A Technique to Functionalize and Self-assemble Macroscopic Nanoparticle-ligand Monolayer Films onto Template-free Substrates

Summary

A simple, robust and scalable technique to functionalize and self-assemble macroscopic nanoparticle-ligand monolayer films onto template-free substrates is described in this protocol.

Abstract

This protocol describes a self-assembly technique to create macroscopic monolayer films composed of ligand-coated nanoparticles^1,2. The simple, robust and scalable technique efficiently functionalizes metallic nanoparticles with thiol-ligands in a miscible water/organic solvent mixture allowing for rapid grafting of thiol groups onto the gold nanoparticle surface. The hydrophobic ligands on the nanoparticles then quickly phase separate the nanoparticles from the aqueous based suspension and confine them to the air-fluid interface. This drives the ligand-capped nanoparticles to form monolayer domains at the air-fluid interface.  The use of water-miscible organic solvents is important as it enables the transport of the nanoparticles from the interface onto template-free substrates.  The flow is mediated by a surface tension gradient^3,4 and creates macroscopic, high-density, monolayer nanoparticle-ligand films.  This self-assembly technique may be generalized to include the use of particles of different compositions, size, and shape and may lead to an efficient assembly method to produce low-cost, macroscopic, high-density, monolayer nanoparticle films for wide-spread applications.

Introduction

The self-assembly of macroscopic nanoparticle films has attracted great attention for their unique properties determined from the geometry and composition of the elements⁵ and may lead to of a wide range of optical, electronic and chemical applications^6-14. To self-assemble such films metallic nanoparticles capped with ligands must be packed into high density, monolayers. However several assembly issues need to be addressed to advance the development of such materials.

First, surfactant stabilized metallic nanoparticles are typically synthesized by wet-chemistry methods in dilute suspensions¹⁵. To prevent ag....

Protocol

1. Self-assembled Ligand-nanoparticle Monolayers

As an illustrative example of the self-assembly technique, macroscopic, thiol-alkane capped gold nanosphere monolayer films are produced as follows:

1. Concentrate 15 nm gold nanospheres (commercially available at a number density: 10¹² particles/ml) to ~10¹³ particles/ml in water.
   1. Place 15 ml of the dilute nanosphere water suspension into an ultra centrifugal filter (100 K nominal molecular weight limit).
   2. Centrifuge the filter/vial at 4,500 x g for 2 min or until only a few ml remain in the filter chamber.
   ....

Results

Figure 1(a) shows a suspension of gold nanospheres, thiol-alkane ligands, tetrahydrofuran and water in a glass vial immediately after mixing. A schematic of the three main self-assembly stages, phase transfer, phase separation, and surface tension gradient mediated film transport is shown in Figure 1(b) as an expanded view at the air-fluid interface near the side of the vial.

The thiol groups on the ligands rapidly bond to the gold nanospheres after mixing, di.......

Discussion

This protocol describes a single ‘one-pot’ self-assembly technique to create macroscopic nanoparticle-ligand monolayer films using phase transfer, phase separation and surface tension gradients.  The advantage of this technique is that it combines three self-assembly processes into a single, low-cost process; by quickly and efficiently phase transferring the nanoparticles, assembling the particles into monolayers at the air-fluid interface and transporting the monolayer films onto template-free substrate.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
1,6-hexanedithiol	Sigma	H12005-5G
1-dodecanethiol	Sigma	471364-100ML
20 ml liquid scintillation vials	Sigma	Z253081-1PAK
Acetone	Sigma	650501-1L
Amicon ultra-15 centrifugal filter	Millipore	100K
Centrifuge	Sorvall	RC5B
Centrifuge	Eppendorf	5810R
Deionized water	In-house	N/A
Glass slides	Sigma	CLS294875X25-72EA
15 nm gold nanospheres	Ted Pella, Inc	15703-1
Hexamethyldisilazane	Sigma	52619-50ML
Hydrogen peroxide (30%)	Sigma	216763-100ML
Scanning electron microscope	Carl Zeiss	Model 55
Polished silicon wafer	Sun Edison	N/A
spectrometer	OceanOptics	USB4000-VIS-NIR
Sulfuric acid	Fisher	A300-212
Tetrahydrofuran	Sigma	401757-100ML