Facile synthesis of nitrogen-doped carbon dots with robust fluorescence in a strongly alkaline solution and a reversible fluorescence ‘off–on’ switch between strongly acidic and alkaline solutions

Abstract

In this study, hydrophilic nitrogen-doped carbon dots (N-CDs) have been hydrothermally prepared using citric acid and various concentrations of ammonium hydroxide. The N-CDs exhibit an excitation-independent blue emission and an optimal quantum yield of 24.2%. Under strongly alkaline conditions adjusted by various alkaline solutions, the N-CDs exhibit robust fluorescence performance, even at an extreme pH value of 13. Although the fluorescence is quenched in strongly acidic solutions (pH = 1), the N-CDs exhibit reversible and repeatable ‘off–on’ fluorescence on modulating the pH value between 1 and 13, resulting from the protonation and deprotonation of the fluorescence-related surface states. Moreover, the N-CDs maintain a bright and stable emission in simulated body fluid (SBF). Thus, the N-CDs have promising applications for dynamic detection of pH switching caused by strongly acidic and strongly alkaline conditions and fluorescence bio-imaging of living organisms even under extreme pH conditions.

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Fluorescent carbon dots (CDs), as an efficient carbon nanomaterial, have attracted considerable interest throughout the world in recent years owing to their unique advantages such as good compatibility, chemical inertness, high photo-stability, non-toxicity, and excellent optical properties, as well as low-cost.^1,2 CDs have broad potential applications in bioimaging,^3,4 optoelectronic devices,^5,6 catalysis^7,8 and sensors.^9,10 Most CDs exhibit pH-dependent emission and stable fluorescence in the mild pH range of about 4–11.^11–13 This optical property has two-sided effects: first, it makes CDs appropriate for use as sensors for analytical detection of metal ions^14,15 and secondly, in the fluorescence bio-imaging of living organisms.^16,17 However, the fluorescence of these CDs is almost quenched by strong acidity and strong alkalinity. Unfortunately, they cannot be successfully applied to fluorescence bio-imaging of acid-tolerant and alkali-tolerant microorganism for real-time investigations of some bio-chemical process^18,19 and for photocatalysis under extreme pH conditions.²⁰ Thus, the highly fluorescent CDs coupled with robust fluorescence to strong acidity and alkalinity are fascinating.

In the past years, many studies have been devoted for the preparation, optimization and utilization of CDs, and certain improvements have been realized.^21,22 However, the fabrication of CDs with high quantum yield (QY) and fascinating optical properties, such as tunable full-color emission, robustness over a large pH range, and resistance to photobleaching, is still a great challenge for their practical application. Recently, it has been demonstrated that a combination of citric acid and nitrogen-containing organic molecules, such as ethylenediamine,^23,24 long-chain polyethylene amines,²⁵ and polyethylenimines,²⁶ can be used as staring materials via hydrothermal or microwave reactions for the facile syntheses of highly fluorescent nitrogen-doped CDs at large scale. The various optical properties of the CDs thus obtained are derived from the different chemical surface compositions. However, the strong photoluminescence signals of these organic nitrogen-doping agents based CDs are only stable in mild pH range and sharply decreased or even almost quenched in extremely low and high pH media, which restrict their application fields, particularly in harsh environments. Therefore, it is desirable to obtain nitrogen-doped CDs with strong fluorescence, simultaneously featuring robust fluorescence performance in strongly acidic/alkaline media by a facile preparation method.

In this study (Scheme 1), we report the hydrothermal preparation of nitrogen doped-CDs (N-CDs) from citric acid and ammonium hydroxide. The N-CDs display an excitation-independent emission and an optimal QY of 24%. It has been demonstrated that the N-CDs exhibit robust fluorescence in strong alkalinity, adjusted by various alkaline solutions, even at the extreme pH value of 13. Although strong acid (pH = 1) leads to the quenching of fluorescence, the fluorescence is recovered again by modulating the N-CDs aqueous solution pH from 1 to 13. Intriguingly, this ‘off–on’ switchable process shows an excellent reversibility and repeatability, probably due to the protonation and deprotonation of the surface groups.

Scheme 1 (a) Illustration of the synthetic procedure for carbon dots and (b) schematic of the reversible fluorescence ‘off–on’ switch, modulated by strong acidic and alkaline media.

This pH-adjusted fluorescence provides a new platform for N-CDs to be utilized as the indicator for real-time detection of pH switching caused by strong acid and strong alkali. Moreover, the detailed fluorescence performance of N-CDs in simulated body fluid (SBF) was also investigated, and their bright and stable fluorescence indicates a potential application for in vivo fluorescence bio-imaging of living organisms, even under extreme pH conditions.

The water-soluble fluorescent N-CDs were prepared using citric acid (CA) and different amounts of ammonium hydroxide via hydrothermal treatment at 200 °C for 6 h. The synthetic procedures are depicted in detail in the Experimental section of the ESI.† As shown in Fig. 1a, the XRD diffraction pattern of the N-CDs displays a broad peak at 2θ = 23° that is similar to the (002) lattice plane of graphite, which indicates a disordered structure of carbon framework with an amorphous surface structure.²⁷ The TEM image in Fig. 1b reveals that the N-CDs are quasi-spherical and well-dispersed with a particle size distribution in the range of 4–9 nm. The HR-TEM inset in Fig. 1b reveals feeble lattice fringes (Fig. S1†), corresponding to the (002) plane of N-CDs.

Fig. 1 (a) XRD pattern and (b) TEM and HR-TEM (inset) images of the N-CDs; (c) UV-vis absorption, excitation and fluorescence emission spectra of the N-CDs in aqueous solution. Insets: photographs of the CDs in water under natural light (left) and UV light (365 nm, right); (d) excitation-independent fluorescence spectra of the N-CDs in an aqueous solution.

Optical features of the fluorescent N-CDs aqueous solution (prepared by utilizing 1800 μL ammonium hydroxide) were measured and are shown in Fig. 1c and d. The UV-vis absorption spectrum of the N-CDs exhibits two absorption peaks at 235 and 333 nm, which is similar to the absorption spectra of previously reported CDs.^28,29 The absorption peak at 235 nm corresponds to the π–π* transition of the formed aromatic sp² domains from the carbon cores, whereas the absorption peak at 333 nm corresponds to the n–π* electronic transition from the surface states.^30,31 Under the identical treatment conditions, these two absorption peaks do not appear in the absorption spectra of CA without the addition of ammonium hydroxide, and a broad absorption below ∼400 nm is observed, without any obvious absorption peaks (Fig. S2†). This demonstrates that nitrogen doping plays an important role in engineering the related surface groups (whose absorption is at 333 nm), and consequently, in the formation of surface state energy gaps of the obtained N-CDs.^32,33 Compared with the absorption peaks, the absorption bands of N-CDs in the photoluminescence excitation (PLE) spectra display red-shift peaks at 244 nm and 352 nm, which were also observed in previously reported CDs.³⁴ Under the optimal excitation wavelength of 350 nm, the as-obtained N-CDs exhibit a maximum emission peak at 440 nm, indicating that the fluorescence emission mainly originates from the lower energy absorption at 333 nm. In comparison to a transparent solution under natural light, a very bright blue emission can be clearly seen under UV light (365 nm; inset of Fig. 1c). On increasing the excitation wavelength from 230 to 410 nm in 20 nm increments, the excitation-independent fluorescence emission is obtained, as shown in Fig. 1d, which suggests the formation of uniform surface states.^35,36 This result also suggests that size effect is not the only limiting factor for excitation-dependent emission because broad size distributions have been evidenced and are shown in Fig. 1b. The influence of the utilized amount of ammonium hydroxide on the fluorescence emission and QY of the resulting N-CDs products was experimentally investigated. As shown in Fig. S3,† when the amount of ammonium hydroxide in the synthesis was 900 μL, the as-obtained N-CDs displayed a slightly red-shift fluorescence emission peak as the excitation wavelength increased from 230 to 410 nm, which further confirms that the amount of utilized ammonium hydroxide strongly influences the optical property of the as-prepared N-CDs. As shown in Fig. S4–8 and Tables S2–6,† the QYs of the N-CDs series were determined using quinine sulfate as the reference (54% in 0.1 M H₂SO₄); an optimal QY 24.2% was obtained when 1800 μL of ammonium hydroxide was used.

With the help of FT-IR spectroscopy, chemical bonds and surface functional groups of the as-prepared N-CDs were analyzed. As shown in Fig. 2, the N-CDs exhibit a distinct FT-IR spectrum in comparison to the CA and ammonium citrate. The FT-IR absorption peaks at 3047, 1567 and 1404 cm⁻¹ are attributed to the stretching and bending vibrations of N–H and the C–N stretching vibration. These nitrogen-related characteristic peaks also indicate that an effective nitrogen-doping creates new groups on the N-CDs surface. The emergent absorption band at 1670 cm⁻¹ is attributed to the stretching vibration of C=O, which is different from those of CA and citrate ammonium (1732 and 1565 cm⁻¹, respectively), indicating the formation of new chemical bonds surrounding the C=O group in the as-obtained N-CDs.³⁷ The absorption peak at 3214 cm⁻¹ is associated with the O–H stretching vibration. The abovementioned active surface groups endow the N-CDs with hydrophilic properties as well as stability in water.

Fig. 2 FT-IR spectra of CA, ammonium citrate and the as-obtained N-CDs.

The pH-dependent fluorescence of the as-obtained N-CDs is directly related to its technical applications;^38,39 therefore, the influence of pH value on the fluorescence performance of the N-CDs was experimentally investigated. As shown in Fig. 3a, the as-obtained N-CDs display the maximum emission intensity at pH 7.0. The fluorescence intensity sharply decreases with the decrease in the value from 5 to 1, adjusted by adding HCl solution drop by drop. However, with an increase in pH value from 7 to 13 (adjusted by adding NaOH solution drop by drop), the fluorescence intensity is retained to ∼87% of the maximum emission intensity. To further confirm the robust fluorescence to strong alkalinity, the fluorescence performance in various strongly alkaline solutions was investigated. As shown in Fig. S9,† the N-CDs can maintain 90% and 76% of fluorescence intensity in concentrated ammonium hydroxide (25 wt%) and saturated Na₂CO₃ (20 °C) solutions, respectively. The high fluorescence performance under strong alkalinity demonstrates that the N-CDs are compatible with extremely harsh pH conditions. As shown in Fig. S10,† in strongly acidic pH (pH = 1), the absorption peak at 333 nm decreases significantly, whereas the one at 235 nm completely disappears. In contrast, at pH 13, the absorption peak at 235 nm decreases to some degree, whereas the absorption peak at 333 nm remains unchanged. Thus, both strong acidic and strong alkaline media can cause a detrimental change to the π–π* transition at 235 nm. Strong acid also leads to a severe decrease in the surface energy transition at 333 nm. Interestingly, as shown in Fig. 3b, the quenched fluorescence in strong acidic condition (pH = 1) is recovered when the pH value is adjusted to 13 by adding an appropriate amount of NaOH. This ‘off–on’ switchable fluorescence process between the pH value 1 and 13 displays excellent reproducibility and reversibility. Even after 4 cycles, as shown in Fig. S11† and 3b, ∼66% of the maximum fluorescence intensity could still be retained. The concomitant absorption spectra were characterized to aid in understanding the cause of the reversible fluorescence. As shown in Fig. 3c, the absorption peak at 333 nm also displays an excellent reversibility and decreases with increase in the reversible cycles; however, as shown in Fig. 3c, the vanished optical absorption peak at 235 nm is not recovered.

Fig. 3 (a) Fluorescence spectra of the N-CDs in aqueous solution of different pH values. Inset: fluorescence intensity of the N-CDs as a function of pH value. (b) The repeatable fluorescence ‘off–on’ switch effect and (c) concomitant evolution of absorption spectra of the obtained N-CDs when pH alternates between 1 and 13. (d) Fluorescence decay curves of the CDs alone and the CDs under extreme pH conditions (pH = 1 and 13).

Herein, we propose that the ‘off–on’ switchable fluorescence induced by the extreme pH values is the result of reversible protonation and deprotonation of the surface groups together with energy transfer. As abovementioned,^16,40 the surface states are responsible for controlling the PL variations. In this study, after nitrogen-doping, the newly formed nitrogen-related groups (–NHR) (as demonstrated before)⁴¹ introduce a strong absorption band at 333 nm (Fig. 1c), which is a fluorescence center.³² These functional groups are also typical Lewis base⁴² and can bond with H⁺ to form [N-CDs + H]⁺ in extremely low pH solutions. Similar to the optical behavior of the reported fluorophore in extremely acidic environments,^19,43 this causes a quenched fluorescence and concomitant decreased absorption. Subsequent addition of OH⁻ can remove H⁺ from the surface groups, thus restoring the fluorescence and absorbance (as evident from Fig. 3b and c) discussed above. As further analyzed by time-resolved fluorescence decay spectroscopy (Fig. 3d), single exponential decays for the N-CDs and the N-CDs/pH = 13 implies the similar PL process, whereas three exponential decays for N-CDs/pH = 1 indicate disruption to radiative recombination of fluorescence centers on the N-CDs surface. Specific fluorescence lifetimes are supplied in Table S7.† The lifetime (5.37 ns) of the N-CDs in pH = 1 solution is lower than that of the N-CDs in aqueous solution (7.99 ns) because H⁺ disturbs the electron–hole radiative recombination of surface states. When H⁺ is introduced, the H⁺ on the CDs surface could trap the electrons and repel the holes; thus, nonradiative recombination will be increased and lifetime of the fluorescence will be decreased.⁴⁴ However, the fluorescence lifetime of the CDs in a strong alkaline solution (pH = 13; 6.86 ns) changes only slightly, which is a possible reason for the robustness exhibited by the N-CDs at a strongly alkaline pH. As demonstrated by the FT-IR analysis (Fig. S12†), the stretching vibration of C=O (1670 cm⁻¹) and bending vibration of N–H (1567 cm⁻¹) remain in N-CDs/pH = 13, whereas these characteristic absorption peaks almost disappear in N-CDs/pH = 1. This result further supports the origin of fluorescence performance variations to strong acidity and strong alkalinity.

By virtue of their bio-compatibility and nontoxicity, one of the significant applications for fluorescent N-CDs is bio-imaging. Herein, a simulated body fluid (SBF) was introduced to better understand the optical properties of the as-obtained N-CDs. SBF solution with ionic strength nearly equals to that of human blood plasma (illustrated in Table 1), which has been widely used for in vivo assessment of bioactivity of artificial materials, was prepared and utilized in this study.⁴⁵ After increasing the N-CDs concentration from 0.02 to 0.08 mg mL⁻¹, no precipitation was detected in the SBF solution, revealing that the obtained N-CDs are highly compatible with the SBF solution. Bright blue fluorescence emission can be observed when the as-obtained SBF solution was excited at 350 nm. As shown in Fig. 4, N-CDs at different concentrations in SBF solution display stable and robust fluorescence performance compared with those in water. High compatibility and stable fluorescence performance in SBF solution demonstrate that the as-obtained N-CDs could be potentially utilized as an excellent bio-imaging reagent for in vivo labeling.

Table 1 Nominal ion concentration of SBF compared with that in human blood

Ion	Blood plasma (mM)	SBF (mM)
Na^+	142.0	12.0
K^+	5.0	5.0
Mg^2+	1.5	1.5
Ca^2+	2.5	2.5
Cl^−	103.0	147.8
HCO3^−	27.0	4.2
HPO4^2−	1.0	1.0
SO4^2−	0.5	0.5
pH	7.2–7.4	7.40

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Fig. 4 Fluorescence intensity of CDs in SBF with different concentrations.

Conclusions

In summary, excitation-independent blue emission nitrogen-doped carbon dots (N-CDs) with quantum yield up to 24.2% have been synthesized by a simple hydrothermal preparation of citric acid and ammonium hydroxide. It has been demonstrated that the as-obtained N-CDs show robust fluorescence performance under strong alkalinity (adjusted by various alkalis) even at an extreme pH value of 13. Although strongly acidic (pH = 1) media quench the fluorescence, the fluorescence is recovered again by modulating the pH value to 13. Strikingly, the N-CDs exhibit a repeatable and reversible ‘off–on’ fluorescence switch between strongly acidic and alkaline conditions (pH = 1 and 13). This repeatable process is due to induced protonation and deprotonation on the N-CDs surface accompanied by energy transfer. In addition, the N-CDs exhibit bright and stable fluorescence performance in simulated body fluid (SBF) solution. The robust fluorescence under strong alkalinity, repeatable ‘off–on’ fluorescence switching between strong acid and strong alkali, and compatibility with SBF solution make the N-CDs highly promising for dynamic detection of pH fluctuations in bio-imaging of living organisms and microorganisms even under extreme pH conditions.

Acknowledgements

The authors are grateful for the financial aid received from the Research Equipment Development Project of Chinese Academy of Sciences (YZ201562), the Youth Innovation Promotion Association CAS (2013150), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20030300), the National Natural Science Foundation of China (51502285, 21521092, 21590794 and 21210001), and the National Key Basic Research Program of China (2014CB643802).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, supported figures and data. See DOI: 10.1039/c6ra21994b

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