Two improvements to the established procedures for synthesis and response detection of ion-selective optical sensors (optodes) were introduced.
The first improvement addresses the drawback of organic dye (optode-localized chromoionophore) photobleaching. This positively impacts fluorescence response and allows for (1) direct measurement of hydrogen ion activity upon binding with the dye, or (2) indirect measurement of sodium activity (when complexed by a second ionophore, highly selective to sodium ions) that competes with protons for ion-exchange sites inside the optode.
To accomplish this, quantum dots (QDs), semiconductor nanocrystals characterized by a high resistivity to photobleaching, were incorporated in the polymer matrix together with all necessary sensing components to serve as "soft" light-source donors (via the inner filter effect) to excite chromoionophores (acceptors). It was shown that this method is appropriate for fluorescent signal (response) measurement while maintaining stability, calibration with respect to pH, and calibration with respect to sodium activity.
The second improvement focused on optimization of the synthesis of optodes. Ion-selective sensors have a relatively short life-time due to photobleaching of the organic dye component of the system. Since the common solvent displacement method employed in the one-step, batch procedure for optode synthesis is not appropriate for "on-demand" synthesis, a microfluidic approach for quasi-continuous synthesis was introduced. An additional benefit of this approach is greater facility for control of particle size distribution.
The microfluidic "chip" was fabricated on a cyclic olefin copolymer (COC) substrate by micromilling the channels to meet the fluid distribution requirements of the process, then sealing the chip by clamping in a home-made fixture. It was discovered that the size of the particle may be determined by the polymer concentration in one of the reacting solutions, while flow rate changes and component ratios were determined not to directly affect the particle size. The microfluidic platform proved to be a convenient tool for optode fabrication.
For the calibration and cell penetration experiments with the optodes, the microfluidic platform was coupled with a second microfluidic "chip" that facilitated optode trapping with the help of laser tweezers.