Y.-T. Zeng et al.
Dyes and Pigments 193 (2021) 109491
Fig. 4. Emission intensity of TPE-POSS in the mixture of Fꢀ and other anions.
Fig. 3. Emission spectra changes of TPE-POSS upon the addition of various
anions. λexc = 340 nm. Inset: the optical images of TPE-POSS with the addition
[TPE-POSS] = 10 μM, [anions] = 40 μM.
of various anions under UV irradiation. [TPE-POSS] = 10
μM; [anions] =
TPE-POSS sensor has a good response speed to Fꢀ (Fig. S6). It should be
noted that TPE-POSS is not soluble in water, but it shows turn-on
response to Fꢀ in the THF-H2O mixed solution (v:v = 1:1) (Fig. S7).
According to the above results, the TPE-POSS sensor exhibits turn-on
fluorescence response to fluoride ions, which avoids the shortcomings
of turn-off sensors that are easily interfered by external factors.
Therefore, the selectivity of TPE-POSS sensor to anions was studied.
The PL spectra were done after adding tetrabutylammonium salts of Clꢀ ,
Brꢀ , Iꢀ , HSOꢀ4 , OHꢀ , ClO4ꢀ , H2POꢀ4 , Acꢀ and Fꢀ to the THF solution of
TPE-POSS. As can be seen in Fig. 3, the fluorescence spectra displayed an
obvious distinction of fluoride ions from other anions. Moreover, the
inset photographs in Fig. 3 show that only the solution with fluoride ion
added has fluorescence turn-on response, and the appearance of green
fluorescence can be detected easily by naked-eye. The visible color
changes did not observed with the addition of other anions. The above
phenomena show that the sensor molecule has excellent specific
recognition performance for fluoride ions.
40 μM.
properties of TPE-POSS were investigated in a mixture of THF/H2O with
different water fractions (fw), in which THF is a good solvent and water is
a poor solvent. As displayed in Fig. S5, the absorption of TPE-POSS
increased dramatically with the addition of water, which is attributed
to the scattering effect of in-situ-generated nanoaggregates [33]. Par-
alleling with the absorption changes, the emission intensity was grad-
ually enhanced with obvious blue shift. As can be seen in Fig. 1a, when
the water fraction (fw) was less than 20%, the solution showed extremely
weak emission because the molecule was in the dispersed state. With the
water content increased, the fluorescence intensity showed significant
enhancement. For instance, compared with the TPE-POSS sample in
pure THF, the PL intensity of TPE-POSS in THF/H2O mixture with fw
=
90% was boosted with 19 folds whereas the emission wavelength was
blue-shifted from 488 nm to 462 nm (Fig. 1b). The obvious blue shift of
the fluorescence spectrum is due to the formation of aggregates. The
internal movement of the molecule was restricted, which caused the
molecule to a more distorted conformation, leading to a decrease in the
conjugation length of the molecule and a blue shift in the spectrum [40].
All of these phenomena showed that TPE-POSS is a typical AIE material,
which can also be seen from the inset photographs in Fig. 1b. Moreover,
it was worth noting that there is almost no overlap between the fluo-
rescence and the absorption spectra. This large Stokes shift helps the
detection system, which can reduce the interference of excitation light
scattering on fluorescence detection and improve the detection
sensitivity.
In order to effectively and specifically the sensing fluoride ions, it is
an essential requirement that other coexisting anions do not interfere
with it. The competition experiments were carried out by measuring the
fluorescence intensity of the sensor TPE-POSS in the presence of the
interfering anions and fluoride ion. As displayed in Fig. 4, the compet-
itive experiments show that there no significant changes in the fluo-
rescence response of TPE-POSS toward Fꢀ with the present of other
anions, which indicates that the sensor TPE-POSS can effectively detect
fluoride ions even in the presence of other competing anions. Therefore,
there is no doubt that TPE-POSS can be used as a potentially highly
selective sensor for detecting Fꢀ .
3.2. Sensing mechanism studies of TPE-POSS to Fꢀ
3.3. Sensing mechanism studies of TPE-POSS to Fꢀ
The intriguing AIE effect and Si–O components of TPE-POSS prompt
us to explore its potential application as fluorescence sensor for
detecting fluoride ions. Fluorescence titration experiments were con-
ducted to quantify the fluoride ions induced fluorescence intensity
changes of TPE-POSS, using n-Bu4NF as the fluoride source. As displayed
In order to elucidate the sensing mechanism between TPE-POSS and
fluoride ions. 29Si NMR spectra and dynamic light scattering (DLS)
measurements were performed. It was found that the peak attributed to
the POSS nanocages disappeared after reacting with Fꢀ (Fig. 5a), which
demonstrated the structural destruction of POSS nanocage. And DLS
measurements further showed that the average size of TPE-POSS (10
in Fig. 2a and b, with the gradual addition of Fꢀ (0–50
μM) to TPE-POSS
(10 μM) solution, the emission intensity at 488 nm increased gradually
and further stabilized. It can be observed a 7.5-fold turn-on ratio when
μM) in the THF solution increased from 22 nm to 142 nm after 4 equiv.
40
μ
M Fꢀ was added. The inset in Fig. 2b shows clearly the change of
fluoride ions added (Fig. 5b). These results indicated that POSS nano-
particles collapse with the addition of Fꢀ , which led to the aggregation
of TPE cores and further promoted the fluorescence enhancement. In the
light of the above results, it can be reasonably inferred that the reaction
mechanism of POSS collapse led to TPE-POSS with excellent sensing
performance (Scheme 2).
fluorescent color after adding fluoride ions under the ultraviolet lamp.
The increase in fluorescence intensity at 488 nm had a good linear
relationship (R2 = 0.99) with the Fꢀ concentration in the range of 5–35
μ
M (Fig. 2b). According to the definition by IUPAC (LD = KSb/m) [41],
the detection limit was calculated to be 1.66 × 10ꢀ 7 M. Moreover, the
4