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Article

Photoinduced Electron Transfer Mediated by Coordination Between Carboxyl on Carbon Nanodots and Cu Quenching Photoluminescence 2+

Cui Liu, Bo Tang, Song Zhang, Miaomiao Zhou, Mengli Yang, Yufei Liu, Zhi-Ling Zhang, Bing Zhang, and Dai-Wen Pang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12681 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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The Journal of Physical Chemistry

Photoinduced

Electron

Transfer

Coordination

between

Carboxyl

Mediated on

by

Carbon

Nanodots and Cu2+ Quenching Photoluminescence Cui Liu,1,

3

Bo Tang,1 Song Zhang,*,

2

Miaomiao Zhou,2 Mengli Yang,1 Yufei Liu,3 Zhi-Ling

Zhang,*, 1 Bing Zhang,2 and Dai-Wen Pang1 1 Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, State Key Laboratory of Virology, The Institute for Advanced Studies, and Wuhan Institute of Biotechnology, Wuhan University, 299 Bayi Road, Wuhan 430072, P. R. China 2 State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, 30 Xiaohongshanxi, Wuhan 430071, P. R. China, University of Chinese Academy of Sciences, Beijing 100049, P. R. China 3 Key Laboratory of Optoelectronic Technology & Systems (Ministry of Education), Centre for Intelligent Sensing Technology, College of Optoelectronic Engineering, Chongqing University, 174 Shazhengjie, Chongqing 400044, P.R. China

ACS Paragon Plus Environment

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ABSTRACT Carbon nanodots (C-dots) have been widely used in sensing, such as detection of ions, small molecules and biomolecules, based on their photoluminescence (PL) quenching by metal ions. Though C-dots prepared by different methods exhibited various sensitives to metal ions, it is labor intensiveness and time-consuming for selecting synthetic route to obtain C-dots that meet requirements of practical applications. Hence, for the high selective and sensitive applications of C-dots, it is the effective approach to reveal the structure-property relationships in the quenching process. Herein, we present an insight into the mechanism of the PL quenching of C-dots by Cu2+. According to the results of PL, UV-vis absorption, time-resolved PL, and femtosecond transient absorbance measurements, we confirmed that the quenching occurs by a photoinduced electron transfer (PET) process from the photoexcited C-dots to the empty d orbits of Cu2+ combining with C-dots. Meanwhile, through separate protecting functional groups on the surface of C-dots, the structure of C-dots coordinating with Cu2+ is revealed to be carboxyl rather than hydroxyl groups. This study leads to a better understanding of the quenching of C-dots and takes an important step toward more rational design of C-dots-based sensor with high performance.

INTRODUCTION

Firstly discovered in 2004,1 carbon nanodots (C-dots) have attracted growing attention due to the excellent photostability, tunable emission, non-blinking, low toxicity, etc.2-6 Because of the above impressing properties, C-dots have been used for various applications, such as sensing, imaging and energy.7-11 Moreover, many research efforts have been devoted to the development


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of fluorescent probes based on C-dots for the detection of numerous kinds of metallic ions and anions, such as Cu2+,12-14 Hg2+,15-17 and S2-,18 while most of these assays were based on the photoluminescence (PL) quenching of C-dots by metal ions. Zhou et al.16 reported a procedure for the sensitive and selective detection of Hg2+ in complex matrices based on the PL quenching of unmodified C-dots by Hg2+. Due to the strong bind preference of biothiols toward Hg2+, the Hg2+ could be removed from the surface of C-dots through forming Hg-S bonds. Thus, the PL of C-dots recovered and accordingly achieved the detection of biothiols. Moreover, different structures of C-dots give rise to verious selectivities for diverse metallic ions. For instance, the C-dots synthesized by pyrolysis can be quenched by Cu2+ but cannot be quenched by Hg2+,12 while, the C-dots prepared by electrochemical method were insensitive to most heavy metal ions including Cu2+.19 However, it is labour intensiveness and time-consuming to search proper synthetic route for obtaining C-dots with high selectivity, hindering the development of C-dotsbased fluorescent probes with high specificity. Hence, understanding the structure-property relationshipes, during the quenching process, would help rational modulating the structure of Cdots to achieve high selectivity. With Cu2+ as an example, we present an in-depth study on the PL quenching mechanism of C-dots by metal ion via excited state dynamics and the separate protection of functional groups. It has been found that Cu2+ coordinates with carboxyl rather than hydroxyl groups on the surface of C-dots, and the photoinduced electron transfer (PET) from excited C-dots to Cu2+ takes place subsequently, which result in the PL quenching. The study and discussion of the PL quenching mechanism of C-dots by metal ions in this paper not only promote the rational design sensor based on C-dots, but also offer new opportunities in potentially using C-dots for electron and light energy conversion.


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EXPERIMENTAL METHODS (1) Synthesis of C-dots: 0.4 g of carbon fiber powder (500 mesh) was added in 10 M nitric acid solution, and the mixture was refluxed for 4 h. The resulting solution containing C-dots was collected, and further neutralized with NaHCO3 till the pH of the solution reached about 3. The resultant solution was first filtered by 0.22 µm BIOSHARPP membrane filters and further dialyzed in a 3500-Da dialysis bag for 7 days. Then the C-dots solution was ultrafiltered through Millipore centrifugal filter devices. Accordingly, the fractions equivalent to < 3 kDa was collected and investigated in this work. (2) Preparation of C-dots-PS: 1 mL of C-dots solution (5 mg/mL) was added to 10 mL of 1, 4dioxane containing 1 g of 1,3-propanesultone (PS). A total of 1 mL triethylamine was then added to this mixture. After stirred for 24 h at 40 ˚C, the mixture was underwent a rotary evaporation to remove solvent. The resulting sample was dispersed in water and dialyzed in 0.1 M NaCl solution for 1 day to remove triethylamine salt through ion exchange, and then further dialyzed in ultrapure water for 3 days (3) Preparation of C-dots-PS-NaOH: 5 mg of C-dots-PS was added to 10 mL of 0.5 M NaOH solution, and then stirred for 24 h at 40 ˚C. The resulting solution was neutralized with hydrochloric acid and further dialyzed for 3 days. (4) Calculation of quenching constant (kq): The corresponding quenching constant (kq) of C-dots

by Cu2+ was evaluated by using the equation of kq= KSV/τ0. Where τ0 is the lifetime of Cdots in the absence of Cu2+, KSV is the Stern-Volmer quenching constant calculated from the equation of I0/I=1+KSV[Q]. 20 I0 and I are the PL intensities of fluorophore in the absence and presence of a quencher, and [Q] is the quencher concentration.


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RESULTS AND DISCUSSION The C-dots in this research were synthesized by using the procedure in our previous report.21 The carbon fibre powder was refluxed in 10 M nitric acid solution for 4 h, followed by neutralization, dialysis, and ultrafiltration. The transmission electron microscopy image and corresponding size distribution of the C-dots (Figure 1A-B) indicate the good dispersion of the C-dots, with an average size of 1.8 ± 0.3 nm (based on statistical analyses of more than 200 dots). The surface structure of C-dots was characterized by C 1s high-resolution X-ray photoelectron spectroscopy (XPS). Shown in Figure 1C, the C 1s XPS of C-dots can be fitted by four peaks at 284.5, 286.0, 287.8 and 289.0 eV, which according to graphitic, alcoholic, carbonyl, and carboxyl carbons, respectively. The existences of functional groups were further confirmed by Fourier-transform infrared spectroscopy (FT-IR) spectrum (Figure 1D). The absorption bands at ~3420, 1729, 1623, and 1224 cm-1 were assigned to hydroxyl (O-H), carboxyl (C=O), carbonyl (C=O) and C-O stretching vibration, respectively. Because of the strong hydrogen bonded stretching vibration, broad absorption band was in the range of 2400-2650 cm-1, which enabled one to distinguish a carboxylic acid from all the other carbonyl compounds.22 For further verifying the existence of carboxyl groups on the surface of C-dots, the sample was alkalified by NH4OH before FT-IR measurement. As shown in Figure S1, the peaks at 1729, 1224, and ~2560 cm-1 disappeared. Instead, two strong bands at 1604 and 1400 cm-1 appeared, which corresponded to the asymmetric and symmetric stretching vibrations of -COO-, respectively, because the carboxyl group (-COOH) was converted to carboxylate anion (-COO-). Moreover, the broad band around ∼3420 cm−1 corresponding to the O-H stretching vibration was still very


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strong. These results demonstrate that the C-dots are covered with abundant carboxyl, carbonyl and hydroxyl groups.

Figure 1. TEM image (A), corresponding size distribution (B), C 1s high-resolution XPS spectrum (C) with identification of peaks by curve fitting and FT-IR spectrum (D) of C-dots. The C-dots exhibited a symmetric PL spectrum with maximum at 522 nm under the excitation of 400 nm, and metal ions such as Hg2+, Cu2+, Ag+, Ni2+ and Mn2+ could decrease the PL of C-dots in different degrees (Figure S2), limiting the applications in detection. Taking the Cu2+ as an example, the quenching mechanism was studied thoroughly in this paper. As demonstrated in Figure 2A, the PL intensity of C-dots decreased gradually as the increase of Cu2+ concentration. The Stern-Volmer quenching constant (KSV) was calculated to be 1.5×105 M1 20

.

The UV-vis absorption spectrum of C-dots showed no change (Figure 2B) when the

concentration of Cu2+ reached 1 mM, while the PL intensity decreased more than 98%, which was different from Shi’s report,23 demonstrating no ground-state complex formation. As displayed in Figure 2C, the PL decay rate of C-dots became faster after Cu2+ added in. The average lifetime of C-dots shortened from 3.4 ns to 2.4 ns (Table S1), indicating that Cu2+


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affected the deexcitation process of the excited C-dots. These results demonstrate that the PL quenching of C-dots by Cu2+ is a dynamic quenching process,20 where Cu2+ acts with the excited C-dots, thus make them return to the ground state without the emission of a photon. The corresponding quenching constant (kq) of C-dots by Cu2+ was evaluated to be 4.4 ×1013 M-1s-1.20

Figure 2. The PL spectra of C-dots with the concentration of Cu2+ increasing from 0 to 7 µM (A), inset shows plots of the PL intensity of C-dots against the concentration of Cu2+; absorption spectra of C-dots and upon addition of Cu2+ before and after irradiation by a mercury lamp with 50 W continuously (B); the time-resolved PL curves of C-dots in the absence and presence of Cu2+ (C); and PL spectra of C-dots (red), C-dots in the presence of Cu2+ (1 mM) before (blue) and after (pink) adding EDTA in (D). The quenching of C-dots by Cu2+ with quenching constant much larger than 107 M-1s-1 is generally attributed either to electron or to energy transfer process.24-25 There is a less efficient overlap in the range of 600-700 nm between the PL spectrum of C-dots and the absorption band of the Cu2+ (Figure S3). As the concentration of Cu2+ increasing from 0 to 7 µM, the PL intensity decreased linearly, but no PL spectrum deformation appeared, which demonstrate that the energy transfer from excited C-dots to Cu2+ could be negligible in this case. Ethylenediaminetetraacetic


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acid disodium salt (EDTA) is an excellent chelating agent which can combine with many kinds of metal ions to form stable and water-soluble coordination complexes. EDTA also has no effect on the PL of C-dots (Figure S4). After adding EDTA to the C-dots quenched by Cu2+, the PL intensity recovered (Figure 2D), which demonstrated that the PL quenching of C-dots by Cu2+ is reversible. Meanwhile, there was no change recorded in the absorption spectrum of C-dots after continuous photoexcitation for 20 min in the presence of Cu2+ (1 mM), excluding the possibility of a photo chemical degradation of C-dots (Figure 2B).24 Besides, Cu2+ is a kind of electron acceptor due to its empty d orbits. Therefore, the quenching mechanism would be mainly due to a PET process from excited C-dots to the empty d orbits of Cu2+. In order to determine the time of PET, the temporal resolution of the excited state dynamics was investigated using femtosecond transient absorption (TA) spectrum. The transient absorption spectra of the C-dots in the absence and presence of Cu2+ are shown in Figure 3A-B. The excited-state absorption (ESA) bands of them are very similar on the shape and position, suggesting that Cu2+ has no obvious effect on the excited state of C-dots. In the presence of Cu2+, the excited state decay of C-dots is much faster, which is consistent with the result of PL lifetimes.


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Figure 3. The two-dimensional transient absorption spectra of C-dots in the absence (A) and presence (B) of Cu2+ within delay time of 100 ps, and the weight of each lifetime component of C-dots in the absence (C) and presence (D) of Cu2+. Cu2+ only increased the number of ways of deexcitation of C-dots, which did not affect the time of intrinsic deexcitation approach. Due to the uncertainty of the PET time, the lifetimes of C-dots in the absence and in the presence of Cu2+ were set as close as possible when the collected data were globally fitted. In this way, the lifetime which ratio of component increasing much obviously should be close to the time of PET. As a result, the relaxation with time constants of ~3 ps, ~35 ps, and 1.5 ns were obtained. The weights of each lifetime component from global fitting are presented in Figure 3C-D. The lifetime component of 1.5 ns can be safely assigned to the radiative recombination, in accordance with previous reports.26-27 The ratio of 1.5 ns component decreased and that of 3 ps component increased, while the ratio of ~35 ps component barely changed. Thus, the time of PET from the excited C-dots to the Cu2+ could be close to 3 ps, agreeing with the time of PET reported in other works.28-29 A molecular contact between the fluorophore and quencher is required for PL quenching. Considering the close distance interaction required for quenching and the highly efficient quenching of C-dots by Cu2+, it should be some structures on the surface of C-dots combining with Cu2+. Liu30 reported that C-dots combined with Cu2+ through their carboxyl groups, while Hou18 declared that Cu2+ bonded with N and S atoms of the ligands on the surface of C-dots rather than carboxyl groups. Meanwhile, Zhang proposed that the formation of complexes between Fe3+ ions and the phenolic hydroxyl groups of C-dots resulted in the PL quenching.31 For the C-dots in this work, there were hydroxyl, carboxyl, and carbonyl groups on the surface. To clarify the definite functional groups combining with Cu2+, the carboxyl and


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hydroxyl groups, which have preferential probability of coordination with Cu2+,32-34 were protected separately. As displayed in Figure S5, 1, 3-propanesultone (PS) can react with carboxyl and hydroxyl groups to form ester and ether, respectively, in mild situation.35-36 Therefore, the reagent was used to react with C-dots for protecting carboxyl and hydroxyl groups simultaneously, and the resulting sample was denoted as C-dots-PS. The changes of functional groups on the surface of C-dots were monitored by the FT-IR (Figure 4A-B). As for C-dots-PS, the band around 2560 cm-1 disappeared and the peak of C=O stretching vibration shifted to 1737 cm-1. Meanwhile, the characteristic absorption peaks of the sulphonic acid at 611, 529, 1150 and 1045 cm-1 appeared (Figure 4A).37 In addition, the proton magnetic resonance (1H-NMR) spectrum of C-dots-PS (blue line), in Figure 4C, shows that the new absorption peaks appeared at 2.1, 2.9 and 4.4 ppm corresponding to 1H nuclei of β, α and γ of –SO3-, respectively. In addition, the XPS spectra (Figure 4D) conveyed that the sulphur-to-carbon ratio of C-dots-PS increased from 0 to 8.7%. These changes demonstrated that PS molecules have reacted with Cdots successfully and carboxyl groups of the C-dots have been sufficiently converted to ester. An ester can hydrolyze under alkaline condition but ethers cannot. After the hydrolysis of C-dots-PS in 0.5 M NaOH solution, C-dots-PS-NaOH was obtained. As shown in Figure 4A-B, the band around 2560 cm-1 of C-dots-PS-NaOH recovered, and the peak of C=O stretching vibration returned to 1729 cm-1, indicating that the ester hydrolysed sufficiently. In the FT-IR spectrum of C-dots-PS-NaOH, the characteristic absorption peaks of the sulphonic acid decreased but were still very obvious (Figure 4A, purple line), and the sulphur-to-carbon ratio was still 6.9% (Figure 4D), indicating the survival of PS residue. At the same time, the absorption peak of 1H nuclei of γ of –SO3- was at 4.3 ppm below that of C-dots-PS (4.4 ppm), indicating that the PS residue of C-dots-PS-NaOH connected with C-dots through ether bond. Hence, the carboxyl and


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hydroxyl groups of C-dots-PS were protected simultaneously, while only the hydroxyl groups of C-dots-PS-NaOH were protected. The contribution between carboxyl and hydroxyl groups on the quenching efficiency can be differentiated through the comparison of the quenching efficiencies among the three kinds of C-dots.

Figure 4. FT-IR spectra (A-B), 1H-NMR (C), and XPS surveys (D) of the C-dots (red), C-dotsPS (blue) and C-dots-PS-NaOH. As shown in Figure 5A, the PL intensity of C-dots-PS decreased slackly, while the concentration of Cu2+ increased by orders of magnitude. The quenching phenomenon may be mainly attribute to the diffusive encounters20 and energy transfer. The diffusive encounters are induced by the collision between C-dots and Cu2+. Meanwhile, the energy transfer originates from the weak overlap in the range of 600-700 nm between the PL spectrum and absorption band of C-dots-PS and Cu2+, respectively, supported by the appearance of PL spectrum deformation of C-dots-PS in the present of Cu2+ (Figure S6A). When the concentration of Cu2+ was 0.1 mM, 60% of the PL intensity of C-dots-PS retained while more than 90% of C-dots PL intensity was quenched. When the concentration of Cu2+ increased to 10 mM, the PL intensity of C-dots-PS decreased 60% while that of C-dots decreased more than 99%. On the other hand, the quenching


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efficiency of C-dots-PS-NaOH by Cu2+ restored with KSV of 1.1×105 M-1, which was commensurate with that of C-dots (Figure 5B). The results elucidate that the structure combine with Cu2+ is mainly carboxyl rather than hydroxyl group.

Figure 5. Normalized PL intensity of C-dots, C-dots-PS and C-dots-PS-NaOH as the concentration of Cu2+ increased (A), PL titrations of C-dots and C-dots-PS-NaOH with Cu2+ (B), and the proposed quenching mechanism of C-dots by Cu2+ (C). To verify the coordination effect between carboxyl groups of C-dots and Cu2+, the FT-IR and linear sweep voltammetry were performed (Figure S7). After the addition of Cu2+, a new peak at 1590 cm-1 appeared (Figure S7A-B), ascribing to the carboxyl groups coordinated with Cu2+. Simultaneously, the two reduction potential peaks of Cu2+ experienced negative shifts in the presence of C-dots (Figure S7C), demonstrating the coordination of Cu2+. The results above revealed that the coordination between Cu2+ and carboxyl group on the surface of C-dots narrows their distance, thus resulting in the efficient quenching of PL (Figure 5C). Conclusion


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In summary, we confirmed that C-dots coordinate with Cu2+ through their carboxyl groups, thus boosting the PET from excited C-dots to the empty d orbits of Cu2+ within picoseconds, and finally result in the quenching of PL, which provides a clear structure-property relationship to guide controlling the structure of C-dots, facilitates the rational designing of C-dots-based fluorescent probes. Moreover, the strategy to modify the surface of C-dots promotes the controlling for the variety and quantity of functional groups on the surface of carbon materials. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. UV-vis and PL spectra; FT-IR spectra; linear sweep voltammetry; lifetimes and

multiexponential

fitting

for

femtosecond

time-resolved

absorption

dynamics.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Author Contributions C. Liu and S. Zhang designed the experiments and analysed the data; C. Liu, M. Zhou, and M. Yang performed the experiments; B. Tang, Z.-L. Zhang, B. Zhang, and D.-W. Pang analysed the data; C. Liu and Y. Liu wrote the paper. Funding Sources This work was supported by the National Natural Science Foundation of China (21535005 and 11674355), the 111 Project (111-2-10), the National Key Research and Development Program of


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China (2016YFE0125200), and Collaborative Innovation Center for Chemistry and Molecular Medicine. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21535005 and 11674355), the 111 Project (111-2-10), the National Key Research and Development Program of China (2016YFE0125200), and Collaborative Innovation Center for Chemistry and Molecular Medicine. The authors would also like to thank Prof. Qu-Quan Wang and Mr. Fan Nan for their assistance of the PL lifetime experiments. REFERENCES (1) Xu, X.; Ray, R.; Gu, Y.; Ploehn, H. J.; Gearheart, L.; Raker, K.; Scrivens, W. A., Electrophoretic Analysis and Purification of Fluorescent Single-Walled Carbon Nanotube Fragments. J. Am. Chem. Soc. 2004, 126, 12736-12737. (2) Bao, L.; Liu, C.; Zhang, Z.-L.; Pang, D.-W., Photoluminescence Tunable Carbon Nanodots: Surface State Energy Gap Tuning. Adv. Mater. 2015, 27, 1663-1667 (3) Baker, S. N.; Baker, G. A., Luminescent Carbon Nanodots: Emergent Nanolights. Angew. Chem. Int. Ed. 2010, 49, 6726-6744. (4) Ding, C.; Zhu, A.; Tian, Y., Functional Surface Engineering of C-Dots for Fluorescent Biosensing and in Vivo Bioimaging. Acc. Chem. Res. 2014, 47, 20-30.


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