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[Mingjun Bai](https://orcid.org/0000-0002-9400-6045), [Hao Wan](https://orcid.org/0000-0002-4487-3538), Ying Zhang, Siqi Chen, Chunyin Lu, [Xiaohe Liu](https://orcid.org/0000-0003-1297-9597), Gen Chen, [Ning Zhang](https://orcid.org/0000-0002-3033-0276), [Renzhi Ma](https://orcid.org/0000-0001-7126-2006)

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[Two-dimensional nanomaterials based on rare earth elements for biomedical applications](https://mdr.nims.go.jp/datasets/28f34a75-656d-4b3d-a85b-dff553f08588)

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Two-dimensional nanomaterials based on rare earth elements for biomedical applicationsChemicalScienceREVIEWTwo-dimensionaaSchool of Materials Science and EngineeriChongqing 400054, P. R. ChinabZhongyuan Critical Metals Laboratory, ZheR. China. E-mail: wanhao@zzu.edu.cn; liuxcSchool of Materials Science and Engineeri410083, P. R. ChinadResearch Center for Materials NanoarchiMaterials Science (NIMS), Tsukuba, Ibaraknims.go.jpCite this: Chem. Sci., 2024, 15, 16887Received 20th April 2024Accepted 15th September 2024DOI: 10.1039/d4sc02625jrsc.li/chemical-science© 2024 The Author(s). Published byl nanomaterials based on rareearth elements for biomedical applicationsMingjun Bai, a HaoWan, *b Ying Zhang,b Siqi Chen,a Chunyin Lu,a Xiaohe Liu, *bGen Chen,c Ning Zhang c and Renzhi Ma *dAs a kind of star materials, two-dimensional (2D) nanomaterials have attracted tremendous attention fortheir unique structures, excellent performance and wide applications. In recent years, layered rare earth-based or doped nanomaterials have become a new important member of the 2D nanomaterial familyand have attracted significant interest, especially layered rare earth hydroxides (LREHs) and layered rareearth-doped perovskites with anion-exchangeability and exfoliative properties. In this review, wesystematically summarize the synthesis, exfoliation, fabrication and biomedical applications of 2D rareearth nanomaterials. Upon exfoliation, the LREHs and layered rare earth-doped perovskites can bedimensionally reduced to ultrathin nanosheets which feature high anisotropy and flexibility. Subsequentfabrication, especially superlattice assembly, enables rare earth nanomaterials with diverse compositionsand structures, which further optimizes or even creates new properties and thus expands the applicationfields. The latest progress in biomedical applications of the 2D rare earth-based or doped nanomaterialsand composites is also reviewed in detail, especially drug delivery and magnetic resonance imaging(MRI). Moreover, at the end of this review, we provide an outlook on the opportunities and challenges ofthe 2D rare earth-based or doped nanomaterials. We believe this review will promote increasing interestin 2D rare earth materials and provide more insight into the artificial design of other nanomaterials basedon rare earth elements for functional applications.1. IntroductionTwo-dimensional (2D) materials are dened as crystallinematerials of single- or few-layer atoms or molecules, in whichthe in-plane interatomic interactions are much stronger thanthose along the stacking direction.1 Aer the exfoliation oflayered graphite into graphene, the 2D materials have grown toa large family including the graphene family (e.g., graphene,graphyne, hexagonal boron nitride (hBN), black phosphorene(BP)),2–6 Xenes (e.g., borophene, phosphorene, silicene),7–10chalcogenides (e.g., MoS2, MoSe2, Sb2Te3),11–13 MXenes (e.g.,Ti3C2, Nb2C, Ta4C3),14–16 metal–organic frameworks (MOFs),17–19covalent organic frameworks (COFs),19–21 2D oxides (e.g., 2Dsemiconductor oxides, 2D perovskites),22–26 2D hydroxides (e.g.,layered double hydroxides (LDHs), and layered rare earthhydroxides (LREHs)).2,27–33 The explosive growth of the 2D familyng, Chongqing University of Technology,ngzhou University, Zhengzhou 450001, P.h@csu.edu.cnng, Central South University, Changshatectonics (MANA), National Institute fori 305-0044, Japan. E-mail: MA.Renzhi@the Royal Society of Chemistryoffers a huge opportunity for the design of crystal structures.The rare earth-based or doped nanomaterials contain a largecollection. This review mainly focuses on LREHs, layered rareearth-containing perovskites and their exfoliated ultrathinnanosheets due to their unique structures and fascinatingphysicochemical properties.Rare earth-based or doped 2D nanomaterials with versatileenergy levels of rare earth/lanthanide (Ln) ions and variousactive sites are widely applied in diverse elds such as lumi-nescence, sensors, catalysis, etc.34–39 Due to the spatial locali-zation of 4f electrons within the RE atoms, which induces anunquenched total angular momentum, rare earth-containingnanomaterials exhibit excellent magnetic properties. Amongthem, rare earth-transition metal nanostructures such asSmCo5 and Nd2Fe14B are promising permanent magnets.40 Inrecent years, ultrathin 2D materials (e.g., graphene, MXene, BP,etc.) have been reported showing great potential in biomedicalelds. For 2D rare earth compounds, in addition to lumines-cence properties, more and more research studies have beendevoted to the biomedical eld such as magnetic resonanceimaging (MRI), drug delivery, photothermal therapy (PTT) andphotodynamic therapy (PDT), especially for Gd3+-containingmaterials, due to the half-lled 4f electron structure, goodexibility and biocompatibility.41–47Chem. Sci., 2024, 15, 16887–16907 | 16887http://crossmark.crossref.org/dialog/?doi=10.1039/d4sc02625j&domain=pdf&date_stamp=2024-10-17http://orcid.org/0000-0002-9400-6045http://orcid.org/0000-0002-4487-3538http://orcid.org/0000-0003-1297-9597http://orcid.org/0000-0002-3033-0276http://orcid.org/0000-0001-7126-2006Fig. 1 Schematic illustration of the properties and biomedical appli-cations of rare earth nanomaterials.Chemical Science ReviewSimilar to LDHs, LREHs are host–guest compounds with rareearth ions occupying the host layers while anions are in theinterlayer gallery. Aer the rst preparation in the 1960s, theLREH family expanded to accommodate nearly all rare earthelements from La to Tm and Y and Lu. Various rare earthelements endow LREHs with color-changeable lumines-cence.32,34,37,48 Recently, in vivo upconversion imaging and MRIperformances based on upconversion luminescence andmagnetic properties have received much attention, especiallyfor Gd3+, Yb3+ and Nd3+ ion-containing host layer LREHs.45,49–53Extensive research has proved that LDHs are good carriers ofmultiple biomedical active species such as deoxyribonucleicacid (DNA), peptides, proteins, vitamins, etc.54–56 With similarlayered structures, LREHs and other layered rare earth mate-rials are also promising carriers of diverse biomedical drugs, forexample, aspirin.44,57,58 Moreover, combined with the specialluminescence and magnetic properties of RE3+, multifunctionaldrug delivery methods can be realized on the rare earth-con-taining layered materials, such as targeted drug delivery andreal-time monitoring of drug release conditions. Comparedwith non-layered materials, the interlayer space and rich ionexchangeability of layered rare earth-containing nanomaterialsare benecial to promoting drug loading and release. Aerexfoliation, the ultra-thin nanosheets expose multiple activesites, ensuring further modication and assembly to facilitatebiomedical applications. In addition, unlike common 3D bulkmaterials, ultra-thin nanosheets are less likely to settle in bio-logical tissues and therefore exhibit higher biocompatibility.The unique layered structure of LREHs enables exfoliation intosingle or several layers. The ultrathin nanosheets obtained aredesirable building blocks to construct articial superlatticestructures due to their large lateral size-to-thickness ratio andhigh exibility.34,59–61 Furthermore, the hydrophilicity andbiocompatibility of the hydroxide nanosheets make thema promising contrast agent for MRI toward tumorvaccination.42,43,62Though the LREH nanosheets show huge applicationpotential, hydroxyl groups and absorbed water molecules in therare earth hydroxide nanosheets weaken the photo-luminescence properties. Also, the inferior stability ofhydroxide nanosheets limits their applications. Therefore,considering the stability, composition and structure variabilityof nanosheets, more efforts are needed to widen the applicationpotential of rare earth nanosheets.In addition to LREHs, layered rare earth doped perovskitesare another class of 2D rare earth-containing nanomaterials.The diverse compositions and structures of perovskites endowthem with various properties and wide applications and havebecome a target topic in recent years. According to the aniontypes, the perovskite materials can be divided into two types,namely halide perovskites and oxides. Compared with bulkperovskite, the perovskite nanosheets present a series ofadvantages, such as higher processability, which is moreconducive to the manufacturing of thin lms and exibledevices; more active sites, which is more conducive for catalysis;and small size effect and quantum effect, which are better foroptical, electrical, and magnetic properties.38,63,64 So chemistry16888 | Chem. Sci., 2024, 15, 16887–16907methods are commonly used for the exfoliation of layeredperovskites, that is, dispersing the pretreated layered rare earthperovskite into an organic solution with continuous agitation orultrasonication. Through this method, diverse perovskitenanosheets were prepared, such as La0.90Eu0.05Nb2O7,Eu0.56Ta2O7, La1−xTbxTa2O7, La0.95Eu0.05Nb2O7 single perov-skite nanosheets and (K1.5Eu0.5)Ta3O10, Gd2−xEuxTi3O10,GdMgWO6:Eu double perovskite nanosheets. Atomic forcemicroscopy (AFM) results indicate a single-layer structure withlateral size up to 4 mm.65–69 Moreover, the layered rare earth-containing perovskite crystal or nanosheet can be applied inphotoluminescence, antibacterial activity studies, photo-catalysis and bioimaging.70–72Therefore, with the combined advantages of exible sheet-like morphology and rich rare earth composition, nanosheetsbased on rare earth elements and their assemblies are prom-ising materials in many elds, especially in biomedical appli-cations (Fig. 1). To date, there are some excellent reviews onLREHs and/or rare earth-doped oxides focusing on their prep-aration, crystal structure, exfoliation and photoluminescenceproperties.31,32,73–75 On the other hand, for 2D materials inbiomedical applications and/or biosensors, the publishedreviews mainly focus on C3N4, BP, transition metal dichalco-genides (TMDs) and MXene.76,77 Few of them have paid suffi-cient attention to the biomedical applications of 2D rare earth-based or doped nanomaterials. Thus, considering the greatpotential of emerging 2D rare earth-based or doped nano-materials, we review the preparation, structural evolution andbiomedical applications of 2D nanomaterials based on rareearth elements. The challenges and future perspectives ofutilizing these emerging 2D nanomaterials in biomedical eldsare also provided.2. Preparation and structuralevolution of 2D rare earth compoundsVarious methods have been developed to prepare 2D materialsand are mainly divided into two strategies, i.e., “bottom-up” and“top-down” approaches. Molecular beam epitaxy (MBE), chem-ical vapour deposition (CVD) and pulsed laser deposition (PLD)are typical bottom-up strategies.78 Diverse 2D rare earth© 2024 The Author(s). Published by the Royal Society of ChemistryReview Chemical Sciencematerials such as Yb-doped WS2, EuC6, GdSi2 and WS2(Er3+)/WSe2(Er3+) were prepared through this bottom-up method,holding potential for uses in next-generation optoelectronicdevices.79–82 But silicon or sapphire wafers are needed for thebottom-up processes, limiting the application in biomedicalelds. On the other hand, mechanical exfoliation and so-chemical exfoliation are well-known top-down methods andwidely used in the preparation of ultra-thin rare earth-containing 2D nanosheets, especially for LREHs and rareearth-containing perovskites.25,33,65,66,682.1 Preparation and exfoliation of LREHsLREHs with a typical formula of RE2(OH)6−m[Ax−]$H2O, inwhich Ax− represents intercalated anions such as Cl− or CO32−,have emerged as one of the most important 2D rare earthcompounds since their rst proposal in 2006.83 Similar to LDHs,the LREHs are composed of positively charged rare earthhydroxide layers and negatively charged interlayer anions. Thehost layers and guest anions are linked through strong chemicalbonds between the RE3+ cations and interlayer anions, resultingin anion-exchangeable properties.83–87 According to the coordi-nation situation, the LREHs can be divided into two types,namely LREH-I and LREH-II. For the former, one RE3+ is coor-dinated by seven OH− groups and one H2O molecule, while forthe later one, each RE3+ is coordinated by six OH− groups, twoH2Omolecules and one anionic ligand, as shown in Fig. 2.31,88 Inaddition, for LREH-I, the interlayer anions, screened from thedirect coordination of RE3+ by H2Omolecules, result in a weakerbond energy and thus a more easier interlayer anion-exchangeprocess.31The commonly used method for the synthesis of LREHs isthe co-precipitation method. Through this method, researchershave synthesized diverse LREHs which contain almost all REelements. With the modication of usingFig. 2 Structure illustrations of LREHs. (I) RE(OH)3, LREH-II and LREH-Istructures. (II) Connectivity of the polyhedral unit for the three struc-tures. (III) In-plane view of the anion-exchangeable LREH-I structureand representative coordination geometries of RE cations. Adaptedfrom ref. 31. Copyright 2023, Elsevier Inc.© 2024 The Author(s). Published by the Royal Society of Chemistryhexamethylenetetramine (HMT) or ammonia as the alkalisource, Geng et al. prepared a series of well-crystallized LREHswith Cl− or NO3− intercalation, and systematically analyzed themorphological and crystal structure (Fig. 3a–e).85,88,89,91,92Through direct synthesis and further ion-exchange processes,researchers expanded the LREHs to Br− and I− intercalatedcounterparts.86,93 Though the morphology and crystal structureof LREHs are fully analyzed, the crystal size of the synthesizedLREHs is small which is adverse to the following exfoliationprocess for the preparation of monolayers or few-layer nano-sheets. In 2012, Li et al. further improved the synthesis processby introducing NH4NO3 as a mineralizer, which expanded theLREH crystal up to 300 mm (Fig. 3f).90,94 Using sodium dodecylsulfate (SDS) as a surfactant, Zhong et al. prepared DS−-inter-calated LREHs (RE = Y, Tb, Er) with a nanocone morphology(Fig. 3g and h).49,95 Therefore, various LREHs were synthesizedwith diverse morphologies.As one of the most important features, LREHs exhibit anion-exchange properties. Through uniformly dispersing the as-prepared LREHs with target anions in deionized water undercontinuous stirring or shaking, LREHs can be topotacticallychanged to target anion-intercalated counterparts, expandingthe LREH family since some functional anionic species areunlikely to intercalate into the interlayer through directsynthetic strategies. There are various interlayer anionicspecies, including negatively charged inorganic anions (e.g.,PO43−, CO32−, HPO42−, ClO4−, etc.), negatively charged organicanions (e.g., oleic acid, phthalic acid, terephthalic acid, etc.),and electrically neutral molecules or polymers (e.g., diclofenac,ibuprofen, naproxen, etc.).57,86,88,93,96–105 Through anionexchange, the interlayer distance can be adjusted and variesfrom 0.8 nm to 5.2 nm depending on the anion species. Inaddition, the anion-exchange property endows LREHs withpotential drug delivery properties.46,57,58,106–108Similar to LDHs, the LREHs can also be exfoliated intonanosheets. In 2009, Lee et al. delaminated the as-prepared Cl−intercalated layered Gd hydroxide (LGdH-Cl), NO3− intercalatedlayered Nd hydroxide (LNdH-NO3) and NO3− intercalatedlayered La hydroxide (LLaH-NO3) crystals into correspondingnanosheets in deionized water under sonication conditions(Fig. 4a and b).41,98 Transmission electron microscopy (TEM)and AFM analysis results show that the lateral size of the ob-tained nanosheets is 50–200 nmwith a thickness of 2–8 nm. Thelarge thickness indicates that the deionized water may not bea suitable medium for exfoliating the LREHs. Considering theoutstanding swelling effect of formamide on LDHs and thesimilar structure of LREHs to that of LDHs, researchers turnedtheir eyes to formamide. Lee et al. dispersed the LGdH-NO3crystal in formamide. Aer 4 days of stirring, a transparentsuspension was obtained. AFM results show that the lateral sizeof the obtained nanosheet is 30–100 nm and the thickness isdominated by two different values (i.e., 0.7 nm and 1.4 nm).111Although the obtained nanosheets are thinner when exfoliatedin formamide, they are not unilamellar (e.g., the crystallo-graphic thickness of 0.65 nm). This may be because of thestrong binding force between the host layers and interlayeranions, thus the formamide molecule is hard to intercalate intoChem. Sci., 2024, 15, 16887–16907 | 16889Fig. 3 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of synthesized LREHs. (a) As-prepared LEuHthrough the homogeneous precipitation method. Adapted from ref. 88. Copyright 2008, John Wiley & Sons, Inc. (b) As-prepared LYH throughthe homogeneous precipitation method. Adapted from ref. 85. Copyright 2008, American Chemical Society. As-prepared (c) LGdH, (d) LDyH, (e)LErH through the homogeneous precipitation method. Adapted from ref. 89. Copyright 2009, American Chemical Society. (f) As-prepared LYHthrough the hydrothermal method with NH4NO3 as a mineralizer. Adapted from ref. 90. Copyright 2012, Elsevier Inc. (g and h) As-prepared LYHthrough the hydrothermal method with SDS as a surfactant. Adapted from ref. 48. Copyright 2017, the Royal Society of Chemistry.Fig. 4 AFM images of LREH nanosheets. (a) LNdH nanosheet through ultrasonication of the NO3− intercalated LNdH powder in deionized water.Adapted from ref. 98. Copyright 2009, JohnWiley & Sons, Inc. (b) LGdH nanosheet through ultrasonication of the Cl− intercalated LGdH powderin deionized water. Adapted from ref. 41. Copyright 2009, the Royal Society of Chemistry. (c) LEuH nanosheet through shaking of the DS−intercalated LEuH powder in formamide. Adapted from ref. 109. Copyright 2010, John Wiley & Sons, Inc. (d) LTmH nanosheet through magneticstirring of the DS− intercalated LTmH powder in formamide. Adapted from ref. 110. Copyright 2017, the Royal Society of Chemistry. (e) LYHnanosheet through magnetic stirring of the anion-exchanged NO3− intercalated LYH powder in toluene. Adapted from ref. 94. Copyright 2015,Springer Nature. (f) LGdH nanosheet through ultrasonication of the DS− intercalated LGdH powder in formamide. Adapted from ref. 61.Copyright 2021, the Royal Society of Chemistry.Chemical Science Reviewthe guest gallery, resulting in inefficient exfoliation. Moreover,the size of the nanosheets is too small for further assembly andapplication. To obtain nanosheets of better quality, DS− anionswere introduced into the interlayer gallery of LEuH through ananion-exchange process, and shaken in formamide for 2 days at16890 | Chem. Sci., 2024, 15, 16887–16907a speed of 170 rpm. Aer ltration, nanosheets with a lateralsize of 500 nm and thickness of 1.6 nm were obtained(Fig. 4c).109 Through a similar two-step exfoliation method,namely, the combination of anion exchange and exfoliation, Liet al. obtained LTmH, LYbH and LYH nanosheets with© 2024 The Author(s). Published by the Royal Society of ChemistryReview Chemical Sciencea thickness of around 2 nm and a lateral size up to 1.5 mm(Fig. 4d).105,110 Moreover, toluene is also used for the exfoliationof oleic acid-intercalated LYH. The as-obtained nanosheetspossess a thickness of 1.55 nm with an ultra-large size of up to20 mm. However, it is difficult to clearly recognize a full nano-sheet by either TEM or AFM observations (Fig. 4e).94 In additionto the anion exchange method, Yapryntsev et al. tried to exfo-liate LYH by a rapid expansion of supercritical suspensions, i.e.,treating the LYH crystals with supercritical CO2 resulted ina drastically increased distance. Aer a following ultra-sonication treatment in toluene solvent, a clear suspension wasobtained.112 However, the authors did not present any TEM orAFM images to conrm the nanosheets. Considering theexcellent exfoliation performance and degradation effect offormamide on DS− intercalated LDHs, Bai et al. modied thetwo-step exfoliation method and proposed a one-step exfolia-tion method, i.e., directly exfoliating the as-synthesized DS−anion intercalated LREHs in formamide under ultrasonicationconditions without any anion-exchange processes.33,113 Thisone-step method avoids the possible breakage of the LREHcrystals during the long-time anion exchange process.Furthermore, the ultrasonic treatment substantially reduces theexfoliation time within 1 hour, greatly shortening the reactiontime with formamide and avoiding degradation. Through thisone-step exfoliation method, a series of LREH nanosheets wereprepared with a lateral size of up to 1 mm while the thicknesswas only 1 nm, indicating a monolayer feature (Fig. 4f).33,61 Thisversatile approach achieves the preparation of relatively largeLREH monolayers, but also offers great potential for thesubsequent construction of various 2D assemblies with richfunctionalities.Although the ion exchange and exfoliation route providesmore possibilities for the synthesis of diverse LREHs andnanosheets, there are some disadvantages. For the ion exchangeprocess, it is hard to completely substitute the original inter-calated ions with the targeted one, which may affect its prop-erties. Moreover, it usually needs a long time (7 days or evenlonger) and continuous shaking or sonication to full ionexchange and exfoliation. The strong shear force or ultrasonicwaves cause the phase and structure damage of layered mate-rials, resulting in reduced size which further affects the exfoli-ation effect.109 As for exfoliation, DS− anion intercalation andusing formamide as the exfoliation solvent have been proven tobe an effective method for the preparation of single-layer LREHnanosheets. But the LREHs undergo degradation or evendissolution in formamide for a long time, especially undershear force/ultrasonic waves. As a result, it is hard to obtainlarge-sized nanosheets. Moreover, the exfoliation yield is stillrelatively low.33,109 In addition, during the exfoliation, there maybe some defects such as oxygen vacancy or metal ion vacancyand/or dangling bonds which may affect the energy level.61However, a systematic study on these aspects is still lacking.In addition to exfoliation, ultrathin rare earth hydroxides canbe prepared by direct synthesis. Through the hydrothermalmethod with triethylamine (TEA) as a hydrolysis reagent, Xiaoet al. prepared a series of Y/Yb/Er-hydroxides in the form ofnanoplatelets with a thickness of 10 nm, the hydroxides© 2024 The Author(s). Published by the Royal Society of Chemistryexhibited obvious upconversion properties (Fig. 5a–c).50 Whileco-doping Bi3+ and Sm3+ into the Y-hydroxides followed by thecalcination process, Y2O3:Bi3+,Sm3+ tetragonal nanoplateletswere obtained. As the emission peaks present differentchanging trends with the temperature changes, theY2O3:Bi3+,Sm3+ composite exhibits three kinds of luminescenceintensity ratio mode, thus making it a promising temperaturesensor with high accuracy (Fig. 5d–f).114 Therefore, variouskinds of 2D rare earth hydroxides with diverse compositionshave been reported till now, which promote their applicationsin diverse elds.2.2 Preparation and exfoliation of layered rare earth-dopedperovskitesPerovskites are a large family with various formulae and struc-tures. Layered perovskites are a sub-set with the generalcomposition of An−1BnX(3n+1), in which negatively charged hostlayers and counter cations in the interlayer space demonstratesome peculiar properties like photocatalytic, conductivity,electronic, and biochemical ones.72,115–119 Layered rare earth-doped perovskites are commonly classied into two types, i.e.,Dion–Jacobson (DJ) type and Ruddlesden–Popper (RP) type withthe general formula of X[An−1BnO(3n+1)] or X2[An−1BnO(3n+1)],respectively, where X and A are alkali or alkali earth elements,and B represents transition metals such as Ti4+, Nb5+ or Ta5+;rare earth elements usually occupy the A site.72,117,118Layered rare earth-doped perovskites with various composi-tions are mainly prepared through the solid-state reactionmethod.65,67,120 The exfoliation of layered rare earth-dopedperovskites is also a two-step process including ion exchangeand exfoliation. Typically, through a replacement of the inter-layer alkali metal ion (e.g., K+ or Rb+) with H3O+ in acid (e.g.,HNO3 or HCl) and the swelling process of perovskite bulk inaqueous TBAOH or TMAOH solution with the help of shaking,the nanosheets are obtained.65,121Based on the successful preparation of LaNb2O7 nanosheets,Ozawa et al. rst synthesized a Eu3+-doped layered perovskite.Aer ion exchange and so chemical exfoliation, La0.90Eu0.05-Nb2O7 monolayers with a thickness of 2 nm and a lateral size of400 nm were nally obtained (Fig. 6a).67,121 The photo-luminescence analysis shows that there are two types of emis-sions of Eu3+ in the bulk, i.e., the direct excited emission andthe host excited emission, whereas only host emission exists forthe nanosheets, which can be attributed to the difference of thedimensionality and the connement of the energy-transfer.67Then, they further studied the role of the Eu3+ dopant in thephotoluminescence properties of Eu0.56Ta2O7 nanosheets(Fig. 6b), and found that the emission intensity originatingfrom the host excitation is much larger than that of the directexcitation of Eu3+.66 The stronger emission intensity of La0.90-Dy0.05Nb2O7 nanosheets than that of the bulk also conrmedthat the host excitation is more efficient than the direct excita-tion.123 Ida et al. expanded the layered rare earth-doped perov-skite nanosheets to Gd1.4Eu0.6Ti3O10 (Fig. 6c) andLa0.7Tb0.3Ta2O7, the nanosheets all have a monolayer feature.Beneting from the energy transfer within the nanosheets fromChem. Sci., 2024, 15, 16887–16907 | 16891Fig. 5 Morphologies of ultrathin rare earth hydroxide or oxide platelets. (a) SEM, (b) TEM and (c) AFM images of Y/Yb/Er hydroxides. Adapted fromref. 50. Copyright 2018, the Royal Society of Chemistry. (d) SEM image, (e) luminescence intensity ratio (LIR) values and (f) relative sensitivity (Sr)values of each mode of the Y2O3:Bi3+,Sm3+. Adapted from ref. 114. Copyright 2021, American Chemical Society.Fig. 6 AFM images of rare earth-doped perovskite nanosheets. (a) La0.90Eu0.05Nb2O7 nanosheets. Adapted from ref. 67. Copyright 2007,American Chemical Society. (b) Eu0.56Ta2O7 nanosheets. Adapted from ref. 66. Copyright 2008, American Chemical Society. (c) Gd1.4Eu0.6Ti3O10nanosheets. Adapted from ref. 65. Copyright 2008, American Chemical Society. (d) Ln2Ti3O10:Tm3+/Er3+ nanosheets. Adapted from ref. 122.Copyright 2021, Elsevier Inc. (e) [K1.5(Tb0.8Sm0.2)]Ta3O10 nanosheets. Adapted from ref. 25. Copyright 2022, the Royal Society of Chemistry. (f)GdMgWO6:60%Eu3+ nanosheets. Adapted from ref. 69. Copyright 2019, Elsevier Inc.16892 | Chem. Sci., 2024, 15, 16887–16907 © 2024 The Author(s). Published by the Royal Society of ChemistryChemical Science ReviewReview Chemical Sciencethe Ti–O network to Gd3+ and then to Eu3+, Gd1.4Eu0.6Ti3O10nanosheets show a much stronger emission than La0.90Eu0.05-Nb2O7 counterparts.65 Moreover, they also found that theemission intensity is related to the excitation light and thedirection of the magnetic eld. Bulk perovskite HLa2Ti2TaO10with a similar crystal structure was also prepared and furtherexfoliated into the corresponding La2Ti2TaO10 nanosheets withan ultra-high conductivity range from 10−9 to 10−5 S cm−1.116Beyond the above down-conversion luminescence nanosheets,up-conversion nanosheets of Yb3+/Er3+, Yb3+/Tm3+, and Tm3+/Er3+ co-doped K2Ln2Ti3O10 perovskites were also prepared, asshown in Fig. 6d.122,124,125In addition to the above single-layered perovskite, researchersalso successfully prepared rare earth-doped double-layeredperovskite nanosheets, e.g., (K1.5Eu0.5)Ta3O10, (K1.5Tb0.5)Ta3O10,and [K1.5(Tb1−xSmx)]Ta3O10 nanosheets (Fig. 6e).25,68 Consideringthe excellent tuneable excitation wavelength and interestingproperty of cascade energy transfer of double-layered perovskites,researchers designed Eu3+-doped double-layered perovskiteGdMgWO6:Eu nanosheets through proton exchange and exfolia-tion in ethylamine solution (Fig. 6f).69,126 Compared with the bulkperovskite, the GdMgWO6:Eu nanosheets showed high concen-tration quenching of up to 60% of Eu3+ and a high quantum yieldof 30%.69 In general, compared with the bulk phase, the ultrathinnanosheets exhibit enhanced properties in photoluminescenceintensity, conductivity and quenching concentration.2.3 Other rare earth-containing 2D materialsAnother attractive 2D material family is rare earth-containing2D metal–organic frameworks (MOFs). Compared withFig. 7 TEM and AFM images of 2DMOF based on rare earth elements. (a)the ligand (Tb-TCPP). Adapted from ref. 130. Copyright 2020, JohnWileyMB/Yb-TCPP-SO4 represents methylene blue (MB) modified porphyrin-Royal Society of Chemistry. (e) TEM and (f) AFM images of rare earth-cAdapted from ref. 132. Copyright 2017, Springer Nature. (g) SEM and (h) Arepresents 2D MOFs with Tb/Eu as nodes and 5-boronoisophthalic acid© 2024 The Author(s). Published by the Royal Society of Chemistrytraditional 3D MOFs, the 2D MOFs exhibit a larger specicsurface area, faster electron transfer and abundant exposedactive sites that are accessible, which enable them withimproved properties such as electrochemiluminescence (ECL),photocatalysis and sensitivity.127,128 With ytterbium as nodesand 4,40,400,4000–(21H,23H-porphine-5,10,15,20-tetrayl) tetrakis-benzoic acid (H2TCPP) as the ligand in dimethylformamide(DMF) solution, 2D Yb-MOFs were synthesized and exhibitedthickness-dependent ECL behavior. In particular, the thinnerthe 2D Yb-MOFs, the stronger the ECL signals, which wasascribed to the large surface area, better electrochemicalconductivity and higher productive ratio of uorescencequantum yield.129 Furthermore, the porphyrin-based 2D Ln-MOFs, especially porphyrin-based 2D Yb-MOF (namely, Yb-TCPP MOF), display outstanding photodynamic activity(Fig. 7a and b).130 By coupling the porphyrin-based 2D Yb-TCPPMOF with methylene blue (MB), Jiang et al. prepared 2D arti-cial light-harvesting-system (MB/Yb-TCPP) nanosheets. TheMB/Yb-TCPP nanosheets showed efficient photon capture,energy transfer and various active centres, which led to excellentphotocatalytic properties (Fig. 7c and d).131 Moreover, rare earthcations such as Eu3+ and Tb3+ are also introduced into the 2DZn(II) MOF to form 2D Ln3+-encapsulated functional materialswith a tuneable emission color.134 With 2,20-thiodiacetic acid(TDA) as a surfactant, a series of rare earth-based layered MOFcan be synthesized, and the layered structures can further beexfoliated into ultrathin nanosheets in EtOH under ultra-sonication conditions (Fig. 7e and f).132 Through simple mix-ing of Tb/Eu salts and 5-boronoisophthalic acid (5-bop) in thepresence of triethylamine (TEA) at room temperature, Wanget al. synthesized three boric acid-functionalized 2D MOFTEM and (b) AFM images of 2DMOFs with Tb as nodes and porphyrin as& Sons, Inc. (c) TEM and (d) AFM images of MB/Yb-TCPP-SO4, in whichbased 2D Yb-TCPP MOF. Adapted from ref. 131. Copyright 2021, theontaining MOF nanosheets with 2,20-thiodiacetic acid as a surfactant.FM images of the 2D Tb/Eu-bop MOF nanosheets, in which Tb/Eu-bopas the ligand. Adapted from ref. 133. Copyright 2022, Elsevier Inc.Chem. Sci., 2024, 15, 16887–16907 | 16893Chemical Science Reviewnanosheets, i.e., Tb-bop, Eu-bop and bimetallic Tb/Eu-bop MOFnanosheets, respectively, expanding the 2D MOF family (Fig. 7gand h).133 Though several 2D MOFs containing rare earths havebeen synthesized in recent years, the ligands used are quitescarce, resulting in limited availability of rare earth-containing2D MOFs, which deserve more attention and exploration.2D rare earth oxyhalides (REOXs) represent another inter-esting class of rare earth-containing 2D compounds. The CVDmethod is widely used for the preparation of the REOXs.Through this method, Chen et al. successfully synthesizedEuOCl with an ultra-narrow linewidth of 1.2 meV at roomtemperature while Tian et al. synthesized DyOCl with strong A-type antiferromagnetism below the Néel temperature TN of 10K.135,136 Zhang et al. designed a facile strategy to prepare a seriesof 2D LnOCl (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy) nanoakesusing the molten salt method assisted by the substrate. Thelateral size of the prepared ultra-thin nanoplate could reach upto 40 mm with a thickness of only 7.5 nm.137 As a gate dielectric,the LnOCl nanoake exhibited competitive device characteris-tics of high on/off ratios up to 107 and low subthreshold swingsdown to 77.1 mV dec−1.2.4 Assembly of rare earth-based or doped nanosheets intofunctional structuresAs one kind of nanosheets showing unique properties, rare earth-based or doped nanosheets can be re-assembled into desirableforms for function exploration. Layer-by-layer (LBL) depositionmethod is a useful method to fabricate the nanosheets on quartzor silicon wafers. As the LREH nanosheets are positively charged,aer introducing the negatively charged poly(sodium-p-styr-enesulfonate) (PSS) ions as counter ions, the LREH nanosheetscan be transferred on the wafer and further LBL assembled toform multilayer lms, such as (LGdH:Eu/PSS)n and (LGdH:Tb/PSS)n lms, where LGdH:Eu represents Eu3+ doped LGdH andLGdH:Tb represents Tb3+ doped LGdH (Fig. 8a and b).60,61Through self-assembly at the hexane/water interface, Eu(OH)2.5-Cl0.5$0.9H2O and Gd(OH)2.5Cl0.5$0.9H2O:0.05Eu can be trans-ferred upon a glass wafer.138,139 Aer an annealing treatment, theFig. 8 Multilayer rare earth hydroxide films. (a) Scheme of the fabricatiofrom ref. 60. Copyright 2010, John Wiley & Sons, Inc. (c) SEM imagesGd2O3:0.05Eu film. Reproduced from ref. 138. Copyright 2009, John W16894 | Chem. Sci., 2024, 15, 16887–16907hydroxide lm can be quasi-topotactically transformed intoGd2O3:0.05Eu (Fig. 8c).138 The self-assembled lm is semi-transparent with a at surface and strong adhesion, andexhibits high orientation, which is benecial for enhancement ofphotoluminescence. This quasi-topotactic transformation andenhanced photoluminescence originating from high orientationwere also found in LYH and Y2O3 lms which were fabricated byspin-coating.140Through the LBL deposition method, the negatively chargedLa0.90Dy0.05Nb2O7 nanosheets can be self-assembled with poly-ethylenimine (PEI) as a connector on a quartz, resulting in (PEI/La0.90Dy0.05Nb2O7)n multilayer lms (Fig. 9a). Photo-luminescence analysis shows that the as-prepared lm displayeda blue-light emission with peaks centred at around 480 and576 nm (Fig. 9b), and it was found that compared with the directexcitation for the bulk structure, the host excitation is ina dominant state in the nanosheet and lm.123 Moreover,multilayer upconversion lms of alternatingly assembled Er3+/Yb3+ co-doped Ln2Ti3O10 (Ln2Ti3O10:Er3+,Yb3+) and Tm3+/Yb3+ co-doped Ln2Ti3O10 (Ln2Ti3O10:Tm3+,Yb3+) nanosheet on indium tinoxide (ITO) glass presented a white emission (Fig. 9c).125Assembling the rare earth-containing nanosheets with other2D nanosheets through the LBL method, superlattice-likenanolms were fabricated. Ida et al. reported a bilayer lmcomposed of Eu(OH)3−x nanosheets and Ti1.81O4 nanosheetswith drastically enhanced luminescence.141 Bai et al. furtherfabricated a series of superlattice lms with LREH nanosheetsand semiconductive nanosheets, such as (LGdH:Eu/Ti0.87O20.52−)n lms, (LGdH:Tb/TaO3−)n lms and (LGdH:Eu/Ti0.87O20.52−/LGdH:Tb/TaO3−)n lms and systematically studiedthe energy transfer process within the superlattice lms, inwhich LGdH:Eu and LGdH:Tb represent Eu3+ or Tb3+ dopedLGdH, respectively (Fig. 10a).61 When LREH nanosheets wereassembled with SiO2 nanoparticles, LREHs/SiO2 porous multi-layer lms were prepared with high transparency (Fig. 10b andc).59,142 The emission light color could be tuned by adjusting thecomposition of the different color light sources. Therefore, theLBL method is a useful way to fabricate multilayer lms withn process and (b) photographs of multilayer LGdH/PSS films. Adaptedand schematic illustration of Gd(OH)2.5Cl0.5$0.9H2O:0.05Eu film andiley & Sons, Inc.© 2024 The Author(s). Published by the Royal Society of ChemistryFig. 9 Rare earth perovskite superlattice films through the LBL fabricationmethod. (a) XRD patterns and (b) photoluminescence emission spectraof (PEI/La0.90Dy0.05Nb2O7)10 films. Adapted from ref. 123. Copyright 2012, Elsevier Inc. (c) CIE diagram and upconversion emission spectra of(Ln2Ti3O10:Er3+,Yb3+/Ln2Ti3O10:Tm3+,Yb3+)60 film. Adapted from ref. 125. Copyright 2022, Elsevier Inc.Fig. 10 LREH superlattice films through the LBL fabrication method. (a) Scheme of the fabrication process of LREHs/semiconducting oxidesuperlattice films. Adapted from ref. 61. Copyright 2021, the Royal Society of Chemistry. (b) Scheme of the fabrication process and (c) photo-graphs of (Gd2O3:Re/SiO2) films under daylight and 254 nm UV irradiation, in which Gd2O3:Re represents Gd2O3 doped with different rare earthions. Adapted from ref. 59. Copyright 2012, John Wiley & Sons, Inc.© 2024 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2024, 15, 16887–16907 | 16895Review Chemical ScienceFig. 11 Rare earth-doped perovskite lamellar flocculation. (a) TEM image and (b) possible model of the energy diagram of Eu3+ flocculatedTi0.92O2 nanosheet (ex-Ti0.92O2/Eu) composite. Adapted from ref. 143. Copyright 2004, American Institute of Physics. (c) TEM image of(Ho0.096Yb0.23Y0.164)Ca1.76Nb3O10 composite. Adapted from ref. 144. Copyright 2014, American Chemical Society. (d) Fabrication process andgrowth inhibition of Ag+ intercalated Tm3+/Er3+ co-doped Ln2Ti3O10 composite. Adapted from ref. 70. Copyright 2022, Elsevier Inc.Chemical Science Reviewcontrollable compositions and structures by using ultra-thinnanosheets. In addition, through a block-by-block epitaxialgrowth of the multistep molten salt method, SmOCl-NdOCl-EuOCl multi-heterostructure and SmOCl-NdOCl-SmOCl-NdOClsuperlattice lms were fabricated, that is, growth of the secondand subsequent layers by precisely sprinkling the powders besidethe existing crystals under the same conditions.137A clean substrate (e.g., quartz glass or silicon wafer) isgenerally needed for the above-mentioned LBL assemblymethod, and the rare earth-containing multilayer lms assem-bled by this method are widely used in the luminescence eld.As rare earth-doped perovskite nanosheets are negativelycharged, they can also be occulated by positively chargedcations to form lamellar structures. This occulation methodendows rare earth-based or doped nanosheets with moreproperties. Xin et al. occulated negatively charged Ti0.91O2nanosheets with Eu3+ or Tb3+ ions to form ex-Ti0.92O2/Eu and ex-Ti0.92O2/Tb composites, respectively, and found that theFig. 12 (a) XRD patterns, (b) TEM image and (c) SEM and elemental mrepresent Eu3+ doped LGdH nanosheets and Ti0.87O20.52− nanosheets, reInc.16896 | Chem. Sci., 2024, 15, 16887–16907occulation was in a lamellar structure with a gallery distance of1.06 nm (Fig. 11a).143 The photoluminescence analysis indicatesthat nonradiative energy transfer from the Ti0.91O2 host to Eu3+can take place in this system (Fig. 11b).143 Similarly, red- andgreen-light-emitting Eu3+ or Tb3+ ion-occulated TiTaO5lamellar aggregates were prepared.145 Through occulatingCa2Nb3O10 nanosheets with Ho3+, Yb3+ and Y3+ ions, Ozawaet al. prepared a new upconversion lamellar material(Ho0.096Yb0.23Y0.164)Ca1.76Nb3O10, in which Ca2Nb3O10 nano-sheets were restacked in a parallel manner with three layers(Fig. 11c).144 While using Ag+ as the counter cations, Günay et al.fabricated an Ag-intercalated Tm3+/Er3+ co-doped Ln2Ti3O10composite, and found that the occulation exhibited efficientantibacterial and antibiolm activity but also low cytotoxicity(Fig. 11d).70 Compared with the LBL fabrication method, thisocculation method shortens the time largely and can enhanceor expand the properties of nanosheets by the introduction ofcounter ions, but it lost the superlattice-like structures.apping images of the GdEu/TiO membrane, in which GdEu and TiOspectively. Adapted from ref. 146. Copyright 2022, John Wiley & Sons,© 2024 The Author(s). Published by the Royal Society of ChemistryReview Chemical ScienceBased on electrostatic absorption, Bai et al. constructeda self-standing superlattice membrane through the directassembly of LREH nanosheets and 2D semiconductive oxidecounterparts in a face-to-face manner (Fig. 12).34 The emissionlight color of the (GdEu/TiO)/(GdTb/TaO) membrane can becontrolled by adjusting the excitation wavelength numbers,where GdEu and GdTb represent Eu3+ or Tb3+ doped LGdHnanosheets, respectively, TiO and TaO represent Ti0.87O20.52−and TaO3− nanosheets, respectively. In addition, the self-standing membrane exhibited stable cathode luminescenceunder continuous electron bombardment. When the LREHnanosheets were assembled with Ti3C2 nanosheets, the LREH/MXene hybrid presented excellent photothermic and MRIproperties.146 This re-assembly method of nanosheets willinspire the structural design development of rare earth-based ordoped 2D materials with multifunction.All the above assembly methods of ultrathin rare earth-basedor doped nanosheets, especially for the superlattices, inspirednew strategies to integrate different nanosheets and promotethe design of rare earth nanomaterials towards novel structuresand interesting functionalities. Further efforts in the assemblyof more functional nanosheets with different compositions/structures are needed to enhance and expand the applicationsof rare earth nanomaterials.3. Biomedical applications based on2D rare earth nanomaterials andcompositesThe properties of rare earth-containing nanomaterials arelargely affected by their structures and doping elements.Tables 1–3 summarize the morphology, rare earth-containingelements and functions of 2D rare earth nanomaterials. It isobvious that the layered structure with interlayer ionexchangeability is benecial in drug release, especially forLREHs. On the other side, due to the half-lled 4f electrons, theexistence of Gd3+ endows the nanomaterials with magneticproperties, resulting in their application as MRI agents. More-over, for ultrathin nanosheets, the abundant active side ensuresthat it can be modied with other functional nanosheets/groups, thus expanding their application scenarios. Thissection summarizes the composition, general properties andapplications of 2D rare earth nanomaterials.Table 1 Summary of 2D rare earth nanomaterials for drug delivery applMaterials Morphology DruY/Tb/Gd-LREH Layered nanocone ASALTbH-Cl Layered plate Dic,LGdH-Cl Layered plate Dic,LTbH-NO3 Layered plate ASALEuH-NO3 Layered plate NapLGdH-NO3 Layered plate NaliLGdH-Cl Layered plate AntiPPF-Gd MOF Layered plate DoxGd&Yb-LDH Nanosheet Che© 2024 The Author(s). Published by the Royal Society of Chemistry3.1 Drug deliveryFor layered structured nanomaterials, the abundant interlayerspace and rich interlayer ion exchangeability greatly promotetheir drug load and release properties, which provide moreideas for the design of drug delivery carriers.Through anion exchange or direct synthesis, various drugs,such as diclofenac (dic), ibuprofen (ibu), naproxen (nap),aspartic acid, glutamic acid, palmitic acid, anti-miRNA oligo-nucleotides and aspirin (ASA), can be intercalated into thegallery of the LREHs.46,57,58,106–108 The non-steroidal anti-inammatory drugs (diclofenac, ibuprofen, and naproxen) canbe inserted into LGdH, and the LGdH-drug intercalates arestable in neutral pH and degrade rapidly under acidic condi-tions.57 Different drugs exhibit diverse release times. Forexample, naproxen-intercalated LREH releases the drug load-ings very rapidly (within 1.5 h), and the diclofenac-intercalatedone shows sustained release time over 4 h, while the ibuprofen-intercalated counterpart shows a continuous release time of24 h and the ASA-loaded LREH extends the release time up to36 h, as shown in Fig. 13a and b.57,58 Furthermore, the LREHs-drug composites show good targeted drug delivery properties,high drug loading and low cytotoxicity, enabling the LREHswith potential applications in drug delivery. Gu et al. prepareda 5-aminolevulinic acid (5-ALA) intercalating LREH-coatedMgFe2O4 particles (5-ALA-MgFe2O4@LREH). With the help ofmagnetic MgFe2O4, the 5-ALA-MgFe2O4@LREH carries drugsefficiently to targeted locations under an external magneticeld.45 Recently, Li et al. proposed Y/Tb/Gd ternary LREHnanocones with drug delivery and simultaneous MRI proper-ties, as shown in Fig. 13c.44 Through anion exchange, ASA canbe loaded into the gallery with a delayed release behavior to 48 h(Fig. 13d), which is favorable for sustained therapy. As theintercalation of ASA can enhance the green light emission at thepeak of 543 nm, the nanocones can also be utilized as a uo-rescence probe to monitor the loading and release process ofASA. Furthermore, the ASA intercalation improves the magneticproperties of the nanocones, which enables an additionalapplication as an MRI contrast agent simultaneously.In addition to inorganic 2D nanomaterials, rare earth-porphyrin MOF nanosheets can also serve as drug deliverymaterials. Through the surfactant-assisted solvothermalmethod, a series of lanthanide-porphyrin MOF nanosheets(denoted as PPF-Ln) with ultrahigh drug loading capacity wereicationsgs Ref.44ibu, nap 46ibu, nap 5758106dixic acid, aspartic acid, glutamic acid, fatty acid 107-miRNA oligonucleotides 108147motherapeutic drug (SN38) 148Chem. Sci., 2024, 15, 16887–16907 | 16897Table 2 Summary of 2D rare earth nanomaterials for bioimaging applicationsMaterials Rare earth elements Bioimaging applications Ref.LGdH Gd MRI 57LGdH/Ti3C2 Gd MRI 146GdDy-LDH Gd MRI 149GdCu-LDH Gd MRI 150LDH-Gd(dtpa) Gd MRI 151 and 152AFGd-LDH Gd MRI 153LGdH:Ce,Tb Gd, Ce, Tb MRI, CT 52UCSP-FeMn-LDH Gd, Yb, Er MRI, CT 154PPF-Gd Gd MRI, uorescence imaging 147FITC/FA-DOX/Gd-LDH Gd MRI, uorescence imaging 155Ce6&AuNCs/Gd-LDH Gd MRI, uorescence imaging 156Gd/MgGa-LDH Gd MRI, CT imaging 157SN38&ICG/Gd&Yb-LDH Gd, Y MRI, CT imaging, NIRF imaging 148ICG/CAC-LDH Gd MRI, photoacoustic imaging 158Table 3 Summary of 2D rare earth nanomaterials for tumor therapyapplicationsMaterials Tumor therapy applications Ref.Phy@PLGdH Radiation therapy 62PPF-Gd/DOX Chemotherapy 147LGdH/Ti3C2 PTT 146SN38&ICG/Gd&Yb-LDH PTT 148BOSC PCT 159 and 160NaYF4:Yb,Tm@NaGdF4 PDT 161ICG/CAC-LDH PTT, CDT 158DOX&ICG/MLDH Chemotherapy, PTT, PDT 162UCSP-FeMn-LDH PTT, PDT, CDT 154Chemical Science Reviewprepared.147 The results show that the loading capacity of PPF-Gd nanosheets can reach 1515% when the doxorubicin (DOX)concentration is 4 mg mL−1 (Fig. 13e). Also, PPF-Gd nanosheetsexhibit pH-responsive biodegradation and persistent drugrelease performance. As shown in Fig. 13f, in the phosphatebuffered saline (PBS) solution (pH = 5.5), about 49% DOX wasreleased over 24 h, while only 15% DOX was released at pH 7.4.The nal release percentage reached 72% and 24% at pH 5.5and 7.4 aer 96 h, respectively. Moreover, the in vivo resultsconrm the evident suppression of the tumor growth by thePPF-Gd/DOX system with negligible side effects. Owing to theunique layered structure, these 2D layered rare earth nano-materials hold controllable drug load and release properties,the release time varies from within 1.5 h to several daysdepending on the type of drug and environment pH, whichenables the 2D rare earth nanomaterials to have promising drugdelivery applications in different in vivo environments.Apart from those described above, 2D rare earth nano-materials can also load and release other therapeutic drugs. Thedrug delivery applications are summarized in Table 1. Despitethe promising commercial prospects, 2D rare earth nano-materials as drug delivery agents still have some deciencies.For instance, LREHs as typical hydroxides are stable in neutraland alkaline systems, but are easily decomposed in acidicsystems. Thus they are scarcely used in acidic environmentssuch as in the stomach, which greatly limits their application16898 | Chem. Sci., 2024, 15, 16887–16907prospects. Moreover, since LREHs and perovskite host layersare positively and negatively charged, respectively, they exhibitgood drug load properties for the negatively and positivelycharged drugs. However, for those drugs with the same elec-trical charge or neutral property, their loading capacity needs tobe improved.3.2 BioimagingMagnetic resonance imaging, computed tomography (CT), B-scan ultrasonography and X-ray radiography are the mostcommon medical imaging diagnostic technologies. The diverse4f electron shell endows rare earth-containing nanomaterialswith unique magnetic properties. For example, Sc3+, Y3+, La3+and Lu3+ are magnetically inert and are mainly employed ashost cations for doping with other ions due to their empty orcompletely lled 4f electrons.163 On the other hand, most rareearth ions are paramagnetic at room temperature due to theunpaired 4f electrons, such as Ce3+, Gd3+, Tb3+ and Eu3+, inwhich Gd3+ possesses the largest magnetic moment of all rareearth ions due to the highly symmetric ground state and sevenunpaired 4f electrons.163–166 Thus, Gd3+-containing nano-materials, with abundant unpaired electrons of the high-spinstates of Gd3+, exhibit signicant magnetic properties and arepromising MRI agents for the detection of pathologicaltissues.41,42,108,167As shown in Fig. 14a, Gd/Y hydroxide nanosheets showexcellent MRI performance with high longitudinal and trans-verse relativities of r1 (103mM−1) and r2 (372mM−1) (Fig. 14a).43When modied with phospholipids, LGdH nanosheets canwork as an efficient MRI contrast agent and show a signicantsignal loss in the T2-weighted MRI of the abdomen (Fig. 14b).42The intercalation of some drugs (e.g., diclofenac, ibuprofen, andnaproxen) can further enhance the MRI performance ofLREHs.57 While MRI contrast agents generally show lowcontrast efficacy for MRI systems with ultrahigh magnetic eldhigher than 7.0 T, Ce and Tb co-doped LGdH (denoted asLGdH:Ce,Tb) shows excellent negative (T2) contrast agent effi-cacy for 7.0 T MRI with a high r2/r1 ratio of 48.80, whereasa lower r2/r1 ratio of 30.60 for 3.0 T MRI, indicating a high-© 2024 The Author(s). Published by the Royal Society of ChemistryFig. 13 Release kinetics of (a) diclofenac, ibuprofen, and naproxen from LGdH. Adapted from ref. 57 Copyright 2018, the Royal Society ofChemistry. (b) Release kinetics of aspirin from LTbH, in which ASA-LTbH-1 : 1 and ASA-LTbH-1 : 2 represent ASA intercalated LTbH at a molarratio of ASA to NaOH of 1 : 1 or 1 : 2, respectively. Reproduced from ref. 58. Copyright 2017, John Wiley & Sons, Inc. (c) Scheme of drug loadingand release process and (d) release kinetics of ASA from Y/Tb/Gd ternary layered rare earth hydroxide nanocones, in which YTG-DS, YTG-ASAand YTG-CL represent DS−, ASA and Cl− intercalated Y/Tb/Gd ternary layered rare earth hydroxide nanocones, respectively. Reproduced fromref. 44. Copyright 2023, John Wiley & Sons, Inc. (e) Drug loading and (f) release capacity of the Gd-based porphyrin paddlewheel framework(PPF-Gd). Adapted from ref. 147. Copyright 2020, Oxford University Press.Review Chemical Scienceperformance T2-weighted contrast agent in ultrahigh-eldMRI.52 Furthermore, the LGdH:Ce,Tb can also work asa contrast agent in CT and uorescence bioimaging. In recentyears, our team proposed several studies on LREH-basednanomaterials with MRI properties. Though the half-lled 4felectron conguration of Gd3+ endows the Gd3+-containingnanomaterials with MRI properties, it's negligible for the purelayered Y/Gd hydroxide. Though the intercalation of ASA canenhance the T1-weighted in vitro MRI performance, the in vivoMRI effects are still not obvious enough.44 Through theconstruction of a superlattice of LGdH nanosheets and Ti3C2nanosheets, Bai et al. proposed an LGdH/Ti3C2 hybrid, whichexhibited obvious enhanced in vitro and in vivo T1-weighted MRIeffect, as shown in Fig. 14c.146Through introducing guests or anion exchange, Gd3+ andGd3+-based contrast agents are doped into LDH to improve theirMR relaxation.168 Gd3+ and Dy3+ co-doped LDH (GdDy-LDH)showed high contrast effect in MRI with r1 = 3.49 mM−1 s−1and r2 = 18.17 mM−1 s−1.149 When Gd3+ and Cu2+ are co-dopedin LDH, the GdCu-LDH showed signicantly higher longitu-dinal relativity compared with those solely doped with Gd3+ orCu2+ (Gd-LDH or Cu-LDH) due to the synergistic T1 shorteningbetween adjacent Gd3+ and Cu2+ in the LDH host layers.150Through ion exchange, Xu et al. prepared Gd-dtpa (dtpa =diethylene triamine pentaacetate) intercalated Mg2Al-Cl-LDH(LDH-Gd(dtpa)).151,152 Compared to Gd(dtpa) free LDH, theLDH-Gd(dtpa) displayed 4 times increase in longitudinal protonrelaxation and a 12 times increase in transverse proton relaxa-tion.151 Guan et al. developed a supramolecular nanomaterial© 2024 The Author(s). Published by the Royal Society of Chemistry(FITC/FA-DOX/Gd-LDH) via co-intercalation of folic acid (FA)and DOX into the Gd-LDH, followed by adsorption of uores-cein isothiocyanate (FITC), FITC/FA-DOX/Gd-LDH exhibitedexcellent T1-weighted MRI and uorescence dual-mode imagingactivity.155 Through adding gold nanoclusters (AuNCs) andChlorin e6 (Ce6) to the Gd-LDH, Mei et al. synthesizedCe6&AuNCs/Gd-LDH. Both in vitro and in vivo results conrmedthat the Ce6&AuNCs/Gd-LDH showed MRI and uorescencedual-mode imaging activity.156Other than inorganic 2D materials, rare earth-based 2DMOFs show promising applications in the bioimagingeld.169,170 The PPF-Gd nanosheets exhibit an obviouslyenhanced T1-weight MR signal with a transverse relativity of10.04 mM−1 s−1.147 As shown in Fig. 14d and e, for both intra-venous injection and subcutaneous injection, the in vivo T1-weighted MRI signal is detected and signicantly higher thanthat before injection. Moreover, the PPF-Gd nanosheets showobvious uorescence imaging properties following subcuta-neous injection aer 72 h.As summarized in Table 2, 2D rare earth nanomaterials canalso work as CT imaging and NIR imaging agents in addition toMRI. Through reverse micelle preparation of LDH and Gd(OH)3,Jung et al. synthesized Gd/MgGa-LDH hybrids, which exhibitedCT and MR dual-modal imaging performance, due to the highX-ray attenuation coefficient and paramagnetic properties ofGd3+ in Gd(OH)3.157 Mei et al. prepared Gd3+ and Yb3+ co-dopedLDH (Gd&Yb-LDH) monolayer nanosheets. The experiments onthe in vivo metabolic pathway indicated that in vivo tri-mode(MR, CT and near-infrared uorescence (NIRF)) imaging wasChem. Sci., 2024, 15, 16887–16907 | 16899Fig. 14 MRI properties of Gd3+-containing 2D nanomaterials. (a) T1-weighted and T2-weighted phantom images of Gd/Y hydroxide nanosheets.Reproduced from ref. 43. Copyright 2020, Multidisciplinary Digital Publishing Institute. (b) In vivo T2-weighted MRI of mouse body with intra-venous injection of LGdH-FS-PEGP, in which LGdH-FS-PEGP represents poly(ethylene glycol)-phospholipid-modified LGdH. Adapted from ref.42. Copyright 2009, JohnWiley & Sons, Inc. (c) In vitro and in vivo T1-weightedMRI effect of LGdH/Ti3C2. Adapted from ref. 146. Copyright 2023,the Royal Society of Chemistry. In vivo T1-weighted MR coronal images of a nude mouse bearing A375 tumor before and after (d) intravenousinjection and (e) subcutaneous injection of PPF-Gd nanosheets, where PPF-Gd represents Gd-porphyrin MOF nanosheets. Adapted from ref.147. Copyright 2020, Oxford University Press. (f) In vivo MR and (g) CT imaging of nude mice bearing HeLa tumors at different time points afterintravenous injection of SN38&ICG/Gd&Yb-LDH. (h) In vivo fluorescence imaging and drug bio-distribution for nude mice bearing HeLa tumorsat different time points after intravenous injection of PBS, SN38&ICG/Gd&Yb-LDH and SN38&ICG, respectively, together with ex vivo imaging ofICG in the tumour and five different organs collected from mice sacrificed at 24 h, in which SN38&ICG/Gd&Yb-LDH represents a chemother-apeutic drug (SN38) and indocyanine green (ICG) co-modified Gd3+ and Yb3+ co-doped LDH. Adapted from ref. 148. Copyright 2018, the RoyalSociety of Chemistry.16900 | Chem. Sci., 2024, 15, 16887–16907 © 2024 The Author(s). Published by the Royal Society of ChemistryChemical Science ReviewReview Chemical Scienceachieved (Fig. 14f–h), enabling a non-invasive visualization ofcancer cell distribution with deep spatial resolution and highsensitivity.148 The Gd&Yb-LDH monolayers exhibited ultrahighloading content of a chemotherapeutic drug (SN38) up to 925%.Furthermore, SN38&ICG/Gd&Yb-LDH was prepared by furthermodifying Gd&Yb-LDH with SN38 and ICG. As shown in Fig. 14fand g, aer intravenous injection of SN38&ICG/Gd&Yb-LDH,the tumor site of nude mouse displayed a gradually enhancedMRI and CT imaging signal due to the enhanced permeabilityand retention (EPR) effects and preferential accumulation,respectively. As shown in Fig. 14h, aer the injection ofSN38&ICG and SN38&ICG/Gd&Yb-LDH for 2 h, signicant NIRFsignals were observed throughout the whole body. For theinjection of SN38&ICG/Gd&Yb-LDH, the signal remained aer24 h while it was rather weak for SN38&ICG aer 6 h.Thus, both the inorganic and organic Gd3+-containing 2Dnanomaterials exhibit high potential in bioimaging applica-tions. Although extensive work has been conducted on themagnetic property modulation and MRI applications of 2D rareearth nanomaterials, it mainly focuses on Gd3+-containingmaterials. Other RE-based nanomaterials have been rarelyexplored, which needs more study.3.3 Tumor therapyOn account of biocompatibility and low toxicity, the applica-tions of 2D rare earth nanomaterials in other biochemical eldsbeyond drug delivery and MRI contrast agents have also beenexplored recently, especially tumor therapy.For instance, poly(ethylene glycol) (PEG) modied LGdH(PLGdH) presents superior X-ray deposition and tumor pene-trability. Moreover, aer further encapsulation of physcion(Phy), the Phy-modied PLGdH (Phy@PLGdH) can furtheramplify PLGdH-sensitized room temperature mediated oxida-tive stress and DNA damage, and synergistically induce thepotent immunogenic death, resulting in highly active tumorinhibition in radiation therapy (RT) (Fig. 15a).62 In addition, Gd-based porphyrin paddlewheel framework (PPF-Gd) MOFshowed signicantly high in vitro and in vivo tumor inhibitionperformance, as illustrated in Fig. 15b and c. Aer subcuta-neous injection, the doxorubicin (DOX) modied PPF-Gd (PPF-Gd/DOX) exhibited a greatly enhanced inhibition efficacy to thetumor growth.147 Meanwhile, the weight of mice was slightlyincreased during the treatment in the PPF-Gd/DOX group,making it an excellent chemotherapy agent candidate in cancertherapy with enhanced therapeutic efficiency and low toxic sideeffects.Though RT and thermotherapy are widely used in tumortherapy, they are harmful to biological tissues. Compared withthese harmful treatments, PTT treatment has gained increasingattention due to its deep penetration and low tissue damage.For photothermal therapy, the LGdH/Ti3C2 hybrid exhibited anenhanced photothermal effect under 2 W cm−2 irradiation for10 min, in which the temperature surpassed 60.4 °C(Fig. 15d).146 The above-mentioned SN38&ICG/Gd&Yb-LDHdemonstrated excellent PTT properties. Aer the injection of3 min, the temperature of the nudemice tumor site increased to© 2024 The Author(s). Published by the Royal Society of Chemistry51 °C, and the dynamic tumor growth experiment alsoconrmed the benet of SN38&ICG/Gd&Yb-LDH to in vivophotothermal therapy.148Other than traditional phototherapy and chemotherapy, 2Drare earth nanomaterials were also used in photocatalytictherapy (PCT), photodynamic therapy (PDT) and chemo-dynamic therapy (CDT).171,172 Zhang et al. synthesized Ce and Sco-doped Bi2O3 (BOSC) nanosheets with enhanced PCT undernear-infrared light irradiation. The introduction of Ce promotesthe generation of reactive oxygen species (ROS), which enhanceslight absorption, introduces oxygen vacancies, reducesbandgap, and facilitates charge separation. The Ce doping alsomodies the band position and Fermi level of BOSC, resultingin increased band bending at the solid–liquid interface,enabling a cascade reaction of ROS.159,160 On the other hand,Wang et al. prepared AFGd-LDH through surface modicationof Gd-LDH with atorvastatin (ATO) and ferritin heavy subunit(FTH). Both in vitro and in vivo experiments veried that theAFGd-LDH shows outstanding MRI performance and alleviatesbrain reperfusion injury issue due to the excellent ROS scav-enging efficiency.153Moreover, PDT has become a promising cancer treatmentapproach with superior advantages.161 Liu et al. synthesizeda tumor target agent through modifying NaYF4:-Yb,Tm@NaGdF4, folic acid (FA), carboxymethyl triphenylphos-phine (TPP) on graphene oxide quantum dots, in which theupconversion nanoparticles serve as a transducer of excitationlight.173 Both in vitro and in vivo experiments show that thesynthesized nanomaterials exhibited signicantly enhancedsubcellular targeting and PDT efficacy for cancer therapy.To further improve the treatment efficiency of tumor cells,researchers have designed 2D rare earth nanomaterials withmultiple treatment pathways. Wang et al. designed Ce-dopedCuAl-LDH (denoted as CAC-LDH) nanosheets and furtherloaded them with indocyanine green (ICG) to synthesize ICG/CAC-LDH.158 The synthesized ICG/CAC-LDH could induceintracellular glutathione (GSH) depletion and reduce Cu2+ andCe4+ to Cu+ and Ce3+, respectively, and further decompose H2O2to cytotoxic $OH through the Fenton reaction. The obviouslyenhanced absorption at 808 nm and efficient NIR light-to-heatconversion enable the ICG/CAC-LDH with photothermal effi-ciency and further increase the Fenton reaction rate. Thus, theICG/CAC-LDH showed remarkable dual-model PTT/CDT effi-cacy against HepG2 cancer cells. Furthermore, ICG/CAC-LDHcan also work as an MRI and photoacoustic imaging (PAI)agent. Moreover, triple-model combined therapy, so-calledchemo/PTT/PDT, can be achieved by co-modication of DOX,ICG and Gd3+ on MgAl-LDH (denoted as DOX&ICG/MLDH).162Jia et al. synthesized UCSP-FeMn-LDH through modifyingFeMn-LDH with mesoporous silica and Ce6 co-coatedNaGdF4:Yb,Er@NaGdF4:Yb (UCSP). The UCSP-FeMn-LDHshowed excellent oxygen-elevated PDT, enhanced PTT andCDT synergistic therapy (Fig. 15e) and real-time monitoring oftherapeutic effect via feasible CT imaging and T1/T2-weightedMRI.154In addition to tumor therapy, 2D rare earth nanomaterialscan also work in other biomedical applications. The Ag+-Chem. Sci., 2024, 15, 16887–16907 | 16901Fig. 15 Tumor therapy, antibacterial activity and living cell sensing properties of 2D rare earth nanomaterials. (a) Images of CT-26 tumor tissuescollected on day 15 after being treated with saline, PLGdH, Phy@PLGdH, saline + RT, Gd-NCPs + RT, PLGdH+ RT, Phy@PLGdH+ RT (group 1–7),in which PLGdH represents PEG-modified LGdH, Phy@PLGdH represents physcion modified PLGdH, Gd-NCPs represents spherical Gd-basednanoscale coordinate polymers and RT represents radiation therapy. Adapted from ref. 62. Copyright 2022, JohnWiley & Sons, Inc. Photographsof (b) tumors from different groups after 14 days treatment and (c) representative tumors in mice from different groups, in which group I to IVrepresent phosphate-buffered saline (PBS), gadolinium-based porphyrin paddlewheel framework (PPF-Gd), doxorubicin (DOX) and DOXmodified PPF-Gd (PPF-Gd/DOX) treatment, respectively. Reproduced from ref. 147. Copyright 2020, Oxford University Press. (d) Heating curvesof the LGdH/Ti3C2 hybrid under NIR laser irradiation. Adapted from ref. 146. Copyright 2023, the Royal Society of Chemistry. (e) Representativedigital photographs of excised tumor invidious treating groups of tumor-bearing mice after 14 days' treatment. (g) The relative tumor volume oftumor-bearing mice treated under different conditions. Adapted from ref. 154. Copyright 2020, John Wiley & Sons, Inc. (f) Antibacterial prop-erties and (g) antibiofilm properties of Tm3+/Er3+ co-doped K2La2Ti3O10 nanomaterials before and after Ag+ intercalation. Adapted from ref. 70.Copyright 2022, Elsevier Inc. (h) Confocal images of intracellular visualization for adenosines after treatment with the dye (tetramethylrhodamine(TAMRA) or fluorescein (FAM))-labelled MOF–La complex after an incubation time of 2 to 6 h and quantification of adenosines by two-colorfluorescence. Adapted from ref. 132. Copyright 2017, Springer Nature.Chemical Science Reviewmodied Tm3+/Er3+ co-doped K2La2Ti3O10 layered compounddemonstrates efficient antibacterial and antibiolm activity(Fig. 15f and g).70,122 Moreover, 2D MOFs with rare earth ions asnodes (MOF-Ln), such as MOF-La, also show some interestingproperties in biomedical applications. Wang et al. found thatthe uorescence of negatively charged uorescein (FAM)-labelled single-stranded DNA (ssDNA) was partially quenched16902 | Chem. Sci., 2024, 15, 16887–16907by MOF-Ln nanosheets, while that of positively charged tetra-methyl rhodamine (TAMRA)-labelled ssDNA showed the samequenching and recovery properties on MOF-Ln nanosheets.Thus the MOF-Ln can be used as a two-color sensing platformfor intracellular detection in living cells (Fig. 15h).132The summary of 2D rare earth nanomaterials for tumortherapy applications is listed in Table 3. The in vitro and in vivo© 2024 The Author(s). Published by the Royal Society of ChemistryReview Chemical Scienceexperiments veried that 2D rare earth-containing nano-materials have low cytotoxicity and great biocompati-bility.43,44,62,153 For example, Phy@LGdH showed no obviouscytotoxicity in CT26 tumor cells and the BOSC showed nosignicant inhibition of 4T1mouse breast cancer cells aer 48 hof incubation, while the LTbH showed no signicant cytotox-icity on Caco-2, HEK 293 cell line, HeLa, broblast and rhab-domyosarcoma cancer cell lines.46,62,148,159 However, only limitedexperiments on in vivo metabolic pathways have been carriedout. More attention in this regard is needed in future clinicalapplications. Therefore, with the combined advantages of highbiocompatibility, low toxicity and the unique biochemicalproperties of 2D rare earth nanomaterials and composites,further investigations of more 2D rare earth nanomaterials inbiomedicine elds, especially in in vivo applications, are highlydesired.4. ConclusionsIn summary, this work provides an overview of the recentprogress in the design, preparation and biomedical applica-tions of 2D nanomaterials based on rare earth elements. Inparticular, focus on the exfoliation of LREHs and rare earth-doped layered perovskites is presented. Through exfoliation,bulk layered LREHs or rare earth-doped perovskites can bedimensionally reduced into ultrathin nanosheets, whichpromotes further assembly and multifunctional applications.Articial superlattice assembly is an effective method toenhance the properties of rare earth-based or rare earth-con-taining nanosheets. Moreover, the application elds can beexpanded with integration with other functional counternanosheets. Progress in diverse biomedical applications (e.g.,drug delivery, MRI, cancer therapy, etc.) implies that the 2D rareearth nanomaterials have great potential in health-related eldapplications. Although signicant breakthroughs have beenmade in these elds, some limitations and challenges still exist.Herein, based on the current progress, several prospects andopportunities for the development of 2D nanomaterials basedon rare earth are proposed.(i) It is desirable to expand the family of rare earth-based ordoped nanosheets. Diverse-high-quality nanosheets based onrare earth elements with tuneable composition are essential forfurther research, although a variety of rare earth nanosheetssuch as LREHs and rare earth-doped perovskite nanosheetshave been proposed. Compared with the large class of similarLDHs and perovskites, the class of rare earth-based or dopednanosheets is still limited. Thus, the design and preparation ofnew 2D nanomaterials based on rare earth elements are of greatimportance.(ii) Further studies on the exfoliation process and in-depthunderstanding of the mechanism are urgent. High quality(i.e., high crystallinity, tuneable lateral size and ultrathinthickness) of rare earth-based or doped nanosheets is vital topromote the assembly and further applications. Although greatbreakthroughs have been made in the preparation of rare earth-based or doped nanosheets recently, the yield is very low.Furthermore, the nanosheets are unstable in the exfoliation© 2024 The Author(s). Published by the Royal Society of Chemistrysolution, that is, the nanosheets are easily restacked to formocculations and/or degrade in the solution upon long timestorage. Thus, more research should focus on the improvementof exfoliation yield and structural stability, such as selectinga more efficient exfoliation solvent and/or surface modicationon the nanosheets. Therefore, further studies and improve-ments in the exfoliation process are needed to propose basicprinciples and key factors which determine the exfoliationefficiency and nanosheet quality.(iii) More attempts at superlattice assembly of rare earth-containing nanosheets and other functional nanosheets areexpected. Molecular-scale assembly enables construction offunctional nanostructures based on 2D rare earth nano-materials with diverse compositions and structures, and is animportant method to improve their properties, especially forrare earth-based or rare earth-containing nanosheets. It isproved that for superlattice-like assembly, the suitable pairing(combination) of rare earth-based or doped nanosheets withother functional nanosheets can signicantly enhance theoriginal properties (e.g., photoluminescence properties) or evencreate new properties (e.g., photothermal properties). Thus,more attempts on the superlattice assembly of rare earth-containing nanosheets and other functional nanosheets (e.g.,MXenes, TMDs, oxides, etc.) may spawn new interesting func-tional materials.(iv) Expansion and acceleration on applications of 2Dnanomaterials based on rare earth elements. 2D rare earth-based or doped nanomaterials are mainly applied to drugdelivery and bioimaging, and have demonstrated excellentperformance and promising application prospects in diversebiomedicine elds. Further investigations of rare earth nano-materials in other frontier biomedical elds such as cancertreatment agents (e.g., photothermal therapy, photodynamictherapy, etc.), biosensors, theragnostic agents, tissue engi-neering and regenerative agents are highly expected.Data availabilityNo primary research results, soware or code has been includedand no new data were generated or analysed as part of thisreview.Author contributionsM. B. and R. M. conceived the idea. H. W., X. L. and R. M.supervised the project. The manuscript was written through thecontribution of all authors. All authors have approved the nalversion of the manuscript.Conflicts of interestThere are no conicts to declare.AcknowledgementsThe authors acknowledge the support of the National NaturalScience Foundation of China (U20A20123). R. M. acknowledgesChem. Sci., 2024, 15, 16887–16907 | 16903Chemical Science Reviewthe support from JSPS KAKENNHI (22H01916, 22K18956). M. B.and S. C. are thankful for the support from the Science andTechnology Research Program of the Chongqing MunicipalEducation Commission (KJQN202301134, KJQN202301133).H. W. is thankful for the support from the Project of ZhongyuanCritical Metals Laboratory (GJJSGFYQ202336).Notes and references1 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang,Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov,Science, 2004, 306, 666–669.2 H. Liu, A. T. Neal, Z. Zhu, Z. Luo, X. Xu, D. Tománek andP. D. Ye, ACS Nano, 2014, 8, 4033–4041.3 Z. 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Sci., 2024, 15, 16887–16907 | 16907 Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications Two-dimensional nanomaterials based on rare earth elements for biomedical applications