Chemoresistive and Photonic Hydrogel Sensors of Transition Metal Ions Via Hofmeister Series Principles

We evaluate the sensory response of hydrogels composed of polyacrylic acid (PAA) co-polymerized with acrylamido-methyl propane sulfonic acid (AMPS) exposed to eleven different transition metal ions, which were reacted with hexacyanoferrate ions. Trends in the chemoresistive responses of the gels to ions of Cu2+, Fe2+, Mn2+, Ni2+, Pb2+, Co2+, Sn2+, Cd2+, Zn2+, Mo5+, and Ag+ due to the formation of metal hexacyanoferrate complexes are shown to depend upon the strength of the gel headgroup-ion interactions based on the Hofmeister series and confirmed using Raman spectroscopy. These hydrogels either showed a decrease in resistivity and swelling with increasing ion concentration or the opposite effect. Sensitivity by colorimetric and conductivity measurements was approximately 100 ppb, which can be adjusted by the gel’s dimensions and density of headgroups within the gel. The photonic and vibrational characteristics of the gels upon ion addition were investigated for use in spectroelectrochemical sensing devices by imaging, electrochemical impedance spectroscopy and Raman spectroscopy