Immediate response for disease control relies on simple, inexpensive, and sensitive diagnostic tests, highly sought after for timely and accurate test of various diseases, including infectious diseases. Composite Fe3O4/Au nanoparticles have attracted considerable interest in diagnostic applications due to their unique physical and chemical properties. Here, we developed a simple coating procedure for gold magnetic nanoparticles (GMNs) with poly(acrylic acid) (PAA). PAA-coated GMNs (PGMNs) were stable and monodispersed and characterized by Fourier transform-infrared spectroscopy (FT-IR), transmission electron microscopy, UV-visible scanning spectrophotometry, thermogravimetric analysis, and Zetasizer methodologies. For diagnostic application, we established a novel lateral flow immunoassay (LFIA) strip test system where recombinant Treponema pallidum antigens (r-Tp) were conjugated with PGMNs to construct a particle probe for detection of anti-Tp antibodies. Intriguingly, the particle probes specifically identified Tp antibodies with a detection limitation as low as 1 national clinical unit/mL (NCU/mL). An ample pool of 1020 sera samples from three independent hospitals were obtained to assess our PGMNs-based LFIA strips, which exhibited substantially high values of sensitivity and specificity for all clinical tests (higher than 97%) and, therefore, proved to be a suitable approach for syphilis screening at a point-of-care test manner.

Composite Fe3O4/Au nanoparticles are attracting considerable interest in developing visual and specific detection of biomolecular due to their unique physical and chemical properties. Here, two localize surface plasmon resonance (LSPR) probes based on Fe3O4/Au nanoparticles (nanoSPR biosensors) were fabricated by exploring 11-mercaptoundecanoic acid (MUA) and poly acrylic acid (PAA) as surface modification agents and subsequently conjugating rabbit IgG with the modified particles' surface. Comparative investigations showed the differences between MUA-particles and PAA-particles, as well as sensitivity of the two as-prepared nanoSPR biosensors when used in target goat anti-rabbit IgG colorimetric detection. Particles coated with PAA were in a better dispersion and showed an ionic independent stability, indicating PAA-particles have a potential application in colorimetric detection. In contrast, the MUA-particle probes revealed a higher sensitivity in SPR detection (50 nmol/L), and further kinetic studies showed the reactions between probes and target followed the second order and the reaction rate of MUA-probes was twice the rate of PAA-probes at the same temperature and condition. Such proof-of concept works reported here demonstrated that the protocol to build nanoSPR biosensors was favored in developing molecular probes, and the novel composite nanoparticles might serve as ideal probes for sensitive, selective and real time detection.