The term asuperficial' has had bad press. If you only look at the surface, it implies, you miss all the detail. But it is not really true. You can find out a great deal from surfaces a either by looking at them, or by exploiting their properties. You just have to know the right way to look, and with which tools. Stuart Nathan reports.
Analytical laboratories have made great strides in the use of surface science. The high magnifications of the electron microscope family has revealed details on the atomic scale, for example. But new techniques, using exotic substances and esoteric properties of materials, are now taking the techniques to a new level.
Since their discovery in 1991, nanotubes have been a Cinderella structure obviously full of potential, but seeking an application. With lengths in the micrometre range, but only a few atoms wide, these molecular carbon tubes have structures related to the football-shaped fullerene carbon molecules.
The strength of nanotubes weight-for-weight, they are six hundred times stronger than steel and their semiconductive properties have given rise to a few applications in materials science and compact electronics. However, laboratory scientists may soon be using them in a very different field analysing the structure of complex biological molecules.
The technique, which has been developed by Stan Wong of the State University of New York at Stony Brook, has grown out of the field of atomic force microscopy. This uses a fine needle-like probe to track the bumps and grooves of a surface, with the probe's movements tracked and mapped by a laser. The finer the probe, the higher the resolution of the surface. Clearly, a nanotube would provide an extremely fine probe, which would be able to detect atomic-level features on the surface. However, Wong's research has not stopped there.
As the tip of the nanotube is comprised simply of carbon atoms, it is accessible for chemical transformations. Wong has found that shortening the tubes in an oxygen environment results in a carboxylic acid group (CO2H) being added onto the end of the tube, making the probe physically as well as chemically sensitive. Most notably, the probe is now sensitive to alkaline groups, such as the amide (NH2) groups which are ubiquitous in proteins and other amino acid-rich substances. This means that an atomic force microscope with a modified nanotube probe is ideal for examining the structure of a folded protein, such as an enzyme.
However, Wong's research is taking him into a different field. Working with biologists at Harvard Medical School, Wong is using the device to investigate the progression of Alzheimer's disease at a molecular level.
Alzheimer's is caused by tangles of a protein called beta-amyloid, which form into dense aplaques' in the brain, disrupting and destroying the normal cellular structure.
This process has several stages, and in all takes decades to progress. First, small aseeds' of the protein appear, which gradually grow into filaments called protofibrils. These twist together to form rope-like strands called fibrils, which tangle together to form the plaques. It is only then that the first symptoms of the disease becomes apparent.
The formation of the fibrils seems to be the first irreversible step in the disease progression, and it is a sudden event, but its trigger which could be the length of the protofibrils, for example, or their density, or a certain type of branching is unknown. Wong's research is allowing Harvard's scientists to distinguish between protofibrils and fibrils in in vitro samples, and to detect defects in their structures, branching, and intertwining.
In the medium term, the ability to detect fibrils could allow the diagnosis of Alzheimer's much earlier currently, it can only be diagnosed clinically, with an absolute diagnosis possible only by an autopsy. In the longer term, understanding how the disease progresses could allow treatments to be developed.
Moreover, if this technique is successful, it could pave the way for investigation into other brain diseases. Parkinson's and
Huntington's diseases are all connected with abnormal protein structures in the brain, as is Creutzfeld-Jakob's disease, the human form of amad cow' disease. None of these conditions are currently understood.
Protein research is also the homeground for surface plasmon resonance technology, an analysis technique which works by surface science phenomena rather than through examining the surface. The applications for the technique, which can detect and measure protein binding, are literally sky-high it is currently being considered for use aboard the International Space Station.
Surface plasmon resonance is a property of metals. When light of a particular wavelength characteristic of individual metals strikes the metal surface, most of the energy of the photons is transferred to the free electrons within the material. This results in an energy wave on the surface of the metal, known as a plasmon, and a 10 per cent reduction in the reflection of the light. The critical wavelength changes if there are molecules bound to the surface of the metal the more molecules, the larger the shift.
This property is the key to using surface plasmon resonance in biosensors. If a piece of gold (used because of its resistance to oxicdation) is coated with antibodies for a particular substance, then exposed to a blood sample, any molecules of the target substance will bind to the antibodies. This will cause a shift in the plasmon resonance frequency proportional to the concentration of the target molecule.
SPR sensors are already finding uses in laboratories. Swedish firm Biocore International has produced a sensor which is proving its worth in the development of a drug to counter kidney conditions associated with the autoimmune disease lupus. The disease causes the body's defence mechanisms to manufacture antibodies which attack essential biological systems; the kidney conditions, for example, are believed to stem from an antibody which attacks doubles-stranded DNA. The drug company, La Jolla Pharmaceuticals, is developing a drug which binds to the offending immune cells, blocking antibody production.
A vital factor in the development of the drug is the binding affinity of each prospective patient's antibodies to the drug molecule. Those with high affinities respond well to the drug, reporting only a third as many kidney flare-ups and needing far fewer doses of the potentially toxic drugs needed to counter the condition. LJP used Biocare's sensor to design a blood test which pinpointed which patients were likely to benefit most from the drug.
Other applications for SPR include high-throughput screening. Different antibodies can be bound to specific sites on the metal surface, which is then exposed to an antigen-containing sample. This would determine the binding affinities of each of the antibodies simultaneously.
The usual method for measuring binding affinities uses radioactive and fluorescent atags' to mark the proteins being studied. As SPR instruments do not need these, the technology is currently being assessed for use on the International Space Station. NASA plans to equip ISS's laboratory so that it can perform the same functions as an Earthbound lab, but needs to track down techniques which are suited to its cramped, zero-gravity environment. SPR, which requires no reagents and performs its analysis in a sealed vessel, would seem to fit the bill perfectly. A Connecticut-based firm, Ciencia, is currently working on compact, rugged version of the SPR sensor, which could soon be bound skywards.
