Design of Hybrid Electrospun Nanofibers Comprised of a Xerogel

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Design of Hybrid Electrospun Nanofibers Comprised of a Xerogel Functionalized With a Fluorescent Dye for Application as Optical Detection Device Juliana Priscila Dreyer, Rafaela I Stock, Vanderlei Gageiro Machado, Hérica Aparecida Magosso Volpato, Ismael Casagrande Bellettini, and Edson Minatti J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01624 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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The Journal of Physical Chemistry

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Design of Hybrid Electrospun Nanofibers Comprised of a Xerogel Functionalized With a Fluorescent Dye for Application as Optical Detection Device Juliana Priscila Dreyer,*a Rafaela Iora Stock,a Vanderlei Gageiro Machado,a Hérica Aparecida Magosso Volpato,b Ismael Casagrande Bellettini c and Edson Minatti a

a

Departamento de Química, Universidade Federal de Santa Catarina, CP 476, Florianópolis, Santa Catarina, Brazil. b

Departamento de Ciências Sociais e Naturais, Universidade Federal de Santa Catarina, Curitibanos, Santa Catarina, Brazil. c

Departamento de Ciências Exatas e Educação, Universidade Federal de Santa Catarina, Blumenau, Santa Catarina, Brazil. *Corresponding author: Departamento de Química, Universidade Federal de Santa Catarina, CP 476, Florianópolis, Santa Catarina, Brazil. Telephone number: +55 48 3721 9854. E-mail: [email protected]. ORCID: 0000-0002-8571-9609

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2 Abstract: The electrospinning technique allows the production of micro- and nanofibers, which can be used to obtain membranes with high surface area and high porosity. These properties are of importance with regard to the use of nanomaterials in the design of optical detection devices. In this paper, electrospun blends comprised of poly(ethylene oxide) [PEO] and sodium alginate [SA], with and without the adsorption of a fluorescent dye (4-[4-(dimethylamino)styryl]-1methypyridinium iodide, [DSMI]), were prepared and characterized. PEO/SA/DSMI nanofibers presented higher fluorescence emission intensity and higher absolute quantum yield compared to DSMI in solution. However, DSMI was leached into the solution during the nanofiber crosslinking process. Thus, in order to avoid this leaching, a xerogel [XSB30] was modified with 4-[4-(dimethylamino)styryl]pyridine [DMASP] to generate covalently-anchored dye units [XSB30-DMASP]. The resulting novel material was then electrospun with PEO/SA. Crosslinking of the electrospun hybrid PEO/SA/XSB30-DMASP nanofibers produced a material exhibiting an increase in both fluorescence emission and absolute quantum yield. Cellulose acetate [CA] was used for comparison because of its solubility in acetone, a less polar solvent which leads to a better distribution of the xerogel. These electrospun systems associated with fluorescent dyes have the potential to be applied in the design of logical gates and chemical sensors.


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Introduction Electrospinning is a technique in which an electric field is applied over a polymer solution or melt polymer, aimed at the production of micro- and nanofibers. This technique has brought great versatility to the design of new polymer-based materials, allowing the synthesis of hybrid organic-inorganic polymeric systems. The membranes obtained from electrospun fibers exhibit high surface area and high porosity, with the potential for application in research on novel materials in the areas of implants and cell growth.1 Other uses include, for instance, the production of filters, drug delivery systems, printed chips, garments and detection devices. 1,2 Several polymers have been assayed for the production of electrospun materials. Poly(ethylene oxide) [PEO] is a non-toxic, water-soluble synthetic polymer, widely used in electrospinning because of its linearity and large amount of available molar weights. 3 However, the direct use of its electrospun nanofibers is limited due to its high solubility in aqueous solutions. Sodium alginate [SA] is a polyelectrolyte which cannot be electrospun due to the fact that intermolecular interactions cause jet instability during the electrospinning process.4 The electrospinning of PEO mixed with SA, followed by crosslinking in the presence of simple divalent cations like Ca 2+, allows the development of water insoluble PEO/SA blend nanofibers.5,6 Another attractive polymer is cellulose acetate [CA], which is obtained from the reaction of cellulose, the most abundant natural polymer, with acetic anhydride and acetic acid in the presence of sulfuric acid. This biodegradable and thermoplastic polymer exhibits some flexibility due to the interaction of hydroxyl groups in its macromolecular structure, which gives it the property of being electrospinnable. 7 The solubility of CA is dependent on its degree of substitution. It is soluble in acetone, dioxane and methyl acetate and insoluble in water, and so CA fibers can be applied directly in aqueous solutions. With regard to the production of chemical detection devices, electrospun nanofibers can be functionalized with detection units that allow the recognition of a particular analyte in combination with a chemical and physical response, identified as a signal. The response corresponding to the presence of an analyte in optical chemical detection devices is provided by a color and/or fluorescence emission change. 8–13 Several fluorescent dyes have been used in combination with polymer systems for the design of optical detection devices. 9,14,15 However, no reports on the use of 4-[4(dimethylamino)styryl]-1-methylpyridinium iodide [DSMI] for the functionalization of polymers could be found in the literature. This fluorescent dye has been used in many applications, for instance, in the analysis of protein aggregation, 16–19 in the identification of the cell nucleus through interaction with DNA.20 Its use as a perichromic probe in the investigation of pure solvents,21,22 solvent mixtures,23–25 and -cyclodextrin in aqueous medium26,27 has also been reported. Compared with organic materials, inorganic nanomaterials have thermal deformation resistance and less flexibility. Thus, hybrid materials offer an alternative approach to obtaining materials with better mechanical and thermal properties. 28,29 Recently, hydrophobic-hydrophilic hybrid nanomaterials, in the form of electrospun nanofibers, have been obtained for fuel filtration, allowing the separation of water from diesel.29 In this study, flexible hybrid materials in the form of electrospun nanofibers, containing physically-adsorbed or covalently-linked DSMI, were fabricated and compared. The nanofibers obtained were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), fluorescence microscopy, dispersive energy spectroscopy (EDS) and contact angle measurements. The fluorescence emissions and the quantum yields of the systems prepared were much higher compared to the values obtained with DSMI in solution. The distribution of the dye in the fibers was greater when the dye was physically-adsorbed rather than covalently bonded to the xerogel, and both systems presented high quantum yield. However, the results showed that the physically adsorbed DSMI was leached when the membranes were used in solution, due to the high surface area of the electrospun fibers and the high porosity of the polymers used. In addition, DSMI is a charged molecule with relative solubility in water, thus the fibers exhibited



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4 greater solubility and larger diameters compared with the values obtained when the probe was covalently bonded to the xerogel. The chemical structures can be observed in Chart 1. + Na O

+ Na O H3C

OH OH

O O

HO O

O

O

HO

O OH OH O

-

O

SA

-

+ Na CH3

O O

O

O

+ Na

O H3C

CH3

O HO

HO

CH3

n-1

N

N

PEO

+

I

-

CH3 O HO H3C

O O

HO

O

HO

O

CA

CH3

O

O O

O CH3

H3C O

N H3C

CH3

DMASP

N CH3

H3C

DSMI

Chart 1: Chemical structures of the polymers sodium alginate [SA], poly (ethylene oxide) [PEO] and cellulose acetate [CA], and for 4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide [DSMI] and its precursor 4-[4-(dimethylamino)styryl]pyridine [DMASP].

Experimental Materials. The poly(ethylene oxide) ([PEO], Sigma-Aldrich, Mw = 1.0 × 10 6 g mol-1), sodium alginate ([SA], Sigma-Aldrich, Mw = 1.2–1.9 × 105 g mol-1, M/G ratio 1.56), and cellulose acetate ([CA], Sigma-Aldrich, Mw = 3.0 × 10 4 g mol-1) were used without prior purification. Distilled water was used to make the aqueous solutions. All other solvents, acetone, ethanol, acetonitrile and N,N-dimethylformamide (DMF), were of analytical grade and used without previous purification. The 4-methylpyridine, 4(dimethylamino)benzaldehyde, iodomethane and piperidine were purchased from SigmaAldrich. The NMR spectra were recorded on 200–MHz Bruker AC–200F and 400–MHz Bruker Avance 400 spectrometers. IR-Attenuated total reflectance (IR/ATR) spectra were obtained on a Perkin Elmer spectrophotometer, model Spectrum 100. The melting points were recorded through DSC analysis using a Shimadzu instrument, model DSC-50. Electrospinning was performed on an electrospinning machine, model 2.0S-500 (Yflow Sistemas y Desarollos S.L.). The characterization of the nanofibers was performed by means of the following techniques: (a) optical microscopy, using a Motic microscope, model SMZ-168, with a BestScope tablet, model BLC-250; (b) scanning electron microscopy (SEM) and dispersive energy spectroscopy (EDS), using a JOEL microscope, model JSM-6390LV; (c) transmission electron microscopy (TEM), with a JOEL microscope, model JEM-1011; (d) confocal laser scanning microscopy, using a Leica microscope, model DMI6000 B; and (e) fluorescence microscopy on a Leica microscope, model DM5500 B. The micrographs were treated using the open platform software for scientific image analysis ImageJ, with the DiameterJ plugin. The fluorescence emission spectra were collected on a Hitachi F4500 spectrofluorimeter, equipped with thermostatted cell compartments at ±0.1 °C, using 1 cm square quartz cuvettes. The spectra were collected for aerated solutions at 25.0 °C, with excitation and emission slit width settings of 5.0 nm. The absolute quantum yields were measured with an integrating sphere model A10094 for Hamamatsu Photonics.


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5 Synthesis of the dye and its precursor. 4-[4-(Dimethylamino)styryl]-1methylpyridinium iodide [DSMI] was synthesized according to the methodology of AlAnsari.21 The product did not need to be recrystallized and was obtained with 46% yield. mp 251.3–255.7 oC (lit.23 251–255 oC). 1H NMR (400 MHz, DMSO-d6) δ (ppm): 3.02 (s, 6H), 4.17 (s, 3H), 6.80 (d, J = 9.0 Hz, 2H), 7.19 (d, J = 16.1 Hz, 1H), 7.60 (d, J = 8.9 Hz, 2H), 7.93 (d, J = 16.1 Hz, 1H), 8.05 (d, J = 6.9 Hz, 2H), 8.69 (d, J = 6.9 Hz, 2H). 4-[4-(Dimethylamino)styryl]pyridine [DMASP] was synthesized according to the methodology described by Lacroix et al.30 The product was purified by recrystallization from ethyl acetate: n-hexane (1:1; v/v) and a yellow solid was obtained, with 32% yield. mp 245-247 oC (lit.30 245-247 oC). 1H NMR (200 MHz, CDCl3) δ (ppm): 3.01 (s, 6H), 6.69 (d, J = 8.5 Hz, 2H), 6.75 (d, J = 16.2 Hz, 1H), 7,20 (d, J = 16.1 Hz, 1H), 7.32 (d, J = 5.4 Hz, 2H), 7.41 (d, J = 8.5 Hz, 2H), 8.49 (d, J = 4.9 Hz, 2H). Formation of xerogel [XSB30]. The xerogel was prepared using an adaptation of previously described methodologies. 31–33 Tetraethylorthosilicate (TEOS; 9.38 mL, 0.042 mol) and ethanol (15 mL, 0.26 mol) were stirred for 15 min in an amber bottle at room temperature. In the next step, NH 4OH (5.30 mL, 0.133 mol) was added, the mixture was left under stirring for 3 h at room temperature and (3-chloropropyl)trimethoxysilane (CPTS; 3.28 mL, 0.018 mol) was then added and allowed to react for 2 h under stirring. After this time the reaction mixture was slowly added to a flask containing 15 mL of an ethanolic solution of cetyltrimethylammonium bromide (CTABr; 0.4 mol L -1) at 50 °C and the solution was homogenized for 15 min. The reaction was then left to stand at 50-55 oC for 65 h, after which time the reaction mixture was poured into a beaker and heated to allow the evaporation of the ethanol. The xerogel was macerated and refluxed with ethanol and concentrated HCl at 70-75 oC for 50 h to remove the surfactant. The solid was washed with ethanol and dried in an oven (90 oC) for 12 h. In the last step, the dry xerogel was passed through a sieve (80 mesh), yielding a white solid. Modification of the xerogel with 4-[4-(dimethylamino)styryl]pyridine [XSB30DMASP]. DMASP (0.0140 g; 0.06 mmol) was dissolved in dry ethanol (15 mL) under magnetic stirring for 2 h at 70 °C. The xerogel (1.0610 g) was then added. The reaction mixture was allowed to reflux for 3 days then filtered using a Büchner funnel and washed with ethanol and dried in an oven (100 oC) for 12 h. A light pink solid was obtained with orange fluorescence. Preparation of the solutions. An amount of mass of sodium alginate [SA] equivalent to 3% (wt/wt) was dissolved in water at 90 °C for 2 h. The poly(ethylene oxide) [PEO] solution was dissolved in 8% (wt/wt) concentration in water for 24 h. The final electrospinning solution, that is, PEO/SA in a 1:1 mixture (wt/wt), was then prepared. For the cellulose acetate [CA], a mass equivalent to 15% (wt/wt) was dissolved in an acetone/water mixture (4:1; wt/wt) for 24 h. The solutions containing the dye and the xerogel were prepared using the polymer solutions, according to the following procedure. For the systems containing the physicallyadsorbed DSMI, a stock solution of the dye was prepared with dried ethanol to give a concentration of 2.6 mmol L-1. Subsequently, 1 mL aliquots of this solution were evaporated, and 10 g of the equivalent polymer solutions were added and the mixture was stirred for 24 h. For the solutions with xerogel, the latter (0.08 g) was added to 10 g of the polymer solutions and vortexed for 5 min, and the mixture was then stirred for 24 h. Electrospinning experiments. The sample injection rates varied from 0.05 mL h -1 to 1.0 mL h-1 and the injector voltage applied varied from 7 kV to 13 kV. The distances from the injector to the collector varied from 10 to 17 cm. The relative humidity was of 50%. The electrospun nanofibers were crosslinked according to the following procedure. First, the nanofibers were carefully wetted in absolute ethanol. After soaking, the nanofibers were placed in a 0.25 mol L -1 solution of CaCl2 in water/ethanol (1:1 wt/wt) for 1 h. Subsequently, the fibers were washed several times, first with CaCl 2 aqueous solution (0.50 mol L-1) and then with distilled water, in a Petri dish. Finally, the samples were dried



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6 in a desiccator over anhydrous CaCl 2 at room temperature. The verification of the crosslinking and washing process was carried out using the IR/ATR technique, while calcium was detected using EDS (Table S4). Results and Discussion Synthesis Scheme 1A shows the route for the two-step synthesis of the dye 4-[4(dimethylamino)styryl]-1-methypyridinium iodide [DSMI], which was performed by alkylation of picoline with methyl iodide followed by aldol condensation with 4(dimethylamino)benzaldehyde in ethanol in the presence of piperidine. The synthesis of the dye precursor 4-[4-(dimethylamino)styryl]pyridine [DMASP] was performed from the aldol condensation of picoline with 4(dimethylamino)benzaldehyde in DMF, using NaH as the base (Scheme 1B). Xerogel modified with dye precursor [XSB30-DMASP] was prepared through the nucleophilic substitution of the xerogel [XSB30] with DMASP (Scheme 1C), by refluxing the reactants in ethanol for 72 h. The color of the suspension changed during the reaction from yellow (due to DMASP) to red and the disperse white solid (xerogel) became light pink at the end of the reaction and exhibited orange fluorescence under exposure to UV light (365 nm) (Scheme 1C). The color was not modified by the washing process, indicating that the DMASP was chemically anchored to the xerogel. SEM and TEM micrographs showed that the chemical modification performed on xerogel to provide XSB30-DMASP did not cause significant changes in the morphology of the material (Figure 1).

Scheme 1: Synthetic routes for the synthesis of (A) DSMI, (B) DMASP and (C) XSB30-DMASP. The photographs show the functionalized xerogel exposed to (a) natural light and (b) UV light (365 nm).


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7

Figure 1: SEM (a and c) and TEM (b and d) micrographs of XSB30 (a and b) and XSB30-DMASP (c and d).

Electrospun nanofibers of poly(ethylene oxide) blend with sodium alginate [PEO/SA] Electrospun nanofibers of PEO/SA (Figure 2a) exhibited an average diameter of 170 nm (Figure 2b), being smaller than PEO nanofibers with lower dispersity. The pKa values for the manuronic and guluronic acid groups in SA are 3.38 and 3.65, respectively. 34 At pH 7, SA is negatively charged, due to deprotonation of the COOH groups in the polymeric chains. Thus, the SA in the mixture favors an increase in the conductivity of the system. With a higher conductivity, the electric charges align with greater homogeneity in the electrospinning jet, decreasing the dispersion compared to PEO solutions. In addition, the strong interactions of PEO with the OH groups of SA contribute to the homogeneity of the system, which decreases the mean diameter, the dispersion and the amount of beads in the electrospun nanofibers.35 Also, the applied voltage needs to be higher as a consequence of the increased surface tension caused by SA. With the addition of dye DSMI to the PEO/SA system (Figure 2c), at the optimized voltage of 10 kV, the presence of the dye caused an increase in the viscosity of the solution due to the interaction of the latter with SA. The addition of the dye also contributed to an increase in the instability of the jet, being responsible for a high repulsion in solution between the molecules, due to: (a) the positive charge of the 4-methylpyridinium group in DSMI; and (b) an increase in the polarity of the applied electrical voltage. As a result, a greater dispersion was verified and consequently the diameter of the electrospun nanofibers was larger (Table 1). It is reported in the literature 36 that in electrospun nanofibers of PEO/SA, the SA is encapsulated in more internal regions. The fluorescence micrograph of the nanofibers with



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8 DSMI (Figure 2d) shows more intense emission in the inner regions of the fibers due to the electrostatic interaction between SA and DSMI.

Figure 2: (a) SEM micrograph of electrospun PEO/SA nanofibers and (b) corresponding diameter distribution. (c) SEM micrograph of electrospun PEO/SA/DSMI nanofibers and (d) fluorescence micrograph (excitation with blue laser ex = 480 nm) overlapped with transmitted light.

DSMI is widely known as a fluorescent dye. 18,23 If a solution of the dye in ethanol is excited at 480 nm, emission occurs at max = 594 nm. When the concentration of DSMI is increased, the emission reaches a maximum at c(DSMI) between 3.0 × 10 -5 and 4.0 × 10-5 mol L-1 (Figure 3a), and above this concentration suppression occurs due to effects such as self-aggregation, self-absorption and the inner filter effect, which are common in fluorescent molecules. 37 DSMI is also a solvatofluorochromic probe (Figure 3b), i.e., the position of its maximum emission changes according to the polarity of the medium. Thus, em = 603 nm was observed in dichloroethane and in isopropyl alcohol the corresponding value is em = 590 nm. In more polar solvents, the energy associated with the fluorescence emission is higher due to the better ability of the medium to stabilize the ground state in comparison with the excited state, which increases the HOMO-LUMO energy gap. In trichloromethane, the highest relative quantum yields are reported for DSMI in solution.16 In the nanofibers, however, an intense increase in the emission of DSMI is verified in comparison to the emission in dichloroethane (Figure 3c), with a hypsochromic shift in the em value from 603 nm in dichloroethane to 563 nm in the nanofibers. This may indicate that the dye molecules are located in a polar microenvironment in the nanofibers and the large increase observed in the quantum yield for the dye in the nanofibers in comparison with dichloroethane can be attributed to the greater rigidity of the fluorophore, causing less vibrational relaxation and favoring radiative relaxation. 16,18,37 Thus, it is possible that electrostatic interactions between the negatively-charged groups of sodium alginate [SA] and the positively-charged 4-methylpyridinyl moiety of DSMI promote enhanced encapsulation of the probe in the nanofibers. The coloration obtained for the PEO/SA/DSMI electrospun nanofibers (Figure S7) and the emission wavelength (Figure 3c) are comparable with those observed when the probe is in more hydrophilic solvents. In more polar solvents, solutions of the dye are yellowish or orange, whereas in non-polar solvents the dye solutions are pink, and the same trend is verified in the case of the fluorescence emission of the dye (Figures S8A and S8B).


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Figure 3: (a) Influence of the DSMI concentration on the maximum emission of the dye in ethanol ( exc = 480 nm); (b) emission spectra for DSMI in different solvents: dichloroethane (__; exc = 517 nm; em = 603 nm); isopropyl alcohol (__; exc = 483 nm; em = 590 nm); ethanol (__; exc = 480 nm; em = 594 nm); and water (__; exc = 448 nm; em = 596 nm); (c) emission spectra for DSMI in dichloroethane (__; 3.6×10-5 mol L-1 exc = 510 nm; em = 603 nm) and electrospun PEO/SA/DSMI nanofibers ( __; exc = 488 nm; em = 563 nm); (d) emission spectra of electrospun PEO/SA/DSMI nanofibers before crosslinking ( __;exc = 488 nm) and after crosslinking (__; exc = 488 nm).

Crosslinking of the electrospun nanofibers of poly(ethylene oxide) with sodium alginate [PEO/SA] Electrospun PEO/SA nanofibers dissolve instantaneously in water, which restricts their use, but SA is easily crosslinked using CaCl2. To avoid a complete change in the morphology of the nanofibers during the crosslinking process, a non-solvent method described in the literature available on electrospun nanofibers 5 was used to afford insoluble and hydrophilic nanomaterials. The crosslinking process of the PEO/SA nanofibers resulted in some coalescence points in the nanofibers and, although a large part of the morphology was retained (Figure 4a and 4b), the average diameter of the fibers decreased. This is due to the loss of PEO mass during the washing (Table 1), which was confirmed by the disappearance of the bands for this polymer on the IR/ATR spectra. The crosslinking of the nanofibers of PEO/SA with dye DSMI [PEO/SA/DSMI] was monitored using the fluorescence technique, which demonstrated that DSMI was absent from the fibers (Figure 3d), since the process involves the immersion of the nanofibers in ethanol and an aqueous solution of CaCl2, causing the leaching of the dye.



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Figure 4: SEM micrographs of electrospun nanofibers of PEO/SA after crosslinking in 3.0- k (a) and 7.0-K (b) magnification.

Electrospinning of cellulose acetate [CA] The preparation of nanofibers of CA was optimized following the same methodology used for PEO and PEO/SA. The CA used has an acetylation degree of 2.3, being soluble in acetone, a solvent suitable for use at a high relative humidity due to its high vapor pressure. Due to the low dielectric constant of acetone, which hinders the fiber formation, an acetone/water mixture (4:1; wt/wt) was used. The CA nanofibers (Figure 5a), in comparison to the PEO and PEO/SA, showed much larger diameters (Table 1) and higher dispersity. This occurs due to the dielectric constant of the solvent mixture aligning the charges in solution and contributing to the homogeneity of the electro-jet.38,39 In the electrospinning of CA, the size variation in relation to the increase in the applied voltage did not show a linear behavior, but an increase in the average diameters with the increase in the pump flow rate was verified. The electrospinning optimization resulted in an applied voltage of 8 kV, flow rate of 0.5 mL h -1, and spinning distance of 10 cm. Under these conditions the nanofibers were homogeneous and with average diameters of 419±112 nm (Figure 5b). The addition of DSMI in the electrospinning solution of CA led to a variation in the stability of the electrospinning jet. Initially, a higher minimum voltage was required to form a Taylor’s cone and, consequently, the electrospinning occurred due to an increase in the viscosity caused by the interaction of CA polymer chains with the DSMI. The electrospun CA/DSMI (Figure 5c) showed greater variations in terms of the morphology of the material, with an increase in the average diameter and dispersity (492±199 nm), due to the application of a more intense electric field with the increase in the voltage (Figure 5d). Once again, an intense increase in the emission was verified for the CA/DSMI nanofibers (Figure 6). When the nanofibers were immersed in water and ethanol, the emission decreased over time. The resulting emission measured for the nanofibers immersed in water (Figure 6a) and ethanol (Figure 6b) is apparently that of the dye still present in the nanofibers, because the maximum emission wavelength is located in the same region, with higher energy than the values for the dye in the pure solvents. The waterimmersed nanofibers showed a continuous hypsochromic shift, which may be related to the more internal location of dye molecules, that is, most were retained in the nanofibers, and even after 24 h (t∞) emission in the system was verified. In ethanol (Figure 6c), a rapid extinction of the emission was verified within 8 min (t∞), suggesting a higher solubility of DSMI in the alcohol in comparison with water. An alternative way to avoid leaching from the DSMI to the solvent is to anchor the dye in the polymer. However, with the modification in the polymer chain, various properties are changed, such as the solubility, conductivity and interaction between the polymer chains, which hinder the electrospinning. Thus, the alternative adopted was the covalent anchorage of DMASP (dye precursor) in an insoluble inorganic polymer (xerogel [XSB30]), which was mixed with the PEO/SA and CA electrospinning solution, to form hybrid nanofibers (organic and inorganic polymer nanofibers).


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Figure 5: micrographs of SEM electrospun nanofibers and diameter distribution of CA (a and b) and CA/DSMI (c and d).

Figure 6: Emission spectra for (a) CA/DSMI electrospun nanofibers (__), immersed in water at t = 0 (__), with DSMI leaching to water (colored spectra) from t 1 to t∞ = 24 h; and (b) CA/DSMI electrospun nanofibers before (__) and after immersion in ethanol in t = 0 (__), with DSMI leaching to ethanol (colored spectra) from t1 or t = 0 to t∞ = 24 h. All spectra were obtained at exc = 488 nm.

Electrospinning of modified xerogel [XSB30-DMASP] with PEO/SA Xerogel modified with DMASP [XSB30-DMASP] with PEO/SA electrospun nanofibers [XSB30-DMASP/PEO/SA] exhibited low dispersity and average diameters of around 184 nm (Table 1), with some agglomeration points, which can be clearly observed on the TEM micrographs (Figure 7). The electrospun nanofibers were made from a dispersion of XSB30-DMASP in an aqueous solution of PEO/SA, because of the XSB30 hydrophobicity, and the observed clusters were considered as XSB30-DMASP (which was later confirmed by confocal microscopy).



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12 The electrospinning was performed at higher voltages due to the lower viscosity of the system. Figure 7a shows the electrospun nanofibers, obtained at a voltage of 10 kV, which have intersections caused by incomplete drying of the solvent (water) during the process. These features decreased with an increase in the voltage to 13 kV (Figure 7e). Table 1: Characteristics of electrospun nanofibers with variations in the composition. standart deviation of diameter distribution. Composition

Injector voltage (kV)

Flow (mL h-1)

Average diameter (nm)

σ (nm)

PEO/SA PEO/SA (crosslinked) PEO/SA/DSMI XSB30-DMASP/PEO/SA

8.0 8.0 10.0 13.0

0.25 0.25 0.25 0.25

170 126 218 184

43 58 51 50

CA CA/DSMI XSB30-DMASP/CA

8.0 11.0 8.0

0.50 0.50 0.50

419 492 487

112 199 151

Electrospinning at different voltages for XSB30-DMASP/PEO/SA did not only influence the morphology as mentioned previously, but also the emission of fluorescence (Figure 8a). Although the quantum yield has been shown to be the same (Table 2), the maximum emission of non-crosslinked nanofibers formed at 10 kV is lower compared to those prepared at 13 kV. However, after crosslinking, the behavior of the nanofibers was very similar (Figure 8b). When the dye was covalently attached to the xerogel the crosslinking process did not promote the leaching of the dye, since the fluorescence intensity was not lost (compare Figure 8b with Figure 3d) After the crosslinking of the XSB30-DMASP/PEO/SA nanofibers, an aggregation of particulate material in the nanofibers was verified (Figures 9a and 9b). This material was present in an orderly and well distributed way, and despite the aggregation the nanofibers retained their morphology (Figures 9a and 9b). The main contributing factor is that the crosslinking process is carried out using the non-solvent method, with aqueous CaCl 2 added to the ethanol containing the nanofibers. As the crosslinking agent (CaCl 2) comes in contact with the nanofibers in the ethanolic medium, the hydrophobic material slowly reorganizes and agglomerates in the nanofiber matrix. The organization of the particulate material can also be verified from the confocal microscopy images (Figures 9c and 9d). Xerogel modified with DMASP [XSB30-DMASP] becomes an extremely hydrophobic material (Figure 10a). However, when electrospun with PEO/SA the resulting material exhibits high wettability (Figure 10b). The low contact angle reported with water indicates that the surface is mainly formed by these hydrophilic polymers, which means that the xerogel is dispersed in the interior of the fibers. This result indicates the possibility of increasing the contact surface of more hydrophobic materials by incorporating hydrophilic matrices. By way of comparison, the spectra in Figure 11 show that the emission occurs at em = 563 nm for PEO/SA with DSMI electrospun nanofibers [PEO/SA/DSMI] and at em = 585 nm for XSB30-DMASP/PEO/SA. This displacement, along with the color obtained from the XSB30-DMASP/PEO/SA nanofibers (Figure S10) indicates that the probe is located in this system in a more hydrophobic microenvironment in comparison with the observation made for PEO/SA/DSMI (Figure S7). Corroborating these data, the excitation wavelengths also showed a shift to less energetic regions, from 488 nm (PEO/SA/DSMI) to 514 nm (XSB30-DMASP/PEO/SA).


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13

Figure 7: SEM micrographs and respective diameter distribution of electrospun XSB30-DMASP/PEO/SA nanofibers formed at 10 kV (a,b) and 13 kV (e and f). TEM micrographs for the same nanofibers formed at 10 kV (c and d) and 13 kV (g), and (h) confocal image for the nanofibers formed at 13 kV, point out: zoom in the confocal micrograph showing fluorescence points for xerogel in nanofiber. The confocal image was obtained with excitation at  = 488 nm.



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Figure 8: Emission spectra (exc = 514 nm) of electrospun XSB30-DMASP/PEO/SA nanofibers formed at 10 kV (__) and 13 kV (__) (a) before and (b) after crosslinking.

Figure 9: (a, b) SEM micrographs of XSB30-DMASP/PEO/SA and (c, d) confocal image after crosslinking. Confocal images were obtained with excitation at ex = 488 nm.

Figure 10: Contact angle images for (a) XSB30-DMASP and (b) crosslinked XSB30-DMASP/PEO/SA electrospun nanofibers.


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15

Figure 11: Emission spectra of PEO/SA/DSMI (__; ex = 488 nm; em = 563 nm) and DMASP/PEO/SA after crosslinking (__; ex = 514 nm; em = 585 nm).

XSB30-

Electrospinning of modified xerogel [XSB30-DMASP] with cellulose acetate [XSB30DMASP/CA] The electrospinning of XSB30-DMASP with CA [XSB30-DMASP/CA] indicated a more homogeneous distribution of XSB30-DMASP (Figures 12a and 12b) than non-crosslinked electrospun nanofibers of PEO/SA with modified xerogel [XSB30-DMASP/PEO/SA], which was

evidenced by TEM and confocal microscopy analysis. TEM micrographs showed dark contrast over the entire length of the nanofibers (Figure 12c), suggesting the presence of homogeneously dispersed xerogel and dark contrast in spot regions indicating spot concentrations of particulate material (Figure 12d). This better distribution of the XSB30DMASP material was also verified in the confocal micrographs (Figure 13) and occurs because the CA solution was prepared in a mixture with acetone, a less polar solvent, which interacts with the XSB30-DMASP during the electrospinning process. The quantum yield of the nanofibers remained the same in different measurements carried out with different samples of the same solution used for the electrospinning. For example, when a higher amount of nanofibers is collected, due to the increase in the membrane layer, the fluorescence emission is higher; however the absorbance is also expected to be proportionally higher, which was verified through the measurement of the absolute quantum yield. In addition, the quantum yield in the nanofibers was significantly higher than that of the dye in solution (Table 2). As a comparison, the absolute quantum yield of DSMI in water (Φ = 8.0 × 10-3) is 76.9 times lower than that in the electrospun nanofibers of PEO/SA with DSMI [PEO/SA/DSMI] (Φ = 0.615) and 68.9 times lower than that in electrospun nanofibers of XSB30-DMASP/PEO/SA after crosslinking (Φ = 0.551). The data show that when the fluorescent species is electrospun with polymers, the emission increases, due to the encapsulation of the fluorophore in the fibers, which makes the system more rigid. In addition, there is no energy loss due to the interaction with the solvent and self-aggregation of the dye molecules. For the CA system, the quantum yield of the DSMI in the fibers is approximately three times lower compared with the PEO/SA system, indicating once again that the probe is located in more internal regions in the latter system, due to the electrostatic interaction of DSMI with SA.



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16

Figure 12: (a) SEM micrograph of electrospun XSB30-DMASP/CA nanofibers and (b) respective diameter distribution. (c, d) TEM micrographs of electrospun XSB30DMASP-CA nanofibers.

Figure 13: Confocal micrographs of XSB30-DMASP/CA electrospun nanofibers. Confocal images were obtained with excitation at ex = 488 nm. (a) corresponds to fluorescence confocal image and (b) corresponds to fluorescence overlapped with transmitted light. Table 2: Absolute quantum yields and standard deviations of DSMI in solution and in the electrospun nanofibers. Experimental conditions DSMI in water

Absolute Φ (%)

Standard deviation

0.80

0.55

DSMI in ethanol

2.30

0.27

XSB30-DMASP PEO/SA/DSMI CA/DSMI XSB30-DMASP/PEO/SA before crosslinking

31.50 61.50 21.90 59.40

0.15 0.67 1.05 0.23

XSB30-DMASP/PEO/SA after crosslinking

55.10

0.13

XSB30-DMASP/CA

44.70

1.30


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17 Conclusions The electrospinning of polymers with fluorescent probes proved to be useful for increasing the fluorescence emission and quantum yield. The use of XSB30-DMASP, which is a more rigid fluorescent system, allowed an increase in the hydrophilicity, emission and quantum yield of the material, without great difficulty being involved in the electrospinning and leaching of the probe. The mixture with solid XSB30 provides a model for other suspensions for electrospinning using a single needle, aimed at observing the distribution of the suspended material during electrospinning. It was noted that the suspension distribution can be improved by modifying the solvent used in the electrospinning process. Electrospun nanofibers and hybrid systems associated with fluorescent dyes can broaden the potential applications in chemical sensors and logical gates. In addition, the high porosity of electrospun membranes will be useful in future studies on the detection and quantification of exosomes, which are chemical messengers of cells that can be used in the detection of various diseases. Associated content Supporting Information The Supporting Information is available free of charge at the ACS Publications website at DOI: 10.1021/ 1 H NMR and IR/ATR spectra; UV-vis results; excitation and emission spectra; images of DSMI solutions in different solvents; and images of the electrospun nanofibers with the dye and xerogel (PDF). Acknowledgements The financial support of the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES – Finance code 001), Fundação de Amparo à Pesquisa e Inovação do Estado de Santa Catarina (FAPESC), Laboratório Central de Microscopia Eletrônica – LCME (UFSC), Central de Análises-EQA-UFSC and UFSC is gratefully acknowledged. Leandro G. Nandi and Juliana Eccher are acknowledged for the help in some experiments. Notes and references (1)

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