Showing papers by "George M. Whitesides published in 2021"

Rectification in Molecular Tunneling Junctions Based on Alkanethiolates with Bipyridine-Metal Complexes.

Abstract: This paper addresses the mechanism for rectification in molecular tunneling junctions based on alkanethiolates terminated by a bipyridine group complexed with a metal ion, that is, having the struc...

Conformation, and Charge Tunneling through Molecules in SAMs.

Abstract: This paper demonstrates that the molecular conformation (in addition to the composition and structure) of molecules making up self-assembled monolayers (SAMs) influences the rates of charge tunneling (CT) through them, in molecular junctions of the form AuTS/S(CH2)2CONR1R2//Ga2O3/EGaIn, where R1 and R2 are alkyl chains of different length. The lengths of chains R1 and R2 were selected to influence the conformations and conformational homogeneity of the molecules in the monolayer. The conformations of the molecules influence the thickness of the monolayer (i.e. tunneling barrier width) and their rectification ratios at ±1.0 V. When R1 = H, the molecules are well ordered and exist predominantly in trans-extended conformations. When R1 is an alkyl group (e.g., R1 ≠ H), however, their conformations can no longer be all-trans-extended, and the molecules adopt more gauche dihedral angles. This change in the type of conformation decreases the conformational order and influences the rates of tunneling. When R1 = R2, the rates of CT decrease (up to 6.3×), relative to rates of CT observed through SAMs having the same total chain lengths, or thicknesses, when R1 = H. When R1 ≠ H ≠ R2, there is a weaker correlation (relative to that when R1 = H or R1 = R2) between current density and chain length or monolayer thickness, and in some cases the rates of CT through SAMs made from molecules with different R2 groups are different, even when the thicknesses of the SAMs (as determined by XPS) are the same. These results indicate that the thickness of a monolayer composed of insulating, amide-containing alkanethiols does not solely determine the rate of CT, and rates of charge tunneling are influenced by the conformation of the molecules making up the junction.

Storing and Reading Information in Mixtures of Fluorescent Molecules

Abstract: The rapidly increasing use of digital technologies requires the rethinking of methods to store data. This work shows that digital data can be stored in mixtures of fluorescent dye molecules, which are deposited on a surface by inkjet printing, where an amide bond tethers the dye molecules to the surface. A microscope equipped with a multichannel fluorescence detector distinguishes individual dyes in the mixture. The presence or absence of these molecules in the mixture encodes binary information (i.e., "0" or "1"). The use of mixtures of molecules, instead of sequence-defined macromolecules, minimizes the time and difficulty of synthesis and eliminates the requirement of sequencing. We have written, stored, and read a total of approximately 400 kilobits (both text and images) with greater than 99% recovery of information, written at an average rate of 128 bits/s (16 bytes/s) and read at a rate of 469 bits/s (58.6 bytes/s).

Estimating the Density of Thin Polymeric Films Using Magnetic Levitation

Abstract: While the density is a central property of a polymer film, it can be difficult to measure in films with a thickness of ∼100 nm or less, where the structure of the interfaces and the confinement of the polymer chains may perturb the packing and dynamics of the polymers relative to the bulk. This Article demonstrates the use of magneto-Archimedes levitation (MagLev) to estimate the density of thin films of hydrophobic polymers ranging from ∼10 to 1000 nm in thickness by employing a substrate with a water-soluble sacrificial release layer to delaminate the films. We validate the performance of MagLev for this application in the ∼1 μm thickness range by comparing measurements of the densities of several different films of amorphous hydrophobic polymers with their bulk values of density. We apply the technique to films < 100 nm and observe that, in several polymers, there are substantial changes in the levitation height, corresponding to both increases and decreases in the apparent density of the film. These apparent changes in density are verified with a buoyancy control experiment in the absence of paramagnetic ions and magnetic fields. We measure the dependence of density upon thickness for two model polymeric films: poly(styrene) (PS) and poly(methyl methacrylate) (PMMA). We observe that, as the films are made thinner, PS increases in density while PMMA decreases in density and that both exhibit a sigmoidal dependence of density with thickness. Such changes in density with thickness of PS have been previously observed with reflectometric measurements (e.g., ellipsometry, X-ray reflectivity). The interpretation of these measurements, however, has been the subject of an ongoing debate. MagLev is also compatible with nontransparent, rough, heterogeneous polymeric films, which are extremely difficult to measure by alternative means. This technique could be useful to investigate the properties of thin films for coatings, electronic devices, and membrane-based separations and other uses of polymer films.

Elastic-instability-enabled locomotion.

Abstract: Locomotion of an organism interacting with an environment is the consequence of a symmetry-breaking action in space-time. Here we show a minimal instantiation of this principle using a thin circular sheet, actuated symmetrically by a pneumatic source, using pressure to change shape nonlinearly via a spontaneous buckling instability. This leads to a polarized, bilaterally symmetric cone that can walk on land and swim in water. In either mode of locomotion, the emergence of shape asymmetry in the sheet leads to an asymmetric interaction with the environment that generates movement--via anisotropic friction on land, and via directed inertial forces in water. Scaling laws for the speed of the sheet of the actuator as a function of its size, shape, and the frequency of actuation are consistent with our observations. The presence of easily controllable reversible modes of buckling deformation further allows for a change in the direction of locomotion in open arenas and the ability to squeeze through confined environments--both of which we demonstrate using simple experiments. Our simple approach of harnessing elastic instabilities in soft structures to drive locomotion enables the design of novel shape-changing robots and other bioinspired machines at multiple scales.

Bio-inspired design of soft mechanisms using a toroidal hydrostat

Abstract: Summary Biology is replete with soft mechanisms of potential use for robotics. Here, we report that a soft, toroidal hydrostat can be used to perform three functions found in both living and engineered systems: gripping, catching, and conveying. We demonstrate a gripping mechanism that uses a tubular inversion to encapsulate objects within a crumpled elastic membrane under hydrostatic pressure. This mechanism produces gripping forces that depend predictably upon the geometric and materials properties of the system. We next demonstrate a catching mechanism akin to that of a chameleon’s tongue: the elasticity of the membrane is used to power a catapulting inversion process (≈400 m/s2) to capture flying objects (e.g., a bouncing ball). Finally, we demonstrate a conveying mechanism that passes objects through the center of the toroidal tube (∼1 cm/s) using a continuous inversion-eversion process. The hybrid hard-soft mechanisms presented here can be applied toward the integration of soft functionality into robotic systems.

Characterizing Chelation at Surfaces by Charge Tunneling.

Abstract: This paper describes a surface analysis technique that uses the "EGaIn junction" to measure tunneling current densities (J(V), amps/cm2) through self-assembled monolayers (SAMs) terminated in a chelating group and incorporating different transition metal ions. Comparisons of J(V) measurements between bare chelating groups and chelates are used to characterize the composition of the SAM and infer the dissociation constant (Kd, mol/L), as well as kinetic rate constants (koff, L/mol·s; kon, 1/s) of the reversible chelate-metal reaction. To demonstrate the concept, SAMs of 11-(4-methyl-2,2'-bipyrid-4'-yl (bpy))undecanethiol (HS(CH2)11bpy) were incubated within ethanol solutions of metal salts. After rinsing and drying the surface, measurements of current as a function of incubation time and concentration in solution are used to infer koff, kon, and Kd. X-ray photoelectron spectroscopy (XPS) provides an independent measure of surface composition to confirm inferences from J(V) measurements. Our experiments establish that (i) bound metal ions are stable to the rinsing step as long as the rinsing time, τrinse ≪ 1 k o f f ; (ii) the bound metal ions increase the current density at the negative bias and reduce the rectification observed with free bpy terminal groups; (iii) the current density as a function of the concentration of metal ions in solution follows a sigmoidal curve; and (iv) the values of Kd measured using J(V) are comparable to those measured using XPS, but larger than those measured in solution. The EGaIn junction, thus, provides a new tool for the analysis of the composition of the surfaces that undergo reversible chemical reactions with species in solution.