Quantum Dots in an Amphiphilic Polyethyleneimine Derivative Platform for Ratiometric and Reversible Nitric Oxide Sensing

Abstract

Nitric oxide (NO) is a diatomic molecule and plays a key role in a variety of biological processes. NO is highly diffusible and extremely labile by oxidation. In order to analyze NO generation and distribution in vivo, a reliable in vivo NO sensing fluorescent probe is still in pursuit. Quantum dots (QDs) have emerged as an alternative to fluorescence proteins or organic dyes in biological applications. QDs have high the extinction coefficient, broad absorption range, high photostability, and resistance against photobleaching that cannot be paralleled by organic fluorophores. Cationic amphiphilic polyethyleneimine derivatives (amPEIs) were synthesized to encapsulate dozens of QDs and NO sensing metal complex. amPEIs successfully wrapped Fe (III) complexes of a tetra-amido macrocyclic ligand and two kinds of QDs (one emits at around 720 nm and the other at around 450 nm). The Fe complex and QDamPEI composite was around 100 nm in hydrodynamic size. The composite was used for NO sensing probe. It had slightly positive outer surface that suited well for cellular internalization. Fe complexes could react with dissolved NO molecules outside of the composites and showed immediate appearance of strong absorption in visible (500~650 nm) and near-IR (700~1000 nm) ranges. When NO was purged out in solutions, absorption spectrum of Fe complex returned to the initial state. QDs-Fe complex-amPEI based NO probe demonstrated accurate and reversible NO sensing by the ratiometric photoluminescence signals. In NO saturated solution, quenching of 720 nm emitting QD PL was observed because Fe complex could absorb part of QD PL upon the binding NO to metal center. As NO was released from the Fe complex, the 720 nm QD emission was recovered. In contrast, 450 nm emitting QD acted as an internal standard. The Fe complex showed similar absorption profile at 450 nm regardless of the binding or leaving of NO, and the 450 nm QD emission intensity was independent of the NO concentration. Reversibility of the QDs-Fe complex-amPEI composites was tested by switching the environment between the nitrogen and nitric oxide conditions for more than 5 cycles. These results showed our QD-based nitric oxide sensing platform technology can realtime monitor NO in cells and in vivo models.