Article pubs.acs.org/IECR
Functionalization of Magnetic Multiwalled Carbon Nanotubes with Mixed Surfactants for Enhancing Protein Adsorption Jian Sun,†,∥ Kun Du,†,∥ Jiang Gao,‡ Ling Li,† Peijun Ji,*,‡ and Wei Feng*,† †
Beijing Key Lab of Bioprocess, Department of Biochemical Engineering and ‡Department of Chemical Engineering, Beijing University of Chemical Technology, Beijing, 100029 China S Supporting Information *
ABSTRACT: Magnetic multiwalled carbon nanotubes (M-MWNTs) have been functionalized with mixed surfactants consisting of a sugar-based surfactant and an ionic surfactant. The synergistic effect of the two surfactants results in more surfactants adsorbed and the formation of a more disordered structure of assembled surfactants. These two aspects facilitate simultaneous hydrogen bonding and electrostatic interactions with proteins. It has been demonstrated that the M-MWNTs functionalized with mixed surfactants adsorbed more proteins than the M-MWNTs functionalized with single surfactants. The M-MWNTs functionalized with mixed surfactants have been tested for the adsorption of two proteins, bovine serum albumin (BSA) with a low isoelectric point and lysozyme with a high isoelectric point, and similar results have been obtained. Hence, carbon nanotubes functionalized with mixed surfactants have a wide range of potential applications for protein adsorption.
1. INTRODUCTION
surfactant. This work demonstrates the synergistic effect of two surfactants on the adsorption of proteins.
Protein-functionalized carbon nanotubes have been extensively investigated for fabrication of biosensors and applications in medical diagnostics. Glucose oxidase-functionalized carbon nanotubes were used for labeling the signal antibodies.1 Biosensor design was based on D-amino acid oxidasefunctionalized carbon nanotubes.2 Horseradish peroxidasecarbon nanotubes bioconjugate can achieve highly sensitive detection of a cancer biomarker in serum and tissue lysates.3 Functionalization makes carbon nanotubes biocompatible and extends their applications.4−7 In comparison to covalent approaches, noncovalent functionalization of CNTs can preserve the intrinsic electronic structure and properties of CNTs.8 Carbohydrates and their derivatives have been used to construct water-soluble, biocompatible CNTs.9−16 Carbon nanotubes wrapped by polysaccharides presented good water solubility, such as starch-wrapped carbon nanotubes,9 dissolution of CNTs by wrapping with single-chain schizophyllan and s-Curdlan.10 Carbohydrate derivatives exhibit amphiphilic properties suitable for the interactions with CNTs, including the pyrene-based glycoconjugates glycolipids and glycopolymers.11−16 Protein/ enzymes can be adsorbed onto CNTs with the assistance of surfactants. CNTs coated with the surfactant Triton X-100 can specifically bind to streptavidin.17 Enzymes and surfactants have been coassembled onto carbon nanotubes,18 and the CNT nanohybrids were demonstrated to facilitate interfacial electron transfer of the proteins with enhanced faradic responses. In our previous work, sugar-based surfactants have been used to functionalize carbon nanotubes,19,20 and the solvent alcohols were investigated for promoting the assembly of the surfactant on CNTs. The functionalized CNTs were applied to adsorb proteins with a good preservation of protein conformation. Herein, we report the functionalization of CNTs using mixed surfactants consisting of a sugar-based surfactant and an ionic © XXXX American Chemical Society
2. MATERIALS AND METHODS 2.1. Materials. Multiwalled carbon nanotubes (MWNTs) were purchased from Nanotech Port Co., Ltd. (Shenzhen, China). The purity is higher than 95%, and the catalyst residue is less than 0.2%. D-Maltose, hexadecylamine, sodium hexadecyl sulfate(SHS), cetyltrimethylammonium bromide (CTAB), bovine serum albumin (BSA), lysozyme, and sucrose were purchased from Sigma-Aldrich Chemical Co., China. Sulfuric acid, nitric acid, ferric nitrate, and isopropyl alcohol were obtained from Sinopharm Chemical Reagent Co. N-HexadecylD-maltosylamine (HDMA) (Scheme S1, Supporting Information) was synthesized according to the method reported in our previous work.20 All chemicals (analytical grade or higher) were used as received without any further purification. Deionized double-distilled water was used in making solutions. 2.2. Preparation of Magnetic MWNTs. MWNTs were purified and oxidized as reported elsewhere.21 As-received MWNTs were purified by refluxing in an aqueous HNO3 of 2.6 M at 70 °C for 45 h. The nanotube suspension was diluted and washed with double-distilled water by filtering through a 0.8 μm polycarbonate membrane. The samples were dried at 80 °C under vacuum. The purified MWNTs were oxidized in the 3:1 concentrated H2SO4/HNO3mixture for 3 h. The nanotube suspensions were diluted and washed with double-distilled water by filtering through a 0.45 μm polycarbonate membrane. The sample was dried at 80 °C under vacuum. Oxidized CNTs were used to prepare magnetic MWNTs (M-MWNTs) in order to promote the interactions between the Received: April 15, 2014 Revised: August 11, 2014 Accepted: August 19, 2014
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on the basis of average values taken from triplicate measurements of three adsorption operations. 2.5. Measurements. Ultraviolet−visible absorption spectroscopy measurements were performed in a Shimadzu UV2550-PC spectrophotometer using 1 cm-path length quartz cuvettes. Spectra were collected within a range of 190−800 nm. Infrared spectra were collected using a Fourier transform infrared (FTIR) spectrometer (Bruker TENSOR 27) equipped with a horizontal, temperature-controlled attenuated total reflectance (ATR) with ZnSe Crystal (Pike Technology). Infrared spectra from 1000 to 4000 cm−1 were collected using a liquid-nitrogen-cooled mercury−cadmium−telluride detector that collected 128 scans per spectrum at a resolution of 2 cm−1. All spectra were corrected by a background subtraction of the ATR element spectrum. Ultrapure nitrogen gas was introduced at a controlled flow rate to purge water vapor. XPS spectra were acquired using a Thermo VG ESCALAB250 X-ray photoelectron spectrometer, which was operated at the pressure of 2 × 10−9 Pa using Mg Kα X-ray as the excitation source. The atomic force microscopy (AFM) experiments were carried out on a Digital Instrument (Veeco, Santa Barbara, CA). A drop of the solution of CNT−protein conjugate (50 μL) was pipetted onto a clean SiO2 substrate and allowed to stand for 30 min. The substrate was then rinsed with a copious amount of distilled and deionized water and dried by a N2 stream. 2.6. Molecular Dynamics Simulations. A coordinate of carbon nanotube with a chiral index (15, 15) was generated by visual molecular dynamics (VMD). The carbon nanotube was 10.0 nm long and had a diameter of 2.03 nm. The parameters for the carbon nanotube were as follows: atom type CB; bond length 0.139 nm; bond energy 435 136 kJ·mol−1nm−2; bond angle 120°; angle energy 527.1840 kJ·mol−1rad−2; dihedral angle 180°; and angle energy 22.8028 kJ·mol−1rad−2. The partial charges for the organic solvent molecule of 2-propanol were downloaded from ATB (http://compbio.chemistry.uq. edu.au/atb). A periodic solvent box was created and equilibrated by MD simulations. All MD simulations were performed with the GROMACS software package (version 4.5.1)23,24 using the Gromos96 45a3 force field. Sufficient water molecules were included in the simulation, ensuring the presence of a water bulk phase around the surfactant/nanotube aggregation. The systems were subjected to the steepest descent energy minimization. MD simulations were performed with a time step of 2 fs, and coordinates were saved every 0.5 ps. Temperature and pressure controls were imposed using a Berendsen-type algorithm with coupling constants of 0.1 and 0.5 ps,25 respectively. The electrostatic interactions were calculated by using the particlemesh-Ewald algorithm,26 with an interpolation order of 6 and a grid spacing of 1.2 Å. The cutoff for van der Waals interactions was 14 Å. Hydrogen-bonding interactions were monitored using 3.5 Å as the donor−acceptor distance cutoff and 60° as the hydrogen−donor−acceptor angle cutoff. During all simulations, the coordinates of the CNT were fixed, while those of SHS, water, and ions were allowed to move. MD simulations for the self-assembly of mixed surfactant on CNT were performed for 60 ns, under constant pressure of 1 bar and constant temperature of 323 K.
magnetic particles with the CNTs through metal-ion interactions. M-MWNTs were prepared according to the methods in the literature.22 Oxidized MWNTs were impregnated with a mixture of 1 g of H2O, 0.1 g of H2SO4 (98%), 0.23 g of sucrose, and 1.5 g of ferric nitrate. This solution was added dropwise until incipient wetness, and then, the impregnated sample was dried at 120 °C. The sample was then thermally treated under N2 at 450 °C for 2 h. 2.3. Adsorption of Surfactants on M-MWNTs. 60 mg of mixed surfactants/single surfactants was added to 40 mL of 2propanol/water solution (volume concentration 15%), and the weight ratios of HDMA to SHS were 1:0, 1:1, 1:2, 1:3, and 0:1, respectively. The mixtures were sonicated for 20 min in an ultrasonic bath (KH-100SP, 40 kHz with rated power output of 70 W), and then, 20 mg of M-MWNTs was added to the mixture, and the sonication was continued for 1 h. The mixture was centrifuged at 12 000 rpm for 30 min, yielding wellsuspended M-MWNTs coated with the surfactants. For measuring the amount of surfactant adsorbed, the functionalized M-MWNTs were separated from the solutions by using a magnet as shown in Figure S1b, Supporting Information. By comparing the amount of surfactants before and after the adsorption, the amount of surfactants adsorbed onto MMWNTs was calculated, on the basis of average values taken from triplicate measurements of three adsorption operations. The amount of surfactants in the solutions was determined on the basis of the calibration curves of single surfactants as shown in Figures S2, S3, and S4, Supporting Information; they were obtained by measuring the UV−vis absorbance at different concentrations. 2.4. Determination of the Saturation Adsorption of Proteins onto the Functionalized M-MWNTs. Protein solutions (referred to as solution A) were prepared by dissolving proteins in a phosphate buffer. Solution A was first sonicated for 30 s and then was shaken under 200 rpm for 2 h at 4 °C. The protein concentrations were from 0.02 to 0.15 mg/mL. The solutions of functionalized M-MWNTs (0.075 mg/mL, referred to as solution B) were prepared by dispersing functionalized M-MWNTs in deionized water under sonication for 10 min and then shaken under 200 rpm for 1 h. For protein adsorption, solution A was added to solution B, and the mixture was then shaken at 4 °C under 150 rpm for 4 h. The protein content was measured using the micro bicinchoninic acid (Micro BCA) assay (Pierce Biotechnology). A protein concentration standard curve was prepared with concentrations ranging from 0.0 to 0.80 mg/mL achieved by serial dilutions of proteins. Twenty μL of each protein standard solution and each unknown sample was pipetted into a 96-well micro plate. 200 μL of the BCA working reagent was added to each well, and the plate was mixed thoroughly on a plate shaker for 30 min. The plate was incubated at 37 °C for 30 min. The plates were read for absorbance at a wavelength of 562 nm using an automated microplate reader (ThermoLab System, Helsinki, Finland). The protein standard curve was used to determine the concentration of each unknown sample. For measuring the amount of protein adsorbed, the functionalized M-MWNTs adsorbed with proteins were separated from the solutions by using a magnet as shown in Figure S1c, Supporting Information.The amount of protein in the solution was determined on the basis of the protein standard curve. By comparing the amount of protein before and after the adsorption, the amount of protein adsorbed onto the functionalized M-MWNTs was calculated,
3. RESULTS AND DISCUSSION 3.1. Assembly of Surfactants on M-MWNTs. MMWNTs were functionalized with mixed surfactants consisting B
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cm−1 decreases, while the intensity of the peak at 1176 cm−1 increases. The functionalized MWNTs were lyophilized to eliminate traces of alcohol and water. When redispersing in water, the dispersibility of the functionalized M-MWNTs was evaluated by measuring UV−vis spectra. A higher UV−vis absorbance means a larger aqueous dispersibility of the functionalized M-MWNTs. It was found that the M-MWNTs functionalized in 15% 2propanol concentration (v/v) had the largest dispersibility in water (Figure S6, Supporting Information). The weight ratio of HDMA to SHS in the adsorbed surfactant has an effect on the dispersibility of the functionalized M-MWNTs in water (Figure 3). Among the measured ratios, with the weight ratio of 0.38:1,
of HDMA and SHS in 2-propanol solutions. Alcohol solutions have been found to promote the assembly of surfactant on CNTs.27 Alcohols, each having a hydroxyl group and a hydrocarbon chain, are cosolvents for alkyl polyglycosides and surfactants.28,29 For nonionic surfactants, due to hydrophilic−hydrophilic and hydrophilic−hydrophobic interactions between alcohols and surfactants, short-chain alcohols can influence the properties of the aggregates of surfactant.29 For ionic surfactants, alcohols can destabilize micelles by displacing water from the surface, therefore decreasing its effective dielectric constant, increasing headgroup repulsions, and disrupting surfactant packing.30 The transmission electron microscopy (TEM) images (Figure 1) show that, compared to the purified CNT (Figure
Figure 1. TEM images. (A) Purified MWNTs. (B) M-MWNTs functionalized with mixed surfactants. (C) Mixed surfactant functionalized M-MWNTs with the adsorption of lysozyme.
1a), a thin layer was formed on the functionalized CNT (Figure 1b), confirming the adsorption of the mixed surfactants. The AFM image (Figure S5b, Supporting Information) also confirms the functionalization by the mixed surfactants. Figure 2 shows the FTIR spectra of functionalized M-MWNTs. The Figure 3. UV−vis spectrum of the functionalized M-MWNTs with various weight ratios of HDMA to SHS. (a) 0.38:1; (b) 0.59:1; (c) 1.1:1; (d) HDMA; (e) SHS.
the mixed surfactant-functionalized M-MWNTs exhibited a better dispersibility in water. Figure 3 also illustrates that the MSF-M-MWNTs exhibited a better dispersibility than single surfactant-functionalized M-MWNTs (SSF-M-MWNTs). A better dispersibility facilitates the adsorption of proteins on the functionalized M-MWNTs. 3.2. Conformational Change of the Assembled Mixed Surfactants. Figure 4 shows that the amount of mixed
Figure 2. FTIR spectra of the functionalized M-MWNTs with single and mixed surfactants.
two intense bands around 2917 and 2850 cm−l were assigned to asymmetric and symmetric stretching vibration of C−CH2 from the methylene chain, respectively. The band at 3360 cm−1 arises from the hydroxyl group from HDMA, and the peak at 1176 cm−1 is due to SO of SHS. It is noted that the two peaks are observed in the IR spectra for the mixed surfactant-functionalized M-MWNTs (MSF-M-MWNTs). With the weight ratio of HDMA to SHS decreasing, the intensity of the peak at 3360
Figure 4. Amount of surfactants adsorbed on M-MWNTs as a function of the weight ratio of HDMA to SHS. C
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Figure 5. FTIR spectra of the νOH region of HDMA of the functionalized M-MWNTs. (a) single HDMA; (b, c, and d) mixed surfactants with weight ratios of 1.1:1, 0.59:1, and 0.38:1, respectively.
surfactants assembled on CNTs is larger than that of single surfactants assembled on CNTs. The method for determining the amount of adsorbed surfactants has been described in detail in Experimental Section 2.3. The conjugate of M-MWNT with the surfactants was easily separated from the solution by utilizing a magnet (Figure S1b, Supporting Information); the separation was carried out without causing any loss of the surfactants remaining in the solution, exhibiting advantages over the methods such as filtration through membranes and precipitation by centrifugation. Thus, the amount of the surfactants remaining in the solution was determined on the basis of the standard curves with high precision. For measuring the amount of mixed surfactants adsorbed on the CNTs, average values were taken from triplicate measurements of three adsorption operations, and the relative deviations are less than 2.10%. The surfactant HDMA has a maltosyl group, and hydrogen bonding interactions between the maltosyl groups contribute to the assembly of HDMA.17 When HDMA assembles together with SHS on M-MWNTs, the hydrogen bonding interactions will be affected. The information on hydrogen bonding interactions is helpful to understand the conformational change
of mixed surfactants when adsorbed on M-MWNTs. Infrared spectroscopy has been proven to be a highly effective means of investigating specific interactions within and between molecules.31 FTIR has also been used to study the mechanism of intra- and intermolecular interactions through hydrogen bonding.32 The conformational change of the assembled mixed surfactants was analyzed by measuring FTIR spectra at the νOH band between 3000 and 3700 cm−1. The spectra in this region have been extensively used to analyze hydrogen bonding interactions.32−34 Figure 5 shows the FTIR spectra of the hydroxyl stretching region (3700 and 3000 cm−1), which is for spectral differences between the single surfactant HDMA and the mixed surfactant adsorbed on M-MWNTs. The peaks were fitted using Peakfit software (version 4.121). The intensities (3430−3470 cm −1 ), which are for multiple hydrogen bonds,32−34 are relatively decreased (Figure 5b−d). This means that the multiple hydrogen bonding interactions between HDMA molecules are decreased. While the intensities (3190−3240 cm−1), which are for intermolecular hydrogen bonds, are increased,32−34 indicating that the intermolecular hydrogen bonding interactions between the maltosyl group and the −SO3− group are increased, the results of FTIR spectra D
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3.3. Adsorption of Proteins on Functionalized MMWNTs. The TEM image in Figure 1c, AFM image in Figure S5c, Supporting Information, and XPS spectra in Figure S7, Supporting Information, confirmed the adsorption of proteins on the functionalized M-MWNTs. The method for determining the saturation adsorption of proteins onto the functionalized M-MWNTs has been described in detail in Experimental Section 2.4. The conjugate of M-MWNT with the protein was easily separated from the solution by utilizing a magnet (Figure S1c, Supporting Information); the separation was carried out without causing any loss of the protein remaining in the solution, exhibiting advantages over the methods such as filtration through membranes and precipitation by centrifugation. Thus, the amount of lysozyme remaining in the solution was determined through the BCA method with a high precision. For measuring the amount of mixed lysozyme adsorbed on the CNTs, average values were taken from triplicate measurements of three adsorption operations, and the relative changes in protein adsorption capacities are less than 2.0%. Figure 7 shows the results of lysozyme adsorption on the
indicate that the self-assembly of single surfactants was reduced and the interaction between the two surfactants was promoted. The synergistic effect of the two surfactants contributed to more mixed surfactants being assembled on CNTs (Figure 4). Molecular dynamics simulations provide powerful tools for the exploration of the conformational change of assembled molecules. Figure 6 shows the structures of the self-assembled
Figure 6. MD simulations of the CNTs with single and mixed surfactants. Top: SHS; middle: HDMA; bottom: mixed surfactant of HDMA/SHS. Left: side views; right: front views.
Figure 7. Amount of lysozyme adsorbed on the M-MWNTs functionalized with single and mixed surfactants.
functionalized M-MWNTs. It can be seen that more lysozyme was adsorbed on MSF-M-MWNTs than on SSF-M-MWNTs. This is due to two reasons. One is that more surfactants were adsorbed on MSF-M-MWNTs (Figure 4), providing more head groups which can interact with the protein through hydrogen bonding and electrostatic interactions. Another reason is that a more disordered structure of the mixed surfactant was formed on MSF-M-MWNTs as explained by Figure 5 and illustrated by Figure 6c. As most of the HDMA are separated by the SHS molecules, the interactions between the maltosyl groups of HDMA have been significantly reduced. This distribution of maltosyl groups and −SO3− groups facilitates the adsorption of more proteins on the functionalized M-MWNTs with multiple interactions. Except using the anionic surfactant SHS to prepare mixed surfactants, the cationic surfactant CTAB, together with HDMA, was used to functionalize M-MWNTs. Similar results were obtained. The FTIR spectra (Figure S8, Supporting Information) confirmed the functionalization by the mixed surfactants. Figure S8, Supporting Information, shows the FTIR spectra of functionalized M-MWNTs. The two intense bands around 2920 and 2850 cm−l were assigned to asymmetric and symmetric stretching vibration of C−CH2 from the methylene chain, respectively. The band at 3360 cm−1 arises from the
surfactants on the CNT based on molecular dynamics simulations. For the single surfactants (Figure 6a,b), the hydrophobic interactions between the hydrophobic chains and the wall of CNT are the driving force for the adsorption of surfactants. The assembly of surfactant molecules on the CNT was driven by chain−chain interactions. The tail groups of the two surfactants are close to the nanotube surface. Most of the head groups of SHS are nearly vertically oriented against the surface and extend to the outer aqueous phase (Figure 6a), while some of the maltosyl groups of HDMA are close to each other due to the hydrogen bonding interactions between the hydroxyl groups (Figure 6b). The front and side view in Figure 6a,b show that the single surfactants are orderly organized through aforementioned interactions. For the adsorption of the mixed surfactants on the CNT (Figure 6c), the hydrophobic interactions between the hydrophobic chains and the wall of CNT and the chain−chain interactions between the two surfactants are the driving force for their assembly. It is also noted that HDMA and SHS are assembled with a more disordered structure. In between the HDMA molecules, the SHS molecules exist; similarly, there are HDMA molecules in between the SHS molecules. As most of the HDMA are separated by the SHS molecules, the interactions between the maltosyl groups of HDMA have been significantly reduced. E
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hydroxyl group from HDMA, and the peak at 1155 cm−1 is due to C−O−C of HDMA. It is noted that the two peaks are observed in the IR spectra for MSF-M-MWNTs. With the weight ratio of HDMA to CTAB increasing, the intensities of the peak at 3360 cm−1 and the peak at 1155 cm−1 increase. Figure S9, Supporting Information, illustrates that the mixed surfactant-functionalized M-MWNTs exhibited a better dispersibility than single surfactant-functionalized M-MWNTs. A better dispersibility facilitates the adsorption of proteins on the functionalized M-MWNTs. Figure S10, Supporting Information, shows that more mixed surfactants were adsorbed on the CNTs compared to the single surfactants. For the mixed surfactants, the chain−chain and hydrophilic−ionic interactions lead to a synergistic effect on the adsorption of the two surfactants, resulting in more surfactants adsorbed on the CNTs. On the surface of the HDMA/CTAB functionalized MMWNTs, sugar head groups and cationic head groups can interact with proteins though hydrogen bonding and ionic interactions. The synergistic effect of the head groups led to more bovine serum albumin (BSA) being adsorbed, as illustrated in Figure S11, Supporting Information.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21376021, 21176025).
4. CONCLUSIONS Mixed surfactants consisting of a sugar-based surfactant and an ionic surfactant have been used to functionalize magnetic multiwalled carbon nanotubes. When assembling on the CNTs, the chain−chain and hydrogen bonding interactions lead to a synergistic effect on the adsorption of the two surfactants. This synergistic effect results in more surfactants adsorbed on the CNTs, providing more head groups available to interact with proteins. Another result that arises from the synergistic effect is that a more disordered structure is formed, reducing the selfassembly of single surfactants, and more maltosyl and ionic head groups are distributed randomly, facilitating the adsorption of proteins on the functionalized M-MWNTs with multiple interactions, including hydrogen bonding and electrostatic interactions. The CNTs functionalized with mixed surfactants HDMA/ SHS and HDMA/CTAB have been tested for the adsorption of two proteins, BSA and lysozyme. At the operating pH condition, BSA with a low isoelectric point is negatively charged and lysozyme with a high isoelectric point is positively charged. Due to the synergistic effect of HDMA and SHS or CTAB, the mixed surfactants functionalized CNTs adsorbed more proteins than the CNTs functionalized with single surfactants. This work presented the advantages of using mixed surfactants over single surfactants for CNT functionalization.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures for determination of the adsorption of surfactants on CNTs; calibration curves, AFM images, and UVvis, XPS, and FTIR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions ∥
REFERENCES
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dx.doi.org/10.1021/ie5015519 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
