Method of measuring external stimulus and volume change using stimulus-responsive gel comprising EDANS

A method of measuring an external stimulus and a volume change in a sensor element, the method comprising:

• (a) creating an analytical curve by plotting logarithms of the volume change against logarithms of fluorescence intensity or absorbance of the visible light or ultraviolet light, by (i) exposing a sensor element to external stimuli of different known ionic concentration, and a liquid, wherein the sensor element comprises a stimulus-responsive gel comprising polyacrylic acid, wherein the stimulus-responsive gel comprises optically active molecules, which are 5-(2′-aminoethyl)aminonaphthalene-1-sulfonate (EDANS), wherein the stimulus responsive gel changes its volume in response to an external stimulus and the sensor element converts into optical data a volume change in the stimulus-responsive gel, and wherein the liquid is absorbed in or discharged out of the sensor element in response to the external stimuli, so as to cause volume changes in the sensor element, (ii) measuring the volume change in the sensor element exposed to the respective external stimuli of different known ionic concentrations, (iii) irradiating light of a particular wavelength on the sensor element exposed to the respective external stimuli of the different known ionic concentrations, and (iv) measuring fluorescence intensity or absorbance of visible light or ultraviolet light therein;
• (b) (i) exposing the sensor element to an external stimulus of an unknown ionic concentration in the presence of the liquid, (ii) irradiating the light of the particular wavelength on the sensor element exposed to the external stimuli of the unknown ionic concentration, and (iii) measuring the fluorescence intensity or the absorbance of the visible light or the ultraviolet light in the sensor element;
• and
• (c) determining how large the volume change in the sensor element and the external stimulus of the unknown ionic concentration are, from the measured fluorescence intensity or the absorbance of the visible light or ultraviolet light by using the analytical curve.