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Probing Energy and Electron Transfer Mechanisms in Fluorescence Quenching of Biomass Carbon Quantum Dots Zicheng Liang, Mijeong Kang, Gregory F. Payne, Xiaohui Wang, and Run-Cang Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04826 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 19, 2016

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ACS Applied Materials & Interfaces

Probing Energy and Electron Transfer Mechanisms in Fluorescence Quenching of Biomass Carbon Quantum Dots Zicheng Liang,† Mijeong Kang,§ Gregory F. Payne, §,* Xiaohui Wang†,* and Runcang Sun†,‡ †

State Key Laboratory of Pulp and Papermaking Engineering, South China University of

Technology, Guangzhou, 510640, P. R. China. §

Fischell Department of Bioengineering and Institute for Bioscience and Biotechnology

Research, University of Maryland, College Park, Maryland, MD 20742, USA. ‡

Institute of Biomass Chemistry and Technology, Beijing Forestry University, Beijing 100083,

P. R. China.

KEYWORDS. Carbon quantum dot, Fluorescence quenching, Förster resonance energy transfer, Spectroelectrochemistry, Chitosan

ABSTRACT. The recent discovery of biomass-derived carbon quantum dots (CQDs) offers the potential to extend the sensing and imaging capabilities of quantum dots (QDs) to applications

ACS Paragon Plus Environment

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that require biocompatibility and environmental friendliness. Many studies have confirmed the exciting optical properties of CQDs and suggested a range of applications, but realizing the potential of CQDs will require a deeper fundamental understanding of their photophysical behavior. Here, biomass-derived CQDs were synthesized by hydrothermal processing methods from the aminopolysaccharide chitosan and their fluorescence quenching behaviors were investigated. A family of nitroaromatics with different ring substituents was used to generate systematically varying CQD-quenching behaviors. Experimental evidence including a correlation between quenching constant and spectral overlap, fluorescence lifetime decay, and donor-acceptor distance all demonstrate that the primary mechanism for QCD-quenching is Förster resonance energy transfer (FRET) and not electron transfer. Spectroelectrochemical studies with redox-dependent quenching molecules and studies with complex dye molecules further support this conclusion. We envision this fundamental understanding of CQDs will facilitate the application of these emerging nanomaterials for sensing and imaging.

1. Introduction The field of nanotechnology was driven in large part by discoveries that nanostructured materials offered unique properties.1,2 For instance, semiconductor quantum dots (QDs) were observed to have size-dependent optical properties that enabled new opportunities for imaging and detection.3-5 These successes spurred efforts to discover new materials that offer similar beneficial optical properties while overcoming some of the limitations of QDs (e.g., toxicity).6,7 In 2004, it was reported that fluorescent carbon-based quantum dots (carbon quantum dots; CQDs) could be formed from carbon nanotubes8 and the synthesis of CQDs (from graphite) was


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first reported in 2006.9 These initial observations were confirmed and extended by numerous reports which demonstrated: (i) CQDs can be prepared from a range of organic materials using a variety of processing methods; (ii) CQDs possess diverse optical (i.e., fluorescence) properties; and (iii) CQDs can perform various functions for a range of applications in detection and imaging.10-15 While previous studies have demonstrated the broad potential of CQDs as a platform material, there has been less progress in understanding the fundamental mechanisms responsible for their optical properties. As illustrated in Figure 1a, it is generally believed that CQDs possess a graphitic core surrounded by a hydrophilic surface.9,16,17 Because of the graphitic core, CQDs have been suggested to be the most recent addition to the fullerene-carbon nanotube-graphene family.10 The hydrophilic surface properties often result from carboxylate moieties which confer favourable solubility properties as well as allow further biochemical functionalization.18,19 While Figure 1a suggests a generalized structure, there appears to be substantial differences in CQDs presumably due to differences in raw materials and fabrication methods.20,21 As a result of these differences, there are often conflicting reports of the intrinsic behavior of CQDs. For instance, two studies reported that the small molecule dopamine can change CQDs’ fluorescence, however one study reported enhanced fluorescence while the other reported quenched fluorescence.22,23 Furthermore, one study on CQDs quenching reported that the CQDs could serve as either an electron donor (with nitroaromatic acceptors) or electron acceptor (with N,N-diethylamine donors).24 Understanding such apparent contradictory behaviors will require standardization materials preparation and characterization, as well as a more fundamental examination of CQDs’ behaviors.


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Here, we prepared CQDs from the aminopolysaccharide chitosan using a somewhat standard fabrication method as illustrated in Figure 1b. Chitosan is emerging as a common biologicallyderived material for preparing CQDs,25-27 and we showed that the chitosan-based CQDs possess the characteristic structures and fluorescent properties. The important finding from this study is that by using a series of nitroaromatic acceptors with different spectral absorption properties it was possible to demonstrate that fluorescence quenching is predominantly due to an energy transfer instead of generally considered electron transfer mechanism.28,29 This finding should facilitate the use of CQDs in wide fields that include sensing, imaging and optoelectronics.

Figure 1. Synthetic route: (a) presumptive structure of CQDs; (b) schematic illustration of chitosan CQD’s synthesis and quenching by nitroaromatics.

2. Experimental Section 2.1. Materials. Chitosan with 50,000 molecular weight was purchased from Haidebei Marine Bioengineering Co. Ltd. Nitroaromatics including 2,4,6-trinitropheonl (TNP), 2,4-dinitrophenol (2,4-DNP), p-nitrophenol (4-NP), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), pnitrotoluene (4-NT), nitrobenzene (NB), toluene and phenol (PhOH) purchased from domestic commercial suppliers or Aladdin Chemistry Co. Ltd were analytical grade and used as received.


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Complex molecules curcumin and amaranth were purchased from Macklin Reagent Biochemical Co. Ltd. Deionized water was used throughout all the experiments. 2.2. Chitosan Carbon Quantum Dots (CQDs) Synthesis. We prepared our chitosan based CQDs by adapting previously reported methods.30 Specifically, we purified chitosan by first dissolving in acetic acid (2% v/v), filtering insoluble materials and then precipitating the solution using NaOH (5% w/w). The precipitate was then washed and freeze dried (2 days at -65°C with vacuum). The freeze dried powder (1 g) was then dissolved in acetic acid (19 ml 2% acetic acid) and added to a 25 mL Teflon container that was packed into a stainless steel autoclave that was then placed in a domestic muffle roaster for hydrothermal carbonization (200°C, 12 h). After this treatment, the autoclave was allowed to cool to room temperature. The resulting semi-solid material was ultrasonicated (100 W, 1 h), and centrifuged at high speed (21000 g, 15 min, with refrigeration) to remove insoluble materials. After dilution, the transparent and yellowish supernatant exhibited bright blue fluorescence under UV radiation. To further purify the crude CQDs from complex product mixture, the CQDs were dialyzed against deionized water using a cellulose ester membrane bag (Molecular weight cut off of 500 Da). The solid-state CQDs were then obtained by freeze drying. The yield from this hydrothermal carbonization method is approximately 10%. The resulting CQDs were well dispersed in aqueous solutions and in alcohols (> 50 mg/mL). No obvious loss of fluorescence was observed after one month. 2.3. Characterization. High-resolution transmission electron microscopy (HRTEM) images were taken on a TECNAI G20 F20 (FEI, USA) electron microscope operating at 200 kV. Atomic-force microscopy (AFM) images were taken on a Nano Surfaces Division Multimode 8 atomic force microscope (Bruker, Germany) under tapping mode by dropping the infinite dilute solution of CQDs on mica substrate. X-ray diffraction (XRD) patterns were measured on a


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D/max-3A X-ray polycrystalline diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 1.54051 Å). The Fourier transform infrared spectroscopy (FTIR) spectra were obtained from a Tensor 27 (Bruker, Germany) spectrometer with KBr tablet (chromatographically pure) powder as background, ranging from 4000 cm-1 to 500 cm-1. X-ray photo-electron spectroscopy (XPS) was investigated on an AMICUS (Shimadzu, Japan) spectrometer with X-ray source Mg Kα (1253.6 eV). The ζ-potential was measured on a Malvern Nano ZS instrument (Malvern, UK). Optical absorption was measured on a UV-3600 (Shimadzu, Japan) spectrometer. Fluorescence spectra were made on a FL-7000 fluorescence spectrometer (Hitachi, Japan) with slit width of 5 nm for excitation and emission at PMT voltage 400 V. The excitation increased by a 20 nm increment starting from 300 nm to 420 nm. All the optical spectra were recorded with a quartz cuvette of 1 cm path length. The quantum yield (QY) of the CQDs was calculated by the following equation:31 Φ = n A FΦ /n AF (1) where n is the refractive index of the solvent, A is the absorbance at corresponding excitation wavelength, F is the integrated intensity of the emission, and Φ is the value of QY. Quinine sulphate in 0.1 M H2SO4 was used as reference (literature QY 54% at 360 nm excitation). Timecorrelated single-photon counting (TCSPC) were investigated on a Fluorolog-3 spectrometer (Horiba Jobin Yvon, France). The energy levels of some nitro compounds were calculated by density functional theory (DFT) method at the B3LYP/6-31G* level in GAUSSIAN 09 software.32 Spectroelectrochemistry was used to simultaneously measure optical signals (either absorbance or fluorescence) and impose/measure electrical signals (voltages/currents). Optical measurements were made using the SpectraMax® M2e (Molecular Devices, USA) with slit width of 9 nm for excitation and emission. Electrical signals were imposed/measured using an electrochemical workstation 620E (CH Instruments, USA) with a gold honeycomb


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spectroelectrochemical cell (Pine Research Instrumentation, USA) serving as the working electrode, an Ag/AgCl reference electrode, and a gold counter electrode. 2.4. Energy Level Calculation of the CQDs. Briefly, cyclic voltammograms of the CQDs were obtained on a CHI660E electrochemical workstation at the sensitivity of 1e-5 A/V with a three electrode system, including an Ag/AgCl electrode as reference electrode, a platinum wire electrode as counter electrode and a glassy carbon electrode (3 mm diameter) as working electrode, using (Bu)4NBF4 (0.1 M) in DMF as electrolyte and kept influx with dry nitrogen flow (Figure S1). Ferrocene/ferrocenium (Fc/Fc+) served as reference (literature HOMO -4.8 eV).33 The energy level of the CQDs and the band gap was determined as follow: HOMO = −[E − E(/) + 4.8] (in eV unit) (2) E = hc/λ = 1240/λ (in eV unit) (3) LUMO = HOMO + E (4) Here, Eox is the onset of oxidation potential of the CQDs, and E(Fc/Fc+) is the onset of the oxidation potential of ferrocene/ferrocenium (literature HOMO -4.8 eV), λ is the onset wavelength in absorption spectrum of the CQDs. 2.5. Donor-acceptor Distance Calculation. The energy transfer efficiency (E) was calculated by following equation. From that, the donor-acceptor distance (r) and Förster distance (R0) between the CQDs and the nitro aromatics could be estimated:34 E = 1 − τ() ⁄τ( (5) R , = 0.211[к n./ ΦJ(λ)]1⁄2 (in Å unit) (6) E = 1⁄[1 + (r⁄R , )2 ] (in Å unit) (7) where τD-A and τD were the average fluorescence lifetime of the CQDs in the absence and in the presence of nitro compounds, к2 is the orientation factor of the donor and acceptor transition


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dipoles and is typically assumed to be 2/3, Φ is the quantum yield of the donor, n is the refractive index of the medium, J(λ) is the integral of overlap values.

3. Result and Discussion 3.1. Morphological and Chemical Characterization of CQDs. The morphology of the CQDs was first characterized by high resolution transmission electron microscopy (HR-TEM) as shown by the images in Figure 2a. Because of the low contrast, it was difficult to discern the shape of these nanoparticles and thus we added white circular outlines of three particles. Also we added two parallel white lines spaced 0.23 nm apart to illustrate the putative lattice spacing. This lattice spacing is consistent with a (100) plane of graphite as reported in other work.35-38 Figure 2b shows that we estimated the size of the particles (n > 100) to be about 5 nm. Figure 2b shows the TEM-based size distribution for the CQDs. We used atomic force microscopy (AFM) as a second method for morphological characterization. Experimentally, we added a dilute solution of CQDs to a mica surface. After drying, the surface was observed to have small spherical particles as illustrated by the white dots in Figure 2c. Figure 2d displays an AFM trace between two particles with heights measured to be 1.5 and 1.2 nm. Figure 2e shows the size distribution for particle heights has a peak near 1.5 nm (n > 100). Differences in the dimensions of the CQDs using these standard methods (i.e., height by AFM and diameter by TEM) have been previously reported for CQDs,39-41 and could originate from a non-spherical (e.g., platelet-like) shape.39


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Figure 2. Morphological characterization of biomass CQDs: (a) a HR-TEM image of the CQDs; (b) size distribution from TEM analysis; (c) an AFM image; (d) height of section trace between two particles; and (e) height distribution from AFM analysis. Next we used X-Ray diffraction (XRD) to compare the starting chitosan powder and the final solid CQDs. Figure 3a shows that the XRD spectra for the starting chitosan powder has a sharp crystalline peak at 2θ=20° and a broader disordered peak at 2θ=10° consistent with an amorphous region. In contrast, the spectra for the CQDs shows a broad amorphous peak around 2θ=23° which is in agreement with previous reports of chitosan-based CQDs.26 These differences in the XRD spectra indicate that the hydrothermal carbonization treatment leads to substantial changes in chitosan’s structures. Initial chemical analysis was performed using FTIR. The spectrum in Figure 3b for chitosan shows characteristic peaks for this aminopolysaccharide, while the spectrum for the CQDs shows substantial differences. Two bands that are substantially weakened for the CQD (compared to chitosan) are the C-H band at 2876 cm-1 and C-C band at 1084 cm-1. This difference is consistent


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with a significant decomposition of the sugar structure. One band that is substantially strengthened for the CQDs is the COO- band at 1402 cm-1. This difference suggests a greater abundance of carboxylate functionality for the CQDs. Our final chemical characterization was by X-Ray photoelectron spectroscopy (XPS). Figure 3c shows that the XPS spectrum for the CQD contains an N1s band. The presence of this N1s band indicates that nitrogen from the starting chitosan material is retained in the final CQDs structures. Figure 3d shows the C1s band could be assigned to five peaks: C-C/C=C bands of graphite or aliphatic carbon at 284.6 eV, C-N bands at 285.6 eV, C-O bands at 286.2 eV, C=O bands at 287.3 eV and N-C=O bands at 288.4 eV.42 Figure 3e shows the N1s band can be solely assigned to –NH2 groups. These results are consistent with the synthesis of CQDs that are composed of a graphitic core and hydrophilic surface with abundant carbonyl, carboxyl and amino functionality.21,43 In summary, the physical and chemical characterization above indicates that the hydrothermal carbonization of chitosan yields nanoscale particles with characteristic CQDs structure illustrated in Figure 1a. Consistent with a hydrophilic, carboxylate-rich surface, we observed the ζ-potential of the CQDs (without pH adjustment) to be -21.3 ± 0.3 mV. The Raman spectrum is unavailable because the Raman signals were badly interfered by extensive fluorescence background.


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Figure 3. Chemical characterization of biomass CQDs: (a) XRD patterns of starting chitosan powder and the solid CQDs; (b) FTIR spectra; (c) XPS spectra; (d) C1s peaks signal and (e) N1s peaks signal from XPS analysis. 3.2. Optical Properties of CQDs. The optical properties of the chitosan-based CQDs were characterized by their optical absorption and fluorescence spectra. The photographs in Figure 4a illustrate the intrinsic fluorescence behavior of the CQDs, which have blue fluorescence emission upon UV light excitation. The absorption spectrum in Figure 4a shows the CQDs has an absorption peak at 284 nm and a weak shoulder at around 320 nm. These observations are reported to result from the π-π* transition of C=C and n-π* transition of C=O, respectively.37 We


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investigated the absorption spectra of CQDs after adding different concentrations of 2,4,6trinitrophenol (TNP). Figure 4b shows that with increasing TNP concentrations, the intensities of the absorption peak at 358 nm (corresponding to TNP) increased but no new absorption peak was observed. We also measured the absorption spectra of CQDs after adding different nitroaromatics (see Figure S2). Compared to the spectral superposition of individual CQDs and corresponding nitroaromatic, no absorption peak appeared or disappeared in the spectra of the mixture of CQDs and nitroaromatic. These results indicated that no non-emissive substance was formed between CQDs and nitroaromatics. We next examined the fluorescence behaviors of the CQDs. In initial studies, we observed an emission peak at 400 nm. Thus, we measured the excitation spectrum for the CQDs at an emission wavelength of 400 nm as shown in Figure 4c. Also shown in Figure 4c are emission spectra for the CQDs excited at 280, 300 and 320 nm. These emission spectra show an increase in intensity but the peak emission occurred at the same 400 nm. Figure 4d shows emission spectra for the CQDs excited at 320, 340, 360, 380, 400 and 420 nm. These emission spectra show both a decrease in intensity and a red shift in the wavelength of peak emission. These excitation-dependent behaviors could be attributed to the optical selection of different-sized nanoparticles or different surface emissive traps.11,12,21 The quantum yield of the chitosan-based CQDs was 13% with quinine sulfate as standard.


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(a)

(b)

(c)

(d)

Figure 4. Optical properties of biomass CQDs: (a) absorption spectra (inset: photograph under day light and UV radiation); (b) absorption spectra in the presence of 2,4,6-trinitrophenol (TNP) with increasing TNP concentration; (c) excitation and excitation-independent emission spectra (280, 300, 320 nm); and (d) excitation and excitation-dependent emission spectra (320, 340, 360, 380, 400, 420 nm). 3.3. Fluorescence Quenching of CQDs. In our initial studies on the quenching of CQDs’ fluorescence, a family of nitroaromatics was explored. Nitroaromatics are commonly used model chemicals for studies in physical organic chemistry because their physicochemical properties are substantially altered by the substituents on the aromatic ring. In the quenching studies, we mixed the CQDs (0.01 mg/mL) with various nitroaromatics (0.05 mM). The photographs in Figure 5a show quenching of the CQDs’ fluorescence and significant differences between the various molecules in this family. Figure 5b shows representative results for studies with 2,4,6-trinitrophenol (TNP). As expected, quenching increased with concentration of this nitroaromatic compound. Large quenching beyond 80% of the initial emission was measured upon addition of 0.1 mM TNP, which could even be seen with the naked eye.


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Comparable quenching could also occur with respective addition of other nitrophenols, such as 2,4-dinitrophenol (2,4-DNP) and p-nitrophenol (4-NP) as shown in Figure S3. The concentration dependence of quenching for TNP and the other nitroaromatics is summarized in the SternVolmer plots of Figure 5c. Good linearity is observed with these Stern-Volmer plots for the various nitroaromatics. The Stern-Volmer quenching constants (Ksv) of the nitroaromatic compounds were calculated by equation 8:44 I, /I = 1 + K 67 [Q] (8) where I0 and I are the fluorescence intensities in the absence and in the presence of the quenching agents [Q]. The S-V quenching constants were calculated with the linear portion of the S-V plot. It is noteworthy that the nitrophenols could generate more effective emission quenching and their Ksv were very close. On the other hand, nitrotoluenes including 2,4,6trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), p-nitrotoluene (4-NT) and nitrobenzene (NB) showed lower quenching efficiencies and their Ksv were inversely proportional to the number of nitro groups on the benzene ring.


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Figure 5. Quenching by nitroaromatics: (a) photograph of the CQDs with different nitroaromatics; (b) quenching spectra of the CQDs with different concentrations of TNP; and (c) Stern-Volmer plot of different nitroaromatics. Fluorescence quenching is typically attributed to either an energy transfer or an electron transfer mechanism. Since nitroaromatics are highly electron-deficient, they are generally believed to be capable of efficient fluorescence quenching through an electron transfer mechanism,29 and the quenching efficiency generally positively correlates with the electron withdrawing ability of nitro groups present in nitroaromatics, which should be in the order of TNT > 2,4-DNT > 4-NT.45,46 However, a reverse trend (TNT < 2,4-DNT < 4-NT) was observed in our study with CQDs. In general the main driving force for electron transfer is the energy gap between the lowest unoccupied molecular orbitals (LUMO) of donor and acceptor.47,48 Thus, if electron transfer were the predominant quenching mechanism, the nitro compounds with lower LUMO should have the largest quenching. We calculated the energy levels of some nitro compounds by density functional theory (DFT) method at the B3LYP/6-31G* level in GAUSSIAN 09 software.32 However, the quenching efficiencies of the nitro compounds are not well corresponding to the energy gaps or dipole moments (see Table S1 for details). All of the above results suggest that the quenching of CQDs by nitroaromatics cannot be simply explained by electron transfer. Energy transfer is often considered to be the predominant quenching mechanism at the surface of nanostructures (e.g., Förster resonance energy transfer, FRET).49 One requirement for an FRET quenching mechanism is that the energy emission spectrum from the donor fluorophore must overlap with the energy absorption spectrum of the acceptor (quenching agent).50 Moreover, the efficiency of energy transfer depends upon the extent of overlap between the donor’s


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emission spectrum and the acceptor’s spectrum.51 Thus, our first line of evidence supporting an FRET mechanism involves a comparison of the observed quenching with the spectral overlap. Figure 6a illustrates the donor-acceptor spectral overlap for the case of TNP. As illustrated, there is significant overlap in the emission of the CQDs donor and the absorption of the TNP acceptor. In general Figure S4 shows that nitrophenols have considerable spectral overlap, while the nitrotoluenes have weaker spectral overlap. The degree of overlap is quantified by the integral of overlap (J(λ)) as calculated by equation 9:34 :

J(λ) = 9, F( (λ) ε) (λ)λ/ dλ (9) where FD(λ) is the corrected fluorescence intensity of the donor in the range of λ to λ+∆λ with the total intensity normalized to unity, and εA (λ) is the extinction coefficient of the acceptor at λ in M-1cm-1. Table S2 in the Supporting Information lists the Ksv and J(λ) values for the various nitroaromatics in this study. The J(λ) values for the family of nitroaromatics were observed to vary by two orders of magnitude. Figure 6b shows a strong correlation between the spectral overlap and the observed CQDs quenching constant. This correlation provides good evidence for an energy transfer quenching mechanism. To further confirm the FRET mechanism, time-correlated single-photon counting technology was used to measure the fluorescence lifetime of the CQDs in the absence (τD) or presence (τD-A) of nitro compounds. The obtained data was fitted by deconvolution as shown in Table S3. Figure 6c shows representative results when 0.05 mM TNP was mixed with the CQDs. In this case, a decrease in fluorescence lifetime is observed (Figure S5 shows analogous results with the other nitroaromatics). Figure 6d shows the decrease in fluorescence lifetime correlates to the SternVolmer quenching constant which provides further evidence of energy transfer mechanism.


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As a final check of the FRET mechanism, we estimated the donor-acceptor distances from the lifetime decay measurements (Table S4). These donor-acceptor distances were estimated to be in the range of 10-100 Å consistent with requirements for a FRET mechanism.52 In summary, all the evidence including the correlation between the quenching constant and corresponding integral of overlap or fluorescence lifetime decay and the donor-acceptor distance indicate that the fluorescence quenching of CQDs in this study is predominantly attributed to an energy transfer mechanism.

Figure 6. Quenching correlates to spectral overlap and fluorescence lifetime decay: (a) spectral overlap between absorption of TNP and emission of the CQDs; (b) correlation between quenching constant (Ksv) and integral of overlap (J(λ)); (c) fluorescence lifetime decay of the CQDs with and without TNP; and (d) correlation between quenching constant (Ksv) and fluorescence lifetime decrease (τD-A/τD) 3.4. Fluorescence Quenching of Optically Complex Molecules. To further validate the energy transfer driven quenching mechanism for the chitosan-based CQDs, we investigated two dye molecules that have complex optical properties and spectral overlap with the emission region


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of the CQDs. Our rationale is that such molecules would be expected to serve as a stringent test of an energy transfer quenching hypothesis. Experimentally we mixed the individual molecules with CQDs (0.01 mg/mL) and measured the resulting fluorescence quenching. Curcumin is a bright yellow natural product from turmeric that is often used as a natural food colorant. Figure 7a illustrates the quenching spectra of CQDs with different concentrations of curcumin. The inset of Figure 7a shows large spectral overlap between the emission of CQDs and the absorption of curcumin. Consistent with the proposed energy transfer mechanism, substantial fluorescence quenching is observed. Interestingly, in addition to the strong decrease in fluorescence at 400 nm, there is a small increase in fluorescence observed at 550 nm. Possibly this increasing fluorescence at 550 nm is due to curcumin’s intrinsic fluorescence which potentially is “turned-on” by absorbing energy from CQDs emission (at 400 nm, see Figure S6).53 Amaranth is a synthetic red-purple azo dye with maximum absorbance at around 520 nm. In contrast to curcumin, there is considerably less spectral overlap between emission of CQDs and the absorption of amaranth. As expected, much less quenching was observed with amaranth as illustrated in Figure 7b. No new peak appears in the curves quenched by amaranth because this dye can't be excited at 400 nm. The results for these molecules are summarized in Figure 7c. Figure 7c illustrates the SternVolmer plots for curcumin and amaranth. In Figure 7d, the results for these molecules are added to the cross-plot of spectral overlap and quenching in Figure 6b (see Table S2 for details). They are very close to the extrapolation of the best fit line from Figure 6b. The agreement indicated that the correlation between Ksv and J(λ) observed for the simple nitroaromatics can be extended


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to the compounds that display more complex photophysical behaviors. These results provide further support that an energy transfer mechanism is responsible for quenching of CQDs.

Figure 7. Quenching by complex molecules: (a) quenching spectra of the CQDs with different concentration of curcumin with corresponding spectral overlap; (b) those of amaranth; (c) SternVolmer plot of curcumin and amaranth; and (d) correlation between quenching constant (Ksv) and integral of overlap (J(λ)) compare to nitroaromatics. 3.5. Spectroelectrochemical Analysis. To provide independent evidence for an energy transfer mechanism, we used spectroelectrochemistry to probe the quenching of CQD fluorescence. For these studies we used two redox active molecules, ferricyanide (Fe(CN)63-/4-: oxidized/reduced species; E° = +0.23 V vs Ag/AgCl) and ferrocene dimethanol (Fc+/Fc: oxidized/reduced species; E° = +0.25 V vs Ag/AgCl), which each have dramatic shifts in their UV-vis absorption depending on their oxidation states.54,55 The left plot of Figure 8a shows that the reduced Fe(CN)64- has little absorbance at 430 nm, while the oxidized Fe(CN)63- has strong absorbance that overlaps with the CQD emission. Similarly, the right plot of Figure 8a shows Fc


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has little UV-vis absorbance in its reduced state but some absorbance in its oxidized state formed after electrochemical oxidation of Fc solution at 0.5 V for 10 min, although the oxidized Fc+ extinction coefficient is considerably lower than that for the oxidized Fe(CN)63-. The left plot in Figure 8b shows that when reduced Fe(CN)64- (5 mM in 0.1 M phosphate buffer, pH = 7.2) was added to the spectroelectrochemical cell and the potential was stepped to an oxidizing voltage (0.3 V vs Ag/AgCl), a dramatic increase in the absorbance at 430 nm was observed. Stepping back to a reducing potential (0.16 V) resulted in a corresponding decrease in absorbance. The right plot in Figure 8b shows similar responses when Fc/Fc+ was stepped between oxidizing (0.5 V) and reducing (0.0 V) voltages although the magnitude of the change in 430 nm absorption was considerably lower than for Fe(CN)63-/4- (the insert shows a magnified absorbance scale). These results indicate that the Fe(CN)63-/4- and Fc/Fc+ provide an interesting test for CQD quenching because an electrical signal can be used to trigger a switch in these redox-active molecules from a state with little spectral overlap (the reduced state) to a state with significant spectral overlap (the oxidized state). We next tested the Fe(CN)63-/4- and Fc/Fc+ molecules for an electrically-induced quenching of CQD fluorescence. Experimentally, solutions were added to the spectroelectrochemical cell and initially no potential was applied. After 300 sec, a constant oxidizing potential was of imposed (either 0.3 V for Fe(CN)63-/4- or 0.5 V for Fc/Fc+) as illustrated by the top plots in Figure 8c. The output electrical responses to these imposed voltage inputs are shown in Figure 8c. Two controls that lack Fe(CN)63-/4- or for Fc/Fc+ are phosphate buffer or buffer containing CQD (0.05 mg/mL) and both controls showed minimal electrical response to these imposed oxidative steps. For solutions containing the Fe(CN)63-/4- or Fc/Fc+, the imposed oxidative potential resulted in significant oxidative currents being drawn and these currents were similar irrelevant to whether


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CQDs were present in the solution. These results suggest that during these experiments the CQDs show limited electron transfer compared to electron transfer associated with the Fe(CN)63/4-

or Fc/Fc+.56 The output absorbance response curves in Figure 8c show that the imposed oxidative potential

leads to a significant increase in 430 nm absorption for the solutions containing Fe(CN)63-/4- or Fc/Fc+. Because of the low extinction coefficient of the oxidized Fc+, an expanded absorption scale is included in Figure 8c. The output fluorescence curves are shown in the bottom curves in Figure 8c. In the absence of Fe(CN)63-/4- or Fc/Fc+ the CQD fluorescence is unaffected by the imposed voltage input. In contrast significant quenching is observed for solutions containing Fe(CN)63-/4-. Considerably less fluorescence quenching is apparent in the Fc/Fc+-containing solutions consistent with the lower extinction coefficient of the oxidized Fc+. In these spectroelectrochemical studies, we used conditions (redox-active molecules plus imposed oxidative potentials) to induce spectral overlap and under these conditions we observed CQD fluorescence quenching consistent with the energy transfer mechanism. While these results support an energy transfer mechanism, they do not definitively exclude an electron transfer mechanism. However, if electron transfer were important for quenching, we might have expected solutions containing only CQDs (without Fe(CN)63-/4- or Fc/Fc+) would have undergone some quenching in response to an imposed oxidative potential. It should also be noted that the observed Fe(CN)63- or Fc+-induced quenching was not always reversible and further studies are required to further assess such irreversibility.


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Figure 8. Spectroelectrochemical investigation of CQD fluorescence quenching by redox-active small molecules: (a) the absorbance spectra overlap with the CQD emission spectrum only for the oxidized Fe(CN)63- form (left) and oxidized Fc+ form (right); (b) changes in the absorbance of Fe(CN)63-/4- (left) and Fc/ Fc+ (right) at 430 nm over time in response to the imposed electrical input (upper panels); (c) electrical input stimuli (top panel) and resulting output responses show a simultaneous change is induced in both absorbance (430 nm) and quenching of CQD fluorescence (415 nm).


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4. Conclusions In conclusion, we synthesized chitosan-based carbon quantum dots (CQDs) and report that fluorescence quenching by nitroaromatics results from a Förster resonance energy transfer (FRET) mechanism (not an electron transfer mechanism). Support for this FRET-based quenching mechanism is provided by (i) a correlation between spectral overlap (CQDs donor and nitroaromatic acceptor) and the Stern-Volmer quenching constant (Ksv), (ii) a correlation between fluorescence lifetime decay and Ksv, (iii) calculated donor-acceptor distances within an appropriate range, and (iv) spectroelectrochemical measurement with redox-dependent quenching capabilities. The correlation between quenching constant and spectral overlap extent is further confirmed by dye molecules, indicating that this correlation can extend to compounds that display more complex photophysical behaviors. We envision these fundamental studies will further enable the use of these emerging nanomaterials in imaging and sensing applications and especially for the applications for detection by portable devices.

ASSOCIATED CONTENT Supporting Information. Detailed data supporting energy transfer mechanism, including quenching spectra of nitroaromatics, spectral overlaps, fluorescence lifetime decay, energy levels calculation, quenching constant and donor-acceptor distances are shown in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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Corresponding Author * Email: [email protected] (G. Payne); [email protected]. (X. Wang).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the the Independent Study Projects of the State Key Laboratory of Pulp and Paper Engineering (2015C08, 2015ZD03), the Science and Technology Program of Guangzhou, China (201504010033), the New Century Excellent Talents in University (NCET13-0215), the Fundamental Research Funds for the Central Universities, SCUT (201522036) and United States National Science Foundation (CBET-1435957).

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