Quartz Crystal Microbalance Electrode Research Articles

Electrophoresis and Quartz Crystal Microbalance Instrumentation to Sense Nanoplastics in Water.

We report a Technical Note on detecting nanoplastics in water samples through electrophoresis and quartz crystal microbalance (QCM) instrumentation. We conducted electrophoresis experiments by immersing a QCM in a sample of ultrapure water containing polyethylene (PE) nanoplastics. It was interesting to observe that nanoplastics were attracted toward the QCM and adhered to one side of the QCM electrode. The attached particles introduced mass loading to the QCM and were characterized by a decrease in resonance frequency of the crystal. Furthermore, when a small region around the center of electrode was alone exposed for direct contact in water and the rest of the electrode was masked using photoresist, the nanoplastics were concentrated only in the exposed electrode region, significantly enhancing detection sensitivity. To further investigate the applicability for real-life water samples, we experimented with the technique with readily available bottled drinking water and mineral water, where we spiked these water samples with nanoplastics. It was observed that the resonance frequency shifts were significantly larger for samples with nanoplastics compared to samples without nanoplastics. In addition, Raman spectroscopy and microscopy imaging were used to further confirm the presence and locations of nanoplastics on the electrode surface. This study highlights the combination of electrophoresis and QCM effectiveness in detecting nanoplastics across different water types and their potential for broader applications in environmental monitoring.

Lithium Electrodeposition Process in Various Hydrogenated Electrolyte Solutions

Introduction: The lithium metal anode, which is attracting attention as a high-capacity next-generation anode material, is hindered in practical application by the formation of dendritic deposits known as lithium dendrites during charging (electrodeposition). One method to achieve dendrite-free electrodeposition of Li involves the addition of a small amount of water to an electrolyte containing LiPF6, leading to the formation of HF through hydrolysis. During electrodeposition, LiF precipitates along with lithium, resulting in the formation of columnar lithium, which is known to inhibit dendrite formation [1]. In this study, therefore, we investigated the influence of solvent properties on the columnar structure of lithium by adding a small amount of water to electrolytes containing LiPF6 in three different solvents with varying properties, comparing the mass changes on the electrode surface and the composition of the deposits during lithium electrodeposition.Experiment: Three solvents (pure dimethyl carbonate (DMC), a mixture of ethyl carbonate (EC) and DMC (v/v = 1:1), and a mixture of EC and diethyl carbonate (DEC) (wt/wt = 1:1)) were used for the experiments after adding 1 M LiPF6 and approximately 50 ppm water, ensuring sufficient progress of hydrolysis. Copper foil or copper-coated quartz crystal microbalance (QCM) electrodes were employed as the working electrode, and lithium foil was used as the counter electrode. Lithium electrodeposition was conducted at a constant current density of -0.5 mA cm-2 for 2 hours, during which the mass changes and impedance variations were monitored using QCM and electrochemical impedance spectroscopy (EIS), respectively. Additionally, composition analysis of the deposited lithium after electrodeposition was performed using X-ray photoelectron spectroscopy (XPS).Results and Discussion: The rate of hydrolysis of LiPF6 by the added water varied significantly depending on the solvent, with durations of 2 days for DMC alone, 8 days for EC/DMC, and 20 days for EC/DEC. This difference is seemed to be due to variations in the dissociation of LiPF6. When a current was passed through the copper electrode, a plateau region of cell voltage due to HF reduction was maintained for 10 to 20 seconds around 2 V, followed by a decrease in voltage and a constant voltage of ca. -0.2 V, indicating the onset of lithium deposition. The ratio of mass increase during lithium deposition (mass per electron; MPE) was in the reverse order of the rate of hydrolysis, with EC/DEC > EC/DMC > DMC. Additionally, the resonant resistance increased significantly in the order of DMC > EC/DMC > EC/DEC. Based on the mass changes and the morphology of the deposition mentioned later, it is highly probable that an increase in viscosity and density of the electrolyte due to side reactions, or an increase in surface area due to dendrite formation, occurred. In EC/DEC and EC/DMC, a characteristic blue-colored precipitate indicative of columnar lithium structures was observed, while in DMC, it was gray, showing lithium dendrite. XPS analysis revealed that LiF was dominant on the surface of the deposited lithium in all electrolytes, but there were significant differences in the amount of surface carbon, with EC/DEC > EC/DMC > EC/DEC. These results suggest that differences in EC content lead to variations in the dissociation and viscosity of the electrolyte, which in turn affect the morphology of the deposited lithium.

(Invited) Zinc Oxide and Titanium Oxide Photocatalysts Fabricated By Electrochemical Methods and Boron Doped Diamond Electrochemistry

Photocatalytic Properties of Transparent ZnO Films Prepared by Anodization of Zinc PlateTransparent zinc oxide films were prepared by anodizing of zinc plate when the electrolyte was kept at low temperature（＜10℃）in low concentration of alkaline solution and under high bias voltage，Obtained transparent film was uniform，and it showed transparency even if it might be thick．Thus obtained transparent film is confirmed to be a pure zinc oxide，and its absorption edge is 380nm，which corresponds to the optical band gap energy of 3.2 eV，Moreover，it had high photocatalytic activity which was superior to the one prepared under no cooling. We have already reported that the photocatalytic properties were closely correlated with their orientation preference of（1010）. In this case，thus obtained transparent film also showed preferred orientation of the（1010），Effects of Electrolytic Condition to Photocatalytic Activity for Cathodically Deposited ZnO Films. Zinc oxide film was prepared easily by cathodic deposition from zinc nitrate aqueous solutions. Deposited film was evaluated as photocatalytic film under different deposition conditions. The Photocatalytic activity was evaluated by measuring acetaldehyde gas degradation. Film prepared in dilute solutions of Zn (NO3)2 showed superior photocatalysis activity. Film prepared under high current density showed superior photocatalysis. We monitored mass changes during cathodic deposition and confirmed that ZnO was completely deposited under the above conditions and that additional reactions took place under other preparation conditions. These additional reaction deactivated photocatalytic activity.Photocatalytic Properties of Titanium Oxide Film Prepared by Electrophoretic Sol－Gel Deposition Electrophoretic Sol-Gel Deposition was introduced to immobilize the TiO2 photocatalysis．Thus prepared coatings showed quite high－photocalytic activities without successive heat treatment．It is suggested that Non-heated coating showed high photocalytic activity because it responds effectively for light，which is attributed to the nano-structure of the deposited coating．Fabrication and Characterization of Diamond Quartz Crystal Microbalance Electrode Boron-doped diamond film on a quartz crystal electrode was prepared by a reflow technique. The mass sensitivity of this diamond-quartz crystal microbalance (QCM) sensor was determined by performing Fe electrodeposition. This novel electrochemical QCM sensor was quite stable under several electrochemical experimental conditions. Its wide potential window and significantly low background current are just the same as the performance of a diamond electrode. We also showed that the fabricated diamond-QCM electrode worked satisfactorily as a useful tool for electrochemical microgravimetry by examining hydrogen evolution, oxygen evolution, and oxidation of the diamond surface in acidic solutions. It was found that evolved H2 inserted into the bulk of as-grown diamond with a storage volume of 0.06 mol/dm3 in 0.5 mol/dm3 HCl solution, after sweeping the potential from 0 to -2.3 V at a scan rate of 10 mV/s, in contrast to O-terminated diamond-QCM, which inhibited hydrogen intercalation.Oxygen reduction on Au nanoparticle deposited boron-doped diamond films Nanoparticle Au was deposited on as grown boron-doped diamond (BDD). The coverage and the morphology of the deposited Au particles were investigated by means of the linear sweep voltammetry and scanning electron microscopy (SEM). From SEM, the gold electrodeposited randomly as small spherical particles with an average diameter of 60nm. The applications of the as grown BDD film deposited by Au for electrocatalytic reduction of the oxygen in acidic solution were investigated. The catalytic efficiency of the Au deposited as grown BDD with the coverage of 0.06 is near 20 times larger than that of polycrystalline gold. The mechanism of the highly electrocatalytic active was investigated by ac impedance and hydrodynamic voltammetric methods.Photoelectrodeposition of Copper on Boron-Doped Diamond Films: Application of Conductive Pattern Formation on Diamond. The Photographic Diamond Surface Phenomenon The photoelectrodeposition of copper on semiconducting B-doped diamond films was investigated. There were clear morphology differences between photoelectrodeposited and electrodeposited copper. Photoelectrodeposition proceeded by a uniform two-dimensional growth process, whereas electrodeposition involved isolated random deposition. By applying this effect we have succeeded in forming a conductive copper pattern on semiconducting B-doped diamond with the aid of a photo-mask. Interestingly, it was further found that changes occurring on the diamond surface during photoelectrochemical polarization in the absence of copper in solution facilitate subsequent copper electrodeposition in the dark, possibly due to the formation of subsurface hydrogen. We refer to this as the `photographic diamond surface phenomenon'.

Performance evaluation of QCM dew point sensors with different wettability electrode

For accurate identification characteristics of dew point sensors based on quartz crystal microbalance (QCM), the performance of QCM with different wettability electrodes was evaluated. With the materials of 1 H, 1 H, 2 H, 2 H-perfluorodecyltrimethoxysilane (PFOTMS) and 3-(methacryloyloxy) propyltrimethoxysilane (MPTMS), and the cleaning methods of plasma and ultrasound, QCM electrodes with different wettability were prepared. The influence of wettability on dew point identification characteristics was analyzed by observing differences in the static contact angle and surface structure of each QCM, and experiments were carried out to study the frequency characteristics of each QCM. The results demonstrated that when the wettability of the QCM electrodes was lower, the dew was close to spherical and its distribution was dense, which was beneficial for accurate identification. The dew point temperature was identified by detecting the temperature of the point when the QCM frequency variation slop changed to 0, and the QCM treated with plasma-PFOTMS had excellent measurement performance as its maximum measurement error was less than 0.38 ℃DP at the dew point range of 0–14 ℃DP in five days. This study provided a new approach for optimizing high-precision QCM dew point sensors.

Measurement of a Bubble-Free Chemical Oscillator Using QCMs Treated with Self-Assembled Monolayers.

The oscillation of a 1,4-cyclohexanedione-bromate (CHD-bromate) system was investigated by using a quartz crystal microbalance (QCM) without or with a self-assembled monolayer (SAM), where gold and platinum were used for QCM electrodes and SAMs were composed of HS(CH2)11CH3, HS(CH2CH2O)5H, and HS(CH2CH2O)5CH3. The CHD-bromate system is well-known as a bubble-free oscillator and oscillates without or with a catalyst. In the CHD-bromate system without a catalyst, the oscillation of a resonant frequency shift (ΔF) of the QCM was observed in the Au-electrode QCMs without a SAM or with SAMs formed from HS(CH2)11CH3 and HS(CH2CH2O)5H. On the other hand, the HS(CH2CH2O)5CH3 SAM suppressed the ΔF oscillation. The results revealed that in the CHD-bromate system without a catalyst, hydrophobic CH3 terminal and helical conformation were important to prevent nonspecific adsorption of substances on a gold surface and its dissolution. In the CHD-bromate system with a catalyst (ferroin), the ΔF oscillation was observed in the Au-electrode QCM with the HS(CH2CH2O)5CH3 SAM. The results suggest evidence that the change of the solution viscosity and density led to the ΔF oscillation. These results for Au-electrode QCMs were also corroborated by those for Pt-electrode QCMs.

Development of gold nanospikes-modified quartz crystal microbalance biosensor for prostate specific antigen detection

Prostate-specific antigen (PSA) screening is vital for the early detection and assessment of prostate cancer treatment efficacy. Integrating nanotechnology into biosensor development allows for targeted biomarker detection. In this study, we present the fabrication of gold nanospikes (AuNS) structures on a quartz crystal microbalance (QCM) using the pulse current electrodeposition (PCD) technique. By varying the duty cycles of PCD, we achieved the desired nanostructure. The characteristics of the AuNS-modified QCM electrode were thoroughly investigated using various micro-structural characterization techniques such as x-ray diffraction (XRD), field emission scanning electron microscope (FESEM), atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). Prior to biomolecule immobilization, each chemical treatment was verified through contact angle and FT-IR analyses. The performance of the developed QCM biosensor was evaluated using PSA at various concentrations. The results revealed that the frequency variation of the QCM biosensor was directly proportional to the PSA concentration. Notably, the AuNS-modified QCM biosensor exhibited the highest frequency shift at a 20 % duty cycle, demonstrating a linear detection range of 0.1–100 ng mL−1 and a limit of detection (LOD) of 24 pg mL−1. The developed QCM biosensor significantly enhances detection efficiency by increasing the number of binding sites for biomolecules through an enlarged surface area. The demonstrated sensitivity of the AuNS-modified QCM biosensor indicates its potential to address the increasing demand for early diagnosis and screening in healthcare systems. This innovative biosensor technology holds promise for improving prostate cancer detection and has broader implications for the development of highly sensitive biosensors for various biomarkers.

Malathion Detection Using Molecularly-Imprinted Polymer Quartz Crystal Microbalance Sensor

The extensive use of pesticides can result in overexposure and soil, water, and produce residues. For instance, residues of malathion were found on some vegetables. Molecularly-imprinted polymers (MIP) have been recently developed for sensing of pesticide residues. This study prepared malathion-imprinted polymers via precipitation polymerization and deposited on quartz crystal microbalance (QCM) electrodes. FTIR spectroscopy proved the incorporation and removal of malathion in the matrix of MIP. SEM images revealed that MIP particles are larger than the non-imprinted polymer (NIP) particles due to the incorporation of malathion. Binding experiments were done using standard malathion solutions of 10 to 60 ppm. The MIP-QCM sensor had a greater response than the NIP-QCM sensor. This is due to the specific binding sites in the MIP matrix. On the other hand, the response of NIP-QCM sensor is attributed to the non-specific adsorption sites in its matrix. A sensitivity and detection limit of 1.62 Hz·L/mg and 5.67 ppm, respectively were determined for the MIP-QCM sensor. Lastly, the MIP-QCM sensor is stable and reusable up to three (3) cycles.

Epitope imprinting of Sip D protein of Salmonella Typhi bacteria through multiple monomers approach

Typhoid fever is an endemic disease in India. An early stage detection of this disease will reduce the burden on healthcare industry and prevent many fatalities. Here a MIP sensor is fabricated to detect Salmonella typhi bacteria through its antigenic epitope sequence identified through immunoinformatic tools. Epitope imprinting through multiple monomers was utilized to deposit the sensing matrix on gold coated electrochemical quartz crystal microbalance (EQCM) electrode. Methacrylic acid, 2-methacryloyloxyethyl phosphorylcholine and benzyl methacrylate were used as monomers while ethylene glycol dimethacrylate as crosslinker and azobisisobutyronitrile was used as initiator. The analytical performance of MIP-EQCM sensor was studied by electrochemical as well as piezoelectric measurements. Selectivity of sensor was examined thorugh mismatched peptide sequences and certain plasma proteins also. The detection limit was found to be 0.27 ng mL−1 by DPV and 0.87 ng mL−1 by piezoelectric measurements. The sensor exhibited high selectivity towards imprinted epitope. Additionally, the sensor demonstrated the specific and selective detection in real sample of typhoid infected patients. Thus, the proposed MIP-EQCM sensor could be proposed as diagnostic tool for typhoid fever.

Electrochemically Controlled Release from a Thin Hydrogel Layer.

In this study, we present a thermoresponsive thin hydrogel layer based on poly(N-isopropylacrylamide), functionalized with β-cyclodextrin groups (p(NIPA-βCD)), as a novel electrochemically controlled release system. This thin hydrogel layer was synthesized and simultaneously attached to the surface of a Au quartz crystal microbalance (QCM) electrode using electrochemically induced free radical polymerization. The process was induced and monitored using cyclic voltammetry and a quartz crystal microbalance with dissipation monitoring (QCM-D), respectively. The properties of the thin layer were investigated by using QCM-D and scanning electron microscopy (SEM). The incorporation of β-cyclodextrin moieties within the polymer network allowed rhodamine B dye modified with ferrocene (RdFc), serving as a model metallodrug, to accumulate in the p(NIPA-βCD) layer through host-guest inclusion complex formation. The redox properties of the electroactive p(NIPA-βCD/RdFc) layer and the dissociation of the host-guest complex triggered by changes in the oxidation state of the ferrocene groups were investigated. It was found that oxidation of the ferrocene moieties led to the release of RdFc. It was crucial to achieve precise control over the release of RdFc by applying the appropriate electrochemical signal, specifically, by applying the appropriate potential to the electrode. Importantly, the electrochemically controlled RdFc release process was performed at a temperature similar to that of the human body and monitored using a spectrofluorimetric technique. The presented system appears to be particularly suitable for transdermal delivery and delivery from intrabody implants.

Room-Temperature SO2 Gas Sensing Properties of the Ag@GO Heterogeneous Nanomaterial

The study proposes SO2 gas sensor based on a quartz crystal microbalance (QCM) coated with Ag@GO heterogeneous nanomaterial at room temperature. The heterogeneous nanomaterial was synthesized by hydrothermal method at 160 C for 90 min. The structure and morphological characteristics were analyzed by using field-emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy. The Ag@GO heterogeneous nanomaterial was dispersed in water and coated on an Au electrode of QCM via spray coating method. The fabricated sensor exhibits a good response to SO2 in the concentration range of 2.5-15 ppm. The highest gas sensitivity of the sensor is 8.98 Hz/ppm corresponding to the adsorbed SO2 mass density of 2.38 μg.cm-2. The gas sensing results indicated that the developed sensor can be applied for monitoring, warning, and controlling SO2 in the environment. Moreover, the gas adsorption properties of Ag@GO nanomaterial can also be considered for a filter to remove toxic gases in respirators and environmental treatment.

Effects of Oscillation Amplitude Variations on QCM Response to Microspheres of Different Sizes

Suspended particulate matter (PMx) is one of the most important environmental pollutants. Miniaturized sensors capable of measuring and analyzing PMx are crucial in environmental research fields. The quartz crystal microbalance (QCM) is one of the most well-known sensors that could be used to monitor PMx. In general, in environmental pollution science, PMx is divided into two main categories correlated to particle diameter (e.g., PM < 2.5 µm and PM < 10 µm). QCM-based systems are capable of measuring this range of particles, but there is an important issue that limits the application. In fact, if particles with different diameters are collected on QCM electrodes, the response will be a result of the total mass of particles; there are no simple methods to discriminate the mass of the two categories without the use of a filter or manipulation during sampling. The QCM response depends on particle dimensions, fundamental resonant frequency, the amplitude of oscillation, and system dissipation properties. In this paper, we study the effects of oscillation amplitude variations and fundamental frequency (10, 5, and 2.5 MHz) values on the response, when particle matter with different sizes (2 µm and 10 µm) is deposited on the electrodes. The results showed that the 10 MHz QCM was not capable of detecting the 10 µm particles, and its response was not influenced by oscillation amplitude. On the other hand, the 2.5 MHz QCM detected the diameters of both particles, but only if a low amplitude value was used.

A Highly Sensitive Toluene and Xylene QCM Nanosensor Using Nanoporous MIL‐101(Cr) as a Sensing Layer

AbstractThis study reports a facile method to fabricate a metal‐organic framework (MOF) MIL‐101(Cr) based quartz crystal microbalance (QCM) for the detection of volatile organic compounds (VOCs) at room temperature. MIL‐101(Cr) as a novel superior material with an extremely large surface area of 2612 m2/g and pore volume of 1.26 cm3/g, can have higher adsorption capacity in comparison with zeolites, carbon nanotubes, and many porous polymers. It was chosen as a sensing thin film coated onto the surface of the QCM electrode by using the drop‐casting method. The synthesized MOF was characterized by scanning electron microscopy, Fourier‐transform infrared spectroscopy, X‐ray diffraction, and so on. The frequency of the crystal changes during the adsorption of gas molecules onto the MOF layer. Adsorption/desorption behaviors of toluene and o‐xylene vapors in different concentrations vs time are considered and sensing properties such as sensor response, response/recovery time, sensitivity, and limit of detection are measured. The sensor exhibits a short response/recovery time, as well as a fully reversible and repeatable sensor response that reflects the high stability of the sensor. The most prominent advantage of this research in comparison with other works is the high sensitivity of the sensor to o‐xylene and toluene with a sensitivity of 26.912 and 11.604 Hz/ppm and the very low detection limits of 1.960, 4.102 ppm for o‐xylene and toluene respectively which are very lower than the reported threshold limit values. π‐π stacking interaction between benzenedicarboxylate groups of MIL‐101(Cr) and aromatic rings may be the effective factor in selective detection among other analytes like methanol, ethanol, acetone, n‐hexane, dichloromethane, tetrahydrofuran (THF).

Development of a QCM-based biosensor for the detection of non-small cell lung cancer biomarkers in liquid biopsies

Lung cancer is the main malignant cancer reported worldwide, with one of the lowest survival rates. Deletions in the Epidermal Growth Factor Receptor (EGFR) gene are often associated with non-small cell lung cancer (NSCLC), a common subtype of lung cancer. The detection of such mutations provides key information for the diagnosis and treatment of the disease; therefore, the early screening of such biomarkers is of vital importance. The need for fast, reliable, and early detection means applied to NSCLC has led to the development of highly sensitive devices that can detect cancer-associated mutations. Such devices, known as biosensors, are a promising alternative to more conventional detection methods and can potentially alter the way cancer is diagnosed and treated. In this study, we report the development of a DNA-based biosensor, namely a quartz crystal microbalance (QCM), applied to the detection of NSCLC, from liquid biopsies samples. The detection, as is the case of most DNA biosensors, is based on the hybridization between the NSCLC-specific probe and the sample DNA (containing specific mutations associated with NSCLC). The surface functionalization was performed with a blocking agent (dithiothreitol) and thiolated-ssDNA strands. The biosensor was able to detect specific DNA sequences in both synthetic and real samples. Aspects such as reutilization and regeneration of the QCM electrode were also studied.

Construction of a QCM sensor for detecting diethylstilbestrol in water based on the computational design of molecularly imprinted polymers

Combining molecular imprinting technology and quartz crystal microbalance (QCM), the diethylstilbestrol-molecularly imprinted polymers (DES-MIPs) were designed. The LC-ωPBE/6-31G(d,p) method was chosen to predict the properties of DES-MIPs in this study. The calculated results showed that the complex formed from DES and methacrylic acid with molar ratio of 1:5 and ethylene dimethacrylate as cross-linking agent had the largest amount of hydrogen bonds, the lowest binding energy, and the optimal stability property. With the guidance of calculations, the DES-MIPs were used to prepare QCM electrode by embedding method to construct the DES-MIPs-QCM sensor. The experimental results displayed that the sensor had a high binding affinity for DES when the DES-MIPs was 15 mg and the coating volume was 10 µL. The minimum detection limit of the sensor for DES was 2.63 ng/mL in the range of 50 to 350 ng/mL. The DES-MIPs-QCM sensor exhibited high recognition capacity for DES compared to its structural analogs. The sensor had been successfully used for the determination of DES in the actual water samples.

Surface Enhancement Using Black Coatings for Sensor Applications.

The resolution of a quartz crystal microbalance (QCM) is particularly crucial for gas sensor applications where low concentrations are detected. This resolution can be improved by increasing the effective surface of QCM electrodes and, thereby, enhancing their sensitivity. For this purpose, various researchers have investigated the use of micro-structured materials with promising results. Herein, we propose the use of easy-to-manufacture metal blacks that are highly structured even on a nanoscale level and thus provide more bonding sites for gas analytes. Two different black metals with thicknesses of 280 nm, black aluminum (B-Al) and black gold (B-Au), were deposited onto the sensor surface to improve the sensitivity following the Sauerbrey equation. Both layers present a high surface roughness due to their cauliflower morphology structure. A high response (i.e., resonant frequency shift) of these QCM sensors coated with a black metal layer was obtained. Two gaseous analytes, H2O vapor and EtOH vapor, at different concentrations, are tested, and a distinct improvement of sensitivity is observed for the QCM sensors coated with a black metal layer compared to the blank ones, without strong side effects on resonance frequency stability or mechanical quality factor. An approximately 10 times higher sensitivity to EtOH gas is reported for the QCM coated with a black gold layer compared to the blank QCM sensor.

Reversible and pH-modulated changes in microgel size triggered by electrochemical stimuli

A novel microgel, the volume of which can be reversibly changed by using an electrochemical trigger, was successfully synthesized. To this end, a monomer based on N-(3-aminopropyl)methacrylamide modified with ferrocenecarboxylic acid was obtained and co-polymerized with acrylic acid and N,N′–methylenebisacrylamide. Using the distillation precipitation polymerization method, spherical, smooth-surfaced microgel particles ca. 500 nm in diameter were obtained. The presence of ferrocene groups in the microgel was shown with nuclear magnetic resonance spectroscopy, energy dispersive X-ray spectroscopy, and cyclic voltammetry methods. Dynamic light scattering experiments showed that the microgel size strongly depends on the oxidation state of the ferrocene groups. Next, cyclic voltammetry and chronoamperometry combined with quartz crystal microbalance measurements, obtained with an Au electrochemical quartz crystal microbalance electrode modified with the microgel, proved that the microgel size could be controlled by an electrochemical trigger. More interestingly, the electro-response was found to strongly depend on pH. In acidic pH, where almost all carboxylic groups are protonated, the oxidation of ferrocene groups led to a decrease in the registered frequency shift, related to the microgel swelling process. In alkaline pH, where many carboxylic groups are ionized, the oxidation of ferrocene led to an increase in frequency, related to the microgel shrinking process. These processes were found to be reversible and relatively fast.

Highly Sensitive and Real-Time Detection of Zinc Oxide Nanoparticles Using Quartz Crystal Microbalance via DNA Induced Conjugation.

With the development of nanotechnology, nanomaterials have been widely used in the development of commercial products. In particular, zinc oxide nanoparticles (ZnONPs) have been of great interest due to their extraordinary properties, such as semiconductive, piezoelectric, and absorbance properties in UVA and UVB (280–400 nm) spectra. However, recent studies have investigated the toxicity of these ZnONPs; therefore, a ZnONP screening tool is required for human health and environmental problems. In this study, we propose a detection method for ZnONPs using quartz crystal microbalance (QCM) and DNA. The detection method was based on the resonance frequency shift of the QCM. In detail, two different complementary DNA strands were used to conjugate ZnONPs, which were subjected to mass amplification. One of these DNA strands was designed to hybridize to a probe DNA immobilized on the QCM electrode. By introducing the ZnONP conjugation, we were able to detect ZnONPs with a detection limit of 100 ng/mL in both distilled water and a real sample of drinking water, which is 3 orders less than the reported critical harmful concentration of ZnONPs. A phosphate terminal group, which selectively interacts with a zinc oxide compound, was also attached at one end of a DNA linker and was attributed to the selective detection of ZnONPs. As a result, better selective detection of ZnONPs was achieved compared to gold and silicon nanoparticles. This work demonstrated the potential of our proposed method as a ZnONP screening tool in real environmental water systems.

Impedance Analysis of Chitin Nanofibers Integrated Bulk Acoustic Wave Humidity Sensor with Asymmetric Electrode Configuration.

This paper fabricated a high-performance chitin nanofibers (ChNFs)-integrated bulk acoustic wave (BAW) humidity sensor with an asymmetric electrode configuration. The ChNFs were successfully prepared from crab shells and used as moisture-sensitive materials to compare the performance of quartz crystal microbalance (QCM) humidity sensors with symmetric and asymmetric electrode structures. The QCM humidity sensor with a smaller electrode area exhibited high sensitivity of 58.84 Hz/%RH, competitive response/recovery time of 30/3.5 s, and low humidity hysteresis of 2.5% RH. However, it is necessary to choose a suitable electrode diameter to balance the stability and sensitivity because the impedance analysis result showed that the reduction of the electrode diameter leads to a sharp decrease in the Q value (stability). Next, the possible humidity-sensitive mechanism of the ChNFs-integrated asymmetric n-m electrode QCM humidity sensor was discussed in detail. Finally, the reasons for the highest sensitivity of the asymmetric n-m electrode QCM humidity sensors having a smaller electrode diameter were analyzed in detail in terms of both mass sensitivity and fringing field effect. This work not only demonstrates that the chitin nanofiber is an excellent potential material for moisture detection, but also provides a new perspective for designing high-performance QCM humidity sensors.

Effect of Electrode Thickness on Quality Factor of Ring Electrode QCM Sensor.

As a key type of sensor, the quartz crystal microbalance (QCM) has been widely used in many research areas. Recently, the ring electrode QCM sensor (R-QCM) with more uniform mass sensitivity has been reported. However, the quality factor (Q-factor) of the R-QCM has still not been studied, especially regarding the effect of electrode thickness on the Q-factor. Considering that the Q-factor is one of crucial parameter to the QCM and it is closely related to the output frequency stability of the QCM, we study the effect of different electrode thicknesses on the Q-factor of the R-QCM in this paper. On the other hand, we clarify the relationship between the electrode thickness and the Q-factor of the R-QCM. The measurement results show that the average Q-factor increases with increases in the thickness of ring electrodes generally; however, the resonance frequency of the QCM resonator decreases with increases in the thickness. The low half-bandwidth (2Γ < 1630 Hz) of the R-QCM shows that the frequency performance is good. Additionally, the R-QCM has a higher Q-factor (Q > 6000), which indicates that it has a higher frequency stability and can be applied in many research areas.

Application of an artificial neural network and QCM sensor coated with γ-Fe2O3 nanoparticles for estimation of SO2 gas sensing characteristics

γ-Fe2O3 nanoparticles (NPs) were synthesized by co-precipitation method and a following annealing treatment at 200 °C in ambient air for 6 hours. A mass-type sensor was prepared by coating γ-Fe2O3 NPs on the active electrode of quartz crystal microbalance (QCM). The obtained results of the γ-Fe2O3 NPs based QCM sensor indicate the high response and good repeatability toward SO2 gas in the range of 2.5 – 20 ppm at room temperature. Moreover, the frequency shift (DF) and change in mass of SO2 adsorption per unit area (Dm) of the γ-Fe2O3 NPs coated QCM sensor have a relationship with the mass density of γ-Fe2O3 NPs and SO2 concentrations. The artificial neural network (ANN) model using Levenberg-Marquardt optimization was used to handle the DF and Dm of the γ-Fe2O3 NPs coated QCM sensor. The results of the model validation proved to be a reliable way between the experiment and prediction values.