Paper-based electrochemical platforms for separation, enrichment, and detection

Paper based analytical devices (PADs) have great potential in the application of point-of-care diagnosis. This dissertation focuses on the design and application of PADs, especially ones that integrate with electrochemical systems, to tackle various problems in analytical chemistry, such as multi-analyte separation, sample enrichment, and sensitive detection. Four types of PADs are described in this dissertation. The first PAD (oPAD-Ep) is designed for multi-analyte separation. The oPAD-Ep is fabricated using the principle of origami to create a stack of connected paper layers as an electrophoresis channel. Due to the thinness of paper, a high electric field can be achieved with low voltage supply. Serum proteins can be separated and the device can be unfolded for post-analysis. The second PAD (oPAD-ITP) is designed on a similar principle as the oPAD-Ep, but it is applied for sample enrichment. The major modification is to adjust electrolyte conditions to enable isotachophoretic enrichment of analytes. DNA with various lengths can be enriched within a few minutes, and can be collected on one of the paper folds. The third PAD (hyPAD) also focuses on sample enrichment. The device is assembled with two different paper materials, nitrocellulose and cellulose. The hyPAD can perform faradaic ion concentration polarization experiments. This technique uses faradaic electrochemistry to create a local electric field gradient in the paper channel and can enrich charged analytes including: DNA, proteins, and nanoparticles. The fourth PAD (oSlip-DNA) focuses on sensitive electrochemical detection of DNA hybridization assays. This method integrates magnetic enrichment and electrochemical signal amplification via silver nanoparticles. Using voltammetry, sensitive detection of Hepatitis B Virus DNA is achieved on the low-cost device.Chemistr

Aspherical supernova explosions and formation of compact black hole low-mass X-ray binaries

It has been suggested that black-hole low-mass X-ray binaries (BHLMXBs) with short orbital periods may have evolved from BH binaries with an intermediate-mass secondary, but the donor star seems to always have higher effective temperatures than measured in BHLMXBs (Justham, Rappaport & Podsiadlowski 2006). Here we suggest that the secondary star is originally an intermediate-mass (\sim 2-5 M_{\sun}) star, which loses a large fraction of its mass due to the ejecta impact during the aspherical SN explosion that produced the BH. The resulted secondary star could be of low-mass (\la 1 M_{\sun}). Magnetic braking would shrink the binary orbit, drive mass transfer between the donor and the BH, producing a compact BHLMXB.Comment: 4 pages, accepted for publication in MNRAS Letter

Collective excitations in low dimensional systems and stochastic control of population growth in a fluctuating environment

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2007.Vita.Includes bibliographical references.In this thesis, I study several problems in the following areas: collective excitations in condensed matter physics, noise in gene network and stochastic control in biophysics. In the first area, I construct an effective field theory to describe Bose-Einstein Condensate (BEC) realized in an external potential. This theory explicitly explores the idea of spontaneous symmetry breaking and its application in the description of phase transitions of confined systems. Based on the effective lagrangian, I calculate the excitation spectrum and Matsubara Green's functions using the method of functional integrals. The theory also shows that in one dimension the collective excitation of a bosonic system can be unified with that of a fermionic system, which is described by Luttinger liquid theory. The unified theory of collective excitations of low dimensional quantum systems motivates my study of collective excitations of interacting classical particles confined in one dimension. It is shown in my paper that the structure of Hamiltonian or Lagrangian for one dimensional constrained systems is uniquely determined by conservation laws. Therefore the excitations of bosonic, fermionic and classical particles are strikingly similar in one dimension.(cont.) In the second area, i. e., noise in gene networks and phenotypic switching in a fluctuating environment, I study the noise propagation in a gene network cascade using the method of master equations which examines the validity of the more popular methods such as the Langevin equation. To further explore the applications of stochastic processes for complex systems, I study phenotypic switches in a fluctuating environment. By combining the techniques of stochastic differential equation and stochastic dynamical programming, I propose a simple framework which can be used to study phenotypic growth dynamics. Another work is to explore the influence of environment on the dynamical properties of small systems is directed to the unusual blinking statistics of semiconductor quantum dots. I show in a model system that a broad spectrum of decay rates is possible when disorder is present in the environment.by Xiang Xia.Ph.D

System regularities in design of experiments and their applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Page 156 blank.Includes bibliographical references (p. 151-155).This dissertation documents a meta-analysis of 113 data sets from published factorial experiments. The study quantifies regularities observed among main effects and multi-factor interactions. Such regularities are critical to efficient planning and analysis of experiments, and to robust design of engineering systems. Three previously observed properties are analyzed - effect sparsity, hierarchy, and heredity. A new regularity on effect synergism is introduced and shown to be statistically significant. It is shown that a preponderance of active two-factor interaction effects are synergistic, meaning that when main effects are used to increase the system response, the interactions provide an additional increase and that when main effects are used to decrease the response, the interactions generally counteract the main effects. Based on the investigation of system regularities, a new strategy is proposed for evaluating and comparing the effectiveness of robust parameter design methods. A hierarchical probability model is used to capture assumptions about robust design scenarios. A process is presented employing this model to evaluate robust design methods.(cont.) This process is then used to explore three topics of debate in robust design: 1) the relative effectiveness of crossed versus combined arrays; 2) the comparative advantages of signal-to-noise ratios versus response modeling for analysis of crossed arrays; and 3) the use of adaptive versus "one shot" methods for robust design. For the particular scenarios studied, it is shown that crossed arrays are preferred to combined arrays regardless of the criterion used in selection of the combined array. It is shown that when analyzing the data from crossed arrays, signal-to-noise ratios generally provide superior performance; although that response modeling should be used when three-factor interactions are absent. Most significantly, it is shown that using an adaptive inner array design crossed with an orthogonal outer array resulted in far more improvement on average than other alternatives.by Xiang Li.Ph.D

Robust transportation policy analysis in Singapore using microscopic traffic simulator

Thesis (S.M. in Transportation)--Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 99-101).One of the main challenges of making strategic decisions in transportation is that we always face a set of possible future states due to deep uncertainty in traffic demand. This thesis focuses on exploring the application of model-based decision support techniques which characterize a set of future states that represent the vulnerabilities of the proposed policy. Vulnerabilities here are interpreted as states of the world where the proposed policy fails its performance goal or deviates significantly from the optimum policy due to deep uncertainty in the future. Based on existing literature and data mining techniques, a computational model-based approach known as scenario discovery is described and applied in an empirical problem. We investigated the application of this new approach in a case study based on a proposed transit policy implemented in Marina Bay district of Singapore. Our results showed that the scenario discovery approach performs well in finding the combinations of uncertain input variables that will result in policy failure.by Xiang Song.S.M.in Transportatio

Controllable Synthesis of Magnesium Oxysulfate Nanowires with Different Morphologies

One-dimensional magnesium oxysulfate 5Mg(OH)2 · MgSO4 · 3H2O (abbreviated as 513MOS) with high aspect ratio has attracted much attention because of its distinctive properties from those of the conventional bulk materials. 513MOS nanowires with different morphologies were formed by varying the mixing ways of MgSO4 · 7H2O and NH4OH solutions at room temperature followed by hydrothermal treatment of the slurries at 150 °C for 12 h with or without EDTA. 513MOS nanowires with a length of 20–60 μm and a diameter of 60–300 nm were prepared in the case of double injection (adding MgSO4 · 7H2O and NH4OH solutions simultaneously into water), compared with the 513MOS with a length of 20–30 μm and a diameter of 0.3–1.7 μm in the case of the single injection (adding MgSO4 · 7H2O solution into NH4OH solution). The presence of minor amount of EDTA in the single injection method led to the formation of 513MOS nanowires with a length of 100–200 μm, a diameter of 80–200 nm, and an aspect ratio of up to 1000. The analysis of the experimental results indicated that the hydrothermal solutions with a lower supersaturation were favorable for the preferential growth of 513MOS nanowires along b axis

Doping evolution of itinerant magnetic excitations in Fe-based oxypnictides

Employing the four-band tight-binding model we study theoretically the doping dependence of the spin response in the normal state of novel Fe-based pnictide superconductors. We show that the commensurate spin density wave (SDW) transition that arises due to interband scattering between the hole $\alpha$-pockets and the electron $\beta$-pockets disappears already at the doping concentration $x \approx 0.04$ reflecting the evolution of the Fermi surfaces. Correspondingly, with further increase of the doping the antiferromagnetic fluctuations are suppressed for $x > 0.1$ and the Im$\chi({\bf Q_{AFM}},\omega)$ becomes nearly temperature independent. At the same time, we observe that the uniform susceptibility deviates from the Pauli-like behavior and is increasing with increasing temperature reflecting the activation processes for the $\alpha$-Fermi surfaces up to temperatures of about T=800K. With increase of the doping the absolute value of the uniform susceptibility lowers and its temperature dependence changes. In particular, it is a constant at low temperatures and then decreases with increasing temperature. We discuss our results in a context of recent experimental data.Comment: 5 pages, accepted in EP

Comparing the effects of sun exposure and vitamin D supplementation on vitamin D insufficiency, and immune and cardio-metabolic function: The Sun Exposure and Vitamin D Supplementation (SEDS) Study

Background: Adults living in the sunny Australian climate are at high risk of skin cancer, but vitamin D deficiency (defined here as a serum 25-hydroxyvitamin D (25(OH)D) concentration of less than 50 nmol/L) is also common. Vitamin D deficiency may be a risk factor for a range of diseases. However, the optimal strategies to achieve and maintain vitamin D adequacy (sun exposure, vitamin D supplementation or both), and whether sun exposure itself has benefits over and above initiating synthesis of vitamin D, remain unclear. The Sun Exposure and Vitamin D Supplementation (SEDS) Study aims to compare the effectiveness of sun exposure and vitamin D supplementation for the management of vitamin D insufficiency, and to test whether these management strategies differentially affect markers of immune and cardio-metabolic function. Methods/Design: The SEDS Study is a multi-centre, randomised controlled trial of two different daily doses of vitamin D supplementation, and placebo, in conjunction with guidance on two different patterns of sun exposure. Participants recruited from across Australia are aged 18-64 years and have a recent vitamin D test result showing a serum 25(OH)D level of 40-60 nmol/L. Discussion: This paper discusses the rationale behind the study design, and considers the challenges but necessity of data collection within a non-institutionalised adult population, in order to address the study aims. We also discuss the challenges of participant recruitment and retention, ongoing engagement of referring medical practitioners and address issues of compliance and participant retention. Trial registration: Australia New Zealand Clinical Trials Registry: ACTRN12613000290796 Registered 14 March 2013

Spintronics: Fundamentals and applications

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes from the published versio