This work was supported by the National Science Foundation (NSF CHE-1847830).

1. Making polyvinyl alcohol (PVA) solution
<ol>
	<li>Measure water and PVA polymer pellets (see Table of Materials) to create a 10 mL solution at a 20% PVA to water ratio by weight.</li>
	<li>Heat the water in the glassware over a hot plate set to 100 &#176;C.</li>
	<li>Place the PVA polymer pellets into the heated water. Insert a magnetic stir bar.</li>
	<li>Reduce the heat to 80 &#176;C and stir until the PVA fully dissolves.</li>
	<li>Cover the top of the glassware to prevent contam...

Exploring Multiphase Polymeric Systems by Nanoscale Infrared Spectroscopy

<ol>
	<li>Dufr&#234;ne, Y. F., Viljoen, A., Mignolet, J., Matheli&#233;-Guinlet, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=AFM+in+cellular+and+molecular+microbiology.">AFM in cellular and molecular microbiology.</a> Cellular Microbiology. 23 (7), e13324 (2021).</li><li>Sharma, A., Rout, C. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Probe-based+techniques+for+2D+layered+materials.">Probe-based techniques for 2D layered materials.</a> Advanced Analytical Techniques for Characterization of 2D Materials. , 1-14 (2022).</li><li>Zhong, J., Yan, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Seeing+is+believing:+atomic+force+microscopy+imaging+for+nanomaterial+research.">Seeing is believing: atomic force microscopy imaging for nanomaterial research.</a> RSC Advances. 6 (2), 1103-1121 (2016).</li><li>Olubowale, O. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=34;May+the+force+be+with+you!&amp;#34;+Force-volume+mapping+with+atomic+force+microscopy.">34;May the force be with you!&amp;#34; Force-volume mapping with atomic force microscopy.</a> ACS Omega. 6 (40), 25860-25875 (2021).</li><li>Dokukin, M. E., Sokolov, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+mapping+of+the+elastic+modulus+of+soft+materials+with+HarmoniX+and+PeakForce+QNM+AFM+modes.">Quantitative mapping of the elastic modulus of soft materials with HarmoniX and PeakForce QNM AFM modes.</a> Langmuir. 28 (46), 16060-16071 (2012).</li><li>Nguyen-Tri, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recent+applications+of+advanced+atomic+force+microscopy+in+polymer+science:+a+review.">Recent applications of advanced atomic force microscopy in polymer science: a review.</a> Polymers. 12 (5), 1142 (2020).</li><li>Vitry, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mode-synthesizing+atomic+force+microscopy+for+3D+reconstruction+of+embedded+low-density+dielectric+nanostructures.">Mode-synthesizing atomic force microscopy for 3D reconstruction of embedded low-density dielectric nanostructures.</a> Nano Research. 8 (7), 2199-2205 (2015).</li><li>Coste, R., Pernes, M., Tetard, L., Molinari, M., Chabbert, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+the+interplay+of+composition+and+environmental+humidity+on+the+nanomechanical+properties+of+hemp+fibers.">Effect of the interplay of composition and environmental humidity on the nanomechanical properties of hemp fibers.</a> ACS Sustainable Chemistry &amp; Engineering. 8 (16), 6381-6390 (2020).</li><li>Sch&#246;nherr, H., Feng, C. L., Tomczak, N., Vancso, G. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Compositional+mapping+of+polymer+surfaces+by+chemical+force+microscopy+down+to+the+nanometer+scale:+reactions+in+block+copolymer+microdomains.">Compositional mapping of polymer surfaces by chemical force microscopy down to the nanometer scale: reactions in block copolymer microdomains.</a> Macromolecular Symposia. 230 (1), 149-157 (2005).</li><li>Holland-Moritz, K., Siesler, H. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Infrared+spectroscopy+of+polymers.">Infrared spectroscopy of polymers.</a> Applied Spectroscopy Reviews. 11 (1), 1-55 (1976).</li><li>Bhargava, R., Wang, S. Q., Koenig, J. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=FTIR+microscpectrscopy+of+polymeric+systems.">FTIR microscpectrscopy of polymeric systems.</a> Liquid Chromatography/FTIR Microspectroscopy/Microwave Assisted Synthesis. , 137-191 (2003).</li><li>Rao, V. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=AFM-IR+and+IR-SNOM+for+the+characterization+of+small+molecule+organic+semiconductors.">AFM-IR and IR-SNOM for the characterization of small molecule organic semiconductors.</a> The Journal of Physical Chemistry C. 124 (9), 5331-5344 (2020).</li><li>Wang, H., Wang, L., Xu, X. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Scattering-type+scanning+near-field+optical+microscopy+with+low-repetition-rate+pulsed+light+source+through+phase-domain+sampling.">Scattering-type scanning near-field optical microscopy with low-repetition-rate pulsed light source through phase-domain sampling.</a> Nature Communications. 7 (1), 13212 (2016).</li><li>Dazzi, A., Prater, C. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=AFM-IR:+Technology+and+applications+in+nanoscale+infrared+spectroscopy+and+chemical+imaging.">AFM-IR: Technology and applications in nanoscale infrared spectroscopy and chemical imaging.</a> Chemical Reviews. 117 (7), 5146-5173 (2017).</li><li>Mathurin, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photothermal+AFM-IR+spectroscopy+and+imaging:+Status,+challenges,+and+trends.">Photothermal AFM-IR spectroscopy and imaging: Status, challenges, and trends.</a> Journal of Applied Physics. 131 (1), 010901 (2022).</li><li>Miller, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Enhanced+surface+nanoanalytics+of+transient+biomolecular+processes.">Enhanced surface nanoanalytics of transient biomolecular processes.</a> Science Advances. 9 (2), 3151 (2023).</li><li>Emin, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Small+soluble+&#945;-synuclein+aggregates+are+the+toxic+species+in+Parkinson's+disease.">Small soluble &#945;-synuclein aggregates are the toxic species in Parkinson's disease.</a> Nature Communications. 13 (1), 5512 (2022).</li><li>Jardine, K., Dove, A., Tetard, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=AFM+force+measurements+to+explore+grain+contacts+with+relevance+for+planetary+materials.">AFM force measurements to explore grain contacts with relevance for planetary materials.</a> The Planetary Science Journal. 3 (12), 273 (2022).</li><li>Sader, J. E., Chon, J. W. M., Mulvaney, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Calibration+of+rectangular+atomic+force+microscope+cantilevers.">Calibration of rectangular atomic force microscope cantilevers.</a> Review of Scientific Instruments. 70 (10), 3967-3969 (1999).</li><li>Higgins, M. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Noninvasive+determination+of+optical+lever+sensitivity+in+atomic+force+microscopy.">Noninvasive determination of optical lever sensitivity in atomic force microscopy.</a> Review of Scientific Instruments. 77 (1), 013701 (2006).</li><li>Smith, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+infrared+spectra+of+polymers+III:+hydrocarbon+polymers.">The infrared spectra of polymers III: hydrocarbon polymers.</a> Spectroscopy. 36 (11), 22-25 (2021).</li><li>Korbag, I., Mohamed Saleh, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Studies+on+the+formation+of+intermolecular+interactions+and+structural+characterization+of+polyvinyl+alcohol/lignin+film.">Studies on the formation of intermolecular interactions and structural characterization of polyvinyl alcohol/lignin film.</a> International Journal of Environmental Studies. 73 (2), 226-235 (2016).</li><li>Tetard, L., Passian, A., Farahi, R. H., Thundat, T., Davison, B. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Opto-nanomechanical+spectroscopic+material+characterization.">Opto-nanomechanical spectroscopic material characterization.</a> Nature Nanotechnology. 10 (10), 870-877 (2015).</li><li>Bai, Y., Yin, J., Cheng, J. -. X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bond-selective+imaging+by+optically+sensing+the+mid-infrared+photothermal+effect.">Bond-selective imaging by optically sensing the mid-infrared photothermal effect.</a> Science Advances. 7 (20), (2021).</li></ol>

AFM combined with nanoIR spectroscopy can provide nanoscale chemical information using a cantilever in contact mode and a pulsed tunable IR light source. Model systems, such as embedding an absorber with finite dimensions in the volume of a polymeric material, are important to improve the understanding of image formation mechanisms and to determine the performance of the tool. In the case of the PS/PVA configuration presented here, optimization was carried out to obtain a stable PS bead positioned above or below the surf...

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.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>10|0 2200 Golden Taklon Round</td>
			<td>Zem</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>5357-8NM Tweezers</td>
			<td>Pelco</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Adhesive Tabs</td>
			<td>Ted Pella</td>
			<td>16079</td>
			<td></td>
		</tr>
		<tr>
			<td>AFM metal specimen disks</td>
			<td>Ted Pella</td>
			<td>16208</td>
			<td></td>
		</tr>
		<tr>
			<td>Binocular</td>
			<td>AmScope</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Cantilever for nanoIR measurements</td>
			<td>AppNano</td>
			<td></td>
			<td>FORTGG</td>
		</tr>
		<tr>
			<td>Cell culture dishes</td>
			<td>Greiner bio-one GmbH</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Desiccator</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Floating optical table</td>
			<td>Newport</td>
			<td>RS 4000</td>
			<td></td>
		</tr>
		<tr>
			<td>Hotplate</td>
			<td>VWR</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Isopropanol&#160;</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Kimwipes</td>
			<td>KIMTECH</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Magnetic stir bar</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Microparticles based on polystyrene size: 5 &#181;m</td>
			<td>SIGMA-ALDRICH</td>
			<td>79633</td>
			<td></td>
		</tr>
		<tr>
			<td>nanoIR2 microscope</td>
			<td>Bruker</td>
			<td></td>
			<td>Contact mode NanoIR2</td>
		</tr>
		<tr>
			<td>Nitrogen Tank</td>
			<td>Airgas</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Petri dishes</td>
			<td>Greiner bio-one GmbH</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Polyvinyl Alcohol</td>
			<td>SIGMA-ALDRICH</td>
			<td>363170</td>
			<td>this polymer was only 87%-89% hydrolyzed, which explains the presence of residual C=O at 1730 cm-1</td>
		</tr>
		<tr>
			<td>Quantum Cascade Laser</td>
			<td>Daylight Solutions</td>
			<td></td>
			<td>1550-1800 cm-1 range</td>
		</tr>
		<tr>
			<td>Silicon wafer</td>
			<td>MEMC St. Peters</td>
			<td>#901319343000</td>
			<td></td>
		</tr>
		<tr>
			<td>Spin coater</td>
			<td>Oscilla</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

advances in nanoscale infrared spectroscopy to explore multiphase polymeric systems

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.

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.

PS ((C8H8)n) beads were deposited on a clean Si substrate (Figure 1A) and on PVA ((CH2CHOH)n) (Figure 1B,C). Due to the poor adhesion of the bead on Si, nanoIR imaging in contact mode could not be acquired for this sample. Instead, using the optical view of the sample on nanoIR, the gold-coated AFM probe was engaged on top of the PS bead in contact mode, with an estimated force of about 100 ...

Watch this Scientific Journal Video about Advances in Nanoscale Infrared Spectroscopy to Explore Multiphase Polymeric Systems at JoVE.com

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 ...

advances-nanoscale-infrared-spectroscopy-to-explore-multiphase