Christine D. Keating

Self-assembly of single electron transistors and related devices

For the past 40 years, since the invention of the integrated circuit, the number of transistors on a computer chip has doubled roughly every 18 months. As the limits of photolithography are rapidly approached, however, it is becoming clear that continued increases in circuit density will require fairly dramatic changes in the way transistors are designed and operated. This review summarizes current strategies for fabricating transistors which operate based on the flow of single electrons through nanometre-sized metal and semiconductor particles; i.e. single electron transistors (SETs). Because the room temperature operation of SETs requires nanoparticles <10 nm in diameter, we focus mainly on devices which have the potential for being assembled from the solution phase (non-lithographic systems). Several applications of SETs are discussed in addition to the major hurdles which must be overcome for their implementation in electronic device technology.

Bottom-up assembly of large-area nanowire resonator arrays

Directed-assembly of nanowire-based devices will enable the development of integrated circuits with new functions that extend well beyond mainstream digital logic. For example, nanoelectromechanical resonators are very attractive for chip-based sensor arrays because of their potential for ultrasensitive mass detection. In this letter, we introduce a new bottom-up assembly method to fabricate large-area nanoelectromechanical arrays each having over 2,000 single-nanowire resonators. The nanowires are synthesized and chemically functionalized before they are integrated onto a silicon chip at predetermined locations. Peptide nucleic acid probe molecules attached to the nanowires before assembly maintain their binding selectivity and recognize complementary oligonucleotide targets once the resonator array is assembled. The two types of cantilevered resonators we integrated here using silicon and rhodium nanowires had Q-factors of approximately 4,500 and approximately 1,150, respectively, in vacuum. Taken together, these results show that bottom-up nanowire assembly can offer a practical alternative to top-down fabrication for sensitive chip-based detection.

DNA-Directed Assembly of Gold Nanowires on Complementary Surfaces**

branes. The tubes are well ordered but somewhat tilted due to their relatively low mechanical strength compared with cylinders. The Raman spectra of the tubes have shown two broad peaks centered on 1345 and 1565 cm ‐1 , indicating that the tubes consist mainly of DLC. All the nanotubes have a uniform outer diameter of about 300 nm. The wall thickness of the tubes is very small, which can be controlled by optimizing the growth conditions. The tube length was about 7 lm, as in the case of cylinders. All the tubes have uniform height and are found to be hollow inside. Such nanotube arrays are probably useful for field emitters as well as for depositing metal catalysts or enzymes in the tubes, which in turn may be useful for new technologies. Other applications presumably include their use as porous electrodes in electrochemistry, as conductive diamond is known to exhibit outstanding electrochemical properties such as low background current, wide electrochemical potential window and high resistance to deactivation. [19] The present technique is a simple one for producing highly ordered diamond nanocylinders and nanotubes in high yield. The dimensions of these nanofibers are easily controllable by varying the pore dimensions of the alumina membrane. This technique enables others to adopt it easily and study the physical properties of these arrays for various applications in fieldemission displays, photonic bandgap materials, composite materials, and electrochemistry.

Nanoscale Characterization of Gold Colloid Monolayers: A Comparison of Four Techniques

Atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and near-field scanning optical microscopy (NSOM) have been used to characterize the nanostructure of Au colloid-based surfaces. Because these substrates are composed of particles whose dimensions are known prior to assembly, they are well-suited for a critical comparison of the capabilities and limitations of each nanoscale imaging technique. The three criteria for this comparison, which are relevant to the field of nanoparticle assemblies in general, are (i) accuracy in establishing particle size, particle coverage, and interparticle spacing; (ii) accuracy in delineating surface topography; and (iii) ease of sample preparation, data acquisition, and image analysis. For colloidal Au arrays, TEM gives the most reliable size and spacing information but exhibits the greatest constraints with regard to sample preparation; in contrast, AFM is widely applicable but yields data that are the least straightforward to interpret. For accurate information regarding nanometer-scale architecture of particle-based surfaces, a combination of at least one scanning probe method (AFM, NSOM) and one accelerated-electron method (TEM, FE-SEM) is required.

Dynamic microcompartmentation in synthetic cells.

An experimental model for cytoplasmic organization is presented. We demonstrate dynamic control over protein distribution within synthetic cells comprising a lipid bilayer membrane surrounding an aqueous polymer solution. This polymer solution generally exists as two immiscible aqueous phases. Protein partitioning between these phases leads to microcompartmentation, or heterogeneous protein distribution within the “cell” interior. This model cytoplasm can be reversibly converted to a single phase by slight changes in temperature or osmolarity, such that local protein concentrations can be manipulated within the vesicle interior.

Aqueous phase separation as a possible route to compartmentalization of biological molecules.

How could the incredible complexity of modern cells evolve from something simple enough to have appeared in a primordial soup? This enduring question has sparked the interest of researchers since Darwin first considered his theory of natural selection. Organic molecules, even potentially functional molecules including peptides and nucleotides, can be produced abiotically. Amphiphiles such as surfactants and lipids display remarkable self-assembly processes including the spontaneous formation of vesicles resembling the membranes of living cells. Nonetheless, numerous questions remain. Given the presumably dilute concentrations of macromolecules in the prebiotic pools where the earliest cells are thought to have appeared, how could the necessary components become concentrated and encapsulated within a semipermeable membrane? What would drive the further structural complexity that is a hallmark of modern living systems? The interior of modern cells is subdivided into microcompartments such as the nucleoid of bacteria or the organelles of eukaryotic cells. Even within what at first appears to be a single compartment, for example, the cytoplasm or nucleus, chemical composition is often nonuniform, containing gradients, macromolecular assemblies, and/or liquid droplets. What might the internal structure of intermediate evolutionary forms have looked like? The nonideal aqueous solution chemistry of macromolecules offers an attractive possible answer to these questions. Aqueous polymer solutions will form multiple coexisting thermodynamic phases under a variety of readily accessible conditions. In this Account, we describe aqueous phase separation as a model system for biological compartmentalization in both early and modern cells, with an emphasis on systems that have been encapsulated within a lipid bilayer. We begin with an introduction to aqueous phase separation and discuss how this phenomenon can lead to microcompartmentalization and could facilitate biopolymer encapsulation by partitioning of solutes between the phases. We then describe primitive model cells based on phase separation inside lipid vesicles, which mimic several basic properties of biological cells: microcompartmentation, protein relocalization in response to stimulus, loss of symmetry, and asymmetric vesicle division. We observe these seemingly complex phenomena in the absence of genetic molecules, enzymes, or cellular machinery, and as a result these processes could provide clues to possible intermediates in the early evolution of cell-like assemblies.

Nanowire sensors for multiplexed detection of biomolecules.

Nanowire-based detection strategies provide promising new routes to bioanalysis that could one day revolutionize the healthcare industry. This review covers recent developments in nanowire sensors for multiplexed detection of biomolecules such as nucleic acids and proteins. We focus on encoded nanowire suspension arrays and semiconductor nanowire-based field-effect transistors. Nanowire assembly and integration with microchip technology is emphasized as a key step toward the ultimate goal of multiplexed detection at the point of care using portable, low power, electronic biosensor chips.

Fabrication of two-dimensional photonic crystals using interference lithography and electrodeposition of CdSe

This letter describes a simple synthetic approach to fabricate two-dimensional midinfrared CdSe photonic crystals (PC) by electrodeposition of CdSe in a polymer template defined using interference lithography. Characterization of the transmission spectra of CdSe PCs with a hexagonal array of 1.3 μm diameter and 2.7 μm pitch air voids showed a well-defined drop in transmission at 4.23 μm. The drop in transmission increased with incident angle, reaching a maximum of approximately 2.6 dB at 40° relative to the surface normal. This two-step synthetic approach can be used to incorporate photonic crystals onto arbitrary substrates for integration into future advanced optical circuits.

RNA catalysis through compartmentalization

RNA performs important cellular functions in contemporary life forms. Its ability to act both as a catalyst and a storage mechanism for genetic information is also an important part of the RNA world hypothesis. Compartmentalization within modern cells allows the local concentration of RNA to be controlled and it has been suggested that this was also important in early life forms. Here, we mimic intracellular compartmentalization and macromolecular crowding by partitioning RNA in an aqueous two-phase system (ATPS). We show that the concentration of RNA is enriched by up to 3,000-fold in the dextran-rich phase of a polyethylene glycol/dextran ATPS and demonstrate that this can lead to approximately 70-fold increase in the rate of ribozyme cleavage. This rate enhancement can be tuned by the relative volumes of the two phases in the ATPS. Our observations support the importance of compartmentalization in the attainment of function in an RNA World as well as in modern biology.

Programmed Assembly of DNA-Coated Nanowire Devices

Combining biomolecular function with integrated circuit technology could usher in a new era of biologically enabled electronics. A key challenge has been coupling different molecular functions to specific chip locations for communication with the circuit. We used spatially confined electric fields to assemble different populations of DNA-coated nanowires to desired positions with an accuracy that enabled postassembly fabrication of contacts to each individual nanowire, with high yield and without loss of DNA function. This combination of off-chip synthesis and biofunctionalization with high-density, heterogeneous assembly and integration at the individual nanowire level points to new ways of incorporating biological functionality with silicon electronics.