Nanodroplets versus Nanofibers Ion-Selective Optodes toward Biocompatible Sensors

This work compared the performance of ion-selective optodes in different nanostructural formats: nanodroplets and nanofibers prepared from biocompatible materials using classical ionophores and ion exchangers. Nanodroplets tested contained minute amounts of plasticizer acetyl tributyl citrate stabilized with poly-(vinyl alcohol) and were suspended in the aqueous phase. It was shown that this system was not directly transferable to the nanofiber format; thus, nanofibers obtained from polycaprolactone were used as the support material. Both nanodroplets and nanofibers were used in Nile blue/Nile red-based systems operating as novel ion-selective sensor optical transducers, allowing sensors to be applied in unbuffered samples. As a model system, calcium sensors were prepared and tested. It was shown that, regardless of format, optodes were characterized by increased emission intensity at the Nile red characteristic wavelength (605 nm) for increasing calcium ion concentration in the sample. Nile blue emission changes for varying analyte concentrations were specific for the format (they changed for nanodroplets, whereas for nanofibers, they were similar within the range of experimental error). For both systems, a linear dependence of the ratio of emission intensity was recorded for Nile red and Nile blue characteristic wavelength on the changes in analyte concentration in the sample within the range from 10^-5 to 10^-1 M. The unique advantages of using nanofiber mats include the determination of optical signals for both sensors present in solution and probes removed from solution; moreover, the same sensor can be transferred between different solutions. This is a clear advantage of the application of nanofiber mats as sensors, especially when used on a dedicated, 3D-made holder.