# Water Soluble Cationic Porphyrin Sensor for Detection of Hg2+, Pb2+, Cd2+, and Cu2+

Authors: Matibur Zamadar; Christopher Orr; Miranda Uherek
Journal: Journal of Sensors
 (2016)

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## Abstract

Here we report the sensing properties of the aqueous solution ofmeso-tetra(N-methyl-4-pyridyl)porphine tetrachloride (1) for simultaneous detection of toxic metal ions by using UV-vis spectroscopy. Cationic porphyrin1 displayed different electronic absorptions in UV-vis region upon interacting with Hg2+, Pb2+, Cd2+, and Cu2+ ions in neutral water solution at room temperature. Quite interestingly, the porphyrin1 showed that it can function as a single optical chemical sensor and/or metal ion receptor capable of detecting two or more toxic metal ions, particularly Hg2+, Pb2+, and Cd2+ ions coexisting in a water sample. Porphyrin1 in an aqueous solution provides a unique UV-vis sensing system for the determination of Cd2+ in the presence of larger metal ions such as Hg2+, or Pb2+. Finally, the examination of the sensing properties of1 demonstrated that it can operate as a Cu2+ ion selective sensor via metal displacement from the1-Hg
2+,1-Pb
2+, and1-Cd
2+.


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## Body

## 1. Introduction

Due to rapid industrialization and fast growing global population, the discharge of transition and heavy metals into water resources has increased globally. Along with other chemicals, heavy metals are extremely toxic and exposure to them causes adverse effects on the environment and human health [1]. For example, a very small concentration of mercury ions (Hg2+) can damage the central nervous system, brain, kidneys, and liver. Additionally, it can also affect the endocrine system and the immune system [2–4]. Lead (Pb2+) ions can cause anemia, kidney damage, brain damage, mental-retardation, and muscle paralysis [5]. Cadmium (Cd2+) ion is another extremely harmful metal; it can damage the kidney’s filtering system, central nervous system, and immune system and causes physiological disorders [6–8]. Copper (Cu2+) ions are widely distributed in plants and animals, including human beings [9]. These ions are involved in various biological processes. However, they are toxic at high concentration causing kidney and liver damage [10].Recently, porphyrin-based probes have shown great promise for detecting heavy metal ions in solution. Porphyrin compounds possess remarkable photophysical properties. They exhibit characteristic electronic absorptions in the Soret band region (~400 nm,S
0
→
S
2) as well as in the Q-bands region (500–700 nm, S
0
→
S
1). Porphyrin solutions have visible color. Most importantly, porphyrins are good functional receptors for various metal ions and can typically form a chelation complex with heavy metal ions. After complexation with a heavy metal ion, metalloporphyrin shows characteristic electronic absorption bands in the visible region which can be readily observed with UV-vis spectroscopy [11–13]. As a consequence, a number of molecular optical sensors and various detection techniques including electronic absorption, fluorescence, colorimetry, and electrochemical methods have been devised to selectively detect heavy metal ions such as Hg2+, Pb2+, Cd2+, and Cu2+. These methods are often found to be very effective. However, reports indicated that most of the porphyrin-based sensors focused on just single metal ion detection [14–21]. Very few examples have been reported for the simultaneous detection of multimetal ions coexisting in a sample. Chen and his coworkers reported the first porphyrin-Bodipy based ratiometric sensor for Fe2+ and Hg2+ ions [22]. The same group (Chen and his coworkers) synthesized otherN,N-bis(2-pyridylmethyl)amine (DPA) modified porphyrin sensors for Pb2+ and Cu2+ ions [23]. Recently, Chen and Wang prepared azacrown[N, S, O]-modified porphyrin sensors for simultaneous detection of Ag+, Pb2+, and Cu2+ ions [24]. However, a porphyrin-based sensor for the simultaneous detection of two or more toxic metal ions, particularly Hg2+, Pb2+, and Cd2+ ions, has not been reported thus far. It is also worth noting that the preparation of reported porphyrin-based optical sensors often requires complex organic synthesis, which is very costly and time consuming. Furthermore, the porphyrin-based sensors are water insoluble, which limits the detection of metal ions in aqueous environment [23, 24]. Thus, finding a simple, inexpensive, water soluble, single optical chemical sensor capable of detecting any or all Hg2+, Pb2+, and Cd2+ ions simultaneously is highly desirable.In this paper, we report a water soluble cationic porphyrin-based optical sensor for the detection of single or multi-heavy metal ions by UV-vis absorption method. The aqueous solution of cationicmeso-tetra(N-methyl-4-pyridyl)porphine tetrachloride1 (Figure 1) was studied and explored as a versatile optical sensor/metal ion receptor for heavy metal ions. Cationic porphyrin1 produced different diagnostic optical absorptions for Hg2+, Pb2+, Cd2+, and Cu2+ in the UV-vis region indicating its ability as a potential multisignal optical sensor. Quite interestingly, we found that porphyrin1 has the potential to behave as a single optical chemical sensor and/or metal ion receptor for detecting two or more toxic metal ions (Hg2+, Pb2+, and Cd2+) simultaneously coexisting in a water sample. Unique sensing ability of1 for Cd2+ ion was investigated in the presence of metallic ions with large ionic radii. Finally, we examine the sensing properties of the aqueous solution of1 for Cu2+ ion. The absorption studies demonstrated that1 can operate as a potential dual-mode Cu2+-selective sensor.Figure 1
Structure of the cationic porphyrin (1).


## 2. Experimental Section

### 2.1. Materials and Sample Preparation

All reagents and chemicals were of the best available analytical reagent and were used as received without purification. Deionized H2O was obtained from a US Filter Corporation deionization system.meso-Tetra(N-methyl-4-pyridyl)porphine tetrachloride was purchased from Frontier Scientific Inc., USA. Mercury (II) nitrate, cadmium (II) chloride, lead (II) nitrate, nickel (II) chloride, and sodium chloride were obtained from Sigma Aldrich, USA. Copper (II) chloride, zinc (II) sulfate, nickel (II) sulfate, iron (II) chloride, and calcium chloride were bought from Flinn Scientific Inc., USA. Potassium chloride was received from Mallinckrodt Pharmaceuticals, USA. UV-vis spectra were recorded on an Agilent 8453 single beam diode array spectrometer (Agilent Technologies, USA, model 8453). All the measurements were performed at room temperature and under an atmospheric pressure. All the metal salt solutions (1.00 × 10−3 M) were prepared in neutral (~pH 7) aqueous solution. Quartz cuvettes with 1.00 cm pathlength and 2 mL volume were used for all measurements. For typical metal-binding titration, microliter amounts of standard solution of metal ion aqueous solutions were added into the cuvette containing 2 mL of solution1 (5.00 × 10−6 M) and thoroughly mixed before a measurement was made.


## 2.1. Materials and Sample Preparation

All reagents and chemicals were of the best available analytical reagent and were used as received without purification. Deionized H2O was obtained from a US Filter Corporation deionization system.meso-Tetra(N-methyl-4-pyridyl)porphine tetrachloride was purchased from Frontier Scientific Inc., USA. Mercury (II) nitrate, cadmium (II) chloride, lead (II) nitrate, nickel (II) chloride, and sodium chloride were obtained from Sigma Aldrich, USA. Copper (II) chloride, zinc (II) sulfate, nickel (II) sulfate, iron (II) chloride, and calcium chloride were bought from Flinn Scientific Inc., USA. Potassium chloride was received from Mallinckrodt Pharmaceuticals, USA. UV-vis spectra were recorded on an Agilent 8453 single beam diode array spectrometer (Agilent Technologies, USA, model 8453). All the measurements were performed at room temperature and under an atmospheric pressure. All the metal salt solutions (1.00 × 10−3 M) were prepared in neutral (~pH 7) aqueous solution. Quartz cuvettes with 1.00 cm pathlength and 2 mL volume were used for all measurements. For typical metal-binding titration, microliter amounts of standard solution of metal ion aqueous solutions were added into the cuvette containing 2 mL of solution1 (5.00 × 10−6 M) and thoroughly mixed before a measurement was made.


## 3. Results and Discussion

### 3.1. The Change of Electronic Absorption of1 by Divalent Metal Cations

To examine the selectivity of metal-free porphyrin1, the absorption spectra were recorded by adding an aqueous aliquot of the metal ions Na+, K+, Zn2+, Fe3+, Co2+, Cd2+, Cu2+, Ni2+, Hg2+, and Pb2+ into an aqueous solution of1. The intensity of the absorption spectra of metal-free porphyrin1 showed almost no change upon addition of Na+, K+, Co2+, and Ni2+. A very small reduction of intensity for the strong Soret band of1 was observed for Zn2+ and Fe3+ metal ions (see Figure S1 in Supplementary Material available online at http://dx.doi.org/10.1155/2016/1905454). As shown in Figure 2, an aqueous solution of1 exhibited a Soret band at 422 nm and Q-bands at 521, 552, and 589 nm. The intensity of the Soret band of metal-free porphyrin1 was decreased with the appearance of a new peak at 458 nm upon addition of Hg2+ ions into the aqueous solution of1, as shown in Figure 3(a). This new peak at 458 nm can be used as a diagnostic absorption for Hg2+ ions. A clear isosbestic point was observed at 432 nm which indicated that the two species (1-Hg
2+ and porphyrin1) were in equilibrium (see Figure S2 in SI). The ratio ofI
458/I
422 was calculated, which increased from 0 to 1.99. The detection limit (LOD) for Hg2+ ions with cationic1 was determined to be 0.5 × 10-6M under the present conditions, which is very comparable with the recent report [25]. Similarly, upon addition of Pb2+ ion, the absorption of the Soret band of1 was greatly decreased and a new peak at 478 nm appeared simultaneously (Figure 3(b)). This new absorption peak is attributed to1-Pb
2+. The ratio ofI
478/I
422 was increased from 0 to 2.07. Well-defined isosbestic points at 374 nm and 447 nm were observed indicating that there was a simple equilibrium involved between metal-free porphyrin1 and metal bound porphyrin,1-Pb
2+ (see Figure S2 in SI). The detection limit for Pb2+ ions with1 was determined to be 0.5 × 10−6 M under the present conditions. Similar detection limit was observed in [23]. As shown in Figure 3(c), upon addition of Cd2+ to the solution of1, the absorbance of all the peaks decreased with a shoulder taking a shape of a peak at 447 nm. The appearance of a new absorption at 447 nm is accounted for1-Cd
2+ complex. The ratio ofI
447/I
422 was increased from 0 to 0.84. Interestingly, copper (Cu2+) ions addition to the aqueous solution of1 showed very marginal change of the absorption spectra of1. It did not produce any separate peak in the Soret band of1. However, an increased absorption at 552 nm was observed upon addition of increasing amount of Cu2+ ions into the aqueous solution of1. The absorption at 552 nm can be used as a diagnostic absorption for Cu2+ ion (see Figure S2). The ratio ofI
552/I
422 was calculated and it showed an increase from 0 to 0.05. The detection limit for Cu2+ ions with1 was determined to be 1.0 × 10−6 M under the present conditions, which is eight times smaller than reported [26]. All the results are tabulated in Table 1. The results indicate that the inexpensive metal-free porphyrin1 can be used as an optical chemical sensor for detecting any of the divalent Hg2+, Pb2+, Cd2+, and Cu2+ ions in water.Table 1
The optical parameter of studied  porphyrin in water.
Compound
UV-bands [nm]
Soret band [nm]
Q-bands [nm]
1
422
521, 553, 589
1
-
H
g
2
+
422, 458
594, 639
1
-
P
b
2
+
375
422, 478
1
-
C
d
2
+
422, 447
521, 585
1
-
C
u
2
+
375
426
521, 552Figure 2
Absorption spectra of1 (5 × 10−6 M) in H2O.Figure 3
Absorption spectra of aqueous solution of (a) 1 × 10−5 M1 and 1 × 10−5 Hg2+ (blue curve), (b) 1 × 10−5M1 and 1 × 10−5 Pb2+ (brown curve), (c) 1 × 10−5 M1 and 1 × 10−5 Cd2+ (pink curve), and (d) 1 × 10−5M1 and 1 × 10−5 Cu2+ (purple curve).


### 3.2. Parallel (or Simultaneous) Multi-Ions (Hg2+, Pb2+, and Cd2+) Detection

To simultaneously (or parallel) detect two or more metal ions coexisting in a sample, an aqueous solution of porphyrin1 was tested against a number of mixed metal ion solutions. An increasing amount of a mixture of different metal ions was added into the porphyrin1 solution. The variation of electronic absorption of porphyrin1 was monitored using UV-vis spectroscopy to detect all metal ions coexisting in solution. As shown in Figure 4, the intensity of absorption of1 exhibited a gradual decrease of the most intense Soret band at 422 nm upon addition of increasing amount of a mixture of Cd2+, Hg2+, and Pb2+. Diagnostic absorptions for1-Hg
2+ at 458 nm,1-Pb
2+ at 478 nm, and1-Cd
2+ at 447 nm were also observed. The results revealed that the metal-free porphyrin1 showed different absorption spectra with metal ions (Hg2+, Pb2+, and Cd2+) coexisting in an aqueous solution. Thus, porphyrin1 represents a single molecular optical sensor which can detect and analyze metal ions, particularly Hg2+, Pb2+, and Cd2+, simultaneously coexisting in a sample.Figure 4
The electronic absorption spectra of aqueous solution of1 (5.0 × 10−6 M) upon addition of increasing amount [0, 1665, 0.333, 0.4995, 0.666, 0.8325, 0.999, 1.1655, 1.332, 1.4985, 1.665, 1.8315, 1.998, 2.1645, 2.331, 2.4975, 2.664, 2.8305, 2.997, 3.1635, 3.33, 3.4965, 3.663, 3.8295, 3.996, 4.1625, 4.329, 4.4955, 4.662, 4.8285, 4.995, 5.8275, 6.66, 7.4925, 8.325, and 9.15 (×10−7 M)] of Hg2+, Pb2+, and Cd2+ ions.


### 3.3. Influence of Larger Metal Ions on the Determination of Cd2+

The detection of Cd2+ ions has always been a challenge due to the very slow complexation reaction in aqueous medium at room temperature [15]. Additionally, the greatest difficulty for determining Cd2+ ions comes from strong interference of larger heavy metal ions. Interestingly, we found that porphyrin1 can detect Cd2+ ions more effectively in presence of larger metal ions, particularly Hg2+ and Pb2+ ions. The experiment was performed by preparing a solution of1-M
2+ complexes by mixing equivalent amounts of1 and M2+ ions in water. The changes of absorption spectra of1-M
2+ complexes were monitored by adding increasing amount of Cd2+ ions to see if porphyrin1 could detect any or both metal ions simultaneously. We effectively employed very slow and more or less the similar metalation rates of metal cations (Hg2+, Pb2+, and Cd2+) with1 to determine the presence of Cd2+ ion coexisting with other metal cations [15]. As shown in Figure 5(a), remarkably, very rapid change of absorption of aqueous solution of1-Pb
2+ was observed upon the addition of Cd2+ ions. The aqueous solution of1-Pb
2+ produced a peak at 447 nm (1-Cd
2+) with marginal change of the absorption at 478 nm (1-Hg
2+) upon addition of 1-2 × 10−6 M of Cd2+. The detection limit for Cd2+ ions with1 was determined to be 0.5 × 10−6 M under the present conditions, which is six times higher than reported [18]. This result indicated that cationic1 can uniquely sense Cd2+ and Pb2+ coexisting in a water sample. Similarly, Figure 5(b) demonstrated the simultaneous detection of1-Hg
2+ at 458 nm and1-Cd
2+ at 447 nm when the [Cd2+] is around 1 × 10−6M. Interestingly, in all above cases, an increased absorption of1-Cd
2+ and a decreased absorption of1-M
2+ were observed upon addition of increasing amount of [Cd2+] (see the inset of Figure 5(a)). Typically, the absorption of1-Cd
2+ complex appeared as a tiny shoulder peak in the Soret band region of1 (see Figure 3). This tiny shoulder peak of1-Cd
2+ rapidly turned into a very clear and sharp peak at 447 nm in the presence of metal ion with a large ionic radius. Similar observations were noticed by other researchers. The rate of complexation for smaller metal ions (Cu(II), Mn(II), and Zn(II)) was found to increase in the presence of metal ions with larger ionic radii [17, 27]. It is attributed to the fact that larger metal ions distort or expand porphyrin and help ease entry for smaller metal ions into its core. The increased absorption for1-Cd
2+ complex clearly revealed its unique sensing ability for Cd2+ ions in presence of metallic ions with large ionic radii, particularly Hg2+ and Pb2+. However, all the metal ions sensing properties strictly depend on the presence of Cu2+ ion, which is discussed next.Figure 5
The electronic absorption spectra of aqueous solution of (a)1-Pb
2+ (5.0 × 10−6 M1 and 5.0 × 10−6 M Pb2+) upon addition of increasing amount [0.50, 1.00, and 1.5 (×10−6 M)] of Cd2+ ions: the inset is the absorption of1-M
2+ versus [Cd2+] (pink curve is the absorption at 447 nm and brown curve is the absorption at 478 nm), and (b)1-Hg
2+ (5.0 × 10−6 M1 and 5.0 × 10−6M Hg2+) upon addition of increasing amount [0.50, 1.00, 1.50, and 2.5 (×10−6 M)] of Cd2+ ions.
(a)
(b)


### 3.4. Strong Binding Affinity of1 with Cu2+ Ions

To determine the effect of divalent Cu2+ ions, an aqueous solution of metal-free porphyrin1 was titrated with a mixture of Hg2+, Pb2+, Cd2+, and Cu2+ aqueous solution. The intensity of absorption of1 exhibited a gradual decrease of the most intense Soret band at 422 nm upon addition of a mixture of Hg2+, Pb2+, Cd2+, and Cu2+. Remarkably, no clear diagnostic absorptions for1-Cd
2+ at 447 nm,1-Hg
2+ at 458 nm, and1-Pb
2+ at 478 nm were observed in UV-visible region (see Figure S3 in SI). However, a shoulder peak (from 455 to 480 nm) in the diagnostic region for Hg2+ and Pb2+ was observed indicating the presence of1-Hg
2+ and1-Pb
2+. A clear diagnostic Q-band absorption for1-Cu
2+ was observed at 552 nm. This observation leads us to postulate that Cu2+ ions are forming stronger complex with1 than other metal ions and that Cu2+ ions are forming the complex with faster rate than the other ions. To see the strong competitive binding of Cu2+ ions with1 a mixture of Cu2+ and other M2+ ions was added into the aqueous solution1. Porphyrin1 exhibited no diagnostic absorptions of1-Hg
2+ or1-Pb
2+ or1-Cd
2+ upon addition of a mixture of Cu2+ and Hg2+ or Cu2+ and Pb2+ or Cu2+ and Cd2+ (see Figure S4 in SI), respectively. Instead, diagnostic absorption of1-Cu
2+ was found increasing at 552 nm in all cases. These results indicated that all the1-M
2+ complexes which were produced almost immediately after addition of a mixture of M2+ ions into the solution of1 are converted into1-Cu
2+ complex. More detailed investigations on possible displacement reactions are discussed next.


### 3.5. Displacement of M2+ from1-M
2+ by Cu2+

To confirm the possible displacement of a Hg2+ from1-Hg
2+ or Pb2+ from1-Pb
2+ or Cd2+ from1-Cd
2+ by Cu2+ to form1-Cu
2+ complex, a number of displacement reactions were carried out. The experiment was performed by adding equivalent amounts of metal cations (Hg2+/Pb2+/Cd2+) into the porphyrin1 solution to allow1 to form1-M
2+ complex in solution. Then, an aqueous Cu2+ ion was added into a1-M
2+ solution to see if Cu2+ ion could displace metal ions from the1-M
2+ complex. Figure 6(a) showed the formation of1-Cu
2+ from1-Hg
2+ complex upon addition of Cu2+. The disappearance of diagnostic absorption of1-Hg
2+ at 458 nm and appearance of1-Cu
2+ were observed. Similarly, Figure 6(b) showed the change of1-Pb
2+ to1-Cu
2+ with the disappearance of the diagnostic absorption at 478 nm and the appearance of the absorption peak at 422 nm. As shown in Figure 6(c), the change of1-Cd
2+ to1-Cu
2+ was observed. Diagnostic absorption of1-Cd
2+ has disappeared with appearance of1-Cu
2+. The displacement of Pb2+, Hg2+, and Cd2+ by Cu2+ ion is attributed to the fact that Cu2+ is smaller in size compared to other metal ions. Larger size Pb2+, Hg2+, and Cd2+ ions cannot fit into the core of the central porphyrin. The larger metal ions locate away from the central core of the porphyrin, perhaps at the periphery of porphyrin1, and form loosely bound1-M
2+ complex, whereas smaller size Cu2+ ions can sit perfectly into the central porphyrin core and form a stable1-Cu
2+ complex. This explains the replacement events described above. Similar observations were noticed by Jiang and Chen groups [23, 24]. Thus, this result indicates that this single optical sensor1 can operate as a dual-mode Cu2+-selective sensor via the absorption nature of1 for Cu2+ as well as the absorption ratio metal displacement from the1-Pb
2+/1-Hg
2+
/1-Cd
2+. This porphyrin1 presented a rare example of a dual-mode multisignal system for detecting toxic metal ions (Pb2+, Hg2+, and Cd2+), particularly for Cu2+ ion [23, 24, 28, 29].Figure 6
The electronic absorption spectra of aqueous solution of (a)1-Hg
2+ (5.0 × 10−6 M1 and 4.0 × 10−6 M Hg2+, blue curve), (b)1-Pb
2+ (5.0 × 10−6 M1 and 4.0 × 10−6 M Pb2+, brown curve), and (c)1-Cd
2+ (5.0 × 10−6 M1 and 4.0 × 10−6 M Cd2+, pink curve) upon addition of Cu2+ ions (5 × 10−6M for1-Hg
2+ and1-Pb
2+, 4 × 10−6 M for1-Cd
2+).
(a)
(b)
(c)


### 3.6. Preliminary Analytical Application

The aqueous solution of1 was challenged with wastewater sample obtained from a rural wastewater treatment facility, Nacogdoches Wastewater Treatment Plant in Nacogdoches City (East Texas), Texas, USA [30]. The wastewater sample was collected from the second treatment stage, namely, aeration chamber. A different amount of wastewater sample was added to the aqueous solution1. Figure 7 showed a gradual decrease of Soret band absorption of1 at 422 nm, which slowly shifted to the red (435 nm) upon addition of increasing amount of wastewater into the aqueous solution of1. Similar observation was found when Ca2+ ion was added into1, shown in Figure S5. Recent report by Onchoke and his coworkers also indicated that wastewater from Nacogdoches plant contains high concentration of Ca2+ ions (100 ppm) and a very small concentration of Cu2+ ions (0.94 ppm) and almost undetectable amount of other toxic cations, particularly cadmium (0.004 ppm), lead (0.07 ppm), and mercury (0.01 pm) [30]. A highly expensive instrument, inductively coupled plasma optical spectrometry (ICP-OES) instrument, was used for metal ions analysis [30]. Our results showed that the inexpensive, water soluble, and commercially available1 can detect Ca2+ ion in wastewater sample with satisfactory performance. To the best of our knowledge, the water soluble porphyrin-based optical sensor for determining Ca2+ ions has not been reported thus far. This will be the first porphyrin-based optical sensor for Ca2+. However,1 was not able to detect Cu2+ due to the strong absorption of1-Ca
2+ complex in the region of1-Cu
2+.Figure 7
The electronic absorption spectra of aqueous solution of (a)1 (5.0
×
10
-
6 M) in DI water (black curve), (b)1 in DI water : wastewater (15 : 1, brown curve), (c)1 in DI water : wastewater (7 : 1, green curve), (d)1 in DI water : wastewater (3 : 1, purple curve), (e)1 in DI water : wastewater (1 : 1, blue curve), (f)1 in DI water : wastewater (1 : 0.33, orange curve), and (g)1 in DI water: wastewater (1 : 0.005, red curve).


## 3.1. The Change of Electronic Absorption of1 by Divalent Metal Cations

To examine the selectivity of metal-free porphyrin1, the absorption spectra were recorded by adding an aqueous aliquot of the metal ions Na+, K+, Zn2+, Fe3+, Co2+, Cd2+, Cu2+, Ni2+, Hg2+, and Pb2+ into an aqueous solution of1. The intensity of the absorption spectra of metal-free porphyrin1 showed almost no change upon addition of Na+, K+, Co2+, and Ni2+. A very small reduction of intensity for the strong Soret band of1 was observed for Zn2+ and Fe3+ metal ions (see Figure S1 in Supplementary Material available online at http://dx.doi.org/10.1155/2016/1905454). As shown in Figure 2, an aqueous solution of1 exhibited a Soret band at 422 nm and Q-bands at 521, 552, and 589 nm. The intensity of the Soret band of metal-free porphyrin1 was decreased with the appearance of a new peak at 458 nm upon addition of Hg2+ ions into the aqueous solution of1, as shown in Figure 3(a). This new peak at 458 nm can be used as a diagnostic absorption for Hg2+ ions. A clear isosbestic point was observed at 432 nm which indicated that the two species (1-Hg
2+ and porphyrin1) were in equilibrium (see Figure S2 in SI). The ratio ofI
458/I
422 was calculated, which increased from 0 to 1.99. The detection limit (LOD) for Hg2+ ions with cationic1 was determined to be 0.5 × 10-6M under the present conditions, which is very comparable with the recent report [25]. Similarly, upon addition of Pb2+ ion, the absorption of the Soret band of1 was greatly decreased and a new peak at 478 nm appeared simultaneously (Figure 3(b)). This new absorption peak is attributed to1-Pb
2+. The ratio ofI
478/I
422 was increased from 0 to 2.07. Well-defined isosbestic points at 374 nm and 447 nm were observed indicating that there was a simple equilibrium involved between metal-free porphyrin1 and metal bound porphyrin,1-Pb
2+ (see Figure S2 in SI). The detection limit for Pb2+ ions with1 was determined to be 0.5 × 10−6 M under the present conditions. Similar detection limit was observed in [23]. As shown in Figure 3(c), upon addition of Cd2+ to the solution of1, the absorbance of all the peaks decreased with a shoulder taking a shape of a peak at 447 nm. The appearance of a new absorption at 447 nm is accounted for1-Cd
2+ complex. The ratio ofI
447/I
422 was increased from 0 to 0.84. Interestingly, copper (Cu2+) ions addition to the aqueous solution of1 showed very marginal change of the absorption spectra of1. It did not produce any separate peak in the Soret band of1. However, an increased absorption at 552 nm was observed upon addition of increasing amount of Cu2+ ions into the aqueous solution of1. The absorption at 552 nm can be used as a diagnostic absorption for Cu2+ ion (see Figure S2). The ratio ofI
552/I
422 was calculated and it showed an increase from 0 to 0.05. The detection limit for Cu2+ ions with1 was determined to be 1.0 × 10−6 M under the present conditions, which is eight times smaller than reported [26]. All the results are tabulated in Table 1. The results indicate that the inexpensive metal-free porphyrin1 can be used as an optical chemical sensor for detecting any of the divalent Hg2+, Pb2+, Cd2+, and Cu2+ ions in water.Table 1
The optical parameter of studied  porphyrin in water.
Compound
UV-bands [nm]
Soret band [nm]
Q-bands [nm]
1
422
521, 553, 589
1
-
H
g
2
+
422, 458
594, 639
1
-
P
b
2
+
375
422, 478
1
-
C
d
2
+
422, 447
521, 585
1
-
C
u
2
+
375
426
521, 552Figure 2
Absorption spectra of1 (5 × 10−6 M) in H2O.Figure 3
Absorption spectra of aqueous solution of (a) 1 × 10−5 M1 and 1 × 10−5 Hg2+ (blue curve), (b) 1 × 10−5M1 and 1 × 10−5 Pb2+ (brown curve), (c) 1 × 10−5 M1 and 1 × 10−5 Cd2+ (pink curve), and (d) 1 × 10−5M1 and 1 × 10−5 Cu2+ (purple curve).


## 3.2. Parallel (or Simultaneous) Multi-Ions (Hg2+, Pb2+, and Cd2+) Detection

To simultaneously (or parallel) detect two or more metal ions coexisting in a sample, an aqueous solution of porphyrin1 was tested against a number of mixed metal ion solutions. An increasing amount of a mixture of different metal ions was added into the porphyrin1 solution. The variation of electronic absorption of porphyrin1 was monitored using UV-vis spectroscopy to detect all metal ions coexisting in solution. As shown in Figure 4, the intensity of absorption of1 exhibited a gradual decrease of the most intense Soret band at 422 nm upon addition of increasing amount of a mixture of Cd2+, Hg2+, and Pb2+. Diagnostic absorptions for1-Hg
2+ at 458 nm,1-Pb
2+ at 478 nm, and1-Cd
2+ at 447 nm were also observed. The results revealed that the metal-free porphyrin1 showed different absorption spectra with metal ions (Hg2+, Pb2+, and Cd2+) coexisting in an aqueous solution. Thus, porphyrin1 represents a single molecular optical sensor which can detect and analyze metal ions, particularly Hg2+, Pb2+, and Cd2+, simultaneously coexisting in a sample.Figure 4
The electronic absorption spectra of aqueous solution of1 (5.0 × 10−6 M) upon addition of increasing amount [0, 1665, 0.333, 0.4995, 0.666, 0.8325, 0.999, 1.1655, 1.332, 1.4985, 1.665, 1.8315, 1.998, 2.1645, 2.331, 2.4975, 2.664, 2.8305, 2.997, 3.1635, 3.33, 3.4965, 3.663, 3.8295, 3.996, 4.1625, 4.329, 4.4955, 4.662, 4.8285, 4.995, 5.8275, 6.66, 7.4925, 8.325, and 9.15 (×10−7 M)] of Hg2+, Pb2+, and Cd2+ ions.


## 3.3. Influence of Larger Metal Ions on the Determination of Cd2+

The detection of Cd2+ ions has always been a challenge due to the very slow complexation reaction in aqueous medium at room temperature [15]. Additionally, the greatest difficulty for determining Cd2+ ions comes from strong interference of larger heavy metal ions. Interestingly, we found that porphyrin1 can detect Cd2+ ions more effectively in presence of larger metal ions, particularly Hg2+ and Pb2+ ions. The experiment was performed by preparing a solution of1-M
2+ complexes by mixing equivalent amounts of1 and M2+ ions in water. The changes of absorption spectra of1-M
2+ complexes were monitored by adding increasing amount of Cd2+ ions to see if porphyrin1 could detect any or both metal ions simultaneously. We effectively employed very slow and more or less the similar metalation rates of metal cations (Hg2+, Pb2+, and Cd2+) with1 to determine the presence of Cd2+ ion coexisting with other metal cations [15]. As shown in Figure 5(a), remarkably, very rapid change of absorption of aqueous solution of1-Pb
2+ was observed upon the addition of Cd2+ ions. The aqueous solution of1-Pb
2+ produced a peak at 447 nm (1-Cd
2+) with marginal change of the absorption at 478 nm (1-Hg
2+) upon addition of 1-2 × 10−6 M of Cd2+. The detection limit for Cd2+ ions with1 was determined to be 0.5 × 10−6 M under the present conditions, which is six times higher than reported [18]. This result indicated that cationic1 can uniquely sense Cd2+ and Pb2+ coexisting in a water sample. Similarly, Figure 5(b) demonstrated the simultaneous detection of1-Hg
2+ at 458 nm and1-Cd
2+ at 447 nm when the [Cd2+] is around 1 × 10−6M. Interestingly, in all above cases, an increased absorption of1-Cd
2+ and a decreased absorption of1-M
2+ were observed upon addition of increasing amount of [Cd2+] (see the inset of Figure 5(a)). Typically, the absorption of1-Cd
2+ complex appeared as a tiny shoulder peak in the Soret band region of1 (see Figure 3). This tiny shoulder peak of1-Cd
2+ rapidly turned into a very clear and sharp peak at 447 nm in the presence of metal ion with a large ionic radius. Similar observations were noticed by other researchers. The rate of complexation for smaller metal ions (Cu(II), Mn(II), and Zn(II)) was found to increase in the presence of metal ions with larger ionic radii [17, 27]. It is attributed to the fact that larger metal ions distort or expand porphyrin and help ease entry for smaller metal ions into its core. The increased absorption for1-Cd
2+ complex clearly revealed its unique sensing ability for Cd2+ ions in presence of metallic ions with large ionic radii, particularly Hg2+ and Pb2+. However, all the metal ions sensing properties strictly depend on the presence of Cu2+ ion, which is discussed next.Figure 5
The electronic absorption spectra of aqueous solution of (a)1-Pb
2+ (5.0 × 10−6 M1 and 5.0 × 10−6 M Pb2+) upon addition of increasing amount [0.50, 1.00, and 1.5 (×10−6 M)] of Cd2+ ions: the inset is the absorption of1-M
2+ versus [Cd2+] (pink curve is the absorption at 447 nm and brown curve is the absorption at 478 nm), and (b)1-Hg
2+ (5.0 × 10−6 M1 and 5.0 × 10−6M Hg2+) upon addition of increasing amount [0.50, 1.00, 1.50, and 2.5 (×10−6 M)] of Cd2+ ions.
(a)
(b)


## 3.4. Strong Binding Affinity of1 with Cu2+ Ions

To determine the effect of divalent Cu2+ ions, an aqueous solution of metal-free porphyrin1 was titrated with a mixture of Hg2+, Pb2+, Cd2+, and Cu2+ aqueous solution. The intensity of absorption of1 exhibited a gradual decrease of the most intense Soret band at 422 nm upon addition of a mixture of Hg2+, Pb2+, Cd2+, and Cu2+. Remarkably, no clear diagnostic absorptions for1-Cd
2+ at 447 nm,1-Hg
2+ at 458 nm, and1-Pb
2+ at 478 nm were observed in UV-visible region (see Figure S3 in SI). However, a shoulder peak (from 455 to 480 nm) in the diagnostic region for Hg2+ and Pb2+ was observed indicating the presence of1-Hg
2+ and1-Pb
2+. A clear diagnostic Q-band absorption for1-Cu
2+ was observed at 552 nm. This observation leads us to postulate that Cu2+ ions are forming stronger complex with1 than other metal ions and that Cu2+ ions are forming the complex with faster rate than the other ions. To see the strong competitive binding of Cu2+ ions with1 a mixture of Cu2+ and other M2+ ions was added into the aqueous solution1. Porphyrin1 exhibited no diagnostic absorptions of1-Hg
2+ or1-Pb
2+ or1-Cd
2+ upon addition of a mixture of Cu2+ and Hg2+ or Cu2+ and Pb2+ or Cu2+ and Cd2+ (see Figure S4 in SI), respectively. Instead, diagnostic absorption of1-Cu
2+ was found increasing at 552 nm in all cases. These results indicated that all the1-M
2+ complexes which were produced almost immediately after addition of a mixture of M2+ ions into the solution of1 are converted into1-Cu
2+ complex. More detailed investigations on possible displacement reactions are discussed next.


## 3.5. Displacement of M2+ from1-M
2+ by Cu2+

To confirm the possible displacement of a Hg2+ from1-Hg
2+ or Pb2+ from1-Pb
2+ or Cd2+ from1-Cd
2+ by Cu2+ to form1-Cu
2+ complex, a number of displacement reactions were carried out. The experiment was performed by adding equivalent amounts of metal cations (Hg2+/Pb2+/Cd2+) into the porphyrin1 solution to allow1 to form1-M
2+ complex in solution. Then, an aqueous Cu2+ ion was added into a1-M
2+ solution to see if Cu2+ ion could displace metal ions from the1-M
2+ complex. Figure 6(a) showed the formation of1-Cu
2+ from1-Hg
2+ complex upon addition of Cu2+. The disappearance of diagnostic absorption of1-Hg
2+ at 458 nm and appearance of1-Cu
2+ were observed. Similarly, Figure 6(b) showed the change of1-Pb
2+ to1-Cu
2+ with the disappearance of the diagnostic absorption at 478 nm and the appearance of the absorption peak at 422 nm. As shown in Figure 6(c), the change of1-Cd
2+ to1-Cu
2+ was observed. Diagnostic absorption of1-Cd
2+ has disappeared with appearance of1-Cu
2+. The displacement of Pb2+, Hg2+, and Cd2+ by Cu2+ ion is attributed to the fact that Cu2+ is smaller in size compared to other metal ions. Larger size Pb2+, Hg2+, and Cd2+ ions cannot fit into the core of the central porphyrin. The larger metal ions locate away from the central core of the porphyrin, perhaps at the periphery of porphyrin1, and form loosely bound1-M
2+ complex, whereas smaller size Cu2+ ions can sit perfectly into the central porphyrin core and form a stable1-Cu
2+ complex. This explains the replacement events described above. Similar observations were noticed by Jiang and Chen groups [23, 24]. Thus, this result indicates that this single optical sensor1 can operate as a dual-mode Cu2+-selective sensor via the absorption nature of1 for Cu2+ as well as the absorption ratio metal displacement from the1-Pb
2+/1-Hg
2+
/1-Cd
2+. This porphyrin1 presented a rare example of a dual-mode multisignal system for detecting toxic metal ions (Pb2+, Hg2+, and Cd2+), particularly for Cu2+ ion [23, 24, 28, 29].Figure 6
The electronic absorption spectra of aqueous solution of (a)1-Hg
2+ (5.0 × 10−6 M1 and 4.0 × 10−6 M Hg2+, blue curve), (b)1-Pb
2+ (5.0 × 10−6 M1 and 4.0 × 10−6 M Pb2+, brown curve), and (c)1-Cd
2+ (5.0 × 10−6 M1 and 4.0 × 10−6 M Cd2+, pink curve) upon addition of Cu2+ ions (5 × 10−6M for1-Hg
2+ and1-Pb
2+, 4 × 10−6 M for1-Cd
2+).
(a)
(b)
(c)


## 3.6. Preliminary Analytical Application

The aqueous solution of1 was challenged with wastewater sample obtained from a rural wastewater treatment facility, Nacogdoches Wastewater Treatment Plant in Nacogdoches City (East Texas), Texas, USA [30]. The wastewater sample was collected from the second treatment stage, namely, aeration chamber. A different amount of wastewater sample was added to the aqueous solution1. Figure 7 showed a gradual decrease of Soret band absorption of1 at 422 nm, which slowly shifted to the red (435 nm) upon addition of increasing amount of wastewater into the aqueous solution of1. Similar observation was found when Ca2+ ion was added into1, shown in Figure S5. Recent report by Onchoke and his coworkers also indicated that wastewater from Nacogdoches plant contains high concentration of Ca2+ ions (100 ppm) and a very small concentration of Cu2+ ions (0.94 ppm) and almost undetectable amount of other toxic cations, particularly cadmium (0.004 ppm), lead (0.07 ppm), and mercury (0.01 pm) [30]. A highly expensive instrument, inductively coupled plasma optical spectrometry (ICP-OES) instrument, was used for metal ions analysis [30]. Our results showed that the inexpensive, water soluble, and commercially available1 can detect Ca2+ ion in wastewater sample with satisfactory performance. To the best of our knowledge, the water soluble porphyrin-based optical sensor for determining Ca2+ ions has not been reported thus far. This will be the first porphyrin-based optical sensor for Ca2+. However,1 was not able to detect Cu2+ due to the strong absorption of1-Ca
2+ complex in the region of1-Cu
2+.Figure 7
The electronic absorption spectra of aqueous solution of (a)1 (5.0
×
10
-
6 M) in DI water (black curve), (b)1 in DI water : wastewater (15 : 1, brown curve), (c)1 in DI water : wastewater (7 : 1, green curve), (d)1 in DI water : wastewater (3 : 1, purple curve), (e)1 in DI water : wastewater (1 : 1, blue curve), (f)1 in DI water : wastewater (1 : 0.33, orange curve), and (g)1 in DI water: wastewater (1 : 0.005, red curve).


## 4. Conclusion

We examined porphyrin1 as an inexpensive, water soluble single optical chemical sensor and/or metal ion receptor for the detection of toxic metal ions. Cationic porphyrin1 produced different electronic absorptions in UV-vis region upon interacting with Hg2+, Pb2+, Cd2+, and Cu2+ ions. This porphyrin can detect two or more toxic metal ions simultaneously coexisting in a water sample. We also found that porphyrin1 can detect Cd2+ easily in the presence of metallic ions with large ionic radius, particularly Hg2+ and Pb2+. Finally, we observed that porphyrin1 can operate as a Cu2+ ion selective sensor via metal displacement from the1-Hg
2+,1-Pb
2+, and1-Cd
2+. The results show that this porphyrin1 is a promising sensor for metals ions in waste water and nutritional sample.



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*Source: 1905454-2016-05-16.xml*