1 Introduction

Heavy metal ions are widely recognized as non-biodegradable micropollutants that accumulate in various environmental compartments, including water bodies and living organisms. Copper, arsenic, lead, iron, cadmium, mercury, and chromium are well-known heavy metal ions, even at trace levels, and they pose significant risks to both human health and environmental integrity. These pollutants are predominantly found in the effluents of industries such as pigment production, acid battery manufacturing, alloy production, printing, herbal product formulation, ceramic and glass production, coating processes, and metal plating [1,2,3,4]. Several techniques have been used to detect heavy metal ions, including spectrophotometry, atomic absorption spectroscopy, electron paramagnetic resonance, electrochemical approaches, and other substances such as organic dyes, nanoclusters, metal oxides, and nanodots [5,6,7]. However, many of these methods suffer from significant drawbacks, including time-consuming procedures.

Counterfeiting is a global issue with serious implications for government bodies, businesses, educational institutions, and consumers. Counterfeit products, spanning commercial goods, microelectronics, automotive components, pharmaceuticals, and official documents, have proliferated in recent years. Carbon nanodots, also known as CNDs, have a bright fluorescence, low cytotoxicity, high quantum yields, and a wide variety of sources, all of which make them very attractive for use in anti-counterfeiting applications. Surprisingly, there is a paucity of study regarding the anti-counterfeiting capability of TWNCNDs (Termite Wing-derived Nitrogen-doped Carbon Nanodots) as luminous materials [8].

Fluorescent nanomaterials have gained prominence due to their rapid detection capabilities, high specificity, solubility, biocompatibility, and ease of use. Semiconductor-based fluorescent quantum dots (FQDs) are frequently employed in metal ion sensing due to their exceptional optical properties. However, FQDs often suffer from toxicity concerns, even at low concentrations, limiting their applicability. Consequently, efforts have been made to develop biocompatible fluorescent QDs. The CNDs are a unique class of zero-dimensional carbon-based nanomaterials with outstanding optical properties suited for a wide range of applications, including optoelectronics, bioimaging, catalysis, and numerous sensor types [9]. Among fluorescent materials, CNDs are the most cost-effective, efficient, and environmentally friendly and have been found to be extensively useful as sensor probes. Various synthesis methods, including laser ablation, electrochemical oxidation, chemical oxidation, arc discharge, hydrothermal, and microwave techniques, have been employed to produce CNDs [10]. Green technology has recently emerged for CND synthesis, offering higher quantum yields, cost-effectiveness, and environmental friendliness. These natural product-derived CNDs, mainly composed of carbon, oxygen, nitrogen, hydrogen, and sulfur, undergo poly-condensation to yield CNDs with abundant surface functional groups [11]. Doping with elements like nitrogen, sulfur, boron, chlorine, and phosphorus is commonly employed to enhance fluorescence characteristics, electron transport, and quantum yields. Nonmetal doping can also lead to shifts in the valence and conduction band positions of carbon nanodots. However, there remains a need for novel approaches to create CNDs with high quantum yields and tailored surface functionalization for specific applications [12].

Termite wings are readily available and abundant in nature, making them a convenient and cost-effective source material for the synthesis of quantum dots. Their accessibility ensures a sustainable and eco-friendly approach to nanoparticle production. Termite wings contain chitin, a natural polysaccharide found in the exoskeletons of insects [13, 14]. Chitin serves as a biocompatible and biodegradable precursor for the synthesis of quantum dots, facilitating the green synthesis process. Chitin-rich biomaterials like termite wings possess inherent chemical reactivity due to functional groups present in their composition. These functional groups can serve as reducing and capping agents during the synthesis of quantum dots, promoting the nucleation and growth of nanoparticles [15, 16]. The unique microstructure of termite wings, including their size, shape, and surface features, may influence the nucleation and growth kinetics of quantum dots [15]. By leveraging these natural properties, researchers can potentially achieve control over the size, morphology, and optical properties of the synthesized nanoparticles [17]. Additionally, the use of renewable biomaterials aligns with sustainable practices and reduces the environmental impact associated with nanoparticle synthesis.

In this investigation, we synthesized luminous TWNCNDs by using a conventional hydrothermal procedure, with termite wings serving as the carbon source and aqueous ammonia playing the role of the nitrogen dopant. In order to characterize the TWNCNDs that we synthesized, we relied heavily on important spectroscopic techniques. These TWNCNDs emit a strong blue fluorescence and have the ability to selectively, visually, and accurately detect Cu2+ ions even when other metal ions are also present. TWNCNDs have the potential to function as powerful anti-counterfeiting agents thanks to their remarkable fluorescence qualities.

2 Result and discussion

2.1 Morphology analysis of TWNCNDs

We performed a thorough characterization of the pristine TWNCNDs, using a variety of approaches to elucidate their structural and morphological characteristics. The TEM analysis unequivocally verified the uniform dispersion of the synthesized TWNCNDs, showcasing their consistent morphology with an average size of 3.3 nm (refer to Fig. 1a, b). The SAED pattern, as depicted in Fig. 1c, exhibited concentric circular rings and a conspicuous bright spot. These features strongly indicate the graphitic structure inherent to TWNCNDs [18, 19]. Furthermore, a high-resolution TEM image (Fig. 1d), corresponding to the (100) crystal facet of graphitic carbon, unveiled well-defined lattice fringes with an interplanar spacing of 0.28 nm. These comprehensive findings collectively underscore the precise structural and morphological characteristics of the TWNCNDs, offering valuable insights into their fundamental properties.

Fig. 1
figure 1

a TEM image of TWNCNDs. b Size distribution graph of TWNCNDs. c SAED pattern of TWNCNDs. d HR-TEM image of TWNCNDs

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2.2 UV–Vis and FT-IR analysis of TWNCNDs

The UV-vis absorption spectrum of TWNCNDs is thoughtfully presented in Fig. 2a for your examination. This spectrum reveals the presence of two distinctive absorption peaks, each conveying essential information about the nanodots’ molecular composition. The first and more prominent absorption peak, observed at 272 nm, can be confidently attributed to the π−π* transition of the C = C bonds nestled within the aromatic rings. This resonance provides valuable insights into the nanodots’ aromatic structural elements. The second discernible peak, situated at 324 nm, aligns with the n-π* transitions associated with C = O bonds. Remarkably, these transitions are intimately linked to the π−π* transition of the C = C bonds found within the aromatic rings, further highlighting the intricate interplay of molecular constituents within the TWNCNDs [20]. When a diluted solution of TWNCNDs in water is exposed to daylight, it appears with a typical color. However, when subjected to UV light at 365 nm, it undergoes a striking transformation, turning vividly blue, as illustrated in the inset of Fig. 2a. This change confirms the successful synthesis of TWNCNDs from termite wings, as corroborated by the UV-Vis data [21].

Fig. 2
figure 2

a UV–Visible absorbance spectrum of TWNCNDs. b XRD pattern of TWNCNDs. c Raman spectrum of prepared TWNCNDs. d FTIR spectra of TWNCNDs

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In terms of structural analysis, we investigated the Raman spectra of TWNCNDs. The D bands observed in the spectra arise from the presence of dangling carbon-atom bonds on disordered graphite termination planes. Conversely, the G bands are associated with the vibration of sp2 carbon atoms in hexagonal lattices in two dimensions (2D). For TWNCNDs, the ratio of ID to IG values was determined to be 0.98, with the intensity of ID being smaller than that of IG (ID/IG). These findings indicate the presence of a mild graphitic structure in the synthesized TWNCNDs and suggest the existence of minor imperfections on the surface of TWNCNDs’ graphitic structure.

Furthermore, we employed FT-IR spectroscopy to delve into the spectrum of functional groups adorning the surface of TWNCNDs. As elucidated in Fig. 2d, the results of this analysis provide critical insights into the molecular composition of these nanodots. A discernible peak at 3384 cm−1 signifies the presence of bending vibrations characteristic of O-H bonds, underscoring the involvement of hydroxyl groups on the TWNCND surface. Simultaneously, a peak observed at 2103 cm−1 corresponds to the stretching vibrations associated with N-H bonds, a pivotal revelation indicative of amino (NH2) groups. Notably, the stretching vibrations originating from C = C/C = O bonds manifest as absorption maxima within the 1635 cm−1 range, while those from C-O bonds are distinctly evident at 1037 cm−1. These findings offer compelling evidence of a high concentration of -NH2 and -COOH functional groups prominently adorning the exterior of TWNCNDs [22, 23].

In Fig. 3a, the comprehensive X-ray photoelectron spectroscopy (XPS) spectra exhibit three distinct peaks situated at 283.6, 399.1, and 532.4 eV, aligning with C1s, N1s, and O1s, respectively. The high-resolution C1s spectra (Fig. 3b) unveil four prominent peaks located at 285.1, 286.6, and 288.4 eV. These peaks can be attributed to specific chemical bonds: C-H bonds, C = C bonds, and C = O bonds, respectively [24]. Figure 3c, the N1s spectra manifest two discernible peaks, one at 398.3 eV and the other at 399.6 eV. These peaks correspond to N-H and C-N-C bonds, respectively, underlining the nitrogen-containing functional groups present on the TWNCNDs’ surface. Notably, these surface components detected by XPS are in close agreement with the findings derived from FTIR spectroscopy [25]. Figure 3d provides further elucidation with the O1s spectrum, which can be effectively deconvoluted into two distinctive peak positions at 531.4 and 532.9 eV. These positions align with C = O and C-OH/C-O-C functionalities, respectively, as illustrated in the figure. These XPS data serve as pivotal evidence of the chemical composition of TWNCNDs, conclusively confirming the presence of specific functional groups on their surface.

Fig. 3
figure 3

a XPS full spectrum, (b) C 1 s spectrum, (c) N 1 s spectrum, (d) O1s spectrum of the TWNCNDs

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The optical properties of TWNCNDs are explored using their PL spectra. The as-prepared TWNCNDs had optimal excitation and emission peak wavelengths of 305 nm and 410 nm, respectively (Fig. 4a). They can also exhibit blue fluorescence when exposed to 365 nm UV light. Figure 4b shows an example of excitation-dependent PL activity. When the excitation wavelength is increased from 345 to 415 nm, the emission evolves to a longer wavelength region with a significant decrease in intensity. The quantum yield of TWNCNDs is 11.8% [26].

Fig. 4
figure 4

a The excitation spectra of TWNCNDs, (b) different excitation wavelengths

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2.3 Metal ion sensing capability of TWNCNDs

TWNCNDs are promising for sensing applications because their oxygen-containing surface groups might potentially interact with transition metal ions [27]. We investigated the feasibility of employing TWNCNDs as fluorescence probes for the quantitative measurement of metal ions. As depicted in Fig. 5a, b, an increase in the concentration of Cu2+ ions led to a gradual reduction in fluorescence intensity [28]. Figure 5c demonstrates a linear relationship (R2 = 0.9897) within the concentration range of 10 to 70 nM, with a detection limit of 0.1 µM established using the 3 σ/slope method. Various methods to detect Cu2+ ions in earlier reports, shown in Table 1.

Fig. 5
figure 5

a Fluorescence spectra for Cu2+ TWNCNDs, at various concentrations (10, 20, 30, 40, 50, 60, 70, 80, 100, 125, 150, 175, 200, 250, 300, 350, 400, and 500 nM) in 20 mM Tris (pH = 7.5) at 37 °C; (b) Fluorescence resolution versus Cu2+ concentrations; (c) linear relationship among fluorescence intensity and Cu2+ concentrations (10–70 nM); (Surprisingly, there is a paucity of study regarding the anti-counterfeiting). d Cu2+ selectivity versus coexisting metal ions. The standard deviation is displayed by the error bars for three different tests

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Table 1 Comparison of different fluorescent probes for sensing Cu2+
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In order to comprehensively investigate the influence of diverse metal ions on TWNCNDs, we conducted an extensive examination of fluorescence intensities both in the presence and absence of specific metal ions. The selected metal ions encompassed a wide range, including Co2+, Cu2+, Ba2+, Fe3+, Ag+, Pb2+, Cr3+, Mg2+, Zn2+, Hg2+, Mn2+, Cr6+, Ca2+, and K+. Remarkably, only the addition of Cu2+ induced an instantaneous and significant decrease in fluorescence intensity when introduced to the untreated TWNCNDs (Fig. 5d). In contrast, introducing other metal ions resulted in negligible or no noticeable alterations in fluorescence intensity. This observed fluorescence quenching phenomenon provided strong evidence for the selectivity of TWNCNDs toward Cu2+ ions. The mechanism underlying fluorescence quenching involves electron or energy transfer mediated by functional groups present on the surface of TWNCNDs. We further explored the sensitivity of this system to Cu2+ ions by examining the variations in fluorescence intensity with increasing Cu2+ concentration [29]. The study involved monitoring changes in fluorescence intensity after adding varying amounts of Cu2+ to a fixed concentration of TWNCNDs solution upon excitation at 305 nm. The results underscored the potential utility of TWNCNDs as a sensitive and selective probe for the quantification of Cu2+ ions.

2.4 Interfering analysis of TWNCNDs

In the study conducted, the selectivity of TWNCNDs as a fluorometric sensor was assessed by examining the emission responses in the presence of various competitive metal ions. The investigation aimed to evaluate the sensor’s ability to detect Cu2+ ions amidst other potentially interfering ions selectively [28]. The emission responses were studied under optimum conditions. The Cu2+ ions at a concentration of 70 nM were introduced into a TWNCNDs dispersion containing other competitive ions 80 nM. The emission spectra were recorded and analyzed to observe any changes in the emission intensity. The emission intensity corresponding to Cu2+ ions was observed to decrease, indicating quenching, as these ions formed complexes with the sensing probe [30]. The influence of other competitive ions on the emission band was investigated at higher concentrations 80 nM. Surprisingly, the presence of other competitive ions had an insignificant effect on the emission band, even at higher concentrations as shown in Fig. 6. The investigation revealed that TWNCNDs exhibited high selectivity and specificity towards Cu2+ ions over other metal ions. This study underscores the importance of understanding the selectivity profile of sensing materials, particularly in environmental monitoring applications, where accurate detection of specific ions is crucial for public health and safety.

Fig. 6
figure 6

Interference experiments of other metal ions

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2.5 Quenching mechanism

In dealing with fluorescent materials in the presence of quenchers, it’s important to understand the two primary mechanisms of fluorescence quenching: dynamic quenching and static-ground state quenching [31]. Dynamic quenching involves introducing the quencher into the fluorescent material solution, where it quickly diffuses and mixes with the fluorescent molecules. As a consequence, the intensity of the fluorescence diminishes following irradiation. This decrease in fluorescence intensity occurs due to the frequent collisions between the excited state fluorescent molecules and the quencher [32]. Think of it as the quencher “stealing” the energy from the excited fluorescent molecules, causing them to return to their ground state and emit fewer photons. Importantly, the degree of fluorescence quenching in dynamic quenching is influenced by the concentration of the quencher; when there’s a higher concentration of quencher molecules, there are more collisions, leading to more pronounced fluorescence quenching. This process is inherently time-dependent, with the rate of quenching related to the diffusion rate of the quencher molecules. On the other hand, static quenching involves a slightly different mechanism. In this scenario, the quencher interacts with the fluorescent material through chemical bonding, forming a complex. This complexation process results in certain fluorescent molecules becoming less susceptible to irradiation effects [33]. When the complex is excited by irradiation, it does not emit photons as it returns to the ground state. This is because energy from the excited state is utilized in the formation of new coordinate chemical bonds within the complex, effectively “locking” the energy and preventing its release as fluorescence [34, 35].

Consequently, static quenching reduces the intensity of fluorescence due to these chemical interactions between the quencher and the fluorescent material. Finally, both dynamic and static quenching mechanisms lead to a reduction in fluorescence intensity, but they do so through different processes. Dynamic quenching relies on collisions and energy transfer between the quencher and fluorescent molecules, while static quenching involves the formation of stable complexes and the subsequent inhibition of fluorescence emission due to chemical bonding within the complex. Understanding these mechanisms is crucial for interpreting and utilizing fluorescence quenching phenomena in various applications, including in your research.

2.6 Fluorescent ink application of TWNCNDs

Counterfeiting is a global issue with significant implications for governments, businesses, and consumers. Consequently, there is a pressing need for the development of anti-counterfeiting agents that facilitate the straightforward identification of counterfeit items such as currency, passports, official documents, and certificates [36]. The synthesized TWNCNDs display distinct fluorescence characteristics that hold immense practical value in real-world anti-counterfeiting applications. This field encompasses diverse materials, including carbon dots, rare-earth metals, polymer dots, perovskite materials, and metal-based nanoparticles. For instance, carbon dots offer advantages such as excellent dispersibility, non-toxicity, environmental friendliness, and cost-effectiveness. In the case of TWNCNDs, we employed polyvinyl alcohol (PVA) as the polymer matrix due to its numerous merits, including high hydrophilicity, biocompatibility, optical transparency, and ease of processing. Additionally, -OH groups within PVA’s molecular structure facilitate hydrogen bonding, enhancing the traceable UV-light recovery response [37]. To illustrate, we created “Letters” using a mixture of TWNCNDs and polyvinyl alcohol (PVA), which could be directly utilized as ink. These letters were allowed to air dry at room temperature, as depicted in Fig. 7a. In the open air, the low TWNCNDs mixture (0.15 wt%) appears colorless; however, it exhibits robust blue fluorescence when exposed to a 365 nm UV lamp (Fig. 7b). The fluorescent ink derived from TWNCNDs presents a cost-effective raw material and a straightforward synthesis process, rendering it a promising candidate for producing fluorescent inks. This innovation has paved the way for the effective utilization of synthetic TWNCNDs as a potent anti-counterfeiting agent.

Fig. 7
figure 7

Fluorescent ink Patterning agents are (a) under daylight and (b) under UV light

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2.7 Polymeric film for anti-counterfeiting

The TWNCNDs/PVA polymeric film has been proven as a suitable lighting material for advanced prevention of counterfeiting (AC) applications. When exposed to 365 nm UV light, this flexible polymeric flim material enables for secure and convenient information encryption and decryption, making it an excellent method of protecting sensitive data [38]. We can see that the synthesized TWNCNDs/PVA polymeric films have high optical transparency and are brightly luminous with various hues when exposed to ultraviolet light (Fig. 8a). The model picture may be seen quite clearly under the produced anti-counterfeit fluorescent polymeric film. TWNCNDs/PVA polymeric films exhibit blue fluorescence when exposed to UV light at 365 nm. We run a 60-day storage test to see how stable the TWNCNDs/PVA polymeric film [39]. Despite this extended interval (Fig. 8c), the emission color of the AC pattern remained constant. This strong and continuous blue emission demonstrates the ecological resilience of TWNCNDs/PVA, confirming its applicability for security applications. These findings give solid evidence that the TWNCNDs/PVA polymeric film is effective and durable in security applications. In Fig. 8b, d show photographic views of TWNCNDs/PVA films under various folds, illustrating their potential for a wide range of applications in the field of flexible electronics. This visual evidence demonstrates that when exposed to UV 365 nm excitation light, they create a vibrant blue glow, demonstrating that PVA does not impair TWNCNDs’ exceptional performance in photoluminescence tests. To broaden their applications, we combined TWNCNDs generated from an abundant and non-toxic source with PVA to form flexible and transparent polymeric films [40]. Bare PVA films were also tested without the addition of TWCNDs UV and naked eye films, as shown in Fig. 8. This technology has promise for lifting and detecting long-term exposed anti-counterfeiting applications. Furthermore, the materials utilized are widely available and safe. This study demonstrates the efficacy of interfacial segregation of TWNCNDs in polymer films, which opens up new possibilities for a variety of applications.

Fig. 8
figure 8

a The light transmission of the developed TWNCNDs/PVA polymeric films, (b) The TWNCNDs/PVA polymeric films are very flexible and easy to fold when exposed to UV 365 nm light, (c) TWNCNDs/PVA polymeric films aged up to 60 days, (d)Twisted films of TWNCNDs/PVA, (e) Without TWNCNDs in PVA polymeric films naked eye(left) and UV light(right)

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3 Conclusions

In conclusion, we found that a simple hydrothermal procedure can be used to manufacture heteroatom-doped carbon nanodots, and our research supports this finding. Termite wings were used as a carbon source, and ammonia solution was used as a nitrogen source. Extensive studies with XPS and FT-IR confirmed the efficient surface functionalization of carbon nanodots (TWNCNDs) with nitrogen heteroatoms. An XRD analysis revealed that the synthesized TWNCNDs were both amorphous and graphitic, with a spherical shape and an average particle size of 3.3 nm. The research results corroborated these observations. The unique optical characteristics of TWNCNDs were determined by fluorescence and UV-Vis spectroscopy. The detection of Cu2+ ions in aqueous solutions was made much simpler and more effective by employing a fluorescence-based approach, and TWNCNDs were utilized as the fluorescence probe. This method has an excellent detection limit of 0.1 µM and a linear detection range from 0 to 0.5 µM. Two types of fluorescence-suppressing mechanisms, one dynamic and one static, helped to promote the interaction between TWNCNDs and Cu2+ ions. The detection of Cu2+ ions in real samples was tested and found to be successful, validating the practical use of our technology. As a bonus, TWNCNDs have been successfully used as fluorescent ink for anti-counterfeiting purposes, where they have proven to be an effective, safe, and low-cost solution. As a result of their wide range of potential uses, TWNCNDs have recently gained attention as a promising choice for a fluorescence probe. Finding and lifting anti-counterfeiting apps that have been exposed for a long time is a potential use case for this technology. Additionally, the materials utilized are both easily accessible and non-toxic. The paper highlights the effective separation of TWNCNDs in polymer films, which can be used for multiple applications.

4 Experimental section

4.1 Materials and methods

Every metal salt was of analytical grade including Calcium nitrate tetrahydrate Ca(NO3)2.4H2O, Mercuric nitrate Hg(NO3)2.H2O, Lead(II) nitrate Pb(NO3)2, Palladium(II) nitrate Pd(NO3)2.2H2O, Manganese(II) nitrate Mn(NO3)2.4H2O, Zinc nitrate Zn(NO3)2, Rhodium(III) nitrate hydrate Rh(NO3).H2O, Nickel(II) nitrate hexahydrate Ni(NO3).6H2O, Iron(III) nitrate Fe2(NO3).9H2O, Copper(II) nitrate trihydrate Cu(NO3)2.3H2O, Arsenic trichloride As(Cl)3, Platinum(IV) Nitrate Pt(NO3)4, Silver nitrate Ag(NO3), Selenium dioxide SeO2, Cadmium nitrate tetrahydrate Cd(NO3)2.4H2O, Ammonium Molybdate (NH4)6Mo7O24, Cobalt(II) nitrate Co(NO3)2.6H2O, Aluminium Nitrate Al(NO3)3.9H2O, Chromium Nitrate Cr(NO3).9H2O, were obtained from avara chemicals. Quinine sulfate as a fluorescence standard reference (98%) was obtained from loba chemicals. Every experiment utilized deionized water.

4.2 Synthesis of TWNCNDs

A specific quantity of termite wings extract (50 mL) was combined with ammonia, then heated at 200 °C for 6 h in a high-pressure reactor for 2 h. The dark-brown mixture was dissolved in deionized water after allowing it to cool to room temperature. After 30 min of vigorous stirring using a magnetic agitator and 10 min of centrifugation at 12,000 rpm to eliminate larger particles, the resultant solution exhibited a dark-brown hue. Subsequently, this solution underwent activation dialysis, employing a molecular weight cutoff bag of 3000 Da, for a duration of 48 h. The outcome of this process was a clear, light brown solution, which was then meticulously labeled as TWNCNDs and appropriately stored at a temperature of 4 °C for further use in our research study.

4.3 Fluorescent quantum yield measurement

To determine the quantum yield (QY) of the TWNCNDs, we conducted a comparison between their absorbance values and integrated photoluminescence intensities in relation to a reference substance, quinine sulfate [17]. Quinine sulfate, known to have a quantum yield (QY) of 0.54, was dissolved in a 0.1 M H2SO4 solution, whereas the carbon dots (CDs) were being dispersed throughout the distilled water. The calculation for quantum yield (Φ) is expressed as follows:

$$\varPhi =\varPhi {\rm{R}}\times {\rm{I}}/{\rm{IR}}\times {\rm{AR}}/{\rm{A}}\times {\eta }^{2}/{\eta }^{2}{\rm{R}}$$
(1)

Here, Φ represents the quantum yield of the TWNCNDs, I is the integrated emission intensity, A is the optical density (OD), and η is the refractive index. This equation allows us to quantitatively assess the quantum yield of the TWNCNDs by comparing their fluorescence properties to those of the reference quinine sulfate.

4.4 Characterization of TWNCNDs

During the course of our study, an extensive array of spectroscopic techniques was employed to characterize the materials under investigation comprehensively. Ultraviolet-visible (UV-vis) spectra were acquired at room temperature using a Jasco UV3600 spectrophotometer, a reputable instrument crafted by Jasco. For fluorescence spectra, we utilized a spectrofluorometer from the same manufacturer, featuring a 10 mm optical path quartz cell and covering a wavelength range of 300–600 nm. The excitation wavelength was systematically varied, incrementally stepping from 290 nm to 360 nm. Fourier-transform infrared (FT-IR) spectra were obtained utilizing a Thermo-manufactured spectrometer, known as the Nicolet 380. In parallel, an X-ray diffractometer (PANalytical Empyrean) was employed to scrutinize the patterns resulting from powder X-ray diffraction (XRD). X-ray photoelectron spectra (XPS) were meticulously collected using a Shimadzu-manufactured spectrometer named the Kratos AXIS SUPRA. These analyses provided invaluable insights into the materials’ elemental composition and chemical states. To delve deeper into the microstructural characteristics, investigations through transmission electron microscopy (TEM) were conducted at room temperature. These examinations were carried out with the aid of a JEOL JEM-1010 transmission electron microscope. The utilization of this diverse array of spectroscopic and microscopic techniques enabled us to thoroughly elucidate the properties of the materials under scrutiny, providing essential insights that significantly contributed to the depth and comprehensiveness of our research.

4.5 Detection for Cu2+

In our experimental setup, we conducted Cu2+ measurements at room temperature (RT) in a PBS buffer solution with a pH of 3.5 and a concentration of 0.1 M. Typically, we filled a 3 mL fluorescence cuvette with 100 µL of TWNCNDs at a concentration of 1 mg/L, added 40 µL of PBS buffer solution, and introduced the corresponding Cu2+ standard solution. The volume was adjusted to a constant 3 mL by adding distilled water. After observing the process for 8 min, we recorded the fluorescence spectra at 305/410 nm for excitation and emission, respectively. The signal output value was determined by taking the difference between the fluorescence intensity of TWNCNDs in the absence of Cu2+ (F0) and in the presence of Cu2+ (F) and dividing by F0. Competing metals were also tested using this method.

4.6 Fluorescent ink of TWNCNDs

Fluorescent ink was made by combining a 10 mL solution of synthetic TWNCNDs with a polyvinyl alcohol (PVA) suspension at a concentration of 10%. After 30 min of magnetic stirring, the PVA was dissolved and the ink’s viscosity was increased. The resulting ink was poured into a dry pen cartridge and used to make notes on office paper. After that, it was exposed to UV light analysis after being air-dried at ambient temperature.

4.7 Flexible TWNCNDs/PVA polymer films

To make the TWNCNDs/PVA polymer matrix, first mix 15 g PVA crystals along with 5 mL TWNCNDs and 100 ml of deionized water (DI) in a three-necked, round-bottomed flask with a magnetic stirrer. The mixed solution was agitated at 60 °C for 4 h to ensure that the added fillers were evenly distributed throughout the polymer matrix. The solution mixture (105 mL) was cast over a glass plate coated with Teflon film (30 cm × 24 cm) for 72 h at 28 °C. The dried films were carefully peeled and stored in the humidity chamber (model FX 1077, Jeio Tech Co. Ltd., Ansan, Korea) at 25 °C and 50% relative humidity for 48 h before further study.