title: Direct production of graphene nanosheets for near infrared photoacoustic imaging.
abstract: Hummers method is commonly used for the fabrication of graphene oxide (GO) from graphite particles. The oxidation process also leads to the cutting of graphene sheets into small pieces. From a thermodynamic perspective, it seems improbable that the aggressive, somewhat random oxidative cutting process could directly result in graphene nanosheets without destroying the intrinsic π-conjugated structures and the associated exotic properties of graphene. In Hummers method, both KMnO4 and NO2(+) (nitronium ions) in concentrated H2SO4 solutions act as oxidants via different oxidation mechanisms. From both experimental observations and theoretical calculations, it appears that KMnO4 plays a major role in the observed oxidative cutting and unzipping processes. We find that KMnO4 also limits nitronium oxidative etching of graphene basal planes, therefore slowing down graphene fracturing processes for nanosheet fabrication. By intentionally excluding KMnO4 and exploiting pure nitronium ion oxidation, aided by the unique thermal and kinetic effects induced by microwave heating, we find that graphite particles can be converted into graphene nanosheets with their π-conjugated aromatic structures and properties largely retained. Without the need of any postreduction processes to remove the high concentration of oxygenated groups that results from Hummers GO formation, the graphene nanosheets as-fabricated exhibit strong absorption, which is nearly wavelength-independent in the visible and near-infrared (NIR) regions, an optical property typical for intrinsic graphene sheets. For the first time, we demonstrate that strong photoacoustic signals can be generated from these graphene nanosheets with NIR excitation. The photo-to-acoustic conversion is weakly dependent on the wavelength of the NIR excitation, which is different from all other NIR photoacoustic contrast agents previously reported.
authors.affiliation: Chemistry Department, Rutgers University , Newark, New Jersey 07102, United States.
authors.forename: Mehulkumar A
authors.initials: MA
authors.lastname: Patel
authors.forename: Hao
authors.initials: H
authors.lastname: Yang
authors.forename: Pui Lam
authors.initials: PL
authors.lastname: Chiu
authors.forename: Daniel D T
authors.initials: DD
authors.lastname: Mastrogiovanni
authors.forename: Carol R
authors.initials: CR
authors.lastname: Flach
authors.forename: Keerthi
authors.initials: K
authors.lastname: Savaram
authors.forename: Lesly
authors.initials: L
authors.lastname: Gomez
authors.forename: Ashley
authors.initials: A
authors.lastname: Hemnarine
authors.forename: Richard
authors.initials: R
authors.lastname: Mendelsohn
authors.forename: Eric
authors.initials: E
authors.lastname: Garfunkel
authors.forename: Huabei
authors.initials: H
authors.lastname: Jiang
authors.forename: Huixin
authors.initials: H
authors.lastname: He
mesh_terms: D054459:Elasticity Imaging Techniques / Q000295:instrumentation / Q000379:methods*; D006108:Graphite / Q000138:chemical synthesis*; D007089:Image Enhancement / Q000295:instrumentation / Q000379:methods; D007259:Infrared Rays; D008422:Materials Testing; D008567:Membranes, Artificial; D053758:Nanoparticles / Q000737:chemistry* / Q000648:ultrastructure; D010316:Particle Size; D019047:Phantoms, Imaging; D061088:Photoacoustic Techniques / Q000295:instrumentation / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical_list: D008567:Membranes, Artificial; D006108:Graphite