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A Schiff base derivative used as sensor of copper through colorimetric and surface plasmon resonance techniques
Accepted Manuscript Title: A Schiff base derivative used as sensor of copper through colorimetric and surface plasmon resonance techniques Author: D. Peralta-Dom´ınguez M. Rodriguez G. Ramos-Ortiz J.L. Maldonado D. Luna-Moreno M. Ortiz-Gutierrez V. Barba PII: DOI: Reference:
S0925-4005(15)30596-7 http://dx.doi.org/doi:10.1016/j.snb.2015.11.013 SNB 19271
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
18-6-2015 30-10-2015 3-11-2015
Please cite this article as: D. Peralta-Dom´inguez, M. Rodriguez, G. RamosOrtiz, J.L. Maldonado, D. Luna-Moreno, M. Ortiz-Gutierrez, V. Barba, A Schiff base derivative used as sensor of copper through colorimetric and surface plasmon resonance techniques, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.11.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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HIGHLINES Low detection limit for copper of 1.25 × 10-7 M in aqueous media Low colorimetric (naked eye) detection limit of 2 × 10-6 M in aqueous media Sensing copper ions through the use of surface plasmon resonance (SPR) technique
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A Schiff base derivative used as sensor of copper through colorimetric and surface plasmon resonance techniques
Centro de Investigaciones en Óptica A.P. 1-948, 37000 León, Gto., México; b Facultad de Cs. Físico-matemáticas, UMSNH, Morelia Mich. México; c Centro de Investigaciones Químicas, UAM, Cuernavaca Morelos, México. * [email protected], [email protected]
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ABSTRACT
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D. Peralta-Domíngueza, M. Rodrigueza,*, G. Ramos-Ortiza,*, J. L. Maldonadoa, D. Luna-Morenoa, M. Ortiz-Gutierrezb, V. Barbac
Organic molecular sensors have the advantage of being used through easy,
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fast, economical and reliable optical methods for detecting toxic metal ions in the environment. In this work, we present a simple but highly specific organic ligand 5-
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Chloro-2-[(1E,2E)-3-(4-(dimethylamino)phenyl)allylidene)amino)]phenol
(S1)
that
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works as a colorimetric sensor of copper ions in aqueous solutions. Binding
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interaction between S1 and various metal ions was established by UV-Vis spectroscopic measurements showing favorable coordination toward Cu2+ and practically no interference with the presence of other metal ions, i.e., Cd2+, Co2+, Cr2+, Fe3+, Mg2+, Ni2+, Hg2+, Pb2+, Mn2+, and Zn2+. S1 exhibited binding-induced color changes from yellow to pink with a detection limit of 1.25 × 10-7 M measured by spectroscopic methods, while colorimetric changes could be observed at naked eye for concentrations as low as 2 × 10-6 M. Furthermore, we also demonstrated that S1 can be useful for sensing copper ions through the use of surface plasmon resonance
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(SPR) technique. This technique allowed detecting the presence of Cu2+ in aqueous solutions at the concentration level of ca 1.5 × 10-6 M due to changes in the refractive index.
1. INTRODUCTION
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Keywords: colorimetric chemosensors, copper (II), surface plasmon resonance.
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Recently, the design and synthesis of intelligent organic molecules to be used
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as chemosensors with high selectivity and sensitivity toward metals has attracted much attention [1-4]. These sensors exploit the colorimetric and fluorescent
environmental sciences [5-6].
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properties of organic molecules and find applications in analytical, biomedical and
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Colorimetric sensors have the ability to produce red- or blue-shifts on their
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absorption bands such that their characteristic color (or absence of color) is modified when they detect specifically any metal ion of interest. The molecular structure of
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these sensors commonly comprise a chromophore and a receptor, which are linked directly or through a π-conjugated chain [7]. An effective colorimetric sensor must convert the event of ion recognition by the receptor into an easily monitored and highly sensitive optical signal (strong change in the UV-Vis absorption band) from the chromophore.
Copper is the third most abundant (after iron and zinc) transition metal in earth. Copper is an essential micronutrient and together with certain proteins produces
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nearly 20 enzymes critical for life [7-10]; it also plays important roles in bone and tissue formation, cellular respiration, immune and brain functions, and gene transcription [11-14]. However, the presence of copper at high concentrations (as a
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consequence of its widespread use for industrial purposes) can produce harmful effects to living organisms due to oxidative stress and the subsequent damage to
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tissue and some organs (kidney and liver). For instance, high concentrations of copper in the human body are associated to Alzheimer, Parkinson, hypoglycemia,
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dyslexia, Menkes and Wilson diseases [11, 15-16]. The normal concentration of
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copper in human blood is limited to 150 µg/dL (23.6 µM) [10, 17]. For natural resources like drinking water, the limit has been established in the range of 15-30 µM
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(1.3 ppm) [18-19]. There are many techniques for detecting Cu2+, i.e., anodic stripping voltammetry, plasma-atomic emission spectroscopy, gravimetric, chromatography,
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ion selective electrodes, etc., but those methods are usually complicated and time-
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consuming, need relatively high cost apparatus, and are not easily adaptable for online monitoring [20-21]. Therefore, the rational design and synthesis of efficient
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chemosensors to easily recognize and quantify copper ions is an important topic in medicine and environmental sciences. The aim of this report is to present the synthesis and evaluation of a novel and
reliable colorimetric chemosensor based in a Schiff-base derivative (S1) for the recognition of copper in aqueous media. This work was motivated by the fact that the majority of organic molecules reported as chemosensors of Cu2+ perform the sensing process in solutions of solvents like acetonitrile (MeCN) [19, 22-26], methanol (MeOH) [9, 27],
Tetrahydrofuran (THF) [28], chloroform (CHCl3) [9], dimethyl
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sulfoxide (DMSO) [29], and others. However, the current interest for biomedical and environmental applications relies in those sensors able to identify Cu2+ in aqueous solutions. Only few Cu2+ chemosensors have been demonstrated to perform sensing
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in aqueous solutions, i.e., mixtures of water with solvents like MeCN [7, 30-34], THF [35], DMSO [17-18, 36-38], ethanol [12, 39], MeOH [5], and ammonia [40] or just in The sensing characteristics of S1 were investigated using UV-Vis
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water [41].
absorption spectroscopy upon the presence of biologically and environmentally
Ours results demonstrated that S1 can be employed for sensing the
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and Zn2+.
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relevant metal ions such as Cd2+, Co2+, Cr2+, Fe3+, Mg2+, Ni2+, Hg2+, Pb2+, Mn2+, Cu2+,
presence of Cu2+ in aqueous media with a spectroscopic detection limit of 1.25 × 10-7
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M. This concentration is among the lowest limits of detection reported for copper in aqueous media. The use of a Schiff base like S1 as chemosensor is attractive
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because of its easy synthesis [27, 42] and its inherent fast and economical
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production. The sensing properties of S1 were also demonstrated using surface plasmon resonance (SPR) technique to monitor the presence of Cu2+ in aqueous
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media with a detection limit in the range of parts per million (1.5 × 10-6 M).
2. EXPERIMENTAL SECTION
2.1 Materials and instrumentation All starting materials for the synthesis of S1 and solvents involved in the
spectroscopic studies were purchased from Sigma-Aldrich distributor and were employed without further purification treatment. NMR experiments were recorded in a
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Jeol Fx 270 spectrometer. Infrared spectrum was measured on a FTIR Varian spectrophotometer with an ATR accessory while UV–Vis absorption spectra were measured using a spectrophotometer (Lambda 900, Perkin-Elmer). Melting point
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(uncorrected data) of S1 was obtained using Electrothermal 9200 apparatus. X-ray
2.2 Synthesis of Schiff base compound (S1)
5-Chloro-2-[(1E,2E)-3-(4-(dimethylamino)phenyl)allyidene)amino]phenol
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Compound
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diffraction analysis was performed by using Bruker Smart Apex diffractometer.
(S1) was prepared using a typical condensation reaction method of imine derivatives.
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S1 was synthesized from the reaction of 4-chloro-2-aminophenol with 4dimethylaminocinnamaldehyde in methanol with catalytic amount of acetic acid. The
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product was obtained as a brown solid with a yield of 85% M.P 156-157°C. Scheme 1 shows the route of synthesis of this novel sensor and its chemical structure.
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Structure of S1 was established in solid state by using Fourier transform infrared
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(FTIR) analysis and in solution through 1H and
13
C NMR experiments. IRνmax (KBr): 1
H NMR
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3450 (OH), 1587 (C=N), 1560, 1515, 1359, 1320, 1230 (C-N), 877 cm−1.
(DMSO-d6, 500 MHz): 8.31 (1H, d, J = 8.6 Hz, H-7), 7.49 (1H, s, H-6), 7.48 (2H, d, J = 8.6 Hz, H-11), 7.29 (1H, d, J = 15.6 Hz, H-9), 7.27 (1H, d, J = 8.2 Hz, H-5), 7.26 (1H, s, H-3), 6.89 (1H, dd, J = 15.6, 8.6 Hz, H-8), 6.67 (2H, d, J = 8.6 Hz, H-12), 3.01(6H, s, CH3) ppm.
13
C NMR (DMSO-d6, 125 MHz) : 166.3 (C-7), 152.0(C-2), 150.1 (C-13),
148.2 (C-9), 145.7 (C-4), 143.7 (C-1), 130.1 (C-11), 123.1 (C-5), 122.3 (C-10), 120.1 (C-6), 116.5 (C-8), 113.2 (C-3), 112.4 (C-12), 31.2 (CH3) ppm.
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3. RESULTS AND DISCUSSION The infrared spectrum of S1 showed an intense absorption assigned to an asymmetric stretching band of C=N at 1587 cm-1. 1H and
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C NMR experiments
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confirm the formation of the imine bond, the signal for iminic proton is at 8.31 ppm while the signal for azomethine carbon is at 166.3 ppm.
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Structure of S1 was corroborated by single crystal X-ray diffraction analysis
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(Figure 1). Crystal data with structure refinement is presented in supplementary information as Table S1. Formation of the imine bond was confirmed by the C=N
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distance value of 1.278(4) Å. The intramolecular hydrogen bond connects the oxygen to nitrogen atoms with a distance O···N of 2.595(4) Å which is shorter than the sum of
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the ionic radios. The supramolecular structure of S1 in the crystal is governed by Cl····Cl intermolecular interactions with distances of 3.465(2) Å. It was noticed that the
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π-backbone skeleton has a planar conformation, which is corroborated by the intense
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intramolecular charger transfer observed by UV-spectroscopy.
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3.1 Sensing properties of S1
For the sensing experiments, stock solutions of metal ions (concentration of 1 ×
10-3 M) were prepared in distilled water at 7 pH. Different levels of pH were also studied. CH3CO2H and CH3CO2Na water solutions were prepared and mixed in appropriate proportions to attain the pH of our interest. The colorimetric chemosensor S1 has the advantage of being soluble in solvents that have miscibility with water, such as MeCN, DMSO and MeOH. The best sensing performance from S1 was
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obtained with the use of MeCN. Stock solution of S1 in MeCN was prepared at the concentration of 2.5 × 10-5 M. Before evaluating the sensing properties of S1 toward metal ions dissolved in
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water, we investigated the stability of this Schiff base in MeCN and MeCN/water (10:1, v/v) mixtures. This effect must be considered because in our work the metals to
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be sensed are always dissolved in water while S1 is dissolved in MeCN. The
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maximum absorption peak of S1 in MeCN was at 407 nm and showed stability for at least 22 hours (see Supporting information, Figure S1).
This absorption peak is
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assigned to a low energy band produced by intramolecular charge transfer (ICT). Such ICT characteristic is ascribed based in bathochromic shifts observed in solution The possibility of ICT has also been
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of S1 using solvents of different polarity.
suggested for other Schiff bases and supported by solvatochromism and
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computational studies [43-46]. It is well known that imines in aqueous solution
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undergo hydrolysis; however, the imine S1 in a mixture MeCN/water remained stable with the condition that MeCN exist in a proportion of at least 80-90 % in volume
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(Figure S2). Furthermore, under that condition a MeCN solution of S1 show an intense color change from faded yellow to intense pink when it is mixed with water containing Cu2+ ions. The colorimetric effect on the MeCN solution of S1 after adding a water solution of Cu2+ is shown in Figure S3 for different proportions of MeCN/water. Once the effect of water on the absorption band of S1 was established, we proceeded to study its colorimetric changes when it is in contact with various metal ions like Cd2+, Co2+, Cr2+, Fe3+, Mg2+, Ni2+, Hg2+, Pb2+, Mn2+, Cu2+, and Zn2+ dissolved in water. The experimental procedure for metal recognition consisted in
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mixing an MeCN solution of S1 with a water solution of the metal under test at the concentration of 1 equiv. (2.5 × 10-5 M). The proportion of MeCN and water was kept at 10:1 % in volume. Unless other way stated, this procedure for sensing metals in
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MeCN/water was used in all the experiments here presented. Figure 2 display the colorimetric changes observed in S1 after adding water solution of different metal
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ions.
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Ours results clearly show that S1 is selective to Cu2+. Figure 2 shows that the metal–ligand (S1–Cu2+) interaction produced the appearance of a new absorption
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peak at 530 nm. This band is red-shifted approximately 123 nm with respect to the band of the free S1. These changes in the absorption spectra are assigned to the
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modification of the ICT in the π electronic system of S1 due to the formation of a bond with the metal ion [43]. On the other hand, S1 displays less color change in
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presence of 1 equiv of Ni2+ (the absorption band is red-shifted only 21 nm) indicating
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a weaker metal–ligand interaction. Weak color change was also observed at different concentration of Ni2+ (see Figure S4 for the titration of S1 with Ni2+). Likewise, the
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response induced by the interaction S1– Ni2+ as a function of ligand concentration was weaker and differentiated in color with respect to that exhibited by the complex S1–Cu2+ (see Figure S5). In any case, the stability of the interaction of S1 with Ni2+ or Cu2+ was of several minutes (see Figure S6). A very weak modification to the absorption was observed for Mn2+, while no significant changes were observed with other metals (Cd2+, Co2+, Cr2+, Fe3+, Mg2+, Hg2+, Pb2+, and Zn2+). The results presented in Figure 2 using distilled water were reproducible when the metals under
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tests were dissolved in tap water (see Fig S7). The ionic strength of the used tap water is regulated by the Mexican Official Norma [47].
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3.2 Effect of pH It is important to consider that the colorimetric properties of S1 in solution can be
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influenced by its protonation states. For instance, in aqueous solutions the level of
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pH can interfere with the functioning of the sensor. In view of this fact, it was necessary to find the pH range where the S1 could perform optimally the sensing
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process. Figure 3 shows the UV-Vis absorption spectra of S1 in MeCN/water at various pH levels. It is observed that the absorption practically remains unaltered in
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neutral and basic conditions (pH in the range 6–8). In this range of pH the metal-free chemosensor S1 exists as a ligand so that it can be used to produce complexation
However, in acidic conditions (pH < 5.5) protonation
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and biomedical sciences.
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with Cu2+. Certainly, such a pH range is of interest for sensing Cu2+ in environmental
causes the coloration of S1 (the absorption peak at 407 nm decreased and the band
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centered al 530 nm increased) limiting its use as colorimetric sensor.
3.3 Effects of multiple presence of ions on the response of S1 Possible interferences in the selectivity of S1 toward Cu2+ were evaluated.
These experiments were carried out by adding into an MeCN solution of S1 a water solution of Cu2+ (at 1 equiv) and a water solution of either Cd2+, Cr2+, Fe3+, Mg2+, Pb2+, Co2+, Hg2+, Ni2+, Mn2+, or Zn2+ (at 2 equiv). Figure 4 shows the optical absorbance values at 530 nm from these combinations, demonstrating that the absorption
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intensity due to the S1–Cu2+ complex was not significantly influenced by the presence of the mentioned ions. These results suggest that S1 could be exploited as a colorimetric sensor for Cu2+ ion, discriminating among the metal ions Ni2+, Hg2+, Fe3+
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or Zn2+, which are sometimes difficult to differentiate from Cu2+ [8, 14, 36, 48-49]. Selectivity of S1 toward Cu2+ was also studied by mixing S1 simultaneously with
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all the studied metals. Figure 5 displays the absorption spectra and a picture obtained
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from mixing S1 with all the metals except copper (procedure A) and from mixing S1 with all metals including copper (procedure B). As it can be observed from this figure,
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the simultaneous presence of Cd2+, Cr2+, Fe3+, Mg2+, Pb2+, Co2+, Hg2+, Ni2+, Mn2+, and Zn2+ modified only slightly the absorption of S1 indicating that these ions have
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negligible interaction with S1, while for the case when Cu2+ is present the spectrum shows clearly an absorption peak at 530 nm distinctive of the S1–Cu2+ complexation.
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Thus, S1 is able to perform a selective detection of copper in MeCN/water solution
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still with the simultaneous presence of other metals.
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3.4 Titration and sensitivity of S1 Titration of S1 (in MeCN) with water solutions of Cu2+ allowed determining the
sensitivity of our chemosensor. Titration was performed in the range 0.02 – 2 equiv of copper (see Figure 6). An increment in Cu2+ concentration leads to a reduction in the absorption intensity at 407 nm and simultaneously produces the apparition of a new characteristic band at 530 nm due to the formation of S1–Cu2+ complex. The presence of a sharp isosbestic point at 454 nm implies that S1 and S1–Cu2+ species are in equilibrium during titration (in contrast, the tritation of S1 with Ni2+ did not
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exhibit any isosbestic point, see Figure S4). The absorption of S1–Cu2+ at 530 nm has initially a rapid increase up to 0.2 equiv, then the absorption increases slower until a plateau is achieved for concentrations higher than 1 equiv. Then the
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stoichiometry formation of S1–Cu2+ results in approximately a 2:1 (Ligand:Metal). We suggest that for the formation of S1–Cu2+, metal is coordinating with oxygen atoms of
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the phenolic and C═N imine groups [27]. The binding ability of imines depends on the electronic distribution over the π-backbone; a high electronic density distribution
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promotes that dents atoms such as oxygen and nitrogen coordinate with metal ions.
electronic density around the dent atoms.
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The presence of chlorine atom in the structure of S1 helps to provide such rich
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Detection limit of the chemosensor S1 was calculated from titration when the first distinctive change in the absorption band was observed by the spectroscopic
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method, corresponding to a concentration of 1.25 × 10-7 M (0.125 µM), which is much
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lower than the maximum value permitted in potable water (<30 µM) [18, 40, 50] and lower than the average concentration of copper in human organism (15.7–23.6 µM)
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[17, 32, 51]. The absorption changes were clearly visible to the naked eye. For instance, the colorless solution of S1 immediately turned to a light orange upon addition of Cu2+ at 0.5 µM while at 1.9 µM it turned reddish (Figure 7), which means that the detection limit of Cu2+ ion by naked eye can be as low as 2 µM. The binding constant K between S1 and the ion can be calculated from the
change in absorbance at 530 nm observed during titration experiments by using the Benessi-Hildebrand equation [52]. K value resulted to be 1.08 × 106 M-1 (Figure S8).
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The K value and the limit of detection exhibited by S1 are among the most competitive values of the sensors used to detect Cu2+ in aqueous media reported recently. Table 1 presents a summary of reported chemosensors of Cu2+ tested in
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aqueous media. 3.5 Sensing of copper with S1 through surface plasmon resonance
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(SPR)
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The change in color induced by the formation of the S1-Cu2+ complex is accompanied by a change in refractive index. To explore this effect, a surface
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plasmon resonance (SPR) sensor based in a standard Kretschmann configuration [53] was implemented (Fig S9). Briefly, a p-polarized He-Ne laser beam (543.5 nm)
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impinges upon a hemi-cylindrical prism (glass BK7) covered with 44.5 nm-thick silver film. We used silver with a purity of 99.99%. The material was evaporated using
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electron gun evaporation (5KV, 26 mA) in a vacuum chamber at an evaporation rate -5
mbar of pressure. The thickness were
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of 5 Å/s in an atmosphere of air at 4 x 10
measured in situ by a quartz crystal microbalance (Leybold Inficon XTC/2 Depositions
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Controllers). This film is in direct contact with a solution of S1 such that the position and width of the SPR curve is monitored upon addition of water solutions of Cu2+ under conditions of attenuated total reflection (ATR). In these experiments the SPR at the silver film results in a dip in the intensity of the reflected light at a specific angle θ. The angle depends on the refractive indexes of the solution and film. Fig 8 shows the SPR curves for a series of aqueous solutions of S1 (3×10-5 M) with different concentrations of Cu2+ in the range 0 – 1 equiv. The angles of minimum reflectance θ
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corresponding to the SPR are calculated by using an algorithm applied to the experimental curves. It is observed that as the concentration of Cu2+ is increased the SPR shifts towards larger angles (due to higher refractive index) and the curves
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become broader. Angle shifts were observed for Cu2+ at concentrations as low as ca 1.5 µM.
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Since the first application of the SPR phenomenon, this technique has been used
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extensively for optical biosensing, in which the analyte (biomolecule) is captured by a ligand permanently immobilized on the metal surface of the SPR. Such approach
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was recently extended for the detection of metal ions using as active layer a modified gold surface with recognition molecules [54]. In contrast to this approach, in our
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sensing process the metal surface of the SPR apparatus is not modified or functionalized, but it is brought in direct contact with a solution containing the ligand
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S1 and the metal ion to be detected. The sensitivity of our experiment to detect
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changes in refractive index in aqueous solution containing Cu2+ is comparable with
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the range of detection 0.1 ppb for Cu2+ obtained with SPR using a metal (gold) surface and an active layer of the peptides of NH2-Gly-Gly-His-COOH (Bachem) and cysteamine 4-methoxyltrityl resin [55].
4 CONCLUSIONS
A new colorimetric sensor S1 was studied by absorption spectroscopy
technique. The chemosensor behavior of this imine was evaluated for metallic ions in a mixture of MeCN/water. The sensor has high affinity and selectivity for Cu2+. It exhibited binding-induced color change from yellow to pink detected even by the
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naked eye without significant interference under the presence of additional transition metal ions. This new colorimetric sensor presented a spectroscopic limit detection of 0.12 µM and colorimetric limit detection evaluated at naked eye of 2 µM. This
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chemosensor offers the possibility to detect Cu2+ in aqueous media. Until now, most of the reported colorimetric sensors for Cu2+ work in pure organic solvents. i.e., Cu2+
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was present in the same solvent in which the sensor was dissolved, limiting their use in practical applications. Finally, a SPR apparatus was implemented to monitor the
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presence of Cu2+ in aqueous media though the use of S1. In this case, the metal
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surface of the SPR apparatus was free of modifications or functionalization with recognition molecules, whereas it was brought in direct contact with an aqueous
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solution of S1 and Cu2+. Our results demonstrate that the sensitivity of the SPR
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ACKNOWLEDGEMENTS
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apparatus to monitor Cu2+ using S1 can be in the range of parts per million.
Authors acknowledge partial economical support from CONACyT (Grants 132946,
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215708 and 183147), Martin Olmos for his technical assistance, and Monica Jenith Castro and Daniel Banuelos Toto for the molecular synthesis.
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Figure Captions
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Scheme 1. Synthesis and molecular structure of S1.
Figure 1. X-ray crystal structure of S1. Thermal ellipsoids are showed at 50 %
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probability level.
Figure 2. (a) Absorption spectra and (b) photograph from S1 (in MeCN) after adding a water solution of different metal ions. In all cases the mixture MeCN/water
d
corresponds to 10:1 volume ratio with S1 concentration of 2.5 × 10-5 M and 1 equiv of
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the corresponding metal, at 7 pH.
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Figure 3. Effect of pH on the absorption of S1 (2.5 ×10−5 M) in MeCN/water. Inset: changes in the absorption peak of S1 (at 407 nm) as a function of pH. In these experiments pH was adjusted with CH3CO2H and CH3CO2Na. Figure 4. Absorbance of S1 (2.5 ×10−5 M) in presence of Cu2+ (1 equiv) and any of the ions Cd2+, Cr2+, Fe3+, Mg2+, Pb2+, Co2+, Hg2+, Ni2+, Mn2+, Zn2+ (2 equiv in each case), in MeCN/water (10:1, v/v) solution.
Figure 5. Absorption of S1 in MeCN/water. Absorption in procedure A: S1 mixed with Cd2+, Cr2+, Fe3+, Mg2+, Pb2+, Co2+, Hg2+, Ni2+, Mn2+, and Zn2+; absorption in procedure
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B: S1 mixed with all the metals of procedure A plus Cu2+. All metals at 1 equiv respect to S1 (2.5 ×10−5 M). Figure 6. Absorption obtained during the titration of S1 with Cu2+. The equivalence of
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copper is denoted.
Figure 7. Colorimetric changes in S1 after addition of different concentrations of
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copper ions. Left to right: 0, 0.5, 1, 1.9, 2.5, 6.4, 12 µM.
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Figure 8. SPR curves from a solution of S1 (3×10-5 M) after mixing it with water containing different concentrations of Cu2+. The arrow indicates the increment of Cu2+
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in the range 0 – 1 equiv. Inset: angle of minimum reflectance θ as a function of metal
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ion concentration.
Table 1. Characteristics of recent chemosensors of Cu2+ in aqueous media Response mode
Testing Media (v/v)
Binding -1 Constant [M ]
Schiff base
Color.
MeCN/water (90:10)
1.0 × 10
0.12 µM
Triazolyl monoazo Aminonaphthoquinone Phenol Nitrophenyl Rhodamine Naphthalimide Rhodamine B
Color. Color.
NA
Color. Color. Color. Color. Color.
DMSO/water (8:2) DMSO/water (9:1) MetOH/water (30:70) MeCN/water (90:10) MeCN/water (50:50)
3.1 × 10 NA 5 2.6 × 10 4 2.1 × 10 5 4.2 × 10
Triphenylamine
Color.
MeCN/water (70/30)
3.3 × 10
Schiff-base Schiff base
Color. Color.
4.9 × 10 5 4.3 × 10
Fluorescein
Color.
Schiff base Rhodamine B Dihydroxyanthraquinone Dye-doped silica
Color. Color.
THF/water (1/4) DMSO/water (9:1) DMSO/water (2% DMSO) DMSO/water (10:0.5) EtOH/water (1:9)
Color.
WATER/AMMONIA
1.5 × 10
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te
d
Structure
Fluor.
MeCN/water (9:1)
EtOH/water (10:90)
Water
Detection Limit
Ref.
13 µM
This work [7]
4
NA
[12]
4
NA 0.10 µM 0.078 µM NA 0.01 µM
[17] [18] [31] [32] [33]
4
∼20µM
[34]
6
0.2 µM 0.80 µM
[35] [36]
6
1.6 × 10
4
3.7 × 10
NA
[37]
NA 5 4.8 × 10
1 µM NA
[38] [39]
5
0.27 µM
[40]
4
NA
[41]
4.1 × 10
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[42]
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NPs response time 60 s. 6 Schiff base Color. MeOH 4.8 × 10 NA NA: Not available; Color. = colorimetric study; Fluor. =Fluorescence study
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Biographies Diecenia Peralta-Dominguez had her M.Sc. in 2009 from Centro de Investigacionesen Optica (CIO), Leon, Mexico. Currently she is pursuing a Ph.D. in CIO. Her research interests are on the design and applications of organic molecules for colorimetric chemosensors. Mario Rodríguez had his Ph.D. from Centro de Investigaciones y de Estudios Avanzados (CINVESTAV) in 2007. Since 2010, he was appointed as an Engineer at Centro de Investigaciones en Óptica (CIO). His current research areas comprise design and synthesis of organic materials with linear and nonlinear optical properties for colorimetric sensor, TPA-biomarkers and organic photovoltaic devices (OPVs). Gabriel Ramos-Ortiz had his B.Sc. degree in physics from the Universidad Nacional Autónoma de México in 1996 and a Ph.D. degree from the University of Arizona, USA, in 2003. In 2004, he became a permanent full time researcher at the Photonics Department of the Optical Research Center (CIO), Leon, Mexico. His researches mostly focus on organic electronics and nonlinear optics, including biophotonics, sensors, solar cells, spectroscopy and ultrafast optics. José-Luis Maldonado completed his Ph.D. in physics at UNAM by 1999 and is a Permanent Full Time Researcher at Centro de Investigaciones en Optica (Optical Research Center) now. During 2001–2002, he was a postdoctoral at the Optical Sciences Center, University of Arizona. Current research areas are on Plastic optoelectronics, Linear and nonlinear optical properties of organic and inorganic materials for photonic devices, Solar cells (OPVs) and OLEDs, within the Group of Optical Properties of Materials GPOM of the Photonics Division, CIO. Donato Luna-Moreno received the BE degree in physics from Universidad de Guadalajara, Mexico in 1988, the MSc degree in optics from the Centro de Investigaciones en Optica, León, México in 1991, and PhD degree in optics from Instituto Nacional de Astrofísica, Optica y Electrónica, Puebla, México in 1997. He is a researcher from Centro de Investigaciones en Optica and his research interest includes thin films and optical sensors. Mauricio Ortiz Gutiérrez has a bachelor´s degree in physics by the Autonomous University of Puebla, México. He is a Master and PhD by the National Institute of Astrophysics, Optics and Electronics, in Puebla, México in 1996 and 1999, respectively. His areas of interest are in spectroscopy, signal modulation, holography and metrology. Since year 2000 he is researcher at the Faculty of Physical and Mathematical Sciences at the Universidad Michoacana de San Nicolás de Hidalgo in Morelia, Michoacán, México.
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Víctor Barba López received the PhD degree in Chemistry from Centro de Investigaciones y de Estudios Avanzados (CINVESTAV) in 2001. He became a permanent full time researcher at Centro de Investigaciones Quimicas at the Universidad Autonoma de Morelos, Morelos México. His current research areas comprise synthesis and structural analysis of macrocyclic compounds derived from representative elements (boron and tin) and their evaluation as molecular receptors.
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S0925-4005(15)30596-7 http://dx.doi.org/doi:10.1016/j.snb.2015.11.013 SNB 19271
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
18-6-2015 30-10-2015 3-11-2015
Please cite this article as: D. Peralta-Dom´inguez, M. Rodriguez, G. RamosOrtiz, J.L. Maldonado, D. Luna-Moreno, M. Ortiz-Gutierrez, V. Barba, A Schiff base derivative used as sensor of copper through colorimetric and surface plasmon resonance techniques, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.11.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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HIGHLINES Low detection limit for copper of 1.25 × 10-7 M in aqueous media Low colorimetric (naked eye) detection limit of 2 × 10-6 M in aqueous media Sensing copper ions through the use of surface plasmon resonance (SPR) technique
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A Schiff base derivative used as sensor of copper through colorimetric and surface plasmon resonance techniques
Centro de Investigaciones en Óptica A.P. 1-948, 37000 León, Gto., México; b Facultad de Cs. Físico-matemáticas, UMSNH, Morelia Mich. México; c Centro de Investigaciones Químicas, UAM, Cuernavaca Morelos, México. * [email protected], [email protected]
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ABSTRACT
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D. Peralta-Domíngueza, M. Rodrigueza,*, G. Ramos-Ortiza,*, J. L. Maldonadoa, D. Luna-Morenoa, M. Ortiz-Gutierrezb, V. Barbac
Organic molecular sensors have the advantage of being used through easy,
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fast, economical and reliable optical methods for detecting toxic metal ions in the environment. In this work, we present a simple but highly specific organic ligand 5-
d
Chloro-2-[(1E,2E)-3-(4-(dimethylamino)phenyl)allylidene)amino)]phenol
(S1)
that
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works as a colorimetric sensor of copper ions in aqueous solutions. Binding
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interaction between S1 and various metal ions was established by UV-Vis spectroscopic measurements showing favorable coordination toward Cu2+ and practically no interference with the presence of other metal ions, i.e., Cd2+, Co2+, Cr2+, Fe3+, Mg2+, Ni2+, Hg2+, Pb2+, Mn2+, and Zn2+. S1 exhibited binding-induced color changes from yellow to pink with a detection limit of 1.25 × 10-7 M measured by spectroscopic methods, while colorimetric changes could be observed at naked eye for concentrations as low as 2 × 10-6 M. Furthermore, we also demonstrated that S1 can be useful for sensing copper ions through the use of surface plasmon resonance
Page 2 of 35
(SPR) technique. This technique allowed detecting the presence of Cu2+ in aqueous solutions at the concentration level of ca 1.5 × 10-6 M due to changes in the refractive index.
1. INTRODUCTION
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Keywords: colorimetric chemosensors, copper (II), surface plasmon resonance.
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Recently, the design and synthesis of intelligent organic molecules to be used
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as chemosensors with high selectivity and sensitivity toward metals has attracted much attention [1-4]. These sensors exploit the colorimetric and fluorescent
environmental sciences [5-6].
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properties of organic molecules and find applications in analytical, biomedical and
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Colorimetric sensors have the ability to produce red- or blue-shifts on their
te
absorption bands such that their characteristic color (or absence of color) is modified when they detect specifically any metal ion of interest. The molecular structure of
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these sensors commonly comprise a chromophore and a receptor, which are linked directly or through a π-conjugated chain [7]. An effective colorimetric sensor must convert the event of ion recognition by the receptor into an easily monitored and highly sensitive optical signal (strong change in the UV-Vis absorption band) from the chromophore.
Copper is the third most abundant (after iron and zinc) transition metal in earth. Copper is an essential micronutrient and together with certain proteins produces
Page 3 of 35
nearly 20 enzymes critical for life [7-10]; it also plays important roles in bone and tissue formation, cellular respiration, immune and brain functions, and gene transcription [11-14]. However, the presence of copper at high concentrations (as a
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consequence of its widespread use for industrial purposes) can produce harmful effects to living organisms due to oxidative stress and the subsequent damage to
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tissue and some organs (kidney and liver). For instance, high concentrations of copper in the human body are associated to Alzheimer, Parkinson, hypoglycemia,
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dyslexia, Menkes and Wilson diseases [11, 15-16]. The normal concentration of
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copper in human blood is limited to 150 µg/dL (23.6 µM) [10, 17]. For natural resources like drinking water, the limit has been established in the range of 15-30 µM
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(1.3 ppm) [18-19]. There are many techniques for detecting Cu2+, i.e., anodic stripping voltammetry, plasma-atomic emission spectroscopy, gravimetric, chromatography,
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ion selective electrodes, etc., but those methods are usually complicated and time-
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consuming, need relatively high cost apparatus, and are not easily adaptable for online monitoring [20-21]. Therefore, the rational design and synthesis of efficient
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chemosensors to easily recognize and quantify copper ions is an important topic in medicine and environmental sciences. The aim of this report is to present the synthesis and evaluation of a novel and
reliable colorimetric chemosensor based in a Schiff-base derivative (S1) for the recognition of copper in aqueous media. This work was motivated by the fact that the majority of organic molecules reported as chemosensors of Cu2+ perform the sensing process in solutions of solvents like acetonitrile (MeCN) [19, 22-26], methanol (MeOH) [9, 27],
Tetrahydrofuran (THF) [28], chloroform (CHCl3) [9], dimethyl
Page 4 of 35
sulfoxide (DMSO) [29], and others. However, the current interest for biomedical and environmental applications relies in those sensors able to identify Cu2+ in aqueous solutions. Only few Cu2+ chemosensors have been demonstrated to perform sensing
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in aqueous solutions, i.e., mixtures of water with solvents like MeCN [7, 30-34], THF [35], DMSO [17-18, 36-38], ethanol [12, 39], MeOH [5], and ammonia [40] or just in The sensing characteristics of S1 were investigated using UV-Vis
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water [41].
absorption spectroscopy upon the presence of biologically and environmentally
Ours results demonstrated that S1 can be employed for sensing the
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and Zn2+.
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relevant metal ions such as Cd2+, Co2+, Cr2+, Fe3+, Mg2+, Ni2+, Hg2+, Pb2+, Mn2+, Cu2+,
presence of Cu2+ in aqueous media with a spectroscopic detection limit of 1.25 × 10-7
M
M. This concentration is among the lowest limits of detection reported for copper in aqueous media. The use of a Schiff base like S1 as chemosensor is attractive
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because of its easy synthesis [27, 42] and its inherent fast and economical
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production. The sensing properties of S1 were also demonstrated using surface plasmon resonance (SPR) technique to monitor the presence of Cu2+ in aqueous
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media with a detection limit in the range of parts per million (1.5 × 10-6 M).
2. EXPERIMENTAL SECTION
2.1 Materials and instrumentation All starting materials for the synthesis of S1 and solvents involved in the
spectroscopic studies were purchased from Sigma-Aldrich distributor and were employed without further purification treatment. NMR experiments were recorded in a
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Jeol Fx 270 spectrometer. Infrared spectrum was measured on a FTIR Varian spectrophotometer with an ATR accessory while UV–Vis absorption spectra were measured using a spectrophotometer (Lambda 900, Perkin-Elmer). Melting point
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(uncorrected data) of S1 was obtained using Electrothermal 9200 apparatus. X-ray
2.2 Synthesis of Schiff base compound (S1)
5-Chloro-2-[(1E,2E)-3-(4-(dimethylamino)phenyl)allyidene)amino]phenol
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Compound
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diffraction analysis was performed by using Bruker Smart Apex diffractometer.
(S1) was prepared using a typical condensation reaction method of imine derivatives.
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S1 was synthesized from the reaction of 4-chloro-2-aminophenol with 4dimethylaminocinnamaldehyde in methanol with catalytic amount of acetic acid. The
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product was obtained as a brown solid with a yield of 85% M.P 156-157°C. Scheme 1 shows the route of synthesis of this novel sensor and its chemical structure.
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Structure of S1 was established in solid state by using Fourier transform infrared
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(FTIR) analysis and in solution through 1H and
13
C NMR experiments. IRνmax (KBr): 1
H NMR
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3450 (OH), 1587 (C=N), 1560, 1515, 1359, 1320, 1230 (C-N), 877 cm−1.
(DMSO-d6, 500 MHz): 8.31 (1H, d, J = 8.6 Hz, H-7), 7.49 (1H, s, H-6), 7.48 (2H, d, J = 8.6 Hz, H-11), 7.29 (1H, d, J = 15.6 Hz, H-9), 7.27 (1H, d, J = 8.2 Hz, H-5), 7.26 (1H, s, H-3), 6.89 (1H, dd, J = 15.6, 8.6 Hz, H-8), 6.67 (2H, d, J = 8.6 Hz, H-12), 3.01(6H, s, CH3) ppm.
13
C NMR (DMSO-d6, 125 MHz) : 166.3 (C-7), 152.0(C-2), 150.1 (C-13),
148.2 (C-9), 145.7 (C-4), 143.7 (C-1), 130.1 (C-11), 123.1 (C-5), 122.3 (C-10), 120.1 (C-6), 116.5 (C-8), 113.2 (C-3), 112.4 (C-12), 31.2 (CH3) ppm.
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3. RESULTS AND DISCUSSION The infrared spectrum of S1 showed an intense absorption assigned to an asymmetric stretching band of C=N at 1587 cm-1. 1H and
13
C NMR experiments
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confirm the formation of the imine bond, the signal for iminic proton is at 8.31 ppm while the signal for azomethine carbon is at 166.3 ppm.
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Structure of S1 was corroborated by single crystal X-ray diffraction analysis
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(Figure 1). Crystal data with structure refinement is presented in supplementary information as Table S1. Formation of the imine bond was confirmed by the C=N
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distance value of 1.278(4) Å. The intramolecular hydrogen bond connects the oxygen to nitrogen atoms with a distance O···N of 2.595(4) Å which is shorter than the sum of
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the ionic radios. The supramolecular structure of S1 in the crystal is governed by Cl····Cl intermolecular interactions with distances of 3.465(2) Å. It was noticed that the
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π-backbone skeleton has a planar conformation, which is corroborated by the intense
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intramolecular charger transfer observed by UV-spectroscopy.
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3.1 Sensing properties of S1
For the sensing experiments, stock solutions of metal ions (concentration of 1 ×
10-3 M) were prepared in distilled water at 7 pH. Different levels of pH were also studied. CH3CO2H and CH3CO2Na water solutions were prepared and mixed in appropriate proportions to attain the pH of our interest. The colorimetric chemosensor S1 has the advantage of being soluble in solvents that have miscibility with water, such as MeCN, DMSO and MeOH. The best sensing performance from S1 was
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obtained with the use of MeCN. Stock solution of S1 in MeCN was prepared at the concentration of 2.5 × 10-5 M. Before evaluating the sensing properties of S1 toward metal ions dissolved in
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water, we investigated the stability of this Schiff base in MeCN and MeCN/water (10:1, v/v) mixtures. This effect must be considered because in our work the metals to
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be sensed are always dissolved in water while S1 is dissolved in MeCN. The
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maximum absorption peak of S1 in MeCN was at 407 nm and showed stability for at least 22 hours (see Supporting information, Figure S1).
This absorption peak is
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assigned to a low energy band produced by intramolecular charge transfer (ICT). Such ICT characteristic is ascribed based in bathochromic shifts observed in solution The possibility of ICT has also been
M
of S1 using solvents of different polarity.
suggested for other Schiff bases and supported by solvatochromism and
d
computational studies [43-46]. It is well known that imines in aqueous solution
te
undergo hydrolysis; however, the imine S1 in a mixture MeCN/water remained stable with the condition that MeCN exist in a proportion of at least 80-90 % in volume
Ac ce p
(Figure S2). Furthermore, under that condition a MeCN solution of S1 show an intense color change from faded yellow to intense pink when it is mixed with water containing Cu2+ ions. The colorimetric effect on the MeCN solution of S1 after adding a water solution of Cu2+ is shown in Figure S3 for different proportions of MeCN/water. Once the effect of water on the absorption band of S1 was established, we proceeded to study its colorimetric changes when it is in contact with various metal ions like Cd2+, Co2+, Cr2+, Fe3+, Mg2+, Ni2+, Hg2+, Pb2+, Mn2+, Cu2+, and Zn2+ dissolved in water. The experimental procedure for metal recognition consisted in
Page 8 of 35
mixing an MeCN solution of S1 with a water solution of the metal under test at the concentration of 1 equiv. (2.5 × 10-5 M). The proportion of MeCN and water was kept at 10:1 % in volume. Unless other way stated, this procedure for sensing metals in
ip t
MeCN/water was used in all the experiments here presented. Figure 2 display the colorimetric changes observed in S1 after adding water solution of different metal
cr
ions.
us
Ours results clearly show that S1 is selective to Cu2+. Figure 2 shows that the metal–ligand (S1–Cu2+) interaction produced the appearance of a new absorption
an
peak at 530 nm. This band is red-shifted approximately 123 nm with respect to the band of the free S1. These changes in the absorption spectra are assigned to the
M
modification of the ICT in the π electronic system of S1 due to the formation of a bond with the metal ion [43]. On the other hand, S1 displays less color change in
d
presence of 1 equiv of Ni2+ (the absorption band is red-shifted only 21 nm) indicating
te
a weaker metal–ligand interaction. Weak color change was also observed at different concentration of Ni2+ (see Figure S4 for the titration of S1 with Ni2+). Likewise, the
Ac ce p
response induced by the interaction S1– Ni2+ as a function of ligand concentration was weaker and differentiated in color with respect to that exhibited by the complex S1–Cu2+ (see Figure S5). In any case, the stability of the interaction of S1 with Ni2+ or Cu2+ was of several minutes (see Figure S6). A very weak modification to the absorption was observed for Mn2+, while no significant changes were observed with other metals (Cd2+, Co2+, Cr2+, Fe3+, Mg2+, Hg2+, Pb2+, and Zn2+). The results presented in Figure 2 using distilled water were reproducible when the metals under
Page 9 of 35
tests were dissolved in tap water (see Fig S7). The ionic strength of the used tap water is regulated by the Mexican Official Norma [47].
ip t
3.2 Effect of pH It is important to consider that the colorimetric properties of S1 in solution can be
cr
influenced by its protonation states. For instance, in aqueous solutions the level of
us
pH can interfere with the functioning of the sensor. In view of this fact, it was necessary to find the pH range where the S1 could perform optimally the sensing
an
process. Figure 3 shows the UV-Vis absorption spectra of S1 in MeCN/water at various pH levels. It is observed that the absorption practically remains unaltered in
M
neutral and basic conditions (pH in the range 6–8). In this range of pH the metal-free chemosensor S1 exists as a ligand so that it can be used to produce complexation
However, in acidic conditions (pH < 5.5) protonation
te
and biomedical sciences.
d
with Cu2+. Certainly, such a pH range is of interest for sensing Cu2+ in environmental
causes the coloration of S1 (the absorption peak at 407 nm decreased and the band
Ac ce p
centered al 530 nm increased) limiting its use as colorimetric sensor.
3.3 Effects of multiple presence of ions on the response of S1 Possible interferences in the selectivity of S1 toward Cu2+ were evaluated.
These experiments were carried out by adding into an MeCN solution of S1 a water solution of Cu2+ (at 1 equiv) and a water solution of either Cd2+, Cr2+, Fe3+, Mg2+, Pb2+, Co2+, Hg2+, Ni2+, Mn2+, or Zn2+ (at 2 equiv). Figure 4 shows the optical absorbance values at 530 nm from these combinations, demonstrating that the absorption
Page 10 of 35
intensity due to the S1–Cu2+ complex was not significantly influenced by the presence of the mentioned ions. These results suggest that S1 could be exploited as a colorimetric sensor for Cu2+ ion, discriminating among the metal ions Ni2+, Hg2+, Fe3+
ip t
or Zn2+, which are sometimes difficult to differentiate from Cu2+ [8, 14, 36, 48-49]. Selectivity of S1 toward Cu2+ was also studied by mixing S1 simultaneously with
cr
all the studied metals. Figure 5 displays the absorption spectra and a picture obtained
us
from mixing S1 with all the metals except copper (procedure A) and from mixing S1 with all metals including copper (procedure B). As it can be observed from this figure,
an
the simultaneous presence of Cd2+, Cr2+, Fe3+, Mg2+, Pb2+, Co2+, Hg2+, Ni2+, Mn2+, and Zn2+ modified only slightly the absorption of S1 indicating that these ions have
M
negligible interaction with S1, while for the case when Cu2+ is present the spectrum shows clearly an absorption peak at 530 nm distinctive of the S1–Cu2+ complexation.
d
Thus, S1 is able to perform a selective detection of copper in MeCN/water solution
te
still with the simultaneous presence of other metals.
Ac ce p
3.4 Titration and sensitivity of S1 Titration of S1 (in MeCN) with water solutions of Cu2+ allowed determining the
sensitivity of our chemosensor. Titration was performed in the range 0.02 – 2 equiv of copper (see Figure 6). An increment in Cu2+ concentration leads to a reduction in the absorption intensity at 407 nm and simultaneously produces the apparition of a new characteristic band at 530 nm due to the formation of S1–Cu2+ complex. The presence of a sharp isosbestic point at 454 nm implies that S1 and S1–Cu2+ species are in equilibrium during titration (in contrast, the tritation of S1 with Ni2+ did not
Page 11 of 35
exhibit any isosbestic point, see Figure S4). The absorption of S1–Cu2+ at 530 nm has initially a rapid increase up to 0.2 equiv, then the absorption increases slower until a plateau is achieved for concentrations higher than 1 equiv. Then the
ip t
stoichiometry formation of S1–Cu2+ results in approximately a 2:1 (Ligand:Metal). We suggest that for the formation of S1–Cu2+, metal is coordinating with oxygen atoms of
cr
the phenolic and C═N imine groups [27]. The binding ability of imines depends on the electronic distribution over the π-backbone; a high electronic density distribution
us
promotes that dents atoms such as oxygen and nitrogen coordinate with metal ions.
electronic density around the dent atoms.
an
The presence of chlorine atom in the structure of S1 helps to provide such rich
M
Detection limit of the chemosensor S1 was calculated from titration when the first distinctive change in the absorption band was observed by the spectroscopic
d
method, corresponding to a concentration of 1.25 × 10-7 M (0.125 µM), which is much
te
lower than the maximum value permitted in potable water (<30 µM) [18, 40, 50] and lower than the average concentration of copper in human organism (15.7–23.6 µM)
Ac ce p
[17, 32, 51]. The absorption changes were clearly visible to the naked eye. For instance, the colorless solution of S1 immediately turned to a light orange upon addition of Cu2+ at 0.5 µM while at 1.9 µM it turned reddish (Figure 7), which means that the detection limit of Cu2+ ion by naked eye can be as low as 2 µM. The binding constant K between S1 and the ion can be calculated from the
change in absorbance at 530 nm observed during titration experiments by using the Benessi-Hildebrand equation [52]. K value resulted to be 1.08 × 106 M-1 (Figure S8).
Page 12 of 35
The K value and the limit of detection exhibited by S1 are among the most competitive values of the sensors used to detect Cu2+ in aqueous media reported recently. Table 1 presents a summary of reported chemosensors of Cu2+ tested in
ip t
aqueous media. 3.5 Sensing of copper with S1 through surface plasmon resonance
cr
(SPR)
us
The change in color induced by the formation of the S1-Cu2+ complex is accompanied by a change in refractive index. To explore this effect, a surface
an
plasmon resonance (SPR) sensor based in a standard Kretschmann configuration [53] was implemented (Fig S9). Briefly, a p-polarized He-Ne laser beam (543.5 nm)
M
impinges upon a hemi-cylindrical prism (glass BK7) covered with 44.5 nm-thick silver film. We used silver with a purity of 99.99%. The material was evaporated using
d
electron gun evaporation (5KV, 26 mA) in a vacuum chamber at an evaporation rate -5
mbar of pressure. The thickness were
te
of 5 Å/s in an atmosphere of air at 4 x 10
measured in situ by a quartz crystal microbalance (Leybold Inficon XTC/2 Depositions
Ac ce p
Controllers). This film is in direct contact with a solution of S1 such that the position and width of the SPR curve is monitored upon addition of water solutions of Cu2+ under conditions of attenuated total reflection (ATR). In these experiments the SPR at the silver film results in a dip in the intensity of the reflected light at a specific angle θ. The angle depends on the refractive indexes of the solution and film. Fig 8 shows the SPR curves for a series of aqueous solutions of S1 (3×10-5 M) with different concentrations of Cu2+ in the range 0 – 1 equiv. The angles of minimum reflectance θ
Page 13 of 35
corresponding to the SPR are calculated by using an algorithm applied to the experimental curves. It is observed that as the concentration of Cu2+ is increased the SPR shifts towards larger angles (due to higher refractive index) and the curves
ip t
become broader. Angle shifts were observed for Cu2+ at concentrations as low as ca 1.5 µM.
cr
Since the first application of the SPR phenomenon, this technique has been used
us
extensively for optical biosensing, in which the analyte (biomolecule) is captured by a ligand permanently immobilized on the metal surface of the SPR. Such approach
an
was recently extended for the detection of metal ions using as active layer a modified gold surface with recognition molecules [54]. In contrast to this approach, in our
M
sensing process the metal surface of the SPR apparatus is not modified or functionalized, but it is brought in direct contact with a solution containing the ligand
d
S1 and the metal ion to be detected. The sensitivity of our experiment to detect
te
changes in refractive index in aqueous solution containing Cu2+ is comparable with
Ac ce p
the range of detection 0.1 ppb for Cu2+ obtained with SPR using a metal (gold) surface and an active layer of the peptides of NH2-Gly-Gly-His-COOH (Bachem) and cysteamine 4-methoxyltrityl resin [55].
4 CONCLUSIONS
A new colorimetric sensor S1 was studied by absorption spectroscopy
technique. The chemosensor behavior of this imine was evaluated for metallic ions in a mixture of MeCN/water. The sensor has high affinity and selectivity for Cu2+. It exhibited binding-induced color change from yellow to pink detected even by the
Page 14 of 35
naked eye without significant interference under the presence of additional transition metal ions. This new colorimetric sensor presented a spectroscopic limit detection of 0.12 µM and colorimetric limit detection evaluated at naked eye of 2 µM. This
ip t
chemosensor offers the possibility to detect Cu2+ in aqueous media. Until now, most of the reported colorimetric sensors for Cu2+ work in pure organic solvents. i.e., Cu2+
cr
was present in the same solvent in which the sensor was dissolved, limiting their use in practical applications. Finally, a SPR apparatus was implemented to monitor the
us
presence of Cu2+ in aqueous media though the use of S1. In this case, the metal
an
surface of the SPR apparatus was free of modifications or functionalization with recognition molecules, whereas it was brought in direct contact with an aqueous
M
solution of S1 and Cu2+. Our results demonstrate that the sensitivity of the SPR
te
ACKNOWLEDGEMENTS
d
apparatus to monitor Cu2+ using S1 can be in the range of parts per million.
Authors acknowledge partial economical support from CONACyT (Grants 132946,
Ac ce p
215708 and 183147), Martin Olmos for his technical assistance, and Monica Jenith Castro and Daniel Banuelos Toto for the molecular synthesis.
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ip t cr us
Figure Captions
an
Scheme 1. Synthesis and molecular structure of S1.
Figure 1. X-ray crystal structure of S1. Thermal ellipsoids are showed at 50 %
M
probability level.
Figure 2. (a) Absorption spectra and (b) photograph from S1 (in MeCN) after adding a water solution of different metal ions. In all cases the mixture MeCN/water
d
corresponds to 10:1 volume ratio with S1 concentration of 2.5 × 10-5 M and 1 equiv of
te
the corresponding metal, at 7 pH.
Ac ce p
Figure 3. Effect of pH on the absorption of S1 (2.5 ×10−5 M) in MeCN/water. Inset: changes in the absorption peak of S1 (at 407 nm) as a function of pH. In these experiments pH was adjusted with CH3CO2H and CH3CO2Na. Figure 4. Absorbance of S1 (2.5 ×10−5 M) in presence of Cu2+ (1 equiv) and any of the ions Cd2+, Cr2+, Fe3+, Mg2+, Pb2+, Co2+, Hg2+, Ni2+, Mn2+, Zn2+ (2 equiv in each case), in MeCN/water (10:1, v/v) solution.
Figure 5. Absorption of S1 in MeCN/water. Absorption in procedure A: S1 mixed with Cd2+, Cr2+, Fe3+, Mg2+, Pb2+, Co2+, Hg2+, Ni2+, Mn2+, and Zn2+; absorption in procedure
Page 23 of 35
B: S1 mixed with all the metals of procedure A plus Cu2+. All metals at 1 equiv respect to S1 (2.5 ×10−5 M). Figure 6. Absorption obtained during the titration of S1 with Cu2+. The equivalence of
ip t
copper is denoted.
Figure 7. Colorimetric changes in S1 after addition of different concentrations of
cr
copper ions. Left to right: 0, 0.5, 1, 1.9, 2.5, 6.4, 12 µM.
us
Figure 8. SPR curves from a solution of S1 (3×10-5 M) after mixing it with water containing different concentrations of Cu2+. The arrow indicates the increment of Cu2+
an
in the range 0 – 1 equiv. Inset: angle of minimum reflectance θ as a function of metal
M
ion concentration.
Table 1. Characteristics of recent chemosensors of Cu2+ in aqueous media Response mode
Testing Media (v/v)
Binding -1 Constant [M ]
Schiff base
Color.
MeCN/water (90:10)
1.0 × 10
0.12 µM
Triazolyl monoazo Aminonaphthoquinone Phenol Nitrophenyl Rhodamine Naphthalimide Rhodamine B
Color. Color.
NA
Color. Color. Color. Color. Color.
DMSO/water (8:2) DMSO/water (9:1) MetOH/water (30:70) MeCN/water (90:10) MeCN/water (50:50)
3.1 × 10 NA 5 2.6 × 10 4 2.1 × 10 5 4.2 × 10
Triphenylamine
Color.
MeCN/water (70/30)
3.3 × 10
Schiff-base Schiff base
Color. Color.
4.9 × 10 5 4.3 × 10
Fluorescein
Color.
Schiff base Rhodamine B Dihydroxyanthraquinone Dye-doped silica
Color. Color.
THF/water (1/4) DMSO/water (9:1) DMSO/water (2% DMSO) DMSO/water (10:0.5) EtOH/water (1:9)
Color.
WATER/AMMONIA
1.5 × 10
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Structure
Fluor.
MeCN/water (9:1)
EtOH/water (10:90)
Water
Detection Limit
Ref.
13 µM
This work [7]
4
NA
[12]
4
NA 0.10 µM 0.078 µM NA 0.01 µM
[17] [18] [31] [32] [33]
4
∼20µM
[34]
6
0.2 µM 0.80 µM
[35] [36]
6
1.6 × 10
4
3.7 × 10
NA
[37]
NA 5 4.8 × 10
1 µM NA
[38] [39]
5
0.27 µM
[40]
4
NA
[41]
4.1 × 10
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[42]
ip t
NPs response time 60 s. 6 Schiff base Color. MeOH 4.8 × 10 NA NA: Not available; Color. = colorimetric study; Fluor. =Fluorescence study
Ac ce p
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Biographies Diecenia Peralta-Dominguez had her M.Sc. in 2009 from Centro de Investigacionesen Optica (CIO), Leon, Mexico. Currently she is pursuing a Ph.D. in CIO. Her research interests are on the design and applications of organic molecules for colorimetric chemosensors. Mario Rodríguez had his Ph.D. from Centro de Investigaciones y de Estudios Avanzados (CINVESTAV) in 2007. Since 2010, he was appointed as an Engineer at Centro de Investigaciones en Óptica (CIO). His current research areas comprise design and synthesis of organic materials with linear and nonlinear optical properties for colorimetric sensor, TPA-biomarkers and organic photovoltaic devices (OPVs). Gabriel Ramos-Ortiz had his B.Sc. degree in physics from the Universidad Nacional Autónoma de México in 1996 and a Ph.D. degree from the University of Arizona, USA, in 2003. In 2004, he became a permanent full time researcher at the Photonics Department of the Optical Research Center (CIO), Leon, Mexico. His researches mostly focus on organic electronics and nonlinear optics, including biophotonics, sensors, solar cells, spectroscopy and ultrafast optics. José-Luis Maldonado completed his Ph.D. in physics at UNAM by 1999 and is a Permanent Full Time Researcher at Centro de Investigaciones en Optica (Optical Research Center) now. During 2001–2002, he was a postdoctoral at the Optical Sciences Center, University of Arizona. Current research areas are on Plastic optoelectronics, Linear and nonlinear optical properties of organic and inorganic materials for photonic devices, Solar cells (OPVs) and OLEDs, within the Group of Optical Properties of Materials GPOM of the Photonics Division, CIO. Donato Luna-Moreno received the BE degree in physics from Universidad de Guadalajara, Mexico in 1988, the MSc degree in optics from the Centro de Investigaciones en Optica, León, México in 1991, and PhD degree in optics from Instituto Nacional de Astrofísica, Optica y Electrónica, Puebla, México in 1997. He is a researcher from Centro de Investigaciones en Optica and his research interest includes thin films and optical sensors. Mauricio Ortiz Gutiérrez has a bachelor´s degree in physics by the Autonomous University of Puebla, México. He is a Master and PhD by the National Institute of Astrophysics, Optics and Electronics, in Puebla, México in 1996 and 1999, respectively. His areas of interest are in spectroscopy, signal modulation, holography and metrology. Since year 2000 he is researcher at the Faculty of Physical and Mathematical Sciences at the Universidad Michoacana de San Nicolás de Hidalgo in Morelia, Michoacán, México.
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Víctor Barba López received the PhD degree in Chemistry from Centro de Investigaciones y de Estudios Avanzados (CINVESTAV) in 2001. He became a permanent full time researcher at Centro de Investigaciones Quimicas at the Universidad Autonoma de Morelos, Morelos México. His current research areas comprise synthesis and structural analysis of macrocyclic compounds derived from representative elements (boron and tin) and their evaluation as molecular receptors.
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