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Covalent organic framework-inspired chromogenic system for visual colorimetric detection of carcinogenic 3, 3′-diaminobenzidine
Sensors & Actuators: B. Chemical xxx (xxxx) xxxx
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Covalent organic framework-inspired chromogenic system for visual colorimetric detection of carcinogenic 3, 3′-diaminobenzidine Rui Ai, Yi He* National Collaborative Innovation Center for Nuclear Waste and Environmental Safety, School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang, 621010, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Covalent organic framework Terephthalaldehyde 3, 3′-diaminobenzidine Colorimetry Visulation
The colorimetric assay is considered to be a simple and reliable analytical approach for visual detection of various analytes. However, the low sensitivity of traditional colorimetric methods greatly restricts their further applications. Herein, inspired by the large π-conjugated system of imine-linked covalent organic frameworks (COFs), we propose a new colorimetric detection methodology by enhancing the extinction coefficient via introduction of large π-conjugated system to effectively improve the sensitivity. As a proof-of-concept demonstration, the classical imine condensation reaction is selected as the model reaction. Terephthalaldehyde (TPA) is employed as the colorimetric probe for detection of mutagenic and carcinogenic 3, 3′-diaminobenzidine (DAB), realizing the sensitive and selective detection of DAB down to 900 nM level. Moreover, this colorimetric assay can detect DAB at as low as 5 μM with the naked eye and achieve satisfactory recoveries in real water samples.
1. Introduction As a very simple yet direct analytical approach, colorimetric assays are extremely attractive thanks to their high accuracy, fast response, low cost, simple experimental operation as well as naked-eye readout [1–5]. The colorimetric assays have been successfully applied for environmental monitoring, food analysis, chemical sensing, medical diagnosis, and public safety, in which a variety of analytes such as metal ions, organic amines, pesticides, biomarkers, biothiols, glucose and explosives are rapidly and facilely determined based on the BouguerLambert-Beer’s law [6–14]. Despite its versatility and simplicity, compared with other optical assays (chemiluminescence and fluorescence), the low sensitivity of colorimetric assays is the bottleneck which restricts its further applications [15,16]. The origin of the unsatisfactory sensitivity for colorimetric methods is mainly ascribed to the low extinction coefficients of various chromogenic substrates. Accordingly, in order to improve the sensitivity of colorimetric assays, developing chromogenic substrates with high extinction coefficients is one of the solutions to the problem of low sensitivity. The extinction coefficients and colors are directly dependent on πconjugated systems that are light-absorbing parts of an organic molecule [17,18]. In general, the π-conjugated system with alternating single and multiple bonds that leads to the electron delocalization lower the energy gap [19,20]. For instance, many pigments have the aid of π-
⁎
conjugated electron systems, resulting in a strong optical absorption [21–23]. Therefore, we rationally suppose that the sensitivity of traditional colorimetric assays can be improved by introduction of π-conjugated systems. Covalent organic frameworks (COFs) represent a new class of porous crystalline materials and have promising application potential in catalysis, optoelectronics, energy transformation and storage as well as molecular adsorption and separation [24–29]. Most of COFs are prepared by condensation reactions between molecular building blocks for formation of B–O, C]C, C]N, N]N linkages, and so on [30–34]. In particular, the COFs synthesized by C]N imine-based linkages are most extensively investigated not only due to high resistance to hydrolysis but also because they are able to be synthesized by the condensation reaction of amines and aldehydes that are easily accessible [35–39]. The imine-linked COFs often feature a number of alternating single and multiple bonds, producing a large π-conjugated system that shows high extinction coefficients [40]. Inspired by the structure characteristic of imine-linked COFs, herein we present a visual colorimetric detection technology for 3, 3′-diaminobenzidine (DAB). It is well-known to be mutagenic and carcinogenic [41]. As a proof of concept study, the classical imine condensation reaction between DAB and terephthalaldehyde (TPA) is taken as an example. As shown in Fig. 1, the TPA is served as a colorimetric probe. In the absence of DAB, the TPA does not shows any absorption in the
Corresponding author. E-mail address: [email protected] (Y. He).
https://doi.org/10.1016/j.snb.2019.127372 Received 16 September 2019; Received in revised form 16 October 2019; Accepted 1 November 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Rui Ai and Yi He, Sensors & Actuators: B. Chemical, https://doi.org/10.1016/j.snb.2019.127372
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Fig. 1. Schematic representation of covalent organic framework-inspired chromogenic system for visual colorimetric detection of DAB.
visible region thanks to the lack of sufficient delocalized π-conjugated electrons. On the contrary, when the target analyte (DAB) is introduced to the TPA solution, the condensation reaction of DAB and TPA proceeds, triggering the production of C]N linkages. This reaction process induces the formation of π-conjugated system, causing the reaction product to be colored. With increasing the DAB concentration, the resulting π-conjugated system become larger owing to the increase in the numbers of C]N bonds, accompanying the absorbance increase and the redshift of absorption bands. As a result, visual colorimetric signals can be generated for DAB detection, which are also quantified by ultraviolet-visible (UV–vis) spectrophotometer.
for detection of DAB in diluted water samples is similar to that for the DAB standards as mentioned above.
3. Results and discussion 3.1. DAB-initiated chromogenic reaction using TPA as the substrate In order to demonstrate the chromogenic reaction, DAB was introduced to the TPA solution. As shown in Fig. 2a, DAB and TPA solutions are colorless because they have no optical absorption in the visible region. However, the mixture solution of TPA and DAB generates a noticeable colorless-to-orange red color change after reaction for 1 h at room temperature. The UV–vis absorption spectra show the appearance of an absorption peak at 560 nm, suggesting that the condensation reaction of DAB and TPA occurs. It should be noted that when TPA is replaced by benzaldehyde (BA) with only one reaction site, the COF-like structure is not formed and there is no obvious absorption in the wavelength range of 400–700 nm (Fig. 2a). These results promulgate that the introduction of a large π-conjugated system is a particularly workable way for enhancing optical absorption. Meanwhile, the structure of the reaction product between DAB and TPA is further investigated by FTIR, MS spectra, XRD, DLS, and theoretical calculation. In the FTIR spectra (Fig. 2b), the characteristic vibrational stretching peak of C]N at 1580 cm−1 is observed in the reaction product of DAB and TPA, confirming the formation of imine [42–46]. In addition, the molecular weight of the reaction product is determined by mass spectroscopy. The ion m/z 1875.8 in Fig. 2c is identified as the covalent organic polymer formed by TPA and DAB (COP-TPA/DAB). The crystal structure of the COP-TPA/DAB is studied by powder XRD (Figs. S1 and S2). As is illustrated in Fig. S1, the XRD data give the peaks at 14.7°, 27.1°, 31.1°, 45.2°, and 66.0°, which corresponding to the (010), (100), (111), (20) and (214) reflection planes. The calculated cell parameters in the 1P1 space group are a = 3.479 Å, b = 14.214 Å, and c = 29.994 Å, with factors of Rp = 22.68 % and Rwp = 7.06% (Fig. S2). These results prove that the COP-TPA/DAB features well-ordered structure and good crystallinity. The hydrodynamic size of COP-TPA/DAB is 1321 ± 12 nm (Fig. S3), and the Zeta potential is +55.85 mV. To elucidate the optical absorption property of COP-TPA/DAB, we calculate its electronic transitions by using B3LYP function. The energy and contribution highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are able to be found in Figs. 2d and S4. Compared with TPA and DAB, the L1UMO-HOMO gap is only 1.49 eV, which is apparently lower than that of TPA (4.76 eV) and DAB (5.41 eV), indicating that there are delocalized π-conjugated electrons in the molecular structure of COP-TPA/ DAB. Consequently, the π - π* transition energy gap becomes narrow, and the corresponding absorption wavelength becomes long, displaying an orange red color and strong absorption in the visible region.
2. Experimental section 2.1. Chemical reagents DAB, TPA, ethanediamine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, nicotinamide, and glucose were obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Other inorganic salts, including potassium bromide (KBr), potassium chloride (KCl), sodium carbonate (Na2CO3), sodium nitrite (NaNO3), sodium phosphate (Na3PO4), and sodium sulfate (Na2SO4), were purchased from Shanghai Sangon Biotechnology Co., Ltd. 2.2. Instruments UV-vis absorption spectra were collected using a UV-1800 spectrophotometer. X-ray powder diffraction (XRD) patterns were recorded with a D/MAX-IIIA X-ray diffractometer. Mass spectra were carried out on a Q-Exactive high-resolution mass spectrometer. A TENSOR spectrometer was applied to acquire Fourier transform infrared spectra (FTIR). Dynamic light scattering (DLS) and Zeta potential were performed on a 90 Plus Particle Size Analyzer. 2.3. Colorimetric assay procedure for DAB Typically, 0.6 mL of 1 mM TPA was putted in a glass vial (5 mL in volume). Then, 2.4 mL DAB dissolved in Britton-Robinson (BR) buffer (pH 4.8) with different concentrations were injected into the above glass reactor. The final DAB concentration is in the range of 0–400 μM. After incubation for 1 h at room temperature (25 °C), the UV–vis absorption spectra and the corresponding optical images were collected. 2.4. Detection procedure for DAB in diluted practical water Two practical water samples (lake water and river water) were collected and filtered, which were diluted 60-fold in BR buffer. Subsequently, the diluted water samples were spiked at three different levels of DAB (20 μM, 50 μM, and 100 μM). The experimental procedure 2
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Fig. 2. (a) UV–vis absorption spectra and the corresponding optical image of 200 μM TPA solution in the absence and presence of 400 μM BA or DAB. Reaction conditions: pH 4.8, and incubation at 25 °C for 1 h. (b) FTIR spectra of DAB, TPA, and the reaction product of DAB and TPA. (c) Mass spectrum of COP-TPA/DAB. The inset is the molecular structure of COP-TPA/ DAB. (d) HOMO-LUMO energy level diagram for DAB, TPA, and COP-TPA/ DAB.
3.2. Optimization of experimental conditions Because the chromogenic reaction between TPA and DAB is affected by various experimental conditions, we optimize the pH, TPA concentration, and reaction time to improve the sensitivity of this colorimetric assay. This chromogenic reaction is performed under different pH values as depicted in Fig. 3a. Here, pH of 4.8 was selected as the optimum pH value, which achieves the maximum absorbance value at 560 nm (A560). In general, the acid is an indispensable catalyst for initiating the imine condensation reaction [47,48], while high level of acid is ready to break the structure of imine. Thus, a modest acid level (pH 4.8) is suitable for this chromogenic reaction. Moreover, to examine the effect of TPA concentration, the absorbance values at 560 nm of the chromogenic reaction in the presence of various concentrations of TPA are collected as indicated in Fig. 3b. It can be seen that absorbance value increases with increasing TPA concentration from 0 to 300 μM. The reason is that more COP-TPA/DAB can be formed with a high concentration of TPA. When the TPA concentration is more than 200 μM, the absorbance value does not increases dramatically (Fig. 3b). To balance the consumption of TPA and colorimetric response, 200 μM TPA is used to detect DAB throughout the experiment. Furthermore, the optimum reaction time is kept at 1 h for the chromogenic reaction because the absorbance value reaches a plateau in most cases after reaction for 60 min (Fig. S5). 3.3. Visual colorimetric detection of DAB Next, the analytical performance of the proposed colorimetric assay is evacuated by using a series of DAB solutions with different concentrations in the range of 0–400 μM. As indicated in Fig. 4a and b, the absorbance value of the detection system at 560 nm increases in proportion to the concentration of the added DAB, accompanying a redshift of absorption band. This phenomenon is due to the fact that the molecule weight of COP-TPA/DAB gradually becomes larger and larger,
Fig. 3. Absorbance values of the TPA-DAB chromogenic system at 560 nm under (a) different pH conditions and (b) various concentrations of TPA. Reaction conditions: 0.2 mM DAB, pH 4.8, and incubation at 25 °C for 1 h.
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Fig. 4. (a) UV-vis absorption spectra of TPA solution for different DAB concentrations (from bottom to up: 0, 5, 20, 40, 60, 80, 100, 150, 200, 300, and 400 μM). (b) The absorbance values of the detection system at 560 nm with different DAB concentrations. (c) Calibration curve for DAB determination. The inset shows the photograph of the detection system in the presence of DAB with different concentrations. (d) Colorimetric response of the detection system in the presence of DAB and possible interfering substances. The concentrations of DAB, ED, NA, PPD, OPD, and MPD are kept at 30 μM, and the concentrations of other substances are maintained at 300 μM. Reaction conditions: pH 4.8, and incubation at 25 °C for 1 h.
which is validated by mass spectra as the DAB concentration increases (Figs. S6–S9), giving rise to the result that π-conjugated electrons are more and more delocalized. A good linear relationship between the absorbance values at 560 nm and DAB concentrations is obtained in the range of 0–150 μM, with a limit of detection (LOD) of 0.9 μM based on 3 σ/k, where σ and k are the standard deviation of blank sample and the slope of the linear equation, respectively. This LOD is much lower than that of the reported colorimetric assays for DAB and other amines as listed in Table S1. Notably, this colorimetric assay allows to visualize the DAB, distinguishable down to 5 μM with the naked eye (the inset of Fig. 4c). Additionally, we also explore the selectivity of this colorimetric assay by introduction of different types of substances, including several amines (ethanediamine (ED), o-phenylenediamine (OPD), m-phenylenediamine (MPD), p-phenylenediamine (PPD), and nicotinamide (NA)), common inorganic anions (Br−, Cl−, PO43-, SO42-, CO32-, I−, and NO3−), and other commonly used industrial organic substances (glucose (Glu)). As illustrated in Fig. 4d, the addition of 30 μM DAB leads to a significantly absorbance response at 560 nm. In contrast, the potentially interfering substances such as 300 μM anions, 300 μM Glu, and 30 μM amines cause virtually no absorbance change at 560 nm. These results testify that the fabricated colorimetric assay has a high sensitivity and selectivity for detection of DAB.
Table 1 Analytical results for detection of DAB in natural water samples. Sample
Added (μM)
Found (μM)
Recovery (%)
RSD (%)
Lake water
20 50 100 20 50 100
22.0 54.4 100.7 20.4 50.2 100.7
110.0 108.9 100.7 102.0 100.4 100.7
2.6 2.1 0.6 2.77 0.57 1.41
River water
extremely promising for determination of DAB in realistically real water samples.
4. Conclusions In summary, we have developed an imine-linked COFs-inspired colorimetric detection methodology by virtue of the resulting large πconjugated system that shows enhanced extinction coefficient. Theoretical simulations reveal that the delocalized π-conjugated electrons decrease the HOMO-LUMO energy gap. As a case investigation, we fabricate a visual colorimetric method for detection of DAB for the first time. The high-efficient imine condensation reaction between DAB and TPA yields large delocalized π-conjugated structures, which enables visible, highly sensitive and selective detection of DAB down to nanomolar levels. Remarkably, the present colorimetric assay is applicable for determining DAB in practical water samples. Encouragingly, based on an overall consideration of experimental operation, detection limit, linear range, cost, and ease-of-use, our COFsinspired colorimetric detection methodology has obvious advantages over other reported assays for DAB and other amines. This work put forward a new avenue for design and realization of chromogenic systems. Using the proposed strategy, it is possible to construct various analytical approaches for visual and highly sensitive detection of several analytes via abundantly available condensation reactions.
3.4. Determination of DAB in diluted real water samples The analysis for DAB in real water samples is very crucial in environmental monitoring by virtue of this colorimetric assay. In order to test whether it can be utilized for detection of DAB in natural water samples or not, three different concentrations of DAB are spiked into diluted lake and river water, which are further determined by the present colorimetric assay. The obtained results in Table 1 clearly reveal that the proposed colorimetric assay for DAB possesses satisfactory recoveries spanning from 100.4% to 110% in the diluted lake and river water. What is more, the corresponding relative standard deviation (RSD) is below 3%, disclosing that the developed colorimetric assay is 4
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Declaration of Competing Interest
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Rui Ai is an undergraduate student in School of National Defence Sciencen& Technology, Southwest University of Science and Technology, Mianyang, China. His research focus is the preparation of covalent organic frameworks and their analytical application. Yi He is a teacher in School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang, China. He received his PhD degree in 2014 from University of Science and Technology of China, his research focus is chemosensor based on nanomaterials.
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Covalent organic framework-inspired chromogenic system for visual colorimetric detection of carcinogenic 3, 3′-diaminobenzidine Rui Ai, Yi He* National Collaborative Innovation Center for Nuclear Waste and Environmental Safety, School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang, 621010, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Covalent organic framework Terephthalaldehyde 3, 3′-diaminobenzidine Colorimetry Visulation
The colorimetric assay is considered to be a simple and reliable analytical approach for visual detection of various analytes. However, the low sensitivity of traditional colorimetric methods greatly restricts their further applications. Herein, inspired by the large π-conjugated system of imine-linked covalent organic frameworks (COFs), we propose a new colorimetric detection methodology by enhancing the extinction coefficient via introduction of large π-conjugated system to effectively improve the sensitivity. As a proof-of-concept demonstration, the classical imine condensation reaction is selected as the model reaction. Terephthalaldehyde (TPA) is employed as the colorimetric probe for detection of mutagenic and carcinogenic 3, 3′-diaminobenzidine (DAB), realizing the sensitive and selective detection of DAB down to 900 nM level. Moreover, this colorimetric assay can detect DAB at as low as 5 μM with the naked eye and achieve satisfactory recoveries in real water samples.
1. Introduction As a very simple yet direct analytical approach, colorimetric assays are extremely attractive thanks to their high accuracy, fast response, low cost, simple experimental operation as well as naked-eye readout [1–5]. The colorimetric assays have been successfully applied for environmental monitoring, food analysis, chemical sensing, medical diagnosis, and public safety, in which a variety of analytes such as metal ions, organic amines, pesticides, biomarkers, biothiols, glucose and explosives are rapidly and facilely determined based on the BouguerLambert-Beer’s law [6–14]. Despite its versatility and simplicity, compared with other optical assays (chemiluminescence and fluorescence), the low sensitivity of colorimetric assays is the bottleneck which restricts its further applications [15,16]. The origin of the unsatisfactory sensitivity for colorimetric methods is mainly ascribed to the low extinction coefficients of various chromogenic substrates. Accordingly, in order to improve the sensitivity of colorimetric assays, developing chromogenic substrates with high extinction coefficients is one of the solutions to the problem of low sensitivity. The extinction coefficients and colors are directly dependent on πconjugated systems that are light-absorbing parts of an organic molecule [17,18]. In general, the π-conjugated system with alternating single and multiple bonds that leads to the electron delocalization lower the energy gap [19,20]. For instance, many pigments have the aid of π-
⁎
conjugated electron systems, resulting in a strong optical absorption [21–23]. Therefore, we rationally suppose that the sensitivity of traditional colorimetric assays can be improved by introduction of π-conjugated systems. Covalent organic frameworks (COFs) represent a new class of porous crystalline materials and have promising application potential in catalysis, optoelectronics, energy transformation and storage as well as molecular adsorption and separation [24–29]. Most of COFs are prepared by condensation reactions between molecular building blocks for formation of B–O, C]C, C]N, N]N linkages, and so on [30–34]. In particular, the COFs synthesized by C]N imine-based linkages are most extensively investigated not only due to high resistance to hydrolysis but also because they are able to be synthesized by the condensation reaction of amines and aldehydes that are easily accessible [35–39]. The imine-linked COFs often feature a number of alternating single and multiple bonds, producing a large π-conjugated system that shows high extinction coefficients [40]. Inspired by the structure characteristic of imine-linked COFs, herein we present a visual colorimetric detection technology for 3, 3′-diaminobenzidine (DAB). It is well-known to be mutagenic and carcinogenic [41]. As a proof of concept study, the classical imine condensation reaction between DAB and terephthalaldehyde (TPA) is taken as an example. As shown in Fig. 1, the TPA is served as a colorimetric probe. In the absence of DAB, the TPA does not shows any absorption in the
Corresponding author. E-mail address: [email protected] (Y. He).
https://doi.org/10.1016/j.snb.2019.127372 Received 16 September 2019; Received in revised form 16 October 2019; Accepted 1 November 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Rui Ai and Yi He, Sensors & Actuators: B. Chemical, https://doi.org/10.1016/j.snb.2019.127372
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Fig. 1. Schematic representation of covalent organic framework-inspired chromogenic system for visual colorimetric detection of DAB.
visible region thanks to the lack of sufficient delocalized π-conjugated electrons. On the contrary, when the target analyte (DAB) is introduced to the TPA solution, the condensation reaction of DAB and TPA proceeds, triggering the production of C]N linkages. This reaction process induces the formation of π-conjugated system, causing the reaction product to be colored. With increasing the DAB concentration, the resulting π-conjugated system become larger owing to the increase in the numbers of C]N bonds, accompanying the absorbance increase and the redshift of absorption bands. As a result, visual colorimetric signals can be generated for DAB detection, which are also quantified by ultraviolet-visible (UV–vis) spectrophotometer.
for detection of DAB in diluted water samples is similar to that for the DAB standards as mentioned above.
3. Results and discussion 3.1. DAB-initiated chromogenic reaction using TPA as the substrate In order to demonstrate the chromogenic reaction, DAB was introduced to the TPA solution. As shown in Fig. 2a, DAB and TPA solutions are colorless because they have no optical absorption in the visible region. However, the mixture solution of TPA and DAB generates a noticeable colorless-to-orange red color change after reaction for 1 h at room temperature. The UV–vis absorption spectra show the appearance of an absorption peak at 560 nm, suggesting that the condensation reaction of DAB and TPA occurs. It should be noted that when TPA is replaced by benzaldehyde (BA) with only one reaction site, the COF-like structure is not formed and there is no obvious absorption in the wavelength range of 400–700 nm (Fig. 2a). These results promulgate that the introduction of a large π-conjugated system is a particularly workable way for enhancing optical absorption. Meanwhile, the structure of the reaction product between DAB and TPA is further investigated by FTIR, MS spectra, XRD, DLS, and theoretical calculation. In the FTIR spectra (Fig. 2b), the characteristic vibrational stretching peak of C]N at 1580 cm−1 is observed in the reaction product of DAB and TPA, confirming the formation of imine [42–46]. In addition, the molecular weight of the reaction product is determined by mass spectroscopy. The ion m/z 1875.8 in Fig. 2c is identified as the covalent organic polymer formed by TPA and DAB (COP-TPA/DAB). The crystal structure of the COP-TPA/DAB is studied by powder XRD (Figs. S1 and S2). As is illustrated in Fig. S1, the XRD data give the peaks at 14.7°, 27.1°, 31.1°, 45.2°, and 66.0°, which corresponding to the (010), (100), (111), (20) and (214) reflection planes. The calculated cell parameters in the 1P1 space group are a = 3.479 Å, b = 14.214 Å, and c = 29.994 Å, with factors of Rp = 22.68 % and Rwp = 7.06% (Fig. S2). These results prove that the COP-TPA/DAB features well-ordered structure and good crystallinity. The hydrodynamic size of COP-TPA/DAB is 1321 ± 12 nm (Fig. S3), and the Zeta potential is +55.85 mV. To elucidate the optical absorption property of COP-TPA/DAB, we calculate its electronic transitions by using B3LYP function. The energy and contribution highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are able to be found in Figs. 2d and S4. Compared with TPA and DAB, the L1UMO-HOMO gap is only 1.49 eV, which is apparently lower than that of TPA (4.76 eV) and DAB (5.41 eV), indicating that there are delocalized π-conjugated electrons in the molecular structure of COP-TPA/ DAB. Consequently, the π - π* transition energy gap becomes narrow, and the corresponding absorption wavelength becomes long, displaying an orange red color and strong absorption in the visible region.
2. Experimental section 2.1. Chemical reagents DAB, TPA, ethanediamine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, nicotinamide, and glucose were obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Other inorganic salts, including potassium bromide (KBr), potassium chloride (KCl), sodium carbonate (Na2CO3), sodium nitrite (NaNO3), sodium phosphate (Na3PO4), and sodium sulfate (Na2SO4), were purchased from Shanghai Sangon Biotechnology Co., Ltd. 2.2. Instruments UV-vis absorption spectra were collected using a UV-1800 spectrophotometer. X-ray powder diffraction (XRD) patterns were recorded with a D/MAX-IIIA X-ray diffractometer. Mass spectra were carried out on a Q-Exactive high-resolution mass spectrometer. A TENSOR spectrometer was applied to acquire Fourier transform infrared spectra (FTIR). Dynamic light scattering (DLS) and Zeta potential were performed on a 90 Plus Particle Size Analyzer. 2.3. Colorimetric assay procedure for DAB Typically, 0.6 mL of 1 mM TPA was putted in a glass vial (5 mL in volume). Then, 2.4 mL DAB dissolved in Britton-Robinson (BR) buffer (pH 4.8) with different concentrations were injected into the above glass reactor. The final DAB concentration is in the range of 0–400 μM. After incubation for 1 h at room temperature (25 °C), the UV–vis absorption spectra and the corresponding optical images were collected. 2.4. Detection procedure for DAB in diluted practical water Two practical water samples (lake water and river water) were collected and filtered, which were diluted 60-fold in BR buffer. Subsequently, the diluted water samples were spiked at three different levels of DAB (20 μM, 50 μM, and 100 μM). The experimental procedure 2
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Fig. 2. (a) UV–vis absorption spectra and the corresponding optical image of 200 μM TPA solution in the absence and presence of 400 μM BA or DAB. Reaction conditions: pH 4.8, and incubation at 25 °C for 1 h. (b) FTIR spectra of DAB, TPA, and the reaction product of DAB and TPA. (c) Mass spectrum of COP-TPA/DAB. The inset is the molecular structure of COP-TPA/ DAB. (d) HOMO-LUMO energy level diagram for DAB, TPA, and COP-TPA/ DAB.
3.2. Optimization of experimental conditions Because the chromogenic reaction between TPA and DAB is affected by various experimental conditions, we optimize the pH, TPA concentration, and reaction time to improve the sensitivity of this colorimetric assay. This chromogenic reaction is performed under different pH values as depicted in Fig. 3a. Here, pH of 4.8 was selected as the optimum pH value, which achieves the maximum absorbance value at 560 nm (A560). In general, the acid is an indispensable catalyst for initiating the imine condensation reaction [47,48], while high level of acid is ready to break the structure of imine. Thus, a modest acid level (pH 4.8) is suitable for this chromogenic reaction. Moreover, to examine the effect of TPA concentration, the absorbance values at 560 nm of the chromogenic reaction in the presence of various concentrations of TPA are collected as indicated in Fig. 3b. It can be seen that absorbance value increases with increasing TPA concentration from 0 to 300 μM. The reason is that more COP-TPA/DAB can be formed with a high concentration of TPA. When the TPA concentration is more than 200 μM, the absorbance value does not increases dramatically (Fig. 3b). To balance the consumption of TPA and colorimetric response, 200 μM TPA is used to detect DAB throughout the experiment. Furthermore, the optimum reaction time is kept at 1 h for the chromogenic reaction because the absorbance value reaches a plateau in most cases after reaction for 60 min (Fig. S5). 3.3. Visual colorimetric detection of DAB Next, the analytical performance of the proposed colorimetric assay is evacuated by using a series of DAB solutions with different concentrations in the range of 0–400 μM. As indicated in Fig. 4a and b, the absorbance value of the detection system at 560 nm increases in proportion to the concentration of the added DAB, accompanying a redshift of absorption band. This phenomenon is due to the fact that the molecule weight of COP-TPA/DAB gradually becomes larger and larger,
Fig. 3. Absorbance values of the TPA-DAB chromogenic system at 560 nm under (a) different pH conditions and (b) various concentrations of TPA. Reaction conditions: 0.2 mM DAB, pH 4.8, and incubation at 25 °C for 1 h.
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Fig. 4. (a) UV-vis absorption spectra of TPA solution for different DAB concentrations (from bottom to up: 0, 5, 20, 40, 60, 80, 100, 150, 200, 300, and 400 μM). (b) The absorbance values of the detection system at 560 nm with different DAB concentrations. (c) Calibration curve for DAB determination. The inset shows the photograph of the detection system in the presence of DAB with different concentrations. (d) Colorimetric response of the detection system in the presence of DAB and possible interfering substances. The concentrations of DAB, ED, NA, PPD, OPD, and MPD are kept at 30 μM, and the concentrations of other substances are maintained at 300 μM. Reaction conditions: pH 4.8, and incubation at 25 °C for 1 h.
which is validated by mass spectra as the DAB concentration increases (Figs. S6–S9), giving rise to the result that π-conjugated electrons are more and more delocalized. A good linear relationship between the absorbance values at 560 nm and DAB concentrations is obtained in the range of 0–150 μM, with a limit of detection (LOD) of 0.9 μM based on 3 σ/k, where σ and k are the standard deviation of blank sample and the slope of the linear equation, respectively. This LOD is much lower than that of the reported colorimetric assays for DAB and other amines as listed in Table S1. Notably, this colorimetric assay allows to visualize the DAB, distinguishable down to 5 μM with the naked eye (the inset of Fig. 4c). Additionally, we also explore the selectivity of this colorimetric assay by introduction of different types of substances, including several amines (ethanediamine (ED), o-phenylenediamine (OPD), m-phenylenediamine (MPD), p-phenylenediamine (PPD), and nicotinamide (NA)), common inorganic anions (Br−, Cl−, PO43-, SO42-, CO32-, I−, and NO3−), and other commonly used industrial organic substances (glucose (Glu)). As illustrated in Fig. 4d, the addition of 30 μM DAB leads to a significantly absorbance response at 560 nm. In contrast, the potentially interfering substances such as 300 μM anions, 300 μM Glu, and 30 μM amines cause virtually no absorbance change at 560 nm. These results testify that the fabricated colorimetric assay has a high sensitivity and selectivity for detection of DAB.
Table 1 Analytical results for detection of DAB in natural water samples. Sample
Added (μM)
Found (μM)
Recovery (%)
RSD (%)
Lake water
20 50 100 20 50 100
22.0 54.4 100.7 20.4 50.2 100.7
110.0 108.9 100.7 102.0 100.4 100.7
2.6 2.1 0.6 2.77 0.57 1.41
River water
extremely promising for determination of DAB in realistically real water samples.
4. Conclusions In summary, we have developed an imine-linked COFs-inspired colorimetric detection methodology by virtue of the resulting large πconjugated system that shows enhanced extinction coefficient. Theoretical simulations reveal that the delocalized π-conjugated electrons decrease the HOMO-LUMO energy gap. As a case investigation, we fabricate a visual colorimetric method for detection of DAB for the first time. The high-efficient imine condensation reaction between DAB and TPA yields large delocalized π-conjugated structures, which enables visible, highly sensitive and selective detection of DAB down to nanomolar levels. Remarkably, the present colorimetric assay is applicable for determining DAB in practical water samples. Encouragingly, based on an overall consideration of experimental operation, detection limit, linear range, cost, and ease-of-use, our COFsinspired colorimetric detection methodology has obvious advantages over other reported assays for DAB and other amines. This work put forward a new avenue for design and realization of chromogenic systems. Using the proposed strategy, it is possible to construct various analytical approaches for visual and highly sensitive detection of several analytes via abundantly available condensation reactions.
3.4. Determination of DAB in diluted real water samples The analysis for DAB in real water samples is very crucial in environmental monitoring by virtue of this colorimetric assay. In order to test whether it can be utilized for detection of DAB in natural water samples or not, three different concentrations of DAB are spiked into diluted lake and river water, which are further determined by the present colorimetric assay. The obtained results in Table 1 clearly reveal that the proposed colorimetric assay for DAB possesses satisfactory recoveries spanning from 100.4% to 110% in the diluted lake and river water. What is more, the corresponding relative standard deviation (RSD) is below 3%, disclosing that the developed colorimetric assay is 4
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Declaration of Competing Interest
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Rui Ai is an undergraduate student in School of National Defence Sciencen& Technology, Southwest University of Science and Technology, Mianyang, China. His research focus is the preparation of covalent organic frameworks and their analytical application. Yi He is a teacher in School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang, China. He received his PhD degree in 2014 from University of Science and Technology of China, his research focus is chemosensor based on nanomaterials.
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