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).