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 aggregation and to control the interparticle spacing of the nanoparticles in the films, the nanoparticles need to be capped with ligand shells. After the nanoparticles have been functionalized with ligands the nanoparticles typically remain in relatively dilute suspensions. A technique is then needed to self-assemble the nanoparticles into macroscopic, high-density, monolayer films^16,17.

Cheng et al.¹⁸ phase transferred gold nanorods using thiolated polystyrene in a water-tetrahydrofuran suspension.  The nanorods where then re-suspended in chloroform and a drop was placed at an air-water interface and evaporated slowly, forming monolayer films. Bigioni et al.¹⁷ created macroscopic monolayers of dodecanethiol capped gold nanospheres using excess ligand and rapid solvent evaporation, but the nanospheres needed to be phase transferred prior to self-assembling. 

Once the monolayer films are formed they typically need to be transported onto a substrate.  Mayya et al.³ confined nanospheres at a water-toluene interface and transferred them onto template-free substrates using surface tension gradients. Similarly, Johnson et al.⁴ suspended silver nanospheres in excess ligand and then translated the nanoparticles up the walls of the vial using surface tension gradients from two immiscible fluids. While assembly techniques exist to address each of these issues the need for more efficient techniques is needed to aid in the development of large-scale nanoparticle film production.

Here we demonstrate a straightforward and robust technique that combines the three self-assembly issues described above in to a single ‘one-pot’ technique, shown in Figure 1. A water miscible organic solvent (e.g. tetrahydrofuran, dimeythl sulfoxide), is used to first quickly and efficiently functionalize thiol-ligands (e.g. thiol-alkane, thiol-ene, thiol-phenol) onto the nanoparticles (e.g. gold nanospheres, nanorods, etc.). The mixture then drives self-assembly of the nanoparticles into macroscopic, high-density, monolayer films at the air-fluid interface using phase separation.  Finally, monolayer films of nanoparticles form onto template-free substrates using surface tension gradients from the water/organic solvent mixture, Figure 2 and Figure 3.

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.
2. Resuspend the nanosphere in approximately 1 ml of deionized (DI) water such that the nanoparticle concentration is 10¹³ particles/ml. The suspension is stable for several hours once resuspended in DI water.
   1. Verify the number density and confirm the nanoparticles have not aggregated. Dilute the concentrated nanoparticle suspension by a factor of 1:10 back to the original concentration by placing 0.150 ml of the suspension into a cuvette (1 cm path length), and to this add 1.35 ml of DI water.
   2. Place the cuvette into a spectrometer and measure the absorbance spectrum of the suspension and the original suspension. Compare the peak position and full width at half maximum to ensure aggregation has not occurred. The magnitude of the absorbance peaks for both samples should be approximately the same, thus ensuring the concentrated sample is denser by a factor of 10.
3. In a separate clean 20 ml borosilicate glass vial add 1 ml of tetrahydrofuran (THF).
4. Add the thiol-alkane ligands (e.g. 5 ml of 1,6-hexanedithiol and 5 ml of 1-dodecanethiol) to the THF and shake the solution to mix uniformly. Enough ligand should be added to cover at least the entire surface area of the suspended nanoparticles. Excess ligand increases the speed and efficiency of the reaction.
5. In a fume hood, pour the contents of the vial containing the gold nanospheres into the vial of THF-ligands.
6. Quickly screw on the lid and shake the vial vigorously for 15 sec.
7. Remove the lid and set the vial down in the fume hood, Figure 1(a). Depending on the ligands used, domains of gold nanoparticle films quickly form at the air-liquid interface, Figure 1(c). The films will then begin to translate up the sides of the vial, Figure 1(d). Nearly all the nanoparticles are capped with thiol-ligand, removed from the suspension, and transported to the sides of the vial within 1 hr, Figure 1(e).

2. Transferring the Monolayers onto Removable Substrates

1. To transfer the films onto removable glass and silicon wafer substrates: cut the substrates into an area of 12.5 mm x 25.4 mm using a scribing pen/wheel.
   1. Glass Substrates: clean using an acetone rinse, followed by an isopropyl alcohol rinse, and finally a DI water rinse. Allow the substrates to dry, proceed to section 2.2.
   2. Silicon Wafer Substrates: in a fume hood prepare Piranha solution (3 parts concentrated sulfuric acid to 1 part 30% hydrogen peroxide, CAUTION: oxidizer, corrosive). Place 15 ml of sulfuric acid into a 20 ml borosilicate glass vial. To this slowly add 5 ml of 30% hydrogen peroxide. Do not cap the vial. Use caution; the mixture is highly exothermic. See reference for more safety information¹⁹.
   3. Carefully submerse the silicon wafer substrates into Piranha solution for 30 min, remove, rinse with DI water and dry with nitrogen.
   4. As an optional step, the vial used for the nanoparticle ligand exchange and self-assembly can be salinized to force all the nanoparticles onto the glass substrate or silicon wafer instead of the walls of the glass vial, otherwise proceed to section 2.2.
   5. Fill the glass vial with piranha solution (CAUTION: oxidizer, corrosive), refer to section 2.1.2.
   6. Allow the vial to soak for 30 min. After 30 min rinse the vial out with DI water.
   7. Fill vial with 1% v/v of hexamethyldisilazane in acetone and cap.
   8. Allow the sealed vial to soak for 24 hr, then rinse with DI water and dry with nitrogen.
2. Prior to shaking (section 1.6) insert the substrate into the vial. Screw on lid and shake.
3. After shaking remove lid and, using tweezers, position the substrate nearly vertical against the vial wall.
4. Use a pipette to coat the reaction mixture onto the substrate. The reaction stops when all the organic solvent has evaporated or all the nanoparticle have been removed from the suspension.

3. Monolayer Analysis

1. Estimate the packing efficiency of the nanospheres in the monolayer quickly by observing the transmission and reflective properties of the film. Illuminate the monolayer on glass substrates from behind with a white-light source. With a white light source, a uniform colored film should be observed for high-density nanoparticle monolayer films in transmission and a gold-like reflection observed in reflection, Figure 2.
2. Use a spectrometer (see section 1.2.2) to quantify the macroscopic absorbance spectrum from the monolayers, Figure 4. Normalize the absorbance spectrum with a clean glass slide. Mount the monolayer film, on a glass substrate, into the beam path of the spectrometer and collect the absorbance spectrum.
   Note: The absorbance peak should be significantly red-shifted several hundred nanometers depending on the ligand used. The quality factor of the absorbance peak should be comparable to the dilute suspension value, but only slightly broadened (Figure 4). If the absorbance peak is very broad or not well defined then the monolayer films are probably of poor quality, proceed to section 3.3 for further characterization.
3. Examine the nanoscopic organization of the nanospheres using scanning electron microscopy (SEM) of monolayers transferred onto silicon wafer substrates (see section 2.1.2) as shown in Figure 3. If the films are on glass substrates connect conductive tape to one corner of the film and ground it to the SEM pedestal to prevent charging and allow imaging.

4. Efficient Phase Transfer Technique for Organic Soluble Nanoparticles

1. To use the technique as an efficient means to functionalize the nanoparticles with thiol-ligands, decant the remaining solution from the bottom of the vial after the reaction is complete, section 1.7, and dry the material in the vial under nitrogen.
2. Add an organic solvent (e.g. chloroform, toluene) to re-suspend the nanoparticles with nearly 100% particle phase transfer and recovery.
3. Repeat section 1.2.1 to ensure the nanoparticles have not aggregated upon re-suspension into the organic solvent. If the absorbance peak is board, relative to the original suspension, sonicate the sample for 15 min to help redisperse the nanoparticles, Figure 4.

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 mixin...

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

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