Nanoporous gold (np-Au) electrode coatings significantly enhance the performance of electrochemical nucleic acid biosensors because of their three-dimensional nanoscale network, high electrical conductivity, facile surface functionalization, and biocompatibility. Contrary to planar electrodes, the np-Au electrodes also exhibit sensitive detection in the presence of common biofouling media due to their porous structure. However, the pore size of the nanomatrix plays a critical role in dictating the extent of biomolecular capture and transport. Small pores perform better in the case of target detection in complex samples by filtering out the large nonspecific proteins. On the other hand, larger pores increase the accessibility of target nucleic acids in the nanoporous structure, enhancing the detection limits of the sensor at the expense of more interference from biofouling molecules. Here, we report a microfabricated np-Au multiple electrode array that displays a range of electrode morphologies on the same chip for identifying feature sizes that reduce the nonspecific adsorption of proteins but facilitate the permeation of target DNA molecules into the pores. We demonstrate the utility of the electrode morphology library in studying DNA functionalization and target detection in complex biological media with a special emphasis on revealing ranges of electrode morphologies that mutually enhance the limit of detection and biofouling resilience. We expect this technique to assist in the development of high-performance biosensors for point-of-care diagnostics and facilitate studies on the electrode structure-property relationships in potential applications ranging from neural electrodes to catalysts.

In this work, sub-terahertz (THz) spectroscopy is applied to characterize lyophilized and in vitro cultured bacterial cells of non-pathogenic species of Escherichia coli (E. coli) and Bacillus subtilis (BG), spores of BG and DNA fro m E. coli. One of the goals of this research is to demonstrate that Fourier Transform (FT) spectroscopy in the frequency region of 10-25 cm -1 is sensitive enough to reveal characteristic spectral features fro m bio-cells and spores in different environment, to verify the d ifferences between species, and to show the response of spores to vacuum and response of cultured cells to heat. The experimental technique was significantly improved for sensitivity and reliab ility. Observed spectra taken with a spectral resolution of 0.25 cm -1 using FT spectrometer with a detector operating at 1.7 K are rich in well resolved features having spectral widths of ~0.5-1 cm -1 . The reproducibility of experimental results was verified and confirmed. Measured spectra from E. coli DNA and from the entire cell have many similarit ies, thus demonstrating that the cellu lar co mponents might contribute to the v ibrational spectrum of the cell. The results of this work confirm that observed spectroscopic features are caused by fundamental physical mechanism of interaction between THz rad iation and biological macro-mo lecules. Particu larly, the analysis of results indicates that the spectroscopic signatures of microorganis ms originate fro m the co mbination of low frequency vibrational modes or group of modes at close frequencies (vibrational bands) within mo lecular co mponents of bacterial cells/spores, with the significant contribution from the DNA. The significance of this study is justified by necessity for a fast and effective, label free and reagent free optical technology to protect against environmental and other biological threats, as well as for general medical research. The obtained results show that THz v ibrational spectroscopy promises to add quantitative genetic information to the characteristic signatures of biological objects, increasing the detection accuracy and selectivity when appropriate spectral resolution is used.

The local electrical characteristics on the surface of MBE-grown Bi2Te3 are probed under ambient conditions by conductive atomic force microscopy. Nanoscale mapping reveals a 10-100× enhancement in current at step-edges compared to that on terraces. Analysis of the local current-voltage characteristics indicates that the transport mechanism is similar for step-edges and terraces. Comparison of the results with those for control samples shows that the current enhancement is not a measurement artifact but instead is due to local differences in electronic properties. The likelihood of various possible mechanisms is discussed. The absence of enhancement at the step-edges for graphite terraces is consistent with the intriguing possibility that spin-orbit coupling and topological effects play a significant role in the step-edge current enhancement in Bi2Te3.

Electrical current is used for tuning pore morphology of nanoporous gold thin films at significantly lower temperatures than previously reported via electrically-assisted mechanisms.This technique allows for precisely controlling the extent and location of pore coarsening and producing a wide range of distinct morphologies on a single substrate for high-throughput studies of structure-property relationships. ABSTRACTNanoporous gold (np-Au) is an emerging nanostructured material that exhibits many desirable properties, including high electrical and thermal conductivity, high surface area-to-volume ratio, tunable pore morphology, well-established surface-binding chemistry, and compatibility with microfabrication. These features made np-Au a popular material for fuel cells, optical and electrical biosensors, drug delivery vehicles, neural electrode coatings, and as a model system for nano-scale mechanics. In each application, np-Au morphology plays an essential role in the overall device operation. Therefore, precise control of morphology is necessary for attaining optimal device performance. Traditionally, thermal treatment in furnaces and on hot plates is used for obtaining np-Au with self-similar but coarser morphologies. However, this approach lacks the ability to create different morphologies on a single substrate and requires high temperatures (>250 °C) that are not compatible with most plastic substrates. Herein, we report electro-annealing as a method that for the first time makes it possible to control the extent and location of pore coarsening on a single substrate with one fast treatment step. In addition, the electro-annealing entails much lower temperatures (<150 °C) than traditional thermal treatment, putatively due to electrically-assisted phenomena that contribute to thermally-activated surface diffusion of gold atoms responsible for coarsening. Overall, this approach can be easily scaled up to display multiple pore morphologies on a single chip and therefore enable high-throughput screening of optimal nanostructures for specific applications.

The ability to fine-tune feature size in nanostructured thin films is critical, as many desirable properties of these materials are dictated by their nanostructure. Accordingly, there is a need for techniques that allow for modifying nanostructure while monitoring the morphological changes in situ. Here, we demonstrate a closed-loop electro-annealing system which enables in situ monitoring of morphology evolution in sub-micron nanoporous gold (np-Au) thin films. Np-Au is produced by a microfabrication-compatible self-assembly process that produces a network of interconnected ligaments with tunable diameter (10 s to 100 s of nanometers), making it a desirable material for numerous applications and fundamental studies alike. We specifically investigate the relationship between np-Au morphology (i.e., ligament diameter) and electrical resistance of the thin film. A strong correlation emerges between ligament size and electrical resistance, which puts forward resistance as an effective parameter for monitoring morphology evolution. Surprisingly, np-Au films with thicker ligaments lead to an increase in electrical resistance, which is unexpected since the extent of charge carrier scattering at the ligament surface should decrease with increasing ligament size. Further examination of np-Au morphology with high-resolution electron microscopy revealed grain growth on the ligaments in highly-annealed np-Au thin films. This suggests that grains act as scattering centers for charge carriers and this becomes the dominant mechanism in dictating electrical resistance in a percolated network of thin conductive ligaments.