Luminescent oxygen probes based on TbIII complexes chemically bonded to polydimethylsiloxane

Sensors & Actuators: B. Chemical 287 (2019) 557–568

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Luminescent oxygen probes based on TbIII complexes chemically bonded to polydimethylsiloxane

T

Rafael D.L. Gaspar, Sofia M.M. Ferraz, Pamela C. Padovani, Paula R. Fortes, Italo O. Mazali, ⁎ Fernando A. Sigoli, Ivo M. Raimundo Jr. Institute of Chemistry, University of Campinas, PO Box 6154, 13083-970, Campinas, Brazil

A R T I C LE I N FO

A B S T R A C T

Keywords: Oxygen sensors Luminescence PDMS Lanthanides

In this work, the functionalization degree of siloxane crosslinkers by allyldiphenylphosphine oxide (adppo) and its effect on the performance of a lanthanide-based optical oxygen sensor were evaluated. Linear poly(dimethylco-hydromethylsiloxane) (PMS) and cyclic tetramethylcyclotetrasiloxane (D4i) crosslinkers were functionalized with different concentrations of adppo to chemically bond the [Tb(dcba)3]·1/2H2O (dcba = dichlorobenzoate) complex. Mechanically stable luminescent silicone membranes were obtained after the curing of the poly(dimethylsiloxane)-vinyl terminated siloxane backbone with the pre-functionalized crosslinkers. Sensing materials exhibit green emission upon 350 nm excitation, and their luminescent properties are dependent of functionalization degree of the crosslinkers. The luminescent sensor materials show fully reversible response and nonlinear Stern-Volmer plots, which are fitted by two quenching parameter model, with highest KSV of 0.08930%−1 when using functionalized D4i as crosslinker and complex concentration of 0.75%.

1. Introduction Oxygen is an important parameter to be determined in several fields, from cellular biology [1,2], industrial chemical process [3], food quality analysis [4] to environmental sciences [5,6]. Amperometric methods are often used to detect and quantify oxygen in various types of samples, despite some disadvantages as low sensitivity, interfering and single point measurements [7,8], which, for some sample applications, these drawbacks disable the use of classic methods for oxygen determination. The use of optical probes is an valuable alternative for oxygen measurement which allows its precise and accurate determination in different types of analytes, showing advantages such as fast response, high sensitivity and facility of miniaturization [8,9]. The development of luminescent oxygen sensor is based on the collisional quenching of a suitable molecular probe by oxygen, often dispersed or anchored in a polymeric matrix. The performance of the sensor is mainly defined by the oxygen permeability of the polymeric matrix and the luminescent properties of the optical probe, as example the emission lifetime [10,11]. Porphyrin-based late metal transition complexes are widely used as luminescent probes, since their visible to infrared long-lifetime emission of the metal to ligand charge transfer (MLCT) state is strongly affected by the oxygen collisional quenching [12]. To improve the low solubility of this type of complexes in

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different polymers, modification of the ligand is required [9]. Lanthanide-based complexes are another class of compound with long-lived emission which has been studied as optical probe for oxygen [13,14]. The lanthanide intraconfigurational f → f transitions yields remarkable optical properties such as narrow emission bands, large pseudo-Stokes shift and emission lifetime in the millisecond regime, providing some advantages for their application as optical probes, e.g., the use of simple time-gated instrumentation for lifetime-based sensors [15,16]. Due to low molar absorption coefficient, the emission of lanthanide ion in the complex is often enhanced by energy transfer from the ligand to the lanthanide ion, known as antenna effect [17]. This effect can result in lanthanide-based optical probes with high brightness, improving the acquisition of the optical signal [18–20]. The polymeric matrix is an important factor to be considered in order to achieve oxygen optical sensor with high sensitivity. The role of polymeric matrix is to act as a solid support for the molecular optical probe and a barrier for potential interfering species. In addition, the polymer interactions may enhance the luminescent properties of the optical probe by, for example, the decrease of concentration quenching [8,11,21]. Thereby, it is desired the use of polymers with high solubility of the optical probe, which also promotes the photostability of the sensor and prevents aggregation. Organic polymers with high oxygen permeability are often used, such as Teflon AF [22], PS [23,24], among

Corresponding author. E-mail address: [email protected] (I.M. Raimundo).

https://doi.org/10.1016/j.snb.2019.02.085 Received 21 July 2018; Received in revised form 29 January 2019; Accepted 19 February 2019 Available online 21 February 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.

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2.2. Apparatus

others, due to chemical and thermal stability, facile deposition and good UV–vis transparency. The choice of the polymer is driven by the interaction of the optical probe and the chemical groups containing in the polymeric material. Suitable optical probe-polymer interaction can decrease aggregation of the solubilized molecule, which leads to a sensitivity enhancement and homogeneous optical response through the material. The polymer functionalization is another possible approach for increasing the optical probe solubility and photostability, but the complicated functionalization of organic polymers with chemical groups that can interact or covalently bond to the optical probe is a disadvantage for its use [25]. Polydimethylsiloxane (PDMS) is an interesting polymer that can be used as a polymeric matrix for optical sensors. It has the highest oxygen permeability value among the polymers, good adhesion properties to several surfaces without prior modification, being hydrophobic and biocompatible [26]. Due to the high hydrophobic character of PDMS, several types of optical probes have poor solubility in this polymer, thus requiring modification of the polymer to disperse or covalently graft the optical probe [27,28]. Functionalization of this polymer can be achieved by several methods, and the hydrosilylation reaction is an interesting route to add molecules containing CeC double or triple bond groups to the polydimethylsiloxane chain [29,30]. This reaction is performed in a non-polar media using an homogeneous Pt catalyst resulting in some advantages, such as formation of low amount of byproducts and control by solvent addition or additives [13]. With this approach, the polydimethylsiloxane chain can graft the optical probe by the insertion of a vinyl or allyl group containing molecule, avoiding issues as aggregation and leaching of the luminescent molecule [31]. The functionalization degree is an important factor that can affects the oxygen permeability in the polymer and the dispersion of the optical probe. In the PDMS elastomers cured by hydrosilylation it is possible to control this degree by modulating the amount of the functionalization molecule and the crosslinker types, enabling the study of the effect of the polymeric matrix on the sensor performance. In this work, the functionalization of two different types of siloxane crosslinkers with allyldiphenylphosphine oxide molecule (adppo) by hydrosilylation was studied to chemicaly attach the TbIII dichlorobenzoate complex optical oxygen probe. Self-supported silicone membranes were obtained by the reaction of the modified crosslinkers containing the indicator using a vinyl-based polydimethylsiloxane as a backbone polymer. Different amounts of the adppo molecule were used in order to evaluate the crosslinker functionalization and consequently the oxygen sensitivity of the obtained optical sensors.

Carbon and hydrogen content of the TbIII complex was obtained by Perkin Elmer 2400 elemental analyzer. Infrared spectra of the solid complex and the polymeric membranes were acquired with a Agilent Cary 630 FTIR spectrophotometer in the range of 4000-400 cm−1 using the ATR mode. 1H and 31P NMR spectra were acquired employing a Bruker Avance 250 MHz for 1H and 31P qualitative measurements and a Bruker Avance 500 MHz for 1H quantitative and 31P qualitative measurements. The solvent peaks of the samples and an 85% phosphoric acid solution were used as internal reference for the acquisition of the 1 H and 31P NMR spectra, respectively. For the 1H quantitative measurements, 1,3,5-trimethoxy-benzene was used as internal standard. The high resolution ESI-QTOF-MS mass spectra of the [Tb (dcba)3(adppo)2] in toluene/ethanol solution acidified with formic acid was obtained in a Waters XEVO-QTOFMS using electrospray in positive ionization mode with 60.0 kV capillary voltage. The purge gas flow as 300 L h−1 and the desolvation gas flow was 600 L hr−1. The desolvation temperature was set to 150 °C. The mass data acquisition was recorded at 100–1500 m/z ratio interval with scan time of 1.0 s. Luminescent characterization of the membranes were obtained by a Horiba Jobin Yvon Fluorolog 3 spectrofluorometer (Horiba FL3-22iHR320), equipped with double-gratings (1200 grooves/mm, 330 nm blazed) in the excitation monochromator and double-gratings (1200 grooves/mm, 500 nm blazed) in the emission monochromator. An ozone-free xenon lamp of 450 W (Ushio) was used as a radiation source and a Hamamatsu R928 P photomultiplier tube as detector. The excitation and emission spectra of the complex and membranes were acquired within 230–500 nm and 450–700 nm ranges, respectively. The emission lifetime curves in the millisecond regime of the luminescent membranes were obtained using a 150 W xenon lamp using a TimeCorrelated Single Photon Count (TCSPC) system equipped in the Horiba FL3-22-iHR320 spectrofluorometer at ambient atmospheric conditions. For the lifetime of the membranes at the nanosecond regime a TimeCorrelated Single Photon Count (TCSPC) Edinburg Analytical Instruments model FL 900 spectrofluorometer was employed, equipped with a multichannel electronic analyzer and a Hamamatsu R3809U-50 MCP-PMT photomultiplier tube. A 335 nm pulsed LED (EPLED-335, 14.4 nm bandwith and 815.2 ps time detection limit) was used as excitation source. The emission decay curves were obtained with 10,000 counts at atmosphere conditions, and the instrument response function (IRF) was acquired using a Ludox colloidal solution. The fitting of emission curves were performed using the FAST software, providing the associated χ² error and the residual distribution. The intensity-based Stern-Volmer plots (SV plots) were obtained from the emission spectra of the membranes acquired in a lab-made fluorometer using a 150 W Xenon lamp (Oriel 66002), a Oriel 77200 single grating excitation monorchromator (1200 grooves/mm, 330 nm blazed) and a Andor Shamrock 500i single grating emission monochromator (600 grooves/ mm, 700 nm blazed) equipped with a iDus 416 CCD camera. Emission spectra of the membranes at different oxygen concentrations were obtained by mixing nitrogen and oxygen using MKS mass flow controllers with lab-made static gas mixing element. The temperature was kept constant at (25 ± 1) oC and the sensor response was measured by fiber optic system (Oriel 77565) in a lab-made gas flow cell. The integrated area of the 5D4 → 7F5 emission was monitored and the SV plot was constructed from the average of 5 measurements.

2. Experimental 2.1. Materials Reagents were used with no prior purification. Poly(dimethylsiloxane) vinyl terminated (PDMS-vinyl, Mw ˜ 17,000 g mol−1), poly (dimethyl-co-hidromethylsiloxane) trymethylsilyl terminated (PDMSPMS, Mw ˜ 3000 g mol−1), 2,4,6,8-tetramethylcyclotetrasiloxane (D4i, 99.5% Mw 240.51 g mol−1), platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane in poly(dimethylsiloxane) vinyl terminated, (Mw 381.48 g mol−1), terbium oxide (Tb4O7 99.5%), 3,5-dichlorobenzic acid (dcba, 97%), allyldiphenylphosphine oxide (adppo, 97%), sodium hydroxide (NaOH 97%) and 1,3,5-trimethoxy-benzene (TraceCERT®) were purchased from Sigma Aldrich (www.sigmaaldrich.com). HPLC grade toluene and ethanol were purchased from Merck (http://www. merckmillipore.com). Deuterated chloroform was purchased from Cambridge Isotope Laboratories (CIL, www.isotope.com). Oxygen (99.5%) and Nitrogen (99.998%) gases were purchased from White Martins (www.praxair.com.br).

3. Procedures 3.1. Synthesis of the [Tb(dcba)3]·1/2H2O complex and the [Tb (dcba)3(adppo)2] in solution The [Tb(dcba)3]·1/2H2O complex was synthesized as described elsewhere [13]. A TbIII chloride solution was obtained from dissolution of 0.125 mmol of Tb4O7 in 25.0 mL of hydrochloric acid 0.07 mol L−1 558

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4. Results and discussion

solution containing 3 drops of 30 vol. hydrogen peroxide. In parallel, 15.0 mL of an aqueous suspension containing 1.5 mmol of 3,5-diclhorobenzoic acid were deprotonated with 3.0 mL of 1.0 mol L−1 sodium hydroxide solution in the 1:1 ligand:base molar ratio under stirring at 80 °C for 30 min. The deprotonated ligand was added to the aqueous TbIII chloride solution in the 1:3 TbIII:ligand molar ratio, yielding a white solid as precipitated. The suspension was kept under stirring for 2.0 h at 80 °C, and the solid was filtered and washed with water and methanol. Elemental Analysis %C found (calc): 33.80 (33.84). %H found (calc): 1.21 (1.19). FT-IR bands: 1545 cm−1, νasym COO-; 1439 / 1383 cm−1, νsym COO-. The synthesis of the [Tb(dcba)3(adppo)2] complex in solution was achieved by the mixture of 10 mL toluene solutions containing 6.70 mg of adppo and 10.10 mg of the [Tb(dcba)3]·1/2H2O complex, respectively. The resulted solution was analyzed by high resolution ESI-MS mass spectra. ESI-MS: m/z: [M+] found (calc) for C44H36Cl4P2O6Tb ([Tb(dcba)2(adppo)2]+): 1022.8087 (1022.9974); found (calc) for C29H21Cl4O5PTb ([Tb(dcba)2(adppo)]+): 780.802 (780.910).

4.1. Evaluation of the crosslinker functionalization and the [Tb (dcba)3(adppo)2] complex in solution The coordination of adppo molecule to the [Tb(dcba)3]·1/2H2O complex was investigated by high-resolution mas spectra (Fig. S2-S4) and it was observed that during the ionization, one molecule of the 3,5dichlorobenzoate ligand is lost from the [Tb(dcba)3(adppo)2] complex. The predominant species in the solution is the bi substituted adppo TbIII complex, according to the respective high intensity signal of the [Tb (dcba)2(adppo)2]+ species at 1022 m/z region. To analyze the functionalization of the PMS and D4i crosslinkers, the 1H NMR spectra were obtained from the solution containing the Karsted catalyst, the crosslinkers and the adppo molecule without addition of vinyl terminated-PDMS. From the NMR spectra of the Figs. 2 and 3, it is possible to observe the presence of the double doublet peak at 3.2 ppm attributed to the allyl group after 4 h of reaction despite the excess of Si-H, indicating that the functionalization of the crosslinkers is not quantitative. However, a triplet and multiplets peaks present at 1.1, 1.7 and 2.3 ppm, respectively, indicate the formation of a propyl chain by Si-C bond by the reaction of allyl group from the adppo molecule with the Si-H group of the crosslinkers. The presence of the peaks attributed to the unreacted allyl group and the propyl chain suggestes that only a fraction of the adppo added to the functionalization is attached by hydrossilylation reaction to the crosslinkers. Comparing the intensity of these peaks in the 1H NMR of the D4i system in Fig. 3, it is possible to note that the propyl/allyl peak intensity ratio increases when the adppo amount added to the reaction increases, revealing that increasing the phosphine oxide concentration in the reaction results in a lower amount of adppo molecule attached to the crosslinker. The same behavior is observed to the PMS system in Fig. 2, although the increase of the propyl/allyl peak intensity ratio as a function of complex concentration is less pronounced for this linear crosslinker, which suggests the attachment of adppo molecule by hydrosilylation reaction to the D4i cyclic crosslinker is more efficient than for the PMS linear one. To proper evaluate the efficiency of the hydrosilylation reaction for D4i and PMS crosslinker, quantitative 1H NMR experiments were performed to obtain the functionalization reaction %yield in the liquid form, and those values are listed in Table 1. The 1H NMR quantitative spectra of the cocktails are showed in Fig. S5-S10. From Table 1 it is possible to observe that at the same concentration of complex the reaction %yield for the PMS crosslinker is higher than for D4i one. The hydrosilylation reaction efficiency is dependent on the solvent nature, the Pt-catalyst employed, the steric effects on the Si-H group and the alkene nature [32–34]. The attachment of an adppo molecule to an adppo-attached D4i crosslinker can be difficult due to steric hindrance caused by the phenyl groups of the already covalent attached adppo molecule in this molecule, which can explain the lower functionalization %yield observed for the D4i system in comparison with the PMS ones. For the PMS crosslinker, the functionalization reaction %yield is higher at higher complex concentration in comparison to the D4i ones, due to the distribution of the adppo attachment over the linear chain of the crosslinker, which decreases the steric hindrance effect in the hydrosilylation reaction. This difference of the functionalization reaction %yield results in a higher amount of unreacted adppo molecules in the D4i membranes. Although the adppo is not quantitatively reacted to the crosslinkers, it is still able to coordinate to the [Tb (dcba)3]·1/2H2O added to the functionalized crosslinker solution, resulting in the [Tb(dcba)3(adppo)x] isolated complex (were x = 1 or 2) soluble in the reactional media after the addition of the PDMS-vinyl backbone. Therefore, the solid membrane obtained after the crosslinking contain both [Tb(dcba)3(pdppo)x] and [Tb(dcba)3(adppo)x] (were x = 1 or 2, and pdppo = Si-propyldiphenylphosphine oxide). The 31P spectra of the functionalized crosslinkers solutions, the mixture of Karsted catalyst and the isolated adppo molecule were

3.2. Synthesis of the sensor membranes The self-supported luminescent membranes were synthesized by the crosslink between allyldiphenylphosphine oxide functionalized PDMSPMS polymer and D4i cyclic oligomer containing the [Tb(dcba)3]·1/ 2H2O complex with PDMS-vinyl backbone polymer. The lanthanide complex was added at ratios of 0.50%, 0.75% and 1.00% in relation to Si-H mol of the PDMS-PMS and D4i crosslinkers, and using allyldiphenylphosphine oxide:TbIII complex at 2:1 molar ratio. To synthesize the different sensor membranes, a specific amount of allyldiphenylphosphine oxide were added in a three-neck round bottom flask, followed by addition of the D4i or PDMS-PMS crosslinker and 2.0 mL of dried toluene. The specific quantities of the reagents used are listed in Table S1 of the supporting information. After stirring the solution for 30 min, three drops of platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane (Karsted catalyst) were added, and the solution was kept under stirring for 4 h under nitrogen atmosphere. Then, different amounts of [Tb(dcba)3]·1/2H2O complex were added to obtain the membranes with desired complex concentration (Table S1). The solution was kept stirring under nitrogen atmosphere for 2 h, yielding a colorless solution. Polymer crosslinking was achieved by the addition of 1.020 g of PDMS-vinyl terminated polymer in the solution under stirring for 30 min at room temperature. After this procedure, the reaction mixture was transferred to circular Teflon dishes (0.5 mm thickness, 45.0 mm diameter) for curing. The self-supported membranes were dried in a vacuum furnace at 40 °C for toluene elimination. For simplicity, these membranes were labeled according to their lanthanide complex content as PMS 0.50%, PMS 0.75% and PMS 1.00% for the PDMS-PMS linear crosslinker and D4i 0.50%, D4i 0.75% and D4i 1.00% for the D4i cyclic one. Fig. 1 depicts the schematic illustration of the synthesis D4i and PMS membranes. 3.3. 1H and

31

P NMR evaluation of the functionalization

The functionalization reaction of the crosslinkers was assessed by 1H and 31P NMR of the products of the PDMS-PMS and D4i hydrosilylation reaction with the allyldiphenylphosphine oxide in solution. To achieve this, the functionalization reaction was carried out using the same amounts of D4i, PDMS-PMS and adppo for the synthesis of the polymeric membranes listed in Table S1. The reagents were added in separated three-neck round bottom flasks with 1.0 mL of CDCl3, and the solution were kept under static N2 atmosphere and stirring for 30 min. After stirring, 1 drop of the platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane was added to the solution and the reaction was kept under stirring for further 4 h in static N2 atmosphere. The resulted solutions were analyzed by 1H qualitative and quantitative NMR spectra and by 31P qualitative NMR spectra. 559

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Fig. 1. Schematic illustration of the synthesis of the D4i (a) and PMS (b) membranes.

membranes are similar, suggesting that the crosslinker type does not significantly alter the energy transfer from ligands to TbIII ion. The excitation of the functionalized membranes D4i and PMS occurs by two ligand bands, the higher intensity band in the region of 230–320 nm assigned to the π → π* transition of the aromatic system of the 3,5diclhorobenzoate and allyldphenylphosphine oxide ligands, and centered 350 nm, possibly assigned to the energy levels of the allyldiphenylphosphine oxide molecule. Regarding the excitation band at 350 nm, it is possible to observe that its intensity increases when the excitation is collected at 544 nm instead of 582 nm, due to the contribution of the adppo ligand emission which overlaps the 5D4 → 7F5 transition. In addition, detailed observation of this excitation band shows an increase of intensity with the increase of the complex concentration in the samples. This can be related to the higher amount of the phosphine oxide coordinated to the TbIII ion, enhancing the energy transfer from the adppo ligand energy levels to the lanthanide ion. This corroborates the observation from the 1H NMR data obtained for the functionalized PMS and D4i crosslinkers in the reactional mixture prior to curing, once the P]O coordination of the free adppo molecule is supposed to be more favorable than the phosphine oxide group of the functionalized crosslinkers. Regarding to the emission spectra of the PMS and D4i membranes, when the emission spectra are collected using λex = 280 nm, only sharp

acquired and are exhibited in Figure S11-S18. Instead of one singlet peak observed at 29 ppm of the 31P NMR spectra of the isolated adppo molecule (Fig. S18), two singlet peaks are observed in the functionalized crosslinkers solutions, and the appearance of another upfield singlet peak is not related to the P]O coordination to the platinum atom of the Karsted catalyst, as seen in the 31P NMR spectra of the Karsted Catalyst and adppo mixture in Fig. S17. This may reveals that the covalent attachment of the phosphine oxide molecule changes the chemical environment of the phosphorous atom [35]. Addition of an alkane group related to the propyl chain formed can deshields the phosphorous atom, however, due to the single-bond nature of the P]O group this effect does not significantly modify the electron density in the oxygen atom of the P]O group. In this sense, it is possible to infer that the steric hindrance from the siloxane chain can decrease of the coordination ability of the P]O group of the crosslinked attached propyldiphenylphosphine oxide to the [Tb(dcba)3]·1/2H2O complex in comparison with the free adppo molecule [36]. 4.2. Luminescent characterization of the membranes Excitation and emission spectra were acquired to investigate the luminescence properties of the membranes, and are displayed in Fig. 4. Overall, the profiles of the excitation spectra of the D4i and PMS 560

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Fig. 2. 1H NMR spectra of the PMS crosslinker functionalized with adppo relative to the 0.50%, 0.75% and 1.00% TbIII complex concentration.

Fig. 3. 1H NMR spectra of the D4i crosslinker functionalized with adppo relative to the 0.50%, 0.75% and 1.00% TbIII complex concentration.

emission bands are observed, and are assigned to 5D4 → 7FJ transitions of intraconfigurational f → f levels of the TbIII ion, being the 5D4 → 7F5 transition at 542 nm the most intense one [37]. At this excitation, the band emission profile of the emission spectra for the D4i and PMS are similar, showing that the crosslinker type does not changes the emission of the TbIII ion, likewise the excitation spectra. However, when the emission spectra are obtained using λex = 350 nm, a broad band is observed between 380–530 nm additionally to the to 5D4 → 7FJ transitions. The same emission profile is observed in the emission spectra of D4i 0.50% crosslinked membrane without TbIII complex obtained at λex = 350 nm exhibited in Fig. S19 of SI, indicating that this emission is assigned to uncoordinated allyldiphenylphosphine oxide molecule in

Table 1 Reaction %yield of the functionalization reaction of the D4i and PMS crosslinkers using 0.50%, 0.75% and 1.00% in relation of Si-H mol concentration. Crosslinker

D4i PMS

Concentration 0.50%

0.75%

1.00%

62% 86%

33% 42%

22% 43%

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Fig. 4. Excitation (λem = 5425 and 582 nm) of the D4i (a) and PMS (d) membranes. Emission spectra under 280 nm excitation of the D4i (b) and PMS (e) membranes and 350 nm excitation of the D4i (c) and PMS (f) with TbIII complex concentration of 0.50%, 0.75% and 1.00%.

phosphine oxide molecule coordinate to the TbIII complex. The decrease of the emission band at 415 nm as function of the amount of complex added is more pronounced to the D4i membranes in comparison to the PMS ones, which is in agreement with the higher amount of adppo covalently attached to the cyclic crosslinker and possibly uncoordinated to the TbIII ion, as seen in the 1H NMR quantitative data for the D4i functionalization. To investigate the different luminescent species throughout the

the polymeric matrix. The intensity of adppo emission in the D4i and PMS membranes decreases with the increase of the complex amount to the polymer in a similar trend observed to the excitation band around 350 nm, indicating that this quenching can occur by an enhancement of the energy transfer from the adppo molecule to the TbIII complex. This behavior is in accordance with the improvement of the energy transfer from the ligand to the TbIII ion suggested by the excitation spectra obtained monitoring the 582 nm emission, due to the increase of

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Fig. 5. Emission decay curves of the 5D4 state obtained of the D4i (a) and PMS (b) membranes in the milisecond regime and the emission decay curves of the D4i (c) and PMS (d) membranes in the nanosecond regime.

crosslinkers. The contribution of the lower lifetime values in the emission decay curves obtained under 280 nm excitation increases with the increase of complex concentration in the D4i and PMS membranes, suggesting an increase of the amount of emitting species containing higher number of P]O group coordinated to the TbIII in the membranes. In fact, the increase of adppo concentration in the membrane results in higher amount of non-attached adppo molecule and a decrease of the attached one, turning more probable the coordination of two P]O groups in the same TbIII ion by the free adppo molecule. In this way, preferential excitation of the TbIII ion by the energy levels of the dichlorobenzoate ligand over the adppo one can occur in the species containing less P]O group coordinated to the TbIII, since the excitation of 280 nm is attributed to π → π* aromatic energy levels of both diclhorobenzoate and addpo ligand. The lifetime values obtained from the emission decay curves monitoring the 5D4 → 7F5 transition at 542 nm under 350 nm excitation can be attributed to the TbIII emitting species with different amount of P]O groups coordinated derived from the free or crosslinked-attached adppo molecule, once at this excitation wavelength, the energy transfer occurs preferentially from the energy levels of the adppo molecules to the TbIII ones. The increase of the complex concentration in the D4i and PMS

membranes, the emission decay curves were acquired for the D4i and PMS samples at different excitation and emission wavelengths to obtain the lifetime values assigned to the TbIII ion and the uncoordinated adppo molecule. The linearized curves are displayed in Fig. 5 and all lifetime values were obtained from a biexponential fit, revealing that two different emitting species are present in the membranes, possibly distinguished by the different amount and P]O coordination mode to the TbIII ion. The decay curves in Fig. 5a and b monitoring the 5D4 → 7 F5 transition at 542 nm under 350 nm excitation exhibits a short emission decay at small time, due to the fast uncoordinated adppo emission in the membranes. To fit those curves, the emission decay in this region were neglected to obtain the data regarding the decay profile derived from the TbIII ion. Examination of the lifetime values of the decay curves in Table 2, monitoring the 5D4 → 7F5 transition at 542 nm under excitation at 280 nm and 350 nm, shows that higher lifetime values are observed for higher excitation energy, which can be explained due to different energy transfer route from ligand to the TbIII ion. The difference in the emission lifetime values can be attributed to the decrease of rigidity of the carboxylate ligands around the TbIII ion by coordination of the P]O group from the attached and/or non-attached adppo molecule to the

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Table 2 Emission lifetime values (τ / ms), standard deviation of the lifetime values (SD), and the contribution of the lifetime values in the emission decay curves (%) of the D4i and PMS membranes with 0.50%, 0.75% and 1.00% complex concentration monitoring the 5D4 → 7F5 transition at 542 nm. λex = 280 nm

Sample

D4i

PMS

0.50 0.75 1.00 0.50 0.75 1.00

λex = 350 nm

%

τ1 / ms

SDτ1

%

τ2 / ms

SDτ2

%

τ1 / ms

SDτ1

%

τ2 / ms

SDτ2

25.3 23.2 59.6 18.9 22.8 28.3

0.792 0.974 1.021 0.920 1.168 1.195

0.273 0.282 0.709 0.231 0.179 0.113

74.7 76.8 40.4 81.1 77.2 71.7

1.756 1.809 1.957 1.869 1.823 1.791

0.042 0.053 0.377 0.032 0.036 0.026

53.8 49.6 39.1 46.5 59.0 34.5

0.504 0.465 0.505 0.495 0.466 0.528

0.008 0.042 0.043 0.027 0.039 0.065

46.2 50.4 60.9 53.5 41.0 65.5

1.589 1.628 1.593 1.689 1.636 1.595

0.005 0.009 0.006 0.008 0.011 0.012

membranes does not significantly alter the emission lifetime values, however the contribution of the lower emission lifetime decreases in the D4i membrane, possibly indicating that more P]O groups coordinates to the TbIII ion, decreasing vibronic coupling and the contribution of this emitting species in the emission decay curves. This trend is also observed from the PMS 0.50% to PMS 1.00% membrane, although the contribution of the lower lifetime value is higher for the PMS 0.75% one. The lifetime decay curves of the adppo emission in the D4i and PMS membranes centered at 415 nm were acquired and are displayed in Fig. 5c and d. The lifetime values were calculated from these curves for the D4i and PMS membranes using a biexpontencial fit function and are listed in Table 3. From the linearized curves it is possible to observe two distinct lifetime decays of the adppo emission, being attributed to the uncoordinated free or crosslinked attached adppo species at lower

Table 3 Emission lifetime values (τ / ms), variation of the lifetime values (Δτ / ns), and the contribution of each lifetime values in the emission decay curves (%) of the D4i and PMS membranes with 0.50%, 0.75% and 1.00% complex concentration, monitoring the transition at 415 nm under 335 nm excitation. Sample D4i

PMS

0.50 0.75 1.00 0.50 0.75 1.00

%

τ1 / ns

SDτ1 / ns

%

τ2 / ns

SDτ2 / ns

χ2

60 43 30 55 43 29

0.42 0.63 0.79 0.59 0.64 0.52

0.079 0.058 0.060 0.061 0.049 0.096

40 57 70 45 57 71

3.40 3.67 3.93 3.06 3.60 3.80

0.008 0.006 0.005 0.013 0.007 0.006

1.154 1.179 1.072 1.093 1.051 1.029

Fig. 6. Epifluorescence images 10x magnification of the luminescent membranes: a) D4i 0.50%, b) D4i 0.75%, c) D4i 1.00%, d) PMS 0.50%, e) PMS 0.75% and f) PMS 1.00%. 564

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Fig. 7. Stern-Volmer plots for the membranes D4i (a) and PMS (b) with 0.50%, 0.75% and 1.00% TbIII complex concentration. The error bars are smaller than the data points.

decay times and to the TbIII coordinate ones at higher decay times. The progressive increase of the contribution of higher lifetime values in the emission decay curves of the D4i and PMS membranes with the increase of complex concentration corroborates with the progressive decrease of the broad emission band centered at 415 nm observed in Fig. 4c and f, indicating an enhancement of the energy transfer from the phosphine oxide ligand to the lanthanide ion. Another evidence of this enhancement is the increase of the higher lifetime values for both D4i and PMS membranes with the increase of complex concentration, suggesting that the emission of the adppo ligand is being interfered possibly by the increase of the P]O coordination group to the TbIII ion. The epifluorescence images were obtained for the membranes with 10x magnification and are exhibited in Fig. 6. It is possible to observe that the TbIII optical probe is more homogeneously dispersed in the D4i 0.50% and PMS 0.75% membranes, resulting in lower amount of aggregates. It is noticeable a blue emission dots in the D4i and PMS

Fig. 8. Response of the membranes D4i (a) and PMS (b) with TbIII complex concentration of 0.50%, 0.75% and 1.00%, monitoring the 542, nm emission under 350 nm excitation.

membranes, which is attributed to the emission of uncoordinated adppo molecule aggregates, confirming the non-quantitative attachment of the adppo molecule to the crosslinkers and the presence of the broad band centered at 415 nm observed in the emission spectra. The presence of this blue emission derived from the adppo aggregates is decreased in the D4i 1.00% and PMS 1.00% membranes, indicating that the increase of complex concentration increases the amount of free adppo molecule coordinated to the TbIII complex. From the 50x magnification epifluorescence images showed at Fig. S20, it is possible to observe that the morphology of the aggregates in the membranes changes with the increase of the complex concentration, indicating different interactions of

Table 4 Stern-Volmer constants (KSV/ %−1), suppression sites fraction (f), response time of 90% intensity decrease (t N2 → O2 ), I0/I100 ratio, thickness (d) and the t N2 → O2 /d ratio for the PMS and D4i membranes with 0.50%, 0.75% and 1.00% TbIII complex concentration. 0 – 100 % O2

Sensor

D4i

PMS

0.50% 0.75% 1.00% 0.50% 0.75% 1.00%

1 KSV / %−1

2 KSV / %−1

f1

f2

R2

0.05912 0.05397 0.0893 0.06421 0.08837 0.06494

4.83 10−4 5.70 10−4 6.56 10−4 8.77 10−4 0.001 4.45 10−4

0.20 0.22 0.11 0.14 0.18 0.22

0.80 0.78 0.89 0.86 0.82 0.78

0.9992 0.9993 0.9975 0.9942 0.9993 0.9983

565

t N2 → O2 / s

I0/ I100

d / mm

t N2 → O2 /d

35 17 51 27 19 58

1.26 1.29 1.18 1.23 1.30 1.28

0.535 0.310 0.688 0.408 0.465 0.670

65 54 74 66 40 86

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Table 5 Stern-Volmer constants (KSV), I0/I100 ratio and response time (t N2 → O2 ) for some selected optical oxygen sensors embedded in different polymeric matrix. Optical probe

Polymer

KSV

I0/I100

t N2 → O2 / s

Ref

Eu(HPhN)3phen Eu(HPhN)3dpp Eu(HPhN)3DDXPO Eu(TTA)3ECIP MIL-100(In)⊃Tb3+ [{(MeMeArO)3tacn}Tb(THF)] nanosheets(TTA) Eu(DBM)3PIP [Tb1−0.01xEu0.01x(Hpidba)] Eu(TTA)3DIQ-Et [Tb(dcba)3(pdppo)2] D4i 0.75% PMS 0.75%

Polystyrene Polystyrene Polystyrene Polystyrene Thin film Polystyrene Polystyrene poly(vinylpyrrolidone) – Poly(vinylpyrrolidone) Silicone rubber (low Mw) Silicone rubber (high Mw) Silicone rubber (high Mw)

0.315 kPa−1 0.278 kPa−1 0.392 kPa−1 0.0228 %−1 14.05 atm−1 0.144 %−1 – 0.0257 %−1 10.3 bar−1 0.0146 %−1 0.23 %−1 0.054 %−1 0.088 %−1

– – – 3.40 15.42 14.9 ˜2.8 ˜3.5 4.17 2.5 8.9 1.29 1.20

– – – 8 4 1.9 – 15 – 12 8.5 17 19

[40] [40] [40] [41] [42] [14] [43] [44] [45] [46] [13] This work

(HPhN = tris-[9-(hydroxy-kO)-1H-phenaleno-1-onato-kO], phen = phenanthroline, dpp = 4,7-diphenyl-1,10-phenanthroline, DDXPO = 1,1′-(9,9-dimethyl-9Hxanthene-4,5-diyl)bis-1,1-diphenyl-phosphine oxide, TTA = tenoyltrifluoroacetonate, ECIP = 1-ethyl-2-(N-ethyl-carbazole-yl-4-)imidazo[4,5-f]1,10-phenanthroline, MIL = 1,3,5-benzenetricarboxylic acid, (MeMeArO)3tacn = 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, DBM = 1,3-diphenyl-propane-1,3dione, PIP = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline, Hpidba = 2-(2-Pyridyl)-1H-imidazol-4,5-di(4-benzoic acid), DIQ-Et = N-ethyl-10H-dipyrido-[f,h]indolo-[3,2-b]-quinoxaline, pdppo = propyldiphenylphosphine oxide).

the aggregated TbIII complex with the polymer. Overall, it seems that different types of crosslinkers have lower direct effect on the aggregation and the morphology of the optical probe in the polymeric membranes than the concentration of the optical probe inserted in the polymeric matrix.

becoming an issue for the oxygen quenching of the optical probe. The decrease of the I0/I100 ratio is more pronounced for the D4i membranes, due to the lowest reaction %yield of the hydrosilylation reaction for the crosslink functionalization, yielding a higher amount of free [Tb (dcba)3(adppo)x] complex in the membrane. Regarding the PMS membranes, the similarity of the Stern-Volmer curve profile for the PMS 0.75% and 1.00% can be linked to the functionalization reaction %yield of this crosslinker in these complex concentrations, which can decreases the influence of the higher aggregation of the free [Tb(dcba)3(adppo)x] complex. On the other hand, the functionalization reaction %yield of the D4i 1.00% membrane is lower in comparison with the PMS one, resulting in a higher influence of the aggregated free [Tb (dcba)3(adppo)x] complex, which decreases the sensitivity to oxygen. Therefore, the 0.75% concentration is the optimum compromise between signal response and aggregation of the optical probe inserted in the polymeric matrix. Comparing the KSV constants obtained for the D4i and PMS membranes from Table 4, one can notice that the membranes with higher I0/ 1 I100 possesses either the highest KSV constant (PMS 0.75%) or the highest f1 fraction (D4i 0.75%). The KSV constant values of the PMS membranes are higher compared to the D4i ones, which may indicates that interaction of the P]O group with the TbIII complex is more efficient in this system, which can be corroborated with the lower emission intensity of the adppo band in the emission spectra obtained for this membrane. Fig. 8 exhibits the normalized luminescence response of the D4i and PMS membranes, and the response values are listed in Table 4. The response times of the membranes were normalized accordingly the membrane thicknesses to allow the comparison between the D4i and PMS samples. It is observed that the response time trends are similar, demonstrating that complex concentration exerts higher influence in this parameter than the crosslinker type, probably due to the observed thickness increase of the membranes. The D4i 0.75% and PMS 0.75% membranes show the lowest response times, despite the thickness difference among them, suggesting that this complex concentration favors the oxygen diffusion in the membrane. The luminescence intensity of the membranes shows reversible response to the changes from N2 to O2 atmosphere, as shown in the emission response graph of Fig. S23. Comparison of the sensitivities and response times obtained from previous lanthanide-based oxygen optical sensors reported in the literature is showed in Table 5. In general, the membranes obtained in this study possess lower KSV constants and I0/I100 ratio, but to the best of our knowledge, no study reports the functionalization of the silicone polymer aiming the immobilization the lanthanide optical probe.

4.3. Performance of the membranes To evaluate the sensitivity of the sensors, emission spectra of the luminescent membranes were collected in the presence of different oxygen concentrations and the Stern-Volmer (I0/I) plots were calculated from the decrease of the integrated area of 5D4 → 7F5 transition at 542 nm, as shown in Fig. 7. It is important to point out that the oxygen quenching of the membranes only occurs when emission is collected under 350 nm excitation, attributed to the energy transfer from the energy levels of the adppo molecule to the TbIII ion. This is possibly due to the energy proximity of the triplet level assigned to this molecule which mediates the energy transfer and can interact with the oxygen molecule [13]. In addition, the excitation of the membranes by 280 nm radiation could imply in a major energy transfer rate from the singlet levels or high energy triplet levels of the ligands to the TbIII ion, which does not interact efficiently with the oxygen molecule. For all membranes, it is possible to observe a non-linear profile of the oxygen response due to the heterogeneity of the optical probe environments throughout the polymer matrix. In this case, the SternVolmer plots can be fitted using the two quenching parameters model [38,39], resulting in two sensitivity values as different Stern-Volmer constants for the different environments where the optical probe is located. In the case of these sensors, the variety of optical probe environments can be ascribed by agglomeration, differences of the polymeric structure and the phosphine oxide coordination to the TbIII complex, caused by the different crosslinkers and quantities of free and adppo attached crosslinkers among the membranes. The values of the calculated Stern-Volmer constants (KSV) and its correspondent fractions are listed in Table 4. From Fig. 7, it is possible to observe that all membranes exhibit higher KSV constants at lower oxygen concentrations. For both D4i and PMS membranes, the increase from 0.50% to 0.75% complex concentration results in membranes with highest I0/I100 value. Further increase of the TbIII complex concentration from 0.75% to 1.00% decreases the I0/I100 ratio. Due to the decrease of crosslinked-attached adppo amount in the D4i and PMS membranes with increase of complex concentration, aggregation of the non-attached [Tb(dcba)3(adppo)x] complex increases as seen in the epifluorescence images of Fig. 6, 566

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Although the membranes synthesized in this work show lower sensitivities than the previous communication report by our group, the change of the PDMS backbone polymer and the crosslinker functionalization revealed to play an important role in the sensitivity of this optical probe.

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5. Conclusion In this work, it was described the study of functionalization of the cyclic and linear siloxane crosslinkers with the allyldiphenylphosphine oxide by catalytic hydrosilylation reaction to give luminescent oxygen sensor materials based on the attached [Tb(dcba)3(adppo)x] complex as optical probe. The silicone materials obtained are mechanically stable, and their luminescent properties as well as the oxygen sensitivity show dependency with the functionalization degree of the crosslinkers and the aggregation of the TbIII complex inserted in the polymeric matrix. The sensor is fully reversible and the response and recovery times are dependent of the TbIII complex concentration for the PDMS-based materials. Acknowledgments R.D.L.G. thanks to CNPq for the fellowship support. P.R.F., F.A.S. and I.M.R.Jr. thank the Brazilian agencies CNPq SWB (Proc. # 407170/ 2013-8, # 151360/2016-2). R.D.L.G thanks Eduardo Maia Paiva for the optical instrumentation assistance, Luis Duarte for the acquisition and discussion of lifetime decay curves in the nanosecond regime and Dr. José Carlos Germino for the epifluorescence images. This work was supported by the National Institute for Advanced Analytical Science and Technology (INCTAA - FAPESP 2008/57808-1 and CNPq 573894/ 2008-6), National Institute of Science and Innovation in Complex Functional Materials (INOMAT - FAPESP 2014/50906-9) and CNPq (Proc. # 421723/2016-5) Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.02.085. References [1] D.B. Papkovsky, R.I. Dmitriev, Biological detection by optical oxygen sensing, Chem. Soc. Rev. 42 (2013) 8700, https://doi.org/10.1039/c3cs60131e. [2] D.B. Papkovsky, R.I. Dmitriev, Imaging of oxygen and hypoxia in cell and tissue samples, Cell. Mol. Life Sci. 75 (2018) 2963–2980, https://doi.org/10.1007/ s00018-018-2840-x. [3] E.I. Prest, M. Staal, M. Kühl, M.C.M. van Loosdrecht, J.S. Vrouwenvelder, Quantitative measurement and visualization of biofilm O2 consumption rates in membrane filtration systems, J. Membr. Sci. 392–393 (2012) 66–75, https://doi. org/10.1016/j.memsci.2011.12.003. [4] S. Banerjee, C. Kelly, J.P. Kerry, D.B. Papkovsky, High throughput non-destructive assessment of quality and safety of packaged food products using phosphorescent oxygen sensors, Trends Food Sci. Technol. 50 (2016) 85–102, https://doi.org/10. 1016/j.tifs.2016.01.021. [5] M. Larsen, P. Lehner, S.M. Borisov, I. Klimant, J.P. Fischer, F.J. Stewart, D.E. Canfield, R.N. Glud, In situ quantification of ultra-low O2 concentrations in oxygen minimum zones: application of novel optodes, Limnol. Oceanogr. Methods 14 (2016) 784–800, https://doi.org/10.1002/lom3.10126. [6] P. Lehner, C. Staudinger, S.M. Borisov, I. Klimant, Ultra-sensitive optical oxygen sensors for characterization of nearly anoxic systems, Nat. Commun. 5 (2014) 4460, https://doi.org/10.1038/ncomms5460. [7] O.S. Wolfbeis, Luminescent sensing and imaging of oxygen: fierce competition to the Clark electrode, BioEssays 37 (2015) 921–928, https://doi.org/10.1002/bies. 201500002. [8] X. Wang, O.S. Wolfbeis, Optical methods for sensing and imaging oxygen: materials, spectroscopies and applications, Chem. Soc. Rev. 43 (2014) 3666–3761, https:// doi.org/10.1039/C4CS00039K. [9] M. Quaranta, S.M. Borisov, I. Klimant, Indicators for optical oxygen sensors, Bioanal. Rev. 4 (2012) 115–157, https://doi.org/10.1007/s12566-012-0032-y. [10] K. Koren, L. Hutter, B. Enko, A. Pein, S.M. Borisov, I. Klimant, Tuning the dynamic range and sensitivity of optical oxygen-sensors by employing differently substituted polystyrene-derivatives, Sens. Actuators B Chem. 176 (2013) 344–350, https://doi. org/10.1016/j.snb.2012.09.057.

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Pamela C. Padovani is an undergraduated student in Chemistry at University of Campinas. She is currently performing her undergraduate research program under supervision of Dr. Ivo Raimundo working with optical oxygen sensors.

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Paula R. Fortes received her Ph.D. in Science at the University of São Paulo, Brazil in 2010. Since then, she has been working as a researcher at the University of Campinas in Brazil. She performed research at the Technical University of Berlin, Germany and the University of Porto, Portugal; and she was a post-doctorate fellow at the University of Ulm, Germany. Her research interests include optical chem/bio sensors, infrared and luminescence spectroscopy, and gas-sensing technologies applied to exhaled breath diagnostics. Italo Odone Mazali is Associate Professor in the Department of Inorganic Chemistry of Institute of Chemistry at University of Campinas (UNICAMP, Brazil). He received the Ph.D. in Sciences (2001) at UNICAMP. He is member of the Laboratory of Functional Materials Group (since 2006) and Laboratory for Advanced Optical Spectroscopy (since 2011) and is currently interested in advanced functional nanostructured materials and highly-sensitive surface-enhanced Raman spectroscopy (SERS)-based chemical sensor using metallic nanoparticles as SERS substrate. Fernando A. Sigoli is associate professor in the Department of Inorganic Chemistry of Institute of Chemistry at UNICAMP (Brazil). He received his PhD in Inorganic Chemistry (2001) from UNESP and worked in a USA-based Optical Sensors Company (California) as a researcher, manager and finally as R&D director from 2001 to 2004. Dr. Sigoli was postdoctorate fellow from December 2004 to August 2006 at UNESP. He is member of the Laboratory of Functional Materials Group since 2006 and Laboratory for Advanced Optical Spectroscopy since 2011. Currently Dr. Sigoli is interested in photophysical properties of Lanthanide ions applied on gas and temperature optical probes and theranostic materials.

Rafael D. L. Gaspar studied chemistry at UNESP and received his PhD degree in science from the University of Campinas (Campinas, Brazil) in 2014. He is currently a post-doc fellow at the same university. His main research interests are the development of luminescent probes for optical sensors, optical sensors materials, molecular and lanthanide luminescence spectroscopy.

Ivo M. Raimundo Jr is associate professor in the Department of Analytical Chemistry of Institute of Chemistry at UNICAMP (Brazil). He received his PhD in Sciences (1995) from UNICAMP and he was post-doctorate fellow (July 1998 to December 1999) at UMIST (Manchester, England). He is member of the Group for Instrumentation and Automation in Analytical Chemistry (UNICAMP) since 1990 and is currently interested in luminescent, near and mid infrared optical sensors for environmental monitoring.

Sofia M. M. Ferraz graduated in Chemistry at University of Campinas, Brazil in 2018. She performed undergraduate research at same university in optical oxygen sensor, and currently works in R&D at coatings lab in Novecare division, at Rhodia Solvay Company.

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