Noninvasive lanthanide‐doped optical thermometers based on fluorescent intensity ratio (FIR) technique have emerged as promising noncontact tools for detecting the inaccessible objects at different scales. Currently, the theoretical and experimental investigations of various influential factors on thermal performances of luminescence thermometers have become one of the hotspots to develop highly sensitive optical thermometers. On the other hand, near‐infrared (NIR) light‐responsive nanothermometers with deep‐tissue penetration have been widely applied for subcutaneous and intracellular thermometry, which could be integrated with optical heating and imaging functions to construct all‐in‐one thermometer‐heater platforms for cancer diagnosis and therapy. In this review, the recent advances in luminescence thermometry based on the thermally coupled levels (TCLs) are elaborately introduced from fundamental aspects to possible biomedical applications, with the perspective and outlook in the emerging challenges of FIR thermometers applied in biomedical science.

Mn-activated oxide phosphors La(MgTi)O (LMT) with far-red emitting were prepared via a sol-gel route. The structures of samples were determined by X-ray diffraction (XRD) and Reitveld refinement. The occupied sites of Mn (d electronic configuration) in host La(MgTi)O were confirmed by ab initio calculations in which the system has the lower formation energy, stable lattice structure, and strong bonding state as Mn enters into Ti site. The luminescent properties of Mn-doped samples were investigated; the samples emit far-red light centered at 708 nm with ultraviolet light (345 nm) or blue light (487 nm) excitation. According to the photoluminescence (PL) and excitation (PLE) spectra, the crystal field strength of the Mn-occupied environment was estimated. The thermal stability of phosphor was also evaluated through temperature-dependent PL intensity in a heating and cooling cycle process. The emission band is well-matched with the absorption band of phytochrome P under the excitation of light in near-ultraviolet to blue, which suggests that the LMT: Mn phosphor has great potential applications in light-emitting diodes (LEDs) for modulating plant growth.

Yb3+/Er3+ codoped Ba5Gd8Zn4O21 up-conversion (UC) phosphors with tunable emission were synthesized using a facile sol–gel method. UC spectra are composed of green emission from 2H11/2/4S3/2 → 4I15/2 transitions and red emission from 4F9/2 → 4I15/2 transition of Er3+ ion with the excitation of 980 nm laser diodes. Modulation of emitting color from green to red could be achieved by adjusting dopant concentrations or pulse width of 980 nm laser. The mechanism of the former strategy was figured out through analyzing visible and near-infrared (NIR) down-conversion emission spectra together with the corresponding green level (4S3/2) lifetimes under excitation of 490 nm light, and the latter method was explained by the non-steady-state up-converison process. Temperature detection range was expanded to low temperature region by utilizing red-emitting stark levels of Er3+ ion as thermally coupled levels. Thermal sensing performances based on green-emitting levels (2H11/2/4S3/2) and red emitting stark levels (4F9/2(1)/ 4F9/2(2)) of Er3+ ion were estimated and the maximum sensitivity are 0.0032 K–1 at 490 and 0.0029 K–1 at 200 K in our experimental range, respectively. Moreover, the effects of UC emission color from different dopant concentrations and pulse widths of lasers on sensor sensitivity were also investigated in detail. Results imply that the present phosphor Ba5Gd8Zn4O21:Er3+/Yb3+ exhibits high and stable sensitivity in a wide temperature detection scope, which makes it an excellent candidate for an optical thermometer.

No abstract

Investigation of the unclear influential factors to thermal sensing capability is the only way to achieve highly sensitive thermometry, which is greatly needed to meet the growing demand for potential sensing applications. Here, the effect from the phonon energy of a matrix on the sensitivity of upconversion (UC) microthermometers is elaborately discussed using a controllable method. Uniform truncated octahedral YF:Er/Yb microcrystals were prepared by a hydrothermal approach, and phase transformation from YF to YOF and YO with nearly unchanged morphology and size was successfully realized by controlling the annealing temperature. The phonon energies of blank matrixes were determined by FT-IR spectra and Raman scattering. Upon 980 nm excitation, phonon energy-dependent UC emitting color was finely tuned from green to yellow for three samples, and the mechanisms were proposed. Thermal sensing behaviors based on the TCLs (H/S) were evaluated, and the sensitivities gradually grew with the increase in the matrix's phonon energy. According to chemical bond theory and first-principle calculations, the most intrinsic factors associated with thermometric ability were qualitatively demonstrated through analyzing the inner relation between the phonon energy and bond covalency. The exciting results provide guiding insights into employing appropriate host materials with desired thermometric ability while offering the possibility of highly accurate measurement of temperature.

Eu(2+) activated fluorophosphate Ba3GdNa(PO4)3F (BGNPF) with blue and red double-color emitting samples were prepared via a solid-state method in a reductive atmosphere. Their crystal structure and cationic sites were identified in light of X-ray diffraction pattern Rietveld refinement. Three different Ba(2+) sites, coordinated by six O atoms referred to as Ba1, two F and five O atoms as Ba2, and two F and six O atoms as Ba3, were partially substituted by Eu(2+). Photoluminescence emission (PL) and excitation (PLE) spectra of phosphor BGNPF:Eu(2+) along with the lifetimes were characterized at the liquid helium temperature (LHT), which further confirm the existence of three Eu(2+) emitting centers resulting in 436, 480, and 640 nm emission from the 5d → 4f transitions of Eu(2+) in three different Ba(2+) crystallographic sites. These emissions overlap with the absorption spectra of carotenoids and chlorophylls from plants, which could directly promote the photosynthesis. Temperature-dependent PL spectra were used to investigate the thermal stability of phosphor, which indicates that the PL intensity of BGNPF:0.9% Eu(2+) with optimal composition at 150 °C still keeps 60% of its PL intensity at room temperature, in which blue emission has higher thermal-stability than the red emission. Furthermore, the approaching white LED devices have also been manufactured with a 365 nm n-UV LED chip and present phosphor, which make operators more comfortable than that of the plant growth purple emitting LEDs system composed of blue and red light. Results indicate that this phosphor is an attractive dual-responsive candidate phosphor in the application n-UV light-excited white LEDs for plant growth.

An optical temperature sensor and optical heater based on Yb3+/Tm3+ co-doped Ba5Gd8Zn4O21 phosphors.

A novel compound Ba3CaK(PO4)3 (BCKP) with new-type structure was synthesized and its structure was determined by X-ray diffraction Rietveld refinement, in which crystal structure consists of Ba1O9, Ba2O12, Ba3O9, CaO8 and KO10 polyhedra, that's five cationic sites. As a phosphor host, Eu2+ doped BCKP emits cold white light with about 90% quantum efficiency (QE) through entering different cationic sites. Based on the results of refinement, three Eu2+ luminescence centers in sites Ca, Ba2 and K were clearly assigned in Eu2+ solely doped BCKP by the time-resolved emission spectra (TRES), Van Uitert equation, but the emissions of Eu2+ at Ba1 and Ba3 sites are not easy to be determined for the same coordination number (CN). According to their different spatial distribution of the coordinated atoms, the first-principles calculation was used to compute the d orbital splitting energy of Eu2+ ions in Ba1 and Ba3 sites to accurately distinguish the ambiguous luminescence centers. In order to meet the requirement of plant growth spectra, Mn2+ was introduced into BCKP: Eu2+ to enhance the red component of spectra, which not only perfectly match with the absorption spectra of carotenoid and chlorophyll-b, but also LEDs fabricated through combining 365 nm near ultraviolet (n-UV) chip with BCKP: Eu2+, Mn2+ phosphor exhibit excellent parameters including high color rendering index (Ra) (92), excellent correlated color temperature (CCT) (4486 K) and outstanding QE up to 65%. Results confirmed that BCKP: Eu2+/Mn2+ phosphor with great potential applications in white LEDs and plant growth. Response to Reviewers: Dear editor, We are pleasure to accept your decision and the reviewers' suggestion. We would like to thank the referees for providing us constructive and valuable queries in bettering our manuscript in the revision. According to the remaining questions of referees, we have answered those questions raised by the referees carefully, and necessary changes have been made in the text. For the received queries, the replies are given below, and the changes in the manuscript have been marked in italic red color letters to show clearly. Reviewers' comments: Reviewer #1: The authors response well to the reviewers' comments and I therefore recommend its publication in Chem. Eng. J. Answer to comments: Thanks for the reviewer's recognition of our answers. Reviewer #3: Compared with the original version, a significant amount of work has gone into the revision of this paper, which has improved it considerably. In my opinion, it is acceptable for publication at its present form. Answer to comments: Thanks for the reviewer's recognition of our answers. Reviewer #9: Question 1: The authors suggested that the results of BCKP structure refinement will convince readers in spite that Biso of cations were over 2 and those for oxygens were over 3. However, as a crystallographer, the suggested crystal structure was less than satisfactory. In addition, the authors discussed the emission properties of BCKP based on the insufficient structure. Answer:...