Surface plasmon resonance study on enhanced refractive index change of an Ag+ ion-sensing membrane containing dithiosquarylium dye Original Research Article

In this study, we have fabricated an Ag+ ion-sensing membrane with a dithiosquarylium (DTSQ) dye containing a polymeric film. The selective sensing signal through the electrostatic interaction between the DTSQ dye and the Ag+ metal cation was effectively transduced to the refractive index (RI) change corresponding to shifts of the surface plasmon resonance (SPR) angle. In addition, a good selective Ag+ ion detection appeared in a wide concentration range from 10−4 to 10−12 M. The resonance angle shift is interpreted with Fresnel equations and Kramers–Kronig relation. In light of these calculations, the enhanced RI increase in this sensing membrane appeared to be caused by the decrease of absorption coefficient of DTSQ dye around the wavelength of SPR probe beam. These results suggest that chromogenic approaches (λmax control of Ag+ ion-sensing membrane with a DTSQ dye by appropriate molecular design) related to SPR phenomena (RI change at the wavelength of probe beam) offer a good strategy for highly sensitive metal ion detection.