Similar to its popular older cousins the fullerene, the carbon nanotube, and graphene, the latest form of nanocarbon, the carbon nanodot, is inspiring intensive research efforts in its own right. These surface-passivated carbonaceous quantum dots, so-called C-dots, combine several favorable attributes of traditional semiconductor-based quantum dots (namely, size- and wavelength-dependent luminescence emission, resistance to photobleaching, ease of bioconjugation) without incurring the burden of intrinsic toxicity or elemental scarcity and without the need for stringent, intricate, tedious, costly, or inefficient preparation steps. C-dots can be produced inexpensively and on a large scale (frequently using a one-step pathway and potentially from biomass waste-derived sources) by many approaches, ranging from simple candle burning to in situ dehydration reactions to laser ablation methods. In this Review, we summarize recent advances in the synthesis and characterization of C-dots. We also speculate on their future and discuss potential developments for their use in energy conversion/storage, bioimaging, drug delivery, sensors, diagnostics, and composites.

Luminescent Carbon Nanodots: Emergent Nanolights

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

Suitable labels are at the core of Luminescence and fluorescence imaging and sensing. One of the most exciting, yet also controversial, advances in label technology is the emerging development of quantum dots (QDs)--inorganic nanocrystals with unique optical and chemical properties but complicated surface chemistry--as in vitro and in vivo fluorophores. Here we compare and evaluate the differences in physicochemical properties of common fluorescent labels, focusing on traditional organic dyes and QDs. Our aim is to provide a better understanding of the advantages and limitations of both classes of chromophores, to facilitate label choice and to address future challenges in the rational design and manipulation of QD labels.

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Quantum dots versus organic dyes as fluorescent labels

Chemically derived graphene oxide (GO) is an atomically thin sheet of graphite that has traditionally served as a precursor for graphene, but is increasingly attracting chemists for its own characteristics. It is covalently decorated with oxygen-containing functional groups - either on the basal plane or at the edges - so that it contains a mixture of sp(2)- and sp(3)-hybridized carbon atoms. In particular, manipulation of the size, shape and relative fraction of the sp(2)-hybridized domains of GO by reduction chemistry provides opportunities for tailoring its optoelectronic properties. For example, as-synthesized GO is insulating but controlled deoxidation leads to an electrically and optically active material that is transparent and conducting. Furthermore, in contrast to pure graphene, GO is fluorescent over a broad range of wavelengths, owing to its heterogeneous electronic structure. In this Review, we highlight the recent advances in optical properties of chemically derived GO, as well as new physical and biological applications.

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Graphene oxide as a chemically tunable platform for optical applications

Two-dimensional graphene offers interesting electronic, thermal, and mechanical properties that are currently being explored for advanced electronics, membranes, and composites. Here we synthesize and explore the biological applications of nano-graphene oxide (NGO), i.e., single-layer graphene oxide sheets down to a few nanometers in lateral width. We develop functionalization chemistry in order to impart solubility and compatibility of NGO in biological environments. We obtain size separated pegylated NGO sheets that are soluble in buffers and serum without agglomeration. The NGO sheets are found to be photoluminescent in the visible and infrared regions. The intrinsic photoluminescence (PL) of NGO is used for live cell imaging in the near-infrared (NIR) with little background. We found that simple physisorption via π-stacking can be used for loading doxorubicin, a widely used cancer drug onto NGO functionalized with antibody for selective killing of cancer cells in vitro. Owing to its small size, intrinsic optical properties, large specific surface area, low cost, and useful non-covalent interactions with aromatic drug molecules, NGO is a promising new material for biological and medical applications.

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Nano-Graphene Oxide for Cellular Imaging and Drug Delivery.

We report that nanoscale carbon particles (carbon dots) upon simple surface passivation are strongly photoluminescent in both solution and the solid state. The luminescence emission of the carbon dots is stable against photobleaching, and there is no blinking effect. These strongly emissive carbon dots may find applications similar to or beyond those of their widely pursued silicon counterparts.

http://www.ecs.umass.edu/ece697mm/Supplement_Clemson_Cbased_Qdots_JAmChemSoc_05_06.pdf

Quantum-sized carbon dots for bright and colorful photoluminescence.

The supercritical fluid-processing technique, rapid expansion of a supercritical solution into a liquid solvent (RESOLV), was applied to the nanosizing of water-insoluble drug particles. Selected for demonstration were antiinflammatory drugs Ibuprofen and Naproxen, for which CO2 and CO2-cosolvent systems were used. The RESOLV process produces exclusively nanoscale (less than 100 nm) Ibuprofen and Naproxen particles suspended in aqueous solutions, and the aqueous suspensions of the drug nanoparticles are protected from particle agglomeration and precipitation by using common polymeric and oligomeric stabilization agents.

Nanosizing drug particles in supercritical fluid processing.

Rapid expansion of a supercritical solution into a liquid solvent (RESOLV) was applied to the nanosizing of ibuprofen particles in aqueous suspension. When the rapid expansion was into neat water, the initially formed nanoscale ibuprofen particles agglomerated into large aggregates. Various water-soluble polymers of different molecular weights were added to the aqueous receiving medium in RESOLV as stabilization agent for the produced nanoparticle suspension, and their effects on the properties of the harvested ibuprofen nanoparticles were evaluated. A conclusion is that the selection of stabilization agent in RESOLV may also be used to alter the sizes and morphology of the nanosized drug particles.

Pankaj Pathak

Papers

Quantum-sized carbon dots for bright and colorful photoluminescence.

Nanosizing drug particles in supercritical fluid processing.

Formation and stabilization of ibuprofen nanoparticles in supercritical fluid processing

Supercritical-fluid processing technique for nanoscale polymer particles.

Metal-coated nanoscale TiO2 catalysts for enhanced CO2 photoreduction