A calix[4]arene based calcium-selective optode membrane: Measuring the absorbance maximum wavelength shift

An absorbance based calcium ion sensor is presented which applies a new imm obilized chromogenic calix[4]arene. The membrane consists of a hydrogel/1,3 -pentanedial mixture and the 1,3-bis(indoaniline)-derived 2,4-bis-[(ethylca rbonyl)methoxy]-calix[4]arene as calcium receptor. The indicator entrapped within the gel was not rapidly washed out. Upon calcium complexation a bath o-chromic shift up to 70 nm was observed along with an increase of the abso rption coefficient. The dynamic range of calcium measurements is from 3 to 10 mM with a point of inflection of the calibration curve at a calcium conc entration of about 7 mM. The concentration of calcium ions can be calculate d from either absorbance at a distinct wavelength or - preferably when indi cator leaching occurs - from the wavelength shift of the absorbance occurri ng when calcium levels are increased.