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Advances in polyoxometalate (POM) self-assembly chemistry are always accompanied by new developments in molecular blocks. The exploration and discovery of uncommon building blocks offer great possibilities for generating unprecedented POM clusters. An intriguing Sb III -W VI -cotemplated antimonotungstate [H 2 N(CH 3 ) 2 ] 11 Na[SbW 9 O 33 ]Er 2 (H 2 O) 2 Sb 2 [SbW VI W 15 O 57 ]·22H 2 O ( 1 ) was synthesized, which comprises a classical trivacant Keggin [SbW 9 O 33 ] 9- ({SbW 9 }) fragment and an unclassical lacunary Dawson-like [SbW VI W 15 O 57 ] 15- ({SbW VI W 15 }) subunit. Notably, the Dawson-like {SbW VI W 15 } subunit is the first example of a [SbO 3 ] 3- and [W VI O 6 ] 6- mixed-heteroatom-directing POM segment. Hexacoordinated [W VI O 6 ] 6- can not only serve as the heteroatom function but its additional oxygen sites can also link to lanthanide, main-group metal, and transition-metal centers to form the innovative structure. {SbW VI W 15 } and {SbW 9 } subunits are joined by the heterometallic [Er 2 (H 2 O) 2 Sb 2 O 17 ] 22- cluster to give rise to an asymmetric sandwich-type architecture. To further realize its potential application in electrochemical sensing, a conductive 1 @rGO composite was obtained by the electrochemical deposition of 1 with graphene oxide (GO). Using a 1 @rGO-modified glassy carbon electrode as the working electrode, an electrochemical biosensor for detecting the antidepressant drug paroxetine (PRX) was successfully constructed. This work can provide a viable strategy for synthesizing mixed-heteroatom-directing POMs and demonstrates the application of POM-based materials for the electrochemical detection of drug molecules.