Recent improvements to the selectivity of extraction-based optical ion sensors

Abstract

Optical sensors continue to demonstrate tremendous potential across a wide range of applications due to their high versatility and low cost. This feature article will focus on a number of recent advances made in improving the performance of extraction-based optical ion sensors within our group. This includes the progress of anchored solvatochromic transduction to provide pH and sample volume independent optical responses in nanoemulsion-based sensors. A recent breakthough is in polyion sensing in biological fluids that uses a novel indirect transduction mechanism that significantly improves the selectivity of dinonylnaphthalenesulfonate-based protamine sensors and its potential applications beyond polyion sensing. The role of particle stabilizers in relation to the response of emulsified sensors is shown to be important. Current challenges in the field and possible opportunities are also discussed.

Fig. 1. Illustrations of reported ion sensing mechanisms. Note that mechanisms depicted with a cationic dye have also been investigated for anionic dyes for analytes of the opposite charge. (A and B) Direct transduction using a analyte specific chromoionophore with co-extraction of an anion or a chromoionophore which deprotonates (C and D) Hydrogen Chromoionophore-based sensors operating through either exchange or co-extraction depending on the charge of the analyte. (E and F) Solvatochromic dye-based sensor signaling a polarity shift as it moves from the hydrophobic sensing matrix to an aqueous environment or vice versa (G) Dye undergoing exchange and changing the absorbance of the sample solution. (H) Dye can also be co-extracted into an organic phase, the absorbance of which can then be measured.

Chart 1. Structures of commonly used optode matrix materials represented in Table 1. Matrices containing PVC are generally among the most hydrophobic resulting in both the highest energy barrier to ion entry and the highest binding constants of incorporated ionophores.

Fig. 2. (a) Reproduced from Xie et al. Multiple pK_as fitted (red) versus a single pK_a (blue) in a nanosensor system (b) highly distinct pK_as in a surfactant stabilized nanosensor system from Soda et al. (c and d) internal polarity of emulsion based optodes.

Fig. 3. Transition of non-ionic surfactant (Trition X-100) into an organic phase stabilizing a metal ion non-specifically in contrast to a zwitterionic sulfobetaine-based surfactant, which does not undergo this interaction.

Fig. 4. Extraction of target (i) or interfering ion (j) into an ion selective optode containing a selective ionophore (L). The total amount of ion that can be transferred is limited by the available ion exchanger not depicted here. (A) The target ion is readily extracted into the membrane, in the presence of surfactant a quality ionophore will still be preferred due to much higher binding constants. (B) As the interfering ion binds only weakly to the ionophore it is excluded, surfactant may have comparable or stronger binding constants to interfering ions than the ionophore, resulting in deteriorated selectivity. (C) Response of target and interfering ion with and without surfactant.

Fig. 5. (A) Anchored solvatochromic dyes provide a pH and volume independent response. (B) Response range can be modulated by altering the hydrophobicity of the solvatochromic dye head group. (C) Solvatochromic dyes can also be paired to another FRET dye in order to provide a larger spectral shift and enable a ratiometric fluorescence readout. (D) FRET interactions with more lipophilic dyes may amplify optical response allowing more precise ratiometric measurements.

Fig. 6. Reproduced from Soda et al. Proposed protamine (polycationic protein) detection mechanisms using a hyperpolarization-based sensor. The highly localised charge of DNNS strongly polarizes the solvatochromic dye, an interaction that is only interrupted by polyions that either localize in (i) or on (ii) the particle.

Fig. 7. (A) Schematic showing non-selective response of sensor using direct optical transduction. (B) Sensor showing no optical response despite non-specific ion exchange. (C) A hyperpolarising organic phase with solvatochromic transducer. (D) Redistribution of DNNS following entry of polycation (protamine).

Kye J. Robinson

Yoshiki Soda

Eric Bakker