Enhanced quality factors and force sensitivity by attaching magnetic beads to cantilevers for atomic force microscopy in liquid (original) (raw)
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An Atomic Force Microscope with Dual Actuation Capability for Biomolecular Experiments
We report a modular atomic force microscope (AFM) design for biomolecular experiments. The AFM head uses readily available components and incorporates deflection-based optics and a piezotube-based cantilever actuator. Jetted-polymers have been used in the mechanical assembly, which allows rapid manufacturing. In addition, a FeCo-tipped electromagnet provides high-force cantilever actuation with vertical magnetic fields up to 0.55 T. Magnetic field calibration has been performed with a micro-hall sensor, which corresponds well with results from finite element magnetostatics simulations. An integrated force resolution of 1.82 and 2.98 pN, in air and in DI water, respectively was achieved in 1 kHz bandwidth with commercially available cantilevers made of Silicon Nitride. The controller and user interface are implemented on modular hardware to ensure scalability. The AFM can be operated in different modes, such as molecular pulling or force-clamp, by actuating the cantilever with the available actuators. The electromagnetic and piezoelectric actuation capabilities have been demonstrated in unbinding experiments of the biotin-streptavidin complex. Atomic force microscopy (AFM) is best known for high-resolution imaging, but it is also a powerful tool for sensitive force measurements. The technology has been demonstrated successfully for various exciting biomolecular measurements, such as receptor/ligand interactions and protein folding/unfolding at single-molecule level 1–4. Structure, function, and energy landscape of biomolecules can be investigated via force spectroscopy experiments. The method is based on actuation of a functionalized microcantilever with attached biomolecules over a functionalized sample surface while monitoring the deflection of the cantilever, which is a measure of the force on the biomolecules. The cantilever is actuated over a wide range of speeds to obtain the energy landscape in detail. At low actuation speeds, drift in the system induces spurious deflection on the cantilever and adversely affects the measurements 5,6. On the other hand, the dynamics of the actuator and the cantilever determine the attainable actuation speed. A fundamental limit for immersed cantilevers at high speeds is the hydrodynamic drag 7,8. Cantilever geometry determines the effect of hydrodynamic drag. Smaller cantilevers tend to have less hydrody-namic drag as compared to larger ones 9. In addition to hydrodynamic drag, indirect actuation of the cantilever using a large-scale piezo actuator has detrimental effects. In a conventional AFM setup, a large piezo actuates the cantilever through its holder and this excites many other modes in the complete mechanical system 10,11. The resonant frequencies of the large mechanical assembly of the actuator and the holder are usually smaller than the resonant frequency of the cantilever. This sets a practical limitation for actuation at high speeds. Direct actuation of cantilevers in liquid using magnetic 12,13 , photothermal 14 , acoustic radiation pressure 15 and capacitive methods 16,17 have been proposed to overcome this problem. A magnetic actuator can be integrated into a conventional AFM setup by coupling an AFM head with an electromagnet or a permanent magnet. Magnetic cantilevers 18,19 , standard cantilevers attached with magnetic particles 13,20,21 or current-carrying cantilevers 22,23 can be used in these setups. Mechanical modification of a conventional AFM head makes it possible to add magnetic actua-tion capability. Rapid prototyping techniques are appealing for constructing customized AFM setups because they simplify the development of such setups according to user needs. Among different techniques, selective laser sintering (SLS) was previously used to develop a new AFM head for force spectroscopy applications 24. The
Journal of Applied Physics, 1998
The detection sensitivity of an atomic force microscope with optical beam deflection for small cantilevers is characterized experimentally and theoretically. An adjustable aperture is used to optimize the detection sensitivity for cantilevers of different length. With the aperture, the signal-to-noise ratio of cantilever deflection measurements is increased by a factor of 1.5 to nearly 3. A theoretical model is set up that generally describes the optical beam deflection detection in an atomic force microscope. This model is based on diffraction theory and includes the particular functional shape of the cantilever.
Highly sensitive AFM using self-excited weakly coupled cantilevers
Applied Physics Letters, 2019
In this article, we propose a method, using weakly coupled cantilevers, to enhance the sensitivity of atomic force microscopy (AFM) by several orders of magnitudes. There are two major dynamics AFM methods, i.e., amplitude modulation AFM and frequency modulation AFM (FM-AFM). In FM-AFM, which is based on the eigenfrequency shift of a single cantilever, the enhancement in sensitivity is restricted because of the limitations of miniaturization in the manufacturing process. By contrast, we used coupled cantilevers based on the eigenmode shift, which corresponds to the amplitude ratio between the cantilevers. This enabled us to increase the sensitivity by reducing the coupling stiffness between cantilevers without relying on further miniaturization. In addition, to detect the eigenmode shift, even in high-viscosity environments, we produced self-excitation in the weakly coupled cantilevers by feedback control. Using this prototype system of coupled macroscale cantilevers subjected to the magnetic force, which simulates the atomic force, we confirmed the high sensitivity of the proposed method.
Monitoring of an atomic force microscope cantilever with a compact disk pickup
Review of Scientific Instruments, 1999
In the present study we test a compact disk pickup as the cantilever position sensor in an atomic force microscope ͑AFM͒. The pickup is placed on top of the optical microscope used for the visual inspection and alignment of the specimen. The AFM is also equipped with its own cantilever movement sensor system. Both the built-in and the new detection devices are simultaneously active for comparison purposes. Two different measurements are performed in sequence on the same sample each using one sensor at a time as the error signal source for the AFM feedback loop. The pickup has demonstrated good sensitivity as well as excellent performance in terms of compactness, reliability, and cost.
Exploiting cantilever curvature for noise reduction in atomic force microscopy
The Review of scientific instruments, 2011
Optical beam deflection is a widely used method for detecting the deflection of atomic force microscope (AFM) cantilevers. This paper presents a first order derivation for the angular detection noise density which determines the lower limit for deflection sensing. Surprisingly, the cantilever radius of curvature, commonly not considered, plays a crucial role and can be exploited to decrease angular detection noise. We demonstrate a reduction in angular detection shot noise of more than an order of magnitude on a home-built AFM with a commercial 450 μm long cantilever by exploiting the optical properties of the cantilever curvature caused by the reflective gold coating. Lastly, we demonstrate how cantilever curvature can be responsible for up to 45% of the variability in the measured sensitivity of cantilevers on commercially available AFMs.
Magnetostriction-driven cantilevers for dynamic atomic force microscopy
Applied Physics Letters, 2009
An actuation mode is presented to drive the mechanical oscillation of cantilevers for dynamic atomic force microscopy. The method is based on direct mechanical excitation of the cantilevers coated with amorphous Fe-B-N thin films, by means of the film magnetostriction, i.e., the dimensional change in the film when magnetized. These amorphous magnetostrictive Fe-B-N thin films exhibit soft magnetic properties, excellent corrosion resistance in liquid environments, nearly zero accumulated stress when properly deposited, and good chemical stability. We present low noise and high resolution topographic images acquired in liquid environment to demonstrate the method capability.
Short cantilevers for atomic force microscopy
Review of Scientific Instruments, 1996
We have designed and tested a family of silicon nitride cantilevers ranging in length from 23 to 203 m. For each, we measured the frequency spectrum of thermal motion in air and water. Spring constants derived from thermal motion data agreed fairly well with the added mass method; these and the resonant frequencies showed the expected increase with decreasing cantilever length. The effective cantilever density ͑calculated from the resonant frequencies͒ was 5.0 g/cm 3 , substantially affected by the mass of the reflective gold coating. In water, resonant frequencies were 2 to 5 times lower and damping was 9 to 24 times higher than in air. Thermal motion at the resonant frequency, a measure of noise in tapping mode atomic force microscopy, decreased about two orders of magnitude from the longest to the shortest cantilever. The advantages of the high resonant frequency and low noise of a short ͑30 m͒ cantilever were demonstrated in tapping mode imaging of a protein sample in buffer. Low-noise images were taken with feedback at a rate of about 0.5 frames/s. Given proper setpoint adjustment, the sample was not damaged, despite this cantilever's high spring constant of 1.3 N/m. Without feedback, images were taken at 1.5 frames/s.
Characteristics and Functionality of Cantilevers and Scanners in Atomic Force Microscopy
Materials
In this paper, we provide a systematic review of atomic force microscopy (AFM), a fast-developing technique that embraces scanners, controllers, and cantilevers. The main objectives of this review are to analyze the available technical solutions of AFM, including the limitations and problems. The main questions the review addresses are the problems of working in contact, noncontact, and tapping AFM modes. We do not include applications of AFM but rather the design of different parts and operation modes. Since the main part of AFM is the cantilever, we focused on its operation and design. Information from scientific articles published over the last 5 years is provided. Many articles in this period disclose minor amendments in the mechanical system but suggest innovative AFM control and imaging algorithms. Some of them are based on artificial intelligence. During operation, control of cantilever dynamic characteristics can be achieved by magnetic field, electrostatic, or aerodynamic f...
Research on Self-Sensing and Self-Actuated Cantilever for Atomic Force Microscopy Probe
Journal of System Design and Dynamics, 2008
Recently, considerable attention of material scientists and mechanical engineers has been devoted to measurement based on atomic force microscopy (AFM). Selfexcitation is known to be an effective excitation method for AFM probe to measure the surface of a biological molecule in liquid. For practical use of a probe in liquid, we must realize a self-sensing and self-actuating AFM probe using PZT instead of using a conventional optical lever method. However, frequency characteristics of the PZT are very complex in applications for probe behavior measurement. For detecting sensor characteristics, the dynamics of the cantilever to which the PZT is attached are extracted from the PZT sensor output signal. To this end, we examine the frequency response of the PZT output signal in the case where the cantilever is excited with constant response amplitude using a PZT actuator. Then, we establish a method to process the signal so that the frequency characteristic of the PZT sensor has no high gain for the frequency range other than the first natural frequency. Finally, we conduct experiments to verify that the resultant signal is suitable to generate van der Pol-type self-excited oscillation.