Synthesis of size monodisperse water-soluble metal nanoclusters for protein quantification by elemental mass spectrometry

The use of metal immunoprobes, defined as recognition molecules (e.g., antibodies) labeled with metal tags, constitutes an interesting strategy for the analysis of proteins in biological samples. Fluorescent and biocompatible metal nanoclusters (MNC) have been recently established as powerful tags for detection by spectrofluorimetry, but also by elemental mass spectrometry (MS). Detection of such immunoprobes by elemental MS allows not only the qualitative analysis of the proteins but also their absolute quantification. However, the deviation associated with the MNCs polydispersity will limit the analytical precision, particularly in those samples where the concentrations of the sought protein are very low (e.g., single cell analysis). In this work the synthesis of size monodisperse gold nanoclusters (AuNCs) is investigated by using different experimental conditions such as reaction time and temperature, solvent, reducing agent, and pH, among others. Characterization of AuNCs was performed by spectrofluorimetry, dynamic light scattering (DLS) and high resolution transmission electron microscopy (HR-TEM) measurements.