Date of Award

Spring 2012

Document Type


Degree Name

Doctor of Philosophy (PhD)


Micro and Nanoscale Systems

First Advisor

Long Que


A cost-effective and highly sensitive portable diagnostic device is needed to enable much more widespread monitoring of health conditions in disease prevention, detection, and control. Miniaturized and easy-to-operate devices can reduce the inherent costs and inefficiencies associated with healthcare testing in central laboratories. Hence, clinicians are beginning to use point of care (POC) testing and flexible clinical chemistry testing devices which are beneficial for the patient.

In our work, a low-cost and simple autonomous microfluidic device for biochemical detection was developed. The pumpless capillary system with capillary stop valves and trigger valves is fabricated on a silicon (Si) wafer and then bonded with the modified polydimethylsiloxane (PDMS) cover. The key point of this study is the change of the surface contact angle of the PDMS to achieve the functionalities such as timing features (capillary-driven stop valve) and basic logical functions (trigger valves). The polydimethylsiloxane-ethylene oxide polymer (PDMS-b-PEO) is utilized as a surfactant additive to make the PDMS hydrophilic. The contact angle of the modified PDMS can be adjusted from 80.9° to 21.5° with different mixing ratios. The contact angles of PEO-PDMS accepted in this work are from 80.9° to 58.5° to bring the capillary channel and valve into effect. This autonomous capillary-driven device with good microfluidic flow manipulation can be widely applied to a number of microfluidic devices and pumpless fluidic actuation mechanisms, which is suitable for cost-effective diagnostic tools in the biomedical analysis and POC testing applications.

Another obstacle for miniaturization of the bio-detection system is the optical detector. We developed a novel, highly sensitive and miniaturized detector. It integrates a light source--light emitting diode (LED), all necessary optical components, and a photodiode with preamplifier into one package about 2 cm x 2 cm x 2 cm, especially for the applications of lab-on-a-chip (LOC), portable bio-detection system and POC diagnostic system. The size of this detector is smaller than the existing miniaturized detector of the size 5 cm x 5 cm x 5 cm. The fluorescence dye 5-Carboxyfluorescein (5-FAM) dissolved into the solvent DMSO (Dimethyl Sulfoxide) and diluted with DI water was used as the testing solution samples. The prototype has been tested to prove a remarkable sensitivity at pico-scale molar, around 1.08 pM, which is the highest sensitivity by now. It is higher than the current limit of detection at 1.96 nm, which will be presented in detail in the latter section.