Bulk polymer nanoparticles containing a tetrakis(3-hydroxyphenyl)porphyrin for fast and highly selective separation of mercury ions
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- Shamsipur, M., Rajabi, H.R., Beyzavi, M.H. et al. Microchim Acta (2013) 180: 791. doi:10.1007/s00604-013-0983-x
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We report on the synthesis of polymeric nanoparticles (PNPs) containing a tetrakis(3-hydroxyphenyl)porphyrin, and their use for the separation of mercury(II) ion. The PNPs were prepared by bulk polymerization from methacrylic acid (the monomer), ethyleneglycol dimethacrylate (the cross-linker), 2,2′-azobisisobutyronitrile (the radical initiator) and the mercury(II) complex of 5,10,15,20-tetrakis(3-hydroxyphenyl)-porphyrin. The Hg(II) ion was then removed by treatment with dilute hydrochloric acid. The PNPs were characterized by colorimetry, FT-IR spectroscopy, and scanning electron microscopy. The material is capable of binding Hg(II) from analyte samples. Bound Hg(II) ions can be eluted with dilute nitric acid and then quantified by cold vapor AAS. The extraction efficiency, the effects of pH, preconcentration and leaching times, sample volume, and of the nature, concentration and volume of eluent were investigated. The maximum adsorption capacity of the PNPs is 249 mg g−1, the relative standard deviation of the AAS assay is 2.2 %, and the limit of detection (3σ) is 8 ng.L−1. The nanoparticles exhibit excellent selectivity for Hg(II) ion over other metal ions and were successfully applied to the selective extraction and determination of Hg(II) ion in spiked water samples.
KeywordsIon-imprinted polymeric nanoparticlesHg2+ ion5,10,15,20-Tetrakis(3-hydroxyphenyl)-porphyrinCold vapor atomic absorption spectrometry
Because of its toxicological and biogeochemical behavior, mercury is one of the most widely studied heavy metals . The determination of trace amounts of mercury is of particular significance in environmental and toxicological studies . However, as the levels of mercury in geological and environmental samples are low, a preconcentrative separation and determination of trace mercury from the natural water is essential and needs much more attention .
Nowadays, molecular imprinting technology has become a powerful strategy for producing chemically selective binding sites, for recognition of a particular molecule, in a polymeric matrix . Ion-imprinting polymers (IIPs) are similar to molecular imprinted polymers, but they recognize metal ions after imprinting and retain all virtues of molecular imprinted polymer. Generally, three steps are involved in the ion-imprinting process: (i) complex formation of target metal ions as template with a suitable ligand, (ii) polymerization of the formed complex and (iii) removal of imprint metal ion from polymeric matrix . After ion imprinting polymerization, the imprint metal ion is removed from the polymeric particles by leaching with mineral acid that leaves cavities or “imprinted sites” in the polymeric particles that are complementary in shape and size of the imprint metal ion . The resultant polymeric particles can be used as efficient sorbents for selective uptake and enrichment of imprint metal ion from dilute aqueous solutions by batch or column experiments.
Porphyrin derivatives have been extensively used in establishment of very interesting analytical methods such as optical sensors , ion selective electrodes , carbon paste electrodes  and flow injection analysis .Up to date, a variety of ligands have been used as complexing agents for preparation of Hg2+ [2, 11–16] and some other metal ion imprinted polymers [17–22]; but, to the best of our knowledge, there is no previous literature report concerning the use of porphyrin derivatives for the synthesis of any metal ion imprinted polymeric nanoparticles. It is worth mentioning that, in recent years a number of different solid phases including nanometer silica functionalized by 2,6-pyridine dicarboxylic acid , magnetic nanoparticles doped with 1,5-diphenylcarbazide  and silica gel modified with diethylenetriamine and thiourea  have also been used for the selective extraction, preconcentration and determination of Hg2+ ions.
In this work, we wish to report the preparation of novel ion-imprinted polymeric nanoparticles by using 5,10,15,20-tetrakis(3-hydroxyphenyl)-porphyrin (T(3-OHP)P) as a non-polymerizable ligand for selective separation and enrichment of imprint Hg2+ ions from aqueous solutions.
Methacrylic acid (MAA), ethyleneglycol dimethacrylate (EGDMA) and 2,2′-azobisisobutyronitrile (AIBN) were supplied by Merck (Darmstadt, Germany, http://www.merck.com/product/home.html). All acids (nitric, sulfuric and hydrochloric acid) and solvents used (acetonitrile, ethanol and dimethylsulfoxide) were of the reagent grade from Merck chemical company (http://www.merck.com/product/home.html) and used as received. Reagent grade HgCl2 · 6H2O and nitrate or chloride salts of other cations were used without any further purification (all from Merck, http://www.merck.com/product/home.html). Solutions of metal ions were prepared in doubly distilled water. The pH adjustment was done by using the nitric acid or sodium hydroxide. 5,10,15,20-Tetrakis(3-hydroxyphenyl)-porphyrin (T(3-OHP)P) was synthesized according to the literature .
Determination of mercury was carried out with a Shimadzu AA-670 atomic absorption spectrometer (http://www.ssi.shimadzu.com/products) equipped with an Hg-hollow cathode lamp and an on-line cold vapor generation system using SnCl2. The wavelength was set at 253.7 nm with a spectral bandwidth of 0.5 nm. A long path quartz cell (2 cm i.d., 10 cm long) connected to the spectrometer was used as a detection system. A digital pH meter, Metrohm model 632 (http://www.metrohm.com/Produkte2/Meters/index.html), equipped with a combined glass calomel electrode was used for the pH adjustments. Scanning electron micrographs were recorded using a Philips XL30 series instrument (http://www.semtechsolutions.com/node/124/philips-xl-30-sem) using a gold film for loading the dried particles on the instrument. Gold films were prepared by a Sputter Coater model SCD005 made by BAL-TEC (Switzerland). The FT-IR spectra (4,500–500 cm−1) were recorded on a Shimadzu FT-IR 8300 spectrophotometer (http://www.shimadzu.com/an/spectro).
Preparation of mercury ion imprinted and non-imprinted polymeric nanoparticles
The synthesis of non imprinted polymer (NIP) was carried out similar to that of the Hg(II)-IIP preparation procedure, but without addition of HgCl2 · 6H2O (template).
An aliquot of mercury solution (e.g., 15.0 mL of 1.0 μg.mL−1) was treated with 30 mg of polymeric nanoparticles at desired pH. The suspension was stirred for pre-selected periods of time using a magnetic stirrer. After centrifugation (5 min, 6,000 rpm), the supernatant solution was removed and the Hg2+ ions preconcentrated onto IIP nanoparticles were then eluted by using 5.0 mL of 1.0 M HNO3, while stirring for 5 min. The suspensions were then centrifuged and eluent solutions containing Hg2+ ions were removed from the nanoparticles. Finally, the mercury contents of the resulted solutions were then determined by CVAAS.
Analysis of water samples
Three different water samples including ground, river and waste waters were collected in PTFE flasks cleaned with a 10 % HNO3 solution overnight, then filtered through 0.45 μm pore-sized membrane filters immediately after sampling. The sample was acidified with a 1 % HNO3 solution and then irradiated for 2 h with a 30-W UV lamp in order to photooxidize the organo-mercury compounds, which could be present in water, and the pH was adjusted at the corresponding optimum value . Finally, the resulting sample solution was treated with synthesized Hg2+ IIPs and the recommended procedure for determination of mercury was followed.
Results and discussion
Characterization of synthesized IIP nanobeads
The prepared imprinted nanoparticles obtained via bulk polymerization method were characterized by colorimetry, FT-IR spectroscopy, and scanning electron microscopy.
An obvious change in the color from dark brown of unleached IIPs to dark green after the leaching process clearly indicated the successful removal of Hg2+ ions from the polymeric matrix. Meanwhile, the adsorption process was associated with an instant color change of the leached dark green nanobeads to a dark brown color, upon fast extraction of mercury ions into the imprinted polymeric matrix brown at the desired pH. According to the literature, the change in the color of solution containing porphyrin after addition of heavy metal ions can be attributed to the complex formation between metal ions and porphyrin, which is a result of metal binding to four N-atoms of the porphyrin ring .
The bands at 3,600–3,400 cm−1 correspond to the free -OH groups, which is present in both unleached and leached IIPs. The presence of bands at 3,500–2,700 cm−1 in both spectra confirms that the intermolecular hydrogen bonding present in the polymeric network of IIP was not collapsed or broken in leaching/unleaching processes [22, 25]. The above observations together with the presence of C-N (1,159 cm−1) and C = N (1,637 cm−1) stretch bands indicated that the ligand T(3-OHP)P has been sufficiently immobilized in the polymer matrix and is not affected during the leaching process. The absorptions due to carbonyl group (1,732 cm−1), C-O stretch (1,263 cm−1) and C-H vibrations (964, 1,456 and 2,956 cm−1) were also observed. However, the C–N stretching vibration at 1164.9 cm−1 in the unleached material is shifted to 1159.1 after leaching, indicating the participation of T(3-OH P)P nitrogen atoms in mercury ion binding.
Effect of pH
The effect of varying pH values in a range of 1.0–11.7 on uptake of Hg2+ ion from its solutions was investigated. Several batch experiments were performed by equilibrating 30 mg of the imprinted nanoparticles with 10 mL of solutions containing 1.0 μg.mL−1 of Hg2+ ion under the desired range of pH. The results revealed that the percent extraction of Hg2+ ions of 70 % was gradually increased by increasing pH of solution to 99 % at a pH of 4.0 and remained constant up to a pH 8.0. The low sorption quantity at lower pH values is most probably due to the competition of proton with mercury ions for binding sites in the IIP. On the other hand, at pH > 8, the Hg2+ extraction was sharply decreased to a value of about 35 % because of the well known hydrolysis of bulk Hg2+ ions, which resulted in the decreased concentration of free Hg2+ ions in sample solution. Thus, a pH range of 4.0–8.0 was chosen as the optimum pH for further experiments.
It is worth mentioning that the optimized pH values reported in previous works [2, 11, 13–16, 29, 30] are in the range of 6.0–8.0. This is most possibly due to the fact that tendency of the mercury ion for the imprinting cavity of IIP and complexation with the ligand used in the polymer is higher than that for the hydroxyl ions present in solution over this pH range.
Choice of eluent
In order to choose a proper eluent for the retained Hg2+ ion, after its uptake from aqueous media, different acidic solutions including HNO3, HCl, H2SO4 and CH3COOH were tested. After leaching of bounded mercury ions with 2.0 M solutions of each acid, the amount of mercury in effluent was determined by CV-AAS method. It was found that 5.0 mL of 2.0 M HNO3 is superior and can accomplish the quantitative elution of the mercury ion from IIP nanobeads. In order to study the optimum concentration of nitric acid solution as a suitable leachant, 5 mL portions of nitric acid solutions with different concentrations (i.e., 0.01, 0.1, 0.5, 1.0 and 2.0 M) were used for leaching of Hg2+ ions from the imprinted sites in the polymer network. The results showed that the desorption of Hg2+ ions is increased from 84 % with increasing concentration of nitric acid until a concentration of 0.5 M is reached. This is most possibly due to the increased protonation of donating nitrogen atoms of ligand T(3-OH P)P. Finally, desorption of mercury ion found to remain quantitative (i.e., >99 %) by using 0.5 to 2.0 M of nitric acid as leachant. Thus, 5 mL of 1.0 M HNO3 solution was selected as a proper leachant solution for further studies.
Adsorption and desorption times
The adsorbed Hg2+ ions were desorbed by treatment with 5.0 mL of 1.0 M HNO3 for different periods of time (i.e., from 2 to 45 min) under continuous stirring. The results shown in Fig. 4b revealed that the Hg2+ ions could be quantitatively eluted for 5 min. Various factors such as the extent of hydration of the metal ions, binding strength between the ligand and the template metal ion and polymer nanostructure are probably involved in determining the rates of Hg2+ desorption [4, 5]. The monodisperse nanoparticles form of the Hg-T(3-OH P)P complex also contributes to the fast adsorption-desorption kinetics established.
Adsorption capacity of the ion imprinted adsorbent for Hg2+ ions
Maximum sample volume and weight of IIP
Different volumes of sample solutions of pH 7.0 (i.e., 50, 100, 250, 400, 600, 800 and 1,000 mL) containing a constant amount of Hg2+ ion (i.e., 1.0 μg) were equilibrated with 30 mg of the sorbent in several batch procedures, upon stirring for 10 min. Then, the Hg2+ ions preconcentrated onto the IIP nanoparticles were stripped by using 5 mL of 1.0 M HNO3, while stirring for 5 min, followed by their measurement with CV-AAS. The results indicated that quantitative mercury recoveries (i.e., 99 % and higher) can achieve up to a sample volume of 800 mL. While, at higher sample volumes, the recovery decreased significantly. Thus, a high preconcentration factor of 160 was obtained for the prepared Hg2+ IIP.
The percent preconcentration of mercury ions with different weights of IIP (i.e., 10, 15, 20, 30, 50 and 80 mg) was also investigated. 25.0 mL of aqueous solution containing 1.0 μg mercury ions at pH 7.0 was used, while other experimental conditions were kept similar to those for the study of the sample volume. The results indicated that while percent extraction by 10 mg of IIP was 95 %, the lead extraction found to be quantitative (i.e., > 99 %) when 15 to 80 mg of IIP nanoparticles was used for the enrichment of mercury ions. Consequently, 30 mg of IIP particles was used for further studies.
Stability and repeated use
In order to examine the reusability of the prepared ion imprinted polymer particles, they were subjected to several loading and elution operations, under the same experimental conditions (i.e., amount of IIP, 30 mg; initial amount of metal ion, 0.1 μg.mL−1; pH, 7.0; treatment period, 30 min). After each use, 5 mL of 1.0 M HNO3 solution was used for elution of the adsorbed Hg2+ ions and regeneration of the IIP. The results obtained for 12 adsorption-desorption cycles by using the same imprinted nanoparticles clearly showed that Hg2+ imprinted particles could be repeatedly used without any significant loss in the initial binding affinity. In fact, the prepared Hg2+ ion-imprinted polymer nanoparticles can be repeatedly used for at least 12 cycles with recoveries not less than 97 %. In addition, these studies indicated the reversible nature of binding sites of the prepared IIP nanoparticles towards the Hg2+ ions in the successive adsorption-desorption cycles.
The prepared ion imprinted polymer nanobeads were repeatedly used and regenerated for at least 3 months. The results revealed no significant decrease in the adsorption-desorption efficiency of the prepared Hg2+ IIP over this period of time. This stability is most possibly due to the strong hydrogen bonding attachment of polymer chains to the monomers and ligand in the polymer network.
The analytical performance data for the batch preconcentration procedure was investigated under optimized experimental conditions. The relative standard deviation (RSD) for seven separate batch experiments with 30 mg of sorbent for the determination of 10 μg Hg2+ in 25 mL water was found to be 2.2 %. The preconcentration procedure showed a linear calibration curve within the Hg2+ concentration range of 1.0–100.0 μg.mL−1. The limit of detection, defined as the concentration of analyte giving a signal equivalent to three times of the blank standard deviation plus the net intensity of the blank for 25 mL of sample volume, was 0.08 μg · L−1.
Due to the presence of uniformly distributed cavities complementary to the imprint ion in size and coordination geometries and selective affinity of the ligand used for the imprint ion, the IIPs are expected to possess good selectivities for the sorption of the metal ion of interest [17, 21].
Distribution ratio (Kd), selectivity coefficient (k) and relative selectively coefficient (k′) values of ion-imprinted polymer (IIP) and control non-imprinted polymer (NIP) material for different cations
Kd (IIP) (mL.g−1)
Kd (NIP) (mL.g−1)
A comparison of the selectivity results of the imprinted and control materials given in Table 1 clearly revealed the ion-imprinting effect of the prepared Hg2+ IIP nanoparticles based on porphyrin derivative T(3-OH P)P as a suitable chelating agent. The increased tendency of the IIPs for complex formation with Hg2+ ion, resulting from the imprinting process, is caused by the energy benefit due to formation of nondistorted complexes in comparison with non-imprinted samples . The selectivity coefficients obtained on ion-imprinted polymeric nanoparticles revealed the fact that the quantitative removal and determination of Hg2+ ions can be achieved even in the presence of excesses of the interfering ions.
The high selectivity coefficients obtained on ion imprinted polymeric nanoparticles are expected to result in the quantitative removal of mercury ion from other metal ions in real samples.
Determination of Hg2+ ions in three different water samples (n = 3)
Comparison of the figures of merit of the method with those of some methods recently reported in literature
The results presented here demonstrate the efficiency of the bulk polymerization procedure for the preparation of novel IIP nanobeads using 5,10,15,20-tetrakis(3-hydroxyphenyl)-porphyrin as a novel sorbent for selective and sensitive determination of mercury ions. Rapid kinetics of the adsorption and desorption of Hg2+ ion on the resulting imprinted sorbent provided a fast preconcentration procedure for mercury ion in aqueous solutions. The new sorbent revealed an excellent selectivity toward mercury ion over a range of strong competing metal ions. The work is in progress in our laboratories to use the prepared imprinted sorbent in selective on-line solid phase extraction, in order to achieve determination of sub-ppb level of mercury ion and increased sampling frequency of the method in analysis of complex aqueous samples. A comparison between the sorbent characteristics, including the polymerization method, porogen solvent, detection system, capacity factor, preconcentration factor, limit of detection, pH, and particle size of the prepared Hg(II)-IIP with those of the previously reported ones [2, 11, 13, 14, 16, 29, 30] (Table 3) clearly revealed that this sorbent can be categorized among the best mercury IIPs ever reported.
The authors acknowledge the financial support of this work by Iran Elites National Foundation (IENF) via the late Allameh Tabatabaei prize.