Synthesis and Characterization of Highly Sensitive pH Probes Based on Fluorescein-Conjugated Gold Nanoclusters

Abstract

Intracellular pH is a critical parameter that regulates the cellular metabolism. Thus, sensitive monitoring of the intracellular pH change is essential in understanding mechanisms governing drug delivery and cell apoptosis. In this work, we synthesized highly sensitive pH probes composed of fluorescein and glutathione-protected gold nanoclusters and characterized their optical properties. The fluorescein was conjugated to the gold nanocluster by using an amide coupling reaction. In steady-state photoluminescence (PL) measurements, fluorescein conjugated to gold nanocluster shows more than 160-fold pH-contrasting emission in the pH range of 4.3-7.8 while glutathione coupled fluorescein shows only 10-fold pH contrast in the same pH range. Transient absorption measurements confirmed that the enhanced PL was caused by the ultrafast energy transfer from the nonthermalized states of gold nanocluster to fluorescein. Furthermore, efficient nonthermalized energy transfer occurring in the fluorescein-conjugated gold nanocluster led to a dramatically pH-contrasting PL. In addition to the intrinsic advantages such as low toxicity and facile surface modification, fluorescein-conjugated gold nanoclusters exhibit high pH sensitivity with wide dynamic range, offering new opportunities for the study of intracellular processes.