A highly sensitive fluorescent chemosensor for simultaneous determination of Ag(I), Hg(II), and Cu(II) ions: Design, synthesis, characterization and application

Article abstract of DOI:10.1016/j.dyepig.2015.01.014

A fluorescent chemosensor based on dimethylaminocinnamaldehyde-aminothiourea (DA) has been designed, synthesized, and applied with a combined theoretical and experimental study. The synthetic path, optimized molecular structure and characteristics of DA were carried out using the calculations at the B3LYP/LanL2DZ level of theory. The experimental investigations have a good agreement with the theoretical results. DA can be used as a chemosensor for simultaneous quantification of silver, copper, and mercury ions in aqueous solution at the ppb level. The stable geometric structures of the complexes between DA and Ag⁺, Hg²⁺, Cu²⁺ with stoichiometry of 1:1, 2:1, and 2:1, have been found and investigated for electronic properties and fluorescence quenching phenomena by using Atoms in Molecules (AIM) and Natural Bond Orbitals (NBO) analyses.

Full text of DOI:10.1016/j.dyepig.2015.01.014

Accepted Manuscript
A highly sensitive fluorescent chemosensor for simultaneous determination of Ag(I),
Hg(II), and Cu(II) ions: design, synthesis, characterization and application
Nguyen Khoa Hien, Nguyen Chi Bao, Nguyen Thi Ai Nhung, Nguyen Tien Trung,
Pham Cam Nam, Tran Duong, Jong Seung Kim, Duong Tuan Quang
PII:
S0143-7208(15)00017-0
10.1016/j.dyepig.2015.01.014
DYPI 4641
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 19 December 2014
Revised Date: 12 January 2015
Accepted Date: 14 January 2015
Please cite this article as: Hien NK, Bao NC, Ai Nhung NT, Trung NT, Nam PC, Duong T, Kim JS,
Quang DT, A highly sensitive fluorescent chemosensor for simultaneous determination of Ag(I), Hg(II),
and Cu(II) ions: design, synthesis, characterization and application, Dyes and Pigments (2015), doi:
10.1016/j.dyepig.2015.01.014.
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Graphical Abstract

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A highly sensitive fluorescent chemosensor for simultaneous determination of
Ag(I), Hg(II), and Cu(II) ions: design, synthesis, characterization and application
Nguyen Khoa Hien a,c, Nguyen Chi Bao a, Nguyen Thi Ai Nhung a, Nguyen Tien Trung b,
Pham Cam Nam d, Tran Duong a, Jong Seung Kim e,*, and Duong Tuan Quang a,*
aDepartment of Chemistry - Hue University, Hue City 8454, Vietnam
bDepartment of Chemistry - Quy Nhon University, Quy Nhon City 8456, Vietnam
cMientrung Institute for Science Research - Vietnam Academy of Science and Technology, Hue City 8454,
Vietnam
dDepartment of Chemistry, Danang University of Science and Technology, Danang City 84511, Vietnam
eDepartment of Chemistry, Korea University, Seoul 136701, Korea
Tel: (+82)-2-3290-3143; Fax: (+82)-2-3290-3121; E-mail: jongskim@korea.ac.kr
Tel: (+84)-54-3826359; E-mail: duongtuanquang@dhsphue.edu.vn
Abstract
A fluorescent chemosensor based on dimethylaminocinnamaldehyde-aminothiourea (DA)
has been designed, synthesized, and applied with a combined theoretical and experimental
study. The synthetic path, optimized molecular structure and characteristics of DA were
carried out using the calculations at the B3LYP/LanL2DZ level of theory. The experimental
investigations have a good agreement with the theoretical results. DA can be used as a
chemosensor for simultaneous quantification of silver, copper, and mercury ions in aqueous
solution at the ppb level. The stable geometric structures of the complexes between DA and
Ag+, Hg2+, Cu2+ with stoichiometry of 1:1, 2:1, and 2:1, have been found and investigated for
electronic properties and fluorescence quenching phenomena by using Atoms in Molecules
(AIM) and Natural Bond Orbitals (NBO) analyses.
Key words: Chemosensor; Fluorescence; AIM analysis; NBO analysis; TD-DFT calculations.
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1. Introduction
Mercury, copper, and silver are three of the least abundant native metals in the earth’s crust
[1]. They often appear simultaneously in minerals containing some chalcogens such as sulfur,
tellurium, and selenium [2]. Mercury can cause erythrism, acrodynia, gingivitis, stomatitis,
digestive system disorders, neurological disorders, and total damage to the brain. It is also
associated with congenital malformation and spontaneous abortion [3]. Copper and silver are
highly toxic to microorganisms such as algae, bacteria, viruses, or larval forms of aquatic
animals. The high concentration of silver ions can have adverse effects on the environment
and is considered hazardous to human health [4]. An excessive intake of copper is thought to
be related to Wilson’s Disease, Infantile Cirrhosis, and disruption of the copper metabolism in
the human body. Some compounds of copper can cause dermal or eye irritation [5].
The development of methods to simultaneously determine the amount of copper, silver and
mercury has always been a challenge to scientists. For simultaneous determination of these
metals at the ppb level, several methods, such as anodic stripping voltammetry and solid
phase extraction combined with ICP-OES or CV-AAS method, have been used [6,7].
However, these methods often require expensive equipment and complicated techniques.
Fluorescence-based analytical methods are gaining popularity because they are sensitive,
simple and less expensive [8-10]. The fluorescent sensors for sensitive determination of
copper, silver and mercury ions are reported [11-16]. In design of fluorescent sensors for
these ions, aminothiourea is a preferred receptor because of its strong affinity [17-19].
It is very costly and time-consuming to perform experimental investigations in order to
design, synthesize, and examine the fluorescence properties of sensors. Currently,
computational chemistry has flourished and contributed significantly to our understanding of
chemical processes and mechanisms. A great deal of important chemicals have been
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synthesized with the aid of theoretical modeling. Many physical and chemical properties can
be predicted or elucidated from theoretical calculated results [20,21].
In this study, dimethylaminocinnamaldehyde-aminothiourea (DA) as a highly sensitive
fluorescent chemosensor for simultaneous determination of mercury, copper, and silver ions
in aqueous solution, has been designed, synthesized, and applied utilizing theoretical
calculations and experimental investigations. This sensor is potentially suitable for detecting
and monitoring these metal ions in drinking water samples according to the regulations of
WHO.
2. Materials and methods
2.1. Instruments
13C and 1H NMR spectra were obtained on a Bruker-400 instrument operating at 100 MHz
13
1
for C and 400MHz for H; mass spectra were acquired on a Finnigan 4021C and analyzed
using Bruker Daltonics FlexAnalysis software; elemental analysis was carried out on a Flash
EA 1112 instrument; absorption and fluorescence spectra were recorded on a Shimadzu UV-
1800 UV-vis spectrophotometer and a Shimadzu RF-5301 PC Series fluorescence
spectrometer, respectively. All theoretical calculations were carried out on a supercomputer
operating system with a 32-cores processor and 72-gigabytes memory at the laboratory of
computation science and modelling of Quy Nhon University, Vietnam.
2.2. Reagents
4-N,N-Dimethylaminocinnamaldehyde (DACA), aminothiourea, and all cations (Zn2+,
Cu2+, Cd2+, Pb2+, Ag+, Fe2+, Cr3+, Co2+, Ni2+, Ca2+, Ba2+, Al3+, Mg2+, Hg2+, K+, and Na+) were
obtained from Aldrich. Perchlorate counterion was used for all metals in the system.
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Tetrasodium ethylenediaminetetraacetate (Na4EDTA), Na2S2O3, and all other solvents were
purchased from Merck and used as received. Spectroscopic studies were performed using
HPLC reagent grade solvents without fluorescent impurities.
2.3. Computational methodology
Geometry optimizations and single point energy calculations of molecules were carried out
using the Gaussian 03 program [22]. The B3LYP density functional was applied using the
LanL2DZ basis set [21,23]. The harmonic vibrational frequency calculations were then
performed at the same level of theory to ensure that all optimized structures were energy
minima on the potential energy surface and to estimate the zero point energies (ZPE). The
interaction energies corrected for ZPE were derived as the difference between the total energy
of the individual components and the energy of each complex. AIM and NBO analyses offer
an effective method to evaluate electronic and chemical bonding properties of weak
interactions [24]. The AIM topological analysis [25] was performed with the AIM2000
software [26] to provide evidence of their presence and to further understand the nature of the
bonds in the compounds. In this method, the electron density (ρ(r)) is used to evaluate the
stability of a bond. Generally, the larger the value of ρ(r), the stronger the bond. The
2
characteristics of the bond are described by the Laplacian (ρ(r)). The bond is said to be
covalent if 2(ρ(r)) < 0; if 2(ρ(r)) > 0 the bond is either said to be ionic, or an interaction
dominated by van der Waals forces or hydrogen bonding. Diagonalization of the Hessian of
the electron density results in three eigenvalues, λ1<λ2<λ3. When one of them is positive,
and the other two are negative, there is a bond critical point (BCP) denoted as (3,-1). There is
a ring critical point (RCP) denoted as (3,+1) when one of them is negative, and the other two
are positive, indicating the existence of a ring structure [27]. NBO analysis [28] was
performed to gain insight into the electronic properties of the free DA and complexes. For
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each donor of natural bond orbital {NBO (i)} and acceptor of a natural bond orbital {NBO
(j)}, the electronic stabilization energy [E(2)], which is related to the delocalization between
the donor and acceptor, is calculated by the following equation:
E(2) = -qi x (Fi,j)2/(εj – εi )
where qi is the orbital occupancy, εi and εj are the diagonal elements, and Fi,j is the off-
diagonal NBO Fock matrix element. These calculations were executed using the NBO 3.1
program implemented in Gaussian 03 at the B3LYP/LanL2DZ level. The excitation energies,
oscillator strengths and molecular orbital plots were calculated using time-dependent density-
functional theory (TD-DFT) at the same level of theory [29].
3. Results and discussion
3.1. The design and synthesis of sensor
3.1.1. The theoretical investigations
Generally, a fluorescent sensor is composed of a fluorophore and a receptor. In addition,
there may be a spacer as a bridge between fluorophore and receptor (Fig. 1). Herein, DACA
was selected as a fluorophore. Aminothiourea and phenyl isothiocyanate were chosen as
receptors due to their strong affinity for Hg2+, Ag+ and Cu2+ ions.
The HOMO and LUMO energy values of the DACA and the receptors were calculated and
shown in Fig. S1 of the Supplementary data. The results indicate that the HOMO energy of
phenyl isothiocyanate (-6.44 eV) and aminothiourea (-5.43 eV) are less than the HOMO
energy of DACA (-5.42 eV). The LUMO energy of phenyl isothiocyanate (-1.56 eV) and
aminothiourea (0.11 eV) are more than the LUMO energy of DACA (-1.90 eV). Therefore,
both aminothiourea and phenyl isothiocyanate are satisfied as receptors for designing an ON-
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OFF fluorescent sensor. This is due to the PET process from the receptor to the fluorophore
not occurring in the obtained sensor. In this study, aminothiourea is selected as a receptor.
The DACA reacts with aminothiourea to form an imine [30]. There are four products that
can be formed from the reaction between DACA and aminothiourea (Fig. 2). The calculated
enthalpies and free energies of reactions to form the most stable structure in each product are
summarized in Table 1 (The geometry optimizations and cartesian coordinates for these
structures are shown in Fig. S2-S7 and Tables S1-S6 of the Supplementary data). The
calculation results showed that the reaction between DACA and aminothiourea to form
products (1) and (2) does not occur (the free energies were positive). This can be explained by
the fact that the lone pair electrons of the thiourea nitrogen (NH2-CS-) are associated with the
C=S bond, facilitating the transfer of charge density to the thiocarbonyl moiety. As a result,
the nucleophilic addition reaction to the electrophilic C of the C=O group does not occur.
Meanwhile, the lone pair electrons of the amino nitrogen (NH2-NH-) are not associated with
the C=S bond. The amino nitrogen is a nucleophile. The free energy (∆G298) of reaction to
form the most stable structure of group (3) and (4) is negative. The enthalpies (∆H298 ) and
free energy (∆G298) of reaction to form the most stable structure of group (3) are the most
negative. Accordingly, the most stable structure of product (3) is the preferred product of the
reaction between DACA and aminothiourea.
3.1.2. The experimental synthesis of sensor
DACA and aminothiourea were mixed in absolute ethanol. The reaction mixture was
heated and refluxed for 6 hours under a nitrogen atmosphere and then stirred for 2 more hours
at room temperature. The resulting precipitate was filtered and washed with ethanol several
times, and then recrystallized from absolute ethanol to obtain the final product DA with
1
83.0% yield. H NMR (DMSO-d6): δ 11.23 (s, 1H), 8.03 (s, 1H), 7.86 (d, 1H), 7.47 (s, 1H),
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7.38 (d, 2H), 6.70 (d, 1H), 6.63 (d, 2H), 6.59 (s, 1H), 2.96 (s, 6H). FAB-MS (M+H+):
m/z=250.1.
3.2. The characterization and application of sensor
3.2.1. The characterization of sensor
The optimized geometry of DA with the numbering scheme of the atoms is shown in Fig.
3(a). The molecular structure of DA obtained from the X-ray diffraction analysis is shown in
Fig. 3(b). The theoretical and experimental structural parameters such as bond lengths, bond
angles and dihedral angles of DA are shown in Table. S7 in Supplementary data.
The paired t-test method in the statistical test [31] was used for comparison between the
calculated values and experimental values. For the bond length, the calculated value of |t| of
0.001 is less than the critical value of |t| of 2.04 (f=n-1=32 degrees of freedom, P=0.05 [31]).
This indicates that there is no significant difference between the calculated and experimental
values for the bond length. Similarly, no significant difference from the calculated values of
bond angles is obtained. The calculated value of |t| of 0.008 is less than the critical value of |t|
of 2.01 (f=n-1=49 degrees of freedom, P=0.05 [31]). These results show that the experimental
results agreed well with the theoretical investigations.
The UV-Vis spectra, the electronic and fluorescent properties of DA were calculated using
the TD-DFT method at the same level optimized structure. Fig. 4 shows the theoretical UV–
Vis spectra of DA and DACA. The theoretical absorption bands of DA and DACA are
predicted at 395.2 and 350 nm, respectively. The calculated results indicate that the
fluorescent properties of DA will be similar with DACA fluorophore.
The electron properties of DA and DACA were also investigated. The main orbital
transitions were calculated using TD-DFT at the B3LYP/LanL2DZ level of theory. The
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results in Table 2 show that the singlet electronic transition of DA and DACA is mainly
contributed by the HOMO→LUMO (S0→S2) transitions with the S0→S2 transition energies
are 3.14 eV (395.15 nm) and 3.53 eV (350.76 nm). The calculated results indicate again that
the fluorescent properties of DA will be similar to DACA fluorophore.
As expected, the free DA exhibits a characteristic absorption band peaked at 390 nm and
gives rise to an extensively green emission at 510 nm in aqueous solution with a quantum
yield of 0.25 and exhibits a good stability under a wide span of pH values from 5 to 9 (Fig. 5).
The experimental study results agreed well with the theoretical investigations.
3.2.2. The application of sensor
Satisfying expectations in the design of fluorescent sensors for HTM ions, the fluorescence
intensity of DA was quenched when Ag+, Cu2+, or Hg2+ ions were added to the solution. In
contrast, no distinct change in the fluorescence spectra was observed when 5 equiv of various
competing metal ions, including Na+, K+, Ca2+, Ba2+, Mg2+, Pb2+, Cd2+, Co2+, Zn2+, Fe2+, Ni2+,
Al3+, and Cr3+ ions, were added to an aqueous solution of DA (Fig. 6).
The possibility of using DA to quantify one of the three ions (Ag+, Hg2+, Cu2+) in presence
of other two ions in an aqueous solution was also investigated. [Hg(S2O3)4]6- and [CuEDTA]2-
are very stable complexes with formation constants of 1033.24 and 1018.70, respectively [32].
Na2S2O3 and Na4EDTA were chosen to form complexes with Hg2+ and Cu2+ ions. Fig. 7
shows that the change in fluorescence intensity of DA by Hg2+ and Cu2+ can be prevented if
10 equiv of Na2S2O3 and 10 equiv of EDTA were added to the solution (against respective
metal ions).
Fig. 8 shows that it is possible to quantify Ag+ in a mixture of metal ions (Ag+, Hg2+, Cu2+)
based on the variation of fluorescence intensity between the free DA solution ((1) solution)
and the DA solution containing the metal ions with Na2S2O3 (100 µM) and EDTA (100 µM)
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((3) solution). Following that the Hg2+ ions concentration can be quantified based on the
variation of fluorescence intensity between the (3) solution and the solution with the removal
of Na2S2O3 (100 µM) ((5) solution). Finally, the Cu2+ ion concentration can be quantified
based on the variation of fluorescence intensity between the (3) solution and the solution with
the removal of EDTA (100 µM) ((7) solution).
As the results, the concentrations of individual ions were quantified from the variation of
fluorescence intensity of DA solution. The amount of silver ions was observed by using both
Na2S2O3 and EDTA to prevent the reaction of Hg2+ and Cu2+ with DA. The amount of
mercury or copper ions was then observed by removing Na2S2O3 or EDTA, respectively.
The change in fluorescence intensity of the chemosensor against pH was also investigated.
The results indicate that the ideal pH range for Ag+, Hg2+, and Cu2+ detection is between 5
and 9 (Fig. S8). The broad pH range makes DA useful for a variety of applications, including
the detection of heavy metal ions in wastewater, industrial trade analysis, and physiological
processes.
There are good linear relationships between the variation of fluorescence intensity of DA
and Ag+, Hg2+ or Cu2+ concentration. The correlation coefficients of the linear regression are
0.999 (Fig. S9). The following equations were developed from calibration curves: ∆I510= (3.1
± 2.1) + (3.4 ± 0.0) × [Ag+], ∆I510= (-6.6 ± 3.2) + (2.6 ± 0.0) × [Hg2+], and ∆I510= (-9.7 ± 2.6)
+ (10.5 ± 0.1) × [Cu2+]. The limits of detection for Hg2+, Cu2+, and Ag+ ions are 2.8, 0.8, and
1.0 ppb, respectively.
3.3. The structures of the complexes between the DA with metal ions
3.3.1. Stable geometry and interaction energies
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The fluorescence titration spectra of DA upon the addition of Ag+, Hg2+, and Cu2+ ions
show that the fluorescence intensity of DA was gradually quenched by increasing the
concentration of the metal ions. Emission was almost completely quenched upon addition of 1
equiv of Ag+, 0.5 equiv of Hg2+ and 0.5 equiv of Cu2+. These results indicate that Ag+, Hg2+,
and Cu2+ reversibly binds to DA to form complexes with stoichiometry of 1:1, 1:2, and 1:2,
respectively.
The two stable configurations (denoted hereafter as S1 and S2) of the 1:1 interaction
between DA and Ag+, one stable configuration (S3) of the 2:1 interaction between DA and
Hg2+, and one stable configuration (S4) of the 2:1 interaction between DA and Cu2+, are
presented in Fig. S10 of the Supplementary data. All stable structures correspond to minima
on the potential energy surface (PES) (Cartesian coordinates for these structures are
summarized in Tables S8-S11 of the Supplementary data).
The Ag N contact distance in S1 is 2.291 Å, slightly longer than in S2 (2.174 Å). The
Ag N contact distances in S1 and S2 are significantly smaller than the sum of the van der
Waals atomic radii of Ag and N (1.720 and 1.550 Å, respectively). The Ag S contact
distances in S1 and S2 are 2.509 and 2.502 Å, respectively. These distances are also
significantly smaller than the sum of the van der Waals atomic radii of Ag and S (1.720 and
1.850 Å, respectively). The distance between the two Ag+ ions was also examined. The
Ag Ag distance in S1 is 3.095 Å, smaller than the sum of the atomic van der Waals radii
(3.440 Å), while the Ag Ag distance in S2 is significantly longer (4.625 Å). It should be
noted that complex formation for both S1 and S2 is energetically favorable. The enthalpy of
complex formation [see reaction (5)] indicates that Ag+ binding is exothermic for both S1 and
S2, with ∆H298 values of -132.4 and -148.5 kcal mol-1, respectively. From this, it can be
concluded that the S2 complex is more stable than the S1 complex.
2Ag+ + 2DA → Ag2DA2 (5)
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∆H298 (S1) = -132.4 kcal mol-1
with
∆H298 (S2) = -148.5 kcal mol-1
The Hg S distance in S3 is 2.740 Å, smaller than the sum of the van der Waals atomic
radii of Hg and S (1.700 and 1.850 Å, respectively), while the Hg N distance in S3 is 4.212
Å, significantly longer than the sum of the van der Waals atomic radii of Hg and N (1.700 and
1.550 Å, respectively). In contrast, the Cu N distance in S4 is 1.977 Å, smaller than the sum
of the van der Waals atomic radii of Cu and N (1.400 and 1.550 Å, respectively), while the
Cu S distance in S4 is 3.450 Å, significantly longer than the sum of the van der Waals
atomic radii of Cu and S (1.400 and 1.850 Å, respectively). The complex formation for both
S3 and S4 is also energetically favorable. The enthalpy of complex formation (∆H298) for S3
and S4, are -319.15 and -405.92 kcal mol-1, respectively.
3.3.2. AIM analysis
The calculated values of ρ
(r) and 2(ρ(r)) at the BCPs of the S1, S2, S3 and S4 complexes
are listed in Table.3. The topological properties of the BCPs are summarized in Fig. S11-S14
of the Supplementary data.
The results indicate the presence of intermolecular interactions in each of the Ag N,
Ag S, Hg S, and Cu N contacts in the S1, S2, S3 and S4 complexes. All values of 2(ρ(r))
at the BCPs for the Ag N, Ag S, Hg S, and Cu N contacts in these complexes are
positive. In addition, the Ag N, Ag S, Hg S, and Cu N contact distances are significantly
smaller than the sum of van der Waals radii of relevant atoms (Ag and S, Ag and N, Hg and
S, Cu and N). Thus, all these interactions are considered as ionic. There are not any BCPs
between Hg and N atom in the S3 complex, or Cu and S atom in the S4 complex. These
results indicate that the Hg N and Cu S bonds are not formed in the S3 and S4 complexes.
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The values of ρ(r) for the Ag S (0.058 au) and Ag N (0.078 au) contacts within S2 are
larger than those of S1 (0.057 au for Ag S and 0.062 au for Ag N). This confirms that the
metal-ligand interaction of S2 is more stable than that of S1. However, there is a BCP for the
Ag Ag contact in the S1 complex that is not present in the S2 complex, suggesting that the
Ag Ag intermolecular interaction plays a negligible role in stabilizing the S1 complex. These
results are also in good agreement with the ∆H298 values calculated for reaction (1). Several
RCPs occur in both the S1 and S2 complexes, indicating that there is a ring structure present
in each complex which is shown in Fig. S10.
3.3.3. NBO analysis
NBO analysis provides more thorough insight into the bond properties and intermolecular
orbital interaction, which aims to explain the fluorescent properties of DA, S1, S2, S3, and
S4. The calculated results are tabulated in Table.4.
Obtained results suggest that the π-electron conjugated system of DA extends throughout
the fluorophore, from the benzene moiety to N13 (see Fig. 3 for numbering scheme), as
evidenced by the existence of double bonds (σ and π bonds) with significantly large degree of
interaction energies (E(2)), including C1=C2, C3=C4, C5=C6, C10=C11, and C12=N13
double bonds. This system also receives electron density from the lone pair of the N7 atom.
The E(2) value for the electron transfer from LP(1)N7 to π*(C3-C4) is 45.27 kcal mol-1. PET
process from the lone pairs of the N31 and N14 atoms of the aminothiourea moiety to the
DACA moiety was not observed. Instead, the electron transfer processes are from LP(1)N31
to σ*(C15-S30) and LP(1)N14 to σ*(C15-S30). The hyper-conjugation energies for these
interactions are 84.26 and 70.25 kcal mol-1, respectively. These findings confirm that DA is a
fluorescent compound with properties similar to DACA.
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According to calculations, the charge values of Ag atoms in S1, S2, Hg atom in S3, and Cu
atom in S4 are +0.615, +0.629, +0.815, and +0.754 e, respectively. The charge values of DA
ligands in S1, S2, S3, and S4 are +0.385, +0.371, +0.593, and +0.623 e, respectively. The
charge of C10 atoms in S1, S2, S3 and S4 are significantly more positive than in free DA.
Therefore, the strong electron density transfer from the LP(1)C6 to the π*(C10–C11) orbitals
was observed in S1, S2, S3 and S4, with associated hyper-conjugation energy of 83.78, 97.05,
71.76, and 25.97 kcal.mol-1, respectively. As a result, the π-bond conjugated system of
benzene rings is broken at C6 atom. This is the most important cause leading to fluorescence
quenching of S1, S2, S3 and S4. In addition, a deficiency of electron on atoms C10 also
resulted in the strong electron density transfer from N7 atom to benzene ring. The double
bond was observed between N7 and C3 atom in S2 and S4. The strong interactions from a
lone pair orbital of the N7 atom, LP(1) N7 to LP*(1)C3 orbitals were also found in S1 and S3
with associated hyper conjugation energies of 130.61 and 128.46 kcal.mol-1, respectively.
These results indicate that the interaction from a lone pair orbital on the N7 atoms to the π-
bond conjugated system, as a interaction between donor and acceptor in fluorophore does not
occur in S1, S2, S3 and S4. This is another important cause leading to fluorescence
quenching.
In S1 and S2, the binding between DA ligands and metal ions is stabilized by the donation
of electron density from a lone pair on both S and N atoms. In contrast to this, the binding is
stabilized by the donation of electron density only from S atoms for S3, or only from N atoms
for S4. The hyper-conjugation energies of the LP(n)S→ LP*(n)Ag and the LP(n)N→
LP*(n)Ag interactions in S2 are 74.76 and 39.22 kcal.mol-1, respectively, significantly higher
than those in S1 (67.60 and 32.15 kcal.mol-1, respectively), again suggesting that the S2
complex is more stable than S1. The hyper-conjugation energies of the LP(n)S→ LP*(n)Hg
and the LP(n)N→ LP*(n)Cu interactions in S3 and S4 are 54.57 and 28.12kcal.mol-1
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respectively, significantly smaller than the LP(n)S→ LP*(n)Ag and LP(n)N→ LP*(n)Ag in
S1 and S2.
4. Conclusions
A new fluorescent chemosensor for simultaneous quantification of silver, copper, and
mercury ions in an aqueous solution has been been designed, synthesized, and applied with a
combined theoretical and experimental investigation. The first studies by quantum chemical
calculations showed that this sensor can be thermodynamically favorable synthesized. The
characteristics and capabilities of sensor applications have also been predicted by quantum
chemical calculations. The experimental investigations are also in a good agreement with
theoretical results.
This system for fluorescent detection of Hg2+, Cu2+, and Ag+ has a number of advantages.
It is highly selective for Hg2+, Cu2+, and Ag+ ions over other competitive ions with high
sensitivity (detection limits = 2.8, 0.8, and 1.0 ppb, respectively) and is useful over a wide
range of pH levels (from 5 to 9), and it requires only a small percentage of organic solvent in
the solution (1:9 EtOH:H2O). Moreover, this system is simple, which offers rapid analysis
time, and inexpensive, making it a promising tool for practical quantification. Especially, it is
potentially suitable for detecting and monitoring these metal ions in drinking water samples
according to the regulations of WHO.
The stable geometric structures of the complexes between DA and Ag+, Hg2+, Cu2+ ions
with stoichiometry of 1:1, 2:1, and 2:1, have been found and investigated for the electronic
properties, the chemical bonding and intermolecular interactions by using the Atoms in
Molecules (AIM) theory and Natural Bond Orbitals (NBO) approach. The results indicate that
the fluorescence quenching of DA upon complexation can be attributed to a transfer of
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electron density from DA to metal ions, which breaks the π-bond conjugated system of the
fluorophore in all complexes.
Acknowledgments
This research is funded by the Vietnam National Foundation for Science and Technology
Development (NAFOSTED) under grant number 104.07-2012.31 (DTQ). This work was also
supported by the Creative Research Initiative program (No. 2009-0081566) of National
Research Foundation of Korea (JSK).
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Figure Captions
Fig. 1. The operation mechanism of the ON-OFF fluorescent sensor type: Molecular orbital energy
diagrams which show the relative energetic dispositions of the frontier orbitals of the fluorophore and
the receptor in (a) the analyte-free situation and (b) the analyte-bound situation.
Fig. 2. The four possible products from the reaction between DACA and aminothiourea.
Fig. 3. The molecular structure of DA: (a) The optimized geometry of DA at the B3LYP/LanL2DZ
level of theory; (b) The molecular structure of DA obtained from the X-ray diffraction analysis
Fig. 4. The UV-Vis spectra of DA (a) and DACA (b) at the B3LYP/LanL2DZ level of theory.
Fig. 5. UV-Vis (DA 15 µM) and fluorescent (DA 3 µM, λex= 390 nm) spectra of DA in C2H5OH/H2O
(1/9, v/v) at pH ~7.
Fig. 6. Changes in fluorescence intensity of DA (3 µM) in EtOH/H2O (1:9, v/v) at pH 7 upon the
addition of different metal ions including Ag+, Hg2+, Cu2+, Pb2+, Cd2+, Cr3+, Zn2+, Fe2+, Co2+, Ni2+,
Ba2+, Al3+, Ca2+, Mg2+, K+ and Na+ ions (10 µM each) with an excitation wavelength of 390 nm.
Fig. 7. Changes in fluorescence intensity of DA (3 µM) in EtOH/H2O (1:9, v/v) at pH 7 upon addition
of Cu2+ and EDTA (a), Hg2+ and Na2S2O3 (b), with an excitation wavelength of 390 nm.
Fig. 8. The possibility of identifying individual ions in the mixture: (1) DA; (2) DA + Ag+ + Hg2+
+
Cu2+; (3) DA + Ag+ + Hg2+ + Na2S2O3 + Cu2+ + EDTA; (4) DA + Ag+; (5) DA + Ag+ + Hg2+ + Cu2+ +
EDTA; (6) DA + Ag+ + Hg2+; (7) DA + Ag+ + Hg2+ + Na2S2O3 + Cu2+; (8) DA + Ag+ + Cu2+; (9) DA +
Hg2+; (10) DA + Cu2+ (the concentrations of DA, metal ions, Na2S2O3, and EDTA: 3 µM, 0.6 µM, 100
µM, and 100 µM, respectively).
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Table Captions
Table 1. The enthalpies and free energies of reactions to form the most stable structure of each
products at the B3LYP/LanL2DZ level of theory (kcal.mol-1)
.
Table 2. Calculated excitation energy (E), wavelength (λ), and oscillator strength (f) for low-laying
singlet state of DA and DACA
Table 3. Electron density (ρ(r), in au) and the Laplacian ( 2(ρ(r)), in au) at the B3LYP/LanL2DZ
level of theory
Table 4. Significant second-order interaction energies (E(2), in kcal mol-1) between donor and acceptor
orbitals in DA, S1, and S2. All values were calculated using the B3LYP/LanL2DZ level of theory.
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Fig. 1.
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Fig. 2.
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(a)
(b)
Fig. 3.
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Fig. 4.
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UV-Vis spectra
Fluorescent spectra
1000
800
600
400
200
0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
250
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 5.
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DA + Pb2+, Cd2+, Cr3+
1000
900
800
700
600
500
400
300
200
100
0
DA
Zn2+, Fe2+, Co2+
Ni2+, Ba2+, Al3+
Ca2+, Mg2+
K+, Na+
DA + Ag+, Hg2+, Cu2+
440 460 480 500 520 540 560 580 600 620 640
Wavelength(nm)
Fig. 6.
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D A
900
800
700
600
500
400
300
200
100
0
(a)
D A + C u2+ + E D T A
E D T A : 2-10 eq uiv
D A + A g++ 10 equ iv E D T A
440 460 480 500 520 540 560 580 600 620 640
W avelength (nm )
(b)
900
800
700
600
500
400
300
200
100
0
D A
N a2S 2O 3: 2-10 equiv
D A + H g2+ + N a2S 2O 3
D A + A g+
+ 10 equiv N a2S 2O 3
440 460 480 500 520 540 560 580 600 620 640
W avelength (nm )
Fig. 7.
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(1 )
9 0 0
8 0 0
7 0 0
6 0 0
5 0 0
4 0 0
3 0 0
2 0 0
1 0 0
0
(4 )
(9 )
(1 0 )
(3 )
(6 )
(5 )
(7 )
(8 )
(2 )
4 4 0 4 6 0 4 8 0 5 0 0 5 2 0 5 4 0 5 6 0 5 8 0 6 0 0 6 2 0 6 4 0
W a v e le n g th (n m )
Fig. 8.
28