Red Upconversion Luminescence Research Articles

Investigation of the sensitization effect of Yb3+ in Yb, Er co-doped Sr5(PO4)3F transparent ceramics: From single-band red upconversion to temperature sensing behavior

Lanthanide-ion-doped red upconversion (UC) fluorescent materials with excellent signal-to-noise ratio, stability, and red-green luminous intensity ratio have proven of tremendous research importance for biomedical, three-dimensional (3D) displays, sensors, and other sectors. In this study, xYb, 1Er: Sr5(PO4)3F (S-FAP) (x=0–5 at%) transparent ceramics with outstanding optical quality were fabricated by hot pressing the co-precipitation powder with high sintering activity. The energy dispersive X-ray (EDX) results showed that the trivalent lanthanide ions were evenly doped into the S-FAP ceramic lattice. Increased Yb3+ doping considerably restricted the growth of ceramic grain size, which should be attributable to a decrease in sintering activity under high Yb doping conditions. The co-doping of Yb3+ successfully resulted in a significant improvement in up/down conversion efficiency via effective energy transfer mechanisms and the optimization of the electron population at the energy level of red-green emission. Furthermore, single-band red upconversion luminescence with the greatest red-green upconversion luminescence integral intensity ratio of 125.74 was discovered under 980 nm laser pumping at room temperature. The energy transition process involved in the phenomenon of the up-conversion luminescence intensity shift of red-green emission has been thoroughly examined. Finally, the noncontact optical thermometric ability based on fluorescence intensity ratio (FIR) technology showed that the maximum absolute sensitivity (Sa) of the 5Yb1Er sample in the temperature range of 320–556 K is 1.1×10−3 K−1, implying the possible use of Yb/Er-doped S-FAP transparent ceramics in the field of optical thermometers.

Upconversion luminescence through dynamically regulating the depletion of Ho3+: 5I6 level for speed sensing

In this work, a strategy for dynamically adjusting the upconversion luminescence (UCL) color of NaGdF4:Yb3+/Ho3+/Ce3+/Sc3+ is reported based on a phosphor wheel. It has been demonstrated that the rotation-dependent UCL mainly originated from the regulation of depletion mode for the Ho3+: 5I6 level. Due to the dominant linear decay, a high-pure red UCL is observed under the steady-state excitation. However, as the proportion of the steady-state excitation decreases, the green–red emission intensity ratio gradually increases, followed by the color conversion from red to green. An approximate physical model is proposed to understand the dependence of IG/IR on rotation speed. We not only report a UCL material that shows potential application in velocity sensing but also provide new insights into wheel-based dynamic UCL regulation.

Large enhancement of red upconversion luminescence in beta Ba2Sc0.67Yb0.3Er0.03AlO5 phosphor via Mn2+ ions doping for thermometry

Here, this study reports single-band red upconversion emission in β-Ba2ScAlO5: Yb3+/Er3+ phosphor by doping Mn2+. The optimum concentration of Mn2+ ions in β-Ba2ScAlO5: Yb3+/Er3+ phosphor was 0.20. The intensity of red and green emissions is increased by 27.4 and 19.3 times, respectively. Compared with the samples without Mn2+ ions, the red-green integral strength ratio of β-Ba2ScAlO5: Yb3+/Er3+/Mn2+ sample was significantly increased by 28.4 times, reaching 110.9. The UCL mechanism was explored by analyzing the down-conversion luminescence spectra, absorption spectra, UCL spectra, and upconversion fluorescence lifetime decay curves of Yb3+/Er3+/Mn2+ co-doped β-Ba2ScAlO5. The enhancement of upconversion red light is achieved through energy transfer between defect bands and Er3+ ions, as well as energy transfer between Mn2+ ions and Er3+ ions. In addition, the Mn2+ doped β-Ba2ScAlO5: Yb3+/Er3+ red UCL phosphors have great potential for ambient temperature sensing in the 298–523 K temperature range. The maximum sensitivity of β-Ba2ScAlO5: Yb3+/Er3+/Mn2+ phosphor as a temperature sensor at 523 K is 0.0247 K−1.

Quantitative Fluorescent Lateral Flow Strip Sensor for Myocardial Infarction Using Purity-Color Upconversion Nanoparticles.

Acute myocardial infarction is a serious cardiovascular disease and poses significant risks to human health. Its early diagnosis and real-time detection are of great importance. Herein, we design a low-cost device that has a high sensitivity of cTnT and cTnI detection. Dual-color upconversion nanoparticles (UCNPs) are prepared as probes, which not only have high-purity red upconversion luminescence (UCL) under 980 or 808 nm excitation but also achieve good temperature sensing. Temperature-dependent multicolor emission excitation is obtained, and the color turns from white to orange and red with increasing temperature. In particular, the maximum SR and SA values based on nonthermally coupled levels are 4.76% K-1 and 8.6% K-1, which are higher than those based on thermally coupled levels. With the UCNPs-based lateral flow strip (LFS), the specific detection of cTnI and cTnT antigens in samples is achieved with a detection limit of 0.001 ng/mL, which is 1 order of magnitude lower than that of their clinical cutoff. The UCNPs-LFS device has a low-cost laser diode and a simplified laser and permits a mobile-phone camera to collect the results, which has an important influence on the field of biomarker sensing.

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

Upconversion nanoparticles (UCNPs), particularly those that are highly doped, possess outstanding properties and have demonstrated tremendous application potential. However, novel fine-tuning strategies are urgently required to enhance the upconversion luminescence (UCL) performance of UCNPs. In this study, a series of core–multishell UCNPs highly doped with Ho3+ and Yb3+ ions were synthesized via the oleate route. The X-ray diffraction (XRD), transmission electron microscopy (TEM), and UCL spectra and decay times, and pump power dependency of the synthesized UCNPs were used to examine their morphology, size, and the UCL properties. Ce3+ ions were introduced to enhance Ho3+-based red UCL, and their doping strategy was also optimized based on summarized UCL trends and detailed UCL mechanism analysis. NaHoF4:Gd3+@NaGdF4:Yb3+,Ce3+@NaGdF4:Yb3+ UCNPs that had 10 mol% Ce3+ dopants in their middle shells exhibited better UCL properties than the larger traditional NaGdF4:Yb3+,Ce3+,Ho3+ UCNPs, probably because of the combined influence of the energy interactions between the Ce3+ ions and surface groups, sensitizing energy enhancement by active shells, and the positive cooperation between the Ce3+ ions and highly-doped core–shell structures. In this study, the phenomenon of UCL decrease resulting from Ce3+ ion doping was explored. Using unique core–multishell structures with highly-doped activators and sensitizers placed within and outside the structures, respectively, the UCL decrease due to Ce3+ ion doping was attributed to a competition for photons. Doping the UCNPs with Ce3+ ions near the UCNP surfaces favors Ho3+-based red UCL. Our study findings can deepen the understanding of the Ho3+-based UCL mechanisms and increase the potential uses of NaHoF4 materials in photo-induced therapy.

Achieving Orthogonal Upconversion Luminescence of a Single Lanthanide Ion in Crystals for Optical Encryption.

Optical encryption shows great potential in meeting the growing demand for advanced anti-counterfeiting in the information age. The development of upconversion luminescence (UCL) materials capable of emitting different colors of light in response to different external stimuli holds great promise in this field. However, the effective realization of multicolor UCL materials usually requires complex structural designs. In this work, orthogonal UCL is achieved in crystals with a simple structure simply by introducing modulator Tm3+ ions to control the photon transition processes between different energy levels of activator Er3+ ions. The obtained crystals emit red and green UCL when excited by 980nm and 808nm lasers, respectively. The orthogonal excitation-emission properties of crystals are shown to be very suitable for high-level optical encryption, which is important for information security and anti-counterfeiting. This work provides an effective strategy for obtaining orthogonal UCL in simple structural materials, which will encourage researchers to further explore novel orthogonal UCL materials and their applications, and has important implications for the development of the frontier photonic upconversion fields.

High quenching concentration of the CaLuAlO4:Er3+ upconversion phosphor with short-range energy migration

Low activator concentration in upconversion phosphors has been a hindrance in improving luminescence performance. Herein, we report an Er3+ doping concentration as high as 30 at. % in the CaLuAlO4 upconversion host. The CaLuAlO4:Er3+ phosphors were prepared by the traditional high-temperature solid-state method. Under the excitation of a 980 nm laser, the optimal concentration of Er3+ approaches 30% according to the total integrated intensity of red and green upconversion luminescence. This quenching concentration is one order of magnitude higher than that in the traditional Er3+ doped upconversion polycrystals. The results obtained from first-principles calculations suggest that there are no in-plane or chain rare-earth ions in the cell. This increases the difficulty of propagating excitation energy to quench centers and is advantageous for high doping concentration. Our work can spark people's interest in the research and application of highly doped upconversion phosphors.

Rapid detection of microalgae cells based on upconversion nanoprobes.

The quantification of microalgae cells is crucial for the treatment of ships' ballast water. However, achieving rapid detection of microalgae cells remains a substantial challenge. Here, we develop a new method for rapid and effective detection of microalgae concentration by utilizing upconversion nanoprobes (UCNPs) of NaYF4:Er3+,Tm3+. Three ligands, carboxylated methoxypolyethylene glycols with 5000 and 2000 molecular weights (mPEG-COOH-5, mPEG-COOH-2) and D-gluconic acid sodium salt (DGAS), were used to convert hydrophobic UCNPs into a hydrophilic state through modification. The results show that the mPEG-COOH-5 modified UCNPs present the highest stability in an aqueous solution. Fourier Transform Infrared Spectroscopy (FTIR) measurements reveal the presence of a significant number of -COOH functional groups on UCNPs after the mPEG-COOH-5 modification. These -COOH groups enhance the hydrophilicity and biocompatibility of UCNPs. The soluble UCNPs were directly mixed with microalgae, and the upconversion luminescence (UCL) spectra of the UCNPs were recorded immediately after thorough shaking. This greatly reduces the measurement time and could realize rapid onboard detection. In this sensing procedure, the UCNPs with red UCL functioned as energy donors, while microalgae with red absorption served as an energy acceptor. The UCL gradually diminishes with an increase in microalgae concentration based on the inner filter effect, thus establishing a relationship between UCL and microalgae concentration. The accuracy of the detection is further validated through the traditional microscope counting method. These findings pave the way for a novel rapid strategy to assess microalgae concentration using UCNPs.

Intense green up/down conversion emission and high-sensitivity optical thermometry of NaYb(WO4)2:Er3+ stoichiometric phosphors

Optical thermometry based on fluorescence intensity ratio (FIR) has garnered significant interest among researchers due to its rapid, non-contact, and high sensitivity characteristics. In this study, stoichiometric NaYb(WO4)2 phosphors, doped with varying concentrations of Er3+, were successfully synthesized using the high-temperature solid-state method. Under both 980 nm and 266 nm excitation, the samples exhibited strong green and very weak red upconversion (UC) and downconversion (DC) luminescence. The structural and morphological properties of the prepared samples were thoroughly characterized using techniques such as X-ray diffraction (XRD), XRD Rietveld refinement, scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy. The phosphors demonstrated high-purity UC and DC emission, with maximum green-to-red ratios of 60.9 and 77.0, respectively. This discovery opens up vast possibilities for their application as pure green color converters. The UC emission of Er3+ ions in the stoichiometric NaYb(WO4)2 phosphor was identified as a two-photon UC process, as evidenced by the relationship between the luminescence intensity and the excitation power of the sample. Moreover, the temperature-dependent emission spectra of UC and DC were monitored, and investigated the fluorescence intensity ratios (FIRs) of 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions to assess the optical temperature sensing behavior of NaYb(WO4)2:Er3+. The experimental results revealed that the maximum absolute sensitivities (SA) of NaYb(WO4)2 phosphors were 0.0120 K-1 (UC) and 0.0077 K-1 (DC), and the maximum relative sensitivities (SR) were 0.0089 K-1 (UC) and 0.0092 K-1 (DC), respectively. These findings highlight the promising of Er3+-doped Yb3+ stoichiometric NaYb(WO4)2 phosphors in optical temperature sensors.

Intense red upconversion luminescence and optical thermometry of a novel Yb3+/Er3+ co-doped Ba3Sc2WO9 phosphor

Yb3+/Er3+ co-doped Ba3Sc2WO9 strong red upconversion luminescent (UCL) phosphors have been achieved by high-temperature solid-phase method. Under 980 nm laser excitation, the phosphor emits strong red light at ∼667 nm and weak green light at ∼560 nm, with the intensity ratio of the red-green integrals up to 23.06. The band gap of the optimally doped Ba3Sc2WO9: 0.1 Yb3+, 0.03 Er3+ sample was calculated to be 3.14 eV. The main excitation process for the red light emission is the transition of Er3+ from the 4I13/2 energy state to the 4F9/2 energy state. In addition, the Yb3+/Er3+ doped Ba3Sc2WO9 red UCL phosphors have great potential for ambient temperature sensing in the temperature range 298–498 K. The Ba3Sc2WO9: 0.1 Yb3+, 0.03 Er3+ phosphor as a temperature sensor has a maximum sensitivity of 0.0096 K−1 at 498 K. This study provides a groundbreaking method for exploring new materials for strong red light upconversion emission.

Upconversion luminescence and optical thermometry behaviors of Yb3+ and Ho3+ co-doped GYTO crystal

Optical thermometry based on the upconversion (UC) luminescence intensity ratio (LIR) has attracted considerable attention because of its feasibility for achievement of accurate non-contact temperature measurement. Compared with traditional UC phosphors, optical thermometry based on UC single crystals can achieve faster response and higher sensitivity due to the stability and high thermal conductivity of the single crystals. In this study, a high-quality 5 at% Yb3+ and 1 at% Ho3+ co-doped Gd0.74Y0.2TaO4 single crystal was grown by the Czochralski (Cz) method, and the structure of the as-grown crystal was characterized. Importantly, the UC luminescent properties and optical thermometry behaviors of this crystal were revealed. Under 980 nm wavelength excitation, green and red UC luminescence lines at 550 and 650 nm and corresponding to the 5F4/5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+, respectively, were observed. The green and red UC emissions involved a two-photon mechanism, as evidenced by the analysis of power-dependent UC emission spectra. The temperature-dependent UC emission spectra were measured in the temperature range of 330–660 K to assess the optical temperature sensing behavior. At 660 K, the maximum relative sensing sensitivity (Sr) was determined to be 0.0037 K−1. These results highlight the significant potential of Yb,Ho:GYTO single crystal for optical temperature sensors.Graphical abstract

Effects of Er3+ and Yb3+ concentrations on upconversion luminescence and thermal sensing characteristics of Sc2O3:Er3+/Yb3+ phosphors

Sc2O3:Er3+(x mol%)/Yb3+(3 mol%) (x = 0.3, 0.5, 0.7) and Sc2O3:Er3+(0.5 mol%)/Yb3+(y mol%) (y = 1, 3, 5) phosphors were synthesized by CO2 laser zone-melting method. The upconversion luminescence (UCL) spectra of Sc2O3:Er3+/Yb3+ fluorescent materials, which were pumped by 980 nm, were measured and analyzed. The results show that the UCL intensity reaches maxima when doped with 0.5 mol% Er3+ and 3 mol% Yb3+, and the increase of Yb3+ concentration leads to reduce the ratio of green UCL to red UCL. The temperature-dependent UCL spectra of Sc2O3:Er3+/Yb3+ fluorescent materials with various Er3+ and Yb3+ concentrations were measured from 290 K to 1023 K, and the thermal sensing characteristics were revealed via the fluorescence intensity ratio (FIR) technique. The maximal relative sensitivity (SR) are almost independent of the Er3+ and Yb3+ concentration, while the maximal absolute sensitivity (SA) increases with Yb3+ concentration in the whole measurement range. Among the prepared samples, the maxima of SA and SR are achieved for the sample of Sc2O3:Er3+(0.5 mol%)/Yb3+(5 mol%)，with the values of 5.34 × 10−3 K−1 (364.1 K) and 12.5 × 10−3 K−1 (290 K), respectively. Interestingly, for Sc2O3:Er3+ (0.5 mol%)/Yb3+ (3 mol%), having the highest UCL intensity, its maximum SA and SR are 4.90 × 10−3 K−1 (371.4 K) and 11.2 × 10−3 K−1 (290 K). The wide temperature sensing range and relatively high sensitivities of the fabricated materials provide excellent potential for temperature sensing.

Understanding and Effective Tuning of Red‐to‐Green Upconversion Emission in Ho‐Based Halide Double Perovskite Microcrystals

AbstractHo‐based lead‐free double perovskite microcrystals (MCs), Cs2NaHo1‐xYbxCl6, are synthesized and demonstrated to be promising optical temperature sensing materials by understanding the concentration‐dependent upconversion (UC) luminescence mechanisms. By optimizing the dopant concentration, a dominant emission shifting from red to green is successfully achieved in an activator‐rich system. During this process, the red to green emission intensity ratio (R/G) can be changed from 12.5 (pure red light) to 0.14 (pure green light). It is clarified that the cross‐relaxation (CR) process between activator and the energy transfer process between Yb3+ and Ho3+ are the key mechanisms of UC luminescence intensity and R/G ratio variation. Moreover, the intensity of red and green UC emission shows strong dependence on temperature, resulting from the competition between CR and energy transfer in the emission levels. The high‐sensitivity Ho‐based thermometer can be obtained by Cs2NaHo0.99Yb0.01Cl6 MCs with bright red UC luminescence, whose fluorescence intensity ratio thermometer gains maximum relative sensitivities reaching 0.68% K−1 at 400 K. The multicolor UC luminescence quantitative modeling mentioned here is crucial for understanding the concentration‐dependence UC luminescence characteristics and to optimize the design of the Ho‐based thermometer. This study can be an extension of the application range for perovskite UC materials.

Ultrapure single-band red upconversion luminescence in Er3+ doped sensitizer-rich ytterbium oxide transparent ceramics for solid-state lighting and temperature sensing.

Achieving single-band upconversion (UC) is a challenging but rewarding approach to attain optimal performance in diverse applications. In this paper, we successfully achieved single-band red UC luminescence in Yb2O3: Er transparent ceramics (TCs) through the utilization of a sensitizer-rich design. The Yb2O3 host, which has a maximum host lattice occupancy by Yb3+ sensitizers, facilitates the utilization of excitation light and enhances energy transfer to activators, resulting in improved UC luminescence. Specifically, by shortening the ionic spacing between sensitizer and activator, the energy back transfer and the cross-relaxation process are promoted, resulting in weakening of green energy level 4S3/2 and 2H11/2 emission and enhancement of red energy level 4F9/2 emission. The prepared Yb2O3: Er TCs exhibited superior optical properties with in-line transmittance over 80% at 600 nm. Notably, in the 980nm-excited UC spectrum, green emission does not appear, thus Yb2O3: Er TCs exhibit ultra-pure single band red emission, with CIE coordinates of (0.72, 0.28) and color purity exceeding 99.9%. To the best of our knowledge, this is the first demonstration of pure red UC luminescence in TCs. Furthermore, the luminescent intensity ratio (LIR) technique was utilized to apply this pure red-emitting TCs for temperature sensing. The absolute sensitivity of Yb2O3: Er TCs was calculated to be 0.319% K-1 at 304 K, which is the highest level of optical thermometry based on 4F9/2 levels splitting of Er3+ known so far. The integration between pure red UC luminescence and temperature sensing performance opens up new possibilities for the development of multi-functional smart windows.

Up-conversion luminescence and defect structures in Sc: Dy: LiNbO3 crystals affected by Sc3+ concentration

Up-conversion luminescent material LiNbO3 crystal has been studied intensively due to its applications in solar cells, medical applications, color displays and so on. LiNbO3 crystal with poor ability of photodamage resistance limits its application in practice. Up-conversion luminescence properties of Dy: LiNbO3 crystals doped with anti-optical damage dopants have a promising future. Four Sc: Dy: LiNbO3 crystal samples were prepared by the Czochralski technique with Sc3+ concentrations of 0, 1, 2, and 3 mol%. Effective concentrations of Sc3+ and Dy3+ in the crystals were measured by ICP-AES. The occupancy rate of the dopant and crystal structure were investigated by XRD and IR transmission spectra. The properties of up-conversion emission were studied in view of up-conversion emission intensity and fluorescence lifetime. Blue (474 nm), yellow (583 nm), and red (655 nm) up-conversion luminescence were achieved under 800 nm excitation. Remarkably, the luminescence intensity increases significantly by several times when the Sc3+ doping concentration was 3 mol%. This indicates that Sc: Dy: LiNbO3 crystals have potential as optical devices.

Uncovering Different Responses and Energy Mechanisms of Sensitizer and Activator in Host Manipulation for Upconversion Nanoparticles.

Agile and efficient upconversion luminescence (UCL) fine-tuning strategies are the most demanded for in the frontier applications of highly doped upconversion nanoparticles (UCNPs). By doping Zn2+ ions into NaHoF4 and NaGdF4:Yb3+ shells using the oleate method, the separate influences of Zn2+ on Ho3+ and Yb3+ ions in UCL-related processes were analyzed in detail, revealing relevant UCL changes and underlying energy mechanisms from a novel but explicit perspective. Different behaviors of green and red UCL before and after Zn2+-ion doping were attributed to the disparities in the energy pathways and features of the sample structures. Herein, the populations of 5S2/5F4 and 5F5 states, not the usually mentioned decay time, decided the UCL intensities of the NaHoF4@NaYbF4-structured highly doped UCNPs. The advantageous small sizes and intense single-band red UCL of these UCNPs were further developed by combining our previous strategies with introducing Zn2+ ions into the NaHoF4 matrix. Overcoming energy loss by surface quenchers and Zn2+-triggered inner defects is the key factor in maximizing 4f-4f transitions. To the best of our knowledge, the current study is the first attempt to date to experimentally reveal separate impacts of the heteroions on activators and sensitizers in UCL-related processes and can deepen the theoretical investigation of Ho-based UCL for the broadened applications of NaHoF4 UCNPs.

An efficient construction model of multi-fluorescence morphologies in oxyfluoride matrix

As the most common up-conversion (UC) activation center now, Er3+ can emit the blue, green and red light under the excitation of 980 nm. It has important meaning how to manipulate the emission wavelength in practical application. In our work, a series of excellent UC luminescence materials Y6O5F8: M3+ (M = Yb, Er, and Fe) are synthesized by the hydrothermal method. Under the condition of different pH, the prepared precursors exhibited different phases of YF3 and Y(OH)1.63F1.37 and various morphologies. The hydrogen bond guides a regular crystal growth process. With the change of pH, OH group could be selectively introduced into the host during this different crystal growth process. The structure change is likely to decrease the rigidity of this lattice and adjusted the cut-off phonon energies, and further influence the f-f transition of Er3+, selectively forming red emission in Y6O5F8: Yb3+, Er3+. The intensity of red UC luminescence is enhanced by 22 times via Fe3+ co-doping into the Y6O5F8: Yb3+, Er3+ crystalline lattice. The introduction of transition metal ions Fe3+, produces an excellent energy transfer from <2F7/2, 4T1g> (Yb3+-Fe3+ dimer) to energy levels of Er3+ as well as the distortion of the crystalline field symmetry. In addition to the enhancing luminescence effect, Fe ions also introduce magnetism from the hysteresis loop curve. The temperature-dependent UC emission of Y6O5F8: Yb3+, Er3+, Fe3+ display significant thermal coupling occurs with a higher absolute sensitivity. Based on this, the anti-fake and temperature-sensitive luminescent applications of this material are investigated in detail.

Upconversion nanoparticle-based fluorescence resonance energy transfer sensing platform for the detection of cathepsin B activity in vitro and in vivo.

A simple fluorescence resonance energy transfer (FRET) sensing platform (termed as USP), comprised of upconversion nanoparticles (UCNPs) as the energy donor and Cy5 as the energy acceptor, has been synthesized for cathepsin B (CTSB) activity detection in vitro and in vivo. When Cy5-modified peptide substrate (peptide-Cy5) of CTSB is covalently linked on the surface of UCNPs, the FRET between the UCNPs (excitation: 980nm; emission: 541nm/655nm) and Cy5 (excitation: 645nm) leads to a reduction in the red upconversion luminescence (UCL) signal intensity of UCNPs. Cy5 can be liberated from UCNPs in the presence of CTSB through the cleavage of peptide-Cy5 by CTSB, leading to the recovery of the red UCL signal of UCNPs. Because the green UCL signal of UCNPs remains constant during the CTSB digestion, it can be considered as an internal reference. The findings demonstrate the ability of USP to detect CTSB with the linear detection ranges of 1 to 100ng·mL-1 in buffer and 2 × 103 to 1 × 105 cells in 0.2mL cell lysates. The limits of detection (LODs) are 0.30ng·mL-1 in buffer and 887 cells in 0.2mL of cell lysates (S/N = 3). The viability of USP to detect CTSB activity in tumor-bearing mice is hasfurtherbeen investigated using in vivo fluorescent imaging.

Enzyme-trigger ratiometric fluorescent nanoplatform for diagnosis and imaging of oral diseases

Oral health is an essential part of overall health. Matrix metalloproteinase-2 (MMP-2) is a potential biomarker for diseases. The ability to accurately detect MMP-2 in vivo and in vitro is of great importance for the early diagnosis and prognosis, as well as the treatment evaluation, of oral diseases. In this study, cyanine 3 (Cy3) polypeptide containing the specific peptide substrate (PLGVR) of MMP-2 was modified onto SiO2-coated upconversion nanoparticles to fabricate a fluorescence resonance energy transfer (FRET)-based ratiometric fluorescent nanoplatform (UCNPs@SiO2@Cy3-pep). The green upconversion luminescence of UCNPs@SiO2 is quenched by Cy3, while its red upconversion luminescence is undisturbed. After Cy3 is cleaved at the PLGVR peptide by MMP-2, it is detached from the surface of UCNPs@SiO2, resulting in the recovery of green luminescence. Based on this principle, we applied UCNPs@SiO2@Cy3-pep to detect MMP-2 activity in different oral disease samples and models. We found that the level of MMP-2 in saliva of patients with oral cancer was 10 times higher than that of healthy individuals. In addition, the MMP-2 level in patients with periodontitis and severe dental caries also increased to varying degrees compared with that in healthy patients. Finally, in vitro and in vivo imaging experiments revealed that the nanoplatform was effective in monitoring MMP-2 level. Together, the developed nanoplatform can be an ideal tool for medical diagnosis of MMP-2-related diseases.

Ratiometric upconversion nanoprobes for turn-on fluorescent detection of hypochlorous acid

Ratiometric fluorescent nanoprobes have been developed for the detection of hypochlorous acid (HOCl) by simply mixing upconversion nanocrystals (UCNCs) with plasmocorinth B (PB) dye molecules, where UCNCs are adopted as the energy donor and dye molecules are introduced as the energy acceptor. Specifically, green upconversion luminescence (UCL) emission of UCNCs can be effectively quenched by PB, resulting from the spectral overlap between the absorption of PB and UCL emission of UCNCs in the green region. Furthermore, owing to the electrostatic repulsion between negatively charged UCNCs and PB dye molecules, the fluorescence quenching mechanism is based on the IFE process. Upon interaction with HOCl, PB dye molecules degrade, resulting in the recovery of green UCL emission through inhibition of the IFE process. The red UCL emission, however, remains unchanged. Therefore, the red UCL emission is employed as an internal reference signal for the ratiometric detection of HOCl. The detection limit of the nanoprobe for HOCl is quantified to be 1.12 µM, which is much lower than that of colorimetric detection. Moreover, the developed sensor has been successfully applied for quantitative monitoring of HOCl in tap water.