Though lanthanoids have been discovered for more than a century, their significance has only been recently discerned. Lanthanoids are used mainly for their electrochemical, optical and magnetic properties derived from their unique f orbitals. There is a plethora of applications based on all these properties, which are dependent on the specific lanthanoid element used.
There are already a vast amount of applications available just based on the lanthanoid luminescence properties alone. In fact, most of the lanthanoid elements are able to emit light provided that they are complexed with a suitable ligand, which will be discussed in further detail in Section 1.4. However, the practical application of such systems involves only specific lanthanoid elements for now.
Some lanthanoid metals, such as europium and terbium, are used for sensory probes with applications in clinical, biological and environmental studies.149 This normally involves a specific lanthanide complex that exhibits a particular sensitivity and selectivity for a particular species, which could then be detected and analysed by luminescence. An example of such application is the antibodies and antigens labels in fluoroimmunoassays.163,164 Besides this, europium and terbium phosphors share the largest commercial market in colour displays (e.g. televisions) for their red, green and blue colours.165,166 Similarly, these lanthanide metals often gain interest for their potential in lighting applications and even security measures in banknotes, documents, visa, passports, identity cards, labels, etc. (luminescent under UV irradiation).167–169 Several lanthanide ions can be used in the application of lasers (light amplification of stimulated emission of radiation),170–173 each operating at their specific frequency range. Among all, the most common is the neodymium laser, which normally involves using Nd3+ ions in yttrium aluminium garnet (YAG; Y3Al5O12).174–176 Also known as the ‘four level’ laser, it functions by a ‘population
inversion’, where the details of the process are explained in this literature.149
Another known use of neodymium, as well as erbium and ytterbium, is their application in telecommunications.177,178 These near infrared (IR) emitting lanthanoids, especially ytterbium with its broad emission around 1000 nm, are also known for their role in solar cells due to having a strong matching spectral response to the c-Si solar cell.179 The other huge area for lanthanoid application is due to their remarkable magnetic properties. They are mainly employed as additives to magnets endowing them to resist demagnetisation at extreme temperatures.180,181 Their high magnetic coercivity can be explained from their strong uniaxial magnetic anisotropy whereby the magnetism within the crystal lattice preferentially orientates along a specific axis pointing in the same direction. Some of the lanthanoid metals can have very high
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magnetic moments due to their incomplete filling of the 4f orbitals resulting in many unpaired electrons up to a maximum of seven (4f7). As these electrons spin in the same direction without any opposite spins to oppose them, they all contribute in generating the strong magnetic field. These lanthanoid permanent magnets, also known as ‘rare earth magnets’, are used in a wide variety of applications, such as in technologies involving renewable energy (e.g. wind turbines, hydropower, wave power, underwater current power, solar updraft tower, geothermal drilling, heat pumps etc.) and electrical generators (e.g. car alternators, jet ignition, tachometers etc.), electrical motors (e.g. aerospace gyros, automobile starters, computer peripherals, cryo-coolers, clocks, textile spinners etc.), electro mechanical actuators (e.g. aircraft flight control, computer printers, industrial robots etc.), electro acoustics (e.g. earphones, microphones, loudspeaker etc.), transducers (e.g. accelerometers and other sensors), measuring instruments (e.g. balances, galvanometers etc.) and electrical switches (e.g. automotive ignition, thermostats, relays etc.).180
Rare earth (RE) magnets are constituted from an alloy of at least one lanthanoid element and one 3d transition metal.180 The RE component is primarily neodymium or samarium, while the 3d transition metal is iron and/or cobalt. Generally, they are separated into two categories which are the RE-iron-boron and RE-cobalt alloys.
Both of these alloys have similar traits but offers different advantages and disadvantages.180 Though the RE-cobalt magnets perform better at higher temperatures, they are brittle causing size limitations and complications in integrating them into certain applications (e.g. motors).182 They are normally utilised in smaller, higher temperature applications such as microwave tubes. The RE-iron- boron magnets are not restricted to their size, thus are suitable for much larger applications such as electrical generators.
These RE magnets are often incorporated with other heavier lanthanoid elements, gadolinium through erbium, to improve their temperature stability but could also hinder their magnetic potentials, thus are normally added in small quantities.180 Other interstitial elements such as boron, copper, hafnium, nitrogen, titanium, silicon or zirconium can also be added to improve its mechanical properties.180,181,183
Nuclear Magnetic Resonance (NMR) shift agents, often referred as Lanthanoid Shift Reagents (LSRs), make use of paramagnetic lanthanoid complexes, such as europium and ytterbium complexes, to increase the structural information displayed in the spectra of organic molecules.149,184–186 This aids in gaining information on similar hydrogen peaks to display signal splitting instead of appearing as a singlet peak.184 Also, it helps to differentiate the enantiomers in a racemic mixture and estimate the yield of each isomer.149 This is done by forming diasteroisomeric complexes with the lanthanoids and identifying the set of peaks associated with each which could then be integrated to find the yield. This methodology was more prevalent in the past during the early 1970s as high frequency spectrometers were not common then.
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Gadolinium complexes are known for their potential in paramagnetic contrast agents for magnetic resonance imaging (MRI).187–192 They take advantage of their large number of unpaired electrons (4f7), enhanced signal intensity, isotropic magnetic properties and relatively long electron spin relaxation time (ca. 10-9 s) that is more ideal than other paramagnetic lanthanoid ions like europium(III), dysprosium(II) and ytterbium(III) (ca. 10-13 s). The results of this non-invasive technique are the improved sensitivity and quality of the MRI images that are generally used in biological studies. The drawback of the toxicity of the free gadolinium(III) ion can be easily overcome by using carefully designed ligands to form complexes with high stability constant.149,187,192 This helps to minimise the quantity of free gadolinium(III) ions in the system. Such complexes, like gadolinium texaphyrin, could also prove usefulness in photoangioplasty and photodynamic therapy for cancer cells due to their effectiveness in being a radiation sensitizer.149 Another application is the electron paramagnetic resonance (EPR) spectroscopy which takes advantage of the electronic properties of gadolinium(III) having the 4f7 configuration enabling the spectra to be obtained at room temperature.149,187
Another area for lanthanoid application is in its therapeutical medicinal value as a metal based drug.193–195 Specific lanthanoid elements can be used as agonist, antagonist or even biological probes to sites of other metal ion with similar ionic radii, such as calcium.193,195 As lanthanoid ions generally have a higher charge (+3), they exhibit stronger affinity to those sites (Ca2+).193,195 Depending on the enzyme system and lanthanoid element used, their effects can activate the receptor (agonist), partially activate the receptor (mixed agonist/antagonist), or does not activate and inhibit its function from other agonist (antagonist).193 Thus far, their applications are used in anticancer, anticarcinogenic, antidiabetic, antimicrobial, cardiovascular diseases, hypercalcemia, hyperphosphatemia, treatment for inflammations and burns etc.193–195
There has been a recent significant increase in the demand for a number of rare earth elements due to future developments and advances in the applications of modern technology.196 Additionally, the price on these rare earth metals has been increasing significantly and is mainly monopolised by China which is the dominant source of the raw material.197 Hence, there are efforts being placed to recycle these rare earth elements to help with this foreseen requirement in supply.