A problem formulation which I modified slightly can be found in [frutos1997demonstration]. This reference also describes the construction of a solution of size 108.
DNA Word Design: A New Constraint Model and New Results
Effective heuristic methods for DNA strand design
Ph.D. thesis, University of British Columbia 2006
Demonstration of a word design strategy for DNA computing on surfaces
Nucleic Acids Research 25(23), 4748–4757, 1997