Many　Pb2+　biosensors　based　on　Pb2+-specific　RNA-cleaving　DNAzyme　have　been　developed　in　the　past　years.　However,　many　of　them　have　limited　practical　use　because　of　high　cost　(e.g.,　enzymes),　complicated　processing　and　the　use　of　unstable　molecules　(e.g.,　RNA).　In　this　study,　a　novel　label-free　fluorescent　biosensor　for　Pb2+　was　proposed　based　on　Pb2+-induced　allosteric　G-quadruplex　(PS2.M).　In　the　presence　of　K+,　N-methyl　mesoporphyrin　IX　(NMM)　could　bind　to　K+-stabilized　G-quadruplexes,　giving　rise　to　high　fluorescence.　On　addition　of　Pb2+,　Pb2+　competitively　binded　to　K+-stabilized　G-quadruplexes　to　form　more　compact　DNA　folds.　The　Pb2+-stabilized　G-quadruplexes　did　not　bind　to　NMM,　which　resulted　in　fluorescence　decrease.　This　allowed　us　to　utilize　PS2.M　for　quantitative　analysis　of　Pb2+　using　the　NMM-G-quadruplex　system　by　convenient　＂mix-and-detect＂　protocol.　The　fluorescence　emission　ratio　(F-0/F)　showed　a　good　linear　response　toward　Pb2+　over　the　range　from　5.0　nM　to　1.0　mu　M　with　a　limit　of　detection　of　1.0　nM.　This　proposed　biosensor　was　simple　and　cost　efficiency　in　design　and　in　operation　with　high　sensitivity　and　selectivity.　We　validated　the　practicality　of　this　biosensor　for　the　determination　of　Pb2+　in　lake　water　samples.　(c)　2012　Elsevier　B.V.　All　rights　reserved.