Nonlinear behaviour of self-excited microcantilevers in viscous fluids

Microcantilevers are increasingly being used to create sensitive sensors for rheology and mass sensing at the micro- and nano-scale. When operating in viscous liquids, the low quality factor of such resonant structures, translating to poor signal-to-noise ratio, is often manipulated by exploiting feedback strategies. However, the presence of feedback introduces poorly-understood dynamical behaviours that may severely degrade the sensor performance and reliability. In this paper, the dynamical behaviour of self-excited microcantilevers vibrating in viscous fluids is characterized experimentally and two complementary modelling approaches are proposed to explain and predict the behaviour of the closed-loop system. In particular, the delay introduced in the feedback loop is shown to cause surprising non-linear phenomena consisting of shifts and sudden-jumps of the oscillation frequency. The proposed dynamical models also suggest strategies for controlling such undesired phenomena

Self-driven soft imaging in liquid by means of photothermal excitation

The use of a laser to induce oscillations of an atomic force microscopy cantilever provides a way to excite the dynamics of the system in a very controlled manner. This excitation scheme has been used to obtain reliable self-sustained oscillations, in air and in liquid environments, and to implement an additional control loop leading to a soft, low-interaction, working mode. The dynamics of the oscillating cantilever has been characterized, both theoretically and experimentally, and the effectiveness of the approach has been verified on a test sample. © 2011 American Institute of Physics.Peer Reviewe

The induction of α-helical structure in partially unfolded HypF-N does not affect its aggregation propensity

The conversion of proteins into structured fibrillar aggregates is a central problem in protein chemistry, biotechnology, biology and medicine. It is generally accepted that aggregation takes place from partially structured states of proteins. However, the role of the residual structure present in such conformational states is not yet understood. In particular, it is not yet clear as to whether the α-helical structure represents a productive or counteracting structural element for protein aggregation. We have addressed this issue by studying the aggregation of pH-unfolded HypF-N. It has previously been shown that the two native α-helices of HypF-N retain a partial α-helical structure in the pH-unfolded state and that these regions are also involved in the formation of the cross-β structure of the aggregates. We have introduced mutations in such stretches of the sequence, with the aim of increasing the α-helical structure in the key regions of the pH-unfolded state, while minimizing the changes of other factors known to influence protein aggregation, such as hydrophobicity, β-Sheet propensity, etc. The resulting HypF-N mutants have higher contents of α-helical structure at the site(s) of mutation in their pH-unfolded states, but such an increase does not correlate with a change of aggregation rate. The results suggest that stabilisation of α-helical structure in amyloidogenic regions of the sequence of highly dynamic states does not have remarkable effects on the rate of protein aggregation from such conformational states. Comparison with other protein systems indicate that the effect of increasing α-helical propensity can vary if the stabilised helices are in non-amyloidogenic stretches of initially unstructured peptides (accelerating effect), in amyloidogenic stretches of initially unstructured peptides (no effect) or in amyloidogenic stretches of initially stable helices (decelerating effect

Mechanisms for the inhibition of amyloid aggregation by small ligands

The formation of amyloid aggregates is the hallmark of systemic and neurodegenerative disorders, also known as amyloidoses. Many proteins have been found to aggregate into amyloid-like fibrils and this process is recognized as a general tendency of polypeptides. Lysozyme, an antibacterial protein, is a well-studied model since it is associated in human with systemic amyloidosis and that is widely available from chicken eggs (HEWL, hen egg white lysozyme). In the present study we investigated the mechanism of interaction of aggregating HEWL with rosmarinic acid and resveratrol, that we verified to be effective and ineffective, respectively, in inhibiting aggregate formation. We used a multidisciplinary strategy to characterize such effects, combining biochemical and biophysical methods with molecular dynamics (MD) simulations on the HEWL peptide 49–64 to gain insights into the mechanisms and energy variations associated to amyloid formation and inhibition. MD revealed that neither resveratrol nor rosmarinic acid were able to compete with the initial formation of the β-sheet structure. We then tested the association of two β-sheets, representing the model of an amyloid core structure. MD showed that rosmarinic acid displayed an interaction energy and a contact map comparable to that of sheet pairings. On the contrary, resveratrol association energy was found to be much lower and its contact map largely different than that of sheet pairings. The overall characterization elucidated a possible mechanism explaining why, in this model, resveratrol is inactive in blocking fibril formation, whereas rosmarinic acid is instead a powerful inhibitor

Opto-mechanical probe for combining atomic force microscopy and optical near-field surface analysis

We have developed a new easy-to-use probe that can be used to combine atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). We show that, using this device, the evanescent field, obtained by total internal reflection conditions in a prism, can be visualized by approaching the surface with the scanning tip. Furthermore, we were able to obtain simultaneous AFM and SNOM images of a standard test grating in air and in liquid. The lateral resolution in AFM and SNOM mode was estimated to be 45 and 160 nm, respectively. This new probe overcomes a number of limitations that commercial probes have, while yielding the same resolution. (C) 2014 Optical Society of Americ

Measuring viscosity with nonlinear self-excited microcantilevers

A viscosity sensor based on the nonlinear behaviour of a microcantilever embedded in a self-excitation loop with an adjustable phase-shifter is proposed. The self-sustained oscillation frequencies of the cantilever are experimentally and theoretically investigated as functions of the fluid viscosity and of the imposed phase shift of the signal along the self-excitation loop. The sensor performance is validated experimentally using different water-glycerol solutions. In contrast to existing rheological sensors, the proposed platform can be tuned to work in two different modes: a high-sensitivity device whose oscillation frequency changes smoothly with the rheological properties of the fluid or a critical viscosity threshold detector, where, for small changes in fluid viscosity, there is a step change in oscillation frequency. The authors wish to acknowledge the support of EPSRC Grant No. EP/N026799/1 on Self-Tuning Advanced Rheology Tool and of the CNR Short Term Mobility grants 2015 and 2016. The authors also wish to thank Elbatech srl for the support in the design and realization of the self-excitation electronics. The data presented in this paper are freely available at https://doi.org/10.5281/zenodo.825797

Ultracompact autocorrelator for multiphoton microscopy

Pulse temporal characterization is a fundamental task when operating a Ti:Sapphire ultrafast laser system for multiphoton microscopy applications. In the present report, an ultracompact autocorrelator setup and a simple procedure is reported to perform pulse width measurements at the focal plane of the microscope objective without the need of any further instrumentation, aside from a few optical elements, since the confocal microscope, detection, data acquisition, processing, and displaying capabilities are used

Modelling and analysis of autonomous micro-cantilever oscillations

Tapping mode atomic force microscopy provides good resolution in imaging applications, but it still requires a time-consuming initial configuration and features quite low scanning velocity. In this paper we present a new dynamic mode in which the cantilever gets excited by a feedback loop containing a saturation function. The proposed scheme is then analysed in the frequency domain and simulated against the standard set-up, showing good performance and elimination of some of the known drawbacks. Preliminary results in experiments confirm the effectiveness of this operating mode