The presence of calcium ions (Ca2+) in the body is vital for regulating various physiological processes. Its imbalance has been associated with increased risk for medical disorders such as osteoporosis, osteopenia, hypocalcemia, etc. Although the various biosensing methodologies (blood, saliva, etc.) offer a low-cost method for monitoring changes in the Ca2+ level, there remains a need to identify robust ionophores with improved sensitivity and selectivity. Oxalate-based organometallic complexes such as potassium titanium oxalate (KTO) have typically been utilized in the areas of textile, wood processing, etc. However, in the presence of Ca2+ ions, the interaction triggers a spontaneous electrochemical reaction, motivating the potential use of these materials as ionophores for the detection of Ca2+ ions. Therefore, the current project proposes taking advantage of the electrostatic interactions of KTO to design a non-traditional ionophore. In this work, a combination of in-vitro and in-vivo studies was performed to demonstrate the sensing capability of KTO. The electrochemical reactivity and selectivity of KTO to common ions (i.e., Ca2+, Na+, K+, and Mg2+) observed in the body were visualized using a microfluidic and electrical resistance tomographic approach. Preliminary XRD, SEM/EDX, and Raman results suggest that a structural transformation is induced from the entrapment of Ca2+ in the KTO lattice. These techniques coupled with in-vitro multiphoton imaging and spectroscopy further demonstrate that this selective entrapment generates distinct electrochemical (conductivity reduction) and optical (fluorescent lifetime quenching) signatures. Finally, in-vivo multiphoton imaging results support the detection of Ca2+ by KTO in biological systems. Overall, these initial results demonstrate that the strong electrostatic interaction observed in oxalate-based organometallic complexes can be leveraged to design dual-modal (electrochemical and optical) sensors for Ca2+ detection, thus contributing to the development of next-generation wearable sensors.

Visualizing electrostatic interactions in oxalate-based organometallic complexes for next-generation multimodal calcium sensors

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