Carlos D. S. Brites

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].

Ratiometric Nanothermometer Based on an Emissive Ln3+-Organic Framework

Luminescent thermometers working at the nanoscale with high spatial resolution, where the conventional methods are ineffective, have emerged over the last couple of years as a very active field of research. Lanthanide-based materials are among the most versatile thermal probes used in luminescent nanothermometers. Here, nanorods of metal organic framework Tb0.99Eu0.01(BDC)1.5(H2O)2 (BDC = 1-4-benzendicarboxylate) have been prepared by the reverse microemulsion technique and characterized and their photoluminescence properties studied from room temperature to 318 K. Aqueous suspensions of these nanoparticles display an excellent performance as ratiometric luminescent nanothermometers in the physiological temperature (300-320 K) range.

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.

Unveiling in Vivo Subcutaneous Thermal Dynamics by Infrared Luminescent Nanothermometers

The recent development of core/shell engineering of rare earth doped luminescent nanoparticles has ushered a new era in fluorescence thermal biosensing, allowing for the performance of minimally invasive experiments, not only in living cells but also in more challenging small animal models. Here, the potential use of active-core/active-shell Nd(3+)- and Yb(3+)-doped nanoparticles as subcutaneous thermal probes has been evaluated. These temperature nanoprobes operate in the infrared transparency window of biological tissues, enabling deep temperature sensing into animal bodies thanks to the temperature dependence of their emission spectra that leads to a ratiometric temperature readout. The ability of active-core/active-shell Nd(3+)- and Yb(3+)-doped nanoparticles for unveiling fundamental tissue properties in in vivo conditions was demonstrated by subcutaneous thermal relaxation monitoring through the injected core/shell nanoparticles. The reported results evidence the potential of infrared luminescence nanothermometry as a diagnosis tool at the small animal level.

Lanthanide Organic Framework Luminescent Thermometers.

Metal-organic frameworks (MOFs) are excellent platforms for engineering luminescence properties as their building blocks, metal ions, linkers, and guest ions or molecules, are all potential sources of light emission. Temperature is one of the most important physical properties affecting the dynamics and viability of natural and engineered systems. Because the luminescence of certain lanthanide-bearing MOFs changes considerably with temperature, in the last few years, these materials have been explored as optical thermometers, especially in temperature sensing based on the intensity ratios of two separate electronic transitions. This review discusses the main concepts and ideas assisting the design of such ratiometric thermometers, and identifies the main challenges presented to this nascent field: develop nanothermometers for bio-applications and nanomedicine; understand the energy transfer mechanisms determining the thermal sensitivity; achieve effective primary thermometers; realize multifunctional nanothermometers; integrate Ln3+ -based thermometers in commercial products.

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.

A cryogenic luminescent ratiometric thermometer based on a lanthanide phosphonate dimer

The first example of a ratiometric Eu/Tb thermometer based on a lanthanide phosphonate is reported, which presents a temperature dependent emission under 393 nm excitation that enables its use as a molecular thermometer with a maximum relative sensitivity of 3.9% K−1 and a minimum temperature uncertainty of 0.15 K, both at 38 K. Additionally, the compound is a competitive luminescent colorimetric probe showing temperature dependent (x, y) CIE color coordinates changing from yellow, (0.48, 0.48) at 18 K, to red, (0.64, 0.37) at 300 K.

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.

Metal-Free Highly Luminescent Silica Nanoparticles

Stable, cost-effective, brightly luminescent, and metal-free organosilica nanoparticles (NPs) were prepared using the Stöber method without any thermal treatment above 318 K. The white-light photoluminescence results from a convolution of the emission originated in the NH(2) groups of the organosilane and oxygen defects in the silica network. The time-resolved emission spectra are red-shifted, relative to those acquired in the steady-state regime, pointing out that the NPs emission is governed by donor-acceptor (D-A) recombination mechanisms. Moreover, the increase of the corresponding lifetime values with the monitored wavelength further supports that the emission is governed by a recombination mechanism typical of a D-A pair attributed to an exceptionally broad inhomogeneous distribution of the emitting centers peculiar to silica-based NPs. These NPs exhibit the highest emission quantum yield value (0.15 ± 0.02) reported so far for organosilica biolabels without activator metals. Moreover, the emission spectra and the quantum yield values are quite stable over time showing no significant aging effects after exposure to the ambient environment for more than 1 year, stressing the potential of these NPs as metal-free biolabels.

Tethering Luminescent Thermometry and Plasmonics: Light Manipulation to Assess Real-Time Thermal Flow in Nanoarchitectures

The past decade has seen significant progresses in the ability to fabricate new mesoporous thin films with highly controlled pore systems and emerging applications in sensing, electrical and thermal isolation, microfluidics, solar cells engineering, energy storage, and catalysis. Heat management at the micro- and nanoscale is a key issue in most of these applications, requiring a complete thermal characterization of the films that is commonly performed using electrical methods. Here, plasmonic-induced heating (through Au NPs) is combined with Tb3+/Eu3+ luminescence thermometry to measure the thermal conductivity of silica and titania mesoporous nanolayers. This innovative method yields values in accord with those measured by the evasive and destructive conventional 3ω-electrical method, simultaneously overcoming their main limitations, for example, a mandatory deposition of additional isolating and metal layers over the films and the previous knowledge of the thermal contact resistance between the heating and the mesoporous layers.