Mannose‐rhodamine (Rh) conjugate (80% yield) was synthesized in a one‐pot reaction and immobilized onto magnetic polymeric nanoparticles (MPNP; 43% magnetic content) of poly(styrene/divinyl benzene/acrylic acid). The resulting nanoparticles contained MPNP as a substrate, mannose as an E. coli receptor and Rh as a fluorescent signaling unit. TEM imaging clearly demonstrated that multiple mannose‐Rh MPNPs could be captured by E. coli strain ORN178. The fluorescent signal from captured nanoparticles was emitted at 580 nm. These results indicate that mannose‐Rh MPNP offers a simple and rapid strategy for bacterial detection. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 40012.