Response Mechanism of Hyperpolarization-Based Polyion Nanosensors

Abstract

The last decade has witnessed a rapid development of nano- and microparticle-based optical ion sensors, including ion-selective optodes (ISOs). While the application of nano-ISOs has shown promising performance for sensing inorganic ions, polyion sensing using nanoscale ISOs has encountered significant interference in complex samples such as blood plasma. Recently, we have reported on a new polyion sensing principle that operates through a novel mechanism to overcome this challenge. The new sensing mechanism showed improved characteristics not observed with conventional ion-exchange type sensors, but the precise mechanism of operation remained thus far unclear. This paper aims to clarify how protamine, the arginine-rich target polycation, behaves during optical signal transduction to give dramatically improved selectivity. Based on thermodynamic data, sensor performance and ζ-potential analysis, two discrete phases of protamine extraction are identified. Initially, protamine extracts into the bulk nanosensor phase, a process that is concurrent with the optical signal change. This is then followed by protamine accumulation onto the nanosensor surface, which starts only upon saturation of the optical signal change. The data indicate that the improved selectivity is due to the inability of small ions to form a sufficiently strong interaction with an active sensing ingredient, DNNS–. Any exchange of one inorganic cation for another therefore remains optically silent, suppressing matrix effects. Moreover, the recognition of protamine is shown to be an exhaustive extraction process, making the response independent of the nature and concentration of the initial small cation in the nanosensor phase.

Optical Detection of Heparin in Whole Blood Samples Using Nanosensors Embedded in an Agarose Hydrogel

“…This new mechanism demonstrated dramatically improved selectivity compared to ion-exchange-type sensors. 14 As the sensor response was linear with the amount of protamine, protamine extraction was determined to operate through a so-called "exhaustive mode". 15 The heparin concentration was successfully determined in human plasma.…”

“…Upon protamine-DNNS – complexation, the dye forms an ion pair with TFPB – containing a delocalized negative charge, resulting in a bathochromic absorbance shift of the dye. This new mechanism demonstrated dramatically improved selectivity compared to ion-exchange-type sensors . As the sensor response was linear with the amount of protamine, protamine extraction was determined to operate through a so-called “exhaustive mode” .…”