Flexoelectric origin of nanomechanic deflection in DNA-microcantilever system

A Genosensor Based on the Modification of a Microcantilever: A Review

When the free end of a microcantilever is modified by a genetic probe, this sensor can be used for a wider range of applications, such as for chemical analysis, biological testing, pharmaceutical screening, and environmental monitoring. In this paper, to clarify the preparation and detection process of a microcantilever sensor with genetic probe modification, the core procedures, such as probe immobilization, complementary hybridization, and signal extraction and processing, are combined and compared. Then, to reveal the microcantilever's detection mechanism and analysis, the influencing factors of testing results, the theoretical research, including the deflection principle, the establishment and verification of a detection model, as well as environmental influencing factors are summarized. Next, to demonstrate the application results of the genetic-probe-modified sensors, based on the classification of detection targets, the application status of other substances except nucleic acid, virus, bacteria and cells is not introduced. Finally, by enumerating the application results of a genetic-probe-modified microcantilever combined with a microfluidic chip, the future development direction of this technology is surveyed. It is hoped that this review will contribute to the future design of a genetic-probe-modified microcantilever, with further exploration of the sensitive mechanism, optimization of the design and processing methods, expansion of the application fields, and promotion of practical application.

Effect of surface charge state on the surface stress of a microcantilever

The surface charge state at a liquid-solid interface is important to the variations in the physical/chemical properties of adsorbate film such as surface stress and the ensuing tip deflection of the microcantilever. The well-known Stoney's equation, derived more than 100 years ago, conceals the film electrical properties with the replacement of substrate deformation induced by adsorptions of particles. This implicit expression provides a shortcut to circumvent the difficulty in identifying some film properties, however, it limits the capacity to ascertain the relation between surface stress variation and the surface charge state. In this paper, we present an analytical expression to quantify the cantilever deflection/surface stress and the film potential difference by combining the piezoelectric theory and Poisson-Boltzmann equation for electrolyte solution. This updated version indicates that the two linear correlations between surface stress and surface charge density or the bias voltage are not contradictory, but two aspects of one thing under different conditions. Based on Parsegian's mesoscopic interaction potential, a multiscale prediction for the piezoelectric coefficient of double-stranded DNA (dsDNA) film is done, and the results show that the distinctive size effect with variations in salt concentration and nucleotide number provides us with an opportunity to obtain a more sensitive potential-actuated microcantilever sensor by careful control of packing conditions.

Influence of disordered packing pattern on elastic modulus of single-stranded DNA film on substrate

Determining mechanical properties of single-stranded DNA film grafted on gold surface is critical for analysis and design of DNA-microcantilever biosensors. However, it remains an open issue to quantify the relations among the disordered packing patterns of DNA chains, the mechanical properties of DNA film and the resultant biodetection signals. In this paper, first, the bending experiment of microcantilever is carried out to provide the basic data for a refined multi-scale model of microcantilever deflection induced by ssDNA immobilization. In the model, the complicated interactions in DNA film (consisting of DNA, water molecules and salt ions) are simplified as effective interactions among coarse-grained soft cylinders, which can reveal the varieties of DNA structure in the circumstances of different lengths and salt concentrations; Ohshima's distribution of net charge density is employed to incorporate compositional variations of salt ions along the thickness direction into the Strey's mesoscopic empirical potential on molecular interactions in DNA solutions, and the related model parameters for ssDNA film on substrate are obtained from the curve fitting with our microcantilever bending experiment. Second, the effect of nanoscopic distribution of DNA chains on elastic modulus of ssDNA film is studied by a thought experiment of uniaxial compression, and the disordered patterns of DNA chains are generated by Monte Carlo method. Simulation results point out that nanoscale ssDNA film shows size effect, gradient and diversity in elastic modulus and can achieve maximum stiffness by preferring a disordered and energetically favorable packing pattern collectively induced by electrostatic force, hydration force and configurational entropy.

Theoretical modeling and experimental validation of surface stress in thrombin aptasensor

Adsorption of target molecules on the immobilized microcantilever surface produced beam displacement due to the differential surface stress generated between the immobilized and non-immobilized surface. Surface stress is caused by the intermolecular forces between the molecules. Van der Waals, electrostatic forces, hydrogen bonding, hydrophobic effect and steric hindrance are some of the intermolecular forces involved. A theoretical framework describing the adsorption-induced microcantilever displacement is derived in this paper. Experimental displacement of thrombin aptamer-thrombin interactions was carried out. The relation between the electrostatic interactions involved between adsorbates (thrombin) as well as adsorbates and substrates (thrombin aptamer) and the microcantilever beam displacement utilizing the proposed mathematical model was quantified and compared to the experimental value. This exercise is important to aid the designers in microcantilever sensing performance optimization.

Approaches to increasing surface stress for improving signal-to-noise ratio of microcantilever sensors

Microcantilever sensor technology has been steadily growing for the last 15 years. While we have gained a great amount of knowledge in microcantilever bending due to surface stress changes, which is a unique property of microcantilever sensors, we are still in the early stages of understanding the fundamental surface chemistries of surface-stress-based microcantilever sensors. In general, increasing surface stress, which is caused by interactions on the microcantilever surfaces, would improve the S/N ratio and subsequently the sensitivity and reliability of microcantilever sensors. In this review, we will summarize (A) the conditions under which a large surface stress can readily be attained and (B) the strategies to increase surface stress in case a large surface stress cannot readily be reached. We will also discuss our perspectives on microcantilever sensors based on surface stress changes.

Cantilever-based sensing: the origin of surface stress and optimization strategies

Many interactions drive the adsorption of molecules on surfaces, all of which can result in a measurable change in surface stress. This article compares the contributions of various possible interactions to the overall induced surface stress for cantilever-based sensing applications. The surface stress resulting from adsorption-induced changes in the electronic density of the underlying surface is up to 2-4 orders of magnitude larger than that resulting from intermolecular electrostatic or Lennard-Jones interactions. We reveal that the surface stress associated with the formation of high quality alkanethiol self-assembled monolayers on gold surfaces is independent of the molecular chain length, supporting our theoretical findings. This provides a foundation for the development of new strategies for increasing the sensitivity of cantilever-based sensors for various applications.

Sensitivity enhancement in DNA hybridization assay using gold nanoparticle-labeled two reporting probes

A simple and sensitive method for DNA detection using gold nanoparticle (AuNP) two-probe detection system (AuNP-TP) was developed. Preliminary experiment was carried out by optimizing slide types, blocking agents and hybridization times. Fluorescent-labeled probes were used along with AuNP-labeled probes to confirm specific binding event between target DNA and probes. The sensitivities between AuNP single-probe (AuNP-SP) and AuNP-TP systems using sandwich-typed assay were compared. The AuNP-TP on epoxide-coated (EP) slides increased sensitivity 1000-fold at the detection limit of 100fM when compared to the AuNP-SP. This result indicates that the assay sensitivity was simply enhanced by simultaneous adding two AuNP labeled probes which selectively recognize different regions of the target DNA. The concept of AuNP-TP could potentially be applied to a macroarray format to detect multiple DNA targets simultaneously; thereby making the assay becomes more affordable and more sensitive.

Nucleic Acid-based Detection of Bacterial Pathogens Using Integrated Microfluidic Platform Systems

The advent of nucleic acid-based pathogen detection methods offers increased sensitivity and specificity over traditional microbiological techniques, driving the development of portable, integrated biosensors. The miniaturization and automation of integrated detection systems presents a significant advantage for rapid, portable field-based testing. In this review, we highlight current developments and directions in nucleic acid-based micro total analysis systems for the detection of bacterial pathogens. Recent progress in the miniaturization of microfluidic processing steps for cell capture, DNA extraction and purification, polymerase chain reaction, and product detection are detailed. Discussions include strategies and challenges for implementation of an integrated portable platform.

Thermodynamics of mechanical transduction of surface confined receptor/ligand reactions

Chemomechanics of surface stress is discussed in terms of interfacial thermodynamics. In the first section the paper shows how to quantitatively describe the chemical equilibrium of a receptor/ligand binding reaction confined at a solid-liquid interface and how the overall work of the reaction splits into chemical and surface work, that appears as a surface pressure. In the second section this thermodynamic model is applied to describe the experimental results of microcantilever bending induced by DNA hybridization occurring onto one of its faces.

A nanomechanical device based on light-driven proton pumps

In this paper, a hybrid device based on a microcantilever interfaced with bacteriorhodopsin (bR) is constructed. The microcantilever, on which the highly oriented bR film is self-assembled, undergoes controllable and reversible bending when the light-driven proton pump protein, bR, on the microcantilever surface is activated by visible light. Several control experiments are carried out to preclude the influence of heat and photothermal effects. It is shown that the nanomechanical motion is induced by the resulting gradient of protons, which are transported from the KCl solution on the cytoplasmic side of the bR film towards the extracellular side of the bR film. Along with a simple physical interpretation, the microfabricated cantilever interfaced with the organized molecular film of bR can simulate the natural machinery in converting solar energy to mechanical energy.

Piezoelectric-excited millimeter-sized cantilever (PEMC) sensors detect Bacillus anthracis at 300spores/mL

Piezoelectric-excited millimeter-sized cantilever (PEMC) sensors consisting of a piezoelectric and a borosilicate glass layer with a sensing area of 2.48 mm2 were fabricated. Antibody specific to Bacillus anthracis (BA, Sterne strain 7702) spores was immobilized on PEMC sensors, and exposed to spores (300 to 3x10(6) spores/mL). The resonant frequency decreased at a rate proportional to the spore concentration and reached a steady state frequency change of 5+/-5 Hz (n=3), 92+/-7 Hz (n=3), 500+/-10 Hz (n=3), 1030+/-10 Hz (n=2), and 2696+/-6 Hz (n=2) corresponding to 0, 3x10(2), 3x10(3), 3x10(4), and 3x10(6) spores/mL, respectively. The reduction in resonant frequency is proportional to the change in cantilever mass, and thus the observed changes are due to the attachment of spores on the sensor surface. Selectivity of the antibody-functionalized sensor was determined with samples of BA (3x10(6)/mL) mixed with Bacillus thuringiensis (BT; 1.5x10(9)/mL) in various volume ratios that yielded BA:BT ratios of 1:0, 1:125, 1:250, 1:500 and 0:1. The corresponding resonance frequency decreases were, respectively, 2345, 1980, 1310, 704 and 10 Hz. Sample containing 100% BT spores (1.5x10(9)/mL and no BA) gave a steady state frequency decrease of 10 Hz, which is within noise level of the sensor, indicating excellent selectivity. The observed binding rate constant for the pure BA and BT-containing samples ranged from 0.105 to 0.043 min-1 in the spore concentration range 300 to 3x10(6)/mL. These results show that detection of B. anthracis spore at a very low concentration (300 spores/mL) and with high selectivity in presence of another Bacillus spore (BT) can be accomplished using piezoelectric-excited millimeter-sized cantilever sensors.

Detection and quantification of proteins using self-excited PZT-glass millimeter-sized cantilever

A composite self-excited PZT-glass cantilever (4mm in length and 2mm wide) was fabricated and used to measure the binding and unbinding of model proteins. A key feature of the cantilever is that its resonant frequency is dependent on its mass. The fabricated cantilever has mass change sensitivity in liquid of 7.2 x 10(-11)g/Hz. Resonant frequency change was measured as protein reacted or bound with the sensing glass cantilever surface. Protein concentrations, 0.1 and 1.0mg/mL, which resulted in nanogram mass change were successfully detected. The mass change sensitivity gave a total mass change of 54+/-0.45 ng for the binding of anti-rabbit IgG (biotin conjugated) to rabbit IgG immobilized cantilever and the subsequent binding of captavidin. The unbinding of anti-rabbit IgG and captavidin gave a total mass change of 54+/-1.70 ng. Fluorescence based assays showed the combined mass of both proteins in the released samples was 54+/-2.24 ng. The binding kinetics of the model proteins is modeled as first order. The initial binding rate constant of anti-rabbit IgG to rabbit IgG was 1.36+/-0.02(min(mg/mL))(-1). The initial binding rate constant of captavidin to biotinylated anti-rabbit IgG was (2.57 x 10(-1))+/-0.003(min(mg/mL))(-1). The significance of the results we report here is that millimeter-sized PZT-actuated glass cantilevers have the sensitivity to measure in real-time protein-protein binding, and the binding rate constant.

Detection of pathogen Escherichia coli O157:H7 using self-excited PZT-glass microcantilevers

Composite self-excited PZT-glass cantilevers (5 and 3 mm in length, 1.8 and 2.0 mm wide) were fabricated and their resonance characteristics were determined in air and at 1 mm liquid immersion. In air, resonance occurred at 65.8 and 63.4 kHz for the two cantilevers used in this paper. Monoclonal antibody (MAb) specific to the pathogen Escherichia coli (E. coli) O157:H7 was immobilized at the cantilever glass tip, and then exposed to pathogen in the concentration range of 7x10(2) to 7x10(7)bacteria/mL. Resonance of the second mode decreased due to pathogen attachment in accordance with a proposed kinetic model. The specific attachment rate constant was found to be 3x10(-9) to 5x10(-9) min-1 (cell/mL)-1. Exposure to a mixed population containing both a pathogenic and non-pathogenic strain showed that the antibody-immobilized cantilever is highly selective, thus demonstrating its usefulness for detecting water-borne pathogens.