Nanoscale systems designed to imitate functions from the macroscopic world lead to a new appreciation of the complexity needed to actuate motion at the limits of miniaturization. A nanoscale 'car' is the latest example. See Letter p.208
A specially designed molecule that has four motorized 'wheels' is reported by Kudernac et al.1 on page 208 of this issue. By depositing the molecules on a surface and providing them with sufficiently energetic electrons from the tip of a scanning tunnelling microscope, the authors were able to drive some of the molecules in a specific direction, much like a car with four-wheel drive. Previous examples of actuated single-molecule motion have been reported2,3,4, but none with the complex action to continue moving in the same direction across a surface like the system devised, synthesized and operated by Kudernac and co-authors.
Biological systems have inspired the design, construction and operation of materials and nanometre-scale devices that have analogous functions on all scales4. In 'top-down' approaches, nanoscale components have been put together whose functions roughly mimic aspects of those of the kidneys, muscles, nose and so on4,5,6. On the smallest scales, molecules have been designed and synthesized to demonstrate various capabilities, and, more importantly, to explore the limits of miniaturization, operation and efficiency for both natural and artificial machines. For example, single-molecule electronic switches and simple molecules that move in response to various stimuli have been demonstrated2,3,4. Typically, these synthetic systems are less flexible than their biological inspirations. This makes it easier to couple experimental results from such systems to theory and simulations ranging from quantum chemistry to mechanical engineering. From such studies, we ultimately hope to learn some of nature's secrets of robust operation, high energy-conversion efficiency and hierarchical function and action.
In such work, the forces that dominate the macroscopic world around us, such as gravity, are less important than the forces that rule the nanoscale and biological worlds, such as van der Waals and capillary forces. Nevertheless, there are similarities between our everyday world and the nanoscale world: a car must have not only the ability to accelerate, but also brakes and traction; likewise, nanoscale actuated motion depends on balancing applied forces with those that hold structures in place. Kudernac et al.1 solve this problem, as others have, by balancing surface interactions and thermal energy with forces that can be applied through actuation, so that their molecules move only on stimulation, rather than by diffusion.
Scanning probe microscopes, especially scanning tunnelling microscopes (STMs), have been used to push, pull and place atoms, molecules and supramolecular assemblies on surfaces7,8,9 (Fig. 1). In complementary studies, mobile atoms and molecules on surfaces have been tracked using STMs to measure nanoscale interaction strengths between molecules, and between molecules and surfaces10,11,12. In studies of both diffusion and manipulation, some of the molecules have been designed to include parts that test modes of motion. For example, do molecular wheels roll or slide? At least in some cases, they roll12. And can synthetic molecules be made to run unidirectionally on tracks, like biomolecular motors? Thus far, they can do so only with carefully programmed tracks and fuel13.
Many of these molecules are analogous to familiar objects, and the similarities are often emphasized in schematic depictions of the molecules that leave out chemical groups and/or structural distortions caused by interactions of the molecules with surfaces. But such groups and distortions have significant consequences for their structures and motion on surfaces that often render our intuition about their behaviour incorrect. For example, although steps just one atom high on a surface can pose significant barriers for atoms, molecules and supramolecular structures moving on that surface, the relatively large molecules used in many studies distort so as to 'climb' up or down such steps with ease12. In some cases, the presence of a moving molecule can even cause the atoms of the steps to reorganize, to make stronger contacts with molecules14.
With their inspired design of a four-wheeled molecule, Kudernac et al.1 have gone a sizeable step further than previous studies. The wheels are actually molecular ratchets (rotary motors that turn in a single direction), some or all of which can be actuated to move when sufficiently energetic electrons are supplied by an STM tip. The authors observed that, on surfaces, the nanoscale car lurches forward by a fraction of a nanometre if each wheel rotates in the same direction. When driven repeatedly in this way, the molecules exhibited net forward motion, although the route taken was not perfectly linear — presumably owing to mismatches between atoms of the surface and those of the wheels — and so the distance travelled was not quite as far as theoretically possible (see Fig. 2 of the paper1). The authors used the STM not only to manipulate their molecules, but also, in a less perturbative mode, to image the progress of the molecules, an approach that has been used in earlier studies7,8,9,11.
A complication of this work is that the chemical synthesis used by Kudernac et al. to make their molecules does not (yet) control the attachment of the wheels to ensure that they all turn in the same direction. In fact, only a fraction of the molecules synthesized had all four wheels optimally connected. Separating these cars from the others using chemical methods prior to deposition on a surface might be desirable, but this is prohibitively difficult at present. However, the properly constructed cars can be differentiated by function (that is, by their directional movement in response to stimulation) once they are deposited on a surface and probed individually.
In addition, the operation of the molecules depends on whether they land in the correct orientation on the surface — akin to having a car factory in which half the fully assembled vehicles are immobilized when they drop off the production line, because they land on their roofs or sides. The key difference is that, in chemical syntheses, trillions of molecules are synthesized in parallel. It has, however, been possible to design asymmetrical molecules in other systems that always orient themselves in a particular way upon deposition15. The orientational and other impediments in the present work1 will, therefore, ultimately be cleared away through molecular-level understanding of the systems and further design constraints.
In developmental and evolutionary biology, 'gains of function' are key points in time. The work of Kudernac et al. represents just such a gain in function for nanotechnology: molecules with multiple motors can now be synthesized, deposited and operated to move directionally across a surface. The authors' system provides a tremendous opportunity to understand how molecular motors can interact, and how molecule–surface interactions affect molecular motion. It also, however, points out many of the complications that will come with creating increasingly complex syntheses and increasingly functional nanostructures. Nonetheless, we can expect further significant gains of function as our understanding of, and intuition for, the nanoscale world grows, and as accessible synthetic structures become ever more complex and sophisticated.
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