Separation and purification of graphene oxide (GO) prepared from chemical oxidation of flake graphite and ultrasonication by capillary electrophoresis (CE) was demonstrated. CE showed the ability to provide high-resolution separations of GO fractionations with baseline separation. The GO fractionations after CE were collected for Raman spectroscopy, atomic force microscopy, and transmission electron microscopy characterizations. GO nanoparticles (unexfoliated GO) or stacked GO sheets migrated toward the anode, while the thin-layer GO sheets migrated toward the cathode. Therefore, CE has to be performed twice with a reversed electric field to achieve a full separation of GO. This separation method was suggested to be based on the surface charge of the GO sheets, and a separation model was proposed. This study might be valuable for fabrication of GO or graphene micro- or nanodevices with controlled thickness.
"Centrifugal purification of graphenes based on their size and/or density   . Another published approach, capillary electrophoresis, relied on the surface charge of the graphene . Salting-out is a distinctive purification method because it relies on a different variable, the dispersibility of the CMGs, which is closely related to both the material's physical properties, for example size, and its chemical properties , such as surface charge. "
[Show abstract][Hide abstract] ABSTRACT: Chemically modified graphenes (CMGs) are used to reinforce composites and as platforms for sensing, electrodes in batteries, conductive films for displays, and others. However, performance of CMG materials is often limited by their physicochemical heterogeneity. Here, we report that simple addition of ammonium sulfate to heterogeneous CMG mixtures, results in effective, scalable, and in-series purification, by salting-out precipitation. The mechanism is based on differences in the dispersibility of the individual graphene oxide (GO). At relatively low ammonium sulfate concentrations (similar to 10 mM), large, GOs with pre-dominant hydrophobic domains are precipitated. As the salt concentration increases, smaller and less GOs with predominant hydrophobic domains are purified, and eventually graphene quantum dots (GQDs) are isolated. The method described herein was inspired by a well-known protein purification process called 'salting-out,' in which proteins of different sizes and surface charge, successively become indispersible over a narrow range of salt concentrations. The salting-out process for GOs is simple and scalable, and can be used to achieve large-scale purification. (c) 2013 Elsevier Ltd. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: The surface microchemical environment of graphene oxide (GO) has so far been oversimplified for understanding practical purposes. The amount as well as the accurate identification of each possible oxygenated group on the GO surface are difficult to describe not only due to the complex chemical nature of the oxidation reactions but also due to several intrinsic variables related to the production and chemical processing of GO-based materials. However, to advance toward a more realistic description of the GO chemical environment, it is necessary to distinguish the oxygenated fragments with very peculiar characteristics that have so far been treated as simply graphene oxide.. In this way, small oxidized graphitic fragments adsorbed on the GO surface, named oxidation debris or carboxylated carbonaceous fragments (CCFs), have been here separated from commercially available GO. Spectroscopy and microscopy results indicated that the chemical nature of these fragments is different from that of GO. By using the decoration of GO with silver nanoparticles as a conceptual model, it was seen that the presence of oxidation debris on the GO surface greatly influences the associated kinetic processes, mainly due to the nucleation and stabilization capacity for silver nanoparticles provided by the oxidation debris fragments. Consequently, when CCFs are present, Ag nanoparticles are significantly smaller and less crystalline. Considering the GO microchemical environment pointed out here, these findings can be qualitatively extrapolated to all other covalent and noncovalent functionalizations of GO.
Chemistry of Materials 09/2012; 24(21):4080–4087. DOI:10.1021/cm301939s · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Capillary electrophoresis (CE) coupled with amperometric detection (AD) method was developed using ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF(4) ) as additive for the simultaneous detection of clenbuterol (CLB), terbutaline (TER) and ractopamine (RAC) in feed. The effects of detection potential, concentration of EMImBF(4) , pH and concentration of the running buffer, separation voltage as well as injection time on the separation and detection of these three β-agonists were investigated in detail. Under the optimum conditions: the detection potential at 1.05 V, 50 mmol/L Tris-HAc at pH 8.0 with 0.6% (v/v) EMImBF(4) , electrokinetic injection 6 s at 16 kV and separation voltage at 16 kV, a baseline separation for these three analytes could be achieved within 11 min. Introduction of EMImBF(4) into the running buffer resulted in significant improvement in separation selectivity and enhancement in peak currents for those β-agonists, especially for TER and RAC, which could not be separated in the running buffer without additive. The method exhibited wide linear range with limit of detection (S/N = 3) of 2 nmol/L, 1 nmol/L and 2 nmol/L for CLB, TER and RAC, respectively. The precision was determined in both intra-day (n = 5) and inter-day (n = 3) assays, and the relative standard derivations (RSDs) for both migration time and peak current were less than 6%. The proposed method was also applied to analyze β-agonists in feed sample.
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