Recent Advances in the Green Synthesis of Lanthanide-Based Organic Compounds for Broad Application Spectrum in Different Sectors: A Review

*Kamna Chaturvedi, Deeksha Malvi, Manish Dhangar, Harsh Bajpai, Ranjan K. Mohapatra, Avanish Kumar Srivastava and Sarika Verma*

#### **Abstract**

The present review highlights the various green method of synthesis and discrete applications of inner transition compounds. Green chemistry's strategies are developing, producing, and using effective, reliable, and eco-friendly chemical products and processes to manage pollution. In this review, the greener or environmentally sound route for synthesizing lanthanide compounds is discussed briefly. The initial section briefs the fundamental principles of greener chemistry. It further emphasizes in-depth studies of synthesis of the different lanthanide-based complexes and their applications in different dimensions. It includes Green Synthesis of (a) lanthanidedoped nanophosphors, (b) rare-earth zirconates, (c) metal oxide nanoparticles, (d) rare-earth ions-doped nanocrystals-based photoluminescent materials, (e) selfassembled nanospherical dysprosium MOFs, and (f) nucleotide-based lanthanide coordination polymers. The last section dedicatedly reports the scope for the future perspective and recommendation in the novel area of research.

**Keywords:** green synthesis, rare earths, nanomaterials, Photoluminescent, MOFs

#### **1. Introduction**

The growing awareness for a thriving environment and the integrity of the industrial applications have paved the way for the emergence of green chemistry as a distinct subject. Years ago, the industrial sector started on a significant scale. Ciamician, a highly foresighted researcher, recognized that it is now essential to produce chemicals remarkably similar to natural ones [1, 2]. Green synthesis has become increasingly important in modern chemistry. Green or sustainable chemistry's primary goal is to make valuable molecules and provide resources to humans while

allowing no environmental damage. Green chemistry contributes to the emergence of novel techniques to prevent ecological degradation by minimizing the number of toxic pollutants and their health effects by commencing with harmless or healthier substances than those now in use. This discipline is presented through a period of rapid advancement and growth. The progression of "green chemistry" has arisen as a surprising move for resolving this challenge and ensuring the environment's security [3]. Green chemistry includes a diverse catalyst, biocatalysis, solvent-free production, and the utilization of microwave and ultrasound, among other things.

Green chemistry has been characterized in a variety of ways by various institutions. According to IUPAC, green chemistry is described as "the discovery, configuration, & application of chemical products and minimizes possible use and generation of toxic chemicals." The Environmental Protection Agency (EPA) defines "chemistry for source reduction" as green chemistry. But, The Organisation for Economic Co-operation and Development (OECD) explained green chemistry as "sustainable chemistry." These concepts of green chemistry appear to be unique in terms of how they are represented, but they all give rise to the same significance and primary objective. Green chemistry's strategies are developing, producing, and using effective, reliable, and eco-friendly chemical products and processes to manage pollution.

Further, the lanthanides belong to the f-block in the modern periodic table, with atomic numbers falling between 57 and 71 (La-Lu). The outermost electron or valence shell electron of lanthanides goes in a 4f subshell. They possess identical chemical and physical properties. The silvery-white soft metals experience various characteristics. With the increase in the atomic numbers, the hardness of the metals also increases due to the rise in their melting and boiling points. They are also paramagnetic. Lanthanides have many scientific and industrial applications. On heating with halogen, lanthanides exhibit fluorescence under UV lights. Its exposure to air gives rise to tarnishing because of oxidation, which is usually very quick in moist air. Lanthanides could burn quickly in the atmosphere and are also highly reactive. They typically react very steadily with O2 and H2O but quickly at high temperatures. Other similar properties of lanthanides include their characteristics as potent reducing agents, making ionic compounds rapidly dissolved in acids. Lanthanides and the different ligands are particularly favorable chemical compounds used in developing functional molecular substances. Lanthanides are an attractive essential matter for synthesizing several divalent or trivalent derivatives of lanthanides with organic compounds. Trivalent lanthanide cations act as "hard" acid, in which the bonding of lanthanides with another element is examined predominantly electrostatic and ionic. Therefore, their affinities toward "hard" ligands with oxygen donor atoms will be decisive. As a suitable fluorescent material, lanthanide-doped nanophosphors gained much interest for biological applications due to their inherent lower cytotoxicity.

The present review highlights the various green route of synthesis of lanthanidebased compounds synthesis and their discrete applications in different dimensions. The initial section gives a brief explanation of the fundamental greener synthesis of organic compounds provided by Paul Anastas & John Warner. It further emphasizes in-depth studies of other lanthanide-based complexes obtained by the green method of their synthesis. It includes green synthesis of a) lanthanide-doped nanophosphors, b) rare-earth zirconates, c) metal oxide nanoparticles, d) rare-earth ions-doped nanocrystals-based photoluminescent materials, e) self-assembled nanospherical dysprosium Metal organic frameworks (MOF), and f) nucleotide-based lanthanide coordination polymers. The last section dedicatedly reports the scope for the future perspective and recommendation in the novel area of research.

*Recent Advances in the Green Synthesis of Lanthanide-Based Organic Compounds for Broad… DOI: http://dx.doi.org/10.5772/intechopen.104716*

#### **2. Basic principles of greener chemistry**

In 1998, Paul Anastas and John Warner put forward the basic principles of green chemistry for the first time [4]. As reported, green chemistry mainly comprises 12 principles, which are widely accepted for explaining the most outstanding everemerging image of green chemistry [5, 6].

	- ix.**Minimize waste by avoiding stoichiometric reagents and using catalysts:** Using catalytic reactions should minimize waste production. Catalysts are more advantageous than stoichiometric reagents, used excessively and employed only once. The catalysts are employed in the least quantity and several times carry out a single reaction.
	- x.**Designing for degradation:** The chemical substances should be prepared to disintegrate the innocuous degradation of products conclusively after their use, and it may not present further in the surroundings.

#### **3. Synthesis of various lanthanide-based complexes with their applications in different dimensions**

#### **3.1 Green synthesis of lanthanide-doped nanophosphors**

Nanocrystal-based semiconductors are seemed to be a useful luminescent material for bio-fluorescence labeling and bio-imaging because of their quantum-sized effects where the absorbance starts, and the emissions shift toward the high energy with the decrease in size. These compounds are prepared under complex, expensive, and dangerous experimental conditions. Recently, studies indicated that they could affect the penetrability of the cell membrane and engender unwanted risky interactions with the human biological system [7]. As a result, the long-term uses of these compounds in the bio-related field have been limited. The higher increase in the recognition of the certainly unfavorable effects of nanomaterials on the well-being of humankind has been directed toward the lanthanide (ln)-doped nanophosphors of lower toxicity in the field of life science. The recent studies introduced a concept of an advanced perspective based on the microplasma approach for the fabrication of crystalline Ln-doped nanophosphors of advanced quality [8]. This approach is perhaps developed for the greener synthesis of different lanthanide-doped/co-doped nanophosphors because it possesses higher elasticity. In 2019, Lin [9] investigated the model of Eu3+-doped yttria for the synthesis of Y2O3:Eu3+ nanophosphors of different sizes and discrete Eu3+ concentrations of doping. The lanthanide (Ln = Tm, Dy, Tb, Eu)-doped nanophosphors are synthesized by the simple microplasma-assisted process. It is demonstrated that the ultra-high pure crystals of Y2O3:Eu3+ nanophosphors are prepared by aq. sol. of Ln(NO3)3·6H2O (Ln = Eu and Y) at meager plasma power consumption (near about 3–5.5 W), except for any involvement of dangerous chemicals. In addition to this, the trivalent Europium ions proved to be efficient, and they are also doped within the yttria matrix homogeneously. The green route's nanophosphors obtained by the electrolytic solution of Ln(NO3)3·6H2O shows the regulated photoluminescence properties.

The advanced microplasma-based method reduces the disturbing obstacles in the present situation of techniques for synthesizing lanthanide-doped nanophosphors. In contrast with the addition of surfactants, stabilizers, or solvents to control the in-homogeneous and vigorous hydrolysis reaction induced by the super-saturated alkali precipitants, this technique generally utilizes H2O as a "soft" source of "OH" for maintaining the milder conditions of hydrolysis [8]. Hence, it is concluded that heating can increase luminescence performance efficiency by regulating the proceeding conditions. Therefore, this method prevents the complex from the time-taking procedures of post-separation/purification and facilitates the complete synthesis system.

*Recent Advances in the Green Synthesis of Lanthanide-Based Organic Compounds for Broad… DOI: http://dx.doi.org/10.5772/intechopen.104716*

#### **3.2 Green synthesis of rare-earth Zirconates**

Rare-earth zirconates (Re2Zr2O7) have been extensively engaged in different areas of research like disposal of nuclear waste, catalyst, diesel engines, and thermalbarrier-coating compounds [10–14]. As yet, a different synthesis method appeared for the manufacturing of Re2Zr2O7. In 2012, Saradhi [15] synthesized the nanocrystalline Eu2Zr2O7 using dil. NH4OH as precipitant by co-precipitation method. In 2017, Zhang and co-workers used glycine as a propellant and ignited to prepare rare-earth zirconates Ni/Ln2Zr2O7 (Ln = Y, Sm, Pr, and La), used as catalysts [16]. Saitzek and co-workers [17] manufactured thin layer of lanthanum zirconate (Ln2Zr2O7) using a sol–gel technique. The other methods of synthesis of the zirconates of lanthanides are comprised of evaporation-induced self-assembly process, thermal impedance process, co-ions complexation, cathode plasma electrolysis, and solid-state reaction [18–20]. At present, the grain size, shape, and the rate of purity have been proved to be the influential factors for the specification of its discrete features and effectivity of the nano-compounds [21–23]. Thus, a substantial number of researchers have concentrated on the composition of the nano-compounds with controlling and modifying the required factors. In this study, Zinatloo-Ajabshir [24] synthesized Ln2Zr2O7 (where, Ln = Pr, Nd)-based ceramic nanostructures by tea leaf extract (green) as new and effective material by green and straightforward path. Remarkably, the effectiveness of Ln2Zr2O7 (Ln = Pr, Nd) was evaluated in the propane-SCR-NOx method. The study introduced the nanostructured Ln2Zr2O7 (Ln = Pr, Nd) as a potent novel variety of catalysts that are preferably efficient for the C3H8-SCR reaction of nitrogen oxides. The catalytic effectiveness of the produced specimen should be considered concerning its capacity of adsorption, specific surface area, crystallinity, and particle size [25]. The catalytic effectiveness of Pr2Zr2O7 for NOx reduction is preferably greater than Nd2Zr2O7 to such an extent that the transformation of nitrogen oxide to nitrogen for Pr2Zr2O7 is approximately 67% and for Nd2Zr2O7 is approximately 56%. The accumulation of carbon monooxide in the exit valve that is generally an unwanted by-product in the operation of SCR-nitrogen oxides-C3H8 for Pr2Zr2O7 is less than Nd2Zr2O7. Therefore, it can be said that Pr2Zr2O7 exhibits preferably greater effectiveness for transformation of NOx to N2.

#### **3.3 Greener synthesis of metal oxide nanoparticles**

MO-based lanthanides nanoparticles have recently achieved more attention because of their particular biological application and unique properties in various fields. At present, metal oxide synthesis demands a time-efficient, ecologically sound, and cost-effective process. The biological synthesis is a preferable alternative for the traditional chemical process of MO nanoparticles [26]. Biological synthesis is an environmentally sound green route using yeast, algae, fungi, and bacteria to synthesize MO nanoparticles. The biological manufacturing route brings forth a reliable, affordable, and less toxic method for synthesizing MO nanoparticles with differing composition, shape, size, and physico-chemical effects [27]. Tin oxide is a prominent semiconductor used biomedically among the different metal oxides. SnO2 has exceptional catalytic properties, a higher charge transfer rate, incredible binding energy, chemical and higher thermal stability, and excellent optical and electrochemical properties. In particular, metal/non-metal-doped SnO2 played a crucial in biomedical applications. S.A. Khan and co-workers suggested that cobalt-doped tin oxide exhibits more significant antimicrobic activity than un-doped SnO2 [28].

Likewise, K. K. Nair described silver-doped tin oxide as exhibiting more significant antibiotic activity than pure SnO2. Recently, biosynthesis is using jujube fruits [29], Ficus Carica [30], and Plectranthusamboinicus [31] for the preparation of SnO2. Solanum nigrum, known as Makoi or Black Nightshade (h) Kakamachi, is an everlasting shrub mainly appearing inside a timbered or forested region. In medicinal chemistry, Makoi/Nightshade has anti-hyperlipidemic, antioxidant, antiherpetic, and anti-inflammatory properties. This herbaceous plant substitutes anodyne, diaphoretics, diuretics, and expectorants. Here, the present study explains the preparation of Y2O3, SnO2, Sn-doped Y2O3, and Y-doped SnO2 and investigates the purpose of doping to increase antibacterial or antimicrobic motility of Y2O3 and SnO2. The outcomes illustrated that the extracted material drawn out from plants and doping is necessary for synthesizing metal nanoparticles and explaining their biomedical implementations (**Table 1)** [32].

#### **3.4 Greener synthesis of rare-earth ions-doped nanocrystals-based Photoluminescent substances**

Photoluminescent substances have been practised globally in the security and anticounterfeiting field because of their eccentric easy identification, excellent reliability, and difficulty with duplication [33, 34]. This paper introduces a simplistic, greener, and cost-efficient method to manufacture biomass composites that comprise cellulose fibers and Ln-doped nanocrystals for anti-counterfeiting applications [35–37]. The photoluminescent substances were synthesized using in situ Chemical Vapour Deposition (CVD) approach of trivalent lanthanides on the bleach hard-wood semi-fluid cellulose fiber (bhpFibers) plane by adding PVP (poly-vinyl-pyrrolidone) to couple the formation of the bhpFibers-PVP@LaF3:Eu3+ composite. In 2018, Qing Wang [38] synthesized the bhpFibers-PVP@LaF3:Eu3+ composite that possesses effective fluorescence. Its luminescence intensity could be easily managed by changing the incorporation of the moral quantity of trivalent lanthanum and europium ions in a solvent.

Moreover, the complex is utilized as a block to form photoluminescence paper through a vacuum filtration method. This synthesized paper shows well writable and printable properties, high flexibility, and good luminescence. Furthermore, the complete process is modest and free from toxic reagents. The bhpFibers-PVP@LaF3:Eu3+ composite exhibited balanced and robust luminescent property for 10–15 days and pH between 2 and 12. Different Ln-doped/fibers nanocrystal composites and various other trivalent lanthanides can be prepared based on this technique. Hence, this simple and greener approach for manufacturing the photoluminescence-active cellulose fibers retains excellent and promising applications in anti-counterfeiting material on a large scale. The detailed schematic representation is explained in **Figure 1**.

#### **3.5 Greener synthesis of self-assembled Nanospherical dysprosium metal: organic frameworks**

Metal–organic frameworks are an advanced category of permeable translucent substances prepared from the multi-dentate organic ligands and metal clusters [39]. An increase in the study of MOFs witnessed in recent times because of its possible applications in magnetism [40], nonlinear optics [41], heterogeneous catalysis [42, 43], gas storage [44], and separation [45], etc. Based on MOF, the sensing materials are predicted to be more favorable than the existing sensor probes because of their exposed active sites, tunable framework compositions, high surface areas, and ease of *Recent Advances in the Green Synthesis of Lanthanide-Based Organic Compounds for Broad… DOI: http://dx.doi.org/10.5772/intechopen.104716*


#### **Table 1.**

*Synthesis of ln-based nanoparticles by ethanolic extraction of solanum nigrum plant.*

**Figure 1.** *Diagrammatic illustration of the fabrication of bhpFibers-PVP@LaF3:Eu3+ [38].*

synthesis [46]. Implementation of MOFs in real-life applications is a significant cause in the present-day research. The main focus was to enhance features like selectivity, sensitivity, latent period, and durability of prepared MOFs to synthesize effective sensory platforms for authentic characteristics. In 2019, Mukherjee and co-workers [47] prepared two new trivalent dysprosium MOFs assimilating πelectron-donating azides performance in ligand vertebrae using the solvothermal method. An ecological and green method has been embraced to scale the prepared substance to nanotechnology. Formerly undetermined Dy (III) carrying nanospherical MOFs thus achieved are used to sense several –NO2-based incendiary devices in the solvent media through fluorescent quenching. On illustrating the potential appropriateness of nanoscale

MOFs for recognizing distinct –NO2-based incendiary devices, the current study also explained the excellent utility of annihilation constants known till date in the permeable substance realm. For attaining the high-performance sensory inquests for reliable and ecological applicative value, ad-libbing the concept of sensors into the vapor state seemed to show excellent outcomes. Hence, researchers considered that the outcomes conferred here obviously expedite a novel approach for the development of green organic–inorganic hybrid nanocomposites to design the new-generation sensory probes. This study illustrates the solvothermal-based preparation of two novel Ln-MOFs, named as [{Dy4(5N3-IPA)6(DMF)3(H2O)4}(DMF)(H2O)2]∞ and [{Dy(2N3-TPA)2(H2O)(CH3OH)}]∞ emanated by two RCOO− ligand named as 2N3- TPA (2-azidoterephthalate) and 5N3-IPA (5-azidoterephthalate) correspondingly, through confined azide components that are directed toward the pore surfaces having two-dimensional sheet-like arrangement with helical metal carboxylates chain. The two synthesized complexes show good aqueous phase and thermal stabilities. A convenient and straightforward method has been embraced to minimize the synthesized compounds' particle size for the preparation of Nanoscale metal organic frameworks (NMOF). These nanoscale metal–organic frameworks are strongly utilized for fluorescence analysis for the identification of several nitro-analytes like picric acid, nitro-toluene, 4-nitrobenzoic acid, 2, 6-dinitrotoluene, and nitrobenzene, etc. Furthermore, selectivity toward picric acid sensing with another elementally identical nitro-analytes and better recyclability of the operating sensing manifesto revealed excellent potential for these substances considering its authentic applications.

The crystal structure and coordination geometry of both compounds are explained in **Figures 2** and **3**.

#### **3.6 Green synthesis of nucleotide-based inner transition coordination polymers**

White-light-emitting substances gained immense recognition due to their applications in various practicable areas, like electrochemical cells, light-emitting diodes,

*Recent Advances in the Green Synthesis of Lanthanide-Based Organic Compounds for Broad… DOI: http://dx.doi.org/10.5772/intechopen.104716*

**Figure 3.** *Crystal structure and coordination geometry of [{Dy4(5N3-IPA)6(DMF)3(H2O)4}(DMF)(H2O)2]∞ [47].*

and several different varieties of light-emitting devices [48–51]. The advanced whitelight-emitting substances noted till today are predominantly concerted on metal– organic hybrid components [52–54], inorganic nanocrystals [55], quantum dots [56], and small-organic molecules [57]. In general, these white-light-emitting materials are achieved by the following three approaches: (1) one-constituent emitting in the entire Vis-region from 400 to 700 nm, (2) two-chromophores carrying constituent emitting orange, yellow or blue, and (3) three-chromophores carrying constituent emitting primary colors [58]. Comparing these three strategies, the outcomes described that the one-constituent white-light-emitting phosphors possess great benefits: lower production cost, higher luminescent efficiency, and higher color displaying index. At the same, the accretion of the one-component substances is efficient of white light fabrication is still difficult. Ln-based phosphors are seemed to be the realm of two-chromophores-assisted white-light-emitting substances during the past decades because of the supremacy of trivalent Ln ions, together with the low toxicity, high photochemical stability, long fluorescence lifetime, sharp emission, and significant Stokes shift.

Presently, the white-light-emitting diodes mainly depend on blue light InGaN chip, and yellow light from Ln-based inorganic materials of Y3Al5O12:Ce3+ (YAG:Ce) is accessible at a large scale [59]. Though, few defects are persisted because of the absence of red-light-emitting constituent, regulating its multifaceted applications. Currently, metal–organic coordination polymers of lanthanide fascinated much interest for white light administration. It is observed that LMOCP (Ln (III) metal–organic coordination polymers) are highly adaptable with the organic matrices for organic LED characteristics than nitride powders and inorganic metal oxides as organic– inorganic hybrid material [60]. Compared with the crystal MOFs, amorphous LMOCP exhibits wide composition diversity, mild reaction conditions, and high

