Synthesis and Evaluation of a Schiff-Based Fluorescent Chemosensors for the Selective and Sensitive Detection of Cu2+ in Aqueous Media with Fluorescence Off-On Responses

Article abstract of DOI:10.1007/s10895-018-2278-4

Copper being an essential nutrient; also pose a risk for human health in excessive amount. A simple and convenient method for the detection of trace amount of copper was employed using an optical probe R1 based on Schiff base. The probe was synthesized by Schiff base condensation of benzyl amine and 2-hydroxy-1-napthaldehyde and characterized by single X-ray diffraction, ¹H NMR and FTIR. By screening its fluorescence response in a mixture of DMSO and H₂O (20:80, v/v) R1 displayed a pronounced enhancement in fluorescence only upon treatment with copper. Other examined metal ions such as alkali, alkaline and transition had no influence. Within a wide pH range 5–12 R1 could selectively detect copper by interrupting ICT mechanism that results in CHEF. From Job’s plot analysis a 2:1 binding stoichiometry was revealed. The fluorescence response was linear in the range 1–10 × 10^?9?M with detection limit 30 × 10^?9?M. Association constant was determined as 1 × 10¹¹?M^?2 by Benesi-Hilderbrand plot. As a fast responsive probe it possesses good reproducibility and was employed for detection of copper in different water samples.

Full text of DOI:10.1007/s10895-018-2278-4

Journal of Fluorescence
https://doi.org/10.1007/s10895-018-2278-4
ORIGINAL ARTICLE
Synthesis and Evaluation of a Schiff-Based Fluorescent Chemosensors
2+
for the Selective and Sensitive Detection of Cu in Aqueous Media
with Fluorescence Off-On Responses
1
1
1
1
Maria Sadia & Robina Naz & Jehangir Khan & Rizwan Khan
Received: 7 May 2018 /Accepted: 9 August 2018
#
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Copper being an essential nutrient; also pose a risk for human health in excessive amount. A simple and convenient method for the
detection of trace amount of copper was employed using an optical probe R1 based on Schiff base. The probe was synthesized by
1
Schiff base condensation of benzyl amine and 2-hydroxy-1-napthaldehyde and characterized by single X-ray diffraction, H NMR
and FTIR. By screening its fluorescence response in a mixture of DMSO and H O (20:80, v/v) R1 displayed a pronounced
2
enhancement in fluorescence only upon treatment with copper. Other examined metal ions such as alkali, alkaline and transition
had no influence. Within a wide pH range 5–12 R1 could selectively detect copper by interrupting ICT mechanism that results in
CHEF. From Job’s plot analysis a 2:1 binding stoichiometry was revealed. The fluorescence response was linear in the range 1–
0 × 10− M with detection limit 30 × 10 M. Association constant was determined as 1 × 10 M by Benesi-Hilderbrand plot.
9
−9
11 −2
1
As a fast responsive probe it possesses good reproducibility and was employed for detection of copper in different water samples.
.
.
.
.
Keywords Fluorescent chemosensor Schiff base Copper CHEF Real-time detection
Introduction
Among the metal ions, after zinc and iron, copper is the
third most abundant vital trace element for life that plays an
important role in various fundamental physiological and bio-
logical processes in organisms [10]. It is also a toxic chemical
that is not biodegradable and bioaccumulative and can accu-
mulate in food chain or human through uptake or consump-
tion and may be hazardous to human health or the environ-
ment [11]. Copper is present in sediments, natural water, and
in the other medium such as air and soil. Industrial effluent are
the potential source of copper contamination such as waste
water from industries manufacturing fungicides, fertilizers,
bactericides, algaecides, electronic goods, copper plumbing,
as well as the use of copper as a mordant for textile dyes, as an
activator in froth floatation of sulfide ores and in plating by
products. Natural sources also contribute to the contamination
of copper that include sea spray, forest fires, decaying vegeta-
tion, volcanoes and windblown. [12–14]. Due to these exten-
sive industrial applications copper can be a significant envi-
ronmental pollutant of worldwide concern [15].
In the last few years, the development of fast, cheap and effi-
cient fluorescent chemosensors with high sensitivity and se-
lectivity for the detection of heavy and transition metal ions
2
+
2+
2+
2+
such as Cu , Cd , Pb , and Hg has attracted wide-spread
interests of chemists, environmentalists, clinical biochemists
and biologists because of their fundamental role in biological,
environmental and medicinal application [1–6].
Fluorescent chemosensors typically consist of two parts:
ionophore (metal chelator) and fluorophore (signaling unit).
The two parts are interconnected through a proper spacer.
Ionophore is required for analyte binding and as a result of
metal binding, photophysical properties of the fluorophore
such as fluorescence intensity, lifetime of the excited state or
absorption wavelength changes, achieving the purpose of
identification [7–9].
*
Maria Sadia
mariasadia@gmail.com
Copper is also an essential micronutrient in the human body.
Because of its redox-active nature it acts as an essential cofactor
of many enzymes such as tyrosinase, cytochrome c oxidase and
superoxide dismutase [16]. It is necessary for the development
of bone, nerve coverings, and connective tissue [17].
1
Department of Chemistry, University of Malakand, Chakdara, Lower
Dir, KPK, Pakistan

J Fluoresc
2
+
Copper exist in three common oxidation states Cu (cu-
weak fluorescence due to ICT mechanism which was inhibited
upon interaction with copper ion and enhancement of fluores-
cence was observed. The proposed fluorescent chemosensors
contain two parts naphthalene group which is the signaling
moiety acting as a fluorophore and imine and hydroxyl group
are the binding sites acting as a receptor. We select 2-
Hydroxynaphthalene as a fluorophore because of its cheap cost,
good binding sites, competitive stability in the environment and
characteristic photophysical properties such as low fluores-
cence quantum yield and short fluorescence lifetime [32, 33].
+
0
pric ion), Cu (cuprous ion) and Cu (metal). Among the three
species the most commonly occurring and toxic to living or-
ganism is the cupric ion. Copper possess high affinity for the
ligand containing sulfur and nitrogen donors thus it effects
enzymes whose activities depend on amino and sulhydryl
groups [18]. Due to its toxicity at cellular level, its distribution
and homeostasis is highly controlled by various factors in-
cluding chaperones and copper transport proteins. Abnormal
level of copper can cause various problems and disorders. For
example, under overloading condition copper can cause dis-
orders such as allergies, migraine headaches, anxiety, depres-
sion, anorexia, premenstrual syndrome, fatigue, kidneys and
blood problems, Wilson’s, Parkinson’s, Alzheimer’s and prion
diseases in humans and its deficiency is associated with mye-
lopathy. In living cells uncontrolled reaction of copper ion
with oxygen result in the production of ROS (reactive oxygen
species) that can damage proteins, nucleic acid and lipids
Excessive absorption of Cu also effects the growth of plants
including fewer leaves, shortening of root length and decrease
in the plants biomass [19–22]. According to World Health
Organization (WHO), the permissible limit of copper in drink-
ing water is 2 ppm (30 μM) or 1.5 mg/L [23, 24]. In view of
such toxic effects of copper, efficient detection of trace
amount of copper in environmental water samples is very
necessary. For these reasons, great effort has been dedicated
to design fluorescent chemosensors for specific recognition
and detection of copper.
Experimental
Materials and Instrumentation
2-hydroxy-1-napthaldehyde, Benzyl amine, Dimethyl sulfox-
ide (DMSO), Hydrochloric acid 37%, Sodium hydroxide,
2
+
2+
+
2+
+
2+
2+
Acetone, and salts of Cu , Na , Mg , K , Zn , Mn ,
2
+
2+
2+
2+
2+
2+
2+
3+
3+
Hg , Ba , Pb , Ni , Cd , Ni , Co , As , Fe and
3
+
Ce were purchased from commercial sources and were used
without further purification. Throughout all experiments dis-
tilled water was used. pH adjustments were made by using
HCl or NaOH. To minimize the release of cations in the solu-
tion and their sorption, the glassware’s were first washed with
acid and then rinsed with distilled water. For fluorometric
2
+
determination of Cu optically four sides’ clear quartz cu-
vettes were used and were washed with acetone before use.
1
Chelation enhanced fluorescent type chemosensor are
more sensitive to metal ions as compared to Chelation en-
The H NMR spectra of the samples were obtained on
Bruker Avance 400 MHz. spectrometer. For FTIR spectra
samples were prepared as KBr pallets and the spectrum was
obtained on FTIR spectrophotometer Pretige 21 Shimadzu
2
+
hanced quenching chemosensors. Cu is known as a fluores-
cence quencher and most of the fluorescent chemosensors
reported for copper so far, detection of copper occur through
a fluorescence quenching process that undergoes a charge or
energy transfer mechanism and Bturn-on^ fluorescent sensors
for recognition of copper are still rare. Thus, there is a great
demand to develop Bturn-on^ fluorescent chemosensors for
specific and sensitive detection of copper [25–27].
Schiff base derivatives have attracted much attention as an
optical chemosensor for the detection of the metal ions because
of their structural flexibility, excellent applicability, high selectiv-
ity, high efficiency and simplicity of synthetic mode. Schiff base
compounds form strong complexes with transition metal ions.
FortheSchiffbase tobe used asafluorescentsensor, thepresence
of a strong fluorophore is required. Schiff base compounds incor-
porating π- conjugated fluorescent fragment are extensively used
as fluorescent chemosensor for the metal ions [28–31].
−1
Japan in the region 400–4000 cm . X-ray structure analysis
of the sample was conducted using Bruker kappa
APEXIICCD diffractometer. For the measurement of melting
points a Bicote Stuart-SMP 10 Japan was used. UV–vis ab-
sorption spectroscopy measurements were conducted on UV-
visible 1800 spectrophotometer. The emission spectra were
monitored using fluorescence spectrophotometer RF 5301
PC Shimadzu Japan equipped with fluorescence free quartz
cuvettes of 1 cm path length. Slit width were 5 nm both for
excitation and emission and a steady state 150 W Xenon lamp
was used as an excitation source. The pH of the test solution
was monitored by a BANTE instrument pH meter. All mea-
surements were conducted at room temperature.
Synthesis of Chemosensor R1
It was within our interests to design a CHEF type Schiff-
base fluorescent sensor for selective detection of copper ion.
Therefore we designed and develop a new fluorescent probe for
copper ion named as (E)-1 ((benzylimino)methyl)naphthalen-
R1 was prepared according to the published method [34, 35].
The synthetic route of R1 is shown in the Scheme 1.2-hy-
droxy-1- napthaldehyde 10 mmol (1.7218 g) was dissolved
in 10 mL methanol and after adding few drop of acetic acid, it
was boiled for a while. After 5 min methanolic solution of
2-ol (R1). The synthesis was simple, single step the product
was highly selective towards copper ion. R1 exhibited very

J Fluoresc
Step 1:
Scheme 1 Synthetic route of
chemosensor R1
H
O
O
HO
CH COOH
3
HO
3
CH OH
Step 2:
H
O
H
O
H
NH2
+
HO
N+
H
HO
Step 3:
H
O
H
N
H
O
H
N
+
H
HO
CH COOH
3
HO
CH COO
3
Step 4:
H
O
H
O
H
N
H
N
H
CH COO
H
3
HO
HO
Step 5:
H
O
3
CH COO
H
N
N
H
CH COOH
3
H O
2
HO
HO
N
H
O
1
benzylamine 10 mmol (1.09 mL) was slowly added to it drop
wise. The mixture was refluxed for about 12 h at 80 °C to give
a clear yellow solution. To get the crystal, solvent was evap-
orated slowly at room temperature by keeping the solution in
air for several days. After evaporation of the solvent rod
shaped yellow color single crystal were obtained which were
recrystallized from ethanol. Yield 80%, mp 120–122 °C, IR
1492, 1435; νaromatic-C-H, 3035. HNMR (400 MHz,
DMSO): δ14.5 s (NHO), 9–10s (-N=CH, 1H), 7.7–8.2d (H-
naphtha, 2H), 6.0–8.0 m (ArH, 9H), 4.5–5.0 s (ArCH , 2H).
2
General Information
Melting point was determined in open mouth capillary and
uncorrected. For spectroscopic investigation analytical reagent
−1
(
KBr, cm ): νC-OH, 1296, 1207; ν C=N, 1614, νaromaticC=C,
O
N
Reflex, 8 h
H
NH2
HO
CH OH
O
3
(E)-1-((benzylimino)methyl)naphthalen-2-ol
phenylmethanamine
2-hydroxy-1-naphthaldehyde
Scheme 2 Chemical structure and synthesis of (E)-1-((benzylimino)methyl)naphthalen-2-ol

J Fluoresc
Table 1 FTIR spectral frequencies for chemosensor R1
IR-band (C=N) cm−
1
IR-band (C-OH) cm−1 IR-band aliphatic (-C-O) cm−1 IR-band aromatic (-C-H) cm−1 IR-band aromatic (C=C) cm−1
1
614
1296, 1207 1138, 1038 3035 1492, 1435
1
grade DMSO and distilled water were used. Fluorescence and
absorption spectra of the chemosensor R1 was recorded in
DMSO:H20 (20:80) at room temperature. Fluorescence-
sensing property of the chemosensor R1 towards metal ions
In the region 400–4000 cm− several absorption bands
were observed in the FTIR spectrum of the chemosensor
R1. Assignments of characteristics bands correspond to vari-
ous functional groups present in the chemosensor were made
by comparison method. The FTIR spectrum confirms the for-
mation of imine bond (-C=N) and no band assigned to (C=O)
was detected. Thus, the disappearance of carbonyl (-C=O)
2+
2+
2+
2+
was conducted using sulphate salt of Cu , Mg , Zn , Mn ;
2
+
2+
2+
+
3+
2+
nitrate salt of Pb , Ba , Cd , K , Ce , Hg ; bromide salt of
2+
+
2+
3+
3+
Co ; chloride salt of Na , Ni , Fe and oxide of As . For
pH adjustment HCL and NaOH were used. From the fluores-
cence intensity data association constant was determined by
Benesi-Hilderbrand Plot. The excitation and emission wave-
length were 320 and 645 nm respectively. All the measure-
ments were conducted at room temperature.
−1
peak in the region of 1700 cm and appearance of the (-
−1
C=N) peak in the region of 1614 cm confirms the formation
of chemosensor R1. The band at 3035 cm corresponds to the
stretching vibration of aromatic-C-H. The spectrum exhibit
absorption band at 1492, 1435 and at 1296, 1207 cm typi-
cally of aromatic-C=C and phenolic-O-H stretching vibration.
−1
−1
−1
The bands at 1138 and 1038 cm correspond to aliphatic-C-O
Results and Discussion
[36–38].
Synthesis and Characterization
Characterization by Single Crystal X-Ray Diffraction
Chemosensor R1 was obtained with good yield by condensing
The synthesis of chemosensor R1 was a single-step straight
forward reaction and no side products were observed or isolated
during the course of reaction. From reaction mixtures, by slow
evaporation of the solvent crystals were separated, Suitable for
single X-ray diffraction analysis. Crystallographic measure-
ment were carried outs with Bruker kappa APEXIICCD dif-
fractometer, with graphite-monochromator (Mo-kα radiation
(λ = 0.71073 A) at ambient temperature. For data collection
ωscan and multi-scan correction were applied. The crystal
structure solution and refinements of chemosensor R1 were
2
-hydroxy-1- napthaldehyde with benzylamine in methanol as
1
shown in scheme 2 and it was well characterized by FTIR, H
NMR and single X-ray crystallography and spectroscopic tools.
Characterization by FTIR
o
In order to determine the functional group in the synthesized
Schiff-based fluorescent chemosensor R1 FTIR spectroscopy
was conducted and the data is given in the Table 1.
1
10
%T
00
Fig. 1 FTIR spectrum of the
chemosensor R1
1
90
80
70
60
50
40
30
4
000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000
800
600
400
1/cm
A

J Fluoresc
geometry is trigonal planner, demonstrating that the lone pair
2
of nitrogen is involved in conjugation without disturbing sp
hybridization. The bond distance between N -C is 1.295 Å
1
11
which is comparable with the double bond character of C=N
bond and bond length of N -C is 1.464 Å and C -O is
1
12
1
1
.267 Å. X-ray diffraction map revealed that imine-N is bond-
+
ed with H and exists as –C=NH – and O of phenolic –OH as
O which is identical with similar reported structure [33].
−
Fig. 2 Molecular structure of chemosensor R1 with partial numbering
scheme
Intramolecular hydrogen bonding stabilized the structure as
shown in the Fig. 3.
1
Characterization by H NMR
handled with SHELXL-97 [39] and publ CIF [40]. Full-matrix
least-squares techniques were done for final refinement on F2.
The molecular view of chemosensor R1 with partial numbering
scheme is shown in Figs. 1 and 2. The crystallographic data
reveals to crystal structure of chemosensor R1 and relevant
refinement parameters are given in Table 2 and the selected
bond angles and bond lengths are listed in Table 2.
1
The HNMR spectrum of the chemosensor R1 displays a pro-
ton at δ14.5 which shows the interaction of imine nitrogen
with the OH proton resulting in zwitter ion as shown in the
Fig. 4. A singlet proton at δ9–10 attributed to -N=CH repre-
sents the imine group (Schiff base). Similarly two doublet
protons exhibited at δ7.7–8.2 are assigned for the naphtha-
lene. Further aromatic group protons at δ6.0–8.0 represent
the benzene and naphthalene rings. A pair of singlet protons
at δ4.5–5.0 are assigned for the ArCH2.
The crystal of R1 is mono-clinic crystal system with space
o
group P2 /n. In the crystal amine moiety through 84.74 is
1
twisted with respect to naphthalene moiety. Around N the
1
Table 2 Crystallographic data
and details of refinements for
Crystal
R1
chemosensor R1
Empirical formula, Formula weight
Crystal system, Space group
Temperature (K)
C
18
H
15NO, 261.31
Monoclinic, P2 /c
296
1
a, b, c, (Å)
25.391 (4), 6.5797 (11), 8.1779 (13)
o
β ( )
92.277 (9)
3
V (Å )
1365.2 (4)
Z
4
Radiation type
Mo Kα
−1
μ (mm )
0.08
Crystal
Yellow, crystal size (0.42 × 0.30 × 0.26)
Data collection
Diffractometer, scan mode
Bruker kappa APEXIICCD, Multi-scan and ωscan
No of measured, independent and
5051, 2746, 1772
observed [I ≥ 2σ(I)] reflections
int
R
0.040
0.651
−1
(
sin θ/λ)max (Å )
Refinements
2
2
2
R [F > 2σ(F )], wR (F ), S
No. of reflections
0.050, 0.126, 0.96
2746
No. of restraints
2
H-atom treatment
H-atom parameter constrained
0.16, −0.17
−3
Δ ρmax, Δρmin (e Å )
Absolute structure
Flack x determined using 598 quotients [(I+)-(I-)]/[(I+)
+
(I-)] (Parsons, Flack and Wagner, Acta Cryst.
B69 (2013) 249–259)
−0.7 (10)
Absolute structure parameter

J Fluoresc
Fig. 3 Part of 3D network of
chemosensor R1 having
Intramolecular hydrogen bonding
Preparation of Stock and Working Solutions of Metals
and Chemosensor R1
were freshly prepared by appropriate dilution of stock solution
to the corresponding desired concentration.
For practical utilization and for detection of toxic metal ions in
water samples it is very necessary for a chemosensor to work
under aqueous condition. Chemosensor R1 was not soluble in
pure water. Therefore 0.001 M stock solution of chemosensor
R1 was prepared in pure DMSO. The chemosensor R1 was
stable in DMSO. Working solutions were prepared by dilution
of a stock solution in DMSO with a mixture of DMSO:H2O
Metal Ion Binding Study by Fluorescence
Spectroscopy
First the fluorescence of chemosensor R1 10 μM was inves-
tigated in mixed aqueous medium DMSO:H O (20:80) by
2
taking 3 mL solution of it in a fluorometric cell. The fluores-
cence emission spectrum of this solution was taken upon ex-
citation at 320 nm. Due to internal charge transfer its fluores-
cence emission was quenched and very weak fluorescence
(
20:80). Stock solutions of metal ions (0.01 M) were prepared
in distilled water. Before spectroscopic analysis, test solutions
1
Fig. 4 H NMR spectrum of chemosensor R1

J Fluoresc
1
1
200
000
a
1
.4
1.35
.3
1.25
.2
.15
.1
R1, Na , K , Mg , Ba2+,
+
+
2+
2+
2+
2+
2+
Ni , Cd ,Co , Pb ,
3+
8
6
4
2
00
00
00
00
0
2+
2+
2+
1
Cu2+
Mn , Hg , Zn , Fe ,
3+
Ce
1
1
1
2
70
290
310
330
350
370
390
5
80
600
620
640
660
680
700
720
Wavelength (nm)
Wavelength (nm)
Fig. 5 Fluorescence emission spectrum of R1 and relative fluorescence
b
0.5
.48
.46
.44
.42
+
+
intensity changes upon the addition of monovalent (Na , K ), divalent
0
0
0
0
2
+
2+
2+
2+
2+
2+
2+
2+
(
(
Hg , Zn , Cd , Pb , Mn , Co , Ni , Cu ) and trivalent cations
3+
3+
3+
Ce , Fe , As ) as their aqueous solutions with an excitation at 320 nm,
temp 25 °C
0
.4
0
0
.38
.36
was observed at 645 nm. To investigate cation sensing
2
+
0.34
.32
.3
ability of R1, 2 equiv. of Cu and 20 equiv. of various
0
+
+
2+
2+
alkali (Na , K ), alkaline (Mg , Ba ) transition metal
0
3
+
3+
3+
2+
2+
2+
2+
2+
ions (Ce , Fe , As , Hg , Zn , Cd , Pb , Mn ,
2
80
290
300
310
320
330
340
350
360
2
+
2+
Co , Ni ) were added separately to the solution of R1
2 mL). The mixture were allowed to equilibrate for 2 min
Wavelength (nm)
(
c 0
0
.48
.47
at room temperature and then transferred separately into
quartz cuvette and the fluorescence emission spectrum
was monitored in the range 250–800 nm with an excita-
tion at 320 nm and emission at 645 nm. The spectrum of
all the examined metal ions was compared with the spec-
trum of R1. It was found that the non-fluorescence behav-
ior become highly fluorescent only in the presence of
0.46
0
0
0
.45
.44
.43
y = 0.006x + 0.412
R² = 0.989
2
+
0.42
.41
Cu ions and no detectable change in the fluorescence
spectra was observed by other metal ions except a signif-
icant increase in the initial fluorescence intensity of R1
0
0
2
4
6
8
10
12
2
+
[Cu2+
]
only upon addition of Cu as shown in Figs. 5 and 6. So
2
+
Fig. 7 a absorbance of 10 μM chemosensor R1. b Shifting of wavelength
upon addition of Cu ions, concentration of chemosensor R1 and Cu
ions is 10 μM. c Enhancement of absorbance at 308 nm upon addition of
Cu ions in the range 1–10 μM
among the tested metal ions only Cu was the one that
readily binds with R1. The increase in the fluorescence
emission of R1 can be explained on the basis of restric-
tion of internal charge transfer (ICT) mechanisms. In the
2+
2+
2+
2
+
absence of Cu ions, due to ICT mechanism, the fluores-
cence emission of the naphthalene fluorophore is greatly
UV-Visible Study
2
+
quenched. But in the presence of Cu ions ICT phenom-
enon is restricted and fluorescence emission was consid-
erably enhanced as result of rigid chelated complex for-
mation [33].
Since chemosensor R1 was not soluble in 100% aque-
ous media therefore DMSO was used as a cosolvent.
UV-vis spectra of chemosensor R1 was taken in a
mixed aqueous media in the absence and presence of
Fig. 6 Turn-On fluorescence
behaviour of R1 and the proposed
binding mode and mechanism
λex= 320 nm
HO
N
N
2+
Cu
N
2+
Cu
OH
OH
λem= 645 nm
Strongly Fluorescent
Non-fluorescent

J Fluoresc
Fig. 8 Changes in the
fluorescence emission spectra of
1200
1
2
3
4
5
6
7
µM Copper
µM Copper
µm Copper
µM Copper
µm Copper
µM Copper
µM Copper
chemosensor R1 10 μM with
1
000
800
2+
increasing amount of Cu ion at
2+
2
5 °C, λex 320 nm, [Cu ]
from left to right (1, 2, 3, 4, 5, 6, 7,
, 9, 10 μM
8
6
4
2
00
00
00
0
8 µM Copper
9
1
µM Copper
0 µM Copper
6
00
620
640
660
680
700
Wavelength (nm)
2
+
2+
different concentration of Cu ions in distilled water.
The UV-vis spectrum of chemosensor R1 exhibited
characteristic main absorption band in the range 260–
DMSO:H O (20:80) as a function of concentration of Cu ion
2
added at room temperature. When excited at 320 nm the fluo-
rescence emission spectrum of chemosensor R1 displayed was
very weak emission at 645 nm due to ICT mechanism in naph-
3
80 nm with a λmax of 302 nm as shown in Fig. 7a.
2
+
2+
Upon addition of Cu ions to chemosensor R1 λmax
shifts to 308 nm. The red-shift of the λmax is indicative
of the coordination involving the hydroxyl and imine as
thalene moiety. However, the addition of Cu ion in the range
1–10 μM led to a rapid increase in the fluorescence emission
intensity at 645 nm as shown in Fig. 8 while no significant
change in position of λ and λ was observed. The enhance-
ment of fluorescence was attributed to the restriction of ICT
mechanism. The increase in fluorescence constitutes the basis
for the detection of Cu ion with the chemosensor R1 pro-
posed in this work. We also determined the linearity by plotting
the emission intensity of chemosensor R1 at 645 nm as a func-
2
+
a receptor and Cu as an analyte. When various con-
ex
em
2
+
centration Cu ions were added to the solution of
chemosensor R1 the absorbance at 308 nm was en-
hanced as shown in the Fig. 7c. Test samples were
prepared by using 10 μM solution of chemosensor R1
and various concentrations of Cu ions. All the mea-
surements were made at room temperature.
2+
2
+
2
+
tion of the Cu ion concentration in the range 1–10 μM.
Chemosensor R1 was sensitive to Cu ion and the increase
in fluorescence had a linear behaviour with correlation coeffi-
2+
2
+
Fluorogenic Detection of Cu
2
cient of R = 0.9978 as shown in Fig. 9.
To investigate the fluorescence-sensing behavior of
chemosensor R1, a quantitative analysis of the binding affinity
Detection Limit
2+
of chemosensor R1 with Cu ion was studied by fluorescence
emission spectroscopy. To address the sensitivity, the fluores-
cence spectral properties of chemosensor R1 were examined in
The detection limit of chemosensor R1 as a fluorescent sensor
for the recognition of Cu ion was determined from the plot
2
+
1
1
200
000
1200
1000
800
600
400
200
0
8
6
4
2
00
00
00
00
0
y = 96.52x + 40.8
R² = 0.997
0
2
4
6
8
10
12
0
0.2
0.4
0.6
0.8
1
[
Cu2+
]
Mole fraction of R1
2
+
Fig. 9 Fluorescence emission profile of chemosensor R1 versus
concentration of Cu ion at 645 nm
Fig. 10 Job’s plot of R1-Cu complex at 645 nm, [R1] + [Cu2+] =
10 μM, λex is 320 nm, showing a binding stoichiometry of 2:1
2+

J Fluoresc
1
1
1
1
6
4
2
0
8
6
4
2
0
800
700
600
R1
5
4
3
2
1
00
00
00
00
00
0
y = 1E-11x + 1.467
R² = 0.984
RI-Cu2+ Complex
0
5E+11
1E+12
1.5E+12
0
5
10
15
20
1
/[Cu2+]2
Time (min)
Fig. 11 Benesi-Hilderbrand plot for determination association constant
2
+
Fig. 13 Time dependent fluorescence emission intensity of chemosensor
R1 in the absence and presence of Cu at 645 nm, λex 320 nm
of Cu (1–10 μM) with chemosensor R1.The excitation wavelength is
20 nm and the emission wavelength is 645 nm
2+
3
prepared and mixed in different ratios by continuous increase of
one constituent with the similar decrease of second constituent
keeping total concentration constant. The sum of concentration
of chemosensor R1 and Cu ion was kept constant at 10 μM
according to the continuous variations, changing the mole frac-
2
+
of fluorescence emission intensity as a function of Cu ion
concentration. The detection limit was calculated based on 3σ/
k [41], where σ is the standard deviation of blank measure-
ment and, and k is the slope of the plot. To determine σ, the
fluorescence emission intensity of chemosensor R1 was mea-
sured six times independently in the absence of copper ions.
2+
tion of chemosensor R1 from 0.1 to 0.9 in a solution of [R1] +
2+
[
Cu ]. Binding stoichiometry was determined by plotting the
−
9
The detection limit calculated was found to be 30 × 10
which is far below the WHO acceptable limit (30 μM or
M
graph between the molar fraction of chemosensor R1 and re-
spective fluorescence emission intensity. When molar fraction
reached to 0.6, maximum fluorescence emission was achieved
2
+
1
.5 mg/L of Cu ) in drinking water. The detection limit of
the proposed chemosensor R1 is lower than the fluorescent
chemosensors of ref. [11, 42–45]. The detection limit was
sufficiently low to recognize Cu in nano molar range. In
term of the detection limit and linear range it can be seen that
the proposed sensor displays more sensitivity and selectivity
2+
at 645 nm, suggesting that the chemosensor R1 and Cu
formed a 2:1 stoichiometry complex as shown in Fig. 10. It
2
+
2+
means one Cu ion bind with two chemosensor R1 molecules.
On the basis of 2:1 stoichiometry as determined by Job’s
plot the association constant was calculated with the help
Benesi–Hildebrand plot from the increasing fluorescence emis-
2
+
for Cu ion by fluorescence spectra.
2+
sion intensity of chemosensor R1 as a function of Cu con-
centration. The association constant of the complex was calcu-
lated through the Benesi-Hildebrand equation stated below
Determination of Binding Stoichiometry
and Association Constant
F
max−
Fo
1
For determining the stoichiometry of chemosensor R1 with
Cu ion the Job’s plot experiment was undertaken in solution
¼
1 þ
Â
Ã
ð1Þ
2+
2
F−Fo
2
þ
K Cu
state using fluorescence spectroscopy. For spectroscopic anal-
2+
−2
ysis equimolar solutions of chemosensor R1 and Cu ion were
Where Ka (M ) is an association constant. F
0
is the
1
1
200
000
1220
1020
820
620
420
220
20
8
6
4
2
00
00
00
00
0
R1
R1-Cu2+
0
2
4
6
8
10
12
14
16
0
2
4
6
8
10
12
14
Schiff base R (10-6 M)
pH
Fig. 12 Fluorescence emission intensity at 645 nm as a function of
concentration of chemosensor R1. Copper concentration was kept
constant (10 × 10− M)
Fig. 14 Fluorescence response of chemosensor R1 in the presence and
2+
absence of Cu as a function of different pH values at room temperature,
6
λex is 320 nm and λem is 645 nm

J Fluoresc
fluorescence emission intensity of chemosensor R1 in the ab-
Effect of pH
2+
sence of Cu , F
chemosensor R1 at [Cu ] in large excess and F is the fluores-
cence emission intensity obtained at different [Cu ] (λ
is the fluorescence emission intensity of
max
2+
For practical utilization the effect of pH on the fluorescence
emission intensity was investigated to determine a suitable pH
range for sensing of chemosensor R1 to Cu . The fluorescence
2+
=
ex
2+
3
20 nm and λem = 645 nm. The association constant Ka was
2+
determined graphically by plotting Fmax-F /F-F versus 1/[Cu ]
emission intensity measurements were performed in the absence
0
0
2
+
as shown in Fig. 11. According to the Eq. (1) data were linearly
and presence of 10 μM Cu . The pH of the solutions was
adjusted using HCl and NaOH and the alteration in fluorescence
emission intensity at 645 nm was monitored in the pH range 2–
12. The fluorescence emission intensity versus pH plot for the
−11
fitted showing a good liner relationship with slope (1 × 10 ),
and intercept = (1.4677) and confirming 2:1 complexation. The
value of Ka value was determined from the slope and intercept
of the line.
2+
chemosensor R1 and R1-Cu complex is shown in Fig. 14. As
shown in Fig, the fluorescence emission intensities of the metal
free chemosensor R1 remain unaffected in the pH range 2–12.
This result indicate that the chemosensor R1, which is organic
solvent-stable and also pH-stable, can be employed for recog-
Effect of Chemosensor R1 Concentration
on Fluorescence Response
2+
The effect of an increasing concentration of chemosensor R1
nition of Cu as a useful potential fluorescent sensing material.
2+
2+
on fluorescence spectra of R1-Cu complex was also investi-
After the addition of Cu to the solution of chemosensor R1 the
gated. Working solution for analysis were prepared in presence
of Cu ion (10 μM) by addition of various concentration of
chemosensor R1 solution in an incremental fashion in the range
fluorescence emission intensity at 645 nm rapidly enhanced in
the pH range 5–12. As it is cleared from the Fig. 14, in the
section of lower pH value (pH < 5) the emission intensity re-
main in the quenched state indicating poor stability of the R1-
2+
1–15 μM. The fluorescence emission intensity of each sample
2+
was monitored separately at 645 nm and was plotted versus
concentration of chemosensor R1 as shown in Fig. 12. The
fluorescence emission intensity enhanced gradually with in-
creasing concentration of chemosensor R1 but at higher con-
centration the fluorescence remain steady. Further replicate
analyses were conducted to figure out reproducibility using
Cu complexes at low pH. This is due to protonation on the
2+
chemosensor R1 that prevents the formation of the R1-Cu
complex. The fluorescence emission intensity was in the turn-
on state for pH > 5 indicating maximum complexation at high
2+
pH values and good fluorescence detection ability to Cu .
2
+
constant concentration of chemosensor R1 and Cu ion
10 μM). Relative standard deviation (RSD) was 0.24% for
0 repeated fluorescence measurements.
Metal Ion Competition Experiment
(
1
Achieving high selectivity for a specific metal ion over a com-
plex background of competitive metals is an important feature to
evaluate the performance of a chemosensor. Therefore the suit-
ability and practical applicability of chemosensor R1 as a selec-
Effect of Time
2+
Response time is very important parameter of a fluorescence
tive fluorescent probe for Cu ion was checked by competition
experiments in the presence of various metal ions. To investigate
the interference from a number of cations the change in fluores-
cence emission intensity at 645 nm was recorded with solution
chemosensor therefore the kinetic studies of chemosensor R1
2+
(
20 μM) both in the absence and presence of Cu (2 μM) was
investigated by fluorescence spectra. For this purpose the fluo-
rescence emission intensities were measured at different inter-
val of time. The time was varied in the range 1–15 min. In the
+
+
2+
2+
3+
3+
of R1 (20 μM) containing Na , K , Mg , Ba , Ce , Fe ,
3+
2+
2+
2+
2+
2+
2+
2+
As , Hg , Zn , Cd , Pb , Mn , Co , Ni (200 μM) and
Hg , Fe (20 μM) followed by the addition of Cu (20 μM).
In recognition of Cu with chemosensor R1 no significant
2
+
2+
3+
2+
absence of Cu at 645 nm almost no changes in the fluores-
cence emission intensity of chemosensor R1 was found as
displayed in Fig. 13, demonstrating stability of chemosensor
R1 in the assay condition. In fluorescent spectroscopy long
response time is undesired. The response of R1 was almost
instantaneous and fluorescence emission intensity turn on re-
2
+
1
200
1000
800
2
+
6
4
2
00
00
00
0
markably at 645 nm upon addition of aqueous solution of Cu
to solution of chemosensor R1, suggesting the rapid reaction of
Metal ion
Metal ion-Cu
2+
2+
chemosensor R1 with Cu . For higher concentration of Cu
chemosensor R1 has a fast response time of less than 30 s. This
result indicates that the recognition was completed immediate-
2
+
ly without any detectable time delay after addition of Cu .
Fig. 15 Fluorescence response of R1 to 200 μM various tested metal ions
grey bar) and to the mixture of 200 μM tested metal ions with 20 μM
Cu (black bar) at room temperature, λex = 320 nm
The chemosensor R1 is highly suitable for real-time detection
(
2
+
2+
of Cu in aqueous solutions in practical analysis.

J Fluoresc
Table 3 Comparison of R1 with previous work [19, 20, 24, 42, 46, 47]
Structure of sensor
Fluorophore Analyte Detection
Testing Detecti Optimal Ref
mode/
media
on limit pH
remarks
Quenching
λex/λem=
M
range
NA
Quinoline
Cu2+
Tris–
HCl
6.66×10
19
−
8
M
N
3
99/514
solution
O
HN
N
[
V(C2H
N
Red shift
and
5OH)/V
(H2O)=
5:5,
O
HO
N
fluorogenic
change
pH=7.2]
Cu2+
Enhanceme Acetonit 2.73 ×
2-8.5
H
N
Pyrene
20
−
6
nt
rile–
water
(v/v =
10 M
N
S
λex/λem=
3
85/468
3
/1, 5
mM
Chromo-
and
HEPES,
pH 7.0)
fluorogenic
changes
2
+
Naphthalene
Cu ,
Enhanceme 10 mM
2.4 ×
5-10
24
CH
−
6
cystein
nt (UV-
vis)
bis-tris
buffer/D
MSO
10 M
NC
N
NH
2
λ
max= 489
HO
(
7:3,
v/v)
Bathoch
romic
shift and
chromog
enic
change
OH
2+
Coumarin
Cu ,
Quenching Aqueous
NA
43
3
+
Fe
λex/λem= solution 1.27 ×
OH
−4
N
340,
(30 mM 10 M,
HEPES
3
64/458,
OH
HO
HO
O
BS1
O
4
37/
buffer,
pH 7.4, 10
1%
1.04 ×
−4
M
N
OH
Change in DMSO)
O
O
fluorescenc
e
BS2
Cu2+
Quenching Methano 0.27×
5-9
47
48
HO
Coumarin
Pyrene
−
6
λex/λem= l–water 10
M
N
N
467/537
(v/v = 1
:
1, 10
mM
N
O
O
Chromo-
and
HEPES,
fluorogenic pH 7.0)
change
Enhanceme Acetonit
Cu2+
NA
5.5-7.5
N
nt
rile-
water
(v/v=
N
λex/λem=
NH
2
3
50/417
N
1
:1, 10
mM
Chromo-
and
fluorogenic
change
HEPES,
pH=7.0)
Cu2+
Enhanceme DMSO: 30×10
-9
5-12
This
work
Naphthalene
nt
H
2
O
M
N
H
λex/λem=
(2:8)
3
20/645
nm
O
Fluorogeni
c change

J Fluoresc
9
8
750
650
550
450
3
2
150
50
50
interference was observed in the presence of most competitive
2+
3+
metal ions except for the Hg and Fe ion as shown in the
2+
3+
Fig. 15. This indicates that only Hg and Fe compete with
2+
Cu for binding with chemosensor R1. The fluorescence en-
2 x 10-6 M Cu
4 x 10-6 M Cu
6 x10 -6 M Cu
2+
hancement caused by the Cu ion remained unaffected with
most examined metal ions. These observations suggests that
R1 can be applied as an outstanding selective and sensitive
fluorescent chemosensors for estimation of trace amount of
50
50
8
x 10-6 M Cu
5
0
10 x 10-6 M Cu
2+
Tap water Well
from our water
laboratory from
Well
water from river
from Swat
Water
Cu ion in real samples in the presence of these metals ions.
Batkhela Thana
Comparison with Pervious Works
2
+
Fig. 17 Respective fluorescence responses of chemosensor R1 to Cu
2+
2+
ion added to the natural water samples at room temperature, [Cu ] 2–
Specific features of the proposed chemosensor R1 towards Cu
was compared with some previously reported chemosensor for
1
0 μM, λex is 320 nm and λem is 645 nm, [R1] 10 μM
2+
Cu in Table 3.
Most of the chemosensor required rigorous testing media
and most of them displayed quenching of fluorescence upon
and its fluorescence emission intensity was monitored at
645 nm after interaction with EDTA. After the addition of
EDTA fluorescence emission signal was restored at 645 nm.
This analysis suggests that the complexation between copper
and chemosensor R1 is really and chemically reversible as
shown in Fig. 16.
2+
interaction with Cu . Our proposed chemosensor R1 presents
a number of attractive analytical features such as one step syn-
2
+
thesis, high selectivity for Cu , sensitivity, enhancement of
fluorescence, good reproducibility, wide pH range and can be
2
+
used for the rapid detection of Cu in natural water samples.
Reversibility of Chemosensor R1
Application to the Analysis of Natural Water Samples
In the development of fluorescent chemosensor, the main fea-
ture to increase their applicability is the detection. The use of a
chemosensor for detection of particular metal ion, reversibility
is an important aspect in practical applications. To investigate
whether the enhancement of fluorescence emission intensity
To investigate the practical applicability and sensitivity of the
new chemosensor R1 in complex matrices, we examined the
ability of chemosensor R1 to detect trace amount of Cu ions
2
+
in three natural water samples using the spike method.
2+
Chemosensor R1 was sensitive enough to detect Cu ion in
natural water samples. For this purpose samples were collect-
ed from three significantly different sourcses: the river water
from river Swat, tap water from laboratory water pipe located
at university of Malakand KPK and well water from Batkhela
2
+
was as a result of indeed Cu ion binding with chemosensor
R1 and not photoactivation of chemosensor R1 or a chemical
reaction reversibility experiment was conducted to study
2
+
whether the complexation of chemosensor R1 with Cu is
reversible or not. For this purpose we use a strong metal ion
chelator EDTA to revert the fluorescence. To the solution con-
2
+
and Thana. Different concentrations of Cu ion were added
into the water samples and assayed within 24 h of the collec-
tion. The fluorescence emission intensity of each sample was
monitored at 645 nm and the results are shown in the Fig. 17.
As it is cleared from the Figure that fluorescence emission
intensity of chemosensor R1 was proportional to the amount
2
+
taining R1-Cu complex 1 equiv. of EDTA was introduced
1
1
200
000
2+
of Cu ion added. These results displayed that relative to
2+
distilled water chemosensor R1 can also detect Cu ion in
the complex media, pointing out its potential applicability in
the field of toxicology and environment sciences.
8
6
4
2
00
00
00
00
0
R1-Copper
R1-Copper-EDTA
Conclusion
In conclusion, we have developed a Schiff based fluorescent
6
20
630
640
650
660
670
2
+
chemosensor for Cu ion and it was characterized by FTIR
Wavelength (nm)
1
and single X-ray diffraction and H NMR. Significant fluores-
Fig. 16 Reversibility of binding interaction between chemosensor R1
10 μM) and copper (20 μM) by the introduction of EDTA (20 μM) to
the solution of complex
cence enhancement was observed by this sensor only in the
(

J Fluoresc
1
5. Yeong KJ, Chang UN, Ha LK, In HH, Cheal K (2013) A Selective
colorimetric and fluorescent chemosensor based-on naphthol for
detection of Al and Cu . Dyes Pigments 99:6–13
6. Cesar GF (2005) Relevance, essentiality and toxicity of trace ele-
ments in human health. Mol Asp Med 26:235–244
presence of Cu2+ ion. The enhancement was due to the re-
striction of ICT phenomenon. The optimal pH range for de-
tection of Cu ion was 5–12. Our proposed chemosensor R1
3+
2+
2
+
1
possess high selectivity, sensitivity, and fast response time
2
+
towards Cu ion over a large number of competing metal
ions. The excellent low limit of detection of this chemosensor
17. Selvaraj R, Younghun K, Cheol KJ, Jongheop Y (2004) Removal of
copper from aqueous solutions by aminated and protonated meso-
porous aluminas: kinetics and equilibrium. J Colloid Interface Sci
2
+
R1 towards Cu ion can be useful in detection of trace
2
73:14–21
2
+
amount of Cu ion in environmental samples. The response
was reversible and the given sensor was successfully applied
for the detection of Cu2+ in natural water sample.
1
1
2
8. Pragnesh ND, Subrahmanyam N, Surendra S (2009) Kinetics and
thermodynamics of copper ions removal from aqueous solution by
use of activated charcoal. Indian J chem Technol 16:234–239
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highly sensitive and selective fluorescent sensor for imaging copper
(II) in living cells. J Fluoresc 24:1331–1336
0. Hsuan-Fu W, Shu-Pao W (2013) A Pyrene-based highly selective
turn-on fluorescent sensor for copper (II) ions and its application in
living cell imaging. Sensors Actuators B 181:743–748
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