Melosh's research is focused on developing methods to detect and control chemical processes on the nanoscale, to create materials that are responsive to their local environment. The research goal incorporates many of the hallmarks of biological adaptability, based on feedback control between cellular receptors and protein expression. Similar artificial networks may be achieved by fabricating arrays of nanoscale devices that can detect and influence their local surroundings through ionic potential, temperature, mechanical motion, capacitance, or electrochemistry. These devices are particularly suited as smart biomaterials, where multiple surface-cell interactions must be monitored and adjusted simultaneously for optimal cell adhesion and growth. Other interests include precise control over self-assembled materials, and potential methods to monitor the diagnostics of complicated chemical systems, such as the effect of drug treatments within patients.
Molecular materials at interfaces
Directed dynamic self-assembly
Controlling molecular or biomolecular assembly and behavior
Influence of local electronic, optical or thermal stimuli
PhD, University of California at Santa Barbara, Materials Science and Engineering (2001)
BS, Harvey Mudd College, Chemistry (1996)
- Introduction to Materials Science, Energy Emphasis
ENGR 50E (Aut)
MATSCI 380 (Win)
- Nanomaterials Laboratory
MATSCI 160 (Spr)
- Re-engineering the energy landscape
MATSCI 84N (Spr)
Independent Studies (8)
- Graduate Independent Study
MATSCI 399 (Aut, Win, Spr, Sum)
- Master's Research
MATSCI 200 (Aut, Win, Spr, Sum)
- Participation in Materials Science Teaching
MATSCI 400 (Win, Spr)
- Ph.D. Research
MATSCI 300 (Aut, Win, Spr, Sum)
- Practical Training
MATSCI 299 (Aut, Win, Spr, Sum)
PHYSICS 490 (Aut, Spr)
- Undergraduate Independent Study
MATSCI 100 (Aut, Win, Spr, Sum)
- Undergraduate Research
MATSCI 150 (Aut, Win, Spr, Sum)
- Graduate Independent Study
Prior Year Courses
- Introduction to Materials Science, Biomaterials Emphasis
ENGR 50M (Win)
- Introduction to Materials Science, Energy Emphasis
ENGR 50E (Aut)
- Nanomaterials Laboratory
MATSCI 160 (Spr)
- Introduction to Materials Science, Energy Emphasis
ENGR 50E (Aut)
MATSCI 380 (Win)
- Nanomaterials Laboratory
MATSCI 160 (Spr)
- Introduction to Materials Science, Biomaterials Emphasis
- Hybrid Group IV Nanophotonic Structures Incorporating Diamond Silicon-Vacancy Color Centers NANO LETTERS 2016; 16 (1): 212-217
- Nanotechnology and neurophysiology CURRENT OPINION IN NEUROBIOLOGY 2015; 32: 132-140
Nanotechnology and neurophysiology.
Current opinion in neurobiology
2015; 32: 132-140
Neuroscience would be revolutionized by a technique to measure intracellular electrical potentials that would not disrupt cellular physiology and could be massively parallelized. Though such a technology does not yet exist, the technical hurdles for fabricating minimally disruptive, solid-state electrical probes have arguably been overcome in the field of nanotechnology. Nanoscale devices can be patterned with features on the same length scale as biological components, and several groups have demonstrated that nanoscale electrical probes can measure the transmembrane potential of electrogenic cells. Developing these nascent technologies into robust intracellular recording tools will now require a better understanding of device-cell interactions, especially the membrane-inorganic interface. Here we review the state-of-the art in nanobioelectronics, emphasizing the characterization and design of stable interfaces between nanoscale devices and cells.
View details for DOI 10.1016/j.conb.2015.03.014
View details for PubMedID 25889532
- Membrane indentation triggers clathrin lattice reorganization and fluidization SOFT MATTER 2015; 11 (3): 439-448
- Thermally-enhanced minority carrier collection in hematite during photoelectrochemical water and sulfite oxidation JOURNAL OF MATERIALS CHEMISTRY A 2015; 3 (20): 10801-10810
Physical properties of materials derived from diamondoid molecules.
Reports on progress in physics. Physical Society (Great Britain)
2015; 78 (1): 016501-?
Diamondoids are small hydrocarbon molecules which have the same rigid cage structure as bulk diamond. They can be considered the smallest nanoparticles of diamond. They exhibit a mixture of properties inherited from bulk cubic diamond as well as a number of unique properties related to their size and structure. Diamondoids with different sizes and shapes can be separated and purified, enabling detailed studies of the effects of size and structure on the diamondoids' properties and also allowing the creation of chemically functionalized diamondoids which can be used to create new materials. Most notable among these new materials are self-assembled monolayers of diamondoid-thiols, which exhibit a number of unique electron emission properties.
View details for DOI 10.1088/0034-4885/78/1/016501
View details for PubMedID 25551840
- Fabrication of sub-cell size "spiky'' nanoparticles and their interfaces with biological cells JOURNAL OF MATERIALS CHEMISTRY B 2015; 3 (26): 5155-5160
Membrane indentation triggers clathrin lattice reorganization and fluidization.
2014; 11 (3): 439-448
Clathrin-mediated endocytosis involves the coordinated assembly of clathrin cages around membrane indentations, necessitating fluid-like reorganization followed by solid-like stabilization. This apparent duality in clathrin's in vivo behavior provides some indication that the physical interactions between clathrin triskelia and the membrane effect a local response that triggers fluid-solid transformations within the clathrin lattice. We develop a computational model to study the response of clathrin protein lattices to spherical deformations of the underlying flexible membrane. These deformations are similar to the shapes assumed during intracellular trafficking of nanoparticles. Through Monte Carlo simulations of clathrin-on-membrane systems, we observe that these membrane indentations give rise to a greater than normal defect density within the overlaid clathrin lattice. In many cases, the bulk surrounding lattice remains in a crystalline phase, and the extra defects are localized to the regions of large curvature. This can be explained by the fact that the in-plane elastic stress in the clathrin lattice are reduced by coupling defects to highly curved regions. The presence of defects brought about by indentation can result in the fluidization of a lattice that would otherwise be crystalline, resulting in an indentation-driven, defect-mediated phase transition. Altering subunit elasticity or membrane properties is shown to drive a similar transition, and we present phase diagrams that map out the combined effects of these parameters on clathrin lattice properties.
View details for DOI 10.1039/c4sm01650e
View details for PubMedID 25412023
- Penetration of Cell Membranes and Synthetic Lipid Bilayers by Nanoprobes BIOPHYSICAL JOURNAL 2014; 107 (9): 2091-2100
- Plasma Membrane and Actin Cytoskeleton as Synergistic Barriers to Nanowire Cell Penetration LANGMUIR 2014; 30 (41): 12362-12367
- Microfabricated Thermally Isolated Low Work-Function Emitter JOURNAL OF MICROELECTROMECHANICAL SYSTEMS 2014; 23 (5): 1182-1187
- Quantification of nanowire penetration into living cells NATURE COMMUNICATIONS 2014; 5
Rough-smooth-rough dynamic interface growth in supported lipid bilayers
PHYSICAL REVIEW E
2014; 89 (1)
The role of lipid bilayer viscoelasticity and the substrate-bilayer interactions on the spreading behavior of supported phospholipid bilayer membranes is studied using fluorescence microscopy. Unlike the monotonic roughening observed on silica or in other dynamic interface growth systems, a unique rough-smooth-rough (RSR) interface transition occurred on chromium oxide with a roughness exponent of 0.45 ± 0.04. This RSR transition is attributed to the elasticity of the lipid bilayer which is initially under compression due to surface interactions, and is well approximated by adding an elastic term to the quenched noise Edwards-Wilkinson equation. A phase diagram depicting the conditions necessary to observe RSR transitions in dynamic interface systems is derived, revealing the classes of dynamically evolving systems is broader than previously thought, and the viscoelastic nature of the lipid bilayer may play a role in supported membrane behavior.
View details for DOI 10.1103/PhysRevE.89.012404
View details for Web of Science ID 000332162500012
View details for PubMedID 24580234
Rheology and simulation of 2-dimensional clathrin protein network assembly
2014; 10 (33): 6219-6227
Clathrin is a three-legged protein complex that assembles into lattice structures on the cell membrane and transforms into fullerene-like cages during endocytosis. This dynamic structural flexibility makes clathrin an attractive building block for guided assembly. The assembly dynamics and the mechanical properties of clathrin protein lattices are studied using rheological measurements and theoretical modelling in an effort to better understand two dynamic processes: protein adsorption to the interface and assembly into a network. We find that percolation models for protein network formation are insufficient to describe clathrin network formation, but with Monte Carlo simulations we can describe the dynamics of network formation very well. Insights from this work can be used to design new bio-inspired nano-assembly systems.
View details for DOI 10.1039/c4sm00025k
View details for Web of Science ID 000340438600010
Mechanical Model of Vertical Nanowire Cell Penetration
2013; 13 (12): 6002-6008
Direct access into cells' interiors is essential for biomolecular delivery, gene transfection, and electrical recordings yet is challenging due to the cell membrane barrier. Recently, molecular delivery using vertical nanowires (NWs) has been demonstrated for introducing biomolecules into a large number of cells in parallel. However, the microscopic understanding of how and when the nanowires penetrate cell membranes is still lacking, and the degree to which actual membrane penetration occurs is controversial. Here we present results from a mechanical continuum model of elastic cell membrane penetration through two mechanisms, namely through "impaling" as cells land onto a bed of nanowires, and through "adhesion-mediated" penetration, which occurs as cells spread on the substrate and generate adhesion force. Our results reveal that penetration is much more effective through the adhesion mechanism, with NW geometry and cell stiffness being critically important. Stiffer cells have higher penetration efficiency, but are more sensitive to NW geometry. These results provide a guide to designing nanowires for applications in cell membrane penetration.
View details for DOI 10.1021/nl403201a
View details for Web of Science ID 000328439200039
View details for PubMedID 24237230
Covalent Attachment of Diamondoid Phosphonic Acid Dichlorides to Tungsten Oxide Surfaces
2013; 29 (31): 9790-9797
Diamondoids (nanometer-sized diamond-like hydrocarbons) are a novel class of carbon nanomaterials that exhibit negative electron affinity (NEA) and strong electron-phonon scattering. Surface-bound diamondoid monolayers exhibit monochromatic photoemission, a unique property that makes them ideal electron sources for electron-beam lithography and high-resolution electron microscopy. However, these applications are limited by the stability of the chemical bonding of diamondoids on surfaces. Here we demonstrate the stable covalent attachment of diamantane phosphonic dichloride on tungsten/tungsten oxide surfaces. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy revealed that diamondoid-functionalized tungsten oxide films were stable up to 300-350 °C, a substantial improvement over conventional diamondoid thiolate monolayers on gold, which dissociate at 100-200 °C. Extreme ultraviolet (EUV) light stimulated photoemission from these diamondoid phosphonate monolayers exhibited a characteristic monochromatic NEA peak with 0.2 eV full width at half-maximum (fwhm) at room temperature, showing that the unique monochromatization property of diamondoids remained intact after attachment. Our results demonstrate that phosphonic dichloride functionality is a promising approach for forming stable diamondoid monolayers for elevated temperature and high-current applications such as electron emission and coatings in micro/nano electromechanical systems (MEMS/NEMS).
View details for DOI 10.1021/la401781e
View details for Web of Science ID 000323014200022
- High-Bandwidth AFM Probes for Imaging in Air and Fluid JOURNAL OF MICROELECTROMECHANICAL SYSTEMS 2013; 22 (3): 603-612
Nanostraw-Electroporation System for Highly Efficient Intracellular Delivery and Transfection
2013; 7 (5): 4351-4358
Nondestructive introduction of genes, proteins, and small molecules into mammalian cells with high efficiency is a challenging, yet critical, process. Here we demonstrate a simple nanoelectroporation platform to achieve highly efficient molecular delivery and high transfection yields with excellent uniformity and cell viability. The system is built on alumina nanostraws extending from a track-etched membrane, forming an array of hollow nanowires connected to an underlying microfluidic channel. Cellular engulfment of the nanostraws provides an intimate contact, significantly reducing the necessary electroporation voltage and increasing homogeneity over a large area. Biomolecule delivery is achieved by diffusion through the nanostraws and enhanced by electrophoresis during pulsing. The system was demonstrated to offer excellent spatial, temporal, and dose control for delivery, as well as providing high-yield cotransfection and sequential transfection.
View details for DOI 10.1021/nn400874a
View details for Web of Science ID 000319856300073
View details for PubMedID 23597131
- Measurement of elastic properties in fluid using high bandwidth atomic force microscope probes APPLIED PHYSICS LETTERS 2013; 102 (10)
- Photon-enhanced thermionic emission from heterostructures with low interface recombination NATURE COMMUNICATIONS 2013; 4
Nanostraw-Mediated Intracellular Delivery: Direct Observation of Cell/Nanotube Interfaces
CELL PRESS. 2013: 194A-194A
View details for Web of Science ID 000316074301491
Microbead-separated thermionic energy converter with enhanced emission current
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
2013; 15 (34): 14442-14446
The efficiency of thermionic energy converters is a strong function of the inter-electrode separation due to space-charge limitations. Here we demonstrate vacuum thermionic energy converters constructed using barium dispenser cathodes and thin film tungsten anodes, separated by size specific alumina microbeads for simple device fabrication and inter-electrode gap control. The current and device efficiency at the maximum power point are strongly dependent on the inter-electrode gap, with a maximum device efficiency of 0.61% observed for a gap on the order of 5 μm. Paths to further reductions in space charge and improved anode work function are outlined with potential for over an order of magnitude improvement in output power and efficiency.
View details for DOI 10.1039/c3cp52895b
View details for Web of Science ID 000322725000036
A semiconductor/mixed ion and electron conductor heterojunction for elevated-temperature water splitting
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
2013; 15 (37): 15459-15469
Photoelectrochemical cells (PECs) have been studied extensively for dissociating water into hydrogen and oxygen. Key bottlenecks for achieving high solar-to-hydrogen efficiency in PECs include increasing solar spectrum utilization, surmounting overpotential losses, and aligning the absorber/electrochemical redox levels. We propose a new class of solid-state PECs based on mixed ionic and electronic conducting (MIEC) oxides that operates at temperatures significantly above ambient and utilizes both the light and thermal energy available from concentrated sunlight to dissociate water vapor. Unlike thermochemical and hybrid photo-thermochemical water-splitting routes, the elevated-temperature PEC is a single-step approach operating isothermally. At the heart of the solid-state PEC is a semiconductor light absorber coated with a thin MIEC layer for improved catalytic activity, electrochemical stability, and ionic conduction. The MIEC, placed between the gas phase and the semiconductor light absorber, provides a facile path for minority carriers to reach the water vapor as well as a path for the ionic carriers to reach the solid electrolyte. Elevated temperature operation allows reasonable band misalignments at the interfaces to be overcome, reduces the required overpotential, and facilitates rapid product diffusion away from the surface. In this work, we simulate the behavior of an oxygen-ion-conducting photocathode in 1-D. Using the detailed-balance approach, in conjunction with recombination and electrochemical reaction rates, the practical efficiency is calculated as a function of temperature, solar flux, and select material properties. For a non-degenerate light absorber with a 2.0 eV band-gap and an uphill band offset of 0.3 eV, an efficiency of 17% and 11% is predicted at 723 and 873 K, respectively.
View details for DOI 10.1039/c3cp52536h
View details for Web of Science ID 000323727800024
View details for PubMedID 23939203
- Nanostraw–Electroporation System for Highly Efficient Intracellular Delivery and Transfection Acs Nano 2013
- Power-independent wavelength determination by hot carrier collection in metal-insulator-metal devices Nat Commun 2013; 4: 1711
- Measurement of elastic properties in fluid using high bandwidth atomic force microscope probes Applied Physics Letters 2013; 102: 103111 (4 pp.)-103111 (4 pp.)
- Photon-enhanced thermionic emission from heterostructures with low interface recombination Nat Commun 2013; 4: 1576
Photon-enhanced thermionic emission from heterostructures with low interface recombination.
2013; 4: 1576-?
Photon-enhanced thermionic emission is a method of solar-energy conversion that promises to combine photon and thermal processes into a single mechanism, overcoming fundamental limits on the efficiency of photovoltaic cells. Photon-enhanced thermionic emission relies on vacuum emission of photoexcited electrons that are in thermal equilibrium with a semiconductor lattice, avoiding challenging non-equilibrium requirements and exotic material properties. However, although previous work demonstrated the photon-enhanced thermionic emission effect, efficiency has until now remained very low. Here we describe electron-emission measurements on a GaAs/AlGaAs heterostructure that introduces an internal interface, decoupling the basic physics of photon-enhanced thermionic emission from the vacuum emission process. Quantum efficiencies are dramatically higher than in previous experiments because of low interface recombination and are projected to increase another order of magnitude with more stable, low work-function coatings. The results highlight the effectiveness of the photon-enhanced thermionic emission process and demonstrate that efficient photon-enhanced thermionic emission is achievable, a key step towards realistic photon-enhanced thermionic emission based energy conversion.
View details for DOI 10.1038/ncomms2577
View details for PubMedID 23481384
Power-independent wavelength determination by hot carrier collection in metal-insulator-metal devices.
2013; 4: 1711-?
Wavelength separation and detection is generally performed by spatial dispersal of incident light onto separate detectors, or by appropriate wavelength-selective filters. Here we demonstrate direct wavelength determination of monochromatic light in a power-independent fashion with a single metal-insulator-metal device. This simple platform allows facile fabrication and scaling, and may be useful for on-chip optical communications. Although a single wavelength is power-independent, with two or more concurrent input signals, the output obeys a simple current sum rule, allowing the output to be tuned by choosing the input wavelengths and power. Finally, we demonstrate real-time deconvolution of three different wavelength asynchronous signals.
View details for DOI 10.1038/ncomms2728
View details for PubMedID 23591878
- Photocathode device using diamondoid and cesium bromide films APPLIED PHYSICS LETTERS 2012; 101 (24)
- A model for emission yield from planar photocathodes based on photon-enhanced thermionic emission or negative-electron-affinity photoemission JOURNAL OF APPLIED PHYSICS 2012; 112 (9)
- Diamondoid coating enables disruptive approach for chemical and magnetic imaging with 10 nm spatial resolution APPLIED PHYSICS LETTERS 2012; 101 (16)
Nanostraws for Direct Fluidic Intracellular Access
2012; 12 (8): 3881-3886
Nanomaterials are promising candidates to improve the delivery efficiency and control of active agents such as DNA or drugs directly into cells. Here we demonstrate cell-culture platforms of nanotemplated "nanostraws" that pierce the cell membrane, providing a permanent fluidic pipeline into the cell for direct cytosolic access. Conventional polymeric track-etch cell culture membranes are alumina coated and etched to produce fields of nanostraws with controllable diameter, thickness, and height. Small molecules and ions were successfully transported into the cytosol with 40 and 70% efficiency, respectively, while GFP plasmids were successfully delivered and expressed. These platforms open the way for active, reproducible delivery of a wide variety of species into cells without endocytosis.
View details for DOI 10.1021/nl204051v
View details for Web of Science ID 000307211000001
View details for PubMedID 22166016
Shape Matters: Intravital Microscopy Reveals Surprising Geometrical Dependence for Nanoparticles in Tumor Models of Extravasation
2012; 12 (7): 3369-3377
Delivery is one of the most critical obstacles confronting nanoparticle use in cancer diagnosis and therapy. For most oncological applications, nanoparticles must extravasate in order to reach tumor cells and perform their designated task. However, little understanding exists regarding the effect of nanoparticle shape on extravasation. Herein we use real-time intravital microscopic imaging to meticulously examine how two different nanoparticles behave across three different murine tumor models. The study quantitatively demonstrates that high-aspect ratio single-walled carbon nanotubes (SWNTs) display extravasational behavior surprisingly different from, and counterintuitive to, spherical nanoparticles although the nanoparticles have similar surface coatings, area, and charge. This work quantitatively indicates that nanoscale extravasational competence is highly dependent on nanoparticle geometry and is heterogeneous.
View details for DOI 10.1021/nl204175t
View details for Web of Science ID 000306296200004
View details for PubMedID 22650417
- Optimal emitter-collector gap for thermionic energy converters APPLIED PHYSICS LETTERS 2012; 100 (17)
Mesoporous Thin-Film on Highly-Sensitive Resonant Chemical Sensor for Relative Humidity and CO2 Detection
2012; 84 (7): 3063-3066
Distributed sensing of gas-phase chemicals is a promising application for mesoporous materials when combined with highly sensitive miniaturized gas sensors. We present a direct application of a mesoporous silica thin film on a highly sensitive miniaturized resonant chemical sensor with a mass sensitivity at the zeptogram scale for relative humidity and CO(2) detection. Using mesoporous silica thin-film, we report one of the lowest volume resolutions and a sensitive detection of 5.1 × 10(-4)% RH/Hz to water vapor in N(2), which is 70 times higher than a device with a nontemplated silica layer. In addition, a mesoporous thin-film that is functionalized with an amino-group is directly applied on the resonant sensor, which exhibits a volume sensitivity of 1.6 × 10(-4)%/Hz and a volume resolution of 1.82 × 10(-4)% to CO(2) in N(2).
View details for DOI 10.1021/ac300225c
View details for Web of Science ID 000302829800006
View details for PubMedID 22372606
MICROFABRICATED SILICON CARBIDE THERMIONIC ENERGY CONVERTER FOR SOLAR ELECTRICITY GENERATION
View details for Web of Science ID 000312912800317
- Diamondoid coating enables disruptive approach for chemical and magnetic imaging with 10 nm spatial resolution Applied Physics Letters 2012; 101: 163101 (5 pp.)-163101 (5 pp.)163101 (5 pp.)
- Optimal emitter-collector gap for thermionic energy converters Applied Physics Letters 2012; 100
- A model for emission yield from planar photocathodes based on photon-enhanced thermionic emission or negative-electron-affinity photoemission Journal of Applied Physics 2012; 112: 094907 (10 pp.)-094907 (10 pp.)094907 (10 pp.)
- Photocathode device using diamondoid and cesium bromide films Applied Physics Letters 2012; 101: 241605 (5 pp.)-241605 (5 pp.)
Plasmonic Energy Collection through Hot Carrier Extraction
2011; 11 (12): 5426-5430
Conversion of light into direct current is important for applications ranging from energy conversion to photodetection, yet often challenging over broad photon frequencies. Here we show a new architecture based on surface plasmon excitation within a metal-insulator-metal device that produces power based on spatial confinement of electron excitation through plasmon absorption. Plasmons excited in the upper metal are absorbed, creating a high concentration of hot electrons which can inject above or tunnel through the thin insulating barrier, producing current. The theoretical power conversion efficiency enhancement achieved can be almost 40 times larger than that of direct illumination while utilizing a broad spectrum of IR to visible wavelengths. Here we present both theoretical estimates of the power conversion efficiency and experimental device measurements, which show clear rectification and power conversion behavior.
View details for DOI 10.1021/nl203196z
View details for Web of Science ID 000297950200056
View details for PubMedID 22023372
- Photoluminescence of diamondoid crystals JOURNAL OF APPLIED PHYSICS 2011; 110 (9)
Molecular Structure Influences the Stability of Membrane Penetrating Biointerfaces
2011; 11 (5): 2066-2070
Nanoscale patterning of hydrophobic bands on otherwise hydrophilic surfaces allows integration of inorganic structures through biological membranes, reminiscent of transmembrane proteins. Here we show that a set of innate molecular properties of the self-assembling hydrophobic band determine the resulting interface stability. Surprisingly, hydrophobicity is found to be a secondary factor with monolayer crystallinity the major determinate of interface strength. These results begin to establish guidelines for seamless bioinorganic integration of nanoscale probes with lipid membranes.
View details for DOI 10.1021/nl200542m
View details for Web of Science ID 000290373000037
View details for PubMedID 21469728
Engineering cell access with nano-functionalized posts
AMER CHEMICAL SOC. 2011
View details for Web of Science ID 000291982803522
- Theoretical analysis of hot electron collection in metal-insulator-metal devices SPIE-INT SOC OPTICAL ENGINEERING. 2011
- Photoluminescence of Diamond Crystals Journal of Applied Physics 2011
- Plasmonic energy collection through hot carrier extraction Nano Letters 2011; 11: 5426-30
- Nanostraws for Direct Fluidic Intracellular Access Nano Letters 2011
Rapid spatial and temporal controlled signal delivery over large cell culture areas
LAB ON A CHIP
2011; 11 (18): 3057-3063
Controlled chemical delivery in microfluidic cell culture devices often relies on slowly evolving diffusive gradients, as the spatial and temporal control provided by fluid flow results in significant cell-perturbation. In this paper we introduce a microfluidic device architecture that allows for rapid spatial and temporal soluble signal delivery over large cell culture areas without fluid flow over the cells. In these devices the cell culture well is divided from a microfluidic channel located directly underneath the chamber by a nanoporous membrane. This configuration requires chemical signals in the microchannel to only diffuse through the thin membrane into large cell culture area, rather than diffuse in from the sides. The spatial chemical pattern within the microfluidic channel was rapidly transferred to the cell culture area with good fidelity through diffusion. The cellular temporal response to a step-function signal showed that dye reached the cell culture surface within 45 s, and achieved a static concentration in under 6 min. Chemical pulses of less than one minute were possible by temporally alternating the signal within the microfluidic channel, enabling rapid flow-free chemical microenvironment control for large cell culture areas.
View details for DOI 10.1039/c1lc20311h
View details for Web of Science ID 000294263400004
View details for PubMedID 21805010
Nanoscale patterning controls inorganic-membrane interface structure
2011; 3 (2): 391-400
The ability to non-destructively integrate inorganic structures into or through biological membranes is essential to realizing full bio-inorganic integration, including arrayed on-chip patch-clamps, drug delivery, and biosensors. Here we explore the role of nanoscale patterning on the strength of biomembrane-inorganic interfaces. AFM measurements show that inorganic probes functionalized with hydrophobic bands with thicknesses complimentary to the hydrophobic lipid bilayer core exhibit strong attachment in the bilayer. As hydrophobic band thickness increases to 2-3 times the bilayer core the interfacial strength decreases, comparable to homogeneously hydrophobic probes. Analytical calculations and molecular dynamics simulations predict a transition between a 'fused' interface and a 'T-junction' that matches the experimental results, showing lipid disorder and defect formation for thicker bands. These results show that matching biological length scales leads to more intimate bio-inorganic junctions, enabling rational design of non-destructive membrane interfaces.
View details for DOI 10.1039/c0nr00486c
View details for Web of Science ID 000287363500006
View details for PubMedID 20931126
- ENZYME ASSAYS Detection by failure NATURE CHEMISTRY 2010; 2 (12): 1006-1007
Effects of tip-induced material reorganization in dynamic force spectroscopy
PHYSICAL REVIEW E
2010; 82 (3)
Dynamic force spectroscopy (DFS) has become a well-established method for characterizing bond strength, yet may also be useful for examining more complex phenomena such as dynamic processes or multiple reaction pathways. Here, we analyze the case where contact between an atomic force microscopy (AFM) tip and the sample induces sample reorganization during testing. Surface contact often causes molecular rearrangement in soft materials, which could also result in an altered reaction energy landscape. We model this situation by allowing the energy barrier position and magnitude to be time-dependent functions with a characteristic time scale ? . We find dynamic energy barriers result in two linear regimes with a dramatic transition near t=? in the DFS analysis. The sharp transition region is a hallmark of a moving energy barrier and indicates the time scale of reorganization. These results illustrate that DFS may be useful to monitor dynamic transitions and also highlight the importance of extending the loading rate range used in DFS studies.
View details for DOI 10.1103/PhysRevE.82.031911
View details for Web of Science ID 000282133400008
View details for PubMedID 21230112
Photon-enhanced thermionic emission for solar concentrator systems
2010; 9 (9): 762-767
Solar-energy conversion usually takes one of two forms: the 'quantum' approach, which uses the large per-photon energy of solar radiation to excite electrons, as in photovoltaic cells, or the 'thermal' approach, which uses concentrated sunlight as a thermal-energy source to indirectly produce electricity using a heat engine. Here we present a new concept for solar electricity generation, photon-enhanced thermionic emission, which combines quantum and thermal mechanisms into a single physical process. The device is based on thermionic emission of photoexcited electrons from a semiconductor cathode at high temperature. Temperature-dependent photoemission-yield measurements from GaN show strong evidence for photon-enhanced thermionic emission, and calculated efficiencies for idealized devices can exceed the theoretical limits of single-junction photovoltaic cells. The proposed solar converter would operate at temperatures exceeding 200 degrees C, enabling its waste heat to be used to power a secondary thermal engine, boosting theoretical combined conversion efficiencies above 50%.
View details for DOI 10.1038/NMAT2814
View details for Web of Science ID 000281178400029
View details for PubMedID 20676086
- Gigaohm resistance membrane seals with stealth probe electrodes APPLIED PHYSICS LETTERS 2010; 97 (3)
An Electrostatic Model for DNA Surface Hybridization
2010; 98 (12): 2954-2963
DNA hybridization at surfaces is a crucial process for biomolecular detection, genotyping, and gene expression analysis. However, hybridization density and kinetics can be strongly inhibited by electric fields from the negatively charged DNA as the reaction proceeds. Here, we develop an electrostatic model to optimize hybridization density and kinetics as a function of DNA surface density, salt concentrations, and applied voltages. The electrostatic repulsion from a DNA surface layer is calculated numerically and incorporated into a modified Langmuir scheme, allowing kinetic suppression of hybridization. At the low DNA probe densities typically used in assays (<10(13)/cm(2)), electrostatics effects are largely screened and hybridization is completed with fast kinetics. However, higher hybridization densities can be achieved at intermediate DNA surface densities, albeit with slower kinetics. The application of positive voltages circumvents issues resulting from the very high DNA probe density, allowing highly enhanced hybridization densities and accelerated kinetics, and validating recent experimental measurements.
View details for DOI 10.1016/j.bpj.2010.03.017
View details for Web of Science ID 000278913500024
View details for PubMedID 20550908
Dynamic actuation using nano-bio interfaces
2010; 13 (6): 14-22
View details for Web of Science ID 000278390600010
Continuum model of mechanical interactions between biological cells and artificial nanostructures
2010; 5 (2): 37-44
The controlled insertion of artificial nanostructures into biological cells has been utilized for patch clamping, targeted drug delivery, cell lysing, and cell mechanics measurements. In this work, an elastic continuum model is implemented to treat the deformation of spherical cells in solution due to their interaction with cylindrical probes. At small deformations, the force varies nonlinearly with indentation due to global deformation of the cell shape. However, at large indentations, the force varies linearly with indentation due to more localized deformations. These trends are consistent with experimental measurements under comparable conditions and can be used to develop design rules for optimizing probe-cell interactions.
View details for DOI 10.1116/1.3431960
View details for Web of Science ID 000282333300003
View details for PubMedID 20831347
Single-Step Process to Reconstitute Cell Membranes on Solid Supports
2010; 26 (7): 4635-4638
A new technique is presented to create supported lipid bilayers from whole cell lipids without the use of detergent or solvent extraction. In a modification of the bubble collapse deposition (BCD) technique, an air bubble is created underwater and brought into contact with a population of cells. The high-energy air/water interface extracts the lipid component of the cell membrane, which can subsequently be redeposited as a fluid bilayer on another substrate. The resulting bilayers were characterized with fluorescence microscopy, and it was found that both leaflets of the cell membrane are transferred but the cytoskeleton is not. The resulting supported bilayer was fluid over an area much larger than a single cell, demonstrating the capacity to create large, continuous bilayer samples. This capability to create fluid, biologically relevant bilayers will facilitate the use of high-resolution scanning microscopy techniques in the study of membrane-related processes.
View details for DOI 10.1021/la100583f
View details for Web of Science ID 000275995100020
View details for PubMedID 20205459
Fusion of biomimetic stealth probes into lipid bilayer cores
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2010; 107 (13): 5815-5820
Many biomaterials are designed to regulate the interactions between artificial and natural surfaces. However, when materials are inserted through the cell membrane itself the interface formed between the interior edge of the membrane and the material surface is not well understood and poorly controlled. Here we demonstrate that by replicating the nanometer-scale hydrophilic-hydrophobic-hydrophilic architecture of transmembrane proteins, artificial "stealth" probes spontaneously insert and anchor within the lipid bilayer core, forming a high-strength interface. These nanometer-scale hydrophobic bands are readily fabricated on metallic probes by functionalizing the exposed sidewall of an ultrathin evaporated Au metal layer rather than by lithography. Penetration and adhesion forces for butanethiol and dodecanethiol functionalized probes were directly measured using atomic force microscopy (AFM) on thick stacks of lipid bilayers to eliminate substrate effects. The penetration dynamics were starkly different for hydrophobic versus hydrophilic probes. Both 5- and 10 nm thick hydrophobically functionalized probes naturally resided within the lipid core, while hydrophilic probes remained in the aqueous region. Surprisingly, the barrier to probe penetration with short butanethiol chains (E(o,5 nm) = 21.8k(b)T, E(o,10 nm) = 15.3k(b)T) was dramatically higher than longer dodecanethiol chains (E(o,5 nm) = 14.0k(b)T, E(o,10 nm) = 10.9k(b)T), indicating that molecular mobility and orientation also play a role in addition to hydrophobicity in determining interface stability. These results highlight a new strategy for designing artificial cell interfaces that can nondestructively penetrate the lipid bilayer.
View details for DOI 10.1073/pnas.0909250107
View details for Web of Science ID 000276159500024
View details for PubMedID 20212151
Lateral fusion of lipid membranes to nanoscale functionalized
AMER CHEMICAL SOC. 2010
View details for Web of Science ID 000208189302023
Clathrin protein assemblies as a biotemplate
AMER CHEMICAL SOC. 2010
View details for Web of Science ID 000208189301895
AFM force spectroscopy on TAT membrane penetration
AMER CHEMICAL SOC. 2010
View details for Web of Science ID 000208189302025
- Gigaohm resistance membrane seals with stealth probe electrodes Applied Physics Letters 2010; 97
- Fusion of Biomimetic ‘Stealth’ Probes into Lipid Bilayer Cores 2010
- Effects of tip-induced material reorganization in dynamic force spectroscopy Phys. Rev. E 2010; 82: 31911
- ENZYME ASSAYS: Detection by failure Nature Chemistry 2010; 2: 1006-1007
Directed Hybridization and Melting of DNA Linkers using Counterion-Screened Electric Fields
2009; 9 (10): 3521-3526
Dynamic self-assembly using responsive, "smart" materials such as DNA is a promising route toward reversible assembly and patterning of nanostructures for error-corrected fabrication, enhanced biosensors, drug delivery and gene therapy. DNA linkers were designed with strategically placed mismatches, allowing rapid attachment and release from a surface in a counterion-screened electric field. These electrostatic fields are inherently highly localized, directing assembly with nanometer precision while avoiding harmful electrochemical reactions. We show that depending on the sign of the applied field, the DNA hybridization density is strongly enhanced or diminished due to the high negative charge density of immobilized DNA. This use of dynamic fields rather than static templates enables fabrication of heterogeneously hybridized electrodes with different functional moieties, despite the use of identical linker sequences.
View details for DOI 10.1021/nl901710n
View details for Web of Science ID 000270670500025
View details for PubMedID 19606816
- Determining orientational structure of diamondoid thiols attached to silver using near-edge X-ray absorption fine structure spectroscopy JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 2009; 172 (1-3): 69-77
Influence of electrostatic fields in self assembly
AMER CHEMICAL SOC. 2009
View details for Web of Science ID 000207857807784
Identification and Passivation of Defects in Self-Assembled Monolayers
2009; 25 (5): 2585-2587
We demonstrate imaging of nanoscale defects in self-assembled monolayers (SAMs). Atomic layer deposition of aluminum oxide (AlO(x)) onto hydrophobic SAMs is followed by imaging using scanning electron microscopy (SEM). The insulating AlO(x) selectively deposits onto the exposed substrate at defect sites and becomes charged during imaging, providing high contrast even for nanometer scale defects. The deposited AlO(x) also acts as a barrier for electron transfer, thereby simultaneously electrically passivating the defects in the SAM as it labels them.
View details for DOI 10.1021/la804162a
View details for Web of Science ID 000263770800009
View details for PubMedID 19437743
An Internally Amplified Signal SOI Nano-bridge Biosensor for Electrical Detection of DNA Hybridization
IEEE. 2009: 67-68
View details for Web of Science ID 000276151400029
- Determining orientational structure of diamondoid thiols attached to silver using near-edge X-ray absorption fine structure spectroscopy Journal of Electron Spectroscopy and Related Phenomena 2009; 172: 69-77
Origin of the Monochromatic Photoemission Peak in Diamondoid Monolayers
2009; 9 (1): 57-61
Recent photoemission experiments have discovered a highly monochromatized secondary electron peak emitted from diamondoid self-assembled monolayers on metal substrates. New experimental data and simulation results are presented to show that a combination of negative electron affinity and strong electron-phonon scattering is responsible for this behavior. The simulation results are generated using a simple Monte Carlo transport algorithm. The simulated spectra recreate the main spectral features of the measured ones.
View details for DOI 10.1021/nl802310k
View details for Web of Science ID 000262519100010
View details for PubMedID 18975993
- Nanopore-Spanning Lipid Bilayers for Controlled Chemical Release ADVANCED MATERIALS 2008; 20 (23): 4423-4427
Formation and Characterization of Fluid Lipid Bilayers on Alumina
2008; 24 (22): 12734-12737
Fluid lipid bilayers were deposited on alumina substrates with the use of bubble collapse deposition (BCD). Previous studies using vesicle rupture have required the use of charged lipids or surface functionalization to induce bilayer formation on alumina, but these modifications are not necessary with BCD. Photobleaching experiments reveal that the diffusion coefficient of POPC on alumina is 0.6 microm (2)/s, which is much lower than the 1.4-2.0 microm (2)/s reported on silica. Systematically accounting for roughness, immobile regions and membrane viscosity shows that pinning sites account for about half of this drop in diffusivity. The remainder of the difference is attributed to a more tightly bound water state on the alumina surface, which induces a larger drag on the bilayer.
View details for DOI 10.1021/la802726u
View details for Web of Science ID 000260874800004
View details for PubMedID 18942863
- Diamondoids as low-kappa dielectric materials APPLIED PHYSICS LETTERS 2008; 93 (17)
Near-edge X-ray absorption fine structure spectroscopy of diamondoid thiol monolayers on gold
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
2008; 130 (32): 10536-10544
Diamondoids, hydrocarbon molecules with cubic-diamond-cage structures, have unique properties with potential value for nanotechnology. The availability and ability to selectively functionalize this special class of nanodiamond materials opens new possibilities for surface modification, for high-efficiency field emitters in molecular electronics, as seed crystals for diamond growth, or as robust mechanical coatings. The properties of self-assembled monolayers (SAMs) of diamondoids are thus of fundamental interest for a variety of emerging applications. This paper presents the effects of thiol substitution position and polymantane order on diamondoid SAMs on gold using near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and X-ray photoelectron spectroscopy (XPS). A framework to determine both molecular tilt and twist through NEXAFS is presented and reveals highly ordered diamondoid SAMs, with the molecular orientation controlled by the thiol location. C 1s and S 2p binding energies are lower in adamantane thiol than alkane thiols on gold by 0.67 +/- 0.05 and 0.16 +/- 0.04 eV, respectively. These binding energies vary with diamondoid monolayer structure and thiol substitution position, consistent with different degrees of steric strain and electronic interaction with the substrate. This work demonstrates control over the assembly, in particular the orientational and electronic structure, providing a flexible design of surface properties with this exciting new class of diamond nanoparticles.
View details for DOI 10.1021/ja711131e
View details for Web of Science ID 000258293800038
View details for PubMedID 18642809
- Interfacial effects in thin films of polymeric semiconductors JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 2008; 26 (4): 1454-1460
Electronically activated actin protein polymerization and alignment
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
2008; 130 (25): 7908-7915
Biological systems are the paragon of dynamic self-assembly, using a combination of spatially localized protein complexation, ion concentration, and protein modification to coordinate a diverse set of self-assembling components. Biomimetic materials based upon biologically inspired design principles or biological components have had some success at replicating these traits, but have difficulty capturing the dynamic aspects and diversity of biological self-assembly. Here, we demonstrate that the polymerization of ion-sensitive proteins can be dynamically regulated using electronically enhanced ion mixing and monomer concentration. Initially, the global activity of the cytoskeletal protein actin is inhibited using a low-ionic strength buffer that minimizes ion complexation and protein-protein interactions. Nucleation and growth of actin filaments are then triggered by a low-frequency AC voltage, which causes local enhancement of the actin monomer concentration and mixing with Mg(2+). The location and extent of polymerization are governed by the voltage and frequency, producing highly ordered structures unprecedented in bulk experiments. Polymerization rate and filament orientation could be independently controlled using a combination of low-frequency (approximately 100 Hz) and high frequency (1 MHz) AC voltages, creating a range of macromolecular architectures from network hydrogel microparticles to highly aligned arrays of actin filaments with approximately 750 nm periodicity. Since a wide range of proteins are activated upon complexation with charged species, this approach may be generally applicable to a variety of biopolymers and proteins.
View details for DOI 10.1021/ja7103284
View details for Web of Science ID 000256962000042
View details for PubMedID 18507467
- Creating large area molecular electronic junctions using atomic layer deposition APPLIED PHYSICS LETTERS 2008; 92 (21)
A nonvolatile plasmonic switch employing photochromic molecules
2008; 8 (5): 1506-1510
We demonstrate a surface plasmon-polariton (SPP) waveguide all-optical switch that combines the unique physical properties of small molecules and metallic (plasmonic) nanostructures. The switch consists of a pair of gratings defined in an aluminum film coated with a 65 nm thick layer of photochromic (PC) molecules. The first grating couples a signal beam consisting of free space photons to SPPs that interact effectively with the PC molecules. These molecules can reversibly be switched between transparent and absorbing states using a free space optical pump. In the transparent (signal "on") state, the SPPs freely propagate through the molecular layer, and in the absorbing (signal "off") state, the SPPs are strongly attenuated. The second grating serves to decouple the SPPs back into a free space optical beam, enabling measurement of the modulated signal with a far-field detector. In a preliminary study, the switching behavior of the PC molecules themselves was confirmed and quantified by surface plasmon resonance spectroscopy. The excellent (16%) overlap of the SPP mode profile with the thin layer of switching molecules enabled efficient switching with power densities of approximately 6.0 mW/cm2 in 1.5 microm x 8 microm devices, resulting in plasmonic switching powers of 0.72 nW per device. Calculations further showed that modulation depths in access of 20 dB can easily be attained in optimized designs. The quantitative experimental and theoretical analysis of the nonvolatile switching behavior in this letter guides the design of future nanoscale optically or electrically pumped optical switches.
View details for DOI 10.1021/nl0808839
View details for Web of Science ID 000255906400042
View details for PubMedID 18412401
- Efficient optical coupling into metal-insulator-metal plasmon modes with subwavelength diffraction gratings APPLIED PHYSICS LETTERS 2008; 92 (11)
- Suspension of nanoparticles in SU-8: Processing and characterization of nanocomposite polymers ELSEVIER SCI LTD. 2008: 228-236
- Diamondoids as low-kappa dielectric materials Applied Physics Letters 2008; 93: 172901
- Efficient optical coupling into metal-insulator-metal plasmon modes with subwavelength diffraction gratings Applied Physics Letters 2008; 92: 113109-1-3
- Creating large area molecular electronic junctions using atomic layer deposition Applied Physics Letters 2008; 92
Lipid bilayer deposition and patterning via air bubble collapse
2007; 23 (18): 9369-9377
We report a new method for forming patterned lipid bilayers on solid substrates. In bubble collapse deposition (BCD), an air bubble is first "inked" with a monolayer of phospholipid molecules and then touched to the surface of a thermally oxidized silicon wafer and the air is slowly withdrawn. As the bubble shrinks, the lipid monolayer pressure increases. Once the monolayer exceeds the collapse pressure, it folds back on itself, depositing a stable lipid bilayer on the surface. These bilayer disks have lateral diffusion coefficients consistent with high quality supported bilayers. By sequentially depositing bilayers in overlapping areas, fluid connections between bilayers of different compositions are formed. Performing vesicle rupture on the open substrate surrounding this bilayer patch results in a fluid but spatially isolated bilayer. Very little intermixing was observed between the vesicle rupture and bubble-deposited bilayers.
View details for DOI 10.1021/la701372b
View details for Web of Science ID 000248886700037
View details for PubMedID 17683151
COLL 108-Lateral fusion of lipid membranes to nanoscale functionalized posts
AMER CHEMICAL SOC. 2007
View details for Web of Science ID 000207593902029
BIOT 53-Nanoscale reservoirs for spatially and temporally controlled biointerfaces
AMER CHEMICAL SOC. 2007
View details for Web of Science ID 000207593903212
Monochromatic electron photoemission from diamondoid monolayers
2007; 316 (5830): 1460-1462
We found monochromatic electron photoemission from large-area self-assembled monolayers of a functionalized diamondoid, tetramantane-6-thiol. Photoelectron spectra of the diamondoid monolayers exhibited a peak at the low-kinetic energy threshold; up to 68% of all emitted electrons were emitted within this single energy peak. The intensity of the emission peak is indicative of diamondoids being negative electron affinity materials. With an energy distribution width of less than 0.5 electron volts, this source of monochromatic electrons may find application in technologies such as electron microscopy, electron beam lithography, and field-emission flat-panel displays.
View details for DOI 10.1126/science.1141811
View details for Web of Science ID 000247066400037
View details for PubMedID 17556579
- Dynamic control of biomolecular activity using electrical interfaces SOFT MATTER 2007; 3 (3): 267-274
- Monochromatic Electron Emission from Negative Electron Affinity Diamondoid Monolayers Science 2007; 315: 1460-1462
Probing molecular junctions using surface plasmon resonance spectroscopy
2006; 6 (12): 2797-2803
The optical absorption spectra of nanometer-thick organic films and molecular monolayers sandwiched between two metal contacts have been measured successfully using surface plasmon resonance spectroscopy (SPRS). The electric field within metal-insulator (organic)-metal (MIM) cross-bar junctions created by surface plasmon-polaritons excited on the metal surface allows sensitive measurement of molecular optical properties. Specifically, this spectroscopic technique extracts the real and imaginary indices of the organic layer for each wavelength of interest. The SPRS sensitivity was calculated for several device architectures, metals, and layer thicknesses to optimize the organic film absorptivity measurements. Distinct optical absorption features were clearly observed for R6G layers as thin as a single molecular monolayer between two metal electrodes. This method also enables dynamic measurement of molecular conformation inside metallic junctions, as shown by following the optical switching of a thin spiropyran/polymer film upon exposure to UV light. Finally, optical and electrical measurements can be made simultaneously to study the effect of electrical bias and current on molecular conformation, which may have significant impact in areas such as molecular and organic electronics.
View details for DOI 10.1021/nl061893h
View details for Web of Science ID 000242786500029
View details for PubMedID 17163708
- Soft deposition of large-area metal contacts for molecular electronics ADVANCED MATERIALS 2006; 18 (12): 1499-?
- Soft Deposition of Large Area Metal Contacts for Molecular Electronics Advanced Materials 2006; 18: 1499-1504
- Probing molecular junctions using surface plasmon resonance spectroscopy Nano Letters 2006; 6: 2797-2803
Silicon chip-based patch-clamp electrodes integrated with PDMS microfluidics
CELL PRESS. 2005: 522A-522A
View details for Web of Science ID 000226378502555
- Fabrication of conducting Si nanowire arrays JOURNAL OF APPLIED PHYSICS 2004; 96 (10): 5921-5923
Silicon chip-based patch-clamp electrodes integrated with PDMS microfluidics
BIOSENSORS & BIOELECTRONICS
2004; 20 (3): 509-517
We report on a silicon wafer-based device that can be used for recording macroscopic ion channel protein activities across a diverse group of cell-types. Gigaohm seals were achieved for CHO-K1 and RIN m5F cells, and both cell-attached and whole-cell mode configurations were also demonstrated. Two distinct intrinsic potassium ion channels were recorded in whole-cell mode for HIT-T15 and RAW 264.7 cells. Polydimethylsiloxane (PDMS) microfluidics were also coupled with the micromachined silicon chips in order to demonstrate that a single cell could be selectively directed to a micropore, and membrane protein currents could subsequently be recorded. These silicon chip-based devices have significant advantages over traditional micropipette approaches, and may serve as combinatorial tools for investigating membrane biophysics, pharmaceutical screening, and other bio-sensing tasks.
View details for DOI 10.1016/j.bios.2004.02.020
View details for Web of Science ID 000225009000016
View details for PubMedID 15494233
- Ultrahigh-density nanowire lattices and circuits Science 2003; 300: 112-15
- Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores Science 1998; 279: 548-552