Differential Surface Stress Research Articles

A Biosensing Platform Based on Metamaterials BioNEMS for Lab-on-Chip Systems.

An optical nanoelectromechanical platform relied on a SRR metamaterial system is presented in this paper as a label-free biosensor. This structure includes a flexible BioNEMS (Bio-Nano-Electro-Mechanical Systems) transducer and a proposed SRR metamaterials for detection of biological changes. Metamaterial cells consist of two parts which are coupled with an air gap distance. A functionalized BioNEMS beam supports one part of the proposed metamaterial cells. When patient samples including target analytes is exposed to the NEMS beam surface, the specific bio-interactions are happened and the energy (surface stress type) is released on the surface. This energy, which is induced only to the one side of the movable beam, causes a differential surface stress and thus displaces the nanomechanical beam. As a result, the air distance between two separated cells of the metamaterial unit is changed. This leads to varying the cell coupling effect which excites plasmon modes in a different wavelength. Therefore, biological quantities can be measured by detecting the resonance wavelength changes. Moreover, analyzing the device by various approaches results its functional characteristics as follows: detection sensitivity of 4251 nm/RIU, figure of merit (FOM) of 500.1 RIU -1 , mechanical sensitivity of [Formula: see text]/Nm -1 and resonant frequency of 17.1 kHz. Consequently, this mechanism is important for label-free biosensing due to its high potential for sensitive and quantitative detection of target analytes which leads to accurate diagnosis of diseases or identification of drugs.

A Novel BioNEMS Sensor Based on Tunable Plasmonic Modes of a Grating MIM Structure

In this paper, a novel optical BioNEMS (Bio Nano-Electro-Mechanical-Systems) sensor using a nanomechanical plasmonic structure is proposed to develop a high performance label-free biosensor. The present device is composed of a functionalized BioNEMS cantilever coated by a gold layer as a part of the proposed plasmonic grating waveguide. This plasmonic system is based on the light interaction with a metallic nanostructure as a metal-insulator-metal (MIM) Bragg grating where an air gap acts as the insulator. Passing patient samples through the functionalized surface of the nanocantilever, the biological interactions release a stress on the surface because of binding events. The cantilever can be thus deflected by inducing a differential surface stress. Therefore, the air gap distance (the insulator width) between two parts of the plasmonic grating structure is changed to excite surface plasmon polaritons in a different wavelength. Furthermore, the present biosensor is analyzed by numerical and analytical approaches and its functional characteristics are obtained as follows: optical sensitivity of 7872 nm/RIU, figure of merit (FOM) of 394 RIU^-1, mechanical sensitivity of 1.2 µm/Nm^-1. According to the results, the proposed label-free identification method can be used as a sensitive platform for detection of biomarkers in point of care testing (POCT) applications.

A label-free biomarkers detection platform relied on a bilayer long-wave infrared metamaterials BioNEMS sensor

A novel approach based on optical Biological-Nano-Electro-Mechanical-Systems (BioNEMS) sensor is presented in this paper to provide highly sensitive and precise detection of biomolecules. The proposed BioNEMS sensor is relied on a bi-layer metamaterials structure, tuned by its wavelength. The presented biosensor consists of a BioNEMS membrane coated by Complementary Split Ring Resonators and an array of Split Ring Resonators cells on the substrate. While the immobilized bioreceptors adsorb the biomarkers (i.e. mRNA or protein), it causes a bending of the suspended membrane. This is due to the differential surface stress which is induced on the Nano-Electro-Mechanical-Systems structure. As a consequence, the coupling strength of two complementary metamaterial layers and thus the electromagnetic response of the biosensor are changed. Furthermore, the proposed device is designed and analyzed by numerical and analytical approaches in order to obtain its functional characteristics as follows: detection sensitivity of 21 967 nm/RIU, figure of merit of 327.8 RIU−1", mechanical sensitivity of 2.6 μm/Nm−1" and resonant frequency of 4.92 kHz. According to the obtained results, the functional characteristics of the proposed label-free biosensor show its high potential for highly sensitive and accurate molecule detections, disease diagnosis as well as drug delivery tests for Lab-On-Chip systems.

A Novel Photonic Crystal BioNEMS Sensing Platform Based on Fano resonances

A novel optical BioNEMS (Bio Nano-Electro-Mechanical-Systems) sensor based on a tunable 2D photonic crystal (PhC) cavity is proposed in this paper. The present device includes a functionalized BioNEMS cantilever and a tunable PhC structure. This optical detection system is a rod-type silicon photonic crystal cavity in which a designed defect allows the transmission of highly confined wave modes. While the immobilized capture probes are exposed to a solution containing the target biomarkers (i.e., DNA, mRNA or proteins), binding events cause a stress on the surface of the nanocantilever because of biological interactions. Therefore, the NEMS movable part is displaced as a result of an induced differential surface stress. This changes the position of the defect nanorods in the PhC cavity which leads to the transmitted wavelength variations. Finally, the concentration of biological quantities is measured by detecting the optical spectrum changes of the proposed biosensor. Furthermore, the presented BioNEMS sensor is analyzed by numerical and analytical approaches to obtain its functional characteristics as follows: optical sensitivity of 5140 nm/RIU, FOM of 10280 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , Q-factor of 3078, mechanical sensitivity of 1.05 µm/Nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and resonant frequency of 23.45 kHz. Based on the obtained results, the proposed structure relied on Fano resonances provides a high-precision biosensor which has a great potential for highly sensitive detection of biomarkers in early diseases diagnosis as well as drug delivery test.

Effect of surface stress induced curvature on the eigenfrequencies of microcantilever plates

Ultrasensitive physical, chemical and biological sensors have emerged in the last decade based on the measurement of the eigenfrequencies of micro- and nanosized cantilever plates. Surface stress is omnipresent in these devices due to a variety of factors such as the fabrication process, temperature variations and analyte adsorption. How surface stress influences on the eigenfrequencies of cantilever plates has remained as an unsolved question in physics that has raised a long debate since first experiments in 1975. Recent theoretical models have shed light on the role of the net surface stress. Still, there exists a discrepancy between theory and some experimental reports, affecting to the capability for quantification of these sensors. In this Letter, we present a theoretical framework that demonstrates that the cantilever bending due to differential surface stress between opposite faces of the cantilever, a neglected effect in classical beam theory, plays a relevant role in the stiffness and eigenfrequencies of cantilevers. We develop a new theoretical framework that provides analytical equations that accurately describe the effect of surface stress on the first three vibration modes of cantilevers. Our findings provide the final piece of the puzzle for solving this long-standing problem in physics.

Hydrogen-bond transition from the vibration mode of ordinary water to the (H, Na)I hydration states: Molecular interactions and solution viscosity

With the aid of differential phonon spectrometrics (DPS) and surface stress detection, we show that HI and NaI solvation transforms different fractions of the HO stretching phonons from the mode of ordinary water centred at ∼3200 to the mode of hydration shell at ∼3500cm−1. Observations suggest that an addition of the H↔H anti-hydrogen-bond to the Zundel notion, [H(H2O)2]+, would be necessary as the HO bond due H3O+ has a 4.0eV energy, and the H↔H fragilization disrupts the solution network and the surface stress. The I− and Na+ ions form each a charge centre that aligns, stretches, and polarize the O:HO bond, resulting in shortening the HO bond and its phonon blue shift in the hydration shell or at the solute-solvent interface. The solute capabilities of bond-number-fraction transition follow: fH=0, fNa∝C, and fI∝1−exp(−C/C0) toward saturation, with C being the solute molar concentration and C0 the decay constant. The fH=0 evidences the non-polarizability of the H+ because of the H↔H formation. The linear fNa(C) suggests the invariance of the Na+ hydration shell size because of the fully-screened cationic potential by the H2O dipoles in the hydration shell but the nonlinear fI(C) fingerprints the I−↔I− interactions at higher concentrations. Concentration trend consistency between Jones–Dole’s viscosity and the fNaI(C) coefficient may evidence the same polarization origin of the solution viscosity and surface stress.

Frequency response of curved bilayer microcantilevers with applications to surface stress measurement

Bilayer microcantilevers are normally curved because of fabrication-induced stresses. When used in biological/chemical sensing applications, the absorption of target agents onto the functionalized surface of the microcantilever creates a surface stress that shifts its resonance frequency. Despite numerous efforts, the mechanisms of surface stress-induced shift in the resonance frequency of microcantilevers remain elusive. To address this problem, this work presents a detailed analysis of the frequency response of microcantilevers, with different width-to-thickness ratios and curvature levels, using classical lamination theory and the Rayleigh–Ritz method. Based on the results of this analysis, a new relationship between resonance frequency shift and curvature variation due to differential surface stress loading is established. By comparing the strain energies associated with the in-plane and out-of-plane displacements of the microcantilever at different curvature levels, a new implicit model for surface stress-induced resonance frequency shift in microcantilevers is presented. Verified against the results of experiments carried out on gold/polysilicon microcantilevers, the new model is expected to provide a better understanding of surface stress-microcantilever resonator interaction, which is critical to systematic optimization of resonance-based micro sensors.

Role of surface morphology on desorption kinetics of water molecules from uncoated silicon microcantilever

Desorption kinetics of water molecules from uncoated silicon microcantilevers (MC) is studied, by measuring resonance frequency, with Relative Humidity (RH) varying in the range of 50–15%. Shift in resonance frequency of MC was used to estimate the desorbed mass, which over-estimated the number of monolayers desorbed between the RH values of interest and followed a single exponential decay model. Moreover, desorption rate is found to be higher for RH>25% compared to RH<25%. However, the Q factor describing the sensitivity of the MC and computed from the resonance spectrum is found to decrease by ∼3%. These results are explained on the basis of complex surface coverage during desorption of water molecules and is supported by topographical image of MC surface clearly showing quasi 3D nanofeatures. The bending of uncoated MC during desorption, due to the generation of differential surface stress, further confirms the presence of these surface features.

Monitoring swelling and deswelling of thin polymer films by microcantilever sensors

Microcantilever based sensors are promising devices because they are fast, low cost, portable and allow measuring extremely small quantities of substances in gas or liquid phase that are not detectable by other methods. In this work we investigate the swelling and deswelling behavior of poly(hydroxyl ethyl methacrylate), PHEMA, films with three different average molecular masses deposited onto bare silicon microcantilevers, a hydrophilic substrate, and onto polystyrene (PS) coated microcantilevers, which is a hydrophobic substrate, under humidity cycles by monitoring the deflections resulting from differential surface stress. The results found in the present study demonstrate that the micromechanical response observed are related not only to the polymer molecular weight, but also to the polymer-interface phenomena and environment–polymer interface that can play a crucial role on the signal transduction. Ellipsometric measurements evidenced that the swelling of PHEMA films with intermediate molecular weight at 80% RH follows Fickian diffusion mechanism. Nevertheless, due to the weak optical contrast or film homogeneity shorter or longer chains than PHEMA-300 could not be investigated by ellipsometry. PHEMA films deposited onto PS surfaces presented dewetting, which limited the ellipsometric measurements. In comparison to ellipsometry, monitoring swelling behavior of PHEMA by nanomechanical responses is more advantageous because it is faster and less restricted.

Structural modifications of gold thin films produced by thiol-derivatized single-stranded DNA immobilization

Recent experiments have reported an opposite sign of the differential surface stress produced on gold-coated cantilevers by a thiol-derivatized single-stranded DNA (SH-DNA) immobilization process. The sign of the surface stress depends on the method used to evaporate the gold thin film, being compressive (negative) or tensile (positive) for e-beam or resistively deposited gold, respectively. This study investigates the origin of this effect by means of a combination of x-ray diffraction and x-ray photoelectron spectroscopy. Both e-beam and resistively grown gold thin films are characterized to find the subtle differences responsible for this intriguing stress behaviour. Somewhat remarkably, these studies show a tight relation between the surface structure of the gold overlayer and the SH-DNA immobilization efficiency. The average grain size variation seems to correlate well with the differential surface stress triggered by the SH-DNA immobilization previously reported. These results suggest that the relation of the probe molecules with the surface structure must be considered to understand surface stress changes.

Development of Robust and Standardized Cantilever Sensors Based on Biotin/Neutravidin Coupling for Antibody Detection

A cantilever-based protein biosensor has been developed providing a customizable multilayer platform for the detection of antibodies. It consists of a biotin-terminated PEG layer pre-functionalized on the gold-coated cantilever surface, onto which NeutrAvidin is adsorbed through biotin/NeutrAvidin specific binding. NeutrAvidin is used as a bridge layer between the biotin-coated surface and the biotinylated biomolecules, such as biotinylated bovine serum albumin (biotinylated BSA), forming a multilayer sensor for direct antibody capture. The cantilever biosensor has been successfully applied to the detection of mouse anti-BSA (m-IgG) and sheep anti-BSA(s-IgG) antibodies. As expected, the average differential surface stress signals of about 5.7 ± 0.8 × 10−3 N/m are very similar for BSA/m-IgG and BSA/s-IgG binding, i.e., they are independent of the origin of the antibody. A statistic evaluation of 112 response curves confirms that the multilayer protein cantilever biosensor shows high reproducibility. As a control test, a biotinylated maltose binding protein was used for detecting specificity of IgG, the result shows a signal of bBSA layer in response to antibody is 5.8 × 10−3 N/m compared to bMBP. The pre-functionalized biotin/PEG cantilever surface is found to show a long shelf-life of at least 40 days and retains its responsivity of above 70% of the signal when stored in PBS buffer at 4 °C. The protein cantilever biosensor represents a rapid, label-free, sensitive and reliable detection technique for a real-time protein assay.

High Sensitivity Microbridge for Molecular Sensing Applications

This paper presents the design and simulation of a SiO2 microbridge which measures the differential surface stress induced by the adsorption of molecules. Materials with smaller Young's modulus would be appropriate to measure the small variation of surface stress on the microbridge and therefore SiO2 is chosen as the material for microbridge. The SiO2 microbridge could be a potential application for the glucose detection if the enzyme glucose oxidase was immobilized on a top surface. The microbridge can be constructed on Silicon on insulator (SOI) substrate using the bulk micromachining technology. The surface stress causes displacement which is measured using the piezoresistors placed on top surface of the microbridge. The piezoresistors are arranged in a Wheatstone bridge configuration to measure the change resistance in terms of voltage. The two piezoresistors are placed at the edges where they experience the maximum stress. The obtained sensitivity, ΔR/R of SiO2 microbridge is 6.15x10-4 which is four times higher than that of existing work [1]. The mechanical behavior of the SiO2 microbridge and the electrical response of the piezoresistors are analyzed using the Finite Element Method (FEM). The dimensions for the microbridge are optimized by considering both the sensitivity and nonlinearity with respect to the deflection. The placement location of the piezoresistors and the effect of geometrical parameters of the piezoresistors on the sensitivity SiO2 microbridge are studied.

Transient deflection response in microcantilever array integrated with polydimethylsiloxane (PDMS) microfluidics

We report the integration of a nanomechanical sensor consisting of 16 silicon microcantilevers with polydimethylsiloxane (PDMS) microfluidics. For microcantilevers positioned near the bottom of a microfluidic flow channel, a transient differential analyte concentration for the top versus bottom surface of each microcantilever is created when an analyte-bearing fluid is introduced into the flow channel (which is initially filled with a non-analyte containing solution). We use this effect to characterize a bare (nonfunctionalized) microcantilever array in which the microcantilevers are simultaneously read out with our recently developed high sensitivity in-plane photonic transduction method. We first examine the case of non-specific binding of bovine serum albumin (BSA) to silicon. The average maximum transient microcantilever deflection in the array is -1.6 nm, which corresponds to a differential surface stress of only -0.23 mN m(-1). This is in excellent agreement with the maximum differential surface stress calculated based on a modified rate equation in conjunction with finite element simulation. Following BSA adsorption, buffer solutions with different pH are introduced to further study microcantilever array transient response. Deflections of 20-100 nm are observed (2-14 mN m(-1) differential surface stress). At a flow rate of 5 μL min(-1), the average measured temporal width (FWHM) of the transient response is 5.3 s for BSA non-specific binding and 0.74 s for pH changes.

Modeling and Comparative Analysis of Carbon Nanotube and Boron Nitride Nanotube Cantilever Biosensors

This paper investigates the bending deformation of a cantilever biosensor based on a single-walled carbon nanotube (CNT) and single-walled boron nitride nanotube (BNNT) due to bioparticle detection. Through 3-D modeling and simulations, the performance of the CNT and BNNT cantilever biosensors is analyzed. It is found that the BNNT cantilever has better response and sensitivity compared to the CNT counterpart. Additionally, an algorithm for an electrostatic-mechanical coupled system is developed. The cantilever (both BNNT and CNT) is modelled by accounting that a conductive polymer is deposited onto the nanotube surfaces. Two main approaches are considered for the mechanical deformation of the nanotube beam. The first one is differential surface stress produced by the binding of biomolecules onto the surface. The second one is the charge released from the biomolecular interaction. Also, different ambient conditions are considered in the study of sensitivity. Sodium Dodisyl Sulphate (SDS) provides better bending deformation than the air medium. Other parameters including length of beam, variation of beam’s location, and chiralities are considered in the design. The results are in excellent agreement with the electrostatic equations that govern the deformation of cantilever.

Stripping voltammetry of Pb and Cu using a microcantilever electrode

Microfabricated silicon microcantilevers coated with gold on one side have been used as working electrode in a three-electrode electrochemical arrangement. In addition to electrochemical current, cantilever bending has been used as a signal for monitoring electrode reactions on the cantilever surface. The microcantilever bending was measured by an optical beam deflection method as the surface potential was scanned and electrochemical reactions occurred on the surface. The microcantilever bending due to differential surface stress was used to sense Pb and Cu using cyclic voltammetry (CV) and linear sweep stripping voltammetry (LSSV).

Stress and DNA Assembly Differences on Cantilevers Gold Coated by Resistive and E-Beam Evaporation Techniques

Changes in the sign of differential surface stress of gold-coated cantilevers produced by thiol-derivatized single-stranded DNA immobilization are observed, depending on the method used to deposit the gold. While the DNA immobilization on e-beam gold-coated cantilevers produces a compressive differential surface stress in the metallic layer, the opposite is observed for resistively coated cantilevers under the same immobilization conditions. The gold films exhibit quite a similar morphology, and the immobilization differences seem to be related to the charge state of the metallic layer surface. This in turn produces a different distribution of the orientation of the DNA strands on the gold layer. A tentative explanation for the observed effect is proposed.

Differential surface stress sensor for detection of chemical and biological species

We report a sensor consisting of two micromachined cantilevers (a sensing/reference pair) that is suitable for detection of chemical and biological species. The sensing strategy involves coating the sensing cantilever with receptors that have high affinities for the analyte. The presence of analyte is detected by determining the differential surface stress associated with its adsorption/absorption to the sensing cantilever. An interferometric technique is utilized to measure the differential bending of the sensing cantilever with respect to reference. Surface stress associated with hybridization of single stranded DNA is measured to demonstrate the unique advantages of the sensor.

Voltammetry of the Pb/Pb2+ Redox Couple using a Gold Microcantilever Electrode

Microfabricated silicon microcantilevers were coated with gold on one side and wired to a potentiostat so that electrochemical measurements could be made using the cantilever surface as an electrode. The microcantilever bending was measured optically as the surface potential was scanned and electrochemical reactions occurred on the surface. The microcantilever bending due to differential surface stress was used to sense Pb(II) using cyclic voltammetry (CV) and linear sweep stripping voltammetry (LSSV).

Wavelength Dependence of Photoinduced Microcantilever Bending in the UV-VIS Range.

Micromechanical devices such as microcantilevers (MC) respond to irradiationwith light by at least two different, photon-mediated processes, which induce MC bendingas a consequence of differential surface stress. The first and slow bending is due to theabsorption of photons, whose energy is transformed into heat and causes bending ofbimetallic microcantilevers due to thermal expansion. The second type of deflection is fastand caused by photons of sufficient energy to promote electrons across the Schottky barrierand thus create charge carriers, resulting in photoinduced stress that causes MC bending. Inthis study, the MC bending response to irradiation with light of wavelengths ranging from250 to 700 nm was investigated. Measurements of the immediate mechanical response tophotoinduced stress as a function of the wavelength of incident light provide an avenue tothe determination of the cut-off wavelength/energy of the Schottky barrier in the MCdevices under investigation. For a gold coated Si3Ni4 microcantilever we measured a cutoffwavelength of 1206 nm, which lies in the range of the literature value of 1100 nm.

A micromachined surface stress sensor with electronic readout

A micromachined surface stress sensor has been fabricated and integrated off chip with a low-noise, differential capacitance, electronic readout circuit. The differential capacitance signal is modulated with a high frequency carrier signal, and the output signal is synchronously demodulated and filtered resulting in a dc output voltage proportional to the change in differential surface stress. The differential surface stress change of the Au(111) coated silicon sensors due to chemisorbed alkanethiols is Deltasigma(s) approximately -0.42+/-0.0028 N m(-1) for 1-dodecanethiol (DT) and Deltasigma(s) approximately -0.14+/-0.0028 N m(-1) for 1-butanethiol (BT). The estimated measurement resolution (1 Hz bandwidth) is approximately 0.12 mN m(-1) (DT: 0.2 pg mm(-2) and BT: 0.8 pg mm(-2)) and as high as approximately 3.82 microN m(-1) (DT: 8 fg mm(-2) and BT: 24 fg mm(-2)) with system optimization.