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This work sheds light on the pump power impact on the performance of luminescent thermometers, which is often underestimated by researchers. An up-converting, inorganic nanoluminophore, YVO4:Yb3+,Er3+ (nanothermometer) was synthesized using the hydrothermal method and a subsequent calcination. This nanomaterial appears as a white powder composed of small nanoparticles (≈20 nm), exhibiting a very intense, green upconverted luminescence (λex = 975 nm), visible to the naked eye. Its emission spectrum consists of four Er3+ bands (500–850 nm) and one Yb3+ band (>900 nm). The obtained compound exhibits temperature-dependent luminescence properties, hence it is used as an optical nanosensor of temperature. The determined band intensity ratios of the non-thermally coupled levels (non-TCLs) of Yb3+/Er3+ and thermally coupled levels (TCLs) of Er3+ are correlated with temperature, and they are used for ratiometric sensing of temperature. The effects of the pump (NIR laser) power on the luminescence properties of the material, including band intensity ratios, absolute and relative sensitivities and temperature resolution are analysed. It was pointed out that the applied laser power has a huge impact on the values of the aforementioned thermometric parameters, and manipulating the laser power can significantly improve the performance of optical nanothermometers.

The use of functional nanomaterials and their combination with organic polymers leads to the formation of advanced composites and hybrid inorganic–organic systems having unique properties. The use of such materials as anti-counterfeiting agents (invisible under ambient conditions) is crucial for modern security systems and for further identification of labelled items and documents in various industrial applications. Here, the fabrication of functional paper by in situ incorporation of luminescent cellulose fibres into its structure, during the formation of paper pulp, is shown. In order to produce luminescent fibres, an in situ modification is made during the spinning process, using lanthanide-doped upconverting nanoparticles (SrF2:Yb3+,Er3+). At first glance (in daylight), the functionalized paper cannot be distinguished from its unmodified form. However, when exposed to invisible near-infrared laser light (975 nm), bright green illumination appears. The modified paper preserves good mechanical durability, which is important for its potential for further processing in documents and other commercially available forms. Protecting documents and various materials with this kind of paper may open new horizons in materials engineering, forensics, and other industrial applications.

Upconverting core@shell type β-NaYF4:Yb3+-Er3+@SiO2 nanorods have been obtained by a two-step synthesis process, which encompasses hydrothermal and microemulsion routes. The synthesized nanomaterial forms stable aqueous colloids and exhibits a bright dual-center emission (λex = 975 nm), i.e., upconversion luminescence of Er3+ and down-shifting emission of Yb3+, located in the first (I-BW) and the second (II-BW) biological windows of the spectral range, respectively. The intensity ratios of the emission bands of Er3+ and Yb3+ observed in the vis-near-infrared (NIR) range monotonously change with temperature, i.e., the thermalized Er3+ levels (2H11/2 → 4I15/2/4S3/2 → 4I15/2) and the nonthermally coupled Yb3+/Er3+ levels (2F5/2 → 2F7/2/4I9/2 → 4I15/2 or 4F9/2 → 4I15/2). Hence, their thermal evolutions have been correlated with temperature using the Boltzmann type distribution and second-order polynomial fits for temperature-sensing purposes, i.e., Er3+ 525/545 nm (max Sr = 1.31% K-1) and Yb3+/Er3+ 1010/810 nm (1.64% K-1) or 1010/660 nm (0.96% K-1). Additionally, a fresh chicken breast was used as a tissue imitation in the performed ex vivo experiment, showing the advantage of the use of NIR Yb3+/Er3+ bands, vs. the typically used Er3+ 525/545 nm band ratio, i.e., better penetration of the luminescence signal through the tissue in the I-BW and II-BW. Such nanomaterials can be utilized as accurate and effective, broad-range vis-NIR optical, contactless sensors of temperature.

Cellulose might be a promising material for surface-enhanced Raman scattering (SERS) substrates due to its wide availability, low cost, ease of fabrication, high flexibility and low optical activity. This work shows, for the first time development of the cellulose-based substrate, that owes its SERS activity to the presence of gold nanorods in its internal structure, and not only on the surface, as it is shown elsewhere, thus ensuring superior stability of the obtained material. This flexible cellulose-based substrate exhibiting plasmonic activity, provide easy and reproducible detection of different analytes via SERS technique. The substrate was prepared by introduction of gold nanorods into the cellulose fibers matrix using an eco-friendly process based on N-Methylmorpholine-N-Oxide. Au-modified cellulose fibers were used for the detection of p-Mercaptobenzoic acid and Bovine Serum Albumin by the SERS method. The obtained results show that this substrate offers large signal enhancement of 6-orders of magnitude, and high signal reproducibility with a relative standard deviation of 8.3%. Additionally, washing tests (90 °C, 20 h) showed superior stability of the as prepared plasmonic fibers, thus proving the good reusability of the substrates and the long shelf life.

Optical thermometers are of great interest due to their non-contact, high-sensitivity and fast measurement characteristics. In this work, a series of dual-center Bi1.96−xMoO6: 0.02Er3+, 0.02Tm3+, xYb3+ (x = 0.10–0.35) upconverting materials were prepared by a sol-gel synthesis method. Upon 975 nm excitation, the prepared materials exhibit bright color-tunable (from yellow to orange) upconversion (UC) emissions, as the Yb3+ content increases. The thermometric properties of the synthesized materials associated with different thermally-coupled and non-thermally coupled levels of Tm3+ and Er3+ were systematically investigated. Based on the temperature-dependent emissions originating from the non-thermally coupled levels of Tm3+ (3F2,3) and Er3+ (4F9/2), i.e., their band intensity ratios 700/670 nm, the developed optical thermometers were found to exhibit an exceptional relative thermal sensitivity (Sr), up to 5.90% K−1 at 293 K. Importantly, in the whole T-range of 293–623 K, the Sr values are larger than 2% K−1. Furthermore, it is revealed that the position of the Tm3+ emission band, centered around 800 nm is highly dependent on temperature, and, so, it can be utilized as a second thermometric parameter, which is important for a multi-parameter temperature sensing in the T-range of 293–623 K. These results suggest that Er3+/Tm3+/Yb3+-doped Bi2MoO6 materials are promising candidates for ultra-sensitive, dual mode optical thermometers and safety sign applications.

Upconverting core@shell type β-NaYF

Counterfeiting of valuable documents, currency and branded products is a growing problem that has serious economic and security implications for governments and companies, as well as for ordinary people/customers. In order to prevent and detect falsification, continuous improvement of anticounterfeiting technology is required. This article presents the fabrication process and properties of the luminescent (upconverting)-magnetic cellulose fibers and their application in textiles. For the first time such functionalized fibers and corresponding luminescent-magnetic textiles are prepared. Cellulose fibers with bifunctional properties were obtained by core/shell type nanostructures based on lanthanide doped fluorides and magnetite nanoparticles. They show a bright red emission under the NIR excitation and magnetic response, along with very good mechanical properties. The fabric with upconverting-magnetic properties made of the as-prepared fibers is a perfect example of advanced, multimodal protection of materials such as textiles and documents against counterfeiting.

This article presents synthesis and properties of the fibers modified with luminescent, inorganic nanoparticles doped with lanthanide(III) ions, i.e. LaF3:Ce3+, Gd3+, Eu3+; CeF3:Tb3+ and CePO4:Tb3+. The fibers with luminescent properties were prepared via so called Lyocell process. This method involves dissolving cellulose in aqueous solution of N-methylmorpholine-N-oxide (NMMO) and a subsequent spinning of the fibers, using a dry-wet method. Thanks to the successful incorporation of the modifier nanoparticles (NPs) into the cellulose matrices, the fibers exhibited bright, multicolor emission upon UV irradiation and good mechanical properties, which allowed further textile processing. This type of fibers, as well as the as-prepared textiles/fabric can be used as an anti-counterfeiting agent for clothes and documents protection.

Novel luminescent–magnetic cellulose microfibers were prepared by a dry–wet spinning method with the use of N-methylmorpholine-N-oxide. The synthesized luminescent–magnetic core/shell type nanostructures, based on the lanthanide-doped fluorides and magnetite nanoparticles (NPs)—Fe3O4/SiO2/NH2/PAA/LnF3, were used as nanomodifiers of the fibers. Thanks to the successful incorporation of the bifunctional nanomodifiers into the cellulose structure, the functionalized fibers exhibited superior properties, that is, bright multicolor emission under UV light and strong magnetic response. By the use of the as-prepared fibers, the luminescent–magnetic thread was fabricated and used to sew and make a unique pattern in the glove material, as a proof of concept for advanced, multimodal cloths’/materials’ protection against counterfeiting. The presence and uniform distribution of the modifier NPs in the polymer matrix were confirmed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis (EDX). The concentration of the modifier NPs in the fibers was determined by inductively coupled plasma mass spectrometry, EDX, and magnetic measurements. The luminescence characteristics of the materials were examined by photoluminescence spectroscopy, and their magnetic field-responsive behavior was investigated by a superconducting quantum interference device.