Resistance switching in large-area vertical junctions of the molecular spin crossover complex [Fe(HB(tz)3)2]: ON/OFF ratios and device stability.

Multilayer crossbar junctions composed of ITO/[Fe(HB(1,2,4-triazol-1-yl)3)2]/M (with M = Al or Ca) were fabricated and investigated for their resistance switching properties. Current-voltage-temperature maps revealed ON/OFF resistance ratios as high as 400, with the ON and OFF states defined, respectively, as the low-resistance, low spin state and the high-resistance, high spin state of the spin crossover layer. Similar results were obtained with Al and Ca cathodes indicating that the charge transport in the insulating spin crossover film is at the origin of the resistance switching instead of electron injection at the electrodes. The reproducibility and stability of the device properties were also studied.

Direct Visualization of Local Spin Transition Behaviors in Thin Molecular Films by Bimodal AFM.

Thin films of the molecular spin-crossover complex [Fe(HB(1,2,4-triazol-1-yl)3 )2 ] undergo spin transition above room temperature, which can be exploited in sensors, actuators, and information processing devices. Variable temperature viscoelastic mapping of the films by atomic force microscopy reveals a pronounced decrease of the elastic modulus when going from the low spin (5.2 ± 0.4 GPa) to the high spin (3.6 ± 0.2 GPa) state, which is also accompanied by increasing energy dissipation. This technique allows imaging, with high spatial resolution, of the formation of high spin puddles around film defects, which is ascribed to local strain relaxation. On the other hand, no clustering process due to cooperative phenomena was observed. This experimental approach sets the stage for the investigation of spin transition at the nanoscale, including phase nucleation and evolution as well as local strain effects.

Finite Size Effects on the Switching Dynamics of Spin-Crossover Thin Films Photoexcited by a Femtosecond Laser Pulse.

Using ultrafast optical absorption spectroscopy, the room-temperature spin-state switching dynamics induced by a femtosecond laser pulse in high-quality thin films of the molecular spin-crossover (SCO) complex [Fe(HB(tz)3 )2 ] (tz = 1,2,4-triazol-1-yl) are studied. These measurements reveal that the early, sub-picosecond, low-spin to high-spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a timescale of tens of nanoseconds, enabling nonlinear amplification. This out-of-equilibrium dynamics is discussed in light of the characteristic timescales associated with the different switching mechanisms, i.e., the electronic and structural rearrangements of photoexcited molecules, the propagation of strain waves at the material scale, and the thermal activation above the molecular energy barrier. Importantly, the additional, nonlinear switching step appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100 ns timescale. These results provide a first milestone toward the assessment of the physical parameters that drive the photoresponse of SCO thin films, opening up appealing perspectives for their use as high-frequency all-optical switches working at room temperature.

Complete Set of Elastic Moduli of a Spin-Crossover Solid: Spin-State Dependence and Mechanical Actuation.

Molecular spin crossover complexes are promising candidates for mechanical actuation purposes. The relationships between their crystal structure and mechanical properties remain, however, not well understood. In this study, combining high pressure synchrotron X-ray diffraction, nuclear inelastic scattering, and micromechanical measurements, we assessed the effective macroscopic bulk modulus ( B = 11.5 ± 1.5 GPa), Young's modulus ( Y = 10.9 ± 1.0 GPa), and Poisson's ratio (ν = 0.34 ± 0.04) of the spin crossover complex [FeII(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl). Crystal structure analysis revealed a pronounced anisotropy of the lattice compressibility, which was correlated with the difference in spacing between the molecules as well as by the distribution of the stiffest C-H···N interactions in different crystallographic directions. Switching the molecules from the low spin to the high spin state leads to a remarkable drop of the Young's modulus to 7.1 ± 0.5 GPa both in bulk and thin film samples. The results highlight the application potential of these films in terms of strain (ε = -0.17 ± 0.05%), recoverable stress (σ = -21 ± 1 MPa), and work density ( W/V = 15 ± 6 mJ/cm3).

A Bistable Microelectromechanical System Actuated by Spin-Crossover Molecules.

We report on a bistable MEMS device actuated by spin-crossover molecules. The device consists of a freestanding silicon microcantilever with an integrated piezoresistive detection system, which was coated with a 140 nm thick film of the [Fe(HB(tz)3 )2 ] (tz=1,2,4-triazol-1-yl) molecular spin-crossover complex. Switching from the low-spin to the high-spin state of the ferrous ions at 338 K led to a reversible upward bending of the cantilever in agreement with the change in the lattice parameters of the complex. The strong mechanical coupling was also evidenced by the decrease of approximately 66 Hz in the resonance frequency in the high-spin state as well as by the drop in the quality factor around the spin transition.

Time resolved and label free monitoring of extracellular metabolites by surface enhanced Raman spectroscopy.

Metabolomics is an emerging field of cell biology that aims at the comprehensive identification of metabolite levels in biological fluids or cells in a specific functional state. Currently, the major tools for determining metabolite concentrations are mass spectrometry coupled with chromatographic techniques and nuclear magnetic resonance, which are expensive, time consuming and destructive for the samples. Here, we report a time resolved approach to monitor metabolite dynamics in cell cultures, based on Surface Enhanced Raman Scattering (SERS). This method is label-free, easy to use and provides the opportunity to simultaneously study a broad range of molecules, without the need to process the biological samples. As proof of concept, NIH/3T3 cells were cultured in vitro, and the extracellular medium was collected at different time points to be analyzed with our engineered SERS substrates. By identifying individual peaks of the Raman spectra, we showed the simultaneous detection of several components of the conditioned medium, such as L-tyrosine, L-tryptophan, glycine, L-phenylalanine, L-histidine and fetal bovine serum proteins, as well as their intensity changes during time. Furthermore, analyzing the whole Raman data set with the Principal Component Analysis (PCA), we demonstrated that the Raman spectra collected at different days of culture and clustered by similarity, described a well-defined trajectory in the principal component plot. This approach was then utilized to determine indirectly the functional state of the macrophage cell line Raw 264.7, stimulated with the lipopolysaccharide (LPS) for 24 hours. The collected spectra at different time points, clustered by the PCA analysis, followed a well-defined trajectory, corresponding to the functional change of cells toward the activated pro-inflammatory state induced by the LPS. This study suggests that our engineered SERS surfaces can be used as a versatile tool both for the characterization of cell culture conditions and the functional state of cells over time.

Superhydrophobic Surfaces Boost Fibril Self-Assembly of Amyloid ß Peptides.

Amyloid ß (Aß) peptides are the main constituents of Alzheimer's amyloid plaques in the brain. Here we report how the unique microfluidic flows exerted by droplets sitting on superhydrophobic surfaces can influence the aggregation mechanisms of several Aß fragments by boosting their fibril self-assembly. Aß(25-35), Aß(1-40), and Aß(12-28) were dried both on flat hydrophilic surfaces (contact angle (CA) = 37.3°) and on nanostructured superhydrophobic ones (CA = 175.8°). By embedding nanoroughened surfaces on top of highly X-ray transparent Si3N4 membranes, it was possible to probe the solid residues by raster-scan synchrotron radiation X-ray microdiffraction (µXRD). As compared to residues obtained on flat Si3N4 membranes, a general enhancement of fibrillar material was detected for all Aß fragments dried on superhydrophobic surfaces, with a particular emphasis on the shorter ones. Indeed, both Aß(25-35) and Aß(12-28) showed a marked crystalline cross-ß phase with varying fiber textures. The homogeneous evaporation rate provided by these nanostructured supports, and the possibility to use transparent membranes, can open a wide range of in situ X-ray and spectroscopic characterizations of amyloidal peptides involved in neurodegenerative diseases and for the fabrication of amyloid-based nanodevices.

Out-of-Plane Plasmonic Antennas for Raman Analysis in Living Cells.

Out-of-plane plasmonic nanoantennas protruding from the substrate are exploited to perform very sensitive surface enhanced Raman scattering analysis of living cells. Cells cultured on three-dimensional surfaces exhibit tight adhesion with nanoantenna tips where the plasmonic hot-spot resides. This fact provides observable cell adhesion sites combined with high plasmonic enhancement, resulting in an ideal system for Raman investigation of cell membranes.

3D plasmonic nanoantennas integrated with MEA biosensors.

Neuronal signaling in brain circuits occurs at multiple scales ranging from molecules and cells to large neuronal assemblies. However, current sensing neurotechnologies are not designed for parallel access of signals at multiple scales. With the aim of combining nanoscale molecular sensing with electrical neural activity recordings within large neuronal assemblies, in this work three-dimensional (3D) plasmonic nanoantennas are integrated with multielectrode arrays (MEA). Nanoantennas are fabricated by fast ion beam milling on optical resist; gold is deposited on the nanoantennas in order to connect them electrically to the MEA microelectrodes and to obtain plasmonic behavior. The optical properties of these 3D nanostructures are studied through finite elements method (FEM) simulations that show a high electromagnetic field enhancement. This plasmonic enhancement is confirmed by surface enhancement Raman spectroscopy of a dye performed in liquid, which presents an enhancement of almost 100 times the incident field amplitude at resonant excitation. Finally, the reported MEA devices are tested on cultured rat hippocampal neurons. Neurons develop by extending branches on the nanostructured electrodes and extracellular action potentials are recorded over multiple days in vitro. Raman spectra of living neurons cultured on the nanoantennas are also acquired. These results highlight that these nanostructures could be potential candidates for combining electrophysiological measures of large networks with simultaneous spectroscopic investigations at the molecular level.

Amyloid ß peptide conformational changes in the presence of a lipid membrane system.

Here we are presenting a comparative analysis of conformational changes of two amyloid ß peptides, Aß(25-35) and Aß(1-42), in the presence and absence of a phospholipid system, namely, POPC/POPS (1-palmitoyl-2-oleoylphospatidylcholine/palmitoyl-2-oleoylphospatidylserine), through Raman spectroscopy, synchrotron radiation micro Fourier-transform infrared spectroscopy, and micro X-ray diffraction. Ringlike samples were obtained from the evaporation of pure and mixed solutions of the proteins together with the POPC/POPS system on highly hydrophilic substrates. The results confirm the presence of a α-helical to ß-sheet transition from the internal rim of the ringlike samples to the external one in the pure Aß(25-35) residual, probably due to the convective flow inside the droplets sitting on highly hydrophilic substrates enhancing the local concentration of the peptide at the external edge of the dried drop. In contrast, the presence of POPC/POPS lipids in the peptide does not result in α-helical structures and introduces the presence of antiparallel ß-sheet material together with parallel ß-sheet structures and possible ß-turns. As a control, Aß(1-42) peptide was also tested and shows ß-sheet conformations independently from the presence of the lipid system. The µXRD analysis further confirmed these conclusions, showing how the absence of the phospholipid system induces in the Aß(25-35) a probable composite α/ß material while its coexistence with the peptide leads to a not oriented ß-sheet conformation. These results open interesting scenarios on the study of conformational changes of Aß peptides and could help, with further investigations, to better clarify the role of enzymes and alternative lipid systems involved in the amyloidosis process of Aß fragments.