George M. Whitesides

Harvard University, Cambridge, Massachusetts, United States

Are you George M. Whitesides?

Claim your profile

Publications (990)4797.59 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Optical metasurfaces-patterned arrays of plasmonic nanoantennas that enable the precise manipulation of light-matter interactions-are emerging as critical components in many nanophotonic materials, including planar metamaterials, chemical and biological sensors, and photovoltaics. The development of these materials has been slowed by the difficulty of efficiently fabricating patterns with the required combinations of intricate nanoscale structure, high areal density, and/or heterogeneous composition. One convenient strategy that enables parallel fabrication of periodic nanopatterns uses self-assembled colloidal monolayers as shadow masks; this method has, however, not been extended beyond a small set of simple patterns and, thus, has remained incompatible with the broad design requirements of metasurfaces. This paper demonstrates a technique-shadow-sphere lithography (SSL)-that uses sequential deposition from multiple angles through plasma-etched microspheres to expand the variety and complexity of structures accessible by colloidal masks. SSL harnesses the entire, relatively unexplored, space of shadow-derived shapes and-with custom software to guide multiangled deposition-contains sufficient degrees of freedom to (i) design and fabricate a wide variety of metasurfaces that incorporate complex structures with small feature sizes and multiple materials and (ii) generate, in parallel, thousands of variations of structures for high-throughput screening of new patterns that may yield unexpected optical spectra. This generalized approach to engineering shadows of spheres provides a new strategy for efficient prototyping and discovery of periodic metasurfaces.
    ACS Nano 09/2014; · 12.03 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper describes the design and fabrication of ion-sensing Electrochemical Paper-based Analytical Devices (EPADs) in which a miniaturized paper reference electrode is integrated with a small ion-selective paper electrode (ISPE) for potentiometric measurements. Ion-sensing EPADs use printed wax barriers to define electrochemical sample and reference zones. Single-layer EPADs for sensing of chloride ions include wax-defined sample and reference zones that each incorporates a Ag/AgCl electrode. In EPADs developed for other electrolytes (potassium, sodium, and calcium ions), a PVC-based ion-selective membrane is added to separate the sample zone from a paper indicator electrode. After the addition of a small volume (less than 10 L) of sample and reference solutions to different zones, ion-sensing EPADs exhibit a linear response, over three orders of magnitude, in ranges of electrolyte concentrations that are relevant to a variety of applications, with a slope close to the theoretical value (59.2/z mV). Ion-selective EPADs provide a portable, inexpensive, and disposable way of measuring concentrations of electrolyte ions in aqueous solutions.
    Analytical chemistry. 09/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Although 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/cm(3)) 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.
    Proceedings of the National Academy of Sciences of the United States of America. 09/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper demonstrates the enrichment of reticulocytes by centrifuging whole blood through aqueous multiphase systems (AMPSs)—immiscible phases of solutions of polymers that form step-gradients in density. The interfaces of an AMPS concentrate cells; this concentration facilitates the extraction of blood enriched for reticulocytes. AMPS enrich reticulocytes from blood from both healthy and hemochromatosis donors. Varying the osmolality and density of the phases of AMPS provides different levels of enrichment and yield of reticulocytes. A maximum enrichment of reticulocytemia of 64 ± 3 % was obtained from donors with hemochromatosis. When used on peripheral blood from normal donors, AMPS can provide a higher yield of enriched reticulocytes and a higher proportion of reticulocytes expressing CD71 than differential centrifugation followed by centrifugation over Percoll. Blood enriched for reticulocytes by AMPS could be useful for research on malaria. Several species of malaria parasites show a preference to invade young erythrocytes and reticulocytes; this preference complicates in vitro cultivation of these species in human blood. Plasmodium knowlesi malaria parasites invade normal human blood enriched for reticulocytes by AMPSs at a rate 2.2 times greater (p-value < 0.01) than they invade unenriched blood. Parasite invasion in normal blood enriched by AMPS was 1.8 times greater (p-value < 0.05) than in blood enriched to a similar reticulocytemia by differential centrifugation followed by centrifugation over Percoll. The enrichment of reticulocytes that are invaded by malaria parasites demonstrates that AMPSs can provide a label-free method to enrich cells for biological research.
    American Journal of Hematology 09/2014; · 4.00 Impact Factor
  • Ju‐Hee So, Alok S. Tayi, Firat Güder, George M. Whitesides
    [Show abstract] [Hide abstract]
    ABSTRACT: This 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.
    Advanced Functional Materials 09/2014; · 10.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Many processes involve the movement of a disordered collection of small particles (e.g., powders, grain, dust, and granular foods). These particles move chaotically, interact randomly among themselves, and gain electrical charge by contact electrification. Understanding the mechanisms of contact electrification of multiple interacting particles has been challenging, in part due to the complex movement and interactions of the particles. To examine the processes contributing to contact electrification at the level of single particles, a system was constructed in which an array of millimeter-sized polymeric beads of different materials were agitated on a dish. The dish was filled almost completely with beads, such that beads did not exchange positions. At the same time, during agitation, there was sufficient space for collisions with neighboring beads. The charge of the beads was measured individually after agitation. Results of systematic variations in the organization and composition of the interacting beads showed that three mechanisms determined the steady-state charge of the beads: (i) contact electrification (charging of beads of different materials), (ii) contact de-electrification (discharging of beads of the same charge polarity to the atmosphere), and (iii) a long-range influence across beads not in contact with one another (occurring, plausibly, by diffusion of charge from a bead with a higher charge to a bead with a lower charge of the same polarity).
    Journal of the American Chemical Society 08/2014; · 10.68 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper describes several noncontact methods of orienting objects in 3D space using Magnetic Levitation (MagLev). The methods use two permanent magnets arranged coaxially with like poles facing and a container containing a paramagnetic liquid in which the objects are suspended. Absent external forcing, objects levitating in the device adopt predictable static orientations; the orientation depends on the shape and distribution of mass within the objects. The orientation of objects of uniform density in the MagLev device shows a sharp geometry-dependent transition: an analytical theory rationalizes this transition and predicts the orientation of objects in the MagLev device. Manipulation of the orientation of the levitating objects in space is achieved in two ways: (i) by rotating and/or translating the MagLev device while the objects are suspended in the paramagnetic solution between the magnets; (ii) by moving a small external magnet close to the levitating objects while keeping the device stationary. Unlike mechanical agitation or robotic selection, orienting using MagLev is possible for objects having a range of different physical characteristics (e.g., different shapes, sizes, and mechanical properties from hard polymers to gels and fluids). MagLev thus has the potential to be useful for sorting and positioning components in 3D space, orienting objects for assembly, constructing noncontact devices, and assembling objects composed of soft materials such as hydrogels, elastomers, and jammed granular media.
    Proceedings of the National Academy of Sciences of the United States of America. 08/2014;
  • Source
    Bone; 08/2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: Self-assembled monolayers (SAMs), prepared by reaction of terminal n-alkynes (HC≡C(CH</sub>2)</sub>nCH</sub>3, n = 5, 7, 9, and 11) with Au(111) at 60</sup>oC were characterized using scanning tunneling microscopy (STM), infra-red reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and contact angles of water. In contrast to previous spectroscopic studies of this type of SAMs, these combined microscopic and spectroscopic experiments confirm formation of highly-ordered SAMs having packing densities and molecular chain orientations very similar to those of alkanethiolates on Au(111). Physical properties-hydrophobicity, high surface order, and packing density-also suggest that SAMs of alkynes are similar to SAMs of alkanethiols. The formation of high-quality SAMs from alkynes requires careful preparation and manipulation of reactants in an oxygen-free environment: trace quantities of oxygen lead to oxidized contaminants and disordered surface films. The oxidation process occurs during formation of the SAM; via oxidation of the -C≡C- group (most likely catalyzed by the gold substrate in the presence of O</sub>2).
    Journal of the American Chemical Society 08/2014; · 10.68 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper describes an inexpensive, handheld device that couples the most common forms of electrochemical analysis directly to "the cloud" using any mobile phone, for use in resource-limited settings. The device is designed to operate with a wide range of electrode formats, performs on-board mixing of samples by vibration, and transmits data over voice using audio-an approach that guarantees broad compatibility with any available mobile phone (from low-end phones to smartphones) or cellular network (second, third, and fourth generation). The electrochemical methods that we demonstrate enable quantitative, broadly applicable, and inexpensive sensing with flexibility based on a wide variety of important electroanalytical techniques (chronoamperometry, cyclic voltammetry, differential pulse voltammetry, square wave voltammetry, and potentiometry), each with different uses. Four applications demonstrate the analytical performance of the device: these involve the detection of (i) glucose in the blood for personal health, (ii) trace heavy metals (lead, cadmium, and zinc) in water for in-field environmental monitoring, (iii) sodium in urine for clinical analysis, and (iv) a malarial antigen (Plasmodium falciparum histidine-rich protein 2) for clinical research. The combination of these electrochemical capabilities in an affordable, handheld format that is compatible with any mobile phone or network worldwide guarantees that sophisticated diagnostic testing can be performed by users with a broad spectrum of needs, resources, and levels of technical expertise.
    Proceedings of the National Academy of Sciences of the United States of America. 08/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: In complex, multicomponent systems, polymers often undergo phase transitions between distinct conformations. This paper reports a millimeter-scale granular model of coil-to-globule transitions: one “polymer” chain—a cylinders-on-a-string “pearl necklace”—and many spheres, all shaken on a horizontal surface. It is possible to describe the behavior of this granular system by using formalisms generally used in statistical physics of polymers. Two sets of experiments allowed the observation of first- and second-order coil-to-globule transitions. The model shows that the competition between long- and short-range interactions leads to a first-order transition. Well-designed granular system represents another kind of approach to the study of polymer phase transitions and might inspire future designs of polymer-like mesoscale systems.
    ChemPlusChem 07/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper demonstrates that the gas-filled compartments in the packing material commonly called "bubble wrap" can be re-purposed in resource-limited regions as containers to store liquid samples, and to perform in-field bioanalyses. The bubbles of bubble wrap are easily filled by injecting the samples into them using a syringe with a needle or a pipette tip, and then sealing the hole with nail hardener. The bubbles are transparent in the visible range of the spectrum, and can be used as "cuvettes" for absorbance and fluorescence measurements. The interiors of these bubbles are sterile and allow storage of samples without the need for expensive sterilization equipment. The bubbles are also permeable to gases, and can be used to culture and store microorganisms. By incorporating carbon electrodes, these bubbles can be used as electrochemical cells. This paper demonstrates the capabilities of the bubbles by culturing E-coli, growing C. elegans, measuring glucose and hemoglobin spectrophotometrically, and measuring ferrocyanide electrochemically, all within the bubbles.
    Analytical chemistry. 07/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper describes the fabrication of 3D soft, inflatable structures from thin, 2D tiles fabricated from elastomeric polymers. The tiles are connected using soft joints that increase the surface area available for gluing them together, and mechanically reinforce the structures to withstand the tensile forces associated with pneumatic actuation. The ability of the elastomeric polymer to withstand large deformations without failure makes it possible to explore and implement new joint designs, for example “double-taper dovetail joints,” that cannot be used with hard materials. This approach simplifies the fabrication of soft structures comprising materials with different physical properties (e.g., stiffness, electrical conductivity, optical transparency), and provides the methods required to “program” the response of these structures to mechanical (e.g., pneumatic pressurization) and other physical (e.g., electrical) stimuli. The flexibility and modularity of this approach is demonstrated in a set of soft structures that expanded or buckled into distinct, predictable shapes when inflated or deflated. These structures combine easily to form extended systems with motions dependent on the configurations of the selected components, and, when fabricated with electrically conductive tiles, electronic circuits with pneumatically active elements. This approach to the fabrication of hollow, 3D structures provides routes to new soft actuators.
    Advanced Functional Materials 07/2014; · 10.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Soft three dimensional, elastomeric structures and composite structures are easy to fabricate using click-e-bricks, and the internal architecture of these structures together with the capabilities built into the bricks themselves provide mechanical, optical, electrical, and fluidic functions.
    Advanced Materials 07/2014; · 14.83 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: doi: 10.1021/cm501596s
    Chemistry of Materials 06/2014; 26(14):4230-4237. · 8.24 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper investigates the influence of the atmosphere used in the fabrication of top electrodes from the liquid eutectic of gallium and indium (EGaIn) (the so-called “EGaIn” electrodes), and in measurements of current density, J(V) (A/cm2), across self-assembled monolayers (SAMs) incorporated into Ag/SR//Ga2O3/EGaIn junctions, on values of J(V) obtained using these electrodes. A gas-tight measurement chamber was used to control the atmosphere in which the electrodes were formed, and also to control the environment in which the electrodes were used to measure current densities across SAM-based junctions. Seven different atmospheres—air, oxygen, nitrogen, argon, and ammonia, as well as air containing vapors of acetic acid or water—were surveyed using both “rough” conical-tip electrodes, and “smooth” hanging-drop electrodes. (The manipulation of the oxide film during the creation of the conical-tip electrodes leads to substantial, micrometer-scale roughness on the surface of the electrode, the extrusion of the drop creates a significantly smoother surface.) Comparing junctions using both geometries for the electrodes, across a SAM of n-dodecanethiol, in air, gave log |J|mean = −2.4 ± 0.4 for the conical tip, and log |J|mean = −0.6 ± 0.3 for the drop electrode (and, thus, Δlog |J| ≈ 1.8); this increase in current density is attributed to a change in the effective electrical contact area of the junction. To establish the influence of the resistivity of the Ga2O3 film on values of J(V), junctions comprising a graphite electrode and a hanging-drop electrode were compared in an experiment where the electrodes did, and did not, have a surface oxide film; the presence of the oxide did not influence measurements of log |J(V)|, and therefore did not contribute to the electrical resistance of the electrode. However, the presence of an oxide film did improve the stability of junctions and increase the yield of working electrodes from 70% to 100%. Increasing the relative humidity (RH) in which J(V) was measured did not influence these values (across methyl (CH3)- or carboxyl (CO2H)-terminated SAMs) over the range typically encountered in the laboratory (20%–60% (RH)).
    Chemistry of Materials. 06/2014; 26(13):3938–3947.
  • [Show abstract] [Hide abstract]
    ABSTRACT: A mid-infrared opto-nanofluidics was developed using a Si-liquid-Si slot-waveguide. Through an optical-field enhancement with a direct interaction between the probe light and analyte, the detection sensitivity is increased by 50 times compared to evanescent-wave-sensing.
    CLEO: Science and Innovations; 06/2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: Crystallization of a solution with high enantiomeric excess can generate a mixture of crystals of the desired enantiomer and the racemic compound. Using a mixture of S-/RS-ibuprofen crystals as a model, we demonstrated that magnetic levitation (MagLev) is a useful technique for analysis, separation and enantioenrichment of chiral/racemic products.
    Chemical communications (Cambridge, England). 05/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Junctions with the structure AgTS/S(CH2)nT//Ga2O3/EGaIn (where S(CH2)nT is a self-assembled monolayer, SAM, of n-alkanethiolate bearing a terminal functional group T) make it possible to examine the response of rates of charge transport by tunneling to changes in the strength of the interaction between T and Ga2O3. Introducing a series of Lewis acidic/basic functional groups (T = -OH, -SH, -CO2H, -CONH2, and -PO3H) at the terminus of the SAM gave values for the tunneling current density, J(V) in A/cm2, that were indistinguishable (i.e., differed by less than a factor of 3) from the values observed with n-alkanethiolates of equivalent length. The insensitivity of the rate of tunneling to changes in the terminal functional group implies that replacing weak van der Waals contact interactions with stronger hydrogen- or ionic bonds at the T//Ga2O3 interface does not change the shape (i.e., the height or width) of the tunneling barrier enough to affect rates of charge transport. A comparison of the injection current, J0, for T = -CO2H, and T = -CH2CH3-two groups having similar extended lengths (in Å, or in numbers of non-hydrogen atoms)-suggests that both groups make indistinguishable contributions to the height of the tunneling barrier.
    Nano letters. 05/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The use of omniphobic "fluoroalkylated paper" as a substrate for inkjet printing of aqueous inks that are the precursors of electrically conductive patterns is described. By controlling the surface chemistry of the paper, it is possible to print high resolution, conductive patterns that remain conductive after folding and exposure to common solvents.
    Advanced Materials 05/2014; · 14.83 Impact Factor

Publication Stats

37k Citations
4,797.59 Total Impact Points

Institutions

  • 1984–2014
    • Harvard University
      • • Wyss Institute for Biologically Inspired Engineering
      • • Department of Chemistry and Chemical Biology
      • • Department of Physics
      Cambridge, Massachusetts, United States
  • 2012
    • Beth Israel Deaconess Medical Center
      • Division of Infectious Diseases
      Boston, MA, United States
    • VU University Amsterdam
      • Department of Molecular Cell Physiology
      Amsterdam, North Holland, Netherlands
    • Stanford University
      • Department of Electrical Engineering
      Palo Alto, California, United States
  • 2011
    • National University of Singapore
      • Department of Chemistry
      Singapore, Singapore
  • 2010
    • Hebrew University of Jerusalem
      Yerushalayim, Jerusalem District, Israel
    • Washington University in St. Louis
      • Department of Energy, Environmental, and Chemical Engineering
      Saint Louis, MO, United States
    • Lawrence Livermore National Laboratory
      • Physical & Life Sciences Directorate
      Livermore, CA, United States
  • 2009
    • Polish Academy of Sciences
      • Instytut Chemii Fizycznej
      Warsaw, Masovian Voivodeship, Poland
  • 2007–2008
    • University of Wisconsin, Madison
      • Department of Biochemistry
      Madison, MS, United States
    • Universiteit Twente
      Enschede, Overijssel, Netherlands
    • University of California, San Diego
      • Department of Mechanical and Aerospace Engineering (MAE)
      San Diego, CA, United States
  • 2002–2008
    • Harvard Medical School
      • Department of Pathology
      Boston, MA, United States
    • Office of Naval Research
      Arlington, Virginia, United States
  • 2005
    • University of Texas at Austin
      • Department of Chemistry and Biochemistry
      Austin, Texas, United States
    • University of Toronto
      • Department of Chemistry
      Toronto, Ontario, Canada
  • 2003
    • University of Illinois, Urbana-Champaign
      • Department of Chemistry
      Urbana, IL, United States
    • University of Washington Seattle
      • Department of Materials Science and Engineering
      Seattle, WA, United States
    • Johns Hopkins University
      • Department of Biomedical Engineering
      Baltimore, MD, United States
  • 1998
    • Universita degli studi di Ferrara
      • Department of Chemical and Pharmaceutical Sciences
      Ferrara, Emilia-Romagna, Italy
  • 1997
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Angeles, CA, United States
  • 1967–1989
    • Massachusetts Institute of Technology
      • Department of Chemistry
      Cambridge, Massachusetts, United States
  • 1985
    • Idenix Pharmaceuticals, Inc.
      Cambridge, Massachusetts, United States