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The vibrational nonlinear activity of films of 2 , 4 - dinitrophenyl phospholipid ( DNP ) at the solid interface is measured by sum - frequency generation spectroscopy ( SFG ) .
 2 , 4 - dinitrophenyl
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 2 , 4 - dinitrophenyl