2.1 Sensing applications
The good water-solubility and low toxicity of nonconventional polysiloxanes make them potential fluorescent probes in molecular biology, analytical chemistry, environmental monitoring and clinical diagnosis. The TSC, which is generated from the aggregation of electron-rich atoms, can be disturbed by electron-deficient compounds and exhibit stimulus responsiveness. For example, P1, which contains conjugated O=C-O-Si-C=C segment, is sensitive to Fe3+.[43] When adding the same amount of Ba2+, Na+, Ca2+, Hg2+, Cd2+, Al3+, Fe3+, Cu2+, Zn2+, Co2+ and Fe2+ to the P1 solutions, the mixture of Fe3+ and P1 showed quenched fluorescence, while other mixtures remain fluorescent (Figure 8A). Within a certain range of Fe3+ concentrations, the fluorescence intensity of P1 solution decrease along with the increase of Fe3+concentrations. A reasonable explanation is the ICT between the aggregated electron-rich atoms and Fe3+ ions. Due to the chelating process between Fe3+ and the electron-rich atoms, a P1-Fe3+ complex is formed when mix Fe3+ with P1. The oxidizing Fe3+ can accept electrons and disturb the TSC of P1, resulting in charge transfer quenching in P1. The addition of Na2EDTA can cooperate with Fe3+ and disassembles the P1-Fe3+ complex, restoring the fluorescence of P1.
Besides local conjugated HBPSi, our group reported that HBPSis contain no conjugates are also sensitive to metal ions, such as Fe3+, Cu2+ and Co2+ (Figure8B and 8C), and they showed varied fluorescent quenching behavior according to the content and relative position of electron-rich groups.[42,47] The Fe3+ responses were also observed in HBPSi synthesized from ethyl orthosilicate,[33]diethanolamine,[47] N-methyldiethyl alcoholamine[38] and diethylene glycol,[44] which may be caused by the chelating process between lone-pair electrons and Fe3+. The chelating effect between metals ions and electron-rich atoms could also generate obvious color change, as shown in Figure 8C. Thus, the HBPSi can be potential candidate for metal ion sensors. Intriguingly, Feng et al. reported the similar Fe3+ response in Si-O-Si consisted linear polysiloxanes. The fluorescence of PMDF can be quenched by Fe3+ and as high as 90% fluorescence quenching was observed.[60] The charge transfer between excited state of PMDF and unfilled orbit of Fe3+ leaded to nonradiative complex and thereby caused fluorescence quenching. As depicted in Figure 8E, the PMDF film is a visible Fe3+detector and the detection limit of Fe3+ is 12.32 μM.
Furthermore, Fent et al. reported the electron transfer from triphenylamine groups in BpaD and BpaP to 4-nitrophenol and developed simple and visualized paper sensors for 4-nitrophenol based on the 4-nitrophenol-caused fluorescence quenching.[61]By coating BpaD or BpaP on a filter paper, paper sensors were obtained. Figure 8D shows the fluorescence quenching of BpaD or BpaP paper by the appearance of 4-nitrophenol. This method provides a portable and visual candidate for 4-nitrophenol detection with a limits of detection (LODs) of 0.6 μM for BpaD and 0.23 μM for BpaP and a wide concentration range of 0-50 μM. Further, they reported the quenching effect of nitrobenzene to n-TPE-AP-PDMS based on charge-transfer mechanism.[59] The n-TPE-AP-PDMS film exhibit high reversibility and the quenching efficiency remained constant after five cycles, which provides a promising method to develop chemosensors with high performance.
Wang et al. developed fluorescent sensors for three nitroaromatic compounds (nitrobenzene, m -dinitrobenzene, and picric acid) based on the electron-rich delocalization structure of TPB functionalized polysiloxane (TPB-P) and porous structure of fluoranthene-modified polysiloxane (FMPS).[54,57] As depicted in Figure 8F and 8G, the unique porous structure of TPB-P aggregates and FMPS nanoparticles enhanced the interactions between nitroaromatic compounds and TPB-P/FMPS, thus gives TPB-P and FMPS sensitive responsiveness. Both TPB-P and FMPS showed highest detection sensitivity and sensitivity towards picric acid, with LODs of 17.8 nM (TPB-P in solution), 4.8 nM (TPB-P in aggregated state) and 69.5 nM (FMPS).