Temperature-Sensitive Chameleon Luminescent Films Based on PMMA Doped with Europium(III) and Terbium(III) Anisometric Complexes

Abstract

The spin-coating technique was used to produce composite films consisting of PMMA polymer doped with anisometric complexes of Eu(III) and Tb(III). It was found that an increase in the content of Tb³⁺ complexes intensifies emission of both ions due to the intermolecular energy transfer from the Tb(III) complex to the Eu(III) complex, which results in the increase in the relative luminescence quantum yield of Eu(III) ion by 36%. The temperature sensitivity of the film luminescence intensity and lifetime in the range of 296–363 K was investigated. The maximum relative sensitivity of the films reaches 5.44% × K⁻¹ and exceeds that of all known lanthanide-containing thermal sensors designed for measuring physiological temperatures. In combination with changing luminescence color, such a sensitivity makes these films promising colorimetric thermal sensors for in situ temperature measurements.

1. Introduction

Luminescent thermometry is an emerging technology that can be used in microelectronics, photonics, microfluidics, medicine and biology [1,2,3,4,5]. Compared to other electronic thermal sensor techniques, the advantages of luminescent thermometry are its ability to monitor surface temperatures with micro- and nanometer-scale spatial resolution (<10 μm), high sensitivity (>1% × K⁻¹), fast response (<1 ms) and measurement accuracy, small size of sensitive elements and resistance to electromagnetic interference [2,6]. The working principle of luminescent thermometers is monitoring the effect of temperature on the luminescence parameters: quenching time, intensity, emission peak area or position and the ratio of intensities of emission bands [7]. Several types of luminophores are already used as temperature sensors, including organic compounds, quantum dots, metal clusters, dye-doped nanoparticles, metal-ligand complexes and lanthanide-containing materials [8,9]. Narrow emission bands of lanthanides make it possible to measure temperature changes with high accuracy. In addition to these properties, lanthanides have a long luminescence lifetime and allow for using temporal methods for increasing signal-to-noise ratios [10]. Materials with two different lanthanide ions are attractive components of ratiometric thermometers [11,12]. Such materials are characterized by high accuracy, photostability and temporal resolution of signals. The accuracy of temperature measurements by an absolute emission intensity is sensitive to external factors such as an excitation power and errors in opto-electronic systems. On the other hand, the luminescence intensity ratio of two luminophores is almost constant, so no additional calibration of a thermal probe is needed for such a double system [13,14].

Among lanthanide coordination compounds, β-diketonate complexes are the most promising materials due to their luminescence with high quantum yield (up to 85% [15,16]), large Stokes shifts, decay time of luminescence in the range of tens to hundreds of microseconds and high temperature sensitivity of luminescence intensity (>1% × K⁻¹) [17]. They also possess a set of attractive chemical properties: a relatively easy synthesis, a good solubility in a wide range of common solvents and an ability to be incorporated into various matrices [18,19,20].

In recent years, the focus of studies in this area is shifting more and more from synthesis of coordination lanthanide compounds to their inclusion into various types of matrices (inorganic, organic or organic-inorganic). A matrix is noticed to enhance not only thermal and chemical stability of complexes and their mechanical strength [21,22] but luminescence efficiency as well [23,24]. Two principal approaches are considered in this aspect: encapsulation of a lanthanide complex into polymer [25,26,27] and synthesis of lanthanide-containing polymers [28,29]. Polymers are ideal candidates for matrices as they possess attractive technological properties such as flexibility, processability, mechanical strength and low cost.

However, to control the distribution of temperature over a surface, we need film materials with intensive luminescence that are transparent in the visible range. The majority of currently known lanthanide(III) β-diketonate complexes have a crystalline structure, so it is difficult to use them for producing homogeneous films. It is necessary to use hard UV radiation for their excitation [30], which makes the resulting thermal sensors expensive and reduces their photostability.

In this work, we examined the possibility of making thermosensitive films using PMMA polymer and anisometric Tb(III) and Eu(III) complexes. The presence of long hydrocarbon substituents at the ends of molecules makes such compounds amorphous, which allows them to form transparent media with no crystalline defects. Good solubility of such complexes in organic solvents and miscibility with polymers [31,32] ensure their uniform dispersal in the film material. Additionally, they absorb light effectively in a wide range from 250 to 400 nm, which eliminates the need to use hard UV radiation for their excitation [33,34,35]. In addition to the advantages listed above, the use of multi-ion lanthanide complexes should enhance an intermolecular energy transfer, according to known theoretical prerequisites and experimental data, thus increasing the efficiency of a thermal sensor [36,37].

2. Results and Discussion

The original 1,10-phenanthroline and β-diketone were used as the ligands for the synthesis of the complexes. The triplet levels of these compounds provide an effective energy transfer to the emitting levels of Eu³⁺ and Tb³⁺ ions (Figure 1). Their composition and structure were confirmed by mass spectrometry and elemental analysis.

These complexes are amorphous powders that are soluble in organic non-polar and weakly polar solvents. They are mutually miscible as well as miscible with PMMA polymer [31,38].

A spin-coating process was used to produce composite films of PMMA polymer doped with 3 w.% of the Eu(III) compound and 1–20% w.% of the Tb(III) compound (3%EuX%Tb) [39]. The films are 300 nm thick (±10%). The films are almost transparent in the infrared and visible ranges (transmittance over 90%), whereas they absorb light strongly in the UV range (Figure S1).

UV irradiation of the PMMA films doped with the individual Tb(III) and Eu(III) ions results in luminescence that is typical for the respective Ln(III) ions (Figure 2).

An increase in the content of the Tb(III) complexes in the 3%EuX%Tb films intensifies emission of both the Tb³⁺ and Eu³⁺ ions due to occurring intermolecular energy transfer (Figure 3). Because of large distances between ions, the most probable energy transfer mechanism is a dipole–dipole interaction (Förster) rather than an exchange energy transfer (Dexter).

It was demonstrated that the emission intensity at λ_em = 613 nm of the 3%Eu15%Tb film increases by 2 times at λ_ex = 270 nm and by 1.4 times at λ_ex = 330 nm, respectively, as compared with the film of the individual Eu(III) compound in PMMA (3%Eu).

The overall luminescence quantum yield of the complex upon excitation of the chromophore is determined by the efficiency of the sensitization (η_sens) and by the emission quantum efficiency (φ^Ln) of the lanthanide luminescence (Equation (1)).

$$\phi ={\eta }_{sens}·{\phi }^{Ln}$$

(1)

Due to the energy transfer from the Tb(III) complex, the relative quantum yield of the Eu(III) ion luminescence increases by 36% at λ_ex = 270 nm and 26% at λ_ex = 330 nm (Equations (2)–(5)) (Tables S1 and S2). A stronger luminescence of the Eu(III) complex is mostly related to the energy transfer from the ligands (η_sens), whereas the contribution of the quantum efficiency of the Eu³⁺ ion luminescence itself (φ^Ln) does not show considerable growth. The Tb(III) complex represents an additional “antenna” that provides a direct energy transfer from the Tb(III) coordination compound to the Eu³⁺ ion. Concentration changes of the Tb(III) complex also lead to changes in the emission color and provide an opportunity to control the color of the films (Figure 3b).

The 3%Eu5%Tb sample was selected to characterize luminescence of the composites at different temperatures. We chose this ratio of luminophores because it provides the optimal color visualization of temperature and almost zero concentration quenching. A relatively low content of luminophores ensures low cost of a resulting thermal sensor. It is known from the literature that luminescence quenching of Tb³⁺ ions is more strongly dependent on temperature than that of Eu³⁺ ions [3,40,41]. A simultaneous presence of Eu(III) and Tb(III) complexes in the composite films allows for a more accurate determination of temperature by evaluating the ratio of their luminescence peaks, which is not sensitive to measurement conditions.

At room temperature, the luminescence spectra of the PMMA films doped with both complexes contain the transition bands of the Eu³⁺ and Tb³⁺ ions (Figure 4). Due to the energy transfer from the Tb(III) complex to the Eu(III) complex, the excitation spectrum of the 3%Eu5%Tb film contains a more intensive excitation peak at 270 nm (Figure 4) as compared with the spectrum of the PMMA film doped with the individual Eu(III) complex. According to the CIE diagram, changing the wavelength does not lead to a considerably different perception of color (Figure 4b).

Figure 5a illustrates the influence of temperature on the luminescence intensity of the analyzed film in the range of 296–363 K at λ_ex = 330 nm and λ_em = 545 and 613 nm (Figure S2 at λ_ex = 270 nm and λ_em = 545 and 613 nm). Decreasing dependences of emissions from the ⁵D₀ → ⁷F₂ (Eu(III), 613 nm) and ⁵D₄ → ⁷F₅ (Tb(III), 545 nm) transitions are adequately described by the exponential functions with the correlation coefficients R² > 0.98.

The maximum value of the relative sensitivity of intensity (S_I) varies from 2.6% × K⁻¹ for the ⁵D₀ → ⁷F₂ transition (Eu(III), 613 nm) to 7.8% × K⁻¹ for the ⁵D₄ → ⁷F₅ transition (Tb(III), 545 nm) (Equation (7)).

Since luminescence of both the Tb³⁺ and Eu³⁺ ions has a specific temperature behavior, it is possible to measure temperature by the ratiometric method from the intensity ratio of the ⁵D₄ → ⁷F₅ (Tb(III), 545 nm) and ⁵D₀ → ⁷F₂ (Eu(III), 613 nm) transitions. The ratio of the transition intensities is almost linear (R² > 0.96) (Figure 5b and Figure S2). Therefore, the ratio of the peaks can be used to determine the surface temperature with high accuracy. The value of the maximum relative sensitivity (S_m) reaches 5.44% × K⁻¹ and exceeds that of all known lanthanide-containing thermal sensors designed for measuring physiological temperatures (

Table 1. Comparative temperature relative sensitivity of various Ln³⁺-based luminescent thermometers.

#	Material	Range, K	Tm, K	Sm, % × K−1	Optical Parameter	Reference
1	3%Eu5%Tb	296–363	363	5.44	Two intensities	This paper
2	Eu0.01Tb0.99(hfa)3(dpbp)	100–450	200	0.83	Two intensities	[42]
3	Eu0.20Tb0.80(bpda)	303–328	328	1.39	Two intensities	[43]
4	Eu0.01Tb0.99(bdc)1.5·(H2O)2	290–320	318	0.31	Two intensities	[44]
5	{Eu0.7Tb0.3(d-cam)(Himdc)2·(H2O)2}3	100–450	450	0.11	Two intensities	[45]
6	Eu0.005Tb0.995@In(OH)(bpydc)	283–333	333	4.47	Two intensities	[46]
7	Eu0.005Tb0.995@Al(OH)(bpydc)	283–333	333	3.00	Two intensities	[46]
8	Eu2(qptca)(NO3)2(DMF)4]·[CH3CH2OH]3 perylene	293–353	293	1.28	Two intensities	[47]
9	Ln-DPA (Ln = Eu, Tb)	293–333	293	1.5	Two intensities	[48]
10	NaGdF4:Yb3+/Ho3+/Ce3+@NaYF4 Yb3+/Tm3+	298–393	298	4.4	Two intensities	[49]
11	NaYF4:Yb3+/Er3+@SiO2	300–900	300	1.0	Two intensities	[50]
12	Gd2O3:Yb3+/Er3+	301–350	301	1.5	Two intensities	[51]
13	Eu3+/RhB-based polymer	300–310	302	3.8	Two intensities	[52]
14	NaGd(MoO4)2:Tb3+/Pr3+	303–483	303	5.3	Two intensities	[53]
15	MPr(PO3)4 (M = Na, Li, K)	298–363	363	0.60	Two intensities	[54]

) [1,17].

The films demonstrate luminescence color changes in the temperature range of 296–363 K. Therefore, they are promising colorimetric thermometers for in situ temperature measurements (Figure 6 and Figure S3). The coordinates at the CIE diagrams were calculated using the respective emission spectra from 296 K to 363 K. The luminescence color of the film changes from yellow (X = 0.411, Y = 0.514) at 296 K to red (X = 0.656, Y = 0.325) at 363 K (Figure 7 and Figure S3).

The luminescence intensity of thermally sensitive films is well-known to depend largely on the characteristics of a sample and measurement conditions [1,2]. Moreover, it is rather difficult to consider the factor of film degradation under UV radiation to avoid a significant error in temperature measurements [55,56,57]. Thermal sensors can also experience changes in their emission intensity when the refractive index of the medium changes (for instance, when water or other liquid is absorbed by the sensor’s surface) or when chemical or biological substances are present. The luminescence quenching time, however, does not depend on the factors mentioned above [1,2]. Therefore, this parameter is often used for more reliable and accurate temperature measurements. Figure 7a illustrates the temperature dependence of the luminescence lifetime demonstrated by the composite film samples.

In the range of 303–363 K, the luminescence lifetime of the samples decreases almost linearly as temperature increases. The slope of the temperature line between 328 K and 363 K is 2.1–2.8 μs/K (Figure 7b). The maximum relative lifetime sensitivity (Equation (7)) reaches 3.3% × K⁻¹. The curves representing the absolute sensitivity of the luminescence lifetime (Equation (6)) have similar profiles and relatively close values of 2.3–3.1 μs × K⁻¹ (Figure 7).

In turn, the ratio of the luminescence lifetimes found for the ⁵D₄ → ⁷F₅ (Tb(III), 545 nm) and ⁵D₀ → ⁷F₂ (Eu(III), 613 nm) transitions is adequately described by a linear function (R²>0.987) in the range of 328-363 K (Figure 7 and Figure S4). Thus, the ratio of the lifetimes of the ⁵D₄ → ⁷F₅ (Tb(III), 545 nm) and ⁵D₀ → ⁷F₂ (Eu(III), 613 nm) transitions allows for a high accuracy determination of a surface temperature by the ratiometric method. The slope of the temperature line representing the ratios in the range of 328–363 K is 0.0049 K⁻¹. The maximum relative lifetime sensitivity reaches 2.67% × K⁻¹. The curves representing the absolute sensitivity of the luminescence lifetime ratio have similar profiles and relatively close values of 0.0062 K⁻¹.

Stability and reversibility of thermosensor readings during heating are critical for its smooth operation. Therefore, the reversibility of intensity readings during cyclic temperature changes was examined. The luminescence intensity of the film is shown to change reversibly in the temperature range of 296–363 K (Figure 8 and Figure S5).

The samples withstand five cycles of heating and cooling under irradiation at 330 nm and demonstrate a relatively small reduction in intensity (less than 2%).

Thus, these composite materials are promising components of luminescent thermometers capable of measuring temperatures in the range of 296–363 K, including the temperature of the human body.

3. Materials and Methods

3.1. Materials

EuCl₃*6H₂O (99.9%), TbCl₃*6H₂O (99.9%), 1,10-Phenanthroline (99%), poly(methyl methacrylate) (PMMA) (M_w~120,000 by GPC) and solvents were purchased from Sigma-Aldrich.

3.2. Characterization Techniques

CHN analysis was performed with an isotope mass spectrometer Delta V Plus Thermo Fisher Scientific. X-ray Fluorescence analysis was performed with a M4 «Tornado» Bruker spectrometer. Absorption and transmission spectra were measured with a UV/Vis spectrophotometer Lambda-35 Perkin–Elmer. Luminescence properties were measured with a Cary Eclipse Varian spectrofluorimeter.

3.3. Synthesis of Complexes

The β-diketone (1-[4-(4-propylcyclohexyl)phenyl]-octane-1,3-dione) was prepared according to the method described in [39,58,59,60].

General procedure used for synthesis of the Ln(III) complexes: EtOH solution (2 mL) of LnCl₃ × 6H₂O (Ln = Tb or Eu) (0.04 mmol) was added dropwise to a stirred hot EtOH solution (10 mL) containing 1,10-Phenanthroline (0.04 mmol), β-diketone (0.12 mmol), and KOH (0.125 mmol). The resulting yellow precipitate was isolated by hot filtration, washed with hot EtOH (25 mL), and dried under vacuum. Further, the product was dissolved in toluene, filtered and the solvent was evaporated. The structural characterization results are shown in Table S3.

3.4. Preparation of PMMA–Ln(III) Films

Spin-coating of films was performed in a spin-coater (WS-650 MZ-23NPP Laurel) by deposition of 0.3 mL solution in toluene on a quartz plate [61].

3.5. Calculation of the Quantum Efficiency and Quantum Yield

To calculate the quantum efficiency of the films doped with the Eu(III) complexes, the following formula was used for Equation (2):

$${\Phi }_{Ln}^{Ln}=\frac{A_{rad}}{A_{rad}+A_{nrad}}=\frac{A_{rad}}{A_{tot}}$$

(2)

where A_rad and A_nrad are the radiative and non-radiative rates, respectively.

The contributions to the A_nrad include the reverse energy transfer to the ligand [62,63,64], electron transfer quenching (mostly for Eu³⁺) [64,65], and, most importantly, quenching by matrix oscillations. Luminescence of lanthanides is effectively quenched by O–H oscillations [66,67,68,69]. In addition, other oscillations of organic molecules can also contribute to A_nrad [69]. Non-radiative processes influence the observed luminescence lifetime (τ_obs = (A_rad + A_nrad)⁻¹). In turn, the lifetime of radiative processes can be defined as (τ_rad = A_rad⁻¹).

Thus, we can calculate Φ^Ln according to the value of the observed luminescence lifetime τ_obs (Equation (3)).

$${\Phi }_{Ln}^{Ln}=\frac{{\tau }_{obs}}{{\tau }_{Rad}}$$

(3)

In turn, τ_rad can be calculated by assuming that the energy of the ⁵D₀ → ⁷F₁ transition and its dipole strength are constant. The resulting Equation (4) connects the form of the Eu³⁺ emission spectrum and its radiation lifetime:

$$\begin{array}{l}\frac{1}{{\tau }_{Rad}}=A_{MD,0}\cdot n^3\cdot \left(\frac{I_{tot}}{I_{MD}}\right)\\ A_{MD,0}=14.65{\mathrm{s}}^{-1}\end{array}$$

(4)

where n is the refraction index of the medium (for a first approximation, we can use the PMMA index value)), A_MD,0 is the probability of a spontaneous emission for the ⁵D₀ → ⁷F₁ transition in vacuum and I_tot/I_MD is the ratio of the total area of the Eu³⁺ emission to the area of the ⁵D₀ → ⁷F₁ band. For the theoretical value of the dipole strength, it was found that A_MD,0 = 14.65 s⁻¹.

The quantum yields of the PMMA–Ln(III) complexes hybrid films were calculated with Equation (5) [70]:

$$\phi =\frac{S_u·A_{st}}{A_u·S_{st}}·\frac{n_u^2}{n_{st}^2}$$

(5)

The subscripts st and u indicate the standard and unknown sample, A—corresponds to the absorbance of the films at the excitation wavelength λ and S is the integrated luminescence spectrum. The standard fluorophore for measurements was Eu(tta)₃phen with φ_std = 53% [70].

3.6. Calculation of the Thermal Sensitivity

The absolute thermal sensitivity (S_a) is calculated as (Equation (6)) [71]:

$$S_a=\frac{\partial \Delta }{\partial T}$$

(6)

where Δ is luminescence lifetime or intensity.

The relative thermal sensitivity (S_r) is calculated as (Equation (7)) [71]:

$$S_r=\frac{\left|\partial \Delta \right|}{\partial T\times \Delta }\times 100\%$$

(7)

4. Conclusions

A spin-coating method was used to produce composite films of PMMA polymer doped with 3 w.% of the Eu(III) compound and 1–20 w.% of the Tb(III) compound. Addition of the Tb³⁺ complex increases the emission intensity of both ions due to the intermolecular energy transfer from the Tb(III) compound to the Eu(III) compound, which leads to the 36% growth of the relative luminescence quantum yield of the Eu(III) ion. We investigated the temperature sensitivity of the luminescence intensity and lifetime of the 3%Eu5%Tb films in the 296–363 K temperature range. The maximum relative sensitivity of intensity (S_I) varies from 2.6% × K⁻¹ for the ⁵D₀ → ⁷F₂ transition (613 nm) to 7.8% × K⁻¹ for the ⁵D₄ → ⁷F₅ transition (545 nm). The maximum relative sensitivity of these films reaches 5.44% × K⁻¹ and exceeds that of all known lanthanide-containing thermal sensors designed for measuring physiological temperatures. Considering their luminescence color changes, the films demonstrate a significant potential as colorimetric thermal sensors for in situ temperature measurements.