Functionalization of Single-walled Carbon Nanotubes with Thermo-reversible Block Copolymers and Characterization by Small-angle Neutron Scattering

Summary

A method for the functionalization of carbon nanotubes with structure-tunable polymeric encapsulation layers and structural characterization using small-angle neutron scattering is presented.

Abstract

We demonstrate a protocol for single-walled carbon nanotube functionalization using thermo-sensitive PEO-PPO-PEO triblock copolymers in an aqueous solution. In a carbon nanotube/PEO105-PPO70-PEO105 (poloxamer 407) aqueous solution, the amphiphilic poloxamer 407 adsorbs onto the carbon nanotube surfaces and self-assembles into continuous layers, driven by intermolecular interactions between constituent molecules. The addition of 5-methylsalicylic acid changes the self-assembled structure from spherical-micellar to a cylindrical morphology. The fabricated poloxamer 407/carbon nanotube hybrid particles exhibit thermo-responsive structural features so that the density and thickness of poloxamer 407 layers are also reversibly controllable by varying temperature. The detailed structural properties of the poloxamer 407/carbon nanotube particles in suspension can be characterized by small-angle neutron scattering experiments and model fit analyses. The distinct curve shapes of the scattering intensities depending on temperature control or addition of aromatic additives are well described by a modified core-shell cylinder model consisting of a carbon nanotube core cylinder, a hydrophobic shell, and a hydrated polymer layer. This method can provide a simple but efficient way for the fabrication and in-situ characterization of carbon nanotube-based nano particles with a structure-tunable encapsulation.

Introduction

Carbon nanotubes (CNTs) are hollow cylindrical nanoparticles formed by rolling a micrometer-scale graphite sheet into a nanotube. Because of their extraordinary mechanical, thermal, and electrical properties, CNTs have been extensively investigated as a novel candidate for functional nanoparticles in therapeutic and bio-sensing applications as well as nano-fillers in self-assembled nanocomposite materials.^1-3 However, their poor solubility and strong preference toward making bundles in commonly used organic and aqueous solvents hinder easy and environmentally-friendly processing as well as advances in biological applications. Therefore, a variety of functionalization methods, such as ultra-sonication, chemical surface modification, and non-covalent functionalization by using surfactants and block copolymers,^4-9 have been developed to modify the CNT surfaces and improve their dispersibility in a wide range of solvents. Non-covalent functionalization methods based on physical surface treatments, in particular, are considered to be a promising and robust strategy, because any surface-modification induced suppression in intrinsic CNT properties can be minimized.¹⁰ To date, there have been numerous efforts to improve the dispersion efficiency of non-covalent functionalization methods by employing various types of dispersive agents including basic surfactants (e.g., SDS, CTAB, NaDDBS),^7,11 amphiphilic block copolymers,⁸ bio-materials (e.g., DNA),^12,13 and synthetic functional polymers (e.g., conjugated polymer, aromatic polymer).^14,15

PEO-PPO-PEO polymers, a kind of triblock copolymer consisting of two hydrophilic poly(ethylene oxide) (PEO) chains at both ends covalently bound to one hydrophobic poly(propylene oxide) (PPO) chain at the center, can extend the potential application of non-covalently functionalized CNTs in aqueous solution. These polymers provide the interface, which is friendly not only to the CNT surfaces but also to aqueous media and other polymer matrices and exhibits tremendous biocompatibility due to the minimal toxicity of the PEO chains. This facilitates easier processing in a wide range of dispersing environments as well as the utilization of polymer-coated CNTs in biomedical applications.^12,16-17 Moreover, the rich thermodynamic phase behavior of these polymers based on their sensitive responses to external stimuli enables the fabrication of the smart block copolymer-CNT hybrid nanostructures in which intra- and inter-particle structures can be reversibly and precisely controlled.^18-21 Here, we present a protocol for the fabrication of CNT-based hybrid nanoparticles with a tunable encapsulation layer of PEO105-PPO70-PEO105 (poloxamer 407). The resulting structure is characterized by small-angle neutron scattering (SANS). This work is expected to introduce the concept of smart functional building blocks and help non-specialists easily prepare block copolymer-functionalized CNT suspensions and use SANS for the detailed characterization at Oak Ridge National Laboratory.

Protocol

Note: This protocol requires special care in the handling of nanomaterials. As-purchased single-walled carbon nanotubes (SWNTs) exist in the form of fine powder and thus, they should be considered as nano-hazardous materials before dispersing them in aqueous solutions. Please use appropriate safety equipment described in the material safety data sheets (MSDS).

1. Preparation of Poloxamer 407/SWNT Aqueous Suspensions

Note: Proceed with all the sample preparation procedures at a lower temperature than the critical micellization temperature (CMT) of the block copolymers used. The poloxamer 407/SWNT samples were prepared at 20 °C, below the CMT of poloxamer 407 (30 °C).²¹

1. Preparation of Poloxamer 407 aqueous solutions (0.25% w/w)
   1. Completely dissolve 0.175 g of poloxamer 407 powder in 70 g D₂O.
      Note: D₂O is used for SANS measurements. 70 g D₂O is approximately 63.2 ml at room temperature. For other purposes, H₂O is recommended for use.
2. Preparation of crude Poloxamer 407/SWNT suspensions
   1. Add 0.01 g SWNT powder to two 50 ml conical centrifuge tubes (tube 1 and tube 2) separately.
   2. Add 31.6 ml of the poloxamer 407 solution (1.1.1), into the tube 1 and 31.6 ml of the remaining solution into the tube 2.
   3. Mix the suspensions in the tube 1 and 2 by vortex-mixing for 5-10 min.
   4. Place the tube 1 in a water bath. Fix the tube position securely. (Figure 1) Dip the tube until the air-suspension interface reaches the surface of water in the bath.
   5. Dip the tip of an ultrasonicator into the suspension of the tube 1. Increase the sonication power gradually from 0% at least until the SWNTs deposited at the bottom of the tube start shattering and spreading due to the ultrasound propagated from the ultrasonicator tip. Treat the suspension with ultrasound for 60 min at 20 °C, while keeping the suspension temperature below 25 °C.
      Note: Do not put the tip end deeper than 1 cm into the suspension. Keep the suspension temperature below 25 °C, either by controlling the temperature of the water reservoir or by refilling the bath appropriately.
   6. Repeat steps 1.2.4 and 1.2.5 for the tube 2.
3. Preparation of Poloxamer 407/SWNT suspensions in the absence and presence of 5-Methylsalicylic acid
   1. Centrifuge the crude suspensions in tubes 1 and 2 at 9,800 × g for 2 hr at 20 °C.
   2. Move 15 ml of the supernatants from each tube to a new tube, separately.
   3. Dissolve 0.015 g of 5-methylsalicylic acid (5MS) into the supernatant taken from tube 2, and label this mixture as Sample #2. Label the other supernatant from the tube 1 as Sample #1.

2. Extended Q-range Small-angle Neutron Scattering (EQ-SANS) Measurements

Note: To work at the beamlines of Spallation Neutron Source (SNS), an accepted beamtime proposal is required. Radiological safety training and other instrument specific training are also required in advance. Access and training details are provided by the SNS User Office and can be found at neutrons.ornl.gov.

1. Sample loading
   1. Load 0.3 ml of Sample #1 of into an amorphous quartz banjo cell and Sample #2 into another banjo cell (Figure 2A-i). Put lids on the two cells and seal them by wrapping tape securely around the lids.
   2. Place one of the sealed cells between spacers (Figure 2A-ii) for an aluminum cell holder (Figure 2A-iii), and assemble the aluminum banjo cell holder (Figure 2B). Assemble the other cell with a different set of banjo cell holder in the same manner.
   3. Load the assembled cells into different sample positions of the EQ-SANS sample paddle (Figure 3A). Make the list of the sample positions of the paddle.
2. Measurements
   1. Set the configurations for the SANS measurements in a script with the help of an instrument scientist, referring to the given example script.
      Note: An example of the script used during the actual SANS measurements is provided in the supplementary material with brief comment. This example is specifically designed for SANS measurements of two samples (sample #1 and #2) using a wavelength band of 9.1 Å < λ < 13.2 Å at the fixed 1.3 m sample-to-detector distance with a 10 mm sample aperture and a 30 mm beam stop. The example script can be used without any modification for the sample #1 and #2.
      1. To cover a q-range of 0.01 - 0.4 Å^-1, select a wavelength band of 9.1 Å < λ < 13.2 Å and a sample-to-detector distance of 1.3 m by inputting 1300 into detector position and 9 into wavelength as shown in the example script.
      2. To use a 10 mm sample aperture and a 30 mm beam stop, set x-y positions of the beam stops and apertures in the script.
      3. Set the sample positions and the corresponding names for both transmission and sample scattering measurements.
      4. Save the script in the folder corresponding to the beam time.
   2. Execute the script to perform measurements by clicking on 'Run Script' on the right side of the PyDAS control window (Figure 3B) and loading the saved script.
   3. Inform the instrument team of the completion of measurements after the experiment is finished.

3. SANS Data Reduction and Analysis

1. SANS Data reduction process
   1. For the reduction of the measured data, use MantidPlot^23-24 software provided at analysis.sns.gov, with the help of the instrument team.
      Note: Detailed instructions to run MantidPlot software can be found at analysis.sns.gov webpage.
   2. Within MantidPlot, open the EQSANS reduction interface from the interfaces menu. (interface>SANS>ORNL SANS). Input all necessary information for the data reduction process.
      Note: Most of important information in the data reduction process is provided by the instrument team. For information, relevant screenshots are also provided in the supplementary materials.
   3. Input all necessary information in the 'Reduction Options' tab.
      1. Input the absolute scale factor from the standard sample measurement. Obtain the absolute intensity from the measurement of a well characterized standard sample whose scattering intensity is known (I(0)=450 cm^-1) and fit with a Debye-Buche scattering model.
      2. Input the dark current file name, which is provided by the instrument team.
      3. Check options for the 'solid angle correction', 'Q resolution', 'use configuration file', 'Correct TOF', and 'user mask from configuration file as applicable'.
      4. Set sample aperture diameter to 10 mm. Set number of Q bins to 200 with a linear Q binning scheme. Input the mask file name, which is also provided by the instrument team.
   4. Input all necessary information to complete the 'Detector' tab.
      1. Check 'Use beam finder' (with fit direct beam option) and 'perform sensitivity correction'. Find a beam center 'data file' using the run number of the empty beam measurement.
      2. Input the sensitivity data file name, which is provided by the instrument scientist. Set the allowed sensitivity range to 0.5 and 2.5 for the min and max, respectively. Check 'Use sample beam center'.
   5. Input all necessary information for the 'Data' tab.
      1. Enter sample scattering run number at 'Scattering data file'. Specify the sample thickness in cm. Select 'Calculate transmission'.
      2. Enter the sample transmission run number at 'Sample direct beam data file'. Enter empty beam run number at 'Empty direct beam data file'.
      3. Check 'Background data file' and enter background scattering run number. Select 'Calculate transmission'. Enter background transmission run number at 'Sample direct beam data file'.
         Note: In this case, the empty banjo cell scattering data file is the background scattering.
      4. Enter empty beam run number at 'Empty direct beam data file'. Typically, this number is same as the empty beam run number (3.1.5.2).
   6. Click on 'Reduce' to execute the data reduction.
      Note: The output is written in the designated folder as #####_Iq.txt where ##### is the run number of the sample scattering file. ASCII format is used for the data files.
2. Model fitting analysis
   Note: SasView is a small-angle scattering analysis software package, which was originally developed as part of the NSF DANSE project, and is currently managed by an international collaboration of facilities (http://www.sasview.org/). The software package can be downloaded at http://sourceforge.net/projects/sasview/files/.
   1. Run SasView, and load a data file by clicking on 'Load Data' from the 'Data Explorer' window.
   2. Click on 'Send To' with the 'Fitting' option, and check the data plot on the popup window.
   3. In the 'Fit Panel', select "Shapes" under the model category, and choose "CoreShellCylinderModel" from the model drop box.
   4. Adjust the parameter values, so the model curve is as close to the data curve as possible.
      Note: Use the SLD (scattering length density) calculator from the Tool menu to calculate scattering length densities.
   5. Select "Use dQ data" and "Use dI Data" in the Fitting panel. Adjust Q range of the data for fitting. Click on 'Fit' to execute data fitting.

4. Real-space Observation Using Atomic Force Microscopy (AFM)

1. Sample preparation on Si-wafers using spin coating
   1. For AFM measurements, take 0.1 ml sample solution from the Sample #1 (1.3.3), and mix it with 1.9 ml de-ionized water.
   2. Place a clean Si-wafer (12 mm × 12 mm) on a spin coater. Fix the wafer position using a vacuum chuck.
   3. Set the rotation speed and the running time at 1,500 revolutions per minute (rpm) and 60 sec, respectively. Wet the exposed surface of the wafer with the diluted sample. Start spin coating.
   4. Turn off the vacuum pump, and remove the coated wafer from the spin coater.
2. AFM measurements
   1. Attach the spin-coated wafer (4.1.4) on an iron disk using a double-sided adhesive carbon tape.
   2. Bring the specimen disk (4.2.1) closer to the edge of the exposed area of the scanner first, and slide the disk toward the center until the bottom surface of the disk completely covers the top of the scanner.
      Note: Avoid sudden contact because a magnet on the top of the scanner strongly attracts the iron disk. Gently make a contact between two edges of the disk and the scanner.
   3. Mount the scanning probe microscope (SPM) head on the scanner, and plug in the cable.
      Note: Extreme care has to be paid while moving the SPM head or docking (removing) it to (from) the scanner. When the head is detached from the scanner stage, keep the bottom surface of the head facing upwards all the time.
   4. Run the instrument-supplied control software, and select the tapping mode in the 'System Configuration' window.
   5. Place the cantilever tip at the center of the monitor window by adjusting the coarse and fine knobs of the optical microscope and by moving the x- and y- optical stages.
   6. Align the laser by adjusting the laser alignment knobs on the SPM head. Locate the red laser dot to the cantilever roughly, and move the dot to the middle of the cantilever tip by tracing the dot shown in the monitor.
      Note: When the laser is properly aligned, the pink reflection spot appears on the laser alignment window.
   7. Align the detector by locating the pink reflection image at the center of the laser alignment window. Adjusting the photodetector knobs on the SPM head until the quadrant photodiode (QPD) signal sum is greater than 2 V at least (2.1 - 2.4 V).
   8. Tune the cantilever using AutoTune in the Cantilever Tuning window. Run AutoTune in a frequency range of 0 - 1,000 kHz.
   9. Bring the wafer surface into focus of the microscope by adjusting the focus knobs.
   10. Drive the cantilever tip slowly toward the wafer surface using the up and down arrows in the 'Motor Stage' window. Stop the movement before the tip touches the sample surface.
       Note: As the tip approaches the wafer, the blurred black image of the tip appears in the monitor and the image becomes clear when the tip and the wafer surface make contact. Never let the tip physically touch the specimen. It damages both sample and instrument. Stop the tip when the dark blurred image is shown.
   11. Click on the engage button on the toolbar.
   12. Select a scan size (5-10 µm), a sampling number (512-1,024), and a scan rate (0.5-0.6 Hz) in the popup window, in order to acquire a large-scale image first.
   13. Start scanning. Gradually adjust P (proportional gain), I (integral gain), and D (vertical deflection) values if the contrast between the particles and the substrate background is too low to clearly recognize particle shapes and boundaries from the scanned image.
       Note: When a new PID value set is entered, the scanning process will be automatically restarted.
   14. If there is any region of interest in the large-scale image, re-run the scan with a proper set of scan size, x-offset, y-offset, and sample number.
   15. Disengage the probe after measurement.
       Note: Raise the probe head to prevent any damage on the cantilever tip and the sample. Remove the probe head first, and then, detach the specimen disk.

Results

Poloxamer 407-coated SWNT nanorod suspensions were fabricated using the sample preparation procedure (Figure 4), which can be divided into two important processes; the physical adsorption process of poloxamer 407 on SWNT surfaces using ultra-sonication, and the fractionation process of individually-stabilized SWNTs from bundled aggregates using centrifugation.

The SANS scattering intensities were obtained for th...

Discussion

SANS and AFM measurements showed that SWNTs have been successfully de-bundled and individually dispersed in aqueous solution using a poloxamer 407 triblock copolymer. In this sample preparation method, ultra-sonication and centrifugation processes are the critical steps determining the characteristics of the final suspension. The strong interaction between the SWNTs, which forces uncoated SWNTs to bundle together in solution, must be overcome to stabilize the individual SWNTs with block copolymers. Providing a sufficient...

Materials

Name	Company	Catalog Number	Comments
HiPco Single-walled carbon nanotubes	Unidym	P2771
Pluronic F127	BASF	9003-11-6	Mw = 12.6 kg/mol
5-methylsalicylic acid	TCI America	C0410
Ultrasonic processor	Cole-Parmer	ML-04714-52
Sorvall 6 plus centrifuge	Thermo Scientific	46910
Innova AFM	Bruker
Si-wafer	Silicon Quest International		150 mm in diameter; N type <1-1-1> cut; 1-10 Ohm/cm; Single-side polyshed (675 ± 25 μm); Diced (12 mm x 12 mm)