Proteins are structurally dynamic macromolecules, and it is challenging to quantify the conformational properties of their native state in solution. Nanopores can be efficient tools to study proteins in a solution environment. In this method, an electric field induces electrophoretic and/or electro-osmotic transport of protein molecules through a nanopore slightly larger than the protein molecule. High-bandwidth ion current measurement is used to detect the transit of each protein molecule. First, our measurements reveal a correlation between the mean current blockade amplitude and the radius of gyration for each protein. Next, we find a correlation between the shape of the current signal amplitude distributions and the protein fluctuation as obtained from molecular dynamics simulations. Further, the magnitude of the structural fluctuations, as probed by experiments and simulations, correlates with the ratio of α-helix to β-sheet content. We highlight the resolution of our measurements by resolving two states of calmodulin, a canonical protein that undergoes a conformational change in response to calcium binding.

Enzymes and motor proteins are dynamic macromolecules that coexist in a number of conformations of similar energies. Protein function is usually accompanied by a change in structure and flexibility, often induced upon binding to ligands. However, while measuring protein flexibility changes between active and resting states is of therapeutic significance, it remains a challenge. Recently, our group has demonstrated that breadth of signal amplitudes in measured electrical signatures as an ensemble of individual protein molecules is driven through solid-state nanopores and correlates with protein conformational dynamics. Here, we extend our study to resolve subtle flexibility variation in dihydrofolate reductase mutants from unlabeled single molecules in solution. We first demonstrate using a canonical protein system, adenylate kinase, that both size and flexibility changes can be observed upon binding to a substrate that locks the protein in a closed conformation. Next, we investigate the influence of voltage bias and pore geometry on the measured electrical pulse statistics during protein transport. Finally, using the optimal experimental conditions, we systematically study a series of wild-type and mutant dihydrofolate reductase proteins, finding a good correlation between nanopore-measured protein conformational dynamics and equilibrium bulk fluorescence probe measurements. Our results unequivocally demonstrate that nanopore-based measurements reliably probe conformational diversity in native protein ensembles.

Molybdenum disulfide (MoS2) flakes can grow beyond the edge of an underlying substrate into a planar freestanding crystal. When the substrate edge is in the form of an aperture, reagent-limited nucleation followed by edge growth facilitate direct and selective growth of freestanding MoS2 membranes. We have found conditions under which MoS2 grows preferentially across micrometer-scale prefabricated solid-state apertures in silicon nitride membranes, resulting in sealed membranes that are one to a few atomic layers thick. We have investigated the structure and purity of our membranes by a combination of atomic-resolution transmission electron microscopy, elemental analysis, Raman spectroscopy, photoluminescence spectroscopy, and low-noise ion-current recordings through nanopores fabricated in such membranes. Finally, we demonstrate the utility of fabricated ultrathin nanopores in such membranes for single-stranded DNA translocation detection.

A hypothesis concerning the exponential buildup of polyelectrolyte multilayer films prepared by layer-by-layer assembly has become widely accepted in the scientific community. This model was first introduced with experimental data in Langmuir. It was subsequently described in Proceedings of the National Academy of Sciences and extended and amended in papers in Langmuir and other journals. According to the "in and out diffusion" hypothesis, as it is called, or "common rule" of exponential multilayer film buildup, as it is widely regarded, "a diffusion-based buildup mechanism ... explains most of the exponential-like growth process of polyelectrolyte multilayers reported in the literature." The present work offers an alternative viewpoint to specific elements of the hypothesis and the model as a whole.

Atomically thin semiconductors, such as the transition metal dichalcogenides, show great potential for nanoscale photodetection, energy harvesting, and nanophotonics. Here, we investigate the efficiency of energy transfer between colloidal quantum dots with a cadmium selenide core and cadmium sulfide shell and monolayer molybdenum diselenide (MoSe 2). We show that MoSe 2 effectively quenches the fluorescence of quantum dots when the two materials are in contact. We then separate the MoSe 2 and quantum dots by inserting variable thickness hexagonal boron nitride (h-BN) spacers and show that the efficiency at which the MoSe 2 quenches fluorescence decreases as the h-BN thickness is increased. For distances d, this trend can be modeled by a 1/d 4 decay, in agreement with theory and recent studies involving graphene. V