A. A. Mani

Study of the water/poly(ethylene glycol) interface by IR-visible sum-frequency generation spectroscopy

We used infrared-visible sum-frequency generation spectroscopy to investigate the vibrational properties of the water/poly(ethylene glycol) interface in the 2800–3800 cm−1 spectral range. The vibrational fingerprint of the interface differs significantly from the one associated with the air/poly(ethylene glycol) interface. It is shown that the poly(ethylene glycol) molecular arrangement, originally relatively well-ordered, becomes disorganised in the presence of water. Moreover, a new OH band is identified demonstrating the strong interaction of water with the polymer.

Development of a two-color picosecond optical parametric oscillator, pumped by a Nd:YAG laser mode locked using a nonlinear mirror, for doubly-resonant sum frequency generation spectroscopy

Abstract We set up a doubly-resonant sum frequency generation (DR-SFG) spectrometer based on the use of an all-solid-state flash-lamp-pumped Nd:YAG laser that synchronously pumps two parametric oscillators. Pulses as short as 12 ps FWHM are generated by mode locking a Nd:YAG oscillator using a frequency doubling nonlinear mirror combined with a two-photon absorber. The available pump power is shared between a LiNbO 3 /AgGaS 2 optical parametric oscillator (OPO), tunable from 3800 to 1100 cm −1 and a BBO OPO tunable from 410 to 2600 nm. Spectral resolution and pulse are 2 and 3 cm −1 in the infrared and visible spectral ranges, respectively. First DR-SFG spectra of self-assembled monolayers on Au are presented.

IR-visible sum-frequency vibrational spectroscopy of Biphenyl-3 methylene thiol monolayer on gold and silver: effect of the visible wavelength on the SFG spectrum

Abstract We measured IR–visible sum-frequency generation spectra of CH3–(C6H4)2–(CH2)3–S–H (Biphenyl-3) self-assembled monolayers on a silver and a gold substrate. For the latter substrate, we observed different interference patterns between the resonant signal of the CH vibration and the non-resonant contribution of the substrate as a function of the visible beam wavelength. The non-linear response of the gold substrate is enhanced around 480 nm corresponding to the s–d interband transition. Such effect is not observed for the silver substrate the interband transition of which is located out of the investigated visible spectral range of 450–700 nm.

Electron–phonon couplings at C60 interfaces: a case study by two-color, infrared–visible sum–frequency generation spectroscopy

Abstract We demonstrate the ability of doubly resonant sum–frequency generation (DR-SFG) to investigate electron–phonon couplings at C 60 –metal interfaces. Due to its coupling to electronic transitions, the totally symmetric A g (2) vibration of C 60 exhibits a huge enhancement of its nonlinear response for sum–frequency energies above the molecular electronic gap. We attribute this resonance to the coupling of the pentagonal pinch mode with the t 1u lowest unoccupied molecular orbital (LUMO) of C 60 .

Density functional theory-based simulations of sum frequency generation spectra involving methyl stretching vibrations: effect of the molecular model on the deduced molecular orientation and comparison with an analytical approach

The knowledge of the first hyperpolarizability tensor elements of molecular groups is crucial for a quantitative interpretation of the sum frequency generation (SFG) activity of thin organic films at interfaces. Here, the SFG response of the terminal methyl group of a dodecanethiol (DDT) monolayer has been interpreted on the basis of calculations performed at the density functional theory (DFT) level of approximation. In particular, DFT calculations have been carried out on three classes of models for the aliphatic chains. The first class of models consists of aliphatic chains, containing from 3 to 12 carbon atoms, in which only one methyl group can freely vibrate, while the rest of the chain is frozen by a strong overweight of its C and H atoms. This enables us to localize the probed vibrational modes on the methyl group. In the second class, only one methyl group is frozen, while the entire remaining chain is allowed to vibrate. This enables us to analyse the influence of the aliphatic chain on the methyl stretching vibrations. Finally, the dodecanethiol (DDT) molecule is considered, for which the effects of two dielectrics, i.e. n-hexane and n-dodecane, are investigated. Moreover, DDT calculations are also carried out by using different exchange-correlation (XC) functionals in order to assess the DFT approximations. Using the DFT IR vectors and Raman tensors, the SFG spectrum of DDT has been simulated and the orientation of the methyl group has then been deduced and compared with that obtained using an analytical approach based on a bond additivity model. This analysis shows that when using DFT molecular properties, the predicted orientation of the terminal methyl group tends to converge as a function of the alkyl chain length and that the effects of the chain as well as of the dielectric environment are small. Instead, a more significant difference is observed when comparing the DFT-based results with those obtained from the analytical approach, thus indicating the importance of a quantum chemical description of the hyperpolarizability tensor elements of the methyl group.

Probing a molecular electronic transition by two-colour sum-frequency generation spectroscopy

Abstract We demonstrate that a new emerging technique, two-colour sum-frequency generation (SFG) spectroscopy, can be used to probe the molecular electronic properties of self-assembled monolayers (SAMs). In the CH spectral range (2800–3200xa0cm−1), we show that the sum-frequency generation signal of a porphyrin alkanethiol derivative adsorbed on Pt(1xa01xa01) reaches a maximum intensity at ∼435xa0nm SFG wavelength. This wavelength corresponds to the porphyrin moiety specific π–π∗ molecular electronic transition which is called the Soret or B band. This resonant behaviour is not observed for 1-dodecanethiol SAMs, which are devoid of molecular electronic transition in the investigated visible spectral range.

Doubling the far-field resolution in mid-infrared microscopy

The spatial resolution in far-field mid-infrared (λ>2.5 µm) microscopy and micro-spectroscopy remains limited with the full-width at half maximum of the point-spread function ca. λ/1.3; a value that is very poor in comparison to that commonly accessible with visible and near-infrared optics. Hereafter, it is demonstrated however that polymer beads that are centre-to-centre spaced by λ/2.6 can be resolved in the mid-infrared. The more than 2-fold improvement in resolution in the far-field is achieved by exploiting a newly constructed scanning microscope built around a mid-infrared optical parametric oscillator and a central solid-immersion lens, and by enforcing the linear polarization unidirectional resolution enhancement with a novel and robust specimen error minimization based on a particle swarm optimization. The method is demonstrated with specimens immersed in air and in water, and its robustness shown by the analysis of dense and complex self-assembled bead islands.

Spatial-domain filter enhanced subtraction microscopy and application to mid-IR imaging

We have experimentally investigated the enhancement in spatial resolution by image subtraction in mid-infrared central solid-immersion lens (c-SIL) microscopy. The subtraction exploits a first image measured with the c-SIL point-spread function (PSF) realized with a Gaussian beam and a second image measured with the beam optically patterned by a silicon π-step phase plate, to realize a centrally hollow PSF. The intense sides lobes in both PSFs that are intrinsic to the SIL make the conventional weighted subtraction methods inadequate. A spatial-domain filter with a kernel optimized to match both experimental PSFs in their periphery was thus developed to modify the first image prior to subtraction, and this resulted in greatly improved performance, with polystyrene beads 1.4 ± 0.1 µm apart optically resolved with a mid-IR wavelength of 3.4 µm in water. Spatial-domain filtering is applicable to other PSF pairs, and simulations show that it also outperforms conventional subtraction methods for the Gaussian and doughnut beams widely used in visible and near-IR microscopy.

Image-Based Tracking of Anticancer Drug-Loaded Nanoengineered Polyelectrolyte Capsules in Cellular Environments Using a Fast Benchtop Mid-Infrared (MIR) Microscope

Drug delivery monitoring and tracking in the human body are two of the biggest challenges in targeted therapy to be addressed by nanomedicine. The ability of imaging drugs and micro-/nanoengineered drug carriers and of visualizing their interactions at the cellular interface in a label-free manner is crucial in providing the ability of tracking their cellular pathways and will help understand their biological impact, allowing thus to improve the therapeutic efficacy. We present a fast, label-free technique to achieve high-resolution imaging at the mid-infrared (MIR) spectrum that provides chemical information. Using our custom-made benchtop infrared microscope using a high-repetition-rate pulsed laser (80 MHz, 40 ps), we were able to acquire images with subwavelength resolution (0.8 × λ) at very high speeds. As a proof-of-concept, we embarked on the investigation of nanoengineered polyelectrolyte capsules (NPCs) containing the anticancer drug, docetaxel. These NPCs were synthesized using a layer-by-layer approach built upon a calcium carbonate (CaCO3) core, which was then removed away with ethylenediaminetetraacetic acid. The obtained MIR images show that NPCs are attached to the cell membrane, which is a good step toward an efficient drug delivery. This has been confirmed by both three-dimensional confocal fluorescence and stimulated emission depletion microscopy. Coupled with additional instrumentation and data processing advancements, this setup is capable of video-rate imaging speeds and will be significantly complementing current super-resolution microscopy techniques while providing an unperturbed view into living cells.