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Preparation of Barbed ZnO Fibers and the Selective Adsorption Behavior for BSA

Preparation of Barbed ZnO Fibers and the Selective Adsorption Behavior for BSA http://pubs.acs.org/journal/acsodf Article Preparation of Barbed ZnO Fibers and the Selective Adsorption Behavior for BSA Liu Liu, Yuxiang Dai,* and Yang Qi* Cite This: ACS Omega 2021, 6, 16438−16445 Read Online Metrics & More Article Recommendations ACCESS ABSTRACT: ZnO electrospun nanofibers can act as seed fibers to fabricate multidentate barbed fibers perpendicular to the growth of the fibers using the chemical bath deposition (CBD) method. Fibers with a multirod morphology have a porous grid structure. The sample is easy to recover, and the nonpolar surface in the sample is sufficiently exposed. In the research of barbed fiber fabrication and adsorption on bovine serum albumin (BSA), the effects of different chemical bath conditions on the growth of ZnO nanorods were discussed. Barbed fibers with large slenderness ratios were obtained at a water content of 60 mL at 75 °C. Each milligram of barbed fibers can quickly adsorb about 162 μg of protein within 30 min. The adsorption activity of BSA between polar and nonpolar ZnO surfaces was also studied. The selective adsorption behavior of BSA on the nonpolar surface was revealed. al. demonstrated for the first time that engineered nanoscale 1. INTRODUCTION ZnO structures can serve as ideal substrates for identifying and Serum albumin in bovine serum (BSA) concentrate is a protein screening the protein−protein interaction. Xie et al. revealed that is commonly used in immunodiagnostic procedures, that the ZnO (10-10) surface showed 2 orders of magnitude clinical reagents, cell culture media, and protein chemistry in 1 higher amounts of surface-bound proteins relative to the biological laboratories. The adsorption of proteins on the (0001), (000-1), and (11-20) planes. Wang et al. also found surface of biological materials is an important step in the basic that compared with other morphologies, ZnO nanorods have biological process. The adsorbed protein will further induce 2,3 the best adsorption performance for BSA. subsequent cell landing, diffusion, and other effects. Thus, it Therefore, we have designed the ZnO material with a porous is necessary to investigate the adsorption activity of BSA. grid structure composed of ZnO fibers. The research work on With the development of biomedical research, the ZnO fiber materials has been widely reported. The application of nanomaterials in disease diagnosis, treatment, electrospun ZnO fibers belong to the polycrystalline wurtzite cell separation, drug carriers, and nanobiochips has attracted structure. However, the polycrystalline fiber mesh is not an increasing attention. In recent years, zinc oxide used for idealized adsorbent biomaterial. In this study, a novel ZnO protein adsorption devices has been used in a variety of barbed fiber material has been synthesized. Perpendicularly biomedical and pharmaceutical fields due to its good orientated nanorods grown on ZnO electrospun fibers by an 5 6,7 biocompatibility, chemical stability, and ease of preparation. eco-friendly process without involving any hazardous, toxic, or In the research field of adsorption of proteins on ZnO, powder highly corrosive chemical reagents. The morphology of ZnO 8,9 10 materials have been widely studied. Song et al. studied the nanorods was modulated to a certain extent. Barbed fibers not adsorption experiments of GST- and His-labeled recombinant only have multidirectional nanorods but also have better proteins on ZnO. The results showed that ZnO had specific recyclability. By observing the adsorption of BSA, crystal adsorption for new and formed proteins. To characterize the planes with better adsorption performance were summarized, adsorption mechanism between the protein and the material and the direction for further research was provided. surface, centrifugal treatment was needed to separate the powders from the protein solution. However, it is difficult to Received: March 17, 2021 recover the powder structure biomaterials with small particle Accepted: June 4, 2021 sizes, and there will be some loss in the recovery process. ZnO Published: June 18, 2021 grown on substrates in the study of protein adsorption has also 11,12 been widely reported. Limited by the morphology of the sample, the two-dimensional ZnO material grown on the substrate cannot have a high specific surface area. Dorfman et © 2021 The Authors. Published by https://doi.org/10.1021/acsomega.1c01454 American Chemical Society ACS Omega 2021, 6, 16438−16445 16438 ACS Omega http://pubs.acs.org/journal/acsodf Article 2. RESULTS AND DISCUSSION pictures correspond to PDF 36-1451. XRD patterns indicate that both seed fibers and barbed fibers are composed of the 2.1. Structural and Morphology Characterizations. wurtzite ZnO structure. In the chemical bath process, the Crystallographic properties of seed fibers and barbed fibers polycrystalline structure of seed fibers provides a large number samples are shown in Figure 1. The diffraction peaks in the of nucleation sites. Therefore, a great quantity of ZnO nanorods can be grown on the seed fibers with a small diameter. For crystalline ZnO, the surface energy of the polar surface is relatively low. This is the reason for the preferential orientation of ZnO crystal growth along the c-axis. The strongest (002) peak at 34.4° in the pattern of barbed fibers indicates that the hexagonal top surface of the nanorods is the polar surface. Because the nanorods grow perpendicularly to seed fibers, the size limitation of the fiber shape is eliminated, and the chemical bath solution has a high ion concentration. The diffraction pattern with observably smaller full width at half-maximum (FWHM) indicates that barbed fibers have a larger grain size than seed fibers. Figure 2a,b shows the morphology of the seed fibers and barbed fibers. The diameter of the seed fiber is about 80 nm. In the process of chemical bath, a great quantity of hexagonal ZnO nanorods grow from nucleation centers on the seed Figure 1. X-ray diffraction patterns of seed fibers and barbed fibers. fibers. After growing the nanorods, the diameter of the barbed fibers reaches a few microns. To reveal the effect of the diameter of seed fibers on the barbed fibers, seed fibers with Figure 2. Scanning electron microscopy (SEM) images of (a) seed fibers by electrospinning, (b) barbed fibers prepared after the chemical bath, and (c) diameter statistics of seed fibers. 16439 https://doi.org/10.1021/acsomega.1c01454 ACS Omega 2021, 6, 16438−16445 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 3. SEM images of barbed fibers in different chemical bath conditions. diameters of 87.6, 81.1, and 76.4 nm were obtained under higher ion concentration, which makes nanorods grow vigorously not only along the a/b-axis but also along the c-axis. different spinning voltages of 12.5, 15, and 17.5 kV, ZnO nanorods with c-axis-preferred orientation have higher respectively. Under the same chemical bath conditions, the surface energy on the nonpolar surface, which is beneficial to diameters of the barbed fibers are 4.17, 4.09, and 4.15 μm. The the adsorption process. To achieve more nonpolar surfaces, the above phenomenon shows that the diameter of seed fibers chemical bath conditions that are more conducive to growth within a certain diameter range has little effect on the along the c-axis are ideal. From Figure 4a, the image clearly morphology of the barbed fibers after the chemical bath. This illustrates that a larger diameter barbed fiber can always be is due to the fact that the size of ZnO nanorods grown on the obtained at 75 °C under each water content condition. This fibers is much larger than that of seed fibers. After chemical reveals that when seed fibers are chemically bathed at this bath deposition (CBD), the diameter of the fiber can reach temperature, the growth conditions of nanorods tend to be several microns, while the diameter of the seed fiber is less than more favorable for growth along the c-axis. When the degree of 100 nm. The diameter distribution of the seed fiber is shown in supersaturation of the chemical bath solution is low, ZnO is Figure 2c. Seed fibers with the polycrystalline structure provide more likely to be heterogeneously nucleated in the buffer nucleation centers at the initial stage of nanorod growth. The solution formed, so the nucleation rate and growth rate of ZnO subsequent growth mode of nanorods is mainly influenced by are slower. In addition, hexamethylenetetramine (HMTA) chemical bath conditions. 2+ forms an amine complex with Zn in the form of a bidentate Sequentially, the morphology of the samples obtained under ligand, and OH generated by slow hydrolysis of HMTA reacts different chemical bath conditions has been characterized. The 2+ with Zn ions to form Zn(OH) . Dehydration of Zn(OH) 2 2 samples of different chemical bath solutions (50, 55, 60, and 65 and hydrolysis of the amine complex form ZnO crystals. Since mL) were observed at bath temperatures of 70, 75, and 80 °C. the hydrolysis rate of the amine compound and HMTA is The diameters of the barbed fibers and nanorods are relatively slow, the morphology of the ZnO nanorods in the statistically obtained by Nano Measurer software. The length uniform and slow reaction is superior to the high concentration of nanorods is approximately equal to half the diameter of chemical bath solution with lower water content. It is obvious barbed fibers. For all samples in the control group, the in Figure 4b that the chemical bath solution with higher water morphologies are shown in Figure 3. Photographs show that content has a positive effect on the preparation of nanorods less water content in the chemical bath solution corresponds to with smaller diameters. 16440 https://doi.org/10.1021/acsomega.1c01454 ACS Omega 2021, 6, 16438−16445 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 4. Diameter distribution of (a) barbed fibers and (b) ZnO nanorods, and (c) slenderness ratio of ZnO nanorods. The slenderness ratio of nanorods was calculated by the statistical analysis of the diameter of barbed fibers and nanorods, as shown in Figure 4c. At the optimum temperature, samples with water contents of 60 and 65 mL in the bath solution have larger slenderness ratios. This achieves the goal of optimizing the preparation conditions because the larger slenderness ratio means that more nonpolar surfaces can be obtained. By comparing these two conditions, it is observed that the growth density of nanorods with a water content of 60 mL is higher. 2.2. BSA Adsorption Behavior Characterization. We Figure 5. Progress sketch of the BSA adsorbing experiment. prepared samples of certain mass for the characterization of protein adsorption properties. Figure 5 shows a schematic according to the standard curve based on the Lambert−Beer diagram of the experimental process for characterizing the law (Figure 6b). protein adsorption properties of the barbed fibers. After At 0.5 h, the light absorption of the protein dyeing solution removing the fibers, the protein concentration of the reached the lowest value, indicating that the protein adsorption unadsorbed BSA solution was used to characterize the capacity of nanorods reached the highest. According to the adsorption properties of the fibers. The micromorphology of calculation by formula (1),40 μg of BSA can be adsorbed protein-adsorbed barbed fibers was observed to discuss the stably per milligram barbed fibers. As the adsorption kinetics adsorption mechanism. curve shown in Figure 7, it takes about 2 h for the samples to The absorption spectra of the unadsorbed protein solution reach the equilibrium of adsorption and desorption, after are shown in Figure 6a. The light absorption of the unadsorbed which stable adsorption can be achieved. In this process, the BSA protein solution decreased at 595 nm, indicating that the maximum adsorption capacity of BSA is 162 μg/mg. protein content in the solution decreased. The protein The morphological characteristics of barbed ZnO fibers with concentration in the unadsorbed solution was calibrated BSA are helpful to study the selectivity of protein adsorption. 16441 https://doi.org/10.1021/acsomega.1c01454 ACS Omega 2021, 6, 16438−16445 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 6. Absorption spectra of the unadsorbed BSA solution (a) according to the standard curve (b). barbed fibers are positively charged because their pl value is larger than the pH value of the adsorption environment. This allows negatively charged proteins to form hydrogen bonds at the lattice oxygen on the nonpolar surfaces of nanorods. ZnO prepared by the CBD method has been widely 23−26 reported as a Zn polar surface. During the hydrothermal crystallization of ZnO, the growth of the [0001] direction has been determined to be the fastest under hydrothermal conditions. This is because the (0001) plane contains a corner of the Zn−O coordination tetrahedron, which can interact more strongly with the solvated Zn species introduced into the solution, while the (000-1) plane has a coordination tetrahedral face, so there are fewer binding sites. The growth habit of the ZnO crystal is similar to that of the idealized hydrothermal crystal model, and the fastest growth direction is along [0001]. This may indicate that the existence of [0001]-oriented crystals is determined by the fastest growing direction in hydrothermal conditions. Therefore, the polar Figure 7. Protein adsorption kinetics curves of BSA on ZnO Barbed surface of nanorods on barbed fibers is most likely to be the Fibers. termination surface of Zn. The hydroxide layer is composed of OH , which is difficult to adsorb proteins on it. In The adsorption forms of BAS on the surface are various. Kozak 17 addition to hydrogen bonding and electrostatic adsorption, et al. studied BSA adsorbed on the surface of hydrogenated Rezek et al. studied the adhesion of BSA to the ZnO surface by and oxidized diamond. For adsorbed BSA, the porous layer atomic force microscopy and atomic-scale computing by the morphology and the small spherical nanoparticle morphology force-field method. The AFM observations were corroborated were observed by atomic force microscopy (AFM). For the by atomic-scale simulations of BSA on the (0001) ZnO surface counted BSA clusters, Wang et al. revealed that the size of using the force-field method and showing rearrangements of BSA was 7.2 ± 0.2 nm, and it was inferred that the white Zn surface atoms. protein substance adsorbed on the barbed fibers should be a Table 1 shows the adsorption properties of BSA on ZnO single or several protein clusters. Through the SEM photos of a nanomaterials with different morphologies. Except for ZnO working voltage of 5 kV, Figure 8 can intuitively show that BSA hollow spheres (ZnO HSs), barded fibers have better is mostly adsorbed on the nonpolar surface of nanorods. adsorption properties compared with powder morphology According to the statistics of the number of BSA clusters in the materials. It is noteworthy that the maximum adsorption low-magnification SEM photos, the amount of BSA clusters −5 capacity of barbed fibers in the adsorption process is higher adsorbed by the polar surface of ZnO nanorods is 2.9 × 10 / than that of other types of nanomaterials. This phenomenon nm , and the amount of BSA clusters adsorbed on the −4 2 may be explained by the morphology of the barbed fibers. nonpolar surface is 1.31 × 10 /nm . This confirms that by Compared with nanoarrays, thin films, and randomly oriented controlling the conditions of the chemical bath, the powder materials, the nonpolar surface with adsorption preparation of barbed fibers with a larger slenderness ratio of advantages is more fully exposed. This allows the barbed nanorods has achieved the purpose of facilitating protein fibers to quickly adsorb a large amount of BSA. Hansda et al. adsorption. confirmed that the alternate multilayer growth of the ZnO/ The adsorption mechanism of BSA on ZnO is considered to 19 20 BSA layer-by-layer film could be deposited via the electrostatic be hydrogen bond adsorption and electrostatic adsorption. interactions, and the BSA molecular domains adsorbed on the When the pH value of the adsorption environment is 7.4, the electronegativity of the barbed fibers and BSA proteins is ZnO surface may have the self-association effect. Therefore, we opposite due to the different isoelectric points (pl). ZnO speculate that due to the bulk effect of a large amount of BSA, 16442 https://doi.org/10.1021/acsomega.1c01454 ACS Omega 2021, 6, 16438−16445 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 8. SEM images of (a) barbed fibers with BSA adsorbed, (b) polar planes, and (c) nonpolar planes. adsorption capacity for the BSA protein. The maximum Table 1. Adsorption Performance of BSA by ZnO with adsorption capacity of BSA on barbed fibers is 162 μg/mg and Different Morphologies finally stabilized at 40 μg/mg. material Q (μg/mg) Q (μg/mg) max ad ZnO hollow sphere 80 4. EXPERIMENTS AND CHARACTERIZATION ZnO nanorods 40 38 4.1. Chemicals. Poly(vinyl alcohol) (PVA, 1750 ± 50; M = ZnO nanosheets 48 39 80000), zinc acetate (Zn(CH COO) ·2H O), hexamethylene- ZnO nanobeams 40 30 3 2 2 tetramine (HMTA), Coomassie Brilliant Blue G-250, phos- this work 162 40 phoric acid (H PO ), BSA protein powder, and phosphate- 3 4 buffered saline (PBS; pH 7.4; Na HPO ,KH PO , NaCl, KCl) 2 4 2 4 the protein on the ZnO surface may be desorbed in avalanche were purchased from Macklin Reagent. All chemicals were of style. This provides a feasible condition for the research and analytical grade and required no further purification, and all development of new applications in the biomedical field. In aqueous solutions were prepared with distilled water. addition, the doping of transition metal ions or irradiation 4.2. Preparation of Seed Fibers. In the course of the modification to the barbed fibers will also be a topic for further experiment, the precursor sol for electrospinning was prepared. in-depth research. PVA was used as a template for the as-spun precursor fibers before sintering. The detailed preparation process of the 3. CONCLUSIONS precursor sol was as follows. An 8 wt % PVA aqueous solution has been obtained by dissolving the swelled PVA aqueous C-axis-oriented ZnO nanorods were grown on electrospun solution at 93 °C for 5 h. After that, 10 mL of a 23% nanofibers by chemical bath deposition. A composite barbed Zn(CH COO) ·2H O aqueous solution was added into 50 g fiber with a grid structure was obtained. Barbed fibers are 3 2 2 easier to recycle than powder materials. Compared with of the PVA aqueous solution at a slow dropping rate. The nanorod arrays, the (100) planes are more sufficiently exposed precursor solution was stirred at 60 °C for at least 2 h. Finally, in the protein solution environment and can grow into a the whole precursor solution was aged for 24 h at room multilayer grid structure. To prepare more nonpolar planes, the temperature. length and diameter of nanorods were controlled by The precursor fluid was electrospun into a fiber shape modulating chemical bath conditions. The optimum chemical through a lab-made electrospinning system. In the course of bath conditions were a water content of 60 mL at 75 °C. The the electrospinning process, the deposition distance between results of adsorption experiments show that BSA has the the collecting plate (connecting ground) and the syringe selective adsorption behavior on the nonpolar surface due to needle (high voltage terminal) was kept at 10 cm. The needle the different surface suspension bonds between the nonpolar type and the environmental conditions (temperature and surface and the Zn-termination polar surface. Therefore, the humidity) were kept constant. According to our previous (100) planes of the ZnO nanorods have the optimum work, 17.5 kV was the optimal spinning voltage among the 16443 https://doi.org/10.1021/acsomega.1c01454 ACS Omega 2021, 6, 16438−16445 ACS Omega http://pubs.acs.org/journal/acsodf Article voltage gradient (7.5−20 kV) in the experiment. After AUTHOR INFORMATION obtaining the PVA and Zn(CH COO) as-spun fibers under 3 2 Corresponding Authors optimized voltage with a 200 μL/h dosing rate, the precursor Yuxiang Dai − Institute of Materials Physics and Chemistry, fibrous sample was subjected to the heat treatment process to School of Materials Science and Engineering, Northeastern sinter the ZnO crystals and eliminate the organic polymer. To University, Shenyang, Liaoning 110819, China; remove the polymer template, the as-spun fibers were sintered orcid.org/0000-0003-4267-5885; Phone: +86- in a well furnace at 447 °C for 5 h. After the furnace cooled, 15943015856; Email: daiyuxiang@mail.neu.edu.cn ZnO polycrystalline seed fibers were prepared. Yang Qi − Institute of Materials Physics and Chemistry, School 4.3. Preparation of Barbed Fibers. The obtained seed of Materials Science and Engineering, Northeastern fibers were subjected to a chemical bath under certain University, Shenyang, Liaoning 110819, China; Key conditions. The solution used for CBD was prepared as Laboratory for Anisotropy and Texture of Materials, follows: to make the solution containing the same molar mass Northeastern University, Shenyang, Liaoning 110819, of zinc ions and hydroxyl ions, we dissolved 0.2744 g of China; orcid.org/0000-0003-1915-474X; Phone: +86- Zn(CH COO) and 0.1752 g of HMTA in a certain amount of 3 2 24-83691993; Email: qiyang@imp.neu.edu.cn deionized water. The bath solution was obtained without Author stirring at room temperature. We immersed the sample upward in the chemical bath for 4 h at different temperatures by a Liu Liu − Institute of Materials Physics and Chemistry, School water bath pot with a condensation tube. In the process of of Materials Science and Engineering, Northeastern preparing barbed fibers by the CBD method, zinc acetate University, Shenyang, Liaoning 110819, China dihydrate and HMTA were added to water for reaction. Complete contact information is available at: HMTA was used as buffer and reactant here because of its slow https://pubs.acs.org/10.1021/acsomega.1c01454 and continuous hydrolysis in aqueous solution. This provided the suitable pH range and sufficient OH for ZnO deposition so Notes that the reaction proceeded gently and smoothly. The whole The authors declare no competing financial interest. reaction process is as follows (CH ) N+↔ 6H O 4NH+ 6HCHO 26 4 2 3 ACKNOWLEDGMENTS NH+↔ H O NH·H O 32 3 2 This work was supported by the Fundamental Research Funds + − NH·↔ H O NH + OH 32 4 for the Central Universities (No. N170203007), the China 2+− Zn +↔ 2OH Zn(OH) 2 Postdoctoral Science Foundation (No. 2018M631801), and Zn(OH) ↔+ ZnO H O the Postdoctoral Foundation of Northeastern University (No. 20180301). 4.4. Protein Adsorption Quantity Measurement. To characterize the adsorption properties of BSA on nanorods, static adsorption experiments were carried out. 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C 2012, 116, 456−460. (26) Goswami, D. Y.; Stefanakos, E.; Batzill, M.; Kislov, N.; Lahiri, J.; Verma, H. Photocatalytic degradation of methyl orange over single crystalline ZnO: orientation dependence of photoactivity and photostability of ZnO. Langmuir 2009, 25, 3310−3315. (27) Li, W. J.; Shi, E. W.; Zhong, W. Z.; Yin, Z. W. Growth mechanism and growth habit of oxide crystals. J. Cryst. Growth 1999, 203, 186−196. (28) Laudise, R. A.; Ballman, A. A. Hydrothermal synthesis of zinc oxide and sulfide. J. Phys. Chem. A 1960, 64, 688−691. (29) Valtiner, M.; Borodin, S.; Grundmeier, G. Stabilization and acidic dissolution mechanism of single-crystalline ZnO(0001) surfaces 16445 https://doi.org/10.1021/acsomega.1c01454 ACS Omega 2021, 6, 16438−16445 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png ACS Omega Pubmed Central

Preparation of Barbed ZnO Fibers and the Selective Adsorption Behavior for BSA

ACS Omega , Volume 6 (25) – Jun 18, 2021

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http://pubs.acs.org/journal/acsodf Article Preparation of Barbed ZnO Fibers and the Selective Adsorption Behavior for BSA Liu Liu, Yuxiang Dai,* and Yang Qi* Cite This: ACS Omega 2021, 6, 16438−16445 Read Online Metrics & More Article Recommendations ACCESS ABSTRACT: ZnO electrospun nanofibers can act as seed fibers to fabricate multidentate barbed fibers perpendicular to the growth of the fibers using the chemical bath deposition (CBD) method. Fibers with a multirod morphology have a porous grid structure. The sample is easy to recover, and the nonpolar surface in the sample is sufficiently exposed. In the research of barbed fiber fabrication and adsorption on bovine serum albumin (BSA), the effects of different chemical bath conditions on the growth of ZnO nanorods were discussed. Barbed fibers with large slenderness ratios were obtained at a water content of 60 mL at 75 °C. Each milligram of barbed fibers can quickly adsorb about 162 μg of protein within 30 min. The adsorption activity of BSA between polar and nonpolar ZnO surfaces was also studied. The selective adsorption behavior of BSA on the nonpolar surface was revealed. al. demonstrated for the first time that engineered nanoscale 1. INTRODUCTION ZnO structures can serve as ideal substrates for identifying and Serum albumin in bovine serum (BSA) concentrate is a protein screening the protein−protein interaction. Xie et al. revealed that is commonly used in immunodiagnostic procedures, that the ZnO (10-10) surface showed 2 orders of magnitude clinical reagents, cell culture media, and protein chemistry in 1 higher amounts of surface-bound proteins relative to the biological laboratories. The adsorption of proteins on the (0001), (000-1), and (11-20) planes. Wang et al. also found surface of biological materials is an important step in the basic that compared with other morphologies, ZnO nanorods have biological process. The adsorbed protein will further induce 2,3 the best adsorption performance for BSA. subsequent cell landing, diffusion, and other effects. Thus, it Therefore, we have designed the ZnO material with a porous is necessary to investigate the adsorption activity of BSA. grid structure composed of ZnO fibers. The research work on With the development of biomedical research, the ZnO fiber materials has been widely reported. The application of nanomaterials in disease diagnosis, treatment, electrospun ZnO fibers belong to the polycrystalline wurtzite cell separation, drug carriers, and nanobiochips has attracted structure. However, the polycrystalline fiber mesh is not an increasing attention. In recent years, zinc oxide used for idealized adsorbent biomaterial. In this study, a novel ZnO protein adsorption devices has been used in a variety of barbed fiber material has been synthesized. Perpendicularly biomedical and pharmaceutical fields due to its good orientated nanorods grown on ZnO electrospun fibers by an 5 6,7 biocompatibility, chemical stability, and ease of preparation. eco-friendly process without involving any hazardous, toxic, or In the research field of adsorption of proteins on ZnO, powder highly corrosive chemical reagents. The morphology of ZnO 8,9 10 materials have been widely studied. Song et al. studied the nanorods was modulated to a certain extent. Barbed fibers not adsorption experiments of GST- and His-labeled recombinant only have multidirectional nanorods but also have better proteins on ZnO. The results showed that ZnO had specific recyclability. By observing the adsorption of BSA, crystal adsorption for new and formed proteins. To characterize the planes with better adsorption performance were summarized, adsorption mechanism between the protein and the material and the direction for further research was provided. surface, centrifugal treatment was needed to separate the powders from the protein solution. However, it is difficult to Received: March 17, 2021 recover the powder structure biomaterials with small particle Accepted: June 4, 2021 sizes, and there will be some loss in the recovery process. ZnO Published: June 18, 2021 grown on substrates in the study of protein adsorption has also 11,12 been widely reported. Limited by the morphology of the sample, the two-dimensional ZnO material grown on the substrate cannot have a high specific surface area. Dorfman et © 2021 The Authors. Published by https://doi.org/10.1021/acsomega.1c01454 American Chemical Society ACS Omega 2021, 6, 16438−16445 16438 ACS Omega http://pubs.acs.org/journal/acsodf Article 2. RESULTS AND DISCUSSION pictures correspond to PDF 36-1451. XRD patterns indicate that both seed fibers and barbed fibers are composed of the 2.1. Structural and Morphology Characterizations. wurtzite ZnO structure. In the chemical bath process, the Crystallographic properties of seed fibers and barbed fibers polycrystalline structure of seed fibers provides a large number samples are shown in Figure 1. The diffraction peaks in the of nucleation sites. Therefore, a great quantity of ZnO nanorods can be grown on the seed fibers with a small diameter. For crystalline ZnO, the surface energy of the polar surface is relatively low. This is the reason for the preferential orientation of ZnO crystal growth along the c-axis. The strongest (002) peak at 34.4° in the pattern of barbed fibers indicates that the hexagonal top surface of the nanorods is the polar surface. Because the nanorods grow perpendicularly to seed fibers, the size limitation of the fiber shape is eliminated, and the chemical bath solution has a high ion concentration. The diffraction pattern with observably smaller full width at half-maximum (FWHM) indicates that barbed fibers have a larger grain size than seed fibers. Figure 2a,b shows the morphology of the seed fibers and barbed fibers. The diameter of the seed fiber is about 80 nm. In the process of chemical bath, a great quantity of hexagonal ZnO nanorods grow from nucleation centers on the seed Figure 1. X-ray diffraction patterns of seed fibers and barbed fibers. fibers. After growing the nanorods, the diameter of the barbed fibers reaches a few microns. To reveal the effect of the diameter of seed fibers on the barbed fibers, seed fibers with Figure 2. Scanning electron microscopy (SEM) images of (a) seed fibers by electrospinning, (b) barbed fibers prepared after the chemical bath, and (c) diameter statistics of seed fibers. 16439 https://doi.org/10.1021/acsomega.1c01454 ACS Omega 2021, 6, 16438−16445 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 3. SEM images of barbed fibers in different chemical bath conditions. diameters of 87.6, 81.1, and 76.4 nm were obtained under higher ion concentration, which makes nanorods grow vigorously not only along the a/b-axis but also along the c-axis. different spinning voltages of 12.5, 15, and 17.5 kV, ZnO nanorods with c-axis-preferred orientation have higher respectively. Under the same chemical bath conditions, the surface energy on the nonpolar surface, which is beneficial to diameters of the barbed fibers are 4.17, 4.09, and 4.15 μm. The the adsorption process. To achieve more nonpolar surfaces, the above phenomenon shows that the diameter of seed fibers chemical bath conditions that are more conducive to growth within a certain diameter range has little effect on the along the c-axis are ideal. From Figure 4a, the image clearly morphology of the barbed fibers after the chemical bath. This illustrates that a larger diameter barbed fiber can always be is due to the fact that the size of ZnO nanorods grown on the obtained at 75 °C under each water content condition. This fibers is much larger than that of seed fibers. After chemical reveals that when seed fibers are chemically bathed at this bath deposition (CBD), the diameter of the fiber can reach temperature, the growth conditions of nanorods tend to be several microns, while the diameter of the seed fiber is less than more favorable for growth along the c-axis. When the degree of 100 nm. The diameter distribution of the seed fiber is shown in supersaturation of the chemical bath solution is low, ZnO is Figure 2c. Seed fibers with the polycrystalline structure provide more likely to be heterogeneously nucleated in the buffer nucleation centers at the initial stage of nanorod growth. The solution formed, so the nucleation rate and growth rate of ZnO subsequent growth mode of nanorods is mainly influenced by are slower. In addition, hexamethylenetetramine (HMTA) chemical bath conditions. 2+ forms an amine complex with Zn in the form of a bidentate Sequentially, the morphology of the samples obtained under ligand, and OH generated by slow hydrolysis of HMTA reacts different chemical bath conditions has been characterized. The 2+ with Zn ions to form Zn(OH) . Dehydration of Zn(OH) 2 2 samples of different chemical bath solutions (50, 55, 60, and 65 and hydrolysis of the amine complex form ZnO crystals. Since mL) were observed at bath temperatures of 70, 75, and 80 °C. the hydrolysis rate of the amine compound and HMTA is The diameters of the barbed fibers and nanorods are relatively slow, the morphology of the ZnO nanorods in the statistically obtained by Nano Measurer software. The length uniform and slow reaction is superior to the high concentration of nanorods is approximately equal to half the diameter of chemical bath solution with lower water content. It is obvious barbed fibers. For all samples in the control group, the in Figure 4b that the chemical bath solution with higher water morphologies are shown in Figure 3. Photographs show that content has a positive effect on the preparation of nanorods less water content in the chemical bath solution corresponds to with smaller diameters. 16440 https://doi.org/10.1021/acsomega.1c01454 ACS Omega 2021, 6, 16438−16445 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 4. Diameter distribution of (a) barbed fibers and (b) ZnO nanorods, and (c) slenderness ratio of ZnO nanorods. The slenderness ratio of nanorods was calculated by the statistical analysis of the diameter of barbed fibers and nanorods, as shown in Figure 4c. At the optimum temperature, samples with water contents of 60 and 65 mL in the bath solution have larger slenderness ratios. This achieves the goal of optimizing the preparation conditions because the larger slenderness ratio means that more nonpolar surfaces can be obtained. By comparing these two conditions, it is observed that the growth density of nanorods with a water content of 60 mL is higher. 2.2. BSA Adsorption Behavior Characterization. We Figure 5. Progress sketch of the BSA adsorbing experiment. prepared samples of certain mass for the characterization of protein adsorption properties. Figure 5 shows a schematic according to the standard curve based on the Lambert−Beer diagram of the experimental process for characterizing the law (Figure 6b). protein adsorption properties of the barbed fibers. After At 0.5 h, the light absorption of the protein dyeing solution removing the fibers, the protein concentration of the reached the lowest value, indicating that the protein adsorption unadsorbed BSA solution was used to characterize the capacity of nanorods reached the highest. According to the adsorption properties of the fibers. The micromorphology of calculation by formula (1),40 μg of BSA can be adsorbed protein-adsorbed barbed fibers was observed to discuss the stably per milligram barbed fibers. As the adsorption kinetics adsorption mechanism. curve shown in Figure 7, it takes about 2 h for the samples to The absorption spectra of the unadsorbed protein solution reach the equilibrium of adsorption and desorption, after are shown in Figure 6a. The light absorption of the unadsorbed which stable adsorption can be achieved. In this process, the BSA protein solution decreased at 595 nm, indicating that the maximum adsorption capacity of BSA is 162 μg/mg. protein content in the solution decreased. The protein The morphological characteristics of barbed ZnO fibers with concentration in the unadsorbed solution was calibrated BSA are helpful to study the selectivity of protein adsorption. 16441 https://doi.org/10.1021/acsomega.1c01454 ACS Omega 2021, 6, 16438−16445 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 6. Absorption spectra of the unadsorbed BSA solution (a) according to the standard curve (b). barbed fibers are positively charged because their pl value is larger than the pH value of the adsorption environment. This allows negatively charged proteins to form hydrogen bonds at the lattice oxygen on the nonpolar surfaces of nanorods. ZnO prepared by the CBD method has been widely 23−26 reported as a Zn polar surface. During the hydrothermal crystallization of ZnO, the growth of the [0001] direction has been determined to be the fastest under hydrothermal conditions. This is because the (0001) plane contains a corner of the Zn−O coordination tetrahedron, which can interact more strongly with the solvated Zn species introduced into the solution, while the (000-1) plane has a coordination tetrahedral face, so there are fewer binding sites. The growth habit of the ZnO crystal is similar to that of the idealized hydrothermal crystal model, and the fastest growth direction is along [0001]. This may indicate that the existence of [0001]-oriented crystals is determined by the fastest growing direction in hydrothermal conditions. Therefore, the polar Figure 7. Protein adsorption kinetics curves of BSA on ZnO Barbed surface of nanorods on barbed fibers is most likely to be the Fibers. termination surface of Zn. The hydroxide layer is composed of OH , which is difficult to adsorb proteins on it. In The adsorption forms of BAS on the surface are various. Kozak 17 addition to hydrogen bonding and electrostatic adsorption, et al. studied BSA adsorbed on the surface of hydrogenated Rezek et al. studied the adhesion of BSA to the ZnO surface by and oxidized diamond. For adsorbed BSA, the porous layer atomic force microscopy and atomic-scale computing by the morphology and the small spherical nanoparticle morphology force-field method. The AFM observations were corroborated were observed by atomic force microscopy (AFM). For the by atomic-scale simulations of BSA on the (0001) ZnO surface counted BSA clusters, Wang et al. revealed that the size of using the force-field method and showing rearrangements of BSA was 7.2 ± 0.2 nm, and it was inferred that the white Zn surface atoms. protein substance adsorbed on the barbed fibers should be a Table 1 shows the adsorption properties of BSA on ZnO single or several protein clusters. Through the SEM photos of a nanomaterials with different morphologies. Except for ZnO working voltage of 5 kV, Figure 8 can intuitively show that BSA hollow spheres (ZnO HSs), barded fibers have better is mostly adsorbed on the nonpolar surface of nanorods. adsorption properties compared with powder morphology According to the statistics of the number of BSA clusters in the materials. It is noteworthy that the maximum adsorption low-magnification SEM photos, the amount of BSA clusters −5 capacity of barbed fibers in the adsorption process is higher adsorbed by the polar surface of ZnO nanorods is 2.9 × 10 / than that of other types of nanomaterials. This phenomenon nm , and the amount of BSA clusters adsorbed on the −4 2 may be explained by the morphology of the barbed fibers. nonpolar surface is 1.31 × 10 /nm . This confirms that by Compared with nanoarrays, thin films, and randomly oriented controlling the conditions of the chemical bath, the powder materials, the nonpolar surface with adsorption preparation of barbed fibers with a larger slenderness ratio of advantages is more fully exposed. This allows the barbed nanorods has achieved the purpose of facilitating protein fibers to quickly adsorb a large amount of BSA. Hansda et al. adsorption. confirmed that the alternate multilayer growth of the ZnO/ The adsorption mechanism of BSA on ZnO is considered to 19 20 BSA layer-by-layer film could be deposited via the electrostatic be hydrogen bond adsorption and electrostatic adsorption. interactions, and the BSA molecular domains adsorbed on the When the pH value of the adsorption environment is 7.4, the electronegativity of the barbed fibers and BSA proteins is ZnO surface may have the self-association effect. Therefore, we opposite due to the different isoelectric points (pl). ZnO speculate that due to the bulk effect of a large amount of BSA, 16442 https://doi.org/10.1021/acsomega.1c01454 ACS Omega 2021, 6, 16438−16445 ACS Omega http://pubs.acs.org/journal/acsodf Article Figure 8. SEM images of (a) barbed fibers with BSA adsorbed, (b) polar planes, and (c) nonpolar planes. adsorption capacity for the BSA protein. The maximum Table 1. Adsorption Performance of BSA by ZnO with adsorption capacity of BSA on barbed fibers is 162 μg/mg and Different Morphologies finally stabilized at 40 μg/mg. material Q (μg/mg) Q (μg/mg) max ad ZnO hollow sphere 80 4. EXPERIMENTS AND CHARACTERIZATION ZnO nanorods 40 38 4.1. Chemicals. Poly(vinyl alcohol) (PVA, 1750 ± 50; M = ZnO nanosheets 48 39 80000), zinc acetate (Zn(CH COO) ·2H O), hexamethylene- ZnO nanobeams 40 30 3 2 2 tetramine (HMTA), Coomassie Brilliant Blue G-250, phos- this work 162 40 phoric acid (H PO ), BSA protein powder, and phosphate- 3 4 buffered saline (PBS; pH 7.4; Na HPO ,KH PO , NaCl, KCl) 2 4 2 4 the protein on the ZnO surface may be desorbed in avalanche were purchased from Macklin Reagent. All chemicals were of style. This provides a feasible condition for the research and analytical grade and required no further purification, and all development of new applications in the biomedical field. In aqueous solutions were prepared with distilled water. addition, the doping of transition metal ions or irradiation 4.2. Preparation of Seed Fibers. In the course of the modification to the barbed fibers will also be a topic for further experiment, the precursor sol for electrospinning was prepared. in-depth research. PVA was used as a template for the as-spun precursor fibers before sintering. The detailed preparation process of the 3. CONCLUSIONS precursor sol was as follows. An 8 wt % PVA aqueous solution has been obtained by dissolving the swelled PVA aqueous C-axis-oriented ZnO nanorods were grown on electrospun solution at 93 °C for 5 h. After that, 10 mL of a 23% nanofibers by chemical bath deposition. A composite barbed Zn(CH COO) ·2H O aqueous solution was added into 50 g fiber with a grid structure was obtained. Barbed fibers are 3 2 2 easier to recycle than powder materials. Compared with of the PVA aqueous solution at a slow dropping rate. The nanorod arrays, the (100) planes are more sufficiently exposed precursor solution was stirred at 60 °C for at least 2 h. Finally, in the protein solution environment and can grow into a the whole precursor solution was aged for 24 h at room multilayer grid structure. To prepare more nonpolar planes, the temperature. length and diameter of nanorods were controlled by The precursor fluid was electrospun into a fiber shape modulating chemical bath conditions. The optimum chemical through a lab-made electrospinning system. In the course of bath conditions were a water content of 60 mL at 75 °C. The the electrospinning process, the deposition distance between results of adsorption experiments show that BSA has the the collecting plate (connecting ground) and the syringe selective adsorption behavior on the nonpolar surface due to needle (high voltage terminal) was kept at 10 cm. The needle the different surface suspension bonds between the nonpolar type and the environmental conditions (temperature and surface and the Zn-termination polar surface. Therefore, the humidity) were kept constant. According to our previous (100) planes of the ZnO nanorods have the optimum work, 17.5 kV was the optimal spinning voltage among the 16443 https://doi.org/10.1021/acsomega.1c01454 ACS Omega 2021, 6, 16438−16445 ACS Omega http://pubs.acs.org/journal/acsodf Article voltage gradient (7.5−20 kV) in the experiment. After AUTHOR INFORMATION obtaining the PVA and Zn(CH COO) as-spun fibers under 3 2 Corresponding Authors optimized voltage with a 200 μL/h dosing rate, the precursor Yuxiang Dai − Institute of Materials Physics and Chemistry, fibrous sample was subjected to the heat treatment process to School of Materials Science and Engineering, Northeastern sinter the ZnO crystals and eliminate the organic polymer. To University, Shenyang, Liaoning 110819, China; remove the polymer template, the as-spun fibers were sintered orcid.org/0000-0003-4267-5885; Phone: +86- in a well furnace at 447 °C for 5 h. After the furnace cooled, 15943015856; Email: daiyuxiang@mail.neu.edu.cn ZnO polycrystalline seed fibers were prepared. Yang Qi − Institute of Materials Physics and Chemistry, School 4.3. Preparation of Barbed Fibers. The obtained seed of Materials Science and Engineering, Northeastern fibers were subjected to a chemical bath under certain University, Shenyang, Liaoning 110819, China; Key conditions. The solution used for CBD was prepared as Laboratory for Anisotropy and Texture of Materials, follows: to make the solution containing the same molar mass Northeastern University, Shenyang, Liaoning 110819, of zinc ions and hydroxyl ions, we dissolved 0.2744 g of China; orcid.org/0000-0003-1915-474X; Phone: +86- Zn(CH COO) and 0.1752 g of HMTA in a certain amount of 3 2 24-83691993; Email: qiyang@imp.neu.edu.cn deionized water. The bath solution was obtained without Author stirring at room temperature. We immersed the sample upward in the chemical bath for 4 h at different temperatures by a Liu Liu − Institute of Materials Physics and Chemistry, School water bath pot with a condensation tube. In the process of of Materials Science and Engineering, Northeastern preparing barbed fibers by the CBD method, zinc acetate University, Shenyang, Liaoning 110819, China dihydrate and HMTA were added to water for reaction. Complete contact information is available at: HMTA was used as buffer and reactant here because of its slow https://pubs.acs.org/10.1021/acsomega.1c01454 and continuous hydrolysis in aqueous solution. This provided the suitable pH range and sufficient OH for ZnO deposition so Notes that the reaction proceeded gently and smoothly. The whole The authors declare no competing financial interest. reaction process is as follows (CH ) N+↔ 6H O 4NH+ 6HCHO 26 4 2 3 ACKNOWLEDGMENTS NH+↔ H O NH·H O 32 3 2 This work was supported by the Fundamental Research Funds + − NH·↔ H O NH + OH 32 4 for the Central Universities (No. N170203007), the China 2+− Zn +↔ 2OH Zn(OH) 2 Postdoctoral Science Foundation (No. 2018M631801), and Zn(OH) ↔+ ZnO H O the Postdoctoral Foundation of Northeastern University (No. 20180301). 4.4. Protein Adsorption Quantity Measurement. To characterize the adsorption properties of BSA on nanorods, static adsorption experiments were carried out. 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ACS OmegaPubmed Central

Published: Jun 18, 2021

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