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Nitrogen-Doped Carbon Nanodots-Ionogels: Preparation, Characterization, and Radical Scavenging Activity Carla Rizzo,†,# Francesca Arcudi,‡,# Luka Đorđević,‡ Nadka Tzankova Dintcheva,§ Renato Noto,† Francesca D’Anna,*,† and Maurizio Prato*,‡,∥,⊥ Downloaded via NORTHWESTERN UNIV on July 11, 2018 at 15:46:11 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
Dipartimento STEBICEF-Sezione di Chimica, Università degli Studi di Palermo, Viale delle Scienze Ed. 17, 90128 Palermo, Italy Dipartimento di Scienze Chimiche e Farmaceutiche, INSTM UdR Trieste, Università degli Studi di Trieste, Via Licio Giorgieri 1, 34127 Trieste, Italy § Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, dei Materiali, Università degli Studi di Palermo, Viale delle Scienze Ed. 8, 90128 Palermo, Italy ∥ Carbon Nanobiotechnology Laboratory CIC biomaGUNE, Paseo de Miramón 182, 20009 Donostia-San Sebastián, Spain ⊥ Basque Fdn Sci Ikerbasque, Bilbao, Spain ‡
S Supporting Information *
ABSTRACT: Hybrid diimidazolium-based ionogels were obtained by dispersing nitrogen-doped carbon nanodots (NCNDs) in ionic liquid (IL) solutions and by using dicationic organic salts as gelators. The properties of the NCND-ionogels were studied in terms of thermal stability, mechanical strength, morphology, rheological, and microscopic analyses. Insights into the formation of the hybrid soft material were attained from kinetics of sol−gel phase transition and from estimating the size of the aggregates, obtained from opacity and resonance light-scattering measurements. We demonstrate that, on one hand, NCNDs were able to favor the gel formation both in the presence of gelating and nongelating ILs. On the other hand, the gelatinous matrix retains and, in some cases, improves the properties of NCNDs. The NCND-ionogels showed the typical fluorescence emission of the carbon dots and a notable antiradical activity, with higher efficiency as compared to the single components. The presented hybrid materials hold great promise for topical applications in antioxidant fields. KEYWORDS: carbon nanostructures, carbon dots, supramolecular gels, ionogels, dicationic organic salts, fluorescence, radical scavenging
C
unpaired electrons on the nanodot surface and has been studied especially for biological purposes. Efforts in embedding nanodots in matrices or in solid-state devices are required, since retaining or even improving the nanomaterial properties in these architectures would expand various technologies that cannot operate in solutions, such as LEDs or photovoltaic devices. Nanocomposite materials in which CNDs are anchored to silica or dispersed in a polymeric matrix, providing superior functionalities, have been recently reported.23−26 Moreover, CND-based organic−inorganic hybrid gel glasses or thin films have been developed as optical materials or sensors.27,28 However, reaching a dispersion uniformity of the dots in the matrices and a high level of
arbon nanodots (CNDs) are the latest discovered carbon nanomaterials comprising quasi-spherical nanoparticles with size below 10 nm.1,2 Their synthesis has been achieved through a number of top-down or bottom-up approaches,3,4 while considerable advancements in improving their functionality, either through their rational design or doping/surface functionalization, have been performed.5−8 CNDs have gained tremendous attention due to their interesting properties in several applications.2 Above all, CNDs are regarded as next-generation luminescent nanomaterials for replacing toxic and expensive semiconductor quantum dots.9,10 Apart from biological applications,11 they hold great promise in photocatalysis, optoelectronics, or electrochemiluminescence.12−14 More recently, their charge-transfer or antiradical activity have widened their applications.15−22 In the latter case, the potential of nanoparticles in quenching reactive radical oxygenated species could be due to the presence of defects and © 2017 American Chemical Society
Received: October 24, 2017 Accepted: December 28, 2017 Published: December 28, 2017 1296
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Figure 1. (a) Schematic representation of NCNDs and structures of ILs and gelators; (b) preparation of NCNDs-based ionogel (for details see the Experimental Section); and (c) images of white opaque gel in normal light and blue emitting gel under irradiation at 365 nm.
control of the final properties of the hybrids is a challenging, as well as a crucial step for high-quality nanocomposites. Supramolecular gels represent a highly versatile functional class of soft materials, which have seen a surge of interest for a wide range of applications.29−33 Various gelators have been studied, including low molecular weight gelators (LMWG) that are able to trap a solvent by capillary forces33,34 and that can self-assemble via multiple supramolecular interactions, resulting in the formation of three-dimensional (3D) networks.35 In the presence of a third component, an additional level of hierarchical control can be introduced in the self-organization process.30 Especially in the case of carbon nanomaterials, the incorporation in supramolecular gel matrices has been widely explored in forming composite materials with improved performances.36 However, few studies of supramolecular gels with CNDs have been reported so far, and they were designed mainly for biological or analytical purposes.37−39 Only recently, Steed et al. reported the first example of CND incorporation within a LMWG-based hydrogel, in which the CNDs were successfully used as hydrophobic components suitable for fluorescent ion sensing in aqueous medium.38 Organo- and hydrogels, however, may suffer solvent volatility, lack of stiffness, or matrix damages, which could limit some of their applications, such as in optoelectronic devices. Ionogels, on the other hand, use ionic liquids (ILs) as solvents, which can guarantee the formation of a robust semisolid matrix and manifest good thermal stability and high conductivity.40,41 Ionogels, besides ILs, require a gelator such as organic salts. The latter are an interesting class since they can be easily prepared and, through small structural modifications, offer the possibility of fine-tuning the properties of the final ionogel.42−44 The use of a series of miscellaneous materials dispersed in gels made of ILs has been reported,45−47 with the “bucky gel” of Aida et al. as a leading example.48−51 In the present work, we prepared and studied nitrogendoped CNDs (NCNDs)-ionogels. In particular, as gelators we selected dicationic organic salts (DOSs) formed by meta- or para-substituted diimidazolium cations, namely 3,3′-di-ndodecyl-1,1′-(1,3-phenylenedimethylene)diimidazolium or 3,3′-di-n-dodecyl-1,1′-(1,4-phenylenedimethylene)diimidazolium terephthalate, from now on m-C12 and p-C12, respectively (Figure 1). Both organic salts are known to exhibit good gelation ability in some IL solutions.52 In order to shed
light on the possible role of NCNDs in the gelation process, we chose 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1-butyl-3-methylimidazolium N-bis-trifluoromethansulfonilimide ([bmim][NTf2]), which are a gelating and a nongelating IL, respectively, for the selected gelators (Figure 1). We explored the effect of NCNDs on the gel-phase formation and properties. Specifically, we determined the critical gelation concentrations and the melting temperatures (Tgel) of the hybrids. The obtained NCND-ionogels were then fully characterized in terms of their photophysical properties, rheological responses, morphology, and ability to self-repair after disruption. Finally, we investigated their radical scavenging activity by using the common 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) test. Notably, the hybrid material consisting of NCNDs and ionogels resulted in an improved antioxidant activity (100% radical scavenger activity) when compared to the pure ionogels, nanodots in solution, or other carbon-based dots already reported. The materials reported herein could find applications relevant to energy and medicine. Besides prominent examples in biology, including preventing cellular oxidative stress and pathological damage,53−56 antioxidant properties are of value as coating materials for plastics (and rubber) or for conservation/preservation of cultural heritage.21,57−59
RESULT AND DISCUSSION Gels Preparation and Gelation Tests. First, DOSs and NCNDs were synthesized by following our previous established procedures.5,52 Briefly, NCNDs were prepared through a simple microwave-assisted hydrothermal method in 180 s at 240 °C by employing arginine and ethylenediamine as precursors. The dots show multiple nitrogen and oxygen functional groups, with a surface rich in primary amino groups and an average size of 2.47 ± 0.84 nm.5 In the second step, NCND-ionogels were prepared by mixing the NCNDs/IL dispersions with diimidazolium salts as gelators. In particular, we studied the effect of NCNDs on the gel phase by exploring different concentrations of both NCNDs (from 0.025 up to 0.1 wt %) and gelators (from 1.1 up to 6.0 wt %) (Tables 1 and S1). The mixtures were heated at 100 °C until clear solutions were observed. After cooling, they were stored at 4 °C, and the gel formation was initially assessed by using the tube inversion 1297
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instead of 3.3 wt %, respectively). On the other hand, CGC values of p-C12 in [bmim][NTf2] at NCNDs concentrations tested are lower than the ones of the corresponding gels in [bmim][BF4] (for example, 2.5 wt % vs 3.7 wt % at 0.025 wt % of NCNDs). To summarize, the largest CGC values were obtained with the ILs having the largest H-bond accepting ability, β (β = 0.376 and 0.243 for [bmim][BF4] and [bmim][NTf2], respectively),62 which is in agreement with previously studied ionogels.52 The above result could be ascribed to the strong cation−anion interactions in [bmim][BF4], which slow down the interactions between the three components (IL-gelator-NCNDs) needed for the gel network formation. The advantageous use of carbon nanomaterials has been previously reported by Aida et al., since the orientation of imidazolium ions by single-wall carbon nanotubes has been found to exert a positive effect on gel formation.47 More recently, the role of carbon dots as centers of hydrophobic nucleation and cross-linking nodes for self-assembled fibrillar network by hydrophobic and π−π stacking interactions was proposed by Steed et al. for hydrogels of bis(urea)derivative salts.38 In our case, we suggest that the amino (and carboxylic) groups present on the carbon dot surface favor H-bond and cation interactions among NCNDs, DOS, and IL structures inducing the gel formation also for solutions in which gelation is normally inhibited (such as, for example, [bmim][NTf2]), vide inf ra (Gel Formation Kinetics and Mechanism). Rheological Properties of NCND-Ionogels. To allow a better comparison, we performed rheological measurements of NCND-ionogels prepared by using 5 wt % of DOS and 0.1 wt % of NCNDs. Moreover, we studied the gel properties at different NCND concentrations by using the p-C12/NTf2 system as a reference. Unfortunately, the rheological properties of m-C12/NTf2-NCND0.1 were difficult to determine because of its highly soft nature and low thermal stability at 25 °C. All the hybrid materials showed a dependence of moduli (G′, storage modulus, and G″, loss modulus) on percentage of applied strain, γ, which is a typical behavior of soft materials (Figures 2a and S1). In particular, G′ were larger than G″ values before reaching the γ value, which is defined as the crossover point (γ at G′ = G″) corresponding to the disruption and collapse of the gel phase, and the trend is then inverted. Frequency sweep measurements confirm the gel nature of our
Table 1. Selected Gelation Tests of Diimidazolium Salts in the Presence of NCNDs [bmim][BF4] DOS m-C12
p-C12
conc. NCNDsa 0 0.05 0.1 0 0.025 0.05 0.1
conc. DOSa e
3.3 5.2e 3.6e 3.3e 3.7e 3.7e 3.5e
appear.b d
OG OG OG OGd OG OG OG
[bmim][NTf2] Tgelc 24 26 31 44 SM 49 49
conc. DOSa 5.0 6.0 5.2e 5.0 2.5e 1.8e 2.2e
appear.b
Tgelc
d
P PG OG Pd OG OG OG
SM 41 34 41
a
Concentration of NCNDs (in ILs or gels) or concentration of DOSs (in gels), (%, w/w). bAppearance: OG = opaque gel; P = precipitate; PG = gel-like precipitate; SM = soft material (the gel did not support the weight of the lead ball for Tgel determination). cTgel determined by the lead ball method, (°C) (values were reproducible within 1 °C). d Data from ref 52. eCritical gelation concentration (CGC) values.
test method.60 As reported in Tables 1 and S1, the majority of gelation tests led to thermoreversible and white opaque gels exhibiting a blue emission typical of NCNDs under UV irradiation (Figure 1b). For purposes of simplicity, from now on, all the gels will be indicated as follows: DOS acronym (mC12 or p-C12)/anion of the IL used as gelation solvent-NCNDx (with x indicating the nanodot concentration). As previously reported, both DOSs were able to form gels in the presence of [bmim][BF4] as solvent, but gave precipitate formation with [bmim][NTf2].52 In the presence of NCNDs, gels from both DOSs in [bmim][BF4] maintain their stability and show comparable properties to the pure gels as evidenced by the critical gelation concentration (CGC) and the melting temperature, that is, the temperature of the phase transition from gel to solution (Tgel). Interestingly, small amounts of NCNDs (0.1 and 0.025 wt % of NCNDs for m-C12 and p-C12, respectively) were able to induce gel formation from solutions in [bmim][NTf2] in the presence of both gelators, which in absence of carbon dots lead to precipitates and no gel formation. Contrary to previously reported hybrid organogels containing carbon nanotubes,61 the presence of NCNDs does not lead to lower CGCs with respect to pure gels. Actually, for the gel formed by m-C12 in [bmim][BF4] with 0.05 wt % of NCNDs, the CGC is higher with respect to pure gel (5.2 wt %
Figure 2. (a) Strain (ω = 1 rad sec−1) and (b) frequency sweeps (γ = 0.025%) for p-C12/NTf2-NCNDs gels prepared by employing 5 wt % of DOS at different NCND concentrations. Full and empty dots indicate G′ and G″, respectively. Black, red, and green dots indicate 0.1, 0.05, and 0.025 wt % of dots, respectively. 1298
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Differently, in the second case, the dots induce a network reorganization by perturbing and weakening the pre-existing interactions. Therefore, when the dots drive the gel formation, this latter phenomenon occurs at lower concentrations (as observed from CGCs investigations) and leads to stronger materials. Self-Repairing Properties of NCND-Ionogels. Significant progress has been achieved in the development of gels with self-repairing properties able to restore their functionalities and structures after damages.67 This property has enormously extended the potential applications of hydrogels or organogels in various fields,68 ranging from biomedicine to sensors, but has been scarcely studied for hybrid ionogels. We evaluated the ability to self-repair of our gels after exposure to magnetic stirring (thixotropy) and ultrasound irradiation (sonotropy). As reported in Table 3, all the gels
Table 2. Elastic and Viscous Moduli, G′ and G″ and γ Value at G′ = G″ by Using 5 wt % DOS Concentrations at 25 °Ca NCNDs-ionogel p-C12/[NTf2]NCND0.1 p-C12/[NTf2]NCND0.05 p-C12/[NTf2]NCND0.025 p-C12/[BF4]c p-C12/[BF4]NCND0.1 m-C12/[BF4]c m-C12/[BF4]NCND0.1
G′ (Pa)
G″ (Pa)
γ at G′ = G″
12400 ± 1600b
4900 ± 1200b
2.49 ± 0.01%
6000 ± 2000b
2400 ± 700b
2.0 ± 0.5%
6600 ± 400
2600 ± 500
2.6 ± 0.1%
b
b
4340 ± 220 180 ± 20d
590 ± 80 90 ± 10d
3.2 ± 0.6% 9 ± 1%
20200 ± 4700 2000 ± 400b
4400 ± 700 500 ± 190b
6.9 ± 1.4% 70 ± 10%
Table 3. Results of Self-Repairing Ability Tests (Sonotropy and Thixotropy) of the Gels Prepared by Employing 5 wt % of DOSa NCNDs-ionogel p-C12/[NTf2]NCND0.1 p-C12/[NTf2]NCND0.05 p-C12/[NTf2]NCND0.025 p-C12/[BF4]NCND0.1 m-C12/[BF4]NCND0.1
a
Error limits are based on average of three different measurements with different aliquots. bValues at γ = 0.025%. cData from ref 52. d Values at a γ = 0.063%.
with increasing the NCND amount was observed. While the disruption of the gel phase occurs at almost the same crossover point (γ at G′ = G″), an elastic modulus two times higher by using 0.1 wt % of dots, with respect to 0.05 wt %, has been measured. On the other hand, a decreasing NCND concentration of 0.025 wt % did not cause significant variations compared to the intermediate dots amount of 0.05 wt %. Despite a proper literature comparison being trivial since similar soft materials have not been reported so far, our results are in agreement with other hybrid organogels or hydrogels.64,65 While the [NTf2]-based gels cannot be formed without the presence of the NCNDs, a rheological comparison for the [BF4]-based NCND-gels with the pure gels showed that, in the presence of dots, the gel is less robust. They are relatively weak, but comparable or, at least, more robust than the ones reported so far by Steed et al. (the most similar material to our ionogels).38 However, the presence of NCNDs is advantageous in terms of crossover point of the [BF4]-based gels, since larger γ percentages were observed in the hybrid systems as compared to the pure gels (≈ 9% vs ≈3% for p-C12/[BF4]-NCND0.1 and corresponding pure gel; γ ≈ 70% vs ≈7% for m-C12/[BF4]NCND0.1 and corresponding pure gel). The observed behavior suggests that the presence of NCNDs increases the resistance of the gel to flow, resulting indeed in a higher percentage strain needed to break the hybrid gel matrix comparing to the one of pure gel. This prolonged LVR may be attributed to a stable NCND dispersion in the gel matrix, in agreement with previously reported carbon nanotube-organogels.61,66 Taken in concert, our data indicate that NCNDs enhance the properties of systems unable to gel in their absence (DOSs in [bmim][NTf2]), but they exert a slightly destabilizing effect on the rheological properties of systems able to self-assemble also in their absence (DOSs in [bmim][BF4]). In the first case, NCNDs act as nucleation agents favoring the organization of different components into fibrils that drive the gelation process.
sonotropy
thixotropy
1st cycle recovery
2nd cycle recovery
stable
yes
19%
9%
stable
yes
27%
26%
stable
yes
67%
41%
stable
no
−
−
yes
no
−
−
a
For thixotropic gels, percentage of G′ recovered after NCNDsionogels disruption performed with rheological tests is also reported.
resisted to the action of ultrasound irradiation with the only exception of m-C12/[BF4]-NCND0.1, which showed sonotropic behavior. Moreover, only the [NTf2]-based hybrid gels recover their viscoelasticity after cessation of destructive mechanical stress, possibly due to their best rheological response (Figures 3 and S2). The degree and time of gel recovery were followed by
Figure 3. G′ (full dots) and G″ (empty dots) (ω = 1 rad sec−1) of pC12/[NTf2]-NCND0.1 as a function of time at 25 °C and at low (G′ > G″ regimes, γ = 0.025%) and destructive strain (G″ > G′ regimes, γ = 25%). 1299
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ACS Nano measuring the temporal evolution of the moduli at low (G′ > G″ regimes, γ = 0.025%) and destructive (G″ > G′ regimes, γ = 25%) strains. When low levels of strain in the LVR were applied after the disruption of the gels, G′ was higher than G″, confirming its recovery. The percentage of gel recovery was estimated by comparing the initial G′ values with the ones obtained in the LVR after disruption (Table 3). A concentration dependence on the viscoelastic recovery was observed. About 67% and 41% of the original G′ value of the gel at 0.025 wt % of nanodots was recovered within the first and the second cycle, respectively. On the other hand, a lower degree of recovery was observed for gels with 0.1 wt % of nanodots and could be due to the significantly higher original G′ value that is probably more difficult to restore. However, the latter is still strong after the first recovery with a G′ value of ca. 2440 Pa. Photophysical Measurements. NCNDs show the typical excitation-wavelength emission, as a result of a distribution of different emissive domains present on their surface.5 In an aqueous solution, they show a broad emission peak at 356 nm when the sample is excited at the optimal excitation wavelength (300 nm) and a fluorescence quantum yield (FLQY) of 17%. The fluorescence peaks shifted from 356 to 474 nm when the excitation wavelength changed from 300 to 420 nm and the fluorescence intensity decreased as the peak red-shifts. The various functional groups present on the surface of NCNDs have different surface state energy levels, which result in a series of emissive traps that dominate the emission at different excitation wavelengths. Herein, we study the fluorescence emission of NCNDs using the ILs as solvents. Very few examples analyzing the emission properties of NCNDs in IL solution have been reported, and an interesting dependence of NCNDs emission intensity from the IL nature has been attributed to the stabilizing layer from charged IL, which could affect the electrostatic potential of the nanoparticle.69 Therefore, we investigated the NCNDs emission in both [bmim][BF4] and [bmim][NTf2] at different dot concentrations (Figure S3). The optimal excitation wavelengths were 340 and 355 nm resulting in emission bands at ca. 420 and ca. 450 nm for [bmim][BF4] and [bmim][NTf2] solutions, respectively. A significant increase in the fluorescence emission by increasing the NCNDs concentration was observed in both ILs, with higher emission intensities in [bmim][BF4], following a linear trend as a function of carbon dot concentration (Figure S3). Interestingly, more significant variations in the emission intensities were observed in the presence of the anion having the highest coordination ability (BF4−),62 underlining how the different nature of the IL anion plays an important role in the interaction with the dots. To further evaluate the matrix effect on the NCNDs emission, we also recorded fluorescence emission spectra of the hot solutions, having the same composition of the gel (5 wt % of DOS and 0.025−0.1 wt % of NCNDs), and, after their overnight equilibration at 4 °C, of the corresponding gel phase (Figures 4 and S4 and Table S2). With the only exception of the m-C12/[NTf2]-NCND0.1 gel, for which we observed a slight increase in the emission intensity, addition of DOSs generally decreases the fluorescence intensity, both in the sol and in the condensed phases, the latter having a more pronounced effect (Figure 4). In gel phases, different from solutions, systems formed in [bmim][NTf2] were more emissive than the ones in [bmim][BF4]. The emission quenching of NCNDs could be
Figure 4. Fluorescence emission spectra of p-C12/[BF4]-NCND0.1 (in gray) and p-C12/[NTf2]-NCND0.1 (in black) as hot solutions (solid line) and hybrid ionogels (short dash) recorded at the optimal excitation wavelength of 340 and 355 nm for [bmim][BF4] and [bmim][NTf2] solutions, respectively.
ascribed to the occurrence of interactions with gelator molecules, which are stronger when [bmim][BF4] was the solvent entrapped in the gel 3D network. Absolute FLQYs measurements of both solution and gel phases confirmed the aforementioned trend (Table S2). A slight decrease of emission in gels compared to sol phases, together with their linear dependence on the dot concentration, is generally observed. FLQYs of NCND-gels range from 2.87% to 6.83%, with the highest values measured from the less opaque m-C12-based hybrid gels. Therefore, both the strong interaction between diimidazolium salts and NCNDs and the matrix opacity play key roles in the emission of the hybrid materials. Our results indicate that the combination of NCNDs with ionogels changes the properties of both pure materials. Although the fluorescence emission is reduced when compared to aqueous solutions, the main feature of the NCNDs is still observable in ILs solution or ionogel phase. Morphology of Hybrid Gels. The realistic hybrid gel morphology could be detected with a technique that allows observing both the nanoparticles and the gel network. Therefore, atomic force microscopy (AFM) analysis was performed on the p-C12/[NTf2]-NCND0.1 gel that has showed the best rheological performance, as a representative system. From height images (Figure 5a,b) it is possible to recognize a gel 3D network, which seems characterized by a thick texture with trunk-like fibers tightly intertwining (Figure 5b). The observed morphology was consistent with the one of pure ionogel,52 and a dense fiber 3D network was already observed for carbon dots-hydrogels.37,38 Both nanodots and gels morphology can be recognized. NCNDs showing nanometric dimensions are arranged in the compact gel network, and a cross-linked structure formed around them can be observed (Figure 5c). Therefore, a tight interaction between the dots and the gelator occurs as the former participates in the gel network formation, in agreement with previously reported graphene- or carbon nanotubes-based organogels.64,66 The closely packed network was observed also by polarizing optical microscopy (POM) and scanning electron microscopy (SEM) images (Figure 5d,e). The needle-like fibers observed from the POM 1300
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gelation solvent. Moreover, p-C12-based hybrid gels generally present larger aggregates than m-C12-based systems, suggesting that the higher fluorescence emission of the latter could be ascribed to the presence of smaller aggregates. The same trend was observed in gel phase. Moreover, interesting insights on the gel network reorganization were obtained by comparing IRLS measured in solution and gel phase as a function of the IL nature. For [bmim][BF4], larger aggregates in solution than in gel phase were observed, while the opposite trend was recognized for the [bmim][NTf2] systems. These findings suggest that, while in the first case larger aggregates rearrange in smaller ones for obtaining the self-assembled fibrillar network (according to pure ionogels),52 in the latter the self-assembly process probably involves small aggregation before the formation of 3D networks (showing larger aggregates). Finally, and in contrast to sol-phase observations, a higher IRLS value was measured employing 0.05 wt % instead of 0.1 wt % of nanodots, suggesting a further gel network reorganization by increasing the nanomaterial concentration, which is in agreement with opacity measurements (vide inf ra). Gels Formation Kinetics and Mechanism. The kinetics of formation were investigated for the NCND-ionogels formed by the organic salt p-C12, while the slow formation kinetics of the m-C12-based hybrid gels did not allow the measurements, as it was observed for the ionogels without NCNDs.52 The gel formation was studied by following the gel opacity (through its absorbance at 568 nm), which is qualitatively related to the number and the size of polydisperse nanostructures in a system and, therefore, to the crystallinity of the gels.72 For all the gels, a two-step formation mechanism was observed (Figure 6). The absorbance rapidly increases and then
Figure 5. Morphological images of NCNDs ionogels at room temperature. (a−c) Tapping mode AFM analysis on a mica substrate with height (a,b) and phase images (c). (d) POM image (crossed-polarizer). (e,f) SEM image of ionogel (left) and xerogel (right).
image (Figure 5d) are complemented by the scanning electron micrograph showing the fibrous nature of the dried xerogel (Figure 5f). Resonance light scattering is a highly sensitive technique for detecting and studying chromophore aggregates, with the intensity being related to the size of the assembly.70,71 In agreement with all the above-reported characterizations, the values of RLS intensity maxima (IRLS), measured for hot solutions and the corresponding gels (Table 4), indicate that Table 4. Maxima of RLS Intensity Peaks for Systems in [bmim][BF4] and in [bmim][NTf2] Imax RLS (au) NCND-ionogels
sol
gel
p-C12/[BF4]-NCND0.1 m-C12/[BF4]-NCND0.1 p-C12/[NTf2]-NCND0.1 p-C12/[NTf2]-NCND0.05 p-C12/[NTf2]-NCND0.025 m-C12/[NTf2]-NCND0.1
365.8 172.9 145.4 86.0 40.6 7.5
242.0 165.8 215.4 320.7 276.6 206.7
Figure 6. Kinetics of gel formation for p-C12/[NTf2] with different amount of NCNDs at 25 °C. Black, red, and green lines indicate 0.1, 0.05, and 0.025 wt % of dots, respectively.
stays constant for ca. 1 h (in the case of NCNDs0.05) or ca. 8 h (in the case of NCNDs0.1 and NCNDs0.025), while, in a second step, it further increases until it reaches a plateau. The observed trend could be attributed to an initial formation of aggregates, followed by their rearrangement to gel phase, which is expected for processes that involve fiber formation (i.e., 1D objects) before their intertwinement into 3D fibrillar networks.73 The comparison between the formation kinetics for p-C12/[BF4]NCND0.1 (Figure S5) and the corresponding pure gel suggested that the NCNDs slow down the gel formation: an
both the gelator/IL nature and the NCND concentration drastically affect the properties of the hybrid materials. For the studied systems, more pronounced differences were detected in sol than in gel phases. In solutions, larger aggregates were observed by increasing the dots concentration and by using the IL with the largest coordination ability ([bmim][BF4]) as 1301
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ACS Nano
Antioxidant Activity of Hybrid Gels. To estimate the gel antioxidant activity, we performed a DPPH radical assay, which is the most common free radical assay for gel system.76−78 The common experimental procedures were suitably adapted, and the dark purple solution of DPPH usually turns light yellow after the contact with an antioxidant. Thus, the efficiency of the scavenger activity was measured by monitoring in time the disappearance of the radical absorbance peak (at 517 nm in the UV−vis spectrum, Figure S8) and comparing to a DPPH control solution. The radical scavenger activity of various carbon dots has been so far reported only in solution,17−21 however their immobilization in a matrix is needed for expanding the application field. At first, we probed the radical scavenger activity of NCNDs in solution, in order to shed light on the components responsible for this property (Table 5, entries 1−3). NCNDs
induction period of nucleation equal to ca. 7 h was observed for the hybrid gels, while an induction period of ca. 1.5 h was observed for the pure gels.52 The observed trend could be ascribed to the occurrence of strong interactions between the gelator and the nanomaterial, the latter showing the presence of complementary sites for H-bond formation, similar to previously reported carbon nanotube-based organogels.61 Moreover, a significant drop in crystallinity values was detected passing from 0.025 or 0.05 wt % to 0.1 wt % of nanodots in [NTf2]-based gels. A lower crystallinity of the gel phase indicates the presence of smaller nanostructures in the system and it complements the RLS measurements (see above), demonstrating that the gel formation is governed by the formation of smaller aggregates in the presence of 0.1 wt % of NCNDs. Although it is not possible to directly ascribe this phenomenon to the percentage of NCNDs, we believe that a larger amount of carbon nanomaterial would make the fiber elongation more difficult and would account for the decreased opacity of the corresponding hybrid gel. In addition, this phenomenon could be hidden by UV−vis analysis if the concentration of NCNDs is too low, as shown by the same absorbance value obtained for 0.025 and 0.05 wt % of NCNDs. In the case of ionogels alone, the self-assembly is guided by H-bonding interactions (as observed previously by powder Xray analysis).52 The surface of the NCNDs, being abundant in (primary) amino groups on their surface,5 could establish Hbond interactions between gelator and solvent and, in cases where the interactions between the latter two are too weak, induce gelation. Further stabilization may arise from other interactions, which include bridges between gelator anions and diimidazolium cations (suggested from DFT calculations)74 or π-stacking (observed for p-C12 gels compared to the m-C12 ones). In the case of hybrid ionogels formation, it is pivotal for the carbon dots to participate in the formation of H-bonds. In order to corroborate the role of NCNDs in the supramolecular organization of the ionogels, we prepared carbon dots bearing different functional groups on their surfaces and tested them for gelation (using the same procedure as for the p-C12/[NTf2]NCND0.1 gel, at the CGC). We focused on carbon dots that, compared to NCNDs, lack the numerous amines around the carbon core (named Arg-CDs)5 or feature a surface rich in carboxylic groups (named Cit-CDs).75 Arg-CDs, without the primary amines shell, were unable to induce the formation of ionogels, while the Cit-CDs exposing −COOH groups succeeded in forming ionogels (Figure S6). Although the two additional carbon dots systems were not optimized for their best rheological performances, they disclose that the presence (on their surface) of groups capable of H-bonding is of outmost importance for the formation of the hybrid ionogels. Additionally, these tests suggested that the incorporation of other types of dots could be achieved, and therefore the hybrid ionogel could be tailored according to specific needs. In this work, we have focused on the radical scavenging properties (vide inf ra), which depend upon the presence of functional groups (such as amines) on the carbon dots surface that are able to interact with the radicals. Finally, the addition of a H-bond disrupting solvent (such as DMSO, 1 μL to ∼250 mg of ionogel) to the p-C12/ [NTf2]-NCND0.1 ionogel causes the disruption of the gel phase, further supporting the importance of H-bonds for the gel formation (Figure S7). Therefore, the NCNDs play a key role in the supramolecular organization, leading to the formation of a 3D network, especially when weak forces between gelator and solvent are not able to self-assemble into gels.
Table 5. Comparison of Percentage of Radical Scavenger Activity (E) for Systems Studied and Some Previously Reported E (%) NCNDs in MeOH Cit-CDs75 in MeOH Arg-CDs5 in MeOH NCNDs in [bmim][NTf2] p-C12/[NTf2]-NCND0.1 p-C12 in [bmim][NTf2] solution L-ascorbic acid in MeOH L-ascorbic acid in [bmim][NTf2] N,S co-doped CNDs N,S co-doped CNDs from garlic CNDs from date molasses
88 (60 min) 55 (60 min) 5 (60 min) 37 (60 min) 100 (60 min) 38 (60 min) 100 (60 min) 100 (60 min) 56 (60 min)20 88 (
