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A single platinum nanowire (PtNW) chemiresistive sensor for ethylene gas is reported. In this application, the PtNW performs three functions: (1) Joule self-heating to a specified temperature, (2) in situ resistance-based temperature measurement, and (3) detection of ethylene in air as a resistance change. Ethylene gas in air is detected as a reduction in nanowire resistance by up to 4.5% for concentrations ranging from 1 to 30 ppm in an optimum NW temperature range from 630 to 660 K. This response is rapid (30-100 s), reversible, and reproducible for repetitive ethylene pulses. A threefold increase in signal amplitude is observed as the NW thickness is reduced from 60 to 20 nm, commensurate with a signal transduction mechanism involving surface electron scattering.

pH sensors that are nanoscopic in all three dimensions are fabricated within a single gold nanowire. Fabrication involves the formation of a nanogap within the nanowire via electromigration, followed by electropolymerization of pH-responsive poly(aniline) (PANI) that fills the nanogap forming the nanojunction. All fabrication steps are performed using wet chemical methods that do not require a clean room. The measured electrical impedance of the PANI nanojunction is correlated with pH from 2.0 to 9.0 with a response time of 30 s. Larger, micrometer-scale PANI junctions exhibit a slower response. The measured pH is weakly influenced by the salt concentration of the contacting aqueous solution. An impedance measurement at two frequencies (300 kHz and 1.0 Hz) enables estimation of the salt concentration and correction of the measured pH value, preserving the accuracy of the pH measurement across the entire calibration curve for salt concentrations up to 1.0 M. The result is a nanoscopic pH sensor with pH sensing performance approaching that of a conventional, macroscopic pH glass-membrane electrode.

In this work, we prepared a well-aligned palladium oxide nanowire (PdO NW) array using the lithographically patterned Pd nanowire electrodeposition (LPNE) method followed by subsequent calcination at 500 °C. Sensitization with platinum (Pt) nanoparticles (NPs), which were functionalized on PdO NWs through a simple reduction process, significantly enhanced the detection capability of the Pt-loaded PdO NWs (Pt-PdO NWs) sensors toward hydrogen gas (H 2 ) at room temperature. The well-distributed Pt NPs, which are known chemical sensitizers, activated the dissociation of H 2 and oxygen molecules through the spillover effect with subsequent diffusion of these products to the PdO surface, thereby transforming the entire surface of the PdO NWs into reaction sites for H 2 . As a result, at a high concentration of H 2 (0.2%), the Pt-PdO NWs showed an enhanced sensitivity of 62% (defined as Δ R/ R air × 100%) compared to that (6.1%) of pristine PdO NWs. The Pt-PdO NWs exhibited a response time of 166 s, which was 2.68-fold faster than that of pristine PdO NWs (445 s). In addition, the Pt-PdO NWs responded to a very low concentration of H 2 (10 ppm) with a sensitivity of 14%, unlike the pristine PdO NWs, which did not exhibit any response at that concentration. These outstanding results are mainly attributed to a homogeneous decoration of Pt NPs on the surface of well-aligned PdO NWs. In this work, we demonstrated the potential suitability of Pt-PdO NWs as a highly sensitive H 2 sensing layer at room temperature.

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 SiO(2)/Si substrates. After electrodeposition, pc-CdSe nanowires were thermally annealed at 300 °C × 4 h either with or without exposure to CdCl(2) in methanol-a grain growth promoter. The influence of CdCl(2) 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 (μ(eff)) by an order of magnitude from 1.94 × 10(-4) cm(2)/(V s) to 23.4 × 10(-4) cm(2)/(V s) for pc-CdSe nanowires subjected to the CdCl(2) treatment. The CdCl(2) 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 CdCl(2)-treated nanowires, μ(eff) 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.

Wafer scale (cm(2)) 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 ≈10(11) 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.

Transformational advances in the performance of nanowire-based chemical sensors and biosensors have been achieved over the past two to three years. These advances have arisen from a better understanding of the mechanisms of transduction operating in these devices, innovations in nanowire fabrication, and improved methods for incorporating receptors into or onto nanowires. Nanowire-based biosensors have detected DNA in undiluted physiological saline. For silicon nanowire nucleic acid sensors, higher sensitivities have been obtained by eliminating the passivating oxide layer on the nanowire surface and by substituting uncharged protein nucleic acids for DNA as the capture strands. Biosensors for peptide and protein cancer markers, based on both semiconductor nanowires and nanowires of conductive polymers, have detected these targets at physiologically relevant concentrations in both blood plasma and whole blood. Nanowire chemical sensors have also detected several gases at the parts-per-million level. This review discusses these and other recent advances, concentrating on work published in the past three years.

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.

The properties of a single heated palladium (Pd) nanowire for the detection of hydrogen gas (H(2)) are explored. In these experiments, a Pd nanowire, 48-98 microm in length, performs three functions in parallel: 1) Joule self-heating is used to elevate the nanowire temperature by up to 128 K, 2) the 4-contact wire resistance in the absence of H(2) is used to measure its temperature, and 3) the nanowire resistance in the presence of H(2) is correlated with its concentration, allowing it to function as a H(2) sensor. Compared with the room-temperature response of a Pd nanowire, the response of the heated nanowire to hydrogen is altered in two ways: First, the resistance change (DeltaR/R(0)) induced by H(2) exposure at any concentration is reduced by a factor of up to 30 and second, the rate of the resistance change - observed at the beginning ("response") and at the end ("recovery") of a pulse of H(2) - is increased by more than a factor of 50 at some H(2) concentrations. Heating nearly eliminates the retardation of response and recovery seen from 1-2% H(2), caused by the alpha --> beta phase transition of PdH(x), a pronounced effect for nanowires at room temperature. The activation energies associated with sensor response and recovery are measured and interpreted.

Surface patterns of single-stranded DNA (ssDNA) consisting of nanoscale lines as thin as 40 nm were fabricated on polymer substrates for nanotechnology and bioaffinity sensing applications. Large scale arrays (with areas up to 4 cm(2)) of ssDNA "nanolines" were created on streptavidin-coated polymer (PDMS) surfaces by transferring biotinylated ssDNA from a master pattern of gold nanowires attached to a glass substrate. The gold nano-wires were first formed on the glass substrate by the process of lithographically patterned nanowire electrodeposition (LPNE), and then "inked" with biotinylated ssDNA by hybridization adsorption to a thiol-modified ssDNA monolayer attached to the gold nanowires. The transferred ssDNA nanolines were capable of hybridizing with ssDNA from solution to form double-stranded DNA (dsDNA) patterns; a combination of fluorescence and atomic force microscopy (AFM) measurements were used to characterize the dsDNA nanoline arrays. To demonstrate the utility of these surfaces for biosensing, optical diffraction measurements of the hybridization adsorption of DNA-coated gold nanoparticles onto the ssDNA nanoline arrays were used to detect a specific target sequence of unlabeled ssDNA in solution.

Lithographically patterned nanowire electrodeposition (LPNE) provides a method for patterning nanowires composed of nanocrystalline cadmium selenide (nc-CdSe) over wafer-scale areas. We assess the properties of (nc-CdSe) nanowires for detecting light as photoconductors. Structural characterization of these nanowires by X-ray diffraction and transmission electron microscopy reveals they are composed of stoichiometric, single phase, cubic CdSe with a mean grain diameter of 10 nm. For nc-CdSe nanowires with lengths of many millimeters, the width and height dimensions could be varied over the range from 60 to 350 nm (w) and 20 to 80 nm (h). Optical absorption and photoluminescence spectra for nc-CdSe nanowires were both dominated by band-edge transitions. The photoconductivity properties of nc-CdSe nanowire arrays containing approximately 350 nanowires were evaluated by electrically isolating 5 microm nanowire lengths using evaporated gold electrodes. Photocurrents, i(photo), of 10-100 x (i(dark)) were observed with a spectral response characterized by an onset at 1.75 eV. i(photo) response and recovery times were virtually identical and in the range from 20 to 40 micros for 60 x 200 nm nanowires.

Parallel arrays of either Au or Pd nanowires were fabricated on glass substrates via the electrochemical process of lithographically patterned nanowire electrodeposition (LPNE) and then characterized with scanning electron microscopy (SEM) and a series of optical diffraction measurements at 633 nm. Nanowires with widths varying from 25 to 150 nm were electrodeposited onto nanoscale Ni surfaces created by the undercut etching of a photoresist pattern on a planar substrate. With the use of a simple transmission grating geometry, up to 60 diffraction orders were observed from the nanowire gratings, with separate oscillatory intensity patterns appearing in the even and odd diffraction orders. The presence of these intensity oscillations is attributed to the LPNE array fabrication process, which creates arrays with alternating interwire spacings of distances d +Delta and d -Delta, where d = 25 microm and the asymmetry Delta varied from 0 to 3.5 microm. The amount of asymmetry could be controlled by varying the LPNE undercut etching time during the creation of the nanoscale Ni surfaces. The Fourier transform of a mathematical model of the nanowire array was used to predict the diffraction intensity patterns and quantitatively determine Delta for any grating. Additional sensitivity and an expanded diffraction order range were obtained through the use of external reflection (ER) and total internal reflection (TIR) diffraction geometries.

Two types of pure palladium (Pd) nanowires, differentiated by microstructure, were electrodeposited: (1) nanocrystalline Pd nanowires (grain diameter approximately 5 nm, henceforth nc5-Pd) and (2) nanocrystalline Pd nanowires with a grain diameter of 15 nm (nc15-Pd). These nanowires were evaluated for the detection of hydrogen gas (H(2)). Despite their fundamental similarities, the behavior of these nanowires upon exposure to H(2) was dramatically and reproducibly different: nc5-Pd nanowires spontaneously fractured upon exposure to H(2) above 1-2%. Fractured nanowires continued to function as sensors for H(2) concentrations above 2%, actuated by the volume change associated with the alpha to beta phase transition of PdH(x). nc15-Pd nanowires, in contrast, withstood repeated exposures to H(2) up to 10% without fracturing. nc15-Pd nanowires showed a rapid (2 s at 10%) increase in resistance in the presence of H(2) and a response that scaled smoothly with [H(2)] spanning 5 orders of magnitude down to 2 ppm.

The 2018 Nobel Prize in Chemistry recognized in vitro evolution, including the development by George Smith and Gregory Winter of phage display, a technology for engineering the functional capabilities of antibodies into viruses. Such bacteriophages solve inherent problems with antibodies, including their high cost, thermal lability, and propensity to aggregate. While phage display accelerated the discovery of peptide and protein motifs for recognition and binding to proteins in a variety of applications, the development of biosensors using intact phage particles was largely unexplored in the early 2000s. Virus particles, 16.5 MDa in size and assembled from thousands of proteins, could not simply be substituted for antibodies in any existing biosensor architectures.Incorporating viruses into biosensors required us to answer several questions: What process will allow the incorporation of viruses into a functional bioaffinity layer? How can the binding of a protein disease marker to a virus particle be electrically transduced to produce a signal? Will the variable salt concentration of a bodily fluid interfere with electrical transduction? A completely new biosensor architecture and a new scheme for electrical transduction of the binding of molecules to viruses were required.This Account describes the highlights of a research program launched in 2006 that answered these questions. These efforts culminated in 2018 in the invention of a biosensor specifically designed to interface with virus particles: the Virus BioResistor (VBR). The VBR is a resistor consisting of a conductive polymer matrix in which M13 virus particles are entrained. The electrical impedance of this resistor, measured across 4 orders of magnitude in frequency, simultaneously measures the concentration of a target protein and the ionic conductivity of the medium in which the resistor is immersed. Large signal amplitudes coupled with the inherent simplicity of the VBR sensor design result in high signal-to-noise ratio (S/N > 100) and excellent sensor-to-sensor reproducibility. Using this new device, we have measured the urinary bladder cancer biomarker nucleic acid deglycase (DJ-1) in urine samples. This optimized VBR is characterized by extremely low sensor-to-sensor coefficients of variation in the range of 3-7% across the DJ-1 binding curve down to a limit of quantitation of 30 pM, encompassing 4 orders of magnitude in concentration.

Single crystal ZnO nanowires are synthesized and configured as field-effect transistors. Photoluminescence and photoconductivity measurements show defect-related deep electronic states giving rise to green-red emission and absorption. Photocurrent temporal response shows that current decay time is significantly prolonged in vacuum due to a slower oxygen chemisorption process. The photoconductivity of ZnO nanowires is strongly polarization dependent. Collectively, these results demonstrate that ZnO nanowire is a remarkable optoelectronic material for nanoscale device applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

We describe the fabrication of arrays of nanowires onglass inwhich a gold core nanowire is encapsulated within a hemicylindricalshell of manganese dioxide. Arrays of linear gold (Au) nanowires arefirst prepared on glass using the lithographically patterned nanowireelectrodeposition (LPNE) method. These Au nanowires have a rectangularcross-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 usedto deposit MnO2by potentiostatic electrooxidation fromMn2늉溶⨧, forming a conformal, hemicylindrical shellwith a controllable diameter ranging from 50 to 300 nm surroundingeach Au nanowire. This MnO2shell is δ-phase andmesoporous, as revealed by X-ray diffraction and Raman spectroscopy.Transmission electron microscopy (TEM) analysis reveals that the MnO2shell is mesoporous (mp-MnO2),consisting of a network of ≈2 nm fibrils. The specific capacitance, Csp, of arrays of gold:mp-MnO2nanowires is measured using cyclic voltammetry.For a mp-MnO2shell thickness of 68 ±3 nm, core:shell nanowires produce a Cspof 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 inaqueous electrolyte, decaying by >90% in 100 scans, but after ovendrying and immersion in dry 1.0 M LiClO4, acetonitrile,dramatically improved cycle stability is achieved characterized bythe absence of Cspfadefor 1000 cycles at 100 mV/s. Core:shellnanowires exhibit true hybridenergy storage, as revealed by deconvolution of Cspinto insertion and noninsertion components. [ABSTRACT FROM AUTHOR]

The formation of a nanometer-scale chemically responsivejunction(CRJ) within a silver nanowire is described. A silver nanowire wasfirst prepared on glass using the lithographically patterned nanowireelectrodeposition method. A 1–5 nm gap was formed in this wireby electromigration. Finally, this gap was reconnected by applyinga 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 changethat was as large as ΔR/R0= ()138% in 7% NH3and observable down to 500ppm NH3. Exposure to water vapor produced a weaker resistanceincrease of ΔR/R0,H2O= ()10–15% (for 2.3% water) while nitrogen dioxide (NO2) exposure induced a stronger concentration-normalized resistancedecrease of ΔR/R0,NO2= (−)10–15% (for 500 ppm NO2). The proposed mechanism of the resistance response for a CRJ, supportedby temperature-dependent measurements of the conductivity for CRJsand density functional theory calculations, is that semiconductingp-type AgxO is formed within the CRJ andthe binding of molecules to this AgxOmodulates its electrical resistance. [ABSTRACT FROM AUTHOR]

Surface patterns of single-stranded DNA (ssDNA) consisting of nanoscale lines as thin as 40 nan were fabricated on polymer substrates for nanotechnology and bioaffinity sensing applications. Iarge scale arrays (with areas up to 4 cm2) of ssDNA "nanolines" were created on streptavidin-coated polymer (PDMS) surfaces by transferring biotinylated ssDNA from a master pattern of gold nanowires attached to a glass substrate. The gold nanowires were first formed on the glass substrate by the process of lithographically patterned nanowire electrodeposition (LPNE), and then "inked" with biotinylated ssDNA by hybridization adsorption to a thiolmodified ssDNA monolayer attached to the gold nanowires. The transferred ssDNA nanolines were capable of hybridizing with ssDNA from solution to form double-stranded DNA (dsDNA) patterns; a combination of fluorescence and atomic force microscopy (AFM) measurements were used to characterize the dsDNA nanoline arrays. To demonstrate the utility of these surices for biosensing, optical diffraction measurements of the hybridization adsorption of DNA-coated gold nanoparticles onto the ssDNA nanoline arrays were used to detect a specific target sequence of unlabeled ssDNA in solution. [ABSTRACT FROM AUTHOR]