Evaluation of N-Alkyl-bis-o-aminobenzamide receptors for the determination and separation of metal ions by fluorescence, UV-visible spectrometry and zeta potential

Article abstract of DOI:10.3390/molecules24091737

This paper presents the synthesis and evaluation of physicochemical behavior of a new series of N-alkyl-bis-o-aminobenzamides (BOABs) in aqueous solution. The study was targeted to the complexing capacity of five metal ions (Fe²⁺, Cu²⁺

Full text of DOI:10.3390/molecules24091737

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molecules
Article
Evaluation of N-Alkyl-bis-o-aminobenzamide
Receptors for the Determination and Separation of
Metal Ions by Fluorescence, UV-Visible Spectrometry
and Zeta Potential
Marisela Martinez-Quiroz 1,2,
* ,
, Xiomara E. Aguilar-Martinez 2
Mercedes T. Oropeza-Guzman 2, Ricardo Valdez 3 and Eduardo A. Lopez-Maldonado 4,
*
1
CETYS Universidad, Centro de Innovación y Diseño, Escuela de Ingenieria Av. CETYS Universidad No. 4
Fracc. El Lago, Tijuana, B.C. CP 22210, México
Tecnologico Nacional de México, Instituto Tecnológico de Tijuana, Blvd. Alberto Limón Padilla s/n,
Mesa de Otay, Tijuana, B.C. CP 22500, México; xiomaraeam@gmail.com (X.E.A.-M.);
oropeza1@yahoo.com (M.T.O.-G.)
Centro de Nanociencias y Nanotecnología CNyN-UNAM, Km 107 Carretera Tijuana-Ensenada, Ensenada,
B.C. CP 22860, México; ricardovc1030@gmail.com
Facultad de Ciencias Químicas e Ingeniería, Universidad Autónoma de Baja California, Tijuana,
B.C. CP 22390, México
2
3
4
*
Correspondence: marisela.martinez@cetys.mx (M.M.-Q.); elopez92@uabc.edu.mx (E.A.L.-M);
Tel: +52-(664)-90318-33 (M.M.-Q.); +52-(664)-979-7500 (E.A.L.-M.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 20 March 2019; Accepted: 13 April 2019; Published: 4 May 2019
Abstract: This paper presents the synthesis and evaluation of physicochemical behavior of a new
series of N-alkyl-bis-o-aminobenzamides (BOABs) in aqueous solution. The study was targeted to the
complexing capacity of five metal ions (Fe2+, Cu2+, Cd2+, Hg2+ and Pb2+) of environmental concern
as the medullar principle of a liquid phase sensor for its application in the determination of these
metal ions due to its versatility of use. Molecular fluorescence, UV-visible and Zeta potential were
measured for five BOABs and the effect of alkyl groups with different central chain length (n = 3,
4, 6, 8 and 10) on physicochemical performance determined. The results have shown that these
derivatives present higher sensibility and selectivity for Cu2+ even in the presence of the other metal
ions. An additional application test was done adding a pectin (0.1 wt %) solution to the BOAB-Cu+2
complex to obtain a precipitate (flocs) as a potential selective separation process of Cu from aqueous
solution. The solid was then lyophilized and analyzed by SEM-EDS, the images showed spheric
forms containing Cu+2 with diameter of approximately of 8 µm and 30 wt %.
Keywords: zeta potential; aminobenzamides; metal titration; fluorescent probes
1. Introduction
The first fluorescent probe was reported in 1987 based on a sulfonamide compound [
sensors for cations of biological or environmental importance using fluorescent probes followed this
work [ ]. These sensors are typically composed of a spacer and a fluorescent compound, and there are
several reported structures [ ]. In recent years, the development of sensors has been expanded for
1]. Different
2
3–5
its broad field of action. Recently, the design and development of selective and sensitive fluorescent
molecules capable of detecting transition and metal ions have attracted considerable attention because
of their use and subsequent impact on the environment and nature [6]. A large number of receptors
has been study for the detection of metal ions. The basis of molecular recognition lies in the interaction
Molecules 2019, 24, 1737; doi:10.3390/molecules24091737
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Molecules 2019, 24, 1737
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of the metal-ligand, which is the weak binding of metal ions to one or more heteroatoms (nitrogen or
oxygen) within the ligand and intramolecular energy transfer from the ligand to the metal ion [6–8].
It is known that bis-o-aminobenzamides has been a subject of study in the recognition of metal
ions in the solid phase in previous cases and the synthesis on Merrifield’s resin and the addition of
different ions obtained good results for Mg2+
characterization of this type of compound [10 11]. Additionally, aminobenzamide derivatives are used
[9]. Other studies show only the synthesis routes and
,
as pharmacological inhibitors on Parkinson’s disease [12], human cancer cells [13], and neurobehavioral
and neurochemical anomalies [14]. The development of fluorescent sensors has received appreciable
attention for detecting biologically important ions, either cations or anions [15].
There are no reports of the use of the bis-o-aminobenzamides derivatives on the determination
and separation in solution of metal ions for environmental applications, only for biological applications.
Hence the importance of the implementation of new bis-o-aminobenzamides compounds capable of
being used in the detection and separation of metal ions in water treatment. It is expected that the use
of these materials provide different properties as chemosensors and the recognition of the metal ions
with advantages for the development of new materials with chelant-coagulant-flocculants properties,
considering the effects and influences observed on diverse spectral changes due to the photophysical
characteristics of these derivatives.
In this paper, the physicochemical behavior of a new series of N-alkyl-bis-o-aminobenzamides
with different central chain lengths were studied in aqueous solution and their complexing capacity
with different metal ions (Fe2+, Cu2+, Cd2+, Hg2+ and Pb2+) of environmental importance evaluated.
The techniques of fluorescence, UV-Vis and zeta potential allowed to systematically complement the
physicochemical evaluation of the BOABs and determine their strategic application.
2. Results and Discussion
2.1. Fluorescence and UV-Vis Properties of Compounds 1a–5a
The fluorescence spectra were obtained for derivatives 1a–5a in order to evaluate how the central
chain length influences the spectral characteristics of these compounds. The excitation and emission
fluorescence spectra of the different derivatives in aqueous solution are presented in Figure 1.
80
1a
2a
60
3a
4a
5a
40
20
0
300
400
500
Wavelength (nm)
Figure 1. Fluorescence spectra of N-alkyl-bis-o-aminobenzamide 1a
pH = 5.4.
–
5a [1
×
10−5 M] in water at
The excitation and emission maximum wavelengths (
λ
exc
= 312 nm,
λ
em
= 413 nm) regardless of
the alkyl chain length) all derivatives showed the same fluorescence peak position. The compound
showing a higher fluorescence intensity is 3a which has a central alkyl chain length of six carbons,

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due to the fact the length of the chain increases the interaction of the molecule with the medium and
the internal conversion velocity increases, there decreasing the fluorescence for the derivatives 4a and
5a, favoring the conformational effect of the more stable complexes, while in the other compounds
it does not appear due to the mobility of the molecule [16–18]. Substituents have large effects on
fluorescence that help to delocalize electrons, such as –NH2 groups, that increase the fluorescence
π
because they tend to increase the probability of transitions between the lowest singlet state and the
base state. Molecular rigidity reduces the interactions of a molecule with its surrounding environment
and, consequently, reduces the speed of deactivation by collisions (external conversion) in the series
of compounds, those that are flatter, more rigid and more hindered, are the most fluorescent. The
derivatives showed the following order 3a
> 1a > 2a > 4a > 5a according to characteristics and structural
changes reflected by fluorescence intensity.
The luminescence properties of N-alkyl-bis-o-aminobenzamide in solution showed similarity to
reported studies in solid phase and the ability to delocalize charge over the carbonyl-benzyl-amine
group [4]. However, in this study it is presented that these BOABs derivatives in aqueous solution
have the same fluorescence response without being supported on the resin. This gives it an advantage
for the application as a sensor in the detection of metal ions and as separation agents in wastewater
treatment. As shown in Figure 2 the fluorescence of these derivatives is sensitive to pH changes. It can
be observed that the fluorescence increases with increasing the pH, this behavior is attributed to the
pka values of the derivatives, which at pH > pka the characteristic functional groups ionize and favor
charge transfer (see Table 1). These derivatives have a negative surface charge at pH > IEP (isoelectric
point) and positive charge at pH < IEP and different limit of
ζ values. On the other hand, the pKa
values for derivatives 1a–5a were calculated using Excel Worksheets for Spectrometry [19].
140
120
100
1a
2a
3a
4a
5a
80
60
40
20
0
2
4
6
8
10
pH
Figure 2. Effect of pH of N-alkyl-bis-o-aminobenzamide 1a–5a at maximum emission [1 × 10−5 M].
Table 1. Isoelectric point and pka’s of bis-o-aminobenzamides.
Compound
IEP
pka Fluorescence Values
1a
2a
3a
4a
5a
6
6
6
7
7
1.83
2.69
2.60
3.32
2.68
In Table 2, the photophysical parameter of the derivatives are shown. The quantum yield or the
quantum efficiency of fluorescence is the ratio between the numbers of molecules that emit radiation
with respect to the total number of molecules excited. Under certain conditions, for highly fluorescent

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molecules the quantum yield approaches unity, while in non-fluorescent molecules it is practically
zero. The quantum yields (ΦF) in water at 1 × 10−5 M were determined following Equation (1):
Int AR n2
ΦF = ΦR X
(1)
IntRAn2
R
where
ΦF is the compound quantum yield, Int is the area under the emission peak, A is the absorbance
at the excitation wavelength, and n is the refractive index of the sample. The subscript R denotes the
respective values of the reference substance. The derivative 3a is the one that had the highest quantum
yield (
ΦF = 0.0315), which enhances its functionality as a fluorescent sensor, unlike the derivative 4a
that had the lowest (ΦF = 0.0015).
Table 2. Photophysical characteristics of 1a–5a derivative.
Derivative/Parameter (nm)
λA
λ
λA − λ
Φ
F
F
F
1a
2a
3a
4a
5a
314
314
312
314
316
416
416
415
416
418
102
102
103
102
102
0.0026
0.0023
0.0315
0.0015
0.0049
The UV-Vis spectra of the derivatives were obtained to complement and corroborate the
photophysical properties determined by the fluorescence studies at the same pH value. In Figure 3 the
absorbance spectra obtained are shown, where the derivatives show the order 3a
> 1a > 2a > 4a > 5a
according to characteristics and structural changes by UV-Vis.
0.12
0.10
0.08
1a
2a
3a
4a
5a
0.06
0.04
0.02
0.00
250
300
350
Wavelength (nm)
Figure 3. Absorbance spectra of N-alkyl-bis-o-aminobenzamides 1a
pH = 5.4.
–
5a [1
×
10−5 M] in water at
2.2. ζ = f (pH) Profiles of the Compounds 1a–5a
The effect of pH on the zeta potential of compounds was studied in a pH range 2–12. Figure 4
shows two groups of zeta potential responses for the derivatives 1a 3a and 4a 5a. The first group
–
–
have a positive charge and the IEP (isoelectric point) at pH~ 6, while the second group have a slightly
positive charge and the IEP showed at pH~7. The change is observed in the pH value of the isoelectric
point (IEP) depending of the long of the central chain. Table 1 shows IEPs for all derivative suggesting
a clear influence of the central alkylic chain length. These derivatives have a negative surface charge
at pH > IEP and positive charge at pH < IEP and different limit of
ζ values. This characteristic