Chromogenic peroxidase substrates

A chromogenic conjugate comprising:

• (a) a chromogenic moiety, and
• (b) a peroxidase substrate moiety,
• wherein the chromogenic moiety and the peroxidase substrate moiety are linked together via a linker,
• wherein the conjugate is a compound of Formula I:
• where X is —OH, —ORX or —NRXRXX,
• where Y is ═O or ═N+RYRYY;
• wherein R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, RX, RXX, RY, and RYY are independently selected from hydrogen and a substituent having less than 40 atoms;
• L is a linker comprising a linear chain of 5 to 29 consecutively connected atoms;
• and
• PS is a peroxidase substrate moiety, wherein the peroxidase substrate moiety has the following formula:
• wherein
• R21 is —H,
• R22 is —H, —O—X, or —N(X)2;
• R23 is —OH;
• R24 is —H, —O—X, or —N(X)2;
• R25 is —H, —O—X, or —N(X)2;
• R26 is —CON—(X)2, —CONH(X), or —COO(X);
• wherein H is hydrogen;
• O is oxygen;
• N is nitrogen;
• and X is H, alkyl or aryl.