a fiber optic based chemical sensor detects chemical species in solution. a fluorogenic indicator is forced through an ultrafiltration membrane into an analyte solution. complex formation between indicator molecules and target ions occurs at the membrane-solution interface where light from a bifurcated fiber optic cable stimulates fluorescence. fluorescent emission from a sample is transmitted back up the fiber optic cable to a linear photo diode detector. the sensor responds linearly to increasing and decreasing concentration changes on time scales of one second. chemical modification of an indicator solution enhanced sensitivity. the present invention directed to providing a fiber optic sensor that uses a pressurized membrane indicator delivery system for remotely detecting chemical species in solution. the apparatus provides an indication of the concentration of a chemical of interest in solution. a source of radiation creates an illuminating wavelength and a detector of radiation at fluorescence wavelengths are appropriately optically coupled to a fiber optic cable. a reservoir of a ligand is included that has the property to fluoresce at the fluorescence wavelength when the chemical of interest is complexed with the ligand and is radiated with the illuminating wavelength