Supramolecular Host–Guest Assembly Based on Phosphotungstate Nanostructures for Pseudocapacitive and Electrochemical Sensing Applications

Flexible ligands and multinuclear complexes were assembled
into
nanoscale {P2W18O62} polyoxometalates
(POMs) to generate two phosphotungstate hybrid derivatives, (H2bip)2{H2P2 W18O62}·2H2O (1) and (bipy)0.5{Ag­(bipy)2}­{Ag4(bipy)7}­{NaP2W18O62}·H2O (2) (bip = 1,5-bis­(imidazole-1-yl)­pentane; bipy = 2,2′-bipy),
via a precursor hydrothermal method. Compound 1 is a
bip ligand modified 4,8-connected supramolecular net with {415;612;8}­{45;6}2 topology. In compound 2, the monocore complexes {Ag­(bipy)2} and tetranuclear
complexes {Ag4(bipy)7} self-associate to generate
two supramolecular chains A and B, respectively, which are further
alternately joined together to form a unique metal–organic
framework with irregular cavities via weak actions and π–π
stacking. The Na-linked {P2W18O62} dipolymers as guest units were embedded into the holes, leading
to a 3D supramolecular host–guest assemblies. The capacitive
performance test reveals that 2-GCE/-CPE presents improved
specific capacitance (802.4 and 752.1 F/g at 3 A/g), cycle efficiency
(93.6 and 94.8% after the 5000th cycle), and electrical conductivity
compared to the precursor and bip ligand modified compound 1. The electrocatalytic comparative data shows that 2-GCE has enhanced electrochemical redox capacity compared to the
precursor and compound 1. Further sensing tests display
that 2-GCE shows strong electrochemical responses to
H2O2/AA at voltages of −0.41/0.36 V with
a lower detection line of 0.33/0.05 μM, a wider linear range
of 1.0 μM to 3.18 mM/0.15 μM to 2.33 mM, merit selectivity,
and reproducibility. The excellent capacitive and sensing performances
are related to increased redox centers, improved ion/electron conversion
efficiency, and host–guest structural stability resulting from
the introduction of multinuclear complexes, irregular holes, and abundant
π–π stacking. This study not only enriches the
variety of Dawson-type POMs but also provides a feasible idea for
enhancing their electrochemical capacitance and sensing performance.