Is it OK to give cash as gifts to people in a professional situation?

These people do not work directly for me but they help make my job easier and I would like to acknowledge that….

I do not know them well enough to give a particular gift….and to “make” something would not be appropriate.

Previously I have given gift cards to Kohls department store in amounts ranging from $25—$100, depending on their level of assistance.

My concern is that gift cards aren’t entirely secure, not everyone may want to shop at Kohl’s and multiple stores are selling gift cards and might not be around next year.

So, is cash OK? Is it tacky? What would you think if you received it..? Would you think I put no thought into the gift? HELP!