Ionic strength assay via polyacrylate–ferriferrous oxide magnetic photonic crystals

Convenient reading out and/or determination of ionic strength (IS) is of great significance for both scientific research and real life applications. We presented here a novel method for the rapid and sensitive IS assay based on the electrolyte-induced sensitive wavelength blueshifts of the reflection spectra of polyacrylate capped Fe3O4 magnetic photonic crystals (PA–Fe3O4-MPCs). For HCl, MgSO4 and the common electrolytes corresponding to the salinity of seawater (including NaCl, KCl, MgCl2, CaCl2, Na2SO4 and their mixtures), the PA–Fe3O4-MPCs displayed wavelength blueshifts identical to the total IS of the aqueous solutions, regardless of the kind of above-mentioned electrolytes in the solutions. Besides, the PA–Fe3O4-MPCs exhibited relatively high sensitivity (an average of 294 nm L mmol−1 in the range of 0.05–0.30 mmol L−1, and an even higher value of 386 nm L mmol−1 at 0.05–0.15 mmol L−1) and fast response (within 8 s) to the IS of aqueous solutions. The relative standard deviation (RSD) for IS (NaCl, 0.1 mmol L−1) was 4.4% (n = 5). The developed method was applied to determine the salinity of seawater samples, and the determined results were validated by the traditional standard chlorinity titration and electric conductimetry method. The recoveries were in the range of 92–104%. The proposed PA–Fe3O4-MPCs based reflectometry method would have great potential for IS and salinity assays.