Small organic sensor molecules were prepared that bind and signal the presence of unlabeled tripeptides in a sequence-selective manner. Sequence-selective peptide binding is a difficult problem because small peptides are highly flexible acid there are no clear rules for designing peptide-binding molecules as there are for the nucleic acids. The signaling system involved the application of fluorescence energy transfer and provided large, real-time fluorescence increases (300 to 500 percent) upon peptide binding. With it, these sensors were sensitive enough to detect unlabeled cognate peptides both in organic solution and in the solid state at low micromolar concentrations.