1. Introduction

Photophysical and photochemical properties of fluorescent nanomaterials are significant for scientific research and various applications of fluorescence spectroscopy. Particularly, photoinduced modification is a simple and effective means to control the optical properties of fluorescent nanomaterials, as well as to realize novel uses [1–6]. For example, using a technique, namely single-molecule high-resolution imaging with photobleaching, Gordon et al. located the position of two fluorescent dyes and determined their separation with 5-nm precision, highly improving the resolving power of conventional light microscopy, which was usually limited by the Rayleigh limit (200 nm) [2]. Besides the photobleaching, the photochemical modification is also employed to enhance the emission and tune the photoluminescence (PL) of fluorescent nanomaterials, such as causing a down shift and an enhancement of PL from the luminescent carbon dots (CDs) and graphene oxide (GO) [3,5–7]. The emergent luminescent CDs usually comprise discrete, quasispherical carbon nanoparticles with the size below 10 nm [8,9]. Their unique size- or surface state-dependent PL, easily subsequent functionality, good biocompatibility, and lower cytotoxicity distinguish themselves from traditional luminescent organic molecules and semiconductor quantum dots [8–15]. The superior properties make luminescent CDs potential candidates for diverse applications, such as bioimaging and optoelectronic devices. Thus far, though, great efforts have been devoted to the studies of luminescent CDs, there are few reports on the ultraviolet (UV) light irradiation induced variation of PL and structures of CDs [6,8–10]. Before using CDs in more science experiments, it is essential to clarify and to control UV light irradiation induced behaviors of the PL (e.g. photobleaching and photoenhancement) and structures. Furthermore, the poorly understood PL mechanism of CDs is still one of the main obstacles, which limits the scope of further progress in the field of CDs. For that, a better understanding on the PL mechanism is vital and necessary.

In this work, effect of UV light (365 nm) irradiation on the PL and surface states of CDs is investigated. After being irradiated by UV light, CDs exhibit red shifted and enhanced green emission, and evidently enhanced emission in the range from green-yellow to red region. The UV emission is decreased and blue shifted. We propose that the anomalous behaviors of PL arise from the UV induced photochemical oxidation of CDs, which is achieved for the first time in CDs. The oxidation degree of CDs is enhanced with increase of the irradiation time from 15 min to 6 h, resulting in significant increase of carbonyl groups. The mechanism of photochemical oxidation is discussed. The photoinduced chemical oxidation offers a simple technique to modulate the PL and investigate the PL mechanism. Our findings will bring new insights into the photophysical and photochemical properties of CDs and also provide a deeper understanding on the PL mechanism.

2. Experimental section

15 mg of glassy carbon powders were dispersed in 50 ml of ethanol. After ultrasonication, about 4 ml of suspension was put into a quartz cell for ablation. Femtosecond (fs) laser pulses were produced by a regenerative amplified mode-locked Ti:sapphire laser (Coherent Inc., wavelength: 800 nm, pulse duration: 100 fs, repetition rate: 1 kHz) and the laser beam was focused by a microscope objective (20 × 0.40 N.A.) into the suspension for about 3 hours. During the femtosecond laser ablation, a magnetic stirrer was utilized to prevent gravitational settling of the glassy carbon powders. The average laser energy is 0.37 mJ/pulse. Centrifugation with a speed of 12 000 rpm was used to separate larger carbon particles and CDs. Pristine CDs (p-CDs) were obtained in the supernatant. After that, the p-CDs were irradiated further by a hand UV lamp with the wavelength of 365 nm for different time. The irradiation powder intensity was about 344 μW/cm². Transmission electron microscopy (TEM) images were recorded by a transmission electron microscope (Tecnai G2 F30 S-Twin). The ESCA 5800 XPS spectrometer was used to record and analyze the X-ray photoelectron spectra (XPS). The PL spectrum measurements were done on the Fluoro Max-3 spectrometer (Horiba Jobin Yvon).

3. Results and discussion

The previous report showed that CDs with typical diamond structure were synthesized by the fs laser ablation of glassy carbon powders in ethanol, as demonstrated in Fig. 1(a) [14]. Herein, after the fs laser ablation and centrifugation, the p-CDs were irradiated further by a hand UV lamp with the wavelength of 365 nm. Figure 1(b) displays the typical TEM image of the CDs after 7 h UV irradiation (7 h-CDs) with the average size of 3.2 nm, which is similar to that of p-CDs [14].

 

Fig. 1 TEM images of p-CDs (a) and 7 h-CDs (b). Inset: size distribution of 7 h-CDs.

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Figure 2 shows the emission spectra of p-CDs (a) and CDs after UV light irradiation for 6 h (6 h-CDs, b), which show obvious distinctions between the emitting behaviors of the two samples. The emission of the p-CDs is mainly in the UV-blue region with a maximum at 375 nm. The 6 h-CDs exhibit two emission maxima at 350 and 506 nm, respectively. Compared to p-CDs, the UV emission maximum (350 nm) of 6 h-CDs is blue shifted with a shorter optimal excitation wavelength of 300 nm and slightly reduced. The new emission peak at 506 nm of the 6 h-CDs exhibits a blue shift of 18 nm and an enhanced intensity compared with that of the p-CDs excited at 400 nm. Furthermore, new emission shoulders emerge when the 6h-CDsare excited by the 300, 320 and 340 nm, as shown in Figs. 2(e) and 2(f). Gaussian fitting is adopted to investigate the details of the dual emission. Dual emission peaks at 370 and 460 nm excited at 320 nm and 396 and 474 nm excited at 340 nm are observed. The dual emission suggests the existence of two radiative recombination routes [14].

 

Fig. 2 Emission spectra of p-CDs (a) and 6 h-CDs (b) excited at different wavelengths. (c, d) Normalized emission spectra of (a, b), respectively. Emission spectra of p-CDs and 6 h-CDs excited at 320 nm (e) and 340 nm (f). Green curves: Gaussian fitting. Red curves: summary of Gaussian fitting. Inset of (e, f): the corresponding normalized emission spectra.

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In order to obtain the details about the effect of UV light irradiation on the PL, emission spectra of the CDs after exposure to UV light for different time are recorded. Figure 3 shows the emission spectra of CDs after the UV light irradiation excited at 300 nm (a), 400 nm (b), and 500 nm (c). Irregular emission changes excited at different wavelengths can be seen clearly. The emission when excited at 300 nm is reduced slowly and blue shifted from 364 nm to 348 nm with the increase of the irradiation time, as demonstrated in Fig. 3(d). The corresponding band gap is increased from 3.41 eV to 3.57 eV (Fig. 3(f)). When excited at 400 nm, a rapid increase of the PL intensity is seen within 15 min of UV exposure, and a slow increase is observed by prolonging the exposure time, as displayed in Fig. 3(f). In addition, the PL peak position (Fig. 3(e)) exhibits a significant red shift after the UV light irradiation for 15 min and a small red shift after longer time irradiation when excited at 400 nm. Correspondingly, the band gap is reduced from 2.53 eV to 2.47 eV after 15 min irradiation, and to 2.45 eV after 7 h (Fig. 3(f)). Different from the emission behaviors exited at 300 and 400 nm, Fig. 3(c) depicts that the PL is significantly enhanced by 3.2 fold and no peak shift can be observed excited at 500 nm. Table 1 demonstrates the quantum yields (QYs) of p-CDs and 7 h-CDs excited at 320, 340 and 360 nm. For p-CDs, the QYs are 3.64, 3.15, 4.39% excited at 320, 340 and 360 nm, respectively. The QY of 7 h-CDs is reduced excited at 320 nm (2.93%) and 340 nm (1.86%), and enhanced excited at 360 nm (4.91%).

 

Fig. 3 Emission spectra of the CDs after the UV light irradiation for different time excited at 300 nm (a), 400 nm (b), and 500 nm (c). The PL peak position and intensity of CDs excited at 300 nm (d) and 400 nm (e) after the UV light irradiation for different time. (f): the relationship between tbe irradiation time and band gap contributing to UV (ex. 300 nm) and green (ex. 400 nm) emissions.

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Table 1. Quantum yields of p-CDs and 7 h-CDs excited at 320, 340 and 360 nm.

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The aforementioned discussions demonstrate that UV light irradiation induces blue shifted and reduced UV emission, red shifted and slightly enhanced green emission, and significantly enhanced yellow emission. In order to shed light on the mechanisms of these anomalous behaviors of the emissions, the excitation spectra are recorded in Fig. 4.The maximum excitation peak for the emission at 370 nm is slightly blue shifted from 316 to 313 nm after exposure of CDs to UV light for 6 h, as revealed in Fig. 4(a), which implies a wider band gap causing UV emission. Moreover, the excitation shoulders, such as 295 and 270 nm are enhanced with increasing the irradiation time. Figure 4(b) shows that for the emission at 500 nm, the excitation peak of p-CDs is at 376 nm, while, the excitation peak at 396 nm becomes stronger than the peak at 376 nm after 15 min irradiation and becomes stronger progressively with increasing irradiation time to 6 h, which indicates change of the band gap contributing to the emission at 500 nm. As the surface defect states play a dominant role in the excitation and emission habits of CDs, Fig. 5 reveals the C1s XPS peaks of p-CDs (a) and 7 h-CDs (b) [14]. Several oxygen containing functional groups are observed. The fitted peaks at 284.8, 285.9, 287.3 eV are assigned to C-C, C-O and C = O bonds, respectively. The detailed contents of these three groups are determined to be about 69.1% (C-C), 23.8% (C-O) and 7.1% (C = O) for p-CDs and 55.5% (C-C), 29.0% (C-O) and 15.5% (C = O) for 7 h-CDs, respectively, based on the calculated value of the integrating fitting curve area. The quantitative analyses show that the contents of the both C-O and C = O groups increase after UV irradiation. Particularly, the percentage of C = O bonds is increased significantly by 2.2 fold. Our previous report suggests that two possible routes can result in emission of CDs: radiative recombination through intrinsic direct transitions between highly localized π states and defect energy trapping (DET) states, which lead to UV and green PL, respectively [14]. In this case, more DET states are created with enhancement of the oxidation degree by increasing the irradiation time, which result in the enhancement and red shift of the emission in the green-red region, consistent with the excitation spectra [14]. In the meanwhile, as the hydroxyl groups related defect states usually lead to blue emission, the DET states-caused green-red emission may be mainly related to the carbonyl groups, which is confirmed by the emission and XPS results [6,15]. Recently, blue shifted and enhanced PL is observed in surface functionalized CDs through the charge transfer effect of functional groups and the CDs [16,17]. Jin et al. suggested that the electron-donating behaviors of the functional groups (NH₂) could result in electron transfer from the functional groups to CDs [17]. As a result, increasing the electron density of CDs can lower the band gap of CDs. Similar effects may be also valid to the carbonyl groups. The electron density of the CDs is increased with more C = O bonds, bringing about a narrower band gap and red shifted PL. Furthermore, the n_(OH)→π*_(CO) interactions (H-O^…C = O) between the hydroxyl groups and carbonyl groups have been proposed to play an important role in stabilizing the excited state, and give rise to a low-lying lowest unoccupied molecular orbital state and a red-shifted fluorescence from organic nanoparticles, recently [18]. Herein, with the increase of hydroxyl groups and carbonyl groups, the n_(OH)→π*_(CO) interactions may become stronger, which will result in a narrower band gap and the red shifted emission in the green region. The higher electron density and stronger n_(OH)→π*_(CO) interactions may also lead to an enhancement of the emission in the region from green-yellow to red. In the meanwhile, the dual emission in Fig. 2(e) and 2(f) and the emission tail in Fig. 3(a) indicate that two kinds of radiative recombination exist when the oxidized CDs are excited at 300-340 nm. With enhancement of oxidation degree, the contribution of the DET states related emission becomes larger. Moreover, the XPS results suggest that the content of C-C bonds decreases after the UV irradiation, which may result in decrease of sp² carbons [13,19]. Consequently, as the band gap between the highly localized π states is determined by the sp² carbons, the decrease of the sp² carbons will lead to an enlarged band gap and attribute to the blue shift and reduction of the UV emission [13,20,21].

 

Fig. 4 Excitation spectra monitoring emissions at 370 nm (a) and 500 nm (b).

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Fig. 5 C1s XPS curves of p-CDs (a) and 7 h-CDs (b).

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Now, we will give a brief investigation on the photoinduced oxidation mechanism of CDs. As all our experiments and characterizations were performed in the atmospheric environment, O₂ and H₂O could be dissolved in the solution [22]. O₂ and H₂O can absorb photons to generate strongly oxidizing singlet oxygen, ozone, hydrogen radical, and oxyhydrogen radical [3,4,6,23]. Furthermore, prior reports have revealed that separated electrons and holes can be produced when CDs is irradiated by UV light, which make CDs are highly active as electron donors [24–26]. Therefore, the CDs behave as nanoreactors that are attacked by the oxidants generated in the solution by the UV light irradiation [3,6]. As a consequence, the CDs are oxidized. Currently, the mechanism of the photochemical oxidation is still not fully understood and merits further study in the future.

4. Conclusions

In summary, the effect of UV light irradiation on the PL of CDs is reported. After exposure to UV light at 365 nm, CDs exhibit red shifted and enhanced green emission, and significantly enhanced emission in the wavelength range from green-yellow to red. The content of carbonyl groups increases significantly with the increase of the irradiation time, resulting in more DET states, the higher electron density of CDs, and stronger n_(OH)→π*_(CO) interactions. As a consequence, the band gap is narrowed and the contributions of the radiative recombination through the DET states increases, resulting in the enhancement of green emission and beyond. In the meanwhile, the UV emission is decreased and blue shifted, resulting from the decrease of sp² carbons. As far as we know, this is the first time to observe UV irradiation induced photochemical oxidation of CDs. Our findings will bring new insights into the photophysical and photochemical properties of CDs, and a deeper understanding on the PL mechanism. In addition, in vivo optical imaging at longer wavelengths is usually preferred due to the improved photon tissue penetration and reduced background autofluorescence, which implies that oxidized CDs can be excellent alternatives for real and long time biological applications.