Summary
A series of N-alkyl-substituted polybenzimidazoles (SPBIs), synthesized by simple condensation and N-alkylation, act as functional materials with tunable microstructures and sensing performance. For their controllable morphologies, the formation of nano-/microspheres is observed at the n(RBr)/n(PBI) feed ratio of 5:1. Products with different degrees of alkylation can recognize metal ions and nitroaromatic compounds (NACs). For example, SPBI-c, obtained at the feed ratio of 1:1, can selectively detect Cu2+, Fe3+, and NACs. By contrast, SPBI-a, obtained at the feed ratio of 0.1:1, can exclusively detect Cu2+ with high sensitivity. Their sensing mechanisms have been studied by FT-IR spectroscopy, SEM, XPS, and DFT calculations. Interestingly, the SPBIs can adsorb Cu2+ in solution and show good recyclability. These results demonstrate that polymeric materials with both sensing and adsorption applications can be realized by regulating the alkylation extent of the main chain, thus providing a new approach for the facile synthesis of multifunctional materials.
Subject areas: organic chemistry, Photonics, polymers, Optical materials
Graphical abstract
Highlights
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Tunable multifunctional sensor materials were synthesized by simple reaction
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Morphology and sensing property can be controlled by the extent of N-alkylation
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SPBIs can selectively and sensitively detect multiple analytes by dual channels
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SPBIs can reversibly and efficiently adsorb Cu2+ as indicated with UV-vis signal
Organic chemistry; Photonics; Polymers; Optical Materials
Introduction
The multianalyte detection concept, which was first proposed by De Silva's group (Magri et al., 2006; Schmittel and Shu, 2012; Chen et al., 2015; Magri, 2021), has become a hotspot in the field of sensors owing to its advantages of high efficiency, rapidity, simultaneous recognition and in-situ detection (Xu et al., 2017; Park et al., 2018; Zhang et al., 2018; Chen et al., 2019a; Zhao et al., 2021; Slenders et al., 2021; Huang et al., 2021b; Nakamitsu et al., 2021). Multianalyte sensors were originally designed for multiple metal ions (Magri et al., 2006), but these systems have subsequently been developed for multiple bioactive molecules (Yin et al., 2018; Zhang et al., 2020b; Thanzeel et al., 2020), bacteria (Zheng et al., 2018), etc. In some cases, multifunctional sensors based on fluorescent gels (Zhang et al., 2018; Ozay et al., 2020) or polymers (Ozay et al., 2020; Liu et al., 2020; Huang et al., 2021a) can simultaneously remove analytes, thus reducing the cost of pollutant treatment in practical applications. However, there are still some challenges in the field of multifunctional sensor materials. For example, when recognizing multiple analytes, the sensitivity of sensor may be reduced to some extent and it is slightly lower than that of a traditional sensor (Schmittel and Shu, 2012; Lochman et al., 2019). Moreover, there is an urgent need to develop a simple, universal method for the preparation of multifunctional sensing materials.
Benzoxazole materials, which show eye-catching fluorescent properties, are not limited to the field of covalent organic frameworks (COFs) (Seo et al., 2019). For example, small benzimidazole fluorescent molecules can sensitively and rapidly detect various analytes through different interactions (Wu et al., 2016, 2018; Ge et al., 2018; Jiang et al., 2019; Chen et al., 2020b). However, the small adsorption capacities of these materials hinder adsorption applications, and thus polymeric or other macromolecular materials are required to achieve simultaneous analyte removal by benzoxazole-based sensors (Rabbani et al., 2017; Lv et al., 2021). Although polybenzimidazole (PBI) has been widely applied in a variety of areas (Liang et al., 2019; Geng et al., 2019; Wang et al., 2019; Shan et al., 2019; Cui et al., 2021; Jin et al., 2021), there are only a few reports on sensing (Park and Gong, 2017; Diao et al., 2018; Kaur et al., 2019) owing to the disadvantages of unmodified PBI, including poor solubility, weak fluorescence, and difficulties in achieving uniform dispersion in sensing systems. We speculated that the introduction of flexible chains into the backbone of PBI through simple N-alkylation would weaken π-π stacking between the polymer chains, thus reducing aggregation-caused quenching phenomena and improving the fluorescence properties of PBI. Moreover, the introduction of alkyl chains could improve the solubility of PBI, which is beneficial for multifunctional sensing (Figure 1).
Figure 1.
Construction of a multifunctional polymeric sensor based on substituted polybenzimidazole (SPBI)
Herein, we report the design and synthesis of a series of substituted polybenzimidazole (SPBI) sensing materials via a metal-free catalytic route (Scheme 1) as well as the applications of these materials. PBI with a linear framework was constructed by dehydration condensation between 3,3′-diaminobenzidine (1) and glutaric acid (2), a dicarboxylic fatty acid. Then, a series of SPBIs with tunable microstructures and sensing performance was obtained by further modification of PBI via simple N-alkylation at different feed ratios (Table 1).
Scheme 1.
Synthetic route for substituted polybenzimidazole (SPBI)
Table 1.
Effect of feed ratio on the basic structure of SPBI
Sample | Feed ratio (RBr/PBI) | R1 | R2 | R3 | R4 | R5 |
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SPBI-a | 0.1:1 | Pentyl- | H | H | H | – |
SPBI-b | 0.5:1 | Pentyl- | H | H | H | – |
SPBI-c | 1:1 | Pentyl- | Pentyl- | Pentyl- | H | – |
SPBI-d | 2:1 | Pentyl- | Pentyl- | Pentyl- | H | – |
SPBI-e | 3:1 | Pentyl- | Pentyl- | Pentyl- | H | – |
SPBI-f | 4:1 | Pentyl- | Pentyl- | Pentyl- | Pentyl- | Pentyl- |
SPBI-g | 5:1 | Pentyl- | Pentyl- | Pentyl- | Pentyl- | Pentyl- |
Results and discussion
Characterization and basic properties of SPBIs
The structures of materials were systematically characterized by 1H NMR (Figures S1–S9 and Table S1), FT-IR spectroscopy (Figure S10 and Table S2), PXRD, and XPS. In 1H NMR spectrum of PBI, the chemical shifts at 7.41–7.48, 7.51–7.61, and 7.65–7.82 ppm were assigned to aromatic hydrogens (Ha, Hb, and Hc) in the repeating unit, and that at 12.35 ppm was assigned to -NH- (Hd) in the benzimidazole ring. Further, the chemical shifts at 2.31–2.36 and 2.92–2.98 ppm corresponded to the alkyl segment (Hf and He) in the repeating unit of PBI. The signals for -CH2- at the end of polymer chain (1.20–1.50 and 2.54–2.63 ppm; Figure S1) were used to estimate the number-average molecular weight (Mn), and the results are summarized in Table S3. Here, we discuss the results for SPBI-c and SPBI-g (Table 1) as representative examples [see the Supplemental Information for further characterization data and spectra of PBI and SPBIs (Figures S1–S8 and Table S1)]. New characteristic signals, including those at 0.69–0.90 ppm (Hk), 1.10–1.32 ppm (Hi and Hj), 1.57–1.75 ppm (Hh), and 4.28 ppm (Hg), were observed in SPBI prepared by the N-alkylation of PBI (Figures 2 and S9). Moreover, the N-H (Hd) signal was weakened gradually, indicating the successful alkylation of PBI.
Figure 2.
1H NMR spectra of PBI, SPBI-c, and SPBI-g
In the FT-IR spectra, SPBI samples exhibited similar characteristic peaks (Figure S10 and Table S2), with a gradual enhancement of the stretching vibrations (at 2957, 2931, and 2855 cm−1) of saturated C-H and its bending vibration (at 1414 cm−1), and the vibrations of the -CH2- groups in alkyl segments (at 724 cm−1) as the proportion of C5H11Br increased. These changes reveal an increase in the alkylation ratio of SPBI.
The XPS spectra of SPBI products were measured according to a previously reported method (Yu et al., 2019; Li et al., 2021). As shown in Figure S11A, the peaks located at 285.1, 397.6, and 528.1 eV were attributed to C1s, N1s, and O1s, respectively, in the backbone of SPBI-c. Moreover, the high-resolution C1s spectrum showed four peaks at 284.6, 285.1, 285.5, and 286.2 eV (Figure S11B), assigned to C-C/C=C, C-N/C-O, C=N, and C=O, respectively, in the terminal group of SPBI-c. The N1s spectrum could be divided into two peaks at 399.9 and 401.6 eV (Figure S11C), corresponding to C-N and C=N, respectively (Jiao et al., 2019; Li et al., 2019b). Thus, the XPS data further confirm the construction of a polymer skeleton.
Using reported methods (Geng et al., 2019; Zhang et al., 2019a, 2019b; To et al., 2020; Fan et al., 2021), the crystallinity and thermal stability of each material were investigated. As depicted in Figure S12, the PXRD pattern of PBI exhibited a broad peak at 15°–35°, indicating an amorphous state. This broad peak remained after alkylation, demonstrating that the SPBI materials are also amorphous. The thermal stabilities of the SPBI were investigated under a N2 atmosphere. As shown in Figure S13, after being modified by alkyl chains, the SPBI began to decompose in the range of 416.6–419.8°C, with the termination of thermal weight loss occurring at 470.0–477.4°C (Table S4), demonstrating the good thermal stability of the SPBIs.
Importantly, the morphologies of materials were evaluated by SEM as reported methods (Shan et al., 2019; Zhang et al., 2019a; To et al., 2020; Fan et al., 2021) (Figure 3). The surface of PBI was uneven and loose powder with stacked micropores was located between the layers. SPBI-a, prepared using an n(C5H11Br)/n(PBI) feed ratio of 0.1:1, showed a morphology similar to that of PBI. In contrast, the morphology of SPBI-b (feed ratio of 0.5:1) began to change into small particles, indicating the effective modification with alkyl chains. Furthermore, SPBI-c (feed ratio of 1:1) formed a network structure. Some small balls were observed in SPBI-d (feed ratio of 2:1) and a regular network was observed for SPBI-e (feed ratio of 3:1). Further increasing the feed ratio to 5:1 (SPBI-g) resulted in the formation of some regular nanospheres, owing to ionization during the N-alkylation of PBI. This salt-type product was affected by electrostatic repulsion to form nanoparticles. Thus, not only the extent of PBI alkylation but also the morphology of the alkylated products can be adjusted by using the feed ratio of the reactants. In particular, nanoparticles can be formed at a high feed ratio when increasing n(C5H11Br).
Figure 3.
SEM images of SPBIs prepared using different feed ratios
Obviously, the N-alkylation of PBI is random and heterogeneous, but this randomness at different feed ratios automatically regulates the polymer structure due to the existence of the steric hindrance. Therefore, for the preparation process of SPBI, it is very simple to wholly regulate the structures and properties of the polymer. And the following researches on the performance and application of SPBI further prove this design.
The photophysical properties of serial SPBIs
The physical and spectral properties of SPBIs were investigated. The solubility was improved by the introduction of alkyl, as expected. The SPBIs could be dissolved in various common organic solvents, such as dichloromethane (DCM), EtOH, and N,N-dimethyl formamide (DMF). The extent of alkylation for the SPBIs is summarized in Table S3. As the feed ratio increased, the alkylation rate and yield increased gradually. When the feed ratio was 4:1, ionization began to occur (Table 1). The accompanying slight color change from gray to brown-red (Table S3) implied that the extent of alkylation may have an effect on the spectral properties of SPBIs.
The UV-vis and fluorescence spectra of PBI and SPBIs were explored in dimethyl sulfoxide (DMSO) or DMF (with increased alkylation, the solubility of the SPBIs in polar DMSO decreases; therefore, DMF was used as the solvent for SPBI-f and SPBI-g) (Ge et al., 2018; Chen et al., 2020b). As shown in Figure S14, the UV-vis absorption spectra of SPBIs were similar, with absorption peaks located at approximately 300 nm. The shoulder peak observed at 265 nm was mainly caused by the π-π∗ electronic transition on the conjugated skeleton. As the degree of alkylation increased, the absorbance of SPBIs decreased slightly. The fluorescence of PBI was so weak under the excitation of 328 nm and there was an obvious enhancement for the fluorescence intensity of SPBIs with the emission peaks red-shifted to approximately 447 nm at the same test conditions. The extent of alkylation had little effect on the emission peak position but changed the fluorescence intensity. The Stokes shifts of SPBI-a ∼ SPBI-g were in the range of 56–87 nm, and the relative fluorescence quantum yields were 34.1%, 31.6%, 32.7%, 43.6%, 28.9%, 50.7%, and 51.2%, respectively. The enhancement of fluorescence might be because of the decrease in π-π interaction between the benzimidazole units in the backbone of PBI (Yang et al., 2017; Xie et al., 2017).
According to the SEM analysis mentioned above, the morphologies of SPBI-c (network structure) and SPBI-g (nano-spheres) were relatively regular. Therefore, they were selected as representative compounds for evaluating the UV-vis and fluorescence spectra in THF, MeCN, EtOH, DMF, and DMSO. As shown in Figures S15 and S16, the UV-vis absorption spectra of SPBI-c and SPBI-g in various solvents showed slight differences. In particular, the absorbance of SPBI-c was largest in DMSO, and an absorption tail appeared in EtOH. For SPBI-g, an absorption tail was observed in MeCN. A comparison of the fluorescence spectra in different solvents showed that the fluorescence intensities of SPBI-c and SPBI-g were relatively strong in DMF and DMSO.
The N atoms in the SPBIs can easily coordinate metal ions, and the products with lower degrees of alkylation (e.g., SPBI-a) have strong π-π interactions and short distances between the polymer chains. Therefore, only metal ions with a suitable size can enter the framework, promoting interactions between analytes and the polymeric sensor. As the extent of alkylation increases, the π-π interactions decrease, accompanying the increase of the distance between the polymer chains. These may affect the interaction of metal ions with SPBIs and will be beneficial for interactions between the SPBIs and nitro-aromatic compounds (NACs) with the larger molecular sizes (Dou et al., 2014; Zeng et al., 2016). To explore the influence of alkylation degree, SPBI-a, SPBI-b, SPBI-c, and SPBI-g were used as representative compounds to investigate the sensing performance.
Sensing performance of SPBIs toward metal ions
Metal ions (e.g., Cu2+) play vital roles in human physiological processes. However, trace metal ions can be amplified through the food chain owing to the non-degradability of metal ions and their accumulation in organisms (Chen et al., 2021). As excess amounts of some metal ions in the body may cause various diseases (Jiang et al., 2018; Pang et al., 2019; Khairy and Duerkop, 2019), it is necessary to develop new sensor materials for the convenient detection of metal ions. The selectivity of SPBI-a (6.0% alkylation) for 16 metal ions was investigated in the DMSO/H2O system.
As shown in Figure 4, the UV-vis absorption peak of SPBI-a was located at 303 nm with a shoulder peak at 265 nm. When Cu2+ was added, the absorbance of the sensing system at 303 nm decreased slightly, and the absorbance at 275 and 350–800 nm increased significantly, causing the solution color change from colorless to indigo (Figure 4C). This result indicated that SPBI-a shows a colorimetric response to Cu2+. To rule out that this change was due to the color of Cu2+ itself, the UV-vis spectrum of an aqueous solution containing only the same amount of Cu2+ was collected. The aqueous solution containing only Cu2+ had an absorbance of less than 0.5 and appeared to be colorless. As shown in Figure 4B, the absorbance of the sensing system at 605 nm increased obviously with the addition of Cu2+, and the absorption of the system at 605 nm was unchanged with the addition of Fe3+. The other metal ions also had few effects on the systems. Therefore, SPBI-a can be used as a Cu2+ colorimetric sensor.
Figure 4.
UV-vis absorption spectra, absorbance changes at 605 nm, and color changes of SPBI-a (DMSO/H2O = 99:1, v/v) after the addition of various metal ions (50 μM)
(A) UV-vis absorption spectra changes.
(B) Absorbance changes at 605 nm.
(C) Color changes.
Interference experiments demonstrated that the UV-vis spectra of this system were not affected significantly by other ions, with the exception of Fe3+, which had only a small effect (Figure S17), indicating the good anti-interference ability of SPBI-a in response to Cu2+. Furthermore, the interaction between SPBI-a and Cu2+ was explored. As shown in Figure S18, with an increase in the Cu2+ concentration, the absorbance of SPBI-a at 303 nm decreased accompanied by a slight of redshift, and the absorbance at 250–280 and 350–800 nm increased gradually, resulting in a change in the solution color from colorless to blue. These observations indicate that there is a strong interaction between SPBI-a and Cu2+ (Aysha et al., 2019). Importantly, the system gradually reached a saturated state when the concentration of Cu2+ is approximately 80 μM. Using a reported method (Kumar and Chae, 2019; Chen et al., 2019a, 2019b; Zhao et al., 2019), the LOD was calculated as 8.76 × 10−7 M (Figure S19), which is equivalent to the sensitivity of some Cu2+ sensors reported recently (Table S6) (Han et al., 2019; Wu et al., 2020b; Wei et al., 2020).
Using reported methods (Imase et al., 2003; Kim et al., 2013), the sensing performance of SPBI materials obtained at other feed ratios were also investigated. For example, using UV-vis absorption spectroscopy, SPBI-b was found to selectively recognize two metal ions (Cu2+ and Fe3+) with LODs of 4.50 × 10−7 and 1.49 × 10−8 M, respectively (Figures S20–S26). With increased alkylation, the distance between the polymer chains might increases and the interactions of the SPBI with analytes might be affected, resulting in changes in sensitivity (Tables S6 and S7). SPBI-c (28.5% alkylation) and SPBI-g (65.0% alkylation) could also discern Cu2+ and Fe3+ but with different sensitivities (Figures S27–S39 and further discussions can be seen in the SI). The signals of SPBI-c were more sensitive to Cu2+ than those of SPBI-g, and its sensitivity was also better than those of some reported metal ion sensors (Table S6) (Kim et al., 2013; Zhang et al., 2020a; Du et al., 2020). For Fe3+, the sensitivity of SPBI-c was also higher than that of SPBI-g as well as those of some reported Fe3+ sensors (Table S7) (Li et al., 2019a, 2019b; Fan et al., 2020; Cui et al., 2020; Zheng et al., 2020; Shia et al., 2020; Jia et al., 2020; Sun et al., 2020). Therefore, the sensing performance of the SPBI is controllable, and the most sensitive material must have a suitable size for metal ions to coordinate with N atoms in SPBI. Herein, SPBI-c obtained at n(C5H11Br)/n(PBI) = 1:1 was found to exhibit the best performance.
Application of SPBIs in cyclic adsorption for Cu2+
At present, there are many types of materials used for adsorption. Especially, recycling these adsorption materials can save the cost of materials to a large extent and is conducive to environmental protection (Liu et al., 2020, 2021; Huang et al., 2021a; Wang et al., 2020b; Chen et al., 2020a), especially for multifunctional material (Chen et al., 2021). Using Cu2+ as an example, the cyclic adsorption application of SPBIs was investigated (Figure S40). Treatment with HCl (pH = 2), EDTA solution, and deionized water promoted Cu2+ desorption from the SPBIs, allowing these materials to be used for the reversible adsorption of Cu2+ under the indication by UV-vis absorbance spectra. The adsorption rates and adsorption capacities are summarized in Table S5.
Owing to the small distance between the polymer chains of SPBI-a with a low alkylation degree, Cu2+ could not easily enter into the polymer framework to coordinate (Imase et al., 2003), but the surface adsorption still occurred. The initial Cu2+ adsorption rate was only 75.07% with poor recyclability. For SPBI-g with a high alkylation degree, the large distance between polymer chains was also not conducive to interactions with metal ions (Imase et al., 2003; Kim et al., 2013), resulting in an adsorption performance similar to that of SPBI-a.
Interestingly, for SPBI-c, the moderate alkylation degree provided a suitable distance between polymer chains that allowed Cu2+ to enter into the framework and coordinate with N atoms, resulting in good adsorption performance with an initial Cu2+ adsorption rate of 96.81%. Moreover, this material maintained an adsorption rate of more than 80% in five cycles (Figure 5). Importantly, the recycling performance of SPBI was similar to those of some reported reusable adsorption materials (Table S9) (Liu et al., 2020; Huang et al., 2021a; Wang et al., 2020b; Chen et al., 2020a).
Figure 5.
Reversible adsorption for Cu2+ by SPBI-c
Sensing performance of SPBIs toward NACs
NACs are important raw materials for some blasting equipment and in the leather industry. Owing to the significant impact of these compounds on public safety, human life, and property, the trace detection of NACs is also a hot topic in the sensor field (Chen et al., 2020c; Wang et al., 2019; Zhuang et al., 2020b; Kasthuri et al., 2019). When the extent of alkylation increases, the π-π interactions in SPBIs decrease, accompanying by an increase in distance between polymer chains (Imase et al., 2003; Kim et al., 2013) These may be beneficial to interactions between C=N in SPBI and NACs with the larger molecular sizes. Thus, SPBI might be applicable to the recognition of electron-deficient NACs, and the selectivity of SPBI-c (28.5% alkylation) toward different NACs (the structures are shown in Figure S41) was tested.
SPBI-c was found to selectively detect PA, DNP, and NP by fluorescence quenching (Figure S42). Using a reported method (Jiao et al., 2019; Zhang et al., 2019a), the fluorescence quenching efficiencies of three typical NACs (PA, DNP, and NP) for SPBI-c were studied (Figure 6 and S43). With 160 μM PA, a maximum quenching efficiency of 96.8% was obtained, whereas higher concentrations of DNP and NP were required to achieve similar quenching efficiencies, demonstrating the high sensitivity of SPBI-c to PA. Further-more, using a reported approach (Gao et al., 2019; Nandi et al., 2020), the LODs of SPBI-c for PA, DNP, and NP were calculated to be 1.81 × 10−7, 2.29 × 10−7 and 2.62 × 10−7 M, respectively, which are equivalent to the LODs of some reported NAC sensors (Gao et al., 2019; Nandi et al., 2020; Ghorai et al., 2019; Dutta et al., 2020).
Figure 6.
Fluorescence spectra, linear plot, and Stern-Volmer plot for SPBI-c (DMSO/H2O = 99:1, V/V) upon the addition of different concentrations of PA
(A) Fluorescence spectra for SPBI-c with the addition of PA (λex = 331 nm).
(B) Linear plot for SPBI-c with the addition of PA.
(C) Stern-Volmer plot for SPBI-c with the addition of PA
The sensitivity of fluorescence-quenching sensors to analytes can also be judged from the Stern-Volmer (S-V) constant, Ksv, which can be calculated as follows: I0/I = 1 + Ksv[Q]. As shown in Figures 6C and S43C, for PA, DNP, or NP, the S-V plot curved upward at higher concentrations, indicating that this process involves a combination of dynamic and static quenching (Goswami et al., 2019; Rajak et al., 2019). In the low concentration range, the S-V plot is linear, and the Ksv values for SPBI-c toward PA, DNP, and NP were 6.5631 × 104, 4.5122 × 104, and 3.8651 × 104 M−1, respectively. These Ksv values are higher than those of the most reported fluorescent polymer sensors for NACs detection (Ghorai et al., 2019; Dutta et al., 2020).
Moreover, SPBI-g was also found to sensitively detect PA, DNP, and NP (Figure S44), with LODs of 1.68 × 10−7, 1.93 × 10−7, and 2.15 × 10−7 M, respectively (Figures S45 and S46), and Ksv values of 3.75 × 104, 3.85 × 104, and 2.68 × 104 M−1, respectively (Figure S47). The sensitivities of SPBI-g and SPBI-c to PA, DNP, and NP were on the same order of magnitude, but SPBI-g showed higher sensitivity for the detection of PA (Chen et al., 2020c; Wu et al., 2020a) (for a detailed comparison, see Table S8).
Sensing mechanism of SPBIs for multianalyte
According to the reported method (Li et al., 2019b; Zhang et al., 2019b; Chen et al., 2021; Chen et al., 2020c; Wang, et al., 2020), the effects of analytes on the FT-IR spectra and morphology of SPBIs were determined. Using SPBI-c as a representative compound, the changes in FT-IR spectra and SEM images before and after the addition of Cu2+ or Fe3+ were observed. As depicted in Figure 7, the stretching vibration of C-N and C=N in SPBI-c are located at 1278 and 1618 cm−1, respectively. After the addition of Cu2+, these stretching vibrations moved to 1272 and 1625 cm−1, respectively, indicating an interaction between the N atoms in SPBI-c and Cu2+ (Wu et al., 2016; Li et al., 2019b). Similarly, these stretching vibrations also moved after the addition of Fe3+ to SPBI-c. In addition, the morphology of SPBI-c changed from a network structure to a dense honeycomb structure, confirming an interaction between SPBI-c and metal ions (Zeng et al., 2016). The interactions of SPBI-a (Figure S48), SPBI-b (Figure S49), SPBI-c (Figure S50), and SPBI-g (Figures S51 and S52) with different analytes (metal ions and NACs) were also examined by FT-IR spectroscopy and SEM.
Figure 7.
FT-IR spectra, SEM images of SPBI-c before and after the addition of Cu2+ or Fe3+
(A) FT-IR spectra of SPBI-c before and after the addition of Cu2+ or Fe3+.
(B) SEM images of SPBI-c.
(C) SEM images of SPBI-c after the addition of Cu2+.
(D) SEM images of SPBI-c after the addition of Fe3+.
In particular, for the detection of NACs, the quenching process at low concentrations can be determined from the fluorescence lifetime changes (Figure S53) (Kasthuri et al., 2019; Zhuang et al., 2020b). The fluorescence lifetime decay curves of SPBI-c and SPBI-g were less affected by NACs. Therefore, the quenching of NACs in the low concentration range can be considered a static quenching process (Jiang et al., 2019; Chen et al., 2020c).
In addition, the interactions between polymeric sensors and analytes can be measured by XPS analysis (Li et al., 2019b; Zhao et al., 2019; Wang et al., 2020c). Using SPBI-c as an example, the C1s and N1s binding energies in the system after the addition of Cu2+, Fe3+, and PA were determined. As shown in Figures 8A and 8D, upon the addition of Cu2+, the C1s binding energy of C=N in SPBI-c moved from 286.1 to 286.6 eV, and the N1s binding energy moved from 401.6 to 402.3 eV. These changes demonstrate the presence of a coordination effect between Cu2+ and C=N in SPBI-c (Li et al., 2019b). Similar interactions were confirmed between SPBI-c and Fe3+ (Figures 8B and 8E) or PA (Figures 8C and 8F).
Figure 8.
C1s and N1s XPS spectra of SPBI-c after the addition of Cu2+, Fe3+, and PA
(A) C1s XPS spectra of SPBI-c after the addition of Cu2+.
(B) C1s XPS spectra of SPBI-c after the addition of Fe3+.
(C) C1s XPS spectra of SPBI-c after the addition of PA.
(D) N1s XPS spectra of SPBI-c after the addition of Cu2+.
(E) N1s XPS spectra of SPBI-c after the addition of Fe3+.
(F) N1s XPS spectra of SPBI-c after the addition of PA.
The structures of the representative compound SPBI-c, its Cu2+ complex and PA were optimized by DFT-B3LYP/6-31G in Gaussian 09 software. As shown in Figure S54, the electron cloud of the frontier orbital in polymer is mainly distributed on the benzimidazole unit and the energy of the highest occupied molecular orbital (HOMO) is −0.39 eV. Although the energy of the HOMO orbital for PA is −4.01 eV, which is lower than the HOMO energy level of the polymer. This facilitates the photoinduced electron transfer (PET) effect produced between the polymers and PA, leading to the fluorescence quenching (Zhuang et al., 2020a, 2020b). Meanwhile, the structure of the metal complex also has been optimized by taking the Cu2+ complex as a representative. It can be found that, when coordinated with Cu2+, the energy gap of HOMO-LUMO orbital for the polymer (SPBI-c) is reduced (Figure S54), forming a more stable metal complex (Park and Gong, 2017; Zeng et al., 2016; Pang et al., 2019). This further proves the coordination between the metal ion and the polymer.
Based on the above analysis, interaction models for the SPBIs with metal ions (Cu2+ or Fe3+) or PA were proposed (Figure 9). The metal ions can coordinate with the C=N bonds in the polymer chains to form a stable metal complex, leading to a change in UV-vis absorption signal of the system and realizing the colorimetric detection of Cu2+ or Fe3+ by SPBIs. For PA, the -OH group can form hydrogen bonds with multiple C=N groups in the polymer backbone (Jiang et al., 2019; Chen et al., 2020c; Kasthuri et al., 2019), inducing fluorescence quenching of the sensing system by photoinduced electron transfer (PET). Thus, the SPBIs can also be used for the recognition of PA.
Figure 9.
Interaction models for SPBI with metal ions (Cu2+, Fe3+) or PA
Conclusions
In summary, PBI, a linear polymer with poor solubility, was prepared by a simple condensation reaction. Subsequently, its solubility was improved via N-alkylation, and a series of SPBI were developed for the first time as colorimetric and ratiometric sensing materials. SPBI with different extents of alkylation were obtained by controlling the feed ratio of C5H11Br and PBI. Furthermore, the morphology of SPBI could be adjusted, and a tendency toward nano-/microsphere formation was observed with increased alkylation. It was found that the interactions between polymer molecules decreased and the distance between polymer chains increased at a high degree of alkylation, causing variations in the sensing performance of SPBI toward metal ions and NACs. Moreover, the SPBI were capable of adsorbing Cu2+ in solution, and good cyclability was achieved with the aid of an acid and a strong coordinating agent. SPBI-c, obtained at the feed ratio of 1:1, exhibited the best adsorption performance. This work provides a new idea for the facile synthesis of multifunctional materials.
Limitations of the study
Reversible experiments need to judge the adsorption effect and desorption degree of the material, mainly through the color changes of the material before and after adsorption, and controlling the time of adsorption and desorption for comparison. Among them, the color discrimination has certain subjective factors, and it may affect the adsorption effect and the degree of desorption in the research process. Therefore, the time was controlled to make the adsorption and desorption effect have certain comparability. Of course, since the adsorption capacity of conjugate organic polymers is influenced by the factors like temperature, pH, adsorption time, and so on, the adsorption experiment may be further optimized.
STAR★Methods
Key resources table
REAGENT or RESOURCE | SOURCE | IDENTIFIER |
---|---|---|
Chemicals, peptides, and recombinant proteins | ||
3,3′-diaminobenzidine | Energy chemical technology | CAS: 91-95-2 |
Glutaric acid | Energy chemical technology | CAS: 110-94-1 |
Bromopentane (n-C5H11Br) | Energy chemical technology | CAS: 110-53-2 |
Polyphosphoric acid (PPA) | Macklin biochemical technology | CAS: 8017-16-1 |
Potassium nitrate (KNO3) | Guangzhou Chemical reagent factory | CAS: 7757-79-1 |
Sodium nitrate (NaNO3) | Guangzhou Chemical reagent factory | CAS: 7631-99-4 |
Silver nitrate (AgNO3) | Guangzhou Chemical reagent factory | CAS: 7761-88-8 |
Barium nitrate [Ba(NO3)2] | Guangzhou Chemical reagent factory | CAS: 10,022-31-8 |
Calcium nitrate [Ca(NO3)2] | Guangzhou Chemical reagent factory | CAS: 10,124-37-5 |
Manganese nitrate tetrahydrate [Mn(NO3)2·4H2O] | Guangzhou Chemical reagent factory | CAS: 20,694-39-7 |
Copper Sulfate (CuSO4) | Guangzhou Chemical reagent factory | CAS: 7758-98-7 |
Ferrous chloride [FeCl2] | Guangzhou Chemical reagent factory | CAS: 7758-94-3 |
Lead nitrate [Pb(NO3)2] | Guangzhou Chemical reagent factory | CAS: 10,099-74-8 |
Mercury nitrate [Hg(NO3)2] | Guangzhou Chemical reagent factory | CAS: 10,045-94-0 |
Zinc nitrate [Zn(NO3)2] | Guangzhou Chemical reagent factory | CAS: 7779-88-6 |
Aluminum trichloride (AlCl3) | Guangzhou Chemical reagent factory | CAS: 7446-70-0 |
Ferric chloride (FeCl3) | Guangzhou Chemical reagent factory | CAS: 7705-08-0 |
Chromium chloride hexahydrate (CrCl3·6H2O) | Guangzhou Chemical reagent factory | CAS: 10,060-12-5 |
Deposited data | ||
Raw and analyzed data | This paper | NA |
Software and algorithms | ||
Gaussian 09 | Gaussian Inc. | Yimo Information Technology Co., Ltd |
Resource availability
Lead contact
Further requests for resources regarding this study will be fulfilled by the corresponding author, Zhao-Yang Wang (wangzy@scnu.edu.cn).
Materials availability
This work did not produce any new unique reagents.
Experimental model and subject details
This work did not need any unique experimental model.
Method details
Materials
3,3′-Diaminobenzidine, glutaric acid, and bromopentane (n-C5H11Br) were purchased from Energy chemical technology (Shanghai) Co. Ltd. Polyphosphoric acid (PPA) was purchased from Macklin biochemical technology Co. Ltd. All soluble metal ion salts and sodium hydroxide (NaOH) are purchased from Guangzhou Chemical reagent factory. All the nitroaromatic compounds purchased from Guangzhou Chemical reagent factory. Tetrahydrofuran (THF), acetonitrile (MeCN), anhydrous ethanol (EtOH), dimethyl formamide (DMF), dichloro-methane (DCM), and dimethyl sulfoxide (DMSO) were purchased from Tianjin Damao chemical reagent factory. DMSO-d6 and CDCl3 were purchased from Energy chemical technology (Shanghai) Co. Ltd. All these reagents were used without further purification.
Apparatus
The scanning electron microscopy (SEM) image was obtained on Phenom Pro X Desktop Scanning Electron Microscope and Energy Spectrum Integrated Machine. The thermogravimetric analysis was tested in TG-209 F3 thermogravimetric analyzer. X-ray photoelectron spectroscopic (XPS) analysis was performed by an Axis Ultra-DLD X-ray photoelectron spectrometer. The fluorescent spectra were obtained with a Hitachi F-4600 spectrophotometer at room temperature using the xenon lamp as light source; the slit width was 5 nm for both excitation and emission and voltage was 400 V. The UV-vis absorption spectra were carried out with an SHIMADZU UV-2700 UV spectrometer. The pH values were recorded by a PHS-25C meter. Fluorescent lifetime was measured by FLS 920 Fluorescence Spectrometer. The metal ion adsorption detection was carried out by AAS-990 atomic absorption spectrophotometer.
Synthesis of intermediate PBI
As the reported method (Wu et al., 2016, 2018; Ge et al., 2018; Jiang et al., 2019; Chen et al., 2020b), 3.0 mmol 3,3′-diaminobenzidine, 3.0 mmol glutaric acid and 20 mL polyphosphoric acid (PPA) were added into a 50 mL round-bottom flask. The mixture was stirred at 120°C for 2 h, and then heated to 170°C for 48 h. Once the reaction was stopped, the pH of the mixture was adjusted to alkaline with NaOH solution till cooling to room temperature. A crude solid product was obtained by vacuum filtration. Subsequently, the crude product was washed several times with water, ethyl acetate, ethanol and other solvents to remove the unreacted raw materials and some by-products with the lower molecular weight. Finally, the product was dried in a vacuum drying oven at 50°C for 24 h.
Synthesis of serial SPBIs
According to reported methods (Wu et al., 2016, 2018; Ge et al., 2018; Jiang et al., 2019; Chen et al., 2020b), 1 mmol PBI, different molar n-C5H11Br, 10 mL MeCN and moderate NaOH were added into the reaction flask. After refluxing for 24 h, the solvent was removed by vacuum distillation. The product was washed with water several times to remove the NaOH. Then, the alkylated product was rinsed alternately with dichloromethane and ethanol, collecting the organic phase and removing the solvent to obtain the purified product. Finally, the anticipated product was dried in a vacuum drying oven at 40°C for 24 h.
General procedure for optical spectral measurements
The SPBI samples were dissolved in DMSO or DMF to acquire a stock solution. Then, the test solution (DMSO/H2O or DMF/H2O as V/V = 99/1) of SPBI was prepared for the experiments of both UV-vis absorption spectra and fluorescence spectra at room temperature. Among them, 1 mg sample was dissolved into 15 mL mixed solvent.
Limit of detection
As the reported (Seo et al., 2014; Giri et al., 2018), the limit of detection (LOD) was measured by the equation: LOD = 3δ/K. Therein, δ is the standard deviation of the blank measurements (n = 12), and the K is the slope of the calibration curve.
SEM analysis
According to the reported method (Zhang et al., 2019a; To et al., 2020), the morphology for SPBI and the morphology changes of SPBI combined with metal ions or nitroaromatic compounds (NACs) were determined by field emission scanning electron microscope.
XPS analysis
As reported method (Wang et al., 2019; Li et al., 2019b), the combining energy of N1s, O1s and C1s in the product and complex were measured by Axis Ultra-DLD X-Ray photoelectron spectrometer
Cyclic adsorption and desorption
According to the literature (Wang et al., 2019; Ozay et al., 2020), to realize the recycle adsorption of SPBI, the coordinated solid was treated with HCl (pH = 2) and EDTA solution, and the mixture was stirred for 30 min. Once the solid color was changed from blue to colourless, the regenerating SPBI solid was obtained by filtering. After neutralizing the acid remained on the surface of SPBI with NaOH solution (pH = 10), and rinsing with the deionized water to a neutral environment, the coordinated N atoms in SPBI were restored to their original state. The concentrations of samples were tested by AAS.
Quantification and statistical analysis
The limit of detection was obtained by the formula “LOD = 3σ/k” and σ was the relative standard deviation. Differences were considered significant at p < 0.05. The statistical analyses were performed with Origin software.
Additional resources
There are no additional resources needed to be declared in this manuscript, additional requests for this can be made by contacting the lead contact.
Acknowledgments
Financial support from the National Natural Science Foundation of China (20772035), Guangdong Basic and Applied Basic Research Foundation (No. 2021A1515012342), the Open Fund of the Key Laboratory of Functional Molecular Engineering of Guangdong Province in SCUT (No. 2017kf01) and Guangdong Provincial Science and Technology Project (No. 2017A010103016) is greatly appreciated.
Author contributions
Conceptualization, C.P. and Z.W.; Investigation, C.P., X.C. and Y.X.; Writing-Original Draft, C.P.; Writing-Review & Editing, C.P., X.C., Y.X., S.L, Q.C., Y.Z. and Z.W.; Funding Acquisition, Z.W. and S.L.; Supervision, S.L. and Z.W.; Project Administration, Z.W.
Declaration of interests
The authors declare no competing interests.
Published: October 22, 2021
Footnotes
Supplemental information can be found online at https://doi.org/10.1016/j.isci.2021.103126.
Contributor Information
Shi-He Luo, Email: pinky_r@163.com.
Zhao-Yang Wang, Email: wangzy@scnu.edu.cn.
Supporting Citations
The following references appear in the Supplemental information: Abbasi et al., 2019; Abuhatab et al., 2020; Ansari et al., 2020; Bora et al., 2019; Chiou et al., 2020; Coldur et al., 2020; Darmayanti et al., 2019; Fang et al., 2020; Feng et al., 2020; Ge et al., 2020; Ghosh et al., 2019; He et al., 2021; Hosseinjani-Pirdehi et al., 2020; Jiang et al., 2020; Jigyasa et al., 2020; Joseph et al., 2020; Kadian and Manik, 2020; Ke et al., 2020; Krishnan and Suneesh, 2019; Landge et al., 2020; Li et al., 2020a; Li et al., 2020b; Ma et al., 2020a; Ma et al., 2020b; Mal et al., 2020; Mathivanan et al., 2020; Miao, 2019; Nan et al., 2020; Podasca et al., 2019; Qian et al., 2019; Qu et al., 2020; Rajalakshmi and Palanisami, 2020; Ran et al., 2020; Roja et al., 2020; Sharma et al., 2019; Tian et al., 2019; Xiao et al., 2020; Xiong et al., 2019; Xu et al., 2019; Zhu et al., 2020.
Supplemental information
Data and code availability
All data are published in this manuscript and supplement; additional requests for data can be made by contacting the lead contact, Zhao-Yang Wang (wangzy@scnu.edu.cn).
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Associated Data
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Supplementary Materials
Data Availability Statement
All data are published in this manuscript and supplement; additional requests for data can be made by contacting the lead contact, Zhao-Yang Wang (wangzy@scnu.edu.cn).