Person: Whitesides, George
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Whitesides
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Whitesides, George
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Publication Stepped Moduli in Layered Composites(Wiley-Blackwell, 2014) So, Ju-Hee; Tayi, Alok S.; Guder, Firat; Whitesides, GeorgeThis paper describes adaptive composites that respond to mechanical stimuli by changing their Young's modulus. These composites are fabricated by combining a shorter layer of elastic material (e.g., latex) and a longer layer of stiffer material (e.g., polyethylene and Kevlar), and fixing them together at their ends. Tension along the layered composite increases its length, and as the strain increases, the composite changes the load-bearing layer from the elastic to the stiff material. The result is a step in the Young's modulus of the composite. The characteristics of the step (or steps) can be engineered by changing the constituent materials, the number of layers, and their geometries (e.g., sinusoidal, hierarchical, two-dimensional web-like, rod-coil, embedded, and ring structures). For composites with more than two steps in modulus, the materials within the composites can be layered in a hierarchical structure to fit within a smaller volume, without sacrificing performance. These composites can also be used to make structures with tunable, stepped compressive moduli. An adaptation of these principles can generate an electronic sensor that can monitor the applied compressive strain. Increasing or decreasing the strain closes or opens a circuit and reversibly activates a light-emitting diode.Publication Density-based separation in multiphase systems provides a simple method to identify sickle cell disease(Proceedings of the National Academy of Sciences, 2014) Kumar, A. A.; Patton, Matthew Reiser; Hennek, J. W.; Lee, S. Y. R.; D, G.; Yang, X.; Kanter, J.; Shevkoplyas, S. S.; Brugnara, Carlo; Whitesides, GeorgeAlthough effective low-cost interventions exist, child mortality attributable to sickle cell disease (SCD) remains high in low-resource areas due, in large part, to the lack of accessible diagnostic methods. The presence of dense (ρ > 1.120 g/cm3) cells is characteristic of SCD. The fluid, self-assembling step-gradients in density created by aqueous multiphase systems (AMPSs) identifies SCD by detecting dense cells. AMPSs separate different forms of red blood cells by density in a microhematocrit centrifuge and provide a visual means to distinguish individuals with SCD from those with normal hemoglobin or with nondisease, sickle-cell trait in under 12 min. Visual evaluation of a simple two-phase system identified the two main subclasses of SCD [homozygous (Hb SS) and heterozygous (Hb SC)] with a sensitivity of 90% (73–98%) and a specificity of 97% (86–100%). A three-phase system identified these two types of SCD with a sensitivity of 91% (78–98%) and a specificity of 88% (74–98%). This system could also distinguish between Hb SS and Hb SC. To the authors’ knowledge, this test demonstrates the first separation of cells by density with AMPSs, and the usefulness of AMPSs in point-of-care diagnostic hematology.Publication Nanoskiving Core–Shell Nanowires: A New Fabrication Method for Nano-optics(American Chemical Society (ACS), 2014) Watson, Douglas C.; Martinez, R; Fontana, Yannik; Russo-Averchi, Eleonora; Heiss, Martin; Fontcuberta i Morral, Anna; Whitesides, George; Loncar, MarkoThis paper describes the fabrication of functional optical devices by sectioning quantum-dotin-nanowires systems with predefined lengths and orientations. This fabrication process requires only two steps: embedding the nanowires in epoxy, and using an ultramicrotome to section them across their axis (“nanoskiving”). This work demonstrates the combination of four capabilities: i) the control of the length of the nanowire sections at the nanometer scale; ii) the ability to process the nanowires after cutting using wet etching; iii) the possibility of modifying the geometry of the wire by varying the sectioning angle; and iv) the generation of as many as 120 consecutive slabs bearing nanowires which have uniform size and approximately reproducible lateral patterns, and which can subsequently be transferred to different substrates. The quantum dots inside the nanowires are functional and of a high optical quality after the sectioning process, and exhibit photoluminescent emission with wavelengths in the range of 650-710 nm.Publication Emergence of reconfigurable wires and spinners via dynamic self-assembly(Nature Publishing Group, 2015) Kokot, Gasper; Piet, David; Whitesides, George; Aranson, Igor S.; Snezhko, AlexeyDissipative colloidal materials use energy to generate and maintain structural complexity. The energy injection rate, and properties of the environment are important control parameters that influence the outcome of dynamic self-assembly. Here we demonstrate that dispersions of magnetic microparticles confined at the air-liquid interface, and energized by a uniaxial in-plane alternating magnetic field, self-assemble into a variety of structures that range from pulsating clusters and single-particle-thick wires to dynamic arrays of spinners (self-assembled short chains) rotating in either direction. The spinners emerge via spontaneous breaking of the uniaxial symmetry of the energizing magnetic field. Demonstration of the formation and disaggregation of particle assemblies suggests strategies to form new meso-scale structures with the potential to perform functions such as mixing and sensing.Publication Odd–Even Effects in Charge Transport across n -Alkanethiolate-Based SAMs(American Chemical Society (ACS), 2014) Baghbanzadeh, Mostafa; Simeone, Felice C.; Bowers, Carleen; Liao, Kung-ching; Thuo, Martin; Baghbanzadeh, Mahdi; Miller, Michael S.; Carmichael, Tricia Breen; Whitesides, GeorgeThis paper compares rates of charge transport across self-assembled monolayers (SAMs) of n-alkanethiolates having odd and even numbers of carbon atoms (nodd and neven) using junctions with the structure MTS/SAM//Ga2O3/EGaIn (M = Au or Ag). Measurements of current density, J(V), across SAMs of n-alkanethiolates on AuTS and AgTS demonstrated a statistically significant odd–even effect on AuTS, but not on AgTS, that could be detected using this technique. Statistical analysis showed the values of tunneling current density across SAMs of n-alkanethiolates on AuTS with nodd and neven belonging to two separate sets, and while there is a significant difference between the values of injection current density, J0, for these two series (log|J0Au,even| = 4.0 ± 0.3 and log|J0Au,odd| = 4.5 ± 0.3), the values of tunneling decay constant, β, for nodd and neven alkyl chains are indistinguishable (βAu,even = 0.73 ± 0.02 Å–1, and βAu,odd= 0.74 ± 0.02 Å–1). A comparison of electrical characteristics across junctions of n-alkanethiolate SAMs on gold and silver electrodes yields indistinguishable values of β and J0 and indicates that a change that substantially alters the tilt angle of the alkyl chain (and, therefore, the thickness of the SAM) has no influence on the injection current density across SAMs of n-alkanethiolates.Publication Open-Source Potentiostat for Wireless Electrochemical Detection with Smartphones(American Chemical Society, 2018) Ainla, Alar; Mousavi, Seyedeh Moloud; Tsaloglou, Maria-Nefeli; Redston, Julia; Bell, Jeffrey; Fernández-Abedul, M. Teresa; Whitesides, GeorgeThis paper describes the design and characterization of an open-source “universal wireless electrochemical detector” (UWED). This detector interfaces with a smartphone (or a tablet) using “Bluetooth Low Energy” protocol; the smartphone provides (i) a user interface for receiving the experimental parameters from the user and visualizing the result in real time, and (ii) a proxy for storing, processing, and transmitting the data and experimental protocols. This approach simplifies the design, and decreases both the size and the cost of the hardware; it also makes the UWED adaptable to different types of analyses by simple modification of the software. The UWED can perform the most common electroanalytical techniques of potentiometry, chronoamperometry, cyclic voltammetry, and square wave voltammetry, with results closely comparable to benchtop commercial potentiostats. Although the operating ranges of electrical current and voltage of the UWED (±1.5 V, ±180 μA) are more limited than most benchtop commercial potentiostats, its functional range is sufficient for most electrochemical analyses in aqueous solutions. Because the UWED is simple, small in size, assembled from inexpensive components, and completely wireless, it offers new opportunities for the development of affordable diagnostics, sensors, and wearable devices.Publication Paper-Based Analytical Device for Electrochemical Flow-Injection Analysis of Glucose in Urine(American Chemical Society (ACS), 2012) Lankelma, Jan; Nie, Zhihong; Carrilho, Emanuel; Whitesides, GeorgeThis article describes a new design for a paper-based electrochemical system for flow-injection analysis. Capillary wicking facilitates a gravity-driven flow of buffer solution continuously through paper and nitrocellulose, from a buffer reservoir at one end of the device to a sink at the other. A difference in height between the reservoir and the sink leads to a continuous and constant flow. The nitrocellulose lies horizontally on a working electrode, which consists of a thin platinum layer deposited on a solid support. The counter and reference electrodes are strategically positioned upstream in the buffer reservoir. A simple pipetting device was developed for reliable application of (sub)microliter volumes of sample without the need of commercial micropipets; this device did not damage the nitrocellulose membrane. Demonstration of the system for the determination of the concentration of glucose in urine resulted in a noninvasive, quantitative assay that could be used for diagnosis and monitoring of diabetes. This method does not require disposable test strips, with enzyme and electrodes, that are thrown away after each measurement. Because of its low cost, this system could be used in medical environments that are resource-limited.Publication Magnetic Levitation as a Platform for Competitive Protein–Ligand Binding Assays(American Chemical Society, 2012) Shapiro, Nathan D.; Soh, Siowling; Mirica, Katherine A.; Whitesides, GeorgeThis paper describes a method based on magnetic levitation (MagLev) that is capable of indirectly measuring the binding of unlabeled ligands to unlabeled protein. We demonstrate this method by measuring the affinity of unlabeled bovine carbonic anhydrase (BCA) for a variety of ligands (most of which are benzene sulfonamide derivatives). This method utilizes porous gel beads that are functionalized with a common aryl sulfonamide ligand. The beads are incubated with BCA and allowed to reach an equilibrium state in which the majority of the immobilized ligands are bound to BCA. Since the beads are less dense than the protein, protein binding to the bead increases the overall density of the bead. This change in density can be monitored using MagLev. Transferring the beads to a solution containing no protein creates a situation where net protein efflux from the bead is thermodynamically favorable. The rate at which protein leaves the bead for the solution can be calculated from the rate at which the levitation height of the bead changes. If another small molecule ligand of BCA is dissolved in the solution, the rate of protein efflux is accelerated significantly. This paper develops a reaction-diffusion (RD) model to explain both this observation, and the physical-organic chemistry that underlies it. Using this model, we calculate the dissociation constants of several unlabeled ligands from BCA, using plots of levitation height versus time. Notably, although this method requires no electricity, and only a single piece of inexpensive equipment, it can measure accurately the binding of unlabeled proteins to small molecules over a wide range of dissociation constants (Kd values within the range from 10 nM to 100 μM are measured easily). Assays performed using this method generally can be completed within a relatively short time period (20 min–2 h). A deficiency of this system is that it is not, in its present form, applicable to proteins with molecular weight greater than approximately 65 kDa.Publication Paper-Based, Capacitive Touch Pads(Wiley-VCH Verlag Berlin, 2012) Mazzeo, Aaron D.; Kalb, William B.; Chan, Lawrence; Killian, Matthew; Bloch, Jean-Francis; Mazzeo, Brian A.; Whitesides, GeorgeMetallized paper is patterned to create touch pads of arrayed buttons that are sensitive to contact with both bare and gloved fingers. The paper-based keypad detects the change in capacitance associated with the touch of a finger to one of its buttons. Mounted on an alarmed cardboard box, the keypad requires the appropriate sequence of touches to disarm the system.Publication Analyzing Forensic Evidence Based on Density with Magnetic Levitation(Wiley-Blackwell, 2013) Lockett, Matthew; Mirica, Katherine A.; Mace, Charles R.; Blackledge, Robert D.; Whitesides, GeorgeThis paper describes a method for determining the density of contact trace objects with magnetic levitation (MagLev). MagLev measurements accurately determine the density (±0.0002 g/cm3) of a diamagnetic object and are compatible with objects that are nonuniform in shape and size. The MagLev device (composed of two permanent magnets with like poles facing) and the method described provide a means of accurately determining the density of trace objects. This method is inexpensive, rapid, and verifiable and provides numerical values—independent of the specific apparatus or analyst—that correspond to the absolute density of the sample that may be entered into a searchable database. We discuss the feasibility of MagLev as a possible means of characterizing forensic-related evidence and demonstrate the ability of MagLev to (i) determine the density of samples of glitter and gunpowder, (ii) separate glitter particles of different densities, and (iii) determine the density of a glitter sample that was removed from a complex sample matrix.