Evaluation of Single Hydrogel Nanofiber Mechanics Using

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Article Cite This: ACS Omega 2018, 3, 18304−18310

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Evaluation of Single Hydrogel Nanofiber Mechanics Using Persistence Length Analysis Angie M. Diaz,† Zeyang Zhang,†,‡ Briana Lee,† Felix M. Hernandez Luna,†,§ Yuen Yee Li Sip,† Xiaoyan Lu,†,‡ James Heidings,† Laurene Tetard,†,∥ Lei Zhai,*,†,‡ and Hyeran Kang*,†,∥ †

NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826, United States Department of Chemistry and ∥Department of Physics, University of Central Florida, Orlando, Florida 32816, United States § Department of Mechanical Engineering, Inter American University of Puerto Rico, Bayamon, Puerto Rico 00957, United States Downloaded via 185.46.87.233 on January 17, 2019 at 17:45:22 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Polyelectrolyte hydrogel fibers can mimic the extracellular matrix and be used for tissue scaffolding. Mechanical properties of polyelectrolyte nanofibers are crucial in manipulating cell behavior, which metal ions have been found to enable tuning. While metal ions play an important role in manipulating the mechanical properties of the fibers, evaluating the mechanical properties of a single hydrated hydrogel fiber remains a challenging task and a more detailed understanding of how ions modulate the mechanical properties of individual polyelectrolyte polymers is still lacking. In this study, dark-field microscopy and persistence length analysis help directly evaluate fiber mechanics using electrospun fibers of poly(acrylic acid) (PAA), chitosan (CS), and ferric ions as a model system. By comparing the persistence length and estimated Young’s modulus of different nanofibers, we demonstrate that persistence length analysis is a viable approach to evaluate mechanical properties of hydrated fibers. Ferric ions were found to create shorter and stiffer nanofibers, with Young’s modulus estimated at a few kilopascals. Ferric ions, at low concentration, reduce the Young’s modulus of PAA and PAA/CS fibers through the interaction between ferric ions and carboxylate groups. Such interaction was further supported by nanoscale infrared spectroscopy studies of PAA and PAA/CS fibers with different concentrations of ferric ions.

1. INTRODUCTION Polyelectrolyte nanofibers produced through electrospinning have been extensively investigated as promising candidates to mimic the fibrous network of extracellular matrix (ECM).1−10 These nanofibers provide cross-linked networks to support cells in differing environments (e.g. pH, temperature, and ionic strength).11−14 The mechanical properties of polyelectrolyte nanofibers and spatial arrangements of the fibrous polymer networks play an important role in manipulating cell behavior, as they can affect cell attachment and spatial distribution.15−17 While much effort has been devoted to investigating the relationship between cell behavior and the mechanical properties of polyelectrolyte nanofiber networks, measuring and controlling the mechanical properties of a single hydrated nanofiber remains an arduous task. Mats of electrospun fibers are commonly used to get an average Young’s modulus and tensile strength. On the other hand, the mechanical properties of individual nanofibers have been probed using a combination of atomic force microscopy (AFM) and optical microscopy techniques.18 Namely, the AFM tip was used to apply pressure on the fiber anchored on the microridges of an optical adhesive substrate. This method makes it possible to determine the viscoelastic properties of single electrospun fibers but requires a tedious experimental setup. Furthermore, the existing literature indicates that studies mostly focus on dry fibers, © 2018 American Chemical Society

despite the fact that the mechanical properties of hydrated fibers would be more in line with the interest in understanding the fibrous network in ECM, which is composed of hydrated polyelectrolyte fibers. Therefore, a simple and versatile approach to measure the mechanical properties of hydrated polyelectrolyte fibers is of prime interest to broaden the applications of electrospun polyelectrolyte fibers in tissue engineering. An indirect, yet effective, approach to evaluate the mechanical properties of hydrogel fibers in solutions (i.e., hydrated fibers) is measuring the bending stiffness in the form of bending persistence length (Lp). Bending Lp is defined as the length along a polymer over which its tangent angle becomes uncorrelated under thermal fluctuations.19−21 The more rigid a polymer fiber is, the larger its bending Lp. The bending Lp of linear biopolymers (e.g., DNA, actin filaments, and microtubules) has been determined from images obtained from various microscopy techniques including electron microscopy and fluorescence microscopy.19−24 In these systems, bending Lp has been determined from various configurations of freely fluctuating polymers in solution or Received: October 15, 2018 Accepted: December 13, 2018 Published: December 26, 2018 18304

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Figure 1. (A) Schematic diagrams of the PAA and PAA/CS nanofiber fabrication process by electrospinning. In both cases, the negatively charged carboxylate functional groups of PAA interact with ferric ions through ionic interactions. SEM images of PAA/CS nanofibers without (B) and with (C) 0.04, (D) 0.07, and (E) 0.1% ferric ions.

properties of PAA and PAA/CS fibers. The bending stiffness and bending length were measured using a dark field microscope. The Young’s moduli of PAA and PAA/CS fibers with different concentrations of ferric ions were calculated using the bending persistence length and the geometry of electrospun PAA and PAA/CS nanofibers. Fourier transform infrared (FTIR) spectroscopy and infrared nanospectroscopy (nanoIR) were used to examine the nanofiber structures and composition.

surface-adsorbed polymers. For example, a recent study by Nakielski and colleagues used a similar methodology to measure bending stiffness of core−shell electrospun fibers in aqueous solutions.24 The bending Lp was obtained from configurations of fibers in solutions where the changes in bending stiffness of the core−shell nanofibers were influenced by Brownian motion and thermal fluctuations as opposed to bending motions influenced mainly by bending stiffness under otherwise controlled parameters. The Young’s moduli of the hydrated fibers obtained through the bending Lp measurement and AFM were comparable (several to 10 kPa). Electrospinning produces polymeric fibers using an external electrostatic field to accelerate and stretch a charged polymer jet.4,25−33 Both modeling and direct observation show that the jet grows rapidly into an electrically driven bending instability before it reaches the collector.28,33,34 We consider that with fixed electrospinning operation parameters (e.g., voltage, spin distance, and polymer feeding rate) and controlled polymer solution properties (i.e., electrical conductivity and polymer concentration), the bending Lp of the collected fibers reflects the modulus of the fibers in a hydrated state. Here, we performed a proof-of-concept evaluation of the mechanical properties of single electrospun hydrogel fibers using dark field microscopy and bending Lp analysis with a recently developed software, persistence.19 A model system of electrospun nanofibers was produced from a solution of poly(acrylic acid) (PAA), chitosan (CS), and metal ions.32 PAA is an anionic polyelectrolyte with carboxylate functional groups on top of a hydrocarbon backbone.35,36 The flexible cationic polysaccharide CS interacts with PAA through ionic cross-links for improved stability in varying pH ranges.37 The metal ions have been shown to provide electrospun nanofiber networks with further structural stability via electrostatic attractions.11−14 Because CS can interact with PAA through electrostatic interactions and metal ions have been demonstrated to improve hydrogel stability and mechanical properties,12,38 it is expected that fibers containing CS and metal ions would have larger bending Lp (i.e., be stiffer) than PAA fibers. Adding ferric ions may further affect the bending Lp of both PAA and PAA/CS fibers. In this study, four types of fiber systems (pure PAA, PAA with increasing concentration of ferric ions (Fe3+), PAA/CS, and PAA/CS with increasing concentration of ferric ions) were used to investigate the impact of ferric ions on the mechanical

2. RESULTS AND DISCUSSION 2.1. Ferric Ions Modulate the Bending Stiffness, Average Length, and Young’s Modulus of Hydrogel Fibers. PAA and PAA/CS fibers with different concentrations of ferric ions (0, 0.04, 0.07, and 0.1 wt % related to polymers) were produced through electrospinning polymer solutions with ferric ions (Figure 1A). The distance between the spinneret and collecting electrode was reduced to collect hydrated hydrogel fibers. Figure 1B is a scanning electron microscopy (SEM) image of dry PAA/CS fibers without ferric ions, where individual fibers attach without being fused. Images of PAA/ CS fibers with different concentrations of ferric ions collected at a reduced collecting distance are provided in Figure 1C−E. The images clearly show that fibers fuse at the contact point because they are hydrated. The diameters of the fibers were obtained by averaging the diameters of 10 fibers from measurements on the SEM images. It was found that the diameter of PAA and PAA/CS fibers increased with ferric ion concentration (Figure S1) because of a combined effect of solution conductivity and viscosity (Table S1). To determine the bending stiffness of the nanofibers, we visualized the electrospun fibers on collectors at varying ferric ion concentrations using dark-field microscopy. On the basis of these images (Figure S1), we measured the bending Lp of the nanofiber from two-dimensional angular correlation analysis that captures semiflexible biopolymer mechanics (Figure 2A).19,39 Pure PAA electrospun fibers were found to be very flexible (evidenced by low bending Lp) with long fiber lengths (i.e., continuous fibers) (Figure 2B,C). Adding CS to PAA (5% in relation to the amount of PAA) produced PAA/CS nanofibers with an average length (Lavg) of about 95.7 μm. The average bending Lp of PAA/CS fibers was higher than that of PAA fibers, indicating that the electrostatic interactions between PAA and CS increase the fiber stiffness. Ferric metal 18305

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by the ferric ions with multiple chains could compensate for the lack of CS in the complex as well as support a steadier increase in bending stiffness, benefitting from low steric hindrance or competition.14,38,40 We estimated the Young’s modulus of PAA and PAA/CS nanofibers with varying concentrations of ferric ions based on bending Lp and nanofiber diameter measurements obtained through SEM images (see Materials and Methods for details) (Figure S4). The Young’s moduli of PAA and PAA/CS fibers were within a range of several kPa to tens of kPa (Figure 2D), which are comparable to that of bulk hydrogels and indicating that our electrospun fibers exist in a hydrated state.46,47 In comparison, the Young’s modulus of a single dry nanofiber was measured in a range of MPa to GPa using AFM through a three-point bend test, where a force was applied to the nanofiber along a small parallel area.48−50 It is interesting to note that the Young’s modulus decreases with increased ferric ion concentration, which is contradictory to the common assumption that ferric ions will cross-link PAA and increase the Young’s modulus (Figure 2D). We believe that ferric ions at low concentration are not able to interact with carboxylate groups from separated PAA chains. Instead, ferric ions interact with carboxylate groups in one PAA chain, reducing the intermolecular interactions of PAA chains and leading to larger nanofiber diameters. The absence of intermolecular interactions of PAA chains is also suggested by viscosity measurements where the viscosity of PAA/ferric ion solutions has negligible increase with ferric ion concentration. On the other hand, PAA chains will be crosslinked by ferric ions at high concentration (i.e., 1%), which is suggested by the gelation of the solution. The Young’s modulus change of PAA/CS fibers with ferric ions reflects the impact of CS on the fiber mechanical properties at low concentration where the Young’s modulus of PAA/CS is higher than that of PAA fibers because of the interaction of PAA and CS through electrostatic interactions. Similar to the effect on PAA fibers, ferric ions interact with carboxylate groups and screen the interactions between carboxylate groups (on PAA) and ammonium groups (on CS), leading to a reduced Young’s modulus with ferric ion concentration. It should be noted that unlike bending stiffness, Young’s modulus is dependent on the fiber geometry (eqs 2 and 3). PAA and PAA/CS nanofibers become thicker with increasing Fe3+ (Figure S1), leading to an overall decrease in the estimated Young’s modulus. Repulsion among the positive charges of CS and metal ions may affect the nanofiber diameter as Fe3+ interacts with carboxylate groups on PAA chains and screens the PAA/CS interactions. This would explain why the nanofiber diameter increases with increasing ferric ions, which results in decreased Young’s modulus, while bending stiffness increases as observed in our results (Figure 2). 2.2. Inhomogeneous Distribution of Ferric Ions Influences the Carboxyl Group Band Shifts of PAA in PAA and PAA/CS Nanofibers. The infrared spectra of the nanofibers are well aligned with previous reports with a signature of the carboxyl group of PAA at ∼1695 cm−1, which experiences a slight shift to 1699 cm−1 in the presence of CS only (Figure 3). For the fibers of PAA interacting with the ferric ions and PAA/CS interacting with ferric ions, the carboxyl band at ∼1710 cm−1 did not shift significantly. We note that the 4 cm−1 spectral resolution limit of the measurement may prevent the identification of smaller shifts. A significant decrease in the amide I and II IR bands of CS at

Figure 2. (A) Average cosine angular correlation of PAA and PAA/ CS nanofibers at varying ferric ion concentrations (0, 0.04, 0.07, and 0.1%). The solid lines represent best fits to the data, yielding bending Lp. (B) Bending persistence length (Lp) of electrospun PAA and PAA/CS nanofibers at varying ferric ion concentrations, (C) average length of PAA and PAA/CS fibers at varying ferric ion concentrations, and (D) estimated nanofiber Young’s modulus of PAA and PAA/CS fibers at varying ferric ion concentrations. Uncertainty bars in (B−D) indicate standard error of the mean.

ions bind with both CS and PAA through electrostatic interactions (Fe3+−COO−) and metal ion/nonpair electron complexing.14,38,40−42 However, the interaction between CS and ferric ions was negligible in our studies because CS was fully protonated in the preparation of the pre-electrospinning solution at low pH (1).11,43 Therefore, we believe that the increment of PAA/CS fiber stiffness upon the addition of ferric ions is attributed to the interactions between the ferric metal ions and PAA. Ferric ions increase the bending Lp and decrease Lavg of both PAA and PAA/CS fibers (Figure 2B,C). This result demonstrates that ferric ions increase the bending stiffness of the fiber through intermolecular interactions between ferric ions and carboxylate groups. While the highest bending Lp and shortest Lavg of both PAA/CS fibers (Lp = 121.96 μm, Lavg = 44 μm) and PAA fibers (Lp = 125.95 μm, Lavg = 89 μm) were obtained with 0.1% ferric ion concentration, the Lp increasing slope of PAA fibers is larger than that of PAA/CS fibers. In other words, ferric ions have a more profound impact on the bending stiffness of the PAA fibers than on the PAA/CS fibers. This may be attributed to the fact that the interactions between CS and PAA screen the interactions between ferric ions and PAA.12,44,45 The presence of CS coincided with higher bending stiffness and shorter nanofibers. Interestingly, the bending Lp converges at 0.10% Fe3+. PAA/CS nanofibers were more resistant to major changes in length and produced shorter fibers, on average, regardless of the level of cross-linking. Presentation of the data in the form of box plots revealed that length variations were the widest at 0.04% concentration of metal ions in the PAA nanofibers without CS. PAA nanofibers with metal ions and no CS showed overall wider distribution of Lavg (Figure S3A). Average bending Lp box plot analysis revealed that both types of nanofibers exhibit a similar variance (Figure S4B, Supporting Information). The electrostatic interactions created 18306

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measures the photothermal expansion of the sample for each wavelength emitted by the laser, which was swept with steps of 1 cm−1. The nanoIR spectra were deconvoluted to assess the variations of the PAA carboxyl band in the fibers. The results indicated that the carboxyl band shifts from 1709 cm−1 in the PAA and PAA/CS fibers without ferric ions to up to 1715 cm−1 in the PAA/CS fiber with 0.1% ferric ions. Hence, the change in Young’s modulus is corroborated by a molecular change in the carboxyl group of PAA in the presence of ferric ions. Comparing to the sub-band at 1680 and 1733 cm−1 (gray curves in Figure 4E−H), the intensity of the carboxyl band also increases in the fibers with higher content in ferric ions. We visualized the variations in the carboxyl band across the fibers by fixing the illumination wavelength of the laser at 1710 cm−1 to obtain a chemical map. The images, presented in Figure 4A−D, reveal inhomogeneities in composition, especially at the cluster sites. Particularly, in Figure 4C, a lower intensity along the axis of the fiber was resolved, which was also noted in other cases (not shown here). This could be due to a difference in the composition of the core of the nanofiber. Overall, the clusters were found to have a higher expansion when excited at 1710 cm−1 specifically at the center. This was not solely due to the higher thickness of the cluster regions, as can be seen in Figure 4C where variations within the cluster are the strongest. The corresponding nanoIR spectra also indicate a large shift of the carboxyl group up to 1715 cm−1 in these regions, suggesting that the distribution of the ferric ions may not be homogeneous throughout the fibers.

Figure 3. FTIR spectra of (A) PAA/CS solutions with increasing concentrations of ferric ions and (B) PAA solution with increasing concentrations of ferric ions.

1649 and 1600 cm−1, respectively, and increase of the absorption of NH3+ at 1640 cm−1 upon PAA/CS interactions have been previously reported,40,51−55 which were not observed in our study, likely due to the full protonation of CS. However, in the PAA/CS fiber, a slight change in the broad band at 2200 cm−1 suggests nonetheless the presence of NH3+ ions from CS in the fibers. Next, we investigated local changes within the fibers by using nanoIR. NanoIR measures the photothermal expansion of the material in contact with the AFM tip at each wavelength emitted with the infrared laser.56 In the range of the laser (1540−1800 cm−1), the ∼1610 cm−1 band of the carboxylate ions and the ∼1650 cm−1 band of CO stretching and ∼1710 cm−1 band of the carboxyl group could be studied. We first collected AFM topography images of the fibers (Figure S5), which revealed the presence of contact points (Figure S5B) and of clusters of various sizes along the fibers (Figure S5C,D) after addition of the ferric ions. The composition of the nanofibers and clusters was examined by collecting nanoIR spectra at different locations (Figure 4). NanoIR is not tight to 4 cm−1 spectra resolution of the FTIR as the cantilever

3. CONCLUSIONS The mechanical properties of PAA and PAA/CS fibers with different concentrations of ferric ions were investigated using persistence length analysis. The Young’s modulus of the hydrated hydrogel fibers calculated from bending Lp and fiber diameter is in the range of several to tens of kPa, which is

Figure 4. NanoIR spectra of PAA and PAA/CS nanofibers at varying ferric concentrations. (A−D) Chemical map (1710 cm−1) of the PAA/CS nanofibers with 0 (A), 0.04 (B), 0.07 (C), and 0.1% (D) of ferric ions. (E−L) nanoIR spectra of the fibers with peak deconvolution made of PAAonly (E), PAA/CS (I), PAA/CA with 0.04% of ferric ions (F), PAA/CA with 0.07% of ferric ions (G), and PAA/CA with 0.1% of ferric ions (H). The nanoIR spectra of the clusters identified on the PAA/CS fibers with ferric ions are presented in (J−L). 18307

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Young’s modulus (E) was calculated by eq 2, where κ and I denote flexural rigidity (or bending stiffness) and moment of inertia of nanofibers, respectively, and kBT is the thermal energy.

consistent with the Young’s modulus measured directly using AFM technology. Increase of the ferric ion concentration decreased the Young’s modulus of PAA and PAA/CS fibers because of the interaction of ferric ions with carboxylate groups. These interactions were confirmed in nanoIR studies. Our work offers a technique of polymer mechanical strength measurement in single nanofibers without requiring complex, time-consuming methods related to the whole nanofiber flexibility as opposed to a single area.

E=

(2)

Moment of inertia (I) of PAA or PAA/CS nanofibers was calculated by eq 3 with the assumption of nanofibers as rodlike structures with radius r based on diameter measurement from SEM images (Figure 1B).24,39,57

4. MATERIALS AND METHODS 4.1. Nanofiber Synthesis by Electrospinning. PAA/CS nanofibers were fabricated by electrospinning as shown in Figure 1A. In the pre-electrospinning solutions, 0.1 M FeCl3 and 6 M HCl were added to 0.568 g of 4.4% CS in 25% acetic acid and stirred for 30 min to create a homogenous solution. Next, 1.43 g of 35% PAA (Mw ≈ 240 000, Sigma-Aldrich) was added to make PAA/CS (mass ratio of 20:1). Different 0.1 M FeCl 3 amounts were used to make Fe 3+ metal ion concentrations 0.04, 0.07, and 0.1% in solution by molar of the carboxylate group on PAA. To avoid gelation between Fe3+ and PAA, the pH was maintained at 1 by addition of 6 M HCl. Addition of deionized water at different amounts maintained the solutions at total polymer concentration of 20%. The solution was loaded into a plastic syringe equipped with a 16 mm stainless steel gauge needle. A high voltage supply was connected to the needle, while a syringe pump supplied solution continuously. The electrospinning process with applied voltages ranging between 10 and 12 kV was conducted in air at room temperature. Continuous, homogeneous nanofibers were obtained by adjusting the voltages applied during the process whenever necessary. The collector was a piece of aluminum foil placed 25 cm away from the tip of the needle, slightly shorter than the collecting distance of the previous report to collect hydrated bend fibers.11 The as-spun nanofibers were dried at 40 °C under vacuum and stored in a desiccator for later use. 4.2. Scanning Electron Microscopy. The morphology of the fibers was characterized by SEM (Zeiss Ultra 55) by placing the electrospun nanofibers on glass substrates. Prior to imaging, all samples were sputtered with a thin layer of Au/Pb (Emitech k550). A working distance of 6−10 mm and accelerating voltage of 5 kV were used for imaging. The scale bar provided on the images was used to determine nanofiber diameter measurements. 4.3. Microscopy Imaging and Bending Mechanics Data Analysis. Electrospun nanofibers were visualized using an Olympus BX51M dark-field microscope equipped with a digital camera (ProgRes GRYPHAX model from Jenoptik) and a 20× objective. Dark field images were processed using ImageJ software (NIH) before persistence length analysis. Persistence19 was used to measure the average length (Lavg) and bending Lp of nanofibers. Bending Lp values were calculated from best fits to two-dimensional average cosine correlation [⟨C(s)⟩] (eq 1) analyses19 of nanofibers (N = 630−2200) using OriginPro 8 software. Average cosine correlation, ⟨C(s)⟩, was obtained from the tangent angles (θ) along the nanofiber segment length (s), with scaling factor (A) (eq 1). ⟨C(s)⟩ = cos[θ(s) − θ(0)] = A e−x /2Lp

LpkBT κ = I I

I=

πr 4 4

(3)

4.4. FTIR Spectroscopy. Infrared spectra of PAA/CS nanofibers were obtained on solutions, prior to electrospinning, with a PerkinElmer Spectrum 100 series FTIR spectrometer in the range of 600−4000 cm−1. Settings for 4 cm−1 spectral resolution were used. Four spectra were averaged for each measurement. Nanofibers could not be used for individual detection of signals; therefore, samples made of vacuum-dried solution (prior to electrospinning) were used. Simulation through use of vacuum-dried membranes assured that no components were lost during electrospinning. IR bands were deconvoluted for composition studies using Fityk and Origin software. 4.5. Infrared Nanospectroscopy. Nanofiber sample preparation was modified for optimal nanoIR measurements. The fibers were deposited onto silicon wafers. The substrate was selected because of the absence of IR bands in the 1530− 1810 cm−1 range. Absolute ethanol was used to clean the wafers through a sonication bath for 2 h before rinsing with deionized water. A half of each substrate was covered with aluminum foil during deposition to maintain a clean area to acquire background reference. Localized IR measurements were acquired on a nanoIR2 platform (Anasys Instrument). Silicon (n-type) cantilevers coated with gold on both sides (PR-EX-nIR2 k ≈ 0.07−0.4 N/m) were used for the nanoIR measurements. The natural resonance of the cantilevers used ranged from 11 to 19 kHz with a spring constant of 0.1−0.6 N/m. A constant scan rate of 1.0 Hz and 500 by 500 points were used for image collection. For each sample, several AFM images and IR spectra at various locations of the fibers were collected. Chemical images were acquired at 1710 cm−1, corresponding to the carboxyl group in PAA. Local IR spectra were obtained on regions of interest in the fibers and analyzed using Fityk and Origin software. 4.6. Ion Conductivity Measurement. Solutions at varying concentrations (0.1 M samples of 6 g) were placed in separate 15 mL EZ Flip conical centrifuge tubes for ionic conductivity studies. The measurements were carried out with a Thermo Scientific Orion 5 STAR A111 Benchtop Meter.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02822. Diameter of PAA and PAA/CS nanofibers with different concentration of ferric ions determined from SEM image measurements; ion conductivity and viscosity of PAA and PAA/CS solutions with different concentrations of

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ferric ions; comparison analysis of nanofiber length and bending stiffness of PAA and PAA/CS electrospun nanofibers with different concentrations of ferric ions; representative dark-field confocal microscopy images of electrospun PAA/CS nanofibers with different concentrations of ferric ions; and topography AFM images of PAA/CS fibers obtained using different concentrations of ferric ions (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +1 (407)882-2847. Fax: +1 (407)882-2819 (L.Z.). *E-mail: [email protected]. Phone: +1 (407)823-2368. Fax: +1 (407)882-2819 (H.K.). ORCID

Lei Zhai: 0000-0002-3886-2154 Hyeran Kang: 0000-0003-2785-3479 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

The financial support from the National Science Foundation Award CMMI 1462895 is greatly appreciated. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Science Foundation Award CMMI 1462895 (Zhai), the UCF start-up fund (Kang), UCF In-House Grant (Kang), and NASA’s 2017 FSGC Dissertation/Thesis Improvement Fellowship (Diaz). The authors are also grateful to the Nanoscience Technology Center and the Materials Characterization Facility at the University of Central Florida.

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REFERENCES

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