If it's only been a few hours, I wouldn't worry. Sometimes people get busy and they don't respond. It doesn't mean you should latch on to the one element of the last message you can spot.

If it's been a couple of days, then maybe you should be concerned, but keep the focus on business, not what they think of you. If they didn't think you were annoying before, overworrying about an emoji would more likely annoy them.