Highly sensitive detection of organophosphorus pesticides by acetylcholinesterase-coated thin film bulk acoustic resonator mass-loading sensor

Surface Engineering of Metal-Organic Framework Prepared on Film Bulk Acoustic Resonator for Vapor Detection

Gravimetric resonators based on micro/nanoelectromechanical systems (M/NEMS) are potential candidates in developing smaller, less expensive, and higher-performance gas sensors. Metal-organic frameworks (MOFs) with high surface areas have recently come into focus as advanced nanoporous sensitive materials in microgravimetric gas sensors. The surface of MOFs on those sensors is critical in offering water stability and varying absorption behaviors. However, the influences of the surface on sensing performance are less explored and the strategy to tune surface properties of MOFs mounted on gravimetric resonators is still rare. In this paper, a straightforward strategy to engineer surface properties of MOFs, specifically Cu₃(benzenetricarboxylate)₂ (known as HKUST-1), is reported and the surface hydrophilicity/hydrophobicity of HKUST-1 is tuned by chemical vapor deposition combined with monolayer self-assembly. It was found that the hybrid inorganic and organic surface engineering strategy not only preserves the absorption capacity of inner MOFs but also significantly enhances the sensor's stability toward water.

Efficient Preservation of Acetylcholinesterase at Room Temperature for Facile Detection of Organophosphorus Pesticide

A simple and inexpensive strategy is reported to facilitate the detection of an organophosphorus pesticide by acetylcholinesterase (AChE). Pullulan is able to preserve AChE at room temperature, but the activity of conserved AChE varies significantly depending on the time, stir and volume of solution to dissolve it. The reason is that AChE entrapped in pullulan tablet remains in an inactive state to avoid denaturalization and deactivation. There is a reactivation process to gradually recover the enzyme activity during dissolution of the tablet. Stirring would interrupt this procedure and lead to a loss of enzyme activity. Dissolution of the tablet for 5 min with a volume of 15 μL could facilitate full recovery of AChE activity. The feasibility of activated AChE for organophosphorus pesticide detection was evaluated using malaoxon. These results contribute to the understanding of preservation mechanism by pullulan and the development of easy-to-use enzyme assays.

Multiple-layer guided surface acoustic wave (SAW)-based pH sensing in longitudinal FiSS-tumoroid cultures

A constitutively increased intracellular pH that is higher than the extracellular pH is emerging as a hallmark of cancer and determining pH could play a significant role in the measurement of drug responsiveness of tumor cells. However, a non-invasive, touch-free and real-time pH sensing as a research tool is lacking and remains a major unmet need. The purpose of the current study is to investigate a microfluidic surface acoustic wave (SAW) sensor platform capable of monitoring pH in cell and tumoroid cultures. A novel multi-layer guided SAW sensor integrated into a microfluidic channel was investigated theoretically and experimentally in detail for pH bio-sensing. Sensitivity and capability of the layer guided Love wave device was modeled using the finite element simulation. The model was verified experimentally, and a study monitoring pH of cell growth media is presented. This novel pH sensor is based on a 13.91 MHz center frequency SAW device coated with ZnO (500 nm) and IrO₂ (30 nm) layers to increase the sensitivity. A change in mechanical and electrical properties of the conductive IrO₂ layer was observed resulting from electrical corrosion induced by pH solutions affecting the charge distribution, SAW phase velocity and attenuation. By measuring the frequency shift induced by the change in SAW phase velocity between the test group and control group, the pH value of cell culture media from H460 cancer cell culture plates from day 0 to day 5 can easily be determined. To improve the sensitivity and stability of the sensor, a finite element method was used to optimize the layer thicknesses. Taken together, the results of experiments show the potential application of this device to be integrated with microfluidic channels and used in determining pH changes in longitudinal tumor cell cultures.

Shear Mode Bulk Acoustic Resonator Based on Inclined c-Axis AlN Film for Monitoring of Human Hemostatic Parameters

Measurement of hemostatic parameters is essential for patients receiving long-term oral anticoagulant agents. In this paper, we present a shear mode bulk acoustic resonator based on an inclined c-axis aluminum nitride (AlN) film for monitoring the human hemostatic parameters. During the blood coagulation process, the resonant frequency of the device decreases along with a step-ladder profile due to the viscosity change during the formation of fibers in blood, revealing the sequential coagulation stages. Two hemostatic parameters with clinical significance, prothrombin time (PT) along with its derived measure of international normalized ratio (INR), are determined from time-frequency curves of the device. Furthermore, the resonator is compared with a commercial coagulometer by monitoring the hemostatic parameters for one month in a patient taking the oral anticoagulant. The results are consistent. In addition, thanks to the excellent potential for integration, miniaturization and the availability of direct digital signals, the proposed device has promising application for point of care coagulation monitoring.

Micro-Electromechanical Acoustic Resonator Coated with Polyethyleneimine Nanofibers for the Detection of Formaldehyde Vapor

We demonstrate a promising strategy to combine the micro-electromechanical film bulk acoustic resonator and the nanostructured sensitive fibers for the detection of low-concentration formaldehyde vapor. The polyethyleneimine nanofibers were directly deposited on the resonator surface by a simple electrospinning method. The film bulk acoustic resonator working at 4.4 GHz acted as a sensitive mass loading platform and the three-dimensional structure of nanofibers provided a large specific surface area for vapor adsorption and diffusion. The ultra-small mass change induced by the absorption of formaldehyde molecules onto the amine groups in polyethyleneimine was detected by measuring the frequency downshift of the film bulk acoustic resonator. The proposed sensor exhibits a fast, reversible and linear response towards formaldehyde vapor with an excellent selectivity. The gas sensitivity and the detection limit were 1.216 kHz/ppb and 37 ppb, respectively. The study offers a great potential for developing sensitive, fast-response and portable sensors for the detection of indoor air pollutions.

Effects of compensating the temperature coefficient of frequency with the acoustic reflector layers on the overall performance of solidly mounted resonators

Thin film acoustic wave resonator based devices require compensation of temperature coefficient of frequency (TCF) in many applications. This work presents the design and fabrication of temperature compensated solidly mounted resonators (SMRs). The characteristics of each material of the layered structure have an effect on the device TCF but depending on the relative position with respect to the piezoelectric material in the stack. The influence of material properties of the different layers composing the device on the TCF is discussed in detail. TCF behavior simulation is done with Mason's model and, to take into account the deterioration of overall performance due to the finite lateral size and shape of the resonator, we have used 2D and 3D finite element modelling of the resonators. The overall behavior of the device for external loads is predicted. SMRs are designed according to simulations and fabricated with different configurations to obtain TCF as near to zero as possible with an optimized response. Resonators are made by depositing Mo/AlN/Mo piezoelectric stacks on acoustic reflectors. As reflector materials, conductive W and insulating WO_x films have been used as high acoustic impedance materials. SiO₂ films are used as low acoustic impedance material.

Streptavidin Modified ZnO Film Bulk Acoustic Resonator for Detection of Tumor Marker Mucin 1

A ZnO-based film bulk acoustic resonator has been fabricated using a magnetron sputtering technology, which was employed as a biosensor for detection of mucin 1. The resonant frequency of the thin-film bulk acoustic resonator was located near at 1503.3 MHz. The average electromechanical coupling factor [Formula: see text] and quality factor Q were 2.39 % and 224, respectively. Using the specific binding system of avidin-biotin, the streptavidin was self-assembled on the top gold electrode as the sensitive layer to indirectly test the MUC1 molecules. The resonant frequency of the biosensor decreases in response to the mass loading in range of 20-500 nM. The sensor modified with the streptavidin exhibits a high sensitivity of 4642.6 Hz/nM and a good selectivity.

Recent advances in biosensors based on enzyme inhibition

Enzyme inhibitors like drugs and pollutants are closely correlated to human and environmental health, thus their monitoring is of paramount importance in analytical chemistry. Enzymatic biosensors represent cost-effective, miniaturized and easy to use devices; particularly biosensors based on enzyme inhibition are useful analytical tools for fast screening and monitoring of inhibitors. The present review will highlight the research carried out in the last 9 years (2006-2014) on biosensors based on enzyme inhibition. We underpin the recent advances focused on the investigation in new theoretical approachs and in the evaluation of biosensor performances for reversible and irreversible inhibitors. The use of nanomaterials and microfluidic systems as well as the applications of the various biosensors in real samples is critically reviewed, demonstrating that such biosensors allow the development of useful devices for a fast and reliable alarm system.

Detecting both the mass and position of an accreted particle by a micro/nano-mechanical resonator sensor

In the application of a micro-/nano-mechanical resonator, the position of an accreted particle and the resonant frequencies are measured by two different physical systems. Detecting the particle position sometimes can be extremely difficult or even impossible, especially when the particle is as small as an atom or a molecule. Using the resonant frequencies to determine the mass and position of an accreted particle formulates an inverse problem. The Dirac delta function and Galerkin method are used to model and formulate an eigenvalue problem of a beam with an accreted particle. An approximate method is proposed by ignoring the off-diagonal elements of the eigenvalue matrix. Based on the approximate method, the mass and position of an accreted particle can be decoupled and uniquely determined by measuring at most three resonant frequencies. The approximate method is demonstrated to be very accurate when the particle mass is small, which is the application scenario for much of the mass sensing of micro-/nano-mechanical resonators. By solving the inverse problem, the position measurement becomes unnecessary, which is of some help to the mass sensing application of a micro-/nano-mechanical resonator by reducing two measurement systems to one. How to apply the method to the general scenario of multiple accreted particles is also discussed.

An acetylcholinesterase-based chronoamperometric biosensor for fast and reliable assay of nerve agents

The enzyme acetylcholinesterase (AChE) is an important part of cholinergic nervous system, where it stops neurotransmission by hydrolysis of the neurotransmitter acetylcholine. It is sensitive to inhibition by organophosphate and carbamate insecticides, some Alzheimer disease drugs, secondary metabolites such as aflatoxins and nerve agents used in chemical warfare. When immobilized on a sensor (physico-chemical transducer), it can be used for assay of these inhibitors. In the experiments described herein, an AChE- based electrochemical biosensor using screen printed electrode systems was prepared. The biosensor was used for assay of nerve agents such as sarin, soman, tabun and VX. The limits of detection achieved in a measuring protocol lasting ten minutes were 7.41 × 10⁻¹² mol/L for sarin, 6.31 × 10⁻¹² mol/L for soman, 6.17 × 10⁻¹¹ mol/L for tabun, and 2.19 × 10⁻¹¹ mol/L for VX, respectively. The assay was reliable, with minor interferences caused by the organic solvents ethanol, methanol, isopropanol and acetonitrile. Isopropanol was chosen as suitable medium for processing lipophilic samples.