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Selective enrichment of submicron pathogens (e.g., bacteria, viruses) from complex matrices containing larger interferents remains a fundamental challenge in rapid biosensing. Conventional thermophoresis-based methods suffer from inherent size-dependent limitations and require fluorescent labeling for specificity, compromising clinical utility. Here, we propose a new strategy by exploiting density-manipulated sedimentation to break this limitation, enabling the selective enrichment of 200 nm targets from 1.3 μm interferents within 15 min. Through aptamer-directed in-situ synthesis of plasmonic nanoparticles (AgNPs) on target pathogens, we increase the apparent density of the targets, while simultaneously covering them with surface-enhanced Raman substrates. When coupled with laser-induced convection, the approach enables label-free detection of target bacteria at clinically relevant concentrations in blood serum with coexisting bacteria, as well as in clinical samples. The synergy of density-enhanced sedimentation and convection competition establishes a new microfluidic principle for particle manipulation. This work not only overcomes a critical bottleneck in point-of-care diagnostics but also provides a versatile platform for the rapid biosensing of diverse pathogens.