Mechanistic studies on the reversible photophysical properties of carbon nanodots at different pH
Introduction
Carbon-based nanomaterials have become the focus of advanced scientific researches as they possess a variety of fascinating properties. Carbon nanodots (C-dots) have attracted great attention in carbon-based fluorescent nanomaterials including nanodiamonds [1], fluorescent carbon nanotubes [2] and fullerene [3], due to their superior physical and chemical properties [4]. Compared with conventional semiconductor quantum dots (QDs) and organic dyes, C-dots possess more outstanding properties, such as superior photoluminescence (PL) properties, favorable biocompatibility, chemical inertness and easy functionalization [5]. Thus, their performance is competitive to the commercially available Cd, Hg and Pb containing QDs [6], [7]. Therefore, these interesting properties make them have potential applications in bioimaging [8], [9], [10], light-emitting diodes [11], sensors [12], [13], [14], drug delivery [15], gene delivery [16], solar cells [17], photocatalysis [18], [19] and mimetics peroxidase [20]. C-dots exhibit several other interesting PL properties, such as excitation wavelength (λex) dependent emission and up-conversion property, which make them quite different from the conventional QDs [5], [6]. In general, highly luminescent C-dots can be achieved through various strategies [21], [22], [23], surface passivation [24], doping [7], coupling to metal-based nanostructures [25] and reducing [26]. Previous studies indicate that the origin of PL in C-dots and graphene quantum dots (GQDs) may arise from quantum size effect, zig–zag sites and defect states (energy traps) [4], [27]. Generally, quantum size effect and zig–zag sites can be considered as intrinsic state emission while the defect effect is classified as emissive traps. In addition, the intrinsic state emission stems from recombination of localized electron–hole pairs [28].
The PL behavior of C-dots has been extensively studied through steady-state and time-resolved PL methods. So far, though much progress has been achieved in the synthesis of highly luminescent C-dots, many issues need to be addressed before we can apply them in practice. The mechanism of PL is still under debate, varying from case to case. However, it is very important for achieving highly luminescent C-dots.
The pH-dependent PL behavior is one of the most important properties existed in C-dots [23], [29]. Pan et al. [30] reported that C-dots emitted strong PL under alkaline conditions, while the PL was nearly completely quenched in acidic conditions. Interestingly, the PL intensity could be restored or quenched by switching pH between 13 and 1, while emission wavelength (λem) was not affected by pH. In parallel, Wang and co-workers [31] reported that when 1.28 mol L−1 HNO3 was added, a new PL sites emerges with an optimal λex at 350 nm and an optimal λem 500 nm, respectively. They attributed this unusual emission transformation to self-assembled aggregation of GQDs. Unfortunately, they did not directly observe the self-assembled aggregation of GQDs by HRTEM. In this work, a similar unusual PL transformation under a milder acidic condition had also been observed, and self-assembled aggregation of C-dots was also directly observed by HRTEM.
To date, the mechanism of reversible pH-dependent PL behavior for C-dots has not been exhaustively studied. Herein, we report the pH-dependent PL behavior of C-dots in detail and explore the possible reason for this interesting phenomenon. The proposed pH-dependent mechanism can provide unique insights into the origin of PL. The findings indicate that pH-dependent PL behavior is associated with two kinds of reactions (fast and slow) occurring at the surface of C-dots. This result further proves that the PL of C-dots is derived from the surface defects states.
Section snippets
Materials
Sucrose, phosphoric acid (H3PO4), hydrochloric acid and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co. (China). All reagents were used as received. The aqueous solutions were prepared with ultra-pure water (18.2 MΩ cm−1, Millipore).
Synthesis of C-dots
The C-dots were prepared by acidic oxidation methods based on our previous report with some modifications [32]. The modifications were given in supporting information.
Characterization
High resolution transmission electron microscopy (HRTEM) images were
pH-dependent behavior of C-dots
The C-dots were prepared by one-pot acidic oxidation methods with some modifications based on our previous report [32]. The as-prepared C-dots were green-emitting with relatively high oxygen content and the PL behavior were pH dependent (Fig. S1), just like the C-dots prepared by other methods [29], [30]. Although the PL intensity of C-dots kept constant in the pH range between 7 and 10, PL intensity notably decreased out of this range.
Excitation dependent PL behavior is very common in C-dots
Conclusion
The pH-dependent PL properties of the C-dots and the possible mechanism for this interesting phenomenon have been exhaustively studied in this work. The PL as well as UV–vis absorption spectra can be reversibly switched between pH range of 3 and 13. Speculatively, the pH-dependent reversibility of the PL behavior is associated with two kinds of reactions (fast and slow) occurring at the surface of C-dots. When the pH is switched from 7 to 3, the C-dots will quickly self-assemble into larger
Acknowledgments
The authors gratefully acknowledge the financial support from Chinese 973 Program (Grant No. 2011CB933600), National Science Fund for Distinguished Young Scholars of China (Grant No. 21225313), National Natural Science Foundation of China (Grant Nos. 21303126, 21473125), Hubei Natural Science Foundation of China (No. 3014CFA003), Fundamental Research Funds for the Central Universities (No. 2042014kf0287). The authors thank Prof. Z.K. He and Dr. C.L. Zhang of Wuhan University for helping with
References (43)
- et al.
A sensitive and reliable dopamine biosensor was developed based on the Au@carbon dots–chitosan composite film
Biosens. Bioelectron.
(2014) - et al.
Hollow luminescent carbon dots for drug delivery
Carbon
(2013) - et al.
Nano-carrier for gene delivery and bioimaging based on carbon dots with PEI-passivation enhanced fluorescence
Biomaterials
(2012) - et al.
One-step ultrasonic synthesis of water-soluble carbon nanoparticles with excellent photoluminescent properties
Carbon
(2011) - et al.
Low temperature synthesis of highly stable phosphate functionalized two color carbon nanodots and their application in cell imaging
Carbon
(2014) - et al.
The properties and applications of nanodiamonds
Nat. Nanotechnol.
(2012) - et al.
A route to brightly fluorescent carbon nanotubes for near-infrared imaging in mice
Nat. Nanotechnol.
(2009) - et al.
Luminescent hollow carbon shells and fullerene-like carbon spheres produced by laser ablation with toluene
J. Mater. Chem.
(2011) - et al.
Surface chemistry routes to modulate the photoluminescence of graphene quantum dots: from fluorescence mechanism to up-conversion bioimaging applications
Adv. Funct. Mater.
(2012) - et al.
Carbon nanodots: synthesis, properties and applications
J. Mater. Chem.
(2012)
Luminescent carbon nanodots: emergent nanolights
Angew. Chem. Int. Ed.
Doped carbon nanoparticles as a new platform for highly photoluminescent dots
J. Phys. Chem. C
Carbon quantum dots for optical bioimaging
J. Mater. Chem. B
One-pot hydrothermal synthesis of highly luminescent nitrogen-doped amphoteric carbon dots for bioimaging from Bombyx mori silk – natural proteins
J. Mater. Chem. B
Large scale synthesis of photoluminescent carbon nanodots and their application for bioimaging
Nanoscale
Color-switchable electroluminescence of carbon dot light-emitting diodes
ACS Nano
Carbon dots and chitosan composite film based biosensor for the sensitive and selective determination of dopamine
Analyst
Ag Nanoparticle-decorated graphene quantum dots for label-free, rapid and sensitive detection of Ag+ and biothiols
Chem. Commun.
Solution phase synthesis of carbon quantum dots as sensitizers for nanocrystalline TiO2 solar cells
J. Mater. Chem.
Near-infrared light controlled photocatalytic activity of carbon quantum dots for highly selective oxidation reaction
Nanoscale
Carbon quantum dots/Cu2O composites with protruding nanostructures and their highly efficient (near) infrared photocatalytic behavior
J. Mater. Chem.
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