Polydopamine-modified nanochannels in vertically aligned carbon

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Polydopamine-modified nanochannels in vertically aligned carbon nanotube arrays for controllable molecule transport Shenglin Zhou, Min Wang, Zhaohui Yang, and Xiaohua Zhang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00619 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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ACS Applied Nano Materials

Polydopamine-modified Nanochannels in Vertically Aligned Carbon Nanotube Arrays for Controllable Molecule Transport Shenglin Zhou, †Min Wang, † Zhaohui Yang,* †,‡, Xiaohua Zhang*†,‡ †

Center for Soft Condensed Matter Physics and Interdisciplinary Research & School of Physical Science and Technology, Soochow University, Suzhou, 215006, P.R. China

‡

Jiangsu Key Laboratory of Thin Films, Soochow University, Suzhou 215006, P.R. China

ABSTRACT: We investigate the transport behavior of hydrophilic calcein molecules in the intertube nanochannels of vertically aligned carbon nanotube arrays (CNTAs). The intertube nanochannels with an average tube-to-tube distance of 18 nm are formed via a capillary-induced densification treatment. During this densification treatment, loosely packed carbon nanotubes (CNTs) in pristine arrays are drawn together like a “zipping” effect. The wettability of nanochannels in the densified CNT array (DCNTA) is greatly improved after a non-covalent modification by incorporating a hydrophilic polydopamine (Pdop) coating on the sidewalls of CNTs through the strong π-π interactions of dopamine and CNTs. Polydopamine incorporated into DCNTA makes the transport of hydrophilic calcein molecules in the nanochannels of DCNTA more efficient. The overall transport rate of calcein in Pdop-modified DCNTA (PdopDCNTA) is significantly enhanced as compared to DCNTA without the chemical modification. Accordingly, the apparent diffusion coefficient of calcein in the Pdop-DCNTA membranes

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significantly increases. After a 24 h modification, the apparent diffusion coefficient of calcein in Pdop-modified DCNTA is 1.20 × 10-4 cm2/s and the corresponding value for unmodified DCNTA is close to 0 because the hydrophilic calcein molecules hardly diffuse through the unmodified DCNTA. The wettability of DCNTA, overall transport rate and apparent diffusion coefficient of calcein in DCNTA can be well controlled by Pdop content in DCNTA or chemical modification time. Moreover, the overall molecule transport rate normalized by the concentration of calcein in feeding solution (Cf) is insensitive to Cf.

KEYWORDS:

polydopamine modification, carbon nanotubes, molecule transport,

hydrophilicity, intertube nanochannel

1. INTRODUCTION The mass transport through nanometer-scale channels plays a crucial role in various biological processes,1-2 energy conversion,3-4 chemical separation5-6 and medicine release.7-8 The nanochannel for efficient mass transport is of particular interest because of its potential applications in ion gate,9-10 energy storage,3, 11 and artificial ion channel.12-13 Recently, the mass transport behaviors in the nanochannels of CNTs have attracted scientific attention because of the outstanding electrical, thermal and mechanical properties of CNTs. Many simulations and experimental investigations on the mass transport behaviors in the nanochannels of CNTs have been reported.14-15 Simulations show an extremely high mass transport rate in the nanochannels of CNTs with sub-1 nm diameter.16-17 Tunuguntla, et al experimentally studied a proton transport behavior in the nanochannels of CNTs with a sub-1 nm diameter.18 Their data indicate that the proton transport in CNTs with a sub-1 nm diameter is ultrafast.18 However, in the relatively large nanochannels of CNTs with the diameter of 1.5 nm, the proton transport is not significantly


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enhanced and the value of the proton transport rate is comparable to bulk water.18 Their results suggest that the spatial nanoconfinement in CNT with a small diameter results in the enhancement of the proton transport.18 Without the spatial nanoconfinement, the wettability of liquid on the sidewall of CNT is a key factor for highly efficient mass transport. Due to the high hydrophobicity of CNT sidewalls, it is extremely challenging for high surface tension liquid like aqueous solution to wet the sidewalls of CNTs.19 However, the efficient transport of hydrophilic molecules or aqueous ions in the nanochannels is often required in many applications. Simulations showed that the aqueous ion transport in the nanochannels of CNTs was enhanced by improving the wettability of CNTs.20-22 A number of approaches have been explored to improve the wettability of CNTs, including chemical modification,23-24 electrowetting,25-26 and electroosmotic flow.27 However, during the chemical modification or the application of an electric field, defects could be introduced into CNTs.28-31 CNT with many defect sites on its sidewall is not desirable in a wide range of applications because the destructed carbon networks in CNTs could cause the deteriorations of the intrinsic excellent properties of CNTs. In addition, many experimental investigations on improving the hydrophilicity of CNTs focus on an improvement in the dispersity of CNTs in solvents, inorganic matrix or polymers.32-34 A few studies have experimentally observed the enhancement of the aqueous ion transport in the interior nanochannels of CNTs after a chemical functionalization at CNT ends.14, 26, 35 However, the overall mass transport in the interior channels of CNTs is not efficient. Recently, the intertube channels (tube-to-tube) in vertically aligned CNT arrays are being considered as potential nanochannels for mass transport.25 Although there have been technical difficulties in fabricating the small-size intertube nanochannels in CNT arrays, the use of the intertube nanochannels to transport aqueous solution is attracting scientific attention. The ion transport


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behaviors in the intertube nanochannels of vertically aligned CNT arrays have been reported.25 After an electric activation treatment, where an electric voltage was applied on sample, the aqueous solution rapidly infiltrated the CNT arrays.25 However, the transport behaviors of the hydrophilic small molecules in the intertube channels of CNT array are not well understood because of the difficulties in improving the wettability of CNTs without destructing the carbon network of the CNT sidewalls, the original orientation and integrity of CNT arrays. In this study, we utilize the intertube nanochannels in highly aligned CNT arrays as transport paths to investigate the transport behaviors of hydrophilic calcein molecules in nanochannels. The intertube nanochannels in CNT arrays were fabricated by a capillary-induced densification treatment. We explore a non-covalent method to functionalize highly aligned CNT arrays by coating polydopamine on the sidewalls of CNTs to improve the wettability of high surface tension liquid on CNTs. This chemical modification via the spontaneous self-polymerization of dopamine and π-π stacking interactions between CNTs and dopamine does not destruct the carbon networks, the original orientation and integrity of CNT arrays, which are of great practical importance. The Pdop-modified intertube channels in DCNTAs might be useful as a nanofluidic system to investigate the transport behaviors of ions, DNAs, proteins, drugs and particles. 2. EXPERIMENTAL DETAILS 2.1. Sample preparation The vertically aligned multi-walled CNT arrays with a height of 0.9 mm was grown on catalysts pre-deposited on substrates through a modified chemical vapor deposition (CVD) method, which was provided in our previous report.36 The ends (or tips) of CNTs obtained by the CVD method are closed.28, 37-38 The inner diameter of multi-walled CNT is 15 ± 2 nm obtained


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from the transmission electron microscope (TEM) image. TEM sample was prepared by collecting CNTs dispersed in ethanol on a copper grid. The densified CNT arrays were prepared by firstly immersing the pristine CNTA in ethanol, and then drying CNTA at room temperature in air. The height of pristine CNT array is 0.9 mm and after the densification treatment, the corresponding value for DCNTA is 0.9 mm. This densification treatment did not cause obvious variations in the height of CNT array. During the liquid immersion and subsequent solvent evaporation, carbon nanotubes in CNTAs were drawn together and the intertube distance decreased because the surface tension of liquids and the strong van der Walls interactions between nanotubes effectively “zip” CNTs together.39 The densification treatment did not cause an obvious variation of the orientation of CNTs because the high alignment of CNTs is an ideal condition for the optimum overlap of van der Waals forces between the nanotubes.39 PdopDCNTA was obtained by firstly immersing CNT arrays in a 1mg/mL dopamine solution in a 95% alcohol and 5 % water mixture at 25 oC, and then dropping Tris-HCl buffer (10 mM, pH=8.5) into the dopamine solution. During this immersion process, the densification of CNT array and the formation of polydopamine coating on the sidewalls of CNTs simultaneously take place. In liquid immersion stage, the capillary forces of liquids draw the nanotubes together, the lateral dimension of CNT array decreases and the CNT array is densified.39 At the same time, the polydopamine coating is formed on CNT walls. After 24 h immersion, the CNTAs are picked up from the dopamine solution, rinsed 3 times using deionized water to remove free dopamine and dried at room temperature. In this liquid evaporation stage, the van der Waals forces between nanotubes further draw the nanotubes together and the fragile pristine CNT array turns into a rigid array with densely packed CNTs.39 Thus, in the liquid immersion stage, the formation of


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polydopamine coating accompanies the densification of CNT array and subsequently, in the liquid evaporation stage, the CNT array is further densified. We separately used 20 nm Au nanosphere colloids and 15 nm Au nanosphere colloids to conduct diffusion measurements in Pdop-DCNTAs to test the integrity of DCNTA membrane. Au colloid diffusion in membranes is a widely used method for checking the integrity of membranes. The 1D objects like nanorods or nanowires have relatively high aspect ratio (length to diameter). The relatively long 1D objects might be stuck in the nanochannels of DCNTAs. Additionally, the detection of Au nanosphere colloids using a UV-vis spectrophotometer is precise and fast. Thus, the Au nanosphere colloids are better candidate for checking the integrity of membranes as compared to 1D objects. The relatively large Au nanospheres with a diameter of > 10nm are commonly used for detecting macroscopic defects or fractures in membranes. The small Au nanospheres with a diameter of ≤ 10nm might pass through the nanochannels in membranes. Thus, the small Au nanospheres are not routinely used to detect the macroscopic detects or fractures in membranes. The schematic of the Au colloid diffusion measurement is shown in Figure S1. UV-Vis spectra show no absorption peaks of 20 nm Au colloids at 525 nm and 15 nm Au colloids at 518 nm in the permeation solutions (Figure S2). It indicates that there are no macroscopic fractures or cracks in DCNTA membranes. 2.2. Characterization Hitachi S-4700 (Hitachi Inc.) scanning electron microscope (SEM) was utilized to image the structures of CNT arrays and to obtain element distribution map using energy dispersive spectroscopy (EDS). TEM images were captured using a Tecnai G220 (FEI) microscope. The contact angle goniometer equipped with a camera (SL200C optical contact angle meter, USA Kino Industry Co., Inc.) was employed to measure the contact angles of DCNTA and Pdop-


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DCNTA membranes. X-ray photoelectron spectroscopy (XPS) data was collected using ESCALAB 250 XI model X-ray photoelectron spectrometer (Thermo Scientific).

Fourier

transform infrared (FT-IR) spectra of the samples were obtained using a Fourier Transform Infrared spectrometer (Nicolet 6700 FT-IR, Thermo Scientific). Thermogravimetric analysis (TGA) data were collected using a thermogravimetric analyzer (TGA/DSC 3+ STAR System, METTLER TOLEDO). The UV-Vis spectra were collected using a UV-3600 230 VCE spectrophotometer (SHIMADZU). The fluorescence microscopy image is obtained using an OLYMPUS LX71 microscope. 2.3. Calcein diffusion measurement The schematic of calcein diffusion measurement setup is shown in Figure 1. Calcein diffusion measurements were conducted at room temperature (25 oC). Calcein exhibits a good solubility in an alkaline solution and is able to fully dissociate in aqueous Na2CO3 solution. Calcein can only sparingly dissolve in water with PH=7.40 In order to obtain a precise measure on the calcein concentration in the permeation solution, we dissolved the calcein in an alkaline solution (aqueous Na2CO3 solution). Permeation solution is 0.1 M sodium carbonate solution. Feeding solutions were prepared by dissolving the calcein in 0.1 M sodium carbonate solution. The PH values of feeding and permeation solutions are same, PH= 10.5. The calcein molecules transport through the intertube (tube-to-tube) nanochannels in CNT arrays. The transport of calcein was monitored by measuring the calcein concentration variations in the permeation solution using a UV-Vis spectrophotometer. The thickness of membrane is 0.9 mm and the membrane diameter is 0.3 cm. No stirrer is used in the transport experiments. The difference of calcein concentrations in the feeding solution and permeation solution drives the calcein to pass through the DCNTA membrane from the feeding solution to the permeation solution. A hydrostatic pressure might


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induce a flow during the calcein diffusion. In our studies, containers with same shape and volume are utilized as the feeding cell and receiving cell. The volumes of the feeding solution and permeation solution are identical. Thus, the free surfaces of feeding solution and permeation solution are at the same level (Figure 1). Also, the volumes of feeding solution and permeation solution are large. The variations in the volumes of feeding solution and permeation solution are negligibly small during the calcein transport. Thus, the hydrostatic pressure exerted on the membrane is small. The flow induced by a hydrostatic pressure is not significant.

Calcein solution

PdopDCNTA

Na2CO3 solution

Kapoton Tape Spacer

Figure 1. The schematic of calcein diffusion measurement setup.


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3. RESULTS AND DISCUSSION (b) Pristine CNTA

(a)

DCNTA

“Zipping” 2μm

(c)Pristine CNTA

cm

2μm

DCNTA

Pristine CNTA

DCNTA

Substrate

cm

(d)

(e)

Pristine CNTA

500nm

DCNTA

500nm

Figure 2. Schematic of the densification of CNT array (a), side-view SEM images of the pristine CNTA and DCNTA (b), pictures of CNT arrays (c), picture of the DCNTA membrane after a pressure of 5 kPa was exerted on the DCNTA membrane (d) and top-view SEM images of the pristine CNTA and DCNTA (e).

Vertically aligned CNT arrays with an inner diameter of 15±2 nm, a length of 0.9 mm and an intertube distance of 55 nm were grown by a thermal chemical vapor deposition method described in our previous studies. 36 Pristine CNT arrays are fragile and exhibit relatively poor mechanical properties due to loosely packed CNTs in arrays. In order to enhance the mechanical strength of CNTAs and modulate the intertube distance, L, of CNTAs, we subjected the pristine


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CNTAs to a densification treatment, where CNT arrays tethered to catalyst substrates were firstly immersed in ethanol, and then dried at room temperature in air. During liquid immersion and solvent evaporation, loosely packed CNTs in pristine arrays were drawn together through a “zipping” effect

39

induced by the surface tension of liquid, capillary forces and van der Waals

interactions between tubes (See Figures 2a - 2c). The average intertube distance of CNT array, L0 , is evaluated from the relation,41 L0= (1.2π r2ρhs /M)1/2 _ 2r, where ρ, r, s, h, and M are the CNT density (1.3 g /cm3), CNT radius (7.5 nm), CNTA area, CNTA thickness (0.9 mm) and total mass of CNTA (0.0044 g), respectively, assuming that the CNTs are hexagonally packed. After this densification treatment, the intertube distance dramatically decreased from 55 nm to 18 nm. Correspondingly, the lateral dimension and area of CNTAs also decreased from 0.95 cm to 0.45 cm, and from 0.90 cm2 to 0.20 cm2, respectively. The pristine arrays with loosely packed CNTs turn into densified arrays with closely packed CNTs. Additionally, the mechanical strength of DCNTAs is enhanced. The densified CNT array does not collapse after a pressure of 5 kPa is applied to the DCNTA due to the improved mechanical strength of DCNTA (Figure 2d). The height of pristine CNT array is 0.9 mm and the corresponding value for DCNTA is 0.9 mm after a pressure of 5 kPa was exerted on the DCNTA membrane. The pressure applied to the DCNTA does not cause a height variation of CNT array. It indicates that the mechanical property of the DCNTA is improved and the DCNTA can sustain a certain level of external force without causing the collapse of CNT array. To check for the mechanical stability of the DCNTA membrane, we examined the DCNTA membrane after a pressure of 5 kPa was exerted on the DCNTA membrane for 24 h to determine if the DCNTA collapsed (see inset in Figure 2d). No obvious variation in the height of DCNTA was observed and the DCNTA did not collapse after a pressure of 5 kPa was exerted on the DCNTA membrane for 24 h. The DCNTA membranes are


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mechanically robust. Enhanced mechanical strength facilitates the removal of CNTA from catalyst substrate without involving any aggressive physical sonication and chemical etching. The representative side-view SEM images of the pristine CNTA and DCNTA show that the integrity and original orientation of CNT arrays, which play crucial roles in the formation of intertube nanochannels in DCNTAs, are retained after this densification treatment (See Figure 2b). The side-view and top-view SEM images of the CNT arrays in Figures 2b and 2e, respectively, show that the CNTs after the densification treatment are packed more densely as compared to the pristine CNTA.

(a)

50nm

(b)

50nm

(c)

200nm

(d)

70nm

Figure 3. TEM images of the pristine CNT (a) and Pdop-modified CNT (b). SEM image (c) and element distribution map (d) of the Pdop-modified CNT. The modification time is 24 h.

The improvement of the CNT hydrophilicity is critical to the effective transport of high surface tension liquids in the nanochannels of DCNTAs since the wettability of liquid on CNT walls is an important parameter in modulating the diffusion of liquid through DCNTA nanochannels. In order to improve the wettability of high surface tension liquid on CNT walls, we altered the chemical properties of CNT walls to increase the average surface energy of CNTs. It is challenging to chemically functionalize CNTA without destructing the integrity and orientation of DCNTAs. In our studies, we modified DCNTAs using a non-covalent modification


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method, where polydopamine was incorporated into DCNTAs by the spontaneous selfpolymerization of dopamine and π-π stacking interactions between dopamine and CNTs.42 This non-covalent method makes the functionalization of CNTAs without destructing the integrity and orientation of CNT arrays possible. After CNTAs were immersed in a dopamine solution at 25 oC for 24 h, CNTAs were rinsed 3 times using deionized water to remove the free dopamine. The sidewall of pristine CNT is smooth as shown in Figure 3a. For polydopamine-modified CNTs, a polydopamine layer with a thickness of 4±1 nm (pointed out by red arrows) is formed on the sidewall of CNT (See Figure 3b). During this immersion stage, free polydopamine which is not tethered on CNT wall is also generated in dopamine solution. The aggregation of free polydopamine around the surface of CNT array might clog the transport path of calcein molecules. Thus, we modified the CNTA for no more than 24 h in order to avoid the formation of the big aggregation of polydopamine. After 24 h immersion, a portion of CNTs are not covered by polydopamine (Figure 3b). CNT sidewalls are not completely coated with polydopamine in the modification time range of our studies. We also checked the morphology and thickness of polydopamine layer coated on the sidewall of CNT after a reasonably long modification time of 48 h (Figure S3). No significant changes in the morphology and thickness of polydopamine layer are observed. The long modification time leads to an increase in the number of effective nanochannels for the calcein diffusion. The side-view SEM image and EDS map show that the polydopamine is uniformly coated on CNT walls (Figures 3c and 3d). To further check the formation of polydopamine layer on the sidewall of CNT, we performed FT-IR and XPS measurements on Pdop-DCNTAs. FT-IR data from Pdop-DCNTA are shown in Figure 4a. The peaks at 1615 cm-1, 1372 cm-1 and 1028 cm-1 associated with aromatic ring, methylene groups and C-N stretching vibration respectively, appeared in the FT-IR spectra of Pdop-


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DCNTA. It is indicative of the formation of Pdop layer by the successful spontaneous selfpolymerization of dopamine and π-π stacking interactions between dopamine and CNTs. An example of XPS data from Pdop-DCNTA is shown in Figure 4b. Pdop-DCNTA shows N 1s peak associated with polydopamine. For the pristine CNT array, there is no N 1s peak in XPS spectra. The N 1s peak in XPS spectra of Pdop-DCNTA provides a reliable measure of polydopamine coated on the sidewalls of CNTs. We also performed TGA measurements on pure polydopamine film and Pdop-DCNTA to quantitatively analyze the polydopamine coated on CNTs (Figure 4c). For pure polydopamine film, a significant weight loss in the temperature range of 200 oC to 535 oC

is associated with the decomposition of polydopamine. The weight loss in the temperature

range of 0-200 oC arises from residual water and volatile impurities in pure Pdop sample.43 Figure 4c shows a two-step TGA profile for Pdop-DCNTA sample. The first step in the temperature range of 0 oC to 200 oC is attributed primarily to the removal of water molecules which are incorporated into DCNTA during chemical modification process and volatile impurities in Pdop layers. As described in a previous study,25 the nanochannels in DCNTAs can provide a protection for the water locked in DCNTAs. A subsequent thermal treatment for removing water in DCNTAs can give rise to a reduction in the content of water in DCNTAs, but it is challenging to remove all water molecules locked in DCNTA.25 It explains the weight loss in the temperature range between 0 oC and 200 oC. The second step in the temperature range of 200 oC

to 535 oC is associated with the decomposition of polydopamine coated on CNTs. The content

of polydopamine incorporated into DCNTA increases with modification treatment time (see Figure 4c). If polydopamine is incorporated into DCNTA, then the wettability of DCNTA by high surface tension liquid should be improved. To make this check, contact angle measurements were conducted. The representative contact angle images of Pdop-DCNTA and untreated


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DCNTA are showed in Figure 4d. The contact angle of DCNTA without polydopamine coating is 133.6o. For Pdop-DCNTA, the contact angle shows a dramatic decrease and corresponding value is close to 0o, indicating a great improvement in the wettability of DCNTA.

(c)

(a)

100

Pure CNT Pure Pdop 4h 8h 12 h 24 h

80 0.01

Weight (%)

Transmittance (A.U.)

0.00

-1

1615 cm -1

1372 cm

0.02

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60

Sample Pure CNT Pdop-DCNTA

40 20

1028 cm

Pure Pdop

0 1000

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(d)

Pristine CNTA Pdop-DCNTA C N

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800

Modification time (h)

4 8 12 24

Weight loss(%) (200-535 oC) 0 4.2 12.9 19.1 30.8 84.2

1000

Temperature ( C)

Wavenumber (cm )

Intensity (A.U.)

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

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133.6 o

O

500

Binding Energy (eV)

600

Untreated DCNTA

Pdop-DCNTA

Figure 4. FT-IR of Pdop-DCNTA (a), XPS spectra of Pdop-DCNTA and pristine CNT array (b), TGA data and the weight loss of pure CNT, Pdop and Pdop-DCNTAs with different modification times (c), and contact angle images of the untreated DCNTA and Pdop-DCNTA (d). The modification time is 24 h.

We next focus our attention on the molecule transport behavior in the intertube nanochannels of Pdop-DCNTAs. Specifically, we consider the transport behavior of hydrophilic calcein


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molecules in high surface tension solvent. In this study, calcein in a 0.1 M sodium carbonate aqueous solution is used as a model hydrophilic molecule. The schematic of experimental setup for calcein transport measurement is shown in Figure 1. The thickness of membrane is 0.9 mm and the membrane diameter is 0.3 cm. No stirrer is used in the transport experiments. The maximum absorption peak for calcein is located at 495 nm. For untreated DCNTA membrane, no obvious signal from calcein is collected after a 1 h transport (Figure 5a), indicating that there is no measurable amount of calcein in the permeation solution. We tested permeation solution after a 24 h transport to determine if the maximum absorption peak for calcein appeared at longer transport time. No absorption peak at 495 nm was observed in UV-Vis spectra after a 24 h transport (Figure 5a). However, for Pdop-DCNTAs, UV-Vis spectra of permeation solutions show an absorption peak at 495 nm after a 1 h transport, indicating that a measurable amount of calcein diffused through Pdop-DCNTAs (Figure 5b). The absorbance intensity of permeation solution at 495 nm increases with the transport time. In order to determine the calcein concentration in permeation cell, a calibration was conducted using a series of calcein solutions with known concentrations. A linear calibration curve of calcein solution is shown in Figure S4. After a 1 h transport, the calcein concentration in permeation cell, Cp, is 1.53×10-5 mol/L and corresponding value increases to 4.85 × 10-5 mol/L after a 5 h transport (see Figure 5c). As expected, the variations in the calcein concentration in the permeation cell (Figure 5c) are all consistent with observations on the absorbance intensity of permeation solution shown in Figure 5b. A linear relationship between the calcein concentration in permeation solution and transport time allows a determination of the overall molecule transport rate, V, through Pdop-DCNTA membrane. The value of V for Pdop-DCNTA membrane after a 24 h modification in the dopamine solution is 1×10-5 mol/L·h-1. The apparent diffusion coefficient of molecules in a


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porous membrane can be evaluated from the relation, D=-N h /(Cp-Cf) s t, based on the Fick’s law of diffusion, where N is the amount of substance that passes through the membrane at time t, h is the thickness of membrane, s is the area of membrane, and Cf is the molecule concentration in the feeding solution. After a 24 h modification, the apparent diffusion coefficient of calcein in Pdop-modified DCNTA is 1.20×10-4 cm2/s and the corresponding value for unmodified CNT array is close to 0 because the hydrophilic calcein molecules hardly diffuse through the unmodified DCNTA. The apparent diffusion coefficient of calcein in DCNTA significantly increases after the polydopamine is incorporated into CNT arrays. It suggests that the improvement of the wettability of DCNTA is crucial to the effective transport of hydrophilic calcein molecules. It is worth noting that there are residual calcein molecules retained in Pdop-DCNTA membrane after the calcein solution diffuses through membrane. The fluorescent image of Pdop-DCNTA after the calcein diffusion experiments shows that the residual calcein molecules are uniformly distributed in the Pdop-DCNTA membrane (Figure S5). The wettability of CNT walls by liquids plays a crucial role in transporting liquid. In liquids like ethanol, dimethylformamide, N-methyl-pyrrolidone, chloroform, etc, the non-modified CNTs exhibit good wettability.44-45 These liquids are able to rapidly infiltrate the non-modified DCNTAs. Figure S6 shows the images of non-modified DCNTAs captured by an optical microscope after ethanol, dimethylformamide, N-methyl-pyrrolidone and chloroform are separately dropped on the surfaces of DCNTAs. No liquids remain on the surfaces of DCNTAs. It indicates that those liquids are able to diffuse through the nanochannels of the non-modified


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DCNTAs due to the good wettability of CNTs by these liquids. Thus, it is possible that molecules with good solubility in liquids that can completely wet the non-modified CNTs, are able to effectively transport in the nanochannels of non-modified DCNTAs. The calcein dissolved in a solution with PH=10.5 carries a net negative charges.46 Polydopamine contains amine groups and phenolic hydroxyl groups. At PH=10.5, the polydopamine molecules are negatively charged because of the deprotonation of the phenolic hydroxyl groups.47-48 We also examined the transport rate of calcein dissolved in an alkaline solution with PH=8.5 to determine if the PH value of alkaline solution had a significant impact on the transport behavior of calcein (Figure S7). The transport rate and the apparent diffusion coefficient of calcein in PH= 8.5 alkaline solution are 1.29×10-6 mol/L·h-1 and 1.14×10-4 cm2/s, respectively and the corresponding values in PH=10.5 alkaline solution are 1.24×10-6 mol/L·h-1 and 1.20 × 10-4 cm2/s, respectively. No significant variations in the transport rate and apparent diffusion coefficient of calcein were observed in PH=8.5 alkaline solution. Moreover, the pore size of nanochannels in DCNTAs is much larger than the Debye length of charge. The net charge of calcein might not have a significant influence on the transport of calcein in the nanochannels.

(b) 1.2

1h 24 h

0.015

0h 1h 2h 3h 4h 5h

Abs

0.9

0.000

-0.015

0.6

0.0 350

400

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Wavelength (nm)

550

600

)

0.030

0.015

0.3

-0.030

(c

0.045

Cp (mmol/L)

(a)

0.030

Abs

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


0.000

350

400

450

500

Wavelength (nm)

550

600

0

1

2

3

Transport time (h)

4

5

Figure 5. UV-Vis spectra of the permeation solutions for untreated DCNTAs (a) and for Pdop-DCNTAs at different transport times (b). The calcein concentration in the


17


permeation cell at different transport times (c). The Pdop modification time is 24 h. The calcein concentration in the feeding solution is 1×10-3 mol/L.

Pdop content in DCNTA is a significant factor controlling the wettability of DCNTA. Thus, this important factor is expected to influence hydrophilic molecule transport in the nanochannels of DCNTA. We modulated the Pdop contents in DCNTAs by altering the modification time. TGA data show that the Pdop contents in DCNTAs increase with modification times (see Figure 4c). To gain insight into the influence of Pdop contents in DCNTAs on hydrophilic molecule transport in the nanochannels of DCNTAs, we conducted measurements on the overall transport rate of calcein molecules in Pdop-DCNTAs with different modification times. The relationship between modification time and overall transport rate is shown in Figure 6. The overall transport rate increases with the modification time or Pdop contents in DCNTAs. As expected, the apparent diffusion coefficient of calcein also increases with the modification time (Figure 6c). The diffusion coefficient of small molecules in aqueous solution is close to 10-5 cm2/s.49 The apparent diffusion coefficient of calcein in nanochannels is greater than the corresponding value of small molecules in aqueous solution.

-4

1.2x10

-3

1.2x10

0.03

-4

6.0x10

350

400

450

500

Wavelength (nm)

550

600

-5

4.0x10

0.0

0.0

0.00

-5

8.0x10

2

V (mmol/L·h-1)

0.06

(c)

(b)

0h 4h 8h 12 h 24 h

D (cm /s)

(a)

Abs

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 31

0

4

8

12

Modification time (h)

24

0

8 12 4 Modification time (h)


18

24

Page 19 of 31

Figure 6. The UV-Vis spectra of calcein solution in the permeation cell for PdopDCNTAs with different modification times (a) and the transport rate (b) and apparent diffusion coefficient (c) of calcein molecules in Pdop-DCNTAs with different modification times. The transport time is 2 h and the calcein concentration in feeding solution is 1×10-4 mol/L.

0.2

0.0

(b) 0.04 0.02

-1

1h 2h 3h 4h 5h

0.4

V/Cf ( h )

(a)

Cp/Cf

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


0.00

-0.02 -0.2 -5

1× 1 0

-5

5× 1 0

-4

1× 1 0

-4

5× 1 0

Cf (mol/L)

-3

1× 1 0

-5

1× 10

-5

0 5× 1

-4

0 1× 1

-4

0 5× 1

Cf (mol/L)

-3

0 1× 1

Figure 7. The normalized calcein concentration in permeation solution as a function of Cf at different transport times (a) and the normalized calcein transport rate as a function of

Cf (b). The modification time of DCNTAs is 24 h. In order to further understand hydrophilic molecule transport in the nanochannels of DCNTAs, it is necessary to decouple the calcein concentration in the feeding solution from its transport behavior. To do so, the calcein concentration in permeation solution was normalized by

Cf. Dimensionless ratios are shown in Figure 7a. We find that the normalized calcein concentrations in permeation solution is insensitive to Cf in the Cf range of our investigation (1× 10-5 mol/L ～100×10-5 mol/L). This dimensionless ratio is 0.01, 0.02, 0.03, 0.04, and 0.05 after calcein solutions diffuse in Pdop-DCNTAs for 1 h, 2 h, 3 h, 4 h, and 5 h, respectively. Figure 7b shows that calcein transport rate normalized by Cf. As expected, the normalized calcein transport


19


Page 20 of 31

rate is fairly constant, 0.01±0.0008 h-1, in the Cf range from 1×10-5 mol/L to 100×10-5 mol/L. It also suggests the concentration of feeding solution does not show an obvious influence on the normalized calcein transport rate in the nanochannels of Pdop-DCNTAs. The wettability of CNT is the key factor for the efficient mass transport in DCNTAs. The disintegration of polydopamine coating from CNT walls should cause the decline of transport rate of calcein in Pdop-DCNTA membrane. We repeated the molecule-transport experiment using same Pdop-DCNTA membrane for 5 times to check the stability of polydopamine coating on CNT walls. The transport time is 5 h for each molecule-transport experiment. The normalized transport rate of calcein is 0.94 × 10-2 h-1 in the 1st transport experiment and the corresponding value is 1.06×10-2 h-1 after the molecule-transport experiment is repeated for 5 times. The normalized transport rate of calcein did not exhibit an obvious decline after the multi-time transport experiments. It indicates that polydopamine coating is relatively stable and calcein transport in nanochannels does not cause the significant disintegration of polydopamine coating in the time range of our measurements. It is worth pointing out that many amphiphilic molecules like sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, poly(styrene)-block-poly(acrylic acid) copolymer, etc, can be used to modify the CNTs to improve the wettability of CNTs by aqueous solution.44,

50

Among these

amphiphilic molecules, poly(styrene)-block-poly(acrylic acid) copolymer has aromatic ring in poly(styrene) block and hydrophilic functional group in poly(acrylic acid) block. Sodium dodecyl benzene sulfonate contains aromatic ring (benzene ring) and hydrophilic functional group (sulfonate group). Thus, it is possible to modify the vertically aligned CNT arrays using molecules having aromatic rings and hydrophilic functional groups. 4. CONCLUSIONS


20

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We have studied in detail the hydrophilic molecule transport in the intertube nanochannels of DCNTAs. The wettability of the nanochannels of DCNTAs is greatly improved by a simple noncovalent modification, in which Pdop is incorporated into DCNTAs by spontaneous selfpolymerization of dopamine and π-π stacking interactions between dopamine and CNTs. The formation of Pdop coating on the sidewalls of CNTs facilitates the infiltration of high surface tension liquid into DCNTAs. The overall transport rate and apparent diffusion coefficient of hydrophilic calcein molecules in the nanochannels of DCNTAs were significantly enhanced. We further found that the overall transport rate and apparent diffusion coefficient can be modulated by altering the Pdop contents in DCNTAs or modification time. By normalizing the transport rate of calcein in the nanochannels of DCNTAs by the calcein concentration in feeding solution, we decouple a significant transport factor, concentration difference between feeding solution and permeation solution, from its transport behavior. The normalized overall calcein transport rate is insensitive to the calcein concentration in feeding solution. The transport of hydrophilic molecules or aqueous ions in nanometer-scale channels plays a crucial role in biological processes, energy conversion and medicine release. The functionalization of the nanochannels of DCNTAs using a simple non-covalent method has a practical benefit to the improvement in the wettability of hydrophobic nanochannels without involving a complex multi-step chemical modification. On the other hand, the manipulation of the wettability of nanochannels is of significant interest in measuring the physical and chemical properties of materials encapsulated in the nanometer-scale channels. Pdop-modified intertube channels in DCNTAs might provide a unique environment for investigating the reaction dynamics of chemical molecules and for measuring thermal, magnetic and mechanical properties of materials incorporated in nanochannels.


21


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ASSOCIATED CONTENT Supporting Information The schematic of the diffusion measurement setup for Au colloids in Pdop-DCNTAs; UV-Vis spectra of feeding and permeation solutions for 20 nm Au colloids and 15 nm Au colloids before and after 120 h diffusion in Pdop-DCNTAs; TEM images of the Pdop-modified CNT after a 48 h modificaiton treatment;The UV-vis spectra of calcein solutions with known concentrations and linear calibration curve of calcein solution; The fluorescence microscopy image of the PdopDCNTA after calcein diffusion measurements; Images of non-modified DCNTAs captured by an optical microscope after different liquids are separately dropped on the surfaces of DCNTAs; The calcein concentration in the permeation solution during the calcein transport in PdopDCNTAs at different PH conditions. AUTHOR INFORMATION *Corresponding Author E-mail: [email protected]; Phone: +86-51265884716 E-mail: [email protected] ORCID Xiaohua Zhang: 0000-0002-3996-702X Zhaohui Yang: 0000-0003-3329-5311 ACKNOWLEDGMENT This work was financially supported by Natural Science Foundation of Jiangsu Province (No.BK20181430). The authors also thank the Specially Appointed Professor Plan in Jiangsu


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Table of Contents

Calcein

Polydopamine

Molecule transport in Pdop-modified CNT array.


30

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Polydopamine-modified nanochannels in vertically aligned carbon

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