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Lanthanide-Organic Gels (LOGs) as a Multifunctional Supramolecular Smart Platform José Yago Rodrigues Silva, Leonis Lourenço da Luz, Filipe Gabriel Martinez Mauricio, Iane Bezerra Vasconcelos Alves, Jamylle Nunes de Souza Ferro, Emiliano Barreto, Ingrid Tavora Weber, Walter Mendes de Azevedo, and Severino Alves Júnior ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017
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ACS Applied Materials & Interfaces
Lanthanide-Organic Gels (LOGs) as a Multifunctional Supramolecular Smart Platform José Yago Rodrigues Silva,a Leonis Lourenço da Luz,b Filipe Gabriel Martinez Mauricio,c Iane Bezerra Vasconcelos Alves,a Jamylle Nunes de Souza Ferro,d Emiliano Barreto,d Ingrid Távora Weber,a,c Walter Mendes de Azevedo,a,b and Severino Alves Júnior* a,b
a
b
Materials Science Program, Federal University of Pernambuco, 50670-901, Recife, PE, Brazil Fundamental Department of Chemistry, Federal University of Pernambuco, 50670-901, Recife, PE, Brazil. c Inorganic and Materials Laboratory, University of Brasília, 70910-000, Asa Norte, Brasília, DF, Brazil. d Laboratory of Cell Biology, Federal University of Alagoas, 57072-970, Maceió, AL, Brazil. * Corresponding author:
[email protected] KEYWORDS: hydrogels, multifunctional material, supramolecular platform, lanthanide-organic gel, smart material ABSTRACT: A multifunctional smart supramolecular platform based on a lanthanide-organic hydrogel is presented. This platform, which provides unique biocompatibility and tunable optical properties, is synthesized by a simple, fast and reproducible ecofriendly microwave-assisted route. Photoluminescent properties enable the production of coated light-emitting diodes (LED), unique luminescent barcodes dependent on the excitation wavelength and thin-films for use in tamper seals. Moreover, piroxicam entrapped in hydrogel acts as a transdermal drug release device efficient in inhibiting edemas compared to a commercial reference.
1. Introduction Photoluminescent multifunctional materials have attracted much attention from the scientific and engineering community as they can be employed in fluorescence imaging1,2, photosensing devices3 and optical devices.4–6 Trivalent lanthanide ions have been increasingly explored for use in multifunctional devices due to their intrinsic photoluminescent properties (4f-4f transitions) such as long lifetimes, defined narrow emission peaks, color purity and high quantum yields.7–9 Among the soft light-emitting materials, multifunctional supramolecular gels are easily tunable hard materials that are able to respond to external stimuli (e.g., light, temperature, pH, pressure) through easy structure modulation. Thus, a supramolecular gel structure combined with luminescent properties can offer a very effective strategy for overcoming problems previously unachievable in soft light emitting diodes (LEDs)10, bioimaging11 or biosensors.12,13 Fluorescent metallohydrogels achieved notoriety in early state, state-of-the-art studies since they presented smart stimuli response, biocompatibility and tunable light emitting in a single material.7,14,15 Recently, we demonstrated a novel class of photochromic hydrogels obtained from lanthanide oxides (Ln2O3) and iminodiacetic acid (H2IDA).16 These eco-friendly Lanthanide-Organic Gels (LOGs) were easily obtained hydrothermally under soft conditions, using only a few reagents at low temperatures without the need for any organic solvent. Even so, they presented remarkable optical properties in the visible region provided by the Eu3+ and Tb3+ ions. Considering potential applications, the combination of the
photoluminescent and structural characteristics of these hydrogels can build a basis for a reliable luminescent multifunctional material. Thus, to further our previous work on these LOGs, in this paper we demonstrate (1) how the luminescent properties of these hydrogels can be easily modulated by different lanthanide ions using a new microwave synthesis method and (2) how these materials work as a multifunctional smart supramolecular platform (MSSP) for different optical and biological applications, namely, a barcode module, a coated-LED, a tamper seal and a drug delivery device. 2. Results and Discussions 2.1. Structural Characterizations and Mechanical Properties The lanthanide hydrogels (G-Ln) were prepared in a twostep hydrothermal microwave-assisted method, as shown in Scheme S1 (supporting information). Our synthesis started with Ln2O3 and H2IDA added to a vial reactor containing water. After microwave heating for 10 minutes, the solution pH was adjusted to 9 and reheated for 10 more minutes, leaving a transparent and fluid hydrogel (Figure S1). For this synthesis, only the metal oxide and organic ligand were used, as well as low temperature and no organic solvents. In addition, this is a rapid synthesis method (20 minutes) using only water as solvent. All features combined to make this synthesis route environmentally friendly.
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Fourier-Transform IR spectra of as-prepared G-Ln indicated that all the compounds in the synthesized series had a similar coordination structure, in agreement with the literature 16,17 (Figure S2 and S3). The X-ray diffraction patterns of the lyophilized hydrogels showed sharp peaks, indicating that the dehydrated gels are crystalline and isostructural with each other (Figure S4). TGA curves of the G-Eu (Figure S5) shows increased swelling water loss below 90°C, corresponding to 97.3% of total weight. After this point, no events can be seen until 150ºC, which indicates that the hydrogels were exhibiting thermal stability. TEM micrographs of dry G-Eu (Figure S6) show interlaced nanofibers. The minimal gelation concentration (MGC) was carried out using the upside-down tube method18, finding 14 mg mL-1 (gelator per solvent) for the G-Eu. Rheological measurements of G-Eu, G-Tb and G-Gd (Figure 1a) indicated a progressive reduction of the solid component below 0.19 rad s-1 occasioned by a polymer crosslink shear. Above this point, an increase in angular frequency (ω) led to the opposite behavior, with regrowth in the solid component and further restoration of the crosslinked polymeric network. Additionally, thixotropy analysis showed a fast loss of viscoelastic solid character (e.g., G’G”) and indicated a fully reversible swelling behavior. Hydrogels show a heavy dependence on physical stimulus that is evidenced when their viscoelastic properties are modified under low stress rates, which characterizes hydrogel as a smart material.19 Above 2.5 Pa, the hydrogel presents a value of tan(δ) = 1 (e.g., G’/G” crossover) (Figure 1c), usually known as the gel-to-sol transition point, indicating a collapse of the gel state and the formation of a quasi-liquid sol state.20 In Figure 1d, the stress rate (τ) dependence of the strain (%) shows an exponential increase, demonstrating that the material exhibits the behavior of a non-Newtonian fluid. Such stimulus dependence has already been reported in the literature and has been used as an external trigger in drug delivery devices.21,22
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2.2. Photoluminescent Behavior 2.2.1. Single-Lanthanide Hydrogels (G-Ln) Photoluminescent measurements data (λexc, observed transitions and CIE coordinates) are compiled in Table S1. By changing Ln3+ ions, we were able to produce different emission patterns in three spectral regions: ultraviolet (UV), visible (VIS) and near-infrared (NIR). Figure 2a shows emission spectra for G-Gd that exhibit the 6P3/2→8S7/2 transition23–25, reported for the first time for a gel material and responsible for the UV emission. In the emission spectra for G-Eu, G-Tb, G-Dy and G-Sm (Figure 2b), very sharp peaks were observed in the VIS region. Remarkably, G-Sm shows near-white light emission (CIE coordinate (0.30; 0.32)), raised from combined 4G5/2→6HJ transitions and a broadband at the blue region (investigation detailed below). The G-Eu and GTb have outstanding optical properties with quantum yields of 32% and 63%, respectively. The main red color of G-Eu is related to its intense peak at 615 nm (5D0→7F2) while the green color of G-Tb is associated with its peak at 544 nm (5D4→7F5). The characteristic cyan color associated with the 4 F9/2→6HJ transition was observed for G-Dy. Figure 2c shows emission spectra in the NIR region for G-Nd, G-Ho and G-Yb, ranging from 982 to 1320 nm. The self-absorption effect was observed in the blue broadband emission of G-Nd and G-Ho with excitation at 370 nm, as shown in Figure S7. All G-Ln samples presented a broad emission band in the blue region, with maximum intensity upon excitation at 360 nm, as exemplify in Figure 2d. To investigate this phenomenon, emission spectra of ligand in solid state and aqueous solution reveals a similar broadband centered at 420 nm when excited at 340 nm (Figure S8). In parallel, SCF calculations (DFT at B3LYP/6-31G level) of the ligand show dislocated HOMO-LUMO densities (Figure S9), which indicates a charge transfer (CT) process. The literature relates that the presence of solvatochromism proof the presence of CT.26–28 Thus, tests were performed adding 2 mL of solvent (Acetone, Water, Isopropanol, Methanol) in 2 mL of G-Lu (Figure S10). Then, a positive solvatochromism in emission spectra of G-Lu were observed, becoming plausible propose that the blue emission in the G-Ln is associated with remaining charge transfer state provoked by the ligand orbitals. Both G-Tb and G-Gd decay curves were fit to a biexponential decay, indicating the existence of two different chemical environments for Tb3+ and Gd3+ ions. Otherwise, GEu exhibits mono-exponential behavior (Figure S11). Different from other rare earth ions, Eu3+ ions have strong radiation deactivation in the presence of O-H stretches, which indicates that at least one chemical site of the gel has water and/or hydroxide in the first coordination sphere leading to a luminescence suppression and a lower quantum yield.29
Figure 1. Rheological analysis of G-Eu, G-Tb and G-Gd measured by (a) plot of storage modulus (G’) and loss modulus (G”) versus oscillation frequency, (b) tan(δ) (G’/G”) versus stress pressure, (c) tension applied versus strain percentage and (d) thixotropy characterization.
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ACS Applied Materials & Interfaces samples are shown in Figures S13 to S19 and their respective lifetimes are compiled in Table S2. 2.3. Platform Applications
Figure 2. Emission and excitation spectra for (a) G-Gd (λex=274 nm) and (b) G-Lu (λex=370 nm). (c) VIS emission spectra for GEu (λex=395 nm), G-Tb (λex=370 nm), G-Dy (λex=365 nm) and GSm (λex=404 nm). (d) NIR emission spectra for G-Yb (λex=370 nm), G-Ho (λex=471 nm) and G-Nd (λex=581 nm). The insets refer to respective CIE resulting coordinates derived from emission spectra.
2.2.2. Mixed-Lanthanide Hydrogels (G-mix) Mixed lanthanide metal hydrogel samples were synthesized using ratios in different proportions to explore luminescent properties and novel emission patterns. In summary, four different forms of G-mix were obtained: i) G-Gd1-xEux, ii) GGd1-xTbx, iii) G-Eu1-xTbx and iv) G-GdxEuyTbz. Remarkably, the G-mix presented easily tunable emission color resulting from the combination of red (Eu3+ ion), green (Tb3+ ion) and blue (LS interaction), providing an RGB emission system that is controlled only by the metal ratio and excitation wavelength adjustment. To demonstrate the photochromic properties of the G-mix, we chose G-Gd0.70Eu0.1Tb0.2 as an example. The spectroscopic data obtained for this gel was plotted as a 3D graph (Figure S12). In Figure 3b, different output colors are observed as a function of the excitation wavelength. The colors are associated with their CIE coordinates, labeled as 1, 2, 3, 4 and 5 (Figure 3a). All of the G-mix samples could generate tunable emission light upon variation of the excitation wavelength. Most state-of-the-art light emitting compounds are monochromatic, usually requiring multiple lighting emitters to achieve white emission.30 However, our hydrogels were capable of producing pure white light emission in a single material. The samples, G-Eu0.75Tb0.25, G-Gd0.7Eu0.1Tb0.2, GGd0.7Eu0.2Tb0.1 and G-Gd0.8Eu0.05Tb0.15, had CIE coordinates of (0.338,0.339); (0.335,0.333); (0.334,0.332) and (0.335,0.330); respectively. The G-mix excitation spectra (Figure S12) exhibited narrow peaks associated with the Gd→Eu,Tb and Tb→Eu energy transfer processes.31 The emission spectra and resulting CIE coordinates of the other twenty-seven G-mix
Up to this point, the hydrogels proved to be synthesized easily with few resources, offering singular tunable UV-VISNIR emitting properties. Additionally, the physical properties of the gels allow for easy material processing to produce coated surfaces or thin-films.32 Thus, to build a multifunctional platform, we tested these LOGs in four different applications. In the first one, we tested the use of these LOGs in luminescent barcodes. In the second one, Light Emitting Diodes (LEDs) were coated with hydrogel films dried at room temperature; in the third one, hydrogel thinfilms were employed in a tamper seal device to assist forensic purposes. As previously demonstrated in the literature, hydrogels have been extensively explored in biocompatible applications (drug delivery and tissue engineering).33–35 Thus, we also tested the potential use of LOGs in an in vivo transdermal drug release assay. 2.3.1. Barcode The tunable emission provided by the G-mix systems allows us to suggest an encoding system based on emission-excitation dependence. Here, each G-mix sample works as a single luminescent barcode system. The interpretation of the encryption can be performed by monitoring the integrated emission or the intensities of the peaks placed at different regions of the spectrum, together with the readout colors represented in CIE coordinates and the visible emission. As a proof-of-concept experiment, we tested the G-Gd0.70Eu0.1Tb0.2 sample. As schematically shown in Figure 3c, barcode readout could be generated by monitoring the absolute emission intensities of peaks at 311, 440, 544 and 615 nm, when excited at 395, 380, 320, 282 and 270 nm. Moreover, it is also possible to obtain barcode modules by monitoring the integrated emission, as previously reported in the literature.36 In a straightforward way, the readout color provides a fast method for encrypting the barcode graph in the VIS region using only the naked eye. We found that different rare earth metal ratios in G-mix matrices led to different output points on the CIE diagram, allowing the generation of numerous distinct barcode graphs and a wide range of unique codes. In the literature, barcode materials are commonly reported emitting a single-channel37–40 or dual-channel36,41 emission. In contrast, we report for the first time a barcode system that provides emission in three different regions of the emission spectra (UV, VIS and NIR) using a single material that can be employed in a more specific readout interpreted by spectroscopy. Another set of encrypted graphs was created for the G-Eu0.75Tb0.25 hydrogel and is displayed in Figure S20.
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LED materials reported in the literature10, even among rigid coated materials such as Metal-Organic Frameworks (MOFs).42,43,45
Figure 4. Hydrogel encapsulated light emission diode (LED) (λex=395 nm) coated with G-Eu under white light (right) and the working device (left).
2.3.3. Tamper Seal
Figure 3. Tunable emission and barcode module application of GGd0.7Eu0.1Tb0.2 excited at 270, 282, 320, 380 and 395 nm (1, 2, 3, 4 and 5, respectively). (a) Resultant color in CIE coordinates, (b) images of cuvettes containing the samples and (c) two types of color-coded schematic barcode readout fashions integrating both visible and invisible information of the respective points.
2.3.2. Coated-LED We investigated the potential of using G-Ln in coated-LED. The main intention here was to obtain an optical material with both moderate luminescence and quantum yield along with easy color tunability. In state-of-the-art studies, encapsulatedLED generally needs several preparation steps for device coating, which makes the process more expensive.42–44 The major benefit of using a soft material is the easy processability of film10 and the thorough cover of the whole UV-VIS-NIR spectrum. To illustrate our concept, we tested G-Eu in LED covering. A simple deposit of the hydrogel followed by room temperature drying was enough to produce a coated-LED. The water loss was sufficient to achieve a rigid film. Figure 4 shows a coated-LED (λex= 395 nm) using G-Eu having red light emission with excellent color quality. The measured luminescence reveals unaltered emission patterns (Figure S21). In this context, any formulations of G-Ln or G-mix can be easily employed to create different optical devices with wide color emission ranges (UV, VIS and NIR), depending only on the excitation wavelengths. We also observed that our hydrogels present quantum yields analogues to other coated-
The Electrospray Deposition (ESD) technique was used to obtain luminescent thin-films with high color quality on glass substrates using G-Eu and G-Tb, as illustrated in Figure S22. Similarly, G-Tb was deposited over a plastic substrate as shown in Figure 5. Films were highly homogeneous, composed of nano-aggregates of approximately 10 nm (Figure S23). This homogeneity was also seen in the optical properties (Figure 5a). Emission spectra obtained from different regions of the thin-film showed high homogeneity and reproducible intensity values. A tamper-indicating device (TID) is designed to leave nonerasable and unambiguous evidence of unauthorized access of evidence, for example. TIDs are widely used in a wide range of government and private sector applications. These include cargo security, inventory control, carrier services, drug enforcements, anti-doping carriers and chain of custody.46 Focusing on forensic purposes, we coated an adhesive substrate with a G-Tb ESD film as a fast way to employ a tamper seal. The seal remains imperceptible under natural light (Figure 5b) and shows characteristic green light emission under 254 nm UV excitation (Figure 5c). Following an infringement test (Figure S24), no visible alterations could be observed under natural light (Figure 5d). However, using UV light, it was possible to identify manipulation, indicating infringement (Figure 5e). Additionally, the hard handling of the film in an unauthorized access attempt provoked the transfer of luminescent material to the handler's fingers, providing a high contrast stain and latent fingerprints on surfaces touched by this individual (Figure 5f). Thus, our method differs from other already-known commercial tamper seals47–53 for assisting in the identification of suspects.
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Figure 5. (a) Emission spectra of different regions (1, 2 and 3) of an ESD covered glass by G-Eu. The insert refers to images of the coated thin-film and the respective measured points 1, 2 and 3. Fabricated tamper seal of G-Tb under (b) natural light and (c) UV light. After the infringement test, fabricated tamper seals under (d) natural light and (e) UV light. (f) Latent fingerprint imprinted on a non-porous wood substrate after contact with the handler finger. (all λexc= 254 nm).
2.3.4. Bioapplication 2.3.4.1. Effect of hydrogel on fibroblasts cell viability As seen before, the application of hydrogels implies that there has been direct handling and contact with the material. Thus, it is very important that the hydrogels do not present any deleterious effect on the skin cells. Based in this fact, and considering hydrogel as a potential for topical application, the fibroblast NIH/3T3 cell line was used to evaluate the cytotoxic effect. For this, an MTT assay was carried out in a 96-well plate. A volume of 200 µL of 2×105 cells/mL were maintained for 24 h in a CO2 chamber. A sufficient biocompatibility could be determined using an MTT assay. The percentage of viability of the piroxicam, GEu, G-EuP, G-Tb or G-TbP with the 3T3 line cell is exhibited in Figure 6. As shown in Figure 6, no significant alterations were observed in the viability of 3T3 cells in any of the concentrations tested for samples. Therefore, the negligible effect of GEu, G-EuP, GTb and G-TbP on cell viability sustains its use for improving the solubility of the host molecule without affecting its biological properties. . 2.3.4.2. Transdermal Drug Release Assay Beside forensic/security and optical applications, we tested the biocompatibility of our hydrogels in a transdermal drug release assay. For our purposes, piroxicam was entrapped inside the hydrogel (G-TbP and G-EuP) and submitted to in vivo appraisal (Figure 7a). Piroxicam is a non-steroidal antiinflammatory drug. We evaluated its anti-inflammatory activity using the carrageenan-induced paw edema model. Experiments were carried out with two gels having different rheological properties, as described in the Structural Characterizations and Mechanical Properties section above. Subcutaneous administration of carrageenan (300 µg paw-1) was performed in the right hind paw and the paw edema volume was evaluated 4 h after the carrageenan injection. The
test group was topically treated with G-Ln containing 25 mg of piroxicam (G-Tb, G-TbP, G-Eu or G-EuP) 1 h after stimulation (Figure 7b). The hydrogel treatment demonstrated increased inhibitory properties compared with the control. The commercial reference group (25 mg piroxicam gel from Feldene®) showed 50% edema inhibition, while G-TbP and G-EuP provided 57% and 72% edema inhibition, respectively. The hydrogel treatment demonstrated inhibitory properties analogous to or better than the reference, depending on the elastic modulus. The G-TbP showed inhibition similar to the reference while G-EuP provided a 22% improvement in performance when compared to the reference.
Figure 6. Effect of piroxicam (Piro), GEu, GEuP, GTb, and GTbP on the viability of NHI-3T3 fibroblast. Each bar represents the mean percentage ± standard error mean (S.E.M.) from three independent experiments comparing to untreated cells (100% viability).
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bioimaging, bio-LED, barcode@LED, and tamperseal@barcode, among others. Overall, this work opens new potential routes for exploiting optical-biological behaviors in a single MSSP device.
ASSOCIATED CONTENT Synthesis scheme of G-Ln, FTIR spectra and XRD patterns of prepared hydrogels, TEM images of G-Eu, Decay curves and lifetimes of G-Eu, G-Tb and G-Gd, Excitation spectra of G-mix, 3D-plot (emission spectra vs. excitation wavelength) and their representative CIE coordinates, additional barcode scheme of GEu0.75Tb0.25, additional ESD coated images and their SEM images.
AUTHOR INFORMATION Prof. Dr. Severino Alves Júnior, Fundamental Department of Chemistry, UFPE, Brazil. E-mail:
[email protected] ACKNOWLEDGMENT This work was supported by CETENE, FACEPE, CAPES and CNPq (INCT/INAME). The authors gratefully acknowledge Euzébio Skovroinski and Fernanda Lira for their support.
REFERENCES
Figure 7. (a) Image of G-Tb application under a mouse paw and a schematic image of transdermal drug release. (b) Effect of entrapped piroxicam hydrogel G-Tb, G-TbP, G-Eu, G-EuP on paw edema induced by carrageenan in mice. This result is representative of three independent assays. *P
