Chapter 23
Ultrasensitive Pathogen Quantification in Drinking Water Using Highly Piezoelectric Microcantilevers 1
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Wan Y.Shih ,G. Campbell, J. W.Yi ,H. Luo R. Mutharasan, and Wei-Heng Shih 1
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Departments of Materials Engineering and ChemicaI Engineering, Drexel University, Philadelphia, PA 10104
Current development of pathogen detection in water relies on filtration culture methods and fluorescence-based methods, e.g., fluorescence probes methods and DNA microarray methods. These techniques, however, do not lend themselves for in-situ, rapid, quantitative measurements. With the filtration culture methods, sample water is passed through a filter that is pretreated for visualization of the target pathogen. Growth of colonies on the filter indicates the presence of the target pathogen in the test water. Both the fluorescently labeled probe methods and the DNA microarray methods rely on detection using fluorescence spectroscopy, which is not quantitative. There is an immediate need for rapid, quantitative, and specific pathogen detection to ensure the safety of natural and manmade water supplies, including source, treated, distributed and recreational waters. Another development of biosensing technologies relies on silicon-based microcantilevers due to their availability and ease of integration with existing silicon based technologies. All silicon-based microcantilevers rely on external optical components for deflection detection. Antibody receptors were coated on the surface of the microcantilevers to bind target DNA, protein molecules, or bacteria. The adsorbed target molecules can be detected by monitoring the mechanical resonance frequency of the microcantilever. The adsorption of target molecules causes a change in the microcantilever's mass, which in turn causes a shift in the resonance frequency. Because of the small sizes of the silicon-based microcantilevers, about 100 μm in length, they exhibit high mass-detection sensitivity. The mass change per Hz is about Δm/Δƒ~10 g/Hz, where Am and Δƒ respectively denote the mass change and the corresponding resonance frequency change due to the binding of the target molecules. However, the required optical components are large and complex, i,ii
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© 2005 American Chemical Society
Karn et al.; Nanotechnology and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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requiring precise aligiment. Moreover, immersing the silicon-based microcantilevers in water reduces the resonance intensity by an order of magnitude,"" reducing the Q factor, defined as the ratio of the resonance peak frequency relative to the resonance peak width at half peak height, to about one, thus prohibiting the use of silicon-based microcantilever for in-water detection. The main reason that silicon based microcantilevers cannot have high resonance signal in water is that the microcantilevers are not piezoelectric. We have demonstrated highly piezoelectric lead zirconate titanate/stainless steel cantilevers for mass detection,** liquid density and viscosity sensing, yeast cell quantification,™ protein detection,™ and protein-antibody specific binding detection.™" Moreover, we showed that unlike a silicon-based microcantilever, a piezoelectric cantilever can still maintains a high Q value in water, making it particularly suitable for in-water detection. As an example, we showed the in-air and in-water resonance spectra of the PZT/stainless steel cantilever that was later used for yeast detection in Fig. 1(a). Detection of yeast cells is shown in Fig. 1(b) where the resonancefrequencyshift versus time of the cantilever immersed in different yeast concentrations is shown. Note that at different yeast concentrations, the resonancefrequencyshift rises differently with time. The kinetics of the resonance frequency shift bears the information of the yeast concentration and can be yeast concentration quantification. xv
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Fig. 1 : (a) First-mode resonancefrequencyspectra of the yeast detecting cantilever, (b) Resonancefrequencyshift versus time after the cantilever was immersed in the lg/L (full diamonds) and 2 g/L (open squares) suspension. Currently we are extending this approach to detect E. coli 0157:H7 in water using a piezoelectric PZT/stainless steel cantilever. The cantilever tip was coated with the antibody of E. coli 0157:H7. The detection of the E. coli 0157:H7 is shown in Fig. 2(a) where the resonance frequency shift with time due to the binding of the Ε coli cells to the cantilever tip. The detachment of the Ε coli 0157:H7 cells from the cantilever tip at pH = 2 in a glycine buffer is shown in
Karn et al.; Nanotechnology and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
181 Fig. 2(b). A scanning electron micrograph of the Ε coli 0157:H7 cells is shown in the insert of Fig. 2(a).
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time (min) Fig, 2: Resonance frequency shift versus time due to the immobilization of E. coli (a) and detachment of Ε coli (b). The insert shows a SEM micrograph of immobilized E. coli.
Examining the dependence of of the mass detection sensitivity, Af /Am, on the length and width of the cantilever and the resonance mode, we showed that the mass detection sensitivity of the cantilever increases with a decreasing cantilever size (see Fig. 3) as Af /Am oc \j wU where L is the length, w the p
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width, signifying that a piezoelectric cantilever of 50 urn in length would reach a detection sensitivity of 10* g/Hz, smaller than the mass of a single bacterium. This offers the potential for unprecedented detection sensitivities in ultra low concentrations. To achieve 10" g/Hz detection sensitivity, currently, we are making highly piezoelectric microcantilevers by integrating highly piezoelectric lead magnesium niobate-lead titanate (PMN-PT) thick layer in the microfabrication process. We have succeeded in making PMN-PT thick layers using a novel nano-layer coating approach and reasonably thick (e.g., 1-10 μηι) layers with excellent piezoelectric properties were made using a modified sol-gel approach. The X-ray diffraction pattern and a SEM micrograph of the PMN-PT thick layer on a titanium foil are shown in Fig. 4(a) and (b). 14
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Karn et al.; Nanotechnology and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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Downloaded by UNIV OF ROCHESTER on June 13, 2018 | https://pubs.acs.org Publication Date: December 14, 2004 | doi: 10.1021/bk-2005-0890.ch023
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