This research was performed at the Pacific Northwest National Laboratory (PNNL), which is operated by Battelle Memorial Institute for the Department of Energy (DOE) under Contract No. DE-AC05-76RL01830. The authors gratefully acknowledge support from the U.S. Department of State through the Key Verification Assets Fund (V Fund) under Interagency Agreement SIAA15AVCVPO10.

1. Material Preparation
<ol>
	<li>Place several 2 in. silicon wafers into a quartz carrier for cleaning and load the silicon wafers into the plasma etching system. Pump the vacuum chamber down until it reaches at least 75 mTorr. This may take a few minutes.</li>
	<li>Begin the flow of O2 (30 sccm) gas and allow the pressure to stabilize. Set the etch time to 15 min. Once the chamber pressure has stabilized initiate the radiofrequency (RF) 13.56 MHz 250 W plasma. 
	NOTE: This step cleans the silicon wafers of any organic contaminates and functionalizes the surface with hydroxylated (-OH) moieties thereby ensuring a hydrophilic surface.</li>
	<li>While waiting for the plasma cleaning step to finish, remove the commercially purchased polystyrene nanospheres (170 nm diameter, 10% solids, 0.5% sodium dodecyl sulfate) from the refrigerator (4 &#176;C). Allow the container to warm to room temperature.</li>
	<li>Briefly vortex (1 min) and sonicate (35 kHz, 1 min) the polystyrene nanospheres to minimize nanosphere agglomeration.</li>
	<li>Into a clean glass vial, measure 1.0 mL of the 170 nm polystyrene nanospheres and add 1.0 mL of H2O to obtain a 5% solids colloidal suspension.</li>
	<li>After 15 min, stop the flow of O2, vent the vacuum chamber, and remove the freshly cleaned wafers.</li>
</ol>
2. Spin-coating of Polystyrene Nanospheres Template
<ol>
	<li>Unload the cleaned silicon wafers from the plasma etcher. Then mount a 2 in. wafer onto the spin-coater. Ensure it is properly centered and that the O-ring is clear of any debris. Initiate the vacuum and ensure that the wafer is securely attached to the stage.</li>
	<li>Set the spin parameters of the spin-coater. These parameters vary based on the nanosphere size. For a solution of 5% 170 nm nanospheres, set the spin-coater to a 1 step process with a spin time of 1 min, a speed of 3,000 rpm, and an acceleration of 2,000 rpm/s.</li>
	<li>Using a disposable syringe, withdraw ~ 1 mL of colloidal suspension from the vial. Set aside the vial. Take a 5 &#181;m syringe filter and place it at the end of the syringe. Depress the syringe until a droplet of suspension clears the tip. The filter removes undesirable aggregates and particulate that can significantly reduce film quality.</li>
	<li>Deposit enough suspension directly on the center of the wafer such that approximately 2/3 of the surface is covered. Try to minimize bubbles because those can impact film quality. Close the spin-coater lid and press Start. During this process, it may be possible to see thin film interference effects on the surface of the wafer as the nanospheres self-assemble. This will vary based on the nanosphere diameter.</li>
	<li>Remove the spin-coated wafer after deactivating the vacuum. Wipe the bowl and lid of the spin-coater to remove excess nanospheres.</li>
</ol>
3. Film Quality Assessment and Preparation for Etching
<ol>
	<li>Visually assess the quality of the self-assembled film by looking for noticeable defects such as streaks or holes that may have been caused by particulate during the spin-coating process.</li>
	<li>Assess the film quality by placing the wafer under an optical microscope. Grain boundaries and some defects are normal. If the wafer has large uncoated areas or obvious multilayers, it is necessary to adjust the spin parameters to obtain a more uniform film. Electron microscopy may also be used to assess film quality.</li>
	<li>Turn on the light source to the microscope and focus onto the surface of the silicon wafer using a 20X objective. Assess the quality at several spots throughout the wafer to ensure uniformity.</li>
	<li>The final film quality check is to use scanning electron microscopy (SEM) to visualize the nanosphere self-assembly at the nanoscale. It is possible to assess the degree of multilayers, holes, and grain boundaries/defects across small portions of the wafer relatively quickly using this technique.</li>
	<li>Once a sufficient film has been obtained, place the wafer into an oven (107 &#176;C) for 2 min to anneal the self-assembled nanospheres. This helps encourage adhesion to the substrate and yields a better nanopatterned surface after etching.</li>
</ol>
4. Etching, Metal Deposition, and Optical Characterization
<ol>
	<li>Load the annealed wafer into the plasma etcher and initiate the pump down process.</li>
	<li>Once the vacuum chamber reaches at least 75 mTorr, begin the flow of O2 (20 sccm) gas and wait for the pressure to stabilize. Initiate the RF plasma (75 W) for 165 s.</li>
	<li>Once the RF plasma cycle has completed, stop the flow of O2 and vent the chamber.</li>
	<li>The substrate is now etched and ready for metal deposition. Transport the sample to a sputter coater and deposit a thin (20 nm) layer of gold. Varying deposition angles may be used to alter the optical properties of the nanocups. In this case, metal deposition normally incident to the substrate was performed.</li>
	<li>After the metal deposition, the substrate may be characterized using optical spectroscopy. Focus the microspectrophotometer onto the surface of the metallized substrate and measure the reflectance spectra. For 170 nm etched nanosphere arrays, the LSPR was at 615 nm.</li>
	<li>Using pressure-sensitive adhesive tape, gently place the film in contact with the substrate. It may be necessary to remove any air bubbles that formed at the interface using a tweezers.</li>
	<li>Once the tape is in contact with the substrate, the tape may be immediately peeled off to remove the nanocups from the substrate surface. Gently peel back the tape and the result is a flexible and conformal film of gold nanocups.</li>
</ol>

<ol>
	<li>Fang, Y., Sun, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoplasmonic+waveguides:+towards+applications+in+integrated+nanophotonic+circuits.">Nanoplasmonic waveguides: towards applications in integrated nanophotonic circuits.</a> Light Sci Appl. 4, e294 (2015).</li><li>Li, J. F., Anema, J. R., Wandlowski, T., Tian, Z. Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dielectric+shell+isolated+and+graphene+shell+isolated+nanoparticle+enhanced+Raman+spectroscopies+and+their+applications.">Dielectric shell isolated and graphene shell isolated nanoparticle enhanced Raman spectroscopies and their applications.</a> Chemical Society Reviews. 44 (23), 8399-8409 (2015).</li><li>Wang, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Large+Area+Plasmonic+Color+Palettes+with+Expanded+Gamut+Using+Colloidal+Self-Assembly.">Large Area Plasmonic Color Palettes with Expanded Gamut Using Colloidal Self-Assembly.</a> ACS Photonics. , (2016).</li><li>Taylor, A. B., Michaux, P., Mohsin, A. S. M., Chon, J. W. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electron-beam+lithography+of+plasmonic+nanorod+arrays+for+multilayered+optical+storage.">Electron-beam lithography of plasmonic nanorod arrays for multilayered optical storage.</a> Optics Express. 22 (11), 13234-13243 (2014).</li><li>Endo, H., Mochizuki, Y., Tamura, M., Kawai, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+and+Functionalization+of+Periodically+Aligned+Metallic+Nanocup+Arrays+Using+Colloidal+Lithography+with+a+Sinusoidally+Wrinkled+Substrate.">Fabrication and Functionalization of Periodically Aligned Metallic Nanocup Arrays Using Colloidal Lithography with a Sinusoidally Wrinkled Substrate.</a> Langmuir. 29 (48), 15058-15064 (2013).</li><li>Wang, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Symmetry+breaking+in+individual+plasmonic+nanoparticles.">Symmetry breaking in individual plasmonic nanoparticles.</a> Proceedings of the National Academy of Sciences. 103 (29), 10856-10860 (2006).</li><li>Wollet, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plasmon+hybridization+in+stacked+metallic+nanocups.">Plasmon hybridization in stacked metallic nanocups.</a> Optical Materials Express. 2 (10), 1384-1390 (2012).</li><li>Duempelmann, L., Casari, D., Luu-Dinh, A., Gallinet, B., Novotny, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Color+Rendering+Plasmonic+Aluminum+Substrates+with+Angular+Symmetry+Breaking.">Color Rendering Plasmonic Aluminum Substrates with Angular Symmetry Breaking.</a> ACS Nano. 9 (12), 12383-12391 (2015).</li><li>King, N. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Angle-+and+Spectral-Dependent+Light+Scattering+from+Plasmonic+Nanocups.">Angle- and Spectral-Dependent Light Scattering from Plasmonic Nanocups.</a> ACS Nano. 5 (9), 7254-7262 (2011).</li><li>Mirin, N. A., Halas, N. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Light-Bending+Nanoparticles.">Light-Bending Nanoparticles.</a> Nano Letters. 9 (3), 1255-1259 (2009).</li><li>Eggleston, M. S., Messer, K., Zhang, L., Yablonovitch, E., Wu, M. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+antenna+enhanced+spontaneous+emission.">Optical antenna enhanced spontaneous emission.</a> Proceedings of the National Academy of Sciences. 112 (6), 1704-1709 (2015).</li><li>Bora, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plasmonic+black+metals+in+resonant+nanocavities.">Plasmonic black metals in resonant nanocavities.</a> Applied Physics Letters. 102 (25), 251105 (2013).</li><li>Akselrod, G. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Efficient+Nanosecond+Photoluminescence+from+Infrared+PbS+Quantum+Dots+Coupled+to+Plasmonic+Nanoantennas.">Efficient Nanosecond Photoluminescence from Infrared PbS Quantum Dots Coupled to Plasmonic Nanoantennas.</a> ACS Photonics. , (2016).</li></ol>

The authors have nothing to disclose.

This protocol demonstrates a low-cost and efficient technique for fabricating periodic arrays of plasmonic gold nanocups. This technique is particularly advantageous because it avoids serial top-down processes such as electron beam lithography or focused ion beam milling. The presented technique shows that commercially available polymeric nanospheres can be self-assembled in a straightforward manner to serve as a nano-sized template for further processing.
Modifications and Troubleshooting:
If the film quality is poor, it may be necessary to pre-filter the nanosphere solution. Here, we used a 5 &#181;m syringe filter but it may be advantageous to use syringe filters down to 0.22 &#181;m, depending on the nanosphere diameter. The etching process can be adjusted to get the desired optical response. The quality of the etch should be evaluated using SEM to ensure non-touching and evenly spaced polymeric nanospheres. Once the etch parameters have been established for a particular system, it is possible to reproducibly manufacture several wafers in a batch with similar plasmon resonances. Metal deposition at varying angles will tune the nanocup&#39;s anisotropic optical properties.
Critical Steps:
The nanospheres must be properly stored and handled to achieve high quality films. Allow the nanospheres to warm to room temperature and briefly vortex followed by sonication to help ensure monodisperse nanospheres. The silicon substrate must be plasma cleaned and used immediately in order to ensure a highly hydrophilic surface. Finally, the self-assembled film should both be inspected by eye as well as through optical microscopy. Minimal defects should be observed, otherwise it will be necessary to adjust spin conditions.
Limitations:
This is a highly scalable technique but it does have several limitations that must be kept in mind. The self-assembly process is excellent at producing large arrays of nanospheres but it is challenging to fabricate nanostructures with three-dimensional anisotropy. Complex nanostructures are best fabricated by electron beam lithography or focused ion beam milling. These nanostructures, however, do not scale well and are exceedingly expensive to manufacture.
Overall, this protocol demonstrates how to fabricate nanoplasmonic films. Nanoplasmonic films have a variety of applications in areas such as nonlinear optical materials7, photovoltaics12, and light emitting diodes13.

The emergence of plasmonics in conjunction with improved nanofabrication and synthesis techniques have brought about a wide variety of exciting technologies such as sub-diffraction limited circuity, enhanced chemical detection, and optical sensing1,2,3. In this protocol, we demonstrate a scalable and relatively low-cost technique capable of fabricating nanopatterned plasmonic substrates using commercially available polymeric nanospheres and an etching step followed by metal deposition. Unlike other techniques for fabricating nanopatterned substrates, such as electron beam lithography4, this technique can quickly and efficiently be scaled to 300 mm wafers and beyond with minimal effort and uses a transfer step to produce flexible and conformal films5.
Since the Roman era, we have known that certain metals such as gold and silver can have brilliant optical properties when they are finely divided. Today, we understand that these metal particles exhibit an effect called the &#34;localized surface plasmon resonance&#34; (LSPR) when their dimensions approach the nanoscale. LSPR is analogous to a standing wave in which weakly bound electrons found in the metal oscillate coherently when light of certain frequencies illuminates the metal particles. Anisotropic nanostructures are of particular interest because unique optical resonances can emerge as a result of symmetry breaking6,7,8.
The illumination of half-shell (nanocup) structures with light can excite electric dipole or magnetic dipole plasmon modes, depending on factors such as the deposition angle of the metal, the orientation of the substrate with respect to the incident light, and the polarization of the incident light9. Nanocups have often been considered analogous to three-dimensional split-ring resonators, in which the resonance frequency can approximated as an LC-oscillator10,11. The resonance frequency for the size of polymeric nanospheres used here (170 nm), the amount of deposited gold (20 nm), and the etch rates yield resonance frequencies spanning the visible and near-IR.
The optical properties of the gold nanocups can be measured either in transmission or reflection, depending on the substrate used for spin-coating. In the presented protocol, we chose to use 2 in. silicon wafers as the substrate and perform reflectance measurements after metal deposition. The measurements were performed using a microscope coupled to a dispersive spectrometer with a halogen light source. We have also had success with using glass substrates, allowing for both transmission and reflection measurements immediately following the metal deposition. Furthermore, this technique can easily be scaled and is not limited to 2 in. wafers. Due to the wide commercial availability of high-quality monodisperse polymeric nanospheres, it is straightforward to tune the optical properties of these structures by simply starting with differently sized nanospheres.
In this protocol, a technique to fabricate anisotropic half-shell (or nanocup) gold nanostructures using a method called colloidal lithography is demonstrated. Colloidal lithography uses self-assembly of highly monodisperse polymeric nanosphere to quickly pattern a substrate that can be further processed into a plasmonic substrate after sputter coating a thin layer of gold. Likewise, it is possible to tune the anisotropy of the substrate by tilting the sample substrate during metal deposition. The resulting structures are polarization-sensitive because of the anisotropy of the formed nanostructure. Here, we demonstrate one particular case and perform optical characterization and lift-off to transfer the structures to a transparent, flexible film.

<table><tbody><tr><td>Polystyrene microspheres</td><td>Bangs Laboratories, Inc.</td><td>PS02N</td><td>170 nm &amp;ndash; 580 nm diameter</td></tr><tr><td>Silicon wafers</td><td>El-CAT, Inc.</td><td>3489</td><td>300 mm thick, one side polished [100]</td></tr><tr><td>Adhesive tape</td><td>3M</td><td>Scotch 600</td><td></td></tr><tr><td>Spin coater</td><td>Laurell</td><td>WS-650-23B</td><td></td></tr><tr><td>Plasma etcher</td><td>Nordson March&amp;nbsp;</td><td>AP-600</td><td></td></tr><tr><td>Microspectrophotometer</td><td>CRAIC</td><td>380-PV</td><td></td></tr><tr><td>Sonicator</td><td>VWR</td><td>97043-932</td><td></td></tr><tr><td>Scintillation vials</td><td>Wheaton</td><td>986734</td><td></td></tr><tr><td>5 um syringe filter</td><td>Millex</td><td>SLSV025LS</td><td></td></tr><tr><td>Oxygen gas</td><td>Oxarc</td><td>PO249&amp;nbsp;</td><td>Industrial Grade 99.5% purity</td></tr><tr><td>Vaccum pump</td><td>Kurt J. Lesker</td><td>Edwards 28</td><td></td></tr><tr><td>Disposable syringes</td><td>Air Tite Products Co.</td><td>14-817-25</td><td>1 mL capacity</td></tr><tr><td>Water</td><td>Sigma-Aldrich</td><td>W4502</td><td></td></tr></tbody></table>

fabrication of periodic gold nanocup arrays using colloidal lithography

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing in combination with new nanofabrication techniques. A plasmon is a quantum of charge density oscillation that lends nanoscale metals such as gold and silver unique optical properties. In particular, gold and silver nanoparticles exhibit localized surface plasmon resonances-collective charge density oscillations on the surface of the nanoparticle-in the visible spectrum. Here, we focus on the fabrication of periodic arrays of anisotropic plasmonic nanostructures. These half-shell (or nanocup) structures can exhibit additional unique light-bending and polarization-dependent optical properties that simple isotropic nanostructures cannot. Researchers are interested in the fabrication of periodic arrays of nanocups for a wide variety of applications such as low-cost optical devices, surface-enhanced Raman scattering, and tamper indication. We present a scalable technique based on colloidal lithography in which it is possible to easily fabricate large periodic arrays of nanocups using spin-coating and self-assembled commercially available polymeric nanospheres. Electron microscopy and optical spectroscopy from the visible to near-infrared (near-IR) was performed to confirm successful nanocup fabrication. We conclude with a demonstration of the transfer of nanocups to a flexible, conformal adhesive film.

Gold nanocups were prepared using 170 nm diameter polystyrene nanospheres. After annealing for 2 min at 107 &#176;C and etching with a 75 W, 20 sccm O2 plasma for 165 s, the resulting film was characterized using SEM (Figure 1). To evaluate the quality of the spin-casted film, optical microscopy-in addition to visual inspection-may be used (Figure 2). High-quality films should be essentially free of defects. Grain boundaries are typically observed even in high-quality films, but with careful attention to detail, it is possible to nearly eliminate point defects. Deposition of 20 nm of gold using sputter coating resulted in a plasmonically-active film and was characterized using optical reflectance spectroscopy (Figure 3). The plasmonic film was transferred from the rigid silicon substrate to a flexible film using commonly available adhesive tape. The tape was placed in contact with the plasmonically-active film and allowed to adhere to the film for 1 min. The tape was then gently removed from the substrate, resulting in a transfer of the gold nanocups to the film (Figure 4).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56204/56204fig1.jpg" /> 
Figure 1: Representative scanning electron micrographs of self-assembled nanostructures fabricated using colloidal lithography. (a) Self-assembled monolayer of a typical array of polystyrene nanospheres before etching, (b) periodically spaced polystyrene nanospheres after annealing and etching (75 W, 20 sccm O2 for 165 s), and (c) periodically spaced gold nanocups with 20 nm of gold (Au) deposited at a normal incidence with respect to the substrate. Scale bar: 100 nm. Magnification: 100 kX. <a href="https://cloudflare-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/56204/56204fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56204/56204fig2.jpg" /> 
Figure 2: Optical microscopy of self-assembled films to evaluate quality. (a) Film with good monolayer coverage and minimal defects. Grain boundaries are observed with minimal defects and holes. (b) Film consisting of monolayer and multilayer regions. (c) Film with major defects and incomplete monolayer coverage. Scale bar: 20 &#181;m. Magnification: 20X. <a href="https://cloudflare-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/56204/56204fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56204/56204fig3.jpg" /> 
Figure 3: Optical reflectance characterization of the fabricated gold nanocup array. Optical reflectance spectra showing a strong plasmonic resonance at ~ 615 nm. <a href="https://cloudflare-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/56204/56204fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56204/56204fig4.jpg" /> 
Figure 4: Resulting flexible, transparent film after peeling gold nanocups from sacrificial silicon (Si) wafer. (a) Schematic of lift-off procedure. (b) Optical image of peeled film. (c) Photograph focused past the film to demonstrate transparency. (d) Representative optical transmission spectra of a film after lift-off. <a href="https://cloudflare-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/56204/56204fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography at JoVE.com

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

We demonstrate the fabrication of periodic gold nanocup arrays using colloidal lithographic techniques and discuss the importance of nanoplasmonic films.

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing ...

fabrication-periodic-gold-nanocup-arrays-using-colloidal

Nanyang Technological University

Research

JoVE Journal

Engineering

7.1K Views.  Energy and Environment Directorate. We demonstrate the fabrication of periodic gold nanocup arrays using colloidal lithographic techniques and discuss the importance of nanoplasmonic films. 

Video: Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

A Technique to Functionalize and Self-assemble Macroscopic Nanoparticle-ligand Monolayer Films onto Template-free Substrates

This protocol describes a self-assembly technique to create macroscopic monolayer films composed of ligand-coated nanoparticles1,2. The simple, robust and scalable technique efficiently functionalizes metallic nanoparticles with thiol-ligands in a miscible water/organic solvent mixture allowing for rapid grafting of thiol groups onto the gold nanoparticle surface. The hydrophobic ligands on the nanoparticles then quickly phase separate the nanoparticles from the aqueous based suspension and confine them to the air-fluid interface. This drives the ligand-capped nanoparticles to form monolayer domains at the air-fluid interface.&#160; The use of water-miscible organic solvents is important as it enables the transport of the nanoparticles from the interface onto template-free substrates.&#160; The flow is mediated by a surface tension gradient3,4 and creates macroscopic, high-density, monolayer nanoparticle-ligand films.&#160; This self-assembly technique may be generalized to include the use of particles of different compositions, size, and shape and may lead to an efficient assembly method to produce low-cost, macroscopic, high-density, monolayer nanoparticle films for wide-spread applications.

A simple, robust and scalable technique to functionalize and self-assemble macroscopic nanoparticle-ligand monolayer films onto template-free substrates is described in this protocol.

This protocol describes a self-assembly technique to create macroscopic monolayer films composed of ligand-coated nanoparticles1,2. The simple, robust and ...

Chemistry

Synthesis, Assembly, and Characterization of Monolayer Protected Gold Nanoparticle Films for Protein Monolayer Electrochemistry

Colloidal gold nanoparticles protected with alkanethiolate ligands called monolayer protected gold clusters (MPCs) are synthesized and subsequently incorporated into film assemblies that serve as adsorption platforms for protein monolayer electrochemistry (PME). PME is utilized as the model system for studying electrochemical properties of redox proteins by confining them to an adsorption platform at a modified electrode, which also serves as a redox partner for electron transfer (ET) reactions. Studies have shown that gold nanoparticle film assemblies of this nature provide for a more homogeneous protein adsorption environment and promote ET without distance dependence compared to the more traditional systems modified with alkanethiol self-assembled monolayers (SAM).1-3 In this paper, MPCs functionalized with hexanethiolate ligands are synthesized using a modified Brust reaction4 and characterized with ultraviolet visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), and proton (1H) nuclear magnetic resonance (NMR). MPC films are assembled on SAM modified gold electrode interfaces by using a "dip cycle" method of alternating MPC layers and dithiol linking molecules. Film growth at gold electrode is tracked electrochemically by measuring changes to the double layer charging current of the system. Analogous films assembled on silane modified glass slides allow for optical monitoring of film growth and cross-sectional TEM analysis provides an estimated film thickness. During film assembly, manipulation of the MPC ligand protection as well as the interparticle linkage mechanism allow for networked films, that are readily adaptable, to interface with redox protein having different adsorption mechanism. For example, Pseudomonas aeruginosa azurin (AZ) can be adsorbed hydrophobically to dithiol-linked films of hexanethiolate MPCs and cytochrome c (cyt c) can be immobilized electrostatically at a carboxylic acid modified MPC interfacial layer. In this report, we focus on the film protocol for the AZ system exclusively. Investigations involving the adsorption of proteins on MPC modified synthetic platforms could further the understanding of interactions between biomolecules and man-made materials, and consequently aid the development of biosensor schemes, ET modeling systems, and synthetic biocompatible materials.5-8

Alkanethiolate stabilized gold colloids known as monolayer protected clusters (MPCs) are synthesized, characterized, and assembled into thin films as an adsorption interface for protein monolayer electrochemistry of simple redox protein like Pseudomonas aeruginosa azurin (AZ) and cytochrome c (cyt c).

Colloidal gold nanoparticles protected with alkanethiolate ligands called monolayer protected gold clusters (MPCs) are synthesized and subsequently ...

Bioengineering

Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic nanoscale patch antennas. This includes a detailed procedure for the fabrication of thin films with a well-controlled thickness over macroscopic areas using layer-by-layer deposition of polyelectrolyte polymers, namely poly(allylamine) hydrochloride (PAH) and polystyrene sulfonate (PSS). These polyelectrolyte spacer layers serve as a dielectric gap in between silver nanocubes and a gold film. By controlling the size of the nanocubes or the gap thickness, the plasmon resonance can be tuned from about 500 nm to 700 nm. Next, we demonstrate how to incorporate organic sulfo-cyanine5 carboxylic acid (Cy5) dye molecules into the dielectric polymer gap region of the nanopatch antennas. Finally, we show greatly enhanced fluorescence of the Cy5 dyes by spectrally matching the plasmon resonance with the excitation energy and the Cy5 absorption peak. The method presented here enables the fabrication of plasmonic nanopatch antennas with well-controlled dimensions utilizing colloidal synthesis and a layer-by-layer dip-coating process with the potential for low cost and large-scale production. These nanopatch antennas hold great promise for practical applications, for example in sensing, ultrafast optoelectronic devices and for high-efficiency photodetectors.

A protocol for the colloidal synthesis of silver nanocubes and fabrication of plasmonic nanoscale patch antennas with sub-10 nm gaps is presented.

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic ...

Microfabrication of Nanoporous Gold Patterns for Cell-material Interaction Studies

Nanostructured materials with feature sizes in tens of nanometers have enhanced the performance of several technologies, including fuel cells, biosensors, biomedical device coatings, and drug delivery tools. Nanoporous gold (np-Au), produced by a nano-scale self-assembly process, is a relatively new material that exhibits large effective surface area, high electrical conductivity, and catalytic activity. These properties have made np-Au an attractive material to scientific community. Most studies on np-Au employ macro-scale specimens and focus on fundamental science of the material and its catalytic and sensor applications. The macro-scale specimens limit np-Au's potential in miniaturized systems, including biomedical devices. In order to address these issues, we initially describe two different methods to micropattern np-Au thin films on rigid substrates. The first method employs manually-produced stencil masks for creating millimeter-scale np-Au patterns, while the second method uses lift-off photolithography to pattern sub-millimeter-scale patterns. As the np-Au thin films are obtained by sputter-deposition process, they are compatible with conventional microfabrication techniques, thereby amenable to facile integration into microsystems. These systems include electrically-addressable biosensor platforms that benefit from high effective surface area, electrical conductivity, and gold-thiol-based surface bioconjugation. We describe cell culture, immunostaining, and image processing techniques to quantify np-Au's interaction with mammalian cells, which is an important performance parameter for some biosensors. We expect that the techniques illustrated here will assist the integration of np-Au in platforms at various length-scales and in numerous applications, including biosensors, energy storage systems, and catalysts.

We report on techniques to micropattern nanoporous gold thin films via stencil printing and photolithography, as well as methods to culture cells on the microfabricated patterns. In addition, we describe image analysis methods to characterize morphology of the material and the cultured cells using scanning electron and fluorescence microscopy techniques.

Nanostructured materials with feature sizes in tens of nanometers have enhanced the performance of several technologies, including fuel cells, biosensors, ...

Large Area Substrate-Based Nanofabrication of Controllable and Customizable Gold Nanoparticles Via Capped Dewetting

Recent scientific advances in the utilization of metallic nanoparticle for enhanced energy conversion efficiency, improved optical device performance, and high-density data storage have demonstrated the potential benefit of their use in industrial applications. These applications require precise control over nanoparticle size, spacing, and sometimes shape. These requirements have resulted in the use of time and cost intensive processing steps to produce nanoparticles, thus making the transition to industrial application unrealistic. This protocol will resolve this issue by providing a scalable and affordable method for the large-area production of nanoparticle films with improved nanoparticle control compared to the current techniques. In this article, the process will be demonstrated with gold, but other metals can also be used.

This protocol details a novel nano-manufacturing technique that can be used to make controllable and customizable nanoparticle films over large areas based on the self-assembly of dewetting of capped metal films.

Recent scientific advances in the utilization of metallic nanoparticle for enhanced energy conversion efficiency, improved optical device performance, and ...

A Femtoliter Droplet Array for Massively Parallel Protein Synthesis from Single DNA Molecules

Advances in spatial resolution and detection sensitivity of scientific instrumentation make it possible to apply small reactors for biological and chemical research. To meet the demand for high-performance microreactors, we developed a femtoliter droplet array (FemDA) device and exemplified its application in massively parallel cell-free protein synthesis (CFPS) reactions. Over one million uniform droplets were readily generated within a finger-sized area using a two-step oil-sealing protocol. Every droplet was anchored in a femtoliter microchamber composed of a hydrophilic bottom and a hydrophobic sidewall. The hybrid hydrophilic-in-hydrophobic structure and the dedicated sealing oils and surfactants are crucial for stably retaining the femtoliter aqueous solution in the femtoliter space without evaporation loss. The femtoliter configuration and the simple structure of the FemDA device allowed minimal reagent consumption. The uniform dimension of the droplet reactors made large-scale quantitative and time-course measurements convincing and reliable. The FemDA technology correlated the protein yield of the CFPS reaction with the number of DNA molecules in each droplet. We streamlined the procedures about the microfabrication of the device, the formation of the femtoliter droplets, and the acquisition and analysis of the microscopic image data. The detailed protocol with the optimized low running cost makes the FemDA technology accessible to everyone who has standard cleanroom facilities and a conventional fluorescence microscope in their own place.

The overall goal of the protocol is to prepare over one million ordered, uniform, stable, and biocompatible femtoliter droplets on a 1 cm2 planar substrate that can be used for cell-free protein synthesis.

Advances in spatial resolution and detection sensitivity of scientific instrumentation make it possible to apply small reactors for biological and ...

Gold Nanoparticle Synthesis

Gold nanoparticles (Au nanoparticles) that are ~12 nm in diameter were synthesized by rapidly injecting a solution of 150 mg (0.15 mmol) of tetrachloroauric acid in 3.0 g (3.7 mmol, 3.6 mL) of oleylamine (technical grade) and 3.0 mL of toluene into a boiling solution of 5.1 g (6.4 mmol, 8.7 mL) of oleylamine in 147 mL of toluene. While boiling and mixing the reaction solution for 2 hours, the color of the reaction mixture changed from clear, to light yellow, to light pink, and then slowly to dark red. The heat was then turned off, and the solution was allowed to gradually cool down to room temperature for 1 hour. The gold nanoparticles were then collected and separated from the solution using a centrifuge and washed three times; by vortexing and dispersing the gold nanoparticles in 10 mL portions of toluene, and then precipitating the gold nanoparticles by adding 40 mL portions of methanol and spinning them in a centrifuge. The solution was then decanted to remove any remaining byproducts and unreacted starting materials. Drying the gold nanoparticles in a vacuum environment produced a solid black pellet; which could be stored for long periods of time (up to one year) for later use, and then redissolved in organic solvents such as toluene.

A protocol for synthesizing ~12 nm diameter gold nanoparticles (Au nanoparticles) in an organic solvent is presented. The gold nanoparticles are capped with oleylamine ligands to prevent agglomeration. The gold nanoparticles are soluble in organic solvents such as toluene.

Gold nanoparticles (Au nanoparticles) that are ~12 nm in diameter were synthesized by rapidly injecting a solution of 150 mg (0.15 mmol) of ...

Versatile Technique to Produce a Hierarchical Design in Nanoporous Gold

The potential to generate variable pore sizes, simplistic surface modification, and a breadth of commercial uses in the fields of biosensors, actuators, drug loading and release, and the development of catalysts have unquestionably accelerated the usage of nanoporous gold (NPG)-based nanomaterials in research and development. This article describes the process of the generation of hierarchical bimodal nanoporous gold (hb-NPG) by employing a step-wise procedure involving electrochemical alloying, chemical dealloying techniques, and annealing to create both macro- and mesopores. This is done to improve the utility of NPG by creating a bicontinuous solid/void morphology. The area available for surface modification is enhanced by smaller pores, while molecular transport benefits from the network of larger pores. The bimodal architecture, which is the result of a series of fabrication steps, is visualized using scanning electron microscopy (SEM) as a network of pores that are less than 100 nm in size and connected by ligaments to larger pores that are several hundred nanometers in size. The electrochemically active surface area of the hb-NPG is assessed using cyclic voltammetry (CV), with a focus on the critical roles that both dealloying and annealing play in creating the necessary structure. The adsorption of different proteins is measured by solution depletion technique, revealing the better performance of hb-NPG in terms of protein loading. By changing the surface area to volume ratio, the created hb-NPG electrode offers tremendous potential for biosensor development. The manuscript discusses a scalable method to create hb-NPG surface structures, as they offer a large surface area for the immobilization of small molecules and improved transport pathways for faster reactions.

Nanoporous gold with a hierarchical and bimodal pore size distribution can be produced by combining electrochemical and chemical dealloying. The composition of the alloy can be monitored via EDS-SEM examination as the dealloying process advances. The material's loading capacity can be determined by studying protein adsorption onto the material.

The potential to generate variable pore sizes, simplistic surface modification, and a breadth of commercial uses in the fields of biosensors, actuators, ...

Large-area Scanning Probe Nanolithography Facilitated by Automated Alignment and Its Application to Substrate Fabrication for Cell Culture Studies

Scanning probe microscopy has enabled the creation of a variety of methods for the constructive (&#39;additive&#39;) top-down fabrication of nanometer-scale features. Historically, a major drawback of scanning probe lithography has been the intrinsically low throughput of single probe systems. This has been tackled by the use of arrays of multiple probes to enable increased nanolithography throughput. In order to implement such parallelized nanolithography, the accurate alignment of probe arrays with the substrate surface is vital, so that all probes make contact with the surface simultaneously when lithographic patterning begins. This protocol describes the utilization of polymer pen lithography to produce nanometer-scale features over centimeter-sized areas, facilitated by the use of an algorithm for the rapid, accurate, and automated alignment of probe arrays. Here, nanolithography of thiols on gold substrates demonstrates the generation of features with high uniformity. These patterns are then functionalized with fibronectin for use in the context of surface-directed cell morphology studies.

Here we present a protocol for wide-area scanning probe nanolithography enabled by the iterative alignment of probe arrays, as well as the utilization of lithographic patterns for cell-surface interaction studies.

Scanning probe microscopy has enabled the creation of a variety of methods for the constructive (&#39;additive&#39;) top-down fabrication of ...

Fabrication of Gradient Nanopattern by Thermal Nanoimprinting Technique and Screening of the Response of Human Endothelial Colony-forming Cells

Nanotopography can be found in various extracellular matrices (ECMs) around the body and is known to have important regulatory actions upon cellular reactions. However, it is difficult to determine the relation between the size of a nanostructure and the responses of cells owing to the lack of proper screening tools. Here, we show the development of reproducible and cost-effective gradient nanopattern plates for the manipulation of cellular responses. Using anodic aluminum oxide (AAO) as a master mold, gradient nanopattern plates with nanopillars of increasing diameter ranges [120-200&#8239;nm (GP 120/200), 200-280&#8239;nm (GP 200/280), and 280-360&#8239;nm (GP 280/360)] were fabricated by a thermal imprinting technique. These gradient nanopattern plates were designed to mimic the various sizes of nanotopography in the ECM and were used to screen the responses of human endothelial colony-forming cells (hECFCs). In this protocol, we describe the step-by-step procedure of fabricating gradient nanopattern plates for cell engineering, techniques of cultivating hECFCs from human peripheral blood, and culturing hECFCs on nanopattern plates.

Here, we present a protocol for the fabrication of gradient nanopattern plates via thermal nanoimprinting and the method of screening responses of human endothelial progenitor cells to the nanostructures. By using the described technology, it is possible to produce a scaffold that can manipulate cell behavior by physical stimuli.

Nanotopography can be found in various extracellular matrices (ECMs) around the body and is known to have important regulatory actions upon cellular ...