Nanocoax-Based Electrochemical Sensor

Suppression of crosstalk in multielectrode arrays with local shielding

Electrical crosstalk can constrain the performance of multielectrode arrays in electro- and neurophysiology, in terms of both stimulation and recording. This is especially so at high electrode density, desirable for spatiotemporal mapping of bioelectrical signals from multiple cells. Channel interference due to crosstalk is currently only partially addressed, via continuous interleaved sampling or post-data acquisition spike sorting. Here, we show that a locally-shielded electrode architecture significantly suppresses crosstalk, and enables multi-site recording at high electrode density without the need for spike sorting. Arrays of shielded electrodes, prepared by micro- and nanofabrication techniques in a vertically-oriented coaxial geometry, demonstrate at least a 400 times improvement in spatial density over the unshielded case.

Facile fabrication and formation mechanism of aluminum nanowire arrays

Anodized alumina membranes (AAMs) have proven effective at making vertically-oriented and well-ordered metal nanowire arrays, which are useful in plasmonics and electrochemistry. Here, we produced Al nanowires via directed AAM pore nucleation: a patterned oxide mask on a flat Al surface directed where pores did and did not form, the pores acting to oxidize Al around the sites without pores. This left Al nanowires embedded in the AAM, and produced freestanding Al nanowires after etching the AAM. The nanowire tops had two distinct contours, smooth bowls and flat rough surfaces-suggesting that nanowires with bowl tops result from slow pore development relative to pattern-nucleated pores, not pore blockage as prior literature suggests. The observed low porosity of ∼2%, as opposed to the more typical 10%, suggests pore nucleation in the electrolyte employed may need greater local variations in electric field or pH, possibly explaining the electrolyte's peculiar ability to make Al nanowires. Finally, a soft nano-imprint lithography process was developed here to pattern the mask without damaging the stamp, avoiding a stamp degradation problem in previous work that utilized hard nano-imprint lithography.

Shielded Coaxial Optrode Arrays for Neurophysiology

Recent progress in the study of the brain has been greatly facilitated by the development of new tools capable of minimally-invasive, robust coupling to neuronal assemblies. Two prominent examples are the microelectrode array (MEA), which enables electrical signals from large numbers of neurons to be detected and spatiotemporally correlated, and optogenetics, which enables the electrical activity of cells to be controlled with light. In the former case, high spatial density is desirable but, as electrode arrays evolve toward higher density and thus smaller pitch, electrical crosstalk increases. In the latter, finer control over light input is desirable, to enable improved studies of neuroelectronic pathways emanating from specific cell stimulation. Here, we introduce a coaxial electrode architecture that is uniquely suited to address these issues, as it can simultaneously be utilized as an optical waveguide and a shielded electrode in dense arrays. Using optogenetically-transfected cells on a coaxial MEA, we demonstrate the utility of the architecture by recording cellular currents evoked from optical stimulation. We also show the capability for network recording by radiating an area of seven individually-addressed coaxial electrode regions with cultured cells covering a section of the extent.

Design of a molecular imprinting biosensor with multi-scale roughness for detection across a broad spectrum of biomolecules

The molecular imprinting technique has tremendous applications in artificial enzymes, bioseparation, and sensor devices.

Aluminum Nanowire Arrays via Directed Assembly

Freestanding and vertically-oriented metal nanowire arrays have potential utility in a number of applications, but presently lack a route to fabrication. Template-based techniques, such as electrodeposition into lithographically defined nanopore arrays, have produced well-ordered nanowire arrays with a maximum pitch of about 2 μm; such nanowires, however, tend to cluster due to local attractive forces. Here, we modify this template fabrication method to produce well-ordered, vertically-oriented, freestanding Al nanowire arrays, etched from an underlying Al substrate, with highly tunable pitch. In addition, optical measurements demonstrated that the nanowires support the propagation of surface plasmon polaritons.

Nanocoaxes for optical and electronic devices

The evolution of micro/nanoelectronics technology, including the shrinking of devices and integrated circuit components, has included the miniaturization of linear and coaxial structures to micro/nanoscale dimensions. This reduction in the size of coaxial structures may offer advantages to existing technologies and benefit the exploration and development of new technologies. The reduction in the size of coaxial structures has been realized with various permutations between metals, semiconductors and dielectrics for the core, shield, and annulus. This review will focus on fabrication schemes of arrays of metal - nonmetal - metal nanocoax structures using non-template and template methods, followed by possible applications. The performance and scientific advantages associated with nanocoax-based optical devices including waveguides, negative refractive index materials, light emitting diodes, and photovoltaics are presented. In addition, benefits and challenges that accrue from the application of novel nanocoax structures in energy storage, electronic and sensing devices are summarized.

A nanocoaxial-based electrochemical sensor for the detection of cholera toxin

Sensitive, real-time detection of biomarkers is of critical importance for rapid and accurate diagnosis of disease for point of care (POC) technologies. Current methods do not allow for POC applications due to several limitations, including sophisticated instrumentation, high reagent consumption, limited multiplexing capability, and cost. Here, we report a nanocoaxial-based electrochemical sensor for the detection of bacterial toxins using an electrochemical enzyme-linked immunosorbent assay (ELISA) and differential pulse voltammetry (DPV) or square wave voltametry (SWV). The device architecture is composed of vertically-oriented, nanoscale coaxial electrodes in array format (~10(6) coaxes per square millimeter). The coax cores and outer shields serve as integrated working and counter electrodes, respectively, exhibiting a nanoscale separation gap corresponding to ~100 nm. Proof-of-concept was demonstrated for the detection of cholera toxin (CT). The linear dynamic range of detection was 10 ng/ml-1 µg/ml, and the limit of detection (LOD) was found to be 2 ng/ml. This level of sensitivity is comparable to the standard optical ELISA used widely in clinical applications, which exhibited a linear dynamic range of 10 ng/ml-1 µg/ml and a LOD of 1 ng/ml. In addition to matching the detection profile of the standard ELISA, the nanocoaxial array provides a simple electrochemical readout and a miniaturized platform with multiplexing capabilities for the simultaneous detection of multiple biomarkers, giving the nanocoax a desirable advantage over the standard method towards POC applications.

Self-assembled large-area annular cavity arrays with tunable cylindrical surface plasmons for sensing

Surface plasmons that propagate along cylindrical metal/dielectric interfaces in annular apertures in metal films, called cylindrical surface plasmons (CSPs), exhibit attractive optical characteristics. However, it is challenging to fabricate these nanocoaxial structures. Here, we demonstrate a practical low-cost route to manufacture highly ordered, large-area annular cavity arrays (ACAs) that can support CSPs with great tunability. By employing a sol-gel coassembly method, reactive ion etching and metal sputtering techniques, regular, highly ordered ACAs in square-centimeter-scale with a gap width tunable in the range of several to hundreds of nanometers have been produced with good reproducibility. Ag ACAs with a gap width of 12 nm and a gap height of 635 nm are demonstrated. By finite-difference time-domain simulation, we confirm that the pronounced dips in the reflectance spectra of ACAs are attributable to CSP resonances excited in the annular gaps. By adjusting etching time and Ag film thickness, the CSP dips can be tuned to sweep the entire optical range of 360 to 1800 nm without changing sphere size, which makes them a promising candidate for forming integrated plasmonic sensing arrays. The high tunability of the CSP resonant frequencies together with strong electric field enhancement in the cavities make the ACAs promising candidates for surface plasmon sensors and SERS substrates, as, for example, they have been used in liquid refractive index (RI) sensing, demonstrating a sensitivity of 1505 nm/RIU and a figure of merit of 9. One of the CSP dips of ACAs with a certain geometry size is angle- (0-70 degrees) and polarization-independent and can be used as a narrow-band absorber. Furthermore, the nano annular cavity arrays can be used to construct solar cells, nanolasers and nanoparticle plasmonic tweezers.

Near-field observation of light propagation in nanocoax waveguides

We report the observation of propagating modes of visible and near infrared light in nanoscale coaxial (metal-dielectric-metal) structures, using near-field scanning optical microscopy. Together with numerical calculations, we show that the propagated modes have different nature depending on the excitation wavelength, i.e., plasmonic TE11 and TE21 modes in the near infrared and photonic TE31, TE41 and TM11 modes in the visible. Far field transmission out of the nanocoaxes is dominated by the superposition of Fabry-Perot cavity modes resonating in the structures, consistent with theory. Such coaxial optical waveguides may be useful for future nanoscale photonic systems.