A problem formulation which I modified slightly
can be found in the following article:
Anthony G. Frutos,
Qinghua Liu,
Andrew J. Thiel,
Anne Marie W. Sanner,
Anne E. Condon,
Lloyd M. Smith, and
Robert M. Corn,
“ Demonstration of a Word Design Strategy for DNA Computing on Surfaces,”
Nucleic Acids Research, 25 4748-4757 (1997).
This reference also describes the construction of a solution of size 108.