Perspectives on weak interactions in complex materials at different length scales

Nanocomposite materials consist of nanometer-sized quantum objects such as atoms, molecules, voids or nanoparticles embedded in a host material. These quantum objects can be exploited as a super-structure, which can be designed to create material properties targeted for specific applications. For electromagnetism, such targeted properties include field enhancements around the bandgap of a semiconductor used for solar cells, directional decay in topological insulators, high kinetic inductance in superconducting circuits, and many more. Despite very different application areas, all of these properties are united by the common aim of exploiting collective interaction effects between quantum objects. The literature on the topic spreads over very many different disciplines and scientific communities. In this review, we present a cross-disciplinary overview of different approaches for the creation, analysis and theoretical description of nanocomposites with applications related to electromagnetic properties.

Next-Generation Nonlocal van der Waals Density Functional

The fundamental ideas for a nonlocal density functional theory-capable of reliably capturing van der Waals interactions-were already conceived in the 1990s. In 2004, a seminal paper introduced the first practical nonlocal exchange-correlation functional called vdW-DF, which has become widely successful and laid the foundation for much further research. However, since then, the functional form of vdW-DF has remained unchanged. Several successful modifications paired the original functional with different (local) exchange functionals to improve performance, and the successor vdW-DF2 also updated one internal parameter. Bringing together different insights from almost 2 decades of development and testing, we present the next-generation nonlocal correlation functional called vdW-DF3, in which we change the functional form while staying true to the original design philosophy. Although many popular functionals show good performance around the binding separation of van der Waals complexes, they often result in significant errors at larger separations. With vdW-DF3, we address this problem by taking advantage of a recently uncovered and largely unconstrained degree of freedom within the vdW-DF framework that can be constrained through empirical input, making our functional semiempirical. For two different parameterizations, we benchmark vdW-DF3 against a large set of well-studied test cases and compare our results with the most popular functionals, finding good performance in general for a wide array of systems and a significant improvement in accuracy at larger separations. Finally, we discuss the achievable performance within the current vdW-DF framework, the flexibility in functional design offered by vdW-DF3, as well as possible future directions for nonlocal van der Waals density functional theory.

Spin Signature of Nonlocal Correlation Binding in Metal-Organic Frameworks

We develop a proper nonempirical spin-density formalism for the van der Waals density functional (vdW-DF) method. We show that this generalization, termed svdW-DF, is firmly rooted in the single-particle nature of exchange and we test it on a range of spin systems. We investigate in detail the role of spin in the nonlocal correlation driven adsorption of H_{2} and CO_{2} in the linear magnets Mn-MOF74, Fe-MOF74, Co-MOF74, and Ni-MOF74. In all cases, we find that spin plays a significant role during the adsorption process despite the general weakness of the molecular-magnetic responses. The case of CO_{2} adsorption in Ni-MOF74 is particularly interesting, as the inclusion of spin effects results in an increased attraction, opposite to what the diamagnetic nature of CO_{2} would suggest. We explain this counterintuitive result, tracking the behavior to a coincidental hybridization of the O p states with the Ni d states in the down-spin channel. More generally, by providing insight on nonlocal correlation in concert with spin effects, our nonempirical svdW-DF method opens the door for a deeper understanding of weak nonlocal magnetic interactions.

Global analysis in fluorescence correlation spectroscopy and fluorescence lifetime microscopy

Fluorescence correlation spectroscopy (FCS) and related fluctuation spectroscopy and microscopy methods have become important research tools that enable detailed investigations of the chemical and physical properties of molecules and molecular systems in a variety of complex environments. Information recovery via curve fitting of fluctuation data can present complicating challenges due to limited resolution and/or problems with fitting model verification. We discuss a new approach to data analysis called τFCS that couples multiple modes of signal acquisition, here specifically FCS and fluorescence lifetimes, with global analysis. We demonstrate enhanced resolution using τFCS, including the capability to recover the concentration of both molecular species in a two-component mixture even when the species have identical diffusion coefficients and molecular brightness values, provided their fluorescent lifetimes are distinct. We also demonstrate how τFCS provides useful tools for model discrimination in FCS curve fitting.

Fibrillogenesis in continuously spun synthetic collagen fiber

The universal structural role of collagen fiber networks has motivated the development of collagen gels, films, coatings, injectables, and other formulations. However, reported synthetic collagen fiber fabrication schemes have either culminated in short, discontinuous fiber segments at unsuitably low production rates, or have incompletely replicated the internal fibrillar structure that dictates fiber mechanical and biological properties. We report a continuous extrusion system with an off-line phosphate buffer incubation step for the manufacture of synthetic collagen fiber. Fiber with a cross-section of 53+ or - 14 by 21 + or - 3 microm and an ultimate tensile strength of 94 + or - 19 MPa was continuously produced at 60 m/hr from an ultrafiltered monomeric collagen solution. The effect of collagen solution concentration, flow rate, and spinneret size on fiber size was investigated. The fiber was further characterized by microdifferential scanning calorimetry, transmission electron microscopy (TEM), second harmonic generation (SHG) analysis, and in a subcutaneous murine implant model. Calorimetry demonstrated stabilization of the collagen triple helical structure, while TEM and SHG revealed a dense, axially aligned D-periodic fibril structure throughout the fiber cross-section. Implantation of glutaraldehyde crosslinked and noncrosslinked fiber in the subcutaneous tissue of mice demonstrated limited inflammatory response and biodegradation after a 6-week implant period.

Fluorescence intensity is a poor predictor of saturation effects in two-photon microscopy: Artifacts in fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) has become an increasingly important measurement tool for biological and biomedical investigations, with the capability to assay molecular dynamics and interactions both in vitro and within living cells. Information recovery in FCS requires an accurate characterization and calibration of the observation volume. A number of recent reports have demonstrated that the calibration of the observation volume is excitation power dependent, a complication that arises due to excitation saturation. While quantitative models are now available to account for these volume variations, many researchers attempt to avoid saturation issues by working with low nonsaturating excitation intensities. For two-photon excited fluorescence, this is typically thought to be achievable by working with excitation powers for which the total measured fluorescence signal maintains its quadratic dependence on excitation intensity. We demonstrate that observing only the power dependence of the fluorescence intensity will tend to underestimate the importance of saturation, and explain these findings in terms of basic physical models.

Excitation saturation in two-photon fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) has become a powerful and sensitive research tool for the study of molecular dynamics at the single-molecule level. Because photophysical dynamics often dramatically influence FCS measurements, the role of various photophysical processes in FCS measurements must be understood to accurately interpret FCS data. We describe the role of excitation saturation in two-photon fluorescence correlation measurements. We introduce a physical model that characterizes the effects of excitation saturation on the size and shape of the two-photon fluorescence observation volume and derive a new analytical expression for fluorescence correlation functions that includes the influence of saturation. With this model, we can accurately describe both the temporal decay and the amplitude of measured fluorescence correlation functions over a wide range of illumination powers.

Temporary articulating methylmethacrylate antibiotic spacer (TAMMAS). A new method of intraoperative manufacturing of a custom articulating spacer

A temporary articulating antibiotic-impregnated cement spacer for use during the first stage of a two-stage revision of a total knee replacement that had failed because of infection was developed by one of us (W.M.G.). It is simply a knee prosthesis made of methylmethacrylate and antibiotics that is manufactured intraoperatively with use of instruments, medications, and supplies that are already available at most hospitals. This construct allows for motion of the knee during treatment of the infection, thereby reducing the risk of loss of motion after subsequent revision. The technique has been successfully utilized in five patients since 1999 and has now become our standard treatment method.

New time-resolved techniques in two-photon microscopy

Microscopy is traditionally a tool for determining biological structures. Many recent advances in optical microscopy involves the incorporation of spectroscopy techniques to monitor biochemical states of microscopic structures in living cells and tissues. By minimizing tissue photodamage, two-photon excitation microscopy provides a new opportunity to study the dynamics of biological systems on time scales from nanoseconds to hours. This review will focus on a number of these new methods: two-photon time-lapse microscopy, two-photon photoactivation, two-photon correlated spectroscopy, two-photon single particle tracking and two-photon lifetime microscopy.