you have to extract the specific pat of medicinal herb first. there are lots of extraction methods and you can find easily in google or pubmed....

second then you have to determine the antioxidant potency of the extract and there are also lots of methods to do that. most widely used one is DPPH free radical scavenging method. it is very simple and easy method.... Just go through some papers

Focus on which extract you want check the antioxidant activity. As you say it is a medicinal herb, obviously the whole plant extract, either the methanol or the aqueous extract will have an antioxidant activity. Separate the whole extract into different fractions by extracting with solvents such as Hexane, Chloroform, Ethyl acetate etc., then try checking the invitro antioxidant capability of each of the separated fractions. If we still require, you can run a column and separate each of the fractions into sub-fractions for precise results. In this way, we can further extend your work to isolation of active antioxidant component from the fraction, ie., the assay guided isolation and purification.

Try initially with in vitro methods such as DPPH and ABTS, these free radical scavenging invitro methods would give better results than the traditional ones. Once the antioxidant activity is confirmed in only one of these two methods, we can further work on other in vitro methods, such as FRAP, FTC or in vivo method

Before you start check the phenolic content of the extract

Thanks to both of your guidance

Will it better, If I choose the topic for Ph.D. such as " Invitro study of antioxidant enzymes, DPPH -Radical scavenging activity and ferric reducing ability of some key medicinal plants". Please give me a proper direction.

You can also check the phenolic content present in research herb which (mostly) have direct correlation with anti-oxidant activity (i.e. higher the phenolic content more the anti-oxidant activity) along with the test suggested by other experts.

One more suggestion I would like to give regarding your PhD work:

Instead of only making in vitro anti-oxidant study, find the therapeutic effectiveness of your herb due to its anti-oxidant activity in various diseases (which are occurred due to oxidative stress like neurodegenerative diseases). So it will increase the importance of your research work as well as it will help to increase the bulk of your PhD thesis.

Best Luck for your PhD!!!

Tejas Ganatra's suggestıion is valid and since it is a Ph D work, it is also appropriate to characterise the antioxidant compounds isolated from the selected medicinal plants. Many of these compounds are phenolic acids. You can use HPLC or LCMS or GCMS to identify those active compounds. Then you will be able to relate the medicinal properties of those phenolic compounds identified.

there are few things that can be done once isolation and characterization is completed. in addition to the DPPH scavenging, you may look at ability of the extract to protect biological macromolecules. you may look at inhibition of protein oxidation, inhibition of the hydroxyl radical production initiated in the fenton herberwiess reaction. you may also look at reducing power of you purified extract or look at metal chelation in-vitro. Niyric oxide radical scavenging can also be done. Antioxidant study in vitro can be taken from various angles.

Thanks to all, it will really helpful for me. .

Thank you Sulaiman...

Thank you Olaniyi Sir, but I am confuse that which medicinal plant I have to select, because almost all the medicinal plant have been tested as an antioxidant activity. Please suggest me to find out any specific plant.

Based on reaction mechanisms involved, antioxidant capacity assays can be divided into two major groups: the one based on hydrogen atom transfer (HAT), and the other based on single electron transfer (SET). Several in vitro assays are preferentially based on a method, on the other or on both.

Folin-Ciocalteu assay, DPPH radical scavenging capacity, Trolox equivalent antioxidant capacity or Ferric reducing antioxidant capacity are typical SET-based assays, while ORAC or beta-carotene linoleic acid bleaching assay are typical examples of HAT-based experiments.

However, due to the complex nature of biological systems, there is no single universal method for measuring antioxidant capacity: for this reason, an examination of various antioxidant assays is required for the development of standard methods. In my opinion, it is desirable to perform at least two different assays for each antioxidant mechanism; after consolidating the in vitro assessment, you cold focus your experiments on other ex vivo antioxidant assays, using e.g. cell cultures (ROS or RNS scavenging detection, SOD activity, an so on), or other biological substrates.

To better study all the different in vitro methods, I suggest you to read this article: "Ndhlala AR, Moyo M, Van Staden J (2010). Natural Antioxidants: Fascinating or Mythical Biomolecules? Molecules 15, 6905-6930".

Best regards, Riccardo.