DNAzyme-Metal-Organic Framework Two-Photon Nanoprobe for in situ Monitoring of Apoptosis-associated Zn2+ in living cells and tissues.

Shi, Xinxin; Meng, Hong-Min; Geng, Xin; Qu, Ling-Bo; Li, Zhaohui

Monitoring Zn2+ in living cells is critical for fully elucidating the biological process of apoptosis. However, the quantitative intracellular sensing of Zn2+ using DNAzyme remains challenging because of issues related to penetration of the signal through tissue, targeted cellular uptake and activation, and susceptibility toward enzymatic degradation. In this study, we developed a novel phosphate ion-activated DNAzyme-metal-organic frameworks (MOFs) nanoprobe for two-photon imaging of Zn2+ in living cells and tissues. The design of this nanoprobe involved the loading of a Zn2+-specific, RNA-cleaving DNAzyme onto the MOFs through strong coordination between the phosphonate O atoms of the DNAzyme backbone and Zr atoms in the MOFs. This coordination restrained the extracellular activity of DNAzyme, however, after cell entry, the DNAzyme was released from the MOFs through competitive bind by the phosphate ions present at a high intracellular concentration. Following their release, the two-photon (TP) fluorophore-labeled substrate strands of DNAzyme was cleaved with the aid of Zn2+, which resulted in a strong fluorescent signal. The incorporation of a TP fluor-ophore into the nanoprobe facilitated near-infrared (NIR) excitation, which allowed the highly sensitive and specific im-aging of Zn2+ in living cells and tissues at greater depths than possible previously. The TP-DNAzyme-MOFs nanoprobe achieved a low detection limit of 3.53 nM, extraordinary selectivity toward Zn2+, and tissue signal penetration of 120 m. More importantly, this nanoprobe was successfully used to monitor cell apoptosis, and this application of the DNAzyme-MOFs probe holds great potential for future use in biological studies and medical diagnostics.