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Supramolecular host-guest system as ratiometric Fe3+ ion sensor based on water-soluble pillar[5]arene Qianfang Yao, Baozhong Lv, Chendong Ji, Yang Cai, and Meizhen Yin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12063 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017
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ACS Applied Materials & Interfaces
Supramolecular Host-guest System as Ratiometric Fe3+ Ion Sensor Based on Water-soluble Pillar[5]arene
Qianfang Yao, Baozhong Lü, Chendong Ji, Yang Cai, Meizhen Yin* State Key Laboratory of Chemical Resource Engineering, Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, 100029 Beijing, China KEYWORDS: Supramolecular host-guest, morphological transformation, photoinduced electron transfer, metal detection
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ABSTRACT: Developing a specific, ratiometric and reversible detection method for metal ions is significant to against the threat of metal-caused environment pollution and organisms poisoning. Here a supramolecular host-guest system (WP5⊃G) based on water-soluble pillar[5]arene (WP5) and water-soluble quaternized perylene diimide derivative (G) was constructed. Morphological transformation was achieved during the process of adding WP5 into G aqueous solution and a fluorescence “turn-off” phenomenon was observed which caused by supramolecular photoinduced electron transfer (PET). Meanwhile, hydrophobic effect and electrostatic interaction played important roles in this supramolecular process, which was confirmed by Isothermal titration calorimeter (ITC) and Zeta potential experiments. Furthermore, the supramolecular host-guest system could be a “turn-on” fluorescent probe for Fe3+ ion detection through the process of interdicting supramolecular PET. Moreover, the Fe3+ ion detection showed specific, ratiometric and reversible performances with detection limit of 2.13×10-7 M, which might have great potentials in biological and environmental monitoring.
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INTRODUCTION Nowadays, supramolecular host-guest systems have been widely investigated for their various advantages, such as low cost,1 convenient synthesis,2 less time-consuming3 with noncovalent interactions and new functions (ion detection and recognition), which are different from host or guest molecule alone.4-5 Pillar[n]arenes, as a new generation of macrocyclic host after crown ethers,6 cyclodextrins,7-8 calixarenes,9 cucurbiturils10 and other hosts,11 have raised significant studying interests in supramolecular host-guest systems with their incorporation properties.12-14 In the family of pillar[n]arenes, pillar[5]arenes and pillar[6]arenes are widely investigated due to their easy synthesis and functionalization.15-18 For example, responsive supramolecular host-guest systems based on pillar[n]arenes for ion sensing and drug controlled release have been developed recently.19-21 In environment and biological systems, metal ions, especially heavy metal ions, have caused severe environment pollutions and toxicity to biological systems. In order to find an effective way to detect metal ions, fluorescent probes have gained extensive attentions due to their high sensitivity and convenience.22-23 For example, perylene diimide (PDI) derivatives are widely used as fluorophores to construct fluorescent probes due to their excellent chemical, thermal, and photoelectronic stability.24-25 However, many reported systems require the participation of organic solvents that is unfavourable for ion detection in biological and environmental systems.26 Furthermore, inherent aggregation caused fluorescence quenching (ACQ) properties of PDI derivatives in aqueous media could further hamper their sensitivity.27 Therefore, it is very important to develop water-soluble PDI derivatives with excellent detection performance in aqueous solution, and supramolecular host-guest system should be one of ideal candidate.
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Scheme 1. Cartoon representation of the formation of supramolecular host-guest system (WP5⊃ G). In this study, we designed a supramolecular host-guest system (WP5⊃G) in aqueous media based on water-soluble hydroxyl modified PDI derivative (G) and water-soluble pillar[5]arene (WP5) (Scheme 1). G alone exhibited irregular aggregates. With the addition of WP5 to G, the aggregates transformed to regular blocks along with fluorescence “turn-off” because of photoinduced electron transfer (PET). Furthermore, the designed supramolecular host-guest system, WP5⊃G, had a specific response to Fe3+ ion through the process of interdicting supramolecular PET. WP5⊃G could promisingly be applied in environmental and biological monitoring with the advantages of selective, ratiometric and reversible Fe3+ ion sensing in aqueous solution. EXPERIMENTAL SECTION Reagents and materials 1,4-Dimethoxybenzene
(98%),
paraformaldehyde
(98%),
borontribromide
(98%),
trifluoromethanesulfonic acid (98%), ethyl bromoacetate (98%), ammonium hydroxide (98%), perylenetetracarboxylic dianhydride (98%), N,N-dimethyl propylene diamine (98%), 2-
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bromoethano (98%), N,N-dimethyl-butylamine (98%) were all obtained from Alfa Aesar. Chlorizated salts of Fe3+, K+, Na+, Li+, Cu+, Zn2+, Mg2+, Cd2+, Hg2+, Fe2+, Sn2+, Cu2+, Ca2+ were purchased from Xiya Reagent with the purity of 99%. All these regents were used without further purification. Characterization 1
H-NMR spectra were observed on a Bruker 400 spectrometer at room temperature. Matrix-
assisted laser-desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) were determined on AXIMA-CFR plus MALDITOF mass spectrometer. Mass spectra (MS) were measured with a XEVO-G2QTOF (ESI) (Waters, USA). UV-visible spectra and Fluorescence (FL) spectra were obtained with a spectrophotometer (Cintra 20, GBC, and Australia) and fluorescent spectrofluorimeter (Horiba Jobin Yvon FluoroMax-4 NIR, NJ, USA) respectively. The morphologies of G and WP5⊃G were observed with HITACHI S-4700 (Japan) scanning electron microscope (SEM) manipulated at an accelerating voltage of 10 kV. Samples (1×10-5 M, T=298.15K) were vacuum sputtered with Pt to increase the contrast before SEM observation. Isothermal titration calorimeter (ITC) experiments were conducted with Nano ITC (TA Instruments Waters, LLC, UT). Dynamic Light Scattering (DLS) and Zeta potential of samples were observed by Malvern Zetasizer Nano instrument with compatible disposable capillary cell (DTS 1070 from Malvern). The electrochemical cyclic voltammetry curves were measured on Zahner IM6e Electrochemical Workstation with Pt disk coated with the samples, Pt plate, and Ag/Ag+ electrode as working electrode counter electrode and reference electrode respectively in a 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution.
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Synthesis and complexation of G with WP5 The detailed synthesis processes of WP5, G and G’ are described in the ESI (Scheme S1-S3). WP5 was synthesized according to previous literature.17 The structures of synthetic intermediate and final products were confirmed by NMR and MS (Figure S1-S4). For the purpose of preparing a 5:1 supramolecular host-guest system (WP5⊃G, 1×10-5 M), 2 µL aqueous solution of WP5 was added into 2 mL aqueous solution of G (1×10-5 M) dropwise. As for 1:1 ([G]/[WP5]), 10 µL aqueous solution of WP5 (2×10-3 M) was added into 2 mL aqueous solution of G (1×10-5 M) dropwise. RESULTS AND DISCUSSION Confirmation of supramolecular host-guest system A water-soluble PDI derivative containing two positive parts in each end was designed as a guest for WP5 to establish a supramolecular host-guest system (WP5⊃G).18, 28-30 To confirm the formation of this supramolecular host-guest system, a simple G’ was used as a model guest. 1H NMR spectra of equimolar WP5 and G’ in aqueous solution was measured. As shown in 1H NMR spectra (Figure S5), the protonic peaks of Ha, Hb, Hc, Hd, He and Hf of G’ shifted upfield and the protonic peaks of H1, H2 and H3 of WP5 shifted downfield,31-32 which indicated the complexation of G’ and WP5. Since MALDI-TOF MS was an effective method to study host-guest complexes,33 MALDI-TOF MS of equimolar aqueous solution WP5 and G was observed to further confirm the complexation (Figure S6), showing the construction of this host-guest system. Meanwhile, in order to estimate the stoichiometry of WP5⊃G, Job’s plot was conducted (Figure S7),8,34-36 indicating that WP5 ⊃ G had a 1:1 stoichiometry. All these characterization results proved that a supramolecular host-guest system was constructed successfully.
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Optical and Assembling properties of WP5⊃ ⊃G
Figure 1. (a) Concentration-dependent UV-Vis absorption spectra of G. (b) UV-Vis absorption spectra and (c) Fluorescence spectra of G aqueous solution with the addition of WP5 (1×10-5 M). (d) Dependence of the optical absorbance intensity at 560 nm on the molar ratio of [G]/[WP5] in aqueous solution. Since PDI derivatives were easily aggregated because of their strong π-π stack, the optical and assembling properties of G were first investigated. As shown in the concentration-dependent UVVis absorption spectra of G (Figure 1a), the featured peaks at 466 nm (S0-2), 499 nm (S0-1) and 540 nm (S0-0) had been limitedly influenced except for an enhancement in the absorbance intensity, which indicated the exist of predominantly monomeric form of G.37-38 The ratio of S0-0 and S0-1 could be used to provide a view into the degree of aggregation in solution. The absorbance intensity ratio of A0-0/A0-1
