Hydrothermal Synthesis of Blue‑ and Green‑Emitting Carbon Quantum Dots from Tofu Wastewater

Abstract

We report a straightforward hydrothermal method for producing fluorescent carbon quantum dots (CQDs) from tofu‑production wastewater. Two CQD variants were obtained: CQDs‑1, synthesized in deionized water, emit blue fluorescence under UV light, while CQDs‑2, prepared in a NaOH solution, emit green light. X‑ray photoelectron spectroscopy (XPS) reveals that the key difference lies in the surface chemistry—CQDs‑1 contain higher concentrations of C–O and C=O groups, whereas CQDs‑2 are enriched in carboxyl functionalities due to NaOH treatment. Photoluminescence (PL) studies confirm the distinct emission colors and provide insight into the role of surface states. With a quantum yield of 54.49 % for CQDs‑1, this approach demonstrates a sustainable route to high‑performance CQDs while valorizing a common food‑processing waste stream.

Background

Tofu, a staple in Asian cuisine, is now produced on an industrial scale, generating large volumes of wastewater rich in soybean yellow serofluid. This effluent, containing carbohydrates, proteins, organic acids, and pigments, poses environmental challenges but also represents a valuable carbon source for nanomaterial synthesis. Carbon quantum dots (CQDs) are sub‑20 nm carbon nanoparticles that combine excellent water solubility, chemical inertness, low toxicity, and biocompatibility with semiconductor‑like electronic structures, making them attractive for optical and biomedical applications. Conventional CQD synthesis relies on top‑down or bottom‑up strategies using diverse precursors; recent work has highlighted biomass (e.g., wheat straw, orange juice) as sustainable carbon feedstocks. Our study adapts the hydrothermal route—known for its simplicity and scalability—to transform tofu wastewater into CQDs, enabling a dual benefit of waste reduction and nanomaterial production.

Methods

Wastewater was collected from the Tofu Industrial Park, Shi Ping County, Yunnan, China. The protocol involved: (i) pyrolyzing 300 mL of yellow serofluid at ~93 °C for 3–5 h until a dry residue formed; (ii) cooling to room temperature and adding 50–200 mL deionized water (for CQDs‑1) or 100 mL NaOH solution (pH ≈ 12.4) (for CQDs‑2); (iii) stirring for 4 min followed by 5 min ultrasonication; (iv) centrifugation at 12 000 rpm for 20 min to isolate the CQD‑laden supernatant. Variations in temperature, time, and pH were used to tune the CQDs’ optical properties. CQDs‑1 appear yellow to the naked eye but fluoresce blue under UV, whereas CQDs‑2 fluoresce green.

Results and Discussion

Morphology and Size. High‑resolution TEM images (Figure 1) show that both CQD variants are monodisperse, spherical nanoparticles with diameters ranging from 2–10 nm; the mean size is 3.5–5.5 nm and a lattice spacing of ~0.21–0.22 nm confirms graphitic ordering.

Surface Chemistry. Full‑scan XPS spectra (Figure 2) confirm the presence of C, N, O, and minor S, P (CQDs‑1) or Na, Cl (CQDs‑2). High‑resolution C 1s spectra (Figure 3) reveal that CQDs‑1 contain C–C/C=C (284.7 eV), C–O (286.1 eV), and C=O (287.9 eV) bonds, whereas CQDs‑2 show an additional COOH peak (289.1 eV). The reduced C–O/C=O content in CQDs‑2 results from hydroxide‑mediated surface passivation, shifting the emission to longer wavelengths.

Photoluminescence. PL spectra (Figure 4) show that CQDs‑1 emit bright blue light with maximum intensity at ~410 nm excitation, while CQDs‑2 emit green light, peaking at ~480 nm excitation. The emission peak shifts to longer wavelengths with increasing excitation wavelength, indicating that surface states, rather than size‑dependent quantum confinement, dominate the PL behavior. The higher nitrogen content (8.5 % vs. 6.8 %) in CQDs‑1 enhances radiative recombination through additional surface states, explaining its superior quantum yield (54.49 %) relative to CQDs‑2.

Emission Mechanism. The data support a surface‑state‑mediated recombination model: photoexcited electrons relax non‑radiatively into surface states (C–O, C=O, COOH) and subsequently recombine radiatively with holes in the valence band. The presence of two distinct surface states in CQDs‑1 accounts for its broader, dual‑peak PL spectrum, while the single COOH‑dominated state in CQDs‑2 yields a narrower, green emission.

Conclusions

This study demonstrates that tofu‑wastewater, a readily available and environmentally problematic by‑product, can be valorized into high‑quality CQDs via a simple hydrothermal process. The resulting blue‑ and green‑emitting CQDs exhibit tunable photoluminescence governed by surface chemistry, with a quantum yield exceeding 50 % for the blue variant. The approach offers a sustainable, low‑cost pathway for large‑scale CQD production, potentially advancing applications in bio‑imaging, sensing, and optoelectronics. Future work will focus on extending the emission spectrum into the red region by further tailoring surface functionalization.