Looking into the ion-selective nanoptode bulk – Alternative mechanism of optical signal transduction for ratiometric ion-selective probes applicable in broad pH range

Abstract

Ion-selective sensors, including optodes, explore ion-exchange at the interface to observe analyte ion binding to neutral ionophore in the probe bulk. Typically the optical transducer used is a charged dye - incorporation of analyte cation requires expulsion of positively charged ion from the probe, the process is giving rise to observable optical signal. We propose an alternative approach focused on the probe bulk and allowing application of uncharged dye as the optical transducer. The neutral solvatochromic dye is not involved in maintaining electroneutrality, but its optical spectrum reflects local changes occurring in the probe bulk in response to analyte incorporation, giving rise to analytical signal dependent on the analyte concentration changes in the sample. The proposed optical transduction mechanism offers increase of emission for increase of analyte contents in the sample in a broad concentration range, a linear dependence of optical signal on logarithm of concentration was observed. Moreover, this approach results in significant benefits: it mitigates the cross sensitivity to hydrogen ions concentration changes observed for classical optodes, allowing application of probes in unbuffered samples as well as in wide pH range, including acidic solution. The system requires the presence of adjunctive, cationic species in the probe, that are exchanged for analyte cations preferred in the system due to interactions with ionophore. Depending on the choice of adjunctive cation ratiometric sensing can be achieved. It is also shown that such system does not require the presence of a surfactant, yet allows formation of stable emulsion of nanoprobes.