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Access Type
WSU Access
Date of Award
January 2023
Degree Type
Dissertation
Degree Name
Ph.D.
Department
Electrical and Computer Engineering
First Advisor
Amar Basu
Abstract
Inline chemical detectors are a critical part of analytical chemistry workflows such as chromatography and flow injection analysis. These detectors typically sense an analyte based on its mass, charge, or optical properties. However, adsorption, partition coefficient, and solubility are equally fundamental properties of an analyte that cannot be sensed by inline detectors. This thesis introduces the Stagnant Cap Hydrodynamic Retardation Effect Detector (SHRED), a novel inline opto-microfluidic sensor modality which detects amphiphilic molecules such as surfactants and proteins based on their adsorption to a liquid-liquid interface. The amphiphilic analytes are rapidly encapsulated in a train of water-in-oil droplets flowing in a microfluidic channel. The molecules adsorb to the liquid-liquid interface and are convected to the rear end of the droplet by recirculating flows that occur naturally as the droplet moves through the channel. The adsorbed molecules aggregate into an immobile, viscoelastic film, i.e., a stagnant cap, which not only deforms the droplet shape but also retards its velocity. These changes are monitored using a light scattering detector. Droplet frequency, which represents relative drop velocity, decreases with analyte concentration, while duty cycle, which represents relative drop length, increases. SHRED offers a label-free, simple, inexpensive, self-renewing surface, real time and continuous monitoring of surfactants and proteins which has widespread applications in pharmaceutical industry, biotechnology, detergent industry, etc. We conducted a detailed study on how surfactant properties impact the formation of the stagnant cap and therefore the ability of the SHRED sensor to detect different type of surfactants. These properties include molecular weight, Critical Micelle Concentration (CMC), Hydrophilic Lipophilic Balance (HLB), charge, adsorption and desorption rates. SHRED successfully detected different concentrations of surfactants Tween 20, Tween 40, Tween 80, Triton X-100, SDS and proteins such as IgG and BSA. Using SHRED with HPLC grade water as dispersed phase and oleic acid as continuous phase, the Limit of Detection (LOD) for BSA and IgG was found to be 606 nM (0.04 mg/ml) and 206 nM (0.031 mg/ml). For non-ionic surfactants in increasing order of molecular weights, Triton-X-100, Tween 20, Tween 40 and Tween 80 the limit of detection was 1.712 mM (1000 ppm), 0.056 mM (62.5 ppm), 0.053 mM (62.5 ppm), and 0.050 mM (62.5 ppm) respectively. Anionic surfactant Sodium Dodecyl Sulfate (SDS) showed a LOD of 0.173 mM while the cationic surfactant CTAB showed no SHRED effect. We then systematically discuss methods to optimize sensor performance by studying the effect of channel geometry and pressure conditions on signal amplitude, speed and stability. To explore the impact of channel geometry, we tested chips of different lengths (20mm,40mm and 50mm) and different aspect ratios (0.67,0.8,1,1.33 and 2). Optimal operating droplet frequency and duty cycle were also determined. To the best of our knowledge, this is the first study to not only leverage the stagnant cap phenomenon in a microfluidic environment for detection, but also to provide detailed fundamental understanding of underlying mechanisms of adsorption, surfactant transport along with the deformation and retardation that occur in the micro droplets due to the presence of surfactants. Our future work involves incorporating selectivity and combining SHRED with a chromatography system to use it for gas detection for environmental pollution monitoring and to perform cell secretion assays for drug testing/discovery and other pharmaceutical purposes. This thesis has laid the framework for our future vision. SHRED has the potential to determine therapy efficacy and personalized therapy regime by monitoring monoclonal antibodies (mAbs) and surfactants that are being considered for fighting COVID-19 and other respiratory infections. Using SHRED sensor, we are successful in detecting IgG antibodies which are indicators of COVID-19 infection. In future we intent to make this cost-effective tool available especially for the drug and pharmaceutical industry to expedite the process of drug discovery. In addition to this, SHRED is being developed towards solving the PFAS contamination issue which is declared as one of the biggest environmental threats.
Recommended Citation
Fatima, Afreen, "Stagnant Cap Hydrodynamic Retardation Effect Detector (shred) For Label-Free And Cost-Effective Sensing Of Surfactants And Proteins" (2023). Wayne State University Dissertations. 3855.
https://digitalcommons.wayne.edu/oa_dissertations/3855