IMAGINE a miniature bucket. Its inner surface is oily and water repellent, but hanging off its outer rim are wettable, oil-repellent strands. This bucket really is tiny 鈥� just big enough to hold one smallish, oily molecule. But that isn鈥檛 all that makes it special. Attached to it is a group of atoms that light up when the bucket is full 鈥� though only when the molecule filling it is of a certain type.
The submicroscopic bucket is an example of what are called 鈥渟upramolecules鈥� 鈥� large molecules that have been specially constructed to be a particular shape. Over the past few years, chemists have learnt to sculpt individual molecules into shapes that range from miniature pagodas to the world鈥檚 smallest bracelets. Intriguing as these molecules are, making them has a practical purpose too. In my research group at Michigan State University (MSU), we have been designing molecular buckets and cradles that can trap and detect a variety of chemicals that are commonly found polluting the environment. We can also use similar supramolecules for tracking complicated airflow patterns over aeroplane wings or within car engines. Other researchers have produced supramolecules that can be built into logic networks for computers, and artificial molecules that mimic the behaviour of natural enzymes.
The miniature bucket was devised in 1992 by Zoe Pikramenou, who is now at the University of Edinburgh but was then a graduate student at MSU. She was looking for a container that would trap and detect a molecule of the pollutant benzene, and realised that a substance called cyclodextrin would fill the bill. A cyclodextrin molecule is made up of six, seven or eight molecules of the common sugar D-glucose. A glucose molecule is itself ring-shaped, and in cyclodextrin the glucose units are joined head-to-tail to form another, larger ring.
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This structure gives cyclodextrin some unusual properties. The inside of the large cyclodextrin ring is formed by the ring-shaped cores of the glucose unit, which are essentially hydrocarbon in character and so repel water and attract oily compounds. Moreover, the cavity is just the right size to hold a benzene molecule. But hanging off the outside of the ring are hydroxyl groups, which have a high affinity for water. The result is that the bucket-shaped cyclodextrin molecule dissolves in water but will fill itself with any hydrophobic compounds such as benzene or similar organic pollutants that happen to be floating past.
So cyclodextrin looks like a promising benzene trap. But building the trap solves only half the problem. Pikramenou wanted the trap to act as a benzene detector, so she needed to know when it had caught a molecule. Supramolecules cannot act as sensors by themselves. They must be combined with some other substance that produces a signal in the presence of the target molecule.
At MSU, we were already working on the problem of building a signalling system like this. When atoms or molecules absorb light energy, they usually re-emit it as heat. But in some cases this does not happen. Instead they 鈥渓uminesce鈥�, re-emitting the energy as light of a different frequency that is characteristic of that particular type of molecule. Unfortunately, there are not many pollutants that emit visible light in this way, but in 1991, Ronald Lessard (now leading a research team at Nalco Chemical Company), Jeong-a Yu (now at Chosun University in South Korea) and I hit on a way of making them do so. Our idea was to build a supramolecular trap in which the pollutant would absorb energy from a light source, and then immediately pass it on to another supramolecule before it could be lost as heat.
Another part of the supramolecular trap would be designed to receive this energy, and to emit a burst of visible light when it arrived, announcing the presence of the pollutant. We reasoned that by shining a continuous beam of light onto the trap, we could make the trapped molecule repeatedly absorb light energy and pass it on to the light-emitting unit, stimulating it to glow at its characteristic visible frequency. The result would be a substance that glowed 鈥� but only when the pollutant was present.
To test our idea we set out to detect benzoate ions 鈥� benzene with a carboxylate group attached 鈥� by using them to trigger light emission from terbium ions built into another part of the trap. Terbium is one of the metallic elements known as lanthanides, and lanthanide ions are well known for re-emitting any energy they absorb as light. They do not normally do this because they are not good at absorbing the energy they need to become excited in the first place. But for us this was not a drawback. In fact it was a positive advantage, because we wanted the lanthanide ion to light up only when something else was absorbing energy and passing it on.
Molecular crown
The first problem was to make sure that the lanthanide ions were protected from water, because any water molecules sticking to an excited lanthanide ion will soak up its energy and prevent it emitting light. We did this by encircling the terbium with a kind of molecular crown called a cryptand ligand. The cryptand crown contains five oxygen atoms and two nitrogens, which bind to seven of the nine available sites on the terbium. This is enough to protect the terbium, but leaves two sites free for the benzoate, which attaches itself via the two oxygen atoms in its carboxylate group.
Having set up our system, we went on to test it by shining laser light onto the system. Sure enough, we found that the benzene ring of the benzoate soaks up energy from the laser, passes it on through its carboxylate arm to the terbium and the terbium glows green. Just to be sure that this was what was happening, Lessard and Yu used ultrafast lasers to track the energy as it passed from the benzoate to the terbium. They saw the absorbed energy flow out of the benzene ring along the carboxylate arm of the benzoate in just 93 picoseconds (million-millionths of a second). When it arrived at the terbium ion, the energy was quickly converted back into light: the green luminescence of the terbium ion appeared just 10 picoseconds later.
This was all very well, but benzoate ions are neither a very common pollutant nor a serious one 鈥� which is where Pikramenou鈥檚 benzene-trapping bucket comes in. To show when the bucket has been filled by a much more serious pollutant such as benzene, she tethered a cryptand ligand and its associated lanthanide ion to the cyclodextrin. The cryptand was attached at just one point, like a handle attached at only one side of the bucket so that it could swing freely. She then added europium ions; she chose europium because it emits red light 鈥� as distinct a colour as possible from benzene鈥檚 normal faint blue fluorescence. Shining blue light on the empty bucket gave no response, but when benzene was added and slipped into the bucket, the europium glowed red.
The next step was to try to improve the sensitivity by putting the europium ion even closer to the benzene molecule. Pikramenou reasoned that this arrangement should allow the energy to flow more quickly and easily between the two. Attaching the cryptand ligand to the bucket in two places, to make it more like a cradle than a handle, achieved the desired arrangement. But it did not have the desired effect: to our surprise, this arrangement turned out to be a much less sensitive benzene detector than the one with the swinging handle. We think that the problem is that forcing the positively charged europium ion to sit very close to the bucket makes the inside of the bucket much less hydrophobic, and hence less likely to take up a benzene molecule. To overcome this effect another researcher in my group, Mark Mortellaro, is now trying to build extra negatively charged binding sites into the cradle to neutralise the positive charge of the europium ion.
Promising though our technique is for picking up the presence of benzene and related compounds, it has an obvious limitation: it will only work if the polluting molecule can absorb light energy and pass it on. Many of the molecules that pollute the environment, including long-chain hydrocarbons, chlorinated hydrocarbons and alcohols, are poor at harvesting light. But inspiration for a way to detect these molecules was at hand, and from an unlikely source: glow-in-the-dark Frisbees. Trapped inside the plastic that these glowing Frisbees are made from is a compound whose molecules absorb the energy from an ordinary light bulb, and then hang on to it for a long time. The molecules in this excited state eventually lose their energy as light -hence the glow. This only happens, however, because the glowing compound is trapped in plastic. In the open air, oxygen would rob the excited molecules of their energy long before they emitted it as light. But because oxygen moves only slowly through the plastic, it can鈥檛 get to the excited molecules in time to deactivate them.
In 1993, Adrian Ponce and Wanda Hartmann, two researchers in my group, decided to borrow the idea to build a detection system for alcohols. Inside a cyclodextrin molecule they placed 1-bromonaphthalene, which has a long-lived excited state that emits green light. But even inside the bucket, the excited state is easily quenched by oxygen, so it stays dark. However, when they added an alcohol and then illuminated the system, it glowed bright green. They found that the alcohol was binding to the rim of the bucket, and its long chain was flipping over the top. In this position it acted as a lid to protect the bromonaphthalene from oxygen, allowing it to glow. The results can be spectacular: adding alcohol increases the intensity of the light emitted by a factor of up to 100 000, depending on the alcohol used and how well it fits onto the bucket.
Dye release
Meanwhile, Mortellaro is trying a different approach for detecting long-chain hydrocarbons. It is especially difficult to know how to deal with these compounds, because, unlike alcohols, they have no active chemical groups that will bind to the cyclodextrins. The idea here is to tether a dye molecule to a cyclodextrin bucket. When there is nothing else around, the dye will sit quietly inside the bucket. But because a long-chain hydrocarbon is much more hydrophobic than the dye, it will kick the dye molecule out in order to get inside. This exposes the dye molecule to water, causing it to lose a proton and begin to glow. The neat thing here is that changing the dye to make it more or less hydrophobic might even make the detection system sensitive to different hydrocarbons.
These applications are just the beginning. Combining different supramolecules and binding them to the tip of an optical fibre could yield a detector that simultaneously picks up many different pollutants. The next step could be a detector on a microchip the size of a pinhead that decides how the pollutants should be destroyed on the basis of the colour and length of the light pulse.
But detecting pollutants is not the only trick that our supramolecular systems have up their sleeves. They can also be used to track the motion of fluids, with potential benefits for designing more efficient car engines, or safer aircraft wings. The conventional way of imaging fluid motion is to seed the fluid with millions of particles. They can be of polystyrene, metal particles, water droplets or anything that scatters light. When the experimenters illuminate a section of the flow with a sheet of laser light, the particles reflect the light to reveal their positions. Shortly after, another sheet of light records their new positions. By comparing photographs from the two exposures, researchers can calculate the flow velocity at different positions.
This technique has many drawbacks, though. First, it only works if the particles stay within the plane of light that is used to illuminate them. This is a serious limitation because most natural flows are three-dimensional with gusts that take the fluid out of any particular plane. Secondly, the particles are generally much heavier than the molecules of the fluid they are monitoring, and this prevents them from responding quickly to flow changes. Their greater inertia means that they may not adequately track the flow, especially when it changes suddenly. Particles do not go into areas of high turbulence or into areas near surfaces, for instance.
Some five years ago I started investigating the possibility of using supramolecules instead of conventional particle markers. Our idea is to set up a pattern of intersecting laser lines that produce a glowing network of supramolecules. Because the supramolecules are almost as small as the fluid molecules, they provide a faithful record of the fluid flow. Although the glowing grid is illuminated for only a fraction of a second, the probe molecules can continue to glow for long enough to follow the flow for at least a few millimetres. Over this period we use special cameras to record the positions of the glowing net at different times.
Using supramolecules as a probe for fluid flow opens up many possibilities. For instance, anything that disturbs the airflow across an aircraft鈥檚 wing can cause it to become turbulent. The drag then increases, and the plane has to burn more fuel to maintain its speed. In extreme cases, such as ice on the wing or a steep change in the wing鈥檚 angle, turbulence at the leading edge of the wing can cause the airflow to detach itself, and the plane can plummet from the sky. Mechanical engineers led by Manooch Koochesfahani at MSU are using the cyclodextrin/1-bromonaphthalene/alcohol system to study flow at the leading edge of aeroplane wings, with the aim of devising safer, more efficient designs.
Fluid flow is also important in liquids. For instance, an inexpensive way to transport coal is to mix it with water and send it in a slurry down a pipeline. The problem is that the solid can separate from the liquid and clog the pipes. In 1993, under contract from the US Department of Energy, Robert Falco of the Mechanical Engineering Department at MSU and I modelled slurry flow using calcium fluoride as the solid and water as the fluid. By replacing around 3 per cent of the calcium ions with europium we tagged the solid with a red grid; the water was tagged with a green grid formed by doping the water with the terbium cryptand supramolecule. When we shone laser light on this system, an orange grid formed from the combination of green and red. Over several tenths of a second, we watched the grids separate as the turbulent motion began.
By changing flow parameters such as the diameter of the pipes and adjusting the flow of the fluid we were able to control the distribution of solid and liquid. We found that under the right conditions the mixture becomes redistributed to form a dense solid core that slides down the centre of the pipe, eased along by a more dilute mixture formed at the pipe walls. This reduces the energy needed to pump the slurry through the pipe.
Supramolecules are also being used at MSU to probe the complex turbulent flow of the mixture of fuel and air inside car engines, with a view to finding out what changes are needed to make them mix more thoroughly, and improve the engine鈥檚 power and fuel efficiency. Going with the flow: tilting an aircraft wing, for example in an acrobatic manoeuvre, can cause the airflow to detach itself from the wing and may even make the plane plummet. Glowing supramolecules can be used to track the airflow and help solve the problem
