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Artificial leaves aim to better mimic nature
Materials Today Volume 17, Number 4 May 2014
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Graphene-based coating produces anticlotting molecules A catalytic coating that could prevent blood clots in implanted biomedical devices has been designed by US scientists. Nitric oxide (NO) is a naturally occurring clot-preventing agent generated by blood vessels. In catheters, heart valves, vascular grafts, and other long-term implanted biomedical devices, blood clots can occur when the supply of NO present in the blood when it entered the device runs out. Devices that gradually release NO into the blood as it flow through them have been developed, but their lifetime is limited by the inevitable depletion of their NO reservoir. Yu Huang at the University of California, Los Angeles in the US and collaborators have developed a catalytic system for the continuous generation of a similar clotpreventing molecule, nitroxyl (HNO). ‘‘Nitroxyl possesses antithrombotic activity analogous to NO,’’ Huang told Materials Today. The system, presented in Nature Communications [Xue, et al., Nat. Commun. (2014), doi:10.1038/ncomms4200], is a sheet of graphene with the iron-containing porphyrin haemin and the enzyme glucose oxidase imbedded in it. ‘‘By simultaneously
Schematic illustration of graphene-haeminglucose oxidase coating. Credit: Teng Xue and Nathan Weiss.
conjugating haemin and glucose oxidase on graphene, we have created an integrated tandem catalyst that can drive a reaction cascade to allow for in situ generation of antithrombotic species,’’ says Huang. Glucose oxide catalyses the oxidation of the glucose naturally present in blood to hydrogen peroxide. The haemin then catalyses the oxidation of the L-arginine naturally present in blood by the hydrogen peroxide, producing HNO. ‘‘Our graphene-haemin-glucose oxide imbedded materials allow sustained generation of
nitroxyl from physiological glucose, L-arginine and blood oxygen,’’ explains Huang. Graphene was chosen as the supporting material due to several unique characteristics. ‘‘Bulk quantities of graphene flakes can be readily prepared through chemical exfoliation of graphite oxide followed by chemical reduction,’’ Huang says. ‘‘The rich surface chemistry of chemically reduced graphene offers excellent potential for coupling multiple distinct catalysts on its surface.’’ Its large open surface area due to its 2D structure offers easy access for substrates. ‘‘Finally, it has been shown that graphene has better biocompatibility than other carbon nanomaterials,’’ he says. To prove the concept, the team then coated a plastic film widely used in biomedical devices – polyurethane – with their catalytic material and exposed it to blood for three days. Clotting on the film remained substantially reduced, compared to an uncoated control film, for the entire test period. ‘‘Next, we are planning to do in vitro tests with animal models and then humans eventually,’’ Huang says. Nina Notman
Artificial leaves aim to better mimic nature US scientists have jumped a developmental hurdle on the path towards an artificial leaf that uses solar energy to cheaply and efficiently split water. This may lead to the technology being used to provide a cheap supply of hydrogen fuel. Early attempts to produce artificial leaves by Ana Moore at the Arizona State University in the US and her coworkers were disappointing, due to the low efficiency of the step where a fast reaction when light energy is converted to chemical energy interacts with the slow reaction when this chemical energy is used to split water in to hydrogen and oxygen. To solve the problem, the researchers turned their attention to Nature. In the study described in Nature Chemistry [Megiatto, et al., Nat. Chem. (2014), doi:10.1038/ nchem.1862] the team probed the part of the photosynthesis process when water is split to yield oxygen. ‘‘We looked in detail and found that nature had used an intermediate step,’’ explains team member Thomas Moore. ‘‘This intermediate step involved a relay
An artificial photosynthetic reaction center containing a bioinspired electron relay (yellow) mimics some aspects of photosynthesis. 161
NEWS
for electrons in which one half of the relay interacted with the fast step in an optimal way to satisfy it, and the other half of the relay then had time to do the slow step of water oxidation in an efficient way.’’ When photosynthesizing organisms in nature absorb light, a powerfully oxidizing chlorophyll complex known as P680+ is generated. A manganese-based catalyst then reduces this complex through an electron
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Materials Today Volume 17, Number 4 May 2014
transfer pathway. This pathway includes an electron transfer relay between two amino acids (tyrosine and histidine). The team found that the key feature in this relay is a very short bond between a hydrogen atom and a nitrogen atom in the amino acid pair. This short bond enabled the reduction of the P680+ to be very rapid, which the team believes limits the prevalence of recombination reactions and ensures a high yield of oxygen.
Next, the team designed an artificial relay incorporating what they had learned about the natural system. The reaction center in the artificial system features a benzimidazole–phenol pair with a short hydrogen bond that mimics the behavior of the very short bond in the natural amino acid pair. When this relay was incorporated in to the team’s artificial leaves a major improvement in efficiency was seen. Nina Notman
NEWS
Graphene-based coating produces anticlotting molecules A catalytic coating that could prevent blood clots in implanted biomedical devices has been designed by US scientists. Nitric oxide (NO) is a naturally occurring clot-preventing agent generated by blood vessels. In catheters, heart valves, vascular grafts, and other long-term implanted biomedical devices, blood clots can occur when the supply of NO present in the blood when it entered the device runs out. Devices that gradually release NO into the blood as it flow through them have been developed, but their lifetime is limited by the inevitable depletion of their NO reservoir. Yu Huang at the University of California, Los Angeles in the US and collaborators have developed a catalytic system for the continuous generation of a similar clotpreventing molecule, nitroxyl (HNO). ‘‘Nitroxyl possesses antithrombotic activity analogous to NO,’’ Huang told Materials Today. The system, presented in Nature Communications [Xue, et al., Nat. Commun. (2014), doi:10.1038/ncomms4200], is a sheet of graphene with the iron-containing porphyrin haemin and the enzyme glucose oxidase imbedded in it. ‘‘By simultaneously
Schematic illustration of graphene-haeminglucose oxidase coating. Credit: Teng Xue and Nathan Weiss.
conjugating haemin and glucose oxidase on graphene, we have created an integrated tandem catalyst that can drive a reaction cascade to allow for in situ generation of antithrombotic species,’’ says Huang. Glucose oxide catalyses the oxidation of the glucose naturally present in blood to hydrogen peroxide. The haemin then catalyses the oxidation of the L-arginine naturally present in blood by the hydrogen peroxide, producing HNO. ‘‘Our graphene-haemin-glucose oxide imbedded materials allow sustained generation of
nitroxyl from physiological glucose, L-arginine and blood oxygen,’’ explains Huang. Graphene was chosen as the supporting material due to several unique characteristics. ‘‘Bulk quantities of graphene flakes can be readily prepared through chemical exfoliation of graphite oxide followed by chemical reduction,’’ Huang says. ‘‘The rich surface chemistry of chemically reduced graphene offers excellent potential for coupling multiple distinct catalysts on its surface.’’ Its large open surface area due to its 2D structure offers easy access for substrates. ‘‘Finally, it has been shown that graphene has better biocompatibility than other carbon nanomaterials,’’ he says. To prove the concept, the team then coated a plastic film widely used in biomedical devices – polyurethane – with their catalytic material and exposed it to blood for three days. Clotting on the film remained substantially reduced, compared to an uncoated control film, for the entire test period. ‘‘Next, we are planning to do in vitro tests with animal models and then humans eventually,’’ Huang says. Nina Notman
Artificial leaves aim to better mimic nature US scientists have jumped a developmental hurdle on the path towards an artificial leaf that uses solar energy to cheaply and efficiently split water. This may lead to the technology being used to provide a cheap supply of hydrogen fuel. Early attempts to produce artificial leaves by Ana Moore at the Arizona State University in the US and her coworkers were disappointing, due to the low efficiency of the step where a fast reaction when light energy is converted to chemical energy interacts with the slow reaction when this chemical energy is used to split water in to hydrogen and oxygen. To solve the problem, the researchers turned their attention to Nature. In the study described in Nature Chemistry [Megiatto, et al., Nat. Chem. (2014), doi:10.1038/ nchem.1862] the team probed the part of the photosynthesis process when water is split to yield oxygen. ‘‘We looked in detail and found that nature had used an intermediate step,’’ explains team member Thomas Moore. ‘‘This intermediate step involved a relay
An artificial photosynthetic reaction center containing a bioinspired electron relay (yellow) mimics some aspects of photosynthesis. 161
NEWS
for electrons in which one half of the relay interacted with the fast step in an optimal way to satisfy it, and the other half of the relay then had time to do the slow step of water oxidation in an efficient way.’’ When photosynthesizing organisms in nature absorb light, a powerfully oxidizing chlorophyll complex known as P680+ is generated. A manganese-based catalyst then reduces this complex through an electron
162
Materials Today Volume 17, Number 4 May 2014
transfer pathway. This pathway includes an electron transfer relay between two amino acids (tyrosine and histidine). The team found that the key feature in this relay is a very short bond between a hydrogen atom and a nitrogen atom in the amino acid pair. This short bond enabled the reduction of the P680+ to be very rapid, which the team believes limits the prevalence of recombination reactions and ensures a high yield of oxygen.
Next, the team designed an artificial relay incorporating what they had learned about the natural system. The reaction center in the artificial system features a benzimidazole–phenol pair with a short hydrogen bond that mimics the behavior of the very short bond in the natural amino acid pair. When this relay was incorporated in to the team’s artificial leaves a major improvement in efficiency was seen. Nina Notman