Albumin-Coated Gold Nanoparticles Functionalised with a Gadolinium-Binding Recombinant Protein for Dual CT/MR Imaging

Gold nanoparticles (AuNPs) are versatile nanomaterial scaffolds for imaging and multifunctional platform development; however, biologically addressable multicomponent coatings often require multistep chemistry that can limit accessibility and reproducibility. Here, we report a one-pot, albumin-mediated synthesis of human serum albumin (HSA)-coated gold nanoparticles (Au–HSA) and their modular functionalisation with F3del-ABD-GFP (FAG), an engineered protein that combines a high-affinity albumin-binding domain with a de novo gadolinium-binding domain. Two preparations, Au–HSA1 and Au–HSA2, with UV–Vis-derived apparent core diameters of approximately 5 and 16 nm, respectively, were obtained by reduction of sodium tetrachloroaurate(III) in the presence of HSA and characterised by UV–Vis spectroscopy, dynamic light scattering, zeta-potential analysis, ICP–AES, SDS–PAGE densitometry, and atomic force microscopy. Recovered-gold yields were 74.8 ± 11.2% (Au–HSA1) and 84.6 ± 9.2% (Au–HSA2). Albumin remained particle-bound in SDS under non-reducing conditions, consistent with thiol-mediated attachment, with an estimated average of approximately 14 HSA molecules per Au–HSA2 particle, calculated using the UV–Vis-derived particle concentration. FAG bound the particle-associated albumin near-stoichiometrically, and pre-loaded FAG:Gd associated with Au–HSA2 under the tested assembly conditions, giving an estimated average batch composition AuNP:HSA:FAG:Gd of 1:14:14:56. In phantom samples, the assembled Au–HSA/FAG:Gd formulation generated qualitative CT and T1-weighted MR signal under the applied acquisition conditions. These imaging findings are qualitative; CT attenuation was not calibrated in Hounsfield units, and MR relaxivity was not determined. No biological, in vivo, safety, or therapeutic studies were performed. Accordingly, the system is presented as a physicochemical proof-of-concept for a modular dual CT/MR imaging platform. Its biological performance, quantitative imaging characteristics, and therapeutic utility require evaluation in future work.