Luminescent Lanthanide Sensors and Lanthanide Doped Upconversion Nanoparticles: Current Status and Future Expectations

Abstract

Lanthanide ions exhibit fascinating optical properties with their potential applications largely governed by their interaction with light. This chapter deals with some relevant aspects concerning the electronic and coordination properties of lanthanides and the basic principles related to the design of efficient luminescent lanthanide complexes. The cleverly designed environment consisting of ligands containing adequate chromophoric groups provide a rigid and protective coordination shell to minimize non-radiative deactivation. Lanthanide doped upconversion nanoparticles (UCNPs) have attracted extensive attention in the field of biomedical applications due to their long luminescence lifetime, narrow emission bandwidth, high quantum yields and low toxicity. In this chapter the upconversion phenomenon is explained with emphasis on the mechanism of upconversion, selection of host materials and impurities in host matrices. The various chemical approaches for the synthesis of lanthanide doped UCNPs have also been discussed. Subsequently, some selected results of our recent work concerning the photoluminescence studies of Eu(III) and Yb (III) complexes are reported which exhibit the characteristic emission bands of Eu(III) ion corresponding to ⁵D_o → ⁷F_J (J = 0–4) transitions with intense red emission at 615 nm due to ⁵D₀ → ⁷F₂ transition of the central Eu(III) ion. These complexes show long radiative lifetime and quantum efficiency which suggest that these complexes can be well utilized as fluorescent probes.

11.1 Introduction

The lanthanide complexes attract intense attention due to their superior optical properties and wide variety of potential applications i.e. biomedical assays and imaging, luminescence devices, ionic conductors and sensors. Majority of Ln³⁺ ions are luminescent, but some lanthanide ions are more emissive than others which depends on the feasibility of its excited state and non radiative de-activation paths. This can be achieved by sensitization through the surrounding ion and the overall quantum yield can be achieved by:

$$ {\boldsymbol{Q}}_{\boldsymbol{L}\boldsymbol{n}}^{\boldsymbol{L}}={\boldsymbol{\eta}}_{\boldsymbol{sens}}\;{\boldsymbol{Q}}_{\boldsymbol{L}\boldsymbol{n}}^{\boldsymbol{L}\boldsymbol{n}} $$

(11.1)

Where ${Q}_{Ln}^L$ and ${Q}_{Ln}^{Ln}$ are the quantum yields from indirect and direct transitions respectively while ${\eta}_{sens}$ represents the efficiency with which electromagnetic energy is transferred from the surroundings to the metal ion [1]. The quantum yield for direct transition ${Q}_{Ln}^{Ln}$ depends upon the energy gap between the emissive state of the metal ion and the highest sublevel of its ground multiplet. If this energy gap is small, the non radiative deactivation process will be faster. In lieu of above, some lanthanide ions such as Eu³⁺, Gd³⁺ and Tb³⁺ ions are the suitable candidates with ΔE = 512,300 (⁵D₀ → ⁷F₆), 32 200 (⁶P_7/2 → ⁸S_7/2), 14,800 (⁵D₄ → ⁷F₀) cm⁻¹ respectively. Out of above these, Gd³⁺ has potential applications for mercury free fluorescent lamps because under vaccum – UV excitation Gd³⁺ can efficiently transfer energy onto Eu³⁺ which results emission of two red photons also known as down conversion effect; but it is not very useful for bioanalysis because it emits in the UV region and its luminescence interferes with the emission or absorption process in organic part of complexes [2–6]. The advantages of Eu³⁺ and Tb³⁺ ions are their tunable energy gap with their sizes, therefore these two ions received much attention. Other lanthanides ions such as Sm³⁺, Dy³⁺ have very low quantum yield hence they appear to be less useful in the applications of luminescence. Pr³⁺ emits both in visible and NIR regions and is considered a suitable component for optical materials because of its ability of generating up conversion process as well. Moreover, Nd³⁺, Ho³⁺ and Yb³⁺ have found applications in the field of lasers and telecommunication devices because they emit in NIR spectral range. Nd³⁺ (lies at 1.06 nm) and Yb³⁺(lies under 1 and 1.6 nm) are less useful than Pr³⁺ but act as an efficient sensitizer of Er³⁺ which emits at 1.55 nm [7, 8].

Recently, three ions Nd³⁺, Er³⁺ and Yb³⁺ have gained popularity because technical developments occurred which facilitated the detection of weak NIR emissions for which efficient sensitizing groups have been explored.

11.2 Principle of Lanthanide Luminescence

The most interesting feature of these lanthanide series is their photoluminescence as these complexes show luminescence in visible or near infrared spectral region [9, 10]. The color of the emitted light depends upon the lanthanide ion viz. Eu³⁺emits red light, Tb³⁺ emits green light, Sm³⁺ orange light, Tm³⁺ blue light. Pr³⁺, Sm³⁺, Dy³⁺, Ho³⁺, Tm³⁺ shows transitions in the near infrared region while Yb³⁺, Nd³⁺, Er³⁺emits near infrared luminescence. Gd³⁺ luminescence can be observed in absence of organic ligands as it emits in ultraviolet region. In lanthanides, the emissions are due to f-f transitions [11, 12]. The partially filled 4f shell is shielded by 5s² and 5p⁶ orbitals and the ligands in the first and second coordination sphere perturb the coordination sphere of lanthanide ions. The narrow band emission and long lifetimes of excited states of lanthanides is observed due to this shielding. Ce³⁺ is a unique case among all lanthanides due to f-d transitions. The emission maxima largely depends upon the ligand environment around Ce³⁺ ion [13–15].

All the lanthanide ions suffer from weak light absorption as the molar absorption coefficients of most of the transitions are smaller than 10 L mol⁻¹cm⁻¹ and a limited amount of radiations is absorbed by direct excitation in the 4f levels because luminescence intensity is proportional to quantum yield and the amount of light absorbed, weak light absorption results in weak luminescence. However, this can be overcome by the antenna effect. Weissman [16] found that upon excitation of lanthanide complexes with organic ligands, intense metal centered luminescence can be observed due to intense absorption of organic chromophores. The excitation energy is transferred from ligand to lanthanide ion via intramolecular energy transfer [17–20]. The same phenomena for the europium complexes with salicylaldehyde, benzoylacetone, and dibenzoylmethane and meta- nitro benzoylacetone has been described.

The mechanism of energy transfer from organic ligands to lanthanide ions is proposed by Crosby and Whan [21]. Upon ultraviolet irradiation, the organic ligands of the complexes are excited to a vibrational level of first excited singlet state (S₁ or S₀), then molecule undergoes to lower vibrational level of S₁ state via internal conversion. The excited singlet state can be deactivated to the ground state (S₁ → S₀) or can undergo nonradiative intersystem crossing from singlet state S₁ to triplet state T₁. The triplet state T₁ can be radiatively to the ground state S₀ by spin forbidden transitions T₁ → S₀. The complex may undergo indirect excitation by a non radiation transition from the triplet state to an excited state of the lanthanide ion which may undergo a radiative transition to a lower 4f state by photoluminescence or may be deactivated by non radiative process [22–28]. According to them, vibronic coupling of lanthanide with the ligand and solvent molecule is the main cause of non radiative deactivation of the lanthanide ion (Fig. 11.1).

Fig. 11.1

Jablonski diagram representing the energy transfer in lanthanide complexes [28, 29]

However, a mechanism was proposed for energy transfer from excited singlet state S₁ to the energy levels of the lanthanide ions. But this theory is not of great importance because it is not efficient due to short lifetime of the singlet excited state [29]. This type of excitation is seen in Tb³⁺ and Eu³⁺. Luminescence by the lanthanide ions is done by resonance levels. The main resonance levels are ⁴G_5/2 for Sm³⁺ ion (17,800 cm⁻¹⁾, ⁵D₀ for Eu³⁺ion (17,250 cm⁻¹), ⁵D₄ for Tb³⁺ (20,430 cm⁻¹) and ⁴F_9/2 for Dy³⁺ (20,960 cm⁻¹) [30]. If the lanthanide is excited to a non emitting level either by direct excitation or indirect excitation, the excitation energy is dissipated via non radiative processes. To achieve the resonance level of the lanthanide, it is mandatory that the lowest triplet state of the complex is located at or equal to the resonance level of the lanthanide ion; if the energy levels of the organic ligands are present below the resonance level of lanthanide, molecular fluorescence or phosphorescence of the ligand is observed. Therefore it is a sensitive function of the lowest triplet level of the complex relative to a resonance level of the lanthanide ion [31–33].

Thus by the variation of ligands, it is possible to control the luminescence intensity for a given lanthanide ion, as the position of the triplet level depends on the nature of the ligands. Further, it is also temperature dependent, the luminescence caused by direct excitation of 4f level is less temperature sensitive than the luminescence caused by indirect excitation by organic ligands. The metal centered emission cannot be observed in case of Gd³⁺ ion as its 4f levels are located above the triplet levels. The presence of heavy paramagnetic ion results in mixing of triplet and singlet states and thereby enhancing the intersystem crossing from singlet state to triplet state [34]. The spin orbit coupling interaction helps the triplet state to acquire a partially singlet character which results in relaxation of selection rules.

The efficiency of energy transfer depends upon the overlap between the phosphorescence spectrum of the ligand and absorption spectrum of lanthanide ion. The back energy transfer can occur from the emitting 4f level of the lanthanide ion to the triplet state of the ligand, if the energy match between these is very close to each other [35]. The fluorescence and phosphorescence of the ligands are quenched if there is efficient energy transfer from ligand to lanthanide ion, but if the energy transfer is not effective it results in observation of ligand emission in addition to the lanthanide emission.

The energy transfer through charge transfer states is another possibility of lanthanide luminescence. In this case, the light is absorbed by the ligand to metal charge transfer states (LMCT) from where the energy can be transferred to the 4f levels of the lanthanide ion, if the LMCT state lies high enough to the lanthanide 4f level. Eu³⁺ ion shows an efficient sensitization through charge transfer state if the LMCT lies above 40,000 cm⁻¹ [36]. If this energy gap is low it results in the quenching of luminescence and when this energy gap is lower than 25,000 cm⁻¹ it leads to total quenching.

11.3 Design of Luminescent Lanthanide Complexes

11.3.1 General Criteria

The lanthanide ions need a cleverly designed environment which consists of ligands containing adequate chromophores, harvesting light and simultaneously providing a rigid and protective coordination shell to minimize non-radiative deactivation . The laporte-forbidden f-f transitions have weak oscillator strengths which results in quenching by the high energy oscillators such as O-H, N-H or C-H groups located in the inner and outer coordination spheres [37]. The design of luminescent lanthanide complexes requires particular care as the electronic, magnetic and photo physical properties of Ln(III) complexes strongly depends on the control of the coordination sphere of the metal . Among various parameters which govern the control over chemical, structural and thermodynamical properties of the complex are ligand topology, (dimensionality, connectivity, shape, size and chirality) and binding sites (electronic properties, nature, number, shape and arrangement), layer properties (rigidity/flexibility and lipophilicity/hydrophilicity ratio, thickness), environment properties and the nature of counter ions present [38]. The luminescent Ln(III) complex represents a multi-component system organized as a supramolecular structure comprising of metal cation and antenna . In general, the complex formation results from the attraction of ligand and metal ion, the resulting complex then undergoes partial to total desolvation [39]. The coordination of lanthanide ion with the ligand is substantially based on the Van der Walls electrostatic forces of attraction. These type of interactions are also observed in alkali-earth cations particularly Ca²⁺. The ligand interacts with the surface of metal cation thus replacing the first solvation sphere either partially or totally. The desolvation step shows an increase in entropy however the enthalpy variation can be positive or negative which can be attributed to the difference in the energy of the bonds formed and the energy of the bonds broken between ligand and cation with solvent respectively [40]. In the complexation of Ln(III) in aqueous solution the dehydration step is endothermic (ΔH > 0) which is an unfavourable energy contribution to the Gibbs free energy [41]. Therefore it is suggested to use polydentate ligands which show chelate effect and afford highly stable complexes in aqueous medium. In order to achieve the overall sensitization efficiency, the design of lanthanide complexes need certain factors to be considered which can be summarized as follows:

11.3.2 Choice of Lanthanide

The lanthanide ions show unique emission properties that cover the ultraviolet (Gd³⁺) to the visible range (Sm³⁺, Eu³⁺, Dy³⁺, Tb³⁺ and Tm³⁺) and the near infrared range (Yb³⁺, Nd³⁺ and Er³⁺). The selection of the lanthanide ion depends on the intrinsic quantum yield which in turn depends on the energy gap between the lowest lying excited emissive state of metal ion and the highest sublevel of its ground multiplet. Accordingly, Eu³⁺, Tb³⁺ and Gd³⁺ ions serve as good candidates for luminescent probes with energy gap values of 12,300 cm⁻¹, 14,800 cm⁻¹ and 32,200 cm⁻¹ however Gd³⁺ emits in ultraviolet region and used as contrast agents . The lanthanide ions possess relatively long lived excited states (microseconds to milliseconds) which can undergo energy transfer to high frequency vibrational oscillators such as O-H, N-H and to a lower extent C-H [42]. As a consequence, the presence of these groups in the proximity of the metal favours thermal dissipation of the energy (vibronic coupling) which gives rise to quenching of the luminescence (Fig. 11.2).

Fig. 11.2

Energy level diagram for the Ln(III) ions showing the main emitting levels and the transitions to the ground state levels [42]

11.3.3 Choice of the Antenna

The term “antenna” refers to the chromophore which promotes the sensitization of lanthanide ion. It can be any aromatic or heteroaromatic highly π-conjugated system characterized by high efficiency of light absorption and high efficiencies of intersystem crossing and energy transfer process . The high yield of energy transfer from the antenna to the metal ion occurs until certain conditions are satisfied such as nature of ion, electronic structure of the donor, their relative positioning and the nature of interaction between metal ion and ligand [43]. The ligand geometric and electronic structure must be taken into account for various energy transfer pathways resulting from energy levels such as singlet ¹S_i(i = 1,2), triplet ³T, intra ligand charge transfer (ILCT), ligand to metal charge transfer (LMCT). Generally, these energy pathways terminate at the emissive level of the lanthanide ion. The potential donor levels of the antenna should be chosen such that these are reasonably above the Ln(III) emissive level to avoid back transfer and is close resonance with one of the higher 4f states. This energy gap should be atleast 1850 cm⁻¹ higher than the lowest emitting level of Ln(III) ion. At higher value, the energy transfer from the triplet flows through the non-radiative excited state of the metal until it reaches the emissive levels and metal centered emission occurs. However, at lower energy gap strong thermal deactivation takes place due to back energy transfer and O₂ –quenching towards the chromophore triplet level [44]. The triplet state energies of some of the commonly used chromophores are reported in Table 11.1. The dependence of the luminescence of Ln(III) ion on the excitation wavelength also suggests that it should be above ca. 350 nm to facilitate the use of inexpensive excitation sources.

Table 11.1 Energy levels of some commonly used chromophores [45]

The lanthanides do not have restricted coordination number and geometry, these can therefore extend their coordination number ranging from 3 to 12, although 8 and 9 are considered the most common ones [46]. This property can be utilized by several auxillary N-or O- donor co-ligands for coordination and to make saturated complexes. In such complexes, the energy transfer is more effective from ligand to metal and complexes show intense photoluminescence which can be attributed to the increased anisotropy around the lanthanide ion. The introduced auxillary coligands make the complex more rigid and asymmetric around the central metal ion which overall improve the photophysical properties of the complexes [47]. The photophysical properties can also be tuned by substituting releasing or donating groups at various positions of the coordinated neutral auxillary ligands. The N-donor neutral auxillary ligands are 1,10-phenanthroline (phen), 2,2′-bipyridine (bipy), terpyridine (terpy) and their analogues form stable ternary complexes with wide range of 1:3 neutral lanthanide complexes [48–52]. The 1,3-diketonate tris complexes and adducts with some auxillary coligands explained the structural behavior of the coligand and its effect towards luminescence intensity stating that a rigid planar structural ligand behaves better in effective energy transfer which is further supported by the comparative study of Eu(tta)₃.phen and Eu(tta)₃.bipy. The important factor to choose a neutral ligand to enhance photoluminescence properties of lanthanides is the value of excited energy levels (singlet and triplet) of the corresponding auxillary ligand [53]. The corresponding singlet and triplet energy levels of bipy(29,900 cm⁻¹, 22,900 cm⁻¹) and phen(31,000 cm⁻¹, 22,100 cm⁻¹) were found suitable for effective energy transfer from ligand to metal and exhibit enhanced metal centered luminescence [54].

1,10-phenanthroline ligand behaves as a weak base in aqueous solution, the protonation constant being 4.95 log units. Its basicity is remarkably lower than aliphatic diamines, such as ethylenediamine(log k = 10.65 and 8.05 for successive addition of acidic protons) which shows its lower donor ability of its nitrogen atoms. [55] Compared to the parent 2,2′-bipyridine (bipy) and 2,2′,6′,6″ terpyridine systems, phen is characterized by two inward-pointing nitrogen donor atoms and therefore pre-organized for strong and entropically favoured metal binding [56]. The photophysical properties of phen have been a subject of a number of studies. Phen is characterized in aqueous solution by UV absorptions at 229 nm and 265 nm, the latter attributed to the Π → Π* transition to the lowest energy excited singlet state. Excitation at 307 nm originates a fluorescence emission band at 380 nm, due to radiative decay of Π → Π* state. These spectroscopic features are modulated by appropriate substituents on the phen framework and pH changes i.e. by protonation of the heteroaromatic nitrogen atoms. The UV-spectrum recorded at pH 4, where phen is monoprotonated form, (phenH)⁺ shows the absorbance 8 nm red shifted to the absorption band at 265 nm attributed to the charge transfer of the Π → Π* transition(i.e. the LUMO has a higher electron density on the nitrogen atoms than the HOMO. Monoprotonation of phen leads to the disappearance of the emission band at 380 nm and to the formation of a new emission broad red shifted band at ca. 410 nm accounting for stabilization of lowest energy state by protonation of the heteroaromatic nitrogen atoms because of its charge transfer character [57]. The electron deficiency of chelating agents phen and bipy is their electron deficiency that makes them excellent acceptors capable of stabilizing metal ions in lower oxidation states. Due to the presence of low energy Π* orbitals of the ligand, metal complexes can be characterized by strong metal-to-ligand charge transfer (MLCT) absorption bands in the visible spectrum and red shifted fluorescent emissions.

Another major class of co-ligands are phosphine oxides and their analogues which are widely studied and used as sensitizers now. The disappearance of broad absorption band in the region of 3000 cm⁻¹ in the IR spectra of lanthanide ternary complexes of pyrazolonates confirms the substitution of water coordination by various phosphine oxide ligands . The increase in photoluminescence intensity of phosphine oxides coordinated Eu(III) complexes can be due to square antiprismatic structures of the complexes which promotes faster radiation rates and an increase in ⁵D₀ → ⁷F₂ emissions related to odd parity. The substitution of auxillary sensitizing ligands in lanthanide complexes also increases the solubility of complexes in organic solvents depending on the substituted neutral ligand used.

The above discussion concludes that the ligand topology , binding sites, layer properties, and environment properties of the antenna plays an important role in the design of lanthanide complexes. In addition to this, the radiative lifetime and photoluminescence quantum yield of lanthanide complexes also influence the design of luminescent sensors.

Among the organic ligands , the pyridine derivatives are important as they form a diversity of coordinating structures including coordination polymers and also have numerous applications in catalysis, non linear optics, luminescence, ion exchange, material chemistry and magnetochemistry, the role of 1,10-phenanthroline as an auxillary ligand is well known and plays a significant role in absorption and transfer of energy to the emission center Eu(III) ion [58–60]. The auxillary ligand reduces the non radiative decay of the excited states of Eu(III) ion and increase the energy transfer efficiency from ligands to Eu(III) ion thereby increasing the stability of europium complexes. Similar observations have been made with the europium complexes [Eu(2-ap)₃(phen)]Cl₃ (1), [Eu(2-ap)₂(phen)₂]Cl₃ (2) and [Eu(2-ap)(phen)₃]Cl₃ (3) synthesized with 2-aminopyridine and 1,10-phenanthroline ligands in different molar ratios [61] (Fig. 11.3).

Fig. 11.3

Structure of europium complexes [Eu(2-ap)₃(phen)]Cl₃(1), [Eu(2-ap)₂(phen)₂]Cl₃(2) and [Eu(2-ap)(phen)₃]Cl₃ (3) (x = 2-aminopyridine, y = 1,10-phenanthroline) [61]

The complexes exhibited characteristic emissions of Eu(III) ion at around 537 nm, 592 nm, 615 nm, 652 nm and 700 nm. These five expected bands are attributed to the ⁵D_o → ⁷F_J (J = 0–4) transitions of Eu(III) ion. The emission bands at 537 nm and 652 nm are very weak since their corresponding transitions ⁵D_o → ⁷F_o and ⁵D_o → ⁷F₃ are forbidden in both magnetic dipole and electric dipole fields . The relative emission of ⁵D_o → ⁷F₁ transition at 592 nm are relatively strong as it is a magnetic dipole transition which is independent of the coordination environment of Eu(III) ion. The ⁵D_o → ⁷F₂ transition at 615 nm is an induced electric dipole transition which is very sensitive to the coordination environment of the Eu(III) ion. This transition is responsible for the strong red emission of europium complexes and also indicates the presence of highly polarizable chemical environment around the Eu(III) ion. The band at 700 nm is relatively weak and corresponds to the ⁵D_o → ⁷F₄ transition. It was also noted that in all the three complexes, the intensity ratio of ⁵D_o → ⁷F₂ transition to ⁵D_o → ⁷F₁ transition was around 4.0 which indicated the location of Eu(III) ion in the environment of low symmetry. The presence of only one line for ⁵D_o → ⁷F_o transition revealed the presence of similar chemical environment around Eu(III) ion. The photoluminescence results showed that the relative emission intensity of ⁵D_o → ⁷F₂ transition was enhanced as the molar ratio of the ligand phen increased from complex (1) to complex (3) which revealed that the sensitization ability of ligand phen is better than that of ligand 2-ap (Fig. 11.4).

Fig. 11.4

Photoluminescence emission spectra of europium complexes in methanol solution (1 × 10⁻⁵ mol dm^-3)[Eu(2-ap)₃(phen)]Cl₃ (1), [Eu(2-ap)₂(phen)₂]Cl₃ (2) and [Eu(2-ap)(phen)₃]Cl₃ (3) at the excitation wavelength of 247 nm [61]

The decay profile of complex (3) shows monoexponential function indicating the presence of a single chemical environment around Eu(III) ion. The lifetime value was found to be 324 μs. According to the emission spectra and the lifetime of the Eu (III) excited state (⁵D₀), the emission quantum efficiency (η) was found to be 10.33, which is good for a ligand-sensitized luminescent europium(III) complex (Fig. 11.5).

Fig. 11.5

Photoluminescence decay curve under laser pulse excitation of [Eu(2-ap)(phen)₃]Cl₃ complex [61]

The above results indicate that the europium complex [Eu(2-ap)(phen)₃]Cl₃ with higher molar ratio of ligand 1,10-phenanthroline shows high luminescence as compared to other complexes [Eu(2-ap)₃(phen)]Cl₃ and [Eu(2-ap)₂(phen)₂]Cl_3. The quantum efficiency is found to be 10.33 which suggest the complex can act as a good fluorescent probe.

The pyridine carboxylates act as good candidates due to existence of N and O mixed donor atoms which can coordinate to the metal ion in a bidentate chelating, bidentate bridging and unidentate manner [62–64]. 2,6-pyridinedicarboxylic acid and α-picolinic acid have been found as promising ligands due to the stability of their Ln(III) complexes, strong fluorescence intensity with long excitation lifetimes [65–68]. The ligand 2,6-pyridinedicarboxylic acid is water soluble, commercially available having nitrogen and oxygen atoms to coordinate with the metal ion. Aminopyridine ligands also act as useful chelating agents for inorganic and organometallic applications and their derivatives can coordinate to the metal ions in a monodentate fashion through the N atom of the ring, however there are several works reported where the amino group also participates in coordination to the metal ion. The hydroxypyridine ligands can also coordinate effectively to the metal ion through the N and O donor atoms and are expected to show good luminescent properties [69–71]. The europium complexes [{Eu(dpa)(α-pc)(CH₃OH)].2CH₃OH}](4), [{Eu(dpa)(2-ap)(CH₃OH)].2CH₃OH}](5) and [{Eu(dpa)(2-hp)(CH₃OH)].2CH₃OH}](6) with dipicolinic acid (dpa) as a primary ligand and α-picolinic acid (α-pc), 2-aminopyridine (2-ap) and 2-hydroxypyridine (2-hp) as secondary ligands [72] shows strong characteristic emission bands of Eu(III) ions in the visible region, which attributes to the electronic transitions from the excited ⁵D_o level to the ground ⁷F_J (J = 0–4) levels of Eu(III) ion . The relative luminescent intensity of ⁵D_o → ⁷F₂ transition is strongest in the complex based on 2-ap as secondary ligand, however the weakest luminescent intensity is observed in complex with 2-hp as secondary ligand. This can be explained by the non-radiative deactivation of energy of excited state as a result of their interaction with high frequency oscillators O-H group which act as efficient quencher of lanthanide ion luminescence in 2-hp ligand . These results showed that –NH₂ group in 2-ap ligand can strongly sensitize the luminescence of Eu(III) ion as compared to the 2-pc and 2-hp ligands (Fig. 11.6).

Fig. 11.6

Luminescent emission spectra of [{Eu(dpa)(α-pc)(CH₃OH)].2CH₃OH}] (4), [{Eu(dpa)(2-ap)(CH₃OH)].2CH₃OH}](5) and [{Eu(dpa)(2-hp)(CH₃OH)].2CH₃OH}](6) [72]

The luminescence decay curves of the complexes obtained at 298 K by monitoring the ⁵D₀ → ⁷F₂ transition (615 nm) show monoexponential function indicating the presence of a single chemical environment around Eu(III) ion. The lifetime values are found to be 725 μs, 825 μs and 675 μs and the quantum efficiency values are found to be 21.60, 27.30 and 17.89 for complexes (4), (5) and (6) respectively (Fig. 11.7).

Fig. 11.7

Photoluminescent lifetime decay measurement of [{Eu(dpa)(α-pc)(CH₃OH)].2CH₃OH}] (4), [{Eu(dpa)(2-ap)(CH₃OH)].2CH₃OH}](5) and [{Eu(dpa)(2-hp)(CH₃OH)].2CH₃OH}] (6) [72]

The complexes show the characteristic emission bands of Eu(III) ion in visible region at 580 nm, 592 nm, 615 nm, 650 nm and 698 nm for ⁵D₀ → ⁷F_J (J = 0–4) transitions respectively. The photoluminescence properties of the complexes are influenced by the secondary ligands as studied by the photoluminescence spectra and decay measurements where the complex [{Eu(dpa)(2-ap)(CH₃OH)].2CH₃OH}] with 2-ap as secondary ligand exhibits strongest emission intensity and relatively longer luminescence lifetime, quantum efficiency as compared to the complex [{Eu(dpa)(α-pc)(CH₃OH)].2CH₃OH}] and complex [{Eu(dpa)(2-hp)(CH₃OH)].2CH₃OH}] (6).

Recently, the luminescent materials with the emissions in the near- infrared region such as Sm(III), Dy(III), Pr(III), Ho(III), Yb(III), Nd(III) and Er(III) have gained much interest due to their applications in telecommunication network as optical signal amplifier, probes for bioassays because human tissue is relatively transparent to near infrared light at 1000 nm. Yb³⁺ ion is usually a prime candidate to be chosen due to its luminescent efficiency close to 100 % and relatively simple electronic structure of two energy level manifolds: the ²F_7/2 ground state and ²F_5/2 excited state around at 10,000 cm⁻¹ in the NIR region [73–77]. The rare earth ion couples such as RE³⁺-Yb³⁺ can be efficiently utilized for near infrared (NIR) quantum cutting (QC) materials [78–80]. The energy gap of ⁵D₂ – ⁷F₀ transition in Eu³⁺ is approximately twice as large as that of ²F_5/2 – ²F_7/2 transition in Yb³⁺ which facilitates the energy transfer between Eu³⁺ ion to Yb³⁺ ion [81]. The room temperature excitation spectra of complex [Eu_0.5Yb_0.5 (sal)₃(phen)] [82] obtained by monitoring the longest emission wavelength of Eu³⁺ ion at 614 nm and Yb³⁺ ion at 980 nm exhibits a broad band at 290 nm attributable to the ligand to metal charge transfer (CT) transitions caused by the interaction between the organic ligands and the Eu³⁺ ions at 614 nm . The strong absorption band centered at 261 nm corresponded to the ligand to metal charge transfer transitions of Yb³⁺ ions. The presence of relatively strong CT absorption band in the excitation spectrum of Yb³⁺ ions reveals the energy transfer from Eu³⁺ ions to Yb³⁺ ions. The excitation spectrum also shows the sharp 4f-4f transitions of Eu³⁺ from the ⁷F₀ ground state to ⁵D_3,4,6,7,1,2 excited states. The difference in energy between the ⁵D₂ – ⁷F₀ transition of Eu³⁺ ions is twice the energy difference between ²F_5/2 – ²F_7/2 transition of Yb³⁺ ions, which means that the ⁵D₂ excited state of Eu³⁺ ion can simultaneously transfer energy to two nearby Yb³⁺ ions and hence the Yb³⁺ ions can emit two infrared photons. This absorption is followed by rapid multi-phonon assisted relaxation from the populated ⁵D₂ energy levels to the metastable energy levels of Eu³⁺ ion (Fig. 11.8).

Fig. 11.8

Photoluminescent excitation spectra of Eu³⁺ emission monitored at 614 nm and Yb³⁺ emission monitored at 980 nm of [Eu _0.5Yb_0.5 (sal)₃(phen)] complex [82]

The emission spectra obtained by the excitation at 384 nm, it was observed that the complexes [Eu(sal)₃(phen)] and [Eu_0.5Yb_0.5(sal)₃(phen)] show the characteristic narrow emission peaks corresponding to the ⁵D₀ – ⁷F_j (J = 0–4) transitions of Eu³⁺ ions. The emission peaks are well resolved and it was observed that the peak at 578 nm and 650 nm were weak since their corresponding transitions ⁵D₀ – ⁷F₀ and ⁵D₀ – ⁷F₃ were forbidden in magnetic and electric dipole fields . The peak at 591 nm is relatively strong and corresponds to ⁵D₀ – ⁷F₁ magnetic transition, the strongest emission observed at 614 nm (⁵D₀ – ⁷F₂) is an induced electric dipole transition sensitive to the coordination environment of Eu³⁺ ion. This transition was responsible for the red emission of the europium complexes. The relative intensity of peaks are stronger in complex [Eu(sal)₃(phen)] as compared to complex [Eu_0.5Yb_0.5(sal)₃(phen)]. The decrease in intensity of all the emission peaks in complex can be considered due to the energy transfer from Eu³⁺ ions to Yb³⁺ ions in the complexes. The emission spectrum of complex [Yb(sal)₃(phen)] which exhibits near infrared emission peaks at 980 and 1030 nm associated with the transition of Yb³⁺ from ²F_5/2 level to ²F_7/2 energy level. The decay curve shows the lifetime quenching of Eu³⁺ emission due to the energy transfer from Eu³⁺ to Yb³⁺ with mean lifetime of 461 ps (Figs. 11.9, 11.10, and 11.11).

Fig. 11.9

Room temperature photoluminescent emission spectra of [Eu(sal)₃(phen)] and [Eu_0.5Yb_0.5(sal)₃(phen)] complexes excited at 384 nm [82]

Fig. 11.10

Room temperature photoluminescent emission spectra of [Yb(sal)₃(phen)] excited at 384 nm [82]

Fig. 11.11

Decay curve of Eu³⁺, ⁵D₀ excited state (excitation wavelength: 384 nm; emission wavelength: 614 nm) of [Eu_0.5Yb_0.5 (sal)₃(phen)] complex [82]

The above results show the study of excitation, emission and decay measurements of [Eu_0.5Yb_0.5 (sal)₃(phen)] complex which reveals the efficient energy transfer from Eu³⁺ to Yb³⁺ by cooperative energy transfer process, which leads to Eu³⁺ emission quenching and occurrence of near infrared emission at 980 and 1030 nm.

11.3.4 Photoluminescence Quantum Yield and Radiative Lifetime

The most important feature of luminescent lanthanide complexes is the photoluminescence quantum yield (PLQY). It is defined as the ratio of number of photons emitted with that of number of photons absorbed when excited at a particular wavelength. The quantum yield of lanthanide complexes involves several factors such as ligand to Ln(III) energy transfer, multiphonon-relaxation, energy back transfer, crossover to charge transfer states etc. [83] Also, there are several competing processes such as fluorescence of antenna (competes with intersystem crossing), quenching of triplet state by dissolved molecular oxygen in case of NIR emitting ions (competes with energy transfer to the lanthanide ion) and the presence of solvent or water molecules which lowers the value of photoluminescence quantum yield. Hence, the overall quantum yield of sensitized emission (Φ_S) is the product of intersystem crossing quantum yield (Φ_ISC), the energy transfer quantum yield (Φ_ET) and the lanthanide luminescence quantum yield (Φ_Lum)

$$ {\boldsymbol{\Phi}}_{\mathbf{S}} = {\boldsymbol{\Phi}}_{\mathbf{ISC}} \ast {\boldsymbol{\Phi}}_{\mathbf{ET}}\ast {\boldsymbol{\Phi}}_{\mathbf{Lum}} $$

(11.2)

The overall quantum yield of the lanthanide complexes with organic ligands can be calculated as:

$$ {\varPhi}_{\boldsymbol{L}\boldsymbol{n}}^{\boldsymbol{L}}={\boldsymbol{\upeta}}_{\mathbf{sens}}.{\boldsymbol{\varPhi}}_{\boldsymbol{L}\boldsymbol{n}}^{\boldsymbol{L}\boldsymbol{n}}={\boldsymbol{\upeta}}_{\mathbf{sens}}\;.\left(\frac{\boldsymbol{Krad}}{\boldsymbol{Kobs}}\right)={\boldsymbol{\upeta}}_{\mathbf{sens}}\;.\left(\frac{\boldsymbol{\tau} \boldsymbol{rad}}{\boldsymbol{\tau} \boldsymbol{obs}}\right) $$

(11.3)

where ${\Phi}_{\mathrm{L}\mathrm{n}}^{\mathrm{L}}$ and ${\varPhi}_{Ln}^{Ln}$ represent the quantum yield resulting from indirect and direct excitation respectively and η_sens is the quantum efficiency with which the electromagnetic energy if transferred from the organic ligands to the lanthanide ion. K_rad is the radiative rate constant, K_obs is the sum total of rates of various deactivation processes and τ_rad is the radiative lifetime. The intrinsic quantum yield ${\Phi}_{\mathrm{Ln}}^{\mathrm{Ln}}$ can be calculated as:

$$ {\varPhi}_{\mathbf{Ln}}^{\mathbf{Ln}}={\mathbf{K}}_{\mathbf{rad}}/\;\left({\mathbf{K}}_{\mathbf{rad}}+{\mathbf{K}}_{\mathbf{nr}}\right) $$

(11.4)

K_nr represents nonradiative rate constant which can be attributed to the back energy transfer to the sensitizer, quenching by matrix vibrations or by electron transfer mainly for the lanthanides having low reduction potential such as Eu(III) and Yb(III) ions [84]. The intrinsic quantum yield can be determined by two proposed mechanisms:

• (a) The rapid diffusion enhanced resonance energy transfer after mixing the lanthanide complexes with a known quantum yield acceptor in solution and the efficiency of energy transfer between them can be calculated from both lifetime and intensity parameters.

• (b) This mechanism is valid only for europium and relies on the fact that the intensity of purely magnetic dipolar transition ⁵D₀ → ⁷F₁ transition is independent of the chemical environment of the lanthanide ion and the radiative lifetime can be calculated from the emission spectrum. The measurement of absolute quantum yield is a sophisticated task, it can be calculated by determination of relative quantum yield with that of standard sample of known absolute quantum yield. The absolute quantum yield of lanthanide complexes in solution can be calculated as:

  $$ {\varPhi}_{\mathbf{S}}={\Phi}_{\mathbf{S}\mathbf{T}}\;\left({\mathbf{I}}_{\mathbf{S}}/\;{\mathbf{I}}_{\mathbf{S}\mathbf{T}}\right)\left(\mathbf{R}{\boldsymbol{I}}_{\boldsymbol{S}}^2/\;\mathbf{R}{\boldsymbol{I}}_{\boldsymbol{S}\boldsymbol{T}}^2\right)\left({\mathbf{F}}_{\mathbf{S}\mathbf{T}}/\;{\mathbf{F}}_{\mathbf{S}}\right) $$

  (11.5)

where Ф_S represents the absolute quantum yield of sample, Ф_ST is the absolute quantum yield of reference compound, RI_S and RI_ST are the refractive index of sample solvent and reference compound respectively and represents the fraction of light absorbed by reference and sample. The accurate determination of quantum yields includes a number of factors e.g. internal filter effects, self-quenching and reliability of the standard value [85]. The other important parameter termed as luminescence lifetime can be defined as the time which an electron spent in excited state after excitation. It can be calculated as:

$$ {\boldsymbol{\uptau}}_{\mathbf{obs}}=1/\;{\mathbf{K}}_{\mathbf{obs}} $$

(11.6)

The radiative rate (K_rad) and non-radiative rate (K_nr) can be calculated as:

$$ {\mathbf{K}}_{\mathbf{rad}}=\boldsymbol{\Phi} /\boldsymbol{\uptau} $$

(11.7)

$$ 1/\boldsymbol{\uptau} ={\mathbf{K}}_{\mathbf{rad}}+{\mathbf{K}}_{\mathbf{nr}} $$

(11.8)

The overall absorbance of the lanthanide complexes is increased by the direct and indirect attachment of the chromophores to the lanthanide ion. The energy transfer process from the ligand to the metal ion is explained by the Forster and Dexter mechanism for direct chromophore attachment, while in case of indirect attachment of chromophore, the energy transfer proceeds through Forster dipole-dipole mechanism . The separation between two chromophores can be accessible through the determination of energy transfer efficiency (η_et) and can be calculated as:

$$ {\boldsymbol{\upeta}}_{\mathbf{et}}=1 - \left({\boldsymbol{\uptau}}_{\mathbf{obs}}/\;{\boldsymbol{\uptau}}_0\right)={\mathbf{K}}_0/\;{\mathbf{K}}_{\mathbf{obs}}=1/{\left(1+{\mathbf{R}}_{\mathbf{DA}}/\;{\mathbf{R}}_0\right)}^6 $$

(11.9)

where τ_obs and τ₀ are the lifetime of donor chromophore in presence and absence of acceptor chromophore respectively, K ₀ and K_obs are the decay rates of acceptor without and with donor respectively. R_DA and R₀ are the distance between donor-acceptor and critical distance respectively. The critical distance depends on the quantum yield of donor Q_D without acceptor, refractive index n of the medium, the overlap integral J_ov between the emission spectrum of donor and absorption spectrum of acceptor and orientation factor κ having isotropic limit of 2/3 and can be calculated as:

$$ {\mathbf{R}}_{06}=8.75\times {10}^{-25}\left({\boldsymbol{\upkappa}}^2.\;{\mathbf{J}}_{\mathbf{ov}}.\;{\mathbf{Q}}_{\mathbf{D}}.{\boldsymbol{n}}^{-4}\right) $$

(11.10)

The Judd-Ofelt analysis is a useful technique to estimate the population of odd-parity electron transitions for Eu(III) complexes. The interaction parameters of the ligand fields can be calculated from the Judd-Ofelt parameter, Ω_λ. The parameter Ω₂ is more sensitive to the symmetry and sequence fields and for anti-symmetrical Eu(III) complexes Ω₂ is large for faster radiative rates. The experimental intensity parameters Ω_λ where λ = 2 and 4, can be determined from the emission spectrum of Eu(III) complexes based on ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transitions where the ⁵D₀ → ⁷F₁ magnetic dipole-allowed transition is taken as reference and can be estimated as:

$$ {\mathbf{A}}_{\mathbf{RAD}}=\left(4{\mathbf{e}}^22{\boldsymbol{\upomega}}^3\;/\;3\mathbf{\hbar}{\mathbf{c}}^3\right)\;\boldsymbol{\upchi} {\sum}_{\uplambda}\;{\boldsymbol{\Omega}}_{\uplambda}<{}^7{\mathbf{F}}_{\mathbf{J}}\;\left|\;{\mathbf{U}}^{\left(\boldsymbol{\uplambda} \right)}\kern0.24em \right|{}^5{\mathbf{D}}_0{>}^2\;.\;\left(1/2\mathbf{J}+1\right) $$

(11.11)

A_RAD is the correspondent coefficient of spontaneous emission , e is the charge on electron, ω is the angular frequency of the transition, ђ is the Planck’s constant over 2л, c is velocity of light and χ is the Lorentz local field correction term which is given by n(n² + 2)²/9 with a refraction index n = 1.43 and < ⁷F_J | U ^(λ) |⁵D₀>² are the squared reduced matrix elements with values of 0.0032 and 0.0023 for J = 2 and 4 respectively. The Ω₆ parameter is difficult to determine as ⁵D₀ → ⁷F₆ transition cannot be experimentally detected in most of Eu(III) complexes.

11.4 Lanthanide Doped Upconversion Nanoparticles

Lanthanide–doped upconversion nanoparticles (UCNPs) are mainly composed of three key component s: a host matrix, sensitizer and activator. Generally, these UCNPs are fabricated by doping the trivalent lanthanides which have metastable excited states into an inorganic crystalline host lattice that can host these dopant ions. In case of lanthanide doped UCNPs, the sensitizer absorbs photons at 980 nm due to which it is excited to the higher energy state. The activator ions now obtain energy from the sensitizers to reach their corresponding excited states and show emission at short wavelengths (Fig. 11.12).

Fig. 11.12

The energy transfer process in up-conversion nanoparticles

The selection of the host materials is important as it determines the lattice between the dopant ions, relative spatial position, coordination numbers and the type of anions surrounding the dopant. Several conditions need to be fulfilled while choosing the host lattice as reviewed by Wang and Liu (2009) [86, 87] and Ong et.al. (2010) [88] are (i) close lattice matches to dopant ions; (ii) low phonon vibration energies and (iii) good chemical stability. The host lattice must be able to accommodate the lanthanide dopant ions as it largely affects the luminescence output and emission intensity ratios of different transitions. The phonon energy of the host lattice must be low to ensure homogeneous doping and minimizing the lattice stress and non-radiative energy loss. The fluoride compounds serve as best host lattices due to their low phonon energy (350 cm^-1) and high chemical stability. These compounds are LaF₄, YF₄, NaYF₄ and BaYF₄ which are widely used as optimal host materials. The longer lifetimes are observed for the excited states of fluorides lattices due to the low phonon energies of the crystal lattice [89]. The lattice impurities increase the multi-phonon relaxation rates between the metastable states, thereby reducing the overall visible emission intensity. The halogenides also have small radiative losses but these materials have low chemical stability. The oxides have high chemical stability but their phonon energy is high (>500 cm⁻¹) due to stretching vibration of host lattice. Therefore fluorides prove to be ideal host candidates for the green and blue upconversion phosphors.

The upconverting luminescence efficiency depends upon nature of absorbers/emitters and the doping concentration of absorbers/emitters. The luminescence efficiency is highest when the doping concentration of absorbers/emitters shows well matching energy level. The improper absorber/emitter doping concentration ratio results concentration related cross-relaxation quenching or disables energy transfer. Among the lanthanide ions, Yb³⁺ ions are frequently used as absorbers due to its simple energy levels ²F_5/2 and ²F_7/2 and high absorption coefficient at 980 nm and are used in relatively high doping ratios (20–30 %) [90]. Some other lanthanide ions like Er³⁺, Tm³⁺ and Ho³⁺ have ladder-like energy levels and are often used as activators with low doping concentrations. The other lanthanide ions (Y³⁺, La³⁺, Gd³⁺, Sc³⁺), transition metal ions (Zr⁴⁺, Ti⁴⁺) and alkaline earth ions (Ca²⁺, Sr²⁺, Ba²⁺) are also suitable ions that can be used. The most practical hosts are the halides (NaYF₄, YF₃, LaF₃), oxides (Y₂O₃, ZrO₂) and oxysulphides (Y₂O₂S, La₂O₂S) due to their low phonon-energy lattice. These hosts reduce multi-phonon relaxation and increase the lifetime of the intermediate states involved in upconversion process [91]. Thus to overcome the limitations of conventional fluoroprobes the lanthanide doped hexagonal phase NaYF₄ upconverting materials are appropriate substituents due to their superior upconverting properties.

The nano lanthanide doped oxide hosts are extensively reported with regard to Y₂O₃ as an important host material. It has broad transparency range (0.2–8 nm) with a band gap of 5.6 eV, high refractive index, better thermal conductivity and low phonon energy that makes it an attractive choice as the host material. The mostly used lanthanide host-dopant systems are listed in Table 11.2.

Table 11.2 Representative UCNPs with different host-dopant systems, excitation wavelengths and emission peaks [92–97]

11.4.1 Mechanism of Upconversion

The conventional fluorophores exhibit the phenomenon of downconversion, i.e. higher energy photons are absorbed while lower energy ones are emitted due to energy loss (IEL). Compared with downconversion, upconversion is a process that causes the emission of higher energy photons through sequential absorption of lower energy photons. The mechanism of upconversion process has been extensively studied and is generally divided into three steps: excited state absorption (ESA), energy transfer upconversion (ETU) and photon avalanche (PA). In comparison with the other two processes, ETU has been widely employed to obtain high upconversion efficiency (emission density versus NIR excitation power), involving the absorption of a pump phonon of the same energy by each of the two neighboring ions. A subsequent non-radiative energy transfer promotes one of the ions to an upper energy level (EL2) while the other ion relaxes back to the ground state (GS). The relaxation from EL2 results in the emission of higher energy photons [98] (Fig. 11.13).

Fig. 11.13

Illustration of (a) downconversion and (b) energy transfer upconversion mechanism. IEL internal energy loss, GS ground state, EL energy level, NRET nonradiative energy transfer, hυl incident light, hυ2 emission light [98]

11.4.2 Enhancing the Up Conversion Efficiency

The lanthanide–doped nanoparticles have great potential as lasers, phosphors and recently biolabels as well. The possibility to enhance the up-conversion luminescence has always been an interesting research topic. These nanoluminophores act as efficient biolabels when compared to organic dyes and quantum dots due to their excellent photostability, efficient near infrared to visible and long luminescence lifetimes suitable for time delayed detection. There is a need to enhance the up-conversion efficiency as these UCNPs suffer from low brightness when compared to conventional biolabels. There are certain ways to modify the upconversion efficiency which includes the dopant concentration, modifying the local chemical and structural environment, controlling the distribution of active ions in host material, varying the composition, structure and morphology of the up-converting nanoparticles. These factors reduce the quenching effect, enhance the absorption of NIR radiation or improve the sensitizer to activator energy transfer.

11.4.2.1 Concentration of Optically Active Ions

The up-conversion quantum yield can be enhanced by increasing the concentration of optically active ions within a single nanoparticle as the upconversion in singly doped NPs is very weak, therefore it is necessary to co-doping the activator ions with the sensitizer ions. The enhanced up-conversion in doubly doped material was first studied by F. Auzel in 1966 [99]. Yb³⁺ ion is most frequently used sensitizing lanthanide ion due to its simple energy levels, large absorption cross section at 940–990 nm in NIR region and large energy gap (10,000 cm⁻¹) and long luminescence lifetimes (1 ms). Yb³⁺ is usually chosen as it absorbes in NIR region and further capable of transferring its energy to the activator ions such as Tm³⁺, Er³⁺ in the energy transfer up-conversion process which requires a single activator ions surrounded by atleast two or three Yb³⁺ ions.

The increase in the activator ions concentration results in the decrease in the effective distance between the ions and when the energy levels are quasi-resonant, the excited ions become non-radiatively depopulated. The concentration quenching manifests itself in shortening the luminescence lifetimes as well as in decreasing the effective luminescence quantum yield enhancement despite the increase in luminescence centres. Therefore the concentration of the activator ions must be carefully optimized. The NIR PL emission from the ultrasmall NaYF₄ : 2 % Tm³⁺ nanocrystals to be 3.6 times more intense than the one from conventional 25–30 nm sized NaYF₄ : 20 % Yb³⁺ / Tm³⁺ nanocrystals. These Yb³⁺ / Tm³⁺ doped UCNPs are particularly attractive due to highly efficient NIR-to-NIR conversion i.e. 980 nm excitation to 800 nm up conversion emission [100].

11.4.2.2 Selection of Host Material

The up conversion efficiency can be enhanced by the proper selection of the host material for lanthanide ion dopants. The low energy phonons cause low non-radiative and multiphonon losses and increase the luminescence lifetimes. Different hosts such as chlorides, bromides and iodides with phonon energies of ~144, 172 and 260 cm⁻¹ and low multiphonon relaxation rates are suitable for up-conversion process as compared to fluorides and oxides [101]. Concluding , we can say that the selection of the host material is a balance between phonon properties and excitation spectra intensity which makes the host material appropriate for energy up-conversion in lanthanide doped materials.

11.4.2.3 Impurities in Host Matrix

The up-conversion efficiency can be enhanced by co-doping passive or active impurities in an efficient way. In case of passive impurities, the doping ions induce distortions to the local symmetry around activators or dissociate the lanthanide clusters in the nanocrystals and leads to increase in the up-conversion efficiency. These impurities do not participate in energy transfer within the matrix. On the other hand, the active impurities modify the energy transfer rates within the system and enhance the up-conversion efficiency. The enhancement of up-conversion efficiency in singly Er³⁺ -doped YAlO₃ phosphor by exchanging 40 % of Y³⁺ ions by larger Gd³⁺ ions (0.1159, 0.1193 nm) resulting in expanding the host lattice and distorting the local symmetry as studied by Li et.al [102]. The co-doping of β-NaGdF₄ : Yb³⁺ /Er³⁺ nanoparticles with different amount of Li⁺ ions also showed a higher up conversion enhancement as studied by Cheng et.al [103]. The green and red up-conversion emission yield intensities of the nanoparticles co-doped with 7 mol % Li⁺ ions were enhanced 47 and 23 times respectively. The alkali ions also have an impact on the structure and spectral properties of up-converting fluoride nanocrystals. The phase, size, shape and stability of the Yb/Er co-doped NaYF₄ also get affected by the Li⁺ and K⁺ content, the green to blue ratio varied in the range ~2–6.5 and ~0.7–1.7 respectively and rose to 20–80 %. The passive impurities affect activator distribution within the UCNP, the active impurities also play a critical role in the energy transfer. This approach leads to the modification of energy distribution as well as the interaction between respective excited energy levels of the co-doping lanthanides. The improvement in the up-conversion quantum yield by the co-doping of Tm³⁺ and Er³⁺ activators with the Yb³⁺ sensitizers serves as an important example.

11.5 Synthesis of Lanthanide Doped UCNPs

The various chemical approaches for the synthesis of Ln-doped UCNPs have been studied by Wang and Liu in 2009 [104], these methods such as co-precipitation, thermal decomposition, sol-gel method and hydro thermal method are discussed as:

11.5.1 Co-precipitation Method

The co-precipitation method is simple as it does not require costly set up, complex procedures and severe reaction conditions. In this method, the nanoparticle growth can be controlled and stabilized by adding capping ligands such as polyvinylpyrrolidone (PVP), polyethylenimine (PEI) and ethylene diaminetetraacetic acid (EDTA) into the solvent. This method generally yields cubic phase NaYF₄ : Yb, Er which is not an efficient up-convertor, subsequent calcinations at high temperatures results in sharpened crystal structure or partial phase transfer to hexagonal –phase NaYF₄ : Yb, Er which has higher upconversion efficiency as studied by Yi et.al [105].

11.5.2 Thermal Decomposition Method

This method involves dissolving of organic precursors in high boiling point solvents (e.g. oleic acid (OA), oleylamine (OM), octadecene(ODE) for the synthesis of highly monodispersed UCNPs. The rare earth trifluoroacetates are thermolyzed in the presence of high boiling point solvents at a temperature usually exceeding 300 °C. Mai et.al [106]. synthesized Er³⁺, Yb³⁺ and Tm³⁺, Yb³⁺ doped monodispersed cubic-phase and hexagonal-phase NaYF₄ nanocrystals by the thermal decomposition of trifluoroacetate precursors in OA/OM/ODE solvents and OA/ODE solvents respectively and also synthesized NaYF₄ based UCNPs with different luminescent properties with similar synthetic methods. Yi and Chow [107] obtained hexagonal-phase NaYF₄ :Yb, Er and NaYF₄ :Yb, Tm nanoparticles with an average particle size of 10.5 nm by decomposing the precursors of Na(CF₃COO), Y(CF₃COO)₃, Yb(CF₃COO)₃ and Er(CF₃COO)₃/Tm(CF₃COO)₃ in OM solvent at 300 °C with much higher up-conversion fluorescence intensity. This method however, has some disadvantages such as use of expensive and air-sensitiv e metal precursors and the generation of toxic by-products.

11.5.3 Sol Gel Method

The sol-gel method is mainly based on the hydrolysis and polycondensation of metal alkoxides or metal acetate precursors to form extended networks with an oxide skeleton. Li et.al [108]. studied the preparation of metal oxide based Ln-doped UCNPs such as YV0₄:Yb, Er, Lu₃Ga₅O₁₂: Er, BaTiO₃: Er and ZrO₂:Er by the sol-gel method. Though this method is good for the synthesis of various Ln-doped UCNPs, however there are certain limitations such as particle size and particle aggregation may occur when dispersed in aqueous solution. Some post treatments are often needed to improve the crystalline phase purity for enhanced luminescence efficiency.

11.5.4 Hydrothermal Method

The synthesis of highly crystalline nanocrystals with tunable size, morphology, optical and magnetic properties via controlled reaction temperature, time, concentration, pH and precursors etc can be done by hydrothermal method. This method improves the solubility of solids under hydrothermal conditions, which accelerates the reactions between solids. The advantage of this method lies in the fact that it requires “one-pot process” with heat resistant polymer (PEI, PVP) added to the solvent. Wang et.al [109]. studied the preparation of uniform-sized nanoparticles with appropriate surface modification which were obtained by single reaction process. The only limitation with the hydrothermal method is the difficulty in observing the nanocrystals growth process.

11.6 Conclusion

Recently the luminescent lanthanide complexes are receiving the attention of scientific fraternity due to their high luminous lifetimes and extremely sharp emission bands arising from the electronic transitions between the 4f energy levels. The lanthanide complexes are widely used in many technologically interesting fields such as photoluminescent materials in display devices, fluorescence probes and labels in biological systems. In order to design the efficient luminescent fluorophores, it is necessary to understand the electronic and spectroscopic properties of rare earth elements, basic principles of photoluminescence which includes the basic concepts of antenna effect, coordination features of Ln³⁺ ions, radiative lifetime and photoluminescence quantum yield of lanthanide complexes. Apart from the luminescent lanthanide complexes, the lanthanide up-converting nanoparticles possess great potential as novel fluorophores for biological applications. These exhibit unique fluorescent property providing tremendous applications which have enormous advantages over conventional fluorophores. This review insights into the main aspects related to design of luminescent lanthanide complexes and lanthanide up-converting nanoparticles and also gives an overview of our recent work on photoluminescent properties of lanthanide complexes with nitrogen donor and oxygen donor ligands. The synthesis and photoluminescence properties of europium and ytterbium complexes with 2-aminopyridine, 1,10-phenanthroline, dipicolinic acid, α-picolinic acid, salicylic acid and 2-hydroxypyridine have been reported. The prepared complexes can act as good candidates for fluorescence probes and optoelectronic devices.