Quality control of automotive engine oils with mass-sensitive chemical sensors – QCMs and molecularly imprinted polymers

Molecularly imprinted polyurethanes were used as sensor materials for monitoring the degradation of automotive engine oils. Imprinting with characteristic oils permits the analysis of these complex mixtures without accurately knowing their composition. Mass-sensitive quartz crystal microbalances (QCMs) coated with such layers exhibit mass effects in addition to frequency shifts caused by viscosity, which can be compensated by an uncoated quartz or a non-imprint layer. Incorporation of degradation products into the imprinted coatings is a bulk phenomenon, which is proven by variation of the sensor layer height. Therefore, the resulting sensor effects are determined by the degradation products in the oil. Received: 5 August 1999 / Revised: 26 October 1999 / Accepted: 1 November 1999     

Molecularly imprinted sol–gel nanoparticles for mass-sensitive engine oil degradation sensing

Titanate sol–gel layers imprinted with midchain carbonic acids have proven highly useful for detecting engine oil degradation processes owing to selective incorporation of oxidised base oil components. Synthesising the material from TiCl₄ in CCl₄ and precipitating with water leads to imprinted TiO₂ nanoparticles with a diameter of 200–300 nm. Replacing the water by a 1 M ammonium hydroxide solution reduces the average particle size to 50–100 nm with retention of the interaction capabilities. Experiments with the latter solution revealed that the 100-nm particles take up substantially more analyte, indicating a size-dependent phenomenon. As the number of interaction sites within each material is the same, this cannot be a consequence of thermodynamics but must be one of accessibility. The sensor characteristic of water-precipitated particles towards engine oil degradation products shows substantially increased sensitivity and dynamic range compared with the corresponding thin films. Coating quartz crystal microbalances with such nanoparticle materials leads to engine oil degradation sensors owing to incorporation of acidic base oil oxidation products. Interaction studies over a large range of layer thicknesses revealed that both the absolute signal and the steepness of the correlation between the sensor signal and the layer height is 2 times higher for the particles. Figure Generation of molecularly imprinted sol–gel nanoparticles     

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。     

Effect of the size of molecularly imprinted polymers sensing materials on piezoelectric quartz crystal sensor performance.

The effect of the size of the molecularly imprinted polymers (MIPs) on the piezoelectric quartz crystal (PQC) sensor performance was investigated. Erythromycin imprinted polymers microspheres with different sizes were synthesized by precipitation polymerization. The size of the MIPs was characterized by using transmission electron microscope (TEM) analysis. Being coated with a poly(vinyl chloride) (PVC) membrane containing MIPs, the proposed PQC sensor can selectively adsorb the template molecule. Investigation of the performance of sensors modified with different sizes of MIPs showed that PQC sensor modified with smaller size MIPs exhibited better performance and excellent selectivity. Other influencing factors on sensor functions modified with different sizes MIPs were also investigated.     

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.     

QCM gas phase detection with ceramic materials—VOCs and oil vapors

Titanate sol–gel layers imprinted with carbonic acids were used as sensitive layers on quartz crystal microbalance. These functionalized ceramics enable us detection of volatile organic compounds such as ethanol, n-propanol, n-butanol, n-hexane, n-heptane, n-/iso-octane, and n-decane. Variation of the precursors (i.e., tetrabutoxy titanium, tetrapropoxy titanium, tetraethoxy titanium) allows us to tune the sensitivity of the material by a factor of 7. Sensitivity as a function of precursors leads to selective inclusion of n-butanol vapors down to 1 ppm. The selectivity of materials is optimized to differentiate between isomers, e.g., n- and iso-octane. The results can be rationalized by correlating the sensor effects of hydrocarbons with the Wiener index. A mass-sensitive sensor based on titanate layer was also developed for monitoring emanation of degraded engine oil. Heating the sensor by a meander avoids vapor condensation. Thus, a continuously working oil quality sensor was designed.     

Ultra-thin SiO2 layers on TiO2: improved photocatalysis by enhancing products' desorption

A study on the photocatalytic degradation of contaminants (salicylic acid and stearic acid) on titanium dioxide films overcoated with a few monolayers of silica is presented herein. A rather uncommon situation was observed, where the presence of the thin silica layers decreased the degradation rate of stearic acid while increasing that of salicylic acid. The results were explained by addressing the effect in the presence of silica films on the desorption of the intermediate products formed in the degradation of salicylic acid. This finding, which may apply also to other aromatic compounds, may have implications on the design and operation of photocatalytic devices for indoor applications, since ultrathin layers of silica are known to be formed over time on the photocatalyst. An interesting effect of the thickness of the silica sub-nanometre layer on the degradation rate of salicylic acid was explained in terms of gradual changes in the isoelectric point. If optimized, this effect can be utilized to precisely control adsorption or desorption and accordingly to induce specificity in the photocatalytic degradation of contaminants. A methodology for preparing a molecularly imprinting photocatalyst with an inert ultrathin layer in between the imprinted sites was presented. It was found that overcoating the area in between the imprinted sites preserved the benevolent effect of imprinting. While at present the imprinting effect was moderate, there is a reason to believe that this effect can be improved considerably by controlling the type of inert overlayer.     

Imprinted sol-gel materials for monitoring degradation products in automotive oils by shear transverse wave

Titania sol-gel layers imprinted with capric acid have been used as synthetic receptors for highly sensitive detection of oxidized products resulting from degradation of automotive engine oil. These layers have been applied as sensitive coating material on shear transverse wave (STW) resonators of frequencies ranging from 100 MHz to 430 MHz. A relatively small size of STW resonators, i.e. about 2 mm for 430 MHz makes these devices extremely useful while considering the concept of miniaturization. It has been proved experimentally that by increasing fundamental resonance frequency of these devices, a very high sensor response i.e. 22 kHz up to 460 kHz can be generated. The geometry of long chain capric acid fits best as recognition element in the synthesis of imprinted TiO₂ network. The thin titania layers coated on transducer surface provide excellent diffusion pathways to oxidized products of waste engine oil for selective and reversible re-inclusion i.e. recovery time of 30 min. Viscosity effects of oxidized engine oil can be minimized by shear waves which do not dissipate considerable amount of energy that ensure smooth liquid phase operation. Different oxidized products i.e. carbonic acids and esters can be characterized in lubricant via infra-red (IR) spectroscopy. The increasing IR absorbance of different waste oil samples is a clear indication of increasing concentration of carbonyl group. The IR absorbance of carbonyl groups is directly correlated to the age of respective waste engine oil samples and a quantitative relationship between sensor responses from STWs and IR absorbance was also developed.     

The fabrication and characterization of a formaldehyde odor sensor using molecularly imprinted polymers

The fabrication and characterization of odor sensors based on molecularly imprinted polymers is reported as the first case of imprinting formaldehyde. A quartz crystal microbalance is employed as a sensitive apparatus of a sensor for the determination of odor formaldehyde. An equation is deduced to characterize the interaction between molecularly imprinted films and the template. A linear relationship between the frequency shifts and the concentration of analyte in the range of 1.25 to 14.25 microM is found. The detection limit is about 20.5 microM. The sensor can selectively distinguish gaseous formaldehyde. It is envisaged that this novel and handy method could be employed to determine formaldehyde gas in the atmosphere.     

A study of a new TSM bio-mimetic sensor using a molecularly imprinted polymer coating and its application for the determination of nicotine in human serum and urine

A new bio-mimetic quartz crystal thickness-shear-mode (TSM) sensor, using an imprinted polymer coating as the sensitive material, has been fabricated and applied to the determination of nicotine (NIC) in human serum and urine. The molecularly imprinted polymer (MIP) was synthesized using NIC as the template molecule and methacrylic acid (MAA) as the functional monomer. The sensor showed high selectivity and a sensitive response to NIC in aqueous system. The linear response range of the sensor was between 5.0 x 10(-8) and 1.0 x 10(-4) M with a detection limit of 2.5 x 10(-8) M. The viscoelasticity of the coating in the air and in liquid has been studied by the impedance spectrum. The MIP sensor was stable and exhibited effective reproducibility. Satisfactory results were achieved in the detection of the real samples.     

新型多壁碳纳米管/白藜芦醇印迹溶胶-凝胶层层组装电化学传感器研究

采用电化学沉积方法将印迹溶胶-凝胶膜沉积到功能化碳纳米管(MWNT-COOH)修饰的碳电极表面,成功研制一种新型多壁碳纳米管/白藜芦醇印迹溶胶-凝胶电化学传感器.采用扫描电镜(SEM),循环伏安法(CV),方波伏安法(SWV)和计时电流法(i-t)详细考察该印迹溶胶-凝胶膜的形态和电化学性能.结果表明该传感器对白藜芦醇具有较高的选择性和亲和性.与无多壁碳纳米管修饰的印迹传感器比较,MWNT层修饰的印迹传感器电流响应信号明显提高.白藜芦醇与印迹溶胶-凝胶膜的特异性结合使该传感器的电流发生变化,电流变化与白藜芦醇浓度在5.0×10-7～8.0×10-5mol?L-1范围内呈良好线性关系,检测限为5.1×10-8mol?L-1,该传感器成功应用于葡萄酒中白藜芦醇含量的检测.     

Molecularly imprinted TiO2 thin film by liquid phase deposition for the determination of L-glutamic acid

For the first time, the feasibility of a molecularly imprinted liquid phase deposition (LPD) thin film has been demonstrated. Thin films of titanium oxide imprinted with L-glutamic acid were prepared by the LPD method on a gold-coated quartz crystal microbalance. The imprinted molecule could be removed upon treatment with immersion in deionized water. A sensor was developed on the basis of this method and showed good sensitivity, selectivity, and reproducibility to the template molecule. An equation was deduced to characterize the interaction between molecularly imprinted films and the template by virtue of Scatchard analysis. X-ray photoelectron spectroscopy was introduced to show the evidence for the molecular imprinting phenomenon. The linear relationship between the frequency shifts and the concentration of analyte in the range of 10-200 microM was obtained. LPD proves to be a powerful method for imprinting titanium oxide thin films.     

Selective piezoelectric odor sensors using molecularly imprinted polymers

Molecular imprinting technique has been used to create sensors with a predetermined selectivity for molecules in the gas phase. Piezoelectric quartz crystals coated with a 2-methylisoborneol (MIB) imprinted polymer gave responses which were consistently 5–10 Hz (1.1–1.3 times) higher than those of sensors coated with a non-imprinted polymer. Geosmin, another tertiary alcohol odorant with an earthy odor resembling, and often accompanying MIB, produced almost equal responses on either imprinted- or non-imprinted sensors. A number of other odorants were examined and their responses to the non-imprinted sensors were found to be similar to or greater than their responses to the imprinted sensors. The responses of MIB to the imprinted sensors were always the highest, while other odorants produced equal or higher responses using the non-imprinted sensor. The sensor has a detection limit of ca. 5 mg l⁻¹ and a dynamic range of at least 1000 mg l^−l. When the time taken for the sensor to stabilize is used as the response, instead of the frequency change, the detection limit is lowered to ca. 200 μg l^−l.     

Enhanced removal of bilirubin on molecularly imprinted titania film

Titania film imprinted by bilirubin molecule at the surface of quartz crystal was prepared using molecular imprinting and surface sol-gel process. The molecularly imprinted titania film was characterized by FTIR spectra, and the interaction between bilirubin and imprinted film was investigated using quartz crystal microbalance (QCM) technique. Compared with pure titania film, the molecularly imprinted titania film exhibits a much higher adsorption capacity for the target molecule, and the adsorption kinetic parameter estimated from the in situ frequency measurement is about 1.6×10(8) M(-1), which is ten times higher than that obtained on pure titania film. The photocatalytic measurements indicate that the bilirubin adsorbed on molecularly imprinted titania film can be completely removed under UV illumination. Moreover, our study indicates that the molecularly imprinted titania film possesses a better stability and reusability.     

A study of a bio-mimetic recognition material for the BAW sensor by molecular imprinting and its application for the determination of paracetamol in the human serum and urine

A new bio-mimetic bulk acoustic wave (BAW) sensor for paracetamol, with a high selectivity and sensitivity, was fabricated by using the molecularly imprinted polymer (MIP) as the sensing material. Non-covalent molecular imprinting polymers were synthesized simultaneously using two different functional monomers, namely the weakly basic 4-vinylpyridine (4-VP) and the acidic and hydrogen binding methacrylic acid (MAA). Such imprinted polymers improved recognition capability as compared with the polymers that were synthesized using only one of the functional monomers. Dynamic quartz crystal impedance analysis was employed to study the effect of the viscoelasticity of the polymer coating on the measurement. There was no variation in the viscoelasticity during the detection. The sensor was stable and exhibited good reproducibility. Satisfactory results were obtained in the determination of paracetamol in real samples.     

Molecular imprinted polymer coated QCM for the detection of nandrolone

An acoustic wave sensor coated with an artificial biomimetic recognition element has been developed to selectively screen for nandrolone in the liquid phase. A highly specific covalently imprinted polymer (MIP) was spin coated onto one electrode of a quartz crystal microbalance (QCM) as a thin permeable film. Selective rebinding of the nandrolone was observed as a frequency shift in the QCM for concentrations up to 0.2 ppm with the sensor binding shown to favour nandrolone over analogous compounds.     

Determination of Parathion‐methyl in Vegetables by Fluorescent‐Labeled Molecular Imprinted Polymer

A novel sensor for the determination of parathion‐methyl based on couple grafting of functional molecular imprinted polymers (MIPs) was fabricated which is developed by anchoring the MIP layer on surfaces of silica particles embedded CdSe quantum dots by surface imprinting technology. The synthesized molecular imprinted silica nanospheres (CdSe@SiO₂@MIP) allow a high selectivity and sensitivity of parathion‐methyl via fluorescence intensity decreasing when the MIP material rebinding the parathion‐methyl molecule. Compared with the MIP fabricated in traditional method, the template of parathion‐methyl was easier to remove from the CdSe@SiO₂@MIP imprinted material. Under optimal conditions, the fluorescence intensity of parathion‐methyl at the imprinted sensor was detected by spectrofluorophotometer. The relative fluorescence intensity of CdSe@SiO₂@MIP decreased linearly with the increasing concentration of parathion‐methyl ranging from 0.013 mg·kg⁻¹ to 2.63 mg·kg⁻¹ with a detection limit (3δ) of 0.004 mg·kg⁻¹ (S/N=3), which is lower than the MIP in tradition. The imprinted film sensor was applied to detect parathion‐methyl in vegetable samples without the interference of other organophosphate pesticides and showed a prosperous application in the field of food safety.     

Development of a rubber elongation factor, surface-imprinted polymer-quartz crystal microbalance sensor, for quantitative determination of Hev b1 rubber latex allergens present in natural rubber latex products

Molecularly imprinted polymers (MIPs) for screening to detect rubber latex allergens (Hev b1) in natural rubber based products were designed as artificial recognition polymeric materials coated onto a quartz crystal microbalance (QCM). The polymers were prepared using a stamp imprinting procedure after mixing optimum amounts of methacrylic acid–vinylpyrrolidone–dihydroxyethylene bisacrylamide and Hev b1 latex allergen proteins, obtained from rubber gloves. QCM measurements showed that the resulting polymer layers after removal of the proteins used in their preparation could incorporate structures and features down to nanometer scale of protein templates into the imprinted polymer much better than a non-specific control polymer under controlled sensor conditions and an optimized polymerization process. This selective polymer but not the non-selective polymer clearly distinguished between the latex allergen Hev b1 and proteins such as lysozyme, ovalbumin and bovine serum albumin, with a selectivity factor of from 2 to 4, and the response of the rubber elongation factors by an astonishing factor of 12. The imprinted cavities recognized specific binding sites and could distinguish among related hevein latex allergenic proteins isolated from fresh natural rubber latex; Hev b1, Hev b2, and Hev b3 with a selectivity factor of from 4 to 6. The different QCM measurements obtained presumably reflected slightly different conformations and affinities to the MIP binding sites. The sensor layers selectively adsorbed Hev b1 within minutes in amounts ranging from 10 to 1500 μg L⁻¹ and with a detection limit of 1 μg L⁻¹. This work has demonstrated that this new sensor provides a fast and reliable response to natural rubber latex protein, even after being extracted from the matrix of rubber gloves.     

Discrimination of peptides by using a molecularly imprinted piezoelectric biosensor

Based on the direct formation of a molecularly imprinted polymer on gold electrodes, we have developed a peptide sensor for the detection of low-molecular-weight peptides. A new cross-linking monomer, (N-Acr-L-Cys-NHBn)(2), was employed to attach the surface of the chip and to copolymerize with other monomers. Interestingly, N-benzylacrylamide participates in the polymerization and recognition is carried out in an aqueous environment. By using quartz crystal microbalance detection, short peptides can be monitored by their interaction with plastic antibodies specific for the target peptides. The selectivity of molecularly imprinted polymers and the sensitivity of such artificial biosensors have been combined to differentiate between traces of oxytocin and vasopressin to the ng mL(-1) scale.     

Biomimetic properties and surface studies of a piezoelectric caffeine sensor based on electrosynthesized polypyrrole

An approach for preparing a chemical sensor for caffeine through the combination of molecularly imprinted polypyrrole and a piezoelectric quartz transducer was proposed. The caffeine-imprinted polymer was synthesized using galvanostatic electropolymerization of pyrrole monomer directly onto one of the gold electrodes of a 9 MHz AT-cut quartz crystal in the presence of caffeine. The optimum conditions for the electrosynthesis of the reagent phase were established. Caffeine molecules were entrapped in the matrix of polymer film, and were removed by subsequent washing with water, leaving behind pores capable of recognizing the target analyte molecule.The caffeine sensor was fixed in a measuring cell and measurement of the resonant frequency of the quartz crystal as it comes in contact with the caffeine solution was carried out in a stopped flow mode. A steady-state response was achieved in about 10 min. The sensor exhibited a linear relationship between the frequency shift and the ln of caffeine concentration in the range of 0.1-10 mg/mL (correlation coefficient, r = 0.9882). The sensitivity of the sensor was about 255 Hz/ln concentration (mg/mL). A good repeatability, R.S.D. = 9 (n = 6) for 0.5 mg/mL caffeine solution was also observed. The use of the sensor can present a potential low-cost option for determining caffeine.Surface analytical techniques such as scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were performed on the polymer coating in order to elucidate the imprinting process and rebinding of caffeine to the polymer matrix during the sensing process. The SEM micrographs and XPS spectra revealed features and structures that could support the imprinting and recognition of caffeine molecule by the imprinted polymer.