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.

1. Forming the Silver Working Electrode
<ol>
	<li>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.</li>
	<li>Install the glass plate into the substrate holder of the metal evaporator, close the chamber, and evacuate to a pressure of below 1.0 &mu;Torr.</li>
	<li>Using a resistive source, evaporate silver pellets (&gt;99.99% purity) onto the substrate at a rate of 0.8 &Aring;/sec until a laye...

<ol>
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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...

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.

<table border="1">
 <tbody>
 <tr>
 <td></td>
 <td></td>
 <td></td>
 <td>Reagent/Material</td>
 </tr>
 <tr>
 <td>UniKera Standard Membrane</td>
 <td>Synkera Technologies Inc.</td>
 <td>SM-X-Y-13</td>
 <td>Anodic aluminum oxide membranes are available from synkera in various pore sizes ranging from 13 - 150 nm, and thicknesses from 50 to 100 &mu;m. We use the 50 &mu;m ones. They are symmetric, meaning the pore size is uniform from top to bottom.</td>
 </tr>
 <tr>
 <td>Anopore Inorganic Membranes</td>
 <td>Whatman</td>
 <td>6809-7023</td>
 <td>13 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</td>
 </tr>
 <tr>
 <td>Silver Pellets %99.99</td>
 <td>Kurt J. Lesker</td>
 <td>EVMAG40EXE-D</td>
 <td></td>
 </tr>
 <tr>
 <td>Copper(II) sulfate pentahydrate</td>
 <td>Sigma-Aldrich</td>
 <td>209189</td>
 <td></td>
 </tr>
 <tr>
 <td>Sulfuric acid</td>
 <td>ACP</td>
 <td>S8780</td>
 <td>Caution: corrosive liquid</td>
 </tr>
 <tr>
 <td>Hydrogen peroxide (30%)</td>
 <td>ACP</td>
 <td>H7000</td>
 <td>Caution: oxidizing liquid</td>
 </tr>
 <tr>
 <td>Nitric Acid</td>
 <td>ACP</td>
 <td>N2800</td>
 <td>Caution: corrosive fuming liquid</td>
 </tr>
 <tr>
 <td>Sodium Hydroxide</td>
 <td>Fisher Scientific</td>
 <td>S318-1</td>
 <td>Caution: caustic powder</td>
 </tr>
 <tr>
 <td>Watts Nickel Pure</td>
 <td>Technic Inc.</td>
 <td>130859</td>
 <td>Product is no longer available from Technic inc., however other commercial nickelplating solutions will work. </td>
 </tr>
 <tr>
 <td>Techni-Gold 434HS</td>
 <td>Technic Inc.</td>
 <td>X6763600</td>
 <td>Contains cyanide, do not acidify</td>
 </tr>
 <tr>
 <td>Boron trifluoride diethyl etherate</td>
 <td>Sigma-Aldrich</td>
 <td>175501-100ML</td>
 <td>Must be stored and used under inert atmosphere</td>
 </tr>
 <tr>
 <td>3-hexylthiophene</td>
 <td>Sigma-Aldrich</td>
 <td>399051-5G</td>
 <td></td>
 </tr>
 <tr>
 <td>Deuterium Oxide</td>
 <td>Sigma-Aldrich</td>
 <td>151880-100G</td>
 <td></td>
 </tr>
 <tr>
 <td>Acetonitrile (anhydrous)</td>
 <td>Sigma-Aldrich</td>
 <td>271004</td>
 <td></td>
 </tr>
 <tr>
 <td>Ethanol (anhydrous)</td>
 <td>Caledon Labs</td>
 <td>1500-1-05</td>
 <td></td>
 </tr>
 <tr>
 <td></td>
 <td></td>
 <td></td>
 <td>Equipment</td>
 </tr>
 <tr>
 <td>EC Epsilon potentiostat/galvanostat</td>
 <td>BASi (Bioanalytical Systems, Inc.)</td>
 <td>N/A</td>
 <td>Reference electrodes and platinum wires were included with the potentiostat, and replacements can be purchaes from BASi <a href="http://www.basinc.com/products/ec/epsilon/features.html" target="_blank">http://www.basinc.com/products/ec/epsilon/features.html</a></td>
 </tr>
 <tr>
 <td>Cary 5000 UV-Vis-NIR spectrophotometer</td>
 <td>Agilent Technologies</td>
 <td>N/A</td>
 <td><a href="http://www.chem.agilent.com/en-US/products-services/Instruments-Systems/Molecular-Spectroscopy/Cary-5000-UV-Vis-NIR/Pages/default.aspx" target="_blank">http://www.chem.agilent.com/en-US/products-services/Instruments-Systems/Molecular-Spectroscopy/Cary-5000-UV-Vis-NIR/Pages/default.aspx</a></td>
 </tr>
 <tr>
 <td>Thermomixer R</td>
 <td>Eppendorf</td>
 <td>N/A</td>
 <td><a href="http://www.eppendorf.com/int/index.php?action=products&amp;contentid=1&amp;catalognode=9832" target="_blank">http://www.eppendorf.com/int/index.php?action=products&amp;contentid=1&amp;catalognode=9832</a></td>
 </tr>
 <tr>
 <td>Branson 2510 Ultrasonic Cleaner</td>
 <td>Bransonic</td>
 <td>Z244810 (From Sigma Aldrich)</td>
 <td><a href="http://www.sigmaaldrich.com/catalog/product/aldrich/Z244910?lang=en&amp;region=CA" target="_blank">http://www.sigmaaldrich.com/catalog/product/aldrich/Z244910?lang=en&amp;region=CA</a></td>
 </tr>
 <tr>
 <td>Covap 2 thermal evaporator</td>
 <td>Angstrom Engineering</td>
 <td>N/A</td>
 <td><a href="http://www.angstromengineering.com/covap.html" target="_blank">http://www.angstromengineering.com/covap.html</a></td>
 </tr>
 <tr>
 <td>Millipore Synergy water purification system</td>
 <td>Millipore</td>
 <td>N/A</td>
 <td><a href="http://www.millipore.com/catalogue/module/c9209" target="_blank">http://www.millipore.com/catalogue/module/c9209</a></td>
 </tr>
 </tbody>
</table>

template directed synthesis of plasmonic gold nanotubes with tunable ir absorbance

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.

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...

Watch this Scientific Journal Video about Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance at JoVE.com

Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance

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.

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) ...

template-directed-synthesis-plasmonic-gold-nanotubes-with-tunable-ir

Research

JoVE Journal

Chemistry

16.1K Views.  University of Toronto. Soluzione-sospendibili nanotubi d'oro con dimensioni controllate possono essere sintetizzati mediante deposizione elettrochimica in anodici porosi di ossido di alluminio (AAO) membrane utilizzando un nucleo polimero idrofobo. Nanotubi d'oro e gli array di nanotubi promettenti per applicazioni in biosensori plasmoniche, superficie-enhanced spettroscopia Raman, foto-termico di riscaldamento, trasporto ionico e molecolare, microfluidica, catalisi e rilevamento elettr...