The present invention relates to G-quadruplex forming isoDNA aptamers and a process for the preparation thereof. The present invention further relates to a stable, non-genetic, guanine rich 2′-5′ linked isoDNA selected from 3′ deoxy 2′-5′ isoDNA and 3′ deoxy 2′-5′ isoDNA-isoRNA hybrid. The instant 2′-5′ linked isoDNA such as the thrombin binding aptamer (isoTBA) can be used in deep vein thrombosis, where prolonged anticoagulant activity is required.