Deep Eutectic Solvent-Assisted Preparation of Nitrogen/Chloride-Doped Carbon Dots for Intracellular Biological Sensing and Live Cell Imaging

Fluorescent carbon dots functionalization

Carbon dots (CDs), as a new type of luminescent zero-dimensional carbon nanomaterial, have been applied in a variety of fields. Currently, functionalization of CDs is an extremely useful method for effectively tuning their intrinsic structure and surface state. Heteroatom doping and surface modification are two functionalization strategies for improving the photophysical performance and broadening the range of applications for fluorescent CDs. Heteroatom doping in CDs can be used to tune their intrinsic properties, which has received significant research interests because of its simplicity. Surface modification can be applied for varying active sites and the functional groups on the CDs surface, which can endow fluorescent CDs with the unique properties resulting from functional ligand. In this review, we summarize the structural and physicochemical properties of functional CDs. We focused our review on the latest developments in functionalization strategies for CDs and discuss the detailed characteristics of different functionalization methods. Ultimately, we hope to inform researchers on the latest progress in functionalization of CDs and provide perspectives on future developments for functionalization of CDs and their potential applications.

One-Step Synthesis of Label-Free Ratiometric Fluorescence Carbon Dots for the Detection of Silver Ions and Glutathione and Cellular Imaging Applications

The construction of ratiometric fluorescence assay has displayed fantastic advantages in improving semi-quantitative visualization capability by presenting successive color changes. Herein, long-wavelength emission nitrogen-doped carbon dots (N-CDs) were developed for intrinsic ratiometric detection of silver ions (Ag⁺) and glutathione (GSH), accompanied by visualization fluorescence variation of orange and green. The label-free N-CDs were favorably obtained through one-step hydrothermal synthesis and displayed single long-wavelength emission at 618 nm under the excitation wavelength of 478 nm. Interestingly, a ratio rising peak emerges at 532 nm and the emission at 618 nm decreases with the introduction of Ag⁺, which exhibits ratiometric fluorescence emission characteristics ( I_618nm/ I_532nm) in the range of 0-140 μM with significant fluorescence varying from orange to green. Furthermore, the fluorescence of CDs@Ag(I) can be effectively ratiometric recovered by virtue of a specific reaction of GSH with Ag⁺, which is accompanied by the fluorescence of the solution returning from green to orange. In addition, the N-CDs hold excellent biocompatibility which can be implemented as the visualization biosensing platform for intracellular determination of Ag⁺ and GSH, demonstrating that proposed N-CDs have tremendous potential in biological systems.

Facile green and one-pot synthesis of purple perilla derived carbon quantum dot as a fluorescent sensor for silver ion

In this work, biomass-derived carbon quantum dots (CQDs) with excellent water solubility, strong fluorescence and favorable biocompatibility were synthesized via one-step hydrothermal treatment of purple perilla for the first time. The functional group composition, morphology, and pH stability of the synthesized CQDs were systematically investigated. And based on fluorescence quenching of CQDs, the as-prepared CQDs were innovatively developed as an effective "signal-off" fluorescent probe for selective and sensitive detection of silver ion (Ag⁺) with two linear ranges of 0-10 and 10-3000 nM, and a detection limit 1.4 nM. The specificity and selectivity of this fluorescent probe were also verified through challenging the detection by using similarmetallic cations or in real water samples. In addition, the as-prepared CQDs exhibit a low cytotoxicity and a good biocompatibility, revealing its potential bioimaging applications in living cells.

One-step synthesis of boron-doped graphene quantum dots for fluorescent sensors and biosensor

Heteroatom doping can endow graphene quantum dots (GQDs) with various new or improved structural, optical and physicochemical properties. In contrast to the widely reported oxygen, nitrogen or sulfur doping in GQDs, simple and scalable synthesis of boron-doped GQDs (B-GQDs) with high yield and quantum yields remains challenge. In this work, B-GQDs are one-step synthesized and serve as the fluorescence probes for the fabrication of sensors towards Fe³⁺ ion or phosphate (Pi) as well as biosensor towards cytochrome C (Cyt C). The B-GQDs are facile synthesized using one-step bottom-up molecular fusion between 1,3,6-trinitropyrene and borax in sodium hydroxide under hydrothermal process. The synthesis can be performed using large volume autoclave (500 ml) with a high yield of 71%, indicating possibility for gram-scale production of B-GQDs. The as-prepared B-GQDs exhibit single or bilayer graphene structure, high crystallinity, uniform size, bright (absolute photoluminescence quantum yield of 16.8%) and excitation-independent green fluorescence (maximum excitation wavelength and emission wavelength of 480 nm and 520 nm, respectively). Successful doping of B atoms in the lattice of GQDs enables high selectivity towards Fe³⁺. Based on quenching of fluorescence of B-GQDs by Fe³⁺ (turn-off model), detection of Fe³⁺ (with limit of detection-LOD of 31.2 nM) and Fe³⁺-rich Cyt C (with LOD of 5.9 μg/ml) are demonstrated. As Pi can recover Fe³⁺-quenched fluorescence of B-GQDs (turn-off-on model), indirect fluorescent detection of Pi is also achieved with LOD of 340 nM. In addition, detection of Fe³⁺, Cyt C and Pi in real samples is achieved.

Interfacial engineering of carbon dots with benzenediboronic acid for fluorescent biosensing

Glucose assay is highly important in clinical diagnostics of diabetes. Herein, we engineered the surface of carbon dots by complexation with functional ligand and constructed fluorescent biosensors for the detection of hydrogen peroxide and glucose. In this study, benzenediboronic acid is conjugated to the surface of citric acid-derived carbon dots through formation of boronate complexes with the nanoparticles. The oxidation of benzenediboronic acid with hydrogen peroxide effectively quenches fluorescence of carbon dots through electron transfer process. The sensing performance of the system according to different engineered surfaces of carbon dots was studied by using carbon dots derived from various precursors and different benzenediboronic acid analogues. As a simple mix-and-detect strategy, this system is facilely applied for glucose sensing as hydrogen peroxide is the product catalyzed by glucose oxidase. The benzenediboronic acid-conjugated carbon dots derived from citric acid act as excellent optical probes for sensitive analysis of glucose with detection limit of 0.4 μM. This sensing system shows great selectivity toward interferent species such as analogues of glucose, and can be used to determine glucose in human serum. Engineering the surface of carbon dots by complexation with ligand of interest provides a feasible way to facilitate the development of biological applications.

Synthesis of highly stable red-emissive carbon polymer dots by modulated polymerization: from the mechanism to application in intracellular pH imaging

Great efforts have been made to develop facile and efficient methods to prepare carbonaceous nanostructures with long-wavelength emission. Herein, we report a low-temperature aqueous strategy to synthesize red-emissive carbon polymer dots (R-CPDs) through the regulation of oxidative polymerization of p-phenylenediamine at 80 °C. The morphology, chemical composition and photophysical properties of the R-CPDs are characterized and analyzed in detail, thereby elucidating their photoluminescence origins from the surface state and crosslink enhanced emission effect. The resulting R-CPDs possess unique features including high pH-sensitivity within pH 4-6 and a wide-range tunable solvent-color effect (λ_em 528-600 nm). Moreover, the R-CPDs show high stability in physiological media with high salinity, and good resistance to photobleaching. In addition to their favorable biocompatibility, the R-CPDs are applied for monitoring the pH fluctuation in HeLa cells. This study not only provides a unique red emissive carbonaceous nanomaterial for cellular imaging and multicolor applications, but also presents a novel perspective for the construction of long-wavelength emission carbon-based nanomaterials by simple and controllable strategies.

Excitation-independent hollow orange-fluorescent carbon nanoparticles for pH sensing in aqueous solution and living cells

We report an ingenious strategy for fabrication of hollow orange fluorescent carbon nanoparticles (HFCNs) and demonstrate their applications for pH biosensing and fingerprint detection. HFCNs have been synthesized using 5-amino salicylic acid as carbon source via one-step hydrothermal treatment without further surface passivation or modification. The as-prepared HFCNs possess excellent hollow structure and bright orange fluorescence. The HFCNs display a remarkable emission enhancement in the orange fluorescence region when the pH is increased from 3.0 to 10.0. The pKa value of HFCNs is found to be 5.97 and a good linearity is shown in the pH range of 5.25-6.75, which makes HFCNs an effective intracellular pH imaging agent for acidic microenvironments. The confocal fluorescent microscopic images of HFCNs-stained latent fingerprint are achieved successfully, suggesting that they have great promise for practical criminal investigations as a simple, fast, and accurate tool.