Fluorescent　nanoparticles　were　prepared　by　encapsulating　carbon　dots　(CDs)　within　silica　spheres　and　then　modifying　these　spheres　with　amino　groups　(CD@SiO2-NH2).　On　the　basis　of　the　silver　mirror　reaction,　Ag+　assembled　on　the　surface　of　CD@SiO2-NH2　is　reduced　to　silver　nanoparticles　(AgNPs)　by　formaldehyde.　The　in-situ　grown　AgNPs　cause　a　visually　distinguishable　fluorescence　enhancement.　This　metal-enhanced　effect　was　investigated　by　transmission　electron　microscopy　and　spectroscopic　characterization,　and　the　relevant　conditions　were　optimized.　CD@SiO2-NH2-Ag+　fluorescent　probes　were　loaded　onto　nano-sponge　pieces　for　the　analysis　of　formaldehyde　gas.　The　blue　fluorescence　emission　(peaking at　466 nm)　in　response　to　formaldehyde　is　greatly　enhanced　(up　to　5.2　times)　over　other　species.　There　is　a　linear　relationship　between　the　fluorescence　enhancement　and　formaldehyde　gas　concentration　in　the　range　of　10 ppb　to　1 ppm,　and　the　detection　limit　is　3　ppb.　The　fluorimetric　assay　needs　30 min　for　the　reaction,　and　the　fluorescent　nano-sponge　pieces　are　disposable.　[Figure　not　available:　see　fulltext.]　©　2020,　Springer-Verlag　GmbH　Austria,　part　of　Springer　Nature.