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We have examined the specificity of electrosynthesized poly- o -phenylenediamine as a kind of material molecularly imprinted with metal chelates. Molecularly imprinted polymers (MIPs) were prepared in situ by an electrochemical method. The EDTA chelate complexes of Cu(II), Zn(II), Fe(III) and Cd(II) ions were used as templates of the MIPs deposited on a gold electrode of a quartz crystal microbalance (QCM). The binding ability and specificity of the MIPs were investigated via the frequency response of the QCM to the adsorption of the template chelates and their analogs. Linear relationships are observed between the frequency shift and the concentration of the template chelates over a typical range of ~10 −6 to ~10 −4 mol·L −1 . The results reveal good binding specificity of such MIPs for their templates over structurally related analogs, but the selectivity coefficients are less significant than that for enantiomers. The results also suggest no significant correlation between binding specificity and the ionic (or atomic) radius of the investigated metal ions. The observed specificity is qualitatively attributed to the overall conformational difference of the metal-EDTA complexes resulting from their difference in both ionic radius and electronic structures. Schematic Representation of the Molecular Imprinting Procedures. The metal ions chelate with EDTA to form metal-EDTA chelates as the template, then polymerization is initiated by an electrochemical method. After the polymerization, the templates are removed to create cavities in the polymer film which have binding ability and specificity to the templates.
MXenes, as a typical graphene-like material, excels in the realm of humidity sensing owing to its two-dimensional layer structure, high electrical conductivity, tunable chemical properties, hydrophilicity, and large specific surface area. This study proposed a quartz crystal microbalance (QCM) humidity sensor using a nanochitin/Ti3C2TxMXene composite as a humidity-sensing material. The morphology, nanostructure, and elemental composition of nanochitin, Ti3C2TxMXene, and nanochitin/Ti3C2TxMXene composite materials were characterized using transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. Compared to the pure Ti3C2TxMXene-coated QCM humidity sensor, the nanochitin/Ti3C2TxMXene-coated QCM humidity sensor exhibited a higher sensitivity (20.54 Hz/%RH) in the humidity range of 11.3% to 97.3%. The nanochitin/Ti3C2TxMxene-coated QCM humidity sensor also demonstrated low humidity hysteresis (2.12%RH), very fast response/recovery times (4.4/4.1 s), a high quality factor (37 k), and excellent repeatability and sustained stability over time. Eventually, a bimodal exponential kinetics adsorption model was utilized for the analysis of the response mechanism of the nanochitin/Ti3C2TxMXene composite material-based QCM humidity sensor. This study provides new ideas for optimizing the moisture-sensitive performance of MXene-based QCM humidity sensors.Keywords:humidity sensor;quartz crystal microbalance (QCM);nanochitin;Ti3C2TxMXene;high quality factor
The effective qualitative and quantitative detection of mixed components of volatile organic compounds (VOCs) with similar molecular structures has always been a challenge and hotpoint in the research. A novel quartz-crystal microbalance (QCM) nanocomposite sensor integrated with a surface-enhanced Raman scattering (SERS) detection platform for multi-component gas analysis was proposed and fabricated in this paper. MIL-100 (Fe)/PAN composite fibers were developed on QCM via electrospinning of polyacrylonitrile (PAN) and hydrothermal synthesis, addressing the integration issues of MIL-100 particles in devices while maintaining high specific surface area. The MIL-100(Fe)/PAN@QCM sensor can simultaneously collect QCM frequency responses and SERS spectra, enabling the quantitative and qualitative detection of toluene and benzaldehyde in mixtures. The linear response relationship between the frequency obtained by the QCM sensor and the concentration of toluene and benzaldehyde gases was in the range of 0–100 ppm, and the limits of detections were 0.57 and 0.86 ppm, respectively. Additionally, an algorithm, that considered the relationship among the QCM frequency response signal, intensity values of the characteristic peaks in the SERS spectra, and the gas concentration, was developed and applied to measure the mixed gases of toluene and benzaldehyde in the concentration range of 0–100 ppm. The relative standard deviations (RSDs) of spiked sample determinations were found to be in the range of 3–8%. This work provided a new method for the efficient and rapid detection of gas molecules with similar structures, which could be extended and applied to detect more multi-component gas mixtures.
Ammonia (NH3) has a significant impact on human health, merely 300 ppm of which could pose a risk of irreversible harm to human life. Size and sensitivity are two primary factors for real-time sensing of NH3. Through dry grinding, we develop a novel 61-MHz quartz crystal microbalance (QCM) sensor with dimensions of only 5.0 mm in length and 3.2 mm in width. In this study, the MXene/ZnO heterojunction solution is created using the liquid phase method, and it is uniformly sprayed onto the surface of the QCM. The large specific surface area of the MXene/ZnO heterojunction provides additional adsorption sites for NH3 detection, enhancing sensitivity across a wide dynamic range and offering superior selectivity toward ammonia. At a temperature of 25° C, the sensitivity of the MXene/ZnO sensor for detecting NH3 is 1.319 Hz/ppm, surpassing that of the ZnO sensor (0.166 Hz/ppm). Furthermore, the sensor’s recovery time is significantly reduced from typically 300 s under normal conditions to 40 s after exposure to ultraviolet (UV) light. The proposed QCM sensor boasts advantages of compact size, high sensitivity, short recovery time, and exceptional selectivity for ammonia, which presents a novel solution for detecting harmful gases such as NH3 at room temperature.
Mustard gas poses a substantial concern for homeland security, and the development of sensors for facile, rapid, highly sensitive, and selective detection of a mustard gas simulant is much desirable. Herein, a new benzimidazole-containing covalent organic framework (COF), termed BABE-TFPy COF, was synthesized by the Schiff base condensation of 1-(4,7-bis(4-aminophenyl)-1H-benzoimidazole-2-yl)ethan-1-ol (BABE) with 1,3,6,8-tetrakis(4-formylphenyl)pyrene (TFPy). By virtue of its crystallinity, high porosity, excellent chemical stability, and abundant accessible benzimidazole sites, an unprecedented COF coated quartz crystal microbalance (QCM) sensor was judiciously fabricated for the detection of various volatile organic compounds (VOCs). Remarkably, the resultant BABE-TFPy COF coated QCM sensor exhibited significantly higher sensitivity to 2-chloroethyl ethyl sulfide (CEES), a typical hazardous mustard gas simulant, than those of the other common VOCs. Moreover, the frequency changes of the COF coated QCM sensor were linearly related to the concentrations of CEES vapor within the range of 5.6–19.7 ppm, and derived a strikingly low detection limit of 0.96 ppm (signal-to-noise ratio = 3), rendering it an extraordinary QCM sensor for the detection of CEES. The enthalpy change indicated by the temperature-varying microgravimetric experiment, along with the Gaussian theoretical calculation, highly suggests a dual-hydrogen bonding formed between the BABE-TFPy COF and the CEES molecule accounts for such an exceptional recognition of CEES.
The mobility of microplastics (MPs) in aqueous media is closely related to their environmental risk. The naturally occurring silica substrate surface in the aquatic environment is easily colonized by microorganisms and forms a biofilm, which may affect the migration and distribution of MPs. Herein, a typical MP, polystyrene (PS), and Pseudomonas fluorescens ( P. fluorescens ) biofilms were selected to study the deposition and release of pristine or ultraviolet (UV)-aged PS MPs on silica and biofilms under different ionic strengths using a quartz crystal microbalance dissipation (QCM-D) system. Statistical analyses of the deposition experiments revealed a significant impact of P. fluorescens biofilms on deposition ( p = 0.0042). The deposition rate of weathered MPs on the biofilms was 4.0 ± 0.1 to 16.3 ± 0.6 times that on silica. A release experiment revealed that the biofilm reduced the release fraction ( f r ) of weathered MPs by 34.5 ± 0.3 % compared to bare silica. In addition, the UV-ageing treatment reduced the deposition mass of MPs on the surface of silica by 27.6 ± 0.21 % compared to pristine microspheres. The analysis of the deposition mechanism revealed that the promotion and inhibition of biofilm or UV-ageing treatment on the deposition of microspheres could be attributed to the non-Derjaguin-Landau-Verwey-Overbeek (DLVO) force and the decreased electrostatic repulsion or the increased hydration repulsion, respectively.
This paper introduces a quartz crystal microbalance sensor (QCM) based on SnO 2 NFs/PDA composite with multi-layer stacked fiber nanostructure, which possesses high sensitivity and selectivity to formaldehyde gas. SnO 2 NFs/PDA composite was synthesized via electrospinning and further oxidative polymerization. The morphology and chemical compositions of the as-prepared composite nanomaterials were confirmed by a series of material characterization technologies. The QCM sensor based on SnO 2 NFs/PDA nanomaterials exhibited excellent sensing characteristics over a wide range of formaldehyde concentration (0.5 − 50 ppm), such as high sensitivity (12.2 Hz/ppm), short response/recovery times (25 s/38 s), high linearity and selectivity. The frequency shift characteristics and electrical parameters of the prepared sensors were confirmed by impedance analysis. The adsorption mechanism of formaldehyde gas could be attributed to hydrogen bonding and the combination of aldehyde ammonia Schiff base interactions. This work indicated that SnO 2 NFs/PDA based QCM possessed great potential in actual formaldehyde sensing, which provided a feasible solution for designing highly sensitive and selective formaldehyde sensor.
An enzyme-amplified magnetic immunosensor was developed for the sensitive and selective detection of ractopamine (RAC) in fodder using a quartz crystal microbalance (QCM). Magnetic capture probes were prepared by immobilizing β-cyclodextrins (β-CD) on Fe 3 O 4 nanoparticles in order to enrich RAC. PowerVision (PV), a kind of enzyme–polymer complex with a high enzyme-to-antibody ratio, served as a signal label. Horseradish peroxidase (HRP) on the immnunocomplex, which arises from sandwich-type immunoreactions, causes 3,3′,5,5′-tetramethylbenzidine (TMB) to produce large amounts of sedimentation with a mass proportional to RAC. The large amounts of HRP on the PV greatly amplify the QCM sensor's signal. Under optimum conditions, the differences of the QCM frequency (Δ f ) are proportional to the concentration of RAC over the range from 0.03 to 25 ng mL −1 . The limit of detection was 0.01 ng mL −1 . More importantly, this assay's results in swine fodder samples were consistent with those of ELISA, with recoveries between 92.3% and 110%. Due to its high sensitivity, portability and selectivity, the immunosensor achieved reliable quantification of RAC in real fodder samples and has the potential to become an on-site screening tool to enhance food safety.
In this work, a piezoelectric immunosensor is developed for the detection of estradiol (E2). The gold-coated chip of quartz crystal microbalance was modified by a 3-mercaptopropionic acid self-assembly monolayer, activated by carbodiimide/n-hydroxylsuccinimide, and then to immobilise the E2 antibody via the interaction with their amine groups. The resonance frequency change resulted by the binding of E2 on immobilised antibody differs from that of a blank (ΔF), was correlated to the antigen concentration. The construction of the sensing interface was confirmed by electrochemical impedance spectroscopy. Both round-off or dynamic modes were explored as analytical detection strategy in this work. Using round-off mode, the output ΔF responds to the concentration of E2 with a detection limit of 0.04 μg mL−1 and a linear range from 0.1 to 10 μg mL−1. Meanwhile, using a dynamic mode, the presence of E2 induced in a real-time sensing output along with the incubation of antigen. The slope of sensing output is in linear relation with the concentration of E2 with a detection limit of 0.06 μg mL−1. Thus, a faster immunesensing technique is established for rapid analysis requiring only several seconds other than the round-off mode which needs a complete incubation period for at least 60 min. This finding will greatly promote the applicability of immunosensor for rapid, real-time or in-site assays.
A Quartz crystal microbalance (QCM) ammonia sensor with ZnO nanoflowers (NFs) doped with cellulose acetate nanofiber (CA) nanofibers and polyaniline-modified nanocomposites (CA/PANI/ZnO) is presented in this paper. The CA/PANI composite was firstly produced by electrostatic spinning and further oxidative polymerization, and then three-dimensional self-assembled ZnO NFs was synthesized by further solvothermal method , and the two were blended to obtain the ternary CA/PANI/ZnO composite with high specific surface area. The morphology and elemental composition of the fabricated composite nanomaterials were tested and analyzed by a series of characterization techniques. The experimental results showed that the QCM sensor based on CA/PANI/ZnO composite nanomaterials at room temperature exhibited good performances over a wide range of ammonia concentrations (1–70 ppm), such as high sensitivity (4.54 Hz/ppm), short response recovery time (15 s/10 s), high linearity and high selectivity. The electrical parameters of the sensor are obtained by the impedance analyzer, and the quality factor is further calculated. The potential synergistic interaction between the composites is the key for ammonia adsorption. The results show that the sensor is expected to be a potential candidate for ammonia detection.
The complexity of in situ volatile organic compounds (VOCs) detection demands high selectivity and anti-interference capability of electronic noses (E-noses). Multivariant virtual sensor array (VSA) is a new generation of E-noses under research to overcome the insufficient VOCs selectivity limitation of existing electronic noses and alleviate the issue of sensor drifts. In this paper, a frequency-amplitude dual-parameter modulation strategy is presented on a wireless electrodeless quartz crystal microbalance with dissipation (WE-QCM-D) to realize the recognition and analysis of VOC mixtures and complex real-world analytes with a single multivariant VSA E-nose, by probing the gas dynamic sorption in a sensitive film at various scales of oscillating shear displacements and obtaining multiple partially independent responses to VOCs. Ten VOC analytes from alcohols, esters, and aromatic hydrocarbons were classified with accuracies of above 95% for both interclass and intraclass discriminations. Additionally, a discriminating accuracy of 95% has been achieved on VOC mixtures, and their component concentrations were predicted with coefficients of determination above 0.9. For practical testing, the system was exposed to the headspace VOCs of banana, pineapple, and mango under different concentrations. It recognized different fruits and identified the ripen state of bananas based on the detection of their volatiles. The dual-parameter modulated WE-QCM-D paves a promising multivariant way to realize online real-time VOCs monitoring with high performance on selectivity and quantitative analysis.
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