For example, Cu(II) toxicity varies over an order of magnitude depending on its degree of complexation with organic matter and competition with other metal ions for binding sites on aquatic organisms [2,3]. The biotic ligand model corrects for these effects and predicts Cu(II) toxicity better than total copper measurements [2,3].While few techniques measure the thermodynamic activity of a metal ion directly, indicators provide a convenient method to quantify this property. Metal ion binding to a ligand (receptor) in an indicator induces measurable changes in the optical properties of a reporting group that may or may not be connected to the ligand. To prevent perturbing the metal ion activity, indicator concentration must remain lower than the total metal ion concentration.

The measurable range of analyte concentration for any indicator depends on the receptor’s affinity for the metal ion of interest, and is defined as log Kf ? 1 to log Kf +1, where Kf is the conditional formation constant for metal ion.The high sensitivity of emission spectroscopy means indicators utilizing fluorescence can be employed at the lowest possible levels. Furthermore, ratiometric fluorescence indicators simplify signal calibration by monitoring changes at different emission wavelengths rather than the absolute intensity. By measuring the intensity ratio at two different wavelengths, the output remains independent of indicator concentration.

Ratiometric indicators are essential for applications such as measuring intracellular metal ion activity where total indicator concentrations cannot be established accurately.

Ratiometric fluorescent indicators have enabled researchers to study the biological Carfilzomib role of Ca(II) [4] and Zn(II) [5]; however, designing Cilengitide ratiometric indicators for metal ions such as Cu(II) that quench fluorescence emission remains challenging.We and others are developing fluorescent metal ion indicators based on the thermal phase transition of poly(N-isopropylacrylamide) (polyNIPAM) [6�C11]. PolyNIPAM undergoes a thermal phase transition at elevated temperatures, which leads to aggregation and precipitation [12]. The temperature at which the phase transition occurs is defined as the lower critical solution temperature (LCST).

Metal ion binding to a polymer-bound ligand can modulate LCSTs by either introducing or neutralizing charge on the macromolecular backbone. With a properly engineered polymer maintained at a specific temperature, metal ion binding can induce the polyNIPAM thermal phase transition. When fluorophores are included in the polyNIPAM formulation, the phase transition can be coupled to emission changes.