A Reversible Plasmonic Probe Sensitive for pH in Micro/nano-spaces Based on i-motif-modulated Morpholino-AuNPs Assembly

Exploring local pH in micro/nano scale is fundamentally important for understanding micro-processes including corrosion of metal and metabolism of cell. Regular fluorescence pH probe and potentionmetric electrode either show low signal intensity or lack of special resolution when being applied in micro/nano environment. Here we developed a nanoscale reversible pH probe based on the plasmonic coupling effect of i-motif modulated gold nanoparticle (AuNP) assembly. The pH probe shows reversible and high sensitive response to pH variation between 4.5 and 7.5. Introduction of morpholino (MO), a neutral analog of DNA, into the assembly endows the pH probe with high stability even under low salt concentration. The intense optical signal of AuNP enables local pH being read out not only in micro/nanofluidic channel but also on a single i-motif-MO-AuNP assembly. Recording of the strong plasmonic resonance scattering spectrum of AuNP provides a promising method for extracting chemical information in nanospace of biological systems.