Quartz Crystal Microbalance-Based Electrospun Nanofiber Sensors For Trace Amounts Of Harmful Substances Detection In Liquid

The quartz crystal microbalance (QCM), as a sensitive piezoelectricity sensor, has been widely used for monitoring very tiny mass changes deposited on the crystal surface. Until fairly recently, it was believed that excessive viscous loading would prohibit use of the QCM in liquid. In fact, operation in liquid is indeed possible, and the response of the QCM is still extremely sensitive to mass changes at the crystal-solution interface. Although the QCM sensor has high sensitivity, its sensitive element has no selectivity. In order to achieve the detection for the target, QCM electrode must be covered by a selective membrane. The sensitivity of a QCM-based sensor has been shown to be positively influenced by the specific surface area (SSA) of the sensing coatings. To increase the sensitivity and lower the detection limit, an increasing number of attempts have been reported to develop nanostructured coatings on the electrode of QCM to achieve the designed functions. The fibrous membranes, which are obtained from the electrospinning technique, have strong potential application as sensors because of their excellent properties such as high SSA, unique netted texture and porosity. Polyelectrolytes deposited as fibrous membranes on the electrode of QCM via electrospinning have shown higher sensitivities in detecting gas (H2S and NH3) and vapors (formaldehyde and moisture) than as cast films. However, water solubility of these polyelectrolytes limits their applications in aqueous environments.Based on the above challenges, the novel sensor for the first time based on electrospun nanofibrous membranes functionalized sputtering and self-assembly monolayer, on a QCM electrode for heavy metal ions, chloramphenicol (CAP), organophosphorous and carbamate pesticide detection were successfully demonstrated. The three-dimensional (3D) hydrophobic polystyrene (PS) fibrous membranes with high SSA were deposited on the electrode of QCM via electrospinning, followed by the functionalization of the physical and chemical modification. The QCM sensors with these fibrous membranes have the high sensitivity and low detection limit for detection in aqueous environments.The PS fibrous membranes with various morphologies, pore width and SSA were obtained by adjusting the concentration of polymer solution and the weight ratios of mixture solvents tetrahydrofuran (THF)/N,N-dimethylformamide (DMF). At the same time, the effect of polymer solution concentration and weight ratios of THF/DMF on PS membranes morphologies, pore width and SSA were investigated in detail by using FE-SEM and BET. As expected, the 10 wt% PS solution yielded 3D beaded fibrous membranes for all samples as a consequence of the Rayleigh instability in the jet because of the low viscosity of the polymer solution. The beaded PS fibrous membranes exhibited a well-developed nanotexture rough structure on the fiber surface and nanopores on the bead surface. With THF, cup-shaped beads with surface pores and several fibers with wrinkled surface were formed. Increasing DMF contents in the solvent mixtures, the average fiber diameters decreased and the beads became elliptical in shape. The highest SSA (43 m2/g) and pore volume (0.144 cm3/g) were achieved at a 1/3 THF/DMF solvent composition. With increasing the PS concentration from 10 wt% to 30 wt%, the beads disappeared gradually while uniform fibers formed, the average fiber diameters increased. But the SSA and pore volume of PS membranes decreased to 14 m2/g and 0.092 cm3/g, respectively.The highly sensitive QCM sensors for heavy metal ions detection in real time in solution were fabricated. The PS fibrous membranes were stabilized on the QCM electrode modified with subsequent gold sputtering, followed by the functionalization of the sensing 3-mercaptopropionic acid and polyethyleneimine. The results exhibited that the MPA-QCM sensors have high sensitivity for Cy2+detection while the PEI-QCM sensors were very sensitive for Cr3+. The effect of PS membranes loading, SSA on QCM electrode and pH on sensors performance was investigated in this study. The results showed that sensors sensitive increased concurrently with both increasing PS loading and SSA on QCM electrodes. The sensitivity of PEI-QCM sensors with optimal sensing membrane structure can be achieved to 427 Hz/ppm, and the detection limit as low to 5 ppb. Meanwhile, the MPA-QCM sensor response was found to be reproducible and showed an increase with increasing pH from 2 to 7.Additionally, QCM immuno and enzyme biosensors were also investigated in detail. A novel CAP biosensor composed of antibody immobilization on electrospun PS membranes and QCM was successfully fabricated. The 3D PS fibrous membranes comprising the porous PS fibers were deposited on the electrode of QCM via electrospinning, followed by the functionalization of the sensing antibody on the membranes. The functionality of the immobilized antibodies with the MPA method was tested with CAP. The frequency decrease was compared with the results of a similar system activated with PEI-glutaraldehyde (GA) immobilization. The MPA method showed the best results in terms of sensitivity. The antibody-functionalized MPA biosensor sensitivity increased concurrently with increasing the PS membranes loading and the SSA on the QCM electrode. At the same time, the morphology and structure of PS membranes also affect the biosensors performance. The biosensor responses showed good linearity in the 5-100 ppb concentration range. The activation time of the immobilization anti-CAP process strongly affected the biosensor performance, and 60 min was an optimum activation time. The resultant biosensors showed fast response (2-3 s) to CAP, with a detection limit of 5 ppb with optimal structure. Moreover, the biosensors also exhibited good capturing selectivity to CAP when tested with other antibiotics at a concentration between 5 and 200 ppb.A nanostructured complex, acetylcholinesterase (AChE) functionalized electrospun PS fibrous membranes, is developed as a novel sensing coating on QCM electrode for organophosphorous and carbamate pesticide detection. Dimethoate and carbaryl were selected as representative of organophosphorus and carbamate pesticides, respectively. The date from reacting between AChE and substrates were fitted to a first-order kinetics equation. The frequency shifts of biosensors and the speed of frequency shift (v0) were strongly dependent on the AChE loading. In fact, a plot of vo vs AChE loading exhibited a linear relationship in the AChE loading range of 0.6-9.6 units. The frequency shift of reaction between AChE and substrates increased with increasing the PS membranes loading and SSA on QCM electrode. For the same carbaryl concentration, the percent inhibition also increased with increasing the PS membranes loading and SSA. The detection limit of resultant enzyme biosensors for carbaryl and dimethoate were 50 and 80 ppb, respectively. Meanwhile, a good linearity was found between pesticide concentration and the percent inhibition in the carbaryl concentration range of 50 ppb-100 ppm (dimethoate concentration range of 80 ppb-100 ppm). Furthermore, the slope of straight line increased with increasing the PS membranes loading and SSA.The research will provide a guideline for improving the sensitivity and lowering detection limit of QCM sensors in solution. The application of the QCM sensor can be extended to detect microorganisms, protein, erythrocyte sedimentation, and gene.