In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Immunofluorescence was used to detect the expression of P2X3 and NK1 receptors in bladder .
### Output Text:
Immunofluorescence was used to detect the expression of P2X3 and NK1 receptors in bladder . 