Read the dose from the rainbow! Visual radiation detection based on changes in the hue of photonic crystals

At 23:00 Beijing time on September 7, 2022, Dr. Wang Yunlong and Professor Ma Jun of the School of Materials Science and Technology of Nanjing University of Aeronautics and Astronautics, Associate Professor Wang Mozhen of the Department of Polymer Science and Engineering of the University of Science and Technology of China and Professor Wang Yinwo of Soochow University jointly published a research paper entitled “Color-phase Readout Radiochromic Photonic Crystal Dosimeter” in the Journal of Content online.

The paper proposes a new radiation dosimeter concept based on the structural color change of photonic crystals, and uses a variety of photonic crystals to verify the concept, obtaining dosimeters with different sensitivities, which is expected to be applied to radiation processing, beam calibration of large scientific devices and radiation medicine dose verification.

Wang Zhihao, phD student of School of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, is the first author, Ge Zhiqing, doctoral student of the Department of Polymer Science and Engineering of the University of Science and Technology of China, is the co-first author, Dr. Wang Yunlong is the corresponding author and co-first author, and Professor Ma Jun and Professor Wang Yinwo are the co-corresponding authors.

With the development and wide application of nuclear technology, radiation processing, radiation medicine, beam detection of large scientific devices, space radiation detection and other demand scenarios have put forward new requirements for radiation dose detection. Radiation discolorogenic films can determine the absolute value of the absorbed radiation dose through the change of color after radiation, and are widely used in the field of radiation detection because of its simple structure, convenient preparation, low cost, and convenient use. However, these traditional film dosimeters use colorimetric/chromaticity methods to determine the absorbed dose, which is difficult to directly identify with the naked eye and requires reading with the help of an optical density meter or spectrometer. In addition, most radiochromic films are sensitive to temperature, humidity or UV exposure and need to be stored in a dark, dry environment. These problems greatly limit their current and future applications.

Similar to butterfly, chameleon, and bird feathers in nature, photonic crystals do not rely on pigments to display color, but instead form an iridescent color through Bragg diffraction of periodic microstructures similar to light wavelengths. Dr. Wang Yunlong combined the radiation effect of polymers with photonic crystals to propose a new dose detection scheme for the first time, that is, to use photonic crystal thin films to achieve full spectral movement of the Prague reflection peak under continuous irradiation of X or γ rays to achieve dose detection. The dose application range of this photonic crystal dosimeter is highly adjustable and has good spatial resolution (better than 30 μm). Compared with conventional radiation discoloration dosimeters, this dosimeter also exhibits good environmental stability for light, temperature and humidity. More importantly, in addition to the traditional spectrometer detection method, this photonic crystal dosimeter can also be quickly and intelligently identified by comparing standard color cards with the naked eye, and even using the hue value obtained by mobile phone photographs, which provides great convenience for its application in industrial irradiation, large scientific devices and radiological medicine.

Figure 1: Schematic of a radiochromic anti-opal photonic crystal (PC) film

The preparation process of the dosage film is shown in Figure 2A. Using the degradation or crosslinking of polymers under irradiation, the research team proposed three dosimeter designs and realized corresponding demonstration examples. The results show that as the absorbed dose increases, the color of the film changes from red to blue in the visible spectrum. When the absorbed dose is increased to 225 kGy, the photon band gap of the original PEDA film (636 nm) gradually decreases to 444 nm, and when the absorbed dose is increased to 250 kGy, the film eventually becomes transparent. The scanning electron microscopy photograph (Figure 2C) shows the collapse of the anti-opal structure during irradiation, indicating that the regular porous structure in the dose film collapses as the radiation oxidation degradation of PEDA occurs. According to the Bragg-Snell equation, the structural color of the film will undergo a blue shift as the layer spacing decreases (Figure 2D). Figure 2E shows that the reflected spectral peak position of the film, λmax, is typically linear with the absorbed dose, so that the film dosimeter can be used to accurately measure the absorbed dose. In addition, the color of the unraveled samples can remain unchanged for at least 62 days in normal light and temperature and humidity environments, confirming the excellent environmental stability of the dose film. In particular, after the film was treated in hot air at 70 °C for 48 hours, its color and brightness did not change significantly. Therefore, its thermal stability is significantly higher than that of existing commercial dosimeters, which greatly facilitates the preservation and use of dosage films. At the same time, by adding a PEG without double bonds to the polymerization precursor to plasticize the dose film, a metastable photonic crystal film can be obtained, so that the sensitivity of the film dosimeter is increased to more than 2 times that of the PEGDA dosimeter alone.

Figure 2: Preparation of an anti-opal dosimeter and its radiant color rendering characteristics

High-energy radiation not only causes chain breakage, but also triggers polymerization and crosslinking of polymer chains at lower doses, while hydrogel materials with unsaturated vinyl react more sensitively. The research team prepared a hydrogel film using methacrylated gelatin (GelMA) and reintroduced the unsaturated vinyl group through chemical modification, and then irradiated the hydrogel using X-rays. As the vinyl group crosslinks during radiation, the anti-opal hydrogel shrinks significantly and causes the color of the hydrogel to shift from red to green. The results showed that under X-ray irradiation, the reflected peak wavelength shift of the anti-opal hydrogel was linearly correlated with the absorption dose from 0 Gy to 60 Gy, and the detection sensitivity reached ~0.61 nm/Gy. As the dose continues to increase, the subsequent crosslinking reaction slows down due to the depletion of the double bonds in the hydrogel, and the movement rate of the reflected peak wavelength decreases accordingly. To test the dosimeter’s resolution, the team used a metal mask to block the X-rays, forming a five-pointed star-shaped X-ray beam to irradiate the film. As can be observed from Figure 3E, there is a clear five-pointed star on the exposed dose film, and the discoloration area of this star shape highly coincides with the shape of the metal mask. Light microscope photographs show that the boundary between the irradiated discoloration area and the irradiated area is clear, and its transition area width is less than 30 μm, indicating that the dosimeter has good spatial resolution. At the same time, GelMa is a well-known equivalent material for human tissues, so this photonic crystal material has great potential as a medical hydrogel radiation dosimeter.

Figure 3: Preparation and radiation discoloration properties of methacrylated gelatin anti-opal hydrogel film

In the dose detection system proposed herein, the absorption dose is measured by the hue/wavelength (λmax) of the dosimeter instead of the absorbance/optical density, making the measurement of the radiation dose more intuitive and convenient. When using color card contrast, changes in hue are not affected by changes in ambient light intensity and stains such as dust on the surface of the material. In cases where accuracy is not required, the radiation dose can be read out by comparing the naked eye with a pre-calibrated standard ribbon. Using photographic equipment such as common digital cameras or mobile phones, more accurate measurement results can be obtained. Although the color of the irradiated sample shows a disordered change in the RGB space (Figure 4D), when converting the color parameters to the HSV color space, its hue value increases monotonically with the absorbed dose and has a linear relationship over a larger range (Figures 4E, F). Therefore, after taking pictures with a camera or mobile phone, the absorbed dose can be more accurately detected by reading the hue value of the dose tablet after irradiation, which allows researchers to make accurate measurements outside the laboratory at a relatively low cost.

Figure 4: Dose reading without spectrometer

Based on the highly tunable nature of the photonic crystal material, the study opens up a general approach to the detection of cumulative doses of radiation. The research team has applied for an invention patent and continues to promote the application of this path in scenarios such as three-dimensional dose verification, spatial radiation measurement, and detection of extreme radiation environments in radiotherapy.

The research was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Jiangsu Province, the Jiangsu Provincial Double Carbon Special Fund, the Basic Scientific Research Business Fund of Central Universities and the New Teacher Start-up Fund of Nanjing University of Aeronautics and Astronautics. In this study γ irradiation experiment was completed in the cobalt-60 source device of the University of Science and Technology of China, and Professor Ge Xuewu and Professor Wang Shangfei of the Department of Polymer Science and Engineering of the University of Science and Technology of China provided guidance and technical support for the design and implementation of the relevant experiments. (Source: Science Network)

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