|Year : 2022 | Volume
| Issue : 1 | Page : 41-47
Development of fluorescein-based dosimeter for radiation processing applications
Sachin Gajanan V. Mhatre, V Sathian, Probal Chaudhury
Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
|Date of Submission||21-Dec-2021|
|Date of Acceptance||23-Mar-2022|
|Date of Web Publication||28-Jun-2022|
Sachin Gajanan V. Mhatre
Bhabha Atomic Research Centre, Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
Dosimetry forms an inseparable part of radiation processing. Many dosimeters are available to cover wide range of doses used in radiation processing. These include the Fricke, ceric-cerous, alanine ESR Alanine (Electron Spin Resonance Technique), dichromate, and radiochromic films. Low-dose radiation processing applications (<2 kGy) such as insect disinfestation, shelf-life extension, and sprout inhibition use imported dosimeters. At present, radiochromic films/waveguides are used for low-dose applications; however, the dosimeters are costly and subject to sanctions and supply restrictions from country of origin. The present work discusses the development of system that can be used as a routine dosimeter for radiation processing facilities in the dose range of 200–2500 Gy. Iodine is generated by the action of radiolysis products of water on iodide and iodate present. Generated iodine halogenates fluorescein to erythrosine with the help of bicarbonate. Increase in absorbance due to the formation of erythrosine can be related to the absorbed dose and used in routine dosimetric applications. As the change is radiochromic, it advantageously provides a visual indication of dose received.
Keywords: Dosimetry, erythrosine, fluorescein, iodine, radiation processing
|How to cite this article:|
V. Mhatre SG, Sathian V, Chaudhury P. Development of fluorescein-based dosimeter for radiation processing applications. Radiat Prot Environ 2022;45:41-7
|How to cite this URL:|
V. Mhatre SG, Sathian V, Chaudhury P. Development of fluorescein-based dosimeter for radiation processing applications. Radiat Prot Environ [serial online] 2022 [cited 2022 Nov 27];45:41-7. Available from: https://www.rpe.org.in/text.asp?2022/45/1/41/348730
| Introduction|| |
One particularly interesting and famous qualitative test for iodine involves formation of erythrosine (red) from fluorescein (fluorescent) and sodium bicarbonate in the presence of acetic acid. The appearance of contrasting color makes this test particularly specific for the detection of iodine.
Electrochemical technique developed for erythrosine synthesis using different electrolyte solutions with varying anode materials and varying current densities works by the same principle. Sodium hydrogen carbonate drives these electrophilic iodination reactions [Figure 1] to completion.
Iodine can be generated using iodide salt and oxidiser mixture, fluorescein iodinates to various degrees (mono, di tri, tetra) depending upon time, temperature and type of oxidizing agent used.
Iodine can also be generated by the reaction between iodide and iodate in the acidic medium. In iodide–iodate dosimeter, triiodide is formed upon exposure to ionizing radiation. The triiodide formed is in equilibrium with iodide ion and iodine molecule.
In the present system, iodine generated by irradiating iodide-iodate solution in alkaline medium  halogenates fluorescein (with the help of bicarbonate) at its electron-rich center, i.e., carbon positions close to hydroxyl group, leading to iodination. The reaction continues at other carbon atoms in the ortho-position of the hydroxyl group. Thus, mono-, di-, tri-, and finally tetra-iodo-fluorescein (erythrosine) are formed. Carbonic acid is formed as a by-product and release of carbon dioxide drives the reaction to completion. Erythrosine (2-[6-hydroxy-2, 4, 5, 7- tetraiodo-3-oxo-xanthen-9-yl] benzoic acid) is a tetra-iodine derivative of fluorescein. It is used as food-coloring agent, printing inks, biological stain, radiopaque mediums, and sensitizers for orthochromatic photographic films. Optimization of the above process was carried out to use the system as radiation dosimeter.
The present work discusses the formation of erythrosine through radiation chemistry pathway and its application as routine dosimeter (200–2500 Gy) for radiation processing applications. The initial color of the solution on irradiation changes from fluorescent yellow to orange and finally red, thus additionally providing a visual indication of the dose received (radiochromism).
The system developed can be used as a routine dosimeter for carrying out dose assurance in radiation processing applications. The dye used, i.e., fluorescein, is simple and relatively nontoxic to general biota, and thus can be easily handled.
All chemicals used were of analytical reagent grade and used as they are without further purification. Fluorescein sodium salt (uranine), potassium iodide, potassium iodate, and ammonium ortho-molybdate were from Sigma Aldrich. Sodium hydrogen carbonate and disodium tetraborate decahydrate (Borax) were from Loba Chemie, India.
Stock solutions of the chemicals were kept at dark to minimize effect of light. All irradiations were carried out either in glass vials or in self-sealing polypropylene tubes. All experiments were carried out at room temperature (25°C–30°C) unless mentioned.
Irradiations and measurements
Polypropylene tubes, used as sample solution containers, were kept in 10% nitric acid for 24 h for degreasing. The degreased tubes were successively washed with tap and distilled water. Containers were preconditioned by filling them with distilled water and irradiating to a dose of about 500 Gy. Jasco V 530 ultraviolet/visible double-beam spectrophotometer was used for all absorbance measurements.
Solution containers along with the Perspex build-up provided required wall thickness (400 mg/cm2) for attaining electronic equilibrium during irradiation, as shown in [Figure 2]. Irradiations were carried out in gamma chambers (GCs), GC-1200 and GC-900, that house 60Co sources indigenously manufactured in India by Board of Radiation and Isotope Technology. All samples were irradiated at the center position of the irradiation volume of the GCs, calibrated using the Fricke dosimetric system by Radiation Safety Systems Division, Bhabha Atomic Research Centre (BARC, National Standard Laboratory in India for ionizing radiations).
Determination of optimum composition
Initial experiments were carried out to test the feasibility of the reaction and subsequent experiments to optimize concentrations of components. The concentrations mentioned in iodide/iodide dosimeter were used as reference in preliminary experiments. Sample solutions of iodide and iodate along with varying concentrations of uranine were irradiated and measured spectrophotometrically. Sodium hydrogen carbonate was added at varying concentrations to study its influence on the reaction. pH of the solutions was varied from 8 to 14 to optimize the yield.
Once the concentration of components was fixed, preirradiation effect, postirradiation effect, dose rate effect, dose fractionation effect, and reproducibility studies were carried out. Effect of catalysts disodium tetraborate decahydrate (borax) and ammonium ortho-molybdate was also studied.
Dose absorbance relation
Samples (in polypropylene tubes) were irradiated at varying doses from 200 Gy to around 2500 Gy in GC, and each dose point was repeated thrice and its average was recorded. All absorbance measurements were performed 15 min postirradiation, to allow reactions to complete. Net changes in absorbance values were recorded. Entire experiment was repeated twice. The experiment was carried out for borax- and molybdenum-catalyzed systems.
Pre- and post-irradiation stability and reproducibility
For preirradiation stability study, freshly prepared solutions were covered with aluminum foils (to avoid exposure to light) and kept at room temperature under standard laboratory conditions; absorbance was recorded for 5 consecutive days.
For postirradiation stability study, solution was irradiated to three known doses so as to cover entire dose range. Irradiated solutions were stored at room temperature (covered with aluminum foils) under standard laboratory conditions.
Reproducibility study was carried out by irradiating 10 different samples each to 200 Gy and 1965 Gy so that it covers entire range of the system.
Dose rate and dose fractionation effect
To study the effect of dose rate on the system, solutions were irradiated to three different doses in GC-900 and GC-1200 having dose rates of 10 Gy/min and 40 Gy/min, respectively. Doses were chosen so that the entire dose range is covered.
In dose fractionation study, dose is fractionated into two equal doses with a gap of 30 min and 60 min between irradiations to simulate conditions of break down in radiation processing facilities. Results of all the exercises are discussed.
| Results & Discussions|| |
The optimum composition of solution was established in two different ways. In the first set of experiments, iodide and iodate concentrations were varied as shown in [Figure 3] and [Figure 4].
After fixing concentration of iodide and iodate, concentration of uranine [Figure 5] and subsequently sodium hydrogen carbonate [Figure 6] was fixed.
5 mM of iodide and 10 mM iodate concentration were found to give optimum results. Fluorescein in excess of 0.5 mM resulted in incomplete iodination. Although 2.5 mM of sodium hydrogen carbonate was found to be optimum, 10 mM was fixed and will be discussed in postirradiation studies.
Borax on irradiation forms perborate that (along with radical species generated) oxidizes iodide–iodate to iodine. Borax was found to enhance the reaction rate with marked increase in sensitivity in threshold dose of detection. Ammonium ortho-molybdate, a well-known catalyst for bromination reactions in organic compounds, showed similar effect (catalytic) in iodination reactions. The catalytic effect of ammonium orthomolybdate is greater than borax with the minimum detectable dose of 100 Gy. 2.5 mm concentration of borax and molybdenum was used separately to study their effect on the above system.
Absorbance of the irradiated solutions is measured to estimate radiation dose (approximate) received. It was experimentally observed that the wavelength of maximum absorption (λmax) shifts from 522 to 545 nm [Figure 7]. Change in wavelength is attributable to different stages of iodination and to the changes in pH occurring with iodination reactions. λmax for erythrosine differs from 532 to 540 nm depending upon the pH of solution. Absorbance measurements were done at 540 nm (λmax) as the dose detection threshold is lowered. 2500 Gy is the saturation dose for all spectrophotometric measurements as above this dose; the increase in absorbance is infinitesimal with respect to the dose received.
[Figure 8] indicates the dose absorption relation of the indicator system with a variation of 5% in the dose values reported for bicarbonate system without any catalysts.
|Figure 8: Dose absorption relation for bicarbonate system without catalyst|
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[Figure 9] and [Figure 10] represent the dose absorbance relation of the bicarbonate system with borax and molybdenum, respectively. Reproducibility study indicates that the percentage standard deviation of the bicarbonate system is below 5% as shown in [Table 1]. The solution is stable up to 25 days when stored in a glass container at a temperature ranging from 5°C to 10°C.
|Figure 9: Dose absorption relation for bicarbonate system with borax as catalyst|
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|Figure 10: Dose absorption relation for bicarbonate system with molybdenum as catalyst|
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|Table 1: Reproducibility for bicarbonate based system with no other catalysts|
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The developed system is unstable due to thermal oxidation of iodide to iodine under normal laboratory conditions (temperature, ozone, etc.) [Figure 11]. Addition of antioxidants such as sulfite, bisulfate, and thiosulfate to the system does improve the preirradiation stability of system by lowering the oxidation of iodide, i.e., these antioxidants lower the rate of iodide oxidation to a considerable degree. However, their presence changes the radiation response [Figure 12]. The threshold dose is increased from 200 Gy to 2 kGy with addition of thiosulfate (2.5 mm). Further, the increase in absorbance with dose is abrupt and nonreproducible. Other antioxidants tried showed similar effects.
|Figure 12: Preirradiation stability with sodium thiosulfate as stabilizer|
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It was observed during concentration-dependency study that 2.5 mM bicarbonate was optimum for iodination reaction. However, the concentration was increased to 10 mm, as experimentally, it was found to greatly improve the postirradiation stability as shown in [Figure 13] and [Figure 14]. It is suggested that all absorbance measurements be performed at least 15 min after irradiation; it is also advisable to complete the measurements within 120 min to minimize the error due to variation in absorbance with time.
|Figure 13: Postirradiation stability before bicarbonate concentration optimized|
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|Figure 14: Postirradiation stability with optimized bicarbonate concentration|
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Two different dose rates 10 Gy/min and 40 Gy/min were used to study the effect of dose rate on the system; however, no significant effect of dose rate was observed.
Preliminary dose fractionation studies suggest negligible effect of dose fractionation at the experimental conditions set.
The un-irradiated solution showed no significant change in absorbance value when stored between 5°C and 10°C (refrigerated conditions). Borax-added solutions have similar storage properties.
Solutions with 5 mm molybdenum are highly unstable, and absorbance of the solution increases exponentially. This solution is not suitable for dosimetric purposes.
The study suggests that the system developed can be used as a routine dosimeter that is easy to prepare, (made from relatively nontoxic chemicals), use, and analyze.
| Conclusions|| |
Preliminary studies suggest that the developed system can be used as dosimeter in radiation processing applications such as insect disinfestations, quarantine treatment of fruits, and shelf-life extension in the dose range of 200 Gy to 2.5 kGy. Further studies such as temperature dependence and energy dependence have to be carried out for establishing the dosimeter for routine dosimetry. As few dosimeters are available in the range, developed system can be used as an alternate to expensive dosimeters and dosimetric systems.
The authors are grateful to Dr. D. K. Aswal, Group Director, Health Safety and Environment Group, BARC, for encouragement in this work. Thanks to all staff members of Radiation Standards Section for their wholehearted support in the arrangement and conduction of several standardization protocols leading to proper dosimetry.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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