Since the initial breakthrough of using BSA and lysozyme, other biomolecules of various sizes have successfully synthesized fluorescent Au NCs, which also exhibit detection capabilities. Au NCs stabilized with the amino acid l-cysteine72 showed fluorescence emission in the blue end of the visible spectrum () and was used as a detection probe for glucose sensing. This study also tested for glucose levels in serum samples with selective detection avoiding interference from other serum proteins. Glutathione was used as a stabilizing biomolecule,84 producing similar red emission to BSA and lysozyme Au NCs but with an emission wavelength further red-shifted, correlating with the larger size of the observed nanocluster (). Glutathione-stabilized Au NCs showed fluorescence quenching from ions and good stability against photobleaching and oxidation, making them potential bio-imaging probes. The iron-binding protein lactoferrin stabilized red-emitting Au NCs while also being useful for detecting ions.58 Fluorescence quenching was minimal with the addition of iron ions, indicating the lactoferrin protein structure was relatively undisturbed in the final product. The lactoferrin-Au NC also exhibited good stability at various pH levels, maintaining strong fluorescence. This study investigated the specific region where the Au NC formed in the lactoferrin by using Förster resonance energy transfer (FRET). This technique detects energy transfers from excited donors to acceptors, which can reveal the energy transfer efficiency and the nature of the bonding between lactoferrin and the Au NC. Another protein, trypsin, can be used as a stabilizing agent for Au NCs59 with red emission at 640 nm, selective detection, and resistance to photobleaching. The stability against photobleaching was comparable with CdSe quantum dots, making them a relatively nontoxic alternative for bio-imaging.