Atomic Force Microscopy-infrared Spectroscopy Research Articles

Advanced Vibrational Spectroscopy and Bacteriophages Team Up: Dynamic Synergy for Medical and Environmental Applications.

Bacteriophages are emerging as a promising alternative in combating antibiotic-resistant bacteria amidst the escalating global antimicrobial resistance crisis. Recently, there has been a notable resurgence of interest in phages, prompting extensive research into their therapeutic potential. Beyond conventional microbiology and virology techniques, such as genomics and proteomics, novel phenotypic and chemical characterization methods are being explored. Among these, there is a growing interest in vibrational spectroscopy, especially in advanced modalities such as surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS), and atomic force microscopy-infrared spectroscopy (AFM-IR), which offer improved sensitivity and spatial resolution. This review explores the spectrum of uses of vibrational spectroscopy for bacteriophages, including its role in diagnostics, biosensing, phage detection, assistance in phage-based therapy, and advancing basic research.

Effects of Wind Speed on Size-Dependent Morphology and Composition of Sea Spray Aerosols.

Variable wind speeds over the ocean can have a significant impact on the formation mechanism and physical-chemical properties of sea spray aerosols (SSA), which in turn influence their climate-relevant impacts. Herein, for the first time, we investigate the effects of wind speed on size-dependent morphology and composition of individual nascent SSA generated from wind-wave interactions of natural seawater within a wind-wave channel as a function of size and their particle-to-particle variability. Filter-based thermal optical analysis, atomic force microscopy (AFM), AFM infrared spectroscopy (AFM-IR), and scanning electron microscopy (SEM) were employed in this regard. This study focuses on SSA with sizes within 0.04-1.8 μm generated at two wind speeds: 10 m/s, representing a wind lull scenario over the ocean, and 19 m/s, indicative of the wind speeds encountered in stormy conditions. Filter-based measurements revealed a reduction of the organic mass fraction as the wind speed increases. AFM imaging at 20% relative humidity of individual SSA identified six main morphologies: prism-like, rounded, core-shell, rod, rod inclusion core-shell, and aggregates. At 10 m/s, most SSA were rounded, while at 19 m/s, core-shells became predominant. Based on AFM-IR, rounded SSA at both wind speeds had similar composition, mainly composed of aliphatic and oxygenated species, whereas the shells of core-shells displayed more oxygenated organics at 19 m/s and more aliphatic organics at 10 m/s. Collectively, our observations can be attributed to the disruption of the sea surface microlayer film structure at higher wind speeds. The findings reveal a significant impact of wind speed on morphology and composition of SSA, which should be accounted for accurate assessment of their climate effects.

Nanoscale colocalized thermal and chemical mapping of pharmaceutical powder aerosols

Inhalation of pharmaceutical aerosol formulations is widely used to treat respiratory diseases. Spatially resolved thermal characterization offers promise for better understanding drug release rates from particles; however, this has been an analytical challenge due to the small particle size (from a few micrometers down to nanometers) and the complex composition of the formulations. Here, we employ nano-thermal analysis (nanoTA) to probe the nanothermal domain of a pharmaceutical aerosol formulation containing a mixture of fluticasone propionate (FP), salmeterol xinafoate (SX), and excipient lactose, which is widely used to treat asthma and chronic obstructive pulmonary disease (COPD). Furthermore, atomic force microscopy-infrared spectroscopy (AFM-IR) and AFM force measurements are performed to provide nanochemical and nanomechanical information to complement the nanothermal data. The colocalized thermal and chemical mapping clearly reveals the surface heterogeneity of the drugs in the aerosol particles and demonstrates the contribution of the surface chemical composition to the variation in the thermal properties of the particles. We present a powerful analytical approach for in-depth characterization of thermal/chemical/morphological properties of dry powder inhaler particles at micro- and nanometer scales. This approach can be used to facilitate the comparison between generics and reference inhalation products and further the development of high-performance pharmaceutical formulations.

Porous, Self-Polarized Ferroelectric Polymer Films Exhibiting Behavior Reminiscent of Morphotropic Phase Boundary Induced by Size-Dependent Interface Effect for Self-Powered Sensing.

Piezoelectric poly(vinylidene fluoride) (PVDF) and its copolymer, poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), have attracted considerable attention due to their potential in flexible, biocompatible energy harvesting and sensing devices. However, their limited piezoelectric performance hinders their widespread application. Inspired by the concept of morphotropic phase boundary (MPB) prevalent in high-performance piezoelectric ceramics, we successfully constructed MPB in the piezoelectric polymer P(VDF-TrFE) through size-dependent interface effects. We provided direct structural evidence using atomic force microscopy-infrared spectroscopy (AFM-IR) and significantly improved the piezoelectric performance of P(VDF-TrFE). The emergence of MPB is attributed to the interface effect induced by electrostatic interactions between ZnO fillers and the -CH2, -CF2, and -CHF groups in P(VDF-TrFE). This interaction drives a concomitant competition between the all-trans β phase (normal ferroelectric) and the 3/1 helical phase (relaxor), resulting in enhanced piezoelectric responses in the transition region. By coupling the MPB effect with a porous structure, we developed a piezoelectric nanogenerator (PENG) that surpasses the electrical output limitation of current P(VDF-TrFE)-based PENGs. The fabricated PENG exhibits superior piezoelectric outputs (6.9 μW/cm2), impressive pressure sensitivity (2.3038 V/kPa), ultrafast response time (4.3 ms), and recovery time (46.4 ms)─notably, without the need for additional poling treatment. In practical applications, the constructed PENG can efficiently generate characteristic signals in response to various human movements and harvest biomechanical energy. This work offers insight into utilizing interface-induced MPB and proposes a simple, scalable approach for developing high-performance self-polarized piezoelectric polymer films for self-powered sensing systems toward human-machine interaction.

Probing the molecular interaction between photoaged polystyrene microplastics and fulvic acid

As emerging contaminants, microplastics (MPs) are becoming a matter of global concern, and they have complex interactions with dissolved organic matter (DOM) widely present in aqueous environments. Here, we investigate the molecular interactions between aged polystyrene microplastics (PS-MPs) and fulvic acid (FA) under neutral conditions using a series of analytical techniques. The structural changes of FA and the binding interactions of PS-MPs with FA at a molecular level were explored by fluorescence and FT-IR combined with two-dimensional correlation spectroscopy (2D-COS). Results showed that photoaging of PS-MPs changed the sequence of structural variations with FA. Atomic force microscopy-infrared spectroscopy (AFM-IR) strongly demonstrated that the surface roughness of both pristine and aged PS-MPs greatly increased after FA addition. Meanwhile, AFM-IR and Raman spectroscopy revealed a stronger interaction between aged PS-MPs and FA. The content of oxygen-containing functional groups in PS-MPs increased after aging and after binding with FA, and surface distribution of these functional groups also changed. XPS analyses indicated that the oxygen content in PS-MPs increased after the interaction with FA and the increase in oxygen content was even greater in aged PS-MPs. Overall, these research findings are useful to understand the environmental impacts of DOM-MPs interactions and to address the uncertainty of MPs aging effect on their environmental behavior in aquatic ecosystems.

Electrochemical Deposition of N-Heterocyclic Carbene on Steels As a Corrosion Protective Layer

Steels play a vital role in every aspect of day-to-day life. They are major components in construction,transportation, medicine, and etc. Despite their larger usability, steels are suffering from inevitablecorrosion. Hence, finding a sustainable solution for corrosion has been a hot topic in the research field forcenturies. One extensively studied strategy is applying a protective layer on top of metals. The recentfindings show that N-heterocyclic carbene (NHC) has the potency to form a carbon-metal bond throughself-assembling1 or electrochemical deposition2 on different metals such as Au, Pt, Pd, and Ag. Herein, weare trying to coat NHC on mild steel (MS) through electrochemical deposition to act as a corrosionprotective layer. Deposition conditions will be optimized by varying the material concentrations, appliedvoltages, time durations, and substrate roughness. The NHC-coated surface is characterized by X-rayPhotoelectron Spectroscopy (XPS), Matrix-Assisted Laser Desorption/Ionization (MALDI), and AtomicForce Microscope-Infrared Spectroscopy (AFM-IR). In addition, the contact angle measurements can beattributed to visualizing the differences in coatings under different conditions. The corrosion studies areutilized to understand the corrosion-resistant properties of the NHC coating. Moreover, the NHC moietycan be modified to perform better to inhibit corrosion and improve the properties of mild steel on theirapplications. Ultimately, this benchtop approach will be utilized and tested on a large scale and industriallevel.(1) Crudden, C. M.; Horton, J. H.; Ebralidze, I. I.; Zenkina, O. V.; McLean, A. B.; Drevniok, B.; She, Z.; Kraatz,H.-B.; Mosey, N. J.; Seki, T. Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold. Nat. Chem. 2014, 6 (5), 409-414.(2) Amit, E.; Dery, L.; Dery, S.; Kim, S.; Roy, A.; Hu, Q.; Gutkin, V.; Eisenberg, H.; Stein, T.; Mandler, D.Electrochemical deposition of N-heterocyclic carbene monolayers on metal surfaces. Nat. Commun. 2020, 11 (1), 1-10.

Nanoscale structural characterization of transthyretin aggregates formed at different time points of protein aggregation using atomic force microscopy-infrared spectroscopy.

Transthyretin (TTR) amyloidosis is a progressive disease characterized by an abrupt aggregation of misfolded protein in multiple organs and tissues TTR is a tetrameric protein expressed in the liver and choroid plexus. Protein misfolding triggers monomerization of TTR tetramers. Next, monomers assemble forming oligomers and fibrils. Although the secondary structure of TTR fibrils is well understood, there is very little if anything is known about the structural organization of TTR oligomers. To end this, we used nano-infrared spectroscopy, also known as atomic force microscopy infrared (AFM-IR) spectroscopy. This emerging technique can be used to determine the secondary structure of individual amyloid oligomers and fibrils. Using AFM-IR, we examined the secondary structure of TTR oligomers formed at the early (3-6 h), middle (9-12 h), and late (28 h) of protein aggregation. We found that aggregating, TTR formed oligomers (Type 1) that were dominated by α-helix (40%) and β-sheet (~30%) together with unordered protein (30%). Our results showed that fibril formation was triggered by another type of TTR oligomers (Type 2) that appeared at 9 h. These new oligomers were primarily composed of parallel β-sheet (55%), with a small amount of antiparallel β-sheet, α-helix, and unordered protein. We also found that Type 1 oligomers were not toxic to cells, whereas TTR fibrils formed at the late stages of protein aggregation were highly cytotoxic. These results show the complexity of protein aggregation and highlight the drastic difference in the protein oligomers that can be formed during such processes.

Complex mixture of organic matter in a xenolithic clast from the Zag meteorite revealed by coordinated analyses using AFM-IR, NanoSIMS and STXM/XANES

Primitive carbonaceous xenolithic clasts found in sturdy metamorphosed meteorites often provide opportunities to reach labile volatile-rich materials which are easily destroyed during atmospheric entry and materials which we do not have sampled as individual meteorites. Among them, a xenolithic carbonaceous clast in the Zag H3–6 ordinary chondrite has been providing us with the opportunity to analyze a possible sample from D/P-type asteroids. Here we performed a new suite of coordinated analyses of organic matter in the Zag clast using the atomic force microscope infrared spectroscopy (AFM-IR) combined with nanoscale secondary ion mass spectrometer (NanoSIMS), X-ray absorption near-edge spectroscopy (XANES) coupled with scanning transmission X-ray macroscope (STXM), Raman, and (scanning) transmission electron microscopy [(S)TEM] on adjacent ultramicrotomed thin sections from a single sample grain. We successfully demonstrated the practicality of coordinated analyses using AFM-IR, Raman and NanoSIMS on the same sample area, as well as STXM/XANES on adjacent (and nearly identical) thin sections to those used for AFM-IR. The AFM-IR map and STXM maps provided consistent and complementary results. We found that at least two types of organics were closely mixed in this specimen. One was deuterium-rich, CO rich organics with likely smaller aromatic domains, possibly originating in relatively oxidized environments from D-rich precursors. The other type was less deuterium-rich, but aromatic-rich organics, possibly produced in relatively reduced and higher temperature environments with less deuterium-rich precursors. These characteristics point to complex mixtures of materials with different origins and sampling a wide heliocentric range of the Solar System before accretion in the parent body of the clast.

Emerging multi-frequency surface strain force microscopy

During the past decade, Scanning Probe Microscopy (SPM) based surface strain detection techniques have been extensively used in the characterization of functional materials, structures, and devices. Here, we refer these techniques as Surface Strain Force Microscopy (SSFM), which mainly includes the Piezoresponse Force Microscopy, Atomic Force Acoustic Microscopy, Atomic Force Microscopy-Infrared spectroscopy (or photothermal induced resonance), Piezomagnetic Force Microscopy, and Scanning Joule Expansion Microscopy. The inception of SSFM opens up a pathway to study the nanoscale physical properties by using a sharp tip to detect the local field-induced surface strain. Through measuring the signals of the surface strain, multiple physical properties, such as the electromechanical, mechanical, photothermal, magnetic, thermoelastic properties, can be characterized with an unprecedented spatial resolution. In order to further develop and overcome the fundamental issues and limitations of the SSFM, the multi-frequency SPM technology has been introduced to the SSFM-based techniques, leading to the emerging of multi-frequency SSFM (MF-SSFM). As a technical breakthrough of the SSFM, MF-SSFM has demonstrated substantial improvements in both performance and capability, resulting in increased attentions and numerous developments in recent years. This Perspective is, therefore, aimed at providing a preliminary summary and systematic understanding for the emerging MF-SSFM technology. We will first introduce the basic principles of conventional SSFM and multi-frequency SPM techniques, followed by a detailed discussion about the existing MF-SSFM techniques. MF-SSFM will play an increasingly important role in future nanoscale characterization of the physical properties. As a result, many more advanced and complex MF-SSFM systems are expected in the coming years.

Bioorganometallic chemistry – the early years

Bioorganometallic chemistry is now a maturing field attracting researchers around the world, and also across the Periodic Table. It has found applications in a broad range of areas such as drug discovery, biocatalysis and imaging. We review here the first experiments carried out in the early 1980′s at the Ecole Nationale Supérieure de Chimie de Paris under the leadership of Gérard Jaouen, a key figure and visionary in the development of this theme. However, they also required important contributions from Canadian collaborators with expertise in NMR and IR spectroscopy, together with the availability of more powerful and versatile instrumentation. This collaboration found its fulfillment in the demonstration of the use of metal carbonyl complexes (exemplified by an estradiol derivative labeled by a Cr(CO)3 unit) as non-isotopic tracers allowing the detection of estradiol receptors, an essential marker for setting up a targeted therapy for breast cancer. This approach was based on the premise that the intense metal carbonyl stretching vibrations are found in a window (2100–1850 cm−1) where the protein itself does not absorb. The field was extended to the arena of non-isotopic immunoassays, including the simultaneous detection of multiple antiepileptic drugs bearing metal carbonyl labels. More recently, taking advantage of the new technique of atomic force microscopy-infrared spectroscopy (AFM-IR) it has been possible to accomplish direct tracing of these complexes in cells. The contributions of the original group of workers in the field are highlighted, and placed in the perspective of today's subsequent emphasis on the management of previously untreatable cancers.

Nanoscale Characterization of Parallel and Antiparallel β-Sheet Amyloid Beta 1-42 Aggregates.

Abrupt aggregation of amyloid beta (Aβ) peptide is strongly associated with Alzheimer's disease. In this study, we used atomic force microscopy-infrared (AFM-IR) spectroscopy to characterize the secondary structure of Aβ oligomers, protofibrils and fibrils formed at the early (4 h), middle (24 h), and late (72 h) stages of protein aggregation. This innovative spectroscopic approach allows for label-free nanoscale structural characterization of individual protein aggregates. Using AFM-IR, we found that at the early stage of protein aggregation, oligomers with parallel β-sheet dominated. However, these species exhibited slower rates of fibril formation compared to the oligomers with antiparallel β-sheet, which first appeared in the middle stage. These antiparallel β-sheet oligomers rapidly propagated into fibrils that were simultaneously observed together with parallel β-sheet fibrils at the late stage of protein aggregation. Our findings showed that aggregation of Aβ is a complex process that yields several distinctly different aggregates with dissimilar toxicities.

Self-assembled microstructures with localized graphene domains in an epoxy blend and their related properties

Locating nanoparticles in selected areas via the mixing of two immiscible polymers is widely studied for achieving nanocomposites with next-level performance, however, the formation of a phase-separated domain constructed with nanofillers from entirely miscible molecules is rarely achieved in the literature. Here we demonstrate a method to fabricate a self-constructed bi-continuous phase structure with localized amine-functionalized graphene nanoplatelets (A-GNPs) in a liquid processable multi-component epoxy blend. Atomic force microscopy infrared spectroscopy (AFM-IR) was employed to identify the compositions of the phase-separated microdomains formed during self-assembly by A-GNP in the multi-component epoxy blend, with incorporating 1-(2-aminoethyl) piperazine (AEPIP) found to be the driving force for the formation of the graphene microdomains. Nanoindentation measurements show that a Young’s modulus of 6.3 GPa for the graphene domain was achieved, which is nearly twice that of the epoxy resin (3.2 GPa). Transient thermoreflectance results indicate that the thermal conductivity of nanocomposite with phase-separated graphene domain reached 0.48 W/mK, exhibiting a significant enhancement (70%) when compared to epoxy resin, while maintaining excellent dielectric properties. Overall, this study provides a simple and effective route to fabricate phase-separated microstructure with nanoparticles from a liquid processable nanocomposite blend, which shows the great potential of this promising new approach to fabricate nanocomposite films with excellent performance for microelectronics applications.

Temporospatial nano-heterogeneity of self-assembly of extracellular polymeric substances on microplastics and water environmental implications

Environmental behavior and ecotoxicity of microplastics (MPs) are significantly influenced by the omnipresent self-assembly of microbial extracellular polymeric substances (EPS) on them. However, mechanisms of EPS self-assembly onto MPs at nanoscale resolution and effects of aging are unclear. For the first time, temporospatial nano-heterogeneity of self-assembly of EPS onto fresh and one-year aged polypropylene (PP) MPs were investigated by atomic-force-microscopy-infrared-spectroscopy (AFM-IR). Natural aging caused high degree nanoscale fragmentation of MPs physically and chemically. Self-assembly of EPS on MPs was aging-dependent. Polysaccharides were assembled on MP surface faster than proteins. Initially, regardless of the fresh or aged MPs, polysaccharides and proteins, with the former being predominant, were successively and separately assembled to different nanospaces because of their competition for binding sites. More and more proteins and polysaccharides were superimposed on each other with assembly time due to intermolecular forces. The nanochemical textural analysis showed that the nano-heterogeneity of EPS assembly to MPs was clearly correlated with the aging-induced nanochemical and nanomechanical heterogeneity of MP surface. The spontaneous self-assembly of EPS with temporospatial nano-heterogeneity on MPs have multiple impacts on behavior, ecotoxicity and fate of MPs and their associated pollutants as well as other key ecological processes in aquatic environment.

Matrix/mineral ratio and domain size variation with bone tissue age: A photothermal infrared study

Atomic force microscopy-infrared spectroscopy (AFM-IR) and optical photothermal infrared spectroscopy (O-PTIR), which feature spectroscopic imaging spatial resolution down to ∼ 50 nm and ∼ 500 nm, respectively, were employed to characterize the nano- to microscale chemical compositional changes in bone. Since these changes are known to be age dependent, fluorescently labelled bone samples were employed. The average matrix/mineral ratio values decrease as the bone tissue matures as measured by both AFM-IR and O-PTIR, which agrees with previously published FTIR and Raman spectroscopy results. IR ratio maps obtained by AFM-IR reveal variation in matrix/mineral ratio-generating micron-scale bands running parallel to the bone surface as well as smaller domains within these bands ranging from ∼ 50 to 700 nm in size, which is consistent with the previously published length scale of nanomechanical heterogeneity. The matrix/mineral changes do not exhibit a smooth gradient with tissue age. Rather, the matrix/mineral transition occurs sharply within the length scale of 100–200 nm. O-PTIR also reveals matrix/mineral band domains running parallel to the bone surface, resulting in waves of matrix/mineral ratios progressing from the youngest to most mature tissue. Both AFM-IR and O-PTIR show a greater variation in matrix/mineral ratio value for younger tissue as compared to older tissue. Together, this data confirms O-PTIR and AFM-IR as techniques that visualize bulk spectroscopic data consistent with higher-order imaging techniques such as Raman and FTIR, while revealing novel insight into how mineralization patterns vary as bone tissue ages.

Retention of Intrafibrillar Minerals Improves Resin–Dentin Bond Durability

The concept of extrafibrillar demineralization involves selective removal of apatite crystallites from the extrafibrillar spaces of mineralized dentin without disturbing the intrafibrillar minerals within collagen. This helps avoiding activation of endogenous proteases and enables air-drying of partially demineralized dentin without causing collapse of completely demineralized collagen matrix that adversely affects resin infiltration. The objective of the present study was to evaluate the potential of quaternized carboxymethyl chitosan (QCMC)–based extrafibrillar demineralization in improving resin–dentin bond durability. Isothermal titration calorimetry indicated that QCMC synthesized by quaternization of O-carboxymethyl chitosan had moderate affinity for Ca2+ (binding constant: 8.9 × 104 M−1). Wet and dry bonding with the QCMC-based demineralization produced tensile bond strengths equivalent to the phosphoric acid (H3PO4)–based etch-and-rinse technique. Those bond strengths were maintained after thermocycling. Amide I and PO43− mappings of QCMC-conditioned dentin were performed with atomic force microscope–infrared spectroscopy (AFM-IR). Whereas H3PO4-etched dentin exhibited an extensive reduction in PO43− signals corresponding to apatite depletion, QCMC-conditioned dentin showed scattered dark areas and bright PO43− streak signals. The latter were consistent with areas identified as collagen fibrils in the amide I mapping and were suggestive of the presence of intrafibrillar minerals in QCMC-conditioned dentin. Young’s modulus mapping of QCMC-demineralized dentin obtained by AFM-based amplitude modulation–frequency modulation recorded moduli that were the same order of magnitude as those in mineralized dentin and at least 1 order higher than H3PO4-etched dentin. In situ zymography of the gelatinolytic activity within hybrid layers created with QCMC conditioning revealed extremely low signals before and after thermocycling, compared with H3PO4-etched dentin for both wet and dry bonding. Confocal laser scanning microscopy identified the antibacterial potential of QCMC against Streptococcus mutans and Enterococcus faecalis biofilms. Taken together, the QCMC-based demineralization retains intrafibrillar minerals, preserves the elastic modulus of collagen fibrils, reduces endogenous proteolytic activity, and inhibits bacteria biofilms to extend dentin bond durability.

Crosslinked polyetherimide nanocomposites with superior energy storage achieved via trace Al2O3 nanoparticles

Polymer dielectrics which operate under elevated temperatures are widely desirable for advanced electric energy storage systems. However, the currently available polymer dielectrics are limited to relatively low working temperatures. Herein, crosslinked nanocomposites with trace wide-bandgap nanofillers alumina (Al2O3) and heat-resistant polyetherimide (PEI) were prepared by in-situ polymerization and solution casting method. Crosslinked networks in the nanocomposites improve the breakdown strength and suppress dielectric loss. Furthermore, we demonstrate trace Al2O3 nanofillers change the microstructure of polymer matrix and facilitate the dipole rotations in the chain segments by atomic force microscopy-infrared spectroscopy (AFM-IR). The dielectric constant of the nanocomposites is enhanced without sacrificing the breakdown and mechanical properties, which compensates for the dielectric constant reduced by crosslinking. All the composite films maintain excellent dielectric stability over a wide range of frequencies and temperatures. Besides, giant discharged energy density and ultrahigh charge-discharge efficiency are simultaneously achieved at high temperatures. Astoundingly, energy storage performance at high temperatures is superior to most of the reported work. This work demonstrates a viable strategy to prepare high-temperature polymer dielectric films for energy storage applications.

Nanoscopic Study of Water Uptake on Glass Surfaces with Organic Thin Films and Particles from Exposure to Indoor Cooking Activities: Comparison to Model Systems.

Water uptake by thin organic films and organic particles on glass substrates at 80% relative humidity was investigated using atomic force microscopy-infrared (AFM-IR) spectroscopy. Glass surfaces exposed to kitchen cooking activities show a wide variability of coverages from organic particles and organic thin films. Water uptake, as measured by changes in the volume of the films and particles, was also quite variable. A comparison of glass surfaces exposed to kitchen activities to model systems shows that they can be largely represented by oxidized oleic acid and carboxylate groups on long and medium hydrocarbon chains (i.e., fatty acids). Overall, we demonstrate that organic particles and thin films that cover glass surfaces can take up water under indoor-relevant conditions but that the water content is not uniform. The spatial heterogeneity of the changes in these aged glass surfaces under dry (5%) and wet (80%) conditions is quite marked, highlighting the need for studies at the nano- and microscale.

Quantification of drug loading in polymeric nanoparticles using AFM-IR technique: a novel method to map and evaluate drug distribution in drug nanocarriers.

Researchers are increasingly thinking smaller to solve some of the biggest challenges in nanomedicine: the control of drug encapsulation. Although recent years have witnessed a significant increase in the development and characterization of polymeric drug nanocarriers, several key features are still to be addressed: Where is the drug located within each nanoparticle (NP)? How much drug does each NP contain? Is the drug loading homogeneous on an individual NP basis? To answer these questions, individual NP characterization was achieved here by using atomic force microscopy-infrared spectroscopy (AFM-IR). A label-free quantification methodology was proposed to estimate with a nanoscale resolution the drug loadings of individual poly(lactic acid) (PLA) NPs loaded with an anticancer drug. First, a drug loading calibration curve was established using conventional IR microspectroscopy employing PLA/drug homogeneous films of well-known compositions. Then, single NPs were investigated by AFM-IR acquiring both IR mappings of PLA and drug as well as local IR spectra. Besides, drug location within single NPs was unravelled. The measured drug loadings were drastically different (0 to 21 wt%) from one NP to another, emphasizing the particular interest of this methodology in providing a simple quantification method for the quality control of nanomedicines.

Attenuated Total Reflection-Fourier Transform Infrared and Atomic Force Microscopy-Infrared Spectroscopic Investigation of Suwannee River Fulvic Acid and Its Interactions with α-FeOOH

Suwannee River fulvic acid (SRFA) and its interaction with α-FeOOH have been investigated using two different spectroscopic techniques─attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy and atomic force microscopy-infrared (AFM-IR) spectroscopy. ATR-FTIR spectroscopy of SRFA thin films yields information on functional groups present within these films prepared from solutions of three different pH values (3, 6, and 8). This technique can also be used to probe the interaction of SRFA with α-FeOOH particle surfaces and the impact that pH has on these surface interactions. AFM-IR spectroscopy offers a different perspective as it probes both film morphology and spectral signals with nanoscale spatial resolution. Herein, we apply AFM-IR spectroscopy to investigate SRFA thin films and the interactions with α-FeOOH. Results from this study show that pH impacts the speciation of SRFA and its interaction with α-FeOOH. Furthermore, there are nanoscale and microscale heterogeneities in these thin films as shown in height images, point spectra, and spectral maps. Overall, these measurements using two different vibrational spectroscopic techniques provide insights into the heterogeneity of natural organic matter and its interactions with mineral surfaces.

AFM-IR and s-SNOM-IR measurements of chemically addressable monolayers on Au nanoparticles.

The performance of catalysts depends on their nanoscale properties, and local variations in structure and composition can have a dramatic impact on the catalytic reactivity. Therefore, probing the localized reactivity of catalytic surfaces using high spatial resolution vibrational spectroscopy, such as infrared (IR) nanospectroscopy and tip-enhanced Raman spectroscopy, is essential for mapping their reactivity pattern. Two fundamentally different scanning probe IR nanospectroscopy techniques, namely, scattering-type scanning near-field optical microscopy (s-SNOM) and atomic force microscopy-infrared spectroscopy (AFM-IR), provide the capabilities for mapping the reactivity pattern of catalytic surfaces with a spatial resolution of ∼20nm. Herein, we compare these two techniques with regard to their applicability for probing the vibrational signature of reactive molecules on catalytic nanoparticles. For this purpose, we use chemically addressable self-assembled molecules on Au nanoparticles as model systems. We identified significant spectral differences depending on the measurement technique, which originate from the fundamentally different working principles of the applied methods. While AFM-IR spectra provided information from all the molecules that were positioned underneath the tip, the s-SNOM spectra were more orientation-sensitive. Due to its field-enhancement factor, the s-SNOM spectra showed higher vibrational signals for dipoles that were perpendicularly oriented to the surface. The s-SNOM sensitivity to the molecular orientation influenced the amplitude, position, and signal-to-noise ratio of the collected spectra. Ensemble-based IR measurements verified that differences in the localized IR spectra stem from the enhanced sensitivity of s-SNOM measurements to the adsorption geometry of the probed molecules.