Conjugated Polyelectrolyte-Based New Strategy for in Situ Detection

Jun 8, 2017 - ... air with a content of ∼400 ppm. Importantly, PFBT is employed to monitor photosynthesis and respiration upon cycling day and night...
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Conjugated Polyelectrolyte-Based New Strategy for in Situ Detection of Carbon Dioxide Yibing Fan,† Chengfen Xing,*,†,‡ Hongbo Yuan,‡ Ran Chai,‡ Linfei Zhao,† and Yong Zhan† †

Key Laboratory of Hebei Province for Molecular Biophysics, Institute of Biophysics, Hebei University of Technology, Tianjin 300401, P.R. China ‡ School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, P.R. China S Supporting Information *

ABSTRACT: A conjugated polymer centered on fluorene and 2,1,3-benzothia-diazole (PFBT) is prepared for sensing CO2 in situ with high sensitivity and low background. Upon introducing CO2, the weaker electrostatic repulsion and stronger hydrophobic interactions between neighboring PFBT molecules enhance the interchain contacts compared to that without CO2, leading to the energy transfer from fluorene to 2,1,3-benzothia-diazole sites and the emission color shift from blue to green, which is sensitive to sensing CO2 in atmospheric air with a content of ∼400 ppm. Importantly, PFBT is employed to monitor photosynthesis and respiration upon cycling day and night in situ. KEYWORDS: in situ detection, CO2, conjugated polymers, energy transfer, photosynthesis

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polydiacetylene.26 We have also realized the CO2 detection in plant photosynthesis by using water-soluble conjugated polythiophene and oligofluorene complex.27 However, these systems are not capable of rapid and in situ28,29 monitoring CO2 in atmospheric air with the content of ∼400 ppm, leading to the relatively high detection limit and long response. Moreover, the elements in these sensing systems must contain one conjugated polymer and additional accessory molecules. It is still needed to explore new strategy only by using single conjugated polymer for rapid and in situ detection of CO2 with low background and high sensitivity. Here, we prepared a water-soluble conjugated polymer centered on fluorene and 2,1,3-benzothia-diazole (PFBT). As shown in Scheme 1, each repeat unit (RU) of PFBT includes two amino and four carboxylic acid groups in the side chain with negative charges in neutral aqueous solution, prohibiting the tight aggregation of PFBT and keeping their backbones separated dominated by the electrostatic repulsion. Following gaseous CO2 (gCO2) or dissolved CO2 (dCO2) is introduced, the protonation of the

arbon dioxide (CO2) is closely related to climatic variation1 and mainly produced by the respiration of animals and plants, 2 volcanic eruption, 3 and industry production.4 The detection of CO2 is important in environmental monitoring, physiological indicating and medical diagnosing.5,6 The detection methods of CO2 including electrochemical technology,7,8 gas chromatography,9,10 and infrared spectroscopy11,12 have been well-established. However, these techniques are always accompanied by the disadvantage of being time-consuming and having low sensitivity, leading to the long response and high detection limit.13,14 An in situ detection approach has been applied to sense the original state of matter in a real-time environment effectively and rapidly, which is helpful for detecting or even preventing accidents.15−19 In this context, a CO2 sensing system based on an in situ detection technique is highly desirable for rapid on-site measurement of CO2 with high sensitivity. Water-soluble conjugated polymers have been extensively developed for chemical or biological sensing systems by taking advantage of their fluorescence signal amplification and high light-harvesting effects.20−25 Recently, Yoon and co-workers developed a polydiacetylene-based optical sensor for detecting CO 2 by applying aqueous solution or solid state of © XXXX American Chemical Society

Received: April 18, 2017 Accepted: June 8, 2017 Published: June 8, 2017 A

DOI: 10.1021/acsami.7b05410 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

for 72h to yield the conjugated polymer PFBT. The thermal gravimetric analysis (TGA) curve in Figure S1 has demonstrated that PFBT is thermally stable up to 205 °C. The photophysical properties of PFBT in aqueous solution are illustrated in Figure S2. The UV−vis absorption spectra of PFBT show a maximum peak at 380 nm, and the fluorescence emission spectra of PFBT illustrate two obvious emission peaks at 414 and 538 nm, respectively. Upon adding the dissolved CO2 into the aqueous solution of PFBT, the fluorene-unit emission at 414 nm was gradually decreased and the emission peak of 538 nm which is characteristic of BT units was enhanced gradually as depicted in Figure 1a. Therefore, the ratio of the emission intensity at 538 and 414 nm (I538 nm/ I414 nm) was gradually enhanced as a function of the concentration of CO2 and reached a plateau after the concentration of CO2 was higher than 45 mM in aqueous solution, which is approximately 8 times higher than that without CO2. Moreover, the color of PFBT solution shifts from blue to green in the presence of dissolved CO2 under UV irradiation as illustrated in Figure 1c. These observations demonstrated that the presence of CO2 enhances the interchain contacts of PFBT and leads to the close proximity between the PFBT molecules in the aqueous solution, allowing for the intramolecular energy transfer from fluorine units to BT units and the emission color shift from blue to green. Figure 2 shows the real-time detection of gaseous CO2 (gCO2) by applying both PFBT in aqueous solutions and in solid state by spraying the solution of PFBT on the surface of cellulose acetate membrane. As shown in Figure 2a, gaseous CO2 was bubbled directly into aqueous solution of PFBT and the fluorescence emission spectra were measured immediately with an excitation wavelength of 380 nm, and the emission intensity of PFBT at 414 nm decreases with the volumes of CO2 bubbled while emission at 538 nm increases gradually. Figure 2b shows the emission intensity ratio of the emission intensity at 538 and 414 nm (I538 nm/I414 nm) was gradually enhanced as a function of volume of gaseous CO2 with a flow rate of 1.0 mL/min and reached a plateau after the volume of CO2 was more than 2.5 mL, in aqueous solution, which is approximately 8 times higher than that without CO2, and the detection limit is 8.0 μL. To check the interference from the air of the CO2 detection strategy, the fluorescence spectra of PFBT upon bubbling with different ratios of CO2/air mixture were examined. The emission intensity ratio (I538 nm/I414 nm) was gradually enhanced as a function of the volume ratio of CO2 in various CO2/air mixtures (Figure 2c) and 0.02% of CO2 can be

Scheme 1. (a) Schematic Representation of PFBT-Based Strategy for CO2 Detection in Plant Photosynthesis and Respiration Process Upon Cycling Day and Night in Situ; (b) Chemical Structure of PFBT

carboxylate and amino groups as depicted in Scheme S1 leads to the weaker electrostatic repulsion and stronger hydrophobic interactions between neighboring PFBT molecules in aqueous solution in relative to that in the absence of CO2, enhancing the interchain contacts of PFBT and forming the tight aggregation, which allows for the intramolecular energy transfer from fluorene units to 2,1,3-benzothia-diazole (BT) units and the emission color shift from blue to green. Furthermore, both PFBT in aqueous solutions and in solid state by spraying the solution of PFBT on the surface of cellulose acetate membrane can be applied for the naked-eye detection of CO2 rapidly and simply. Moreover, PFBT is employed to detect the variation of CO2 concentrations in real time caused by plants in greenhouse and monitor the plant photosynthesis and respiration upon cycling day and night in situ. The synthetic route of PFBT is depicted in Scheme S2. Suzuki coupling of monomers 1 and 2 with the molar feed ratio of 19:1 in the presence of 1,4-phenyldiboronic ester in aqueous solution of Na2CO3 (2.0 M) with Pd(dppf)Cl2 in toluene affords crude polymer, which was purified and dialyzed against water using a membrane with a molecular weight cutoff of 3500

Figure 1. (a) Fluorescence emission spectra of PFBT in water solution with various concentrations of dissolved CO2. Experiments were taken at 4 °C with excitation of 380 nm in deionized water. (b) Emission intensity ratio (I538 nm/I414 nm) of PFBT with different concentrations of CO2 in deionized water. (c) Fluorescence images of PFBT in deionized water in the presence and absence of CO2 with the UV irradiation (λmax = 365 nm). Error bars were obtained by the standard deviations of data in three independent experiments. B

DOI: 10.1021/acsami.7b05410 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 2. (a) Fluorescence emission spectra of PFBT upon bubbling with various volumes of CO2 by using different bubbling time with a flow rate of 1 mL/min. (b) The emission intensity ratio (I538 nm/I414 nm) of PFBT as a function CO2 volume. (c) Fluorescence spectra of PFBT upon bubbling with different ratios of CO2/air for 1.0 min. The flow rate is 1.5 mL/min. (d) The emission intensity ratio (I538 nm/I414 nm) of PFBT as a function of the ratio of CO2/air. (e) Fluorescence spectra of PFBT upon bubbling with air for 2.0 min. The flow rate is 5.0 mL/min f) Emission spectra of PFBT upon bubbling with CO2-free air for 2.0 min. The flow rate is 5.0 mL/min. (g) Fluorescence image of PFBT in the absence of CO2 on the solid film of ester composite film under UV light (λ = 365 nm). (h) Fluorescence image of PFBT with CO2 on the solid film of ester composite film under UV light (λ = 365 nm). [PFBT] = 10 μg/mL, the volume ratios of CO2/air (V/V %) range (0−100.0%). Error bars were obtained by the standard deviations of data in three independent experiments. Fluorescence emission spectra were measured at 4 °C with excitation of 380 nm.

detected (Figure 2d). It is demonstrated the PFBT-based CO2 detection strategy is high sensitive and satisfies the minor interference from air. To address the in situ while real-time detection of carbon dioxide (CO2) under realistic conditions, fluorescence spectra of PFBT upon bubbling with air and CO2free air for 2.0 min were examined. The CO2-free air was obtained from a CO2 absorption equipment with alkaline substances was made to remove CO2 from the air. As shown in Figure 2e, the emission intensity ratio (I538 nm/I414 nm) is enhanced after bubbling with air including CO2 with the content of ∼400 ppm. However, there is no obvious change of fluorescence spectra of PFBT upon bubbling with CO2-free air as indicated in Figure 2f, demonstrating that PFBT is applied for sensing CO2 in situ with high sensitivity and low background. Furthermore, PFBT in solid state by spraying the solution of PFBT on the surface of cellulose acetate film can be applied for the naked-eye detection of CO2 rapidly and simply under UV irradiation. As demonstrated in Figure 2g, h, the color of the film shifts from blue to green in the presence of CO2 under UV irradiation. Therefore, both PFBT in aqueous solutions and in solid state can be used for the CO2 sensing for in situ measuring CO2 with high sensitivity. To gain more insight into the working mechanism of the CO2 detection system, we conducted dynamic light scattering (DLS) measurements to check the assembly of PFBT in aqueous solution in the absence and presence of CO2. As indicated in Figure S3a, the average hydrodynamic radius of PFBT in the presence of CO2 is 10 times higher than that in the absence of CO2, demonstrating the PFBT forms tight aggregation induced in aqueous solution at 4 °C and a similar result was obtained at 25 °C as shown in Figure S3b. Furthermore, the AFM measurements gave more evidence that CO2 promotes PFBT aggregation forming. Figure 3 shows that PFBT forms the larger aggregates in the presence of CO2 with a topographic height which is approximately 10 times higher than that without CO2, which is consistent with the DLS analysis.

Figure 3. Atomic force microscopy (AFM) images of PFBT in the (a) absence and (b) presence of CO2 at 25 °C. The line plots of the aggregates are present in the right. [PFBT] = 5 μg/mL.

Moreover, the scanning electron microscopy (SEM) examination also indicates that the presence of CO2 leads to larger aggregates of PFBT in aqueous solution (Figure S4) and in solid state (Figure S5) in comparison with PFBT itself. ζ potentials of PFBT with and without CO2 were examined as well to check the effect of CO2 on the surface charges of PFBT assembly in aqueous solution. As shown in Table S1, ζ potentials of PFBT became more cationic promoted by CO2, confirming the decrease of the negative charges with CO2 and resulting in the weaker electrostatic repulsion in relative to that of PFBT itself. These results revealed that the weaker electrostatic repulsion between PFBT molecules in aqueous C

DOI: 10.1021/acsami.7b05410 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 4. Emission intensity ratio of the emission intensity at 538 and 414 nm (I538 nm/I414 nm) of PFBT measured at different times in the greenhouse from 21:00 to 10:00 during (a) 1 day and night and (b) 3 days and nights. [PFBT] = 10 μg/mL. Error bars were obtained by the standard deviations of data in three independent experiments. Fluorescence emission spectra were measured at 4 °C with excitation of 380 nm.

solution caused by CO2 make the proximity become closer than that in the absence of CO2, which leads to more tight hydrophobic interactions between the backbones of PFBT, inducing the intramolecular energy transfer from fluorene units to BT units and offering a emission ratio-based strategy for sensing CO2. To apply PFBT to realize the real-time detection of CO2 produced by plants in greenhouse and monitor the plant photosynthesis and respiration upon cycling day and night in situ, the emission intensity ratios of the emission intensity at 538 and 414 nm (I538 nm/I414 nm) of PFBT were measured at different times in the greenhouse from 21:00 to 10:00 during 1 day and night. As shown in Figure 4a, the ratio of I538 nm/I414 nm during the night without light from 21:00 to 6:00 was enhanced gradually, resulting from the plant respiration that exists in the night without light. However, when the sunlight increases from 6:00 am to 10:00 am, the ratio of I538 nm/I414 nm was decreased gradually because the CO2 was consumed by green plants in the greenhouse during the daytime under the sunshine. Moreover, the ratios of I538 nm/I414 nm were measured at different times in the greenhouse from 21:00 to 10:00 upon cycling day and night in situ as illustrated in Figure 4b, exhibiting circulatory change during the day and night. In conclusion, the water-soluble conjugated polymer centered on fluorene and 2,1,3-benzothia-diazole (PFBT) has been designed and synthesized for sensing gaseous CO2 (gCO2) and dissolved CO2 (dCO2) in situ with high sensitivity and low background. CO2 enhances the interchain contacts of PFBT and leads to the close proximity between neighboring PFBT molecules, resulting in the intramolecular energy transfer from fluorene to the 2,1,3-benzothia-diazole (BT) units and the emission color shift from blue to green. Furthermore, the emission ratio-based strategy by applying PFBT realizes the real-time detection of CO2 in atmospheric air with the volume content of ∼400 ppm. Moreover, PFBT is explored to sense CO2 produced by plants in a greenhouse and monitor the photosynthesis and respiration upon cycling day and night in situ. Thus, the CO2 detection strategy based on PFBT has great significance in environmental monitoring and as a physiological indicator.

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CO2 in water; detection of gaseous CO2; detection of CO2 by composite film; detection of CO2 in photosynthesis in situ; morphology study of PFBT in the absence and presence of CO2; scheme of the charge changes of PFBT upon bubbling with CO2; schematic preparation of PFBT; ζpotentials of PFBT in ddH2O and saturated carbon dioxide solution; the thermal gravimetric analysis (TGA) curve of PFBT; UV−vis absorption and emission spectra of PFBT in ddH2O; dynamic light scattering analysis of PFBT in aqueous solution in the absence and presence of CO2 at 4 and 25 °C; SEM images of PFBT in the absence and presence of CO2 at room temperature; SEM images of composite film in the absence and presence of CO2 at room temperature (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.X.). ORCID

Chengfen Xing: 0000-0002-1939-2265 Author Contributions

C.X. designed experiments and organized the manuscript; Y.F. performed experimental part and analyzed data; H.Y., R.C.. and L.Z. performed experimental part; Y.Z. analyzed data. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support of the National Natural Science Foundation of China ( 21574037), the “100 Talents” Program of Hebei Province, China (No. E2014100004), the Natural Science Foundation of Hebei Province (B2015202330 and B2017202051), the Program for Top 100 Innovative Talents in Colleges and Universities of Hebei Province (SLRC2017028) and the Tianjin Natural Science Foundation (15JCYBJC17500).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05410. Materials and measurements; synthesis of PFBT; preparation of CO2 saturated ddH2O; detection of D

DOI: 10.1021/acsami.7b05410 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.7b05410 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX