In-situ visualization of the enzymatic growth of surface-immobilized DNA block copolymer micelles by scanning force microscopy

The enzymatic growth of diblock copolymer micelles consisting of single-stranded DNA and poly(propylene oxide) (PPO) segments was studied with scan ning force microscopy (SFM). DNA- b -PPO aggregates adsorbed randomly on a mica surface exhibiting a height of 4-5 nm. The size of the micelles was increased by terminal deoxynucleotidyl transferase (TdT), which catalyzes the repetitive addition of deoxyribonucleotides to the 3 ' -hydroxyl end of DNA. The onset of the micelle growth was cont rolled by the addition of 2 ' -deoxythymidine 5 ' -triphosphate (dTTP) mononucleotide to the reaction solution or by increasing the temperature to 37 ° C. We determined the mean heights of the micelles from statistica l analysis of SFM pictures recorded at different reaction times in-situ. The arrangements, heights, and lateral sizes of individual micelles were also followed. After 60 min a plateau in micelle height was observed (6-8 nm). In contrast, micelles that were grown in solution, i.e., in the absence of a mica surface, showed no saturation effect for the same reaction time. The interaction of DNA with the surface and the hindered exchange of the block copolymers are proposed as possible reasons for the saturation in the growth of micelles adsorbed on mica. From a simple geometrical model, we estimated the number of extended mononucleotides.