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Frise Et Al. 2010 Protein Dispersant Binding on Nanotubes Studied by NMR Self-Diffusion and Cryo-TEM Techniques

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  • 1. pubs.acs.org/JPCLProtein Dispersant Binding on Nanotubes Studied by NMRSelf-Diffusion and Cryo-TEM TechniquesAnton E. Frise,† Eran Edri,‡ Istvn Furo,† and Oren Regev*,‡ a ‡ Department of Chemical Engineering and the Ilse Katz Institute for Meso and Nanoscale Science and Technology, Ben-GurionUniversity of the Negev, 84105 Beer-Sheva, Israel, and †Division of Physical Chemistry, Department of Chemistry,Royal Institute of Technology, SE-10044 Stockholm, Sweden ABSTRACT Carbon nanotubes can be dispersed by a variety of molecules. We investigate the dynamics of protein-assisted carbon nanotube dispersion in water. We find that in equilibrium, only a small fraction of the dispersants is indeed adsorbed to the nanotube surface, while there is a fast exchange process between the adsorbed and free protein molecules. Self-diffusion NMR spectroscopy in combination with cryo-transmission electron microscopy imaging are employed. SECTION Nanoparticles and Nanostructures arbon nanotubes (CNTs) possess unique electrical,C on the nanotubes surface remains, among other issues, mechanical, and optical properties and are hence ambiguous.27,28 implemented in applications ranging from nanoelec- As concerns dispersion mechanisms, for polymer-typetronics1-3 through biosensors4 to drug delivery.5 A major dispersants, it was suggested that a weak, long-ranged en-drawback of as-produced CNTs is their bundling due to strong tropic repulsion among polymer-decorated tubes acts as avan-der-Waals interactions. Therefore, the advantages with the steric barrier preventing the tubes from approaching thehigh aspect ratio (>1000) and other unique properties of attractive part of the intertube potential.7,29 Others suggestedindividual single-wall carbon nanotubes (SWNTs) are lost. a wrapping-type mechanisms.12,30 However, while there isHence, substantial effort has been devoted to dispersing and plenty of data on the conformation of the dispersing agent indebundling CNTs through either covalently grafting dispersing dry samples,31,32 there is lack of experimental information onmolecules onto the CNT surface6 or noncovalently adsorbing it the dispersant configuration or the density of dispersanton the CNT surface.7 To preserve the original CNT structure molecules on the nanotube in situ. For surfactants, varioustogether with the associated electrical and mechanical proper- anchoring possibilities on nanotubes were suggested, primar-ties, the hybridization of the carbon atoms should be minimally ily involving hydrophobic interactions.33perturbed during the debundling process, which calls for Interactions involved in protein adsorption on NT sur-noncovalent dispersion. Moreover, the noncovalent approach, faces can be electrostatic and hydrophobic. While someinvestigated here, does not require chemical reactions. researchers suggest that hydrophobic interactions20,34 While the noncovalent dispersion and exfoliation of CNTs dominantly control protein adsorption, others35 claim thatwith surfactants,8-10 polysaccharides,7,11 and polymers12,13 is the electrostatic contribution is the major one. Recently, thewell-established, biomaterials such as proteins are rather new distribution of the dispersant-dispersant distance oncandidates for dispersing NTs. Nonetheless, proteins gain a the nanotubes (L) (Figure 1) has been reported for BSA asrapidly growing interest as an alternative route for the de- the dispersant.36bundling and dispersion of CNTs, especially for biomedical Despite significant research, little is known on the nature ofapplications. This stems from the hope that they also enhance noncovalent dispersion of nanotubes by protein or otherthe biocompatibility of SWNTs.14-16 Lately, an efficient up- dispersants. How much dispersant is adsorbed on a nano-take of protein (drug)-dispersed SWNT by cells, probably via tube? What is the fraction of surface area covered? Are thean endocytosis17 pathway, has been reported. Here, the CNT-adsorbed molecules in equilibrium with the freely dis-SWNTs are employed as molecular transporters for various persed ones? These questions are extremely relevant tocargos.18 applications where nanotubes are exploited as transporters Indeed, natural water-soluble proteins are reported of drugs (proteins) to cells.17 In many studies, one assumed ato successfully disperse NTs through noncovalent inter- full coverage,28,33 which is, however, difficult to prove experi-actions.19-22 Short synthetic peptides were also shown to mentally.successfully exfoliate, debundle, and disperse nanotubes viaspecifically designed adsorption23,24 or helical-like wrap-ping.25 Indeed, the use of peptides and proteins has opened Received Date: March 16, 2010a door to many potential applications.26 However, the dis- Accepted Date: April 8, 2010persant (e.g., surfactant, polymer, or protein) configuration Published on Web Date: April 14, 2010 r 2010 American Chemical Society 1414 DOI: 10.1021/jz100342c |J. Phys. Chem. Lett. 2010, 1, 1414–1419
  • 2. pubs.acs.org/JPCL BSA in an aqueous solution has been found to adsorb to within the approximately (2% statistical error) water diffu-CNTs.36 The proposed mechanism is that the protein partly sion coefficients in all measured solutions (for more details,unfolds and exposes some of its hydrophobic amino acids, see Experimental Methods section).which interact with the surface of the CNTs.37,38 The binding The validity of the assumption of fast exchange wasof protein to the surface introduces steric repulsion and will confirmed by comparing the BSA spectral integrals obtaineddisperse CNTs. in conventional single-pulse experiments for solution A on In this paper, we show that in the dispersed state, the one hand and those for solutions B1 and B2 on the other hand.protein molecules cover only a very small fraction of the In the case of slow exchange, proteins binding to the NTsnanotube surface and exchange quickly with the pool of would experience very slow tumbling that would lead to theunadsorbed proteins. loss of their signal because of the very large broadening. Samples were prepared by mixing protein and nanotubes, However, no such reduction in the signal could be found, andsonicating and extracting the supernatant by centrifugation the assumption of fast exchange should therefore hold for this(for details, see Supporting Information). The concentrations system. (Alternatively, if there were slow exchange, the frac-of NTs in the dispersion were determined by UV-vis-based tion of NT-bound BSA should be lower that the 2-3%chemometric techniques22 and are presented in Table 1 along accuracy of the NMR intensity measurements). As an addi-with the initial mixing concentrations. As concerns the protein tional indication, the BSA line widths were slightly larger incomponent, BSA concentrations as provided by NMR spectral solutions B1 and B2 as compared to those in solution A. Thisintensities of BSA were identical (within the experimental behavior is as expected for BSA molecules in fast exchangeerror of a few percent) before and after centrifugation. between sites with slow (in solution) and fast (bound to NTs) We first present the results of our self-diffusion measure- transverse relaxation.ments and then relate to our cryo-TEM micrographs, including In fast exchange, the observed BSA diffusion coefficientcalculations of area per adsorbed protein molecule. (Dobs) in solutions B1 and B2 becomes the population average Self-Diffusion Study. Previous studies of self-diffusion of NT of the diffusion coefficients in the bound and free states.42systems concerned the diffusion of gases and liquids confined Dobs ¼ Dfree 3 pfree þ Dbound 3 ð1 - pfree Þ ð1Þinside of the NTs.39,40 Our use of the molecular diffusion here isentirely different since we exploit self-diffusion NMR to report Considering that the diffusion of BSA in its NT-bound state ison the ratio of BSA bound to CNTs versus that free in the negligibly small, Dfree . Dbound, this expression simplifies tosolution. It is assumed that these are the only states that the Dobs = Dfree 3 pfree, from which the fraction of bound BSAprotein takes and also that the exchange between the two molecules can be estimated asstates is fast on the NMR time scale. A third assumption is that Dobswhen the protein is bound to CNTs, its diffusion becomes pbound ¼ 1 - pfree ¼ 1 - ð2Þ Dfreenegligible. This is reasonable considering that the NTs are100-1000 times larger than the proteins, and their movement The fractions and resulting absolute concentrations Cbound ofshould therefore be correspondingly slower. Finally, we as- NT-bound BSA as provided by eq 2 are presented in Table 1.sume that the obstruction41 on diffusion by the NTs is negli- For both SWNTs and MWNTs, we find that the bound fractionsgible, which is validated by the identical (1.90 Â 10-9 m2/s are small, on the order of only a few percent. Unfortunately, this and the comparable magnitude of the precision of the diffusion data also mean that the obtained relative error of pbound is high. We use the obtained Cbound and the total dispersed con- centration of NT, CNT, from chemometric measurements22 to calculate the area per one BSA molecule on NT, ABSA(NT). First, the number of bound BSA molecules per volume unit is obtained as CboundFigure 1. The dispersant-dispersant distance (L) in the arrange- NBSA ¼ NA ð3Þment of dispersant molecules (dotted) adsorbed on a nanotube. MWBSATable 1. Initial NT and BSA Mixing Weights and Final (for solutions B1 and B2) NT Concentrations after Sonication and Centrifugationasample compound initial conc. [mg/mL] CNT [mg/mL]b Dobs [Â10-11 m2/s] Pboundc Cboundd [mg/mL] A BSA 4 5.22 ( 0.13 B1 BSA 4 5.03 ( 0.05 0.04 ( 0.03 0.16 ( 0.12 SWNT 2 0.24 ( 0.01 B2 BSA 4 4.74 ( 0.13 0.09 ( 0.05 0.36 ( 0.20 MWNT 2 0.02 a NT concentrations were obtained by chemometric-based experiments on UV-vis spectroscopy,22 and NMR spectral intensities indicate identicalBSA concentrations before and after centrifugation. b Dispersed concentration in the supernatant. c Fraction of NT-bound BSA. d The concentration ofNT-bound BSA. r 2010 American Chemical Society 1415 DOI: 10.1021/jz100342c |J. Phys. Chem. Lett. 2010, 1, 1414–1419
  • 3. pubs.acs.org/JPCL it seems that the surface density should, if anything, be higher for MWNTs than that for SWNTs and that low values of R provide peculiarly high surface density values (more than monolayer coverage, under the assumption of adsorption on the external surface of the outermost tube). Electron Microscopy Study. The BSA molecules have insuffi- ciently low contrast in cryo-TEM measurements (Figure 2A). To enhance the contrast, we attached an 8 nm gold nanopar- ticle (GNP) to the individual BSA molecules (Figure 2B). We have previously shown that the GNP-BSA complex disperses the SWNTwithout changing the conformation of the protein.36 The measured distribution of the dispersant- dispersant distance (L; see Figure 1) for the BSA-GNP com- plex as imaged by cryo-TEM indicates an average distance of ÆLæ=77 ( 55 nm for SWNT solutions36 (Figure 2B) in order-of- magnitude agreement with the NMR findings above. In the cryo-TEM image (Figure 2B), L has a rather wide polydisper- sity. Similar images in the corresponding MWNT system demonstrate binding, but we refrain from a quantitative analysis (see discussion above). The area per adsorbed BSA molecule, ABSA(NT), is obtained via eq 6 as ABSA(SWNT) = 360 ( 260 nm2.36 Since the concentration of the GNP-BSA was lower than the BSA concentration in the NMR experiments (see discussion above), this value is consistent with that derived from the NMR experiments.Figure 2. Cryo-TEM micrographs of SWNTs (A,B) and MWNTs First, it should be noted that due to the low BSA contrast in(C,D) dispersed by unlabeled BSA (A,C) and by BSA labeled by 8 nmgold nanoparticles (B,D). It is impossible to see any unlabeled TEM, the NMR and cryo-TEM measurements are not con-adsorbed BSA molecules. The spots in A and C marked by arrows ducted on exactly the same system; (1) the GNP introducedare due to surface contamination during vitrification and transfer into the cryo-TEM preparation are not added in the NMRto the electron microscope. Bar = 100 nm. experiments, and (2) the BSA concentration in the cryo-TEMwhere MWBSA=67100 Da is the molar mass of BSA and NA is measurement is much lower than that in the NMR experi-the Avogadro number. The total surface area of NTs (per ments (see Experimental Methods section). Therefore, involume) in the solution, ANT, is obtained as Figure 2, we mostly see GNP-BSA that are bound (in close ANT ¼ CNT 3 SSA ð4Þ proximity) to the SWNT, which is in contrast to the NMR finding of most BSA being not bound. This contradiction canwhere SSA is the reported specific surface area of the NTs. be resolved by considering the low GNP-BSA concentration. For SWNT, available values are SSA(SWNT) = 24843,44 or Here, the NMR findings can be considered as quantitatively283 m2/g.45 Hence, the area per adsorbed BSA molecule is accurate, while the cryo-TEM observations can primarily be ANT seen as rough verifications of the finding of the small amount ABSA ðSWNTÞ ¼ ¼ 80 ( 60 nm2 ð5Þ of CNT-bound protein. NBSA In this context, there is no easy way to perform the NMRThis area yields, via and cryo-TEM experiments on the same system because ABSA ðNTÞ ¼ πDNT ÆLæ ð6Þ binding the gold nanoparticles broadens the NMR signal to close undetectability. Even if the 1H NMR signal fromand the known diameter of SWNT (DNT=1.5 nm), the average GNP-BSA were detectable, the diffusion coefficient for themdispersant-dispersant distance (L; see Figure 1) on the order in the free state would be much lower, which would decreaseof ÆLæ ≈ 20 nm, in agreement with reported values in the the sensitivity of NMR experiments to binding. Since ourliterature.31 current sensitivity is barely enough to detect binding, diffu- The active surface area per weight (available for ad- sion experiments with GNP-BSA are not feasible.sorption) for a MWNT is plausibly lower than the one obtained Despite all possible shortcoming of our experiments, ourby BET measurements (233 m2/g46) if we assume that only current observations together with some earlier ones36 pro-the external surface of the outermost tube is accessible for vide two significant findings. First, the most striking feature isadsorption of a protein molecule. Since (i) the reported values that only a very small fraction of the dispersing moleculesfor the ratio of the active surface area/total surface area, R, for (a few percent; see Table 1) are actually adsorbed on the NTMWNTs are scattered within a very wide range of surface at a given instance. This is true for both the SWNTand0.1-0.001.47,48 and (ii) there is a large polydispersity in MWNT (Table 1). However, if one attempts to disperse the NTsMWNT width and the number of layers, we cannot provide by initially adding only that much BSA that corresponds to thea surface density estimate for the MWNT solutions. However, pbound (that is, for example, preparing a SWNT dispersion by r 2010 American Chemical Society 1416 DOI: 10.1021/jz100342c |J. Phys. Chem. Lett. 2010, 1, 1414–1419
  • 4. pubs.acs.org/JPCLFigure 3. Schematics of the dynamic equilibrium between adsorbed and free BSA molecules (red). According to our results, most BSAmolecules are in the free state at any given instance, and the exchange is fast, on a time scale <50 ms.adding 0.2 mg/mL BSA instead of 4 mg/mL; see Table 1) withthe same initial amount of NT (2 mg/mL), no SWNT will bedispersed.21 Therefore, BSA molecules seem to be in adynamic equilibrium and exchange between bound and freestates, similar to surfactant molecules in a micelle above theCMC and as illustrated in Figure 3. This process can take placeand can take place quickly (<50 ms, which is the time scale ofour NMR diffusion experiments) since the BSA adsorbs on theNT via a nonwrapping mechanism.49 Indeed, for other pro-teins or other dispersants with different adsorption energies,the situation could be completely different, especially forcertain polymers as dispersants. Our second finding is that the area per adsorbed BSAmolecule seems to differ for SWNTs and MWNTs. Unfortu-nately, we could not quantify the extent of this difference. Onepossible reason for such a difference is different curvatures for Figure 4. Normalized signal amplitudes of stimulated echo de-SWNTs and MWNTs. cays versus the Stejskal-Tanner factor (b = (γgδ)2(Δ - δ/3), Upon dispersing NTs by a dispersant, part of the NTs are where γ is the magnetogyric ratio of 1H) of BSA in solutionsindeed dispersed, but some precipitate. After centrifugation, without (A; see Table 1) and with added carbon nanotubes (B1we decant the supernatant,9 where a stable NT dispersion is and B2). Spectra obtained in pulsed field gradient spin-echo experiments in aqueous solutions of BSA. The solid lines are linearobtained. It has been recently reported that there is a thresh- fits to the decay data with adjusted R2 values higher than 0.99.old of dispersant concentration, above which a dispersion isobtained.28 This value depends on the dispersant as well as would therefore be to study different dispersants to clarifythe solvent. Increasing the dispersant concentration above these issues.this threshold value for a given NTconcentration will not bringmore nanotubes to the dispersed solution50 and may even EXPERIMENTAL METHODSreduce the concentration of the dispersed NT.21 Intuitively, itcould then be assumed that in the dispersed state, most The diffusion coefficients of BSA were measured withdispersing molecules are indeed adsorbed on the NT surface. pulsed gradient spin-echo (PGSE) NMR using the stimulatedThis is clearly not the case, as we have shown in this Letter. echo pulse sequence.51 The gradient strength ( g) was in- The protein molecules bound to the dispersed nanotubes creased in 10 steps from 0.5 to 3.9 T m-1. At each step, 128are only a very small fraction of the available protein mole- scans were acquired. The diffusion time (Δ) was set constantcules in the system. The free and bound protein molecules are to 50 ms and gradient pulse duration (δ) was set toin fast exchange between these two states. These findings 1 ms, with 1 ms stabilization time between the gradient andprovide new insights and an alternative to the previously radio frequency pulses. The diffusion coefficients were ex-depicted scenario of persistent dispersant-NT complexes in tracted by fitting the conventional Stejskal-Tanner expres-solution. For BSA adsorbed on SWNTs, a protein-to-protein sion52 to the observed diffusional decays (see Figure 4). Thedistance of 5-30 nm is estimated from NMR measurements, longitudinal relaxation time (T1) of BSA was estimated byand its order of magnitude is confirmed by cryo-TEM mea- inversion recovery to ∼0.5 s, and the recycle delay betweensurements. two acquisitions was therefore set to 3 s. The diffusion We cannot tell if the reported dynamic equilibrium picture measurements were repeated three times for the free proteininferred in this system is true as well for other systems with (solution A) and five times for protein in the NT dispersiondifferent adsorption energies or curvatures. Our next step (solutions B1 and B2); the results given in Table 1 are r 2010 American Chemical Society 1417 DOI: 10.1021/jz100342c |J. Phys. Chem. Lett. 2010, 1, 1414–1419
  • 5. pubs.acs.org/JPCLarithmetic means, and the error bars are estimates of preci- (4) Chen, R. J.; Bangsaruntip, S.; Drouvalakis, K. A.; Kam, N. W. S.;sion (1 standard deviation) from the observed scatter. We Shim, M.; Li, Y. M.; Kim, W.; Utz, P. J.; Dai, H. J. Noncovalentnote that the diffusion coefficient that we obtained for Functionalization of Carbon Nanotubes for Highly Specificfree BSA molecules (Dfree) in solution A is (5.22 ( 0.13) Â Electronic Biosensors. Proc. Natl. Acad. Sci. U.S.A. 2003, 100,10-11 m2/s, in agreement with the literature value of 5.31 Â 4984–4989. (5) Bianco, A.; Kostarelos, K.; Prato, M. Applications of Carbon10-11 m2/s.53 Nanotubes in Drug Delivery. Curr. Opin. Chem. Biol. 2005, 9, It is known that paramagnetic impurities (residual catalysts 674–679.Ni and Y) may be present in the used SWNT. The 1H longi- (6) Liu, P. Modifications of Carbon Nanotubes with Polymers. Eur.tudinal relaxation time T1 of water (or HDO) is longer than Polym. J. 2005, 41, 2693–2703.that for BSA and should therefore be affected more by (7) Bandyopadhyaya, R.; Nativ-Roth, E.; Regev, O.; Yerushalmi-paramagnetic materials. Indeed, we observed T1(HDO) = Rozen, R. Stabilization of Individual Carbon Nanotubes in10.8 s in pure BSA solution and T1(HDO)=9.8 s in the MWNT Aqueous Solutions. Nano Lett. 2002, 2, 25–28.dispersion, while T1(HDO)=3.2 s was observed in the SWNT (8) Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Journet,dispersion. This indicates that there might be some paramag- C.; Bernier, P.; Poulin, P. Macroscopic Fibers and Ribbons ofnetic impurity in the SWNT solutions which was not removed Oriented Carbon Nanotubes. Science 2000, 290, 1331–1334.by centrifugation. At the same time, because of the shorter 1H (9) Regev, O.; ElKati, P. N. B.; Loos, J.; Koning, C. E. Preparation ofrelaxation in BSA (T1 ≈ 0.5 s, in all solutions), this will have no Conductive Nanotube-Polymer Composites Using Latex Technology. Adv. 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