Andrew D. L. Humphris

A mechanical microscope: High-speed atomic force microscopy

An atomic force microscope capable of obtaining images in less than 20ms is presented. By utilizing a microresonator as a scan stage, and through the implementation of a passive mechanical feedback loop with a bandwidth of more than 2MHz, a 1000-fold increase in image acquisition rate relative to a conventional atomic force microscope is obtained. This has allowed images of soft crystalline and molten polymer surfaces to be collected in 14.3ms, with a tip velocity of 22.4cms−1 while maintaining nanometer resolution.

Chemical sensors and biosensors in liquid environment based on microcantilevers with amplified quality factor

A new technique is presented for bio/chemical sensors, based on microcantilevers, for detection in liquid environment. The low quality factor of the cantilever in liquid is increased up to three orders of magnitude by using Q-control. This enables AC detection that is immune to the long-term drift of the DC cantilever response in liquids, and to temperature variations. This technique has been applied for the detection of ethanol in aqueous solution by using the microbalance method, and for antibody/antigen recognition by the surface stress method. The results show the feasibility and very high sensitivity of these novel devices.

Piconewton regime dynamic force microscopy in liquid

In this work, a cantilever in a fluid is driven by a mixed signal composed of a standard driving signal and a feedback signal consisting of the amplified and phase shifted oscillation signal. This mimics the oscillation of a cantilever with a quality factor up to three orders of magnitude higher than its natural Q (∼1). This technique allows the identification of the resonance frequency of the cantilever by mechanical excitation of the fluid. The improved sensitivity has been checked by imaging a very soft sample of 1% agarose gel in the dynamic mode. A force smaller than 50 pN could be applied to the sample, improving the spatial resolution and the phase contrast significantly. This technique provides a major improvement in atomic force microscopy/spectroscopy in liquids.

Ultrahigh-speed scanning near-field optical microscopy capable of over 100 frames per second

Scanning near-field optical microscopy is a powerful technique offering subdiffraction-limit optical resolution. However, the range of applications is limited by slow image acquisition rates. In this letter we demonstrate an implementation of a near-field optical microscope capable of scan speeds of 150 mm/s producing images of an area 20 μm2 in less than 10 ms, i.e., over 100 frames/s. To achieve this, a method of measuring the optical near-field intensity with a high bandwidth of greater than 1 MHz has been developed. A second original aspect is that the scan system uses a mechanical resonance of the probe to address the sample. The presented microscope is over 1000 times faster than a conventional scanning near-field optical microscope and ∼10 times faster than any scanning probe microscope to date.

Observation of molecular layering in a confined water film and study of the layers viscoelastic properties

A transverse dynamic force microscope, more commonly known as shear force microscope, has been used to investigate confined water films under shear. A cylindrically tapered glass probe was mounted perpendicularly to the sample surface. Pure water was confined between the probe and a freshly cleaved mica surface and a sinusoidal shear strain was applied by setting the probe into transverse oscillation. Repeated measurements of the probe oscillation amplitude and relative phase lag, at different tip-sample separations, exhibited a clear step-like behavior. The periodicity, recorded over several curves, ranged between 2.4 and 2.9 A, which is similar to the diameter of the water molecule. The in-phase (elastic) and the out-of-phase (viscous) stress response of the confined water film was evaluated (from the experimental data) by assuming a linear viscoelastic behavior. Finally, by modeling the water film with the Maxwell mechanical model, the values for the shear viscosity and shear rigidity were obtained.

High-Q Dynamic Force Microscopy in Liquid and Its Application to Living Cells

We present a new dynamic force microscopy technique for imaging in liquids in the piconewton regime. The low quality factor (Q) of the cantilever is increased up to three orders of magnitude by the implementation of a positive feedback control. The technique also includes a phase-locked loop unit to track the resonance of the cantilever. Experiments and computer simulations indicate that the tip-sample forces are below 100 pN, about two orders of magnitude lower than in conventional tapping mode atomic force microscopy. Furthermore, the spectroscopic ability is greatly enhanced. Either the phase shift or the resonant frequency shows a high sensitivity to variations in either the energy dissipation or conservative interactions between the tip and the sample, respectively. The potential of this technique is demonstrated by imaging living cells.

Atomic Force Microscopy (AFM) Study of Interactions of HMW Subunits of Wheat Glutenin

ABSTRACT Atomic force microscopy (AFM) has been used to study the noncovalent interactions of alkylated HMW subunit 1Dx5 and a M r 58,000 peptide derived from the central repetitive domain. Both protein and peptide align side-by-side to form fibrils, the HMW subunit forming a branched network, and the peptide forming linear rods. The N- and C-terminal domains of the subunit would, therefore, appear to contain regions that interact through noncovalent interactions in the absence of disulfide bond formation. These regions may be of importance in facilitating disulfide bond formation during protein body development.

Large scan area high-speed atomic force microscopy using a resonant scanner

A large scan area high-speed scan stage for atomic force microscopy using the resonant oscillation of a quartz bar has been constructed. The sample scanner can be used for high-speed imaging in both air and liquid environments. The well-defined time-position response of the scan stage due to the use of resonance allows highly linearized images to be obtained with a scan size up to 37.5 mum in 0.7 s. The scanner is demonstrated for imaging highly topographic silicon test samples and a semicrystalline polymer undergoing crystallization in air, while images of a polymer and a living bacteria, S. aureus, are obtained in liquid.

Some recent developments in SPM of crystalline polymers

Some recent advances in the application of atomic force microscopy to crystalline polymers are detailed. Ultra-high resolution imaging of crystal surfaces, combined with the analysis of computer generated Connolly surfaces, enables the unambiguous identification of features on the cellulose crystal surface at near-atomic resolution. The electronic enhancement of the quality factor of the cantilever when tapping in liquids enables a considerable improvement in force sensitivity to be obtained, allowing the fully saturated surface of an isotactic polystyrene gel to be imaged under the solvating molecule, at nanometre resolution. A series of experiments are detailed in which processes such as crystallization, crystal thickening and crystal deformation are followed in situ, in real time, providing significant new insights into long standing problems in polymer science.

Modeling of cylindrically tapered cantilevers for transverse dynamic force microscopy (TDFM).

In transverse dynamic force microscopy a cylindrically tapered cantilever is mounted perpendicularly to the sample surface and set into transversal oscillation. The dynamics of the cantilever has been studied using the continuum mechanical model with discrete element analysis. A viscoelastic model has been used to describe the tip-sample interaction. In this way an in-phase and an out-of-phase component of the force has been extracted from the experimental data. Two different techniques, involving two experimental setups and two corresponding data analysis routines, have been developed to calculate the two components of the force at different tip-sample separations. In one case the change in resonant frequency and corresponding oscillation amplitude is measured whereas in the second case the usual way of recording amplitude and phase signal at a fixed driving frequency is applied. The results from these two methods are shown to be completely consistent and produce almost identical force curves.