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 3þ /Tb 3þ lanthanide complexes. The low thermometer heat capacitance (0.021 3 K À1 ) and heater/thermometer resistance (1 K 3 W À1 ), the high temperature sensitivity (5.8% 3 K À1 at 296 K) and uncertainty (0.5 K), the physiological working temperature range (295À315 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 3þ /Tb 3þ intensities. Time-resolved thermometry under an ac magnetic field evidences the failure of using macroscopic thermal parameters to describe heat diffusion at the nanoscale.

We show that the coercive field in ferritin and ferrihydrite depends on the maximum magnetic field in a hysteresis loop and that coercivity and loop shifts depend both on the maximum and cooling fields. In the case of ferritin, we show that the time dependence of the magnetization also depends on the maximum and previous cooling fields. This behavior is associated to changes in the intraparticle energy barriers imprinted by these fields. Accordingly, the dependence of the coercive and loop-shift fields with the maximum field in ferritin and ferrihydrite can be described within the frame of a uniform-rotation model considering a dependence of the energy barrier with the maximum and the cooling fields.

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 r 1 is well described by the heuristic model of 1 H nuclear relaxation induced by superparamagnetic nanoparticles proposed by Roch and co-workers. The contrast efficiency parameter, i.e., the nuclear transverse relaxivity r 2 , 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 5 15 nm. The contrast efficiency results are confirmed by in vitro MRI experiments at n 5 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 66:1715-1721,

Biomedical applications of superparamagnetic iron oxide particles have been of interest for quite a number of years. Recent developments show that multifunctionality can be efficiently achieved using polymers to coat the particles and to provide anchoring elements to their surface. This leads to the formation of nanobeads with a reduced number of particles trapped by the polymeric structure. While the magnetothermic behavior of isolated nanoparticles has been a subject of interest over the past several years, multicore magnetic nanobeads have thus far not received the same attention. The influence of structural and magnetic properties in the hyperthermia performance of a series of magnetic fluids designed for biomedical purposes is studied here. The fluids are made of maghemite multicore polymeric beads, with variable nanoparticle size and hydrodynamic size, dispersed in a buffer solution. The specific loss power (SLP) was measured from 5 to 100 kHz with a field intensity of 21.8 kA/m. SLP increases with increasing magnetic core size, reaching 32 W/g Fe 2 O 3 at 100 kHz for 16.2 nm. Within the framework of the linear response theory, a graphical construction is proposed to describe the interplay of both size distributions and magnetic properties in the heating performance of such fluids in a given frequency range. Furthermore, a numerical model is developed to calculate the spare contribution of Néel and Brown relaxation mechanisms to SLP, which gives a fair reproduction of the experimental data.