A selective proton transfer optical sensor for copper II based on chelation enhancement quenching effect (CHEQ)

Article abstract of DOI:10.1016/j.dyepig.2020.108566

A novel selective fluorescent sensor for Cu²⁺ in solution has been developed by means of chelation enhancement quenching effect (CHEQ). The compound N,N′-bis(salycilidene)-(2-(3′,4′-diaminophenyl)benzothiazole (BTS) is reactive to intramolecular proton transfer on the electronic excited state (ESIPT) presenting fluorescence emission in the blue region with very large Stokes shift (8794 cm^?1). Spectroscopic titration experiments indicate that not only the fluorescence quenching but also of the ESIPT reaction allowed naked eye detection as a paper strip-based sensor. This chemosensor showed to be selective to Cu²⁺ among 13 other cations. The sensor:Cu²⁺ ratio was determined by both Job and Benesi-Hildebrand plots as 1:2. In addition, the binding between sensor and metal was investigated by ¹H NMR, FTIR and HRMS in order to elucidate the formed complex and corroborate the results of BTS:Cu²⁺ ratio. The binding constant was also found as 3.0 × 10¹⁰ mol^?2 L² at 25 °C. The detection and quantification limits were also obtained as 32 ppb and 108 ppb, respectively.

Full text of DOI:10.1016/j.dyepig.2020.108566

Journal Pre-proof
A selective proton transfer optical sensor for copper II based on chelation
enhancement quenching effect (CHEQ)
Luís Gustavo Teixeira Alves Duarte, Felipe Lange Coelho, José Carlos Germino,
Gabriela Gamino da Costa, Jônatas Faleiro Berbigier, Fabiano Severo Rodembusch,
Teresa Dib Zambon Atvars
PII:
S0143-7208(20)30304-1
DOI:
https://doi.org/10.1016/j.dyepig.2020.108566
DYPI 108566
Reference:
To appear in: Dyes and Pigments
Received Date: 3 February 2020
Revised Date: 18 May 2020
Accepted Date: 18 May 2020
Please cite this article as: Teixeira Alves Duarte LuíGustavo, Coelho FL, Germino JoséCarlos, Gamino
da Costa G, Berbigier JôFaleiro, Rodembusch FS, Zambon Atvars TD, A selective proton transfer
optical sensor for copper II based on chelation enhancement quenching effect (CHEQ), Dyes and
Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2020.108566.
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Author contributions
Luís Gustavo Teixeira Alves Duarte: Conceptualization; Methodology; Formal
analysis; Roles/Writing - original draft.
Felipe Lange Coelho: Methodology; Formal analysis; Roles/Writing - original
draft.
José Carlos Germino: Formal analysis; Roles/Writing - original draft.
Gabriela Gamino da Costa: Formal analysis; Roles/Writing - original draft.
Jônatas Faleiro Berbigier: Methodology; Formal analysis; Roles/Writing -
original draft.
Fabiano Severo Rodembusch: Supervision; Writing - review & editing.
Teresa Dib Zambon Atvars: Funding acquisition; Project administration;
Conceptualization; Supervision; Writing - review & editing.

A selective proton transfer optical sensor for copper II based on chelation
enhancement quenching effect (CHEQ)
Luís Gustavo Teixeira Alves Duarte,a† Felipe Lange Coelho,b† José Carlos Germino,a
Gabriela Gamino da Costa,b Jônatas Faleiro Berbigier,b Fabiano Severo Rodembuschb
and Teresa Dib Zambon Atvarsa*
aChemistry Institute, University of Campinas, Brazil. E-mail: tatvars@unicamp.br
bGrupo de Pesquisa em Fotoquímica Orgânica Aplicada, Universidade Federal do Rio
Grande do Sul, Porto Alegre, Brazil.
† Authors contributed equally to this work.
Abstract:
A novel selective fluorescent sensor for Cu2+ in solution has been developed by means
of chelation enhancement quenching effect (CHEQ). The compound N,N’-
bis(salycilidene)-(2-(3’,4’-diaminophenyl)benzothiazole (BTS) is reactive to
intramolecular proton transfer on the electronic excited state (ESIPT) presenting
fluorescence emission in the blue region with very large Stokes shift (8794 cm-1).
Spectroscopic titration experiments indicate that not only the fluorescence quenching
but also of the ESIPT reaction allowed naked eye detection as a paper strip-based
sensor. This chemosensor showed to be selective to Cu2+ among 13 other cations. The
sensor:Cu2+ ratio was determined by both Job and Benesi-Hildebrand plots as 1:2. In
addition, the binding between sensor and metal was investigated by 1H NMR, FTIR and
HRMS in order to elucidate the formed complex and corroborate the results of
BTS:Cu2+ ratio. The binding constant was also found as 3.0 x 1010 mol-2 L2 at 25oC. The
detection and quantification limits were also obtained as 32 ppb and 108 ppb,
respectively.
Keywords: benzothiazole; ESIPT; salycilidene, optical sensor; naked eye
1. Introduction
In recent decades, the design and synthesis of different Schiff base-metal
complexes was applied mainly in catalysis, biomedicinal sciences and material
chemistry [1]. In particular, Salen or Salophen-type, that are tetradentate ligands
1

showing an almost planar geometry, are able to stabilize different metals, including
titanium, zirconium, vanadium, manganese, iron, copper, cobalt, nickel, rhodium,
palladium, among others, in various oxidation states [2]. These complexes successfully
promoted reactions of carbon-carbon and carbon-heteroatom bond formation, C-H
functionalization and a diverse set of chemical transformations [3]. On the other side,
the structural features of Salen or Salophen compounds make them suitable for metal
cation recognition. Once these compounds present interesting photophysical properties
and great affinity for metals, the interaction between sensor and analyte can result in
luminescence changes which can be monitored [4]. Among these metals, the present
study focus on copper identification, which is still widely used in construction for
electrical applications, pipes for industrial flow of fuels, water purification and
beverages preparations, or in machine production as an alloy to enhance materials
resistance [5,6]. In nature, copper regards significant biochemical processes on every
living organism [7,8,9], with a role mainly connected to its redox activity or chemical
reactivity where the balance between Cu1+ and Cu2+ increases its availability due to
solubility reasons [10]. Furthermore, it is a cofactor to enzymes that participate on
cascade reactions [11] such as respiration [9], photosynthesis [12], neurotransmitter [13]
and ATP [14,15] synthesis. Despite being an important micronutrient, the excess of
Cu2+ or its disbalance between mono and bivalent cations can promote, for instance,
neurodegenerative diseases [16,17]. In addition, in food crops such as wheat and rice,
depending on the concentration, copper turns a pollutant responsible for growth
inhibition and oxidative damage [18]. Thus, to monitor copper presence becomes a
relevant issue, where its detection in solution is currently performed by fluorescent
probes [19-22]. This metal can be detected as a coordination compound of different
ligands, where Schiff bases showed to be useful for this purpose, not requiring
complicated synthetic routes and generally using relatively low-cost precursors [33].
These compounds can be envisaged as ligand frameworks with structures easily tuned
to bind to cations in solution by mechanisms that might quench (Chelation
Enhancement Quenching Effect – CHEQ) or enhance (Chelation Enhanced
Fluorescence Effect – CHEF) fluorescence emission [19]. In this context, fluorescence
spectroscopy comprises a field in constant development due to its high sensitivity and
the possibility of application in many areas including chemistry, biology and medicine
[10,19,23-41].
2

There are several processes that can be responsible for the emission dependence
on Cu2+ coordination. Specifically, copper can be detected by CHEQ or CHEF
mechanisms mainly because of the creation of electronic transitions based on charge
transfer from ligand to metal (LMCT – Ligand-Metal Charge Transfer).[19] Moreover,
those transitions may be related to either photoinduced electron transfer [23] or to
energy transfer processes (resonant or not) [42]. Regardless the process, the
fluorescence quenching is described by Yang et al. [43] by CHEQ mechanism in the
presence of copper due to its paramagnetic electronic structure [19] and the appearance
of a faster and competitive nonradiative deactivation path between LMCT state and the
local electronic transition of the ligand.
Herein, we move one step further on the potential technological applications of
Schiff bases based on benzothiazole and Salophen moieties that undergoes Excited State
Intramolecular Proton Transfer reaction (ESIPT) [44]. In this study, we describe a
Salophen sensor selective for Cu2+ detection in a high sensibility. Additionally, we also
dedicate our efforts to elucidate the complex generated between sensor metal cation by
Benesi-Hildebrand and Job plots, as well as spectroscopic analysis.
2. Experimental Procedure
2.1 Materials and Methods
Commercially available reagents were used without further purification. The
solvents were purified using standard procedures [45] or used as received.
Spectroscopic grade solvents (Merck or Aldrich) were used for the photophysical
characterization (UV-Vis absorption and fluorescence emission). All salts were stored
under vacuum with dry silica gel for a week before the preparation of the working
solutions. Electronic absorption spectra were recorded with the Hewlett-Packard 8452A
diode array spectrophotometer. Fluorescence emission spectra were performed by ISS
single-photon counting spectrofluorimeter model PC1 in a quartz cuvette with 1 cm of
optical path, respectively. Samples were placed in the Hewlett-Packard 89090A Peltier
temperature controller set at 25 oC for 5 min before analysis and metal ions addition. A
stock solution of BTS 4.0 µmol L-1 in a mixture of acetonitrile and deionised water
(MeCN/H2O) 7:3 v/v was employed. Along with solutions of chloride salts from the
cations sodium, potassium, lithium, barium, calcium, magnesium, cadmium, tin (II),
nickel, aluminium, copper (II), mercury (II) and silver nitrate 5.0 mmol L-1 in deionised
3

water or pure acetonitrile for the last two. Deionised water was obtained from Direct-Q3
UV deionizer from Millipore® with 18.2 mΩ of conductivity.
13
1H NMR and C NMR spectra were recorded in CDCl3 or DMSO-d6 solutions
on a Varian 400 MHz and Bruker 400 MHz spectrometers. Chemical shifts (δ) are given
in part per million from the peak of tetramethylsilane (δ=0.00 ppm) as internal standard
1
13
in H NMR or from the solvent peak of CDCl3 (δ=77.23 ppm) in C NMR. Data are
reported as follows: chemical shift (δ), multiplicity, coupling constant (J) in Hertz and
integrated intensity. High resolution mass spectra (HMRS) were recorded on a
Micromass Q-Tof spectrometer, using electrospray ionization (ESI). FTIR spectra were
recorded on a Varian 640-IR using KBr disc or at ATR mode. Purification by column
chromatography was carried out on silica gel 60 (230−400 mesh). Analytical thin-layer
chromatography (TLC) was conducted on aluminium plates with 0.2 mm of silica gel
60F-254. Melting points were determined on a Büchi Melting Point M-545.
2.2 Synthesis and spectroscopic characterization
Bis(2-aminophenyl)disulfide (2)
A suspension of benzo[d]thiazol-2-amine (1) (numbers for compounds
identification were shown in Scheme 1) (2.0 g, 11.1 mmol) in 15 mL of aqueous
solution of NaOH (10 M) was refluxed for 24 h. After cooling, mixture was filtered and
filtrate was acidified with concentrated HCl to pH 2-3 and stirred for 10 min. After,
solids were separated out and solution was neutralized with K2CO3 and kept
crystallizing overnight. Filtration gave 1.03 g (60 % yield) of desired product as yellow
1
crystals. H NMR (400 MHz, CDCl3): δ 7.19-7.12 (m, 4H), 6.70 (d, J=7.09 Hz, 2H),
13
6.58 (t, J=7.09 Hz, 2H), 4.32 (s, 4H); C NMR (100.7 MHz, CDCl3): δ 148.5, 136.9,
131.6, 118.7, 118.3, 115.1.
2-(3’,4’-diaminophenyl)benzothiazole (4)
In 50 mL of dry toluene was dissolved disulfide 2 (1.0 mmol) and PBu3 (2.0
mmol) and stirred for 10 minutes. After 3,4-diaminobenzoic acid (1.0 mmol) was added
and the mixture was refluxed for 48 h. After cooling, reaction was diluted with 40 mL
of ethyl acetate and washed with concentrated NaHCO3 solution (2x45 mL) and water
(2x45 mL). The organic phase was separated, dried with Na2SO4 and concentrated
under vacuum. The residue was purified by column chromatography on silica gel using
dichloromethane/ethyl acetate (50:50) as eluent. The product was obtained as white
4

1
purple crystals in 82% yield. H NMR (400 MHz, DMSO-d6): δ 8.00 (d, J=8.05 Hz,
1H), 7,88 (d, J=8.05 Hz, 1H), 7.44 (t, J=7.61 Hz, 1H), 7.33 (t, J=7.61 Hz, 1H), 7.32 (d,
J=2.00 Hz, 1H), 7.17 (dd, J=8.00 Hz, 2.00 Hz, 1H), 6.60 (d, J=8.22 Hz, 1H), 5.24 (s,
2H), 4.81 (s, 2H). 13C NMR (100.7 MHz, DMSO-d6): δ 169.2, 154.3, 139.7, 135.2,
134.2, 126.6, 124.7, 122.2, 122.1, 122.0, 118.5, 114.1, 113.1.
N,N’-bis(salycilidene)-(2-(3’,4’-diaminophenyl)benzothiazole (6)
In a Schlenk tube, 2-(3’,4’-diaminophenyl)benzothiazole (2 mmol) and
salicylaldehyde (8 mmol) were dissolved in ethanol and kept at reflux temperature for
24 h. The desired product (6) was purified using a Soxhlet extractor with ethanol as the
solvent and isolated with 90% yield as an orange powder. The product (6) was
recrystallized from hot ethanol solution followed by drop-wise addition of hot
1
tetrahydrofuran (THF) until complete solubilization. M.p.: 224–226 °C. H NMR (400
MHz, DMSO-d6): δ 12.76-12.80 (s, 1H), 12.72-12.76 (s, 1H), 9.08-9.10 (s, 1H), 9.01-
9.03 (s, 1H), 8.15-8.19 (d, 1H, J=8.0 Hz), 8.07-8.13 (t, 3H, J=7.5 Hz), 7.75-7.78 (dd,
1H, J=7.5 Hz and J=1.5 Hz), 7.70-7.73 (dd, 1H, J=7.5 Hz and J=1.5 Hz), 7.62-7.66 (d,
1H, J=8.5 Hz), 7.55-7.60 (t, 1H, J=7.5 Hz), 7.47-7.51 (t, 1H, J=7.5 Hz), 7.42-7.47 (t,
13
2H, J=8.0 Hz), 6.97-7.02 (m, 4H). C NMR (100.7 MHz, DMSO-d6): δ 207.2, 166.9,
165.3, 164.9, 160.9, 160.9, 154.0, 145.2, 143.7, 135.1, 134.4, 134.2, 133.0, 132.9,
132.5, 127.3, 127.0, 126.2, 123.3, 122.9, 121.2, 120.0, 119.8, 119.7, 118.6, 117.2,
117.2. FTIR (cm-1): 3058, 1609-1592, 755. HRMS (ESI) calcd for C27H20N3O2S
requires 450.1232 m/z, found 450.1229 m/z (M+H)+.
2.3 Determination of limits of detection (LD) and quantification (LQ)
The determination of BTS:Cu2+ ratio and the binding constant for the
coordination compound formation were performed in terms of Benesi-Hildebrand and
Job plots. The limits of detection (LD) and quantification (LQ) were calculated based on
fluorescence titration of BTS to the linear response region of the plot F/F0 vs [Cu2+],
according to the Equations (1) and (2):
3ꢂ
ꢀꢁ =
(1)
(2)
ꢃ
10ꢂ
ꢃ
ꢀꢄ =
5

where S is the standard deviation of the curve linear coefficient and b is the angular
coefficient [46]. To visualize the quenching process and evaluate the possibility of the
use as a sensor, paper filter strips of ± 1.0 cm² were immersed initially in BTS
MeCN/H2O (7:3 v/v) solution dried in air and subsequently immersed in the respective
salt solutions. Samples were irradiated by a commercial UV-lamp before and after the
addition of ions.
3. Results and Discussion
3.1 Synthesis
The synthetic route for obtaining the compound N,N’-bis(salycilidene)-(2-(3’,4’-
diaminophenyl)benzothiazole (6), so-called BTS, is presented in Scheme 1. The
synthesis starts from the hydrolysis of the benzothiazole 1 with aqueous sodium
hydroxide to produce the disulfide 2. Compound 2 was reacted with 3,4-diaminobenzoic
acid (3) on reflux in presence of tributylphosphine that promotes disulphide cleavage
and coupling with the carboxylic acid to afford the compound 4 [47]. The condensation
between diamine derivative 4 and salicylaldehyde (5) in ethanol under reflux leads to
formation of the product 6 in 90% yield, which was first purified using a Soxhlet
extraction system using ethanol as solvent, followed by recrystallization with
ethanol/tetrahydrofuran.
Scheme 1. Synthetic methodology for obtaining BTS (6).
6

3.2 Photophysical characterization
As previously described for dichloromethane (DCM) solution [44], BTS is
highly reactive to ESIPT process. In MeCN/H2O (7:3 v/v) solution (4.0 µmol L-1), the
ligand kept its absorption band with maximum centered at 340 nm. It was observed a
slight blue shift due to media polarity increase when compared to DCM, once
intramolecular hydrogen bond stability between the iminic nitrogens and hydroxyls
moieties are committed (Figure 1a). Fluorescence emission profile (λexc = 340 nm) in
MeCN/H2O (7:3 v/v) solution is similar to the spectrum obtained in DCM solution,
presenting a triple emission profile with maxima centered at 390 nm, 485 nm and 520
nm related to the enol form N*, tautomeric T* and anionic A* forms, respectively. N*
is the locally excited species, T* form arises from the ESIPT reaction (Figure 1b) and
A* is due to deprotonation at electronic ground state [44].
Figure 1. (a) Normalised electronic UV-Vis absorption and fluorescence emission
spectra of BTS in MeCN/H2O (7:3 v/v) solution (4.0 µmol L-1, λexc = 340 nm) and (b)
ESIPT photochemical cycle.
7

3.3 Metal Ion Recognition
BTS in MeCN/H2O (7:3 v/v) solution was applied as optical sensor for metal ion
detection. To the BTS solution (4.0 µmol L-1) 11 equivalents from the ion solutions (5.0
mmol L-1) were independently added. The fluorescence spectra of these mixtures were
acquired with λexc = 340 nm and their intensities (F) were related to pure BTS emission
in the absence of the ions (F0) as presented in Figure 2. It can be observed that all
studied ions presented F/F0 ratio higher than 0.5. However, in presence of copper, the
BTS fluorescence intensity decreases significantly presenting an F/F0 ratio close to zero,
indicating the sensitivity and selectivity of BTS to copper (II).
Figure 2. Histogram of BTS normalised fluorescence response upon the addition of
various cations in MeCN/H2O (7:3 v/v) solution where BTS is the normalised emission
intensity (F0) in the absence of ions and F the emission after metal addition.
3.4 Spectroscopic titrations
Based on the photophysical results of BTS in presence of copper II, its ability to
bind to BTS was evaluated by spectroscopic titration with metal ion addition. In this
sense, spectroscopic titration experiments were employed in terms of Job and Benesi-
Hildebrand plots by the acquisition of electronic absorption and emission spectra,
respectively. Regarding the absorption spectra along the addition of Cu2+ (Figure 3a), it
was observed a decrease of BTS absorption intensity at the maxima wavelength (340
nm).
8

Using the normalised absorption spectra, the Job plot was constructed with the
λabs = 334 nm against the molar fraction of Cu2+ (ΧCu2+) and offered the proportion
between ligand and metal ion as 1:2 (Figure 3b).
Figure 3. (a) Electronic absorption spectra dependence on the addition of Cu2+ and (b)
Job plot using the normalised absorbance at 334 nm to ΧCu2+ = 0.00 to 0.88 Eq. BTS
concentration: 4.0 µmol L-1.
The fluorescence emission spectra upon the addition of Cu2+ (Figure 4) indicate
the occurrence of CHEQ process. The quenching occurs mostly at the emission region
of the tautomeric species (T*) of BTS (λem = 485 nm). It is worth mentioning that A*
species is still present after Cu2+ addition with λem = 540 nm and an additional band
centered at λem = 425 nm was observed, which will be addressed later on. Hence, it was
assumed that Cu2+ is not inducing deprotonation of BTS prior to coordination in
MeCN/H2O (7:3 v/v) solution, but only disfavouring the ESIPT balance. To the
construction of Benesi-Hildebrand plot, it was considered the following chemical
reaction:
ꢅꢆꢂ ꢇ ꢈꢉꢊꢋꢌ → ꢍ, (3)
where Y is the coordination compound and n stands for the stoichiometric proportion
between ligand and Cu2+. Hence, the binding constant Kbind of Y can be determined by
Equation (4).
9

ꢓ ꢔ
ꢍ
( )
4
ꢎꢏꢐꢑꢒ
=
ꢓ
ꢔꢓ ꢋꢌꢔꢑ
ꢅꢆꢂ ꢉꢊ
Figure 4. Electronic emission spectra dependence on the addition of Cu2+ until 55.0 Eq.
of BTS initial concentration (4.0 µmol L-1, λexc = 340 nm). The inset presents two
cuvettes with and without the addition of Cu2+ under a UV-lamp light for visual
comparison.
Assuming that the concentration of free ions is equal to its total amount in
solution and the fluorescence intensities (F) are directly proportional to the
concentrations of Y and BTS, Kbind can be deduced using Equation (4) as follows:
ꢓ ꢔ
ꢍ
ꢕ − ꢕꢖ
ꢓ
ꢋꢌꢔꢑ ( )
5
=
= ꢎꢏꢐꢑꢒ ꢉꢊ
ꢓ
ꢔ
ꢕ − ꢕ
ꢅꢆꢂ
ꢗ
where F0, F and Ff are the fluorescence intensities without Cu2+ addition, along Cu2+
addition and at final Cu2+ addition in the titration experiment using λem = 485 nm,
respectively [36,48]. After taking the module and linearizing Equation (5), the
logarithmic form of Benesi-Hildebrand plot was finally constructed (R2 = 0.98671,
Figure 5a) to the region where the plot F/F0 vs [Cu2+] is a straight line (R2 = 0.98002).
The linear plot resulted in n = 2.0 and Kbind= 3.0 x 1010 mol-2 L2 at 25oC, in agreement
with the absorption experiments. From Figure 5b we determined both the detection and
10

quantification limits using Equations (1) and (2) as LD = 32 ppb and LQ = 108 ppb,
which are comparable to the values found in the literature [36].
Figure 5. (a) Benesi-Hildebrand plot (R2 = 0.98671) to the range of concentration of
Cu2+ from 7.4 x 10-7 mol L-1 to 8.0 x 10-6 mol L-1 and (b) F/F0 vs [Cu2+] (R2 = 0.98002)
to the range of concentration of Cu2+ from 0.74 µmol L-1 to 220 µmol L-1 using the
emission intensity at 485 nm. BTS concentration: 4.0 µmol L-1.
In order to understand the unusual proportion between BTS and Cu2+, found as
2:1, respectively, for Salophen-type ligands [49,50], and the extra emission band (λem
=
425 nm) additional experiments were performed in attempt to characterize the generated
complex. In this way, spectroscopic and spectrometric analysis were performed in
1
samples containing 0.25 to 2.0 Eq. of Cu2+. Firstly, from the H NMR analysis (Figure
6, and Figures S1-S3), it was clearly observed that even at low Cu2+ concentration (0.25
Eq.) the imine group of the BTS hydrolyses. The presence of hydrogens of aldehyde C-
1
H bond and hydroxyl group of salicylaldehyde in the H NMR spectrum corroborate
with the proposed hydrolysis. Furthermore, the intensity of corresponding signals
increased as the amount of Cu2+ is raised. Also, the presence of a broad singlet located
at 5.7-5.4 ppm, as well as the vibrational band located at 4300 cm-1 in FTIR spectrum
(Figure S4), assigned to secondary amine symmetric stretching, overlapped by O-H
stretching, confirm the presence of NH2 group.
11

Figure 6. 1H NMR spectra (DMSO-d6, 400 MHz) of compounds 4 (a) and 6 (b) and 6
with 0.5 (c), 1.0 (d) and 2.0 (e) equivalents of Cu2+.
Upon high amounts of Cu2+ the emission shifts drastically to higher energies,
below 450 nm. In this case, it is believed that the ligand no longer presents the original
structure and the emission is due to the hydrolysed benzothiazole. Further photophysical
experiments were performed with the precursor 4 (Table S1, Figures S5 and S6)
corroborates with this affirmation, since this compound presents fluorescence emission
maxima in the violet to blue regions (430-459 nm) depending on the solvent.
Additionally, ESI-MS analysis were also performed to better understand the formed
complex between Cu2+ and BTS presenting the molar stoichiometry of 1:2 obtained
from the Job plot. In this sense, the signal of m/z = 604.9068 related to the [M+Na] +
(Figure S8) in which can be observed the isotopic pattern of copper complexes is
assigned to the complex illustrated in Scheme 2. Moreover, the product of hydrolysis of
BTS, ion peak of m/z = 346.1014 (Figure S9) is also observed. Thus, the presence of an
amino group in the ligand structure allows a better interaction of the ligand with the
second species of copper. It is worth mentioning that in this proposal it was assumed the
reduction of copper(II) during the electrospray ionization step and the formation of
copper(I) complex, as already been reported in the literature [51,52].
12

Scheme 2. Proposed chemical mechanism to CHEQ phenomena induced by Cu2+
binding, where the blue and green spheres represent acetonitrile molecule and chloride
atoms, respectively.
Finally, in order to study a potential application of this process as a CHEQ
sensor-based, paper filter strips 1.0 cm² were immersed in BTS solution, dried in air
and then immersed in all metal ions salt solutions. After drying in air again for a few
minutes, samples were exposed to UV-lamp illumination (Figure 7). Fluorescence
quenching was observed only on the strip immersed in Cu2+ solution.
Figure 7. Picture of the emission of paper strips after their immersion in the BTS
solution (top) and in each salt solution (bottom) with the exposure to UV-lamp light.
Conclusions
In conclusion, it was described a new application of the ESIPT reactive
compound BTS. The synthesised compound presented great sensitivity to copper (II)
ions in solution due to a chelation mechanism that inhibits proton transfer on the
electronic excited state. This quenching was also evaluated for several metallic ions
tested in the mixture MeCN/H2O (7:3 v/v). In this condition, only Cu2+ coordinated
successfully and offered pronounced changes of spectral emission, originated from
coordination compound with one molecule of BTS partially hydrolysed and two Cu2+
1
ions. This was, further corroborated by H NMR, FTIR and ESI-MS experiments. The
13

photophysical experiments also offered the binding constant in the order of 1010 mol-2
L2 and the limits of detection and quantification of copper (II) as 32 ppb and 108 ppb.
Appendix A. Supporting Information
1H NMR, FTIR and HRMS spectra, Supplementary photophysical data.
Author Information
Corresponding Author
*tatvars@unicamp.br
Authors Contribution
The manuscript was written through contributions of all authors. All authors
have given approval to the final version of the manuscript.
Acknowledgements
The authors are thankful for financial support and scholarships from CNPq, CAPES -
Financial Code 001, FAPESP 2013/16245-2, FAEPEX and INEO.
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19

Highlights
•
Novel fluorescent sensor for Cu2+ by Chelation Enhancement Quenching
Effect;
•
•
•
Cu2+ detection by quenching of ESIPT process;
The detection limits for Cu2+ were 32 ppb;
Experimental photophysical properties further compared with 1H NMR
and HRMS.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: