Exploiting the Physiochemical Interactions between Single-Walled

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Exploiting the Physiochemical Interactions between Single-Walled Carbon Nanotubes and Hydrogel Microspheres To Afford Chirally Pure Nanotubes Brennan P. Watts, Cameron H. Barbee, and Kevin Tvrdy* Department of Chemistry and Biochemistry, University of Colorado Colorado Springs, Colorado Springs, Colorado 80918-3733, United States Downloaded via RUTGERS UNIV on August 8, 2019 at 10:31:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Single-walled carbon-nanotubes (SWNTs) exhibit unique electronic, optical, and mechanical properties; however, a narrow availability of chirally pure (single electronic structure) material can limit effective integration within novel devices and schemes. This work focuses on understanding, modeling, and advancing methodology used to generate preparative quantities of single-chirality SWNT: the iterative adsorption/desorption of SWNT from aqueous surfactant suspensions to/from hydrogel microspheres. Commercially available hydrogel microspheres (Sephacryl S200) were sorted by radius and exposed to SWNT, affording a direct correlation between microsphere surface area and quantity of SWNT adsorbed using differential absorbance spectroscopy. This relationship elucidates a SWNT/gel purification scheme interaction mechanism exclusively involving the gel surface. High-concentration surfactant was used to elute SWNT from the gel with desorption efficiencies dependent on both SWNT chirality and hydrogel microsphere radius, ranging from 25−45%. A thermodynamic model for SWNT desorption that accounts for hydrogel microsphere curvature effects is presented and suggests that (when compared with experimental data) SWNT with greater than ∼41% of their length adsorbed to a hydrogel surface bind irreversibly, while others are desorbed in the presence of high-concentration surfactant. These findings inspired the generation and application of mechanically fractured hydrogel microspheres (exhibiting greater surface area) for use as SWNT purification media in per-iteration quantities far less than traditional gels. A 10-iteration SWNT purification procedure demonstrated a marked improvement in process efficiency, as mechanically fractured gels afford a 10-fold reduction in gel-media use (the major expense of the process) while yielding equivalent SWNT purification. KEYWORDS: single-walled carbon nanotubes, Sephacryl S200, single chirality SWNT separation, carbon nanotube purification efficiency, SWNT/hydrogel interactions

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INTRODUCTION In recent decades, single-walled carbon nanotubes (SWNTs) have been demonstrated as active components within a variety of applications, including nanostructured electronics,1,2 energy harvesting,3−6 energy storage,7−9 composites,10−12 flexible thermoelectrics,13,14 environmental remediation,15 and biological sensors.16,17 However, widespread use of SWNTs remains limited by a problem of chiral impurity: the varied SWNT geometries (defined by (n,m), chiral wrapping vector) produced within common bulk-scale SWNT syntheses exhibit inherently inhomogeneous electronic and structural characteristics.18 In light of this, efforts have been made to design processes that enrich a single chirality of SWNT.19−21 Some such methods include density gradient ultracentrifugation,22−25 DNA-based chromatography,26−29 and selective adsorption of SWNTs to hydrogel materials,30−32 which is arguably the most scalable of these methods.33 The work presented herein focuses on SWNT/hydrogel interactions and © 2019 American Chemical Society

aims to provide fundamental insight into the physiochemical impact of hydrogel morphology on SWNT purification. Furthermore, we report how said morphology may be manipulated to improve process efficiency and reduce process costs. Hydrogel-based SWNT separation was first reported by Kappes and co-workers,34 in which the mesoporous hydrogel Sephacryl was employed for the electronic type separation of SWNTs suspended in aqueous solutions of sodium dodecyl sulfate (SDS). This application of Sephacryl is somewhat unique because such hydrogels are typically employed in sizeexclusion chromatography for the purification of large biomolecules,35,36 where porosity and pore size are of great importance. Improvements to this method were discussed Received: March 26, 2019 Accepted: May 7, 2019 Published: May 7, 2019 3615

DOI: 10.1021/acsanm.9b00567 ACS Appl. Nano Mater. 2019, 2, 3615−3625

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ACS Applied Nano Materials shortly after by Kataura and co-workers,30 who reported that iterative application of this method achieves separation of SWNTs by chirality. More recently it has been shown that hydrogel schemes can also achieve enantiomeric separation of both metallic and semiconducting SWNTs, which may aid in their implementation within biomedical schemes.37 Despite notable successes in Sephacryl-based purification of SWNT, the specific chemical and mechanistic natures of SWNT/ hydrogel interactions remain unclear. For example, the SWNTexclusive geometric property of the smallest bond curvature radius38,39 has been shown to exhibit a strong correlation with SWNT (n,m) separation order within gel purification schemes,30 but neglects SWNT-gel, SWNT-SDS, and gelSDS interactions within the system. With an ever-increasing interest in the implementation of single-chirality SWNT within industrial and lab scale devices/schemes, there exists continued justification for focus on understanding and improving gelbased SWNT purification methods. This study is based on the hypothesis that SWNTs do not rely on hydrogel porosity to afford single-chirality separation because the hydrodynamic radii of SWNTs are physically larger than porous interiors of gels. Specifically, the hydrogel employed for SWNT purification here and in the aforementioned studies (Sephacryl S200) has an exclusion limit of 400 kDa for globular proteins and hence a maximum pore diameter of approximately 10 nm.40 This work demonstrates evidence supporting a physiochemical SWNT purification mechanism driven by hydrogel surface area in which smaller hydrogel beads provide a greater normalized surface area available for SWNT binding and correlatively result in a larger uptake and subsequent release of bound SWNTs. Furthermore, we show that the fraction of SWNT irreversibly adsorbed to Sephacryl increases with increasing gel bead diameter, and that this process may be modeled by consideration of the portion of rigid SWNT length in physical contact with a curved microsphere surface. Consequently, we demonstrate that the efficiency of hydrogel-based SWNT purification schemes may be significantly improved by utilizing gels with a smaller size as the ratio of SWNT purified per hydrogel volume utilized can be increased 10-fold, which is an especially significant finding given that the hydrogel medium constitutes approximately 75% of procedural expenses (not including the cost of unpurified SWNT). These results both advance fundamental understanding of the physiochemical relationship between SWNTs and hydrogel morphology and represent a novel means by which the production of chirally pure SWNT may be enhanced.

Figure 1. Comparison of microscopic characterization and resultant size-distributions of native and processed Sephacryl S200 hydrogel media. (A) The microsphere form of native Sephacryl S200 enables ease of homogeneous packing within size selective protein purification columns. (B) The iterative process of size selective sedimentation converts a homogeneous aqueous suspension of microspheres within 70 mM SDS to a bed graded roughly by microsphere radius from which size-purified aliquots can be isolated by selective removal of vertical fractions of settled gel. (C) Microscopy and size distribution characterization resulting from two iterations of size selective sedimentation of Sephacryl microspheres with radial distributions and normalized surface areas determined to be 15.1 ± 3.3 μm and 2157 cm2/g; 18.1 ± 4.7 μm and 1826 cm2/g; 22.2 ± 5.6 μm and 1482 cm2/g; and 23.9 ± 6.0 μm and 1442 cm2/g, respectively.

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RESULTS AND DISCUSSION Correlation between Hydrogel Surface Area and SWNT Uptake. The commercially available hydrogel Sephacryl S200 (GE Healthcare, hereafter Sephacryl) exists natively in the form of microspheres with radial size distributions of 18.5 ± 5.6 μm (Figure 1A), a property that allows for its uniform loading within size exclusion chromatography columns.35 To test the hypothesis that SWNT binding occurs exclusively at the hydrogel microsphere surface, a means of segregating native Sephacryl based on microsphere radius was developed. Specifically, native Sephacryl was homogenized via gentle mixing and allowed to settle over several hours, during which gel microspheres with larger diameters settled more quickly than those with smaller diameters due to the inherently greater diffusion of smaller

radii particles. This procedure afforded the formation of a gel bed vertically graded by mean microsphere diameter (Figure 1B). To isolate said microspheres according to their diameter, 3616

DOI: 10.1021/acsanm.9b00567 ACS Appl. Nano Mater. 2019, 2, 3615−3625

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ACS Applied Nano Materials Table 1. Summary of Quantitative Analysis Following Best-Fit of Absorbance Spectra in Figure 2a SWNT Adsorbed to Gel (Differential Spectra) 67.2 cm2

pre-gel-interaction SWNT chirality (5,4) (7,3) (6,4) (6,5) (7,5) (9,1) total

# SWNT/1011 0.12 5.19 2.40 3.68 3.90 0.42 15.7

purity 0.8 33.0 15.3 23.4 24.8 2.7

# SWNT/1011 0.17 1.88 1.79 4.03 2.33 0.24 10.4

56.9 cm2 purity 1.6 18.0 17.1 38.6 22.4 2.3

# SWNT/1011 0.14 1.53 1.64 3.58 2.04 0.19 9.12

46.2 cm2 purity 1.6 16.8 18.0 39.1 22.3 2.1

# SWNT/1011 0.09 1.13 1.41 2.92 1.68 0.10 7.33

44.9 cm2 purity 1.2 15.5 19.2 39.7 22.9 1.4

# SWNT/1011 0.09 1.35 1.38 2.62 1.60 0.09 7.12

purity 1.2 19.0 19.4 36.6 22.4 1.3

a The # SWNT (units of number/1011) columns are calculated using chirality-dependent SWNT molar absorptivity along with an assumed SWNT length of 300 nm for each chirality and the purity (units of %) columns represent the fractional contribution of each chirality to the total SWNT present.

sonication treatment (yielding individualized SWNT within solution) and ultracentrifugation (removing SWNT bundles) was divided into four equivalent volume aliquots, which were subsequently passed through each of the four Sephacryl beds with varied surface area. A total SWNT concentration of 7.21 × 1011 nanotubes/mL (assuming all SWNT to be 300 nm in length)41 within an aqueous 70 mM (2 wt %) sodium dodecyl sulfate (SDS) solution was utilized to afford sufficient SWNT to saturate hydrogel binding sites while also allowing for spectroscopic analysis without dilution. Quantitative spectroscopic analysis of the pre-gel-interaction SWNT suspension employed here is provided in Table 1, whereas spectroscopic deconvolution methods used to afford per-chirality quantification are detailed in Supporting Information, Section S2. To ensure that each hydrogel microsphere bed achieved full SWNT saturation, SWNT aliquots were collected after passing through each bed and iteratively added to the top of that gel bed, which allowed for constant interaction between the SWNT suspension and the gel for 60 min. Although significantly longer than a SWNT/gel interaction period used within a standard purification scheme,30,31 this prolonged interaction time was deemed necessary to both achieve full saturation at the SWNT concentration utilized and afford quantitative comparison of SWNT uptake between the gel beds of varying surface area. Following completion of SWNT/gel interactions, each postgel-interaction SWNT suspension was collected for spectroscopic analysis. The absorbance spectrum of the SWNT suspensions prior to gel interaction, as well as post-interaction with hydrogel beds exhibiting surface areas of 67.2, 56.9, 46.2, and 44.9 cm2 are shown in Figure 2A. The near-infrared region of these spectra exhibits the convolution of several lowest energy SWNT electronic transition (E11 ) peaks with corresponding phonon sidebands42 and background contributions from amorphous carbon.43 These spectra demonstrate that less SWNT exists within the post-gel-interaction samples (due to the occupation of hydrogel binding sites by SWNT) and that less SWNT is present in suspensions that interacted with gel beds exhibiting greater surface area. To further quantify the SWNT bound to each gel aliquot, the absorbance spectrum of each post-gel-interaction SWNT suspension was subtracted from that prior to gel exposure, Figure 2B. Assuming that the hydrogel does not permanently alter the chemical nature of the SWNT that passes through it, these differential spectra represent the spectroscopic signature of the material removed from the original SWNT suspension by each of the four Sephacryl beds of varied surface area. To

sequential additions of solvent and gentle agitation were employed to generate a gel microsphere suspension consisting of only the top portion of the bed, which was then removed via decanting. Iterative application of this method afforded further size segregation of the material (see Supporting Information, Figure S1), which allowed the selective combination of fractions to produce Sephacryl aliquots with mean radii of 15.1 ± 3.3, 18.1 ± 4.7, 22.2 ± 5.6, and 23.9 ± 6.0 μm (Figure 1C). While these four aliquots span less than an order of magnitude in radius-space, achievable range is inherently limited by the predetermined radial distribution of commercially available Sephacryl, the exclusive investigation of which is justified by its widespread use in the generation of preparative scale single chirality SWNT. Additionally, the nonzero radiusspace overlap among the samples represents that afforded by iterative application of two rounds of size selective sedimentation. Although additional sedimentation is likely to produce gel microsphere aliquots with more narrow size distributions, such would have also significantly reduced the amount of usable material. For the purpose of this study, two iterations of this method were determined to strike a reasonable balance between size segregation and the generation of sufficient material to afford study of SWNT/ gel interactions. To afford comparison with the amount of SWNT bound during a hydrogel SWNT purification procedure, the total surface area normalized to gel mass was determined for each aliquot by AN =

∫0

∞

3 P(r )104 dr rρ

(1)

where r is microsphere radius in μm; ρ is hydrogel density (taken to be 1 g/cm3); P(r) is the normalized radial distribution of gel microspheres determined from microscopy; and the factor of 104 yields normalized gel area AN in units of cm2/g. Application of the radial distributions shown in Figure 1C to eq 1 yields normalized hydrogel surface areas of 2157, 1826, 1482, and 1442 cm2/g for aliquots with mean radii of 15.1, 18.1, 22.2, and 23.9 μm, respectively. Equal masses (see Supporting Information, Section S1.2.3) of each hydrogel microsphere fraction in its hydrated form was loaded into a fritted column, similar to the setup utilized in a standard gel-based SWNT purification scheme.30,31 This resulted in gel beds of equivalent volume and height but different total surface area: 67.2, 56.9, 46.2, and 44.9 cm2. A multichirality SWNT suspension produced by tip-horn 3617

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demonstrates first that the material bound to Sephacryl is SWNT and second that the approach of using differential absorbance plots to provide quantitative information about the chirality-dependent quantity of SWNT taken up by a Sephacryl bed of known gel surface area is viable. To identify the chirality-dependent adsorption of SWNT onto each of the four gel beds, the results of the respective best-fits were converted into chirality-dependent number of SWNT by translating the height of each best-fit chiralitydependent E11 contribution to the number of SWNT using previously reported chirality-dependent molar absorptivities,44 chirality-dependent unit cells,45 and the assumption of 300 nm SWNT length.41 The chirality-specific number of SWNT bound (for the six fitted SWNT chiralities) is enumerated for each of the differential spectra within Figure 2B in Table 1. Also shown in Table 1 is the percent composition of each SWNT chirality relative to all SWNT bound to the gel bed, which ranges from ∼1% for the (5,4) chirality to ∼40% for the (6,5) chirality. These data are significant because the relative purity of SWNT chiralities exhibiting high-affinity for Sephacryl (as measured by their relatively large binding rate constants)31 is greater than in unpurified solutions; (6,5) accounts for ∼25% of the SWNT within the pre-gel-interaction suspension and is increased to ∼40% of all SWNT bound to the gel, confirming the ability of Sephacryl to serve as a chirality-sensitive SWNT purification medium through selective adsorption. Additionally, the SWNT material bound to each of the four gel beds exhibiting different surface area is similar in terms of chiral fraction, which demonstrates that any chirality-dependent adsorption effects that may distort surfacearea-dependent behavior (the focus of this work) may be ignored. To gain insight into the mechanism for SWNT/gel binding, the number of SWNT bound to each of the four gel beds was calculated and compared with the total surface area exhibited by each gel, Figure 3. The strong correlation between SWNT uptake and hydrogel surface area across four hydrogel beds exhibiting unique surface areas clearly demonstrates that SWNT binding events within a Sephacryl-based purification scheme occur exclusively at the surfaces of hydrogel microspheres. This observation is significant because the commer-

Figure 2. Spectroscopic analysis of SWNT suspensions before and after interaction with hydrogel microsphere beds of varying surface area. (A) Spectroscopic comparison between SWNT suspensions before and after interaction with gel beds demonstrates a reduced SWNT concentration post-gel-interaction. (B) Differential absorbance spectra taken by subtracting the post-gel-interaction spectra from the pre-gel-interaction spectrum yields the spectroscopic nature of the material adsorbed to the gel, exhibiting clear SWNT character within the near-infrared region of the spectra. (C) Fitting of differential spectra to a sum of contributions from the E11 peaks and phonon sidebands of six SWNT chiralities, as well as a carbonaceous background, yields a strong fit and affords quantization of a number of SWNTs adsorbed to each Sephacryl bed of varying surface area, as listed in Table 1.

demonstrate the identification of these spectra as originating from SWNT, the E11 region of each spectrum was fit to a sum of six Lorentzian line shapes of constant width (fwhm best-fit of 60 meV), six complementary phonon sidebands (best-fits blue-shifted by 0.24 eV relative to corresponding E11, fwhm of 0.12 eV, and height of 10.8% of corresponding E11), and a sum of background contributions from carbonaceous fractions originating from sonication. Both the experimental and bestfit spectra corresponding to material bound to the Sephacryl bed of 56.9 cm2 are shown in Figure 2C, while those corresponding to the material bound to Sephacryl beds of 67.2, 46.2, and 44.9 cm2 are shown in Supporting Information, Figure S2. The strength of the fit of a multichirality SWNT absorbance model to that of the differential absorbance plot

Figure 3. Correlation between total number of SWNT adsorbed by Sephacryl gel beds and gel bed surface area. The strong agreement between these parameters suggests a SWNT/gel interaction mechanism whereby SWNT binding sites are limited to the surface of the gel microspheres, which are well modeled as spheres of smooth surface. 3618

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ACS Applied Nano Materials cially intended use for Sephacryl is as a size selective purification medium, where molecules capable of sampling the interior porosity of the hydrogel exhibit a longer diffusive path length resulting in longer elution times. Clearly, although Sephacryl is manufactured for its ability to differentiate proteins based on their hydrodynamic size,35 the chemical mechanism driving its ability to purify SWNT based on chirality is unique and worthy of further investigation. Hydrogel Curvature Effects on Efficiency of SWNT Recovery. The partial reversibility of the SWNT-to-Sephacryl binding of each of the four gel aliquots was demonstrated through the introduction of 175 mM (5 wt %) aqueous SDS (after rinsing each bed with neat 70 mM aqueous SDS to remove unbound SWNT present in the interspherical space), which was collected following a single pass through each gel bed. The general method of complete loading of Sephacryl binding sites with a SWNT in aqueous 70 mM SDS solution, rinsing with neat 70 mM SDS, and subsequent release and collection of chirally purified SWNT from Sephacryl via addition of 175 mM SDS represents the iterative hydrogelbased purification of SWNT, which is both scalable30 and relatively cost-effective compared to other methods. 46 Absorbance spectra of the SWNT suspensions collected from the gel beds presenting 67.2, 56.9, 46.2, and 44.9 cm2 are shown in Figure 4A, whereas the best-fit results and deconvoluted contributions from constituent SWNT chiralities for each spectrum is shown in Supporting Information, Figure S4. Additionally, the quantitative results of the best-fit analysis of these spectra in terms of the number of SWNT eluted and fractional chiral purity are presented in Table 2. Both the overall efficiency of SWNT eluted as well as the dependence of this efficiency on hydrogel microsphere diameter are worthy of further discussion. First, a quantitative comparison of the data presented in Tables 1 and 2 affords calculation of per-gel-bed SWNT elution efficiency (summed over all chiralities), which is presented graphically in Figure 4B. This efficiency ranges from 28.9% to 37.3%, indicating that approximately two-thirds of all chirally purified material bound to Sephacryl is not recovered in the elution process, significantly limiting process efficiency. Others have noted that attempts to recover such materials (the presence of which has been noted elsewhere but is quantified for the first time here) using either saturated SDS solution or other surfactants known to interact strongly with the SWNT sidewall have not been successful. 31 Although the general scalability of Sephacryl-based SWNT purification methods makes this strategy the most desirable option for preparative scale production of chirally pure SWNT, significant room for improvement exists in terms of identifying new hydrogels and/ or new surfactants to improve the overall efficiency of the approach. Second, it is important to note that the four hydrogel size distributions studied here show a correlation between elution efficiency and microsphere size. Mechanistic insight into this correlation should also necessarily consider the thermodynamically driven nature of the SWNT desorption process, which has been demonstrated experimentally by the irreversible nature of SWNT/gel binding in the presence of 70 mM SDS. Specifically, SWNT bound to Sephacryl is not released upon the addition of neat 70 mM SDS but rather only in the presence of additional dielectric screening (introduction of 175 mM SDS or NaCl/SDS mixtures) does a fraction of the SWNT desorb from the gel. 32,47−50 This observation

Figure 4. Spectroscopic analysis of SWNT desorbed from hydrogel microsphere beds upon introduction of 175 mM SDS. (A) Absorbance spectra of SWNT desorbed from gel beds exhibiting 67.2, 56.9, 46.2, and 44.9 cm2 demonstrate a qualitative correlation between gel surface area and number of SWNT released. (B) Quantitative analysis of total SWNT released from each gel bed demonstrates the relative efficiency of elution associated with gel beds of different surface area and exhibiting different microsphere diameter distributions. (C) Per-chirality consideration of the fraction of SWNT desorbed from each gel bed demonstrates that, in addition to geometric effects investigated here, additional dependence on chirality-specific SWNT binding reversibility plays a role in desorption efficiency.

demonstrates that a thermodynamic stability, as opposed to a kinetic equilibrium due to a gradient in SWNT concentration, is responsible for driving the process of SWNT desorption from gels. Additional insight into the nature of the SWNT/gel elution process can be gained by examining the chirality dependence of the fraction of SWNT eluted from each gel bed, as shown in Figure 4C for the three chiralities constituting at minimum 3619

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ACS Applied Nano Materials Table 2. Summary of Quantitative Analysis Following Best-Fit of Absorbance Spectra in Figure 4Aa SWNT Desorbed from Gel 67.2 cm2

56.9 cm2

46.2 cm2

44.9 cm2

SWNT chirality

# SWNT/1011

purity

# SWNT/1011

purity

# SWNT/1011

purity

# SWNT/1011

purity

(5,4) (7,3) (6,4) (6,5) (7,5) (9,1) Total

0.09 0.30 0.67 2.06 0.68 0.08 3.88

2.3 7.7 17.4 53.1 17.5 2.0

0.07 0.18 0.61 1.73 0.57 0.06 3.23

2.3 5.5 18.9 53.6 17.7 2.0

0.05 0.16 0.50 1.21 0.41 0.04 2.38

2.1 6.9 21.1 50.9 17.4 1.6

0.05 0.19 0.44 1.00 0.35 0.04 2.06

2.3 9 21.6 48.5 16.8 1.8

a The # SWNT (units of number/1011) columns are calculated using chirality-dependent SWNT molar absorptivity along with an assumed SWNT length of 300 nm for each chirality and the purity (units of %) columns represent the fractional contribution of each chirality to the total SWNT present.

data presented herein, the enhancement in purity of the (6,5) chirality, a net change from 23% to ∼52% purity, can be attributed to both the chiral selectivity of the adsorption event (∼65% increase in purity) as well as that of the desorption event (further ∼34% increase in purity). This finding can potentially be used in the development of next-generation SWNT purification media that engineer chiral selectivity in either the SWNT/gel binding event, the elution event, or both, improving the scope and scale of single-chirality SWNT purification.52 Thermodynamically Driven Model of SWNT Desorption Efficiency. As a means to capture both the thermodynamically driven nature of the SWNT desorption process as well as the dependence of desorption efficiency on hydrogel microsphere size, we describe the SWNT/gel microsphere interaction in terms of the fraction of a SWNT length that exists at distances from the curved gel surface over which SWNT/gel interactions are expected to occur, the geometry of which is illustrated in Figure 5A. Such SWNT/gel interaction distances were previously estimated to range from 0.127 to 0.313 nm, the width of the potential well that drives SWNT/gel binding.32 Taking the SWNT to be a rigid cylinder of length LSWNT and midpoint aligned with the surface normal to the gel microsphere (exhibiting radius r), the fraction of the total length of SWNT lying between the minimum and maximum distances of energetically favorable SWNT/gel interaction (Dmin and Dmax, respectively) is given by

10% of the total SWNT content eluted. Each of the (6,4), (6,5), and (7,5) chiralities demonstrate the general trend observed in Figure 4B in that gel beds exhibiting larger average microsphere diameter result in reduced efficiency of SWNT elution. Further, the chirality-dependent elution efficiency trend of (6,5) > (6,4) > (7,5) spans from over 45% (the (6,5) chirality interacting with relatively small diameter gel) to 25% (the (7,5) chirality interacting with relatively large diameter gel), which is a wide range. This observation is especially interesting, as the noted chirality dependence in irreversible binding between SWNT and Sephacryl does not correlate with order-of-separation, (6,4) before (6,5) before (7,5), with SWNT-diameter or SWNT smallest bond curvature radius, both (6,4) < (6,5) < (7,5), or with SWNT chiral angle, (6,4) < (7,5) < (6,5), each of which has been postulated to be responsible for mechanistically driving interactions between SWNT and S200.30 Other interactions, potentially chemical in nature and not modeled by gel particle size effects or the aforementioned SWNT parameters, are likely responsible for the trend in SWNT elution efficiency shown in Figure 4C. Although the present study focuses on the elucidation and modeling of the SWNT/gel physiochemical relationship in a chirally independent manner, chirally dependent physiochemical interactions clearly still exist. Perhaps more importantly, such chirally dependent interactions are responsible for the ability of Sephacryl to purify SWNT and form the basis of justifying the work presented here. Future investigation, both experimental (requiring preparative quantities of chirally pure SWNT) and computational, are necessary to fully understand the chirality-dependent nature of the SWNT/gel physiochemical interactions and represent a logical pathway toward eventual optimization of hydrogel-based SWNT purification. Before introduction of a model to describe gel curvature effects on SWNT elution efficiency, it is worthwhile to examine the chiral purity of the SWNT eluted from each of the four gel beds listed in Table 2. Specifically, SWNT separation by chirality is achieved in each of the two gel interaction steps examined in detail here: the adsorption event and the desorption event. For example, the (6,5) chirality constitutes 23% of all SWNT within the pre-gel-interaction suspension, 37−40% of that adsorbed to the hydrogel beds, and 49−54% of that desorbed from those beds. This finding is significant because the differential absorbance method introduced in this work is capable of deconvoluting the roles of the binding and elution processes in terms of achieving chiral separation, a result that was previously assigned exclusively to chiral selectivity during the binding event.30−32,51 In terms of the

fL =

2r LSWNT

1−

(r + Dmin − Dmax )2 r2

(2)

a full derivation of which is provided within Supporting Information, Section S3. Because of the nature of the linear SWNT and curved hydrogel microsphere, system geometry prevents the entirety of a SWNT’s length from interacting at a fixed distance from the gel surface (see Supporting Information Figure S6 for a plot of f L versus microsphere radius). This model hypothesizes that with larger fractions of SWNT length interacting with the gel surface comes a more thermodynamically stable SWNT/gel interaction and a decreased likelihood of SWNT desorption in the presence of 175 mM SDS. A detailed discussion of all model parameters and the validity of assumptions made can be found in Supporting Information, Section S3. Taking SWNT length to be 300 nm (following prolonged tip-horn sonication41) and minimum and maximum distance over which SWNT/gel interaction is energetically favorable as 3620

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0.127 and 0.313 nm,32 respectively, eq 2 affords direct correlation between hydrogel microsphere diameter and the fraction of SWNT length interacting with a hydrogel surface. The hydrogel diameter distributions shown in Figure 1C were translated from diameter space to f L (fraction of length bound) space, shown in Figure 5B. Because this model is predicated on the assumption that the ability of a SWNT to desorb from a curved gel surface is governed by the thermodynamic stability of the adsorbed state, the parameter of interest becomes the critical fraction of the SWNT length interacting with the gel surface, f L,crit, above which results in irreversible SWNT/gel binding interaction (driven by the thermodynamic stability of the SWNT/gel interaction) and below which results in SWNT elution (driven by the thermodynamic stability of the SWNT/ SDS interaction at 175 mM SDS concentration). A schematic energy diagram relating the relative stability of SWNT/gel/ SDS systems over the span of 0 ≤ f L ≤ 1 is shown in Figure 5C. The four gel size distributions shown here in f L space can be translated into the physically relevant parameter of fraction of SWNT predicted to be irreversibly bound to the gel surface using the following relationship fb =

∫0

1

P(fL )σ(f L; fL,crit , w)dfL

(3)

where P(f L) is the probability distribution of hydrogel spheres in terms of fraction of SWNT length interacting with the gel surface and σ(f L; f L,crit, w) is a sigmoid function centered at f L,crit and exhibiting fwhm of w. The use of a sigmoid function (as opposed to a stepwise cutoff) to represent the f L above which SWNT become irreversibly bound allows for a distribution of chemical behavior to be captured within a single parameter, f L,crit ± w. Some system properties that may contribute to an inhomogeneous distribution of effective f L,crit (nonzero w) include hydrogel density and chemical functionality along the SWNT/gel interaction path, SWNT/gel orientation, and SWNT chirality and length. For example, although this model assumes a spherically homogeneous hydrogel surface, in reality that surface contains pores of approximately 10 nm (Sephacryl is used for size selective protein purification) that render it inherently inhomogeneous on the scale of SWNT diameters (∼1 nm), yielding inhomogeneity in SWNT/gel interactions across many binding events. Additionally, this model treats SWNT/gel desorption as chirality independent, whereas this event has been shown to exhibit strong chiral dependency (Figure 4C). Using a sigmoid function of nonzero fwhm allows for effects due to environmental and chiral inhomogeneities to be collectively captured by f L,crit ± w, maintaining a relatively straightforward model (dependent on a single curvature parameter associated with the purification media, gel microsphere radius) with clear and relevant physical interpretation. Interestingly, the desorption event is also predicted to be SWNT length dependent. Longer SWNT will have a smaller length fraction (f L) interacting with the hydrogel resulting in a greater probability of desorption upon addition of higherconcentration SDS. Such SWNT-length sorting using hydrogel microspheres has been previously demonstrated,53,54 and yields length-dependence in SWNT/gel interaction consistent with the model introduced here. Specifically, SWNT eluted from gels exhibit longer average lengths than pre-gel-interaction SWNT, likely due (according to this model) to preferential

Figure 5. Summary of thermodynamically driven model correlating hydrogel microsphere radius with SWNT desorption efficiency. (A) The curved surfaces of hydrogel microspheres in conjunction with the rigid nature of SWNT affords consideration of a fraction of SWNT length present at distances from gel surface over which SWNT/gel interactions are expected to occur, spanning Dmin to Dmax. (B) The four hydrogel microsphere size distributions considered in this work converted from physical (radial) space to fraction of SWNT length interacting with gel surface between Dmin and Dmax space, or f L space. (C) Experimental observations of SWNT desorption suggest a thermodynamically driven model, whereby a critical fraction of SWNT length interacting with the gel surface, f L,crit governs the reversibility (f L, < f L,crit) or irreversibility (f L, > f L,crit) of the SWNT/ gel interaction. (D) General agreement between experimentally measured fraction of SWNT irreversibly bound to gel beds of varying radial distribution and the model presented here suggests that development of SWNT purification schemes utilizing either longer SWNT length or smaller gel microsphere radius would increase reversibility of SWNT/gel interaction, improving process efficiency. 3621

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ACS Applied Nano Materials irreversible adsorption of shorter SWNT which are subjected to larger fractions of SWNT/gel interaction along their length. Using the probability distributions shown in Figure 5B and a sigmoid parameter of f L,crit = 0.41 ± 0.27 yields the fraction of gel bed exhibiting SWNT interactions at or above the critical parameter f L,crit of f b = 0.61, 0.67, 0.74, and 0.75 for the gel beds presenting surface areas of 67.2, 56.9, 46.2, and 44.9 cm2, respectively. A comparison between this model and experiment (the fraction of adsorbed SWNT released from each gel bed) is presented in Figure 5D. The strong agreement between experimental observations of SWNT desorption efficiency and this model suggests that SWNTs (in the presence of 70 mM SDS) are thermodynamically stabilized in a bound state to Sephacryl because of some fraction of their length interacting with the surface of that gel. Further, when exposed to 175 mM SDS to desorb SWNTs from their bound state, those with less than ∼41% of their length lying within 0.127−0.313 nm from the gel surface will elute (and be available for collection as purified material) whereas those with greater fractions of their length exhibiting SWNT/gel interaction will remain irreversibly bound, contributing to process inefficiency. This observation indicates that gel-based SWNT purification procedures may be improved substantially (efficiency increased by up to a factor of 2) by either the use of gel microspheres with smaller diameter and/or the purification of SWNT with lengths greater than 300 nm, both of which would decrease the fraction of the SWNT length in direct contact with the surface of hydrogel microsphere media (and therefore promote elution following the addition of 175 mM SDS). Before turning to a demonstration of how the results presented herein can be used to improve SWNT/gel purification schemes, it is worthwhile to provide context for the fundamental aspects of this work. Specifically, the works of others have demonstrated chirality-dependent interactions between SWNT and Sephacryl based on both kinetic31 and thermodynamic55 principles. Here, it is demonstrated that a third factor also significantly impacts the behavior and efficiency of SWNT/gel purification: the microscale formfactor of the hydrogel purification media, an aspect complementary to previous reports on the chirality-dependent kinetic and thermodynamic behaviors. While SWNT/gel systems used in single-chirality purification schemes are dynamic and complex, the development of a comprehensive model incorporating the fundamental knowledge of this system will serve as both an achievement in advancing fundamental knowledge as well as an invaluable guide in the ongoing quest to achieve tailored, more efficient gel-based SWNT purification methods. Enhancement of SWNT Purification Efficiency Using Mechanically Fractured Hydrogel. Given the demonstrated correlation between hydrogel surface area and SWNT uptake (Figure 3), there exists an opportunity to physically modify native Sephacryl (which accounts for ∼75% of the cost of consumables necessary to perform gel-based SWNT purification) to significantly increase the efficiency of its use as a SWNT purification medium. Specifically, as-purchased Sephacryl was stirred over the course of 24 h using a magnetic stir bar, which provided sufficient agitative force to mechanically fracture most of the microspheres. This process yielded hydrogel media with significantly larger surface area than what could be produced by subjecting Sephacryl to size selective sedimentation alone, given the limitation of the inherent size distribution of native Sephacryl (Figure 1A).

Figure 6. Demonstration of the ability to achieve equivalent SWNT purification using Sephacryl modified to exhibit higher surface area via mechanical fractionation, significantly reducing the cost of gel-based SWNT purification methods. (A) Microscope images of native Sephacryl S200 before and after mechanical fractionation reveal the transformation of pristine hydrogel microspheres into a blend of hydrogel bits of varying shape and size with a net increase in exhibited gel surface area accompanied by a loss of uniform gel geometry. (B) Comparison of the absorbance characteristics of each aliquot eluted from a 10-iteration gel-based SWNT purification procedure utilizing both 21 mg of native S200 gel and 2.1 mg mechanically fractured S200 gel per iteration, respectively. Note that these are the massequivalents of dried gel per iteration, however, fully hydrated and never-dried gels were used in the procedure. (C) Resulting periteration content of each SWNT aliquot in terms of chirality provides further evidence of the ability to achieve comparable purification of SWNT using an order-of-magnitude less hydrogel by control of gel surface area.

A side-by-side comparison of the morphological differences between native Sephacryl and mechanically fractured Sephacryl is shown in Figure 6A. An important difference between the native and fractionated samples is seen in their relative size and shape distributions in that native Sephacryl presents as spherical particles with a relatively narrow size distribution, whereas fractionated Sephacryl presents as widely distributed in size and highly inhomogeneous in shape. Attempts were made to segregate fractured gel according to size using both porous membrane and glass fritted filters. In every case, the presence of fractured gel at or below the pore size of the 3622

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ACS Applied Nano Materials filtration media irreversibly clogged the filters used, thereby necessitating the implementation of as-prepared mechanically fractured gel within SWNT/gel schemes. Although the inherent nonspherical nature of the fractured gel prevents accurate quantification of hydrogel surface area using microscopy, a preliminary study based on SWNT eluted and corresponding differential absorbance spectra suggests that the mechanically fractured Sephacryl exhibits an effective surface area of approximately 10 times that of native Sephacryl. Problems similar to the aforementioned clogging of filtration media were encountered when loading mechanically fractured Sephacryl within fritted columns typically employed within SWNT purification schemes. Specifically, obstruction of flow through those frits prevented the use of column-based methods for study of mechanically fractured gels. This challenge was circumvented through the implementation of a centrifugation/decanting based method to achieve phase separation between the SWNT-suspension and the hydrogel, as detailed in Supporting Information, Section S1.2.5. To demonstrate the ability of mechanically fractured gel to achieve SWNT separation by chirality, an iterative multicolumn purification was performed by parallel passing of equivalent volumes of a multichirality SWNT suspension through a series of 10 columns containing 21 mg of native Sephacryl S200 per iteration (at 1867 cm2/g, totaling 39.2 cm2) as well as 2.1 mg of mechanically fractured Sephacryl per iteration (estimated at ∼18 000 cm2/g, totaling approximately 40 cm2). These amounts were chosen to normalize each series according to per iteration gel surface area, thus showcasing that mechanical fractionation of Sephacryl significantly reduces the amount of gel needed to achieve SWNT purification equivalent to that obtained from native Sephacryl. Utilizing the aforementioned centrifuge-based phase-separation method over the course of 10 iterations yielded per-iteration eluants containing chirally purified SWNT similar to those reported with column-based phase-separation, Figure 6B.30 A general agreement between column-based and centrifuge-based purification methods was also established, see Supporting Information, Figures S7 and S8. To afford quantitative comparison between SWNT purified by native and mechanically fractured Sephacryl, the absorbance spectrum of each elution was best-fit and deconvoluted according to SWNT chirality,56,57 the per-iteration per-chirality quantity of which can be analyzed against iteration number, Figure 6C. While requiring only one-tenth of the amount of purification media (2.1 mg/column versus 21 mg/column), the substitution of mechanically fractured, high surface area hydrogel yields approximately the same chirally purified SWNT in each column over the course of a 10-column purification. This advancement demonstrates the utility of the fundamental findings presented herein in the achievement of equivalent chirally purified SWNT while significantly reducing the amount of hydrogel consumed, by far the most expensive component of the method.

differential absorbance between SWNT suspensions before and after gel interaction. The fraction of bound SWNT released from the gel medium upon addition of higher concentration surfactant is significantly less than unity and decreases with increasing average microsphere diameter, data that was modeled by consideration of the fraction of a rigid SWNT length in contact with a curved microsphere surface. The combination of these data inspired the creation of mechanically fractured Sephacryl for use within gel-based SWNT purification schemes with an estimated factor of 10 times the surface area of native materials for overall reduction of purification media used. Creation of such a material and its equivalent performance alongside native Sephacryl demonstrates the marked advantages of using high surface area hydrogel within SWNT purification schemes. These results are useful to both those interested in understanding and improving gel-based SWNT purification methods as well as those needing to efficiently generate preparative scale quantities of chirally pure SWNTs.

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EXPERIMENTAL SECTION

Detailed procedure descriptions along with a list of all materials and equipment used are provided in Supporting Information, Section S1, whereas the most relevant experimental details are provided here, as generalized into two categories: hydrogel manipulation/characterization and SWNT/gel interaction. First, as purchased Sephacryl S200 was exchanged from its native solvent (80:20 water/ethanol) by decanting of solvent after gravitational settling and replacing with neat 70 mM aqueous sodium dodecyl sulfate solution (SDS), performed five times in succession. Hydrogel microspheres were segregated by size by homogenizing gently by hand a 9:1 by volume mixture of 70 mM SDS/settled gel in 15 mL batches within conical tubes of 17 mm diameter. Gravitational settling over 24 h produced a gel bed of approximately 1.5 mL, which was divided into five vertical portions by iterative addition, gentle agitation, and decanting of 0.5 mL aliquots of 70 mM SDS. Size distributions of hydrogel microsphere fractions were determined by analysis of images collected via upright microscope using home written software within Mathematica. High surface area mechanically fractured Sephacryl was generated by placing 50 mL of Sephacryl equilibrated with 70 mM SDS at ∼80% concentration (settled gel by volume) within a 500 mL Erlenmeyer flask and stirring at 150 rpm for 24 h with a magnetic rod-shaped stir bar of dimensions 7.5 × 1 cm (stir bar extending across most of the flask bottom). To ensure accurate comparison between different gel aliquots, the mass of dried hydrogel per milliliter of SDS/gel suspension was determined for each gel used by drying under vacuum and normalizing the used gel accordingly; however, gel aliquots used within SWNT/gel interaction schemes maintained hydration at all times. Suspensions of individualized SWNT were generated by tip horn sonication (14W, 1/2 in. tip) of a 75 mL solution of 1 mg SWNT (Signis SG65i, enriched in (6,5) chirality) per milliliter of 70 mM SDS for 15 h followed by ultracentrifugation (210 k×g) for 2 h and collection of the top 90% of each centrifuge tube. Enriched nanotubes were used in this study because of increased accuracy of spectral fitting/deconvolution while still demonstrating enhancement of chiral purity by hydrogel methods. Interaction of gels with SWNT was performed using both a fritted column and centrifuge (21 k×g for 5 min)/decant method to achieve gel−liquid phase separation, which yielded similar purification results (see Supporting Information Figures S7 and S8). The latter allowed for the use of mechanically fractured gels that were found to clog traditionally utilized porous frits. Studies of SWNT uptake by differential absorption spectroscopy involved the continuous passing/interaction of aqueous SWNT within 70 mM SDS through a gel bed for 60 min, which afforded saturation of hydrogel SWNT binding sites while still allowing spectroscopic analysis of SWNT suspensions pre- and post-gel-interaction without

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CONCLUSION The work presented herein expands understanding of hydrogel/SWNT interactions and further demonstrates how that knowledge can be applied to significantly improve current gelbased SWNT purification methods. Specifically, sorting of the commercially available hydrogel Sephacryl based on microsphere radius afforded demonstration of strong correlation between gel surface area and SWNT uptake, as quantified by 3623

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ACS Applied Nano Materials

tube Composites for High-Performance K-Organic Batteries. ACS Nano 2019, 13, 3600−3607. (8) Puttaswamy, R.; Suresh, G. S.; Mahadevan, K. M.; Arthoba Nayaka, Y. Carbon-Nanotube-Encapsulated LiTiOPO 4 Composite Electrode for Aqueous Rechargeable Battery Applications. ChemistrySelect 2018, 3 (11), 3056−3069. (9) Liu, S.; Wang, L.; Liu, J.; Zhou, M.; Nian, Q.; Feng, Y.; Tao, Z.; Shao, L. Na 3 V 2 (PO 4) 2 F 3 − SWCNT: A High Voltage Cathode for Non-Aqueous and Aqueous Sodium-Ion Batteries. J. Mater. Chem. A 2019, 7 (1), 248−256. (10) Green, A. A.; Hersam, M. C. Colored Semitransparent Conductive Coatings Consisting of Monodisperse Metallic SingleWalled Carbon Nanotubes. Nano Lett. 2008, 8 (5), 1417−1422. (11) Gnanaseelan, M.; Chen, Y.; Luo, J.; Krause, B.; Pionteck, J.; Pötschke, P.; Qi, H. Cellulose-Carbon Nanotube Composite Aerogels as Novel Thermoelectric Materials. Compos. Sci. Technol. 2018, 163, 133−140. (12) Ma, J.; Larsen, R. M. Effect of Concentration and Surface Modification of Single Walled Carbon Nanotubes on Mechanical Properties of Epoxy Composites. Fibers Polym. 2014, 15 (10), 2169− 2174. (13) Nishimura, K.; Ushiyama, T.; Viet, N. X.; Inaba, M.; Kishimoto, S.; Ohno, Y. Enhancement of the Electron Transfer Rate in Carbon Nanotube Flexible Electrochemical Sensors by Surface Functionalization. Electrochim. Acta 2019, 295, 157−163. (14) Wu, R.; Yuan, H.; Liu, C.; Lan, J.-L.; Yang, X.; Lin, Y.-H. Flexible PANI/SWCNT Thermoelectric Films with Ultrahigh Electrical Conductivity. RSC Adv. 2018, 8 (46), 26011−26019. (15) Rocha, J.-D. R.; Rogers, R. E.; Dichiara, A. B.; Capasse, R. C. Emerging Investigators Series: Highly Effective Adsorption of Organic Aromatic Molecules from Aqueous Environments by Electronically Sorted Single-Walled Carbon Nanotubes. Environmental Science: Water Research & Technology 2017, 3 (2), 203−212. (16) Lee, K.; Nojoomi, A.; Jeon, J.; Lee, C. Y.; Yum, K. NearInfrared Fluorescence Modulation of Refolded DNA AptamerFunctionalized Single-Walled Carbon Nanotubes for Optical Sensing. ACS Applied Nano Materials 2018, 1 (9), 5327−5336. (17) Liu, H.; Li, Y.; Dykes, J.; Gilliam, T.; Burnham, K.; Chopra, N. Manipulating the Functionalization Surface of Graphene-Encapsulated Gold Nanoparticles with Single-Walled Carbon Nanotubes for SERS Sensing. Carbon 2018, 140, 306−313. (18) Nikolaev, P. Gas-Phase Production of Single-Walled Carbon Nanotubes from Carbon Monoxide: A Review of the HiPco Process. ChemInform 2004, 35 (44), 307−316. (19) Hersam, M. C. Progress towards Monodisperse Single-Walled Carbon Nanotubes. Nat. Nanotechnol. 2008, 3 (7), 387−394. (20) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Sorting Carbon Nanotubes by Electronic Structure Using Density Differentiation. Nat. Nanotechnol. 2006, 1 (1), 60−65. (21) Zheng, M.; Semke, E. D. Enrichment of Single Chirality Carbon Nanotubes. J. Am. Chem. Soc. 2007, 129 (19), 6084−6085. (22) Arnold, M. S.; Stupp, S. I.; Hersam, M. C. Enrichment of Single-Walled Carbon Nanotubes by Diameter in Density Gradients. Nano Lett. 2005, 5 (4), 713−718. (23) Green, A. A.; Hersam, M. C. Nearly Single-Chirality SingleWalled Carbon Nanotubes Produced via Orthogonal Iterative Density Gradient Ultracentrifugation. Adv. Mater. 2011, 23 (19), 2185−2190. (24) Feng, Y.; Miyata, Y.; Matsuishi, K.; Kataura, H. High-Efficiency Separation of Single-Wall Carbon Nanotubes by Self-Generated Density Gradient Ultracentrifugation. J. Phys. Chem. C 2011, 115 (5), 1752−1756. (25) Li, P.; Kumar, A.; Ma, J.; Kuang, Y.; Luo, L.; Sun, X. Density Gradient Ultracentrifugation for Colloidal Nanostructures Separation and Investigation. Science Bulletin 2018, 63 (10), 645−662. (26) Zheng, M. Structure-Based Carbon Nanotube Sorting by Sequence-Dependent DNA Assembly. Science 2003, 302 (5650), 1545−1548. (27) Huang, X.; Mclean, R. S.; Zheng, M. High-Resolution Length Sorting and Purification of DNA-Wrapped Carbon Nanotubes by

dilution. The general procedure used to perform iterative purification of SWNT followed that which has been detailed elsewhere31 and involved the adsorption of SWNT to gel (in 70 mM SDS), the rinsing of excess (unbound) SWNT from gel (with neat 70 mM SDS), and the desorption/collection of SWNT from gel (with neat 175 mM SDS). Additionally, both the total volume of hydrogel bed and the SWNT volume passed through those beds were scaled according to the reduced column diameter used here (7 mm) in comparison with other studies (15 mm).30,31

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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/acsanm.9b00567. Detailed methods for the generation of sorted and fractionated Sephacryl materials. Spectroscopic deconvolution of SWNT absorbance spectra. Derivation of a geometric model of SWNT interaction with curved hydrogel microsphere surface. Comparison between column and centrifuge-based methodology within SWNT purification schemes.(PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kevin Tvrdy: 0000-0003-4949-3806 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Caleb Rolsma and Kathryn Prescott for helpful discussions and the collection of preliminary data leading to this study. K.T. would like to acknowledge the UCCS College of Letters, Arts, & Sciences for provided startup funds and NSF Award #1156932 for supporting preliminary data collection.

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