Impact of Excitation Intensity-Dependent Fluorescence Intensity Ratio of Upconversion Nanoparticles on Wide-Field Thermal Imaging

Abstract

Erbium ion (Er 3+ )-doped upconversion nanoparticles (UCNPs) are frequently used for nanothermometry because their fluorescence intensity ratio (FIR) between two green emission bands at ∼525 and ∼545 nm is sensitive to temperature variation. One of the prerequisites for nanothermometry is that the FIR be independent of excitation intensity at constant temperature. In this work, the effect of excitation intensity on the FIR of core−double-shell NaYF 4 :Yb 3+ ,Er 3+ @NaYF 4 :Yb 3+ ,Nd 3+ @NaYF 4 UCNPs was investigated in two environments. The first environment is in aqueous solution, and the second is a monolayer of UCNPs on top of a silica−silicon substrate in air. The experimental results showed that the FIR decreases with the excitation intensity at constant temperature in each case. We further found that the excitation intensity-dependent FIR indeed deteriorated the thermal images acquired by wide-field upconversion fluorescence microscopy, in which a Gaussian laser beam was used to excite UCNPs uniformly coated on a silica−silicon substrate. The nonuniform excitation intensity of the incident laser beam resulted in thermal images that showed nonuniform temperature distributions in a 100 μm range field of view, even though the whole sample was maintained at constant temperature in air. To tackle this problem, we first measured the excitation intensity and temperature dependence of the FIR and the excitation laser intensity distribution on the sample. We then developed a correction scheme to correct the thermal images. With our correction process, the temperature distribution on the sample can be accurately mapped even with nonuniform illumination.

Luminescence Thermometry Beyond the Biological Realm

“…Complementary rate equation modeling instead showed that a combination of radiative and nonradiative relaxation from higher-lying Er 3+ energy levels that become heavily populated at high single-particle excitation intensities increases the temperature-dependent ratio. Working in a lower excitation intensity range, Nguyen et al instead identified a decrease in the ratio with excitation intensity that was attributed to decreased population of the 2 H 11/2 excited state. Several works have identified emission from the 2 H 9/2 to 4 I 13/2 transition of Er 3+ , which is frequently observed at higher excitation intensities and overlaps spectrally with the 4 S 3/2 to 4 I 15/2 Er 3+ transition used for ratiometric thermometry, as a source of measurement artifacts. ,− Because the 2 H 9/2 to 4 I 13/2 transition results from a three-photon upconversion process, the emission from this transition displays a different excitation intensity scaling than the Er 3+ transitions used for ratiometric thermometry that originate from two-photon upconversion processes, leading to errors in temperature measurements where the excitation intensity is varied (Figure d).…”

“…Some excitation intensity-dependent artifacts can be avoided or mitigated by working at fixed or low excitation intensities. Another approach is to correct the ratio by subtracting or dividing out an excitation intensity-dependent term. , Martins et al also demonstrated the disappearance of the emission from the 2 H 9/2 to 4 I 13/2 transition when the excitation wavelength was switched from 980 to 808 nm. Alternatively, deconvolution or experimental procedures , can be applied to separate and subsequently remove the emission originating from the 2 H 9/2 to 4 I 13/2 transition prior to calculating the ratio.…”

“…Complementary rate equation modeling instead showed that a combination of radiative and nonradiative relaxation from higher-lying Er 3+ energy levels that become heavily populated at high single-particle excitation intensities increases the temperature-dependent ratio. Working in a lower excitation intensity range, Nguyen et al 257 instead identified a decrease in the ratio with excitation intensity that was attributed to decreased population of the 2 H 11/2 excited state. Several works have identified emission from the 2 H 9/2 to 4 I 13/2 transition of Er 3+ , which is frequently observed at higher excitation intensities and overlaps spectrally with the 4 S 3/2 to 4 I 15/2 Er 3+ transition used for ratiometric thermometry, as a source of measurement artifacts.…”

Ultrasensitive Upconversion Nanoparticle Immunoassay for Human Serum Cardiac Troponin I Detection Achieved with Resonant Waveguide Grating

“…UCNPs typically consist of a crystalline host, such as sodium yttrium tetrafluoride (NaYF 4 ), codoped with trivalent lanthanide ion sensitizers (ytterbium, Yb 3+ , and neodymium, Nd 3+ ) and activators (erbium, Er 3+ , thulium, Tm 3+ , and holmium, Ho 3+ ). UCNPs have excellent photophysical properties, such as narrow emission lines, high photostability, no photobleaching, no autofluorescence, and low background signal. − …”