Nano water Demonstration: Light Interactions with Dye Molecules and Nanomaterials

Transcription

1 Nano water Demonstration: Light Interactions with Dye Molecules and Nanomaterials Key Concepts: In this module, students are expected to learn: How the light can be absorbed or scattered by small objects such as nanoparticles; How light can be used to monitor the water cleaning process; How to compare light absorption, fluorescence, and light scattering; and Color sensitivity of the absorption, scattering and fluorescence of a materials. What are the learning objectives you want them to learn? (List here). National Science Education Standards: Grades 5-8 Introduction Abilities necessary to do scientific inquiry Transfer of Energy Light transmission, absorption, and scattering. Energy as a property of many substances and its association with heat. light, electricity, mechanical motion, sound, atomic, nuclei, and the nature of a chemical Energy transfer This module was developed to introduce principles of light and concepts of emerging water quality issues to be considered with the development of new man-made processes and products. Specifically this demonstration introduces to students to different materials that may be in water and how light can be used to explain property differences of the material. Three concepts of light that will be introduced are Tyndall Effect, Rayleigh Scattering, and light absorption and fluorescence of dye molecule: Tyndall Effect: Light shows particles of different sizes (larger than incident light wavelengths) dispersed or separated throughout water. In this module, milk is used to demonstrate how particles of varied size may be suspended in colloidal forms in liquids. Students will learn how light interacts with a colloidal suspension. Rayleigh Scattering: Light interferes as it passes through a liquid reflecting particles at a size equal or near the wavelength of the light. In this module, silver coagulate nanoparticles (approximately 400nm in size) are used absorb and scatter a green laser light (approximately 500nm wavelength) (what is the laser used for). Students will find that nanoparticles are about the size of the wavelength of the green light. In the typical case of silver nanoparticle light absorption and scattering, the color of the solution and optical properties is caused by a plasmonic effect (which is?) as metal nanoparticles made by silver and gold refers to strong light absorption and scattering properties when these particles in the size range from a few nanometer to tens of nanometer interact with visible light. When

2 metallic materials have small sizes like this they tend to be transparent to visible light. This is different from bulk silver and gold. Absorption and Fluorescence: Light shines brightly because chemicals in the water are excited. In this module a fluorescent Rhodamin dye is introduced into a water sample where the chemicals in the dye are excited with varied laser lights (red, blue, green). Students will learn light can be produced by energy and produced chemically. Different from nanoparticles and micrometer particles, light absorption and fluorescence are mainly due to the electronic transitions of molecules with well-defined molecule structures at the atomic/molecular level. The varying samples created are made to explore how red and green laser light and light properties, including wavelength, differ between samples with water, milk, and dye. Students will learn how nanomaterials are made, how they used in every day products, and how they could potentially enter the environment. This short demonstration provides guidance as to how one might visualize different light-material interaction, and understand the underlying fundamental physics and chemistry concepts. Building on the Nano Water Demonstration, the Seeing Nano Demonstration was designed Figure 1. Materials included in this module around the use of the 5E s instructional model with phases known as Engage, Explore, Explain, Elaborate and Evaluate.(Bybee, 1983). Materials Check List (cf. Figure 1) Wash bottle (1) Ultra Silver coagulate nanoparticle (2) Whole milk(3) Salt (4) Rodamine dye (5) Green laser (6)) Red laser (7) Blue/violet laser (8) Pipettes (students may make cuvette samples or samples provided) (not shown in Figure 1)

3 Gloves (not shown in Figure 1) Red safety glasses if available (students to be warned not to shine laser light in eyes!) Spoon (9) Five capped glass vials (10) Engage Holding up a container of clean looking water, such as from the tap or water bottle, students should be asked Do you think this water is clean? Would you drink it? Is all water clean? How can you tell? What can be in water and not visible to the eye? Have students (in partners) write their response to these questions and then discuss with the larger group. Ask students to consider what are ways you can visually see what is in water? Then introduce the three concepts about light that they will learn: Tyndall Effect, fluorescence, and Rayleigh Scattering. Providing them the descriptions above, ask them to examine each phenomena. Explore The purpose of the following activity is to understand how blue, green and red) light from handhold laser pointers interact with different materials, including nanoparticles, fluorescence dye and milk. A. Procedure Label the five provided glass vials with numerical numbers 1-4. Making the glass vial mixtures by follow the steps below: a. Silver Nanoparticle Mixture: Fill vial No. 1 with distilled water to its 4/5 total volume using wash bottle containing water. Ultra Silver will come with a clean pipette in the bottle at the shared chemical station, place one drop of ultra silver (2) in the cuvette. Cap. Rotate to shake. Record the color of this vial below: b. Fluorescence dye: Fill glass vial No.2 to the line with distilled water to its 4/5 total volume using wash bottle containing water. Use a clean pipette to take up ~0.5 ml Rhodamine dye (5) from the shared chemical station, place it in the glass vial. Cap it and then gently rotate to shake. Record the color of this vial below: c. Milk: Fill vial No. 3 to its 4/5 total volume using wash bottle containing water. Use a clean pipette to take up milk (3) from the shared chemical station. Place three drops of milk into vial No. 3, rotating gently to mix. Record the color of this vial below:

4 d. Transfer half spoon salt into vial No. 4, fill the glass vial with water to its 4/5 total volume using wash bottle containing water. Cap it and then gently rotate to shake to completely dissolve the salt. Record the color of this vial below: All four No. 1 No. 2 No. 3 No. 4 All Silver nanoparticles Rhodamin dye Milk Salt Above glass vials are arranged lest to right as shown in Figure 2 for tests below. Provide students with a light spectrum scale: Red light = approximately 630 nm Green light = approximately 530 nm Blue light = approximately 405 nm Testing the light: Figure 2. Schematic of four glass vials containing different samples. a. Shine each light through the No.1 vial containing silver nanoparticle mixture to see if students can see the passage when each of light source travels through the glass vial, and let students to record the relative intensity of the light passage using words of strong, normal, weak. Red: Green: Blue: Students should be asked to identify which principle of light this mixture demonstrates and explain. Hints you may provide to your student: Nanoparticles are small, if one of the light sources is able to help you see nanoparticles in the water, what would they look like? (students should be able to see small shiny silvery sparkles) What light best detects nanoparticles in the mixture?

5 Red light has the longest wavelength and should not be as visible in this mixture as green light. What can you say about the size of the silver nanoparticles? (students should understand the size of the green light is close to the size of the nanoparticles) b. Shine each light through the vial No. 2 with the fluorescent dye mixture. Record the color of the light passage for each of the laser color below: Red: Green: Blue: Students should be asked to again identify which principle of light this mixture demonstrates and explain. Hints you may provide to your student: The chemicals in the dye absorb light. Why do you see the color of the light passage in the solution different from the laser color that travels into the solution? Chemicals in the dye absorb the light. Students do not see light, but instead see energy produced from the particles in the chemicals vibrating so fast as to create light from energy. Is one light easier to see than another? Can you explain why? c. Shine each light through Glass vial No. 3 with the milk mixture. Students should once again be asked to identify which principle of light this mixture demonstrates and explain. Hints you may provide your student: Students should be able to list some of the constituents of whole milk: calcium, fat, etc. Students should understand there are large and small particles in suspension and in colloidal form. The varied sizes of particles in whole milk will interact with all light of all wavelengths. Particles range from small to large. All light should be visible. d. Finally, repeat step c for vial No. 4 and see if the light passages are different from each other for the three different light. Then transfer one drop of silver nanoparticles (2) into the salt solution in vial No. 4, cap and then rotate gently to mix the solution. recoard the color of the silver nanoparticle solution below: Explain why the color is different from the color of vial No. 1 and what the difference can salt make to silver nanoparticle suspension. You may extend the experiment to clean water. See Elaborate below.

6 Explain 1. What differences did you note between the red, green, and blue light in the milk, dye, and nanoparticle mixtures? Rank which was best at showing the green light, the red light, and the blue light. Explain each. 2. Which demonstrated Rayleigh Scattering? Which demonstrated fluorescence? 3. What have you learned about wavelength dependence of the light-matter interaction process? Teacher Background and Discussion of the Explain: Rayleigh is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. Rayleigh scattering is used in this module to describe the light interference with the silver nanoparticle mixture. The particles may be individual atoms or molecules. It can occur when light travels through transparent solids and liquids, but is most prominently seen in gases. Rayleigh scattering results from the electric polarizability of the particles. The oscillating electric field of a light wave acts on the charges within a particle, causing them to move at the same frequency. The particle therefore becomes a small radiating dipole whose radiation we see as scattered light. Fluorescence of dye is emission of light from molecules upon photoexcitation. Longer wavelength light emission is expected when the molecule is excited using a short wavelength as short wavelength light has higher energy to excite electrons of the molecule from their ground state to excited states. These excited electrons relax and return to ground states by releasing energy in forms of light. The Tyndall effect, also known as Tyndall scattering, is light scattering by particles in a colloid or particles in a fine suspension. It is named after the 19th century physicist John Tyndall. It is similar to Rayleigh scattering, in that the intensity of the scattered light depends on the fourth power of the frequency, so blue light is scattered much more strongly than red light. An example in everyday life is the blue color sometimes seen in the smoke emitted by motorcycles, particularly two stroke machines where the burnt engine oil provides the particles. Under the Tyndall effect, the longer-wavelength light is more transmitted while the shorterwavelength light is more reflected via scattering. An analogy to this wavelength dependency is that longwave electromagnetic waves such as radio waves are able to pass through the walls of buildings, while shortwave electromagnetic waves such as light waves are stopped and reflected by the walls. The Tyndall effect is seen when light-scattering particulate-matter is dispersed in an otherwise light-transmitting medium, when the cross-section of an individual particulate is the range of roughly between 40 and 900 nanometers, i.e., somewhat below or near the wavelength of visible light ( nanometers).

7 It is particularly applicable to colloidal mixtures and suspensions; for example, the Tyndall effect is commercially exploited to determine the size and density of particles in aerosols and other colloidal matter (see ultramicroscope and turbidimeter). Elaborate: Students may want to compare light in clean water. Using the blue light, we should not see the light in clean water, blue is the shortest wavelength. Note, tap water, and some distilled bottle water may still contain small particles detectable by the light. If possible, a laboratory distilled water should be used. Students may also be asked to clean water and then detect light after cleaning. Processes for emerging chemists and engineers are available for mixing non-toxic chemicals for making clean water. To aid visualization, mud may be added to water. Removing particles from water involves addition of Ca(OH) 2 and Alum, or use of a polymer. Instructions are available upon request. As fun facts, the following may be used: Discussion items taken from the internet include: Skin pigmentation permanent blue coloration from drinking large doses of silver nanoparticle based arthritis treatment remedies. Nanoparticles readily enter the blood stream resulting in faster concentration doses.