Aggregation-Induced Room-Temperature Phosphorescence Obtained from Water-Dispersible Carbon Dot-Based Composite Materials

Abstract

Room-temperature phosphorescence (RTP) materials are desirable in chemical sensing because of their long emission lifetime and they are free from background autofluorescence. Nevertheless, the achievement of RTP in aqueous solution is still a highly challenging task. Herein, a molten salt method to prepare carbon dot (CD)-based RTP materials is presented by direct calcination of carbon sources in the presence of inorganic salts. The resultant CD composites (CDs@MP) exhibit bright RTP with a quantum yield of 26.4% and a lifetime of 1.28 s, which lasts for about 6 s to the naked eye. Importantly, their aqueous dispersion also has good RTP characteristics. This is the first time that the long-lived CDs@MP with RTP are achieved in aqueous solution owing to the synergistic effect of crystalline confinement and aggregation-induced phosphorescence. Further investigations reveal that three key processes may be responsible for the observed RTP of the composite materials: (1) The rigid crystalline salt shell can preserve the triplet states of CDs@MP in water and suppress the nonradiative deactivation; (2) The addition of high-charge-density metal ions Mg­(II) and phosphorus element in the composite facilitates the singlet-to-triplet intersystem crossing process and enhances the RTP emission; (3) The aggregation of CDs@MP nanocomposites enables the matrix shell to self-assemble into a network, which further improves the rigidity of the shell and prevents the intermolecular motions, hence prolonging the RTP lifetime. The unique RTP feature and good water dispersibility allow the CD-based composite materials to be applicable in detection of temperature and pH in the aqueous phase. Our approach for producing long-lived RTP CDs@MP is effective, simple, and low-cost, which opens a new route to develop RTP materials that are applicable in aqueous solution.

Carbon Dot−NaCl Crystals for White‐Light Generation and Fabry‐Perot Lasing

“…[2a,3] The metal-doped materials with long lifetime generally have a high cost, cytotoxicity and low stability. [4] Meanwhile, the organic afterglow materials are less efficient because of their low lightfastness. [2b,5] These materials also often demand complicated synthesis procedures (high temperature, long duration, strong, redox material).…”

“…However, long lifetime materials have several limitations [2a,3] . The metal‐doped materials with long lifetime generally have a high cost, cytotoxicity and low stability [4] . Meanwhile, the organic afterglow materials are less efficient because of their low lightfastness [2b,5] .…”

Afterglow Carbon Dots: From Fundamentals to Applications

“…The proposed aggregation-induced RTP mechanism was supported by TEM imaging in which randomly oriented CD aggregates were observed (Figure 8(i)). Another example was reported by Wang et al in 2020, who have found that enhancing ethanol fraction in an acidic CD dispersion higher than 30% can also trigger the appearance of yellow RTP [52]. In this case, enhancing the ethanol fraction in the mixture first led to the formation of CDs@MS nanocomposites which underwent further aggregation and gave rise to bright yellow RTP in the resulting solution (Figure 8(m)).…”

“…Despite the attractiveness, the use of MS in the preparation of CD-based afterglow nanocomposites is hindered by the hygroscopic nature of the used MS, which enables the weak photoluminescence stability of the resultant CDs@MS nanocomposites. This concern can be partially addressed by doping MgCl 2 and KH 2 PO 4 into the MS in the carbonization synthesis of CDs, as supported by their follow-up work [52]. Owing to the poor solubility of Mg-PO 4 salts in water, the tolerance of the afterglow luminescence toward water has been considerably improved, even leading to the observation of a bright long-lived yellow RTP in an aqueous solution of the resulting CD@MP after ceasing the UV excitation (Figures 5(b)-5(d)).…”

“…For fluorescence, aggregation of CDs can largely decrease the distance between the fluorescent centers, leading to a redshift in the emission profile or even the quenching of the emission due to the nonradiative recombination of charge carriers [104,105]. In contrast, recent studies suggest that aggregation of CDs resulting from their π-π stacking not only holds promise to stabilize the T 1 excited state but also possibly leads to the formation of an additional triplet excited state (T 1 * ) (Figure 8 (adapted and copyright permission [52], American Chemical Society). Research prepared via the hydrothermal treatment of trimellitic acid (TA) at 260°C for 12 h [40].…”