Paper-Based Heavy Metal Sensors from the Concise Synthesis of an

Laboratory Experiment pubs.acs.org/jchemeduc

Paper-Based Heavy Metal Sensors from the Concise Synthesis of an Anionic Porphyrin: A Practical Application of Organic Synthesis to Environmental Chemistry Jutamat Prabpal, Tirayut Vilaivan, and Thanit Praneenararat* Department of Chemistry, Faculty of Science, Chulalongkorn University, Phayathai Road, Pathumwan, Bangkok 10330, Thailand S Supporting Information *

ABSTRACT: Tetrakis(4-sulfonatophenyl)porphyrin (TSPP) was immobilized on patterned paper and used as a sensor for heavy metal ions in an advanced organic chemistry course. The resulting sensor could detect Hg2+ and Cd2+ ions colorimetrically, while Cu2+ ion resulted in fluorescence quenching, thus demonstrating a multiplex capability of this paper-based sensor. From the synthesis of the porphyrin to the fabrication of the sensor, advanced undergraduate chemistry students gained firsthand experience with a classical organic synthesis, and also witnessed its application within 2−3 full-day laboratory sessions. This experiment is a good demonstration of how an easily synthesized compound can be exploited to tackle an important problem in real-world situations. KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Environmental Chemistry, Organic Chemistry, Hands-On Learning/Manipulatives, Applications of Chemistry, Fluorescence Spectroscopy, Qualitative Analysis, Synthesis, Transition Elements

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o serve the advancement of modern technologies, transition metals are heavily used in a variety of products such as batteries and electronics. Although these metals are essential to the manufacturing process of these products, it is well-established that they can pose serious hazards to human health and the environment. Therefore, there have been several attempts to reduce, recycle, or reuse the metals to avoid creating excessive waste.1,2 As a first measure, the ability to detect and quantify these metals would be beneficial for the overall management. Therefore, several methods have been developed to allow detection and quantification of these metals with great sensitivity and selectivity.3 Nevertheless, a majority of these methods, e.g., atomic absorption spectroscopy, require sophisticated instruments and highly skilled operators, and are thus considered to be inappropriate in field tests where resources are limited. Driven by the aforementioned problem, a number of simpler sensing devices recognizing heavy metals were developed in recent years,4,5 with a focus on simple readouts such as colorimetric (naked-eye) changes,6,7 or fluorescence from an excitation at a “black light” wavelength.8 These sensors usually needed organic chromophores that can coordinate to transition metals with concomitant spectroscopic changes. Moreover, in recent studies these molecules were also embedded or immobilized on solid supports to enhance their versatility and applicability.9−11 A good example of this application is the use of microfluidic paper-based analytical devices (μPADs) where heavy metal ions could be easily and conveniently detected.12,13 © XXXX American Chemical Society and Division of Chemical Education, Inc.

Among diverse classes of chromophores exerting spectroscopic changes upon metal bindings, porphyrins gained only little attention as a chromophore for metal sensing due to certain limitations. Although porphyrins are well-known for the ability to coordinate with various metal ions, some of these complexations were shown to be relatively slow.14,15 More importantly, most of these metal−porphyrin complexes did not exhibit significantly different absorption spectra from the free ligand, thus being deemed unusable as metal sensors. Nevertheless, recent studies showed that certain charged porphyrins could exhibit substantial colorimetric changes that can be detected by naked eyes.7,16,17 On the basis of these studies, an interdisciplinary advanced undergraduate chemistry laboratory experiment for a Practical Organic Chemistry course was developed. Students synthesize and characterize a charged porphyrin derivative, and use it for a qualitative study to sense metal ions both in solution and on a paper support. In fact, undergraduate-level experiments related to identification and quantification of heavy metals have gained much attention for several years.13,18−21 For example, two reports highlighted the use of atomic absorption spectroscopy (AAS) to perform quantitative analysis of heavy metals, mainly mercury.20,21 Although these studies efficiently showcased an Received: December 7, 2016 Revised: June 6, 2017

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DOI: 10.1021/acs.jchemed.6b00943 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

executed this experiment in the last two academic years. The overall pedagogical goal of this experiment is to expose advanced undergraduate chemistry students to a combination of learning skills that leads to a practical tool for real-life problems. First, as a practical organic chemistry course, students were required to synthesize their own batches of TSPPthis is to make the students appreciate the process of getting a useful compound that they will utilize later. Although this does not lead to a significant enhancement of synthesis skills, it serves as practice for students who may have various backgrounds. The characterization also serves as first-hand experience of a unique feature in the nuclear magnetic resonance (NMR) spectra of porphyrins, namely, the highly upfield chemical shift of pyrrole N−H protons. It is relatively rare to observe protons in this region for typical compounds at the undergraduate level. Thereafter, they received a hands-on practice of basic analytical techniques such as reagent preparation, which sets the stage for of a more specialized topic, namely, paper-based microfluidic devices, through construction of a simple paper-based sensor. Last but not least, they were trained on proper safety for laboratory setups and waste disposals.

adaptation of a conventional instrument to teaching laboratory classes, they were not designed to be adopted by institutions that lack a sophisticated instrument such as an atomic absorption spectrometer. A more conventional experiment using a titration technique to selectively detect mercury was also reported.22 This, on the other hand, was designed to be a more traditional experiment (requiring more reagents with a higher detection limit), and is thus considered to be different from the work reported herein. Another approach in designing a teaching experiment that involves an analysis of metal ions is to utilize organic chemosensors. For instance, Paddock et al. illustrated the use of a polymeric material as a support for incorporating a chelating agent that is selective to Cu2+ ion.18 This format, however, did not allow a multiplex analysis where more than one metal ion can be tested. Another study by Espinosa et al. showcased the utility of an organic chemosensor that is selective to Cu2+ and Hg2+.19 Although the changes after metal complexation were very clear in the fluorescence mode, the change on solid support (a PTFE membrane) was quite ambiguous, thus preventing a broader use of this molecule on solid support. In addition, the molecule is highly hydrophobic, thus requiring the use of a hydrophobic membrane and an organic solvent to dissolve the compound, which renders it more difficult to connect with real-world applications. A water-soluble, anionic porphyrin, 5,10,15,20-tetrakis(4sulfonatophenyl)porphyrin (TSPP), has been used to detect Fe3+ and Al3+.23 However, the study did not include other metal ions, and only discussed an observation of a change in the colorimetric mode. Herein, an experiment is described that begins with the synthesis of TSPP (Scheme 1).24,25 Thereafter, TSPP is added to patterned papers made from hydrophobic inks, and the resulting paper-based sensor is used to detect Hg2+, Cd2+, and Cu2+ ions by naked eyes by observing colorimetric and fluorescence changes. The whole experiment required 3 days of 6 h laboratory sessions, and 15 students have

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MATERIALS AND METHODS

Synthesis and Characterization of Porphyrins

5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin (TSPP, isolated as a sodium salt) was synthesized by a well-known method of porphyrin synthesis (the Adler−Longo method),24 and subsequent sulfonation (Scheme 1, details can be found in the student instructions in the Supporting Information). Filtrations with pH adjustment and salt formation by neutralization with NaOH were the key purification steps. Students’ samples were taken for 1 H and 13C NMR experiments, which indicated sufficient purity (see Supporting Information) for both reactions. Metal Sensing of TSPP in Solution Phase

Scheme 1. Two-Step Synthesis of 5,10,15,20-Tetrakis(4sulfonatophenyl)porphyrin (TSPP, as a Tetrasodium salt)

A variety of metal ion solutions (K+, Na+, Ca2+, Mg2+, Cr3+, Fe2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Pb2+, and Hg2+) were prepared, either by the students or staff (depending on available time of the class). The concentration of all metal ions tested was 0.10 mM. Solutions were prepared in 2 mM HEPES buffer pH 7.5. There were 15 drops of TSPP solution (0.10 mM in 2 mM HEPES pH 7.5) added into each of a set of standard test tubes. The number of test tubes in the set depends on how many “inactive” metal ions the instructor needs. In this case, 14 tubes, inclusive of a blank solution, were used. Thereafter, each metal ion solution (0.10 mM in 2 mM HEPES pH 7.5) was added into each tube for 15 drops. For colorimetric tests, the changes were seen instantaneously after gentle shaking. For fluorescence tests, a 5-min waiting period was required for clear fluorescence reduction by the active metal ion (Cu2+ ion). The fluorescence test would require a hand-held 365-nm UV lamp or a UV transilluminator. If photographs need to be taken, a customized box made from cardboard is needed unless the light of the room can be controlled. Fabrication of Patterned Papers

Patterned papers (Whatman No. 1 filter papers) were fabricated by drawing patterns (a pattern was shown in the instructions for students) with permanent markers. For avoidance of excessive liquid spreading, the circle areas were B




gave the sodium salt of TSPP with an average yield of 45% after workup and purification. The spectra of both TPP and TSPP were found to match with the reported data.25 If a UV−vis spectrophotometer is available, this is a good opportunity for students to observe the characteristic absorption peaks of porphyrins in TSPP, namely, the Soret band (major absorption peak at 413 nm), along with multiple Q bands (Figure 1). A

lifted by folding the borders of the paper (see Supporting Information for details). Template rulers can be used to create more neat and equal patterns. Students were warned to avoid vivid color markers as many of them had fluorescent color additives that would interfere with observation of fluorescence from TSPP. Metal Sensing of TSPP Immobilized on Patterned Papers

One drop of TSPP solution (0.10 mM in 2 mM HEPES pH 7.5) from a Pasteur pipet was added onto a prepatterned paper and allowed to air-dry. Each metal solution (0.10 mM in 2 mM HEPES pH 7.5) was added (one drop) directly into a circle, and colorimetric change was observed immediately. It was found that the color change was more easily observed while the liquid drop of metal solution was still intact. For fluorescence measurement, changes can be observed after drying.

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HAZARDS Safety goggles and other standard protective equipment must be worn all the time. Care should be taken when handling organic compounds and solvents, most of which are flammable and irritating. Students were particularly advised to avoid contact with propionic acid, which has an unpleasant smell that persists on skin and cloths. For other chemicals, specific hazards can be found in the Supporting Information. Because the experiment involves the use of diverse types of chemicals (organic, inorganic, heavy metals), instructors are advised to use this as an opportunity to educate students about proper waste management and categorization.

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Figure 1. UV−vis (violet ) and fluorescence (orange ---, excited at 413 nm) spectra of TSPP (0.01 mM in 2 mM HEPES pH 7.5). Inset: Photos of TSPP under white light (left) and 365-nm UV light (right). These data were obtained by our laboratory students, who also synthesized the TSPP sample used in this measurement.

RESULTS AND DISCUSSION

fluorescence spectrophotometer, if available, would also provide additional insight as emission peaks are clearly visible at 643 and 704 nm (when excited at 413 nm). A bright pink fluorescence could be observed by naked eyes when the solution was illuminated with a 365-nm UV lamp or transilluminator (Figure 1, inset).

Synthesis and Characterization of Porphyrins

The development of synthetic methods to prepare porphyrins has been a subject of extensive studies in the past few decades. Although the synthetic method pioneered by Lindsey et al.26 is considered the most efficient and high-yielding method, the Adler/Longo method24 was chosen due to its simpler reaction setup, lower costs, greater availability of reagents, and ease of purification. In this method, freshly distilled pyrrole (which could be prepared by students for the practice of distillation technique) and benzaldehyde were mixed in propionic acid under reflux condition for 1 h (Scheme 1). Thereafter, vacuum filtration and methanol washing of the resultant purple solid provided the product, TPP. Typical yields obtained by students averaged around 15%, which, although being rather mediocre, provided ample material for further testing. Also, only filtration and washing were required as a workup step, which was wellsuited for undergraduate experiments with limited time and resources. The TPP obtained from this method was sufficiently pure for further reactions, as illustrated by 1H NMR and 13C NMR spectroscopy (Figure S1). A chemical shift at −2.71 ppm in the 1 H NMR spectrum is noteworthy as an example for an anisotropic phenomenon to the upfield direction, and the student should be able to identify the proton that gives rise to this signal and explain the reason. The concept of aromaticity and how to count the number of π-electrons can be discussed here. Subsequently, sulfonation of TPP to TSPP could be achieved by using concentrated sulfuric acid as both solvent and reagent. Heating the solution at 140 °C for 4 h25 provided a deep green solution (Scheme 1). Neutralization with NaOH

Metal Sensing of TSPP in Solution Phase

Porphyrins with permanent positive or negative charges could provide noticeable spectral changes upon binding with metals.7,16,17 Hence, they were used as a basis in this experiment, where TSPP was expected to exert spectroscopic changes on certain metal ions. For students, a basic introduction about porphyrins and their complexation ability was provided. With the porphyrin being fixed as TSPP, two factors are considered to be the main contributors of complexation behavior, all of which may instead be used as lead questions for students’ discussion (see examples in the Supporting Information). First, the kinetics of complexation is crucial to the applicability of using a porphyrin as a sensor. Each metal ion has a different rate of complexation with porphyrin, and those metal ions that take impractically long times, i.e., hours, are deemed unusable. Second, although the rate of complexation is rapid enough, a complexation may not necessarily lead to a discernible (naked-eye) change in UV and/or fluorescence properties. Therefore, these restrictions led to only a few metal ions that can exhibit significant changes in spectroscopic properties when complexed with TSPP, leading to the observable selectivity. After an initial discussion, students were then asked to mix TSPP (0.10 mM in 2 mM HEPES pH 7.5) with each metal ion (0.10 mM in 2 mM HEPES pH 7.5) and observe the changes (at the final concentration of 0.05 mM for both species). Both C




educational laboratory, the experiment herein was performed in a buffer solution. The effect of concentration was another factor that needed to be accounted for to reduce the complexity in designing the experiment, where the aim was to provide a qualitative platform that can selectively and unambiguously detect certain metal ions via UV and fluorescence methods. At first glance, using a high concentration of TSPP may be desired to ensure the visibility of color or fluorescence from TSPP, i.e., concentrations that are too low will not result in a detectable color or fluorescence by human eyes. Nevertheless, this has to be balanced by the fact that the extent of spectroscopic change is equally important. The use of high concentrations of TSPP will result in a decrease of the ratio of bound TSPP/free TSPP at a given metal ion concentration. As a consequence, this will make the observed change more subtle. Using the paper-based format as discussed below, the issue was clearly seen in the fluorescence quenching of Cu2+, where greater than 0.10 mM of TSPP resulted in incomplete quenching (at 0.10 mM metal ions). Since the goal was to get an unambiguous change for a simpler experimental setup, the concentration of 0.10 mM was eventually selected. On the other hand, the use of even higher concentrations of metal ions is not preferred, for the obvious reason that higher concentrations are less likely to be realistic. Moreover, higher concentrations of Ag+, at 1 mM and beyond, started to show some decrease of fluorescence of TSPP, thereby complicating the analysis of fluorescence quenching from Cu2+. Also, Ag+ and Pb2+ were found to interfere with the colorimetric mode at 1 and 0.5 mM, respectively. In terms of the rate of reaction, it was found that all concentrations tested (0.01−1 mM of Hg2+ or Cd2+) gave an instantaneous color change in both the solution phase and the paper support. In the fluorescence mode, a complete quenching of fluorescence from Cu2+ ion (0.1 mM) was achieved at about 15 min in the paper format.28 Although higher concentrations did lead to faster quenching, such as 5 min for complete quenching at 10 mM, it led to the aforementioned interferences from other metal ions. Therefore, the use of concentrations of 0.10 mM for both TSPP and metal ions was justified. At the instructors’ discretion, the issues discussed in this section may be directly provided to students, or some parts of the explanation can be left to students for contemplation, as guided by sample questions in the Supporting Information.

solutions were added with approximately equal volume so that the final mole ratio of TSPP/metal ion was approximately 1:1 for simplicity. As expected from previous reports,7,16 Cd2+ and Hg2+ gave discernible changes where the original pink color turned greenish yellow (Figure 2), with Hg2+ being more

Figure 2. Colorimetric changes of TSPP (0.10 mM in 2 mM HEPES pH 7.5) when complexed with a variety of metal ions (0.10 mM in 2 mM HEPES pH 7.5; taken from student’s results). The final concentrations of both species were approximately 0.05 mM.

obvious. Cu2+ and Pb2+ also showed some changes in color hue although these changes were quite subtle compared to those of Cd2+ and Hg2+. If more time is available, all of these solutions can be taken for UV−vis measurement, which will then result in some quantitative data for further discussion. In the fluorescence detection mode, Cu2+ could significantly quench the red fluorescence of TSPP (Figure 3) within 5 min,

Figure 3. Fluorometric changes of TSPP (0.10 mM in 2 mM HEPES pH 7.5) when complexed with a variety of metal ions (0.10 mM in 2 mM HEPES pH 7.5). The photos were taken after a 5-min waiting period (from student’s results). The final concentrations of both species were approximately 0.05 mM.

allowing a possibility of using this change as a means to detect Cu2+. It was noticed that Cd2+ and Hg2+ could also quench the fluorescence, but to a much lesser degree than did Cu2+.

Metal Sensing of TSPP Immobilized on Patterned Papers

Effects of pH and Concentrations to the Complexation Behavior

Even though the cost of patterning papers with hydrophobic solid inks, i.e., wax printing, has become relatively low, a few reports have shown that permanent markers containing hydrophobic resin can also be a useful tool for paper patterning.29,30 In this experiment, instead of using patterns created from computer software, students were instructed to draw patterns by hand as a way to simplify the process and to demonstrate the practicality of the whole fabrication. Simple circle patterns, acting as reaction “wells”, were drawn on a round Whatman No. 1 filter paper. The borders of the paper were folded to serve as a stand for the sensing area; this is to avoid dropping liquids directly to the paper lying on the lab bench, which would result in uncontrollable liquid spreading (Figure S3). In this pattern, the testing solution was dropped into circles directly. It is also worth mentioning that the laboratory time can also be allocated to the fabrication of more sophisticated and robust (yet still economical and practical)

It is worth mentioning that pH has a profound effect on the complexation behavior of TSPP. Specifically, TSPP showed some pH-dependent color changes, which could be observed by the naked eye. In fact, students undertaking this experiment could witness this change during the workup process of the sulfonation step in the synthesis of TSPP (green in highly acidic solution and pink in neutral or basic solution). Several metal ions with the oxidation state of +3 are well-known to be prone to hydrolysis,27 yielding an acidic solution. Thus, it is likely that the color change of TSPP in the presence of Fe3+ and Al3+ observed in an aforementioned study23 was due to a pH change resulting from the hydrolysis from each metal ion rather than actual complexation. The fact that all changes were in the hue of green corroborated well with this hypothesis. Thus, in order to avoid such complexity in setting up an D




Finally, the paper-based sensor was tested for its ability to identify unknown samples. For single metal samples, all students had no problem in finding the sample containing Cu2+. Interestingly, if both Hg2+ and Cd2+ were used in the experiment, answers from students can reflect their logical thinking very well. For example, some students simply answered either Hg2+ or Cd2+ in a sample containing Hg2+, while some students acknowledged the possibility of having both. In addition, unknown samples can also be a mixture of two metal ions (with some exceptions, see instructor’s notes in the Supporting Information). For instance, all students correctly identified that a mixture contained Hg2+ although Ca2+ and Pb2+ were also present. On the other hand, some students (about 30%) could not identify samples containing Cd2+ (when Hg2+ was also absent). Given a weaker color change from Cd2+, instructors may opt to exclude Cd2+ for more clear-cut answers from students. As discussed above, however, instructors may still choose to provide both to promote some logical thinking from students. Notably, although the working concentration of the sensor was deemed relatively high compared to other sophisticated instruments, some improvement could make it more applicable to real applications. The Environmental Protection Agency (EPA) imposed an upper limit of copper in drinking water to be 1.3 ppm,31 which is 0.02 mM (vs 0.1 mM used in this work). Thus, some modifications, such as the use of a camera with image analysis software, could improve this sensitivity further, and may result in a practical project or simply an additional discussion with students. Last but not least, it is worth noting that the experiment outlined herein can also be split to suit different laboratory courses. For example, the synthesis of TSPP can serve as a standalone experiment in a basic-level organic chemistry laboratory. On the other hand, the compound can be obtained from the instructors, and an experiment focused on the photophysical properties and complexation behavior with metal ions can be created.

paper-based microfluidic devices, as highlighted by the Remcho group.13 In addition, it was found that some markers with vivid colors can interfere with the observation of fluorescence, and some may not have proper chemical composition to retain water on paper. In our case, students were asked to bring their own markers without a prior guideline. As expected, some students brought markers that did not work; they were then provided with a correct type of markers. Interestingly, this failure can be used as a side discussion topic about the effect of chemical composition on the physical properties of markers. Nevertheless, instructors may choose to provide a correct type of marker to all students to save some experiment time. Thereafter, students were then advised to investigate complexation behaviors in solid-phase format. In this experiment, only one drop of TSPP (0.10 mM) was deposited on each patterned circle on the paper. After the drop was dried, each metal ion solution (one drop at 0.10 mM) was added. The results showed that TSPP still exhibited a colorimetric change when complexed with Cd2+ and Hg2+ (Figure 4A), with Hg2+

Figure 4. Patterned circles on paper containing one drop of TSPP (0.10 mM in 2 mM HEPES pH 7.5) and various metal ions (one drop, 0.10 mM in 2 mM HEPES pH 7.5) under (A) white light and (B) 365nm UV light. All images were from student results.

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CONCLUSION

This experiment showcases how a known porphyrin can be utilized in a novel way that connects with practical applications. Within three laboratory sessions, students could synthesize their own batches of the compound, revisiting their organic laboratory skills and gaining appreciation of how the compound can be obtained for the subsequent experiments. Later, they also had hands-on experience in UV and fluorescence spectroscopy, along with a chance to revise their basic analytical laboratory skills. Finally, they were introduced to the field of paper-based microfluidic devices in which they were allowed to create a simple device for real applications: metal ion sensing. Overall, the pedagogical goal was met as it was evident that students showed much more enthusiasm when they could utilize their own synthesized compound as a practical tool to solve another chemistry problem. In essence, this experiment could be a fascinating addition to an advanced chemistry laboratory, or an integrated-style chemistry laboratory where students learn to realize why doing research in an interdisciplinary fashion is crucial for modern chemical research.

being more prominent. On the contrary, multiple experiments confirmed that Cu2+ and Pb2+ could not exert any clear color change in this paper-based format. It was also found that the color change was more pronounced and reliable when the droplet of the solution was still intact. Fluorescence quenching, on the other hand, could be observed by naked eyes in the paper format after the solution dried out. Upon illumination with 365-nm light, the fluorescence of TSPP was completely quenched when Cu2+ was added (Figure 4B), whereas Cd2+ and Hg2+ showed a slight decrease of fluorescence intensity, which was deemed to be insufficient for use as a simple qualitative identification. In brief, the paper-based sensor can be utilized as a portable and convenient sensor for the three metal ions Hg2+, Cd2+, and Cu2+ using two complementary modes of detection. Interestingly, TSPP exhibited nearly identical spectroscopic properties between solution and solid phasesthis thus allowed clear identification when a suitable metal ion analyte was added. This is in contrast to the sensor developed by Espinosa et al.,19 where the change in the solid-phase format was substantially more ambiguous compared to its own solution-phase version. E



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(13) Koesdjojo, M. T.; Pengpumkiat, S.; Wu, Y. Y.; Boonloed, A.; Huynh, D.; Remcho, T. P.; Remcho, V. T. Cost Effective Paper-Based Colorimetric Microfluidic Devices and Mobile Phone Camera Readers for the Classroom. J. Chem. Educ. 2015, 92, 737−741. (14) Biesaga, M.; Pyrzyńska, K.; Trojanowicz, M.; et al. Porphyrins in analytical chemistry. A review. Talanta 2000, 51, 209−224. (15) Valicsek, Z.; Horváth, O. Application of the electronic spectra of porphyrins for analytical purposes: The effects of metal ions and structural distortions. Microchem. J. 2013, 107, 47−62. (16) El-Safty, S. A.; Prabhakaran, D.; Ismail, A. A.; Matsunaga, H.; Mizukami, F. Nanosensor design packages: a smart and compact development for metal ions sensing responses. Adv. Funct. Mater. 2007, 17, 3731−3745. (17) Liu, X.; Liu, X.; Tao, M.; Zhang, W. A highly selective and sensitive recyclable colorimetric Hg2+ sensor based on the porphyrinfunctionalized polyacrylonitrile fiber. J. Mater. Chem. A 2015, 3, 13254−13262. (18) Paddock, J. R.; Maghasi, A. T.; Heineman, W. R.; Seliskar, C. J. Making and using a sensing polymeric material for Cu2+: an introduction to polymers and chemical sensing. J. Chem. Educ. 2005, 82, 1370−1371. (19) Espinosa, A.; Otón, F.; Martínez, R.; Tárraga, A.; Molina, P. A multidimensional undergraduate experiment for easy solution and surface sensing of mercury(II) and copper(II) metal cations. J. Chem. Educ. 2013, 90, 1057−1060. (20) Finch, L. E.; Hillyer, M. M.; Leopold, M. C. Quantitative analysis of heavy metals in children’s toys and jewelry: a multiinstrument, multitechnique exercise in analytical chemistry and public health. J. Chem. Educ. 2015, 92, 849−854. (21) Kristian, K. E.; Friedbauer, S.; Kabashi, D.; Ferencz, K. M.; Barajas, J. C.; O’Brien, K. A simplified digestion protocol for the analysis of Hg in fish by cold vapor atomic absorption spectroscopy. J. Chem. Educ. 2015, 92, 698−702. (22) Romero, M.; Guidi, V.; Ibarrolaza, A.; Castells, C. Complexometric Determination of Mercury Based on a Selective Masking Reaction. J. Chem. Educ. 2009, 86, 1091−1093. (23) Li, L.; Xiang, H.; Zhou, X.; Li, M.; Wu, D. Detection of Fe3+ and Al3+ by Test Paper. J. Chem. Educ. 2012, 89, 559−560. (24) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour, J.; Korsakoff, L. A simplified synthesis for meso-tetraphenylporphine. J. Org. Chem. 1967, 32, 476−476. (25) Dong, Z.; Scammells, P. J. New methodology for the Ndemethylation of opiate alkaloids. J. Org. Chem. 2007, 72, 9881−9885. (26) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.; Marguerettaz, A. M. Rothemund and Adler-Longo reactions revisited: synthesis of tetraphenylporphyrins under equilibrium conditions. J. Org. Chem. 1987, 52, 827−836. (27) Whitten, K. W.; Davis, R. E.; Peck, M. L.; Stanley, G. G. Chemistry, 9th ed.; Brooks/Cole: Belmont, 2010; pp 734−736. (28) This incubation time was longer than that of the solution phase, where 5 min of incubation time was sufficient for complete quenching at 0.1 mM (see above). (29) Rajendra, V.; Sicard, C.; Brennan, J. D.; Brook, M. A. Printing silicone-based hydrophobic barriers on paper for microfluidic assays using low-cost ink jet printers. Analyst 2014, 139, 6361−6365. (30) Xu, C.; Cai, L.; Zhong, M.; Zheng, S. Low-cost and rapid prototyping of microfluidic paper-based analytical devices by inkjet printing of permanent marker ink. RSC Adv. 2015, 5, 4770−4773. (31) Environmental Protection Agency. Lead and Copper Rule. https://www.epa.gov/dwreginfo/lead-and-copper-rule (accessed Apr 2017).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00943. Additional figures, notes for instructors, and student instructions (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Thanit Praneenararat: 0000-0001-9165-2642 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Thailand Research Fund (MRG5980037), and the Ratchadapiseksomphot Endowment Fund, Chulalongkorn University [Grant for Young Researchers and Outstanding Research Performance Program (Sci Super II)]. The Graduate School, Chulalongkorn University, is gratefully acknowledged for the scholarship to commemorate the 72nd anniversary of his Majesty King Bhumibol Adulyadej given to J.P. for her M.Sc. study.

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Paper-Based Heavy Metal Sensors from the Concise Synthesis of an

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