Christine Dolan Keating

Functional Architectures on the Nano- to Mesoscale

The Keating lab is interested in construction of functional materials from the bottom up, by control of their nanoscale and mesoscale features. Controlling the composition of matter at these length scales can lead to materials with entirely new and tailorable optical, electronic, and structural properties. Such materials will find applications in medicine, biotechnology, sensors, nanoscale electronics, and in a variety of other fields. Finding inspiration in cell biology and materials science, our research aims to bring new building blocks and new assembly tools to this task.

Cytomimetic chemistry: synthesis of "artificial cells"

The living cell can be thought of as a highly functional supramolecular assembly. The intricately complex physical structure and diversity of functions carried out by cells are simply extraordinary. How does the physical and chemical structure of the cell contribute to its properties? Can synthetic cells be assembled to perform functions of our own design? This project is aimed at attainable progress towards the ultimate goal of learning how to use all of biology's tricks, with an eye towards understanding life and redirecting these tools to assemble novel functional materials.

We are designing an entirely new class of synthetic cells to mimic not only the plasma membrane but also the cytoplasm and internal structure found in biological cells. This work is motivated by research implicating the cytoplasm itself as a key element in spatial localization and control of functionality within cells. The cytoplasm is 30 percent by weight proteins and nucleic acids; the high concentration of macromolecules leads to dramatic changes in the thermodynamic activities of dissolved proteins due to volume exclusion (macromolecular crowding). This effect can be studied in simple polymeric systems. Our first goal is to understand the phase behavior of mixed-polymer aqueous solutions within small volume reactors. We will then add complexity to the system in the form of synthetic "organelles" and cytoskeletal fibers. It is anticipated that volume exclusion effects, even short of phase separation, will play an important role in the spatial arrangement of macromolecules, vesicles, and the polymerizable proteins of the cytoskeleton.

An important effect of intracellular organization can be metabolite channeling, in which coupled biochemical reactions are spatially localized and the product from one enzyme is "handed off" to another as the reactant, without diffusing throughout the cell. If phase separation occurs in living cells, it is one possible mechanism for metabolite channeling. Synthetic cells can be used to investigate these types of reactions, and to create conditions under which even nonbio-logical reactions might be coupled. This approach could be used to design sensors based on macromolecule association/dissociation, in analogy to many biological sensing motifs.

"Barcoded" metal particles for bioanalysis

The goal of this project is to develop tiny "barcodes" (analogous to those used in retail industries) for use in bioanalytical chemistry. The barcode pattern is built into metal rods during their synthesis via template-directed electrochemical deposition. The particles can be synthesized in a wide variety of lengths (up to several microns), widths (tens to hundreds of nanometers), and compositions (e.g., Au, Ag, Pt, Cu, Co, CdSe). Importantly, their composition can be varied along the length of the rod to yield "stripes" of different materials. This leads to the ability to both optically and chemically distinguish the different segments. For example, Au and Pt segments have different optical reflectivities and different surface chemistries.

Because the striping pattern can be readily determined via optical reflectance microscopy, striped metal rods can be used as uniquely identifiable substrates for bioanalysis. The number of distinguishable barcode rods which can, in principle, be prepared is staggering (i.e., a pattern for every gene of the human genome is within the realm of possibility). These particles are therefore extremely interesting for multiplexed bioanalytical applications. Key areas of fundamental investigation involve characterization of the metal-metal interfaces, development of surface attachment chemistries, and investigations of the optical properties of these particles. In addition, we are investigating their application in multiplexed DNA assays.

DNA-directed assembly of metal "nanowires"

As a result of recent advances in preparation, functionalization, and assembly of a wide variety of nanoscale and microscale building blocks, researchers can construct nanostructured materials via self-assembly that are difficult or impossible to prepare by conventional technology. The trouble with self-assembly is that the ultimate superstructure is largely a function of the system itself, rather than of conveniently controlled variables. (That is, the particles go where they want, not where we want them, to go). To date, most work has relied upon relatively nonspecific interactions to drive assembly of spherical particles. The goal of this project is to use the high selectivity of biomolecular recognition to direct assembly of striped nanorods into predetermined architectures. We have begun to use DNA hybridization to assemble segmented metallic rods onto surfaces and in solution. The advantages of DNA as "glue" include its exquisite selectivity, reversibility, and the essentially limitless number of different sequences (and therefore directed interactions) possible. Segmented metal nanorods are an ideal building block in that they are simple to prepare, monodisperse, and can be synthesized in a wide variety of sizes, aspect ratios, and compositions. In addition, these particles are amenable to thiol modification and have tunable electronic and optical properties. In this research, we take advantage of both the several forms of anisotropy (shape, composition, and surface chemistry) available in nanorods and the selectivity of nucleic acid base-pairing interactions to prepare complex, deterministic nanostructures. One potential application of such structures is as "nanowiring" in nanoscale electronic devices.