Nitrogen-Doped Carbon Nanodots-Ionogels: Preparation

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Nitrogen-Doped Carbon Nanodots-Ionogels: Preparation, Characterization and Radical Scavenger Activity Carla Rizzo, Francesca Arcudi, Luka #or#evi#, Nadka Tzankova Dintcheva, Renato Noto, Francesca D'Anna, and Maurizio Prato ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07529 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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Nitrogen-Doped Carbon Nanodots-Ionogels: Preparation, Characterization and Radical Scavenger Activity Carla Rizzo,†‡ Francesca Arcudi,§‡ Luka Ðorđević,§ Nadka Tzankova Dintcheva,⊥ Renato Noto,† Francesca D’Anna†* and Maurizio Prato§,▽,º*

† Dipartimento STEBICEF-Sezione di Chimica, Università degli Studi di Palermo, Viale delle Scienze Ed. 17, 90128 Palermo (Italy). E-mail: [email protected] § Dipartimento di Scienze Chimiche e Farmaceutiche, INSTM UdR Trieste, Università degli Studi di Trieste, Via Licio Giorgieri 1, Trieste 34127 (Italy). E-mail: [email protected] ⊥

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). º Basque Fdn Sci Ikerbasque, Bilbao 48013 (Spain).

ACS Paragon Plus Environment

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KEYWORDS carbon nanostructures; carbon dots; supramolecular gels; ionogels; dicationic organic salts; fluorescence; radical scavenging.

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 non-gelating 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 anti-radical activity, with higher efficiency as compared to the single components. The presented hybrid materials hold great promise for topical applications in anti-oxidant fields.

Carbon Nanodots (CNDs) are the latest discovered carbon nanomaterials comprising quasispherical 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


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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 widen their applications.15–21 In the latter case, the potential of nanoparticles in quenching reactive radical oxygenated species could be due to the presence of defects and 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.22–25 Moreover, CNDbased organic-inorganic hybrid gel glasses or thin films have been developed as optical materials or sensors.26,27 However, reaching a dispersion uniformity of the dots in the matrices and a high level of 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.28–32 Various gelators have been studied, including low molecular weight gelators (LMWG) able to trap a solvent by capillary forces,32,33 and that can self-assemble via multiple supramolecular interactions, resulting in the formation of three-dimensional networks.34 In the presence of a third component, an additional level of hierarchical control can be introduced in the self-organization process.29 Especially in the case of


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carbon nanomaterials, the incorporation in supramolecular gel matrices has been widely explored in forming composite materials with improved performances.35 However, few studies of supramolecular gels with CNDs have been reported so far and they were designed mainly for biological or analytical purposes.36–38 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 component suitable for fluorescent ion sensing in aqueous medium.37 Organoand hydrogels, however, may suffer of 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 semi-solid matrix and manifest good thermal stability and high conductivity.39,40 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.41–43 The use of a series of miscellaneous materials dispersed in gels made of ILs has been reported,44–46 with the “bucky gel” of Aida et al. as leading example.47–50 In the present work, we prepared and studied nitrogen-doped (NCNDs)-ionogels. In particular, as gelators we selected dicationic organic salts (DOSs) formed by meta- or para-substituted diimidazolium

cations

–

phenylenedimethylene)diimidazolium

namely

3,3’-di-n-dodecyl-1,1’-(1,3-

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.51 In order to shed light on the possible role of NCNDs in the gelation process, we chose 1-butyl-3methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1-butyl-3-methylimidazolium N-bis-


4

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trifluoromethansulfonilimide ([bmim][NTf2]), which are a gelating and a non-gelating 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,52–55 antioxidant properties are of value as coating materials for plastics (and rubber) or for conservation/preservation of cultural heritage.21,56–58


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Figure 1. a) Schematic representation of NCNDs and structures of ionic liquids and gelators; b) preparation of NCNDs-based ionogel (for details see the experimental section); c) images of white opaque gel in normal light and blue emitting gel under irradiation at 365 nm.

Result and Discussion Gels preparation and gelation tests. Firstly, DOSs and NCNDs were synthesized by following our previous established procedures.5,51 Briefly, NCNDs were prepared through a simple microwave-assisted hydrothermal method in 180 seconds at 240 °C by employing arginine and ethylenediamine as precursors. The dots show multiple nitrogen and oxygen functional groups, with the 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


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initially assessed by using the tube inversion test method.59 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 follow: DOS acronym (m-C12 or p-C12)/anion of the ionic liquid used as gelation solvent-NCNDx (with x indicating the dots concentration). Table 1. Selected gelation tests of diimidazolium salts in the presence of NCNDs. [bmim][BF4] DOS m-C12

p-C12

a

[bmim][NTf2]

conc. NCNDsa conc. DOSa appear.b Tgelc conc. DOSa appear.b Tgelc 0

3.3*

OGd

24

5.0

Pd

0.05

5.2*

OG

26

6.0

PG

0.1

3.6*

OG

31

5.2*

OG

0

3.3*

OGd

44

5.0

Pd

0.025

3.7*

OG

SM

2.5*

OG

41

0.05

3.7*

OG

49

1.8*

OG

34

0.1

3.5*

OG

49

2.2*

OG

41

SM

concentration of NCNDs (in ILs or gels) or concentration of DOSs (in gels), (%, w/w);

b

appearance: 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); ddata from ref. 51; *critical gelation concentration (CGC) values.

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].51 In the presence of NCNDs, gels from both DOSs in [bmim][BF4] maintain their stability and show comparable properties to the


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pure gels as evidenced by the critical gelation concentration (CGC) and the melting temperature, i.e. temperature of 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,60 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% 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),61 which is in agreement with previously studied ionogels.51 The above result could be ascribed to the strong cation-anion interactions in [bmim][BF4], which slow down the interactions between the three components (ionic liquidgelator-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.46 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.37 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


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structures inducing the gel formation also for solutions in which gelation is normally inhibited (such as, for example, [bmim][NTf2]) – vide infra (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/NTf2NCND0.1 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 samples since storage moduli G′ were found always greater than loss moduli G′′ (Figures 2b and S1).62 Moreover, the moduli were invariant with the angular frequency by fixing a value of strain within the linear viscoelastic region (LVR) (Figures 2b and S1).


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Figure 2. a) Strain (ω = 1 rad sec-1) and b) frequency sweeps (γ = 0.025%) for p-C12/NTf2NCNDs 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 wt%, 0.05 wt% and 0.025 wt% of dots respectively.

All the measured rheological parameters are summarized in Table 2. An increased gel strength for the p-C12/NTf2 systems 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.63,64


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Table 2. Elastic and viscous moduli, G′ and G′′; and γ value at G′ = G′′ by using 5 wt% DOS concentrations at 25 °C. Error limits are based on average of three different measurements with different aliquots.

a

NCNDs-ionogel

G′′ (Pa)

G′′′ (Pa)

γ at G′′ = G′′′

p-C12/[NTf2]-NCND0.1

12400 ± 1600a

4900 ± 1200a

2.49 ± 0.01%


6000 ± 2000a

2400 ± 700a

2.0 ± 0.5%


6600 ± 400a

2600 ± 500a

2.6 ± 0.1%

p-C12/[BF4]b

4340 ± 220

590 ± 80

3.2 ± 0.6%

p-C12/[BF4]-NCND0.1

180 ± 20c

90 ± 10c

9 ± 1%

m-C12/[BF4]b

20200 ± 4700

4400 ± 700

6.9 ± 1.4%

m-C12/[BF4]-NCND0.1

2000 ± 400a

500 ± 190a

70 ± 10%

values at γ = 0.025%; bdata from ref. 51; cvalues at a γ = 0.063%.

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).37 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 mC12/[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


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prolonged LVR may be attributed to a stable NCND dispersion in the gel matrix, in agreement with previously reported carbon nanotube-organogels.60,65 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. 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 lead 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.66 This property has enormously extended the potential applications of hydrogels or organogels in various fields,67 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 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 recoveries were followed by measuring the


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temporal evolution of the moduli at low (G′ > G′′ regimes, γ = 0.025%) and destructive (G′′ > G′ regimes, γ = 25%) strain. 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 comparing the initial G′ values with the ones obtained in the LVR after disruption (Table 3). A concentration dependence on the viscoelastic recovery has been 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.

Table 3. Results of self-repairing ability tests (sonotropy and thixotropy) of the gels prepared by employing 5 wt% of DOS; for thixotropic gels percentage of G′ recovered after NCNDs-ionogels disruption performed with rheological tests are also reported.

NCNDs-ionogel

Sonotropy

1st cycle

2nd cycle

recovery

recovery

Thixotropy


Stable

Yes

19%

9%


Stable

Yes

27%

26%


Stable

Yes

67%

41%

p-C12/[BF4]-NCND0.1

Stable

No

/

/

m-C12/[BF4]-NCND0.1

Yes

No

/

/


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Figure 3. G′ (full dots) and G′′ (empty dots) (ω = 1 rad sec-1) of p-C12/[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%).

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 nm 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


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the stabilizing layer from charged IL, which could affect the electrostatic potential of the nanoparticle.68 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 nm 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-),61 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 gels phase (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, S4 and Table S2). With the only exception of the mC12/[NTf2]-NCND0.1 gel, for which we observed a slight increase in the emission intensity, addition of DOSs generally decrease the fluorescence intensity, both in the sol and in the condensed phase, the latter having a more pronounced effect (Figure 4). In gel phases, differently from solutions, gels formed in [bmim][NTf2] were more emissive than the ones in [bmim][BF4]. The emission quenching of NCNDs could be ascribed to the occurrence of interactions with gelator molecules, which are stronger when [bmim][BF4] was the solvent entrapped in the gel three-dimensional network.


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Figure 4. Fluorescence emission spectra of p-C12/[BF4]-NCND0.1 (in grey) 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.

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 ionic liquids solution or ionogel phase.


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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.

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).


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From height images (Figures 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 thick texture morphology was consistent with the one of pure ionogel,51 and a dense fiber 3D-network was already observed for carbon dots-hydrogels.36,37 Both the morphology of the nanodots and gels 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.63,65 The closely packed network was observed also by polarizing optical microscopy (POM) and scanning electron microscopy (SEM) images (Figures 5d,e). The needle-like fibers observed from the POM image (Figure 5d) are complemented by the scanning electron micrograph showing the fibrous nature of the dried xerogel (Figure 5f).

Table 4. Maxima of RLS intensity peaks for systems in [bmim][BF4] and in [bmim][NTf2]. Imax RLS (a.u.) NCND-ionogels SOL

GEL

p-C12/[BF4]-NCND0.1

365.8

242.0

m-C12/[BF4]-NCND0.1

172.9

165.8


145.4

215.4


86.0

320.7


40.6

276.6

m-C12/[NTf2]-NCND0.1

7.5

206.7


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ACS Nano

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.69,70 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 both the gelator/IL nature and the NCNDs 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 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),51 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 infra).


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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 mC12-based hybrid gels did not allow the measurements, as it was observed for the ionogels without NCNDs.51 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.71

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 wt%, 0.05 wt% and 0.025 wt% of dots respectively.

For all the gels, a two-step formation mechanism was observed (Figure 6). The absorbance rapidly increases, then staying constant for ca. 1 h (in case of NCNDs0.05) or ca. 8 h (in case of NCNDs0.1 and NCNDs0.025), while, in a second step, it further increases until 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


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ACS Nano

objects) before their intertwinement into 3D fibrillar networks.72 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 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.51 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, similarly to previously reported carbon nanotubes-based organogels.60 Moreover, a significant drop in crystallinity values was detected passing from 0.025 wt% 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 presence of 0.1 wt% of NCNDs. Although it is not possible to directly ascribe this phenomenon to the percentage of NCNDs, however 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 X-ray analysis).51 The surface of the NCNDs, being abundant in (primary) amino groups on their surface,5 could establish H-bonds 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)73 or π-stacking (observed for p-C12


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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).74 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 infra), 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.


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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.75–77 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 showed significantly higher antioxidant ability when compared to carbon dots without an amine-rich shell or when coated with carboxylic groups (5% and 55% efficiency, respectively for Arg-CDs and Cit-CDs, Figure S9). This activity, therefore, was ascribed to the presence of primary amino terminations, similarly to previously reported (benzyl)amines, with the hydrogen donating ability able to stabilize the free radicals.78 Next, we investigated the radical scavenging efficiency of our systems and we focused, in particular, on the most fluorescent p-C12/[NTf2]-NCND0.1 ionogel, since an interesting relation between FLQY and radical scavenger efficiency has been recently reported for carbon dots solutions.21 The efficiency of the gel was notably high (88%) after only 5 minutes of contact between the radical solution and the gel, while it reached the maximum value by achieving 100% of efficiency after 1 h (Table S3). Interestingly, the gel phase remains stable even after this exposure to the radical solution (Figure S10), showing a great potential as coating material (also


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for prolonged contacts). The integrity of the gel after the exposure to the radical solution was further confirmed by the tube inversion test (Figure S10). In order to shed light on the components responsible of the antiradical efficiency, we evaluated the radical scavenging activity of both NCNDs and DOS-IL solutions (Table 5). To favor the NCNDs solubilization in the ethereal DPPH solution, nanodots were firstly dissolved in methanol and a radical scavenger activity of 88% after 1 h was observed. On the other hand, both NCNDs-IL solution and DOS-IL solution showed a moderate activity (37% and 28% after 1 h). On the basis of these combined pieces of evidence, the NCNDs not only improved the moderate radical scavenger activity of the pure gel, but also benefited from the immobilization in the ionogel phase, showing higher antioxidant efficiency than in solution. Moreover, a comparison with previously reported carbon dots, as well as L-ascorbic acid as a standard radical scavenger (Table 5),18–20 further confirms the prospects of the present system. On one hand, NCNDs presented a prominent antioxidant activity in solution when compared to other carbon dots, being only less effective than L-ascorbic acid. On the other hand, the hybrid gels showed higher efficiency as compared to the reference nanomaterials in solution and comparable activity only with the L-ascorbic acid (either in methanol or in ionic liquid), thus offering a valuable alternative when using solutions is not suitable from an applicative point of view. Table 5. Comparison of Percentage of Radical scavenger activity (E) for systems studied and some ones previously reported. E (%) NCNDs in MeOH

88 (60 min)

Cit-CDs74 in MeOH

55 (60 min)


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Arg-CDs5 in MeOH

5 (60 min)

NCNDs in [bmim][NTf2]

37 (60 min)


100 (60 min)

p-C12 in [bmim][NTf2] solution

38 (60 min)

L-ascorbic acid in MeOH

100 (60 min)

L-ascorbic acid in [bmim][NTf2]

100 (60 min)

N,S co-doped CNDs

56 (60 min)20

N,S co-doped CNDs from garlic

88 (

Nitrogen-Doped Carbon Nanodots-Ionogels: Preparation

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