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Method Article

Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance

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DOI:

10.3791/50420

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April 1st, 2013

 ,  ,  , 
1Department of Chemistry, University of Toronto

In This Article

Summary

Solution-suspendable gold nanotubes with controlled dimensions can be synthesized by electrochemical deposition in porous anodic aluminum oxide (AAO) membranes using a hydrophobic polymer core. Gold nanotubes and nanotube arrays hold promise for applications in plasmonic biosensing, surface-enhanced Raman spectroscopy, photo-thermal heating, ionic and molecular transport, microfluidics, catalysis and electrochemical sensing.

Abstract

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) membranes have been under development since the 1990's and have become a common method to template the synthesis of high aspect ratio nanostructures, mostly by electrochemical growth or pore-wetting. Recently, these membranes have become commercially available in a wide range of pore sizes and densities, leading to an extensive library of functional nanostructures being synthesized from AAO membranes. These include composite nanorods, nanowires and nanotubes made of metals, inorganic materials or polymers 3-10. Nanoporous membranes have been used to synthesize nanoparticle and nanotube arrays that perform well as refractive index sensors, plasmonic biosensors, or surface enhanced Raman spectroscopy (SERS) substrates 11-16, as well as a wide range of other fields such as photo-thermal heating 17, permselective transport 18, 19, catalysis 20, microfluidics 21, and electrochemical sensing 22, 23. Here, we report a novel procedure to prepare gold nanotubes in AAO membranes. Hollow nanostructures have potential application in plasmonic and SERS sensing, and we anticipate these gold nanotubes will allow for high sensitivity and strong plasmon signals, arising from decreased material dampening 15.

Introduction

When their dimensions approach the penetration depth of light (~50 nm; the nanoscale), noble metals, and most importantly gold, exhibit exquisite size, shape and environment dependent optical properties 24, 25. On this scale, direct illumination causes a coherent oscillation of conduction electrons known as the surface plasmon resonance (SPR). SPR is highly dependent on nanostructure size, shape, and the dielectric properties of the surrounding medium. There is great interest in characterizing SPR properties in new materials, as SPR-based devices are emerging for use in sub-wavelength optics, SERS substrates, and ultra-sensitive optical sensors 11-16, 26-29. As such, developing computational methods to more accurately predict how size and structure can vary plasmonic response remains a major goal. The use of AAO membranes affords a convenient way to vary the particle diameter or length, and several important studies use this to correlate measured and calculated plasmonic response with varying particle diameter, length, and aspect ratio 30, 31. Perhaps the most studied and successful use of plasmonic materials is as refractive index based biosensors. For this, resonances in the red to near infrared (NIR) range (~800 - 1,300 nm) are desirable since they are more sensitive to refractive index change, and lie in the "water window" such that they are transmitted through both water and human tissues. Solution-suspendable nanostructures with SPR peaks in this range open intriguing possibilities for in vivo plasmonic biosensing.

Porous AAO has been used to prepare polymer nanotubes or nanowires by electrochemical synthesis or template wetting, and proven to be applicable to a wide variety of materials. AAO membranes are now being used to synthesize solution-suspendable high aspect ratio nanorods and nanostructured arrays that function as high performance plasmonic biosensors or SERS substrates. While AAO membranes have mostly been used as templates for synthesizing solid rods, in some cases it may be desirable for the structure to be hollow. Plasmonic and SERS sensing applications, for example, are surface based, and hollow structures with large surface-area-to-volume ratios may lead to stronger signal generation and higher sensitivity 14, 15, 32. With respect to this, gold nanotubes have been synthesized from various methods including galvanic replacement reactions on silver nanorods 33, electroless plating 34, 35, surface modification of the template pores 36, 37, sol-gel methods 38, and electrodeposition 39-41. These syntheses typically leave poorly formed, porous nanotubes or allow for little control over the size and morphology. Syntheses have also been reported wherein a metallic shell is deposited over a polymer core in an AAO membrane 42, 43. These synthesis leave the gold nanotubes bound to the substrate and rely on template etching to allow for growth of gold around the polymer, thus they cannot be studied in solution. Moreover, template etching has some potential drawbacks. First, non-uniform pore etching along the template wall may lead to a non-uniform gold shell thickness. Second, significant etching (i.e. to make very thick wall tubes) may dissolve pore walls completely.

Very recently, Bridges et al. reported an etchant free method to synthesize gold nanotubes in AAO membranes that uses a sacrificial poly(3-hexyl)thiophene core and yields solution-suspendable gold nanotubes with extremely high refractive index sensitivity 15. From that and subsequent work, it was discovered that in order to deposit gold shells around the polymer core without chemical etching, the polymer must be tubular such that there is interior space for it to collapse, and the polymer must be hydrophobic such that it will collapse onto itself rather than adhere to the template pore walls 16. When hydrophilic polymers are used, a gold "sheath" partially covering the polymer core is observed, indicating the polymer core adheres to one of the walls of the template during gold deposition 44. Herein, the detailed protocol for the synthesis of hollow gold nanotubes that allows for control over length and diameter is described (Figure 1). These solution-suspendable gold nanotubes are promising materials for a wide range of applications including plasmonic biosensing or SERS substrates.

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Protocol

1. Forming the Silver Working Electrode

  1. Secure the AAO membrane substrate top side up on a glass plate using a 2-sided adhesive. Note: minimize the membrane area in contact with the adhesive, as it will clog the pores.
  2. Install the glass plate into the substrate holder of the metal evaporator, close the chamber, and evacuate to a pressure of below 1.0 μTorr.
  3. Using a resistive source, evaporate silver pellets (>99.99% purity) onto the substrate at a rate of 0.8 Å/sec until a layer thickness of 100 nm is reached, then increase the evaporation rate to 1.5 Å/sec until a final thickness of 250 nm is reached.
  4. Release the AAO membranes by wiping the adhesive layer with a cotton swab wetted with dichloromethane to dissolve the adhesive.

2. Electrodepositing Copper and Nickel

  1. Steps 2-3 use a custom two-piece open-face Teflon electrochemical cell designed to hold the AAO membranes in contact with a conductive foil that serves as the working electrode (Figure 2). The details of the cell design can be found elsewhere 45. Clean a Teflon cell by rinsing 3 x 10 sec with acetone, ethanol, then 18.2 MΩ deionized water. Allow the cell to dry in the ambient laboratory air.
  2. Place the membrane silver-side down onto a piece of smooth aluminum foil in the Teflon electrochemical cell, sealing the working electrode area with a Viton O-ring (Figure 2).
  3. Add 3.0 ml of copper plating solution (0.95 M CuSO4(5H2O), 0.21 M H2SO4) to the Teflon cell. Connect a platinum counter electrode, aqueous reference electrode and the aluminum foil working electrode to a potentiostat using a conventional 3 electrode set-up. Apply a potential of -90 mV vs. Ag/AgCl for 15 min.
  4. Disconnect and remove the reference and auxiliary electrodes, keeping the two piece cell and AAO membrane intact with the foil, then rinse the cell under running 18.2 MΩ deionized water. Let the cell soak for 30 min in 5 ml 18.2 MΩ deionized water to remove excess copper plating solution from within the pores.
  5. Empty the cell, and add 3.0 ml of commercial nickel plating solution (Watt's Nickel Pure from Technic inc.) and connect the counter reference, and working electrodes as described in step 2.3. Apply a potential of -900 mV vs. Ag/AgCl for 20 min.
  6. Disconnect and remove the reference and auxiliary electrodes keeping the two-piece cell and AAO membrane and foil intact. Rinse the cell 3 x 10 sec with 18.2 MΩ deionized water, then let it soak for 30 min in 5 ml 18.2 MΩ deionized water to remove excess plating solution from the pores. Allow the cell to thoroughly dry in the ambient laboratory air overnight.

3. Electropolymerizing the Polymer Core

  1. Bring the intact Teflon cell assembly into an inert atmosphere glove box equipped with external connections to a potentiostat.
  2. Prepare a solution of 30 mM 3-hexylthiophene in 3.0 ml of 46% boron trifluoride in diethyl ether and add it to the Teflon electrochemical cell.
  3. Connect the counter and working electrodes to a potentiostat as described in step 2.3. Add a Ag/AgNO3 acetonitrile reference electrode and connect as described in step 2.3. Apply a potential of +1,500 mV vs. Ag/AgNO3 for 10 min. Currents on the order of 0.1 mA indicate a successful deposition (Figure 3).
  4. Disconnect and remove the reference and auxiliary electrodes keeping the two-piece cell and AAO membrane and foil intact and rinse the cell with 5 ml of acetonitrile in the glove box to remove excess boron trifluoride. Remove the cell from the glove box and rinse with a 5 ml aliquot of ethanol, and then allow the cell to soak in fresh ethanol for 20 min. Rinse the cell again with 5 ml milli-Q 18.2 MΩ deionized water and then allow the cell to soak in 18.2 MΩ deionized water for 20 min. Allow it to dry in the ambient laboratory air.

4. Electrodepositing the Gold Shell

  1. Add 3.0 ml of commercial gold plating solution (Orotemp 24 RTU from Technic Inc.) to the Teflon cell, mix with a pipette for 2 min to allow the gold plating solution infiltrate the pores completely and induce hydrophobic collapse of the polymer core.
  2. Connect the working electrode, counter electrode, and aqueous reference electrode to a potentiostat as in step 2.3, and apply -920 mV vs. Ag/AgCl for varying times (5 min to 5 hr). Currents on the order of 0.5 mA indicate a successful deposition (Figure 3). The length of the gold nanotube is determined by the deposition time (Figure 4).
  3. Rinse the cell under a stream of 18.2 MΩ deionized water and allow it to dry.

5. Removing Sacrificial Material and Isolating the Gold Nanotubes

  1. Remove the membrane from the Teflon cell assembly, and dissolve the silver, copper and nickel with a few drops of conc. Nitric acid (> 68%) on the silver coated side. Remove the acid and rinse the membranes 3 x 10 sec with 18.2 MΩ deionized water.
  2. Etch the polymer core by immersing the membrane overnight in a 3:1 v/v solution of sulfuric acid and 30% hydrogen peroxide (Caution! This solution is a strong oxidizer and should be handled with care).
  3. Remove the acid solution and rinse the membrane under a stream of 18.2 mΩ deionized water. Break the membrane into small pieces and place in a 3.0 ml centrifuge vial, and add 2 ml of an aqueous 3.0 M NaOH solution. Agitate the vial in a heated mixer operating at 1,000 rpm and 40 °C for 3 hr or until the membrane is dissolved.
  4. Centrifuge the mixture for 10 min at 21,000 x g, remove the supernatant liquid and replace it with 18.2 MΩ deionized water. Repeat this cycle 3 times. The vial now contains gold nanotubes that can be suspended by gentle sonication. Upon sonication and suspension the solution should appear light purple.

6. Optical Characterization of Gold Nanotubes

  1. To measure the optical spectra, centrifuge the solution of gold nanotubes for 10 min at 21,000 x g, remove the supernatant liquid and replace it with D2O. Repeat this process 3 times.
  2. Sonicate the mixture for 30 sec until the solution becomes clear, and transfer the solution into a 1 ml quartz cuvette.
  3. Obtain the extinction spectra from 200 nm to 2,000 nm in a UV/vis spectrophotometer, operating in dual beam mode using a cuvette with D2O as the reference cell. Two absorbances should be present, corresponding to the transverse and longitudinal plasmon modes (Figure 5).
  4. To measure the solid state spectra, proceed to step 5.2. Stop and place the intact membrane on a glass slide.
  5. Wet the membrane and glass slide with D2O to increase transparency.
  6. Attach the membrane secured to a glass slide and mount it in a thin film sample holder for a UV/vis spectrophotometer. Operating in dual beam mode, obtain an extinction spectra from 200 nm to 1,300 nm using a glass slide as the reference.

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Results

After each step, one can visibly determine whether or not the synthesis is successful by observing the color of the membrane. After copper deposition (step 2.3) the template will appear purple. During nickel deposition (step 2.5) the template will slowly turn black. After the polymer deposition (step 3.3) the template should appear darker purple/black and more glossy (Figure 2). Typical chronoapmerograms of successful polymer and gold are included (Figure 3). During the final etching ste...

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Discussion

Template directed synthesis of nanorods in AAO membranes has become increasingly popular, however syntheses of nanorods tend to be very sensitive towards minor changes in material and synthesis conditions. Here, a comprehensive understanding of the advantages and limitations of using AAO membranes is outlined, as well as a general guideline for using AAO membranes for electrochemical synthesis of nanostructures.

When purchasing AAO membranes, there are two general types available: asymmetric a...

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Acknowledgements

This work was supported by the University of Toronto, the Natural Sciences and Engineering Research Council of Canada, the Canadian Foundation for Innovation, and the Ontario Research Fund. DSS thanks the Ontario Ministry for an Early Researcher Award.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Reagent/Material
UniKera Standard MembraneSynkera Technologies Inc.SM-X-Y-13Anodic aluminum oxide membranes are available from synkera in various pore sizes ranging from 13 - 150 nm, and thicknesses from 50 to 100 μm. We use the 50 μm ones. They are symmetric, meaning the pore size is uniform from top to bottom.
Anopore Inorganic MembranesWhatman6809-702313 mm diameter, 200 nm pore size. These membranes are very fragile. The pore diameters are not uniform throughout, so it is important to always use the bottom of the membrane as the working electrode
Silver Pellets %99.99Kurt J. LeskerEVMAG40EXE-D
Copper(II) sulfate pentahydrateSigma-Aldrich209189
Sulfuric acidACPS8780Caution: corrosive liquid
Hydrogen peroxide (30%)ACPH7000Caution: oxidizing liquid
Nitric AcidACPN2800Caution: corrosive fuming liquid
Sodium HydroxideFisher ScientificS318-1Caution: caustic powder
Watts Nickel PureTechnic Inc.130859Product is no longer available from Technic inc., however other commercial nickelplating solutions will work.
Techni-Gold 434HSTechnic Inc.X6763600Contains cyanide, do not acidify
Boron trifluoride diethyl etherateSigma-Aldrich175501-100MLMust be stored and used under inert atmosphere
3-hexylthiopheneSigma-Aldrich399051-5G
Deuterium OxideSigma-Aldrich151880-100G
Acetonitrile (anhydrous)Sigma-Aldrich271004
Ethanol (anhydrous)Caledon Labs1500-1-05
Equipment
EC Epsilon potentiostat/galvanostatBASi (Bioanalytical Systems, Inc.)N/AReference electrodes and platinum wires were included with the potentiostat, and replacements can be purchaes from BASi http://www.basinc.com/products/ec/epsilon/features.html
Cary 5000 UV-Vis-NIR spectrophotometerAgilent TechnologiesN/Ahttp://www.chem.agilent.com/en-US/products-services/Instruments-Systems/Molecular-Spectroscopy/Cary-5000-UV-Vis-NIR/Pages/default.aspx
Thermomixer REppendorfN/Ahttp://www.eppendorf.com/int/index.php?action=products&contentid=1&catalognode=9832
Branson 2510 Ultrasonic CleanerBransonicZ244810 (From Sigma Aldrich)http://www.sigmaaldrich.com/catalog/product/aldrich/Z244910?lang=en®ion=CA
Covap 2 thermal evaporatorAngstrom EngineeringN/Ahttp://www.angstromengineering.com/covap.html
Millipore Synergy water purification systemMilliporeN/Ahttp://www.millipore.com/catalogue/module/c9209

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Plasmonic AbsorbanceAnodic Aluminum OxideElectrodepositionPolymer CoreInfrared TuningHollow NanostructuresSolution SuspensionOptical SpectroscopyNanotube Synthesis
Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance
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