Some TMNT stuff was always meant for grownups.
Donatello is the most intellectual of all his brothers.
In the comics, Donatello is originally presented along with Leonardo as one of the two calmer Turtles. While the comics' portrayal of the team has no official command structure, in the early stories he is depicted as second-in-command. In the first issue, he is the one that killed the Shredder by knocking him and his grenade off the roof.
The second issue elaborated more on each of the Turtles' personalities and opened with Donatello soldering a circuit. Later in the issue, Donatello states that he is "familiar with some computer systems" and helps April O'Neil deactivate the Mousers.
In the Donatello one shot, Donatello encounters an artist called Kirby (an homage to the comic artist Jack Kirby) whose mysterious crystal brings his drawings to life before disappearing. The two newfound friends journey to a dimension inhabited by Kirby's creations and help the heroes defeat the invading monsters.
During the Turtles' exile to Northampton, Donatello becomes obsessed with fixing up and repairing the many broken things within the farmhouse where they were living. Most notably, he spent days and nights fixing the boiler to give his family hot running water as well as building a windmill and a water wheel to provide electricity. He also finds an old typewriter and writes his own personal credo.
In the Shades of Grey storyline, Casey Jones encounters the Turtle by a ravine as he was pondering "the fractal structure of natural patterns". Casey accuses the Turtle of using big words and acting better than everyone else. Donatello suggests they should continue the conversation when Casey is sober. Grabbing a stick, an angry Jones continually pokes the Turtle until he loses his temper and sends Casey careening into the water.
City at War
In the City at War storyline, the Turtles return to New York to put an end to the Foot Clan's civil war. During a battle with Shredder's Elite Guards in the ruins of the Second Time Around Shop, Donatello falls through the floor and breaks his leg. Seeing their ally Karai subdued and about to be killed, Donatello grabs one of the Foot's machine guns and repeatedly shoots the Foot Elite. Donatello is visibly shaken by the violence and throws the gun away.
At the end of the story the Turtles, April and Casey move back to New York save for Donatello who chooses to stay in Northampton with Master Splinter to heal from his injury as well as reflect on everything that had happened.
While meditating with Splinter, Donatello receives a vision of the future where he is in the village of Chihaya in Japan. When Donatello asks what he would be doing there, his master simply replies "To bury me."
In the volume 4 comics, Donatello finds an armored truck in the sewers which apparently had been part of a bank robbery in the sixties. Along with Raphael and Casey Jones, he undertakes the task of fixing up the vehicle.
After the death of Splinter, Donatello secretly goes to his body and asks forgiveness before clipping something off of him. Shortly after, Donatello decides to accompany the Utroms on a mission to Tepui to search for two missing research teams. The group is attacked by strange wooden creatures who shrink them to miniature size. The creatures turn out to be a group of Utroms which had been stranded in the Jungle during their first stay on Earth and had been living in secret thanks to their Quantum Inversion Redimensioning device which could alter their size. The process could not be reversed on Donatello, possibly due to his mutation, leaving him the size of an action figure.
While the Utroms work on returning him to normal, Donatello put his new size to use infiltrating a terrorist organization's warehouse. He's also made a robotic body for himself made to resemble a turtle to help him move around.