such UV-Vis, fluorescence, quantum yield, and energy bandgap, in addition to the influence on the photoactivity of the CNDs as photocatalysts for the degradation of methylene blue (MB). CNDs were prepared from olive solid wastes (OSWs) by pyrolysis at different temperatures (300-900 °C) for conversion to carbonized material, and then oxidized chemically in the presence of hydrogen peroxide (H2O2). It was found that an increase in the carbonization temperature of the OSWs leads to an increase in the product yield with a maximum value at 500 °C, and it then decreased dramatically. On the other hand, a decrease in fluorescence due to the diminishment of oxygen groups and the destruction of the surface of the CNDs was observed. The higher quantum yield (5.17%) and bandgap (2.77 eV) were achieved for CNDs prepared from OSWs that carbonized at 300 °C. The rate and degradation efficiency of MB were studied with the different synthesized CNDs, and it was found that an increase in the carbonization temperature leads to a decrease in the rate and degradation efficiency of MB, with the highest degradation rate of 0.0575 min-1 and degradation efficiency of 100% after 120 minutes of light irradiation being realized for the sample carbonized at 300 °C.

Rapid detection of silver ions based on luminescent carbon nanodots for multicolor patterning, smartphone sensors, and bioimaging applications

Photoluminescent nitrogen and sulfur co-doped carbon nanodots (N,S-CNDs) were prepared via single-step hydrothermal carbonization using 2,4-diaminobenzenesulfonic acid (2,4-DABSA) as the sole precursor. The synthesized N,S-CNDs are easily dispersed in aqueous solution and have an average particle size of 5.0 ± 0.2 nm, showing a high quantum yield of 23.1% with excellent stability. The surface states of the N,S-CNDs were confirmed by Fourier-transform infrared spectroscopy, powder X-ray diffractometry, Raman spectroscopy, and X-ray photoelectron spectroscopy techniques. These N,S-CNDs were applied for the rapid visual sensing detection of Ag⁺ ions, which can be identified by their photoluminescent color change under ultraviolet (UV) light illumination at 365 nm within 5 s. Furthermore, a linear correlation coefficient between P₀/P and Ag⁺ ions was observed in the linear range of 0-1.2 μM with a detection limit of 7.88 nM. The proposed method was successfully used for the sensitive detection of Ag⁺ ions in real samples with satisfactory recoveries and relative standard deviation. The photoluminescence properties of N,S-CND and N,S-CNDs/Ag⁺ aqueous solutions were demonstrated by their invisible inks that can only be seen when irradiated with UV light. The RGB values of N,S-CND and N,S-CNDs/Ag⁺ aqueous solutions were measured using a color selector smartphone application. In addition, N,S-CND and N,S-CNDs/Ag⁺ aqueous solutions were further used for the multicolor imaging of HCT-116 cancer cells due to the low toxicity of N,S-CNDs.

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

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

Photoluminescent Nanoparticles for Chemical and Biological Analysis and Imaging

Research related to the development and application of luminescent nanoparticles (LNPs) for chemical and biological analysis and imaging is flourishing. Novel materials and new applications continue to be reported after two decades of research. This review provides a comprehensive and heuristic overview of this field. It is targeted to both newcomers and experts who are interested in a critical assessment of LNP materials, their properties, strengths and weaknesses, and prospective applications. Numerous LNP materials are cataloged by fundamental descriptions of their chemical identities and physical morphology, quantitative photoluminescence (PL) properties, PL mechanisms, and surface chemistry. These materials include various semiconductor quantum dots, carbon nanotubes, graphene derivatives, carbon dots, nanodiamonds, luminescent metal nanoclusters, lanthanide-doped upconversion nanoparticles and downshifting nanoparticles, triplet-triplet annihilation nanoparticles, persistent-luminescence nanoparticles, conjugated polymer nanoparticles and semiconducting polymer dots, multi-nanoparticle assemblies, and doped and labeled nanoparticles, including but not limited to those based on polymers and silica. As an exercise in the critical assessment of LNP properties, these materials are ranked by several application-related functional criteria. Additional sections highlight recent examples of advances in chemical and biological analysis, point-of-care diagnostics, and cellular, tissue, and in vivo imaging and theranostics. These examples are drawn from the recent literature and organized by both LNP material and the particular properties that are leveraged to an advantage. Finally, a perspective on what comes next for the field is offered.

Fluorescent nanoparticles as tools in ecology and physiology

Fluorescent nanoparticles (FNPs) have been widely used in chemistry and medicine for decades, but their employment in biology is relatively recent. Past reviews on FNPs have focused on chemical, physical or medical uses, making the extrapolation to biological applications difficult. In biology, FNPs have largely been used for biosensing and molecular tracking. However, concerns over toxicity in early types of FNPs, such as cadmium-containing quantum dots (QDs), may have prevented wide adoption. Recent developments, especially in non-Cd-containing FNPs, have alleviated toxicity problems, facilitating the use of FNPs for addressing ecological, physiological and molecule-level processes in biological research. Standardised protocols from synthesis to application and interdisciplinary approaches are critical for establishing FNPs in the biologists' tool kit. Here, we present an introduction to FNPs, summarise their use in biological applications, and discuss technical issues such as data reliability and biocompatibility. We assess whether biological research can benefit from FNPs and suggest ways in which FNPs can be applied to answer questions in biology. We conclude that FNPs have a great potential for studying various biological processes, especially tracking, sensing and imaging in physiology and ecology.

Controlled synthesis of fluorescent carbon materials with the assistance of capillary electrophoresis

Carbon nanodots (CNDs) have been widely applied in variety of fields, while some evidences indicate their components may be complicated. In this work, capillary electrophoresis (CE) was used to evaluate the effect of synthetic conditions of fluorescent CNDs prepared through the hydrothermal method using citric acid (CA) and Triaminoguanidinium chloride (TG_Cl) as the starting materials. The results indicated that the fluorescent components of the products were affected by the ratio of the starting materials, the reaction temperature and reaction time. Under selected conditions, a ratio of TG_Cl to CA of 1:6, the reaction at 180 °C for 3 h, the product contains more than 4 fluorescent components with similar optical properties. CNDs were used for the determination of Cr(VI) in environmental samples with recoveries ranging in 95.3-107%, and the mechanism was also confirmed.

Design of Multifunctional Fluorescent Hybrid Materials Based on SiO2 Materials and Core-Shell Fe3 O4 @SiO2 Nanoparticles for Metal Ion Sensing

Hybrid fluorescent materials constructed from organic chelating fluorescent probes and inorganic solid supports by covalent interactions are a special type of hybrid sensing platform that has gained much interest in the context of metal ion sensing applications owing to their excellent advantages, recyclability, and solubility/dispersibility in particular, as compared with single organic fluorescent molecules. In recent decades, SiO₂ materials and core-shell Fe₃ O₄ @SiO₂ nanoparticles have become important inorganic solid materials and have been used as inorganic solid supports to hybridize with organic fluorescent receptors, resulting in multifunctional fluorescent hybrid systems for potential applications in sensing and related research fields. Therefore, recent progress in various fluorescent-group-functionalized SiO₂ materials is reviewed, with a focus on mesoporous silica nanoparticles and core-shell Fe₃ O₄ @SiO₂ nanoparticles, as interesting fluorescent organic-inorganic hybrid materials for sensing applications toward essential and toxic metal ions. Selective examples of other types of silica/silicon materials, such as periodic mesoporous organosilicas, solid SiO₂ nanoparticles, fibrous silica spheres, silica nanowires, silica nanotubes, and silica hollow microspheres, are also mentioned. Finally, relevant perspectives of metal-ion-sensing-oriented silica-fluorescent probe hybrid materials are provided.

Surface Sensitive Photoluminescence of Carbon Nanodots: Coupling between the Carbonyl Group and π-Electron System

The functional groups and π-electron system of carbon dots (C-dots) were carefully controlled by several innovative chemical methods, without any changes in size, to unravel the relationship between the surface structure and photoluminescence (PL). The results of experiments and theoretical calculations reveal that the PL of C-dots is related to the surface state. The energy gap is determined by the coupling of the π-electron system and carbonyl group, and the quantum yield (QY) is dependent on the carbonyl group. The carbonyl group is the main factor increasing the ratio of nonradiation to radiation recombination, thereby leading to the low QY of C-dots. This work provides a strategy for effectively tuning the structure of C-dots, giving rise to the tunable PL emission wavelength and highly desirable QY, which enables us to further unravel the PL mechanism.

Synthesis of ginsenoside Re-based carbon dots applied for bioimaging and effective inhibition of cancer cells

BACKGROUND

Fluorescent carbon-based nanomaterials have promising properties such as biosensing, cell imaging, tracing and drug delivery. However, carbon dots (CDs) with specific inherent biological functions have not been investigated. Ginsenosides are the components with multiple bioactivities found in plants of the genus Panax, which have attracted a lot of attention for their anticancer effect.

MATERIALS AND METHODS

In this study, we prepared a kind of novel photoluminescent CDs from ginsenoside Re by one-step hydrothermal synthesis method. The conventional methods including transmission electron microscopy, Fourier transform infrared spectroscopy, HPLC and fluorescence spectrum were used for characterization of CDs. In vitro anticancer effect was investigated by cytotoxicity assay, flow cytometry and Western blot analysis.

RESULTS

The as-prepared Re-CDs had an average diameter of 4.6±0.6 nm and excellent luminescent properties. Cellular uptake of Re-CDs was facilitated by their tiny nanosize, with evidence of their bright excitation-dependent fluorescent images. Compared with ginsenoside Re, the Re-CDs showed greater inhibition efficiency of cancer cell proliferation, with lower toxicity to the normal cells. The anticancer activity of Re-CDs was suggested to be associated with the generation of large amount of ROS and the caspase-3 related cell apoptosis.

CONCLUSION

Hopefully, the dual functional Re-CDs, which could both exhibit bioimaging and anticancer effect, are expected to have great potential in future clinical applications.

Facile Fluorescence "Turn on" Sensing of Lead Ions in Water via Carbon Nanodots Immobilized in Spherical Polyelectrolyte Brushes

Heavy metal detection has become very important for the protection of water resource. In this work, a novel controllable probe is presented for the sensitive detection of Pb2+ in aqueous solutions. The probe was synthesized via the immobilization of surface functionalized carbon dots (named as CAEA-Hs) into the shell of the spherical polyelectrolyte brushes (SPB). The fluorescence of CAEA-H was firstly "turned off" via electrostatic interaction induced quenching. Based on the aggregation induced emission enhancement (AIEE), the fluorescence of the immobilized CAEA-H could be specifically turned on via the aggregation of the SPB particles. This fluorescence "turn on" sensor could selectively detect Pb2+ among five different metal ions with a relatively wide detecting range (0-1.67 mM) and good linear relationship (R 2 = 0.9958). Moreover, the aggregating behavior and nano-structure of CAEA-H loaded SPB have been systematically analyzed via small angle X-ray scattering, turbidity titration, and Zeta-potential measurement. Based on a series of control experiments, we finally gain an insight into the sensing mechanism of this novel sensing probe. This contributed a proof of concept demonstration that sensitive and selective chemical detection can be achieved via a C-dot/SPB synergistic platform.

Capacitively Coupled Plasma Discharge of Ionic Liquid Solutions to Synthesize Carbon Dots as Fluorescent Sensors

Oxygen and nitrogen capacitively coupled plasma (CCP) was used to irradiate mixtures of aliphatic acids in high boiling point solvents to synthesize fluorescent carbon dots (C-dots). With a high fluorescence intensity, the C-dots obtained from the O₂/CCP radiation of a 1-ethyl-3-methylimidazolium dicyanamide ionic liquid solution of citric acid were characterized with an average diameter of 8.6 nm (σ = 1.1 nm), nitrogen and oxygen bonding functionalities, excitation-independent emissions, and upconversion fluorescence. Through dialysis of the CCP-treated C-dots, two emissive surface states corresponding to their respective functionalities and emissions were identified. The fluorescence spectrum of the CCP-treated C-dots was different from that of the microwave irradiation and possessed higher intensity than that of hydrothermal pyrolysis. By evaluation of the fluorescence quenching effect on flavonoids and metal ions, the CCP-treated C-dots showed a high selectivity for quercetin and sensitivity to Hg²⁺. Based on the Perrin model, a calibration curve (R² = 0.9992) was established for quercetin ranging from 2.4 μM to 119 μM with an LOD (limit of detection) = 0.5 μM. The quercetin in the ethanol extract of the sun-dried peel of Citrus reticulata cv. Chachiensis was determined by a standard addition method to be 4.20 ± 0.15 mg/g with a matrix effect of 8.16%.

One-step synthesis of nitrogen, sulfur co-doped carbon nanodots and application for Fe3+ detection

Carbon nanodots (CDs) are novel forms of zero-dimensional carbonaceous nanomaterials, which have attracted the attention of researchers.

Dramatic photoluminescence quenching in carbon dots induced by cyclic voltammetry

We disclose an electrochemically activated quenching mechanism that is dominant in certain types of C-dots.

A Ratiometric Fluorescence Universal Platform Based on N, Cu Codoped Carbon Dots to Detect Metabolites Participating in H2O2-Generation Reactions

In this work, a new kind of N, Cu codoped carbon dots (N/Cu-CDs) was prepared via a facile one-pot hydrothermal method by using citric acid monohydrate, copper acetate monohydrate and diethylenetriamine. The prepared N/Cu-CDs with a high quantum yield (50.1%) showed excitation-independent emission at 460 nm. The structure and fluorescence properties of N/Cu-CDs were characterized by high-resolution transmission electron microscopy, fluorescence spectrofluorometer, FT-IR spectrometer, UV-visible spectrophotometer and X-ray photoelectron spectroscopy. N/Cu-CDs were applied to establishing a ratiometric fluorescence probe toward H₂O₂ based on the inner filter effect (IFE) between N/Cu-CDs and DAP (2,3-diaminophenazine, the oxidative product of o-phenylenediamine (OPD)), and provided a ratiometric fluorescence universal platform for detection of the metabolites participating in H₂O₂-generation reactions (cholesterol and xanthine). The proposed method was demonstrated to be ultrasensitive and highly selective for cholesterol and xanthine assay with detection limits of 0.03 and 0.10 μM, respectively. The fluorescence probe built was applied to the determination of cholesterol and xanthine in human serum with satisfactory results.

Folic acid-conjugated carbon dots as green fluorescent probes based on cellular targeting imaging for recognizing cancer cells

Green fluorescent CDs using the natural product dandelion as a carbon source were covalently conjugated with folate to differentiate cancer cells from normal cells.