One-step synthesis of chiral carbon quantum dots and their enantioselective recognition

Abstract

Chiral carbon quantum dots (L-carbon quantum dots, L-CQDs; and D-carbon quantum dots, D-CQDs) were synthesized through the facile hydrothermal treatment of carbonated citric acid and L-cysteine (or D-cysteine). The chirality in L-CQDs and D-CQDs was demonstrated by circular dichroism. Both L-carbon quantum dots and D-carbon quantum dots could exhibit good enantioselective recognition ability.

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1 Introduction

Carbon quantum dots (CQDs)¹ show a wide range of applications, such as, bioimaging,^2–4 chemical sensing,^5–12 photocatalysis^13–16 and electrocatalysis,^17,18 which are attributed to singular features including excellent photoluminescence, biocompatibility, low toxicity, photoinduced electron transfer properties and so on.^19,20 On the other hand, chirality exists massively in nature, not only in macroscopic proteins and DNA, but also in supramolecular and nanoscopic structures. Up to now, the chiral modifications of fullerenes and carbon nanotubes have been extensively investigated^21–23 and exhibit great potential in organocatalysis,^24,25 enantioselective recognition^26,27 as well as chiral sensing.²⁸ However, only few researches based on chirality of CQDs have been reported.²⁹ Notably, the addition of chiral properties into CQDs further enhances their versatile properties, which is extremely important both fundamentally and technologically. Here, we demonstrate the facile synthesis of chiral carbon quantum dots (L-CQDs and D-CQDs). As Scheme 1 shown, the chiral carbon quantum dots were synthesized by hydrothermal method with the presence of citric acid (CA) and chiral cysteine (L or D) in a Teflon-sealed autoclave and samples were purified by cellulose dialysis.^30–33 We further testify that both L-CQDs and D-CQDs exhibit good enantioselective recognition ability.

Scheme 1 Synthesis strategy of L-CQDs and D-CQDs.

2 Experimental

2.1 Materials

Citric acid, L- and D-cysteine, L- and D-tartaric acid were obtained from Sigma-Aldrich, which were used without any further purification. Cellulose dialysis bag with Da 500–1000 was purchased from Shanghai Yuanye Biotechnology Co. Ltd.

2.2 Preparation of CQDs

The achiral CQDs were prepared in a typical electrochemical etching method developed by our group.³⁴ While, the chiral CDQs were typically synthesized through pyrolysis of citric acid and cysteine illustrated in Scheme 1. In the process of synthesis, citric acid (1.92 g, 0.01 mol), L-cysteine (or D-cysteine) (2.42 g, 0.02 mol) were sufficiently dissolved and dispersed into 5 mL deionized water by ultrasonic for 30 min. Then the mixed solution was transferred into a 25 mL Teflon-sealed autoclave at 180 °C for 1 h. When the reaction was completed, the Teflon-sealed autoclave cooled naturally in the air and the dark brown solution was acquired. After the solution dialyzed against a 500–1000 Da cellulose dialysis membrane for four days, the obtained brown sample was L(D)-CQDs. Carbon skeleton was acquired though the same condition with citric acid (1.92 g, 0.01 mol) and deionized water (5 mL) in the Teflon-sealed autoclave.

2.3 Electrochemical measurement

All the electrochemical measurements were carried out in a conventional three electrode cell with modified carbon paste electrode as the working electrode, platinum wire as the counter electrode and saturated Ag/AgCl (saturated KCl) electrode as the reference electrode.³⁵ To prepare the working electrode, 0.2 g graphite powder was mixed with 20 mg L-CQDs or D-CQDs and grounded together with the help of agate mortar to obtain homogeneous phase. Then 0.66 mL liquid paraffin was added to the above mixture and grounded again to make them adhesive. Subsequently, the prepared electrode material was packed into the glass tubes and the surface was wiped smoothly with weighing paper. The bare carbon paste electrode was prepared in a similar way without adding any chiral carbon quantum dots. For achiral electrode, the achiral carbon quantum dots were used instead of L-CQDs in electrode material. The enantioselective recognition abilities of L-CQDs or D-CQDs were analysed through the electrochemical impedance spectroscopy (EIS) in Na₂SO₄ solution. Here 0.5 M Na₂SO₄ solution was used as electrolyte. Electrochemical properties were also tested in Na₂SO₄ solution at the scan rates of 50 mV s⁻¹.

2.4 Characterization

Transmission electron microscopy (TEM) images were obtained using a FEI/Philips Tecnai G² F20 transmission electron microscope. Atomic force microscopy (AFM) measurements were performed on a Veeco Multimode V atomic force microscope. Energy dispersive X-ray spectroscopy (EDS) measurements were conducted on a FEI-quanta 200 scanning electron microscope with an acceleration voltage of 10 kV. Photoluminescence study was carried out with Fluromax 4 (France JY company). Fourier transform infrared (FT-IR) spectra were collected on a FT-IR spectrometer (Spectrum One, Perkin Elmer). X-ray photoelectron spectroscopy (XPS) data were collected using an Axis Ultra instrument under ultrahigh vacuum (<10⁻⁸ Torr). The circular dichroism (CD) spectra and the ultraviolet-visible spectroscopy (UV-vis) absorption spectra were recorded on a JASCO J-815 spectropolarimeter. The electrochemical impedance spectroscopy (EIS) measurement was obtained on a CHI 832 electrochemical instrument (CHI Inc., USA). The linear sweep voltammogram (LSV) tests were carried out with a CHI 660E workstation (CHI Instruments, Chenhua, Shanghai, China).

3 Results and discussion

Fig. 1a shows the TEM image of L-CQDs, indicating the obtained L-CQDs are uniform with the size in the range of 2–5 nm. The AFM image and corresponding height analysis of L-CQDs shown in Fig. 1b confirm the height of them is about 2.8 nm. And the TEM and AFM images of D-CQDs are exhibited in Fig. S1a and b.† The D-CQDs have the similar size (2–5 nm) with that of L-CQDs, and the height of them about 2.7 nm. In addition, optical properties of CQDs were also investigated shown in Fig. S2.† It turns out that both of L-CQDs and D-CQDs have similar photoluminescence.

Fig. 1 (a) TEM and (b) AFM images of L-CQDs.

The FT-IR spectra were used to identify the functional groups on the carbon skeleton, cysteine and L-CQDs or D-CQDs. The red trace in Fig. 2 shows the FT-IR spectrum of carbon skeleton, in which the absorption signals at 3456, 1729 and 1399 cm⁻¹ are attributed to O–H, C=O and C–O stretching vibrations, respectively.^36,37 The black trace in Fig. 2, representing the FT-IR spectrum of cysteine (Cys), exhibits one special peak at 1620 cm⁻¹ attributed to N–H.³¹ The FT-IR spectra of L-CQDs and D-CQDs are the blue trace in Fig. 2 and the black trace in Fig. S3† separately. As shown, after the polymerization of the carbon skeleton and cysteine, the L-CQDs and D-CQDs show a red shift of peak at 1700 cm⁻¹ compared with the carbon skeleton, which may be due to the interaction between carbon skeleton and cysteine. In addition, the broad peak at 3456 cm⁻¹ is corresponding to O–H vibration mode inheriting from carbon skeleton and cysteine. The other intense peak at 1620 cm⁻¹ is the N–H from the amino in cysteine. Based on the above results, L-CQDs and D-CQDs are synthesized successfully through the linkage of carbon skeleton and cysteine.

Fig. 2 FT-IR spectra of cysteine (Cys, black line), carbon skeleton (red line) and L-CQDs (blue line).

To further investigate the structures of L-CQDs or D-CQDs, XPS was used to analyze the elemental composition and chemical state of them. Fig. S5† demonstrates the full scan XPS survey spectra of L-CQDs and D-CQDs, showing the samples possess the C, O, N, S elements without other impurity, where the doping concentrations of N is 10.19% (atomic ratio) in L-CQDs and 10.52% in D-CQDs (Fig. S4†). Fig. 3a describes the high-resolution C 1s XPS spectra of L-CQDs, which can be divided into five peaks. The peak located at 284.8 eV is corresponding to graphite carbon. Four fitted peaks located at 285.1 eV, 285.7 eV, 286.6 eV and 289 eV can be ascribed to the alkylcarbon atoms in C–S and C–N moieties, the linkage of carbon, the carboxyl or hydroxyl oxygen and the carbonyl C in the C=O group, respectively.^38,39 Fig. 3b shows the binding energies of N 1s. The main signal displays occurrence of C–N group at 400.3 eV and amino N at 401.6 eV.⁴⁰ The peak at 405 eV is ascribed to π-excitations caused by charging effects.^16,41 The main O 1s peak is located at 531.9 eV attributed to the O–H bond; two more components were individuated at higher binding energy values: the component at 532.7 eV is associated with C–O bond; another one at 533.5 eV may be the carboxyl oxygen in Fig. 3c.⁴² Fig. 3d displays the high-resolution S 2p XPS spectra of L-CQDs, in which the peaks at 163.46 eV ^43,44 and 164.62 eV ⁴⁵ are corresponding to sulfur atoms of free thiol terminal groups. The binding energies of D-CQDs are similar to L-CQDs shown in Fig. S6.†

Fig. 3 High-resolution XPS spectra of (a) C 1s, (b) N 1s, (c) O 1s and (d) S 2p of L-CQDs.

The UV-vis absorption and circular dichroism (CD) spectra of samples are shown in Fig. 4 and S7.† Fig. 4a displays the UV-vis absorption spectra of L-cysteine (L-cys), carbon skeleton and L-CQDs. As exhibited, the L-CQDs have three obvious absorption peaks, which are located at 210 nm, 245 nm and 350 nm, respectively. The typical absorption band at 245 nm is ascribed to π–π* transition of conjugated system.⁴⁶ And the absorption band at 350 nm is attributed to n–π* transition,⁴⁷ which is caused by surface carboxyl amino or other moieties that passivate the surface traps. In addition, the strong absorption peaks at 210 nm is assigned to the preservation of cysteine or the effect of carbon skeleton compared to the UV-vis absorption of pure cysteine and carbon skeleton respective. Similar UV-vis absorption spectra to L-cys and L-CQDs are observed in the D-cysteine (D-cys) and D-CQDs in Fig. S7a.† Although the UV-vis absorption of L-CQDs or D-CQDs is identical, opposite cotton effects are still obtained in the CD spectra in the UV-vis absorption range.⁴⁸ Fig. 4b shows the CD spectra of L-CQDs and D-CQDs. The CD spectra are mirror images of one another, which conclusively demonstrate that L-CQDs and D-CQDs are enantiomers. Fig. S7b† reveals the CD spectra of L-cys and D-cys. With respect to pure cysteine, the L-CQDs or D-CQDs present two additional CD signals at 245 nm and 350 nm. Apart from preservation of chirality from cysteine, chirality can be also induced in the L-CQDs or D-CQDs cores through chiral inheritance and transfer.^49,50

Fig. 4 (a) UV-vis adsorption spectra of L-cys (black line), carbon skeleton (blue line) and L-CQDs (red line), respectively; (b) CD spectra of L-CQDs and D-CQDs (black line and red line for L-CQDs and D-CQDs, respectively).

In the followed studies, the electrochemical properties of L-CQDs and D-CQDs were explored by electrochemical impedance spectroscopy (EIS) measurement and linear sweep voltammogram (LSV) test. Fig. 5 depicts the Nyquist plots and electrochemical curves of different processes at the electrode surface of bare-CPE, L-CQDs or D-CQDs. As a proof of concept, the semicircle in the high frequency corresponds to charge transfer, generated by redox reaction at the interface between electrode and electrolyte. The straight line in the low frequency is attributed to impedance of current as a result of diffusion from solution to interface.⁵¹ As described in Fig. 5a, when we use bare-CPE as electrode, there are only straight lines in the Nyquist plots no matter in the 40 mL, 0.5 M Na₂SO₄ with 3 mL, 0.1 M L-tartaric acid (L-tart) or D-tartaric acid (D-tart), showing only diffusion process occurs between electrode and target molecules. Also, the LSV test was further carried out with the same electrolyte to observe the reaction activities between electrode materials and target molecules. From Fig. 5b, we can see that bare-CPE almost has no activity for L-tart or D-tart with so small current.

Fig. 5 EIS spectra of bare-CPE (a), L-CQDs (c) and D-CQDs (e) toward L-tart and D-tart at a scan rate of 50 mV s⁻¹ in 0.5 M Na₂SO₄. LSV curves for oxidation of L-tart and D-tart on bare-CPE (b), L-CQDs (d) and D-CQDs (f) with a sweep rate of 50 mV s⁻¹ in 0.5 M Na₂SO₄.

With L-CQDs as electrode in the same electrolyte shown in Fig. 5c, smaller semicircle appears toward L-tart than that in D-tart in high-frequency region, indicating lager availability for a redox reaction exists between L-CQDs and L-tart. Similarly, it is obvious that when D-CQDs are served as catalyst in Fig. 5e, the more rapid electron transfer emerges on the electrode surface in the case of Na₂SO₄ with D-tart applied as electrolyte, as a result of smaller semicircle than that in L-tart. This result may be attributed to a preferential interaction of L-CQDs toward L-tart and vice versa.⁵² Further, L-CQDs are more active for oxidation of L-tart with about 20% of higher current than D-tart in Fig. 5d. Beyond doubt, D-CQDs show higher ability for oxidation of D-tart analogously with earlier onset potential [∼1.0 V vs. the reversible hydrogen electrode (RHE)] than L-tart (∼1.2 V) and about two-fold larger current in Fig. 5f. Taking above critical results into consideration, it is deduced that due to chiral surrounding on electrode surface, L-CQDs and D-CQDs have preferential oxidation ability toward small molecules that have the right configuration. In addition, chiral CQDs on the electrode surface can also improve the ability of electron transfer, giving rise to enhancing recognition ability.⁵³

In further control experiments, we use achiral-CQDs as electrode to further corroborate that it is chirality rather than other factors to contribute to the recognition. As shown in Fig. S8,† there are just straight lines in the Nyquist plots regardless of in L-tart or D-tart, showing no enantioselective recognition toward target molecules. Combined with electrochemical measurements, no obviously different activity for oxidation of D-tart or L-tart further manifests the above result. In conclusion, based on EIS and LSV test, L-CQDs or D-CQDs possess intriguing enantioselectivity recognition and electrocatalysis ability toward L-tart or D-tart.

4 Conclusions

We demonstrate a one-step hydrothermal method to synthesize chiral carbon quantum dots, where citric acid was carbonized and polymerized with chiral amino acid. Through chiral inheritance and transfer, L-CQDs and D-CQDs exhibit well opposite and symmetrical circular dichroism signals. More interestingly, chiral carbon quantum dots can be used as a good catalyst for chiral recognition and electrocatalysis toward small molecules such as L-tart and D-tart. Our studies shed light on the designing and fabricating new types of carbon-based nanodots which may be applied in chiral catalysis, chiral sensing and so on.

Acknowledgements

This work is supported by the Collaborative Innovation Center of Suzhou Nano Science and Technology, the National Basic Research Program of China (973 Program) (2012CB825803, 2013CB932702), the National Natural Science Foundation of China (51422207, 51132006, 51572179, 21471106, 21501126), the Specialized Research Fund for the Doctoral Program of Higher Education (20123201110018), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12420h

‡ These authors contributed equally to this work.

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