Carlos M. Quintero

A novel approach for fluorescent thermometry and thermal imaging purposes using spin crossover nanoparticles

Temperature plays a fundamental role in all fields of science; hence the development of methods for measuring this property remains in vogue. Within this vast field, fluorescent thermometry appears as a simple, noninvasive and cost-effective method for providing good spatial, temporal and thermal resolution in both solid and liquid phases, even in distant or inaccessible environments. Here we describe the properties of a two-component fluorescent thermometry system comprised of Fe(II)-triazole type spin-crossover nanoparticles (temperature sensor) and an appropriate fluorophore (signal transducer). The primary advantage of this system is that the nanoparticles are modified easily, which enables fine control of the thermometric properties, while the optical properties (i.e. the signal detection) remain virtually unchanged. This system could thus be adapted in a straightforward manner to various problems where the use of fluorescent thermometry would be beneficial.

Molecular actuators driven by cooperative spin-state switching

Molecular switches have great potential to convert different forms of energy into mechanical motion; however, their use is often limited by the narrow range of operating conditions. Here we report on the development of bilayer actuator devices using molecular spin crossover materials. Motion of the bilayer cantilever architecture results from the huge spontaneous strain accompanying the spin-state switching. The advantages of using spin crossover complexes here are substantial. The operating conditions used to switch the device can be manipulated through chemical modification, and there are many existing compounds to choose from. Spin crossover materials may be switched by diverse stimuli including light, temperature, pressure, guest molecules and magnetic field, allowing complex input combinations or highly specific operation. We demonstrate the versatility of this approach by fabricating actuators from four different spin crossover materials and by using both thermal variation and light to induce motion in a controlled direction.

Soft lithographic patterning of spin crossover complexes. Part 1: fluorescent detection of the spin transition in single nano-objects

The investigation of size-reduction effects on the spin crossover properties of a class of transition metal complexes has recently become an area of intensive research. However, to avoid inter-particle interactions, ensemble averaging and matrix effects it is necessary to develop methods for the systematic study of individual nano-objects. To this aim thin films and nano-patterns of the compound [FeII(hptrz)3](OTs)2 doped with acridine orange were elaborated by spin coating and soft-lithography, respectively. The luminescence intensity was found to change significantly upon the spin transition even in isolated nano-objects of ca. 150 nm size, allowing us to monitor in a massively parallel way the spin crossover phenomenon in large arrays viafluorescence microscopy.

Synthesis of Spin‐Crossover Nano‐ and Micro‐objects in Homogeneous Media

New methods are proposed for the synthesis of spin-crossover nano- and micro-objects. Several nano-objects that are based upon the spin-crossover complex [Fe(hptrz)(3)](OTs)(2) (hptrz=4-heptyl-1,2,4-triazole, Ts=para-toluenesulfonyl) were prepared in homogeneous media. The use of various reagents (Triton X-100, PVP, TOPO, and PEGs of different molecular weights) as stabilizing agents yielded materials of different size (6 nm-2 μm) and morphology (nanorods, nanoplates, small spherical particles, and nano- and micro-crystals). In particular, when Triton X-100 was used, a variation in the morphology from nanorods to nanoplates was observed by changing the nature of the solvent. Interestingly, the preparation of the nanorods and nanoplates was always accompanied by the formation of small spherical particles. Alternatively, when PEG was used, 200-400 nm crystals of the complex were obtained. In addition, a very promising polymer-free synthetic method is discussed that was based on the preparation of relatively stable Fe(II)-triazole oligomers in CHCl(3). Their specific treatment led to micro-crystals, small nanoparticles, or gels. The size and morphology of all of these objects were characterized by TEM and by dynamic light scattering (DLS) where possible. Their spin-crossover behavior was studied by optical and magnetic measurements. The spin-transition features for large particles (>100 nm) were very similar to that of the bulk material, that is, close to room temperature with a hysteresis width of up to 8 K. The effects of the matrix and/or size-reduction led to modification of the transition temperature and an abruptness of the spin transition for oligomeric solutions and small nanoparticles of 6 nm in size.

Spin crossover composite materials for electrothermomechanical actuators

Composites of the spin crossover complex [Fe(trz)(H-trz)2](BF4) (H-trz = 1,2,4-4H-triazole and trz = 1,2,4-triazolato) dispersed in a poly(methylmethacrylate) (PMMA) matrix were synthesized and investigated for their spin crossover properties by optical reflectivity, Raman spectroscopy and calorimetry. These composite films were used to fabricate bilayer cantilevers that can perform efficient and tuneable mechanical actuation based on the spin transition. A prototype device that uses the spin transition phenomenon to convert electrical energy into mechanical motion through Joule heating is described. This device is used to perform oscillatory actuation driven by a modulated current. The ability to tune the performance of this electromechanical system is demonstrated by varying the working temperature, the applied ac current and its frequency.

Current Switching Coupled to Molecular Spin-States in Large-Area Junctions

The fabrication of large-area vertical junctions with a molecular spin-crossover complex displaying concerted changes of spin degrees of freedom and charge-transport properties is reported. Fabricated devices allow spin-state switching in the spin-crossover layer to be triggered and probed by optical means, while detecting associated changes in electrical resistance in the junctions.

Hybrid spin-crossover nanostructures.

Summary This review reports on the recent progress in the synthesis, modelling and application of hybrid spin-crossover materials, including core–shell nanoparticles and multilayer thin films or nanopatterns. These systems combine, often in synergy, different physical properties (optical, magnetic, mechanical and electrical) of their constituents with the switching properties of spin-crossover complexes, providing access to materials with unprecedented capabilities.

Detection of molecular spin-state changes in ultrathin films by photonic methods

Ultrathin films of molecular spin crossover materials exhibit very appealing properties for a variety of photonic applications because the spin-state switching is accompanied by a spectacular change of the complex refractive index in a wide spectral range. After examining different optical spectroscopic approaches for the detection of spin-state changes in nanometric films, we found that conventional light absorption measurements can be used down to the nanometer thickness if the oscillator strength of the transition is high, which is often the case for charge transfer transitions in the ultraviolet range. Methods based on fluorescence energy transfer provide a straightforward means for detecting spin-state changes in films in the visible wavelength range, even if photobleaching may be a problem for certain luminophores. Alternatively, changes in the refractive index accompanying the spin transition can be conveniently determined by surface plasmon resonance spectroscopy, which can also provide very accurate film thickness determination. Plasmonic effects were also used to investigate spin-crossover films by means of surface-enhanced Raman spectroscopy. We found that this technique can provide information not only on the spin state of the molecules in very thin layers, but also on their chemical composition and structure.

AFM Imaging of Molecular Spin‐State Changes through Quantitative Thermomechanical Measurements

Quantitative atomic force microscopy is used in conjunction with microwire heaters for high-resolution imaging of the Youngs modulus changes across the spin-state transition. When going from the high spin to the low spin state, a significant stiffening is observed.

High Spatial Resolution Imaging of Transient Thermal Events Using Materials with Thermal Memory

The working principle of a new kind of nanothermometer is experimentally demonstrated using bistable materials with thermal memory. This thermometry approach allows for acquiring sub-wavelength resolution images of fast, transient heating events.