Near-Infrared Photothermally Activated DNAzyme-Gold Nanoshells for Imaging Metal Ions in Living Cells

Abstract

DNAzymes have enjoyed success as metal ion sensors outside cells. Their susceptibility to metal-dependent cleavage during delivery into cells has limited their intracellular applications. To overcome this limitation, a near-infrared (NIR) photothermal activation method is presented for controlling DNAzyme activity in living cells. The system consists of a three-stranded DNAzyme precursor (TSDP), the hybridization of which prevents the DNAzyme from being active. After conjugating the TSDP onto gold nanoshells and upon NIR illumination, the increased temperature dehybridizes the TSDP to release the active DNAzyme, which then carries out metal-ion-dependent cleavage, resulting in releasing the cleaved product containing a fluorophore. Using this construct, detecting Zn²⁺ in living HeLa cells is demonstrated. This method has expanded the DNAzyme versatility for detecting metal ions in biological systems under NIR light that exhibits lower phototoxicity and higher tissue penetration ability.

Keywords: DNAzymes; HeLa cells; gold nanoshells; metal ions; photothermal activation.

Figure 1

Native PAGE of TSDP formation and stability in the presence of metal ion. The gel was stained with ethidium bromide for visualization. M: DNA ladder; Lane 1: Linker DNA (the ethidium bromide does not stain short single stranded Linker DNA); Lane 2: Enzyme strand; Lane 3: Substrate strand; Lane 4: Linker DNA+ Enzyme strand; Lane 5: Enzyme strand+ Substrate strand; Lane 6: Linker DNA+ Enzyme strand+ Substrate strand; Lane 7: lane 5+ Zn²⁺; Lane 8: lane 6+ Zn²⁺. The dotted lines (from top to bottom) indicate TSDP, DNAzymes and cleaved DNAzymes.

Figure 2

Characterization of AuNS and TSDP-AuNS. a) TEM image of bare AuNS; b). Photothermal response of bare AuNS at different concentrations.; c) TEM image of TSDP-AuNS; d). Absorption spectra of bare AuNS in water (black line), bare AuNS in buffer (blue line) and TSDP-AuNS in buffer (red line). Scale bar: 200 nm.

Figure 3

DNAzyme release study from TSDP-AuNS under NIR irradiation. a). Fluorescence spectra of TSDP-AuNS before NIR irradiation and supernatant of TSDP-AuNS after NIR irradiation under different conditions; b). 12% PAGE. M: DNA ladder; Lane 1 and 4: TSDP-AuNS without NIR irradiation or addition of Zn²⁺; Lane 2: NIR irradiation for 15 min, with Zn²⁺; Lane 3: 70 °C water bath for 15 min, with Zn²⁺; Lane 5: NIR irradiation for 15 min, without Zn²⁺; Lane 6: 70 °C water bath for 15 min, without Zn²⁺. The dotted lines (from top to bottom) indicate released active DNAzymes and cleaved DNAzymes.

Figure 4

Confocal microscopy images of HeLa cells. Scale bar: 20 μm.

Scheme 1

Design of NIR activated TSDP-AuNS for intracellular metal ion sensing.