UC Davis

Biosensors are highly selective and sensitive towards target molecules and should be convenient for on-site toxicant detection. However, the conventional biosensors, including enzyme-linked immunoassay (ELISA), are lacking high sensitivity and hard to reveal a significant colorimetric signal for detecting a trace amount of toxicant. Thus, the detection requires analytical instruments, which limit the application of biosensors for on-site detection. In this dissertation, highly sensitive and portable colorimetric biosensors for the detection of trace amounts of toxicants were designed and fabricated by covalently immobilizing antibodies onto controlled microporous and nanofibrous membranes. The high specific surface of the nanofibers significantly increased the number of immobilized antibodies and the binding capacity with the target toxicant. Thus, the sensitivity of the nanofibrous membrane biosensor was dramatically increased, and a trace number of toxicants could reveal a naked-eye detectable color. Additionally, the diffusion of large biomolecules inside nanofibrous membranes was investigated, revealing the heterogeneous structures of electrospun nanofibrous membranes significantly hinder the diffusion of antibodies into the membrane and dramatically limit the sensitivity of nanofibrous membrane biosensors. Such structural drawbacks of regular electrospun nanofibrous membranes could be overcome by increasing hydrophilicity and controlling microporous structure. Moreover, an ultra-highly sensitive and portable biosensor was fabricated after optimization.Specifically, chapter 1 summarizes the background information of biosensor and toxicant detection. Chapter 2 reviews the conventional toxicant detection methods, the technique of fabricating electrospun nanofibrous membrane, and the development of biosensors. In chapter 3, the nanofibrous membrane ELISA biosensors were fabricated for the detection of residual antibiotics in foods. The biosensors exhibited a significant naked-eye distinguishable color at chloramphenicol at 0.3 ng/mL, revealing the potential for on-site detection applications. The novel nanofibrous membrane ELISA biosensors revealed several times increased sensitivities. However, the sensitivity was lower than the expectation based on the design of nanofibrous membrane sensors, which could be attributed to the hindered diffusion behavior of antibodies inside nanofibrous membranes. In chapter 4, the diffusion and partition behavior of proteins inside nanofibrous membranes were studied. Different from other micro-porous materials, electrospun nanofibrous membranes possess layer-by-layer accumulative heterogeneous structures. The effective pore sizes of the nanofibrous membranes were much smaller than the measured pore sizes, affecting the diffusion of proteins through the system and limiting the sensitivity of the biosensors. The results provided insights into the design of proper nanofibrous materials for optimizing the performance of biosensors. In chapter 5, an ultra-highly sensitive and portable colorimetric biosensor was fabricated by controlling microporous structure and increasing hydrophilicity. The protein could rapidly diffuse through the membrane, and the sensitivity of the membrane-based ELISA was dramatically improved. Lastly, chapter 6 summarizes the achievements of the dissertation.