In TERS, a metallic scanning probe tip is brought into proximity to a sample surface. When irradiated with laser light, the tip acts as an optical antenna that enhances the localized electromagnetic field at the tip apex via surface plasmon resonance. This leads to enhanced Raman scattering from molecules in the tip-sample nanogap region, providing chemical information with spatial resolution on the order of 10-20 nm.
Tip-enhanced Raman spectroscopy (TERS) utilizing 4-nitrobenzene-thiol (4NBT) on gold nanoparticles allows for ultrasensitive characterization and imaging of single molecules with unprecedented spatial resolution.
4-Nitrobenzenethiol as an Ideal TERS Probe Molecule
An ideal TERS probe molecule should have a large Raman cross-section and form stable and reproducible self-assembled monolayers (SAMs) on metal surfaces. 4-Nitrobenzenethiol) satisfies both criteria, making it a widely utilized TERS probe.
4NBT contains an aromatic ring and a thiol group (-SH). The aromatic ring produces strong resonant Raman enhancement while the thiol allows 4NBT to bind to gold substrates via gold-thiol chemistry reliably. These attributes render 4NBT an excellent model system for TERS analysis and mapping at the single-molecule level.
4-NBT Enhances Nanoscale Imaging and Spectroscopy of Single Molecules Using TERS
A major advantage of TERS is its ability to identify and investigate individual molecules, which is challenging for conventional diffraction-limited techniques. By using a sharp gold or silver TERS tip, the 4NBT Raman signal can be spatially confined to detect the vibrational signature of just a few or even single molecules within the tip-enhanced near-field volume.
TERS allows simultaneous topographic and hyperspectral Raman characterization of a surface with nanoscale resolution. Consequently, the structure, orientation, and interactions of single 4NBT molecules adsorbed on surfaces can be studied at the molecular scale. This enables unique insights into nanoscale environmental influences on molecular properties and processes.
Applications of 4NBT TERS
The ultrahigh sensitivity of 4NBT TERS has enabled investigations into diverse nanoscience phenomena at the single molecule level:
Studying Plasmonic Hotspots
TERS can map plasmonic hotspots between nanoparticle dimers by probing spatial variations in 4NBT Raman intensity. This reveals how nanogap dimensions affect electromagnetic field confinement and enhancement.
Probing Molecular Junctions
By binding to metallic nanoparticle junctions, single 4NBT molecules can act as Raman reporters of quantum conductance in nanoscale junctions. TERS has elucidated the structure-function relationship in single-molecule junctions.
Imaging Chemical Reactions
TERS has tracked the stepwise photoreduction of single 4NBT molecules into 4-aminothiophenol, visualizing reaction intermediates and dynamics at the molecular scale.
Analyzing Nanoparticle-Molecule Interfaces
The SERS response of 4NBT bound to different metal nanoparticles reveals how nanoparticle composition, size, and shape influence electromagnetic enhancement mechanisms in plasmonic nanostructures.
The Future Of 4NBT TERS for Other Emerging Applications
The molecular sensitivity and nanoscale resolution of 4NBT TERS are poised to impact diverse areas such as molecular electronics, metamaterials, nanophotonics, quantum optics, and single-molecule surface science.
Ongoing TERS developments like tip-on-tip geometries, ultrafast TERS, and TERS combined with scanning tunneling microscopy promise even greater nanoscale characterization and manipulation capabilities. 4NBT will continue to be instrumental as a platform for pioneering new tip-enhanced techniques and applications. Ongoing advances promise exciting possibilities in diverse disciplines including surface science, nanophotonics, quantum physics, molecular electronics, and materials research.