The ABTS free radical scavenging rate test is a widely adopted spectrophotometric method for evaluating the total antioxidant capacity of plant extracts, foods, cosmetics, and biological samples. Based on the ability of antioxidants to decolorize the pre-formed ABTS radical cation, this assay delivers rapid, reproducible, and quantitative results that support product development, quality control, and scientific research.
Technical Overview
The ABTS assay measures the capacity of a sample to scavenge the ABTS radical cation (ABTS•+), a stable, blue-green chromophore generated by the oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) with potassium persulfate. When an antioxidant compound donates a hydrogen atom or an electron to the radical cation, the extended conjugation of the chromophore is disrupted, resulting in a measurable decrease in absorbance at 734 nm. The degree of decolorization is directly proportional to the antioxidant activity of the sample.
The method was popularized by the protocol of Re et al. (1999) and has since become a benchmark technique alongside the DPPH assay. One of the key advantages of the ABTS test is that it is applicable to both hydrophilic and lipophilic antioxidant systems, allowing the analysis of aqueous extracts, oils, and emulsions within the same analytical framework. Because detection is performed at 734 nm in the near-visible region, interference from the natural color of many plant and food matrices is minimized.
Results are typically reported as a scavenging percentage at a defined concentration, an IC50 value representing the concentration required to inhibit 50% of the radical, or as Trolox equivalent antioxidant capacity (TEAC) derived from a calibration curve of the water-soluble vitamin E analog Trolox.
Testing Samples
Plant Extracts and Herbal Products
Crude botanical extracts, fractionated phytochemical preparations, essential oils, and traditional herbal formulations are routinely screened using the ABTS assay to identify promising antioxidant leads and to standardize herbal raw materials.
Food and Beverage Products
Fruit and vegetable juices, teas, wines, honeys, cereals, edible oils, fermented products, and functional beverages are analyzed to characterize antioxidant profiles, compare processing methods, and support nutritional labeling claims.
Cosmetic and Personal Care Ingredients
Plant-derived actives, peptides, vitamins, polyphenols, and finished skincare formulations are tested to substantiate antioxidant efficacy claims and to evaluate the protective stability of formulations against oxidative degradation.
Pharmaceutical and Biological Samples
Pure compounds, synthetic molecules, serum, plasma, and tissue homogenates can be evaluated to assess radical-scavenging potency in drug discovery pipelines and in oxidative stress-related biomedical studies.
Testing Items
- ABTS free radical scavenging rate (%) at one or more specified sample concentrations
- IC50 value: the sample concentration required to achieve 50% radical inhibition
- TEAC value: antioxidant capacity expressed as mmol Trolox equivalents per liter or per gram of sample
- Trolox standard calibration curve with linearity verification (R² typically greater than 0.99)
- Comparative antioxidant ranking of multiple samples or formulation variants
Testing Methods
The assay is performed using a validated microplate or cuvette-based spectrophotometric procedure. The general workflow is as follows:
- Reagent preparation: ABTS is dissolved at 7 mM and oxidized with 2.45 mM potassium persulfate. The mixture is incubated for 12 to 16 hours at room temperature in the dark to allow complete radical cation formation.
- Working solution adjustment: The stock ABTS•+ solution is diluted with ethanol or phosphate-buffered saline to an absorbance of 0.70 ± 0.02 at 734 nm, ensuring a consistent radical concentration across batches.
- Standard curve generation: Serial dilutions of Trolox are reacted with the ABTS working solution to construct a calibration curve covering the analytical range.
- Sample reaction: A defined volume of test sample or blank solvent is mixed with the ABTS working solution and incubated for 6 minutes at room temperature under light-protected conditions.
- Absorbance measurement: The absorbance at 734 nm is recorded for control, blank, and sample groups using a UV-Vis spectrophotometer or microplate reader.
- Data calculation: The scavenging rate is calculated using the formula: Scavenging rate (%) = [1 − (Asample − Ablank) / Acontrol] × 100, where Acontrol is the absorbance of the radical solution with solvent, Ablank corrects for sample color, and Asample is the absorbance after reaction with the antioxidant sample.
- IC50 determination: Concentration-response curves are fitted by regression analysis, and the IC50 is interpolated from the resulting inhibition curve.
All assays are performed in replicate with appropriate quality controls. Deionized water, analytical-grade reagents, and freshly prepared working solutions are used throughout to guarantee the accuracy and reproducibility of the results.
Applications
- Screening natural product libraries and medicinal plants for antioxidant lead compounds
- Comparing antioxidant performance across cultivars, growing conditions, processing techniques, and storage durations
- Supporting functional food development and antioxidant content claims for labeling purposes
- Validating the antioxidant efficacy of cosmetic actives in anti-aging and skin protection research
- Evaluating the synergistic or antagonistic effects of antioxidant combinations in formulation studies
- Monitoring oxidative stress biomarkers and antioxidant status in clinical and preclinical research
FAQ
What is the difference between the ABTS and DPPH assays?
The DPPH assay uses an organic nitrogen radical soluble only in organic solvents, which limits its applicability to hydrophilic samples. In contrast, the ABTS radical cation is soluble in both aqueous and organic media, making the ABTS test suitable for a broader range of sample types. The two methods are frequently run in parallel to provide complementary antioxidant capacity data.
How should samples be prepared before testing?
Solid samples such as plant powders or finished products are typically extracted with ethanol, methanol, water, or aqueous-organic mixtures, followed by centrifugation and filtration. The extract concentration must be documented so that results can be normalized. Highly colored or turbid samples require a matched blank correction to eliminate matrix interference at 734 nm.
How are the results reported?
The primary deliverables are the percentage scavenging rate at specified concentrations, the IC50 value, and the TEAC value when a Trolox calibration curve is included. A lower IC50 indicates stronger antioxidant activity, while a higher TEAC value corresponds to greater antioxidant capacity.
What are the limitations of the ABTS assay?
The ABTS radical cation is a synthetic radical that does not fully replicate the reactive oxygen species found in living systems, so results should be interpreted as an in vitro indicator of antioxidant capacity rather than a direct measure of in vivo efficacy. Combining the ABTS assay with ORAC, FRAP, or cellular antioxidant assays provides a more comprehensive antioxidant evaluation.
