Synthesis of fluorescent BSA-Au NCs for the detection of Hg²⁺ ions

In this study, we present a simple heating approach for preparation of gold nanoclusters (Au NCs) using bovine serum albumin (BSA) as a template. At 70 C, the reaction for the preparation of BSA-Au NCs is completed within 20 min. By conducting matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), we have found that the main product is BSA-Au₂₀ NCs that emit at 660 nm when excited at 330 nm due to "molecular-like" behavior. The X-ray photoelectron spectroscopy data reveal that there are Au⁺ ions and Au atoms coexisting in the BSA-Au NCs. The as-prepared Au NCs show excellent stability over a wide pH range (2.0-10.0). The fluorescence and MALDI-MS data reveal that the changes in their fluorescence properties are due to the formation of various sizes of BSA-Au NCs for different periods of reaction time. The as-prepared BSA-Au NCs are selective and sensitive (limit of detection of 4 nM at a signal-to noise ratio 3) for the detection of Hg²⁺ ions through the d¹⁰-d¹⁰ metallophilic interaction of Au⁺ and Hg²⁺ that leads to a decrease in fluorescence. The present assay has been validated for the detection of Hg²⁺ ions in real water samples, with a result being in good agreement with that from inductively coupled plasma mass spectrometry.