Surface-Oriented Assembly of Cyclodextrin Metal–Organic Framework Film for Enhanced Peptide-Enantiomers Sensing

Abstract

Open AccessCCS ChemistryCOMMUNICATION7 Nov 2022Surface-Oriented Assembly of Cyclodextrin Metal–Organic Framework Film for Enhanced Peptide-Enantiomers Sensing Li-Mei Chang, Qiao-hong Li, Peter Weidler, Zhi-Gang Gu, Christof Wöll and Jian Zhang Li-Mei Chang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 College of Chemistry, Fuzhou University, Fuzhou 350108 , Qiao-hong Li State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 , Peter Weidler Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen 76344 , Zhi-Gang Gu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108 , Christof Wöll Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen 76344 and Jian Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108 https://doi.org/10.31635/ccschem.022.202101708 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Oriented metal–organic framework (MOF) films are attracting great attention due to their fascinating physicochemical properties and unique functionalities. Here, we report an [110]-oriented biomolecular γ-cyclodextrin (γCD) MOF film with arrayed CD channels running perpendicular to the substrate surface. This sophisticated architecture was realized by combining liquid phase epitaxial layer-by-layer (lbl) methods with a γCD-based thiol self-assembled monolayer (SAM) functionalized surface. This first demonstration of the lbl method for MOF growth from aqueous conditions yielded oriented, highly homogeneous, and chiral γCD-SURMOFs (surface-coordinated MOFs) with tunable thickness. Using a quartz crystal microbalance (QCM) to monitor adsorption of biomolecules, we demonstrated that γCD-SURMOF provides highly-efficient recognition of tripeptide enantiomers (Tyr-(l-Ala)-Phe vs Tyr-(d-Ala)-Phe), clearly outperforming γCD(SH)8 SAMs as well as polycrystalline, mixed-orientations γCD-MOF film. The presence of well-aligned γCD-channels enables highly efficient transport channels with large adsorption capacity and fast loading, along with high enantioselectivity. In addition, the fast and highly specific loading rates allow for the realization of highly specific sensors for biomolecules with short response times. Download figure Download PowerPoint Introduction Metal–organic frameworks (MOFs), a class of strictly periodic reticular materials, are constructed from organic ligands and metal ions or clusters.1–4 Their very diverse structures and tunable functionalities have attracted great attention regarding a broad spectrum of potential applications.5–9 Among the plethora of different multifunctional MOFs, network materials constructed from cyclodextrin (CD)-based ligands and alkali metal salts are particularly appealing with regard to life-science applications. Such CD-MOFs (MOFs constructed from CD based ligands)10–14 are renewable and completely nontoxic. Since Stoddart and coworkers15 reported the first series of CD-MOFs from γ-cyclodextrin (γCD) and alkali metal salts in 2010, CD-MOFs, particularly γCD-MOFs with K2CO3, have emerged as a kind of homochiral, edible, and porous material. Such crystalline, supramolecular CD arrangements exhibit a rich host–guest chemistry, and the straightforward fabrication of such compounds triggered much research on the applications for adsorption and separation,14,16,17 proton conductivities,12,18 purification of petrochemicals,16 drug delivery,19–21 electrical memristors,22 and biomedicine.23 Particularly, peptides, as biomolecules constructed from two or more amino acids, serve as potential high-value and broadly applicable compounds in drug discovery. However, the standard powder form of MOFs largely prohibits applications where anisotropic transport of guest molecules is required. CD-channels, which allow transport, are of pronounced importance in the biomedical area, especially for realizing CD-based ion channels.24 However, the macroscopic alignment of CD channels for biomolecule transport when using powder particulate forms of MOFs is difficult to achieve and has not been reported to date. Similar challenges are encountered in other contexts, for example, when integrating MOFs into devices.25–28 In many cases, the availability of well-defined, oriented MOF thin films has played a key role in the areas of adsorption and separation,29–32 catalysis,33,34 as well as fabrication of optical and electronic sensors and devices.35–45 In addition, the possibility of realizing highly oriented thin films containing arrayed nanochannels to realize anisotropic transport is very appealing,46,47 for example, with regard to lowering diffusion path lengths48 and decreasing the contribution from interfacial defects.49,50 Regarding the fabrication of anisotropic, oriented, high-quality MOF thin films, the liquid-phase quasi-epitaxial (LPE) layer-by-layer (lbl) method plays a special role. Surface-coordinated MOF thin films, SURMOFs,51–53 grown on modified substrates (e.g., via adsorption of self-assembled monolayers, SAMs) have exhibited a high degree of orientation, tunable thickness, and high structural quality often exceeding that of bulk MOF materials.54–56 Although the application of the lbl strategy to realize highly oriented γCD-MOF appears to be straightforward, no successful realization has been reported so far. A first challenge in this context is that a successful application of the lbl method using aqueous conditions has not been reported. We were unable to grow oriented γCD-SURMOF using previously reported substrate-functionalization strategies.57 We overcame this problem by choosing a special type of SAM fabricated from a CD-based monomer, octathiolated γCD (γCD(SH)8) (Scheme 1), which is anchored to the Au surface via eight thiol groups installed on the rim of the CD. Applying the LPE lbl method to γCD(SH)8 SAMs functionalized Au substrate was successful. Immersing the substrate into aqueous solutions of K2CO3 and γCD in an alternating fashion yielded a highly oriented γCD-MOF film with the crystallographic [110] direction that was perpendicular to the substrate (γCD-SURMOF, see Scheme 1). Scheme 1 | Schematic illustration for the preparation of γCD-SURMOF on octathiolated γCD (γCD(SH)8) SAMs modified Au surface by using LPE lbl method. Download figure Download PowerPoint The high accessibility of the arrayed γCD channels to guest molecules was demonstrated by using a quartz crystal microbalance with dissipation (QCM-D). These data showed that γCD-SURMOF with ∼350 nm exhibited a large adsorption capacity (∼111.8 μg/cm2) for Tyr-(l-Ala)-Phe. The pronounced enantioselectivity of the γCD-SURMOF was demonstrated when the d-form enantiomer, Tyr-(d-Ala)-Phe, was adsorbed to a lesser extent (∼21.5 μg/cm2). The beneficial effect of the orientated channels becomes evident when compared with the results obtained from mixed-orientation γCD-MOF film prepared by the solution vapor method. γCD-SURMOF shows 2–4 times higher absorption capacity and 2–3 times larger uptake rate (and thus the diffusion coefficient) of the tripeptide enantiomers than that of polycrystalline γCD-MOF films. We attributed this finding to the γCD-channels perpendicular to the γCD-SURMOF surface, which can be loaded efficiently because the obstruction imposed by grain-boundaries between differently oriented crystals in polycrystalline MOF films is absent. Results and Discussion To overcome the mismatching issue between γCD-MOF and the frequently-used SAMs, such as 16-mercaptohexadecanoic acid (MHDA), during the γCD-MOF film growth, we naturally considered a γCD-based SAM for continuous and homogeneous γCD-SURMOF film growth. The γCD derivative thiol γCD(SH)8 molecules were attached to SAMs (see the model in Figure 1a and Supporting Information Figure S1) by immersing clean Au substrates in an aqueous solution of γCD(SH)8 (1 mM) for 24 h, followed by rinsing with deionized water and nitrogen gas purging. The successful assembly of Au(γCDS8) SAMs was demonstrated by IR spectroscopy and X-ray photoelectron spectroscopy (XPS) as well as water contact angle measurements. Infrared reflection–absorption spectroscopy (IRRAS) data of the obtained Au(γCDS8) SAMs showed the existence of the coupled C–O–C stretching and O–H bending vibrations (1000–1200 cm−1), and the O–H stretching vibration (∼3300 cm−1), which is consistent with that of γCD(SH)8 ( Supporting Information Figure S2). The XPS analysis confirmed the expected chemical composition (Figures 1b and 1c and Supporting Information Figures S3 and S4). The S 2p spectra from the resulting γCD(SH)8 SAMs exhibited the shape typical for thiolate-based SAMs, a doublet with the S 2p3/2 peak located at 161.5 eV. According to these results, all eight anchoring thiol groups of the γCDS8 reacted with the Au substrate to form thiolate bonds. In addition, we determined the water contact angle for the γCD(SH)8 SAMs. The contact angle observed was 26.5° on the Au(γCDS8) surface (γCD(SH)8 SAMs) as seen in Figure 1d, fully consistent with the presence of hydroxyl-terminated γCD(SH)8 monolayers. Figure 1 | (a) Surface adsorption model of γCD(SH)8 on Au(111) surface. (b) XPS spectra of Au 4f in bare Au surface and Au(γCDS8) surface. (c) XPS spectra of S 2p in γCD(SH)8 and Au(γCDS8) surface. (d) contact angles of Au(γCDS8) surface and γCD-SURMOF. Download figure Download PowerPoint The γCD-functionalized substrates were then used to grow γCD-SURMOFs by a homemade automatic dipping instrument operated at room temperature.58,59 The Au(γCDS8) substrate was subsequently immersed in K2CO3 (180 mM) and γCD (8 mM) aqueous solutions (containing 5% (v/v) methanol) for 30 min, respectively. Note, that in contrast to SURMOF grown from ethanolic and methanolic solutions, no solvent washing step was applied after dipping into the separate solutions. It is worth emphasizing that this was the first successful assembly of SURMOFs using aqueous solution. SURMOF growth was carried out for 25, 50, 100, 150, and 200 cycles. In all cases the formation of homogeneous γCD-SURMOF was observed. Out-of-plane X-ray diffraction (XRD) patterns (Figure 2a and Supporting Information Figure S5) showed three clear diffraction peaks located at 3.81°, 7.62°, 11.43°, corresponding to the (110), (220), and (330) diffraction peaks, respectively, in the XRD patterns simulated for this particular γCD-MOF.15 The in-plane XRD patterns showed distinct peaks at 5.38°, 7.62°, 10.76°, and 14.78°, which are related to the (200), (220), (400), and (521) peaks, respectively. The high-quality out-of-plane and in-plane XRD patterns as well as the small width of the diffraction peaks clearly reveal the successful growth of highly crystalline γCD-SURMOF with its [110]-crystallographic direction perpendicular to the substrate surface. As noted above, we attribute this high degree of orientation to the directed nucleation induced by the Au(γCDS8), with the γCD toroids forced into an orientation parallel to the substrate by the thiolate anchoring groups (Figure 1a). The IRRAS results recorded for the γCD-SURMOF were fully consistent with the presence of homogeneous γCD-MOF films ( Supporting Information Figure S6). Figure 2 | (a) Out-of- and in-plane XRD patterns. Surface (b) and cross-sectional SEM image (c) of γCD-SURMOF with 150 cycles. (d) The thickness of γCD-SURMOF with 25, 50, 100, 150, and 200 preparation cycles. Download figure Download PowerPoint Scanning electron microscopy (SEM) images recorded from the top and cross-sections (Figures 2b and 2c) demonstrated the presence of homogeneous and compact thin films. The thickness (as determined from the cross-sectional SEM images) amounted to ∼80, ∼112, ∼215, ∼350, and ∼395 nm for 25, 50, 100, 150, and 200 cycles, respectively (Figure 2d and Supporting Information Figure S7). For comparison, a polycrystalline γCD-MOF film was prepared by the so-called solution vapor method.57 The XRD peaks showed a mixed-orientation γCD-MOF film was obtained ( Supporting Information Figure S8a), and the SEM image ( Supporting Information Figure S8b) revealed the presence of a relatively rough and incompact adlayer. In addition, due to the abundant hydroxyl groups in the MOF, the water contact angle (Figure 1d) of the γCD-SURMOF surface changed from 18.5° from the Au(γCDS8) surface to 26.5°. We attributed the decreased contact angle to the high porosity and abundant γCD groups in γCD-MOF that lead to a pronounced hydrophilic character of the γCD-MOF film. The circular dichroism spectra ( Supporting Information Figure S9) of γCD-SURMOF showed a strong positive peak at 285 nm, indicating an obvious chiral character. CD-based materials provide excellent potential for sensing biomolecules with high selectivity, including enantiomer separation. We demonstrated the impressive performance of our γCD-SURMOF for biomolecular sensing of short peptides constructed from two or more amino acids, which are emerging as model compounds in drug discovery,60 and therefore, the specific recognition of peptides is crucial for drug development. In addition, biocompatible thin films presently receive pronounced attention with regard to detecting and monitoring enantiomers at very low concentrations.61–63 In the following, we demonstrate that γCD-SURMOF is particularly attractive in this regard. To investigate the enantiomers recognition and separation of peptides of γCD-SURMOF, the enantiomeric tripeptide pair Tyr-(l-Ala)-Phe and Tyr-(d-Ala)-Phe ( Supporting Information Figure S10) were chosen because of the low cost of both d- and l-forms of raw reagents and the ease of enantiomer sythesis, as well as their suitable size. The as-synthesized tripeptides were characterized by liquid chromatography-mass spectrometry ( Supporting Information Figures S11 and S12) and circular dichorism ( Supporting Information Figure S13). The recorded circular dichorism spectra of Tyr-(l-Ala)-Phe revealed two positive signals at 245 and 322 nm, and two negative ones at 225 and 270 nm. Tyr-(d-Ala)-Phe showed the circular dichorism signals with opposite polarity but the same intensity as Tyr-(l-Ala)-Phe, as expected for this enantiomeric pair of tripeptides. In addition, the size of Tyr-(Ala)-Phe is calculated to be about 0.5 Í 1.5 nm, which is smaller than that of the pore size (1.7 nm) and channel diameter (0.9 nm) of γCD-MOF. It reveals that the tripeptide-enantiomers can be loaded in γCD-SURMOF. The changes in photoluminescence of γCD-SURMOF upon loading with the two different tripeptides were studied. While spectra recorded for neat (empty) γCD-SURMOF showed no obvious fluorescence, after loading, intense fluorescence with the maximum emissions at ∼400 nm (excited at 290 nm) was observed, which clearly originated from the guest tripeptides ( Supporting Information Figures S14–S16). When applying the same loading conditions, Tyr-(l-Ala)-Phe loaded γCD-SURMOF (namely Tyr-(l-Ala)[email protected]γCD-SURMOF) had much more intense emission than that of Tyr-(d-Ala)-Phe loaded γCD-SURMOF, thus demonstrating that the adsorption capacity ( Supporting Information Figure S17) of Tyr-(d-Ala)-Phe is much larger than for its enantiomer for the γCD-SURMOF. To further characterize the enantiomeric selectivity of the γCD-SURMOF, the uptake of the tripeptides was studied using QCM-D, a powerful tool for the quantitative investigation of biomolecule recognition and enantiomeric discrimination by thin films.64 Herein, ethanol was used as the solvent for performing the QCM-D adsorption experiment because the γCD-SURMOF is stable in ethanol solution. By measuring the resonance frequency change of the QCM-D chip (Figure 3a), a slight mass change caused by the uptake of chemical species can be monitored with high sensitivity. To systematically study the enantiomeric recognition of tripeptides Tyr-(l-Ala)-Phe and Tyr-(d-Ala)-Phe, three different samples were investigated, γCD(SH)8 SAMs, mixed-orientation γCD-MOF film, and γCD-SURMOF (Figures 3b–3d). For the γCD(SH)8 SAMs, the adsorption uptakes of Tyr-(l-Ala)-Phe and Tyr-(d-Ala)-Phe amounted to 1.6 (ML) and 1.15 μg/cm2 (MD). The mixed-orientation γCD-MOF film (average thickness, ∼350 nm) showed substantially larger values of ML = 26.5 and MD = 9.4 μg/cm2, as expected. Figure 3 | (a) The chiral sensor structure based on QCM-D chip; the mass uptakes of Tyr-(l-Ala)-Phe and Tyr-(d-Ala)-Phe on Au(γCDS8) surface (γCD(SH)8 SAMs) (b), mixed-orientation γCD-MOF film (c), and γCD-SURMOF (d). (e–g) The comparison mass uptakes of Tyr-(l-Ala)-Phe and Tyr-(d-Ala)-Phe on Au(γCDS8) surface, mixed-orientation γCD-MOF film and γCD-SURMOF, respectively. (h) The enantioselectivities (ee) of Au(γCDS8) surface, mixed-orientation γCD-MOF film and γCD-SURMOF. The time constants comparison for the adsorption of Tyr-(l-Ala)-Phe (i) and Tyr-(d-Ala)-Phe (j) in Au(γCDS8) surface, mixed-orientation γCD-MOF film and γCD-SURMOF. (k) The adsorption model of tripeptides enantiomers between mixed-orientation γCD-MOF film (left) and γCD-SURMOF (right). Download figure Download PowerPoint The largest values were obtained for the γCD-SURMOF (150 cycles; thickness, ∼350 nm), with ML = 111.8 and MD = 21.5 μg/cm2. A compilation of the results for the adsorption experiments is shown in Figures 3e–3g. The different adsorption capacities of enantiomers allow for the calculation of the enantioselectivity (ee) by the equation ee = |ML − MD|/(ML + MD) × 100. The enantioselectivity of γCD(SH)8 SAMs, mixed-orientation γCD-MOF film, and γCD-SURMOF for Tyr-Ala-Phe were 16.4, 47.6, and 67.6%, respectively (Figure 3h). The oriented γCD-SURMOF exhibited a substantially larger adsorption capacity than the polycrystalline MOF films. This can be attributed to the aligned channels in the γCD-SURMOF structure running perpendicular to the surface. As a result, there is a larger accessibility of the MOF pores (1.7 nm), whereas in the polycrystalline film domain boundaries are likely to obstruct indiffusion of guest molecules (Figure 3k). In addition to the total uptake, the time dependence of the QCM-D data also allows for a determination of the diffusion coefficient into MOF thin films.65–67 To this end we model the uptake as M(t) = M0 + A*exp(−t/τ) ( Supporting Information Table S1), where τ denotes the time constant, M0 is the adsorption constant and t is the adsorption time. The results of fitting these data to the curves in Figures 3b–3d showed that the time constant (τ) of Tyr-(l-Ala)-Phe adsorption for mixed-orientation γCD-MOF film and γCD-SURMOF were 483 and 200, respectively, while that of Tyr-(d-Ala)-Phe adsorption were 1136 and 552, respectively (Figures 3i and 3j and Supporting Information Table S1). Using the equation D = (l)2/τ (D is the diffusion coefficient; l is the film thickness),68,69 the diffusion coefficients can be determined: for γCD-SURMOF diffusion coefficients of 6.1 × 10−16 and 2.2 × 10−16 m2s−1 were found for Tyr-(l-Ala)-Phe and Tyr-(d-Ala)-Phe, respectively, whereas for the mixed-orientation γCD-MOF film substantially lower values were observed, (2.5 × 10−16 m2s−1 for Tyr-(l-Ala)-Phe and 1.1 × 10−16 m2s−1 for Tyr-(d-Ala)-Phe) ( Supporting Information Figure S18). The faster adsorption kinetics indicated that γCD-SURMOF shows fast rates for indiffusion, due to the straight adsorption nanopaths (as drawn in Figure 3k (right)). The above QCM-D results confirmed that the γCD-MOF material is well suited for the enantiomeric sensing of tripeptides. To understand the relationship between the enantiomeric peptides recognition and γCD-MOF, we determined the thermodynamic stability of the peptide enantiomers bound to γCD(SH)8 SAMs and to the pores of γCD-MOF (Figures 4a–4d). The results revealed that the adsorption energy (Eads) of Tyr-(l-Ala)-Phe (Eads-L) and Tyr-(d-Ala)-Phe (Eads-D) on γCD(SH)8 SAMs amounted to −1.36 and −1.23 eV, while in the MOF pores the binding energies increased to −6.13 and −3.93 eV for Eads-L and Eads-D, respectively. These results illustrated the l-configuration of the peptides is more stable on both γCD(SH)8 and γCD-MOF. The normalized adsorption energy difference (|Eads-L − Eads-D|/|Eads-L|) was much larger for the MOF (0.39) than for the SAMs (0.096), revealing a more specific binding in the MOF pores than on the CD toroids exposed by the SAMs. Figure 4 | The stable structure models of the enantiomeric tripeptides Tyr-(l-Ala)-Phe (a, c) and Tyr-(d-Ala)-Phe (b, d) adsorbed in γCD(SH)8 SAMs (a, b) and γCD-MOF (c, d), respectively. Download figure Download PowerPoint Conclusion We have developed a new class of highly oriented SURMOFs fabricated from CD ligands under aqueous conditions, which show excellent enantioselectivity for tripeptides. By using a thiol-modified cyclodextrin γCD(SH)8 for surface functionalization, homogeneous γCD-SURMOF with [110]-orientation were grown on the substrate via an LPE lbl assembly strategy. The porous, nontoxic γCD-SURMOF exhibited a pronounced hydrophilic character, which is beneficial for the recognition and separation of biomolecules. Two model biomolecules, tripeptides of Tyr-(l-Ala)-Phe and Tyr-(d-Ala)-Phe were chosen as probe enantiomers models for a detailed QCM-D adsorption study. A thorough analysis of the results revealed that the oriented channels in the γCD-SURMOF led to a pronounced increase in uptake and diffusion coefficient compared to polycrystalline γCD-MOF films. This first surface-oriented growth of CD SURMOF by using CD based SAMs provides an effective avenue to prepare oriented MOF film of high quality and also promotes a new choice of chiral, nontoxic, and porous crystalline network materials for biomolecule recognition and separation applications. Supporting Information Supporting Information is available and includes additional experimental details, XRD patterns, IR spectra, circular dichroism spectra, XPS spectra, photoluminescent spectra, SEM images, and density functional theory calculations and description, The comparison for the diffusion coefficients, the comparison for the adsorption capacities and adsorption rates. Conflict of Interest There is no conflict of interest to report. Funding Information This work was supported by the National Natural Science Foundation of China (grant nos. 21872148 and 92161105), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (grant no. 2018339), and Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (grant no. 2021ZR131). C.W. acknowledges support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under the Germany Excellence Strategy via the Excellence Cluster 3D Matter Made to Order (grant no. EXC-2082/1-390761711). References 1. Hanikel N.; Pei X.; Chheda S.; Lyu H.; Jeong W.; Sauer J.; Gagliardi L.; Yaghi O. M.Evolution of Water Structures in Metal-Organic Frameworks for Improved Atmospheric Water Harvesting.Science2021, 374, 454–459. Zhang X.; X.; L.; X.; Li O. of the and of Metal-Organic J.; J.; the of Metal-Organic Frameworks and as Li L.; L.; H.; Li S.; Li J.; W.; of from Using a Metal-Organic X.; S.; J.; J.; L.; Metal-Organic Framework with for S.; Yaghi O. to Frameworks to S.; Zhang with W.; in Metal-Organic L.; L.; S.; J.; Zhang of a MOF Using a Stoddart Metal-Organic Frameworks and Their W.; X.; N.; Metal-Organic Structure to Li W.; X.; H.; H.; O. J.; Stoddart of Metal-Organic Frameworks on a O. Stoddart of in Cyclodextrin Metal-Organic Frameworks by J.; S.; Stoddart A S.; H.; J.; Yaghi O. Stoddart Frameworks from Natural J.; S.; Stoddart of J.; H.; S.; Yaghi O. 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