An electrochemical sensor comprising: a working electrode; a counter electrode; wherein the working electrode comprises an analyte-responsive enzyme and a mediator complex, the mediator complex having the formula: wherein c is a negative, neutral, or positive charge represented by -1 to -5, 0, or +1 to +5, inclusive, respectively; d is a number of counter ions, X, from 1 to 5, inclusive; M is osmium or ruthenium; L 1 is a substituted or an unsubstituted heterocyclic nitrogen-containing ligand; L 2 is a negatively charged ligand; and L and L' are independently: wherein R' 1 is a substituted or an unsubstituted alkyl, alkenyl or aryl; each of R a and R b is independently -H, -F, -Cl, -Br, -I, -NO 2, -CN, -CO 2 H, -SO 3 H,-NHNH 2, -SH, -OH, -NH 2, or substituted or unsubstituted alkoxycarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, alkoxy, alkylamino, dialkylamino, alkanoylamino, arylcarboxamido, hydrazino, alkylhydrazino, hydroxylamino, alkoxylamino, alkylthio, alkenyl, aryl, or alkyl; each of R e and R d is independently -H, -F, -Cl, -Br, -I, -NO 2, -CN, -CO 2 H,-SO 3 H, -NHNH 2, -SH, -OH, -NH 2, or substituted or unsubstituted alkoxycarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, alkoxy, alkylamino, dialkylamino, alkanoylamino, arylcarboxamido, hydrazino, alkylhydrazino, hydroxylamino, alkoxylamino, alkylthio, alkenyl, aryl, or alkyl, or a combination of R e and R d forms a saturated or an unsaturated 5- or 6-membered ring; the term "alkyl" includes both alkyl and cycloalkyl groups; the term "alkoxy" includes both alkoxy and cycloalkoxy groups; and each of R' 3 and R' 4 is independently -H, -F, -Cl, -Br, -I, -NO 2, -CN, -CO 2 H,-SO 3 H, -NHNH 2, -SH, -OH, -NH 2, or substituted or unsubstituted alkoxycarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, alkoxy, alkylamino, dialkylamino, alkanoylamino, arylcarboxamido, hydrazino, alkylhydrazino, hydroxylamino, alkoxylamino, alkylthio, alkenyl, aryl, or alkyl, or a combination of R' 3 and R' 4 forms a saturated or an unsaturated 5- or 6-membered ring. a working electrode; a counter electrode; wherein the working electrode comprises an analyte-responsive enzyme and a mediator complex, the mediator complex having the formula: wherein c is a negative, neutral, or positive charge represented by -1 to -5, 0, or +1 to +5, inclusive, respectively; d is a number of counter ions, X, from 1 to 5, inclusive; M is osmium or ruthenium; L 1 is a substituted or an unsubstituted heterocyclic nitrogen-containing ligand; L 2 is a negatively charged ligand; and L and L' are independently: wherein R' 1 is a substituted or an unsubstituted alkyl, alkenyl or aryl; each of R a and R b is independently -H, -F, -Cl, -Br, -I, -NO 2, -CN, -CO 2 H, -SO 3 H,-NHNH 2, -SH, -OH, -NH 2, or substituted or unsubstituted alkoxycarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, alkoxy, alkylamino, dialkylamino, alkanoylamino, arylcarboxamido, hydrazino, alkylhydrazino, hydroxylamino, alkoxylamino, alkylthio, alkenyl, aryl, or alkyl; each of R e and R d is independently -H, -F, -Cl, -Br, -I, -NO 2, -CN, -CO 2 H,-SO 3 H, -NHNH 2, -SH, -OH, -NH 2, or substituted or unsubstituted alkoxycarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, alkoxy, alkylamino, dialkylamino, alkanoylamino, arylcarboxamido, hydrazino, alkylhydrazino, hydroxylamino, alkoxylamino, alkylthio, alkenyl, aryl, or alkyl, or a combination of R e and R d forms a saturated or an unsaturated 5- or 6-membered ring; the term "alkyl" includes both alkyl and cycloalkyl groups; the term "alkoxy" includes both alkoxy and cycloalkoxy groups; and each of R' 3 and R' 4 is independently -H, -F, -Cl, -Br, -I, -NO 2, -CN, -CO 2 H,-SO 3 H, -NHNH 2, -SH, -OH, -NH 2, or substituted or unsubstituted alkoxycarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, alkoxy, alkylamino, dialkylamino, alkanoylamino, arylcarboxamido, hydrazino, alkylhydrazino, hydroxylamino, alkoxylamino, alkylthio, alkenyl, aryl, or alkyl, or a combination of R' 3 and R' 4 forms a saturated or an unsaturated 5- or 6-membered ring.