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Advanced Fiber Materials (2020) 2:204–211https://doi.org/10.1007/s42765-020-00028-w

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RESEARCH ARTICLE

Chirality Transfer in Supramolecular Co‑assembled Fibrous Material Enabling the Visual Recognition of Sucrose

Nabila Mehwish1 · Xiaoqiu Dou1 · Changli Zhao1 · Chuanliang Feng1  · Qiang Fu2

Received: 14 November 2019 / Accepted: 13 January 2020 / Published online: 4 February 2020 © Donghua University, Shanghai, China 2020

Abstract Molecular recognition of simple sugars is crucial due to their essential roles in most living organisms. However, it remains extremely challenging to achieve a visual recognition of simple sugars like sucrose in water media under physiological conditions. In this article, the visual recognition of sucrose is accomplished by a chiral supramolecular hydrogel formation through the co-assembly of a two-component fibrous solution (l-phenylalanine based gelator co-diaminopyridine, LDAP) and sucrose. H-bonding between the amino group of LDAP and the hydroxyl group of sucrose facilitates the gelation by loading sucrose into the LDAP solution. The formed hydrogel showed an amplified inversion of circular dichroism (CD) signals as compared to the corresponding LDAP solution. In addition, the effective chirality transfer was accompanied by a bathochromic shift in UV–Vis and FL spectra of the gel. Such a simple and straightforward chiral co-assembled strategy to visually recognize sucrose will have the potential use of smart gelators in saccharides separation and proteomics to be further applied in medical diagnostics and cell imaging.

Keywords Supramolecular hydrogels · Chirality inversion · Molecular recognition · Sugars · Sucrose · Glucose

Introduction

Molecular recognition of simple sugars has been an ongoing field of research for the past few decades [1, 2]. Numerous processes at the cellular level, ranging from protein fold-ing to disease development are mediated by a sugar based molecular information system [3]. However, a little knowl-edge is there about the essential aspects of sugars and molec-ular recognition of simple sugars (mono and disaccharides) like sucrose in water media. Sucrose, a common table sugar

[4], usually gets reduced to glucose and fructose upon diges-tion leading to high glucose level, which in turns may cause diabetes and other high sugar level disease like blood sugar spike [5, 6]. Consequently, the recognition of sucrose and the study of the behavior by sugar molecules in water under physiological conditions are highly in demand.

Simple sugars are uncharged, highly hydrophilic and pos-sess no potential chromophores, leading to a difficulty in analyzing them by spectroscopic or electrochemical tech-niques [7, 8]. To date, there are two main types of recep-tors developed for sugar recognition, (1) boronic acid-sug-ars covalent interactions based via pH control [9–11], (ii) receptors-sugar non-covalent interactions based [12–14]. Although a lot of earlier researches for detection of sug-ars have made progress [3, 15–19], there is scarcely any report on how to visually recognize sucrose and study the behavior of simple sugars in water media under physiologi-cal conditions.

Recently, the principle of chirality transfer in hybrid hydrogels has been developed for molecular recognition of structural isomers [20]. Inspired by this finding, a sucrose triggered chiral supramolecular assembly containing l-phe-nylalanine based gelator (LPF) [21], and diaminopyridine (DAP) is rationally designed to address the challenge of

Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s4276 5-020-00028 -w) contains supplementary material, which is available to authorized users.

* Xiaoqiu Dou douxiaoqiu@sjtu.edu.cn

* Chuanliang Feng clfeng@sjtu.edu.cn

1 State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiaotong University, Dongchuan Rd 800, Shanghai 200240, China

2 State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China

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visual recognition. The solution of LPF and DAP mixture (LDAP) can be observed to transfer into stable hydro-gel when the loading ratios of sucrose was 0.05–7 mM (Scheme 1). Interestingly, four other tested sugars (glucose, maltose, galactose, and fructose) could not form gel with LDAP because of unmatchable interaction between these sugars and LDAP, as shown in Figure S1 in supporting infor-mation (represented by glucose in Scheme 1). Moreover, the supramolecular chirality of LDAP assemblies was inversed from right-handed (P) fibers to left-handed (M) with the addition of sucrose (Scheme 1). This technique of visually recognizing sucrose along with regulation of supramolecular chirality triggered by simple sugars may provide a general-ized strategy which will have great potential for amino acid or peptide-based supramolecular chiral gelators in the field of sugar sensors [22, 23] and medical diagnostics.

Experimental Section

Materials

1,4-Benzenedicarbonyl dichloride (98%), l-phenylalanine methyl ester hydrochloride (97%), diaminopyridine (DAP), glucose, sucrose, maltose, galactose, fructose were pur-chased by Tansoole company Shanghai, China.

Synthesis

LPF was synthesized as previously reported method [24].

Gel Preparation

The LPF-DAP (1:1 molar) hydrogel is used as an example to explain the hydrogel preparation procedure. A 5 mL glass vial was used to suspend 2 mg of LPF and 0.5 mg DAP in 1 mL water as solvent. The mixture was subjected to gentle heating at 80–100 °C until gelator was completely dissolved, followed by spontaneous cooling at room temperature. Vial inversion method was used to confirm gel formation (Figure S1 in supporting information).

Morphological Characterization

Samples for scanning electron microscopy (SEM) were pre-pared by diluting the gels up to 0.03 wt%, dropped on a sili-con wafer, air dried, sprayed with gold and imaged by using FEI Quanta 250 scanning electron microscope.

Structural Characterization

Circular dichroism (CD) spectra of assemblies were recorded using a JASCO J-815 CD spectrometer operated at a bandwidth of 0.5 nm covering the wavelength range of 190–600 nm. 0.1 mm cuvette was used to determine CD at molar ratio of 1:1 for all samples while to check the selectiv-ity of sucrose by LDAP system, 1:0.05–1:10 mM for LDAP and sucrose concentration was used. The total gelator con-centration was 0.2 wt%.

Scheme 1 a Chemical structures of the components involved in co-assembled hydrogels: l-phenylalanine based gelators (LPF) and diaminopyridine (DAP), b Schematic representation of regulation

of chirality by adding selected sugars (represented by glucose) and molecular recognition test of sucrose by LPF and LDAP (LPF co DAP) system

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UV–Vis Experiments

UV–Vis absorbance spectra of assemblies were recorded using Evolution 201 UV–Vis spectrophotometer operated at a bandwidth of 0.5 nm, covering the wavelength range of 190–600 nm. A 0.1-mm cuvette was used to determine UV–Vis spectra at a molar ratio of 1:1 for LDAP-sugar concentration.

Fluorescence Experiments

LS 55 instrument was used to carry out the fluorescence studies with excitation wavelength (λex) of 263 nm and a scan speed of 400 nm min−1, covering the wavelength range of 190–600  nm. A 0.1-mm cuvette was used to determine FL spectra at a molar ratio of 1:1 for LDAP-sugar concentration.

Contact Angle Measurements

Contact angle measurement system Powereach JC2000D2 instrument was used to measure the contact angles (CAs) at ambient atmosphere and temperature. The gel was dropped on a glass plate, air-dried and a water drop was made on the dried sample. Images of water drop were recorded by a camera, with analyzation from the software supplied by the manufacturer. Ten different points were measured for each sample.

FTIR Experiments

Fourier transform infrared spectroscopy (FTIR) of xerogels was performed using Thermo scientific FTIR spectropho-tometer with KBr pellet method over the frequency range of 400–4000 cm−1. 16 scans were performed for each sample with a resolution of 4 cm−1.

Results and Discussion

In this study, a chiral co-assembled hydrogel strategy focused on the molecular recognition ability of LDAP hav-ing amino terminus group, for sucrose, possessing hydroxyl groups is investigated. To demonstrate the recognition of sucrose by the self-assembly of LDAP (LDAP-S), self-sup-porting and homogenous hydrogels were formed for a vari-ety of molar ratios of sucrose ranging from 0.05 to 7 mM, while glucose (representing four other simple sugars tested) couldn’t form gel (LDAP-G). Moreover, a phenomenon of supramolecular chirality inversion observed in sugar-based

co-assemblies as compared to the supramolecular chirality of the corresponding individual components.

SEM Studies

In order to investigate the nanoscale morphological fea-tures of the self-assemblies, scanning electron microscopy (SEM) experiments of the xerogels was conducted. Figure 1 shows the SEM images of LPF, LPF-G, LPF-S, LDAP, LDAP-G and LDAP-S. Supramolecular chirality of obvi-ous left-handed nanofibers was observed in LPF hydrogels with an average diameter of 150 nm and helical pitch of around 1.3 µm (Fig. 1a). Remarkably, for LPF-G, and LPF-S unexpected right-handed helical nanofibers obtained with the twist diameter of 120-150 nm and pitch falling in the range of 0.10-0.25 µm (Fig. 1b, c) [25]. LDAP assembled into right-handed twisted nanofibers with the twist diameter and pitch ranging from 200 to 300 nm and 1.0 to 1.1 µm, respectively (Fig. 1d). Highly twisted and intertwined fibers of LDAP-S resulted in higher twist pitch and higher diam-eter in comparison to fibers obtained by LDAP-G. LDAP-G showed left-handed fibers of pitch around 0.8–1 µm with diameter of about 150–200 nm (Fig. 1e). These fibers were changed to less twisted big nanofibers upon addition of sucrose with diameter in the range of 270–300 nm and twist pitch of about 5–6 µm (Fig. 1f). These results indicated that the nanoscale twists of opposite handedness generated by the LDAP-S caused sucrose recognition by LDAP system. The SEM images show that the handedness of the co-assembled structures can be switched by adding sugars. This inversion of chirality upon addition of sugars can be mainly attributed to the hydrogen bonding interactions between OH group of sugars and COOH/NH2 group of LPF/LDAP respectively.

CD Activity of Co‑assembled Hydrogels

Circular dichroism (CD) spectra were employed for further characterization of the nanoscale chirality. As shown in Fig. 2, the hydrogel of LPF demonstrated negative cotton effects at 219 nm and 257 nm. LDAP exhibited inversion of chirality with a small negative cotton effect at 218 nm and a broad positive peak at 262 nm. By adding the selected sug-ars to the LPF or LDAP, inversion in chirality was observed which is in accordance with the SEM images and due to non-covalent interactions. Herein, glucose representing four other sugars tested showed a negative peak at 220 nm and a positive peak at 252 nm for LPF-sugar co-assemblies. While in the case of LDAP-sugar co-assemblies, one positive peak appeared at 216 and a negative peak at 255 nm [26].

In complement to SEM images, the spectra of the samples by adding sugars were found to be equal and opposite thus an apparent chirality inversion occurred from LPF to LDAP. However, for the same concentration used in the co-gel

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system, there was no characteristic peak in the CD spectrum of glucose, sucrose, DAP, DAP-glucose, and DAP-sucrose (Figure S2 in supporting information). Since the aminopyri-dine host is achiral but chromophoric and sugar is chiral

but non-chromophoric, only their complex is (induced) CD active [27]. An interesting phenomenon was observed with the addition of sucrose to LDAP sol, i.e. LDAP-S hydrogel, which showed one positive cotton effect at 218 nm and a negative cotton effect at 265 nm with a redshift in signals attributed to higher interactions among co-assembly [18]. However, LDAP-G behaved more or less like LPF, and there was no redshift in the spectrum except the inversion in com-parison to LDAP observed, which can be attributed to the lesser affinity of glucose to LDAP system.

Insight to Sucrose‑LDAP Interaction with Spectroscopy

The UV–Vis and FL spectra of assemblies give further infor-mation about stacking mode of nanostructures. In spite of the high solubility of LDAP in water, the addition of sucrose lead to a gel that contains self-assembled left-handed twisted nanofibers of LDAP-S (Fig. 1f). LDAP in water at a concen-tration of 10 µg/mL exhibits two absorption peaks at 241 and 330 nm and a fluorescence maximum of 389 nm (Fig. 3a, b). Upon the addition of sucrose solution, unequivocal changes to both the UV–Vis absorbance and the fluorescence spectra can be clearly observed.

Fig. 1 SEM images of: a left-handed nanofibers made up of LPF, b right-handed helical nanofibers made up of LPF and G, c right-handed helical nanofibers made up of LPF and S, d right-handed twisted nanofibers of LDAP = 1:1, e left-handed twisted nanofib-

ers made of LDAP and G = 1:1:1 and f left-handed loosely twisted nanofibers of LDAP and S = 1:1:1 xerogels. The concentration of LPF in all samples was 4 µM, and the molar ratio of LPF to DAP, G and S in co-assembled gels was 1:1 mM

Fig. 2 CD spectra of LPF, LPF-G, LPF-S and LDAP-S hydrogels along with LDAP and LDAP-G sol. The concentration of LPF in all samples was 4 µM, and the molar ratio of LPF to DAP, G and S in co-assembled hydrogels was 1:1 mM

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The UV–Vis spectra LDAP-S show peaks at 244 and 332 nm exhibiting bathochromic shift in peaks of LDAP by adding sucrose. The strong dependency of the absorption on the sucrose concentration indicates the interaction of LDAP with sucrose and the redshift can be attributed to the π–π stacking mode of co-assembly [28]. It should be noted here that this behavior is unlikely to be the result of altered sugars only, as for example, the addition of glucose to LDAP led to the absorption at same peaks of 241 and 330 nm, which was explained by lesser interactions between glucose and LDAP (Fig. 3a). There was no characteristic change in UV–Vis of LPF and co sugar systems as illustrated by Figure S3 in supporting information.

Complementing the UV–Vis results are fluorescence studies, which revealed bathochromic fluorescence peak at 392 nm (attributed to LDAP), and two new peaks at 487 and 528 nm upon addition of sucrose to LDAP. The addi-tion of glucose has no significant change in fluorescence except the decrease in intensity, which can be ascribed to the changes in the microenvironment. Usually, a sign of increased hydrophilicity (Figure S5 e in supporting infor-mation) and hydrogen bonding, results in a decline in flu-orescence intensity (Fig. 3b) and this bathochromic shift with a decrease in intensity is also indicative of potential aggregation [29]. Addition of other sugars, including closely competitive glucose, fructose, maltose, and galactose did not show a significant fluorescence change (Figure S4 in supporting information).

Selective Binding Tests

We studied the selectivity of the LDAP towards sucrose by evaluating its interaction with selected sugars (glucose, galactose, maltose and fructose). These selected sugars (1.5 mM each) were dissolved in 10 µg of LDAP mL−1 of

water. The CD and fluorescence spectra of the co-assembled LDAP after dissolution with different sugars and varied con-centration of sucrose are shown in Fig. 4a, b respectively. CD cotton effect of LDAP (B in Fig. 4a) is inverted after adding four selected sugars namely glucose, galactose, malt-ose and fructose, but the peak intensity is very low and not very clear. However, by adding sucrose resulted in good peak inversion compared to LDAP (containing mixture of selected sugars), and there’s a decrease in the peak intensity with further increase in the concentration of sucrose as a result of selective binding of sucrose causing aggregation [30].

Similarly, the fluorescence intensity of LDAP (Fig. 4b) is reduced in the presence of the four selected sugars compar-ing to the spectrum obtained after the LDAP alone reflecting the binding of sugars other than sucrose by the LDAP. Inter-estingly, the fluorescence intensity is decreased, and two new bumps appeared when sucrose was added into the LDAP containing other sugars (see traces C–F in Fig. 4b). The data apparently suggest the ability of the LDAP to discriminate sucrose from other sugars and the possibility of using the LDAP for the detection of sucrose in aqueous media. This bathochromic shift of CD peak and hypochromic shift with the appearance of clear new FL peaks by adding sucrose into LDAP in the presence of other sugars confirmed the good selectivity (Fig. 4).

Contact Angle Measurement

To further support the higher affinity and gelation of LDAP with sucrose than glucose, herein, the hydrophilicity evaluation [31] as detected by contact angle measurements was made, and the results for LPF, LG, LS, LDAP, LDAP-G and LDAP-S xerogel films are shown in Figure S5 in supporting informa-tion. As shown in Figure S4 in supporting information, the

Fig. 3 a UV–Vis absorbance and b FL spectra of; DAP, DAP-G, DAP-S and LDAP-S hydrogels along with LDAP and LDAP-G sol. The con-centration of LPF in all samples was 4 µM, and the molar ratio of LPF to DAP, G and S in co-assembled hydrogels was 1:1 mM

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original film of LPF was hydrophobic with a contact angle (CA) of 74 ± 2°. After being immersed in a glucose and sucrose solution (concentration: 20 mM) for 30 min, followed by the removal of any remaining excess liquid by a N2 flow, the film became more hydrophobic with a CA of about 130 ± 1o and 101 ± 5° respectively (Figure S5b, c). By comparison, the film formed by LDAP was hydrophilic with CA 43 ± 1° (Figure S5d). Interestingly, the LDAP film becomes hydro-philic by immersing in glucose with CA (41 ± 3°), and higher hydrophilicity was observed for LDAP immersed in sucrose with CA 33 ± 2° (Figure S5e, f). Remarkably, the addition of sucrose into LDAP film selectively produced a hydrophilic film showing stronger bonding and smoother surface which is in accordance with the higher interactions of sucrose with LDAP supporting the visual recognition of sucrose [32–34].

FTIR

To investigate the gel behaviors and the valuable underlying information about the molecular-level interactions of the co-assembly process, FTIR spectroscopy was used to charac-terize molecular structures and supramolecular assemblies (Fig. 5). The xerogel of LPF showed characteristic stretching vibration bands of C=O from carboxyl groups at 1740 cm−1 which become weaker in the hybrid material, i.e. LDAP because of non-covalent interactions with the carbonyl group of LPF. Furthermore, LDAP shows a characteristic N–H stretching vibration peak (amide I band) at 3443 cm−1, an amide II at 1643 cm−1, and amide III band at 1542 cm−1 [35], which suggests that the amide–amide H-bonds were not disturbed. Furthermore, upon co-assembly with glucose and sucrose, the amide II peak at 3421 and 3317 cm−1 is shifted from higher wavenumber (3443 cm−1) respectively,

which means that the amide groups of gelators and the –OH groups of sugars are involved in the H-bonds.

In compliment with other results, the peak shift is very high ~ 100 cm−1 for the case of sucrose binding in the LDAP system, elucidating the multiple hydrogen bonding interac-tions and selectivity for sucrose. The observation implied that well-developed amide hydrogen bonds networks formed in the nanofibers, which can also be confirmed by amide I, and amide II bands, appearing at around 1643 and 1542 cm−1, respectively. Herein, FTIR suggests that a good balance of the polar (hydrogen bonding and solvation) and apolar (π–π stacking or hydrophobic) interactions (Fig. 3)

Fig. 4 Selective binding studies by a CD and b Fluorescence spec-tra of; LDAP (A) and in the presence of 1.5  mM each of glucose, maltose, galactose and fructose (B). With the addition of varied con-centration of d-sucrose, the fluorescence intensity is reduced propor-

tionally, indicating binding of sucrose with LDAP (C–F). The con-centration of LPF in all samples was 4  µM, and the molar ratio of LPF to DAP in co-assembled hydrogels was 1:1 mM

Fig. 5 FTIR spectra of DAP, LPF, LDAP, LDAP-G, and LDAP-S. The concentration of LPF in all samples was 4  µM, and the molar ratio of LPF to DAP, G and S in co-assembled gels was 1:1 mM

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among sucrose and LDAP seemed to be playing an impor-tant role for visual recognition of sucrose in water.

Conclusions

In summary, a visual recognition towards sucrose is discov-ered by the employment of the chiral co-assembled strategy with the indicative sol-to-gel-transformation. The mixture “LDAP” [containing chiral gelator (LPF) and achiral molecule (DAP)] can transfer from solution to stable hydrogel by load-ing sucrose. H-bonding between the amino group of LDAP and the hydroxyl group of sucrose facilitates the supramolecu-lar co-assembly to gelation. CD spectra showed the amplified inversion in the optical signals of the sucrose triggered gel as compared to the corresponding LDAP solution; meanwhile, a bathochromic shift of the CD signals is accompanied by the addition of sucrose. Such chiral co-assembled strategy can be valuable and potentially useful on the subject of visual dis-crimination of sucrose for further application of smart gelators in saccharides separation and cell imaging.

Acknowledgements This work was supported by the Innova-tion Program of Shanghai Municipal Education Commission (201701070002E00061), the NSFC (51833006, 51573092), Program for Professors of Special Appointment (Eastern) at the Shanghai Insti-tutions of Higher Learning, Science and Technology Commission of Shanghai Municipality (STCSM, No. 19441903000, 19ZR1425400), and Shanghai Jiao Tong University Interdisciplinary (Biomedical Engi-neering) Research Fund (No. ZH2018QNA12).

Compliance with Ethical Standards

Conflicts of Interest There are no conflicts to declare.

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Nabila Mehwish received her master’s degree in Chemistry from Quaid.i.Azam University, Islamabad, Pakistan in 2014. Since 2016, she has been a doc-toral student in school of materi-als science and engineering (SMSE), Shanghai Jiao Tong University (SJTU), China. Her research interest includes supra-molecular hydrogels for bio-medical applications.

Xiaoqiu Dou received her BSc degree in Polymeric Materials and Engineering from Ocean University of China in 2009, Ph.D degree in Materials Sci-ence and Engineering from Shanghai Jiaotong University in 2015. Since 2016, she is a post-doctor in the Department of Chemistry and Biology at Uni-versity of Siegen (Siegen, Ger-many), and awarded the Hum-boldt Research Fellowship for postdoctoral researchers. Her research interests include devel-opment of stimuli-responsive

supramolecular hydrogels for application in biomedical and biotechnol-ogy fields.

Changli Zhao received his PhD degree from Shanghai Jiao Tong University in 2012. Now he is an assistant research fellow at Shanghai Jiao Tong University. His research interests include metallic biomaterials and supra-molecular hydrogels in biomedi-cal application.

P r o f . D r . C h u a n l i a n g Feng received Doctor Degree from University of Twente (the Netherlands) in 2005, then he worked in Max-Planck Institute for Polymer Research as postdo-cotor (Mainz, Germany). From April 1998 to July 2009, he was a research scientist in Biomade Technology Foundation (Gronin-gen, the Netherlands). In August 2009, he was appointed as a full professor in School of Material Sciences and Technology, Shanghai Jiaotong University. His research mainly focuses on

functionalized polymeric nanomaterials, bioadhesion materials, supra-molecular hydrogels. Important topics are synthesis and characteriza-tion of chiral hydrogels as well as applications of biomaterials in bio-medical and tissue engineering study.

Qiang Fu received his PhD degree from Sichuan University in 1993. Now he is a full profes-sor at the College of Polymer Science and Engineer ing, Sichuan University. His research interests include polymeric mater ials processing and modification.