Fluorescent linear polyurea based on toluene diisocyanate: Easy preparation, broad emission and potential applications

Abstract

In contrast to conventional fluorescent polymers featured by large conjugation structures, a new class of fluorescent polymers without above conjugations are gaining constant interest owing to their significant academic importance and promising applications in diverse fields. These unconventional fluorescent polymers are in general composed of heteroatoms (e.g. N, O, P, and S) under different forms. Here we report our recent study on polyurea, prepared by a very simple one step precipitation polymerization of toluene diisocyanate in a binary solvent of water-acetone. This polyurea, basically consisting of phenyl ring and urea group, shows fluorescent emission in a broad concentration range, from very low (10^-5 mg/mL) to its solubility limit (50 mg/mL), and in a wide range of emission wavelength from UV to visible regions of up to 500 nm under varied excitation wavelength. The emission behaviors were fully studied under different concentrations and excitations. It was concluded that the emission in UV region was intrinsic due to the conjugation between the phenyl and the adjacent urea unit; while the emission in visible region, strongly excitation dependent, was caused by the cluster formation of the molecular chains, in accordance with the cluster-triggered-emission (CTE) mechanism. The formation of the cluster was tested through dynamic light scattering, FTIR and UV absorbance. Tested in presence of different metal ions, Fe³⁺ demonstrated a quenching effect with high selectivity. Based on this study, different paper-based sensors were designed to detect Fe³⁺, H₂O₂ in bioanalysis and for data encryption. This work provides a simple way to prepare a polyurea, a novel type of unconventional fluorescent polymer, with high emission performance distinct from its known analogues.

Keywords: Cluster formation; Ferric ion quenching; Fluorescent emission; Polyurea; Precipitation polymerization.

Graphical abstract

Fig. 1

Photos of TPU powder taken under daylight (A, left) and 365 nm UV irradiation (A, right); Photos of TPU solutions (B) in DMSO at different concentrations under λ_ex = 365 nm.

Fig. 2

Fluorescent emission spectra of TPU in DMSO at concentration varied from 10⁻⁵ to 0.4 mg/mL under λ_ex = 300 nm (A); the spectra at concentration from 0.004 to 10 mg/mL under λ_ex = 320 nm (B); those at concentration from 0.1 to 20 mg/mL under λ_ex = 350 nm (C); Emission intensity as function of TPU concentration from 10⁻⁵ to 0.4 mg/mL under λ_ex = 300 nm (red line), and that from 0.004 to 10 mg/mL under λ_ex = 320 nm (black line) (D). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Fluorescent spectra of TPU in DMSO solution (5 mg/mL) at different excitation: λ_ex from 320 to 400 nm (A) and from 400 to 440 nm (B).

Fig. 4

Chemical structures of the four model compounds and TPU.

Fig. 5

Particles size detected by DLS in TPU solution at different concentrations.

Fig. 6

FTIR spectra of TPU solutions in DMSO at different concentrations (A); Enlarged spectra of the absorbance zones from 1640 to 1740 cm⁻¹ (B); from 1510 to 1570 cm⁻¹ (C); from 1170 to 1250 cm⁻¹ (D).

Fig. 7

Emission spectra of TPU solution (10 mg/mL) in DMSO in the presence of different metal ions (0.3 mM) under excitation of λ_ex = 350 nm (A), and those in the presence of Fe³⁺ solution of different concentrations under excitation of λ_ex = 350 nm (B).

Fig. 8

Emission intensity of TPU solution (10 mg/mL) as function of Fe³⁺ concentration (mM) under λ_ex = 350 nm (A), and linear relationship of the emission intensity versus Fe³⁺ concentration from 0 to 0.15 mM (B).

Fig. 9

Photo of a paper strip dipped in DMSO solution of TPU (30 mg/mL) under UV lamp (A), the same paper after drawing letter K using aqueous solution of Fe³⁺ ions (0.03 mM, B).

Fig. 10

Under UV lamp, photo of a paper strip dipped in DMSO solution of TPU (30 mg/mL) (A), after writing “COVID-19” using aqueous solution of Na₂EDTA (B), and the same paper strip brushed with aqueous solution of FeCl₃ (1.0 mM, C).