Polypeptide functionalized gold nanoprobes for SERS-fluorescence detection and imaging of caspase-9 during apoptosis.

Background: Deoxynivalenol is one of the common fungal toxins in processed grain foods. It has the characteristic of high temperature resistance. Dietary intake of DON contaminated food can cause adverse symptoms. Its cytotoxicity is mainly associated with the expression of apoptosis and interfering with protein synthesis. Among which, caspase family proteases play a crucial role in different types of apoptosis signaling pathways. Thus, it is important to develop a platform for real-time and in situ monitoring of caspase in living cells.

Results: In this paper, a polypeptide functionalized gold nanoprobe was designed for real-time and in situ detection of caspase-9 in living cells during DON induced apoptosis. Highly anisotropic gold nanostars (AuNSs) with good LSPR effect were synthesized. It could either serve as the surface enhanced Raman scattering (SERS) substrate or quench fluorescence through fluorescence resonance energy transfer (FRET). Polypeptide containing the LEHD (Leu-Glu-His-Asp) sequence was connected to AuNSs through Au-S bonds. During DON induced cell apoptosis, caspase-9 was activated, which could specifically cleave the recognition site LEHD, causing the polypeptide chain modified with Rhodamine B (Rb) signal group to fall off and move away from AuNSs, ultimately reducing the SERS signal and enhancing the fluorescence signal in the system. The experimental results showed that the nanoprobe had high sensitivity, with a linear range of 5 ng/mL to 400 ng/mL and a minimum detection limit of 0.38 ng/mL.

Significance: This method achieved dual signal quantification and visualization imaging of fluorescence and SERS for caspase-9 in living cells. The application of nanomaterials has been broadened and the assay was well versatile in different human cell lines. It provided a new platform in studying the relationship between food safety and cellular homeostasis mechanisms.