Professional development courses help teachers integrate new content knowledge into the high-school curriculum. Designing practical online courses for this purpose is challenging, particularly in emerging fields such as nanotechnology. In this study, we evaluated such a course in three complementary dimensions: (1) knowledge, (2) the complexity of learners' understanding, and (3) learners' difficulties during the course. We evaluated the first aspect using a pre--post-knowledge test, the second using the Structure of the Observed Learning Outcome (SOLO) taxonomy applied to the course's assignments, and the third by analyzing online activity reports and semi-structured interviews. Three cohorts comprising 95 teachers participated in this course from 2016 to 2019. Our multi-dimensional analysis revealed that teachers' knowledge of nanotechnology concepts improved. They were able to connect the advanced content they learned to the high-school chemistry curriculum at different levels of complexity. The evaluation process also enabled us to identify difficult sections of the course. These findings can contribute to designing the curriculum of future professional development online science courses in terms of the intended learning outcomes.