Nanoplasmonic gene switches enable temporal and spatial regulation of intracellular genetic activity. Using remote-controlled NIR light as a trigger to release free oligonucleotides and “activate” their functionality, endogenous intracellular genes can be silenced on-demand. In addition to the inhibitory effects, exogenous foreign genes can also be introduced and expressed on-demand.
Because of their large surface-to-volume ratio, nanoplasmonic gene switches are ideal carriers of oligonucleotides, such as single-stranded DNA (ssDNA), short interfering RNA (siRNA), and plasmid DNA. For example, short ssDNA, otherwise known as antisense DNA, can be hybridized to thiolated complementary sense DNA and bound to the switch’s surface through the gold-thiol covalent bond [3
]. While attached to their carriers, oligonucleotides are rendered inactive due to steric hindrances between the tightly-packed DNA. In the presence of continuous-wave incident light that is matched to their plasmon resonance wavelength, antisense DNA is photothermally dehybridized from its carrier to freely interact with the local environment. Antisense DNA has also been photothermally dehybridized from other antenna structures, such as gold nanoshells [27
] and gold nanoprisms [28
]. Release of circular plasmid DNA [29
], linearized plasmid DNA [4
], siRNA [2
], and directly conjugated single-stranded DNA [31
] has also been demonstrated by photothermally melting the carrier. This strategy of photothermal dehybridization offers several notable advantages. Firstly, no chemical modifications are made to the antisense DNA strand itself since a thiolated complementary strand is used to directly conjugate to the switch’s surface. Because chemical modifications can interfere with nucleic acid functionality and gene silencing efficacy, unmodified antisense DNA is highly desirable. Secondly, gold-thiol covalent bonds are stable after illumination, such that the switch’s surface remains covered with the thiolated complementary sense strands. With respect to cytotoxicity, this surface coating of complementary strands after illumination is critical. While the gold core is widely accepted as being biocompatible, bare nanoparticles have been shown to interact with proteins and induce mis-folding at physiological conditions [32
]. Maintaining surface coverage with complementary strands after illumination also prevents reattachment of antisense DNA strands back onto the switch since rehybridization events are thermodynamically unfavorable due to steric hindrances and electrostatic repulsive forces at the switch’s surface [33
]. Finally, the structural integrity of the switches is uncompromised after illumination. Maintaining structure after illumination allows unique nano-scale optical properties to be retained, thereby enabling the same incident light wavelength to be used. Repetitive or finely-graded release of cargo is conceivable for future applications requiring precise temporal patterns of cargo release. Maintaining structure after illumination is also crucial for in vivo applications, where the size, geometry, coating material, and core material of nanoparticles are precisely designed and carefully characterized for proper biodistribution [34
] and eventual environmental distribution [35
Using antisense DNA-conjugated nanoplasmonic gene switches, endogenous intracellular genes can be silenced on-demand. On-demand silencing of ERBB2 expression has been qualitatively demonstrated using immunofluorescence imaging and quantitatively shown using flow cytometry [3
]. Intracellular genes can also be silenced on-demand using siRNA-conjugated nanoplasmonic gene switches [2
]. In addition to the inhibitory effects of interfering oligonucleotides, exogenous foreign genes can also be expressed on-demand using plasmid-conjugated nanoplasmonic gene switches [4
]. In this way, nanoplasmonic gene switches can enable spatially precise regulation of intracellular activity to give rise to location-specific function.