Graduate Thesis Or Dissertation


Semiconductor Nanocrystals as Detection Components in Microfluidic Analytical Systems Public Deposited

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  • Miniaturized and portable microfluidic analytical platforms have been widely explored in the broad field of chemical analysis. The concept of microfluidics offer a number of important advantages, including low reagent consumption, low-cost detection, high sample throughput, and shorter analysis time. Semiconductor nanocrystals or quantum dots have been extensively utilized in a wide variety of manners including as emitters in consumer products (light bulbs) and, more importantly here, as elements for chemical and biological sensing. These applications are possible because of their unique optical and electronic properties. The goal of this dissertation was to integrate two fields of study – microfluidics for chemical and biological analysis, and semiconductor nanocrystals for detection – into a single analytical device. In this work, the development of microfluidic analytical devices incorporating semiconductor nanocrystals for chemical sensing is presented. The research effort started with the synthesis of a dual emission fluorescent quantum dot pair. CdTe quantum dots of two different sizes, emitting green and red light, were utilized as fluorophores. The green dot was used as a constant emitter and was encapsulated in a silica shell. The red dot was immobilized on the silica surface and selectively quenched in the presence of copper. The wavelength-ratiometric sensor thus constructed resulted in a fluorescence color change from red to green, identified visually, corresponding to the absence and presence of copper. The wavelength-ratiometric sensor was transferred to a microfluidic format made of poly(methyl methacrylate) (PMMA) and assembled using polycaprolactone (PCL). Red and green intensity values from the RGB colorspace were used as analytical signals to generate a calibration curve. A Stern-Volmer plot in the range of 1-30 mg/L was used to quantify copper in water samples. For serum samples, absolute intensity values of green and red were plotted to mitigate non-linearity in the Stern-Volmer plot arising from the formation of copper-protein complexes. The research continued with the development of a quantum dot assay performed in a paper-based well plate format for cyanide detection. Chitosan modified CdTe quantum dots (CS-QDs) were synthesized and used as a fluorophore. The CS-QDs were quenched with copper and deposited onto a glass microfiber filter (GF/B). The paper-based well plate resembled a conventional plastic well plate in its x and y dimensions, to facilitate detection in a plate reader, and was designed to contain three layers: an opaque floor, a middle layer containing the reagents in a GF/B matrix, and an upper layer that retained the reagent pads while providing direct access to the individual wells for sample application and optical interrogation. The dried assay of quenched CS-QDs on GF/B was assembled on a paper-based well plate using PCL as a hydrophobic barrier and adhesive for each individual assay. The introduction of cyanide triggered recovery of a fluorescent signal from the CS-QDs in proportion to cyanide concentration, arising from copper-cyanide complex formation and depletion of copper from the QD surface. Linear calibration was measured in the range of 0-200 µM within 30 min. A paper-based well plate using CS-QDs made rapid, high sample throughput, low-cost testing feasible and convenient for cyanide detection. The color change was visually perceptible (making field use feasible) and the fluorescence output change was readily and sensitively measured, with greater accuracy and precision, using an automated plate reader. In the final component of the research effort described here, a novel method of constructing dsDNA, aimed at total gene synthesis on a shortened timescale and with a reduced error rate, was developed. Short (28-mer) oligo fragments from a library were assembled through successive annealing and ligation processes, followed by polymerase chain reaction (PCR) amplification. First, two oligo fragments annealed to form a double-stranded DNA molecule (dsDNA). The dsDNA was immobilized onto magnetic beads as solid support via streptavidin-biotin binding. Next, singlestranded oligo fragments were successively added through ligation to form the complete DNA molecule. More than 97% of the nucleotides matched the expected sequence. Coupled with an automated dispensing system and libraries of short oligo fragments, this novel dsDNA synthesis method offers an efficient and cost-effective method for dsDNA production. The longer-term aim of the project is to utilize inkjet deposition technology to fully automate the assembly process.
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  • 2017-08-09 to 2018-03-26



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