ReviewLuminescent graphene quantum dots as new fluorescent materials for environmental and biological applications
Introduction
Carbon-based nanomaterials, known for their different morphologies and unique properties, have inspired intensive research efforts over the past few decades. Since zero-dimensional fullerene [1], [2], one-dimensional carbon nanotubes (CNTs) [3], [4], [5] and two-dimensional graphene [6], [7] have been discovered in succession, they have been applied in the biological, chemical, material and environmental fields. Graphene, a novel single-atom-thick and two-dimensional nano graphitic carbon sheet, exhibits characteristics including large surface area, high carrier transport mobility, superior mechanical flexibility and excellent thermal and chemical stability, which are suitable for various applications in optoelectronic devices, energy-storage media and drug-delivery vehicles [8], [9], [10]. In addition to the superlative electronic properties, recent theoretical and experimental studies show that the distinctive properties of graphene can be realized by tuning the band gap of graphene via altering their size, destroying the integrity of the π system and controlling the chemical structures or layers of graphene [11], [12], [13], [14]. Consequently, graphene quantum dots (GQDs) were discovered very recently as a class of zero-dimensional graphitic nanomaterials with lateral dimensions less than 100 nm in single layer, double layers and a few layers (3 to <10) [15], [16], [17]. They are superior in terms of chemical inertness, ease of production, resistance to photobleaching, low cytotoxicity and excellent biocompatibility in comparison to conventional semiconductor QDs, thus making them promising in sensors, bioimaging, optoelectronic devices and so on [18], [19]. Moreover, similar to graphene, GQDs have excellent characteristics of large surface area, large diameter, fine surface grafting using the π–π conjugated network or surface groups and other special physical properties. Furthermore, the carboxyl and hydroxyl groups at their edge enable them to display excellent water solubility and suitability for successive functionalization with various organic, inorganic, polymeric or biological species [20], [21], [22], [23].
In this article, we aim to review the recent development of a protocol and a methodology for the preparation of GQDs and their applications based on their excellent properties. We summarize the currently available approaches to the preparation of GQDs, and reveal their characteristics associated with their size, shape, doping, surface modification, and reaction to pH and solvents. Subsequently, we describe the applications of GQDs in environmental and biological fields. In addition, we speculate on some critical issues for further exploration and possible ongoing developments of GQDs.
Section snippets
Approaches to synthesis
So far, the synthesis of GQDs with excellent photoluminescence (PL) properties can be divided into two strategies: size tuning and surface chemistry. Approaches to size tuning can generally be classified into top-down and bottom-up methods. Top-down methods include cutting large graphene-based materials into nanosized GQDs. But, bottom-up approaches involve preparing GQDs with organic molecules as a carbon source. Surface chemistry-tuned strategies include surface functionalization [20], [21],
Absorbance and PL
The absorption-peak position of GQDs depends on the preparation method and their size for a quantum-confinement effect. GQDs exhibited a characteristic absorption around 230 nm, attributed to π-π∗ transition of CC within the graphene structure, with a tail extending out into the visible range [15], [32], [36], [38], [45]. Some of the GQDs obtained had an absorption peak around a wavelength in the range 270–360 nm due to the n → π∗ transition of CO [23], [26], [30], [35], [37], [44], [55], [56]. For
Environmental
Recently, various optical sensors were constructed based on signal-off or signal-on processes because of the excellent luminescent properties of GQDs. A fluorescence-sensing platform based on GQDs was designed for ultrasensitive detection of 2,4,6-trinitrotoluene (TNT) using fluorescence resonance-energy transfer (FRET) quenching via the π–π stacking interaction between GQDs and the aromatic rings of TNT [64]. TNT bound on surface of the GQDs strongly suppressed the fluorescence emission from
Summary and outlook
As presented above, GQDs, as a new graphene-based nanomaterial, have inspired intensive research in environmental, biological and other fields because of their low cytotoxicity, excellent stability and resilience of PL in vivo. There are many available methods for preparing GQDs, which have special properties, such as absorption, PL and electroluminescence, which can be obtained by monitoring the band gap with unique size-tuning and functional-modification methods.
However, some issues remain
Acknowledgments
This research work was financially supported by the National Nature Scientific Foundation of China (No. 21175112, No. 21375112) and NFFTBS (No. J1030415), which are gratefully acknowledged. Furthermore, we would like to extend our thanks to Professor John Hodgkiss of The University of Hong Kong for his assistance with English.
References (78)
- et al.
Blue luminescent graphene quantum dots and graphene oxide prepared by tuning the carbonization degree of citric acid
Carbon
(2012) - et al.
Fluorescence resonance energy transfer quenching at the surface of graphene quantum dots for ultrasensitive detection of TNT
Talanta
(2012) - et al.
Chemically tailoring graphene oxides into fluorescent nanosheets for Fe3+ ion detection
Carbon
(2012) - et al.
Graphene quantum dots as a new substrate for immobilization and direct electrochemistry of glucose oxidase: application to sensitive glucose determination
Biosens. Bioelectron.
(2013) - et al.
Graphene quantum dots-based platform for the fabrication of electrochemical biosensors
Electrochem. Commun.
(2011) - et al.
Medicinal applications of fullerenes
Int. J. Nanomed.
(2007) - et al.
Fullerene for organic electronics
Chem. Soc. Rev.
(2009) - et al.
Carbon nanotubes: present and future commercial applications
Science
(2013) - et al.
A review of fabrication and applications of carbon nanotube film-based flexible electronics
Nanoscale
(2013) Science and technology of the twenty-first century: synthesis, properties, and applications of carbon nanotubes
Annu. Rev. Mater. Res.
(2003)