Polymer Gels - ACS Publications


Jan 7, 2019 - KEYWORDS: carbon dots, self-healing gels, fluorescent gels, dynamic covalent bonds, ... determining the overall emission color of the ge...
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Fluorescent Self-Healing Carbon Dot / Polymer Gels Sagarika Bhattacharya, Ravindra Suresh Phatake, Shiran Nabha Barnea, Nicholas Zerby, Jun-Jie Zhu, Rafi Shikler, Norberto Gabriel Lemcoff, and Raz Jelinek ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07087 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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Fluorescent Self-Healing Carbon Dot / Polymer Gels Sagarika Bhattacharya,1 Ravindra Suresh Phatake,1 Shiran Nabha Barnea,2 Nicholas Zerby,1 Jun-Jie Zhu,3 Rafi Shikler,2 Norberto Gabriel Lemcoff, 1,4 and Raz Jelinek*,1,4 1

Department of Chemistry, Ben Gurion University of the Negev, Beer Sheva 84105, Israel. E-

mail: [email protected]; Fax: (+) 972-8-6472943 2

Department of Electrical and Computer Engineering, Ben Gurion University of the Negev, Beer

Sheva 84105, Israel. 3

State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and

Chemical Engineering, Nanjing University, Nanjing 210023, China 4

Ilse Katz Institute for Nanotechnology, Ben Gurion University of the Negev, Beer Sheva 84105,

Israel

* [email protected]

KEYWORDS: carbon dots; self-healing gels; fluorescent gels; dynamic covalent bonds; imines; aldehydes.

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ABSTRACT

Multicolor, fluorescent self-healing gels were constructed through reacting carbon dots produced from different aldehyde precursors with branched polyethylenimine. The self-healing gels were formed through Schiff base reaction between the aldehyde units displayed upon the carbon dots' surface and primary amine residues within the polyethylenimine network, generating imine bonds. The dynamic covalent imine bonds between the carbon dots and polymeric matrix endowed the gels with both excellent self-healing properties, as well as high mechanical strength. Moreover, the viscoelastic properties of the gels could be intimately modulated by controlling the ratio between the carbon dots and polymer. The distinct fluorescence emissions of the gels, originating from the specific carbon dot constituents, were employed for fabrication of light emitters at different colors, particularly generating white light.

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Carbon dots (C-dots) have attracted significant interest in recent years due to their fluorescence and optical properties, biocompatibility, and simple synthetic routes from readily-available substances.1-6 The multicolor fluorescence properties, photo-stability, low toxicity, and ease of surface functionalization have made C-dots promising conduits for varied applications, from sensing and bioimaging to optical devices and photo-catalysts. Importantly, immobilization of Cdots within supramolecular frameworks has been pursued, designed to prevent aggregationinduced fluorescence quenching and produce fluorescent materials for potentially practical applications.7-11 C-dots have been incorporated, for example, in epoxy resin8,12,13 poly (methylmethylacrylate),13-16

polyurethane,16

polyvinylalcohol,11,17-19

and

methyl/phenyl-

triethoxysilane (MTES/PTES),20,21 and some of these matrixes have been examined as optical platforms. C-dots were embedded as guest species within ionogels,10,22 hydrogels,9,23-25 and organogels26 and these hybrid materials have been used for sensing, optical, and other applications. Self-healing gels have garnered broad scientific and technological interest due to their intrinsic ability to repair after enduring damage. Self-healing gels have been categorized as physical selfhealing gels, formed through non-covalent interactions among the molecular gelating agents (e.g. hydrophobic interactions, hydrogen bonding), and chemical self-healing gels assembled via dynamic covalent bond formation.27-31 Self-healing covalent gels generally exhibit greater resilience and mechanical stabilities compared to the non-covalent counterparts. Such gels have been mostly synthesized through disulfide exchange,32,33 boronic acid condensation,27, 34 and imine chemistry.35,36 Immobilization of various nanomaterials in self-healing gels has been reported, including metal nanoparticle,37,38 graphene oxide (GO),37, 39,40 and carbon nanotubes (CNT).37,39,40 In a recent work, supramolecular hydrogels were constructed by an unprotected tripeptide serving as a host matrix

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and oxidized nanocarbon guests.39 The nanomaterials in these hybrid gel systems have been shown to modulate the physico-chemical properties of the gels. Importantly, such nanomaterials have been embedded in the gel matrix by weak, non-covalent interactions. C-dots have been also encapsulated within self-healing gels. Specifically, self-healing diimidazolium-based ionogels were doped with C-dots, in which dicationic organic salts were used as gelators.10 C-dot-induced gelation of histidine-based gels was also described.41 While, as indicated above, C-dots have been incorporated as guest species within self-healing gels, the use of C-dots as actual gel building blocks have not been reported yet. Here, we present synthesis of fluorescent self-healing gels through reaction between polyethylenimine (PEI) and Cdots prepared from aldehyde species as the carbonaceous building blocks. The self-healing gels were formed through Schiff base reaction between the aldehyde residues upon the C-dots' surfaces and the primary amines of PEI, generating dynamic imine bonds. Importantly, the C-dots served here both as covalent cross-linkers in the gel framework, as well as fluorophores determining the overall emission color of the gels. In effect, the C-dots serve as the actual gelator species in selfhealing gels. Importantly, the ratio between the C-dot and polymer constituents determined the viscoelastic properties of the gels. In addition, the multicolor fluorescent properties and transparent nature of the C-dot/PEI films were also exploited for generating differently-colored light through illumination with a blue light emitting diode (LED).

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RESULTS / DISCUSSION

Experimental strategy. Figure 1 illustrates the synthesis schemes and assembly of the fluorescent C-dot/polymer self-healing gel. The thrust of the experimental strategy is the construction of aldehyde-displaying C-dots employed as the cross-linker units in the self-healing gels. Figure 1A depicts the three aldehyde building blocks and experimental conditions utilized to generate the differently-colored C-dots. Specifically, the aliphatic dialdehyde glutaraldehyde yielded green-fluorescence C-dots denoted G-C-dots (Figure 1A,i); benzaldehye, an aromatic aldehyde was used to produce blue-fluorescence C-dots (B-C-dots); a cyclooctadiene-aldehyde polymeric derivative was the carbonaceous precursor for construction of yellow C-dots (CoAP-Cdots). As indicated in Figure 1A, the distinct C-dot fluorescence emission properties (e.g. different colors) are determined by the different carbonaceous precursors employed.8, 42,43

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Figure 1. Synthesis of the aldehyde-C-dots and C-dot/PEI gels. A. Synthesis of aldehyde containing C-dots starting from an aliphatic dialdehyde (glutaraldehyde, i), an aromatic aldehyde (benzaldehyde, ii), and a ROMP-derived cyclooctadiene-aldehyde polymer (iii). B. Assembly of the C-dot/PEI gel through Schiff base reaction between the aldehyde units upon the C-dots' surface and the amines within the PEI framework. C. Self-healing properties of the gel are attained through

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reversible imine bond formation. Representative fluorescence images of the self-healing phenomenon are shown.

Figure 1B depicts the gel synthesis strategy, particularly the crucial structural role of the aldehyde residues. The C-dots displayed aldehydes on their surfaces due to the mild reaction conditions (specifically low reaction temperatures), which did not pyrolyze the carbonaceous precursor molecules.44 Upon mixing of the C-dots and polyethylenimine (PEI), Schiff base reaction between the aldehyde residues and the abundant amines within the branched PEI framework yielded imine bonds which stabilized the resultant C-dot/PEI gel (Figure 1B).45 The gel was prepared in an ethanol / chloroform mixture as the C-dots were not soluble in water. Notably, the C-dot/PEI gels exhibited fluorescence colors that echo the fluorescence emission wavelengths of the aldehyde C-dot building blocks (i.e. Figure 1A). The dynamic covalent imine bonds are the core structural element responsible for the self-healing properties of the gel, schematically shown in Figure 1C. Specifically, the imine bonds can be readily broken and reconstituted in mild conditions, essentially allowing mechanical disruption and repair of the fluorescent C-dot/PEI gel (Figure 1C).35, 46,47

Characterization of the aldehyde C-dots and C-dot//PEI gels. The aldehyde C-dots were characterized by several microscopic and spectroscopic techniques (Figure 2). The high-resolution transmission electron microscopy (HR-TEM) images recorded for the three C-dot species (Figure 2A) reveal well-resolved lattice planes confirming the formation of sp2 graphitic cores.8 The d spacing calculated for the G-C-dots was 0.28 nm (Figure 2A,i) corresponding to the (020) plane of graphitic carbon44 whereas the interplanar distances in case of both B-C-dots and CoAP-C-dots

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were 0.24 and 0.34 nm, ascribed to the (100) plane of graphene and (001) of graphite.8 X-ray diffraction (XRD) patterns recorded for the three C-dots (Figure S4) confirm the crystallinity of the carbon cores. Based upon the HR-TEM analyses the average C-dot particle sizes were 6 ± 3 nm, 3 ± 1 nm, and 7 ± 2 nm for G-C-dots, B-C-dots and CoAP-C-dots, respectively (Figure S5). Atomic force microscopy experiments further attest to the quasi-spherical morphology of the Cdots and the relatively uniform size distribution (Figure S6).

Figure 2. C-dot characterization. A. High resolution transmission electron microscopy (HRTEM) images of the G-C-dots (i), B-C-dots (ii), and CoAP-C-dots (iii). Lattice fringes within the graphitic cores of the C-dots are indicated. Scale bars correspond to 2 nm. B. X-ray photoelectron spectroscopy (XPS) analysis of C 1s (i) and O 1s (ii) of CoAP-C-dots. The aldehyde C=O peaks are highlighted.

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X-ray photoelectron spectroscopy (XPS) data in Figure 2B disclose the different atomic species in the CoAP-C-dots, particularly confirming the presence of aldehyde units at the C-dot surface [qualitatively similar results, displaying slight precursor-dependent spectral shifts, were obtained for G-C-dots (Figure S7) and B-C-dots/G-C-dots (Figure S8)]. The deconvoluted high resolution C1s spectra in Figure 2B,i reveal four carbon species, corresponding to sp2 (C=C) at 284.8 eV, sp3 (C ̶ C/C ̶ H) at 285.3 eV, C ̶ O at 286.5 eV, and C=O at 287.7 eV.42 The deconvoluted O1s spectrum shows three Gaussian peaks ascribed to quinone C=O at 531.9 eV, carbonyl (C=O) at 532.5 eV, and etheric oxygen at 533.08 eV.48,49 Together, the C1s peak at 287.7 eV and O1s signal at 532.5 eV confirm the display of aldehyde residues at the CoAP-C-dots' surface. Application of the purpald-test50 lent further evidence for the presence of abundant aldehyde units upon the C-dots (Figure S9 and Scheme S5). Figure 3 presents experimental data attesting to formation of the imine bond-supported Cdot/PEI gel (i.e. Figure 1B). The excitation-dependent emission spectra of CoAP-C-dots (dissolved in chloroform) prior to, and after gel formation are depicted in Figure 3A. The as-synthesized CoAP-C-dots display a maximum emission peak at around 580 nm (excitation 470 nm) accounting for the yellow appearance of the C-dots (Figure 3A,i). Notably, changes in the excitationdependent emission spectra are apparent following mixing the CoAP-C-dots with PEI, leading to gel formation (Figure 3A,ii). Specifically, the emission peaks undergo experimentally-significant wavelength shifts. For example, the maximal intensity peak shifted from 580 nm (for soluble CoAP-C-dots) to ~565 nm in the CoAP-C-dot/PEI gel (Figure 3A). The spectral transformations reflect the distinct C-dots' chemical environments in the C-dot/PEI gel framework compared to the soluble state prior to gel formation. Similar spectral modulations were recorded for the G-C-dots

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and B-C-dots upon reaction with PEI (Figures S10-S11). The shifts in maximal fluorescence emission peak positions for the three C-dot/PEI gels are summarized in Table S1. The lack of aggregation-induced fluorescence quenching, widely-observed in C-dot systems in the solid phase, should be also emphasized. This is directly related to immobilization of the cross-linked C-dots within the gel matrix, thereby preventing their close proximity and concomitant self-quenching.

Figure 3. Experimental evidence for coupling between the aldehyde C-dots and PEI, generating self-healing gels. A. Excitation-dependent fluorescence emission spectra of CoAP-Cdots in chloroform solution (1 mg/ml, i) and CoAP-C-dot/PEI gel (ii). The different excitation

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wavelengths are indicated. B. FT-IR spectra of soluble CoAP-C-dots (black) and CoAP-C-dot/PEI gel (red). C. Uv-vis spectra of soluble CoAP-C-dots (black), CoAP-C-dot/PEI gel (red) and pure polyethylenimine (blue). D. XPS N 1s spectra of PEI alone (i) and CoAP-C-dot/PEI gel (ii).

To elucidate the specific molecular transformations associated with gel formation, we carried out Fourier transform-infrared (FT-IR) spectroscopy experiments (Figure 3B). The FT-IR analysis reveals that the aldehyde peak at ~1700 cm-1,35, 51 recorded in the soluble CoAP-C-dots (black spectrum in Figure 3B) was virtually eliminated in the CoAP-C-dot/PEI gel (red spectrum in Figure 3B). The dramatic attenuation of the 1700 cm-1 signal is ascribed to the Schiff base reaction between the aldehydes on the C-dots' surface and the primary amines within the PEI matrix, generating imine bonds displaying the FT-IR band at 1650 cm-1. Transformations of the FT-IR spectra were similarly observed following formation of G-C-dot/PEI gel and B-C-dot/PEI gel (Figure S12). The ultraviolet-visible (uv-vis) absorbance spectra of CoAP-C-dots and CoAP-C-dot/PEI gel, respectively, in Figure 3C fall in line with the FT-IR data, providing further evidence for imine bond formation accounting for the assembly of the C-dot/PEI gel. Soluble CoAP-C-dots gave rise to an absorbance peak at 290 nm corresponding to π–π* transition of the aromatic C=C sp2 carbons, while the shoulder at 340 nm is attributed n–π* transition of the C=O bonds (Figure 3C, black spectrum).44 Following reaction between the CoAP-C-dots and PEI, however, the π–π* transition peak broadened, together with appearance of a new peak at 380 nm (Figure 3C, red spectrum), which corresponds to imine absorption.52 The significant broadening and red shift of the π–π* peak reflect the immobilization of the C-dots in the gel.53 Similar gel-associated transformations of the uv-vis spectra were recorded in case of G-C-dot/PEI and B-C-dot/G-C-dot/PEI gels (Figure S13).

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The XPS N 1s data in Figure 3D provide additional experimental evidence for the occurrence of Schiff base reaction between the C-dot-displayed aldehydes and the amine residues in PEI. The N 1s peak of PEI was deconvoluted, yielding two peaks at 398.5 eV assigned to secondary and the tertiary nitrogen atom of the polymer matrix, and at 400.8 eV, corresponding to the primary amines (–NH2) of PEI (Figure 3D,i).51 Specifically, the XPS peak at 400.8 eV completely disappeared upon gel formation (Figure 3D,ii), accounting for the covalent imine bonds in the C-dot/PEI gel.54 The deconvolution of high resolution N 1s peak of the CoAP-C-dots/PEI gel reveals a new species at 399.4 eV attributed to the –C=N peak.44 Similar XPS N 1s results were recorded for the G-Cdot/PEI gel and B-C-dot/PEI gel (Figure S14). The relative percentages of carbon and oxygen in the C-dots and the gel, based upon XPS, are given in Table S2. Overall, both the spectral transformations recorded in the XPS analyses (Figures 3D and S14) and FT-IR data (Figure 3B and Figure S12) provide experimental evidence for the formation of imine bonds in the composite gel assembly. Figure 4 presents morphological and rheological analyses of the CoAP-C-dot/PEI gel. A representative scanning electron microscopy (SEM) image in Figure 4A shows oriented wrinkles on the surface of the C-dot/PEI composite gel. This aligned wrinkle morphology may be ascribed to the directionality of the elongated PEI network. SEM images depicting similar surface appearances were recorded for gels assembled from the other two aldehyde C-dot precursors (Figure S15). Interestingly, the film topography was modulated depending upon the C-dot species used. Specifically, the width of the wrinkles, calculated from the SEM images, was between 0.250.33 µm for B-C-dots/G-C-dot/PEI gel, 0.65-0.8 µm in case of the CoAP-C-dots/PEI and G-Cdots/PEI gels (Figure S15). Corrugated surface morphologies were reported for other self-healing gels.55-56

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The rheology analysis presented in Figure 4B-D highlights the viscoelastic properties of the CoAP-C-dot/PEI gel. In the frequency range of 0.1-100 rad/s at constant oscillation stress of 15 Pa, the higher storage modulus (Gʹ) values were higher than the loss modulus (Gʹʹ), confirming that CoAP-C-dot/PEI adopted a gel organization (Figure 4B). The Gʹ value reached a maximum of 3.0 kPa with increasing frequency, indicating that our C-dot crosslinked gel covalent imine gel exhibit high mechanical strength. Figure 4C reveals that the CoAP-C-dots/PEI gel exhibits a dependence of Gʹ and Gʹʹ on percentage of applied strain (), accounting for gel assembly. Notably, Figure 4C demonstrates that the gel is resilient up to a 500 % strain at fixed angular frequency of 1 Hz before the occurrence of gel collapse (Gʹʹ > Gʹ). The extraordinary mechanical strength of the gel reflects the contribution of the imine bond network within the aldehyde C-dot / PEI gel.

Figure 4. Morphology and rheological properties of the C-dot/PEI gel. A. Scanning electron microscope image of CoAP-C-dots/PEI gel. Scale bar corresponds to 10 µm. B. Frequency sweep

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measurement for CoAP-C-dots/PEI gel (oscillation stress 15 Pa). C. Strain sweep experiment of a 10% CoAP-C-dot/PEI gel (angular frequency 1 Hz). D. Dependence of Gʹ and Gʹʹ upon C-dot concentration in the CoAP-C-dot/PEI gel (at angular frequency of 1 rad/s and oscillation stress 15 Pa), recorded at room temperature.

The bar diagram in Figure 4D underscores the intimate relationship between C-dot concentration within the gel framework and the gels' viscoelastic properties. Specifically, G' was significantly lower upon reducing the weight ratio between the CoAP-C-dots and PEI, while G" appeared unaffected. This result further attests to the covalent incorporation of the C-dots within the gel framework, echoing the spectroscopic data in Figure 3. Furthermore, Figure 4D indicates that the mechanical strength of the gel can be modulated by varying the C-dot concentration. Rheology profiles similar to those depicted in Figure 4 were recorded for the G-C-dot/PEI gel and B-Cdot/G-C-dot/PEI gel (Figure S16-17), confirming the generality of the C-dot/PEI viscoelastic properties. The ratio between C-dot and polymer constituents also affected the fluorescence intensity (but not the fluorescence peak position), as shown in Figure S18. Self-healing properties of the aldehyde-C-dot/PEI gels. Figure 5 highlights the self-healing properties of the aldehyde-C-dot/PEI gels, attributed to the dynamic covalent nature of the imine bonds between the aldehyde-displaying C-dots and the amine residues of PEI. The strain alternation experiment in Figure 5A demonstrates viscoelasticity recovery of the CoAP-C-dot/PEI gel. Initially the gel was placed under a low 0.1 % strain for 180 sec at room temperature for which Gʹ > Gʹʹ. Figure 5A shows that upon applying a high strain of 300 % for 180 s the gel became viscous (i.e. Gʹʹ > G'). Notably, after returning the strain back to 0.1 % (for 240 sec), the gel rapidly healed (within 10 sec), almost returning to its initial viscoelastic profile. Figure 5A demonstrates

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that the alternating strain experiment could be repeated four times, demonstrating the self-healing properties of the C-dot/PEI gel. Notably, application of 300% strain at the 5th cycle resulted in a pronounced, unrecoverable decrease in Gʹʹ and Gʹ values of the CoAP-C-dot/PEI gel, rendering the 5th cycle irreversible (Figure 5A). Similar thixotropic properties of the G-C-dots/PEI and B-Cdot/G-C-dots/PEI gels were also recorded (Figure S19). The G-C-dots/PEI gel however, could completely self-heal even after 5 cycles than CoAP-C-dot/PEI gel (Figure S19). It should be noted that the mechanical resilience of the aldehyde-C-dot/PEI gels depicted in Figure 5A is significantly higher than previously reported C-dot-containing ionogel (applied strain 25 %),10 or a Cdot/hydrogel (applied strain 100 %),41 reflecting the covalent bonding between the C-dots and PEI. Self-healing gels are highly sensitive to external stimuli. Indeed, addition of water accelerated the self-healing rates (as the gel swelled). Similarly, heating also enhanced the self-healing process.57

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Figure 5. Self-healing behavior of the C-dot/PEI gels. A. Strain recovery experiment: Gʹ (■) and Gʹʹ (●) values of CoAP-C-dots/PEI gel at a fixed angular frequency of 1 Hz. Strain percentage values () are indicated. B. G-C-dots/PEI gel film (i) cut into two pieces (ii). The two pieces fused together within 60 sec after manually pushed together (iii). The top row presents conventional photographs, while the bottom row shows the fluorescence images (excitation 365 nm). C. Photographs showing a smooth surface of a CoAP-C-dot/PEI gel (i) impacted by a tweezer tip (ii), iii and iv show the gel surface after 30 min and 1 hour, respectively. D. Merging of a CoAP-Cdot/PEI and B-C-dot/G-C-dot/PEI gels. The two films were manually interfaced for 3 minutes.

The photographs and fluorescent images in Figure 5B-D provide a vivid demonstration of the self-healing properties of the aldehyde-C-dot/PEI gels. Figure 5B depicts conventional photographed (top row) and fluorescent images (bottom row, exc. 365 nm) images of a G-Cdot/PEI film, cut in the middle, and recovered through self-healing. Notably, the two separate pieces of the gel shown in Figure 5B,ii re-attached upon attaining a physical contact within less than 60 seconds (Figure 5B,iii), attesting to the rapid room temperature reconstitution of the imine bonds at the interface between the two films. Indeed, the border between the two fused pieces could hardly be deciphered in the recovered film, both in the conventional photographs nor in the fluorescence images (Figure 5B,iii). Figure 5C underscores a self-healing effect upon applying a local deformation (scratch) upon the surface of a CoAP-C-dot/PEI gel. Specifically, Figure 5C shows that a gash created upon the gel surface gradually disappeared (within less than an hour). Figure 5D shows that self-healing can be exploited for attaching gels exhibiting different C-dot compositions. The conventional (left) and fluorescent (right) images in Figure 5D show a spontaneously-fused film comprising CoAP-

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C-dots/PEI gel and G-C-dot/B-C-dot/PEI gel after attaining a physical contact for 3 minutes. As depicted in Figure 5D, the resultant film was tightly fused and resilient, effective fusion was confirmed through lifting the film from one side by the tweezer without disintegration. The reversibility of the imine bonds formed was confirmed by detecting pH-induced gel disintegration and assembly (Figure S20).35,46,47 The pH dependency of C-dots intrinsic fluorescence inside the gel framework was also monitored for the G-C-dots/PEI gel (Figure S21). Notably, gels could not be assembled when C-dots comprising citric acid as the molecular building block were employed (Figure S22), attesting to the critical role of imine bonds in stabilizing the gel structure through Cdot cross-linking. Optical properties of the aldehyde-C-dot/PEI gels. The multicolor properties of the aldehydeC-dot/PEI gels can be exploited for optical applications (Figure 6). In the experiments presented in Figure 6, the light emissions of different aldehyde-C-dot/PEI gel films deposited upon quartz slides were evaluated upon irradiation with a light emitting diode (LED) emitting blue light (403 nm). The photoluminescent quantum yields of the G-C-dots/PEI gel, B-C-dot/G-C-dot/PEI gel, and CoAP-C-dot/PEI gel were 4%, 2% and 1.9%, respectively. The thicknesses of the B-C-dot/GC-dot/PEI film and CoAP-C-dot/PEI film were 85 µm (measured by Vernier calliper) and 40 µm, respectively (Figure S23). The visible spectra and associated digital photographs in Figure 6A, and corresponding Commission Internationale de I’Eclairage (CIE) chromaticity plot in Figure 6B, reflect the distinct colors emitted by the films (the quantitative optical parameters are indicated in Table S3). The digital images of the films and their photoluminescent images under 365 nm UV lamp are depicted in Figure S24. As highlighted in the CIE plot in Figure 6B, the emission spectra of the fluorescent gel films exhibited the distinct colors of the specific aldehyde-C-dot building blocks: blue in case of G-C-dot/PEI (i); green for the B-C-dot/G-C-dot/PEI film (ii), and yellow

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for the CoAP-C-dot/PEI gel (iii). Notably, generation of white light was accomplished through irradiation of the B-C-dot/G-C-dot/PEI and CoAP-C-dot/PEI films that were stacked horizontally on opposite sides of a quartz glass slide (iv). The data presented in Figure 6 underscore the potential applicability of the aldehyde-C-dot/PEI gels in optical devices, and the intrinsic color tunability available through selection of the C-dot components of the gels.

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Figure 6. Optical properties of the aldehyde-C-dot/PEI gels. A. Emission spectra recorded upon illumination of thin gel films with a 403 nm light emitting diode (LED). The insets show the respective photographs of the LED-illuminated films. i) G-C-dot/PEI gel; ii) B-C-dot/G-C-dot/PEI gel; iii) CoAP-C-dots gel; iv) CoAP-C-dots gel + B-C-dot/G-C-dot/PEI gel. B. CIE (1931) plot exhibiting the chromaticity coordinates of the gels.

CONCLUSIONS

We synthesized fluorescent self-healing gels through reacting aldehyde-containing C-dots and PEI. The gels exhibited multicolor fluorescence, high mechanical strength, and self-healing properties. We showed that the distinct physico-chemical properties of the aldehyde C-dot/PEI gels are due to dynamic covalent imine bonds formed between the C-dots' aldehydes and primary amines of PEI. The C-dots played key roles in determining gel properties. In essence, the C-dots constitute both the fluorophore and the gelator; moreover, the ratio between the C-dots and PEI significantly affected the mechanical profiles of the C-dot / PEI gels formed. Importantly, the absence of aggregation-induced quenching of the C-dot's fluorescence, a well-known phenomenon associated with C-dots in solid phases, is one of the features of the C-dot/polymer gels, endowing them with the multicolor fluorescence properties. Indeed, this observation is ascribed to the fact that the C-dots are cross-linked with the polymer network, together supporting the gel framework; thus the immobilized C-dots maintain sufficient distance among them, blocking aggregationinduced quenching. We also demonstrate that the fluorescence properties of the gels, intrinsically dependent upon the C-dot precursors employed, could be exploited for construction of multicolor light emissive devices; in particular, generation of white light was achieved through usage of C-

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dot/PEI films exhibiting different fluorescence emissions. The tunable fluorescent aldehyde Cdot/PEI self-healing gels could be used in diverse applications, including optical devices, strain sensing, controlled drug release through skin patches, and others.

METHODS/EXPERIMENTAL

Materials. Benzaldehyde ≥ 99%, cis-1,5-cyclooctadiene (99+%), meta-chloroperoxybenzoic acid (77%), lithium aluminium hydride (95%), 4-carboxybenzaldehyde (97%), N,N'dicyclohexylcarbodiimide (99%), 4-dimethylaminopyridine (99%), Grubbs catalyst™ 2nd generation, ethyl vinyl ether (99%), 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (≥99%) (purpald), sodium hydroxide, and branched polyethylenimine, (average Mw ~25,000) were purchased from Sigma Aldrich, St. Louis, MO, USA. Glutaraldehyde, 50% aqueous solution was bought from Alfa Aesar. All the chemicals were used without further purification. Methanol, ethanol, dichloromethane, tetrahydrofuran, chloroform and triethylamine were purchased from Bio-Lab Ltd. (Jerusalem, Israel). Glutaraldehyde C-dots (G-C-dots) / PEI gel. Glutaraldehyde (300 µL) was mixed with ethanol (600 µL) in a Teflon tightened 20 ml glass vial and heated in an oven (kept on oven floor) at 150 °C for 1 hr. After the reaction, the vial was cooled down and the resultant brown solid was redispersed in acetone and chloroform and centrifuged at 10,000 rpm for 15 min to remove highweight carbon aggregates. This was repeated for three times for each solvent and was evaporated under reduced pressure to obtain a brown solid. Next the brown solid was redissolved in ethanol for further characterization and use. G-C-dots (100 mg in 5 ml ethanol) were added dropwise to a solution containing 1 g of polyethylenimine (PEI) in 15 ml ethanol, stirred vigorously overnight

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in which the volume was reduced to 4 ml. The suspension was then kept at room temperature for 24 hours for gelation (as tested from the “stable to inversion” method confirming gel viscosity).23 Benzaldehyde-C-dot (B-C-dot) / PEI gel. Benzaldehyde (300 µL) was dissolved in 50 mL ethanol and the solution was transferred to a poly(tetrafluoroethylene) (Teflon)-lined autoclave (100 mL) and heated at 180 °C for 20 hours. Next it was cooled down to room temperature and the solvent was evaporated under reduced pressure. The brown solid was purified by a similar method to the stated above. Next 100 mg of B-C-dots and 30 mg of G-C-dots in 5 ml ethanol solution was added drop by drop to an ethanolic solution of PEI (1 g) under continuous stirring with constant heating at 80 °C for overnight. Subsequently, excess ethanol was evaporated under reduced pressure down to 3 mL volume and the suspension was kept at room temperature for gelation. Cyclooctadiene-aldehyde-polymer (CoAP)-C-dot / PEI gel. Detailed procedure for synthesis of the ring opening metathesis polymerization (ROMP)-derived cyclooctadiene-aldehyde polymer (CoAP) and characterization are provided in the Supporting Information (Figure S1-3 and Scheme S1-4). CoAP (50 mg) was dissolved overnight in 50 ml chloroform, transferred to a poly(tetrafluoroethylene) (Teflon)-lined autoclave (100 mL), and heated at 180 °C for 20 hours resulting in a brown solution. After filtration, the excess solvent was evaporated under reduced pressure. This procedure was repeated for purification. Subsequently, 100 mg of CoAP-C-dots in 5 ml chloroform were added to a PEI (1 g) / chloroform solution (15 ml) under continuous stirring overnight. Excess chloroform was subsequently evaporated under reduced pressure up to 4 mL volume and kept at room temperature for gelation. Characterization.

Fluorescence

emission

spectra

were

recorded

on

an

FL920

spectrofluorimeter. G-C-dots, B-C-dots and CoAP-C-dots were dissolved in ethanol and

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chloroform, respectively, with concentration of 1 mg/ml. High resolution transmission electron microscopy (HR-TEM) experiments were carried out on a 200 kV JEOL JEM-2100F microscope (Japan) with a drop of C-dots solution was added a on a graphene-coated copper grid and it was dried for 12 h. X-ray photoelectron spectroscopy (XPS) measurements were performed on an Xray photoelectron spectrometer ESCALAB 250 ultrahigh vacuum (1 × 10−9 bar) apparatus with an AlKα X-ray source and a monochromator. Concentrated solution of the C-dots and the corresponding gel were drop-casted on silicon wafers and after drying the experiment were monitored. The X-ray beam size was 500 μm and survey spectra was recorded with pass energy (PE) 150 eV and high energy resolution spectra were recorded with pass energy (PE) 20 eV. Processing of the XPS results was carried out using AVANTGE program. Fourier transforminfrared (FT-IR) measurements were performed on a Thermo Scientific Nicolet 6700 spectrometer. Ultraviolet-visible (UV-Vis) spectra were acquired on a Thermo Scientific Evolution 220 spectrophotometer. Scanning electron microscopy (SEM) images were recorded on a JEOL (Tokyo, Japan) model JSM-7400F scanning electron microscope. The dilute solutions of gel material were dried on a glass cover slip and Gold sputter coating was carried out. Rheological experiments were carried out on an Advanced Rheometer AR 2000 (TA Instruments) by cone and plate geometry in a Peltier plate. The cone diameter was 20 mm, cone angle 1°, and truncation 27 µm. Frequency sweep and strain sweep measurement were performed from 0.1-100 rad/s and 0.11300 %, respectively. The thickness of the B-C-dot/G-C-dot/PEI film was measured by Vernier scale while for the thickness of CoAP-C-dot/PEI film was monitored by using Zygo New View 200 interferometer. Optical measurements. For the optical analysis, aldehyde-C-dot/PEI films were placed on quartz glass and then illuminated with a uv-light emitting diode (LED, 403 nm wavelength

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emission). The current of the LED was controlled by a Keithley Source Meter, operating voltage 2.8 V. An optical fiber cable connected to a Labsphere CDS 2600 detector (USA) was placed above the films to collect the emitted light. For the photoluminescent quantum yield measurements, samples coated on quartz glass slides were placed inside an integrating sphere connected to a Labsphere CDS 2600 detector fitted with an optical cable and irradiated with a 473 nm laser. Chromaticity points (x,y) and correlated color temperature (CCT) were calculated by the Labsphere software from the CIE 1931 coordinate diagram. Color rendering index (CRI) was calculated by monitoring the ratio of the C-dots integrated emissive area with the total area of UVLED and C-dot’s emission.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed synthetic procedure and characterization for COD-aldehyde polymer, Average size distribution by HR-TEM, AFM and XPS of C-dots, Photoluminescence spectra of C-dots, Photoluminescence spectra of C-dots/PEI gel, Excitation and emission wavelength shift of Cdots in solution and in gel matrix, Optical spectra, FT-IR, XPS, SEM, Rheological measurements, Table for optical properties of C-dot/PEI gel film, thickness measurement. (Figures S1- S24, Scheme S1-S5, Table S1-S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Fax: (+) 972-8-6472943 ORCID Raz Jelinek: 0000-0002-0336-1384

ACKNOWLEDGMENT

We are grateful to the Ministry of Science and Technology, Israel, for financial support under the China-Israel grant program. J. Zhu thanks the support from International cooperation foundation from Ministry of Science and Technology of China (2016YFE0130100). We thank Mr. Ahiud Morag for help with the digital images and Mr. Juergen Jopp for assistance with the interferometer and AFM experiments.

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SYNOPSIS

Multicolor fluorescent self-healing gels were constructed through dynamic imine bond formation between aldehyde-containing carbon dots and primary amine and branched polyethylenimine.

Table of Contents (TOC) graphics

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