Academic Journal of Science and Technology ISSN: 2771-3032 | Vol. 13, No. 3, 2024 315 Bayberry Tannin‐modified SiO2 used for 68Ge/68Ga Generator Xiang Gong, Jiali Liu, Sheng Hu CGN Isotope (Mian yang) Co., Ltd., Mianyang, Sichuan 621000, PR China Abstract: A synthetic method of modified SiO₂ grafted with bayberry tannin (SiO₂-BT) was reported. When the prepared SiO₂-BT was used for the treatment of the mixed solution of Ge and Ga, it could achieve a 99.99% Ge adsorption performance with hardly Ga adsorb. In addition, this SiO₂-BT also exhibited excellent acid resistance, in 0.1-0.5 M HCl solutions, it maintaining over 99.8% Ge adsorption efficiency. The results of radioactive experiments showed that with 10 μCi ⁶⁸Ge loaded, this SiO₂-BT could achieve an elution efficiency of ⁶⁸Ga of 75% while the breakthrough of ⁶⁸Ge maintained below 5×10-6. Overall, this SiO₂-BT has excellent acid resistance, highly efficient ⁶⁸Ga elution, and the characteristic of long-term low ⁶⁸Ge breakthrough, making it a potential adsorbent for ⁶⁸Ge/⁶⁸Ga generators. Keywords: 68Ge/68Ga Generator; Absorbent; PET; SiO2. 1. Introduction PET (positron computed tomography) is a high-resolution, highly sensitive and non-invasive imaging technology, plays an important role in the field of medical image diagnosis. Positrons emitted by radionuclides such as 18F[1]、89Zr[2]、124I[3]、 11C[4]、68Ga[5], etc., annihilated with electrons around tissues to emit two γ-photons with energy of 511 keV. And the γ- photons were detected by the instrument to obtain high- resolution and high-sensitivity images[6]. As an important positron radionuclide, 68Ga has a short half-life (67.7 minutes), matches the pharmacokinetics of many peptides and other small molecules, and emits maximum β+ emission energy of 1899.1 keV (87.7%). It maintains long-term radioactive equilibrium with its long half-life parent nuclide 68Ge (half-life 271 days) [7-9]. The short half-life of 68Ga allows healthcare professionals to perform PET scans with a suitable 68Ga radiotracer label while keeping the radiation doses received by patients within acceptable limits. The generator-accessible properties of 68Ga makes it an excellent source of ready-to-use positron nuclides, enabling PET imaging in hospitals without on-site cyclotrons. In addition, Ga3+, as a Lewis hard acid, is easy to form thermodynamically stable complexes with Lewis hard base ligands containing N and O (such as DOTA and NOTA). Therefor, 68Ga has a high labeling efficiency[10]. As a diagnostic radionuclide, 68Ga can pair with therapeutic isotopes such as 177Lu or 225Ac which realizing the integration of diagnosis and treatment[11]. Due to the excellent properties of 68Ga, researchers have been developing and researching 68Ge/68Ga generators for many years. At present, a large number of studies have reported the use of metal oxides SnO2, TiO2, Al2O3, Ta2O5, CeO2, ZrO2, etc., and non-metallic oxides modified SiO2 as adsorbents[12-13]. And there are numbers of 68Ge/68Ga generators such as SnO2 of iThemba Labs (South Africa), TiO2 of Cyclotron corporation (Russia), and modified SiO2 of ITG (Germany) has entered the market. However, these adsorbents still have some defects, such as high 68Ge breakthrough in the eluent, high content of metal impurities, large eluent volume (4-10 mL), high concentration of HCl (>0.5mol/L), and short service life. In addition, repeated elution will significantly reduce the elution efficiency of 68Ga, increase the 68Ge breakthrough, and shorten the service life of generator. Some generator eluents need to be pre-purified to meet the labeling requirements, which makes the labeling process complicated. Therefore, it is urgent to develop a 68Ge/68Ga generator absorbents which can achieve low concentration of hydrochloric acid elution, high elution rate of 68Ga, low leakage of 68Ge and long service life. This study reports the surface modification method of SiO₂ employed as an adsorbent for the 68Ge/68Ga generator. The synthesized SiO₂ adsorbent (SiO₂-BT) fabricated via this approach exhibits high acid resistance and enables rapid and efficient separation of Ge and Ga. When applied to the 68Ge/68Ga generator, the SiO₂-BT adsorbent can achieve an 68Ga elution efficiency of 75% with 10 μCi of 68Ge loaded, while limited the 68Ge breakthrough 5×10-6. In summary, the prepared SiO2-BT is a potential 68Ge/68Ga generator adsorbent, which provides a new way for the selection of 68Ge/68Ga generator adsorbent. 2. Experiment 2.1. Chemicals and materials Bayberry tannin (Guangxi Baise Forest Chemical Plant), C6H12 (Cyclohexane, AR, content ≥99.7%, Chengdu Jinshan Chemical Reagent Co., LTD.), CH3(CH2)5OH (N-hexyl alcohol, AR, Chengdu Jinshan Chemical Reagent Co., LTD.), APTES (≥98%, Sigma), SiO2 (AR, 99.5%, 8 μm, Aladdin), C5H8O2 (Glutaraldehyde, AR, Content ≥50%, Chengdu Jinshan Chemical Reagent Co., LTD.) 2.2. The Synthesis of SiO2-BT 20 ml cyclohexane and 5 ml N-hexanol were added to 200- ml deionized water, and a emulsified solution was formed after magnetic stirring for 5-10 min. Then, 15 ml of APTES and 10 g SiO2 were added into the emulsion and stirred for 24 h. The amino functionalized SiO2 was obtained after washing, filtering, and dried in 80 ℃ oven overnight. Dissolve 10 g of bayberry tannin (BT) in 200 ml of water. Subsequently, add 10 g of amino-functionalized SiO2 and stir for 12 hours. After that, introduce 40 ml of Glutaraldehyde solution and continue to stir at 30 ℃ for 24 hours. The resultant product is then 316 washed with deionized water, filtered, and dried in an oven at 80 ℃, thus BT modified SiO2 was obtained and marked as SiO2-BT. 2.3. Characterizations The crystal structure of SiO2-BT was detected by X-ray diffractometer (XRD, X'Pert pro, PANalytical) at a velocity of 2° min-1 in the range of 3°-80° and the surface functional groups were identified by Fourier infrared spectroscopy (FT- IR, Nicolet iS 50). The morphology of SiO2-BT was studied by scanning electron microscopy (SEM UItra55, Carl zeissNTS GmbH). And Inductively coupled plasma mass spectrometer (ICP-MS, PerkinElmer NexION 300X; iCAP6300) was used to measure the concentration of Ge and Ga ions in the supernatant. The breakthrough efficiency of 68Ge was measured with a scaler (FH463A), and the radioactivity of 68Ga was measured with an activity meter (CRC-15R,Capintec). 2.4. Non-radioactive Ge/Ga adsorption separation experiment In this study, SiO2 (SiO2-BT) modified with myriceberry tannin was used as the adsorbent. Firstly, 1 g SiO2-BT was weighed and added into 25 mL glass bottle, and 20 mL 0.1/0.2/0.3/0.4/0.5 M HCl solution containing 1 ppm Ge and Ga was added. Placed the glass bottless on the magnetic stirrer for stirring. After absorbing, 1 mL of supernatant was filtered by 0.22 μm membrane every 5 min, absorbing 0.5 mL of supernatant and diluting it to 5 mL for ICP-MS measurement. The adsorption efficiency was calculated by measuring the concentration of Ge and Ga, and the adsorption efficiency of Ge and Ga was calculated as: Ge/Ga adsorption efficiency (%) = 1 ×100% Among them, Ct is the concentration of Ge or Ga in solution after absorption by SiO2-BT, and C0 is the initial concentration of Ge and Ga in solution, that is, 1ppm. 2.5. Radioactive elution experiment 68Ge loading: Dilute 68Ge with an activity of 3.7×105 Bq (10 μCi) into 5 mL solution (mother liquor) with a concentration of 0.1 M HCl, connect the peristaltic pump system, and set the pump speed to 0.1mL /min, until the solution is fully collected into a 10 ml centrifuge tube. After loaded, 50 mL 0.1 M HCl solution was used to quickly wash the adsorption column at 1 mL/min pump rate. Collecting the washing liquid and after loading liquid for the 68Ge upload efficiency measurement after 24 h. The calculation formula of 68Ge column ratio is as follows: 𝐺𝑒 𝑢𝑝𝑙𝑜𝑎𝑑 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 % 1 𝐴𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑎𝑓𝑡𝑒𝑟 𝑙𝑜𝑎𝑑𝑖𝑛𝑔 𝑎𝑛𝑑 𝑤𝑎𝑠ℎ𝑖𝑛𝑔 𝑓𝑜𝑟 24ℎ 𝐴𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑚𝑜𝑡ℎ𝑒𝑟 𝑙𝑖𝑞𝑢𝑖𝑑 The elution process of 68Ga is as follows: Put 5 ml of HCl with a certain concentration into the elution container and adjust the pump speed to 1 ml/min until 5 ml solution is completely collected. To calculate the elution efficiency of 68Ga, the collected eluent is immediately measured with an activity meter. The formula for calculating 68Ga elution efficiency is as follows: 𝐺𝑎 𝑒𝑙𝑢𝑡𝑖𝑜𝑛 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 % 𝐺𝑎 𝑎𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑒𝑙𝑢𝑎𝑡𝑒 𝐺𝑎 𝑎𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑐𝑜𝑙𝑢𝑚𝑛 The activity of 68Ge is measured with a scaler, and the 68Ge breakthrough is calculated as follows: 𝐺𝑒 breakthrough % 𝐺𝑎 𝑎𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑒𝑙𝑢𝑎𝑡𝑒 𝑎𝑓𝑡𝑒𝑟 24 ℎ 𝐺𝑎 𝑎𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑐𝑜𝑙𝑢𝑚𝑛 3. Results and Discussion 3.1. Synthesis and characterization Figure 1. XRD pattern of SiO2-BT material Firstly, the amino functionalized SiO2 was obtained by condensation of the silica hydroxyl group produced by the hydrolysis of APTES with the hydroxyl group on the surface of SiO2, and then the BT was crosslinked with the amino functionalized SiO2 by glutaraldehyde to obtain SiO2-BT. Subsequently, the phase of SiO2-BT was characterized by XRD. As shown in Figure 1, the Broad peak of SiO2 at 2θ degree of 24.4° indicates that the oxides of SiO2-BT prepared are amorphous[14]. Figure 2. FT-IR spectrum of SiO2-BT material In order to verify the successful grafting of myricetannin (BT) on SiO2, FT-IR was further used to characterize SiO2- BT adsorbent. As shown in Figure 2, the peak around 3400 cm-1 was attributed to the -OH group of Bayberry tannin and the stretching vibration peak of -OH group on carrier SiO2. Peaks in the range of 1620 to 1450 cm-1 (1620 cm-1, 1543 cm- 1 and 1450 cm-1) indicate the presence of tannin aromatic rings. The peak at 1356 cm-1 belongs to the in-plane flexural vibration of the tannin hydroxyl group[15]. The infrared spectrum analysis of SiO2-BT material showed that Bayberry tannin had been successfully grafted onto amino functionalized silica material, which confirmed the successful preparation of SiO2-BT material. 317 Figure 3. SEM image of SiO2-BT The microstructure of SiO2-BT was analyzed by SEM. As shown in Figure 3, SiO2-BT material is granular, with loose particles and rough surface. The loose and rough surface characteristics of the SiO2-BT particles are conducive to enhancing its Ge adsorption performance. 3.2. Adsorption experiment In order to study the Ge and Ga adsorption properties of SiO2-BT, 1g SiO2-BT was placed in 20 mL of 1 ppm Ge and Ga solution containing HCl with a concentration of 0.1/0.2/0.3/0.4./05 M and stirred on a magnetic stirrer. Every 5 minutes, the supernatant was taken for concentration ICP- MS test to calculate the adsorption efficiency of Ge and Ga. In this case, the time-efficiency curves of adsorption of Ge and Ga by SiO2-BT are obtained. Figure 4. Time-adsorption efficiency curves of SiO2-BT adsorption of (a) Ge and (b) Ga Figure 4a shows the time-adsorption efficiency curve of SiO2-BT at different HCl concentrations. The Ge adsorption efficiency of SiO2-BT improves with the increase of adsorption time, and the adsorption equilibrium is basically reached at 35 minutes. Among them, SiO2-BT can achieve the highest Ge adsorption efficiency of 99.99% at 0.1 M HCl concentration, and The stable Ge adsorption efficiency decreases to 99.98%, 99.92%, 99.87% and 99.85% with the increase of hydrochloric acid concentration. The results showed that SiO2 grafted with bayberry tannin could achieve high Ge adsorption efficiency in a wide acidity range. Figure 4b shows the Ga time-adsorption efficiency of SiO2-BT. As shown in the figure, the saturation Ga adsorption efficiency of SiO2-BT decreases continuously with the increase of HCl concentration. With the increase of HCl concentration, the values decreased to 10.88%, 7.3%, 5.65% and 3.25%. The above results show that the synthesized SiO2-BT can achieve the selective adsorption of Ge in the mixed solution of Ge and Ga under non-radioactive experimental conditions. Based on the experimental results, the Ge and Ga distribution coefficients (Kd) of SiO2-BT material were calculated by the saturation Ge and Ga adsorption efficiency at various HCl concentrations, as shown in Table 1. Table 1. Distribution coefficients of Ge and Ga of the synthesized SiO2-BT adsorbent absorbent HCl concentration (M) Kd Ge Ga SiO2-BT 0.1 9999 0.136 0.2 4999 0.122 0.3 1249 0.079 0.4 768 0.060 0.5 665 0.034 It can be seen from Table 1 that the distribution coefficient of Ge on SiO2-BT is extremely high, reaching 9999 and 4999 respectively at 0.1 and 0.2 M HCl solution. And the distribution coefficient of Ge decreases with the increase of acidity of the solution. On the contrary, in the range of HCl concentration, the distribution coefficient of Ga on SiO2-BT decreases continuously, indicating that high acidity is conducive to the elution of Ga. However, due to the increase of 68Ge breakthrough in the 68Ge/68Ga generator eluate decreases the PET image quality, the optimal upper column leaching condition is 0.1 M HCl. For evaluating the 68Ga elution efficiency and 68Ge breakthrough of SiO2-BT on the radioactive 68Ge column, radioactive elution experiments were carried out. The HCl concentration of 68Ge with an activity of 3.7×105 Bq (10 μCi) was adjusted to 0.1 M, and then the mother liquid was pumped into the 68Ge/68Ga generator by peristaltic pump for 68Ge loading. After loading, the adsorption column was quickly washed and removed with 50 mL of 0.1 M HCl solution at 1 mL/min pump rate. After 24 hours, the activity of mother liquor and the washing solution were tested for calculating the 68Ge upload efficiency. 318 After the column loading completed, the SiO2-BT generator was placed in the hot chamber for 24 hours until radioactive equilibrium was reached. Then, 5 mL 0.1 M HCl was used to elute the generator at the speed of 1 mL/min. The solution was collected immediately after the elution for activity test to calculate the 68Ga elution efficiency, and then the solution was placed for 24 hours for 68Ge breakthrough test. The test results show that the 68Ge upload efficency is as high as 99.99% with the absorbent of SiO2-BT. In addition, the elution efficency of 68Ga in the eluate reaches 75% while the 68Ge breakthrough is only 5×10-6. 4. Conclusion In summary, this experiment successfully synthesized the surface modified SiO2 grafted with Bayberry tannin. The static adsorption experiment confirmed that the synthesized SiO2-BT could achieve highly Ge selective adsorption efficiency in the mixed solution of Ge and Ga in 0.1-0.5 M HCl (the adsorption efficiency of Ge was greater than 99.8%) while only less than 12% of the Ga in the solution is adsorbed. Subsequent radioactive elution experiments show that SiO2- BT can achieve 99.99% 68Ge upload efficiency, and ensure 75% of 68Ga elution efficiency while limited 68Ge breakthrough below 5×10-6. The above results show that SiO2-BT has high research value and provides a new choice for commercial 68Ge/68Ga generator column absorbent. References [1] Chiappiniello, A., Iacco, M., Rongoni, A., et al. 2020. Assessment of radionuclide impurities in [18F] fluoromethylcholine ([18F]FMCH). Physica Medica. 78, 150- 155. doi:10.1016/j.ejmp.2020.09.025. [2] Friberger, I., Nilsson, J. N., Lu, L., et al. 2023. Comparative in vivo biodistribution of cells labelled with [89Zr]Zr-(oxinate)4 or [89Zr]Zr-DFO-NCS using PET. EJNMMI Research. 13. doi:10.1186/s13550-023-01021-1. [3] Cho, S. Y., Rowe, S. P., Jain, S. K., et al. 2020. Evaluation of Musculoskeletal and Pulmonary Bacterial Infections With [124I]FIAU PET/CT. Molecular Imaging. 19. doi:10.1177/1536012120936876. [4] Anders, D. A., Bongarzone, S., Fortt, R., et al. 2017. Electrochemical [11C]CO2 to [11C]CO conversion for PET imaging. Chemical Communications. 53, 2982-2985. doi:10.1039/c7cc00319f. [5] Amor-Coarasa, A., Schoendorf, M., Meckel, M., et al. 2016. Comprehensive Quality Control of the ITG 68Ge/68Ga Generator and Synthesis of 68Ga-DOTATOC and 68Ga-PSMA- HBED-CC for Clinical Imaging. Journal of Nuclear Medicine. 57, 1402-1405. doi:10.2967/jnumed.115.171249. [6] Fuchigami, T., Ono, H., Oyadomari, K., et al. 2017. Development of a ⁶⁸Ge/⁶⁸Ga Generator System Using Polysaccharide Polymers and Its Application in PET Imaging of Tropical Infectious Diseases. ACS Omega. 2, 1400-1407. doi:10.1021/acsomega.7b00147. [7] Lee, J. Y., Vyas, C. K., Kim, B.-R., et al. 2016. Acid resistant zirconium phosphate for the long term application of 68Ge/68Ga generator system. Applied Radiation and Isotopes. 118, 343- 349. doi:10.1016/j.apradiso.2016.09.025. [8] Sammartano, A., Migliari, S., Scarlattei, M., et al. 2022. Performance and long-term consistency of five Galliform 68Ge/68Ga generators used for clinical Ga-68 preparations over a 4 year period. Nuclear Medicine Communications. 43, 568- 576. doi:10.1097/mnm.0000000000001545. [9] Wang, J., Gao, R., Cao, S., et al. 2023. Production of medical isotope 68Ge based on a novel chromatography separation technique and assembling of 68Ge/68Ga generator. Applied Radiation and Isotopes. 192. doi:10.1016/j.apradiso.2022.110599. [10] Burke, B. P., Clemente, G. S., Archibald, S. J. 2014. Recent advances in chelator design and labelling methodology for 68Ga radiopharmaceuticals. Journal of Labelled Compounds and Radiopharmaceuticals. 57, 239-243. doi:10.1002/jlcr.3146. [11] Velikyan, I. 2018. Prospective of 68Ga Radionuclide Contribution to the Development of Imaging Agents for Infection and Inflammation. Contrast Media & Molecular Imaging. 2018, 1-24. doi:10.1155/2018/9713691. [12] Amor-Coarasa, A., Gruka, M., Hurez, S., et al. 2019. Comparison of the IGG-100 vs the ITG 68Ge/68Ga generators: impact of impurities on radiolabeling. Nuclear Medicine and Biology. 72-73, S47-S48. doi:10.1016/s0969-8051(19)30319- 1. [13] Chakravarty, R., Chakraborty, S., Ram, R., et al. 2016. Detailed evaluation of different 68Ge/68Ga generators: an attempt toward achieving efficient 68Ga radiopharmacy. Journal of Labelled Compounds and Radiopharmaceuticals. 59, 87-94. doi:10.1002/jlcr.3371. [14] Xin Huang., Li Lia., Xuepin Liao., et al. 2010. Preparation of platinum nanoparticles supported on bayberry tannin grafted silica bead and its catalytic properties in hydrogenation. 320, 40-46. doi:10.1016/j.molcata.2009.12.013. [15] Rui-lin Yang., Ya-chun Liu., Qian Liu., et al. 2016. Tannin- grafted aminated silicon adsorbents: adsorption performance of rare earth ions coexistence. 1-10. doi: 10.1080/19443994.2016.1169948