Radiogenic dental damage is a common and crucial problem in patients receiving radiotherapy for malignancies in the head and neck region. Unfortunately, little is known about the development of complications after radiation therapy on the microstructure profiles of the human tooth. Therefore, we propose a novel method in which the primary focus is to investigate, in vitro, the direct influences of di↵erent radiation doses on elastic properties of enamel and dentin of human tooth by Scanning Acoustic Microscopy (SAM) at the microscale. We obtain two-dimensional (2D) acoustic impedance images from twenty-five sound human third molars each of which is cut into a 1 mm thick cross-sectional slices. Acoustic impedance by SAM operating at 320 MHz are recorded from the sections comprising enamel and dentin before and after every irradiation dose to a cumulative dose of 60 Gy. The findings of our study reveal that radiation therapy changes the micro-elastic features of enamel and dentin accompanied by the decreased acoustic impedance. We establish a relationship between cumulative irradiation doses and the measured acoustic impedance. The quantified acoustic impedance values for the different irradiation doses might be helpful in in vitro assays for the determination of the safe dose limits to prevent severe tooth damage in the treatment plan of the individuals having head and neck cancer.
OPTICAL PROPERTIES OF WATER BASED COLLOIDAL SUSPENSIONS OF GRAPHENE QUANTUM DOTS
Melita Parlak1, Elif Kayaaslan2, Dilan Ugus2, Bukem Bilen1, Hakan Erturk2
1Bogazici University, Department of Physics, Istanbul, 34342, Turkey 2Bogazici University, Department of Mechanical Engineering, Istanbul, 34342, Turkey
Graphene quantum dots (GQDs) and their colloidal suspensions have been attracting attention for various applications such as biomedical imaging or energy harvesting. Optical properties need to be calculated for quantum dots’ effective use in different applications. Spectral absorbance for dilute colloidal suspensions of graphene quantum dots in water is predicted theo- retically by several methods including Rayleigh theory, Rayleigh Debye Gans approximation for fractal aggregates (RDG-FA) and discrete dipole approx- imation (DDA) within visible range of light (450- 700 nm). Confinement effect is observed while shrinking from 3 dimensional bulk material to quan- tum dots, and when the size of GQD is smaller than 10 nm, confinement effect becomes remarkable.The sizes of the inspected colloidal suspensions of quantum dots in water are reported to be 5 nm in average. After calculating optical properties of graphene quantum dots by using these approximations, the predicted spectral absorbances are compared to the experimental results. Absorption spectra of colloidal suspensions of GQDs in water with concen- trations ranging from 10000 ppm to 80000 ppm are obtained by Avantes Spectrometer using halogen lamp illumination and AvaSoft software. The size distribution and therefore the aggregation level of the quantum dots are identified through comparisons of the predicted and the measured spectral absorbances.
The radiative decay rates of perylene dye molecules, attached to silicon nano-rods are investigated by means of timeresolved
fluorescence experiments. The decay rates of dye molecules in the vicinity of silicon nano-rods are inhibited
due to their various diameters and therefore the modification of the surrounding environment. Inhibition is caused by an
increased nonradiative rate due to resonant energy transfer described by the Gersten-Nitzan model.
Humidity induced changes in the refractive index and thickness of polyethylene glycol (PEG) thin films are in situ
determined by optical waveguide spectroscopy. PEG brushes are covalently attached to the surface of a thin gold film on
a borosilicate crown glass (BK7) using a grafting-from chemical synthesis technique. The measurements are carried out
in an attenuated total internal reflection setup. At low humidity levels, both the refractive index and the thickness change
gradually due to swelling of the PEG thin films upon water intake. At around 80% relative humidity, a steep decrease in
the refractive index and a steep increase in the thickness are observed as a result of a phase change from a
semicrystalline state to a physical gel state. The hydrogenation of PEG films causes a less pronounced phase change
from a semicrystalline state to a gel state. Due to fewer ether oxygen atoms available for the water molecules to make
hydrogen bonding, the polymer has a more stable structure than before and the phase change is observed to shift to
higher humidity levels. It is discussed that such a humidity induced change in the index of refraction can be utilized in
constructing of a PEG based humidity sensor.
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