Nicotinate-Modified\nLanthanide-Substituted Selenotungstate\nand Its Catalase-like Activity for the Detection of H<sub>2</sub>O<sub>2</sub> and Ascorbic Acid

A rigid nicotinate-modified lanthanide-substituted\nselenotungstate\n[H<sub>2</sub>N­(CH<sub>3</sub>)<sub>2</sub>]<sub>6</sub>Na<sub>3</sub>H­[La<sub>4</sub>­SeW<sub>8</sub>­(H<sub>2</sub>O)<sub>16</sub>(nica)<sub>2</sub>O<sub>28</sub>]​​[SeW<sub>9</sub>O<sub>33</sub>]<sub>2</sub>·32H<sub>2</sub>O (<b>1</b>, Hnica = nicotinic acid) was synthesized and consists of two trivacant\nKeggin [B-α-SeW<sub>9</sub>O<sub>33</sub>]<sup>8–</sup> fragments and one unusual [SeW<sub>4</sub>O<sub>18</sub>]<sup>8–</sup> fragment bridged by a heterometallic [La<sub>4</sub>W<sub>4</sub>(H<sub>2</sub>O)<sub>16</sub>­(nica)<sub>2</sub>O<sub>28</sub>]<sup>18–</sup> cluster. In the heterometallic cluster, two\ncarboxyl O atoms in two nicotinate ligands directly coordinate with\ntwo W atoms in a stable C–O–W–O–W–O\nsix-membered ring fashion. According to its catalase-like activity, <b>1</b> was utilized to catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine\n(TMB) by H<sub>2</sub>O<sub>2</sub> to produce blue oxidized TMB (ox-TMB),\nwhich can be used to establish a colorimetric sensing method for the\ndetection of ascorbic acid. This work not only provides a promising\nplatform for detecting H<sub>2</sub>O<sub>2</sub> and ascorbic acid\nbut also expands the application potential of polyoxometalate-based\nmaterials in biological and clinical analyses.