Single-nanocrystal sensitivity achieved by enhanced upconversion luminescence

Journal name:
Nature Nanotechnology
Year published:
Published online


Upconversion nanocrystals convert infrared radiation to visible luminescence, and are promising for applications in biodetection1, 2, 3, bioimaging4, 5, 6, 7, solar cells8, 9, 10 and three-dimensional display technologies8, 9, 11. Although the design of suitable nanocrystals has improved the performance of upconversion nanocrystals10, 12, 13, 14, their emission brightness is limited by the low doping concentration of activator ions needed to avoid the luminescence quenching that occurs at high concentrations15, 16. Here, we demonstrate that high excitation irradiance can alleviate concentration quenching in upconversion luminescence when combined with higher activator concentration, which can be increased from 0.5 mol% to 8 mol% Tm3+ in NaYF4. This leads to significantly enhanced luminescence signals, by up to a factor of 70. By using such bright nanocrystals, we demonstrate remote tracking of a single nanocrystal with a microstructured optical-fibre dip sensor. This represents a sensitivity improvement of three orders of magnitude over benchmark nanocrystals such as quantum dots17.

At a glance


  1. Highly Tm3+-doped NaYF4 nanocrystals exhibit enhanced upconversion in a suspended-core fibre.
    Figure 1: Highly Tm3+-doped NaYF4 nanocrystals exhibit enhanced upconversion in a suspended-core fibre.

    a, Transmission electron microscopy images of monodispersed NaYF4:Yb/Tm nanocrystals at different doping levels. All nanoparticles have a similar average size with a narrow size distribution. b, Schematic of the experimental configuration for capturing upconversion luminescence of NaYF4:Yb/Tm nanocrystals using a suspended-core microstructured optical-fibre dip sensor. The continuous-wave 980-nm diode laser is targeted at the suspended core. Light propagates along the length of the fibre and interacts with the upconversion nanocrystals located within the surrounding holes. The excited upconversion luminescence is coupled into the fibre core and the backward-propagating light is captured by a spectrometer. Inset: scanning electron microscope images showing a cross-section of the F2 suspended-core microstructured optical fibre at different magnifications. The fibre outer diameter is 160 µm with a 17 µm hole and 1.43 µm core. c, Upconversion spectra of a series of NaYF4:Yb/Tm nanocrystals with varied Tm3+ concentrations under an excitation irradiance of 2.5 × 106 W cm−2, showing a steady increase in upconversion luminescence with increasing Tm3+ content from 0.2 mol% to 8 mol%.

  2. Analysis of power-dependent multiphoton upconversion.
    Figure 2: Analysis of power-dependent multiphoton upconversion.

    a, Simplified energy-level scheme of NaYF4:Yb/Tm nanocrystals indicating major upconversion processes. Dashed lines indicate non-radiative energy transfer, and curved arrows indicate multiphonon relaxation. b, Typical evolution of spectra for 1 mol% Tm3+ as a function of excitation, showing substantial growth of emissions from the 1G4 and 1D2 levels with increasing excitation from 1 × 104 W cm−2 to 2.5 × 106 W cm−2. c, Decomposition of the spectra into individual Gaussian peaks. Integrated intensities are given by Iλ where λ is the peak wavelength. Different transitions are indicated by the colours shown in the energy-level scheme in a. For example, the shaded area represents the 3H4 right arrow 3H6 transitions. d, Intensity ratios of the 1D2 to 3H4 classes (I455 + I514 + I744 + I782)/I802 and 1G4 to 3H4 classes (I480 + I660)/I802) as a function of excitation irradiance. e, Diagram illustrating energy transfer between the ensemble of Yb3+ and Tm3+ ions and subsequent radiative and non-radiative pathways. Top (bottom) panels: low (high) Tm3+/Yb3+ ratio. In the case of a low Tm3+/Yb3+ ratio, the limited number of Tm3+ ions creates an energy transfer bottleneck, due to the limited capacity of Tm3+ to release energy from the 3F4 and 3H4 states. Thus, at increasing excitation, alternative energy loss channels (radiative and non-radiative) involving higher states 1G4 and 1D2 progressively switch on. Brown, excitation light; green, simplified energy levels; red, blue and purple, radiative energy flux; grey, radiative flux not observed in this work; black, non-radiative energy loss.

  3. Analysis of power-dependent upconversion efficiency.
    Figure 3: Analysis of power-dependent upconversion efficiency.

    a, Integrated upconversion luminescence intensity (~400–850 nm) as a function of excitation irradiance for a series of Tm3+-doped nanocrystals. All samples have the same volume and number of nanocrystals. b, As in a, but divided by the concentration of Tm3+ ions. Under an excitation irradiance of 2.5 × 106 W cm−2, 2 mol% Tm3+ has the highest relative upconversion efficiency, whereas the strongest upconversion signal is observed in 8 mol% Tm3+ due to the larger number of activators available with sufficient excitation.

  4. Detecting a single nanocrystal in a suspended-core microstructured fibre dip sensor.
    Figure 4: Detecting a single nanocrystal in a suspended-core microstructured fibre dip sensor.

    a, Results of 10 trials of loading 3.9 fM nanocrystal solution into the fibre dip sensor. Four positive trials, shown in red, magenta, dark red and orange, show comparable ~800–810 nm emission peaks, and six trials result in consistent background noise baselines (presented in the remaining colours). The baseline level is due to scattering of 980-nm excitation. b, Normalized nanocrystal emission integrated from ~800 to 810 nm. The four positive trials shown in red, magenta, dark red and orange produce intensities of ~250 with a low coefficient of variation (CV) of 4.7%, and high signal-to-noise ratio of >8. c, Time-dependent dynamics of three independent trials. Red: trial with no nanocrystals observed (only background is observed). Blue: one nanocrystal appears shortly after the start of the trial. Black: single nanocrystal appears in the fibre after 2 min, followed by a second at ~5 min; one of the nanocrystals then exits the observation volume.

  5. Proof-of-principle experiments demonstrating a broad spectrum of applications.
    Figure 5: Proof-of-principle experiments demonstrating a broad spectrum of applications.

    a, Images of Giardia lamblia cells labelled with antibody-conjugated 4 mol% Tm3+ upconversion nanocrystals under transmission (top) and luminescence (bottom) modes. The 980-nm wide-field excitation and upconversion detection yield negligible autofluorescence background, so absolute signal intensities of each single microorganism (see histogram in c) provide quantification of the level of surface antigens. b, Individual cells localized on a glass slide by a scanning cytometry system (top), and its schematic (bottom). Targeted cells are symbolized by blue dots. c, Histogram showing the quantification results of the population of nanocrystal-labelled Giarida lamblia (CV, coefficient of variation). d, Demonstrations of security inks using the power-dependent optimal Tm3+ concentration. Low-concentration (0.2 mol% Tm3+) nanocrystals were used to stain the masking pattern (University of Adelaide logo), which is visible under both low-power illumination (top) and high-power illumination (bottom). High-concentration (4 mol% Tm3+) nanocrystals were used to stain the hidden pattern (Macquarie University logo), which is over 10 times brighter than the masking pattern. At this dynamic range the masking pattern is almost unnoticeable. e, Nanocrystal solution ‘security inks’ were used in an inkjet printer with 0.5 mol% Tm3+ nanocrystals as a rectangular mask to confound the signal image from 8 mol% Tm3+ nanocrystals. At laser scanning confocal setting (>1 × 106 W cm−2), the hidden trademark image of the 8 mol% Tm3+ nanocrystals becomes visible and dominant.


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Author information


  1. Advanced Cytometry Laboratories, MQ Photonics Research Centre and MQ BioFocus Research Centre, Macquarie University, Sydney, New South Wales 2109, Australia

    • Jiangbo Zhao,
    • Dayong Jin,
    • Yiqing Lu,
    • Yujia Liu,
    • Andrei V. Zvyagin,
    • Lixin Zhang,
    • Judith M. Dawes,
    • James A. Piper &
    • Ewa M. Goldys
  2. Institute of Photonics and Advanced Sensing and School of Chemistry and Physics, University of Adelaide, Adelaide, South Australia 5005, Australia

    • Erik P. Schartner &
    • Tanya M. Monro
  3. School of Biomedical Engineering, Shanghai Jiao Tong University, Shanghai, China

    • Yujia Liu &
    • Peng Xi
  4. Department of Biomedical Engineering, College of Engineering, Peking University, Beijing 100871, China

    • Peng Xi


D.J. and T.M. conceived the project, designed the experiments and supervised the research. J.Z., E.S., Y.Lu and D.J. were primarily responsible for data collection and analysis. D.J., E.G., J.Z. and T.M. prepared figures and wrote the main manuscript text. J.Z., E.G., A.Z. and D.J. were primarily responsible for supporting information and numerical simulations. All authors contributed to data analysis, discussions and manuscript preparation.

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