Fluorescent carbon nanoparticles or carbon quantum dots (CQDs) are a new class of carbon nanomaterials that have emerged recently and have garnered much interest as potential competitors to conventional semiconductor quantum dots. In addition to their comparable optical properties, CQDs have the desired advantages of low toxicity, environmental friendliness low cost and simple synthetic routes. Moreover, surface passivation and functionalization of CQDs allow for the control of their physicochemical properties. Since their discovery, CQDs have found many applications in the fields of chemical sensing, biosensing, bioimaging, nanomedicine, photocatalysis and electrocatalysis. This article reviews the progress in the research and development of CQDs with an emphasis on their synthesis, functionalization and technical applications along with some discussion on challenges and perspectives in this exciting and promising field.

Carbon quantum dots and their applications

Carbon nanodots (C-dots) have generated enormous excitement because of their superiority in water solubility, chemical inertness, low toxicity, ease of functionalization and resistance to photobleaching. In this review, by introducing the synthesis and photo- and electron-properties of C-dots, we hope to provide further insight into their controversial emission origin (particularly the upconverted photoluminescence) and to stimulate further research into their potential applications, especially in photocatalysis, energy conversion, optoelectronics, and sensing.

Carbon nanodots: synthesis, properties and applications

At present, the actual mechanism of the photoluminescence (PL) of fluorescent carbon dots (CDs) is still an open debate among researchers. Because of the variety of CDs, it is highly important to summarize the PL mechanism for these kinds of carbon materials; doing so can guide the development of effective synthesis routes and novel applications. This review will focus on the PL mechanism of CDs. Three types of fluorescent CDs were involved: graphene quantum dots (GQDs), carbon nanodots (CNDs), and polymer dots (PDs). Four reasonable PL mechanisms have been confirmed: the quantum confinement effect or conjugated π-domains, which are determined by the carbon core; the surface state, which is determined by hybridization of the carbon backbone and the connected chemical groups; the molecule state, which is determined solely by the fluorescent molecules connected on the surface or interior of the CDs; and the crosslink-enhanced emission (CEE) effect. To give a thorough summary, the category and synthesis routes, as well as the chemical/physical properties for the CDs, are briefly introduced in advance.

The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots, and polymer dots): current state and future perspective

Carbon quantum dots (CQDs, C-dots or CDs), which are generally small carbon nanoparticles (less than 10 nm in size) with various unique properties, have found wide use in more and more fields during the last few years. In this feature article, we describe the recent progress in the field of CQDs, focusing on their synthetic methods, size control, modification strategies, photoelectric properties, luminescent mechanism, and applications in biomedicine, optronics, catalysis and sensor issues.

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Carbon quantum dots: synthesis, properties and applications

Carbon nanodots as fluorescence probes for rapid, sensitive, and label-free detection of Hg2+ and biothiols in complex matrices

In this work, chemiluminescent (CL) property of the carbon dots in the presence of peroxynitrous acid was studied. Peroxynitrous acid is formed by online mixing of nitrite and acidified hydrogen peroxide. The CL intensity was increased linearly with nitrite concentration in the range from 1.0 × 10–7 M to 1.0 × 10–5 M, and the detection limit was 5.3 × 10–8 M (signal-to-noise ratio of S/N = 3). This method has been successfully applied to the determination of nitrites in pond water, river water, and pure milk, with recoveries in the range of 98%–108%. The CL mechanism of the peroxynitrous acid–carbon dots system was investigated using the CL, ultraviolet–visible light (UV–vis), and electron paramagnetic resonance (EPR) spectra. The electron-transfer annihilation of hole-injected and electron-injected carbon dots could mainly account for the CL emission, which sheds new light on the optical properties of the carbon dots.

Peroxynitrous-acid-induced chemiluminescence of fluorescent carbon dots for nitrite sensing.

Point of care testing for infectious diseases

Production of superoxide anion radicals as evidence for carbon nanodots acting as electron donors by the chemiluminescence method

In this work, carbon nanodots were synthesized through a novel solvothermal route, and the effects of carbon nanodots on the ultraweak chemiluminescence (CL) reaction of hydrogen peroxide (H2O2) and sodium bisulfite (NaHSO3) were explored for the first time. It was found that the CL emission intensity of H2O2–HSO3– was significantly enhanced by carbon nanodots: about 60-fold increase in the CL intensity was obtained. The enhanced CL was induced by the excited-state carbon nanodots (CD*), which could be produced from the electron-transfer annihilation of positively charged carbon nanodots (CD•+) and negatively charged carbon nanodots (CD•–). Radical scavengers such as nitro blue tetrazolium chloride (NBT), sodium azide, thiourea, 5,5-dimethyl-1-pyrroline N-oxide, and ascorbic acid were used to study the intermediate species. The intermediate radicals generated during the reaction of H2O2 and NaHSO3, such as hydroxide radical (•OH), sulfate anionic radical (SO4•–), superoxide anionic radical (•O2–), and sul...

Enhancement of Ultraweak Chemiluminescence from Reaction of Hydrogen Peroxide and Bisulfite by Water-Soluble Carbon Nanodots

The ultraweak chemiluminescence (CL) from the reaction of hydrogen peroxide and carbonate is strongly enhanced by the branched NaYF(4):Yb(3+)/Er(3+) nanoparticle (NP) in the presence of aqueous ammonia. It was explained that ammonia catalyzes the decomposition of peroxymonocarbonate, which is the product of hydrogen peroxide mixing with bicarbonate, making the formation of (CO(2))(2)*, (O(2))(2)*, and (1)O(2). The excitation energy, carried by these emitter intermediates, can be transferred to NaYF(4):Yb(3+)/Er(3+) NP. The CL intensity is directly proportional to the concentration of ammonia present in the solution. A flow-injection CL system with high sensitivity, selectivity, and reproducibility is proposed for the determination of aqueous ammonia. The proposed method exhibited advantages in a larger linear range from 0.5 μmol L(-1) to 50 μmol L(-1) and a lower detection limit of 1.1 × 10(-8) mol L(-1) (S/N = 3). This method has been successfully applied to the evaluation of ammonia in water samples with recoveries from 95% to 108%. The relative standard deviations are 1.8% and 4.1% for intra-assay and inter-assay precision, respectively.

Determination of ammonia in water based on chemiluminescence resonance energy transfer between peroxymonocarbonate and branched NaYF4:Yb3+/Er3+ nanoparticles.

Hui Chen

Papers

Peroxynitrous-acid-induced chemiluminescence of fluorescent carbon dots for nitrite sensing.

Point of care testing for infectious diseases

Production of superoxide anion radicals as evidence for carbon nanodots acting as electron donors by the chemiluminescence method

Enhancement of Ultraweak Chemiluminescence from Reaction of Hydrogen Peroxide and Bisulfite by Water-Soluble Carbon Nanodots

Determination of ammonia in water based on chemiluminescence resonance energy transfer between peroxymonocarbonate and branched NaYF4:Yb3+/Er3+ nanoparticles.