A novel chromone and rhodamine derivative as fluorescent probe for the detection of Zn(II) and Al(III) based on two different mechanisms

Article abstract of DOI:10.1016/j.saa.2018.06.076

In this study, a novel fluorescent probe, 6?hydroxychromone?3?carbaldehyde?(rhodamine B carbonyl) hydrazine (L), for Zn²⁺ and Al³⁺ was designed and synthesized. Initially, this probe L exhibited inferior fluorescence emission peak centered at 488 nm in EtOH/HEPES solution (3/1, 10.0 μM HEPES, pH 7.4) when excited at 421 nm. After the addition of Zn²⁺, this probe L displayed excellent selectivity towards Zn²⁺ with obvious fluorescence color change from colorless to yellow, which might be attributed to the formation of a 1:1 ligand-metal complex resulting in the inhibition of photo-induced electron transfer phenomenon. Whereas, the prepared Zn²⁺ complex of L could be used as a ratiometric fluorescent probe to detect Al³⁺ on the basis of fluorescence resonance energy transfer mechanism. This ligand-metal complex of Zn²⁺ (LZn) showed high selectivity towards Al³⁺ with obvious enhancement in fluorescence emission intensity at 580 nm and remarkable decrease in fluorescence emission intensity at 488 nm, and the fluorescence color also changed from yellow to pink. Furthermore, the detection limit of the probe L, LZn towards Zn²⁺, Al³⁺ were 1.25 × 10^?7 M and 3.179 × 10^?6 M, respectively. Additionally, the complexation properties of L towards Zn²⁺ and LZn towards Al³⁺ were studied in detail.

Full text of DOI:10.1016/j.saa.2018.06.076

Accepted Manuscript
A novel chromone and rhodamine derivative as fluorescent probe
for the detection of Zn(II) and Al(III) based on two different
mechanisms
Bing-jie Pang, Chao-rui Li, Zheng-yin Yang
PII:
DOI:
Reference:
S1386-1425(18)30618-8
doi:10.1016/j.saa.2018.06.076
SAA 16235
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
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Received date: 7 April 2018
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20 June 2018
Please cite this article as: Bing-jie Pang, Chao-rui Li, Zheng-yin Yang , A novel chromone
and rhodamine derivative as fluorescent probe for the detection of Zn(II) and Al(III) based
on two different mechanisms. Saa (2018), doi:10.1016/j.saa.2018.06.076
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ACCEPTED MANUSCRIPT
A novel chromone and rhodamine derivative as fluorescent
probe for the detection of Zn(Ⅱ) and Al(Ⅲ) based on two
different mechanisms
Bing-jie Pang, Chao-rui Li, Zheng-yin Yang*
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied
Organic Chemistry, Lanzhou University, Lanzhou 730000, PR China
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Abstract
In this study, a novel fluorescent probe, 6-hydroxychromone-3-carbaldehyde-
(rhodamine B carbonyl) hydrazine (L), for Zn2+ and Al3+ was designed and synthesized.
Initially, this probe L exhibited inferior fluorescence emission peak centered at 488 nm
in EtOH/HEPES solution (3/1, 10.0 μM HEPES, pH 7.4) when excited at 421 nm. After
the addition of Zn2+, this probe L displayed excellent selectivity towards Zn2+ with
obvious fluorescence color change from colorless to yellow, which might be attributed
to the formation of a 1 : 1 ligand-metal complex resulting in the inhibition of photo-
induced electron transfer phenomenon. Whereas, the prepared Zn2+ complex of L could
be used as a ratiometric fluorescent probe to detect Al3+ on the basis of fluorescence
resonance energy transfer mechanism. This ligand-metal complex of Zn2+ (LZn)
showed high selectivity towards Al3+ with obvious enhancement in fluorescence
emission intensity at 580 nm and remarkable decrease in fluorescence emission
intensity at 488 nm, and the fluorescence color also changed from yellow to pink.
Furthermore, the detection limit of the probe L, LZn toward Zn2+, Al3+ were 1.25 × 10-
7
M and 3.179 × 10-6 M, respectively. Additionally, the complexation properties of L
towards Zn2+ and LZn towards Al3+ were studied in detail.
Keywords: Fluorescent probe; Rhodamine; Chromone; Zn2+; Al3+; FRET
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Introduction
Zinc is considered to be the second most abundant transition metal ion in the human
body and plays a crucial role in a variety of physiological and pathological processes
including DNA synthesis, gene expression, enzyme regulation structure and neuronal
signal transmission and so on [1-3]. Thus, an appropriate amount of Zn2+ is beneficial
for the human health. However, excessive amount of Zn2+ exhibits toxicity and causes
many severe diseases such as Alzheimer’s disease, Parkinson’s disease [4-7]. Therefore,
it is worthwhile to develop new methods for the detection of Zn2+.
Aluminum is the most abundant metal element in the earth's crust and aluminum
compounds are extensively utilized in defense scientific researches, manufacturing, and
other fields, bringing great convenience to human society [8-10]. Nevertheless,
aluminum also has some adverse effects on people’s life. Al3+ can accumulate in the
human body through food chains and enrich in the body eventually, which can lead to
a variety of health issues, such as impairment of memory, Alzheimer’s disease,
Parkinson’s disease and bone softening [11-17]. Since Al3+ is closely related to human
health, it is significantly important to develop several highly selective and convenient
tools for the detection of Al3+.
In recent years, fluorescent probes have been widely used for the detection of
various metal ions due to their advantages like real-time detection, high selectivity and
versatility [18-30]. Especially, the ratiometric fluorescent probes can avoid the effects
of a series of external factors, such as intensity of the light source, scattering and
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coloration of the media, and the development of them has attracted the attention of
researchers [31, 32]. Meanwhile, among various mechanisms already proposed for
metal ions sensing on a basis of ratiometric fluorescence response, fluorescence
resonance energy transfer (FRET) is more advantageous than internal charge transfer
(ICT) [33]. More importantly, to the best of our knowledge, there have been relatively
few studies focused on using of ligand-metal complexes as probes to identify metal
ions.
Rhodamine and its derivatives have been widely utilized for designing of
fluorescent probes due to their excellent photophysical properties such as high
extinction coefficient, excellent quantum yields and great photostability. It is well
known that rhodamine spirolactam derivatives have no fluorescence emission and no
fluorescence color. Nevertheless, when a specific metal ion is added, it triggers the ring-
opening reaction of rhodamine and emits its characteristic emission peak [30, 34-36].
Due to the excellent spectroscopic and pharmacological properties, chromone derived
compounds have drawn special attention in the study of fluorophores and antitumoral
agents [37].
Herein, for these reasons, we have presented the design and synthesis of a new type
of rhodamine and chromone derived fluorescent probe called 6-hydroxychromone-3-
carbaldehyde-(rhodamine B carbonyl) hydrazine (L) for the differential detection of
Zn2+ and Al3+. Upon the addition of Zn2+, this probe L showed a significant
enhancement in fluorescence emission intensity at 488 nm in EtOH/HEPES solution
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(3/1, 10.0 μM HEPES, pH 7.4), which might be attributed to the formation of a 1 : 1
ligand-metal complex resulting in the inhibited the photo-induced electron transfer
(PET) process. More importantly, the ligand-metal complex LZn could detect Al3+ in
a ratiometric way based on FRET mechanism. This complex LZn displayed high
selectivity towards Al3+ in Ethanol/MeCN/HEPES solution (6/1/1, 10.0 μM HEPES,
pH 7.4) with significant enhancement in fluorescence intensity emission at 580 nm
while remarkable decrease in fluorescence emission intensity at 488 nm.
Experimental
Materials and instrumentation
All chemicals and reagents were obtained from commercial suppliers and used as
received without further purification. 1H NMR spectra were measured on the JNM-ECS
400 MHz instruments using DMSO-d6 as a solvent and TMS as an internal standard.
ESI-MS spectra were determined on a Bruker esquire 6000 spectrometer in absolute
ethanol. UV–vis absorption spectra were determined on a UV-240 spectrophotometer
(Shimadzu). Fluorescence emission spectra were recorded on a RF-5301
spectrophotometer (Hitachi) equipped with quartz cuvettes of 1 cm path length. The
melting points were recorded on a Beijing XT4-100 microscopic melting point
apparatus without correction.
Synthesis
6-Hydroxyl-3-formyl chromone (compound 1) [38] and rhodamine B hydrazide
(compound 2) [39] were obtained according to the reported procedures. Synthesis of
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fluorescent probe L was based on the following method (Scheme 1): an ethanol solution
(25 mL) of rhodamine B hydrazide (0.457 g, 1 mmol) was added to another solution of
6-Hydroxyl-3-formyl chromone (0.190 g, 1 mmol) in ethanol (25 mL). Then the
mixture was refluxed for 12 h under stirring, during which time some light pink
precipitant appeared. After the reaction was completed, the reaction mixture was cooled
to room temperature. Subsequently, the precipitant was filtered under reduced pressure,
washed 3 times with ethanol (10 mL). Then the precipitant was dried in vacuo and pink
solid was obtained. Yield: 72.0%. M.P.: 264-266 ℃. 1H NMR (DMSO-d6, 400 MHz): δ
10.12 (s, 1H, H13), 8.70 (s, 1H, H8), 8.33 (s, 1H, H12), 7.91 (d, 1H, J = 6.8 Hz, H7), 7.61
(td, 1H, J = 7.2 Hz, J = 1.2 Hz, H5), 7.55 (td, 1H, J = 7.6 Hz, J = 0.8 Hz, H6), 7.51 (d,
1H, J = 8.8 Hz, H9), 7.26 (d, 1H, J = 2.8 Hz, H11), 7.21 (dd, 1H, J = 8.8 Hz, J = 2.8 Hz,
H10), 7.07 (d, 1H, J = 7.6 Hz, H4), 6.45 (d, 2H, J = 2.4 Hz, H1), 6.41 (d, 2H, J = 8.8 Hz,
H3), 6.33 (dd, 2H, J = 8.8 Hz, J = 2.6 Hz, H2), 3.31 (q, 8H, J = 14 Hz, J = 6.8 Hz, H14),
1.07 (t, 12H, J = 6.8 Hz, H15), (Fig. S1). MS (ESI): m/z 629.39 [M + H] +, (Fig. S2).
Analysis
Stock solutions of various metal ions (5 mM) were prepared in water from Al(NO3)3,
AgNO3, BaCl2, Ca(NO3)2, Cd(NO3)2, CrCl3, CoCl2, Cu(NO3)2, FeCl3, HgCl2, KNO3,
LiNO3, Mn(NO3)2, NaNO3, Ni(NO3)2, PB(NO3)2 and Zn(NO3)2. Stock solution of
fluorescent probe L (5 mM) was prepared in DMSO. Test solutions were prepared by
placing 20 μL of the probe L and LZn stock solution into cuvettes, adding an
appropriate aliquot of various metal ions solutions, and diluting the solution to 2 mL
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with EtOH/HEPES solution (3/1, 10.0 μM HEPES, pH 7.4) and EtOH/MeCN/HEPES
solution (6/1/1, 10.0 μM HEPES, pH 7.4) respectively. After mixing fluorescent probe
L and LZn with metal ions for 10 min, the UV-vis absorption spectra and fluorescence
emission spectra were recorded at room temperature. For all fluorescence
measurements, the excitation and emission slit widths were both set at 5 nm.
Results and discussion
Sensing behavior of L towards Zn2+
In order to study the binding ability between L and Zn2+, the UV-vis absorption
spectra of L in the absence and presence of various metal ions were recorded in
EtOH/HEPES solution (3/1, 10.0 μM HEPES, pH 7.4), and the results were illustrated
in Fig. 1. Upon addition of Zn2+ (1.0 equiv.), the absorption bands centered at 421 nm
and 450 nm assigned to the chromone moiety enhanced and their absorbance reached a
maximum in the presence of 1.0 equiv. Zn2+ (Fig. 1a), while almost no significant
enhancement in these two absorption bands were observed in the presence of other
metal ions (1.0 equiv.) (Fig. 1b). In addition, in the presence of Zn2+, the probe L did
not exhibit an absorption band from 485 nm to 600 nm, which indicated that rhodamine
moiety of L was in the ring-closed isomeric form. Nevertheless, the addition of Al3+,
Fe3+ and Cr3+ (1.0 equiv.) to L solution led to the appearance of two new absorption
bands centered at 520 nm and 560 nm, which was attributed to the ring opening reaction
of the rhodamine core. In the presence of Cu2+ and Co2+, the probe L only showed one
absorption band centered at 560 nm, which might be attributed to the color change of
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L solution. These results suggested that L could form a stable complex with Zn2+.
The fluorescence emission spectra of L in the absence and presence of various metal
ions demonstrated a similar behavior to UV-vis absorption spectra. As shown in Fig. 2,
when it was excited at 421 nm, the free probe L solution displayed a weak fluorescence
emission band and was colorless. However, after adding a variety of metal ions to L
solution, this probe L did not give any visible responses to these metal ions except for
Zn2+ at 488 nm. With the increasing concentration of Zn2+, the fluorescence emission
intensity of L at 488 nm enhanced gradually, and reached a maximum in the presence
of 1.0 equiv. of Zn2+. The quantum yield of L increased from 0.02 in the absence of
Zn2+ to 0.17 for the L-Zn2+ complex. Simultaneously, the color of probe L solution
changed from colorless to light yellow（Fig. 4）. Additionally, almost no enhancement
in the fluorescence emission intensity of L-Zn2+ was observed at 580 nm, but the
addition of Al3+, Fe3+ and Cr3+ (1.0 equiv.) to probe L solution engendered significant
fluorescence enhancement at 580 nm and the color of L solution changes from colorless
to pink were observed, which showed that the presence of Zn2+ did not interfere with
the spirolactam form of the rhodamine core, and the rhodamine core was in the ring-
opened isomeric form in the presence of Al3+, Fe3+ and Cr3+. Therefore, the mechanism
for the investigation of Zn2+ by probe L should be attributed to the interaction of the
chromone moiety with Zn2+ which inhibited the PET process rather than the ring
opening reaction of the rhodamine spirolactam.
In addition, we performed competition experiments by adding Zn2+ (1.0 equiv.) to
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probe L solution in the presence of other completing metal ions (1.0 equiv.) to explore
the property of this probe L. As shown in Fig. 3, we could find that the other metal ions
except for Cu2+, Cr3+, Fe3+, Al3+ and Ni2+ did not cause any significant interferences
with the detection of Zn2+. The fluorescence emission at 488 nm of L was almost
quenched in the presence of Cu2+, Cr3+ and Fe3+, which might be related to the magnetic
properties of these three metal ions. The response of probe L to Zn2+ in the presence of
Ni2+ and Al3+ were relatively low but could be clearly detectable. Therefore, L was
shown to be a promising fluorescent probe for the detection of Zn2+ in the presence of
most other metal ions.
Subsequently, to investigate the binding stoichiometry between this probe L and
Zn2+, the Job’s plot experiment was carried out. As depicted in Fig. 5, the maximum
point appeared at a mole fraction of 0.5, which proved that probe L formed a 1 : 1
ligand-metal complex with Zn2+. In addition, the binding stoichiometry between probe
L and Zn2+ was further confirmed by the appearance of a peak at m/z 754.1179 that was
- +
assigned to [L + Zn2+ + NO3 ] in the ESI-MS spectra (Fig. S2). Furthermore, the
fluorescence intensity of probe L at 488 nm afforded a good linear relationship with the
concentration of Zn2+ (5.0-50.0 mM). According to the fluorescence titration
experiments of L towards increasing amounts of Zn2+, the detection limit of probe L
for sensing Zn2+ was calculated to be 1.25 × 10-7 M (R2 = 0.968) based on an equation
of LOD = 3σ/k [40, 41] (Fig. S3) and the association constant Ka between probe L and
Zn2+ was found to be 3.850 × 104 M-1 (R2 =0.996) with a good linear relationship by
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the Benesi-Hildebrand method (Fig. S4) [42].
1H NMR titration experiments were recorded in DMSO-d6 solution to further
explore the combination mode of L with Zn2+. As illustrated in Fig. 6, upon addition of
Zn2+ to L solution, the proton signals of H7 and H12 were shifted upfield (Δδ= 0.0195
ppm and 0.105 ppm, respectively), and the proton signals of H8 and H11 were shifted
downfield (Δδ= 0.182 ppm and 0.0265 ppm, respectively). These results clearly
suggested that the nitrogen atom of -CH=N- group, the oxygen atom of the carbonyl
group from chromone unit and the oxygen atom of the carbonyl group from rhodamine
moiety in L participated in the coordination with Zn2+. Therefore, we proposed the
binding mode of the complex LZn as shown in Scheme 2.
Sensing behavior of LZn towards Al3+
As described above, in the presence of Al3+, Cr3+ and Fe3+, this probe L exhibited a
characteristic fluorescence emission band of rhodamine centered at 580 nm and the
color of probe L solution changed from colorless to pink. Nevertheless, the addition of
Zn2+ (1.0 equiv.) to probe L solution engendered significant enhancement in
fluorescence emission intensity at 488 nm which belonged to the chromone moiety and
we could advert that almost no emission bands ranging from 500 nm to 600 nm emerged.
According to previously reported literatures, 3-carbaldehyde chromone could
coordinate with Zn2+ and emit fluorescence, the rhodamine core was in the ring-closed
isomeric form and the spirolactam form of rhodamine was not disturbed in the presence
of Zn2+. However, when Al3+ or Fe3+ was present, it easily led to the ring-opening
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reaction of rhodamine and displayed a characteristic fluorescent emission of rhodamine
core [32, 43-46]. Therefore, the spirolactam structure of the rhodamine unit in such
ligand-metal complex might be further transferred into ring-opened isomeric form,
giving a characteristic fluorescent emission that belonged to rhodamine in the presence
of Al3+, Cr3+ and Fe3+. Thus, the ligand-metal complex LZn might be used as a probe
to identify other metal ions, such as Al3+. To verify this conjecture, we prepared this
ligand-metal complex LZn by mixed L (50 μM) with 1.0 equiv. of Zn2+ in
Ethanol/MeCN/HEPES solution (6/1/1, 10.0 μM HEPES, pH 7.4) and carried out the
further experiments.
The selectivity of LZn towards Al3+ was investigated in Ethanol/MeCN/HEPES
solution (6/1/1, 10.0 μM HEPES, pH 7.4). As shown in Fig. 8, this ligand-metal
complex LZn showed yellow fluorescence emission at 488 nm corresponding to
chromone moiety when it was excited at 467 nm. However, the addition of Al3+ to LZn
solution immediately triggered a much more significant red-shift of 92 nm in
fluorescence emission spectra and a new emission peak centered at 580 nm emerged,
which might be ascribed to a FRET process from excited 6-Hydroxyl-3-formyl
chromone moiety to ring-opened rhodamine unit [45, 46]. Nevertheless, upon the
addition of other various metal ions such as Ag+, Ba2+, Ca2+, Co2+, Cr3+, Cu2+, Fe3+,
Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+ and Pb2+ to the solution of LZn, almost no
observable responses at 580 nm were obtained except for Cr3+ and Fe3+, only Cr3+ and
Fe3+ caused a relatively weak emission band centered at 580 nm, it did not remarkably
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interfere with the detection of Al3+ by LZn. Additionally, the presence of Cu2+ caused
the fluorescence emission of LZn at 488 nm to almost completely quenched, which
was due to the magnetic property of Cu2+. These results strongly supported the
conclusion that LZn could be utilized as a selective ratiometric fluorescent probe based
on the FRET mechanism for sensing Al3+.
The interaction between LZn and Al3+ was then investigated by measuring the
change in UV-vis absorption spectra of LZn (50 μM) with addition of Al3+ (1.0 equiv.).
As shown in Fig. 7, upon the addition of Al3+ (1.0 equiv.) into LZn solution, the
absorption bands centered at 405 nm, 421 nm and 450 nm obviously decreased with
concomitant formation of two new absorption bands centered at 520 nm and 560 nm,
and the color of the LZn solution changed from yellow to pink, which further suggested
that the rhodamine core of LZn was in the ring-closed isomeric form and the
spirolactam form of rhodamine could be transformed into the ring-open isomeric form
in the presence of Al3+. Furthermore, the fluorescence titration experiments of LZn
were also recorded upon excitation at 467 nm. As shown in Fig. 8, the addition of Al3+
(1.0 equiv.) to LZn solution aroused a significant increase in fluorescence emission
intensity at 580 nm with a decrease in the emission peak of LZn at 488 nm, and the
quantum yield of LZn increased to 0.44. the ratio of fluorescence emission intensity at
580 nm and 488 nm (F580/F488) changed about 331.4 times (from 0.137 to 45.4) and
reached a maximum in the presence of 1.0 equiv. of Al3+, which was consistent with the
results from UV-vis titration experiments, suggesting that the recognition molar ratio
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between LZn and Al3+ might be 1:1.
One of the significant properties of a good probe is its high selectivity towards the
target metal ion exceeding other competing metal ions. Therefore, the competition
experiments of LZn were carried out in Ethanol/MeCN/HEPES solution (6/1/1, 10.0
μM HEPES, pH 7.4). For this purpose, competitive ions were firstly added to the LZn
solution, and then Al3+ (1.0 equiv.) was added after 10 min. The results shown in Fig.
10 confirmed that most of other competitive metal ions showed relatively low
interference with the detection of Al3+, only Cu2+ had posed an effect on the
fluorescence response of LZn to Al3+ that was due to the magnetic property of Cu2+.
Moreover, Cr3+ decreased the fluorescence emission intensity ratio (F580/F488) of LZn
in the presence of Al3+ to a certain degree but the effect was negligible. Therefore, LZn
was shown to be a promising ratiometric fluorescent probe for sensing Al3+ in the
presence of most competing metal ions.
On a basis of fluorescence titration experiments, relative fluorescence intensity ratio
(F580/F488, λex = 467 nm) afforded a good linear relationship versus the concentration of
Al3+ (10.0 – 50.0 μM). The detection limit of LZn for sensing Al3+ was also calculated
according to the equation of LOD = 3σ/k, and the result was found to be 3.179 × 10-6
M (R2 = 0.983) (Fig. S6). Furthermore, the association constant Ka between LZn and
Al3+ was calculated to be 1.611 × 104 M-1 (R2 = 0.997) with a good linear relationship
by the Benesi-Hildebrand method (Fig. S7).
The binding stoichiometry between LZn and Al3+ was confirmed by the ESI-MS
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spectra in ethanol. As can be seen from Fig. S8, the peak at m/z 779.1659 corresponding
- +
to [LZn - Zn2+ + Al3+ + 2NO3 ] and the peak at m/z 762.2143 corresponding to [LZn
-
- Zn2+ + Al3+ + NO3 + EtOH – H+]+ were clearly observed in the ESI-MS spectra, which
indicated that LZn reacted with Al3+ in a 1 : 1 ratio and Zn2+ in LZn was replaced by
1
Al3+ during this reaction process. In addition, H NMR studies provided additional
evidence of the interaction between LZn and Al3+. For this purpose, 1H NMR spectra
of LZn were recorded in DMSO-d6 solution upon addition of Al3+. As shown in Fig.
11, when Al3+ was added to LZn in DMSO-d6 solution, the proton signal of imino group
(-NH-) emerged at 9.18ppm. Furthermore, the peaks of H7 and H12 were shifted
downfield (Δδ = 0.0225 ppm and 0.100 ppm, respectively), and the peaks of H8 and H11
were shifted upfield (Δδ = 0.207 ppm and 0.0265 ppm, respectively). These results
suggested that the imino N, rhodamine carbonyl O, and chromone carbonyl O atoms
participated in the coordination with Al3+. More importantly, the appearance of the
proton signal for imino group (-NH-) clearly proved that the rhodamine core in LZn
was in the ring-opened isomeric form in the presence of Al3+. On a basis of the results
above, the proposed binding mode of LZn with Al3+ was deduced and illustrated in
Scheme 2.
Application of L as a solid state probe
According to the previous experiments, this probe L solution emitted yellow
fluorescence after adding Zn2+. Inspired by this, we tested the practical applicability of
utilizing L as a solid state probe for the detection of Zn2+. For this purpose, L was
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adsorbed on the filter papers by immersing filter papers in the EtOH/HEPES solution
(3/1, 10.0 μM HEPES, pH 7.4) of L (1 mM) and then drying them in the air for 12
hours. Subsequently, the solutions of various metal ions (1 mM) at the same
concentration were dropped in these test strips, and their photographs under a 365 nm
UV lamp were shown in Fig. 12. The color of the coated test paper changed from black
to yellow with the addition of Zn2+ under illumination using a 365 nm UV lamp,
indicating that this probe L could also recognize Zn2+ in the solid state. Nevertheless,
only Al3+ caused a color changed of coated test paper from black to light pink under a
365 nm UV lamp but it could be clearly distinguished from that in the presence of Zn2+.
Furthermore, the introduction of other competitive metal ions did not lead to any color
changes of coated test paper. As a result, this probe L exhibited excellent fluorescence
sensing performance even in the solid state and could be applied for the detection of
Zn2+ in practical samples [47].
Conclusion
In conclusion, a novel chromone and rhodamine derivative as fluorescent probe for
the detection of Zn2+ and Al3+ based on two different mechanisms was reported. This
probe L exhibited high selectivity towards Zn2+ (excitation at 421 nm and emission at
488 nm) over other metal ions and high sensitivity for Zn2+ with the detection limit
reaching 10-7 level in the EtOH/HEPES solution (3/1, 10.0 μM HEPES, pH 7.4). The
selective recognition of L towards Zn2+ should be attributed to the interaction of the
chromone moiety with Zn2+ which inhibited the PET process. Furthermore, we found
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that the ligand-metal complex LZn could be utilized as a ratiometric fluorescent probe
to selectively sense Al3+ (excitation at 467 nm and emission at 580 nm) on a basis of
the FRET mechanism. The addition of Al3+ to LZn solution triggered a much more
remarkable red-shift of 92 nm in fluorescence emission spectra and a new emission
peak centered at 580 nm emerged. To the best of our knowledge, the Zn2+ complex of
chromone derivatives which could be used as new FRET fluorescent probes are
relatively rare (Table. 1.). More importantly, this probe L displayed an excellent
fluorescence sensing performance even in the solid state. This strategy might provide a
general way for designing new FRET probes for detecting other environmentally and
biologically relevant species.
Acknowledgments
This work is supported by the National Natural Science Foundation of China
(21761019)
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Figure captions
Scheme 1. Synthetic route of L.
Fig. 1. (a) UV-vis absorption spectra of L (50.0 μM) in EtOH/HEPES solution (3/1,
10.0 μM HEPES, pH 7.4) after addition of Zn2+ (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8,
0.9, 1.0 equiv.). Insert: the color changes of L solution (50.0 μM) in the absence and
presence of Zn2+ (1.0 equiv.); (b) Absorption changes of L upon addition of different
metal ions (50.0 μM).
Fig. 2. Fluorescence emission spectra of L (50 μM) upon the addition of various metal
ions (1.0 equiv.) of Ag+, Al3+, Ba2+, Ca2+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, K+, Li+, Mg2+,
Mn2+, Na+, Ni2+, Pb2+ and Zn2+ in EtOH/HEPES solution (3/1, 10.0 μM HEPES, pH 7.4)
(λex = 421 nm, Slit widths: 5 nm/5 nm). Inset: color of L (left) and L + Zn2+ (right)
system under UV lamp.
Fig. 3. Fluorescence emission intensity at 488 nm of L and its complexation with Zn2+
in the presence of other various metal ions (Ag+, Al3+, Ba2+, Ca2+, Co2+, Cr3+, Cu2+, Fe3+,
Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+ and Pb2+) in EtOH/HEPES solution (3/1, 10.0 μM
HEPES, pH 7.4). Black bar: L (50.0 μM) in the presence of 1.0 equiv. of various metal
ions. Red bar: L (50.0 μM) with 1.0 equiv. of Zn2+ in the presence of 1.0 equiv. of other
various metal ions (λex = 421 nm).
Fig. 4. Fluorescence emission spectra of L (50 μM) in EtOH/HEPES solution (3/1, 10.0
μM HEPES, pH 7.4) upon gradual addition of Zn2+ (0-1.2 equiv.) (λex = 421 nm, slit
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widths: 5 nm/5 nm).
Fig. 5. Job’s plot for determining the stoichiometry between probe L and Zn2+ in
EtOH/HEPES solution (3/1, 10.0 μM HEPES, pH 7.4). XZn = [Zn2+]/([Zn2+] + [L]), the
total concentration of L and Zn2+ was 50 μM (λex = 421 nm, slit widths: 5 nm/5 nm).
Fig. 6. 1H NMR (DMSO-d6, 400 MHz) spectra of L only and L with 1 equiv. of Zn2+.
Fig. 7. UV-vis absorption spectra of LZn (50.0 μM) in EtOH/MeCN/HEPES solution
(6/1/1, 10.0 μM HEPES, pH 7.4) after addition of Al3+ (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6,
0.7, 0.8, 0.9, 1.0 equiv.). Insert: the color changes of LZn solution (50.0 μM) in the
absence and presence of Al3+ (1.0 equiv.).
Fig. 8. (a) Fluorescence emission spectra of LZn (50 μM) in EtOH/MeCN/HEPES
solution (6/1/1, 10.0 μM HEPES, pH 7.4) upon gradual addition of Al3+ (0-1.0 equiv.)
(λex = 467 nm, slit widths: 5 nm/5 nm). Inset: color of LZn (left) and LZn + Al3+ (right)
system under UV lamp; (b) Plot of relative fluorescence intensity ratio (F580/F488, λex =
467 nm) as a function of [Al3+].
Fig. 9. (a) Fluorescence emission spectra of LZn (50 μM) upon the addition of various
metal ions (Ag+, Ba2+, Ca2+, Co2+, Cr3+, Cu2+, Fe3+, Hg+, K+, Li+, Mg2+, Mn2+, Na+, Ni+ ,
Pb2+ and Al3+) (1.0 equiv.) in EtOH/MeCN/HEPES solution (6/1/1, 10.0 μM HEPES,
pH 7.4); (b) Relative fluorescence intensity ratio (F580/F488, λex = 467 nm) of LZn (50
μM) in the absence and presence of various metal ions in EtOH/MeCN/HEPES solution
(6/1/1, 10.0 μM HEPES, pH 7.4) (λex = 467 nm, slit widths: 5 nm/5 nm).
Fig. 10. The ratio of fluorescence emission intensity at 580 nm and 488 nm (F580/F488)
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of LZn and its complexation with Al3+ in the presence of other various metal ions (Ag+,
Ba2+, Ca2+, Co2+, Cr3+, Cu2+, Fe3+, Hg+, K+, Li+, Mg2+, Mn2+, Na+, Ni+ and Pb2+) in
EtOH/MeCN/HEPES solution (6/1/1, 10.0 μM HEPES, pH 7.4). Black bar: LZn (50.0
μM) in the presence of 1.0 equiv. of various metal ions. Red bar: LZn (50.0 μM) with
1.0 equiv. of Al3+ in the presence of 1.0 equiv. of other various metal ions (λex = 467 nm,
slit widths: 5 nm/5 nm).
Fig. 11. 1H NMR (DMSO-d6, 400 MHz) spectra of L with 1 equiv. of Zn2+ (LZn); LZn
with 1 equiv. of Al3+.
Fig. 12. The photographs of coated test papers after addition of different metal ions
under illumination using a 365 nm UV lamp.
Table 1. Comparison of different properties of L and LZn with recently reported probes
based on rhodamine derivatives.
Scheme 2. Proposed binding mechanism of L with Zn2+ and LZn with Al3+.
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Scheme 1. Synthetic route of L.
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Fig. 1. (a) UV-vis absorption spectra of L (50.0 μM) in EtOH/HEPES solution (3/1,
10.0 μM HEPES, pH 7.4) after addition of Zn2+ (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8,
0.9, 1.0 equiv.). Insert: the color changes of L solution (50.0 μM) in the absence and
presence of Zn2+ (1.0 equiv.); (b) Absorption changes of L upon addition of different
metal ions (50.0 μM).
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Fig. 2. Fluorescence emission spectra of L (50 μM) upon the addition of various metal
ions (1.0 equiv.) of Ag+, Al3+, Ba2+, Ca2+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, K+, Li+, Mg2+,
Mn2+, Na+, Ni2+, Pb2+ and Zn2+ in EtOH/HEPES solution (3/1, 10.0 μM HEPES, pH 7.4)
(λex = 421 nm, Slit widths: 5 nm/5 nm). Inset: color of L (left) and L + Zn2+ (right)
system under UV lamp.
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Fig. 3. Fluorescence emission intensity at 488 nm of L and its complexation with Zn2+
in the presence of other various metal ions (Ag+, Al3+, Ba2+, Ca2+, Co2+, Cr3+, Cu2+, Fe3+,
Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+ and Pb2+) in EtOH/HEPES solution (3/1, 10.0 μM
HEPES, pH 7.4). Black bar: L (50.0 μM) in the presence of 1.0 equiv. of various metal
ions. Red bar: L (50.0 μM) with 1.0 equiv. of Zn2+ in the presence of 1.0 equiv. of other
various metal ions (λex = 421 nm).
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Fig. 4. Fluorescence emission spectra of L (50 μM) in EtOH/HEPES solution (3/1, 10.0
μM HEPES, pH 7.4) upon gradual addition of Zn2+ (0-1.2 equiv.) (λex = 421 nm, slit
widths: 5 nm/5 nm).
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Fig. 5. Job’s plot for determining the stoichiometry between probe L and Zn2+ in
EtOH/HEPES solution (3/1, 10.0 μM HEPES, pH 7.4). XZn = [Zn2+]/([Zn2+] + [L]), the
total concentration of L and Zn2+ was 50 μM (λex = 421 nm, slit widths: 5 nm/5 nm).
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Fig. 6. 1H NMR (DMSO-d6, 400 MHz) spectra of L only and L with 1 equiv. of Zn2+.
29