Keith C. Donavan

20 μs Photocurrent Response from Lithographically Patterned Nanocrystalline Cadmium Selenide Nanowires

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.

The surface scattering-based detection of hydrogen in air using a platinum nanowire.

The performance of a single platinum (Pt) nanowire for detecting H(2) in air is reported. A Pt nanowire shows no resistance change upon exposure to H(2) in N(2), but H(2) exposure in air causes a reversible resistance decrease for H(2) concentrations above 10 ppm. The amplitude of the resistance change induced by H(2) 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 H(2) 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 H(2) in air of single Pt and Pd nanowires of similar size are compared in this study. Pt nanowires have a limit-of-detection for H(2) (LOD(H(2))) 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 [H(2)] ≈ 1%.

Virus–Polymer Hybrid Nanowires Tailored to Detect Prostate-Specific Membrane Antigen

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.

Tunable photoconduction sensitivity and bandwidth for lithographically patterned nanocrystalline cadmium selenide nanowires.

Nanocrystalline cadmium selenide (nc-CdSe) nanowires were prepared using the lithographically patterned nanowire electrodeposition method. Arrays of 350 linear nc-CdSe nanowires with lateral dimensions of 60 nm (h) × 200 nm (w) were patterned at 5 μm pitch on glass. nc-CdSe nanowires electrodeposited from aqueous solutions at 25 °C had a mean grain diameter, d(ave), of 5 nm. A combination of three methods was used to increase d(ave) to 10, 20, and 100 nm: (1) The deposition bath was heated to 75 °C, (2) nanowires were thermally annealed at 300 °C, and (3) nanowires were exposed to methanolic CdCl(2) followed by thermal annealing at 300 °C. The morphology, chemical composition, grain diameter, and photoconductivity of the resulting nanowires were studied as a function of d(ave). As d(ave) was increased from 10 to 100 nm, the photoconductivity response of the nanowires was modified in two ways: First, the measured photoconductive gain, G, was elevated from G = 0.017 (d(ave) = 5 nm) to ∼4.9 (100 nm), a factor of 290. Second, the photocurrent rise time was increased from 8 μs for d(ave) = 10 nm to 8 s for 100 nm, corresponding to a decrease by a factor of 1 million of the photoconduction bandwidth from 44 kHz to 44 mHz.

Sub-nanomolar Detection of Prostate-Specific Membrane Antigen in Synthetic Urine by Synergistic, Dual-Ligand Phage

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.

Virus-poly(3,4-ethylenedioxythiophene) composite films for impedance-based biosensing.

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 (Z(RE)) and imaginary (Z(IM)) 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 (Z(total)) with p-Ab concentration conforms to a Langmuir adsorption isotherm over the concentration range from from 6 to 66 nM, yielding a value for K(d) = 16.9 nM at 1000 Hz.

A chemically-responsive nanojunction within a silver nanowire.

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 (NH(3)) induced a rapid (<30 s) and reversible resistance change that was as large as ΔR/R(0) = (+)138% in 7% NH(3) and observable down to 500 ppm NH(3). Exposure to water vapor produced a weaker resistance increase of ΔR/R(0,H(2)O) = (+)10-15% (for 2.3% water) while nitrogen dioxide (NO(2)) exposure induced a stronger concentration-normalized resistance decrease of ΔR/R(0,NO(2)) = (-)10-15% (for 500 ppm NO(2)). 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 Ag(x)O is formed within the CRJ and the binding of molecules to this Ag(x)O modulates its electrical resistance.

Virus-poly(3,4-ethylenedioxythiophene) biocomposite films.

Virus-poly(3,4-ethylenedioxythiophene) (virus-PEDOT) biocomposite films are prepared by electropolymerizing 3,4-ethylenedioxythiophene (EDOT) in aqueous electrolytes containing 12 mM LiClO(4) 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.

Wafer-Scale Fabrication of Nanofluidic Arrays and Networks Using Nanoimprint Lithography and Lithographically Patterned Nanowire Electrodeposition Gold Nanowire Masters

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.