Change In Resonant Frequency Research Articles

QCM Sensors Combining Highly Nanostructured Metal-Blacks Sublayers and Active Self-Assembled Monolayers

Introduction Quartz Crystal Microbalance (QCM) sensors are characterized by many potential applications. They are being widely used as biosensors, in pharmacological applications, for combustion control, environmental pollution monitoring, etc. [1]. Regardless the specific application, the recognition resolution of quartz chips is usually determined and restrained according to the Sauerbray equation (For example for 10 MHz QCM, the change of resonance frequency in 1 Hz corresponds to 4.4 ng/cm2 of adsorbed mass per surface). This fact is crucial mainly for gas sensor application where we need to detect concentration down to ppm or even ppb levels. However, there are some possibilities on how to enhance the QCM sensor properties and amplify their sensitivity, for instance by increasing the specific surface area of QCM electrodes, and thus providing more bonding sites for the analyte [2]. In this contribution, we propose a novel method that uses nanostructured materials called metal-blacks as a sublayer for the increase of active sensor surface, in combination with self-assembled monolayers (SAMs) that serves as a receptor layer. Metal-Blacks Metal-blacks – MBs are highly nanoporous and nanostructured materials with a large surface area. Their name originates in black colour, caused by the absorption of incident light on their surface. The incident light penetrates into the pores and channels where it undergoes multiple reflections in cavities and where it is almost completely absorbed. Due to the very large surface area, MBs are ideal materials for enhancement of QCM gas sensors properties [3]. Experimental The nanostructured layers of black gold (BAu) and black palladium (BPd) were prepared by evaporation from a tungsten boat at an elevated pressure of 200 Pa. Prepared layers of MBs were then characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SAMs of zinc octacarboxylphthalocyanine ZnPc(COOH)8 were prepared by two-step grafting method, where ZnPc(COOH)8 is covalently bonded to the molecule of aminothiophenol, which serves as an surface anchor via its gold/palladium-sulfur bond. The preparation procedure was as follows. At first, the QCM electrodes with MB layers were degreased in a sonicated ethanol bath during a few minutes and then dried under a nitrogen flow. Second, the QCM samples were cleaned by UV-ozone treatment and immediately immersed into 10-4 M solution of aminothiophenol in ethanol/DMF solvent for 24h. Third, after the 24h incubation time, the samples were moved into 10-4 M solution of the macrocycles and left there for another 24h. In the end, the samples were carefully rinsed with the solvent to eliminate adsorbed molecular overlayers and subsequently washed with ethanol in order to remove solvent residues and dried under nitrogen flux.Once prepared, QCM sensor impedance spectra were obtained by impedance analyzer Agilent 4294A. Responses towards different analytes (EtOH, CH4, H2O, NO2, toluene, acetaldehyde) were measured in a glass chamber that enables the measurement of up to 6 sensors at the same time. Sensors were measured at a constant gas flow and room temperature. The sensor resonant frequencies were measured by oscillattor circuits in connection with an NI PCI-6602 card used as a precise counter Results and Conclusions In this contribution, we have proved that highly nanoporous and nanostructured materials (such as metal-blacks), which have a large specific surface area, provide more binding sites for analytes and, hence, increase the response of the QCM sensors. As an example measurement, we present the response of QCMs with BAu towards ethanol vapours (Fig. 1). The curves illustrate that the response of the sensor with the BAu electrode is 10-times bigger than the one of the standard QCM (152Hz/15Hz). For the combined layer (BAu + ZnPc(COOH)8), the response amplification is not so large but it is still significant. In this case, the response is 3-times bigger than the one from the QCM with ZnPc(COOH)8 only (without BAu). However, in contrast with QCM with just bare BAu, the surface modification with SAMs layers influences the sensor selectivity. The combined layers of MBs and SAMs then result in a selective sensor with increased sensitivity. According to the presented results metal-blacks are promising materials that can significantly boost the sensitivity of QCM sensors and still preserve the selectivity of its active layers.

Micromechanical Resonators for Ultrasound-Based Sensors

The current field of sensor platforms is substantial, and ever more creative approaches are envisioned and realized. Herein, we showcase a new platform technology for continuous sensing that relies on the resonant absorption of ultrasound waves in a microfabricated array of smart hydrogel structures (see Figure 1).[1] Smart hydrogels are biocompatible and can be tailored to respond to different analytes. [2,3] These selective materials undergo a volume-phase transition in response to changing levels of biomarkers. In addition, smart hydrogels can be based on compositions that include poly(ethylene glycol), poly(methyl methacrylate), or hyaluronic acid to be designed to biodegrade. By employing smart hydrogels as resonator structures and probing them using ultrasound will provide an implantable sensor platform that is minimally invasive, non-destructive, and free of transcutaneous wires.This approach makes use of the fact that hydrogels can be molded into mechanical microresonators that are tuned to a specific ultrasound frequency. As they swell or contract in response to the presence or absence of the target analyte, their resonance frequency changes, leading to a shift in the ultrasound absorption spectrum. Measuring these changes allows for the reconstruction of the analyte concentration. In addition to that, tailoring the resonance frequency of the smart hydrogel resonator structures to different frequency ranges allows for multiplexing. We successfully demonstrate the use of this platform technology using glucose as a target metabolic marker in-vitro and in-vivo in rats (see Figure 2).Florian Solzbacher declares financial interest in Blackrock Microsystems LLC and Sentiomed, Inc. Jules Magda declares financial interest in Applied Biosensors LLC.[1] N. Farhoudi, H.-Y.L., J. Magda, F. Solzbacher and C.F. Reiche, A Biomedical Sensor Based on Resonant Absorption of Ultrasound Waves in Hydrogel-based Resonators. TechConnect Briefs 2019, 2019: p. 412.[2] Buenger, F. Topuz, and J. Groll, “Hydrogels in sensing applications,” Prog. Polym. Sci., vol. 37, no. 12, pp. 1678–1719, 2012.[3] A. Peppas, J. Z. Hilt, A. Khademhosseini, and R. Langer, “Hydrogels in biology and medicine:From molecular principles to bionanotechnology,” Adv. Mater., vol. 18, no. 11, pp. 1345–1360, Jun. 2006. Figure 1

Surface Plasmon Resonance of Au Thin Films Modified By Catalyst Monolayer

In this talk, we present the results and modeling o the catalyst monolayer modification effects on plasmon resonance of Au thin films. The experiments are conducted using an attenuated total reflection arrangement in which p-polarized light is incident through a prism onto a thin (-400 A) Au film deposited on the mica substrate. (Kretschmann configuration [[1]]). The angle of incidence on the base of the prism is greater than the critical angle for the prism: electrolyte interface and SPs are excited by the evanescent wave generated under these conditions of total internal reflection. The excitation is manifested by a decrease in the intensity of reflected light. The Pd monolayer on Au were electrodeposited using procedure reported by Kolbe et al [[2],[3]], while the Pt submonolayer/monolayers were deposited on Au using Surface Limited Redox Replacement (SLRR) of Pb UPD monolayer [[4]]. The plasmon resonance frequency of Au(hkl), PdML/Au(hkl) and PtML/Au(hkl) and PtSML/Au(hkl) were investigated as a function of the applied potential in the range between -0.24 V to 0.4 V vs. SCE. The comparative analysis, between Au and catalyst monolayer modified Au(hkl) samples indicate that SP resonant frequency change is induced by both; surface charge and dipole moment of the surface as well as he adsorption of H on the catalyst monolayer surface in the UPD region. The latter effect is quantified using subtractive analysis between the Au and catalyst modified Au surface and solutions with different pH. In conclusion, we present a simple model describing the catalyst monolayer effect on plasmon resonance of Au films. [1] E. Kretschmann and H. Raether, Z. Natutforsch. 23a (1968) 2135. [2] L.A. Kibler, M. Kleinert, R. Randler, D.M. Kolb, Surface Science, 443 (1999) 19. [3] J. Tang, M. Petri, L.A. Kibler, D.M. Kolb, Electrochimica Acta, 51, (2005) 125 [4] K. Sasaki et al, Electrochim. Acta, 55, (2010) 2645.

Static and Dynamic Analysis of Carbon Nano Tube Cantilever for Nano Electro Mechanical Systems Based Applications

The cantilever is the most basic form of the MEMS and NEMS-based applications. It may include actuators and sensors in multiple domains for the detection of deflections due to change in mass and charge from atomic to sub-atomic levels. The main constraints in the real-time environment for sensing applications are good sensitivity and selectivity along with good response time and wide dynamic range. CNTs (Carbon Nano Tubes) possess excellent electrical characteristics with good mechanical strength like elasticity, flexibility, tensile strength and good thermal conductivity. These properties make use of CNTs in NEMS that gives rise to good sensing platforms. In this paper we propose, design and analysis of CNT based Cantilever structure using FEM to determine the change in deflection due to varying load under static analysis. Similarly from dynamic analysis change in resonance frequency for change in thickness of Cantilever is performed. From the experimental results using COMSOL simulation, it is observed that the CNT based Cantilever deflection sensitivity is 9.85 x 10-8 m under maximum stress of 6.71 x 10-9 N/m2. From the dynamic analysis, resonant frequency occurs at 1.8 x 107 Hz for 10 nm thickness and for 50 nm is 7.5 x 107 Hz respectively.

Monitoring and Modeling of Living Cell Responses in the Attachment Process and Reaction to the Antitumor Reagent Cisplatin Studied by a Quartz Crystal Microbalance Combined with a Microscope

The attachment process and response to an antitumor reagent for cultured cells were monitored with a quartz crystal microbalance (QCM) combined with a microscope. To fit the experimentally obtained curves of the resonant frequency, model equations of resonant frequency curves were built, and parameters of time constants and scale coefficients were determined. For the cell attachment process, a first-order lag response curve well fit the experimental curves. For the response to cisplatin, two response steps were observed in both QCM data and microscopic images, where the cells loosened in the first step and shrank in the second step. Resonant frequency responses for both processes were well fit by two logarithmic normal distribution functions. In addition, the dependence of the resonant frequency change on the cell number was also studied, and a cell-cell interaction model for attached cells was proposed to explain the saturation of the resonant frequency change in high density cell seeding.

Study on magnetorheological damper stiffness shift

Electrical current is usually used to change the damping force of Magnetorheological Dampers (MRDs). However, changing the electrical current could shift the stiffness of the system, the phenomenon that was not considered carefully. This study aims to evaluate this shift. A typical MRD was designed, optimized, and fabricated to do some accurate and detailed experimental tests to examine the stiffness variation. The damper is equipped with a circulating system to prevent the deposition of particles when it is at rest. Besides that, a vibration setup was developed for the experimental study. It is capable of generating vibration with either constant frequency or frequency sweep and measure the amplitude of vibration. The damper was tested by the vibrating setup, and it was concluded that with a change in electrical current from 0 to 1.4 A, resonant frequency would change from 13.8 Hz to 16 Hz. Considering the unchanging mass of 85.1 kg, the change in resonant frequency translates as a shift in stiffness, which changes from 640 kN/m to 860 kN/m.

Effect of clamped beam pattern on resonant frequency shift of microresonator under near-infrared laser irradiation

In this study, we investigated the effect of the resonator beam shape on the thermal stress and the resonance frequency change when providing heat to the resonator beam by near-infrared laser irradiation. This investigation provides crucial data for optical and heat sensing application using microresonators. The microresonator beam with an optical absorber for near-infrared region was fabricated, varying the numbers of beams and corners. When irradiating the optical absorber with a laser, the resonant frequency decreased. It is because the photothermal effect provides compressive stress in the beam, resulting in resonant frequency decrease. The resonant frequency change rate increased with decreasing the number of beams. According to the analytical results, we confirmed that the analytical thermal stress determined the resonant frequency change rate in the same beam length. The corner remarkably decreased the resonant frequency change rate. The straight beams bend to the orthogonal direction by the thermal expansion of another straight part of the beam, resulting in deformation of the beam in the axial direction. Therefore, the reaction force and compressive stress in the axial direction reduces.

A magnetic sensor using a 2D van der Waals ferromagnetic material

Two-dimensional (2D) van der Waals ferromagnetic materials are emerging as promising candidates for applications in ultra-compact spintronic nanodevices, nanosensors, and information storage. Our recent discovery of the strong room temperature ferromagnetism in single layers of VSe2 grown on graphite or MoS2 substrate has opened new opportunities to explore these ultrathin magnets for such applications. In this paper, we present a new type of magnetic sensor that utilizes the single layer VSe2 film as a highly sensitive magnetic core. The sensor relies in changes in resonance frequency of the LC circuit composed of a soft ferromagnetic microwire coil that contains the ferromagnetic VSe2 film subject to applied DC magnetic fields. We define sensitivity as the slope of the characteristic curve of our sensor, df0/dH, where f0 is the resonance frequency and H is the external magnetic field. The sensitivity of the sensor reaches a large value of 16 × 106 Hz/Oe, making it a potential candidate for a wide range of magnetic sensing applications.

A quartz crystal microbalance based study reveals living cell loading rate via [formula omitted] integrins

Cellular interactions with the microenvironment are mediated by ligand-receptor bonds. Such ligand-receptor bond dynamics is known to be heavily dependent on the loading rate. However, the physiologically-relevant loading rate of living cells remains unknown. Here, using a quartz crystal microbalance (QCM), we developed a bulk-force sensing platform to semi-quantitatively detect the rate of cellular force application during early stages of cell adhesion and spreading. Atop an Au-coated quartz crystal, covalently linked self-assembled monolayers (SAM) were used to immobilize cyclic-RGDfK peptides that can interact with the αvβ3 integrins on cells. The QCM detects the changes in resonant frequency of the vibrating crystal due to the cellular activity/probing (force application) on the QCM surface. The corresponding changes in mass on the surface, proportional to the rate of force application, arise from the cellular interactions with the functionalized surface were calculated. The loading rate of living cells was found to be ∼80–115 pN/s. Collectively, our results revealed a fundamental feature of cell adhesion and spreading providing valuable information regarding cellular interactions with the extracellular matrix.

Wire-based triboelectric resonator for a self-powered crack monitoring system

In this study, we investigated a wire-type triboelectric resonator (WTER) for monitoring cracks. Vibration of a metal wire on a dielectric film produces a resonant frequency via triboelectric outputs. We found that the tension, linear density, and wire length significantly affect the resonant frequency measured by the WTER. For a 10-cm-long wire, the WTER's sensitivity, as defined by a resonant frequency change for an elongation (i.e., Hz/mm), reached approximately 1,020 Hz/mm; however, this changed to 115 Hz/mm after an elongation of 1.5 mm. When a 40-cm-long wire was used, the sensitivity changed to about 105 Hz/mm, but it was very stable because there was no plastic deformation. We developed a stand-alone, self-powered crack monitoring system by assembling a WTER with an Arduino board (A-WTER); this setup demonstrated monitored frequency value errors of less than 1%. Our A-WTER successfully monitored an elongation of 100 μm with a resonant frequency change of approximately 30 Hz. We investigated the A-WTER behavior under varying humidity and temperature conditions in order to understand its environmental stability. We found it was highly stable through a broad humidity range (RH 10%–80%) while its thermal stability was controlled by changing the tension and length of the wire.

Temperature-Modulated Micromechanical Thermal Analysis with Microstring Resonators Detects Multiple Coherent Features of Small Molecule Glass Transition.

Micromechanical Thermal Analysis utilizes microstring resonators to analyze a minimum amount of sample to obtain both the thermal and mechanical responses of the sample during a heating ramp. We introduce a modulated setup by superimposing a sinusoidal heating on the linear heating and implementing a post-measurement data deconvolution process. This setup is utilized to take a closer look at the glass transition as an important fundamental feature of amorphous matter with relations to the processing and physical stability of small molecule drugs. With an additionally developed image and qualitative mode shape analysis, we are able to separate distinct features of the glass transition process and explain a previously observed two-fold change in resonance frequency. The results from this setup indicate the detection of initial relaxation to viscous flow onset as well as differences in mode responsivity and possible changes in the primary resonance mode of the string resonators. The modulated setup is helpful to distinguish these processes during the glass transition with varying responses in the frequency and quality factor domain and offers a more robust way to detect the glass transition compared to previously developed methods. Furthermore, practical and theoretical considerations are discussed when performing measurements on string resonators (and comparable emerging analytical techniques) for physicochemical characterization.

Linking mass measured by the quartz crystal microbalance to the SI

Despite ubiquitous implementation of the quartz crystal microbalance (QCM) for measuring thin film thickness throughout industry and academia, a direct link to the SI (International System of Units) does not exist. Confidence in QCM measurements relies on over a half-a-century of academic and industrial research used to understand the resonant frequency change due to loading mass onto a quartz crystal. Here, we use before and after gravimetric mass measurements, linked directly to the SI, to measure mass change. A custom vacuum metal deposition system is used to deposit gold films of various masses onto a series of quartz crystals while the mass dependent frequency change is monitored in real time. The gravimetric (known) mass changes are compared to three analytical methods (frequency, time and energy) used to convert resonant frequency shifts to mass changes, none of which rely on the material properties of the deposited material. Additionally, we evaluate the reversible and irreversible contributions to mass change from the loading into, and removal from, the vacuum environment. We find the energy-based method for frequency to mass conversion has the best accuracy over the longest range, at 0.36 % to >1 mg. Only for mass changes below 100 are deviations >2 % observed. A complete uncertainty budget is provided.

Detection Principles of Temperature Compensated Oscillators with Reactance Influence on Piezoelectric Resonator.

This review presents various ways of detection of different physical quantities based on the frequency change of oscillators using piezoelectric crystals. These are influenced by the reactance changes modifying their electrical characteristics. Reactance in series, in parallel, or a combination of reactances can impact the electrical crystal substitute model by influencing its resonant oscillation frequency. In this way, various physical quantities near resonance can be detected with great sensitivity through a small change of capacitance or inductance. A piezoelectric crystal impedance circle and the mode of frequency changing around the resonant frequency change are shown. This review also presents the influence of reactance on the piezoelectric crystal, the way in which the capacitance lost among the crystal’s electrodes is compensated, and how the mode of oscillators’ output frequency is converted to lower frequency range (1–100 kHz). Finally, the review also explains the temperature–frequency compensation of the crystals’ characteristics in oscillators that use temperature–frequency pair of crystals and the procedure of the compensation of crystals own temperature characteristics based on the method switching between the active and reference reactance. For the latter, the experimental results of the oscillator’s output frequency stability (fout = ±0.002 ppm) at dynamical change of environment temperature (0–50 °C) are shown.

Novel Bi2O3-added Al2Mo3O12 composite microwave dielectric ceramics for ULTCC applications

Ultra-low temperature co-fired ceramics (ULTCC) technology has been extensively developed in modern microwave communications, for which microwave dielectric ceramics with ultra-low sintering temperature play a crucial role. Among them, Al2Mo3O12 is a promising candidate with excellent microwave dielectric properties, but its sintering temperature (roughly 790 °C) is still too high for ULTCC. In this work, novel Bi2O3-added Al2Mo3O12 composite microwave dielectric ceramics were designed and prepared by conventional solid-state sintering reaction. The addition effects of Bi2O3 on the phase composition, microstructure and microwave dielectric properties of the obtained composite ceramics were investigated. The added Bi2O3 will react with Al2Mo3O12 to form a new Bi2Mo3O12 phase with an even lower melting point than Bi2O3, effectively reducing the sintering temperature of the composite ceramics to below 660 °C. As the extra addition amount of Bi2O3 increases from 0 to 5 wt%, for the composite ceramics sintered at the designed different temperatures (620–710 °C) for 12 h, the dielectric constant (εr) increases from 5.5 to 5.98, quality factor (Q×f) could be effectively enhanced, reaching a maximum of 49900 GHz, and temperature coefficient of the resonant frequency (τf) changes from −36.4 to −42.8 ppm/°C. With 4 wt% Bi2O3, the composite ceramics sintered at 650 °C for 12 h present good microwave dielectric properties of εr = 5.91, Q×f = 49600 GHz and τf = −41.2 ppm/°C, and excellent chemical stability with the electrode metal Al, indicating that it is a promising candidate for ULTCC applications.

A Non-Invasive Method for Hydration Status Measurement With a Microwave Sensor Using Skin Phantoms

Fluid balance is important for a healthy human being. In this paper, a method to measure hydration status was developed and tested towards non-invasive measurement from human skin. Measurement of hydration status was performed by a microwave sensor utilizing a complementary split ring resonator (CSRR). The sensor was modeled, manufactured and then characterized by measuring tailor made skin phantoms based on the realistic electrical properties of skin with different degrees of hydration status. Qualitative longer term (>24 h) evaluation of the sensor was also performed by measuring polyester tissue that was drying over the time. Hydration status, represented by dehydrated, normal and hydrated skin phantoms, based on polyurethane with carbon and ceramic additives, was measured successfully by monitoring the changes in resonance frequency around 5.52 GHz. All results were compared to the dielectric reference measurements done by a commercial laboratory instrument.

A study of surface stress and flexural rigidity of symmetrically and asymmetrically biofunctionalized microcantilevers

Dynamic sensing using microcantilevers has held the centre stage for a long time in biomolecular detection. In this work, we have reported a comparative study of dynamic responses of asymmetric (top or bottom surface) and symmetric (both top and bottom surfaces) modifications (biofunctionalization) of silicon nitride/oxide microcantilevers. For the first time, surface stress (in terms of surface energy/area) has been shown to govern the change in resonant frequency by studying the dynamic responses of asymmetrically and symmetrically modified microcantilevers using a conventional laser doppler vibrometer. The resonant frequency of asymmetrically modified microcantilever was found to be lower than that of symmetrically modified one by a magnitude of ~0.4 kHz. Also, it was observed that the amplitude of oscillation increased from symmetrically modified to asymmetrically modified microcantilever. Consequentially, we have reported (by means of mathematical calculations and nanoindentation experiments) the increase in flexural rigidity as well as surface stress from symmetric modification to the asymmetric one. Owing to the decrease in the resonant frequencies as well as downward bending of microcantilevers, we conclude that compressive surface stress is developed after surface modifications. This work will be of immense interest to the researchers working in the field of dynamic sensing using MEMS-based sensors.

Magnetoelastic Humidity Sensors with TiO2 Nanotube Sensing Layers.

In this study, TiO2 nanotubes (TiO2-NTs) are coated with a drop-casting method on Fe40Ni38Mo4B18 amorphous ferromagnetic ribbons and the humidity response of the prepared magnetoelastic sensors (MES) is investigated. The synthesis of TiO2-NTs is performed using a hydrothermal process. Sample characterization is carried out using X-ray diffraction and scanning electron microscopy. The results show that the sensors can measure moisture values in the range of 5% to 95% with very high precision and very low hysteresis. The humidity variation between 5% and 95% shows a change in the sensor resonance frequency of ~3180 Hz, which is a significant change compared to many magnetoelastic humidity sensors developed so far.

A Langasite Crystal Microbalance Coated with Graphene Oxide-Platinum Nanocomposite as a Volatile Organic Compound Sensor: Detection and Discrimination Characteristics.

We propose a novel langasite crystal microbalance (LCM) sensor with a graphene-based sensing medium to detect and discriminate volatile organic compounds (VOCs) at room temperature. A thin film of graphene oxide embedded with Pt nanostructures (GO-Pt nanocomposite) was deposited on the electrode surface of the LCM, a thickness-shear acoustic wave resonator. Ethyl acetate, acetic acid, and ethanol were chosen as typical VOCs for this study. Sensitivity and selectivity of coated LCM were investigated for different concentrations of the VOCs by analysing the resonant properties of the sensor. When exposed to VOCs, a negative shift in series resonance frequency was observed due to the mass loading of VOC molecules. Simultaneously, changes in equivalent resistance and parallel resonance frequency of the sensor were also observed due to the interaction of VOCs with charge carriers on the GO-Pt nanocomposite film surface. This dual measurement of both series and parallel resonance frequencies allowed for detection and discrimination of VOCs. Moreover, the high thermal stability of langasite makes the proposed sensor suitable even for harsh environmental conditions.

MEMS Gas Flow Sensor Based on Thermally Induced Cantilever Resonance Frequency Shift

This paper reports a novel MEMS gas flow sensor that relies on the temperature drop induced when the gas flows over an electrically heated MEMS triple-beam resonator. Modelling, simulation and characterization of the sensor has been undertaken to quantify the temperature-induced shift of resonance frequency of the resonator, which can be directly related to the rate of gas flow over the heated resonator. The MEMS resonator was actuated into mechanical resonance through application of an AC voltage to an aluminum nitride (AlN) piezoelectric layer coated on the central beam of the triple-beam resonator. A reversible change in resonance frequency was measured experimentally for nitrogen flow rates up to 5000 ml/min. At 5 V operating voltage the linear response fit measured from experiments yielded a 67 ml/min per Hz slope over a flow rate range from 0 ml/min to 4000 ml/min.

Integrated Multichannel Electrochemical–Quartz Crystal Microbalance Sensors for Liquid Sensing

This paper highlights the design, simulation and fabrication of an array of twelve integrated electrochemical – quartz crystal microbalance (IEQCM) sensors on a single substrate for liquid sensing. Integration of both measurement techniques is made possible by combining the three electrode electrochemical device with the top and bottom electrodes for the microbalance. Important design parameters such as the working electrode radius and gap spacing, were studied using both theoretical calculations and COMSOL Multiphysics R finite element simulations. The sensor’s working electrode radius affects the magnitude of the frequency response while the gap affects the capacitance and current density which are important for electrochemical measurements. It was found that the best values for the working electrode radius was 2 mm and gap spacing was 0.5 mm. The sensors were fabricated using microfabrication techniques for the gold electrode and screen printing techniques for the reference electrode. Water contact angle, atomic force microscopy, and scanning electron microscope were utilized to study the surface roughness of the IEQCM sensor. IEQCM has a low contact angle of 53.0 ± 1 ◦ and low surface roughness of 1.92nm. For liquid sensing, an array of circular chambers were fabricated using polydimethylsiloxane (PDMS) and placed on top of the quartz substrate for liquid testing. Electrochemical measurements and cyclic voltammetry were performed using the sensor in ferri-ferrocyanide and phosphate buffered saline solution to study the function of scan rates on the peak current with respect to the potential difference. For mass sensing measurements, liquid water droplets of 1uL – 10 uL were placed onto the sensing surface and the change in resonance frequencies of the sensors were measured. These resonance frequency changes can be converted in mass change/area in accordance to the advanced Sauerbrey equation. The multichannel IEQCM sensor shows good potential as a parallel sensor for both biosensing and environmental applications.