Synthetic tips

SIR-Capabilities in scanning tunnelling and atomic force microscopy (STM and AFM) depend on probe-tip properties, but tip properties are at present poorly controlled; suitable tips have poor reproducibility at the atomic level. AFMs with tips characterized by a small (~ 1 N/m) vertical spring constant6 and a large (⅝103 N m"1) transverse spring constant (T. Albrecht, personal communication) can be positioned with atomic precision in a liquid medium6. Since the spring constants for deforming multinanometer scale globular proteins and for bending single bonds are both >10 N m-1, there seems no obstacle to positioning bound reactive moieties with a transverse spring constant £ N m-1. At 300 K, a thermally excited structure with a 1 N m-1 spring constant will be characterized by a gaussian positional uncertainty with sigma 103. [...]strong and adjustable positional control of effective concentrations (and hence of reaction rates) on an atomic distance scale appears physically possible in a class of systems that may be practically realizable. 1.