Measuring Nano-particle Fluorescence in Caramelized Glass William R. Heffner1 and Donald Wright III2 1 IMI-NFG, Lehigh University, Bethlehem, PA , 2 Oakwood University. Huntsville, AL

Abstract: Nano-particle fluorescence is a stimulating topic for introducing students to contemporary experimental physics. We present her the unexpected fluorescence observed in simple glasses made Carbon Nano-dot Fluorescence from (hard ), requiring only kitchen stove processing. The fluorescence is easy to see with UV or green laser pointer illumination. We characterize the observed emission using a home-built Certain semiconductor quantum dots are known to exhibit WL dependent fluorescence monitoring system consisting of high intensity LEDs for the excitation and the student grade Ocean Optics Red Tide Spectrometer to resolve the emission. The fluorescence was found to span fluorescence. Sun, et al. [3] was the first to observe bright visible between about 470 nm and 650 nm with a marked dependence on excitation wavelength (WL). The fluorescence and absorption also increase as the glass caramelizes (browns) with further heat treatment photoluminescence from surface passivated carbon nano-particles (CNP) in 2006. (cooking). Recent literature has shown similar fluorescence in caramelized carbohydrates and sugars to be due to the production of carbon nanoparticles. We propose the experiment as a cross-disciplinary These CNPs have generated considerable interest as environmentally friendly bio- and open-ended investigation for an undergraduate lab in physics, chemistry or material science. This is part of a larger collection of experiments to explore glass and optical science through sugar glass [1,2]. compatible fluorescent probes[4]. In 2012 Sk, et al. [5] found that the same CNPs are produced during the caramelization of foods, establishing the CNP origin of our own observations. Experimental Results Sugar Glass Preparation and the Excited Emission Carbon Quantum Dots In Caramelized Bread and Sugars The luminescence is shown (yellow) on left, superimposed Cane sugar, corn and water are heated to first RT Emission from Various LED Sources with the exciting LED light spectra. Note the strong dissolve all crystals and then remove most of the 300 dependence on source wavelength, with essentially no water. Boiling to ~ 150 °C leaves about 1-2% water 290 340 emission for yellow or red LEDs. Violet led provides the 280 Violet LED (397 nm) 320 Blue LED (470 nm) and will make a glassy candy on cooling. 270 300 best separation between source and fluorescence. 260 280 Radiation. For green, the overlapping excitation peak can 250 240 260 be fit and removed (subtracted out) in Excel. 230 240 220 220 210 200 200 350 450 550 650 750 350 400 450 500 550 600 650 700 750

300 Yellow LED (594 nm)

A half disc was molded for a Snell’s law demo; the 340 Blue /Gr LED (500 nm) 280 260 320 yellowish scatter from green laser suggested something more 240 interesting. Under UV (375 nm) light, the same candy glass sample 300 220 280 200 emits blue. 350 450 550 650 750 260 350 Low Cost Approach for Investigation 240 Red LED (627 nm) 300 220 Excel scaled luminescence shows a distinct dependence Sk, et. al. 2012 [5] provide TEM analysis of carbon nano dots in caramelized High brightness LEDs are used to obtain a range of 200 250 350 450 550 650 750 of fluorescence peak on the excitation wavelength, foods and provide similar, yet more thorough, measurements of the excitation excitation wavelength sources 200 350 450 550 650 750 characteristic of NP fluorescence. from UV (violet) to RED. dependent fluorescence. LEDs alone are sufficient for qualitative observation of the WL dependence of the fluorescence. Summary For quantitative examination: Fluorescence monitoring system was assembled using the Fluorescence Decrease with Temperature Increased Absorption with Caramelization We provide a simple and highly accessible system to engage students in Ocean Optics Red Tide USB Spectrometer available in many high school nano-particle optics within glassy materials. and undergraduate labs. A nichrome wire was wrapped around an 300 • The fluorescence arises from the formation of surface stabilized carbon nano- outer glass tube to provide a simple heater for temperature control. 290 T0 RT spheres in caramelized sugar glass 280 T2 • The fluorescence can be examined qualitatively with simple, LED multi The Fluorescence Monitoring System 270 T3 wavelength sources or quantitatively with the addition of a low-cost student 260 T4 spectrometer. 250 RT T5 • Example of how exploring even common materials like candy, with some basic

Intensity 240 T6 tools such as a simple student spectrometer, can bring the curious student to 230 T7 150 discovery of real, contemporary science. 220 • Appropriate for the undergraduate lab and especially as an open-ended 210 introductory research project. A gateway project. 200 350 450 550 650 750 Wavelength References: Gradual decrease of the fluorescence peak near 550 nm as 1. W. R. Heffner and H. Jain, “Building a Low Cost, Hands-on Learning Curriculum on Glass the temperature increases from RT (T0) to T7 (near boiling). Science and Engineering using Candy Glass” in MRS Proceedings 1233 , Boston, 2009, (Materials After heating the room temperature fluorescence has Research Society, 2010). increased appreciably, associated with a deeper yellow Transmission for sugar glass heated to increasingly 2. W. R. Heffner and H Jain, “Low-Cost, Experimental Curriculum in Materials Science Using color in the sample from caramelization. higher temperatures from 160 to 200 ⁰C. The highly Candy Glass Part 2: Home-Built Apparatuses” in MRS Proceedings 1657, Boston, 2014, (Materials caramelized samples are quite absorbing in the Research Society, 2014). blue, resulting in a reabsorption of any fluorescence. 3. Y. Sun, et al., “Quantum-Sized Carbon Dots for Bright and Colorful Photoluminescence”, J. Am. Chem. Soc. 128, 7756 (2006),. 4. S. Yang, et al., "Carbon dots as nontoxic and high-performance fluorescence imaging agents." The Journal of Physical Chemistry C 113, 18110 (2009). 5. M. Sk, et al., "Presence of Amorphous Carbon Nanoparticles in Food ." Scientific Reports 2, (2012), article 383.

Acknowledgement: This work has been supported by IMI-NFG, Lehigh University through National Science Foundations (NSF) Grant : DMR-0844014

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