Fluorene and Tetraphenylsilane Based Conjugated Microporous Polymer Nanoparticles for Highly Efficient Nitroaromatics Detection in Aqueous Media

Conjugated microporous polymers (CMPs) are considered promising sensing materials for the detection of nitroaromatics. Unfortunately, the photoluminescence (PL) sensing application in water media is limited, owing to the poor solubility/dispersion in aqueous media. Here, two novel CMPs named poly(dimethylfluorene‐co‐tetrakis(4‐phenyl)silane)s (P4SiF) and poly(dimethylfluorene‐co‐tetrakis(3‐phenyl)silane)s (P3SiF) nanoparticles based on fluorene group and tetraphenylsilane unit are prepared using Suzuki coupling in water/toluene miniemulsion. P4SiF and P3SiF possess spherical particle morphology with sizes of 45 and 60 nm, respectively, and porous structure with pore diameters of both 1.9 nm. PL sensing experiments based on P4SiF and P3SiF nanoparticles in aqueous media show a sensitive and selective detection toward 2,4,6‐trinitrophenol (TNP). It is worth noting that the PL quenching effect toward TNP is not significantly changed after the addition of other nitro compounds, proving the excellent anti‐interference characteristic and highly selective detection of the polymers in water. Moreover, the t‐test indicates that the PL sensing method using P4SiF and P3SiF nanoparticles as sensing materials realizes the reliable detection of TNP in actual water samples at a 95% confidence level. Finally, the PL sensing mechanism is investigated, showing that the inner filter effect is the main factor in the PL quenching process of TNP in aqueous media.


Introduction
3] In a variety of nitroaromatic explosives, 2,4,6-trinitrotoluene DOI: 10.1002/adsr.202300139[9] So, it is necessary to develop a highly sensitive, selective, and trustworthy assay for the detection of TNP in aqueous media.So far, researchers have developed many detection methods, such as surfaceenhanced Raman spectroscopy, [10] energy dispersive X-ray diffraction, [11] nuclear magnetic resonance, [12] ion mobility spectrometry, [13] electrochemical method, [14] and so on.The above methods require complex operations, or expensive equipment, or are inconvenient to carry. [15][35] CMPs are composed of conjugated units cross-linked by covalent bonds, possessing several merits, such as good chemical stability, [36] Scheme 1.The synthetic routes of P4SiF and P3SiF.
high specific surface area, [37] low density [38] and permanent porosity, [39] which is beneficial for the detection of nitroaromatics.For example, Li and co-workers [40] synthesized the thiophenebased and carbazole-based CMPs (PTPATTh and (PTPATCz), which could detect TNP in a sensitive and selective manner.Pal and co-workers [41] reported three luminescent CMPs based on truxene core (Tx-CMP-1, Tx-CMP-2, and Tx-CMP-3), which realized the detection limit in the nanomolar range toward TNP.Samanta and co-workers [42] prepared a triphenylamineanthracene-based CMP, which exhibited a good PL sensing performance toward various nitroaromatics.Unfortunately, traditional CMPs are insoluble or cannot be uniformly dispersed in water media owing to the highly cross-linked structure, thus limiting their application in aqueous media.
Herein, two novel CMPs named poly(dimethylfluorene-cotetrakis(4-phenyl)silane)s (P4SiF) and poly(dimethylfluorene-cotetrakis(3-phenyl)silane)s (P3SiF) nanoparticles based on fluorene group and tetraphenylsilane unit were designed and synthesized by Suzuki coupling in water/toluene miniemulsion.The prepared P4SiF and P3SiF nanoparticles have porous structures and small particle sizes.The PL sensing performance of P4SiF and P3SiF nanoparticles on nitroaromatic compounds in aqueous media was studied by PL titration experiment, exhibiting relatively high sensitivity and selectivity of TNP detection.The Stern-Volmer constant (K SV ) values of P4SiF and P3SiF nanoparticles toward TNP achieve 6.43 × 10 4 and 4.97 × 10 4 m −1 , respectively, and the limits of detection (LODs) and limits of quantification (LOQs) are 0.45 and 1.50 μm for P4SiF, 0.74 and 2.48 μm for P3SiF, respectively.It is worth noting that TNP detection could be recognized by P4SiF and P3SiF nanoparticles in the presence of other nitro compounds and common ions, indicating their excellent selectivity and anti-interference performance.Further spike/recovery tests were conducted for TNP detection in actual water samples, and the accuracy of the PL sensing method at a 95% confidence level was demonstrated by t-tests.Finally, the PL sensing mechanism of the sensitive and selective TNP detection based on P4SiF and P3SiF nanoparticles was further discussed by cyclic voltammetry (CV), UV−visible (UV−vis) absorption, excitation and PL spectra, and PL decay curve, respectively.

Synthesis and Characterization
The preparation routes of P4SiF and P3SiF by Suzuki crosscoupling polymerization in miniemulsion are described in Scheme 1.The monomers 2,2′-(9,9-dimethyl-9H-fluorene-2,7diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (M0), tetrakis(4bromophenyl)silane (M1) and tetrakis(3-bromophenyl)silane (M2) were prepared according to the literature, [43,44] and the detailed procedures and structural characterizations are given in the Scheme S1 and Figures S1−S6 (Supporting Information).P4SiF and P3SiF nanoparticles were synthesized by Suzuki-type coupling in water/toluene miniemulsion, using sodium dodecyl sulfate (SDS) as a surfactant, tetrakis(triphenylphosphine)palladium (Pd(PPh 3 ) 4 ) as a catalyst and K 2 CO 3 as a base.The chemical structures of the prepared P4SiF and P3SiF were characterized by Fourier Transform infrared (FT-IR) spectroscopy and 1 H Nuclear Magnetic Resonance ( 1 H NMR) spectra (Figure 1).P4SiF and P3SiF have almost identical peaks in the FT-IR spectra (Figure 1a) because their chemical structures are similar.The peaks ≈2974 cm −1 come from the ─CH 3 stretching.The Si−Ar stretching occurs at 1420 and 1142 cm −1 .The 810 cm −1 is the para-disubstituted peak of the phenyl ring, and peaks at 747 cm −1 are attributed to the orthodisubstituted peak of the phenyl ring, while 690 cm −1 is the metadisubstituted peak on the phenyl ring.The successful preparation of P4SiF and P3SiF was further confirmed via 1 H NMR spectra in Figure 1b.The monomers M0, M1, and M2 exhibit definite peaks with precise assignment of hydrogen atoms in 1 H NMR spectra (Figures S1, S3, and S5, Supporting Information), and compared with the monomers, the 1 H NMR peaks of the polymers are widened and sophisticated owing to their highly crosslinking structure, proved the successful preparation of the polymers P4SiF and P3SiF.

Morphology and Porous Structure
The morphology of P4SiF and P3SiF nanoparticles was obtained by scanning electron microscopy (SEM).Obviously, P4SiF and P3SiF display homogeneous spherical morphologies of nanoparticles from the SEM images (Figure 2a,c), and the diameters ranges are from 35 to 55 nm and 45 to 75 nm for P4SiF and P3SiF nanoparticles, respectively, as shown in Figure 2b,d.In addition, the nanosized particles of the polymers were also confirmed by dynamic light scattering (DLS) experiments shown in Figure S7 (Supporting Information).Consistent with the SEM results, the average diameters of P4SiF and P3SiF nanoparticles were determined to be 45 and 60 nm, respectively.As shown in Figure S8 (Supporting Information), the photographs of P4SiF and P3SiF in water show uniform dispersity and the obvious Tyndall effect.The small nano-sizes of P4SiF and P3SiF nanoparticles allow them to disperse well in water, which facilitates the detection of nitroaromatic explosives in aqueous media.Porosities and surface areas of P4SiF and P3SiF nanoparticles were derived by N 2 adsorption and desorption isotherms (Figure 3).The Brunauer-Emmett-Teller (BET) surface areas of P4SiF and P3SiF nanoparticles were measured to be 102 and 129 m 2 g −1 , respectively (Figure 3a,c), and the pore sizes are both focused on 1.9 nm (Figure 3b,d), suggesting the microporosity of P4SiF and P3SiF.The large specific surface area and porosity of P4SiF and P3SiF nanoparticles are beneficial for the adsorption and diffusion of nitroaromatic explosives, which is favorable for PL sensing.

Photophysical Property
As shown in Figure 4, the UV−vis absorption, excitation, and PL spectra of P4SiF and P3SiF nanoparticles in deionized water were recorded.P4SiF and P3SiF nanoparticles exhibit similar UV−vis absorption spectra, with a wide absorption band at ≈320 nm, which is considered as ─* transition of the P4SiF and P3SiF backbones.The maximum excitation wavelengths of P4SiF and P3SiF nanoparticles are ≈282 nm.The PL spectrum of P4SiF nanoparticles centers at 396 nm with a peak width of 58 nm at half-height, while P3SiF nanoparticles centers at 392 nm with a peak width of 67 nm at half-height, indicating a dark blue emission (Figure S9, Supporting Information).The emission peak of P3SiF nanoparticles shows a blueshift of 4 nm compared with P4SiF, which may be due to the weakened spatial conjugative interaction between fluorene and tetraphenylsilane in P3SiF.

Electrochemical Property
The electrochemical characteristics of P4SiF and P3SiF nanoparticles were examined by CV method in Figure 5. Obviously, both polymers exhibit obvious oxidation and reduction processes.The initial oxidation potentials were determined to be 1.13 and 1.11 V, and the initial reduction potentials were −2.20 and −2.30V, respectively, for P4SiF and P3SiF nanoparticles, corresponding to the oxidation and reduction of polymer backbones.According to the equation HOMO (highest occupied molecular orbital) = −(4.36+ E Ox onset ) eV and LUMO (lowest unoccupied   molecular orbital) = −(4.36+ E Red onset ), the HOMO and LUMO energy levels were measured to be ≈−5.51 and −2.18 eV for P4SiF, and −5.49 and −2.08 eV for P3SiF, respectively.The relatively high LUMO energy levels are beneficial to realize the photoinduced electron transfer by LUMO-LUMO offset from electronrich polymers to electron-deficient nitroaromatics. [45,46]

PL Sensing in Aqueous Media
The PL spectra and Stern-Volmer plots of P4SiF and P3SiF nanoparticles in aqueous media with different TNP contents are recorded in Figure 6.Obviously, the PL intensities of the two polymers gradually decrease with the increase of TNP concentration, indicating their sensing ability toward TNP.In addition, the peak position and shape of the PL spectra do not show significant changes upon TNP addition, indicating no new substances formed in the PL quenching process.As the TNP concentration increases from 0 to 49.76 μm, the PL of P4SiF nanoparticles in aqueous dispersion was significantly quenched, with a quenching degree of 82% (Figure 6a).When the TNP concentration is lower than 35 μm, the K SV value of P4SiF is 6.43 × 10 4 m −1 , proving a high sensitivity in detecting of TNP (Figure 6b).In the PL sensing process based on P3SiF nanoparticles, after adding 49.76 μm TNP, the PL quenching degree of P3SiF reached 78% (Figure 6c), and the calculated K SV value is 4.97 × 10 4 m −1 (Figure 6d).The confidence intervals for the K SV values of P4SiF and P3SiF were determined to be 6.32 × 10 4 -6.54 × 10 4 and 4.69 × 10 4 -5.26 × 10 4 m −1 , and the relative standard deviations (RSDs) were determined as 0.68% and 2.34%, respectively.LOD and LOQ as important analytical parameters for PL sensing, were analyzed via the equations of LOD = 3 K −1 and LOQ = 10 K −1 , [47] which are 0.45 and 1.50 μm for P4SiF nanoparticles, 0.74 and 2.48 μm for P3SiF nanoparticles, respectively (Figures S10 and S11, Tables S1 and S2, Supporting Information).The confidence intervals of LOD and LOQ are 0.41-0.46μm and 1.35-1.54μm for P4SiF, 0.72-0.77μm and 2.38-2.57μm for P3SiF, respectively.Table 1 lists the PL sensing analysis parameters of the prepared polymers toward TNP.0]   DNT and TNB, respectively.The LODs of P4SiF nanoparticles toward 4-NP, DNP, DNT, and TNB were determined to be 0.44, 0.62, 7.25, and 3.73 μm, while LOQs were determined to be 1.45, 2.07, 24.17, and 12.43 μm, respectively (Figures S16-S19, Tables  In environmental water samples, there are various ions exist which may affect the PL sensing process; therefore, the ion anti-interference experiment was carried out.By adding potential interfering ions (Ba 2+ of 10 mg mL −1 ; Br − and OH − of 1 mg mL −1 ; Cl − , Ca 2+ , K + , Cu 2+ , Fe 3+ , Fe 2+ , Na + , Zn 2+ , Mn 2+ , Mg 2+ and NO 2 − of 2 mg mL −1 ) into P4SiF and P3SiF aqueous dispersions followed by TNP (25 μm) addition, the PL quenching was recorded in Figure S28 (Supporting Information).Significantly, the interfering ions only have little impact on the PL sensing results of P4SiF and P3SiF nanoparticles toward TNP, which suggests the TNP detection in water based on P4SiF and P3SiF has high ion anti-interference characteristics.

Application
In order to test the practical application of P4SiF and P3SiF nanoparticles, PL sensing measurements for TNP were conducted in environmental water samples.Based on the relationship between the relative PL intensity of P4SiF and P3SiF nanoparticles (0.07 mg mL −1 ) and the concentration of corresponding TNP (mg L −1 ), the linear regression equations were determined to be y = 0.2798[TNP] − 0.0905 and y = 0.1895[TNP] − 0.0560, respectively, as the calibration equations for the detection of TNP in real water samples (Figure S29, Supporting Information).The spike/recovery test was conducted in deionized water, mineral water, and tap water, and the spiked amount was 5.00 mg L −1 .The quantitative recovery rates of TNP in various water samples were determined to be 95.06-107.06% and 97.07-103.23%based on P4SiF and P3SiF, respectively (Table 2).The RSDs of TNP analysis are in the range of 1.99-4.11%and 0.98-1.65%based on P4SiF and P3SiF, respectively.The t-test data were calculated to evaluate the accuracy of our sensing method, as shown in Tables S11 and S12 (Supporting Information).The results are all acceptable with t ref. values less than 4.30 at a 95% confidence level, thus demonstrating the reliable detection of TNP with high sensitivity and selectivity by the PL sensing using P4SiF and P3SiF nanoparticles as sensing materials.
In order to explore the on-site practical application of the polymers in the detection of nitroaromatic explosives, P4SiF and P3SiF paper strips were made by immersing the filter paper into the individual polymer solutions followed by drying in the air.As shown in Figure 8, by dropping TNP solutions of different concentrations onto the polymer paper, gradual PL quenching can be observed as the amount of TNP increases.It is important to note that only 0.05 mg mL −1 TNP solution is required to identify the obvious PL quenching of the two polymers.Therefore, this result demonstrates the practical application of P4SiF and P3SiF nanoparticles as PL solid-state sensors toward TNP detection.

Quenching Mechanism
P4SiF and P3SiF nanoparticles exhibit high sensitivity toward TNP detection in water media, owing to the microporosity and electron-rich -conjugated polymer backbones that promote the adsorption and diffusion of TNP within the nanoparticles and the interaction with TNP.In order to further explore the reasons for the sensing property of P4SiF and P3SiF nanoparti-cles toward TNP, the sensing mechanism was investigated.The LUMO energy levels of the two polymers and the nitroaromatics are recorded in Figure 9, [51][52][53] and it is evident that the LUMO levels of the detected nitroaromatics are lower than those of the polymers.[56] It is worth noting that the LUMO energy level of TNP is significantly lower than that of DNP, DNT, and TNB, which may be one of the reasons for the higher sensitivity toward TNP detection.Besides, the UV−vis absorption spectra of the nitroaromatic compounds, the excitation spectra, and the PL spectra of P4SiF and P3SiF nanoparticles in water are shown in  dispersions toward TNP were carried out, as shown in Figure S30 (Supporting Information).It can be seen that no additional absorption bands are observed in the UV−vis absorption spectra, which conforms to the inner filter effect mechanism and excludes the possibility of static quenching. [57,58]The time-resolved PL spectra of P4SiF and P3SiF are shown in Figure 11.The pristine PL lifetimes of P4SiF and P3SiF in deionized water were calculated to be 0.89 and 0.96 ns, respectively.Upon the TNP addition (21 μm), the PL lifetimes remain almost unchanged, with 0.89 and 0.88 ns, respectively.In addition, time resolved PL titration experiments were conducted based on P4SiF and P3SiF aqueous dispersions with different TNP concentrations (Figure S31, Supporting Information).The  0 / of P4SiF and P3SiF aqueous dispersions can be linear fitted against TNP concentration, which is almost parallel to the x-axis, indicating the substantially invariable PL lifetimes during TNP addition.Therefore, it demonstrates that the fluorescence inner filter effect is the main mechanism for the better sensitivity and selectivity of P4SiF and P3SiF nanoparticles toward TNP detection.The nitrophenolic molecules, such as 4-NP, have similar absorption spectra and show similar sensing sensitivity despite their lower amounts of nitro group; however, TNB that has the same amount of nitro group with TNP but a very different absorption spectrum causes a much lower quenching effect with one order of magnitude lower K SV , which is consistent with the proposed sensing mechanism.

Conclusion
Two novel conjugated microporous polymers P4SiF and P3SiF nanoparticles containing fluorene and tetraphenylsilane were successfully synthesized by Suzuki miniemulsion polymerization, which possess porous structure and nano-spherical morphology.The PL sensing process of P4SiF and P3SiF nanoparticles toward nitroaromatics was studied in aqueous media by PL titration experiments, and highly sensitive and selective detection performance toward TNP was realized, with K SV values of 6.43 × 10 4 and 4.97 × 10 4 m −1 , LODs of 0.45 and 0.74 μm, LOQs of 1.50 and 2.48 μm, respectively.Furthermore, TNP was detected in environmental water samples using P4SiF and P3SiF nanoparticles by spike/recovery test, and the result shows that the detection method using polymer nanoparticles as sensing materials has high accuracy at a 95% confidence level (t-test).The PL sensing mechanism study reveals that the inner filter effect is the main reason for the sensitive and selective PL quenching toward TNP.

Experimental Section
Characterization of Materials: The details for the characterization of materials are listed in the Supporting Information.
Synthesis of P4SiF Nanoparticles: M0 (268 mg, 0.60 mmol), M1 (196 mg, 0.30 mmol), and Pd(PPh 3 ) 4 (10.4 mg, 0.009 mmol) were dissolved in toluene (2 mL), then ultrasonically treated for 10 min.SDS (200 mg) and K 2 CO 3 (331 mg, 2.40 mmol) were mixed in deionized water and sonicated for 15 min.Then the two solutions were mixed and sonicated for 10 min to obtain a stable water/toluene miniemulsion, which was heated at 85 °C in N 2 atmosphere for 24 h.At the end of the polymerization, the solution was evaporated to remove toluene and dialyzed in deionized water to remove the unreacted monomers, surfactant, catalyst, base, and low molecular weight oligomers.Finally, the mixture was centrifuged to obtain P4SiF nanoparticles.Yield: 147 mg (68%).

Figure 1 .
Figure 1.a) The FT-IR spectra of P4SiF and P3SiF.b) The 1 H NMR spectra of P4SiF, P3SiF and the corresponding monomers.

Figure 2 .
Figure 2. SEM images and sizes of a, b) P4SiF and c, d) P3SiF nanoparticles.

Figure 3 .
Figure 3.The N 2 isothermal adsorption and desorption curves of a) P4SiF and c) P3SiF nanoparticles.The pore size distributions of b) P4SiF and d) P3SiF nanoparticles.

Figure 4 .
Figure 4. UV−vis absorption, excitation, and emission spectra of a) P4SiF and b) P3SiF nanoparticles in deionized water.

Figure 6 .
Figure 6.PL spectra of a) P4SiF and c) P3SiF nanoparticles in aqueous media upon TNP addition.Stern-Volmer plots of relative PL intensities (I 0 /I-1) of b) P4SiF and d) P3SiF nanoparticles versus TNP concentration.

Figure 7 .
Figure 7. K SV values of a) P4SiF and c) P3SiF nanoparticles in aqueous media toward various nitroaromatic compounds.PL quenching degrees of b) P4SiF and d) P3SiF nanoparticles upon the individual addition of 25 μm of various nitroaromatic compounds, followed by adding the same amount of TNP.

Figure 10 .
Figure 10.a) The UV−vis absorption spectra of TNP, 4-NP, DNP, DNT, and TNB in water.b) The UV−vis absorption spectra of TNP and 4-NP, the excitation and PL spectra of P4SiF and P3SiF nanoparticles in water.

Figure 11 .
Figure 11.The time-resolved PL decay spectra of a) P4SiF and b) P3SiF nanoparticles in aqueous media with and without TNP addition.

Table 1 .
Analytical parameters of P4SiF and P3SiF nanoparticles for TNP detection.
a) The confidence interval was calculated on the basis of x ± t S √ n , x is sample mean, n is number of experiments, S is sample standard deviation, confidence level is 95% and t is 4.30.

Table 2 .
Spike and recovery tests for detecting TNP in environmental water samples.