Aggregation of poly(N-isopropylacrylamide) homopolymer by Cu2+ and Zn2+: Significance for ratiometric metal ion indicators

Abstract

Aqueous transition metal dications Cu2+ and Zn2+ bind to the amide groups of the responsive polymer poly(N-isopropylacrylamide) (PNIPAm). Ratiometric detection by the response of PNIPAm is based on the effect of metal ion binding to ligands that are copolymerized into PNIPAm. Metal ions bound to copolymerized ligands cause a polarity change that shifts the ratio of coil and globule polymer phases, whereby the coil:globule state is monitored by the FRET process of a pair of fluorophores distributed throughout the polymer. This work shows that aqueous Cu2+ or Zn2+ ([M2+] = 2.5 × 10−2 M to 2.5 × 10−4 M) cause aggregation of the PNIPAm homopolymer (Mn = 8400 and 10,300) in the globule phase above 32 °C, as determined by second-order and dynamic light-scattering. Conditional formation constants (pH 6.3) for Cu2+ binding PNIPAm were measured as log Kc = 2.64 ± 0.07 and 4.02 ± 0.05 for PNIPAm concentrations of 1 mg/mL and 0.05 mg/mL, respectively. The interaction of Cu2+ and Zn2+ with PNIPAm is therefore in competition with the copolymerized ligands and when detecting millimolar to high micromolar concentrations, this interaction poses a potential interference to metal ion detection. Several aggregation processes are implicated based on Cu2+ or Zn2+ from 8 × 10−5 to 2.5 × 10−2 M. These processes are fully reversible by reducing the sample temperature.