Pierre Farger

Cédric Leuvrey

Mathieu Gallart

Pierre Gilliot

Guillaume Rogez

João Rocha

Duarte Ananias

Pierre Rabu
email: pierre.rabu@ipcms.unistra.fr

Émilie Delahaye

Emilie Delahaye
email: emilie.delahaye@ipcms.unistra.fr

Magnetic and luminescent coordination networks based on imidazolium salts and lanthanides for sensitive ratiometric thermometry

Keywords: coordination network, imidazolium salt, lanthanides, magnetism, thermometry

Introduction

Metal-organic coordination networks have been the subject of considerable research in the last years as evidenced by the increasing number of papers published in the field [1]. Indeed, the possibility of combining different properties by judicious choice of the organic and inorganic moieties makes these systems good candidates for the elaboration of (multi)functional architectures [2,3]. Among the various functionalities that can be envisioned for this class of hybrid compounds, the elaboration of luminescent networks is interesting in term of potential applications in lighting, display, sensing, biomedicine and for optical devices [4][5][6][7][8][9][10][11][12].

Luminescent coordination networks can be obtained either by the use of specific luminescent organic ligands or by the use of main-group elements, d 10 transition metals or of trivalent lanthanide ions for the inorganic moiety [13,14]. The luminescent properties of the trivalent lanthanide ions are particularly interesting since they cover a large range of emission from the ultraviolet (Gd 3+ ) to near-infrared (Pr 3+ , Nd 3+ , Ho 3+ , Er 3+ , Yb 3+ ) through the visible domain (Pr 3+ , Sm 3+ , Eu 3+ , Tb 3+ , Dy 3+ , Tm 3+ ). It confers to lanthanide-based networks a large tunability of emission properties, which is very useful for the elaboration of light-emitting devices or for biomedical applications [15]. Moreover, due to the narrowness and the hypersensitivity of their transitions, lanthanide-based networks can also find utility for the sensing of gases, vapors or small molecules [9,[START_REF] Fordham | Lanthanide Metal-Organic Frameworks: Syntheses, Properties, and Potential Applications[END_REF]. In the case of mixed lanthanide coordination networks, the luminescent properties can be used to synthesize temperature probes with possible applications in the aerospace area, safety and health [START_REF] Rocha | [END_REF]18].

Beside luminescent properties, lanthanide ions exhibit large magnetic moment and strong magnetic anisotropy, which might have potential applications of lanthanide-based networks in information storage, quantum computing and spintronics [19][20][21][22][23].

Most of these lanthanide-based networks are obtained with neutral organic ligands such as benzene-1,4-dicarboxylate (1,4bdc) [24], benzene-1,3,5-tricarboxylate (TMA) [25], pyridine-2,5-dicarboxylate (2,5-H 2 pdc) [24] or 1H-2-propyl-4,5-imidazoledicarboxylate (pimda) [26]. Only few examples of lanthanide-based networks obtained with charged ligands are reported in the literature [27][28][29][30]. Following this last point, we have chosen to synthesize lanthanide-based networks from positively charged imidazolium dicarboxylate salts [31][START_REF] Farger | Synthèse Ionothermale de Réseaux Hybrides Multiferroïques[END_REF][START_REF] Farger | [END_REF][34]. Contrarily to the classical imidazolium salts or ionic liquids used in ionothermal syntheses [35][36][37][38][39][40], the functionalization of imidazolium moieties with coordinating functions reduces the influence of the imidazolium salt on the ligand for a better rationalization of the synthesis [31][START_REF] Farger | Synthèse Ionothermale de Réseaux Hybrides Multiferroïques[END_REF][START_REF] Farger | [END_REF][34].

We report in this paper the synthesis and the characterization of six new networks obtained from an imidazolium dicarboxylate salt, oxalic acid and lanthanide ions. The structure of these networks has been solved by single crystal X-ray diffraction and their physical properties (magnetism and luminescence) have been investigated. We establish that these networks show antiferromagnetic interactions. The study of the luminescent properties evidences the presence of well-defined transitions characteristic for the considered lanthanide. These results have prompted us to extend our strategy to the synthesis of mixed lanthanide networks with four different ratios Tb 3+ /Eu 3+ . The powder X-ray diffraction analysis indicates that these mixed lanthanide networks are isostructural to the parent homolanthanide compounds. The temperature-resolved photo-luminescent properties of the latter indicate possible applications in thermometry.

Results and Discussion

Synthesis

The 1,3-bis(carboxymethyl-)-imidazolium ligand [HL] was synthesized according to protocols published in the literature [41,42].

Single crystals and homogeneous powders of [Ln(L)(ox)(H 2 O)]

were obtained with Ln = Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ and Yb 3+ by reacting a water/ethanol solution of the lanthanide nitrate and oxalic acid (H 2 ox) with [HL]. The mixture was sealed in a Teflon-lined stainless steel autoclave and heated at 393 K for 72 h. After cooling to room temperature, the autoclaves were opened and crystals were filtered and washed with ethanol. The yields of the reactions range from 36 to 59 %. Similar reactions were carried out with Nd 3+ and Sm 3+ ions leading to different structures [31]. In addition, in the case of Nd 3+ and Sm 3+ , various crystalline compounds were obtained depending on whether oxalic acid was added or not. When oxalic acid was not added in situ formation of the oxalate ligand has been observed. The peculiar behavior of these two ions compared to others can be explained by their place in the first part of the lanthanide series [43]. In the case of Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ and Yb 3+ , described in the present work, the direct reaction between lanthanide nitrate and [HL], without addition of oxalic acid, did not give crystalline compounds.

Characterization of the homolanthanide [Ln(L)(ox)(H 2 O)] compounds with Ln = Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ and Yb 3+ Single crystal X-ray analysis of the [Ln(L)(ox)(H 2 O)] compounds with Ln = Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ and Yb 3+ reveal that the six compounds are isostructural. All compounds are obtained as colorless crystals and crystallize in the monoclinic space group P2 1 /a (no. 14). Crystal data for these series of compounds are collected in Table 1 andTable 2. The asymmetric unit contains one Ln 3+ ion, one [L] -ligand, two half-oxalate ligands and one coordinating water molecule (Figure 1). Ln 3+ ions are surrounded by nine oxygens with four a The relatively high values of the residual density can be explained by the difficulty to isolate single crystals. Indeed SEM images reveal the presence of relatively small and entangled crystals (see Figure S3, Supporting Information File 1). a The relatively high values of the residual density can be explained by the difficulty to isolate single crystals. Indeed SEM images reveal the presence of relatively small and entangled crystals (see Figure S3, Supporting Information File 1). oxygen atoms coming from one and same carboxylate function of two different [L] -ligands, one from the water molecule and four from two different oxalate ligands. The coordination environment of Ln 3+ ions is a tricapped trigonal prism (Figure S1, Supporting Information File 1) with Ln-O distances similar to those observed in structurally related compounds [44,45]. These distances decrease progressively with the size of the lanthanide ion in agreement with the lanthanide contraction effect (Table S1, Supporting Information File 1). The same tendency is observed with the shortest Ln-Ln distances, which correspond to two Ln 3+ ions connected by an oxalate ligand (Table S1, Supporting Information File 1).

Two separate Ln 3+ ions are connected through an oxalate ligand in a bis-bidentate bridging coordination mode forming undulating chains along the a-axis (Figure 2). The Ln 3+ ions are connected to the carboxylate functions of the [L] -ligand in a bidentate chelate mode. The cohesion between these chains is realized through H bonding between H atoms of the coordinated water molecules and O atoms of the carboxylate functions.

Beside the single crystal analysis, the homogeneity of the six samples was checked by powder X-ray diffraction. As shown in Figure S2 (Supporting Information File 1), the experimental powder patterns fit well with the patterns calculated from the single crystal structure and show no additional phases.

In order to investigate the thermal stability, thermogravimetric analysis (TGA) was realized under air stream from 25 to 900 °C (Figure S4, Supporting Information File 1). The weight loss corresponding to the elimination of the coordinated water molecule occurs between 130 and 310 °C (step 1). The second weight loss between 310 and 750 °C (step 2) is associated to the combustion of the organic moieties (oxalate and [L] -ligands), concomitant with the formation of oxide (Ln 2 O 3 was identified by powder X-ray diffraction in the final product). The total weight loss is in good agreement with the calculated values (Table S2, Supporting Information File 1).

The infrared spectra of the six compounds are similar (Figure S5, Supporting Information File 1). The broad band around 3250 cm -1 and the one at 1672 cm -1 are ascribed to the coordinated water (stretching and bending vibration modes, respectively). The vibration bands of the aromatic and aliphatic C-H bonds are observed in the range 3150-3050 cm -1 and 3050-2950 cm -1 , respectively. The characteristic frequencies of the coordinating carboxylate functions are observed at 1627 and 1571 cm -1 (antisymmetric vibration bands) and at 1411 and 1431 cm -1 (symmetric vibration bands). It leads to Δν (Δν = ν antisym -ν sym ) equal to 216 and 140 cm -1 in agreement with a bis-bidentate bridging coordination mode of the oxalate ligand and a bidentate chelate coordination mode of the carboxylate functions of the [L] -ligand, respectively [46,47].

Magnetic properties

The magnetic behavior of the six compounds [Ln(L)(ox)(H 2 O)] have been studied in the temperature range of 1.8-300 K under a 0.5 T dc magnetic field. The magnetic susceptibilities and products χT are presented as functions of the temperature in Figure 3.

At 300 K, the value of χT for the compound [Gd(L)(ox)(H 2 O)] is 7.88 emu•K•mol -1 , which agrees well with the theoretical value for spin-only S = 7/2 Gd 3+ ions. The χT product remains almost constant above 30 K and then decreases down to 6.7 emu•K•mol -1 at 1.8 K. This decrease suggests the occurrence of antiferromagnetic coupling between neighboring gadolinium centers. Since [Gd(L)(ox)(H 2 O)] is constituted of linear chains of Gd 3+ ions with large spin moment, S = 7/2, we evaluated the magnetic coupling, J, between neighboring Gd 3+ ions by using the Fisher expression for classical spin chains [START_REF] Kahn | Molecular Magnetism[END_REF][START_REF] Fisher | [END_REF]

:
In the above expression, N is the Avogadro number, g is the Landé factor, β is the Bohr magneton, k is the Boltzmann constant, S is the spin moment, J is the magnetic coupling constant and T is the temperature. The simultaneous fitting of the suscep-tibility and the χT product with the Fischer expression above lead to refined values of g = 2.00(1) and J = -0.026 cm -1 . The g value was left free during fitting and is in line with the tabulated g values for Gd ions [50]. The absolute value and the sign of J support the presence of weak antiferromagnetic interactions in [Gd(L)(ox)(H 2 O)], in agreement with results reported in previous works [51][52][53]. The magnetic exchange coupling between lanthanide ions is usually weak, due to limited extension of the 4f orbitals.

For [Eu(L)(ox)(H 2 O)], the χT product decreases continuously from 1.4 emu•K•mol -1 at 300 K to 0 emu•K•mol -1 at 1.8 K.
This behavior is typical for Eu 3+ ions for which the 7 F ground term is split in seven 7 F J (0 ≤ J ≤ 6) states because of spin-orbit coupling [54,55]. The spin-orbit coupling constant, λ, can be evaluated considering isotropic isolated Eu 3+ ions parametrized with the appropriate expression (E1 in Supporting Information File 1) for the isotropic susceptibility of Eu 3+ ions [54,55].

A very good fit of the experimental susceptibility and the χT product of [Eu(L)(ox)(H 2 O)] was obtained above 25 K giving the refined value of λ = 309.00(4) cm -1 . This value is consistent with the value determined from the luminescence measurements (see below) and confirms that considering only the isotropic component of the susceptibility is a good approximation to analyze the magnetic data [54]. The behavior of [Tb(L)(ox)(H 2 O)] is also typical for quasi-isolated Tb 3+ ions with χT = 11.75 emu•K•mol -1 at 300 K (expected value: 11.82 emu•K•mol -1 with g = 1.5) [START_REF] Kahn | Molecular Magnetism[END_REF]50]. When decreasing the temperature, the χT product remains constant until 100 K and then decreases to 4.50 emu•K•mol -1 at 1.8 K. This decay is due to the depopulation of the low-lying J states arising from the splitting of the 7 F ground term under spin-orbit coupling. In order to determine the spin-orbit coupling, λ, it was necessary to take into account an antiferromagnetic interaction between neighboring Tb 3+ ions using a mean-field approach, in addition to the intrinsic behavior of isolated Tb 3+ ions (E5 in Supporting Information File 1) [START_REF] Boča | Theoretical Foundations of Molecular Magnetism[END_REF].

Subsequently, a good fit of the magnetic data was obtained on the whole temperature range with λ = -303(75) cm -1 and zJ′ = -0.106(1) cm -1 . The obtained λ value is consistent with other values reported in the literature for compounds containing isolated Tb 3+ ions [START_REF] Amar | [END_REF][START_REF] Liu | [END_REF]. , the χT product decreases slowly between 300 and 100 K, and a steeper decrease is observed from 13.67 emu•K•mol -1 at 100 K to 2 emu•K•mol -1 at 1.8 K. Finally, for the Yb analogue [Yb(L)(ox)(H 2 O)], the χT product decreases slowly as the temperature decreases to reach 1.61 emu•K•mol -1 at 1.8 K. This behavior is ascribed to the depopulation of the low lying states (m J states) arising from the 6 H 5/2 (Dy 3+ ), 5 I 8 (Ho 3+ ) and 2 F 7/2 (Yb 3+ ), ground states split through the action of the crystal field (for these ions, the ground state is well below the first excited J state). Using the free-ion approach and the isotropic (z) component of the susceptibility, the value of the zero-field splitting (ZFS), Δ, was evaluated for each ion using the expressions E2, E3 and E4 in Supporting Information File 1, leading to Δ = 0.169(3), 0.284(4) and 3.25(1) cm -1 for Dy, Ho and Yb, respectively. These values are in the range of those reported in the literature [51]. It can be noticed that the introduction of a zJ′ term to fit the magnetic curves down to low temperatures for [Dy(L)(ox)(H 2 O)], [Ho(L)(ox)(H 2 O)] and [Yb(L)(ox)(H 2 O)] compounds did not lead to better results. partially superimposed with the intra-4f 6 7 F 0,1 → 5 D 1-4 , 5 L 6 , 5 G 2-6 , 5 H 3-7 and 5 F 1-5 transitions of Eu 3+ , which dominate the corresponding excitation spectra. Finally, the [Tb(L)(ox)(H 2 O)] excitation spectra feature a strong and broad UV band ranging from 220 to ca. 300 nm, with a maximum at 267 nm, which has no counterpart in the [Gd(L)(ox)(H 2 O)] excitation spectra. Thus, this band is attributed to the inter-configurational spinforbiden 4f 8 →4f 7 5d 1 transition of Tb 3+ because its energy is similar to the energy reported for layered Tb 3+ silicates [60]. The additional sharp lines in the spectra of [Tb(L)(ox)(H 2 O)] are ascribed to the intra-4f 8 7 F 6 → 5 D 2-4 , 5 G J and 5 H 7 transitions of Tb 3+ . Although with a lower relevance, the excited states of the ligands also contribute to the entire excitation spectra of Tb 3+ , as shown below.

Luminescence properties in the solid state

The emission spectra of [Gd(L)(ox)(H 2 O)], [Eu(L)(ox)(H 2 O)] and [Tb(L)(ox)(H 2 O

)] recorded at 297 K and 12 K are given in Figure 5. [Gd(L)(ox)(H 2 O)] displays two broad bands from 390 to ca. 650 nm attributed to the S 1 →S 0 (peaking at ca. 415 nm) fluorescence and T 1 →S 0 (peaking at 503 nm) ligand phosphorescence. This assignment is supported by the time-resolved emission spectra recorded at 12 K excited at 350 nm (Figure S6, Supporting Information File 1), which demonstrates a much faster time dependence of the S 1 →S 0 transition com-pared to the transition T 1 →S 0 . Under 364 nm excitation, a relative minimum for the Eu 3+ and Tb 3+ auto-absorption, [Eu(L)(ox)(H 2 O)] and [Tb(L)(ox)(H 2 O)] show mainly the typical sharp Eu 3+ and Tb 3+ emission lines assigned to the 5 D 0 → 7 F 0-4 and 5 D 4 → 7 F 6-0 transitions, respectively. In addition, both compounds also exhibit a broad band from 400 to ca. 550 nm, particularly weak in the case of the former, attributed to the S 1 →S 0 transition of the ligand. Accordingly, as exemplified in Figure S7 The emission of Eu 3+ is highly sensitive to slight changes in the first coordination sphere of the metal, and because of this it is widely used as a local probe [START_REF] Bünzli | Lanthanide Probes in Life, Chemical and Earth Sciences[END_REF]. For [Eu(L)(ox)(H 2 O)], the emission spectra recorded at 297 K and 12 K show i) a single 5 D 0 → 7 F 0 transition and a local-field splitting of the 7 F 1,2 levels into three and five Stark components, respectively; ii) and the predominance of the 5 D 0 → 7 F 2 transition relatively to the 5 D 0 → 7 F 1 one, witnessing the presence of a single lowsymmetry Eu 3+ environment, in accordance with the crystal structure. Additionally, the room-temperature Eu 3+ 5 D 0 and Tb 3+ 5 D 4 decay curves were well fitted by single exponential functions, yielding lifetimes of 0.60 ± 0.01 and 0.98 ± 0.01 ms for [Eu(L)(ox)(H 2 O)] and [Tb(L)(ox)(H 2 O)], respectively (Figure S9, Supporting Information File 1), corroborating the presence of a unique Ln 3+ crystallographic site.

Consideration of these luminescence results prompted the study of isostructural mixed lanthanide networks. In particular, our interest was focused on networks bearing Tb 3+ and Eu 3+ in view of their potential application in optical thermometry [START_REF] Rocha | [END_REF][START_REF] Cui | [END_REF]. Accordingly, four Tb 3+ /Eu 3+ mixed lanthanide networks of formula [Tb 1-x Eu x (L)(ox)(H 2 O)] with x = 0.01, 0.03, 0.05 and 0.10 have been synthesized using the same protocol but varying the molar ratio of Tb(NO 3 ) 3 •6H 2 O and Eu(NO 3 ) 3 •6H 2 O. As expected, these mixed lanthanide networks are isostructural with the parent compound [Ln(L)(ox)(H 2 O)] (Figure S10, Supporting Information File 1) and show the presence of Tb and Eu in the expected ratio and homogeneously distributed in the crystals (Figure S11, Supporting Information File 1).

Consider the emission spectra of the four Tb 3+ /Eu 3+ mixed lanthanide networks measured at room temperature (Figure S12, Supporting Information File 1). To maximize the relative poor ligand emission, 364 nm excitation was used since it corresponds to a maximum of the ligand excitation and to relative minima of both Eu 3+ and Tb 3+ auto-absorptions, as demonstrated by the selective 12 K excitation spectra of [Tb 0.90 Eu 0.10 (L)(ox)(H 2 O)] (Figure S13, Supporting Information File 1). The room-temperature emission spectra of [Tb 0.90 Eu 0.10 (L)(ox)(H 2 O)] under ambient pressure and after exposure to high vacuum (5 × 10 -3 mbar, Figure S14, Supporting Information File 1) demonstrates the good stability of the emission of the sample against UV irradiation and pressure change.

Among the four mixed Tb 3+ /Eu 3+ mixed lanthanide networks, [Tb 0.97 Eu 0.03 (L)(ox)(H 2 O)] presents at room temperature the best balance between the emissions of ligand, Tb 3+ and Eu 3+ . Based on the integrated areas of the ligand (I L ), Tb 3+ 5 D 4 → 7 F 5 (I Tb ) and Eu 3+ 5 D 0 → 7 F 2 (I Eu ) emissions, three distinct thermometric parameters may be defined, Δ 1 = I Tb /I Eu , Δ 2 = I Tb /I L and Δ 3 = I Eu /I L , allowing for the conversion of the emission intensities into absolute temperature values. The temperature dependence of the [Tb 0.97 Eu 0.03 (L)(ox)(H 2 O)] emission in the range of 250-340 K is presented in Figure 6a. Four consecutive emission spectra were collected for each temperature and used to determine the average thermometric parameter, with the errors calculated from the corresponding standard deviation (95% confidence). I L , I Tb and I Eu were determined by integrating the emission spectra in the ranges of 392-478 nm, 536-556 and 606-630 nm, respectively. Figure 6b depicts the temperature dependence of the three integrated emissions. The emission of the ligand decreases by 58% from 250 to 340 K, the Tb 3+ and Eu 3+ emissions decrease by 31% and by 20%, respectively.

The temperature dependence of the thermometric parameters Δ 1 , Δ 2 and Δ 3 in the range of 250-340 K is shown in Figure 7a. The corresponding relative sensitivity, defined as S r = |∂Δ/∂T|/Δ [63], a figure of merit used to compare the performance of ratiometric luminescent thermometers, is plotted in Figure 7b. Δ 2 and Δ 3 exhibit very good sensitivities in the range of 250-340 K with maximum relative sensitivities, S m , of 1.14% and 1.38%•K -1 at 340 K, respectively. For Δ 1 , S m is only 0.18%•K -1 . The S r values obtained for Δ 3 , in particular, are among the highest reported for metal-organic frameworks or MOF-based luminescent thermometers operative in the physiological range. Indeed, to the best of our knowledge, only eight such visible luminescent ratiometric LnMOF thermometers have been reported [64][65][66][67][68][69][70][71], among which two outperform our material [Tb 0.97 Eu 0.03 (L)(ox)(H 2 O)]: Tb 0.995 Eu 0.005 @In(OH)(2,2′-bipyridine-5,5′-dicarboxylate) with S m = 4.47%•K -1 at 333 K [65] and Eu@UiO-(2,2′-bipyridine-5,5′-dicarboxylate) with S m = 2.19%•K -1 at 293 K [67] (value recalculated and corrected using the published calibration curve). Two other thermometers have a performance similar to ours, Eu 0.089 Tb 0.9911 [2,6-di(2′,4′-dicarboxylphenyl)pyridine] with S m = 1.39%•K -1 at 328 K [70] and [(Eu 0.231 Tb 0.769 (adipate) 0.5 (phthalate)(H 2 O) 2 ] with S m = 1.21%•K -1 at 303 K [71]. These systems are, thus, appealing for potential application as biological sensors [63,72]. The Tb 3+ -to-Eu 3+ energy transfer plays an important role in the higher sensitivity of Δ 3 (I Eu /I L ). On the one hand, the Tb 3+ lifetimes obtained for [Tb 0.97 Eu 0.03 (L)(ox)(H 2 O)] from single exponential functions (Figure S15, Supporting Information File 1) decrease from 0.98 ± 0.01 ms obtained for the Tb 3+ -only sample at 297 K to 0.62 ± 0.01 ms (250 K) and 0.57 ± 0.01 ms (340 K). On the other hand, the Eu 3+ lifetimes for the mixed compound (Figure S15, Supporting Information File 1), 0.86 ± 0.02 ms (250 K) and 0.78 ± 0.01 ms (340 K), increases relatively to the one obtained at 297 K for the Eu 3+ -only compound (0.60 ± 0.01 ms). In addition, the Eu 3+ decay curves also exhibit a rise, to 0.81 ± 0.08 ms (250 K) and 0.93±0.06 ms (340 K), most probably originating from the population of the Eu 3+ 5 D 0 emitting level trough the Tb 3+ 5 D 4 donor level.

Conclusion

Six novel coordination networks based on an imidazolium dicarboxylate 1,3-(biscarboxymethyl)imidazolium and Ln 3+ ions (Ln = Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ and Yb 3+ ) in the presence of oxalate have been obtained by solvothermal reaction and totally characterized. These coordination networks are isostructural and present a monoclinic structure (space group P2 1 /a). They exhibit magnetic and luminescent properties that are characteristic for the considered lanthanide ions (except for compounds based on Gd 3+ ions). The possibility to obtain Tb 3+ /Eu 3+ mixed lanthanide networks has been exploited for potential application in thermometry. Accordingly, four mixed lanthanide networks [Tb 1-x Eu x (L)(ox)(H 2 O)] (x = 0.01, 0.03, 0.05 and 0.10) were synthesized with different Tb 3+ /Eu 3+ ratios. Using as the thermometric parameter the ratio between the Eu 3+ , 5 D 0 → 7 F 2 transition, and the ligand emissions, [Tb 0.97 Eu 0.03 (L) 2 (ox)(H 2 O)] was found to be one of the best three luminescent ratiometric LnMOF thermometers, operative in the physiological range with a maximum sensitivity of 1.38%•K -1 at 340 K. The fact that the structure and properties of these coordination networks can be predicted by design constitutes a promising approach to new multifunctional materials, especially magnetic and luminescent, materials.

Experimental Synthesis

Glycine, paraformaldehyde, oxalic acid, Nd(NO 3 ) 3

•6H 2 O, Sm(NO 3 ) 3 •6H 2 O, Eu(NO 3 ) 3 •6H 2 O, Gd(NO 3 ) 3 •6H 2 O, Tb(NO 3 ) 3 •6H 2 O, Dy(NO 3 ) 3 •5H 2 O, Ho(NO 3 ) 3 •5H 2 O
and Yb(NO 3 ) 3 •xH 2 O were purchased from Alfa Aesar and were used as received.

[HL] was synthesized according protocols published in the literature [41,42]. Synthesis method and characterizations (elemental analysis, 1 H and 13 C NMR) can be found in a previously published paper [31].

[Ln(L)(ox)(H 2 O)] compounds with Ln = Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ and Yb 3+ were prepared by solvothermal reaction by mixing [HL] (0.5 mmol), lanthanide nitrate (0.5 mmol) and oxalic acid (0.25 mmol) in a water/ethanol solution (1.5 mL). The solution was sealed in a Teflon-line stainless steel bomb (6 mL) and heated at 393 K for 72 h. After cooling to room temperature, the bomb was opened and colorless crystals were filtered and washed with ethanol and dried at room temperature. Yields were between 41% and 59%. [Tb 1-x Eu x (L)(ox)(H 2 O)] compounds with x = 0.01, 0.03, 0.05 and 0.10 were prepared in a similar manner but terbium nitrate and europium nitrate were introduced with the adapted stoichiometry. Yields were between 32% and 34%.

Physical measurements

Elemental analyses for C, H, N were carried out at the Service de Microanalyses of the Institut de Chimie de Strasbourg. The SEM images were obtained with a JEOL 6700F (scanning electron microscope (SEM) equipped with a field-emission gun (FEG), operating at 3 kV in the SEI mode instrument. FTIR spectra were collected on a Perkin Elmer Spectrum Two UATR-FTIR spectrometer. TGA-TDA experiments were performed using a TA instrument SDT Q600 (heating rates of 5 °C•min -1 under air stream). NMR spectra in solution were recorded using a Bruker AVANCE 300 (300 MHz) spectrometer. The emission and excitation spectra were recorded on a modular double grating excitation spectrofluorometer with a TRIAX 320 emission monochromator (Fluorolog-3, Horiba Scientific) coupled to a R928 or a H9170 Hamamatsu photomultiplier, for the detection on the visible and near-infrared spectral ranges, respectively, using the front-face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorometer and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. Time-resolved measurements were carried out with the pulsed Xe-Hg lamp excitation, in front-face acquisition mode. The temperature was controlled with a helium closed-cycle cryostat with vacuum system (ca. 5 × 10 -6 mbar) and a Lakeshore 330 auto-tuning temperature controller with a resistive heater. The temperature can be adjusted from ca. 12 to 450 K with a maximum accuracy of 0.1 K. The sample temperature was fixed to a particular value using the auto-tuning temperature controller; after waiting 5 min to thermalize the sample, four consecutive steady-state emission spectra were measured for each temperature; the maximum temperature difference detected during the acquisitions was 0.1 K, the temperature accuracy of the controller. Magnetic measurements were performed using a Quantum Design SQUID-VSM magnetometer. The static susceptibility measurements were performed in the temperature range of 1.8-300 K with an applied field of 0.5 T. Samples were blocked in eicosane to avoid orientation under magnetic field. Magnetization measurements at different fields and at given temperature confirm the absence of ferromagnetic impurities. Data were corrected for the sample holder and eicosane and diamagnetism was estimated from Pascal constants. The powder XRD patterns were collected with a Bruker D8 diffractometer (Cu Kα 1 , λ = 1.540598 Å) operating at 40 kV and 40 mA equipped with a LynxEye detector. The X-ray diffraction data on single crystal were collected with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) with a Kappa Nonius CCD diffractometer at room temperature. Intensity data were corrected for Lorenzpolarization and absorption factors. The structures were solved by direct methods using SIR92 [73], and refined against F 2 by full-matrix least-squares methods using SHELXL-2014 with anisotropic displacement parameters for all non-hydrogen atoms [74,[START_REF]Program for Crystal Structure Solution[END_REF]. All calculations were performed by using the crystal structure crystallographic software package WINGX [START_REF] Farrugia | [END_REF]. The structure was drawn using Mercury or Diamond [77,[START_REF] Diamond | Crystal Impact GbR[END_REF]. Hydrogen atoms were located on a difference Fourier map and introduced into the calculations as a riding model with isotropic thermal parameters. Crystallographic data for the structures re

Figure 1 :

 1 Figure 1: Ellipsoid view of the asymmetric unit of [Gd(L)(ox)(H 2 O)] (Gd in green, C in grey, O in red, N in blue and H in white).

Figure 2 :

 2 Figure 2: Selected packing view of the crystal structure of [Gd(L)(ox)(H 2 O)] along the c-axis (Gd in green, C in grey, O in red, N in blue). H atoms have been omitted for clarity.

Figure 3 :

 3 Figure 3: Plots of χ (closed circles) and χT (open circles) as functions of T for [Ln(L) 2 (ox)(H 2 O)] with Ln = Gd 3+ , Eu 3+ , Tb 3+ , Dy 3+ , Ho 3+ and Yb 3+ . The full lines correspond to the fit of the data using the expressions discussed in the text and given in Supporting Information File 1.

Figure 4 :

 4 Figure 4: Excitation spectra of [Gd(L)(ox)(H 2 O)] (λ Em = 520 nm), [Eu(L)(ox)(H 2 O)] (λ Em = 619.6 nm) and [Tb(L)(ox)(H 2 O)] (λ Em = 542 nm) recorded at 297 K (black lines) and 12 K (red lines). The intensity is only comparable for the variation of the temperature in each compound.

  In the case of [Dy(L)(ox)(H 2 O)], [Ho(L)(ox)(H 2 O)] and [Yb(L)(ox)(H 2 O)] the χT values at 300 K are 13.82, 13.61 and 2.48 emu•K•mol -1 , in line with the theoretical values for isolated Dy 3+ ions (14.17 emu•K•mol -1 with g = 1.33) [50] Ho 3+ ions (14.07 emu•K•mol -1 with g = 1.25) [50,59], and Yb 3+ ions (theoretical value of 2.57 emu•K•mol -1 with g = 1.14) [48,50]. Upon cooling, the χT product of [Dy(L)(ox)(H 2 O)] remains nearly constant until 170 K and then decreases to 9.90 emu•K•mol -1 at 1.8 K. For the Ho analogue [Ho(L)(ox)(H 2 O)]

  The excitation spectra of [Gd(L)(ox)(H 2 O)], [Eu(L)(ox)(H 2 O)] and [Tb(L)(ox)(H 2 O)] were recorded at room temperature (ca. 297 K) and 12 K monitoring the ligand emission at 520 nm, and the strongest Eu 3+ 5 D 0 → 7 F 2 and Tb 3+ 5 D 4 → 7 F 5 transitions (Figure4). The [Gd(L)(ox)(H 2 O)] excitation spectra consist of three distinct broad UV bands, ranging from 230 to 400 nm, attributed to the S 0 →S 3,2,1 excited transitions of the organic ligand. For [Eu(L)(ox)(H 2 O)], these ligand transitions are

Figure 5 :

 5 Figure 5: Emission spectra of [Gd(L)(ox)(H 2 O)] (λ Exc = 350 nm), [Eu(L)(ox)(H 2 O)] (λ Exc = 364 nm) and [Tb(L)(ox)(H 2 O)] (λ Em = 364 nm) recorded at 297 K (black lines) and 12 K (red lines). The intensity is only comparable for the variation of the temperature in each compound. For [Eu(L)(ox)(H 2 O)], the negative peak at 465 nm is due to the Eu 3+ auto-absorption from the 7 F 0 → 5 D 2 excited transition.

  (Supporting Information File 1) with the [Tb(L)(ox)(H 2 O)] time-resolved 12 K emission spectra, the broad band has a very fast time dependence totally suppressed by a time delay of only 0.05 ms. The suppression of the lowenergy T 1 →S 0 ligand emission denotes an energy transfer from the triplet excited state to the Eu 3+ and Tb 3+ excited levels. This energy transfer is more effective for the Eu 3+ compound, which almost suppresses also the S 1 →S 0 emission. Under excitation at their corresponding maxima, 270 and 395 nm for Tb 3+ and Eu 3+ , respectively, both [Eu(L)(ox)(H 2 O)] and [Tb(L)(ox)(H 2 O)] show only the respective sharp emission lines (Figure S8; Supporting Information File 1).

Figure 6 :

 6 Figure 6: (a) Emission spectra of [Tb 0.97 Eu 0.03 (L)(ox)(H 2 O)] in the range of 250-340 K with the excitation fixed at 364 nm, and (b) corresponding temperature dependence of I L (blue), I Tb (green) and I Eu (red).

Figure 7 :

 7 Figure 7: (a) Temperature dependence of Δ 1 (black), Δ 2 (green) and Δ 3 (red) in the range of 250-340 K for [Tb 0.97 Eu 0.03 (L)(ox)(H 2 O)]. The solid lines are the calibration curves, resulting from the fit considering a linear function for Δ 1 , Δ(T) = Δ 0 + mT (r 2 = 0.998) and second-order polynomial functions, Δ(T) = Δ 0 + a 1 T + a 2 T 2 , for Δ 2 (r 2 = 0.999) and Δ 3 (r 2 = 0.999). The bars depict the errors in the thermometric parameter resulting from the propagation of the errors determined for I L , I Tb and I Eu and (b) corresponding relative thermal sensitivities in the same temperature range.

  ported have been deposited in the Cambridge Crystallographic Data Centre with CCDC reference numbers 1541843, 1541844, 1541845, 1541846, 1541847, 1541848 for [Gd(L)(ox)(H 2 O)], [Yb(L)(ox)(H 2 O)], [Dy(L)(ox)(H 2 O)], [Ho(L)(ox)(H 2 O)], [Tb(L)(ox)(H 2 O)], [Eu(L)(ox)(H 2 O)].

Table 1 :

 1 Crystallographic data for [Ln(L)(ox)(H 2 O)] compounds with Ln = Eu 3+ , Gd 3+ and Tb 3+ . a

		[Eu(L)(ox)(H 2 O)]	[Gd(L)(ox)(H 2 O)]	[Tb(L)(ox)(H 2 O)]
	chemical formula	C 9 H 9 N 2 O 9 Eu	C 9 H 9 N 2 O 9 Gd	C 9 H 9 N 2 O 9 Tb
	molar mass [g•mol -1 ]	441.14	446.43	448.10
	crystal system	monoclinic	monoclinic	monoclinic
	space group	P2 1 /a	P2 1 /a	P2 1 /a
	a [Å]	9.212(3)	9.224(4)	9.246(3)
	b [Å]	13.228(4)	13.226(4)	13.219(9)
	c [Å]	10.9893(17)	10.950(2)	10.904(3)
	α [°]	90	90	90
	β [°]	111.491(18)	111.48(2)	111.63(2)
	γ [°]	90	90	90
	Z	4	4	4
	T [K]	293(2)	293(2)	293(2)
	μ (Mo Kα) [mm -1 ]	5.044	5.366	5.716
	reflection collected	11538	6602	14582
	independent reflections	2854	2825	2840
	data/restraints/parameters	2854/3/196	2825/3/196	2840/3/196
	R1, wR2 [I > 2σ(I)]	0.0358, 0.0690	0.0268, 0.0458	0.0427, 0.0769
	R1, wR2 [all data]	0.0579, 0.0770	0.0430, 0.0501	0.0627, 0.0850
	GOOF	1.063	1.092	1.094
	largest diff. peak and hole (e•Å -3 )	1.297, -1.365	0.681, -0.689	1.868, -1.838

Table 2 :

 2 Crystallographic

	data for [Ln(L)(ox)(H 2 O)] compounds with Ln = Dy 3+ , Ho 3+ and Yb 3+ . a
		[Dy(L)(ox)(H 2 O)]	[Ho(L)(ox)(H 2 O)]	[Yb(L)(ox)(H 2 O)]
	chemical formula	C 9 H 9 N 2 O 9 Dy	C 9 H 9 N 2 O 9 Ho	C 9 H 9 N 2 O 9 Yb
	molar mass [g•mol -1 ]	451.68	454.11	462.22
	crystal system	monoclinic	monoclinic	monoclinic
	space group	P2 1 /a	P2 1 /a	P2 1 /a
	a [Å]	9.191(4)	9.228(10)	9.193(2)
	b [Å]	13.188(4)	13.185(4)	13.097(3)
	c [Å]	10.85(5)	10.862(8)	10.721(5)
	α [°]	90	90	90
	β [°]	111.63(3)	111.95(6)	112.19(3)
	γ [°]	90	90	90
	Z	4	4	4
	T [K]	293(2)	293(2)	293(2)
	μ (Mo Kα) [mm -1 ]	6.137	6.479	7.843
	reflection collected	13140	13992	7321
	independent reflections	2802	2814	2735
	data/restraints/parameters	2802/3/196	2814/3/196	2735/3/196
	R1, wR2 [I > 2σ(I)]	0.0336, 0.0573	0.0494, 0.0751	0.0516, 0.1119
	R1, wR2 [all data]	0.0517, 0.0624	0.0854, 0.0840	0.0914, 0.1319
	GOOF	1.122	1.133	1.031
	largest diff. peak and hole (e•Å -3 )	0.742, -1.382	1.164, -1.034	2.751, -2.662

  Elemental analysis confirmed the composition of each compounds. [Eu(L)(ox)(H 2 O)]: Anal. calcd for C 9 H 9 N 2 O 9 Eu (440.96 g•mol -1 ): C, 24.49; H, 2.04; N, 6.35; found: C, 24.01; H, 2.06; N, 6.00; [Gd(L)(ox)(H 2 O)]: anal. calcd for C 9 H 9 N 2 O 9 Gd (446.25 g•mol -1 ): C, 24.20; H, 2.02; N, 6.27; found: C, 23.96; H, 2.05; N, 6.24; [Tb(L)(ox)(H 2 O)]: anal. calcd for C 9 H 9 N 2 O 9 Tb (447.92 g•mol -1 ): C, 24.11; H, 2.01; N, 6.25; found: C, 23.68;

	H, 2.07; N, 6.10; [Dy(L)(ox)(H 2 O)]: anal. calcd for
	C 9 H 9 N 2 O 9 Dy (451.50 g•mol -1 ): C, 23.92; H, 1.99; N, 6.20;
	found: C, 23.47; H, 2.05; N, 6.15; [Ho(L)(ox)(H 2 O)]: anal.
	calcd for C 9 H 9 N 2 O 9 Ho (453.93 g•mol -1 ): C, 23.79; H, 1.98; N,
	6.17; found: C, 23.16; H, 2.03; N, 6.05; [Yb(L)(ox)(H 2 O)]:
	anal. calcd for C 9 H 9 N 2 O 9 Yb (462.04 g•mol -1 ): C, 23.37; H,
	1.95; N, 6.06; found: C, 23.08; H, 2.10; N 5.95.

Acknowledgements

The authors thank the Centre National de la Recherche Scientifique (CNRS), the Université de Strasbourg (Idex), the Labex NIE (ANR-11-LABX-0058_NIE within the Investissement d'Avenir program ANR-10-IDEX-0002-02), the Agence Nationale de la Recherche (ANR contract no. ANR-15-CE08-0020-01) and the icFRC (http://www.icfrc.fr) for funding. Part of the present work comes from the thesis of Pierre Farger [32] and is part of the research activity supported by the European COST action MP1202: HINT (Rational design of hybrid organic-inorganic interfaces: the next step towards advanced functional materials. http://www.cost-hint.cnrs.fr). This work was also developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (Fundação para a Ciência e a Tecnologia, FCT, Ref. UID/CTM/ 50011/2013), financed by national funds through the FCT/MEC (Ministério da Educação e Ciência) and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. D.A. (SFRH/BPD/95032/2013) thanks FCT for a postdoctoral grant. The authors are grateful to Didier Burger for technical assistance.

ORCID ® iDs

Guillaume Rogez -https://orcid.org/0000-0001-9006-7273 Duarte Ananias -https://orcid.org/0000-0003-4394-045X Emilie Delahaye -https://orcid.org/0000-0001-9114-1682

Supporting Information

Supporting Information contains a representation of the coordination polyhedron, a table of selected bonds, a comparison of the experimental powder X-ray diffraction patterns of the different compounds and the simulated pattern from single crystals X-ray data, SEM analysis, TGA/TDA analysis, a summary of the weight loss values for the different compounds, infrared spectra, luminescence measurement and magnetic expression.

Supporting Information File 1

Additional experimental data.

[https://www.beilstein-journals.org/bjnano/content/ supplementary/2190-4286-9-259-S1.pdf]

License and Terms

This is an Open Access article under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0). Please note that the reuse, redistribution and reproduction in particular requires that the authors and source are credited.

The license is subject to the Beilstein Journal of Nanotechnology terms and conditions: (https://www.beilstein-journals.org/bjnano)

The definitive version of this article is the electronic one which can be found at: doi:10.3762/bjnano.9.259


Periklis Panagopoulos

C Minero

E Boonen

P Lakuzny

Florent Neumann

Sylvie Lacombe-Lhoste

Mickael Le Béchec

C Guillard

C Hill

Development of standardized method for the determination of the degradation of nitric oxide (NO) in the air by photocatalytic materials: Inter-laboratory validation tests

Keywords: NO photocatalysis, building materials CEN Technical Specification, inter-laboratory tests

    Development of standardized method for the determination of the degradation of nitric oxide (NO) in the air by photocatalytic materials: Inter-laboratory validation tests

Introduction

Heterogeneous photocatalysis is an efficient and clean technology for the degradation of air pollutants. It represents an emerging environmental control option for the efficient removal of chemical pollutants in air. This process involves a semiconductor catalyst, regularly titanium dioxide (TiO2), which is activated with ultraviolet light of the appropriate wavelength.

Over the last years, heterogeneous photocatalysis has received great attention and has become a very promising technology for the remediation of environmental pollution owing to the fact that it can lead to the complete destruction of a great number of organic and inorganic compounds with low energy costs. In addition, particular interest has been given to the titanium dioxide (TiO 2 ) photocatalysis, which is found to exhibit unique advantages in efficiently decomposing low concentrations of inorganics and VOCs under mild conditions in comparison to other available remediation technologies, such as the thermal and catalytic incineration technologies (Maggos T.), Fu, Jacobi, Blanco). For example, while one NOx control methodology is to reduce NOx back to N 2 (Zhang), another approach is to oxidize NO to NO 2 and HNO 3 along the general direction of nitrogen fixation. Overall, the NOx photocatalytic oxidation offers the following distinctive advantages: (1) no extra reactants are required and (2) NOx is recycled or recovered as nitric acid, which constitutes a potential raw material for fertilizers.

The use of the TiO 2 photocatalyst in cementitious and other construction materials has become an interesting approach over the last decades for the removal of air pollutants in urban areas (Fujishima). Concrete pavement surfaces and external building surfaces constitute optimal media for the application of photocatalytic materials than can reduce air pollution in indoor and outdoor environments. A number of laboratory studies (Strini; Poon and Cheung,; Demeestere) as well as several pilot projects can nowadays affirm the depollution effectiveness CEST2017_00850 of such photocatalytic building materials. Furthermore, it is proven that even in real complex site conditions, the photocatalytic air purification effect of the (photocatalytic) construction materials (cementitious layer) remains effective (Guerrini and Peccati; Beeldens).

In addition to the materials' construction quest, in the literature of photocatalysis, special care has also been given to the optimal reactor configuration. Several different laboratory reactor designs are nowadays reported in the literature, including fixed bed annular reactor configurations, batch reactor, semi-batch reactor with quartz flat window, circulating fluidized bed (CFB) reactor, microchannel reactor, honey comp monolith reactor, TiO2-coated fibre-optic cable reactor, annular venture reactor etc. (Tomasic). In the majority of studies, a continuous-flow air pollutant stream is employed (dynamic systems), and the photo-efficiency of the materials is evaluated by the difference in the air pollutant concentrations, which are recorded at the steady states before and after the system's irradiation. However, further investigation on the matter is needed since the relationship between the employed photocatalytic reactor configuration, the adopted operational conditions and the photocatalysts' nature still remains quite unexplored.

It is the purpose of this study to evaluate through an interlaboratory comparison the TS (technical specification) prepared by the European Standardisation Committee for assessing the performance of photocatalytic inorganic materials contained in cement mortars and limes, ceramicbased matrices, paints and materials deposited as thin films or coatings on a variety of substrates. The photocatalytic abatement rate was calculated from the observed rate by eliminating the effects of mass transfer.

Scope and Method

Labs from 6 European countries (Torino University/Italy, Fraunhofer Institut/Germany, Belgian Road Research Center (BRRC)/Belgium, Cristal Corporate/U.K, TERA Group and IPREM (Institut des sciences analytiques et de physico-chimie pour l'environnement et les matériaux (PAU)/France, NCSR "Demokritos"/ Greece ) were participated in an inter-laboratory validation exercise where the experiments had to be done in a special design CEN reactor using LED system. The experimental procedure and conditions are described hereafter.

The principle of the current method consists in measuring the photocatalytic abatement of nitric oxide (NO) by photocatalytic building materials using a Continuous Stirred-Tank Reactor (CSTR) with flow tangential to the specimen testing materials were provided by CRISTAL Company (CC) and Torino University (TU). The residual NO and NOx concentration at the CSTR outlet was measured by a chemiluminescence analyzer (EN ISO 7996). The photocatalytic activity test was carried out using chromatographic grade air to which NO was added in such an amount as to simulate a high degree of air pollution.

Apparatus

The NO concentration was set to (0.50  0.05) ppmv. The mass flow controllers, calibrated and traceable, ensure a maximum flow consistent with that needed for a correct test execution. The gas mixture preparation system ensure a relative humidity of (40  5) % inside the CSTR reactor. The relative humidity was measured inside the reactor R by means of a hygro-thermometer (Table 1)

Irradiation was provided by Omicron FSLED device, being located horizontally, at a distance of approximately ~45mm from the photocatalytic material. The total UV light intensity measured on the samples' surface was on average 10 Wm -2 .

The reaction chamber was built with NO inert materials (polymethylmethacrylate) and is gas-tight. On top, the chamber has a flat window that is transparent to the light emitted by the illumination system and vertically incident the specimen. This flat window, transparent to the wavelengths used is made up of borosilicate glass and it is 4 mm thick to ensure sufficient strength and also removable to allow for specimen positioning. The fan inside the reactor ensures perfect mixing inside the reaction chamber. The fan is placed on a support having a hole with diameter equal to the fan opening and axial to the fan axis. The support has lateral holes that ensure full air mixing inside the chamber. The fan provides nominal flows at the nominal supply voltage of 70 m 3 h -1 . The fan flow shall be varied by varying the supply voltage by an appropriate power supply of continuous variable output voltage

The NO x concentration is measured with a chemiluminescence NOx analyzer (Environment S.A AC42M), which was connected in line to the outlet of the reactor.

Calculation of Photocatalytic Performance

The following values are measured in sequential order in order to calculate the conversion in the absence of the specimen.  Furthermore the photocatalytic rates are calculated as the difference between the abatement rates and the rates observed in the dark.

(4)

Results and Discussion

The results of the % NO photocatalytic conversion and photocatalytic rate from the 2 samples used in the inter-laboratory test (CC and TU) are presented in figure 1. The results obtained for CC samples showed a variation between 15% and 35% on the photocatalytic NO conversion while the mean value was 25%. The corresponding values for TU samples were 19% up to 31% with a mean value 25%, while the corresponding photocatalytic rate was ranged

C

  PHOTO lamp NO,

Figure 1 .

 1 PHOTO lamp NO

Table 1 .

 1 Experimental conditions

	Parameter		Symbol	Measurement unit	Nomimal value	Variation allowed
	Average irradiance	I	W m -2	10	5%
	Inlet concentration	NO 2		ppmv	<0.025	-
	Inlet NO concentration		ppmv	0.50	10%
	Temperature inside the reactor	T	o C	25	±5 o C
	Gas relative humidity
	at 20 o C inside the	RH	%		±5%
	reactor				40%
	Flow		F	dm 3 min -1	1.6	10%
	Reactor net volume	V r	dm 3	3.2	20%


Julien Nicolas

Shiyong Liu

Dahui Zhao

Frank Caruso

Elsa Reichmanis

Jillian Buriak

Best Practices for New Polymers and Nanoparticulate Systems

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Best Practices for New Polymers and Nanoparticulate Systems

How not to be soft with reporting standards

Soft materials include a broad swath of flexible or deformable molecules, polymers, gels, colloids, and composites, many of which are inspired or derived from natural systems and materials. Due to the myriad of applications, from organic electronics to biomedical applications, these materials are of great interest to the materials chemistry community. We have contributed to an ongoing series of editorials and perspectives on best practices to help outline the basic information and data needed to properly report many flavors of new materials and devices, but this will be the first for new polymers and soft nanoparticle-based materials. The goal is to be constructive, and thus eliminate frustration of both authors and reviewers, and to help clarify the standards necessary for publishing in Chemistry of Materials.

In addition to the items indicated below, we also encourage authors to include videos in their Supplementary Information, 1 and to ensure that their experimental sections contain sufficient information to reproduce the work. Authors can integrate a direct link from the main text to the video to make it easy for readers to connect.

New polymers

If you are reporting a new polymer, below is a list of prerequisites that should be included in your manuscript to ensure complete characterization and reproducibility.

 Detailed experimental section to allow the monomer and polymer synthesis, and their purifications to be repeated. The characterization and data presentation of all small molecule intermediates should follow those adopted by conventional organic chemistry practices. Videos are strongly encouraged to assist with experimental reproducibility. Final monomer conversion and, ideally, synthesis yield (obtained after purification), should also be reported.

 Size exclusion chromatography (SEC) to determine the number-average molar mass (M n ), the weight-average molar mass (M w ) and the dispersity (Ð = M w /M n ). SEC chromatograms for all polymer precursors and final products should be given in the SI.  Proton NMR spectroscopy with peak assignments. This information is also useful to determine the M n (via integration of chain-end protons, if possible). Carbon NMR spectroscopic analysis should also be provided. Whenever possible or appropriate, the report of MALDI-TOF MS data of key polymers is the most ideal.  If it is a copolymer: the final composition (molar ratio of the different monomer units) must be determined, usually by proton or carbon NMR.  If it is a block copolymer: the chain lengths of the different blocks must be determined, by proton NMR and SEC.  Direct proof of coupling and yield are required if it is a functional copolymer that is obtained by coupling/grafting or if a polymer is functionalized. Typical methods would be NMR spectroscopy and SEC and/or MALDI-TOF MS. For instance, if click chemistry is performed, direct proof is the appearance of the proton of the triazole ring.

New soft materials nanoparticulate systems

The requirements for inorganic colloids have already been described, and differ from soft materials. 2 The following characterization should be included to substantiate claims of both composition and morphology.

 Detailed formulation method (indicate the volumes of organic/aqueous phases, concentration, pH, buffer composition and strength, vortex time, ultrasound power, duty cycle and time, magnetic stirring rate, speed of the dropwise addition of one of the phases, dialysis molecular weight cutoff and duration, etc.). Videos taken with a smartphone and added to the Supplementary Information are strongly encouraged.  Average diameters (dynamic light scattering (DLS) and TEM/SEM, etc.) with statistics regarding particle size distribution. For DLS, the intensity distribution is generally given as it is considered always correct (the volume and number distributions can be given as well but this should be justified). Be sure to include original data in the SI.  Data regarding morphology/supramolecular organization. If you are using electron microscopy (TEM, SEM), please be sure to include both low and high magnification images. SAXS data is also encouraged.  For porous materials, be sure to include porosimetry and/or physisorption isotherms.  Colloidal stability (in water, PBS, cell culture medium if biological applications are envisioned). Indicate the evolution of the average diameter and of the particle size distribution with time. The colloidal stability should be acceptable for at least the duration of the biological evaluation.  Zeta potential should be given to elucidate surface charge and its evolution with time should be determined.  For surface-functionalized particles, be sure to include quantification of (macro)molecules/ligands/dyes/drugs linked to the surface of the nanoparticle, the coupling yield, direct proof of coupling, and surface functionalization.

 For composite systems (ie, co-self-assembly of different materials like two different block copolymers), please provide proof that the nanoparticles are indeed composites and not a mixture of nanoparticles each composed of the individual materials that comprise the composite. We recognize that this characterization may be difficult, but some consideration and necessary supporting data have to be provided in the main text.  When a molecule is encapsulated, a clear measure of the drug loading (%DL = [entrapped drug/nanoparticles weight] * 100) and the encapsulation efficiency (%EE = [(drug added -free "unentrapped drug")/drug added] * 100) needs to be provided, providing the measuring technique and detailed procedures in addition to standard calibration curve in the SI, if applicable.

The goal of this list of items is to help authors submit papers that are convincing with respect to the soft materials being reported. As scientists ourselves, we understand that not all materials will enable all boxes to be checked, but careful consideration of these experiments will help raise the standards of your paper, and lead to better outcomes with respect to peer review. In this context, a 'minimum information standard' for studies investigating bio-nano interactions has also been recently proposed.

  3

	Julian Nicolas*
	Associate Editor
	Shiyong Liu
	Associate Editor
	Dahui Zhao
	Associate Editor
	Frank Caruso
	Executive Editor
	Elsa Reichmanis
	Executive Editor
	Jillian M. Buriak
	Editor-in-Chief

Björnmalm, M.; Faria, M.; Caruso, Advancing Research Using Action Cameras Chem. Mater.

2016, 28, 8441-8442. 2 Murphy, C. J.; Buriak, J. M. Best Practices for the Reporting of Colloidal Inorganic Nanomaterials. Chem. Mater. 2015, 27, 4911-4913.3 Faria, M.; Björnmalm, M.; Thurecht, K. J.; Kent, S. J.; Parton, R. G.; Kavallaris, M.; Johnston, A. P. R.; Gooding, J. J.; Corrie, S. R.; Boyd, B. J.; Thordarson, P.; Whittaker, A.