This work was sponsored by the Army Research Laboratory and was accomplished under USARL Cooperative Agreement Number W911NF&#8208;17&#8208;2&#8208;0057 awarded to G.T.F. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.

1. Allocation of Au nanorods
NOTE: Cetyltrimethylammonium bromide (CTAB)-covered AuNR may be synthesized by wet-chemistry (step 1.1) or purchased commercially (step 1.2) according to the reader&#8217;s preference, with each yielding similar results. Results in this work were based on commercially-sourced, AuNR with penta-twinned crystal structure. Impact of AuNR seed crystal structure (i.e., monocrystalline vs. penta-twinned) on ultimate morphology of the secondary metal shell remains unclear within the scope of plasmonic photodeposition, but has been of keen interest in both wet-14,15 and similar photo-chemical12 syntheses. Alternative surfactants to CTAB may be employed so long as Zeta-potential is positive, although final Pd morphology could change.
<ol>
	<li>Synthesis Techniques: Synthesize aqueously-dispersed AuNR at 0.5 mM Au using the silver-assisted method by Nikoobakht et al.16,17 (yielding monocrystalline structure) or the surfactant-assisted method by Murphy et al.18,19 (yielding penta-twinned crystal structure). Wash the AuNR via centrifugation20,21 to remove excess, free CTAB to a final concentration of 1-10 mM.</li>
	<li>Commercial Sources: Purchase aqueous AuNR dispersions at 0.5 mM Au with the following specifications: 40 nm diameter, 808 nm LSPR, and CTAB ligand (5 mM concentration) in DI water. Wash the AuNR via centrifugation20,21 to remove excess, free CTAB if the CTAB concentration exceeds 1-10 mM upon receipt. 
	NOTE: Aqueous AuNR dispersions with CTAB surfactant at a variety of sizes, aspect ratios, and particle number densities may be purchased from many commercial vendors and used successfully in this protocol.</li>
</ol>
2. Plasmonic photodeposition of Pd onto Au nanorods
<ol>
	<li>Preparation of Pd precursor
	<ol>
		<li>Prepare a 20 mM HCl solution. First, make 0.1 M HCl by diluting 830 &#956;L of stock concentrated HCl (37%, 12 M) with water to 100 mL. Second, make 0.02 M HCl by diluting 4 mL of 0.1 M HCl with water to 20 mL.</li>
		<li>Pipette 10 mL of 20 mM HCl into appropriate glassware and place in a bath sonicator (no sonication) with water temperature set to 60 &#176;C.</li>
		<li>Add 0.0177 g of PdCl2 into the 10 mL of 20 mM HCl and mix via sonication until all PdCl2 is dissolved. The resultant 10 mM H2PdCl4 solution should exhibit a dark orange color.</li>
	</ol>
	</li>
	<li>Preparation of photodeposition reaction mixture 
	NOTE: The procedure described assumes a 3 mL total volume for use in a cuvette to allow real-time feedback into plasmonic photodeposition process. The cited masses/volumes were selected for compatibility with typical chemicals/materials/reagents while allowing facile washing/recovery of the Pd-decorated AuNR. It is anticipated that similar results may be achieved if scaled to other volumes and/or alternative reaction vessels are used (e.g., glass beaker).
	<ol>
		<li>Degas stock AuNR solution and methanol (MeOH) in a bath sonicator for 30 min.</li>
		<li>Pipette 2.5 mL of aqueously-suspended AuNR (from step 2.2.1) into a 1 cm path length, macrovolume cuvette with a magnetic stir bar. Place the cuvette on a stir plate. 
		NOTE: Typical volume of a macrovolume cuvette is 3.5 mL. Quartz may be substituted with UV-transparent plastics.</li>
		<li>Pipette 475 &#956;L of degassed MeOH (from step 2.2.1) into the cuvette while gently stirring for approximately 15-30 min. Periodically remove any bubbles by gently tapping the bottom of the cuvette against a rigid surface as needed; removing solvated gasses can prolong the stability of the metal halide salt.</li>
		<li>Pipette 5 &#956;L of stock concentrated HCl (37%, 12 M) into the cuvette and let mix for 15 min. 
		NOTE: Tuning concentration of HCl support could influence final morphology/rate of Pd deposition, but concentrations less than 20 mM in the reaction mixture will allow H2PdCl4 to progressively hydrolyze and oxolate, leading to eventual PdOx formation after ~3 h.</li>
	</ol>
	</li>
	<li>Plasmonic photoreduction of [PdCl4]2- onto AuNR1,13
	<ol>
		<li>Inject 25 &#956;L of 10 mM H2PdCl4 into the reaction mixture for a 1:5 Pd:Au atomic ratio. Let the solution complex in dark for 1 h while stirring. 
		NOTE: This quantity may be adjusted according the desired Pd:Au ratio as the expense of altering the final molarities of Au, [PdCl4]2-, HCl, and MeOH of the reaction mixture. Reference22 illustrates example Pt-AuNR morphologies at different Pt:Au ratios- similar results can be expected with Pd.</li>
		<li>Irradiate the reaction mixture with an un-polarized, 715 nm long-pass filtered tungsten-halogen lamp at 35 mW/cm2 intensity for 24 h. 
		NOTE: Different light filters (or sources, e.g., laser) may be chosen according to unique LSPR wavelength for different Au nanostructure seeds. For example, a 420 nm long-pass filter may be used for plasmonic seed structures exhibiting LSPR at 450 nm. Light intensity may be decreased with neutral density filtration at the expense of a slower [PdCl4]2- reduction rate, leading to a longer total reaction time. Light intensity may be increased to reduce reaction time at the expense of potential for thermal reduction of [PdCl4]2- (onset is ~360 &#176;C via Reference23). An appropriate intensity can be calculated a priori to mitigate thermal reduction via calculation of nanoparticle surface temperature in isolation and/or collective ensembles24. Effects on ultimate Pd-AuNR morphology from varying irradiation intensity have not been explored.</li>
		<li>Wash the residual chemicals/reagents from the Pd-AuNR two times, each by: centrifugation at 9,000 x g, removing the supernatant with a pipette, re-suspending the Pd-AuNR pellet in water, and immersing the vial into a bath sonicator for 1-2 min to disperse20,21.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

Monitoring changes in optical absorbance using transmission UV-vis spectroscopy is useful to assess status of the photocatalytic reaction, with particular attention to the LMCT features of H2PdCl4. Wavelength maxima of LMCT features after injection of H2PdCl4 at step 2.3.1 (going from solid black to solid blue in Figure 1) provide insights into the local &#8220;environment&#8221; of the [PdCl4]2- molecules1 (e.g., electrostatic coordination with N+ headgroups of CTAB followed by transport to the AuNR surface1 and/or molecular speciation consequent of hydrolysis and/or oxolation31,32,33). Magnitude of LMCT features during irradiation (dark red through yellow in Figure 1) quantifies the concentration of H2PdCl4 remaining in solution as the precursor is progressively photoreduced to Pd0 during LSPR irradiation. If the LMCT features do not decrease in magnitude during irradiation, then the photocatalytic reaction is not taking place (CTAB concentration could be too high and additional washing is recommended). A flattening of the long-wavelength tail on the Lorentzian LSPR feature should occur around 950 nm (see &#8220;MeOH consumption&#8221; label in Figure 1) during LSPR irradiation as a result of the sacrificial MeOH scavenging hot holes at the AuNR surfaces12 to maintain charge neutrality1. The SPR modes may be monitored during the reaction, but their wavelengths and intensities appear to hold little quantitative information with regards to the progressive status of the reaction1. This is due to the multitude of convoluting effects from parallel changes in (i) the precursor electrolyte environment over time (e.g., effective solvent refractive index and/or tail of precursor d&#8594;d band) vs. (ii) morphological changes (e.g., rod elongation). If the solution exhibits a dark brown/orange color after ~3 h with broad, feature-less UV absorbance, then it is likely PdOx has formed. Any residual, unconsumed H2PdCl4 will be evident in XPS analysis where the divalent Pd 3d lines (i.e., Pd2+) will occur approximately 2.5 eV higher in binding energy than the metallic lines shown in Figure 3.
Minute changes in final LSPR wavelength after Pd photodeposition, as shown in Figure 2, are typical of the plasmonic photodeposition process when using NR seeds1. Other seed structures or Pd:Au atomic ratios, however, may result in more drastic shifts and remain to be examined. A core-shell growth mechanism, where LSPR is governed by the overall rod aspect ratio,1,34 appears to be responsible for the minutely changed LSPR. For example, a mean length growth of 4.7 nm was recently reported for Pt photodeposited onto AuNR under similar conditions which lead to an AR increase from 4.4 to 4.7 (&#177;1.0) and followed an anisotropic core-shell growth mechanism1. This is in stark contrast to wet-chemical methods reporting dumbbell-like morphologies that yield 50-250 nm LSPR red-shifts for nanorods22,35,36,37. Ultimate Pd thickness can be increased by&#160;adding additional H2PdCl4 in protocol step 2.3.1 (e.g., total of 62.5 &#956;L of 10 mM H2PdCl4 for a 1:2 Pd:Au atomic ratio). FWHM expansions in the LSPR appears to predominantly be consequent of Pd deposition polydispersity38, as opposed to a damping signature1.
The penultimate structural morphology resultant from the plasmon-driven photoreduction of metal salts, such as H2PdCl4, is hypothesized to be governed by the spatial distribution of plasmonic hot electrons under LSPR excitation whose absorbed energy exceeds the reduction potential of the precursor1,22,39. Although only yet demonstrated for Pd and Pt1,13, the technique is anticipated to be amenable to other metals, such as Ag, Ni, Ir, Cu, Co, Ru, etc. This makes it a potentially powerful and flexible technique for synthesizing heterometallic plasmonic structures with sub-5 nm features- in particular, for plasmonically-sensitized photocatalysts. At its current stage, the technique is limited to solution-phase deposition onto colloidally-suspended plasmonic metals. The potential exists to perform reductive plasmonic photodeposition in gaseous-phase environments (e.g., in a chemical vapor deposition furnace) for high-volume processing, but remains to be explored.

Guiding metal deposition onto plasmonic substrates via plasmonic hot carriers generated from a resonant external field could support 2-step formation of heterometallic, anisotropic nanostructures at ambient conditions with new degrees-of-freedom1,2,3. Conventional redox chemistry, vapor deposition, and/or electrodeposition approaches are ill-suited for high-volume processing. This is primarily due to excess/sacrificial reagent waste, low throughput 5+ step lithography processes, and energy intensive environments (0.01-10 Torr and/or 400-1000 &#176;C temperatures) with little or no direct control over resultant material characteristics. Immersion of a plasmonic substrate (e.g., Au nanoparticle/seed) into a precursor environment (e.g., aqueous Pd salt solution) under illumination at the localized surface plasmon resonance (SPR) initiates externally-tunable (i.e., field polarization and intensity) photochemical deposition of the precursor via plasmonic hot electrons and/or photothermal gradients3,4. For example, protocol parameters/requirements for plasmonically-driven photothermal decomposition of Au, Cu, Pb, and Ti organometallics and Ge hydrides onto nanostructured Ag and Au substrates have been detailed5,6,7,8,9. However, utilization of femtosecond plasmonic hot electrons to directly photoreduce metal salts at a metal-solution interface remains largely undeveloped, absent processes employing citrate or poly(vinylpyrrolidone) ligands acting as intermediary charge relays to direct nucleation/growth of the secondary metal2,10,11,12. Anisotropic Pt-decoration of Au nanorods (AuNR) under longitudinal SPR (LSPR) excitation was recently reported1,13 where the Pt distribution coincided with the dipole polarity (i.e., the assumed spatial distribution of hot carriers).
The protocol herein expands upon recent Pt-AuNR work to include Pd and highlights key synthesis metrics that can be observed in real-time, showing the reductive plasmonic photodeposition technique is applicable toward other metal halide salts (Ag, Ni, Ir, etc.).

<table><tbody><tr><td>Aspheric Condenser Lens w/ Diffuser</td><td>Thorlabs</td><td>ACL5040U-DG15</td><td>f=40 mm, NA=0.60, 1500 grit, uncoated</td></tr><tr><td>Deuterium + Tungsten-Halogen Lightsource</td><td>StellarNet</td><td>SL5</td><td></td></tr><tr><td>Gold Nanorods, AuNR</td><td>NanoPartz</td><td>A12-40-808-CTAB</td><td>CTAB surfactant, 808 nm LSPR, 40 nm diameter</td></tr><tr><td>Ground Glass Diffuser</td><td>Thorlabs</td><td>DG20-1500</td><td>1500 grit, N-BK7</td></tr><tr><td>Hydrochloric acid, HCl</td><td>J.T. Baker</td><td>9539-03</td><td>concentrated, 37%</td></tr><tr><td>Low Profile Magnetic Stirrer</td><td>VWR</td><td>10153-690</td><td></td></tr><tr><td>Macro Disposable Cuvettes, UV Plastic</td><td>FireFlySci</td><td>1PUV</td><td>10 mm path length</td></tr><tr><td>Methanol, MeOH</td><td>J.T. Baker</td><td>9073-05</td><td>&amp;ge;99.9%</td></tr><tr><td>Palladium (II) chloride, PdCl2</td><td>Sigma Aldrich</td><td>520659</td><td>&amp;ge;99.9%</td></tr><tr><td>Plano-Convex Lens</td><td>Thorlabs</td><td>LA1145</td><td>f=75 mm, N-BK7, uncoated</td></tr><tr><td>Quartz Tungsten-Halogen Lamp</td><td>Thorlabs</td><td>QTH10</td><td></td></tr><tr><td>UV-vis Spectrometer</td><td>Avantes</td><td>ULS2048L-USB2-UA-RS</td><td>AvaSpec-ULS2048L</td></tr></tbody></table>

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).

Transmission UV-vis spectra, X-ray photoelectron spectroscopy (XPS) data, and transmission electron microscopy (TEM) images were acquired for the CTAB-covered AuNR in the presence/absence of H2PdCl4 in dark and under resonant irradiation at their longitudinal SPR (LSPR) to catalyze nucleation/growth of Pd. Transmission UV-vis spectra in Figure 1 and Figure 2 provide insights into the reaction dynamics according to changes in: (a) precursor ligand-metal charge transfer (LMCT) feature intensity and wavelength and (b) nanorod SPR intensity, full width at half maximum (FWHM), and wavelength (&#955;). XPS is used to confirm presence of metallic Pd and covalent Pd-Au bonding. XPS is also used to characterize the composite valence band density-of-states (DOS) of the bimetallic nanostructures, shown in Figure 3. TEM images and energy dispersive spectroscopy (EDS) maps in Figure 4 determine the structural morphology and size distribution of the Pd-decorated AuNR.
Figure 1 shows representative UV-vis-NIR absorbance trends upon sequential, step-by-step addition of each chemical component comprising the reaction mixture, beginning with 2.5 mL of stock 0.5 mM AuNR (dashed black). Addition of 475 &#956;L of MeOH as a sacrificial hole scavenger and 5 &#956;L of 12 M HCl (solid black) decreases absorbance magnitude across the UV and visible spectrum due to simple dilution. A ~5-8 nm blue-shift in the longitudinal SPR (LSPR) wavelength upon HCl addition is typical, which likely arises from screening by the solvated Cl- anions25. Addition of 25 &#956;L of 10 mM H2PdCl4 (dashed and solid blue) causes high intensity UV absorbance features to emerge, which correspond to LMCT bands of [PdCl4]2-. LMCT bands are characteristic of metal halide salts26,27. After equilibrating in the dark for 1 h with the CTAB-covered AuNR in 20 mM HCl, the [PdCl4]2- molecules exhibit LMCT features at approximately 247 nm and 310 nm. Upon light irradiation resonant with the AuNR LSPR (dark red), the [PdCl4]2- LMCT bands respectively blue-shift to 230 nm and 277 nm within a few minutes, and their molar absorptivity appears to decrease. Absorbance magnitude of the L&#960;MCT band decreases from 1.7 to approximately 0.47 over the course of 24 h due to progressive photoreduction of [PdCl4]2- (dark red through yellow) by the excited AuNR via plasmonic hot electrons1,13. Precursor LMCT features in the UV region disappear after 24 h (yellow), which indicates full consumption of [PdCl4]2-. Transverse SPR (TSPR) and LSPR features begin red-shifting as the [PdCl4]2- LMCT bands lower simultaneously. Temperature of the reaction vessel may be monitored concomitantly (e.g., via thermocouple) to ensure plasmonic photothermal damping does not increase the bulk temperature above the ~360&#176; C onset temperature for [PdCl4]2- reduction23. Typical steady-state temperatures range from 26-32 &#176;C under these experimental conditions without ambient convection.
Figure 2 shows the TSPR and LSPR of the doubly washed particles before (black) and after (red) resonant irradiation in the presence of adsorbed [PdCl4]2-. The LSPR wavelength red-shifts from 807 nm to 816 nm along with a 5% FWHM expansion. The TSPR remains unchanged. Absorbance magnitude at wavelengths below ~400 nm is increased by ~40-55%, due to both changes in and accrued interband metal absorption after apparent photodeposition of Pd.
XPS analysis in Figure 3A confirm presence of metallic Pd by the emergence of Pd 3d lines at 335 eV and 340 eV binding energies. Note that Au exhibits convoluting 4d photoelectron line in this binding energy region as well, but is suppressed after photoreduction of [PdCl4]2- that covers AuNR with Pd. A ~0.5 eV shift in the Au 4f photoelectron lines to lower binding energies in Figure 3B is indicative of covalent Au-Pd interaction28,29. The valence band DOS after Pd photodeposition in Figure 3C exhibit a higher DOS near the Fermi level, EF (i.e., binding energy of 0 eV) and moves the d-band onset toward the EF13. These are typical characteristics of metallic Pd and may be calculated a priori using density functional theory (DFT)13.
TEM analysis in Figure 4A,B reveal the respective structural morphologies of the AuNR mixed with H2PdCl4 in the dark (Figure 4A, blue) and under LSPR irradiation (Figure 4B, red). Sharp-tipped Pd-AuNR are observed as a result of Pd photoreduction by plasmonic hot electrons generated under LSPR irradiation. These sharp nanorod tips coincide with the end Au (111) facets that are characteristic of the penta-twinned AuNR seeds30. Such exacerbated end facets are not observed for AuNR mixed with H2PdCl4 in the dark. Size distribution analysis of rod lengths in Figure 4C indicates LSPR irradiation expands mean rod length from 127 nm to 129 nm, due to presence of photoreduced Pd. An apparent sub-2 nm Pd thickness is confirmed in an energy dispersive spectroscopy (EDS) map of a representative Pd-AuNR, shown in Figure 4D. No change in rod diameter is observed (39.1 nm under dark condition versus 39.2 nm under LSPR irradiation). Overall rod AR increases from 3.27 to 3.30 (&#177;0.34) due to the increase in nanorod length. These size population metrics are consistent with the small 7 nm LSPR red-shift measured in Figure 2.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60041/60041fig01.jpg" /> 
Figure 1: Transmission UV-vis spectroscopy analysis of the AuNR-H2PdCl4 reaction mixture. 
The spectra showing typical LMCT and SPR absorbance features upon sequential addition of MeOH (solid black) and H2PdCl4 (dashed blue) to a stock 0.5 mM AuNR solution (dashed black). After 1 h equilibration in the dark (solid blue), broadband LSPR irradiation with a 715 nm long-pass filter (35 mW/cm2; red shaded area) catalyzes photoreduction over a 24 h timespan (solid red &#8594; yellow, 2 h time-steps). MeOH consumption as the reaction progresses is observable around 950 nm. Arrows guide the eye to show trends in LMCT wavelength shifts with time. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60041/60041fig01large.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/60041/60041fig02.jpg" /> 
Figure 2: Transmission vis-NIR spectroscopy analysis of SPR modes of doubly washed AuNR before (black) and after addition + photoreduction of H2PdCl4 (red). 
Respective shifts in resonant wavelength (&#916; &#955;) and bandwidth expansion (&#916; FWHM) of the TSPR and LSPR modes after photoreduction of H2PdCl4 are inset. Accrued interband Pd absorption is evident below ~480 nm.
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60041/60041fig03.jpg" /> 
Figure 3: XPS analysis of AuNR before (black) and after LSPR irradiation in presence of H2 PdCl4 (red). 
(A) Au 4d and Pd 3d region showing respective spin-orbit split 5/2 and 3/2 lines. (B) Au 4f region showing spin-orbit split 7/2 and 5/2 lines. (C) Valence band DOS region, where 0 eV binding energy is the Fermi level (EF).
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/60041/60041fig04.jpg" /> 
Figure 4: TEM analysis of AuNR in the presence of H2 PdCl4 in dark versus LSPR illumination. 
(A) TEM micrographs of AuNR mixed with H2PdCl4 in dark for 24 h and washed 2x. (B) TEM micrographs of AuNR mixed with H2PdCl4 under LSPR excitation for 24 h and washed 2x. (C) Cumulative distribution function (CDF) of nanorod lengths, where blue and red correspond to the dark and light conditions, respectively. (D) EDS mapping of Au (purple) and Pd (green) signals at the tip of on representative nanorod that was resonantly irradiated in the presence of H2PdCl4.

Watch this Scientific Journal Video about Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation at JoVE.com

Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation

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 ...

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Nanyang Technological University

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7.4K Views.  U.S. Army Research Laboratory. A protocol for anisotropic photodeposition of Pd onto aqueously-suspended Au nanorods via localized surface plasmon excitation is presented.

Video: Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation

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) ...

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 ...

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 ...

Engineering

In Situ Synthesis of Gold Nanoparticles without Aggregation in the Interlayer Space of Layered Titanate Transparent Films

Combinations of metal oxide semiconductors and gold nanoparticles (AuNPs) have been investigated as new types of materials. The in situ synthesis of AuNPs within the interlayer space of semiconducting layered titania nanosheet (TNS) films was investigated here. Two types of intermediate films (i.e., TNS films containing methyl viologen (TNS/MV2+) and 2-ammoniumethanethiol (TNS/2-AET+)) were prepared. The two intermediate films were soaked in an aqueous tetrachloroauric(III) acid (HAuCl4) solution, whereby considerable amounts of Au(III) species were accommodated within the interlayer spaces of the TNS films. The two types of obtained films were then soaked in an aqueous sodium tetrahydroborate (NaBH4) solution, whereupon the color of the films immediately changed from colorless to purple, suggesting the formation of AuNPs within the TNS interlayer. When only TNS/MV2+ was used as the intermediate film, the color of the film gradually changed from metallic purple to dusty purple within 30 min, suggesting that aggregation of AuNPs had occurred. In contrast, this color change was suppressed by using the TNS/2-AET+ intermediate film, and the AuNPs were stabilized for over 4 months, as evidenced by the characteristic extinction (absorption and scattering) band from the AuNPs.

In Situ Synthesis of Gold Nanoparticles without Aggregation in the Interlayer Space of Layered Titanate Transparent Films

Here, we present a protocol for the in situ synthesis of gold nanoparticles (AuNPs) within the interlayer space of layered titanate films without the aggregation of AuNPs. No spectral change was observed even after 4 months. The synthesized material has expected applications in catalysis, photo-catalysis, and the development of cost-effective plasmonic devices.

Combinations of metal oxide semiconductors and gold nanoparticles (AuNPs) have been investigated as new types of materials. The in situ synthesis of AuNPs ...

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 ...

Rapid Nanoprobe Signal Enhancement by In Situ Gold Nanoparticle Synthesis

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

Rapid Nanoprobe Signal Enhancement by In Situ Gold Nanoparticle Synthesis

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

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

Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles

The size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution of capping ligands during the synthesis reaction, their role in regulating the nucleation and growth rates of colloidal nanoparticles is not well understood. In this work, we demonstrate a mechanistic investigation of the role of trioctylphosphine (TOP) in Pd nanoparticles in different solvents (toluene and pyridine) using in situ SAXS and ligand-based kinetic modeling. Our results under different synthetic conditions reveal the overlap of nucleation and growth of Pd nanoparticles during the reaction, which contradicts the LaMer-type nucleation and growth model. The model accounts for the kinetics of Pd-TOP binding for both, the precursor and the particle surface, which is essential to capture the size evolution as well as the concentration of particles in situ. In addition, we illustrate the predictive power of our ligand-based model through designing the synthetic conditions to obtain nanoparticles with desired sizes. The proposed methodology can be applied to other synthesis systems and therefore serves as an effective strategy for predictive synthesis of colloidal nanoparticles.

The main goal of this work is to elucidate the role of capping agents in regulating the size of palladium nanoparticles by combining in situ small angle x-ray scattering (SAXS) and ligand-based kinetic modeling.

The size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution ...

A Continuous-flow Photocatalytic Reactor for the Precisely Controlled Deposition of Metallic Nanoparticles

In this work, a novel photocatalytic reactor for the pulsed and controlled excitation of the photocatalyst and the precise deposition of metallic nanoparticles is developed. Guidelines for the replication of the reactor and its operation are provided in detail. Three different composite systems (Pt/graphene, Pt/TiO2, and Au/TiO2) with monodisperse and uniformly distributed particles are produced by this reactor, and the photodeposition mechanism, as well as the synthesis optimization strategy, are discussed. The synthesis methods and their technical aspects are described comprehensively. The role of the ultraviolet (UV) dose (in each excitation pulse) on the photodeposition process is investigated and the optimum values for each composite system are provided.

For a continuous and scalable synthesis of noble-metal-based nanocomposites, a novel photocatalytic reactor is developed and its structure, operation principles, and product quality optimization strategies are described.

In this work, a novel photocatalytic reactor for the pulsed and controlled excitation of the photocatalyst and the precise deposition of metallic ...

Preparation and Photoacoustic Analysis of Cellular Vehicles Containing Gold Nanorods

Gold nanorods are attractive for a range of biomedical applications, such as the photothermal ablation and the photoacoustic imaging of cancer, thanks to their intense optical absorbance in the near-infrared window, low cytotoxicity and potential to home into tumors. However, their delivery to tumors still remains an issue. An innovative approach consists of the exploitation of the tropism of tumor-associated macrophages that may be loaded with gold nanorods in vitro. Here, we describe the preparation and the photoacoustic inspection of cellular vehicles containing gold nanorods. PEGylated gold nanorods are modified with quaternary ammonium compounds, in order to achieve a cationic profile. On contact with murine macrophages in ordinary Petri dishes, these particles are found to undergo massive uptake into endocytic vesicles. Then these cells are embedded in biopolymeric hydrogels, which are used to verify that the stability of photoacoustic conversion of the particles is retained in their inclusion into cellular vehicles. We are confident that these results may provide new inspiration for the development of novel strategies to deliver plasmonic particles to tumors.

We show the preparation and address the feasibility of cellular vehicles containing gold nanorods for the photoacoustic imaging of cancer.

Gold nanorods are attractive for a range of biomedical applications, such as the photothermal ablation and the photoacoustic imaging of cancer, thanks to ...

Bioengineering

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.

Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation

Summary

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Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance

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

Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

In Situ Synthesis of Gold Nanoparticles without Aggregation in the Interlayer Space of Layered Titanate Transparent Films

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

Rapid Nanoprobe Signal Enhancement by In Situ Gold Nanoparticle Synthesis

Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles

A Continuous-flow Photocatalytic Reactor for the Precisely Controlled Deposition of Metallic Nanoparticles

Preparation and Photoacoustic Analysis of Cellular Vehicles Containing Gold Nanorods

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