Name: Creating Topographical and NanoIR Images of the Multipolymer Sample
Uploaded: 2023-06-23T14:55:00
Description: Watch this Scientific Journal Video about Creating Topographical and NanoIR Images of the Multipolymer Sample at JoVE.com

creating topographical and nanoir images of the multipolymer sample

Exploring Multiphase Polymeric Systems by Nanoscale Infrared Spectroscopy

Atomic force microscopy (AFM) has become essential to image and characterize the morphology of a wide variety of samples with nanoscale resolution1,2,3. By measuring the deflection of an AFM cantilever resulting from the interaction of the sharp tip with the sample surface, nanoscale functional imaging protocols for local stiffness measurements and tip-sample adhesion have been developed4,5. For soft condensed matter and polymer analysis, AFM measurements exploring the nanomechanical and nanochemical properties of local domains are highly sought after6,7,8. Before the emergence of nanoscale infrared (nanoIR) spectroscopy, AFM tips were chemically modified to assess the presence of different domains from the AFM force curve and deduct the nature of the tip-sample interaction. For instance, this approach was used to unveil the transformation of microdomains of poly(tert-butyl acrylate) at the surface of cyclohexane-treated polystyrene-block-poly(tert-butyl acrylate)block copolymer thin films at the sub 50 nm level9.
The combination of infrared (IR) light with AFM has had a significant impact on the field of polymer science6. Conventional IR spectroscopy is a widely used technique for studying the chemical structure of polymeric materials10,11, but it fails to provide information on individual phases and interphase behavior, as the regions are too small compared to the size of the IR beam used to probe the sample. The problem holds with IR microspectroscopy, as it is restricted by the optical diffraction limit6. Such measurements average the contributions of the entire region excited by the IR light; the signals resulting from the presence of nanoscale phases inside the probed region either exhibit complex fingerprints that should be deconvoluted during post-processing or are lost due to a signal level below the detectable level. Hence, it is essential to develop tools capable of nanoscale spatial resolution and high IR sensitivity to explore nanoscale chemical features in complex media.
Schemes to achieve nanoIR spectroscopy have been developed, first using a metallic AFM tip as a nanoantenna12,13, and more recently exploiting the AFM cantilever&#39;s ability to monitor changes in the photothermal expansion incurred during IR illumination of the sample12,14,15. The latter uses a pulsed, tunable IR light source tuned to an absorption band of the material probed, which causes the sample to absorb radiation and undergo photothermal expansion. This approach is well-suited for organic and polymeric materials. The pulsed excitation makes the effect detectable by the AFM cantilever in contact with the sample surface in the form of an oscillation. The amplitude of one of the contact resonances of the system observed in the frequency spectrum is then monitored as a function of illumination wavelength, which constitutes the nanoIR absorption spectrum of the material beneath the AFM tip15. The spatial resolution of nanoIR imaging and spectroscopy is limited by various effects of the photothermal expansion of the material. It has been evaluated that photothermal nanoIR spectroscopy using contact mode AFM can acquire the vibrational absorption spectra properties of materials with sub 50 nm scale spatial resolution14, with recent work demonstrating the detection of monomers and dimers of &#945;-synuclein16,17. However, quantitative studies of the performance of nanoIR measurements on heterogeneous polymeric materials assembled in various configurations, such as the case of absorbers of finite dimensions embedded in the volume of various polymeric films, remain limited.
This article aims to create a polymeric assembly with an embedded feature of a known dimension to evaluate the sensitivity of photothermal expansion and spatial resolution of nanoIR during surface analysis. The protocol covers the preparation of a polyvinyl alcohol (PVA) polymer thin film on a silicon substrate and the placement of a three-dimensional polystyrene (PS) bead onto or embedded in the PVA film, which constitutes the formation of the model system. NanoIR imaging and spectroscopy measurements are described in the context of evaluating the signals generated by the same PS bead positioned on or beneath the PVA film. The influence of the bead position on the nanoIR signals is evaluated. Methods to assess the spatial footprint of the bead in the nanoIR map are discussed, and the effects of several parameters are considered.

Author Spotlight: Advances in Nanoscale Infrared Spectroscopy to Explore Multiphase Polymeric Systems  

Advances in Nanoscale Infrared Spectroscopy to Explore Multiphase Polymeric Systems

Our group focuses on advancing the field of nanoscale functional imaging and spectroscopy to study complex systems with an emphasis on nanoscale infrared spectroscopy. Our research aims to develop methods that allows us to make sense of the behavior of heterogeneous systems by studying their nanoscale properties. Understanding the role of actuations at different frequencies to extract information about the sample is essential.As this is complicated, developing samples that can be modeled with physics simulation packages is also important. A nanoscale infrared spectroscopy determining the penetration depth of the measurement and understanding how subsurface features affect the signal detected by the AFM tip is very important. This protocol proposes a simple approach to create a model system that enables quantifying the penetration depth of nanoscale infrared spectroscopy, and the effect of JUUL heating on the sample response involved in the measurement of multi-phase polymer materials.Our team will continue to focus on developing new ideas to improve nanoscale infrared spectroscopy and other nanoscale functional tools to explore real-life systems at a small scale.

Watch this Scientific Journal Video about Creating Topographical and NanoIR Images of the Multipolymer Sample at JoVE.com

Creating Topographical and NanoIR Images of the Multipolymer Sample  

For NanoIR measurements, position the atomic force microscope, or AFM, tip on the feature of interest identified from the topography image. Select the tuning fork icon in the NanoIR control panel to determine the contact resonance frequencies of the cantilever. Then set an illumination wave number to excite photothermal expansion in the material.Next, set a range of laser pulse frequency to sweep and set the duty cycle of the NanoIR laser. Select acquire within the laser pulse tune window. Position the marker bar at the peak to select the second contact resonance of the tip sample system for NanoIR measurements.Click on the optimized button to align the center of the IR laser focal region with the position of the cantilever tip. Acquire IR laser illumination background. Select the wave number range, step size, and number of averages for the NanoIR spectrum.Then perform a background correction of the spectra by dividing the photothermal amplitude by the attenuated background. Enable phase locked loop, or PLL, auto tune in the laser pulse tune window. Then adjust the maximum and minimum frequency to create a sweep range centered at the second resonance mode in the general control panel.Click on zero in the PLL control panel, and then click on okay in the laser pulse tune window. Enable IR imaging by selecting the checkbox of IR imaging enabled in the NanoIR control panel. In the imaging view control panel, choose height, amplitude two, and phase two to acquire the topographical and chemical images of the sample.Then set the acquisition direction to trace or retrace. Select the scan icon in the AFM scan control panel. Then select the now or end of frame icon in the capture control panel to save the image.To export the data, right click on the image or spectrum file names within the data lists. Select export, and then choose the file format to export. Finally, save the file in the desired computer folder.The NanoIR spectrum of polystyrene yielded two IR bands corresponding to the stretch mode of the phenyl moiety at 1, 600 wave number and a subset of the ring stretching at 1, 730 wave number. The NanoIR spectrum of PVA exhibited better agreement with the FDIR spectrum, with the predominant absorption band centered at 1, 730 wave number. The NanoIR spectra were used to select the illumination wave numbers for chemical imaging of the polystyrene bead deposited at the surface of PVA and of the polystyrene bead coated with PVA.Next, spectra were collected on top of a polystyrene bead covered with PVA at different laser powers. The polystyrene signal at 1, 600 wave number was significantly lower than that of PVA at 1, 750 wave number. However, it was noted that increasing the laser power led to a higher ratio.

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Research

JoVE Journal

Engineering

Multiphase polymeric systems encompass local domains with dimensions that can vary from a few tens of nanometers to several micrometers. Their composition is commonly assessed using infrared spectroscopy, which provides an average fingerprint of the various materials contained in the volume probed. However, this approach does not offer any details on the arrangement of the phases in the material. Interfacial regions between two polymeric phases, often in the nanoscale range, are also challenging to access. Photothermal nanoscale infrared spectroscopy monitors the local response of materials excited by infrared light with the sensitive probe of an atomic force microscope (AFM). While the technique is suitable for interrogating small features, such as individual proteins on pristine gold surfaces, the characterization of three-dimensional multicomponent materials is more elusive. This is due to a relatively large volume of material undergoing photothermal expansion, defined by the laser focalization onto the sample and by the thermal properties of the polymeric constituents, compared to the nanoscale region probed by the AFM tip. Using a polystyrene (PS) bead and a polyvinyl alcohol (PVA) film, we evaluate the spatial footprint of photothermal nanoscale infrared spectroscopy for surface analysis as a function of the position of PS in the PVA film. The effect of the feature position on the nanoscale infrared images is investigated, and spectra are acquired. Some perspectives on the future advances in the field of photothermal nanoscale infrared spectroscopy are provided, considering the characterization of complex systems with embedded polymeric structures.

This protocol describes the application of atomic force microscopy and nanoscale infrared spectroscopy to evaluate the performance of photothermal nanoscale infrared spectroscopy in the characterization of three-dimensional multi-polymeric samples.

Multiphase polymeric systems encompass local domains with dimensions that can vary from a few tens of nanometers to several micrometers. Their composition ...

Placing Polystyrene (PS) Beads onto the PVA-Coated Surface

To begin, clean a silicon substrate using isopropyl alcohol, and let it air dry. Using a pipette, place one microliter of polystyrene beads suspended in water onto the center of the substrate. Place the polystyrene beads added substrate in a storage compartment containing bentonite clay dessicant to allow the water to evaporate.Examine the dried polystyrene beads substrate and a previously prepared polyvinyl alcohol or PVA coated substrate under an optical microscope. Using ultra fine tweezers, gently losing the beads, then collect a few beads using a fine hair paintbrush and gently tap the hairs of the paintbrush over the freshly PVA coated wafer. Repeat collecting the beads until individual polystyrene beads adhere to the PVA surface as confirmed by optical microscopy.Deposition of the polystyrene beads was confirmed by scanning electron microscopy imaging of the five micrometer bead on top of a pristine silicon substrate on top of a PVA film and covered with PVA.