Enantioselective sensor based on microgravimetric quartz crystal microbalance with molecularly imprinted polymer film

We report a novel quartz crystal microbalance sensor that provides enantioselectivity to dansylphenylalanine enantiomers by using a molecularly imprinted polymer film as a recognition element. The polymeric recognition thin film, imprinted with chiral dansyl-L-phenylalanine, was immobilised on a gold electrode modified with a photoactive precursor monolayer via a self-assembly process using photopolymerization. The fabricated sensor was able to discriminate between L- and D-dansylphenylalanine enantiomers in solution owing to the enantioselectivity of the imprinted sites. The enantiomeric composition of L- and D-enantiomeric mixtures could be quantitatively determined by the fabricated sensor. The detection limit is 5 micrograms mL-1 with a response range of 5-500 micrograms mL-1 at pH 10.0. The influence of the template concentration on the sensitivity and selectivity of the synthesised polymer membranes was investigated and optimised. The surface characteristics of the polymer coating were studied by varying the pH value of the buffer solution, and a convenient regeneration process was proposed to increase the reproducibility and reusability of the sensor by flushing with pH 2.0 buffer. The selectivity and recognition mechanism of the imprinted polymer film were studied with compounds that are structurally related to the template. The method presented in this work provides a novel means of preparing highly selective and sensitive chemical sensors via self-assembly and molecularly imprinting techniques.     

Determination of coupled solvent mass in quartz crystal microbalance measurements using deuterated solvents

A simple method is described for determining of the contribution of hydrodynamically coupled solvent to the adsorbed film mass determined in a quartz crystal microbalance (QCM) when operated in liquid. The method requires no additional apparatus and utilizes the change in QCM resonant frequency response between measurements made in non-deuterated and deuterated solvents. The mass of coupled water in a polymer film has been determined and is found to agree with that determined by XPS analysis of the dried polymer film.     

Measurement of sorption in polymer membranes with a quartz crystal microbalance

An application of the oscillating quartz crystal microbalance (QCM) in the measurements of sorption rates and equilibria in polymer—vapor systems is presented. The sorption isotherms of methanol, ethanol and n-heptane in cellulose acetate and poly(hexamethylene adipamide) were measured with the QCM at 298K. The results agree reasonably with the Flory model except at low absorbate activities where appreciable deviations due to localized absorption were observed, as verified with IR spectroscopy. The diffusion coefficient of methanol in cellulose acetate was also calculated from the sorption rate data measured with the QCM. The values obtained at different methanol concentrations correlate well with the published diffusion coefficients.     

Liposome layers characterized by quartz crystal microbalance measurements and multirelease delivery

Intact liposomes have been immobilized onto solid surfaces by a NeutrAvidin-biotin link. The construction of these layers has been followed up by X-ray photoelectron spectroscopy (XPS) and quartz crystal microbalance (QCM) measurements with energy dissipation monitoring. Also, the simultaneous release of two fluorescent probes from these liposome layers has been investigated with the aim to validate this method in multirelease delivery systems. XPS showed the successful immobilization of the different layers. XPS results also point out the importance of the deactivation method used to reveal the presence of the specific NeutrAvidin-biotin attachment. QCM measurements allowed the buildup of the different layers to be followed in real time and in situ and suggest that biotinylated liposomes stay intact upon surface attachment on NeutrAvidin-covered surfaces and had viscoelastic behavior. QCM experiments also demonstrated that surface-immobilized liposomes were able to resist irreversible adsorption from fetal bovine serum. Release kinetic profiles were studied by monitoring the release of two different fluorescent probes, namely, carboxyfluorescein and levofloxacin, from these liposome layers. These studies showed that it was possible to modulate to some extent the release rates of the two molecules by using different configurations of liposome layers.     

A study of glycoprotein-lectin interactions using quartz crystal microbalance

A study of biospecific interactions between lectins and glycoproteins using a quartz crystal microbalance biosensor with dissipation monitoring (QCM-D) was reported. Four lectins were covalently immobilised on the thiol-modified gold electrode of the QCM chips in order to obtain sensing surfaces. The frequency shift served as analytical signal and the dissipation shift provided additional information about the viscoelastic properties of the glycoprotein-lectin complex formed on the surface of the QCM chip. The working conditions of the assay were optimised. The interaction between different lectins and glycoproteins was characterised by specific frequency shifts and each glycoprotein displayed its own unique lectin-binding pattern. This lectin pattern can serve as a finger print for the discrimination between various glycoproteins. The biosensor enabled quantitative determination of glycoproteins in the concentration range of 50 μg mL⁻¹ to 1 mg mL⁻¹ with good linearity and R.S.D. of less than 6.0%. An additional advantage of the proposed biosensor was the possibility to re-use the same lectin surfaces during a long period of time (2 month) without changes in analytical response. This was experimentally achieved by the application of a proper regeneration solution (10 mM glycine-HCl, pH 2.5). The lectin-based quartz crystal microbalance technique is suitable both for rapid screening and for quantitative assay of serum glycoproteins.     

基于分子印迹技术的丙溴磷压电石英晶体微天平研制

介绍了一种用于检测丙溴磷农药的分子印迹压电生物传感器的构建方法。采用沉淀聚合法合成了农药丙溴磷的分子印迹聚合物,将其固定于石英晶体微天平电极表面构建传感器;采用环境扫描电镜以及原子力显微镜对聚合物形貌、传感器电极表面形貌特征进行分析,并利用传感器对丙溴磷农药进行检测分析,其质量浓度在10~1000 ng/mL范围内,传感器频率改变与丙溴磷浓度之间的响应呈线性关系,线性方程为y=0.139ρ+2.26(r=0.9984)。结果表明,构建的分子印迹压电生物传感器能够对农药进行初步检测,具有较高的灵敏性和较好的特异识别能力。     

The quantification of potassium using a quartz crystal microbalance

N,N'-dibenzyl-4,13-diaza-18-crown-6 (A) and bis[(benzo-15-crown-5)-4'-ylmethyl] pimelate (B) were tested as coatings for two piezoelectric crystals for potassium quantification. Both sensors showed stability, reversibility and sensitivity characteristics that allowed their use in quantitative analysis. However, compound A is much more sensitive to potassium than B. Compound A also shows a larger relative sensitivity for potassium with regard to sodium than B. A pharmaceutical sample of known composition was analysed both by an acoustic wave sensor with a crystal coated with compound A and by conductivity. No statistically significant difference in the median of the results was found (alpha = 0.05), although precision is superior for the conductivity methodology. Performance of the sensor in terms of frequency stability and selectivity was improved by the incorporation of PVC, a plasticizer and a lipophilic salt in the coating composition. Limits of detection found for potassium were 1.92 ppm, or 1.75 ppm for a crystal with a frequency decrease due to coating of 2.9 kHz or 3.9 kHz, respectively. Selectivity coefficients (fixed interference 80 ppm) for potassium over Na, Ca, Al, Zn, Mg, and Fe ranged from 0.103 to 0.332.     

Determination of 4-aminophenol using the quartz crystal microbalance sensor

In this paper we report a method for the determination of 4-aminophenol (4-AP) in solution using a quartz crystal microbalance (QCM) sensor. 4-AP reacts with (para-unsubstituted) phenols to form hydrophobic indophenol dye species that precipitate out and adsorb to the surface of the crystal to produce a shift in the crystal resonant frequency. This frequency change, due to in-situ indophenol mass adsorption, can be related to the initial 4-AP concentration. A range of phenols (namely o-cresol, 1-naphthol, resorcinol, catechol and 8-hydroxyquinoline) and their reaction with 4-AP were tested. Ammonium persulfate (APS) and potassium periodate were used as initiators to improve the speed of the reaction and the rate of formation of the precipitate. APS elicited improved signal in terms of response times and frequency shifts compared with KIO4. Of the phenols studied, resorcinol gave the best response time of 6 min for 4-AP determination. The reaction of resorcinol with 4-AP gave extended response times and signal size with decreasing concentration of 4-AP (in the range 2-5 mM).     

Design and properties of a flow-injection analysis cell using potassium-selective ion-sensitive field-effect transistors as detection elements

The combination of flow-injection analysis (FIA) and chemically modified ion-sensitive field-effect transistors (CHEMFETs) is described. In a wall-jet cell, two identical potassium-selective CHEMFETs were used for a differential measurement using a platinum (pseudo-)reference electrode. Silicone-rubber membrane materials, chemically bound to the SiO₂ gate oxide, were used with valinomycin as the ionophore. The optimized FIA system showed a linear response of 56 mV per decade for potassium concentrations above 5 × 10^?5 M. Preliminary results of potassium determinations in human serum and urine samples are presented.     

Surface immobilization of bio-functionalized cubosomes: sensing of proteins by quartz crystal microbalance

A strategy for tethering lipid liquid crystalline submicrometer particles (cubosomes) to a gold surface for the detection of proteins is reported. Time-resolved quartz crystal microbalance (QCM-D) was used to monitor the cubosome-protein interaction in real time. To achieve specific binding, cubosomes were prepared from the nonionic surfactant phytantriol, block-copolymer, Pluronic F-127, and a secondary biotinylated lipid, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[biotinyl(polyethyleneglycol)-2000], which enabled attachment of the particles to a neutravidin (NAv)-alkanethiol monolayer at the gold surface of the QCM sensor chip. A second set of cubosomes was further functionalized with addition of the glycolipid (G(M1)) to facilitate a specific binding uptake of the protein, cholera toxin B subunit (CT(B)), from solution. QCM-D confirmed the specificity of the cubosome-NAv binding. The analysis of titration experiments, also performed with QCM, suggests that an optimal concentration of cubosomes is required for the efficient packing of the particles at the surface: high cubosome concentrations lead to chaotic cubosome binding onto the surface, sterically inhibiting surface attachment, or require significant reorganization to permit uniform cubosome coverage. The methodology enabled the straightforward preparation of a complex nanostructured edifice, which was then used to specifically capture analyte proteins (cholera toxin B subunit or free NAv) from solution, supporting the potential for development of this approach as a biosensing platform.     

Humidity sensing characteristics of Sn doped Zinc oxide based quartz crystal microbalance sensors

The electrical, optical and humidity sensor properties of nanostructured ZnO samples were investigated. The structural properties of Sn doped ZnO samples were characterized by X-ray diffraction and atomic force microscopy. It was found that the all samples have a hexagonal crystal structure. The electrical conductivity of the samples indicates that undoped and Sn doped ZnO samples exhibit the semiconducting behavior. The optical absorption method was used to determine the optical band gaps of the samples. The optical band gap and activation energy values of the ZnO samples were changed with Sn doping. The ZnO based on quartz crystal microbalance humidity sensors were prepared and sensing properties of the sensors were changed with Sn doping. The response time required to reach 70 % is about 13–16 s, while the recovery time from 70 to 30 % RH is about 13–15 s. The fast response of the sensors is due to easy diffusion of water molecules between ZnO nanopowders. The prepared sensors have a high reproducibility and sensitivity for humidity sensing applications.     

The importance of temperature and viscosity effects for surfactant adsorption measurements made using the electrochemical quartz crystal microbalance

The measurement of adsorbed surfactant is important to fields such as corrosion inhibition, metal cleaning, and separation technologies. The electrochemical quartz crystal microbalance (EQCM) is an important tool that can be used to measure adsorbed surfactant. However, such measurements are subject to significant temperature and viscosity effects that must be appropriately considered. This paper discusses the effect of temperature and viscosity on EQCM measurements in solution environments and the ability of the EQCM to measure surfactant adsorption.     

The study of electrochemical palladium behavior using the quartz crystal microbalance

The electrochemical quartz crystal microbalance (EQCMB) method has been used to evaluate the processes which occur in/on the palladium electrode in basic solutions. Hydrogen electrosorption in palladium is accompanied by an additional frequency shift that can be attributed to the stresses generated inside the Pd metal. A non-linear dependence between the mass change and the charge consumed during hydrogen oxidation in the Pd electrode has been found for hydrogen absorbed in the α- and β-phases. This effect precludes the objective estimation of the amount of hydrogen absorbed inside the Pd electrode. The EQCMB method has been used, however, for studying the surface electrode processes on the Pd electrode, i.e. specific anion adsorption, surface oxidation and dissolution. Also, the structure of the palladium oxide formed on the Pd surface during electrochemical oxidation is discussed in this paper and the effect of the anodic limiting potential on the oxide structure is reported. Received: 10 August 1999 / Accepted: 24 September 1999     

一种通用型电化学石英晶体微天平电解池的设计

电化学石英晶体微天平(EQCM)同时结合了电化学检测的高灵敏度和石英晶体微天平(QCM)可实时检测电极表面质量变化及阻尼的特点,在电化学研究中具有非常好的应用前景,已得到越来越广泛的应用.本文设计了一种通用型的EQCM电解池,用恒电流电沉积铜的方法测定了QCM Pt电极的质量灵敏因子Cf,分析了Cf实验测定值与理论值偏差的原因,并讨论了在所设计的EQCM电解池中QCM Pt电极的使用范围,为进一步开展EQCM的应用研究提供可靠的基础.     

Acoustic quantification of ATP using a quartz crystal microbalance with dissipation

A quartz crystal microbalance with a dissipation monitoring (QCM-D) sensor was developed for highly sensitive and specific detection of adenosine-5'-triphosphate (ATP) by using an aptamer. The binding of ATP molecules on the aptamer films could be calculated as accurate mass changes using multiple frequency and dissipation measurements. The detection is achieved by calculating the mass changes from conformational rearrangements of the sensor surface upon interaction with the target. The sensor was demonstrated to respond to changes in ATP concentrations in real time suitable for continuous monitoring applications. This sensor showed excellent selectivity toward ATP compared with other chemically similar nucleotide GTP. The feasibility of the sensor was demonstrated by analyzing ATP concentrations in cell culture media with serum. The maximum frequency change was about -2 Hz after injection of 500 μM ATP. The affinity constant of the aptamer was determined to be 49 ± 7.59 μM. The proposed sensor can extend the application of the QCM-D system in medical diagnosis, and could be adopted for the detection of other small molecules with the use of specific aptamers.     

Selective separation of magnolol using molecularly imprinted membranes

Molecularly imprinted membranes (MIMs) for selective separation of magnolol were prepared by thermal polymerization using magnolol as the template, ethylene glycol dimethacrylate (EGDMA) as the cross‐linker, 2,2‐azobisisobutyronitrile (AIBN) as the initiator, organic solvent as the porogen, methacrylamide (MAM) and acrylic acid (AA) as the functional monomers and cellulose acetate as the agglutinant. Commercial filter paper was used as the supporting material. The effects of different porogens and the ratio of functional monomers on the binding and recognition capacity of MIMs were investigated, and the morphology of the membranes was examined by scanning electron microscopy (SEM). The results showed that the MIMs have the highest selectivity to magnolol when the ratio of MAM/AA was 1:4 and tetrahydrofuran (THF) with dimethyl sulfoxide (DMSO) was used as the porogen. The morphology of the imprinted membranes after template extracting is much rougher with big cavities than that of the non‐imprinted membranes (NIMs) and the imprinted membranes before template extracting. The MIMs can selectively separate the magnolol.     

The quantification of sodium in mineral waters using a quartz crystal microbalance

A sensor for sodium based on a piezoelectric quartz crystal is proposed. The quartz crystal was coated with 5% of bis[(12-crown-4)methyl] dodecylmethylmalonate, 33% of PVC and 62% of NPOE to which KTpClPB in a 22% molar proportion to the ionophore was added. Coating amounts producing a frequency decrease around 18 kHz show optimum linear calibration ranges for the analysis of sodium in commercial mineral waters. Besides sensitivity, coating stability and selectivity of the sensor over other cations were adequate for those analyses. The results obtained analysing commercial mineral waters by the proposed method are not significantly different (α=0.05) from the ones obtained by atomic spectrometry.     

Viscoelastic, mechanical, and dielectric measurements on complex samples with the quartz crystal microbalance

Piezoelectric resonators have been in use as mass-sensing devices for almost half a century. More recently it was recognized that shifts in frequency and bandwidth can come about by a diverse variety of interactions with the sample. The classical "load" consists of a thin film, which shifts the resonance frequency due to its inertia. Other types of loading include semi-infinite viscoelastic media, rough objects contacting the crystal via isolated asperities, mechanically nonlinear contacts, and dielectric films. All these interactions can be analyzed within the small-load approximation. The small-load approximation provides a unified frame for interpretation and thereby opens the way to new applications of the QCM.     

Desolvation of BSA-ligand complexes measured using the quartz crystal microbalance and dual polarization interferometer

By taking advantage of their unique difference in hydration sensitivity, we have shown that dual polarization interferometer (DPI) and quartz-crystal microbalance with dissipation monitoring (QCM-D) measurements can be used together to explore the degree of desolvation involved in the binding of small drug molecules to an immobilized bovine serum albumin film in real time. Results with DPI and QCM-D show significantly different mass values for three ligands of varying hydrophobicities that may be attributed to changes in the degree of hydration of the ligand-protein complexes in accordance with the physicochemical properties of the ligands. Furthermore, our data suggest that masses measured by QCM-D can be overwhelmed by changes in water content of ligand-protein, binary complexes, which has important consequences for future studies using mechanical resonators to study protein-binding events.