Fluorescent Self-Healing Carbon Dot/PolymerGelsSagarika Bhattacharya,† Ravindra Suresh Phatake,† Shiran Nabha Barnea,‡ Nicholas Zerby,†

Jun-Jie Zhu,§ Rafi Shikler,‡ Norberto Gabriel Lemcoff,†,∥ and Raz Jelinek*,†,∥

†Department of Chemistry, Ben Gurion University of the Negev, Beer Sheva 84105, Israel‡Department of Electrical and Computer Engineering, Ben Gurion University of the Negev, Beer Sheva 84105, Israel§State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University,Nanjing 210023, China∥Ilse Katz Institute for Nanotechnology, Ben Gurion University of the Negev, Beer Sheva 84105, Israel

*S Supporting Information

ABSTRACT: Multicolor, fluorescent self-healing gels were constructed through reacting carbon dots produced fromdifferent aldehyde precursors with branched polyethylenimine. The self-healing gels were formed through Schiff basereaction between the aldehyde units displayed upon the carbon dots’ surface and primary amine residues within thepolyethylenimine network, generating imine bonds. The dynamic covalent imine bonds between the carbon dots andpolymeric 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 thecarbon dots and polymer. The distinct fluorescence emissions of the gels, originating from the specific carbon dotconstituents, were employed for fabrication of light emitters at different colors, particularly generating white light.KEYWORDS: carbon dots, self-healing gels, fluorescent gels, dynamic covalent bonds, imines, aldehydes

Carbon dots (C-dots) have attracted significant interestin recent years due to their fluorescence and opticalproperties, biocompatibility, and simple synthetic

routes from readily available substances.1−6 The multicolorfluorescence properties, photostability, low toxicity, and ease ofsurface functionalization have made C-dots promising conduitsfor varied applications, from sensing and bioimaging to opticaldevices and photocatalysts. Importantly, immobilization of C-dots within supramolecular frameworks has been pursued,designed to prevent aggregation-induced fluorescence quench-ing and produce fluorescent materials for potentially practicalapplications.7−11 C-dots have been incorporated, for example,in epoxy resin8,12,13 poly(methylmethyl acrylate),13−16 polyur-ethane,16 poly(vinyl alcohol),11,17−19 and methyl/phenyl-triethoxysilane (MTES/PTES),20,21 and some of thesematrixes have been examined as optical platforms. C-dots

were embedded as guest species within ionogels,10,22 hydro-gels,9,23−25 and organogels,26 and these hybrid materials havebeen used for sensing, optical, and other applications.Self-healing gels have garnered broad scientific and

technological interest due to their intrinsic ability to repairafter enduring damage. Self-healing gels have been categorizedas physical self-healing gels, formed through noncovalentinteractions among the molecular gelating agents (e.g.,hydrophobic interactions, hydrogen bonding), and chemicalself-healing gels assembled via dynamic covalent-bondformation.27−31 Self-healing covalent gels generally exhibitgreater resilience and mechanical stabilities compared to the

Received: September 16, 2018Accepted: January 7, 2019Published: January 7, 2019

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© XXXX American Chemical Society A DOI: 10.1021/acsnano.8b07087ACS Nano XXXX, XXX, XXX−XXX

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noncovalent counterparts. Such gels have been mostlysynthesized through disulfide exchange,32,33 boronic acidcondensation,27,34 and imine chemistry.35,36

Immobilization of various nanomaterials in self-healing gelshas been reported, including metal nanoparticle,37,38 grapheneoxide (GO),37,39,40 and carbon nanotubes (CNT).37,39,40 In arecent work, supramolecular hydrogels were constructed by anunprotected tripeptide serving as a host matrix and oxidizednanocarbon guests.39 The nanomaterials in these hybrid gelsystems have been shown to modulate the physicochemicalproperties of the gels. Importantly, such nanomaterials havebeen embedded in the gel matrix by weak, noncovalentinteractions. C-dots have been also encapsulated within self-healing gels. Specifically, self-healing diimidazolium-based

ionogels were doped with C-dots, in which dicationic organicsalts were used as gelators.10 C-dot-induced gelation ofhistidine-based gels was also described.41

While, as indicated above, C-dots have been incorporated asguest species within self-healing gels, the use of C-dots asactual gel building blocks has not been reported yet. Here, wepresent synthesis of fluorescent self-healing gels throughreaction between polyethylenimine (PEI) and C-dots preparedfrom aldehyde species as the carbonaceous building blocks.The self-healing gels were formed through Schiff base reactionbetween the aldehyde residues upon the C-dots’ surfaces andthe 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

Figure 1. Synthesis of the aldehyde-C-dots and C-dot/PEI gels. (A) Synthesis of aldehyde containing C-dots starting from an aliphaticdialdehyde (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 thePEI framework. (C) Self-healing properties of the gel are attained through reversible imine-bond formation. Representative fluorescenceimages of the self-healing phenomenon are shown.

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determining the overall emission color of the gels. In effect, theC-dots serve as the actual gelator species in self-healing gels.Importantly, the ratio between the C-dot and polymerconstituents determined the viscoelastic properties of thegels. In addition, the multicolor fluorescent properties andtransparent nature of the C-dot/PEI films were also exploitedfor generating differently colored light through illuminationwith a blue light-emitting diode (LED).

RESULTS/DISCUSSIONExperimental 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 theconstruction of aldehyde-displaying C-dots employed as thecross-linker units in the self-healing gels. Figure 1A depicts thethree aldehyde building blocks and experimental conditionsutilized to generate the differently colored C-dots. Specifically,the aliphatic dialdehyde glutaraldehyde yielded green-fluo-rescence C-dots denoted G-C-dots (Figure 1A,i), benzaldehye,an aromatic aldehyde, was used to produce blue-fluorescenceC-dots (B-C-dots), and a cyclooctadiene-aldehyde polymericderivative was the carbonaceous precursor for construction ofyellow C-dots (CoAP-C-dots). As indicated in Figure 1A, thedistinct C-dot fluorescence emission properties (e.g., differentcolors) are determined by the different carbonaceousprecursors employed.8,42,43

Figure 1B depicts the gel synthesis strategy, particularly thecrucial structural role of the aldehyde residues. The C-dotsdisplayed aldehydes on their surfaces due to the mild reactionconditions (specifically low reaction temperatures), which didnot pyrolyze the carbonaceous precursor molecules.44 Uponmixing of the C-dots and PEI, Schiff base reaction between thealdehyde residues and the abundant amines within thebranched PEI framework yielded imine bonds which stabilized

the resultant C-dot/PEI gel (Figure 1B).45 The gel wasprepared in an ethanol/chloroform mixture as the C-dots werenot soluble in water. Notably, the C-dot/PEI gels exhibitedfluorescence colors that echo the fluorescence emissionwavelengths of the aldehyde C-dot building blocks (i.e., Figure1A). The dynamic covalent imine bonds are the core structuralelement responsible for the self-healing properties of the gel,schematically shown in Figure 1C. Specifically, the iminebonds can be readily broken and reconstituted in mildconditions, essentially allowing mechanical disruption andrepair 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 severalmicroscopic and spectroscopic techniques (Figure 2). Thehigh-resolution transmission electron microscopy (HR-TEM)images recorded for the three C-dot species (Figure 2A) revealwell-resolved lattice planes confirming the formation of sp2

graphitic cores.8 The d spacing calculated for the G-C-dots was0.28 nm (Figure 2A,i) corresponding to the (020) plane ofgraphitic carbon,44 whereas the interplanar distances in case ofboth B-C-dots, and CoAP-C-dots were 0.24 and 0.34 nm,ascribed to the (100) plane of graphene and (001) ofgraphite.8 X-ray diffraction (XRD) patterns recorded for thethree C-dots (Figure S4) confirm the crystallinity of the carboncores. Based upon the HR-TEM analyses, the average C-dotparticle 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 (AFM) experiments further attest tothe quasi-spherical morphology of the C-dots and the relativelyuniform size distribution (Figure S6).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

Figure 2. C-dot characterization. (A) HR-TEM images of the G-C-dots (i), B-C-dots (ii), and CoAP-C-dots (iii). Lattice fringes within thegraphitic cores of the C-dots are indicated. Scale bars correspond to 5 nm. (B) XPS analysis of C 1s (i) and O 1s (ii) of CoAP-C-dots. Thealdehyde CO peaks are highlighted.

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precursor-dependent spectral shifts, were obtained for G-C-dots (Figure S7) and B-C-dots/G-C-dots (Figure S8)]. Thedeconvoluted high-resolution C 1s spectra in Figure 2B,i revealfour carbon species, corresponding to sp2 (CC) at 284.8 eV,sp3 (C−C/C−H) at 285.3 eV, C−O at 286.5 eV, and CO at287.7 eV.42 The deconvoluted O 1s spectrum shows threeGaussian peaks ascribed to quinone CO at 531.9 eV,carbonyl (CO) at 532.5 eV, and etheric oxygen at 533.08eV.48,49 Together, the C 1s peak at 287.7 eV and O 1s signal at532.5 eV confirm the display of aldehyde residues at theCoAP-C-dots’ surface. Application of the purpald test50 lentfurther evidence for the presence of abundant aldehyde unitsupon the C-dots (Figure S9 and Scheme S5).Figure 3 presents experimental data attesting to formation of

the imine-bond-supported C-dot/PEI gel (i.e., Figure 1B). Theexcitation-dependent emission spectra of CoAP-C-dots (dis-solved in chloroform) prior to and after gel formation aredepicted in Figure 3A. The as-synthesized CoAP-C-dotsdisplay a maximum emission peak at around 580 nm(excitation 470 nm), accounting for the yellow appearance ofthe C-dots (Figure 3A,i). Notably, changes in the excitation-dependent emission spectra are apparent following mixing the

CoAP-C-dots with PEI, leading to gel formation (Figure 3A,ii).Specifically, the emission peaks undergo experimentallysignificant wavelength shifts. For example, the maximalintensity 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 frameworkcompared to the soluble state prior to gel formation. Similarspectral modulations were recorded for the G-C-dots and B-C-dots upon reaction with PEI (Figures S10−S11). The shifts inmaximal fluorescence emission peak positions for the three C-dot/PEI gels are summarized in Table S1. The lack ofaggregation-induced fluorescence quenching, widely observedin 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 closeproximity and concomitant self-quenching.To elucidate the specific molecular transformations

associated with gel formation, we carried out Fouriertransform-infrared (FT-IR) spectroscopy experiments (Figure3B). The FT-IR analysis reveals that the aldehyde peak at∼1700 cm−1,35,51 recorded for CoAP-C-dots (black spectrum

Figure 3. Experimental evidence for coupling between the aldehyde C-dots and PEI, generating self-healing gels. (A) Excitation-dependentfluorescence emission spectra of CoAP-C-dots in chloroform solution (1 mg/mL, i) and CoAP-C-dot/PEI gel (ii). The different excitationwavelengths are indicated. (B) FT-IR spectra of soluble CoAP-C-dots (black) and CoAP-C-dot/PEI gel (red). (C) UV−vis spectra of solubleCoAP-C-dots (black), CoAP-C-dot/PEI gel (red), and pure PEI (blue). (D) XPS N 1s spectra of PEI alone (i) and CoAP-C-dot/PEI gel (ii).

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in Figure 3B), was virtually eliminated in the CoAP-C-dot/PEIgel (red spectrum in Figure 3B). The dramatic attenuation ofthe 1700 cm−1 signal is ascribed to the Schiff base reactionbetween the aldehydes on the C-dots’ surface and the primaryamines within the PEI matrix, generating imine bondsdisplaying the FT-IR band at 1650 cm−1. Transformations ofthe FT-IR spectra were similarly observed following formationof 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 Figure3C fall in line with the FT-IR data, providing further evidencefor imine-bond formation accounting for the assembly of theC-dot/PEI gel. Soluble CoAP-C-dots gave rise to anabsorbance peak at 290 nm corresponding to π−π* transitionof the aromatic CC sp2 carbons, while the shoulder at 340nm is attributed n−π* transition of the CO bonds (Figure3C, 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 Thesignificant broadening and red shift of the π−π* peak reflectthe immobilization of the C-dots in the gel.53 Similar gel-associated transformations of the UV−vis spectra wererecorded in case of G-C-dot/PEI and B-C-dot/G-C-dot/PEIgels (Figure S13).The XPS N 1s data in Figure 3D provide additional

experimental evidence for the occurrence of Schiff basereaction between the C-dot-displayed aldehydes and theamine residues in PEI. The N 1s peak of PEI wasdeconvoluted, yielding two peaks at 398.5 eV assigned tosecondary and the tertiary nitrogen atom of the polymer matrixand at 400.8 eV, corresponding to the primary amines (−NH2)of PEI (Figure 3D,i).51 Specifically, the XPS peak at 400.8 eVcompletely disappeared upon gel formation (Figure 3D,ii),accounting for the covalent imine bonds in the C-dot/PEIgel.54 The deconvolution of the high-resolution N 1s peak of

the CoAP-C-dot/PEI gel reveals a new species at 399.4 eVattributed to the −CN peak.44 Similar XPS N 1s results wererecorded for the G-C-dot/PEI gel and B-C-dot/PEI gel(Figure S14). The relative percentages of carbon and oxygen inthe C-dots and the gel, based upon XPS, are given in Table S2.Overall, both the spectral transformations recorded in the XPSanalyses (Figure 3D and Figure S14) and FT-IR data (Figure3B and Figure S12) provide experimental evidence for theformation 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 electronmicroscopy (SEM) image in Figure 4A shows orientedwrinkles on the surface of the C-dot/PEI composite gel.This aligned wrinkle morphology may be ascribed to thedirectionality of the elongated PEI network. SEM imagesdepicting similar surface appearances were recorded for gelsassembled from the other two aldehyde C-dot precursors(Figure S15). Interestingly, the film topography was modulateddepending upon the C-dot species used. Specifically, the widthof the wrinkles, calculated from the SEM images, was between0.25 and 0.33 μm for B-C-dot/G-C-dot/PEI gel, 0.65−0.8 μmin case of the CoAP-C-dot/PEI and G-C-dot/PEI gel (FigureS15). Corrugated surface morphologies were reported forother self-healing gels.55,56

The rheology analysis presented in Figure 4B−D highlightsthe viscoelastic properties of the CoAP-C-dot/PEI gel. In thefrequency range of 0.1−100 rad/s at constant oscillation stressof 15 Pa, the higher storage modulus (G′) values were higherthan the loss modulus (G″), confirming that CoAP-C-dot/PEIadopted a gel organization (Figure 4B). The G′ value reached amaximum of 3.0 kPa with increasing frequency, indicating thatour C-dot cross-linked gel covalent imine gel exhibits highmechanical strength. Figure 4C reveals that the CoAP-C-dot/PEI gel exhibits a dependence of G′ and G″ on percentage ofapplied strain (γ), accounting for gel assembly. Notably, Figure4C demonstrates that the gel is resilient up to a 500% strain at

Figure 4. Morphology and rheological properties of the C-dot/PEI gel. (A) Scanning electron microscope image of CoAP-C-dot/PEI gel.Scale bar corresponds to 10 μm. (B) Frequency sweep measurement for CoAP-C-dot/PEI gel (oscillation stress 15 Pa). (C) Strain sweepexperiment of a 10% CoAP-C-dot/PEI gel (angular frequency 1 Hz). (D) Dependence of G′ and G″ 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.

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fixed angular frequency of 1 Hz before the occurrence of gelcollapse (G″ > G′). The extraordinary mechanical strength ofthe gel reflects the contribution of the imine-bond networkwithin the aldehyde C-dot/PEI gel.The bar diagram in Figure 4D underscores the intimate

relationship between C-dot concentration within the gelframework and the gels’ viscoelastic properties. Specifically,G′ was significantly lower upon reducing the weight ratiobetween the CoAP-C-dots and PEI, while G″ appearedunaffected. This result further attests to the covalentincorporation of the C-dots within the gel framework, echoingthe spectroscopic data in Figure 3. Furthermore, Figure 4Dindicates that the mechanical strength of the gel can bemodulated by varying the C-dot concentration. Rheologyprofiles similar to those depicted in Figure 4 were recorded forthe G-C-dot/PEI gel and B-C-dot/G-C-dot/PEI gel (FiguresS16−17), confirming the generality of the C-dot/PEIviscoelastic properties. The ratio between C-dot and polymerconstituents also affected the fluorescence intensity (but notthe 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 thealdehyde-C-dot/PEI gels, attributed to the dynamic covalentnature of the imine bonds between the aldehyde-displaying C-dots and the amine residues of PEI. The strain alternationexperiment in Figure 5A demonstrates viscoelasticity recoveryof the CoAP-C-dot/PEI gel. Initially the gel was placed under alow 0.1% strain for 180 s at room temperature for which G′ >G″. Figure 5A shows that upon applying a high strain of 300%for 180 s, the gel became viscous (i.e., G″ > G′). Notably, afterreturning the strain back to 0.1% (for 240 s), the gel rapidlyhealed (within 10 s), almost returning to its initial viscoelasticprofile. Figure 5A demonstrates that the alternating strainexperiment could be repeated four times, demonstrating theself-healing properties of the C-dot/PEI gel. Notably,application of 300% strain at the fifth cycle resulted in a

pronounced, unrecoverable decrease in G″ and G′ values of theCoAP-C-dot/PEI gel, rendering the fifth cycle irreversible(Figure 5A). Similar thixotropic properties of the G-C-dot/PEIand B-C-dot/G-C-dot/PEI gels were also recorded (FigureS19). The G-C-dot/PEI gel, however, could completely self-heal even after 5 cycles than CoAP-C-dot/PEI gel (FigureS19). It should be noted that the mechanical resilience of thealdehyde-C-dot/PEI gels depicted in Figure 5A is significantlyhigher than previously reported C-dot-containing ionogel(applied strain 25%),10 or a C-dot/hydrogel (applied strain100%),41 reflecting the covalent bonding between the C-dotsand PEI. Self-healing gels are highly sensitive to externalstimuli. Indeed, addition of water accelerated the self-healingrates (as the gel swelled). Similarly, heating also enhanced theself-healing process.57

The photographs and fluorescent images in Figure 5B−Dprovide a vivid demonstration of the self-healing properties ofthe aldehyde-C-dot/PEI gels. Figure 5B depicts conventionalphotographed (top row) and fluorescent images (bottom row,exc. 365 nm) of a G-C-dot/PEI film, cut in the middle, andrecovered through self-healing. Notably, the two separatepieces of the gel shown in Figure 5B,ii reattached uponattaining a physical contact within <60 s (Figure 5B,iii),attesting to the rapid room temperature reconstitution of theimine bonds at the interface between the two films. Indeed, theborder between the two fused pieces could hardly bedeciphered in the recovered film, both in the conventionalphotographs 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 createdupon the gel surface gradually disappeared (within <1 hour).Figure 5D shows that self-healing can be exploited forattaching gels exhibiting different C-dot compositions. Theconventional (left) and fluorescent (right) images in Figure 5Dshow a spontaneously fused film comprising CoAP-C-dot/PEI

Figure 5. Self-healing behavior of the C-dot/PEI gels. (A) Strain recovery experiment: G′ (black ■) and G″ (red ●) values of CoAP-C-dot/PEI gel at a fixed angular frequency of 1 Hz. Strain percentage values (γ) are indicated. (B) G-C-dot/PEI gel film (i) cut into two pieces (ii).The two pieces fused together within 60 s after manually pushed together (iii). The top row presents conventional photographs, while thebottom 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 h, respectively. (D) Merging of a CoAP-C-dot/PEI andB-C-dot/G-C-dot/PEI gels. The two films were manually interfaced for 3 min.

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gel and G-C-dot/B-C-dot/PEI gel after attaining a physicalcontact for 3 min. As depicted in Figure 5D, the resultant filmwas tightly fused and resilient, and effective fusion wasconfirmed through lifting the film from one side by the tweezerwithout disintegration. The reversibility of the imine bondsformed was confirmed by detecting pH-induced gel disintegra-tion and assembly (Figure S20).35,46,47 The pH dependency ofC-dots intrinsic fluorescence inside the gel framework was alsomonitored for the G-C-dot/PEI gel (Figure S21). Notably, gelscould not be assembled when C-dots comprising citric acid asthe molecular building block were employed (Figure S22),attesting to the critical role of imine bonds in stabilizing the gelstructure through C-dot cross-linking.Optical Properties of the Aldehyde-C-dot/PEI Gels.

The multicolor properties of the aldehyde-C-dot/PEI gels canbe exploited for optical applications (Figure 6). In theexperiments presented in Figure 6, the light emissions ofdifferent aldehyde-C-dot/PEI gel films deposited upon quartzslides were evaluated upon irradiation with a LED emittingblue light (403 nm). The photoluminescent quantum yields of

the G-C-dot/PEI gel, B-C-dot/G-C-dot/PEI gel, and CoAP-C-dot/PEI gel were 4%, 2% and 1.9%, respectively. Thethicknesses of the B-C-dot/G-C-dot/PEI film and CoAP-C-dot/PEI film were 85 μm (measured by Vernier calliper) and40 μm, respectively (Figure S23). The visible spectra andassociated digital photographs in Figure 6A, and correspondingCommission Internationale de I’Eclairage (CIE) chromaticityplot in Figure 6B, reflect the distinct colors emitted by thefilms (the quantitative optical parameters are indicated inTable S3). The digital images of the films and theirphotoluminescent images under 365 nm UV lamp are depictedin Figure S24. As highlighted in the CIE plot in Figure 6B, theemission spectra of the fluorescent gel films exhibited thedistinct 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 for the CoAP-C-dot/PEI gel(iii). Notably, generation of white light was accomplishedthrough irradiation of the B-C-dot/G-C-dot/PEI and CoAP-C-dot/PEI films that were stacked horizontally on opposite sidesof a quartz glass slide (iv). The data presented in Figure 6

Figure 6. Optical properties of the aldehyde-C-dot/PEI gels. (A) Emission spectra recorded upon illumination of thin gel films with a 403nm 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-dot/PEI gel; (iv) CoAP-C-dot/PEIgel + B-C-dot/G-C-dot/PEI gel. (B) CIE (1931) plot exhibiting the chromaticity coordinates ofthe gels.

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underscore the potential applicability of the aldehyde-C-dot/PEI gels in optical devices and the intrinsic color tunabilityavailable through selection of the C-dot components of thegels.

CONCLUSIONS

We synthesized fluorescent self-healing gels through reactingaldehyde-containing C-dots and PEI. The gels exhibitedmulticolor fluorescence, high mechanical strength, and self-healing properties. We showed that the distinct physicochem-ical properties of the aldehyde C-dot/PEI gels are due todynamic covalent imine bonds formed between the C-dots’aldehydes and primary amines of PEI. The C-dots played keyroles in determining gel properties. In essence, the C-dotsconstitute both the fluorophore and the gelator; moreover, theratio between the C-dots and PEI significantly affected themechanical profiles of the C-dot/PEI gels formed. Importantly,the absence of aggregation-induced quenching of the C-dot’sfluorescence, 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 fluorescenceproperties. Indeed, this observation is ascribed to the fact thatthe C-dots are cross-linked with the polymer network, togethersupporting the gel framework; thus the immobilized C-dotsmaintain sufficient distance among them, blocking aggregation-induced quenching. We also demonstrate that the fluorescenceproperties of the gels, intrinsically dependent upon the C-dotprecursors employed, could be exploited for construction ofmulticolor light emissive devices; in particular, generation ofwhite light was achieved through usage of C-dot/PEI filmsexhibiting different fluorescence emissions. The tunablefluorescent aldehyde C-dot/PEI self-healing gels could beused in diverse applications, including optical devices, strainsensing, controlled drug release through skin patches, andothers.

METHODS/EXPERIMENTALMaterials. Benzaldehyde ≥99%, cis-1,5-cyclooctadiene (99+%),

meta-chloroperoxybenzoic acid (77%), lithium aluminum hydride(95%), 4-carboxybenzaldehyde (97%), N,N′-dicyclohexylcarbodii-mide (99%), 4-dimethylaminopyridine (99%), Grubbs catalyst secondgeneration, ethyl vinyl ether (99%), 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (≥99%) (purpald), sodium hydroxide, and branchedPEI, (average Mw ∼25,000) were purchased from Sigma-Aldrich, St.Louis, MO, USA. Glutaraldehyde, 50% aqueous solution was boughtfrom Alfa Aesar. All the chemicals were used without furtherpurification. Methanol, ethanol, dichloromethane, tetrahydrofuran,chloroform, and triethylamine were purchased from Bio-Lab Ltd.(Jerusalem, Israel).Glutaraldehyde C-Dots (G-C-Dot)/PEI Gel. Glutaraldehyde

(300 μL) was mixed with ethanol (600 μL) in a Teflon tightened20 mL glass vial and heated in an oven (kept on oven floor) at 150 °Cfor 2 h. After the reaction, the vial was cooled down, and the resultantbrown solid was redispersed in acetone and chloroform andcentrifuged at 10,000 rpm for 15 min to remove high-weight carbonaggregates. This was repeated for three times for each solvent and wasevaporated under reduced pressure to obtain a brown solid. Next thebrown solid was redissolved in ethanol for further characterization anduse. G-C-dots (100 mg in 5 mL ethanol) were added dropwise to asolution containing 1 g of PEI in 15 mL ethanol and stirred vigorouslyovernight in which the volume was reduced to 4 mL. The suspensionwas then kept at room temperature for 24 h for gelation (as testedfrom the “stable to inversion” method confirming gel viscosity).23

Benzaldehyde-C-Dot (B-C-Dot)/PEI Gel. Benzaldehyde (300μL) was dissolved in 50 mL ethanol, and the solution was transferred

to a poly(tetrafluoroethylene) (Teflon)-lined autoclave (100 mL) andheated at 180 °C for 20 h. Next it was cooled down to roomtemperature, 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 ethanolsolution was added drop by drop to an ethanolic solution of PEI (1 g)under continuous stirring with constant heating at 80 °C forovernight. Subsequently, excess ethanol was evaporated underreduced pressure down to 3 mL volume, and the suspension waskept at room temperature for gelation.

Cyclooctadiene-Aldehyde Polymer (CoAP)-C-Dot/PEI Gel.Detailed procedure for synthesis of the ring-opening metathesispolymerization (ROMP)-derived CoAP and characterization areprovided in the Figures S1−S3 and Schemes S1−S4). CoAP (50mg) was dissolved overnight in 50 mL chloroform, transferred to apoly(tetrafluoroethylene) (Teflon)-lined autoclave (100 mL), andheated at 180 °C for 20 h, resulting in a brown solution. Afterfiltration, the excess solvent was evaporated under reduced pressureand was purified by above mentioned method. Subsequently, 100 mgof CoAP-C-dots in 5 mL chloroform was added to a PEI (1 g)/chloroform solution (15 mL) under continuous stirring overnight.Excess chloroform was subsequently evaporated under reducedpressure up to 4 mL volume and kept at room temperature forgelation.

Characterization. Fluorescence emission spectra were recordedon an FL920 spectrofluorimeter. G-C-dots, B-C-dots, and CoAP-C-dots were dissolved in ethanol and chloroform, respectively, withconcentration of 1 mg/mL. HR-TEM experiments were carried outon a 200 kV JEOL JEM-2100F microscope (Japan) with a drop of C-dots solution added on a graphene-coated copper grid, and it wasdried for 12 h. XPS measurements were performed on an X-rayphotoelectron spectrometer ESCALAB 250 ultrahigh vacuum (1 ×10−9 bar) apparatus with an Al Kα X-ray source and amonochromator. Concentrated solution of the C-dots and thecorresponding gel were drop-casted on silicon wafers, and afterdrying, the experiment were monitored. The X-ray beam size was 500μm, and survey spectra were recorded with pass energy (PE) 150 eVand high-energy resolution spectra were recorded with PE 20 eV.Processing of the XPS results was carried out using AVANTGEprogram. FT-IR measurements were performed on a ThermoScientific Nicolet 6700 spectrometer. UV−vis spectra were acquiredon a Thermo Scientific Evolution 220 spectrophotometer. Scanningelectron microscopy (SEM) images were recorded on a JEOL(Tokyo, Japan) model JSM-7400F scanning electron microscope. Thedilute solutions of gel material were dried on a glass coverslip, andgold sputter coating was carried out. Rheological experiments werecarried out on an Advanced Rheometer AR 2000 (TA Instruments)by cone and plate geometry in a Peltier plate. The cone diameter was20 mm, cone angle 1°, and truncation 27 μm. Frequency sweep andstrain sweep measurements were performed from 0.1 to 100 rad/s and0.1 and 1300%, respectively. The thickness of the B-C-dot/G-C-dot/PEI film was measured by Vernier scale, while for the thickness ofCoAP-C-dot/PEI film was monitored by using Zygo New View 200interferometer.

Optical Measurements. For the optical analysis, aldehyde-C-dot/PEI films were placed on quartz glass and then illuminated with aUV-LED (403 nm wavelength emission). The current of the LED wascontrolled by a Keithley Source Meter, operating voltage 2.8 V. Anoptical fiber cable connected to a Labsphere CDS 2600 detector(USA) was placed above the films to collect the emitted light. For thephotoluminescent quantum yield measurements, samples coated onquartz glass slides were placed inside an integrating sphere connectedto a Labsphere CDS 2600 detector fitted with an optical cable andirradiated with a 473 nm laser. Chromaticity points (x,y) andcorrelated color temperature (CCT) were calculated by theLabsphere software from the CIE 1931 coordinate diagram. Colorrendering index (CRI) was calculated by monitoring the ratio of theC-dots integrated emissive area with the total area of UV-LED and C-dot’s emission.

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ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.8b07087.

Detailed synthetic procedure and characterization forCOD-aldehyde polymer, average size distribution byHR-TEM, AFM and XPS of C-dots, photoluminescencespectra of C-dots, photoluminescence spectra of C-dots/PEI gel, excitation and emission wavelength shift of C-dots in solution and in gel matrix, optical spectra, FT-IR,XPS, SEM, rheological measurements, table for opticalproperties of C-dot/PEI gel film, thickness measure-ment. Figures S1−S24, Schemes S1−S5, and Tables S1−S3 (PDF)

AUTHOR INFORMATIONCorresponding Author*E-mail: razj@bgu.ac.il.ORCIDSagarika Bhattacharya: 0000-0003-2641-0246Ravindra Suresh Phatake: 0000-0001-6046-1042Jun-Jie Zhu: 0000-0002-8201-1285Norberto Gabriel Lemcoff: 0000-0003-1254-1149Raz Jelinek: 0000-0002-0336-1384NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTSWe are grateful to the Ministry of Science and Technology,Israel, for financial support under the China-Israel grantprogram. J.-J.Z. thanks the support from Internationalcooperation foundation from Ministry of Science andTechnology of China (2016YFE0130100). We thank Mr.Ahiud Morag for help with the digital images and Mr. JuergenJopp for assistance with the interferometer and AFMexperiments.

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