Michel Kuhl

Soft substrates are interesting for a range of applications from mimicking cellular micro-environment to implants. Conductive electrodes on soft substrates open a broad spectrum of possibilities such as electrical and electrochemical sensing coupled with the flexibility, elasticity and transparency of the underlying substrate. Single layer graphene on a soft substrate as a candidate for such flexible electrodes brings the additional 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). Specifically, we have systematically studied the effect of surface energy before, during and after the transfer of graphene. Based on this, 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 in millimeter-scale on PDMS, which can withstand continuous measurements in cell culture media for several days. From local nanomechanical measurements with an AFM, we infer that the softness of the substrate is slightly affected after graphene transfer. However, we can modulate the stiffness using PDMS of differing composition. Finally, we show that graphene sensors on PDMS can be successfully used as electrodes for real-time monitoring of cell adhesion kinetics on a soft substrate. The routine availability of single layer graphene electrodes on a flexible soft substrate with tunable stiffness will open a new avenue for a range of studies, where the PDMS-liquid interface is made conducting with minimal alteration of the intrinsic material properties such as softness, flexibility, elasticity and transparency.

Artificial van der Waals heterostructures, obtained by stacking layered two-dimensional materials, represent a novel material platform for investigating physicochemical phenomena and applications. Here, the electrochemistry at the one-dimensional edge of a graphene sheet, which is sandwiched between two hexagonal boron nitride (hBN) multilayer flakes, is reported. When such an hBN/graphene/hBN heterostructure is immersed in a solution, the basal plane of graphene is protected and isolated by the hBN stack, and the edge of the graphene sheet is exclusively available in the solution. This forms an electrochemical nanoelectrode, which enabled the investigation of electron transfer using several redox probes, e.g., ferrocene(di)methanol, hexaammineruthenium, methylene blue, dopamine and ferrocyanide. The relatively low capacitance of the van der Waals edge electrode facilitates cyclic voltammetry at very high scan rates (up to 1000 V/s). Using fast scan cyclic voltammetry imaging, redox species could be detected voltammetrically down to micromolar concentrations with sub-second time resolution at the sandwiched graphene edge, promoted by the rapid equilibration of analyte species in the diffusion layer. Furthermore, the nanoband nature of the edge electrode allows its operation directly in water in the absence of added electrolyte. Finally, two adjacent edge electrodes could be realized in a redox-cycling format. In all, the van der Waals edge electrode is unique among nanoelectrodes as it enables investigations of all the above-mentioned phenomena in the same device. Due to its versatility, it constitutes a new avenue for nanoscale electrochemistry, which will be useful for studying electron transfer mechanisms as well as for the detection of analyte species in ultralow sample volumes.

Chiral and achiral vibrational sum-frequency generation (VSFG) spectroscopy was performed in the 1400–1700 and 2800–3800 cm⁻¹ range to study the interfacial structure of photoactive yellow protein (PYP) adsorbed on polyethyleneimine (PEI) and poly-l-glutamic acid (PGA) surfaces. Nanometer-thick polyelectrolyte layers served as the substrate for PYP adsorption, with 6.5-pair layers providing the most homogeneous surfaces. When the topmost material was PGA, it acquired a random coil structure with a small number of β₂-fibrils. Upon adsorption on oppositely charged surfaces, PYP yielded similar achiral spectra. However, the VSFG signal intensity increased for PGA surfaces with a concomitant redshift of the chiral C^α-H and N–H stretching bands, suggesting increased adsorption for PGA compared to PEI. At low wavenumbers, both the backbone and the side chains of PYP induced drastic changes to all measured chiral and achiral VSFG spectra. Decreasing ambient humidity led to the loss of tertiary structure with a re-orientation of α-helixes, evidenced by a strongly blue-shifted chiral amide I band of the β-sheet structure with a shoulder at 1654 cm⁻¹. Our observations indicate that chiral VSFG spectroscopy is not only capable of determining the main type of secondary structure of PYP, i.e., β-scaffold, but is also sensitive to tertiary protein structure.

Chiral and achiral vibrational sum-frequency generation (VSFG) spectroscopy was performed in the 1400-1700 and 2800-3800 cm-1 range to study the interfacial structure of photoactive yellow protein (PYP) adsorbed on polyethyleneimine (PEI) and poly-L-glutamic acid (PGA) surfaces. Nanometer-thick polyelectrolyte layers served as the substrate for PYP adsorption, with 6.5-pair layers providing the most homogeneous surfaces. When the topmost material was PGA, it acquired a random coil structure with a small number of ß2-fibrils. Upon adsorption on oppositely charged surfaces, PYP yielded similar achiral spectra. However, the VSFG signal intensity increased for PGA surfaces with a concomitant redshift of the chiral C-H and N-H stretching bands suggesting increased adsorption for PGA compared to PEI. At low wavenumbers, both the backbone and the side chains of PYP induced drastic changes to all measured chiral and achiral VSFG spectra. Decreasing ambient humidity led to the loss of tertiary structure with a re-orientation of a-helixes, evidenced by a strongly blue-shifted chiral amide I band of the ß-sheet structure with a shoulder at 1654 cm-1. Our observations indicate that chiral VSFG spectroscopy is not only capable of determining the main type of secondary structure of PYP, i.e., ß-scaffold, but is also sensitive to tertiary protein structure.