sensors Article
Embedded Ceria Nanoparticles in Crosslinked PVA Electrospun Nanofibers as Optical Sensors for Radicals Nader Shehata 1,2,3, *, Effat Samir 2,4 , Soha Gaballah 2,5 , Aya Hamed 1,2 and Asmaa Elrasheedy 2,5 1 2
3 4 5
*
Department of Engineering Mathematics and Physics, Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt; [email protected] Center of Smart Nanotechnology and Photonics (CSNP), Smart CI Research Center, Alexandria University, Alexandria 21544, Egypt; [email protected] (E.S.); [email protected] (S.G.); [email protected] (A.E.) USTAR Bioinnovations Center, Utah State University, Logan, UT 84341, USA Department of Electrical Engineering, Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt Department of Chemical Engineering, Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt Correspondence: [email protected]; Tel.: +20-109-116-5300
Academic Editor: Ki-Hyun Kim Received: 24 February 2016; Accepted: 11 April 2016; Published: 26 August 2016
Abstract: This work presents a new nanocomposite of cerium oxide (ceria) nanoparticles embedded in electrospun PVA nanofibers for optical sensing of radicals in solutions. Our ceria nanoparticles are synthesized to have O-vacancies which are the receptors for the radicals extracted from peroxide in water solution. Ceria nanoparticles are embedded insitu in PVA solution and then formed as nanofibers using an electrospinning technique. The formed nanocomposite emits visible fluorescent emissions under 430 nm excitation, due to the active ceria nanoparticles with fluorescent Ce3+ ionization states. When the formed nanocomposite is in contact with peroxide solution, the fluorescence emission intensity peak has been found to be reduced with increasing concentration of peroxide or the corresponding radicals through a fluorescence quenching mechanism. The fluorescence intensity peak is found to be reduced to more than 30% of its original value at a peroxide weight concentration up to 27%. This work could be helpful in further applications of radicals sensing using a solid mat through biomedical and environmental monitoring applications. Keywords: ceria nanoparticles; electrospinning; crosslinking; fluorescence quenching; radicals
1. Introduction Radical sensing is important in different applications including cancer treatment [1,2] and water quality monitoring [3]. One source of radicals in experiments is peroxides, due to the unstable oxygen-oxygen chemical bond and consequently the creation of active radicals [4]. Regarding the techniques for sensing radicals, the optical fluorescence-based sensing mechanism is better than other electrochemical ones as it does not require a reference electrode and it is immune to exterior electromagnetic field interference [5–7]. Cerium oxide nanoparticles (ceria NPs) can be considered one of the most promising nanostructures which can capture the radicals depending on their formed O-vacancies associated to the active trivalent ionization state of cerium [8–11]. In this novel work, we aimed to use ceria nanoparticles as an optical sensor for peroxides or the corresponding radicals. However, our ceria NPs would be in the form of a solid host instead of being colloidal solutions so that they could be flexibly used in a wide variety of applications. Therefore, our synthesized nanoparticles
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are embedded in-situ with polyvinyl alcohol (PVA) nanofibers formed by an electrospinning technique. Then, the formed nanocomposite is used as an optical sensing mat for peroxide or radicals in solution. PVA was selected as a non-toxic biodegradable water soluble polymer [12,13]. Regarding the formation of PVA nanofibers, an electrospinning technique is selected as the fabrication method for the proposed nanocomposite because of the simplicity of operation, the feasibility to embed ceria NPs in the resulting PVA nanofibers, and the potential for scale-up to manufacture large volumes [14,15]. In electrospinning, a high strength electric field is applied between a metallic needle and a metallic target. The strength of the electric field forces the polymer droplets at the needle tip to stretch and to be deposited into fibers on the surface of the target. To make the nanofibers (NFs) suitable for sensing radicals in solution, the formed nanocomposite has been crosslinked via a chemical esterification method, therefore, the produced nanofibers (NFs) can resist solubilization and became a hydrophobic material. Different characterizations of the formed nanocomposite are presented in this work, including absorbance dispersion, bandgap calculation, fluorescence spectroscopy, SEM, and FTIR spectroscopy. The studied optical characterizations prove that the synthesized nanocomposite has some trivalent cerium ions which can be active receptors for radicals adsorption through the associated formed O-vacancies [16]. Then, the fluorescence emissions of the formed nanocomposite in the presence of different concentrations of peroxide in water solution are studied with calculating the Stern-Volmer constant in the linear region of the relative intensity change graph. The visible fluorescence emission is obtained under near-UV excitation in a home-made fluorescence spectroscopy setup. The fluorescence intensity is found to be reduced with increasing radical concentration due to a fluorescence quenching mechanism. 2. Materials and Methods 2.1. Nanoparticle and Polymer Synthesis Ceria nanoparticles were synthesized using a chemical precipitation technique, similar to that described in [17,18], as it is a relatively simple procedure using low cost precursors. During ceria preparation, cerium (III) chloride heptahydrate (0.5 g, 99.9%, Sigma-Aldrich, St. Louis, MO, USA) is added to deionized (DI) water (40 mL), then ammonia (1.6 mL) is added as a catalyst. The solution is stirred over a magnetic stirrer for 24 h in an open container at 500 rpm. During the first 2 h of the stirring, the container is held in 50 ◦ C water bath and then the solution is allowed to cool to room temperature for the remainder of the stirring period. The long duration of the stirring helps in fracturing nanorods into nanoparticles. The solution is then centrifuged and washed twice with both deionized water and ethanol to remove any unreacted cerium and ammonia. PVA solution is prepared by mixing PVA pellets(10 g, Mw = 205,000 g/mol, Sigma Aldrich, St. Louis, MO, USA) with distilled water to make 100 mL of solution. The solution is heated to 100 ◦ C for 30 min then stirred overnight. Different weight percentages of ceria NPs are added in-situ to the PVA solution. The mixture is stirred for 30 min before it is used in the electrospinning process. 2.2. Electrospinning and Crosslinking The electrospinning setup consists of high voltage power supply (model CZE1000R, Spellman High Voltage Electronics Corporation, Hauppauge, New York, NY, USA), a syringe pump (NE1000-Single Syringe Pump, New Era, Farmingdale, New York, NY, USA) which is used to regulate the feed rate of polymer solution, a 5 mL plastic syringe with an 18 gauge metallic needle to hold the polymer solution, and a circular metallic collector of radius 10 cm covered with aluminum foil is used as a target. The voltage power supply is connected to the needle while the collector is grounded. The distance between the needle tip and the collector is fixed at 15 cm. The voltage difference between the needle and target is 25 kV. The flow rate of the polymer solution is fixed at 2 mL/h. The running time of electrospinning process per sample is about 30 min. A high voltage is applied to the syringe’s needle; the droplet of polymer solution takes the shape of a cone known as Taylor cone [19]. Then,
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Taylor cone [19]. Then, a jet is erupted from the cone and is accelerated towards the collector. As the jet travels through the air, the solvent evaporates leaving behind fibers to be collected randomly on a jetgrounded is eruptedtarget. from the and is accelerated towards the collector. the jet travels through the the Forcone crosslinking of the formed nanofibers, VaporAs phase esterification process air, the solvent evaporates leaving behind fibers to be collected randomly on the grounded target. was done in the oven on two subsequent steps. In the first step, electrospun nanofibers of PVA or For crosslinking the formed Vapor phase esterification process was done theg) oven PVA-doped ceriaofwere placed nanofibers, in a container along with a small amount of malic acidin (1–2 andon a two subsequent steps. In the first step, electrospun nanofibers of PVA or PVA-doped ceria were placed few drops of HCl were added to the malic acid. The container was sealed from the ambient moisture in a container withata 80 small of malic acid (1–2 g)occurred and a few of HCl were added to and placed inalong an oven °C amount for 15 min. Esterification viadrops a heterogeneous reaction ◦ the malic container from the moisture placed during 15acid. min. The In the second was step,sealed the sample wasambient cured for 20 minand in an oven in at an 120oven °C. at 80 C for 15 min. Esterification occurred via a heterogeneous reaction during 15 min. In the second step, the sample was cured Characterizations for 20 min in an oven at 120 ◦ C. 2.3. Nanocomposite PVA nanofibers with embedded ceria NPs are optically characterized by measuring theoptical 2.3. Nanocomposite Characterizations absorbance, and fluorescence intensity curves. Optical absorbance in the wavelength range from 300 PVA nanofibers with embedded ceria NPs are optically characterized by measuring theoptical to 700 nm was measured by using a T92+UV-Vis spectrophotometer (PG instruments, Beijing, absorbance, and fluorescence intensity curves. Optical absorbance in the wavelength range from 300 China). From the absorbance curves, the corresponding NFs band gap can be determined, as to 700 nm was measured by using a T92+UV-Vis spectrophotometer (PG instruments, Beijing, China). explained in the Results and Discussion sections. The mean synthesized nanoparticle size was From the absorbance curves, the corresponding NFs band gap can be determined, as explained in determination of observed by JEM-2100F transmission electron microscopy (JEOL, Tokyo, Japan) the Results and Discussion sections. The mean synthesized nanoparticle size was determination of with an accelerating potential of 80KV. Surface morphology and diameter size of electrospun observed by JEM-2100F transmission electron microscopy (JEOL, Tokyo, Japan) with an accelerating nanofibers before and after esterification were investigated using scanning electron microscopy potential of 80 KV. Surface morphology and diameter size of electrospun nanofibers before and after (Quanta 200, FEI, Hillsboro, OR, USA). After sputter-coating with gold, the fiber size distribution of esterification were investigated using scanning electron microscopy (Quanta 200, FEI, Hillsboro, randomly selected SEM micrographs was measured using the Image-J software. The formed OR, USA). After sputter-coating with gold, the fiber size distribution of randomly selected SEM nanocomposites have been characterized using IR tracer-100 FTIR spectroscopy (Shimadzu, Kyoto, micrographs was measured using the Image-J software. The formed nanocomposites have been Japan). characterized using IR tracer-100 FTIR spectroscopy (Shimadzu, Kyoto, Japan). 2.4. Fluorescence Quenching Experiment 2.4. Fluorescence Quenching Experiment Fluorescence intensity measurements are done by a home-built fluorescence spectroscopy Fluorescence intensity measurements are done by a home-built fluorescence spectroscopy setup, setup, shown in Figure 1. The fluorescence setup is composed of an ultraviolet (UV) LED with 430 shown in Figure 1. The fluorescence setup is composed of an ultraviolet (UV) LED with 430 nm nm excitation wavelength, a Cornerstone 130 Monochromator (Newport, Irvine, CA, USA); which is excitation wavelength, a Cornerstone 130 Monochromator (Newport, Irvine, CA, USA); which is set to set to obtain the fluorescence intensity at wavelengths from 500 to 700 nm, an Oriel photomultiplier obtain the fluorescence intensity at wavelengths from 500 to 700 nm, an Oriel photomultiplier tube tube (Newport PMT77340) as a fluorescence intensity detector, and a Newport 1918-R power meter (Newport PMT77340) as a fluorescence intensity detector, and a Newport 1918-R power meter to to display PMT detection readings. Fluorescence intensity is measured by positioning the NFs solid display PMT detection readings. Fluorescence intensity is measured by positioning the NFs solid sample holder at a 45◦ between the UV-LED and the input port of the monochromator. The output sample holder at a 45 between the UV-LED and the input port of the monochromator. The output port of the monochromator was directly connected with the PMT, which was directly connected to port of the monochromator was directly connected with the PMT, which was directly connected to the the 1918-R power meter. 1918-R power meter.
Figure 1. Fluorescenceintensity intensityhome-made home-madespectroscopy spectroscopy setup. setup. Figure 1.Fluorescence
To test the the ability ability of of our our synthesized synthesized nanocomposite To test nanocomposite to to attract attract free free radicals, radicals, hydrogen hydrogen peroxide peroxide solution (35 wt %, Sigma-Aldrich) samples with different volume percent areused. The solution (35 wt %, Sigma-Aldrich) samples with different volume percent areused. The samplesample holder holder with the synthesized nanocomposite mat, aofpiece of × 0.51.5 cmcm, × 1.5 cm, is inserted a 100 mL with the synthesized nanocomposite mat, a piece 0.5 cm is inserted in a 100inmL beaker beaker which contains the radicals at different weight concentrations. At each peroxide weight which contains the radicals at different weight concentrations. At each peroxide weight concentration, concentration, theintensity fluorescence peakthe emitted from nanocomposite the contacted nanocomposite the fluorescence peak intensity emitted from contacted with error barswith has error been bars has been detected and by the power meter. detected and monitored by monitored the power meter.
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3. Results 3. Results 3. Results 3.1. Nanocomposite Charcterization 3.1. Nanocomposite Charcterization The absorbance dispersion curves of our synthesized nanocomposite PVA NFs with embedded The absorbance dispersion curves of our synthesized nanocomposite PVA NFs with embedded ceria NPs with some weight concentrations are shown in Figure 2a. The corresponding direct NPswith withsome some weight concentrations are shown in Figure The corresponding direct ceria NPs weight concentrations are shown in Figure 2a. The2a. corresponding direct bandgap bandgap can be calculated from the absorbance dispersion curve through Equation (1) [20]: bandgap can be calculated from the absorbance curveEquation through (1) Equation can be calculated from the absorbance dispersiondispersion curve through [20]: (1) [20]:
( E) A(E Eg )11 22 ( E) A(E E ) α( E) = A( E − Eg )g1/2
(1) (1) (1) where α is the absorbance, A is a material-dependent constant which includes both the effective where α is the absorbance, A is a material-dependent constant which includes both the effective massesαofiselectrons and holesAand constant of the nanocomposite, E is theboth absorbed photon where the absorbance, is adielectric material-dependent constant which includes the effective masses of electrons and holes and dielectric constant of the nanocomposite, E is the absorbed photon energy (E hc/λ) as and λ is the optical absorbedconstant wavelength, and Eg is the allowed bandgap of the masses of =electrons holes and dielectric of the nanocomposite, E isdirect the absorbed photon energy (E = hc/λ) as λ is the optical absorbed wavelength, and Eg is the allowed direct bandgap of the 2 and E is presented in Figure 2b. The synthesized nanocomposite. Then, the relation between (αE) energy (E = hc/λ) as λ is the optical absorbed wavelength, and 2Eg is the allowed direct bandgap of the synthesized nanocomposite. Then, the relation between2 (αE) 2 and E is presented in Figure 2b. The intersection of the extrapolation of the partbetween of (αE) (αE) curve and withEE-axis is equalin toFigure allowed synthesized nanocomposite. Then, thelinear relation is presented 2b.direct The intersection of the extrapolation of the linear part of (αE)22 curve with E-axis is equal to allowed direct bandgap Eg,ofthat particular composition ceria NPs with concentration of tri-valent intersection the extrapolation of the linearofpart of (αE) curvemoderate with E-axis is equal to allowed direct bandgap Eg, that particular composition of ceria NPs with moderate concentration of tri-valent cerium ionization states with associated O-vacancies, is within accepted range biased to bandgap Eg, that particular composition of ceria NPs withwhich moderate concentration of tri-valent cerium cerium ionization states with associated O-vacancies, which is within accepted range biased to 3 eV [16,21]. ionization states with associated O-vacancies, which is within accepted range biased to 3 eV [16,21]. 3 eV [16,21].
Figure 2. PVA NFs with in-situ in-situ embedded ceria ceria NPs. (a) (a) Absorbance curve; curve; (b) Band Band gap curve. curve. Figure 2. 2. PVA NFs Figure PVA NFs with with in-situ embedded embedded ceria NPs. NPs. (a) Absorbance Absorbance curve; (b) (b) Band gap gap curve.
Visible fluorescence intensity spectra, under 430 nm optical excitation, are shown in Figure 3 for fluorescence intensity spectra, under 430 nm optical excitation, are shown in Figure 3 for Visible fluorescence different concentrations of ceria NPs which are embedded in-situ in crosslinked PVA NFs. concentrationsofof ceria which are embedded in crosslinked NFs. different concentrations ceria NPsNPs which are embedded in-situ in-situ in crosslinked PVA NFs.PVA Generally, Generally, our synthesized nanocomposite mat displays the same optical activity properties as ceria Generally, our synthesized nanocomposite the same optical activity properties as ceria our synthesized nanocomposite mat displaysmat the displays same optical activity properties as ceria nanoparticles nanoparticles in a biodegradable solid and flexible host of PVA nanofibers for a wide variety of nanoparticles in a solid biodegradable flexible host of nanofibers a wide variety of in a biodegradable and flexiblesolid host and of PVA nanofibers forPVA a wide variety offor possible applications. possible applications. possible applications.
Figure 3. Fluorescence intensity PVA NFs with in-situ in-situ embedded different different concentrations ceria ceria NPs. Figure 3. 3. Fluorescence Fluorescence intensity intensity PVA PVA NFs Figure NFs with with in-situ embedded embedded different concentrations concentrations ceria NPs. NPs.
Figure 4a shows both TEM of ceria NPs with mean diameter less than 10 nm. SEM images of Figure 4a NPs with mean diameter lessless thanthan 10 nm. SEMSEM images of both 4a shows showsboth bothTEM TEMofofceria ceria NPs with mean diameter 10 nm. images of both PVA nanofibers with embedded ceria NPs before and after crosslinking are shown in both PVA with embedded ceria NPs before after and crosslinking are shown are in both Figure 4b,c, both nanofibers PVA nanofibers with embedded ceria NPsand before after crosslinking shown in both Figure 4b,c, with a focused image of some agglomerated nanoparticles inside the fiber shown in Figure 4b,c, with a focused image of some agglomerated nanoparticles inside the fiber shown in Figure 4d. The synthesized nanofibers, in both cases, have mean diameters around 130 nm but with Figure 4d. The synthesized nanofibers, in both cases, have mean diameters around 130 nm but with
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with focused image of some agglomerated nanoparticles inside the fiber shown in Figure 4d. 5The Sensorsa2016, 16, 1371 of 8 Sensors 2016, 16, 1371 5 of 8 synthesized nanofibers, in both cases, have mean diameters around 130 nm but with higher numbers of formed beads of in formed the crosslinked Some ceria NPs expected to beare located on the surface of higher numbers beads incase. the crosslinked case.are Some ceria NPs expected to be located higher numbers of formed beads in the crosslinked case. Some ceria NPs are expected to be located the electrospun especially before the crosslinking effect. In addition, aeffect. macroscopic photoa on the surface ofnanofibers, the electrospun nanofibers, especially before the crosslinking In addition, on the surface of the electrospun nanofibers, especially before the crosslinking effect. In addition, a of the formedphoto crosslinked is shown innanofibers Figure 4e to that formed nanofibers matthe is macroscopic of thenanofibers formed crosslinked is show shown inthe Figure 4e to show that macroscopic photo of the formed crosslinked nanofibers is shown in Figure 4e to show that the formed nanofibers is uniform with onthe theFTIR sides. Figure 5 shows theof FTIR uniform with some mat minor defects on the some sides.minor Figuredefects 5 shows spectroscopy pattern the formed nanofibers mat is uniform with some minor defects on the sides. Figure 5 shows the FTIR spectroscopy pattern ofwith the crosslinked nanofibers crosslinked nanofibers embedded ceria NPs. with embedded ceria NPs. spectroscopy pattern of the crosslinked nanofibers with embedded ceria NPs.
(a) (a)
(c) (c)
(b) (b)
(d) (d)
(e) (e)
Figure 4. (a) TEM image of ceria NPs; (b) SEM image of our synthesized nanocomposite of PVA Figure (a) TEM TEM image image of of ceria ceria NPs; NPs; (b) (b) SEM SEM image image of of our our synthesized synthesized nanocomposite nanocomposite of of PVA PVA Figure 4. 4. (a) nanofibers embedded embedded with with ceria ceria NPs; NPs; (c) SEM image of the nanocomposite after crosslinking; nanofibers (c) SEM image of the nanocomposite after crosslinking; nanofibers embedded with ceria NPs; (c) SEM image of the nanocomposite after crosslinking; (d) STEM image of some agglomerated nanoparticles inside the fiber; and (e) macroscopic photo of (d) image of of some someagglomerated agglomeratednanoparticles nanoparticlesinside insidethe thefiber; fiber;and and macroscopic photo (d) STEM STEM image (e)(e) macroscopic photo of the synthesized electrospun nanofibers. of synthesized electrospun nanofibers. thethe synthesized electrospun nanofibers.
Figure 5. FTIR spectroscopy pattern of crosslinked PVA with embedded ceria NPs. Figure embedded ceria ceria NPs. NPs. Figure 5. FTIR FTIR spectroscopy spectroscopy pattern of crosslinked PVA with embedded
3.2. Optical Sensing of Peroxide Using Fluorescence Quenching Mechanism 3.2. Optical Using Fluorescence Fluorescence Quenching Quenching Mechanism Mechanism 3.2. Optical Sensing Sensing of of Peroxide Peroxide Using The variation of the fluorescence intensity peaks emitted from our synthesized nanocomposite The variation variation of of thefluorescence fluorescenceintensity intensitypeaks peaksemitted emittedfrom from our synthesized nanocomposite The synthesized at at different peroxidethe weight concentrations is shown in Figureour 6. This proves nanocomposite the fluorescence at different peroxide weight concentrations is shown in Figure 6. This proves the fluorescence different peroxide weight concentrations is shown in Figure 6. This proves the fluorescence quenching quenching mechanism is due to the radical quenching sensed by the active ceria NPs embedded in quenching mechanism due toquenching the radical quenching by the active ceria NPs embedded in mechanism is due to theisradical sensed byfluorescence thesensed active ceria NPs embedded in the the crosslinked nanofibers. The relative change of intensity as a function ofcrosslinked the molar the crosslinked nanofibers. The relative change of fluorescence intensity as a function of the molar concentration of peroxide in water solution is presented in Figure 7. concentration of peroxide in water solution is presented in Figure 7.
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nanofibers. The relative change of fluorescence intensity as a function of the molar concentration of of 88 66 of solution is presented in Figure 7.
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Figure 6. 6. Fluorescence peak degradation degradation with with increased increased peroxide peroxide weight weight concentrations concentrations Figure Fluorescence intensity intensity peak (normalized to to the the fluorescence fluorescence intensity intensity peak peak of of the the synthesized synthesized nanocomposite nanocomposite in in the the presence presence (normalized of peroxide). peroxide). of peroxide).
Figure 7. 7. Relative Relative intensity intensity change, change, compared compared to to the the fluorescence fluorescence intensity intensity peak peak of of nanocomposite nanocomposite in in Figure absence of of peroxide, peroxide, versus versus the the variable molar concentrations of peroxide. peroxide. absence versus thevariable variablemolar molar concentrations concentrations of
4. 4. Discussion Discussion The The bandgap bandgap values values found found from from Figure Figure 22 shows shows evidence evidence for for the the fluorescence fluorescence spectra spectra shown shown in in Figure 3, which proves the formation of a visible emission peak at 520 nm under near-UV excitation Figure 3, which proves the formation of a visible emission peak at 520 nm under near-UV excitation 3+ ions in Ce22O33, which results in a which confirms confirms the the relaxation relaxation via viathe the5d-4f 5d-4ftransition transitionof ofexcited excitedCe Ce3+3+ which ions in Ce2 O3 , which results in a 3+ 3+ 3+ photon emission [17]. Therefore, the higher concentration of Ce states in non-stoichiometric non-stoichiometric CeO CeOxxx photon emission [17]. Therefore, the higher concentration of Ce states in along with an increase in the associated O-vacancies results in a stronger fluorescence emission along with an increase in the associated O-vacancies results in a stronger fluorescence emission [16]. [16]. The ceria NPs in the hosthost material, but but the The authors authors tried triedhigher higherweight weightconcentrations concentrationsofofembedded embedded ceria NPs in the material, fluorescence intensity peak becomes lower, because the static quenching of higher nanoparticle the fluorescence intensity peak becomes lower, because the static quenching of higher nanoparticle concentrations becomesdominant. dominant.In Inaddition, addition,the thehigher higherconcentrations concentrationsofof ceria nanoparticles lead concentrations becomes ceria nanoparticles lead to to some droplets in the nanofiber due to lower viscosity. That us leads us to conclude that 1 wt be % some droplets in the nanofiber mat mat due to lower viscosity. That leads to conclude that 1 wt % may may be the optimum for ourstudied current nanocomposite studied nanocomposite and its application. targeted application. Inspectrum the FTIR the optimum for our current and its targeted In the FTIR spectrum shown5,inmost Figure 5, most of the original peaks both PVAacid, andincluding malic acid, including OH shown in Figure of the original peaks of both PVAofand malic OH alcohol, free alcohol, free hydroxyl, C-H alkane, C-O carboxylic acid, ester and C=O carboxylic bonds, are not hydroxyl, C-H alkane, C-O carboxylic acid, ester and C=O carboxylic bonds, are not affected through affected through embedding ceria NPs. Supported by Figure 4b, the FTIR analysis is important to embedding ceria NPs. Supported by Figure 4b, the FTIR analysis is important to show that ceria show that ceria nanoparticles with theirvacancies corresponding vacancies have higher possibility to be nanoparticles with their corresponding may have highermay possibility to be on the surface of on the surface of the nanofibers with better sensitivity to adsorb and sense the radicals in the the nanofibers with better sensitivity to adsorb and sense the radicals in the aqueous medium. aqueous medium. Figure 6 shows the reduction of the fluorescence intensity emitted from our synthesized nanocomposite more than 30% with increase of radicals or peroxide weight concentration up to 27%. The relation between the amplitude of the fluorescence signal and the quencher concentration; the
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Figure 6 shows the reduction of the fluorescence intensity emitted from our synthesized nanocomposite more than 30% with increase of radicals or peroxide weight concentration up to 27%. The relation between the amplitude of the fluorescence signal and the quencher concentration; the peroxide or the corresponding radicals, is clarified in Figure 7. The linear part is described by the Stern-Volmer equation, as follows [22]: Io = 1 + KSV [ Q] I
(2)
where Io and I represent the peak intensities of the steady-state fluorescence in the absence and presence, respectively, of the peroxide quencher, KSV is the Stern-Volmer quenching constant which is an indication for the sensitivity of the nanocomposite to sense the radicals [7], and [Q] is the quencher concentration. The results of the analysis of the fluorescence data as functions of radicals concentration are shown in Figure 7. The KSV value of the synthesized nanoparticles is calculated from the slope of the fitted line, and found to be 0.037 M−1 . 5. Conclusions This work introduces a novel nanocomposite of ceria NPs embedded in crosslinked PVA electrospun nanofibersas an optical sensing mat for peroxide and the corresponding radicals. Our synthesized nanocomposite proves the activity of the embedded ceria NPs to have some Ce3+ ionization states and the associated charged O-vacancies to be receptors for radical adsorption. In addition, the synthesized nanocomposite is found to be fluorescent with visible emission under near-UV excitation. The visible fluorescence intensity peak is found to be reduced with increasing peroxide concentration through a fluorescence quenching mechanism. Through optical fluorescence, the intensity could be reduced clearly up to 30% of its original value in the absence of radicals with peroxide concentrations up to 27 wt %. This study could be extensively helpful in further applications in environmental monitoring and cancer detection. Acknowledgments: The authors appreciate the partial financial support of Smart Critical Infrastructure (SmartCI) in Alexandria University for supplying some facilities used in the prepared fluorescence spectroscopy. Also, the authors would like to thank both Nazly Hassan from Scientific Research City in Borg Al-Arab, Alexandria, Egypt and Ibrahim Hassounah from the USTAR Bioinnovations center at Utah State University for their help in FTIR measurements and analysis. Alexandria University is responsible for the open access fees payment through the internal rules for supporting the affiliated academic staff through publishing open access journal papers. Author Contributions: Nader Shehata is the main supervisor of the research group with giving the proper guidance in the nanocomposite formation procedure and fluorescence experiment. Effat Samir is the main person responsible for optical characterizations including both absorbance and fluorescence spectroscopy. Soha Gaballah is responsible for TEM, SEM, and FTIR analysis, in addition to nanoparticles synthesis. Aya Hamed is the contact person for the electrospinning process. Asmaa Elrasheedy helped in the crosslinking process via esterification. Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations The following abbreviations are used in this manuscript: Ceria NPs PVA SEM TEM
Cerium oxide nanoparticles Polyvinyl alcohol Scanning Electron Microscope Transmission Electron Microscope
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Hoigné, J. Inter-calibration of OH radical sources and water quality parameters. Water Sci. Technol. 1997, 35, 1–8. [CrossRef] Middleburgh, S.; Lagerlof, K.; Grimes, R. Accommodation of Excess Oxygen in Group II Monoxides. J. Am. Ceram. Soc. 2013, 96, 308–311. [CrossRef] Griffiths, J.; Robinson, S. The Oxy Lite: A fibre-optic oxygen sensor. J. Radiol. 1999, 72, 627–630. [CrossRef] [PubMed] Braun, R.; Lanzen, J.; Snyder, S.; Dewhirst, M. Comparison of tumor and normal tissue oxygen tension measurements using OxyLite or microelectrodes in rodents. Am. J. Physiol. Heart Circ. Physiol. 2001, 280, H2533–H2544. [PubMed] Shehata, N.; Meehan, K.; Leber, D. Fluorescence quenching in ceria nanoparticles: A dissolved oxygen molecular probe with a relatively temperature insensitive Stern-Volmer constant up to 50 ◦ C. J. Nanophotonics 2012, 6. [CrossRef] Feng, N.; Xie, J.; Zhang, D. Synthesis, characterization, photophysical and oxygen-sensing properties of a novel europium(III) complex. Spectrochim. Acta A 2010, 77, 292–296. [CrossRef] [PubMed] Shehata, N.; Meehan, K.; Hassounah, I.; Hudait, M.; Jain, N.; Clavel, M.; Elhelw, S.; Madi, N. Reduced erbium-doped ceria nanoparticles: One nano-host applicable for simultaneous optical down- and up-conversions. Nanoscale Res. Lett. 2014, 9. [CrossRef] [PubMed] Borisov, S.; Klimant, I. New luminescent oxygen-sensing and temperature-sensing materials based on gadolinium(III) and europium(III) complexes embedded in an acridone/polystyrene conjugate. Anal. Bioanal. Chem. 2012, 404, 2797–2806. [CrossRef] [PubMed] Quaranta, M.; Borisov, S.; Klimant, I. Indicators for optical oxygen sensors. Bioanal. Rev. 2012, 4, 115–157. [CrossRef] [PubMed] Matsumura, S.; Shimura, Y.; Terayama, K.; Kiyohara, T. Effects of Molecular Weight and Stereoregularity on Biodegradation of Poly(Vinyl Alcohol) by Alcaligenes Faecalis. Biotechnol. Lett. 1994, 16, 1205–1210. [CrossRef] Mori, T.; Sakimoto, M.; Kagi, T.; Sakai, T. Isolation and Characterization of a Strain of Bacillus Megaterium that Degrades Poly(vinyl alcohol). Biosci. Biotechnol. Biochem. 1996, 60, 330–332. [CrossRef] [PubMed] Larrondo, L.; St. John Manley, R. Electrostatic Fiber Spinning from Polymer Melts. I. Experimental Observations on Fiber Formation and Properties. J. Polym. Sci.: Polym. Phys. Ed. 1981, 19, 909–920. [CrossRef] Shin, Y.M.; Hohman, M.M.; Brenner, M.P.; Rutledge, G.C. Electrospinning: A Whipping Fluid Jet Generates Submicron Polymer Fibers. Appl. Phys. Lett. 2001, 78, 1–3. [CrossRef] Shehata, N.; Meehan, K.; Hudait, M.; Jain, N. Control of oxygen vacancies and Ce3+ concentrations in doped ceria nanoparticles via the selection of lanthanide. J. Nanoparticle Res. 2012, 14, 1173–1183. [CrossRef] Shehata, N.; Meehan, K.; Hudait, M.; Jain, N.; Gaballah, S. Studyof Optical and Structural Characteristicsof Ceria Nanoparticles Dopedwith Negative and Positive Association Lanthanide Elements. J. Nanomater. 2014, 2014, 1–7. [CrossRef] Chen, H.; Chang, H. Homogeneous precipitation of cerium dioxide nanoparticles in alcohol/water mixed solvents. Coll. Surf. A 2014, 242, 61–69. [CrossRef] Hassounah, I.; Shehata, N.; Hudson, A.; Orler, B.; Meehan, K. Characteristics and 3D formation of PVA and PEO electrospun nanofibers with embedded urea. J. Appl. Polym. Sci. 2014, 131. [CrossRef] Pankove, J. Optical Processes in Semiconductors, 1st ed.; Dover Publications Inc.: New York, NY, USA, 1971. Goharshadi, E.K.; Samiee, S.; Nancarrow, P. Fabrication of cerium oxide nanoparticles: Characterization and optical properties. J. ColloidInterface Sci. 2011, 356, 472–480. [CrossRef] [PubMed] Chu, C.; Lo, Y. A plastic optical fiber sensor for the dual sensing of temperature and oxygen. IEEE Photonics Tech. Lett. 2008, 20, 63–65. [CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
Embedded Ceria Nanoparticles in Crosslinked PVA Electrospun Nanofibers as Optical Sensors for Radicals Nader Shehata 1,2,3, *, Effat Samir 2,4 , Soha Gaballah 2,5 , Aya Hamed 1,2 and Asmaa Elrasheedy 2,5 1 2
3 4 5
*
Department of Engineering Mathematics and Physics, Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt; [email protected] Center of Smart Nanotechnology and Photonics (CSNP), Smart CI Research Center, Alexandria University, Alexandria 21544, Egypt; [email protected] (E.S.); [email protected] (S.G.); [email protected] (A.E.) USTAR Bioinnovations Center, Utah State University, Logan, UT 84341, USA Department of Electrical Engineering, Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt Department of Chemical Engineering, Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt Correspondence: [email protected]; Tel.: +20-109-116-5300
Academic Editor: Ki-Hyun Kim Received: 24 February 2016; Accepted: 11 April 2016; Published: 26 August 2016
Abstract: This work presents a new nanocomposite of cerium oxide (ceria) nanoparticles embedded in electrospun PVA nanofibers for optical sensing of radicals in solutions. Our ceria nanoparticles are synthesized to have O-vacancies which are the receptors for the radicals extracted from peroxide in water solution. Ceria nanoparticles are embedded insitu in PVA solution and then formed as nanofibers using an electrospinning technique. The formed nanocomposite emits visible fluorescent emissions under 430 nm excitation, due to the active ceria nanoparticles with fluorescent Ce3+ ionization states. When the formed nanocomposite is in contact with peroxide solution, the fluorescence emission intensity peak has been found to be reduced with increasing concentration of peroxide or the corresponding radicals through a fluorescence quenching mechanism. The fluorescence intensity peak is found to be reduced to more than 30% of its original value at a peroxide weight concentration up to 27%. This work could be helpful in further applications of radicals sensing using a solid mat through biomedical and environmental monitoring applications. Keywords: ceria nanoparticles; electrospinning; crosslinking; fluorescence quenching; radicals
1. Introduction Radical sensing is important in different applications including cancer treatment [1,2] and water quality monitoring [3]. One source of radicals in experiments is peroxides, due to the unstable oxygen-oxygen chemical bond and consequently the creation of active radicals [4]. Regarding the techniques for sensing radicals, the optical fluorescence-based sensing mechanism is better than other electrochemical ones as it does not require a reference electrode and it is immune to exterior electromagnetic field interference [5–7]. Cerium oxide nanoparticles (ceria NPs) can be considered one of the most promising nanostructures which can capture the radicals depending on their formed O-vacancies associated to the active trivalent ionization state of cerium [8–11]. In this novel work, we aimed to use ceria nanoparticles as an optical sensor for peroxides or the corresponding radicals. However, our ceria NPs would be in the form of a solid host instead of being colloidal solutions so that they could be flexibly used in a wide variety of applications. Therefore, our synthesized nanoparticles
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are embedded in-situ with polyvinyl alcohol (PVA) nanofibers formed by an electrospinning technique. Then, the formed nanocomposite is used as an optical sensing mat for peroxide or radicals in solution. PVA was selected as a non-toxic biodegradable water soluble polymer [12,13]. Regarding the formation of PVA nanofibers, an electrospinning technique is selected as the fabrication method for the proposed nanocomposite because of the simplicity of operation, the feasibility to embed ceria NPs in the resulting PVA nanofibers, and the potential for scale-up to manufacture large volumes [14,15]. In electrospinning, a high strength electric field is applied between a metallic needle and a metallic target. The strength of the electric field forces the polymer droplets at the needle tip to stretch and to be deposited into fibers on the surface of the target. To make the nanofibers (NFs) suitable for sensing radicals in solution, the formed nanocomposite has been crosslinked via a chemical esterification method, therefore, the produced nanofibers (NFs) can resist solubilization and became a hydrophobic material. Different characterizations of the formed nanocomposite are presented in this work, including absorbance dispersion, bandgap calculation, fluorescence spectroscopy, SEM, and FTIR spectroscopy. The studied optical characterizations prove that the synthesized nanocomposite has some trivalent cerium ions which can be active receptors for radicals adsorption through the associated formed O-vacancies [16]. Then, the fluorescence emissions of the formed nanocomposite in the presence of different concentrations of peroxide in water solution are studied with calculating the Stern-Volmer constant in the linear region of the relative intensity change graph. The visible fluorescence emission is obtained under near-UV excitation in a home-made fluorescence spectroscopy setup. The fluorescence intensity is found to be reduced with increasing radical concentration due to a fluorescence quenching mechanism. 2. Materials and Methods 2.1. Nanoparticle and Polymer Synthesis Ceria nanoparticles were synthesized using a chemical precipitation technique, similar to that described in [17,18], as it is a relatively simple procedure using low cost precursors. During ceria preparation, cerium (III) chloride heptahydrate (0.5 g, 99.9%, Sigma-Aldrich, St. Louis, MO, USA) is added to deionized (DI) water (40 mL), then ammonia (1.6 mL) is added as a catalyst. The solution is stirred over a magnetic stirrer for 24 h in an open container at 500 rpm. During the first 2 h of the stirring, the container is held in 50 ◦ C water bath and then the solution is allowed to cool to room temperature for the remainder of the stirring period. The long duration of the stirring helps in fracturing nanorods into nanoparticles. The solution is then centrifuged and washed twice with both deionized water and ethanol to remove any unreacted cerium and ammonia. PVA solution is prepared by mixing PVA pellets(10 g, Mw = 205,000 g/mol, Sigma Aldrich, St. Louis, MO, USA) with distilled water to make 100 mL of solution. The solution is heated to 100 ◦ C for 30 min then stirred overnight. Different weight percentages of ceria NPs are added in-situ to the PVA solution. The mixture is stirred for 30 min before it is used in the electrospinning process. 2.2. Electrospinning and Crosslinking The electrospinning setup consists of high voltage power supply (model CZE1000R, Spellman High Voltage Electronics Corporation, Hauppauge, New York, NY, USA), a syringe pump (NE1000-Single Syringe Pump, New Era, Farmingdale, New York, NY, USA) which is used to regulate the feed rate of polymer solution, a 5 mL plastic syringe with an 18 gauge metallic needle to hold the polymer solution, and a circular metallic collector of radius 10 cm covered with aluminum foil is used as a target. The voltage power supply is connected to the needle while the collector is grounded. The distance between the needle tip and the collector is fixed at 15 cm. The voltage difference between the needle and target is 25 kV. The flow rate of the polymer solution is fixed at 2 mL/h. The running time of electrospinning process per sample is about 30 min. A high voltage is applied to the syringe’s needle; the droplet of polymer solution takes the shape of a cone known as Taylor cone [19]. Then,
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Taylor cone [19]. Then, a jet is erupted from the cone and is accelerated towards the collector. As the jet travels through the air, the solvent evaporates leaving behind fibers to be collected randomly on a jetgrounded is eruptedtarget. from the and is accelerated towards the collector. the jet travels through the the Forcone crosslinking of the formed nanofibers, VaporAs phase esterification process air, the solvent evaporates leaving behind fibers to be collected randomly on the grounded target. was done in the oven on two subsequent steps. In the first step, electrospun nanofibers of PVA or For crosslinking the formed Vapor phase esterification process was done theg) oven PVA-doped ceriaofwere placed nanofibers, in a container along with a small amount of malic acidin (1–2 andon a two subsequent steps. In the first step, electrospun nanofibers of PVA or PVA-doped ceria were placed few drops of HCl were added to the malic acid. The container was sealed from the ambient moisture in a container withata 80 small of malic acid (1–2 g)occurred and a few of HCl were added to and placed inalong an oven °C amount for 15 min. Esterification viadrops a heterogeneous reaction ◦ the malic container from the moisture placed during 15acid. min. The In the second was step,sealed the sample wasambient cured for 20 minand in an oven in at an 120oven °C. at 80 C for 15 min. Esterification occurred via a heterogeneous reaction during 15 min. In the second step, the sample was cured Characterizations for 20 min in an oven at 120 ◦ C. 2.3. Nanocomposite PVA nanofibers with embedded ceria NPs are optically characterized by measuring theoptical 2.3. Nanocomposite Characterizations absorbance, and fluorescence intensity curves. Optical absorbance in the wavelength range from 300 PVA nanofibers with embedded ceria NPs are optically characterized by measuring theoptical to 700 nm was measured by using a T92+UV-Vis spectrophotometer (PG instruments, Beijing, absorbance, and fluorescence intensity curves. Optical absorbance in the wavelength range from 300 China). From the absorbance curves, the corresponding NFs band gap can be determined, as to 700 nm was measured by using a T92+UV-Vis spectrophotometer (PG instruments, Beijing, China). explained in the Results and Discussion sections. The mean synthesized nanoparticle size was From the absorbance curves, the corresponding NFs band gap can be determined, as explained in determination of observed by JEM-2100F transmission electron microscopy (JEOL, Tokyo, Japan) the Results and Discussion sections. The mean synthesized nanoparticle size was determination of with an accelerating potential of 80KV. Surface morphology and diameter size of electrospun observed by JEM-2100F transmission electron microscopy (JEOL, Tokyo, Japan) with an accelerating nanofibers before and after esterification were investigated using scanning electron microscopy potential of 80 KV. Surface morphology and diameter size of electrospun nanofibers before and after (Quanta 200, FEI, Hillsboro, OR, USA). After sputter-coating with gold, the fiber size distribution of esterification were investigated using scanning electron microscopy (Quanta 200, FEI, Hillsboro, randomly selected SEM micrographs was measured using the Image-J software. The formed OR, USA). After sputter-coating with gold, the fiber size distribution of randomly selected SEM nanocomposites have been characterized using IR tracer-100 FTIR spectroscopy (Shimadzu, Kyoto, micrographs was measured using the Image-J software. The formed nanocomposites have been Japan). characterized using IR tracer-100 FTIR spectroscopy (Shimadzu, Kyoto, Japan). 2.4. Fluorescence Quenching Experiment 2.4. Fluorescence Quenching Experiment Fluorescence intensity measurements are done by a home-built fluorescence spectroscopy Fluorescence intensity measurements are done by a home-built fluorescence spectroscopy setup, setup, shown in Figure 1. The fluorescence setup is composed of an ultraviolet (UV) LED with 430 shown in Figure 1. The fluorescence setup is composed of an ultraviolet (UV) LED with 430 nm nm excitation wavelength, a Cornerstone 130 Monochromator (Newport, Irvine, CA, USA); which is excitation wavelength, a Cornerstone 130 Monochromator (Newport, Irvine, CA, USA); which is set to set to obtain the fluorescence intensity at wavelengths from 500 to 700 nm, an Oriel photomultiplier obtain the fluorescence intensity at wavelengths from 500 to 700 nm, an Oriel photomultiplier tube tube (Newport PMT77340) as a fluorescence intensity detector, and a Newport 1918-R power meter (Newport PMT77340) as a fluorescence intensity detector, and a Newport 1918-R power meter to to display PMT detection readings. Fluorescence intensity is measured by positioning the NFs solid display PMT detection readings. Fluorescence intensity is measured by positioning the NFs solid sample holder at a 45◦ between the UV-LED and the input port of the monochromator. The output sample holder at a 45 between the UV-LED and the input port of the monochromator. The output port of the monochromator was directly connected with the PMT, which was directly connected to port of the monochromator was directly connected with the PMT, which was directly connected to the the 1918-R power meter. 1918-R power meter.
Figure 1. Fluorescenceintensity intensityhome-made home-madespectroscopy spectroscopy setup. setup. Figure 1.Fluorescence
To test the the ability ability of of our our synthesized synthesized nanocomposite To test nanocomposite to to attract attract free free radicals, radicals, hydrogen hydrogen peroxide peroxide solution (35 wt %, Sigma-Aldrich) samples with different volume percent areused. The solution (35 wt %, Sigma-Aldrich) samples with different volume percent areused. The samplesample holder holder with the synthesized nanocomposite mat, aofpiece of × 0.51.5 cmcm, × 1.5 cm, is inserted a 100 mL with the synthesized nanocomposite mat, a piece 0.5 cm is inserted in a 100inmL beaker beaker which contains the radicals at different weight concentrations. At each peroxide weight which contains the radicals at different weight concentrations. At each peroxide weight concentration, concentration, theintensity fluorescence peakthe emitted from nanocomposite the contacted nanocomposite the fluorescence peak intensity emitted from contacted with error barswith has error been bars has been detected and by the power meter. detected and monitored by monitored the power meter.
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3. Results 3. Results 3. Results 3.1. Nanocomposite Charcterization 3.1. Nanocomposite Charcterization The absorbance dispersion curves of our synthesized nanocomposite PVA NFs with embedded The absorbance dispersion curves of our synthesized nanocomposite PVA NFs with embedded ceria NPs with some weight concentrations are shown in Figure 2a. The corresponding direct NPswith withsome some weight concentrations are shown in Figure The corresponding direct ceria NPs weight concentrations are shown in Figure 2a. The2a. corresponding direct bandgap bandgap can be calculated from the absorbance dispersion curve through Equation (1) [20]: bandgap can be calculated from the absorbance curveEquation through (1) Equation can be calculated from the absorbance dispersiondispersion curve through [20]: (1) [20]:
( E) A(E Eg )11 22 ( E) A(E E ) α( E) = A( E − Eg )g1/2
(1) (1) (1) where α is the absorbance, A is a material-dependent constant which includes both the effective where α is the absorbance, A is a material-dependent constant which includes both the effective massesαofiselectrons and holesAand constant of the nanocomposite, E is theboth absorbed photon where the absorbance, is adielectric material-dependent constant which includes the effective masses of electrons and holes and dielectric constant of the nanocomposite, E is the absorbed photon energy (E hc/λ) as and λ is the optical absorbedconstant wavelength, and Eg is the allowed bandgap of the masses of =electrons holes and dielectric of the nanocomposite, E isdirect the absorbed photon energy (E = hc/λ) as λ is the optical absorbed wavelength, and Eg is the allowed direct bandgap of the 2 and E is presented in Figure 2b. The synthesized nanocomposite. Then, the relation between (αE) energy (E = hc/λ) as λ is the optical absorbed wavelength, and 2Eg is the allowed direct bandgap of the synthesized nanocomposite. Then, the relation between2 (αE) 2 and E is presented in Figure 2b. The intersection of the extrapolation of the partbetween of (αE) (αE) curve and withEE-axis is equalin toFigure allowed synthesized nanocomposite. Then, thelinear relation is presented 2b.direct The intersection of the extrapolation of the linear part of (αE)22 curve with E-axis is equal to allowed direct bandgap Eg,ofthat particular composition ceria NPs with concentration of tri-valent intersection the extrapolation of the linearofpart of (αE) curvemoderate with E-axis is equal to allowed direct bandgap Eg, that particular composition of ceria NPs with moderate concentration of tri-valent cerium ionization states with associated O-vacancies, is within accepted range biased to bandgap Eg, that particular composition of ceria NPs withwhich moderate concentration of tri-valent cerium cerium ionization states with associated O-vacancies, which is within accepted range biased to 3 eV [16,21]. ionization states with associated O-vacancies, which is within accepted range biased to 3 eV [16,21]. 3 eV [16,21].
Figure 2. PVA NFs with in-situ in-situ embedded ceria ceria NPs. (a) (a) Absorbance curve; curve; (b) Band Band gap curve. curve. Figure 2. 2. PVA NFs Figure PVA NFs with with in-situ embedded embedded ceria NPs. NPs. (a) Absorbance Absorbance curve; (b) (b) Band gap gap curve.
Visible fluorescence intensity spectra, under 430 nm optical excitation, are shown in Figure 3 for fluorescence intensity spectra, under 430 nm optical excitation, are shown in Figure 3 for Visible fluorescence different concentrations of ceria NPs which are embedded in-situ in crosslinked PVA NFs. concentrationsofof ceria which are embedded in crosslinked NFs. different concentrations ceria NPsNPs which are embedded in-situ in-situ in crosslinked PVA NFs.PVA Generally, Generally, our synthesized nanocomposite mat displays the same optical activity properties as ceria Generally, our synthesized nanocomposite the same optical activity properties as ceria our synthesized nanocomposite mat displaysmat the displays same optical activity properties as ceria nanoparticles nanoparticles in a biodegradable solid and flexible host of PVA nanofibers for a wide variety of nanoparticles in a solid biodegradable flexible host of nanofibers a wide variety of in a biodegradable and flexiblesolid host and of PVA nanofibers forPVA a wide variety offor possible applications. possible applications. possible applications.
Figure 3. Fluorescence intensity PVA NFs with in-situ in-situ embedded different different concentrations ceria ceria NPs. Figure 3. 3. Fluorescence Fluorescence intensity intensity PVA PVA NFs Figure NFs with with in-situ embedded embedded different concentrations concentrations ceria NPs. NPs.
Figure 4a shows both TEM of ceria NPs with mean diameter less than 10 nm. SEM images of Figure 4a NPs with mean diameter lessless thanthan 10 nm. SEMSEM images of both 4a shows showsboth bothTEM TEMofofceria ceria NPs with mean diameter 10 nm. images of both PVA nanofibers with embedded ceria NPs before and after crosslinking are shown in both PVA with embedded ceria NPs before after and crosslinking are shown are in both Figure 4b,c, both nanofibers PVA nanofibers with embedded ceria NPsand before after crosslinking shown in both Figure 4b,c, with a focused image of some agglomerated nanoparticles inside the fiber shown in Figure 4b,c, with a focused image of some agglomerated nanoparticles inside the fiber shown in Figure 4d. The synthesized nanofibers, in both cases, have mean diameters around 130 nm but with Figure 4d. The synthesized nanofibers, in both cases, have mean diameters around 130 nm but with
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with focused image of some agglomerated nanoparticles inside the fiber shown in Figure 4d. 5The Sensorsa2016, 16, 1371 of 8 Sensors 2016, 16, 1371 5 of 8 synthesized nanofibers, in both cases, have mean diameters around 130 nm but with higher numbers of formed beads of in formed the crosslinked Some ceria NPs expected to beare located on the surface of higher numbers beads incase. the crosslinked case.are Some ceria NPs expected to be located higher numbers of formed beads in the crosslinked case. Some ceria NPs are expected to be located the electrospun especially before the crosslinking effect. In addition, aeffect. macroscopic photoa on the surface ofnanofibers, the electrospun nanofibers, especially before the crosslinking In addition, on the surface of the electrospun nanofibers, especially before the crosslinking effect. In addition, a of the formedphoto crosslinked is shown innanofibers Figure 4e to that formed nanofibers matthe is macroscopic of thenanofibers formed crosslinked is show shown inthe Figure 4e to show that macroscopic photo of the formed crosslinked nanofibers is shown in Figure 4e to show that the formed nanofibers is uniform with onthe theFTIR sides. Figure 5 shows theof FTIR uniform with some mat minor defects on the some sides.minor Figuredefects 5 shows spectroscopy pattern the formed nanofibers mat is uniform with some minor defects on the sides. Figure 5 shows the FTIR spectroscopy pattern ofwith the crosslinked nanofibers crosslinked nanofibers embedded ceria NPs. with embedded ceria NPs. spectroscopy pattern of the crosslinked nanofibers with embedded ceria NPs.
(a) (a)
(c) (c)
(b) (b)
(d) (d)
(e) (e)
Figure 4. (a) TEM image of ceria NPs; (b) SEM image of our synthesized nanocomposite of PVA Figure (a) TEM TEM image image of of ceria ceria NPs; NPs; (b) (b) SEM SEM image image of of our our synthesized synthesized nanocomposite nanocomposite of of PVA PVA Figure 4. 4. (a) nanofibers embedded embedded with with ceria ceria NPs; NPs; (c) SEM image of the nanocomposite after crosslinking; nanofibers (c) SEM image of the nanocomposite after crosslinking; nanofibers embedded with ceria NPs; (c) SEM image of the nanocomposite after crosslinking; (d) STEM image of some agglomerated nanoparticles inside the fiber; and (e) macroscopic photo of (d) image of of some someagglomerated agglomeratednanoparticles nanoparticlesinside insidethe thefiber; fiber;and and macroscopic photo (d) STEM STEM image (e)(e) macroscopic photo of the synthesized electrospun nanofibers. of synthesized electrospun nanofibers. thethe synthesized electrospun nanofibers.
Figure 5. FTIR spectroscopy pattern of crosslinked PVA with embedded ceria NPs. Figure embedded ceria ceria NPs. NPs. Figure 5. FTIR FTIR spectroscopy spectroscopy pattern of crosslinked PVA with embedded
3.2. Optical Sensing of Peroxide Using Fluorescence Quenching Mechanism 3.2. Optical Using Fluorescence Fluorescence Quenching Quenching Mechanism Mechanism 3.2. Optical Sensing Sensing of of Peroxide Peroxide Using The variation of the fluorescence intensity peaks emitted from our synthesized nanocomposite The variation variation of of thefluorescence fluorescenceintensity intensitypeaks peaksemitted emittedfrom from our synthesized nanocomposite The synthesized at at different peroxidethe weight concentrations is shown in Figureour 6. This proves nanocomposite the fluorescence at different peroxide weight concentrations is shown in Figure 6. This proves the fluorescence different peroxide weight concentrations is shown in Figure 6. This proves the fluorescence quenching quenching mechanism is due to the radical quenching sensed by the active ceria NPs embedded in quenching mechanism due toquenching the radical quenching by the active ceria NPs embedded in mechanism is due to theisradical sensed byfluorescence thesensed active ceria NPs embedded in the the crosslinked nanofibers. The relative change of intensity as a function ofcrosslinked the molar the crosslinked nanofibers. The relative change of fluorescence intensity as a function of the molar concentration of peroxide in water solution is presented in Figure 7. concentration of peroxide in water solution is presented in Figure 7.
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nanofibers. The relative change of fluorescence intensity as a function of the molar concentration of of 88 66 of solution is presented in Figure 7.
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Figure 6. 6. Fluorescence peak degradation degradation with with increased increased peroxide peroxide weight weight concentrations concentrations Figure Fluorescence intensity intensity peak (normalized to to the the fluorescence fluorescence intensity intensity peak peak of of the the synthesized synthesized nanocomposite nanocomposite in in the the presence presence (normalized of peroxide). peroxide). of peroxide).
Figure 7. 7. Relative Relative intensity intensity change, change, compared compared to to the the fluorescence fluorescence intensity intensity peak peak of of nanocomposite nanocomposite in in Figure absence of of peroxide, peroxide, versus versus the the variable molar concentrations of peroxide. peroxide. absence versus thevariable variablemolar molar concentrations concentrations of
4. 4. Discussion Discussion The The bandgap bandgap values values found found from from Figure Figure 22 shows shows evidence evidence for for the the fluorescence fluorescence spectra spectra shown shown in in Figure 3, which proves the formation of a visible emission peak at 520 nm under near-UV excitation Figure 3, which proves the formation of a visible emission peak at 520 nm under near-UV excitation 3+ ions in Ce22O33, which results in a which confirms confirms the the relaxation relaxation via viathe the5d-4f 5d-4ftransition transitionof ofexcited excitedCe Ce3+3+ which ions in Ce2 O3 , which results in a 3+ 3+ 3+ photon emission [17]. Therefore, the higher concentration of Ce states in non-stoichiometric non-stoichiometric CeO CeOxxx photon emission [17]. Therefore, the higher concentration of Ce states in along with an increase in the associated O-vacancies results in a stronger fluorescence emission along with an increase in the associated O-vacancies results in a stronger fluorescence emission [16]. [16]. The ceria NPs in the hosthost material, but but the The authors authors tried triedhigher higherweight weightconcentrations concentrationsofofembedded embedded ceria NPs in the material, fluorescence intensity peak becomes lower, because the static quenching of higher nanoparticle the fluorescence intensity peak becomes lower, because the static quenching of higher nanoparticle concentrations becomesdominant. dominant.In Inaddition, addition,the thehigher higherconcentrations concentrationsofof ceria nanoparticles lead concentrations becomes ceria nanoparticles lead to to some droplets in the nanofiber due to lower viscosity. That us leads us to conclude that 1 wt be % some droplets in the nanofiber mat mat due to lower viscosity. That leads to conclude that 1 wt % may may be the optimum for ourstudied current nanocomposite studied nanocomposite and its application. targeted application. Inspectrum the FTIR the optimum for our current and its targeted In the FTIR spectrum shown5,inmost Figure 5, most of the original peaks both PVAacid, andincluding malic acid, including OH shown in Figure of the original peaks of both PVAofand malic OH alcohol, free alcohol, free hydroxyl, C-H alkane, C-O carboxylic acid, ester and C=O carboxylic bonds, are not hydroxyl, C-H alkane, C-O carboxylic acid, ester and C=O carboxylic bonds, are not affected through affected through embedding ceria NPs. Supported by Figure 4b, the FTIR analysis is important to embedding ceria NPs. Supported by Figure 4b, the FTIR analysis is important to show that ceria show that ceria nanoparticles with theirvacancies corresponding vacancies have higher possibility to be nanoparticles with their corresponding may have highermay possibility to be on the surface of on the surface of the nanofibers with better sensitivity to adsorb and sense the radicals in the the nanofibers with better sensitivity to adsorb and sense the radicals in the aqueous medium. aqueous medium. Figure 6 shows the reduction of the fluorescence intensity emitted from our synthesized nanocomposite more than 30% with increase of radicals or peroxide weight concentration up to 27%. The relation between the amplitude of the fluorescence signal and the quencher concentration; the
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Figure 6 shows the reduction of the fluorescence intensity emitted from our synthesized nanocomposite more than 30% with increase of radicals or peroxide weight concentration up to 27%. The relation between the amplitude of the fluorescence signal and the quencher concentration; the peroxide or the corresponding radicals, is clarified in Figure 7. The linear part is described by the Stern-Volmer equation, as follows [22]: Io = 1 + KSV [ Q] I
(2)
where Io and I represent the peak intensities of the steady-state fluorescence in the absence and presence, respectively, of the peroxide quencher, KSV is the Stern-Volmer quenching constant which is an indication for the sensitivity of the nanocomposite to sense the radicals [7], and [Q] is the quencher concentration. The results of the analysis of the fluorescence data as functions of radicals concentration are shown in Figure 7. The KSV value of the synthesized nanoparticles is calculated from the slope of the fitted line, and found to be 0.037 M−1 . 5. Conclusions This work introduces a novel nanocomposite of ceria NPs embedded in crosslinked PVA electrospun nanofibersas an optical sensing mat for peroxide and the corresponding radicals. Our synthesized nanocomposite proves the activity of the embedded ceria NPs to have some Ce3+ ionization states and the associated charged O-vacancies to be receptors for radical adsorption. In addition, the synthesized nanocomposite is found to be fluorescent with visible emission under near-UV excitation. The visible fluorescence intensity peak is found to be reduced with increasing peroxide concentration through a fluorescence quenching mechanism. Through optical fluorescence, the intensity could be reduced clearly up to 30% of its original value in the absence of radicals with peroxide concentrations up to 27 wt %. This study could be extensively helpful in further applications in environmental monitoring and cancer detection. Acknowledgments: The authors appreciate the partial financial support of Smart Critical Infrastructure (SmartCI) in Alexandria University for supplying some facilities used in the prepared fluorescence spectroscopy. Also, the authors would like to thank both Nazly Hassan from Scientific Research City in Borg Al-Arab, Alexandria, Egypt and Ibrahim Hassounah from the USTAR Bioinnovations center at Utah State University for their help in FTIR measurements and analysis. Alexandria University is responsible for the open access fees payment through the internal rules for supporting the affiliated academic staff through publishing open access journal papers. Author Contributions: Nader Shehata is the main supervisor of the research group with giving the proper guidance in the nanocomposite formation procedure and fluorescence experiment. Effat Samir is the main person responsible for optical characterizations including both absorbance and fluorescence spectroscopy. Soha Gaballah is responsible for TEM, SEM, and FTIR analysis, in addition to nanoparticles synthesis. Aya Hamed is the contact person for the electrospinning process. Asmaa Elrasheedy helped in the crosslinking process via esterification. Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations The following abbreviations are used in this manuscript: Ceria NPs PVA SEM TEM
Cerium oxide nanoparticles Polyvinyl alcohol Scanning Electron Microscope Transmission Electron Microscope
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