Fe 3 O 4 -Au Core–Satellite Nanoparticles as Direct Colorimetric Reporters in a Smartphone-Quantified Lateral Flow Immunoassay for Prostate-Specific Antigen

<title>Abstract</title> <p>Lateral flow assays (LFAs) combine rapid operation, low cost, and direct visual detection, but their quantitative performance is often limited by the optical signal generated by conventional gold nanoparticle reporters. Sensitivity enhancement commonly relies on external excitation, dedicated detectors, or post-assay amplification, thereby increasing assay complexity. Here, Fe₃O₄/Au core–satellite magnetic particles (CSMPs) were investigated as high-payload direct colorimetric reporters in a smartphone-quantified sandwich LFA for prostate-specific antigen (PSA) determination in undiluted human serum. Commercially available PEGylated dextran-coated Fe₃O₄ magnetic clusters were used as scaffolds, decorated with multiple gold nanoparticles, and subsequently functionalized with anti-PSA antibodies through photochemical immobilization. Magnetic recoverability facilitated particle isolation during synthesis and bioconjugation, whereas assay transduction remained exclusively optical, without magnetic actuation, external excitation, or post-assay signal amplification. Systematic variation of reporter extinction revealed a non-monotonic relationship between particle loading and analytical performance. Low particle loading produced weak optical signals, whereas excessive loading caused early saturation and compression of the test-to-control response. An intermediate extinction provided the best balance between signal intensity, low-concentration detectability, and usable dynamic response. Under optimized conditions, the assay provided an estimated limit of detection of 0.25 ng·mL⁻¹ within 20 min. The assay showed limited interference from human kallikrein 2, intra- and inter-assay coefficients of variation below 10%, and preliminary agreement with a reference chemiluminescent immunoassay in seven human serum samples. These results demonstrate that Fe₃O₄/Au core–satellite particles can serve as direct high-contrast colorimetric reporters for quantitative LFA while preserving a simple smartphone-based readout.</p>