An effective dual-DNAzyme-based unimolecular probe design employing intramolecular signal transduction is demonstrated. The probe is composed of three domains: a DNA-cleaving DNAzyme, a substrate, and an HRP-mimicking DNAzyme. When the probe meets its target, cleavage of the substrate by the DNA-cleaving DNAzyme activates the HRP-mimicking DNAzyme, producing a colorimetric signal. The Cu(2+)-dependent DNAzyme engineered to demonstrate this design revealed a sensitivity corresponding to 65 ppb, which is sufficient to detect Cu(2+) in drinking water. The new probe has excellent selectivity toward Cu(2+). This three-component design is simple and easy to engineer. It may provide the basis for future development of other nucleic acid-based probes for toxicological and environmental monitoring.