Minimizing color interference from biological samples in optode-based measurements

• We quantify interference from colored samples in colorimetric optode measurements.
• We quantify performance of sequential LED illumination + monochrome CCD based optode imaging.
• Interference from colored samples and illumination-detection errors is effectively minimized with white bead-dispersion within the sensor and a LED-monochrome measurement system.

Optodes contain an ionophore, chromoionophore, and lipophilic anion in a plasticized membrane. The chromoionophore has a neutral and a protonated form with partially overlapping absorption peaks. The ratio of these forms determines the color of the optode membrane, which depends on the ratio of the activities of protons and the ion that the ionophore is selective for in the sample. In most physiological fluids, sodium is well buffered and the color of a pH/Na+ optode depends predominantly on pH alone. However, colored samples can interfere with the measurement. We explore suppression of color interference by using white beads immobilized in a hydrogel layer between the sample and the optode membrane, and TiO2 nanoparticles dispersed within the optode membrane in reflectance based measurements. Both reduce color interference to very low levels, but the latter approach leads to more stable color response. Other physical interferences in optode-based measurements are due to instability in illumination, and partial overlap of the absorption peaks. Both problems are addressed here using a set of red, green and blue narrow-band LEDs and grayscale CCD detector, as opposed to a color camera with white illumination. LED-illuminated and color-insulated optodes remove substantial hurdles that stood in the way of biomedical and other applications of this technology.