Multi-timescale infrared quantum cascade laser ellipsometry.

We recently introduced a novel, to the best of our knowledge, infrared laser ellipsometer for sub-decisecond spectroscopy [Opt. Lett.44, 4387 (2019)10.1364/OL.44.004387] and 0.03 mm2 spot-sized hyperspectral imaging [Opt. Lett.44, 4893 (2019)10.1364/OL.44.004893]. Here we report on the next device generation for thin-film sensitive simultaneous single-shot amplitude and phase measurements. The multi-timescale ellipsometer achieves 10 µs time resolution and long-term stability over hours at high spectral resolution (0.2 cm-1). We investigate the temporal stages (from minutes to milliseconds) of fatty acid thin-film formation upon solvent evaporation from acetone-diluted microliter droplets. Optical thickness variations, structure modifications, and molecular interactions are probed during the liquid-to-solid phase transition. Multi-timescale ellipsometry could greatly impact fields like in situ biosensing, microfluidics, and polymer analytics, but also operando applications in membrane research, catalysis, and studies of interface processes and surface reactions.

Sensing and structure analysis by in situ IR spectroscopy: from mL flow cells to microfluidic applications.

In situ mid-infrared (MIR) spectroscopy in liquids is an emerging field for the analysis of functional surfaces and chemical reactions. Different basic geometries exist for in situ MIR spectroscopy in milliliter (mL) and microfluidic flow cells, such as attenuated total reflection (ATR), simple reflection, transmission and fiber waveguides. After a general introduction of linear optical in situ MIR techniques, the methodology of ATR, ellipsometric and microfluidic applications in single-reflection geometries is presented. Selected examples focusing on thin layers relevant to optical, electronical, polymer, biomedical, sensing and silicon technology are discussed. The development of an optofluidic platform translates IR spectroscopy to the world of micro- and nanofluidics. With the implementation of SEIRA (surface enhanced infrared absorption) interfaces, the sensitivity of optofluidic analyses of biomolecules can be improved significantly. A large variety of enhancement surfaces ranging from tailored nanostructures to metal-island film substrates are promising for this purpose. Meanwhile, time-resolved studies, such as sub-monolayer formation of organic molecules in nL volumes, become available in microscopic or laser-based set-ups. With the adaption of modern brilliant IR sources, such as tunable and broadband IR lasers as well as frequency comb sources, possible applications of far-field IR spectroscopy in in situ sensing with high lateral (sub-mm) and time (sub-s) resolution are considerably extended.

Enhanced neuronal differentiation of neural stem cells with mechanically enhanced touch-spun nanofibrous scaffolds.

We studied NE-4C neural cells differentiation on 2D polycaprolactone (PCL) nanofibrous scaffolds with systematically varied mechanical characteristics of nanofibers while retaining an unchanged fiber alignment, diameter, and chemical composition. Our experiments demonstrated that the nanofibers with enhanced mechanical properties are beneficial for the preferential development of neuronal cells vs. glial cells. Electrospun (ES) and touch-spun (TS) nanofibers were fabricated with Young's modulus in the range of 10 MPa to 230 MPa and a fraction of crystallinity from 30% to 80%. The TS fibers undergo a greater drawing ratio and thus approach a greater polymer chain stretching and alignment that resulted in an increased crystallinity. The TS scaffolds demonstrated improved stability in the aqueous cell culture environment, resisting misalignment and entanglement after a period of 2 weeks of swelling followed by 14 days of neural differentiation. The results confirmed that the neurites on the TS fibers have a preferred orientation even after swelling.

Hyperspectral infrared laser polarimetry for single-shot phase-amplitude imaging of thin films.

We recently presented a novel laser-based infrared (IR) spectroscopic phase-amplitude polarimeter for sub-decisecond and sub-mm2 measurements of organic thin films [Opt. Lett.44, 4387 (2019)OPLEDP0146-959210.1364/OL.44.004387]. Here we report on the hyperspectral-imaging capabilities of this device. The single-shot polarimeter employs a broadly tunable mid-IR (1318-1765 cm-1) quantum cascade laser (QCL) and a four-channel beam-division design for simultaneous phase and amplitude measurements. Fast QCL tuning speeds of up to 1500 cm-1/s enable hyperspectral mapping of large sample areas (50×50 mm2) within several tens of minutes, achieving 120 µm spatial and <0.5 cm-1 spectral resolution. We apply the instrument for imaging both the heterogeneous chemical and structural properties of sub-100 nm thin polymer and fatty-acid films. Our polarimeter opens up new applications regarding laterally resolved IR analyses of complex thin films.

Sub-second infrared broadband-laser single-shot phase-amplitude polarimetry of thin films.

We report on the first, to the best of our knowledge, sub-second, sub-mm2 infrared (IR) spectroscopic measurements of thin organic films employing a laser-based phase-amplitude polarimeter in reflection geometry. The polarimeter uses a broadband mid-IR quantum cascade laser tunable between 1318 cm-1 and 1765 cm-1, as well as a single-shot beam division scheme for simultaneous single-pulse phase and amplitude measurements. The instrument achieves 120 µm spatial and <0.5 cm-1 spectral resolution, while providing unrivaled performance in terms of acquisition times. Spectral measurements within 100 ms and single-wavelength tracking at 16 µs are now possible. Investigating the vibrational properties accessible in the mid-IR, the polarimeter was applied for monitoring changes in molecular interactions of a 150 nm thin myristic acid film during its thermal phase transition around 55°C.

Thin Composite Carbon Molecular Sieve Membranes from a Polymer of Intrinsic Microporosity Precursor.

Ultra-thin composite carbon molecular sieve (CMS) membranes were fabricated on well-defined inorganic alumina substrates using a polymer of intrinsic microporosity (PIM) as a precursor. Details of the pyrolysis-related structural development were elucidated using focused-beam, interference-enhanced spectroscopic ellipsometry (both in the UV-vis and IR range), which allowed accurate determination of the film thickness, optical properties as well as following the chemical transformations. The pyrolysis-induced collapse of thin and bulk PIM-derived CMS membranes was compared with CMS made from a well-known non-PIM precursor 6FDA-DABA. Significant differences between the PIM and non-PIM precursors were discovered and explained by a much larger possible volume contraction in the PIM. In spite of the differences, surprisingly, the gas separation properties did not fundamentally differ. The high-temperature collapse of the initially amorphous and isotropic precursor structure was accompanied by a significant molecular orientation within the formed turbostratic carbon network guided by the laterally constraining presence of the substrate. This manifested itself in the development of uniaxial optical anisotropy, which was shown to correlate with increases in gas separation selectivity for multiple technologically important gas pairs. Reduction of CMS skin thickness significantly below ∼1 µm induced large losses in permeability coefficients with only small to moderate effects on selectivity. Remarkably, skin thickness reduction and physical aging seemed to superimpose onto the same trend, which explains and strengthens some of the earlier fundamental insights.

Broadband infrared Mueller-matrix ellipsometry for studies of structured surfaces and thin films.

We present a high-optical-throughput infrared Mueller-matrix (MM) ellipsometer for the characterization of structured surfaces and ultrathin films. Its unprecedented sensitivity of about 10-4 in the normalized MM elements enables studies of the complex vibrational fingerprint of thin organic films under different ambient conditions. The ellipsometer acquires quadruples of MM elements within a few 10 s to min, rendering it interesting for process and in-line monitoring. It uses retractable achromatic retarders for increased signal to noise, and tandem wire-grid polarizers for improved polarization control. We demonstrate several scientific and industry-related applications. First, we determine the 3D profile of µm-sized trapezoidal SiO2 gratings on Si from azimuth-dependent MM measurements. Data modeling based on rigorous coupled-wave analysis is employed to quantify grating structure and orientation. We then monitor polymer relaxation processes with a time resolution of 47 s. Measurements of polymer films as thin as 7.7 nm illustrate the sensitivity of the device. We finally couple a liquid flow cell to the ellipsometer, highlighting the prospects for in situ infrared MM studies of thin films at solid-liquid interfaces.

Hybrid molecularly imprinted polymer for amoxicillin detection.

The potential adverse effects of the environmental presence of antibiotics on the ecosystem demands the development of new methods suitable for accurate detection of these micropollutants in various aquatic media. An analytical method exploiting the synergistic effect of a label-free sensing platform combined with a molecularly imprinted polymer (MIP) as robust recognition element could represent an efficient tool for the real-time monitoring of antibiotics. In this work, a hybrid organic-inorganic MIP film (AMO-MIP) selective towards amoxicillin (AMO) was synthesized and integrated with a surface plasmon resonance (SPR) sensor. The film was prepared by sol-gel using methacrylamide (MAAM) as organic functional monomer, tetraethoxysilane (TEOS) as inorganic precursor, and vinyltrimethoxysilane (VTMOS) as coupling agent. The AMO-MIP film characterized with the SPR system demonstrated about 16 times higher binding capacity to AMO than corresponding reference non-imprinted polymer (NIP). AMO-MIP-modified SPR sensors could detect AMO with LoD down to 73 pM and discriminate AMO among structurally similar molecules both in buffer and in tap water. Good reproducibility was achieved for several rebinding-regeneration cycles. The sensor could be stored at room temperature for up to 6 months without losing stability.

Nanoliter Sensing for Infrared Bioanalytics.

Nondestructive label-free bioanalytics of microliter to nanoliter sample volumes with low analyte concentrations requires novel analytic approaches. For this purpose, we present an optofluidic platform that combines surface-enhanced in situ infrared spectroscopy with microfluidics for sensing of surface-immobilized ultrathin biomolecular films in liquid analytes. Submonolayer sensitivity down to surface densities of few ng/cm2 is demonstrated for the adsorption of the thiolate tripeptide glutathione and for the recognition of streptavidin on a biotinylated enhancement substrate. Nonfunctionalized and functionalized metal island films on planar oxidized silicon substrates are used for signal enhancement with quantifiable enhancement properties. A single-reflection geometry at an incidence angle below the attenuated-total-reflection (ATR) regime is used with ordinary planar, IR-transparent windows. The geometry circumvents the strong IR absorption of common polymer materials and of aqueous environments in the IR fingerprint region. This practice enables straightforward quantitative analyses of, e.g., adsorption kinetics as well as chemical and structural properties in dependence of external stimuli.

Mixed-Penetrant Sorption in Ultrathin Films of Polymer of Intrinsic Microporosity PIM-1.

Mixed-penetrant sorption into ultrathin films of a superglassy polymer of intrinsic microporosity (PIM-1) was studied for the first time by using interference-enhanced in situ spectroscopic ellipsometry. PIM-1 swelling and the concurrent changes in its refractive index were determined in ultrathin (12-14 nm) films exposed to pure and mixed penetrants. The penetrants included water, n-hexane, and ethanol and were chosen on the basis of their significantly different penetrant-penetrant and penetrant-polymer affinities. This allowed studying microporous polymer responses at diverse ternary compositions and revealed effects such as competition for the sorption sites (for water/n-hexane or ethanol/n-hexane) or enhancement in sorption of typically weakly sorbing water in the presence of more highly sorbing ethanol. The results reveal details of the mutual sorption effects which often complicate comprehension of glassy polymers' behavior in applications such as high-performance membranes, adsorbents, or catalysts. Mixed-penetrant effects are typically very challenging to study directly, and their understanding is necessary owing to a broadly recognized inadequacy of simple extrapolations from measurements in a pure component environment.

Molecular Interactions and Hydration States of Ultrathin Functional Films at the Solid-Liquid Interface.

We significantly improve the infrared analysis of ultrathin films in aqueous environments by employing in situ infrared ellipsometry. Combining it with rigorous optical modeling avoids otherwise typical misinterpretations of spectral features and enables the simultaneous quantification of chemical composition, hydration states, structure, and molecular interactions. We apply this approach to study covalently end-grafted, nanometer-thin brushes of poly(N-isopropylacrylamide), a thermoresponsive model polymer for proteins at solid-liquid interfaces. Quantitative analyses are based on a dielectric layer model that accounts for film swelling and deswelling, hydration of hydrophilic amide and hydrophobic isopropyl side groups, as well as molecular interactions of the polymer's amide moieties. We thereby quantify the hydration and structure dependence of intra- and intermolecular C═O···H-N and C═O···H2O hydrogen bonds, elucidating their role in the brush's temperature-induced phase separation. The presented method is directly applicable to functional and biorelated films like polymer and polypeptide layers, which is of topical interest for interface studies, such as membrane processes and protein unfolding.

Supramolecular Orientation in Anisotropic Assemblies by Infrared Nanopolarimetry.

We report on the experimental characterization of anisotropic supramolecular assemblies by infrared (IR) nanopolarimetry. The presented IR absorption anisotropy imaging method simultaneously provides nanoscale-resolved insights into internal composition, intermolecular interactions, and supramolecular orientation in a label-free and noninvasive fashion. Our study of porphyrin aggregates demonstrates that their morphology can be correlated with stable J-type and metastable H-type stacking-induced anisotropic organization, revealing different oriented attachment growth mechanisms supported by theory. This analysis establishes the broad applicability of IR nanopolarimetric studies to supramolecular polymerization and biomolecular assemblies, opening up new routes in polymer science and macromolecular research.

Probing carbonyl-water hydrogen-bond interactions in thin polyoxazoline brushes.

Temperature-responsive oxazoline-based polymer brushes have gained increased attention as biocompatible surfaces. In aqueous environment, they can be tuned between hydrophilic and hydrophobic behavior triggered by a temperature stimulus. This transition is connected with changes in molecule-solvent interactions and results in a switching of the brushes between swollen and collapsed states. This work studies the temperature-dependent interactions between poly(2-oxazoline) brushes and water. In detail, thermoresponsive poly(2-cyclopropyl-2-oxazoline), nonresponsive hydrophilic poly(2-methyl-2-oxazoline), as well as a copolymer of the two were investigated with in situ infrared ellipsometry. Focus was put on interactions of the brushes' carbonyl groups with water molecules. Different polymer-water interactions could be observed and assigned to hydrogen bonding between C=O groups and water molecules. The switching behavior of the brushes in the range of 20-45 °C was identified by frequency shifts and intensity changes of the amide I band.

Molecularly Imprinted Polymer Integrated with a Surface Acoustic Wave Technique for Detection of Sulfamethizole.

The synergistic effect of combining molecular imprinting and surface acoustic wave (SAW) technologies for the selective and label-free detection of sulfamethizole as a model antibiotic in aqueous environment was demonstrated. A molecularly imprinted polymer (MIP) for sulfamethizole (SMZ) selective recognition was prepared in the form of a homogeneous thin film on the sensing surfaces of SAW chip by oxidative electropolymerization of m-phenylenediamine (mPD) in the presence of SMZ, acting as a template. Special attention was paid to the rational selection of the functional monomer using computational and spectroscopic approaches. SMZ template incorporation and its subsequent release from the polymer was supported by IR microscopic measurements. Precise control of the thicknesses of the SMZ-MIP and respective nonimprinted reference films (NIP) was achieved by correlating the electrical charge dosage during electrodeposition with spectroscopic ellipsometry measurements in order to ensure accurate interpretation of label-free responses originating from the MIP modified sensor. The fabricated SMZ-MIP films were characterized in terms of their binding affinity and selectivity toward the target by analyzing the binding kinetics recorded using the SAW system. The SMZ-MIPs had SMZ binding capacity approximately more than eight times higher than the respective NIP and were able to discriminate among structurally similar molecules, i.e., sulfanilamide and sulfadimethoxine. The presented approach for the facile integration of a sulfonamide antibiotic-sensing layer with SAW technology allowed observing the real-time binding events of the target molecule at nanomolar concentration levels and could be potentially suitable for cost-effective fabrication of a multianalyte chemosensor for analysis of hazardous pollutants in an aqueous environment.

Fast IR laser mapping ellipsometry for the study of functional organic thin films.

Fast infrared mapping with sub-millimeter lateral resolution as well as time-resolved infrared studies of kinetic processes of functional organic thin films require a new generation of infrared ellipsometers. We present a novel laboratory-based infrared (IR) laser mapping ellipsometer, in which a laser is coupled to a variable-angle rotating analyzer ellipsometer. Compared to conventional Fourier-transform infrared (FT-IR) ellipsometers, the IR laser ellipsometer provides ten- to hundredfold shorter measurement times down to 80 ms per measured spot, as well as about tenfold increased lateral resolution of 120 µm, thus enabling mapping of small sample areas with thin-film sensitivity. The ellipsometer, equipped with a HeNe laser emitting at about 2949 cm(-1), was applied for the optical characterization of inhomogeneous poly(3-hexylthiophene) [P3HT] and poly(N-isopropylacrylamide) [PNIPAAm] organic thin films used for opto-electronics and bioapplications. With the constant development of tunable IR laser sources, laser-based infrared ellipsometry is a promising technique for fast in-depth mapping characterization of thin films and blends.

In Situ Infrared Ellipsometry for Protein Adsorption Studies on Ultrathin Smart Polymer Brushes in Aqueous Environment.

The protein-adsorbing and -repelling properties of various smart nanometer-thin polymer brushes containing poly(N-isopropylacrylamide) and poly(acrylic acid) with high potential for biosensing and biomedical applications are studied by in situ infrared-spectroscopic ellipsometry (IRSE). IRSE is a highly sensitive nondestructive technique that allows protein adsorption on polymer brushes to be investigated in an aqueous environment as external stimuli, such as temperature and pH, are varied. These changes are relevant to conditions for regulation of protein adsorption and desorption for biotechnology, biocatalysis, and bioanalytical applications. Here brushes are used as model surfaces for controlling protein adsorption of human serum albumin and human fibrinogen. The important finding of this work is that IRSE in the in situ experiments in protein solutions can distinguish between contributions of polymer brushes and proteins. The vibrational bands of the polymers provide insights into the hydration state of the brushes, whereas the protein-specific amide bands are related to changes of the protein secondary structure.

Gyrotropy in achiral materials: the coupled oscillator model.

A coupled oscillator model is developed to explain the observation of gyrotropy in achiral metamaterials. By the action of distinct excitation modes, which only combine under oblique incidence, the measurement of circular birefringence in a split-ring resonator (SRR) array is explained. The symmetry of the SRR resembles the water molecule, and parallels between the systems are drawn.