Measuring Nano-particle Fluorescence in Caramelized Sugar Glass

Measuring Nano-particle Fluorescence in Caramelized Sugar 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
from sugars (hard candy), 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
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
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
(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
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].
Results
Experimental
Sugar Glass Preparation and the Excited Emission
Cane sugar, corn syrup and water are heated to first
dissolve all crystals and then remove most of the
water. Boiling to ~ 150 °C leaves about 1-2% water
and will make a glassy candy on cooling.
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Blue LED (470 nm)
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Violet LED (397 nm)
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Certain semiconductor quantum dots are known to exhibit WL dependent
fluorescence. Sun, et al. [3] was the first to observe bright visible
photoluminescence from surface passivated carbon nano-particles (CNP) in 2006.
These CNPs have generated considerable interest as environmentally friendly biocompatible 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.
Carbon Quantum Dots In Caramelized Bread and Sugars
The luminescence is shown (yellow) on left, superimposed
with the exciting LED light spectra. Note the strong
dependence on source wavelength, with essentially no
emission for yellow or red LEDs. Violet led provides the
best separation between source and fluorescence.
Radiation. For green, the overlapping excitation peak can
be fit and removed (subtracted out) in Excel.
RT Emission from Various LED Sources
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Carbon Nano-dot Fluorescence
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Low Cost Approach for Investigation
Blue /Gr LED (500 nm)
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For quantitative examination:
Fluorescence monitoring system was assembled using the
Ocean Optics Red Tide USB Spectrometer available in many high school
and undergraduate labs. A nichrome wire was wrapped around an
outer glass tube to provide a simple heater for temperature control.
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Red LED (627 nm)
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High brightness LEDs are used to obtain a range of
excitation wavelength sources
from UV (violet) to RED.
LEDs alone are sufficient for qualitative observation
of the WL dependence of the fluorescence.
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Yellow LED (594 nm)
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A half disc was molded for a Snell’s law demo; the
yellowish scatter from green laser suggested something more
interesting. Under UV (375 nm) light, the same candy glass sample
emits blue.
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Excel scaled luminescence shows a distinct dependence
of fluorescence peak on the excitation wavelength,
characteristic of NP fluorescence.
Sk, et. al. 2012 [5] provide TEM analysis of carbon nano dots in caramelized
foods and provide similar, yet more thorough, measurements of the excitation
dependent fluorescence.
Summary
Fluorescence Decrease with Temperature
Increased Absorption with Caramelization
• The fluorescence arises from the formation of surface stabilized carbon nanospheres in caramelized sugar glass
• The fluorescence can be examined qualitatively with simple, LED multi
wavelength sources or quantitatively with the addition of a low-cost student
spectrometer.
• Example of how exploring even common materials like candy, with some basic
tools such as a simple student spectrometer, can bring the curious student to
discovery of real, contemporary science.
• Appropriate for the undergraduate lab and especially as an open-ended
introductory research project. A gateway project.
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Intensity
The Fluorescence Monitoring System
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T0
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T2
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T3
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T4
RT
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RT
T5
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T6
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T7
We provide a simple and highly accessible system to engage students in
nano-particle optics within glassy materials.
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Wavelength
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Gradual decrease of the fluorescence peak near 550 nm as
the temperature increases from RT (T0) to T7 (near boiling).
After heating the room temperature fluorescence has
increased appreciably, associated with a deeper yellow
color in the sample from caramelization.
References:
Transmission for sugar glass heated to increasingly
higher temperatures from 160 to 200 ⁰C. The highly
caramelized samples are quite absorbing in the
blue, resulting in a reabsorption of any fluorescence.
Acknowledgement: This work has been supported by IMI-NFG, Lehigh University through National Science Foundations (NSF) Grant : DMR-0844014
International Materials Institute for New Functionality in Glass
Sponsored by US National Science Foundation
www.lehigh.edu/imi
1. W. R. Heffner and H. Jain, “Building a Low Cost, Hands-on Learning Curriculum on Glass
Science and Engineering using Candy Glass” in MRS Proceedings 1233 , Boston, 2009, (Materials
Research Society, 2010).
2. W. R. Heffner and H Jain, “Low-Cost, Experimental Curriculum in Materials Science Using
Candy Glass Part 2: Home-Built Apparatuses” in MRS Proceedings 1657, Boston, 2014, (Materials
Research Society, 2014).
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 Caramels." Scientific
Reports 2, (2012), article 383.