Hydrogel
is a unique family of biocompatible materials with growing
applications in chemical and biological sensors. During the past few
decades, various hydrogel-based optical ion sensors have been developed
aiming at point-of-care testing and environmental monitoring. In this
Perspective, we provide an overview of the research field including
topics such as photonic crystals, DNAzyme cross-linked hydrogels,
ionophore-based ion sensing hydrogels, and fluoroionophore-based optodes.
As the different sensing principles are summarized, each strategy
offers its advantages and limitations. In a nutshell, developing optical
ion sensing hydrogels is still in the early stage with many opportunities
lying ahead, especially with challenges in selectivity, assay time,
detection limit, and usability.
Blood electrolyte measurements play important roles in clinical diagnostics. Optical ion sensors as simple and elegant as a mercury thermometer are in high demand. We present here an analytical method to quantify potassium ions in undiluted human blood and plasma by measuring the distance or the rate of the color propagation. The sensor was composed of K-selective nanospheres embedded in an agarose hydrogel where mass transport was diffusion controlled. The sensor's color-changing rate and the distance of color propagation depended linearly on the logarithm of K activity. A theoretical model was established and fully supported the experimental findings. This work lays the foundation of a new family of optical ion sensors for direct determination of common blood electrolytes.
Nanoscale ionophore-based ion-selective optodes (nano-ISOs) are effective sensing tools for in situ and real time measurements of ion concentrations in biological and environmental samples. While searching for novel sensing materials, nano-ISOs free of plasticizers are particularly important for biological and environmental applications. This work described plasticizer-free nano-ISOs based on Si-containing particles including PEGylated organosilica nanoparticles, PDMS nanospheres, and SiO microspheres, with diameters around 50 nm, 100 nm, and 5 μm, respectively. The platform enabled the use of highly selective ionophores, where the nanomatrices played important roles in tuning the ion-carrier complex formation constants and led to better selectivity for the PEGylated organosilica nano-ISOs than those based on PDMS. With use of the versatile silica chemistry, pH and ion dual sensing was achieved on SiO microspheres. In addition, increasing the cross-linking degree of the PDMS nano-ISOs extended the linear response range, and cellular uptake experiments showed that the nano-ISOs could readily enter HeLa cells with very low cytotoxicity.
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