Solid-phase organic synthesis of sensing sorbent materials for copper and lead recovery

Journal of the Brazilian Chemical Society

Castillo, M.¹  Technological Institute of Tijuana, México¹  Pina-Luis, G.¹  University of Oviedo, Oviedo, Spain¹  Díaz-García, M. E.¹  Rivero, I. A.¹ 

关键词: solid phase synthesis;    metal sorbents;    fluorescence sensing;    continuous monitoring;   

DOI  :  10.1590/S0103-50532005000300015

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Heavy metals contamination is recognized as a priority problem in environmental protection. Metal ions such as copper, lead, mercury, cadmium, cobalt etc, usually represent an environmental concern when present in uncontrolled and high concentrations. For example, elevated copper concentrations are frequently associated with leaching from antifouling paints and pressure-treated docks pilings, discharges from power and desalination plants and runoff from other land-based sources.1-4 Due to their persistence in the environment and the relatively rapid uptake and accumulation in living organisms, heavy metal ions are polluting water resources and may cause long-term risk to humans and ecosystems. The direct determination of metals in effluents or in saline matrices (e.g seawater) by atomic spectrophotometry such as flame or plasma techniques is difficult due to the nebulizer blockage, the high background, transport and chemical interferences which result in a decrease in precision and sensitivity. Therefore, separation and preconcentration methods have played a fundamental role in solving these problems.Solid-phase extraction is nowadays one of the interesting areas in analytical chemistry for sample clean-up. Anchoring a reactive motif to a solid support provides an immobilized active surface capable of selective and quantitative separation of cations from aqueous solution. These systems have important advantages over traditional liquid-liquid extraction or precipitation methods.For example, they exhibit higher preconcentration factors, greater efficiency, higher reproducibility and simple handling. Among the solid materials used as sorbents for metal ions, chelating resins are extensively used due to their high selectivity.5 Although there are various chelating groups, those derivated from thiol and amine groups have high affinities towards Hg(II), Cu(II) and Pb(II).6 For a rational design of a metal ion fluorescent sensing sorbent the system can be disassembled into three components as shown in Figure 1: the metal ion recognition/retention moiety, the optical reporter and the solid support.    With this in mind, Merrifield, Wang and Argogel resins, extensively used in the solid phase synthesis of many molecules,7-11 were evaluated for preparation of immobilized recognition units. Due to its strong luminescence and stability, the anthracene unit has been used as the light-emitting group and a phosphine sulfide recognition domain, with selective metal binding properties, was attached to the 9-position on anthracene, according to the synthetic route outlined in Scheme 1.12    By exploiting the selective metal binding properties of the phosphine sulphide domain, we have sought to combine the advantegeous aspects of both fluorosensing within selective metal immobilization. In this paper insight into the sorbent and signalling properties of these materials for Cu(II) and Pb(II) in a continuous flow approach, the influence of the solid support used and their complexation behaviour towards these metal ions are described.   ExperimentalMaterialsAll resins used in this work were purchased from Argonaut Technologies Inc. PS Wang (polystyrene backbone lightly cross-linked with divinylbenzene) have loading of 1.21 mmoles g-1, Argogel Wang (polystyrene backbone (1-2%) cross-linked with divinylbenzene grafted with polyethylene glycol) have loading of 0.47 mmoles g-1, and Merrifield resins (polystyrene backbone cross-linked with divinyilbenzene) have loading of 1.19 mmoles g-1. All other reagents, if not specified, were purchased from Aldrich. All other chemicals, buffers and solvents were of analytical reagent grade and were used without further purification. All aqueous solutions were prepared using deionized water. Stantards solution of Cu(II) and Pb(II) at different concentrations were prepared in the buffer solution.InstrumentationAll fluorescence intensity measurements were made with a Shimadzu RF-5301 PC spectrometer which has a xenon discharge excitation source (pulse width at peak half-height < 10 µs). The 3 nm slit width for both excitation and emission intensities were used. Instrumental parameters and processing data are controlled by the Fluorescence Data manager software. A single flow injection system equipped with a Hellma Model 176.52 flow cell (25 µL) was used. Beads were washed free of any starting materials and solvents and were packed into the flow-through cell. Measures of fluorescence intensity were obtained directly from the resin beads. All experiments were carried out at 20 ± 2º. pH measurements were made with an Orion Model 710A pH/ISE Meter. Lead and copper were estimated by Atomic Absorption spectrometer using a Varian Spectrometer AA-5 model with acetylene-air flame. Mass spectra were obtained on Agilent 1100 Series LC/MSD Trap 9. Elemental Analyses were performed at Numega Resonance Labs. San Diego, CA. Combinatorial Chemistry was carried out in a Quest Reactor Argonaut model SLN-210.Flow injection manifold and general procedureThe experimental set up for the system is shown in Figure 2. A Minipuls 2 four-channel peristaltic pump (Gilson, Worthington, OH, USA) was used to generate the flow stream. A type 50 PTFE four-way rotatory valve (Omnifit, Cambridge, UK) provided with a 125 µL sample loop was used for solution standard introduction. PTFE tubing (0.8 mm id) and fittings were used for connecting the flow-through cell, the valve and the carrier solution. The carrier buffers consisted of 0.1 mol L-1 tris (pH 7.7) for Cu(II) determination and 0.1 mol L-1 buffer (pH 2.4) for Pb(II) determination. Flow rate was 0.8 mL min-1.    CalibrationStandard solutions of metal ions (consisting of carrier solution plus a known volume of the standard metal solution) were injected through the valve into the carrier flow stream. This stream was merged with the carrier mixed in the reaction coil and driven to the flow cell packed with immobilized resins (4a-c). Typical quenching fluorescence signals were obtained and the Stern-Volmer relationship was plotted against molar metal ion concentration. Results and Discussion Our synthetic route (Scheme 1),12 involves the initial reaction of anthraquinone with Me3SI in the presence of NaH and DMSO as the solvent at 93 ºC to obtain 9,10-anthracenediepoxide (yield 82%). This intermediate reacts with KI/LiBr to generate 10-methanol-9-anthracene carboxyaldehyde 1 (yield 75%). The next reaction involves the coupling of 1 to the Merrifield, Argogel-Cl and Wang-Cl resins through a nucleophilic displacement of chloride in the presence of THF/NaH, followed of a reduction with NaBH4. The chlorination reaction is carried out in the presence of triphosgene (BTC)/triphenylphosphine13 to obtain the chlorinated resin derivatives PS-10-oxymethyl-9-chloromethyl-anthracene (2a-c). Polymer-supporting yields were calculated according to Volhard titration of the residual chlorine content in the resin14 (92-99 % conversion). The chlorinated resins were treated with phosphine Grignard previously prepared in THF to obtain PS-10-oxymethyl-9-(diisobutyl-phosphine oxide-methyl)-anthracene (3a-c) (82-91% conversion). Finally, reaction of the phosphine resins with P4S10 in DMA as solvent to replace the oxygen by sulphur gave the desired immobilized anthracene-phosphine sulfide resins (4a-c).The final products were washed with THF/ DCM/ MeOH and dried under high vacuum for 12 h. Elemental analysis of final product were realized.Yields and IR data are presented for Merrifield, Argogel and PS Wang-based anthracene-phosphine-sulfide final materials. 4ax. 0.47 g (yield 95 %); 76 % conversion; IR(KBr) nmax/cm-1: 3058, 1602, 590 (P=S); ESIMS (m/z): 377 [M(C23H37PS) + H]+. Anal. calc. for polymer-supported: C, 87.91; H, 7.06; S, 2.08. Found; S, 1.58.4bx. 0.47 g (yield 96 %); 68 % conversion; IR(KBr) nmax/cm-1: 3052, 1109, 592 (P=S); ESIMS (m/z): 377 [M(C23H37PS) + H]+. ]+. Anal. calc. for polymer-supported: C, 88.00; H, 7.01; S, 2.05. Found; S, 1.39.4cx. 0.37 g (yield 74 %); 65 % conversion; IR(KBr) nmax/cm-1: 3052, 1109, 592 (P=S); ESIMS (m/z): 377 [M(C23H37PS) + H]+. ]+. Anal. calc. for polymer-supported: C, 77.54; H, 7.91; S, 1.29. Found; S, 0.84.The sulphur percentage indicates the presence of total content of isobutyl-phosphine sulfide bound to the resins. The immobilized anthracene-phosphine sulfides were cleaved from the resins by treatment with a solution of TFA:DCM (1:10) during 30 min in an ultrasonic bath. The resins were filtered and 1 µL of the washing solution was directly injected on an ES-MS. The cuasi molecular ions MH+ = 369 were obtained, verifying the presence of the phosphine sulfide group. In Table 1 the solid supports and phosphine sulfide substituents used in the synthesis are shown.    Spectral characteristics of the fluorescent sorption materialsThe fluorescence spectra of 4ax, 4bx and 4cx were obtained in order to evaluate the influence of the solid support on the spectral characteristics of the fluorescent materials. As observed in Figure 3, the spectra of the Argogel-based materials do not present the pattern of the structured bands of the anthracene moiety. A possible explanation could be the formation of intramolecular excimers among anthracene moieties.15 On the other hand, the Wang- and the Merrifield-based material show the structured bands typical of the anthracene moiety. An important red shift (ca. 220 nm) was observed for the Merrifield resin.    Spectral fluorimetric data are collected in Table 2 in which we can observe that excitation and emission maximum wavelengths differ from that of "blank" (unmodified) resins, indicating an effective incorporation of the anthracene-phosphine sulfide onto the resins.    When considering the nature of the phosphine sulfide-alkyl substituent, in the

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