Modeling the Response Function of Dual-Enzyme Microbiosensors

A general theoretical model for competitive dual-enzyme microbiosensors based on self-assembled monolayers (SAM) is presented. The model is derived for amperometric dual-enzyme ATP sensors and provides excellent agreement with experimental ATP measurements at 25 μm diameter microelectrodes. In this model, the statistical probability of a glucose molecule in competition between two enzymes, glucose oxidase (GOD)/hexokinase (HEX), at the ATP sensor surface is combined with the enzymatic reaction rate. Thereby, a simple model predicting the sensor signal for varying surface concentrations of GOD and HEX, glucose concentration, and ATP concentration is obtained. Excellent agreement of the predicted current signal with experimentally obtained sensor signals was achieved at ATP concentrations between 10 and 300 μM in a buffer containing glucose at physiologically relevant levels. Consequently, the development time for new dual-enzyme biosensors can be reduced, and an analytical model for the sensor response function is provided facilitating the calibration of enzymatic biosensors

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

Supplemental material for Optimizing the Analytical Performance of Substrate-Integrated Hollow Waveguides: Experiment and Simulation

Supplemental Material for Optimizing the Analytical Performance of Substrate-Integrated Hollow Waveguides: Experiment and Simulation by L. Tamina Hagemann, Sonja Ehrle and Boris Mizaikoff in Applied Spectroscopy</p

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

Imprinted Polymeric Materials. Insight into the Nature of Prepolymerization Complexes of Quercetin Imprinted Polymers

Molecular imprinting techniques have proved to be a highly accessible method for producing molecule-specific recognition materials for a variety of applications, ranging from sensing to catalysis and separations. In noncovalent imprinting, it is anticipated that polymerizable complexes are created in the prepolymerization solution via self-assembly of functional monomers and template molecules resulting from inherent chemical complementarity, which will ideally form binding sites within the cross-linked matrix after polymerization. On the basis of 1H NMR data and X-ray crystallographic evidence, we now infer a more important role for template self-association for the recognition properties of quercetin-imprinted polymers. While directly applicable to fundamental understanding of the molecular imprinting mechanism of this polyphenol, on a more generic scale, this work also demonstrates the utility of this strategy toward analyzing complex noncovalent interaction mechanisms between small molecules. These interactions are of particular interest for quercetin and other members of the flavone/flavonoid class of compounds, which are radical-scavenging polyphenols of substantial interest to biomedicine

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

Batch Fabrication of Atomic Force Microscopy Probes with Recessed Integrated Ring Microelectrodes at a Wafer Level

A batch fabrication process at the wafer-level integrating ring microelectrodes into atomic force microscopy (AFM) tips is presented. The fabrication process results in bifunctional scanning probes combining atomic force microscopy with scanning electrochemical microscopy (AFM−SECM) with a ring microelectrode integrated at a defined distance above the apex of the AFM tip. Silicon carbide is used as AFM tip material, resulting in reduced mechanical tip wear for extended usage. The presented approach for the probe fabrication is based on batch processing using standard microfabrication techniques, which provides bifunctional scanning probes at a wafer scale and at low cost. Additional benefits of batch fabrication include the high processing reproducibility, uniformity, and tuning of the physical properties of the cantilever for optimized AFM dynamic mode operation. The performance of batch-fabricated bifunctional probes was demonstrated by simultaneous imaging micropatterned platinum structures at a silicon dioxide substrate in intermittent (dynamic) and contact mode, respectively, and feedback mode SECM. In both, intermittent and contact mode, the bifunctional probes provided reliable correlated electrochemical and topographical data. In addition, simulations of the diffusion-limited steady-state currents at the integrated electrode using finite element methods were performed for characterizing the developed probes