Fabrication of gold tips by chemical etching in aqua regia.

We present a method to produce sharp gold tips for applications in apertureless near-field optical microscopy and spectroscopy. Thin gold wires are tapered by chemical etching in aqua regia, covered by an isooctane protective layer. Tips with apical radii of curvature of <50 nm are obtained with a 40% yield. The tip performances have been checked by shear-force imaging of amyloid fibrils samples and compared to optical fiber probes. The analysis of the tip morphology, carried out by scanning electron microscopy, shows the existence of two different etching processes occurring in bulk and at the liquid-liquid interface. A simple analytical model is presented to describe the dynamics of the tip formation at the liquid-liquid meniscus interface that fits remarkably well the experimental results in terms of tip shape and length.