Study Of Novel Piezoelectric Sensor Based On Molecularly Imprinted Technique

Owing to its physical robustness, mechanical strength, durability to heat and pressure, high affinity and selectivity, molecularly imprinted polymers (MIP) have been widely applied to numerous fields including solid-phase extraction; pharmaceutical analysis; separation in high performance liquid chromatography (HPLC); and sensors. During the last few years, a big boost in the use of mass-sensitive acoustic transducers based on the MIP has been observed. However, most of these studies limit to traditional imprinting formulations such as polyacrylates or methacrylic acid as the functional monomer in organic solvent. These formulations employ free radical polymerization and rely on the use of hydrogen-bonding interactions between the template and functional monomers. Commonly, most of the resulting polymers are irregular. The aim of this work was to develop novel piezoelectric sensors based on molecularly imprinting technique and discuss in detail the interfering factors. The main work could be summarized as follows: 1. Study on a novel piezoelectric biomimetic sensor based on imprinted microspheres was described. Dipyridamole imprinted microspheres were synthesized by using precipitation technique. The result of Scatchard analysis shows that the equilibrium dissociation constant (KD) and the apparent maximum number (Qmax) of the imprinted microspheres were 3.70 mM and 9.11 μmol?g-1 of dry microspheres, respectively. Compared with other molecularly imprinted polymerized particles, this kind of imprinted microspheres exhibits higher selectivity and sensitivity to the template molecule (dipyridamole). The recoveries of the sensor were 95.1%-105.4%. Studies show that the stability of this sensor is excellent. The sensor has been applied successfully to the determination of dipyridamole in human urine. 2. A dipyridamole imprinted post-microspheres piezoelectric sensor was developed successfully. These dipyridamole molecularly imprinted post-microspheres were synthesized by using precipitation technique. Transmission electron microscopy (TEM) indicates that their average diameter is 150 nm and a cavity exists in each post-microphere. Compared with other molecularly imprinted polymerized particles, this kind of post-microspheres exhibits higher selectivity and sensitivity to dipyridamole than that of the microspheres. Excellent properties suggested that this kind of post-microspheres will find its application in many aspects. 3. A piezoelectric sensor coated with a thin molecularly imprinted sol-gel film was developed for the determination of L-histidine in the liquid phase. Without pre-protection, L-histidine was imprinted directly into silica sol-gel films consisted of a hybrid mixture of functionalized organosilicon precursors (phenyltrimethoxysilane and methyltrimethoxysolane). The viscoelasticity of the film in the air and in buffer solution were studied by piezoelectric quartz crystal impedance (PQCI) technique. The binding of L-histidine to the imprinted film in the liquid phase was investigated by piezoelectric microgravimetry and electrochemical impedance technique. Scatchard analysis showed that the maximum binding site (Qmax) of the L-histidine imprinted sol-gel film is about 23.7 μmol/g. A linear range from 5.0 × 10?8 to 1.0 × 10?4 M for the detection of L-histidine was observed with a detection limit of 2.5×10?8 M (S/N =3). The proposed imprinted sol-gel sensor exhibits good stability, high specificity and excellent stereoselectivity. 4. A new method of combining sol-gel and self-assembly technology to prepare a kind of novel human serum albumin (HSA) imprinted film on the surface of PQC Au electrode modified with thioglycolic acid was described. The template molecule could be removed from the film with hot distilled water (40 oC). The performance of the sol-gel-imprinted film was investigated by using PQCI technique and electrochemical impedance technique. The results showed that the imprinted film can selectively recognize the template molecule. Other impacting factors (temperature and pH) have also been investigated. The self-assembly sol-gel imprinting process provides an alternative method for the preparation of the biomolecule imprinted thin film. 5. A thin molecularly imprinted sol-gel film with specific recognition for nucleotide (cytidine) was electrosynthesized on the surface of PQC Au-electrode. In this method, a sufficiently negative potential was applied to the electrode surface to generate hydroxyl ions, which play the role of the catalyst for the hydrolysis and condensation of 3-aminopropyltrimethoxysilane (APTS). Au nanoparticles were modified on the surface of the PQC Au-electrode during the self-assembly process to avoid the deposited film flaking off and increase the conductance of the PQC. The process of the preparation of the imprinted film was detailedly investigated by using the PQCI technique and cyclic voltammetry. The morphology of the sol-gel imprinted film on the PQC Au-electrode was characterized by using SEM. The change of the electrochemical performance of the PQC Au-electrode during the self-assembly and electrodeposition, the binding capacity and the selectivity of the electrodepositedimprinted sol-gel film were also studied detailedly. The electrodeposited imprinted sol-gel film exhibited high selectivity toward cytidine in comparison to interfering substances. The dissociation constant (Kd) at different pHs indicated a strong imprinted interaction existing between the electrodeposited sol-gel-imprinted film and the template cytidine. These results prove the proposed electrodeposited sol-gel imprinting technique as a promising method. 6. The performance of the piezoelectric sensor modified with the MIP, which were synthesized in the presence of different amount of cross-linker (EDMA) and constant amount of functional monomer (MAA) and template (niacinamide), was investigated detailedly. The morphology of these polymers was analyzed by using scanning electron microscope (SEM). Scatchard analysis shows that the dissociation constant (KD) and the apparent maximum number of affinity binding sites (Qmax) increase with the increment of the ratio of EDMA to MAA until the ratio reaches 20. The selectivity of the MIP-PQC sensors decreased with the decrease of the ratio of cross-linking in the polymers. The frequency response of the MIP-PQC sensor increases with the increment of the molar ratio of EDMA to MAA until it reaches 20 over the sample concentration range from 5×10-9 to 5 ×10-6 M. 7 The influence of the extraction methods (conventional immersion extraction, ultrasonic assisted extraction, microwave-assisted extraction,and Soxhlet extraction) on the MIP-PQC sensor's performance was investigated. The MIPs were synthesized by template analyte (theophylline) copolymerized with methacrylic acid (MAA), and then cross-linked with ethylene glycol dimethacrylate (EDMA) in tetrahydrofuran. Response performance, selectivity, and recovery of the different sensors were compared and influencing factors were studied. Compared with the other extraction methods, microwave-assisted extraction is the fastest method (The total time of the actual microwave irradiation was 10 min, with a maximum extraction efficiency of 90%) and the Soxhlet extraction gives the highest extraction efficiency, although the latter requires a much longer extraction time (24 h with the maximum extraction efficiency reached up to 98%). The performances of the sensors modified with the MIPs extracted by different extraction methods were investigated. The results showed that all sensors possess excellent selectivity to theophylline and durability to acid and base. Nevertheless, the frequency response of the sensor modified with the MIP extracted by Soxhlet method is slightly higher than those two other extraction methods. 8. The effect of the size of the MIPs on the PQC sensor performance wasinvestigated. Erythromycin imprinted polymers microspheres with different sizes were synthesized by precipitation polymerization under different centrifugal rate. The size of the MIP was characterized by using TEM. Being coated with a poly (vinyl chloride) (PVC) membrane containing MIP; the proposed PQC sensor can selectively adsorb the template molecule. Investigation of the performance of sensors modified with different sizes of MIP showed that PQC sensor modified with smaller size MIPs exhibited better performance and excellent selectivity. Other influencing factors on sensor performance modified with different sizes MIP were also investigated.