A Simple Approach for Synthesizing of Fluorescent Carbon Quantum Dots from Tofu Wastewater

Abstract

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.

Material and Optical Properties of Fluorescent Carbon Quantum Dots Fabricated from Lemon Juice via Hydrothermal Reaction

“…The green fluorescent CQDs also show a broad PL peak that shifts with the change of excitation wavelength, which is related to the quantum-confinement effect and edge defects. With the standard PL measurement [ 22 ], the fluorescence quantum yield of the CQDs is 16.7% with an excitation wavelength of 410 nm, where quinine sulfate had been used as the reference. This value is significantly better than the QY (8.95%) of CQDs made from lemon juice in the previous report [ 24 ].…”

“…However, we know that NaBH 4 is toxic. Very recently, we have fabricated the green- and blue-emitting CQDs from tofu wastewater without adding any toxic substances [ 22 ]. The CQDs made from food products and/or by-products are considered be safe for biological applications because there is almost no known toxicity in these natural carbon resources.…”

Influence of Group Modification at the Edges of Carbon Quantum Dots on Fluorescent Emission

“…Furthermore, the corresponding fluorescent QY has not yet reached the requirements for the application as practical devices. The results obtained from experimental [27–29] and theoretical [30, 33] research have confirmed that the edge passivation can effectively improve the electronic optical properties of CQDs. Passivation agents are widely used to adjust the fluorescent properties of CQDs [21–23].…”

Biomass‐Derived Carbon Dots and Their Applications

“…used dried shrimp as the source material of BCDs and were reasonably made into a traceable drug delivery system for targeted delivery of crude in MCF‐7 cells. N. Mehta et al . synthesized BCDs by using pasteurized milk as a carbon source.…”

“…The electron‐donor capabilities of photoexcited BCDs have clear potential in reduction reactions. Zhang et al . presented an investigation of BCDs synthesized from wastewater from the production of tofu.…”