Letter pubs.acs.org/Langmuir
Length Fractionation of Boron Nitride Nanotubes Using Creamed Oilin-Water Emulsions Yiu-Ting R. Lau,† Maho Yamaguchi,†,‡ Xia Li,† Yoshio Bando,† Dmitri Golberg,*,†,‡ and Françoise M. Winnik*,†,§ †
World Premier International (WPI) Research Center Initiative, International Center for Materials Nanoarchitectonics (MANA), and National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan ‡ Graduate School of Pure and Applied Science, University of Tsukuba, Tennodai 1-1-1, Tsukuba 3058577, Japan § Faculté de Pharmacie and Département de Chimie, Université de Montréal, CP 6128 Succursale Centre Ville, Montréal, QC H3C 3J7, Canada S Supporting Information *
ABSTRACT: The fractionation by length of multiwalled boron nitride nanotubes (BNNTs) was achieved by emulsification and creaming of an oil/water/surfactant mixture. The length separation is based on the fact that nanoparticle-coated oil droplets polydisperse in size move toward the upper surface or the bottom of an emulsified mixture depending on the density of the droplets, such that droplets of different sizes are located at different heights. By sampling heightwise an unstable hexane/water/Tween 20/ BNNT (1−20 μm long) emulsion, we observed that the lengths of the BNNTs adsorbed on the droplets display a strong correlation with the droplets sizes, thus leading to selective separation of the BNNT lengths, as confirmed by dark-field optical imaging and dynamic light scattering. This method may potentially be extended to other high aspect ratio nanomaterials exhibiting emulsification properties similar to those of BNNTs.
H
stabilization of emulsions by nanoparticles (Pickering emulsion). Creaming of emulsions is the result of gravity on an emulsion when the densities of the droplets (disperse phase) and the medium (continuous phase) are not equal. The inclination of the droplets to move up (creaming) or down (sedimentation) is governed by the ratio between Fg, as a result of the gravitational force (eq 1), and FB, as a result of the Brownian diffusion (eq 2):
igh aspect ratio nanoparticles, such as carbon nanotubes (CNT),1 boron nitride nanotubes (BNNT),2 and cellulose nanocrystals3 are expected to play key roles in the areas of electronics, actuators, membranes, diagnostics, and nanomedical devices, among many others. Although unfractionated NTs are employed in most current studies, a growing number of reports indicate that NTs monodisperse in size can exhibit new properties that may trigger further innovation. Nanotubes of uniform size are also needed for investigations of their size-dependent physical, chemical, and biochemical properties. Moreover, toxicological studies indicate that long nanotubes can cause serious cellular damage, whereas short ones are much safer.4,5 Because it is difficult to control the size of either CNTs or BNNTs during their synthesis,1,2 postsynthesis size control must be carried out. A number of purification and sorting approaches with regard to dimensions have been established in the case of CNTs and other tubular particles,6−8 including iterative centrifugation,9,10 density gradient centrifugation,11,12 field-flow fractionation,13 electrophoresis,14 chromatography, 15−18 filtration, 19 phase separation20,21 and more.22−24 These methods require specialized equipment and multiple fractionations in order to collect sufficient nanoparticles monodisperse in size for further use. The method described here employs two well-known phenomena in colloidal science: emulsion creaming and the © 2014 American Chemical Society
Fg =
4 3 πr ΔρgH 3
(1)
and FB = kT
(2)
where k, T, r, Δρ, H, and g are the Boltzmann constant, temperature, droplet radius, density difference between the droplets and the aqueous medium, height of the column, and acceleration due to gravity, respectively.25 Creaming is significant when the ratio satisfies the condition Fg/FB > 1. The creaming velocity, v, generally increases with the droplet Received: December 26, 2013 Revised: February 3, 2014 Published: February 6, 2014 1735
dx.doi.org/10.1021/la404961p | Langmuir 2014, 30, 1735−1740
Langmuir
Letter
Figure 1. Dark-field micrographs of emulsion droplets and surface-adsorbed BNNTs as a function of the height position of a creamed emulsion. Columns a and b are micrographs of BNNTs before and after drying of the emulsion.
size as approximated by the Stokes relation (eq 3), where η is the medium viscosity.25 2gr 2Δρ v=− 9η
and Ed =
(3)
3πE BD4 ⎛ 2 2⎞ δ ⎜1 + δ ⎟ 3 ⎝ L 3D2 ⎠
E I = 4πr 2γow
Fdδ 2
(6)
(7)
The separation process is demonstrated here for the size fractionation of a BNNT sample with a broad NT length distribution (1−20 μm). We used BNNTs because currently there is no efficient method to fractionate them, whereas clearly BNNTs of a narrow length distribution are needed to address important issues related to their cytotoxicity5 and other aspects in device fabrication,31 energy conversion,32 transport,33 and separation technology.34 Some techniques, such as cutting via extended-time sonication, were used to reduce the size of BNNTs from 10 μm to 500 nm,35 but so far there have been no reports on the length fractionation of polydisperse BNNTs. The general procedure was performed on multiwalled BNNTs synthesized by boron oxide chemical vapor deposition.2 Pristine BNNTs (0.45 mg) were dispersed in ethanol and ground gently in a marble mortar in order to debundle the tubes. The resulting suspension was sonicated in a water bath (38 kHz, 60W) at room temperature until the BNNTs were completely dispersed (∼5 min). The resulting dispersion was subjected to a 1 min centrifugation at 15 000 rpm. The clear ethanolic supernatant was removed. Aliquots of the recovered ethanolic supernatant (∼20 μL) were deposited on a cleaned
(4)
where EB, D, L, and δ are, respectively, the bending modulus of the tube, its diameter, its length, and its deflection, defined by eq 5, where θ is the bending angle, defined by θ = L/r. For the sake of simplicity, we assume here that there is a linear relationship between the applied force and the deflection. The strain energy is then given by eq 6. ⎛ θ⎞ δ = r ⎜1 − cos ⎟ ⎝ 2⎠
Fdδ dδ ≈
In an emulsion at equilibrium, the free energy gained by tube bending is equal to the energy, EI, necessary to form a stable oil droplet in water. The energy EI is given by eq 7, where γow is the oil/water interfacial tension. From these considerations it follows that nanotubes of a given length preferentially adsorb onto droplets of commensurate radius; consequently, the length fractionation of NTs should take place vertically along a creamed emulsion.
Hence, creaming leads to a vertical fractionation of the droplets in an emulsion as a function of their size, with the largest ones being found in the topmost section of the emulsion if the oil is lighter than water. It is well known that nanotubes can be trapped at the oil/water interface.26 In addition, both CNTs27 and cellulose nanocrystals28 stabilize emulsions to form socalled Pickering emulsions to distinguish them from standard emulsions that owe their stability to the presence of surfactants adsorbed at the oil/water interface. The stability of Pickering emulsions is attributed, in part, to the irreversible nature of the nanoparticle adsorption on a droplet. Because of the droplet curvature, a long, straight nanotube adheres to a small spherical droplet via a single contact point. This situation is energetically unfavorable, hence the nanotube needs to bend.29 The force required to bend a nanotube to a given transverse displacement can be calculated using the Heidelberg-Boland model. Treating the NT as a solid cylinder, this model leads to the following relationship (eq 4)30 Fd =
∫
(5) 1736
dx.doi.org/10.1021/la404961p | Langmuir 2014, 30, 1735−1740
Langmuir
Letter
Figure 2. Optical microscopy images showing the dependence of the oil droplet diameter on the length separation of the surface-adsorbed BNNTs, collected from the top to the bottom (a−d) of a creamed emulsion; e and f are consecutive time-sequence micrographs following b and d, respectively. For the time sequences, the focusing point was fixed and scanned along the surface of the droplet, following the lateral movement of the droplet.
disturbing the suspension. To minimize as much as possible the extent of intermixing between layers of the suspension during the process, fraction sampling started from the topmost layer in a single withdrawal, avoiding any agitation. Each aliquot was spread between two glass coverslips for observation with an optical microscope (Nikon Eclipse ME600L) equipped with a halogen white light source and a digital camera (DXM1200, Nikon). Dark-field micrographs of the liquid mixture, displayed in Figure 1, were acquired before drying (Figure 1a) and after complete drying of the emulsion under ambient conditions (Figure 1b). We observed a significant decrease in the mean droplet size as a function of sampling height (Figure 1a). The droplet diameter ranges from >50 μm in the cream layer to about ∼2 μm in the middle layer to ∼1 μm or less in the serum. The surface of the droplets is decorated with BNNTs revealed as lasing rods.26 Dark-field images of the dried aliquots (Figure 1b) indicate that BNNTs are found in the three emulsion layers and that they are scrupulously length-fractionated in the aliquots taken at the three height positions of the creamed emulsion. The topmost fraction contains mostly long tubes (>10 μm); BNNTs found in the middle layer are 2−8 μm in length, whereas in the bottom layer their length is
