19733
Our study revealed that there was negative correlation between the particle size and the toxicity : small AgNPs ( 5 - 10nm ) had higher toxicity than large AgNPs ( 15 - 25nm ) to Tetrahymena pyriformis ( T . pyriformis ) under dark condition .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
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