Real-time naked-eye recognizable temperature monitoring based on Ho3+ (or Tm3+)-activated NaYF4 upconversion nanowires via visual multicolor alteration

Dongdong Lia, Wen-Yong Lai*ab, Xiaoqin Shena, Qiyue Shaoc and Wei Huangab
aKey Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts and Telecommunications, 9 Wenyuan Road, Nanjing 210023, China. E-mail: iamwylai@njupt.edu.cn
bShaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an 710072, Shaanxi, China
cJiangsu Key Laboratory of Advanced Metallic Materials, Department of Materials Science and Engineering, Southeast University, Nanjing 211189, China

Received 26th November 2018 , Accepted 3rd January 2019

First published on 4th January 2019


Non-contact thermometry for real-time temperature monitoring is a challenging research topic. Advances in microelectronics and biotechnology demand precise temperature monitoring with novel materials and approaches, where conventional thermometers are burdensome because of employing expensive additional equipment (e.g. spectrometers) and further data processing. Lanthanide-doped upconversion nanomaterials that can convert single near-infrared excitation into multicolor visible emissions open the door for a novel strategy to thermometry. Herein, a real-time naked-eye recognizable color change was achieved based on Ho3+ (or Tm3+)-activated NaYF4 upconversion nanowires, depending on the different spectral sensitivities of the blue, green and red upconversion emissions to temperature. Furthermore, the luminescence color can be also directly modulated by only using 975 nm laser radiation, which extended their application scope. These desirable properties make upconversion nanomaterials promising for temperature monitoring, anti-counterfeiting, and multicolor temperature probing applications with the advantages of being simple, convenient and unreplicable.


Non-contact temperature monitoring, sensing and thermal imaging approaches have attracted considerable attention and are considered irreplaceable for their specific scope of applications, including nanoscale integrated circuits, fluidic channels, and electronic and biological devices.1,2 Various temperature sensing strategies and methods have been developed by utilizing the physical properties of different materials such as thermal-optical response, volume expansion/contraction, and temperature-dependent dimensional changes.3,4 For example, Wang et al. have designed a nanosized thermometer using Ga-filled carbon nanotubes (diameter, 40–150 nm), in which Ga serves as a temperature indicator and can expand/contract inside the nanotube in the range of 50–500 °C.5 Zhang et al. reported a novel luminescent thermometer based on carbon dots (CDs), taking advantage of the temperature-induced aggregation of CDs and corresponding fluorescence quenching.6 However, there are inherent limitations associated with these materials with regard to harsh test conditions (e.g. microscopic imaging), high autofluorescence background and potential toxicity, making them not optimal as the optical probes to be used in specific fields (e.g. deep tissue imaging).7

Lanthanide-doped upconversion nanomaterials that can convert long-wavelength near-infrared (NIR) excitation into visible or NIR emission have proven to be excellent alternatives to overcome these limitations.8 The manifold energy-levels of lanthanide ions also enable them to offer multicolor upconversion emissions by using single NIR excitation, rendering them particularly attractive for potential multicolor optical probing applications.9 Based on these advantages, optical upconversion nanothermometers have thus been extensively investigated depending on the fluorescence intensity ratio (FIR) technique, which explores the potentials of different fluorescence intensities from two thermally coupled energy levels for accurate temperature sensing with high sensitivity and low environmental dependence.10 For instance, Li et al. built up a carbon-coated core–shell upconversion nanostructure of NaLuF4:Yb,Er@NaLuF4@Carbon, and obtained the eigen temperature of tissue phantom and solutions using the FIR technique between the upconversion luminescence (UCL) peaks centered at 525 and 545 nm.11 Capobianco et al. have shown the biological applications of the nanothermometers based on nanocrystalline NaYF4:Yb3+,Er3+ by monitoring the temperature variation of an individual cancer cell until its thermal-induced death.12 Liu and Carlos et al. studied the temperature mapping of a nanofluid by using NaYF4:Yb3+,Er3+ upconversion nanothermometers in both aqueous and organic solvents, exhibiting the advantages of high spatial resolution (<1 μm) and excellent thermal sensitivity (1.15% K−1 at 296 K).13 Despite great success being achieved in the field of upconversion-based nanothermometry, additional expensive equipment (e.g. spectrophotometer) and further data processing (e.g. integral curve fitting) are usually required in this technique, which increase the complexity and the cost of utilization. The development of a simple and convenient temperature monitoring approach is thus extremely imperative and highly desirable for real-time temperature monitoring, anti-counterfeiting, and multicolor temperature probing applications.

With continuing enthusiasm in developing upconversion nanomaterials for novel utilization,13–17 we report herein for the first time on developing a naked-eye recognizable color change responding to external temperature stimuli by using upconversion-based nanomaterials of Ho3+ (or Tm3+)-activated NaYF4 upconversion nanowires. Based on the different sensitivities of the blue, green and red upconversion emissions to temperature, a visual color change has been manifested. In addition, UCL color switching was also simply realized by commercially available laser radiation. This represents the first example of real-time naked-eye recognizable temperature monitoring via visual multicolor alteration based on upconversion nanowires.

Hexagonal-phase NaYF4:Yb3+,Ho3+,Tm3+ upconversion nanowires with different rare-earth (RE) doping proportions (Y/Yb/Ho/Tm = 79[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]0.3[thin space (1/6-em)]:[thin space (1/6-em)]0.7; Y/Yb/Ho/Tm = 79[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]0.5) were synthesized based on the reported procedure.18 In a typical synthesis process, 1.5 mmol RE nitrates were added to a 10 mL aqueous solution with 1.5 mmol citric acid to give a metal–citrate complex. After stirring for 5 min, sodium dodecyl sulfonate (1.5 mmol) was introduced and stirred for another 15 min. Then, NaF (15 mmol) was added into the mixture with stirring for 30 min, and the resulting solution was transferred to a 50 mL stainless-steel autoclave. The mixture was heated at 180 °C (or 200 °C) for 24 h. After cooling down, the products were washed three times with ethanol before re-dispersing in 5 mL ethanol.

SEM and TEM images of the as-prepared NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ upconversion nanowires are shown in Fig. 1(a and b). It can be seen that the wire-like nanomaterials have an average length of 9.1 μm and an average diameter of 640 nm. The high-resolution TEM (HRTEM) image in Fig. 1(c) reveals that the upconversion nanowires are of a single-crystalline nature. The lattice distance of 0.30 nm can correspond to the d-spacing of the (11[2 with combining macron]0) lattice plane of the hexagonal NaYF4 structure. The selected-area electron diffraction (SAED) pattern in Fig. 1(d) further confirms that the NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ nanowires are of pure hexagonal crystal structures. The energy dispersive spectroscopy (EDS) mapping images (Fig. 1e) indicate that each element (Y, Yb, Ho, Tm) has a uniform distribution throughout the whole nanowire area, offering premise for a high UCL efficiency.


image file: c8qm00608c-f1.tif
Fig. 1 (a and b) SEM and TEM images of NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ upconversion nanowires. (c) HRTEM image of a NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ upconversion nanowire. (d) SAED pattern of NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ upconversion nanowires. (e) EDS mapping (area distribution of Y, Yb, Ho and Tm elements) of a single nanowire.

Fig. 2a and Fig. S1 and S2 (ESI) show the corresponding UCL spectra of the Yb/Ho/Tm-doped NaYF4 upconversion nanowires. Under 975 nm laser excitation, green and red UCL centered at around 540 and 645 nm are observed, corresponding to the 5F4, 5S25I8 and 5F55I8 transitions of Ho3+ ions, respectively (Fig. 2c). The blue and red UCL centered at around 475, 645 and 695 nm can be attributed to the 1G43H6, 1G43F4, and 3F33H6 transitions of Tm3+ ions, respectively (Fig. 2c). As the temperature increases, the UCL intensities decrease gradually (Fig. 2a and Fig. S1, S2, ESI), which is due to the enhanced nonradiative transition processes at higher temperatures. The enhanced 3F33H6 transition (695 nm) of Tm3+ ions can be also explained by the increased nonradiative relaxation probabilities of higher energy levels. The reduction in UCL intensities indicates that the thermal quenching mechanism dominates the UCL processes at higher temperatures. It is worth noting that the blue, green and red upconversion emissions decrease (or increase) at different levels in intensity with increasing temperature (Fig. 2b and Fig. S1, S2, ESI): e.g. the green (540 nm) and red (645 nm) upconversion emission of NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ nanowires decrease by 85% and 36% as the temperature increases from 35 to 295 °C, respectively, while the red upconversion emission (695 nm) increases by 619% under the same conditions (Fig. 2b). The less UCL reduction at 645 nm and the significant UCL enhancement at 695 nm can be attributed to their increased energy level population because of the nonradiative relaxation processes from higher energy levels (blue and green). The different changing levels of the UCL intensities at different wavelengths (blue, green and red) in Fig. 2(b) would eventually lead to significant changes in UCL color with increasing temperature. This inspired us to develop a new type of upconversion-based nanomaterial that can respond to external temperature stimuli and simultaneously emit different colors of upconversion fluorescence with increasing temperature.


image file: c8qm00608c-f2.tif
Fig. 2 (a) Temperature-dependent UCL spectra of NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ upconversion nanowires. (b) The corresponding integrated UCL intensities at different wavelengths as a function of temperature. Integrated intensities at various temperatures were normalized to that at 35 °C. (c) Excitation and luminescence scheme in Yb/Ho/Tm systems: energy transfer (dashed), radiative processes (full), multiphonon relaxation (dotted).

Fig. 3(a) shows the chromaticity coordinate shifts of NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ upconversion nanowires with the increase of temperature. The chromaticity coordinate in CIE 1931 color space shifts from (0.29, 0.45) to (0.42, 0.36) as the temperature raises from 35 to 295 °C. The corresponding UCL color can be quantitatively expressed by the following equation:19

 
image file: c8qm00608c-t1.tif(1)
where u′ = 4x/(3 − 2x + 12y) and v′ = 9y/(3 − 2x + 12y) are the chromaticity coordinates in uv′ uniform color space, x and y are the chromaticity coordinates in CIE 1931 color space, o and t are the chromaticity shift at room temperature and a set temperature, respectively, and w′ = 1 − u′ − v′. According to the equation, the chromaticity shift (ΔE) at 295 °C was calculated to be 0.12, suggesting a wide color span easily allowing naked eye recognition. This prediction is confirmed by the inset of Fig. 3(a) where the UCL color changes from green to white, and finally to red, which is sufficiently easy and convenient for visual identification. Fig. 3(b) gives a quantitative interpretation of this real-time naked-eye recognizable UCL color change. The initial green UCL originates from the stronger upconversion emission at 540 nm relative to other wavelengths (475 nm, 645 nm and 695 nm). As the temperature increases, the UCL intensities at 475 nm, 540 nm and 645 nm decrease at different levels, while the UCL intensity at 695 nm increases significantly. This eventually led to an obvious change in relative UCL intensity, and therefore a noticeable UCL color alteration switching from green to white, and finally to red. Similarly, a visible color variation from blue to red (or green to red) was also achieved by adjusting the original RE doping proportion and/or the synthesis temperature of the nanowires (Fig. S3 and S4, ESI). This represents a novel thermometry technique that can act as a promising candidate for future non-contact optical temperature monitoring and sensing with a naked-eye recognizable color change within a low temperature span <100 °C, offering the advantages of being simple, convenient and unreplicable. In contrast to the commonly employed non-contact thermometry methods such as thermal imaging and FIR techniques (Fig. 3c and d), this technique is handy and low-cost without additional expensive equipment (e.g. thermal imager and spectrophotometer) and further data processing (e.g. integral curve fitting).


image file: c8qm00608c-f3.tif
Fig. 3 (a) Chromaticity coordinate shifts and the corresponding color change of NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ upconversion nanowires with increasing temperatures. (b) Integrated UCL intensities at different wavelengths (normalized to that of 540 nm) as a function of temperature. (c) Thermal images of NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ upconversion nanowires at 150 °C using an expensive infrared thermal imager. (d) Non-contact temperature sensing based on the FIR (I695/I475) technique of Tm3+ which related to an expensive spectrophotometer and further data processing.

This upconversion-based multicolor alteration that can respond to external temperature stimuli represents an excellent choice for novel non-contact temperature monitoring and sensing applications in various fields, such as nanoscale integrated circuits, computer chips, and electronic and biological devices. It is well known that a chip would yield high Joule heat in operation, which is unfavorable for its stable operation. By using this upconversion-based non-contact temperature monitoring technique, the chip temperature in Fig. 4(a) can be monitored in real time via visual UCL multicolor alteration (from green to red, Fig. 4b), which can prevent possible overheating failure of the chip. More importantly, the change of ambient temperature can be also directly reflected by the UCL color variation, and therefore such novel upconversion-based nanowires show great promise for real-time temperature monitoring and facile multicolor temperature probing applications over a wide scope of use. The recoverable UCL intensities after a heating–cooling cycle (Fig. 4c) and multiple integral UCL intensities at 35 °C and 295 °C (Fig. 4d) demonstrate high thermal stability of the upconversion nanowires, suggesting their good repeatability on applications over a broad temperature range.


image file: c8qm00608c-f4.tif
Fig. 4 (a and b) Real-time temperature monitoring of a chip in operation via visual multicolor alteration of the NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ upconversion nanowires. (c) UCL spectra before and after a heating–cooling cycle (35 °C → 295 °C → 35 °C) of upconversion nanowires. (d) Reversibility experiments of the upconversion nanowires for temperature monitoring.

In addition, this temperature-dependent multicolor alteration based on upconversion nanowires also presents very promising applications in other important fields, especially in novel security and anti-counterfeiting techniques. Considering portability needs, the efficient photothermal conversion effect of the upconversion nanowires was employed to substitute the traditional heating method for this kind of application (Fig. 5a). As shown in Fig. 5(b), the temperature of the upconversion nanowires increased remarkably from 35 to 71.4 °C under a low excitation power of 0.27 W, indicating the feasibility of the laser induced thermal approach. Fig. 5(c) shows a cash (100 RMB) that combined with NaYF4:20%Yb3+,0.5%Ho3+,0.5%Tm3+ nanowires. A visible color change from green to white can be clearly observed by using 975 nm laser radiation. Similarly, a commercial label that combined with NaYF4:20%Yb3+,0.3%Ho3+,0.7%Tm3+ nanowires also presents an obvious color change from blue to red under 975 nm laser radiation (Fig. 5d). This real-time naked-eye recognizable color change by only using 975 nm laser radiation allows the upconversion nanowires to offer more secure anti-counterfeiting applications with the rather attractive advantages of being simple, convenient and unreplicable.


image file: c8qm00608c-f5.tif
Fig. 5 (a) A visual color alteration can be achieved through the photothermal conversion process of Ho3+ (or Tm3+)-activated NaYF4 upconversion nanowires. (b) Temperature increases of Ho3+ (or Tm3+)-activated NaYF4 upconversion nanowires as a function of excitation time. Real-time naked-eye recognizable color change of (c) a cash and (d) a commercial label that combined with Ho3+ (or Tm3+)-activated NaYF4 upconversion nanowires for anti-counterfeiting applications.

In summary, Ho3+ (or Tm3+)-activated NaYF4 upconversion nanowires have been synthesized and the effects of temperature on their UCL properties were investigated in detail. The temperature-dependent UCL spectra revealed that the blue, green and red upconversion emissions change in different levels with increasing temperatures, leading to a real-time naked-eye recognizable color change. In addition, the UCL color alteration was also manifested by using 975 nm laser radiation, depending on the intrinsic photothermal conversion effect of upconversion nanowires. This study opens a promising avenue for naked-eye recognizable real-time temperature monitoring, anti-counterfeiting, and multicolor temperature probing applications by exploring the great potentials of upconversion nanowires to induce visual multicolor alteration based on the temperature variations.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge financial support from the National Key Basic Research Program of China (973 Program, 2014CB648300, 2017YFB0404501), the National Natural Science Foundation of China (61704085, 21835003, 21422402, and 21674050), the Natural Science Foundation of Jiangsu Province (BK20160073, BK20140060, BK20140865, and BM2012010), Program for Jiangsu Specially-Appointed Professors (RK030STP15001), the Leading Talent of Technological Innovation of National Ten-Thousands Talents Program of China, the Excellent Scientific and Technological Innovative Teams of Jiangsu Higher Education Institutions (TJ217038), the Synergetic Innovation Center for Organic Electronics and Information Displays, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the NUPT “1311 Project” and Scientific Foundation (NY217169), the 333 Project of Jiangsu Province (BRA2017402), and the Natural Science Foundation of Universities from Jiangsu Province (17KJD510004). This work was also sponsored by NUPTSF (NY216025 and NY217073).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qm00608c

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