The oxygen, O2, in air interferes with the detection of H2 by palladium (Pd)-based H2 sensors, including Pd nanowires (NWs), depressing the sensitivity and retarding the response/recovery speed in air—relative to N2 or Ar. Here, we describe the preparation of H2 sensors in which a nanofiltration layer consisting of a Zn metal–organic framework (MOF) is assembled onto Pd NWs. Polyhedron particles of Zn-based zeolite imidazole framework (ZIF-8) were synthesized on lithographically patterned Pd NWs, leading to the creation of ZIF-8/Pd NW bilayered H2 sensors. The ZIF-8 filter has many micropores (0.34 nm for gas diffusion) which allows for the predominant penetration of hydrogen molecules with a kinetic diameter of 0.289 nm, whereas relatively larger gas molecules including oxygen (0.345 nm) and nitrogen (0.364 nm) in air are effectively screened, resulting in superior hydrogen sensing properties. Very importantly, the Pd NWs filtered by ZIF-8 membrane (Pd NWs@ZIF-8) reduced the H2 response amplitude slightly (ΔR/R0 = 3.5% to 1% of H2 versus 5.9% for Pd NWs) and showed 20-fold faster recovery (7 s to 1% of H2) and response (10 s to 1% of H2) speed compared to that of pristine Pd NWs (164 s for response and 229 s for recovery to 1% of H2). These outstanding results, which are mainly attributed to the molecular sieving and acceleration effect of ZIF-8 covered on Pd NWs, rank highest in H2 sensing speed among room-temperature Pd-based H2 sensors.Keywords: electrodeposition; hydrogen gas sensor; metal−organic framework; Pd nanowire; recovery; response;

The influence of hexamethylenetetraamine (HMTA) on the morphology of δ-MnO2 and its properties for electrical energy storage are investigated—specifically for ultrathick δ-MnO2 layers in the micron scale. Planar arrays of gold@δ-MnO2, core@shell nanowires, were prepared by electrodeposition with and without the HMTA and their electrochemical properties were evaluated. HMTA alters the MnO2 in three ways: First, it creates a more open morphology for the MnO2 coating, characterized by “petals” with a thickness of 6 to 9 nm, rather than much thinner δ-MnO2 sheets seen in the absence of HMTA. Second, the electronic conductivity of the δ-MnO2 is increased by an order of magnitude. Third, δ-MnO2 prepared in HMTA shows a (001) interlayer spacing that is expanded by ≈30% possibly accelerating Li transport. The net effect of “HTMA doping” is to dramatically improve high rate performance, culminating in an increase in the specific capacity for the thickest MnO2 shells examined here by a factor of 15 at 100 mV/s.

Pd based alloy materials with hollow nanostructures are ideal hydrogen (H2) sensor building blocks because of their double-H2 sensing active sites (interior and exterior side of hollow Pd alloy) and fast response. In this work, for the first time, we report a simple fabrication process for preparing hollow Pd–Ag alloy nanowires (Pd@Ag HNWs) by using the electrodeposition of lithographically patterned silver nanowires (NWs), followed by galvanic replacement reaction (GRR) to form palladium. By controlling the GRR time of aligned Ag NWs within an aqueous Pd2+-containing solution, the compositional transition and morphological evolution from Ag NWs to Pd@Ag HNWs simultaneously occurred, and the relative atomic ratio between Pd and Ag was controlled. Interestingly, a GRR duration of 17 h transformed Ag NWs into Pd@Ag HNWs that showed enhanced H2 response and faster sensing response time, reduced 2.5-fold, as compared with Ag NWs subjected to a shorter GRR period of 10 h. Furthermore, Pd@Ag HNWs patterned on the colorless and flexible polyimide (cPI) substrate showed highly reversible H2 sensing characteristics. To further demonstrate the potential use of Pd@Ag HNWs as sensing layers for all-transparent, wearable H2 sensing devices, we patterned the Au NWs perpendicular to Pd@Ag HNWs to form a heterogeneous grid-type metallic NW electrode which showed reversible H2 sensing properties in both bent and flat states.Keywords: alloy metal; galvanic replacement reaction; hollow nanowire; LPNE; sensor;

Recently we demonstrated that symmetric, all Au@δ-MnO2 core@shell nanowire capacitors can achieve cycle stability to 100 000 cycles and beyond in a poly(methyl methacrylate) (PMMA) gel electrolyte. Here we examine the limits of the PMMA gel to confer this extraordinary stability, in terms of the accessible maximum voltage, Vmax, and the thickness of the PMMA gel electrolyte layer. Two conclusions are (1) the PMMA gel permits the Vmax to be increased by 50% from 1.2 V to 1.8 V, allowing the specific energy to be increased 5–6 fold, and (2) the PMMA gel layer thickness can be reduced from 180 μm (previously) to 2 μm while simultaneously utilizing two layers of nanowires and patterning nanowires in each layer at 5× higher density. For this nanowire “sandwich” architecture, a net increase in volumetric capacity of 600× up to 500 mF/cm3 can be achieved while retaining cycle stability to 100 000 cycles.

ConspectusHydrogen gas (H2) is odorless and flammable at concentrations above 4% (v/v) in air. Sensors capable of detecting it rapidly at lower concentrations are needed to “sniff” for leaked H2 wherever it is used. Electrical H2 sensors are attractive because of their simplicity and low cost: Such sensors consist of a metal (usually palladium, Pd) resistor. Exposure to H2 causes a resistance increase, as Pd metal is converted into more resistive palladium hydride (PdHx). Sensors based upon Pd alloy films, developed in the early 1990s, were both too slow and too insensitive to meet the requirements of H2 safety sensing.In this Account, we describe the development of H2 sensors that are based upon electrodeposited nanomaterials. This story begins with the rise to prominence of nanowire-based sensors in 2001 and our demonstration that year of the first nanowire-based H2 sensor. The Pd nanowires used in these experiments were prepared by electrodepositing Pd at linear step-edge defects on a graphite electrode surface. In 2005, lithographically patterned nanowire electrodeposition (LPNE) provided the capability to pattern single Pd nanowires on dielectrics using electrodeposition. LPNE also provided control over the nanowire thickness (±1 nm) and width (±10–15%). Using single Pd nanowires, it was demonstrated in 2010 that smaller nanowires responded more rapidly to H2 exposure. Heating the nanowire using Joule self-heating (2010) also dramatically accelerated sensor response and recovery, leading to the conclusion that thermally activated H2 chemisorption and desorption of H2 were rate-limiting steps in sensor response to and recovery from H2 exposure.Platinum (Pt) nanowires, studied in 2012, showed an inverted resistance response to H2 exposure, that is, the resistance of Pt nanowires decreased instead of increased upon H2 exposure. H2 dissociatively chemisorbs at a Pt surface to form Pt–H, but in contrast to Pd, it stays on the Pt surface. Pt nanowires showed a faster response to H2 exposure than Pd nanowires operating at the same elevated temperature, but they had a surprising disadvantage: The resistance change observed for Pt nanowires was exactly the same for all H2 concentrations. Electron surface scattering was implicated in the mechanism for these sensors. Work on Pt nanowires lead in 2015 to the preparation of Pd nanowires that were electrochemically modified with thin Pt layers (Pd@Pt nanowires). Relative to Pd nanowires, Pt@Pd nanowires showed accelerated response and recovery to H2 while retaining the same high sensitivity to H2 concentration seen for sensors based upon pure Pd nanowires.A new chapter in H2 sensing (2017) involves the replacement of metal nanowires with carbon nanotube ropes decorated with electrodeposited Pd nanoparticles (NPs). Even higher sensitivity and faster sensor response and recovery are enabled by this sensor architecture. Sensor properties are strongly dependent on the size and size monodispersity of the Pd NPs, with smaller NPs yielding higher sensitivity and more rapid response/recovery. We hope the lessons learned from this science over 15 years will catalyze the development of sensors based upon electrodeposited nanomaterials for gases other than H2.

The label-free detection of human serum albumin (HSA) in aqueous buffer is demonstrated using a simple, monolithic, two-electrode electrochemical biosensor. In this device, both millimeter-scale electrodes are coated with a thin layer of a composite containing M13 virus particles and the electronically conductive polymer poly(3,4-ethylenedioxy thiophene) or PEDOT. These virus particles, engineered to selectively bind HSA, serve as receptors in this biosensor. The resistance component of the electrical impedance, Zre, measured between these two electrodes provides electrical transduction of HSA binding to the virus-PEDOT film. The analysis of sample volumes as small as 50 μL is made possible using a microfluidic cell. Upon exposure to HSA, virus-PEDOT films show a prompt increase in Zre within 5 s and a stable Zre signal within 15 min. HSA concentrations in the range from 100 nM to 5 μM are detectable. Sensor-to-sensor reproducibility of the HSA measurement is characterized by a coefficient-of-variance (COV) ranging from 2% to 8% across this entire concentration range. In addition, virus-PEDOT sensors successfully detected HSA in synthetic urine solutions.

Palladium (Pd) nanoparticle (NP)-decorated carbon nanotube (CNT) ropes (or CNT@PdNP) are used as the sensing element for hydrogen gas (H2) chemiresistors. In spite of the fact that Pd NPs have a mean diameter below 6 nm and are highly dispersed on the CNT surfaces, CNT@PdNP ropes produce a relative resistance change 20–30 times larger than is observed at single, pure Pd nanowires. Thus, CNT@PdNP rope sensors improve upon all H2 sensing metrics (speed, dynamic range, and limit-of-detection), relative to single Pd nanowires which heretofore have defined the state-of-the-art in H2 sensing performance. Specifically, response and recovery times in air at [H2] ≈ 50 ppm are one-sixth of those produced by single Pd nanowires with cross-sectional dimensions of 40 × 100 nm Pd. The LODH2 is <10 ppm versus 300 ppm, and the dynamic range (10 ppm −4%) is nearly twice that afforded by the Pd nanowire. CNT@PdNP rope sensors are prepared by the dielectrophoretic deposition of a single semiconducting CNT rope followed by the electrodeposition of Pd nanoparticles with mean diameters ranging from 4.5 (±1) nm to 5.8 (±3) nm. The diminutive mean diameter and the high degree of diameter monodispersity for the deposited Pd nanoparticles are distinguishing features of the CNT@PdNP rope sensors described here, relative to prior work on similar systems.Keywords: dielectrophoresis; dynamic range; electrodeposition; limit-of-detection; monodisperse; response time;

The preparation by electrodeposition of transverse nanowire electroluminescent junctions (tn-ELJs) is described, and the electroluminescence (EL) properties of these devices are characterized. The lithographically patterned nanowire electrodeposition process is first used to prepare long (millimeters), linear, nanocrystalline CdSe nanowires on glass. The thickness of these nanowires along the emission axis is 60 nm, and the width, wCdSe, along the electrical axis is adjustable from 100 to 450 nm. Ten pairs of nickel–gold electrical contacts are then positioned along the axis of this nanowire using lithographically directed electrodeposition. The resulting linear array of nickel–CdSe–gold junctions produces EL with an external quantum efficiency, EQE, and threshold voltage, Vth, that depend sensitively on wCdSe. EQE increases with increasing electric field and also with increasing wCdSe, and Vth also increases with wCdSe and, therefore, the electrical resistance of the tn-ELJs. Vth down to 1.8(±0.2) V (for wCdSe ≈ 100 nm) and EQE of 5.5(±0.5) × 10–5 (for wCdSe ≈ 450 nm) are obtained. tn-ELJs produce a broad EL emission envelope, spanning the wavelength range from 600 to 960 nm.Keywords: cadmium selenide; light emission; photolithography; quantum efficiency; threshold voltage; transport

We demonstrate reversible cycle stability for up to 200 000 cycles with 94–96% average Coulombic efficiency for symmetrical δ-MnO2 nanowire capacitors operating across a 1.2 V voltage window in a poly(methyl methacrylate) (PMMA) gel electrolyte. The nanowires investigated here have a Au@δ-MnO2 core@shell architecture in which a central gold nanowire current collector is surrounded by an electrodeposited layer of δ-MnO2 that has a thickness of between 143 and 300 nm. Identical capacitors operating in the absence of PMMA (propylene carbonate (PC), 1.0 M LiClO4) show dramatically reduced cycle stabilities ranging from 2000 to 8000 cycles. In the liquid PC electrolyte, the δ-MnO2 shell fractures, delaminates, and separates from the gold nanowire current collector. These deleterious processes are not observed in the PMMA electrolyte.

We describe the preparation and properties of a coaxial, three-layer, gold-CdSe-gold nanowire 30 μm in length that functions as a monolithic photodetector. The gold (Au) electrode core of this sandwich structure is prepared using the lithographically patterned nanowire electrodeposition (LPNE) method on a glass surface. A CdSe shell of defined thickness, dCdSe, from 200 to 280 nm is then electrodeposited on this Au nanowire. Finally, a conformal gold layer is electrodeposited on top of the CdSe shell. The two concentric gold electrodes within this architecture measure the photoconductivity of the ultrathin CdSe absorbing layer in the direction orthogonal to the nanowire axis. This architecture enables accelerated response/recovery of the nanowire to light while simultaneously maximizing the photoconductive gain without relinquishing any of the photoresponsive area of a ”bare” nanowire. Characterization by scanning electron microscopy (SEM) of focused ion beam (FIB) cross sections together with electron dispersive X-ray spectroscopy (EDS) reveal the distinct core–multishell nanostructure, layer thicknesses, and layer compositions. The position-dependent photoresponse along the axis of the nanowire, probed using a laser spot, shows that the Au nanoshell significantly enhances the photocurrent. The performance of Au–CdSe–Au core–multishell nanowire photodetectors depend sensitively on the thickness of CdSe nanoshell over the range of from 200 nm < dCdSe < 280 nm. The highest performance was obtained for the dCdSe = 250 nm this device, which showed a photoconductive gain of 2172, a responsivity of 209 A·W–1, a response time of 17 μs, and a recovery time of 96 μs.

Manganese oxide, MnO2, excels as a hybrid electrical energy storage material: The manganese centers in MnO2 are capable of undergoing a reduction from 4+ to 3+ balanced by the intercalation of lithium ions to form LixMnO2 while its conductive surfaces simultaneously store energy as an electrical double layer capacitor. The highest capacitance and power performance for MnO2 has been obtained for ensembles of nanowires that are 200 nm or less in width and many microns in length. Typically such MnO2 nanowires are attached to a current collector at just one end, and electrical conductivity of the nanowire is therefore required in order to maintain a consistent redox and charge state along its axis. The electrical conductance of the nanowire therefore plays a very important role, and yet this parameter has been measured in few previous studies. In this work, we directly measure the electrical conductance of δ-MnO2 nanowires in situ in 1 M LiClO4, acetonitrile as a function of the equilibrium Li content for nanowires with varying lateral dimensions. This measurement is accomplished using arrays of 200 MnO2 nanowires that are 40–60 nm in height and 275–870 nm in width and which span a 10 μm gap between two gold contacts. Nanowires of fully oxidized MnO2 are first prepared at +0.60 V vs MSE in acetonitrile. As the equilibrium electrode potential is decreased from 0.60 V to −0.80 V and lithium is intercalated, the electrical conductivity of MnO2 nanowires increases by up to 1 order of magnitude. The measured change in conductivity is dependent on the equilibrium potential, which in turn is related to the Li content, and also depends on the width of nanowires. After doping at −0.80 V vs MSE, the conductivity increases by 30% for a 870 nm wide nanowire array and 880% for a 275 nm wide nanowire array. TEM investigations implicate the nanowire porosity in this difference.

Arrays of nanowires of an electronically conductive polymeric affinity medium tailored to the detection of Fe(III) are prepared, and their properties for detecting Fe(III) are evaluated. This polymeric affinity medium consists of poly(3,4-ethylenedioxythiophene) (PEDOT) into which an iron chelator, deferoxamine (DFA), has been doped during the polymerization process. PEDOT–DFA nanowires are potentiostatically deposited from a solution containing both EDOT and DFA using lithographically patterned nanowire electrodeposition (LPNE). The through-nanowire electrical resistance of PEDOT–DFA nanowires is measured as a function of the Fe(III) concentration. In parallel with measurements on PEDOT–DFA nanowire arrays, the electrochemical impedance of PEDOT–DFA films is characterized as a function of the Fe(III) concentration and the frequency of the impedance measurement in order to better understand the mechanism of transduction. PEDOT–DFA nanowires detect Fe(III) from 10–4 to 10–8 M with a limit of detection of 300 pM (calculated) and 10 nM (measured).

Platinum (Pt)-modified palladium (Pd) nanowires (or Pd@Pt nanowires) are prepared with controlled Pt coverage. These Pd@Pt nanowires are used as resistive gas sensors for the detection of hydrogen gas in air, and the influence of the Pt surface layer is assessed. Pd nanowires with dimensions of 40 nm (h) × 100 nm (w) × 50 μm (l) are first prepared using lithographically patterned nanowire electrodeposition. A thin Pt surface layer is electrodeposited conformally onto a Pd nanowire at coverages, θPt, of 0.10 monolayer (ML), 1.0 ML, and 10 ML. X-ray photoelectron spectroscopy coupled with scanning electron microscopy and electrochemical measurements is consistent with a layer-by-layer deposition mode for Pt on the Pd nanowire surface. The resistance of a single Pd@Pt nanowire is measured during the exposure of these nanowires to pulses of hydrogen gas in air at concentrations ranging from 0.05 to 5.0 vol %. Both Pd nanowires and Pd@Pt nanowires show a prompt and reversible increase in resistance upon exposure to H2 in air, caused by the conversion of Pd to more resistive PdHx. Relative to a pure Pd nanowire, the addition of 1.0 ML of Pt to the Pd surface alters the H2 detection properties of Pd@Pt nanowires in two ways. First, the amplitude of the relative resistance change, ΔR/R0, measured at each H2 concentration is reduced at low temperatures (T = 294 and 303 K) and is unaffected at higher temperatures (T = 316, 344, and 376 K). Second, response and recovery rates are both faster at all temperatures in this range and for all H2 concentrations. For higher θPt = 10 ML, sensitivity to H2 is dramatically reduced. For lower θPt = 0.1 ML, no significant influence on sensitivity or the speed of response/recovery is observed.Keywords: catalytic; chemiresistor; electrodeposition; lithography; palladium; safety; sensor;

A symmetrical hybrid capacitor consisting of interdigitated, horizontal nanowires is described. Each of the 750 nanowires within the capacitor is 2.5 mm in length, consisting of a gold nanowire core (40 × ≈200 nm) encapsulated within a hemicylindrical shell of δ-phase MnO2 (thickness = 60–220 nm). These Au@δ-MnO2 nanowires are patterned onto a planar glass surface using lithographically patterned nanowire electrodeposition (LPNE). A power density of 165 kW/kg and energy density of 24 Wh/kg were obtained for a typical nanowire array in which the MnO2 shell thickness was 68 ± 8 nm. Capacitors incorporating these ultralong nanowires lost ≈10% of their capacity rapidly, during the first 20 discharge cycles, and then retained 90% of their maximum capacity for the ensuing 6000 cycles. The ability of capacitors consisting of ultralong Au@δ-MnO2 nanowires to simultaneously deliver high power and high capacity with acceptable cycle life is demonstrated.Keywords: electrochemical capacitors; electrodeposition; lithium ion; manganese oxide; photolithography; pseudo-capacitance;

We report the investigation of thermophones consisting of arrays of ultralong (mm scale) polycrystalline gold nanowires. Arrays of ∼4000 linear gold nanowires are fabricated at 5 μm pitch on glass surfaces using lithographically patterned nanowire electrodeposition (LPNE). The properties of nanowire arrays for generating sound are evaluated as a function of frequency (from 5–120 kHz), angle from the plane of the nanowires, input power (from 0.30–2.5 W), and the width of the nanowires in the array (from 270 to 500 nm). Classical theory for thermophones based on metal films accurately predicts the measured properties of these gold nanowire arrays. Angular “nodes” for the off-axis sound pressure level (SPL) versus frequency data, predicted by the directivity factor, are faithfully reproduced by these nanowire arrays. The maximum efficiency of these arrays (∼10–10 at 25 kHz), the power dependence, and the frequency dependence is independent of the lateral dimensions of these wires over the range from 270 to 500 nm.

Prior work involving the detection and emission of light from semiconductor nanostructures has involved single crystalline nanomaterials. Here we review the use of electrodeposited, polycrystalline (pc), cadmium selenide (CdSe) in nanowires and nanogap device structures for photonics. The photodetectors and photon emitters we describe are symmetrical metal–semiconductor–metal (M-S-M) devices prepared either by the evaporation of two gold contacts onto linear arrays of pc-CdSe nanowires prepared using lithographically patterned nanowires electrodeposition (LPNE), or by the electrodeposition of pc-CdSe directly onto gold nanogaps. The properties of these devices for detecting light using photoconductivity, and for generating light by electroluminescence, are described.

The sensitive detection of cancer biomarkers in urine could revolutionize cancer diagnosis and treatment. Such detectors must be inexpensive, easy to interpret, and sensitive. This report describes a bioaffinity matrix of viruses integrated into PEDOT films for electrochemical sensing of prostate-specific membrane antigen (PSMA), a prostate cancer biomarker. High sensitivity to PSMA resulted from synergistic action by two different ligands to PSMA on the same phage particle. One ligand was genetically encoded, and the secondary recognition ligand was chemically synthesized to wrap around the phage. The dual ligands result in a bidentate binder with high-copy, dense ligand display for enhanced PSMA detection through a chelate-based avidity effect. Biosensing with virus–PEDOT films provides a 100 pM limit of detection for PSMA in synthetic urine without requiring enzymatic or other amplification.

Electroluminescent (EL) metal-semiconductor-metal nanojunctions are prepared by electrodepositing nanocrystalline cadmium selenide (nc-CdSe) within ∼250 nm gold (Au) nanogaps prepared by focused ion beam milling. The electrodeposition of nc-CdSe is carried out at two temperatures: 20 °C (“cold”) and 75 °C (“hot”), producing mean grain diameters of 6 ± 1 nm and 11 ± 2 nm, respectively, for the nc-CdSe. Light-emitting nanojunctions (LEnJs) prepared at both temperatures show a low threshold voltage for light emission of <2 V; just above the 1.74 eV bandgap of CdSe. The EL intensity increases with the injection current and hot-deposited LEnJs produced a maximum EL intensity that is an order of magnitude higher than the cold-deposited LEnJs. Emitted photons are bimodal in energy with emission near the band gap of CdSe, and also at energies 200 meV below it; consistent with a mechanism of light emission involving the radiative recombination of injected holes with electrons at both band-edge and defect states. The quantum yield for “hot” electrodeposited nc-CdSe LEnJs is comparable to devices constructed from single crystalline nanowires of CdSe, and the threshold voltage of 1.9 (±0.1) V (cold) and 1.5 (±0.2) V (hot) is at the low end of the range reported for CdSe nanowire based devices.Keywords: cadmium selenide; electrodeposition; focused ion beam (FIB) milling; light emitting diodes (LEDs); metal−semiconductor−metal (M−S−M) junction; polycrystalline;

The efficacy of laser annealing for the thermal annealing of nanocrystalline gold nanowires is evaluated. Continuous laser illumination at 532 nm, focused to a 0.5 μm diameter spot, was rastered perpendicular to the axis of nanocrystalline gold nanowire at ∼2 kHz. This rastered beam was then scanned down the nanowire at velocities from 7 to 112 nm/s. The influence on the electrical resistance of the gold nanowire of laser power, polarization, translation speed, and nanowire width were evaluated. Nanocrystalline gold nanowires were prepared on glass surfaces using the lithographically patterned nanowire electrodeposition (LPNE) method. These nanowires had a rectangular cross section with a height of 20 (± 3) nm and widths ranging from 76 to 274 nm. The 4-contact electrical resistance of the nanowire is measured in situ during laser annealing and a real-time decrease in electrical resistance of between 30 and 65% is observed, depending upon the laser power and scan rate along the nanowire. These resistance decreases are associated with an increase in the mean grain diameter within these nanowires, measured using transmission electron microscopy, of up to 300%. The observed decrease in the electrical resistance induced by laser annealing conforms to classical predictions based upon the reduction in grain boundary scattering induced by grain growth.Keywords: electrical resistivity; electrodeposition; grain growth; photolithography; zone;

Development of electrocatalysts for the conversion of water to dioxygen is important in a variety of chemical applications. Despite much research in this field, there are still several fundamental issues about the electrocatalysts that need to be resolved. Two such problems are that the catalyst mass loading on the electrode is subject to large uncertainties and the wetted surface area of the catalyst is often unknown and difficult to determine. To address these topics, a cobalt monolayer was prepared on a gold electrode by underpotential deposition and used to probe its efficiency for the oxidation of water. This electrocatalyst was characterized by atomic force microscopy, grazing-incidence X-ray diffraction, and X-ray photoelectron spectroscopy at various potentials to determine if changes occur on the surface during catalysis. An enhancement of current was observed upon addition of PO43– ions, suggesting an effect from surface-bound ligands on the efficiency of water oxidation. At 500 mV overpotential, current densities of 0.20, 0.74, and 2.4 mA/cm2 for gold, cobalt, and cobalt in PO43– were observed. This approach thus provided electrocatalysts whose surface areas and activity can be accurately determined.

Electroluminescence (EL) from nanocrystalline CdSe (nc-CdSe) nanowire arrays is reported. The n-type, nc-CdSe nanowires, 400–450 nm in width and 60 nm in thickness, were synthesized using lithographically patterned nanowire electrodeposition, and metal–semiconductor–metal (M–S–M) devices were prepared by the evaporation of two gold contacts spaced by either 0.6 or 5 μm. These M–S–M devices showed symmetrical current voltage curves characterized by currents that increased exponentially with applied voltage bias. As the applied biased was increased, an increasing number of nanowires within the array “turned on”, culminating in EL emission from 30 to 50% of these nanowires at applied voltages of 25–30 V. The spectrum of the emitted light was broad and centered at 770 nm, close to the 1.74 eV (712 nm) band gap of CdSe. EL light emission occurred with an external quantum efficiency of 4 × 10–6 for devices with a 0.60 μm gap between the gold contacts and 0.5 × 10–6 for a 5 μm gap—values similar to those reported for M–S–M devices constructed from single-crystalline CdSe nanowires. Kelvin probe force microscopy of 5 μm nc-CdSe nanowire arrays showed pronounced electric fields at the gold electrical contacts, coinciding with the location of strongest EL light emission in these devices. This electric field is implicated in the Poole–Frenkel minority carrier emission and recombination mechanism proposed to account for EL light emission in most of the devices that were investigated.Keywords: electrodeposition; grain growth; Kelvin probe force microscopy; photolithography; thermal annealing

The formation of a nanometer-scale chemically responsive junction (CRJ) within a silver nanowire is described. A silver nanowire was first prepared on glass using the lithographically patterned nanowire electrodeposition method. A 1–5 nm gap was formed in this wire by electromigration. Finally, this gap was reconnected by applying a voltage ramp to the nanowire resulting in the formation of a resistive, ohmic CRJ. Exposure of this CRJ-containing nanowire to ammonia (NH3) induced a rapid (<30 s) and reversible resistance change that was as large as ΔR/R0 = (+)138% in 7% NH3 and observable down to 500 ppm NH3. Exposure to water vapor produced a weaker resistance increase of ΔR/R0,H2O = (+)10–15% (for 2.3% water) while nitrogen dioxide (NO2) exposure induced a stronger concentration-normalized resistance decrease of ΔR/R0,NO2 = (−)10–15% (for 500 ppm NO2). The proposed mechanism of the resistance response for a CRJ, supported by temperature-dependent measurements of the conductivity for CRJs and density functional theory calculations, is that semiconducting p-type AgxO is formed within the CRJ and the binding of molecules to this AgxO modulates its electrical resistance.

The performance of a single platinum (Pt) nanowire for detecting H2 in air is reported. A Pt nanowire shows no resistance change upon exposure to H2 in N2, but H2 exposure in air causes a reversible resistance decrease for H2 concentrations above 10 ppm. The amplitude of the resistance change induced by H2 exposure and the time rate of change of the nanowire resistance both increased with increasing temperature from 298 to 550 K. This resistance decrease of the Pt nanowire in the presence of H2 results from reduced electron diffuse scattering at hydrogen-covered Pt surfaces as compared with oxygen-covered platinum surfaces, we hypothesize. The properties for the detection of H2 in air of single Pt and Pd nanowires of similar size are compared in this study. Pt nanowires have a limit-of-detection for H2 (LODH2) of 10 ppm; 3 orders of magnitude lower than for Pd nanowires of the same size, as well as a response time that is 1/100th of Pd for [H2] ≈ 1%.

We describe the fabrication of arrays of nanowires on glass in which a gold core nanowire is encapsulated within a hemicylindrical shell of manganese dioxide. Arrays of linear gold (Au) nanowires are first prepared on glass using the lithographically patterned nanowire electrodeposition (LPNE) method. These Au nanowires have a rectangular cross-section with a width and height of ≈200 and 40 nm, respectively, and lengths in the 1 mm to 1 cm range. Au nanowires are then used to deposit MnO2 by potentiostatic electrooxidation from Mn2+ solution, forming a conformal, hemicylindrical shell with a controllable diameter ranging from 50 to 300 nm surrounding each Au nanowire. This MnO2 shell is δ-phase and mesoporous, as revealed by X-ray diffraction and Raman spectroscopy. Transmission electron microscopy (TEM) analysis reveals that the MnO2 shell is mesoporous (mp-MnO2), consisting of a network of ≈2 nm fibrils. The specific capacitance, Csp, of arrays of gold:mp-MnO2 nanowires is measured using cyclic voltammetry. For a mp-MnO2 shell thickness of 68 ± 3 nm, core:shell nanowires produce a Csp of 1020 ± 100 F/g at 5 mV/s and 450 ± 70 F/g at 100 mV/s. The cycle stability of this Csp, however, is extremely limited in aqueous electrolyte, decaying by >90% in 100 scans, but after oven drying and immersion in dry 1.0 M LiClO4, acetonitrile, dramatically improved cycle stability is achieved characterized by the absence of Csp fade for 1000 cycles at 100 mV/s. Core:shell nanowires exhibit true hybrid energy storage, as revealed by deconvolution of Csp into insertion and noninsertion components.Keywords: acetonitrile; birnessite; cyclic voltammetry; electrodeposition; microfabrication;

Field-effect transistors (NWFETs) have been prepared from arrays of polycrystalline cadmium selenide (pc-CdSe) nanowires using a back gate configuration. pc-CdSe nanowires were fabricated using the lithographically patterned nanowire electrodeposition (LPNE) process on SiO2/Si substrates. After electrodeposition, pc-CdSe nanowires were thermally annealed at 300 °C × 4 h either with or without exposure to CdCl2 in methanol—a grain growth promoter. The influence of CdCl2 treatment was to increase the mean grain diameter from 10 to 80 nm as determined by grazing incidence X-ray diffraction and to convert the crystal structure from cubic to wurtzite. Measured transfer characteristics showed an increase of the field effect mobility (μeff) by an order of magnitude from 1.94 × 10–4 cm2/(V s) to 23.4 × 10–4 cm2/(V s) for pc-CdSe nanowires subjected to the CdCl2 treatment. The CdCl2 treatment also reduced the threshold voltage (from 20 to 5 V) and the subthreshold slope (by ∼35%). Transfer characteristics for pc-CdSe NWFETs were also influenced by the channel length, L. For CdCl2-treated nanowires, μeff was reduced by a factor of eight as L increased from 5 to 25 μm. These channel length effects are attributed to the presence of defects including breaks and constrictions within individual pc-CdSe nanowires.Keywords: annealing; channel length; electrodeposition; lithography; mobility; NWFET;

Wafer scale (cm2) arrays and networks of nanochannels were created in polydimethylsiloxane (PDMS) from a surface pattern of electrodeposited gold nanowires in a master-replica process and characterized with scanning electron microscopy (SEM), atomic force microscopy (AFM), and fluorescence imaging measurements. Patterns of gold nanowires with cross-sectional dimensions as small as 50 nm in height and 100 nm in width were prepared on silica substrates using the process of lithographically patterned nanowire electrodeposition (LPNE). These nanowire patterns were then employed as masters for the fabrication of inverse replica nanochannels in a special formulation of PDMS. SEM and AFM measurements verified a linear correlation between the widths and heights of the nanowires and nanochannels over a range of 50 to 500 nm. The PDMS replica was then oxygen plasma-bonded to a glass substrate in order to create a linear array of nanofluidic channels (up to 1 mm in length) filled with solutions of either fluorescent dye or 20 nm diameter fluorescent polymer nanoparticles. Nanochannel continuity and a 99% fill success rate was determined from the fluorescence imaging measurements, and the electrophoretic injection of both dye and nanoparticles in the nanochannel arrays was also demonstrated. Employing a double LPNE fabrication method, this master-replica process was also used to create a large two-dimensional network of crossed nanofluidic channels.

We demonstrate the de novo fabrication of a biosensor, based upon virus-containing poly(3,4-ethylene-dioxythiophene) (PEDOT) nanowires, that detects prostate-specific membrane antigen (PSMA). This development process occurs in three phases: (1) isolation of a M13 virus with a displayed polypeptide receptor, from a library of ≈1011 phage-displayed peptides, which binds PSMA with high affinity and selectivity, (2) microfabrication of PEDOT nanowires that entrain these virus particles using the lithographically patterned nanowire electrodeposition (LPNE) method, and (3) electrical detection of the PSMA in high ionic strength (150 mM salt) media, including synthetic urine, using an array of virus–PEDOT nanowires with the electrical resistance of these nanowires for transduction. The electrical resistance of an array of these nanowires increases linearly with the PSMA concentration from 20 to 120 nM in high ionic strength phosphate-buffered fluoride (PBF) buffer, yielding a limit of detection (LOD) for PSMA of 56 nM.

Nanocrystalline cadmium selenide (nc-CdSe) was electrodeposited within a sub-50 nm gold nanogap, prepared by feedback-controlled electromigration, to form a photoconductive metal–semiconductor–metal nanojunction. Both gap formation and electrodeposition were rapid and automated. The electrodeposited nc-CdSe was stoichiometric, single cubic phase with a mean grain diameter of ∼7 nm. Optical absorption, photoluminescence, and the spectral photoconductivity response of the nc-CdSe were all dominated by band-edge transitions. The photoconductivity of these nc-CdSe-filled gold nanogaps was characterized by a detectivity of 6.9 × 1010 Jones and a photosensitivity of 500. These devices also demonstrated a maximum photoconductive gain of ∼45 and response and recovery times below 2 μs, corresponding to a 3 dB bandwidth of at least 175 kHz.Keywords: cadmium selenide; electrodeposition; electromigration; nanogap; photodetector

Virus-poly(3,4-ethylenedioxythiophene) (virus-PEDOT) biocomposite films are prepared by electropolymerizing 3,4-ethylenedioxythiophene (EDOT) in aqueous electrolytes containing 12 mM LiClO4 and the bacteriophage M13. The concentration of virus in these solutions, [virus]soln, is varied from 3 to 15 nM. A quartz crystal microbalance is used to directly measure the total mass of the biocomposite film during its electrodeposition. In combination with a measurement of the electrodeposition charge, the mass of the virus incorporated into the film is calculated. These data show that the concentration of the M13 within the electropolymerized film, [virus]film, increases linearly with [virus]soln. The incorporation of virus particles into the PEDOT film from solution is efficient, resulting in a concentration ratio of [virus]film:[virus]soln ≈ 450. Virus incorporation into the PEDOT causes roughening of the film topography that is observed using scanning electron microscopy and atomic force microscopy (AFM). The electrical conductivity of the virus-PEDOT film, measured perpendicular to the plane of the film using conductive tip AFM, decreases linearly with virus loading, from 270 μS/cm for pure PEDOT films to 50 μS/cm for films containing 100 μM virus. The presence on the virus surface of displayed affinity peptides did not significantly influence the efficiency of incorporation into virus-PEDOT biocomposite films.

The Seebeck coefficient, S, and the electrical conductivity, σ, of electrodeposited poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires and thin films are reported. PEDOT nanowires were prepared by electropolymerizing 3,4-ethylenedioxythiophene (EDOT) in aqueous LiClO4 within a template prepared using the lithographically patterned nanowire electrodeposition (LPNE) process. These nanowires were 40−90 nm in thickness, 150−580 nm in width, and 200 μm in length. σ and S were measured from 190 K to 310 K by fabricating heaters and thermocouples on top of arrays of 750 PEDOT nanowires. Such PEDOT nanowire arrays consistently produced S values that were higher than those for PEDOT films: up to −122 μV/K (310 K) for nanowires and up to −57 μV/K (310 K) for films. The sample-to-sample variation in S for 14 samples of PEDOT nanowires and films, across a wide range of critical dimensions, is fully explained by variations in the carrier concentrations in accordance with the Mott equation. In spite of their higher |S| values, PEDOT nanowires also had higher σ than films, on average, because electron mobilities were greater in nanowires by a factor of 3.

Composite films composed of poly(3,4-ethylenedioxythiophene), PEDOT, and the filamentous virus M13-K07 were prepared by electrooxidation of 3,4-ethylenedioxythiophene (EDOT) in aqueous solutions containing 8 nM of the virus at planar gold electrodes. These films were characterized using atomic force microscopy and scanning electron microscopy. The electrochemical impedance of virus−PEDOT films increases upon exposure to an antibody (p-Ab) that selectively binds to the M13 coat peptide. Exposure to p-Ab causes a shift in both real (ZRE) and imaginary (ZIM) impedance components across a broad range of frequencies from 50 Hz to 10 kHz. Within a narrower frequency range from 250 Hz to 5 kHz, the increase of the total impedance (Ztotal) with p-Ab concentration conforms to a Langmuir adsorption isotherm over the concentration range from from 6 to 66 nM, yielding a value for Kd = 16.9 nM at 1000 Hz.

The electrical resistance, R, of an array of 30 palladium nanowires is used to detect the concentration of dissolved hydrogen gas (H2) in transformer oil over the temperature range from 21 to 70 °C. The palladium nanowire array (PdNWA), consisting of Pd nanowires ∼100 nm (width), ∼20 nm (height), and 100 μm (length), was prepared using the lithographically patterned nanowire electrodeposition (LPNE) method. The R of the PdNWA increased by up to 8% upon exposure to dissolved H2 at concentrations above 1.0 ppm and up to 2940 ppm at 21 °C. The measured limit-of-detection for dissolved H2 was 1.0 ppm at 21 °C and 1.6 ppm at 70 °C. The increase in resistance induced by exposure to H2 was linear with [H2]oil1/2 across this concentration range. A PdNWA sensor operating in flowing transformer oil has functioned continuously for 150 days.

Despite the fact that polymer surfaces are soft, they are notoriously difficult to pattern over large areas on the nanoscale. Two previously described methods, nanoimprint lithography (NIL) and nanosphere lithography (NSL), can be used to nanopattern polymer surfaces, but both of these methods involve many (>6) processing steps. Laser interference patterning (LIP) is a maskless surface nanopatterning technology that has been around for more than 10 years. In this issue of ACS Nano, Wang and co-workers demonstrate that LIP can form the basis for a simplified nanopatterning scheme that is general for a wide variety of polymer surfaces. As reported in this issue of ACS Nano, laser interference patterning (LIP) has been adapted to the problem of nanopatterning polymer surfaces rapidly and over wafer-scale areas.

Arrays of mesoporous manganese dioxide, mp-MnO2, nanowires were electrodeposited on glass and silicon surfaces using the lithographically patterned nanowire electrodeposition (LPNE) method. The electrodeposition procedure involved the application, in a Mn(ClO4)2-containing aqueous electrolyte, of a sequence of 0.60 V (vs MSE) voltage pulses delineated by 25 s rest intervals. This “multipulse” deposition program produced mp-MnO2 nanowires with a total porosity of 43–56%. Transmission electron microscopy revealed the presence within these nanowires of a network of 3–5 nm diameter fibrils that were X-ray and electron amorphous, consistent with the measured porosity values. mp-MnO2 nanowires were rectangular in cross-section with adjustable height, ranging from 21 to 63 nm, and adjustable width ranging from 200 to 600 nm. Arrays of 20 nm × 400 nm mp-MnO2 nanowires were characterized by a specific capacitance, Csp, of 923 ± 24 F/g at 5 mV/s and 484 ± 15 F/g at 100 mV/s. These Csp values reflected true hybrid electrical energy storage with significant contributions from double-layer capacitance and noninsertion pseudocapacitance (38% for 20 nm × 400 nm nanowires at 5 mV/s) coupled with a Faradaic insertion capacity (62%). These two contributions to the total Csp were deconvoluted as a function of the potential scan rate.Keywords: battery; cathode; electrodeposition; electrodeposition; lithium ion; photolithography; pseudocapacitance

The separate fields of conducting polymer-based electrochemical sensors and virus-based molecular recognition offer numerous advantages for biosensing. Grafting M13 bacteriophage into an array of poly (3,4-ethylenedioxythiophene) (PEDOT) nanowires generated hybrids of conducting polymers and viruses. The virus incorporation into the polymeric backbone of PEDOT occurs during electropolymerization via lithographically patterned nanowire electrodeposition. The resultant arrays of virus-PEDOT nanowires enable real-time, reagent-free electrochemical biosensing of analytes in physiologically relevant buffers.

We report the fabrication of silver nanowires using lithographically patterned nanowire electrodeposition (LPNE). The LPNE synthesis of silver nanowires proceeds by lithographically patterning, and then etching an evaporated nickel film to produce a nickel nanoband 20–80 nm in height. This nanoband, which traces the perimeter of the exposed region, is recessed by ≈500 nm into the photoresist producing a horizontal trench. A silver nanowire, of controlled height and width, is formed within this trench by electrodepositing silver from either of two aqueous solutions at the nickel nanoband. Silver nanowires with controlled widths ranging from 100 to 400 nm were obtained and the height of silver nanowires was independently controllable over the range from 20 to 80 nm. The LPNE process is wafer-scale and continuous silver nanowires millimeters in length are readily obtained. Data for the characterization of these nanowires using AFM, TEM, and XRD is presented.

Palladium nanowires prepared using the lithographically patterned nanowire electrodeposition (LPNE) method are used to detect hydrogen gas (H2). These palladium nanowires are prepared by electrodepositing palladium from EDTA-containing solutions under conditions favoring the formation of β-phase PdHx. The Pd nanowires produced by this procedure are characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. These nanowires have a mean grain diameter of 15 nm and are composed of pure Pd with no XPS-detectable bulk carbon. The four-point resistance of 50−100 μm segments of individual nanowires is used to detect H2 in N2 and air at concentrations ranging from 2 ppm to 10%. For low [H2] < 1%, the response amplitude increases by a factor of 2−3 with a reduction in the lateral dimensions of the nanowire. Smaller nanowires show accelerated response and recovery rates at all H2 concentrations from, 5 ppm to 10%. For 12 devices, response and recovery times are correlated with the surface area/volume ratio of the palladium detection element. We conclude that the kinetics of hydrogen adsorption limits the observed response rate seen for the nanowire, and that hydrogen desorption from the nanowire limits the observed recovery rate; proton diffusion within PdHx does not limit the rates of either of these processes.Keywords: electrodeposition; hydride; hydrogen; palladium; sensor

A high-throughput method for measuring the Seebeck coefficient, S, and the electrical conductivity, σ, of lithographically patterned nanowire arrays is described. Our method involves the microfabrication of two heaters and two Ag/Ni thermocouples, literally on top of an array of polycrystalline PbTe nanowires synthesized on a Si3N4 wafer using the lithographically patterned nanowire electrodeposition (LPNE) method. This strategy eliminates the transfer and manipulation of nanowires as a prerequisite for carrying out measurements on these wires of thermoelectric metrics. With these devices, we have measured the influence of the thermal annealing temperature on the thermoelectric properties of nine arrays of 60 nm ×200 nm × 200 μm PbTe nanowires, and we find that at an optimum annealing temperature of 453 K, the S at 300 K is increased from −41 μV/K for unannealed wires to −479 μV/K, 80% larger in magnitude than the S (−260 μV/K) of bulk PbTe.Keywords (keywords): grain growth; lead telluride; oxidation; polycrystalline nanowires; thermoelectric; thermopower;

The diffraction limit, d≈λ/2, constrains the resolution with which structures may be produced using photolithography. Practical limits for d are in the 100 nm range. To circumvent this limit, photolithography can be used to fabricate a sacrificial electrode that is then used to initiate and propagate the growth by electrodeposition of a nanowire. We have described a version of this strategy in which the sacrificial electrode delimits one edge of the nascent nanowire, and a microfabricated “ceiling” constrains its height during growth. The width of the nanowire is determined by the electrochemical deposition parameters (deposition time, applied potential, and solution composition). Using this method, called lithographically patterned nanowire electrodeposition (LPNE), nanowires with minimum dimensions of 11 nm (w) × 5 nm (h) have been obtained. The lengths of these nanowires can be wafer-scale. LPNE has been used to synthesize nanowires composed of bismuth, gold, silver, palladium, platinum, and lead telluride.

Nanowires of lead telluride (PbTe) were patterned on glass surfaces using lithographically patterned nanowire electrodeposition (LPNE). LPNE involved the fabrication by photolithography of a contoured nickel nanoband that is recessed by ≈300 nm into a horizontal photoresist trench. Cubic PbTe was then electrodeposited from a basic aqueous solution containing Pb2+ and TeO32− at the nickel nanoband using a cyclic deposition/stripping potential program in which lead-rich PbTe was first deposited in a negative-going potential scan and excess lead was then anodically stripped from the nascent nanowire by scanning in the positive direction to produce near stoichiometric PbTe. Repeating this scanning procedure permitted PbTe nanowires 60−400 nm in width to be obtained. The wire height was controlled over the range of 20−100 nm based upon the nickel film thickness. Nanowires with lengths exceeding 1 cm were prepared in this study. We report the characterization of these nanowires using X-ray diffraction, transmission electron microscopy and electron diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The surface chemical composition of PbTe nanowires was monitored by XPS as a function of time during the exposure of these nanowires to laboratory air. One to two monolayers of a mixed Pb and Te oxide are formed during a 24 h exposure. The electrical conductivity of PbTe nanowires was strongly affected by air oxidation, declining from an initial value of 2.0(±1.5) × 10 4 S/m by 61% (for nanowires with a 20 nm thickness), 55% (for 40 nm), and 12% (for 60 nm).Keywords: electrodeposition; lead telluride; nanowire; thermoelectric

We describe the preparation by electrodeposition of arrays of lead telluride (PbTe) nanowires using the lithographically patterned nanowire electrodeposition (LPNE) method. PbTe nanowires had a rectangular cross-section with adjustable width and height ranging between 60−400 nm (w) and 20−100 nm (h). The characterization of these nanowire arrays using X-ray diffraction, transmission electron microscopy and electron diffraction, scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy (XPS) is reported. PbTe nanowires were electrodeposited using a cyclic electrodeposition-stripping technique that produced polycrystalline, stoichiometric, face-centered cubic PbTe with a mean grain diameter of 10−20 nm. These nanowires were more than 1 mm in length and two additional processing steps permitted their suspension across 25 μm air gaps microfabricated on these surfaces. The LPNE synthesis of lithographically patterned PbTe nanowires was carried out in unfiltered laboratory air. Nanowires with lengths of 70−100 μm showed an electrical resistivity comparable to bulk PbTe. XPS reveals that exposure of PbTe nanowires to air causes the formation on the nanowire surface of approximately one monolayer of a mixed lead oxide and tellurium oxide within a few minutes.

We describe the preparation of three types of silver/nickel thermocouples (TCs) based upon electrodeposited wires with diameters at, or just below, 1.0 μm: Type 1—wire/thin film TCs consisting of an electrodeposited submicrometer diameter wire and an evaporated metal thin film; Type 2—wire-wire TCs consisting of end-butted and electrochemically welded silver and nickel submicrometer wires; Type 3—wire-wire TCs for which the silver wires were electrochemically etched prior to the electrodeposition of the nickel wires. The metal wires in all devices were prepared using the electrochemical step edge decoration method. The properties of these TCs for measuring temperature in the range from ambient to 110 °C were evaluated. Output voltage, VTC, versus temperature was linear for these devices yielding Seebeck coefficients of 19-22 μV/°C—within 95% of the expected bulk value. Temperature excursions across a 90 °C range caused no measurable hysteresis in VTC for these devices. All three types of devices retained the ability to accurately measure temperature for months after exposure to ambient laboratory air. The response times for these TCs were measured using two different laser heating methods.

Electrochemical impedance spectroscopy is used to detect the binding of a 148.2 kDa antibody to a “covalent virus layer” (CVL) immobilized on a gold electrode. The CVL consisted of M13 phage particles covalently anchored to a 3 mm diameter gold disk electrode. The ability of the CVL to distinguish this antibody (“p-Ab”) from a second, nonbinding antibody (“n-Ab”) was evaluated as a function of the frequency and phase of the measured current relative to the applied voltage. The binding of p-Ab to the CVL was correlated with a change in the resistance, reducing it at low frequency (1−40 Hz) while increasing it at high frequency (2−140 kHz). The capacitance of the CVL was virtually uncorrelated with p-Ab binding. At both low and high frequency, the electrode resistance was linearly dependent on the p-Ab concentration from 20 to 266 nM but noise compromised the reproducibility of the p-Ab measurement at frequencies below 40 Hz. A “signal-to-noise” ratio for antibody detection was computed based upon the ratio between the measured resistance change upon p-Ab binding and the standard deviation of this change obtained from multiple measurements. In spite of the fact that the impedance change upon p-Ab binding in the low frequency domain was more than 100 times larger than that measured at high frequency, the S/N ratio at high frequency was higher and virtually independent of frequency from 4 to 140 kHz. Attempts to release p-Ab from the CVL using 0.05 M HCl, as previously described for mass-based detection, caused a loss of sensitivity that may be associated with a transition of these phage particles within the CVL from a linear to a coiled conformation at low pH.

Lithographically patterned nanowire electrodeposition (LPNE) is a new method for fabricating polycrystalline metal nanowires using electrodeposition. In LPNE, a sacrificial metal (M1 = silver or nickel) layer, 5−100 nm in thickness, is first vapor deposited onto a glass, oxidized silicon, or Kapton polymer film. A (+) photoresist (PR) layer is then deposited, photopatterned, and the exposed Ag or Ni is removed by wet etching. The etching duration is adjusted to produce an undercut ≈300 nm in width at the edges of the exposed PR. This undercut produces a horizontal trench with a precisely defined height equal to the thickness of the M1 layer. Within this trench, a nanowire of metal M2 is electrodeposited (M2 = gold, platinum, palladium, or bismuth). Finally the PR layer and M1 layer are removed. The nanowire height and width can be independently controlled down to minimum dimensions of 5 nm (h) and 11 nm (w), for example, in the case of platinum. These nanowires can be 1 cm in total length. We measure the temperature-dependent resistance of 100 μm sections of Au and Pd wires in order to estimate an electrical grain size for comparison with measurements by X-ray diffraction and transmission electron microscopy. Nanowire arrays can be postpatterned to produce two-dimensional arrays of nanorods. Nanowire patterns can also be overlaid one on top of another by repeating the LPNE process twice in succession to produce, for example, arrays of low-impedance, nanowire−nanowire junctions.Keywords: electrodeposition; nanowire; noble metal; photolithography

Metal nano- and microparticles that are narrowly dispersed in diameter can be electrodeposited on graphite basal plane surfaces using the two-step method: First, a voltage pulse with a duration of 5 ms and an overpotential, η=−500 mV was used to nucleate metal particles on the graphite surface. Then, a growth pulse with an overpotential, η, of −20 to −250 mV was applied to grow the metal particles obtained in step 1 to the desired final diameter. For a variety of metals, including silver, gold, platinum, molybdenum, and nickel, this ‘slow-growth’ method yielded dispersions of particles ranging in diameter from 50 nm to 2 μm having a relative standard deviation (RSDdia=σdia/〈dia〉) as low as 7%.

The diffraction limit, d≈λ/2, constrains the resolution with which structures may be produced using photolithography. Practical limits for d are in the 100 nm range. To circumvent this limit, photolithography can be used to fabricate a sacrificial electrode that is then used to initiate and propagate the growth by electrodeposition of a nanowire. We have described a version of this strategy in which the sacrificial electrode delimits one edge of the nascent nanowire, and a microfabricated “ceiling” constrains its height during growth. The width of the nanowire is determined by the electrochemical deposition parameters (deposition time, applied potential, and solution composition). Using this method, called lithographically patterned nanowire electrodeposition (LPNE), nanowires with minimum dimensions of 11 nm (w) × 5 nm (h) have been obtained. The lengths of these nanowires can be wafer-scale. LPNE has been used to synthesize nanowires composed of bismuth, gold, silver, palladium, platinum, and lead telluride.