Simultaneous Nanomechanical and Electrochemical Mapping: Combining Peak Force Tapping Atomic Force Microscopy with Scanning Electrochemical Microscopy

Soft electronic devices play a crucial role in, e.g., neural implants as stimulating electrodes, transducers for biosensors, or selective drug-delivery. Because of their elasticity, they can easily adapt to their environment and prevent immunoreactions leading to an overall improved long-term performance. In addition, flexible electronic devices such as stretchable displays will be increasingly used in everyday life, e.g., for so-called electronic wearables. Atomic force microscopy (AFM) is a versatile tool to characterize these micro- and nanostructured devices in terms of their topography. Using advanced imaging techniques such as peak force tapping (PFT), nanomechanical properties including adhesion, deformation, and Young’s modulus can be simultaneously mapped along with surface features. However, conventional AFM provides limited laterally resolved information on electrical or electrochemical properties such as the activity of an electrode array. In this study, we present the first combination of AFM with scanning electrochemical microscopy (SECM) in PFT mode, thereby offering spatially correlated electrochemical and nanomechanical information paired with high-resolution topographical data under force control (QNM-AFM-SECM). The versatility of this combined scanning probe approach is demonstrated by mapping topographical, electrochemical, and nanomechanical properties of gold microelectrodes and of gold electrodes patterned onto polydimethylsiloxane

Versatile Analytical Platform Based on Graphene-Enhanced Infrared Attenuated Total Reflection Spectroscopy

Graphene, with its unique properties including atomic thickness, atomic uniformity, and delocalized π bonds, has been reported as a promising alternative material versus noble metals for surface-enhanced spectroscopies. Here, a simple and effective graphene-enhanced infrared absorption (GEIRA) strategy was developed based on infrared attenuated total reflection spectroscopy (IR-ATR). The IR signals of a broad range of molecules were significantly enhanced using graphene-decorated diamond ATR crystal surfaces versus conventional ATR waveguides. Utilizing rhodamine 6G (R6G) as the main model molecule, the experimental conditions were optimized, and potential enhancement mechanisms are discussed. Aqueous sample solutions were directly analyzed utilizing graphene dispersions, which eliminates harsh experimental conditions, tedious sample pretreatment, and sophisticated fabrication/patterning routines at the ATR waveguide surface. The GEIRA approach presented here provides simple experimental procedures, convenient operation, and excellent reproducibility, promoting a more widespread usage of graphene in surface-enhanced infrared absorption spectroscopy

Versatile Analytical Platform Based on Graphene-Enhanced Infrared Attenuated Total Reflection Spectroscopy

Graphene, with its unique properties including atomic thickness, atomic uniformity, and delocalized π bonds, has been reported as a promising alternative material versus noble metals for surface-enhanced spectroscopies. Here, a simple and effective graphene-enhanced infrared absorption (GEIRA) strategy was developed based on infrared attenuated total reflection spectroscopy (IR-ATR). The IR signals of a broad range of molecules were significantly enhanced using graphene-decorated diamond ATR crystal surfaces versus conventional ATR waveguides. Utilizing rhodamine 6G (R6G) as the main model molecule, the experimental conditions were optimized, and potential enhancement mechanisms are discussed. Aqueous sample solutions were directly analyzed utilizing graphene dispersions, which eliminates harsh experimental conditions, tedious sample pretreatment, and sophisticated fabrication/patterning routines at the ATR waveguide surface. The GEIRA approach presented here provides simple experimental procedures, convenient operation, and excellent reproducibility, promoting a more widespread usage of graphene in surface-enhanced infrared absorption spectroscopy

In Situ Trace Analysis of Oil in Water with Mid-Infrared Fiberoptic Chemical Sensors

The determination of trace amounts of oil in water facilitates the forensic analysis on the presence and origin of oil in the aqueous environment. To this end, the present study focuses on direct sensing schemes for quantifying trace amounts of oil in water using mid-infrared (MIR) evanescent field absorption spectroscopy via fiberoptic chemical sensors. MIR transparent silver halide fibers were utilized as optical transducer for interrogating oil-in-water emulsions via the evanescent field emanating from the waveguide surface, and penetrating the surrounding aqueous environment by a couple of micrometers. Unmodified fibers and fibers surface-modified with grafted epoxidized polybutadiene layers enabled the direct detection of crude oil in a deionized water matrix at the ppm level to ppb concentration level, respectively. Thus, direct chemical sensing of crude oil IR signatures without any sample preparation as low as 46 ppb was achieved with a response time of a few seconds

Fingerprinting Oils in Water via Their Dissolved VOC Pattern Using Mid-Infrared Sensors

An infrared attenuated total reflection (IR-ATR) method for detecting, differentiating, and quantifying hydrocarbons dissolved in water relevant for oil spills by evaluating the “fingerprint” of the volatile organic compounds (VOCs) associated with individual oil types in the mid-infrared spectral range (i.e., 800–600 cm^–1) is presented. In this spectral regime, these hydrocarbons provide distinctive absorption features, which may be used to identify specific hydrocarbon patterns that are characteristic for different crude and refined oils. For analyzing the “VOC fingerprint” resulting from various oil samples, aqueous solutions containing the dissolved hydrocarbons from different crude oils (i.e., types “<i>Barrow</i>”, “<i>Goodwyn</i>”, and “<i>Saladin</i>”) and refined oils (i.e., “<i>Petrol</i>” and “<i>Diesel</i>”) were analyzed using a ZnSe ATR waveguide as the optical sensing element. To minimize interferences from the surrounding water matrix and for amplifying the VOC signatures by enrichment, a thin layer of poly­(ethylene-<i>co</i>-propylene) was coated onto the ATR waveguide surface, thereby enabling the establishment of suitable calibration functions for the quantification of characteristic concentration patterns of the detected VOCs. Multivariate data analysis was then used for a prelininary classification of various oil-types via their VOC patterns

Impact of Urea on Monoclonal Antibodies: Multiple Destabilization and Aggregation Effects for Therapeutic Immunoglobulin G Proteins

We performed nano differential scanning fluorimetry (nanoDSF) measurements of immunoglobulin G (IgG) in urea gradient solutions under thermal unfolding. Our results show that the denaturing effect of urea on individual IgG domains can be monitored via a linear mapping of thermal shift curves to the corresponding urea concentrations. Assignment of IgG domains to each thermal shift curve allows for a reliable differentiation of the underlying mechanisms. Further results show a decisive influence of salt-induced electrostatic screening effects. We are able to explain all findings by preferential binding mechanisms in combination with electrostatic effects. The results of our study shed more light on the complex interaction mechanisms between buffer solutions and complex proteins, which are important for improving the shelf life of protein therapeutic formulation

Infrared Attenuated Total Reflection Spectroscopy for the Characterization of Gold Nanoparticles in Solution

In situ synthesis of bare gold nanoparticles mediated by stainless steel as reducing agent was monitored via infrared attenuated total reflection (IR-ATR) spectroscopy. Gold nanoparticles were directly synthesized within the liquid cell of the ATR unit taking immediate advantage of the stainless steel walls of the ATR cell. As nanoparticles were formed, a layer of particles was deposited at the SiO₂ ATR waveguide surface. Incidentally, the absorption bands of water increased resulting from surface-enhanced infrared absorption (SEIRA) effects arising from the presence of the gold nanoparticles within the evanescent field. Next to the influence of the Au­(III) precursor concentration and the temperature, the suitability of IR-ATR spectroscopy as an innovative tool for investigating changes of nanoparticles in solution, including their aggregation promoted by an increase of the ionic strength or via a pH decrease, and for detailing the sedimentation process of gold nanoparticles was confirmed

iHWG-ICL: Methane Sensing with Substrate-Integrated Hollow Waveguides Directly Coupled to Interband Cascade Lasers

The development of a compact iHWG-ICL gas sensor combining innovative substrate-integrated hollow waveguides (iHWG) with mid-infrared emitting type-II interband cascade lasers (ICL) is presented. Hence, tunable laser absorption spectroscopy (TLAS) with iHWGs in direct absorption mode is enabled. Using a room-temperature distributed feedback (DFB) ICL emitting at approximately 3.366 μm, quantitative sensing of methane was demonstrated. Wavelength scanning was obtained via current tuning for monitoring an isolated line in the v3 fundamental band of CH₄. The obtained spectra were compared to calculated spectra derived from the HITRAN2012 database. Furthermore, the performance of iHWGs simultaneously serving as miniaturized gas cell and as efficient optical waveguide at various absorption path lengths was tested and optimized. Calibration functions in the concentration range of 50 to 400 ppm_v were established enabling limits of detection ranging from 6 to 28 ppm_v. Hence, the combination of iHWGs with ICLs facilitates a new generation of compact optical sensor devices for rapid gas diagnostics in low sample volumes

Mid-Infrared Spectroscopic Method for the Identification and Quantification of Dissolved Oil Components in Marine Environments

The use of mid-infrared sensors based on conventional spectroscopic equipment for oil spill monitoring and fingerprinting in aqueous systems has to date been mainly confined to laboratory environments. This paper presents a portable-based mid-infrared attenuated total reflectance (MIR-ATR) sensor system that was used to quantify a number of environmentally relevant hydrocarbon contaminants in marine water. The sensor comprises a polymer-coated diamond waveguide in combination with a room-temperature operated pyroelectric detector, and the analytical performance was optimized by evaluating the influence of polymer composition, polymer film thickness, and solution flow rate on the sensor response. Uncertainties regarding the analytical performance and instrument specifications for dissolved oil detection were investigated using real-world seawater matrices. The reliability of the sensor was tested by exposition to known volumes of different oils; crude oil and diesel samples were equilibrated with seawater and then analyzed using the developed MIR-ATR sensor system. For validation, gas chromatographic measurements were performed revealing that the MIR-ATR sensor is a promising on-site monitoring tool for determining the concentration of a range of dissolved oil components in seawater at ppb to ppm levels

Enhanced Selectivity by Passivation: Molecular Imprints for Viruses with Exceptional Binding Properties

Inspired by the recognition processes found in biology such as enzyme–substrate and antibody–antigen interactions, synthetic systems with comparable molecular recognition properties have been investigated during recent years based on molecular imprinting strategies. While materials with recognition capabilities for small molecules (i.e., with low molecular weight) have achieved substantial advancements, the synthesis of molecularly imprinted materials with virus recognition properties remains challenging to date. Likewise, protein–surface and protein–protein interactions are essential for a wide variety of biological applications in biotechnology. In biological sensor technology the coating of surfaces to prevent nonspecific adsorption interactions plays an important role. Particularly, polyethylene glycol (PEG) stands out for its high performance in preventing proteins from nonspecifically interactions. However, blocking agents such as the protein bovine serum albumin (BSA) can also be useful as unspecific binding prevention agents for passivation, without modification of the surface. Herein the influence of blocking agents as unspecific reaction components is investigated on the enhancements of selectivity from adenovirus-imprinted particles, whereas adenovirus was used as target species in molecular imprinting. Furthermore, quantitative polymerase chain reaction (qPCR) was used for the first time as virus quantification approach in this context