(679f) Ultrathin Hollow Graphene Oxide Membranes for Use As Nanoparticle Carriers for Energy and Biomedical Applications

We synthesize hollow spherical particles/membrane (*) sacks of graphene oxide loaded with nanoparticles to be used as nanoparticles carriers, through a new method based on emulsion precipitation and sublimation of the cores. We vary the synthesis parameters, such as shear rate, pH, and graphene oxide and oil concentration ratios. Our results show a concentration dependent membrane thickness that varies between 3 and 25 nm depending on the concentration, and their mean diameters vary between 500 nm and 70 μ m. In addition, polymeric nanoparticles are loaded inside the graphene oxide shells forming core− shell particles demonstrating that they can be used as carriers for nanoparticles. In our work we use a range of GO sheet sizes. To synthesize our larger Hollow Graphene Oxide Membrane (HGOM) particles we use larger graphene oxide sheets of approximately 4µm obtained with a bath sonication method and to synthesize the smaller micron sized HGOM particles, we use smaller GO sheets of the order of 0.52 µm obtained with a higher power probe sonication method. We have identified processing parameters, such as GO concentration, shear rate and pH that produce graphene oxide stabilized Pickering emulsions. When these emulsions are subsequently cooled they produced stable suspensions of a solid oil phase wrapped in graphene oxide membranes. Proper adjustment of the processing parameters can provide control over the size of the templated membranes and the thickness of the graphene oxide layer. When high shear rates and high concentrations of graphene oxide are used, particles of smaller diameters with thick membranes are produced. HGOM particles with diameters as small as 500− 3000 nm are produced for use as carriers for silicon and PTFE nanoparticles, respectively. By decreasing the concentration of graphene oxide, membranes as thin as 3 nm can be produced. Plotting the various processing parameters and the resulting particle characteristic results in three distinctive regions, which appear to be characteristic to this process. Region I consists of particles with nearly constant GO membrane thickness and varying particle diameter. Region II consists of a transition region in which the GO concentration aff ects both membrane thickness and particle size. Region III consists of particles with constant diameter as the GO concentration is increased for a fixed shear rate. Decreasing the pH assists in forming multiple layers of graphene oxide, since lower pH values favor the interactions between individual graphene oxide sheets and these interactions are more favorable than the interactions with the aqueous phase. We found that pH conditions which resulted in zeta potentials of approximately −20 to −30 mV provided very stable emulsions. It is worth mentioning that stable emulsions using graphene oxide and naphthalene or other aromatic compounds can also be obtained without further decreasing the pH of the emulsion. However, by adding extra acid, membranes become more hydrophobic, and hence, the amount of GO that migrates to the water phase is minimized. Thus, in this way GO sheets tend to be more attracted to the oil phase and stack to each other. Our particles are characterized via laser diffraction, zeta potential, FE-SEM, TEM, BET, and AFM. Potential applications of this work include applications that benefit from core− shell structures and nanoparticle carriers, including drug formulation, catalysis, and electrochemical applications.

(*) Langmuir March 2017. Kurt B. Smith, M. Silvina Tomassone, DOI: 10.1021/acs.langmuir.6b04583