Chemical characterization of materials at the nanoscale provides insights into their compositions and organizations. Infrared spectroscopy has been a powerful tool that directly indicates the identity and amount of functional groups of molecules by measuring the absorption of infrared light. However, nanoscale spatial resolution is hard to achieve for conventional Fourier-transform infrared (FTIR) spectroscopy because of the optical diffraction limit. Herein we reported a recently developed infrared microscopy and spectroscopy technique also based on infrared absorption of molecules – peak force infrared microscopy – that combines atomic force microscopy and infrared laser illumination. Sub 10 nanometer spatial resolution has been demonstrated on various samples, including block copolymers, hexagonal boron nitride flakes, and amyloid fibrils. Simultaneous chemical and mechanical mapping can be obtained with peak force infrared microscopy in that both information is encoded in the cantilever deflection curves during peak force tapping cycles. The high spatial resolution and multimodal measurement capability render peak force infrared microscopy a label-free chemical imaging technique for explorations of nanoscale across broad disciplines.
KEYWORDS: Boron, Phonons, Polaritons, Near field optics, Infrared imaging, Infrared radiation, Optical microscopy, Near field, Absorption, Atomic force microscopy
The excitation of surface phonon-polariton (SPhP) modes in polar materials using scattering type near-field optical microscopy (s-SNOM) has recently become an area of interest because of its potential for application as naturally occurring meta-materials and in low-loss energy transfer. Within this area, hexagonal boron nitride (h-BN) and boron nitride nanotubes (BNNTs) are the primary structures under investigation. Here we present pump-probe continuous wave (CW) scattering-type scanning near-field optical microscopy (s-SNOM) - a novel two color pump-probe infrared technique which uses two continuous wave tunable light sources and is based on s-SNOM. The technique allows us to spatially resolve coupling of the longitudinal optical and surface phonon polariton modes in BNNTs. However, no similar coupling is observed in two-dimensional h-BN crystals.
The ease and flexibility of functionalization and inherent light scattering properties of plasmonic nanoparticles make
them suitable contrast agents for measurement of cell surface markers. Immunophenotyping of lymphoproliferative
disorders is traditionally undertaken using fluorescence detection methods which have a number of limitations. Herein,
surface-enhanced Raman scattering (SERS) gold nanoparticles conjugated to monoclonal antibodies are used for the
selective targeting of CD molecules on the surface of chronic lymphocytic leukemia (CLL) cells. Raman-active
reporters were physisorbed on to the surface of 60 nm spherical Au nanoparticles, the particles were coated with 5kDa
polyethylene glycol (PEG) including functionalities for conjugation to monoclonal IgG1 antibodies. A novel method for
quantifying the number of antibodies bound to SERS probes on an individual basis as opposed to obtaining averages
from solution was demonstrated using metal dots in transmission electron microscopy (TEM). The specificity of the
interaction between SERS probes and surface CD molecules of CLL cells was assessed using Raman spectroscopy and
dark field microscopy. An in-depth study of SERS probe targeting to B lymphocyte marker CD20 was undertaken, and
proof-of-concept targeting using different SERS nanoparticle dyes specific for cell surface CD19, CD45 and CD5
demonstrated using SERS spectroscopy.
60 nm diameter gold nanoparticles (AuNP) were coated with a ternary mixture of lipids and targeted to human
lymphocytes. Previously, the versatility, stability and ease of application of the lipid coating was demonstrated by the
incorporation of three classes of Raman-active species. In the present study, lipid encapsulated AuNPs were conjugated
to two targeting species, namely whole antibodies and antibody fragments (Fab), by two methods. Furthermore, in vitro
targeting of lipid-encapsulated Au nanoparticles to patient-derived chronic lymphocytic leukemia (CLL) cells was
demonstrated by Raman spectroscopy, Raman mapping, and darkfield microscopy. These results further demonstrate
the versatility of the lipid layer for imparting stability, SERS activity, and targeting capability, which make these
particles promising candidates for biodiagnostics.
Electronic and mechanical studies of metal-molecules-metal junction have been accomplished to evaluate how bias-induced adhesion forces influence the charge transport efficiency of these junctions. The conducting probe atomic force microscope (CP-AFM) measures the current through an organic film sandwiched between two metal electrodes as a function of the tip-sample separation simultaneously with detection of the force between the probe and the surface. By applying a voltage between the sample and the tip, an attractive electrostatic capacitance force is added to the adhesion force. This paper describes probe-sample capacitance forces in conducting probe (CP) microscopy of polythiophene and alkanethiol monolayers, both theoretically and experimentally. The importance of taking into account an offset in the interaction force determined by the bias-induced adhesion force in the electronic measurements is demonstrated using current-voltage (I-V) characteristics of the polythiophene monolayer and the dependence of the adhesion force as a function of applied tip bias.
Technical developments and applications in infrared near-field microscopy for chemical imaging are discussed. A method to amplify the weak scattered signal in an apertureless microscope is demonstrated.
KEYWORDS: Near field, Near field scanning optical microscopy, Finite-difference time-domain method, Signal detection, Near field optics, Metals, Semiconductor lasers, Polarization, Ion beams, Free space
We have investigated nano-apertures with different geometries on VSALs using far-field measurements, near-field measurements, and finite difference time domain (FDTD) simulation methods. We were able to quantitatively verify the aperture geometry dependent power throughput in all three methods. From both far-field measurements and FDTD simulation results, we conclude that for the apertures of the same area, a rectangular aperture with the long side perpendicular to the active layer has the largest throughput, while a circular aperture has the second largest, and the rectangular aperture with the long side parallel to the active layer has the least throughput among the three. We have attempted to correlate the relationship between far-field power and near-field power. Employing an apertureless near-field scanning optical microscopy (NSOM), we found that for the two rectangular apertures being studied, the near-field power throughput results was consistent to that of far-field measurement. Using VSALs as a near-field aperture testbed was also proposed and demonstrated.
Recent infrared spectroscopic studies of the ultrafast responses of optically triggered changes in proteins are discussed. Examples from research on bacteriorhodospin and reaction centers illustrate the potential of time resolved infrared spectroscopy in the field of protein dynamics.
We have made ultrafast time resolved pump probe measurements on the intramolecular electron transfer (ET) of the betaines, specifically betaine3O and penta-t-butyl betaine. The data have been analyzed to determine the ET rate in a range of solvent environments, at various temperatures and at a variety of pump and probe wavelengths. In all cases, the observed ET rate is fast, often faster than predicted by common ET theories. As a result, we have extended some common theories to successfully predict the measured ET rates. In addition to the ET dynamics, the data also display evidence for local heat deposition in the immediate vicinity of the betaine molecule which our extended model qualitatively predicts.
This paper describes new ultrafast fluorescence measurements on the excited state
dynamics of polar molecules in polar solvents. Two processes are studied, excited state solvation
dynamics and excited state intramolecular electron transfer.
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