Letter Cite This: Org. Lett. 2018, 20, 2972−2975
pubs.acs.org/OrgLett
Coupling Reagent for UV/vis Absorbing Azobenzene-Based Quantitative Analysis of the Extent of Functional Group Immobilization on Silica Ra-Young Choi,§ Chang-Hee Lee,§ and Chul-Ho Jun* Department of Chemistry, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea S Supporting Information *
ABSTRACT: A methallylsilane coupling reagent, containing both a N-hydroxysuccinimidyl(NHS)-ester group and a UV/vis absorbing azobenzene linker undergoes acid-catalyzed immobilization on silica. Analysis of the UV/vis absorption band associated with the azobenzene group in the adduct enables facile quantitative determination of the extent of loading of the NHS groups. Reaction of NHS-groups on the silica surface with amine groups of GOx and rhodamine can be employed to generate enzyme or dye-immobilized silica for quantitative analysis.
M
olecular level engineering of organic functional groups on inorganic solid surfaces, such as glass and silica, has received considerable attention in the material science field.1 The increased interest in surface engineering technologies, including cell adhesion,2 enzyme immobilization,3 DNA probe films,4 and chemical and biosensor,5 has led to a demand for methods to measure concentrations of chemical functional groups on solid surfaces quantitatively and simply.6 Thus, far, some studies have been conducted to develop techniques for quantification of surface functional groups, such as those relying on X-ray photoelectron,7 elemental analysis,8 infrared spectroscopy,8 and contact angle measurements.9 However, these approaches have limitations, the most significant of which is difficulty associated with precisely determining the extent of the loading.6b,d Among them elemental analysis is a very reliable but destructive method, and it requires a highly pure sample to determine the loading efficiency. In previous efforts, we developed a catalytic grafting method, using methallyl-10 and vinylsilane11 derivatives, for facile, room temperature modification of silica or glass surfaces with organic functional groups (Figure 1a). Unlike alkoxysilanes1a,b and chlorosilanes,12 typically used for this purpose, methallylsilane derivatives are stable and readily purifiable, thus, enabling them to be useful for introduction of functional groups in a quantitative manner. Despite their high utility, the extent of organic functional group incorporation into silica using the methallylsilane derivatives can be determined by using timeconsuming and troublesome elemental analysis. To overcome this limitation, we envisioned that inclusion of a chromophore, such as an azobenzene moiety, into a methallylsilane coupling reagent would lead to a simple, nondestructive, and rapid UV/ vis spectroscopic method to assess loading efficiencies without being effected by nonchromophoric impurities. Based on this proposal, we designed a new methallylsilane coupling reagent, which contains a UV/vis absorbing azobenzene group and a N© 2018 American Chemical Society
Figure 1. (a) Previous approach using methallylsilane derivatives. (b) Conceptual basis of the new method.
hydroxysuccinimidyl (NHS) ester group (Figure 1b). In the studies described below, we want to report that the new reagent can be utilized to assess the loading efficiencies of NHS-ester groups onto silica by using simple UV/vis absorption spectral analysis. Moreover, dyes and enzymes can be immobilized on the generated NHS-functionalized silica. Finally, the activities and efficiencies for detecting metal ions by these conjugates were found to correlate linearly with the extent of the loading of the coupling reagent. The synthetic sequence used to prepare the new azobenzene and NHS group containing methallylsilane coupling reagent 7 is given in Scheme 1. The sequence begins with an azocoupling reaction between ethyl 4-aminobenzoate (1) and phenol to produce the diazo-product 2, which reacts with Received: March 30, 2018 Published: April 30, 2018 2972
DOI: 10.1021/acs.orglett.8b01016 Org. Lett. 2018, 20, 2972−2975
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Organic Letters Scheme 1. Preparation of Coupling Reagent 7
Figure 3. (a) Immobilization of coupling reagent 7 on the silica surface. (b) Extent of loading 7 on NHS-Azo@Si. (c) Correlation between the extent of loading 7 on NHS-Azo@Si and UV/vis absorbance of NHS-Azo@Si at 360 nm.
Figure 4. (a) Immobilization of GOx on NHS-ester-grafted silica. (b) Correlation between the UV/vis absorbance NHS-Azo@Si at 360 nm and the activity of GOx-Azo@Si (based on 7.8 × 10−2 mg silica). Figure 2. (a) Immobilization of 7 and modification of NHS-ester on the silica surface with 10. (b) UV/vis spectrum of DABS-Azo@Si.
7 (6 mg) with silica (100 mg) in the presence of TfOH (9, 10 mol % based on 7) at room temperature for 12 h produces the surface-modified silica NHS-Azo@Si. The loading extent of the NHS-ester-linked azobenzene moiety in NHS-Azo@Si was determined by using elemental analysis to be 0.10 mmol/g.14 The UV/vis spectrum of NHS-Azo@Si contains a λmax at 360 nm (Figure 2b). Reaction of NHS-Azo@Si with dabsyl amine 10 (3 equiv based on the loading of 7) takes place at room temperature to form DABS-Azo@Si (method A), whose UV/ vis spectrum contains a new dabsyl group derived absorption peak at 480 nm along with one at 360 nm attributed to the azobenzene moiety. The dabsyl and azobenzene group functionalized methallylsilane 11, prepared by reaction of 7 with sulfonamide 10, reacts with silica in the presence of TfOH to form DABS-Azo@Si (method B), whose UV/vis spectrum is identical to that of the substance prepared by reaction of NHS-Azo@Si with 3 equiv of 10. The results confirm that
propargyl bromide in the presence of K2CO3 to form the propargylic ether 3. Hydrolysis of 3 by using LiOH generates the free acid 4, which undergoes a Cu(I)-promoted [3 + 2]cycloaddition reaction13 with 11-azidoundecyl-dimethallylmethylsilane (5) to form the triazole 6 in 76% yield. Finally, reaction of 6 with N-hydroxysuccinimide in the presence of 1ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC) produces the coupling reagent 7 in 73% isolated yield after chromatographic purification (see the Supporting Information (SI)). The methallysilyl group in 7 readily reacts with hydroxyl groups on the silica surface (8a, particle/pore size: 10 μm/10 nm) under acid conditions to form UV/vis absorbing NHS group-immobilized silica (Figure 2a).10a Specifically, reaction of 2973
DOI: 10.1021/acs.orglett.8b01016 Org. Lett. 2018, 20, 2972−2975
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Organic Letters
Figure 5. (a) Immobilization of 12 (1.8 × 10−2 mmol, 3 equiv based on the loading of NHS-Azo(1.13)@Si) on NHS-ester-grafted silica and the rhodamine ring opening reaction promoted by using Sn2+. (b) Fluorescence spectra of Sn2+-Rhod-Azo@Si. (c) Correlation between UV/vis absorbance of NHS-Azo@Si at 360 nm and the fluorescence intensities of Sn2+-Rhod-Azo@Si at 580 nm. (d) Fluorescence intensities at 580 nm of Rhod-Azo(X)@Si (X = 0.33, 0.63, 1.13) with various metal cations (3 × 10−3 mmol, 10 equiv based on the loading of Rhod-Azo(1.13)@Si).
coupling reagent 7 can be immobilized on silica without destroying the NHS-ester group, which can readily react with the amine group of dabsyl amine on the surface to form an amide bond. To investigate the relationship between UV/vis absorbance and the extent of loading of the NHS-ester-linked azobenzene moiety in NHS-Azo@Si, different amounts of the NHSfunctionalized methallylsilane derivative 7 were utilized in the immobilization reaction with silica 8a. The coupling reagent 7 (2, 6, 10, 40, 100, 130, 150, 250 mg) was reacted with silica (100 mg) in the presence of TfOH (9, 10 mol % based on 7) at room temperature for 12 h to give NHS-Azo@Si with different extents of loading (Figure 3a). When 2 mg of 7 are used, the loading extent of NHS-Azo@Si is 0.04 mmol/g (Figure 3b), and when the amount of 7 employed in the reaction is increased to 150 mg, the loading extent of NHS-Azo@Si increases to 0.60 mmol/g. A further increase in the amount of 7 (250 mg) does not lead to a significant increase in the extent of loading. The results of UV/vis spectroscopic analysis shows that the intensity of absorbance at 360 nm, associated with the azobenzene chromophore in NHS-Azo@Si with a loading extent of 0.04 mmol/g, is 0.25 (Figure 3c). When the loading extent in NHS-Azo@Si is increased to 0.60 mmol/g, the intensity of the UV/vis absorbance band at 360 nm increases to 1.13. Moreover, a linear correlation was found to exist between the loading extent and the UV/vis absorbance intensity at 360 nm. Therefore, the amount of the NHS-ester groupimmobilized on silica can be directly determined in a quantitative manner by measuring the UV/vis absorbance of the resulting modified silica. It was also found that there was a similar correlation when the immobilization reaction was carried out with silica 8b (particle/pore size: 10 μm/30 nm) having a pore size different from that of silica 8a (Figure 3b and 3c, entries 9 and 10).
Because of their interesting binding and catalytic properties, enzymes are interesting substances to immobilize on silica.3 These enzymatic properties can be used to show the correlation between the UV/vis absorbance and the loading extent of NHS-Azo@Si. In the current effort, we focused our attention on immobilization of glucose oxidase (GOx), which contains a lysine amine group that reacts with the NHS-ester group of NHS-Azo@Si to form an amide bond (Figure 4a). Specifically, treatment of 5 mg of NHS-Azo(X)@Si (UV/vis absorbance, X = 0.25, 0.33, 0.45, 0.63, 0.84, 0.99, 1.13) with 5 mg of GOx at 0 °C for 2 h leads to production of GOx-Azo@Si. The enzymatic activity of GOx-Azo@Si was measured by a spectrometric assay for determining the amount of H2O2 generated from glucose and O2.10c,e The activity of GOx-Azo(0.25)@Si derived from NHS-Azo(0.25)@Si was found to be 25 μM (Figure 4b). As the extent of loading in the NHS-Azo@Si, determined by using UV/vis absorbance measurements, gradually increases up to 1.13, the enzyme activity of the produced GOx-Azo@Si increases to 112 μM in a linear manner. This result shows that the UV/vis absorbance of NHS-Azo@Si also linearly correlates with the amount of GOx immobilized on the silica surface. Therefore, the loading extent of GOx immobilized on silica can be experimentally controlled by simply adjusting the UV/vis absorbance of NHS-Azo@Si. To assess the metal ion sensing properties of dyes immobilized on silica,15 Rhod-Azo@Si was prepared by reacting rhodamine ethylenediamine 12 (1.8 × 10−2 mmol, 3 equiv based on the loading of NHS-Azo(1.13)@Si) with NHSAzo(X)@Si (UV/vis absorbance, X = 0.25, 0.33, 0.45, 0.63, 0.84, 0.99, 1.13) (Figure 5a). Addition of Sn2+ (3 × 10−3 mmol, 10 equiv based on the loading of NHS-Azo(1.13)@Si) to Rhod-Azo(X)@Si was found to generate the rhodamine ringopened product Sn2+-Rhod-Azo(X)@Si, in conjunction with fluorescence “turn-on” at 580 nm (Figure 5b).16 The intensity 2974
DOI: 10.1021/acs.orglett.8b01016 Org. Lett. 2018, 20, 2972−2975
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of fluorescence of Sn2+-Rhod-Azo(0.25)@Si was determined to be 55 au. As the loading extent in NHS-Azo(X)@Si used to generate Rhod-Azo(X)@Si is gradually increased up to 1.13, the intensity of fluorescence emission from Sn2+-Rhod-Azo(X) @Si also increases to 634 au in a linear manner (Figure 5c). A study was carried out to determine if Rhod-Azo@Si can be used as a selective sensor for Sn2+. For this purpose, various metal ions, including Sn2+, Zn2+, Hg2+, Cu2+, Ni2+, Mn2+, Ca2+, and Na+, were added to solutions of Rhod-Azo(0.33)@Si in methanol while recording fluorescence emission intensities (Figure 5d). Among various metal ions, Sn2+ promotes the strongest enhancement in the emission intensity associated with its promotion of ring opening of rhodamine B. When the same amount of Sn2+ is added to solutions of Rhod-Azo(0.63) @Si and Rhod-Azo(1.13)@Si, the intensities of fluorescence emission dramatically increase. In contrast, other metal ions do not promote this change. This result shows that the RhodAzo(1.13)@Si can be used as a selective sensor for Sn2+. Moreover, the sensitivity of Rhod-Azo@Si for Sn2+ can be controlled by experimentally adjusting the amount of immobilized NHS-ester groups. In the investigation described above, we developed a new methallylsilane coupling reagent that contains an NHS-ester group and a UV/vis absorbing azobenzene linker. This reagent enables facile quantitative analysis of the extent of immobilization of organic functional groups on the silica surface by simply measuring the intensity of the azobenzene derived UV/vis absorption band. We also showed that a linear correlation exists between the UV/vis absorbance of the immobilized linker and both the enzymatic activity of GOx and the metal cation detection efficiency of rhodamine immobilized on silica. The sensitivity of rhodamine-immobilized silica for detecting Sn2+ can be adjusted by simply controlling the loading extent of NHS-ester groups, which can be monitored by using UV/vis absorption spectroscopy.
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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/acs.orglett.8b01016. Compound characterization data, 1H and 13C NMR spectra (PDF)
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chul-Ho Jun: 0000-0002-5578-2228 Author Contributions §
R.-Y.C. and C.-H.L. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a grant from the National Research Foundation of Korea (NRF) (Grant 2016-R-1A2b4009460). 2975
DOI: 10.1021/acs.orglett.8b01016 Org. Lett. 2018, 20, 2972−2975
