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Molecular abacus
RESEARCH NEWS
Molecular abacus INFORMATION STORAGE Materials systems for information storage require the ability to be switched between states of comparable free energy. A group of researchers from the University of Edinburgh, UK, and the Institute for Nanostructured Materials Studies and Universitá degli Studi di Bologna, Italy have investigated thin films of rotaxanes, believing that the structure of these molecules will allow switchable molecular motion. They show that rotaxane films can be induced to change structure and so have potential for novel information storage systems [Science (2003) 299, 531]. Rotaxanes consist of a molecular thread that runs through the center of a cyclical molecule or macrocycle. Bulky groups on each end of the thread lock the macrocycle onto the thread. The authors compare this arrangement to an abacus, with the macrocycle able to move up and down the thread. Atomic force microscopy (AFM) of the thin films of rotaxane molecules reveals that, above a threshold load force, continuously scanning the AFM tip along a line results in the production of a series of regularly spaced dots of uniform size. The number of dots depends only on the scan length, and so it is possible to write information as strings of bits. Molecular modeling shows the rotaxane can switch between two nearly degenerate structures with a small activation energy. The researchers believe the AFM provides this energy. The ease of rotaxane rearrangement then allows nuclei of reorganized molecules to coarsen and give the dots observed. “With such an approach, information storage on a thin film could reach densities of 1-10 Gb/in2,” claim the authors.
14
March 2003
Switching surfaces SURFACE SCIENCE
Using an electrical potential to trigger a conformational change in a self-assembled monolayer (SAM), researchers from the Massachusetts Institute of Technology (MIT), University of California at Santa Barbara, and at Berkeley have shown the reversible switching of surface wettability [Science (2003) 299, 371]. The group believes the dynamic control of other surface properties, such as adhesion, friction, and biocompatibility, may be possible. “This opens the door to a variety of applications, including novel drug-delivery systems and smart templates for the bioseparation of one molecule from another,” says Robert Langer of MIT, who led the team. The alkanethiolate (16-mercapto)hexadecanoic acid (MHA) has a hydrophobic chain capped by a hydrophilic carboxylate group and forms a SAM on a Au surface. The chains have a straight, upright equilibrium conformation, presenting the carboxylate groups to the surrounding medium. Under an applied electric potential, the carboxylate groups are attracted to the Au surface, causing the molecules to rearrange and expose the hydrophobic chains. This approach relies on obtaining a low density monolayer on the Au surface to give enough space for the conformational change. Synthesizing an MHA derivative with a bulky endgroup gives a self-assembled monolayer with the required density. This end-group is then removed. Sum-frequency generation spectroscopy confirms the reversible molecular switching in response to the electrical potential and contact angles for the SAM showed switching of the macroscopic hydrophilicity. “This is the first time to our knowledge that anyone has created a truly reversible switch of a surface’s property exploiting monomolecular layers,” says Joerg Lahann of MIT.
Doctoring nanotubes NANOBIOTECHNOLOGY There is currently much interest in the use of nanoparticles for biomedical applications, from drug and biomolecule delivery to biosensors. While many groups have investigated the use of spherical particles, researchers from the University of Florida and VTT Biotechnology, Finland are putting the case for nanotubes. Charles R. Martin and coworkers have developed a template synthesis method that produces nanotubes with a number of advantages. The template allows control of the dimensions of the nanotubes, and enables them to be made out of nearly any material. Importantly for biomedical applications, the nanotubes have inner voids that can be filled with small molecules or large proteins, and the inner and outer surfaces can be modified separately to give distinct functionalities. Using silica nanotubes, the group has performed a series of proof-of-concept experiments that show the potential of this approach [JACS (2002) 124, 11864]. First, by derivatizing the outer
Membrane-less fuel cell
surface of the nanotubes, the
ELECTROCHEMISTRY
the nanotubes into either an organic or
A millimeter-scale fuel cell without the membrane that normally separates the anode and cathode compartments has been designed and fabricated by chemists at Harvard University [JACS (2002) 124, 12930]. Instead, the laminar flow at low Reynolds number of two liquids, one oxidizing and one reducing, is exploited to stop the mixing of the two fuels. Fuel cells with this feature have a channel with two inlets and one outlet running past graphite electrodes. V(II) and V(V) aqueous solutions flow into the channel to give the redox couples V(III)/V(II) at the anode and V(V)/V(IV) at the cathode. Three of these cells in series were able to power a light-emitting diode (LED). The group, led by George M. Whitesides, also demonstrated some of the advantages and disadvantages of these membrane-less fuel cells. The system shows poor efficiency in terms of the ratio of fuel consumed at the electrodes to that delivered to the cell. However, it eliminates the ohmic losses and fouling of a membrane, is comparable in performance to macroscopic fuel cells, and could prove a general format for electrochemistry in microsystems.
researchers controlled partitioning of aqueous phase. Second, nanotubes with hydrophilic outer surfaces and hydrophobic inner surfaces efficiently extracted lipophilic molecules from an aqueous solution. Third, immobilizing an antibody fragment to the nanotubes allowed the recognition and moleculespecific extraction of an enantiomer from a racemic mixture. Finally, the group was able to attach enzyme molecules to give biologically active nanotubes. The researchers are keen to stress the applicability of their method to most materials, which means that biodegradable nanotubes for in vivo conditions could be made.
Molecular abacus INFORMATION STORAGE Materials systems for information storage require the ability to be switched between states of comparable free energy. A group of researchers from the University of Edinburgh, UK, and the Institute for Nanostructured Materials Studies and Universitá degli Studi di Bologna, Italy have investigated thin films of rotaxanes, believing that the structure of these molecules will allow switchable molecular motion. They show that rotaxane films can be induced to change structure and so have potential for novel information storage systems [Science (2003) 299, 531]. Rotaxanes consist of a molecular thread that runs through the center of a cyclical molecule or macrocycle. Bulky groups on each end of the thread lock the macrocycle onto the thread. The authors compare this arrangement to an abacus, with the macrocycle able to move up and down the thread. Atomic force microscopy (AFM) of the thin films of rotaxane molecules reveals that, above a threshold load force, continuously scanning the AFM tip along a line results in the production of a series of regularly spaced dots of uniform size. The number of dots depends only on the scan length, and so it is possible to write information as strings of bits. Molecular modeling shows the rotaxane can switch between two nearly degenerate structures with a small activation energy. The researchers believe the AFM provides this energy. The ease of rotaxane rearrangement then allows nuclei of reorganized molecules to coarsen and give the dots observed. “With such an approach, information storage on a thin film could reach densities of 1-10 Gb/in2,” claim the authors.
14
March 2003
Switching surfaces SURFACE SCIENCE
Using an electrical potential to trigger a conformational change in a self-assembled monolayer (SAM), researchers from the Massachusetts Institute of Technology (MIT), University of California at Santa Barbara, and at Berkeley have shown the reversible switching of surface wettability [Science (2003) 299, 371]. The group believes the dynamic control of other surface properties, such as adhesion, friction, and biocompatibility, may be possible. “This opens the door to a variety of applications, including novel drug-delivery systems and smart templates for the bioseparation of one molecule from another,” says Robert Langer of MIT, who led the team. The alkanethiolate (16-mercapto)hexadecanoic acid (MHA) has a hydrophobic chain capped by a hydrophilic carboxylate group and forms a SAM on a Au surface. The chains have a straight, upright equilibrium conformation, presenting the carboxylate groups to the surrounding medium. Under an applied electric potential, the carboxylate groups are attracted to the Au surface, causing the molecules to rearrange and expose the hydrophobic chains. This approach relies on obtaining a low density monolayer on the Au surface to give enough space for the conformational change. Synthesizing an MHA derivative with a bulky endgroup gives a self-assembled monolayer with the required density. This end-group is then removed. Sum-frequency generation spectroscopy confirms the reversible molecular switching in response to the electrical potential and contact angles for the SAM showed switching of the macroscopic hydrophilicity. “This is the first time to our knowledge that anyone has created a truly reversible switch of a surface’s property exploiting monomolecular layers,” says Joerg Lahann of MIT.
Doctoring nanotubes NANOBIOTECHNOLOGY There is currently much interest in the use of nanoparticles for biomedical applications, from drug and biomolecule delivery to biosensors. While many groups have investigated the use of spherical particles, researchers from the University of Florida and VTT Biotechnology, Finland are putting the case for nanotubes. Charles R. Martin and coworkers have developed a template synthesis method that produces nanotubes with a number of advantages. The template allows control of the dimensions of the nanotubes, and enables them to be made out of nearly any material. Importantly for biomedical applications, the nanotubes have inner voids that can be filled with small molecules or large proteins, and the inner and outer surfaces can be modified separately to give distinct functionalities. Using silica nanotubes, the group has performed a series of proof-of-concept experiments that show the potential of this approach [JACS (2002) 124, 11864]. First, by derivatizing the outer
Membrane-less fuel cell
surface of the nanotubes, the
ELECTROCHEMISTRY
the nanotubes into either an organic or
A millimeter-scale fuel cell without the membrane that normally separates the anode and cathode compartments has been designed and fabricated by chemists at Harvard University [JACS (2002) 124, 12930]. Instead, the laminar flow at low Reynolds number of two liquids, one oxidizing and one reducing, is exploited to stop the mixing of the two fuels. Fuel cells with this feature have a channel with two inlets and one outlet running past graphite electrodes. V(II) and V(V) aqueous solutions flow into the channel to give the redox couples V(III)/V(II) at the anode and V(V)/V(IV) at the cathode. Three of these cells in series were able to power a light-emitting diode (LED). The group, led by George M. Whitesides, also demonstrated some of the advantages and disadvantages of these membrane-less fuel cells. The system shows poor efficiency in terms of the ratio of fuel consumed at the electrodes to that delivered to the cell. However, it eliminates the ohmic losses and fouling of a membrane, is comparable in performance to macroscopic fuel cells, and could prove a general format for electrochemistry in microsystems.
researchers controlled partitioning of aqueous phase. Second, nanotubes with hydrophilic outer surfaces and hydrophobic inner surfaces efficiently extracted lipophilic molecules from an aqueous solution. Third, immobilizing an antibody fragment to the nanotubes allowed the recognition and moleculespecific extraction of an enantiomer from a racemic mixture. Finally, the group was able to attach enzyme molecules to give biologically active nanotubes. The researchers are keen to stress the applicability of their method to most materials, which means that biodegradable nanotubes for in vivo conditions could be made.