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: In group - 1 ( n = 10 ) hippocampal DBS was off and in the group - 2 ( n = 10 ) hippocampal DBS was on ( 185 Hz , 0 . 5V , 1V , 2V , and 5V for 60 sec ) following penicillin G injection intracortically .
### Output Text:
In group - 1 ( n = 10 ) hippocampal DBS was off and in the group - 2 ( n = 10 ) hippocampal DBS was on ( 185 Hz , 0 . 5V , 1V , 2V , and 5V for 60 sec ) following <span class="chemical">penicillin G</span> injection intracortically . 