Educational Light-POD: An Activity for Middle and High School

May 24, 2013 - In an effort to entice more pre-college students to become the future scientists, a simple multicomponent device, the Educational Light...
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Educational Light-POD: An Activity for Middle and High School Students To Explore the Principles of Analog Transmission Using Photoacoustic Modulation of Fluorescence Lorenzo Echevarria†,‡ and Florencio Eloy Hernandez†,§,* †

Department of Chemistry, University of Central Florida, Orlando, Florida 32816-2366, United States Departamento de Química, Universidad Simón Bolívar, Caracas 1020A, Venezuela § The College of Optics and Photonics, CREOL, University of Central Florida, Orlando, Florida 32816-2366, United States ‡

S Supporting Information *

ABSTRACT: In an effort to entice more pre-college students to become the future scientists, a simple multicomponent device, the Educational Light-Pod (ELP), was developed to introduce the principles of analog transmission using photoacoustic modulation of fluorescence in an illustrative manner. This eye-catching activity combines concepts of chemistry, physics, optics, and engineering. The ELP can be built by middle and high school students under the supervision of their science teacher; however, if the cost associated with the students individually building the apparatus is prohibitive, the ELP can be used for group demonstrations. KEYWORDS: Elementary/Middle School Science, High School/Introductory Chemistry, First-Year Undergraduate/General, Demonstrations, Laboratory Instruction, Hands-On Learning/Manipulatives, Fluorescence Spectroscopy, Lasers, Molecular Properties/Structure, UV-Vis Spectroscopy

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lasers, light can transfer large quantities of data through great distances, quickly and without loss of information. In addition to the wow-factor of this activity, which can help spark the curiosity of the so-called generation Z, the ELP illustrates, in a tangible fashion, the technological application of light to transfer and exchange information using radiation− matter interaction. Assisting the young generation to understand the concepts behind the technology and the devices that they use or will use on a regular basis for communicating is paramount. As recently stated in “The Global Information Technology Report 2012”,7 we live in a hyperconnected world, where the immediate accessibility to the Internet, from a vast variety of devices, is transforming our society.

any of our colleagues in the scientific and academic community are developing programs for teachers and precollege students to persuade the latter to become part of the future generation of scientists. Through these programs, basic scientific concepts and their practical applications are disseminated across high, middle, and elementary schools, exposing students to the latest achievements in science and technology worldwide. However, if a change is to occur in the future, a greater effort has to be made to develop catchy experiments and activities that inspire the upcoming generation to pursue careers in sciences, technology, engineering, and math (STEM). Herein, we present a simple multicomponent device that demonstrates the principles of analog transmission using photoacoustic modulation of fluorescence: the Educational Light-Pod (ELP). This eye-catching activity combines fundamentals of lasers,1 electronic transitions,2 the Franck− Condon principle,3 light absorption and the Beer−Lambert law,4 fluorescence,5 photonics, and principles of optical communication,6 as well as signal modulation, transmission, and processing.6 However, the most perceptible phenomena on the proposed scientific project are the visualization of the Beer−Lambert law, fluorescence, and optical communication that work synergistically for the transmission of a song emanated from a portable media player or a cellular phone, or to allow a conversation with a hands-free system. The selection of diode lasers for data transmission in this activity was motivated by the importance of this source device in modern free-space channel telecommunication networks. Using © XXXX American Chemical Society and Division of Chemical Education, Inc.

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OBJECTIVES AND OVERVIEW OF THE ACTIVITY The central educational goal of the activity is to expose middle and high school students to the latest achievements in science and technology through an exciting hands-on science activity. The main objectives are (i) to motivate middle and high school students to complete a research project in an independent manner, (ii) to help them understand the process of developing an analytical method through building an instrument for chemical analysis, and (iii) to introduce the principles of analog transmission using photoacoustic modulation of fluorescence. In addition, these students will be introduced to the scientific method. In the activity, students work in groups of two to promote teamwork. To help students develop interpersonal skills and

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students in this exciting scientific project. The estimated cost per ELP is less than $30.

leadership, project partners are chosen randomly. After concluding the project students are expected to answer questions following the template in the student handout (see the Supporting Information). The ELP was designed to be implemented in existent science course such as middle school science, physical sciences, advanced physical sciences, chemistry and physics, honors chemistry and physics, as well as advance placement chemistry and physics. The estimated time for building the circuits is approximately 10 h. Therefore, the first part of activity should be assigned as a home project. Afterward, students set up the ELP in the classroom to fully develop the proposed activity under the supervision of an instructor. The estimated in-class time to run the activity is 2 h. If the cost associated with the students building the apparatus becomes prohibitive or if there is a desire to promote the interest for STEM among K−5 students, ELP can be used for group demonstrations in existent science courses (science K−5 and physical sciences). The estimated average time required to set up the activity and explain the principles behind ELP in an illustrative manner is of 30 min plus questions. The activity was presented in an elementary school to 4th graders and to first year-college students.

Table 1. Electronics Parts for ELP

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ELP DESIGN The general schematic of the ELP setup is shown in Figure 1. Its main components are (A) portable media player or a cellular Figure 2 and 3 show the TIM and RIM circuits (schematic and pictorial diagrams). Whereas the TIM module is based on

Figure 1. Schematic (top) and photo (bottom) of the Educational Light-Pod: (A) portable media player or a cellular phone (stereo male−male cable); (B) transmitter interface module (TIM) with a diode laser; (C) 1 cm four-sides-clear cuvette with a fluorescent dye; (D) receiver interface module (RIM) with a photodiode; (E) speaker; (F) bandpass filter; and (G) shield panel.

Figure 2. Diagram of the transmitter interface module (TIM) used in ELP: schematic diagram (top) and pictorial diagram (bottom).

an LM317 voltage regulator, the RIM module is based on an LF353 amplifier. The circuits given here are typical applications of components used in data transmission.8 The list of the electronic components given in Table 1 can be purchased in an electronic store or online at Digikey.com or Jameco.com. The blue laser and photodiode can be purchased on eBay.com.

phone; (B) transmitter interface module (TIM) with a diode laser; (C) 1 cm four-sides-clear cuvette with a fluorescent dye; (D) receiver interface module (RIM) with a photodiode; (E) speaker; (F) bandpass filter (clear yellow plastic or lens); and (G) shield panel. All the elements required to build the different components of the ELP are commercially available and inexpensive. With the specific electronic circuit diagrams and the list of electronic elements necessary to assemble the TIM and RIM modules (Table 1), teachers can work with

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STOCK SOLUTIONS The chosen dye for this activity is a natural nontoxic food spice, turmeric (Curcuma longa), which is an orange-yellow powder frequently present in curries. Turmeric can be purchased at a supermarket. The fluorencence solution is made by dissolving B

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FUNDAMENTAL PRINCIPLES OF OPERATION The ELP functioning principles reside in photonics, that is, the science that studies photons (light), their interaction with matter, and their application to signal transmission, modulation, processing, switching, amplification and detection. Data transmission (DT) is the physical transfer of information over a point-to-point communication channel. DT can be digital or analog. Whereas the former refers to breaking an audio or video signal into a binary format, the latter consist of a continuous signal with a time varying feature that changes a specific parameter. For this specific activity, analog DT based on amplitude modulation (AM) was chosen due to its relative simplicity and low cost. In addition, AM is commonly used in transmitting information using radio waves, for example, the changes in signal strength are used to indicate the sounds to be reproduced by a speaker far away. Combining these principles with those to describe the basis of lasers and spectroscopy, one can cover all the concepts blended in ELP. It is well-known that the probability of an electronic transition at the excitation wavelength (typically in the ultraviolet−blue region of the electromagnetic spectrum) and in the linear regime is directly proportional to the intensity of the incoming radiation. As a result, the number of molecules promoted from their ground to their first excited state through a vertical transition can be modulated by controlling the strength of the external electric field. Once a molecule has lived in its excited state for certain time (lifetime of the excitedstate), the molecule relaxes back to its ground state dissipating energy through two main mechanisms: radiative (fluorescence) or nonradiative (internal conversion). In molecules with a high fluorescence quantum yield, the former becomes the dominant mechanism. Because the fluorescence intensity is proportional to the population of molecules in the excited state, which at the same time is directly proportional to that of the excitation radiation, the intensity of the emitted light can be easily modulated by controlling that of the excitation. The former is valid as long as the concentration of the dye in solution is low enough to avoid quenching effects. According to Beer−Lambert law, the absorption as well as the emission of a dye in solution is proportional to the concentration of the dye. Another interesting point about fluorescence that deserves attention is the fact that the wavelength of the emitted light is typically redshifted with respect to that of the peak absorption. This effect, known as the Stokes shift, is a consequence of the dissipation of energy within an electronic state or between electronic states, that is, vibrational relaxation or internal conversion, respectively. The excitation process in the proposed apparatus is produced by a laser (light amplification by stimulated emission of radiation). Lasers have become firmly established tools in traditional and multidisciplinary research fields due to their many technological applications to the study of matter and its transformation, chemical and biological processes at different time scales, and the propagation and modulation of light through different media. Laser light is monochromatic, directional, and coherent. Whereas monochromatic refers to one specific wavelength (single color), directional implies that the beam spreads slowly as it propagates over long distances. The light output of a laser diode, such as a commercial, inexpensive laser pointer, is directly proportional to the current flowing through it. Because this current depends on the voltage applied to the laser diode, its intensity can be tuned by

Figure 3. Diagram of the Receiver interface module (RIM) used in ELP: schematic diagram (top) and pictorial diagram (bottom).

25.0 mg of turmeric in 5.0 mL of a 75% by volume rubbing alcohol (2-propanol/water solution), a solvent typically used as hand sanitizer. Turmeric is a highly fluorescent dye with emission maximum at approximately 540 nm (green) in 2propanol.

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PROCEDURE A portable media player or a cellular phone is first connected to the TIM that modulates a low-power (5 mW) laser diode with output wavelength of 405 nm. The RIM is placed at approximately 30 cm from the light source (laser diode) and perpendicular to the propagating beam direction. To collect part of the fluorescence generated in the sample, the photodiode in the RIM should be facing the beam propagation axis (see Figure 1). Next, the cuvette containing the turmeric/ isopropanol solution is placed in front of the emitted radiation at the same distance from the source as the RIM. A clear yellow-colored filter is placed in front of the photodiode (phototransistor), between the cuvette and the RIM, to block any scattered radiation from the source. Finally, the RIM is plugged into a set of speakers. A shield panel (see Figure 1) placed in front of the laser beam is utilized as a safety measure, to block the path of the laser beam in the forward direction. To reinforce and to assess the understanding of the most perceptible concepts in ELP the effect of changing the following parameters are qualitatively examined: (i) intensity of the laser; (ii) type of dye (chlorophyll, food dye); (iii) concentration of turmeric in solution; (iv) induction of modulation, by periodically blocking the beam path at different frequencies; and (v) distance TIM−cuvette. Instructions for the specific modifications to the setup and questions are available in the Supporting Information. The teaching aspects of ELP are (a) assembly of the circuits, (b) setup of the activity, (c) basic spectroscopic information about turmeric, (d) preparation of the sample, (e) discussion about the theoretical aspects of ELP, (f) testing the apparatus, (g) interpretation of the observations, and (h) supplementary activities. Finally, the students answer questions about the activity and their experience throughout the activity (see the Supporting Information).

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HAZARDS The laser beam should never be directed at anyone in the room because it can cause eye damage. C

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(7) INSEAD: The Bussiness School for the World and World Economic Forum. The Global Information Technology Report 2012: Living in a Hyperconnected World; Dutta, S. , Bilbao-Osorio, B., Eds.; World Economic Forum and INSEAD: Geneva, 2012; pp 182−183. (8) Mazzolini, A. Module 5. In Active Learning in Optics and Photonics: Training Manual; Sokoloff, D. R., Ed.; UNESCO-SPIEICTP: Paris, France, 2006; pp 175−198.

supplying small voltage variations. By means of TIM (Figure 2), the electrical signal generated by a portable media player or a cellular phone can be manipulated to drive the intensity of the laser traveling through the dye solution. As a result, the signal modulation is then transferred to the emission intensity of the dye. Using RIM (Figure 3), the modulated fluorescence is converted back into an electrical signal via a phototransistor, that is, the photodiode, with high gain and sufficient bandwidth for the modulated sound signal. The signal produced by the phototransistor is amplified and de-codified as sound through a set of speakers.

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CONCLUSION In summary, this versatile activity successfully demonstrates the principles of lasers, molecular excitation and relaxation mechanisms, and the ability of carrying information through long distances using modulated laser radiation. Significant educational value is associated with the construction of a complete working instrument that can be used, for instance, as an analytical sensor for the qualitative determination of heavy metals in solution or as a photoacoustic titration technique, among others. If the cost associated with building the apparatus becomes prohibitive, the ELP can be used for group demonstrations. However, if funding is available, it can be implemented as a scientific project for students. ELP can also be used as a demonstration or experiment in college to persuade undecided freshmen students to opt for STEM careers.

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ASSOCIATED CONTENT

S Supporting Information *

Questions for the students and details about the setup. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: fl[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge Sean Campbell for his contribution to the development of many educational experiments during Summer 2010. This work was partially supported by the National Science Foundation Research Experience for Teacher (NSF-RET) through grant number CHE-0832622.

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REFERENCES

(1) Coleman, W. F. LasersAn introduction. J. Chem. Educ. 1982, 59, 441−445. (2) Foss, J. G. Absorption, dispersion, circular dichroism, and rotatory dispersion. J. Chem. Educ. 1963, 40, 592−597. (3) Schwartz, S. E. Franck-Condon principle and duration of electronic transitions. J. Chem. Educ. 1973, 50, 608−610. (4) Wentworth, W. E. Dependence of the Beer−Lambert absorption law on monochromatic radiation: An experiment of spectrometry. J. Chem. Educ. 1966, 43, 262−264. (5) Blitz, J. P.; Sheeran, D. J.; Becker, T. L. Classroom Demonstration of Concepts in Molecular Fluorescence. J. Chem. Educ. 2006, 83, 758−760. (6) Teacher’s Notes 2004, Explaining the principles of Photonics and Optical communications, www.photonicseducationsystems.com. D

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