Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance

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 environments^{1, 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, ....}

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 t....

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.......

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Reagent/Material
UniKera Standard Membrane	Synkera Technologies Inc.	SM-X-Y-13	Anodic 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 Membranes	Whatman	6809-7023	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
Silver Pellets %99.99	Kurt J. Lesker	EVMAG40EXE-D
Copper(II) sulfate pentahydrate	Sigma-Aldrich	209189
Sulfuric acid	ACP	S8780	Caution: corrosive liquid
Hydrogen peroxide (30%)	ACP	H7000	Caution: oxidizing liquid
Nitric Acid	ACP	N2800	Caution: corrosive fuming liquid
Sodium Hydroxide	Fisher Scientific	S318-1	Caution: caustic powder
Watts Nickel Pure	Technic Inc.	130859	Product is no longer available from Technic inc., however other commercial nickelplating solutions will work.
Techni-Gold 434HS	Technic Inc.	X6763600	Contains cyanide, do not acidify
Boron trifluoride diethyl etherate	Sigma-Aldrich	175501-100ML	Must be stored and used under inert atmosphere
3-hexylthiophene	Sigma-Aldrich	399051-5G
Deuterium Oxide	Sigma-Aldrich	151880-100G
Acetonitrile (anhydrous)	Sigma-Aldrich	271004
Ethanol (anhydrous)	Caledon Labs	1500-1-05
Equipment
EC Epsilon potentiostat/galvanostat	BASi (Bioanalytical Systems, Inc.)	N/A	Reference 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 spectrophotometer	Agilent Technologies	N/A	http://www.chem.agilent.com/en-US/products-services/Instruments-Systems/Molecular-Spectroscopy/Cary-5000-UV-Vis-NIR/Pages/default.aspx
Thermomixer R	Eppendorf	N/A	http://www.eppendorf.com/int/index.php?action=products&contentid=1&catalognode=9832
Branson 2510 Ultrasonic Cleaner	Bransonic	Z244810 (From Sigma Aldrich)	http://www.sigmaaldrich.com/catalog/product/aldrich/Z244910?lang=en®ion=CA
Covap 2 thermal evaporator	Angstrom Engineering	N/A	http://www.angstromengineering.com/covap.html
Millipore Synergy water purification system	Millipore	N/A	http://www.millipore.com/catalogue/module/c9209