Impedance and plasmonic based MEMS sensors for chemical and biological applications
Authors:   , Jasim Ibrahem 
Published: August 2021

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI-COLUMBIA AT REQUEST OF AUTHOR.] The first project is implementing a biosensor for simultaneous detection of Salmonella serogroups. Foodborne pathogens cause millions of infections annually in the United States alone. Those infections are dangerous that may lead to death. Salmonella is one of the public foodborne bacterial pathogens and results in severe illness such as diarrhea, abdominal cramps, and fever. The economic impact of foodborne pathogen testing cost the food industry huge cost. The food industry is seeking new pathogen detection techniques that can perform the task rapidly while maintaining high accuracy by eliminating the enrichment step. In this research, the design and fabrication of a highly sensitive impedance based biosensor was reported for the rapid and simultaneous detection of three Salmonella serogroups or other pathogen types. The microfluidic device starts with a single microchannel that splits into three micro-channels. Each channel includes the following innovative features: (1) a region for focusing the Salmonella cells using a ramp down vertical electrode pair along with tilted finger pairs (45o) with a ramp down channel that generates p-DEP forces to focus and concentrate the Salmonella pathogens into the center of the microchannel, and direct them toward a narrower channel where the sensing region is located. This technique resulted in highly concentrated samples. (2) A region for trapping the targeted pathogen cells over the detection region, to be captured by the immobilized antibodies, consists of a pair of elliptical shaped vertical electrodes that generates p-DEP across the detection region. (3) A region for bacteria sensing consists of interdigitated electrode array (IDE array) with 10-finger pairs. The device was fabricated using microfabrication techniques such as photolithography, wet etching and surface plasma treatment. Initially three types of Salmonella antibodies (type B, D and E) were mixed separately with the cross-linker (Sulfo-LC-SPDP). The electrode surfaces were then functionalized with the three mixtures, one for each channel and without causing any cross contamination. The Salmonella samples were spiked with Salmonella type B and introduced into the biosensor via the sample inlet towards the focusing region. An optimum AC voltage 6 Vp-p was applied to the focusing electrode at 6 MHz to focus and concentrate the Salmonella pathogens to the center of the microchannel and directed toward the sensing region, where they bind to the immobilized antibody sites. This binding caused a change in the impedance, which was measured using an impedance analyzer. The performance of the devices was excellent as evidenced by the focusing capabilities that increased the strength of the measured signal by a factor between 4 and 6.25, high sensitivity of 3 cell/ml and detection time of less than 1 hour. The second project is implementing a fiber optic based chemical sensor, which consists of a thin film metal patterned with holes, used for extraordinary optical transmission (EOT). Those patterned metal holes provide an efficient chemical sensing mechanism because of their dependence on the resonance wavelength of the EOT on the surrounding medium refractive index (RI). The sensing technique is based on the change of the effective index at the interface of the metal to the surrounding environment which produces a wavelength shift depending on the refractive index of the ambient medium. In this research, the integration of EOT with a single mode fiber optic was reported to form smaller, more sensitive and cheaper chemical sensor. The patterned aluminum (Al) holes were fabricated on top of the fiber tip of the single mode fiber using microspheres lithography (MPL). MPL is a nanopatterning technique that utilizes a self-assembled microspheres single layer as a mask optical element to create highly ordered periodic nano/microstructures. The reflection spectrums of the fabricated fiber were recorded. The sensor can be designed to operate at a specific wavelength by controlling different parameters such as holes diameter, periodicity, number of holes in the core, and the thickness of the patterned Al. The sensor was tested in different mediums. Whenever the medium was changed, a wavelength shift was obtained. For glucose sensing application, the sensor was tested in different glucose concentrations. The lowest detection limit was 6.7 mg/ml with a sensitivity of 632 nm/RIU. By changing the angle-of-incident UV illumination (off-axis exposure) and map the ability to steer the photonic jet around the microsphere array combined with etching/ lift-off, complex nano structures were successfully fabricated. Using the same assembled microspheres monolayer, multiple UV illumination jets were projected to create multiple holes groups from each assembled sphere. The RI based fiber sensor with three-hole group and four-hole group patterns were fabricated and tested in different concentrations of glucose and salt in water with sensitivities of 946 nm/RIU and 735 nm/RIU, respectively. The lowest tested glucose and salt concentration was 5 mg/ml. Microsphere lithography with off-axis exposure technique has increased the fiber probe sensitivity by 40 [percent] compared to single exposure technique, while it still provides a cost-effective, flexible, high-throughput fabrication of the fiber sensor. In this research, MPL was used as an alternative fabrication technique to Focus Ion Beam (FIB) and other expensive nanofabrication techniques for performing photolithography and thus creating nanodisks on the endface of the fiber optic to create a Surface Enhanced Raman Spectroscopy (SERS) sensor. The SERS sensor consists of an array of periodic disks patterned on the endface of a multi-mode fiber optic of core diameter of 62. The fabricated disks are made of gold because it provides a good reflected Raman signals with high intensities. The research results show that despite the lack of alignment to the fiber core, viable sensors can be created at low cost. MPL with off sure technique was used to create 3-disks and 4-disks groups which increased the number of disks in the core area of the fiber endface, increasing the periodicity of the fabricated disks. The sensor was successfully tested in different concentrations of Rhodamine 6G (R6G) dye solutions.