Q. Zhao et al.
Dyes and Pigments 184 (2021) 108875
Fig. 7. a) Writing and erasing of a natural light invisible image on the NDG film; b) schematic for patterning fluorescent hydrogel by photolithography.
NDG-Fe enhanced when Fꢀ was added. In addition, the fluorescence
titration experiments was carried out (Fig. 6c), the LOD of NDG-Fe for
Fꢀ was 1.61 × 10ꢀ 8 M (Fig. 6d). Meanwhile, the addition of other anions
did not cause a significant change on the fluorescence intensity of the
NDG-Fe (Fig. S19). Thus, the NDG-Fe was able to selectively detect Fꢀ in
aqueous solution.
Therefore, the results support the possible NDG continuity recognition
mechanism proposed in Fig. 1 [59]. In addition, the chelator EDTA
(ethylenediaminetetraacetic acid) has strong affinity with Fe3+. In order
to confirm the binding competition mechanism, we use EDTA instead of
Fꢀ . As a result, the EDTA made the fluorescence of NDG-Fe recovery,
which indicated that Fꢀ competitively bound with Fe3þ (Fig. S21).
For application purposes, the ingestion capacity of the NDG for Fe3+
in aqueous solution was assessed by inductively coupled plasma (ICP)
analysis. The xerogel of NDG (1 mg, 6 × 10ꢀ 7 mol) was suspended in
aqueous solution of Fe3+ (1 × 10ꢀ 5 M, 5.0 mL) and stirred for 12 h. Then
the suspension removed by centrifuging at 10,000 rpm for 10 min.
Finally, the ICP was used to assess the ingestion capacity of NDG for
Fe3+. As shown in Table S5 and Table S6, after the adsorption Fe3+ by
the xerogel of NDG in aqueous solution, the residual concentration of
Fe3+ less than 1 × 10ꢀ 7 M. The absorption rate of NDG for Fe3+ was
approximately 99.26%, and the absorption capacity was about 23 mg/g.
Experimental analysis results indicated that the NDG efficiently
removed Fe3+ even in extremely dilute solutions. Compared with other
adsorbent materials, supramolecular hydrogel NDG not only can carry
out ultrasensitive and selective fluorescence detects Fe3+, but also effi-
ciently adsorbs Fe3+ in extremely dilute solutions.
Supramolecular polymer hydrogel NDG was constructed by host-
guest interactions. Nowadays, the host-guest interactions have been
employed to hold two or more ions or molecules together in certain
structures. The combination of host-guest interactions with metal-ligand
coordination is an efficient approach for the hierarchical self-assembly
of functional complex architectures [52]. We all know that Fe3+ is an
ion of a single electron and has high ionic strength [53], which is easy to
induce the
cation-
π
electrons on naphthalene group transfer to Fe3+ and form
π
interactions, so that Fe3+ can be identified by cation-
π in-
teractions. Based on this, the continuity detection mechanism of Fe3+
and Fꢀ by NDG was researched via FT-IR, XRD, SEM and HRMS ana-
lyses. First, the HRMS (Fig. S20) spectrum of NDG exhibits a parent peak
at m/z 523.6006 corresponding to the [NTS + DTB + 3Fe + 6Cl]3+
fragment. In the corresponding IR spectra (Fig. S16b), the stretching
– –
appeared at 3046, 1534, 1708 and 3195 cmꢀ 1, respectively. With the
vibration absorption peaks of = C–H, C C, –C O and –N–H groups
– –
The conformation, packing and intermolecular interactions of su-
pramolecular organic fluorophores play an important role in the fluo-
rescence emission of molecules. Supramolecular organic fluorescent
materials are easy to prepare, low cost, and environmentally friendly.
Therefore, fluorescent materials with good stability can be developed to
produce an erasable fluorescent anti-counterfeit label, and supramo-
lecular organic fluorescent materials can be used as ion-controlled
fluorescent switch excellent candidates. The NDG-based film was pre-
pared by pouring heated DMSO-H2O solution of NDG onto a clean glass
surface, and then drying in air. As shown in Fig. 7a, the NDG film
exhibited white fluorescence emission under UV (365 nm) light. When
writing on the film with a writing brush dipped in Fe3+ aqueous solu-
tion, a clear dark writing image was observed under irradiation at 365
nm using a UV lamp, which was invisible under natural light. In addi-
tion, when aqueous solution of Fꢀ was added, the fluorescence effec-
tively restored. Thus, we can control the fluorescence by Fe3+ and Fꢀ
competitive coordination to realize the patterning of hydrogels for
protected information display. As shown in Fig. 7b, more complex pat-
terns be facilely obtained by designing the photomask based on ions
sensing.
addition of Fe3+ into NDG and formed NDG-Fe, the vibration absorption
– –
– –
peak of = C–H, C C, –C O and –N–H red shifted (the vibration ab-
sorption peak shifted to 2930, 1512, 1697 and 3188 cmꢀ 1), which
demonstrated that the NDG combined with Fe3+ via cation-
π in-
teractions [54]. The electrons transferred from NDG to Fe3+ due to the
existence of cation-π and metal coordination interactions. The electron
cloud density of NDG decreases, causing the proton peaks move to low
wavenumbers, which induced the fluorescence quenching of NDG. As
we expected, after the addition of Fꢀ into the NDG-Fe, the –C O, =C–H
–
–
and –N–H absorption peaks recovered to their initial positions. These
results attributed to the strong coordination ability of Fe3+ with Fꢀ , and
the competitive binding of Fꢀ with Fe3+ [55].
In the XRD patterns (Fig. S17), the xerogel of NDG exhibited a peak
of 3.41 Å at 2θ = 26.08, which indicated the existence of π-π stacking
interactions in NDG [48]. However, the peak disappeared when Fe3+
was added to NDG, and formed metallogel NDG-Fe. A new peak at 2θ =
31.36 (d = 2.85 Å) appeared, indicating that the NDG combined with
Fe3+ via cation-
π
interactions [54,56,57]. After addition of Fꢀ into
NDG-Fe, the signal peak of 3.47 Å at 2θ = 25.62 appeared again. These
results indicated that Fꢀ competitively bound with Fe3+, and the
π-
π
The time-dependency on the Fe3+-induced change in fluorescence of
the NDG was then studied. Generally, reaction-based chemosensors
suffer from a long response time, however, the transient response
property of NDG to Fe3+ and NDG-Fe to Fꢀ was found in this work. The
prepared films based on NDG and NDG-Fe could instantaneous detec-
tion of Fe3+ and Fꢀ . The NDG film was prepared by pouring heated
stacking interactions formed again [58]. As shown in SEM images, the
graceful rod structure of NDG (Fig. 4c) became fibrous structure of
NDG-Fe (Fig. 4d), and after adding Fꢀ to NDG-Fe, the fibrous structure
was approximately restored to rod structure (Fig. 4e). Indicating Fꢀ
competitively bound with Fe3+ induce the change of morphology.
6