The financial support of this work was provided by Department of Energy DOE- Laboratory Directed Research &#38; Development (LDRD) Strategic Initiative Program. We thank Mr. Henry Sessions, and Mr. Charles Shick for providing their time and expertise to assist us with our experiments.

1. Nanomaterials Synthesis Protocol
<ol>
	<li>Prepare a stock solution of Fe2O3 of 25 mM. 
	Note: All stock solutions are prepared using deionized water unless stated otherwise.</li>
	<li>Take a 25 ml conical flask.</li>
	<li>Add 10 ml deionized (DI) water and a stir bar, and place it on a heating block.</li>
	<li>Add 100 &#181;l of Fe2O3 stock solution (25 mM) to this flask.</li>
	<li>Heat the solution while stirring for approximately 5 min.</li>
	<li>Prepare 10 ml 1% sodium citrate by dissolving 0.1 g of sodium citrate to 10 ml of water.</li>
	<li>Add 1 ml of the 1% sodium citrate solution to the 25 ml flask containing the Fe2O3 aqueous solution.</li>
	<li>Bring the solution to a boil (100 &#176;C).</li>
	<li>Add 250 &#181;l of 0.01 M chloroauric acid.</li>
	<li>Continue heating the solution at 100 &#176;C for 10 min. After several minutes (2-3 min), the solution turns red/brownish indicating that Au nanoparticles are being produced.</li>
	<li>Remove the solution from the heating block and allow it to cool off at RT (approximately 20 &#176;C) (1-2 hr).</li>
	<li>Purify the samples by centrifugation for 7 min at 4,700 &#215; g.</li>
	<li>Remove the supernatant from the centrifuged samples.</li>
	<li>Re-disperse the centrifuged nanoparticles in DI water, up to 10 ml.</li>
</ol>
2. Nanoparticles Characterization
<ol>
	<li>SEM/EDX characterization:
	<ol>
		<li>Place 1-2 &#181;l of centrifuged nanoparticles on a copper grid and allow it to dry for 1 hr.</li>
		<li>Place sample in a clean container and take it to the SEM/EDX for characterization.11,12</li>
	</ol>
	</li>
	<li>UV-Vis characterization:
	<ol>
		<li>Turn on the UV-Vis and allow it to warm up for 10-15 min.</li>
		<li>Record a reference DI water spectrum.</li>
		<li>Place 1 ml of the nanoparticle&#39;s aqueous solution in a methacrylate cuvette and record the UV-Vis spectra over wavelengths &#955; = 300 - 1,000 nm.</li>
		<li>Avoid saturation of the signal by keeping the maximum absorbance lesser than ~1.2. If the observed maximum absorbance is larger, reduce the peak height by diluting the sample or using a shorter path length cuvette. 
		Note: Surface plasmon band of Au (&#955; &#8776; 525 nm) should be easily observed.</li>
	</ol>
	</li>
	<li>Magnetic manipulation
	<ol>
		<li>Place 3 ml of the red/brownish aqueous samples of magnetic/plasmonic nanostructures in methacrylate cuvettes.</li>
		<li>Place a commercially purchased magnet (~100 Gauss) in the close proximity of the cuvette. 
		Note: Within minutes, all magnetic/plasmonic nanoparticles are &#34;attached&#34; to the methacrylate cuvette side where the magnet was placed. Solution turned from brown to colorless indicating that the nanoparticles retained their magnetic properties even after Au was deposited on Fe2O3 surface.</li>
	</ol>
	</li>
	<li>Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis.13
	<ol>
		<li>Use aqueous samples of nanoparticle solutions in this analysis.</li>
		<li>Digest purified nanoparticle samples in nitric acid to transform them to an ionic form prior to mass analysis experiments by transferring all samples in tubes with a final volume of 10 ml of 2% nitric acid. Allow 30 min for digestion to take place.</li>
		<li>Create a calibration curve with known concentrations of analytes of interest (e.g., Au, Fe).</li>
		<li>Spike samples with an internal standard solution containing 10 ppb Rh and In and analyze in the semi-quantitative mode of the ICP-MS according to manufacturer&#39;s instructions. This technique entails the analysis of a NIST traceable multi-element standard (10 ppb In and 100 ppb Li, Mn, Fe, Co, Sr, Cd, Bi, and U).</li>
		<li>Compare the determined intensities for the standard with the intensities for the other samples to yield approximate concentrations for selected elements. To account for plasma and instrument drifts, all samples should have a minimum of 10 ppb concentration for In that was added to all samples.</li>
		<li>Determine elemental concentration of the analytes of interests for the prepared solutions by following these steps:
		<ol>
			<li>Perform an initial calibration validation sample of the multi-element standard (10 ppb In and 75 ppb Li, Mg, Fe, Co, Sr, Cd, Bi, and U).</li>
			<li>Perform initial calibration blank of deionized water.</li>
			<li>Perform ICP-MS analysis on two sample of interest.</li>
			<li>Continue performing calibration validation sample (10 ppb In and 75 ppb Li, Mg, Fe, Co, Sr, Cd, Bi, and U) of the multi-element standard.</li>
			<li>Continue calibration blank of deionized water. 
			Note: According to the vendor specifications, the ICP-MS measurements have an uncertainty of 20%. Nanomaterial laboratory work was performed under a fume hood. PPE (lab coat, apron, thin mil nitrile gloves for incidental contact, and goggles) and a face shield should be used if hood sash is above chin level. Minimum PPE required when working with nanoscale materials; disposable lab coat, thin mil nitrile gloves for incidental contact and safety glasses with side shields will be worn in the lab when handling nanomaterials. Nanomaterial bearing waste shall not be put in regular trash or down the drain.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
3. Laser Heating Experiment
<ol>
	<li>Turn on the laser power supply and balance. 
	Note: The laser wavelength used in this experiment (&#955; = 532 nm) is chosen to match the LSPR absorbance peak as closely as possible. However, photothermal effects can be induced using any wavelength that overlaps with the absorbance of the nanoparticles. The heating efficiency is just greater when illuminated on resonance.</li>
	<li>Position the balance windows so they do not obstruct the laser path or block the infrared (IR) thermocouples. The IR thermocouples are non-contact temperature probes and must have a clear line of sight to the measurement surface. Figure 1 shows a schematic of the experimental setup.</li>
	<li>Remove the protective covers from the IR thermocouples.</li>
	<li>Open the data collection software program and run, naming the measurement, &#34;warmup.&#34; The custom software program collects the balance and thermocouple resistance values as a function of time, and when the program is running it logs these values into a data file.</li>
	<li>Run the measurement for at least 20 min to allow the system to warm up.</li>
	<li>While the system is warming up, prepare the sample by pipetting the appropriate amount (3 ml) of the desired solution into a methacrylate cuvette. The amounts used here are 3 ml of solution for standard cuvettes, and 1 ml for semi-micro cuvettes.</li>
	<li>Adjust the laser power to the lowest setting that produces a barely visible beam, which is 1.5 A for the laser system used here. Check to make sure that the laser beam spot is unobstructed and remains at the focal point of the IR thermocouple.</li>
	<li>Place the sample on the balance arm such that the side of the cuvette is perpendicular to the IR measurement beam of the thermocouple and the laser beam spot strikes the center of the solution.</li>
	<li>Reduce the laser power until the beam is no longer visible, but do not turn off the power supply.</li>
	<li>After 20 min the warm-up is complete. Stop the measurement program and exit out of the software.</li>
	<li>Re-zero the balance. Open the data collection software program, click run, and then create a name for the data file. The experiment will run after naming the file and clicking &#34;Save.&#34; The exact experimental routine will depend on the information desired, but a model routine is provided here.
	<ol>
		<li>Start the data collection. After 120 sec, turn up the laser power to desired setting (1.2 W for these experiments, which when focused into a ~20 &#181;m spot corresponds with ~3.8 &#215; 105 W/cm2). Collect data for another 1,000 sec, then adjust laser power to the minimum setting and turn off laser power supply. Continue to collect data for another 1,000 sec before halting the measurement.</li>
	</ol>
	</li>
	<li>After the experimental routine is complete, exit out of the program, turn everything off, and re-cover all equipment. Save the experimental data in an ASCII format and further process and analyze using additional software.</li>
</ol>

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The authors have nothing to disclose.

The use of optically tunable gold nanoparticles for hyperthermia studies is an attractive option as it capitalizes on plasmonic heating of gold nanoparticles tuned to absorb light strongly in the VIS-NIR region. The plasmonic heating studies described here were examined by using laboratory prepared and commercially available iron oxide-gold hybrid nanomaterials. One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to neutral gold Au(0) by reducing agents, such as sodium citrate, sodium borohydride, etc.15,16 The synthesis of the gold nanoparticles on iron oxide nanoparticles is straightforward. One could easily control the Au metal size, shape, and loading by changing experimental parameters, e.g., reactants concentrations, reducing agents, surfactants, etc.17 This approach yields good control over Au nanoparticle dimensions and uniform nanoparticle loading onto Fe2O3. Other noble metals can also be prepared by this procedure, including Ag, Pt, and Pd.18 A distinct advantage of this procedure is that the reaction procedure can be done in air or water, and, in principle, is amenable to scaling up. Using commercial nanomaterials and/or scalable-wet chemical procedures is ideal for large-scale treatment applications or biological applications because these materials are readily available and more economical than custom synthesized materials and procedures. Surface modifications of these metallic nanostructures are also of interest in the scientific community. A number of organic (surfactants, bifunctional thiols, polymers, amino acids, proteins, DNA) and inorganic materials (silica, other metals, metal oxides, etc.)19 can be further loaded or functionalized onto these surfaces to create nanocomposite materials with various designs, geometries, compositions and multifunctional capabilities, for biological targeting, drug delivery, sensing, imaging, environmental applications, etc.
Additionally, the photothermal technique described here is well-suited to characterize the plasmonic properties of different materials, as bulk temperature and mass measurements are relatively easy to perform using readily available equipment. The ease of sample preparation and measurement is a distinct advantage over other plasmonic techniques/applications. For example, techniques such as surface-enhanced Raman spectroscopy and LSPR sensing are highly sensitive to the preparation of both the substrate and the target,20,21 which makes repeatability and comparison across samples more challenging. One possible drawback to the photothermal measurements described above is that temperature is measured on the bulk scale and not on the surface of the nanoparticle where the heat is transduced. There are thermometry techniques that can provide this local temperature information,22-24 but these require more complicated sample preparation, making them more challenging to implement. Finally, the measurements described here could easily be combined with other techniques (e.g., photocatalytic degradation)9 to assess photothermal effects on different processes.
In summary, we have described the synthesis of hybrid Fe2O3-Au nanoparticles solutions and their photothermal characterization. Even with a 20&#215; smaller concentration of Au, these Fe2O3-Au nanoparticles are able to photothermally heat aqueous solutions as efficiently as AuNPs, demonstrating the advantages of hybrid materials. Furthermore, the hybrid structures retain the properties of both materials, creating a multifunctional structure with magnetic and plasmonic properties. Such structures are interesting for biomedical applications,8 but many additional uses can be envisioned.

Beginning with their use in ancient dichroic glass,1 gold nanoparticles (AuNPs) have often contributed to the development of new technologies.2,3 More modern examples of these technologies include cloaking devices and particles that can both detect and treat cancer.4,5 AuNPs have many remarkable properties, but the most notable among these is the presence of localized surface plasmon resonances (LSPRs), which occur when incident electromagnetic radiation resonantly drives free electrons into collective oscillations, creating intense and highly confined electromagnetic fields.6 An intriguing aspect of LSPRs is that they are tunable. That is, the resonance energy can be adjusted by modifying the shape and size of the AuNPs or by changing the refractive index of the ambient environment. Another property of AuNPs, and gold in general, is that they are relatively expensive. While this might make gold more attractive from a luxury standpoint, for technological applications, this is a drawback and could be an obstacle to general use. Two potential solutions for this problem are searching for less-expensive alternative materials that exhibit similar properties as gold, or finding a way to combine gold with another material to create a composite material with similar properties but smaller amounts of the precious metal. The latter solution is perhaps more interesting as it allows for the possibility of creating a multifunctional hybrid nanostructure with the physicochemical properties of two or more materials.7
Iron(III) oxide, Fe2O3, is an excellent candidate for one component of such a mixture because it is widely available, inexpensive, and non-toxic. Furthermore, the maghemite phase, &#947;-Fe2O3, is ferrimagnetic, and the hematite phase, &#945;-Fe2O3, is weakly ferromagnetic. Thus, the combination of gold with Fe2O3 could potentially yield nanoparticles that exhibit plasmonic properties and also interact with external magnetic fields, yet are significantly less expensive than pure gold. Such a hybrid nanostructure could find interesting real world applications. For example, Fe2O3-Au nanoparticles have proven useful for both cancer diagnosis and treatment through magnetic resonance imaging and photothermal therapy.8 In this case, Fe2O3 functions as an MRI contrast agent, while the Au portion locally converts incident light to heat through dissipation of electromagnetic energy absorbed during LSPR. Additionally, Fe2O3-Au nanoparticles have demonstrated plasmonic enhancement of the catalytic conversion of CO into CO2 under visible light illumination, and such structures could also be used for photothermal solar energy conversion.9,10
This report describes the synthesis of Fe2O3-Au nanoparticles using a simple wet chemical method. The hybrid structure consists of a Fe2O3 core that is decorated with smaller AuNPs. Importantly, the obtained Fe2O3-Au nanoparticles retain both magnetic and plasmonic properties of the constituent materials, which creates a multifunctional particle that could be useful for a variety of applications. In order to illustrate the plasmonic applications of these hybrid nanoparticles, photothermal characterization of the nanoparticles using a laser heating system is also described. The photothermal measurements demonstrate that the hybrid Fe2O3-Au nanoparticles are able to heat aqueous solutions as efficiently as pure AuNPs, even with a significantly smaller concentration of the noble metal. These results validate the method of using composite or hybrid materials to reduce costs and achieve greater functionality.

<table border="0" cellpadding="0" cellspacing="0" width="614">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Gold(III) chloride trihydrate&#160;</td>
			<td style="width:112px;">Sigma-Aldrich</td>
			<td style="width:119px;">520918&#160;</td>
			<td style="width:167px;">&#8805;99.9% trace metals basis</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Iron(III) oxide</td>
			<td style="width:112px;">Sigma-Aldrich</td>
			<td style="width:119px;">544884</td>
			<td style="width:167px;">nanopowder, &#60;50&#160;nm particle size (BET)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Sodium citrate tribasic dihydrate</td>
			<td style="width:112px;">Sigma-Aldrich</td>
			<td style="width:119px;">S4641</td>
			<td>ACS reagent, &#8805;99.0%&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">SEM</td>
			<td style="width:112px;">Hitachi&#160;</td>
			<td style="width:119px;">S8200</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">TEM</td>
			<td style="width:112px;">Hitachi&#160;</td>
			<td style="width:119px;">H95000</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">EDX</td>
			<td style="width:112px;">Oxford Instruments&#160;</td>
			<td style="width:119px;">SDD - X-Max</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">DLS</td>
			<td style="width:112px;">Brookhaven Instruments</td>
			<td style="width:119px;">NanoBrook Omni</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">ICP-MS</td>
			<td style="width:112px;">Agilent&#160;</td>
			<td style="width:119px;">7500s</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">UV-Vis-NIR spectrometer</td>
			<td style="width:112px;">Tec5 MultiSpec</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Laser, &#955; = 532 nm&#160;</td>
			<td style="width:112px;">Del Mar Photonics</td>
			<td style="width:119px;">DMPV-532-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Microgram Balance</td>
			<td style="width:112px;">Mettler Toledo&#160;</td>
			<td style="width:119px;">XP205</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Infrared Thermocouples</td>
			<td style="width:112px;">Omega Engineering</td>
			<td style="width:119px;">OS801-HT</td>
			<td></td>
		</tr>
	</tbody>
</table>

multifunctional hybrid fe2o3-au nanoparticles for efficient plasmonic heating

One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to neutral gold Au(0) by reducing agents, such as sodium citrate or sodium borohydride. The extension of this method to decorate iron oxide or similar nanoparticles with gold nanoparticles to create multifunctional hybrid Fe2O3-Au nanoparticles is straightforward. This approach yields fairly good control over Au nanoparticle dimensions and loading onto Fe2O3. Additionally, the Au metal size, shape, and loading can easily be tuned by changing experimental parameters (e.g., reactant concentrations, reducing agents, surfactants, etc.). An advantage of this procedure is that the reaction can be done in air or water, and, in principle, is amenable to scaling up. The use of such optically tunable Fe2O3-Au nanoparticles for hyperthermia studies is an attractive option as it capitalizes on plasmonic heating of gold nanoparticles tuned to absorb light strongly in the VIS-NIR region. In addition to its plasmonic effects, nanoscale Au provides a unique surface for interesting chemistries and catalysis. The Fe2O3 material provides additional functionality due to its magnetic property. For example, an external magnetic field could be used to collect and recycle the hybrid Fe2O3-Au nanoparticles after a catalytic experiment, or alternatively, the magnetic Fe2O3 can be used for hyperthermia studies through magnetic heat induction. The photothermal experiment described in this report measures bulk temperature change and nanoparticle solution mass loss as functions of time using infrared thermocouples and a balance, respectively. The ease of sample preparation and the use of readily available equipment are distinct advantages of this technique. A caveat is that these photothermal measurements assess the bulk solution temperature and not the surface of the nanoparticle where the heat is transduced and the temperature is likely to be higher.

Material composition is an important consideration for hybrid materials. Energy dispersive X-ray analysis (EDX) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) can provide this information. EDX analysis provides semi-quantitative data (Figure 2) while ICP-MS provides accurate, quantitative information regarding the elements of interest. It is found that the hybrid Fe2O3-Au nanoparticles have Fe and Au concentrations of &#961;Fe = 150 ppb and &#961;Au = 49 ppb. In comparison, pure Au nanoparticles, which are used as a control for photothermal heating, have much higher Au concentrations of &#961;Au = 1,100 ppb.
SEM analysis reveals the morphology of the Fe2O3-Au nanoparticles (Figure 3), showing aggregates of rounded, irregular particles that appear functionalized with smaller, bright, and rounded nanoparticles. The larger nanoparticles are identified as Fe2O3, while the smaller, brighter nanoparticles are identified as Au. This type of morphology is often referred to as &#34;decorated&#34; nanoparticles.14 In this case, the surface of the supporting particle, Fe2O3, is adorned with smaller, isolated Au nanoparticles. Statistical analysis of the nanoparticles reveals that Fe2O3 nanoparticles have an average diameter of d = 40 &#177; 10 nm. The functionalizing Au nanoparticles have a wider range of sizes, with d = 20 &#177; 20 nm. Dynamic Light Scattering (DLS) measurements can quantify the aggregation behavior, and it is found that the hybrid Fe2O3-Au nanoparticles have an average hydrodynamic radius of dh = 243 nm with population bins at dh = 61 nm (13%) and dh = 310 nm (87%). Additionally, the zeta potential is found to &#950; = -16 mV, which might help to limit the aggregation behavior.
The UV-vis-NIR spectrum of the hybrid Fe2O3-Au nanoparticles is shown in Figure 4A. A distinct absorbance peak is observed at wavelength &#955; &#8776; 520 nm, and is attributed to the LSPR mode of the Au nanoparticles functionalizing the Fe2O3. The wavelength of the LSPR is consistent with literature values for AuNPs with similar morphologies.11,12 The plasmonic behavior of the hybrid structures is due to AuNP formation on the Fe2O3 supports. This can be directly observed by in-situ UV-vis spectroscopy. Figure 4B shows the UV-vis absorbance spectra of the reactant solution at various times during the reaction. Initially, there is some slight visible light absorbance attributed to the Fe2O3 nanoparticles dispersed in the solution. As the reaction proceeds, the absorbance increases, and at 1.5 min, a peak begins to form, which becomes better defined as the reaction goes on. This peak results from LSPR absorbance and corresponds with the formation of AuNPs and their deposition on the Fe2O3 support surface. The magnetic behavior of the Fe2O3-Au nanoparticles is readily observed through manipulation with an external magnetic field. Initially, the Fe2O3-Au solution has a brownish color (Figure 5B). However, after placing the solution in an external magnetic field, the solution gradually turns clear over several minutes as the entirety of the magnetic hybrid nanoparticles is collected by the field (Figure 5C). The magnetic collection is reversible, and the multifunctional nanoparticles can be re-dispersed by agitating the solution, as shown in Figures 5D and 5E.
Photothermal heating measurements are shown in Figure 6A, which plots the bulk temperature change in irradiated solution, &#916;T, as a function of time for the hybrid Fe2O3-Au nanoparticles, AuNPs, and pure deionized water (DI H2O). The Fe2O3-Au and Au nanoparticles exhibit an almost identical temperature profile, with temperatures increasing by more than 40 &#176;C. Clearly, the plasmonic absorbances of both nanoparticles types are able to transduce light into heat very efficiently, but the Fe2O3-Au do so with a considerably lower concentration of Au, as discussed above. On the other hand, the DI H2O experiment shows no change in temperature, which demonstrates that the temperature rise in the nanoparticle solutions is solely due to the dissipation of absorbed electromagnetic energy in the nanoparticles. &#916;T in Figure 6A describes the bulk change temperature, and temperatures in the irradiated region and near the nanoparticle surfaces can be much higher.13 The change in the mass of the solution, &#916;m, that arises from steam generation is one indicator of these higher temperatures. Figure 6B plots &#916;m versus time for the hybrid Fe2O3-Au nanoparticles and for DI H2O. &#916;m for the nanoparticle solution is much greater than the background evaporation rate, indicating sufficiently high surface temperatures to generate steam at a significant rate.
<img alt="Figure 1" src="/files/ftp_upload/53598/53598fig1.jpg" /> 
Figure 1. Schematic of the laser heating setup. A cuvette is placed on a microgram scale and illuminated by a laser beam from above. Two IR thermocouples measure the temperature of the cuvette and ambient, respectively. All measurements are synchronized and logged in a data collection program. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53598/53598fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53598/53598fig2.jpg" /> 
Figure 2. Representative EDX spectrum of the hybrid Fe2O3-Au nanoparticles. The abscissa axis corresponds with the energy and the ordinate axis corresponds with the number of counts. Peaks have been labeled with the corresponding element.<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53598/53598fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53598/53598fig3.jpg" /> 
Figure 3. SEM image of the hybrid Fe2O3-Au nanoparticles. The larger, darker regions are Fe2O3 particles, which are decorated with smaller brighter Au nanoparticles. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53598/53598fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53598/53598fig4.jpg" width="400" /> 
Figure 4. Optical properties. (A) UV-vis absorbance spectra of the hybrid Fe2O3-Au nanoparticles, showing the broad visible light absorbance of Fe2O3and the plasmonic peak attributed to the Au nanoparticles near 530 nm. (B) The UV-vis absorbance spectra of the reactant solution at various times during the reaction, showing the LSPR absorbance arising from AuNP formation in the solution and on the Fe2O3 nanoparticles. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53598/53598fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/53598/53598fig5.jpg" /> 
Figure 5. Magnetic Properties. Photographs of Au-Fe2O3 nanoparticles; (A) dispersed in aqueous solution; (B) magnetic manipulation (time = 0 sec); (C) magnetic manipulation (time = 2 min); (D) magnet removed; (E) Au-Fe2O3 nanoparticles following magnetic manipulation, showing that they can be easily re-dispersed in the aqueous solution. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53598/53598fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/53598/53598fig6.jpg" /> 
Figure 6. Photothermal experiments. Plots showing the (A) change in solution temperature, &#916;T, and (B) mass loss, &#916;m, as functions of time. Under laser illumination, the nanoparticles (black and red curves) generate sizeable &#916;T and &#916;m values that are significantly larger than those occurring for pure DI H2O under identical conditions (blue curve). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53598/53598fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating at JoVE.com

Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

We describe the synthesis and properties of multifunctional Fe2O3-Au nanoparticles produced by a wet chemical approach and investigate their photothermal properties using laser irradiation. The composite Fe2O3-Au nanoparticles retain the properties of both materials, creating a multifunctional structure with excellent magnetic and plasmonic properties.

Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to ...

multifunctional-hybrid-fe2o3-au-nanoparticles-for-efficient-plasmonic

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13.6K Views.  Savannah River National Laboratory. We describe the synthesis and properties of multifunctional Fe2O3-Au nanoparticles produced by a wet chemical approach and investigate their photothermal properties using laser irradiation. The composite Fe2O3-Au nanoparticles retain the properties of both materials, creating a multifunctional structure with excellent magnetic and plasmonic properties.

Video: Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

Optical Trapping of Nanoparticles

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping and manipulating small biological particles. Ashkin and co-workers first demonstrated optical tweezers using a single focused beam1. The single beam trap can be described accurately using the perturbative gradient force formulation in the case of small Rayleigh regime particles1. In the perturbative regime, the optical power required for trapping a particle scales as the inverse fourth power of the particle size. High optical powers can damage dielectric particles and cause heating. For instance, trapped latex spheres of 109 nm in diameter were destroyed by a 15 mW beam in 25 sec1, which has serious implications for biological matter2,3.
A self-induced back-action (SIBA) optical trapping was proposed to trap 50 nm polystyrene spheres in the non-perturbative regime4. In a non-perturbative regime, even a small particle with little permittivity contrast to the background can influence significantly the ambient electromagnetic field and induce a large optical force. As a particle enters an illuminated aperture, light transmission increases dramatically because of dielectric loading. If the particle attempts to leave the aperture, decreased transmission causes a change in momentum outwards from the hole and, by Newton's Third Law, results in a force on the particle inwards into the hole, trapping the particle. The light transmission can be monitored; hence, the trap can become a sensor. The SIBA trapping technique can be further improved by using a double-nanohole structure.
The double-nanohole structure has been shown to give a strong local field enhancement5,6. Between the two sharp tips of the double-nanohole, a small particle can cause a large change in optical transmission, thereby inducing a large optical force. As a result, smaller nanoparticles can be trapped, such as 12 nm silicate spheres7 and 3.4 nm hydrodynamic radius bovine serum albumin proteins8. In this work, the experimental configuration used for nanoparticle trapping is outlined. First, we detail the assembly of the trapping setup which is based on a Thorlabs Optical Tweezer Kit. Next, we explain the nanofabrication procedure of the double-nanohole in a metal film, the fabrication of the microfluidic chamber and the sample preparation. Finally, we detail the data acquisition procedure and provide typical results for trapping 20 nm polystyrene nanospheres.

The following setup approach details low power optical trapping of dielectric nanoparticles using a double-nanohole in metal film.

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping ...

Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on photoconduction, which has been one of the most commonly used techniques for terahertz generation 1-8. Terahertz generation in a photoconductive emitter is achieved by pumping an ultrafast photoconductor with a pulsed or heterodyned laser illumination. The induced photocurrent, which follows the envelope of the pump laser, is routed to a terahertz radiating antenna connected to the photoconductor contact electrodes to generate terahertz radiation. Although the quantum efficiency of a photoconductive emitter can theoretically reach 100%, the relatively long transport path lengths of photo-generated carriers to the contact electrodes of conventional photoconductors have severely limited their quantum efficiency. Additionally, the carrier screening effect and thermal breakdown strictly limit the maximum output power of conventional photoconductive terahertz sources. To address the quantum efficiency limitations of conventional photoconductive terahertz emitters, we have developed a new photoconductive emitter concept which incorporates a plasmonic contact electrode configuration to offer high quantum-efficiency and ultrafast operation simultaneously. By using nano-scale plasmonic contact electrodes, we significantly reduce the average photo-generated carrier transport path to photoconductor contact electrodes compared to conventional photoconductors 9. Our method also allows increasing photoconductor active area without a considerable increase in the capacitive loading to the antenna, boosting the maximum terahertz radiation power by preventing the carrier screening effect and thermal breakdown at high optical pump powers. By incorporating plasmonic contact electrodes, we demonstrate enhancing the optical-to-terahertz power conversion efficiency of a conventional photoconductive terahertz emitter by a factor of 50 10.

We describe methods for the design, fabrication, and experimental characterization of plasmonic photoconductive emitters, which offer two orders of magnitude higher terahertz power levels compared to conventional photoconductive emitters.

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on ...

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 ...

Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A single gold nanoparticle held by an optical tweezer is used as a sensor to quantify the rhythmic motion of a Nauplius larva (Artemia salina) in a water sample. This is achieved by monitoring the time dependent displacement of the trapped nanoparticle as a consequence of the Nauplius activity. A Fourier analysis of the nanoparticle&#39;s position then yields a frequency spectrum that is characteristic to the motion of the observed species. This experiment demonstrates the capability of this method to measure and characterize the activity of small aquatic larvae without the requirement to observe them directly and to gain information about the position of the larvae with respect to the trapped particle. Overall, this approach could give an insight on the vitality of certain species found in an aquatic ecosystem and could expand the range of conventional methods for analyzing water samples.

Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

We use optical tracking of plasmonic nanoparticles to probe and characterize the frequency movements of aquatic organisms.

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A ...

Energy Dispersive X-ray Tomography for 3D Elemental Mapping of Individual Nanoparticles

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial resolution, and is even capable of providing atomically resolved elemental maps. In this technique, a highly focused electron beam is incident upon a thin sample and the energy of emitted X-rays is measured in order to determine the atomic species of material within the beam path. This elementally sensitive spectroscopy technique can be extended to three dimensional tomographic imaging by acquiring multiple spectrum images with the sample tilted along an axis perpendicular to the electron beam direction.
Elemental distributions within single nanoparticles are often important for determining their optical, catalytic and magnetic properties. Techniques such as X-ray tomography and slice and view energy dispersive X-ray mapping in the scanning electron microscope provide elementally sensitive three dimensional imaging but are typically limited to spatial resolutions of &gt; 20 nm. Atom probe tomography provides near atomic resolution but preparing nanoparticle samples for atom probe analysis is often challenging. Thus, elementally sensitive techniques applied within the scanning transmission electron microscope are uniquely placed to study elemental distributions within nanoparticles of dimensions 10-100 nm.
Here, energy dispersive X-ray (EDX) spectroscopy within the STEM is applied to investigate the distribution of elements in single AgAu nanoparticles. The surface segregation of both Ag and Au, at different nanoparticle compositions, has been observed.

The use of energy dispersive X-ray tomography in the scanning transmission electron microscope to characterize elemental distributions within single nanoparticles in three dimensions is described.

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial ...

Measurement of Scattering Nonlinearities from a Single Plasmonic Nanoparticle

Plasmonics, which are based on the collective oscillation of electrons due to light excitation, involve strongly enhanced local electric fields and thus have potential applications in nonlinear optics, which requires extraordinary optical intensity. One of the most studied nonlinearities in plasmonics is nonlinear absorption, including saturation and reverse saturation behaviors. Although scattering and absorption in nanoparticles are closely correlated by the Mie theory, there has been no report of nonlinearities in plasmonic scattering until very recently. 
Last year, not only saturation, but also reverse saturation of scattering in an isolated plasmonic particle was demonstrated for the first time. The results showed that saturable scattering exhibits clear wavelength dependence, which seems to be directly linked to the localized surface plasmon resonance (LSPR). Combined with the intensity-dependent measurements, the results suggest the possibility of a common mechanism underlying the nonlinear behaviors of scattering and absorption. These nonlinearities of scattering from a single gold nanosphere (GNS) are widely applicable, including in super-resolution microscopy and optical switches. 
In this paper, it is described in detail how to measure nonlinearity of scattering in a single GNP and how to employ the super-resolution technique to enhance the optical imaging resolution based on saturable scattering. This discovery features the first super-resolution microscopy based on nonlinear scattering, which is a novel non-bleaching contrast method that can achieve a resolution as low as l/8 and will potentially be useful in biomedicine and material studies.

Saturable and reverse saturable scattering were discovered in isolated plasmonic particles and adopted as a novel non-bleaching contrast method in super-resolution microscopy. Here the experimental procedures of detecting and extracting nonlinear scattering are explained in detail, as well as how to enhance resolution with the aid of saturated excitation microscopy.

Plasmonics, which are based on the collective oscillation of electrons due to light excitation, involve strongly enhanced local electric fields and thus ...

Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few can observe immobilized particles. Moreover, a limited number of studies have experimentally measured the exertable forces on the particles. The designs can be classified as the protruding nanodisk type or the suppressed nanohole type. For the latter, microscopic observation is extremely challenging. In this paper, a new plasmonic tweezer system is introduced to monitor particles, both in directions parallel and orthogonal to the symmetric axis of a plasmonic nanohole structure. This feature enables us to observe the movement of each particle near the rim of the nanohole. Furthermore, we can quantitatively estimate the maximal trapping forces using a new fluidic channel.

A microchip fabrication process that incorporates plasmonic tweezers is presented here. The microchip enables the imaging of a trapped particle to measure maximal trapping forces.

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

Subsurface Defect Localization by Structured Heating Using Laser Projected Photothermal Thermography

The presented method is used to locate subsurface defects oriented perpendicularly to the surface. To achieve this, we create destructively interfering thermal wave fields that are disturbed by the defect. This effect is measured and used to locate the defect. We form the destructively interfering wave fields by using a modified projector. The original light engine of the projector is replaced with a fiber-coupled high-power diode laser. Its beam is shaped and aligned to the projector&#39;s spatial light modulator and optimized for optimal optical throughput and homogeneous projection by first characterizing the beam profile, and, second, correcting it mechanically and numerically. A high-performance infrared (IR) camera is set up according to the tight geometrical situation (including corrections of the geometrical image distortions) and the requirement to detect weak temperature oscillations at the sample surface. Data acquisition can be performed once a synchronization between the individual thermal wave field sources, the scanning stage, and the IR camera is established by using a dedicated experimental setup which needs to be tuned to the specific material being investigated. During data post-processing, the relevant information on the presence of a defect below the surface of the sample is extracted. It is retrieved from the oscillating part of the acquired thermal radiation coming from the so-called depletion line of the sample surface. The exact location of the defect is deduced from the analysis of the spatial-temporal shape of these oscillations in a final step. The method is reference-free and very sensitive to changes within the thermal wave field. So far, the method has been tested with steel samples but is applicable to different materials as well, in particular to temperature sensitive materials.

This method aims at locating vertical subsurface defects. Here, we couple a laser with a spatial light modulator and trigger its video input to heat a sample surface deterministically with two anti-phased modulated lines while acquiring highly resolved thermal images. The defect position is retrieved from evaluating thermal wave interference minima.

The presented method is used to locate subsurface defects oriented perpendicularly to the surface. To achieve this, we create destructively interfering ...

Controllable Nucleation of Cavitation from Plasmonic Gold Nanoparticles for Enhancing High Intensity Focused Ultrasound Applications

In this study, plasmonic gold nanoparticles were simultaneously exposed to pulsed near-infrared laser light and high intensity focused ultrasound (HIFU) for the controllable nucleation of cavitation in tissue-mimicking gel phantoms. This in vitro protocol was developed to demonstrate the feasibility of this approach, for both enhancement of imaging and therapeutic applications for cancer. The same apparatus can be used for both imaging and therapeutic applications by varying the exposure duration of the HIFU system. For short duration exposures (10 &#181;s), broadband acoustic emissions were generated through the controlled nucleation of inertial cavitation around the gold nanoparticles. These emissions provide direct localization of nanoparticles. For future applications, these particles may be functionalized with molecular-targeting antibodies (e.g. anti-HER2 for breast cancer) and can provide precise localization of cancerous regions, complementing routine diagnostic ultrasound imaging. For continuous wave (CW) exposures, the cavitation activity was used to increase the localized heating from the HIFU exposures resulting in larger thermal damage in the gel phantoms. The acoustic emissions generated from inertial cavitation activity during these CW exposures was monitored using a passive cavitation detection (PCD) system to provide feedback of cavitation activity. Increased localized heating was only achieved through the unique combination of nanoparticles, laser light and HIFU. Further validation of this technique in pre-clinical models of cancer is necessary.

This protocol demonstrates the controllable nucleation of cavitation in gel phantoms, through simultaneous exposure to both near-infrared pulsed laser light and high intensity focused ultrasound (HIFU). The cavitation activity can then be used for enhancing imaging and/or therapeutic uses of HIFU.

In this study, plasmonic gold nanoparticles were simultaneously exposed to pulsed near-infrared laser light and high intensity focused ultrasound (HIFU) ...