Bacterial pathogens are important targets for detection and identification in medicine, food safety, public health, and security. A rapid diagnostic of water quality is fundamental to avoid population exposure to pathogens. Traditional bacterial detection methods need large incubation times (between 18 and 96 hours) during which time people are at risk. Biosensors are easy to use and no complex equipment are needed to use them. Moreover, they are easy to integrate with fluidic systems and the results can be achieved instantaneously. For this reason, this thesis considers the use of electrical biosensors based on non-faradaic impedance to achieve the objective of detecting bacteria in aqueous media. Such biosensors allow direct detection without adding any additional reagent and they permit the distinction between live and dead bacteria. This work is based on a previous work in which was proposed the detection of bacteria by measurements of the capacity of the solution (Cs). The differentiation between dead and living cells was attributed to the difference of volumes and electric properties of each type of cell. This thesis is a rethinking of these studies and it is proposed that in the difference in the conductivity of its internal medium lies the distinction between dead and living cells. Indeed, as a result of its structure, living cells can act as conductive or insulating particles depending on the measured frequency. Based on these assumptions the following studies are performed. Using FEA simulations, we can evaluate the effect of bacteria positioning and the conductivity of measurement medium. The optimum relation between bacteria and interdigitated electrodes size is studied as well. Moreover, measures to detect E. coli with 1.5x1.5 µm IDEs and S. cerevisiae with IDEs de 6x6 µm are performed. A differential impedance spectrum representation is used to study the unique fingerprint that arises when microorganisms attach to the surface of IDEs. For E. coli, that fingerprint shows the dual electrical behavior, insulating and conductive, at different frequency ranges. However, in the case of yeast, it is observed that it acts as purely conductive particle. The design and manufacture of a new geometry of IDE to detect sub-micrometer microorganisms is performed. This new chip is designed with a microfluidic channel and a magnet integrated in the encapsulation to perform immunomagnetic catchment. All settings are optimized using techniques such as confocal microscopy and, finally, direct and indirect measurements of bacteria are carried out and the reason for the different results obtained are analyzed.
|Date of Award||2 Sept 2016|
- Institute of Microelectronics of Barcelona (IMB-CNM, CSIC)
|Supervisor||Antonio Baldí Coll (Director) & César Sánchez Fernández (Director)|
- Microorganisms detection
- Interdigitated electrodes