Non-conjugated polyurethane polymer dots based on crosslink enhanced emission (CEE) and application in Fe³⁺ sensing

Abstract

Non-conjugated polymer dots (NCPDs) based on crosslink enhanced emission were synthesized via polyurethane polymerization of triazine derivatives. These non-conjugated polymer dots can be applied in recognizing Fe³⁺.

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Polymer dots (PDs) have drawn increasing attention in the area of functional materials due to their broad applications in photoconducting devices, cell imaging, sensors and so on.¹ Generally, PDs refer to the conjugated PDs, which are obtained from an assembly of fluorescent conjugated polymers. In these polymer dots, carbon-based π electrons or carbon–carbon double/triple bonds are regarded as the main building unit for the photoluminescence (PL) center. Non-conjugated polymers have little fluorescence due to their unconjugated structures. Interestingly, in recent years, researchers have found that some non-conjugated polymer dots (NCPDs) showed good emission that could be observed by naked eye due to their crosslink enhanced emission (CEE) effect.² These NCPDs possess high fluorescence efficiency and extraordinary light-gathering capability. Moreover, compared with conjugated polymers, they are more stable for practical applications. However, the synthetic methods of NCPDs are limited and most are obtained from carbonization or dehydration of commercial polymers, such as polyethyleneimine (PEI), poly(vinyl alcohol) (PVA), polyethylene glycol (PEG), poly(L-proline), chitosan and so on.³ Similar to the synthesis of C-dots, these methods inevitably needed high reaction temperature and long reaction time. Thus, it is difficult to preserve the chemical properties of the precursors. Therefore, it is highly desired to develop a simple method to synthesize NCPDs in mild conditions.

In 2012, Pan's group reported a new non-conjugated fluorescent polymer based on hyperbranched polymerization of tertiary aliphatic amines.⁴ Via Michael addition between sulfydryl (–SH) and unsaturated double bonds (–CH=CH–), a large number of unconjugated nitrogen atoms were fixed in the polymer network. This method could retain high fluorescence efficiency (Φ = 2.5% in CHCl₃) of the tertiary amines which can be easily quenched by solvents. Nevertheless, the poor dispersibility in water limits their wide application and development. After that, this hyperbranched crosslink enhanced emission rarely appears in literatures and has been no further breakthrough.

To set out the above problems in hyperbranched NCPDs, we consider polyurethane reaction as a better alternative (Scheme 1).⁵ Similar to those nanoparticles (nano Au, Pt, Fe₃O₄, TiO₂ etc.) modified by hydrophilic substance (PEG, EGM etc.),⁶ we selected triethylene glycol monomethyl ether (TGME) as the hydrophilic end-capping agent for our NCPDs. As a result, our strategy significantly improves the dispersity of hyperbranched polymers in water. Another advantage is that the fluorescence of our NCPDs in 90% H₂O is better than that in pure DMSO. The reason for this is that the hydrophobic PL center in our NCPDs is compressed and get closer in 90% H₂O, which is conducive to CEE (Fig. S1†). And their dispersion states can be maintained over 6 months.

Scheme 1 Synthesis of HPUs-Tx NCPDs.

As the CEE unit in our polyurethane, hexahydro-1,3,5-tris(hydroxyethyl)-s-triazine (HTHT) is an inexpensive industrial product, widely applied in adhesive, bactericide, desulfurizer, metal working fluid and so on.⁷ It is also worth to mention that there are three tertiary aliphatic amines in such a simple non-conjugated triazine structure that could meet the condition of CEE monomer. In addition, the polymer based non-conjugated triazine ring has been becoming a research hotspot in the area of thermosets recently.⁸ However, their photoluminescence has not been explored. Herein, we chose HTHT to form CEE unit and synthesized hyperbranched polyurethanes (HPUs) through one-pot step in 90 °C (Scheme 1b). Different to carbonization and dehydration, our synthesis is simple, mild and rapid. During the synthesis process, heating and stirring, the reaction solution became thicker and thicker. After 15 min, gelation behavior was observed, and curing process occurred after 20 min. We suggest that adding end-capping agent is the key step. For comparison, with different time of adding end-capping agent TGME, compounds HPUs-Tx (x = 1, 5, 10, 15, 20) were synthesized. Meanwhile, with methanol as end-capping agent, compound HPUs-T0 was designed. As we expected, HPUs-T0 is hydrophobic and could be not dispersed in water/organic solvent (DMSO, DMF and so on).

Fig. 1a illustrates the TEM image of the obtained polyurethane PDs. In contrast to HPUs-T0, it can be seen that these HPUs-T1–20 NCPDs are well dispersed in 90% water. Same as previous NCPDs, their dimensions are about in the range of 50 nm (Fig. 1b and S2†). As shown in Fig. 1c, these NCPDs exhibit characteristic absorption bands of C–N, O=C–NH–, C–H and N–H. Also these group could be observed in NMR (Fig. S3 and S4†). The above results show that there are various functional groups in HPUs-Tx. Moreover, the TGA curves of HTHT and HPUs-Tx are described in Fig. 1d, indicating that crosslink polymerization is occurred. The polymerization time (1–20 min) has little influence on their thermal stability. Similar TGA curves could be found in other polymers based on non-conjugated triazine ring.

Fig. 1 (a) TEM image of NCPDs by HPUs-T15; (b) size distribution histogram of NCPDs by HPUs-T15. Inset: photo of HTHT (left) and HPUs-T15 (right) in DMSO under a UV lamp (365 nm); (c) FTIR spectra of HPUs-Tx (x = 1, 5, 10, 15, 20); (d) TGA curves of HTHT and HPUs-Tx (x = 1, 5, 10, 15, 20).

Our prepared NCPDs solutions could emit blue luminescence under a UV lamp but their monomer HTHT couldn't (365 nm, inset in Fig. 1b). Due to free rotations in triazine structure, the tertiary amines in HTHT display weak peak in DMSO solution. In NCPDs, these tertiary amines are fixed in the crosslink polymers that could restrict their vibration and rotation.^2d Due to this crosslink enhanced emission (CEE) effect, our NCPDs have good fluorescence properties. The absorption spectrum shows a narrow peak at 325 nm (Fig. 2a left). When our NCPDs are excited at 340 nm, a maximum emission peak at 430 nm is observed. Like other NCPDs, the excitation-dependent emission properties also could be observed (Fig. 2a right). As the excitation wavelength gradually red-shifts from 320 nm to 460 nm, the emission wavelength changes with the increase of excitation wavelength. The property of excitation induced tunable emission is very obvious and reflects the different molecule states in the NCPDs. The quantum yields (Φ) of NCPDs by HPUs-Tx (DMSO : H₂O = 1 : 9) are about 1.1–2.2% (Table S1†). These results are slightly higher than that in pure DMSO and at the same level as other common NCPDs. Among them, the best performance is occurred in NCPDs by HPUs-T15 (Fig. 2b). The reason is that the number of that the tertiary amines in NCPDs increases when the first polyurethane reaction is in process. This helps to enhance the fluorescence of NCPDs but this improvement effect is not endless. The large size of hydrophobic polyurethane core is unfavorable for the dispersity of NCPDs in water. Thus, adding end-capping agent TGME after 15 min to stop polyurethane reaction seems to be the best choice. Better than previous hyperbranched NCPDs, the tertiary amine in HPUs-Tx polymers can retain high fluorescence efficiency in 90% water with the help of surrounding hydrophilic TGME. The pH behavior of our NCPDs was explored (Fig. 2c). The NCPDs have high stability against pH variation in the range of 2–12. In the previous report, the polymer based non-conjugated triazine ring could be digested at low pH (<2) to recover the monomers (Scheme S1†).^8a,b We also have tried to add our NCPDs into pH = 1 solution and found their solution became turbid immediately (Fig. S5†). We infer that the triazines are gradually decomposed (pH = 1) and the outside hydrophilic TGME are stripped. As we expected, we have found this decomposition product of NCPDs in MS spectra (Fig. S6†). With non-conjugated triazine ring, our polymer dots are easily degraded by strong acid after using. To our knowledge, this new property of NCPDs has been not reported in other conjugated polymer dots. Additionally, no obvious photobleaching phenomenon was observed with a continuous exposure under UV excitation for 6 hours (Fig. 2d). Thus, our NCPDs has excellent photostability.

Fig. 2 The optical properties of NCPDs at a concentration of 0.5 mg mL⁻¹ in DMSO–H₂O (1 : 9, v/v). (a) The absorption and fluorescence spectra of NCPDs by HPUs-T15; (b) the PL peak intensity (430 nm) of NCPDs by HPUs-Tx (x = 1, 5, 10, 15, 20) λ_ex = 340 nm. Inset: photos of NCPDs by HPUs-Tx (x = 1, 5, 10, 15, 20) in DMSO–H₂O (1 : 9, v/v) under a UV lamp (365 nm); (c) the PL intensity of pH variation (pH = 2–12); (d) photostability of NCPDs by HPUs-T15 under UV excitation at various time.

Although selective detection of metal ions using fluorescence materials has been the subject of many studies,⁹ there have been very few reports in NCPDs. In Fig. 3a, it is found that its fluorescence could be quenched by Fe³⁺ in PBS buffer (pH = 7.4). Meanwhile, there are no obvious changes when other metal ions are added. These results reveal that our Fe³⁺ detection method has high selectivity against the other metal ions. Fig. 3b represents the relation of the relative intensity ((F₀ − F)/F) with different Fe³⁺ concentrations. The quenching efficiency can be further described by the Stern–Volmer plot with a perfect linear behavior (the linear correlation coefficient is 0.996) (Fig. 3b). The Stern–Volmer equation is F₀/F = 1 + 0.0078[Fe³⁺], where F₀ and F are the fluorescence intensities of our NCPDs in the absence and presence of Fe³⁺. And their UV absorption spectra is shown in Fig. S7 and fluorescence lifetimes in Fig. S8.† To our knowledge, the application of NCPDs in Fe³⁺ sensing has not been reported.

Fig. 3 (a) Fluorescence changes of NCPDs by HPUs-T15 (0.5 mg mL⁻¹) upon the addition of 50 μM various metal ions (λ_ex = 340 nm) in DMSO–PBS buffer (10 mM) (pH = 7.4, 1 : 9, v/v); (b) PL spectra of NCPDs with different Fe³⁺ concentrations (0 to 125 μM). Inset: plot of (F₀ − F)/F with different Fe³⁺ concentrations (0 to 125 μM); (c) structures and HOMO/LUMO orbitals of NCPDs and NCPDS–Fe³⁺.

Next, we also need to study the mechanism of our NCPDs vs. Fe³⁺. Similar to other Fe³⁺ sensors based on N-doped carbon dots and graphene quantum dots,¹⁰ we deduce that Fe³⁺ ions surrounding our NCPDs quench the CEE fluorescence (Scheme S2†). It could be proved by the followings: (1) fluorescence recovery of NCPDs is occurred after adding EDTA (Fig. S9†); (2) in addition of Fe³⁺, the decomposition product of NCPDs has not been found in MS spectra (Fig. S6†). Then, we study the NCPDs–Fe³⁺ complex by NMR (Fig. S10†). After the addition of Fe³⁺ to NCPDs in a d-DMSO solution, the proton signals of –O–CO–NH– in polyurethane and –CH₂– in triazine display apparent downfield shifts from the peaks centered at 7.09/6.95 ppm and 4.01/3.95 ppm to broad peaks centered at 7.13 ppm and 4.06 ppm, respectively. The reason lies in the paramagnetic property of Fe³⁺ ion and its complex with NCPDs can affect the proton signals that are close to the Fe³⁺ binding site. According to the literature reports, keto–enol tautomerism in amide is the key to capture Fe³⁺ ions.¹¹ Therefore, the combination of NCPDs and Fe³⁺ ion we infer is shown in Fig. 3c.

To better understand the coordination of Fe³⁺ and NCPDs, we further perform theoretical calculations at DFT//B3LYP/6-31G(d) level using Gaussian 09 program.¹² From Fig. 3c, it can be found that the Gibbs free energy of the complex NCPDs–Fe³⁺ is smaller than that of NCPDs. It reveals that this reaction can be carried out spontaneously. Moreover, in the complex NCPDs–Fe³⁺, the electron cloud distribution of triazine CEE unit has changed remarkably. We could reason it should be responsible for its fluorescence quenching. Our calculated results agree with the experimental results.

Conclusions

In summary, we successfully developed pH-stable (pH = 2–11), water-dispersible non-conjugated polymer dots (NCPDs) by using inexpensive industrial hexahydro-1,3,5-tris(hydroxyethyl)-s-triazine (HTHT). Although HTHT has little fluorescence on account of their non-conjugated structures, our new NCPDs show so good emission that could be observed by naked eye (50 nm; Φ = 2.2%; τ = 2.5 ns) due to crosslink enhanced emission (CEE) effect. Moreover, our NCPDs is not only a kind of nanoparticle but also a sort of polymer. For one thing, compared with N-doped carbon dots and graphene quantum dots, the chemical structures and properties of precursors in our NCPDs are well retained without carbonization or dehydration. For another, compared with conjugated polymers, our NCPDs are low-cost, easy to be synthesized and possess excellent photostability. And better than conjugated polymers, our NCPDs could be easily decomposed in the strong acid condition after using. In our work, not only using tertiary aliphatic amines to synthesis NCPDs has been improved by the introduction of hydrophilic end-capping agent triethylene glycol monomethyl ether (TGME), but also new performance of the polymer based on non-conjugated triazine ring has been developed.

Acknowledgements

We thank Dr Shoujun Zhu for the discussion and assistance. This work was supported by “Thousand Talents Program”, National Natural Science Foundation of China (Grand No. 21507139), China Postdoctoral Science Foundation (Grand No. 2015M581971), Zhejiang Postdoctoral Sustentation Fund, China (Grand No. BSH1502162) and the Natural Science Foundation of Ningbo (Grant No. 2016A610265).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, NMR and HRMS spectra, optical spectra and other details. See DOI: 10.1039/c6ra17068d

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