And if you use Senpy in your research, please cite Senpy: A Pragmatic Linked Sentiment Analysis Framework (BibTex):

Sánchez-Rada, J. F., Iglesias, C. A., Corcuera, I., & Araque, Ó. (2016, October).
Senpy: A Pragmatic Linked Sentiment Analysis Framework.
In Data Science and Advanced Analytics (DSAA),
2016 IEEE International Conference on (pp. 735-742). IEEE.

Senpy uses Onyx for emotion representation, first introduced in:

Sánchez-Rada, J. F., & Iglesias, C. A. (2016).
Onyx: A linked data approach to emotion representation.
Information Processing & Management, 52(1), 99-114.

Senpy uses Marl for sentiment representation, which was presented in:

Westerski, A., Iglesias Fernandez, C. A., & Tapia Rico, F. (2011).
Linked opinions: Describing sentiments on the structured web of data.

The representation models, formats and challenges are partially covered in a chapter of the book Sentiment Analysis in Social Networks:

Iglesias, C. A., Sánchez-Rada, J. F., Vulcu, G., & Buitelaar, P. (2017).
Linked Data Models for Sentiment and Emotion Analysis in Social Networks.
In Sentiment Analysis in Social Networks (pp. 49-69).