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 layer thickness of 100 nm is reached, then increase the evaporation rate to 1.5 &Aring;/sec until a final thickness of 250 nm is reached.</li>
	<li>Release the AAO membranes by wiping the adhesive layer with a cotton swab wetted with dichloromethane to dissolve the adhesive.</li>
</ol>
2. Electrodepositing Copper and Nickel
<ol>
	<li>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 M&Omega; deionized water. Allow the cell to dry in the ambient laboratory air.</li>
	<li>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).</li>
	<li>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.</li>
	<li>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 M&Omega; deionized water. Let the cell soak for 30 min in 5 ml 18.2 M&Omega; deionized water to remove excess copper plating solution from within the pores.</li>
	<li>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.</li>
	<li>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 M&Omega; deionized water, then let it soak for 30 min in 5 ml 18.2 M&Omega; deionized water to remove excess plating solution from the pores. Allow the cell to thoroughly dry in the ambient laboratory air overnight.</li>
</ol>
3. Electropolymerizing the Polymer Core
<ol>
	<li>Bring the intact Teflon cell assembly into an inert atmosphere glove box equipped with external connections to a potentiostat.</li>
	<li>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.</li>
	<li>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).</li>
	<li>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 M&Omega; deionized water and then allow the cell to soak in 18.2 M&Omega; deionized water for 20 min. Allow it to dry in the ambient laboratory air.</li>
</ol>
4. Electrodepositing the Gold Shell
<ol>
	<li>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.</li>
	<li>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).</li>
	<li>Rinse the cell under a stream of 18.2 M&Omega; deionized water and allow it to dry.</li>
</ol>
5. Removing Sacrificial Material and Isolating the Gold Nanotubes
<ol>
	<li>Remove the membrane from the Teflon cell assembly, and dissolve the silver, copper and nickel with a few drops of conc. Nitric acid (&gt; 68%) on the silver coated side. Remove the acid and rinse the membranes 3 x 10 sec with 18.2 M&Omega; deionized water.</li>
	<li>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).</li>
	<li>Remove the acid solution and rinse the membrane under a stream of 18.2 m&Omega; 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 &deg;C for 3 hr or until the membrane is dissolved.</li>
	<li>Centrifuge the mixture for 10 min at 21,000 x g, remove the supernatant liquid and replace it with 18.2 M&Omega; 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.</li>
</ol>
6. Optical Characterization of Gold Nanotubes
<ol>
	<li>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.</li>
	<li>Sonicate the mixture for 30 sec until the solution becomes clear, and transfer the solution into a 1 ml quartz cuvette.</li>
	<li>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).</li>
	<li>To measure the solid state spectra, proceed to step 5.2. Stop and place the intact membrane on a glass slide.</li>
	<li>Wet the membrane and glass slide with D2O to increase transparency.</li>
	<li>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.</li>
</ol>

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A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On-wire+lithography:+synthesis,+encoding+and+biological+applications.">On-wire lithography: synthesis, encoding and biological applications.</a> Nature Protocols. 4 (6), 838-848 (2009).</li></ol>

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 and symmetric. Asymmetric membranes have pore diameters that vary from the top to the bottom. The bottom of the templates typically consists of a branched network of pores, which eventually leads into an aligned, parallel array. Symmetric membranes are also available, and are typically higher quality, with uniform aligned pore diameters along the entire thickness of the membrane. Membranes of this type are preferred if the goal is to create an array of nanostructures bound to a substrate.
As purchased, AAO membranes are open at each end. The purpose of the evaporated silver layer is to form a working electrode that seals one end of the membrane. This allows each pore to act as an individual electrochemical cell during the synthesis. The following step is the electrodeposition of metal, and is required in asymmetric membranes to fill in the branched area of the membrane with non-uniform pore diameters. This step is important because without it, branched and irregular nanostructures are formed. The choice of metal is not important and depends on your desired etching conditions. Copper was used because of its high conductivity, low cost, and ease of removal, however silver, nickel, and gold can also be used.
The nickel layer is important for the electropolymerization step. The goal of this step is to form a 200-500 nm nickel coating on top of the copper to form a layer for the polymer to adhere to. Only gold and nickel have suitably high work functions to support oxidative polymerization. Gold however, cannot be etched separately from the nanotube (also composed of gold), thus using gold would result in tubes that are sealed at one end. Nickel is the only metal that can be used in this step if you require solution suspendable gold nanotubes that are open on both ends.
The polymer acts as a sacrificial core for the gold nanotube shell, however the choice of polymer and its nanotube morphology are very important. The polymer must be hydrophobic, such that it collapses onto itself upon addition of the aqueous gold plating solution rather than adhere to the template wall. This hydrophobic collapse provides a space for the gold nanotube to be deposited between the polymer core and template walls and hydrophilic polymers subjected to the same synthesis conditions do not allow for full gold tubes to form. The polymer must also form a tube rather than a rod, as polymer rod cores (hydrophobic or hydrophilic) cannot collapse, thus not allow for gold nanotube shell deposition. The morphology of the polymer core is also affected by the solvent/electrolyte used for electropolymerization, which also affects the wall thickness of the resulting gold nanotube. A more detailed description of the mechanism of core collapse and how to control wall thickness of the resulting gold nanotubes has been recently described in literature 16. In this study, we chose 3-hexylthiophene as the monomer and 46% boron trifluoride in diethyl ether as our solvent/electrolyte since it is known to produce thin walled, highly hydrophobic poly-3-hexylthiophene nanotubes 7, 10.
The final step is electrodepositing the gold shell. At this point it is crucial to ensure the pores of the membrane are not clogged, which prevents electrodeposition. This can be accomplished by thorough, gentle rinsing after each step, and by allowing the gold plating solution several minutes to permeate the membrane entirely before applying a potential. The easiest indication that a membrane has become clogged is a low current (below 1 &mu;amp/sec for the diameter of membranes described here, 13 mm). The length of the gold nanotube can be varied by increasing the deposition time.
After acid etching the base metals and polymer core, the gold nanotubes are left in the membrane. At this point their optical properties can be studied as an array, or the template can be dissolved and their homogeneous solution optical properties can be observed. When conducting optical measurements it is important to ensure all traces of water are removed and replaced with deuterium oxide, as water will interfere with the near-IR portion of the spectra where the longitudinal plasmon mode occurs. Another important consideration for optical measurements is the aggregation of gold nanotubes in solution. Unmodified gold nanotubes will aggregate if left in solution, thus brief sonication fully reverses aggregation of these nanotubes, and is required to freely suspend them prior to extinction measurements. Solutions of these gold nanotubes remain stable over periods of minutes to hours, depending on their size, before requiring further sonication.
In summary, solution-suspendable gold nanotubes can be prepared in AAO membranes. AAO membranes are useful for synthesizing arrays of high aspect ratio nanorods, and have advantages over solution based syntheses in that it is very easy to control nanoparticle dimensions. While solution based syntheses can yield more material, synthesizing complex composite or hollow nanoparticles is much more controlled using AAO membranes, and allows for the synthesis of ordered arrays.

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 step (5.2), the template should appear purple and opaque (Figure 2) due to the gold nanotubes SPR. After the membrane is dissolved (step 5.4), the gold nanotubes can be visualized using electron microscopy (Figure 6). The gold nanotubes can either be imaged from solution by drop casting onto a copper TEM grid, or as an aligned array grown off a gold base by mounting a sample onto an SEM stage prior to template dissolution. The membrane pore size determines the diameter, which varies between 10 and 250 nm according to the manufacturer's specifications. The length of gold nanotubes depends on the deposition time, which can be tuned from 150 nm to several microns. The standard deviation of lengths is expected to be around 15% (Figure 4).
Representative optical spectra for 55 nm diameter structures are included (Figure 5). The 55 nm diameter structures exhibit two plasmon modes in solution: the transverse mode lying in the visible region (520 nm) and longitudinal mode lying in the near IR region (~1,200 nm). The position of the transverse mode will vary depending on the length of the nanotube. Nanostructures synthesized in 200 nm pore size templates will appear turbid and brown in solution, and scatter heavily across all wavelengths.
<img alt="Figure 1" fo:content-src="/files/ftp_upload/50420/50420fig1highres.jpg" fo:content-width="6.5in" src="/files/ftp_upload/50420/50420fig1.jpg" /> 
Figure 1. Scheme depicting the procedure for preparing gold nanotubes. One side of the AAO membrane is coated with silver, followed by electrodeposition of the copper and nickel layers within the pores (A). The polymer core is deposited (B). The polymer core collapses when exposed to water (C). The gold shell is deposited (D). All sacrificial materials are etched yielding a hollow gold nanotube (E). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/50420/50420fig1large.jpg" target="_blank"> Click here to view larger figure</a>.
<img alt="Figure 2" src="/files/ftp_upload/50420/50420fig2.jpg" /> 
Figure 2. Digital pictures of the Teflon electrochemical cell with a silver coated AAO membrane face down on the aluminum foil before (A) and after (B) assembly. Image of an AAO membrane after copper deposition (C), nickel deposition (D), polymer deposition (E) and gold nanotube deposition after the sacrificial metals and polymer have been etched (F).
<img alt="Figure 3" fo:content-src="/files/ftp_upload/50420/50420fig3highres.jpg" fo:content-width="4in" src="/files/ftp_upload/50420/50420fig3.jpg" /> 
Figure 3. Chronoamperograms of gold nanotube electrodeposition at -920 mV (red) and polymer core electropolymerization at +1,500 mV (blue).
<img alt="Figure 4" fo:content-src="/files/ftp_upload/50420/50420fig4highres.jpg" fo:content-width="4in" src="/files/ftp_upload/50420/50420fig4.jpg" /> 
Figure 4. Graph of gold nanotube length versus electrodeposition time at -920 mV for 200 nm gold nanotubes. A linear correlation between length and time is observed. Error bars represent 1 standard deviation in length, based off 100 measurements.
<img alt="Figure 5" fo:content-src="/files/ftp_upload/50420/50420fig5highres.jpg" fo:content-width="6in" src="/files/ftp_upload/50420/50420fig5.jpg" /> 
Figure 5. Representative extinction spectra of an aligned array of 55 nm diameter gold nanotubes (A). Representative extinction spectra of solution suspended gold nanotubes as length (L) increases (B).
<img alt="Figure 6" src="/files/ftp_upload/50420/50420fig6.jpg" /> 
Figure 6. An SEM image of an aligned array of gold nanotubes grown off a gold substrate prepared in a 55 nm pore template (A). A TEM image of gold nanotubes prepared in a 55 nm pore template (B). A TEM cross section of a gold nanotube prepared in a 200 nm pore template (C). A TEM image of a gold nanotube prepared in a 200 nm pore template (D). Red arrows highlight the lighter contrast area of the nanotube, indicating its cavity size.

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

Nanyang Technological University

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Chemistry

16.1K Views.  University of Toronto. 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.

Video: Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance

Synthesis of Immunotargeted Magneto-plasmonic Nanoclusters

Magnetic and plasmonic properties combined in a single nanoparticle provide a synergy that is advantageous in a number of biomedical applications including contrast enhancement in novel magnetomotive imaging modalities, simultaneous capture&#160;and detection of circulating tumor cells (CTCs),&#160;and multimodal molecular imaging combined with photothermal therapy of cancer cells. These applications have stimulated significant interest in development of protocols for synthesis of magneto-plasmonic nanoparticles with optical absorbance in the near-infrared (NIR) region and a strong magnetic moment. Here, we present a novel protocol for synthesis of such hybrid nanoparticles that is based on an oil-in-water microemulsion method. The unique feature of the protocol described herein is synthesis of magneto-plasmonic nanoparticles of various sizes from primary blocks which also have magneto-plasmonic characteristics. This approach yields nanoparticles with a high density of magnetic and plasmonic functionalities which are uniformly distributed throughout the nanoparticle volume. The hybrid nanoparticles can be easily functionalized by attaching antibodies through the Fc moiety leaving the Fab portion that is responsible for antigen binding available for targeting.

Here, we describe a protocol for synthesis of magneto-plasmonic nanoparticles with a strong magnetic moment and a strong near-infrared (NIR) absorbance. The protocol also includes antibody conjugation to the nanoparticles through the Fc moiety for various biomedical applications which require molecular specific targeting.

Magnetic and plasmonic properties combined in a single nanoparticle provide a synergy that is advantageous in a number of biomedical applications ...

Synthesis and Catalytic Performance of Gold Intercalated in the Walls of Mesoporous Silica

As a promising catalytically active nano reactor, gold nanoparticles intercalated in mesoporous silica (GMS) were successfully synthesized and properties of the materials were investigated. We used a one pot sol-gel approach to intercalate gold nano particles in the walls of mesoporous silica. To start with the synthesis, P123 was used as template to form micelles. Then TESPTS was used as a surface modification agent to intercalate gold nano particles. Following this process, TEOS was added in as a silica source which underwent a polymerization process in acid environment. After hydrothermal processing and calcination, the final product was acquired. Several techniques were utilized to characterize the porosity, morphology and structure of the gold intercalated mesoporous silica. The results showed a stable structure of mesoporous silica after gold intercalation. Through the oxidation of benzyl alcohol as a benchmark reaction, the GMS materials showed high selectivity and recyclability.

Here, we present a protocol with a sol-gel process to synthesize gold intercalated in the walls of mesoporous materials (GMS), which is confirmed to possess a mesoporous matrix with gold intercalated in the walls imparting great stability and recyclability.

As a promising catalytically active nano reactor, gold nanoparticles intercalated in mesoporous silica (GMS) were successfully synthesized and properties ...

A Simple Method for the Size Controlled Synthesis of Stable Oligomeric Clusters of Gold Nanoparticles under Ambient Conditions

Reducing dilute aqueous HAuCl4 with sodium thiocyanate (NaSCN) under alkaline conditions produces 2 to 3 nm diameter nanoparticles. Stable grape-like oligomeric clusters of these yellow nanoparticles of narrow size distribution are synthesized under ambient conditions via two methods. The delay-time method controls the number of subunits in the oligoclusters by varying the time between the addition of HAuCl4 to alkaline solution and the subsequent addition of reducing agent, NaSCN. The yellow oligoclusters produced range in size from ~3 to ~25 nm. This size range can be further extended by an add-on method utilizing hydroxylated gold chloride (Na+[Au(OH4-x)Clx]-) to auto-catalytically increase the number of subunits in the as-synthesized oligocluster nanoparticles, providing a total range of 3 nm to 70 nm. The crude oligocluster preparations display narrow size distributions and do not require further fractionation for most purposes. The oligoclusters formed can be concentrated &#62;300 fold without aggregation and the crude reaction mixtures remain stable for weeks without further processing. Because these oligomeric clusters can be concentrated before derivatization they allow expensive derivatizing agents to be used economically. In addition, we present two models by which predictions of particle size can be made with great accuracy.

We describe a simple method for producing highly stable oligomeric clusters of gold nanoparticles via the reduction of chloroauric acid (HAuCl4) with sodium thiocyanate (NaSCN). The oligoclusters have a narrow size distribution and can be produced with a wide range of sizes and surface coats.

Reducing dilute aqueous HAuCl4 with sodium thiocyanate (NaSCN) under alkaline conditions produces 2 to 3 nm diameter nanoparticles. Stable grape-like ...

Hydroquinone Based Synthesis of Gold Nanorods

Gold nanorods are an important kind of nanoparticles characterized by peculiar plasmonic properties. Despite their widespread use in nanotechnology, the synthetic methods for the preparation of gold nanorods are still not fully optimized. In this paper we describe a new, highly efficient, two-step protocol based on the use of hydroquinone as a mild reducing agent. Our approach allows the preparation of nanorods with a good control of size and aspect ratio (AR) simply by varying the amount of hexadecyl trimethylammonium bromide (CTAB) and silver ions (Ag+) present in the "growth solution". By using this method, it is possible to markedly reduce the amount of CTAB, an expensive and cytotoxic reagent, necessary to obtain the elongated shape. Gold nanorods with an aspect ratio of about 3 can be obtained in the presence of just 50 mM of CTAB (versus 100 mM used in the standard protocol based on the use of ascorbic acid), while shorter gold nanorods are obtained using a concentration as low as 10 mM.

This paper describes a protocol for the synthesis of gold nanorods, based on the use of hydroquinone as reducing agent, plus the different mechanisms for controlling their size and aspect ratio.

Gold nanorods are an important kind of nanoparticles characterized by peculiar plasmonic properties. Despite their widespread use in nanotechnology, the ...

Synthesis and Characterization of Fe-doped Aluminosilicate Nanotubes with Enhanced Electron Conductive Properties

The goal of the protocol is to synthesize Fe-doped aluminosilicate nanotubes of the imogolite type with the formula (OH)3Al2-xFexO3SiOH. Doping with Fe aims at lowering the band gap of imogolite, an insulator with the chemical formula (OH)3Al2O3SiOH, and at modifying its adsorption properties towards azo-dyes, an important class of organic pollutants of both wastewater and groundwater.
Fe-doped nanotubes are obtained in two ways: by direct synthesis, where FeCl3 is added to an aqueous mixture of the Si and Al precursors, and by post-synthesis loading, where preformed nanotubes are put in contact with a FeCl3&#8226;6H2O aqueous solution. In both synthesis methods, isomorphic substitution of Al3+ by Fe3+ occurs, preserving the nanotube structure. Isomorphic substitution is indeed limited to a mass fraction of ~1.0% Fe, since at a higher Fe content (i.e., a mass fraction of 1.4% Fe), Fe2O3 clusters form, especially when the loading procedure is adopted. The physicochemical properties of the materials are studied by means of X-ray powder diffraction (XRD), N2 sorption isotherms at -196 &#176;C, high resolution transmission electron microscopy (HRTEM), diffuse reflectance (DR) UV-Vis spectroscopy, and &#950;-potential measurements. The most relevant result is the possibility to replace Al3+ ions (located on the outer surface of the nanotubes) by post-synthesis loading on preformed imogolite without perturbing the delicate hydrolysis equilibria occurring during nanotube formation. During the loading procedure, an anionic exchange occurs, where Al3+ ions on the outer surface of the nanotubes are replaced by Fe3+ ions. In Fe-doped aluminosilicate nanotubes, isomorphic substitution of Al3+ by Fe3+ is found to affect the band gap of doped imogolite. Nonetheless, Fe3+ sites on the outer surface of nanotubes are able to coordinate organic moieties, like the azo-dye Acid Orange 7, through a ligand-displacement mechanism occurring in an aqueous solution.

Here, we present a protocol to synthesize and characterize Fe-doped aluminosilicate nanotubes. The materials are obtained by either sol-gel synthesis upon addition of FeCl3&#8226;6H2O to the mixture containing the Si and Al precursors or by post-synthesis ionic exchange of preformed aluminosilicate nanotubes.

The goal of the protocol is to synthesize Fe-doped aluminosilicate nanotubes of the imogolite type with the formula (OH)3Al2-xFexO3SiOH. Doping with Fe ...

Grafting Multiwalled Carbon Nanotubes with Polystyrene to Enable Self-Assembly and Anisotropic Patchiness

We demonstrate a straightforward protocol to graft pristine multiwalled carbon nanotubes (MWCNTs) with polystyrene (PS) chains at the sidewalls through a free-radical polymerization strategy to enable the modulation of the nanotube surface properties and produce supramolecular self-assembly of the nanostructures. First, a selective hydroxylation of the pristine nanotubes through a biphasic catalytically mediated oxidation reaction creates superficially distributed reactive sites at the sidewalls. The latter reactive sites are subsequently modified with methacrylic moieties using a silylated methacrylic precursor to create polymerizable sites. Those polymerizable groups can address further polymerization of styrene to produce a hybrid nanomaterial containing PS chains grafted to the nanotube sidewalls. The polymer-graft content, amount of silylated methacrylic moieties introduced and hydroxylation modification of the nanotubes are identified and quantified by Thermogravimetric Analysis (TGA). The presence of reactive functional groups hydroxyl and silylated methacrylate are confirmed by Fourier Transform Infrared Spectroscopy (FT-IR). Polystyrene-grafted carbon nanotube solutions in tetrahydrofuran (THF) provide wall-to-wall collinearly self-assembled nanotubes when cast samples are analyzed by transmission electron microscopy (TEM). Those self-assemblies are not obtained when suitable blanks are similarly cast from analogous solutions containing non-grafted counterparts. Therefore, this method enables the modification of the nanotube anisotropic patchiness at the sidewalls which results into spontaneous auto-organization at the nanoscale.

A procedure for the synthesis of polystyrene-grafted multiwalled carbon nanotubes using successive chemical modification steps to selectively introduce the polymer chains to the sidewalls and their self-assembly via anisotropic patchiness is presented.

We demonstrate a straightforward protocol to graft pristine multiwalled carbon nanotubes (MWCNTs) with polystyrene (PS) chains at the sidewalls through a ...

Rapid Nanoprobe Signal Enhancement by In Situ Gold Nanoparticle Synthesis

The use of nanoprobes such as gold, silver, silica or iron-oxide nanoparticles as detection reagents in bioanalytical assays can enable high sensitivity and convenient colorimetric readout. However, high densities of nanoparticles are typically needed for detection. The available synthesis-based enhancement protocols are either limited to gold and silver nanoparticles or rely on precise enzymatic control and optimization. Here, we present a protocol to enhance the colorimetric readout of gold, silver, silica, and iron oxide nanoprobes. It was observed that the colorimetric signal can be improved by up to a 10000-fold factor. The basis for such signal enhancement strategies is the chemical reduction of Au3+ to Au0. There are several chemical reactions that enable the reduction of Au3+ to Au0. In the protocol, Good&#39;s buffers and H2O2 are used and it is possible to favor the deposition of Au0 onto the surface of existing nanoprobes, in detriment of the formation of new gold nanoparticles. The protocol consists of the incubation of the microarray with a solution consisting of chloroauric acid and H2O2 in 2-(N-morpholino)ethanesulfonic acid pH 6 buffer following the nanoprobe-based detection assay. The enhancement solution can be applied to paper and glass-based sensors. Moreover, it can be used in commercially available immunoassays as demonstrated by the application of the method to a commercial allergen microarray. The signal development requires less than 5 min of incubation with the enhancement solution and the readout can be assessed by naked eye or low-end image acquisition devices such as a table-top scanner or a digital camera.

Rapid Nanoprobe Signal Enhancement by In Situ Gold Nanoparticle Synthesis

In this work, a protocol for signal enhancement of nanoprobe-based biosensing is presented. The protocol is based on the reduction of chloroauric acid onto the surface of existing nanoprobes that consist of gold, silver, silica or iron-oxide nanoparticles.

The use of nanoprobes such as gold, silver, silica or iron-oxide nanoparticles as detection reagents in bioanalytical assays can enable high sensitivity ...

Synthesis and Characterization of Amphiphilic Gold Nanoparticles

Gold nanoparticles covered with a mixture of 1-octanethiol (OT) and 11-mercapto-1-undecane sulfonic acid (MUS) have been extensively studied because of their interactions with cell membranes, lipid bilayers, and viruses. The hydrophilic ligands make these particles colloidally stable in aqueous solutions and the combination with hydrophobic ligands creates an amphiphilic particle that can be loaded with hydrophobic drugs, fuse with the lipid membranes, and resist nonspecific protein adsorption. Many of these properties depend on nanoparticle size and the composition of the ligand shell. It is, therefore, crucial to have a reproducible synthetic method and reliable characterization techniques that allow the determination of nanoparticle properties and the ligand shell composition. Here, a one-phase chemical reduction, followed by a thorough purification to synthesize these nanoparticles with diameters below 5 nm, is presented. The ratio between the two ligands on the surface of the nanoparticle can be tuned through their stoichiometric ratio used during synthesis. We demonstrate how various routine techniques, such as transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and ultraviolet-visible (UV-Vis) spectrometry, are combined to comprehensively characterize the physicochemical parameters of the nanoparticles.

Amphiphilic gold nanoparticles can be used in many biological applications. A protocol to synthesize gold nanoparticles coated by a binary mixture of ligands and a detailed characterization of these particles is presented.

Gold nanoparticles covered with a mixture of 1-octanethiol (OT) and 11-mercapto-1-undecane sulfonic acid (MUS) have been extensively studied because of ...

Assembly of Gold Nanorods into Chiral Plasmonic Metamolecules Using DNA Origami Templates

The inherent addressability of DNA origami structures makes them ideal templates for the arrangement of metal nanoparticles into complex plasmonic nanostructures. The high spatial precision of a DNA origami-templated assembly allows controlling the coupling between plasmonic resonances of individual particles and enables tailoring optical properties of the constructed nanostructures. Recently, chiral plasmonic systems attracted a lot of attention due to the strong correlation between the spatial configuration of plasmonic assemblies and their optical responses (e.g., circular dichroism [CD]). In this protocol, we describe the whole workflow for the generation of DNA origami-based chiral assemblies of gold nanorods (AuNRs). The protocol includes a detailed description of the design principles and experimental procedures for the fabrication of DNA origami templates, the synthesis of AuNRs, and the assembly of origami-AuNR structures. In addition, the characterization of structures using transmission electron microscopy (TEM) and CD spectroscopy is included. The described protocol is not limited to chiral configurations and can be adapted for the construction of various plasmonic architectures.

We describe the detailed protocol for the DNA origami-based assembly of gold nanorods into chiral plasmonic metamolecules with strong chiroptical responses. The protocol is not limited to chiral configurations and can be easily adapted for the fabrication of various plasmonic architectures.

The inherent addressability of DNA origami structures makes them ideal templates for the arrangement of metal nanoparticles into complex plasmonic ...

Synthesis of Information-bearing Peptoids and their Sequence-directed Dynamic Covalent Self-assembly

This protocol presents the use of Lewis acidic multi-role reagents to circumvent kinetic trapping observed during the self-assembly of information-encoded oligomeric strands mediated by paired dynamic covalent interactions in a manner mimicking the thermal cycling commonly employed for the self-assembly of complementary nucleic acid sequences. Primary amine monomers bearing aldehyde and amine pendant moieties are functionalized with orthogonal protecting groups for use as dynamic covalent reactant pairs. Using a modified automated peptide synthesizer, the primary amine monomers are encoded into oligo(peptoid) strands through solid-phase submonomer synthesis. Upon purification by high-performance liquid chromatography (HPLC) and characterization by electrospray ionization mass spectrometry (ESI-MS), sequence-specific oligomers are subjected to high-loading of a Lewis acidic rare-earth metal triflate which both deprotects the aldehyde moieties and affects the reactant pair equilibrium such that strands completely dissociate. Subsequently, a fraction of the Lewis acid is extracted, enabling annealing of complementary sequence-specific strands to form information-encoded molecular ladders characterized by matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS). The simple procedure outlined in this report circumvents kinetic traps commonly experienced in the field of dynamic covalent assembly and serves as a platform for the future design of robust, complex architectures.

A protocol is presented for the synthesis of information-encoded peptoid oligomers and for the sequence-directed self-assembly of these peptoids into molecular ladders using amines and aldehydes as dynamic covalent reactant pairs and Lewis acidic rare-earth metal triflates as multi-role reagents.

This protocol presents the use of Lewis acidic multi-role reagents to circumvent kinetic trapping observed during the self-assembly of information-encoded ...