Ultra-trace determination of hexavalent chromium in a wide pH range triggered by heterometallic Cu-Mn centers modified reduced phosphomolybdate hybrids

Ultra-trace determination of the carcinogen chromium(VI) in wide pH water environment via electrochemical sensing technology is of great significance, in which the key point is to develop high sensitive and selective electrocatalyst. Herein, two heterometallic centers modified reduced phosphomolybdate hybrids with formula of [CuI(BBTZ)]4[MnII(H2O)3]2[CuII(P4MoV6O31H7)2]⋅H3PO4⋅3H2O (1) and NaI2(H2BBTZ)[NaI(H2O)(BBTZ)]2[MnII(H2O)2]2[MnII(P4MoV6O31H6)2]⋅2H2O (2) (BBTZ = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene) were synthesized as electrocatalysts, in which the reduced phosphomolybdate [P4MoV6O31]12- (abbr. {P4Mo6}) clusters cooperated with different kinds of heterometallic centers (CuI, CuII and MnII) achieve an efficient electrochemical detection of ultra-trace Cr(VI) in wide pH ranges of 0–5 for the first time. In electrolyte of pH = 0, hybrids 1–2 displayed prominent sensitivities of 111.08 μA·μM−1 and 119.87 μA·μM−1, along with ultra-low LODs towards Cr(VI) of 1.59 and 2.91 nM, respectively, which fully satisfy the WHO standards for drinking water. In the pH range of 1–5, good sensitivities and low LODs (<25 nM) are also achieved by hybrids 1–2, realizing the ultra-trace Cr(VI) detection in wide pH ranges. Moreover, hybrids 1–2 exhibited high selectivity of 87.38 μA·μM−1 and 83.62 μA·μM−1 and low LODs of 2.03 nM for 1 and 3.5 nM for 2 in the actual water sample along with excellent anti-interference ability and electrochemical stability. The activity origin of hybrids 1–2 for impressive electrochemical behaviours were investigated that originated from the synergistic effect between reduced {P4Mo6} cluster and heterometallic centers at the molecular level. This work firstly explored the wide-pH-response electrochemical sensors for Cr(VI) detection and proposed an important guidance for designing efficient electrochemical sensors towards the ultra-trace detection of heavy metal ions in practical environment.