This journal is © the Owner Societies 2016 Phys. Chem. Chem. Phys. Cite this: DOI: 10.1039/c6cp01528j Graphene quantum dots from graphite by liquid exfoliation showing excitation-independent emission, fluorescence upconversion and delayed fluorescence† Suprabhat Sarkar, ab Dayakar Gandla, ab Yeduru Venkatesh, bc Prakriti Ranjan Bangal, bc Sutapa Ghosh,* ab Yang Yang d and Sunil Misra e Facile synthesis of 2–10 nm-sized graphene quantum dots (GQDs) from graphite powder by organic solvent-assisted liquid exfoliation using a sonochemical method is reported in this study. Synthesized GQDs are well dispersed in organic solvents like ethyl acetoacetate (EAA), dimethyl formamide (DMF) and also in water. MALDI-TOF mass spectrometry reveals its selective mass fragmentation. Detailed characterizations by various techniques like X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and high resolution transmission electron microscopy (HRTEM) confirm the formation of disordered, functional GQDs. Density functional theory (DFT) calculation confirms HOMO–LUMO energy gap variation with changing size and functionalities. Photoluminescence (PL) properties of as-prepared GQDs were studied in detail. The ensemble studies of GQDs showed excellent photoluminescence properties comprising normal and upconverted fluorescence, delayed fluorescence and room-temperature phosphorescence. PL decay dynamics of GQDs has been explored using time-correlated single-photon technique (TCSPC) as well as femtosecond fluorescence upconversion technique. In vitro cytotoxicity study reveals its biocompatibility and high cell viability (491%) even at high concentration (400 mg mL À1 ) of GQDs in Chinese Hamster Ovary (CHO) cells. 1. Introduction A tremendous amount of experimental and theoretical research work has been done to introduce band gaps into graphene. 1 It can be noted that the band gap of graphene is opened up by reducing its dimensionality due to quantum confinement. 2,3 Stable zero-dimensional graphene quantum dots (GQDs) (band gap, 3 eV) can be made by reducing its dimension (band gap, 0.4 eV). 4,5 The newest member of the quantum dot family was discovered only in 2010 and has the prospect of utilization in light-emitting diodes (LEDs), 6,7 electroluminescence, 8 organic photovoltaic diodes (OPDs), 9 solar cells, 10,11 bio-imaging, 12 sensors, 13 photodetector applications 14 and optically addressable qubits in quantum information science. 15,16 The photophysics of GQDs is vital for various applications, 17 and thus it is essential to have a detailed understanding of their photophysical properties. 18,19 Blue or green fluorescent GQDs can be produced by simple oxidative cutting of carbon sources (e.g., graphite, carbon nano- tubes, carbon fibers, C 60 , etc.) 20–24 using modified Hummers’ methods, hydrothermal treatment, oxygen plasma treatment, one-pot electrochemistry method, separation of carbon sources or by using organic synthesis starting from small molecules. 25–27 Solution-based synthetic routes to obtain GQDs with better stability and more uniform structure are reported by Mullen’s and Li’s groups. 28,29 Luminescence mechanism can be explained from quantum size effect, edge effect and defect effect (energy traps). 30–32 In general, PL of GQDs can be analyzed by combining the effects of intrinsic emission and defect emission, which requires detailed analysis and experiments. Device applications of fluorescent GQDs require further investigations and optimizations of its fundamental properties. Many researchers have tried to make water-soluble GQDs, 33,34 which are eco-friendly, chemically inert 35 and with low toxicity compared to traditional quantum dots (QDs) of toxic heavy a Nanomaterials Laboratory, Inorganic and Physical Chemistry Division, CSIR-IICT, Hyderabad-500007, T.S., India. E-mail: sghosh@iict.res.in b Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, New Delhi-110001, India c Inorganic and Physical Chemistry Division, CSIR-IICT, Hyderabad-500007, T.S., India d Chemistry and Biochemistry Division, Rowan University, Glassboro, New Jersey, 08028, USA e Biology Division, CSIR-IICT, Hyderabad-500007, T.S., India † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cp01528j Received 5th March 2016, Accepted 1st June 2016 DOI: 10.1039/c6cp01528j www.rsc.org/pccp PCCP PAPER Published on 02 June 2016. Downloaded by Indian Institute of Chemical Technology (IICT), Hyderabad on 15/06/2016 13:08:30. View Article Online View Journal