Innovations in ion-selective optodes: a comprehensive exploration of modern designs and nanomaterial integration

Abstract

In recent years, ion-selective optodes (ISOs) have remarkably progressed, driven by innovative modern designs and nanomaterial integration. This review explored the development of modern ISO by describing state-of-the-art strategies to improve their sensitivity, selectivity, and real-time monitoring capacity. The review reported the traditional membrane based-optodes, and investigated the latest research, current design principles, and the use of essential components, such as ionophores, indicator dyes, polymer membranes, and nanomaterials, in ISO fabrication. Special attention was given to nanomaterials (e.g., quantum dots, polymer dots, nanospheres, nanorods and nanocapsules) and particularly on how rare earth elements can further enhance their potential. It also described innovative ISO designs, including wearable optodes, smartphone-based optodes, and disposable paper-based optodes. As the pursuit of highly sensitive, selective, and adaptable ion sensing devices continues, this summary of the current knowledge sets the stage for upcoming innovations and applications in different domains (pharmaceutical formulations, medical diagnosis, environmental monitoring, and industrial applications).

Keywords: disposable paper-based optodes; indicator dyes; ionophores; nanomaterials; optodes; polymer membranes; smartphone-based optodes; wearable optodes.

FIGURE 1

Different designs of ISOs: (A) Preparation of the membrane optode. (B) Response mechanism in chromoionophore-based membrane optodes to an analyte cation. (C) The sigmoidal shape of the calibration curve of the membrane selective optode.

FIGURE 2

Illustration of reversible magnetic ion-selective colorimetric microsensors using surface-modified polystyrene beads (Apichai et al., 2020).

FIGURE 3

Illustration of the process to fabricate ultrasmall fluorescent ion-exchanging nanospheres that incorporate the oxazinoindoline chromoionophore and sodium-selective ionophores. The fabrication involves integrating a non-ionic and biocompatible surfactant, Pluronic^® F-127 (F127), into the solution. F127 has a central hydrophobic poly(propylene oxide) chain flanked by two hydrophilic poly(ethylene glycol) (PEG) chains (Xie et al., 2013). THF, tetrahydrofuran. Copyright © Open Access (ACS, 2013).

FIGURE 4

Visual sensing system for milk freshness monitoring using fluorescence sensors based on nitrogen-doped carbon QDs (N-CQDs): (A) Easy-to-use visual sensing device of milk freshness; (B) N-CQD fabrication; and (C) Fluorescence quenching of sensors by rancid milk (Hu et al., 2022). Copyright MDPI (2022).

FIGURE 5

A polymer-dot transducer (PD4Gx) for wireless glucose monitoring with a smartphone. In vivo glucose monitoring in mice with the PD4Gx transducer and a smartphone: (A) Live glucose measurement. (B) Image decomposition for blood glucose monitoring. (C) True-color images before and after glucose administration. (D) Magnified images highlighting glucose concentration changes over time after glucose infusion. (E) Calibration curve showing the correlation between red/blue intensity ratio and glucose concentration (R ² > 0.99), and (F) Real-time dynamic glucose monitoring with the implanted PD4Gx and a smartphone, including the fluctuations after glucose and insulin administration; red scattered points indicate glucose measurements in tail blood using a commercial glucose meter (Sun et al., 2018). Copyright © ACS (2018).

FIGURE 6

Deposition of 3,4-ethylenedioxythiophene (EDOT)-decorated hollow nanocapsules onto PEDOT films for optical and electrochemical sensing: (A) Schematic representation of a nanocapsule and a scanning electron microscopy image. (B) Overview of the synthesis steps for creating EDOT-decorated nanocapsules. (1) Anionic and cationic surfactants self-assemble into vesicles in the presence of acrylate monomers. These vesicles encapsulate acrylate monomers and a photoinitiator within their hydrophobic region that can also accommodate a pore-forming template. (2) Vesicle extrusion through a track-etch membrane, followed by (3) UV polymerization of the monomer within the hydrophobic region. (4) To complete the synthesis process, the surfactant vesicle template and the pore-forming template are eliminated (Hambly et al., 2020). Copyright © 2020 American Chemical Society.

FIGURE 7

Smartphone-based POC optical device to detect HPV DNA in saliva samples. (A) The top panel shows the four-chamber microfluidic chip and the bottom panel the smartphone-equipped smart cup. (B) Photographs of the application user interfaces, including settings (left) and readout (right). (C) Detection of HPV DNA in spiked or not (control) saliva samples; ***p < 0.001 (t-test); error bars indicate the standard deviation (n = 3) (Yin et al., 2019). Figure reused from Yin et al., 2019 under the Creative Commons Attribution-NonCommercial (CC BY-NC) license.

FIGURE 8

Paradigmatic example of a wearable optical sensor designed for real-time monitoring of sweat. (A) Wearable band seamlessly integrated onto the skin for continuous sweat monitoring. (B) Cross-sectional view showing the efficient pumping of sweat from sweat glands into indicator-modified super-hydrophilic microwells. Sweat samples are analyzed using a cellphone-assisted RGB screening system (He et al., 2019). Copyright © 2019 American Chemical Society.