SYNTHESIS OF SILICA-COATED Au WITH Ag, Co, Cu, AND Ir BIMETALLIC RADIOISOTOPE NANOPARTICLE RADIOTRACERS

Radioisotopes are useful tracers because their chemical properties are the same as those of stable isotopes and they show higher detection sensitivity than other tracers. Their usefulness has been demonstrated in fluorescence experiments in diverse areas including life science, medicine, and engineering. SiO2-coated radioisotope Au coreshell nanoparticles (Au@SiO2 NPs) were first synthesized by sol-gel reactions of aqueous Au NPs induced by γ irradiation without reduction agents [1]. Their SiO2 shells were physically stable in high-energy gamma environments. Radioisotope Au@SiO2 NPs with Au-198 exhibited a half life, T1/2, of 15 days and emitted energy, Γ, of 0.412 –1.088 MeV, making them unsuitable for use as radiotracers for long periods in high-temperature petrochemical and refinery processes. Bimetallic NPs have exhibited better electronic, optical, catalytic, and magnetic properties than the corresponding monometallic NPs and can improve sensors' stability and selectivity [2-5]. Ni-based NPs have potentially useful catalytic and magnetic properties. Completely miscible Cu-Ni NPs can selectively inhibit hydrogenolysis [6]; Pt-Ni intermetallic NPs have demonstrated enhanced electrocatalytic activity towards oxygen reduction compared with Pt alone [7]; and Sn-Ni alloys have shown unique catalytic selectivity in the dehydrogenation of cyclohexane [8]. PVP-protected bilayered Pd-Ni NPs have exhibited enhanced magnetic susceptibility at low temperatures due to quantum size effects [9]. However, radioisotope NPs containing two gamma emitters for use as radiotracers, have yet to be reported. Silica NPs have useful properties such as inertness and suitability for encapsulation and surface modification [10]. They are generally 20 to 300 nm in size and can be prepared at room temperature by the Stöber method, using sol-gel. Quantum dots (QDs) and magnetite (Fe3O4) NPs have been encapsulated in silica NPs for use as highbrightness and magnetically guided biomarkers for living cancers [11]. To prevent nanoparticles' aggregation at high temperatures, SiO2 NPs can be encapsulated onto radioisotope NPs by sol-gel reactions at room temperature [1]. This work reports the synthesis of radioactive silicacoated bimetallic NPs (Au-Ag@SiO2, Au-Co@SiO2, AuCu@SiO2, and Au-Ir@SiO2) by neutron irradiation after coating SiO2 onto the bimetallic NPs by the Stöber method. The resulting particles could act as durable tracers for high-temperature use. The bimetallic core nanoparticles were synthesized at room temperature by the radiationinduced reduction of aqueous metal ions without a reducing agent. The core-shell Au-Ag@SiO2, Au-Co@SiO2, AuSilica-coated Au with Ag, Co, Cu, and Ir bimetallic radioisotope nanoparticles were synthesized by neutron irradiation, after coating SiO2 onto the bimetallic particles by the sol-gel Stöber process. Bimetallic nanoparticles were synthesized by irradiating aqueous bimetallic ions at room temperature. Their shell and core diameters were recorded by TEM to be 100 – 112 nm and 20 – 50 nm, respectively. The bimetallic radioisotope nanoparticles' gamma spectra showed that they each contained two gamma-emitting nuclides. The nanoparticles could be used as radiotracers in petrochemical and refinery processes that involve temperatures that would decompose conventional organic radioactive labels.