Principle and Overview

3,3′,5,5′-Tetramethylbenzidine (TMB) is a chromogenic substrate for horseradish peroxidase (HRP). In the presence of HRP and hydrogen peroxide, TMB undergoes oxidation to produce a blue-colored reaction product. Upon addition of a stop solution (typically dilute sulfuric acid), the color shifts from blue to yellow, with a peak absorbance at 450 nm. This colorimetric change allows quantitative measurement of the HRP signal, which is proportional to the amount of antigen captured in the assay.

The Supersensitive formulation (Aladdin T755518, referenced to Sigma T4444) is specifically optimized to provide enhanced signal intensity compared to standard TMB substrates, enabling reliable detection of low-abundance analytes that may fall below the detection limit of conventional formulations.

Materials Required

• 3,3′,5,5′-Tetramethylbenzidine Liquid Substrate, Supersensitive (Aladdin T755518)
• ELISA stop solution (e.g., 1 M H₂SO₄ or 2 N H₂SO₄)
• HRP-conjugated detection antibody
• 96-well microplate (ELISA-grade)
• Microplate reader capable of absorbance measurement at 450 nm and 650 nm
• Multichannel pipette and reagent reservoirs
• Timer
• Distilled or deionized water for washing steps

Note: Bring all reagents to room temperature (18–25 °C) before use. Avoid exposure of TMB substrate to light during storage and handling.

Step-by-Step Protocol

1. Preparation: Complete all prior ELISA steps (coating, blocking, sample and primary antibody incubation, washing). Ensure the plate has been thoroughly washed after the HRP-conjugated detection antibody step to minimize background signal.
2. Substrate addition: Add 100 µL of TMB Supersensitive Liquid Substrate (T755518) to each well. Use a multichannel pipette to ensure consistent timing across all wells.
3. Incubation: Incubate the plate at room temperature (18–25 °C) in the dark for 5–30 minutes. Monitor color development visually. Stop the reaction when sufficient color has developed in positive control wells and background in negative control wells remains low.
4. Stopping the reaction: Add 100 µL of stop solution (e.g., 1 M or 2 N H₂SO₄) to each well in the same order and at the same rate as the substrate was added. The solution should turn from blue to yellow upon acidification.
5. Absorbance reading: Read the plate within 30 minutes of stopping the reaction. Measure absorbance at 450 nm (primary wavelength). A reference wavelength of 570 nm or 630 nm may be used for background correction. If no stop solution is used (kinetic assay), measure at 650 nm.

Note: If any wells remain blue or show a green color after stop solution addition, this may indicate incomplete stop or very high signal. Verify stop solution concentration and ensure even mixing.

Troubleshooting and Tips

For optimal results, avoid freeze-thaw cycles of the substrate and do not use substrate that has turned blue prior to use, as this indicates oxidation and loss of sensitivity.

Storage and Safety

Storage: Store TMB Supersensitive Liquid Substrate (T755518) at 2–8 °C, protected from light. Do not freeze. Shelf life should be verified on the product label and Certificate of Analysis. Discard any substrate that appears blue prior to use.

Safety: TMB is considered to have low acute toxicity; however, standard laboratory safety practices should be followed. Wear appropriate personal protective equipment (gloves, lab coat, eye protection). The stop solution (H₂SO₄) is corrosive — handle with care and refer to the Safety Data Sheet (SDS) for full hazard information. Dispose of waste according to local institutional and regulatory guidelines.