Engines of Creation: The Coming Era of Nanotechnology. Probably in the early 90's. I remember being really impressed with the book, which outlines the possibilities (and risks) of precision manufacturing at the scale of individual molecules. It never seemed that far-fetched to me, since every living thing on this planet is a proof of concept of molecular machines building other molecular machines such as marigolds, whales, and human beings. With massive parallelism, you can start with a single cell and grow a small human in about nine months. I didn't think it would be easy, but whether it would take 20 years or 100 years, it seemed pretty clear that if DNA and RNA and other molecular level machines in cells could build precise and complex machines (e.g., us), humans could eventually figure out how to do this too. Physics doesn't rule it out, and there's plenty of room at the bottom, as Richard Feynman pointed out in 1959.
Of course we have lots of "nanotechnology" these days - but sunscreen and microfiber cleaning cloths are not what Drexler had in mind in Engines. He was talking about molecular manufacturing, and he specifically talked about nanorobotic "assemblers" that could replicate themselves. He later backed away from that idea as necessary for efficient molecular manufacturing, but not before the idea of self-replicating nanobots running amok and turning everything into "gray goo" had taken hold in the popular imagination. Although progress was made, the misconceptions really limited nanotechnology development for a few years (for more on this, see Drexler's "Letter from the Author" at the start of the 2006 "2.0" edition of Engines, which you can read online for free, or purchase a PDF for 99 cents here).
this Technology Review article, based on a new paper in Nature. The title and subtitle give the general idea: "DNA Robots on the Move: Machines made of DNA could one day assemble complex--and tiny--electrical and mechanical devices." Whoa.