This work was supported by the Air Force Office of Scientific Research Young Investigator Research Program (AFOSR, Grant. No. FA9550-15-1-0301).

Caution: Several chemicals (such as concentrated nitric acid (15.698 M HNO3) and hydrochloric acid (6 M HCl)) used in these procedures are hazardous. Proper gloves, eye protection and other safety equipment must be used. Please refer to the materials safety datasheets (MSDS) of all chemicals before use.
1. Nanocube Synthesis
<ol>
	<li>Preparation of reagents 
	Note: Ethylene glycol (EG) must be anhydrous. Close the EG container&#39;s cap whenever it is not used to prevent water absorption. Silver trifluoroacetate (AgC2F3O2) is very sensitive to light therefore the AgC2F3O2 solution is prepared in the last step.
	<ol>
		<li>Prepare 1.3 mM sodium hydrosulfide hydrate (NaSH) solution by dissolving 1 mg of NaSH in 13.5 ml of EG.</li>
		<li>Prepare 20 mg/ml poly vinylpyrrolidone (PVP) solution by dissolving 0.1 g of PVP in 5 ml of EG.</li>
		<li>Prepare 3 mM hydrochloric solution by mixing 2.5 &#181;l of 6.0 M liquid HCl solution with 4.9975 ml of EG.</li>
		<li>Prepare AgC2F3O2 solution by dissolving 0.1 g of AgC2F3O2 in 0.8 ml of EG.</li>
	</ol>
	</li>
	<li>Equipment setup
	<ol>
		<li>Clean the round-bottom flask (RBF) and its cap with concentrated (70%, 15.698 M) nitric acid HNO3. Fill the RBF with HNO3 and put the cap on for 30 min. Ensure the cap touches the acid.</li>
		<li>After HNO3 acid, clean the RBF and cap again with clean de-ionized (DI) water. Use clean nitrogen gas to dry the RBF and cap afterwards. The RBF and its cap must be clean and dry.</li>
		<li>Clean a magnetic stirring bar by immersing it in HNO3 for 30 min. After HNO3, clean it again with DI water and dry it with clean nitrogen gas.</li>
		<li>Prepare a heating bath. Place a silicone fluid bath (shown in Figure 1A) on top of a stirring hotplate with a well-controlled temperature. Use an external thermometer to monitor the fluid bath temperature. Set the temperature to 150 &#176;C and the stirring speed to 260 rpm.</li>
		<li>Mount the RBF with a clamp as shown in Figure 1B. Place the magnetic stirring bar (prepared in step 1.2.3) into the RBF.</li>
	</ol>
	</li>
	<li>Synthesis procedure
	<ol>
		<li>Dip the RBF into the heating bath (approximately 10 mm deep into the liquid; see Figures 1A-1B).</li>
		<li>Use a micropipette to place 10 ml of EG solution into the RBF. Put the cap on the RBF and wait for 20 min. The purpose of this step is to clean the RBF again, this time with EG.</li>
		<li>After 20 min, remove the cap and then lift the RBF out of the heating bath, pour the 10 ml of EG into a disposal container. Note: The EG solution is hot (150 &#176;C) and it is recommended to take the entire clamp out (Figure 1B). Make sure that the magnetic stirring bar (see step 1.2.5) does not fall off.</li>
		<li>Put the RBF back into the heating bath (see step 1.3.1).</li>
		<li>Use a micropipette to place 5 ml of EG into the RBF and put the cap on.</li>
		<li>Wait for 5 min.</li>
		<li>Take the RBF&#39;s cap off, use a micropipette to place 60 &#181;l of NaSH (as prepared in step 1.1.1 above) into the RBF. Put the cap back on.</li>
		<li>Wait for 2 min.</li>
		<li>Take the RBR&#39;s cap off, use a micropipette to place 500 &#181;l of the HCl solution (as prepared in step 1.1.3 above) into the RBF.</li>
		<li>Immediately after the previous step, use a micropipette to place 1.25 ml of the PVP solution (as prepared in step 1.1.2 above) into the RBF. Put the cap back on.</li>
		<li>Wait for 2 min.</li>
		<li>Take the RBF&#39;s cap off, use a micropipette to place 400 &#181;l of the AgC2F3O2 solution (as prepared in step 1.1.4 above) into the RBF. Put the cap back on.</li>
		<li>Wait for 2.5 hr. Ag nanocubes are forming during this step. If possible, during this time, reduce the room light to a minimum.</li>
		<li>After 2.5 hr, turn the heater off but leave the stirring on to avoid the fluid burning on the bottom. Use the clamp (shown in Figure 1B) to raise the RBF above the heating bath. Remove the cap.</li>
		<li>Remove the RBF from the heating bath such that it will cool off faster. After ~20 min, add 5 ml of acetone into the RBF. Vortex it in order to mix the solutions well. In the end, the total volume of the solution is 12 ml. See Figure 2A.</li>
		<li>Use a micropipette and transfer the final solution to eight smaller 1.5 ml plastic tubes.</li>
		<li>Centrifuge these eight tubes at a speed of 5150 x g for 10 min. As a result, all the Ag nanocubes will be at the bottom of the tubes. Use a micropipette to remove the top supernatant, leaving ~100 &#181;l at the bottom of each tube.</li>
		<li>Fill 1 ml of DI water into each of these tubes (obtained from step 1.3.17). Vortex and sonicate (5 min) the tubes. Nanocubes are now suspended in mainly DI water.</li>
		<li>Centrifuge again the eight tubes prepared in step 1.3.18 at 5,150 x g for 5 min. All the Ag nanocubes will be at the bottom of the tubes. Use a micropipette to remove the top supernatant, leaving around 100 &#181;l at the bottom of each tube.</li>
		<li>Fill 1 ml of DI water into each of the tubes obtained from step 1.3.19. Vortex and sonicate the tubes. Nanocubes are now suspended in DI water. The final nanocube solution obtained from this synthesis is shown in Figure 2B as an example.</li>
	</ol>
	</li>
</ol>
2. Gold Film Evaporation
Note: An electron-beam evaporator was used to deposit gold (Au) films onto purchased cleanroom cleaned slides, with chromium (Cr) acting as an adhesion layer. The evaporation process takes place inside a vacuum chamber, enabling the molecules to evaporate freely in the chamber and then sublimate on the substrate. The operation procedure is:
<ol>
	<li>Vent the chamber, by pressing &#34;Auto Vent&#34;.</li>
	<li>Open chamber door and load substrates in the dome.</li>
	<li>Close the door and pump down by pressing &#34;Auto Pump&#34;, it takes approximately 1 hr for the chamber to pump down until the pressure is below 5&#215;10-6 Torr.</li>
	<li>Edit the recipe. Layer#1: Cr, thickness: 5 nm, deposition rate: 1 &#197;/sec; layer#2: Au, thickness: 50 nm, deposition rate: 2 &#197;/sec.</li>
	<li>Upon reaching the desired vacuum level, the deposition process of the first metal will automatically start by pressing &#34;Auto Run&#34;. 
	Note: During the deposition, the high voltage module is turned on and the voltage is 10 kV. Gun rotation module is turned on, and fixture rotation is 20 rpm. After the first layer finishes, the system will automatically move to the pocket location of the second metal and start deposition.</li>
	<li>After the entire process is complete, press &#34;Auto Vent&#34; to vent the chamber and take the sample out. 
	Note: The total thickness of the Au film was 50 nm and the surface roughness was measured using an atomic force microscope (AFM) yielding a typical root mean square (RMS) of 0.7 nm. No special treatment was performed of the purchased glass substrates before Au film deposition.</li>
</ol>
3. Deposition of PE Layers
<ol>
	<li>Preparation of reagents
	<ol>
		<li>For the sodium chloride (NaCl) solution, mix 29 g of NaCl powder with 500 ml of DI water.</li>
		<li>For the polystyrene sulfonate (PSS) solution, mix 29 g of NaCl powder with 500 ml of DI water then add 1.5 ml of the PSS stock solution.</li>
		<li>For the poly(allylamine) hydrochloride (PAH) solution, mix 29 g of NaCl powder with 500 ml of DI water then add 132 mg of PAH.</li>
	</ol>
	</li>
	<li>Layer-by-layer deposition 
	Note: PAH is slightly positively charged while PSS is slightly negatively charged. As the Au film fabricated in Section 2 above is slightly negatively charged, a PAH layer will be deposited first. The steps below will demonstrate in details how to deposit five PE layers: PAH/PSS/PAH/PSS/PAH.
	<ol>
		<li>First, deposit a PAH layer by immersing the gold film (fabricated in Section 2 above) into a PAH solution (prepared in step 3.1.3) for 5 min. This results in a PAH layer on top of the Au film with a thickness of ~1 nm.</li>
		<li>After 5 min, rinse the Au film + 1 PAH layer with clean DI water.</li>
		<li>Immerse the Au film + 1 PAH layer into a NaCl solution (prepared in step 3.1.1) for 1 min.</li>
		<li>Immerse the Au film + 1 PAH layer (after step 3.2.3) into a PSS solution for 5 min. This results in a PSS layer with a thickness of ~1 nm on top of the PAH layer.</li>
		<li>After 5 min, rinse the Au film + 1 PAH layer +1 PSS layer with clean DI water.</li>
		<li>Immerse the Au film + 1 PAH layer + 1 PSS layer into the NaCl solution for 1 min.</li>
		<li>Immerse the Au film + 1 PAH layer + 1 PSS layer into the PAH solution for 5 min. This results in another PAH layer with a thickness of ~1 nm on top of the PSS layer (prepared in step 3.2.4 above).</li>
		<li>After 5 min, rinse the Au film + 1 PAH +1 PSS + 1 PAH layers with clean DI water.</li>
		<li>Immerse the Au film + 1 PAH + 1 PSS + 1 PAH layers into the NaCl solution for 1 min.</li>
		<li>Immerse the Au film + 1 PAH + 1 PSS + 1 PAH layers into the PAH solution for 5 min. This results in a second PSS layer with a thickness of ~1 nm on top of the PAH layer (which was prepared in step 3.2.7 above).</li>
		<li>After 5 min, rinse the Au film + 1 PAH +1 PSS + 1 PAH + 1 PSS layers with clean DI water.</li>
		<li>Immerse the Au film + 1 PAH + 1 PSS + 1 PAH +1 PSS layers into the NaCl solution for 1 min.</li>
		<li>Immerse the Au film + 1 PAH + 1 PSS + 1 PAH +1 PSS layers into the PAH solution for 5 min. This results in another PAH layer with a thickness of ~1 nm on top of the PSS layer (which was prepared in step 3.2.10 above).</li>
		<li>Finally, rinse the Au film + 1 PAH + 1 PSS + 1 PAH +1 PSS + 1 PAH with DI water and dry the sample using clean nitrogen gas. 
		Note: The total thickness of the five PE layers was measured in air using a spectroscopic ellipsometer at incidence angles of 65&#176;, 70&#176;, and 75&#176;, yielding a thickness of 5.0 &#177; 0.1 nm.</li>
	</ol>
	</li>
</ol>
4. Deposition of Cy5 Dye Molecules
<ol>
	<li>Prepare a 25 &#181;M Cy5 solution with DI water as the solvent.</li>
	<li>Expose the surface of the sample (which has a series of five PE layers, as described in Section 3 above) to 100 &#181;l of a 25 &#181;M Cy5 solution for 10 min. First drop cast 100 &#181;l of the Cy5 solution (prepared in step 4.1) onto the sample surface and then place a cover slip on top of the solution drop. Cy5 molecules will incorporate into the top PE layers uniformly.</li>
	<li>After 10 min, rinse the sample with DI water and dry it with clean nitrogen gas.</li>
</ol>
5. Deposition of Nanocubes to Form Nanopatch Antennas (NPAs)
<ol>
	<li>Dilute the nanocube solution obtained from Section 1 by a factor of 1/100 using DI water to enable the optical study of individual NPAs.</li>
	<li>Use a micropipette to place a drop of 20 &#181;l of diluted nanocube solution (prepared in step 5.1) onto a clean cover slip. Place the sample (prepared in Section 4) in contact with the cover slip for 2 min. This results in the Ag nanocubes to be immobilized on the top terminal PAH layer because the nanocubes synthesized here are negatively charged and the top PAH layer is positively charged.</li>
	<li>After 2 min, rinse the sample with DI water and dry with clean nitrogen gas. 
	Note: Steps 5.1 - 5.3 describe a procedure to prepare a sample for optical studies of single NPAs using a dark field microscope (dark field scattering). To prepare a sample for reflectivity measurements, a similar procedure is applied except that in step 5.1 the original nanocube solution is diluted by a factor of 1/10 instead of 1/100.</li>
</ol>
6. Optical Measurements
Note: A custom built optical bright-/dark-field microscope is used in these measurements. The NPAs are illuminated by a white light source through a long working distance bright-/dark-field objective. The reflected/scattered light from the NPAs is collected by the same objective. A pinhole aperture (50 &#956;m diameter) is used at an image plane to select signal from an individual nanoantenna. A digital camera is used to capture a color image. A spectrometer and a charge coupled device (CCD) camera are used to acquire spectral data. For fluorescence measurements, a 633 nm continuous wave HeNe laser is used for excitation and the signal was spectrally filtered by a long pass filter.
<ol>
	<li>Dark field scattering spectrum of single NPAs
	<ol>
		<li>Under white light illumination, identify single NPAs on the sample that was prepared in Section 5. Under white light illumination, individual NPAs appear as bright, red or pink colored dots as shown in Figure 4C.</li>
		<li>Align a single NPA with the pinhole aperture using a translation stage. Ensure that the dark field scattering image of the NPA is still observed after the pinhole aperture.</li>
		<li>Acquire a spectrum of the scattered light from the NPA using the spectrometer and CCD camera with a 1 sec integration time. Because the aperture area (50 &#181;m) is much bigger than the physical size of the NPA (~ 75 nm) the spectrum contains scattered light from the NPA in addition to signal from the area surrounding the NPA.</li>
		<li>Move the sample to an area without any NPAs and acquire another spectrum with a 1 s integration time. This spectrum represents scattered light from the background.</li>
		<li>Remove the sample with NPAs and place a certified reflectance standard sample in the setup. Acquire a spectrum of the scattered light with a 0.1 sec integration time in order to normalize the signal from the NPA.</li>
		<li>Close the pinhole aperture and acquire a spectrum with a 0.1 sec integration time without any input signal. This spectrum represents the CCD dark counts.</li>
		<li>Calculate the final scattering spectrum of an NPA as follows: 
		<img alt="Equation 1" src="/files/ftp_upload/53876/53876eq1.jpg" /> 
		where INPA+background, Ibackground, Iwhite light, ICCD dark are the scattering spectra measured by steps 6.1.3, 6.1.4, 6.1.5 and 6.1.6, respectively.</li>
		<li>Extract the plasmon resonance of the NPA by calculating the centroid of the scattering resonance peak.16</li>
	</ol>
	</li>
	<li>Fluorescence enhancement of Cy5 molecules by single NPAs
	<ol>
		<li>Under white light illumination, identify single NPAs from the sample prepared in Section 5. In dark field, individual NPAs appear as bright, red or pink colored dots as shown in Figure 4C.</li>
		<li>Align a single NPA with the pinhole aperture using a translation stage. Ensure that the dark field scattering image of the NPA is detected by the camera placed after the pinhole aperture.</li>
		<li>Turn off the white light illumination and turn on the 633 nm continuous wave HeNe laser used for excitation.</li>
		<li>Place a 633 nm long pass laser filter in the optical path right before the entrance to the spectrometer in order to block any scattered laser light.</li>
		<li>Acquire a fluorescence spectrum of the emission from the Cy5 molecules using a 1 sec integration time. Because the aperture area (50 &#181;m) is much larger than the physical size of the NPA (~ 75 nm) this spectrum contains emission from both molecules embedded in the NPA as well as molecules surrounding the NPA.</li>
		<li>Move the sample to an area without any NPAs and acquire another spectrum with a 1 sec integration time. This spectrum represents the emission from molecules in the background, without any NPAs.</li>
		<li>Prepare a separate sample, which will be used as a control sample, following the procedure in Sections 3 and 4 where Cy5 molecules are incorporated with PE layers on top of a glass slide (without a Au film and Ag nanocubes).</li>
		<li>Acquire a fluorescence spectrum of the emission from Cy5 molecules on the control sample prepared in the previous step using a 10 sec integration time.</li>
		<li>Determine the fluorescence enhancement factor by using the fluorescence spectra measured in steps 6.2.5, 6.2.6 and 6.2.8, taking into account the CCD dark counts, normalization by unit area and acquisition times.12,14</li>
	</ol>
	</li>
</ol>

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The authors declare that they have no competing financial interests.

Silver nanocubes were chemically synthesized using reaction conditions similar to previously reported syntheses.2,12,17-20 This synthesis enables the fabrication of nanocubes with side lengths ranging from 50 to 100 nm. For example, a typical heating time of 2.5 hr will result in nanocubes with side lengths of ~75 nm. A longer synthesis time (&#62; 3 hr) will lead to larger nanoparticles, however, this may also result in different shapes such as truncated nanocubes or octahedrons. The final solution was centrifuged and re-suspended in deionized water, and may be stored for at least one month in a fridge at 4 &#176;C without any noticeable changes in the scattering spectra of the plasmon resonances.12
The size and shape of the Ag nanocubes from the process presented in the above protocol are very sensitive to the cleaning of the RBF, its cap and the stirring bar as well as the quality of the EG solution. Nanoparticles with different shapes such as rounded or elongated nanoparticles is a sign that there is likely an issue with one of these steps in the synthesis. It is therefore advised that steps 1.1.1-1.1.4 and 1.2.1-1.2.2 are critically important.
In Figure 4b the scattering spectrum collected from a single nanopatch antenna is shown which exhibits a strong plasmon resonance at 650 nm. Such a resonance indicates an excellent mode confinement in the gap region between the Ag nanocube and Au film made possible by high-quality nanocubes. Additionally, to obtain such a spectrum, it is also required that the sample is clean, the spacer layers (PE layers) have a uniform thickness and that the underlying Au film is smooth. The strong plasmon resonance is further confirmed by the data presented in Figure 4c where individual nanopatch antennas can be observed in the dark field image and in Figure 4d where large fluorescence enhancement is observed of Cy5 molecules located in the gap region. It should also be noted that the Ag nanocubes oxidize over time despite the PVP coating when exposed to air and thus it is recommended that optical measurements should be performed on the day the sample is prepared or within 1 to 3 days. To minimize oxidation, it is recommended that the nanopatch antenna samples be stored in vacuum or nitrogen gas.
The method presented in this paper enables the fabrication of Ag nanocubes and plasmonic nanopatch antennas with well-controlled dimensions utilizing colloidal synthesis and a layer-by-layer dip-coating process. Compared with other techniques such as optical or electron beam lithography, the technique presented here offers the potential for low cost and large-scale production while producing a narrow size distribution of nanoparticles.
The plasmonic nanopatch antennas presented in this paper also hold great promise for new nanomaterials by design exhibiting unique properties which may not exist in their macroscopic counterparts. In particular, these nanoantennas have shown record-high fluorescence enhancement of embedded dye molecules exceeding 30,000;12 spontaneous emission rate enhancements of 1,000; ultrafast spontaneous emission and high quantum yield.13,14 Additionally, it has been shown that emitters coupled to these nanopatch antennas exhibit highly directional emission which is critical for applications where coupling to an external detector or single mode fiber is required. Future applications of the nanoscale patch antennas may range from ultrafast optoelectronic devices, such as light emitting diodes, to high efficiency photodetectors and photovoltaic devices, sensing and quantum information processing technologies.12-14

In recent years, colloidal synthesis of nanoparticles and the assembly of these into advanced structures have attracted great interests both in research and industrial developments.1-4 Colloidal synthesis of nanoparticles have several advantages over lithographically fabricated nanostructures including superior size homogeneity, low cost and the possibility of large-scale, parallel production.
Metal nanoparticles such as silver (Ag) and gold (Au) can support localized surface plasmon polaritons and have the ability to confine light in a volume much smaller than the diffraction limit.1,3-5 The resulting high field intensity creates an enhanced local density of states enabling light-matter interactions to be tailored at the nanoscale. Recent efforts have demonstrated procedures to synthesize Ag and Au nanoparticles in a wide range of sizes and shapes, including triangles,4,6 cages,3,4 and rods4,7,8 in addition to the nanocubes discussed here. Nanostructures composed of several Ag or Au nanocomponents have also been fabricated demonstrating tailored properties.1,9-11
Here, we demonstrate a procedure to synthesize Ag nanocubes and more importantly, to couple these Ag nanocubes with an underlying Au film in order to form plasmonic nanopatch antennas. The distance between Ag nanocubes and the Au film can be controlled with ~1 nm resolution by using a series of polyelectrolyte spacer layers. We also demonstrate how to incorporate an active medium, such as an organic dye, into the plasmonic nanopatch antennas. Due to the strongly confined electromagnetic fields in the gap region between the nanocubes and the Au film, the nanopatch antennas can be used for highly enhanced fluorescence and spontaneous emission of the embedded dye molecules.12,13 The methods presented in this paper can be generalized to other emitters, such as colloidal solid-state quantum dots14 or two-dimensional semiconductor materials,15 and the plasmon resonance can be tuned over a wide spectral range by varying the size of the nanocubes or the gap.

<table><tbody><tr><td colspan="4">&lt;strong&gt;Reagents&lt;/strong&gt;</td></tr><tr><td>Ethylene glycol&amp;nbsp;</td><td>J.T. Baker</td><td>9300</td><td>Must be anhydrous</td></tr><tr><td>Sodium hydrosulfide hydrate&amp;nbsp;&amp;nbsp;</td><td>Sigma Aldrich</td><td>161527</td><td></td></tr><tr><td>Poly vinylpyrrolidone&amp;nbsp;</td><td>Sigma Aldrich</td><td>856568</td><td></td></tr><tr><td>Hydrochroric acid BDH ARISTAR PLUS</td><td>VWR International</td><td>7647-01-0</td><td></td></tr><tr><td>Silver trifluoroacetate&amp;nbsp;</td><td>Sigma Aldrich</td><td>482307</td><td>Store in dark place</td></tr><tr><td>Acetone</td><td>Sigma Aldrich</td><td>48358</td><td></td></tr><tr><td>Nitric acid</td><td>Sigma Aldrich</td><td>7697-37-2</td><td>concentrated (70%), for cleaning</td></tr><tr><td>Poly(allylamine) hydrochloride (PAH)</td><td>Sigma-Aldrich</td><td>283215</td><td></td></tr><tr><td>Polystyrene sulfonate&amp;nbsp; (PSS)</td><td>Sigma-Aldrich</td><td>561223</td><td></td></tr><tr><td>Sodium Chloride&amp;nbsp;</td><td>Macron Inc.</td><td>7647</td><td></td></tr><tr><td>Sulfo-Cyanine5 carboxylic acid (Cy5)</td><td>Lumiprobe</td><td>13390</td><td>Fluorescent dye (molecular weight: 664.76 g/mol)</td></tr><tr><td colspan="4">&lt;strong&gt;Equipments&lt;/strong&gt;</td></tr><tr><td>Stirring hotplate with temperature control</td><td>VWR International</td><td>89000-338</td><td></td></tr><tr><td>Vortex mixers</td><td>VWR International</td><td>10153-834</td><td></td></tr><tr><td>Microcentrifuge</td><td>Thermoscientific</td><td>Model 59A</td><td></td></tr><tr><td>Silicone fluid&amp;nbsp;</td><td>Sigma-Aldrich</td><td>63148-62-9</td><td></td></tr><tr><td>Micro-scale</td><td>Mettler Toledo</td><td>Model ML 104/03</td><td></td></tr><tr><td>Electron-beam metal evaporator&amp;nbsp;</td><td>CHA Industries</td><td>E-beam evaporator</td><td>Located inside a clean room</td></tr><tr><td>Pre-cleaned glass slides</td><td>Schott North America, Inc.</td><td>Nexterion Glass B&amp;nbsp;</td><td>Clean room pre-cleaned</td></tr><tr><td>25-ml 24/40 round-bottom flask</td><td>VWR International</td><td>60002-290</td><td></td></tr><tr><td>Magnetic stirring bar</td><td>VWR International</td><td>58948-116</td><td></td></tr><tr><td>Micropipettes (1&amp;ndash;10&amp;nbsp;ml, 10&amp;ndash;100&amp;nbsp;ml and 100&amp;ndash;1,000&amp;nbsp;ml)</td><td>VWR International</td><td></td><td></td></tr><tr><td>Ultrasonic cleaning bath</td><td>Branson Ultrasonic</td><td>Model 1510R-DTH</td><td></td></tr><tr><td>Stopwatch</td><td>VWR International</td><td></td><td></td></tr><tr><td>Eppendorf centrifugation tubes (1.5&amp;nbsp;ml)</td><td>VWR International</td><td>22364111</td><td></td></tr><tr><td>Poly(propylene) coning tubes (50&amp;nbsp;ml)</td><td>VWR International</td><td></td><td></td></tr><tr><td>Home built bright/darkfield microscope</td><td></td><td></td><td>75 W Xenon white light source, Nikon BF/DF 50X ELWD&lt;br/&gt; 0.55 NA, 8.2 mm WD objective, Nikon D90 digital camera, Acton 2300i spectrometer, Photometrics CoolSnap HQ charge coupled device (CCD) camera</td></tr><tr><td>He Ne laser (633&amp;nbsp;nm), 5&amp;nbsp;mW</td><td>Newport Corp.</td><td>R-30990</td><td></td></tr><tr><td>Reflectance standard</td><td>Lab Sphere</td><td>Model SRS-99-010</td><td></td></tr><tr><td>Laser long pass filter 633&amp;nbsp;nm</td><td>Semrock</td><td>LP02-633RU-25</td><td></td></tr></tbody></table>

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.

Here, we show representative results of the characteristics of the plasmonic nanopatch antennas, including SEM images of the sample structure, a reflectivity spectrum of a collection of nanopatch antennas and a scattering spectrum from a single nanopatch antenna. The energy of the plasmon resonance of the nanopatch antennas depends on the size of the nanocubes, the thickness of the dielectric gap region, i.e., the number of PE layers, as well as the dielectric material. In the procedure presented above we obtained Ag nanocubes with an average side length of 75 nm and slightly rounded corners (radius of curvature ~10 nm) coated in a PVP layer with an estimated thickness of 1-3 nm. In combination with the 5 PE layers and gold film, this result in a plasmon resonance centered at ~650 nm with a full-width-at-half-maximum (FWHM) of ~50 nm. This in turn has good spectral overlap with the absorption and emission wavelength of the Cy5 molecules which is centered at 646 and 662 nm, respectively.
Figure 3A shows a SEM image of a sample with a high concentration of nanocubes. These nanocubes were deposited on top of a Au film with 5 PE layers. Such SEM images are used to verify the overall quality of the nanocube synthesis; however, these samples are not utilized for further optical measurements as the density of the nanocubes is too high. Additionally, due to the high density, some nanocubes do not lie on the surface which is essential to form the plasmonic nanopatch antenna structure.
Figure 3B shows a SEM image of a sample of nanocubes fabricated using a nanocube solution which has been diluted by a factor of 1/10. This sample is used for measurements where the reflectivity of white light from an ensemble of nanopatch antennas was measured to determine the overall plasmon resonance. Figure 3C shows a SEM image of a sample of nanocubes fabricated using a nanocube solution which has been diluted by a factor of 1/100. This sample is used for scattering measurements of an individual nanopatch antenna. Using the diluted nanocube solution enables individual nanopatch antennas to be spatially isolated at an image plane by using a small pinhole.
Figure 4A shows a reflectivity spectrum, after normalization with the white light background, measured from a similar sample to the one shown in the SEM image in Figure 3B. Figure 4B shows a scattering spectrum from a single nanopatch antenna similar to the sample shown in the SEM image in Figure 3C.
Figure 4C shows a dark field image of a nanopatch antenna sample (prepared by a 1/100 diluted nanocube solution dispersed on a gold film with 5 PE layers) taken by a Nikon D90 digital camera. The observed bright red dots are due to scattering of white light from individual nanopatch antennas. A few spots are observed to have colors other than red, which is a result of nanocubes with different sizes or larger nanoparticles with non-cubic shapes.
Figure 4D shows two fluorescence spectra, one measured from a single nanopatch antenna (from a sample similar to the one shown in Figure 3C) and the other from a control sample consisting of a glass slide with the same number of PE layers and density of Cy5 dye molecules. The fluorescence intensity from Cy5 molecules coupled to the nanopatch antenna is much stronger than on the glass slide. This results from an enhanced excitation rate as well as a modified radiation pattern and increased quantum efficiency of the dye molecules.12 After correcting for background fluorescence and normalizing per unit area by dividing the area under the nanocube with the excitation spot size,12 we obtain an enhancement factor of ~ 12,000 from the data shown in Figure 4D. This enhancement factor is smaller compared with the previously reported value of 30,00012 likely due to the use of a Au instead of Ag film, increasing non-radiative losses.
<img alt="Figure 1" src="/files/ftp_upload/53876/53876fig1.jpg" /> 
Figure 1. Equipment setup for Ag nanocube synthesis.&#160;(A) A photograph of the equipment setup showing the heating bath on top of the stirring hotplate with temperature control. (B) A close up of the round-bottom&#160;flask (RBF) containing the nanocube solution during the synthesis. The setup is located inside a fume hood with proper air ventilation. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/53876/53876fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53876/53876fig2.jpg" /> 
Figure 2. Pictures of the nanocube solution.&#160;(A) Nanocube solution after the 2.5-hr synthesis and (B) after transfer to smaller tubes and re-suspended in de-ionized water. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/53876/53876fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53876/53876fig3highres.jpg" width="700" /> 
Figure 3. SEM characterizations of Ag nanocubes.&#160;(A) SEM image of a concentrated nanocube sample, (B) a diluted (1/10) nanocube sample, and (C) a diluted (1/100) nanocube sample. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/53876/53876fig3highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53876/53876fig4highres.jpg" width="700" /> 
Figure 4. Optical characterizations of nanopatch antennas. (A) Normalized reflection spectrum measured from an ensemble of nanopatch antennas (non-diluted nanocube solution). (B) Scattering spectrum from a single nanopatch antenna (1/100 diluted nanocube solution). (C) A dark-field image of a nanopatch antenna sample (1/100 diluted nanocube solution) taken under white light illumination. Each bright red dot corresponds to an individual plasmonic nanopatch antenna. (D) Fluorescence from Cy5 dye molecules embedded in a nanopatch antenna (red solid line) compared to that from a glass slide with an identical concentration of Cy5 dyes (dashed black line). <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/53876/53876fig4highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics at JoVE.com

Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

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 ...

colloidal-synthesis-nanopatch-antennas-for-applications-plasmonics

Universidad de Cartagena UDEC

Research

JoVE Journal

Engineering

11.0K Views.  Duke University. A protocol for the colloidal synthesis of silver nanocubes and fabrication of plasmonic nanoscale patch antennas with sub-10 nm gaps is presented.

Video: Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

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.

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) ...

Chemistry

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 ...

Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information support1,2,3. In this context, metal nanowires are especially desirable for realizing dense routing networks4. A prerequisite to operate such shared nanowire-based platform relies on our ability to electrically contact individual metal nanowires and efficiently excite surface plasmon polaritons5 in this information support. In this article, we describe a protocol to bring electrical terminals to chemically-synthesized silver nanowires6 randomly distributed on a glass substrate7. The positions of the nanowire ends with respect to predefined landmarks are precisely located using standard optical transmission microscopy before encapsulation in an electron-sensitive resist. Trenches representing the electrode layout are subsequently designed by electron-beam lithography. Metal electrodes are then fabricated by thermally evaporating a Cr/Au layer followed by a chemical lift-off. The contacted silver nanowires are finally transferred to a leakage radiation microscope for surface plasmon excitation and characterization8,9. Surface plasmons are launched in the nanowires by focusing a near infrared laser beam on a diffraction-limited spot overlapping one nanowire extremity5,9. For sufficiently large nanowires, the surface plasmon mode leaks into the glass substrate9,10. This leakage radiation is readily detected, imaged, and analyzed in the different conjugate planes in leakage radiation microscopy9,11. The electrical terminals do not affect the plasmon propagation. However, a current-induced morphological deterioration of the nanowire drastically degrades the flow of surface plasmons. The combination of surface plasmon leakage radiation microscopy with a simultaneous analysis of the nanowire electrical transport characteristics reveals the intrinsic limitations of such plasmonic circuitry.

Silver nanowires can simultaneously transport electrons and optical information in the form of surface plasmons. A procedure is described here to realize such a shared circuitry and the limitations at propagating both information carriers are evaluated.

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information ...

Utilization of Plasmonic and Photonic Crystal Nanostructures for Enhanced Micro- and Nanoparticle Manipulation

A method to manipulate the position and orientation of submicron particles nondestructively would be an incredibly useful tool for basic biological research. Perhaps the most widely used physical force to achieve noninvasive manipulation of small particles has been dielectrophoresis(DEP).1 However, DEP on its own lacks the versatility and precision that are desired when manipulating cells since it is traditionally done with stationary electrodes. Optical tweezers, which utilize a three dimensional electromagnetic field gradient to exert forces on small particles, achieve this desired versatility and precision.2 However, a major drawback of this approach is the high radiation intensity required to achieve the necessary force to trap a particle which can damage biological samples.3 A solution that allows trapping and sorting with lower optical intensities are optoelectronic tweezers (OET) but OET's have limitations with fine manipulation of small particles; being DEP-based technology also puts constraint on the property of the solution.4,5
This video article will describe two methods that decrease the intensity of the radiation needed for optical manipulation of living cells and also describe a method for orientation control. The first method is plasmonic tweezers which use a random gold nanoparticle (AuNP) array as a substrate for the sample as shown in Figure 1. The AuNP array converts the incident photons into localized surface plasmons (LSP) which consist of resonant dipole moments that radiate and generate a patterned radiation field with a large gradient in the cell solution. Initial work on surface plasmon enhanced trapping by Righini et al and our own modeling have shown the fields generated by the plasmonic substrate reduce the initial intensity required by enhancing the gradient field that traps the particle.6,7,8 The plasmonic approach allows for fine orientation control of ellipsoidal particles and cells with low optical intensities because of more efficient optical energy conversion into mechanical energy and a dipole-dependent radiation field. These fields are shown in figure 2 and the low trapping intensities are detailed in figures 4 and 5. The main problems with plasmonic tweezers are that the LSP's generate a considerable amount of heat and the trapping is only two dimensional. This heat generates convective flows and thermophoresis which can be powerful enough to expel submicron particles from the trap.9,10 The second approach that we will describe is utilizing periodic dielectric nanostructures to scatter incident light very efficiently into diffraction modes, as shown in figure 6.11 Ideally, one would make this structure out of a dielectric material to avoid the same heating problems experienced with the plasmonic tweezers but in our approach an aluminum-coated diffraction grating is used as a one-dimensional periodic dielectric nanostructure. Although it is not a semiconductor, it did not experience significant heating and effectively trapped small particles with low trapping intensities, as shown in figure 7. Alignment of particles with the grating substrate conceptually validates the proposition that a 2-D photonic crystal could allow precise rotation of non-spherical micron sized particles.10 The efficiencies of these optical traps are increased due to the enhanced fields produced by the nanostructures described in this paper.

Plasmonic tweezers and photonic crystal nanostructures are shown to produce useful enhancements in the efficiency and orientation control of optically trapping micro- and nano-particles.

A method to manipulate the position and orientation of submicron particles nondestructively would be an incredibly useful tool for basic biological ...

Bioengineering

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 ...

Using Polystyrene-block-poly(acrylic acid)-coated Metal Nanoparticles as Monomers for Their Homo- and Co-polymerization

We present a template-free method for &#8220;polymerizing&#8221; nanoparticles into long chains without side branches. A variety of nanoparticles are encapsulated in polystyrene-block-poly(acrylic acid) (PSPAA) shells and then used as monomers for their self-assembly. Spherical PSPAA micelles upon acid treatment are known to assemble into cylindrical micelles. Exploiting this tendency, the core-shell nanoparticles are induced to aggregate, coalesce, and then transform into long chains. When more than one type of nanoparticles are used, random and block &#8220;copolymers&#8221; of nanoparticles can be obtained. Detailed procedures are reported for the PSPAA encapsulation of nanoparticles, homo- and co-polymerization of the core-shell nanoparticles, separation and purification of the resulting nanoparticle chains. Transformations of single-line chains into double- and triple-line chains are also presented. The synergy between the polymer shell and the embedded nanoparticles leads to an unusual chain-growth polymerization mode, giving long nanoparticle chains that are distinct from the products of the traditional step-growth aggregation process.

Using Polystyrene-block-polyacrylic acid-coated Metal Nanoparticles as Monomers for Their Homo- and Co-polymerization

We report protocols for &#8220;polymerizing&#8221; various types of polymer-encapsulated metal nanoparticles into long chains of &#8220;homo-&#8220; and &#8220;co-polymers&#8221;.

We present a template-free method for &#8220;polymerizing&#8221; nanoparticles into long chains without side branches. A variety of nanoparticles are ...

Synthesis, Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.

The synthesis of uniform gold nanoparticles coated with semiconductor shells of CdS or ZnS is performed. The semiconductor coating is conducted by first depositing a silver sulfide shell and exchanging the silver cations for zinc or cadmium cations.

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large ...

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.

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 ...

Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation

A protocol is described to photocatalytically guide Pd deposition onto Au nanorods (AuNR) using surface plasmon resonance (SPR). Excited plasmonic hot electrons upon SPR irradiation drive reductive deposition of Pd on colloidal AuNR in the presence of [PdCl4]2-. Plasmon-driven reduction of secondary metals potentiates covalent, sub-wavelength deposition at targeted locations coinciding with electric field &#8220;hot-spots&#8221; of the plasmonic substrate using an external field (e.g., laser). The process described herein details a solution-phase deposition of a catalytically-active noble metal (Pd) from a transition metal halide salt (H2PdCl4) onto aqueously-suspended, anisotropic plasmonic structures (AuNR). The solution-phase process is amenable to making other bimetallic architectures. Transmission UV-vis monitoring of the photochemical reaction, coupled with ex situ XPS and statistical TEM analysis, provide immediate experimental feedback to evaluate properties of the bimetallic structures as they evolve during the photocatalytic reaction. Resonant plasmon irradiation of AuNR in the presence of [PdCl4]2- creates a thin, covalently-bound Pd0 shell without any significant dampening effect on its plasmonic behavior in this representative experiment/batch. Overall, plasmonic photodeposition offers an alternative route for high-volume, economical synthesis of optoelectronic materials with sub-5 nm features (e.g., heterometallic photocatalysts or optoelectronic interconnects).

A protocol for anisotropic photodeposition of Pd onto aqueously-suspended Au nanorods via localized surface plasmon excitation is presented.

A protocol is described to photocatalytically guide Pd deposition onto Au nanorods (AuNR) using surface plasmon resonance (SPR). Excited plasmonic hot ...

Preparation of Silver-Palladium Alloyed Nanoparticles for Plasmonic Catalysis under Visible-Light Illumination

Localized surface plasmon resonance (LSPR) in plasmonic nanoparticles (NPs) can accelerate and control the selectivity of a variety of molecular transformations. This opens possibilities for the use of visible or near-IR light as a sustainable input to drive and control reactions when plasmonic nanoparticles supporting LSPR excitation in these ranges are employed as catalysts. Unfortunately, this is not the case for several catalytic metals such as palladium (Pd). One strategy to overcome this limitation is to employ bimetallic NPs containing plasmonic and catalytic metals. In this case, the LSPR excitation in the plasmonic metal can contribute to accelerate and control transformations driven by the catalytic component. The method reported herein focuses on the synthesis of bimetallic silver-palladium (Ag-Pd) NPs supported on ZrO2 (Ag-Pd/ZrO2) that acts as a plasmonic-catalytic system. The NPs were prepared by co-impregnation of corresponding metal precursors on the ZrO2 support followed by simultaneous reduction leading to the formation of bimetallic NPs directly on the ZrO2 support. The Ag-Pd/ZrO2 NPs were then used as plasmonic catalysts for the reduction of nitrobenzene under 425 nm illumination by LED lamps. Using gas chromatography (GC), the conversion and selectivity of the reduction reaction under the dark and light irradiation conditions can be monitored, demonstrating the enhanced catalytic performance and control over selectivity under LSPR excitation after alloying non-plasmonic Pd with plasmonic metal Ag. This technique can be adapted to a wide range of molecular transformations and NPs compositions, making it useful for the characterization of the plasmonic catalytic activity of different types of catalysis in terms of conversion and selectivity.

Presented here is a protocol for the synthesis of silver-palladium (Ag-Pd) alloy nanoparticles (NPs) supported on ZrO2 (Ag-Pd/ZrO2). This system allows for harvesting energy from visible light irradiation to accelerate and control molecular transformations. This is illustrated by nitrobenzene reduction under light irradiation catalyzed by Ag-Pd/ZrO2 NPs.