Unlocking the Essence of Lignin: High-Performance Adhesives That Bond via Thiol-Catechol Connectivities and Debond on Electrochemical Command
Authors
K. Walter, D. Hoch, L. Hertweck, K. Balasubramanian, J. Geisler, M. Röllig, T. Neubert, H. Börner
Year of publication
Published in:
Advanced Materials
The next generation of adhesives requires effective debonding capabilities that can be triggered on demand to enable advanced circular repair and recycling strategies. A new class of lignin-inspired, two-component (2K) structural adhesives offers bonding strengths of up to 20 MPa and clean, on-command electrochemical debonding within 5–30 min. The debonding is induced by a distinct electrochemical oxidation of thiol-catechol connectivities (TCCs) within the entire adhesive network, enforcing rapid and clean adhesive failure on the cathodic substrate side. The TCC-functionalities are formed during curing by a thiol-quinone Michael-type polyaddition, reacting polyester-based trithiols with tris-quinones as lignin-inspired minimal building blocks. The structural adhesive can be fine-tuned by adjusting the formulation. The addition of carbon black and ionic liquids facilitates the desired electrochemical transformation of TCC-catechols to TCC-quinones. Applying only 9 V for 5–30 min, leads to clean debonding with 72–86% loss of shear strength. A comprehensive study of curing, bonding, and debonding behavior by rheological, spectroscopic, and electrochemical investigations reveals the debonding mechanism by correlating catechol oxidation to adhesive performance. The electrochemical debonding capability of TCC-structural adhesives is demonstrated in a functional prototype, where on-command detachment of a cover glass from a display device is achieved within 6.5 min.
Electrosynthesis of Mussel-inspired Adhesive Polymers as a Novel Class of Transient Enzyme Stabilizers
Authors
T. Neubert, M. Hielscher, K. Walter, C. Schröter, M. Stage, R. Rosencrantz, F. Panis, A. Rompel, K. Balasubramanian, S. Waldvogel, H. Börner
Year of publication
Published in:
Angewandte Chemie: International Edition
Multifunctional ortho-quinones are required for the formation of thiol-catechol-connectivities (TCC) but can be delicate to handle. We present the electrochemical oxidation of the dipeptide DiDOPA, achieving up to 92 % conversion efficiency of the catechols to ortho-quinones. Graphite and stainless steel could be employed as cost-efficient electrodes. The electrochemical activation yields quinone-solutions, which are free of undesired reactive compounds and eliminates the challenging step of isolating the reactive quinones. The DiDOPA quinones were employed in polyaddition reactions with multi-thiols, forming oligomers that functioned as transient enzyme stabilizers (TES). These TCC-TES-additives improved the thermal stability and the activity of tyrosinase in heat stress assays.
Quantum Dot Modification of Large Area Graphene Surfaces via Amide Bonding
Authors
T. Neubert, F. Rösicke, K. Hinrichs, N. Nickel, J. Rappich
Year of publication
Published in:
Advanced Materials Interfaces
Large-area graphene grown by chemical vapor deposition is functionalized by p-aminophenyl groups via the diazonium-route. Subsequently, carboxylic acid-modified CdS X Se ₁₋X /ZnS dots (QDs) with maximum emission wavelengths of 540 nm and 630 nm are immobilized via amidation reaction to the amino-functionalized graphene on the copper growth substrate forming stable covalent bonds in all functionalization steps. After immobilization of the QDs, the functional QD/graphene stack is available for transfer from the growth substrate onto any other substrate. In this study, the QD-modified graphene is transferred to a Si wafer with surface oxide without losing the QD modification. The successful binding of the QDs onto the functionalized graphene is characterized by their vibrational signature using Raman backscattering and infrared-spectroscopic ellipsometry and by their specific light emission as measured by photoluminescence (PL) spectroscopy.
Electrochemistry at the Edge of a van der Waals Heterostructure
Authors
A. Plačkić, T. Neubert, K. Patel, M. Kuhl, K. Watanabe, T. Taniguchi, A. Zurutuza, R. Sordan, K. Balasubramanian
Year of publication
Published in:
Small : nano micro
Artificial van der Waals heterostructures, obtained by stacking two-dimensional (2D) materials, represent a novel platform for investigating physicochemical phenomena and applications. Here, the electrochemistry at the one-dimensional (1D) edge of a graphene sheet, sandwiched between two hexagonal boron nitride (hBN) flakes, is reported. When such an hBN/graphene/hBN heterostructure is immersed in a solution, the basal plane of graphene is encapsulated by hBN, and the graphene edge is exclusively available in the solution. This forms an electrochemical nanoelectrode, enabling the investigation of electron transfer using several redox probes, e.g., ferrocene(di)methanol, hexaammineruthenium, methylene blue, dopamine and ferrocyanide. The low capacitance of the van der Waals edge electrode facilitates cyclic voltammetry at very high scan rates (up to 1000 V s −¹ ), allowing voltammetric detection of redox species down to micromolar concentrations with sub-second time resolution. The nanoband nature of the edge electrode allows operation in water without added electrolyte. Finally, two adjacent edge electrodes are realized in a redox-cycling format. All the above-mentioned phenomena can be investigated at the edge, demonstrating that nanoscale electrochemistry is a new application avenue for van der Waals heterostructures. Such an edge electrode will be useful for studying electron transfer mechanisms and the detection of analyte species in ultralow sample volumes.
Redox-Triggered Debonding of Mussel-Inspired Pressure Sensitive Adhesives: Improving Efficiency Through Functional Design
Authors
T. Neubert, K. Walter, C. Schröter, V. Guglielmotti, K. Hinrichs, S. Reinicke, A. Taden, K. Balasubramanian, H. Börner
Year of publication
Published in:
Angewandte Chemie: International Edition
Debondable pressure-sensitive adhesives (PSAs) promise access to recyclability in microelectronics in the transition toward a circular economy. Two PSAs were synthesized from a tetravalent thiol star-polyester forming thiol-catechol-connectivities (TCC) with either the biorelated DiDopa-bisquinone (BY*Q) or the fossil-based bisquinone A (BQA). The PSAs enable debonding by oxidation of TCC-catechols to quinones. The extent of debonding efficiency depends on the interaction modes, which are determined by the chemical structure differences of both TCC-motifs. BY*Q-TCC-PSA debonds with exceptional loss of 99 % of its approx. 2 MPa shear strength in glass-on-glass junctions. For BQA-TCC-PSA, a debonding efficiency of only approx. 60 % was found, irrespective of its initial shear strength, which could be tuned up to approx. 7 MPa. The efficiency of debonding for BY*Q-TCC-PSA after TCC-oxidation is linked to the loss of synergistic interactions without strongly affecting the bulk glue properties, outperforming the purely catechol-based BQA-analogue.
Real-Time Monitoring of Cell Adhesion onto a Soft Substrate by a Graphene Impedance Biosensor
Authors
V. Guglielmotti, E. Fuhry, T. Neubert, M. Kuhl, D. Pallarola, K. Balasubramanian
Year of publication
Published in:
ACS Sensors
Soft substrates are interesting for many applications, ranging from mimicking the cellular microenvironment to implants. Conductive electrodes on such substrates allow the realization of flexible, elastic, and transparent sensors. Single-layer graphene as a candidate for such electrodes brings the advantage that the active area of the sensor is transparent and conformal to the underlying substrate. Here, we overcome several challenges facing the routine realization of graphene cell sensors on a canonical soft substrate, namely, poly(dimethylsiloxane) (PDMS). We have systematically studied the effect of surface energy before, during, and after the transfer of graphene. Thus, we have identified a suitable support polymer, optimal substrate (pre)treatment, and an appropriate solvent for the removal of the support. Using this procedure, we can reproducibly obtain stable and intact graphene sensors on a millimeter scale on PDMS, which can withstand continuous measurements in cell culture media for several days. From local nanomechanical measurements, we infer that the softness of the substrate is slightly affected after the graphene transfer. However, we can modulate the stiffness using PDMS with differing compositions. Finally, we show that graphene sensors on PDMS can be successfully used as soft electrodes for real-time monitoring of the cell adhesion kinetics. The routine availability of single-layer graphene electrodes on a soft substrate with tunable stiffness will open a new avenue for studies, where the PDMS-liquid interface is made conducting with minimal alteration of the intrinsic material properties such as softness, flexibility, elasticity, and transparency.
Statistical Copolymers that Mimic Aspects of Mussel Adhesive Proteins: Access to Robust Adhesive-Domains for Non-Covalent Surface PEGylation
Authors
S. Peplau, T. Neubert, K. Balasubramanian, J. Polleux, H. Börner
Year of publication
Published in:
Macromolecular rapid communications : MRC
Reconstructing functional sequence motifs of proteins, using statistical copolymers greatly reduces the information content, but simplifies synthesis significantly. Key amino acid residues involved in the adhesion of mussel foot proteins are identified. The side-chain functionalities of Dopa, lysine, and arginine are abstracted and incorporated into acrylate monomers to allow controlled radical polymerization. The resulting Dopa-acrylate (Y*-acr), arginine-acrylate (R-acr), and lysine-acrylate (K-acr) monomers are polymerized in different monomer ratios and compositions by reversible addition fragmentation transfer polymerization with a poly(ethylene glycol) (PEG) macrochain transfer agent. This results in two sets of PEG-block-copolymers with statistical mixtures and different monomer ratios of catechol/primary amine and catechol/guanidine side-chain functionalities, both important pairs for mimicking π-cation interactions. The coating behavior of these PEG-block-copolymers is evaluated using quartz crystal microbalance with dissipation energy monitoring (QCM-D), leading to non-covalent PEGylation of the substrates with clear compositional optima in the coating stability and antifouling properties. The coatings prevent non-reversible albumin or serum adsorption, as well as reduce cellular adhesion and fungal spore attachment.
Infrared and Raman spectroscopic analysis of functionalized graphene
Authors
K. Hinrichs, T. Neubert, C. Kratz, T. Shaykhutdinov, J. Rappich, K. Balasubramanian
Year of publication
Published in:
Applied research
In this mini-review, we discuss our work in the analysis of material properties of electrochemically functionalized graphene using infrared (IR) and Raman techniques as thin film sensitive vibrational spectroscopies. Multiscale characterization is demonstrated using a combination of IR spectroscopic ellipsometry (IRSE), confocal Raman spectroscopy (RS) and photothermal atomic force microscopy coupled IR analysis (AFM-IR). IRSE is used for spots with dimensions of a few mm, RS for spots with a diameter of a few micrometer and AFM-IR at the sub-100 nm scale. In the first part of the article, functionalized large-area graphene sheets electrochemically modified by ultrathin oligomer films of maleimidophenyl or 4-aminophenyl acetic acid are studied. Characteristic molecular vibrations of the functional layers are analyzed by the IR methods, while the graphene-related phonons are characterized by RS. This dual approach allowed for the determination of the attached functional groups as well as the identification of the nature of chemical coupling of the functional moieties to graphene. The thickness of the deposited layers extracted from IRSE data correlates very well with AFM measurements. In the second part of the article, functionalized graphene sheets are characterized by correlative vibrational optical spectroscopy directly at a device level.
Sensing Mechanisms in Graphene Field‐Effect Transistors Operating in Liquid
Authors
T. Neubert, K. Balasubramanian
Year of publication
Published in:
Graphene Field-Effect Transistors : advanced bioelectronic devices for sensing applications
Field-effect transistors (FETs) based on graphene are highly promising as chemical and biological sensors, due to the high sensitivity of the two-dimensional material to analyte molecules in its environment. A reliable operation of these devices in a liquid environment is mandatory in applications such as biosensing, where the concentration of biomolecules needs to be determined in their native form. For practical applications, it is necessary to have a clear idea of the mechanisms behind the detection of molecular species. In this chapter, we focus on the various sensing mechanisms that play a role when graphene FETs are deployed in solution. On the one hand, we draw parallels to known mechanistic information from sensors operating in air, while on the other, we present an electrochemical perspective of the possible molecular interactions at the graphene–liquid interface. Finally, using selected examples, we show how the measured data have been interpreted based on this understanding.
A highly durable graphene monolayer electrode under long-term hydrogen evolution cycling
Authors
M. Wehrhold, T. Neubert, T. Grosser, M. Vondráček, J. Honolka, K. Balasubramanian
Year of publication
Published in:
Chemical Communications
Achieving long term stability of single graphene sheets towards repeated electrochemical hydrogen evolution reaction (HER) cycling has been challenging. Here, we show through appropriate electrode preparation that it is possible to obtain highly durable isolated graphene electrodes, which can survive several hundreds of HER cycles with virtually no damage to the sp ² -carbon framework and persistently good electron transfer characteristics.
