Nonspecific cleavage of proteins using graphene oxide.

In this article, we report the intrinsic catalytic activity of graphene oxide (GO) for the nonspecific cleavage of proteins. We used bovine serum albumin (BSA) and a recombinant esterase (rEstKp) from the cold-adapted bacterium Pseudomonas mandelii as test proteins. Cleavage of BSA and rEstKp was nonspecific regarding amino acid sequence, but it exhibited dependence on temperature, time, and the amount of GO. However, cleavage of the proteins did not result in complete hydrolysis into their constituent amino acids. GO also invoked hydrolysis of p-nitrophenyl esters at moderate temperatures lower than those required for peptide hydrolysis regardless of chain length of the fatty acyl esters. Based on the results, the functional groups of GO, including alcohols, phenols, and carboxylates, can be considered as crucial roles in the GO-mediated hydrolysis of peptides and esters via general acid-base catalysis. Our findings provide novel insights into the role of GO as a carbocatalyst with nonspecific endopeptidase activity in biochemical reactions.

Increased thermal stability of cold-adapted esterase at ambient temperatures by immobilization on graphene oxide.

In this study, the effect of graphene oxide (GO) on the thermal stability of a recombinant esterase from cold-adapted Pseudomonas mandelii, rEstKp, was investigated. The complex GO-rEstKp was formed by cross-linking. Both free rEstKp and GO-rEstKp complex showed similar optimum pH and temperatures. GO-rEstKp complex exhibited enhanced thermal stability at ambient temperatures than rEstKp, which prevented the denaturation of the enzyme by hydrophilic interactions. However, the catalytic efficiency of GO-rEstKp complex was lowered to approximately 40% of that of free rEstKp. This study provides an insight into the addition of GO for industrial applications of cold-adapted enzymes at ambient temperatures.

Laser thinning for monolayer graphene formation: heat sink and interference effect.

Despite the availability of large-area graphene synthesized by chemical vapor deposition (CVD), the control of a uniform monolayer graphene remained challenging. Here, we report a method of acquiring monolayer graphene by laser irradiation. The accumulation of heat on graphene by absorbing light, followed by oxidative burning of upper graphene layers, which strongly relies on the wavelength of light and optical parameters of the substrate, was in situ measured by the G-band shift in Raman spectroscopy. The substrate plays a crucial role as a heat sink for the bottom monolayer graphene, resulting in no burning or etching. Oscillatory thinning behavior dependent on the substrate oxide thickness was evaluated by adopting a simple Fresnel's equation. This paves the way for future research in utilizing monolayer graphene for high-speed electronic devices.

Breaking AB stacking order in graphite oxide: ab initio approach.

Different bulk structures of graphite oxide were systematically investigated using density functional theory (DFT). Our model consisted of a hexagonal in-plane structure of graphene with hydroxyl and epoxide groups, and different oxidation levels and water content. The graphitic AB stacking order was stable in anhydrous graphite oxide, independent of oxidation levels. The hydrogen bonding interaction of layers became weaker as the oxidation level increased to the saturation limit. When water molecules were present in highly oxidized graphite oxide, the AB stacking order was broken due to entropic disorder. The interlayer distances increased with the oxidation level: the interlayer distance was 5.1 A for low oxidation graphite oxide and 5.8 A for high oxidation graphite oxide. The calculated interlayer distance of hydrated graphite oxide was 7.3 A, which is in excellent agreement with experimental observations.

Enhanced electric double layer capacitance of graphite oxide intercalated by poly(sodium 4-styrensulfonate) with high cycle stability.

We propose a new material for high power and high density supercapacitors with excellent cycle stability. Graphite oxide (PSS-GO) intercalated with poly(sodium 4-styrensulfonate) showed high performance of electric double layer capacitance (EDLC) compared to that of the pristine graphite oxide. Specific capacitance of the PSS-GO reached 190 F/g, and the energy density was much improved to 38 Wh/kg with a power density of 61 W/kg. Cycle test showed that the specific capacitance decreased by only 12% after 14860 cycles, providing excellent cyclic stability. The high EDLC performance of PSS-GO composite was attributed to the wide interlayer distance and simple pore structures accommodating fast ion kinetics.

Evidence of graphitic AB stacking order of graphite oxides.

Graphite oxide (GO) samples were prepared by a simplified Brodie method. Hydroxyl, epoxide, carboxyl, and some alkyl functional groups are present in the GO, as identified by solid-state 13C NMR, Fourier-transform infrared spectroscopy, and X-ray photoemission spectroscopy. Starting with pyrolytic graphite (interlayer separation 3.36 A), the average interlayer distance after 1 h of reaction, as determined by X-ray diffraction, increased to 5.62 A and then increased with further oxidation to 7.37 A after 24 h. A smaller signal in 13C CPMAS NMR compared to that in 13C NMR suggests that carboxyl and alkyl groups are at the edges of the flakes of graphite oxide. Other aspects of the chemical bonding were assessed from the NMR and XPS data and are discussed. AB stacking of the layers in the GO was inferred from an electron diffraction study. The elemental composition of GO prepared using this simplified Brodie method is further discussed.

Quantized electron accumulation states in indium nitride studied by angle-resolved photoemission spectroscopy.

Electron accumulation states in InN have been measured using high resolution angle-resolved photoemission spectroscopy (ARPES). The electrons in the accumulation layer have been discovered to reside in quantum well states. ARPES was also used to measure the Fermi surface of these quantum well states, as well as their constant binding energy contours below the Fermi level E(F). The energy of the Fermi level and the size of the Fermi surface for these quantum well states could be controlled by varying the method of surface preparation. This is the first unambiguous observation that electrons in the InN accumulation layer are quantized and the first time the Fermi surface associated with such states has been measured.