Antenna Enhanced Infrared Photoinduced Force Imaging in Aqueous Environment with Super-Resolution and Hypersensitivity

Abstract

Tip enhanced IR spectra and imaging have been widely used in cutting-edge studies for the indepth understanding of the composition, structure and function of interfaces at the nanoscale. However, molecular monolayer sensitivity has only been demonstrated at solid/gas interfaces. In aqueous environment, the reduced sensitivity limits the practical applications of tip enhanced IR nanospectroscopy. Here, we demonstrate an approach to hypersensitive nanoscale IR spectra and imaging in aqueous environment with the combination of photoinduced force (PiF) microscopy and resonant antennas. The highly confined electric field inbetween the tip and antenna amplifies the PiF, while the excitation via plasmon internal reflection mode minimizes the environmental absorption. A polydimethylsiloxane (PDMS) layer (~1-2 nm thickness) functionalized on the AFM tip and self-assembled monolayer of bovine hemoglobin on antenna has been successfully identified in water with antennas of different sizes. Sampling volume of ~604 chemical bonds from PDMS was demonstrated with sub-10 nm spatial resolution confirmed by electric field distribution mapping on antennas. This platform demonstrates for the first time the application of PiF microscopy in aqueous environments, providing a brand-new configuration to achieve highly enhanced nanoscale IR signals, which is promising for future research of interfaces and nanosystems in aqueous environments.

Revealing the Hydrogen Bonding Interaction of DNA with Unnatural Bases via Plasmonic Antenna Enhanced Infrared Spectroscopy

“…Figure d shows the PiF image simultaneously recorded at 1580 cm –1 . If compared with the PiF images recorded at nonresonant wavenumbers of 1100 and 800 cm –1 (Figure S4), a much higher PiF signal appears at the antenna tip than on the antenna face and the Si substrate when illumination with the antenna resonance wavenumber is used, as we revealed previously. ,, This is further confirmed by the numerical simulation using COMSOL. The simulated electric field is extremely confined at the tip end of the antenna in water with back excitation and decays rapidly within a short distance from the antenna surface (Figure e).…”

“…Surface enhanced infrared absorption spectroscopy (SEIRAS) has shown a promising candidate technique since it provides fingerprint information relating to vibrations and rotations of chemical bonds and obtains a dynamic variation of molecular structures and functions. − In conventional SEIRAS, metal island and metal nanofilm have been used as enhanced substrates that provide mild signal amplification with an enhancement factor of ∼100, allowing IR detection with monolayer level sensitivity. − Nevertheless, this technique is hardly used to monitor DNA behavior with low concentrations owing to the limitation of the small infrared cross section of the targets and the strong background water absorption interference. Recently, plasmonic antenna enhanced IR (PAEIR) spectroscopy, also known as resonant SEIRAS, has been proposed to realize highly sensitive IR measurement. − The metallic antennas can resonant with incident light and confine the electric field ( E ) within “hot spots”, amplifying IR signals of targets with an enhancement factor up to 10 6 . The huge absorption of water can be efficiently reduced by exciting the antenna array with plasmonic internal reflection (PIR) mode or attenuated total reflection (ATR) mode (Figure S1).…”

Amplitude- and Phase-Resolved Infrared Nanoimaging and Nanospectroscopy of Polaritons in a Liquid Environment

“…Graphene plasmons and phonon polaritons in polar vdW materials can be imaged and studied in great detail by scanning probe techniques, such as scattering-type scanning near-field optical microscopy (s-SNOM), photothermal expansion (PTE) microscopy and photo-induced force microscopy (PiFM). So far, polariton imaging in the aqueous phase has been demonstrated only by PTE 19 and PiFM 20 and considered not being possible by s-SNOM. On the other hand, the interferometric detection scheme of s-SNOM offers many advantages over PTE and PiFM, including direct near-field phase measurements for retrieving phase and group velocities of polaritons 21 , reconstruction of complex-valued dielectric functions 22,23 , and tomographic sample reconstruction 24 .…”

“…Graphene plasmons and phonon polaritons in polar vdW materials can be imaged by scattering-type scanning near-field optical microscopy (s-SNOM), photothermal expansion (PTE) microscopy, and photoinduced force microscopy (PiFM). So far, polariton imaging in the aqueous phase has been demonstrated only by PTE and PiFM, although the interferometric detection scheme of s-SNOM offers many advantages, including direct near-field phase measurements for retrieving phase and group velocities of polaritons, reconstruction of complex-valued dielectric functions, , and tomographic sample reconstruction . Further, s-SNOM allows for nanoimaging and nanospectroscopy in a broad spectral range from visible to sub-THz frequencies, using continuous wave (CW) and ultrafast lasers, , synchrotrons, , or free electron lasers .…”

Liquid-Phase Peak Force Infrared Microscopy for Chemical Nanoimaging and Spectroscopy

“…Moreover, the s-SNOM apparatus has a higher instrumental complexity and often requires two separate types of light sources for IR imaging and spectroscopy. Recently, PiFM has also been implemented in the liquid; however, the high hydraulic drag of the fluid damps the mechanical oscillations of the cantilever, reducing the efficiency for the heterodyne force detection scheme …”