The potential of a nucleic acid-based optical bioprobe for environmental measurements and drug monitoring is described. The sensor employs longwavelength intercalating fl uorophores like TO-PRO-3 (TP3). Compounds that interact with the TP3-DNA complex are indirectly detected by a decrease in the fl uorescence intensity. We found that the confi guration and length of the DNA dramatically affected the intensity of the fl uorescence emitted from the TP3-DNA complex. We have compared nucleic acids from different sources and optimized the system. In one proposed method, we attempt to combine broad-range detection with rapid and simple operation. A fi ber-optic capillary fl uorescence system was used to analyze toxic aromatic amines, antibiotics, and several kinds of antitumor drugs, using small amounts of sample, down to 10 m L, with sensitivity comparable to that of current electrochemical methods. The detection limit can be as low as a few ppb or submicromolar. This approach is useful for routine screening in environmental monitoring or for controlling cytotoxic drug administration. The ease of operation and the rapid response allow high-throughput screening. To enable suffi ciently high sensitivity to detect substances that may be used for bioterrorism concentration enhancing methods such as SPE can be used for sample collection.