Abrin L. Schmucker

DNA-mediated nanoparticle crystallization into Wulff polyhedra

Crystallization is a fundamental and ubiquitous process much studied over the centuries. But although the crystallization of atoms is fairly well understood, it remains challenging to predict reliably the outcome of molecular crystallization processes that are complicated by various molecular interactions and solvent involvement. This difficulty also applies to nanoparticles: high-quality three-dimensional crystals are mostly produced using drying and sedimentation techniques that are often impossible to rationalize and control to give a desired crystal symmetry, lattice spacing and habit (crystal shape). In principle, DNA-mediated assembly of nanoparticles offers an ideal opportunity for studying nanoparticle crystallization: a well-defined set of rules have been developed to target desired lattice symmetries and lattice constants, and the occurrence of features such as grain boundaries and twinning in DNA superlattices and traditional crystals comprised of molecular or atomic building blocks suggests that similar principles govern their crystallization. But the presence of charged biomolecules, interparticle spacings of tens of nanometres, and the realization so far of only polycrystalline DNA-interconnected nanoparticle superlattices, all suggest that DNA-guided crystallization may differ from traditional crystal growth. Here we show that very slow cooling, over several days, of solutions of complementary-DNA-modified nanoparticles through the melting temperature of the system gives the thermodynamic product with a specific and uniform crystal habit. We find that our nanoparticle assemblies have the Wulff equilibrium crystal structure that is predicted from theoretical considerations and molecular dynamics simulations, thus establishing that DNA hybridization can direct nanoparticle assembly along a pathway that mimics atomic crystallization.

Dispersible Gold Nanorod Dimers with Sub-5 nm Gaps as Local Amplifiers for Surface-Enhanced Raman Scattering

We report the synthesis of solution-dispersible, 35 nm diameter gold nanorod dimers with gaps as small as ∼2 nm for surface-enhanced Raman scattering (SERS). Using on-wire lithography (OWL), we prepared tailorable dimers in high yield and high monodispersity (∼96% dimers) that produce both large and reproducible SERS signals with enhancement factors of (6.8 ± 0.7) × 10(8) for single dimers in air and 1.2 × 10(6) for ensemble-averaged solution measurements. Furthermore, we show that these structures, which are the smallest ever made by OWL, can be used to detect molecules on flat surfaces and in aqueous solutions. When combined, these attributes with respect to sensitivity, reproducibility, and tailorability lead to a novel and powerful local amplification system for SERS applications.

Cellular Response of Polyvalent Oligonucleotide−Gold Nanoparticle Conjugates

Nanoparticles are finding utility in myriad biotechnological applications, including gene regulation, intracellular imaging, and medical diagnostics. Thus, evaluating the biocompatibility of these nanomaterials is imperative. Here we use genome-wide expression profiling to study the biological response of HeLa cells to gold nanoparticles functionalized with nucleic acids. Our study finds that the biological response to gold nanoparticles stabilized by weakly bound surface ligands is significant (cells recognize and react to the presence of the particles), yet when these same nanoparticles are stably functionalized with covalently attached nucleic acids, the cell shows no measurable response. This finding is important for researchers studying and using nanomaterials in biological settings, as it demonstrates how slight changes in surface chemistry and particle stability can lead to significant differences in cellular responses.

Correlating nanorod structure with experimentally measured and theoretically predicted surface plasmon resonance

The extinction spectra of Au nanorods electrochemically synthesized using anodic aluminum oxide templates are reported. Homogeneous suspensions of nanorods with average diameters of 35, 55, 80, and 100 nm and varying lengths were synthesized, and their resultant surface plasmon resonances were probed by experimental and theoretical methods. Experimental extinction spectra of the nanoparticles exhibit good overall agreement with those calculated using the discrete dipole approximation. We determine the dependence of the dipole plasmon wavelength on both rod length and diameter, and we then utilize these results to derive an equation for predicting longitudinal dipole resonance wavelength for nanorod dimensions beyond the quasistatic limit. On average, the equation allows one to predict plasmon resonance maxima within 25 nm of the experimentally measured values. An analysis of factors that are important in determining the plasmon width is also provided. For long rods, the width decreases with increasing length in spite of increased radiative damping due to increased frequency dispersion in the real part of the metal dielectric function.

Desktop Nanofabrication with Massively Multiplexed Beam Pen Lithography

The development of a lithographic method that can rapidly define nanoscale features across centimeter-scale surfaces has been a long standing goal of the nanotechnology community. If such a ‘desktop nanofab’ could be implemented in a low-cost format, it would bring the possibility of point-of-use nanofabrication for rapidly prototyping diverse functional structures. Here we report the development of a new tool that is capable of writing arbitrary patterns composed of diffraction-unlimited features over square centimeter areas that are in registry with existing patterns and nanostructures. Importantly, this instrument is based on components that are inexpensive compared to the combination of state-of-the-art nanofabrication tools that approach its capabilities. This tool can be used to prototype functional electronic devices in a mask-free fashion in addition to providing a unique platform for performing high throughput nano- to macroscale photochemistry with relevance to biology and medicine.

One-Dimensional Nanorod Arrays: Independent Control of Composition, Length, and Interparticle Spacing with Nanometer Precision

We report the synthesis of solution dispersible, one-dimensional metal nanostructure arrays as small as 35 nm in diameter using on-wire lithography, wherein feature thickness and spacing in the arrays is tailorable down to approximately 6 and 1 nm, respectively. Using this unique level of control, we present solution-averaged extinction spectra of 35 nm diameter Au nanorod dimers with varying gap sizes to illustrate the effect of gap size on plasmon coupling between nanorods. Additionally, we demonstrate control over the composition of the arrays with Au, Ni, and Pt segments, representing important advances in controlling the ordering of sub-100 nm nanostructures that are not available with current synthesis or assembly methods.

A cantilever-free approach to dot-matrix nanoprinting

Scanning probe lithography (SPL) is a promising candidate approach for desktop nanofabrication, but trade-offs in throughput, cost, and resolution have limited its application. The recent development of cantilever-free scanning probe arrays has allowed researchers to define nanoscale patterns in a low-cost and high-resolution format, but with the limitation that these are duplication tools where each probe in the array creates a copy of a single pattern. Here, we report a cantilever-free SPL architecture that can generate 100 nanometer-scale molecular features using a 2D array of independently actuated probes. To physically actuate a probe, local heating is used to thermally expand the elastomeric film beneath a single probe, bringing it into contact with the patterning surface. Not only is this architecture simple and scalable, but it addresses fundamental limitations of 2D SPL by allowing one to compensate for unavoidable imperfections in the system. This cantilever-free dot-matrix nanoprinting will enable the construction of surfaces with chemical functionality that is tuned across the nano- and macroscales.

Hybrid semiconductor core-shell nanowires with tunable plasmonic nanoantennas

Multi-segmented nanowires with optically active hybrid core-shell regions are fabricated between two metal nanoantennas. These nanowires generate significant photocurrent under illumination and are solution-dispersible.

Nanotube-Bridged Wires with Sub-10 nm Gaps

We report a simple but efficient method to synthesize carbon nanotube-bridged wires (NBWs) with gaps as small as 5 nm. In this method, we have combined a strategy for assembling carbon nanotubes (CNTs) inside anodized aluminum oxide pores and the on-wire lithography technique to fabricate CNT-bridged wires with gap sizes deliberately tailored over the 5-600 nm range. As a proof-of-concept demonstration of the utility of this architecture, we have prepared NBW-based chemical and biosensors which exhibit higher analyte sensitivity (lower limits of detection) than those based on planar CNT networks. This observation is attributed to a greater surface-to-volume ratio of CNTs in the NBWs than those in the planar CNT devices. Because of the ease of synthesis and high yield of NBWs, this technique may enable the further incorporation of CNT-based architectures into various nanoelectronic and sensor platforms.

OWL-Based Nanomasks for Preparing Graphene Ribbons with Sub-10 nm Gaps

We report a simple and highly efficient method for creating graphene nanostructures with gaps that can be controlled on the sub-10 nm length scale by utilizing etch masks comprised of electrochemically synthesized multisegmented metal nanowires. This method involves depositing striped nanowires with Au and Ni segments on a graphene-coated substrate, chemically etching the Ni segments, and using a reactive ion etch to remove the graphene not protected by the remaining Au segments. Graphene nanoribbons with gaps as small as 6 nm are fabricated and characterized with atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. The high level of control afforded by electrochemical synthesis of the nanowires allows us to specify the dimensions of the nanoribbon, as well as the number, location, and size of nanogaps within the nanoribbon. In addition, the generality of this technique is demonstrated by creating silicon nanostructures with nanogaps.