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

Box-Behnken design: An alternative for the optimization of analytical methods

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

Tracing dissolved organic matter in aquatic environments using a new approach to fluorescence spectroscopy

Carbon dots (CDs) comprise a recently discovered class of strongly fluorescent, emission-color-tuning and non-blinking nanoparticles with great analytical and bioanalytical potential. Raw CDs can be obtained by laser ablation or electrochemical exfoliation of graphite, from soot, or thermal carbonization, acid dehydration or ultrasonic treatment of molecular precursors. Passivation of raw CDs makes them fluorescent and their functionalization confers reactivity towards selected targets. CDs can be excited by single-photon (ultraviolet or near-ultraviolet) and multi-photon (red or near-infrared) excitation, and their luminescence properties are due to surface defects. CDs are being proposed as bioimaging probes because they comprise non-toxic elements and are biocompatible. Passivated and functionalized CDs can be made to sense pH, metal ions and molecular substances.

Analytical and bioanalytical applications of carbon dots

The synthesis of carbon nanoparticles obtained by direct laser ablation [UV pulsed laser irradiation (248 nm, KrF)] of carbon targets immersed in water is described. Laser ablation features were optimized to produce carbon nanoparticles with dimensions up to about 100 nm. After functionalization with NH 2 –polyethylene-glycol (PEG 200 ) and N-acetyl- l -cysteine (NAC) the carbon nanoparticles become fluorescent with excitation and emission wavelengths at 340 and 450 nm, respectively. The fluorescence decay time was complex and a three-component decay time model originated a good fit ( χ  = 1.09) with the following lifetimes: τ 1  = 0.35 ns; τ 2  = 1.8 ns; and τ 3  = 4.39 ns. The fluorescence of the carbon dots is sensitive to pH with an apparent p K a  = 4.2. The carbon dots were characterized by 1 H NMR and HSQC and the results show an interaction between PEG 200 and the carbon surface as well as a dependence of the chemical shift with the reaction time. The fluorescence intensity of the nanoparticles is quenched by the presence of Hg(II) and Cu(II) ions with a Stern–Volmer constant (pH = 6.8) of 1.3 × 10 5 and 5.6 × 10 4  M −1 , respectively. As such the synthesis and application of a novel biocompatible nanosensor for measuring Hg(II) is presented.

/pdf/hg-ii-sensing-based-on-functionalized-carbon-dots-obtained-1vyhq0bs32.pdf

Hg(II) sensing based on functionalized carbon dots obtained by direct laser ablation

Summary Luciferase is a general term for enzymes catalyzing visible light emission by living organisms (bioluminescence). The studies carried out with Photinus pyralis (firefly) luciferase allowed the discovery of the reaction leading to light production. It can be regarded as a two-step process: the first corresponds to the reaction of luciferase’s substrate, luciferin (LH2), with ATPMg 21 generating inorganic pyrophosphate and an intermediate luciferyl-adenylate (LH2-AMP); the second is the oxidation and decarboxylation of LH2-AMP to oxyluciferin, the light emitter, producing CO2, AMP, and photons of yellow-green light (550– 570 nm). In a dark reaction LH2-AMP is oxidized to dehydroluciferyl-adenylate (L-AMP). Luciferase also shows acyl-coenzyme A synthetase activity, which leads to the formation of dehydroluciferyl-coenzyme A (L-CoA), luciferyl-coenzyme A (LH2CoA), and fatty acyl-CoAs. Moreover luciferase catalyzes the synthesis of dinucleoside polyphosphates from nucleosides with at least a 3 0 -phosphate chain plus an intact terminal pyrophosphate moiety. The LH2 stereospecificity is a particular feature of the bioluminescent reaction where each isomer, D-LH2 or L-LH2, has a specific function. Practical applications of the luciferase system, either in its native form or with engineered proteins, encloses the analytical assay of metabolites like ATP and molecular biology studies with luc as a reporter gene, including the most recent and increasing field of bioimaging. 2008 IUBMB IUBMB Life, 61(1): 6–17, 2009

https://iubmb.onlinelibrary.wiley.com/doi/pdf/10.1002/iub.134

Firefly bioluminescence: a mechanistic approach of luciferase catalyzed reactions.

https://recipp.ipp.pt/bitstream/10400.22/3034/1/ART_HelenaGoncalves_2010_GRAQ.pdf

Joaquim C.G. Esteves da Silva

Papers

Analytical and bioanalytical applications of carbon dots

Hg(II) sensing based on functionalized carbon dots obtained by direct laser ablation

Firefly bioluminescence: a mechanistic approach of luciferase catalyzed reactions.

Optical fiber sensor for Hg(II) based on carbon dots.

The degradation products of UV filters in aqueous and chlorinated aqueous solutions.