Recently,the　newly　emerging　lead　halide　perovskite　nanocrystals(PNCs)　have　been　intensely　researched　in　many　fields,　such　as　light-emittingdiodes　(LEDs),　solar　cells,　lasers,　and　display　devices.　The　extremelyhigh　fluorescence　quantum　yield　(near　100%)　of　PNCs　over　classic　fluorescentmaterials　would　enable　good　applications　of　PNCs　in　sensing.　However,the　study　on　PNCs　for　bio-　and　chemical　sensing,　especially　for　detectingtargets　that　exist　in　aqueous　medium,　faces　great　challenges　due　tothe　well-known　instability　of　PNCs　in　polar　solvents,　especially　water.Although　the　encapsulation　of　polymers　or　inorganic　materials　canefficiently　protect　PNCs　from　decomposition　in　aqueous　solution,　thesensing　based　on　the　interaction　between　PNCs　and　targets　is　severelyhindered　by　the　compact　protection　coating　at　the　surfaces　of　PNCs.In　this　work,　novel　water-dispersed　PNCs　(W-PNCs),　i.e.,　CsPbBr3@CsPb2Br5　PNCs,　with　strong　fluorescenceand　excellent　water　stability　were　synthesized　from　OAm-capped　CsPbBr3　PNCs　by　a　simple　＂oil-solid-water＂phase　transition.　The　W-PNCs　without　being　encapsulated　with　compactpolymers　or　inorganic　materials　can　sensitively　and　stably　sense　targetsin　the　pure　water　phase　via　direct　chemical　reactions.　For　the　firsttime,　ion　exchanges　between　PNCs　and　halide　ions　and　their　effectson　the　fluorescence　wavelength　of　PNCs　were　investigated　in　the　purewater　phase,　on　the　basis　of　which　a　new,　visualized,　selective,　andsensitive　smartphone-based　sensing　platform　for　halide　ions　has　beenestablished　by　the　integration　of　the　conveniently　prepared　W-PNCnanoprobe　and　the　portable　mobile　phone.　It　is　envisioned　that　theuncoated　but　extremely　water-stable　and　highly　fluorescent　W-PNCshave　promising　applications　in　chemical　sensing,　biosensing,　and　bioimagingof　targets　in　aqueous　medium.