Chemical Communications Blog

Graphene, a two-dimensional single-layer graphite sheet, has aroused worldwide attention since the last decade. Its ultrahigh electrical and thermal conductivities, high mechanical stiffness and unique band structure have attracted extensive research efforts to develop graphene-based electronics, photonics, printing materials etc. Currently, among various strategies, the wet-chemical method still remains the most practical protocol for large-scale production of graphene in laboratories. This process in general involves two steps: the oxidative exfoliation of graphite, a.k.a. Hummers’ method, followed by reduction of the oxidized graphite sheets. Graphite oxide (GO), possessing a layered structure analogous to graphite but with rich oxygen functionalities (such as hydroxyl and carboxyl groups) anchored on each layer, is the product of the first step and thus serves as a precursor of graphene.

As the aforementioned wet chemical method is usually carried out in water, GO is primarily stored as aqueous-based colloidal dispersions. However, GO is reported to be chemically unstable in water since water molecules can react with electropositive carbons of GO. Though the reaction is not rapid, it partially removes the oxygen functionalities and breaks the carbon matrix, which eventually forces GO to precipitate and reduces the shelf life of the GO precursor.

Recently, Shi and coworkers from Tsinghua University have successfully prolonged the lifetime of GO by dispersing it in organic solvents. During the last purification step of the Hummers’ method, instead of using de-ionized water, anhydrous ethanol was utilized to rinse the GO product and obtain ethanol-wetted GO. X-ray diffraction revealed that ethanol molecules existed in the inter-layer space between adjacent layers. The ethanol-wetted GO could be readily dissolved in propylene carbonate, an organic solvent, for concentrations ranging from 0.1 mg mL^-1 to 40 mg mL^-1 (Figures a and b). More importantly, GO could be stored in propylene carbonate for at least a month without a colour change, whilst the colour of aqueous GO dispersion discernibly darkened (Figure c). Spectroscopic studies indicated that the colour change was attributed to the loss of oxygen functionalities. The results unambiguously prove that GO in propylene carbonate is much more stable than GO in water.

Figure. (a) Dissolution of ethanol-wetted GO in propylene carbonate. (b) GO colloidal dispersions with various concentrations. (c) Color evolution of GO dispersions (1 mg mL^-1) with water and propylene carbonate as solvents before and after storing for 28 days under ambient conditions.

Aside from propylene carbonate, dimethyl sulfoxide, ethylene glycol and N,N-dimethylformamide are solvents that can dissolve the ethanol-wetted GO. The successful stabilization of GO colloidal dispersions could ensure the steady production of graphene in laboratories, as well as reveal new opportunities to develop GO-based devices.

To find out more please read:

Organic Dispersions of Graphene Oxide with Arbitrary Concentrations and Improved Chemical Stability

Wencheng Du, Mingmao Wu, Miao Zhang, Guochuang Xu, Tiantian Gao, Liu Qian, Xiaowen Yu, Fengyao Chi, Chun Li and Gaoquan Shi

DOI: 10.1039/c7cc04584k

About the author:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web blog writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.

People who love the taste of garlic are often annoyed by its lingering smell. While there are various tips to get rid of this unpleasant odor, have you ever thought that this garlic aroma brings you chemical compounds that are potent anti-cancer reagents?

Diallyl trisulfide, one of the natural occurring components rendering the flavor of garlic, is able to release hydrogen sulfide (H₂S) upon contacting with thiol compounds (i.e., organic molecules with –SH functional groups). H₂S is a pungent gas that one might never forget after sniffing a rotten egg. However, this “notorious” gas, when at low concentrations, is reported to be friendly to our bodies. It relaxes vascular smooth muscle, reduces blood pressure, lowers risk associated with cancer as well as protects gastrointestinal, nervous and immune systems. All the aforementioned benefits of H₂S have aroused worldwide efforts in developing H₂S-releasing and bio-compatible materials that mimic the natural products for pharmaceutical applications.

Davis, Quinn and co-workers from Monash University, Australia and University of Warwick, United Kingdom, recently published a paper in Chemical Communications that reports a trisulfide-linked organic polymer capable of releasing H₂S when meets –SH groups. As shown in the scheme below, the synthesized polymer is composed of three parts: a polyethylene glycol (PEG) unit on the left (in blue), a cholesterol (CHOL) group on the right (in orange), and a linker (in black) joining the two ends. PEG and CHOL are chosen mainly due to their bio-compatibility. By changing the structure of the middle linker, the authors obtained three types of polymers that behave differently when mixing with thiol compounds. The trisulfide linker (denoted as T) enables release of H₂S gas and initiates polymer degradation. The disulfide linker (denoted as D) allows polymer degradation only. The amide linker (denoted as C) containing no sulfide atoms is inert to the thiol exposure.

Scheme. The chemical structure of the synthesized polymers with different linkers.

Experiments showed that the T-linked polymers are capable of releasing H₂S both in vitro and in vivo.

A fluorescent probe, which can be reduced by H₂S and becomes fluorescent, is applied to detect the existence of H₂S. As shown in Figure a, the trisulfide linked polymers tested in vitro exhibited the highest fluorescence when mixing with L-cysteine (a thiol compound to trigger H₂S generation). For the in vivo measurements, the authors incubated HEK293 cells with the polymers and the probe. Similar as the in vitro results, the fluorescence intensity of the cells containing the T-linked polymers is the highest (Figure b). Both the in vitro and in vivo results unambiguously proved that the presence of the T-linker was responsible for generating H₂S. Additionally, another set of tests using Nile Red confirmed the biodegradability of the T-linked polymers.

Figure. (a) Fluorescence spectra collected from different systems in vitro. The inset shows the chemical reaction between the probe (SF4) and H2S that displays fluorescence. (b) Fluorescence intensity of different polymers over time in HEK293 cells.

The developed tri-sulfide linker may allow the mimicry of endogenous biosynthesis, the initiation of discrete signaling events and the synthesis of next-generation pharmaceutical excipients.

To find out more please read:

Garlic-inspired Trisulfide Linkers for Thiol-stimulated H₂S Release
Francesca Ercole, Michael R. Whittaker, Michelle L. Halls, Ben J. Boyd, Thomas P. Davis and John F. Quinn
DOI: 10.1039/c7cc03820h

About the author:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.

Research associated with batteries is gaining increasing attention and extensive efforts in recently decades, partly due to the development of sustainable energy to combat a series of problems including fossil fuel depletion, environmental pollution and global warming. Batteries are indispensable energy storage devices for the utilization of sustainable energy (e.g., solar and wind energy). One of the battery’s cutting-edge research topics is to achieve novel batteries with higher capacity (a figure-of-merit to measure how much electrical energy a battery can store) and better reliability than the lithium-ion batteries that currently dominate the battery market.

In the past decade, batteries based on magnesium ions, termed as magnesium-ion batteries, are emerging. The magnesium-ion batteries possess at least two advantages over lithium-ion batteries. Firstly, their typical anode material, magnesium metal, has a theoretical capacity of 3833 mAh/cm³. This value is much higher than that of graphite, a conventional anode material for lithium-ion batteries. Secondly, the formation of metal dendrite on anode surface can be avoided by replacing lithium metal with magnesium metal. Metal dendrites grow from anodes can eventually touch cathodes, causing electric short circuits and triggering fire and explosion. Therefore, magnesium-ion batteries are safer than lithium-ion batteries. However, nothing can be perfect. The limited mobility of Mg²⁺ of cathode materials greatly reduces the capacity (particularly at fast charging rates) and practicability of the magnesium-ion batteries.

Now Rong et al. has published an article in Chemical Communications stating that a promising cathode material capable of fast conducting Mg²⁺ for magnesium-ion batteries has been identified. The material is a molybdenum phosphate compound with a chemical formula of Mo₃(PO₄)₃O. It is composed of several edge-sharing MoO₆ octahedra, corner-sharing MoO₅ trigonal bipyramids, MoO₄ tetrahedra, and PO₄ tetrahedra. Using advanced simulation and computation techniques (i.e., the first-principles density functional theory), the authors first proved that Mg²⁺ can stably reside in some interstitial sites among the aforementioned polyhedra, indicating the identified compound is active for Mg²⁺ storage. In addition, the authors plotted two possible pathways for Mg²⁺ diffusion during charge and discharge processes (shown in the Figure). As illustrated in Figure a1, the first one is an inner-channel path along the b-axis. The second one is an inter-channel path along the c-axis.

The most striking feature of the path #1 is its ultra-low activation barrier (i.e., the highest potential energy that a Mg²⁺ need to overcome when diffusing) of only ~80 meV (Figure a2). Such a low diffusion barrier is expected to allow facile Mg²⁺ diffusion within the bulk of Mo₃(PO₄)₃O, which can boost the capacity of the magnesium-ion batteries particularly at elevated charging rates. On the contrary, the activation barrier of the path #2 is as high as ~1200 meV. The authors claimed that the Mg²⁺ diffusion along the path #2 “should be ~10¹⁸ times less frequent than” the path #1.

Figure (a1) schematic of the Mg2+ diffusion path #1 and (a2) its corresponding diffusion potential barrier distribution along the way. (b1) Schematic of the Mg2+ diffusion path #2 and (a2) its corresponding diffusion potential barrier distribution along the way.

At last, the authors estimated the theoretical average potential that Mo₃(PO₄)₃O can reach is 1.98 V, corresponding to a promising energy density of 173 Wh/kg. Although the proposed phosphate is hypothetical, the investigation of its stability reveals the possibility that this material can be experimentally synthesized.

To find out more please read:

Fast Mg²⁺ Diffusion in Mo₃(PO₄)₃O for Mg Batteries
Ziqin Rong, Penghao Xiao, Miao Liu, Wenxuan Huang, Daniel C. Hannah, William Scullin, Kristin A. Persson and Gerbrand Ceder
DOI: 10.1039/c7cc02903a

About the author:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.