Near-infrared excited dual-mode nanoprobes for background-free and on-site detection of 5-hydroxymethylfurfural in food.

Lanthanide-based upconversion nanoparticles (UCNPs) offer new strategies for luminescence-based sensing and imaging. One of the best studied materials are hexagonal 𝛽-NaYF4 UCNPs doped with 20 % Yb3+ and 2 % Er3+, which efficiently convert 976 nm light to photons emitted at 540 nm, 655 nm, and 845 nm, respectively, reveal long luminescence lifetimes (> 100 μs), and are very photostable and chemically inert.[1,2] The properties of their upconversion (UC) luminescence (UCL) are, however, strongly influenced by particle size, concentration and spatial arrangement of dopant ions, surface chemistry, and microenvironment.[3,4] In addition, the multiphotonic absorption processes responsible for UCL render UCL dependent on excitation power density (𝑃). The rational design of brighter UCNPs particle architectures encouraged us to assess systematically the influence of these parameters on UCL for differently doped UCNPs relying on the commonly used -NaYF4 matrix using steady state and time resolved fluorometry as well as integrating sphere spectroscopy for P varied over almost three orders of magnitude. This includes comprehensive studies of the influence of size and shell, Yb3+ and Er3+ dopand concentrations, and energy transfer processes from UCNPs to surface-bound organic dyes or vice versa [5]. Our results underline the need for really quantitative luminescence studies for mechanistic insights, the potential of high P to compensate for UCL surface quenching, and the matrix- and P-dependence of the optimum dopand concentration. Key words: upconverting nanoparticles, size, FRET, fluorescence, absolute fluorescence quantum yield, fluorescence decay kinetics, power density dependence References. [1] Haase, M.; Schafer, H., Angew. Chem. Int. Ed. 2011, 50, 5808-5829. [2] Liu, G. K., Chem. Soc. Rev. 2015, 44, 1635-1652. [3] Wurth, C.; Kaiser, M.; Wilhelm, S.; Grauel, B.; Hirsch, T., Resch-Genger, U.; Nanoscale 2017, 9, 4283-4294. [4] Kaiser, M.; Wurth, C.; Hyppanen I.; Soukka T;, Resch-Genger, Nanoscale 2017, on line (10.1039/C7NR02449E). [5] Kraft, M.; Wurth, C.; Muhr, V.; Hirsch, T.; Resch-Genger, U., Nanoscale, submitted.

The realization of primary red, green, and blue (RGB) luminescence from a single upconversion nanoparticle (UCNP) enables multicolor fine-tuning of the upconversion luminescence through the combination of RGB emissions, ultimately making it suitable for color volumetric display applications. However, excitation-wavelength-orthogonal RGB-emitting UCNPs require complex nanostructures and the synthesis of such RGB-emitting single UCNPs remains a great challenge. Here, we report a core@quadruple-shell (C@4S) single UCNP that exhibits primary tricolor upconversion luminescence under near-infrared (NIR) excitations. We first synthesize NaYbF 4 :Ho which emits green and red light under low- and high-power 980 nm NIR light. Then the NaYbF 4 :Ho core is coated with a NaYF 4 shell followed by the successive growth of NaYF 4 :Yb,Tm, NaYF 4 :Nd,Yb, and NaYF 4 shells. The synthesized NaYbF 4 :Ho@NaYF 4 @NaYF 4 :Yb,Tm@NaYF 4 :Nd,Yb@NaYF 4 C@4S UCNPs emit blue light under 800 nm excitation. By adjusting the power and wavelength of NIR light, the C@4S UCNPs exhibit various emission colors, including blue, sky-blue, green, yellow-green, orange, red, white, and more. Finally, the transparent C@4S UCNP-polydimethylsiloxane composite is prepared and various color images are displayed by simply scanning NIR light within the composite, indicating the high potential of the C@4S UCNPs for full-color display applications. • Core@quadruple-shell (C@4S) upconversion nanoparticles (UCNPs) are facily synthesized. • The C@4S UCNPs show primary tricolor red/green/blue luminescence. • Full-color tuning is achieved from the C@4S UCNPs by adjusting the excitation condition. • Various color images are displayed in the C@4S UCNP-polydimethylsiloxane composite.

Upconverting nanoparticles (UCNPs) offer significant potential for highly sensitive biosensing due to their background-free detection and excellent photostability. However, their intrinsically low upconversion efficiency limits their usage in practical applications. To reach ultra-high sensitivity with a simplified readout system, the excitation field, and consequently the upconversion luminescence, can be strengthened by using plasmon-based mechanisms. For qualitative detection, the optical field inhomogeneities in plasmon-enhanced upconversion do not generally cause practical limitations, but in quantitative detection, a homogeneous signal across the biosensor surface is required. With UCNPs this is particularly challenging since the upconversion efficiency is highly non-linear depending on the excitation field intensity. The motivation for this work is to compare the digital and analog readouts for quantitative biosensing applications. UCNPs are imaged via diverging surface plasmon polaritons, and the images are analyzed using both methods: digital detection counts individual UCNPs, whereas analog detection quantifies upconversion luminescence by integrating the pixel intensity across the image. Our results show that analog detection exhibits greater variability than digital detection. This indicates that digital detection is likely to provide better repeatability when employing plasmon-enhanced UCNPs for quantitative biosensing.

Exploring energy mechanisms involved in the luminescence reduction due to Ce3+ doping via core-multishell upconversion nanoparticles highly doped with Ho3+ and Yb3+

The up-conversion luminescence tuning of rare-earth ions is an important research topic for understanding luminescence mechanisms and promoting related applications. In this paper, we experimentally study the up-conversion luminescence tuning of Er3+-doped ceramic glass excited by the unshaped, V-shaped and cosine-shaped femtosecond laser field with different laser powers. The results show that green and red up-conversion luminescence can be effectively tuned by varying the power or spectral phase of the femtosecond laser field. We further analyze the up-conversion luminescence tuning mechanism by considering different excitation processes, including single-photon absorption (SPA), two-photon absorption (TPA), excited state absorption (ESA), and energy transfer up-conversion (ETU). The relative weight of TPA in the whole excitation process can increase with the increase of the laser power, thereby enhancing the intensity ratio between green and red luminescence (I547/I656). However, the second ETU (ETU2) process can generate red luminescence and reduce the green and red luminescence intensity ratio I547/I656, while the third ESA (ESA3) process can produce green luminescence and enhance its control efficiency. Moreover, the up-conversion luminescence tuning mechanism is further validated by observing the up-conversion luminescence intensity, depending on the laser power and the down-conversion luminescence spectrum under the excitation of 400-nm femtosecond laser pulse. These studies can present a clear physical picture that enables us to understand the up-conversion luminescence tuning mechanism in rare-earth ions, and can also provide an opportunity to tune up-conversion luminescence to promote its related applications.

Upconversion nanoparticles (UCNPs) are well-reported for bioimaging. However, their applications are limited by low luminescence intensity. To enhance the intensity, often the UCNPs are coated with macromolecules or excited with high laser power, which is detrimental to their long-term biological applications. Herein, we report a novel approach to prepare co-doped CaF2:Yb3+ (20%), Er3+ with varying concentrations of Er (2%, 2.5%, 3%, and 5%) at ambient temperature with minimal surfactant and high-pressure homogenization. Strong luminescence and effective red emission of the UCNPs were seen even at low power and without functionalization. X-ray diffraction (XRD) of UCNPs revealed the formation of highly crystalline, single-phase cubic fluorite-type nanostructures, and transmission electron microscopy (TEM) showed co-doped UCNPs are of ~12 nm. The successful doping of Yb and Er was evident from TEM-energy dispersive X-ray analysis (TEM-EDAX) and X-ray photoelectron spectroscopy (XPS) studies. Photoluminescence studies of UCNPs revealed the effect of phonon coupling between host lattice (CaF2), sensitizer (Yb3+), and activator (Er3+). They exhibited tunable upconversion luminescence (UCL) under irradiation of near-infrared (NIR) light (980 nm) at low laser powers (0.28-0.7 W). The UCL properties increased until 3% doping of Er3+ ions, after which quenching of UCL was observed with higher Er3+ ion concentration, probably due to non-radiative energy transfer and cross-relaxation between Yb3+-Er3+ and Er3+-Er3+ ions. The decay studies aligned with the above observation and showed the dependence of UCL on Er3+ concentration. Further, the UCNPs exhibited strong red emission under irradiation of 980 nm light and retained their red luminescence upon internalization into cancer cell lines, as evident from confocal microscopic imaging. The present study demonstrated an effective approach to designing UCNPs with tunable luminescence properties and their capability for cellular imaging under low laser power.

Upconversion luminescence tracking of gene delivery via multifunctional nanocapsules

Background-free luminescent and chromatic assay for strong visual detection of creatinine.

We assessed the influence of Yb3+ and Er3+ dopant concentration on the relative spectral distribution, quantum yield (ΦUC), and decay kinetics of the upconversion luminescence (UCL) and particle brightness (BUC) for similarly sized (33 nm) oleate-capped β-NaYF4:Yb3+,Er3+ upconversion (UC) nanoparticles (UCNPs) in toluene at broadly varied excitation power densities (P). This included an Yb3+ series where the Yb3+ concentration was varied between 11%-21% for a constant Er3+ concentration of 3%, and an Er3+ series, where the Er3+ concentration was varied between 1%-4% for a constant Yb3+ concentration of 14%. The results were fitted with a coupled rate equation model utilizing the UCL data and decay kinetics of the green and red Er3+ emission and the Yb3+ luminescence at 980 nm. An increasing Yb3+ concentration favors a pronounced triphotonic population of 4F9/2 at high P by an enhanced back energy transfer (BET) from the 4G11/2 level. Simultaneously, the Yb3+-controlled UCNPs absorption cross section overcompensates for the reduction in ΦUC with increasing Yb3+ concentration at high P, resulting in an increase in BUC. Additionally, our results show that an increase in Yb3+ and a decrease in Er3+ concentration enhance the color tuning range by P. These findings will pave the road to a deeper understanding of the energy transfer processes and their contribution to efficient UCL, as well as still debated trends in green-to-red intensity ratios of UCNPs at different P.

Hyperspectral imaging (HSI) is a robust and nondestructive method that can detect foreign particles such as microbial, chemical, and physical contamination in food. This review summarizes the work done in the last two decades in this field with a highlight on challenges, risks, and research gaps. Considering the challenges of using HSI on complex matrices like food (e.g., the confounding and masking effects of background signals), application of machine learning and modeling approaches that have been successful in achieving better accuracy as well as increasing the detection limit have also been discussed here. Foodborne microbial contaminants such as bacteria, fungi, viruses, yeast, and protozoa are of interest and concern to food manufacturers due to the potential risk of either food poisoning or food spoilage. Detection of these contaminants using fast and efficient methods would not only prevent outbreaks and recalls but will also increase consumer acceptance and demand for shelf-stable food products. The conventional culture-based methods for microbial detection are time and labor-intensive, whereas hyperspectral imaging (HSI) is robust, nondestructive with minimum sample preparation, and has gained significant attention due to its rapid approach to detection of microbial contaminants. This review is a comprehensive summary of the detection of bacterial, viral, and fungal contaminants in food with detailed emphasis on the specific modeling and datamining approaches used to overcome the specific challenges associated with background and data complexity.

Synthesis and characterization of HA/YVO4: Yb3+, Er3+ up-conversion luminescent nano-rods

Development of multicolor-emitting upconversion nanoparticles (UCNPs) is of significant importance for applications in optical encoding, anti-counterfeiting, display, and bioimaging. However, realizing the orthogonal three-primary color (TPC) upconversion luminescence in a single nanoparticle remains a huge challenge. Herein, we have rationally designed core-multishell-structured NaYF4 UCNPs through regulating the dopant concentration, composition of luminescent layers, and shell position and thickness, which are capable of emitting red, green, and blue luminescence with high color purity in response to ternary near-infrared quadrature excitations (1560/808/980 nm). Moreover, their high color purity is well retained with varying excitation power densities. This orthogonal TPC emissions property of such UCNPs endows them with great promise in the field of security. As a proof-of-concept, we have demonstrated the feasibility of combining such UCNPs with MnO2 nanosheets for information encryption and decryption. This work not only offers a new way to achieve TPC upconversion luminescence at a single nanoparticle level but also broadens the scope of application for security protection.

Contamination of foods by mycotoxins is a common yet serious problem. Owing to the increase in consumption of fresh produce, consumers have become aware of food safety issues caused by mycotoxins. Therefore, rapid and sensitive mycotoxin detection is in great demand in fields such as food safety and public health. Here we report a single-step luminescence resonance energy transfer (LRET) aptasensor for mycotoxin detection. To accomplish the single-step sensor, our sensor was constructed by linking a quencher-labeled aptamer through a linker to the surface of upconversion nanoparticles (UCNPs). Our LRET aptasensor is composed of Mn2+-doped NaYF4:Yb3+,Er3+ UCNPs as the LRET donor, and black hole quencher 3 (BHQ3) as the acceptor. The maximum quenching efficiency is obtained by modulating the linker length, which controls the distance between the quencher and the UCNPs. Our distinctive design of LRET aptasensor allows detection of mycotoxins selectively in colored food samples within 10 min without multiple bioassay steps. We believe our single-step aptasensor has a significant potential for on-site detection of food contaminants, environmental pollutants, and biological metabolites.

Solvothermal synthesis and upconversion luminescence of ultra-small Sc2O3: Yb, Er nanoparticles

Brightening heavily doped upconversion nanoparticles by tuning characteristics of core-shell structures