Quartz Crystal Microbalance Surface Research Articles

THE EFFECT OF SPIN COATING’S SPEED ON GRAPHENE-OXIDE’S LAYER TOPOGRAPHY AND QCM VISCOELASTICITY

Spin coating speed controls the deposition process, affecting the formation of the layer’s topography of graphene oxide and the viscoelastic properties of quartz crystal microbalance (QCM). In this work, the graphene oxide was deposited on the QCM surface by spin coating method with speeds ranging from 500 to 800 rpm. The layer’s surface topography was subsequently measured and analyzed using a roughness multiparameter, which included the area average (Sa), area roughness deviation (Sq), localized surface roughness (Sz), surface skewness (Ssk), and surface kurtosis (Sku). The Sa, Sq, and Sz, which were decreased as the spinning speed increased. The measurements indicated that the graphene oxide layer deposited at 500 rpm had a rough and clumpy texture, while at a high speed of 800 rpm, the layer had a much smoother texture. A detailed analysis using the Sku and Ssk parameters revealed that the clumpy texture in the 500 rpm sample contains tapered sharp graphene oxide structures and a small amount of porosity. On the other hand, the sharp structures were significantly reduced, and porosity was dominant in the sample deposited at high speeds of 700 rpm and 800 rpm. The effect of the spinning speed and the topography on the viscoelasticity was studied using an impedance analyzer. The impedance of the layer deposited at 500 rpm was relatively higher than that at 800 rpm. The frequency shift was also relatively significant for the sample deposited at low speed and was reduced at higher speed. The impedance analysis demonstrated that the layer’s viscosity is affected by its mass, roughness, texture, and porosity.

Formaldehyde gas sensors based on a quartz crystal microbalance modified with aniline-doped polyvinyl acetate nanofibers.

Real-time detection of formaldehyde in the atmosphere remains challenging. The available gaseous formaldehyde sensing methods offer limited sensitivity, selectivity, and robustness. We modified a quartz crystal microbalance (QCM) system for selective detection of formaldehyde in air. The QCM surface was functionalized with polyvinyl acetate (PVAc) nanofibers and doped with 2, 4, and 6 wt% aniline to improve the selectivity and sensitivity of the sensor. The chemical content and morphological structure of PVAc nanofibers doped with aniline were confirmed by Fourier-transform infrared (FTIR) spectroscopy, energy-dispersive X-ray (EDX) spectroscopy, and scanning electron microscopy (SEM). The results showed that the modified QCM sensor had a sensitivity of 0.056 Hz ppm-1 with a response and recovery times of 200 s and 90 s, respectively. It gave limits of detection (LOD) and limit of quantification (LOQ) of 28 ppm and 96 ppm, respectively. Moreover, the modified QCM was selective towards formaldehyde compared to the other gases. The current workplace exposure limit (WEL) for formaldehyde is 2 ppm, with a time-weighted average over eight hours. Future work will focus on improving the reported QCM sensor to meet the required LOD for formaldehyde detection in the environment and industrial sites.

MOF-composite sensors to eliminate the QCM positive frequency shift

In a quartz crystal microbalance (QCM) Sauerbrey equation can usually be applied in the ideal scenario and when the QCM frequency shift goes to the negative direction. In some cases, however, the requirements to achieve the ideal condition are not fulfilled and thus leading to QCM positive frequency shift phenomenon. Herein we have also observed that a porphyrin-based metal organic frameworks (MOF), namely MOF-525(Zn), QCM sensor also exhibits the positive frequency shift phenomenon when exposed to xylene isomers, which could be caused by the loose attachment of the MOF nanoparticles on the QCM surface. Therefore, in the presence of the analytes, the bonding between the MOF and the QCM surface is disturbed and higher analyte concentration also leads to the increase of the sensitive layer stiffness, resulting in higher positive frequency shift. However, such a phenomenon can be completely eliminated by simply adding polymer to MOF nanoparticle sensors to strengthen the bonding of MOF nanoparticles to the QCM surface. Through this study we have shown that adjusting the amount of polymer is very crucial to completely reverse the positive frequency response. Otherwise, it appears a critical analyte concentration of gas analytes where the QCM frequency shift changes its direction from negative to positive response. Lastly, the same strategy has also been used to other MOF-based QCM sensor systems where their sensitive layer is built from other MOFs, namely MOF-525 and NU-902, and ethanol is used as the analyte to prove the generalizability of this simple approach.

Quartz Crystal Microbalance Application and In Silico Studies to Characterize the Interaction of Bovine Serum Albumin with Plasma Polymerized Pyrrole Surfaces: Implications for the Development of Biomaterials.

Plasma polymerized pyrrole/iodine (PPPy/I) microparticles and bovine serum albumin (BSA) protein have shown interesting results in experimental models for the treatment of traumatic spinal cord injury. By studying the interaction between BSA and PPPy/I by a quartz crystal microbalance (QCM) and docking, we obtained important results to elucidate possible cellular interactions and promote the use of these polymers as biomaterials. These measurements were also used to characterize the adsorption process using an equilibrium constant. In addition, atomic force microscopy (AFM) was used to obtain images of the QCM surface sensors before and after BSA adsorption. Furthermore, we carried out molecular dynamics simulations and molecular docking to characterize the molecular recognition between BSA and the previously reported PPPy/I structure. For this study, we used two combinatorial models that have not been tested. Thus, we could determine the electrostatic (ΔGele) and nonelectrostatic (ΔGnonelec) components of the free binding energy (ΔGb). We demonstrated that BSA is adsorbed on PPPy/I with an adsorption constant of K = 24.35 μ-1 indicating high affinity. This observation combined with molecular docking and binding free energy calculations showed that the interaction between BSA and both combinatorial models of the PPPy structure is spontaneous.

Implications of the Electrodeposition Scan Rate on the Morphology of Polyaniline Layer and the Impedance of a QCM Sensor

This work aims to determine how the scan rate affects PANI layers' morphology and resonance parameters deposited on quartz crystal microbalance (QCM). The PANI layer was produced using the electrodeposition method with a scan rate variation of 10 to 50 mV/s. The PANI layer deposited at a scan rate of 10 mV/s has an aggregate morphology, whereas the morphology becomes more scattered at a higher scan rate. On the other hand, the homogeneity of the distribution of PANI particles increases due to the increase in the scan rate. The QCM impedance profile also changes with PANI layer deposition at different scan rates. The QCM impedance profile with the PANI layer that is most similar to the uncoated QCM impedance profile is at a scan rate of 10, 40 and 50 mV/s. The resonance parameters, which represent the inertial mass and the dissipative characteristics of the PANI layer, were investigated. The PANI layer with the smallest resonance parameter has a scan rate of 10 and 40 mV/s. Therefore, the 2 scan rates are the best candidates to be applied further. HIGHLIGHTS The electrodeposition scan rate influences the morphology of the PANI layer deposited above QCM The amount of carbon distributed across the QCM surface represents how effective the PANI deposition was The resulting morphology will impact the PANI layer's impedance characteristic and resonance parameter values GRAPHICAL ABSTRACT

Fabrication of Gold Nanostructures on Quartz Crystal Microbalance Surface Using Nanoimprint Lithography for Sensing Applications.

A quartz crystal microbalance (QCM) is a sensor that uses the piezoelectric properties of quartz crystals sandwiched between conductive electrodes. Localized surface plasmon resonance (LSPR) is an analytical technique that uses the collective vibration of free electrons on metal surfaces. These measurements are known as analysis techniques that use metal surfaces and have been applied as biosensors because they allow for the label-free monitoring of biomolecular binding reactions. These measurements can be used in combination to analyze the reactions that occur on metal surfaces because different types of information can be obtained from them. However, as different devices are used for these measurements, the results often contain device-to-device errors and are not accurately evaluated. In this study, we directly fabricated gold nanostructures on the surface of a QCM to create a device that can simultaneously measure the mass and refractive index information of the analyte. In addition, the device could be easily fabricated because nanoimprint lithography was used to fabricate gold nanostructures. As a proof of concept, the nanoparticle adsorption on gold nanostructures was evaluated, and it was observed that mass and refractive index information were successfully obtained without device-to-device errors.

High-Sensitivity and Environmentally Friendly Humidity Sensors Deposited with Recyclable Green Microspheres for Wireless Monitoring.

The reliable, high-sensitive, wireless, and affordable requirements for humidity sensors are needed in high-precision measurement fields. Quartz crystal microbalance (QCM) based on the piezoelectric effect can accurately detect the mass changes at the nanogram level. However, water-capture materials deposited on the surface of QCM generally show disadvantages in either cost, sensitivity, or recyclability. Herein, novel QCM-based humidity sensors (NQHSs) are developed by uniformly depositing green microspheres (GMs) of natural polymers prepared by the chemical synthesis of the emulsification/inner gel method on QCM as humidity-sensitive materials. The NQHSs demonstrate high accuracy and sensitivity (27.1 Hz/% RH) owing to the various hydrophilic groups and porous nano-3D deposition structure. Compared with the devices deposited with a smooth film, the frequency of the NQHSs shows almost no changes during the cyclic test and exhibits long-term stability. The NQHSs have been successfully applied to non-contact sensing human activities and remote real-time humidity monitoring via Bluetooth transmission. In addition, the deposited humidity-sensitive GMs and QCM substrate are fully recycled and reused (72% of the original value). This work has provided an innovative idea to construct environmental-friendly, high-sensitivity, and wireless humidity sensors.

Synthesis TiO2 using sonochemical method and responses the CO2 gas of the nanoparticle TiO2 layers on the QCM sensor surfaces

TiO2 synthesis using sonochemical methods and their coating on the surface of the Quartz Crystal Microbalance (QCM) sensor utilizing the spin coating method has been carried out. The synthesis started by mixing 7 ml of titanium (IV) isopropoxide (TTIP) (97%) in 70 ml of ethanol (96%). The mixture of TTIP solution was stirred for 30 minutes. After ward, the sonication was carried out for 4 hours at a frequency of 40 kHz and heated for 16 hours at 80 °C. To produce TiO2 powder, the precursor was calcined at 500°C for 3 hours. The TiO2 nanoparticle was analyzed using XRD, and the particle size was observed using SEM/Eds. Based on the XRD results, the crystal structure of Anatases was obtained at the 2θ (deg) of 25.503°(101), 29.56° (103), 36.20° (004), 37.977° (112), 48. 25.9° (200), 54.100° (105), 55.272° (211), 62.89°. The particle size from the SEM results showed the average particle size in the range of ±7μm-±54 nm with the majority in ±96 nm. The obtained TiO2 was dissolved with ethanol then the solvent was deposited on the QCM surface using the spin coating method. A homogeneous and porous surface structure is obtained from the SEM observation. The coated QCM was tested for its ability to adsorb CO2 gas. QCM was exposed to CO2 gas for 100 seconds with a constant exposure discharge. A positive response was obtained for the adsorbability of the coated QCM to adsorb CO2 gas with time variations. TiO2’s response against CO2 gas adsorption indicates that TiO2 can be further investigated as a CO2 gas sensor. The measuring response of the TiO2 layer on the QCM sensor was conducted to adsorb CO2 gas.

Real-time selective measurement of combustion origin hydrophobic PM using QCM

Combustion origin particulate matter (PM) has different characteristics such as hydrophilicity and hydrophobicity depending on the combustion method, however it is difficult to separate each PM characteristics. In this study, to develop real-time selective PM sensing system using a quartz crystal microbalance (QCM), PM adsorption characteristics on the QCM surface was investigated. PM emitted from a pool flame of hydrocarbon fuel such as cyclohexane and toluene was supplied to the QCM, and the frequency shift of the QCM was measured. In addition, polystyrene coated QCM with hydrophobic and acetylcellulose coated QCM with hydrophilic characteristics were also investigated.

QCM-based assay designs for human serum albumin

Solid-phase synthesis is an elegant way to create molecularly imprinted polymer nanoparticles (nano-MIPs) comprising a single binding site, i.e. mimics of antibodies. When using human serum albumin (HSA) as the template, one achieves nano-MIPs with 53 ± 19 nm diameter, while non-imprinted polymer nanoparticles (nano-NIPs) reach 191 ± 96 nm. Fluorescence assays lead to Stern–Volmer plots revealing selective binding to HSA with selectivity factors of 1.2 compared to bovine serum albumin (BSA), 1.9 for lysozyme, and 4.1 for pepsin. Direct quartz crystal microbalance (QCM) assays confirm these results: nano-MIPs bind to HSA immobilized on QCM surfaces. This opens the way for competitive QCM-based assays for HSA: adding HSA to nanoparticle solutions indeed reduces binding to the QCM surfaces in a concentration-dependent manner. They achieve a limit of detection (LoD) of 80 nM and a limit of quantification (LoQ) of 244 nM. Furthermore, the assay shows recovery rates around 100% for HSA even in the presence of competing analytes.

In-Situ Monitoring of Real-Time Loop-Mediated Isothermal Amplification with QCM: Detecting Listeria monocytogenes

Functionalized DNA sequences are promising sensing elements to combine with transducers for bio-sensing specific target microbes. As an application example, this paper demonstrates in situ detection of loop-mediated isothermal amplification products by hybridizing them with thiolated-ssDNA covalently anchored on the electrodes of a quartz crystal microbalance (QCM). Such hybridization leads to a frequency signal, which is suitable for monitoring real-time LAMP amplification based on mass-sensing: it detects interactions between the complementary nucleobases of LAMP products in solution and the thiolated-ssDNA probe sequence on the gold surface. Target DNA LAMP products cause irreversible frequency shifts on the QCM surfaces during hybridization in the kHz range, which result from both changes in mass and charge on the electrode surface. In order to confirm the LAMP assay working in the QCM sensing system at elevated temperature, the sky blue of positive LAMP products solution was achieved by using the Hydroxy Naphthol Blue (HNB) and agarose gel electrophoresis. Since on-QCM sensing of DNA hybridization leads to irreversible sensor responses, this work shows characterization by X-ray photoelectron spectroscopy (XPS) core spectra of S2p, N1s, Mg1s, P2p and C1s. XPS results confirmed that indeed both DNA and by-products of LAMP attached to the surface. Listeria monocytogenes DNA served to study in-situ detection of amplified LAMP products on DNA-functionalized surfaces.

An Investigation of Factors Affecting Measurement Accuracy of Nanoparticles Real-time Sensor using Corona Discharge integrated with Quartz Crystal Microbalance

A nanoparticles real-time sensor in the present study was designed using corona discharge integrated with quartz crystal microbalance (QCM) or Nano-QCM detector. We investigated the following factors affecting the measurement accuracy of the detector; needle to plate distance, flow rate, and number of particles on QCM. DC high voltage power supply of 7 kV was used to maintain the positive corona voltages difference in the charger. The mass sensitivity of 10 MHz AT-cut QCMs coated by plate rigid gold film were investigated. The monodisperse aerosol particles smaller than 300 nm were initially used for the testing of the system. They were charged with negative corona voltages and then deposited on the surface of QCM electrode. Sauerbrey equation was used to describe the frequency-mass relationship at the QCM surface. Results show that the above factors significantly affect the accuracy of the detector measurement. The particles can be collected on a QCM electrode using the partial breakdown high voltage (reach to breakdown points) in terms of needle-to-plate corona discharge. The optimal needle-to-plate distance is 6 mm with maximum mass change (frequency shift) on QCM electrode of 0.607μg/min. The QCM collection efficiency of nanoparticles on the QCM electrode surface using corona discharge technique can be as high as 95% of total particles. Collection efficiency of PM0.1 is under investigation.

Signal Amplification of a Gravimetric Glucose Biosensor Based on the Concanavalin A–Dextran Affinity Assay

The mass and viscoelastic properties associated with the binding of the Concanavalin A (Con A) - dextran (M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</sub> = 80 kDa) affinity assay as a function of glucose concentration, was investigated using a quartz crystal microbalance (QCM). The Con A was immobilized on the gold-coated QCM surface using a thiol linker molecule, resulting in a densely packed layer monitored by cyclic voltammetry. The resulting mass change (Am) from the immobilization of Con A ranged from 0.35 to 0.45 ± 0.09 μg cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> depending on the viscoelastic properties of the lectin layer. The subsequent mass change from the interaction with dextran (0.16 ± 0.04 μg cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ) suggests that the polysaccharide also stacks in a well-packed layer, which dissociatesin the presence of glucose (2 - 40 mM) due to competitive binding with the monosaccharide to Con A. A maximum frequency change of 10 ± 2 Hz observed in the presence of 40 mM glucose, suggests that the system has a resolution equivalent to the noise baseline of ± 0.4 mM. Cyclic tests demonstrated that the assay is reversible with time, exhibits a higher sensitivity at lower glucose concentrations, but with a relative signal magnitude that decreases for each cycle. The system was also sensitive for solution or temperature induced viscosity changes that requires compensation by an appropriate algorithm or an unmodified sensor acting as a reference.reference.

2D material attached quartz crystal microbalance for sensing SF 6 gas flow under humid condition

The authors report quartz crystal microbalance (QCM) integrated with hexagonal boron nitride–graphene (hg) monolayer heterostructure as a sensor to detect mass flow of sulphur hexafluoride (SF 6 ), an important electrical insulating gas, under humid conditions. Their recent work demonstrated an enhancement in QCM sensing characteristics upon attaching monolayer 2D materials onto the QCM electrode surface and its application for improved relative humidity measurement. Here, they extend this approach to detect flow of SF 6 gas causing variation in relative humidity. They fabricated a hg-QCM sensor by transferring the heterostructure thin film onto the QCM electrode surface using a standard wet transfer technique and measured return loss of the QCM under SF 6 gas flow at a selected RH condition. The mass flow of the humid SF 6 gas effects perturbation in damping at the QCM surface as indicated by changes in the motional resistance of the QCM. The motional resistance changes introduced significant variation in loaded Q -factor, the magnitude of which was gas species-dependent, thus allowing to distinguish SF 6 from other common electronics gases.

Rheology of the Electric Double Layer in Electrolyte Solutions.

Electric double layers (EDLs) are ionic structures formed on charged surfaces and play an important role in various biological and industrial processes. An extensive study in the past decade has revealed the structure of the EDL in concentrated electrolyte solutions of both ordinary salts and ionic liquids. However, how the EDL structure affects their material properties remains a challenging topic due to technical difficulties of these measurements at nanoscale. In this work, we report the first detailed characterization of the viscoelasticity of the EDL formed over a wide range of ion concentrations, including concentrated electrolyte solutions. Specifically, we investigate the complex shear modulus of the EDL by measuring the resonant frequency and the energy dissipation of a quartz crystal microbalance (QCM), a surface-sensitive device, immersed in aqueous solutions containing three types of solutes: an ionic liquid, 1-butyl-3-methylimidazolium chloride (BmimCl); an ordinary salt, sodium chloride (NaCl); and a nonelectrolyte, ethylene glycol (EG). For the two electrolyte solutions, we observe a monotonic decrease in the resonant frequency and a monotonic increase in the energy dissipation with increasing ion concentrations due to the presence of the EDL. The complex shear modulus of the EDL is estimated through a wave propagation model in which the density and shear modulus of the EDL decay exponentially toward those of the bulk solution. Our results show that both the storage and the loss modulus of the EDL increase rapidly with increasing ion concentrations in the low ion concentration regime (<1 M) but reach saturation values with similar magnitude at a sufficiently high ion concentration. The shear viscosity of the EDL near the charged QCM surface is approximately 50 times for NaCl solutions and 500 times for BmimCl solutions of the bulk solution value at the saturation concentration. We also demonstrate that QCM can be utilized for analyzing the rheological properties of the EDL, thus providing a complementary, low-cost, and portable alternative to conventional laboratory instruments such as the surface force apparatus. Our results elucidate new perspectives on the viscoelastic properties of the EDL and can potentially guide device optimization for applications such as biosensing and fast charging of batteries.

High-Performance QCM Humidity Sensors Using Carbon Nanostructure Material as Sensing Film

Introduction Nowadays, sensors have become increasingly indispensable. It is particularly worth mentioning that the humidity has gradually become the standard for measuring environmental state and physical health. From the perspective of manufcturing, the performance and technical index of humidity sensors can vary greatly on materials, structures and processes. For instance, multiple sensing technologies including acoustic [1], resistance [2], optical [3], impedance [4],capacitance [5], thermal [6], and resonance [7], etc. have been explored to monitor the humidity. quartz crystal microbalance (QCM) has been proven to be an excellent mass sensing platform for humidity detection according to Sauerbrey’s equation [8].Sensitive material is the key of a QCM sensor. Among various materials, a variety of sensitive materials has been deposited on the electrode surface of QCM for detecting moisture, such as polymer, metal oxides, carbonic materials and so on. In this study, a series of high performance QCM humidity sensors were developed which using multi-walled CNTs (MWCNTs), Graphene quantum dots(GQDs), Chitosan(CS) and their composites as sensing film. The humidity sensing characteristics, such as dynamical response, reproducibility, and hysteresis characteristics were measured and discussed. Furthermore, the adsorption dynamics of water vapor molecules mechanism was also analyzed. Method Characterization Raman spectra were obtained on J-YT64000 Raman spectrometer with 633 nm wavelength incident laser light. The X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALAB MK II Xray photoelectron spectrometer using Mg as exciting source. The Fourier transform infrared spectroscopy (FT-IR) spectra of as-prepared were obtained on a WQF-510AFTIR spectrometer, using KBr pellets as the reference. The transmission electron microscopy (TEM) images were taken by a JEM-2100F electron microscope (JEOL, Japan). Fabrication of QCM humidity sensor The QCM substrates used in this work were purchased from Beijing Chenjing Electronic Co., Ltd. China. The QCM was consisted of AT-cut quartz crystal (8 mm diameter) with a fundamental frequency of 10 MHz and silver electrodes (5 mm diameter) were covered on both sides, the electrode leaded two pins for testing purposes. As a typical procedure, the QCM substrates were ultrasonic cleaned by acetone, ethanol and deionized water for 10 min successively, and then dried in an air-tight container which filled with inert nitrogen gas for the whole night at room temperature. The QCM humidity sensor was obtained via dropping the as-prepared composites dispersion by micropipette on the Ag-electrodes surface of the QCM. Then, the modified QCM sensor was put into the foregoing container for at least 12 h. Results and Conclusions The humidity sensing characteristics of prepared QCM sensors were quantified by transforming the RH level ambient and measuring the shift of the vibration frequency. The humidity hysteresis performance, reproducibility and reversibility were also examined respectively. After further study, a plausible mechanism analysis for the above excellent sensitivity was given. In conclusion, an excellent QCM humidity sensor was obtained to detect full RH range. The oscillating circuit and impedance analysis methods have been adopted to examine the dynamic response and recovery behaviors and static humidity sensing performances, respectively. In addition, the intrinsic hydrophilicity is able to provide more water adsorption sites. The optimized QCM sensor demonstrated the highest humidity sensitivity and long-term stability, and tiny humidity hysteresis. The prepared QCM sensor is reliable for monitoring water vapor content in wide humidity range.

Superhydrophobic Hierarchical Porous Divinylbenzene Polymer for BTEX Sensing and Toluene/Water Selective Detection

As shown in the Fig.1, this work reports a superhydrophobic divinylbenzene polymer with hierarchical porous structure as sensing material to modify the quartz crystal microbalance (QCM) to detect benzene, toluene, ethylbenzene, and xylene (BTEX) vapor. Notably, sensing results toward toluene vapor in different relative humidities indicates that this superhydrophobic polymer has favorable toluene/water selective detection performance. Besides, the limit of detection toward toluene is lower than 1 ppm.IntroductionAs typical hazardous chemicals, benzene series, especially BTEX, are easily dispersed in the air because of their extremely easy volatile nature. BTEX can paralyze the central nervous system of humans, causing neurasthenia, liver disease, and even cancer. Therefore, using gas sensor for real-time detection of BTEX is necessary.Compared with other sensors, QCM sensor has the advantages of low cost and working at room temperature. By modifying sensitive materials on the surface of QCM, the target gas can be adsorbed and the surface quality of QCM changes. This change can be further transformed into frequency shift by Sauerbrey equation, and recorded by computer. Based on QCM platform, some benzene sensors have been reported. For example, Kumar et al. used tetra-tert-butyl copper phthalocyanine to realize the sensing of BTEX. The response to 500 ppm of toluene can reach 60 Hz. But none of these reports discussed the effect of humidity on the sensor response. Our research group detected the BTEX vapor via using superhydrophobic polyoctadecylsiloxane (PODS) based on QCM. Besides, PODS can stably detect toluene vapor under different relative humidities. However, when the concentration of toluene is 20 ppm, the response value is only about 7 Hz. It is indicating that the detection limit of PODS to toluene cannot be less than 20 ppm. Figure 1

The Effect of Solution Concentration and Deposition Time on Viscoelasticity and Morphology of Polyaniline Coating

As one of the most studied conductive polymers, polyaniline (PANi) has a variety of applications, one of which is an active gas sensor, due to the presence of amine and imine groups in PANi that can easily react with gas. Quartz crystal microbalance (QCM) is a piezoelectric sensor with the working principle of frequency changes as a result of changes in mass on its surface. As an active gas sensor material on the QCM substrate, PANi must be deposited in the form of a thin layer. A good quality of thin film is if the film does not give loading the effect on QCM and has a homogeneous morphology. This research has succeeded in depositing PANi thin films by surface polymerization method by varying the parameters of PANi solution concentration and deposition time. Deposition parameters that are varied can be seen its effect on the viscoelasticity and morphology of PANi thin film formed. The viscoelasticity of the thin film was interpreted from the measurement of the thin layer impedance using an impedance analyser, while the morphology of the thin film was characterized using SEM. From the results of this study it was found that with the surface polymerization method, the PANi thin layer impedance value changes with the deposition time which is getting longer, but the change in impedance value is not significant and does not give a loading effect on QCM. At the same deposition time with different solution concentrations, the impedance value of the formed layer also does not give a loading effect on QCM. Although it has successfully deposited PANi with a surface polymerization method, the morphology of the PANi film formed is not yet homogeneous, as seen from the formation of PANi agglomeration on the QCM surface.

MEMS biosensor for monitoring water toxicity based on quartz crystal microbalance.

This paper presents the use of a commercial quartz crystal microbalance (QCM) to investigate live-cell activity in water-based toxic solutions. The QCM used in this research has a resonant frequency of 10 MHz and consists of an AT-cut quartz crystal with gold electrodes on both sides. This QCM was transformed into a functional biosensor by integrating with polydimethylsiloxane culturing chambers. Rainbow trout gill epithelial cells were cultured on the resonators as a sensorial layer. The fluctuation of the resonant frequency, due to the change of cell morphology and adhesion, is an indicator of water toxicity. The shift in the resonant frequency provides information about the viability of the cells after exposure to toxicants. The toxicity result shows distinct responses after exposing cells to 0.526 μM of pentachlorophenol (PCP) solution, which is the Military Exposure Guidelines concentration. This research demonstrated that the QCM is sensitive to a low concentration of PCP and no further modification of the QCM surface was required.

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.