Dynamic Measurements with Scanning Probe Microscopy: Surface Studies Using Nanostructured Test Platforms of Metalloporphyrins, Nanoparticles and Amyloid Fibrils
A hybrid imaging mode for characterization of magnetic nanomaterials has been developed, using atomic force microscopy (AFM) combined with electromagnetic sample actuation. Instead of using a coated AFM probe as a magnetic sensor; our strategy is to use a nonmagnetic probe with contact mode AFM to characterize the vibration of magnetic and superparamagnetic nanomaterials responding to the flux of an AC electromagnetic field. We refer to the hybrid imaging mode as magnetic sample modulation (MSM-AFM). An oscillating magnetic field is produced by applying an AC current to a wire coil solenoid placed under the sample stage for tuning selected parameters of driving frequency and strength of the magnetic field. When the AC field is on, the AFM probe is scanned in contact with the sample to sense periodic changes in the force and motion of vibrating nanomaterials. With MSM, responses of both the amplitude and phase signal along with spatial maps of the topography channel can be collected simultaneously. A requirement for MSM is that the samples can be free to vibrate, yet remain attached to the surface. Particle lithography was used to prepare well-defined test platforms of ring structures of magnetic or superparamagnetic nanomaterials. Capillary filling of polydimethylsiloxane (PDMS) molds was applied to generate stripes of FeNi3 nanoparticles with microscale dimensions as test platforms. The MSM-AFM imaging mode was used successfully to characterize nanomaterials of FeNi3 nanoparticles, cobalt nanoparticles, octa-substituted porphyrin nanocrystals and ionic liquid nanoGUMBOS with dimensions ranging from 1 to 200 nm. Dynamic MSM-AFM measurements can be obtained by placing the tip on a vibrating nanoparticle and sweeping the frequency or field strength. Changes in frequency spectra and vibrational amplitude can be mapped for nanoparticles of different sizes, shapes and composition. The MSM-AFM imaging mode provides a useful tool for investigating changes in size dependent magnetic properties of materials at the nanoscale. Samples of designed amyloid proteins were characterized ex situ using scanning probe microscopy. The progressive growth and fibrillization of amyloid â over extended time intervals was visualized with high resolution using AFM
