Original Research PaperRetained fluorescence of aggregation-caused quenched Rhodamine grafted in the hierarchical mesopores of silica MCM-41 at solid-state
Graphical abstract
Introduction
Recently, luminescence especially fluorescence and phosphorescence, which arise from the spontaneous light emission from a material after absorption of UV/visible light, has received great attention in different research disciplines, such as material science, chemistry, physics, and biological medicine [1], [2], [3], [4]. Different materials, in different states of matter fluorescence or phosphorescence in a unique manner. Organic compounds in dilute solutions are known to show different photophysical properties compared to their solid states or at high concentrations in solutions [5], [6]. Focusing on the photophysical properties of Xanthenes, particularly Rhodamines, they are characterized to follow aggregation-caused quench (ACQ) effect phenomena [7]. For example, their luminescence is weakened at solid state. The observed concentration-caused quenching is reported to arise from the formation of aggregates due to dynamic intramolecular rotations which result in non-radiative annihilation of its excited states [8]. Rhodamines have a stable structure, good light stability and high fluorescence quantum yield characteristics in dilute solution [9], [10], [11]. They are widely used as fluorescent probes [12], chemosensors in the detection of small molecules [12], sensitizers in dye-sensitized solar cells [13] and as laser dyes [14]. However, due to their ACQ effect, Rhodamine dyes’ light emission at solid state is significantly quenched, which seriously limit their applications [15], [16].
Various approaches have been utilised in order to overcome this deficiency. These include, a straightforward approach of avoiding the use of high-concentration of Rhodamines in solution, creating electric double layers with the help of latexes adsorbed onto Rhodamine molecules which tends to prevent their aggregation [17], [18], and compositing Rhodamine with Aggregation-Induced Emission (AIE) chromophores whose fluorescence behaviors are opposite to ACQ at solid state [19], [20]. Mesoporous materials such as porous silica have previously been utilized to induce new functionalities to various chromophores in the past [21], [22], [23]. These include (but not limited to) stability to chemical degradation, new organized arrangements [24] and photophysical properties, novel host-guest interactions, and ordered transfer of electromagnetic energy in chromophore dipoles around inorganic pores [21]. With regards to silica, the liquid-assisted introduction of RhB chromophores to sol-gel prepared porous silica in the previous past has been reported to yield new photophysical and photochemical properties to both silica and RhB [25], [26], [27]. This is attributed to the fact that the process aids inhibition of undesirable migration and aggregation of RhB [21]. Inspired by these previous works on chromophores in porous silica, herein we report and study a way to inhibit the RhB ACQ effect with the help of different kind of pre-amino treated mesoporous silica having a hierarchical structure known as MCM-41. MCM-41 exhibits a novel kind of regular cylindrical mesopores arrangement held in a one-dimensional pore morphology [28], [29] fitting for proposed strategy.
In this current work, it is interesting to note that introduction of RhB to amino-functionalized MCM-41, followed by subsequent washing inhibits its ACQ effect and improves its photoluminescence (PL) significantly at solid state. Briefly, this has been mainly ascribed to the hierarchical and one-dimensional pores of MCM-41 which provides uniform dispersion space for the RhB across its pore diameters. This, in turn tends to inhibit the obvious aggregation and undesirable migrations of RhB in the resultant composite, hence showing improved luminescence even at solid state just like the Aggregation induced emission-based chromophores. Also, the amino-functionalization of MCM-41 with 3-Aminopropyltriethoxysilane (APTES) and the post-washing of composites after grafting were studied. Beyond these, some applications of these obtained materials were probed.
Section snippets
Materials
All chemicals were used as received without further purification. 3-Aminopropyltriethoxysilane (APTES, Sigma–Aldrich), cetyltrimethylammonium bromide (CTAB, Shanghai Wyeth Chemical Co., Ltd.), tetraethoxysilane (TEOS, Sinopharm Chemical Reagent Co., Ltd.), sodium hydroxide (NaOH, Sinopharm Chemical Reagent Co., Ltd.), Rhodamine B (RhB, Sigma–Aldrich), deionized water was generated using a Millipore Milli-Q plus system.
Synthesis of different MCM-41 mesoporous silica powders
MCM-41 mesoporous silica powders (mSiP) with a hierarchical structure was
Characterization of fluorescent mesoporous silica powders
Fig. 1a presents FTIR spectra of neat mSiP and the modified samples of mSiP/RhB, A-mSiP-2-1 and A-mSiP-2-1/RhB. The neat mSiP showed distinct peaks at 3440 cm−1 and 1640 cm−1 which are ascribed to the overlaid asymmetric stretching vibration and bending vibration of hydroxyl of the SiOH and the absorbed H2O. The peaks 1085 cm−1, 800 cm−1 and 464 cm−1 are all ascribed to the symmetrical stretching vibration of SiOSi of a silicon-oxygen tetrahedron whereas 960 cm−1 is ascribed to the symmetrical
Conclusions
In this paper, the method to limit the very high stacking interactions of ACQ-based RhB has been proposed. During then, RhB was introduced to a highly porous (BET surface area of 903.18 m2/g) MCM-41 type of silica (mSiP) with one-dimensional hierarchical structure with the help of APTES. The morphological structure of mSiP, APTES modification of mSiP, and the subsequent washing of the composites were studied in detail to understand their effects to the observed optical behaviours of the
Acknowledgements
This work was funded by the National Natural Science Foundation of China (Grant No. 51673088).
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