Heavy atom-induced quenching of fluorescent organosilicon nanoparticles for iodide sensing and total antioxidant capacity assessment

We present a novel approach for iodide sensing based on the heavy-atom effect to quench the green fluorescent emission of organosilicon nanoparticles (OSiNPs). The fluorescence of OSiNPs was significantly quenched (up to 97.4% quenching efficiency) in the presence of iodide ions (I-) through oxidation by hydrogen peroxide. Therefore, OSiNPs can serve as a fluorescent probe to detect I- with high selectivity and sensitivity. The highly selective response is attributed to the hydrophilic surface enabling good dispersion in aqueous solutions and the lipophilic core allowing the generated liposoluble I2 to approach and quench the fluorescence of OSiNPs. The linear working range for I- was from 0 to 50 μM, with a detection limit of 0.1 μM. We successfully applied this nanosensor to determine iodine content in edible salt. Furthermore, the fluorescent OSiNPs can be utilized for the determination of total antioxidant capacity (TAC). Antioxidants reduce I2 to I-, and the extent of quenching by the remaining I2 on the OSiNPs indicates the TAC level. The responses to ascorbic acid, pyrogallic acid, and glutathione were investigated, and the detection limit for ascorbic acid was as low as 0.03 μM. It was applied to the determination of TAC in ascorbic acid tablets and fruit juices, indicating the potential application of the OSiNP-based I2 sensing technique in the field of food analysis.

Supramolecular structures from structurally persistent and surface active carbon dots in water

Carbon dots (CDs) have developed into an important class of nanomaterials that have attracted increasing attention during the past decades. Despite numerous types of CDs reported to date, research on their self-assembly is still limited. Herein, we report for the first time the self-assembly of CDs in water, which show concentration-dependent aggregation behavior. The CDs used have a structural motif of a fully carbonized core surrounded by a highly condensed, polymeric network, to which triethylene glycol monomethyl ether (TGME) chains are grafted. When dissolved in water, they show a low critical aggregation concentration (cac) of 0.07 mg mL^-1 with the lowest surface tension of ∼37 mN m^-1. Above this cac, nanoclusters and vesicles are observed at relatively low and high concentrations, respectively. At an intermediate concentration, polymorphism is noticed where nanotubes coexist with nanorods. At an elevated temperature, the CDs become more hydrophobic due to the dehydration of peripheral TGME, which decreases the cac and triggers phase transfer from water to toluene. These surface active CDs were used to disperse and stabilize multi-walled carbon nanotubes in water, which showed much better performance than that of both traditional ionic and nonionic surfactants. Our work indicates that with a careful structural design, CDs can be developed into a new type of amphiphiles with properties superior to those of traditional surfactants in specific aspects.

Dual emissive amphiphilic carbon dots as ratiometric fluorescent probes for the determination of critical micelle concentration of surfactants

The sensitive determination of the critical micelle concentration (CMC) of surfactants is very important for their practical application. Due to their good sensitivity and simple operation, pyrene and its derivatives have been widely used as fluorescent probes to detect the CMC. However, their virulent and poor water-soluble nature has limited their wide employment. In the present work, environmentally friendly amphiphilic carbon dots (Cdots) with dual-color emission and absolute quantum yield (PLQY) values higher than 50% have been fabricated through a solvothermal process, which could successfully serve as self-calibrative, ratiometric fluorescent probes to estimate the CMC of both non-ionic and ionic surfactants. This work not only provides a new strategy to design green ratiometric fluorescent probes for the CMC measurement of surfactants but also expands the application of Cdots in the colloidal field.

Multiple fluorescence response behaviours to proteins/bacteria and selective antibacterial activity of cetylpyridinium chloride (CPC)-based cationic carbon dots

Direct interaction between carbon dots (CDs) and biomolecules leads to changes in the chemical and physical status as well as properties of CDs, which can have various biological and biomedical applications. In this work, the surface of CDs was modified with cetylpyridinium chloride (CPC) to facilitate interactions between CDs and biomolecules. Multiple fluorescence response behaviours of CPC-based CDs were observed towards several proteins (bovine serum albumin, lysozyme, protamine, and hemoglobin) and bacterial cells (Escherichia coli and Staphylococcus aureus). Electrostatic attraction and hydrogen bonding were involved in inducing aggregation of CDs and fluorence enhancement. An inner filter effect might also occur to reduce fluorescence of CDs when interacting with proteins. Selective antibacterial activity of CPC-based CDs was observed towards Gram positive bacterium Staphylococcus aureus. This work provides potential to develop CD-based techniques for detecting and visualizing proteins/bacteria as well as selective antibacterial agents towards Gram-positive bacteria.

Aggregation of cetylpyridinium chloride-based cationic carbon dots (CPC-based CDs) inducing by proteins or bacteria could led to multiple fluorescence response behaviours. The CPC-based CDs also showed selective antibacterial activity.

Application of carbon dots and their composite materials for the detection and removal of radioactive ions: A review

Radioactive ions with high-heat release or long half-life could cause long-term influence on environment and they might enter the food chain to damage human body for their toxicity and radioactivity. It is of great importance to develop methods and materials to detect and remove radioactive ions. Carbon dots and their composite materials has been applied widely in many fields due to their plentiful raw materials, facile synthesis and functional process, unique optical property and abundant functional groups. This comprehensive review focuses on the preparation of CDs and composite materials for the detection and adsorption of radioactive ions. Firstly, the recent-developed synthetic methods for CDs were summarized briefly, including hydrothermal/solvothermal, microwave, electrochemistry, microplasma, chemical oxidation methods, focusing on the influence of CDs properties. Secondly, the synthetic methods for CDs composite materials were classified to four categories and summarized generally. Thirdly, the application of CDs for radioactive ions detection and adsorption were explored and concluded including uranium, iodine, europium, strontium, samarium et al. Finally, the detection and adsorption mechanism for radioactive ions were searched and the perspective and outlook of CDs for detection and adsorption radioactive ions have been proposed based on our understanding.

Carbon dots: synthesis, properties and biomedical applications

Carbon dots (CDs) are a new type of carbon nanomaterial that have unique physical and chemical properties, good biocompatibility, low toxicity, and easy surface functionalization, making them widely used in biological imaging, environmental monitoring, chemical analysis, targeted drug delivery, disease diagnosis, therapy, etc. In this review, our content is mainly divided into four parts. In the first part, we focused on the preparation methods of CDs, including arc discharge, laser ablation, electrochemical oxidation, chemical oxidation, combustion, hydrothermal/solvent thermal, microwave, template, method etc. Next, we summarized methods of CD modification, including heteroatom doping and surface functionalization. Then, we discussed the optical properties of CDs (ultraviolet absorption, photoluminescence, up-conversion fluorescence, etc.). Lastly, we reviewed the common applications of CDs in biomedicine from the aspects of in vivo and in vitro imaging, sensors, drug delivery, cancer theranostics, etc. Furthermore, we also discussed the existing problems and the future development direction of CDs.

A study on human serum albumin corona formed on photoluminescent carbon dots

Fluorescence nanostructures have been widely applied in the biomedical field as therapeutic agents and as novel tools for labeling, imaging, and sensing. However, the protein corona will dramatically influence the predesigned properties of nanostructures in serum. Therefore, it is important to understand the mechanism of protein corona formation on nanostructures. Photoluminescent carbon dots have been widely applied in the biomedical field since their discovery. Due to the large overlap between the absorption spectra of proteins and the fluorescence spectra of photoluminescent carbon dots, herein we investigate the mechanism of human serum albumin corona formed on photoluminescent carbon dots using fluorescence resonance energy transfer. By employing spectroscopic methods, the binding constants and the number of binding sites between human serum albumin and photoluminescent carbon dots have been determined, and the corresponding thermodynamics are also discussed as well for the interaction between photoluminescent carbon dots and human serum albumin. In addition, we successfully demonstrate the photoluminescent carbon dots in labeling bean sprouts. We believe that the current research cannot shed light on the mechanism of protein corona formation on nanostructures, but also could benefit the design of hybrid nanomaterial which will be applied to serum environments.

Evolution and Synthesis of Carbon Dots: From Carbon Dots to Carbonized Polymer Dots

Despite the various synthesis methods to obtain carbon dots (CDs), the bottom-up methods are still the most widely administrated route to afford large-scale and low-cost synthesis. However, as CDs are developed with increasing reports involved in producing many CDs, the structure and property features have changed enormously compared with the first generation of CDs, raising classification concerns. To this end, a new classification of CDs, named carbonized polymer dots (CPDs), is summarized according to the analysis of structure and property features. Here, CPDs are revealed as an emerging class of CDs with distinctive polymer/carbon hybrid structures and properties. Furthermore, deep insights into the effects of synthesis on the structure/property features of CDs are provided. Herein, the synthesis methods of CDs are also summarized in detail, and the effects of synthesis conditions of the bottom-up methods in terms of the structures and properties of CPDs are discussed and analyzed comprehensively. Insights into formation process and nucleation mechanism of CPDs are also offered. Finally, a perspective of the future development of CDs is proposed with critical insights into facilitating their potential in various application fields.

Carbon dots functionalized papers for high-throughput sensing of 4-chloroethcathinone and its analogues in crime sites

Sensitive and selective assays are demanded for quantitation of new psychoactive substances such as 4-chloroethcathinone that is a π-conjugated keto compound. Carbon dots (C-dots) prepared from L-arginine through a hydrothermal route have been used for quantitation of 4-chloroethcathinone in aqueous solution and on C-dot-functionalized papers (CDFPs). To prepare CDFPs, chromatography papers, each with a pattern of 8 × 12 circles (wells), are first fabricated through a solid-ink printing method and then the C-dots are coated into the wells. π-Conjugated keto or ester compounds induce photoluminescence quenching of C-dots through an electron transfer process. At pH 7.0, the CDFPs allow screening of abused drugs such as cocaine, heroin and cathinones. Because of poor solubility of heroin and cocaine at pH 11.0, the C-dot probe is selective for cathinones. The C-dots in aqueous solution and CDFPs at pH 11.0 allow quantitation of 4-chloroethcathinone down to 1.73 mM and 0.14 mM, respectively. Our sensing system consisting of a portable UV-lamp, a smartphone and a low-cost CDFP has been used to detect cathinones, cocaine and heroin at pH 7.0, showing its potential for screening of these drugs in crime sites.

Facile and large-scale synthesis of green-emitting carbon nanodots from aspartame and the applications for ferric ions sensing and cell imaging

A facile, economical and green strategy to prepare green-fluorescent nitrogen-doped carbon nanodots (N-CDs) with a quantum yield (QY) of approximately 31.91% has been built up, while aspartame was employed as the carbon-nitrogen source for the first time. The prepared N-CDs exhibited ultrahigh brightness, favorable strong photostability and negligible cytotoxicity. The outstanding optical properties are mainly derived from the their robost composition and steric distribution of the doped nitrogen atoms, which have been characterized detailedly. The obtained N-CDs showed highly selective and sensitive response toward ferric ions (Fe³⁺) through a fluorescence static quenching process in a wide linear range of 0.005-60µmol/L. The detection limit was as low as 1.43nmol/L, allowing the analysis of Fe³⁺ in a very simple method. The excitation-dependent luminescent behavior of the obtained N-CDs guaranteed the multicolor emissive property when they were used in cell imaging. And the application for intracellular Fe³⁺ sensing further verified this novel N-CDs may open more opportunities in biosensor, bioimaging and biological assay.

Cationic Carbon Dots for Modification-Free Detection of Hyaluronidase via an Electrostatic-Controlled Ratiometric Fluorescence Assay

Carbon dots (CDs) emerge as excellent fluorescent nanomaterials, but the full exploitation and application of their exceptional properties in the development of fluorescence assay are still rare. In this work, cationic carbon dots (C-CDs) covered with plenty of positive charges on the surface were synthesized through a facile ultrasonic method. Negatively charged hyaluronic acid (HA) caused the aggregation of positively charged C-CDs and neutral red (NR) along its linear chain via electrostatic adsorption, leading to a remarkable Förster resonance energy transfer (FRET) from C-CDs to NR. However, the presence of hyaluronidase (HAase) resulted in the enzymolysis of HA, as well as the liberation of C-CDs and NR. The corresponding change of fluorescence color from red to green-yellow afforded a reliable ratiometric assay for HAase. Also the ratio of fluorescence intensity for C-CDs (I525) to that for NR (I630) was used for quantitative detection of HAase. The proposed sensing system was easily operated in aqueous media with a detection limit of 0.05 U/mL. This strategy provides a new approach for the wider application of some special CDs in detecting biomolecules.

N-Doped carbon dots: green and efficient synthesis on a large-scale and their application in fluorescent pH sensing

A green and efficient anhydrous method was developed to synthesize nitrogen doped carbon dots (N-CDs) on a large-scale, and the N-CDs exhibited excellent reversible pH-sensitive property.