Today,there is an increasing demand for chemical sensors to detect a variety of analytes for a range of applications,including environment monitoring,homeland security guarding,and food quality controlling. Typically,chemical sensors can bind selectively and reversibly the analytes of interest with a concomitant change in electrical,optical,or mechanical properties. Among all the chemosensors,luminescence-based ones present many advantages in terms of definite fluorescence spectrum,visible signal by naked eye,high sensitivity and selectivity,and ease of operation. Even so,there still remain great challenges to realize better sensory performance including increased brightness,higher quantum yield,excellent photo-stability under UV irradiation,and multifunctional systems. Here,a particular class of materials wellknown as metal-organic frameworks (MOFs) will be presented to construct a robust platform to deal with these challenges.

MOFs,a promising class of hydrid materials constructed from straight self-assembly of metal cations/clusters and organic ligands through coordination bonds,have been extensively studied over the past two decades. The variety of metal and organic ligand building units makes it possible to prepare virtually unlimited MOFs to utilize many potential applications in numerous areas, including gas adsorption ^[1],separation ^{[2, 3]},catalysis ^{[4, 5]}, chemical sensing ^{[6, 7, 8]},opto-electronics ^[9],clean energy ^[10], bio-imaging and drug delivery ^[11]. Among them,MOFs are utilized for chemical sensing mostly based on the fluorescence emission ^[12],because all of metal ions,organic ligands,metal- organic charge transfer,and luminescent guest molecules can generate luminescence ^{[13, 14]}. And corresponding changes act in response to the uptake of analyte guests,as a mean of signal transduction. Furthermore,the functional sites in MOFs such as Lewis basic/acidic sites and unsaturated metal sites can be used for selective recognition of targeted molecules/ions. The tunable porosity can enable the reversible storage of guest species,making MOFs both as detection and pre-concentrator medium.

However,MOFs in the common form of bulk crystals cannot meet the specific requirements for analytical applications. In view of this,MOF materials are needed to be miniaturized at a nanometer scale or immobilized on selected substrates for practical applications. Nanoscale MOFs (NMOFs) have been proved to be a kind of effective sensory materials ^[15]. First,NMOF materials exhibit not only the rich diversity of compositions, structures,and properties as the bulk MOFs,but also the high dispersity in aqueous solution and the intrinsic biocompatibility, which facilitate the wide applications of NMOFs in the fields of chemical sensing ^{[16, 17]}. Second,NMOF materials display extremely high surface areas to enhance the sensory performance in both response time and detection sensitivity due to a preconcentration effect,so the analytes are absorbed and concentrated inside the NMOF channels. The combination of nanoscale processability,intrinsic luminescent properties,and permanent porosity of NMOFs will definitely create a bright prospect for designing novel luminescent sensory nanomaterials with enhanced desired multifunctionalities.

Like macroscopic counterparts,NMOFs are mainly prepared through the "bottom-up" synthetic strategy based on straight selfassembly of inorganic and organic components. The synthesis of nanoscale metal-organic materials and porous materials has recently been reviewed ^{[17, 18]},so general synthetic strategies for luminescent NMOFs are outlined. In order to clearly interpret various synthetic methods,these NMOF materials are classified into three broad classes: class I,direct luminescent MOF nanoparticles; class II,host NMOFs containing luminescent guests; class III,2-dimension (2-D) NMOF luminescent films.

Three different strategies are generally applied for synthesizing MOF nanoparticles: (i) the nano-precipitation method; (ii) the confinement at nanoscopic regime by using emulsions or templates; (iii) microwave or ultrasound assisted synthesis.

First of all,nano-precipitation is the simplest method for the preparation of MOF nanoparticles. This method usually involves two steps: (i) the dissolution of reaction precursors in a good solvent,(ii) the addition of a poor solvent to stop the assembly process,resulting in the formation of particles at a nanometer scale. Mirkin and Wang have done pioneering work to develop this methodology ^{[19, 20]}. Lin applied this strategy to synthesize MOF nanoparticles constructed from the anticancer drug c,c,t-(diamminedichlorodisuccinato) Pt(IV) (DSCP) and Tb³⁺ ions ^[21]. Typically, a precursor solution of TbCl₃ (15 mmol) and DSCP (10 mmol) was mixed in H₂O,and the pH value was adjusted to 5.5. After that, methanol was rapidly added with vigorous magnetic stirring, which resulted in the formation of nanoparticles. However,this nano-precipitation method is not appropriate for crystalline NMOFs.

The second family of strategies for miniaturizing MOFs is the nanoscopic confinement effect assisted by surfactant. Lin developed a reverse microemulsion methodology to synthesize NMOFs, such as Gd(BDC)_1.5(H₂O)₂,[Gd(BTC)(H₂O)₃]·H₂O ^[22],and Gd₂(BHC)(H₂O)₆ ^[23]. Reverse microemulsions were formed by using surfactants to stabilize water droplets within a nonpolar organic phase. Two separate microemulsions containing either Ln³⁺ ions or organic linkers were mixed and reacted for a given period of time to form crystalline MOF nanoparticles. Eu³⁺ and Tb³⁺-doped Gd(BDC)_1.5(H₂O)₂ nanorods were also prepared and proved to be luminescent.

The third family of strategies for miniaturizing MOFs is microwave or ultrasound assisted synthesis. Generally speaking, microwave irradiation can accelerate the crystal nucleation and produce nanocrystals with narrower size distributions than conventional solvothermal approach. Nanoscale Tb-MOF-76 was prepared through a rapid microwave-assisted method with proline and glycine as capping agents ^[24]. It exhibited a typical emission of Tb³⁺ ions,which was sensitive to acetone in aqueous solution. Similarly,ultrasonic irradiation can generate a homogenous nucleation and a substantial reduction in crystallization time. Well-defined Mg-MOF-74 nanocrystals with diameters of 100 nm were successfully synthesized in one hour in the presence of triethylamine as a deprotonating agent ^[25].

As for conventional solvothermal synthesis,various additives have been proved to suppress NMOF growth through a coordination modulation route. First,alkylamine additives such as triethylamine ^[25],n-butylamine and diethylamine ^[26] effectively deprotonate the metal complex and accelerate crystal nucleation, resulting in an obvious decrease of crystal sizes. Second, carboxylate and N-heterocycle additives,such as benzoic acid ^[27], acetate acid ^[28],sodium formate,sodium acetate ^[29],and 1- methylimidazole ^[30],have the same chemical functionality as the bridging linkers to impede the coordination interaction between the metal ions and the bridging linkers. Luminescent Eu_1-xTb_x- MOFs nanocrystals around 100 nm were synthesized with the addition of sodium formate or sodium acetate,and further fabricated to form continuous films via spin-coating deposition ^[29]. The films exhibited strong luminescent properties and thus showed potential applications in the field of luminescent sensors. Other additives such as p-perfluoroethylbenzoic acid ^[31], dodecanoic acid ^[32],4n-decylbenzoic acid ^[33] and polymers such as poly(acrylic acid sodium salt) ^[34] and poly(diallyldimethylammonium chloride) ^[35] were also used to control the sizes and morphologies of NMOFs.

Confinement of different guests,especially luminescent guests, into pores of NMOFs makes them versatile candidates to develop multifunctional sensory materials. One of the ideal methods is the direct incorporation of luminescent guests during the process of NMOF synthesis. Maspoch prepared metal-organic nanospheres constructed from Zn²⁺ and 1,4-bis(imidazol-1-ylmethyl)benzene (bix) by a fast precipitation ^[36]. These Zn(bix) nanospheres were used as functional matrices for the encapsulation of a variety of substances,such as luminescent quantum dots (QDs),fluorescein, rhodamine B,or two kinds of substances. The formed nanoparticles retained the intrinsic luminescent properties of guest species. Recently,a great breakthrough was the introduction of polyvinylpyrrolidone (PVP) as capping agents into the synthetic system of NP@MOF core-shell nanostructures ^[37]. A variety of PVPcapped nanoparticles (NP) was successively adsorbed on the continuously forming surfaces of ZIF-8 nanocrystals to form NP/ ZIF-8 hybrid nanocrystals (Fig. 1). Imparting luminescent properties to ZIF-8 nanocrystals was demonstrated through separate encapsulation of lanthanide-doped NaYF4 nanorods and CdTe QDs. Remarkably,the luminescent features of different cores were well maintained. Chi reported a highly-luminescent NMOF material through encapsulating the branched poly-(ethylenimine)-capped carbon quantum dots into ZIF-8 ^[38].

Fig. 1

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Fig. 1.Schematic presentation of the controlled encapsulation of various PVPcapped nanoparticles into ZIF-8 nanocrystals. Reprinted with permission from Ref. [37], Copyright 2012, Nature Publishing Group.

Postsynthetic loading strategy,usually ion-exchange method,is another effective method to introduce luminescent ions or molecules into NMOF pores. For example,different lanthanide cations were introduced into cationic bio-MOF-1 pores through soaking bio-MOF-1 samples in DMF solution of Tb³⁺,Sm³⁺,Eu³⁺, Yb³⁺ salts,respectively ^[39]. The resulted Tb@bio-MOF-1,Sm@bio- MOF-1,and Eu@bio-MOF-1 samples emitted their distinctive colors: green,orange-pink,and red,respectively. The resulted Yb@bio-MOF-1 emitted NIR luminescence and was used as oxygen sensors. Besides lanthanide cations,pyridinium hemicyanine dye was encapsulated into bio-MOF-1 pores to realize a new synergistic two-photon-pumped lasing functionality ^[40].

Luminescent NMOFs can be fabricated into thin films for their straightforward sensing applications. So far,the general strategy is the direct nucleation and growth of MOFs on selected substrates, e.g. SiO₂,Al₂ O₃,Au,etc ^{[41, 42]}. However,the bare substrate surfaces usually inhibited the nucleation of NMOFs. Caro ^[43],Qiu ^[44] and Ameloot ^[45] reported the chemical modification of substrates through the oxidation of substrate surfaces to create more surface nucleation points to obtain dense coatings. In addition,the addition of crystal seeds is another proposed way to induce a better crystallization process to fabricate robust NMOF membranes ^{[46, 47]}.

Recently,Terfort developed an effective ink-jet printing technology to realize the controlled deposition of HKUST-1 onto plastics,papers,and textiles (Fig. 2) ^[48]. The "ink" was a precursor solution of Cu(NO₃)₂·3H₂O and BTC in mixed solvents of dimethyl sulfoxide,ethanol,and ethylene glycol. The desired patterns were simply obtained by printing-drying process and the thickness was directly controlled by multiple "printing-drying" cycles. Textiles with ink-printed HKUST-1 could detect several toxic gases (NH₃, H₂S,HCl, etc.) through a unique color change.

Fig. 2

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Fig. 2.(a) The applicability of HKUST-1 for large area patterning; (b) various graphic patterns printed onto PET foil; (c) SEM images of printed HKUST-1 after 8 printingdrying cycles; (d) adsorption capacities of HKUST-1 films on QCM for various gases. Reprinted with permission from Ref. [48], Copyright 2013, Wiley-VCH.

The luminescent properties of NMOFs are very sensitive to subtle structural changes including coordination environment of metal ions,interactions between host framework and guest species,porosity,and energy transfer between ligands and metal centers. Therefore,the stimuli-responsive properties of NMOFs make it an ideal candidate working as chemosensory materials. Over the past few years,luminescent NMOF materials have been reported to detect ion species,organic molecules,noxious gases, explosives,pH value,and temperature.

Rational design of MOF with special sites to selectively bind analytes is of significant importance for sensing applications. Oh developed an excellent strategy to design a chemosensory particle, as named CPP-16,consisting of a crystalline framework,a fluorophore part,and a Cu²⁺ detecting part (Fig. 3) ^[49]. CPP-16 was fluorescent due to the incorporation of fluorescent pyrenefunctionalized building blocks. With the addition of Cu²⁺,the luminescence intensity of CPP-16 decreased. The detection mechanism was attributed to the specific interaction between Cu²⁺ and amide group of CPP-16,causing pyrene moieties to undergo conformation changes from the stacked excimer state to the quenched excimer state.

Fig. 3

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Fig. 3.Schematic representation of CPP-16 as a "turn-off" fluorescent chemosensor to detect Cu2+ ion. Reprinted with permission from Ref. [49], Copyright 2014, American Chemical Society.

By using thiol-functionalized 2,5-dimercapto-1,4-benzenedicarboxylic acid (H₂DMBD) as the organic linker,a Zr-DMBD MOF with size of 200 nm was prepared ^[50]. Zr-DMBD was capable of effective uptake of Hg²⁺ ions from aqueous solution and gaseous phase,because of the strong binding between thiol groups and Hg²⁺. After mercury uptake,the emission at 500 nm was largely suppressed and hardly visible to naked eye. Chen prepared coordination polymer nanoparticles composing of adenine (Ad), Tb³⁺ ion,and dipicolinic acid (DPA) (Fig. 4) ^[51]. The DPA molecule was expected to sensitize the fluorescence of Tb³⁺. However,the fluorescence of nanoparticles was very weak because intramolecular energy transfer (ET) from DPA to Tb³⁺ was impeded by photoinduced electron transfer (PET) from adenine to DPA. When Hg²⁺ was added,a significant enhancement in emission intensity was observed because of the suppression of PET process through the coordination between Hg²⁺ with adenine. These nanoparticles exhibited excellent selectivity and ultra-high sensitivity up to the 0.2 nM detection limit.

Fig. 4

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Fig. 4.Schematic illustration of Ad/Tb/DPA nanoparticles for sensing of Hg2+ ions. Reprinted with permission from Ref. [51], Copyright 2012, American Chemical Society.

Yan reported the exploration of luminescent nMOF-253 nanomaterial for highly selective and sensitive detection of Fe²⁺ ions in aqueous solution ^[52]. The detection mechanism was likely attributed to PET process and the coordination between Fe²⁺ and bipyridine. Moreover,nMOF-253 was successfully applied in luminescent bioimaging and intracellular Fe²⁺ sensing in HeLa cells.

Luminescent MOF nanocrystals were prepared through encapsulating Eu³⁺ ions into MIL-53-COOH (Al) pores,showing highly selective and sensitive detection of Fe³⁺ in aqueous solution ^[53]. The possible sensing mechanism was based on ion exchange between the analyte Fe³⁺ and the framework Al³⁺.

Solvatochromism is the ability of chemosensory material to change its color in response to a change of solvent polarity. For example,{[(WS₄Cu₄)I₂(dptz)₃]·DMF}_n (dptz = 3,6-di(pyridin-4-yl)- 1,2,4,5-tetrazine) MOF displayed different colors after immersed in solventswith different polarity ^[54]. Kitagawa proposed amolecular decoding methodology to prove that [Zn₂(bdc)₂(dpNDI)]_n (dpNDI = N,N'-di(4-pyridyl)-1,4,5,8-naphthalenediimide) MOF distinguished different guests confined in the nanopores by transducing a particular host-guest interaction into a corresponding color change ^[55]. And {[Tb₂(NO₃)₂(edc)(DMF)₄]·DMF·H₂O}_n with minimal diameters of 70 nm acted as a luminescent probe to recognize cyclohexane and nitrobenzene with high sensitivity and quick regeneration ability ^[56].

Dipicolinic acid (DPA) is an important molecular marker in spore producing bacteria,which constitutes up to 15% of dry mass of spores. Lin developed a novel method to functionalize silicacoated NMOFs for the luminescence sensing of DPA ^[57]. Eu-doped Gd(bdc)_1.5(H₂O)₂ nanoparticles were coated orderly with PVP molecules,silica nanoshell,and silylated Tb-EDTM monoamide derivative to form silica-coated NMOFs. Under excitation at 278 nm,the composite only gave Eu³⁺ luminescence because Tb-EDTM was essentially non-emissive. When DPA was added,the Tb³⁺ luminescence became clearly visible due to the formation of Tb-EDTM-DPA complex. Therefore,the Tb³⁺ emission provided a sensitive probe for DPA,while the Eu³⁺ emission in the NMOF core acted as a non-interfering internal calibration. Afterward,Chen and Qian reported Eu²(FMA)²(OX)(H²O)⁴·4H²O (FMA = fumarate, OX = oxalate) of about 200-400 nm,whose size and morphology was simply tailored by the addition of different amount of CTAB ^[58]. The as-made NMOF exhibited highly sensitive,selective and instant "turn-on" sensing of DPA,because the coordination between DPA molecules and Eu³⁺ significantly enforced the intramolecular energy transfer.

Chemical sensors for rapid detection of explosives are attracting many attentions due to homeland security,anti-terrorism,and humanitarian implications. Luminescent NMOF materials have been utilized for effective detection of high explosive molecules. Chen and Qian employed the water-in-oil microemulsion strategy to synthesize Eu₂(BDC)₃(H₂O)₂(H₂O)₂ nanorods of less than 100 nm ^[59]. The ethanol dispersion of MOF nanorods exhibited typical emission of Eu³⁺ ions. The nitroaromatic compounds such as 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT) showed a significant quenching effect on the emission intensity of the NMOF dispersion,maybe because of a competitive absorption of the light source energy and the electronic interaction between the nitroaromatic compounds and ligands. Zang reported a Zn-bcpa NMOF with cubic shape of 500-1000 nm in size (bcpa = 9,10-bis(p-carboxyphenyl)anthracene) ^[60]. Its fluorescence emission was quenched upon interaction with the nitroaromatic explosives (e.g. DNT,TNT) and nitroaliphatic explosives (e.g. nitromethane). Even when the concentration of nitromethane was diluted down to 1% of the saturated vapor,the emission was still quenched. The detection limit of nitromethane was down to ppm level. The quenching mechanism was likely attributed to the PET process from the excited NMOFs to the adsorbed explosive molecules. Qiu used a self-sacrificing template strategy to synthesize luminescent CdBTC-MOF nanotubes as efficient chemical sensors for trace-level detection of DNT vapor ^[61].

Gas sensors have a wide range of applications in the fields of aerodynamics,environmental monitoring,analytical chemistry, and biochemistry. Luminescent NMOFs as gas sensors have been exploited because the luminescent properties of NMOFs can be perturbed by toxic gases and vapors.

Lu has fabricated a novel type of highly luminescent Cu(I)-NCPs through a nano-precipitation method ^[62]. These Cu(I)-NCPs have provided a rapid and sensitive platform to detect H₂S vapor because H₂S selectively quenched Cu(I)-NCPs’ fluorescence. Moreover,Cu(I)-NCPs served as "inks" for writeable detection of H2S. Chen reported a coordination polymer nanoparticle composing of Tb³⁺ ions,Ag⁺ ions,and adenosine monophosphate,where the fluorescence of Tb³⁺ ions was sensitized by Ag⁺ ions ^[63]. When H₂S was introduced,a fluorescence quenching would be observed because of strong affinity of H₂S to Ag⁺ ions.

Highly-sensitive sensors for trace oxygen analytes are demanded in many oxygen-free environments,such as medical environment,chemical industry,and so on. Some MOF materials with the incorporation of phosphorescent Ru/Ir-complexes have shown high oxygen-sensing efficiencies ^{[64, 65]}. However,bulk MOF crystals cannot be directly used as gas sensors,unless they are immobilized into solid devices or membranes. Chen synthesized a Cu(I)-triazolate [Cu(detz)] (Hdetz = 3,5-diethyl-1,2,4-trizole) NMOF,which was not luminescent in air due to the quenching effect by oxygen ^[66]. Indeed,Cu(detz) in vacuum showed luminescence,indicating that Cu(detz) exhibited O2-sensing properties. In order to realize its practical application,Cu(detz) was composited with silicon rubbers to fabricate soft and robust membranes by a counter-diffusion crystal-growth method. The membrane not only maintained original luminescence and oxygen-sensing properties,but also further improved the chemical stability of the Cu(detz) NMOF.

HKUST-1 framework contains a high density of unsaturated acid sites which can coordinate small gas molecules. When HKUST- 1 was printed on textiles,the textiles showed potential applications to capture hazardous gases ^[48]. Meanwhile,HKUST-1 rapidly changed its colors from turquoise to dark blue,yellow,and brown after the exposure to NH3,HCl,and H2S vapors,respectively. This color-change phenomenon,not only indicated the capture process,but also presented the possibility of inkjet-printed HKUST-1 to fabricate practical gas sensors.

One of the significant subjects in MOF-based chemical sensors is the detection of pH values in aqueous solution,especially for monitoring subtle pH changes in biological environments and living cells. Yan exploited a post-synthetic method to simultaneously introduce two types of Eu³⁺ ions with different excitation wavelengths into nanoscale MOF-253 ^[67]. One of Eu³⁺ was sensitive to pH changes while the other was not,resulting in a ratiometric pH response in the pH range of 5.0-7.2. Recently,a novel type of NMOF based pH sensors has been explored in realtime sensing and monitoring of intracellular pH changes inside live cells ^[68]. Fluorescein isothiocyanate (FITC) is pH sensitive and exhibits pH-dependent ratiometric fluorescence changes. However, the utility of FITC in live cell imaging is severely limited by its rapid release from cells. In order to address the challenges,Lin designed a novel NMOF sensor (called F-UiO) through covalent linkage of FITC to amino-modified UiO-MOF (Fig. 5). The F-UiO nanocomposites with a diameter of 100 nm and a thickness of 30 nm exhibited structural stability,high fluorescent efficiency,pH response sensitivity,and efficient cellular uptake. The ratiometric pH-sensitivity of F-UiO was determined with pH value range of 4.0-8.0 by a fluorometer. The ratios of 520 nm emission intensity under excitation at 488 and 435 nm (I_488/520/I_435/520) were calculated,and the correlation of emission intensity ratio to pH was established by nonlinear curve fitting. Upon rapid and efficient endocytosis,F-UiO maintained the structural integrity inside endosomes. Live cell imaging was performed to monitor the endocytosis of F-UiO and endosome acidification process in real time.

Fig. 5

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Fig. 5.(Left) (a) Schematic illustration of F-UiO synthesis; (b) PXRD patterns of UiO, F-UiO, and F-UiO after incubating in HBSS for 12 h; (c) TEM images of F-UiO; and (d) F-UiO after incubating in HBSS for 12 h. (Right) (a) Correlation between FITC absorbance and fluorescence at various FITC loadings; (b–d) pH calibration curves of free FITC (b) and FUiO acquired by fluorimetry (c) and by CLSM (d). (e) CLSM images showing the colors of F-UiO particles in HBSS buffers with different pH values. Reprinted with permission from Ref. [68], Copyright 2014, American Chemical Society.

The mixed-lanthanide (usually Eu³⁺ and Tb³⁺ ions) MOF materials show unique temperature-dependent luminescent properties,giving rise to their potential applications as luminescent thermometers. In 2010,Palacio and Carlos have done the pioneering work to develop a unique Eu³⁺/Tb³⁺ luminescent nanothermometer ^[69]. The thermometer consisted of [Eu(btfa)₃(- MeOH)(bpeta)] and [Tb(btfa)₃(MeOH)(bpeta)] β-diketonate chelates (btfa = 4,4,4-trifluoro-l-phenyl-1,3-butanedione,bpeta = 1,2- bis(4-pyridyl)ethane),which were embedded into organic-inorganic hybrid nanoclusters formed by a magnetic core and a TEOS/ APTES organosilica shell. As the temperature in the range of 10- 350 K increased,the emission intensity of Tb³⁺ strongly decreased, while that of Eu³⁺ started to increase. The mechanism was assumed to be a two-steps process of Tb³⁺→host→Eu³⁺ energy transfer. Although the thermometer showed temperature sensitivity up to 4.9% per K,the synthesis procedure was complicated and two-steps energy transferwas obviously inefficient. Qian and Chen have improved this strategy to fabricate luminescent Ln-MOF thermometers,which was based on direct energy transfer from Tb³⁺ to Eu³⁺ within the same framework ^[70]. They designed an organic ligand H2DMBDC to sensitize both Eu³⁺ and Tb³⁺ emissions and constructed a mixed-lanthanide MOF (Eu_xTb_1-x)2(DMBDC)₃(H₂O)₄·DMF·H₂O,showing two characteristic emissions of Tb³⁺ ions at 545 nm and Eu³⁺ ions at 613 nm,as expected. Taking x = 0.0069 as an example,at 10 K the emission peaks of 545 nm (Tb³⁺) and 613 nm (Eu³⁺) exhibited comparable intensity. However,when the temperature was increasing,the emission intensity of the Tb³⁺ ions decreased,while that of the Eu³⁺ increased. At 300 K,the emission band was almost at 613 nm and that at 545 nm was nearly invisible. The unique behavior suggested an efficient energy transfer from Tb³⁺ to Eu³⁺ with increasing temperature. Furthermore,the emission intensity ratio (I_Tb/I_Eu)was linearly related to the temperature in awide range from 10 K to 300 K,highlighting an ideal luminescent thermometer.

Afterwards,Rocha and Carlos have prepared nanorods of Tb_0.99Eu_0.01(BDC)_1.5·(H₂O)₂ through a reversemicroemulsion method (Fig. 6) ^[71]. The resulted nanorods in an aqueous suspension displayed an excellent performance as ratiometric luminescent nanothermometers in the physiological temperature range of 300-320 K. The temperature-dependence of the ⁵D₀→⁷F₂ (Eu³⁺) and ⁵D₄→⁷F₅ (Tb³⁺) energy transfer was used to define the ratiometric thermometric parameter. Recently,they prepared [(Tb_0.914Eu_0.086)₂(PDA)₃(H₂O)]·2H₂O (PDA = 1,4-phenylenediacetic acid) nanoparticles by the spray-drying method. It is an excellent nano-thermometer in the range of 10-325 K with high sensitivity (up to 5.96 ± 0.04% K^-1 at 25 K),high reproducibility (>97%,at room temperature),and low-temperature uncertainty (0.02 K at 25 K) ^[72].

Fig. 6

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Fig. 6.Schematic illustration of luminescent Tb0.99Eu0.01(BDC)1.5(H2O)2 nanorods as a ratiometric nanothermometer in the physiological temperature range of 300– 320 K. Reprinted with permission from Ref. [71], Copyright 2013, American Chemical Society.

Luminescent NMOFs are an exactly fascinating class of multifunctional chemosensors through the rational design of NMOF materials with tailorable particle sizes,suitable porosity, specific functional groups,strong electron and energy transfer,and tunable luminescent properties. Although at the early stage of their development,luminescent NMOFs have been a versatile and remarkable type of chemical sensors for the detection of ions, molecules,gases,pH value,and temperature.

These achievements are just a tip of iceberg and there are still many challenges in the area of chemical detection. First of all,a major challenge is to pre-design novelNMOFmaterials with desired properties. Any specific application can theoretically be realized through the rational design of NMOF materials in the terms of compositions,sizes,shapes,nanoporosity,and luminescent properties. Second,the majority of NMOF sensors are based on the monitoring of emission intensity changes in a single emission peak by a fluorometer. The accuracy can be heavily affected by the optoelectronic drifts of excitation power and detectors. Ratiometric or self-calibrated chemosensory materials can avoid the drawbacks of those based on single-emission intensity,making precise chemical detection possible. Third,the targeted analytes are limited to a few types of guests,such as strong electron receptors or donors. An improved approach is to couple luminescent NMOFs with other methods of signal transduction,such as interferometry,localized surface plasmon resonance (LSPR),surface-enhanced Raman spectroscopy (SERS),and electrical/electrochemical/mechanical schemes,developing multifunctional NMOFs for the diverse chemosensory applications. Forth,other synthetic methodologies (e.g. patterned deposition ^[73],microfluidics ^[74/span>],supercritical conditions,etc.) and lithographic techniques (e.g. photolithographic, dip-pen nanolithographic ^[75],fountain-pen lithographic,etc.) can be implemented to expand the variety of synthesized NMOFs. Briefly,nanoscale luminescentMOFs will be extensively pursued for their practical applications in chemical detection.

This work was supported by National Natural Science Foundation of China (No. 21303178) and Jilin Province Youth Foundation (No. 20140520091JH).