Electrochemical Redox Switchable Dispersion of Single-Walled

Subscriber access provided by GAZI UNIV

Article

Electrochemical Redox Switchable Dispersion of Single-Walled Carbon Nanotubes in Water Anchao Feng, Liao Peng, Bowen Liu, Senyang Liu, Shanfeng Wang, and Jinying Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12864 • Publication Date (Web): 30 Mar 2016 Downloaded from http://pubs.acs.org on April 1, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Electrochemical Redox Switchable Dispersion of Single-Walled Carbon Nanotubes in Water Anchao Fenga, Liao Penga, Bowen Liua, Senyang Liua, Shanfeng Wangb,*, Jinying Yuana,*

a

Key Lab of Organic Optoelectronic & Molecular Engineering, Department of Chemistry,

Tsinghua University, Beijing, 100084, China. b

Department of Materials Science and Engineering, The University of Tennessee, Knoxville,

Tennessee 37996, United States.

KEYWORDS Single-walled carbon nanotubes (SWNTs), stimuli-responsive polymer, electrochemical redox switch, dispersion of SWNTs, host-guest interaction

ABSTRACT

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

We present a new, efficient approach to achieve superior dispersibility of single-walled carbon nanotube (SWNTs) in water by integrating reversible host-guest interaction and π-π stacking. In this approach, β-cyclodextrin (β-CD) was first modified with a pyrene group to be adsorbed onto the wall of pristine SWNTs via π-π stacking, followed by further functionalization with Ferrocene (Fc)-terminated water-soluble poly(ethylene glycol) (PEG) through supramolecular host-guest interaction between β-CD and Fc. Upon alternate electrochemical oxidative/reductive stimuli, the reversible host-guest pair enabled the PEG-Fc@Py-CD@SWNTs to exhibit switchable conversion between dispersion and aggregation states. Electric field controllable PEG-Fc@Py-CD@SWNTs with good reversibility and intact nanotube structure may find potential applications in selective screening of SWNTs, biosensors, and targeted drug delivery.


2

Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Single-walled carbon nanotubes (SWNTs) with excellent optoelectronic, mechanical and thermal properties1 have received considerable interests in applications such as electronics,2 sensors,3-6 high-efficient catalysts,7 composite fibers8, and medicinal materials.9-11 However, SWNTs tend to aggregate into bundles by strong intertubular van der Waals interaction and thus individual tubes or small-sized bundles desirable for excellent material performance are extremely difficult to achieve. There have been numerous methods to improve the dispersion and individualization of SWNTs.12-13 Among these methods, covalent functionalization of SWNTs results in carbon hybridization on the tube surface that may disrupt the π-π conjugation and impair the mechanical and electronic properties.14-16 In comparison, noncovalent modification such as through π-π stacking is more attractive as it can preserve the electronic structure and properties of SWNTs.17-23 Despite the past efforts, the improvement in the water dispersibility of SWNTs is still unsatisfactory. Moreover, it is highly desirable to make the dispersibility environmentally responsive.24-26

A

variety

of

stimuli-responsive

water-soluble

polymers

including

temperature-responsive poly(N-isopropylacrylamide) and CO2-sensitive dispersant/polymer with amidine groups have been developed to modify the surface properties of SWNTs for achieving switchable dispersion/aggregation in water.27-31,32,33-34

In addition, reversible light-controlled

carbon nanotubes/polymer nanocomposites based on reversible photo-isomerization between trans and cis forms in azobenzene has also been reported recently.35-36 Temperature, CO2, and light are considered as “green” triggers that do not contaminate SWNTs or the dispersion media. Here we present a novel “green” stimulation approach using electrochemical redox to control the


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 26

dispersion of SWNTs. This method can readily realize redox reactions through inducing electron transfer reactions without accumulation of redox reagents. There exist electrochemical redox responsive supramolecular systems, in particular, the ones based on host-guest interactions, for example, the pair of β-cyclodextrin (β-CD) and ferrocene (Fc).37-41 Uncharged Fc or its derivatives can be strongly bound in the cavity of β-CD, whereas the charged one (Fc+), as a cation, dissociates rapidly from the cavity.42-43 We previously fabricated electrochemical redox responsive micelles, vesicles, and nanofibers based on polymers containing β-CD and Fc moieties.37-39, 42 A supramolecular hydrogel with a sol-gel transition induced by electrochemical redox stimuli was prepared by another research group using polyacrylic acids (PAA) with these two moieties (PAA-6-β-CD and PAA-Fc).43 In this study, the electrochemical redox system was also prepared using the host-guest pair of β-CD and Fc, in which the former was modified with a pyrene group (Py-CD) to adsorb onto SWNTs via π-π interaction while the latter was used to end-cap poly(ethylene glycol) (PEG) into PEG-Fc.35, 44 The hydrophobic inner β-CD cavities covering the tube surface were used as the host for interacting with Fc guest end-caps in PEG-Fc. Through the inclusion/exclusion processes between PEG-Fc and Py-CD, which was greatly enhanced by the excellent electrical conductivity of SWNTs, the water solubility of SWNTs could be adjusted or controlled by applying proper redox potentials (Scheme 1) and thus reversible dispersion/aggregation of SWNTs in water could be achieved, offering strategies for developing more versatile carbon nanotubes based devices in electroactive medium or sensory applications.45-46


4

Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. SWNTs modified with Py-CD and PEG-Fc and the responses to the stimuli of positive and negative electrical potentials.

2. EXPERIMENTAL SECTION 2.1. Materials SWNTs with length of 5-30 μm and outer diameter of 2-3 nm used here were purchased from Timesnano Ltd. (Chengdu, China) without further treatment. Deionized water was always used in this study and simplified as “water” in the discussion. Methoxy poly(ethylene glycol) (mPEG-OH, Mn,GPC = 5000 g mol-1) purchased from Aldrich was dried through azeotropic distillation in toluene. β-CD was purchased from Kermel (China) and purified by recrystallization from water before use. 2-Pyrenebutyric acid (Aldrich, 97%), tosyl chloride (TsCl, Acros, 99%), dicyclohexylcarbodiimide (DCC, Alfa Aesar, 99%), N-hydroxysuccinimide


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

(NHS, Alfa Aesar, 99%), 4-dimethylaminopyridine (DMAP, Alfa Aesar, 98%) and Fc carboxylic acid (FcA, Acros Organics, 99%) were utilized as received. Tetrahydrofuran (THF), dichloromethane (DCM), methanol, hexane, N,N-dimethylformamide (DMF), and acetonitrile were received from Beijing Chemical Reagent Co. Ltd (China) and purified. All other chemicals of analytical grade were used directly after purchases.

2.2. Synthesis of PEG-Fc mPEG-OH (3.34 g, 0.66 mmol), FcA (0.32 g, 1.53 mmol), DCC (0.327 g, 1.60 mmol), and DMAP (0.041 mg, 0.33 mmol) were dissolved in 30.0 mL anhydrous DCM, and the solution was degassed three times and sealed under nitrogen. The reaction was performed at room temperature for 48 h and the precipitated solid byproduct (dicyclohexylcarbodiurea) was removed by filtration.

Then

precipitated

the

solution

into

diethyl

ether,

and

repeated

the

dissolving-precipitation twice, the obtained light yellow product was dried in vacuum for 24 h to get PEG-Fc.

2.3. Synthesis of Amino-β-CD (NH2-CD) NH2-CD was synthesized according to our previous work.38,39 Specifically, 60 g of β-CD was dissolved in 500 mL of water, and 20 mL of NaOH solution containing 6.5 g NaOH was added dropwise in 6 min with violent stirring. 10 g of TsCl was dissolved in 30 mL of acetonitrile and added dropwise in 8 min into the above solution in an ice bath. Then the mixture was stirred at room temperature for 2 h and stored at 4 °C overnight for precipitation. After filtration and recrystallization in boiling water, the precipitate was collected for further usage. 5 g of the precipitate was added into 30 mL of ethylene diamine and heated at 75 °C for 4 h. After being


6

Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cooled down, the mixture was added into 300 mL acetone dropwise with violent stirring to precipitate. The precipitate was dissolved in water-methanol mixture (1:1, v) and precipitated in acetone three times. The product was dried at 50 °C for 24 h in a vacuum oven and labeled as NH2-CD.

2.4. Synthesis of Pyrene-Labelled β-CD (Py-CD) A mixture of NH2-CD (0.25 g, 0.22 mmol), 2-pyrenebutyric acid (0.06 g, 0.25 mol), NHS (28 mg, 0.26 mmol), and DMAP (13.4 mg, 0.11 mmol) was dissolved in 4 mL dry DMF. The reaction mixture was stirred at 0 °C, and DCC (50 mg, 0.24 mmol) was added dropwise to the stirred solution. After DCC was completely added, the reaction mixture was moved to room temperature and kept stirring overnight. Insoluble salts were removed through filtration and the filtrate was poured into acetone. The obtained precipitate was washed with water three times to remove excess NH2-CD. The product was dried at 50 °C for 24 h in a vacuum oven, and Py-CD was obtained as a yellow solid (yield = 43.2 %).

2.5. Synthesis of Py-CD@SWNTs Py-CD (160.0 mg, 0.11 mmol) was dissolved in 40.0 mL of DMF to prepare a solution of Py-CD. SWNTs (40.0 mg) was added into the prepared solution and reacted under sonication for 30 min at room temperature. Then the Py-CD@SWNTs hybrids was obtained by filtering the solution through a polytetrafluoroethylene (PTFE) micro-porous membrane (0.22 μm). The black solid left on the membrane were washed with DMF several times and dried at 50°C for 48 h in a vacuum oven.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

2.6. Synthesis of PEG-Fc@Py-CD@SWNTs Dispersion Py-CD@SWNTs (30.0 mg) and PEG-Fc (300.0 mg) were added into 30.0 mL water, followed by sonication for 20 min at room temperature. The solution was centrifuged at 6000 rpm for 10 min to remove unreacted polymers and redispersed in water. After several rounds of centrifuge and wash, homogeneous dispersion of SWNTs was obtained for further uses.

2.7. Electrochemical Redox Response Experiments PEG-Fc@Py-CD@SWNTs were dispersed in a solution of saturated KCl, and an electrochemical workstation (CHI760E, Shanghai Huachen Instrument, China) combined with platinum sheets (working and counter electrodes) and Hg/Hg2Cl2 electrode (reference electrode) were used to apply the electric treatment. According to our previous work,39 an oxidative potential of +0.80 V (vs. saturated calomel electrode, SCE) was applied to the solution with gentle agitation for 4 h. In the meantime, a control sample was immersed in the same solution separately without electric treatment. The reductive potential of -0.40 V was conducted in a similar way.

2.8. Characterization 1

H Nuclear Magnetic Resonance (NMR) spectra were recorded at 25 °C on a JEOL

JNM-ECA300 (300 MHz). Electrospray Ionization Mass (ESI-MS) spectra were collected using a Micro TOF-QII Bruker. UV-Vis spectra were acquired on a HITACHI U-3010 spectrophotometer with double beams of light. Raman spectra were recorded on HORIBA Evolution Raman Microscope, using He-Ne laser as the light source. Thermogravimetric analysis (TGA) was performed on a TA Q50 thermogravimetric analyzer from 25 to 600 °C at a heating


8

Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rate of 20 °C min-1 in nitrogen. Transmission electron microscope (TEM) images were obtained from a JEM-2010 microscope at an accelerating voltage of 120 kV. The samples for TEM measurements were prepared by placing one drop of sample suspension on microgrates. Scanning electron microscopic (SEM) imaging was conducted using a JSM-7401 microscope (JEOL, Japan) with accelerating voltages between 0.1-30 kV. UV-Vis-NIR characterization was performed using a Hitachi UV-Vis 3900 spectrophotometer (Japan) with a dual beam. Cyclic voltammetric (CV) experiments were carried out using a CHI760E potentiostat (Shanghai Chenhua, China) with a three-electrode system at ambient temperature, in which two platinum sheets worked as a working electrode and a counter electrode with a Hg/Hg2Cl2 (saturated KCl) reference electrode.

Figure 1. Synthesis of a) Py -CD and b) PEG-Fc.

3. RESULTS AND DISCUSSION 3.1. Synthesis The synthesis of Py -CD and PEG-Fc was shown in Figure 1. PEG-Fc was characterized using

1

H NMR spectrum in CDCl3 (Figure S1). The signals at 4.79 ppm (s, 2H,

=CHC(COOH)CH=), 4.41 ppm (s, 2H, -CH=CH-), and 4.28 ppm (s, 5H, another cyclopentyl)


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

are attributed to the protons in the Fc moiety. Based on the integral areas compared with protons on PEG (3.68 and 3.36 ppm), the degree of terminal functionalization was calculated as ~100%. The chemical structure of Py-CD was confirmed using 1H NMR spectrum (Figure S2) and ESI-MS data (Figure S3). The characteristic NMR resonance peaks of the aromatic pyrene side groups between 8.50 and 7.70 ppm, as well as β-CD protons at 5.82, 4.87, 4.49, 3.68, and 3.37 ppm could be assigned. ESI-MS data further demonstrated that β-CD was mono-modified with a pyrene group, as the molecular ion peak at m/z 1448.9 [M+H+] accorded with the theoretical value of 1447.4.

3.2. Immobilization of Py-CD onto SWNTs To form supramolecular linking between the nanotubes and PEG chains, the nanotube surface were first attached by the host functional groups. The pre-decorated pyrene group in Py-CD can be adsorbed onto the outer wall of the nanotubes via π-π interaction after 20 min of sonication treatment. The π-π interaction and surface functionalization were characterized using Raman spectroscopy. As shown in Figure 2b, pristine SWNTs revealed the representative second disordered band (D∗) and the tangential (G) band at 2581.4 and 1581.0 cm−1, respectively. In contrast with the pristine nanotubes, the corresponding bands of the Py-CD@SWNTs upshifted to 2593.2 and 1589.3 cm−1 because of the transfer of the charges from the nanotubes to the pyrene groups immobilized on them via π-π stacking. TGA thermograms in Figure 2c were to quantify the amount of Py-CD grafted onto the nanotubes. Pristine SWNTs had little weight loss up to 600 °C, while Py-CD underwent about ~100% weight loss at nearly 570 °C. Based on the weight loss of ~20% in the TGA thermogram of Py-CD@SWNTs, there existed 1.43 × 10-4 mol of Py-CD in 1 g Py-CD@SWNTs or roughly 3 Py-CD molecules per 10 nm2 nanotube surface if


10

Page 11 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a specific surface area of 380 m2/g was used for SWNTs, abundant for further modification through host-guest interaction.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

Figure 2. a) Schematic illustration of immobilization of Py-CD onto SWNTs. b) Raman spectra of pristine SWNTs and Py-CD@SWNTs. c) TGA thermograms of pristine SWNTs, Py-CD@SWNTs, Py-CD, PEG-Fc, and PEG-Fc@Py-CD@SWNTs.

3.3. Dispersibility of PEG-Fc@Py-CD@SWNTs in Water The host-guest complex between PEG-Fc and Py-CD@SWNTs was conducted with assistance of sonication in water, followed by further purification. The amount of PEG-Fc decorated on the surface of SWNTs was measured using TGA (Figure 2c). The final weight loss of PEG-Fc@Py-CD@SWNTs increased greatly to 53% from ~20% for Py-CD@SWNTs, accompanied with a similar downtrend to that in PEG-Fc in the temperature range of 165-400 °C. Based on the above results, there was 0.47 g SWNTs in 1 g of PEG-Fc@Py-CD@SWNTs, adsorbing 0.1175 g Py-CD, and then there was 0.4125 g PEG-Fc in the 0.53 g weight loss. The mass ratio of PEG-Fc and Py-CD in PEG-Fc@Py-CD@SWNTs was nearly 7:2, in agreement with the molar mass ratio of the compounds (PEG-Fc, Mn = 5000 g·mol-1; Py-CD, Mn =1447 g·mol-1). It suggested that each CD group interacted with one PEG-Fc chain, as a typical host-guest pair. 1H NMR was conducted to further characterize the host-guest interaction in PEG-Fc@Py-CD@SWNTs as the spectrum of a mixture reflects and quantifies the intermolecular association.47 Compared with the spectrum of PEG-Fc before adding equimolar Py-CD, a clear down-field shift was observed at characteristic resonance peak of the Fc moiety at 4.28 ppm (Figure S4), indicating the formation of an inclusion complex between β-CD and Fc. The dispersibility of SWNTs in water was first observed with naked eyes. Both the pristine and Py-CD@SWNTs were unable to efficiently disperse in water, as shown as precipitates at the bottom of the vials (photos 1 and 2 in Figure 3a). In contrast, PEG-Fc@Py-CD@SWNTs were


12

Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


readily dispersed in water (photo 3 in Figure 3a) and no sedimentation was observed even after one month. The resulted composites were further dispersed in D2O and characterized using 1H NMR spectroscopy. Characteristic signals of PEG with a chemical shift at 3.63 ppm and Fc with chemical shifts at 4.39 and 4.08 ppm were observed. The characteristic resonance peaks of the Py moieties down shifted slightly, which may be attributed to the π-π interaction between Py side groups and the nanotube surface. The characteristic signals of CD were always inevident and screened by other strong signals. These results suggested that SWNTs were decorated with PEG successfully (Figure S5). The appearance of pristine SWNTs and SWNTs hybrids in typical organic solvents (Figure S6) was also examined. We found that the pristine SWNTs were non-dispersed in THF and hexane, and settled at the bottom of the vials. In case of SWNTs hybrids modified by PEG, good dispersibility was observed in both THF and DMF, but still clear phase separation in hexane. It demonstrated that after successfully modified with PEG, the dispersibility of SWNTs hybrids was dominated by the solubility of polymer, which offer more possibilities for our dispersion method applying in many fields. The micro-morphology of dispersed SWNTs in water was characterized using TEM, as shown in Figure 3b. Most of the SWNTs presented as debundled or in a small cluster and the average diameter of the individual nanotubes was 7-8 nm, larger than the value of 1-2 nm supplied by the manufacturer. The TEM image in Figure 3c confirmed the successful coating of polymer layers surrounding the nanotubes. More importantly, no obvious damages were observed in the nanotube structure, suggesting that the electronic characteristics of SWNT were preserved after surface modification. Surface functionalization of SWNTs with polymer was also verified by using SEM. Unlike the TEM results, the polymer coating film was observed evidently in Figure 3d, but the relatively complete structure of the nanotubes only appeared at the


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

cross-sections. Because of the polymer coating, the SWNTs presented as dispersed individual ones.

Figure 3. a) SWNTs (1), Py-CD@SWNTs (2), and PEG-Fc@Py-CD@SWNTs (3) in water. b,c) Representative TEM and d) SEM images of PEG-Fc@Py-CD@SWNTs dispersed in water.

3.4. Electrochemical Redox-Switchable Behaviors of PEG-Fc@Py-CD@SWNTs Because the host-guest interaction between Fc and β-CD can be regulated electrochemical redox stimulus, the water dispersibility of PEG-Fc@Py-CD@SWNTs can be reversibly controlled by electrochemical potential. Before the electrochemical redox stimulus, PEG-Fc@Py-CD@SWNTs was well dispersed in water. An oxidative potential of +0.80 V could make PEG-Fc@Py-CD@SWNTs gradually precipitate within 2 h. TEM was utilized to accurately characterize the micromorphological changes in response to the oxidative potential. SWNTs piled up with certain order and displayed a heavily bundled and networked


14

Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


microstructure (Figure 4a). The UV-Vis-NIR spectra in Figure 4b were further used to indicate the switchable dispersibilities of PEG-Fc@Py-CD@SWNTs. The spectra reflected a superposition of electronic transitions from various fullerene structures in the nanotubes, in which a series of relatively sharp interband transition peaks corresponded to the energies dependent on tube diameter. In other words, such sharp peaks in absorption spectra, associated with those van Hove singularities, revealed the dispersion state of the hybrid suspension.48 A series of strong absorbances was observed in the wavelength range of 850-900 nm before electrochemical redox stimuli, indicating a well dispersibility of PEG-Fc@Py-CD@SWNTs in water. After an oxidative potential of +0.80 V was applied, the characteristic absorption peaks were not detectable, indicating the aggregation and bundling among SWNTs. The absorption intensity decreased significantly to the level of pristine SWNTs as PEG-Fc@Py-CD@SWNTs precipitated. Then we attempted to answer a more challenging question: can we reverse this dispersion to aggregation process by applying a reductive potential? Pristine SWNTs cannot recover their water dispersibility under such stimulus, but Py-CD adsorbed on the nanotube surface offered such possibility. We used UV-Vis spectroscopy to expediently demonstrate the alteration in transmittance at the wavelength of 600 nm to reflect the dispersibility (Figure 4c). When a reductive potential of -0.40 V was exerted to the separated suspension (Figure 4d) for 4 h with continuous

mild

stirring

and

sonication,

PEG-Fc@Py-CD@SWNTs

again

dispersed

homogeneously in water (Figure 4e). Accordingly the transmittance decreased sharply from more than 90% to near 0. This aggregation-to-dispersion conversion was because β-CD and reduced Fc formed the host-guest inclusive composite again. Merely stirring and sonication without a reductive potential could not re-disperse PEG-Fc@Py-CD@SWNTs in water. More


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

interestingly, such a reversible conversion was effective even beyond some cycles of alternative electrochemical redox potentials. It is worth mentioning that the applied oxidative potential of +0.80 V was optimized based on the CV curve of PEG-Fc (see Figure S7). The half-wave oxidative potential of PEG-Fc was 0.44 V, meaning an oxidative potential over 0.64 V (0.44 V + 0.20 V) could totally oxidize it in theory. In our experiments, +0.80 V was the lowest operative potential to realize the dispersion/aggregation transitions within several hours and this value was lower than that we employed for our former electrochemical redox responsive vesicle and micelle systems.38-39 Such a decreased operative potential should be attributed to a higher efficiency in electronic conductivity in the present SWNT system and it is of practical significance in terms of being energy efficient and avoiding potential decomposition of aqueous solution that may occur at +1.25 V. The present method can be further extended to surface modification of other nanomaterials and surfaces, such as magnetic nanoparticles and graphene. Electric field controllable PEG-Fc@Py-CD@SWNTs with properties of good reversibility and intact nanotube structure may find potential applications in selective screening of SWNTs, biosensors, and targeted drug delivery.


16

Page 17 of 26


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

Figure 4. a) TEM images of PEG-Fc@Py-CD@SWNTs after an oxidative potential stimulus. b) UV-Vis-NIR spectrum of pristine SWNTs and PEG-Fc@Py-CD@SWNTs before and after the electrochemical redox stimuli for 2 h. c) Reversible change of the transmittance of PEG-Fc@Py-CD@SWNTs in water at the wavelength of 600 nm upon alternative redox potentials. d,e) Photos of PEG-Fc@Py-CD@SWNTs in water at corresponding conditions.

4. CONCLUSIONS We have developed an electrochemical redox method for the first time to modify the SWNT surface with PEG via a non-covalent host-guest interaction between CD and Fc. The modified SWNTs demonstrated stable, homogeneous dispersibility in water. A switchable conversion between

dispersion

and

aggregation

was

achieved

upon

alternate

electrochemical

oxidative/reductive stimuli. Compared with conventional methods, the non-covalent strategy described here for surface modification of CNT was very neat, facile, and efficient, offering more opportunities for conveniently developing other functional polymeric systems.

ASSOCIATED CONTENT Supporting Information: It includes

1

H NMR spectra of PEG-Fc, Py-CD, and

PEG-Fc@Py-CD@SWNTs, ESI-MS of Py-CD, and CV curve of PEG-Fc. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


18

Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


*Corresponding author. Tel.:+86 10 62783668; Fax: +86 10 62771149. E-mail: [email protected] or [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (51573086, 21428401, and 21174076), and Tsinghua University Initiative Scientific Research Program (2012z023998).

REFERENCES (1) Hersam, M. C. Progress Towards Monodisperse Single-Walled Carbon Nanotubes. Nat. Nano 2008, 3, 387-394. (2) Wang, W.; Ruderer, M. A.; Metwalli, E.; Guo, S.; Herzig, E. M.; Perlich, J.; Müller-Buschbaum, P. Effect of Methanol Addition on the Resistivity and Morphology of PEDOT:PSS Layers on Top of Carbon Nanotubes for Use as Flexible Electrodes. ACS Appl. Mater. Inter. 2015, 7, 8789-8797. (3) Zhang, Y. B.; Kanungo, M.; Ho, A. J.; Freimuth, P.; van der Lelie, D.; Chen, M.; Khamis, S. M.; Datta, S. S.; Johnson, A. T. C.; Misewich, J. A.; Wong, S. S. Functionalized Carbon Nanotubes for Detecting Viral Proteins. Nano Lett. 2007, 7, 3086-3091.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

(4) Barone, P. W.; Strano, M. S. Reversible Control of Carbon Nanotube Aggregation for a Glucose Affinity Sensor. Angew. Chem. Int. Ed. 2006, 45, 8138-8141. (5) Zeng, S.; Baillargeat, D.; Ho, H. P.; Yong, K. T. Nanomaterials Enhanced Surface Plasmon Resonance for Biological and Chemical Sensing Applications. Chem. Soc. Rev. 2014, 43, 3426-3452. (6) Barone, P. W.; Baik, S.; Heller, D. A.; Strano, M. S. Near-Infrared Optical Sensors Based on Single-Walled Carbon Nanotubes. Nat. Mater. 2005, 4, 86-92. (7) Wang, S.; Yu, D.; Dai, L. Polyelectrolyte Functionalized Carbon Nanotubes as Efficient Metal-Free Electrocatalysts for Oxygen Reduction. J. Am. Chem. Soc. 2011, 133, 5182-5185. (8) Yan, Y.; Cui, J.; Pötschke, P.; Voit, B. Dispersion of Pristine Single-Walled Carbon Nanotubes Using Pyrene-Capped Polystyrene and its Application for Preparation of Polystyrene Matrix Composites. Carbon 2010, 48, 2603-2612. (9) Chen, J.; Chen, S.; Zhao, X.; Kuznetsova, L. V.; Wong, S. S.; Ojima, I. Functionalized Single-Walled Carbon Nanotubes as Rationally Designed Vehicles for Tumor-Targeted Drug Delivery. J. Am. Chem. Soc. 2008, 130, 16778-16785. (10) Bhirde, A. A.; Patel, V.; Gavard, J.; Zhang, G.; Sousa, A. A.; Masedunskas, A.; Leapman, R. D.; Weigert, R.; Gutkind, J. S.; Rusling, J. F. Targeted Killing of Cancer Cells in Vivo and in Vitro with EGF-Directed Carbon Nanotube-Based Drug Delivery. ACS Nano 2009, 3, 307-316. (11) Liu, J.; Wang, C.; Wang, X.; Wang, X.; Cheng, L.; Li, Y.; Liu, Z. Mesoporous Silica Coated Single-Walled Carbon Nanotubes as a Multifunctional Light-Responsive Platform for Cancer Combination Therapy. Adv. Funct. Mater. 2015, 25, 384-392.


20

Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Zhang, X.; Zeng, G.; Tian, J.; Wan, Q.; Huang, Q.; Wang, K.; Zhang, Q.; Liu, M.; Deng, F.; Wei, Y. PEGylation of Carbon Nanotubes via Mussel Inspired Chemistry: Preparation, Characterization and Biocompatibility Evaluation. Appl. Surf. Sci. 2015, 351, 425-432. (13) Deng, Y.; Hu, Q.; Yuan, Q.; Wu, Y.; Ling, Y.; Tang, H. One-Pot Synthesis of Molecular Bottle-Brush Functionalized Single-Walled Carbon Nanotubes with Superior Dispersibility in Water. Macromol. Rapid Commun. 2014, 35, 97-102. (14) Dionisio, M.; Schnorr, J. M.; Michaelis, V. K.; Griffin, R. G.; Swager, T. M.; Dalcanale, E. Cavitand-Functionalized SWCNTs for N-Methylammonium Detection. J. Am. Chem. Soc. 2012, 134, 6540-6543. (15) Hong, C.Y.; Pan, C.Y. Functionalized Carbon Nanotubes Responsive to Environmental Stimuli. J. Mater. Chem. 2008, 18, 1831-1836. (16) Gao, C.; Muthukrishnan, S.; Li, W.; Yuan, J.; Xu, Y.; Müller, A. H. E. Linear and Hyperbranched Glycopolymer-Functionalized Carbon Nanotubes: Synthesis, Kinetics, and Characterization. Macromolecules 2007, 40, 1803-1815. (17) Khadem, M.; Zhao, Y. Tetrathiafulvalene Vinylogue-Fluorene Co-oligomers: Synthesis, Properties, and Supramoleclar Interactions with Carbon Nanotubes. J. Org. Chem. 2015, 80, 7419-7429. (18) Yu, G.; Li, J.; Yu, W.; Han, C.; Mao, Z.; Gao, C.; Huang, F. Carbon Nanotube/Biocompatible Bola-Amphiphile Supramolecular Biohybrid Materials: Preparation and Their Application in Bacterial Cell Agglutination. Adv. Mater. 2013, 25, 6373-6379. (19) Pochorovski, I.; Wang, H.; Feldblyum, J. I.; Zhang, X.; Antaris, A. L.; Bao, Z. H-Bonded Supramolecular Polymer for the Selective Dispersion and Subsequent Release of Large-Diameter Semiconducting Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2015, 137, 4328-4331.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

(20) Schopf, E.; Broyer, R.; Tao, L.; Chen, Y.; Maynard, H. D. Directed Carbon Nanotube Assembly Using a Pyrene-Functionalized Polymer. Chem. Commun. 2009, 32, 4818-4820. (21) Yu, G.; Xue, M.; Zhang, Z.; Li, J.; Han, C.; Huang, F. A Water-Soluble Pillar[6]arene: Synthesis, Host-Guest Chemistry, and its Application in Dispersion of Multiwalled Carbon Nanotubes in Water. J. Am. Chem. Soc. 2012, 134, 13248-13251. (22) Yang, R.; Wei, Y.; Yu, Y.; Gao, C.; Wang, L.; Liu, J. H.; Huang, X. J. Make it Different: The Plasma Treated Multi-Walled Carbon Nanotubes Improve Electrochemical Performances Toward Nitroaromatic Compounds. Electrochim. Acta 2012, 76, 354-362. (23) Chang, D. W.; Jeon, I. Y.; Baek, J. B.; Dai, L. Efficient Dispersion of Single-Walled Carbon Nanotubes by Novel Amphiphilic Dendrimers in Water and Substitution of the Pre-Adsorbed Dendrimers with Conventional Surfactants and Lipids. Chem. Commun. 2010, 46, 7924-7926. (24) Chang, S.; Singamaneni, S.; Kharlampieva, E.; Young, S. L.; Tsukruk, V. V. Responsive Hybrid Nanotubes Composed of Block Copolymer and Gold Nanoparticles. Macromolecules 2009, 42, 5781-5785. (25) Liang, S.; Zhao, Y.; Adronov, A., Selective and Reversible Noncovalent Functionalization of

Single-Walled

Carbon

Nanotubes

by

a

pH-Responsive

Vinylogous

Tetrathiafulvalene-Fluorene Copolymer. J. Am. Chem. Soc. 2014, 136, 970-977. (26) Xue, C.; Birel, O.; Xue, Y.; Dai, L.; Urbas, A.; Li, Q. pH and Temperature Modulated Aggregation of Hydrophilic Gold Nanorods with Perylene Dyes and Carbon Nanotubes. J. Phys. Chem. C 2013, 117, 6752-6758. (27) Liang, S.; Chen, G.; Peddle, J.; Zhao, Y. Reversible Dispersion and Releasing of Single-Walled Carbon Nanotubes by a Stimuli-Responsive TTFV-Phenylacetylene Polymer. Chem. Commun. 2012, 48, 3100-3102.


22

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Umeyama, T.; Kawabata, K.; Tezuka, N.; Matano, Y.; Miyato, Y.; Matsushige, K.; Tsujimoto, M.; Isoda, S.; Takano, M.; Imahori, H. Dispersion of Carbon Nanotubes by Photoand Thermal-Responsive Polymers Containing Azobenzene Unit in the Backbone. Chem. Commun. 2010, 46, 5969-5971. (29) Wang, D.; Chen, L., Temperature and pH-Responsive Single-Walled Carbon Nanotube Dispersions. Nano Lett. 2007, 7, 1480-1484. (30) Soll, S.; Antonietti, M.; Yuan, J., Double Stimuli-Responsive Copolymer Stabilizers for Multiwalled Carbon Nanotubes. ACS Macro Letters 2012, 1, 84-87. (31) Kuzmicz, D.; Prescher, S.; Polzer, F.; Soll, S.; Seitz, C.; Antonietti, M.; Yuan, J. The Colloidal Stabilization of Carbon with Carbon: Carbon Nanobubbles as both Dispersant and Glue for Carbon Nanotubes. Angew. Chem. Int. Ed. 2014, 53, 1062-1066. (32) Etika, K. C.; Jochum, F. D.; Theato, P.; Grunlan, J. C. Temperature Controlled Dispersion of Carbon Nanotubes in Water with Pyrene-Functionalized Poly(N-cyclopropylacrylamide). J. Am. Chem. Soc. 2009, 131, 13598-13599. (33) Ding, Y.; Chen, S.; Xu, H.; Wang, Z.; Zhang, X.; Ngo, T. H.; Smet, M. Reversible Dispersion of Single-Walled Carbon Nanotubes Based on a CO2-Responsive Dispersant. Langmuir 2010, 26, 16667-16671. (34) Guo, Z.; Feng, Y.; He, S.; Qu, M.; Chen, H.; Liu, H.; Wu, Y.; Wang, Y. CO2-Responsive "Smart" Single-Walled Carbon Nanotubes. Adv. Mater. 2013, 25, 584-590. (35) Guo, Z.; Feng, Y.; Zhu, D.; He, S.; Liu, H.; Shi, X.; Sun, J.; Qu, M. Light-Switchable Single-Walled Carbon Nanotubes Based on Host-Guest Chemistry. Adv. Funct. Mater. 2013, 23, 5010-5018.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

(36) Basuki, S. W.; Schneider, V.; Strunskus, T.; Elbahri, M.; Faupel, F. Light-Controlled Conductance Switching in Azobenzene-Containing MWCNT-Polymer Nanocomposites. ACS Appl. Mater. Inter. 2015, 7, 11257-11262. (37) Peng, L.; Feng, A.; Zhang, H.; Wang, H.; Jian, C.; Liu, B.; Gao, W.; Yuan, J. Voltage-Responsive Micelles Based on the Assembly of two Biocompatible Homopolymers. Polym. Chem. 2014, 5, 1751-1759. (38) Feng, A.; Yan, Q.; Zhang, H.; Peng, L.; Yuan, J. Electrochemical Redox Responsive Polymeric Micelles Formed from Amphiphilic Supramolecular Brushes. Chem. Commun. 2014, 50, 4740-4742. (39) Yan, Q.; Yuan, J.; Cai, Z.; Xin, Y.; Kang, Y.; Yin, Y. Voltage-Responsive Vesicles Based on Orthogonal Assembly of two Homopolymers. J. Am. Chem. Soc. 2010, 132, 9268-9270. (40) Kim, H.; Jeong, S. M.; Park, J. W. Electrical Switching Between Vesicles and Micelles via Redox-Responsive Self-Assembly of Amphiphilic Rod-Coils. J. Am. Chem. Soc. 2011, 133, 5206-9. (41) Sun, Z.; Li, Z.; He, Y.; Shen, R.; Deng, L.; Yang, M.; Liang, Y.; Zhang, Y. Ferrocenoyl Phenylalanine: A New Strategy Toward Supramolecular Hydrogels with Multistimuli Responsive Properties. J. Am. Chem. Soc. 2013, 135, 13379-13386. (42) Yan, Q.; Feng, A.; Zhang, H.; Yin, Y.; Yuan, J. Redox-Switchable Supramolecular Polymers for Responsive Self-Healing Nanofibers in Water. Polym. Chem. 2013, 4, 1216-1220. (43) Nakahata, M.; Takashima, Y.; Yamaguchi, H.; Harada, A. Redox-Responsive Self-Healing Materials Formed from Host-Guest Polymers. Nat. Commun. 2011, 2, 487-502.


24

Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44) Ogoshi, T.; Takashima, Y.; Yamaguchi, H.; Harada, A. Chemically-Responsive Sol-Gel Transition of Supramolecular Single-Walled Carbon Nanotubes (SWNTs) Hydrogel Made by Hybrids of SWNTs and Cyclodextrins. J. Am. Chem. Soc. 2007, 129, 4878-4879. (45) Mercante, L. A.; Pavinatto, A.; Iwaki, L. E.; Scagion, V. P.; Zucolotto, V.; Oliveira, O. N., Jr.; Mattoso, L. H.; Correa, D. S. Electrospun Polyamide 6/Poly(allylamine hydrochloride) Nanofibers Functionalized with Carbon Nanotubes for Electrochemical Detection of Dopamine. ACS Appl. Mater. Inter. 2015, 7, 4784-4790. (46) Gao, C.; Guo, Z.; Liu, J. H.; Huang, X. J. The New Age of Carbon Nanotubes: an Updated Review of Functionalized Carbon Nanotubes in Electrochemical Sensors. Nanoscale 2012, 4, 1948-1963. (47) Fielding, L. Determination of Association Constants (Ka) from Solution NMR Data. Tetrahedron 2000, 56, 6151-6170. (48) Grossiord, N.; Loos, J.; Meuldijk, J.; Regev, O.; Miltner, H. E.; Mele, B. V.; Koning, C. E. Conductive Carbon-Nanotube/Polymer Composites: Spectroscopic Monitoring of the Exfoliation Process in Water. Compos. Sci. Technol. 2007, 67, 778-782.


25


Page 26 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

1

Electrochemical Redox Switchable Dispersion of Single-Walled

Recommend Documents