The present invention relates to compounds that are substrates for an enzyme, and upon reaction with the enzyme provide a detectable response, such as an optically detectable response. In particular, the compounds have utility in detecting the presence of a β-lactamase in a sample. In addition to the compounds, methods are disclosed for analyzing a sample for the presence of a β-lactamase, for example, as an indicator of expression of a nucleic acid sequence including a sequence coding for a β-lactamase. Kits are disclosed that include the disclosed compounds and additional components, for example, cells, antibodies, a β-lactamase or instructions for using the components in an assay.