QCM Electrode Research Articles

A sub-second QCM humidity sensor with low hysteresis base on Nafion thin film

In this paper, we developed a QCM humidity sensor with Nafion thin film coated on its electrode. An impedance-based QCM measurement system proposed in our previous work was used for measurement without oscillators. The experimental results indicate that the obtained QCM sensor has an ultrafast response speed (0.6/0.9 s) and very low hysteresis (1.61% RH). This may be related to the low amount of Nafion on the QCM electrode and the thin enough film generated on it.

QCM Electrode Configurations for Enhanced Mass Distribution and Sensitivity

QCM Electrode Configurations for Enhanced Mass Distribution and Sensitivity

Selection of Highly Specific DNA Aptamer for the Development of QCM-Based Arsenic Sensor.

Heavy metal arsenic is a water pollutant that affects millions of lives worldwide. A novel aptamer candidate for specific and sensitive arsenic detection was identified using Graphene Oxide-SELEX (GO-SELEX). Eleven rounds of GO-SELEX were performed to screen As(III) specific sequences. The selected aptamer sequences were evaluated for their binding affinity. The dissociation constant of the best aptamer candidate, As-06 was estimated by fluorescence recovery upon target addition, and it was found to be 8.15 nM. A QCM-based biosensing platform was designed based on the target-triggered release of aptamer from the QCM electrode. An rGO-SWCNT nanocomposite was adsorbed on the gold surface, and the single-stranded probe was stacked on the rGO-CNT layer. Upon addition of the target to the solution, a concentration-dependent release of the ssDNA probe was observed and recorded as the change in the electrode frequency. The developed QCM sensor showed a dynamic linear range from 10 nM to 100 nM and a low detection limit of 8.6 nM. The sensor exhibited excellent selectivity when challenged with common interfering anions and cations.

Preparation of a quartz microbalance sensor based on molecularly imprinted polymers and its application in formaldehyde detection.

Quartz crystal microbalances (QCMs) have been widely used in the food industry, environmental monitoring, and biomedicine. Here, a molecularly imprinted QCM sensor was prepared and used for formaldehyde detection. Using polyvinyl chloride as the embedding material and tetrahydrofuran as the solvent, a QCM electrode was modified with HCHO molecularly imprinted polymers (HCHO-MIPs). The detection conditions of the sensor were optimized, and its selectivity was investigated. The theoretical calculation results revealed that the acrylamide and pentaerythritol triacrylate were potential candidate functional monomer and cross-linking agent, respectively, in the preparation of HCHO-MIPs with high adsorbability, superselectivity, and stability. According to the calculated results, a sensor had been prepared. When the pH was 7, the added mass of the HCHO-MIPs (or NIPs) was 20 mg, and the amount of PVC coating was 20 μL, the sensor exhibited good adsorption, selectivity, repeatability, high sensitivity, high accuracy, and a short response time. The lowest detection limit was 10.72 ng mL−1. The sensor exhibited higher selectivity for HCHO than for propionaldehyde and benzaldehyde. The HCHO contents in fresh shrimp samples were detected using the sensor for four cycles, and the detection rates were in the range of 97.56–98.60%. This study provided a theoretical and experimental basis for the rapid detection of HCHO.

Physico-chemical study of complexation of silver ion (Ag+) by macrocyclic molecules (hexa-Helicenes) based on statistical physics theory: new description of a cancer drug

In recent papers, it is found that the silver-[6]Helicene complex can be used as a cancer drug but the interaction silver-hexaHelicene has not yet proven. The idea of this paper is to investigate the complexation process of the [6]Helicene by the silver metal (Ag+) using three types of adsorbates. Indeed, the adsorption of silver chloride, silver nitrate and silver sulfide into the sensor films deposited on the QCM electrode are measured at three temperatures (293–333 K). Films of the [6]Helicene were deposited on the QCM resonators using spin coating method in order to obtain uniform and homogenous sensor surface. Experimental results indicated that the [6]Helicene can form a stable complex with the silver ion and that the AgCl is the appropriate adsorbate for the complexation achievement. Actually, an advanced modeling analysis by means of statistical physics adsorption models is applied to explore the new vision of the complextion system. The values of the models parameters are deduced from fitting the experimental data with the developed models. They result in confirming the experimental findings by comparing the complexation energies of the three examined systems. In particular, for the silver nitrate, the Van-der-Waals parameters explained the isotherms drop at high concentration through the lateral interactions between the adsorbates. The adsorption energies analysis showed the highest interaction AgCl-[6]Helicene. Density functional theory (DFT) simulations showed that chemical bonds take place during the adsorption of silver chloride on hexaHelicene which confirms that the [6]Helicene can function as a chiral molecular tweezer of the univalent cationic silver.

Quartz Crystal Microbalance Aptasensor for the Molecular Detection of Dengue Virus

Introduction Dengue fever is the major public health concern in the Philippines and is endemic in all regions of the country in which all serotypes are present[1]. Despite the widespread nature of the virus transmitted by mosquitos, molecular diagnostics have significant limitations. An early diagnosis of dengue may prevent severe clinical complications associated with the infection. A novel genosensor using an aptamer for the detection of dengue virus serotypes 1-4 was demonstrated in this study. Method This genosensor composed of a thiolated aptamer immobilized on a 10MHz gold electrode QCM targeting the 3’ untranslated region of all complete genome sequences of dengue virus available in GenBank[2]. Aptamer concentration, buffer pH and ionic strength of buffer solutions were optimized to achieve the highest sensitivity of the QCM genosensor. Atomic force microscopy [3], electrochemical impedance spectrum (EIS) measurements and cyclic voltammetry (CV) were used to characterize the surface of the QCM electrode prepared[4]. Evaluation of binding kinetics and affinity study of the Aptamer-dengue genome was determined by surface plasmon resonance (SPR)[5]. Results and Conclusions Atomic Force Microscopy (AFM), Electrochemical Impedance (EIS) and Cyclic Voltammetry (CV) have been used to characterize the surface of the PQC electrode. The AFM results indicate that the thiolated aptamer onto the electrode has decreased the roughness of gold surface by forming a uniform layer on the electrode surface. The EIS and CV results indicate that the charge transfer resistance is increased after immobilization of the aptamer onto the bare gold electrodes, indicating that direct S-Au bonding via disulfide moieties and the negative charge of protein have blocked the electrode reaction between Fe (II) and Fe (III). The results showed that the developed aptamer QCM sensor has an optimum concentration range of the target dengue genome is 20–100 mg/ml for hybridization. This work demonstrated the possibility of using an immobilized aptamer on a QCM and other sensor platforms for the direct detection of the RT-PCR products of dengue virus serotypes 1-4.

Label-free analytical performances of a peptide-based QCM biosensor for trypsin.

A label-free biosensor was fabricated for the detection of trypsin by using a peptide-functionalized quartz crystal microbalance gold electrode. The synthetized peptide chains were immobilized tightly on the QCM electrode via a self-assembly method, which formed a thin and approximate rigid layer of peptides. The detection signal was achieved by calculating the mass changes on the QCM electrode because the peptide chains could be specifically cleaved in the carboxyl terminuses of arginine and lysine by trypsin. When gold nanoparticles were coupled to the peptide chains, the sensing signal would be amplified 10.9 times. Furthermore, the sensor interface shows a lower resonance resistance change when the peptide chain is immobilized horizontally. Independent detections in parallel on different electrodes have a wide linear range. Under the optimum conditions, the signal-amplified biosensor allowed the measurement of trypsin over the range of 0-750 ng mL-1 with a detection limit of 8.6 ng mL-1. Moreover, for screening the inhibitor of trypsin, the IC50 values were obtained to be 1.85 μg mL-1 for benzamidine hydrochloride and 20.5 ng mL-1 for the inhibitor from soybean.

Electrospun polyvinyl acetate nanofiber modified quartz crystal microbalance for detection of primary alcohol vapor

Analytical parameters including sensitivity in the QCM sensing system are critical to understand its sensing mechanism. We investigated the QCM analytical parameters after the QCM electrode surface was modified with polyvinyl acetate (PVAc) nanofiber for primary alcohols’ vapor detection. The PVAc nanofibers were prepared by electrospinning method and were observed using a scanning electron microscope (SEM). The modified QCM sensing system was used to test the vapor of primary alcohols. The QCM sensitivity for methanol, ethanol, n-propanol, n-butanol, and n-pentanol was found to be 6.8 ± 0.4, 13.1 ± 0.5, 25.2 ± 1.1, 39.7 ± 1.9, and 51.9 ± 2.6 Hz mg−1 L, respectively. The limit of detection for n-pentanol vapor was 0.023 mg/L, which the highest of all. The shortest response time was 4 s, when it was exposed to methanol vapor. The intermolecular interaction between the active surface and gaseous molecules is believed to responsible for frequency shift. The concentration of the analyte in the air is linearly correlated to the compound vapor pressure. The results could provide a new promising way to detect vapor of structurally similar of volatile organic compounds (VOCs) by QCM.

In-situ polymerization of metal organic frameworks-derived ZnCo2O4/polypyrrole nanofilm on QCM electrodes for ultra-highly sensitive humidity sensing application

A metal organic frameworks (MOFs)-derived ZnCo2O4/polypyrrole (ZnCo2O4/PPy) nanocomposite coated QCM humidity sensor with high performance was demonstrated in this paper. The element composition, morphology and nanostructure of the ZnCo2O4/PPy nanocomposite were characterized by energy dispersive spectroscopy (EDS), FT-IR spectrums, X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). All humidity sensing tests of ZnCo2O4/PPy QCM sensor were performed under room temperature environment. The experimental results revealed that the ZnCo2O4/PPy sensor exhibits excellent humidity sensing behavior. Comparing with pure PPy and pure ZnCo2O4 sensor, ZnCo2O4/PPy film coated QCM humidity sensor displays higher sensitivity (58.4 Hz/%RH) and better response/recovery properties (8 s/7 s). Besides, the equivalent circuit elements parameters of ZnCo2O4/PPy coated QCM sensor at various RH environments were determined by the modified Butterworth-Van-Dyke (BVD) equivalent circuit model. And the adsorption kinetics of water molecules was investigated by bimodal exponential kinetic adsorption model.

Evaluation of Three Peptide Immobilization Techniques on a QCM Surface Related to Acetaldehyde Responses in the Gas Phase.

The quartz-crystal microbalance is a sensitive and universal tool for measuring concentrations of various gases in the air. Biochemical functionalization of the QCM electrode allows a label-free detection of specific molecular interactions with high sensitivity and specificity. In addition, it enables a real-time determination of its kinetic rates and affinity constants. This makes QCM a versatile bioanalytical screening tool for various applications, with surface modifications ranging from the detection of single molecular monolayers to whole cells. Various types of biomaterials, including peptides mapping the binding sites of olfactory receptors, can be deposited as a sensitive element on the surface of the electrodes. One of key ways to ensure the sensitivity and accuracy of the sensor is provided by application of an optimal and repeatable method of immobilization. Therefore, effective sensors operation requires development of an optimal method of deposition. This paper reviews popular techniques (drop-casting, spin-coating, dip-coating) for coating peptides on piezoelectric crystals surface. Peptide (LEKKKKDC-NH2) derived from an aldehyde binding site in the HarmOBP7 protein was synthesized and used as a sensing material for the biosensor. The degree of deposition of the sensitive layer was monitoring by variations in the sensors frequency. The highest mass threshold for QCM measurements for peptides was approximately 16.43 µg·mm−2 for spin coating method. Developed sensor exhibited repeatable response to acetaldehyde. Moreover, responses to toluene was observed to evaluate sensors specificity. Calibration curves of the three sensors showed good determination coefficients (R2 > 0.99) for drop casting and dip coating and 0.97 for the spin-coating method. Sensors sensitivity vs. acetaldehyde were significantly higher for the dip-coating and drop-casting methods and lower for spin-coating one.

In situ chemical deposition of PPy/NDSA and PPy/DBSA layers on QCM electrodes: synthesis, structural, morphological and ammonia sensing performances study

Aiming to detect ammonia vapor, polypyrrole (PPy) thin layers were in situ coated on AT-cut 10 MHz quartz crystal microbalance QCM electrode by a facile chemical polymerization process using two organic acids as dopants, i.e. dodecylbenzene sulfonic acid (BDSA) and 1–5 naphthalene disulfonic acid (NDSA). Then after, polymers structural and morphological features were determined by FTIR, Raman spectroscopy and Scanning Electron Microscopy (SEM) techniques. Ammonia vapor sensing tests were related to QCM frequency changes recorded upon vapor adsorption and desorption on PPy films, it was found that frequency shifts varied linearly with both vapor concentration expressed in part per million (ppm) and polymer’s thin layer thickness given in nanometer (nm). This fact has been assumed to be mainly related to the electrostatic interactions established between ammonia vapor molecules and the polymer dopant agents. Tests have shown that films based PPy/NDSA exhibit high sensitivity around 3 ppm and detection limit of 4 ppm over films based PPy/DBSA. Interestingly, an excellent recovery time less than 3 min has been also recorded with PPy/NDSA thin layers. Moreover, when applying Fick’s second law, they have also shown a high diffusion constant.

P-Hexafluoroisopropanol phenyl functionalized graphene for QCM based detection of dimethyl methylphosphonate, a simulant of the nerve agent sarin.

The detection of DMMP (dimethyl methylphosphonate, a simulant of nerve agent sarin) was performed by using p-hexafluoroisopropanol phenyl (HFIPP) functionalized graphene (GR) via hydrogen bond interactions. For this, the HFIPP moiety was covalently functionalized on the surface of GR by a diazo reaction. The HFIPP-GR film-modified QCM electrodes were fabricated and their sensing characteristics towards DMMP were investigated. The proposed sensor showed good response towards sensing DMMP vapor at room temperature. In order to see the effect of HFIPP derivatives on DMMP vapor sensing, a comparative study was also conducted with unfunctionalized graphene. The sensitivity and detection limit of the HFIPP-GR sensor against DMMP vapors were 12.24 Hz ppm−1 and 150 ppb respectively. The HFIPP-GR coated sensors showed good selectivity towards sensing DMMP vapors when compared with common organic vapors.

High performance formaldehyde detection based on a novel copper (II) complex functionalized QCM gas sensor

In order to construct highly sensitive and stable QCM gas sensor, we present an innovative strategy. Firstly, we employ a physical vapor deposition (PVD) method to coat a thin layer of copper on the surface of QCM silver electrode. Then, a novel metal complex as sensing material, [Cu(DDS)2(Cl)2(MeOH)2], is grown in-situ on the copper layer to ensure a strong coupling between metal complex sensing materials and QCM electrode. The sensing material is characterized through following instruments: elemental analysis, single crystal XRD, SEM, PXRD, IR spectra and TGA. The sensing test results indicate that the QCM sensor exhibit high sensitivity and selectivity toward gaseous formaldehyde because of a reversible interaction between amino group and HCHO molecule. The detection limit reaches down to 50ppb. This implies the facile-fabricated sensor has a great potential in the area of rapid, stable, sensitive, and selective formaldehyde on-site detection. Based on the temperature-varying micro-gravimetric experiment, enthalpy change is quantitatively obtained, which indicates that the interaction between the complex and HCHO molecules belongs to chemical adsorption. The values of ΔH calculated by Gaussian 09 indicate that chemical adsorption between NH2 group and aldehyde group play a leading role in detecting selectively formaldehyde gas.

Leveraging Local pH Gradients to Optimize Electrochemical Lanthanum Accumulation

Lanthanum accumulation on a polarized unmodified electrode has been shown to be under local pH control. Cyclic voltammograms in unbuffered aqueous solutions have shown that the local pH varies up to 10 pH units compared to the bulk solution, while buffered solutions vary ~ 6 pH units. This is due to lanthanum’s theoretical deposition potential being well outside the working region of water, resulting in the consumption of proton-equivalents during water electrolysis. Previous work has shown that high pH induces lanthanum to become insoluble and thus unavailable for electrochemical reactions. The large background current from water electrolysis hides lanthanum’s electrochemical response during cyclic voltammetry experiments, so the electrochemical quartz crystal microbalance (eQCM) was used to track the frequency shift resulting from attached mass on the surface of carbon-coated QCM electrodes. Varying levels of buffering agent (acetate buffer) resulted in modulation of the onset deposition potential as well as the length of the stable film region during anodic stripping. This shows that bulk parameters (pH, buffer strength) can be used to determine the potentials at which lanthanum will deposit and strip on unmodified carbon electrodes.

Detection and kinetics of methylamine on chitosan film coated quartz crystal microbalance electrode

Chitosan (CHS) coated-quartz crystal microbalance (QCM) electrode was utilized as a sensor for the recognition of methylamine. A homogeneous smooth thin film of CHS coating on the QCM electrode was prepared by the drop casting of CHS solution. The frequency shifts (Δf) of the QCM due to the adsorption of methylamine on the CHS film were measured as a function of concentrations. Calibration curve was plotted which shows a linear relationship of Δf (Hz) versus the methylamine concentrations in the range of 0.5–2.3 mg L−1. In addition to the linearity (R2 = 0.994) and short response time, the sensor shows a high sensitivity, reproducibility and fast reversibility. The kinetics of methylamine adsorption onto CHS film were calculated. Based on the dynamic analysis of adsorption, the association constant of methylamine vapor molecules with CHS films was estimated to be 11386.3 M−1. The diffusion coefficients of various aliphatic amines were calculated and showed that methylamine exhibits the highest diffusion coefficient value compared to dimethylamine and diethylamine.

Binding interaction and gelation in aqueous mixtures of poly(N-isopropylacrylamide) and hectorite clay.

The binding interaction between poly(N-isopropylacrylamide) (PNIPAm) chains and hectorite clay platelets was directly detected by quartz crystal microbalance with dissipation (QCM-D) to explore the cross-linking mechanism in the strong nanocomposite hydrogel (NC gel), which is in situ polymerized with NIPAm in the clay suspension. PNIPAm chains were allowed to be adsorbed on the gold surface of the QCM electrode. A large frequency shift Δf in the QCM as introducing the clay indicated that a large amount of clay platelets were adsorbed on the deposited PNIPAm layer. The relationship between the dissipation shift ΔD and Δf revealed that the adsorption included two steps of fast initial buildup and following densification of the clay platelets. In dilute aqueous mixtures, the PNIPAm chain and clay formed aggregates as observed from the hydrodynamic diameter. A gelation state diagram was established for concentrated aqueous PNIPAm-clay mixtures. The Raman spectrum pointed out the conformation change of the PNIPAm chains in aqueous solutions when the clay was added, which would be caused by the adsorption of PNIPAm chains to the clay platelets.

Real-time multianalyte biosensors based on interference-free multichannel monolithic quartz crystal microbalance.

In this work, we design, fabricate and characterize a new interference-free multichannel monolithic quartz crystal microbalance (MQCM) platform for bio-sensing applications. Firstly, interference due to thickness-shear vibration mode coupling between channels in MQCM array is effectively suppressed by interposing a polydimethylsiloxane wall between adjacent QCM electrodes on a quartz substrate to form inverted-mesa-like structure. In addition, the electrical coupling due to the electrical impedance of solution is diminished by extending the flow path between them with an extended-design flow channel. The electrical testing results show that individual QCM signal is unaffected by those of adjacent channels under liquid loading, signifying the achievement of interference-free MQCM. The MQCM is applied for multi-analyte biosensing of IgG and HSA. The anti-IgG and anti-HSA are separately immobilized on two adjacent QCM electrodes, which are subsequently blocked with BSA to avoid unspecific binding. The MQCM biosensors are tested with single- and double-analyte solutions under continuous flow of buffer. The IgG and HSA QCM sensors only show frequency shift responses to their corresponding analytes and there are very small cross frequency shifts due to remnant unspecific binding. Moreover, MQCM sensors show approximately linear frequency shift response with analyte concentration. Therefore, the developed MQCM platform is promising for real-time interference-free label-free detection and quantification of multiple bio-analytes.

One-step fabrication of ammonia sensor by electrospinning PS-b-PMA nanofibers on quartz crystal microbalance

One-step process of poly (styrene-block-maleic acid) (PS-b-PMA) nanofibers/quartz crystal microbalance (QCM) combined system was proposed as a novel ammonia detection in this research. The PS-b-PMA nanofibrous membranes were deposited on the QCM electrode via electrospinning technique directly, which could retain the intrinsic high specific surface area of PS-b-PMA nanofibers, and much simplify ammonia detection process. Experimental results showed that the specific surface area (SSA) of the PS-b-PMA membranes was controllable by adjusting the weight ratios of the mixture solvents of acetone/N,N-dimethylformamide. The PS-b-PMA nanofibers/QCM sensor system exhibited excellent ammonia sensing performances, such as rapid response, excellent reversibility and high sensitivity (low detection limit at 1.5ppm) at room temperature. The frequency responses showed a linear growth with the injected ammonia concentration ranging from 1.5ppm to 50ppm. In addition, a linear relationship between the frequency responses per unit injected gas concentration (Hz/ppm) and different SSAs was found. These sensing results indicated that QCM sensor on the basis of PS-b-PMA nanofibers had good potential for further application in ammonia detection and remote monitoring in real time.

One-Step Process of NH<sub>3 </sub>detection Based on Nanofibrous Membranes/QCM System

One-step process of poly(styrene-co-maleic acid)(PS-PMA) nanofibers/QCM combination system was proposed as a novel ammonia detection in this study. Nano-sized PS-PMA fibers were deposited on the QCM electrodes via electrospinning technique directly, which can retain the original high specific surface area of PS-PMA nanofibers, and simplify NH3detection process significantly. Experimental results showed that this new PS-PMA nanofibers/QCM sensor exhibited excellent ammonia sensing performances, such as rapid response and good reproducibility. Moreover, the carboxyl content on the surface of PS-PMA membranes was also determined as well as the specific surface area in order to study the structure-properties relationships.

Atmospheric pressure plasma polymers for tuned QCM detection of protein adhesion

Our efforts have been concentrated in preparing plasma polymeric thin layers at atmospheric pressure grown on Quartz Crystal Microbalance—QCM electrodes for which the non-specific absorption of proteins can be efficiently modulated, tuned and used for QCM biosensing and quantification. Plasma polymerization reaction at atmospheric pressure has been used as a simple and viable method for the preparation of QCM bioactive surfaces, featuring variable protein binding properties. Polyethyleneglycol (ppEG), polystyrene (ppST) and poly(ethyleneglycol–styrene) (ppST–EG) thin-layers have been grown on QCM electrodes. These layers were characterized by Atomic Force Microscopy (AFM), Contact angle measurements, Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). The plasma ppST QCM electrodes present a higher adsorption of Concanavalin A (ConA) and Bovine Serum Albumin (BSA) proteins when compared with the commercial coated polystyrene (ppST) ones. The minimum adsorption was found for ppEG, surface, known by their protein anti-fouling properties. The amount of adsorbed proteins can be tuned by the introduction of PEG precursors in the plasma discharge during the preparation of ppST polymers.