Molecularly Imprinted Polymer-Based Electrochemical Sensor for the Detection of Azoxystrobin in Aqueous Media.

This work presents an electrochemical sensor detecting a fungicide-azoxystrobin (AZO) in aqueous environments. This AZO sensor utilizes a thin-film metal electrode (TFME) combined with an AZO-selective molecularly imprinted polymer (AZO-MIP). The AZO-MIP was directly generated on TFME through electrochemical polymerization from the solution containing two functional monomers: aniline (Ani) and m-phenylenediamine (mPD), and the template: AZO, which was afterwards removed to form AZO-selective cavities in the polymer matrix. The AZO-MIP preparation was characterized by electrochemical and ellipsometry measurements. Optimization of the synthesis parameters, including the charge density applied during electrodeposition, the monomer-to-template ratio, was performed to enhance the sensor's performance. The results demonstrated that the AZO sensor achieved a low limit of detection (LOD) of 3.6 nM and a limit of quantification (LOQ) of 11.8 nM in tap water, indicating its sensitivity in a complex aqueous environment. The sensor also exhibited satisfactory selectivity for AZO in both ultrapure and tap-water samples and achieved a good recovery (94-119%) for the target analyte. This study highlights the potential of MIP-based electrochemical sensors for the rapid and accurate detection of fungicide contaminants in water, contributing to the advancement of analytical tools for water-quality monitoring and risk assessment.

Surface-Mediated Charge Transfer of Photogenerated Carriers in Diamond.

Solvated electrons are highly reductive chemical species whose chemical properties remain largely unknown. Diamond materials are proposed as a promising emitter of solvated electrons and visible light excitation would enable solar-driven CO2 or N2 reductions reactions in aqueous medium. But sub-bandgap excitation remains challenging. In this work, the role of surface states on diamond materials for charge separation and emission in both gaseous and aqueous environments from deep UV to visible light excitation is elucidated. Four different X-ray and UV-vis spectroscopy methods are applied to diamond materials with different surface termination, doping and crystallinity. Surface states are found to dominate sub-bandgap charge transfer. However, the surface charge separation is drastically reduced for boron-doped diamond due to a very high density of bulk defects. In a gaseous atmosphere, the oxidized diamond surface maintains a negative electron affinity, allowing charge emission, due to remaining hydrogenated and hydroxylated groups. In an aqueous electrolyte, a photocurrent for illumination down to 3.5 eV is observed for boron-doped nanostructured diamond, independent of the surface termination. This study opens new perspectives on photo-induced interfacial charge transfer processes from metal-free semiconductors such as diamonds.

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.

Interference with ERK-dimerization at the nucleocytosolic interface targets pathological ERK1/2 signaling without cardiotoxic side-effects.

Dysregulation of extracellular signal-regulated kinases (ERK1/2) is linked to several diseases including heart failure, genetic syndromes and cancer. Inhibition of ERK1/2, however, can cause severe cardiac side-effects, precluding its wide therapeutic application. ERKT188-autophosphorylation was identified to cause pathological cardiac hypertrophy. Here we report that interference with ERK-dimerization, a prerequisite for ERKT188-phosphorylation, minimizes cardiac hypertrophy without inducing cardiac adverse effects: an ERK-dimerization inhibitory peptide (EDI) prevents ERKT188-phosphorylation, nuclear ERK1/2-signaling and cardiomyocyte hypertrophy, protecting from pressure-overload-induced heart failure in mice whilst preserving ERK1/2-activity and cytosolic survival signaling. We also examine this alternative ERK1/2-targeting strategy in cancer: indeed, ERKT188-phosphorylation is strongly upregulated in cancer and EDI efficiently suppresses cancer cell proliferation without causing cardiotoxicity. This powerful cardio-safe strategy of interfering with ERK-dimerization thus combats pathological ERK1/2-signaling in heart and cancer, and may potentially expand therapeutic options for ERK1/2-related diseases, such as heart failure and genetic syndromes.

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.

Gradient metal nanoislands as a unified surface enhanced Raman scattering and surface enhanced infrared absorption platform for analytics.

In the last few decades, the use of plasmonics in vibrational spectroscopy has expanded the scope of (bio)analytical investigations. Nevertheless, there is a demand for a combined platform that can be simultaneously efficient for Surface Enhanced Raman Scattering (SERS) and Surface Enhanced Infrared Absorption (SEIRA). Here, we present a solution on the basis of a plasmonic Ag nanoparticle layer with a thickness gradient. The optical resonance along the layer varies from the visible to the infrared range offering optimal and intermediate sites for SERS and SEIRA of the analyte molecule (mercaptobenzonitrile). Enhancement factors for the same mode were determined to be ca. 104 and 170 for SERS and SEIRA, respectively. We present a full optical and vibrational characterization and demonstrate further tunability. The platform resolves reproducibility and comparability issues by a combination of the two methods. It also offers individualized solutions for different investigation conditions, i.e. a choice between excitation wavelengths and resonant Raman molecules. The multiple applicabilities of the presented unifying substrate can contribute to the expansion of the vibrational spectroscopic field and to analytics.

Biofouling-Resistant Porous Membranes with a Precisely Adjustable Pore Diameter via 3D Polymer Grafting.

A facile route to biofouling-resistant porous thin-film membranes that can be fine-tuned for specific needs in diverse bioseparation, mass flow control, sensors, and drug delivery applications is reported. The proposed approach is based on combining two distinct macromolecular systems-a cross-linked poly(2-vinyl pyridine) network and a 3D-grafted polyethylene oxide (PEO) layer-in one robust porous material whose porosity can be adjusted within a wide range, covering the macroporous and mesoporous size regimes. Notably, this reconfigurable material maintains its antifouling properties throughout the entire range of pore size configurations because of a dense surface carpet of PEO chains with self-healing properties that are immobilized both onto the surface and inside the polymer network through what was termed 3D grafting. Experimental results are supplemented by computer simulations of a coarse-grained model of a porous membrane that shows qualitatively similar pore swelling behavior.

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.

Polarization-Dependent Atomic Force Microscopy-Infrared Spectroscopy (AFM-IR): Infrared Nanopolarimetric Analysis of Structure and Anisotropy of Thin Films and Surfaces.

Infrared techniques enable nondestructive and label-free studies of thin films with high chemical and structural contrast. In this work, we review recent progress and perspectives in the nanoscale analysis of anisotropic materials using an extended version of the atomic force microscopy-infrared (AFM-IR) technique. This advanced photothermal technique, includes polarization control of the incoming light and bridges the gap in IR spectroscopic analysis of local anisotropic material properties. Such local anisotropy occurs in a wide range of materials during molecular nucleation, aggregation, and crystallization processes. However, analysis of the anisotropy in morphology and structure can be experimentally and theoretically demanding as it is related to order and disorder processes in ranges from nanoscopic to macroscopic length scales, depending on preparation and environmental conditions. In this context IR techniques can significantly assist as IR spectra can be interpreted in the framework of optical models and numerical calculations with respect to both, the present chemical conditions as well as the micro- and nanostructure. With these extraordinary analytic possibilities, the advanced AFM-IR approach is an essential puzzle piece in direction to connect nanoscale and macroscale anisotropic thin film properties experimentally. In this review, we highlight the analytic possibilities of AFM-IR for studies on nanoscale anisotropy with a set of examples for polymer, plasmonic, and polaritonic films, as well as aggregates of large molecules and proteins.

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.

Functionalization of any substrate using covalently modified large area CVD graphene.

We report a novel route for the functionalization of any substrates, including chemically inert substrates. CVD grown graphene is electrochemically functionalized with p-(N-maleimido)phenyl residues and consecutively transferred to various substrates. The transfer process is shown to be without noticeable loss. The functional layer exhibits a thickness of appx. 4.5 nm.

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.

Mussel-Inspired Polymer Carpets: Direct Photografting of Polymer Brushes on Polydopamine Nanosheets for Controlled Cell Adhesion.

2D mussel-inspired polydopamine (PDA) nanosheets are prepared and exploited as a functional surface for grafting various polymer brushes. The PDA nanosheet and its polymer-brush derivatives show lateral integrity and are robust; therefore, they can be detached from their substrates. Cell-adhesion tests show that the PDA nanosheet promotes cell growth and attachment, while a PDA-based poly(3-sulfopropyl methacrylate) carpet exhibits nonfouling behavior.

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.