Metal binding colloidal gold particles: A versatile scanning force microscope tip calibrator for fluid imaging

One of the primary difficulties of imaging under aqueous solution with the scanning force microscope (SFM) is a lack of reliable tip characterization. The requirements of imaging under solution greatly limit the utility of available standards. We report the synthesis of metal binding colloidal gold (MBG) which allows direct deconvolution of tip shape during sample imaging. Divalent metal ions have been used extensively for binding samples to mica for SFM in fluid, especially biological samples such as DNA. The MBG synthesized here can be imaged bound to mica, demonstrating their usefulness for fluid SFM. Their diameter has been measured and is consistent with the expected value. They are shown to be robust to repeated imaging and stable in a variety of conditions. Their ability to be imaged is inhibited by the presence of a metal chelator. Finally, these gold particles, in conjunction with computational image restoration methods, have been used to analyze plasmid DNA images obtained in aqueous solution. Enabling colloidal gold particles to bind metals through the attachment of a synthesized polypeptide allows simpler and faster tip calibration in aqueous solution than calibrations obtained by previously described methods.