Luminescent Materials with Turn-on and Ratiometric Sensory Response Based on Coordination Compounds of Lanthanides

Abstract

Luminescent lanthanide complexes serve as a unique set of tools for creating sensory materials. The most significant types of sensory response in such materials are the turn-on/off response, when the analyte causes an increase or decrease in the emission intensity, respectively, as well as the ratiometric response, which manifests itself as a change in the ratio of luminescence intensities at different wavelengths. In this paper, we consider two of the most technologically advanced types of luminescent sensor materials based on lanthanide compounds—“turn on” and ratiometric sensors. The production of such materials is not only of importance per their possible application but is especially interesting from a fundamental point of view, since their design requires the implementation of non-trivial solutions.

1. Introduction

The lanthanide (Ln) family, due to the features of the electron shell, forms a large number of complexes with a set of luminescent properties that is unique among all non-radioactive elements of the Periodic Table. The lanthanide compounds combine effective luminescence in the UV, visible, or near-IR ranges with narrow spectral lines and constant wavelengths. In addition, in the case of lanthanides, it is especially convenient to drive and control luminescence, which opens the way to the creation of new emerging technologies in a chemical sensorics. In the accompanying paper, we have described the principle of luminescence materials with turn-off sensory response, and here we complete our work by proponing both “turn on” and “ratiometric” sensors. This manuscript also concentrates on the role of elements of the Lanthanide Series in this emerging technology.

2. “Turn on” sensors

There are few publications on “turn on” materials with respect to “turn off” sensors, due to several factors: the rarity of processes accompanied by an increase in luminescence requires a rational design of materials, and at the same time, impurities in analytes can cause quenching processes that level the effect of luminescence enhancement. However, examples of materials with such a response have been growing in recent years, and the described mechanisms can serve as an inspiration for new researchers.

Like quenching, luminescence enhancement can be described by the Stern-Volmer equations, but the definition of the constants is given only in a small number of papers.

The most common “turn on” response mechanisms are:

1. The analyte acts as an antenna, its coordination leads to an increase in the luminescence of lanthanide (Direct Analyte Antenna Function, DAAF).

2. An analyte (anionic or neutral) does not directly act as an antenna, but displaces quencher molecules (usually H₂O) from the lanthanide coordination sphere (solvent-quencher displacement, SQD).

3. The analyte, without destroying the molecular structure of the complex, forms or destroys weak bonds (usually hydrogen), reducing the effectiveness of vibrational quenching control (VQC).

4. The analyte leads to partial destruction of the material, removing the quenching fragment not directly bound to the Ln³⁺ ion (quenching fragment removal, QFR).

5. The analyte suppresses luminescence quenching by PET, FRET, or DEE mechanisms (energy transfer quenching control, ETQC).

6. The analyte affects the electronic structure of the sensor by changing the position of the triplet level (triplet level control, TLC), the effectiveness of sensitization (sensitization control, SC), changing the absorption and excitation spectra (excitation wavelength control, EWC).

An analysis of the literature shows that the SQD strategy (Figure 1) is the most often implemented. In this case, the organic solvent molecules or other ligands that do not have their own absorption in the same excitation region of the complex displace water molecules from the coordination sphere of lanthanide. Quenching of Ln³⁺-centered luminescence through interactions with OH, CH, and NH bonds is caused by the dissipation of the energy of the Ln³⁺ excited state into high-energy stretching vibrations of several neighboring molecules [1, 2]. The efficiency of vibrational quenching on these bonds depends on the energy of the excited state Ln³⁺ and the number of vibrational modes of the X-H bond that cover this energy (Figure 2).

Figure 1.

Substitution of the solvent quencher by the analyte (SQD: solvent-quencher displacement).

Figure 2.

Nonradiative deactivation of excited states of terbium and europium by multiphonon relaxation on O-H and O-D bonds.

For europium ions the required number of OH bonds vibration modes (4) is less than for terbium ions (5), which determines a much more efficient quenching of the luminescence of Eu³⁺. In the transition to O-D bonds, the number of modes for both ions increases (6 and 5, respectively). As a result, the observed lifetime of Eu³⁺_(aq) is shorter than that of Tb³⁺_(aq), and when the medium is replaced by D₂O, the difference is noticeably leveled (see Part 1, Table 1). Even greater is the effect of vibrations of these bonds on the emission of IR-emitting ions. The difference in τ_obs of various REE ions in protic and deuterated water allows to estimate the number of water molecules in the near coordination sphere of lanthanide according to empirical formulas $q_{\mathit{Ln}}=k\times \left(\frac{1}{{\tau }_{H_{2}O}}-\frac{1}{{\tau }_{D_{2}O}}-b\right)$, where q_Ln is the number of water molecules in the coordination sphere of lanthanide, τ_H2O and τ_D2O are the observed lifetimes of excited REE states in water and D₂O. k and b parameters are given in Table 1. This makes it possible to evaluate the difference in sensitivity for materials based on ions with lower slope values (Nd³⁺, Eu³⁺, and Yb³⁺) and larger ones (Tb³⁺, Sm³⁺, and Dy³⁺). Most of the papers are devoted specifically to europium derivatives, which is not accidental. Neodymium compounds have been studied as sensor materials with less SQD response, but appear to be very promising [7].

Ln^3+	Slope (k)	Intercept (b)	Ref.
Eu	1.11	0.31	[3, 4]
Eu	1.05	0.44	[5]
Tb	5.0	0.06	[3]
Tb	4.03	0.87	[5]
Sm	2.54	0.37	[5]
Dy	2.11	0.60	[5]
Nd	0.358	1.97	[6]
Yb	1.0	0.20	[3]

Table 1.

Slope (k) and intercept (b) in the Ln³⁺ hydration number formulae.

An interesting example of this type of sensor is presented in [8], which shows the selectivity of the response with respect to methanol against the background of ethanol and propanol-1. The effect is due to the lock-and-key matching of the channel diameters in the MOF sensor structure with the sizes of the indicated alcohol molecules.

The QFR mechanism (Figure 3) has been often implemented using not-luminescent copper-lanthanide heterometallic complexes capable of “donating” copper ions under the action of various analytes, especially those containing sulfur [9, 10, 11, 12] and nitrogen [12, 13, 14] donor atoms with a high affinity for copper ions. This approach makes it possible to achieve greater selectivity with respect to background ions not strongly interacting with Cu²⁺ ions. In other cases, an analyte with strong oxidizing (ClO⁻, [15]) or reductive (ascorbic acid, [16]) power removes the quencher fragment. Other analytes having the same redox properties are expected to show a similar effect.

Figure 3.

The quencher fragment removing mechanism (QFR).

The interaction of some analytes with the sensor material leads to a change in the electronic structure, which increases the efficiency of sensitization of lanthanide ions. The detailed mechanism of such enhancement is difficult to determine, but a suppression of the non-radiative relaxation of the singlet and triplet states of the ligands [17], an increase in the energy transfer constant from the ligand to the metal, and a change in the position of the triplet level [18], which affects the efficiency of the reverse transfer, can also make a contribution. These sensor materials showed a response with respect to s- and p-metal cations, as they generally contain a crown-containing fragment [17] or suitable coordination sites determining the material selectivity. The response to gases has been studied much less frequently than the response to analytes in solution; a paper [18] describing the turn-on sensor for NO₂ is of particular interest, two luminescent Eu and Tb complexes being investigated: the Tb complex shows that a reversible sorption of NO₂ leads to a “turn on” response, while for europium, a “turn off” response is observed. The interaction between the sensor with the analyte leads to an increase in the energy of the triplet level by ∼260 cm⁻¹, which is favorable for energy transfer to a higher resonance level of terbium, but reduces the efficiency for the low-lying level of europium (Figure 4).

Figure 4.

Jabłoński diagram featuring the ground singlet (S⁰), first excited singlet (S¹), and lowest triplet (T¹) states of the ligand together with the relevant atomic levels of Gd³⁺,Tb³⁺, and Eu³⁺. Values for the energy levels are given in cm⁻¹. Represented with a permission from Ref. [18].

A sensor material with a positive luminescent response to Cu²⁺ ions has been recently described [19]. Upon addition of up to 4 equivalents of 3d-metal salts (especially Cu²⁺) an intense band appears in the excitation spectrum associated with intraligand energy transfer. The appearance of this band makes the luminescence excitation more efficient when using the corresponding wavelength, which leads to a more than twofold increase in the quantum yield of Eu³⁺. This work is a unique example of a turn-on sensor for a d-metal cation.

Finally, the direct antenna function of the analyte (Figure 5) can take place if the analyte contains a suitable conjugate system [20, 21]. A not-high selectivity has been showed by such sensors, but the use of a well-defined excitation wavelength, coinciding with the absorption of the analyte, can increase it (Table 2).

Figure 5.

Direct antenna analyte function mechanism (DAAF).

Material	Analyte	Media	Linearity range	LOD	Response time	Mechanism	Ref.
Cu-Eu or Cu-Tb heterometallic MOF	S^2−	H2O; HeLa cells		0.01 μM	8–12 min	QFR (Cu^2+)	[9]
Cu-Tb heterometallic MOF	S^2−	H2O		0.13 μM		QFR (Cu^2+)	[10]
Hybrid material Eu3+ complex-covered laponite	Glutathione	H2O	0.5–30 μM	0.162 μM	10 min	QFR (Cu^2+)	[11]
Hybrid material: Tb^3+ covered Cu^2+-BTC MOF	amyloid β-peptide	H2O	1–550 nM; 5–490 nM	0.3 nM		QFR (Cu^2+)	[13]
Composite material: carbon quantum dots covered with a H2DPA. Eu^3+ and Cu^2+	H2DPA	H2O	0–10 μM	0.137 μM		QFR (Cu^2+)	[12]
Tb@duoble hydroxid Ni-Fe	H2DPA	H2O	0–12 μM	0.36 μM		SQD+DAAF	[22]
Eu@ duoble hydroxid Ni-Fe	H2DPA	H2O	0–12 μM	1.03 μM		SQD+DAAF
(Cu^2+@Tb-MOFs)	Uric acid	H2O	0–104 μM	0.650μM		QFR (Cu^2+)	[14]
Tb-MOF	DMSO	H2O	1.0–100%	1. 1.68%	90s	SQD	[23]
Eu-MOF	MeOH	CH2Cl2		<0.03 vol.%		SQD	[8]
Eu-complex nanoparticles	Cipro-floxacin	H2O	1–40 μM	780 nM	15 min	SQD	[24]
Eu-MOF	Diethyl phosphorochloridate	CH3CN	15–90 μM		<2 min	SQD	[25]
Eu-MOF	AMF	H2O		2 μM		SQD	[26]
nanoparticles [Tb(Cit)(H2O)]	Guanosine-5-mono-phosphate	H2O	0.15–20 μM	0.1 μM		SQD	[27]
Tb(HCOO)3	l-kynurenine	H2O + 5%DMF	1–10 μM	1 nM		SQD	[28]
Eu-MOF	NH3CH3NH2	Air	(1) 5–350 ppm; (2) 0.7–15.5ppm		500–600 s	SQD	[29]
Eu-MOF	NH3	Air		>28ppm	100–250 s	SQD	[30]
Nd-MOF	F-	CH3CN				VQC	[7]
Eu(L)-(UIO-67)	ClO^−	H2O	0.1–5 μM	16nM	5 s	QFR	[15]
Composite material: Eu^3+ complex in a PMMA membrane reacted with a KMnO4	Ascorbic acid	H2O	0–35 μM	48 nM	1 min	QFR	[16]
Eu-MOF	Zn^2+	MeOH	2–8 mM			VQC	[31]
Eu-MOF	Cd^2+	H2O			2 h	VQC	[32]
EuL; TbL; EuL^’; TbL^’; ML = crown-containing ligands	K^+	MeOH	0–10eq; 0–10eq; 0–2eq; 0–1.7eq			ETQC	[17]
Tb-Zn heterometallic MOF	Mg^2+	DMF	10^−5–3·10^−3 mM	1.38·10^−5 M; KSV = 1.81·10^3		n./a.; probably SC	[33]
Eu-MOF	Cd^2+	H2O	0.2–2.2 μM	1.1761×10^7		n./a.; probably SC	[34]
Hybrid material: Eu^3+ covered Zr-MOF Uio-66(Zr)–(BTEC-COOH)2	Cd^2+	H2O	0^−500 μM	0.06 μM	>2 min	SC	[35]
Tb-MOF	Pb^2+	EtOH	10^−8–10^−3 M			SC	[36]
Eu-MOF	Pb^2+	H2O	0–0.1 mM KSV=2970 M^−1	8.22 μM		n/a	[37]
Tb-MOF	NO2		1–5 ppm	2 ppm	1 min	TLC	[18]
Eu-MOF	Cu^2+	CH3CN				EWC	[19]
[Tb-MOF	Tryptophan	H2O pH=4	2.5 10^−5–2.5 10^−4	1 μM	2 h	DAAF	[20]
Nanoparticles of Na[Gd0.88Tb0.12F4]	Dopamine	H2O	0–10 μM	47 nM	5 min	DAAF	[21]
[Eu-MOF	Al^3+	DMF				n/a	[38]
Tb^3+-functionalized COF	Ochratoxin A	H2O	0–10 μM	13.5 nM	10 s	DAAF+SQD	[39]
Tb^3+ MOF	Acetyl-acetone	H2O	0–1750 ppm	0.129 ppm		DAAF	[40]
	Aspartic acid	H2O	0–25 μM	0.025 ppm		DAAF	[40]

Table 2.

Turn-on sensors.

To determine the correct mechanism of the observed “turn on” response, we propose the following algorithm, which is also relevant for the rational design of such materials:

1. A chemical analysis of the “analyte + sensory material” system to answer the question “whether a new chemical compound is formed” during their interaction.

2. A study in the case of a positive reply on the elimination of molecules or quenching ions (water, transition metals cations, etc.) occurs. In the case of a positive response, a mechanism such as SQD, QFR, and DAAF can be assumed. The usually observed increase in the τ_obs of lanthanide ion is evident in favor of such an assumption. In the case of vibrational quenching, one should keep in mind the significant temperature and isotope sensitivity of the phonon relaxation efficiency, so measurements of τ_obs in a deuterated solvent and/or upon cooling are the crucial experiment to confirm the SQD mechanism.

3. An IR study, in the case of a preserved molecular structure of the sensor to verify the breaking or formation of weak bonds (hydrogen, etc.), which could support a VQC mechanism.

4. A study confirming that a new ligand (usually an analyte) is coordinated to the sensor and is capable to perform an antenna function. The DAAF mechanism is, in fact, confirmed by the combination of analyte absorption and sensor excitation spectra in the presence and absence of the analyte, as well as measurements of τ_obs of Ln³⁺.

5. A measure of the energy of the triplet level from the phosphorescence spectra of the Gd³⁺ complex in the presence of a chemical interaction between the sensor and the analyte. The change in the energy of the triplet level indicates the implementation of a relatively rare TLC mechanism. Similarly, the appearance of a new intense band in the excitation spectrum makes it possible to implement the EWC mechanism.

6. A combined analysis of both absorption and excitation spectra of the sensor material, which can be used to prove the ETQC mechanism. For a more accurate determination of the latter, a calculation of the energy of the orbitals is required.

3. “Ratiometric” sensors

The third important sensor type is the “ratiometric” one. In materials showing these properties, the signal is estimated as the ratio between luminescence values at two different wavelengths, either affecting the effect of Ln³⁺ ions or affecting both the Ln³⁺ and the organic ligand.

$$\mathit{\text{Sensory Resonse}}\left(S\right)=\frac{I_1}{I_2}=\frac{{\int }_{{\lambda }_{1\left(\mathit{min}\right)}}^{{\lambda }_{1\left(\mathit{max}\right)}}\mathit{Id\lambda }}{{\int }_{{\lambda }_{2\left(\mathit{min}\right)}}^{{\lambda }_{2\left(\mathit{max}\right)}}\mathit{Id\lambda }},$$E1

where λ_1,2(min/max) -are the initial and final coordinates of the bands in the spectrum involved in the integration.

“Ratiometric” sensors do not have the disadvantage to determine the presence of internal standards as in the case of “turn off” and “turn on” sensors. An unusual uniqueness of ratiometric sensory visibility materials is the naked eye color change of luminescent properties when they are in presence of an analyte.

The possible response mechanisms are similar to the “turn off” and “turn on” signals considered above. In addition, if two different Ln³⁺ ions are simultaneously present in a compound (Ln-Ln’ sensors), energy transfer between them is possible, the efficiency of which depends on the Ln-Ln distances and other factors [41]; the analyte can affect the transfer efficiency constant. This type of response is defined as Metal-to-Metal Energy Transfer (MMET).

Ratiometric materials generally use the Eu³⁺/Tb³⁺ bimetallic pair, since these ions have the most efficient luminescence. Other lanthanide-based systems were used only in a small number of cases, for example, Ce³⁺/Tb³⁺ [42], Dy³⁺/Eu³⁺ [43], and Eu³⁺/Yb³⁺ [44] systems. To achieve greater accuracy, as a rule, it is better to employ the most intense lines in the emission spectra, for example, those corresponding to the europium ⁵D₀-⁷F₂ (∼612 nm) and terbium ⁵D₄-⁷F₅ (∼544 nm) transitions, also if this is not entirely correct, because the europium transition ⁵D₀-⁷F₂ is partially superimposed on the low-intensity transition of terbium ⁵D₄-⁷F₅ (∼620 nm). The sensor response can be calculated using the europium ⁵D₀-7F₄ (∼700 nm) transition, which in many cases is also very intense and does not overlap with any terbium line, but lies in the region of reduced sensitivity of common photomultipliers [45]. The usual molar fractions ratio of lanthanides with a predominance of terbium ions is caused by the transfer of energy between lanthanide ions.

O₂ in the triplet state can act as a luminescence quencher [46, 47]. The emission of Tb³⁺ ions is quenched by O₂ molecules more efficiently than by Eu³⁺ due to the smaller difference in the energies of ³T level of O₂ and the excited state of corresponding Ln³⁺ ions. This principle is the basis for a sensor for gaseous oxygen, which is a mechanical mixture of terbium and europium complexes immobilized on a quartz surface [48].

The possibility to determine, using “ratiometric sensors”, small H₂O impurities against the background of organic solvents (for which Karl Fischer titration is usually used), which requires the use of toxic reagents and “capricious” equipment, is of great interest. In addition, “ratiometric” sensors can detect the mixture of light water in and D₂O, which is impossible with Fischer titration and requires expensive mass spectrometers or precision IR spectrometers [49, 50]. Such materials are based on MOF structures containing intra-sphere water molecules. The sensor material is activated by heat treatment in vacuum, after which the obtained anhydrous powder is dispersed in the medium of the investigated solvent. This approach is not accidental: polymeric MOFs are insoluble in most organic solvents, which makes them easy to regenerate and reuse. OH quenching is the basis for the detection of methanol in ethanol and in the form of vapors in air [51], and a similar principle can be extended to CH oscillations in an elegant DMSO impurity sensor in deuterated DMSO-d₆ [44]. Substitution of an OH group [45] or a water molecule [52] in the coordination sphere of lanthanide with F^– ions also suppresses Eu³⁺ quenching more effectively than Tb³⁺, which was used in the development of fluoride-sensitive sensors.

Mixed-metal MOFs with a statistical distribution of Ln³⁺ ions are commonly used as material for “ratiometric” sensors. However, Tscelykh et al proposed in [53] to use solutions of pentafluorobenzoates or even Ln³⁺ chlorides, since the sensitivity of the sensor is directly dependent on the number of water molecules in the environment of Eu³⁺. The disadvantages of this approach include the complexity of material regeneration and contamination of the analyzed media, which makes flow analysis impossible and increases its cost.

Moisture sensors are closely related to pH sensor materials. “Ratiometric” pH sensors based on carboxylate MOFs have been also considered [54, 55, 56]. In the first two cases, as the pH increases, the color of the luminescence changes from green to red; the relative intensity of Eu³⁺ emission increases. This is explained by the strong pH dependence of the excitation transfer rate constant from Tb³⁺ to Eu³⁺, which was confirmed by measurements of τ_obs in monometallic and bimetallic complexes [55] (in the analysis of kinetic data, certain caution is required due to the non-monoexponential nature of the Eu³⁺ decay curve in the presence of luminescence sensitization by Ln ions [41, 50]). It has been described [56] that as the pH increases, the luminescence color, on the contrary, changes from red to green, that is, the relative intensity of Tb³⁺ luminescence increases. It was shown by the DFT method that the energy of the ³T* triplet level increases from ∼24.400 to ∼26.400 cm⁻¹ upon transition from the protonated to the deprotonated form of the ligand. The resonant level of europium is much lower than both of these values, so the sensitization of europium luminescence (17.200 cm⁻¹) does not change its efficiency (τ_obs(Eu) also changes slightly with pH). The resonant level of terbium (20.500 cm⁻¹) lies higher, and an increase in the triplet level energy weakens the back transfer of excitation from Tb³⁺ to the ligand. This leads to a significant increase in τ_obs(Tb) and in the intensity of Tb³⁺ emission.

The MMET mechanism can be further confirmed by measuring the response in a bimetallic complex and in a mechanical mixture of complexes of two REEs. The decrease or disappearance of the response in the second case indicates the partial or complete participation of the MMET mechanism. A similar approach was used [57] where the MMET mechanism is related to the response to hydrosulfide anions. Interestingly, the other two analyzed analytes (THF and Ag⁺ cations) exhibit different response mechanisms.

The highly selective sensor for a potassium ions, as in the case of the “turn-on” material described above [13], contains diaza-18-crown-6 in the structure, which makes it selective to the tested s-metal cations [58]. It has been shown that the capture of the potassium ion by the crown fragment leads to an increase in the triplet level energy from ∼22.400 cm⁻¹ to ∼23.400 cm⁻¹. This effect enhances the emission of Eu³⁺ to a greater extent than that of Tb³⁺. In addition, it was found that the efficiency of energy transfer from Tb³⁺ to a Eu³⁺ increases with increasing K⁺ concentration, which leads to an increase in the contribution of europium bands in the spectrum.

For Ln-Ln’ “ratiometric” sensors, in contrast to the “turn off” and “turn on” materials considered above, the response to conjugated organic analytes has been poorly studied. The sensory response of Eu-Tb bimetallic BTC-MOF film to a number of drugs [59] has been also investigated. The nature of the emission of this material changes significantly in the presence of a number of compounds, in particular, in the presence of coumarin and caffeine. The nature of the response to caffeine is not discussed, and for coumarin, an increase in k^Ln_ET is assumed, which is confirmed by kinetic measurements, but the corresponding quantum chemical studies are not provided.

In the presence of only one lanthanide luminescent center, the “ratiometric” response can still be realized if the ligand is fluorescent or phosphorescent (Ln-L sensors). This is possible if the antenna sensitization efficiency is low, which can be caused by too large or too small energy gap between ³T* and the REE resonant level, as well as by a large metal-ligand distance. Possible mechanisms for the occurrence of this type of sensory response include binding of the lanthanide ion caused by decomposition of the parent complex, which weakens the efficiency of antenna sensitization (lanthanide ion ejection, LIE), and direct antenna function of the analyte.

The detection of Hg²⁺ ions was possible by using a composite material containing a luminescent terbium coordination polymer impregnated with a coumarin solution [60]. The response is selective to Hg²⁺ ions with respect to a wide range of s-, p-, and d-metal ions and is associated with the displacement of Tb³⁺ ions from the adenosine monophosphate environment, leading to a decrease in the Tb³⁺ luminescence intensity and to a weakening of its sensitization by coumarin. The driving force behind the displacement reaction of the lanthanide ion into a complex with a lower luminescence intensity can be not only the strength of the “cationic analyte-ligand” bond, but also the formation of a stable “analyte-lanthanide cation” complex. A similar strategy was implemented for phosphate ions [61] and for alkaline phosphatase [62]. In some cases, the response occurs when the ligand is destroyed, for example, in sensors for the determination of HClO [63] or formaldehyde [64]. The vulnerability of these two strategies is the impossibility of a sensor regeneration.

Radiative relaxation by the PET or FRET process can also lead to a response if the analyte has the appropriate LUMO energy. This approach was implemented in a sensor material for the detection of nitrofuranose and furazolidone against the background of other antibiotics in a dysprosium-containing sensor [65]. Trinitrophenol (TNP) similarly blocks the sensitization of terbium luminescence in specifically designed Tb-MOF via PET and FRET mechanisms [66].

As found for Ln-L sensors, the response manifests itself as a drop in the relative efficiency of lanthanide luminescence. The opposite is also possible if the ligand displaces quencher molecules (SQD) and exhibits antenna properties itself (DAAF). An example of this approach is in a study [67, 68, 69] describing the response to dipicolinic acid, an important biological marker associated with antrax disease.

Bisphenol-A, an important industrial reagent in the production of plastics, can act as an effective antenna ligand for europium cations. This underlies the importance of the production of a highly sensitive sensor material based on a composite of carbon quantum dots and europium 5’-adenosine monophosphate [70].

Finally, a response can also be caused by an internal filter effect (IFE). This effect can be the main mechanism, as in the sensor for tetracyclines [71], or manifest itself simultaneously with quenching through ET mechanisms in sensors for Fe³⁺ [72] or for the aforementioned nitrofuranose and furazolidone (Table 3) [65].

Material	Analyte	Media	Response wavelengths λ1, λ2 (nm)	Linearity range	LOD	Mechanism	Ref.
Ln-Ln’ materials
[Eu0.167Tb0.833 (L)(H2O)3] (1,4-dioxane) MOF	H2O	D2O; dioxane DMF	543Tb-616Eu	10–120000 ppm		SQD	[49]
{[Eu0.1Tb0.9(L) (H2O)3] MOF	H2O	D2O	615Eu-545Tb	0.5–100%		SQD	[50]
		dioxane		0–100%.		SQD
		CH3CN		0–60%.		SQD
Tb0.9711Eu0.0289 (L)0.5 MOF	H2O	CH3CN	543Tb-615Eu	0–2.5 v%		SQD	[73]
{[(Eu0.02Dy0.98)2; (L)2]·2NH2 (CH3)2·H2O} MOF	H2O	EtOH	416 Dy-614Eu	0–10%	0.1%	SQD	[43]
[Tb0.3Eu0.7](L) MOF	H2O (vapors)	Air	619Eu-546Tb	20–100%	4.3%	SQD	[74]
10Eu(C6F5COO)3+1Tb (C6F5COO)3 solution	H2O	D2O	612 Eu, 545 Tb	0–50%		SQD	[53]
Eu0.022Tb0.978 MOF	H2O	EtOH	544Tb-614Eu	0–0.67%	0.016%	SQD	[75]
Eu0.2Tb0.8MOF	H2O	EtOH	546Tb-616Eu	10–100%		SQD	[76]
Eu0.2Tb0.8MOF	H2O	EtOH	546Tb-616Eu	0–10%	0.01%	SQD
[Eu0.05Tb0.95(L) (H2O)Cl] MOF	pH, H2O	DMF	547Tb-616Eu	pH 3–11; 0–0.8 v% H2O		SQD	[54]
Eu0.034Tb0.966(L)2(C2O4)(H2O)4 MOF	pH	H2O	618Eu-545Tb	pH 3–7		SQD	[55]
Eu0.205Tb0.795(L)(H2O)4·0.5H2O MOF	pH	H2O	616Eu-543Tb	pH 4.0–7.5		SQD	[56]
Eu0.4Tb0.6 Ciprofloxacin complex + dextran aldehyde (DEX) + chitosan hydrogel	pH	H2O	615Eu-545Tb	pH 5.5–8.0		SQD	[77]
{(Me2NH2)[Tb0.9; Eu0.1(L)2]·(L^76)·; (H2O)1.5}n MOF	MeOH	EtOH, air	545Tb-618Eu	0–1 v%		SQD	[51]
Eu0.1Yb0.9(dbm)3x(BPhen) solution	DMSO	DMSO-d6	612Eu-975Yb	0–50 v%		SQD	[44]
[Eu0.1Tb1.9(FDC)3(DMF)2]·2DMF	CH3NHCO	DMF	544Tb-614Eu	0–100 v%			[78]
[Eu0.5Tb1.5(FDC)3]	ethylene glycol (EG)	EG-dioxane mixture	546Tb-616Eu	0–100 v%		SQD	[79]
Hybrid material: mixture of solid Ln(L) complexes (Ln=Tb and Eu) immobilized on glass surface	O2	Air/N2	546Tb-616Eu	0–0.2 atm		ET quenching	[48]
EuTbL complex with a crown-containing ligand	K^+	MeOH	545Tb-617Eu	0.5–6 eq K^+/Ln		TLC	[58]
[Eu0.47Tb0.53(L) (H2O)3] MOF	Styrene (vapors)	Air	545Tb-617Eu			PET+FRET quencing	[80]
7:1 Tb:Eu complex with a 3,5-di-carboxy-benzene boronic acid	Ciprofloxacin	H2O	545Tb-615Eu	0.3–24 μM	90 nM	DAAF	[81]
[Eu0.5Tb0.5L(H2O)]Cl MOF	F-	H2O	700Eu-545Tb	0–2 eq F-/Ln	17.7 nM	SQD	[45]
[Tb0.97Eu0.03(L) (H2O)] MOF	F^−		547Tb-617Eu	0–1.9 ppm	96 ppb	SQD	[52]
Hybrid material: luminescent complex [TbL] MOF enclosed in SiO2 nanoparticles, surface coated with luminescent europium complex Eu(L)	HClO	EtOH	539Tb-607Eu	0–40 μM	0.27 μM	Ligand destruction	[63]
Eu0.05Tb0.95BTC0.9(L)0.1 MOF	CrO4^2−		545Tb-618Eu		1μM	IFE+ET	[82]
Tb0.6Eu0.4-MOF	Hg^2+	H2O	617Eu-543Tb	0–40 μM	4.83 nM	MMET	[83]
Hybrid material: complex Eu4Tb6(acac) on the surface of NZL zeolite	Et3N, tBuNH2,; n-BuNH2, BnNH2, en		545Tb-618Eu			SC	[84]
[La0.88Eu0.02Tb0.1 (L)(DMF)2]n ·H2O^.0.5DMF MOF	HS-	DMF/ H2O	544Tb-616Eu	0–400 μM		MMET	[57]
	THF	DMF		0–8%		TLC
	Ag^+	DMF/ H2O				TLC
Tb0.833Eu0.167(L^77)3 MOF	H2DPA	H2O	549Tb-620Eu	2–16 μM	96 nM	MMET+DAAF	[85]
[Tb0.833Eu0.167BTC] ^.6H2O MOF	H2DPA	H2O	545Tb-613Eu	0–700 nM	4.55 nM	MMET+DAAF	[86]
[Eu0.06Tb0.04Gd0.9 (L)1.5(H2O) (DMF)] MOF	MnO4^−	H2O	613Eu-544Tb	0–0.2 mM	0.02 μM	IFE+FRET	[87]
Ce^3+/Tb^3+ bimetallic guanosine monophosphate complex	alkaline phosphatase	H2O	552Tb-384Ce	0.2–60 mU mL^−1	0.12 mU/mL	Ligand destruction	[42]
[Eu0.1Tb0.9BTC(H2O)2].3DMF.H2O MOF	Coumarin		613Eu-544Tb	1–100 μM	2.3 μM	MMET	[59]
	Caffeine		613Eu-544Tb	1–100 μM		n/a
Ln-L materials
Hybrid material: cerium/terbium adenosine monophosphate impregnated with coumarin solution	Hg^2+	H2O	445–548Tb	0.08–1000 nM	0.03 nM	LIE	[60]
Hybrid material: CsPbBr3@Eu-BTC MOF	Hg^2+	H2O	520L-616Eu	0–1 μM	0.116 nM	PET	[88]
Hybrid material: Eu^3+@ Ca-MOF	Hg^2+	H2O	381L-590Eu	0.02–200 μM	2.6 nM	TLC	[89]
{[Eu(L)(H2O)1.35(DMF)0.65]·1.9DMF}n MOF	PO4^3−	H2O	614Eu-368L	0.1–15 μM	52 nM	LIE	[61]
Solution: Eu^3+ complex with H2DPA + luminol	PO4^3−	H2O	615Eu—423L	0.5–50 μM	0.12 μM	QFR (luminol)	[90]
Eu-2′-amino- [1,1′:4′,1″-terphenyl]-3,3″,5,5″-tetracarboxylic acid MOF	F-	H2O	615Eu-397L	0−5120 μM	11.26 μM	TLC	[91]
Composite material: Tb^3+ guanosine 5’-disodium@Cu-nanoclusters	alkaline phosphatase	H2O	545Tb-425L	0.002–2 U mL^−1	0.002 U mL^−1	LIE	[62]
Eu0.7Gd0.3(L)1.5 (Phen)^.H2O complex	Fe^3+	H2O	614Eu-415L	0–60 μM	91 nM	IFE+ET	[72]
Composite material Eu0.7Gd0.3(tfbdc)1.5 (Phen).H2O complex +FeCl3	Ascorbic acid			0–60 μM	0.184 μM	QFR (Fe^3+)
Composite material Zn-MOF impregnated with a TbCl3 solution	H2DPA	H2O	544Tb-330L	0–1 μM	3.6 nM	SQD+DAAF	[67]
EuCl3 + sodium polycarylate + pyranine +	H2DPA	H2O	615Eu-510L	0–90 μM	10.3 nM	SQD+DAAF	[68]
Eu^3+@Zr-MOF UiO-66-(COOH)2-NH2	H2DPA	H2O	621Eu-453L	0–40 μM	25 nM	SQD+DAAF	[69]
[Tb2(L^2)1.5(NMP)2]n MOF	TNP	H2O	546Tb-400L	0–50 μM	0.11 μM	FRET and PET quenching	[66]
[Tb0.02Gd1.98L1.5 (NMP)2]n; MOF		H2O		0–80 μM	0.41 μM	FRET and PET quenching
Composite material: Eu MOF in a PMMA matrix	Form-aldehyde	H2O, Air	453L-616Eu	0.05–1%		Ligand destruction	[64]
[Dy(L^9)(DMF)3]n MOF	Nitro-furanose	H2O	385L-573Dy	0–60 μM	47.6 nM	PET+IFE	[65]
	Furazolidone	H2O		0–60 μM	48.2 nM	PET+IFE
[NMe2H2]2[Tb9 (μ^3-OH)8(μ^2-OH)3 (H2O)3(L)6] ·11DMF·23H2O MOF	Tetra-cycline	H2O	543Tb-345L	0.14–23 μM	0.18nM	IFE	[71]
Eu^3+ complex with a luminol (LML) and guanosine 50-monophosphate (GMP) nanoparticles	Tetra-cycline	H2O	617Eu-430L	0–60 μM	23.2 nM	IFE+ DAAF	[92]
Eu-adenosine-monophosphate@CD	Bisphenol-A	H2O	429L-623Eu	0–100 μM	20nM	DAAF	[70]
Hybdrid material: Tb@Bi-based MOF	Serotonin	H2O	545Tb-350L	0–200 μM	0.57 μM	IFE	[93]
Eu^3+-functionalized hydrogen-bonded organic framework	CH3NH2	H2O	425L-615Eu	10^−8–10^−2 M	0.87 ppm	IFE	[94]
[Eu(DMTP-DC)1.5(H2O)3]·DMF MOF	arginine	H2O	441L-617Eu	0–250 μM	24.38 μM	PET	[95]
	lysine	H2O	441L-617Eu	0–250 μM	9.31 μM	PET
Eu^3+@Al-based MOF	β-glucuro nidase	H2O	450L-617Eu	0.1–50 U L^−1	0.03 U L^−1	IFE	[96]

Table 3.

“Ratiometric” sensors.

As in the previous cases, the key stages in studying the mechanism of the “ratiometric” sensory response are the chemical analysis of the reaction product between the sensor and analyte; comparison of the excitation and emission spectra of the sensor and the absorption of the analyte; kinetic studies of excited state lifetimes and quantum chemical modeling. In general, the“ratiometric” response mechanisms usually coincide with those for“turn off” and“turn on” systems. An exception is the mechanism associated with changing the MMET constant (k^Ln_ET). This mechanism must be reliably established by kinetic measurements, as well as by studying the response in a mechanical mixture of complexes of two lanthanides.

4. Conclusions

The classification of sensory response mechanisms reported in this review, although perhaps not complete, allows us to successfully classify most of the cited works and make appropriate generalizations, moving from a descriptive style to a debatable one.

The reported examples show the significant progress in the field of lanthanide-based luminescent sensors achieved in recent years. A wide variety of analytes can be qualitatively or quantitatively determined using suitable lanthanide compounds, which requires rational system design. “Turn off” sensors can have a niche application in the analysis of nitroaromatic compounds, while more popular “Turn on” and “ratiometric” materials can be produced using fairly simple strategies: different quenching efficiency of various REE ions by bond vibrations, binding and removal of a quenching fragment by a suitable analyte (e.g., binding Cu²⁺ ions with sulfur-containing ligands), etc.

Progress has touched not only the field of materials design, but also a reliable determination of the sensory response mechanism, which requires several spectroscopic and kinetic measurements, and in some cases quantum chemical calculations of orbital energies. The number of papers containing these studies has been increasing in recent years, and we hope that just such a systematic approach will become the standard in the future works.

Acknowledgments

University of Camerino is gratefully acknowledged.

Conflict of interest

The authors declare no conflict of interest.