Other types of nanosensors, including quantum dots and gold nanoparticles, are currently being developed to detect pollutants and toxins in the environment. These take advantage of the localized surface plasmon resonance (LSPR) that arises at the nanoscale, which results in wavelength specific absorption. This LSPR spectrum is particularly sensitive, and its dependence on nanoparticle size and environment can be used in various ways to design optical sensors. To take advantage of the LSPR spectrum shift that occurs when molecules bind to the nanoparticle, their surfaces can be functionalized to dictate which molecules will bind and trigger a response. For environmental applications, quantum dot surfaces can be modified with antibodies that bind specifically to microorganisms or other pollutants. Spectroscopy can then be used to observe and quantify this spectrum shift, enabling precise detection, potentially on the order of molecules. Similarly, fluorescent semiconducting nanosensors may take advantage of fluorescence resonance energy transfer (FRET) to achieve optical detection. Quantum dots can be used as donors, and will transfer electronic excitation energy when positioned near acceptor molecules, thus losing their fluorescence. These quantum dots can be functionalized to determine which molecules will bind, upon which fluorescence will be restored. Gold nanoparticle-based optical sensors can be used to detect heavy metals very precisely; for example, mercury levels as low as 0.49 nanometers. This sensing modality takes advantage of FRET, in which the presence of metals inhibits the interaction between quantum dots and gold nanoparticles, and quenches the FRET response. Another potential implementation takes advantage of the size dependence of the LSPR spectrum to achieve ion sensing. In one study, Liu et al. functionalized gold nanoparticles with a Pb2+ sensitive enzyme to produce a lead sensor. Generally, the gold nanoparticles would aggregate as they approached each other, and the change in size would result in a color change. Interactions between the enzyme and Pb2+ ions would inhibit this aggregation, and thus the presence of ions could be detected.