Article pubs.acs.org/JPCC
Attractive Interactions between DNA−Carbon Nanotube Hybrids in Monovalent Salts Xiangyun Qiu,*,† Fuyou Ke,‡ Raju Timsina,† Constantine Y. Khripin,§ and Ming Zheng§ †
Department of Physics, George Washington University, Washington, DC 20052, United States College of Material Science and Engineering & State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China § Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ‡
S Supporting Information *
ABSTRACT: DNA−carbon nanotube (DNA-CNT) hybrids are nanometersized, highly charged, rodlike molecules with complex surface chemistry, and their behaviors in aqueous solutions are governed by multifactorial interactions with both solvent and cosolutes. We have previously measured the force between DNA-CNTs as a function of their interaxial distance in low monovalent salts where interhybrid electrostatic repulsion dominates. The characteristics of DNACNT forces were further shown to closely resemble that of double-stranded DNA (dsDNA) in low salts. However, contrasting behaviors emerge at elevated monovalent salts: DNA-CNT condenses spontaneously, whereas dsDNA remains soluble. Here we report force−distance dependencies of DNA-CNTs across wide-ranging monovalent salt concentrations. DNA-CNT force curves are observed to deviate from dsDNA curves above 300 mmol/L NaCl, and the deviation grows with increasing salts. Most notably, DNA-CNT forces become net attractive above 1 mol/L NaCl, whereas dsDNA forces are repulsive at all salt concentrations. We further discuss possible physical origins for the observed DNA-CNT attraction in monovalent salts, in consideration of the complex surface chemistry and unique polyelectrolyte properties of DNACNT hybrids.
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INTRODUCTION DNA and carbon nanotubes represent two of the most diverse members of their respective biological and inorganic worlds. Single-stranded DNA are flexible chains of a set of four nucleotides, and their combinatorial sequences, in astronomical numbers, encode the blueprint of life and afford practically unlimited variations as the favorite building blocks for nanobiotechnology.1,2 Single-wall carbon nanotubes (CNTs) are rigid hollow cylinders enclosed by one-atom-thick graphene sheets, and the myriad rolling vectors of graphene give rise to chirality-dependent CNT structures.3 As such, noncovalent conjugates of DNA and CNTs are promising bioabiotic hybrids of extraordinary biological, chemical, and physical properties. Early studies of DNA-CNT hybrids focused on overcoming two major barriers of the CNT technology: water-insolubility and inhomogeneous chirality distribution of as-synthesized CNTs. Aqueous dispersion of CNTs is a significant step because it allows uses of effective solution-based processing techniques such as characterization and assembly.4 Likewise, sorting by CNT chirality is essential for exploring the chiralityspecific electronic, optical, and thermal properties of CNTs.5 Remarkably, DNA-assisted dispersion and sorting of CNTs have attained CNT solutions among the highest concentrations and, most importantly, a growing list of CNT types of highest single-chirality purities.6−8 © XXXX American Chemical Society
Given the chemically homogeneous and hydrophobic CNT surface, its high affinity to DNA necessarily arises from surface stacking with hydrophobic DNA motifs (i.e., aromatic planes of nucleobases) via a combination of interdependent hydrophobic, electronic π−π, and van der Waals interactions.9,10 The highly charged DNA backbones exposed outside subsequently confer high solubility. Initial sorting studies utilized ion-exchange chromatography (IEX) to separate DNA-CNT hybrids by CNT chirality.6−8 It was observed that CNTs of specific chirality are best sorted with DNA of specific sequence, indicating peculiar couplings between CNT chirality and DNA sequence in determining the hybrid surface characteristics (e.g., charge density and hydration strength). Knowledge of the exact DNA structure on the CNT surface is key to unraveling the underlying mechanisms but remains unknown. Recently, a breakthrough in CNT sorting demonstrated that DNA-CNT hybrids partition differentially in neutral-polymer-based aqueous two-phase (ATP) systems,11 and such partition is strongly modulated by DNA sequence, CNT chirality, ATP composition, and salt conditions. It is worth noting that the ATP-based CNT sorting is also applicable to surfactant-dispersed CNTs, Received: May 7, 2016 Revised: June 1, 2016
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DOI: 10.1021/acs.jpcc.6b04623 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C though the exact partition parameters and outcomes differ.12−15 While it is apparent that electrostatic interactions play an important role in IEX separation, the mechanism of ATP separation highlights the importance of DNA-CNT solvation energy which is under delicate control by a number of internal (DNA and CNT) and external (e.g., polymer, salt, and pH) factors.11 Given the ability to readily modulate these factors and quantify their effects through CNT partitioning, the system of DNA-CNT hybrids presents a unique opportunity to probe the intricate interactions between an inorganic surface, a charged biopolymer, and their aqueous and ionic environment. We have previously studied the electrostatic and hydration interactions of DNA-CNT hybrids in low monovalent salts (up to 500 mmol/L NaCl) by measuring the interhybrid force as a function of the interhybrid distance.16,17 The main finding is that DNA-CNT hybrids exhibit force−distance dependencies very similar to that from double-stranded DNA (dsDNA) in low salts, once their difference in diameter is taken into account. Consistent with their highly charged nature, electrostatic repulsion was observed to dominate at low salts. At close surface−surface separations (1 mol/L monovalent salts,21,22 while dsDNA remains soluble (e.g., up to 3 mol/L NaCl). This qualitative difference indicates a source of attractive force present in DNA-CNT only, and a new mode of DNA-CNT interaction is suggested. To this end, this study aims to quantify the force characteristics in high monovalent salts and to investigate likely physical origins for DNA-CNT attraction. By measuring the force curves of DNA-CNT and dsDNA under identical conditions, we report conclusive evidence for the presence of attraction in DNA-CNT in monovalent salts. We further parametrize the measured force− distance dependencies based on the hydration force formalism and estimated the magnitudes of the DNA-CNT attraction. Possible mechanisms for DNA-CNT attraction are discussed in light of the distinct surface chemistry and polyelectrolyte properties of DNA-CNT.
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distance curve was obtained by varying the osmotic pressure via osmolyte concentration.
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RESULTS The highly charged nature of DNA-CNT hybrids results in strong electrostatic interactions that dominate in low salt concentrations. Increasing salt concentration consequently provides a convenient way to weaken electrostatic forces and makes it possible to discern the existence of other types of interactions. We show the force between DNA-CNT hybrids as a function of interaxial distance in Figure 1 under salt
Figure 1. Force−distance curves of the DNA-CNT hybrid and dsDNA at different salts. The same symbol is used for a specific Na+ concentration as denoted in the legend. The dsDNA curves in black are shown with a positive offset of 13 Å in interaxial distance to aid visualization. Solid lines are guides to the eye to illustrate the “concave” and “convex” shapes. Note that the 300 and 500 mmol/L NaCl DNA-CNT curves have been shown previously,17 though no comparison or analysis with dsDNA curves was carried out.
conditions ranging from 300 mmol/L to 2 mol/L NaCl, noting that the force is shown as the osmotic pressure that is directly varied in experiments. The force−distance curves of dsDNA under the same salt conditions are also shown to illustrate the force characteristics from a charged and fully hydrated surface. The force curves in even lower salts of 50 and 150 mmol/L NaCl have been analyzed previously,16 where DNA-CNT and dsDNA show the same shapes once differences in their diameters are taken into account. Given that a different batch of CNT (CoMoCAT SG65) was used in our earlier study,16 the results are reproduced in Figure S2a-d for the CNT (CoMoCAT SG65i) studied herein, noting that a difference of 2 Å in average DNA-CNT diameters was found. Upon increasing salts, both sets of curves show decreasing forces, consistent with salt screening of the electrostatic repulsion. However, the shapes of the DNA-CNT and dsDNA curves begin to differ. Qualitatively, all the dsDNA force curves can be described by a fast-decaying repulsion dominating at the shortrange and a slow-decaying repulsion at the long-range, giving rise to “concave” shapes under all salt conditions. In contrast, the DNA-CNT force curves above 1.0 mol/L NaCl are “convex”: while the fast-decaying short-range repulsion persists, a long-range attraction force necessarily exists to explain the “downturn” of the curves with increasing interaxial distances. We next seek to provide an analytical formulation of the measured DNA-CNT forces and further extract the magnitude of the attraction between DNA-CNTs. In our previous analysis of DNA-CNT forces in low salts (50 and 150 mmol/L NaCl),16 we showed that the hydration force formalism,24 proposed by Parsegian and Rau for dsDNA forces at the last nanometer of separation,25 is able to quantitatively describe the
MATERIALS AND METHODS
Preparation of DNA-CNT hybrids followed the protocols as described in refs 16 and 23, and force measurements were carried out with the osmotic stress method (OSM).16,24 We provide key specifics below, and additional details can be found in the Supporting Information. Single-wall CNT (CoMoCAT SG65i) was obtained from South West Nano Technologies, and single-stranded (GT)20 DNA was obtained from Integrated DNA Technologies; genomic dsDNA (Salmon, >10 kilo base pairs) was purchased from Sigma. To probe intermolecular forces at close separations, condensed pellets of DNA-CNT or dsDNA were first obtained via polymer-induced formation of liquid crystalline phases in low salts. It is worth noting that low salts (
