Polymer-Mediated Gold Deposition on Magnetic Nanoparticles: Structure–Property Relationships for Ultra-Sensitive Electrochemical Interfaces Detection of Leptospira DNA

Gold-decorated magnetic particles (GMPs) were successfully synthesized through the coprecipitation of oleic acid-stabilized magnetic particles followed by citrate reduction of HAuCl4. Here, we reported the impact of the polymer on the formation of gold-decorated magnetic particles. Transmission electron microscopy (TEM) revealed uniformly distributed gold nanoparticles on the surface of the magnetic cores in GMP, whereas seed-mediated synthesis produced GMP-seed with larger particles due to further reduction and growth of gold on pre-existing gold nanoparticles, and more homogeneously distributed gold nanoparticles. The estimated gold mass fraction increased from 47.35% to 58.67% for GMP-seed1 and GMP-seed2, respectively, confirming increased gold deposition during seed-mediated synthesis. Five polymers aminodextran, poly(acrylic acid), poly(vinyl alcohol), poly(ethylenimine), and poly(vinylpyrrolidone)─were first adsorbed onto the magnetic particles prior to gold deposition to investigate polymer-mediated gold nanoparticle formation. Zeta potential and TEM analyses verified successful polymer coating and gold deposition, with variations in surface charge confirming distinct polymer functionalities. The incorporation of gold-decorated magnetic nanostructures into the electrochemical platform enhanced the measured electrochemical response compared with magnetic particles alone, as supported by the quantitative CV peak-current comparison. The GMPs showed successful functionalization with an ssDNA probe complementary to a conserved Leptospira interrogans gene sequence, enabling selective hybridization-based detection by electrochemical assay. The platform demonstrated detection of Leptospira ssDNA down to the lowest tested concentration of 0.1 ng/mL within 20 min, without cross-reactivity with nontarget DNA, demonstrating potential for rapid, field-deployable leptospirosis diagnostics. This nanobiosensor trades extreme sensitivity for rapid, point-of-care operation without thermal cycling─making it far more viable for field deployment in resource-limited, endemic tropical regions.