Novel　ratiometric　fluorescent　silica　nanoparticles　with　high　selectivity　towards　Hg2+　were　synthesized　for　the　detection　of　Hg2+,　Hg2+　promoted　the　ring　opening　of　spirolactam　in　the　rhodamine　moiety　grafted　onto　the　silica　nanoparticles,　resulting　in　a　change　in　the　fluorescence　intensity.　The　fluorescence　intensity　was　proportional　to　the　Hg2+　concentration,　and　the　detection　limit　(S/N　=　3)　for　Hg2+　was　found　to　be　2.59　x　10(-9)　mol　L-1　with　a　linear　range　from　0.4　to　8　x　10(-7)　mol　L-1.　In　addition,　the　morphology　of　the　silica　nanoparticles,　the　effects　of　pH　and　co-existing　substances,　and　the　reversibility　were　investigated.　The　proposed　approach　was　successfully　applied　to　the　determination　of　Hg2+　in　water　samples.　(C)　2010　Elsevier　B.V.　All　rights　reserved.