Replacing precious platinum with earth-abundant materials for the oxygen reduction reaction (ORR) in fuel cells has been the objective worldwide for several decades. In the last 10 years, the fastest-growing branch in this area has been carbon-based metal-free ORR electrocatalysts. Great progress has been made in promoting the performance and understanding the underlying fundamentals. Here, a comprehensive review of this field is presented by emphasizing the emerging issues including the predictive design and controllable construction of porous structures and doping configurations, mechanistic understanding from the model catalysts, integrated experimental and theoretical studies, and performance evaluation in full cells. Centering on these topics, the most up-to-date results are presented, along with remarks and perspectives for the future development of carbon-based metal-free ORR electrocatalysts.

Carbon-Based Metal-Free ORR Electrocatalysts for Fuel Cells: Past, Present, and Future.

Carbon quantum dots from natural resource: A review

Distinct from conventional carbon nanostructures, such as fullerene, graphene, and carbon nanotubes, carbon nanodots (C-dots) exhibit unique properties such as strong fluorescence, high photostability, chemical inertness, low toxicity, and biocompatibility. Various synthesis routes for C-dots have been developed in the last few years, and now intense research efforts have been focused on improving their functionality. In this aspect, doping and surface functionalization are two major ways to control the chemical, optical, and electrical properties of C-dots. Doping introduces atomic impurities into C-dots to modulate their electronic structure, and surface functionalization modifies the C-dot surface with functional molecules or polymers. In this review, we summarize recent progress in doping and surface functionalization of C-dots for improving their functionality, and offer insight into controlling the properties of C-dots for a variety of applications ranging from biomedicine to optoelectronics to energy.

Improving the functionality of carbon nanodots: doping and surface functionalization

A review on nanostructured carbon quantum dots and their applications in biotechnology, sensors, and chemiluminescence.

In recent years, nano carbon quantum dots (CQDs) have received increasing attention due to their properties such as small size, fluorescence emission, chemical stability, water solubility, easy synthesis, and the possibility of functionalization. CQDs are fluorescent 0D carbon nanostructures with sizes below 10 nm. The fluorescence in CQDs originates from two sources, the fluorescence emission from bandgap transitions of conjugated π-domains and fluorescence from surface defects. The CQDs can emit fluorescence in the near-infrared (NIR) spectral region which makes them appropriate for biomedical applications. The fluorescence in these structures can be tuned with respect to the excitation wavelength. The CQDs have found applications in different areas such as biomedicine, photocatalysis, photosensors, solar energy conversion, light emitting diodes (LEDs), etc. The biomedical applications of CQDs include bioimaging, drug delivery, gene delivery, and cancer therapy. The fluorescent CQDs have low toxicity and other exceptional physicochemical properties in comparison to heavy metals semiconductor quantum dots (QDs) which make them superior candidates for biomedical applications. In this review, the synthesis routes and optical properties of the CQDs are clarified and recent advances in CQDs biomedical applications in bioimaging (in vivo and in vitro), drug delivery, cancer therapy, their potential to pass blood–brain barrier (BBB), and gene delivery are discussed.

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Carbon quantum dots and their biomedical and therapeutic applications: a review

In this work, nitrogen-doped carbon quantum dots (N-CQDs) with high fluorescence quantum yield (46.2%) were successfully synthesized through one-pot hydrothermal carbonization approach assisted by ethylenediamine as surface passivation agent, wherein alanine acted as a single carbon source, ethylenediamine as the surface passivation agent. The obtained biocompatible N-CQDs could effectively label both the MCF-7 cell membranes and the nucleus, suggesting the potential application in biolabeling and bioimaging. Furthermore, the addition of dihydronicotinamide adenine dinucleotide (NADH) could dramatically quench the fluorescence of N-CQDs. Therefore, a novel protocol for NADH determination was established and its mechanism was discussed. This proposed fluorescence sensor displayed an enhanced performance for NADH determination with the linear range up to 80 μM and the detection limit down to 25.1 nM.

Ethylenediamine-assisted hydrothermal synthesis of nitrogen-doped carbon quantum dots as fluorescent probes for sensitive biosensing and bioimaging

One-pot synthesis of nitrogen-rich carbon dots decorated graphene oxide as metal-free electrocatalyst for oxygen reduction reaction

Here we report a novel solid-state ECL sensor for ultrasensitive sensing of glutathione (GSH) based on ferrocyanide-ferricyanide redox couple (Fe(CN)6(3-/4-)) induced electrochemiluminescence (ECL) amplification of carbon dots (C-dots). The electropolymerization of C-dots and (11-pyrrolyl-1-yl-undecyl) triethylammonium tetrafluoroborate (A2) enabled immobilization of the hydrophilic C-dots on the surface of glassy carbon electrode (GCE) perfectly, while the excellent conductivity of polypyrrole was exploited to accelerate electron transfer between them. The Fe(CN)6(3-/4-) can expeditiously convert the C-dots and S2O8(2-) to C-dot(•-) and SO4(•-), respectively. High yields of the excited state C-dots (C-dots*) were obtained, and a ∼10-fold ECL amplification was realized. The C-dots* obtained through the recombination of electron-injected and hole-injected processes may be impeded due to the interference of GSH to K2S2O8. Therefore, the constructed sensor for GSH showed a detection limit down to 54.3 nM (S/N = 3) and a wide linear range from 0.1-1.0 μM with a correlation coefficient of 0.997.

Ferrocyanide-Ferricyanide Redox Couple Induced Electrochemiluminescence Amplification of Carbon Dots for Ultrasensitive Sensing of Glutathione.

The invention discloses a carbon quantum dot/aurum cluster ratiometric fluorescent probe for detection of cadmium ion and ascorbic acid. A preparation method comprises the following steps: preparing CQDs (Carbon Quantum Dots) from alanine and histidine through a one-step hydrothermal method; performing amino-functionalization on the CQDs by using 3-aminopropyltriethoxysilane, wherein the CQDs which are subjected to the amino-functionalization serve as a reference chromophore; reducing chloroauric acid through sodium borohydride by taking 11-sulfydryl undecanoic acid as a surfactant to obtain MUA-modified AuNCs (Aurum Nano Clusters), wherein the MUA-modified AuNCs serve as a main fluorophore of the ratiometric fluorescent probe; finally, coupling the CQDs and the AuNCs through an amidation reaction to obtain the CQDs/AuNCs ratiometric fluorescent probe. On the basis of static quenching and inter-filtering effects, the fluorescence of the CQDs/AuNCs can be quenched by Cd ; the invention discloses the application of the CQDs/AuNCs ratiometric fluorescent probe in Cd detection. The fluorescence of the CQDs/AuNCs which is quenched by the Cd can be recovered by the ascorbic acid, so that the ratiometric fluorescent probe can also be used for the detection of AA (Ascorbic Acid). The ratiometric fluorescent probe disclosed by the invention has the lower detection limit of 32.5 nM to the Cd , has the lower detection limit of 0.105 muM to the AA, and has an application value in the detection of the cadmium ion and the ascorbic acid.

Wen-Jun Niu

Papers

Ethylenediamine-assisted hydrothermal synthesis of nitrogen-doped carbon quantum dots as fluorescent probes for sensitive biosensing and bioimaging

One-pot synthesis of nitrogen-rich carbon dots decorated graphene oxide as metal-free electrocatalyst for oxygen reduction reaction

Ferrocyanide-Ferricyanide Redox Couple Induced Electrochemiluminescence Amplification of Carbon Dots for Ultrasensitive Sensing of Glutathione.

Carbon quantum dot/aurum cluster ratiometric fluorescent probe for detection of cadmium ion and ascorbic acid

Ferricyanide confined into the integrative system of pyrrolic surfactant and SWCNTs: The enhanced electrochemial sensing of paracetamol