Abstract
Microparticles consisting of the thermal responsive polymer N-isopropyl acrylamide (polyNIPAM), a metal ion-binding ligand and a fluorophore pair that undergoes fluorescence resonance energy transfer (FRET) have been prepared and characterized. Upon the addition of Cu(II), the microparticles swell or contract depending on whether charge is introduced or neutralized on the polymer backbone. The variation in microparticle morphology is translated into changes in emission of each fluorophore in the FRET pair. By measuring the emission intensity ratio between the FRET pair upon Cu(II) addition, the concentration of metal ion in solution can be quantified. This ratiometric fluorescent indicator is the newest technique in an ongoing effort to use emission spectroscopy to monitor Cu(II) thermodynamic activity in environmental water samples.
Highlights
The effects of metal ions in biological and environmental systems depend on thermodynamic activity rather than total concentration [1]
The lower critical solution temperature (LCST) of 32 °C, pure polyNIPAM remains soluble in water because of hydrogen bonding between water and the amide groups; the polymer collapses and precipitates from solution at higher temperatures when the hydrogen bonds are broken [15]
Excitation of the donor fluorophore leads to donor-based emission if the fluorescence resonance energy transfer (FRET) pair is separated in space
Summary
The effects of metal ions in biological and environmental systems depend on thermodynamic activity rather than total concentration [1]. 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
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