Graphene is a monolayer of tightly packed carbon atoms that possesses many interesting properties and has numerous exciting applications. In this work, we report the antibacterial activity of two waterdispersible graphene derivatives, graphene oxide (GO) and reduced graphene oxide (rGO) nanosheets. Such graphene-based nanomaterials can effectively inhibit the growth of E. coli bacteria while showing minimal cytotoxicity. We have also demonstrated that macroscopic freestanding GO and rGO paper can be conveniently fabricated from their suspension via simple vacuum filtration. Given the superior antibacterial effect of GO and the fact that GO can be mass-produced and easily processed to make freestanding and flexible paper with low cost, we expect this new carbon nanomaterial may find important environmental and clinical applications.

Graphene is a single layer of sp(2)-bonded carbons that has unique and highly attractive electronic, mechanical, and thermal properties. Consequently, the potential impact of graphene and its derivatives (e.g., graphene oxide, GO) on human and environmental health has raised considerable concerns. In this study, we have carried out a systematic investigation on cellular effects of GO nanosheets and identified the effect of fetal bovine serum (FBS), an often-employed component in cell culture medium, on the cytotoxicity of GO. At low concentrations of FBS (1%), human cells were sensitive to the presence of GO and showed concentration-dependent cytotoxicity. Interestingly, the cytotoxicity of GO was greatly mitigated at 10% FBS, the concentration usually employed in cell medium. Our studies have demonstrated that the cytotoxicity of GO nanosheets arises from direct interactions between the cell membrane and GO nanosheets that result in physical damage to the cell membrane. This effect is largely attenuated when GO is incubated with FBS due to the extremely high protein adsorption ability of GO. The observation of this FBS-mitigated GO cytotoxicity effect may provide an alternative and convenient route to engineer nanomaterials for safe biomedical and environmental applications.

We report the effect of molecular weight and comonomer content on melt crystallization of model random ethylene 1-butene copolymers. A large set of narrowly distributed linear polyethylenes (PE) was used as reference of unbranched molecules. The samples were crystallized from a melt state above the equilibrium melting temperature and cooled at a constant rate. The exothermic peaks of the melt-solid transition are reported as the crystallization temperatures (T c ). Following expectations, the T c of unbranched PE samples was constant and independent of the initial melt temperature. The same independence was observed for copolymers (2.2 mol % ethyl branches) with molar mass below 4500 g/mol. Moreover, the T c of copolymers with higher molar mass depends on the temperature of the initial melt, T c increases as the temperature of the melt decreases. We attribute the increase in T c to a strong crystallization memory in the melt above the equilibrium melting, and correlate this phenomenon with remains in the melt of the copolymer’s crystallizable sequence partitioning. Albeit molten, long crystallizable sequences remain in the copolymer’s melt at a close proximity, lowering the change in free energy barrier for nucleation. The residual sequence segregation in the melt is attributed to restrictions of the copolymer crystalline sequences to diffuse upon melting and to reach the initial random topology of the copolymer melt. Erasing memory of the prior sequence selection in copolymer melts requires much higher temperatures than the theoretical equilibrium value. The critical melt temperature to reach homogeneous copolymer melts (T onset ), and the comonomer content at which melt memory above the equilibrium melting vanishes are established. The observed correlation between melt memory, copolymer crystallinity and melt topology offers strategies to control the state of copolymer melts in ways of technological relevance for melt processing of LLDPE and other random olefin copolymers.

In general, when a crystal is molten, all molecules forget about their mutual correlations and long-range order is lost. Thus, a regrown crystal does not inherit any features from an initially present crystal. Such is true for materials exhibiting a well-defined melting point. However, polymer crystallites have a wide range of melting temperatures, enabling paradoxical phenomena such as the coexistence of melting and crystallization. Here, we report a self-seeding technique that enables the generation of arrays of orientation-correlated polymer crystals of uniform size and shape ('clones') with their orientation inherited from an initial single crystal. Moreover, the number density and locations of these cloned crystals can to some extent be predetermined through the thermal history of the starting crystal. We attribute this unique behaviour of polymers to the coexistence of variable fold lengths in metastable crystalline lamellae, typical for ordering of complex chain-like molecules.

The kinetic folding of ribonuclease H was studied by hydrogen exchange (HX) pulse labeling with analysis by an advanced fragment separation mass spectrometry technology. The results show that folding proceeds through distinct intermediates in a stepwise pathway that sequentially incorporates cooperative native-like structural elements to build the native protein. Each step is seen as a concerted transition of one or more segments from an HX-unprotected to an HX-protected state. Deconvolution of the data to near amino acid resolution shows that each step corresponds to the folding of a secondary structural element of the native protein, termed a "foldon." Each folded segment is retained through subsequent steps of foldon addition, revealing a stepwise buildup of the native structure via a single dominant pathway. Analysis of the pertinent literature suggests that this model is consistent with experimental results for many proteins and some current theoretical results. Two biophysical principles appear to dictate this behavior. The principle of cooperativity determines the central role of native-like foldon units. An interaction principle termed "sequential stabilization" based on nativelike interfoldon interactions orders the pathway.D o proteins fold through varied and multiple tracks, or do they fold through predetermined intermediates according to understandable biophysical principles (1)? This question is fundamental for the interpretation of a large amount of biophysical and biological research. The question could be resolved if it were possible to define the intermediate structures and pathways that unfolded proteins move through on their way to the native state. Unfortunately, transient intermediates cannot be studied by the usual crystallographic and NMR methods. The range of kinetic and spectroscopic methods has been applied to many proteins, but these methods do not yield the necessary structural information.We used a developing technology, hydrogen exchange pulse labeling measured by MS (HX MS), to study the folding of a cysteine-free variant of Escherichia coli ribonuclease H1 (RNase H), a mixed α/β protein that has served as a major proteinfolding model (2-5). Previous studies showed that RNase H folds in a fast, unresolved burst phase (15 ms dead time) to an intermediate termed "I core " and then much more slowly (in seconds) to the native state (3). HX pulse-labeling and equilibrium native-state HX experiments monitored by NMR showed that I core comprises a continuous region of the protein between helix A and strand 5 and that β-strands 1, 2, and 3 and helix E acquire protection much later, consistent with mutational analysis (2-4). Single-molecule and mutational studies indicated that the intermediate is obligatory, on-pathway, and folds first even when I core is not observably populated (6, 7).The HX MS technique used here is able to follow the entire folding trajectory of RNase H in considerable structural and temporal detail. The analysis monitors every amide site, evaluates the folding coopera...

Graphene is an increasingly important nanomaterial that has shown great promise in the area of nanotechnology. In this study, fluorescein-functionalized graphene oxide (GO) is synthesized via a polyethylene glycol (PEG) bridge and its application in intracellular imaging is explored. GO is an oxide form of graphene that provides an ideal platform to prepare graphene-based functional nanomaterials via chemical modification. The PEG bridge was introduced to prevent GO-induced quenching of conjugated fluorescein. The fluorescein-PEG-GO conjugate thus prepared exhibits excellent pH-tunable fluorescent properties and, more significantly, can be efficiently taken up by cells and serve as a fluorescent nanoprobe for intracellular imaging.

Investigations of the interactions between carbon nanomaterials (CNMs) and living organisms and their subsequent biological responses are of fundamental significance for toxicity assessment and further biomedical applications. In this work, the cellular uptake and cytotoxicity of multi-walled carbon nanotubes (MWCNTs), graphene oxide (GO) and nanodiamond (ND) were examined and compared. We demonstrated that all of the CNMs were readily internalized by HeLa cells through nonspecific cellular uptake. Their cell uptake ratios showed significant differences in the following order: ND > MWCNTs > GO. A series of biological assays were used to evaluate the cytotoxicity of CNMs. It was found that CNMs showed dose- and time-dependent cytotoxicity to HeLa cells. However, cytotoxicity of CNMs was not associated with their cell uptake ratios. Among them, ND exhibited the highest cell uptake ratio and the least cytotoxicity. To the best of our knowledge, this is the first study which has quantitatively determined and compared the cell uptake ratios and cytotoxicities of MWCNTs, GO and ND. And we expect that these results described here could provide useful information for the development of new strategies to design efficient drug delivery nanocarriers and therapeutic systems as well as deep insights into the mechanism of CNMs' cytotoxicity.

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