Micromechanical mass sensors for biomolecular detection in a physiological environment

A microwave scattering spectral method to detect the nanomechanical vibrations embedded in a superconducting qubit

Nanomechanical resonators (NMRs), as the quantum mechanical sensing probers, have played the important roles for various high-precision quantum measurements. Differing from the previous emission spectral probes (i.e., the NMR modified the atomic emission), in this paper we propose an alternative approach, i.e., by probing the scattering spectra of the quantum mechanical prober coupled to the driving microwaves, to characterize the physical features of the NMR embedded in a rf-SQUID based superconducting qubit. It is shown that, from the observed specifical frequency points in the spectra, i.e., either the dips or the peaks, the vibrational features (i.e., they are classical vibration or quantum mechanical one) and the physical parameters (typically such as the vibrational frequency and displacements) of the NMR can be determined effectively. The proposal is feasible with the current technique and should be useful to design the desired NMRs for various quantum metrological applications.

Hemolysis-Inspired, Highly Sensitive, Label-Free IgM Detection Using Erythrocyte Membrane-Functionalized Nanomechanical Resonators

Immunoglobulin detection is important for immunoassays, such as diagnosing infectious diseases, evaluating immune status, and determining neutralizing antibody concentrations. However, since most immunoassays rely on labeling methods, there are limitations on determining the limit of detection (LOD) of biosensors. In addition, although the antigen must be immobilized via complex chemical treatment, it is difficult to precisely control the immobilization concentration. This reduces the reproducibility of the biosensor. In this study, we propose a label-free method for antibody detection using microcantilever-based nanomechanical resonators functionalized with erythrocyte membrane (EM). This label-free method focuses on the phenomenon of antibody binding to oligosaccharides (blood type antigen) on the surface of the erythrocyte. We established a method for extracting the EM from erythrocytes and fabricated an EM-functionalized microcantilever (MC), termed EMMC, by surface-coating EM layers on the MC. When the EMMC was treated with immunoglobulin M (IgM), the bioassay was successfully performed in the linear range from 2.2 pM to 22 nM, and the LOD was 2.0 pM. The EMMC also exhibited excellent selectivity compared to other biomolecules such as serum albumin, γ-globulin, and IgM with different paratopes. These results demonstrate that EMMC-based nanotechnology may be utilized in criminal investigations to identify blood types with minimal amounts of blood or to evaluate individual immunity through virus-neutralizing antibody detection.

Multimodal real-time frequency tracking of cantilever arrays in liquid environment for biodetection: Comprehensive setup and performance analysis

We present a nanomechanical platform for real-time quantitative label-free detection of target biomolecules in a liquid environment with mass sensitivity down to few pg. Newly fabricated arrays of up to 18 cantilevers are integrated in a micromachined fluidic chamber, connected to software-controlled fluidic pumps for automated sample injections. We discuss two functionalization approaches to independently sensitize the interface of different cantilevers. A custom piezo-stack actuator and optical readout system enable the measurement of resonance frequencies up to 2 MHz. We implement a new measurement strategy based on a phase-locked loop (PLL), built via in-house developed software. The PLL allows us to track, within the same experiment, the evolution of resonance frequency over time of up to four modes for all the cantilevers in the array. With respect to the previous measurement technique, based on standard frequency sweep, the PLL enhances the estimated detection limit of the device by a factor of 7 (down to 2 pg in 5 min integration time) and the time resolution by more than threefold (below 15 s), being on par with commercial gold-standard techniques. The detection limit and noise of the new setup are investigated via Allan deviation and standard deviation analysis, considering different resonance modes and interface chemistries. As a proof-of-concept, we show the immobilization and label-free in situ detection of live bacterial cells (E. coli), demonstrating qualitative and quantitative agreement in the mechanical response of three different resonance modes.

Nanomotion Detection-Based Rapid Antibiotic Susceptibility Testing

Rapid antibiotic susceptibility testing (AST) could play a major role in fighting multidrug-resistant bacteria. Recently, it was discovered that all living organisms oscillate in the range of nanometers and that these oscillations, referred to as nanomotion, stop as soon the organism dies. This finding led to the development of rapid AST techniques based on the monitoring of these oscillations upon exposure to antibiotics. In this review, we explain the working principle of this novel technique, compare the method with current ASTs, explore its application and give some advice about its implementation. As an illustrative example, we present the application of the technique to the slowly growing and pathogenic Bordetella pertussis bacteria.

Nanotechnological immunoassay for rapid label-free analysis of candidate malaria vaccines

Malaria is a life-threatening epidemic disease with half of the world's population at risk. Although its incidence rate has fallen since 2010, this ratio dramatically stalled between 2014 and 2018. New fast and optimized tools in vaccine analysis and seroconversion testing are critically needed. We developed a clinical diagnostic device based on piezo-actuated nanoresonators that perform as quantitative in situ calibrated nano-bio sensors for specific detection of multiple target molecules in serum samples. The immunoassay successfully diagnoses humoral immune responses induced by malaria vaccine candidates and reveals the timeline and stage of the infection. We applied the newly developed strategy to a variety of different samples, from pure antibody/vaccine solutions, to blood samples from clinical trials on both naïve and pre-exposed malaria volunteers from sub-Saharan countries. Our nanomechanical assay provides a direct one-step label-free quantitative immunoassay that is on par with the gold-standard, multi-step enzyme-linked immunosorbent assay (ELISA). We achieve a limit of detection of few pg ml-1, or sub-pM concentrations. The 6 μl sample volume allows more than 50 experiments from one finger prick. Furthermore, we simultaneously detected multiple analytes by differential functionalization of multiple sensors in parallel. The inherent differential read-out with in situ controls reduces false positive results. Due to the faster turnaround time, the minimal volume required and the automatized handling system, this technique has great potential for miniaturization and routine diagnostics in pandemic emergencies.

Realization and direct observation of five normal and parametric modes in silicon nanowire resonators by in situ transmission electron microscopy

Mechanical resonators have wide applications in sensing bio-chemical substances, and provide an accurate method to measure the intrinsic elastic properties of oscillating materials. A high resonance order with high response frequency and a small resonator mass are critical for enhancing the sensitivity and precision. Here, we report on the realization and direct observation of high-order and high-frequency silicon nanowire (Si NW) resonators. By using an oscillating electric-field for inducing a mechanical resonance of single-crystalline Si NWs inside a transmission electron microscope (TEM), we observed resonance up to the 5th order, for both normal and parametric modes at ∼100 MHz frequencies. The precision of the resonant frequency was enhanced, as the deviation reduced from 3.14% at the 1st order to 0.25% at the 5th order, correlating with the increase of energy dissipation. The elastic modulus of Si NWs was measured to be ∼169 GPa in the [110] direction, and size scaling effects were found to be absent down to the ∼20 nm level.

Flow force augmented 3D suspended polymeric microfluidic (SPMF3 ) platform

Detection and study of bioelements using microfluidic systems has been of great interest in the biodiagnostics field. Microcantilevers are the most used systems in biodetection due to their implementation simplicity which have been used for a wide variety of applications ranging from cellular to molecular diagnosis. However, increasing further the sensitivity of the microcantilever systems have a great effect on the cantilever based sensing for chemical and bio applications. In order to improve further the performance of microcantilevers, a flow force augmented 3D suspended microchannel is proposed using which microparticles can be conveyed through a microchannel inside the microcantilever to the detection area. This innovative microchannel design addresses the low sensitivity issue by increasing its sensitivity up to 5 times than the earlier reported similar microsystems. Moreover, fabricating this microsystem out of Polydimethylsiloxane (PDMS) would eliminate external exciter dependency in many detection applications such as biodiagnostics. In this study, the designed microsystem has been analyzed theoretically, simulated and tested. Moreover, the microsystem has been fabricated and tested under different conditions, the results of which have been compared with simulation results. Finally, its innovative fabrication process and issues are reported and discussed.

Nanomechanical clinical coagulation diagnostics and monitoring of therapies

Clinical coagulation diagnostics often requires multiple tests. Coagulation times are a first indication of an abnormal coagulation process, such as a coagulation factor deficiency. To determine the specific deficient factor, additional immuno- and/or enzyme assays are necessary. Currently, every clinical laboratory has to normalize their assays (international normalized ratio, INR), and therefore, certain variability within the clinical analytics exists. We report a novel strategy for a quick, reliable and quantitative diagnosis of blood coagulation diseases (e.g. haemophilia) and for monitoring factor replacement and anticoagulant therapies (e.g. heparin treatment). We exploit nano-oscillations of microcantilevers for real-time measurements of the evolving blood plasma clot strength (viscosity). The sensors are oscillated at multiple high resonance mode numbers, in order to minimise the oscillation amplitude (a few nanometers), to provide direct internal control and to increase the quality factor. Along with the activated thromboplastin time (aPTT) and prothrombin time (PT) other parameters important for thrombosis diagnostics can be obtained, including the final clot strength and the fibrinolysis time. We demonstrate the dependence of the parameters on factor deficiencies and we diagnose a specific factor deficiency through an integrated and quantitative in situ immunoassay. This approach does not require continuous calibration since it delivers an absolute quantity (clot strength). The low sample volume required (a few μl) and the ability to measure different parameters within the same test (PT, aPTT and global coagulation assay) make the presented technique a versatile point-of-care device for clinical coagulation diagnostics.

A microcantilever-based silver ion sensor using DNA-functionalized gold nanoparticles as a mass amplifier

Silver ions have been used to sterilize many products, however, it has recently been demonstrated that silver ions can be toxic. This toxicity has been studied over many years with the lethal concentration at 10 μM. Silver ions can accumulate through the food chain, causing serious health problems in many species. Hence, there is a need for a commercially available silver ion sensor, with high detection sensitivity. In this work, we develop an ultra-sensitive silver ion sensor platform, using cytosine based DNA and gold nanoparticles as the mass amplifier. We achieve a lower detection limit for silver ions of 10 pM; this detection limit is one million times lower than the toxic concentration. Using our sensor platform we examine highly selective characteristics of other typical ions in water from natural sources. Furthermore, our sensor platform is able to detect silver ions in a real practical sample of commercially available drinking water. Our sensor platform, which we have termed a 'MAIS' (mass amplifier ion sensor), with a simple detection procedure, high sensitivity, selectivity and real practical applicability has shown potential as an early toxicity assessment of silver ions in the environment.

Nanomechanical Sensors

This chapter introduces nanomechanical sensors and their applications. All molecules have “volume” and “mass.” Direct measurement of these fundamental parameters can realize label-free and real-time measurements. Nanomechanical sensors have been emerging as a key device for such label-free and real-time measurements with their multiple operation modes; static and dynamic modes for detecting volume- and mass-related features, respectively. A cantilever array sensor is a representative example among various geometries, while structural optimization can enhance the scope of nanomechanical sensors in both academic and industrial applications. One of the most advanced sensing platforms is a membrane-type surface stress sensor (MSS), which realizes both high sensitivity and compact system at the same time. The MSS is also expected to contribute to addressing nanomechanical behavior of living cells and their network.

Toward Higher-Order Mass Detection: Influence of an Adsorbate's Rotational Inertia and Eccentricity on the Resonant Response of a Bernoulli-Euler Cantilever Beam

In this paper a new theoretical model is derived, the results of which permit a detailed examination of how the resonant characteristics of a cantilever are influenced by a particle (adsorbate) attached at an arbitrary position along the beam’s length. Unlike most previous work, the particle need not be small in mass or dimension relative to the beam, and the adsorbate’s geometric characteristics are incorporated into the model via its rotational inertia and eccentricity relative to the beam axis. For the special case in which the adsorbate’s (translational) mass is indeed small, an analytical solution is obtained for the particle-induced resonant frequency shift of an arbitrary flexural mode, including the effects of rotational inertia and eccentricity. This solution is shown to possess the exact first-order behavior in the normalized particle mass and represents a generalization of analytical solutions derived by others in earlier studies. The results suggest the potential for “higher-order” nanobeam-based mass detection methods by which the multi-mode frequency response reflects not only the adsorbate’s mass but also important geometric data related to its size, shape, or orientation (i.e., the mass distribution), thus resulting in more highly discriminatory techniques for discrete-mass sensing.

Tunable micro- and nanomechanical resonators

Advances in micro- and nanofabrication technologies have enabled the development of novel micro- and nanomechanical resonators which have attracted significant attention due to their fascinating physical properties and growing potential applications. In this review, we have presented a brief overview of the resonance behavior and frequency tuning principles by varying either the mass or the stiffness of resonators. The progress in micro- and nanomechanical resonators using the tuning electrode, tuning fork, and suspended channel structures and made of graphene have been reviewed. We have also highlighted some major influencing factors such as large-amplitude effect, surface effect and fluid effect on the performances of resonators. More specifically, we have addressed the effects of axial stress/strain, residual surface stress and adsorption-induced surface stress on the sensing and detection applications and discussed the current challenges. We have significantly focused on the active and passive frequency tuning methods and techniques for micro- and nanomechanical resonator applications. On one hand, we have comprehensively evaluated the advantages and disadvantages of each strategy, including active methods such as electrothermal, electrostatic, piezoelectrical, dielectric, magnetomotive, photothermal, mode-coupling as well as tension-based tuning mechanisms, and passive techniques such as post-fabrication and post-packaging tuning processes. On the other hand, the tuning capability and challenges to integrate reliable and customizable frequency tuning methods have been addressed. We have additionally concluded with a discussion of important future directions for further tunable micro- and nanomechanical resonators.

Mass Detection in Viscous Fluid Utilizing Vibrating Micro- and Nanomechanical Mass Sensors under Applied Axial Tensile Force

Vibrating micro- and nanomechanical mass sensors are capable of quantitatively determining attached mass from only the first three (two) measured cantilever (suspended) resonant frequencies. However, in aqueous solutions that are relevant to most biological systems, the mass determination is challenging because the quality factor (Q-factor) due to fluid damping decreases and, as a result, usually just the fundamental resonant frequencies can be correctly identified. Moreover, for higher modes the resonance coupling, noise, and internal damping have been proven to strongly affect the measured resonances and, correspondingly, the accuracy of estimated masses. In this work, a technique capable of determining the mass for the cantilever and also the position of nanobeads attached on the vibrating micro-/nanomechanical beam under intentionally applied axial tensile force from the measured fundamental flexural resonant frequencies is proposed. The axial force can be created and controlled through an external electrostatic or magnetostatic field. Practicality of the proposed technique is confirmed on the suspended multi-walled carbon nanotube and the rectangular silicon cantilever-based mass sensors. We show that typically achievable force resolution has a negligibly small impact on the accuracy of mass measurement.

Giant resonance tuning of micro and nanomechanical oscillators

We present a method to tune the resonance frequency and the Q-factor of micro and nano-metric mechanical oscillators. A counteracting loop drives a capacitive force applied to the oscillator. The proportional and differential gains are used to shift the resonance frequency up to 75% and to tune the Q-factor of the oscillator, by changing its effective stiffness and damping ratio. The oscillator position is monitored in a large bandwidth with a fiber-optic based interferometer. We applied this simple operational scheme with different oscillators for modifying easily their dynamical properties. Compared to alternative methods requiring external fields, our method can either increase or decrease the resonance frequency in a frequency range much more extended. This opens up a wide range of applications, from force sensors with extremely low elastic constants but high quality factor to tunable energy harvesters or to high-frequency tuning of radio frequency filters. The control scheme can work in different media, and is then suitable to be applied to biological sensors and actuators.

Hybrid spin-microcantilever sensor for environmental, chemical, and biological detection

Nowadays hybrid spin-micro/nanomechanical systems are being actively explored for potential quantum sensing applications. In combination with the pump-probe technique or the spin resonance spectrum, we theoretically propose a realistic, feasible, and an exact way to measure the cantilever frequency in a hybrid spin-micromechanical cantilever system which has a strong coherent coupling of a single nitrogen vacancy center in the single-crystal diamond cantilever with the microcantilever. The probe absorption spectrum which exhibits new features such as mechanically induced three-photon resonance and ac Stark effect is obtained. Simultaneously, we further develop this hybrid spin-micromechanical system to be an ultrasensitive mass sensor, which can be operated at 300 K with a mass responsivity 0.137 Hz ag(-1), for accurate sensing of gaseous or aqueous environments, chemical vapors, and biomolecules. And the best performance on the minimum detectable mass can be [Formula: see text] in vacuum. Finally, we illustrate an in situ measurement to detect Angiopoietin-1, a marker of tumor angiogenesis, accurately with this hybrid microcantilever at room temperature.

Real-time viscosity and mass density sensors requiring microliter sample volume based on nanomechanical resonators

A microcantilever based method for fluid viscosity and mass density measurements with high temporal resolution and microliter sample consumption is presented. Nanomechanical cantilever vibration is driven by photothermal excitation and detected by an optical beam deflection system using two laser beams of different wavelengths. The theoretical framework relating cantilever response to the viscosity and mass density of the surrounding fluid was extended to consider higher flexural modes vibrating at high Reynolds numbers. The performance of the developed sensor and extended theory was validated over a viscosity range of 1-20 mPa·s and a corresponding mass density range of 998-1176 kg/m(3) using reference fluids. Separating sample plugs from the carrier fluid by a two-phase configuration in combination with a microfluidic flow cell, allowed samples of 5 μL to be sequentially measured under continuous flow, opening the method to fast and reliable screening applications. To demonstrate the study of dynamic processes, the viscosity and mass density changes occurring during the free radical polymerization of acrylamide were monitored and compared to published data. Shear-thinning was observed in the viscosity data at higher flexural modes, which vibrate at elevated frequencies. Rheokinetic models allowed the monomer-to-polymer conversion to be tracked in spite of the shear-thinning behavior, and could be applied to study the kinetics of unknown processes.

Biosensors based on nanomechanical systems

The advances in micro- and nanofabrication technologies enable the preparation of increasingly smaller mechanical transducers capable of detecting the forces, motion, mechanical properties and masses that emerge in biomolecular interactions and fundamental biological processes. Thus, biosensors based on nanomechanical systems have gained considerable relevance in the last decade. This review provides insight into the mechanical phenomena that occur in suspended mechanical structures when either biological adsorption or interactions take place on their surface. This review guides the reader through the parameters that change as a consequence of biomolecular adsorption: mass, surface stress, effective Young's modulus and viscoelasticity. The mathematical background needed to correctly interpret the output signals from nanomechanical biosensors is also outlined here. Other practical issues reviewed are the immobilization of biomolecular receptors on the surface of nanomechanical systems and methods to attain that in large arrays of sensors. We then describe some relevant realizations of biosensor devices based on nanomechanical systems that harness some of the mechanical effects cited above. We finally discuss the intrinsic detection limits of the devices and the limitation that arises from non-specific adsorption.

High-sensitivity detection of silver ions using oligonucleotide-immobilized oscillator

With the remarkable developments in the fields of nanoscale research and industry, nanotoxicity is gaining importance from the viewpoint of its potential impact on human health and the environment. Herein, we report on the label-free, high-sensitivity detection of Ag(+), a representative nanotoxic material, by using a silver-specific nucleotide-coated oscillator. The detection is based on the measurement of the resonant frequency shift arising from constitution of the cytosine-Ag(+)-cytosine bonding. We amplify the resonant frequency shift by using single cytosine molecules. It is shown that a silver-specific DNA-immobilized oscillator enables the capture of silver ions at concentrations below 1 nM. Remarkably, the nucleotide-based oscillator enables an insight into the coordination chemistry, which plays an important role in the early detection of toxicity. This implies that the bio-conjugated sensor could be used to set the reference point for water quality.

Challenges for nanomechanical sensors in biological detection

Nanomechanical biosensing relies on changes in the movement and deformation of micro- and nanoscale objects when they interact with biomolecules and other biological targets. This field of research has provided ever-increasing records in the sensitivity of label-free detection but it has not yet been established as a practical alternative for biological detection. We analyze here the latest advancements in the field, along with the challenges remaining for nanomechanical biosensors to become a commonly used tool in biology and biochemistry laboratories.

Atomic force microscopy as a tool applied to nano/biosensors

This review article discusses and documents the basic concepts and principles of nano/biosensors. More specifically, we comment on the use of Chemical Force Microscopy (CFM) to study various aspects of architectural and chemical design details of specific molecules and polymers and its influence on the control of chemical interactions between the Atomic Force Microscopy (AFM) tip and the sample. This technique is based on the fabrication of nanomechanical cantilever sensors (NCS) and microcantilever-based biosensors (MC-B), which can provide, depending on the application, rapid, sensitive, simple and low-cost in situ detection. Besides, it can provide high repeatability and reproducibility. Here, we review the applications of CFM through some application examples which should function as methodological questions to understand and transform this tool into a reliable source of data. This section is followed by a description of the theoretical principle and usage of the functionalized NCS and MC-B technique in several fields, such as agriculture, biotechnology and immunoassay. Finally, we hope this review will help the reader to appreciate how important the tools CFM, NCS and MC-B are for characterization and understanding of systems on the atomic scale.

Energy dissipation in microfluidic beam resonators: effect of Poisson's ratio

Dissipation of mechanical energy underlies the sensitivity of many nanomechanical devices, with environmental effects often having a significant effect. One case of practical relevance is the interaction of elastic beam resonators with fluid, which is known to dramatically increase energy dissipation. Recently, we investigated energy dissipation in a different class of elastic beam resonator that embeds a microfluidic channel in its interior. In this paper, we examine the effect of the beam material Poisson ratio on these devices and discover that it can strongly affect energy dissipation--this is in direct contrast to conventional cantilever beams immersed in fluid. Increasing the Poisson ratio in these microfluidic devices is found to decrease energy dissipation, with the incompressible material limit providing minimum energy dissipation. Our paper establishes that, in this limit, placement of the fluid channel away from the beam neutral axis has negligible effect on energy dissipation in many cases of practical interest. The physical implications of these findings are discussed, and a detailed comparison with available experimental results is provided.

Nanodevices in diagnostics

The real-time, personalized and highly sensitive early-stage diagnosis of disease remains an important challenge in modern medicine. With the ability to interact with matter at the nanoscale, the development of nanotechnology architectures and materials could potentially extend subcellular and molecular detection beyond the limits of conventional diagnostic modalities. At the very least, nanotechnology should be able to dramatically accelerate biomarker discovery, as well as facilitate disease monitoring, especially of maladies presenting a high degree of molecular and compositional heterogeneity. This article gives an overview of several of the most promising nanodevices and nanomaterials along with their applications in clinical practice. Significant work to adapt nanoscale materials and devices to clinical applications involving large interdisciplinary collaborations is already underway with the potential for nanotechnology to become an important enabling diagnostic technology.

Finite size effect on nanomechanical mass detection: the role of surface elasticity

Nanomechanical resonators have recently been highlighted because of their remarkable ability to perform both sensing and detection. Since the nanomechanical resonators are characterized by a large surface-to-volume ratio, it is implied that the surface effect plays a substantial role on not only the resonance but also the sensing performance of nanomechanical resonators. In this work, we have studied the role of surface effect on the detection sensitivity of a nanoresonator that undergoes either harmonic vibration or nonlinear oscillation based on the continuum elastic model such as an elastic beam model. It is shown that the surface effect makes an impact on both harmonic resonance and nonlinear oscillations, and that the sensing performance is dependent on the surface effect. Moreover, we have also investigated the surface effect on the mechanical tuning of resonance and sensing performance. It is interestingly found that the mechanical tuning of resonance is independent of the surface effect, while the mechanical tuning of sensing performance is determined by the surface effect. Our study sheds light on the importance of the surface effect on the sensing performance of nanoresonators.

Comparative advantages of mechanical biosensors

Mechanical interactions are fundamental to biology. Mechanical forces of chemical origin determine motility and adhesion on the cellular scale, and govern transport and affinity on the molecular scale. Biological sensing in the mechanical domain provides unique opportunities to measure forces, displacements and mass changes from cellular and subcellular processes. Nanomechanical systems are particularly well matched in size with molecular interactions, and provide a basis for biological probes with single-molecule sensitivity. Here we review micro- and nanoscale biosensors, with a particular focus on fast mechanical biosensing in fluid by mass- and force-based methods, and the challenges presented by non-specific interactions. We explain the general issues that will be critical to the success of any type of next-generation mechanical biosensor, such as the need to improve intrinsic device performance, fabrication reproducibility and system integration. We also discuss the need for a greater understanding of analyte-sensor interactions on the nanoscale and of stochastic processes in the sensing environment.

Energy dissipation in microfluidic beam resonators: Dependence on mode number

Energy dissipation experienced by vibrating microcantilever beams immersed in fluid is strongly dependent on the mode of vibration, with quality factors typically increasing with mode number. Recently, we examined energy dissipation in a new class of cantilever device that embeds a microfluidic channel in its interior-the fundamental mode of vibration only was considered. Due to its importance in practice, we examine the effect of mode number on energy dissipation in these microfluidic beam resonators. Interestingly, and in contrast to other cantilever devices, we find that the quality factor typically decreases with increasing mode number. We explore the underlying physical mechanisms leading to this counterintuitive behavior, and provide a detailed comparison to experimental measurements for which good agreement is found.

A cantilever sensor with an integrated optical readout for detection of enzymatically produced homocysteine

Microcantilever sensors have been recognized as a promising sensor platform for various chemical and biological applications. One of their major limitations is that the measurement of cantilever displacement typically involves elaborate off-chip setups with free-space optics. An improved device, known as the optical cantilever, has been proposed recently to eliminate the external optics. The response of the optical cantilever is measured on-chip through integrated waveguides. However, this method has been previously demonstrated only for devices operating in air, whereas most chemical and biological samples are in solution state. We present the first optical cantilever capable of operation in liquid. We test it with the detection of homocysteine with a minimal concentration of 10 muM. The minimal measurable cantilever displacement and surface stress are 5 nm and 1 mN/m, respectively. The presented device will be used in studies of a homocysteine-producing bacterial pathway for the purpose of drug discovery. It can also be extended to various other chemical- or biological-sensing applications by selecting an appropriate surface coating.

Experimental and computational characterization of biological liquid crystals: a review of single-molecule bioassays

Quantitative understanding of the mechanical behavior of biological liquid crystals such as proteins is essential for gaining insight into their biological functions, since some proteins perform notable mechanical functions. Recently, single-molecule experiments have allowed not only the quantitative characterization of the mechanical behavior of proteins such as protein unfolding mechanics, but also the exploration of the free energy landscape for protein folding. In this work, we have reviewed the current state-of-art in single-molecule bioassays that enable quantitative studies on protein unfolding mechanics and/or various molecular interactions. Specifically, single-molecule pulling experiments based on atomic force microscopy (AFM) have been overviewed. In addition, the computational simulations on single-molecule pulling experiments have been reviewed. We have also reviewed the AFM cantilever-based bioassay that provides insight into various molecular interactions. Our review highlights the AFM-based single-molecule bioassay for quantitative characterization of biological liquid crystals such as proteins.

Nanomechanical in situ monitoring of proteolysis of peptide by Cathepsin B

Characterization and control of proteolysis of peptides by specific cellular protease is a priori requisite for effective drug discovery. Here, we report the nanomechanical, in situ monitoring of proteolysis of peptide chain attributed to protease (Cathepsin B) by using a resonant nanomechanical microcantilever immersed in a liquid. Specifically, the detection is based on measurement of resonant frequency shift arising from proteolysis of peptides (leading to decrease of cantilever's overall mass, and consequently, increases in the resonance). It is shown that resonant microcantilever enables the quantification of proteolysis efficacy with respect to protease concentration. Remarkably, the nanomechanical, in situ monitoring of proteolysis allows us to gain insight into the kinetics of proteolysis of peptides, which is well depicted by Langmuir kinetic model. This implies that nanomechanical biosensor enables the characterization of specific cellular protease such as its kinetics.

Stochastic dynamics of micron-scale doubly clamped beams in a viscous fluid

We study the stochastic dynamics of doubly clamped micron-scale beams in a viscous fluid driven by Brownian motion. We use a thermodynamic approach to compute the equilibrium fluctuations in beam displacement that requires only deterministic calculations. From calculations of the autocorrelations and noise spectra we quantify the beam dynamics by the quality factor and resonant frequency of the fundamental flexural mode over a wide range of experimentally accessible geometries. We consider beams with uniform rectangular cross section and explore the increased quality factor and resonant frequency as a baseline geometry is varied by increasing the width, increasing the thickness, and decreasing the length. The quality factor is nearly doubled by tripling either the width or the height of the beam. Much larger improvements are found by decreasing the beam length, however this is limited by the appearance of additional modes of fluid dissipation. Overall, the stochastic dynamics of the wider and thicker beams are well predicted by a two-dimensional approximate theory beyond what may be expected based upon the underlying assumptions, whereas the shorter beams require a more detailed analysis.

Nonmonotonic energy dissipation in microfluidic resonators

Nanomechanical resonators enable a range of precision measurements in air or vacuum, but strong viscous damping makes applications in liquid challenging. Recent experiments have shown that fluid damping is greatly reduced in fluidic embedded-channel microcantilevers. Here we report the discovery of nonmonotonic energy dissipation due to the fluid in such devices, which leads to the intriguing prospect of enhancing the quality factor upon miniaturization. These observations elucidate the physical mechanisms of energy dissipation in embedded-channel resonators and thus provide the basis for numerous applications in nanoscience and biology.

Quantitative time-resolved measurement of membrane protein-ligand interactions using microcantilever array sensors

Membrane proteins are central to many biological processes, and the interactions between transmembrane protein receptors and their ligands are of fundamental importance in medical research. However, measuring and characterizing these interactions is challenging. Here we report that sensors based on arrays of resonating microcantilevers can measure such interactions under physiological conditions. A protein receptor--the FhuA receptor of Escherichia coli--is crystallized in liposomes, and the proteoliposomes then immobilized on the chemically activated gold-coated surface of the sensor by ink-jet spotting in a humid environment, thus keeping the receptors functional. Quantitative mass-binding measurements of the bacterial virus T5 at subpicomolar concentrations are performed. These experiments demonstrate the potential of resonating microcantilevers for the specific, label-free and time-resolved detection of membrane protein-ligand interactions in a micro-array format.

Development of atomic force microscope with wide-band magnetic excitation for study of soft matter dynamics

In order to probe dynamical properties of mesoscopic soft matter systems such as polymers, structured liquid, etc., a new atomic force microscopy apparatus with a wide-band magnetic cantilever excitation system was developed. Constant-current driving of an electromagnet up to 1 MHz was implemented with a closed-loop driver circuit. Transfer function of a commercial cantilever attached with a magnetic particle was measured in a frequency range of 1-1000 kHz in distilled water. Effects of the laser spot position, distribution of the force exerted on the cantilever, and difference in the detection scheme on the obtained transfer function are discussed in comparison with theoretical predictions by other research groups. A preliminary result of viscoelasticity spectrum measurement of a single dextran chain is shown and is compared with a recent theoretical calculation.

Comprehensive characterization of molecular interactions based on nanomechanics

Molecular interaction is a key concept in our understanding of the biological mechanisms of life. Two physical properties change when one molecular partner binds to another. Firstly, the masses combine and secondly, the structure of at least one binding partner is altered, mechanically transducing the binding into subsequent biological reactions. Here we present a nanomechanical micro-array technique for bio-medical research, which not only monitors the binding of effector molecules to their target but also the subsequent effect on a biological system in vitro. This label-free and real-time method directly and simultaneously tracks mass and nanomechanical changes at the sensor interface using micro-cantilever technology. To prove the concept we measured lipid vesicle (approximately 748*10(6) Da) adsorption on the sensor interface followed by subsequent binding of the bee venom peptide melittin (2840 Da) to the vesicles. The results show the high dynamic range of the instrument and that measuring the mass and structural changes simultaneously allow a comprehensive discussion of molecular interactions.

Hydrodynamic methods for calibrating the normal spring constant of microcantilevers

Knowledge of the spring constants of microcantilevers is vital in atomic force microscopy and for cantilever-based devices that are, for example, employed as probes in biomedical applications. We compare two recently developed hydrodynamic methods for the determination of the normal spring constant of microcantilevers. Both approaches are non-invasive when determining the spring constant and require only knowledge of the thermal noise response of the cantilever in a fluid and its plan view dimensions. The methods do not bear the risk of damaging the cantilever and are therefore attractive for example in mass sensing applications in cases where the cantilever has been modified, e.g. with a coating. The specific strengths of the methods are discussed and the results for a variety of cantilevers are presented and compared.

Microcantilever sensors

Microfabricated cantilevers have been used in atomic force microscopy for the topography imagingof non-conductive surfaces for more than 20 years. Cantilever beams without tips have proved theirapplicability in recent years as miniaturized, ultrasensitive, and fast-responding sensors for applicationsin chemistry, physics, biochemistry, and medicine. Microcantilever sensors respond by bending dueto the absorption of molecules. A shift in resonance frequency also occurs. They can be operatedin different environments such as gaseous environment, liquids, or vacuum. In gas, microcantileversensors can be operated as an artificial nose, whereby the bending pattern of a microfabricatedarray of eight polymer-coated silicon cantilevers is characteristic of the different vapors from solvents,flavors, and beverages. When operated in a liquid, microcantilever sensors are able to detectbiochemical reactions. Each cantilever is functionalized with a specific biochemical probe receptor,sensitive for detection of the corresponding target molecule. Applications lie in the fields of label-and amplification-free detection of DNA hybridization, the detection of proteins as well as antigen-antibodyreactions, and the detection of larger entities, such as bacteria and fungi.

Nanobiosensors: optofluidic, electrical and mechanical approaches to biomolecular detection at the nanoscale

Next generation biosensor platforms will require significant improvements in sensitivity, specificity and parallelity in order to meet the future needs of a variety of fields ranging from in vitro medical diagnostics, pharmaceutical discovery and pathogen detection. Nano-biosensors, which exploit some fundamental nanoscopic effect in order to detect a specific biomolecular interaction, have now been developed to a point where it is possible to determine in what cases their inherent advantages over traditional techniques (such as nucleic acid microarrays) more than offset the added complexity and cost involved constructing and assembling the devices. In this paper we will review the state of the art in nanoscale biosensor technologies, focusing primarily on optofluidic type devices but also covering those which exploit fundamental mechanical and electrical transduction mechanisms. A detailed overview of next generation requirements is presented yielding a series of metrics (namely limit of detection, multiplexibility, measurement limitations, and ease of fabrication/assembly) against which the various technologies are evaluated. Concluding remarks regarding the likely technological impact of some of the promising technologies are also provided.

Micro- and nanomechanical sensors for environmental, chemical, and biological detection

Micro- and nanoelectromechanical systems, including cantilevers and other small scale structures, have been studied for sensor applications. Accurate sensing of gaseous or aqueous environments, chemical vapors, and biomolecules have been demonstrated using a variety of these devices that undergo static deflections or shifts in resonant frequency upon analyte binding. In particular, biological detection of viruses, antigens, DNA, and other proteins is of great interest. While the majority of currently used detection schemes are reliant on biomarkers, such as fluorescent labels, time, effort, and chemical activity could be saved by developing an ultrasensitive method of label-free mass detection. Micro- and nanoscale sensors have been effectively applied as label-free detectors. In the following, we review the technologies and recent developments in the field of micro- and nanoelectromechanical sensors with particular emphasis on their application as biological sensors and recent work towards integrating these sensors in microfluidic systems.

Diamond and biology

A summary of photo- and electrochemical surface modifications applied on single-crystalline chemical vapour deposition diamond films is given. The covalently bonded formation of amine and phenyl linker molecular layers is characterized using X-ray photoelectron spectroscopy, atomic force microscopy (AFM), cyclic voltammetry and field-effect transistor characterization experiments. Amine and phenyl layers are very different with respect to formation, growth, thickness and molecular arrangement. We deduce a sub-monolayer of amine linker molecules on diamond with approximately 10% coverage of 1.510(15) cm(-2) carbon bonds. Amine is bonded only on initially H-terminated surface areas. In the case of electrochemical deposition of phenyl layers, multilayer properties are detected with three-dimensional nitrophenyl growth properties. This leads to the formation of typically 25 A thick layers. The electrochemical bonding to boron-doped diamond works on H-terminated and oxidized surfaces. After reacting such films with heterobifunctional cross-linker molecules, thiol-modified ss-DNA markers are bonded to the organic system. Application of fluorescence and AFM on hybridized DNA films shows dense arrangements with densities up to 10(13) cm(-2). The DNA is tilted by an angle of approximately 35 degrees with respect to the diamond surface. Shortening the bonding time of thiol-modified ss-DNA to 10 min causes a decrease in DNA density to approximately 10(12) cm(-2). Application of AFM scratching experiments shows threshold removal forces of approximately 75 and 45 nN for the DNA bonded to the phenyl and the amine linker molecules, respectively. First, DNA sensor applications using Fe(CN6) 3-/4- mediator redox molecules and DNA field-effect transistor devices are introduced and discussed.

Microresonator mass sensors for detection of Bacillus anthracis Sterne spores in air and water

Towards the goal of developing a real-time monitoring device for microorganisms, we demonstrate the use of microcantilevers as resonant mass sensors for detection of Bacillus anthracis Sterne spores in air and liquid. The detection scheme was based on measuring resonant frequency decrease driven by thermally induced oscillations, as a result of the added mass of the spores with the use of a laser Doppler vibrometer (LDV). Viscous effects were investigated by comparing measurements in air and deionized (DI) water along with theoretical values. Moreover, biological experiments were performed which involved suspending spores onto the cantilevers and performing mass detection in air and water. For detection of spores in water, the cantilevers were functionalized with antibodies in order to fix the spores onto the surface. We demonstrate that as few as 50 spores on the cantilever can be detected in water using the thermal noise as excitation source. Measurement sensitivity of 9.23 Hz/fg for air and 0.1 Hz/fg for water were obtained. These measurements were compared with theoretical values and sources of improvement in cantilever sensitivity in a viscous medium were also discussed. It is expected that by driving the cantilevers and using higher order modes, detection of a single spore in liquids should be achievable.