Dynamics of Individual Single-Walled Carbon Nanotubes in Water by Real-Time Visualization Rajat Duggal and Matteo Pasquali * Department of Chemical and Biomolecular Engineering, Department of Chemistry, Carbon Nanotechnology Laboratory, Center for Biological and Environmental Nanotechnology, The Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, Texas 77005, USA (Received 13 December 2005; published 23 June 2006) Individual single-walled carbon nanotubes (SWNTs) in aqueous suspension are visualized directly by fluorescence video microscopy. The fluorescent tagging is simple, biocompatible, and does not modify the SWNTs. The dynamics of individual SWNTs in water are observed and quantified for the first time. We measure the confined rotational diffusion coefficient and find it in reasonable agreement with predictions based on confined diffusion of dilute Brownian rods. We determine the critical concentration at which SWNTs in suspensions start interacting. By analyzing the fluctuating shape of SWNTs in the 3 to 5 m range, we determine that their persistence length ranges between 32 and 174 m, in agreement with theoretical estimates; thus, commonly available SWNTs in liquids can be considered as rigid Brownian rods in the absence of imposed external fields or self-attractive forces. DOI: 10.1103/PhysRevLett.96.246104 PACS numbers: 81.07.De, 82.70.Kj, 83.85.Ei The behavior of single-walled carbon nanotubes (SWNTs) in liquids is still poorly understood. This is perhaps surprising, because SWNTs are commonly de- scribed as high aspect ratio rodlike particles, and slender rigid objects in liquids have been studied for decades [1– 3]. The difficulty in dispersing SWNTs in liquids and the lack of viable techniques for observing their dynamics in suspensions have slowed fundamental progress on liquid- phase behavior of SWNTs. SWNTs in liquids are impor- tant in the physical, material, and life sciences; real-time visualization of SWNTs in liquids can impact each of these areas of research. Liquid-phase processing is key to devel- oping scalable techniques for directed assembly or self- assembly of SWNTs, e.g., production of SWNT fibers [4] and films [5], length and type separation of SWNTs. The high aspect ratio and stiffness of SWNTs may enable more efficient delivery of genes and drugs through cell mem- branes [6]; directly visualizing SWNTs in water would yield detailed information on the interaction of SWNTs with cells [6,7] and biomolecules—DNA and proteins can provide unique and selective building blocks for directed assembly of SWNTs into functional nanoscale and micro- scale structures, e.g., sensors [8]. Visualization can also be important for controlled manipulation of SWNTs into nanostructures, e.g., by optical trapping and tweezing [9]. From a fundamental viewpoint, it is not known whether the theoretical predictions of rotational diffusivity and persis- tence length stemming from the assumption that SWNTs are homogeneous hollow cylinders [10,11] can be applied to SWNTs in liquids because the diameter of a SWNT ( 1 nm) is close to the size of the solvent molecules, and any imperfections in the sidewalls of the nanotubes could affect dynamic properties. High resolution techniques like transmission electron microscopy and atomic force mi- croscopy yield SWNT length and diameter [12,13], but do not provide real-time dynamics in liquids and cannot be applied to living systems. The dynamics of small objects, including SWNTs, in liquids have been studied using bulk techniques such as light and neutron scattering [14,15] and birefringence [16]; these techniques involve large ensem- bles of molecules and the interpretation of results is diffi- cult when samples are poorly characterized and polydis- perse, as is always the case with SWNTs. These difficulties hinder greatly progress on characterization and certifica- tion of SWNT samples, posing problems both to producers and users; direct determination of SWNT rotational and bending properties in liquids would remove these barriers to widespread commercial use. Single molecule fluores- cence video microscopy has provided dynamic information on molecules and small particles like F-actin [17], micro- tubules [17], DNA [18,19], and wormlike micelles [20]. Recent progress has been reported on fluorescence visual- ization of SWNTs [6,21–25]; the most promising method developed so far relies on the spontaneous infrared fluo- rescence of SWNTs [25]. These techniques do not have sufficient temporal and spatial resolution to yield the dy- namics of SWNTs in liquids. Here we present a simple and convenient SWNT fluorescent tagging procedure; with this technique, we study the dynamics of individual SWNTs in water by video microscopy. As-synthesised SWNTs exist as bundles or aggregates bound tightly by strong van der Waals forces. The SWNTs were dispersed as individuals in an aqueous solution of sodium dodecyl sulphate (SDS) [12]. In water, SDS above the critical micelle concentration (8.1 mM) exists as spheri- cal micelles up to a concentration of 810 mM [26]. During sonication in water with SDS, individual SWNTs get en- cased in SDS micelles [Fig. 1(a)] whose estimated diame- ter is 7 nm [27]. Raman and fluorescence spectroscopy [12,27] of the dispersion showed that the SWNTs in solu- tion were dispersed individually (Fig. S1 in Ref. [28]). The first moment and cube root of the third moment of the length distribution in the polydisperse sample were hLi 250 nm and hL 3 i 1=3  440 nm (determined by atomic PRL 96, 246104 (2006) PHYSICAL REVIEW LETTERS week ending 23 JUNE 2006 0031-9007= 06=96(24)=246104(4) 246104-1  2006 The American Physical Society