Fluorescent carbon dots and their sensing applications Xiangcheng Sun a, * , Yu Lei b a Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, USA b Department of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT 06269, USA article info Article history: Available online 13 February 2017 Keywords: Fluorescent carbon dots Sensing mechanism Sensor design Sensing applications Future directions abstract Fluorescent carbon dots (CDs) are a new class of carbon nanomaterials and have demonstrated excellent optical properties, good biocompatibility, great aqueous solubility, low cost, simple synthesis, etc. Since their discovery, various synthesis methods using different precursors have been developed, and are mainly classified as top-down and bottom-up approaches. For the mechanistic origin of CDs photo- luminescence, three mechanisms have been proposed such as quantum confinement effect, surface state and molecule state. CDs have presented many applications, and this review article mainly focuses on the development of CDs based fluorescent sensors in recent 5 years. The sensing mechanisms, senor design and sensing properties to various targets are summarized. A broad range of analytes including cations, anions, small molecules, macromolecules, cells and bacteria have been discussed. In addition, the challenges and future directions for CDs as sensing materials are also presented. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Fluorescence (FL) based sensing has received much attention in recent years due to the benefits such as excellent sensitivity, short response time, and low cost. Various fluorescent sensory materials have been designed and developed including organic dyes, quan- tum dots, metal-organic frameworks, fluorescent proteins, etc. While each material offers advantages, their use is not without problem. Carbon dots (CDs, also called as carbon quantum dots, or carbon nanoparticles (CNPs)), which are one kind of newly discovered fluorescent nanomaterials, have become increasingly popular in the past decade due to their unique optical properties, good biocom- patibility, low toxicity, great aqueous stability, facile synthesis, etc. Carbon dots are generally quasi-spherical nanoparticles comprising amorphous to nanocrystalline and always consist of sp 2 /sp 3 carbon, oxygen/nitrogen based groups, and post-modified chemical groups [1,2]. To possess fluorescent properties, the size and surface chem- ical groups of carbon dots should be carefully modulated. Especially, the emission of CDs could be adjusted by controlling the conden- sation reaction, chemical manipulations, or most commonly by doping in of other elements [3e6]. Nitrogen (N) is by far the most prominent doping candidate, while boron (B), sulfur (S) and phos- phorus (P) were also used in combination with nitrogen. Ironically, CDs were accidentally discovered in 2004, after sep- aration and purification of single walled carbon nanotubes syn- thesized by arc-discharge methods [7]. Since then, the simplicity of synthesis and unique properties of CDs has resulted in an explosion in the number of publications employing different kinds of pre- cursors and synthesis methods, which were mainly classified as top-down and bottom-up approaches. In top-down methods, the nanoparticles are formed by breaking down larger pieces of materials into the desired nanostructures through arc discharge, laser ablation, chemical, or electrochemical oxidation. A variety of carbon materials have been applied in this approach such as nanodiamonds, graphite, carbon nanotubes, car- bon soot, activated carbon. For example, Sun and co-workers pre- pared CDs through laser ablation of a carbon target (such as graphite and cement) in the presence of water vapor with argon as a carrier gas under 900  C and 75 kPa [8]. Du's group reported one- step synthesis of fluorescent carbon dots by laser irradiation of a suspension of carbon materials in an organic solvent, which helped modify and achieve tunable light emission [9]. They claimed that the origin of the luminescence was attributed to the surface states related to the carboxylate ligands on the surface of the CDs. Elec- trochemical method for carbon dots synthesis was firstly described by Zhou et al. through growing multi-walled carbon nanotubes on a carbon paper in an electrochemical cell [10]. In bottom-up approaches, the CDs are obtained from assembling molecular precursors (such as, citrate, carbohydrates, biomaterials, polymer-silica nanocomposite, etc.) under a range of different * Corresponding author. E-mail address: xs266@cornell.edu (X. Sun). Contents lists available at ScienceDirect Trends in Analytical Chemistry journal homepage: www.elsevier.com/locate/trac http://dx.doi.org/10.1016/j.trac.2017.02.001 0165-9936/© 2017 Elsevier B.V. All rights reserved. Trends in Analytical Chemistry 89 (2017) 163e180