I'm a bit confused on what the に particle is doing here in this sentence, まじめに して下さい. I know that the sentence means, "Please take this seriously" but I'm not sure why に is used here. I've seen it used most often as a time or target marker but I'm not sure what it is doing here nor can I think of another particle that would be better used here. If anyone could let me know its usage in this scenario, I'd really appreciate it.
Thanks