Carbon nanomaterials had extensive studied and received great attention since the discovery of Buckminster fullerene [1]. Carbon dots (CDs) consist, graphene, carbon quantum dots (CQDs), carbon nanotubes (CNTs), carbon nano onions (CNOs) and now it’s a new, important fluorescent carbon nanomaterial with below than 10 nm in size. These carbon nano materials had strong luminescent property and highly soluble in aqueous medium [2]. An experiment designed to understand the procedure of the preparation of long chain carbon moieties interstellar space, took the place of fullerene. The characterization of carbon nanotubes (CNTs) was made after this [3]. After a year of the discovery of CNT, another member of carbon family, carbon nano onion (CNO) or carbon quantum dot (CQD) was discovered [4], an ideal member of the carbon allotropes, which made up of concentric graphitic shells [5]. It was first synthesized and obtained by Iijima in 1980 [6], in vacuum deposited amorphous carbon films by high resolution TEM (Fig. 1). After discovery CNO, Ugarte observed a transformation of CNT to CNO [4]. CNO had a quasi-spherical onion like structure composed of concentric graphitic layers with an inner core, which either hollow or encapsulated with metal. Because of the ideal structure, potential applications of CNO for lubricants [7, 8], magnetic storage materials [9], devices [10] and electrochemical capacitors [11]. Carbon quantum dots (CQDs) had amorphous sp²-conjugated nanocrystalline core with sp²/sp³ hybridization and consist highly abundant oxygen groups (like carboxyl, hydroxyl, aldehyde etc) on the surface [12], which consider those nano materials as water soluble CQDs (Fig. 2). They had wonderful electron donor-acceptor induced photoluminescence and fluorescence properties. Due to photoluminescence property, critical to re-size and change the surface chemical groups of carbon dots [13]. Doping with other elements also used to increase their properties [14]. Carbon quantum dots or carbon dots, consider as next generation material in nanomaterial technology which had unique optical and electronic properties.

In comparison of other quantum dot semiconductors, organic agents, and other fluorescent sensors, carbon quantum dots (CQD) had very interesting properties like unique emissions with various fluorescence excitation, chemical compositions, cheap and simple synthesis, functionalization of surface and modification, photo-chemical stability [2], which proved CQDs or CDs could be apply in different technical, medical applications. Carbon dots easily played favourable roles in applications due to their simple and cheap synthesis, easily water soluble and biocompatible [15-18]. Those fluorescent carbon  nano materials attract the interest of researchers due to their wide variety of applications like bioimaging, bioanalysis, sensor, drug delivery, photocatalysis, optoelectronics, electrocatalysis, fuel cell and many more [19].

Many fluorescent quantum dots like graphene quantum dots (GQDs) [20-24], polymer dots (PDs) [25, 26], carbon nanotube (CNT) dots [27, 28], nano diamonds (NDs) [29-33], and CQDs synthesised previously. According to density of states (DOS), quantum dots had distinct materials with compact atoms. When the size of QD particles decreased upto below the size, bandgap energy increased. Because of electron-hole radiative recombination, effect of quantum-confinement did not relevant to carbon dots. Carbon quantum dots consist small size with high surface defects. Size-dependent optical properties because of various surface defect and surface to volume ratio [34]. Here, we described the synthesis, characterization and properties of carbon quantum dots with their application in various areas.

Fig.1. Spherical particles of graphitic carbon nano-onions found in the amorphous carbon films, (a – d). (scale bar 2 nm) [6]

Fig.2. Structure of carbon quantum dots (CQDs) [35]. 

2. Synthesis of nano carbon quantum dots (CQDs):

Carbon quantum dots (CQDs), serendipitously determined in the  process of single wall carbon nano tubes (SWCNTs) purification [36]. Suspension of SWCNTs on gel electrophoresis, differentiated into three important classes of materials, included a fast moving band of high fluorescent material which showed  size dependent fluorescent properties. It was  first discovery of carbon nanoparticles or carbon dots. CDs contain contained lower amount of carbon with high oxygenated groups, also called carbogenic nanodots [17]. Baker et al. described different synthesis process and properties of carbon nanoparticles. Synthesis of CDs divided into two classes of methods; top-down and bottom-up. Top down method included synthesis of nanoparticles from large carbon structure. And other hand, in bottom-up method CDs prepared by molecular precursor.

2.1 Top-down methods:

Firstly, fluorescence nano materials CDs stable graphene sheets, carbon fibres, CNTs, CNOs, carbon soots etc, classified as following methods:

(i) Arc-discharge method: Bottini et al. [37] used electric arc technology to synthesized luminescence CDs through pristine CNTs and oxidized (with nitric acid) CNTs.

(ii) Laser ablation method: Sun's et al. [38] synthesized CDs via Q-switched Nd : YAG laser ablation at 900 °C and 75 kPa, by graphite and cement mixture hot pressing. Carbon dot’s surface were passivated via diamine-terminated oligomeric PEG_1500N with passing acid; which attached with organic moieties on the surface of CDs obtained (Fig. 3a).

(iii) Hydrothermal cutting method: Pan et al. [39] synthesized CDs via  hydrothermal method, in it cutting graphene sheets into 10 nm sized functionalized graphene quantum dots. These functionalized GQDs exhibited blue fluorescence which induced through edge effect.

(iv) Chemical oxidation method: Zhou et al. [40] reported that carboxylic group attached onto carbon nano materials or GQDs surface, synthesized through oxidation reaction under UV irradiation, in between graphene oxide (GO) and Fenton reagent (Fe²⁺/Fe³⁺/H₂O₂).

(v) Electrochemical oxidation method: Zhou et al. described [41], synthesis of  CD through multiwall carbon nanotubes which fabricated by electrochemical oxidation method (Fig. 3b). 

Fig.3. Schematic view of synthesis of (a) fluorescent carbon nano dots (CDs) via laser ablation and attached with PEG on the surface [38]