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# Transforming waste rubber gloves into value-added fluorescent carbon quantum dots for direct ink writing

Yanqing Liao, Haitao Ren, Shuhan Liu, Hong Kang, Meng Luo, Tian Luo, Yiyin Zhu, Baoyang Lu, Eric Lichtfouse, Chuanyi Wang

► To cite this version:

Yanqing Liao, Haitao Ren, Shuhan Liu, Hong Kang, Meng Luo, et al. Transforming waste rubber gloves into value-added fluorescent carbon quantum dots for direct ink writing. Journal of Environmental Chemical Engineering, 2025, 13, pp.115729. 10.1016/j.jece.2025.115729. hal-04939572

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Submitted on 11 Feb 2025

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# Transforming waste rubber gloves into value-added fluorescent carbon quantum dots for direct ink writing

Yanqing Liao<sup>a,1</sup>, Haitao Ren<sup>b,1</sup>, Shuhan Liu<sup>c</sup>, Hong Kang<sup>a</sup>, Meng Luo<sup>a</sup>, Tian Luo<sup>a</sup>, Yiyin Zhu<sup>a</sup>, Baoyang Lu<sup>c,*</sup>, Eric Lichtfouse<sup>d>ID</sup>, Chuanyi Wang<sup>a,* ID</sup>

Keywords:

Carbon quantum dots
Waste disposal
Photoluminescence
Direct ink writing

## ABSTRACT

About 10 million tons of waste rubber are generated annually in China, which has caused potential environmental pollution and resource waste due to improper disposal. In the circular economy context, there is a need for methods to make rubber waste into valuable carbon materials. Here, we report a simple acid-assisted hydrothermal process for the first time to convert nitrile gloves into value-added carbon quantum dots (CQDs), and their use in ink-writing hydrogels. The product yield of CQDs is 36 % under optimized conditions, surpassing previously reported results of CQDs derived from plastics. Moreover, the CQDs exhibit high fluorescence stability from pH 2–7, under ultraviolet irradiation during 3 h, and treated by H₂O₂ at concentrations of 0–500 µmol/L. CQDs were near-spherical with an average particle size of 2.45 nm and the lattice fringe spacing measured at 0.21 nm. The oxygen-containing groups on the surface of CQDs endowed them high hydrophilicity and excitation-dependent fluorescence behavior with a quantum yield of 9.7 % and a lifetime of 6.94 ns. Given their favorable fluorescence and hydrophilic properties, the direct ink-writing hydrogels prepared with CQDs and polyvinyl alcohol display unique patterns on polyethylene terephthalate (PET) substrates, exhibiting high dimensional accuracy and the ability to show fluorescent colors.

## 1. Introduction

In accordance with incomplete statistics, the annual production of waste rubber in China exceeds 10 million tons, but 70 % of the rubber is recycled, and the rest is leaked into the environment [1]. These rubbers that cannot be naturally degradable cause varying degrees of pollution in ecological environments such as cities, rural areas, oceans, and lakes. It is necessary to develop a simple and convenient recycling method to increase the added value of these waste rubbers. As demonstrated in Scheme 1, the predominant techniques for rubber disposal encompass landfills, pyrolysis, and combustion, etc [2]. The landfill is extensively utilized, given that waste rubber is non-biodegradable, it tends to consume precious land resources. Aside from that, it carries a significant risk of releasing toxic molecules such as aromatic hydrocarbons and benzene rings, which ultimately give rise to serious pollution of soil and water resources [3,4]. Pyrolysis primarily involves breaking down the

macromolecular structure of waste rubber at high temperatures, to recover carbon black, pyrolysis gas, and mixed oil derived from the decomposition of waste rubber [5,6]. Nevertheless, this approach is typically featured by substantial investment costs, produces chemicals and fuels of suboptimal quality, and encompasses a relatively intricate process. Combustion processing denotes the utilization of waste rubber as an energy source, which comprises the generation of electricity and heat. Nonetheless, the incineration of waste rubber generates enormous detrimental gases, including SO₂ and CO₂, which not only brings about alarming environmental pollution, but also doesn't coincide with the current spirit of recycling waste resources advocated by the country. As a consequence, it is urgent to explore more effective strategies to recycle waste rubber from the natural environment.

Carbon quantum dots (CQDs) are a class of zero-dimensional carbon nanomaterials that have captured extensive attention on account of their remarkable fluorescence properties, coupled with outstanding

<footnote><sup>a</sup> School of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, PR China</footnote>

<footnote><sup>b</sup> Technological Institute of Materials & Energy Science (TIMES), Xijing University, Xi'an 710123, PR China</footnote>

<footnote><sup>c</sup> Jiangxi Province Key Laboratory of Flexible Electronics, School of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330038, PR China</footnote>

<footnote>* Corresponding authors.
E-mail addresses: luby@jxstnu.edu.cn (B. Lu), wangchuanyi@sust.edu.cn (C. Wang).</footnote>

<footnote n="1">1 Yanqing Liao and Haitao Ren contributed equally to this work.</footnote>
biocompatibility, high water solubility, excellent chemical stability, minimal toxicity, exceptional photobleaching, and tunable emission wavelengths [7,8]. These advantages enhance the versatile applications of CQDs in multiple domains, which encompass bioimaging, sensing, environmental monitoring, energy conversion, chemical analysis, photo-catalysis, electro-catalysis, and anti-counterfeiting [9–14]. The synthesis strategy of CQDs encompasses both top-down and bottom-up methodologies [15]. The top-down approach involves the disaggregation of larger carbonaceous sources into smaller CQDs through physical or chemical processes, which include techniques like arc discharge, laser ablation, electrochemical synthesis, and high-energy ball milling [16–18]. The bottom-up synthesis strategy is to synthesize CQDs from organic molecules, which is predominantly actualized by solvothermal treatment, hydrothermal method, and microwave synthesis thermal decomposition [19–21]. Nevertheless, the majority of methods involve expensive and harmful precursors, thereby limiting the large-scale preparation of CQDs [22,23]. For this reason, it is necessary to develop a vast array of sources, simple operation, and low-cost CQDs synthesis approach.

The high carbon content within waste rubber serves as an optimal precursor for the synthesis of CQDs. Nonetheless, this characteristic has not received conspicuous attention in previous research. With the purpose of effectively utilizing waste rubber, this study extracted fluorescent CQDs from waste rubber gloves by adopting an acid-assisted hydrothermal method (Scheme 1). This investigation represents the first demonstration that CQDs derived from waste rubber possess abundant functional groups, exceptional hydrophilicity, and high fluorescence stability. More importantly, CQDs synthesized by employing this method have a high yield of 36 %, surpassing previously reported CQDs derived from plastics. Direct ink writing (DIW) is an extrusion-based additive manufacturing method that uses a small nozzle to dispense ink to create prints [24,25]. CQDs are considered a promising material for DIW technology attributable to their exceptional water solubility and superior fluorescence properties [26]. Consequently, we uniformly blend the prepared CQDs with polyvinyl alcohol for value-added fabrication, specifically in the printing context, to fabricate flexibility and complexity patterns.

## 2. Experimental

### 2.1. Materials

Nitrile glove was sourced from Shaanxi University of Science &

Technology (Xi'an, China). Polyvinyl alcohol (PVA) and sulfuric acid ($H_2SO_4$, 98 %) were acquired from Chemical Reagent Co., Ltd (Shanghai, China). Quinine sulfate (99 %) was obtained from Shanghai Boer Chemical Reagent Co., Ltd (Shanghai, China). Sodium hydroxide (95 %) was acquired from Shanghai Macklin Biochemical Technology Co., Ltd. Hydrogen peroxide (30 %) was acquired from Tianjin Damao Chemical Reagent Factory. Sodium chloride (99.5 %) was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd.

### 2.2. Preparation of carbon quantum dots

High-value CQDs were obtained from laboratory discarded nitrile gloves through hydrothermal and calcination strategies. The specific preparation process is as follows: First, cut the discarded nitrile gloves into small pieces (1.0 × 1.0 cm), and placed 5 g of these pieces into a ceramic dry pot. Then, the sample glove pieces were processed in a muffle furnace (KSL-1100X) with a temperature rise rate of 2 °C/min, and the carbonization temperature was maintained at 300 °C for 2 h. After the carbonization process, allowed the product to cool to room temperature in the air. Grinded the resulting black powder and sieved it to obtain fine black carbon powder. 0.12 g of the resulting black powder was added to 29 mL of pure water and 1 mL of concentrated sulfuric acid, stirred for 1 hour to achieve dispersion, and then transferred to a Teflon-lined autoclave. React the mixture at temperatures of 160, 180, 200, 220, and 240 °C for 12 h, respectively. The black liquid after the reaction was filtered 5 times through 0.05 µm membrane, and a yellow-brown transparent CQDs suspension was obtained. The concentration of CQDs suspension prepared at 240 °C was 0.388 mg/mL, corresponding to a pH value of 2.7. Under the irradiation of a 365 nm ultraviolet lamp, the CQDs suspension emits bright blue fluorescence. The above preparation process is illustrated in Fig. 1, within which samples display dissimilar images at distinct stages.

### 2.3. Preparation of direct ink printing material

2-gram polyvinyl alcohol (PVA) was dissolved in 20 mL of aqueous solution of CQDs, with magnetic agitation at 90 °C for 90 min. Subsequently, the solution was permitted to cool down to ambient temperature and was subjected to high-speed centrifugation to obtain a homogeneous mixture of CQDs and PVA. At this time, the final concentration of CQDs was 0.353 mg/mL. This mixture was then utilized in direct ink-writing technology for information encryption.

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Strategies to dispose of waste rubber gloves.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 1. Steps to synthesize carbon quantum dots from waste rubber gloves.</figcaption></figure>

## 2.4. Characterization

Transmission electron microscopy (TEM) images were acquired by employing a JEM-2100F microscope operating at 200 kV. X-ray Photoelectron Spectroscopy (XPS) measures the amount or relative concentration of elements and atoms in a sample by adopting a monochromatic Kα source aluminum excitation Thermal Science Kα spectrometer calibrated using a C-C key in C1s (284.8 eV). Fourier transform infrared (FTIR) spectra are measured by an in situ photochemical vacuum infrared spectrometer (Vertex 70 V). KBr powder was used as a blank reference, with a scanning range between 4000 and 700 cm⁻¹. Photoluminescence (PL) spectra were recorded on an Edinburgh Instruments FS5 fluorescence spectrometer, and fluorescence lifetime measurements were carried out on the same FS5 Edinburgh instrument, which was equipped with an EPLED-340 pulse source. Raman analysis was performed using a micro-confocal laser Raman spectrometer with an excitation wavelength of 532 nm and a scattering range of 1000–2000 cm⁻¹.

## 2.5. Calculation of yield

The relative quantum yield method was utilized to determine the quantum yield of fluorescent CQDs. As previously reported by relevant literature, quinine sulfate solution (quantum yield = 54 %) was utilized as a reference to evaluate the quantum yield of CQDs. The relative quantum yield was calculated by referring to the following formula [27, 28].

$$QY_{CQDs} = QY_{ref} (I_{CQDs} A_{ref} / A_{CQDs} I_{ref}) (\eta_{CQDs} / \eta_{ref})^2$$

Where "I" denotes the emission intensity, "η" represents the refractive index of the solvent, and "A" signifies the absorbance at the optimal excitation wavelength of 360 nm, which is typically below 0.1. In these suspensions, the ratio of η$_{CQDs}$ to η$_{ref}$ approaches unity. We define production yield as the ratio of the mass of pure (dry) product relative to the mass of the starting material.

# 3. Results and discussion

## 3.1. Optimization of experimental conditions

The photochemical properties of CQDs were examined by adopting photoluminescence spectroscopy and ultraviolet-visible spectroscopy (Fig. 2a-e). The five CQDs synthesized at varying temperatures exhibited distinct optimal excitation and emission wavelengths. It's particularly noteworthy that the optimal excitation wavelengths were detected at 327, 335, 338, 332, and 334 nm, while the corresponding optimum emission wavelengths were observed at 423, 406, 405, 393, and 389 nm, respectively. Within the temperature ranging from 160 to 240 °C, the emission peaks of CQDs began to shift towards the blue end of the spectrum, resulting in reduced wavelengths. This shift indicates an increase in photon energy, correlating with an augmented band gap in the CQDs. Apart from that, their suspension exhibited intense blue fluorescence when exposed to a 365 nm ultraviolet light source.

At temperatures of 160 and 180 °C, few fluorescent products were produced, with quantum yields of 5.35 % and 5.4 %, and product yields of 5.6 % and 16 %, respectively (Figs. 2f and S1b). Heightening temperatures to 200, 220 and 240 °C triggered an elevation in the fluorescence of products, thereby giving rise to an augment in production yield to 36 % and a quantum yield of 9.7 %. On this basis, 240 °C was chosen as the optimal temperature in that it materialized a desirable balance between higher product yields and reasonable PL. As illustrated in Fig. S1a, the CQDs exhibit similar FTIR spectra, characterized by stretching vibrations in the following regions: O-H/N-H (3200–3400 cm⁻¹), C=O (1637–1702 cm⁻¹), C=C/C-N (1424 cm⁻¹), C-C (1231 cm⁻¹), and C-O (1074 cm⁻¹) [29–35].

## 3.2. Structure analysis of carbon quantum dots

The CQDs synthesized from nitrile glove waste were analyzed by utilizing TEM and Raman spectroscopy. As depicted in Fig. 3a, the TEM images evidently demonstrate that the synthetic CQDs are uniformly distributed with a near-spherical morphology, without any signs of aggregation. Fig. 3b illustrates that the lattice spacing of CQDs is 0.21 nm, corresponding to the (100) plane of graphite [15,36,37]. This suggests that the structure of CQDs is comparable to that of graphite. The
<figure><img src="image_4.png" /><figcaption>Fig. 2. Ultraviolet-visible spectroscopy absorption and photoluminescence excitation and emission spectra of carbon quantum dots in aqueous medium. (a) Reaction temperature is 160 °C, (b) reaction temperature is 180 °C, (c) reaction temperature is 200 °C, (d) reaction temperature is 220 °C, (e) reaction temperature is 240 °C, (f) and quantum yield of CQDs.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. (a) TEM, (b) HR-TEM micrograph images, (c) size distribution histogram, (d) and Raman spectrum of the CQDs.</figcaption></figure>

histogram in Fig. 3c illustrates the size distribution pertinent to the 245 nanoparticles, which, upon statistical analysis, reveals a distribution ranging from 0.75 to 4.5 nm, with an average size of 2.45 nm for the CQDs. The structural characteristics of CQDs were investigated by employing Raman spectroscopy, principally centering around the

disordered (D band) and graphitic (G band) configurations. As depicted in Fig. 3d, two distinct peaks are evident, marking the presence of two classic Raman features at 1571 and 1353 cm⁻¹, which correspond to the disordered sp² G band and the D graphite band [20,38,39]. The intensity ratio of the D band to the G band is measured at 0.84. This ratio suggests
a satisfactory degree of graphitization, which is attributed to the defects within the sp² bonding of the CQDs, particularly those containing heteroatoms and functional groups [40–43].

The compositional elemental analysis and surface configuration of CQDs were investigated via XPS and FTIR spectroscopy. Fig. 4a presents the XPS spectrum of the CQDs, which apparently reveals that the CQDs are primarily composed of 41.89 % carbon (C), 46.71 % oxygen (O), 7.74 % sulfur (S), and 3.65 % nitrogen (N). In the range of 10–270 eV and after the N1s peak, these peaks are mainly attributed to impurities, while S is the residue remaining after the purification process. The high-resolution XPS spectrum of C1s, as depicted in Fig. 4b, reveals three distinct peaks at 284.71, 285.91, and 288.51 eV, which is in alignment with the chemical bonds C—C/C=C, C—N/C—O, and C=O, with a goodness of fit value of 0.67478 [44,45]. Moreover, the high-resolution XPS spectrum of O1s, demonstrated in Fig. 4c, reveals two signals at 530 eV and 532.9 eV, which are stemmed from C=O and C-O bonds, respectively [46,47]. The goodness of fit for this analysis is 2.212. Fig. 4d presents the high-resolution XPS spectrum of the N1s, which exhibits two peaks at 399.4 and 401.4 eV, and these peaks are identified as C—N and N—H [48,49]. The goodness of fit for this analysis is 0.9369.

The FTIR spectrum of CQDs at 240 °C corroborates this assertion. As depicted in Fig. S1a, the broad absorption band observed around 3208 cm⁻¹ indicates the stretching vibrations involving hydroxyl (-OH) and nitrogen-hydrogen (N-H) functional groups. The distinct absorption features at 1645, 1427, 1203, and 1076 cm⁻¹ correspond to the stretching vibrations of the carbonyl group (C=O), C=C, C-C, and C-O bonds, respectively. XPS and FTIR spectra have confirmed the presence of nitrogen-containing functional groups within the synthesized CQDs, indicating the incorporation of nitrogen into the CQDs. The CQDs possess a myriad of functional groups, including the -carboxyl (-COOH), the hydroxyl (-OH), and the -amino (-NH₂) groups, which are capable of enhancing the hydrophilicity and stability of the CQDs within aqueous systems, thereby facilitating further functionalization and applications of CQDs.

### 3.3. Fluorescence properties of synthesized CQDs

The fluorescence properties of CQDs synthesized from discarded rubber gloves under optimal hydrothermal conditions at 240 °C were systematically studied. Figs. 5a and S2a illustrate the fluorescence emission spectra of CQDs at varying excitation wavelengths. The graph demonstrates a distinct dependence of CQDs on the excitation wavelength. This illustrates the presence of multiple emission centers, demonstrating multiple deactivation pathways. These pathways normally arise from a combination of quantum confinement effects and the distribution of distinct emission states [50]. As suggested in Fig. 5b, a distinct, narrow emission peak resembling concentric circles is observed in the three-dimensional fluorescence spectrum of CQDs. This signifies the presence of a homogeneous substance. The peak maximum for emission occurs at approximately 390 nm, while the excitation maximum is located at around 334 nm. As shown in Fig. 5c, the coordinates of the International Commission on Illumination (CIE) (0.1876, 0.1772) further corroborate the blue fluorescence exhibited by CQDs under excitation data elicited at 334 nm. As depicted in Fig. 5d, the fluorescence lifetime of CQDs primarily consists of t₁, t₂, and t₃. Notable, t₁ is associated with the electron transition within the carbon core, which t₂ and t₃ reveal the surface state of this electron transition. This demonstrates the existence of a non-radiative emission pathway in CQDs, namely, the surface defect state [51]. As delineated in Table S1, the empirical data derived from the PL decay curve illustrates that the PL lifetime of CQDs is predominantly governed by t₂ and t₃. This signifies that the surface defect state serves as the primary emitter for PL, which consequently dictates the photoluminescence characteristics of CQDs, as referenced in literature [52–55]. The aforementioned research findings underscore that the blue-fluorescent CQDs derived from waste latex glove not only exhibit fluorescence properties dependent on the excitation wavelength, but also display single-exponential decay characteristics with a lifetime (τ) of 6.94 nanoseconds.

<figure><img src="image_6.png" /><figcaption>Fig. 4. (a) XPS survey spectrum, (b) high-resolution C1s XPS spectrum, (c) high-resolution O1s XPS spectrum, (d) high-resolution N1s XPS spectrum.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. (a) Excitation-emission photoluminescence spectra of CQDs. (b) Excitation-emission matrix for CQDs. (c) International Commission on Illumination coordinates of CQDs. (d) Time-resolved decay and fitting curves of CQDs.</figcaption></figure>

3.4. Stability of CQDs

A systematic exploration was carried out to analyze the

photostability and chemical stability of CQDs, along with the influences
of pH, ionic strengths, exposure duration to irradiation, and oxidation
concentrations on fluorescence intensity. As illustrated in Figs. 6a and

<figure><img src="image_8.png" /><figcaption>Fig. 6. (a) Effect of Irradiation time on the fluorescence intensity of the CQDs, (b) influence of diverse NaCl on the stability of CQDs, (c) different H₂O₂ concentrations on the antioxidant capacity of CQDs, (d) influence of pH on the fluorescence intensity of the CQDs.</figcaption></figure>
S3a, the fluorescence intensity of the CQDs exhibited dramatic stability following 180 min of continuous irradiation under a 365 nm ultraviolet light source, which illustrates an excellent resistance to photobleaching in the prepared CQDs. As illustrated in Figs. 6b and S3b, upon the transition of sodium chloride concentration from 0.1 to 0.5 mol/L, the fluorescence intensity of CQDs remains unchanged, exhibiting profound salt resistance of CQDs under varying ionic strengths [56]. To investigate the antioxidant potential of CQDs, varying concentrations of hydrogen peroxide were incrementally introduced into an aqueous solution of CQDs. As illustrated in Figs. 6c and S3c, the findings indicate that the prepared CQDs exhibit a robust antioxidant capacity, as evidenced by the negligible alteration in fluorescence intensity. Furthermore, pH values exert a pronounced influence on the optical characteristics of CQDs. Aqueous suspensions of CQDs were prepared across a pH range of 1–14, and their photoluminescence spectra were recorded (Figs. 6d and S3d). Notably, the fluorescence intensity is starkly intense when the pH is equal to or greater than 2.0. On the contrary, a considerable decline in fluorescence intensity is observed when the pH exceeds 7.0, with a particularly notable reduction occurring within the pH range of 8.0–14.0. The reason behind this phenomenon resides in a fundamental fact that the emission from carboxyl-functional groups is inhibited in alkaline environments, which disrupts the surface functional groups of CQDs. In this regard, the electronic transitions in the luminous center are affected, leading to a decrement in fluorescence intensity. Hence, it can be deduced that an acidic or neutral environment is most conducive to the optical properties of CQDs [57].

Table 1 delineates the specifics of CQDs derived from various plastic wastes, from which a pertinent conclusion can be drawn that the methodologies for synthesizing CQDs from plastic materials entail the utilization of massive organic solvents and potent oxidants. Such reagents are not only costly and deleterious but also pose substantial risks to environmental and human health. Concurrently, the quantum yield associated with CQDs is not so satisfactory as anticipated. In contrast, this study uses waste rubber as a precursor for synthesizing CQDs, which offers multiple advantages, such as easy collection of raw materials, no use of organic reagents, lowered use of strong oxidants, and elevated fluorescence quantum yield. Attributable to the excellent fluorescence stability and hydrophilicity of CQDs, their combination with polyvinyl alcohol holds promising application prospects in printing. Meanwhile, we briefly discuss the reasons for the high yield of CQDs synthesized from simple small molecules and the low yield of CQDs synthesized from plastic and rubber. Plastic and rubber are typically composed of a variety of polymers and additives, constituting high molecular weight polymers that exhibit particular structural and thermal stability, making

them resistant to conversion into CQDs. By comparison, small molecules normally possess relatively simple structure. Thus, it is easier to form CQDs with specific structures and properties under specific conditions. In the reaction process, it's challenging to break the C-C main chain on the rubber and plastic structure, and the carbonization is incomplete, which augments the difficulty in controlling the reaction of generating CQDs, and simultaneously lessens the yield. The synthesis of CQDs through small molecules involves a relatively straightforward reaction pathway and mechanism, which affords researchers greater accuracy in predicting and controlling the reaction process. As a result, the quantum yield and product yield of CQDs can be enhanced.

### 3.5. Direct ink writing

The exceptional optical characteristics of CQDs offer intriguing photoluminescence properties, which are ideal for printing applications. The 3D printing system utilized in this experiment is a direct-writing 3D printer (Model DB100, Shanghai Mifang Technology Co., LTD.), which is composed of an air compressor and a distribution needle, featured by high-precision air pressure control. Throughout the printing process, the precision of the printed sample can be enhanced by adjusting several factors, namely, the diameter of the dispensing needle, the distance between the dispensing needle and the substrate, the printing speed, and the air supply pressure (Fig. S4 and S5). Consequently, we have combined CQDs with polyvinyl alcohol to create transparent CQDs and PVA composite hydrogels. Fig. 7 illustrates the two-dimensional code of CQDs and PVA composite hydrogel printing. The dimensions of the two-dimensional code are 36 × 36 mm, and its right-angle edge presents a high level of resolution. Under ambient lighting conditions, the hydrogel is translucent, precluding the visibility of any information. Nevertheless, fluorescent two-dimensional code can be recognized, under 254 and 365 nm ultraviolet light. The two-dimensional code obtains the stored information "CQDs" by scanning the mobile device, demonstrating that the information can be encrypted multiple times. Furthermore, the excellent fluorescence properties of CQDs are fully guaranteed on the basis of the strong hydrogen bond between CQDs and PVA. Under such circumstance, CQDs can maintain high fluorescence intensity and excellent fluorescence quantum yield (9.7 %) in fluorescent ink. PVA is soluble in water, the surface of the printed object is coated with a waterproof film to prevent absorption and dehydration when exposed to air and water. This precaution ensures the synthetic CQDs and PVA composite hydrogel maintains its structural integrity upon exposure to water, thereby heightening utility and stability in the realm of anti-counterfeiting printing applications. Grounded in the above analysis, synthesized CQDs possess exceptional fluorescent properties, rendering

<table><caption>**Table 1**
Comparison of carbon quantum dots derived from different plastics with the results of this study.</caption><thead><tr><th>Raw material</th><th>Solvent/catalyst</th><th>Method</th><th>Wavelength/nm</th><th>QYs</th><th>Application</th><th>Ref.</th></tr></thead><tbody><tr><td>Polyolefin</td><td>H<sub>2</sub>SO<sub>4</sub> and HNO<sub>3</sub></td><td>Hydrothermal method at 120 °C for 12 h</td><td>490/540</td><td>4.84 %</td><td>Detection of Cu<sup>2+</sup></td><td>[7]</td></tr><tr><td>Soft drink bottles</td><td>Dimethyl sulfoxide</td><td>Hydrothermal synthesis 250 °C for 5 h</td><td>300/435</td><td>9.1 %</td><td>Sensitive detection of Fe<sup>3+</sup> and Cu<sup>2+</sup></td><td>[14]</td></tr><tr><td>Polyvinyl chloride</td><td>H<sub>2</sub>SO<sub>4</sub> and H<sub>2</sub>O<sub>2</sub></td><td>Oxidation approach at 150 °C for 2 h</td><td>330/446</td><td>8 %</td><td>O<sub>2</sub> harvesting (from the air)</td><td>[15]</td></tr><tr><td>Polyethylene</td><td>H<sub>2</sub>O<sub>2</sub></td><td>Acid-Mediated carbonization process 150 °C for 3 h</td><td>500/534</td><td>7 %</td><td>Photocatalysis</td><td>[18]</td></tr><tr><td>Polypropylene</td><td>H<sub>2</sub>SO<sub>4</sub> and KMnO<sub>4</sub></td><td>Hydrothermal method, 150 °C for 6 h</td><td>350/430</td><td>3.0 %</td><td>NA</td><td>[21]</td></tr><tr><td>Polystyrene</td><td>HNO<sub>3</sub></td><td>Solvothermal method at 200 °C for 5 h</td><td>420/470</td><td>5.2 %</td><td>LED</td><td>[35]</td></tr><tr><td>Polyethylene terephthalate</td><td>H<sub>2</sub>O<sub>2</sub></td><td>Hydrothermal method 180 °C for 12 h</td><td>340/434</td><td>5.2 %</td><td>Detection of Fe<sup>3+</sup></td><td>[39]</td></tr><tr><td>Plastic bottles</td><td>Octadecene</td><td>Solvothermal method 250 °C for 5 h</td><td>370/430</td><td>6 %</td><td>LED</td><td>[43]</td></tr><tr><td>Polyethylene</td><td>H<sub>2</sub>O<sub>2</sub> and H<sub>2</sub>SO<sub>4</sub></td><td>Chemical oxidation approach at 140–150 °C for 1 h</td><td>395/510</td><td>6.9 %</td><td>Photocatalysis</td><td>[47]</td></tr><tr><td>Polymeric membrane</td><td>H<sub>2</sub>O<sub>2</sub></td><td>Hydrothermal method at 180 °C for 12 h</td><td>320/450</td><td>6.24 %</td><td>Detection of Fe<sup>3+</sup></td><td>[56]</td></tr><tr><td>Waste rubber gloves</td><td>H<sub>2</sub>SO<sub>4</sub></td><td>Carbonization and hydrothermal method</td><td>334/389</td><td>9.7 %</td><td>Direct ink writing</td><td>This work</td></tr></tbody></table>

NA: Not Available, LED: light emitting diode.
<figure><img src="image_9.png" /><figcaption>Fig. 7. CQDs and PVA composite hydrogel print two-dimensional coding under indoor lighting and 254 and 365 nm illumination.</figcaption></figure>

them to be a prioritized choice for direct ink writing inks.

## 4. Conclusion

In conclusion, blue fluorescent CQDs were synthesized from waste rubber via a hydrothermal approach, utilizing no costly reagents during the preparation process. This methodology boasts the benefits of being cost-effective, possessing a straightforward purification procedure, and turning waste into treasure, thereby diminishing environmental pollution. The synthesized CQDs have a graphite-like structural configuration, with lattice stripe spacings at 0.21 nm and an average particle diameter of 2.45 nm. The CQDs exhibit wavelength-dependent fluorescence characteristics with the quantum yield reaching up to 9.7%. Moreover, the CQDs exhibit high fluorescence stability under high ion strength, prolonged ultraviolet radiation, and strong oxidants. Owing to the commendable performance of CQDs, they can be synergistically integrated with PVA to fabricate hydrogels demonstrating superior direct ink writing characteristics. The resulting object exhibits robust fluorescence stability and high resolution, emitting vibrant blue light under excitation at 254 and 365 nm. This study not only provides an important solution for the value-added recycling of waste rubber but also serves as a reference for the development of effective direct ink writing materials.

### CRediT authorship contribution statement

**Lichtfouse Eric:** Writing – review & editing, Visualization. **Lu Baoyang:** Supervision. **Zhu Yiyin:** Software. **Luo Tian:** Resources. **Luo Meng:** Methodology. **Kang Hong:** Data curation. **Liu Shuhan:** Validation, Supervision. **Ren Haitao:** Writing – review & editing, Visualization, Validation. **Liao Yanqing:** Writing – original draft, Formal analysis, Data curation. **Wang Chuanyi:** Methodology, Funding acquisition.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This work was supported by the National Natural Science Foundation of China (52161145409), and SAFEA of China (“Belt and Road” Innovative Exchange Foreign Expert Project, DL2023041004L).

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2025.115729.

### Data availability

Data will be made available on request.

### References

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**Supporting Information**

**Transforming waste rubber glove into value-added fluorescent carbon
quantum dots for direct ink writing**

Yanqing Liao<sup>a,1</sup>, Haitao Ren<sup>b,1</sup>, Shuhan Liu<sup>c</sup>, Hong Kang<sup>a</sup>, Meng Luo<sup>a</sup>, Tian Luo<sup>a</sup>, Yiyin Zhu<sup>a</sup>, Baoyang Lu<sup>c*</sup>, Eric Lichtfouse<sup>d</sup>, Chuanyi Wang<sup>a,*</sup>

<footnote><sup>a</sup> School of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xian 710021, P.R. China</footnote>

<footnote><sup>b</sup> Technological Institute of Materials & Energy Science (TIMES), Xijing University, Xi'an 710123, P.R. China</footnote>

<footnote><sup>c</sup> Jiangxi Province Key Laboratory of Flexible Electronics, School of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang 330038, Jiangxi, P.R. China</footnote>

<footnote><sup>d</sup> State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049, China</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: wangchuanyi@sust.edu.cn (C. Wang).</footnote>

<footnote n="1"><sup>1</sup> Yanqing Liao and Haitao Ren contributed equally to this work.</footnote>
<figure><img src="image_10.png" /><figcaption>Fig. S1. (a) FTIR spectra and (b) production yields of various CQDs.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. S2. Excitation-emission PL spectra of CQDs prepared from 240 °C.</figcaption></figure>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. S3.** (a) Irradiation time on the fluorescence intensity of the CQDs, (b) effect of different concentrations of NaCl on the stability of CQDs, (c) different $H_2O_2$ concentrations on the antioxidant capacity of CQDs, (d) effect of pH on the fluorescence intensity of the CQDs.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. S4.** Direct ink writing 3D printer.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. S5.** (a) Different content of CQDs/PVA composite hydrogel, (b) CQDs/PVA (20:2), printing needle 160um, printing speed and line width relationship.</figcaption></figure>

<table><caption>**Table S1.** Fluorescence lifetime test parameters of CQDs prepared from 240 °C.</caption><thead><tr><th>t<sub>1</sub> (ns)</th><th>Rel<sub>1</sub> (%)</th><th>t<sub>2</sub> (ns)</th><th>Rel<sub>2</sub> (%)</th><th>t<sub>3</sub> (ns)</th><th>Rel<sub>3</sub>(%)</th><th>t<sub>ave</sub> (ns)</th></tr></thead><tbody><tr><td>1.4896</td><td>22.42</td><td>4.7774</td><td>58.35</td><td>19.7148</td><td>19.24</td><td>6.94</td></tr></tbody></table>