An enzymatic polymerization-activated silver nanocluster probe for in situ apoptosis assay

As an emerging category of fluorophores, nucleic acid-stabilized silver nanoclusters (DNA/AgNCs) have attracted a great deal of interest and have been widely applied for interdisciplinary research. In this work, we have constructed a novel DNA/AgNC probe for cell apoptosis detection and imaging based on an enzyme-polymerized polyadenylic acid (poly-dA) DNA chain and a toehold strand displacement reaction. This method can effectively "tag" intracellular genomic DNA fragments, a biochemical hallmark of apoptosis, with poly-dA DNA chains up to 400-bases produced by terminal deoxynucleotidyl transferase (TdT)-activated polymerization. The strand displacement initiated by the target poly-dA DNA chain releases the quencher labeled-DNA from the DNA/AgNC probe, leading to a significant fluorescence lighting-up of DNA/AgNCs for the sensitive detection of cell apoptosis, with a high signal-to-background ratio (S/B = 58). Using the DNA/AgNC-based assay, as few as 20 apoptotic cells can be detected in vitro. Furthermore, the feasibility of our approach was demonstrated by the in situ quantitative analysis of apoptosis in HepG2 cells without the need for tedious washing and separation steps.