We present an investigation on carbon quantum dots (CQDs) synthesized from wastewater induced during the production of tofu. We find that tofu wastewater is a good source of raw material in making fluorescent CQDs. The corresponding CQDs can be fabricated simply via hydrothermal reaction to carbonize the organic matter in the yellow serofluid of tofu wastewater. Two sorts of CQDs can be obtained within the deionized water and NaOH solution, respectively, where the CQDs in water (NaOH solution) can emit blue (green) light under the UV irradiation. It is found from X-ray photoelectron spectroscopy (XPS) that the basic difference between these two sorts of CQDs is the contents of C–O and C=O bonds on the surface of the CQDs. This difference can cause different features of the photoluminescence (PL) spectra of the CQDs. On the basis of the obtained results from the XPS and PL measurements, we propose a mechanism in understanding and explaining the photon-induced light emission from CQDs. This study is relevant to the fabrication and application of fluorescent CQDs as, e.g., light display materials. Keywords: Fluorescence, Carbon quantum dots, Tofu wastewater, Hydrothermal reaction Background concentrated with organic matters and contains carbohy- Tofu, made from soybean, is the daily food in China and in drates, proteins, organic acids, functional oligosaccharides, the Asian community. In the past, tofu and related prod- water-soluble non-protein nitrogen and vitamins, lipids, ucts were mainly made by families and small factories in and other pigment substances. Therefore, it is a good relatively small amount. With the vegetarian being more source of raw material in fabricating carbon quantum dots and more popular worldwide, the demand of tofu products (CQDs) for optics, biomedicine, and other applications. has been rapidly increasing in the last two decades since Thus, applying tofu wastewater to make CQDs can reuse the big international supermarkets such as WalMart and the wastes from mass production of tofu and largely reduce Carrefour sold these as health-foods. Nowadays, tofu and environmental pollution. These become the prime motiv- related products are mainly mass-produced by big factories ation of our present study. in the industrial park in China. However, one of the envir- Carbon quantum dots is a new class of carbon-based onmental issues of mass production of tofu in the indus- nanomaterial normally with spatial size of 20 nm or less trial park is the wastewater. The production of soybean [1, 2]. It has been found that the CQDs are of good water products would result in wastewater mixed with soybean solubility, high chemical inertness, low toxicity, and excel- yellow serofluid. This wastewater can cause environmental lent biocompatibility [3, 4]. From a viewpoint of physics, pollution. On the other hand, tofu yellow serofluid is highly the electronic energy spectrum for a CQD is akin to a direct band-gap semiconductor. Thus, the CQDs have been proposed as fluorescent materials for advanced * Correspondence: email@example.com; firstname.lastname@example.org optical and optoelectronic devices [5, 6]. In recent years, School of Physics and Astronomy and International Joint Research Center for Optoelectronic and Energy Materials, Yunnan University, Kunming the CQDs have been rather intensively investigated. A var- 650091, People’s Republic of China iety of fabrication methods and different sources of raw Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Zhang et al. Nanoscale Research Letters (2017) 12:611 Page 2 of 7 materials have been applied to realize the CQDs for optical is magnetically stirred for 4 min to achieve the uniform applications [5–7]. In general, the synthesis of CQDs can and full mix of the matters and water. (iv) The mixture is be achieved through top-down and bottom-up approaches taken for 5 min ultrasonic shock to break the loosing clus- . The top-down method is mainly a physical approach ters. Thus, we can obtain the supernatant which contains in forming carbon dots by breaking or peeling larger car- carbon dots. (v) The supernatant is further centrifuged at a bon material structures, including arc discharge , elec- speed of 12,000 r/min for 20 min, and the further super- trochemical oxidation , chemical oxidation , laser natant can be obtained. As a result, the CQDs can be fi- ablation , etc. The bottom-up method is to employ the nally acquired within deionized water. It is found that the small molecules as precursors to obtain CQDs through heating temperature, the heating time, and the pH value of chemical reactions, including combustion , microwave the yellow pulp water in the synthesis process can affect , and ultrasonic  approaches along with chemical rather strongly the growth of the CQDs. Therefore, the solution synthesis , hydrothermal reaction , etc. CQDs can be fabricated with certain fluorescent features In recent years, biomass such as wheat straw  and through varying the above synthesis conditions. We notice plant leaves  has been widely used as carbon sources from bare eye observation with daylight that the super- for synthesis of the CQDs. Moreover, water-soluble natant with CQDs prepared under abovementioned ex- fluorescent CQDs have been prepared by hydrothermal perimental conditions looks yellow. However, it can look treatments of orange juice  and Jinhua bergamot blue under the UV irradiation. We name this sort of fluor-  which are taken as carbon sources. Such a simple escent CQDs as CQDs-1 in this article. approach has been applied for the large-scale synthesis By taking the similar synthesis approach, we can pro- of water-soluble CQDs from many sorts of food waste- duce the CQDs via using NaOH as solution for burning derived sources . dry tofu yellow serofluid after pyrolysizing, instead of In this study, we take tofu yellow serofluid as carbon using deionized water discussed above. We add 100 ml source to synthesize the CQDs via employing the hydro- of NaOH solution with a pH value about 12.4. Following thermal method to carbonize the organic matters in the the same processes of magnetic stirring, ultrasonic yellow serofluid. It has been pointed out  that the shocking, and centrifuging as stated above, we can also hydrothermal method is an easy and low-cost approach acquire the CQDs within the NaOH solution. These which can be applied to large-scale and one-step synthesis CQDs also look yellow from bare eye observation with of water-soluble fluorescent CQDs. For optical application daylight. However, they can look green under the UV of the CQDs, especially as light display materials, it is de- irradiation. We name this sort of fluorescent CQDs as sirable to be able to produce the fluorescent CQDs which CQDs-2 in this article. can emit blue, green, and red radiation. Our current re- In this work, we have made two types of CQDs which search work is being conducted along this direction. In the can emit green and blue lights under the UV irradiation. present study, we prepare a series of fluorescent CQDs for The further investigation of the present work is conducted investigation. The transmission electron microscopy and mainly for these two types of CQDs realized from tofu the X-ray photoelectron spectroscopy are applied for the wastewater. characterization of the fabricated CQDs. The photolumi- nescence experiment is employed to measure the optical Results and Discussions properties of the CQDs. For characterization of the CQDs synthesized from tofu wastewater, we first carry out the morphological analysis Methods forthese CQDs.InFig.1,weshow the typicalimage of the In this study, the wastewater from tofu production is CQDs within deionized water and NaOH solution (CQDs- taken from the Tofu Industrial Park in Shi Ping County, 1 and CQDs-2), obtained from high-resolution transmis- Yunnan, China. The general processes to synthesize the sion electron microscopy (TEM). As we can see, the pre- CQDs from yellow serofluid in tofu wastewater can be pared CQDs are spherical and mono-dispersive within the described as follows: (i) We prepare the carbon precur- deionized water (for CQDs-1) or NaOH solution (for sory materials via pyrolysis of the tofu yellow pulp in CQDs-2). Through a statistical average of the TEM image, wastewater. Here, 300 ml of tofu yellow syrup is put into theparticlesizeoftheseCQDs is in therangefrom 2to the 500-ml beaker and placed onto the heating platform 10 nm. We find that these CQDs are highly crystallized for constant heating. We find that when heating with typical lattice structure of carbon. The lattice fringes temperature is at about 93 °C and the heating time is for are clear and the corresponding lattice spacing is about 3 to 5 h, the tofu yellow serofluid in the beaker can be- 0.22 and 0.21 nm, respectively. We would like to note that come burning dry. (ii) We let the stuff in the beaker cool the results shown in Fig. 1 are very similar to those down naturally till room temperature and add 50– reported previously for the N- and S-doping content in N- 200 ml deionized water into the beaker. (iii) The mixture and S-CQDs with high yield [23, 24]. Moreover, we find Zhang et al. Nanoscale Research Letters (2017) 12:611 Page 3 of 7 0.22 nm 5 nm 10 nm 10 nm 2 nm Fig. 1 a TEM images for CQDs in deionized water (CQDs-1) and b TEM images for CQDs in NaOH solution (CQDs-2). c, d Zoomed-in image of a single CQD of a and b, respectively that the size distribution of the CQDs in deionized water that the basic difference between CQDs-1 and CQDs-2 is (CQDs-1) or in NaOH solution (CQDs-2) is mainly located the contents of C–O and C=O bonds on the surface of the around 3.5–5.5 nm and the thickness of these CQDs is CQDs within water and NaOH solution, respectively. It is about 3.5 nm. known that the OH in NaOH solution can couple with As we know, the X-ray photoelectron spectroscopy C–O and C=O bonds on the surface of the CQDs to form (XPS) is a powerful tool for the measurement and un- COOH and the carboxyl group and, thus, to reduce the derstanding of the elemental compositions and the con- tent of the CQDs, especially for the examination of surface-modified features of the CQDs such as the func- tional groups on the surface of the CQDs . In Fig. 2, the XPS full spectra for CQDs-1 and CQDs-2 are pre- sented and the corresponding findings are indicated. We notice that the CQDs measured here contain mainly C (with a typical binding energy C ls = 284.8 eV), N (with a typical binding energy N ls = 400 eV), and O (with a typ- ical binding energy O ls = 532 eV). The other elements such as S and P (Na and Cl) can also be found in CQDs- 1 (CQDs-2). As a result, we see that CQDs-1 is mainly composed of C, N, O, S, and P elements, in which the atomic ratio of these elements is C1s:O1s:N1s:S2p:P2p = 61.0:29.6:8.5:0.5:0.4. We also see that CQDs-2 is mainly composed of C, O, N, Na, and Cl elements. The atomic ratio of these elements is C1s:O1s:N1s:Na1s:Cl2p = 66.7:26.2:6.8:0.1:0.1. Because the tofu wastewater itself contains chloride and sulfate induced by the process in making tofu, there are rather broad spectra of S and Cl signals in Fig. 2. Moreover, because CQDs-2 is for CQDs in NaOH solution in which NaOH can play a role as passivation of the CQDs, there is a Na signal in the lower panel of Fig. 2. In Fig. 3, we show the high resolution C1s spectra for CQDs-1 and CQDs-2, respectively, fitted by a binding en- ergy Cls. It can be seen from the C1s spectrum in the upper panel of Fig. 3 that three chemical bonds C–C/C=C at 284.7 eV, C–O at 286.08 eV, and C=O at 287.86 eV present in CQDs-1. There are four chemical bonds C–Cat Fig. 2 The XPS full spectrum for CQDs-1 (upper panel) and CQDs-2 284.8 eV, C–O at 286.16 eV, C=O at 288 eV, and COOH at (lower panel), respectively, where the obtained contents of elements 289.14 eV present in CQDs-2, as shown in the lower panel are indicated of Fig. 3. From the XPS results shown in Fig. 3, we learn Zhang et al. Nanoscale Research Letters (2017) 12:611 Page 4 of 7 Fig. 4 The PL spectrum for CQDs-1 in upper panel and CQDs-2 in Fig. 3 The high resolution C1s spectrum for CQDs-1 (upper panel) lower panel at different excitation wavelengths λ . In the upper ex and CQDs-2 (lower panel), respectively, fitted by a binding panel, λ are 370 nm (red), 380 nm (green), 390 nm (blue), 400 nm ex energy C1s (light blue), 410 nm (deep pink), 420 nm (yellow), 430 nm (light green), 440 nm (dark green), 450 nm (light red), and 490 nm (dark olive green). In the lower panel, λ are 420 nm (orange), 440 nm ex contents of the C–O and C=O groups in CQDs-2. This is (blue), 460 nm (yellow), 480 nm (red), 490 nm (green), 500 nm (pink), the main reason why the contents of C–O and C=O bonds and 510 nm (dark olive green). The inserts show the peak wavelength in CQDs-1 are markedly higher than those in CQDs-2. in the PL spectrum, λem, as the function of excitation wavelength In this study, we take a standard experimental setup to measure the photoluminescence (PL) emission from CQDs realized from tofu wastewater in visible bandwidth. The HORIBA fluorescence system (USA) is applied for the then decreases with increasing excitation wavelength. The measurement, where a xenon lamp is taken as broadband strongest PL emission can be observed at about λ ~ ex excitation light source, the GEMIMI 180 mono-chromator 410 nm for CQDs-1 and 480 nm for CQDs-2, respectively. is used for choosing the optical pumping wavelength, and (ii) The peak wavelength position λ in the PL spectrum em the iHR320 grating spectrometer together with a photo- varies with altering the excitation wavelength for both electric multiplier tube (PMT) detector is used for record- CQDs-1 and CQDs-2. In the inserts of Fig. 4, we show λ em ing the spectrum of the light emission from samples. The as a function of λ so we can see more clearly how the PL ex measurements are carried out at room-temperature. In peaks shift with excitation wavelength. As shown in Fig. 4, Fig. 4, we show the PL emission spectra for CQDs-1 in the λ increases monotonously with λ for both CQDs-1 em ex upper panel and CQDs-2 in the lower panel at different and CQDs-2. (iii) In relatively shorter excitation wave- excitation wavelengths λ . For the PL measurement, the length regime, two PL peaks can be observed for CQDs-1, ex recording of the intensity of the emission light often starts whereas only one PL peak can be seen for CQDs-2 over after the excitation wavelength to void the damage of the the 420–510 nm wavelength regime. (iv) CQDs-1 can re- PMT detector. Thus, there have been cutoffs in the curves sult in a more broadened PL spectrum than CQDs-2 can. of the PL spectra in Fig. 4. We notice the following fea- (v) The PL peak wavelength induced by CQDs-1 is shorter tures: (i) The intensity of the PL emission first increases than that induced by CQDs-2. At 410 nm excitation Zhang et al. Nanoscale Research Letters (2017) 12:611 Page 5 of 7 wavelength, the blue fluorescence can be achieved by KOH is much stronger than that prepared by NaOH. CQDs-1, whereas at 480 nm excitation wavelength, the With the same excitation wavelength, we find that the green fluorescence can be seen for CQDs-2. (vi) The fluor- alkali ion in alkaline solutions do not affect significantly escence of the CQDs-1 with 8.5% N-doping content is the position of the PL emission wavelength. higher than that of the CQDs-2 with 6.8% N-doping con- For the case where the CQDs are in water (CDQs-1), tent. The reason why the PL emission increases with N- there are two intermediate states induced by surface doping content of CQDs is that N-doping can introduce a states of the C–O and C=O bonds and related functional new kind of surface state. Electrons trapped by the new groups. These two surface states are with different en- formed surface states are able to facilitate a high yield of ergy levels and corresponding selection rules for radia- radiation recombination . The PL results obtained tive electronic transitions, which are responsible for the from this study indicate that the blue and green light emis- emission of PL with two emission wavelengths under sion can be achieved by CQDs-1 and CQDs-2, respect- relatively short wavelength light excitation. The photoex- ively, under optical pumping. cited electrons in the higher energy states in the conduc- At present, the physical mechanism for photon- tion band of CQDs first quickly relax into the surface induced light emission from CQDs is still unclear. How- states via non-radiative relaxation mechanism such as ever, the results obtained from related investigations [12, electron-phonon scattering and electron-electron inter- 26, 27] have shown that the surface modification of the action. When the non-radiative electronic relaxation CQDs by amino and carboxyl functional groups can play time for electrons in the surface states is longer or larger an important role for the PL emission from CQDs. The than the radiative electronic relaxation time, these elec- features of the PL spectrum of CQDs are determined trons can go back to the valence band and emit photons. not only by the particle size of the CQDs  but also by With decreasing pumping wavelength, more states in the surface properties of CQDs [26, 27]. Based on our the valence band and especially in the conduction band XPS and PL results obtained from the present study, we can take part in this pumping, relaxation, and light emis- now discuss the physical mechanism behind the experi- sion process and, thus, the peak wavelength in the light mental findings shown in Fig. 4 for CQDs realized from emission spectrum decreases with excitation wavelength. tofu wastewater. We know that the electronic band Therefore, the wavelength of the light emission depends structure of CQDs is very much similar to that in a dir- on the excitation light wavelength. The increase in the ect band gap semiconductor. However, for CQDs syn- peak wavelength of light emission with excitation wave- thesized from tofu wastewater in different solutions such length implies that the non-radiative electronic relax- as water and NaOH, there are C–O, C = O, and COOH ation time increases with lowering energy levels in the bond-based functional groups on the surface of the surface states. For relatively long wavelength light excita- CQDs, as shown by the XPS results in Fig. 3. The energy tion, the photoexcited electrons in CQDs are quickly re- states of these functional groups are surface states which laxed from the conduction band to the lower energy are located in between the conduction and valence levels of the surface states and emit photons. The possi- bands of the CQDs. They play a role like intermediate bility for the emission of photons from higher energy states, very similar to impurity states in a direct band levels of the surface states becomes low enough so that gap semiconductor. In the presence of excitation light the effect cannot be markedly measured. field, the electrons in the valence band of the CQDs are For the case where the CQDs are in NaOH solution pumped into the conduction band via optical absorption (CDQs-2), there is only one intermediate state for the mechanism. Because the position of the PL peak in the radiative electronic transitions. Because the contents of spectrum depends on the excitation wavelength, the PL the C–O and C=O bonds and related functional group emission via excitonic mechanism  is not the case are relatively low in this case, the radiative surface states for these CQDs. The photon-induced light emission are mainly induced COOH based groups for CQDs-2. from CQDs is therefore a consequence of the direct As a result, only one peak of the PL emission can be ob- photoemission induced by electronic transitions from served. Since the energy levels of the surface states in- higher energy levels to lower energy states. As we know, duced by C–O and C=O bonds and related functional the electrons are normally with a quicker or smaller re- groups are normally higher than those induced by laxation time in the higher energy states than that in the COOH groups, the shorter wavelength PL emission can lower energy states. The results from our XPS and PL be observed for CQDs-1. This is the main reason why measurements suggest that the radiative electronic tran- CQDs-1 can emit blue light whereas CQDs-2 can emit sition in CQDs is mainly achieved via relaxation of elec- green light under optical excitation. trons from the surface states to the valence band of the The quantum efficiency Q of the fluorescence for CQDs. The obtained experimental results show that the CQDs-1 can be evaluated from the experimental data intensity of the PL emission from CQDs prepared by via [29, 30] Zhang et al. Nanoscale Research Letters (2017) 12:611 Page 6 of 7 Authors’ Contributions I A η JZ proposed the research work and designed the experiments. HW carried Q ¼ Q ð1Þ out the major work of sample preparation. YMX assisted with the I A η measurements of the PL spectra and corresponding theoretical analyses. JT participated in the fabrication of the samples and in the quantum efficiency Here Q is the quantum efficiency of the fluorescence measurement. CNL conducted the PL measurements. FYL took part in the fabrication of the samples. HMD worked on the theoretical analyses of the for a standard sample for reference. Under a fixed exci- experimental results. WX supervised the PL measurements and participated tation wavelength at, e.g., 364 nm, I and I are the inte- in the analyses of the experimental results and in the preparation of the grated emission intensities of the CQDs-1 sample and manuscript. All authors read and approved the final manuscript. the standard sample, respectively. A and A are respect- Competing Interests ively the absorbance of the prepared sample and stand- The authors declare that they have no competing interests. ard sample at the same excitation wavelength. η and η are respectively the refractivity of the prepared sample Publisher’sNote and standard sample. It is found that the fluorescent Springer Nature remains neutral with regard to jurisdictional claims in quantum efficiency of CQDs-1 is about 54.49%. Because published maps and institutional affiliations. we cannot find the reference sample for CQDs-2, the Author details fluorescent quantum efficiency of CQDs-2 is not evalu- School of Physics and Astronomy and International Joint Research Center ated in the present study. for Optoelectronic and Energy Materials, Yunnan University, Kunming 650091, People’s Republic of China. Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, Conclusions People’s Republic of China. Department of Physics, China University of In this study, we have fabricated the carbon quantum Mining and Technology, Xuzhou 221116, People’s Republic of China. dots (CQDs) from wastewater induced during the pro- Received: 22 September 2017 Accepted: 12 November 2017 duction of tofu. We have demonstrated that the tofu wastewater is a good source of raw material in making CQDs. The fluorescent CQDs can be fabricated simply References via hydrothermal reaction to carbonize the organic mat- 1. Sun YP, Zhou B, Lin Y, Wang W, Fernando KAS, Pathak P et al (2006) Quantum-sized carbon dots for bright and colorful photoluminescence. J ters in the yellow serofluid of tofu wastewater. The aver- Am Chem Soc 128(24):7756–7757 age size of the CQDs synthesized from tofu wastewater 2. Baker SN, Baker GA (2010) Luminescent carbon nanodots: emergent can be up to 3.5 nm. We have obtained two sorts of nanolights. Angew Chem Int Ed 49(38):6726–6744 3. 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