Upconversion Fluorescence Research Articles

Efficiency Enhancement Strategies for Stable Bismuth-Based Perovskite and Its Bioimaging Applications.

Lead-free perovskite is one of the ideal solutions for the toxicity and instability of lead halide perovskite quantum dots. As the most ideal lead-free perovskite at present, bismuth-based perovskite quantum dots still have the problem of a low photoluminescence quantum yield, and its biocompatibility also needs to be explored. In this paper, Ce3+ ions were successfully introduced into the Cs3Bi2Cl9 lattice using a modified antisolvent method. The photoluminescence quantum yield of Cs3Bi2Cl9:Ce is up to 22.12%, which is 71% higher than that of undoped Cs3Bi2Cl9. The two quantum dots show high water-soluble stability and good biocompatibility. Under the excitation of a 750 nm femtosecond laser, high-intensity up-conversion fluorescence images of human liver hepatocellular carcinoma cells cultured with the quantum dots were obtained, and the fluorescence of the two quantum dots was observed in the image of the nucleus. The fluorescence intensity of cells cultured with Cs3Bi2Cl9:Ce was 3.20 times of that of the control group and 4.54 times of the control group for the fluorescence intensity of the nucleus, respectively. This paper provides a new strategy to develop the biocompatibility and water stability of perovskite and expands the application of perovskite in the field.

Dual functions of nitrogen and phosphorus co-doped carbon dots for drug-targeted delivery and two-photon cell imaging

Heteroatom-doping carbon dots (CDs) have excellent fluorescence properties. In the paper, nitrogen (N) and phosphorus (P) co-doped CDs (N, P-CDs) were prepared through the hydrothermal method and characterized by TEM, PXRD, FT-IR, 13C NMR, XPS, UV–vis, and fluorescence spectra. The optical properties and drug (doxorubicin) - delivery performance of N, P-CDs were also studied. The results showed that the average size of N, P-CDs was 3.64 nm and the quantum yield was 30 %. The N, P-CDs had down- and up-conversion fluorescence properties, and the luminescence mechanism of the N, P-CDs was explained. The drug loading capacity of N, P-CDs was 39.11 %, and the drug delivery system (N, P-CDs-DOX) had low cytotoxicity, sustained release, and pH-targeted properties. The in vitro release of N, P-CDs-DOX belonged to the Weibull model and Fick diffusion, exhibiting the same release model and mechanism with free DOX. The N, P-CDs could deliver DOX successfully, which was demonstrated by the co-collocation of N, P-CDs and DOX in SH-SY5Y cells through single/two-photon imaging. N, P-CDs could potentially be used in drug-targeted delivery and cell imaging.

Developing boron carbon nitride/boron carbon nitride-citric acid quantum dot metal-free photocatalyst and evaluating the degradation performance difference of photo-induced species for tetracycline via theoretical and experimental study

For opening a way to synthesize novel metal-free catalysts and clarifying the photodegradation performance difference of photoactive species (such as ·O2−, h+), a series of metal-free photocatalysts have been synthesized by using different existing forms of the same materials (boron carbon nitride (BCN) and boron carbon nitride-citric acid quantum dot (BCQD)) as precursors via calcinating their mixture at 350 °C. BCQD has good fluorescence and up-conversion fluorescence performance. BCN/BCQD-350 has the highest removal efficiency (90%, including adsorption 60% and photodegradation 30%) for tetracycline (TC) among all samples under visible light irradiation. TC adsorption by BCN/BCQD-350 conforms to pseudo-second-order kinetic and Langmuir isotherm models. TC photodegradation by BCN/BCQD-350 conforms to type II heterojunction mechanism. Photoactive species capture experiments suggest that·O2− makes a higher contribution for TC photodegradation, followed by h+, ·OH, 1O2 and e−. From LC-MS results, TC photodegradation is initiated by the dehydration step. TC dehydration activated by ·O2− has the lowest barrier (43.4 kcal/mol) than that (50.1 kcal/mol) activated by h+, that (64.8 kcal/mol) without the activation by photoactive species. TC removal rate of BCN/BCQD-350 (0.01563 min−1) is higher than that of g-C3N4, P25 (TiO2), BNPA, BCNPA, etc. Furthermore, BCN/BCQD-350 can also photodegrade TC under infrared light irradiation (λ > 800 nm).

Research Progress on Up-Conversion Fluorescence Probe for Detection of Perfluorooctanoic Acid in Water Treatment.

Perfluorooctanoic acid (PFOA) is a new type of organic pollutant in wastewater that is persistent, toxic, and accumulates in living organisms. The development of rapid and sensitive analytical methods to detect PFOA in environmental media is of great importance. Fluorescence detection has the advantages of high efficiency and low cost, in which fluorescent probes have excellent fluorescence properties, excellent bio-solubility, and remarkable photostability. It is necessary to review the fluorescence detection routes for PFOA. In addition, the up-conversion of fluorescent materials (UCNPs), as fluorescent materials to prepare fluorescent probes with, has significant advantages and also attracts the attention of researchers, however, reviews related to their application in detecting PFOA and comparing them with other routes are rare. Furthermore, there are many strategies to improve the performance of up-conversion fluorescent probes including SiO2 modification and amino modification. These strategies can enhance the detection effect of PFOA. Thus, this work reviews the types of fluorescence detection, the design, and synthesis of UCNPs, their recognition mechanism, properties, and their application progress. Moreover, the development trend and prospects of these detection probes are given.

Low-noise synchronized mid-infrared upconversion spectrometer with a large spectral coverage

Mid-infrared (MIR) frequency upconversion presents a powerful tool for MIR photon detection at room temperature. However, the introduction of strong pump light will exacerbate the limitations of upconverted thermal radiation and upconverted parametric fluorescence on detectability in broadband MIR frequency upconversion. Here, a broadband synchronized MIR upconversion spectrometer system based on a step-chirped poled LiNbO3 crystal is demonstrated. This system is integrated with spontaneous parametric downconversion (SPDC) and sum-frequency generation (SFG) to form a broadband MIR light generation and synchronous pulsed upconversion, which are simultaneously driven by the same pulsed pump laser. The spectral coverage is close to 1.4 μm from 2450 to 3850 nm and the resolution of the system is about 4 cm−1 after deconvolution, as confirmed by a Fourier transform infrared spectrometer. In conjunction with synchronous pumping-based temporal gating, the system noise is suppressed to less than 10 counts per second per nm. This broadband synchronized SPDC–SFG configuration can be extended to different infrared wavelengths for low-noise broadband spectroscopic measurement.

A spectroscopic and thermometric comparison of α- and β-phase KYF4:Yb3+/Er3+ nanoparticles

We report absorption, fluorescence, and thermometric studies of both (α) cubic and (β) hexagonal phase KYF4:Yb/Er nanoparticles. The cubic phase of KYF4:Yb/Er nanoparticles was achieved using a hydrothermal approach, and the cubic to hexagonal phase transformation was successfully realized by controlling the annealing temperature at 500 °C. Both nanoparticle systems exhibit similar maxima in the Yb3+ absorption spectra at 10 268 cm−1 (974 nm). The 2H11/2, 4S3/2, and 4F9/2 → 4I15/2 transitions are recorded in the Er3+ upconversion fluorescence spectrum using 974 nm laser excitation. The green and red fluorescence of the hexagonal phase were approximately 100 and 2000 times more intense, respectively, than that of the cubic phase. The red to green ratio (R/G) was 50:1 and 2:1 for β-KYF4:Yb/Er and α-KYF4:Yb/Er nanoparticles, respectively. A three-photon process is involved in green upconversion for β-KYF4:Yb/Er, as described by 2F5/2 (Yb3+) + 4F9/2 (Er3+) → 2F7/2 (Yb3+) + 4G11/2 (Er3+). Using the fluorescence intensity ratio technique from thermally coupled Er3+ 2H11/2 and 4S3/2 levels, a very high thermal sensitivity of 1.76 and 1.49% K−1 at ambient temperatures was achieved for α- and β-phase materials, respectively.

The visible light photocatalytic H2 evolution and degradation enhancement of g-C3N4/Er:CdSe/SiO2 ternary core-shell heterojunction with up-conversion fluorescence auxiliary

The g-C3N4/Er:CdSe/SiO2 ternary core-shell heterojunction with up-conversion fluorescence auxiliary is fabricated via a simple chemical-calcination method. As revealed, the g-C3N4/Er:CdSe/SiO2 ternary core-shell heterojunction (g-C3N4/Er:CdSe/SiO2-3) exhibits remarkable visible light photocatalytic hydrogen evolution and photodegradation tetracycline hydrochloride enhancement than that of single CdSe (∼35 folds of HER, ∼2 folds of photodegradation) and single g-C3N4 (∼70 folds of HER, ∼2 folds of photodegradation), and achieves decent photocatalytic stability. It can be mainly attributed to that, the SiO2 spheres act as optical lens and Er-doping with up-conversion fluorescence can increase solar efficiency, and the formed g-C3N4/Er:CdSe heterojunction with appropriate potential gradient can improve carrier efficiency, including increasing transportation, prolonging lifetime and decreasing recombination. What’s more, the ternary core-shell spheroidal structure can increase active sites and photocatalytic stability.

Dual-mode up/down-conversion optical thermometry of Pr<sup>3+</sup>-regulated Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup> phosphors

Photothermal sensing is crucial in developing smart wearable devices. However, designing and synthesizing luminescent materials with suitable multi-wavelength emission and constructing multiple sets of probes in a single material system is a huge challenge for constructing sensitive temperature sensors with a wide temperature range. In this paper, Pr<sup>3+</sup>, Er<sup>3+</sup> single-doped and double-doped Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub> phosphors are successfully prepared by high temperature solid phase method, and their structures, morphologies, excitation wavelengths and temperature-dependent fluorescence properties are characterized by XRD, SEM, fluorescence spectrometer and self-made heating device. Firstly, the photoluminescences of the synthesized series of samples are investigated. The results show that comparing with the single-doped Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>: Er<sup>3+</sup> sample, the up/down-conversion spectra of Pr<sup>3+</sup>, Er<sup>3+</sup> co-doped phosphors under 808 nm/380 nm excitation show that the green fluorescence emission of Er<sup>3+</sup> is enhanced. In addition, under 980 nm excitation, Pr<sup>3+</sup> can effectively regulate the fluorescence energy level population pathway, so that the electrons are more effectively arranged in the <sup>2</sup>H<sub>11/2</sub> and <sup>4</sup>S<sub>3/2</sub> energy levels in the excitation process. The red emission is weakened and the green emission is enhanced, which improves the signal resolution of the fluorescent material and has a significant influence on the optical temperature measurement. Secondly, the up-conversion fluorescence property of Er<sup>3+</sup> under 808 nm/980 nm laser excitation in Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup> and Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Pr<sup>3+</sup>,Er<sup>3+</sup> phosphors are investigated. The results show that the red and green fluorescence emissions of Er<sup>3+</sup> are two-photon processes. Finally, the up/down-conversion dual-mode temperature sensing properties of Er<sup>3+</sup> in Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup> and Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Pr<sup>3+</sup>, Er<sup>3+</sup> phosphors are investigated. It is found that both materials have good optical temperature measurement performances. The Pr<sup>3+</sup> doping optimizes the dual-mode optical temperature measurement performances of Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup> phosphors derived from the thermal coupling energy level of Er<sup>3+</sup> ions. In addition, the up/down-conversion fluorescence mechanism of Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup> and Li<sub>0.9</sub>K<sub>0.1</sub>NbO<sub>3</sub>:Er<sup>3+</sup>, Pr<sup>3+</sup> phosphors are proposed, and the enhanced green fluorescence by Pr<sup>3+</sup> co-doping is attributed to the energy transfer from Pr<sup>3+</sup> ions to Er<sup>3+</sup> ions, leading to the increase of green fluorescence level population and the decrease of red fluorescence level population of the Er<sup>3+</sup> ions. This new dual-mode optical temperature measurement material provides a material basis and optical temperature measurement technology for exploring other temperature measurement materials.

Infrared imaging of polydimethylsiloxane-doped upconversion particle composites.

Short-wave infrared imaging is playing an increasingly important role in present scientific research. However, the high experimental cost limits its application. Based on the upconversion fluorescence effect, we prepared a low-cost composite film for short-wave infrared imaging using upconversion particles and polydimethylsiloxane. The imaging quality and stability of the prepared composite films are examined using an infrared laser. The fluorescence density fluctuation of the composite films at 100 is only 1.2%, and the maximum value of the fluorescence power percentage error caused by the inhomogeneity is only 4.5%. Due to the affordable and easy accessibility, a typical optical laboratory can prepare and use the aforementioned technique in experiments in a short time.

Magnetic modification of lanthanide-based upconversion nanocrystals for fingerprint information recognition

The development of new magnetic fluorescent materials is of great significance for identification and criminal investigation. Since the photosensitive elements used in conventional cameras have exhibited the highest quantum efficiency in the range of 500–700 nm, lanthanide-based upconversion nanoparticles (UCNPs) with main emission peaks at 507–533 nm, 533–568 nm and 637–683 nm are suitable for constructing magnetic fluorescent materials. In this work, we demonstrate a type of magnetic upconversion nanoparticle (MUCNP) of NaGdF<sub>4</sub>:Yb,Er-Fe<sub>3</sub>O<sub>4</sub> by a ligand-linked method. After optimizing the reaction parameters, the composite particles possess remarkable magnetic properties and upconversion fluorescence intensity and achieve high contrast for latent fingerprint recognition on various substrates. The combination of upconversion luminescence and magnetism contributes to good fingerprint recognition sensitivity and universality.

Focusing of surface plasmon polaritons propagating at the SiO2/Ag interface with 2-level and 4-level Fresnel phase zone pad structures.

We propose and demonstrate dielectric Fresnel phase zone pad (FPZP) structures for focusing surface plasmon polaritons (SPPs) propagating at the SiO2/Ag interfaces. We exploited up-conversion fluorescence microscopy to characterize the SPP focusing. We first report on the SPP focusing with 2-level FPZP structures that introduced a π-phase shift in the SPP wavefront between adjacent zones. We optimized the SPP focusing by fine-tuning the longitudinal width of the FPZP structure. This led to the enhancement of the peak intensity of the SPP focal spot and the reduction of the focal spot size in both the longitudinal and transverse directions. Such focusing was also demonstrated with different focal lengths. To further improve the SPP focusing, we developed a 4-level FPZP structure, which introduced a π/2-phase shift in the SPP wavefront between adjacent zones. With the optimized 4-level FPZP structure, the SPP focal spot peak intensity is further improved, and the spot size is reduced. To assist the design of the FPZP structures, we carried out theoretical analysis and numerical calculations to determine the SPP wavelengths at various oxide/Ag interfaces. We also carried out finite difference time domain (FDTD) calculations to simulate the SPP focusing with the FPZP structures. The results of the FDTD simulation agree with the experimental results qualitatively.

Alkylated green fluorescent protein chromophores: dynamics in the gas phase and in aqueous solution.

Fluorescent labelling of macromolecular samples, including using the green fluorescent protein (GFP), has revolutionised the field of bioimaging. The ongoing development of fluorescent proteins require a detailed understanding of the photophysics of the biochromophore, and how chemical derivatisation influences the excited state dynamics. Here, we investigate the photophysical properties associated with the S1 state of three alkylated derivatives of the chromophore in GFP, in the gas phase using time-resolved photoelectron imaging, and in water using femtosecond fluorescence upconversion. The gas-phase lifetimes (1.6-10 ps), which are associated with the intrinsic (environment independent) dynamics, are substantially longer than the lifetimes in water (0.06-3 ps), attributed to stabilisation of both twisted intermediate structures and conical intersection seams in the condensed phase. In the gas phase, alkylation on the 3 and 5 positions of the phenyl ring slows the dynamics due to inertial effects, while a 'pre-twist' of the methine bridge through alkylation on the 2 and 6 positions significantly shortens the excited state lifetimes. Formation of a minor, long-lived (≫ 40 ps) excited state population in the gas phase is attributed to intersystem crossing to a triplet state, accessed because of a T1/S1 degeneracy in the so-called P-trap potential energy minimum associated with torsion of the single-bond in the bridging unit connecting to the phenoxide ring. A small amount of intersystem crossing is supported through TD-DFT molecular dynamics trajectories and MS-CASPT2 calculations. No such intersystem crossing occurs in water at T = 300 K or in ethanol at T ≈ 77 K, due to a significantly altered potential energy surface and P-trap geometry.

Unveiling the double triplet nature of the 2Ag state in conjugated stilbenoid compounds to achieve efficient singlet fission.

In this investigation, the excited-state evolution in a series of all-trans stilbenoid compounds, displaying a low-lying dark singlet state of 2Ag-like symmetry nearly degenerate with the bright 1Bu state, was unveiled by employing advanced ultrafast spectroscopies while probing the effect of solvent polarizability. Together with the dual emission, femtosecond transient absorption and broadband fluorescence up-conversion disclosed the double nature of the 2Ag-like state showing both singlet features, a lifetime typical of a singlet and the ability to emit, and a triplet character, exhibiting a triplet-like absorption spectrum. The ultrafast formation (in hundreds of femtoseconds) from the non-relaxed upper singlet state led to the identification of 2Ag as the correlated triplet pair of singlet fission. The spectral difference obtained by comparison of transient absorption peaks of the 2Ag (1TT) and the triplet states was found to be in remarkable agreement with the observed triplet yield and the 1(TT) separation rate constant. Indeed, this spectral shift provided an experimental method to gain qualitative insight into the ease of separation of the 1(TT) and the relative SF efficiency. The highly conjugated polyene-like structures enable the ultrafast formation of the double triplet, but then the large binding energy prevents the triplet separation and thus the effective completion of singlet fission. Even though thermodynamically feasible for all the investigated stilbenoids according to TD-DFT calculations, singlet fission resulted to occur efficiently in the case of 1-(pyridyl-4-ylethenyl)-4-(p-nitrostyryl)benzene and nitro-styrylfuran with the triplet yield reaching 120% and 140%, respectively, triggered by their greatly enhanced intramolecular charge transfer character relative to the other compounds in the series.

Structural change dynamics of heteroleptic Cu(I) complexes observed by ultrafast emission spectroscopy.

[Cu(I)(dmp)(P)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline derivatives; P = phosphine ligand) is one of the most promising photosensitizers used in a photo-catalytic system for reducing CO2, for which the quantum yield is as high as 57%. In this work, time-resolved emission spectra of Cu(I) complexes in solutions were investigated using femtosecond fluorescence up-conversion and nanosecond time-resolved emission spectroscopic systems. The temporal profiles of emission intensities less than 10 ps in acetonitrile solution were reproduced using a tri-exponential function with three-time constants of 0.040 ps, 0.78 ps and 8.0 ps. We found that only the second time constant is dependent on the solvent (acetonitrile: 0.78 ps, butyronitrile: 1.4 ps), indicating that the 0.78 ps spectral change is attributed to the structural change of the Cu(I) complex. The oscillator strengths of transition species are derived from the intensities in the time-resolved emission spectra (species-associated spectra). Based on the oscillator strengths, we concluded that the 0.040 ps process is the Sn → S1 internal conversion and the 0.78 ps process is a structural change in the S1 state. The final time constant of 8.0 ps is assigned to the S1 → T1 intersystem crossing because the 3MLCT state (τT1 = 97 ns) is generated after the decay. The DFT calculation showed that the 0.78 ps spectral change (∼600 cm-1 redshift) is attributed to Jahn-Teller distortion around the metal center, and there is a large structural change in the ligand, which results in a large Stokes shift in the Sn state (7.3 × 103 cm-1).

Fluorescence of Bimolecular Guanine Quadruplexes: From Femtoseconds to Nanoseconds

The paper deals with the fluorescence of guanine quadruplexes (G4) formed by association of two DNA strands d(GGGGTTTTGGGG) in the presence of K+ cations, noted as OXY/K+ in reference to the protozoon Oxytricha nova, whose telomere contains TTTTGGGG repeats. They were studied by steady-state and time-resolved techniques, time-correlated single photon counting, and fluorescence upconversion. The maximum of the OXY/K+ fluorescence spectrum is located at 334 nm, and the quantum yield is 5.8 × 10-4. About 75% of the photons are emitted before 100 ps and stem from ππ* states, possibly with a small contribution of charge transfer. Time-resolved fluorescence anisotropy measurements indicate that ultrafast (<330 fs) excitation transfer, due to internal conversion among exciton states, is more efficient in OXY/K+ compared to previously studied G4 structures. This is attributed to the arrangement of the peripheral thymines in two diagonal loops with restricted mobility, facilitating the interaction among them and with guanines. Thymines should also be responsible for a weak intensity excimer/exciplex emission band, peaking at 445 nm. Finally, the longest living fluorescence component (∼2.1 ns) is observed at the blue side of the spectrum. So far, high-energy long-lived emitting states had been reported only for double-stranded structures but not for G4.

NaYF4:Yb3+/Tm3+@NaYF4:Yb3+ Upconversion Nanoparticles for Optical Temperature Monitoring and Self-Heating in Photothermal Therapy

The core–shell NaYF4:Yb3+/Tm3+@NaYF4:Yb3+ upconversion nanoparticles were successfully prepared by a solvothermal method, and a layer of mesoporous silica (mSiO2) was successfully coated on the periphery of the core–shell nanoparticles to transform their surface from lipophilic to hydrophilic, further expanding their applications in biological tissues. The physical phase, morphology, structure, and fluorescence properties were characterized by X-ray diffraction (XRD), field emission transmission electron microscopy (TEM), Fourier infrared spectroscopy (FT-IR), ζ potential analysis, and fluorescence spectroscopy. It was found that the material has a hexagonal structure with good hydrophilicity and emits intense fluorescence under 980 nm pump laser excitation. The non-contact temperature sensing performance of nanoparticles was evaluated by analyzing the upconversion fluorescence of Tm3+ (1G4 → 3F4 and 3F3 → 3H6) in the temperature range of 284–344 K. The absolute and relative sensitivities were found to be 0.0067 K–1 and 1.08 % K–1, respectively, with high-temperature measurement reliability and good temperature cycling performance. More importantly, its temperature measurement in phosphate-buffered saline (PBS) solution is accurate. In addition, the temperature of the cells can be increased by adjusting the laser power density and laser irradiation time. Therefore, an optical temperature sensing platform was built to realize the application of real-time monitoring of cancer cell temperature and the dual function of photothermal therapy.

A strategy for the accurate detection of glucose in human serum based on the IFE effect of up-transformed nanoparticles

Here, the inner filter effect (IFE) was applied to construct a “turn off” mode up-conversion fluorescence and colorimetric dual-reading detection platform consisting of the chromogenic substance p-phenylenediamine (PPD) and NaYF4:Yb/Er@NaYF4 core–shell up-conversion nanoparticles (UCNPs) for the detection of glucose and hydrogen peroxide (H2O2) with high sensitivity and selectivity. In this strategy, hydrogen peroxide is produced in the process of glucose oxidation by catalyzing glucose oxidase (GOx). PPD is oxidized by H2O2 under the action of horseradish peroxidase (HRP), resulting in the transformation into its oxidation product PPDox, whose absorption peak coincides with the emission peak of UCNP at 540 nm, which can significantly quench the fluorescence of UCNP. The quenching efficiency is related to PPDox production, so the relationship between the glucose content and the fluorescence changes of UCNP can be established to realize the sensitive detection of glucose. UCNPs modified by –COOH and the oxidation product of PPD were negatively charged, which increased the distance between donor and recipient due to electrostatic repulsion. It was not conducive to the occurrence of distance-dependent fluorescence resonance energy transfer (FRET), which was also verified by fluorescence lifetime test. Under the optimal experimental conditions, the linear ranges of hydrogen peroxide and glucose were 2.5 ∼ 50 and 5 ∼ 80 μmol/L with the detection limit (LOD) of 0.32 and 0.83 μmol/L, respectively, which were lower than of some reported approaches. Furthermore, the system could be applied to the detection of glucose in actual serum sample and the results were consistency of commercial glucose meters which indicated that the protocol was reliable and potential applications for clinical diagnosis.

A novel ADA-coated UCNPs@NB sensing platform combined with nucleic acid amplification for rapid detection of Escherichia coli

In this study, we reported a novel sensing platform based on fluorescence quenching composed of alendronic acid (ADA) coated upconversion nanoparticles (UCNPs) and Nile Blue (NB) combined with polymerase chain reaction (PCR) for rapid, sensitive, and specific detection of Escherichia coli (E. coli). As a fluorescence acceptor, NB has a broad absorption band and can quench upconversion fluorescence intensity at 544 nm and 658 nm based on IFE. PCR is a double-stranded DNA (dsDNA) amplification technique with high specificity. The NB-dsDNA complex can be formed by intercalation of NB between base pairs and groove of dsDNA, leading to upconversion fluorescence recovery. The ADA-coated UCNPs@NB sensing platform achieved to detect E. coli in 1.5 h, with a lower limit of detection (33 CFU mL−1). In addition, the sensitivity of the ADA@UCNPs-NB fluorescence sensor under different PCR cycle numbers was discussed. The results showed that the proposed sensor could effectively shorten the assay time (1.0 h) while maintaining excellent sensitivity. This study demonstrated a rapid and sensitive analytical method for detecting E. coli in chicken, providing a reference for constructing PCR fluorescence sensors.

Novel colorimetric, photothermal and up-conversion fluorescence triple-signal sensor for rosmarinic acid detection

Rosmarinic acid (RA) is promising as a natural and nontoxic food additive. However, many analysis methods for RA generally depend on large instruments and single signals for quantitative detection. A new up-conversion fluorescence, colorimetric and photothermal multi-modal sensing strategy is developed for the quantification of RA. β-cyclodextrin (CD) modified citric acid (Cit) wrapped NaYF4:Yb/Er-Cit-CD (Y:Yb/Er-Cit-CD) up-conversion nanocomposite has been synthesized, which emits green fluorescence at 550 nm under 980 nm near-infrared (NIR) excitation. In the presence of oxidized 3,3′,5,5′-tetramethylbenzidine (oxTMB), the green fluorescence is significantly quenched attributed to the fluorescence inner filter effect (IFE) between oxTMB and Y:Yb/Er-Cit-CD. When RA is intervened, blue oxTMB is reduced to colorless 3,3′,5,5′-tetramethylbenzidine (TMB) inducing the recovery of up-conversion fluorescence. At the same time, colorimetric and photothermal signals readout can be easily achieved thanks to the color indication and photothermal effect of the oxTMB. The constructed Y:Yb/Er-Cit-CD/oxTMB sensor displays high sensitivity, visibility and simplicity for RA, and the limits of detection (LOD) for fluorescence, colorimetric and photothermal were 0.004 µmol/L, 0.036 µmol/L and 0.043 µmol/L, respectively. This sensing system is successfully performed for the detection of RA in food samples.

Ultrasensitive hairpin mediated upconversion fluorescence biosensor for Staphylococcus aureus detection in foods and waters exploiting g–C3N4–assisted catalysis

A novel g-C3N4 nanosheets (g-C3N4 NSs)-assisted upconversion fluorescent aptasensor was proposed for Staphylococcus aureus (S. aureus) detection by adopting hybridization chain reaction (HCR) as a sensitizer. Two hairpin (H1 and H2) structured DNA probes were engineered predicated on the partial complementary sequence (cDNA) of S. aureus aptamer and modified on the exterior of the upconversion nanoparticles (UCNPs), respectively. The presence of S. aureus initiated the HCR system and activated H1 and H2 probes to form a double-helix away from the g-C3N4 NSs vicinity. This led to the decrease in peroxidase-like activity (PA) of the g-C3N4 NSs and corresponding fluorescence recovery proportional to the concentration of S. aureus (10-106 cfu mL−1). The method was applied to real food samples with acceptable recoveries (91.1–101.6%) and further validated by traditional plate counting method (p > 0.05).