MHz Quartz Crystal Microbalance Research Articles

First Direct Gravimetric Detection of Perfluorooctane Sulfonic Acid (PFOS) Water Contaminants, Combination with Electrical Measurements on the Same Device—Proof of Concepts

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are pollutants of concern due to their long-term persistence in the environment and human health effects. Among them, perfluorooctane sulfonic acid (PFOS) is very ubiquitous and dangerous for health. Currently, the detection levels required by the legislation can be achieved only with expensive laboratory equipment. Hence, there is a need for portable, in-field, and possibly real-time detection. Optical and electrochemical transduction mechanisms are mainly used for the chemical sensors. Here, we report the first gravimetric detection of small-sized molecules like PFOS (MW 500) dissolved in water. A 100 MHz quartz crystal microbalance (QCM) measured at the third harmonic and an even more sensitive 434 MHz two-port surface acoustic wave (SAW) resonator with gold electrodes were used as transducers. The PFOS selective sensing layer was prepared from the metal organic framework (MOF) MIL-101(Cr). Its nano-sized thickness and structure were optimized using the discreet Langmuir–Blodgett (LB) film deposition method. This is the first time that LB multilayers from bulk MOFs have been prepared. The measured frequency downshifts of around 220 kHz per 1 µmol/L of PFOS, a SAW resonator-loaded QL-factor above 2000, and reaction times in the minutes’ range are highly promising for an in-field sensor reaching the water safety directives. Additionally, we use the micrometer-sized interdigitated electrodes of the SAW resonator to strongly enhance the electrochemical impedance spectroscopy (EIS) of the PFOS contamination. Thus, for the first time, we combine the ultra-sensitive gravimetry of small molecules in a water environment with electrical measurements on a single device. This combination provides additional sensor selectivity. Control tests against a bare resonator and two similar compounds prove the concept’s viability. All measurements were performed with pocket-sized tablet-powered devices, thus making the system highly portable and field-deployable. While here we focus on one of the emerging water contaminants, this concept with a different selective coating can be used for other new contaminants.

Real-time monitoring of dephosphorylation process of phosphopeptide and rapid assay of PTP1B activity based on a 100 MHz QCM biosensing platform

The misregulation of protein phosphatases is a key factor in the development of many human diseases, notably cancers. Here, based on a 100 MHz quartz crystal microbalance (QCM) biosensing platform, the dephosphorylation process of phosphopeptide (P-peptide) caused by protein tyrosine phosphatase 1B (PTP1B) was monitored in real time for the first time and PTP1B activity was assayed rapidly and sensitively. The QCM chip, coated with a gold (Au) film, was used to immobilized thiol-labeled single-stranded 5′-phosphate-DNAs (P-DNA) through Au–S bond. The P-peptide, specific to PTP1B, was then connected to the P-DNA via chelation between Zr4+ and phosphate groups. When PTP1B was injected into the QCM flow cell where the P-peptide/Zr4+/MCH/P-DNA/Au chip was placed, the P-peptide was dephosphorylated and released from the Au chip surface, resulting in an increase in the frequency of the QCM Au chip. This allowed the real-time monitoring of the P-peptide dephosphorylation process and sensitive detection of PTP1B activity within 6 min with a linear detection range of 0.01–100 pM and a detection limit of 0.008 pM. In addition, the maximum inhibitory ratios of inhibitors were evaluated using this proposed 100 MHz QCM biosensor. The developed 100 MHz QCM biosensing platform shows immense potential for early diagnosis of diseases related to protein phosphatases and the development of drugs targeting protein phosphatases.

Design and development of a portable low-cost QCM-based system for liquid biosensing

Quartz crystal microbalance (QCM) is a versatile sensing platform that has gained increasing attention for its use in bioapplications due to its high sensitivity, real-time measurement capabilities, and label-free detection. This article presents a portable QCM system for liquid biosensing that uses a modified Hartley oscillator to drive 14 mm-diameter commercial QCM sensors. The system is designed to be low-cost, easy to use, and highly sensitive, making it ideal for various bioapplications. A new flow cell design to deliver samples to the surface of the sensor has been designed, fabricated, and tested. For portability and miniaturization purposes, a micropump-based pumping system is used in the current system. The system has a built-in temperature controller allowing for accurate frequency measurements. In addition, the system can be used in benchtop mode. The capability of the present system to be used in liquid biosensing is demonstrated through an experimental test for sensitivity to changes in the viscosity of glycerol samples. It was found to have a sensitivity of 263.51 Hz/mPa.s using a 10 MHz QCM sensor. Future work regarding potential applications was suggested.Graphical

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.

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.

Direct and controlled device integration of graphene oxide on Quartz Crystal Microbalance via electrospray deposition for stable humidity sensing

Integration of advanced and functional materials onto conventional sensing platforms can improve the device performances and even discover new applications. For piezoelectric resonant sensors, an addition of sensing materials can induce damping and hinder a stable device operation. Hence, the development of efficient method for materials integration is important to ensure high-performance and reliable sensor operation. This work presents a direct and precisely controlled integration of graphene oxide (GO) using the electrospray deposition (ESD) onto a 10 MHz Quartz Crystal Microbalance (QCM) for humidity sensing. The proposed ESD method achieves a high mass resolution of a few nanograms. Moreover, the GO uniformly coats across the sensing electrode region as it acts as a ground electrode during ESD. The proposed ESD method also works for a wide range of nanomaterials, such as carbon nanotubes, tin oxide, and silicon carbide micro-and nano-powders. Compared to the conventional drop-casting and dip coating approaches, our method ensures minimal GO agglomeration, resulting in a stable QCM-oscillator operation in a wide range of relative humidity from 11% to 97%. The measurement sensitivity increases with an amount of GO, but less GO results in better noise and detection limit performances. The results shed light on the importance of selecting an optimal amount of sensing materials for stable sensor operations.

A Hairpin DNA-Based Piezoelectric E-Nose: Exploring the Performances of Heptamer Loops for the Detection of Volatile Organic Compounds

A hairpin DNA (hpDNA) piezoelectric gas sensors array with heptamer loops as sensing elements was designed, realized, and challenged with pure volatile organic compounds VOCs and real samples (beer). The virtual binding versus five chemical classes (alcohols, aldehydes, esters, hydrocarbons, and ketones) of the entire combinatorial library of heptamer loops (16,384 elements) was studied by molecular modelling. Six heptamer loops, having the largest variance in binding the chemical classes, were selected to build the array. The six gas sensors were realized by immobilizing onto gold nanoparticles (AuNPs) via a thiol spacer the hpDNA constituted by the heptamer loops and the same double helix stem of four base pairs (GAAG at 5′ and CTTC at 3′ end). The HpDNA-AuNP was used to modify the surface of 20 MHz quartz crystal microbalances (QCMs). The realized E-nose was able to clearly discriminate among 15 pure VOCs of different chemical classes, as demonstrated by hierarchical cluster analysis. The analysis of real beer samples during fermentation was also carried out. In such a challenging matrix consisting of 23 different VOCs, the hpDNA E-nose with heptamer loops was able to discriminate among different fermentation times with high success rate. Class assignment using the Bayes theorem gave an excellent 98% correct beer samples classification in cross-validation.

Studying the Bulk and Contour Ice Nucleation of Water Droplets via Quartz Crystal Microbalances.

Due to the stochastic and time-dependent character of the ice embryo formation and growth (i.e., a process that can be analyzed statistically, but cannot be predicted precisely), the heterogeneous ice nucleation on atmospheric aerosols or macroscopic solid surfaces is still shrouded in mystery, regardless of the extremely active research and exponential progress within this scientific field. For instance, whether the icing appears from outside-in or inside-out is a subject of intense controversy, with practicability in designing passive icephobic coatings or improving the effectiveness of the cryopreservation technologies. Here, we propose an artful technique for quantitative analysis of the different modes of water freezing using super-nonwettable soot-coated quartz crystal microbalances (QCMs). To achieve this goal, a set of 5 MHz QCMs are loaded one at a time with a 50 μL droplet, whose bulk or contour solidification is detected in real-time. The obtained experimental results show that our sensor devices recognize explicitly if the ice nuclei form predominantly at the liquid–solid interface or spread along the droplet’s entire outer shell by triggering individual reproducible responses in terms of the direction of signal evolution in time. Our results may serve as a foundation for the future incorporation of QCM devices in different freezing assays, where gaining information about the ice adhesion forces and ice layer’s thickness is mandatory.

Direct Electron Transfer of Fungal Pyrroloquinoline Quinone-Dependent Dehydrogenase Lacking the Cytochrome Domain

Bioelectrocatalysis is the use of biomaterials as catalysts for redox reactions combined with electrode reactions. Electrodes modified with enzymes have been extensively studied in the field of bioelectrochemistry. Since electron transfer between the enzyme and the electrode is the most important phenomenon, the discussion about it has dominated the study of bioelectrocatalysis. Direct electron transfer (DET) between oxidoreductases and electrodes is important not only for understanding the fundamental features of redox proteins, but also for developing mediator-free bioelectronics devices. Among the enzymes with various coenzymes, the pyrroloquinoline quinone (PQQ)-dependent dehydrogenases are promising biocatalysts for both biosensors and biofuel cells. However, a limited number of studies have reported DET between PQQ in the enzyme and the electrode.We discovered a fungal PQQ-dependent pyranose dehydrogenase from the basidiomycete Coprinopsis cinerea (CcPDH) as the first example of a eukaryotic PQQ-dependent enzyme [1-3]. This enzyme comprises three domains: an N-terminal cytochrome b domain, a central PQQ dependent catalytic domain (PQQ domain), and a C-terminal family 1 carbohydrate-binding module (CBM1), and thus, it is regarded as quinohemoprotein. A previous study demonstrated that full-length CcPDH exhibited the direct bioelectrocatalysis based on DET on a glassy carbon (GC) electrode [4]. The catalytic currents were observed at a lower potential than the redox potential of heme b in the cytochrome domain, suggesting that both PQQ in the catalytic domain and heme b in the cytochrome domain are capable of DET. In the present study, the PQQ domain (containing only the PQQ cofactor) which was prepared through genetic engineering, was studied in DET with various electrodes.A 2-mercaptoethanol self-assembled monolayer (SAM)-coated polycrystalline gold electrode was found to be superior for the DET of PQQ in the domain, and the catalytic current density was higher than that of the bare GC electrode. The catalytic current density has increased 10 times. The amount of immobilized PQQ domain was determined to be 8.2 ± 0.4 pmol/cm2 by using a 27 MHz quartz-crystal microbalance (QCM), suggesting an approximate protein monolayer formation on the SAM modified gold surface. To improve current density, gold nanoparticles (AuNPs) were modified on top of polycrystalline gold electrodes. Importantly, a high catalytic current density of 1.6 mA/cm2 for the oxidation of L-fucose was achieved under optimized conditions. These results show highly efficient DET to PQQ in the active site of the fungal PQQ-dependent dehydrogenase. Thus, the PQQ-domain/SAM/AuNP coated polycrystalline gold electrode could serve as a highly sensitive biosensor and a highly active sugar oxidizing anode for a biofuel cell.Cyclic voltammetry using a AuNP modified electrode in the absence of substrate showed two pairs of redox peaks with midpoint potentials of E 1 = -45 mV and E 2 = -216 mV, indicating the redox reaction of the bound PQQ in the catalytic domain is composed of two one-electron steps via a semiquinone radical state. This was confirmed by the detection of an EPR signal with a g-value of 2.0065, which was assigned to a PQQ semiquinone radical. The details of DET of the PQQ domain will be discussed.[1] H. Matsumura, et al.,PLoS One, 9 (2014) e104851.[2] K. Takeda, et al., PLoS One, 10 (2015) e0115722.[3] K. Takeda, et al., Curr Opin Chem Biol, 49 (2019) 113-121.[4] K. Takeda, et al., Biochem Biophys Res Commun, 477 (2016) 369-373.

Bioelectrocatalysis based on direct electron transfer of fungal pyrroloquinoline quinone-dependent dehydrogenase lacking the cytochrome domain

Direct electron transfer (DET) between oxidoreductases and electrodes is crucial for understanding the fundamental features of redox proteins as well as developing mediator-free bioelectronics devices. Bioelectrocatalysis with pyrroloquinoline quinone (PQQ)-dependent dehydrogenases is promising for both biosensors and biofuel cells. However, a limited number of studies have reported DET between PQQ in the enzyme and the electrode. Here, the PQQ domain (containing only the PQQ cofactor) of the fungal PQQ-dependent pyranose dehydrogenase from Coprinopsis cinerea, was studied in DET with various electrodes. A 2-mercaptoethanol self-assembled monolayer (SAM)-coated polycrystalline gold electrode was found to give excellent DET for the PQQ domain, resulting in 10-times the catalytic current density compared to bare glassy carbon. The amount of immobilized PQQ domain was determined to be 8.2 ± 0.4 pmol/cm2 by using a 27 MHz quartz-crystal microbalance (QCM), suggesting an approximate protein monolayer formation on the SAM modified gold surface. To improve current density, gold nanoparticles (AuNPs) were modified on top of polycrystalline gold electrodes. Importantly, high catalytic current densities of 1.6 mA/cm2 for the oxidation of l-fucose were achieved under optimized conditions. Together, these results demonstrate highly efficient DET to PQQ in the active site of the fungal PQQ-dependent dehydrogenase. Thus, our PQQ-domain/SAM/AuNPs coated polycrystalline gold electrode could serve as an exquisitely sensitive biosensor and a highly active sugar oxidizing anode for biofuel cells.

Graphene derivative coated QCM-based gas sensor for volatile organic compound (VOC) detection at room temperature

<span>Volatile organic compounds (VOCs) affect our daily life through their emission from very common sources such as plants, building materials, paints, pesticides, and fossil fuel burning. The detection of VOCs at room temperature is a prime requirement. The graphene-based gas sensor has the potential to detect these VOC gases due to its attractive features such as high mobility and large surface area. In this work, a graphene-derivative is prepared as a sensing material in order to detect acetone. The thin film of graphene-derivative is prepared by a drop-cast method on a quartz crystal microbalance (QCM) sensor followed by drying in the room environment conditions. The prepared graphene-derivative and thin films are characterized structurally and morphologically by standard microscopic techniques such as FESEM, EDX, and Raman spectroscopy. The electrical parameters such as mobility and resistivity are measured using Hall-effect measurements at room temperature. The response and recovery time of the graphene-derivative based 10 MHz QCM sensor are found to be 23 s and 20 s, respectively. This highly sensitive graphene-based gas sensor with good reversibility can be employed for human health and environment safety applications. </span>

QCM Sensors Combining Highly Nanostructured Metal-Blacks Sublayers and Active Self-Assembled Monolayers

Introduction Quartz Crystal Microbalance (QCM) sensors are characterized by many potential applications. They are being widely used as biosensors, in pharmacological applications, for combustion control, environmental pollution monitoring, etc. [1]. Regardless the specific application, the recognition resolution of quartz chips is usually determined and restrained according to the Sauerbray equation (For example for 10 MHz QCM, the change of resonance frequency in 1 Hz corresponds to 4.4 ng/cm2 of adsorbed mass per surface). This fact is crucial mainly for gas sensor application where we need to detect concentration down to ppm or even ppb levels. However, there are some possibilities on how to enhance the QCM sensor properties and amplify their sensitivity, for instance by increasing the specific surface area of QCM electrodes, and thus providing more bonding sites for the analyte [2]. In this contribution, we propose a novel method that uses nanostructured materials called metal-blacks as a sublayer for the increase of active sensor surface, in combination with self-assembled monolayers (SAMs) that serves as a receptor layer. Metal-Blacks Metal-blacks – MBs are highly nanoporous and nanostructured materials with a large surface area. Their name originates in black colour, caused by the absorption of incident light on their surface. The incident light penetrates into the pores and channels where it undergoes multiple reflections in cavities and where it is almost completely absorbed. Due to the very large surface area, MBs are ideal materials for enhancement of QCM gas sensors properties [3]. Experimental The nanostructured layers of black gold (BAu) and black palladium (BPd) were prepared by evaporation from a tungsten boat at an elevated pressure of 200 Pa. Prepared layers of MBs were then characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SAMs of zinc octacarboxylphthalocyanine ZnPc(COOH)8 were prepared by two-step grafting method, where ZnPc(COOH)8 is covalently bonded to the molecule of aminothiophenol, which serves as an surface anchor via its gold/palladium-sulfur bond. The preparation procedure was as follows. At first, the QCM electrodes with MB layers were degreased in a sonicated ethanol bath during a few minutes and then dried under a nitrogen flow. Second, the QCM samples were cleaned by UV-ozone treatment and immediately immersed into 10-4 M solution of aminothiophenol in ethanol/DMF solvent for 24h. Third, after the 24h incubation time, the samples were moved into 10-4 M solution of the macrocycles and left there for another 24h. In the end, the samples were carefully rinsed with the solvent to eliminate adsorbed molecular overlayers and subsequently washed with ethanol in order to remove solvent residues and dried under nitrogen flux.Once prepared, QCM sensor impedance spectra were obtained by impedance analyzer Agilent 4294A. Responses towards different analytes (EtOH, CH4, H2O, NO2, toluene, acetaldehyde) were measured in a glass chamber that enables the measurement of up to 6 sensors at the same time. Sensors were measured at a constant gas flow and room temperature. The sensor resonant frequencies were measured by oscillattor circuits in connection with an NI PCI-6602 card used as a precise counter Results and Conclusions In this contribution, we have proved that highly nanoporous and nanostructured materials (such as metal-blacks), which have a large specific surface area, provide more binding sites for analytes and, hence, increase the response of the QCM sensors. As an example measurement, we present the response of QCMs with BAu towards ethanol vapours (Fig. 1). The curves illustrate that the response of the sensor with the BAu electrode is 10-times bigger than the one of the standard QCM (152Hz/15Hz). For the combined layer (BAu + ZnPc(COOH)8), the response amplification is not so large but it is still significant. In this case, the response is 3-times bigger than the one from the QCM with ZnPc(COOH)8 only (without BAu). However, in contrast with QCM with just bare BAu, the surface modification with SAMs layers influences the sensor selectivity. The combined layers of MBs and SAMs then result in a selective sensor with increased sensitivity. According to the presented results metal-blacks are promising materials that can significantly boost the sensitivity of QCM sensors and still preserve the selectivity of its active layers.

Adsorption behaviors of salivary pellicle proteins onto denture base metals using 27-MHz quartz crystal microbalance.

The adsorption of salivary pellicle proteins onto the material surface is key for denture plaque formation. We aimed to investigate the adsorption of bovine serum albumin (BSA) and mucin (MCN) onto denture base metal materials using a 27-MHz quartz crystal microbalance (QCM) method. A gold (Au), titanium (Ti), and cobalt chromium alloy (Co-Cr) sensors were employed for QCM measurements. Adsorbed amounts of BSA or MCN were calculated by observing the frequency decrease, and the apparent reaction rate, kobs, was obtained by the curve fitting of the frequency shift against the adsorption time. The adsorbed amounts of BSA on Ti were significantly lower than those on Au and Co-Cr. For MCN adsorption, Au showed significantly greater amounts of adsorption than Co-Cr. The kobs of Ti for BSA adsorption was significantly smaller than for the Co-Cr. The kobs of Ti, and Co-Cr for MCN adsorption were significantly smaller than for the Au. A clear correlation was not determined between adsorbed amounts of BSA or MCN onto each sensor and the surface topography or contact angles. The difference of denture base metals and the difference of salivary proteins influences the adsorption behavior of salivary proteins.

Uneven Coating Influences on Electrical Impedance of Quartz Crystal Microbalance

High shear modulus coating material does not provide loading effect to the Quartz Crystal Microbalance sensor. The condition is met when the coating material was glassy and the layer was homogeneous. Polystyrene is considered as a glassy coating material when used on 10MHz Quartz Crystal Microbalance. Homogeneous and compact polystyrene coatings that are glassy can be obtained using the spin coating method. The film’s rigid condition is shown by the sensor impedance value at the series resonance frequency after coating. However, using ultrasonic spray coatings, it was known that polystyrene films are formed not as homogeneous as when coated using spin coatings. We observed that Quartz Crystal Microbalance was damped by the polystyrene film when the coating film was not evenly distributed over the sensor surface, especially on the sensor surface above the electrode. The minimum impedance of the QCM sensor increased significantly. The unevenness of the coating causes the damping effect on the QCM sensor. The minimum sensor impedance before coating was less than 15 Ohms. When the polystyrene layer was uneven, we found that the impedance increased by more than 25 Ohms. The results showed that the distribution of polymer layers on the sensor surface is one of the important factors that must be considered to avoid the effect of damping on the sensor. The measurement results showed that the sensor has a negative mass effect.

A High-Q Quartz Crystal Microbalance with Mass Sensitivity up to 1017 Hz/kg**Supported by the National Natural Science Foundation of China under Grant No 61871098.

A high-Q quartz crystal microbalance (QCM) sensor with a fundamental resonance frequency of 210 MHz is developed based on inverted mesa technology. The mass sensitivity reaches 5.332 × 1017 Hz/kg at the center of the electrode, which is 5–7 orders of magnitude higher than the commonly used 5 MHz or 10 MHz QCMs (their mass sensitivity is 1010–1012 Hz/kg). This mass sensitivity is confirmed by an experiment of plating 1-ng rigid aluminium films on the surface of the QCM sensor. By comparing the changes in QCM equivalent parameters before and after coating the aluminum films, it is found that the QCM sensor maintains the high-Q characteristics of the quartz crystal while the mass sensitivity is significantly improved. Therefore, this QCM sensor may be used as a promising analytical tool for applications requiring high sensitivity detection.

An innovative approach for in-situ detection of postejaculatory semen coagulation and liquefaction using superhydrophobic soot coated quartz crystal microbalances

At present, the abnormal viscosity of human ejaculate is a crucial parameter often associated with the male factor infertility. However, the existing techniques for seminal viscosity evaluation are prone to fair criticism, as most of them are semi-quantitative or too ineffective in monitoring the processes of semen coagulation and liquefaction. Within this article, we report a novel method for quantitative in-situ assessment of the postejaculatory dynamics and peculiarities of human seminal fluids (viscosity, volume, sperm concentration) using superhydrophobic soot coated quartz crystal microbalances (QCMs). A detailed analysis of the real-time sensor signal of two soot coated 5 MHz QCMs, generated by the raw ejaculates of normozoospermic and mild-moderate oligozoospermic patients, reveals that the QCMs can quantify the seminal viscosity (e.g. as normal, increased or hyperviscous) and detect in a highly repeatable manner the phases of semen coagulation and liquefaction. In addition, these devices seem useful in establishing correlative dependencies among the fluid volume, spermatozoa concentrations and observed signal fluctuations from patient-to-patient and measurement-to-measurement. Our findings open future opportunities for rapid, cost-effective and reliable determination of the basic parameters and reproductive potential of human spermatozoa based on the physicochemical properties of the seminal plasma.

QCM immunosensor for the determination of Staphylococcus aureus antigen

Staphylococcus aureus is a causative agent of infectious diseases and a producer of a group of highly toxic proteins called Staphylococcal enterotoxins. Currently, the presence of S. aureus in a sample can be determined by a wide spectrum of genetic, immunochemical and microbiological techniques. Despite availability of standard methods and devices, they are not fully suitable for a simple, label-free mode of use by an unskilled worker. In this paper, a biosensor based on quartz crystal microbalance (QCM) sensor is performed as a tool for a simple and reliable determination of S. aureus. The biosensor contained an antibody specific to S. aureus immobilized through protein A on 10 MHz QCM sensors and the interdigitated layer was stabilized by iron nanoparticles. The assay was performed as label free and sample was applied without any pretreatment. Limit of detection 145 µg/ml (5.18 × 108 CFU/ml) was achieved. When considered standard size of a sample 50 μl, the limit of detection was equal to 7.25 μg (2.59 × 107 CFU). The biosensors kept more than 90% sensitivity to the analyte after storage in laboratory conditions for 90 days. Good correlation to standard enzyme-linked immunosorbent assay was also proved. The biosensor appears as a simple but reliable tool for S. aureus assay and the QCM platform would provide further applications in microbial analyses.

In-Silico Prediction on the MSAMS-Assisted Immobilization of Bovine Serum Albumin on 10MHz Piezoelectric Immunosensors

The biological sensing interface on the active area of a piezo transducer is responsible for the sensitivity, specificity, reusability, and reproducibility of these devices. Among the approaches used to functionalize piezo transducers, mixed self-assembled monolayers (MSAMs) are one of the most successful, given that they allow the obtaining of semi-crystalline molecular arrays and the arrangement of a bioreceptor on the surface. But, to deploy MSAMs on a substrate effectively, one must optimize and characterize the structural ratio between them and the bioreceptor. In this paper, we developed a molecular model of the interaction between Bovine Serum Albumin (BSA) and MSAMs-functionalized gold substrates. First, we evaluated the conditions for the functionalization of the substrates and found that a 50:1 16-mercaptohexadecaonic acid (MHDA) to 11 mercapto-1-undecanol (MUA) ratio produced the best features on the surface. We also evaluated the specific conditions to immobilize BSA on MSAMs (using the afore-established ratio) via Atomic Force Microscopy (AFM), and then on a 10[Formula: see text]MHz quartz crystal microbalance (QCM), and with the data obtained we concluded that a structural ratio of 0.005 (MSAM/BSA) is obtained when 1[Formula: see text][Formula: see text]M MHDA and 200[Formula: see text][Formula: see text]g/mL BSA were used, provided the most suitable conditions for the functionalization of a piezo transducer.

Superhydrophobic Soot Coated Quartz Crystal Microbalances: A Novel Platform for Human Spermatozoa Quality Assessment.

The functionality of human spermatozoa is a key factor for the success rate of natural human reproduction, but unfortunately the infertility progressively increases due to multifarious environmental factors. Such disquieting statistics requires the employment of sophisticated computer-assisted methods for semen quality analysis, whose precision, however, is unreliable in cases of patients with low sperm concentrations. In this study, we report a novel quartz crystal microbalance (QCM) based biosensor for in-situ quality assessment of male gametes, comprising a superhydrophobic soot coating as an interface sensing material. The soot deposition on the surface of a 5 MHz QCM eliminates the noise that normally arises upon immersion of the uncoated sensor in the test liquid environment, allowing the detection of human spermatozoa down to 1000–100,000 units/mL (1–100 ppb). Furthermore, the soot coated QCM delimitates in a highly repeatable way the immotile and motile sperm cells by inducing fundamentally distinct responses in respect to sensor sensitivity and signal trends. The obtained results reveal the strong potential of the superhydrophobic QCM for future inclusion in diverse laboratory analyses closely related to the in vitro fertilization procedures, with a final aim of gaining practical approaches for diagnoses and selection of male gametes.

Early stage anti-bioadhesion behavior of superhydrophobic soot based coatings towards Pseudomonas putida

Biofilm development, associated with the adherence of microbial cells on various solid substrates, has an adverse economic impact and is the main reason for the spreading of microbial infections. The present article reports novel findings about the anti-bioadhesion performance of four types of superhydrophobic soot coatings, deposited via combustion flame synthesis and further functionalized using plasma polymerization and/or fluorination, towards a Gram-negative bacterial strain Pseudomonas putida. The real-time sensor response of four representative soot coated 5 MHz quartz crystal microbalances, along with scanning electron microscopy, fluorescence microscopy and contact angle measurements on the model soot surfaces, reveal reversible and irreversible bioadhesion on the soot during the first hour of cell colonization. Each adhesion mode is related to the presence and distribution of morphological features within the size of bacteria, but the prolonged 7-day exposure to the bacterial suspensions unifies the number of attached cells (only 3 times difference from coating-to-coating). Nevertheless, the soot coatings inhibit the proliferation of Pseudomonas species and reduce their quantity by two orders of magnitude compared to an uncoated glass slide, which demonstrates the importance of surface characteristics for precise control of the initial cell attachment and development of multifunctional soot coatings with anti-bioadhesion properties.