SCANNING-TUNNELING-MICROSCOPY OF COLLOIDAL GOLD BEADS

Biological molecules have been imaged by scanning tunneling microscopy, although the reliability and reproducibility of such images have been questioned. Colloidal gold beads are good conductors and are of well defined size and shape, hence they provide a convenient positive control for scanning tunneling microscopy (STM) studies of biomolecules. Aggregates of 5 and 20 nm beads can be reproducibly imaged on graphite, provided that the tunneling current be kept low (around 30 pA). Isolated beads are unstable under these conditions, suggesting that even if biomolecules do adequately conduct current, they must be attached securely to the substrate in order to be imaged. The height of 20 nm beads was accurately measured by STM, while the width was overestimated due to tip convolution. Platinum/carbon shadowing appeared to have a stabilizing effect and allowed the imaging of isolated 20 nm beads. Shadowing resulted in a 20% decrease in height measurements, whereas the measured width was increased by 20%.