Non-invasive precise 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. This has been strongly stimulated by the numerous challenging requests arising from nanotechnology and biomedicine. This critical review offers a general overview of recent examples of luminescent and non-luminescent thermometers working at nanometric scale. Luminescent thermometers encompass organic dyes, QDs and Ln3+ions as thermal probes, as well as more complex thermometric systems formed by polymer and organic–inorganic hybrid matrices encapsulating these emitting centres. Non-luminescent thermometers comprise of scanning thermal microscopy, nanolithography thermometry, carbon nanotube thermometry and biomaterials thermometry. Emphasis has been put on ratiometric examples reporting spatial resolution lower than 1 micron, as, for instance, intracellular thermometers based on organic dyes, thermoresponsive polymers, mesoporous silica NPs, QDs, and Ln3+-based up-converting NPs and β-diketonate complexes. Finally, we discuss the challenges and opportunities in the development for highly sensitive ratiometric thermometers operating at the physiological temperature range with submicron spatial resolution.

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Thermometry 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].

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

The effects of rare element Ga on solderability, microstructure, and mechanical properties of Sn–0.5Ag–0.7Cu lead-free solder were investigated. The experimental results show that Ga plays a positive role in improving the wettability and the microstructure of the solder. When the content of Ga is at 0.5 wt%, the grain size of the solder is smaller and the shear force is enhanced greatly. It is also found that the thickness of the IMCs at the solder/Cu interface is reduced with proper addition of Ga. The increase of mechanical properties may be related to the refining of IMCs of the solder due to Ga addition.

Effects of Ga addition on microstructure and properties of Sn–0.5Ag–0.7Cu solder

The focus of this paper is centered on the thermal reduction of KMnO4 at controlled temperatures of 400 and 800 °C. The materials under study were characterized by atomic absorption spectroscopy, thermogravimetric analysis, average oxidation state of manganese, nitrogen adsorption–desorption, and impedance spectroscopy. The structural formulas, found as a result of these analyses, were K 0.29 + ( M n 0.84 4 + M n 0.16 3 + ) O 2.07 · 0.61 H 2 O and K 0.48 + ( M n 0.64 4 + M n 0.36 3 + ) O 2.06 · 0.50 H 2 O . The N2 adsorption–desorption isotherms show the microporous and mesoporous nature of the structure. Structural analysis showed that synthesis temperature affects the crystal size and symmetry, varying their electrical properties. Impedance spectroscopy (IS) was used to measure the electrical properties of these materials. The measurements attained, as a result of IS, show that these materials have both electronic and ionic conductivity. The conductivity values obtained at 10 Hz were 4.1250 × 10−6 and 1.6870 × 10−4 Ω−1cm−1 for Mn4 at 298 and 423 K respectively. For Mn8, the conductivity values at this frequency were 3.7074 × 10−7 (298) and 3.9866 × 10−5 Ω−1cm−1 (423 K). The electrical behavior was associated with electron hopping at high frequencies, and protonic conduction and ionic movement of the K+ species, in the interlayer region at low frequencies.

Structural and Electrical Studies for Birnessite-Type Materials Synthesized by Solid-State Reactions

In this paper, Copper Phthalocyanine (CuPc) nanostructures are the focus of an electrochemical investigation. This Phthalocyanine is nitrogen enriched having metal redox centers that facilitate the redox reaction. CuPc nanowires are synthesized by a simple room temperature process from bulk CuPc powder and their electrochemical properties are explored for the first time. The as prepared samples are characterized by X-ray Diffraction, high resolution transmission electron microscopy, field emission scanning electron microscopy, etc. They are then tested electrochemically in a three electrode system. Cyclic voltammogram (CV) and charge discharge (CD) profiles have been recorded. The CuPc nanowire electrode exhibits 406 F/g capacitance at 5 mVS−1 scan rate and 260 F/g at 0.3 A/g current density. Long cycling stability up to 3000 cycles at 6 A/g has been carried out. The results show that this material can be used as the potential candidate for positive electrode in supercapacitor applications.

Solution processed Copper Phthalocyanine nanowires: A promising supercapacitor anode material

Impedance spectroscopic studies of nickel nanocluster in silica matrix synthesized by sol–gel method

The reaction kinetics of NiCl2 in silica-alumina gel matrix

Molybdenum carbide has been formed in the silica gel matrix by the in situ reduction of ammonium molybdate. The average particle size of molybdenum carbide is in the range of 5-15 nm. The detailed XRD analyses coupled with thermodynamic arguments show that reduction of ammonium molybdate is predominantly by in situ generated hydrogen.

Sathi Mallick

Papers

Impedance spectroscopic studies of nickel nanocluster in silica matrix synthesized by sol–gel method

The reaction kinetics of NiCl2 in silica-alumina gel matrix

Preparation of silica-molybdenum carbide nanocomposite by in situ reduction