Characterizing Upconversion Nanoparticles: Spectra, Lifetime and Quantum Yield

Photon upconversion (UC) occurs when two or more lower-energy photons are converted into higher-energy ones. Upconversion nanoparticles (UCNPs) are a key type of upconversion material.

UCNPs are nanoparticles consisting of rare-earth elements, including erbium and ytterbium, lodged in a host matrix. They may be utilized to translate near-infrared light into higher-energy light in both the visible and ultraviolet spectral regions.

UCNPs carry a broad spectrum of potential uses. In solar energy harvesting, they can boost photovoltaic device performance and solar cell efficiency by translating unutilized solar spectrum segments into higher-energy photons. Meanwhile, UCNPs can be used for non-invasive imaging and diagnostics in biomedicine.

As imaging contrast agents, they facilitate high-resolution imaging by translating infrared light into visible light, which facilitates deeper tissue penetration and diminishes background signal. In drug delivery, their distinct optical characteristics can be utilized for the controlled release of therapeutic agents.¹

Proper understanding of upconversion luminescence characteristics that belong to UCNPs is critical for customizing their features to specified use cases and maximizing their performance. In this report, an Edinburgh Instruments FS5 Spectrofluorometer was utilized for a full upconversion luminescence characterisation of UCNPs consisting of spectral, time-resolved, and quantum-yield assessments.

Materials and Methods

The sample used for study was NaYF₄:Yb,Er UCNPs equipped with a polyethylenimine (PEI) polymer coating dispersed in deionised water at a concentration of 10 mg/mL. The UCNP dispersion was kept in a 10 mm × 10 mm quartz cuvette and characterized via an FS5 Spectrofluorometer.

For excitation, the FS5 was fitted with a 2W 980 nm laser diode with a pulse modulation box (PM-2), facilitating CW as well as pulsed operation. For detection, the FS5 was fitted with two photodetectors: a PMT-900 and PMT-1010 (FS5-NIR upgrade) and multichannel scaling (MCS) lifetime electronics.

The PMT-900, which has a spectral range of 200-900 nm, was utilized for spectral and lifetime assessments. Meanwhile, the PMT-1010, with its enlarged spectral range spanning to 1010 nm, was utilized for determining quantum yield. The sample cuvette was kept in the SC-05 Standard Cuvette Module for both spectral and lifetime assessments, while the SC-30 Integrating Sphere Module was utilized for quantum yield assessments.

Figure 1. Edinburgh Instruments FS5 Spectrofluorometer. Image Credit: Edinburgh Instruments

Upconversion Spectrum

The nanoparticles’ emission spectrums were acquired (Figure 2) via the 980 nm laser diode in CW mode for excitation and the PMT-900 for detection.

Figure 2. Upconversion emission spectrum of the UCNPs acquired using a 2 W 980 nm laser diode in CW mode and the PMT-900. Image Credit: Edinburgh Instruments

An emission spectrum may be utilized to distinguish emissive energy transitions in materials. Emission arises from the absorption of multiple low-energy 980 nm photons by many Yb³⁺ ions (sensitisers), which direct their energy to a single emissive Er³⁺ ion (emitter).

The energy transfer is carried out through a process of non-radiative multi-ion upconversion. The emission peak centered around 660 nm is the ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺, while the additional emission peaks at 521 nm and 545 nm originate from the ⁴H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions (Figure 3).^1,2

Figure 3. Energy level diagram showing the upconversion process within the NaYF₄:Yb,Er nanoparticles. Image Credit: Edinburgh Instruments

Upconversion Lifetime

Upconversion decay of the ⁴F^9/2 → 4I_15/2 transition at 653 nm was then assessed, as shown in Figure 4. The 980 nm laser diode was put into pulsed mode with a repetition rate of 500 Hz and a pulse width of 10 µs. Meanwhile, the upconversion decay was acquired via MCS single photon counting. The decay was next put in Fluoracle^® via a two-exponential model, bringing about an intensity average lifetime of 125 µs.

The lifetime is smaller than past findings from similar materials, which reported lifetimes above 440 µs.³ A potential reason for the discrepancy in lifetime values may lie in the deionized water utilized for dilution, the presence of which may have introduced significant quenching effects, leading to a shorter lifetime.

Figure 4. Upconversion decay of the UCNPs at 653 nm acquired using a 2 W 980 nm laser diode in pulsed mode at 500 Hz and the PMT-900. Image Credit: Edinburgh Instruments

Upconversion Quantum Yield

Assessing quantum yield is crucial for examining the efficiency of UCNPs for generating upconverted light. UCNPs typically have lower quantum yields than their bulk alternatives.

Multiple reasons may be behind this, including the forbidden 4f – 4f transition, and unavoidable challenges such as poor crystallinity and surface defects.⁴ Developing UCNPS with elevated quantum yields is an ongoing research pursuit.

To determine the upconversion quantum yield, the FS5 SC-30 Integrating Sphere Module was utilized. The SC-30 is directly inserted into the FS5 without alignment or optical fibers. The FS5's secondary PMT-1010 detector was utilized to detect light.

The expanded spectral range PMT-1010 to 1010 nm facilitates a precise assessment of the 980 nm excitation light and is needed for 980 nm excitation upconversion quantum yield assessments.

The emission and excitation scatter from the UCNP sample cuvette and a reference cuvette containing deionized water, which was only measured in the SC-30, were used to quantify the quantum yield.

Figure 5 showcases the emission and scattering peaks of the UCNPs and deionized water reference. The quantum yield was next quantified through the quantum yield wizard of the Fluoracle software, which is based on the following formula:

(1)

Figure 5. Scattering and emission spectra of the UCNPs sample and deionized water reference were acquired using a 2 W 980 nm laser diode in CW mode and the PMT-1010. Image Credit: Edinburgh Instruments

Conclusion

This report showcases a complete upconversion luminescence characterization of upconversion nanoparticles utilizing the Edinburgh Instruments FS5 Spectrofluorometer.

The flexible source, detector, and sample module upgrade options of the FS5 enable upconversion spectral, lifetime and quantum yield assessments to be carried out in a single compact tool.

Acknowledgments

Edinburgh Instruments is grateful to Dr Lewis MacKenzie and his team from the University of Strathclyde for synthesizing the UCNPs utilized in this report.

References and Further Reading

1. MacKenzie, L.E., Alvarez-Ruiz, D. and Róbert Pál (2022). Low-temperature open-air synthesis of PVP-coated NaYF ₄:Yb,Er,Mn upconversion nanoparticles with strong red emission. Royal Society Open Science, 9(1). https://doi.org/10.1098/rsos.211508.
2. Birch, R.B., et al. (2023). Influence of polyvinylpyrrolidone (PVP) in the synthesis of luminescent NaYF₄:Yb,Er upconversion nanoparticles. Methods and Applications in Fluorescence, 11(3), pp.034001–034001. https://doi.org/10.1088/2050-6120/acd837.
3. Qin, H., et al. (2018). Tuning the upconversion photoluminescence lifetimes of NaYF₄:Yb³⁺, Er³⁺ through lanthanide Gd³⁺ doping. Scientific Reports, 8(1). https://doi.org/10.1038/s41598-018-30983-9.
4. Callum, Gakamsky, A. and Marques-Hueso, J. (2021). The upconversion quantum yield (UCQY): a review to standardize the measurement methodology, improve comparability, and define efficiency standards. Science and technology of advanced materials, 22(1), pp.810–848. https://doi.org/10.1080/14686996.2021.1967698.

This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.

For more information on this source, please visit Edinburgh Instruments.