Cyclodextrin-Grafted TiO2 Nanoparticles: Synthesis ... - MDPI

nanomaterials Article

Cyclodextrin-Grafted TiO2 Nanoparticles: Synthesis, Complexation Capacity, and Dispersion in Polymeric Matrices Pablo Monreal-Pérez 1 , José Ramón Isasi 1, *, Javier González-Benito 2 , Dania Olmos 2 Gustavo González-Gaitano 1, * ID 1 2

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Department of Chemistry, Facultad de Ciencias, Universidad de Navarra, 31080 Pamplona, Spain; [email protected] Department of Materials Science and Engineering and Chemical Engineering, Instituto de Química y Materiales Álvaro Alonso Barba (IQMAA), Universidad Carlos III de Madrid, 28911 Leganés, Spain; [email protected] (J.G.-B.); [email protected] (D.O.) Correspondence: [email protected] (J.R.I.); [email protected] (G.G.-G.); Tel.: 34+948-425-600 (J.R.I.); 34+948-425-600 (G.G.-G.)

Received: 30 July 2018; Accepted: 19 August 2018; Published: 22 August 2018

Abstract: The modification of the surface of titanium dioxide nanoparticles (TiO2 NPs) by the incorporation of cyclodextrins (CDs), cyclic oligosaccharides with a hydrophobic cavity, can largely improve the functionality of TiO2 by lodging molecules of interest in the CD to act directly on the surface of the nanoparticles or for further release. With this aim, we have synthesized βCD-modified nanoparticles (βCDTiO2 NPs) by a two-step reaction that involves the incorporation of a spacer and then the linking of the macrocycle, and characterized them by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The capacity of the functionalized structures to trap model compounds (Rhodamine and 1-naphthol) has been compared to that of bare TiO2 NPs by fluorescence and Ultraviolet-visible (UV-visible) spectroscopy. The presence of the CDs on the surface of the TiO2 avoids the photo-degradation of the guest, which is of interest in order to combine the photocatalytic activity of TiO2 , one of its most interesting features for practical purposes, with the delivery of compounds susceptible of being photo-degraded. The βCDTiO2 NPs have been dispersed in polymeric matrices of frequently used polymers, polyethylene (LDPE) and polyethylene oxide (PEO), by cryogenic high energy ball milling to produce nanocomposites in the form of films. The surface modification of the nanoparticles favors the homogenization of the filler in the matrix, while the nanoparticles, either in bare or functionalized form, do not seem to alter the crystallization properties of the polymer at least up to a 5% (w/w) load of filler. Keywords: TiO2 nanoparticles; cyclodextrins; polymer nanocomposites; surface modification; high energy ball milling (HEBM)

1. Introduction Nanocomposites are composite materials in which at least one of the components has dimensions of less than 0.1 µm [1]. In their simplest form, they consist of a nanoscale filler, like nanoparticles [2], fibres [3], or sheets [4], dispersed homogeneously in the bulk component (matrix). Due to the high surface to volume ratio of the nanofiller, the large interface can produce significant effects on the macroscale properties of the material. Specifically, nanoparticles present an elevated surface area that contributes to dramatically change the matrix characteristics even when present in low proportion. Thus, nanocomposite materials are obtained and utilized in a vast number of applications, Nanomaterials 2018, 8, 642; doi:10.3390/nano8090642

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Nanomaterials 2018, 8, 642

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such as rheology [5], lubrication and nano-manufacturing [6], prosthesis [7], or for active delivery in pharmacy [8], amongst others. Focusing on polymeric-matrices nanocomposites, the production and properties of these materials containing TiO2 as the filler have been described in the literature [9,10]. TiO2 is a ceramic compound appearing in three crystalline structures: rutile (the most common form, tetragonal), anatase (metastable tetragonal), and brookite (orthorhombic). Amongst other many uses, TiO2 is used as a photocatalyst [11–13], capable of degrading organic pollutants [14] and bacteria [15]. In this later case, bacteria generate a self-destructing mechanism providing TiO2 with a “self-cleaning” ability [16,17] which can actually be enhanced by the light. The properties and applications of nanocomposites based on TiO2 nanoparticles can be expanded in combination with other components, for example, electrochemical properties when Niobium oxide is utilized [18], antimicrobial activity when appearing with chitosan in food packaging [19], or the ability to degrade antibiotics enlarged by mesoporous carbon [20]. Reaching the level of desired functionality most often leads to the addition of multiple fillers into the same matrix, with the implicit limitations of chemical and physical compatibility that this involves. A solution to this issues can be the synthesis of multifunctional particles by modification of the nanoparticle surface, incorporating active molecules that can play different roles. To this effect, the use of cyclodextrins (CDs) has been described [21–23]. Cyclodextrins (CDs) are cyclic oligosaccharides composed of several units of D-glucopyranose: αCD (formed by 6 units), βCD (7 units), and γCD (8 units). Their ability to form inclusion complexes with a variety of guest molecules is their most important feature, which is possible due to their hollow, truncated cone-shaped morphology, with a relatively hydrophobic inner cavity that contrasts with the hydrophilic external surface [24–26]. The number of glucose units in the oligosaccharide produces a range of cavity sizes which enables CDs to complex different compounds that fit into the macrocycle. To this regard, CDs are good candidates to complex active principles, which could be subsequently released in specific body locations for therapeutic purposes, for example [27]. In addition, CDs are easily functionalized, making them versatile and the right choice when it comes to attaching to different systems in order to modify the substrate physio-chemical properties. Within this framework, one of the objectives of this work has been to produce multifunctional TiO2 NPs, by incorporating CDs on their surface, in order to grant complexing and delivery functionalities. Thus, to the intrinsic photocatalytic activity of TiO2 , it would add up the function of the guest lodged in the cavity of the macrocycle. The second objective has been to study the feasibility of the βCDTiO2 NPs being dispersed within polymeric matrixes to produce nanocomposites in the form of films, as a proof of concept for potential applications as functional bio-nanomaterials. We have focused on low-density polyethylene (LDPE) and polyethylene oxide (PEO), both of them biocompatible. Polyethylene (PE) is one of the most widely used polymers [28], as it shows very good chemical resistance and mechanical properties [29]. PEO shares with PE some features, like flexibility and low toxicity and presents some other properties like hydrophilicity and water-solubility [30], and is frequently employed in a number of uses, from skin creams or toothpastes to technical ceramics or solid polymer electrolytes [31], and in drug delivery [32], among other diverse commercial applications [33]. The added value of the materials here described lies in the surface modification of titania nanoparticles with CDs which could extend the potential applications of the nanocomposites for the controlled delivery of a molecule of interest, previously lodged in the cavity of the macrocycles. 2. Materials and Methods 2.1. Materials Titanium(IV) oxide nanoparticles (TiO2 NPs, ρ = 4.26 g·cm−3 , 99.5% purity, and 21 nm size) were supplied by Sigma-Aldrich (batch 718467, St. Louis, MO, USA). N,N-dimethylformamide (DMF, 99.8% purity), polyethylene (ρ = 0.925 g·cm−3 ; low density, melt index 25 g/10 min),


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and poly(ethylene oxide) (PEO, ρ = 1.13 g·cm−3 ; Mw = 100,000 g·mol−1 ) were also from Sigma-Aldrich. Hexamethylene diisocyanate (HMDI) (ρ = 1.47 g·cm−3 , 98% purity) was supplied by Fluka (Morris Planes, NJ, USA). β-Cyclodextrin (βCD) was supplied by Wacker as Cavamax W7 (97% purity, Münich, Germany). Rhodamine B (RhB), 99% purity, was supplied by Acros Organics (Daltham, MA, USA), and 1-naphthol from Merck (99% purity, Darmstadt, Germany). Methanol (99.85% purity) was supplied by Oppac (Noain, Spain), and acetone (99.7% purity) by Quimipur (Camporeal, Spain). All the reactants were used as received. 2.2. Sample Preparation 2.2.1. Nanoparticles Surface Modification The grafting of cyclodextrins to the surface of TiO2 was based on a reported method [34]. Firstly, a linear spacer with a length of 12 atoms was covalently attached to TiO2 NPs by reaction of 1.5 mL of HMDI with 2 g of TiO2 NPs in 100 mL of DMF. This demanded DMF and the nanoparticles to be previously heated up under mild conditions (75 ◦ C and 60 ◦ C, respectively, in order to remove the water without eliminating hydroxyl groups from the Nanoparticles surface). Reaction proceeded under nitrogen atmosphere for 72 h at 100 ◦ C under vigorous stirring. Then, the product was centrifuged (8000 rpm, 30 min) and washed three times with acetone. In a second stage, βCD was covalently bonded to the HMDI spacer already attached to the nanoparticles by adding the product of the first step to 100 mL of dry DMF and 3 g of dry βCD (65 ◦ C for 24 h). The reaction took place during 24 h at 100 ◦ C under vigorous stirring and nitrogen atmosphere. The product was centrifuged (8000 rpm, 30 min) and then washed three times with methanol. Finally, the solid product was let to dry. Three batches of βCD-grafted nanoparticles were produced. The first two batches were synthetized as described, and for the third batch 4.5 mL of HMDI and 9 g of βCD were used (a three-fold proportion with respect to the synthesis described). 2.2.2. Production of Polymeric Nanocomposites The nanoparticles were uniformly dispersed in polymeric matrices of either low-density polyethylene (LDPE) or polyethylene oxide (PEO) by cryogenic high-energy ball milling, using a Retsch MM400 mill (Haan, Germany). The as-received TiO2 NPs or βCD-grafted nanoparticles were introduced with the previously ground polymers (5% by weight of the nanofiller) into two 50 mL steel jars with one stainless steel ball (Ø 2.5 cm) and immersed for 5 min in liquid nitrogen prior milling. The milling conditions were 30 Hz for 1 min, repeating three times the cooling–milling cycle. The resulting fine powders were processed in the form of films by hot pressing in an aluminium mould using a Specac Mini-Film Maker (Orpington, UK), at a constant load of 0.5 ton at 150 ◦ C for 1 min. Then, the samples were let to cool down at room temperature under the same constant pressure. 2.3. Techniques The sorption capacity of βCDTiO2 NPs was studied using the intrinsic fluorescence of Rhodamine B (RhB). The emission spectra were recorded using an Edinburgh Instruments FLS920 spectrofluorometer (Livingston, UK). The excitation was set to 554 nm and the emission recorded from 560 to 700 nm at 1 nm steps and 0.1 s dwell time, with excitation and emission slits of 1 nm and 2 nm, respectively. An amount of 100 mg of each batch of βCD-modified nanoparticles were added to a 4 × 10−6 M RhB aqueous solution and stirred for 2 h. Then, the mixtures were centrifuged at 8000 rpm for 30min, and the nanoparticles isolated from the supernatant and dried. The spectra of the solids were recorded using a front-face sample holder, while for the supernatants the sample was contained in 10 mm path length quartz cuvettes and the spectra collected under constant magnetic stirring. Thermogravimetric analysis of the samples (TGA), was performed in a TGA-SDTA 851 Mettler Toledo (Columbus, OH, USA). The samples were weighed in platinum crucibles and the weight recorded in the range from 25 ◦ C to 1000 ◦ C at 10 ◦ C/min under N2 atmosphere.


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Fourier transform infrared spectroscopy characterization was carried out in attenuated total reflectance mode (ATR) utilizing an FTIR Nicolet Avatar 360 spectrometer (Waltham, MA, USA), coupled to a Specac Golden Gate ATR. Spectra were recorded with a resolution of 2 cm−1 and 32 scans per spectrum. Post processing consisted of manual base-line correction to the averaged spectra. For the sorption equilibrium experiments, 50 mg of each type of nanoparticles (native and CD modified) were placed in 10 mL vials containing 1-naphthol of different concentrations (ranging between 75 and 200 ppm). The samples were placed on a magnetic stirrer for at least 5 h, to ensure sorption equilibrium was reached. The samples were then filtered by 0.1 µm polyvinylidene difluoride (PVDF) membranes (centrifugation was required prior to filtering in the case of unmodified nanoparticles) and the supernatants measured by UV-visible spectroscopy with a diode array spectrophotometer Agilent 8454 (Santa Clara, CA, USA). Optical microscopy of the films was performed using a Zuzi polarizing microscope (Beriain, Spain). The morphologies of the nanoparticles and nanocomposites were imaged using the backscattered electron (BSE) signal in a Philips XL30 scanning electron microscope, scanning electron microscopy (SEM), (Eindhoven, Netherlands). To avoid charge accumulation, the samples were gold coated by sputtering using a low-vacuum coater Leica EM ACE200 (Wetzlar, Germany). Differential scanning calorimetry (DSC) of LDPE, PEO, and their mixtures with the as-received and βCD-modified nanoparticles were carried out using a Mettler Toledo 822E analyser (Greifensee, Switzerland). All samples, of about 1.5–3 mg, were subjected to the following thermal steps under a nitrogen atmosphere: (i) A first heating scan from 30 ◦ C to 150 ◦ C (for LDPE) or 100 ◦ C (for PEO) at 10 ◦ C/min to investigate thermal transitions of the materials; (ii) a stabilization step at 150 ◦ C (100 ◦ C for PEO) for 5 min to erase thermal history; (iii) a cooling scan from that temperature down to 30 ◦ C at 10 ◦ C to study thermal transitions of the relaxed nanocomposite system; and (iv) a second heating scan from 30 ◦ C to 150 ◦ C (100 ◦ C for PEO) to study the thermal transitions with the same thermal history. Crystallization and melting temperatures were obtained from the cooling and the second heating scan respectively. Also, the enthalpies of each thermal transition were analysed in each case. In composite materials, enthalpies were corrected and referred to the total amount of polymer dividing the raw value obtained from the DSC trace by (1 − x), where x corresponds to the fraction of particles (in a 1/1 ratio). 3. Results and Discussion 3.1. Characterization of the Nanoparticles Surface Surface modification of the TiO2 NPs has been characterized by FTIR-ATR spectroscopy and TGA. The infrared spectra corresponding to βCD, the as-received nanoparticles, and three batches of the βCD-grafted nanoparticles are shown in Figure 1. Some characteristic bands coming from all the reactants can be found in the spectra of modified nanoparticles, as well as other due to the new functional groups created in the reaction, which prove the actual linking of the macrocycles to the surface of TiO2 . For example, in the group vibration zone of the mid IR spectrum, βCD presents a broad band due to the stretching of the primary and secondary –OH groups at the rims of the macrocycle, while in this same region TiO2 shows a broad and much less intense band, which indicates a certain extent of hydration. By contrast, the spectrum of the βCDTiO2 NPs presents a narrow band centered at ca. 3300 cm−1 corresponding to the amino groups (–NH) resulting from the reaction with the cross-linker. Regarding the characteristic vibrations from diisocyanate, we can find bands of –CH groups at 3350 cm−1 and the band of urethane carbonyl groups (–C=O) at ≈1650 cm−1 . In addition, the band of N–H bending appears at ≈1550 cm−1 and the C–O vibration from the urethane group at ≈1250 cm−1 . In the case of the vibrational modes corresponding to βCD, those of the C–O–C group can be found at ≈1150 cm−1 . As a matter of fact, the region between 900 and 1200 cm−1 is of particular interest in order to confirm and evaluate the βCD modification step. Even though both batches 1 and 2 have been prepared according to the same procedure, the comparison of the intensities


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of the 1050 cm−1 band indicate that batch 2 must have a higher βCD content than batch 1 (Figure 1, Nanomaterials 2018, 8,Batch x FOR 3PEER REVIEW 5 of 14 zoomed view). shows, in turn, a considerable higher intensity for this band, as expected. Nanomaterials 2018, 8, x FOR PEER REVIEW 5 of 14 The reactions involving isocyanate groups are very sensitive to the experimental conditions (such as conditions (such the humidity) and must taken out step. In of batch 3, the the humidity) andasspecial care must be special taken tocare carry outbe this step.toIncarry batch 3, this the amount reactants conditions (such as the humidity) and special care must be taken to carry out this step. In batch 3, the amount of reactants was orderaddition to attainofthe highest addition of grafted cyclodextrin was increased in order to increased attain the in highest cyclodextrin moieties to themoieties surface amount of reactants was increased in order to attain the highest addition of cyclodextrin moieties grafted to the surfaceFinally, of the with nanoparticles. with regard to the with the TiO2 of the nanoparticles. regard to Finally, the similarities with the TiOsimilarities 2 spectrum, the tendency grafted to the the tendency surface ofofthe nanoparticles. Finally, with regard to the similarities with theinTiO −1 can spectrum, increasing absorbance starting at ≈850 cm be also detected the2 − 1 of increasing absorbance starting at ≈850 cm can be also detected in the modified nanoparticles. −1 spectrum, the tendency of increasing absorbance starting at ≈850 cm can be also detected in the modified nanoparticles. The differences in intensity between the spectra of modified The differences in intensity between the spectra of modified nanoparticles from batchesnanoparticles 1 and 2 with modified nanoparticles. The differences in intensity between the spectra of modified nanoparticles −1 from batches and 23with respect to that batch 3cm in−the interval are due to theused higher 1 are respect to that1batch in the interval 2800–3000 due to2800–3000 the highercm ratio of reactants in from batches 1 and 2 with respect tocase. that batch 3 in the interval 2800–3000 cm−1 are due to the higher ratio of reactants used in the latter the latter case. ratio of reactants used in the latter case. 0.5

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Figure 1. (Left) Full Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) Figure 1. (Left) Full Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) Figure (Left) Full Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) 2 NPs (batches 1, 2, and spectra.1.(Right) Zoomed view of the as-received TiO2 NPs, βCD, and βCDTiO spectra. (Right) Zoomed view of the as-received TiO2 NPs, βCD, and βCDTiO2 NPs (batches 1, 2, and 2 NPs, βCD, and βCDTiO2 NPs (batches 1, 2, and spectra. (Right) Zoomed view of the as-received TiO 3). 3). 3).

The functionalization of the nanoparticles has been checked by SEM. Figure 2 shows the The functionalization functionalization of the nanoparticles nanoparticles has has been checked checked by SEM. SEM. Figure Figure 22 shows shows the The micrographs correspondingof tothethe as-received and been functionalizedbynanoparticles, in which the the micrographs corresponding toto thethe as-received and and functionalized nanoparticles, in which the grafting micrographs as-received functionalized nanoparticles, which the grafting of thecorresponding CD produces somewhat larger nanoparticles than the original ones andinwith uneven, of the CD somewhat larger nanoparticles than thethan original ones and with uneven, ‘softer’ grafting ofproduces the CD produces somewhat larger nanoparticles the original ones and with uneven, ‘softer’ surfaces, due to the functionalization. surfaces, due to the ‘softer’ surfaces, duefunctionalization. to the functionalization.

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Figure 2. Scanning electron microscopy (SEM) micrographs corresponding to (a) as-received TiO2 Figure 2. (b) Scanning electron microscopy (SEM) micrographs corresponding to (a) as-received TiO2 NPs and βCDTiO 2 NPs. microscopy (SEM) micrographs corresponding to (a) as-received TiO NPs Figure 2. Scanning electron 2 NPs and (b) βCDTiO 2 NPs. and (b) βCDTiO NPs. 2

Thermogravimetric analysis was performed on samples of βCD and the as-received and Thermogravimetric analysis(Figure was performed on samples of 2βCD the as-received surface-modified nanoparticles 3). In the TGA plots, TiO NPs and show a flat profile asand its Thermogravimetric analysis was performed on samples of βCD and the as-received and surface-modified nanoparticles (Figure 3). In the TGA plots, TiO 2 NPs show a flat profile as its decomposition temperature is not reached. of plots, the βCD two weight losses can be surface-modified nanoparticles (Figure 3). InInthe thecase TGA TiOsample, 2 NPs show a flat profile as its decomposition temperature reached. In ends the case of 100 the βCD sample, two weight can be noticed. The first one starts is atnot 40–50 °C and at ca. °C, attributed to the loss losses of hydration noticed. Thesecond first one at 40–50to°C and ends at ca. decomposition 100 °C, attributed the loss hydration water. The onestarts corresponds the βCD thermal thatto starts at ca.of320 °C [35]. water. The second one corresponds to the βCD thermal decomposition that starts at ca. 320 °C [35]. When the βCD-grafted nanoparticles are analysed, the losses due to hydration are much smaller in When theterms, βCD-grafted areloss analysed, thea losses to hydration much smaller in absolute and thenanoparticles second weight starts at lower due temperature (ca. are 30 °C below). Some absolute terms, have and the second weight loss decomposing starts at a lower temperature (ca. 30might °C below). Some polyurethanes been reported to start at 300 °C [36], which explain the


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decomposition temperature is not reached. In the case of the βCD sample, two weight losses can be noticed. The first one starts at 40–50 ◦ C and ends at ca. 100 ◦ C, attributed to the loss of hydration water. The second one corresponds to the βCD thermal decomposition that starts at ca. 320 ◦ C [35]. When the βCD-grafted nanoparticles analysed, the losses due to hydration are much smaller in absolute Nanomaterials 2018, 8, x FOR PEERare REVIEW 6 of 14 terms, and the second weight loss starts at a lower temperature (ca. 30 ◦ C below). Some polyurethanes ◦ C [36], slightbeen differences thermograms of βCD andwhich the βCDTiO NPs, asthe theslight formed urethane have reportedbetween to start the decomposing at 300 might 2explain differences groups after reaction areofconsiderably stable2 NPs, than as pure βCD. Inurethane addition, the decomposition between the thermograms βCD and theless βCDTiO the formed groups after reaction does not occur inless a single it happens the cyclodextrin, and thedoes decomposition are considerably stablestep thanaspure βCD. Inwith addition, the decomposition not occur inofabatch single3 ◦ Cthe starting ca. 260 °C is clearly separated in two (the secondof ofbatch which3 is barely at detected step as itat happens with the cyclodextrin, and thesteps decomposition starting ca. 260for is nanoparticles modified in batches 1 and of 2).which Batches 1 and 2detected show similar losses even though clearly separated in two steps (the second is barely for theweight nanoparticles modified in their corresponding βCD1amounts might be different, as FTIR results have shown. The weightβCD loss batches 1 and 2). Batches and 2 show similar weight losses even though their corresponding in batch might 3 is higher than that of the otherhave two shown. batches,The accordingly the higher proportion of amounts be different, as FTIR results weight losswith in batch 3 is higher than that spacer and two βCDbatches, used. accordingly with the higher proportion of spacer and βCD used. of the other

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3.2. 3.2.Characterization Characterizationofofthe theComplexation ComplexationAbility Abilityofofthe theβCDTiO βCDTiO22 NPs NPs The Theresults resultsobtained obtainedby byFTIR FTIRand andTGA TGA indicate indicate that that the the surface surface modification modification has has taken taken place. place. However, ofof thethe βCD layer covering thethe nanoparticle needs yet to This This has However,the thefunctionality functionality βCD layer covering nanoparticle needs yetbetoproven. be proven. been carried out making use of two different model compounds that form stable inclusion complexes has been carried out making use of two different model compounds that form stable inclusion with the βCD: Rhodamine (RhB), to test the complexing of the CD-grafted nanoparticles, complexes with the βCD: BRhodamine B (RhB), to test theability complexing ability of the CD-grafted and 1-naphthol,and to analyse the sorption equilibria. Rhodamine B forms a sTable 1:1 stoichiometry nanoparticles, 1-naphthol, to analyse the sorption equilibria. Rhodamine B forms a sTable 1:1 complex with βCD [37,38]. of The the βCD grafted the grafted nanoparticles complex this stoichiometry complex withThe βCDability [37,38]. ability of thetoβCD to the to nanoparticles to fluorophore canfluorophore be extendedcan to other molecules, bestowwhich extra could functionality the complex this be extended to which other could molecules, bestowtoextra nanoparticles, or antibiotics,drugs for instance. In order to carry out this functionality tolike theanti-inflammatory nanoparticles, likedrugs anti-inflammatory or antibiotics, for instance. In order − 6 test, non-modified and βCD-grafted were dispersed in a 4were × 10dispersed M RhB solution to carry out this test, non-modifiednanoparticles and βCD-grafted nanoparticles in a 4 × under 10−6 M constant stirringunder and the precipitates andand supernatants were analysed by fluorescence RhB solution constant stirring the precipitates and supernatants werespectroscopy. analysed by Figure 4 shows the emissionFigure of the4solids, being in of contact with after the solution to the fluorescence spectroscopy. showsafter the emission the solids, being inaccording contact with the procedure describedtoabove. It can be seen how the modified nanoparticles from batches 2 and 3 solution according the procedure described above. It can be seen how the modified nanoparticles provided the highest similar each other,very and similar closely followed byand the closely nanoparticles from from batches 2 and 3emission, providedvery the highest emission, each other, followed by batch 1, while the as-received TiO showed a virtually null showed response. The fact that 2 and the nanoparticles from batch 1, while the as-received TiO2 NPs a virtually nullbatches response. The 2 NPs 3fact show the2 same intensity would point intensity to a similar amount βCDs on thatnearly batches and 3emission show nearly the same emission would pointoftofunctional a similar amount of their surface, despite the different proportions used in proportions the synthesisused thatin confirm the grafting of the functional βCDs on their surface, despite the different the synthesis that confirm macrocycle to the surface, making it available for a further complexation with the grafting of nanoparticles the macrocycle to the nanoparticles surface, making it available fora molecule a further complexation with a molecule of interest that may fit into the cavity of the CD. Apart from the differences in intensity, a slight shift towards higher wavelengths was observed in batches 2 and 3 with respect to batch 1. This spectral shift to higher wavelengths may be due to either an increase of the polarity in the vicinity of the probe fluorophore or a decrease in the rigidity of the immediate surroundings where the probe is immersed. As explained above, batch 1 seems to be somewhat


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of interest that may fit into the cavity of the CD. Apart from the differences in intensity, a slight shift towards higher wavelengths was observed in batches 2 and 3 with respect to batch 1. This spectral shift to higher wavelengths may be due to either an increase of the polarity in the vicinity of the probe fluorophore or a decrease in the rigidity of the immediate surroundings where the probe is immersed. As explained above, batch 1 seems to be somewhat anomalous. A defective βCD substitution in batch Nanomaterials 2018, 8, x deactivation FOR PEER REVIEW 7 offor 14 1 (caused by some of the reactive end of the isocyanate linkers) might be responsible the differences in the polarity of the nanoparticle surfaces. Thus, batch 1 would have produced a less surfaces. Thus, batch 1 would have2 and produced a less surface than those batchesand 2 and 3, in polar surface than those of batches 3, in line withpolar the comparatively lowerofemission slightly line with the comparatively lower emission and slightly blue-shifted band. blue-shifted band. 5

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(batches 2, Figure 4. Solid phase fluorescence fluorescencespectra spectraof ofthe theas-received as-receivedTiO TiO NPsand andβCDTiO βCDTiO 2 NPs (batches1, 1, 2 2NPs 2 NPs −6−6 M RhB solution. and 3) 3) after equilibration with a 4a × 2, and after equilibration with 4 ×1010 M RhB solution.

It It is is interesting interesting to to compare compare these these results results with with the the emission emission of of the the solutions solutions coming coming from from the the −7 supernatants, which are are shown shownin inFigure Figure5.5.In Inthis thiscase, case,the theemission emissionisiscompared comparedtotoa a4 4××10 10−7 M supernatants, which M RhB RhB reference reference solution solution which which was was not not put put in in contact contact with with any any of of the the nanoparticles. nanoparticles. It It can can be be seen seen how how the the substituted nanoparticles produce in all cases the reduction in the emission of the solutions, substituted nanoparticles produce in all cases the reduction in the emission of the solutions, mirroring mirroring qualitatively the fluorescence of and, the solids and,case as in of the precipitates, qualitatively the fluorescence behaviour behaviour of the solids as in the ofthe thecase precipitates, the higher the higher the βCD grafted to the nanoparticles, the lower the amount of fluorophore left in the the βCD grafted to the nanoparticles, the lower the amount of fluorophore left in the supernatant supernatant solution. the However, the as-received TiO2 NPs reduce significantly the emission in the solution. However, as-received TiO2 NPs reduce significantly the emission in the solution, solution, which seems striking when compared to the fluorescence of the corresponding solid which seems striking when compared to the fluorescence of the corresponding solid nanoparticles. nanoparticles. This must be attributed to the intrinsic photocatalytic activity of TiO 2, which in fact is This must be attributed to the intrinsic photocatalytic activity of TiO2 , which in fact is utilized to utilized degrade organic compounds, dyes among Theshown resultshere shown here degradetoorganic compounds, including including dyes among them [15].them The [15]. results are then are then particularly interesting since the RhB complexed with the βCD on the surface of the particularly interesting since the RhB complexed with the βCD on the surface of the nanoparticles does nanoparticles does not get photo-degraded. Thus, using appropriate component ratios, a not get photo-degraded. Thus, using appropriate component ratios, a multifunctional nanoparticle multifunctional nanoparticle couldboth be synthetized, which would bothand, present photocatalytic activity could be synthetized, which would present photocatalytic activity at the same time, efficiently and, at the same time, efficiently deliver compounds susceptible of being degraded by the TiO 2. deliver compounds susceptible of being degraded by the TiO2 . An additional fact comes into play if we consider that, according to the literature, TiO 2 can also An additional fact comes into play if we consider that, according to the literature, TiO2 can also degrade βCD [39]. [39]. In In their their work, work, the theauthors authorsshowed showedthe theaffinity affinityofofCDs CDsfor for the photoactive surface degrade βCD the photoactive surface of of TiO 2 and their subsequent degradation under UV irradiation. We hypothesize that in our case this TiO2 and their subsequent degradation under UV irradiation. We hypothesize that in our case this effect contacting the the TiO TiO2 surface effect must must not not be be that that important, important, since since the the cyclodextrin cyclodextrin moieties moieties are are not not contacting 2 surface but separated by the spacers in a certain extent. Nevertheless, additional investigations should but separated by the spacers in a certain extent. Nevertheless, additional investigations should be be carried carried out out to to evaluate evaluate in in full full the the photocatalytic photocatalytic activity activity of of the the modified modified nanoparticles, nanoparticles, topic topic which which is is out of the scope of this work. out of the scope of this work.

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1.8

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TiO2 NPs RhB 0.4 μM TiO2 NPs batch 1 RhB 0.4 batch 2 μM batch batch13 batch 2 batch 3

emission intensity (a.u.)(a.u.) emission intensity

1.5 1.2 1.2 0.9 0.9 0.6 0.6 0.3 0.3 0.0 0.0 560

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Figure 5. Fluorescence spectra of supernatants in contact with the as-received TiO2 NPs and βCDTiO2 M RhB included for NPs (batches 1, 2, andspectra 3). The of the reference 4 × 10−7TiO Figure 5. Fluorescence Fluorescence of spectrum supernatants in contact with the βCDTiO supernatants contact withsolution the as-received as-received TiO NPsisand and βCDTiO 22 NPs 22 − 7 comparison purposes. −7 M RhB RhB is is included for NPs (batches 1, 2, 2, and and 3). 3). The The spectrum spectrum of of the the reference reference solution solution44×× 10 10 M comparison purposes. comparison purposes.

The sorption capacity of the βCDTiO2 NPs as a function of the amount of sorbate is another interesting property to beoftested for practical purposes. In oforder to characterize the The sorption capacity 22 NPs a function the amount of sorbate is sorption the βCDTiO NPs as as function another equilibrium at room temperature, both βCD-grafted and as-received nanoparticles were placed in interesting propertyto to be tested for practical purposes. to characterize sorption interesting property be tested for practical purposes. In orderInto order characterize the sorptionthe equilibrium contact with 1-naphthol solutions at different sorbate concentrations. This guest molecule has been equilibrium at room temperature, both βCD-grafted andnanoparticles as-received nanoparticles were placed in at room temperature, both βCD-grafted and as-received were placed in contact with frequently used as a model molecule to study the complexation behaviour of cyclodextrin polymers contact with 1-naphthol solutions at different sorbate concentrations. This guest molecule has been 1-naphthol solutions at different sorbate concentrations. This guest molecule has been frequently used [40] given used its high to the βCD, derived its adequate size and hydrophobicity Once frequently as aaffinity model molecule to studyfrom thebehaviour complexation behaviour ofpolymers cyclodextrin polymers as a model molecule to study complexation of cyclodextrin [40][41]. given its equilibrium was reached, the supernatants were tested by UV-visible spectroscopy and the sorption [40] given its high affinity to βCD, derived from its adequate size and hydrophobicity [41]. Once high affinity to βCD, derived from its adequate size and hydrophobicity [41]. Once equilibrium was isotherms were obtained to by standard As canand bespectroscopy seen in Figure 6,the thesorption sorption equilibrium was reached, according the supernatants were procedures. tested by UV-visible and reached, the supernatants were tested UV-visible spectroscopy the sorption isotherms were capacity of the βCDTiO 2 NPs is quite remarkable when compared to the bare ones, which show isotherms were obtained according to standard AsFigure can be6,seen Figure capacity 6, the sorption obtained according to standard procedures. As procedures. can be seen in the in sorption of the practically no towards this sorbate, in line with the results obtained by ones, the fluorescence of capacity theisaffinity βCDTiO 2 NPs is quite compared to the bare which show βCDTiO2of NPs quite remarkable when remarkable compared towhen the bare ones, which show practically no affinity the solid product in contact with RhB. As occurs in most cases, the higher the concentration of practically affinityintowards in line with results obtained by the fluorescence of towards thisno sorbate, line withthis the sorbate, results obtained by thethe fluorescence of the solid product in contact sorbate in equilibrium, the higher the sorption capacity is. In these experiments it was observed that the solid inincontact with the RhB. As occurs in most cases, the higher the concentration of with RhB.product As occurs most cases, higher the concentration of sorbate in equilibrium, the higher as received TiO 2 NPs formed more stable colloidal dispersions than the βCD-modified ones, most sorbate in equilibrium, higher the sorptionit capacity is. In these that the sorption capacity is.the In these experiments was observed that asexperiments received TiOit2 was NPsobserved formed more likely due toTiO the2 hydrophilic nature of the outer partdispersions of the CDs, than with athe high density of non-reacted – as received NPs formed more stable colloidal ones, most stable colloidal dispersions than the βCD-modified ones, most likely due toβCD-modified the hydrophilic nature of OH groups. likely duepart to the nature ofdensity the outer of the CDs, with a high density of non-reacted – the outer of hydrophilic the CDs, with a high of part non-reacted –OH groups. OH groups. 0.014 0.014 0.012

βCDTiO2 NPs

TiO2 NPs

βCDTiO2 NPs

q (mg /mgNP ) q naph (mg /mg ) naph NP

0.012 0.010

TiO2 NPs

0.010 0.008 0.008 0.006 0.006 0.004 0.004 0.002 0.002 0.000 0.000 0 0

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Figure Figure6.6.Sorption Sorptionequilibrium equilibriumof of1-naphthol 1-naphtholdata dataat atroom roomtemperature temperaturecorresponding correspondingto toas-received as-received (blue squares) and βCDTiO NPs (red circles). Trend lines are included for comparison purposes. 2 (red circles). Trend lines temperature are includedcorresponding for comparisontopurposes. (blue squares) andequilibrium βCDTiO2 NPs Figure 6. Sorption of 1-naphthol data at room as-received (blue squares) and βCDTiO2 NPs (red circles). Trend lines are included for comparison purposes.


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If we could consider the βCDTiO2 NPs as a homogeneous sorbate, a Langmuir fitting of the sorption equilibrium data would yield the maximum coverage for 1-naphthol, which in this case corresponds to ca. 20x FOR mgPEER of REVIEW sorbate per gram of modified nanoparticle. In a previous Nanomaterials 2018, 8, 9 of 14 work, we reported the sorption behaviour of βCD polymers crosslinked with HMDI and other cross we could consider theisotherms βCDTiO2 NPs as a homogeneous sorbate, a Langmuir fitting of linkers [42].IfThe corresponding showed that, for those CD-based networks, thethesorption sorption equilibrium data would yield the maximum coverage for 1-naphthol, which in this case capacity could be above 100 mg of 1-naphthol per gram of cyclodextrin polymer, in which the sorption corresponds to ca. 20 mg of sorbate per gram of modified nanoparticle. In a previous work, we takes place boththe within thebehaviour CD cavities and by association with the somewhat hydrophilic reported sorption of βCD polymers crosslinked with HMDI and other cross linkerspockets created [42]. in the polymeric networks (i.e.,showed the crosslinking chains between the βCD moieties). In the case The corresponding isotherms that, for those CD-based networks, the sorption capacity of the composite materials in this is expectedpolymer, that the can be attributed could be above 100 mg studied of 1-naphthol perwork, gram ofitcyclodextrin inassociation which the sorption takes place within the CD cavities and by association the somewhat hydrophilic createdcapacity. mainly to theboth CDs, the isocyanate crosslinking bridgeswith playing just a minor role in pockets the sorption in theifpolymeric networks (i.e., the crosslinking betweenofthe In the case of the In any case, we consider the TGA data as a validchains estimation theβCD NPmoieties). organic/TiO 2 ratio, above 40% composite materials studied in this work, it is expected that the association can be attributed mainly of the weight of the nanoparticles (batch 3) correspond to the inorganic TiO2 component, with a scarce to the CDs, the isocyanate crosslinking bridges playing just a minor role in the sorption capacity. In influence theifsorption of the 1-naphthol. this correction, the βCD anyincase, we consider TGA data asWith a valid estimation of thethe NPadsorption organic/TiO2 efficiency ratio, above of 40% organic of shell the modified nanoparticles would come closer what is observed for typical the of weight of the nanoparticles (batch 3) correspond to thetoinorganic TiO2 component, with hydrogel a scarce influence in the sorption of 1-naphthol. With this correction, the adsorption efficiency of the networks based on βCD [42]. βCD organic shell of the modified nanoparticles would come closer to what is observed for typical hydrogel networks [42]. 3.3. Characterization of thebased EffectonofβCD the Nanoparticles Dispersion within Polymeric Matrices

Two crystalline selected disperse the nanoparticles within 3.3.ubiquitous Characterization of the Effectthermoplastics of the Nanoparticleswere Dispersion withintoPolymeric Matrices a polymericTwo matrix: low-density polyethylene (LDPE) and polyethylene oxide (PEO). Focusing on ubiquitous crystalline thermoplastics were selected to disperse the nanoparticles within a LDPE, Figure 7 shows simple mortar grinding is inefficient to homogenize the Focusing dispersion. polymeric matrix:how low-density polyethylene (LDPE) and polyethylene oxide (PEO). on On the other hand, milling cryogenic produces homogeneous LDPE,ball Figure 7 showsunder how simple mortarconditions grinding is inefficient to considerably homogenize the more dispersion. On theIn other hand, ball under cryogenic conditions produces considerably more homogeneous samples. addition, themilling optical micrographs show that the βCDTiO seem to yield a better 2 NPs samples. In addition, the optical micrographs show that the βCDTiO 2 NPs seem to yield a better dispersion than the bare ones. SEM micrographs obtained with BSE (not shown) confirm the optical dispersion than the bare ones. SEM micrographs obtained with BSE (not shown) confirm the optical microscopy analysis. microscopy analysis.

(a)

(b)

(c)

(d)

Figure 7. Optical micrographs (100×) of polyethylene films: (a) LDPE; (b) LDPE + 5% βCDTiO2 NPs

Figure 7. Optical micrographs (100×) of polyethylene films: (a) LDPE; (b) LDPE + 5% βCDTiO2 NPs (mortar grinding); (c) LDPE + 5% TiO2 NPs (cryo-milled mixture); (d) LDPE + 5% βCDTiO2 NPs (mortar(cryo-milled grinding);mixture). (c) LDPE + 5% TiO2 NPs (cryo-milled mixture); (d) LDPE + 5% βCDTiO2 NPs (cryo-milled mixture).


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A homogeneous dispersion of nanoparticles produces a considerable increase in the interphase between the polymeric matrix and the filler and this will be reflected in the properties of the bulk material. However, the calorimetry results corresponding to the crystallization and melting of LPDE and its mixtures with TiO2 NPs did not show any significant differences. As can be seen in Table 1, Nanomaterials 2018, 8, x FOR PEER REVIEW 10 of 14 in the the crystallization temperatures are nearly the same in all cases. There is a small decrease value of theAcrystallization enthalpy that seems somewhat more important in the case of the bare homogeneous dispersion of nanoparticles produces a considerable increase in the interphase nanoparticles. Coincidentally, the melting are be equal in allincases within experimental between the polymeric matrix and the temperatures filler and this will reflected the properties of the bulk error, and the material. diminution of thethe melting enthalpies is also small.toIn the thermogravimetric However, calorimetry results corresponding theaddition, crystallization and melting of LPDE curves and loaded its mixtures with TiO2 either NPs did not show any As can be seen(data in Table of both 5% LDPE (with as-received or significant βCDTiO2differences. NPs are also identical not1,shown). crystallization arethe nearly the same inthermodynamic all cases. There is point a smallofdecrease in the It can bethe concluded that,temperatures at least from macroscopic view, the influence value of the crystallization enthalpy that seems somewhat more important in the case of the bare of the nanofiller is too small to be unambiguously detected. In order to check if these effects are also nanoparticles. Coincidentally, the melting temperatures are equal in all cases within experimental negligible at aand molecular scale,ofan study of theissamples crystallized different conditions error, the diminution theFTIR melting enthalpies also small. In addition, under the thermogravimetric has alsocurves been of performed. Infrared and Raman spectra can be used to characterize both 5% loaded LDPE (with either as-received or βCDTiO2 NPs are also identicalsemi (datacrystalline not shown). It can be concluded that,toatthe least from the macroscopic thermodynamic point of view, polymers because of their sensitivity conformation and packing of macromolecules [43].theWe have of the nanofiller is too for small to study. be unambiguously detected. Inbands order to check in if these focusedinfluence on the 735–715 cm−1 region our Three rocking mode appear this region, effects are also negligible−1at a molecular scale, an FTIR study of the samples crystallized under two of them (730 and 722 cm ) are associated with the crystalline fraction of the samples, and the different conditions has also been performed. Infrared and Raman spectra can be used to one corresponding to the amorphous fraction appears at 723 cm−1 . Thus, the latter overlaps one of characterize semi crystalline polymers because of their sensitivity to the conformation and packing the former, although this[43]. is aWe low-intensity broad band under the two narrow of macromolecules have focused and on the 735–715 cm−1partially region forhidden our study. Three rocking −1 crystalline bands. tests pure film samples. It was confirmed that, mode bandsSeveral appear in this were region,performed two of themusing (730 and 722LDPE cm ) are associated with the crystalline fraction of the samples, and the one corresponding the amorphous fraction at 723modes cm−1. were using different crystallization conditions, the relativetointensities of these twoappears crystalline Thus, the latter overlapsthe oneresults of the former, thiscrystallization is a low-intensityexperiments. and broad bandItpartially different. Figure 8 presents of twoalthough different is remarkable hidden under the two narrow crystalline bands. Several tests were performed using pure LDPE film that, in both cases, there is a significant difference (especially for the band at 730 cm−1 ) between LDPE samples. It was confirmed that, using different crystallization conditions, the relative intensities of and the these polymer loaded with the TiO2 NPs, while the film produced with βCDTiO2 NPs yields a very two crystalline modes were different. Figure 8 presents the results of two different similar result to that of pure LDPE. crystallization experiments. It is remarkable that, in both cases, there is a significant difference (especially for the band at 730 cm−1) between LDPE and the polymer loaded with the TiO2 NPs, while

Table Crystallization melting (peak) enthalpies of LDPE and its mixtures the1. film produced withand βCDTiO 2 NPs yieldstemperatures a very similar and result to that of pure LDPE. with 5% TiO2 NPs and 1%, 3% and 5% βCDTiO2 NPs (values between parentheses correspond to Tablecrystallization 1. Crystallization and melting (peak) temperatures and enthalpies of LDPE and its mixtures a secondary peak). with 5% TiO2 NPs and 1%, 3% and 5% βCDTiO2 NPs (values between parentheses correspond to a secondary crystallization peak).

SAMPLE

SAMPLE

LDPE LDPE LDPE + 5% TiO 2 NPs LDPE + 5% TiO2 NPs LDPE + 1% βCDTiO 2 NPs2 NPs LDPE + 1% βCDTiO LDPE + 3% βCDTiO 2 NPs2 NPs LDPE + 3% βCDTiO LDPE + 5% βCDTiO 2 NPs2 NPs LDPE + 5% βCDTiO 0.2

Crystallization

Melting

Crystallization −1 ∆Hc (J·g PE) Tc (°C) ΔHc (J·g−1 PE) 99.0 (60.7) 87.7 (6.0) 99.0 (60.7) 87.7 (6.0) 100.1 (61.4) 80.4 (5.2) 100.1 (61.4) 80.4 (5.2) 99.2 79.1 (4.7) 99.2 (60.4) (60.4) 79.1 (4.7) 99.0 (60.6) 83.3 (4.8) 99.0 (60.6) 83.3 (4.8) 100.8 (61.6) 82.5 (5.5) 100.8 (61.6) 82.5 (5.5)

Melting ∆Hm (J·g−1 PE) Tm (°C) ΔHm (J·g−1 PE) 111.3 111.3 109.7109.7 111.6 111.6 101.0101.0 112.0 112.0 97.1 97.1 112.2 112.2 105.6105.6 111.6 104.7 111.6 104.7

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Figure 8. FTIR-ATR spectra in the 750–680 cm−1 region of LDPE, LDPE + 5% TiO2 NPs, and LDPE +

Figure 8. FTIR-ATR spectra in the 750–680 cm−1 region of LDPE, LDPE + 5% TiO2 NPs, and LDPE + 5% 5% βCDTiO2 NPs films: (a) Crystallized at 123 °C for 2 h; (b) annealed at 123 °C for 24 h from a ◦ ◦ βCDTiOmolten films: quenched (a) Crystallized 2 NPssample at −70 °C.at 123 C for 2 h; (b) annealed at 123 C for 24 h from a molten ◦ sample quenched at −70 C.

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The influence of bare nanoparticles and surface modified ones on the crystallization and The influence ofalso barebeen nanoparticles and surface ones onhave the crystallization melting of PEO has considered (Table 2).modified Similar results been found inand thismelting case to of PEO has also been considered (Table 2). Similar results have been found in this case to those those of LDPE. Firstly, the thermodynamic parameters corresponding to the phase changeofofLDPE. PEO Firstly, the thermodynamic parameters corresponding to the phase change of PEO are slightly altered are slightly altered in the presence of both additives. Secondly, additional optical microscopy studies in of bothpatterns additives. additional microscopy the crystallization onthe thepresence crystallization of Secondly, these samples showoptical that the behaviourstudies of the on βCDTiO 2 NPs in the patterns of these samples show that the behaviour of the βCDTiO NPs in the spherulite morphology 2 spherulite morphology of PEO is not as important as the one corresponding to bare nanoparticles. In of PEO is not as important as the one corresponding to bare nanoparticles. In other words, the strong other words, the strong nucleation effect found for the TiO2 NPs is not as relevant in the case of the nucleation effect found forwhere the TiO NPs isnumber not as relevant in the grow case ofup thetoβCDTiO PEO, 2 NPs βCDTiO2 NPs with PEO, a2small of crystallites a similar size with to that of where a small number crystallites up to a similar sizeβCD to that of pure PEOnanoparticles (see Figure 9).isWe can pure PEO (see Figureof9). We can grow hypothesize that the shell of the more hypothesize that the of in thethe nanoparticles more with the PEO chainsfor in the compatible with the βCD PEO shell chains liquid-like is state, socompatible they are not acting as nuclei the liquid-like state, so they are not acting as nuclei for the crystallization processes. crystallization processes. and enthalpies of PEO andand its mixtures withwith 1% and Table Table 2.2. Crystallization Crystallizationand andmelting meltingtemperatures temperatures and enthalpies of PEO its mixtures 1% 3% TiO NPs and βCDTiO NPs. 2 2 and 3% TiO2 NPs and βCDTiO2 NPs.

SAMPLE SAMPLE PEO PEO PEO + 1% TiO 2 NPs PEO + 1% TiO 2 NPs PEO + 1% βCDTiO NPs PEO + 1% βCDTiO2 2NPs PEO + 3% TiO NPs PEO + 3% TiO2 2NPs PEO + 3% βCDTiO NPs PEO + 3% βCDTiO2 2NPs

Crystallization Crystallization −1 1 ◦ TTc c(°C) ΔH ( C) ∆Hc c(J·g (J·g−PEO) PEO) 39.6 133.9 39.6 133.9 38.9 129.8 38.9 129.8 39.5 138.7 39.5 138.7 40.7 140.4 40.7 140.4 38.9 132.1 38.9 132.1

Melting Melting ◦ C) TTmm ((°C) 64.5 64.5 63.5 63.5 63.8 63.8 65.2 65.2 64.4 64.4

−1 PEO) m (J·g PEO) ∆HΔH m (J·g −1

125.9

125.9 124.8 124.8 130.8 130.8 132.7 132.7 125.8

125.8

Figure 9. 9. Polarized Polarized optical optical micrographs micrographs (100 (100×) corresponding to to Figure ×) of of poly(ethylene poly(ethylene oxide) oxide) crystallites crystallites corresponding 2 NPs and PEO + 5% βCDTiO2 NPs. (from left to right): pure PEO, PEO + 5% TiO (from left to right): pure PEO, PEO + 5% TiO2 NPs and PEO + 5% βCDTiO2 NPs.

4. Conclusions 4. Conclusions The surface of TiO2 NPs has been successfully modified with βCD in a two-step reaction that The surface of TiO2 NPs has been successfully modified with βCD in a two-step reaction that involves firstly the incorporation of a spacer (HMDI) and then the grafting of the CD moieties. The involves firstly the incorporation of a spacer (HMDI) and then the grafting of the CD moieties. surface modification has been characterized by TGA, FTIR, and SEM, which prove the validity of the The surface modification has been characterized by TGA, FTIR, and SEM, which prove the validity of procedure. The grafted nanoparticles are capable of incorporating guest molecules that remain the procedure. The grafted nanoparticles are capable of incorporating guest molecules that remain lodged in the CD cavities, according to the results obtained with model compounds, Rhodamine B lodged in the CD cavities, according to the results obtained with model compounds, Rhodamine B and 1-naphthol, obtaining adsorption efficiencies comparable to those of other sorbents based on and 1-naphthol, obtaining adsorption efficiencies comparable to those of other sorbents based on βCD. In addition, the presence of the CD attached to the surface, avoids the photo-degradation of the βCD. In addition, the presence of the CD attached to the surface, avoids the photo-degradation of guest, which is of interest in order to combine the photocatalytic activity of TiO2 and, at the same the guest, which is of interest in order to combine the photocatalytic activity of TiO2 and, at the same time, achieve an efficient delivery of compounds susceptible of being photo-degraded. The time, achieve an efficient delivery of compounds susceptible of being photo-degraded. The feasibility feasibility of the use of the functionalized nanoparticles for biomedical applications has also been of the use of the functionalized nanoparticles for biomedical applications has also been explored by explored by testing its dispersion in polymeric matrices of polyethylene (LDPE) and polyethylene testing its dispersion in polymeric matrices of polyethylene (LDPE) and polyethylene oxide (PEO), oxide (PEO), to produce nanocomposites in the form of films. Up to a 5% by weight load of filler, the to produce nanocomposites in the form of films. Up to a 5% by weight load of filler, the range studied, range studied, the nanoparticles do not seem to alter the crystallization properties of the polymer the nanoparticles do not seem to alter the crystallization properties of the polymer and, at the same time, and, at the same time, the surface modification helps to produce a better homogenization of the filler the surface modification helps to produce a better homogenization of the filler in the polymer matrix. in the polymer matrix.


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Author Contributions: Conceptualization, P.M.-P., J.R.I. and G.G.-G.; Data curation, P.M.-P. and J.R.I.; Funding acquisition, G.G.-G.; Investigation, P.M.-P., J.R.I., J.G.-B., D.O. and G.G.-G.; Methodology, P.M.-P., J.R.I., J.G.-B. and G.G.-G.; Project administration, J.G.-B. and G.G.-G.; Supervision, J.R.I., J.G.-B. and G.G.-G.; Writing—original draft, P.M.-P., J.R.I. and G.G.-G.; Writing—review & editing, P.M.-P., J.R.I., J.G.-B., D.O. and G.G.-G. Funding: This research was funded by Ministerio de Ciencia e Innovación, project MAT2014-59116-C2-2-R. Acknowledgments: The technical assistance of M. Domeño in the experiments is acknowledged. P.M.-P. is grateful to the Asociación de Amigos de la Universidad de Navarra for his doctoral grant. Conflicts of Interest: The authors declare no conflict of interest.

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6. 7. 8. 9. 10.

11.

12. 13. 14. 15.

16.

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