Quartz Crystal Microbalance Signal Research Articles

Coverage Effects in Quartz Crystal Microbalance Measurements with Suspended and Adsorbed Nanoparticles.

Quartz crystal microbalance (QCM) biosensors often deal with nanoparticles suspended in the solvent at tens of nanometers above the resonator while being linked to some molecular receptor (DNA, antibody, etc.). This work presents a numerical analysis based on the immersed boundary method for the flow and QCM impedance created by an ensemble of spherical particles of radius R at varying surface coverage Θ and particle-surface gap distance Δ. The trends for the frequency Δf and dissipation ΔD shifts against Θ and Δ are shown to be determined by modifications in the structure of the perturbative flow created by the analytes. Simulations are in good agreement with a relatively large experimental database collected from the literature. Qualitative differences between the adsorbed (Δ ≈ 0) and suspended states (Δ > 0) are highlighted. In the case of adsorbed particles, deviations from the linear scaling Δf ∝ Θ are observed above Θ > 0.05 and largely depend on the specific analyte-substrate combination. Moreover, in general, ΔD(Θ) is not monotonous and usually presents a maximum around Θ ∼ 0.2. In the case of suspended analytes, the agreement with the numerical results is quantitative, indicating that the predicted scalings are universal and determined by hydrodynamics. Up to high coverage, the suspended particles present Δf ∼ Θ and ΔD ∼ Θβ, where β ≈ 0.85 is not largely dependent on R. The present findings should help forecast molecular configurations from QCM signals and have implications on QCM analyses, e.g., in the case of suspended ligands (Δf ∝ Θ), it is safe to use Δf to build Langmuir isotherms and estimate equilibrium constants. Open questions on the transition from the suspended-to-adsorbed state are discussed.

Precision Temperature Control System with Low EMI for Applications in Analyzing Thermal Properties of Highly Sensitive Piezoelectric Sensors.

A low electromagnetic interference (EMI), precision temperature control system for sensitive piezoelectric sensors stabilization and their thermal characteristics research was proposed. Quartz crystal microbalance (QCM) was chosen as the device to be tested. Recently, QCMs found use in many fields of study such as biology, chemistry, and aerospace. They often operate in harsh environments and are exposed to many external factors including temperature fluctuations, to which QCMs are highly susceptible. Such disturbances can cause undesirable resonant frequency shifts resulting in measurement errors that are difficult to eliminate. The proposed solution enables measurements of QCMs thermal characteristics, effectiveness evaluation of temperature compensation methods, and testing of the frequency stability. As a part of the developed solution, two independent temperature regulators were used: first to maintain the QCM crystal at desired temperature, and second to keep the QCM oscillator circuit at fixed temperature. The single regulator consists of a thermoelectric module (TEC) used for both heating and cooling. Two considered TEC driving methods were compared in terms of EMI and their impact on the QCM signal quality. The proposed system was examined for its temperature stabilization capability showing high stability of 11 mKp-p for one hour and the setpoint accuracy of ±15 mK in the full temperature range.

Bifunctional Magnetic Fe3O4@Cu2O@TiO2 Nanosphere-Mediated Dual-Mode Assay of PTP1B Activity Based on Photocurrent Polarity Switching and Nanozyme-Engineered Biocatalytic Precipitation Strategies.

Dysregulation of protein phosphatases is associated with the progression of various human diseases and cancers. Herein, a photoelectrochemical (PEC)-quartz crystal microbalance (QCM) dual-mode sensing platform was developed for protein tyrosine phosphatase 1B (PTP1B) activity assay based on bifunctional magnetic Fe3O4@Cu2O@TiO2 nanosphere-mediated PEC photocurrent polarity switching and QCM signal amplification strategies. The PTP1B-specific phosphopeptide (P-peptide) with a cysteine end was designed and immobilized onto the QCM Au chip via the Au-S bond. Subsequently, the Fe3O4@Cu2O@TiO2 nanosphere was connected to the P-peptide via the specific interaction between the phosphate group on the P-peptide and TiO2. After incubation with PTP1B, the dephosphorylation of the P-peptide occurred, causing some Fe3O4@Cu2O@TiO2 nanospheres to be released from the chip surface. The released magnetic Fe3O4@Cu2O@TiO2 nanospheres (labeled as R-Fe3O4@Cu2O@TiO2) were quickly separated via magnetic separation technology and attached to the Bi2S3-decorated magnetic indium-tin oxide (Bi2S3/MITO) electrode by magnetic force, inducing the switch of the photocurrent polarity of the electrode from anodic current (the Bi2S3/MITO electrode) to cathodic current (the R-Fe3O4@Cu2O@TiO2/Bi2S3/MITO electrode). Also, the nondephosphorylated P-peptide linked Fe3O4@Cu2O@TiO2 nanospheres as nanozymes with horseradish peroxidase activity to catalyze the formation of precipitation on the surface of the Au chip, leading to a frequency change of the QCM. Thus, the proposed PEC-QCM dual-mode sensing platform achieved accurate and reliable assay of PTP1B activity because of the different mechanisms and independent signal transductions. In addition, this dual-mode sensing platform can be easily extended for other protein phosphatase activity analysis and shows great potential in the early diagnosis of the protein phosphatase-related diseases and the protein phosphatase-targeted drug discovery.

Strategies for the Accurate Measurement of the Resonance Frequency in QCM-D Systems via Low-Cost Digital Techniques.

In this paper, an FPGA (Field Programmable Gate Array)-based digital architecture for the measurement of quartz crystal microbalance (QCM) oscillating frequency of transient responses, i.e., in QCM-D (QCM and Dissipation) applications, is presented. The measurement system is conceived for operations in liquid, with short QCM transient responses due to the large mechanical load. The proposed solution allows for avoiding the complex processing systems typically required by the QCM-D techniques and grants frequency resolutions better than 1 ppm. The core of the architecture is a reciprocal digital frequency meter, combined with the preprocessing of the QCM signal through mixing operations, such as a step-down of the input frequency and reducing the measurement error. The measurement error is further reduced through averaging. Different strategies are proposed to implement the proposed measurement solution, comprising an all-digital circuit and mixed analog/digital ones. The performance of the proposed architectures is theoretically derived, compared, and analyzed by means of experimental data obtained considering 10 MHz QCMs and 200 μs long transient responses. A frequency resolution of about 240 ppb, which corresponds to a Sauerbrey mass resolution of 8 ng/cm2, is obtained for the all-digital solution, whereas for the mixed solution the resolution halves to 120 ppb, with a measurement time of about one second over 100 repetitions.

A Point-of-Care Quartz Biosensor Chip for Detection of PF4/Heparin Complexes

BACKGROUNDOur recent studies indicate that circulating PF4/heparin complexes activate complement and contribute to a subsequent immune response seen in heparin-induced thrombocytopenia (HIT) (Khandelwal, Blood 2016). We undertook studies to develop a sensitive assay for measuring circulating PF4/heparin. To do so, we utilize a quartz crystal microbalance (QCM) device, which is a piezoelectric acoustic resonator that detects decreases in the resonance frequency of the quartz crystal due to the increased mass caused by molecules binding to the electrode surface. We hypothesize that a novel label-free QCM biosensor functionalized with PF4/heparin capture antibodies can provide for robust quantitative antigen detection and physical characterization of antigen-antibody interactions. METHODSWe formed a mixed self-assembled monolayer by using thiol-functionalized polyethylene glycols (PEGs) with -OH and biotin terminated head-groups on gold electrodes of quartz biosensor (Nelson; Langmuir 2001). PEG-backfilling of this surface provided protein non-fouling effects (1% BSA adsorbed only 10 ng/cm2), while biotin end-groups enabled close to full surface coverage of streptavidin (180 ng/cm2). This surface became our platform to immobilize biotinylated KKO (anti-PF4/heparin IgG2Κb monoclonal antibody, developed in Arepally Lab) and TRA (isotype IgG2Κb control antibody; developed in Arepally Lab).To characterize the sensor's detection ability, we flowed recombinant PF4 (provided by M. Poncz, Children's Hospital of Philadelphia) with a range of heparin (Elkins-Sinn, NJ) concentrations. The solutions are introduced to the biosensors 10 minutes after mixing the heparin and flowed through the system until the frequency readout stabilized. We determined the amount of captured PF4/heparin complexes and the layer thickness by monitoring the sensor's resonance frequency shifts and acoustic dissipation.The surface distribution of PF4/heparin complexes on KKO-functionalized surfaces was imaged by tapping-mode atomic force microscopy (AFM). The imaging results are used to verify layer thickness calculated by acoustic modeling of the QCM signal. RESULTSIn initial experiments, KKO showed no changes in the presence of buffer or PF4 alone. However, when PF4 (10 μg/ml) was infused with increasing amounts of heparin (0.2, 1 or 10 U/ml) significant binding was detected starting at 0.2 U/ml heparin (Figure A). The acoustic modeling of the KKO-bound PF4 complexes with 0.2 U/ml heparin suggested that the macromolecular complexes were nearly 25 nm high. These results are consistent with the neutralization model for the role of PF4 in heparin-induced thrombocytopenia, in which the amount of PF4/ heparin complexes peaks at a specific molar ratio of PF4:heparin (Kowalska; Thrombosis Research 2010). Comparatively, almost no binding occurred when the quartz crystal was functionalized with TRA. This result not only demonstrates the protein non-fouling capacity of the PEG backfilling of the surface, but also shows the specificity of the surface-immobilized assay.To gauge the sensitivity of the QCM biosensor to lower concentrations of PF4 and heparin that may be found in vivo, we tested 1µg/mL PF4 with lower heparin concentrations (0.02-0.05 U/ml). As shown in Figure B, a nearly linear KKO binding response was observed in this range capable of distinguishing heparin concentrations that differed by as little as 0.01 U/ml. The highest adsorption was observed at heparin concentration of 0.02 U/ml with associated atomic force microscopy (AFM) images again showing spherical particles with 25 nm diameter on the KKO immobilized surface. CONCLUSIONSHighly specific binding to PF4 complexes demonstrates the sensitivity of the QCM platform and its ability to distinguish differences between heparin concentrations of even 0.01 U/ml. Since QCM is a mass-sensing biosensor, it has a unique sensitivity advantage while sensing high molecular weight macromolecular complexes. This, combined with possible miniaturization of the sensor and integration with lab-on-a-chip systems, creates the potential for point-of-care bed-side detection of PF4/heparin. [Display omitted] DisclosuresArepally:Biokit: Patents & Royalties.

Applicability of QCM-D for Quantitative Measurements of Nano- and Microparticle Deposition Kinetics: Theoretical Modeling and Experiments.

A new theoretical model is formulated for the quantitative analysis of quartz crystal microbalance (QCM) response for heterogeneous loads consisting of nano- and microparticles. The influence of particle coverage and structure is described using a universal correction function in an ab initio manner. Explicit analytical expressions for the frequency and dissipation shifts are derived for the entire range of particle size under the rigid contact regime. The solvent coupling functions are also calculated to determine the dry coverage using the QCM measurements. These expressions furnish the upper limit of the QCM signal, which can be attained for a sensor providing perfect adhesion of particles. Correction functions accounting for the finite adhesion strength (soft contact regime) are also derived. The theoretical results are confronted with QCM and atomic force microscopy measurements of positively charged polymer particle deposition on silica sensors. The main features of the theoretical model are confirmed, especially the abrupt decrease in the QCM wet mass with the particle coverage and the overtone number. The latter effect is especially pronounced for microparticles under the soft contact regime, where the higher-number overtones produce a negligible QCM signal. These results represent a useful reference data for the interpretation of protein and bioparticles, for example, virus and bacteria attachment processes to various substrates.

Investigating Adsorbing Viscoelastic Fluids Using the Quartz Crystal Microbalance.

There is little research on using the quartz crystal microbalance (QCM) with adsorbing viscoelastic fluids. These fluids are widely encountered but often difficult to study as many are opaque and highly viscous. Since the QCM does not involve any scattering or reflection of input radiation, it has the potential to study these complex fluids to determine the relative viscoelasticity of the bulk fluid and surface adsorption of active species onto different substrates. In the current study, both Newtonian (sucrose) and viscoelastic (sodium polystyrene sulfonate (NaPSS)) fluids were introduced into the QCM, and the sensor responses were compared. QCM responses of Newtonian sucrose solutions matched the Kanazawa and Gordon model (KG model), as expected. The QCM responses with viscoelastic NaPSS solutions were well below those described by the KG model. A viscoelastic model was used to determine the fluid viscosity and shear modulus at a very high frequency. It was found that the viscosity of NaPSS did not change much compared with low-frequency rheometer measurements, but a significant increase in the shear modulus of several orders of magnitude was found at the QCM frequencies. Modifying the KG model frequency shifts by multiplying by the QCM shear wave decay length ratio, X = δV/δN, we were able to match the measured QCM values in viscoelastic NaPSS solutions. The QCM dissipation values for NaPSS were matched in a similar way by multiplying the KG model by X1/3. By changing the QCM sensor from silica (no NaPSS adsorption) to alumina (NaPSS adsorption), it was shown that the adsorption isotherm of NaPSS on alumina could be recovered and fitted with a Langmuir isotherm despite the frequency response being only a small fraction of the total measured QCM signal.

Hydrodynamics of Quartz-Crystal-Microbalance DNA Sensors Based on Liposome Amplifiers

The quartz crystal microbalance (QCM) is widely used to study surface adsorbed molecules, often of biological significance. However, the relation between raw acoustic response (frequency shift $\Delta f$ and dissipation factor $\Delta D$) and mechanical properties of the macromolecules still needs to be deciphered, particularly in the case of suspended discrete particles. We study the QCM response of suspended liposomes tethered to the resonator wall by double stranded DNA, with the other end attached to surface-adsorbed neutravidin through a biotin linker. Liposome radius and dsDNA contour length are comparable to the wave penetration depth ($\delta\sim 100\ \mathrm{nm}$). Simulations, based on the immersed boundary method and an elastic network model for the liposome-DNA complex, are in good agreement with experimental results for POPC liposomes. We find that the added stress at the resonator surface, i.e. the impedance Z sensed by QCM, is dominated by the flow-induced liposome surface-stress, which propagates towards the resonator by viscous forces. QCM signals are extremely sensitive to the liposome's height distribution P(y) which depends on the actual number and mechanical properties of the tethers, in addition to the usual local attractive/repulsive chemical forces. Our approach helps in deciphering the role of hydrodynamics in acoustic sensing and revealing the role of parameters hitherto largely unexplored. A practical consequence would be the design of improved biosensors and detection schemes.

A new QCM signal enhancement strategy based on streptavidin@metal-organic framework complex for miRNA detection

Sensitive and selective detection of miRNA is of great significance for the early diagnosis of human diseases, especially for cancers. Quartz crystal microbalance (QCM) is an effective tool for detecting biological molecules; however, the application of QCM for miRNA detection is still very limited. One of the great needs for QCM detection is to further improve the QCM signal. Herein, for the first time, we promote a new signal enhancement strategy for the detection of miRNA by QCM. First, a hairpin biotin-modified DNA was used as a probe DNA, which exposes the biotin site when interacting with target miRNA. Then, a streptavidin@metal-organic framework (SA@MOF) complex formed by electrostatic attractions between SA and a MOF was introduced into the QCM detection system. The SA@MOF complexes serve as both a signal amplifier and a specific recognition element via specific biotin-SA interactions. The strategy was applied to the detection of a colorectal cancer marker, miR-221, by using a stable Zr(IV)-MOF, UiO-66-NH2. The detection linear range was 10 fM-1 nM, the detection limit was 6.9 fM, and the relative standard deviation (RSD) (n = 5) was lower than 10% in both simulated conditions and the real serum environment. Furthermore, the detection limit reached 0.79 aM when coupled with the isothermal exponential amplification reaction (EXPAR).

Strategic Approaches for Highly Selective and Sensitive Detection of Hg2+ Ion Using Mass Sensitive Sensors.

Here we present a quartz crystal microbalance (QCM) sensor for the highly selective and sensitive detection of Hg2+ ion, a toxic chemical species and a hazardous environmental contaminant. Hg2+ ion can be quantitatively measured based on changes in the resonance frequency of QCM following mass changes on the QCM sensor surface. The high selectivity for Hg2+ ion in this study can be obtained using a thymine-Hg2+-thymine pair, which is more stable than the adenine-thymine base pair in DNA. On the other hand, gold nanoparticles (AuNPs) and their size-enhancement techniques were used to amplify the QCM signals to increase the sensitivity for Hg2+ ion. With this strategic approach, the proposed QCM sensor can be used to quantitatively analyze Hg2+ ion with high selectivity and sensitivity. The detection limit was as low as 98.7 pM. The sensor failed to work with other metal ions at concentrations 1000-times higher than that of the Hg2+ ion. Finally, the recovery does not exceed 10% of the original value for the detection of Hg2+ ion in tap and bottled water. The results indicate acceptable accuracy and precision for practical applications.

Specific responses of surface plasmon resonance and quartz crystal microbalance techniques to the mass of adsorbates at solid-liquid interfaces

Surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) techniques are become increasingly popular in probing behaviors and properties of adsorbates at solid-liquid interfaces. Despite the numerous studies focusing on revealing various relations between their output signals and adsorbed mass, however, the mechanics of these specific relations remains elusive. In this paper, we showed that the universality of a proportional relation between SPR signal and surface mass stems from the catholicity of a linear relation between surface refractive index and surface concentration, and a small refractive index shift. The partial validity of a proportional relation between QCM signal and surface mass is due to that a linear relation between surface viscosity and surface concentration, and a small viscosity shift are not always true. The ratio of SPR and QCM signals to the surface mass is dominated by the refractive index increment and intrinsic viscosity, respectively. The weak sensitivity of refractive index increment to conformation, architecture, etc., makes the mass estimation from SPR technique very convenient. The strong sensitivity of intrinsic viscosity to these factors makes the mass evaluation from QCM technique very difficult, whereas endowing a powerful method in quantitatively probing the conformation of adsorbates at solid-liquid interfaces.

Novel quartz crystal microbalance immunodetection of aflatoxin B1 coupling cargo-encapsulated liposome with indicator-triggered displacement assay

A simple and sensitive quartz crystal microbalance (QCM) immunosensing platform was designed for the high-efficient detection of aflatoxin B1 (AFB1) in foodstuff. Initially, phenoxy-derived dextran molecule was immobilized on the surface of QCM gold substrate by using thiolated β-cyclodextrin based on the supramolecular host-guest chemistry between phenoxy group and cyclodextrin. Then, AFB1-bovine serum albumin (AFB1-BSA)-conjugated concanavalin A (Con A) was assembled onto the QCM probe through the dextran-Con A interaction. Glucose-loaded nanoliposome, labeled with monocolonal anti-AFB1 antibody, was used for the amplification of QCM signal. Upon target AFB1 introduction, the analyte competed with the immobilized AFB1-BSA on the probe for the labeled anti-AFB1 antibody on the nanoliposome. Based on specific antigen-antibody reaction, the amount of the conjugated nanoliposomes on the QCM probe gradually decreased with the increment of target AFB1 in the sample. Upon injection of Triton X-100 in the detection cell, the carried nanoliposome was lysed to release the encapsulated glucose molecules. Thanks to the stronger affinity of Con A toward glucose than that of dextran, AFB1-BSA-labeled Con A was displaced from the QCM probe, resulting in the change of the local frequency. Under the optimum conditions, the shift of the functionalized QCM immunosensing interface in the frequency shift was proportional to the concentration of target AFB1 within a dynamic range from 1.0 ng kg−1 to 10 μg kg-1 at a low detection limit of 0.83 ng kg−1. In addition, the acceptable assayed results on precision, reproducibility, specificity and method accuracy for the analysis of real samples were also acquired. Importantly, our strategy can provide a signal-on competitive immunoassay for the detection of small molecules, e.g., mycotoxins and biotoxins, thereby representing a versatile sensing schemes by controlling the corresponding antibody or hapten in the analysis of food safety.

Modified Sauerbrey equation: a facile method to quantitatively probe the conformation of isolated molecules at solid-liquid interfaces.

Despite the increasingly popular application of the quartz crystal microbalance (QCM) technique in monitoring phenomena taking place at solid-liquid interfaces, ranging from changes in mass to changes in conformation, a simple, direct relationship between QCM signal and surface mass remains elusive. In this paper, we report that the proportional relationship between the QCM signal and the surface mass arises from the linear relationship between the viscosity of the layer adsorbed at the solid-liquid interface and the surface coverage, as well as a small viscosity shift. The proportionality coefficient depends on the intrinsic viscosity of adsorbates, solvent density, and quartz crystal thickness. The intrinsic viscosity is dominated by the conformation of the entire molecular chain and the adsorption blob for end-grafted and physisorbed molecules, respectively. Using this modified Sauerbrey equation, the phenomena relating to the conformation of discrete chains at the solid-liquid interfaces can be semi-quantitatively described.

A sensitive quartz crystal microbalance assay of adenosine triphosphate via DNAzyme-activated and aptamer-based target-triggering circular amplification

In this work, a simple and novel quartz crystal microbalance (QCM) assay is demonstrated to selectively and sensitively detect the adenosine triphosphate (ATP). The amplification process consists of circular nucleic acid strand-displacement polymerization, aptamer recognition strategy and nanoparticle signal amplification. With the involvement of an aptamer-based complex, two amplification reaction templates and AuNP-functionalized probes, the whole circle amplification process is triggered by the target recognition of ATP. As an efficient mass amplifier, AuNP-functionalized probes are introduced to enhance the QCM signals. As a result of DNA multiple amplification, a large number of AuNP-functionalized probes are released and hybridized with the capture probes on the gold electrode. Therefore the QCM signals are significantly enhanced, reaching a detection limit of ATP as low as 1.3nM. This strategy can be conveniently used for any aptamer-target binding events with other biological detection such as protein and small molecules. Moreover, the practical determination of ATP in cancer cells demonstrates the feasibility of this QCM approach and potential application in clinical diagnostics.

Real Time Analysis of Binding between Rituximab (Anti-CD20 Antibody) and B Lymphoma Cells

CD20, expressed on greater than 90% of B-lymphocytic lymphomas, is an attractive target for antibody therapy. Rituximab is a chimeric murine/human-engineered monoclonal antibody which can selectively deplete CD20-expressing cells in peripheral blood and lymphoid tissues. The immobilization of B-lymphoblast-like Burkitt's lymphoma Raji cells on the quartz crystal microbalance (QCM) gold electrode surface using arginine-glycine-aspartic acid (RGD) tripeptide was electrochemically confirmed. The real-time processes of attachment of Raji cells on the gold electrode and the subsequent binding of Rituximab to the cells were studied using a QCM biosensor. The interaction between Rituximab and Raji cells led to the increased resonant frequency shifts (Δf0) in the studied antibody concentration range from 5 to 250 μg mL(-1) following the Langmuir adsorption model. From these observations, the apparent binding constant between a single-layer of Rituximab and Raji cells was calculated to be 1.6 × 10(6) M(-1). Control experiments using other therapeutic antibodies (i.e., Trastuzumab and Bevacizumab) and different cells (i.e., T cells and endothelial cells) proved the specific interaction between Rituximab and B cells. The effects of Ca(2+) and Mn(2+) ions on the Rituximab-Raji cell interaction were also studied providing the enhanced QCM signals, in particular with Ca(2+), further indicating that CD20 is a calcium ion channel that can transport these metal ions into the cells and accelerate the cell lysis induced by Rituximab. Thus, the real time capability of QCM and its simplicity of operation are shown to be highly suitable for multipurpose studies on living cells including cell-immobilization, cytotoxicity of drugs, and the cell action mechanisms.

Acoustic detection of cell adhesion to a coated quartz crystal microbalance – implications for studying the biocompatibility of polymers

Biocompatibility of polymers is an important parameter for the successful application of polymers in tissue engineering. In this work, quartz crystal microbalance (QCM) devices were used to follow the adhesion of NIH 3T3 fibroblasts to QCM surfaces modified with fibronectin (FN) and poly-D-lysine (PDL). The variations in sensor resonant frequency (Δf) and motional resistance (ΔR), monitored as the sensor signal, revealed that cell adhesion was favored in the PDL-coated QCMs. Fluorescence microscopy images of seeded cells showed more highly spread cells on the PDL substrate, which is consistent with the results of the QCM signals. The sensor signal was shown to be sensitive to extracellular matrix (ECM)-binding motifs. Ethylenediaminetetraacetic acid (EDTA) and soluble Gly-Arg-Gly-Asp-Ser (GRGDS) peptides were used to interfere with cell-ECM binding motifs onto FN-coated QCMs. The acquired acoustic signals successfully showed that in the presence of 30 mM EDTA or 1 mM GRGDS, cell adhesion is almost completely abolished due to the inhibition/blocking of integrin function by these compounds. The results presented here demonstrate the potential of the QCM sensor to study cell adhesion, to monitor the biocompatibility of polymers and materials, and to assess the effect of adhesion modulators. QCM sensors have great potential in tissue engineering applications, as QCM sensors are able to analyze the biocompatibility of surfaces and it has the added advantage of being able to evaluate, in situ and in real time, the effect of specific drugs/treatments on cells.

Acoustic detection of cell adhesion on a quartz crystal microbalance

An acoustic quartz crystal microbalance (QCM) was used to signal and follow the cell‑adhesion process of epithelial cells [human embryonic kidney(HEK)293T and cervical cancer (HeLa) and fibroblasts [African Green Monkey kidney cells (COS-7)] onto gold surfaces. Cells were applied on the sensor and grown under serum-free and serum-supplemented culture media. The sensor resonance frequency (Δf) and motional resistance (ΔR) variations were measured during cell growth to monitor cell adhesion processes. Fingerprints of the adhesion processes, generated using the QCM signal, were found to be specific for each cell type while enabling the identification of the phases of the adhesion process. Under serum-free conditions, the deposition of HEK293T and HeLa cells was characterized by a decrease of Δf with constant ΔR, whereas for COS‑7 cells, this initial deposition was signaled by variations of ΔR at constant Δf. Toward the end of the adhesion process, fingerprints were characterized by a continuous increase of ΔR consistent with the increase in viscoelasticity. The morphology of adherent cells was visualized by fluorescent microscopy, enabling the association of the cell morphology with QCM signals.

Highly Selective Detection of Single-Nucleotide Polymorphisms Using a Quartz Crystal Microbalance Biosensor Based on the Toehold-Mediated Strand Displacement Reaction

Toehold-mediated strand displacement reaction (SDR) is first introduced to develop a simple quartz crystal microbalance (QCM) biosensor without an enzyme or label at normal temperature for highly selective and sensitive detection of single-nucleotide polymorphism (SNP) in the p53 tumor suppressor gene. A hairpin capture probe with an external toehold is designed and immobilized on the gold electrode surface of QCM. A successive SDR is initiated by the target sequence hybridization with the toehold domain and ends with the unfolding of the capture probe. Finally, the open-loop capture probe hybridizes with the streptavidin-coupled reporter probe as an efficient mass amplifier to enhance the QCM signal. The proposed biosensor displays remarkable specificity to target the p53 gene fragment against single-base mutant sequences (e.g., the largest discrimination factor is 63 to C-C mismatch) and high sensitivity with the detection limit of 0.3 nM at 20 °C. As the crucial component of the fabricated biosensor for providing the high discrimination capability, the design rationale of the capture probe is further verified by fluorescence sensing and atomic force microscopy imaging. Additionally, a recovery of 84.1% is obtained when detecting the target sequence in spiked HeLa cells lysate, demonstrating the feasibility of employing this biosensor in detecting SNPs in biological samples.

On-line monitoring of parylene film deposition with quartz crystal microbalance (QCM)

The parylene is a polymer of p-xylene and it is polymerized by pyrolysis of the parylene dimer called di-p-xylene. Until now the thickness of parylene has been controlled by adjusting the amount of the parylene dimer. However, the parylene film is required of more accurate and reproducible control method. This work presents the quartz crystal microbalance (QCM) for the on-line monitoring of parylene thickness. QCM was installed at the deposition chamber and the QCM signal monitoring was carried out during the whole deposition process. The QCM sensorgrams were recorded and the resonance frequency shift by the deposition of parylene on the QCM was correlated to the thickness of parylene film from the AFM. From the linear correlation parameters between the resonance frequency shift analysis and the film thickness, the QCM monitoring was determined to be feasible for the monitoring of parylene film thickness during the parylene deposition process.

A peptide cross-linked polyacrylamide hydrogel for the detection of human neutrophil elastase

Polyacrylamide hydrogel films cross-linked with the peptide sequence AAPVAAK were synthesised on piezoelectric quartz crystals. Degradation of the hydrogel films was monitored using a combination of quartz crystal microbalance (QCM) and electrochemical impedance measurements. The films were shown to degrade in the presence of human neutrophil elastase (HNE). The rate of degradation was directly related to the enzyme activity making the system suitable for the detection of HNE in the activity range from 0.72 to 30 U mL −1. The films were not affected by non-specific adsorption. Film degradation was only accompanied by changes in the QCM signal, while no significant impedance change was observed during degradation.