Fe

Fe₃O₄-Au cluster-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 CSMPs can serve as direct high-contrast colorimetric reporters for quantitative LFA while preserving a simple smartphone-based readout.