Frequency Shift Of Quartz Crystal Microbalance Research Articles

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.

An Adaptive Measurement System for the Simultaneous Evaluation of Frequency Shift and Series Resistance of QCM in Liquid.

In this paper, a novel measurement system based on Quartz Crystal Microbalances is presented. The proposed solution was conceived specifically to overcome the measurement problems related to Quartz Crystal Microbalance (QCM) applications in dielectric liquids where the Q-factor of the resonant system is severely reduced with respect to in-gas applications. The QCM is placed in a Meacham oscillator embedding an amplifier with adjustable gain, an automatic strategy for gain tuning allows for maintaining the oscillator frequency close to the series resonance frequency of the quartz, which is related in a simple way with the physical parameters of interest. The proposed system can be used to monitor simultaneously both the series resonant frequency and the equivalent electromechanical resistance of the quartz. The feasibility and the performance of the proposed method are proven by means of measurements obtained with a prototype based on a 10-MHz AT-cut quartz.

Topology Challenge for the Assessment of Living Cell Deposits with Shear Bulk Acoustic Biosensor.

Shear bulk acoustic type of resonant biosensors, such as the quartz crystal microbalance (QCM), give access to label-free in-liquid analysis of surface interactions. The general understanding of the sensing principles was inherited from past developments in biofilms measurements and applied to cells while keeping the same basic assumptions. Thus, the biosensor readouts are still quite often described using ‘mass’ related terminology. This contribution aims to show that assessment of cell deposits with acoustic biosensors requires a deep understanding of the sensor transduction mechanism. More specifically, the cell deposits should be considered as a structured viscoelastic load and the sensor response depends on both material and topological parameters of the deposits. This shifts the paradigm of acoustic biosensor away from the classical mass loading perspective. As a proof of the concept, we recorded QCM frequency shifts caused by blood platelet deposits on a collagen surface under different rheological conditions and observed the final deposit shape with atomic force microscopy (AFM). The results vividly demonstrate that the frequency shift is highly impacted by the platelet topology on the bio-interface. We support our findings with numerical simulations of viscoelastic unstructured and structured loads in liquid. Both experimental and theoretical studies underline the complexity behind the frequency shift interpretation when acoustic biosensing is used with cell deposits.

Isolation and detection of exosomes via AAO membrane and QCM measurement

Increasing evidence suggests that exosomes are secreted by the majority of cancer cells and present in myriad biological fluids. They have been recognized as a potential new class of biomarkers for disease prognosis and diagnosis. The isolation and quantification of exosomes are crucial in facilitating exosome research. This paper presents a filtration method to isolate exosomes via anodic aluminum oxide (AAO) membrane and to quantify them using a quartz crystal microbalance (QCM). The filtration chip was designed to have two nano-porous AAO membranes sandwiched in between PDMS layers. In this study, fetal bovine serum (FBS) was adopted as the analyte for exosomal bioassays. The experimental results show that AAO membrane has a hydrophilic surface able to sieve exosomes by gravity. The spectrophotometric measurement and the result of gel electrophoresis confirmed that most of proteins in FBS were removed, leaving pure and undamaged exosomes for further analyses. Then, QCM measurement of the filtered FBS solution can give a quantitative analysis of exosome concentration. The relationship between the exosome concentration and its corresponding QCM's frequency shift is linear and monotonic. QCM is still sensitive to measure low concentration of exosomes.

A Numerical Study on the Effect of Particle Surface Coverage on the Quartz Crystal Microbalance Response.

The quartz crystal microbalance (QCM) is a surface-sensitive measurement technique to characterize adsorption processes at solid-fluid interfaces. While QCM measurements are routinely applied to study homogeneous thin films, characterizing heterogeneous films of adsorbed particles remains challenging because QCM is sensitive to not only the mass of adsorbed particles but also to that of hydrodynamically coupled fluid. To extract information about adsorbed particles, it is necessary to model these hydrodynamic effects, however, current QCM models are restricted to the limit of either a very low surface coverage or to the extrapolated limit of saturation coverage. Herein, we investigated QCM measurement responses in the intermediate surface coverage regime, by conducting lattice Boltzmann simulations of monodisperse, spherical particles that are attached to an oscillating surface. From the simulations, we relate the overtone-dependent QCM frequency and bandwidth shifts to particle size, interparticle distance, and the relevant hydrodynamic length scale. The corresponding results are in qualitative agreement with experimental QCM data for sub-100 nm, gel-phase liposomes. Furthermore, the data provide a theoretical basis for extracting particle sizes from QCM data in the high surface coverage limit.

Quartz Crystal Microbalance Model for Quantitatively Probing the Deformation of Adsorbed Particles at Low Surface Coverage.

Characterizing the deformation of nanoscale, soft-matter particulates at solid-liquid interfaces is a demanding task, and there are limited experimental options to perform quantitative measurements in a nonperturbative manner. Previous attempts, based on the quartz crystal microbalance (QCM) technique, focused on the high surface coverage regime and modeled the adsorbed particles as a homogeneous film, while not considering the coupling between particles and surrounding fluid and hence resulting in an underestimation of the known particle height. In this work, we develop a model for the hydrodynamic coupling between adsorbed particles and surrounding fluid in the limit of a low surface coverage, which can be used to extract shape information from QCM measurement data. We tackle this problem by using hydrodynamic simulations of an ellipsoidal particle on an oscillating surface. From the simulation results, we derived a phenomenological relation between the aspect ratio r of the absorbed particles and the slope and intercept of the line that fits instantaneous, overtone-dependent QCM data on (δ/a, -Δf/n) coordinates where δ is the viscous penetration depth, a is the particle radius, Δf is the QCM frequency shift, and n is the overtone number. The model was applied to QCM measurement data pertaining to the adsorption of 34 nm radius, fluid-phase and gel-phase liposomes onto a titanium oxide-coated surface. The osmotic pressure across the liposomal bilayer was varied to induce shape deformation. By combining these results with a membrane bending model, we determined the membrane bending energy for the gel-phase liposomes, and the results are consistent with literature values. In summary, a phenomenological model is presented and validated in order to show for the first time that QCM experiments can quantitatively measure the deformation of adsorbed particles at low surface coverage.

Ultrasensitive Detection of DNA and Ramos Cell Using In Situ Selective Crystallization Based Quartz Crystal Microbalance.

Herein we introduce the first successful assay of biomolecule by in situ selective crystallization based quartz crystal microbalance (QCM). Selective crystallization of CaCO3 on QCM sensor surface was utilized as an efficient mass amplification strategy and enhanced the sensitivity of QCM significantly. High specificity is guaranteed by the cooperation of two functional groups: -N(CH3)3 and -COOH. Passivation of sensor surface is realized by self-assembly of -N(CH3)3, which effectively inhibited the nonspecific crystallization. The DNA target is detected through hybridization of probe DNA labeled with -COOH, which can effectively promot the in situ surface crystallization of CaCO3. The concentration of target DNA is reflected by the frequency shift of QCM which is directly induced by the surface crystallization. The selective crystallization based QCM platform is simple, straightforward, high sensitive, and high specific. We demonstrate the excellent LOD (2 aM DNA) and a linear range of 10aM to 1 nM for DNA. Detection of Ramos cells are also realized with a LOD of five cells and a linear range of 5-6000 cells.

A quartz crystal microbalance sensor for endotoxin assay by monitoring limulus amebocyte lysate protease reaction

Endotoxin is able to trigger strong innate immune responses by interacting with specific receptors on immune cells. Therefore, accurate and rapid detection of endotoxin is of primary importance. In this study, endotoxin induced viscosity variation of limulus amebocyte lysate (LAL) reagent is monitored by a quartz crystal microbalance (QCM) sensor with enhanced signal. Based on the analysis of the relationship between endotoxin concentration and QCM frequency shift with time, an effective sensing strategy is developed for endotoxin assay, which shows excellent sensitivity and specificity in the linear detection range from 0.005 to 10 EU mL−1. Moreover, this QCM sensor could be reused after a simple regeneration procedure. Therefore, it has potential practical utility for endotoxin determination in various applications.

A dual usage smart sorbent/recognition element based on nanostructured conducting molecularly imprinted polypyrrole for simultaneous potential-induced nanoextraction/determination of ibuprofen in biomedical samples by quartz crystal microbalance sensor

A selective detection strategy presented for ibuprofen (IBP) as a model anionic drug based on potential-induced uptake/release of the drug by a smart conducting molecularly imprinted polymer (CMIP) as sorbent/recognition element for simultaneous nanoextraction and determination of IBP in plasma samples by quartz crystal microbalance (QCM) without any additional treatment. The nanostructured CMIP film was directly synthesized by cyclic voltammetry method on the QCM gold electrode and applied for selective uptake of IBP from sample solution by the aid of positive potential and its successive release by negative potential. The frequency shifts of QCM in uptake and release steps were monitored and used as sensor signal for IBP sorption/determination. In the biomedical samples, the frequency shift of release step exhibited more linear and interference free behavior. Several parameters like synthesis conditions, potential, time, pH of uptake and release and exchange anion type were explored to achieve the optimal sorption/determination conditions. The calibration graph was linear in the ranges of 1–10 and 10–100μmolL−1, the limit of detection was 0.05μmolL−1 and RSD% (n=5) was lower than 3%. The proposed method was successfully applied for quantification of trace amounts of IBP in biomedical samples and validated by HPLC method. The CMIP/QCM sensor shows a good long-term stability and the sensor response drops 10% after 80 cycles of measurement in 8th week.

Molecular imprinted polyacrylic acids based QCM sensor array for recognition of organic acids in body odor

In present research, a novel volatile acids imprinted 3-element quartz crystal microbalance (QCM) sensor array has been designed for the selective recognition of organic acids odor. The target organic acids have been previously assigned as significant biomarker in human body odor and human waste odor. This is also confirmed with solid phase micro-extraction gas chromatography-mass spectrometer (SPME-GC-MS) characterization of body odor samples in present study. Polyacrylic acid (PAA) polymer has been used to prepare three selective molecular imprinted polymers (MIPs). MIP films were coated on the surface of QCM electrodes. Performance of MIP based QCM sensor array has been studied by exposing different concentrations of propenoic acid, hexanoic acid, and octanoic acid in singly and their binary mixtures. The response time and recovery time were 5s and 14s, respectively for one of the MIP coated QCM sensor to a typical binary mixture of acids odor. Frequency shift of QCM due to odor molecules adsorption in thin layer of MIP was measured as sensor signal. Sensor signal response matrix has been analyzed with principal component analysis (PCA) and support vector machine (SVM) methods for the odor recognition. The acid odors were identified effectively in principal component (PC) space. SVM classifier results 70–76% recognition rate for three classes of binary odor. While excluding the binary mixture of propenoic acid and octanoic acid the average recognition rate approaches to 95% in 3-fold cross validation of SVM classifier.

Aliphatic amines vapours detection by quartz crystal microbalance sensor

Polymerized castor oil based quartz crystal microbalance (QCM) sensors are fabricated for the detection of aliphatic amines vapours. The sensors are fabricated by solution dip-dry method followed by heating at 100°C under argon atmosphere to get cross linked film on QCM surface. A series of sensors with various weight percentages of benzoyl peroxide added to the solution are fabricated for the optimization of the sensitivities of the polymeric films. It is found that the sensor fabricated with a solution having 1.40% (w/v) of benzoyl peroxide has high sensitivity for vapours of organic compounds and are very effective for aliphatic amines vapours. The adsorption of the vapours to the surface of the QCM sensor is determined by the frequency shift of QCM. The frequency shift linearly increases with the gas vapour concentrations from 5 to 250ppm at room temperature. The sensitivity-values of methylamine, ethylamine, tert-butylamine, diethylamine, triethylamine and ammonia vapours are 0.917, 0.657, 0.655, 0.543, 0.407 and 0.397Hz/ppm respectively. The structure of the polymeric film is studied by FTIR spectra. The surface morphologies of the QCM sensor before and after gas adsorption are characterized by AFM, which indicates mild swelling of the surface after adsorption and it is reversible.

Note: Sensitivity multiplication module for quartz crystal microbalance applications

In this Note, a novel sensitivity multiplication module was added to classical quartz crystal microbalance (QCM). The purpose is to increase QCM frequency shift without changing nominal frequency of the quartz crystal resonator or nominal frequency value delivered to the frequency counter. Allan deviance measurement confirms that the multiplication of the frequency shift is limited by the quartz crystal loads with direct effect in quartz crystal quality factor and oscillator stability. An experimental implementation of this new sensitivity multiplication module that can increase up to six times the frequency shift of the QCM was experimentally investigated using different load conditions.

Environmental Monitoring of Acetic Acid Gas by Thin Film Polyaniline Sensor

The acetic acid gas, as a kind of organic pollutants, can cause severe damage to both people and materials, and has received much attention as a problem. In this paper, a QCM sensor coated with the polyaniline film was employed as a gas sensor to detect the content of the acetic acid gas. The polyaniline film was electrochemical polymerized in situ on the silver electrode surface of quartz crystal microbalance (QCM) sensor and employed as sensing film for detection of acetic acid gas. Responses of the QCM sensors coated with the polyaniline film to different concentrations of the acetic acid gas as a function of time was presented. And the frequency shifts of QCM showed a linear correlation of the concentration of acetic acid gas. The result of our experiment fully indicated that the QCM sensor with the polyaniline film appear to respond effectively and sensitively to the acetic acid gas within the range of experimental concentrations. Such sensor will have potential application in our living environment for real-time monitoring.

Solid-phase PEGylation of an immobilized protein cage on polyelectrolyte multilayer

We used a quartz crystal microbalance (QCM) to quantitatively characterize solid-phase poly(ethylene glycol) modification (PEGylation) of apoferritin that was electrostatically immobilized on the surface of a polyelectrolyte multilayer. The solid-phase PEGylation processes were monitored by analyzing QCM frequency shifts, which showed that the PEG chains were covalently introduced onto the surface of the immobilized apoferritin. We investigated the effect of PEG concentration, PEG molecular weight, and two-dimensional coverage of the immobilized apoferritin on the solid-phase PEGylation process in addition to the surface properties of the PEGylated apoferritin film, such as wettability and protein adsorption capacity. Since the reaction field is more spatially restricted in solid-phase PEGylation than in traditional aqueous-phase PEGylation, this study shows that a ferritin protein cage is potentially useful as a tailored building block, one that has well-defined structures different from the PEGylated ferritin prepared by an aqueous-phase approach.

Microgel-based etalon coated quartz crystal microbalances for detecting solution pH: The effect of Au overlayer thickness

Poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgels were “painted” on the Au electrode of a quartz crystal microbalance (QCM). Another Au layer (overlayer) was subsequently deposited on the microgel layer. This structure is known as a microgel-based etalon. These devices have been shown to exhibit optical properties (i.e., color) that depend on solution pH and temperature, among other things. Previously, we measured QCM frequency shifts that are a result of solution pH changes; the frequency shifts are a direct result of the pH dependent solvation state of the microgels that make up the etalon. In fact, the shifts observed for the etalons were much greater in magnitude than just a microgel layer immobilized on the QCM crystal without the Au overlayer. We reasoned that the Au overlayer lead to an enhancement of the observed frequency change due to its mass. In this submission we investigate how the Au overlayer thickness (mass) affects the observed sensitivity to solution pH. We found that the change in QCM resonant frequency depended dramatically on the mass of the Au overlayer.

Scanning tunneling microscope-quartz crystal microbalance study of temperature gradients at an asperity contact

Investigations of atomic-scale friction frequently involve setups where a tip and substrate are initially at different temperatures. The temperature of the sliding interface upon contact has thus become a topic of interest. A method for detecting initial tip-sample temperature differences at an asperity contact is described, which consists of a scanning tunneling microscope (STM) tip in contact with the surface electrode of a quartz crystal microbalance (QCM). The technique makes use of the fact that a QCM is extremely sensitive to abrupt changes in temperature. In order to demonstrate the technique's capabilities, QCM frequency shifts were recorded for varying initial tip-substrate temperature differences as an STM tip was brought into and out of contact. The results are interpreted within the context of a recent model for thermal heat conduction at an asperity contact, and it is concluded that the transient frequency response is attributable to small changes in temperature close to the region of contact rather than a change in the overall temperature of the QCM itself. For the assumed model parameters, the results moreover reveal substantial temperature discontinuities at the boundary between the tip and the sample, for example, on the order of 10-15 °C for initial temperature differences of 20 °C.

Determination of functionalized gold nanoparticles incorporated in hydrophilic and hydrophobic microenvironments by surface modification of quartz crystal microbalance

In this study, plasma deposition methods were used to immobilize Au electrode of a quartz crystal microbalance (QCM) to create different microenvironments for mass measurement of various modified Au nanoparticles (AuNPs). AuNPs were modified by 11-mercaptoundecanoic acid (MUA) and 1-decanethiol (DCT) for potential applications to drug release, protective coatings, and immunosensors. We aimed to develop a highly sensitive and reliable method to quantify the mass of various modified AuNPs. The surface of AuNPs and Au electrode was coated with polymer films, as determined by Fourier transform infrared spectroscopy and atomic force microscopy. Measurements obtained for various AuNPs and the plasma-treated surface of the Au electrode were compared with those obtained for an untreated Au electrode. According to the resonant frequency shift of QCM, a linear relationship was observed that significantly differed for AuNPs, MUA-AuNPs, and DCT-AuNPs (R2 range, 0.94–0.965, 0.934–0.972, and 0.874–0.9514, respectively). Compared to inductively coupled plasma and micro-computerized tomography, the QCM method with plasma treatment has advantages of real-time monitoring, greater sensitivity, and lower cost. Our results demonstrate that surface modifications measured by a QCM system for various modified AuNPs were reliable.

Humidity sensing properties of ZnO colloid crystals coated on quartz crystal microbalance by the self-assembly method

Abstract ZnO colloid crystals were coated on quartz crystal microbalance (QCM) by the self-assembly method. Field emission scanning electron microscopy and X-ray diffraction were used to analyze the morphology and crystal structure of ZnO colloid crystals. The sensing behavior was examined by measuring the resonance frequency shifts of QCM. The device exhibits excellent humidity sensing properties in the whole humidity range from 11% to 95%, such as good linearity, fast response time and recovery time, excellent reliability, and long-term stability. We also discussed the adsorption and desorption of water molecules on ZnO colloid crystals at different humidity conditions.

Deliquescence Phase Transition Measurements by Quartz Crystal Microbalance Frequency Shifts

Measurements of the hygroscopic properties of aerosols are needed to better understand the role of aerosols as cloud condensation nuclei. Several techniques have been used to measure deliquescence (solid to liquid) phase transitions in particular. In this study, we have measured the deliquescence relative humidity (DRH) of organic and inorganic salts, organic acids (glutaric and succinic acid), and mixtures of organic acids with ammonium sulfate using a quartz crystal microbalance (QCM). The QCM allows for measurement of the deliquescence phase transition due to inherent measurement differences between solids and liquids in the oscillation frequency of a quartz crystal. The relative humidity dependent frequency measurements can be used to identify compounds that adsorb monolayer amounts of water or form hydrates prior to deliquescence (e.g., lithium chloride, potassium and sodium acetate). Although the amount of water uptake by a deliquescing material cannot be quantified with this technique, deliquescence measurements of mixtures of hygroscopic and nonhygroscopic components (e.g., ammonium sulfate and succinic acid (DRH > 95%)) show that the mass fraction of the deliquescing portion of the sample can be quantitatively determined from the relative change in oscillation frequency at deliquescence. The results demonstrate the use of this technique as an alternative method for phase transition measurements and as a direct measurement of the mass fraction of a sample that undergoes deliquescence. Further, deliquescence measurements by the QCM may provide improved understanding of discrepancies in atmospheric particle mass measurements between filter samples and the tapered element oscillating microbalance given the similar measurement principle employed by the QCM.

Rapid Detection of Staphylococcal Enterotoxin B by Two-Dimensional Molecularly Imprinted Film-Coated Quartz Crystal Microbalance

We fabricated a two-dimensional (2D) molecularly imprinted sol-gel thin film-coated quartz crystal microbalance (QCM) for the rapid detection of staphylococcal enterotoxin B (SEB) by combining organosilanes and the template protein SEB on the surface of piezoelectric quartz crystal (PQC) Au-electrode by in-situ immobilization. The detection process was monitored by the QCM's frequency shift (Δf). The working range of this method was 1.0 × 10−1–1.0 × 103 µg/mL. The detection limit was 6.1 ng/mL, which was lower than that of the PQC immunosensor, and the detection period was within 0.5 h. The reproducibility of the imprinted film-coated QCM was satisfactory due to no significant statistical difference (P > 0.05) in the rapid detection of SEB between intra- and inter-batch. The selectivity of the imprinted sol-gel film showed that it could discriminate the template molecule from its analogues and other guest molecules. Compared with immunochip, the imprinted film-coated QCM is more advantageous in terms of simplicity, rapidity, low cost, and sensitivity. Moreover, in real sample analysis, the recoveries of this method were 89.4–106.63%, which can be considered a favorable and applicable method for the rapid determination of SEB in real samples.