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

Design strategies for water-soluble small molecular chromogenic and fluorogenic probes.

Carbon dots (CDs) possess unique optical properties such as tunable photoluminescence (PL) and excitation dependent multicolor emission. The quenching and recovery of the fluorescence of CDs can be utilized for detecting analytes. The PL mechanisms of CDs have been discussed in previous articles, but the quenching mechanisms of CDs have not been summarized so far. Quenching mechanisms include static quenching, dynamic quenching, Forster resonance energy transfer (FRET), photoinduced electron transfer (PET), surface energy transfer (SET), Dexter energy transfer (DET) and inner filter effect (IFE). Following an introduction, the review (with 88 refs.) first summarizes the various kinds of quenching mechanisms of CDs (including static quenching, dynamic quenching, FRET, PET and IFE), the principles of these quenching mechanisms, and the methods of distinguishing these quenching mechanisms. This is followed by an overview on applications of the various quenching mechanisms in detection and imaging.

The quenching of the fluorescence of carbon dots: A review on mechanisms and applications

Carbon dots: synthesis, formation mechanism, fluorescence origin and sensing applications

Carbon and graphene quantum dots (CQDs and GQDs), known as zero-dimensional (0D) nanomaterials, have been attracting increasing attention in sensing and bioimaging. Their unique electronic, fluores...

Review of Carbon and Graphene Quantum Dots for Sensing

Based on two kinds of hetero atom doping carbon dots (CD-1, CD-2) with brilliant fluorescent quantum yield and other fluorescence properties, two new sensors were prepared and characterized as novel fluorescent probes for detecting mercury ions in pure aqueous solution with a broad pH. Mercury ions can be captured by the carboxyl groups of these two kinds of carbon dots to form nonfluorescent complexes, resulting in a strong quenching. It suggested that both CD-1 and CD-2 exhibited high sensitivity and selectivity toward mercury ions: the linear ranges of CD-1 and CD-2 were estimated to be 1–12 μM and 1–15 μM while the limit of detection (LOD) was calculated to be 226 nM and 845 nM, respectively. Furthermore, these two kinds of CDs were applied to intracellular sensing and imaging of mercury ions as a consequence of the fluorescence properties and the established low cytotoxicity.

Highly photoluminescent carbon dots-based fluorescent chemosensors for sensitive and selective detection of mercury ions and application of imaging in living cells

Carbon dots (CDs) display tunable photoluminescence and excitation-wavelength dependent emission. The color of fluorescence is affected by electronic bandgap transitions of conjugated π-domains, surface defect states, local fluorophores and element doping. In this review (with 145 refs.), the studies performed in the past 5 years on the relationship between the fluorescence mechanism and modes for modulating the emission color of CDs are summarized. The applications of such CDs in sensors and assays are then outlined. A concluding section then gives an outlook and describes current challenges in the design of CDs with different emission colors. Graphical abstract Schematic representation of the relationship between the color-emitting (blue, green, yellow, red and multicolor) modulation of carbon dots and fluorescence mechanism including bandgap transitions of conjugated π-domains and surface defect states.

The fluorescence mechanism of carbon dots, and methods for tuning their emission color: a review

Surface functional groups strongly affect the properties of carbon dots (CDs). Amino, carboxy, and hydroxy groups are most commonly encountered in CDs, and they can be introduced via covalent and noncovalent modification. This article (with 116 refs.) reviews the progress made in the past few years. Following an introduction into the field, a large section covers methods for covalent modification (via amide coupling reactions, silylation, and other reactions including esterification, sulfonylation and copolymerization). Next section reviews methods for noncovalent modifications (π interactions, complexation/chelation, and electrostatic interactions). The resulting modified CDs are powerful nanomaterials for targeting and extracting analytes, and in drug release. The modification of the surface also affects fluorescence quantum yields, complexation capacity, the color of fluorescence, and their quenching capability. Current challenges are critically assessed in the concluding section.

Fanyong Yan

Papers

The quenching of the fluorescence of carbon dots: A review on mechanisms and applications

Highly photoluminescent carbon dots-based fluorescent chemosensors for sensitive and selective detection of mercury ions and application of imaging in living cells

The fluorescence mechanism of carbon dots, and methods for tuning their emission color: a review

Surface modification and chemical functionalization of carbon dots: a review

Formation of N, S-codoped fluorescent carbon dots from biomass and their application for the selective detection of mercury and iron ion.