Using protein-encapsulated gold nanoclusters as photoluminescent sensing probes for biomolecules

In this study, we generated gold nanoclusters (AuNCs) using inexpensive chicken egg white proteins (AuNCs@ew) as reagents. AuNCs@ew were generated by reacting aqueous tetrachloroauric acid with diluted chicken egg white under microwave heating (90W) through subsequent heating cycles (5 min/cycle). Within 10 cycles, red photoluminescent AuNCs@ew with maximum emission wavelength at ~640 nm (λex=370 nm) were obtained. The quantum yield of the as-generated AuNCs was ~6.6%. The intact and the tryptic digest of AuNCs@ew were characterized by mass spectrometry. The results showed that the AuNCs@ew were mainly derived from ovalbumin, i.e., the major protein in egg white, encapsulated AuNCs. The AuNCs@ew also has the common features found in AuNCs@protein, which is sensitive to the presence of heavy metal ions such as Cu(2+). The photoluminescence of the AuNCs@ew was quenched with the addition of Cu(2+). Furthermore, the photoluminescence of the quenched AuNCs@ew can be restored in the presence of the molecules containing phosphate functional groups because of the strong binding affinity between Cu(2+) and phosphates. We used the AuNCs@ew-Cu(2+) conjugates as switch-on sensing probes for the detection of phosphate containing metabolites such as adenosine-5'-triphosphate (ATP) and pyrophosphate (PPi). The results showed that the photoluminescence of the sensing probes increased as the concentration of the phosphate-containing molecules in the sample solution increased. The limits of detection achieved using the AuNCs@ew-Cu(2+) for ATP and PPi were ~19 and ~5 μM, respectively. Additionally, we also demonstrated the feasibility of using the AuNCs@ew as the sensing probes for lectins such as concanavalin A (Con A) based on the molecular recognitions between the glycan ligands on the AuNCs@ew and glycan binding sites on Con A.