A water-soluble europium(III) complex (1) has been synthesized and demonstrated to be a specific fluorescence probe for the paralytic shellfish toxin saxitoxin, a neurotoxin that blocks the voltage-gated sodium channels on cell membranes. Saxitoxin binds to the europium complex (KB = 6.1 脳 104 M鈥?) and triggers a two-photon induced f鈥揻 emission enhancement by over 100% and increases the two-photon absorption cross-section from 9 to 36 GM.