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Abstract
The Förster resonance energy transfer (FRET) from the β-NaYF4:Yb3+, Er3+/NaYF4 upconversion nanoparticles (UCNPs) to ZnCdSe/ZnS quantum dots (QDs) as a function of temperature (77–427 K) is demonstrated. With an increasing of temperature, both the intensity and peak position of QDs emission variated, which is attributed to the combining of the FRET and thermal quenching effect. By analyzing the dependence of the photoluminescence (PL) spectra on temperature, the UCNP + QD sample can be considered as dual thermal probes with high sensitivity based on either the UCL or the spectral shift of QD emission under 980 nm excitation. The lifetime of the UCNP and UCNP + QD samples are collected to investigate the dynamics of the FRET at various temperatures, showing a decrease and then an increase of the FRET efficiency from UCNPs to QDs with temperature from 77 to 427 K. This result is mainly attributed to the variation of the Förster distance R0 with the increasing of temperature. The work will be significant to detect the nanoscale interaction and it can be widely applied in biomedical, sensing and imaging.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Förster resonance energy transfer (FRET) is a non-radiative energy transfer process from a donor to an acceptor through intermolecular dipole–dipole coupling [1]. It is a phenomenon usually observed in short range of 1−10 nm between the donor and acceptor and it can transduce the interaction between molecules into the fluorescence signal with high sensitivity [2]. Hence, FRET is a powerful tool to detect the nanoscale interaction and has been widely applied in biomedical, sensing and imaging [3–5]. To satisfy diverse demands, various promising fluorescent materials are used in the FRET technic, such as organic dyes, semiconductor quantum dots (QDs), lanthanide-doped upconversion nanoparticles (UCNPs) and so on [6–8]. Among many donor-acceptor pairs, the UCNPs-donor and QDs-acceptor pair attracts great attention for its unique properties. Compared with traditional organic dyes, lanthanide-doped UCNPs have long fluorescent lifetime, large Stokes-shift and great photo-stability [9]. Besides, as the donor, UCNPs are excited by the near infrared (NIR) light which can reduce the damage to biological samples [3,9]. It can also decrease the auto-fluorescence of samples by using the NIR excitation, thus decrease the background signal. Hence, UCNPs are suitable for use as an energy donor. On the other hand, the QDs have broad absorption and size-tunable photoluminescence (PL) bands, which make them very suitable as acceptor [7,10]. These properties make the UCNP + QD system drawn wide attention in recent years. A few works studied the UCNP + QD system based on the FRET from UCNPs to QDs and the corresponding results were used in bio-sensing, immunoassay, biological analysis, detection and so on [11–17]. However, the above researches were performed at room temperature. To our knowledge, none work was about its dependence on temperature. Since the FRET from UCNPs to QDs will also be affected by temperature due to the temperature-dependent fluorescence properties of both UCNPs and QDs [18–21], it is of great significance to study the FRET from UCNPs to QDs as the function of temperature. Herein, the UCNP + QD sample was fabricated using a spin-coating method. By changing temperature from 77 to 427 K, the PL spectra and lifetimes of the sample under different temperature were collected to investigate the temperature-dependent FRET from UCNPs to QDs. The fluorescence properties of the sample and the FRET efficiency from UCNPs to QDs as the function of temperature were demonstrated. The result is conducive to expand the applications of the UCNP + QD system in term of temperature, such as temperature detection, temperature sensing and imaging.
2. Experiment
2.1 Fabrication of UCNP + QD sample
For their excellent luminescence properties, β-NaYF4:Yb3+ (2%), Er3+ (20%)/NaYF4 core/shell nanoparticles were chosen as the UCNPs used in our experiment, which are purchased from Hefei Fluonano Biotech Co., Ltd. The QDs chosen in our experiment are ZnCdSe/ZnS core/shell QDs, which were purchased from Wuhan Jiayuan Quantum Dots Co., Ltd. First, the UCNPs and QDs acetone solutions were respectively diluted to a concentration of 0.3 mg/mL. Second, The UCNPs and QDs solutions were mixed with a mass ratio of 1:2 and the mixed solution was kept under magnetic stirring at 25°C for 2 hours to ensure its uniformity. Third, a few drops of the mixture was spread on a clean silicon wafer and then spin-coated at 1,000 rpm for 20 s to form a uniform film of UCNP + QD. Finally, the sample was left in air at room temperature for 20 mins to allow the evaporation of acetone such that the UCNP + QD sample was fabricated.
2.2 Apparatus and measurements
The PL spectra and decay curves of the UCNP and UCNP + QD samples were measured using a phosphorescence spectrometer (FLS980, Edinburgh) equipped with an adjustable 980 nm diode laser (MDL-III-980-1W) as excitation source. A pulse modulator is used to switch the operating mode of the laser. The sample temperature were changed by a liquid nitrogen-cooled cryostat (OptistatDN, Oxford Instruments) from 77 to 427 K with an accuracy of ± 0.1 K. The highest temperature is set as 427 K to ensure a stable crystal phase of the NaYF4. Transmission electron microscope (TEM, JEM-200CX) was used to observe the surface morphologies of the UCNPs and QDs structure. In this work, all the intensities of QD and UCNP emissions are the integral intensity.
3. Results and discussions
3.1 Characteristics of the UCNPs and QDs
Figure 1(a) shows the transmission electron microscopy (TEM) image of the UCNPs. It can be seen that, the UCNPs are spherical in shape and uniform in size with a diameter ∼25 nm. The inset is the corresponding high resolution transmission electron microscopy (HRTEM), which shows distinguished lattice fringes with a spacing of 0.482 nm, indicating the high crystallinity of the UCNPs. Figure 1(b) shows the TEM image of the QDs with an average size of 10 nm. The distinguished lattice fringes spacing of 0.302 nm in the corresponding HRTEM image (the inset) also indicate their good crystallinity. Both the normalized upconversion luminescence (UCL) spectrum of the UCNPs under 980 nm excitation (red) and the absorption spectrum of QDs in the visible range (blue) were collected at room temperature, as show in Fig. 1(c). It can be seen that, for the UCNPs, there are three major emission bands with center wavelengths at 520, 540 and 655 nm, which correspond to the transitions 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 of Er3+, respectively. The QDs present an obvious absorbance in the green band, which matches well with the green emission of UCNPs. Such overlap of the spectra provides a prerequisite for the happening of the FRET from UCNP to QDs. In addition, the normalized PL spectrum of the QDs under 540 nm excitation (black) was also shown in Fig. 1(c). The QDs present one emission band with peak position at 620 nm, which can be easily distinguished from the UCL emission bands. This largely facilitates the acquisition and analysis of the fluorescent signals.
Fig. 1. (a) TEM images of the NaYF4:Yb3+ (20%), Er3+ (2%)/NaYF4 core/shell nanoparticles and (b) the ZnCdSe/ZnS core/shell QDs. The insets are the HRTEM images of the corresponding individual nanocrystals which were marked with yellow dotted boxes. (c) The normalized UCL spectrum of the UCNPs under 980 nm excitation (red curve), the absorption (blue curve) and PL (black curve) spectrum of QDs. The PL spectrum of QDs was collected under 540 nm excitation.
3.2 FRET from UCNPs to QDs
Figure 2(a) is the HRTEM image of the UCNP + QD sample, in which the representative UCNP and QDs are marked by the yellow and red dotted line circles, respectively. It shows that the QDs were adsorbed on the surface of the UCNP and there is no obvious gap at the interface between the QDs and UCNPs, which are beneficial for the generation of the FRET from UCNPs to QDs. At room temperature, the PL spectra of the UCNP and UCNP + QD samples under 980 nm excitation were collected and shown in Fig. 2(b) as the red and black curves, respectively. For UCNP + QD sample, four emission bands are observed with center wavelengths at 520, 540, 655 and 620 nm, in which the first three are from the UCNPs and the last one is from the QDs. Compared to UCNP sample, the green emission intensity of the UCNP + QD sample decreased by approximately 60% due to the FRET. The detailed mechanism of the FRET process is schematically illustrated in Fig. 2(c). The excitation light at 980 nm was absorbed by Yb3+ ions and then transferred to multiple excited levels of the Er3+ ions in the UCNP. Then, because the energy of these excited levels (2H11/2, 4S3/2) fits the energy gap of the QD, the UCNP transferred a portion of photon energy of the excited levels to the QD through FRET, leading to the appearance of the 620 nm emission. The rest of the energy in the UCNP was emitted as green (2H11/2, 4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) emissions through the non-radiative relaxation. To further prove this FRET process, the PL decay curves of green emission for both UCNP + QD (blue) and UCNP (pink) samples were measured [(Fig. 2(d)]. Because the energy levels of 2H11/2 and 4S3/2 is close and the intensity of 540 nm emission is dominated, the PL decay at 540 nm was chosen to represent the decay of green emissions in UCNP [16]. The decay curves were well fitted with a double exponential function:
in which the average decay lifetime τ can be calculated by the following expression:Fig. 2. (a) The HRTEM image of the UCNP + QD sample. The representative UCNP and QDs are marked by the yellow and red dotted line circles, respectively. (b)The normalized PL spectra of the UCNP and UCNP + QD sample under 980 nm excitation. (c) Energy transfer process in the UCNP + QD sample. It indicates absorption (solid black arrow), emission (solid colored arrows), energy transfers between the energy levels in UCNP (dashed black arrows) and that from UCNP to QD (purple wavy line), non-radiative relaxation (dotted arrows) processes. CB and VB represent the conduction and valence bands of QDs, respectively. (d) The PL decay for 540 nm emission in UCNPs with (blue) and without (pink) the presence of QDs. The emission of the QDs (red) was monitored at 620 nm.
The calculated lifetime τ of the UCNP sample is 267 µs, while that of the UCNP + QD sample is decreased to 260 µs due to the FRET to QDs. In addition, the PL decay curve of the QD emission at 620 nm for the UCNP + QD sample was also measured, as shown in Fig. 2(d) (red curve). The corresponding lifetime is calculated as 257 µs, much higher than that of the light-excited QD emission (< 100 ns) [22]. This also confirms the existence of the FRET from UCNPs to QDs. The FRET efficiency ηFRET can be calculated based on the value of the donor lifetimes with (τDA) and without (τD) the acceptor, expressed as:
In our experiment, τDA and τD represent the lifetime of 540 nm emission in UCNP and UCNP + QD samples, respectively. Hence, the FRET efficiency is 2.7% for the UCNP + QD sample at room temperature. The low FRET efficiency may result from the fact that the donor luminescence originates from all Er3+ ions evenly distributed in the volume of the UCNP, whereas only superficial ions take part in the energy transfer to QDs [16,23].In order to get a more efficient FRET coupling between UCNPs and QDs nanoparticles, an additional experiment for conjugating particles has been performed through electrostatic attraction method [23]. The surface of UCNPs and QDs were functionalized by amine groups (NH2) and carboxy groups (COOH), respectively and UCNPs and QDs were conjugated by electrostatic attraction, as the TEM image shown in Fig. 3(a). It can be seen that, the QDs were adsorbed around the UCNPs with a ∼1 nm gap between them. The FRET efficiency from UCNPs to QDs was measured as 10.5 ± 0.3%, which is higher than that (2.7%) measured in the UCNP + QD sample fabricated by direct mixing. To further study the distance dependence of the FRET efficiency from UCNPs to QDs, an additional Polyethylene glycol (PEG) layer with a controlled thickness has been covered the surface of the UCNPs, shorted as UCNP@PEG−NH2. As an example, Figs. 3(b)–3(d) shows the TEM images of UCNPs@PEG−NH2 with PEG layer thicknesses of 2.7, 5.6 and 8.2 nm, respectively. The corresponding fluorescence lifetime (λ = 540 nm) of UCNPs with and without QDs were measured and the FRET efficiency as a function of PEG layer thickness were calculated, as plotted in Fig. 3(e). It can be seen that, the FRET efficiency decreases with the increasing of the PEG layer thickness, indicating that the FRET efficiency decreases with the increasing of the distance between the donor and accepter.
Fig. 3. (a) TEM image of the UCNP + QD sample conjugated by electrostatic attraction. The representative UCNP and QDs are marked by the yellow dotted circle and red arrows, respectively. (b−d) TEM images for UCNPs@PEG−NH2 with PEG layer thicknesses of 2.7, 5.6 and 8.2 nm, respectively. The scale bar is 20 nm.
3.3 Temperature-dependent FRET
To investigate the effect of temperature on the FRET from UCNPs to QDs, the PL spectra of the UCNP and UCNP + QD samples at different temperatures (77–427 K) were collected and shown in Figs. 4(a) and 4(b), respectively. It can be seen that, with an increasing temperature, the peak positions of the UCL emissions have no change, while that of QDs emission presents a 34 nm red shift (from 599 to 633 nm), which is almost the same as the trend of the QD emission with temperature in the pure QD sample as shown in Fig. 4(c). It indicates that the red shift here is resulting from the intrinsic thermal quenching effect of QDs [20,22]. The PL peak intensities for the green emission in the UCNP sample and the green and QDs emission in the UCNP + QD sample at different temperatures were extracted from the measured spectra and were plotted in Fig. 4(d). It is observed that as the temperature increases, the green emission intensity of the UCNP sample presents an increase from 77 to 157 K and a decrease from 157 to 427 K, resulting from the temperature-induced population redistributions effect [24,25]. Both the green and QD emission intensities of the UCNP + QD sample show the same variation trend, which are caused by the combined effect of the thermal effect and the FRET from the UCNPs to QDs.
Fig. 4. PL spectra of UCNP (a), UCNP + QD (b) and pure QD (c) samples at different temperature. (d) The re-plotted green emission intensity of the UCNP sample (blue dots) and the green (red dots) and QDs emission (black dots) intensities of the UCNP + QD sample as functions of temperature.
The temperature-dependent FRET is promising to expand the applications of the UCNP + QD system in thermal imaging and temperature sensing. Here, the UCNP + QD sample can be used as the dual thermal probe. First, the QDs in the UCNP + QD sample can be used as nanothermometer under NIR excitation for their temperature-variation peak position, as shown in Fig. 5(a). It can be seen that, the peak position variation as a function of temperature followed a linear relationship. Thus, a variation coefficient of 0.10 nm/K can be estimated through a linear fit (red line). It is comparable to previously reported values in different QDs under a visible excitation [26,27], indicating the achieving of the nanothermometer based on QDs under NIR excitation. Besides, the intensity ratio of the 2H11/2→4I15/2 (520 nm) and 4S3/2→4I15/2 (540 nm) transition emissions is also an important parameter in the UCNPs as a referenced signal for optical sensing of temperature. Herein, for the UCNP + QD sample, the intensity ratio of I520/I540 as a function of temperature was achieved and shown in Fig. 5(b). It can be seen that, the experimental data can be well fitted by the Boltzmann equation. Further, the relative sensitivity (S) was calculated [28] and has the maximum sensitivity of 0.47% K−1 at 427 K. These results indicate that the UCNP + QD sample can be considered as dual thermal probes based on either the spectral shift of QD emission or the intensity ratio variation of the UCL. In addition, in an FRET system, as an analytical signal, the FRET-ratio, which is defined as the PL intensity ratio of acceptor to donor, is very advantageous for bio-sensing applications [11,29]. In the UCNP + QD system, the FRET-ratio is the intensity ratio of the QDs emission to UCNPs emissions (including 520, 540 and 655 nm emissions). The FRET-ratio as a function of temperature was shown in Fig. 5(c). It can be seen that, the FRET-ratio overall appears a decrease with the increasing of the temperature. The result indicates the sensitivity of the FRET-ratio to temperature, which is conducive to expanding its applications in biomedicine and temperature sensing.
Fig. 5. (a) The wavelength shift of QDs emission and (b) the intensity ratio I520/I540, (c) the FRET-ratio of the UCNP + QD sample as the function of temperature.
3.4 FRET efficiency as the function of temperature
To further quantitatively evaluate the temperature dependence of the FRET process, the FRET efficiency ηFRET as a function of temperature is further explored. To get the lifetimes, the PL decay curves of 540 nm emission for UCNP and UCNP + QD samples at different temperature were measured as shown in Figs. 6(a) and 6(b), and the calculated lifetimes was shown in Fig. 6(c). It can be seen that, the lifetime of UCNP + QD sample is smaller than that of UCNP at a certain temperature because of the FRET and both of them presents a decrease and then an increase with the increasing temperature [30,31]. According to the Eq. (3) and Fig. 6(c), ηFRET as a function of temperature was achieved and was shown in Fig. 6(d). It can be seen that, the ηFRET decreases with temperature from 77 to 317 K and then increases with temperature from 317 to 427 K. It has a minimum value of 2.1% at 317 K and a maximum value of 10.9% at 77 K.
Fig. 6. PL decay curves of 540 nm emission for UCNP (a) and UCNP + QD (b) samples at different temperature. Inset is a partial enlargement of the decay curves. (c) The lifetime at 540 nm for UCNP + QD (black) and UCNP (red) samples under different temperature. (d) The FRET efficiency from UCNPs to QDs as a function of temperature.
The variation of the FRET efficiency with temperature is mainly caused by the temperature-variation Förster distance R0 [16], which is defined as the distance between the donor and the acceptor when the ηFRET is equal to 50%. The changing of R0 with the temperature is considered to be the result of a combination of the fluorescence quantum yield of the Er3+ Q0 and the spectral overlap integral J for that they are temperature-dependent. On the one hand, Q0 will be decreased with an increasing temperature, because of the increasing non-radiative transitions [32]. On the other hand, J is related with the normalized absorption spectral of the acceptor and the spectral profile of the donor emission. To investigate the effect of the QDs absorption on the FRET efficiency at different temperature, the normalized absorption spectra of the QDs at different temperature were measured and shown in Fig. 7. It can be seen that, with the temperature increasing from 177 to 327 K, the QDs absorption peaks in the wavelength range from 550 to 625 nm present a red shift of 7.5 nm. However, the normalized absorption spectra of the QDs change slightly for the wavelength range from 500 to 550 nm (the absorbance decrease 0.025 and 0.021 at 520 and 540 nm, respectively), which is negligible for the change of J. Hence, the effect of the QDs absorption with temperature on the FRET efficiency is negligible, thus, the value of J is mainly affected by the spectral profile of UCL and will increase with the temperature for the increasing intensity ratio of I520/I540. With temperature from 77 to 317 K, the intensity ratio of I520/I540 increased slowly indicating J increased slowly [Fig. 5(b)] and thus the decreasing Q0 was domination resulting in the decreasing R0 with the temperature. So the ηFRET decreases within the temperature range. Then, with temperature from 317 to 427 K, the intensity ratio of I520/I540 increased fast and thus the increasing J was domination, therefore, the R0 appeared an increase trend and the ηFRET increases correspondingly.
4. Conclusion
In our study, the temperature-dependent FRET from β-NaYF4:Yb3+: Er3+ / NaYF4 UCNPs to ZnCdSe/ZnS QDs was demonstrated by analyzing the PL spectra and lifetime of the UCNP + QD sample variation with temperature from 77 to 427 K. The result shows that, the UCNP + QD sample has great temperature-sensing properties based on either the spectral shift of QD emission with a variation coefficient of 0.10 nm/K, or the UCL with a maximal sensitivity of 0.47% K−1 at 427 K. Besides, the decrease of the FRET-ratio with the increasing of temperature also confirmed the temperature-sensitivity of the UCNP + QD sample. Further, the lifetime analyzing shows a decrease and then an increase of the FRET efficiency with the temperature increasing, resulting in the maximum value of 10.9% at 77 K and the minimum value of 2.1% at 317 K. The results provide new ideas for improving the FRET performance from UCNPs to QDs, which is conducive to a better understanding of the FRET process and promising to expand the applications of the UCNP + QD system in biomedicine and temperature sensing.
Funding
National Natural Science Foundation of China (11274395, 11974435, 21703083); Natural Science Foundation of Guangdong Province (2018A030313498); the Open Fund of the Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications (Jinan University) (CZ156091).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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