Ángel Millán

A luminescent molecular thermometer for long-term absolute temperature measurements at the nanoscale

Temperature is a fundamental thermodynamic variable, the measurement of which is crucial in countless scientific investigations and technological developments, accounting at present for 75%–80% of the sensor market throughout the world.[1] Traditional liquid-filled and bimetallic thermometers, thermocouples, pyrometers, and thermistors are generally not suitable for temperature measurements at scales below 10 μm. This intrinsic limitation has encouraged the development of new non-contact accurate thermometers with micrometric and nanometric precision, a challenging research topic increasingly hankered for [1–2].

Lanthanide-based luminescent molecular thermometers

Non-invasive accurate thermometers with high spatial resolution and operating at sub-micron scales, where the conventional methods are ineffective, are currently a very active field of research strongly stimulated in the last couple of years by the challenging demands of nanotechnology and biomedicine. This perspective offers a general overview of recent examples of accurate luminescent thermometers working at micrometric and nanometric scales, particularly those involving advanced Ln3+-based functional organic–inorganic hybrid materials.

Role of calcium-phosphate deposition in vascular smooth muscle cell calcification

In this work we are studying whether calcium phosphate deposition (CPD) during vascular calcification is a passive or a cell-mediated mechanism. Passive CPD was studied in fixed vascular smooth muscle cells (VSMC), which calcify faster than live cells in the presence of 1.8 mM Ca²(+) and 2 mM P(i). CPD seems to be a cell-independent process that depends on the concentration of calcium, phosphate, and hydroxyl ions, but not on Ca × P(i) concentration products, given that deposition is obtained with 2 × 2 and 4 × 1 Ca × P(i) mM² but not with 2 × 1 or 1 × 4 Ca × P(i) mM². Incubation with 4 mM P(i) without CPD (i.e., plus 1 mM Ca) does not induce osteogene expression. Increased expression of bone markers such as Bmp2 and Cbfa1 is only observed concomitantly with CPD. Hydroxyapatite is the only crystalline phase in both lysed and live cells. Lysed cell deposits are highly crystalline, whereas live cell deposits still contain large amounts of amorphous calcium. High-resolution transmission electron microscopy revealed a nanostructure of rounded crystallites of 5-10 nm oriented at random in lysed cells, which is compatible with spontaneous precipitation. The nanostructure in live cells consisted of long fiber crystals, 10-nm thick, embedded in an amorphous matrix. This structure indicates an active role of cells in the process of hydroxyapatite crystallization. In conclusion, our data suggest that CPD is a passive phenomenon, which triggers the osteogenic changes that are involved in the formation of a well organized, calcified crystalline structure.

Production of magnetic nanoparticles in a polyvinylpyridine matrix

AbstractA commercial polyvinylpyridine polymer has been used for the in situ preparation of magnetic nanocomposites. Several procedures havebeen followed in the preparation of the nanoparticles based on the formation of polymer–metal complex gels. The use of mixtures of proticand aprotic solvents for the reaction of the polymer with the metal salts is discussed. Superparamagnetic nanoparticles of CoO and Fe 2 O 3 have been obtained with an average size of 50 and 10 nm, respectively. The utility of nitrogen base polymers for the fabrication of magneticnanocomposites is emphasized. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polymer nanocomposites; Magnetic nanoparticles; Metal oxides 1. IntroductionMagnetic nanocomposites have many possible technolo-gical applications [1,2]. Several solid matrixes have beenused in this kind of material including gels, glasses, zeolites,metals, etc. Although, the most suitable matrix will alwaysdepend on the future application of the material, organicpolymers have been often a suitable choice because oftheir high processability and versatility. The in situ precipi-tation of the magnetic particles in a polymeric matrix is anexcellent method to control the mean size and size disper-sion of a nanoparticle population. Polymers employed forthis purpose in the past have been selected from variousconsiderations. Sulfonated polymers have often been useddue to the capability of the sulfonic groups to retain metalions [3–5]. Polymers with a tridimensional structure, suchas ion interchange resins, have rigid pores that set an upperlimit to the size of particles that grow inside [6]. Diblockcopolymers, composed of a part with charged radicals andanother part with hydrophobic groups, contain polar nanor-egions of a controllable size and shape [7] that serve asexcellent sites for the encapsulation of inorganic particles[8]. Cross-linking iron ions in polysaccharide complex gelsare known to act as nucleation centers for iron oxide nano-particles [9]. Polypirrole polymers have been chosen aselectrical conductive matrixes [10]. Phenolic polymerspermit the fabrication of submicron spherical composites[11]. In general, polymers used as a matrix for the precipita-tion of magnetic nanoparticles are polyelectrolytes becauseof their capability to absorb metal ions. However, neutralpolymers might be more advantageous concerning proces-sability and versatility. Recently, we have successfullygrown several kind of magnetic nanoparticles in N-basedpolymeric matrixes[12]. In this work, a commercial poly(4-vinylpyridine) (PVP) polymer has been used to producenanocomposites of iron and cobalt oxides. As a precedent,Mo¨ssbauer spectroscopy studies had already revealed ananomalous magnetic behavior in polyvinylpyridine–iron-chloride compounds [13]. Later, magnetic nanoparticleswere grown in a PVP matrix by a laborious method invol-ving precipitation, reduction to metallic iron and furtheroxidation to magnetite [14]. Some of the advantages ofPVP as a matrix for magnetic nanocomposites are thefollowing: easily available; soluble in slightly acidicaqueous media and in polar organic solvents; resistant todegradation by acids, alkalis, reductors and oxidants; itmelts congruently; and it is thermally stable. In this work,complexes of PVP with several metal ions have beenprepared by various procedures.2. ExperimentalHigh purity reagents were purchased from Aldrich andFluka. Solvents were dried following the standard procedureand deoxygenated under an argon flow. PVP, of averagemolecular weight of 50,000 Da was supplied by Sigma.Polymer–metal coordination compounds were obtained

Joining Time-Resolved Thermometry and Magnetic-Induced Heating in a Single Nanoparticle Unveils Intriguing Thermal Properties

Whereas efficient and sensitive nanoheaters and nanothermometers are demanding tools in modern bio- and nanomedicine, joining both features in a single nanoparticle still remains a real challenge, despite the recent progress achieved, most of it within the last year. Here we demonstrate a successful realization of this challenge. The heating is magnetically induced, the temperature readout is optical, and the ratiometric thermometric probes are dual-emissive Eu(3+)/Tb(3+) lanthanide complexes. The low thermometer heat capacitance (0.021·K(-1)) and heater/thermometer resistance (1 K·W(-1)), the high temperature sensitivity (5.8%·K(-1) at 296 K) and uncertainty (0.5 K), the physiological working temperature range (295-315 K), the readout reproducibility (>99.5%), and the fast time response (0.250 s) make the heater/thermometer nanoplatform proposed here unique. Cells were incubated with the nanoparticles, and fluorescence microscopy permits the mapping of the intracellular local temperature using the pixel-by-pixel ratio of the Eu(3+)/Tb(3+) intensities. Time-resolved thermometry under an ac magnetic field evidences the failure of using macroscopic thermal parameters to describe heat diffusion at the nanoscale.

Temperature dependence of antiferromagnetic susceptibility in ferritin

Departamento de F´isica and CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal(Dated: February 6, 2009)We show that antiferromagnetic susceptibility in ferritin increases with temperature between 4.2K and 180 K (i. e. below the N´eel temperature) when taken as the derivative of the magnetization athigh fields (30×10

Surface and core magnetic anisotropy in maghemite nanoparticles determined by pressure experiments

In magnetic nanoparticles, anisotropy energy has extra contributions compared to that of the bulk counterparts, being the most relevant surface anisotropy. Here we use pressure to separate core from surface anisotropy in one system of maghemite nanoparticles dispersed in a polymer. The core anisotropy is Kcore=7.7×105 erg/cm3 while the surface anisotropy is KS=4.2×10−2 erg/cm2. This in-one-sample separation is possible due to changes in structurally ordered and disordered ratio, which induce changes in the average magnetic anisotropy energy.

Magnetic and relaxation properties of multifunctional polymer-based nanostructured bioferrofluids as MRI contrast agents

A series of maghemite/polymer composite ferrofluids with variable magnetic core size, which show a good efficiency as MRI contrast agents, are presented. These ferrofluids are biocompatible and can be proposed as possible platforms for multifunctional biomedical applications, as they contain anchoring groups for biofunctionalization, can incorporate fluorescent dyes, and have shown low cellular toxicity. The magnetic properties of the ferrofluids have been determined by means of magnetization and ac susceptibility measurements as a function of temperature and frequency. The NMR dispersion profiles show that the low frequency behavior of the longitudinal relaxivity r1 is well described by the heuristic model of 1H nuclear relaxation induced by superparamagnetic nanoparticles proposed by Roch and co‐workers. The contrast efficiency parameter, i.e., the nuclear transverse relaxivity r2, for samples with d > 10 nm assumes values comparable with or better than the ones of commercial samples, the best results obtained in particles with the biggest magnetic core, d = 15 nm. The contrast efficiency results are confirmed by in vitro MRI experiments at ν = 8.5 MHz, thus allowing us to propose a set of optimal microstructural parameters for multifunctional ferrofluids to be used in MRI medical diagnosis. Magn Reson Med, 2011.

Organic–Inorganic Eu3+/Tb3+ codoped hybrid films for temperature mapping in integrated circuits

The continuous decrease on the geometric size of electronic devices and integrated circuits generates higher local power densities and localized heating problems that cannot be characterized by conventional thermographic techniques. Here, a self-referencing intensity-based molecular thermometer involving a di-ureasil organic-inorganic hybrid thin film co-doped with Eu3+ and Tb3+ tris (β-diketonate) chelates is used to obtain the temperature map of a FR4 printed wiring board with spatio-temporal resolutions of 0.42 μm/4.8 ms.

Magnetic properties of maghemite nanoparticles in a polyvinylpyridine matrix

Abstract Magnetic measurements have been carried out on nanocomposites consisting of individual maghemite (γ-Fe 2 O 3 ) particles distributed throughout a polyvinylpyridine matrix with a range of particle sizes and concentrations. Magnetic measurements and particle size measurements have been compared to investigate particle size effects upon magnetic properties. Magnetisation increases slowly with increasing iron content up to approximately 18%, at which point the increase becomes sharper. Magnetic moment, as derived from a modified Langevin equation, is found to be less than the value expected from the particle, indicating possible surface effects. Blocking temperatures have been observed to increase with increasing iron concentration. Assuming no significant particle interactions it is possible to consider the effective anisotropy of individual particles. Effective anisotropy has been observed to increase with decreasing particle size, implying an increase in contribution from surface effects, and increases with the presence of rod shaped particles, implying a contribution from shape anisotropy.