Cantilever-based Sensors Research Articles

Sensing volatile organic compounds in aquatic samples: a review

ABSTRACT Volatile organic compounds (VOCs) are toxic and harmful to human health; thus, their detection in water samples is critical for ensuring safe drinking water and protecting public health. The aim of this review was to explore the various techniques available in the literature for detecting VOCs in water samples and discuss their features, advantages, and limitations in facilitating fast and rapid VOC analysis. Established techniques such as gas chromatography and mass spectrometry offer high sensitivity and selectivity but are often hindered by complexity and cost. In contrast, emerging technologies like microfluidic gas sensors and cantilever-based sensors provide portable and real-time monitoring capabilities, although they may be susceptible to matrix effects. Overcoming these challenges requires advanced sample preparation techniques and method optimization. Additionally, the integration of machine learning algorithms and nanotechnology shows promise for enhancing sensitivity and precision in the detection of VOCs. By overcoming these limitations and leveraging technological advancements, researchers can improve water quality monitoring and help protect the environment.

Pt-Black-Modified (Hemi)spherical AFM Sensors: In Situ Imaging of Light-Driven Hydrogen Peroxide Evolution.

In this work, we present (hemi)spherical atomic force microscopy (AFM) sensors for the detection of hydrogen peroxide. Platinum-black (Pt-B) was electrodeposited onto conductive colloidal AFM probes or directly at recessed microelectrodes located at the end of a tipless cantilever, resulting in electrocatalytically active cantilever-based sensors that have a small geometric area but, due to the porosity of the films, exhibit a large electroactive surface area. Focused ion beam-scanning electron microscopy tomography revealed the porous 3D structure of the deposited Pt-B. Given the accurate positioning capability of AFM, these probes are suitable for local in situ sensing of hydrogen peroxide and at the same time can be used for (electrochemical) force spectroscopy measurements. Detection limits for hydrogen peroxide in the nanomolar range (LOD = 68 ± 7 nM) were obtained. Stability test and first in situ proof-of-principle experiments to achieve the electrochemical imaging of hydrogen peroxide generated at a microelectrode and at photocatalytically active structured poly(heptazine imide) films are demonstrated. Force spectroscopic data of the photocatalyst films were recorded in ambient conditions, in solution, and by applying a potential, which demonstrates the versatility of these novel Pt-B-modified spherical AFM probes.

NDE of Concrete Strength and Elasticity Modulus Based on Jerk Measurement

The quality assessment of concrete material for new or existing buildings is critical to civil and structural engineers. In this scope, the development of nondestructive testing methods (NDT) has received significant attention and becomes essential for enhancing the on-site assessment of concrete performance. The Schmidt hammer (SH) test is one of the most widely used NDT for quantifying concrete strength. However, the reliability of this test and its accuracy are questioned by many researchers. In this study, the measurement quality of SH has been improved by attaching a developed cantilever-based jerk sensor to the SH mass. The theoretical model of the proposed measurement technique has been formulated and discussed. The experimental approach, jerk sensor design, SH body modification, electronic reading unit, testing, and construction of the calibration curves have been presented. Experimental results revealed that the accuracy of the developed SH has increased by 13% for concrete strength measurement, and the correlation coefficient of the strength calibration curve is 0.94. At the same time, the calibration curve of the concrete elasticity modulus showed a 0.93 correlation coefficient with 92% measurement accuracy. Analytical and experimental results have confirmed the applicability of the modified Schmidt hammer since it is reliable and more accurate compared to the traditional Schmidt Hammer.

Scanning electrochemical probe microscopy: towards the characterization of micro- and nanostructured photocatalytic materials.

Platinum-black (Pt-B) has been demonstrated to be an excellent electrocatalytic material for the electrochemical oxidation of hydrogen peroxide (H2O2). As Pt-B films can be deposited electrochemically, micro- and nano-sized conductive transducers can be modified with Pt-B. Here, we present the potential of Pt-B micro- and sub-micro-sized sensors for the detection and quantification of hydrogen (H2) in solution. Using these microsensors, no sampling step for H2 determination is required and e.g., in photocatalysis, the onset of H2 evolution can be monitored in situ. We present Pt-B-based H2 micro- and sub-micro-sized sensors based on different electrochemical transducers such as microelectrodes and atomic force microscopy (AFM)-scanning electrochemical microscopy (SECM) probes, which enable local measurements e.g., at heterogenized photocatalytically active samples. The microsensors are characterized in terms of limits of detection (LOD), which ranges from 4.0 μM to 30 μM depending on the size of the sensors and the experimental conditions such as type of electrolyte and pH. The sensors were tested for the in situ H2 evolution by light-driven water-splitting, i.e., using ascorbic acid or triethanolamine solutions, showing a wide linear concentration range, good reproducibility, and high sensitivity. Proof-of-principle experiments using Pt-B-modified cantilever-based sensors were performed using a model sample platinum substrate to map the electrochemical H2 evolution along with the topography using AFM-SECM.

Highly sensitive low-frequency-detectable acoustic sensor using a piezoresistive cantilever for health monitoring applications

This study investigates a cantilever-based pressure sensor that can achieve a resolution of approximately 0.2 mPa, over the frequency range of 0.1–250 Hz. A piezoresistive cantilever with ultra-high acoustic compliance is used as the sensing element in the proposed pressure sensor. We achieved a cantilever with a sensitivity of approximately 40 times higher than that of the previous cantilever device by realizing an ultrathin (340 nm thick) structure with large pads and narrow hinges. Based on the measurement results, the proposed pressure sensor can measure acoustic signals with frequencies as low as 0.1 Hz. The proposed pressure sensor can be used to measure low-frequency pressure and sound, which is crucial for various applications, including photoacoustic-based gas/chemical sensing and monitoring of physiological parameters and natural disasters. We demonstrate the measurement of heart sounds with a high SNR of 58 dB. We believe the proposed microphone will be used in various applications, such as wearable health monitoring, monitoring of natural disasters, and realization of high-resolution photoacoustic-based gas sensors. We successfully measured the first (S1) and second (S2) cardiac sounds with frequencies of 7–100 Hz and 20–45 Hz, respectively.

Nanodiamonds enable femtosecond-processed ultrathin glass as a hybrid quantum sensor

The quantum properties of fluorescent nanodiamonds offer great promise for fabricating quantum-enabled devices for physical applications. However, the nanodiamonds need to be suitably combined with a substrate to exploit their properties. Here, we show that ultrathin and flexible glass (thickness 30 microns) can be functionalized by nanodiamonds and nano-shaped using intense femtosecond pulses to design cantilever-based nanomechanical hybrid quantum sensors. Thus fabricated ultrathin glass cantilevers show stable optical, electronic, and magnetic properties of nitrogen-vacancy centers, including well-defined fluorescence with zero-phonon lines and optically detected magnetic resonance (ODMR) near 2.87 GHz. We demonstrate several sensing applications of the fluorescent ultrathin glass cantilever by measuring acoustic pulses, external magnetic field using Zeeman splitting of the NV centers, or CW laser-induced heating by measuring thermal shifting of ODMR lines. This work demonstrates the suitability of the femtosecond-processed fluorescent ultrathin glass as a new versatile substrate for multifunctional quantum devices.

Determination of Staphylococcus Enterotoxin B Using a Flexible Aptamer-Based Biosensor

We demonstrate a fully flexible micromachined cantilever-based aptasensor for sensitive and specific sensing of Staphylococcus Enterotoxin B (SEB). The proposed flexible cantilever was fabricated using parylene-C as surface passivation layers and poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) as strain gauge for characterization of resistance. A spring constant of prepared cantilever was measured to be 12 nN/μm together with a deflection sensitivity of 1.42×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> /μm. With assembled 78-mer 5’-biotinylated aptamers on the sensing cantilever surface, the fully flexible aptasensor is capable of specific recognition SEB with a limit detection of 0.3 ng/mL. To evaluate the applicability of the proposed aptasensor, detections on raw milk spiked with SEB were implemented. The comparison indicates the prepared aptasensor could be qualified for SEB recognition in food contaminants detections, and the fully flexible cantilever-based sensors have tremendous potential for applications in food safety evaluation.

Multi-Position Measurable Flow Velocity Sensor for Microfluidic Applications

Here we developed an innovative flow sensor using ultrathin (i.e., <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4~\mu \text{m}$ </tex-math></inline-formula> ) glass sheet fabricated by femtosecond laser processing. The sensor can quantify the flow velocity of liquid flows by measuring the displacement and vibration frequency of the ultrathin glass sheet induced by flows in the microchannel. The developed flow sensor is transparent, chemically inert, and can measure water flows with a velocity between 0.067 and 0.804 m/s (Reynolds numbers 3–36). The displacement of the ultrathin glass sensor varying from 0.4 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.1~\mu \text{m}$ </tex-math></inline-formula> has negligible influence on the fluid conditions in the microchannel. Moreover, the sensitivity and dynamic range of the sensor can be readily adjusted by optimizing geometric parameters, such as the aspect ratio and thickness of the ultrathin glass sheet. Besides, the sensor is capable of multiposition measurement to investigate the differences in localized flow velocity within the microchannel. Compared with existing cantilever-based flow velocity sensors, our developed sensor has a higher sensitivity [409 mV/(m/s)], a larger measurement range (0.067–0.804 m/s), and a smaller displacement range (0.4– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.1~\mu \text{m}$ </tex-math></inline-formula> ), which is desirable for high-throughput microfluidics-based applications, such as cell separation and cell-based therapeutics.

Contact bump attachment and stress layer formation method for fabrication yield improvement and individual difference reduction in cantilever-based tactile sensor

Contact bump attachment and stress layer formation method for fabrication yield improvement and individual difference reduction in cantilever-based tactile sensor

Terahertz radiation detection with a cantilever-based photoacoustic sensor.

We report the photoacoustic (PA) response in the terahertz (THz) range by employing a detection process actuated with a silicon cantilever pressure sensor and a carbon-based radiation absorber. The detection relies on the mechanical response of the cantilever, when the volume of the carrier gas inside the PA cell expands with the heat produced by the radiation absorber. The detector interferometrically monitors the movement of the cantilever sensor to generate the PA signal. We selected the absorber material with the highest THz responsivity for detailed studies at 1.4 THz (214 µm wavelength). The observed responsivities of two different radiation absorbers are nearly the same at 1.4 THz and agree within 10% with responsivity values at 0.633 µm wavelength. The results demonstrate the potential of covering with a single PA detector a broad spectral range with approximately constant responsivity, large dynamic range, and high damage threshold.

Design and characterization of Screen-Printed Piezoresistive Cantilever for Gas Sensor Application

In this paper, the design, and the characterization of screen-printed cantilever-based humidity sensor are described. The project investigated in detail the sensitivity, and fabrication process of screen-printing based sensor with coated active humidity material on cantilever’s free end. The main aim of this paper is to compare the screen-printed technology with other existing technologies like micro-electro-mechanical system (MEMS), and low temperature cofired circuit (LTCC). The sensor has been simulated under different level of relative humidity with two different temperature value: 35°C, and 45°C. This paper also represents an analytical and mathematical modeling approach for frequency shift and thick film resistance change of the selected sensor. The analytical results are compared with the simulated results.

ZIF-Based Metal-Organic Frameworks for Cantilever Gas Sensors

Among the different gas sensing platforms, cantilever-based sensors have attracted considerable interest in recent years thanks to their ultra-sensitivity and high-speed response. The gas sensing mechanism in a dynamic cantilever sensor is based on its resonance frequency shift upon adsorption of a gas molecule on the sensor. In order to sensitize the surface of a cantilever, a sensitive receptor material with large surface area is required. Metal-organic frameworks (MOFs) are a class of nanoporous crystalline materials composed of metal ions coordinated to organic linkers. MOFs are promising for gas sensing applications as they have large surface area, rich porosity with adjustable pore size and excellent selective adsorption capability for various gasses.[1] Zeolite imidazole frameworks (ZIFs) are a class of MOFs where metals with tetrahedral coordination (i.e. Zn, Co, Fe, Cu) are the central node and the ligands are imidazolate-based organic molecules.In this work, we developed a ZIF-based thin film for dynamic cantilever gas-sensing applications. We employed a novel atmospheric pressure spatial atomic layer deposition (AP-SALD)[2][3] technique to deposit a ZnO sacrificial layer on the silicon cantilevers. This technique allows the deposition of high-quality films at atmospheric pressure, faster than conventional ALD. The ZnO layer was then converted to a particular ZIF film with desired porosity and size, through a MOF-CVD process.[4] A gas-sensing bench setup was developed for the cantilever actuation and read-out. We present the chemical and morphological properties of the ZIF, as well as the frequency response of the sensor to various gases. The device showed reliable sensitivity to humidity, CO2 and several VOCs.

Anisotropy of the ΔE Effect in Ni-Based Magnetoelectric Cantilevers: A Finite Element Method Analysis.

In recent investigations of magnetoelectric sensors based on microelectromechanical cantilevers made of TiN/AlN/Ni, a complex eigenfrequency behavior arising from the anisotropic ΔE effect was demonstrated. Within this work, a FEM simulation model based on this material system is presented to allow an investigation of the vibrational properties of cantilever-based sensors derived from magnetocrystalline anisotropy while avoiding other anisotropic contributions. Using the magnetocrystalline ΔE effect, a magnetic hardening of Nickel is demonstrated for the (110) as well as the (111) orientation. The sensitivity is extracted from the field-dependent eigenfrequency curves. It is found, that the transitions of the individual magnetic domain states in the magnetization process are the dominant influencing factor on the sensitivity for all crystal orientations. It is shown, that Nickel layers in the sensor aligned along the medium or hard axis yield a higher sensitivity than layers along the easy axis. The peak sensitivity was determined to 41.3 T−1 for (110) in-plane-oriented Nickel at a magnetic bias flux of 1.78 mT. The results achieved by FEM simulations are compared to the results calculated by the Euler–Bernoulli theory.

Virtual Sensor Array Based on Piezoelectric Cantilever Resonator for Identification of Volatile Organic Compounds.

Piezoelectric cantilever resonator is one of the most promising platforms for real-time sensing of volatile organic compounds (VOCs). However, it has been a great challenge to eliminate the cross-sensitivity of various VOCs for these cantilever-based VOC sensors. Herein, a virtual sensor array (VSA) is proposed on the basis of a sensing layer of GO film deposited onto an AlN piezoelectric cantilever with five groups of top electrodes for identification of various VOCs. Different groups of top electrodes are applied to obtain high amplitudes of multiple resonance peaks for the cantilever, thus achieving low limits of detection (LODs) to VOCs. Frequency shifts of multiple resonant modes and changes of impedance values are taken as the responses of the proposed VSA to VOCs, and these multidimensional responses generate a unique fingerprint for each VOC. On the basis of machine learning algorithms, the proposed VSA can accurately identify different types of VOCs and mixtures with accuracies of 95.8 and 87.5%, respectively. Furthermore, the VSA has successfully been applied to identify the emissions from healthy plants and "plants with late blight" with an accuracy of 89%. The high levels of identifications show great potentials of the VSA for diagnosis of infectious plant diseases by detecting VOC biomarkers.

A Magnetic Tuning Mechanism to Compensate Frequency Ratio Deviation in Coupled Resonators for Improving Sensing Resolution With High Sensitivity

A magnetic tuning mechanism to compensate the frequency ratio deviation in coupled resonators is proposed in this paper for fulfilling the potential of nonlinear internal resonance to sensing applications. A magnetic tuning theory is constructed to deduce theoretical expressions of 1:3 internal resonance with lumped parameter model and multi-scale method. The effectiveness of the proposed magnetic tuning mechanism is experimentally demonstrated by taking a two-cantilever resonator system as an example, with 1:3 internal resonance characteristics under different magnetic forces. The possibility to compensate frequency ratio deviation caused by manufacturing, assembling and other errors generated in production and service processes is further analytically and experimentally studied to improve sensing resolution and increase detection accuracy, achieving an adjustment range from 164.3Hz to 146.5Hz in frequency and 1:2.83 to 1:3.17 in frequency ratio. The proposed compensation approach is believed to be universally applicable to cantilever-based sensors with the need for frequency matching.

Direct Color Observation of Light-Driven Molecular Conformation-Induced Stress.

Although usually complex to handle, nanomechanical sensors are exceptional, label-free tools for monitoring molecular conformational changes, which makes them of paramount importance in understanding biomolecular interactions. Herein, a simple and inexpensive mechanical imaging approach based on low-stiffness cantilevers with structural coloration (mechanochromic cantilevers (MMC)) is demonstrated, able to monitor and quantify molecular conformational changes with similar sensitivity to the classical optical beam detection method of cantilever-based sensors (≈4.6×10-3 Nm-1 ). This high sensitivity is achieved by using a white light and an RGB camera working in the reflection configuration. The sensor performance is demonstrated by monitoring the UV-light induced reversible conformational changes of azobenzene molecules coating. The trans-cis isomerization of the azobenzene molecules induces a deflection of the cantilevers modifying their diffracted color, which returns to the initial state by cis-trans relaxation. Interestingly, the mechanical imaging enables a simultaneous 2D mapping of the response thus enhancing the spatial resolution of the measurements. A tight correlation is found between the color output and the cantilever's deflection and curvature angle (sensitivities of 5×10-3 Hueµm-1 and 1.5×10-1 Hue(°)-1 ). These findings highlight the suitability of low-stiffness MMC as an enabling technology for monitoring molecular changes with unprecedented simplicity, high-throughput capability, and functionalities.

A Cantilever-Based Flow Sensor for Domestic and Agricultural Water Supply System

Most of the existing flow sensors are expensive and limited in their capabilities for sensing bidirectional flow. Low-cost and accurate flow sensors with bidirectional sensing capability have numerous applications in the residential and irrigation sectors. Evaluation of a low-cost, cantilever-based sensor, suitable for measuring flow rates under turbulent flow conditions is presented in this article. Such sensors are reported for micro-fluidic applications but its potential application in large diameter pipes under turbulent flow has not been studied yet. A cantilever formed using a thin stainless-steel strip is used as the sensing element in the proposed sensor. One of the ends of the cantilever is firmly fitted to the inner wall of the pipe, and it bends or deflects towards the direction of the flow as a function of the flow rate. To experimentally evaluate the sensor in detail, the mean deflection angle of the cantilever is measured using a camera, and an image processing algorithm. In practice, the angle can be sensed using simpler methods. The performance of the prototype sensor has been evaluated after building an appropriate regression model. The results are subsequently expressed in terms of the mean flow velocity, thereby providing its potential utility in pipes of other dimensions. The shape of the mean flow velocity with respect to the mean angle of deflection characteristic of the proposed sensor matched well with the theoretical deflection computed. The sensor developed has given an accuracy of 3 % of full scale, for flow rates in the range of 2–15.5 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> /hr. The proposed sensing mechanism can realize cost-effective, simple, and reliable flow sensors. Such sensors will find applications in residential and industrial domains.

A Miniature Fiber-Optic Silicon Cantilever-Based Acoustic Sensor Using Ultra-High Speed Spectrum Demodulation

A silicon cantilever-based fiber-optic acoustic sensor (FOAS) is presented in this work. A rectangular cantilever is fabricated upon a silicon-on-insulator (SOI) wafer using a micro-electro-mechanical system (MEMS). The length, width and thickness of the silicon cantilever are 530 μm, 200 μm and 3 μm, respectively. The resonant frequency of the silicon cantilever is 14820 Hz with a sensitivity of 950 nm/Pa. An ultra-high speed absolute cavity length demodulation method is adopted using a complementary metal oxide semiconductor (CMOS) spectrometer and an 850 nm superluminescent light emitting diode (SLED). A modified Buneman frequency estimation and total phase demodulation algorithm is adopted. The frequency response curve shows a relatively flat trend from 20 Hz to 13 kHz. The sensor exhibits good linearities at different frequencies while the applied acoustic pressure is increased from 0 Pa to 2.5 Pa. Experimental results indicate that the minimum detectable pressure (MDP) of the proposed FOAS is calculated to be 25.68 μPa/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> at the frequency of 13 kHz. The silicon cantilever-based FOAS achieves broad operating bandwidth, high sensitivity, small size, and good stability.

Cantilever-based differential pressure sensor with a bio-inspired bristled configuration

Inspired by the bristled wing configuration of tiny insects, we proposed a novel polyimide (PI) cantilever-based differential pressure (DP) sensor. This bristled PI cantilever with a thin metallic piezoresistor was designed to detect the pressure difference that induced the aerodynamic loading on the surface of the cantilever. Owing to the aerodynamic characteristics of the bristled cantilever, the DP-sensor with the bristled cantilever could not only retain a comparable sensitivity with that of the paddle cantilever under low differential pressures but also achieve a higher theoretical upper detection limit due to the enhanced leakage of the bristles. Experimental results indicated that the DP-sensor with bristled cantilevers extended the detection range by ∼30% in comparison with the DP-sensor with paddle cantilevers. The high sensitivity, wide detection range, and facile fabrication process of these bio-inspired DP-sensors make them promising for future applications.

All-optical high-sensitivity resonant photoacoustic sensor for remote CH4 gas detection.

This paper presents an all-optical high-sensitivity resonant photoacoustic (PA) sensor to realize remote, long-distance and space-limited trace gas detection. The sensor is an integration of a T-type resonant PA cell and a particular cantilever-based fiber-optic acoustic sensor. The finite element simulations about the cantilever vibration mode and the PA field distributions are carried out based on COMSOL. The all-optical high-sensitivity resonant PA sensor, together with a high-speed spectrometer and a DFB laser source, makes up of a photoacoustic spectroscopy (PAS) system which is employed for CH4 detection. The measured sensitivity is 0.6 pm/ppm in the case of 1000 s average time, and the minimum detection limit (MDL) reaches 15.9 parts per billion (ppb). The detective light source and the excitation light source are all transmitted by optical fibers, therefore remote and long-distance measurement of trace gas can be realized. Furthermore, the excitation light source and the acoustic sensor are designed at the same side of the PA cell, the sensor may be used for space-limited trace gas detection.