Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance

The overall goal of this procedure is to synthesize solution suspendable plasmonic gold nanotubes with tunable infrared absorbances. This is accomplished by first electrode depositing base metals within the pores of a A o membranes, which serve as sacrificial substrates to support the gold nanotubes. The second step is to electro polymerize a hydrophobic polymer core, which serves as the core for the gold nano tube to deposit around.

Next, the gold shell is electrode deposited around the hydrophobic polymer core. The final step is to etch the sacrificial polymer core base metals and membrane, releasing the gold nanotubes into solution gold nanotubes, exhibit tunable plasmonic absorbances in the infrared, which may be applied in a variety of fields including biosensing, photovoltaics or optics. The main advantage of this technique over existing methods like avan replacement reactions and electros plating, is that we're able to synthesize non-porous solutions, suspendable gold nanotubes with strong absorbances in the visible and infrared regions.

Using our procedure, we're able to control the length and both inner and outer diameter of the nanotubes, allowing us to tune the infrared absorbance. The implications of this technique extend toward optical biosensing due to the sensitivity of the plasmonic absorbent to the refracted index surrounding the nano structure. Cold nanotubes can also be applied as substrates for microfluidics perm selective transport, photothermal therapy, and photovoltaic cells.

The synthesis and study of gold nanotubes can provide insight into how hollow nanostructures can increase the refractive index sensitivity of plasmonic biosensors. Visual representation of this method is critical as it is highly multidisciplinary involving customized equipment and a range of techniques which are not adequately described by written instruction. To begin this procedure, secure the anodic aluminum oxide membrane substrate with its topside up on a glass plate using two-sided adhesive.

It is important to minimize the membrane area in contact with the adhesive as it will clog the pores. Next, place the glass plate into the substrate holder of a metal evaporator. Close the chamber and evacuate the chamber to below 1.0 E minus six tor Using a resistive source, evaporate silver pellets onto the substrate at a rate of 0.8 angstroms per second until a layer thickness of 100 nanometers is reached.

Then increase the evaporation rate to 1.5 angstroms per second until a final thickness of 250 nanometers is reached. Once finished, remove the sample from the evaporator. Wet a cotton swab with di chloro methane and use it to dissolve the adhesive in order to release the a a o membrane.

All electro deposition steps occur in a custom two piece open face Teflon electrochemical cell, the cell, as described by Ban Holzer. Etal is designed to hold the a a o membranes in contact with a conductive foil that serves as the working electrode. To begin copper and nickel deposition, clean the Teflon cell by rinsing it for 10 seconds, three times with acetone ethanol.

And finally, 18.2 mega deionized water. Allow the cell to dry in the ambient laboratory air. Next, place the membrane silver side down onto a piece of smooth aluminum foil placed in the Teflon electrochemical cell and seal the working electrode area with a viton O-ring.

Then at 3.0 milliliters of copper plating solution to the Teflon cell. Connect the aluminum foil working electrode, a platinum counter electrode and the aqueous reference electrode to a potential stat using a conventional three electrode setup. Apply a potential of negative 90 millivolts versus the silver, silver chloride redox couple for 15 minutes following copper deposition, the membrane will appear purple.

Once finished, disconnect and remove the reference and auxiliary electrodes while keeping the two piece cell and a a o membrane intact. Then rinse the cell three times for 10 seconds each with 18.2 mega deionized water. Let the cell soak for 30 minutes in five milliliters of 18.2 mega deionized water to remove excess copper plating solution from within the pores.

Next, empty the cell. Then add 3.0 millimeters of commercial nickel plating solution and reconnect the counter reference and working electrodes. Apply a potential of negative 900 millivolts versus the silver, silver chloride redox couple for 20 minutes during nickel deposition.

The template will slowly turn black. Once nickel deposition is complete. Disconnect and remove the reference and auxiliary electrodes keeping the two piece cell and a a o membrane assembly intact.

Then rinse the cell three times 10 seconds each with 18.2 mega deionized water before letting it soak in the water for 30 minutes. To remove excess plating solution from the pores, allow the cell to thoroughly dry in the ambient laboratory air Overnight. Transfer the intact Teflon cell assembly into an inert atmosphere glove box equipped with external connections to a potential stat.

Next, prepare a solution of 30 millimolar three heyl opine in 3.0 milliliters of 46%boron tri fluoride in dyl ether, and add it to the Teflon electrochemical cell. Then connect the counter electrode, working electrode and silver, silver nitrate acetyl nitrile reference electrode to the potentials stat. Apply a potential of plus 1500 millivolts versus the silver, silver nitrate redox.

Couple for 10 minutes. Currents on the order of 0.1 milliamps after 10 minutes indicate a successful deposition. The membrane will appear dark, purple and glossy after electro polymerization.

Once completed, disconnect and remove the reference and auxiliary electrodes keeping the two piece cell and a a o membrane and foil intact. Next, rinse the cell with five milliliters of acetyl nitrile in the glove box. To remove excess boron tri fluoride, remove the cell from the glove box and rinse with five milliliters of ethanol.

Then soak the cell in fresh ethanol for 20 minutes. Rinse the cell again with five milliliters of 18.2 mega deionized water and soak in fresh water for 20 minutes. Allow the cell to dry in ambient laboratory air.

Begin gold shell deposition by adding 3.0 milliliters of commercial gold plating solution to the Teflon cell. Mix the solution gently with a pipette for two minutes to help the gold plating solution infiltrate the pores completely and induce hydrophobic collapse of the polymer core. Then connect the working electrode, counter electrode and aqueous reference electrode to a potential stat and apply negative 920 millivolts versus the silver, silver chloride redox couple.

The length of a gold nano tube is determined by the deposition time. An initial current of around 0.5 milliamps indicates a successful deposition. Following deposition, rinse the cell under a stream of 18.2 mega deionized water and allow it to dry.

Remove the membrane from the Teflon cell assembly and dissolve the silver, copper and nickel with a few drops of concentrated nitric acid on the silver coated side. Then remove the acid and rinse the membranes three times for 10 seconds with 18.2 mega deionized water next, etch the polymer core by immersing the membrane overnight in a three to one volume, volume solution of sulfuric acid and 30%hydrogen peroxide. After this step, the membrane will appear purple and translucent.

The next day, remove the acid solution and rinse the membrane under a stream of 18.2 mega deionized water. Then break the membrane into small pieces and place them into a 3.0 milliliter centrifuge.Vial. Add two milliliters of an aqueous 3.0 molar sodium hydroxide solution to the vial and agitated in a heated mixer operating at 1000 RPM and 40 degrees Celsius for three hours or until the membrane is dissolved.

Once dissolved, centrifuge the mixture for 10 minutes at 21, 000 times gravity. Finally, remove the supernatant liquid and replace it with 18.2 mega deionized water. Repeat this cycle three times.

The vial now contains gold nanotubes that can be suspended by gentle son upon son and suspension. The solution will appear like purple. To measure the optical spectra of the gold nanotubes, centrifuge them in solution for 10 minutes at 21, 000 times gravity.

Then remove the supernatant liquid and replace it with D two O.Repeat this process three times. Next, sonicate the mixture for 30 seconds until the solution becomes clear and transfer the solution into a one milliliter quartz vete. Obtain the extinction spectra from 200 to 2000 nanometer in a spectrophotometer that is operating in dual beam.

Mode two absorbances will be present corresponding to the transverse and longitudinal plasmin modes. Next, measure the solid state spectra by placing the intact membrane on a glass slide and wet it with D two O to increase transparency. Then mount the slide on a thin film sample holder and place it into a UV to visible range capable spectrophotometer operating in dual beam mode.

Obtain an extinction spectrum from 200 nanometers to 1, 300 nanometers using a glass slide as the reference. The measurement of the extinction spectra from 500 to 800 nanometers shown here is reflective of the 55 nanometer diameter of the gold nanotubes that were formed. The length can be varied based on deposition time and three different trials are shown here.

Each representing a different deposition, time scanning and transmission electron microscopy can also be used to measure the physical characteristics of the gold nanotubes. Shown here is a scanning electron microscope image of the cross section of a gold nano tube made using a 55 nanometer PO template transmission. Electron microscopy gives similarly high resolution when measuring physical dimensions such as diameter and length of various gold nanotubes.

In this graph, 100 nanotubes were measured for seven different deposition times. This resulted in a linear correlation of deposition time and length. Following this procedure, the gold nanotubes can be functionalized with analytes such as DNA or other biomolecules, and their utility as biosensors can be investigated by measuring the shift in plasma resonance induced by analyte binding events.

This technique will allow researchers in the field of plasmas and nanotechnology further explore how shape can affect optical properties. Gold nanotubes can also act as refractive index sensors, which can more accurately detect molecular binding events. After watching this video, you should have a good understanding of how to electrode deposit metals and polymers within the pores of anodic aluminum oxide membranes synthesize both composite and single component nanotubes and measure their optical properties.