7506
Determining the existence of any direct spectral relationship between the far - field scattering properties and the near - field Raman - enhancing properties of surface - enhanced Raman spectroscopy ( SERS ) substrates has been a challenging task with only a few significant results to date .
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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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