AIE based “on-off” fluorescence probe for the detection of Cu2+ ions in aqueous media

Article abstract of DOI:10.1016/j.inoche.2018.11.001

A simple hydrazine carboxamide derivative with intramolecular charge transfer and aggregation induced emission (AIE) properties have been designed and synthesized. The derivative (Probe 1) shows typical AIE characteristic emission in THF-water mixtures and emits yellowish orange color on reaching the water fraction 98%. The probe molecule show selective sensing response for Cu²⁺ ions via fluorescence turn-off mechanism in aqueous media over other metal ions. The emission changes towards Cu²⁺ ions that could be clearly observed by the naked eyes under 365 nm UV lamp promote the probe molecule as a promising candidate for practical utilization. Furthermore, from the fluorescence titration data, the detection limit for Cu²⁺ ions was found to be 0.44 μM.

Full text of DOI:10.1016/j.inoche.2018.11.001

Accepted Manuscript
AIE based “on-off” fluorescence probe for the detection of Cu2+
ions in aqueous media
Mohammed A. Assiri, Abdullah G. Al-Sehemi, Mehboobali
Pannipara
PII:
S1387-7003(18)30894-3
DOI:
Reference:
https://doi.org/10.1016/j.inoche.2018.11.001
INOCHE 7165
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
30 September 2018
31 October 2018
1 November 2018
Please cite this article as: Mohammed A. Assiri, Abdullah G. Al-Sehemi, Mehboobali
Pannipara , AIE based “on-off” fluorescence probe for the detection of Cu2+ ions in
aqueous media. Inoche (2018), https://doi.org/10.1016/j.inoche.2018.11.001
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AIE based “on-off” fluorescence probe for the detection of Cu2+ ions in aqueous media
Mohammed A. Assiri a, b, Abdullah G. Al-Sehemi a,b, Mehboobali Pannipara* a,b
a Department of Chemistry, Faculty of Science, King Khalid University, P.O. Box 9004, Abha
61413, Saudi Arabia.
b Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box
9004, Abha 61413, Saudi Arabia.
Corresponding Authors Address:
E-mail: mpannipara@kku.edu.sa; mhbb25@gmail.com (Mehboobali Pannipara)
Tel.: +966 553956503
Fax: 0096672418426

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Abstract
A simple hydrazine carboxamide derivative with intramolecular charge transfer and aggregartion
induced emission (AIE) properties have been designed and synthesized. The derivative (Probe 1)
shows typical AIE characteristic emission in THF-water mixtures and emits yellowish orange
color on reaching the water fraction 98%. The probe molecule show selective sensing response
for Cu2+ ions via fluorescence turn-off mechanism in aqueous media over other metal ions. The
emission changes towards Cu2+ ions could be clearly observed by the naked eyes under 365 nm
UV lamp promote the probe molecule as a promising candidate for practical utilization.
Furthermore, from the fluorescence titration data, the detection limit for Cu2+ ions was found to
be 0.44 µM.
Keywords: AIE; Cu2+ ions; visual sensing

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Molecular architectures possessing donor-acceptor group with extended pi-conjugated system
attracts massive scientific interest due to intramolecular charge transfer (ICT) characteristics that
found applications as fluorescent sensors for biological environments, pH sensors, nonlinear
optical (NLO) materials [1-3]. Hence, design and synthesis of ICT compounds have witnessed
tremendous growth over past decade the as it can be tuned and tailored by structural modification
for their potential applications. The nature of ICT and degrees of charge transfer in the ground
and excited state in these compounds strongly depend on the polarity of the surrounding
medium, temperature and nature of the electron donating and accepting substituents [4, 5].
Since after the discovery of the exciting phenomenon termed as aggregation induced emission
(AIE) by Tang and co-workers, where compounds exhibit strong luminescence in the
aggregation state/ in poor solvents and weakly emit in dilute solution, brought renaissance to the
application of organic luminescent materials [6]. As most of practical applications of the
luminescent materials are relied on solid state rather than in solution state, AIE active
luminescent materials brought a new platform for researchers to explore the photophysical
properties associated with solid state luminescent materials and are widely used as fluorescent
chemosensors, nonlinear optical materials and laser dyes [7-15].
Molecular probes based on fluorescence signaling has witnessed a tremendous growth over the
last decades for the detection of heavy metal ions owing to its sensitivity, specificity, cost-
effectiveness and fast response time [16, 17]. Stimuli responsive molecular material are
considered to be ideal candidates for the use for chemical sensor as the change in molecular
configuration brought out by the external stimulus or analyte can be detected either by
colorimetric or fluorimetric approach [18, 19]. Aggregation-induced emission (AIE) has been
found versatile applications for the detection of Cu (II) ions in environmental and biological
specimens due to the high sensitivity, ease of manipulation, and visual observation [20-22]. AIE
molecules can aggregate and emit strong fluorescence or quench the emission on coordination
with metal ions that have great potential for the highly selective and sensitive detection of copper
ions in pure and real water, especially in living tissue containing water. Moreover, such kind of
AIE probes for detection of Cu (II) ions can easily be prepared on site just by the addition of
water to the solution of probe in THF. In case of Cu2+ ion sensing by AIE compounds, the
probe molecule mostly work in aqueous media for detection of copper ions as water is

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responsible for aggregation in most of the case and a very low amount of probe can detect
copper ions having very less concentration [23-25]. In continuation with our ongoing research in
the field of single molecule based metal sensors, the present work report a simple hydrazine-
caboxamide derivate (Probe 1) having intramolecular charge transfer and AIE characteristics.
The Probe molecule show distinct sensing property towards Cu2+ ions via fluorescence turn-off
manner with lower detection limit. The reported molecule enjoy greater practical interest as it
work in aqueous media for detection of copper ions and with the help of a 365 nm UV lamp,
fluorescence changes imparted by the metal ion could be visualized easily.
Scheme 1outlines the synthetic procedure for Probe 1. Cyano acetohydrazide 1 was obtained by
the reaction between ethyl cyanoacetate and hydrazine hydrate in ethanol. Condensation reaction
of cyano acetohydrazide 1 with salicylaldehyde yielded 2-cyano-2-[(2-hydroxyphenyl)
methylene]hydrazide 2. The Knoevenagel condensation of 2 with 4-dimethylamino bezaldehyde
lead to the target molecule Probe 1. All the compounds were characterized by various spectral
methods (see supporting information).
The photophysical properties of Probe1 (20µM) in solvents of varying polarity has been
investigated by applying UV-vis absorption and emission techniques and are displayed in Fig. 1.
As inferred from Fig. 1a, the electronic absorption spectra of Probe 1 shows broad absorption
band for all solvents except for water where the absorption band was found to be more structured
and red shifted which might be due to the formation of aggregates in water (explained in detail in
the forthcoming section). However, the absorption band did not show any significant regularity
on changing the solvent polarity, the emission profile of Probe 1 show remarkable influence of
solvent polarity as seen from Fig. 1b. On exciting at 400 nm, the emission maxima undergo a red
shift on increasing the polarity of the solvents indicating larger charge transfer in the excited
state due to stabilization of a highly dipolar excited state in polar solvents as observed in other
related molecules [26-28]. Furthermore, as seen from Fig. 2, correlation of solvent polarity
parameter f with fluorescence peak energies (fl in cm-1) follow reasonably good linear
relationship with f and the plot between fl and f show large negative slop, indicating that the
emission state is strongly dipolar in nature due to strong intramolecular charge transfer (ICT)
[29, 30]. However, contrary to the general trend observed for ICT molecules, the emission
intensity in ethylene glycol and water were found to be exceptionally high which might arise due
to aggregation induced emission enhancement (AIE) [31-33].

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In order to investigate the AIE behavior of Probe 1 in detail, the emission spectra of Probe 1
were obtained in water/ THF mixtures with varying water volume fractions (fw, vol %) from 0 to
98% by fixing the concentration of Probe 1 (20 μM). Fig. 3a and 3b shows the photographic
image of Probe 1 and the corresponding emission profile at different water volume fractions. As
evident from Fig. 3, on progressive addition of water fraction, the photoluminescence intensity of
Probe 1 remains unchanged in spectral change features up to fw = 50%, but starts to emit intense
yellowish to orange fluorescence on reaching the water fraction fw =70%. The emission intensity
was found to be continuously intensified on selected increment of water fractions and reaches its
maximum at fw = 98%. Further, a 78-fold enhancement (expressed as I-I0/I0) in the emission
intensity were observed in comparison with pure solvents; verifying the typical AIE
characteristics of the Probe molecule. Surprisingly, the Stokes shift was found to be
exceptionally large as 130 nm for Probe 1 at higher water fraction with strong emission which
might arise as a result of ESIPT coupled AIE process. At lower water volume fractions, ESIPT
process of Probe 1 was suppressed due to non-radiative way of excited states and free rotation
around C-N bond. However, in high water volume fraction, rotation was restricted due to close
packing of the molecule via intramolecular hydrogen bonding and predominate high quantum
yield of the keto form in the aggregate state which might cause larger stokes-shift in its PL
spectrum [34, 35]. Further, to investigate the formation of nanoparticles during aggregation at
98% water fraction, Scanning electron microscope (SEM) study was exploited. As seen from
Fig. 4, SEM image of Probe 1 found morphology of agglomerated spherical shaped
nanoaggregates with average size of 210 nm.
Owing to its potential metal ion coordinating site, Probe 1 was screened for the sensing
properties towards various metal ions using fluorescence measurements. The sensing ability of
Probe 1 (20 μM ) towards metals ions were carried out in THF-water fraction (fw = 98%,), in
order to maintain the AIE characteristics, by treating it with 10 equiv. of varying metal ions
namely Ag1+, Al3+, Cd2+, Co2+, Cu2+, Ni2+, Hg2+, Zn2+, Mn2+ and Pb2+. As evident from Fig. 5 (a)
and (b), the addition of metal ions to Probe 1 resulted in changes in intensity on the emission
profile. However, highly efficient fluorescence quenching was observed for Cu2+ ions as
compared to other tested metal ions, which could be even visualized with the help of UV lamp,
demonstrating the selectivity of Probe 1 towards Cu2+ ions.

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Furthermore, fluorescence titration were performed with varying concentration of Cu2+ , to get
more insight into the quenching efficiency of metal ions. As expected, increasing the
concentration of Cu2+ ions resulted in the gradual decrease of the intense fluorescence of Probe 1
as inferred from Fig. 6. It is obvious to note that addition of 10 µM of Cu2+ ions has resulted
almost complete quenching of emission intensity, suggesting highly selective sensing ability of
Probe 1 towards the detection of Cu2+ ions. The observed quenching of fluorescence by Cu2+ ions
may be ascribed due to the paramagnetic nature of these ions, which result in chelation-enhanced
quenching (CHEQ), by metal to ligand charge transfer reaction between metal ions and Probe 1
[36]. In order to gain understanding of the binding mode, we determined the stoichiometry of the
interaction between Probe 1 and Cu2+ ions using Job's plot method and it show 1:1 binding
stoichiometry (Fig. 7).
The detection limit of Probe 1 for Cu2+ ion was calculated from the fluorescence titration data
was found to be 4.39×10-7 M; using equation (1),
Detection limit: 3 σ/ k
(1)
where σ is the standard deviation of ten blank measurment and k is the slope of fluorescence
intensity versus metal ion concentration [37].
The fluorescence quenching data were analyzed using Stern-Volmer equation (2) in order to
establish the extent and nature of quenching by Cu2+ ions [38].
퐼
표
= 1 + 퐾푠푣 [Cu]
(2)
퐼
where Io and I are the original fluorescence intensity and quenched intensity, respectively and
KSV denotes the Stern–Volmer quenching constant. The value for KSV obtained from the plot of
I0/I values versus the concentrations of Cu2+ ions was found to be 1.67x106 M-1, indicating much
significant interaction between the Cu2+ ions fluorophore. However, the positive deviation from
the linearity of the plot with an upward curvature indicates that quenching process
simultaneously follows combination of static and dynamic quenching mechanism (Fig. 8) [39].
In conclusion, a simple hydrazine carboxamide AIE active fluorescent probe has been
successfully designed and synthesized. The probe molecule exhibit AIE emission characteristics

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in THF-water mixtures and intense yellowish orange emission were observed on reaching the
water fraction at 98%. The probe molecule exhibited selective and sensitive sensor for Cu2+ ions
through a turn-off fluorescence manner on compared to other tested metal ions. The probe
molecule could be used as a promising candidate for practical utilization, as it works in aqueous
media and the emission changes towards Cu2+ ions could be clearly observed by the naked eyes
under 365 nm UV lamp. Moreover, owing to its higher sensitivity and selectivity, the reported
probe molecule have potential practical interest and could be applied for the intracellular
detection of Cu (II) ions in living systems.
Acknowledgements
The authors extend their appreciation to the Deanship of Scientific Research at King Khalid
University for funding this work through General Research Project under grant number (G. R. P-
294-39).
Electronic supplementary information (ESI) available: The complete details on Experimental
section, materials and methods, synthesis of Probe 1 and spectral values.
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Scheme 1. Synthesis of Probe 1. (a) Ethanol, 0 0C, 5 h; (b) Ethanol, 1 h, 70 0C; (c) Ethanol,
piperidine, p-Dimethyl aminobezaldehyde, 2 h, reflux.

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Fig. 1 Electronic absorption spectra (a) and emission spectra (b) of Probe1 (20 µM) in different
solvents.
Fig. 2 Plot of emission frequency as function of solvent polarity parameter (Δf).

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Fig. 3 Photographic image (a) and corresponding of emission spectra (b) of Probe 1 (20 µM)in
THF/ water mixtureswith different water fractions (fw).
Fig. 4 SEM images of nanoaggregates of Probe 1 in THF/ water (fw = 98%).

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Fig. 5 (a) Photographic image of Probe 1(20 µM) under 365 nm UV lampand the corresponding
(b) emission spectra in THF/ water (fw = 98%, λex= 400 nm) after addition of different metal
ions.

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Fig. 6 PL spectra of Probe 1 as a function of [Cu2+].[1] =20 µM in THF:water, fw=98%
(λex=400 nm).

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Fig. 7. Jobs plot of Probe 1 with Cu 2+ showing maxima at 1:1

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Fig. 8. Stern-Volmer plot for Probe 1 (20µM) at various concentration of Cu2+ ions.
.

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Graphical abstract

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Highlights
A simple caboxamide derivative displaying aggregation induced emission has been synthesized.
It can act as fluorescent probe for Cu2+ions.
The sensing ability of the derivative could be visualized using 365 nm UV lamp.

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