The earth is a planet of water where diverse living organisms have flourished and gone extinct. In any activity of living organisms, ions, electrically charged species, play an important role and therefore the sensing of ionic species has always been of great importance. Ion-selective optical sensors which derived form ion-selective electrodes (ISEs), or optodes are one of the most powerful tools to quantitatively visualize ionic species. Optodes typically contain an ionophore (selective ligand of ions), an ion-exchanger and an optical reporter and take a form of membrane or particles. Optodes continue to demonstrate tremendous potential across a wide range of applications due to their high versatility and low cost. In this thesis, both polymeric membrane and nanoparticle optodes will be presented.

Ever since the 90’s, optodes have upon ion-exchange reactions involving an analyte and an optical transducer. Unfortunately, due to the limited selectivity of the available ionophores for polyions, this mechanism has suffered from severe interference in complex sample matrices. Chapter 2 will describe a new type of nanosensor based on the discovery of a “hyper-polarizing lipophilic phase” in which dinonylnaphthalenesulfonate (DNNS) polarizes a solvatochromic dye much more than even an aqueous environment. We have found that the apparent polarity of the organic phase is only modulated when DNNS binds to large polyions such as protamine. Our new sensing mechanism allows solvatochromic signal transduction without the transducer undergoing ion exchange. This is the first optode which does not utilize ion exchange for optical signal transduction. The result is significantly improved sensitivity and selectivity, enabling for the first time the quantification of protamine and heparin in human plasma using optical nanosensors. Chapter 3 will discuss on the precise mechanism of the hyperpolarization-based sensor. More specifically how protamine, the arginine-rich target polycation, behaves during optical signal transduction and the origin of the dramatically improved selectivity will here be clarified. Based on thermodynamic parameters and zeta potential analysis, we have observed two discrete phases of protamine extraction: protamine diffusion into the bulk, concurrent with optical signal change, followed by surface accumulation starting only upon saturation of the optical signal change. Data also indicate that the improved selectivity is due to the inability of small ions to form a strong complex with DNNS that would inhibit the polarizing interaction between DNNS and a solvatochromic dye, providing a specificity which cannot be achieved by classical ion-exchange type sensors.

Chapter 4 shows the development of a capillary-based optode with microfluidic paper-based analytical devices (μPADs) as a readout platform. Plasticized poly(vinyl chloride) (PVC) containing valinomycin (K⁺-selective ionophore), ion exchange and a cationic dye (thioflavin T: ThT) was solvent-cast on the inner wall of a capillary, forming the capillary-based optode. Upon introduction of the sample, K⁺ in the aqueous sample solution is quantitatively extracted i