In the Classroom
Tested Demonstrations
Light Emission at Electrodes: An Electrochemiluminescence Demonstration submitted by:
Ed Bolton and Mark M. Richter* Department of Chemistry, Southwest Missouri State University Springfield, MO 65804; *
[email protected] checked by:
Robert Eierman Department of Chemistry, University of Wisconsin–Eau Claire, Eau Claire, WI 54701
Overview Electrochemiluminescence, electrogenerated chemiluminescence, or ECL involves the production of light near an electrode surface by the generation of species that can undergo highly energetic electron transfer reactions (1–3). This demonstration uses Ru(bpy)32+ (bpy = 2,2′-bipyridine) as the light-emitting molecule and tri-n-propylamine (C9H21N) as a coreactant (i.e., a species capable of generating a strong reductant upon oxidation). Upon application of ~3 V to the Ru(bpy)32+/ C9H21N solution an orange glow, readily visible in a semidarkened room, is produced at a platinum electrode. Materials and Equipment Ru(bpy) 3Cl 2 (e.g., Strem Chemical Co. or Aldrich Chemical Co.) 0.2 M KH2PO4 buffer solution Tri-n-propylamine (C9H21N; Avocado) Acetonitrile 100-mL beaker 100-mL volumetric flask Stir plate Magnetic stir bar Parafilm Platinum electrodes (~5 cm straight, ~9 cm coiled to ~5 cm) Flashlight (Everready) 2 leads with alligator clips (e.g., Radio Shack) 2 D alkaline batteries
Preparation of ECL Solution Transfer 0.075 g of Ru(bpy)3Cl2 into a 100-mL volumetric flask with 25 mL of buffer solution. Add 0.95 mL of the coreactant (C9H21N) to the volumetric flask. Dilute to mark with acetonitrile. Mix thoroughly. Although ECL can be generated in purely aqueous media, the addition of acetonitrile results in much higher emission intensity and a more striking demonstration. Electrode Design and Hookup Place approximately 80 mL of ECL (Ru(bpy) 32+ – C9H21N) solution into a 100-mL beaker and add a Tefloncovered magnetic stir bar. Cover the top of the beaker with
Parafilm. The electrodes are made of platinum wire. The working electrode can be made into a coil to enhance the visual effect. Wrap a strip of Parafilm around the top of the electrodes several times so they will not fall through the Parafilm covering the beaker. Insert the electrodes into the solution through the layer of Parafilm. For repeated use a Teflon top with holes for electrodes may be substituted for the Parafilm. Teflon tops with holes are available commercially (e.g., Bioanalytical Systems), and most machine shops can fabricate these for electrochemical cells. Disassemble a standard D-cell flashlight. Remove the light bulb assembly from the flashlight, since this will not be needed. Attach leads to the positive and negative connections using black electrical tape.1 Alternatives such as a dc power supply or adjustable voltage source can be substituted for the flashlight assembly. Attach the alligator clips of the leads to the electrodes in solution. The lead connected to the positive end should be attached to the working electrode (i.e., the electrode where light emission is desired). Place the beaker on a stir plate. Stir the solution so that the reagents are constantly flowing past the electrode surfaces, flip the on/off switch on the “flashlight” assembly, and the chemistry is in motion. Alternately, the demonstrator may swirl the beaker manually for a similar effect. When the demonstration is complete, rinse the electrodes with water and place them in a beaker containing ~6 M sulfuric acid. Attach the leads to the electrode and turn on the cell for about 10 seconds. Turn it off and reverse the leads. Repeat several times. This effectively cleans the electrode by producing reactive intermediates in the oxidation cycle (e.g., hydroxyl radicals) that oxidize organics (4 ). The extreme potentials also desorb species from the electrode surface. The oxidation and reduction of the Pt surface in H2SO4 also breaks it up to produce a more active surface. Hazards Ru(bpy)3Cl2 is a heavy metal complex and its toxicity and carcinogenicity are not known. C9H21N is a flammable toxic irritant. Therefore, goggles should always be worn when preparing solutions and performing the demonstration. Gloves should be worn when preparing solutions and C9H21N should be handled in a fume hood. Sulfuric acid is a strong acid and dehydrating agent. Spills should be neutralized with an agent such as sodium bicarbonate and then rinsed clean. Please dispose of materials in a manner consistent with local, state, and federal regulations.
JChemEd.chem.wisc.edu • Vol. 78 No. 5 May 2001 • Journal of Chemical Education
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In the Classroom
Discussion Since its discovery in the mid-1960s, ECL has moved from being a laboratory curiosity to being a useful analytical technique. ECL is a means of converting electrical energy into light (radiative energy). It involves the formation of electronically excited states by energetic electron transfer reactions of electrochemically generated species (1, 2, 5). Traditionally, ECL was generated via annihilation in an electron transfer reaction between an oxidized and reduced species, both of which are generated at an electrode by alternate pulsing of the electrode potential (2). A + e᎑ → A᎑ D–
e᎑
→
D+
(reduction at electrode)
(1)
(oxidation at electrode)
(2)
A + D → A* + D (excited state formation) ᎑
+
A* → A + hν
(light emission)
(3) (4)
For example, the potential of the working electrode is quickly changed between two values in order to generate the reduced, A᎑, and oxidized, D+, species (eqs 1 and 2, respectively) that will react near the electrode surface to form the emissive state, A* (eq 3). These types of reactions generally involve the use of rigorously purified and deoxygenated nonaqueous solvents (e.g., dimethylformamide and acetonitrile), since the available potential range in water is too narrow to generate the required energetic precursors. Many ECL reactions of this type have been investigated and their mechanisms are well understood (1, 2, 6 ). Of more interest to practical applications, ECL can also be generated in a single step utilizing a coreactant (i.e., a species capable of forming strong oxidants or reductants upon bond cleavage) (7–9). In the Ru(bpy)32+/C9H21N system demonstrated here (9), ECL is produced upon concomitant oxidation of Ru(bpy) 32+ and C9H21N: Ru(bpy)32+ – e᎑ → Ru(bpy)33+ (oxidation at electrode)
C9H21N – e᎑ → [C9H21N]+ → C9H20N + H+ (oxidation at electrode)
(5) (6)
C9H20N + Ru(bpy)33+ → *Ru(bpy)32+ + products (7) (excited state formation)
*Ru(bpy)3 → Ru(bpy)32+ + hν (~2.0 eV, 610 nm) (8) 2+
(light emission)
2+
where *Ru(bpy)3 is the electronically excited species capable of undergoing emission. The ECL mechanism and subsequent signal generation in this system are believed to occur through an “oxidative–reductive” pathway. This involves production of a strong reducing agent (presumably C9H20 N ) (10) by an initial oxidation sequence. This radical can then reduce Ru(bpy)33+ to *Ru(bpy)32+ (9, 11). Alternately, C9H20 N may reduce Ru(bpy)32+ to Ru(bpy)31+ and this is followed by annihilation (8, 12, 13): Ru(bpy)31+ + Ru(bpy)33+ → *Ru(bpy)32+ + Ru(bpy)32+ (9) Although the details of the coreactant ECL mechanism (eqs 5–8) to generate light emission are still under study, the
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origin of the light emission has been well documented (1, 6, 14). The excited state formed in the ECL reaction is the same as that formed during photoexcitation. In photoexcitation (i.e., photoluminescence), an electron is excited from metalbased dπ orbitals to ligand-based π* orbitals (a metal-to-ligand charge transfer [MLCT] transition). The excited electron then undergoes intersystem crossing to the lowest triplet state of *Ru(bpy)32+ from where emission occurs (15). Since the photoluminescent and ECL spectra are nearly identical (1, 2, 5, 6 ) the emission process in ECL involves the MLCT state of Ru(bpy)32+. This state may be formed if the reducing agent (i.e., C9H20N ) transfers an electron to the π* orbital of one of the bipyridine ligands. *Ru(bpy)32+ can then decay to the ground state, producing the same luminescence as obtained from photoluminescence spectroscopy. ECL has found use in studying the properties of both organic and inorganic systems (16 ). These include polyaromatic hydrocarbons (17 ), polymer assemblies (18 ), and transition metal complexes incorporating such metals as Ru, Os, and Pt (16, 19, 20), as well as rare earth systems (21). The studies include characterizing the emitting states, discerning the mechanisms by which these states are formed, and determining the efficiency of excited-state formation. Coreactant ECL has become increasingly attractive for the detection of numerous chemical and biological molecules, and many of its applications have been reviewed (22, 23). There has also been interest in using ECL reactions as the basis for highly sensitive and selective analysis. In such schemes the ECL luminophore (e.g., Ru(bpy)32+) is used as a label on the molecule of interest (e.g., DNA or an antibody), in the same way that photoluminescent (fluorescent) or radioactive labels are employed (23, 24 ). ECL has the advantage over radioactive labels of not producing radioactive waste. Unlike fluorescent methods, ECL does not require an excitation source and therefore is immune to interferences from luminescent impurities and scattered light, making it more sensitive and selective. ECL appears very promising and is being commercially developed and marketed for use in the clinical analysis of biomolecules (23, 24 ). Unfortunately, such developments don’t happen overnight (this one took ~30 years) or without laying solid foundations of the underlying science (since the first detailed studies in the 1960s more than 500 papers, patents, and book chapters have appeared on ECL, ranging from the fundamental to the very applied). Therefore, this demonstration facilitates discussions about the importance of basic science and pursuing laboratory curiosities. This demonstration also facilitates discussions of redox reactions, reaction kinetics and energetics, and the formation of excited states using ECL, chemiluminescence, and photoluminescence. The bright orange emission characteristic of *Ru(bpy)32+ may also be generated using chemiluminescence—for example, by reducing Ru(bpy)32+ and S2O82᎑ with Mg(s) to form Ru(bpy)31+ and SO4᎑, which then undergo electron transfer to form *Ru(bpy)32+, or by reacting Ru(bpy)33+ with the powerful reducing agent tetrahydridoborate ion (BH4᎑) (25). Other demonstrations such as producing luminol chemiluminescence using an electrogenerated oxidant (14 ) also complement the demonstration presented here.
Journal of Chemical Education • Vol. 78 No. 5 May 2001 • JChemEd.chem.wisc.edu
In the Classroom
Acknowledgment We gratefully acknowledge the financial support of Southwest Missouri State University. Note 1. Alternately, one can use duct tape to hold the D-cell batteries in proximity to one another. Place the positive end of one battery to the negative terminal of the other and hold them in place using a piece of duct tape. Attach one lead to the positive end and another to the negative end of the assembly using electrical tape. When the alligator clips are attached to the electrode, ECL should be generated at the electrode connected to the positive lead. If no light is visible, the connection between the two batteries is probably not complete. Simply press the batteries together and hold them in place so that a connection is made, and the demonstration will work.
Literature Cited 1. Glass, R. S.; Faulkner, L. R. J. Phys. Chem. 1981, 85, 160. 2. Faulkner, L. R.; Bard, A. J. In Electroanalytical Chemistry, Vol. 10; Bard, A. J., Ed.; Dekker: New York, 1977; pp 1–95. 3. Alexander, C.; McCall, J.; Richter, M. M. Chem. Educ. 1998, 3, 1. 4. Woods, R. In Electroanalytical Chemistry, Vol. 9; Bard, A. J., Ed.; Dekker: New York, 1979; p 1. 5. Tokel, N.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 2862. 6. Faulkner, L. R.; Glass, R. S. In Chemical and Biological Generation of Excited States; Waldemar, A.; Giusseppe, C., Eds; Academic: New York, 1982; Chapter 6. 7. Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 512. 8. White, H. S.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 6891. 9. Leland, J. K.; Powell, M. J. J. Electrochem. Soc. 1990, 137, 3127–3131. 10. Smith, P. J.; Mann, C. K. J. Org. Chem. 1969, 34, 1821. 11. McCord, P. M.; Bard, A. J. J. Electroanal. Chem. 1991, 318, 91. 12. Richter, M. M.; Debad, J. D.; Striplin, D. R.; Crosby, G. A.; Bard, A. J. Anal. Chem. 1996, 68, 4370.
13. Boletta, F.; Rossi, A.; Balzani, V. Inorg. Chim. Acta 1981, 53, L23. Boletta, F.; Ciano, M.; Balzani, V.; Serpone, N. Inorg. Chim. Acta 1982, 62, 207. 14. Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 1; University of Wisconsin Press: Madison, WI, 1983; p 216. Bolton, E.; Richter, M. M. J. Chem. Educ. 2001, 78, 47. 15. See, for example, Meyer, T. Acc. Chem. Res. 1989, 22, 163– 170. Roundhill, D. M. Photochemistry and Photophysics of Metal Complexes; Plenum: New York, 1994; Chapter 5. Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic: New York, 1992; Chapter 1. 16. Knight, A. W.; Greenway, G. M Analyst 1994, 119, 879. 17. Richards, T. C.; Bard, A. J. Anal. Chem. 1995, 34, 3140. 18. Rubinstein, I.; Martin, C. R.; Bard, A. J. Anal. Chem. 1983, 55, 1580. Downey, T.-M.; Nieman, T. A. Anal. Chem. 1992, 64, 261. Richter, M. M.; Fan, F.-R. F.; Klavetter, F.; Heeger, A. J.; Bard, A. J. Chem. Phys. Lett. 1994, 226, 115. 19. See, for example, Tokel, N.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 2862. Glass, R. S.; Faulkner, L. R. J. Phys. Chem. 1981, 85, 1160. 20. Vogler, A.; Kunkeley, H. Angew. Chem., Int. Ed. Engl. 1984, 23, 316. Kim, J.; Fan, F.-R. F.; Bard, A. J.; Che, C.-M.; Gray, H. B. Chem. Phys. Lett. 1985, 121, 543. 21. Hemingway, R. E.; Park, S.-M.; Bard, A. J. J. Am. Chem. Soc. 1975, 95, 200. Richter, M. M.; Bard, A. J. Anal. Chem. 1996, 68, 2641. 22. Knight, A. W.; Greenway, G. M. Analyst 1994, 119, 879. 23. Yang, H.; Leland, J. K.; Yost, D.; Massey, R. J. Biotechnology 1994, 12, 193. Blackburn, G. F.; Shah, H. P.; Kenten, J. H.; Leland, J.; Kamin, R. A.; Link, J.; Pterman, J.; Powell, M. J.; Shah, A.; Talley, D. B.; Tyagi, S. K.; Wilkins, E.; Wu, T-G.; Massey, R. J. Clin. Chem. 1991, 37, 1626. 24. Bard, A. J.; Whitesides, G. M. Luminescent Metal Chelate Labels and Means for Their Detection; U.S. Patents 5 221 605, Jun 22, 1993; 5 238 808, Aug 24, 1993; 5 310 687, May 10, 1994. 25. Gafney, H. D.; Adamson, A. W. J. Chem. Educ. 1975, 52, 480–481.
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