A Multidimensional Undergraduate Experiment for Easy Solution and

Laboratory Experiment pubs.acs.org/jchemeduc

A Multidimensional Undergraduate Experiment for Easy Solution and Surface Sensing of Mercury(II) and Copper(II) Metal Cations Arturo Espinosa,* Francisco Otón, Rosario Martínez, Alberto Tárraga, and Pedro Molina Departamento de Química Orgánica, Universidad de Murcia, 30100-Murcia, Spain S Supporting Information *

ABSTRACT: The recently reported bis(1-pyrenyl)azine, easily prepared in a one-step reaction from 1-pyrenecarboxaldehyde and hydrazine hydrate, is used as a receptor for the visual detection of Hg2+ and Cu2+ ions in solution in an undergraduate laboratory experiment. The receptor was also supported on both filter paper and a hydrophobic PTFE (polytetrafluoroethylene) membrane, and the resulting probe-coated sheets were successfully used for selective sensing of the same metal ions using optical and fluorescent spectroscopies. This project provides an opportunity to organic chemistry students to learn about the challenging problem of selectively detecting toxic metal ions, such as Cu2+ and Hg2+, in solution.

KEYWORDS: Second-Year Undergraduate, Analytical Chemistry, Environmental Chemistry, Laboratory Instruction, Organic Chemistry, Hands-On Learning/Manipulatives, Fluorescence Spectroscopy, Qualitative Analysis, Synthesis, Transition Elements

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monomer,10 with a wavelength for the maximum of the excimer emission (λE) generally falling in the range 475−485 nm. Based on this knowledge, the synthesis, characterization, and cation-coordination properties of receptor bis(1-pyrenyl)azine, 1, that brings together the photoactive pyrene ring with the 2,3diaza-butadiene group11 (azine) as both cation binding and quencher site (Scheme 1), was reported.12 This receptor showed interesting dual optical- and fluorescent-sensing abilities toward Hg2+ and Cu2+ cations. Fixation on solid substrates of colorimetric or fluorimetric reagents for HTM detection has been attempted by several

ational design and preparation of new sensors for heavyand transition-metal (HTM) ions is a task of prime importance for chemical, environmental, or health applications.1 In particular, mercury is one of the most toxic elements to microorganisms and the environment2 that can be released from various sources, such as fossil fuel combustion or the electronic industry. Copper toxicity can occur from eating acid foods cooked in uncoated copper cookware, exposure to excess copper in drinking water, or other environmental sources.3 It generates reactive oxygen species that damage proteins, lipids, and DNA.4 Among the sensors for HTM ions, optical detectors that allow on-site, real-time, and nondestructive qualitative or semiquantitative detection without the use of any complex spectroscopic instrumentation have received much attention.5 In particular, fluorescent sensors allow fast detection of HTM ions by simple enhancement or quenching of the fluorescence response6 and usually display low detection limits. Sensors must combine an ionophore unit that selectively binds the cation and a closely connected fluorophore that signals detection via off−on fluorescent switching. In the ionophore, nitrogen-binding sites are often the best choice for the selective recognition of soft HTM ions, as exemplified with azacrown ethers.7 Recent computational studies have shown that N binding sites display stronger interactions to Hg2+ in comparison to the kinetically favored S donor sites.8 Pyrene is an effective fluorescence probe due to its high detection sensibility9 and its emission wavelength is sensitive to the polarity of the local environment. Formation of the selfassembled complex results in a remarkable change in the fluorescence emission intensities of pyrene excimer and © XXXX American Chemical Society and Division of Chemical Education, Inc.

Scheme 1. One-Step Synthesis of Bis(1-pyrenyl)azine, 1, from 1-Pyrenecarboxaldehyde and Hydrazine Hydrate

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dx.doi.org/10.1021/ed300571s | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

the filter paper, and the results were recorded for visual and UV detection. (iv) PTFE membrane-supported sensing: a disc of PTFE membrane was impregnated with a solution of 1, the membrane was cut into equal pieces, a solution of metal ion was spotted on each piece, and the color change was recorded.

techniques.13 However, they have some drawbacks: complicated synthetic procedures, insufficient sensitivity, and requirements for auxiliary additives. Cellulose is an abundant, inexpensive, biodegradable, renewable biopolymer exhibiting good mechanical properties and water-absorbing behavior and can be modified to tailor its chemical and physical properties.14 The use of cellulose-based materials, including filtering paper, could be extended to new applications by incorporation of functional detecting molecules onto the fiber surface. Thus, paper strips have recently been used in biomedical assays15 and HTM ions detection.16 Also, hydrophobic PTFE (polytetrafluoroethylene) membranes are becoming an attractive option as multipurpose solid support due to their high thermal, mechanical, and chemical stability.17 Toxic HTM cation pollutants have generated growing interest at the student laboratory level, as indicated by recent experiments devoted to removal18 or analysis of copper(II)19 or mercury(II)20 cations. A series of lab experiments were designed with the aim of sensing certain analytes. Among them, several metal cations are toxic in different degrees (Hg2+, Cu2+, Cd2+, Pb2+, and Ni2+) and offer an additional valuable opportunity to show students appropriate handling and waste removal for hazardous species. A simple undergraduate-adapted synthesis with spectroscopic characterization of receptor 1 and its use in optical and fluorescence metal ion sensing either in solution or supported in solid materials have been successfully incorporated into a second-year undergraduate organic chemistry lab course. Thirty pairs of students executed this lab over consecutive two, 3 h lab periods during the last three academic years. The solid-support versions are a filter paperand a PTFE-membrane-based visual system for mercury and copper that employ a simple procedure for on-site selective metal-ion detection. Underlying concepts regarding the coordination chemistry of Cu2+ and Hg2+, as well as others, such as excimer formation, could be additionally proposed to students based on the analysis of reported geometries for model complexes.12

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Identification of Metal Ions in an Unknown Solution

As a final test for checking the acquired skills, students were provided with samples (three to five, depending on time) to be analyzed and containing unknown solutions of (a) Cu2+, (b) Hg2+, (c) both Cu2+ and Hg2+, or (d) none of them, in addition to any of the other silent metal cations.

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HAZARDS Safety goggles and gloves are worn at all times in the laboratory. Hydrazine hydrate is toxic (by inhalation and skin absorption), corrosive to tissue, potentially flammable (contact with oxidizing materials may cause spontaneous ignition), and must be handled under a fume hood. Ethanol, acetone, and acetonitrile are highly flammable and irritating to eyes. 1Pyrenecarboxaldehyde is irritating to eyes, respiratory system, and skin. Acute exposure to isopropyl alcohol might cause irritation of the eyes and the mucus membranes in the face, skin eczema, and tenderness. Dimethyl sulfoxide-d6 is flammable and an irritant by inhalation or skin contact. Caution is also required for handling all metal ion solutions, especially those of Hg2+, Cu2+, Cd2+, Pb2+, and Ni2+. In particular, perchlorates interfere with iodide uptake into the thyroid gland and can be explosive. All wastes, including receptor 1 and deuterated solvents, must be collected in suitably labeled waste containers for appropriate subsequent processing and elimination.

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RESULTS AND DISCUSSION

Synthesis and Characterization of Sensing Ligand 1

The synthesis of the stable ligand 1 was started at the end of a previous lab session and refluxed for 12 h with a timer socket (see the Supporting Information for other time constraints). It crystallized from the reaction mixture on cooling with sufficient purity. Additional recrystallization from ethanol was not necessary. Students obtained a yield of 40−65% (normally the higher reaction time, the larger yield obtained). Depending on time limitations, the characterization of 1 could be either skipped or accomplished by providing students with appropriate spectra (see the Supporting Information).

EXPERIMENT OVERVIEW

Synthesis and Characterization of Sensing Ligand 1

The beginning of this experiment was scheduled at the end of the previous lab session (see timing details in the Supporting Information). The synthesis of 1 was completed overnight by refluxing 1-pyrenecarboxaldehyde and a slight excess of hydrazine hydrate in ethanol; on cooling, a yellow solid was collected by filtration and can be recrystallized from ethanol (a detailed procedure is provided in the Supporting Information). The product can be characterized by IR, UV−vis, and 1H NMR spectroscopies (student data in the Supporting Information).

Sensing Metal Ions with Ligand 1 in Solution

The next part of this experiment dealt with the use of the ligand for metal ion sensing. Preparation of a large number of different metal-ion solutions could be conveniently distributed among several groups of students, for example, 15 groups could prepare solutions of 15 different proposed metal cations in two solvents (acetone and acetonitrile). In a small test tube or vial, metal-ion solutions in acetonitrile were added to an acetonitrile solution of 1. No color change, besides dilution, was observed upon addition of any of the metal-ion solutions (Figure 1, only divalent metal cations shown), except in the cases of Cu2+ and Hg2+ that afforded reddish solutions. This allowed easy, visual detection of these two metal ions. The fluorescent emission of receptor 1 in acetonitrile is almost quenched, probably due to a photoinduced electron transfer process from nitrogen electron pairs. The fluorescent emission of the prepared solutions of 1 and the set of metal

Sensing Metal Ions with Ligand 1

Acetonitrile and acetone solutions of 1, as well as of a set of the following metal ions (perchlorate or triflate salts, one single metal cation in each solution), or a selection of them, were prepared: Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+, Sm3+, Eu3+, Yb3+, and Lu3+. These solutions were used in the following experiments. (i) Visual detection: equal volumes of ligand 1 and metal ion were mixed together and the color change for each solution was recorded. (ii) Fluorescence emission: each solution in (i) was viewed under a UV lamp (λexe = 365 nm) and the results were recorded. (iii) Filter paper-supported sensing: a disc of filter paper was impregnated with a solution of 1, solutions of the metal ions were spotted on B




Figure 1. Color displayed by CH3CN solutions of 1 after addition of 1 equiv of different metal cations (taken from students results).

cations showed changes in the fluorescence behavior only in the cases of Cu2+ and Hg2+ (Figure 2), which displayed a red-shift Figure 4. Color displayed by 1 supported on a hydrophobic PTFE membrane upon addition of different metal cation solutions (taken from students results).

case only, mercury caused the change whereas the copper cation hardly interfered. Identification of Metal Ions in an Unknown Solution

Figure 2. Fluorescence emission (λexc = 365 nm) in CH3CN solutions of 1 after addition of 1 equiv of different metal cations (taken from students results).

Once the students mastered the four analysis techniques (i−iv), unknown solutions containing one or several metal cations, prepared by the instructor (or even proceeding from a “real sample”),18 were supplied to students to perform the detection of copper and mercury. Students tended to use the simplest methods in solution through both visible and fluorescence channels. Unsuccessful results were given only when more diluted samples were provided (intentionally or not) because color intensity did not allow a clear differentiation. In these cases, a second chance made students turn to a solid-supported (filter paper or PTFE) technique as they enabled signal amplification by using larger volumes of the solutions. Under these circumstances, the correct answer concerning the existence of (a) Cu2+, (b) Hg2+, (c) both, or (d) none of them in the unknown samples was almost always successfully achieved (>95% of cases).

with a large intensity enhancement. Excimer formation can be assumed for the Hg2+ complex.11 Competitive experiments showed no interference with other metal cations. Solid Phase-Supported Metal-Ion Sensing

Acetone solutions of the dication metal ions were added as a single spot within the appropriate sector of filter paper impregnated with 1. Visual inspection of the filter paper did not show any observable change, but, under UV light, only Cu2+ caused a remarkable change in the receptor’s fluorescence (Figure 3). With this methodology, Cu2+ was easily detected even in mixtures with other metal cations.

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CONCLUSION The selective complexing behavior of bis(1-pyrenyl)azine, 1, toward Cu2+ and Hg2+ metal ions was the basis for a set of simple undergraduate experiments that included (a) the synthesis of the ligand, (b) the visual colorimetric, and (c) the fluorimetric selective sensing of these two metal cations in solution. The experiment was also adapted for the preparation of solid-supported sensors by using either filter paper or hydrophobic PTFE discs. The project was easily, quickly, and successfully conducted in all constituent individual steps and the final analysis of unknown samples contributed to stimulate student’s enthusiasm. Students showed good acceptance of this experiment not only in terms of understanding of the underlying concepts, but also from the reliability and easiness of the protocol.

Figure 3. Fluorescence of 1 supported on a filter paper after addition of solutions of different metal cations: using UV light of (A) λexc = 254 and (B) 365 nm (taken from students results).

Receptor 1 was also supported at the central part of a commercially available hydrophobic PTFE filter disc that was then cut into eight sections to give individual sensor units. Every acetone metal solution was dropped onto a sensor section and dried with hot air. No color change developed for any of the metals ions tested, except in the case of the Hg2+ solution that markedly darkened the section (Figure 4), indicating formation of a different chemical species. This analysis was complementary to the filter paper because, in this

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ASSOCIATED CONTENT

* Supporting Information S

Student lab manual including background; instructor notes including timing, materials, hazards, and possible variations; experimental methods; IR and NMR spectra. This material is available via the Internet at http://pubs.acs.org. C



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by MICINN-Spain (Project CTQ 2011/ 27175) and Fundación Séneca (Project 04509/GERM/06). REFERENCES

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A Multidimensional Undergraduate Experiment for Easy Solution and

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