Design of Lanthanide-Doped Colloidal Nanocrystals

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Design of Lanthanide-Doped Colloidal Nanocrystals: Applications as Phosphors, Sensors, and Photocatalysts Debashrita Sarkar, Sagar Ganguli, Tuhin Samanta, and Venkataramanan Mahalingam*

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Department of Chemical Sciences and Center for Advanced Functional Materials (CAFM), Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, 741246, West Bengal, India ABSTRACT: The unique optical characteristics of lanthanides (Ln3+) such as high color purity, long excited-state lifetimes, less perturbation of excited states by the crystal field environment, and the easy spectral conversion of wavelengths through upconversion and downconversion processes have caught the attention of many scientists in the recent past. To broaden the scope of using these properties, it is important to make suitable Ln3+-doped materials, particularly in colloidal forms. In this feature article, we discuss the different synthesis strategies for making Ln3+-doped nanoparticles in colloidal forms, particularly ways of functionalizing hydrophobic surfaces to hydrophilic surfaces to enhance their dispersibility and luminescence in aqueous media. We have enumerated the various strategies and sensitizers utilized to increase the luminescence of the nanoparticles. Furthermore, the use of these colloidal nanoparticle systems in sensing application by the appropriate selection of capping ligands has been discussed. In addition, we have shown how the energy transfer efficiency from Ce3+ to Ln3+ ions can be utilized for the detection of toxic metal ions and small molecules. Finally, we discuss examples where the spectral conversion ability of these materials has been used in photocatalysis and solar cell applications. The characteristic sharp luminescence signals from Ln3+ ions are broadly divided into two processes, Stokes and anti-Stokes shifted emissions. The Stokes shift is the conversion of highenergy photons to lower-energy photons. This process has been further classified into two categories, namely, downshifting and downconversion. In energy terms, downshifting is the shift in the emission energy toward lower energy (higher wavelength) compared to the excitation energy and is shown by most Ln3+ ions such as Ce3+, Tb3+, Eu3+, etc. Downconversion is the process of converting one high-energy photon into two or more lower-energy photons (usually displayed by Pr3+, Eu3+, and Tb3+ with the support of Gd3+).3 Typically, UV energy photons are converted into two photons of visible energy. This property of the Ln3+ ions is useful in the conversion of harmful radiation such as X-rays or γ-rays into visible radiation. The second category called anti-Stokes emission is unique on its own. It is the process of converting two or more lower-energy photons into higher-energy ones. This type of emission, also termed upconversion, is typically observed from Ln3+ like Er3+, Tm3+, and Ho3+ in the presence of Yb3+, which acts as a sensitizer. Such emission is often utilized for imaging purposes in biomedical applications. Figure 2 shows the schematic illustration differentiating the luminescence originating from the three different above-mentioned processes.4 The above features are possible due to the long excited-state lifetimes as well as the presence of

1. INTRODUCTION 1.1. Lanthanide Luminescence. Luminescence has become an inevitable tool for understanding many physical, chemical, and biological processes. The broader scope of luminescence in various applications led to the design and synthesis of several luminescent materials, such as quantum dots, metal clusters, transition metal- and lanthanide (Ln3+)doped materials, and so forth. Among these, Ln3+-doped materials are particularly interesting due to their unique optical characteristics, which originate from intra 4f → 4f transitions. Though, these transitions are forbidden according to the Laporte rule, yet Ln3+ luminescence is still observed. This is attributed to the admixing of states and relaxation in symmetry, which increases the probability of transition.1 The transitions related to Ln3+ ions possess unique characteristics such as a narrow bandwidth leading to high color purity of emission, longer excited-state luminescence lifetimes typically in the microsecond to millisecond range, and large Stokes shifts. Most importantly, because of the shielding effect of outer 5s and 5d orbitals, the 4f → 4f transitions are largely unaffected by the ligand field around them, resulting in a unique unperturbed characteristic peak for each individual ion. In addition, the luminescence from the Ln3+ ions can span a wide spectral window, from the ultraviolet (UV) to near-infrared (NIR) region (Figure 1). This is possible due to the presence of multiple energy levels that arise from splitting owing to spin− orbit coupling. Although emissions from multiple energy levels are feasible, the selection rules limit the observation to only a few transitions with reasonable strength.2 © XXXX American Chemical Society

Received: May 14, 2018 Revised: August 25, 2018 Published: August 27, 2018 A

DOI: 10.1021/acs.langmuir.8b01593 Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

Figure 1. Fluorescence emission of different lanthanide (Ln3+) ions spanning the ultraviolet to near-infrared region. Reproduced with permission from ref 31. Copyright 2010 Elsevier.

suitable and well-researched hosts for Ln3+ ions. In fact, both Tb3+- and Eu3+-doped Y2O3 are well-known green and red phosphors and are extensively used in cathode ray tubes (CRT).6 In fact, Vetrone and coworkers have prepared Ln3+doped Y2O3 with quite small crystallite sizes ( 1000 °C solid state at T > 1000 °C combustion at T < 1000 °C combustion at T < 1000 °C sol−gel at T < 1000 °C sol−gel at T < 1000 °C hydrothermal method coprecipitation microwave assisted microwave assisted mechanochemical mechanochemical

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of phosphors and corresponding synthesis techniques employed for their preparation. Furthermore, we point readers to excellent articles on this topic from Capobianco’s group as well as others for further information.8−12 In most of the above-mentioned literature, the oxide-based phosphors were synthesized using a high-temperature solid-state method. However, the increasing scope of interest toward using Ln3+-doped materials for real world applications demands the growth of Ln3+-doped nanoparticles in colloidal forms. This is mainly due to the easier processability of colloidal dispersions in preparing films or for use in biological experiments. Moreover, colloidal nanoparticles have the advantage of easy integration with other materials to expand the application window. The past decade noted phenomenal growth in the design and synthesis of several dispersible Ln3+-doped nanocrystals. Because of their ease of synthesis, the literature on Ln3+-doped colloidal nanoparticles is dominated by fluoride nanocrystals. In addition, the lower phonon energy of fluorides (∼350 cm−1) compared to that of oxides (∼600 cm−1) makes fluorides advantageous for achieving better radiative emissions. The lower phonon energy helps in reducing the probability of nonradiative relaxations of the excited state, leading to a better luminescence quantum yield. Thanks to the development of thermal decomposition and hydrothermal methods, a variety of fluoride nanocrystals such as B


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telecommunications applications as well as deep tissue imaging. In fact, there are only a few reports on NIR light-emitting materials. Moreover, preparing nanocrystals with such properties in colloidal form is particularly challenging. Thus, it is important to develop sensitizers which can strongly enhance the luminescence from Ln3+ ions emitting over the large spectral window. Second, to widen the scope of applications it is important to develop different synthesis strategies for making water-dispersible Ln3+-doped nanocrystals. This is particularly challenging becauase the high phonon energies of water are detrimental to radiative relaxation of the Ln3+ excited states. However, research in this direction is rewarding as increase in the luminescence quantum yield along with suitable ligand choices are expected to push the detection limit down further in the sensing applications using these nanoparticles. Third, the wide absorption and emission window of the Ln3+ ions can be further explored for energy storage and environmental remediation applications, such as in photocatalysis and solar cells. In fact, to our knowledge, the Ln3+-doped nanocrystals have been relatively less explored for these applications. Fourth, batch-to-batch reproducibility is one major challenge in the field of colloidal nanocrystal synthesis and is a major roadblock for scaling up, which is important for commercializing colloidalnanocrystal-based techniques. The aforementioned points are some of the important aspects that scientists in this field are endeavoring to address. In this feature article, we attempt to collate and present some of this interesting work along with the effort from our group in addressing the pressing aspects listed above.

NaYF4, LiYF4, LaF3, YF3, and GdF3 have been synthesized with various morphologies in the recent past.25−29 For example, Zhao and co-workers have designed β-NaYF4 nanocrystals in hexagonal nanotube, flower-patterned hexagonal disk, and hexagonal nanorod shapes, which are shown in Figure 3. In

Figure 3. TEM images of β-NaYF4 with (a) arrays of flower-patterned hexagonal disks and a corresponding electron diffraction pattern (inset), (b) hexagonal nanorods and a corresponding electron diffraction pattern (inset), (c) hexagonal nanotubes with a corresponding electron diffraction pattern (inset), and (d) HRTEM images of the hexagonal nanotubes of β-NaYF4. Reproduced with permission from ref 29. Copyright 2007 Wiley.

2. STRATEGIES TO ENHANCE THE LUMINESCENCE OF Ln3+-DOPED NANOCRYSTALS 2.1. Sensitization of Stokes-Shifted Luminescence. One of the major limitations of Ln3+ ions arises from their low absorption coefficients, due to which the formation of excited states is severely hindered. This leads to the observance of weak luminescence signals from Ln3+ ions. One way to address this issue is through the use of sensitizers. The sensitizers possessing significantly higher absorption coefficients and quantum yields transfer their energy to the Ln3+ ions via the luminescence resonance energy transfer process (LRET), thereby promoting a relatively larger number of Ln3+ ions into their excited states.30 As a result of a higher population of Ln3+ in excited states, the intensity of the luminescence signal from Ln3+ ions increases significantly compared to that from direct excitation. This kind of effect is well documented in complexes with Ln3+ ions and is referred as the “antenna effect”.31 This process is efficient when the emission of the sensitizer (dye in many cases) effectively overlaps with the excitation profile of Ln3+ ions. Furthermore, LRET strongly depends on the distance between the sensitizer and the Ln3+ ions. For efficient energy transfer, the sensitizers and Ln3+ ions must be within the limit of the 10 nm distance. The efficiency of this process is given by E = 1 − (FDA/FD), where E is the transfer efficiency and FD and FDA are the fluorescence intensities of the donor in the absence and presence of acceptor, respectively. The Ce3+ ion has been extensively used as a sensitizer for the Ln3+ ions for a long time. Because of the similarity in ionic radii, they can be easily incorporated in place of Ln3+ ions in the host matrixes. It has been considered to be a promising sensitizer due to its unique 4f5d energy level, the position of which is largely dependent upon the geometry of the host matrix and thus can be tuned easily.32 Because of the mixing with d orbitals, transitions

the thermal decomposition method, the fluoride precursors of the Ln3+ salts are decomposed at a temperature above 300 °C in the presence of a capping ligand. The latter plays a decisive role in controlling both the size and shape of the nanocrystals. Because of the requirement of a high reaction temperature, this method demands the use of organic molecules, particularly longchain molecules which are stable at such high temperature. These necessities make oleic acid and oleylamine indispensable for the synthesis of Ln3+-doped fluoride nanocrystals. Although there are several studies on colloidal fluoride nanocrystals, there are fewer reports on corresponding Ln3+-doped oxide nanocrystals (vide infra). 1.3. Challenging Tasks. From the above discussion, it is clear that there is continuous research attention focused on developing colloidal Ln3+-doped nanocrystals due to their unique optical characteristics and the ease with which they can be prepared. Despite numerous reports and systematic studies, there is still room for improvement to expand the scope of applications of these nanocrystals. First, enhancing the luminescence quantum yield of the colloidal Ln3+-doped nanocrystals in both downshifting and upconversion processes is vital because it has been found to be quite low in most cases. For example, the calculated absolute quantum yields for upconversion are less than 5% even in bulk phosphors. In the case of nanoparticles the high surface free energy and the presence of more surface-related defects increase the probability of nonradiative relaxation leading to a decrease in the quantum yield. In this context, we mention that the maximum theoretical luminescence quantum yields for two-photon and three-photon processes are 50 and 33.3%, respectively. In addition, there is strong interest in developing materials which emit in the longerwavelength (e.g., NIR) region due to their use in optical C


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Figure 4. Energy-level diagram of different Ln3+ ions. The energy gap for the 4f → 4f5d transition in Ce3+ closely matches the higher energy states of Sm3+, Tb3+, Dy3+, and Tm3+ ions.

Figure 5. Digital images of the Ce3+/Ln3+-doped CaF2 nanoparticles (Ln = Tm/Tb/Sm)−PVA nanocomposite (A) showing high transparency of the composite film and (B) the film under UV light. (C) The emission spectrum of Ce3+/Ln3+-doped CaF2 nanoparticles show emissions in the entire visible region. The inset shows white light emission from an aqueous dispersion of the nanocrystals upon UV excitaion. Reproduced with permission from ref 43. Copyright 2016 Royal Society of Chemistry.

Tb3+-doped LaPO4 synthesized using a hydrothermal method was found to form nanofibers, microwave synthesis produced spherical nanoparticles.35 Lin’s group has made redispersible Tb3+-doped CeF3 nanoparticles with an average size of 7 nm in diethylene glycol at 200 °C.36 In fact, the authors coated an inert shell of LaF3 over the Tb3+-doped nanoparticles and observed an increase in the luminescence quantum yield. Ce3+/Tb3+-doped CaF2 hollow spheres synthesized by a hydrothermal method have been explored for drug (Ibuprofen) delivery application.37 One of the drawbacks of using Ce3+ arises from its tendency to readily oxidize to Ce4+.To minimize this effect, Capobianco et al. have prepared Ce3+/Tb3+-doped NaGdF4 nanocrystals with a suitable NaYF4 shell using a thermal decomposition method.33 Next to Ce3+ sensitization of Tb3+ ions, energy transfer to Dy3+ ions has also been well studied. For instance, Ln3+-doped CeF3 nanoparticles (Ln = Tb, Eu, and Dy) synthesized using a facile polyol method showed white light emission due to the combination of these Ln3+ ions.38 In another report, a roomtemperature synthesis route was developed to prepare water-

between the 4f5d level and energy states arising from 4f orbitals are allowed. In fluoride matrixes, this band lies between 30 000 and 35 000 cm−1.This broad 4f5d band matches a few excited energy levels of Ln3+ ions (such as Tb3+, Dy3+, Sm3+, and Tm3+) as shown in Figure 4. For example, both Tb3+ and Dy3+ ions possess excited energy levels close in energy to the 4f5d band of Ce3+ ions, and thus efficient energy transfer is observed from Ce3+ to Tb3+ and Dy3+ ions.33 In fact, there are several reports available on Ce3+- and Tb3+- or Dy3+-doped systems, only few representation examples of which will be discussed. For example, Haase et al. prepared monodispersed colloidal Tb3+-doped CePO4 in the size range of 5 nm. The nanocrystals were synthesized using Ce(NO3)3 and tris(ethylhexyl) phosphate under a nitrogen atmosphere at 180 °C. An inert atmosphere is required to prevent the oxidation of Ce3+ to Ce4+. The nanocrystals exhibited intense green emission with a relative quantum yield of 11% (only Tb3+ emission) with respect to rhodamine 6G.34 The synthesis method also plays a pivotal role in the morphology of nanocrystals. For example, while Ce3+/ D


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doped into the host matrix.49 This motivated scientists to develop other sensitizers such as semiconductor quantum dots, suitable organic chromophores, and carbon dots possessing strong absorption coefficients to sensitize the Ln3+ luminescence.50 For example, quantum dots such as CdSe and ZnS have been explored for the sensitization of Ln3+ luminescence. Later, Pteoud and coworkers reported the synthesis of Tb3+-doped CdSe nanocrystals and observed intense Tb3+ luminescence via CdSe excitation.51 The same group later developed a postmodification strategy and successfully doped Tb3+ and Eu3+ in ZnS nanocrystals.52 In their seminal work, Meijering’s group have reported the incorporation of Yb3+ ions in CdSe nanocrystals. Their results suggest that Yb3+ ions first get adsorbed on the CdSe surface followed by shell growth with Se ions.53 Despite the above reports, the incorporation of Ln3+ ions inside quantum dots is a challenge because of the differences in charge and coordination number compared to those of the host cation. In the recent past, the development of carbon-based quantum dots (C dots) has received a huge amount of attention because of the strong absorption and emission in the near-UV to visible region. We took advantage of the strong absorption of the C dots and developed a Eu3+-doped LaF3/C dots composite. Upon UV excitation at 340 nm, intense sharp emissions characteristic of Eu3+ ions were observed from the colloidal dispersion of the nanocrystals.54 The enhanced intensity was ascribed to the energy transfer from C dots to Ln3+ ions and was verified by the presence of a broad band in the excitation spectrum of the composite. The other strategy which has been less well explored is the use of organic molecules as both capping ligands and sensitizers. For example, Petoud and coworkers have shown the sensitization of the NIR emission of Nd3+ and Yb3+ in Ln3+-doped NaYF4 nancrystals using tropolone as a sensitizer.31 Upon UV excitation, tropolonate ions absorb the light and transfer energy to Yb3+ and Nd3+ ions, which in turn emit in the NIR region. We have developed a ligand-exchange strategy to bind a sensitizer such as 4,4,4-trifluoro-1-phenyl-1,3-butanedione (TPB) to Eu3+-doped LiYF4 nanocrystals. The sensitization of Eu3+ ions greatly enhanced the quantum yield of Eu3+ ions (∼31%) compared to the ∼5% obtained via the direct excitation of Eu3+ ions (λex = 394 nm) in Eu3+-doped LiYF4 nanocrystals. We utilized these nanocrystals for the enhancement of photocurrent in the solar cell. This was achieved by the deposition of the nanocrystals on top of a Si solar cell and subsequent excitation with UV light.55 2.2. Synthesis of Core−Shell Nanoparticles. Another way to increase the luminescence efficiency of lanthanides is through developing core−shell nanocrystals. Usually the vibrational modes of different functional groups, such as −OH and −NH2, on the surface of nanocrystals reduce the Ln3+ luminescence efficiency as they promote nonradiative relaxation of the excited state. This problem can be overcome by covering the surface of the nanocrystals with appropriate shell material. The synthesis of shell over nanocrystals decreases surface defects, which are major promoters of energy loss via a nonradiative pathway. One of the most used materials for shell formation is silica, which is not only easy to coat but is also biocompatible. Furthermore, it provides both water dispersibility and porosity to the material. Zhang et al. have synthesized Yb3+/Tm3+- doped NaYF4 core nanocrystals with a silica shell and have used the resulting highly crystalline hexagonal nanocrystals as fluorescent probes for cell imaging. However, the presence of the silica shell decreased the luminescence intensity of the nanocrystals.56

dispersible Ln3+-doped KGdF4 (Ln = Ce, Eu, Tb, and Dy). The nanoparticles were prepared in a water−diethylene glycol solvent mixture and were found to be quite monodisperse in water.39 Most of the studies on Ce3+ sensitization were focused on Tb3+ and Dy3+ ions. In fact, the sensitization of other Ln3+ ions using Ce3+ ions has been less explored, particularly in colloidal nanoparticles. Recently, Sun et al. developed a Ce3+-sensitized quantum cutting phosphor with core/shell nanostructured NaGdF4:Ce3+@NaGdF4:Nd3+@NaYF4. This material was found to be efficient in converting one UV photon to one visible and NIR photon.40 In another report, water-dispersible Nd3+- and Tb3+-doped CeF3 nanocrystals were made at room temperature. This material was explored as a bimodal probe in fluorescence and CT imaging.41 Our group has worked extensively in studying energy transfer from Ce3+ to other Ln3+ ions such as Tm3+, Nd3+, and Sm3+. For example, we prepared citric acid-capped Tm3+-doped NaYF4 nanocrystals, which displayed intense single-band blue emission at 450 nm from a 0.1% colloidal nanoparticle dispersion. The nanoparticles were uniformly distributed in a polymer (poly(vinyl alcohol)) to develop a transparent film. In fact, by depositing these films onto a commercial UV LED, a strong blue light was obtained upon electrical excitation.42 We extended the idea to develop singlecomponent white-light-emitting colloidal nanoparticles with single excitation. This was achieved by preparing Ce3+/Ln3+doped CaF2 nanocrystals (Ln = Tm/Tb/Sm). Upon exciting the Ce3+ ion, energy gets efficiently transferred to other codoped Ln3+ ions. This results in white-light emission originating from the combination of blue, green, and red emission from Tm3+, Tb3+, and Sm3+ ions, respectively (as shown in Figure 5).43 In another study, we have replaced Tb3+ and Sm3+ with Mn2+ ions to produce white-light emission in NaYF4 nanocrystals.44 Replacing Ln3+ ions with transition-metal ions is cheaper because of their abundance. As is clear from the above discussion, most of the studies report the sensitization of visible emission peaks of Ln3+ ions. However, it is quite challenging to sensitize NIR emissions because they are more prone to nonradiative relaxation leading to smaller luminescence quantum yields. Stouwdam et al. have reported strong emissions in the NIR region from an Ln3+-doped LaVO4 matrix (Ln = Nd, Dy, and Ho). In this material, the sensitization of the Ln3+ ions occurred through energy transfer from the excited vanadate (VO43−) groups to the Ln3+ ions.45 We have shown that both Nd3+ and Sm3+ when incorporated in citric acid-capped CeF3 nanocrystals are able to display strong emission peaks centered at 1054 and 980 nm, respectively. In addition, the nanocrystals were highly dispersible in water due to surface functionalization with citric acid.46 Recently, we have shown strong NIR emissions, spanning a wide spectrum (900 to 2000 nm), from a series of Ln3+ (Ln = Sm, Nd, Dy, Tm, Er, and Ho) ions from water-dispersible Ln3+-doped GdVO4 nanocrystals.47 The calculated energy-transfer efficiency for most of the studied Ln3+ ions is about 90%. In fact, the observed enhancement in the NIR emission intensity varies between 3-fold (for Sm3+) and 53fold (Nd3+) compared to direct excitation. Similar energy transfer has been explored in Dy3+- and Tm3+-doped LaVO4 core−shell nanocrystals for the development of white light.48 Though the sensitization of Ln3+ ions with Ce3+is effective, one of the major limitations is also the high propensity of Ce3+ to emit as a result of the allowed nature of the 4f5d → 4f transition. In addition, Ce3+ ions possess a high probability for concentration quenching, which limits the amount of Ce3+ E


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The shell was further doped with Eu3+ and Tb3+ ions because the emission of both are temperature-dependent. The authors were able to show a sensitivity of 1.2% per Kelvin in the broad range of 125 to 300 K.63 The above-stated examples imply that the formation of a shell around an active core is one of the techniques for enhancing the luminescence intensity of the Ln3+doped nanomaterials. 2.3. Sensitization of Luminescence Obtained via Upconversion. Some of the Ln3+ ions possess a unique optical property called upconversion. In this process, upon NIR excitation at 980 nm, emission is observed in the visible region. Initially, Ln3+ ions in their ground state absorb a NIR photon and get promoted to an excited state. Because they have long excited-state lifetimes, another photon is absorbed by the system before relaxation and a higher excited state is achieved. Thereafter, radiative relaxation occurs, resulting in emission in the visible region. To improve the efficiency of this process, Yb3+ ions are often used as a sensitizer because they have a relatively higher absorption coefficient compared to those of other Ln3+ ions. In general, Yb3+ ions are codoped with other suitable Ln3+ ions (such as Er3+ and Tm3+) in a host matrix. Upon 980 nm excitation, the Yb3+ ions are excited and transfer their energy to a neighboring Ln3+ ion having a long excited-state lifetime. Subsequent energy transfer from Yb3+ to the same Ln3+ ion promotes it to a further higher-energy excited state that radiatively relaxes back to produce emission in the visible region. Figure 7 shows the upconversion emission spectra and

It has been observed that the formation of either a passive shell, i.e., undoped host material or an active shell such as Ln3+doped host material, lead to the improvement of luminescence intensity. Furthermore, materials with the same or similar lattice parameters are preferred as shell material because they reduce luminescence quenching due to less lattice mismatch. Ballato and coworkers developed Eu3+-doped LnF3 core−LaF3 shell nanocrystals that showed enhanced emission.57 Along similar lines, Lin et al. have developed a Tb3+-doped CeF3 core encapsulated in a LaF3 shell. The synthesis of shell around an active core reduced the nonradiative processes and displayed an enhanced luminescence and lifetime over those of bare Tb3+doped CeF3 nanocrystals.58 Haase and co-workers synthesized Tb3+-doped CePO4 core with LaPO4 shell showing a quantum yield of 80% which is pretty high for nanocrystalline phosphors.59 Capabianco and co-workers prepared a Yb3+/Er3+-doped NaGdF4 active core and a Yb3+-doped NaGdF4 shell which showed enhanced emission in the red (nearly 3×) and green (nearly 10×) regions compared to that of the active core and passive shell.60 Emission from several activator lanthanide ions was demonstrated using a core−shell−shell approach by Wang et al. The Nd3+-doped NaYbF4 as core was designed with an Er3+/Ho3+/Tm3+-doped Na(Yb/Er)F4 shell and NaGdF4 as a shell over shell using epitaxial growth.61 The TEM image of the above-discussed nanocrystals with the core−shell−shell approach is shown in Figure 6. The results clearly demonstrated

Figure 6. (a) TEM image of NaYbF4:Nd@Na-(Yb,Gd)F4:Er@ NaGdF4 core−shell−shell nanocrystals. (b) High-resolution TEM image of a nanocrystal. (c) TEM image of randomly selected nanocrystals for compositional analysis. (d−f) Elemental maps of Nd, Yb, and Gd in the nanocrystals. Reproduced with permission from ref 61. Copyright 2013 Wiley.

Figure 7. UC luminescence spectra of β-NaYF4 codoped with (a) 2 wt % Er3+ and 20 wt % Yb3+ or (b) 2 wt % Tm3+ and 20 wt % Yb3+ (excited with a 978 nm laser diode; power density of 80 W cm2). (c) Photographs of the UC luminescence of 0.2 wt % (1) and 1 wt % (2) hexane solutions of β-NaYF4 codoped with 2 wt % Er3+ and 20 wt % Yb3+ and of 0.2 wt % (3) and 1 wt % (4) hexane solutions of β-NaYF4 codoped with 2 wt % Tm3+ and 20 wt % Yb3+ (excited with a 978 nm laser diode with a laser power of 800 mW). Reproduced with permission from ref 29. Copyright 2007 Wiley.

the presence of Nd3+ in the core and Gd3+ in the shell. Other interesting work from Liu’s group reports that upconversion energy can be tuned with energy migration via the careful design of the core−shell structure. They synthesized a Gd3+-based sublattice which was able to show upconversion from a variety of lanthanide ions as activators. NaGdF4@NaGdF4 core−shell nanoparticles were prepared with the Yb3+/Tm3+ system as a sensitizer and Tb3+, Dy3+, Sm3+, and Eu3+ ions as separate activators. This energy-transfer mechanism from core to shell reduced the cross-relaxation processes and long-lived intermediate energy states which act as barriers for lanthanide emission.62 Qiu’s group has developed a NaGdF4 core NaGdF4 shell as a noncontact reference probe for temperature sensing. In this work, Yb3+/Tm3+ absorbed the energy and transferred it to Gd3+ in the shell through a two-step energy-transfer process.

the digital image of the cuvette displaying visible emission from Yb3+/Er3+- and Yb3+/Tm3+-doped NaYF4 nanocrystals, respectively, upon 978 nm NIR excitation. However, the enhancement of upconversion emission is a challenge compared to Stokesshifted luminescence. Please note that although Yb3+ ions are used as a sensitizer we exclude it in this discussion because it is codoped with other activator Ln3+ ions in most of the upconversion studies.64−66 We highlight only some of the representative examples. In fact, sensitization of the luminescence occurring via upconversion is a challenge because it requires a suitable material which has absorption in the NIR F


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Figure 8. General depiction of the active-core/active-shell nanoparticle architecture showing the absorption of NIR light by the Yb3+-rich shell (represented in red) and subsequent energy transfer to the Er3+/Yb3+ codoped core (represented in green), which leads to upconverted blue, green, and red emissions. Photographs of colloidal solutions of NaGdF4:Er3+/Yb3+ active-core/NaGdF4 inert-shell nanoparticles and active-core/active-shell NaGdF4:Er3+/Yb3+/NaGdF4:Yb3+ nanoparticles in toluene (1 wt %) following excitation with 980 nm. Reproduced with permission from ref 60. Copyright 2009 Wiley.

region and is able to efficiently transfer energy to the Ln3+ ions. There are, in fact, meagre reports on the enhancement of upconversion luminescence by the sensitization technique. Hummelen et al. have shown an almost 3200-fold enhancement of Er3+ emission via sensitization with an IR dye in Er3+/Yb3+doped NaYF4 nanocrystals.67 Recently, Chanchal et al. have employed water-dispersible NIR-II dye (IR-1061) to sensitize core/active shell upconverting nanocrystals and have achieved a sufficiently higher upconversion luminescence intensity in aqueous media.68 Although NIR dyes are a good choice, their stability is an issue of concern. To overcome this, Vetrone et al. have developed an active core/active shell strategy which provides a huge enhancement in upconversion luminescence (Figure 8).60 Recently, it was observed that the integration of upconverting nanocrystals with noble metal nanocrystals resulted in an enhancement of the upconversion process.69 For example, Sudheendra et al. synthesized core/shell nanocrystals by developing a thin gold shell over Tm3+/Yb3+-doped NaYF4 nanocrystals. The presence of the gold shell enhanced the upconversion emission intensity of Tm3+ ions by about 8fold. They attributed the observed enhancement to the following two reasons: first, the change in the crystal field of the dopant ions induced by the coupling of gold plasmon with the local electric field and second, the prediction that the rates of the nonradiative decay might be decreased.70

high temperature and thus require the use of high-boiling-point solvents and organic capping agents, which make them hydrophobic and render them dispersible only in organic media.60,76,77 Though the thermal decomposition method produces Ln3+-doped nanomaterials that have excellent optical properties, the fluorinated byproducts often formed during these reactions are extremely hazardous in nature. The coprecipitation technique is an alternative synthesis method but generally produces nanocrystals with poor optical properties. The high-temperature methods reported above make use of long-chain hydrocarbon molecules such as oleic acid and oleylamine containing a −COOH or −NH2 group on one end and a methyl group on the other end. This −COOH or −NH2 group attaches to the nanoparticle surface, and the alkyl groups dangling outward give rise to the hydrophobic nature. Though these hydrophobic nanocrystals are easily dispersible in nonaqueous solvents such as toluene, cyclohexane, and hexane, they must be dispersible in water in order to explore their potential for biological applications. Most of the synthesis methods use oleic acid as the surface capping agent. To make the oleic acid-capped nanocrystals hydrophilic, Capobianco’s group oxidized the double bond of the oleic acid, thereby breaking it and converting it to azelaic acid.78 However, the intensity of the upconversion emission decreased, unlike for the C18 oleic acid, as the C9 carbon shield was unable to protect the excited Ln3+ ions from solvent-induced quenching. The steric hindrance of the hydrophobic groups of oleic acid can be stabilized by the introduction of amphiphilic molecules, which also leads to the improvement of the water dispersibility.79 Chow et al. developed Yb3+/Er3+-doped NaYF4 nanocrystals coated with an amphiphilic group (25% octylamine- and 40% isopropylamine-modified poly(acrylic acid)).80 Though the water dispersibility of the nanocrystals was improved, the introduction of additional polymer layers resulted in an increase in their hydrodynamic size. The encapsulation of the nanocrystal surface with a SiO2 layer is another approach to make nanoparticles water-dispersible. Although the silica coating is biocompatible and widely explored, the presence of strong Si−O stretching vibrations decreases the overall luminescence quantum yield.81 Zhang and coworkers have synthesized silica-coated Yb3+/Er3+-doped NaYF4 nanocrystals which were surface coated with polyvinyl pyrrolidine.82 Along with the decrease in emission, controlling the size is also a

3. CHEMICAL APPROACHES OF MAKING WATER-DISPERSIBLE Ln3+-DOPED COLLOIDAL NANOCRYSTALS 3+ Ln -doped nanocrystals, especially upconverting nanocrystals, have huge potential in biological applications such as in sensing, imaging, and drug delivery, just to mention a few.71,72 A NIR light source has many advantages over UV or visible light sources for use in biology- and biomedicine-related fields. First, deep tissue penetration is possible via a NIR source.73 Second, there is less autofluorescence, which accounts for a better signal-to-noise ratio. Third, the Ln3+ ions used in the nanocrystals have longer luminescence lifetimes as compared to quantum dots and organic dyes which are used for the same purpose.74,75 All of these factors make Ln3+-doped upconverting nanocrystals an ideal material for biological applications. Most of the syntheses methods (such as thermal decomposition, solvothermal, and others) reported for preparing upconverting materials involve G


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green ratio increases with increase in the polarity of the solvent. Thus, the red to green ratio can be utilized as a probe for distinguishing protic and aprotic solvents.86 To further increase the reactivity of the double bond present in the surface capping agent, 10-undecenoic acid was chosen because it has a terminal double bond that can be functionalized relatively easily. 10-Undecenoic acid-capped Yb3+/Tm3+-doped LiYF4 nanocrystals were synthesized using a thermal decomposition method at 300 °C.87 The resulting nanocrystals crystallized in the tetragonal phase and were diamond-shaped with high crystallinity. The surface modification was done by thiol−ene click chemistry using various thiol-containing molecules such as cysteine, cysteamine hydrochloride, 3mercaptopropanoic acid, and 2,3-dimercapto-1-propanol. Surface modification of the nanocrystals imparted to them −COOH, −NH2, and − OH groups, thereby making the surface of the nanocrystals hydrophilic in nature. Prior to surface modification, the Yb3+/Tm3+-doped LiYF4 nanocrystals were dispersible only in toluene, whereas after thiol−ene click chemistry was performed, their dispersibility in water was improved. Chow and co-workers have used bifunctional organic molecules (poly(ethylene glycol) 600 diacid) to replace the amine ligand of the oleylamine-capped Yb3+/Er3+-doped NaYF4 nanocrystals.80 To widen the ligand choice for functionalization, we developed a ligand exchange method using methyl oleatecapped nanocrystals. The idea of using methyl oleate as a capping ligand was due to their weak ability to coordinate to the surface of the nanocrystals. This property enables easier removal of the ligands and allows the nanocrystals to further bind to water-soluble ligands to convert the surface from hydrophobic to hydrophilic. Methyl oleate-capped Yb3+/Er3+-doped NaYF4 nanocrystals synthesized using a thermal decomposition method were pure cubic phase with an average particle size of around 35 nm.88 The ligand exchange was performed by simple sonication in the presence of a series of dicarboxylic acids such as oxalic acid, malonic acid, and adipic acid. After successful ligand exchange, it was observed that the nanocrystals were noticeably dispersible in water. Furthermore, aqueous dispersions of dicarboxylic acid capped nanocrystals were subjected to continuous irradiation under 980 nm laser and the photoluminescence was recorded at regular intervals (up to 12 hours). The results showed no significant change in the luminescence behavior, indicating excellent photostability of Ln3+ emission under continuous excitation. This may be attributed to the wellprotected nature of 4f orbitals that are the origin of Ln3+ luminescence. A recent report from Shuck and Talapin’s group indicated that upconverting nanoparticles are extremely photostable when kept under a 10 mW 980 nm CW laser for 1 h. During this time, no photobleacing or photodamage was observed while studying the single-molecule spectroscopy of upconverting nanoparticles.89 From the above discussions, we may conclude that with the proper choice of ligands and surface modification techniques one can easily tune the surfaces of nanocrystals and their luminescence behavior. An overall scheme depicting different routes used by our group to synthesize water-dispersible nanocrystals is shown in Figure 10.

challenge due to the difficulty in forming a uniform coating of the SiO2 layer on the nanocrystal surface. Alternatively, water dispersibility can also be achieved by oxidation of the double bond of the oleic acid on the surface of the nanocrystal. This method results in the formation of −COOH groups, but the time taken to complete the reaction is generally quite long (∼48 h).78,83 Apart from the above method, the synthesis of ligandfree nanocrystals is another way to achieve water dispersibility. This can be done by the removal of oleic acid molecules from the nanocrystal surface using a 0.1 M HCl solution.84 Though the ligand removal successfully makes the nanocrystals waterdispersible, the presence of H3O+ reduces the luminescence quantum yield because of their high vibrational states that nonradiatively depopulate the excited states of Ln3+ ions. Ligand exchange of oleic acid with hydrophilic polymers such as poly(vinylpyrrolidone), poly(acrylic acid), and poly(allylamine) has been used to make nanocrystals water-dispersible. It is worth pointing out here that the complete removal of oleic acid via a ligand-exchange pathway is difficult because oleic acid binds very strongly to the nanocrystal surface.85 Motivated by the work of Li et al. in converting hydrophobic nanocrystals to hydrophilic ones by oxidation of the double bond of oleic acid by the Lemieux−von Rudolff reagent, we have used ricinoleic acid as a surface capping agent.83 Ricinoleic acid is a long-chain hydrocarbon with the carboxylic group at one terminal and a double bond at the C9 position with a −OH group adjacent to it. Ricinoleic acid-capped Yb3+/Er3+-doped BaLuF5 was synthesized using a thermal decomposition method with an average particle size of 10 nm having a pure tetragonal phase. Hydroxylation of the double bond was performed with epoxidation using H2O2/HCOOH followed by ring opening of the epoxy bond using NaOH in water. This process converted the hydrophobic nanoparticles to hydrophilic ones. The higher water dispersibility of the hydroxylated ricinoleic acid-capped BaLuF5 nanocrystals was attributed to the bent structure which exposed multiple hydroxide groups near the surface of the nanocrystals facing the solvent medium. A comparison between the dispersibility of the nanocrystals in H2O, DMF, DMSO, and THF as a dispersion medium showed that the nanocrystals have the highest luminescence in water as shown in Figure 9. The red to green ratio in upconversion emissions of the Yb3+/Er3+-doped BaLuF5 nanocrystals was different in different solvents. The ratios obtained were 1.40, 0.87, 0.69, and 0.27 for H2O, THF, DMF, and DMSO, respectively. The trend shows that the red to

4. Ln3+-DOPED COLLOIDAL NANOCRYSTALS IN DETECTION APPLICATIONS Detection or sensing based on Ln3+-based colloidal nanocrystals has several advantages. Their higher luminescence lifetimes added with sharp emission peaks unperturbed by the crystal

Figure 9. Photoluminescence spectra from hydroxylated ricinoleic acid-capped BaLuF5 nanoparticles dispersed in H2O, DMF, DMSO, and THF. The results show that the nanoparticles show superior luminescence in water. Reproduced with permission from ref 86. Copyright 2013 Wiley. H


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Figure 10. Scheme of different routes used by our group to synthesize water-dispersible nanoparticles.

field, and their large Stokes shift and high resistance to photobleaching make them ideal candidates to be used as optical probes. These properties complement other luminescent nanocrystals such as quantum dots, which display a strong size effect on optical properties. Furthermore, most of the other luminescent markers such as quantum dots and organic molecules suffer from shorter lifetimes and poor stability. Detection using Ln3+-doped nanocrystals generally follows the LRET mechanism. For effective LRET, there are two conditions that must be satisfied. The first is the overlap of the absorption band of the acceptor with the emission from the Ln3+-doped nanocrystals. The second is the optimum distance between the donor and acceptor molecules. Different Ln3+ ions show emission in different regions of the electromagnetic spectrum (UV−NIR). This property enables Ln3+-based colloidal systems to be used for sensing applications because the effective overlap between donor and target analytes can be controlled by the proper choice of Ln3+ ions. Also, time-resolved detection is easier owing to the longer excited-state lifetimes of Ln3+ ions. Our group has developed versatile and easy techniques for the fast and efficient detection of some toxic metal ions and molecules. 4.1. Metal Ion Detection. 4.1.1. Pb2+ and Hg2+ Ions. Pb2+ is one of the most toxic heavy metal ions which have caused substantial environmental pollution due to their extensive use in industrial sectors for several decades. Their persistent accumulation in the water, soil, and air leading to health hazards related to respiratory, breathing, endocrine, reproductive, and developmental disorders has already been proven. On the other hand, Hg2+ is another hazardous heavy metal ion which has been known to cause deadly ailments such as Minamata disease. The

simultaneous detection of Pb2+ and Hg2+ in bodies of water is very crucial as more and more reports are showing that bodies of water are contaminated by both of these toxic heavy metal ions. In the recent past, Wang and co-workers have developed an upconversion-based system for the simultaneous detection of Pb2+ and Hg2+.90 The upconverting nanocrystals were acting as donors and were decorated with Au nanocrystals acting as energy acceptors. The nanocrystals were fabricated with a special sequence of DNA which was responsive to Pb2+ and Hg2+ by forming a G quadruplex and a hairpinlike structure upon addition. Upon interaction with Pb2+ and Hg2+ as analytes, a detachment of Au nanocrystals from upconverting nanocrystals takes place, followed by the recovery of luminescence. Li et al. have developed Ln3+-coordination polymer nanocrystals which act as a turn-on fluorescent sensor for the detection of Hg2+.91 Saleh et al. have systematically reviewed the effects of metal ions on the upconversion luminescence from Er3+/Yb3+-doped NaYF4 nanocrystals.92 Our group has developed ultrasmall glutathione-capped Ce3+/Tb3+-doped SrF2 nanocrystals via microwave irradiation. The nanocrystals displayed selective quenching of green luminescence of Tb3+ ions upon addition of Pb2+ and Hg2+ ions. This quenching was attributed to the strong interaction between the heavy metal ions and the thiol group present in glutathione. This strong interaction led to the detachment of S− H from the surface of the nanocrystals, leaving defect sites which are responsible for the quenching of Tb3+ luminescence. The limit of detections was found to be 20 and 30 μM for Pb2+ and Hg2+, respectively. The sensing application of the luminescent probe was also tested in real water samples (lake water and pond water) for the detection of the heavy metal ions in water. The I


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This leads to quenching of the intensity of green emission from Tb3+ ions, which is used as a response to the presence of H2O2 in the dispersion of nanocrystals. 4.2.2. Picric Acid. In recent years, the development of selective, sensitive, rapid, low-cost, and portable methods for the detection of explosives, particularly nitro derivatives, has received much attention. Among them, the detection of trinitrotoluene (TNT) has been extensively researched. Unfortunately, a similar effort to detect picric acid, which is a far stronger nitro-based explosive and extensively used in conventional bombs, grenades, and improvised explosive devices has not received as much attention. Lately, Sudesna et al. have developed a fluorescent marker consisting of scutellarin, hispiduloside, and curcumin with green solvent glycerol for picric acid sensing.102 Dynamic covalent imine chemistry has been utilized by Koushik et al. for selective picric acid detection through fluorescence. They have prepared a self-assembled nanoscopic organic cage that is sensitive to picric acid even at 6.4 ppb concentration.103 In their seminal work, Sarkar et al. have developed a luminescent probe for the selective and sensitive detection of picric acid based on the luminescence quenching of Ce3+ and Tb3+ codoped Sr2GdF7 nanocrystals. The use of Ln3+luminescence helped in pushing the detection limit down to 4 nM. This value was approximately 3 orders of magnitude lower than that in previous reports, where the limit of detection was in the micromolar range.104 4.2.3. Aromatic Amino Acids. Aromatic amino acids such as tryptophan, tyrosine, and phenylalanine are essential components in the production of hormones. It has been observed that during gastric cancer the levels of aromatic amino acids increase. Therefore, its detection inside the body is essential for diagnostic purposes. Au and Ag nanoparticles (specifically their surface plasmon resonance), screen-printed electrodes, and CdS nanocrystals are some of the reported materials for the detection of amino acids.105 Our group has successfully developed sodium dodecyl sulfate (SDS)-coated Ce3+/Tb3+-CaMoO4 based nanocrystals for the simultaneous detection of aromatic amino acids. The mechanism of detection is based on the LRET method. Aromatic amino acids absorb around 280 nm, which overlaps with the emission spectra of Ce3+ ions. In the presence of amino acids, the energy-transfer efficiency between Ce3+ and Tb3+ ions is reduced. The distance between Ce3+ and amino acids was calculated to be 4 nm, which is equal to the SDS bilayer formed. This distance is within the LRET distance, and hence the effective energy transfer was quite high. The limit of detection was calculated to be 20.9 nM, 0.27 μM, and 9 μM for tryptophan, tyrosine, and phenylalanine, respectively.106 To examine the feasibility of the developed nanocrystals for use in a sensing device, the nanocrystals were deposited onto positively charged glass slides. The system showed a remarkable sensing property, which was active even after five uses. A schematic of the aforementioned system is presented in Figure 11. 4.2.4. Melamine. Melamine is an important industrial material which was used in plastic engineering and the agricultural sector as a fertilizer. However, recently it has been found to be used as an adulterant in many baby and pet foods to increase the protein content. To determine the content of melamine present in milk samples, cyanuric acid-derived Au nanocrystals have been developed.107 Lin et al. have developed a turn-on fluorescent sensor based on fluorescein and Au nanocrystals for the sensitive detection of melamine.108 As discussed before, upconverting nanocrystals can play a major

material could be reused because the recovery of luminescence was possible by the addition of EDTA to the nanoparticles dispersion in water.93 4.1.2. Cu2+ Ions. Cu2+ (cupric ion) is one of the essential trace metals found in biological systems, but often excess Cu2+ can lead to neurodegenerative conditions such as Alzheimer’s, Parkinson’s, and Wilson’s diseases.94 The determination of Cu2+ concentration is thus useful in the early detection of diseases and their prevention. For biological applications, ultrasmall, waterdispersible nanocrystals are essential so that they can cross the cell-wall barrier. Jiang and coworkers have designed a dyeloaded upconverting nanocrystal that is able to sense Cu2+ ions.95 In another report, Er3+/Yb3+-doped NaYF4 functionalized with branched polyethylenimine was found to selectively detect Cu2+ ions.96 Liong and coworkers have shown that silicacoated Eu3+-doped GdVO4 nanocrystals are efficient detectors of Cu2+ ions. Strong reduction in the Eu3+ emission intensity is noted in the presence of Cu2+ ions. The authors proposed that the matching of the absorbance of Cu2+ with the Eu3+ luminescene is responsible for the quenching of the emission intensity.97 Our group has synthesized polyacrylic acid-capped Eu3+doped KZnF3 nanocrystals and used them for the selective detection of Cu2+ ions. The red emission of the nanocrystals was found to quench in the presence of Cu2+ ions. This is possibly due to energy transfer of Eu3+ to surface-attached Cu2+ ions. The limit of detection that was calculated was about 0.48 μM.98 To improve the sensitivity of Cu2+ detection, another set of waterdispersible nanocrystals was developed by our group. Ce3+/ Tb3+-doped SrF2 nanocrystals were synthesized using microwave irradiation. The synthesized nanocrystals were waterdispersible due to surface capping using poly(acrylic acid). It was observed that the green emission of nanocrystals showed selective quenching only in the presence of Cu2+ ions whereas the luminescence behavior remained unchanged in the presence of other interfering ions such as Ca2+, Cd2+, Co2+, Pb2+, Ag+, Mn2+, and Fe3+. The limit of detection was calculated to be 2.2 nM. The effective overlap of the absorption of Cu2+ with the emission spectrum of Ce3+ is mainly responsible for the selective quenching of Tb3+ ion emission intensity upon addition of Cu2+ because LRET can occur between the two ions.99 4.2. Molecular Detection. 4.2.1. Hydrogen Peroxide. Among the many oxidative species present inside our body, hydrogen peroxide (H2O2) is one of the prime components. It is one of the most important reactive oxygen species and is often the byproduct of a number of biological transformations. Excess accumulation of H2O2 inside the body often leads to disorders such as cardiovascular diseases and Parkinson’s and Alzheimer’s diseases. All of these demand biocompatible optical markers which can be used to detect the presence of H2O2 even at trace levels. Changjian et al. have reported the use of Tb3+-doped CePO4 colloidal nanoparticles for the detection of H2O2.This occurred as a result of the oxidation of Ce3+ to Ce4+, resulting in less absorption of Ce3+.100 Our group has synthesized Ce3+/ Tb3+-doped NaYF4 microrods which were able to detect H2O2 even at the 0.5 nM level.101 The sensing was due to the presence of para-phenylenediamine which was used as a capping agent for the nanocrystals. In the presence of H2O2, para-phenylenediamine is converted to a trimeric derivative called Brandrowski’s base. The absorption of the latter overlaps with the absorption range of Ce3+ ions. When excited at 290 nm, the energy gets largely absorbed by Brandrowski’s base, and hence the energy transfer between Ce3+ and Tb3+ is greatly reduced. J


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leading to quenching. A schematic of the quenching mechanism is presented in Figure 12. For practical applications, commercially available milk samples spiked with melamine were used to check the sensitivity of the sensor. While quenching was observed for a melamine mixed milk sample, no noticeable change was observed for pure milk samples. This shows that the sensor can be efficiently used for the detection of melamine in adulterated milk samples. The limit of detection was calculated to be 9.1 nM. 4.3. Gas and pH. 4.3.1. H2S. Among the various signaling gases in our body, hydrogen sulfide (H2S) is one of the most abundant ones. Various diseases such as Alzheimer’s disease, Down’s syndrome, and diabetes are in some ways related to abnormal levels of H2S present in the body. To understand its role in the physiological system, systematic detection is required. Huang’s group has developed an upconverting-material-based detector which was used for the ratiometric sensing of H2S. They developed Yb3+/Er3+/Tm3+-doped NaYF4 and coated it with mesoporous silica. A silica layer was used as an adsorbent for loading merocyanine (MC), which is a sensitive dye for H2S. Because the dye has a spectral overlap with the green emission of the Er3+ions, the luminescence from Er3+ was quenched. Interestingly, in the presence of HS− (the most stable physiological form of H2S in the body), the dye gets chemically converted to MCSH. This reduced the LRET between the dye and the Ln3+ emission, leading to a restoration of the luminesnce behavior. The limit of detection was calculated to be close to 0.58 μM. The upconverting nanocrystals were employed for the bioimaging of intracellular HS− as well.110 4.3.2. pH and Ammonia. Upconverting materials have also been used as a pH sensor. To achieve this, a pH probe such as phenol red has been used with Yb3+/Er3+-NaYF4 nanocrystals. Phenol red is unique because its absorbance spectra shift in the range of 460 to 540 nm with a change in pH. At high pH, when the color of the indicator is pink, the green emission of Er3+ is quenched, whereas at acidic pH where the color of the probe is yellow, the upconversion spectrum remains unaffected. The red emission of Er3+ is barely affected by the inner filter effect of the

Figure 11. Schematic illustration of the attachment of nanocrystals onto a positively charged glass slide and the luminescence changes of the same after dipping into aromatic amino acid solution followed by immersion in a ninhydrin solution. Reproduced with permission from ref 105. Copyright 2014 Royal Society of Chemistry.

role in the detection of bioconjugated molecules. Melamine, being an electron-rich donor, can interact efficiently with an electron acceptor. Dinitrobenzoic acid, which is an electron acceptor, was used as a surface capping molecule for the upconverting nanocrystals. Yb3+/Er3+-doped NaYF4 nanocrystals of average size of around 8 nm were synthesized using microwave irradiation. The electron-rich amino groups of melamine have a charge-transfer interaction with the electrondeficient aromatic nucleus of the dinitrobenzoic acid attached to the surface of the nanocrystals. Along with charge transfer, H bonding was also possible between the O atom of the nitro group from the dinitrobenzoic acid and the H atom of the amino group of the melamine.109 The strong favorable interaction between dinitrobenzoic acid and melamine weakens the attachment of the capping ligand from the nanocrystal’s surface. This leads to the formation of multiple defect surface sites

Figure 12. (Left) Schematic illustration of the quenching mechanism of Ln3+ luminescence in the presence of melamine. (Right) The bar diagram shows the selectivity of melamine compared to that of other nitrogen-rich molecules. Reproduced from ref 109. Copyright 2014 American Chemical Society. K


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Figure 13. (Left) Photocatalytic degradation rates of rhodamine B under NIR irradiation for different photocatalyst solutions. (Right) Schematic illustration of the photocatalysis mechanism of the MoS2−NaYF4:Yb3+,Er3+ nanocomposite under NIR excitation. Reproduced with permission from ref 116. Copyright 2016 Royal Society of Chemistry.

pH probe.111 The same probe along with the addition of a polystyrene matrix was used as an ammonia gas sensor. The mechanism relies on the fact that protons cannot permeate the polystyrene matrix whereas ammonia gas can. Thus, when a film of a composite involving the pH probe, upconverting material, and polystyrene was used as a sensor for ammonia, the probe color immediately changed from yellow to pink in the presence of ammonia. This was associated with a decrease in the intensity of the green emission. The pH at lower concentration has hardly any effect on the intensity of the red emission of Er3+ ions. Upconverting materials have a huge potential to be used as nanothermometers in temperature sensing,72 which can be used to analyze the temperature of nanoelectronic devices and cancer cells for the optimization of hyperthermal tumor treatment.

aforementioned materials. However, reports on the use of lanthanide ion (Ln = Ln3+ and Ln2+)-doped materials to improve the photocatalytic activity are scarce. In the recent past, Guo et al. have investigated the effects of α and β NaYF4:Yb3+,Tm3+ structures on the NIR photocatalytic performance of ZnO.113 The nanocomposites were employed to evaluate the degradation of rhodamine B, and the results suggest the interaction between NaYF4 and oxygen vacancies of ZnO to be the primary factor behind differential photocatalytic activities. Tang et al. have prepared NaYF4:Yb3+,Tm3+ core− TiO2 shell particles that showed promising photocatalytic activity under NIR irradiation.114 Similarly, Huang et al. have synthesized a NIR-active photocatalyst comprising a Ln3+ ion pair (Yb3+ and Er3+)-doped CaF2 core and a TiO2 shell.115 The energy of 408 nm emission originating from the upconversion process from Ln3+-doped particles gets transferred to the TiO2 shell, leading to the creation of excitons that are responsible for the degradation of organic molecules. NaGdF4 as a host matrix for Ln3+ ions has also been explored by Chen et al. in the recent past. A composite of these nanocrystals with TiO2 has shown the efficient degradation of methylene blue dye under NIR illumination.116 The above reports motivated us to synthesize materials that will be able to harness the NIR region, which is about 46% of the solar spectrum, for photocatalytic application. We mulled over the possibility of using upconverting Ln3+ ion pairs as a NIR harvester and prepared a MoS2−NaYF4:Yb3+,Er3+ nanocomposite via a hydrothermal method.117 The superior photocatalytic activity of MoS2 under visible light has already been reported by several research groups. However, it is unable to harvest the NIR region for photocatalysis. In the nanocomposite, the doped Yb3+ ions absorb in the NIR region (980 nm) and transfer the energy to Er3+ ions, which are able to emit in the visible region via an upconversion process. In the nanocomposite, the emissions from Er3+ get quenched by subsequent energy transfer to MoS2. This results in the generation of activated MoS2 with an electron in the conduction band and a hole in the valence band. These in turn generate active species leading to the degradation of dye molecules as shown in Figure 13 (61% after 12 h under only NIR irradiation). In another report, we have prepared bilayer-stabilized Ln3+ (Eu3+/Yb3+,Er3+)-doped CaMoO4 nanocrystals with a uniform size distribution. The nanocrystals were prepared within a 10 min reaction time at 90 °C using a microwave technique.118 The photocatalytic activity of these nanocrystals under UV excitation was then evaluated by the degradation of rhodamine B in an aqueous solution. About 80% of rhodamine B was degraded in 4

5. Ln3+-DOPED NANOCRYSTALS AS EFFICIENT PHOTOCATALYSTS Considering the rich optical properties of Ln3+ ions, particularly their ability to absorb a wide spectrum of solar energy (UV to NIR) and transfer it efficiently to other materials, we find that they are quite promising candidates for photocatalysis. However, they must be used in combination because no single Ln3+ is able to harvest light over a wide region. Photocatalysis is one of the practical methods for the environmental remediation of pollutants. For example, the dye-containing effluents that are directly discharged into bodies of water by the textile industries are one of the primary pollutants of water because the carcinogenicity of these dyes makes them a threat to aquatic life.112 However, biological oxidation and chemical treatment are generally employed to remove such dyes from bodies of water, and the use of heterogeneous semiconductor-based photocatalysts is gaining traction nowadays as a cost-effective and easy-to-use alternative. Although TiO2 and ZnO are very efficient materials for photocatalysis, they are wide-band semiconductors and work only in the high-energy UV region, which comprises less than 6% of the solar spectrum. On the other hand, several semiconductor materials possess band gaps in the visible region that covers about 47% of the solar spectrum. However, most of the semiconductor materials possess a high recombination rate, resulting in the reduction of the photocatalytic activity. Although most of the hosts used for doping Ln3+ ions are insulators, integrating them with suitable materials such as wide band gap semiconductors and layered materials can enhance the photocatalytic activity of the latter. For example, the NIR region covers about 46% of the solar energy spectrum. This can be utilized for enhancing the photocatalysis efficiency by integrating upconverting nanocrystals with one of the L


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h with a degradation rate constant of 0.0274 min−1, which is comparable to the reported value of the ZnO photocatalyst.119 To make nanocrystals water-dispersible, organic molecules such as sodium dodecyl sulfate (SDS), citric acid, and cetyl trimethylammonium bromide (CTAB) are generally used as capping agents. However, these surfactants inhibit the transfer of photogenerated holes and electrons from the catalyst surface. This inspired us to develop water-dispersible ligand-free Ln3+doped nanocrystals for photocatalysis. To achieve water dispersibility without ligands, ultrasmall Eu3+-doped PbMoO4 nanocrystals with excess residual charge (both positive and negative) on their surfaces were synthesized by a microwave method (10 min, 150 °C).120 The electrostatic repulsion between charged surfaces prevents their agglomeration in water. These nanocrystals were then used as a photocatalyst to degrade rhodamine B dye in an aqueous solution. In this case, about 70% of the dye degraded in 100 min with a rate constant of 0.010 min−1. Interestingly, the residual charges on the nanocrystal surface had no effect on the photocatalytic performance of the nanocrystals. Similar studies were conducted with microwavesynthesized Eu3+-doped BiOX (X = Cl, Br, and I) nanocrystals. To check the photocatalytic activity, aqueous solutions of rhodamine B dye were exposed to sunlight in the presence of these nanocrystals, which were able to efficiently degrade the dye with high rate constants. Among the three materials, BiOCl has shown the best catalytic activity (100% in just 20 min; rate constant, 0.27 min−1) followed by BiOBr (rate constant, 0.23 min−1) and BiOI (rate constant, 0.10 min−1).121 In all three aforementioned studies, though Ln3+ ions had no significant effect on the photocatalytic activity of the nanocrystals, the presence of Eu3+ as a dopant provides paramount information about the symmetry of its occupying site. Interestingly, Eu2+ ions are known to undergo a downshifting process (i.e., they absorb in the UV region (250 nm) and emit at ∼368 nm, which in turn is the absorption region of C dots that has been reported to be an efficient photocatalyst). Polyacrylic acid-coated Eu2+-doped BaSO4 nanocrystals and CTAB-capped C dots were synthesized by a microwave technique.122 While the separate use of Eu2+doped BaSO4 nanocrystals and C dots degrades rhodamine B in an aqueous solution under irradiation bore no fruitful results, excellent photocatalytic activity was observed when they were used together (95% in 3 h; rate constant, 0.0142 min−1). The results indicate that upon UV excitation the Eu2+ present in BaSO4 nanocrystals efficiently transfers energy to C dots, which in turn degrades organic pollutants. Here, the downshifting property of the Eu2+ ion was employed to harvest the UV region of the solar spectrum to improve the photocatalytic activity of C dots. Synthesizing a photocatalyst that is able to harvest all three regions of the solar spectrum, i.e., UV, visible, and NIR, to degrade organic pollutants has always been an active area of research. The careful selection of materials led us to the development of a Ln3+ (Yb3+, Tm3+)-doped BiPO4/BiVO4 nanocomposite, where the BiPO4 and BiVO4 components are photocatalytically active in the UV and visible regions, respectively.123 The Yb3+ ions absorb in the NIR region. In the nanocomposite, the strong blue emission from Tm3+ ions via upconversion is nonradiatively transferred to BiVO4, resulting in the production of excitons. This in turn generates reactive oxygen species and efficiently degrades methylene blue dye in the aqueous medium (Figure 14). The nanocomposite also shows high photocatalytic activity in both the visible region (0.010 min−1) and over the full solar spectrum (0.047 min−1).

Figure 14. Schematic illustration of the photocatalysis mechanism of the (Yb3+,Tm3+)-doped BiPO4/BiVO4 nanocomposite under UV− vis−NIR excitation. Reproduced from ref 123. Copyright 2014 American Chemical Society.

Similarly, Vetrone and co-workers have synthesized a core−shell based visible to near-infrared photocatalyst with Yb3+/Er3+NaGdF4 as the core and bismuth ferrite as the shell material. The nanohybrid composite was able to degrade methyl orange and 4chlorophenol under visible and NIR light illumination.124 After the initial success in improving the photocatalytic activity of semiconductors by incorporating Ln3+ ions in their matrixes, the prospect of further enhancement by utilizing broadband-sensitized Ln3+ emission is currently under investigation in our research group.

6. PHOTOVOLTAICS The ability of Ln3+ ions to down-convert as well as up-convert electromagnetic radiation has motivated researchers to use them as spectral converters in photovoltaic devices. This is particularly important because the widely used semiconductors are unable to efficiently harvest both high-energy and low-energy photons compared to their band gap energy. To overcome this problem, Ln3+ up-converters have been used as a layer at the back of GaAs, c-Si, a-Si, and dye-sensitized solar cells and were found to successfully enhance their efficiency. However, down-converting nanocrystals are more promising compared to upconverting ones as the latter require the use of a laser. Recently, by using a layer of Eu3+-doped Y(OH)3 nanotubes, Chin-Lung et al. increased the conversion efficiency of a screen-printed monocrystalline silicon solar cell (SPMSSCs) by 2% (from 15.2 to 17.2%). For dye-sensitized solar cells, the incorporation of suitable Ln3+ ions in TiO2 hosts has also shown improvements in their photovoltaic performance. Recently, Carlos et al. have developed solar concentrators based on Eu3+-complexed N,N′diureido-2,2′-bipyridine isomers and Eu3+-based triureasil hybrids that achieved quantum efficiencies of 34 and 82%, respectively.125,126 The same group has developed a NIRemitting luminescence solar concentrator using silicon 2,3naphthalocyanine bis(trihexylsilyloxide) (SiNc or NIR775) immobilized in an organic−inorganic triureasil matrix (tU(5000)). The photophysical properties of the SiNc dye incorporated into the triureasil host closely resembled those of SiNc in THF solution (an absolute emission quantum yield of 17% and a fluorescence lifetime of ∼3.6 ns). The luminescence solar concentrator coupled to a Si-based photovoltaic device achieved an optical conversion efficiency of ∼1.5%, which was among the largest values known in the literature for NIRemitting luminescence solar concentrators.127 Our group has recently used ligand-sensitized Eu3+ emissions to enhance the M


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Figure 15. TEM image of (A) oleic acid-capped Eu3+-doped LiYF4 nanocrystals, (B) bare LiYF4 nanocrystals, and (C) TPB-capped Eu3+-doped LiYF4 nanocrystals. (D) Schematic representation of the device set up with down-shifting nanocrystals. Reproduced with permission from ref 55. Copyright 2017 Royal Society of Chemistry.

Figure 16. Digital image of the Ce3+/Tm3+/Mn2+-doped NaYF4/PVA nanocomposite film (A), after bending (B, C and D), and the film under UV light before (E) and after bending (F). Reproduced with permission from ref 44. Copyright 2017 Wiley.

solar-to-current conversion efficiency of Si solar cells. Eu3+ ions doped in LiYF4 nanocrystals were sensitized by 4,4,4-trifluoro-1phenyl-1,3-butanedione (TPB) ligands, which also served the role of capping agents. The attachment of ligands to the

nanocrystals was achieved through a ligand exchange mechanism (Figure 15a−c).55 Thereafter, a device was fabricated by embedding the nanocrystals on a Si solar cell (as shown in Figure 15d). Upon exposure to UV light, the nanocrystal-embedded Si N


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crystal−polymer composite that have recently shown promising activity in various fields has been included. We strongly believe that there is ample scope for further research on these materials. For example, improving the luminescence quantum yield of these colloidal nanocrystals is indispensable if these materials need to find a place in technology development. Although to some extent enhancement is achieved for down-conversion luminescence, there is not much that has been done to improve the quantum yield of upconverting materials. This is because of limited sensitizers available with strong absorbance in the NIR region (such as IR dyes). Even those sensitizer molecules are not very stable. We believe robust materials such as semiconductor quantum dots with long luminescence lifetimes could be efficient. The development of novel hybrids with those materials would be promising as sensitizers to increase the upconversion luminescence. The synthesis protocols need improvements such as a reduction in the reaction temperature, the use of green solvents, the minimization of toxic reactants, and so forth. Some excellent phosphors such as garnets are still prepared using sol− gel or solid-state methods. Designing synthesis methods to make them in the colloidal state is still challenging yet rewarding. With respect to sensing applications, we strongly envision that the use of a suitable capping ligand and optimum tuning of the Ln3+ to the analyte distance would lower the limit of detection in sensing applications. All of these require concerted efforts from scientists working in different disciplines.

solar cell shows relative enhancements of 9 and 2% in the photocurrent upon excitation under UV radiation and overall solar radiation, respectively. We point readers to excellent reviews on this topic by Lugo et al. and Schropp et al.128,129

7. Ln3+-DOPED NANOCRYSTAL−POLYMER NANOCOMPOSITES In recent times, Ln3+-doped nanocrystal−polymer composites have gained attention for their potential applications as optical amplifiers, waveguides, laser materials, and implantable medical devices. Several research groups are actively pursuing the synthesis and application of such systems. For example, Riman et al. have prepared transparent composites of CeF3:Yb3+,Er3+ with polystyrene and poly(methyl methacrylate) (PMMA). They observed that at higher loadings of nanocrystals the optical transparency was significantly affected by the PMMA nanocomposite compared to polystyrene due to a larger mismatch of the refractive index between the polymer and nanocrystal.130 Recently, Burunkova et al. has used a mixture of acrylate and urethane as the polymer matrix for preparing a Ln-oxide polymer composite via the UV-curing method. They have introduced silica nanoparticles during nanocomposite formation and have observed the excellent homogeneity of the nanocrystals in the matrix. Furthermore, they added Au nanocrystals that increase the luminescence performance due to their surface plasmon resonance.131 Li et al. have reported a nanocomposite of NaLuF4:Yb3+,Er3+ with a thermoresponsive poly(N-isopropylacrylamide-co-acrylic acid) copolymer by using a ligand exchange strategy. The resulting nanocomposite exhibited an 80-fold increase in upconversion luminescence intensity at 31 °C. The low toxicity and significant signal-to-noise ratio of this material make it a potential candidate for in vitro imaging.132 As has been discussed earlier, our group has prepared a polymer nanocomposite with poly(vinyl alcohol), and the resulting transparent film had an excellent phosphor property under UV excitation, the digital images of which are shown in Figure 16.44

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AUTHOR INFORMATION

ORCID

Venkataramanan Mahalingam: 0000-0003-1414-805X Notes

The authors declare no competing financial interest.

8. SUMMARY AND OUTLOOK This feature article is an attempt to convince the readers about unique optical properties of Ln3+ ions and the need for the development of Ln3+-doped colloidal nanocrystals to explore their potential in different applications such as sensing and environmental remediation. The section on the sensitization of Ln3+ luminescence discusses the various methods developed to improve the luminescence quantum yield of the Ln3+ ions in Ln3+-doped colloidal nanocrystals. In addition to Ce3+ ions, quantum dots, ZnO, dyes, and organic molecules were found to be efficient sensitizers in transferring their excited energy to Ln3+ ions. The section on water dispersibility enumerates the different chemical and surface modification approaches to improving the water dispersibility of the Ln3+-doped colloidal nanocrystals to widen the scope of utilizing these materials in environmental applications. It is quite unambiguous from the discussion that Ce3+ to Ln3+ ion energy transfer not only enhances the Ln3+ luminescence but can be explored for the detection of various toxic metal ions and small molecules including explosive materials. In addition, it is possible to selectively detect analytes using Ln3+-doped nanocrystals with the appropriate choice of capping ligands. Furthermore, we have shown how the spectral converting properties of the Ln3+-doped nanocrystals can be explored for photocatalysis and photovoltaic applications. Finally, a brief discussion on Ln3+-doped nano-

Biographies

Debashrita Sarkar received her bachelor’s degree from Guru Nanak Dev University, Amritsar, India in 2013. She joined the Indian Institute of Science Education and Research, Kolkata, India as an integrated M.S.− Ph.D. student under the supervision of Dr. Venkataramanan Mahalingam. During her master’s thesis, she worked on the application of lanthanide-doped nanomaterials. Her research interest focuses on environmental remediation using silicate nanomaterials. O


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University of Victoria and Concordia University. He joined IISER Kolkata as an assistant professor in 2009 and was promoted to associate professor in 2013. His research interests include lanthanide-doped nanoparticles, 2D layer materials, and porous polymers and exploring their properties for sensing, energy, and environmental remediation applications. He has authored about 70 publications in peer-reviewed journals.

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ACKNOWLEDGMENTS V.M. thanks all of the present and past Ph.D. students, postdoctoral researchers, and undergraduate students who have worked very hard and contributed to the work discussed in this feature article. V.M. sincerely acknowledges funding support from both the DST and CSIR. D.S, S.G., and T.S. thank IISER-Kolkata, CSIR, and UGC, respectively, for their fellowships. We greatly acknowledge the immense support from IISER-Kolkata for proving infrastructural facilities and generous funding. Finally, we acknowledge all of the reviewers of our papers whose valuable comments and constructive criticism helped us in improving our understanding of this subject.

Sagar Ganguli received his B.Sc. in chemistry from Chandernagore College (Burdwan University) in 2012. Thereafter, he joined the Indian Institute of Science Education and Research Kolkata as an integrated M.S.−Ph.D. fellow. For his master’s project, he joined Dr. Venkataramanan Mahalingam’s group and continued his Ph.D. in the same group as a CSIR fellow. His research is focused on the development of energy storage systems and analyzing lanthanide luminescence.

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REFERENCES

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Tuhin Samanta received his B.Sc. in chemistry from Ramakrishna Mission Residential College (University of Calcutta) and his M.Sc. from the Indian Institute of Technology Madras in 2012. He joined the Indian Institute of Science Education and Research Kolkata as a graduate student in 2013 and completed his Ph.D. in 2018. Currently, he is working as a postdoctoral research fellow at Karlsruhe Institute of Technology on spectral conversion materials.

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