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

Summary

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.

Abstract

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.

Introduction

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 r....

Protocol

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).

1. 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" × 12") inside the tube as the substrate for product collection. Seal the quartz tube using homemade stainless steel caps with built-in gas and liquid injection connections/tubes.
2. Make a solution of liquid precursor containing 0.75 wt% ferrocene, 10 wt% acetonitrile and 89.25 wt% x....

Results

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.......

Discussion

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.......

Materials

List of materials used in this article

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