Journal of Materials Chemistry C FEATURE ARTICLE View Article Online View Journal | View Issue Carbon quantum dots: synthesis, properties and applications Cite this: J. Mater. Chem. C,2014,2, 6921 Youfu Wang and Aiguo Hu* Carbon quantum dots (CQDs, C-dots or CDs), which are generally small carbon nanoparticles (less than Received 13th May 2014 10 nm in size) with various unique properties, have found wide use in more and more fields during the Accepted 16th June 2014 last few years. In this feature article, we describe the recent progress in the field of CQDs, focusing on DOI: 10.1039/c4tc00988f their synthetic methods, size control, modification strategies, photoelectric properties, luminescent www.rsc.org/MaterialsC mechanism, and applications in biomedicine, optronics, catalysis and sensor issues. 1. Introduction quantum dots because of their good solubility and strong luminescence, for which they are referred to as carbon Carbon-based quantum dots consisting of graphene quantum nanolights.3 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. dots (QGDs) and carbon quantum dots (CQDs, C-dots or CDs) During the past few years, much progress has been achieved are a new class of carbon nanomaterials with sizes below 10 nm. in the synthesis, properties and applications of carbon-based They were rst obtained during the purication of single-walled quantum dots, as reviewed by Baker et al.,3 Lee et al.,4 and Zhu carbon nanotubes through preparative electrophoresis in 2004,1 et al.5 Compared to traditional semiconductor quantum dots and then via laser ablation of graphite powder and cement in and organic dyes, photoluminescent carbon-based quantum 2006.2 Carbon-based quantum dots with fascinating properties dots are superior in terms of high (aqueous) solubility, robust have gradually become a rising star as a new nanocarbon chemical inertness, facile modication and high resistance to member due to their benign, abundant and inexpensive photobleaching. The superior biological properties of carbon- nature.3 Carbon is commonly a black material, and was gener- based quantum dots, such as low toxicity and good biocom- This article is licensed under a ally considered to have low solubility in water and weak uo- patibility, entrust them with potential applications in bio- rescence. Wide attention has been focused on carbon-based imaging, biosensor and biomolecule/drug delivery. The outstanding electronic properties of carbon-based quantum Open Access Article. Published on 17 June 2014. Downloaded 10/1/2021 4:06:05 PM. dots as electron donors and acceptors, causing chem- Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials iluminescence and electrochemical luminescence, endow them Science and Engineering, East China University of Science and Technology, with wide potentials in optronics, catalysis and sensors. Shanghai, 200237, China. E-mail: [email protected]; Fax: +86-21-64253037; Tel: +86-21-64253037 Youfu Wang was born in Anqing Aiguo Hu received his Ph.D. City, Anhui province of China. degree from Nankai University He received his B.Sc. degree in 2001 (with Prof. Jitao Wang). from East China University of Aer working as postdoctoral Science and Technology fellow in the University of North (ECUST, China) in 2011. Then, Carolina at Chapel Hill (USA) he was admitted to pursue his (with Prof. Wenbin Lin), he Ph.D. degree in ECUST under returned to China and kicked off the supervision of Professor his independent research at Aiguo Hu. His current research ECUST in 2007. He was focuses on synthetic polymer promoted as an “Eastern chemistry and rationally Scholar Professor” in 2011. His designed carbon nanomaterials main research interests are for electronic applications. synthetic polymer chemistry, functional nanostructure, rationally designed carbon materials for electronic applications, and func- tionalized polymers for pharmaceutical applications. This journal is © The Royal Society of Chemistry 2014 J. Mater. Chem. C,2014,2, 6921–6939 | 6921 View Article Online Journal of Materials Chemistry C Feature Article can be tuned during preparation or post-treatment. We will discuss the main methods for CQDs synthesis, the size control via conned pyrolysis and the modication of CQDs, including functionalization, doping and nanohybrids. The features of different synthetic methods for the preparation of CQDs are summarized in Table 1. 2.1. Synthetic methods 2.1.1. Chemical ablation. Strong oxidizing acids carbonize small organic molecules to carbonaceous materials, which can Fig. 1 CQDs with unique properties have great potential in biomedi- be further cut into small sheets by controlled oxidation.9–14 This cine, optronics, catalysis and sensors. method may suffer from harsh conditions and drastic processes. Peng and Travas-Sejdic reported a simple route to prepare luminescent CQDs in an aqueous solution by dehy- As the properties and applications of GQDs have been drating carbohydrates with concentrated H2SO4, followed by – systematically summarized elsewhere,5 7 we will focus our breaking the carbonaceous materials into individual CQDs with horizons on CQDs due to their unique properties and great HNO3, and nally passivating with amine-terminated potential in various applications (Fig. 1). In this feature article, compounds (4,7,10-trioxa-1,13-tridecanediamine).13 The surface we describe the recent progress in the eld of CQDs, focusing passivation was essential for the photoluminescence (PL) of on their synthetic methods, size control, modication strate- these CQDs. The emission wavelength of these CQDs can be gies, optical properties, luminescent mechanism, and applica- tuned by differing the starting material and the duration of the tions in biomedicine, optronics, catalysis and sensor issues. Creative Commons Attribution-NonCommercial 3.0 Unported Licence. nitric acid treatment. The multicolor emission capabilities and nontoxic nature of these CQDs enable them to be applied in life 2. Synthesis, size control and science research. modification Photoluminescent CQDs were synthesized in one-pot using polyethylenimine (PEI), a cationic branched polyelectrolyte, as Many methods have been proposed to prepare CQDs during the both a carbon source and passivating agent via HNO3 oxida- 14 last decade, which can be roughly classied into “Top-down” tion. In contrast to the commonly reported pH-insensitive and “Bottom-up” approaches, and they can be modied during CQDs, the PL of these CQDs was highly pH-sensitive, i.e., the PL preparation or post-treatment (Fig. 2). Three problems facing intensity decreased with increasing pH from pH 2 to 12. In This article is licensed under a CQDs preparation need to be noticed: (i) carbonaceous aggre- addition, the pH response of the PL behaviour was reversible. gation during carbonization, which can be avoided by using This property endows them the potential to serve as proton electrochemical synthesis, conned pyrolysis or solution sensors in monitoring cell metabolization processes with Open Access Article. Published on 17 June 2014. Downloaded 10/1/2021 4:06:05 PM. chemistry methods, (ii) size control and uniformity, which is proton release. When incubated with HeLa cells, the CQDs important for uniform properties and mechanistic study, and could readily penetrate the cell membrane and exhibit low can be optimized via post-treatment, such as gel electropho- cytotoxicity and favorable biocompatibility, which is essential resis, centrifugation, and dialysis and (iii) surface properties for HeLa cell imaging. that are critical for solubility and selected applications, which 2.1.2. Electrochemical carbonization. Electrochemical soaking is a powerful method to prepare CQDs using various bulk carbon materials as precursors.15–21 However, there are only a few reports about electrochemically carbonizing small molecules to CQDs. Preparation of CQDs via the electro- chemical carbonization of low-molecular-weight alcohols is proposed by Zhang and co-workers.22 Two Pt sheets were used as the working and auxiliary electrode, and a calomel electrode mounted on a freely adjustable Luggin capillary was used as the reference electrode. The alcohols were transformed into CQDs aer electrochemical carbonization under basic conditions.21 The sizes and graphitization degrees of these CQDs increase with the increasing applied potential. The resultant CQDs with amorphous core exhibited excellent excitation- and size- dependent PL properties without complicated purication and passivation procedures. Note that the quantum yields (QYs) of Fig. 2 Schematic illustration of CQDs preparation via “top-down” and “bottom-up” approaches, and modification including functionaliza- these CQDs can reach 15.9%. CQDs can be prepared from tion, doping and nanohybrids. Adapted with permission.8 Copyright different small molecular alcohols showing low toxicity to 2013, Royal Society of Chemistry. human cancer cells. 6922 | J. Mater. Chem. C,2014,2, 6921–6939 This journal is © The Royal Society of Chemistry 2014 View Article Online Feature Article Journal of Materials Chemistry C Table 1 The features of the different synthetic methods used for the preparation of CQDs Synthetic Methods Advantages Disadvantages Ref. Chemical ablation Most accessible, various sources Harsh conditions, drastic processes, 9–14 multiple-steps, poor control over sizes Electrochemical carbonization Size and nanostructure are controllable, Few small molecule precursors 15–22 stable, one-step Laser ablation Rapid, effective, surface states tunable