Two Dimensional Host−Guest Metal−Organic Framework Sensorwith High Selectivity and Sensitivity to Picric AcidMinoo Bagheri,†,§ Mohammad Yaser Masoomi,†,§ Ali Morsali,*,† and Alexander Schoedel‡

†Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, Tehran 14117-13116, Iran‡Department of Chemistry, University of California, Berkeley; Materials Sciences Division, Lawrence Berkeley National Laboratory;Kavli Energy Nanoscience Institute, Berkeley, California 94720, United States

*S Supporting Information

ABSTRACT: A dye-sensitized metal−organic framework, TMU-5S, was synthesized based on introducing the laser dyeRhodamine B into the porous framework TMU-5. TMU-5S wasinvestigated as a ratiometric fluorescent sensor for the detectionof explosive nitro aromatic compounds and showed four timesgreater selectivity to picric acid than any state-of-the-artluminescent-based sensor. Moreover, it can selectively discrim-inate picric acid concentrations in the presence of other nitroaromatics and volatile organic compounds. Our findings indicatethat using this sensor in two dimensions leads to a greatly reducedenvironmental interference response and thus creates exceptionalsensitivity toward explosive molecules with a fast response.

KEYWORDS: ratiometric fluorescent sensor, metal−organic frameworks, picric acid, sensitivity, selectivity

■ INTRODUCTION

Metal−organic frameworks (MOFs) are a relatively new classof highly porous crystals that are composed of metal oxide unitsstitched together covalently by organic linkers. Thesearchitecturally stable extended structures not only supportpermanent porosity, but both inorganic and organic constitu-ents can be varied by design to fine-tune the pore size and thepore chemistry.1−3 In terms of synthesis and modification, theadvantages of MOFs over conventional porous materials havebeen underlined by many practical applications including gasstorage and separation or catalysis, among others.4 MOFs havealso been widely used as chemical sensors because of theprecision with which they can be designed to create favorableinteractions between the pore interior and diverse analytes.5−11

The various MOF sensors reported thus far operate throughluminescence, solvatochromic/vapochromic, interferometry,localized surface plasmon resonance, with colloidal crystals orconductivity and electromechanical detection.8,12−19 Moststudies were carried out on luminescence-based MOF sensorsfor the detection of hazardous materials and high explo-sives.20−23 The characteristics of a good sensor are generallysummarized as the “4S”: sensitivity, selectivity, stability, andspeed of response and recovery times.24,25 In most luminescentMOF sensors, the response is based on quenching, orenhancement of the emission intensity upon guest adsorption.However, only selectivity and not sensitivity can be achieved bythis type of signal transduction. The resulting sensor responsecan be observed by introduction of any analyte to anycompound as a sensor that is usually insufficient for accurateand sensitive detection of a specific analyte.5,26−28 In these

cases, the introduction of an additional sensing/detectionparameter such as a shift in emission frequency (wavelength) isexpected to greatly enhance signal transduction from one-dimensional (1D) to two-dimensional (2D) and remove false-positive responses; however, this has not yet been realized.29−31

We herein report a ratiometric luminescent sensor, based ona MOF being able to create sensitivity for detection of picricacid (PA). According to the U.S. Environmental ProtectionAgency, PA is widely recognized as an environmentalcontaminant and as harmful to humans and wildlife.32 It iswidely used as a staining agent and a reagent in laboratoryprocedures as well as in the manufacture of rocket fuels,fireworks, matches, and so forth. Apart from its acidicproperties, PA is unstable and readily reacts with othermaterials to create explosive compounds. Its explosive poweris somewhat superior when compared to that of TNT.33,34

In contrast to most luminescent MOF sensors that rely onquenching or enhancement of emission intensity upon guestadsorption (1D), we follow a different strategy to significantlyimprove accuracy and selective detection of a specific analyte.Our 2D signal transduction approach is achieved byintroducing an appropriate guest to the interior of the MOFwith a different photoinduced emission wavelength than thehost framework. Thereby a ratiometric fluorescent sensor with2D signal transduction was created in which the response nowdepends on the ratio of emission intensities at two different

Received: June 10, 2016Accepted: August 1, 2016Published: August 1, 2016

Research Article

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© 2016 American Chemical Society 21472 DOI: 10.1021/acsami.6b06955ACS Appl. Mater. Interfaces 2016, 8, 21472−21479

wavelengths, for example, the host and the guest.35 This type ofsignal transduction eliminates environmental interference suchas drift of the light source or detector and concentrationchanges of competing analytes.27,36

This strategy is based on creating a sensor with convertibilityof diverse responses from different analytes in variousconcentrations to unit signal and producing a special signalto specific concentrations of a target analyte. In contrast tomost reported MOF sensors, and in order to obtain animproved sensor, the effective parameters (4S) were consideredand investigated.8,26,37,38 Interestingly, for the first time in thiskind of sensor, differentiation between the response and thesensitivity has been observed, and it shows particular sensitivityto a certain analyte with a specific concentration.TMU-5, [Zn(oba)(4-bpdh)0.5]n·1.5DMF (H2oba = 4,4-

oxybisbenzoic acid; 4-bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene; DMF = N,N-dimethylformamide), is a Znpaddlewheel-based MOF in which narrow, interconnected,and azine-decorated pores generate favorable interactions withacidic analytes.39,40 We detail how dye-sensitized MOF TMU-5can act as a ratiometric fluorescent sensor for selectivedetection of low concentrations of PA in the presence ofnitro aromatic compounds (4-nitroaniline (NA), nitrobenzene(NB), and 4-nitrophenol (NP)) and representatives of volatileorganic compounds (VOCs) (acetone, toluene, methanol, andethanol). The TMU-5S sensor has great selectivity to PA thatarises from existence of basic azine groups (fluorophores) in thenarrow pore walls. In addition, 2D signal transduction caneliminate environmental interference and enables selectiveresponse to a specific concentration of PA.

■ EXPERIMENTAL SECTIONMaterials and Physical Techniques. All reagents for the

synthesis and analysis were commercially available from Aldrich andMerck and used as received.Apparatus. IR spectra were recorded using a Thermo Nicolet IR

100 FT-IR. Thermal behavior was measured with a PL-STA 1500apparatus at a rate of 10 °C min−1 under a static atmosphere of argon.Powder X-ray diffraction (PXRD) measurements were performedusing a Philips X’pert diffractometer with monochromated Cu Kαradiation. Elemental analyses were collected on a CHNS ThermoScientific Flash 2000 elemental analyzer. The UV−visible (UV−vis)absorption spectra were collected on a Cecil Aquarius CE 7200 doublebeam UV−vis spectrophotometer equipped with 1.0 cm in widthquartz cells. Fluorescence microscopic images were taken on an AxiomBM-600 LED fluorescence microscope. The diffuse reflectancespectroscopy (DRS) UV−vis spectra were recorded on a Lambda1050 using BaSO4 as a reference. The samples were characterized withfield-emission scanning electron microscopy (FE-SEM) on a TescanMira microscope. Sorption studies on TMU-5 and TMU-5S wereperformed using the AutosorbIQ from Micrometrics Instruments: N2at 77 K.Synthesis of TMU-5 Single Crystals. Yellow crystals of TMU-5

were synthesized according to previously reported methods,39 bydissolving Zn(NO3)2·6H2O, H2oba, and 4-bpdh in DMF. The reactionmixture was placed into an oven at 80 °C for 3 days. The crystallineproduct was washed with DMF and dried in air.Synthesis of TMU-5 Powder. In a typical synthesis, a mixture of

H2oba (0.128 g, 0.5 mmol), 4-bpdh (0.110 g, 0.5 mmol), andZn(OAc)2·2H2O (0.07 g, 0.3 mmol) in 20 mL of DMF was reacted inan ultrasonic bath at ambient temperature and atmospheric pressurefor 60 min. The resulting yellow powder was isolated bycentrifugation, washed with 3 mL of DMF three times, and dried at80 °C for 24 h. Yield: 0.221 g (81% based on oba). IR data (KBrpellet, ν/cm−1): selected bands: 652(s), 779(m), 873(m), 1021(m),1092(m), 1162(s), 1233(vs), 1397(vs), 1499(m), 1631(vs), 1671(vs),

and 3414(w-br). Elemental analysis (%) calculated for [Zn(C14O5H8)-(C14H14N4)0.5]·(C3NOH7)1.5: C, 55.6; H, 4.7; N, 8.9; found: C, 54.8;H, 4.2; N, 8.8.

Synthesis of TMU-5S Single Crystals. Single crystals of TMU-5Swere obtained from a mixture of Zn(NO3)2·6H2O (0.225 g, 0.76mmol), H2oba (0.248 g, 1 mmol), 4-bpdh (0.220 g, 1 mmol), andRhodamine B (RhB, 0.054 g, 0.11 mmol) in 40 mL of DMF. Thismixture was sonicated until all solids were dissolved (∼3 min). Theresulting solution was divided into 10 vials that were placed in an ovenat 80 °C. After 72 h, red crystals of TMU-5S were collected andwashed with DMF (4 × 5 mL). Yield: 0.401 g (78% based on oba). IRdata (KBr pellet, ν/cm−1): selected bands: 577(m), 650(s), 776(s),833(m), 871(m), 1020(m), 1087(m), 1160(s), 1229(vs), 1395(vs),1498(m), 1633(vs), 1674(vs), and 3414(w-br). Elemental analysis(%): C, 54.6; H, 4.3; N, 8.9.

Synthesis of TMU-5S Powder. Red powder of TMU-5S wasobtained under the same conditions as TMU-5 powder by adding0.054 g (0.11 mmol) of RhB into the solution. The product waswashed with DMF until no fluorescence was observed in the washings(∼9 × 5 mL) and then dried at 80 °C for 24 h. Yield: 0.195 g (71%based on oba). IR data (KBr pellet, ν/cm−1): selected bands: 439(w),526(w), 576(w), 656(m), 780(m), 835(w), 874(m), 1020(w),1092(w), 1162(m), 1234(vs), 1399(vs), 1498(m), 1609(vs),1633(vs), 1672(vs), and 3421(w-br). Elemental analysis (%): C,54.5; H, 4.4; N, 8.9.

Activation Method. All as-syntesized samples were directly heatedunder vacuum in an oven at 140 °C for 48 h to remove guest DMFmolecules in the pores of the MOFs. The structure remained intactupon removal of guest DMF molecules as indicated by unalteredPXRD patterns (Supporting Information, Figure S1).

Determination of Dye Contents. A powder sample of TMU-5S(10 mg) was digested with NaOH (5 mL, 2 M), and the resultingclear, light red solution was diluted to 10 mL and adjusted to a pH of 7with HCl. The concentration of RhB was determined by comparingthe UV−vis absorption with a standard curve.

Electrochemical Measurements. Electrochemical measurementsof TMU-5, TMU-5S, and the analytes were carried out at roomtemperature in a three-electrode cell. An EG&G Princeton AppliedResearch model 273A instrument was used in all electrochemicalmeasurements. A glassy carbon (GC) electrode (geometric area0.0314 cm2) was used as the working electrode, and platinum foil (1.0cm2) and Ag/AgCl electrodes served as counter and referenceelectrodes, respectively. For the preparation of working electrodes, 2.5mg of TMU-5 or TMU-5S was sonicated in a 2.5 mL mixture ofisopropyl alcohol/water (2:1) and 0.1764 mg/mL Nafion solution as abinder until a uniform suspension was achieved (approximately 10min). Then, 10 μL of this suspension was coated quantitatively onto aGC electrode, and the electrode was dried in air at room temperature.The modified electrode was immersed in 30 mL of 0.1 Mtetraetylammonium bromide in acetonitrile and cycled several timesbetween potentials of −2.0 and 1.0 V. The electrochemicalmeasurements of the analytes were carried out through cyclicvoltammetry by dissolving 20 mg of the analytes in 30 mL of 0.1 Mtetraetylammonium bromide in acetonitrile. The reduction potentialsfor all compounds were calculated from the voltammograms andcorrected with the reduction potentials of ferrocene as the internalstandard (Supporting Information, Table S1).

Fluorescence Quenching Titrations in Dispersed Medium.The fluorescence of TMU-5 and TMU-5S in different solventemulsions was recorded on a PerkinElmer-LS55 fluorescencespectrometer at room temperature. After activation, 1 mg of theMOF was dispersed in 4 mL of the desired solvent with ultrasonicationfor 10 min. All titrations were carried out by gradually adding 40−200μL of the 1.0 × 10−3 M target analyte solutions in acetonitrileincluding PA, NA, NB and NP in an incremental fashion to achieve theconcentration of 1−5 × 10−5 M of target analytes. For interferenceexperiments, 10−100 μL of methanol, ethanol, acetone, and toluene(HPLC grade), as typical representatives of VOCs, were added to theTMU-5S suspension in acetonitrile (Supporting Information, TableS2). Each titration was repeated at least four times for consistency

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purposes. For all measurements, disperse solutions of the MOFs wereexcited at λex = 355 nm, and their corresponding emission wavelengthwas monitored from λem = 380−680 nm. The slit widths for bothexcitation and emission were set to 10 nm. The fluorescencequenching efficiency is calculated using [(I0 − I)/I0] × 100%, whereI0 is the initial fluorescence intensity of the soaked MOF sample inacetonitrile at λ = 485 nm and I is the fluorescence intensity in thepresence of the desired nitro aromatic compounds at the same

wavelength. For TMU-5, the sensor response (R) is defined as I0/I.The quenching constant (KQ) (M−1) is calculated by the Stern−Volmer equation, (I0/I) = KQ[A] + 1, where [A] is the molarconcentration of analyte in the presence of TMU-5. The sensorresponse (R) for TMU-5S is defined as the emission intensity ratio ofλ583 to λ485 of the sensor (I583/I485, ratiometric response). Theresponse time was recorded for acetonitrile-dispersed TMU-5S asprobe fluorescence in the presence of 4.0 × 10−5 M PA at any given

Figure 1. Single-crystal X-ray structure of TMU-5 (top) and of the azine-functionalized pores; the interaction between the azine moiety of 4-bpdhand picric acid is highlighted (bottom). Color code: O, red; N, blue; C, black; and Zn, blue polyhedra.

Figure 2. Photographs of (a, b) single crystals and (c, d) powder of pure TMU-5 and (e, f) single crystals and (g, h) powder of TMU-5S (RhB@TMU-5) under ambient light (a, c, e, g) and UV irradiation (b, d, f, h) observed using a fluorescence microscope.

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time. Also, the recovery time was investigated by refreshing acetonitrileat given intervals. Sensor stability tests of sensitive TMU-5S to PAwere performed in the same way as the sensing tests. The fluorescenceof TMU-5S in the presence of 4.0 × 10−5 M PA was also recorded.The material was recovered by centrifugation after each quenchingtitration and then washed several times with acetonitrile. The materialwas subsequently dried and used for six further cycles.

■ RESULTS AND DISCUSSIONCharacterization. TMU-5 is fast and easy to prepare on a

large scale at low cost and can be obtained via various methodssuch as conventional heating, mechanosynthesis, and sono-chemistry.39,41 This MOF is based on Zn2(−COO)4(-PY)2pillared paddlewheel secondary building units that are six-coordinate and linked through oba and 4-bpdh. The overallstructure can therefore be regarded as a square grid (rhombic),connected into a three-dimensional framework. The 4-bpdhlinker provides for azine-functionalized pores with an aperturesize of 4.4 Å × 6.2 Å and 34.6% void space (nitrogen probe)per unit cell (Figure 1).Single crystals and powders of dye-sensitized TMU-5, TMU-

5S, were synthesized using RhB (Figure 2e, g and SupportingInformation, Figure S2). Comparison between the simulatedand experimental PXRD patterns of TMU-5 and TMU-5Srevealed that they are structurally identical (SupportingInformation, Figure S1).Dye-uptake measurements by UV−vis absorption spectros-

copy show uptakes of RhB as much as 0.19% with respect tothe framework weight (2 mmol/mol) (Supporting Information,Figure S3).42,43 The thermogravimetric analysis (TGA) curvesof TMU-5 and TMU-5S demonstrate that dye uptake has noinfluence on the residual mass but changes the decompositionstep. (Supporting Information, Figure S4).44 Fluorescencemicroscopy images of TMU-5 and TMU-5S show incorpo-

ration of RhB dye in the framework (Figure 2f, h). The DRSUV−vis spectrum of TMU-5 exhibits a strong absorption in thevisible light region at approximately 550 nm with an opticalband gap at roughly 2.25 eV (Supporting Information, FigureS5) that is related to the absorption caused by excitation ofelectrons from the valence band to the conduction band inTMU-5. The absorption edge of TMU-5S shifts to longerwavelengths (at about 615 nm, band gap = 2.02 eV). Thisexemplifies the influence of RhB addition on the reduction ofthe band gap and also confirms incorporation of RhB into theframework. (Supporting Information, Figure S5). TheBrunauer−Emmett−Teller specific surface area (N2, 77 K) ofTMU-5 and TMU-5S powders synthesized by the sonochem-ical method are 582 and 520 m2/g, respectively, indicating thatthe loaded dye (0.19%) has only a marginal effect on porosity(Supporting Information, Figure S6).

Fluorescence Properties of TMU-5 and TMU-5S.Comparison of emission spectra of TMU-5 and TMU-5Srevealed that, in addition to an emission peak centered ataround 485 nm (framework emission), loading of RhB resultsin an additional emission peak centered at around 583 nm(RhB emission) (Figure 3a). Fluorescence emission spectra ofTMU-5 and TMU-5S in different solvents indicate no shift inthe emission maxima with increasing polarity of the solvent(Supporting Information, Figures S7 and S8). In the case ofacetonitrile, an increase in the intensity, possibly due tostronger interactions with the framework, is observed.TMU-5S samples were washed five times with acetonitrile

before the sensing tests, and no fluorescence was observed afterthe washings. There is also no decrease in fluorescence intensityof TMU-5S samples (Supporting Information, Figures S9 andS10).

Figure 3. (a) Emission spectra of TMU-5 and TMU-5S dispersed in acetonitrile upon excitation at 355 nm. (b) Emission spectra of TMU-5dispersed in acetonitrile with incremental addition of picric acid solution in acetonitrile; (inset) corresponding Stern−Volmer plot of picric acid. (c)Emission spectra of TMU-5S dispersed in acetonitrile with incremental addition of picric acid solution in acetonitrile. (d) Emission intensity ratios(R = I583/I485 = 5.1) of TMU-5S in the presence of 4−5 × 10−5 M picric acid.

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Effects of Different Analytes and Their Concentra-tions. Fluorescence emission spectra of TMU-5 dispersed inacetonitrile were collected with a gradual increase in theconcentration of nitro aromatics (PA, NA, NB, and NP) andrevealed quenching of fluorescence intensities of the MOF(Figure3b and Supporting Information, Figures S11−S13). Inaddition, the UV−vis spectra of all analytes were measured toexclude any absorption effects. Figure S14 (SupportingInformation) indicates that the maximum absorption intensitiesare indeed different from that of TMU-5. The percentage offluorescence quenching efficiency and KQ obtained for differentanalytes are given in Table S3 (Supporting Information). Theresults show that quenching efficiency increases in the orderNP < NB < NA < PA and that TMU-5 is selective to PA.However, the changes of sensor responses with respect to theanalyte concentration remain irregular and are of very limitedpractical utility as sensitivity cannot be observed (Figure 4a, b).This type of 1D signal transduction (“turn-off” sensing) istherefore not a generalizable approach to distinguish particularsensitivity of a certain type of analyte with a specificconcentration.We addressed this problem by introducing RhB into TMU-5

rendering it into a 2D sensor (TMU-5S). Fluorescencedetection experiments were carried out on TMU-5S in thepresence of different concentrations of PA, NA, NB, and NP(Figure 3c and Supporting Information, Figures S15−S17 andTable S4). The quenching effect at 485 nm is similar to that ofTMU-5. The sensor response is equal (R = 2.3) for all analytesand concentrations except for 4 and 5 × 10−5 M of PA with R =5.1 (Figure 4c). Comparison between response changes from 3to 4 × 10−5 M PA in TMU-5 (R = 3.5 to 5.1, 1.4 times) andthose in TMU-5S (R = 2.3 to 5.1, 2.2 times) indicates thatTMU-5S can differentiate between sensor response andsensitivity. In MOF-based sensors, the word “sensitivity” hasoccasionally been confused with “response”. Sensitivity (S) isdefined as changes of response in the presence of analyte over

initial response ( = −S R RR

f i

i, where Ri is the initial response of

the sensor and Rf is the response of the sensor in the presenceof the desired nitro aromatic compounds). TMU-5S shows

sensitivity of 0 ( = =−S 02.3 2.32.3

) for all concentrations except

for 4−5 × 10−5 M whose sensitivity is 1.2 ( = =−S 1.25.1 2.32.3

).

In fact, in this study sensitivity was also observed in addition tothe response. By using TMU-5S, the irregular quenching ofTMU-5 in the presence of various analytes is turned into onesingle response, which only changes at a specific concentrationof PA. Therefore, introduction of RhB to the framework causesTMU-5S to be selectively sensitive to PA in the presence ofother analytes (NA, NP, NB) at different concentrations. Thismeans that the ratiometric fluorescent sensor acts as an outputsignal modulator while eliminating response-influencing envi-ronmental factors and amplifying sensitivity.The selectivity of MOFs to specific analytes can be explained

by donor−acceptor electron-transfer mechanisms. Generally,this behavior arises from specific interactions between the guestand MOF fluorophores (depending on size and type ofinteraction) as well as the position of the molecular orbital andthe band structure of the MOF and the analyte.26

The KQ of TMU-5 for PA (∼1.3 × 105 M−1) is 3.8 timesgreater when compared to the KQ of the fluorescent MOF[Cd(ndc)0.5(pca)] (ndc = 2,6-napthalenedicarboxylic acid, pca= 4-pyridinecaboxylic acid),37 with KQ = 3.5 × 104 M−1. This

value is also comparable with the best conjugate polymersensors.27,45,46 A structural comparison of these two MOFsreveals that the greater selectivity of TMU-5 can be attributedto the existence of basic azine groups (fluorophores) in thenarrow pore walls37,47 providing enhanced interactions with thehighly acidic phenol group of PA.Cyclic voltammetry measurements (Supporting Information,

Table S1) demonstrate that the reduction potential of PA ismore positive than that of TMU-5S. Consequently, TMU-5Scan act as an electron donor to PA leading to turn-off sensing.

Figure 4. (a) Percentage of fluorescence quenching and (b) responsesof TMU-5 in the presence of different nitro aromatics at differentconcentrations. (c) Comparison of responses of TMU-5S in thepresence of different nitro aromatic analytes at different concen-trations. (λex = 355 nm).

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Therefore, strong electrostatic interactions between fluoro-phores of TMU-5 and PA, along with favorable electrontransfer, facilitate a higher fluorescence quenching response.Investigation of Response and Recovery Times. On

the basis of sensitivity results, the response and recovery timesof TMU-5S to 4 × 10−5 M PA were investigated (SupportingInformation, Figure S18). The time needed to reach 90% finalresponse value (R = 5.1) of the sensor upon exposure to atarget analyte (PA) and switching off from the target aredefined as response (τres) and recovery (τrecov) times,respectively.48 The response and recovery times for TMU-5Sare 45 and 460 s, respectively, indicating fast response of thissensor to PA. The recovery time from a certain target is usuallylonger than the response time to the same analyte;49 however,extremely faster response times are indicative of stronginteractions between the sensor and the target analyte.50 Wetherefore conclude that strong interactions between thefluorophore of TMU-5S and PA are present.Stability of the TMU-5S Sensor. The sensor stability in

water and durability through sensing plays a main role in termsof its practical utility.51,52 As previously reported, TMU-5 isstable in water, even after 24 h as exemplified by unalteredPXRD patterns (Supporting Information, Figure S19).39,40

Water stability tests of TMU-5S also confirm its stability inwater after 24 h (Supporting Information, Figure S19). Thefluorescence intensity of TMU-5S in the presence of water alsoshows no change even after 180 min (Supporting Information,Figure S20). In addition, the sensor stability parameters,response, response times, and recovery times after six cyclesremain unaltered, indicating a high photostability of TMU-5Sin the presence of 4 × 10−5 M PA (Figure 5). Moreover,

comparison between the PXRD patterns of TMU-5S beforeand after six repeated reactions clearly indicate that theframework remained intact (Supporting Information, FigureS21).Investigation of Selectivity of TMU-5S in the Presence

of Other Nitro Aromatics and VOCs. As our newlydeveloped 2D sensor showed exceptional selectivity for PA,we aimed to test it in the presence of other nitro aromatics andVOCs. For this purpose, simultaneous selectivity experiments

were carried out by adding 160 μL of PA (aqueous 1 mM) toTMU-5S dispersed in 4 mL of acetonitrile, followed by 40 μLof saturated aqueous solutions of NA, NB, and NP (Figure 6).

After the response of TMU-5S to PA, the addition of saturatedaqueous solutions of other nitro aromatics showed no effect onthe ratio of the two fluorescence intensities. Subsequently, uponaddition of another 40 μL of aqueous PA (final concentrationof 5 × 10−5 M PA), a significant quenching effect with the ratioof intensities of I583/I485 = 5.1 is observed (Figure 6). Thus,TMU-5S as a fluorescent probe can simultaneously act selectiveto PA, even in the presence of other nitro aromatics.Since VOCs exist at higher levels in the external atmosphere,

their interfering effects on selectivity need to be addressed.15,53

The selectivity of TMU-5S in the presence of some competingVOCs at different concentrations including acetone, toluene,methanol, and ethanol (Supporting Information, Table S2) ispresented in Figures S22−26 (Supporting Information).Results indicate that there is no or negligible change in thepresence of VOCs as well as on the quenching effect.Therefore, this sensor is highly selective to PA, not only inthe presence of other nitroaromatics but also VOC targetanalytes (Supporting Information, Figure S27).

■ CONCLUSIONSTMU-5 and its dye-sensitized compound, TMU-5S, have beensynthesized and employed as sensing probes to detect explosivenitro aromatic compounds including NA, NB, NP, and PA atdifferent concentrations. By comparing sensor responses, wehave demonstrated that the dye-sensitized MOF can differ-entiate between sensor response and sensitivity. By usingTMU-5S, the variable quenching of TMU-5 in the presence ofvarious analytes is turned into a single response that onlychanges at a specific concentration of PA. Selectivity experi-ments in the presence of all tested analytes and VOCs showthat TMU-5S is highly selective to PA. Fast response andpractical recovery times as well as stability in water turn TMU-5S into an excellent candidate to be used in production ofratiometric fluorescent sensors with selectively sensitivedetection of low concentrations of PA by eliminatingenvironmental interferences.

Figure 5. Comparison of stability parameters of TMU-5S, response,response time, and recovery time in the presence of 4 × 10−5 M picricacid during six cycles.

Figure 6. Simultaneous selectivity tests of TMU-5S dispersed inacetonitrile exposed to 4−5 × 10−5 M picric acid (PA) followed byaddition of solutions of 40 μL of 4-nitroaniline (NA), nitrobenzene(NB), and 4-nitrophenol (NP).

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■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b06955.

XRPD patterns, FE-SEM images, UV−vis spectra,thermogravimetric profiles, DRS UV−vis spectra, N2

isotherms at 77 K, emission spectra, time-dependentratiometric fluorescence plots, selectivity evaluation, andFTIR spectra (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: morsali_a@modares.ac.ir; phone: (+98) 21-82884416.Author Contributions§M.B. and M.Y.M. contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSSupport of this investigation by Tarbiat Modares University isgratefully acknowledged. A.S. gratefully acknowledges theGerman Research Foundation (DFG, SCHO 1639/1-1) forfinancial support. The authors would like to thank Zhe Ji (UCBerkeley) for producing the TOC graphics.

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