β-Cyclodextrin Functionalized Nanoporous Graphene Oxides for Efficient Resolution of Asparagine Enantiomers

Article abstract of DOI:10.1002/asia.201800970

Efficient resolution of racemic mixture has long been an attractive but challenging subject since Pasteur separated tartrate enantiomers in 19^th century. Graphene oxide (GO) could be flexibly functionalized by using a variety of chiral host molecules and therefore, was expected to show excellent enantioselective resolution performance. However, this combination with efficient enantioselective resolution capability has been scarcely demonstrated. Here, nanoporous graphene oxides were produced and then covalently functionalized by using a chiral host material-β-cyclodextrin (β-CD). This chiral GO displayed enantioselective affinity toward the l-enantiomers of amino acids. In particular, >99 % of l-asparagine (Asn) was captured in a racemic solution of Asn while the adsorption of d-enantiomer was not observed. This remarkable resolution performance was subsequently modelled by using an attach-pull-release dynamic method. We expect this preliminary concept could be expanded to other chiral host molecules and be employed to current membrane separation technologies and finally show practical use for many other racemates.

Full text of DOI:10.1002/asia.201800970

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: β-Cyclodextrin functionalized nanoporous graphene oxides for
efficient resolution of asparagine enantiomers
Authors: Fengxiang Qie, Jiahui Guo, Bin Tu, Xing Zhao, Yuchun
Zhang, and Yong Yan
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Asian J. 10.1002/asia.201800970
Link to VoR: http://dx.doi.org/10.1002/asia.201800970
A Journal of
A sister journal of Angewandte Chemie
and Chemistry – A European Journal

10.1002/asia.201800970
Chemistry - An Asian Journal
β-Cyclodextrin functionalized nanoporous graphene oxides for
efficient resolution of asparagine enantiomers
Fengxiang Qiea,§, Jiahui Guoa,b,§, Bin Tua,§, Xing Zhaoa, Yuchun Zhanga, and Yong
Yana,*
aCAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for
Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing
100190, China
bUniversity of Chinese Academy of Sciences, Beijing, 100049, China
§These authors contributed equally to this work
*Correspondence to: yany@nanoctr.cn
1
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
Efficient resolution of racemic mixture has long been the most attractive but
challenging subject since Pasteur separated tartrate enantiomers in 19th century.
Graphene oxide (GO) could be flexibly functionalized by using a variety of chiral
host molecules and therefore, was expected to show excellent enantioselective
resolution performance. However, this combination with efficient enantioselective
resolution capability has been scarcely demonstrated. Here, Nanopores graphene
oxides were firstly produced and then covalently functionalized by using a chiral
host material—-cyclodextrin (-CD). This chiral GO displayed enantioselective
affinity toward the L-enantiomers of amino acids. Particularly, > 99% of L-
asparagine (Asn) was captured in a racemic solution of Asn while the adsorption
of D-enantiomer was not observed. This remarkable resolution performance was
subsequently modelled by using an attach-pull-release dynamic method. We
expect this preliminary concept could be expanded to other chiral host molecules
and be employed to current membrane separation technologies and finally show
practical use for many other racemates.
2
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
The two enantiomers of chiral molecule are non-superimposable mirror images but
have almost the same chemical and/or physical properties and therefore, are usually
hard to separate.[1] However, enantioselective resolution of these two enantiomers is of
particular importance because distinct biological and/or pharmacological effects were
commonly found in chiral biomolecules and pharmaceuticals.[2] Many enantioselective
resolution techniques including chromatography, crystallization, and membrane-based
separations such as chiral membranes and/or molecularly imprinted membranes have
been developed.[3] Although these techniques exhibit various advantages, they still face
important technical limitations, such as high energy consumption, high cost, low
efficiency, discontinuous operation, and difficult to scale-up.[4] On the other hand,
graphene oxide contains plenty of reactive oxygen groups (-COOH, -OH, carbonyl, and
epoxy), which provides possibility for chemical modification and produces functional
materials for a wide range of applications.[5] In particular, these GOs have been
demonstrated to efficiently separate various materials such as cations,[6] gases,[7] and
organic molecules[8] because of their properties such as large surface area, tunable
interactive sites, as well as adjustable interlayer spacing.[9]
Chiral molecules, including polymers[10] and biomolecules,[11] have also been
utilized to functionalize these graphene oxides and show typical enantioselective
interaction toward guest enantiomers.[12] In this way, chiral GO enantioselective sensors
were demonstrated.[13] However, this general recognition phenomenon has been rarely
applied to separate enantiomers or racemates because of great challenges in fabricating
chiral GOs with efficient enantioselective resolution capabilities.[14] This low efficiency
3
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
is probably due to insufficient chiral host molecules and mismatch between chiral host
and guest pairs.[15] Moreover, instead of investigating the “response/signal” difference
between pure D- and L-enantiomers in these chiral sensors, separating one enantiomer
from a mixture of racemic enantiomers is of particular difficult because of fierce
competition between the two enantiomers.[16] Furthermore, sites for molecular
interaction is also important for the host-guest recognition. Ariga et al conducted a
systematic investigation of a great variety of combinations of the host monolayers and
aqueous guests where the molecular recognition is performed at the air-water
interface.[17] Here, our method is, therefore, covalently bonding nanoporous GOs with
chiral host materials as much as possible and performing the host-guest molecular
recognition in aqueous solution. Meanwhile, we choose a classic host-guest pair of β-
CD and racemic amino acid solutions where enantioselective interaction was found for
many other systems but seldom for GOs.[18]
Figure 1a shows a schematic illustration of enantioselective resolution of amino
acids by using chiral graphene oxide. β-CDs as chiral host molecules were covalently
connected on GO through an amidation reaction between -COOH and -NH2 groups. To
increase the density of chiral host molecules, nanopores in GO were created. This
nanoporous chiral GOs could selectively bind the L-enantiomers in an amino acid
racemates solution. The D-enantiomers were then collected by isolating these GOs with
simple centrifuging or filtrating process. Here, five amino acid racemates were mixed
with nanoporous chiral GOs and apparent enantioselective resolution phenomenon has
been observed (Figure 1b, HPLC results). Specifically, the ratio of L-enantiomer for all
4
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
five amino acids was lower than 50%, which indicated that these L-enantiomers were
selectively adsorbed. In addition, more than 99% of L-Asn was captured by these chiral
GOs, leaving almost enantiopure D-Asn solution. This surprising efficient resolution
capability (~99% enantiomeric excess, ee) was hard to achieve especially for those
using chiral graphene oxides.
To synthesize these nanoporous chiral GOs, commercial graphene oxides (Figure
2a, no apparent nanopores) were firstly oxidized by using nitric acid due to the
relatively higher chemical activities intrinsically associated with the defect sites of GOs
(see details in the Supporting information). This produced nanoporous GO (N-GO)
where the density of –COOH group has increased and reached the maximum value at
ca. 4 hrs oxidation (Figure 2d, statistics and S1, XPS data). Specifically, ~30% more –
COOH groups were produced and its ratio increased to ~10%. Further oxidation led to
a decrease of –COOH density which is probably because as the increase of nanopore
diameter, the effective surface area and total edge length have reduced. These
nanoporous structures were confirmed by comparing the TEM images where these 4~5
nm nanopores (Figure 2c, 4 hrs oxidation, the inset show the distribution of nanopores)
were gradually expanded to ca. 80 nm nanopores (Figure S2, 10 hrs oxidation). The
pore size distribution and surface area of N-GO were subsequently investigated by
using TEM and nitrogen adsorption-desorption isotherms (Figure S3 and S4, Table S1,
see details in the Supporting information). On the other hand, it must emphasize that
these nanoporous GOs could be readily dispersed and stabilized in aqueous solution for
more than one year (Figure 1b photographs, ca. 1 mg∙mL-1). This indicated that these
5
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
nanoporous GOs might be monodispersed and was then confirmed by AFM result
(Figure S5). We believe this monodisperse characteristic is beneficial for the
performance of following resolution performance because any agglomeration would
reduce the effective density of active sites (-COOH) and subsequent chiral host
molecules.
Next, chiral host molecules (β-CD) were then covalently bonded onto these
nanoporous GOs by using the EDC/NHS method.[19] The feed ratio between β-CD and
GO was determined by recording the changes of the zeta potential because of their
distinct surface charge characteristics (Figure 2e, green and orange histograms). As
more host molecules bonded, the Zeta potential of the complex gradually decreased and
finally stabilized at a steady value (Figure 2e, blue histograms), which indicated that
the majority of -COOH was transformed into -CONH- groups. This amidation reaction
was further confirmed by carrying out FTIR and XPS spectra measurements. The FTIR
spectra of β-CDs, nanoporous GOs, and their complex were shown and compared in
Figure 2f. The nanoporous GOs displayed characteristic stretching vibrational modes
at 1730, 1625, 1216, and 1039 cm-1, corresponding to C=O (carbonyl), C=C (aromatic),
C–OH (hydroxyl), and C–O (epoxy) respectively (Figure 2f, orange curve).[20] After
functionalization (Figure 2f, blue curve), the stretching band of the amide C-N (1240
cm-1), a sharp stretching vibration of -CH2 (2930 cm-1), as well as typical features of
glucose units and glucose rings (500-1200 cm-1) were appeared in these nanoporous
chiral GOs.[21] Moreover, the characteristic peaks of amide group were also found in
these complex where a broad peak at 1605 cm-1 was due to bending vibration of N-H
6
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
group and a band at 1660 cm-1 corresponded to the stretching of C=O groups.[22] The
XPS spectra of nanoporous GOs before and after functionalization were subsequently
recorded (Figure 2g). C-N (399.7 eV) and N-H (401.7 eV) peaks were observed in these
functionalized GOs while no apparent peak was found for nanoporous GOs (Figure 2g,
orange curve).[22-23]
The amount of connected β-CD on these N-GOs was subsequently quantified by
using thermogravimetric analysis. As shown in Figure S6, β-CD-nanoporous-GOs
show a 3.6% weight loss below 120 ºC as a result of the release of absorbed water, and
a further rapid weight loss (13.1%) from 140 to 250 ºC which is attributed to the
removal of the oxygen-containing functional groups. Comparing to non-functionalized
nanoporous GOs, β-CD-nanoporous-GOs displayed a sharp drop in the range of 250-
340 ºC, with a mass loss of 23.1 wt%, should be attributed to the decomposition of β-
CD moieties, which is consistent with the thermal decomposition profile of pure β-CDs.
These nanoporous chiral GOs were then applied for resolving amino acid
enantiomers. In a racemic mixture solution, chiral GOs were added to capture the
enantiomers and then isolated. It was found that excess D-enantiomers of asparagine
were left in the solution where a negative peak at ca. 200 nm appeared in the circular
dichroism spectrum (Figure 3a). This indicated that the nanoporous chiral GOs
selectively captured L-Asn and the host-guest interaction between L-Asn and GOs was
stronger comparing to the other one. In addition, more D-enantiomers were collected as
the increase of incubation time (Figure 3a, and b, red square left y-axis). This was
further confirmed by using HPLC measurements. As the increase of interaction time,
7
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
the peak of L-Asn gradually decreased and finally disappeared while no apparent
changes have been observed for D-Asn (Figure 3c, and b, black triangle and blue circle,
right y-axis). Specifically, more than 99% of L-Asn was adsorbed by chiral GOs and
almost enantiopure D-Asn was obtained.
Four additional amino acids, phenylalanine (Phe), tryptophan (Trp), leucine (Leu),
and histidine (His) were subsequently tested. Similarly, L-enantiomers of these acids
were also captured by nanoporous chiral GOs, but the enantiomeric excess values were
much lower comparing to asparagine (Figure 1b, S9). Such an efficient resolution
capability (~99% ee) toward Asn enantiomers was scarcely found and we believe both
nanoporous GOs and β-CD-Asn host-guest pair were indispensable. Nanoporous
structures provided sufficient chiral host sites while the specific but not strong host-
guest interactions guaranteed enantioselectivity.
The enantioselective resolution capabilities of these chiral GOs were then evaluated
by using an attach-pull-release dynamic model.[24] In this model, enantiomers were
forced to penetrate though the cavity of β-CD in four steps, specifically, bound step,
attached step, pulled step, and unbound step (Figure 4a-d). In these steps, 60
consecutive interaction sites were calculated. At each site, the enantiomers were
allowed to stay for ca. 2.5 ns which is enough to find their most stable conformation.
The magnitude of applied force and corresponding work were determined by the non-
covalently weak interactions (e.g. H-bonding, Van der Waals force) between the host
and two enantiomeric guest molecules. Finally, the overall binding free energies were
calculated from
∆퐺푏표ꢀ푛푑 = −(푊푎푡푡푎푐ℎ + 푊푝푢푙푙 + 푊푟푒푙푒푎푠푒−푐표푛푓 + 푊
)
푟푒푙푒푎푠푒−푠푡푑
8
This article is protected by copyright. All rights reserved.

was calculated by using 퐾푏ꢀ푛푑 = exp
(∆퐺푏ꢀ푛푑 푅푇) equation (ca. 1.9E8).
10.1002/asia.201800970
Chemistry - An Asian Journal
equation.[24] Here, the free energies of both asparagine and tryptophan were calculated
and compared. For Asn, the binding energies of L- and D-enantiomers were -6.78
kcal∙mol-1 and -6.36 kcal∙mol-1 respectively which were much lower than Trps (L-Trp,
-11.29 kcal∙mol-1 and D-Trp -11.28 kcal∙mol-1). This indicated that the host-guest
interaction between Trp and β-CD was much stronger where a higher binding constant
표
⁄
However, the enantioselective resolution performance should be determined by the
binding energy difference between two enantiomers rather than absolute binding energy
between host and guest molecules. The energy differences of two enantiomers of
asparagine and tryptophan were calculated as 0.42 kcal∙mol-1 and 0.01 kcal∙mol-1
respectively (Figure 4e, blue histograms, left y-axis). In this way, Asn with lower
binding constant should display stronger enantioselective resolution capability than Trp
which agreed well with experimental observations (Figure 1b). The binding constant
⁄
ratios (퐾퐿 퐾퐷) between two enantiomers were also calculated. This ratio for Asn was
ca. 2 while for Trp was very close to 1 (Figure 4e, orange histograms, right y-axis),
further confirmed that these chiral molecules displayed stronger enantioselective
resolution capability toward asparagines.
Next, we inferred that the main reasons for the stronger interactions between L-
asparagines and β-CD-nanoporous-GOs are attributed to 1) The structures---the
dominant interaction between amino acids (tryptophan, phenylalanine, histidine) and
β-CD-nanoporous-GOs is the π-π interaction. While for leucine and asparagine, the
main interaction with β-CD-nanoporous-GOs is hydrogen bonding interaction.
9
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
Comparing to π-π interaction (~10 KJ.mol-1), hydrogen bonding (~25-40 KJ.mol-1)
possesses a higher binding energy. Besides, asparagine has more interaction sites for
hydrogen bonding. 2) The chiral selectivity is dependent on the β-CD which is in favor
of L- over D-type.
Finally, it is important to compare the recognition efficiency of our β-CD-
nanoporous-GOs with other GO-based techniques. Deng has synthesized chiral 3D
porous hybrid foams by using optically active helical-substituted polyacetylene and
reduced graphene oxide (RGO). This foam was used as additive to induce
enantioselective crystallization of racemic alanine. The ee reached the maximum value
(88%) at third cycle with 96 hrs interaction.[25] Hong et al developed a novel GO-
modified affinity capillary monoliths by employing human serum albumin (HSA) or
pepsin as chiral selector. Amino acid enantiomers were chosen as the model analytes
and the resolution (Rs) and separation factor (α ~ 1.2) are used to evaluate the chiral
separation capability.[11b] Meng et al fabricated graphene-based chiral membranes and
the enantioseparation performance toward 3, 4-Dihydroxy-D or L-phenylalanine were
studied. Notable separation factor (α ~ 1.25) was achieved even at flux as high as 250
nmoles∙cm-1∙h-1.[12a] With different evaluating parameters, it is hard to compare the
enantioseparation performance of these GO-based chiral materials. However, in the
context of ee, our method is competitive where 99% ee has observed for asparagine
enantiomers.
To conclude, we have produced a new type of nanoporous chiral graphene oxides.
These chiral GOs show efficient enantioselective resolution capability toward
10
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
asparagines. A molecular dynamic model was proposed to explain these observations.
Looking forward, we wish these nanoporous chiral GOs could be combined with
current membrane and/or column chromatography (as stationary phase materials)
separation technologies and show real application in resolving important chiral
materials such as pharmaceutical intermediates, agrochemicals, and fragrances.[26]
Supplementary information
Electronic supplementary information (ESI) available: Materials and methods,
Theoretical details, and additional figures.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Ministry of Science and Technology of China (No
2016YFA0203400) and the Chinese Academy of Sciences.
References
[1] G. Subramanian, Chiral Separation Techniques: A Practical Approach, Wiley-
VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2006, p.
[2] a) S. Ahuja, Chirality of Biomolecules and Biotechnology Products, John Wiley &
Sons, Inc., Hobken, New Jersey, 2011, p; b) F. Jamali, R. Mehvar and F. M. Pasutto, J.
Pharm. Sci. 1989, 78, 695-715.
[3] a) T. J. Ward and K. D. Ward, Anal. Chem. 2012, 84, 626-635; b) W. H. Pirkle and
T. C. Pochapsky, Chem. Rev. 1989, 89, 347-362; c) A. Svang-Ariyaskul, W. J. Koros
and R. W. Rousseau, Chem. Eng. Sci. 2009, 64, 1980-1984; d) R. Xie, L.-Y. Chu and
J.-G. Deng, Chem. Soc. Rev. 2008, 37, 1243-1263; e) M. Ulbricht, J. Chromatogr. B
2004, 804, 113-125.
[4] N. M. Maier, P. Franco and W. Lindner, J. Chromatogr. A 2001, 906, 3-33.
[5] a) Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts and R. S. Ruoff, Adv.
Mater. 2010, 22, 3906-3924; b) V. Georgakilas, J. N. Tiwari, K. C. Kemp, J. A. Perman,
A. B. Bourlinos, K. S. Kim and R. Zboril, Chem. Rev. 2016, 116, 5464-5519.
11
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
[6] A. Esfandiar, B. Radha, F. C. Wang, Q. Yang, S. Hu, S. Garaj, R. R. Nair, A. K.
Geim and K. Gopinadhan, Science 2017, 358, 511-513.
[7] H. W. Kim, H. W. Yoon, S.-M. Yoon, B. M. Yoo, B. K. Ahn, Y. H. Cho, H. J. Shin,
H. Yang, U. Paik, S. Kwon, J.-Y. Choi and H. B. Park, Science 2013, 342, 91-95.
[8] A. Morelos-Gomez, R. Cruz-Silva, H. Muramatsu, J. Ortiz-Medina, T. Araki, T.
Fukuyo, S. Tejima, K. Takeuchi, T. Hayashi and M. Terrones, Nat. Nanotechnol. 2017,
12, 1083-1088.
[9] L. Chen, G. Shi, J. Shen, B. Peng, B. Zhang, Y. Wang, F. Bian, J. Wang, D. Li and
Z. Qian, Nature 2017, 550, 380-383.
[10] W. Li, B. Wang, W. Yang and J. Deng, Macromol. Rapid Commun. 2015, 36, 319-
326.
[11] a) A. W. Hauser, N. Mardirossian, J. A. Panetier, M. Head-Gordon, A. T. Bell and
P. Schwerdtfeger, Angew. Chem. Int. Ed. 2014, 53, 9957-9960; b) T. Hong, X. Chen,
Y. Xu, X. Cui, R. Bai, C. Jin, R. Li and Y. Ji, J. Chromatogr. A 2016, 1456, 249-256.
[12] a) C. Meng, Y. Sheng, Q. Chen, H. Tan and H. Liu, J. Membr. Sci. 2017, 526, 25-
31; b) I. Fuchs, N. Fechler, M. Antonietti and Y. Mastai, Angew. Chem. Int. Ed. 2016,
55, 408-412.
[13] a) E. Zor, I. H. Patir, H. Bingol and M. Ersoz, Biosens. Bioelectron. 2013, 42, 321-
325; b) W. Wei, K. Qu, J. Ren and X. Qu, Chem. Sci. 2011, 2, 2050-2056; c) Q. Han,
Y. Wang, Y. Huang, L. Guo and Y. Fu, Analyst 2013, 138, 2051-2056; d) E. Zor, M.
Esad Saglam, S. Alpaydin and H. Bingol, Analytical Methods 2014, 6, 6522-6530; e)
S. Ates, E. Zor, I. Akin, H. Bingol, S. Alpaydin and E. G. Akgemci, Analytica Chimica
Acta 2017, 970, 30-37; f) X. Mao and H. Li, Journal of Materials Chemistry B 2013, 1,
4267-4272; g) J. Ou, Y. Zhu, Y. Kong and J. Ma, Electrochemistry Communications
2015, 60, 60-63; h) Y.-f. Hu, Z.-h. Zhang, H.-b. Zhang, L.-j. Luo and S.-z. Yao, Talanta
2011, 84, 305-313; i) X. Juanjuan, W. Qinghong, X. Chunzhi, X. Qiao, L. Xia and F.
Yingzi, Electroanalysis 2016, 28, 868-873; j) Y. Tao, J. Dai, Y. Kong and Y. Sha,
Analytical Chemistry 2014, 86, 2633-2639.
[14] Y. Xiao, H.-Q. Wang, H. Zhang, Z.-Q. Jiang, Y.-Q. Wang, H. Li, J. Yin, Y.-Y.
Zhu and Z.-Q. Wu, . Polym. Sci. A 2017, 55, 2092-2103.
[15] a) B. Kesanli and W. B. Lin, Coord. Chem. Rev. 2003, 246, 305-326; b) T. H.
Webb and C. S. Wilcox, Chem. Soc. Rev. 1993, 22, 383-395.
[16] a) H. J. Federsel in 2.17 - Chiral Drug Discovery and Development – From
Concept Stage to Market Launch A2 - Taylor, John B, (Ed. D. J. Triggle), Elsevier,
Oxford, 2007, pp. 713-736; b) X. Lin, R. Gong, J. Li, P. Li, J. Yu and A. E. Rodrigues,
Journal of Chromatography A 2016, 1467, 347-355.
[17] a) K. Ariga, T. Michinobu, T. Nakanishi and J. P. Hill, Curr. Opin. Colloid
Interface Sci. 2008, 13, 23-30; b) T. Michinobu, S. Shinoda, T. Nakanishi, J. P. Hill, K.
Fujii, T. N. Player, H. Tsukube and K. Ariga, J. Am. Chem. Soc. 2006, 128, 14478-
14479; c) K. Ariga, G. J. Richards, S. Ishihara, H. Izawa and J. P. Hill, Sensors 2010,
10, 6796-6820.
[18] R.-P. Liang, C.-M. Liu, X.-Y. Meng, J.-W. Wang and J.-D. Qiu, Journal of
Chromatography A 2012, 1266, 95-102.
12
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
[19] N. Suzuki, Y. Wang, P. Elvati, Z. B. Qu, K. Kim, S. Jiang, E. Baumeister, J. Lee,
B. Yeom, J. H. Bahng, J. Lee, A. Violi and N. A. Kotov, Acs Nano 2016, 10, 1744-
1755.
[20] X. T. Zheng and C. M. Li, Molecular Pharmaceutics 2012, 9, 615-621.
[21] W. Feng, C. Liu, S. Lu, C. Zhang, X. Zhu, Y. Liang and J. Nan, Microchim. Acta
2014, 181, 501-509.
[22] I. Kaminska, M. R. Das, Y. Coffinier, J. Niedziolka-Jonsson, J. Sobczak, P. Woisel,
J. Lyskawa, M. Opallo, R. Boukherroub and S. Szunerits, ACS Appl. Mater. Interfaces
2012, 4, 1016-1020.
[23] H. J. Kim, I.-S. Bae, S.-J. Cho, J.-H. Boo, B.-C. Lee, J. Heo, I. Chung and B. Hong,
Nanoscale Res. Lett. 2012, 7, 1-7.
[24] N. M. Henriksen, A. T. Fenley and M. K. Gilson, J. Chem. Theory Comput. 2015,
11, 4377-4394.
[25] B. Wang, W. Li and J. Deng, J. Mater. Sci. 2017, 52, 4575-4586.
[26] E. Sanganyado, Z. Lu, Q. Fu, D. Schlenk and J. Gan, Water Research 2017, 124,
527-542.
13
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
Figure 1. (a) The scheme of enantioselective resolution by using nanoporous chiral
graphene oxides. β-CDs were covalently bonded on nanoporous GO and selectively
bound the L-enantiomers in amino acid racemic solution. (b) Resolution capability of
nanoporous chiral GO (0.8 mg.mL-1) toward five amino acid racemates (3 mM),
specifically, phenylalanine (Phe), tryptophan (Trp), leucine (Leu), histidine (His), and
asparagine (Asn). The nanoporous chiral GO selectively bound the L-enantiomer for all
five amino acids and displayed the best performance toward Asn racemates (D-Asn% >
99%). Note: 1) The ratio is calculated by comparing the HPLC peak area of D- and L-
enantiomers. 2) The error bar is based on four independent experiments. 3) The upper
solution is used for HPLC experiment (see details in experimental section) and
therefore, the ratio of L-enantiomer is less than D-enantiomer indicates that the
nanoporous GO selectively binds L-enantiomer.
14
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
Figure 2. Synthesis of nanoporous graphene oxides (N-GO) and functionalization of
these GOs by using β-CDs. TEM images and photographs of the aqueous dispersions
of commercial GO (a, b-left) and nanoporous GO (oxidation for 4 hrs, c, b-right). The
concentration of nanoporous GO is ca. 1 mg∙mL-1. Nanopores (ca. 4~5 nm in diameter)
were created and more –COOH groups were produced on GOs. (d) The ratios of –
COOH group on GOs oxidized for different time (calculated from XPS data). (e) to (g)
Zeta potential, FTIR spectra, and N1s XPS spectra of nanoporous GOs before and after
functionalization. The zeta potential was monitored by changing the feed ratio between
β-CD and GO (0.8 mg∙mL-1). The feed ratio is then fixed at 40 for FTIR, XPS and the
following resolution experiment.
15
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
Figure 3. Enantioselective resolution of Asn. (a) The circular dichroism spectra of the
upper solution after enantioselective interaction between nanoporous chiral GOs and
Asn racemates for different time. The inset shows the spectra of enantiopure D-Asn, L-
Asn, and r-Asn, which indicated that D-Asn was accumulating in the upper solution
while L-Asn was gradually adsorbed by nanoporous chiral GOs. (b) The statistics of
circular dichroism intensity (red square, left y axis) and HPLC peak area (D-Asn and L-
Asn, black triangle and blue circle, right y axis) of the upper solution. The error bar is
based on four independent experiments. (c) Typical HPLC result of the upper solution
at different time.
16
This article is protected by copyright. All rights reserved.

10.1002/asia.201800970
Chemistry - An Asian Journal
Figure 4. (a) to (d) The enantioselective interaction between β-CD and amino acids
was modelled by using an attach-pull-release method. The enantiomers were forced to
penetrate though the cavity of β-CD and the overall binding energies were calculated.
(e) The difference of binding energy (blue histogram, left y axis) and corresponding
binding constant ratio (orange histogram, right y axis) between D- and L-enantiomer of
Trp and Asn.
17
This article is protected by copyright. All rights reserved.