Two-color interferometric scattering (iSCAT) microscopy reveals structural dynamics in discrete plasmonic molecules

Area of Science:

• Nanotechnology
• Optical Nanosensors
• Plasmonics

Background:

• Plasmonic molecules, assemblies of noble metal nanoparticles (NPs), are crucial for optical nanosensors.
• Larger NPs (∼40 nm) were preferred for optical detection due to limitations with smaller NPs in conventional microscopy.
• Detecting smaller NPs and their assemblies is challenging but essential for advanced nanosensing.

Purpose of the Study:

• To apply two-color interferometric scattering (iSCAT) microscopy for characterizing smaller plasmonic molecules.
• To investigate polyethylene glycol (PEG) tethered dimers of 10 nm and 20 nm silver (Ag) nanoparticles (NPs).
• To demonstrate iSCAT's capability in distinguishing NP dimers from monomers and analyzing their dynamics.

Main Methods:

• Utilized 405 nm and 445 nm two-color interferometric scattering (iSCAT) microscopy.
• Characterized polyethylene glycol (PEG) tethered dimers and monomers of 10 nm and 20 nm Ag NPs.
• Analyzed iSCAT contrast changes and temporal fluctuations to identify dimer formation and structural dynamics.

Main Results:

• Dimers of both 10 nm and 20 nm Ag NPs were successfully distinguished from monomers via iSCAT contrast.
• Dimer formation in 20 nm Ag NPs induced a sign change in iSCAT contrast, signaling binding events.
• Different PEG spacer lengths (0.4 kDa and 3.4 kDa) in 20 nm Ag NP dimers resulted in distinct iSCAT contrast distributions.

Conclusions:

• Two-color iSCAT microscopy effectively characterizes smaller plasmonic molecules, overcoming limitations of conventional methods.
• iSCAT contrast provides a reliable metric for detecting dimer formation and structural dynamics in NP assemblies.
• This work paves the way for developing new interferometric plasmon rulers and advanced optical nanosensors.