Bryan Kaehr

Chemically exfoliated MoS2 as near-infrared photothermal agents.

The near-infrared (NIR) window refers to a range of wavelengths (700–1300 nm) in which biological tissues are highly transparent.[1] Consequently, biological imaging and therapy modalities employ light at these wavelengths for the monitoring[1] and triggering[2] of biological events in vitro and in vivo. For instance, photothermal ablation takes advantage of NIR absorbing materials for transducing light into heat.[2] The resultant thermal energy can be used for a number of applications, such as tissue ablation and drug release. Despite the intense interest in NIR photothermal agents, their development has suffered from considerable challenges. In particular, few nanomaterials display the requisite absorption profiles required for NIR photothermal transduction.

Multiphoton fabrication of chemically responsive protein hydrogels for microactuation

We report a method for creating stimuli-responsive biomaterials in which scanning nonlinear excitation is used to photocrosslink proteins at submicrometer 3D coordinates. Proteins with differing hydration properties can be combined to achieve tunable volume changes that are rapid and reversible in response to changes in chemical environment. Protein matrices having arbitrary 3D topographies and definable density gradients over micrometer dimensions provide the ability to effect rapid (<1 sec) and precise mechanical manipulations by means of changes in hydrogel size and shape, and applicability of these materials to cell biology is shown through the fabrication of responsive bacterial cages.

Microreplication and Design of Biological Architectures Using Dynamic-Mask Multiphoton Lithography

A strategy for rapidly printing three-dimensional (3D) microscopic replicas using multiphoton lithography directed by a dynamic electronic mask is reported. Morphological descriptions of 3D structures are encoded as stacks of 2D slices created from tomographic and computer-designed instruction sets. In this manner, digital images serve as input for a sequence of reflective photomasks on a digital micromirror device to direct replication of a structure. By scanning a laser focus across the face of the intrinsically aligned masks, tomographic and computed data can be translated into protein-based 3D reproductions with submicrometer feature sizes within 1 min. This straightforward and highly versatile approach may provide improved routes for the development of 3D cellular scaffolds, rapid prototyping of microanalytical devices, and production of custom tissue replacements.

Understanding catalysis in a multiphasic two-dimensional transition metal dichalcogenide

Establishing processing–structure–property relationships for monolayer materials is crucial for a range of applications spanning optics, catalysis, electronics and energy. Presently, for molybdenum disulfide, a promising catalyst for artificial photosynthesis, considerable debate surrounds the structure/property relationships of its various allotropes. Here we unambiguously solve the structure of molybdenum disulfide monolayers using high-resolution transmission electron microscopy supported by density functional theory and show lithium intercalation to direct a preferential transformation of the basal plane from 2H (trigonal prismatic) to 1T′ (clustered Mo). These changes alter the energetics of molybdenum disulfide interactions with hydrogen (ΔGH), and, with respect to catalysis, the 1T′ transformation renders the normally inert basal plane amenable towards hydrogen adsorption and hydrogen evolution. Indeed, we show basal plane activation of 1T′ molybdenum disulfide and a lowering of ΔGH from +1.6 eV for 2H to +0.18 eV for 1T′, comparable to 2H molybdenum disulfide edges on Au(111), one of the most active hydrogen evolution catalysts known.

Controlling the Metal to Semiconductor Transition of MoS2 and WS2 in Solution

Lithiation-exfoliation produces single to few-layered MoS2 and WS2 sheets dispersible in water. However, the process transforms them from the pristine semiconducting 2H phase to a distorted metallic phase. Recovery of the semiconducting properties typically involves heating of the chemically exfoliated sheets at elevated temperatures. Therefore, it has been largely limited to sheets deposited on solid substrates. Here, we report the dispersion of chemically exfoliated MoS2 sheets in high boiling point organic solvents enabled by surface functionalization and the controllable recovery of their semiconducting properties directly in solution. This process connects the scalability of chemical exfoliation with the simplicity of solution processing, ultimately enabling a facile method for tuning the metal to semiconductor transitions of MoS2 and WS2 within a liquid medium.

Protein-Directed Assembly of Arbitrary Three-Dimensional Nanoporous Silica Architectures

Through precise control of nanoscale building blocks, such as proteins and polyamines, silica condensing microorganisms are able to create intricate mineral structures displaying hierarchical features from nano- to millimeter-length scales. The creation of artificial structures of similar characteristics is facilitated through biomimetic approaches, for instance, by first creating a bioscaffold comprised of silica condensing moieties which, in turn, govern silica deposition into three-dimensional (3D) structures. In this work, we demonstrate a protein-directed approach to template silica into true arbitrary 3D architectures by employing cross-linked protein hydrogels to controllably direct silica condensation. Protein hydrogels are fabricated using multiphoton lithography, which enables user-defined control over template features in three dimensions. Silica deposition, under acidic conditions, proceeds throughout protein hydrogel templates via flocculation of silica nanoparticles by protein molecules, as indicated by dynamic light scattering (DLS) and time-dependent measurements of elastic modulus. Following silica deposition, the protein template can be removed using mild thermal processing yielding high surface area (625 m(2)/g) porous silica replicas that do not undergo significant volume change compared to the starting template. We demonstrate the capabilities of this approach to create bioinspired silica microstructures displaying hierarchical features over broad length scales and the infiltration/functionalization capabilities of the nanoporous silica matrix by laser printing a 3D gold image within a 3D silica matrix. This work provides a foundation to potentially understand and mimic biogenic silica condensation under the constraints of user-defined biotemplates and further should enable a wide range of complex inorganic architectures to be explored using silica transformational chemistries, for instance silica to silicon, as demonstrated herein.

Cellular complexity captured in durable silica biocomposites

Tissue-derived cultured cells exhibit a remarkable range of morphological features in vitro, depending on phenotypic expression and environmental interactions. Translation of these cellular architectures into inorganic materials would provide routes to generate hierarchical nanomaterials with stabilized structures and functions. Here, we describe the fabrication of cell/silica composites (CSCs) and their conversion to silica replicas using mammalian cells as scaffolds to direct complex structure formation. Under mildly acidic solution conditions, silica deposition is restricted to the molecularly crowded cellular template. Inter- and intracellular heterogeneity from the nano- to macroscale is captured and dimensionally preserved in CSCs following drying and subjection to extreme temperatures allowing, for instance, size and shape preserving pyrolysis of cellular architectures to form conductive carbon replicas. The structural and behavioral malleability of the starting material (cultured cells) provides opportunities to develop robust and economical biocomposites with programmed structures and functions.

Measuring the Thermal Conductivity of Porous, Transparent SiO2 Films With Time Domain Thermoreflectance

Nanocomposites offer unique capabilities of controlling thermal transport through the manipulation of various structural aspects of the material. However, measurements of the thermal properties of these composites are often difficult, especially porous nanomaterials. Optical measurements of these properties, although ideal due to the noncontact nature, are challenging due to the large surface variability of nanoporous structures. In this work, we use a vector-based thermal algorithm to solve for the temperature change and heat transfer in which a thin film subjected to a modulated heat source is sandwiched between two thermally conductive pathways. We validate our solution with time domain thermoreflectance measurements on glass slides and extend the thermal conductivity measurements to SiO 2 -based nanostructured films.

Multiphoton Lithography of Nanocrystalline Platinum and Palladium for Site-Specific Catalysis in 3D Microenvironments.

Integration of catalytic nanostructured platinum and palladium within 3D microscale structures or fluidic environments is important for systems ranging from micropumps to microfluidic chemical reactors and energy converters. We report a straightforward procedure to fabricate microscale patterns of nanocrystalline platinum and palladium using multiphoton lithography. These materials display excellent catalytic, electrical, and electrochemical properties, and we demonstrate high-resolution integration of catalysts within 3D defined microenvironments to generate directed autonomous particle and fluid transport.

Direct Writing and Actuation of Three‐Dimensionally Patterned Hydrogel Pads on Micropillar Supports

Many biological organisms employ microand nanoscale systems to actuate structural components with a high degree of spatial control. The resulting patterned or predetermined movement of the components gives rise to versatile biological materials with locally reconfigurable features and regionspecific dynamic properties. On the molecular level, biological systems may regulate the availability of catalytic sites on enzymes by local reconfiguration of the protein structure, such as in allosteric modulation. On the microscale, echinoderms use actuating pedicellariae for particle capture and release, and body cleaning, and bacteria employ the movement of flagella to generate directional locomotion. Squid use the mechanical expansion and contraction of chromatophores to reversibly change color and pattern for camouflage and communication. These systems provide inspiration for the development of artificial “smart” materials and surfaces with similar properties that respond autonomously and reversibly to environmental cues. Recently, such reversibly responsive materials, particularly those patterned or manipulated on the nanoand microscale, have been the subject of intense research because of their promising impact in areas including sensors and actuators, microfluidic systems, microelectromechanical systems, and switchable surfaces with adaptive wettability, optical, mechanical, or adhesive properties. In particular, hydrogels can be tailored to respond volumetrically to a wide variety of stimuli including temperature, pH, light, and biomolecules (e.g., glucose), and there has been a significant amount of research and applications devised for this class of materials in areas ranging from tissue engineering to responsive photonics. We recently described a responsive and reversibly actuating surface based on a hybrid architecture consisting of passive polymeric structural (“skeletal”) elements embedded in and under the control of a responsive hydrogel layer (“muscle”) attached to a solid support. While the volume change of the polymer muscle enables large-area, directional movement of skeletal elements, anchoring to a solid support imposes a serious constraint on the capacity for hydrogel expansion or contraction, thus limiting the extent of induced actuation of the structural elements. Moreover, this approach does not allow the formation of hydrogel islands that would induce localized actuation of selected areas and the associated regional changes in surface properties. To expand the opportunities for integration of hydrogels in such composite systems, it would be advantageous to tailor not only the chemistry and swelling properties of the hydrogels but also the size, shape, and placement of the gel in relation to other system components. For example, welldefined, three dimensionally patterned, responsive hydrogel pads placed at the tips of micropillars with microscale control would enable nearly unrestricted gel swelling, both in and out of plane, which would locally actuate the pillars with more precise control over the movement of individual elements. While extensive research has been devoted to tailoring the swelling, chemical properties, and responsive behavior of hydrogels, less attention has been paid to the development of patterning protocols that would offer area-specific synthesis and 3D control over the microor nanoscale features of the gel. Many routes to defining hydrogel patterns have been explored including photolithography, soft lithography, and masking techniques, but these 2D approaches lack [*] Prof. C. J. Brinker, Dr. B. Kaehr Advanced Materials Laboratory, Sandia National Laboratories 1001 University Blvd. SE, Albuquerque, NM 87106 (USA) and Department of Chemical and Nuclear Engineering and Center for Micro-engineered Materials, University of New Mexico Albuquerque, NM 87106 (USA) E-mail: bjkaehr@sandia.gov