Supramolecular oligourethane gel as a highly selective fluorescent ‘‘on–off–on’’ sensor for ions †

Stimuli-responsive supramolecular gels (SRSGs) are an important class of smart materials. It is of practical importance to develop an SRSG which can both detect and remove toxic metal ions. We have designed and synthesized an aggregation induced emission (AIE)-active oligourethane (OU) gelator which self-assembles into a supramolecular gel ( OUG ), through hydrogen-bonding, p – p stacking and van der Waals interactions. By taking advantage of the weak and dynamic nature of these non-covalent bonds, OUG shows stimuli-response to multiple factors. Importantly, OUG has the capacity for real-time detection and high selectivity for Fe 3+ , HSO 4 (cid:2) and F (cid:2) . The lowest detection limits are in the range of 5.89 (cid:3) 10 (cid:2) 9 to 8.17 (cid:3) 10 (cid:2) 8 M, indicating high sensitivity. More importantly, OUG is shown to adsorb and separate Fe 3+ from aqueous solution, with an absorbing rate of up 97.5%. A simple writing board was fabricated, which could be written repeatedly and reused. OUG acts as a reversible and recyclable ‘‘on–off–on’’ fluorescence sensor via competitive cation– p and cation–anion interactions. OUG has great potential as an environmentally sustainable probe for ions.


Introduction
Stimuli-responsive supramolecular gels (SRSGs) have the ability to respond to a chemical substance, 1 light, 2 heat, 3 pH 4 or pressure. 5 They have been applied in chemical sensors, 6 displays, 7 drug deliveries 8 and other fields. 9 Responsive behavior can be achieved by a gel-sol state transition or by changing the luminescence. 7,10 The latter response works by changing the gel's fluorescence intensity or color, and can be free from the influence of temperature, 2 pH, 11 an oxidizing agent, 8 and other factors. 12 Therefore luminescence detection has considerably higher sensitivity and more reliable real-time response. [13][14][15] Traditional conjugated gelators usually suffer from aggregationcaused quenching (ACQ), which sharply weakens the emission behavior in aggregation or solid states, thereby limiting their applications. 16 The emergence of polymers/oligomers with aggregation-induced emission (AIE) properties has been a breakthrough in the field. 17,18 In addition to their excellent emission characteristics, AIE-active supramolecular gels show strong absorption activity and synergistic effects because of their large contact area with analytes. 19 Recently, our group has explored AIE-active poly/oligourethane-based unconventional luminophores, which are without typical polycyclic p-conjugated units. These materials show obvious advantages like environmental friendliness, excellent hydrophilicity, chain flexibility, ease of synthesis and structural versatility compared with traditional organic luminescent materials. [20][21][22] Fe 3+ is an indispensable element in the process of oxygen uptake and metabolism. 23 However, an excess of Fe 3+ might cause pathological diseases like cancer and organ dysfunction. 24 F À and HSO 4 À also play essential roles in human biological processes, 25,26 although undue fluoride may cause kidney problems, dental and skeletal fluorosis. 27 HSO 4 À can produce poisonous SO 4 2À under acidic conditions, which will stimulate the skin and eyes and can even cause respiratory paralysis. 28 Thus, methods to efficiently detect these ions have received extensive attention. The established detection techniques, such as inductively coupled plasma spectroscopy, 29 high performance liquid chromatography (HPLC) 30 and electrochemical methods, 31 all require tedious sample preparations, sophisticated instruments and professional operators. However, fluorescent sensor molecules, which convert and amplify the signals into a visible and easily recognized fluorescent output, offer a more significant practical method. 6,32 Herein, we report an AIE-active supramolecular oligourethane gel (OUG) and demonstrate its usage as a specific Fe 3+ sensor in an aqueous environment. The material is based on the following design criteria: (i) inserting benzophenone into an oligourethane (OU) backbone provides CQO units with prominent hydrogen-bonding sites for self-assembly, and formation of oxygen clusters, which could enhance fluorescent emission. 33 (ii) Inserting linear 1,6-diisocyanatohexane offers strong van der Waals interactions among alkyl chains, limiting internal rotation of the molecular chains, thereby blocking the non-radiative pathways and favoring AIE. Taking advantage of the rich hydrogen bond acceptors/donors (CQO/N-H) among the oligourethane skeleton, [34][35][36][37][38][39][40][41] we introduced solvents with hydrogen-bonding acceptor units (CQO or SQO) as external crosslinking agents, to self-assemble a supramolecular oligourethane gel (OUG) relying on multiple hydrogen bonds.

Synthesis and characterization
The OU was synthesized through a facile procedure as shown in Scheme S1 (ESI †), by the reaction of 4,4 0 -dihydroxybenzophenone, hexamethylene diisocyanate and DABCO in anhydrous tetrahydrofuran and end-capping with polyethylene glycol monomethyl ether to give a viscous solution. The product was purified by a counter precipitation method. 1 H NMR and FTIR characterization data are given in Fig. 1a and in the ESI, † confirming the structure of OU. The M n value of 1814 g mol À1 , calculated from the 1 H NMR data, established that OU should be classified as an oligomer. 42 The FTIR spectra (Fig. S1, ESI †) showed absorbance bands at 3323 cm À1 and 1706 cm À1 , assigned to stretching vibrations of N-H and CQO, indicating the formation of amide bonds. Absorbance bands at 2936 cm À1 and 2860 cm À1 correspond to v(-CH 2 -) and at 1163 cm À1 correspond to v(C-O-C) stretching vibrations. The UV-Vis absorption spectrum of OU in the solid-state (Fig. S2, ESI †) showed a major peak at 277 nm from a p-p* transition of the aromatic rings. 43 Self-assembly gelation OU spontaneously self-assembles in certain solvents (notably dimethyl formamide and dimethylsulfoxide) transforming into a supramolecular gel (Table S1, ESI †). The lowest critical gelation concentration (CGC) of OU is 4% (w/v, 10 mg mL À1 = 1%), and the corresponding gel-sol transition temperature (T gel ) is 85-87 1C. In order to gain an insight into the selfassembly mechanism, 1 H NMR, FTIR, XRD and urea addition experiments were conducted. 1 H NMR spectra were recorded for different concentrations of OU in DMSO-d 6 (Fig. 1b). The H a and H f proton signals are shifted ca. 0.04 and 0.03 ppm upfield compared to pure OUG upon adding 25 mM Fe 3+ . Meanwhile, the signals of protons H d (the NH groups) shifted slightly downfield ca. 0.01 ppm. 44 These results confirmed the H-bonding interactions between amide groups and van der Waals interactions between alkyl chains. Comparing the FTIR data before and after gelation (Fig. S1, ESI †), the N-H stretching absorbance bands of OUG are broader and move to significantly higher wavenumbers (3323 to 3361 cm À1 ) in the solid state compared to the gel state: these data suggest hydrogen bonds play a critical role in the gelation process. 45,46 It is well known that adding urea, which has a high propensity to form hydrogen bonds, can disrupt existing hydrogen bonds in a supramolecular structure. 47,48 Accordingly, adding urea (10 equiv.) into OUG and heating the gel, led to the formation of a sol. It was observed that after adding urea, the sol did not revert back to gel, even when the OUG-urea mixture was cooled at 15 1C for several days, indicating that the gelation is driven by hydrogen bonds among OU molecular chains (Fig. 2a). Besides, the X-ray diffraction (XRD) peaks of OUG  This journal is © The Royal Society of Chemistry 2020 2y = 20.541, 23.221 corresponding to d-spacings of 4.32 Å and 3.83 Å, respectively, also indicated the presence of p-p stacking interactions (Fig. S11b, ESI †), further promoting the selfassembly behavior. OUG showed weak fluorescence in the sol state, however, after transforming to the gel state, the emission intensity of OUG at 439 nm increased 6 times (Fig. S3, ESI †), indicating that OU is an AIE-active gelator. 49 Stimuli-responsive behaviors OUG exhibits a high selectivity to Fe 3+ over other metal ions. By monitoring the change of fluorescence, we investigated the recognition characteristics of OUG towards metal ions. Using nitrate salts as the cation sources, an aqueous metal ion solution of Na + , Ca 2+ , Co 2+ , Cu 2+ , Mn 2+ , Ni 2+ , Cr 3+ , La 3+ , Fe 3+ , Sr 2+ , Ce 3+ , Ag + , Al 3+ , Mg 2+ , Cd 2+ , Pb 2+ or Fe 2+ (c = 0.2 M) was added to the OUG to generate the corresponding metal-gels. [50][51][52][53] As shown in Fig. 3a and c, initially, the OUG had a strong blue fluorescence emission. When different metal ions were added, only Fe 3+ quenched the fluorescence of OUG. Thus, the OUG could effectively and selectively detect Fe 3+ . To further evaluate the sensitivity of OUG for Fe 3+ , the fluorescence behavior of OUG was monitored by continuous titrations with Fe 3+ . As shown in Fig. S5a (ESI †), with the increasing addition of Fe 3+ (0-1.1 equiv.), the emission intensity of the corresponding metal-gels (OUFeG) at 439 nm gradually decreased. The limit of detection (LOD) of OUG towards Fe 3+ was calculated to be 5.89 Â 10 À9 M based on the 3d/S method 54 (Fig. S4 and S5a, ESI †), confirming the high selectivity of OUG as a sensor for Fe 3+ compared with other reported sensor systems (Table S2, ESI †). The high selectivity of OUG to Fe 3+ is attributed to two reasons: firstly, unpaired electrons in Fe 3+ cause a paramagnetic effect, prompting energy dissipation of excited states through non-radiative pathways. 55 Secondly, the high ionic strength of Fe 3+ could easily induce the transfer of p-electrons from the urethane backbone to Fe 3+ through cation-p interactions. 56 both of these effects will cause the fluorescence quenching of OUG.
A simple regeneration treatment verified the recyclability of OUG. An anion solution (F À or HSO 4 À , 2 Â 10 À5 mol L À1 ; 10 mL) was added into metal-gel OUFeG, stirring the mixture for 5 min, centrifuging and recycling OUG for again detecting ions. As shown in Fig. S10 (ESI †), after five consecutive cycles, the intensity of the OUG signal is essentially unchanged, indicating the excellent recyclability and reversibility of the OUG for the detection of Fe 3+ and HSO 4 À or F À .  (Fig. 4a), indicating that the OUG combined with Fe 3+ via cation-p interactions between the urethane groups and Fe 3+ . 56,61 As Fig. 4b   2.95 ppm), which indicated the cation-anion interactions between Fe 3+ and F À or HSO 4 À could release the p-electrons of urethane groups, thus recovering the fluorescence of OUG.

Mechanism of cation-anion sensor
In the FTIR experiments (Fig. S11a, ESI †), when Fe 3+ was added into OUG to form OUFeG, the stretching absorbance bands of N-H, CQO and C-O-C shifted from 3361 cm À1 , 1708 cm À1 and 1161 cm À1 to 3480 cm À1 , 1673 cm À1 and 1158 cm À1 respectively, which further confirmed that Fe 3+ interacts with p-electrons of the urethane groups, thus influencing H-bonds between the amide groups. 6,57 After the addition of F À or HSO 4 À into the OUFeG, the CQO, N-H and C-O-C all reverted to their initial positions (Fig. S11a, ESI †). These observations suggested that F À and HSO 4 À competitively bound to Fe 3+ rather than to OUG. Moreover, the XRD peaks of OUG moved with adding Fe 3+ into OUG, and recovered when F À or HSO 4 À was added into OUFeG (Fig. S11b, ESI †).
To get further insight into the mechanism of cation-anion sensing, as shown in Fig. 5, the SEM studies were carried out. Fig. 5a demonstrates that gel OUG shows a lamellar stacking structure with a smooth surface. This structure was converted into a honeycomb structure in the metal-gel OUFeG (Fig. 5b), while in gel OUFeG + HSO 4 À and OUFeG + F À , the image again showed a smooth lamellar stacking structure ( Fig. 5c and d).
Such morphological change is attributed to the cation-p interactions between OUG and Fe 3+ , breaking hydrogen bonding between the OUG chains and modifying the supramolecular structure. 57,61 After adding F À or HSO 4 À into OUFeG, p-electrons of OUG were released, hydrogen bonds are rebuilt and the morphology is recovered. These experimental results indicated that the fluorescence of OUG can be reversibly switched by Fe 3+ (off) and then by F À or HSO 4 À (on), through repeated competition between cation-p and cation-anion interactions (Fig. 2b).
Application in the rapid removal of Fe 3+ The development of new sorbents for the sensing and extraction of metal ions from environmental and biological samples is of current importance. 62,63 The performance of OUG to effectively remove Fe 3+ from aqueous solution was analyzed by atomic absorption spectrometry (

Application as a writing display material
Based on the above-mentioned ''on-off-on'' properties, the OUG has a great potential as a rewritable fluorescent display material. As a proof-of-concept a rewritable board was constructed (Fig. 6). The detailed steps are described as follows: (i) OUG sol (10%) was poured onto a clean quartz plate surface and dried under ambient conditions to give a film emitting strong blue fluorescence under ultraviolet radiation (365 nm).
(ii) Writing the symbol ''Fe'' on the film with a brush dipped in   This journal is © The Royal Society of Chemistry 2020 aqueous Fe 3+ solution (0.3 M), a dark ''Fe'' image was clearly displayed due to the fluorescence quenching effect of Fe 3+ on OUG. (iii) The whole OUG film was transformed into a nonfluorescent display board by brushing with Fe 3+ solution. (iv) Two new letters ''S'' and ''F'' could be written again with the same brushing method using HSO 4 À and F À solutions (0.3 M), respectively. Visually, the letters emitted blue fluorescence under a UV lamp. Combining these practically very simple processes with the excellent recyclability of OUG (discussed above; Fig. S10, ESI †) means that OUG has promising applications as a fluorescent writing display material.

Conclusion
In conclusion, a novel supramolecular AIE gel, OUG, was designed and synthesized by a straightforward ''one-pot'' procedure. The dynamic and reversible noncovalent interactions endow OUG with distinct advantages of a reversible and highly sensitive response to Fe 3+ , HSO 4 À , and F À , acting as an ''onoff-on'' fluorescent sensor for these cationic and anionic species. Importantly, OUG can absorb up to 97.5% Fe 3+ from a water environment. This rapid, simple, low cost and highly sensitive material has great potential for practical applications in intelligent sensing, handling heavy metal ion pollution and environmental remediation.

Conflicts of interest
There are no conflicts to declare.