Preparation of Graphene Quantum Dots from Pyrolyzed Alginate Pedro Atienzar, †,§ Ana Primo, †,§ Cristina Lavorato, †,‡ Raffaele Molinari, ‡ and Hermenegildo García* ,† † Instituto Universitario de Tecnología Química CSIC-UPV, Univ. Politecnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain ‡ Department of Environmental and Chemical Engineering, University of Calabria, Via P. Bucci, 44/A, 87036 Rende, CS, Italy ABSTRACT: Pyrolysis at 900 °C under an inert atmosphere of alginate, a natural widely available biopolymer, renders a graphitic carbon that upon ablation by exposure to a pulsed 532 nm laser (7 ns, 50 mJ pulse -1 ) in acetonitrile, water, and other solvents leads to the formation of multilayer graphitic quantum dots. The dimensions and the number of layers of these graphitic nanoparticles decrease along the number of laser pulses from 100 to 10 nm average and from multiple layers to few layers graphene (1-1.5 nm thickness), respectively, leading to graphene quantum dots (GQDs). Accordingly, the emission intensity of these GQDs increases appearing at about 500 nm in the visible region along the reduction of the particle size. Transient absorption spectroscopy has allowed detection of a transient signal decaying in the microsecond time scale that has been attributed to the charge separation state. ■ INTRODUCTION Graphene (G) being one atom thick layer of extended sp 2 carbons represents the limit of the thinnest possible 2D conductive surface, 1,2 exhibiting very fast electron mobility and high charge carrier density. 3 One of the most convenient procedures for the preparation of G suspensions consists in the deep oxidation of graphite flakes followed by exfoliation of the resulting graphite oxide by sonication in the appropriate solvent and reconstitution of G by reduction of the dispersed graphene oxide (GO). 4-6 While this methodology enjoys high reproducibility and can produce highly concentrated aqueous suspensions of GO (about 0.1 mg L -1 ), the materials present in the suspensions are typically constituted by micrometric sheets of nanometric thickness. Because of the remarkable properties derived from 2D confinement at the nanoscale caused by the effect of the edges, G quantum dots (GQDs) exhibit new properties such as emission and their behavior as spin qubit with collective spin states. 7,8 Recent reviews have summarized the unique properties of GQDs. 9-14 It is, therefore, very important to develop reliable and efficient procedures for the preparation of GQDs to exploit their unique properties arising from confinement. Unfortunately, graphite oxidation and exfoliation is not a suitable procedure to obtain GQDs, since it fails to provide nanometric sheets that have a large tendency to undergo complete oxidation to CO 2 and other alternative procedures have to be implemented. For this reason GQDs have to be prepared in alternative ways, such as for instance by acid-catalyzed microwave pyrolysis of carbon precursors. 15 Because of their size, GQDs form persistent dispersions in different solvents and may have large biomedical application due to the possibility to cross cellular membranes. 16 Also, GQDs have attracted considerable attention as emerging fluorescent dots for bioimaging, sensing, 17 pollultant removal, 18 and even in photovoltaic devices. 19 GQDs also hold promise in catalysis due to their large surface area and accessibility of the active sites. 20 In the present work we report an innovative and efficient preparation of GQDs of tunable dimensions and describe the unique photophysical properties GQD suspen- sions. ■ RESULTS AND DISCUSSION We have recently reported that pyrolysis of natural chitosan can form G films on various supports. 21 On the basis of this finding, we performed the pyrolysis of millimetric alginate beads at 900 °C to form graphitic carbon residues. XRD patterns of these alginate-derived carbons show that pyrolyzed alginate beads have a tendency to form graphitic carbons, although the broadness of the peak at 23° indicates low crystallinity and loose z-stacking compared to graphite (Figure 1). Similar observations that pyrolysis of alginate form graphitic carbons have been already reported in the literature. 22 When carbon residues from alginate were submitted to ablation in a solvent using 532 nm laser pulses, an increasing darkening of the liquid phase is observed. Figure 2 shows the increasing intensity of the optical UV-vis absorption spectra of acetonitrile upon 532 nm laser ablation of the alginate derived carbon in contact with this solvent, suggesting that some carbon residue is detached from the submillimetric solid carbon beads, becoming suspended in the liquid phase. We noticed that the region 200-300 nm (not shown in Figure 2) contain some fine structure in the optical spectra that could correspond to the liberation of small condensed polycyclic aromatic Received: February 16, 2013 Revised: April 18, 2013 Published: April 26, 2013 Article pubs.acs.org/Langmuir © 2013 American Chemical Society 6141 dx.doi.org/10.1021/la400618s | Langmuir 2013, 29, 6141-6146