Gold nanostructures and processes for their preparation

Abstract

An electroless process for depositing gold (Au.sup.0) from a solution, comprising allowing gold (Au.sup.0) place from a solution of gold thiocyanate complex dissolved in a mixture of water-miscible organic solvent and water, or the deposition of gold (Au.sup.0) takes place on a deposition-directing layer comprising positively charged organic groups, said layer being provided on at least a portion of a surface of a substrate sought to be gold-coated.

Claims

1. An electroless process for depositing gold from a solution, comprising providing a solution of a source of of gold thiocyanate complex comprising M.sup.+[Au(SCN).sub.4].sup.1, or M.sub.+[Au(SCN).sub.2].sup.1, wherein M is a metal, and combinations thereof, and subjecting a substrate sought to be gold-plated to said solution to deposit Au.sup.0 from said solution, wherein said deposition is a spontaneous reduction of gold of said complex by thiocyanate carried out in the absence of an auxiliary reducing agent, further wherein 1) said solution is said gold thiocyanate complex dissolved in a mixture of water-miscible organic solvent and water, and said spontaneous reduction occurring upon evaporation of said solution, or 2) at least a portion of said surface comprises a deposition-directing layer comprising positively charged non-metallic groups.

2. A process according to claim 1, wherein said gold thiocyanate complex dissolved in a said mixture of water-miscible organic solvent and water is [Au(SCN).sub.4].sup.1, and wherein said process comprising crystallizing Au.sup.0 wires from said solution.

3. A process according to claim 2, wherein the crystallization is induced by gradually removing the solvent mixture.

4. A process according to claim 3, wherein the gradual solvent removal is achieved by allowing the solvent mixture to evaporate slowly.

5. A process according to claim 2, wherein the Au.sup.0 wires contain Au.sup.3+ compound.

6. A process according to claim 5, further comprising the step of subjecting the wires to a reductive environment, increasing the content of Au0 in the wires.

7. A process of claim 2, wherein the water-miscible organic solvent is aprotic solvent.

8. A process according to claim 7, wherein the solvent is dimethyl sulfoxide.

9. A process according to claim 1, wherein said Au.sup.0 deposition takes place onto said deposition-directing layer upon contacting said gold thiocyanate complex, with said deposition-directing layer.

10. A process according to claim 9, wherein the positively charged non-metallic groups are organic groups.

11. A process according to claim 10, wherein the positively charged organic groups include positively charged amine groups.

12. A process according to claim 9, wherein the substrate is either planar or curved, non-metallic substrate.

13. A process according to claim 9, further comprising a step of enhancing the electrical conductivity of the film.

14. A process according to claim 13, comprising one or more of the following steps: (i) subjecting the film to a reductive environment, thereby increasing the content of Au.sup.0 in the film; (ii) treating the film with a conductive polymer.

15. An electroless process for depositing gold from a solution comprising providing a solution of a source of gold thiocyanate complex selected from the group consisting of M.sup.+[Au(SCN).sub.4].sup.1, M.sup.+[Au(SCN).sub.2].sup.1, wherein M is a metal, and combinations thereof, and and subjecting a substrate sought to be gold-plated to said solution to deposit Au.sup. from said solution, wherein said deposition is a spontaneous reduction of gold of said complex by thiocyanate carried out in the absence of an auxiliary reducing agent, and further wherein said solution is said gold thiocyanate complex dissolved in a mixture of water-miscible organic solvent and water, and said spontaneous reduction occurring upon evaporation of said solution.

16. An electroless process for depositing gold from a solution onto a substrate, comprising providing a solution of of a source of gold thiocyanate complex selected from the group consisting of M.sup.+[Au(SCN).sub.4].sup.1, M.sup.+[Au(SCN).sub.2].sup.1, wherein M is a metal, and combinations thereof, and subjecting a substrate sought to be gold-plated to said solution to deposit Au.sup.0 from said solution, wherein said deposition is a spontaneous reduction of gold of said complex by thiocyanate carried out in the absence of an auxiliary reducing agent upon evaporation of said solution, and further wherein at least a portion of said surface comprises a deposition-directing layer comprising positively charged non-metallic groups upon evaporation of said solution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a SEM image showing the network arrangement of the individual gold wires deposited on a glass substrate, using a solution of gold thiocyanate complex in DMSO/water as the electroless deposition solution.

(2) FIGS. 2a and 2b are SEM images illustrating the morphology of the gold wire (deposited from a solution of gold thiocyanate complex in DMSO/water) before and after plasma reduction, respectively.

(3) FIGS. 3a and 3b are XRD obtained for the gold wires (deposited from a solution of gold thiocyanate complex in DMSO/water) before and after plasma reduction, respectively.

(4) FIG. 4 is the transmittance spectrum of the gold wires (deposited from a solution of gold thiocyanate complex in DMSO/water) after plasma reduction.

(5) FIG. 5 is a current/voltage curve recorded for a conductivity experiment for the gold nanowires (deposited from a solution of gold thiocyanate complex in DMSO/water).

(6) FIGS. 6a and 6b are the XPS spectra depicting the relative abundance of Au species in the film (deposited from a solution of gold thiocyanate complex in DMSO/water) before and after plasma reduction, respectively (Au.sup.0 is indicated by the darker line and the arrow).

(7) FIG. 7 is a SEM image showing individual gold wires deposited on a glass substrate, using a solution of gold thiocyanate complex in DMF/water as the electroless deposition solution.

(8) FIG. 8 is a SEM image showing individual gold wires deposited on a glass substrate, using a solution of gold thiocyanate complex in THF/water as the electroless deposition solution.

(9) FIG. 9 is a SEM image showing individual gold wires deposited on a glass substrate, using a solution of gold thiocyanate complex in ethylene glycol/water as the electroless deposition solution.

(10) FIGS. 10A-10E demonstrates the morphology and dimensional characteristics of a gold film deposited on amino-functionalized substrate. FIG. 10A shows a template mask employed for creating an amine-functionalized surface upon a silicon-oxide substrate. FIGS. 10B-10C are scanning electron microscopy (SEM) images showing the selective growth of gold nanostructures on an amine functionalized substrate. FIG. 10D shows a height measurement trace based on atomic force microscopy (AFM) images along the edge of a region of deposited gold that was scratched off, exposing the amine-functionalized surface. FIG. 10E is an SEM image showing a cross-section of the gold deposit.

(11) FIG. 11 is a graph showing the ratio of Au(I) to Au(0) on the deposited gold based XPS analysis.

(12) FIG. 12A is a high resolution transmission electron microscopy (HRTEM) image of a gold nano-ribbon (black). FIG. 12B is the x-ray diffraction (XRD) spectrum of the gold nano-ribbons grown on silicon oxide for 60 hours.

(13) FIG. 13A is a photograph showing an image on a piece of paper seen through a piece of gold-deposited glass. FIG. 13B is a trace showing the transmittance of light at or near the visible spectrum through gold-deposited glass at a range of wavelengths between 350 nm and BOO nm wavelength. FIG. 13C is an I-V trace showing the current passed through the gold deposit at a range of voltages, between 4V and 4V, in a pH 5.5 environment. FIG. 13D is an I-V trace showing the current passed through the gold deposit at a range of voltages, between 4V and 4V, in a pH 7.7 environment.

(14) FIG. 14A is HRTEM image showing the peptides sheets bound (darker regions) to the substrate. FIGS. 14B-C are HRTEM images showing the gold deposits (black strips) formed on the peptides sheets. FIG. 14D is a SEM image showing the nano-ribbon structure of the deposited gold (white).

(15) FIG. 15A is a photograph of a PDMS substrate without amine functionalization after incubation with Au(SCN).sub.2.sup.1. FIG. 15B, is a photograph of a PDMS substrate with amine functionalization after incubation with Au(SCN).sub.2.sup.1. FIG. 15C is an SEM image showing the gold thin film nanostructure on the amine functionalized PDMS.

(16) FIG. 16 shows Surface morphology of Au-coated PDMS. Scanning electron microscopy (SEM) images of Au-coated planar PDMS surface (A-B) and Au-coated wrinkled PDMS (C-D).

(17) FIG. 17 provides the structural characterization of the Au films grown on PDMS: (A) XPS spectra in the Au(4f) region; (B) Powder XRD pattern.

(18) FIG. 18 presents the results of electrical conductivity in different PDMS surface morphologies. (A) Planar PDMS. Optical microscopy image of the electrode configuration (picture showing three bright square electrodes deposited on the surface) (top), and corresponding I-V curve (bottom); (B) Wrinkled PDMS. Optical microscopy image of the electrode configuration (picture showing three square electrodes) (top), and corresponding I-V curve (bottom); (C) Physical bending of coated PDMS. Picture of the experimental setup, showing the two electrodes in contact with the bent PDMS (the arrow points to the PDMS slab wrapped around a glass tube) (left); the corresponding I-V curve (right). The Ohmic (linear) behavior apparent in all I-V curves indicates electrical conductivity.

EXAMPLES

Methods

(19) Scanning Electron Microscopy (SEM):

(20) (i) For SEM images, gold nano-ribbons were grown on silicon, with thermal oxide layer of 100 nm, the wafer being modified with a 3-aminopropyltriethoxy silane self-assembled monolayer. SEM images were recorded using JEOL JSM-7400F Scanning Electron Microscope (JEOL LTD, Tokyo, Japan). (ii) For SEM images, 20 L of a 24 h incubated solution of KAu(SCN).sub.4 (24 mg mL.sup.1) was drop cast on a silicon piece (2.5*1.0 cm.sup.2) and the solvent was left to evaporate at room temperature. SEM images were recorded on a JEOL JSM-7400F Scanning Electron Microscope (JEOL LTD, Tokyo, Japan) at an acceleration voltage of 3 kV.

(21) High Resolution Transmission Electron Microscopy (HRTEM):

(22) samples were prepared as follows. Dodecylamine films, compressed to surface pressure of 25 mN/m, on a Langmuir trough at 20 C. were transferred horizontally onto 400 mesh copper formvar/carbon grids (Electron Microscope Sciences, Hatfield, Pa., USA). The grids were allowed to float on solution of Au(SCN).sub.2.sup.1 for 1 h after which the sample left to dry and were plasma cleaned prior to analysis. HRTEM images were recorded on a 200 kV JEOL JEM-2100F. SEM analysis of grid left for 24 hours in the same solution was done to confirm the formation of nanoribbons.

(23) Powder X-Ray Diffraction (XRD):

(24) XRD patterns were obtained using Panalytical Empyrean Powder Diffractometer equipped with a parabolic mirror on incident beam providing quasi-monochromatic Cu K radiation (=1.54059 ) and X'Celerator linear detector. Data were collected in the grazing geometry with constant incident beam angle equal to 1 in a 2 range of 10-80 with a step equal to 0.05.

(25) X-Ray Photoelectron Spectroscopy (XPS):

(26) XPS analysis was carried out using Thermo Fisher ESCALAB 250 instrument with a basic pressure of 2.Math.10.sup.9 mbar. The samples were irradiated in 2 different areas using monochromatic Al K, 1486.6 eV X-rays, using a beam size of 500 m. The high energy resolution measurements were performed with pass energy of 20 eV. The core level binding energies of the Au4f peaks were normalized by setting the binding energy for the C1s at 284.8 eV.

(27) Infrared Measurements:

(28) IR measurements were done in the following way: a solution of Au(SCN).sub.4.sup.1 was placed to incubate in 25 C. for 72 h to get oxidation of thiocyanate. After 72 h the solution was separated from the precipitation (solid gold) by filtration and solid Ba(NO.sub.3).sub.2 was added in excess to the solution for the formation of BaSO.sub.4. The solution was centrifuge and the precipitation was placed on a silicon wafer and left to dry in room temperature prior to analysis. Control samples were prepared by adding Ba(NO.sub.3).sub.2 to 2 M H.sub.2SO.sub.4 solution and 2 M KSCN solution. The solution with KSCN shows no precipitation. The H.sub.2SO.sub.4 with add Ba(NO.sub.3).sub.2 was centrifuge and the precipitation was placed on a silicon wafer and left to dry prior to analysis. The data was recorded by FTIR microscopy, Nicolet iN10.

(29) Atomic Force Microscopy (AFM):

(30) AFM measurements were performed at ambient conditions using a Digital Instrument Dimension3100 mounted on an active anti-vibration table. A scratch on the deposited gold was made and the height difference on the edge of the scratch was measured. A second scratch perpendicular to the first was done in order to check that the scratch removed only the gold thin film and did not harm the surface of the substrate.

(31) UV-vis spectra (i.e. Plasmon transmittance) were recorded using a JASCO V-550 UV-vis spectrophotometer.

(32) Conductivity measurements were conducted as follows: a 10 nm layer of chromium follow by a 90 nm of gold was deposited on glass surface with gold thin film, using thermal evaporation, in order to create electrodes. The evaporation was done selectively using a mask with desirable gaps (100 m). Room temperature electrical measurements were carried out in a two-probe configuration using a probe-station equipped with a Keithley 4200SCS semiconductor parameter analyzer.

Example 1

Preparation and Characterization of Gold Nanowires

(33) 1 mL of HAuCl.sub.4.3H.sub.2O dissolved in water (24 mg mL.sup.1) was added to mL aqueous solution of KSCN (60 mg mL.sup.1). The precipitate formed was separated by centrifugation at 4000 g for 10 min in order to separate the complex from the solution which contains KCl and excess of KSCN. The precipitate was dried and dissolved in 2 mL mixture of DMSO and water (4:1 v:v). The solution was left to incubate for 24 h after which 100 L, of solution was drop cast on a 1.0 cm*2.5 cm, ozone exposed glass slide, and left to evaporate at room temperature.

(34) The glass was inserted to a plasma cleaner, PDC-32G, Harrick Plasma, and the vacuum pump was turned on and work for 90 s. After 90 s the sample was exposed to plasma, at high RF (18 W), for 3 min, effectively reducing Au.sup.3+ to Au.sup.0.

(35) SEM image shown in FIG. 1 demonstrates a network structure consisting of individual long wires. FIG. 2a is the SEM image of a single wire (before plasma treatment), showing that the wire's surface is highly smooth. FIG. 2b is the SEM image of a single wire following plasma reduction, showing that the surface of the wire becomes coarse.

(36) The X-ray powder diffraction patterns of the wires, before and after the reduction step, are presented in FIGS. 3a and 3b, respectively. The as-prepared wires exhibit X-ray powder diffraction pattern having peaks at 2 positions corresponding to d-spacings of 8.34 , 6.11 and 2.90 , assigned to KAu(SCN).sub.4, and minor peaks at 2 positions of 38 & 44 assigned to Au.sup.0. A comparison with FIG. 3b illustrates the efficacy of the plasma reduction: the diffraction peaks assigned to the KAu(SCN).sub.4 crystalline species are significantly diminished in intensity following, plasma reduction, with the XRD peaks assigned to crystalline Au.sup.0 becoming the prominent peaks. The ratio Au/Au.sup.3+ is quantifiable through XPS and was found to be 77:23. FIGS. 6a and 6b are the XPS spectra depicting the relative abundance of Au species in the film before and after plasma reduction, respectively (Au.sup.0 is indicated by the darker line to which the arrow points).

(37) The essentially metallic, glass-supported film consisting of gold nanowires was also tested to determine its optical and electrical properties.

(38) Optical transmittance: UV-Vis transmittance measurements in the range of 300-900 nm were conducted on a Carla 5000, Varian Analytical Instruments, Melbourne. FIG. 4 shows the transmittance spectrum, indicating that approximately 80% of the incident light was retained after passing through the nanowire film, demonstrating its excellent transparency.

(39) Electrical conductivity: Cr and Au electrodes were thermally evaporated on glass substrate onto which the Au film was deposited. Each electrode consisted of 10 nm thick Cr layer, and on top of it 90 nm thick Au layer. The length and width of each Cr/Au electrode were 100 m100 m. In one experiment, the electrodes were spaced 100 m apart and in another experiment, the electrodes were spaced 1 mm apart, with the gold film being deposited in the spacing between the electrodes. Room temperature conductivity measurements were carried out in a two-probe configuration using a probe-station equipped with a Keithley 2400 SMU, and the current passing through the wires across the electrodes was measured. Data is presented in the form of current/voltage curves shown in FIG. 5, indicating that the network of gold nanowires exhibits good electrical conductivity.

Example 2

Preparation of Gold Nanowires

(40) 14 mg KAu(SCN).sub.4 was dissolved in 2 mL of DMF and water (4:1 v:v). The solution was left to incubate for 24 h after which 20 L of solution was drop cast on a 1.0 cm*2.5 cm, ozone exposed glass slide, and left to evaporate at room temperature. SEM images were recorded on a JEOL JSM-7400F Scanning Electron Microscope (JEOL LTD, Tokyo, Japan) at an acceleration voltage of 3 kV. The SEM image shown in FIG. 7 illustrates the formation of gold nanowires.

Example 3

Self-Assembly of Gold Nanowires

(41) 14 mg KAu(SCN)4 was dissolved in 2 mL of THF and water (4:1 v:v). The solution was left to incubate for 24 h after which 20 L of solution was drop cast on a 1.0 cm*2.5 cm, ozone exposed glass slide, and left to evaporate at room temperature. SEM images were recorded on a JEOL JSM-7400F Scanning Electron Microscope (JEOL LTD, Tokyo, Japan) at an acceleration voltage of 3 kV. The SEM image shown in FIG. 8 illustrates the formation of gold nanowires.

Example 4

Self-Assembly of Gold Nano-Wires

(42) 14 mg KAu(SCN)4 was dissolved in 2 mL of ethylene glycol and water (4:1 v:v). The solution was left to incubate for 24 h after which 20 L of solution was drop cast on a 1.0 cm*2.5 cm, ozone exposed glass slide, and left to evaporate at room temperature. SEM images were recorded on a JEOL JSM-7400F Scanning Electron Microscope (JEOL LTD, Tokyo, Japan) at an acceleration voltage of 3 kV. The SEM image shown in FIG. 9 illustrates the formation of gold nanowires.

Example 5

Deposition of a Gold Film on Amino-Functionalized Substrate and Characterization of the Film

(43) Glass or silicon wafers with an amine terminal group deposition-directing layer were prepared as follows: The substrates were in a 70 C. piranha solution, 70% concentrated sulfuric acid and 30% hydrogen peroxide, for 30 min and another 30 min under sonication. The substrates were then rinsed with double distilled water and dried with compressed air stream. The dried substrates were immersed in a 1% (volume) of 3-aminopropyltriethoxy silane in heptane solution for 1 h which after the substrates were rinsed in cyclohexane and were left to dry prior to use. Silicon substrates were put in ozone oven for 30 min prior to the immersion in the amino silane solution. Patterned substrates were prepared by placing a mask on the substrate and exposing it to plasma for 1 min.

(44) Au(SCN).sub.4.sup.1 complex was prepared as follows: 1 mL of HAuCl.sub.4.3H.sub.2O in water (24 mg/mL) was added to a 1 mL solution of KSCN in water (60 mg/mL). The precipitation formed was separated by centrifuge (4000 g) for 10 min. X-ray photoelectron spectroscopy (XPS) analysis was done to confirm the existence of the complex.

(45) Thin gold films were prepared as follows: The Au(SCN).sub.4.sup.1 (gold complex was transferred to 40 mL of water and sonicated in a sonication bath for 30 minutes. At this stage, the Au(SCN).sub.2.sup.1 complex is spontaneously formed. The concentration of the Au(SCN).sub.2.sup.1 complex was 1.5 mM. The substrate was inserted to the solution for 60 hours at 4 C. The substrate was oriented perpendicular to the ground, in order to prevent the fall of pre-formed aggregates on the substrate surface due to gravity. After 60 hours, the samples were rinsed with water and left to dry at room temperature.

(46) The so formed gold film deposited on the amino-functionalized substrate was investigated and characterized as follows.

(47) The morphology of the surface-deposited pattern was examined by scanning electron microscopy (SEM). FIG. 10A depicts an example of a template mask employed for creating an amine-functionalized surface upon a clean silicon-oxide substrate.

(48) The SEM image of the resultant gold thin film in FIG. 10B (scale bar=100 microns) demonstrates that gold deposition (light) occurred exclusively within the surface areas in which NH.sub.2 was displayed, and the surfaces not displaying NH.sub.2 was essentially free of gold deposition (dark). Closer examination of the surface, as shown in FIG. 10C (scale bar=200 nanometers), reveals that the gold deposit has a complex structure. The gold is assembled into an aggregation of nano-ribbons that protrude from the substrate surface and interlace with other nearby nano-ribbons, creating a layer of gold nano-ribbon mesh. The nano-ribbons appear to be approximately 25 nm thick. The mesh is dense, such that the length of individual nano-ribbons cannot be determined. However, the mesh is loose enough such that small gaps are present in the mesh. Further, the presence of the deposited gold was specific to the amine-functionalized portion of the silicon-oxide surface, leaving a clear demarcation, even at the nanometer scale, between the portion of the surface with deposited gold and the portion without. The gold nano-ribbon assembly was not removed through prolonged washing and sonication, attesting to high stability and strong attachment to the surface.

(49) The thickness of the deposited gold was determined by AFM (FIG. 10D) and SEM (FIG. 10E). As shown in FIG. 10D, AFM height measurements were made near a scratch made on gold deposited on amine-functionalized glass. The distance between the glass substrate surface (measured at points 1 and 2) and the top of the gold deposit (measured at points 3 and 4), i.e., the thickness of the gold deposit, was determined to be 152.19 nm. A second scratch perpendicular to the first was done in order to check that the scratch removed only the gold structure. Separately, as shown in FIG. 10E, an SEM cross section image of a gold thin film created on amine-functionalized silicon similarly showed that the thickness of the gold deposit was uniform and approximately 150 nm.

(50) To evaluate the gold species deposited upon the amine-displaying surface, we carried out x-ray photoelectron spectroscopy (XPS) experiments at different incubation times (FIG. 11). FIG. 11 shows that the XPS spectra in all time-points comprise superimposed peaks from Au(0) and Au(I). The XPS analysis demonstrates that most of the gold within the deposited nano-ribbon film is metallic, and the ratio between Au(0) and Au(I) remains almost constant, at 3:1, respectively, throughout the entire deposition process, as shown by the results set out in Table 1.

(51) TABLE-US-00001 TABLE 1 Au species over time (based on data of FIG. 11) Time (h) Au(I) Au(0) 1 0.27 0.73 2 0.19 0.81 4 0.24 0.76 60 0.24 0.76

(52) This result indicates that spontaneous reduction of the gold thiocyanate complex occurs rapidly following binding and crystallization at the amine-functionalized surface. In order to further confirm that a reduction/oxidation reaction had taken place during incubation, we analyzed the used gold thiocyanate solution following incubation for oxidation residue by treating the used solution with Ba(NO.sub.3).sub.2 and assaying for the formation of BaSO.sub.4. We found that the used gold thiocyanate solution had significantly higher levels of oxidation residue compared to controls, which, as expected, contained no oxidation residues (data not shown).

(53) As the XPS data point to rapid reduction of Au(I) to Au(0), one needs to determine whether the nano-ribbon gold structures (visualized in FIG. 10) and rapid reduction indeed occurred after adsorption of the gold complex to the surface. Several lines of evidence attest to this scenario. First, while some Au(0) colloids do form spontaneously in aqueous solution during the initial preparation of the Au(SCN).sub.4.sup.1 complex, such aggregates are generally structurally amorphous and lack the nano-ribbon structures (data not shown). Furthermore, while few Au(0) aggregates (pre-formed through spontaneous reduction pathways in the buffer solution) did bind to immersed surfaces, they were easily removed upon rinsing, in contradistinction to the gold nano-ribbons that were strongly bound to the substrate.

(54) To analyze the molecular structures and crystallinity of the gold nanostructures we applied high resolution transmission electron microscopy (HRTEM, FIG. 12A), and X-ray diffraction (XRD, FIG. 12B). The HRTEM image of FIG. 12A (scale bar=10 nanometers) depicts the growth of a single gold nano-ribbon. Plasma cleaning was used prior to analysis. Crystal organization of both metallic gold and the Au(SCN).sub.2.sup. complex are clearly apparent in the XRD pattern (FIG. 12B). The existence of both metallic gold, e.g., at (111) and (200), and gold organic hybrid structure, e.g., at d=2.53, 3.00 and 5.12 , are shown. The distances recorded in the XRD spectrum indicate that aurophilic interactions are pre-dominant in promoting gold crystallization upon the NH.sub.2-functionalized surface.

(55) FIG. 13A shows the optical transparency of the gold thin film that is prepared according to the above methods. Gold thin film was deposited upon the entire surface of an amine-functionalized glass panel of approximately 1 cm in width, according to the methods described above. The glass panel was placed on a piece of paper having an image of a university logo printed on it. Even with the gold deposited upon it, the glass panel is highly transparent, such that the logo is clearly visible. FIG. 13B is a graph showing the relationship between the level of light transmittance through the deposited gold and the wavelength of the light. Except for the sharp dip in transmittance for short wavelength like (less than approximately 450 nm), transmittance of light in the visible spectrum ranged from about 55% to about 80% at pH 7.7 (in 10 mM phosphate buffer). Transmittance of light was even better at 5.5 pH, ranging from about 80% to about 90%. At pH 7.7, as at pH 5.5, there was an overall trend of transmittance being worse for lower wavelength light, and transmittance sharply fell for light of wavelengths less than approximately 450 nm.

(56) FIGS. 13C-D shows the current-voltage relationship of a current being passed across the deposited gold film, showing that the gold is conductive. The higher conductivity at pH 7.7 compared to pH 5.5 is due to the release of H+ ion in the gold reduction reaction induced by the application of current. For example, at pH5.5, the application of a 4 V electrical potential resulted in a current of +1.009891E-7 A. At pH 7.7, an application of a 4 V potential resulted in a current of 5.367393E-7 A

(57) Both physical properties are related to the configuration of the gold structures. Specifically, the protruding orientation of the nano-ribbons and resultant large empty surface areas enables optical transparency. Similarly, conductivity depends upon the interface/contact between the individual gold nanostructures.

Example 6

Deposition of a Gold Film on Amino-Functionalized Substrate

(58) Transmission electron microscopy (TEM) grids (400 mesh copper formvar/carbon grids; Electron Microscope Sciences, Hatfield, Pa., USA) with amine-rich peptide deposition-directing layer were prepared as follows: A solution of proline-(lysine-phenylalanine).sub.5-lysine-proline (PKFKFKFKFKFKP) peptide in methanol/chloroform (1:9 v/v) was prepared at a concentration of approximately 0.1 mg/mL. An appropriate amount of the peptide solution was spread over a KCl (1 M) subphase in a Langmuir trough (KSV minitrough). Following evaporation of the methanol/chloroform solvent, the barriers of the trough were compressed at a rate of 4 mm/min. The surface pressure-area isotherm was recorded and was stopped at the required surface pressure. A monolayer of the peptides was transferred to the TEM grids at the desired surface pressure using the Langmuir-Schaefer method.

(59) Gold growth over the peptide-treated TEM grids: For gold crystallization over the peptides, the TEM grids were kept floating over an aqueous solution of K[Au(SCN).sub.2] (pH5.5). After the desired duration of incubation in the gold complex solution, the grids were taken out and floated over deionized water to remove the unbound moieties and unreacted reagents. Samples were analyzed after drying.

(60) FIG. 14A (scale bar=500 nanometers) is a TEM image showing the peptides sheets bound (darker regions) to the substrate. FIG. 14B (scale bar=100 nanometers) shows a TEM image showing the gold deposits (black strips) formed on the peptides sheet after being incubated for 2 days with 1.5 mM Au(SCN).sub.2.sup.1 in aqueous solution. FIG. 14C (scale bar=10 nanometers) is a higher magnification TEM image showing a boundary between a section with gold deposition (black) and a section without gold deposition (white). FIG. 14D (scale bar=500 nanometers) is a SEM image showing the nano-ribbon structure of the deposited gold (white). Note that the deposited gold appears black in the TEM images (FIGS. 14B-C) and as white in the SEM images (FIG. 14D).

Example 7

Deposition of a Gold Film on Amino-Functionalized Substrate

(61) The above method of spontaneous gold thin film deposition may be conducted on a variety of substrates. FIG. 15A-C shows the successful deposition of gold thin film on amine functionalized polydimethylsiloxane (PDMS) using essentially the same methods. FIG. 15A is a photograph of a PDMS substrate (of about 1 cm in width) without amine functionalization after incubation with Au(SCN).sub.2.sup.1 in aqueous solution, showing that gold deposition did not take place. By contrast, as shown in FIG. 15B, incubating an amine (NH.sub.2) functionalized PDMS substrate (of about 1 cm in width) with Au(SCN).sub.2.sup.1 in aqueous solution resulted in gold deposition, as evidenced by the presence of a brownish red coating. FIG. 15C shows an SEM image of the gold nanostructure film on the amine functionalized PDMS.

Example 8

Deposition of Gold Films on Amino-Functionalized Planar and Non-Planar Substrates and Characterization of the Films

(62) Planar PDMS samples were prepared as per the instructions provided by the supplier (Sylgard 184 kit, including monomer and curing agent, was purchased from Dow Corning). The monomer and curing agent were mixed in a ratio 10:1 and cured at 70 C. for 2 hours on a hydrophobic surface. After curing, samples were peeled off from the supporting surface.

(63) Wrinkled PDMS was made using a reported procedure [Lee et al., Adv. Mater 25, p. 2162 (2013)]. Briefly, PDMS films were initially prepared by mixing the elastomer and curing agent in a ratio of 20:1. These PDMS films were then mechanically pulled with uniaxial strain in a custom-made device and kept in an UVO oven for 40 minutes. Wrinkles were produced on the PDMS surface after releasing of the strain.

(64) Amine modification of the PDMS surfaces was carried out as follows. The PDMS surfaces were first treated in plasma for 3 min and subsequently immersed in a solution containing ethanol, water and 3-aminopropyl triethoxy silane (APTES) in a ratio of 200:20:1 (v/v/v) for 2 hours. Following this treatment, the substrates were washed consecutively with ethanol and water and then dried in flow of compressed air.

(65) KAu(SCN).sub.4 complex was prepared as described in the foregoing examples. 1 mL aqueous solution of HAuCl.sub.4.3H.sub.2O (24 mg.Math.mL.sup.1) was added to 1 mL solution of KSCN in water (60 mg.Math.mL.sup.1). The precipitate formed was separated by centrifugation at 4000 g for 10 min. The supernatant was decanted and the residue was dried in room temperature.

(66) Growth of Au films on PDMS substrates was accomplished as follows (the same procedure was used for gold film formation upon both the planar and wrinkled PDMS surfaces. Aqueous solutions of Au(SCN).sub.4.sup.1 (0.7 mg.Math.mL.sup.1) was prepared in slightly acidic water (pH5.5) and the amine-modified PDMS substrates were vertically immersed in the solution and kept at 4 C. for 3 days. After the gold growth was completed, the substrates were taken out of the growth solution and washed thoroughly with water for removing unreacted materials, and subsequently dried in room temperature.

(67) Au/PDMS samples were treated in plasma for 40 to ensure complete reduction of the gold layer. 50 L of a 1:2 v/v dispersion of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) [PEDOT:PSS] in isopropanol was then dropped over the substrate and spin-coated for 1 minute at 1000 rpm.

(68) The so formed gold films were investigated and characterized as follows.

(69) The morphology of the surface-deposited pattern was examined by scanning electron microscopy (SEM). The SEM images in FIGS. 16A and 16C underscore the uniform gold coverage of both the planar PDMS surface (FIG. 16A) or the wrinkled surface (FIG. 16C). Closer examination of the surface reveals a dense nano-ribbon morphology of the gold films (FIGS. 16B,16D), similar to films produced upon incubation of Au(SCN).sub.4.sup.1 with amine-modified glass surfaces. Atomic force microscopy (AFM) analysis implied a thickness of approximately 300 nm of the gold film deposited upon the PDMS.

(70) Chemical species and crystalline properties of the Au films grown at the PDMS surface was carried out through application of X-ray photoelectron spectroscopy (XPS) and powder x-ray diffraction (XRD) (FIG. 17A, 17B). The XPS spectrum in FIG. 17A shows two peaks corresponding to binding energies of 88.2 eV and 84.3 eV, respectively, ascribed to the 4f.sub.5/2 and 4f.sub.7/2 peaks of Au(0). This result confirms that the Au film predominantly comprises of Au(0). The XRD pattern in FIG. 17B highlights the crystallinity of the metallic Au(0) film, showing signals ascribed to Au (111), Au (200), Au (220) and Au (311) crystal planes, respectively. Additional peaks at 5.12 , 3 and 2.6 are assigned to Au(SCN).sub.2.sup.1 crystallites formed through aurophilic interactions. XPS and XRD analyses performed on Au films grown over the wrinkled PDMS surface gave similar results. Together, the XPS and XRD data in FIG. 17 demonstrate that the self-assembled films grown at the amine-derivatized PDMS surfaces mostly comprise of metallic, crystalline gold.

(71) FIG. 18 presents the conductivity profiles of planar and non-planar PDMS surfaces. The linear current-voltage (I-V) curves recorded for the different surface morphologies in FIG. 18 underscore the significant electrical conductivity attained by the film fabrication according to the invention both for the planar and non-planar surfaces. It should be noted that PEDOT:PSS spin coating was carried out following gold deposition in order to enhance electron transport within the Au films. Addition of PEDOT:PSS gave rise to higher conductivity likely by filling the grooves on the Au/PDMS surface (which are apparent in the SEM images in FIG. 10C), as well as through nano-soldering of the interspersed Au nano-ribbons, overall eliminating possible gaps in electron transport. FIG. 18A (top) presents an optical microscopy image of the experimental setup for measuring conductivity in the planar Au/PEDOT:PSS/PDMS surface configuration, showing the square-shaped gold electrode pads deposited on the coated PDMS surface. The linear I-V curve recorded between adjacent electrodes corresponding to spacing of approximately 50 m (FIG. 18A, bottom graph) indicates Ohmic behavior and reasonably good sheet resistance of 610.sup.3 .Math.sq.sup.1. A remarkable conductivity profile was apparent for the wrinkled PDMS surface, FIG. 18B. The optical image in FIG. 18B, top, demonstrates that conductivity was measured over several ridges between adjacent electrode pads. Indeed, the I-V curve in FIG. 18B (bottom) demonstrates that electrical conductivity was retained even in this non-planar surface morphology. The wrinkled Au/PDMS sheet resistance of 1410.sup.3 .Math.sq.sup.1 is the same order of magnitude as the value obtained for the planar Au/PDMS surface (FIG. 18A), recorded in higher electrode spacingsunderscoring the capability of the new approach to achieve effective coating of three-dimensional objects with a conductive layer. Notably, the planar PDMS sample was conductive up to 500 m electrode spacings, while the wrinkled surfaces exhibited conductivity in up to 1 mm of electrode separation (not shown).

(72) To further test the feasibility of the process of the invention for achieving conductivity in flexible, bent surface configurations, we examined the effect of mechanical modification of surface curvature (FIG. 18C). As shown in the photograph in FIG. 18C (left), the flat Au-coated PDMS slab (complemented with PEDOT:PSS surface treatment) was bent around a low-diameter glass tube and the conductivity was measured between two electrodes placed upon the bent PDMS surface. The I-V curve in FIG. 18C (right) clearly demonstrates that even in the bent configuration (around 2.2 cm.sup.1 curvature) the coated PDMS retained its conductivity. Indeed, the sheet resistance measured810.sup.3 .Math.sq.sup.1was comparable to the value recorded in the initial, planar configuration. It should be emphasized that conductivity in all cases was directly related to the deposition of the Au film upon the PDMS surface. Specifically, control experiments demonstrated that PDMS or amine-modified PDMS that were not incubated with the Au thiocyanate complex were not conductive even after treatment with PEDOT:PSS.