Gold glitters because it is highly reflective, a quality once considered important for precision measurements made with gold-coated probes in atomic force microscopy (AFM). In reality, the usual gold coating on AFM probes is a major cause of force instability and measurement imprecision, according to research done by the Perkins group. The group has shown that gold-coated probes are a particular problem for high-precision measurements of the tiny forces involved in the folding and stretching of large biomolecules, such as proteins and DNA, in liquid.
The best commercial AFM probes are very thin diving-board-shaped cantilevers made of silicon or silicon nitride. Since these thin structures barely reflect the laser beams used to detect their positions, they are coated with gold to enhance reflectivity.
The Perkins Group uses the technique of AFM-based force spectroscopy in liquid environments – precision measurement of piconewton-level forces – to learn about the dynamics of large biomolecules that are crucial to normal physiology and disease. Force spectroscopy requires exquisite control of both the position of the AFM tip on the atomic scale, and stable force measurements.
The Perkins team previously made great advances in stabilizing AFM position. But their research on force spectroscopy was limited by substantial drifts in force measurements that plagued all users of gold-coated AFM cantilevers. Although the gold coating gives a much stronger optical signal, the coating moves around slightly on the cantilever, causing the cantilever to move unpredictably. So instead of the AFM cantilever moving only in response to the molecule being studied, the gold coating itself was causing cantilever motion and creating significant measurement errors.
Not surprisingly, the group decided to go after the gold that was interfering with their research. Graduate student Allison Churnside, research associate Ruby May Sullan, undergraduates Duc Nguyen, Sara Case, and Matthew Bull, former research associate Gavin King, and Fellow Tom Perkins used a short chemical treatment to strip the gold off a commercial cantilever. Then, they compared this cantilever’s behavior with that of a traditional gold-coated one.
The Perkins group's comparison showed that measurements made with the uncoated cantilever were ten times more precise in a measurement time of one second. It also showed that the gold coating was a primary source of force drift over hours. Force drift is a measure of how far the probe’s starting position drifts during a series of experiments. With the normal gold-coated cantilevers, this drift was about 1,000 nm; it was only 70 nm when the gold had been stripped away.
The improvements in stability and precision were evident just 30 minutes after the uncoated probe was readied for an experiment. In contrast, gold-coated probes often require more than two hours or overnight to settle down enough to allow an experiment to proceed.
The Perkins group anticipates that the advantages of doing away with gold coatings on AFM cantilevers will benefit all sorts of experiments in biophysics and nanoscience. — Julie Phillips and Tom O’Brian