Neurotransmitters play a central role in complex neural networks by serving as chemical units of neuronal communication. Quantitative optical methods for the detection of changes in neurotransmitter levels has the potential to profoundly increase our understanding of how the brain works. Therapeutic drugs that target neurotransmitter release are used ubiquitously to treat a vast array of brain and behavioral disorders. For example, new methods in this sphere could provide a new platform by which to validate the function of drugs that alter modulatory neurotransmission, or to screen antipsychotic and antidepressant drugs. However, currently in neuroscience, few optical methods exist that can detect neurotransmitters with high spatial and temporal resolution in vitro or in vivo. Brain tissue also readily scatters visible wavelengths of light currently used to perform biological imaging, and neuronal tissue and has an abundance of biomolecules that are chemically or structurally similar and therefore hard to specifically distinguish. Furthermore, neurotransmission relevant processes occur at challenging spatial and temporal scales. UC Berkeley investigators have developed polymer-functionalized carbon nanotubes for in vitro and in vivo quantification of extracellular modulatory neurotransmitter levels using optical detectors. The method uses the fluorescent optical properties of polymer-functionalized carbon nanotubes to selectively report changes in concentration of specific neurotransmitters. The scheme is novel in that the detection method applies to wide variety of specific neurotransmitters, it is an optical method and therefore gives greater spatial information, and enables the potential for imaging of one or more neurotransmitters. The optical method also produces less damage to the surrounding tissue than methods that implant electrodes or cells and allows high resolution localization with other methods of optical investigation. The invention takes advantage of favorable fluorescence properties of carbon nanotubes, such as carbon nanotube emission in the near infrared and infinite fluorescence lifetime. The near infrared emission scatters less than shorter wavelengths, enabling greater signal recovery from deeper tissue, and allows greater compatibility with other techniques. The optical properties also enable long term potentially even chronic use.