Rationale of 3,3',5,5'-Tetramethylbenzidine as the Chromogenic Substrate in Colorimetric Analysis

Area of Science:

• Analytical Chemistry
• Biochemistry
• Enzyme Assays

Background:

• Horseradish peroxidase (HRP) assays are valued for their simple colorimetric detection.
• 3,3",5,5"-tetramethylbenzidine (TMB) is the most common chromogenic substrate for HRP and HRP-mimicking enzymes like nanozymes and DNAzymes.
• The chemical rationale for TMB's widespread use over other benzidine derivatives is not fully understood.

Purpose of the Study:

• To investigate the chemical basis for 3,3",5,5"-tetramethylbenzidine (TMB) being the preferred substrate in horseradish peroxidase (HRP)-based colorimetric assays.
• To compare the performance of TMB against other benzidine derivatives in HRP-involved chromogenic systems.

Main Methods:

• Investigated nine benzidine derivatives with varying substitution patterns in four TMB-based chromogenic systems.
• Synthesized and compared two novel tetrasubstituted benzidine derivatives (4OCH and 2OCH2CH) with TMB.
• Evaluated substrate performance based on sensitivity, color purity of products, and stability of oxidation products.

Main Results:

• 3,3",5,5"-tetramethylbenzidine (TMB) demonstrated higher sensitivity, better color purity, and more stable oxidation products compared to other tested benzidine derivatives.
• Synthesized tetrasubstituted benzidine derivatives did not outperform TMB.
• TMB's superior performance was chemically confirmed across the evaluated chromogenic systems.

Conclusions:

• 3,3",5,5"-tetramethylbenzidine (TMB) is chemically confirmed as the optimal substrate for HRP-based colorimetric assays due to its superior sensitivity, color purity, and product stability.
• The findings provide a chemical basis for TMB's established role in enzymatic assays.
• TMB remains the "chosen substrate" for colorimetric assays involving HRP and HRP-mimics.