Article pubs.acs.org/cm
Boranephosphonate DNA-Mediated Metallization of Single-Walled Carbon Nanotubes Saheli Ganguly,† Sibasish Paul,† Omer Yehezkeli,‡ Jennifer Cha,‡ and Marvin H. Caruthers*,† †
Department of Chemistry and Biochemistry, and ‡Department of Chemical and Biochemical Engineering, University of Colorado, Boulder, Colorado 80303, United States S Supporting Information *
ABSTRACT: Single-walled carbon nanotubes (SWNTs), when dispersed in DMSO with boranephosphonate DNA (bpDNA), were efficiently metalized with silver, gold, and palladium nanoparticles (NPs). This was possible by first adsorbing boranephosphonate DNA onto the surface of SWNTs and then bathing with silver, gold, and palladium metal salts, which form the corresponding nanoparticles by reduction of their respective ions without addition of any external reducing agent. Reduction of a redox dye, 2,6-dichlorophenolindophenol (DCPIP), by Pd nanoparticle conjugates (PdNP/bpDNA/SWNT) disclosed the efficient electron transfer properties of these metallized SWNTs. These PdNP/bpDNA/SWNT conjugates were also successfully used to catalyze Heck and Suzuki coupling reactions. Boranephosphonate DNA-mediated metallization of SWNTs therefore provides a new method for fabricating well-defined SWNT-based nanostructures. This discovery should reveal unexpected applications in various research areas ranging from nanoelectronic devices to nanoscale SWNT supported multimetallic catalysts having different compositions.
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disperse, assemble, and pattern SWNTs for microelectronics.13 Molecular dynamic simulations enabled the exploration of the physical properties of these multifunctional nanomaterials.14−16 Oligonucleotides containing boranephosphonate internucleotide linkages were first synthesized by the Shaw laboratory.17 These compounds contain a borane (BH3) moiety in place of one of the nonbridging phosphate oxygens (Figure 1). Our group has recently reported a new method for synthesizing boranephosphonate DNA and has shown that this analogue reduces metal ions such as Au(III), Pt(II), and Ag(I).18,19 This research was further extended by constructing a two-dimensional DNA array that was metalized via incorporation of boranephosphonate DNA.19 These arrays were very fragile and observed to roll-up into cylinders that formed both simple and complex structures. Similar results were reported previously with nonmetalized two-dimensional DNA arrays12,20
INTRODUCTION Because of the unique combination of one-dimensional structure and topology, single-walled carbon nanotubes exhibit outstanding structural, mechanical, and electronic properties. Since their discovery two decades ago,1 carbon nanotubes (CNTs) have been the focal point for research directed toward several potential applications such as field effect transistors (FET), electrodes for lithium ion batteries and scanning probes,2−4 and biosensors.5 Several investigations have been conducted on the use of DNA as a fabrication material for designing new SWNT applications. This is because singlestranded DNA (ssDNA), through π-stacking interactions with the sidewalls of CNTs, offers intriguing possibilities for wrapping SWNTs with DNA. For example, considerable research has been carried out with sequence specific ssDNA in attempts to disperse, via sonication, SWNTs in aqueous solution.6−11 Attempts have also been made to metalize DNA by sequence specific scaffolding with reducing agents including glutaraldehyde.7 In other research, Han et al. have reported depositing linear arrays of gold particles onto SWNTs wrapped with thiolated DNA.12 Keren et al. have used ssDNA to © 2017 American Chemical Society
Received: December 7, 2016 Revised: February 7, 2017 Published: February 10, 2017 2239
DOI: 10.1021/acs.chemmater.6b05182 Chem. Mater. 2017, 29, 2239−2245
Article
Chemistry of Materials
SpeedVac. bpDNA was then dissolved in an appropriate solvent for further use. Preparation of bpDNA Wrapped SWNTs. bpDNA was purified using acrylamide gel (20%) electrophoresis and then used for dispersing SWNTs. SWNTs (0.1 mg) in dimethyl sulfoxide (DMSO) and bpDNA in water were dispersed in a total volume of 500 μL (30 μM bpDNA, 10% water, 90% DMSO). DNA wrapped SWNTs were then prepared following a published procedure.11 The solution was sonicated at 250 W for 20 min and subsequently centrifuged at 12 000g for 10 min. The precipitate containing nondispersed SWNTs was discarded. The supernatant solution containing dispersed SWNTs and unbound DNA was further centrifuged for 10 min at 12 000g. The precipitate containing DNA wrapped SWNTs was collected and suspended in DMSO. This solution was used for metallization. Metalization of bpDNA/SWNTs. Solutions (10 mM) of AgNO3, HAuCl4, and Na2PdCl4 in Millipore water were prepared. These salt solutions (25 μL each) were added to 75 μL aliquots of bpDNA wrapped SWNTs suspended in DMSO, and the solution was incubated at 50 °C overnight with continuous shaking in a MultiTherm Heat Shaker. The solution was centrifuged at 12 000g, and the precipitate containing MetalNP/bpDNA/SWNTs was suspended in DMSO for subsequent characterizations and use in various experiments. Catalysis Reactions with PdNP/bpDNA/SWNTs. Heck Coupling. 5-Iodo-2′-deoxyribouridine (0.005 g, 0.014 mmol) and styrene were added to a solution of the PdNP/bpDNA/SWNT catalyst (1 × 10−9 M solution of Pd NPs as calculated using an inductively coupled plasma (ICP) measurement) and triethylamine (0.003 mL) in 1methyl-2-pyrrolidone (NMP, 1.0 mL). This reaction mixture, under an argon atmosphere, was stirred for 5 min at room temperature and then for 3 h at 120 °C. At this time, TLC indicated complete conversion to product (Rf 0.3, 9:1 chloroform:methanol). The reaction mixture was evaporated to dryness, diluted with dichloromethane (5.0 mL), and washed with brine. The organic layer was dried over sodium sulfate and analyzed by ESI (mass found for C17H18N2O5H+, 331.1485; calculated mass, 331.1494). The same protocol was used for the reaction of iodobenzene, styrene, and the PdNP/bpDNA/SWNT catalyst. The product was analyzed by ESI (mass found for C14H12, 180.1024; calculated mass, 180.1039). Suzuki Coupling. 5-Iodo-2′-deoxyribouridine (0.005 g, 0.0015 mmol), pyrene-1-ylboronic acid (0.007 g, 0.03 mmol), and K2CO3 (0.006 g, 0.044 mmol) were placed in a 5.0 mL vial containing water (2.0 mL) and acetonitrile (0.5 mL). A solution of the PdNP/bpDNA/ SWNT catalyst (1 × 10−9 M) in DMSO was centrifuged, the DMSO was removed, and the reaction mixture was added to the catalyst. The vial was flushed with argon, closed, and heated at 55 °C. After 5 h, TLC analysis showed almost complete conversion (∼90%) of starting material to product (Rf 0.56, 1:1 hexane:ethyl acetate). This reaction was also monitored using reverse phase HPLC (buffer A, triethylammonium bicarbonate, 0.05 M; buffer B, acetonitrile; 0− 100% B in 50 min; 55 °C; 1.5 mL/min flow rate). Aliquots of the reaction mixture (100 uL) at the indicated time intervals were filtered using 0.2 um centrifugal filters and injected onto the HPLC column. Following this analysis, after 5 h the reaction mixture was allowed to cool to room temperature and extracted with dichloromethane (2 × 4.0 mL). The combined organic phases were washed with brine, dried over Na2SO4, and solvent was removed in vacuo. The ESI mass for the product was 430.1438 (calculated mass for C25H22N2O5: 430.1429). Pyrene-1-ylboronic acid and iodobenzene were also condensed using the Suzuki reaction toward complete conversion. ESI mass found for 1-phenylpyrene was 278.1214 (calculated mass for C22H14: 278.1196).
Figure 1. Chemical structures of DNA and boranephosphonate DNA.
as well as with DNA arrays that were metalized by other procedures.21−23 This Article reports a strategy for organizing noble metal nanoparticles (Ag, Au, Pd) onto bpDNA wrapped SWNTs. Transmission electron microscopy, scanning electron microscopy, energy dispersive spectroscopy, and atomic force microscopy were used to characterize the metal NP/bpDNA/ SWNT conjugates. The electrochemical and chemical catalytic properties of PdNP/bpDNA/SWNTs were studied by reduction of 2,6-dichlorophenolindophenol (DCPIP) and by catalyzing both Heck and Suzuki reactions.
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EXPERIMENTAL SECTION
General. SWNTs (7,6-chirality) with a diameter of 0.7−1.1 nm were purchased from Sigma-Aldrich. Other chemicals and buffers were purchased from Sigma-Aldrich unless noted. TEM imaging was carried out using CM-10 and FEI Technai-12 electron microscopes. CM-10 is a 100 kV TEM equipped with a 1K × 1K GatanBioscanDigital camera and a traditional film camera. FEI Technai-12 is a 120 kV TEM equipped with two digital cameras: a wide-angle, side-mounted 2K × 2K AMT camera and a bottom-mounted 4K × 4K FEI Eagle camera. Carbon-coated gold grids (400 mesh, Electron Microscopy Science) were used for imaging. Prior to being used, these grids were cleaned by a plasma treatment. AFM imaging was performed using NanosurfEasyscan. SEM imaging with EDS was performed using a JEOL6480LV microscope. Samples for SEM imaging were deposited on 4” diameter silicon wafers in a 5 × 5 mm chip (TED Pella Inc.). Cyclic voltametric measurements were performed in a three-electrode system consisting of metal NP/bpDNA/SWNT deposited on a glassy carbon electrode (GCE), a platinum counter electrode, and a Ag/AgCl reference electrode. Data were recorded with an Autolabpotentiostat interfaced with a computer. All of the electrochemical experiments were performed at room temperature with pH 7 phosphate buffer (0.1 M) under an argon atmosphere. UV−vis measurements were completed using a CARY 100 Bio spectrophotometer. The ICP measurements were performed by a ARL 3410+ inductively coupled optical emission spectrometer (ICP-OES). A blank and three standards were used for calibration. Standards were made by accurately diluting certified standards. Synthesis of Boranephosphonate DNA. Oligodeoxynucleotides (ODNs) containing boranephosphonate linkages were synthesized as described previously.19 Following synthesis, each resin linked ODN was treated with disodium-2-carbamoyl-2-cyanoethylene-1,1-dithiolate in N,N-dimethylformamide (1.0 M) for 45 min to remove the methyl protecting groups from phosphorus, washed repeatedly with anhydrous DMF and acetonitrile, and dried under a stream of argon. The resin was next treated overnight with a solution containing 940 μL of DMF, 470 μL of Et3N, and 630 μL of Et3N·(HF)3 to remove the silyl protecting groups from the 2′-deoxyribonucleoside bases. The resulting resin was washed repeatedly first with DMF and then with anhydrous acetonitrile. The bpDNA was hydrolyzed from the resin by treatment with 1 mL of 37% ammonium hydroxide in water for 30 min, and ammonia was removed by evaporation in a
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RESULTS Preparation of bpDNA Wrapped SWNTs. Previous research has demonstrated that the relative binding energies of the DNA bases onto SWNTs follow the hierarchy G > T > A > C.24 Moreover, Zheng et al. reported that poly(A) and poly(C) strongly self-stack in aqueous solutions, which leads to 2240
DOI: 10.1021/acs.chemmater.6b05182 Chem. Mater. 2017, 29, 2239−2245
Article
Chemistry of Materials a minimum free energy of binding to carbon nanotubes and therefore a lower dispersion efficiency.6 These observations coupled with other studies that report wrapping SWNTs with d(GT)n in water using vigorous probe sonication12 led us to prepare d(TbpT)10 and d(GbpT)10 and to observe their interactions with carbon nanotubes.25 Our initial studies followed the research of others where we attempted to use water or methanol/water solutions coupled with vigorous pulse probe sonication at 750 W for 1 h to disperse the carbon nanotubes wrapped with these bpDNAs. Generally, we observed considerable aggregation and minimal dispersion (Figure S1). Because it is known that boranephosphonate DNA is relatively nonpolar when compared to DNA,17 we chose to study the aggregation of bpDNA wrapped carbon nanotubes using a series of polar organic solvents having high dielectric constants and solubility for bpDNA. These included dimethyl sulfoxide, N,N-dimethylformamide, and formamide. With N,Ndimethylformamide (Figures S2 and S3) and formamide (Figures S4 and S5), ultra sonication was not efficient for dispersion of SWNTs in the presence of bpDNA. The SWNTs remained either aggregated or bundled in these solvents. However, SWNTs in the presence of bpDNA were readily dispersed in DMSO (Figure S6). Previous studies26 on SWNT dispersion efficiencies relative to sonication times with (dT)30 having natural phosphate diester linkages showed that high sonication time increased dispersion efficiency but reduced the length of SWNTs (90 min probe sonication at 130 W). However, in the present study, 20 min ultrasonication at 250 W was found to be sufficient for bpDNA mediated dispersion of SWNTs in DMSO. When measured on a TEM grid, the length and thickness of these bpDNA dispersed SWNTs from DMSO varied between 0.5 and 1.5 μm and 6−12 nm, respectively. Because the SWNT thickness as specified by the manufacturer was ∼1 nm, these results suggest that the bpDNA wrapped nanotubes were present as thin bundles in DMSO, although we occasionally isolated SWNTs with thickness
