This work was supported by the National Research Foundation of Korea (2013R1A1A1061387) and KU-KIST research fund.

1. Preparation of Precursor
<ol>
	<li>Preparation of graphene oxide (GO):
	<ol>
		<li>GO preparation using modified Hummer&#8217;s method14
		<ol>
			<li>Add 3.0 g of graphite flakes to a mixture of concentrated H2SO4/H3PO4 (360:40 ml) at RT. (Note: Special care has to be taken while using strong acids H2SO4 and H3PO4.)</li>
			<li>Add KMnO4 (18.0 g) slowly with stirring and cooling in an ice bath to maintain the temperature of reaction mixture at &#60; 35 &#176;C. (The solution become sticky with increased reaction time, need to use proper method to maintain efficient stirring.) (Note: Special care has to be taken while adding KMnO4 due to exothermic reaction.)</li>
			<li>Stir for 12 hr at 50 &#176;C and then cool to RT and pouring reaction mixture onto ice (400 ml) containing 30% H2O2 (3 ml).</li>
			<li>Filter the reaction mixture using a metal U.S. Standard testing sieve (300 &#181;m) to remove unreacted graphite and centrifuge the filtrate (4,722 x g speed for 2 hr) to remove the supernatant.</li>
			<li>Repeat the centrifugation step with 200 ml of water, 200 ml of 30% HCl, 200 ml of ethanol, and distilled water again until pH of solution reach at 5.0&#8211;6.0.</li>
			<li>Lyophilize the final solutions to produce a fluffy GO powder.</li>
			<li>In order to make nanosized GO solution, dissolve 20 mg of GO powder in 40 ml of triple distilled water (&#62; 18 M&#937;), and then exfoliate by prolonged sonication (35% amplitude, 500 W, 2 hr) until the entire size distribution become below 150 nm, then centrifuge it two times (10,625 x g speed, 15 min) to remove precipitates (un-exfoliated large GO sheets).&#160;&#160;</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Preparation of plasmonic nanoparticle
	<ol>
		<li>Preparation of AuNPs
		<ol>
			<li>Citrate-stabilized spherical shape gold nanoparticle (AuNPs, O.D. = 1.0) of 30 nm particles size was used for r-GO reduction.</li>
		</ol>
		</li>
		<li>Preparation of AuNRs15
		<ol>
			<li>Prepare the seed solution by adding a freshly prepared 0.6 ml ice-cold solution of NaBH4 solution (0.01 M) into an aqueous mixture solution compose of 0.25 ml of HAuCl4 (0.01 M) and 9.75 ml of cetyltrimethylammonium bromide (CTAB, 0.1 M).</li>
			<li>Stir the resulting mixture vigorously for 0.5 min and then keep it at 28 &#176;C for 3 hr.</li>
			<li>Prepare the growth solution by mixing 475 ml of CTAB (0.1 M), 3 ml of AgNO3 (0.01 M) and 20 ml of HAuCl4 (0.01 M).</li>
			<li>Then add freshly prepared 3.2 ml of ascorbic acid (0.01 M) to the mixture followed by the addition of 0.8 ml of an aqueous HCl (1.0 M) solution.</li>
			<li>In the final step add 3.2 ml of seed solution to the growth solution at 28 &#176;C and subject the reaction mixture to quick inversion for few seconds. Finally, keep the resulting mixture undisturbed for at least 6 hr.</li>
			<li>Analyze the prepared AuNRs with UV-Visible spectroscopy for absorption maxima (&#955;max) and TEM analysis (typically the &#955;max and aspect ratio was found to be 730 nm and 3.5, respectively).</li>
		</ol>
		</li>
		<li>Preparation of AuNSs16
		<ol>
			<li>Prepare aqueous stock solution of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) with concentration of 100 mM, and adjust the pH to 7.4 at 25 &#176;C by adding 1.0 M NaOH solution.</li>
			<li>Mix 20 ml of phosphate buffer (100 mM) with 30 ml of 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (100 mM).</li>
			<li>Then add 500 &#181;l of gold(III) chloride trihydrate (20 mM) to the above mixture and keep at 28.5 &#176;C for 30 min in water bath. Solution color changes from light yellow to greenish blue after 30 min could be observed.</li>
			<li>Centrifuge the solution at 8,928 x g speed for 30 min and disperse the precipitates in distilled water.</li>
			<li>Finally, analyze the prepared AuNSs with UV-Visible spectroscopy for absorption maxima (&#955;max) and TEM analysis for particles size confirmation which is found to be 740 nm and 30 nm, respectively.&#160;&#160;</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
2. Preparation of r-GO Using Visible Light and AuNPs
<ol>
	<li>Add 1 ml of plasmonic nanoparticles (Abs 1.0 at 520 nm for AuNPs, Abs 1.0 at 750 nm for AuNRs, and Abs 1.0 at 730 nm for AuNSs, respectively) and 100 &#181;l of ammonium hydroxide (28%, w/v%) to 10 ml of GO solution (OD 1.0 at 230 nm, 0.125 mg ml-1) placed in a Pyrex glass reactor equipped with a water-circulating jacket.</li>
	<li>Irradiate the mixture with Xe lamp (power density of 1.56 W cm-2) for 30 min with water circulation through water-circulating jacket to maintain the temperature at 25 &#176;C and then centrifuge the solution at 10,625 x g speed for 15 min to remove gold nanoparticles.</li>
	<li>Take the supernatant containing the prepared r-GO to analyze with UV-Visible spectrophotometer (r-GO should show the distinctive absorption band at 270 nm) in the range of 200&#8211;900 nm.</li>
</ol>
3. Target DNA Detection Using r-GO Solution17
<ol>
	<li>For fluorescence quenching, add 20 &#181;l of 10-6 M Cy3-modified ssDNA (5&#39;-ATC CTT ATC AAT ATT TAA CAA TAA TCC CTC-Cy3-3&#39;) into GO or r-GO solution containing 25 &#181;l of GO (0.125 mg ml-1) or r-GO (0.125 mg ml-1) in 1,955 &#181;l of 0.3 M PBS solution (10 mM phosphate buffer, 0.3 M NaCl) and incubate for 10 min at RT.</li>
	<li>Measure the fluorescence intensity of these samples with spectrofluorometer (&#955;ex= 529 nm).</li>
	<li>For Target Detection, add 200 &#181;l of target oligonucleotide solution (5&#39;- GAG GGA TTA TTG TTA AAT ATT GAT AAG GAT- 3&#39;) in three different concentrations (10-6 M, 10-7 M, 10-8 M) into the GO or r-GO solution containing 20 &#181;l of 10-6 M ssDNA-Cy3, 25 &#181;l of GO or r-GO (0.125 mg ml-1), and 1,755 &#181;l of 0.3 M PBS for fluorescence recovery experiment.17 
	Notes: 
	Light Sources &#38; Reactor 
	Visible light (400&#8211;780 nm) source. Visible light irradiate through Pyrex glass reactor (window diameter = 1.1 cm) containing GO solution using Xe lamp (1.56 W/cm2 power). The photon energy applied to the reactor is calculated to be 4.8 &#215; 1021 photons per min (Figure 2A-2C). 
	Near-infrared (NIR) laser. NIR laser (window diameter = 13.2 cm) with power density of 0.36 W/cm2, and operating wavelength of 808 nm has been used as the source of near-infrared light for GO reduction reactions (Figure 2E). The photon energy is calculated to be 2.43 &#215; 1021 photons per min. 
	Reactor: Pyrex glass reactor (window diameter = 1.1 cm; reaction volume = 10 ml) equipped with a water-circulating jacket is used for both visible light and NIR light irradiated GO reduction reactions (Figure 2F).</li>
</ol>

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We have nothing to disclose.

Visible light irradiation onto GO solution for 30 min with gold nanoparticles (AuNPs, AuNSs &#38; AuNRs) showed the rapid color changes from light yellow-brown to black color (Figure 1). To obtain highly pure r-GO product in high yield, there are two important factors need to follow. One is the use of AuNPs as an efficient plasmonic catalyst, since AuNPs can strongly absorb the visible light among other structures (i.e., AuNRs, AuNSs). Another is the use of nanosized GO solution to obtain nanoparticle-free highly pure r-GO product. The used plasmonic nanoparticles as a photocatalyst should be completely removed, which is easily achievable by applying simple centrifugation steps at 10,625 x g speed for 15 min. But in doing this, the large sheet size of r-GO (&#62; 500 nm) can be centrifuged down with gold nanoparticles which lead to the great loss of the product (r-GO). Therefore, using nanosized GO solution (average sheet size &#60; 150&#8211;200 nm) is very important, because the nanosized r-GO is not possible to be centrifuged down using such a usual centrifugation conditions (i.e., 10,625 x g speed for 15 min).
Therefore, the use of AuNPs and the use of nanosized GO solution are critical factors to obtain highly pure r-GO solution in a high yield. To get powder form of product, it is required to perform additional freeze drying steps. To confirm the successful formation of r-GO, the measurement of UV spectra will be one of simple method. The red shift in UV-Vis spectra from 230 nm to 270 nm is one of clear indication of the successful conversion of GO into r-GO (Figure 3C). To verify the complete removal of used gold nanoparticles, the measurement with UV-Vis and HR-TEM analysis are also required as shown in Figure 3B &#38; 3C.
The disappearance of graphite peak at 26.48 in GO and the (001) peak at 10.2 corresponding to GO in the XRD spectrum showed the successful formation of r-GO (Figure 4A). GO and r-GO prepared by a chemical method or a light-induced method were analyzed qualitatively by Raman spectrometry as displayed in Figure 4B. The D band corresponding to disordered structures and edge planes and the G band corresponding to ordered sp2 bonded carbon appeared at 1,327 cm-1 and 1,590 cm-1, respectively, in the Raman spectrum of GO.18 The D and G bands at 1,336 and 1,592 cm-1 were also present in the Raman spectrum of the chemically reduced GO, visible light irradiated reduced GO and plasmon assisted visible light irradiated reduction of GO. The D/G intensity ratios (ID/IG) were found to be 1.03, 1.13, 1.12, 1.13, 1.13 and 1.13 for GO and r-GO prepared by a chemical method or a light-induced method without or with NPs (AuNRs, AuNPs, and AuNSs), respectively. The XPS analysis is the most convincing and quantitative analytical method to verify the successful conversion of GO into r-GO product. The C/O ratios (based on the intensity of each element (carbon and oxygen) were found to be 1.95, 4.81, 3.74, and 5.19 for GO, r-GO (hydrazine), r-GO (hv only), and r-GO (hv + AuNPs), respectively (Figure 5).
The possible limitation of the current method for r-GO synthesis is the required light source such as Xe-lamp for reactions. But one possible, promising and ultimate solution for this limitation is the use of sun light as a light source since the sun light are mainly composed of UV and visible light. But in this case, longer illumination time is expected to be the one possible problem.
There are numerous potential applications of r-GO,19-24 one of important properties for bio application is the fluorescence quenching effect of r-GO. In this protocol, we described simple application of using r-GO for sensitive DNA detection scheme. As described the results in Figure 6, the r-GO prepared using visible light and plasmonic nanoparticles showed excellent properties for fluorescence quenching and recovery in the presence of the target DNA as compared to r-GO prepared by chemical method (Figure 6B).
In this protocol, we described the simple synthetic method for r-GO using visible light and its analytical method and applications. As discussed, the future modifications of this method will be the use of sun light which regarded as the most eco-friendly energy sources for reactions.

The first developed scotch-tape based method1 and chemical vapor deposition2 were excellent methods to produce the pristine state of a graphene, but the large scale graphene synthesis or graphene layer formation on the surface with wide area have been regarded as a key limitation of previous methods.3 One of possible solution for large scale r-GO synthesis will be wet-chemical synthetic method which first requires the reactions with strong oxidants, extensive physical treatment such as sonication to produce GO sheet, and finally the reduction of oxygen functionalities such as hydroxy, epoxide and carbonyl groups in GO is essential in order to recover its original physical properties.4 Mostly, the reduction of GO was carried out with either chemical method using hydrazine or its derivatives5 or by thermal treatment method (550&#8211;1,100 &#176;C) in an inert or reducing atmosphere.6
These processes require the toxic chemicals, long reaction time and high temperature which increased total energy demand for r-GO synthesis.7 While the photo-irradiating reduction processes such as UV-induced,8 photo-thermal process using a pulsed xenon flash,9 pulsed laser assisted10 and photo-thermal heating with camera flash lights11 have also been reported for the preparation of r-GO. In general, the low conversion efficiency of the photo-induced methods propagated to the use of UV or pulsed laser irradiation that can deliver high photon energy. The low photon energy of visible light limits its use and not attracted much for r-GO synthesis. Excellent light absorption properties of plasmonic nanoparticles in the visible and/or NIR regions can greatly improve the current drawbacks of the use of visible light for r-GO synthesis.12,13 Mild reaction conditions, short reaction time and limited use of toxic chemicals could make the visible light induced plasmon assisted photocatalytic reduction of GO as a useful alternative method.
In present method, we describe the efficient and simple r-GO synthetic method using plasmonic nanoparticles and visible light. The reaction progress was found to be strongly dependent on the structures of plasmonic nanoparticles such as spherical gold nanoparticles (AuNPs), gold nanorods (AuNRs), and gold nanostars (AuNSs). The use of AuNPs showed the most efficient reduction of GO and the nanoparticles are easily removable and recyclable for the repeated use (Figure 1). The r-GO synthesized using visible light and AuNPs showed almost equal quality compared with the r-GO prepared by well-known chemical method (hydrazine) as demonstrated by use of various analytical measurements and the fluorescence quenching/recovery based DNA detection method.

<table><tbody><tr><td>Cy3 modifeid ssDNA</td><td>IDT(Iowa, USA)</td><td></td><td>HPLC purified by IDT</td></tr><tr><td>Gold nanoparticles (30&amp;#160;nm)</td><td>Ted Pella, Inc(Redding, CA, USA).</td><td>15706-20</td><td>colloidal solution</td></tr><tr><td>4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid (HEPES) (99.5%)</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>7365-45-9</td><td></td></tr><tr><td>Gold(III) Chloride Hydrate (99.999%)</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>&amp;nbsp;27988-77-8</td><td>strongly hygroscopic</td></tr><tr><td>Sodium Borohydride (99.99%)</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>16940-66-2</td><td></td></tr><tr><td>Hexadecyltrimethylammonium bromide (&amp;ge;99%)</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>57-09-0</td><td></td></tr><tr><td>L-Ascorbic Acid(&amp;ge;99.0%)</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>50-81-7</td><td></td></tr><tr><td>Sodium Chloride (99.5%)</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>7647-14-5</td><td></td></tr><tr><td>Silver Nitrate&amp;nbsp; (&amp;ge;99.0%)</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>7761-88-8</td><td></td></tr><tr><td>Graphite</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>7782-42-5</td><td></td></tr><tr><td>Sulfuric acid</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>7664-93-9</td><td></td></tr><tr><td>Phophoric acid</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>7664-38-2</td><td></td></tr><tr><td>Potassium permanganate</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>7722-64-7</td><td></td></tr><tr><td>Hydrogen peroxide</td><td>JUNSEI</td><td>23150-0350</td><td></td></tr><tr><td>Ammonium hydroxide</td><td>Sigma-Aldrich (St. Louis, MO, USA)</td><td>1336-21-6</td><td></td></tr><tr><td>Xe-lamp</td><td>&amp;nbsp;Cermax, Waltham, USA</td><td></td><td></td></tr><tr><td>NIR Laser</td><td>Class-IV, Sanctity Laser, Shanghai, China&amp;nbsp;</td><td></td><td>6W (output power)</td></tr><tr><td>UV-Vis spectrophotometer&amp;nbsp;</td><td>S-3100, SINCO, South Korea</td><td></td><td></td></tr><tr><td>Transmission Electron Microscopy</td><td>H-7650, Hitachi, Japan</td><td></td><td></td></tr><tr><td>Spectro Fluorometer</td><td>Jasco FP-6500, Tokyo, Japan</td><td></td><td></td></tr><tr><td>X-ray Photoelectron Spectrometer</td><td>AXIS&amp;ndash;NOVA, KRATOS Inc., UK</td><td></td><td></td></tr></tbody></table>

visible-light induced reduction of graphene oxide using plasmonic nanoparticle

Present work demonstrates the simple, chemical free, fast, and energy efficient method to produce reduced graphene oxide (r-GO) solution at RT using visible light irradiation with plasmonic nanoparticles. The plasmonic nanoparticle is used to improve the reduction efficiency of GO. It only takes 30 min&#160;at RT by illuminating the solutions with Xe-lamp, the r-GO solutions can be obtained by completely removing gold nanoparticles through simple centrifugation step. The spherical gold nanoparticles (AuNPs) as compared to the other nanostructures is the most suitable plasmonic nanostructure for r-GO preparation. The reduced graphene oxide prepared using visible light and AuNPs was equally qualitative as chemically reduced graphene oxide, which was supported by various analytical techniques such as UV-Vis spectroscopy, Raman spectroscopy, powder XRD and XPS. The reduced graphene oxide prepared with visible light shows excellent quenching properties over the fluorescent molecules modified on ssDNA and excellent fluorescence recovery for target DNA detection. The r-GO prepared by recycled AuNPs is found to be of same quality with that of chemically reduced r-GO. The use of visible light with plasmonic nanoparticle demonstrates the good alternative method for r-GO synthesis.

Figure 1 shows the overall scheme for visible light and plasmonic nanoparticle based r-GO reduction reaction. Figure 2 shows the instrumental setup for the reactions. After reaction, it is required the centrifugation step to remove the used photocatalyst (AuNSs, AuNRs, or AuNPs) as shown in Figure 3A. The HRTEM analysis shows the complete removal of nanoparticles in the supernatant (r-GO) (Figure 3B), which is also possible to confirm with UV-Visible analysis as shown in Figure 3C, the absorption band around 500&#8211;800 nm from the r-GO and nanoparticle mixture solution was disappeared after centrifugation step indicating the complete removal of plasmonic nanoparticles in r-GO product. The structural changes in the r-GO have been analyzed by XRD technique. Disappearance of GO peak at 10.2 clearly indicated the formation of r-GO as shown in Figure 4A. The D/G intensity ratios (ID/IG) of GO and r-GO produced by a chemical method or a light-induced method without or with NPs (AuNRs, AuNPs, and AuNSs) was measured by Raman analysis as shown in Figure 4B. The formation of r-GO was confirmed more quantitatively by comparing the C/O ratios in XPS analysis between samples as shown in Figure 5. By dividing area % of carbon (C) with the area % of oxygen (O), the C/O ratios of the prepared r-GO could be calculated, the higher number of C/O ratio indicate the higher degree of reduced state of r-GO. As shown in Figure 5, the C/O ratio of GO, r-GO (chemically reduced with hydrazine), r-GO (visible light only), and r-GO (visible light and plasmonic nanoparticle) were 1.95, 4.81, 3.74, and 5.19. These results show the usefulness of visible light and plasmonic nanoparticle based method for the preparations of r-GO.
The fluorescence quenching efficiency and recovery for target DNA detection have been performed to demonstrate the potentials of r-GO for bio applications. Figure 6A is the summarized fluorescence emission spectra of Cy3-modified DNA after incubating with GO, r-GO solutions in 0.3 M PBS, the decreased intensity indicates the efficient quenching efficiency of GO, r-GO. The r-GO prepared with AuNPs and visible light showed the most efficient quenching efficiency. When the Cy3-modified DNA bound with target DNA (anthrax DNA in this paper), the Cy3-modified DNA could form duplex and separated from the r-GO sheet, which result in the fluorescence recovery (Figure 6B). It is believed that the prepared r-GO using visible light and plasmonic nanoparticle shows as excellent as physical properties of chemically reduced r-GO (Figure 6).
<img alt="Figure 1" src="/files/ftp_upload/53108/53108fig1.jpg" /> 
Figure 1. Reaction scheme for r-GO synthesis. Schematic description of r-GO synthesis using plasmonic nanoparticle and visible light. (Re-print with the permission from ref. 13) <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53108/53108fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53108/53108fig2.jpg" /> 
Figure 2. The reactor and light sources for r-GO preparation. The photographs of (A) reactor with water circulation jacket in the box equipped with visible light source (Xe lamp), (B) enlarged images of reactor, (C) the hole to guide visible light into the reactor, (D) the spectrum of visible light from Xe-lamp, (E) NIR laser with reaction apparatus, (F) the side view of reactor (Pyrex, window diameter = 11 mm). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53108/53108fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53108/53108fig3.jpg" /> 
Figure 3. Photographs, HR-TEM images and UV-Vis spectrum of r-GO. (A) The photographs of r-GO, r-GO + AuNPs, r-GO + AuNRs, r-GO + AuNSs solution before and after centrifugation, (B) HR-TEM images of r-GO solution and precipitates, (C) the UV-Vis spectra of r-GO + AuNPs mixture, r-GO + AuNRs mixture, and r-GO + AuNSs mixture before and after centrifugation. (Re-print with the permission from ref. 13) <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53108/53108fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53108/53108fig4.jpg" /> 
Figure 4. Qualitative analysis of GO and r-GO prepared. (A) XRD data; (B) Raman spectra of GO and r-GO produced by a chemical method and a light-induced method with or without AuNPs, AuNRs, and AuNSs. (Re-print with the permission from ref. 13) <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53108/53108fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/53108/53108fig5.jpg" /> 
Figure 5. The XPS analysis of r-GO. The XPS analysis of GO solution (A), r-GO solutions prepared with chemical method (B), and light-induced method without AuNPs (C) or with AuNPs (D). (Re-print with the permission from ref. 13) <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53108/53108fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/53108/53108fig6.jpg" /> 
Figure 6. Fluorescence quenching and fluorescence recovery analysis. (A) Fluorescence quenching of ssDNA-Cy3 using GO and r-GO chemically reduced or produced using visible light and plasmonic nanoparticles, (B) fluorescence recovery using varying concentrations of target DNA (10-7 M, 10-8 M and 10-9 M). Data are means &#177; standard deviations, N=4. (Re-print with the permission from ref. 13) <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53108/53108fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Visible-light Induced Reduction of Graphene Oxide Using Plasmonic Nanoparticle at JoVE.com

Visible-light Induced Reduction of Graphene Oxide Using Plasmonic Nanoparticle

A simple protocol for the preparation of reduced graphene oxide using visible light and plasmonic nanoparticle is described.

Present work demonstrates the simple, chemical free, fast, and energy efficient method to produce reduced graphene oxide (r-GO) solution at RT using ...

visible-light-induced-reduction-graphene-oxide-using-plasmonic

Nanyang Technological University

Research

JoVE Journal

Engineering

14.4K Views.  Korea University.  Een eenvoudig protocol voor het bereiden van verminderde grafeen oxide met zichtbaar licht en plasmonische nanodeeltjes wordt beschreven. 

Video: Visible-light Induced Reduction of Graphene Oxide Using Plasmonic Nanoparticle

Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) membranes have been under development since the 1990's and have become a common method to template the synthesis of high aspect ratio nanostructures, mostly by electrochemical growth or pore-wetting. Recently, these membranes have become commercially available in a wide range of pore sizes and densities, leading to an extensive library of functional nanostructures being synthesized from AAO membranes. These include composite nanorods, nanowires and nanotubes made of metals, inorganic materials or polymers 3-10. Nanoporous membranes have been used to synthesize nanoparticle and nanotube arrays that perform well as refractive index sensors, plasmonic biosensors, or surface enhanced Raman spectroscopy (SERS) substrates 11-16, as well as a wide range of other fields such as photo-thermal heating 17, permselective transport 18, 19, catalysis 20, microfluidics 21, and electrochemical sensing 22, 23. Here, we report a novel procedure to prepare gold nanotubes in AAO membranes. Hollow nanostructures have potential application in plasmonic and SERS sensing, and we anticipate these gold nanotubes will allow for high sensitivity and strong plasmon signals, arising from decreased material dampening 15.

Solution-suspendable gold nanotubes with controlled dimensions can be synthesized by electrochemical deposition in porous anodic aluminum oxide (AAO) membranes using a hydrophobic polymer core. Gold nanotubes and nanotube arrays hold promise for applications in plasmonic biosensing, surface-enhanced Raman spectroscopy, photo-thermal heating, ionic and molecular transport, microfluidics, catalysis and electrochemical sensing.

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) ...

Chemistry

Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

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

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

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

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

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

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

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

Bioengineering

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.

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.

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

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 ...

Determination of the Excitation and Coupling Rates Between Light Emitters and Surface Plasmon Polaritons

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising from metallic periodic arrays without involving time-resolved techniques. We have formulated the rates by quantities that can be measured by simple optical measurements. The instrumentation based on angle- and polarization-resolved reflectivity and photoluminescence spectroscopy will be described in detail here. Our approach is intriguing due to its simplicity, which requires routine optics and several mechanical stages, and thus is highly affordable to most of the research laboratories.

This protocol describes the instrumentation for determining the excitation and coupling rates between light emitters and Bloch-like surface plasmon polaritons arising from periodic arrays.

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising ...

Performing Spectroscopy on Plasmonic Nanoparticles with Transmission-Based Nomarski-Type Differential Interference Contrast Microscopy

Differential interference contrast (DIC) microscopy is a powerful imaging tool that is most commonly employed for imaging microscale objects using visible-range light. The purpose of this protocol is to detail a proven method for preparing plasmonic nanoparticle samples and performing single particle spectroscopy on them with DIC microscopy. Several important steps must be followed carefully in order to perform repeatable spectroscopy experiments. First, landmarks can be etched into the sample substrate, which aids in locating the sample surface and in tracking the region of interest during experiments. Next, the substrate must be properly cleaned of debris and contaminants that can otherwise hinder or obscure examination of the sample. Once a sample is properly prepared, the optical path of the microscope must be aligned, using Kohler Illumination. With a standard Nomarski style DIC microscope, rotation of the sample may be necessary, particularly when the plasmonic nanoparticles exhibit orientation-dependent optical properties. Because DIC microscopy has two inherent orthogonal polarization fields, the wavelength-dependent DIC contrast pattern reveals the orientation of rod-shaped plasmonic nanoparticles. Finally, data acquisition and data analyses must be carefully performed. It is common to represent DIC-based spectroscopy data as a contrast value, but it is also possible to present it as intensity data. In this demonstration of DIC for single particle spectroscopy, the focus is on spherical and rod-shaped gold nanoparticles.

The goal of this protocol is to detail a proven approach for the preparation of plasmonic nanoparticle samples and for performing single particle spectroscopy on them with differential interference contrast (DIC) microscopy.

Differential interference contrast (DIC) microscopy is a powerful imaging tool that is most commonly employed for imaging microscale objects using ...

Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation

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

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

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

Graphene-Assisted Quasi-van der Waals Epitaxy of AlN Film on Nano-Patterned Sapphire Substrate for Ultraviolet Light Emitting Diodes

This protocol demonstrates a method for graphene-assisted quick growth and coalescence of AlN on nano-pattened sapphire substrate (NPSS). Graphene layers are directly grown on NPSS using catalyst-free atmospheric-pressure chemical vapor deposition (APCVD). By applying nitrogen reactive ion etching (RIE) plasma treatment, defects are introduced into the graphene film to enhance chemical reactivity. During metal-organic chemical vapor deposition (MOCVD) growth of AlN, this N-plasma treated graphene buffer enables AlN quick growth, and coalescence on NPSS is confirmed by cross-sectional scanning electron microscopy (SEM). The high quality of AlN on graphene-NPSS is then evaluated by X-ray rocking curves (XRCs) with narrow (0002) and (10-12) full width at half-maximum (FWHM) as 267.2 arcsec and 503.4 arcsec, respectively. Compared to bare NPSS, AlN growth on graphene-NPSS shows significant reduction of residual stress from 0.87 GPa to 0.25 Gpa, based on Raman measurements. Followed by AlGaN multiple quantum wells (MQWS) growth on graphene-NPSS, AlGaN-based deep ultraviolet light-emitting-diodes (DUV LEDs) are fabricated. The fabricated DUV-LEDs also demonstrate obvious, enhanced luminescence performance. This work provides a new solution for the growth of high quality AlN and fabrication of high performance DUV-LEDs using a shorter process and less costs.

A protocol for graphene-assisted growth of high-quality AlN films on nano-patterned sapphire substrate is presented.

This protocol demonstrates a method for graphene-assisted quick growth and coalescence of AlN on nano-pattened sapphire substrate (NPSS). Graphene layers ...

Visible-light Induced Reduction of Graphene Oxide Using Plasmonic Nanoparticle

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