Redox Induced Fluorescence OnâOff Switching Based on Nitrogen

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Redox induced fluorescence on-off switching based on nitrogen enriched graphene quantum dots for formaldehyde detection and bioimaging Huijun Li, Xiong Sun, Fengfeng Xue, Nanquan Ou, Bo-wen Sun, DongJin Qian, Meng Chen, Ding Wang, Junhe Yang, and Xianying Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02941 • Publication Date (Web): 30 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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ACS Sustainable Chemistry & Engineering

Redox induced fluorescence on-off switching based on nitrogen enriched graphene quantum dots for formaldehyde detection and bioimaging Hui-Jun Lia1, Xiong Suna1, FengFeng Xueb, Nanquan Oua, Bo-Wen Sun b, Dong-Jin Qianb, Meng Chenb, Ding Wanga, JunHe Yang*a and XianYing Wang*a a

School of Materials Science and Technology, University of Shanghai for Science and

Technology, No. 516 Jungong Rd., Shanghai 200093, P.R. China. b

Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative

Materials, Fudan University, No. 220 Handan Rd., Shanghai 200433, P. R. China.

Corresponding Author Prof. Xianying Wang: [email protected]; Prof. Junhe Yang: [email protected].

KEYWORDS: graphene quantum dots; formaldehyde detection; gas sensor; bioimaging; stimuliresponsive

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ABSTRACT

Tunable optical and fluorescent properties of graphene quantum dots (GQDs) by heteroatom doping and surface functionalization provide tremendous advantages for practical applications. One of these notable issues is the construction of stimuli-responsive GQDs which can be used as smart green non-metal materials in the field of sensor and biotechnology. In this work, N-doped graphene quantum dots (NGQDs) encapsulated by organic species were obtained through hydrothermal method, of which the quantum yield achieved 22.9%, much higher than that of bare NGQDs (1%). Interestingly, it was found that the functionalized NGQDs exhibited a color oscillation in open air and its fluorescence emission can be reversibly switched “on-off” via redox reactions. Meanwhile, the PL emission would be transformed from excitation-independent to excitation-dependent through modifying the surface states of NGQDs. The origin of PL is then studied, emphatically distinguishing the role of different C-N configurations and surface functional groups. The NGQDs of distinct optical and luminescent characteristic can serve as fluorescent sensors for detection of CH2O and probes for bioimaging.


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INTRODUCTION Formaldehyde (HCHO), one of the most common toxic volatile compounds, emitted from widely used decorative materials, oils and textiles, etc., is harmful to human health even at a very low concentration level. Up to now, various approaches have been utilized to detect CH2O including gas chromatography, titrimetric measurement and conductometry. Nevertheless, these methods are limited by expensive instruments or high operating temperature, which are not desirable to satisfy human demand.1-3 Recently, optical sensors have been developed for monitoring the concentration of formaldehyde based on the principle of dynamic fluorescence quenching, emerging as a promising sensing technique owing to their handheld capability and room temperature operation.4,5 Meanwhile, many materials have been attempted to realize quick and efficient gas sensing. Among them, environmentally friendly carbon-based nanomaterials have attracted much attention due to their superior physico-chemical properties.6,7 The functionalized carbon-based materials have been reported as smart and green analytic tools in gas sensing and biological applications. Some excellent works even revealed of dual/multi-functional nanomaterials based on stimuli-responsive carbon nanodots.8,9 One series of the carbon-based materials, nitrogen-doped graphene quantum dots (NGQDs) were demonstrated great potential in designing the stimuli-responsive materials. Nitrogen doping not only retain primary properties of GQDs such as large specific surface areas, quantum size effects, good biocompatibility, but also lead to distinctive optical and electronic characteristics. The electron delocalization and charge-carrier density of NGQDs can be drastically altered by tuning N doping degree, C-N configurations or surface functional groups.7,10-13 The superior properties will further endow NGQDs more possibility being used as effective dual/ sensor or probe. New approaches have been continuously developed to prepare all sorts of NGQDs.14 In


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most cases, GQDs obtained by top-down methods generally exhibit poor optical properties of weak visible absorption.15,16 In contrast, bottom-up routes have obvious advantages in modulating band gap and fluorescent property of dots or providing more active sites through selection of organic precursors and carbonization conditions.17-19 It is worth noting that NGQDs have been successfully synthesized using cheap and environmental hydrothermal reaction of citric acid and urea.19 Following that, studies have been focused on improving the light absorption ability and quantum yields of NGQDs. Besides, understanding the emission origins of NGQDs and realizing control over the property has also drawn wide attention.11-13 The fluorescent property of GQDs is generally taken as an integrated effect of free zig-zag sites, sp2 skeleton, intrinsic defects or surface functional groups, based on size-confinement effect, radiation recombination of excitons, energy trapping and charge transfer effect.18,20,21 These mechanisms could then lead to excitation-independent/ excitation-dependent emissions or fluorescent quenching phenomenon. In spite of the common consensus of PL origins in GQDs, distinguishing the roles or synergistic effects of these factors in the emission properties of functionalized NGQDs still remain to be specified for rational designing materials in the future, due to its importance in excavating further applications. Herein, CH2O-responsive NGQDs were prepared using citrate and urea as carbon and nitrogen precursor. On account of surface functionalization, the obtained NGQDs exhibited an obviously spontaneous color oscillation between blue and green in open air under ambient conditions, which was proved to be a redox reaction. Meanwhile, the material displayed oxidative/reductive switchable fluorescence and owned higher quantum yields (QY), compared to bare NGQDs. Further varying surface functional groups of NGQDs can also contribute to the transformation of PL property from excitation-independent emission to excitation-dependent emission. To the best


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of our knowledge, researches regarding CH2O-responsive NGQDs haven’t been reported yet. The switchable fluorescence of NGQDs was then studied by means of PL spectra. Based on this novel property, the smart and green NGQDs have been demonstrated as a promising fluorescent sensor for formaldehyde and also fluorescent probe in cell image. RESULTS AND DISCUSSION Synthesis The primary obtained samples were gold yellow, as shown in Figure 1. After being transferred into a bottle covered with a lid, the sample endured a gradual color transformation from yellow to green and then blue, starting from the surface of solution, seen from Figure S1. If the lid was removed, the color change would occur very quickly. The blue solution was stable, but it would slowly turn into green, starting from the bottom of the solution. When converted back into yellow color, the solution would endure a second round of color change and become blue solution again. One round of this color change lasted for about 1.5~2 days without any disturbance. After 3~5 cycles, color change stopped and the solution remained as lime green. This repetitive variation in time between two different states was defined as oscillation, which was similar with the famous Belousov–Zhabotinsky reactions.22,23 We also found that the oscillation period gradually prolonged over time. The extending of oscillation period is a common phenomenon in chemical oscillation. Since an obvious variable in above-mentioned phenomenon was oxygen, we supposed that the color oscillation could be an O2-responsive reaction. The blue sample was then defined as ONGQDs (Oxidized-NGQDs). If we added reducer (Na2S2O4) into the blue solution, yellow solution was prepared, which was defined as R-NGQDs (Reduced-NGQDs). It can be found that


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the blue color would change into green and final yellow when increasing the adding amount of Na2S2O4, shown in Figure S2. The prepared R-NGQDs could also be easily oxidized without isolation from the air. When the O-NGQDs precipitates were dried in vacuum at 80 ºC for several hours, the final solids can be easily re-dispersed into water, displaying as blue solution. However, if the O-NGQDs solids were further dried in vacuum at 80 ºC for several days, black powder was obtained, which could still be well dispersed in water while demonstrating as yellow solution. This sample was defined as B-NGQDs (Bare NGQDs). The B-NGQDs sample didn’t present any color oscillation.

Figure 1. Picture description of different NGQDs samples. The reaction pressure or temperature is supposed to be the main reason of forming bluecolored solutions. An intermediate of blue color has also been observed in a microwave-assisted method of preparing carbon nanodots.24 Higher temperature (200 ºC) was attempted in our experiment, and yellow solution without any color change was obtained, proving the importance of reaction conditions. According to previous reports, citric acid amide formed firstly, and selfassembly of the sample into a nanosheet structure was induced under hydrothermal conditions, during which intramolecular dehydration and deamination among the C=O, -OH and amino


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groups occurred.19,25 The functional groups contributed to the good water-dispersibility of products. Morphology and structural characterization As shown in Figure 2, the morphology of different samples exhibited obvious difference. When TEM samples of O-NGQDs were prepared using the obtained solutions, nanodots of clear lattice structure were found to be dispersed uniformly with diameters in a narrow range of 5~8 nm (Figure 2A). Seen from HRTEM image (Figure 2B), the lattice fringe was measured to be 0.21 nm, corresponding to the crystal phase of graphene (100) planes.10,17,26,27 When redispersing the O-NGQDs solids into water by sonication treatments, TEM image presented that most of the nanodots still dispersed well (Figure 2C). However, if the O-NGQDs solids were redispersed by simple shaking, aggregates could be clearly observed shown in Figure 2D. TEM images of higher magnification (inset in Figure 2D) showed that the aggregates consisted of several dots with a size around 5 nm in diameter. The dark part around the nanodots was amorphous materials which can be assigned to organic species. The size distribution of ONGQDs was also measured by using dynamic light scattering (DLS), shown in Figure S3. Both small particles with a size of around 6~7 nm and large aggregates with a size of around 230~240 nm were detected. The size distribution provided by DLS is larger than that shown in TEM images, possibly due to solvent effect in the hydrated state.28 For sample B-NGQDs, no aggregates of nanodots were demonstrated, seen from Figure 2E. The size distribution of re-dispersed nanodots by sonication treatment was similar with that of ONGQDs, while the average size of B-NGQDs which were re-dispersed in water by simple shaking was slightly enlarged probably due to surface instability (Figure 2F). The comparison


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suggested the outer organic species can be removed via overheating and its existence prohibited grain growth during post-heating process.

Figure 2. Representative TEM images of (A) O-NGQDs, (B) the corresponding HR-TEM image of the particle marked by a red circle in (A); re-dispersed O-NGQDs (C) by sonication treatment and (D) by simple shaking; re-dispersed B-NGQDs (E) by sonication treatment and (F) by simple shaking. The structure is then characterized by XRD and Raman spectra to study the distinction in the TEM images of these samples. For O-NGQDs, many unidentified XRD peaks were recognized, which should be attributed to the organic species encapsulating nanodots (Figure S4A). On the


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other hand, the XRD pattern of B-NGQDs shows one peak at around 26.8 º corresponding to the (002) planes of graphite.17,19,29 The Raman spectra (λex=532 nm) of B-NGQDs revealed two prominent peaks, where the G band at 1580 cm-1 is stronger than the D band at 1340 cm-1. The disorder extent of samples could be measured by the intensity ratio of the G and D bands (IG/ID), which was 2.11 indicative of relatively good-quality GQDs, consistent with the TEM images.17,30,31 For O-NGQDs, new Raman peaks were observed (Figure S4B). Based on these analyses, it was supposed that the organic species encapsulating the nanodots leaded to the formation of aggregation. Through further drying treatment, the organic systems might be decomposed. Sonication can also contribute to the destruction of these aggregates. FT-IR spectra of O-NGQDs and B-NGQDs showed similar characteristic peaks (Figure 3A). Besides a broad absorption peak including C=C vibrations at ~1625 cm-1 and C=O vibrations at ~1650 cm-1, carboxyl C-OH vibrations at ~1400 cm-1 and hydroxyl C-OH vibrations at ~1080 cm-1 had also been observed. Other peaks at ~3400 cm-1 and ~2974 cm-1 could be ascribed to the O-H vibrations and C-H vibrations, respectively. In addition, there was another peak attributed to N-H vibrations at ~3200 cm-1 in O-NGQDs. The peak was consistent with the rich organic species encapsulating outside the carbogenic cores. It revealed that the O-NGQDs should be comodified with hydroxyl and amine groups, thus leading to good water-dispersibility.13,32,33 Furthermore, the O-NGQDs showed a negative zeta potential of –30.8 mV, indicating the existence of rich hydroxyl groups on the surface of O-NGQDs which tended to lose a proton in the basic solution.34 The chemical composition of two samples was measured by XPS spectroscopy.7,11,13,25,35 In full spectra, three peaks centered at ~284.6 eV, 400.6 eV, 531.6 eV were shown in Figure 3B, assigned to C 1s, N 1s, and O 1s, respectively. The percent of N atom in sample O-NGQDs and


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B-NGQDs were 5.4% and 3%. The percent of O atom in sample O-NGQDs and B-NGQDs were 39.19% and 29.36%. Another small peak at ~523.6 eV was ascribed to Si substract. Decrease in the amount of N atoms was attributed to the decomposition of outer organic species. Specific CN configurations were analyzed through high-resolution spectrum of C 1s, N 1s and O 1s. The C spectra shown in Figure S5 were deconvoluted into four peaks, including a C=C peak with a binding energy of ~284.6 eV, a C-N/C=N peak with a binding energy of ~285.8 eV, a C=O peak with a binding energy of ~286.8 eV and an O-C=O peak with a binding energy of ~289 eV. The percentage of C=C bonding increased in B-NGQDs, corresponding to the removal of surface organic species rich of nitrogen and oxygen atoms. The high-resolution spectra of N 1s seen from Figure 3C then revealed two types of N atoms, pyridinic type (399.8 eV), pyrrolic type (400.5 eV), and graphite type (401.6 eV). The calculated atomic percentages in the fitting data of pyridinic N and pyrrolic N are 63.16% and 39.26% for O-NGQDs and B-NGQDs, respectively. For graphite N, the percentages are 36.84% and 60.74%. As shown in Figure 3D, both main peaks of O 1s could be deconvoluted into three Gaussian– Lorentzian peaks. The three new core levels at 531.8 eV, 532.6 eV and 533.1 eV were contributed from C=O, absorbed water and C–OH groups, respectively.36-38 These signals suggest that the oxygen species are present not simply as absorbed water but as various functional groups on the surface. The calculated atomic percentage in the fitting data of ONGQDs for C–OH at 533.1 eV is 47.7%, C=O at 531.8 eV is 43.0% and absorbed water at 532.6 eV is 9.3%. The calculated atomic percentage in the fitting data of B-NGQDs for C–OH at 533.1 eV is 29.2%, C=O at 531.8 eV is 24.7% and absorbed water at 532.6 eV is 46.0%. Combined with the high-resolution N spectra, there is an obvious increase in the total amount of organic Oand N- containing groups on O-NGQDs surface.


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Figure 3. (A) FT-IR spectra, (B) XPS survey spectra, (C) N 1s high-resolution XPS spectra, (D) O 1s high-resolution XPS spectra of O-NGQDs and B-NGQDs. Optical properties The optical properties of NGQDs were emphatically studied. In the UV-vis spectra shown in Figure 4, absorption peak at 233 nm was attributed to π-π* transition of aromatic C=C domains, and the absorption at 344 nm (3.60 eV) might correspond to n-π* transition of C=O or C=N.20,31 These two peaks remained the same in different samples, when light absorption measurements were carried out using dilute solutions. Furthermore, we noticed that the absorption ascribed to n-π* transition in sample B-NGQDs had a blue shift to 333 nm (3.72 eV), compared to 340 nm (3.65 eV) of O-NGQDs. Generally, increasing in band gap can be caused by the decrease of electron density.39-41 Based on above-mentioned XPS analysis, the percentage of pyridinic and pyrrolic N was much lower in B-NGQDs since outer organic species were decomposed. Generally, pyridinic and pyrrolic-N could be a good electron donator, contributing one or two electrons to π orbital, while graphite N is regarded as electron acceptor due to the higher electronegativity of nitrogen than carbon.42,43 Thus, higher percentage of pyridinic and pyrrolic N


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would contribute to band gap narrowing through increasing the electron density of the delocalized system band. It is noteworthy that when the samples were blue or green, that is, in oxidized state, an obvious absorption peak around 640~660 nm appeared, and the absorption intensity increased by increasing the concentration of solutions. Besides, after reducer Na2S2O4 was added into the solution, blue solution turned into yellow with the disappearance of absorption peak in the long wavelength region. After being agitated in open air for a while, the solution showed as blue with reappearing of the absorption peak. This phenomenon hasn’t been reported as far. One suspect was that the absorption was caused by the rich surface organic groups functioned as chromophores which could be oxidized easily by dissolved O2 in the solution, since no UV-vis absorption was observed for B-NGQDs.44,45 The whole process can be analogous to the “blue bottle” experiment, in which the outer organic species of NGQDs should own similar properties as methyl blue dye. 46

Figure 4. UV-vis spectra of (A) O-NGQDs and B-NGQDs after drying treatment, (B) ONGQDs, intermediate between O-NGQDs and R-NGQDs. For O-NGQDs, the PL emission band showed excitation-independent property, seen from Figure 5. Through removal of surface organic species, the PL emission of B-NGQDs was demonstrated as excitation-dependent. This phenomenon testified the importance of surface


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passivation in emission property.19,47 In addition, for O-NGQDs, there were two PLE peaks while it changed into one broad PLE peak with a shoulder peak for B-NGQDs. The splitting of excitation peaks could be attributed to the appearance of aggregates, which was probably induced by surface organic species, corresponding to the previous TEM image.48 When we increase the concentration of O-NGQDs solution, it can also be found that one PLE peak split into two peaks (seen from Figure S6), which provided further evidence of the existence of aggregates. The PL QYs of the O-NGQDs and B-NGQDs samples were 22.9% and 1% for the emission at 380 nm, respectively. The prominent increase of QY is due to the existence of surface functional species, which have been proven by many studies.39,49,50 The quantum yield (φ) of O-NGQDs is higher than some previous reports (shown in Table S1).51,52 The average lifetimes were 6.67 and 4.78 ns, respectively. The extending of lifetime can be contributed to the mid-gap states of O-NGQDs which can serve as the intermediate relaxation orbitals for the π*→π electron transfer.53

Figure 5. PL spectra of (A) O-NGQD, (B) B-NGQDs at different excitation wavelengths, PL lifetimes of (C) O-NGQDs and (D) B-NGQDs.


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Proposed PL mechanism Based on these results, the invariable and variable PL maxim under different excitation wavelengths of the samples suggested that there were different emissive mechanisms. In this system, the PL emission peaks might involve the contribution of different components including aromatic cores, intrinsic defects, and functional groups. Combining UV absorption and PL spectrum, it could be inferred that the emissions mainly originated from π→π* transition as shown in Figure 6. The delocalized π electron was excited to π* bonds of sp2 C, and could be deactivated, undergoing radiative recombination. For N doped GQDs, C-N configurations as the intrinsic defects could also contribute one or two unpaired electrons to the HOMO orbital, leading to a n→π* transition, which appears frequently in some fluorescent dyes.11 In B-NGQDs, trap-mediated functional groups (n→π* transition) is another possible key factor for the PL properties.53 Different surface functional groups bonding with sp2 carbons would induce local distortion, creating various energy gaps located in the band tail of π-π* gap. These functional groups would predominate in the lower energy gaps for bare NGQDs, thus leading to a red shift of the emission peak when the excitation wavelength was increased.17,20,54 On the other hand, when the surface states were passivated, which means that the effect of defect energy levels was hindered, radiative transition of sp2 carbon was believed to be the primary mechanism. Then no red shift of emission peaks was observed. Previous reports have also shown that surface passivation via nitrogen containing organic species can achieve excitationindependent PL property. Moreover, since more delocalized electrons existed in N-rich ONGQDs, the PL intensity was much stronger than that of B-NGQDs.55-57


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Figure 6. Scheme of possible PL mechanism in O-NGQDs and B-NGQDs based on experimental results. CH2O responsive fluorescence Inspired by the optical properties, O2-responsive fluorescence of NGQDs under excitation of 360 nm was studied, found to present periodic changes during redox reactions shown in Figure 7A. For yellow R-NGQDs solution, a weak blue emission at 440 nm was observed. After oxidation by O2, the intensity of emission peak greatly increased without shifting of emission center. Further changing the redox states would lead to reversible variation of PL intensity. In the light of the redox characteristic of organic dye MB, we inferred that oxidation of the structure might allow a lengthened electronic delocalization. N atoms doped in carbon cores would endure a transfer of valence, during which the primarily hindered free electrons of nitrogen were then allowed to be injected into the delocalization system due to the molecular configuration


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transformation. Therefore, growth in the numbers of delocalized electrons leaded to increase in PL intensity.11,40 Based on this phenomenon, the O-NGQDs were then utilized for CH2O detection since CH2O possesses strong reducibility. To lower the detection limit of CH2O, the pristine O-NGQDs solution was diluted 3000 times, and the final solution displayed as transparent and colorless. 15 ml diluted O-NGQDs solution were then stored in five degased, closed glass bottles, respectively. Formaldehyde was injected into the solution with its final concentrations ranging from 0 µM to 300 µM. The fluorescent emission spectrum of the NGQDs towards different volumes of CH2O liquid was measured. The highly diluted CH2O solution was injected into the O-NGQDs solution in a closed system. All the PL emission peaks of those samples were centered at around 440 nm, while an obvious decrease in the PL emission intensity was observed with increasing the concentration of formaldehyde, shown in Figure 7B. A wide dynamic range from 10-6 to 10-4 M was achieved in this system, and the fluorescence intensity had a good linear relationship at low CH2O concentration, seen from Figure 7C. With further addition of CH2O with concentration above 150 µM, no obvious decrease in PL intensity could be observed. The linear regression equation was calculated as I=－0.29 × Cformaldehyde + 22.67 (R2=0.99). The limit of dectection (LOD) of the CH2O sensor was estimated to be 0.15 µM (4.5 ppb) according to the 3σ rule, which is lower than the health standard limitation (100ppb). Compared to other reported results, the O-NGQDs showed great advantages of improved selectivity and lowered cost due to its simple design and easy operation (Table S2). The reductive formaldehyde may react with the abundant N-containing species on O-NGQDs, and


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the redox-induced charge transfer can cause a perturbation of the electronic states of the NGQDs as well as non-radiative transitions, thus leading to a significant fluorescence quenching.

Figure 7. (A) PL emission spectra of NGQDs switched between oxidized and reduced state. Inset: Photograph of the O-NGQDs and R-NGQDs aqueous solution taken under UV light in a fluorescence spectrophotometer. (B) PL emission spectra of the O-NGQD aqueous solution in the presence of different concentrations of CH2O. (C) Variation in PL intensity of O-NGQDs with the addition of different amounts of CH2O. (D) The linear fit of the PL intensity plotted against molarity of CH2O at low concentration range. The selectivity of the system has also been demonstrated. No obvious change in PL intensity was observed when disturbing substances including C2H5OH, CH3COCH3, NH3·H2O, CO were added, seen from Figure S7. The concentrations of these substances have been raised to 200 mg/L (the injection amount of CO was 30 mL), which suggests the O-NGQDs sample has excellent selectivity for HCHO sensing. In addition, the PL stability of O-NGQDs under different pH conditions and the existence of different ions were also measured. As shown in


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Figure S8A, the dots remained stable in alkaline environment, which can be attributed to the rich nitrogen containing organic groups on the surface.58 On the other hand, the PL intensity of the particles remained stable in various ions solution, such as: Na+, K+, Ag+, Pb+, Ni+, Cu2+, Al3+, Co2+, Cr3+, SO42-, Cl-, NO3-, L-Cysteine, Sarcosine and Glycine, shown in Figure S8B. These disruptors are representative of typical environment. The good selectivity in presence of most of the chosen analytes and amino acids is possibly attributed to the weak coordination ability of NGQDs in both “on” and “off” state.59,60 These results demonstrated that the obtained NGQDs could be utilized as a promising fluorescent probe for the detection of CH2O, meeting the requirement of practical applications. Cell imaging Due to strong photoluminescent intensity, the NGQDs have been evaluated for its application in biolabeling and bioimaging for HepG2 cells. It has been shown in Figure 8 clearly that both samples exhibited extremely low cytotoxicity with cell viability above 90%, even at a concentration of 400 µg/mL after incubating for 12 h and 24 h, respectively.

Figure 8. Cell viability assay with the HepG2 cells treated with different concentration of (A) ONGQDs and (B) B-NGQDs in culture medium for 12 h (black column) and 24 h (red column), respectively.


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For fluorescence probes, the cells were treated with O-NGQDs or B-NGQDs for 30 min, and irradiated under confocal microscope. The cells exhibited enhanced fluorescence in cytoplasm, indicative of that nanodots have penetrated into the cells and could still remain emissive.19,50 As seen in Figure 9, the profiles of HepG2 cells under excitation demonstrated a good potential in visualization of living cells by PL by using O-NGQDs, compared to that of B-NGQDs. In addition, Z-axis fluorescent microscopy (Figure S9) further proved that most of the NGQDs located in nucleus regions.61 Based on above analysis, the synthesized O-NGQDs encapsulated with functional organic species can be used as safe and efficient fluorescence nanoprobes for cell imaging.

Figure 9. The HepG2 cells treated without (A-C), with O-NGQD (E-G) and B-NGQDs (H-K) suspension: (A, E, H) confocal fluorescence photomicrographs under 405 nm excitation; (B, F, I) bright-field images; (C, G, K) Overlay images.


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CONCLUSION In summary, CH2O-responsive NGQDs encapsulated by N-rich organic species were synthesized via a facile hydrothermal method. The obtained NGQDs show superior re-dispersity in water. Its particular structure provided itself reversibly switchable “on-off” fluorescence via redox reactions. Also, O-NGQDs can be utilized as a fluorescent sensor for CH2O calibration. Removal of the outer organic layer can terminate the color oscillation phenomenon and alter emission property. Moreover, the functionalized NGQDs were demonstrated to show good biocompatibility, which could be used as excellent fluorescent probes for cell bioimage. The strategy in our work might provide a new vision for preparing smart green GQDs in the fields of gas sensing and bioimaging. EXPERIMENTAL Materials All the chemicals are commercial available. All the chemicals were used without further purification.

Citric

acid

monohydrate,

urea,

formaldehyde,

NaCl,

KCl,

AgNO3,

Pb(CH3COO)2·3H2O, NiSO4·6H2O, CuCl2·2H2O, Al2(SO4)3·18H2O, CoCl2·6H2O CrCl3 were purchased from Sinopharm Chemical Reagent Ltd. L-Cysteine, Sarcosine, Glycine was purchased from Sigma Aldrich .Na2S2O4 was purchased from J&K Scientific Ltd. Synthesis of NGQDs NGQDs were synthesized through a hydrothermal route. The details of the synthesis have been explained elsewhere.11,18 Typically, 1.68 g (8 mmol) citric acid and 1.44 g (24 mmol) urea were dissolved in 40 mL water, and stirred for 5 min until the solution was clear. The solution was


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then transferred into a 50 mL Teflon lined stainless autoclave of good air tightness. The sealed autoclaves were heated to 180 ºC in 20 min and kept for 8 hours. After finishing the reaction, the autoclaves were taken out from the oven and cooled to room temperature naturally in open air. It is noticed that urea from different companies can lead to different phenomena. If the urea was manufactured by Aladdin Industrial Corporation (Shanghai, China), the obtained product was light brown color. If the urea was provided by Sinopharm Chemical Reagent Ltd (Shanghai, China), the obtained product was bright yellow, which would further endure a color oscillation process. The primary products could be collected by adding excess ethanol into the obtained solutions and centrifuged at 10000 rpm for 20 min. Finally, the blue precipitates were dried in vacuum at 80 ºC for several hours or days. Sensing and Selectivity Protocols A dispersion of the O-NGQDs in water has been prepared with a concentration of 10 µg/mL, and stored in a sealing bottle which was then vacuumed. To 15 ml of this dispersion, different volume of CH2O solution was injected through the rubber sealing of the lid. The quantitative estimation of CH2O was carried out by measuring the PL spectra. For demonstrating the selectivity of the sensor, a series of analytes including heavy metals ions, normal ions, amino acids and biomolecule has been added into the O-NGQDs solution and the PL spectra were measured. PL intensities of the O-NGQD aqueous solution in the presence of different concentrations of solvents or gases (C2H5OH: 200 mg/L, CH3COCH3: 200 mg/L, NH3•H2O: 200 mg/L, CO: 30 mL) were also tested. Characterization


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The X-ray powder diffraction (XRD) patterns were recorded using a Rigaku D/Max 2550 Xray diffractometer with Cu-Ka radiation (γ = 0.15418 nm) in the range of 10–70 degree at room temperature. Raman spectra were carried out by using Raman Microscopy (Horiba, LabRAM HR Evolution, France) with an excitation wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) experiments were carried out with a RBD 147 upgraded PerkinElmer PHI 5000C ESCA system equipped with a hemispherical electron energy analyzer. The Mg Kα anode is operated at 14 kV and 20 mA. The spectra were recorded in the constant pass energy mode with a value of 46.95 eV, and all binding energies were calibrated using the carbonaceous C 1s line at 284.6 eV as reference. The Fourier transform infrared (FT-IR) experiment was carried out on a Nicolet Nexus 470 FT-IR spectrometer and used a wavenumber range of 400−4000 cm-1. The samples were prepared as pellets together with KBr. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on a JEOL JEM-2100F/CESCOR electron microscope equipped with a field-emission gun operating at 200 kV. All TEM samples were made using aqueous colloids of the samples directly without size selection. The UV−vis absorption spectroscopy was taken at room temperature on a UV-3150 spectrophotometer (Shimadzu, Kyoto, Japan). The photoluminescence (PL) spectroscopy was performed using a RF-5301pc fluorescence spectrometer (Shimadzu, Kyoto, Japan). The time-resolved fluorospectroscopy of the sample was measured by a fluorescence lifetime spectrometer (QM 40, PTI). The absolute quantum yield was acquired by using a calibrated integrating sphere on the QM40 Fluorescence Lifetime Spectrometer. Confocal imaging was performed on an Olympus FV1000 confocal fluorescence microscope with a 60 × oil-immersion objective lens. Cell culture


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The HepG2 cells were provided by the Institute of Biochemistry and Cell Biology, China and were grown in RPMI 1640 media supplemented with 10% fetal bovine serum at 37 ºC in a humidified atmosphere containing 5% CO2. Cell viability Cytotoxicity was assessed by performing MTT assay with the HepG2 cells. Cells were seeded into a 96-well plate at 2 × 103/well and were cultured at 37 °C and 5% CO2 for 24 h. Different concentrations of NGQDs (0, 12.5, 25, 50, 100, 200 and 400 µg/mL) were then added to the wells. After incubation for 12 or 24 h, MTT (0.5 mg/mL) was added to each well, and the plate was incubated for another 4 h. The optical densities at 490 nm were measured. Confocal laser scanning microscope (CLSM) studies Cells were plated on 14 mm glass coverslips and were incubated overnight. The cells were incubated with 100 µg/mL O-NGQDs or B-NGQDs in RPMI 1640 for 3 h at 37 ºC washed with and then RPMI 1640 media. After washing three times, the cells were subjected to CLSM imaging with the excitation wavelength at 405 nm. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website: XRD patterns, Raman spectra, DLS, high-resolution spectrum of C 1s, PLE spectra, selectivity, comparison of quantum yields and detection limit, and Z-axis bright field confocal images. Notes The authors declare no competing financial interest.


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Author Contributions Hui-Jun Li and Xiong Sun contributed equally to this work. ACKNOWLEDGMENT We greatly appreciate the financial supports from Shanghai Municipal Science and Technology Commission (16060502300, 16JC1402200, 15520720300), the National Natural Science Foundation of China (Grant Number: 51402193, 51572173, 51602197 and 11402149), Shanghai Eastern Scholar Program (Grant Number: QD2016014) and Shanghai Pujiang Talent Program (Grant Number: 16PJ1407700). REFERENCES (1) Chung, P.R.; Tzeng, C.T.; Ke, M.T.; Lee, C.-Y. Formaldehyde gas sensors: a review. Sensors, 2013, 13, 4468−4484. (2) Mirzaei, A.; Leonardi, S. G.; Neri, G. Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review. Ceram. Int. 2016, 42, 15119−15141. (3) Yan, D.; Xu, P.; Xiang, Q.; Mou, H.; Xu, J.; Wen, W.; Zhang, Y. Polydopamine nanotubes: bio-inspired synthesis, formaldehyde sensing properties and thermodynamic investigation. J Mater. Chem. A 2016, 4, 3487. (4) Descamps, M. N.; Bordy, T.; Hue, J.; Mariano, S.; Nonglaton, G.; Schultz, E.; VignoudDespond, S. Real-time detection of formaldehyde by a sensor. Sensors Actuat. B-Chem. 2012, 170, 104−108.


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FOR TABLE OF CONTENTS USE ONLY

The NGQDs of distinct optical and luminescent characteristic can serve as fluorescent sensors for detection of CH2O.


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Redox Induced Fluorescence OnâOff Switching Based on Nitrogen

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