Name: synthesis and functionalization of nitrogen-doped carbon nanotube cups with gold nanoparticles as cork stoppers
Uploaded: 2013-05-13T12:00:00
Description: Watch this Scientific Journal Video about Synthesis and Functionalization of Nitrogen-doped Carbon Nanotube Cups with Gold Nanoparticles as Cork Stoppers at JoVE.com

This work was supported by an NSF CAREER Award No. 0954345.

1. CVD Synthesis of Nitrogen-doped Carbon Nanotube Cups (NCNCs)
NCNCs were synthesized employing chemical vapor deposition (CVD) technique on quartz substrate using liquid precursors (Figure 1A).
<ol>
	<li>Place a 3 ft long quartz tube (2.5 cm i.d.) in a Lindberg/Blue tube furnace as the reaction chamber. Place a quartz plate (1" &times; 12") inside the tube as the substrate for product collection. Seal the quartz tube using homemade stainless steel caps with built-in gas an...

<ol>
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The primary goal of our experiments was to effectively produce graphitic nanocups from nitrogen-doped CNTs. However, nitrogen-doping in the CVD synthesis does not guarantee the formation of the stacked cup-shaped structure. Depending on the chemical composition of the precursor and other growth conditions, the morphology of the resulted product may vary a lot.19 The concentration of nitrogen source is the primary factor influencing the structure because the compartmented structure results from the incompatibil...

With their inherent inner cavities and versatile surface chemistry, hollow carbon-based nanomaterials, such as carbon nanotubes (CNTs), are considered to be good nanocarriers in drug delivery applications.1,2 However, the fibril structure of pristine CNTs has rather inaccessible hollow interiors and may cause severe inflammatory response and cytotoxic effects in biological systems.3,4 Nitrogen-doped CNTs, on the other hand, have been found to possess higher biocompatibility than undoped multiwalled carbon nanotubes (MWCNTs)5,6 and may have better drug delivery performance. Doping of nitrogen atoms into the nanotube graphitic lattices results in a compartmented hollow structure resembling stacked cups which can be separated out to obtain individual nitrogen-doped carbon nanotube cups (NCNCs) with typical length under 200 nm.7,8 With their accessible interiors and nitrogen functionalities which allow for further chemical functionalization, these individual graphitic cups are highly advantageous for drug delivery applications.
Among different synthetic methods for nitrogen-doped CNTs including arc-discharge9 and dc magnetron sputtering,10 chemical vapor deposition (CVD) has been the most prevalent method due to several advantages such as higher yield and easier control over nanotube growth conditions. The vapor-liquid-solid (VLS) growth mechanism is commonly employed to understand the CVD growth process of nitrogen-doped CNTs.11 Generally there are two different schemes to use metal catalyst seeds in the growth. In the "fixed-bed" scheme, iron nanoparticles with defined sizes were first synthesized by thermal decomposition of iron pentacarbonyl and then plated on quartz slides by spin coating for subsequent CVD growth.12 In the "floating catalyst" scheme, iron catalyst (typically ferrocene) was mixed and injected with carbon and nitrogen precursors, and the thermal decomposition of ferrocene provided in situ generation of iron catalytic nanoparticles on which the carbon and nitrogen precursors were deposited. While fixed-bed catalyst provides better size control over the resultant NCNCs, the yield of product is typically lower (&lt;1 mg) compared to the floating catalyst scheme (&gt;5 mg) for the same precursor amount and growth time. As the floating catalyst scheme also provides fairly uniform size distribution of NCNCs, it was adopted in this paper for CVD synthesis of NCNCs.
CVD method affords as-synthesized NCNCs which exhibit fibril morphology comprised of many stacked cups. Although there is no chemical bonding between adjacent cups,8 challenges remain in effective isolation of the individual cups because they are firmly inserted into each other's cavities and held by multiple noncovalent interactions and an outer layer of amorphous carbon.8 Attempts to separate the stacked cups include both chemical and physical approaches. While oxidation treatments in a mixture of strong acids is a typical procedure to cut CNTs and introduce oxygen functionalities,13,14 it can also be applied to cut NCNCs into shorter sections. Microwave plasma etching procedures have been also shown to separate the NCNCs.15 Compared to the chemical approaches, physical separation is more straightforward. Our previous study showed that by simply grinding with a mortar and pestle individual NCNCs can be partially isolated from their stacked structure.7 In addition, high-intensity probe-tip sonication, which was reported to effectively cut single-walled carbon nanotubes (SWCNTs),16 was also shown to have a significant effect on separation of NCNCs.8 The probe-tip sonication delivers high-intensity ultrasonic power to the NCNC solution that essentially "shakes" the stacked cups and disrupts the weak interactions that hold the cups together. While other potential separation methods are either inefficient or destructive to the cup structure, probe-tip sonication provides a highly effective, cost-efficient and less-destructive physical separation method to obtain individual graphitic cups.
The as-synthesized fibril NCNCs were first treated in concentrated H2SO4/HNO3 acid mixture prior to their separation with probe-tip sonication. The resultant separated NCNCs were highly hydrophilic and effectively dispersed in water. We have previously identified nitrogen functionalities such as amine groups on NCNCs and utilized their chemical reactivity for NCNCs functionalization.7,8,17 Compared to our previously reported method of corking NCNCs with commercial nanoparticles,8 in this work, gold nanoparticles (GNPs) were effectively anchored to the surface of the cups by citrate reduction from chloroauric acid. Due to the preferential distribution of nitrogen functionalities on the open rims of NCNCs, the GNPs synthesized in situ from the gold precursors tended to have better interaction with the open rims and form GNP "cork stoppers" on the cups. Such synthesis and functionalization methods have resulted in a novel GNP-NCNC hybrid nanomaterial for potential applications as drug delivery carriers.

<table border="1">
 <tbody>
 <tr>
 <td>Reagent Name</td>
 <td>Company</td>
 <td>Catalogue Number</td>
 <td>Comment</td>
 </tr>
 <tr>
 <td></td>
 <td></td>
 <td>&nbsp;</td>
 <td>Reagents</td>
 </tr>
 <tr>
 <td>H2</td>
 <td>Valley National Gases</td>
 <td>Grade 5.0</td>
 <td></td>
 </tr>
 <tr>
 <td>Ar</td>
 <td>Valley National Gases</td>
 <td>Grade 5.0</td>
 <td></td>
 </tr>
 <tr>
 <td>Ferrocene</td>
 <td>Sigma-Aldrich</td>
 <td>F408-500G</td>
 <td></td>
 </tr>
 <tr>
 <td>Xylenes</td>
 <td>Fisher Scientific</td>
 <td>X5-500</td>
 <td></td>
 </tr>
 <tr>
 <td>Acetonitrile</td>
 <td>EMD</td>
 <td>AXO149-6</td>
 <td></td>
 </tr>
 <tr>
 <td>H2SO4</td>
 <td>Fisher Scientific</td>
 <td>A300-500</td>
 <td></td>
 </tr>
 <tr>
 <td>HNO3</td>
 <td>EMD</td>
 <td>NX0409-2</td>
 <td></td>
 </tr>
 <tr>
 <td>DMF</td>
 <td>Fisher Scientific</td>
 <td>D119-500</td>
 <td></td>
 </tr>
 <tr>
 <td>Ethanol</td>
 <td>Decon</td>
 <td>2716</td>
 <td></td>
 </tr>
 <tr>
 <td>Phenol</td>
 <td>Sigma-Aldrich</td>
 <td>P1037-100G</td>
 <td></td>
 </tr>
 <tr>
 <td>Pyridine</td>
 <td>EMD</td>
 <td>PX2020-6</td>
 <td></td>
 </tr>
 <tr>
 <td>Hydridantin</td>
 <td>Sigma-Aldrich</td>
 <td>H2003-10G</td>
 <td></td>
 </tr>
 <tr>
 <td>Ninhydrin </td>
 <td>Alfa Aesar</td>
 <td>43846</td>
 <td></td>
 </tr>
 <tr>
 <td>HAuCl4</td>
 <td>Sigma-Aldrich</td>
 <td>52918-1G</td>
 <td></td>
 </tr>
 <tr>
 <td>Sodium Citrate</td>
 <td>SAFC</td>
 <td>W302600</td>
 <td></td>
 </tr>
 <tr>
 <td></td>
 <td></td>
 <td>&nbsp;</td>
 <td>Equipment</td>
 </tr>
 <tr>
 <td>CVD Furnace</td>
 <td>Lindberg/Blue</td>
 <td>&nbsp;</td>
 <td></td>
 </tr>
 <tr>
 <td>TEM (low-resolution)</td>
 <td>FEI Morgagni</td>
 <td>&nbsp;</td>
 <td></td>
 </tr>
 <tr>
 <td>TEM (high-resolution)</td>
 <td>JOEL</td>
 <td>2100F</td>
 <td></td>
 </tr>
 <tr>
 <td>Probe-tip Sonicator</td>
 <td>Qsonica</td>
 <td>XL-2000</td>
 <td></td>
 </tr>
 <tr>
 <td>UV-Vis Spectrometer</td>
 <td>Perkin-Elmer</td>
 <td>Lambda 900</td>
 <td></td>
 </tr>
 <tr>
 <td>Zeta Potential Analyzer</td>
 <td>Brookheaven</td>
 <td>ZetaPlus</td>
 <td></td>
 </tr>
 <tr>
 <td>EDX spectroscopy</td>
 <td>Phillips</td>
 <td>XL30 FEG</td>
 <td></td>
 </tr>
 </tbody>
</table>

synthesis and functionalization of nitrogen-doped carbon nanotube cups with gold nanoparticles as cork stoppers

Nitrogen-doped carbon nanotubes consist of many cup-shaped graphitic compartments termed as nitrogen-doped carbon nanotube cups (NCNCs). These as-synthesized graphitic nanocups from chemical vapor deposition (CVD) method were stacked in a head-to-tail fashion held only through noncovalent interactions. Individual NCNCs can be isolated out of their stacking structure through a series of chemical and physical separation processes. First, as-synthesized NCNCs were oxidized in a mixture of strong acids to introduce oxygen-containing defects on the graphitic walls. The oxidized NCNCs were then processed using high-intensity probe-tip sonication which effectively separated the stacked NCNCs into individual graphitic nanocups. Owing to their abundant oxygen and nitrogen surface functionalities, the resulted individual NCNCs are highly hydrophilic and can be effectively functionalized with gold nanoparticles (GNPs), which preferentially fit in the opening of the cups as cork stoppers. These graphitic nanocups corked with GNPs may find promising applications as nanoscale containers and drug carriers.

The as-synthesized NCNCs from CVD growth appeared as a carpet of black material on quartz substrate. Thick films of NCNCs weighing about several mg were obtained by peeling with a razor blade (Figure 1B). TEM images show the morphology of as-synthesized NCNCs at different magnifications (Figure 1). At the lower magnification (Figure 1C), the as-synthesized NCNCs all showed a fibril structure with lengths of typically several micrometers and diameters of 20 - 30 nm. Unlik...

Watch this Scientific Journal Video about Synthesis and Functionalization of Nitrogen-doped Carbon Nanotube Cups with Gold Nanoparticles as Cork Stoppers at JoVE.com

Synthesis and Functionalization of Nitrogen-doped Carbon Nanotube Cups with Gold Nanoparticles as Cork Stoppers

We discussed the synthesis of individual graphitic nanocups using a series of techniques including chemical vapor deposition, acid oxidation and probe-tip sonication. By citrate reduction of HAuCl4, the graphitic nanocups were effectively corked with gold nanoparticles due to the chemically reactive edges of the cups.

Nitrogen-doped carbon  nanotubes consist of many cup-shaped graphitic compartments termed as  nitrogen-doped carbon nanotube cups (NCNCs).  These ...

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