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In this study, we investigated the effect of the thickness of metal-dioxide thin films on silicon (Si) micro-electromechanical systems (MEMS) for sensitive, accurate, and reproducible detection of gamma-radiation dose. Silicon wafers and microcantilevers were coated with various thicknesses of titanium dioxide (TiO2) thin films and controlled by the number of cycles in atomic layer deposition (ALD) under 17-mbar pressure and temperature of 200 °C. All samples were exposed to different doses of gamma (0, 10, and 20 kGy) using a60Co source. Before and after gamma irradiation, the optical and mechanical properties of the TiO2 thin films on Si wafers were studied by spectroscopic ellipsometry (SE) and atomic force microscopy (AFM). The resonance frequency shift (RFS) resulting from exposing different thicknesses of TiO2 thin films on MEMS-based cantilevers to gamma-radiation doses were evaluated by AFM. SE results revealed the film thicknesses of 11.91, 21.77, 62.91, and 218.23 nm at 250, 500, 1250, and 2500 coating cycles, respectively. Through SE, other optical constants, such as surface roughness (SR) and refractive index (n), were obtained. The root-mean-square (RMS) SR obtained from AFM images and SE measurements on Si wafers showed the same behavior under gamma radiation according to the TiO2 thin-films thicknesses. The RFS results show that the best film thickness for reproducible and sensitive gamma-radiation detection was 218.23 nm. The frequency shift as the gamma dose increase from 0 to 20 kGy was ∼60 Hz. It is a very palpable and linear shift; hence, it can be used as a dosimeter sensor. The results were verified by the statistical correlation coefficient method to find the correlation between RFS and the gamma dose at different film thicknesses. Correlation results are consistent with other results.

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This study generally relates to nuclear sensors and specifically to detecting nuclear and electromagnetic radiation using an ultrasensitive quartz tuning fork (QTF) sensor. We aim to detect low doses of gamma radiation with fast response time using QTF. Three different types of QTFs (uncoated and gold coated) were used in this study in order to investigate their sensitivity to gamma radiations. Our results show that a thick gold coating on QTF can enhance the quality factor and increase the resonance frequency from 32.7 to 32.9 kHz as compared to uncoated QTF. The results also show that increasing the surface area of the gold coating on the QTF can significantly enhance the sensitivity of the QTF to radiation. We investigated the properties of gold-coated and uncoated QTFs before and after irradiation by scanning electron microscopy. We further investigated the optical properties of SiO2 wafers (quartz) by spectroscopic ellipsometry (SE). The SE studies revealed that even a small change in the microstructure of the material caused by gamma radiation would have an impact on mechanical properties of QTF, resulting in a shift in resonance frequency. Overall, the results of the experiments demonstrated the feasibility of using QTF sensors as an easy to use, low-cost, and sensitive radiation detector.

RESUMEN

Despite their advantageous chemical properties for nuclear imaging, radioactive sodium-22 ((22)Na) tracers have been excluded for biomedical applications because of their extremely long lifetime. In the current study, we proposed, for the first time, the use of (22)Na radiotracers for pre-clinical applications by efficiently loading with silica nanoparticles (SiNPs) and thus offering a new life for this radiotracer. Crown-ether-conjugated SiNPs (300 nm; -0.18±0.1 mV) were successfully loaded with (22)Na with a loading efficacy of 98.1%±1.4%. Noninvasive positron emission tomography imaging revealed a transient accumulation of (22)Na-loaded SiNPs in the liver and to a lower extent in the spleen, kidneys, and lung. However, the signal gradually decreased in a time-dependent manner to become not detectable starting from 2 weeks postinjection. These observations were confirmed ex vivo by quantifying (22)Na radioactivity using γ-counter and silicon content using inductively coupled plasma-mass spectrometry in the blood and the different organs of interest. Quantification of Si content in the urine and feces revealed that SiNPs accumulated in the organs were cleared from the body within a period of 2 weeks and completely in 1 month. Biocompatibility evaluations performed during the 1-month follow-up study to assess the possibility of synthesized nanocarriers to induce oxidative stress or DNA damage confirmed their safety for pre-clinical applications. (22)Na-loaded nanocarriers can thus provide an innovative diagnostic agent allowing ultra-sensitive positron emission tomography imaging. On the other hand, with its long lifetime, onsite generators or cyclotrons will not be required as (22)Na can be easily stored in the nuclear medicine department and be used on-demand.

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