A Flexible Chemosensor Based on Colorimetric and Fluorescent Dual Modes for Rapid and Sensitive Detection of Hypochlorite Anion

A flexible chemosensor has been developed based on colorimetric and fluorescent dual modes using tetraphenylethylene-centered tetraaniline (TPE4A) for rapid and sensitive detection of hypochlorite anion. The fluorescent probe TPE4A exhibits a unique aggregation-induced emission (AIE) character which is proved by a blue shift of the fluorescent peak from 544 to 474 nm with the water equivalents increasing. With the addition of hypochlorite in solution, the absorbance of the probe changes and the responding fluorescence color can be observed to change from light green to purple. The detection limit of hypochlorite is 1.80 × 10−4 M in solution, and the visual detection limit is 1.27 µg/cm2 with the naked eye for the flexible paper-based chemosensor. The proposed flexible chemosensors show a good selectivity and sensitivity which has great potential for effective detection of hypochlorite anions without any spectroscopic instrumentation.


Introduction
Hypochlorite anion (ClO − ) plays an essential role in biological organisms and environmental monitors, which not only is produced by H 2 O 2 and chloride ions in activated neutrophils [1][2][3][4][5] but is also a kind of disinfector which can kill the coronavirus [6][7][8]. However, excessive hypochlorite could lead to diseases and even cancer including cardiovascular diseases, neuron degeneration, and arthritis [9,10]. Therefore, quantitative detection of hypochlorite with highly sensitive and selective methods becomes more and more crucial.
Many various analytical methods are available for the detection of hypochlorite (for example, colorimetric and fluorescent chemosensors) [11,12]. Recent reviews have summarized the advances of hypochlorite probes [13,14]. There are some successful designs for quantitative detection of hypochlorite based on the principle of colorimetric, chromatographic, electrochemical, and luminescent methods. Specifically, the fluorescent probe is a promising method with advantages such as low cytotoxicity, high selectivity, and fast response time for ClO − detection. Moreover, understanding visual detection is an unceasingly thorough process for rapid and sensitive detection of hypochlorite anions [15][16][17][18][19][20][21][22][23][24].
Recently, dual modes [25][26][27] bioanalysis has become a popular research area, wherein researchers are studying scrambly as well. When the environment changes in the direction of complexity, single-switch optical detection may no longer meet the reliability of data and the diversity of application scenarios. Therefore, it is very important to construct a dual-mode optical detection method. The optical detection based on colorimetric and fluorescent dual modes could not only improve the accuracy of results but also lead to of data and the diversity of application scenarios. Therefore, it is very important to construct a dual-mode optical detection method. The optical detection based on colorimetric and fluorescent dual modes could not only improve the accuracy of results but also lead to higher efficiency. In recent years, flexible thin-film devices [28,29] have been developed vigorously, especially in material, chemical, biological, physical fields, due to their unique advantages such as their low-cost, porosity, ready availability, mechanical flexibility, etc. Therefore, the design of paper-based chemosensors for simple and rapid detection is of great significance. In this work, we present a commercially available material TPE4A for fabricating a flexible paper-based chemosensor based on colorimetric and fluorescent dual modes in Scheme 1. Scheme 1. The chemical structure of tetraphenylethylene-functionalized molecule TPE4A.

Aggregation-Induced Fluorescent Behavior
With the enhancement of volume fraction of water phase (fw), UV-Vis and fluorescent spectra have been shown for compound TPE4A in Figure 1. A step-by-step fluorescent turn-on is observed from fw = 0 to 70%, due to the controlled rotational motion in the molecule. Interestingly, a remarkable color change occurs from orange when fw = 70% to green when fw = 80% mainly due to the formation of aggregates. Simultaneously, the maximum emission wavelength λmax is from 544 nm when fw = 70% to 474 nm when fw = 80%. Absorption and emission spectra show a clear AIE character for compound TPE4A as expected. The dominant luminescence is from the emissive solution to solid fluorescence. The same phenomenon is observed in the different concentration measurements. According to restriction of intramolecular vibrations (RIV) and restriction of intramolecular rotations (RIR), the possess of twisted intramolecular charge transfer (TICT) also is blocked, resulting in decreasing red-shift and increasing blue-shift.

Titration Experiment of Probe TPE4A
Compared to other chemical anions, molecule TPE4A has been designed as a highly selective fluorescent probe for ClO − anion in Figure 2

Aggregation-Induced Fluorescent Behavior
With the enhancement of volume fraction of water phase (f w ), UV-Vis and fluorescent spectra have been shown for compound TPE4A in Figure 1. A step-by-step fluorescent turnon is observed from f w = 0 to 70%, due to the controlled rotational motion in the molecule. Interestingly, a remarkable color change occurs from orange when f w = 70% to green when f w = 80% mainly due to the formation of aggregates. Simultaneously, the maximum emission wavelength λ max is from 544 nm when f w = 70% to 474 nm when f w = 80%. Absorption and emission spectra show a clear AIE character for compound TPE4A as expected. The dominant luminescence is from the emissive solution to solid fluorescence. The same phenomenon is observed in the different concentration measurements. According to restriction of intramolecular vibrations (RIV) and restriction of intramolecular rotations (RIR), the possess of twisted intramolecular charge transfer (TICT) also is blocked, resulting in decreasing red-shift and increasing blue-shift.

Titration Experiment of Probe TPE4A
Compared to other chemical anions, molecule TPE4A has been designed as a highly selective fluorescent probe for ClO − anion in Figure 2. After adding 1.00 mM of analytes, such as SCN − , CO 3 2− , S 2− , NO 2 − , H 2 O 2 , F − , and ClO − , UV-Vis absorption, and fluorescent intensities of probe TPE4A (40 µM) have been shown in mixed solution (f w = 80%). The absorption peak of TPE4A is from 398 nm to 429 nm (also from 3.12 eV to 2.89 eV) in the presence of ClO − , which is consistent with the results of calculated energy gaps. absorption peak of TPE4A is from 398 nm to 429 nm (also from 3.12 eV to 2.89 eV) in the presence of ClO − , which is consistent with the results of calculated energy gaps.   More importantly, a fluorescent quenching occurs from light green to dark, while the emissive color of this probe remains unchanged after the adding of other chemical species. Quantitative titration experiment shows a clear fluorescent turn-off in the presence of different concentrations of hypochlorite anion from 0 to 25 equivalent in mixed solution (f w = 80%) with 100% of quenching percentage in Figure 3. It is noted that the intensity of  Figure 3. It is noted that the intensity of TPE4A at 480 nm does not significantly change under H2O2 or ClO4 − oxidants, since this compound avoids being oxidized in this case.

Paper-Based Sensor
To effective detection of ClO − in the state of aggregation, we prepared a test paper as the paper-based sensor. Then, we dipped the test paper into a solution containing chromophore test paper. As shown in Figure 4, the SEM diagram indicates that TPE4A can be attached to paper-based fibers. The diameter of particulate matter is approximately 100 µ m, while the diameter of melt-blown fibers is 20~50 µ m, showing a size matching on a micron scale. We carried out the naked-eye recognition matrix of paper-based sensor TPE4A for different concentrations of ClO − under visible and UV light in Figure 5. The fluorescence of the paper-based sensor is quenched gradually with the increase of the concentration of ClO − . At the same time, the color of the sensor changes from white to yellow under visible light, while from green to purple with the light of the UV lamp at 365 nm. Quantitative analysis demonstrates that the detection limit of hypochlorite is 1.80 × 10 −4 M in solution based on the IUPAC definition 139 (CDL = 3 Sb m −1 ) [34], and the visual detection limit is 1.27 µ g/cm 2 with the naked eye. Therefore, tetraphenylethylene-centered tetraaniline materials have a rapid and highly sensitive character for more convenient and visual detecting hypochlorite in a few seconds.

Paper-Based Sensor
To effective detection of ClO − in the state of aggregation, we prepared a test paper as the paper-based sensor. Then, we dipped the test paper into a solution containing chromophore test paper. As shown in Figure 4, the SEM diagram indicates that TPE4A can be attached to paper-based fibers. The diameter of particulate matter is approximately 100 µm, while the diameter of melt-blown fibers is 20~50 µm, showing a size matching on a micron scale. We carried out the naked-eye recognition matrix of paper-based sensor TPE4A for different concentrations of ClO − under visible and UV light in Figure 5. The fluorescence of the paper-based sensor is quenched gradually with the increase of the concentration of ClO − . At the same time, the color of the sensor changes from white to yellow under visible light, while from green to purple with the light of the UV lamp at 365 nm. Quantitative analysis demonstrates that the detection limit of hypochlorite is 1.80 × 10 −4 M in solution based on the IUPAC definition 139 (C DL = 3 Sb m −1 ) [34], and the visual detection limit is 1.27 µg/cm 2 with the naked eye. Therefore, tetraphenylethylene-centered tetraaniline materials have a rapid and highly sensitive character for more convenient and visual detecting hypochlorite in a few seconds.

Possible Mechanism of Probe TPE4A
According to the references [16,35,36], as well as experimental and theoretical calculations [37], we have investigated the possible mechanism of TPE4A in Scheme 2. First of all, the tetraaniline structure may be oxidized to the azo counterpart under an appropriate oxidant in this case. It is noted that perchloride is a stronger oxidant meant it could oxidize the aniline group to form the azo intermediate. However, the solubility of perchloride is limited in f w = 80%. The mechanism of fluorescent turn-off quenching is a reaction from strong chromophore TPE4A to weak chromophore TPE4A-NNCl, which could be speculated that owing to the oxidation of TPE4A. Furthermore, the C-N bond could be easy to cleavage in the azo intermediate and further to form a radical which could combine with chlorine radicals to form the compound TPE4A-NNCl. The energy gap is 3.44 eV for TPE4A, while the energy gap is 3.17 eV for TPE4A-NNCl (see Supporting Information).

Possible Mechanism of Probe TPE4A
According to the references [16,35,36], as well as experimental and theoretical calculations [37], we have investigated the possible mechanism of TPE4A in Scheme 2. First of all, the tetraaniline structure may be oxidized to the azo counterpart under an appropriate oxidant in this case. It is noted that perchloride is a stronger oxidant meant it could oxidize the aniline group to form the azo intermediate. However, the solubility of perchloride is limited in fw = 80%. The mechanism of fluorescent turn-off quenching is a reaction from strong chromophore TPE4A to weak chromophore TPE4A-NNCl, which could be specu-

Possible Mechanism of Probe TPE4A
According to the references [16,35,36], as well as experimental and theoretical calculations [37], we have investigated the possible mechanism of TPE4A in Scheme 2. First of all, the tetraaniline structure may be oxidized to the azo counterpart under an appropriate oxidant in this case. It is noted that perchloride is a stronger oxidant meant it could oxidize the aniline group to form the azo intermediate. However, the solubility of perchloride is limited in fw = 80%. The mechanism of fluorescent turn-off quenching is a reaction from strong chromophore TPE4A to weak chromophore TPE4A-NNCl, which could be speculated that owing to the oxidation of TPE4A. Furthermore, the C-N bond could be easy to cleavage in the azo intermediate and further to form a radical which could combine with chlorine radicals to form the compound TPE4A-NNCl. The energy gap is 3.44 eV for TPE4A, while the energy gap is 3.17 eV for TPE4A-NNCl (see Supporting Information). In addition, we carried out the ESI-MS analysis to explore the complete reaction mixture of the probe with ClO − in Figure S3. ESI-MS analysis that a peak at m/z = 837.1954 corresponding to [TPE4A-NNCl-2Cl+Na] + (calcd for [C 50 H 32 Cl 2 N 8 Na] + : 837.2019) is clearly observed with other identical isotopic peaks. This promoted a possible method to design fluorescent probes for hypochlorite with the inorganic/organic composites containing amino groups. In addition, we carried out the ESI-MS analysis to explore the complete reaction mixture of the probe with ClO − in Figure S3. ESI-MS analysis that a peak at m/z = 837.1954 corresponding to [TPE4A-NNCl-2Cl+Na] + (calcd for [C50H32Cl2N8Na] + : 837.2019) is clearly observed with other identical isotopic peaks. This promoted a possible method to design fluorescent probes for hypochlorite with the inorganic/organic composites containing amino groups.

Conclusions
In summary, we have designed and developed a flexible paper-based material for hypochlorite detection. The quantitative analysis demonstrates that with the enhancement of hypochlorite equivalents, the UV-Vis peaks have been decreasing from 398 nm to 429 nm, while the fluorescent peak at 480 nm has been decreasing more remarkably. Furthermore, the detection limit of hypochlorite is 1.80 × 10 −4 M in solution, and the visual detection limit is 1.27 µ g/cm 2 with the naked eye. The mechanism of fluorescent turn-off quenching is a reaction from strong chromophore TPE4A to weak chromophore TPE4A-NNCl, which has been confirmed by experimental and theoretical results, as well as support from references. This study can provide a flexible paper-based portable chemosensor based on colorimetric and fluorescent dual modes for hypochlorite before conventional chemical synthesis.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1. Figure S1: 1 H NMR of TPE4A; Figure S2: The selectivity of TPE4A for ClO − , SCN − , CO3 2− , S 2− , NO2 − , H2O2, F − and ClO4 − ; Figure S3: Fitting plot for compound 1 with the addition of different contentions of TPE4A in THF solution to calculate the limit of detection; Figure S4: The experimental and theoretical ESI-MS of the product obtained by mixing probe NaOCl; Table S1

Conclusions
In summary, we have designed and developed a flexible paper-based material for hypochlorite detection. The quantitative analysis demonstrates that with the enhancement of hypochlorite equivalents, the UV-Vis peaks have been decreasing from 398 nm to 429 nm, while the fluorescent peak at 480 nm has been decreasing more remarkably. Furthermore, the detection limit of hypochlorite is 1.80 × 10 −4 M in solution, and the visual detection limit is 1.27 µg/cm 2 with the naked eye. The mechanism of fluorescent turn-off quenching is a reaction from strong chromophore TPE4A to weak chromophore TPE4A-NNCl, which has been confirmed by experimental and theoretical results, as well as support from references. This study can provide a flexible paper-based portable chemosensor based on colorimetric and fluorescent dual modes for hypochlorite before conventional chemical synthesis.