Introduction

As invaluable treasures of human civilization, grottoes reflect the advanced construction techniques and exceptional artistic accomplishments of ancient civilizations. They serve as witnesses to the exchange and mutual understanding between various cultures and document the prosperity of human societies throughout history. Grottoes are invaluable historical and cultural heritage with rich in artistic and academic significance, representing the shared wealth of all humanity. However, over time, grottoes suffer varying degrees of damage due to natural weathering and human activities, resulting in complex and diverse forms of deterioration (Fig. 1). Among the various types of damage, water seepage is one of the most severe and widespread, causing incalculable harm to murals and statues and significantly threatening their safe preservation and long-term preservation.

Fig. 1: Different deterioration types of the Leshan Giant Buddha.
figure 1

a Water seepage; b Delamination (c) Crust; d Microbial colonization. The reference scale used in the figure is a multi-functional reference ruler employed during fieldwork.

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Identifying the sources and paths of the water seepage responsible for this damage is fundamental to resolving this issue. However, due to the unique nature of cultural heritage conservation, only non-destructive methods can be used to study it. Consequently, methods of examining water damage in these structures are limited. Commonly used methods include geophysical techniques such as electrical resistivity tomography1, self-potential surveys2,3, ground-penetrating radar4, electrical sounding5, acoustic exploration6, and surface nuclear magnetic resonance7. However, these techniques were originally developed for detecting oil reservoirs, mineral deposits, and hydrogeological formations, which differ considerably from the preservation requirements of cultural heritage sites such as grottoes. Furthermore, many geophysical exploration studies on the conservation of grottoes and other cultural heritage sites have failed to meet expectations due to limitations in resolution, detection range, and parametrization. Therefore, addressing the shortcomings of existing technologies is an urgent task for researchers, prompting the exploration of new methods suitable for complex environments and the specific protection requirements of grottoes.

The tracer method is a direct and accurate approach to identifying the sources and paths of groundwater. A tracer into the potential water source or seepage path and tracks its movement with the water flow, thereby providing a clear, intuitive, and timely illustration of the seepage process8. Studies on revealing groundwater movement via tracer methods mainly focus on two aspects: On the one hand, groundwater movement is investigated using well-established tracers based on specific working conditions9, covering groundwater tracing in natural environments and engineering scenarios. For tracing groundwater movement in natural environments, substances inherently present in the environment are typically utilized as tracers, such as inorganic salts (e.g., Cl−, Br−, SO42−), halogenated hydrocarbons, alcohols, radioisotopes (e.g., 14C, 131I, 3H), stable isotopes (e.g., 2H,18O), trace elements (e.g., rare earth elements), or existing pollutants in such environments. These tracers are employed to study groundwater recharge, runoff, and discharge, as well as pollutant transport, among other processes10. In specific engineering projects, it is mainly used for identifying leakage channels in levees11, detecting landfill leakage12, locating outflow points and liquid production profiles of oil and gas wells13,14, evaluating the capacity of oil and gas reservoirs15, and assessing the closure performance of gas reservoirs. Besides natural tracers, artificial tracers are also used: artificial tracers are injected at potential groundwater sources, the content of the corresponding tracer in seepage water is detected, and seepage paths are inferred based on changes in the tracer content16.On the other hand, for the development of new types of tracers, laboratory experiments are conducted to determine whether the developed tracers can be applied to groundwater tracing, in accordance with the requirements for qualifying as a tracer. The research focus is primarily on investigating the water-borne migration characteristics of tracers by setting up laboratory test columns17. However, the potential environmental impact of certain tracers has restricted the use of tracer-based methods for assessing water seepage in cultural heritage sites like grottoes. The main challenge in examining water seepage in grottoes using the tracer method is the development of environmentally friendly tracers that do not harm grottoes.

Carbon quantum dots (CQDs) synthesized from green precursors are mainly composed of carbon (C) and primarily consist of the elements C, H, O, and N. Owing to their environmental friendliness and good biocompatibility, they have been widely applied in drug delivery, analytical chemistry, bioimaging, biosensing, food safety, chemical detection, and nanomedicine18. Nevertheless, on the one hand, the relatively high synthesis cost of CQDs has constrained their widespread application in tracing water seepage in grottoes. To address this limitation, this study adopted a biomass-based hydrothermal synthesis method to produce CQDs in large quantities on-site, which significantly reduces production costs and provides a basis for the practical application of CQDs in tracing water seepage pathways in grottoes19.

However, the uniqueness of cultural relics dictates that tracers must not only be environmentally friendly but also non-damaging to grotto temples. Since the minerals constituting the rocks of grotto temples are mainly alkaline compounds, they are prone to corrosion in acidic environments. Furthermore, if the tracer undergoes chemical reactions with the rock during the tracing process or deposits and crystallizes in seepage channels, it may increase or alter the rock’s porosity and fracture structure, thereby causing damage to grotto temples. Therefore, for the developed CQDs, further research is required on their morphological structure and chemical reaction characteristics with rocks. In practical tracer applications, the migration characteristics of a tracer with water are a key indicator for evaluating its performance, and the diversity of rock types and the complexity of mineral compositions can significantly affect the performance of fluorescent tracers20.

To summarize, in order to confirm whether the developed CQDs can be used for tracing seepage-induced deterioration in grotto temples, this study assessed the applicability of the synthesized CQDs for tracing seepage in grotto environments. This was achieved by sampling rock from the vicinity of the Leshan Giant Buddha (LGB) and conducting laboratory experiments to examine potential chemical interactions between the CQDs solution and the LGB sandstone. Additionally, the transport behavior and retention characteristics of the CQDs were evaluated using a laboratory-scale column packed with crushed sandstone collected from the LGB site.

Methods

CQD synthesis and characterization

Among the synthesis methods for CQDs, the hydrothermal method is environmentally friendly and easy to operate; thus, this study selected this method for CQD synthesis21,22. The synthesis methods of fluorescent CQDs can generally be divided into two categories: top-down approaches, which include arc discharge, laser ablation, electrochemical oxidation, and hydrothermal synthesis23,24,25,26, and bottom-up approaches, which mainly include combustion and pyrolysis methods27,28,29. Among these, the hydrothermal method was selected in this study due to its environmental friendliness, operational simplicity, and ability to produce CQDs in large quantities at low cost, which is critical for practical application in tracing water seepage pathways in grottoes21,30. To meet the requirements for large-scale preparation in the field, an autoclave (with a pressure value of 70 kPa) was employed as the preparation equipment. The specific operation was conducted as follows: Fresh ginkgo biloba (1300 g) was rinsed several times with deionized (DI) water; subsequently, it was placed into a 15 L autoclave containing 13 L of DI water, followed by heating the mixture at 120 °C for 3 h. After the mixture was cooled to room temperature, the liquid fraction was separated via filtration. The separated liquid fraction was then centrifuged at 12,000 rpm for 10 min to remove fine impurities. The resulting supernatant of fluorescent-containing CQDs was dialyzed over DI water for 2 days to remove all inorganic ions and molecules and produce a pure CQDs solution.

The purified CQDs solution was diluted 10 times with DI water to obtain a CQDs solution for tracing. For comparative analysis, two commonly used fluorescent tracers (rhodamine B and fluorescein sodium salt) were used to assess the suitability of the synthesized CQDs for tracing groundwater movement. These tracers were selected due to their widespread use in groundwater studies and water seepage control in buildings, as well as their commercial availability from chemical suppliers. Rhodamine B and fluorescein sodium salt were purchased from Aladdin and dissolved in DI water to form a 1 mg/L tracer solution.

The characteristics of CQDs were characterized (Fig. 2). A transmission electron microscope (TEM, JEM-2100F, Japan; accelerating voltage: 200KV, maximum magnification: 1.5M times) was used to characterize the morphology of the quantum dots, spacing of the crystal surfaces, and degree of crystallinity.

Fig. 2: Characterization of carbon quantum dots (CQDs).
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Detailed analysis and characterization methods for CQDs.

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The functional groups in the CQDs were identified using Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS). The powder samples obtained from drying the CQDs solution were mixed with potassium bromide (KBr) powder and pressed into pellets, which were tested using a spectrometer (Nicolet Nexus FTIR model 670, USA, measuring range 400–4000 cm−1). For XPS analysis, the dried CQDs powders were mounted on conductive adhesive tape and examined using an optoelectronic spectrometer (ESCALAB 250Xi, USA) equipped with a monochromatized Al Kα X-ray source (X-ray energy = 1486.6 eV). The binding energy scale was calibrated with the high-resolution C1s peak at 284.8 eV to ensure accuracy in chemical state determination. The fluorescence intensity of the CQDs was detected using a steady-state/transient fluorescence spectrometer (FLS-1000, UK, with 450 Watt Xenon Xe 900 as the light source).

Rock sample collection and analysis

In the peripheral area of the LGB rock carvings, samples were collected through core drilling using horizontal drill holes of ~25 mm in diameter after stripping the weathered surface. Fresh rock was drilled to a depth of ~40 cm, and the collected samples are presented in Fig. 3a. The samples were sent to the National Geological Testing Center for analysis. X-ray fluorescence spectrometry (XRFS) was used to quantify the major oxide contents, and X-ray diffraction (XRD) was employed to identify the primary mineral compositions. Then, the samples were brought back to the laboratory, crushed into granular form, and used for column experiments.

Fig. 3: Illustration of the Leshan Giant Buddha (LGB) sampling process.
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a Sampling point; b Collected samples; c Batch experiment showing the reaction of three tracer solutions (fluorescein sodium salt, rhodamine B, and CQDs) and DI water with rocks; d Packed column experiments.

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Batch experiment

To increase the contact surface area between rocks and the tracer, ensure full contact between the rocks and the tracer solution, accelerate the reaction rate between the tracer solution and the rocks, and increase the adsorption capacity of the rocks for the tracer, the collected rock samples were first crushed into particles (average particle size: 0.15 mm) for the experiments. The experimental setup included control and experimental groups, all of which were conducted under the same conditions (Fig. 3c). The prepared tracer solutions (CQDs, rhodamine B, fluorescein sodium salt) and DI water (with concentrations of Ca2+, Mg2+, and Na+ <0.5 mg/L, and that of K+ <1.0 mg/L) were added to four 500 mL conical flasks. Crush sandstone (100 g) was added to each flask, and the mouths of the flasks were sealed with parafilm. Samples were collected every 24 h using a pipette (sampling volume: 10 mL), with 14 samples collected over 14 d.

Cations in the collected samples were detected using an ICP mass spectrometer with a testing error of less than 5%. Prior to analysis, the water samples were filtered through a 0.22 μm membrane to remove insoluble impurities and transferred to clean bottles for ion content analysis.

Packed column experiments

The collected rock samples were pretreated via crushing and sieving (passed through a 100-mesh sieve, with an average particle size of 0.15 mm), and then uniformly packed into a simulation device—a Plexiglas column with an inner diameter of 5 cm and a length of 30 cm (Fig. 3d). A glass wool mesh sieve was placed at the bottom of the column to prevent the loss of rock particles through the bottom mesh, and a layer of gravel particles (~3 cm in thickness) was laid on top of the packed rock particles to avoid particle migration and volume changes during the experiment.

In addition to the Plexiglas column (used for holding the crushed sandstone), the experimental setup also included a water tank placed above the column and connecting infusion tubes between the water tank and the column (Fig. 3). A hole was drilled on the side of the water tank to ensure that the water level inside the tank could not exceed the height of this hole. During the experiment, water was continuously added manually to the tank at a low flow rate to maintain a constant water level in the tank.

The dried crushed sandstone was fully saturated with deionized (DI) water and then gradually deposited into the glass columns in 1.5 cm increments, ensuring that a 0.5–1 cm thick water layer remained at the top. After packing, all the crushed sandstone in the column was completely removed, dried, and then placed into a 1000 mL graduated cylinder filled with water. The volume of the crushed sandstone was determined based on the change in the water level scale of the graduated cylinder; by combining this volume with the internal volume of the glass column (device), the porosity of the packed column was calculated to be 37%. Subsequently, the dried crushed sandstone was repacked into the glass column following the same procedure as the initial packing.

Three experiments were conducted in the pre-saturated sandstone columns to assess the transport and retention properties of the CQDs (Fig. 2). First, DI water was injected, and the column was flushed for 24 h to ensure a reliable baseline before starting the fluorescent experiment. This step prevented interference with the detection of the target fluorescent tracer and stabilized the flow rate.

Subsequently, the tracer solutions (CQDs, rhodamine B, and fluorescein sodium salt) were injected. The specific operation was as follows: Flow clamps were used to clamp the effluent collection tubes and the lower part of the tubes connecting the water tank and the column. The water in the tubes was drained, and the water tank was replaced with one containing the tracer solution. When the tubes above the flow clamps were filled with the tracer solution (if air bubbles were present, tap the infusion tubes by hand to expel the bubbles), the flow clamps were opened. After the tracer solution was injected at the top of the column, a sample bottle was placed at the bottom to collect the effluent. Based on the volume and porosity of the sandstone column, the volume corresponding to one pore volume (PV) was calculated to be ~240 mL, and four samples were collected per pore volume. The pore volume (PV) of the packed column was calculated as the product of the column bulk volume and porosity, resulting in ~240 mL for the experimental setup. The injection volume of the tracer solution was set to five pore volumes (5 PV) to ensure sufficient mass recovery and reliable breakthrough curve characterization. Using a smaller injection volume may lead to incomplete elution, thereby underestimating the adsorption and transport parameters. Slightly larger injection volumes have minimal impact on the transport parameters, making 5 PV a robust choice for tracer tests.

To ensure the accuracy and reliability of the experimental results, a total of 1200 mL of tracer solution (equivalent to 5 pore volumes, PV) was injected, followed by an equal volume of deionized (DI) water, which was added using the same method described above.

The experimentally collected samples were labeled and analyzed using a FLS. The fluorescence intensities of the CQDs, rhodamine B, and fluorescein sodium salt in each sample were measured as the arithmetic means of the emission wavelengths: 429–439 nm for CQDs, 571–581 nm for rhodamine B, and 508–518 nm for fluorescein sodium salt.

Results

Characterization of the properties and environmental stability of CQDs

In terms of size and structure, TEM and high-resolution TEM analyzes showed that the CQDs had a size range of 2–6 nm and an average size of ~3 nm (Fig. S1). This small size enabled the CQDs to migrate easily through cracks and pores, with minimal aggregation and sedimentation during the tracing process. Additionally, selected area electron diffraction revealed a circular halo for the CQDs, confirming their amorphous structure—this nature mitigated the risks associated with crystalline tracers (which can recrystallize in microcracks of artifacts and exacerbate structural damage). Meanwhile, the CQDs exhibited excellent fluorescence performance (Fig. S2): their fluorescence signal remained detectable even after dilution, with a strong photoluminescence (PL) response observed at concentrations as low as 2.0 ng/L19.

In terms of chemical composition and functional groups, FTIR spectroscopy confirmed that the CQDs contained O–H, C–H, C=C, carbonyl (C=O), N–H, C–O–C, and C–O functional groups (Fig. S3). Furthermore, XPS provided complementary verification: the full XPS spectrum showed the CQDs primarily consisted of carbon, oxygen, and a small amount of nitrogen, while the fitted high-resolution C1s spectrum identified five bonding types (C–C at 282.6 eV, C–N at 285.9 eV, C=C at 284.2 eV, C–O at 285.2 eV, and C=O at 286.5 eV), consistent with the functional groups detected by FTIR19. Notably, the CQDs also possessed hydrophilic groups (e.g., carboxyl, hydroxyl, and a small number of amino groups), which contributed to their excellent water solubility. The CQDs in this study are classified as inorganic substances with organic characteristics. Compared with CQDs containing cycloalkane structures, they exhibit superior performance in terms of biocompatibility and environmental friendliness31.

Different fluorescent agents exhibit varying sensitivities to pH and are therefore suitable for different pH environments. Notably, the CQDs in this study maintain stable fluorescence under different pH conditions: their fluorescence intensity reaches a maximum of 19.82 in a neutral environment; while the intensity decreases in alkaline and acidic environments, the degree of reduction is small, with values of 18.76 and 17.93, respectively. Generally, the fluorescence behavior of CQDs is sensitive to pH, and such sensitivity varies across different types of CQDs.

Compared with CQDs synthesized using 1-naphthylamine and trichloromethane as precursors (which exhibit an ultrasensitive spectral redshift property at pH < 5.632) and those prepared via hydrothermal treatment of glucose (Glc) (which show the maximum fluorescence intensity at pH = 3, with a significant decrease in intensity as pH increases33), the CQDs developed by our team exhibit superior fluorescence performance in natural water environments (neutral and weakly alkaline water bodies). Thus, they demonstrate greater advantages as tracers for seepage water in grotto temples.

Temperature is also one of the key factors affecting the fluorescence intensity of CQDs (Fig. 4). The CQDs developed by our team maintain stable fluorescence within the temperature range of 0–50 °C: the fluorescence intensities at 25 °C and 50 °C show a small difference, with values of 19.82 and 19.7, respectively; while in an ice-water mixture at near 0 °C, the fluorescence intensity is slightly lower, at 19.01.

Fig. 4: Fluorescence intensity of CQDs under varying aquatic environmental conditions.
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a Different pH values; b Different temperatures; c Different salinities.

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Compared with CQDs prepared via hydrothermal treatment of glucose (which exhibit a significant linear decrease in fluorescence intensity as temperature increases within the range of 15–60 °C33), the CQDs in this study exhibit more stable fluorescence under natural water temperatures. Their temperature stability is comparable to that of CQDs synthesized using ascorbic acid and p-phenylenediamine as precursors (the fluorescence intensity remains unchanged below 30 °C, and above 30 °C, the intensity decreases with increasing temperature but with a small reduction amplitude34). However, the latter are far inferior to the CQDs developed by our team in terms of environmental friendliness and biocompatibility.

The fluorescence intensity of the CQDs in calcium bicarbonate (Ca(HCO3)2) solution is lower than that in deionized water, with a value of 18.15, but the reduction amplitude is small. Generally, the fluorescence intensity of CQDs decreases in solutions with high concentrations of metal cations and may even quench; different metal ions exhibit varying effects on the fluorescence intensity of CQDs. Among these ions, Ca2+ and Mg2+ have relatively minor effects, while Cu2+, Fe2+, and Co2+ exert more significant effects on the fluorescence intensity of CQDs35. Notably, the cations in groundwater and seepage water in the LGB area are dominated by Ca2+, and the developed CQDs maintain stable fluorescence intensity in Ca2+-dominated aqueous solutions.

CQDs prepared using ginkgo biloba leaves as the precursor exhibit high stability under different pH conditions and temperatures. Compared with CQDs synthesized using citric acid, xylitol, and other substances as precursors, they show better stability in natural aquatic environments and are suitable for tracing the seepage paths and sources of groundwater in different aquatic environments dominated by Ca2+36.

Chemical reactions of tracer solutions with rock

The CQDs contained a significant number of carboxyl functional groups, whereas the rocks were primarily composed of carbonate, silicate, and rock salt minerals. Therefore, the CQDs tracer may react with surrounding rocks, and the resulting ions depend on the type of rock minerals.

The primary rock stratum at the LGB site is the Upper Cretaceous Jieguan Formation, which is extensively distributed across the LGB area and its surroundings. It stretches from Jiufeng Town in the south to Foguang Park in the north, encompassing Greenheart Park within the urban zone. The predominant lithology consists of thick brownish-red and purple-red layers of feldspar sandstone and feldspar quartz sandstone, with minor interlayers of purple-red muddy siltstone37. Purple–red conglomerates and conglomerate-bearing sandstones are commonly found at the base of the Jieguan Formation. The stratigraphic structure is well-developed, featuring large parallel laminations, plate-like interlayers, oblique laminations, wave marks, microfine horizontal textures, salmon laminations, and other sedimentary features.

The rock mainly contained Si, O, Ca, Al, Mg, Na, K, C through XRFS. The most abundant component was SiO2, accounting for 55.69%. Al2O3 was the second most prevalent oxide, accounting for 17.44%, followed by CO3 (11.29%). Other oxides, including CaO, Na2O, and K2O, were present at lower concentrations, accounted for 3.69%, 1.22% and 2.69%, respectively. In addition, the lowest amount of MgO (0.96%) was observed.

The sedimentary rocks were predominantly composed of quartz, feldspar, and sedimentary rock fragments through XRD (Fig. 5). Furthermore, the mineral composition of the rock samples was analyzed, revealing that the minerals mainly included quartz, sodium feldspar, micro plagioclase feldspar, calcite, montmorillonite, and dolomite. Among these, silicate minerals were dominant. Sodium feldspar, quartz, micro plagioclase feldspar and montmorillonite were 35.17%, 33.9%, 9.98% and 5.07%, respectively. Followed by carbonate minerals, and dolomite and calcite accounted for 7.99% and 7.86%.

Fig. 5: Mineralogical composition of the rock sample.
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a Overall mineral composition; b Enlarged view showing Qz (quartz), Pl (plagioclase feldspar), and Lc (lithic clast).

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Based on the mineral composition of the rock samples, the CQDs would be react with the silicate and carbonate minerals in the rocks during the water seepage tracing process. Quartz was chemically stable, whereas montmorillonite, a secondary mineral, was typically formed in response to chemical reactions. Thus, the primary minerals that underwent the chemical reactions were sodium feldspar, micro plagioclase feldspar, and calcite.

Under natural conditions, chemical reactions between rock and water generate metal cations. Therefore, the chemical reactions between the tracer solution and rock can be divided into two components: (1) the reactions between the tracer and rock; (2) the reactions between water and rock. The major elements in natural water are directly associated with the rock minerals that participate in chemical reactions and with the rate of those reactions. The LGB is situated in a geological stratum composed primarily of silicate and carbonate minerals. Carbonate minerals, such as MgCO3, CaCO3, and CaMg(CO3)2, react with water to produce Mg2+ and Ca2+ ions. Silicate minerals react under acidic conditions to produce Na+ and K+. Therefore, as water interacts with rock, the concentrations of Ca2+, Na+, and K+ in the water increase. The extent of this increase is controlled by the mineral composition and the rate of the water–rock reactions.

Among the four major cations produced by the chemical reactions between DI water and rock sample, the increase in Ca2+ was the most significant, increasing from <0.5 mg/L at baseline to 16.9 mg/L, with an average concentration of 12.9 mg/L thorugh ICP mass spectrometer. The increases in Mg2+ and Na+ were lower, with concentrations rising from <0.5 mg/L to 1.4 mg/L and 3.5 mg/L, respectively. The average concentrations were 1.0 mg/L for Mg2+ and 3.3 mg/L for Na+. The concentration of K+ exhibited little variation, remaining below 1.0 mg/L in DI water, and the final measured concentration was 1.0 mg/L, with an average of 1.1 mg/L in the samples.

The chemical reaction between calcite and water was the most significant during the experiment. Although calcite constituted a smaller portion of the rock composition than feldspar, the rate of reaction between carbonate minerals and water was significantly (approximately one million times) faster than that of silicate minerals. Therefore, in rocks containing carbonate minerals, the chemical reactions primarily involved carbonate minerals, which resulted in a higher concentration of ions from carbonate minerals in the water than those from silicate minerals. The small change in the K+ concentration suggested that potassium, which was less involved in the chemical reaction, had a minimal impact on the overall process (Table 1).

Table 1 Difference in ion concentrations between the tracer solution and blank sample, and corresponding error averages
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As the time of reactions between the tracer solution and rock increased, the concentrations of Ca2+, Mg2+, and Na+ in the solution also gradually increased, whereas the K⁺ concentration fluctuated with minimal change. The trend of ion concentration variation mirrored that observed in the chemical reaction between water and rock (Fig. 6). This suggested that the cations generated during the chemical interaction were mainly a result of the water-rock reactions, with additional cations possibly formed by the reaction between the tracer solution and rock.

Fig. 6: Variation of cation concentrations in different tracer solutions.
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a Ca2+; b Mg2+; c Na+; d K+.

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The concentrations of Ca2+, Mg2+, Na+, and K+ in the collected samples of the CQDs solution ranged from 9.4 to 16.8 mg/L, 0.7 to 1.4 mg/L, 2.9 to 3.5 mg/L, and 0.9 to 1.2 mg/L, respectively. The mean concentrations of these cations were 13.2 mg/L, 1.1 mg/L, 3.5 mg/L, and 1.0 mg/L. Among samples collected at the same time, the differences in ion concentrations between the CQDs solution and DI water were 0.6 mg/L for Ca2+, 0.1 mg/L for Mg2+, 0.2 mg/L for Na+, and 0.0 mg/L for K+. However, due to measurement errors in the ion concentration determination, the observed differences in ion concentrations could not be definitively attributed to the chemical reaction between the CQDs tracer and rock. The errors associated with these measurements were considered. Using an error propagation formula, the mean errors in the differences in concentrations of Ca2+, Mg2+, Na+, and K+ generated by the chemical reaction between the CQDs solution and rock, compared to that between DI water and rock, were 0.9 mg/L, 0.1 mg/L, 0.2 mg/L, and 0.1 mg/L, respectively. As these errors exceeded the observed concentration differences, it was concluded that the CQDs solution did not significantly react chemically with the surrounding rocks.

Similarly, the differences in ion concentrations resulting from the interactions between rhodamine B or fluorescein sodium salt solutions and the rock, when compared to those with DI water, also fell within the margin of measurement error. This indicated that neither rhodamine B nor fluorescein sodium salt underwent significant chemical reactions with the rock.

Transport properties of CQDs in packed columns

In the CQDs tracer solution experiment, the fluorescence intensity of the exudate collected at the bottom of the sample column was 60 nm before tracer injection. This baseline fluorescence intensity was likely due to natural fluorescent compounds in the rock samples, despite a 24-h DI water wash to remove potential contaminants. The fluorescence intensity at this stage reflected the background value. At the beginning of the CQDs solution injection, no fluorescence peaks from the CQDs were observed in the exudate. As the injection continued, the fluorescence peaks of the CQDs gradually appeared and intensified. After injecting 1200 mL (5PV) of the tracer, the injection was stopped, and DI water was began. Initially, the fluorescence intensity remained high; however, it decreased sharply as DI water was injected (Fig. 7a, b).

Fig. 7: Fluorescence spectral characteristics of exudates in migration tests.
figure 7

(a, b) CQDs solution, (c, d) Rhodamine B solution, (e, f) Fluorescein sodium salt solution as tracers. (a, c, e injected tracer solution; b, d, f injected DI water).

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Before injecting the rhodamine B solution, no significant wave peaks corresponding to rhodamine B were observed in the exudate. The background fluorescence intensity was low, at ~100 a.u through FLS-1000. No wave peaks appeared until the tracer injection reached less than 3 PV. After 3PV of the tracer solution were injected, the fluorescence peaks corresponding to rhodamine B began to appear, and the intensity of these peaks gradually increased. After 1200 mL (5PV) of the tracer was injected, the injection was stopped, and DI water was introduced. Initially, the fluorescence intensity of the exudate remained high, peaking at 1589 a.u.; however, it decreased with further DI water injection. Even after injecting 1200 mL (5PV) of DI water, the fluorescence peaks of rhodamine B remained in the exudate (Fig. 7c, d).

In the fluorescein sodium salt solution, no fluorescence peaks were detected before the tracer was injected, indicating the absence of interfering natural fluorescent compounds in the rock samples. Initially, no fluorescence peaks of fluorescein sodium salt were detected in the exudate. However, as the injection continued, the fluorescence intensity of the exudate rapidly increased. During DI water injection, the fluorescence intensity of the exudate was initially high but gradually decreased. After ~5PV of DI water were injected, fluorescein sodium salt fluorescence peaks were no longer observed in the exudate (Fig. 7e, f).

Among the three tracers, rhodamine B exhibited the weakest migration ability, whereas fluorescein sodium salt and CQDs showed similar and comparatively stronger migration abilities. No fluorescence peaks for any of the tracers were detected in the exudate until less than one PV of the tracer was injected. When one PV of the tracer was injected, the fluorescence intensity of CQDs and fluorescein sodium salt in the exudate increased sharply, with the increase in fluorescence OF CQDs being more pronounced. After injecting 2–5PV of the tracer, the fluorescence intensities of the CQDs and fluorescein sodium salt remained relatively stable. In contrast, the fluorescence intensity of rhodamine B in the exudate gradually increased after injecting 3 PV of the tracer, exhibiting a background level of intensity between 0 and 3 PV of the tracer solution, which gradually increased. The fluorescence intensity of fluorescein sodium salt and CQDs remained at background levels between 0 and 0.75 PV, then sharply increased between 0.75 and 1.75 PV and stabilized between 1.7 and 5 PV.

During DI water injection, the fluorescence intensity of the CQDs and fluorescein sodium salt remained relatively stable and was high between 0 and 0.75 PV. However, the fluorescence intensity of rhodamine B continued to increase, peaking at 0.75 PV. Between 0.75 and 1.25 PV, the fluorescence intensity of the CQDs and fluorescein sodium salt decreased sharply, and their intensity continued to decrease between 1.25 and 2 PV, although at a slower rate. After further DI water injection, no significant fluorescence peaks of the CQDs and fluorescein sodium salt remained in the exudate, indicating that most of the tracers had been flushed out of the sample column. During DI water flushing, rhodamine B fluorescence displayed a two-phase trend: an increase from 0.75 to 1.25 PV, followed by a decline from 1.25 to 5 PV. Even after five PV of DI water had been injected, rhodamine B fluorescence remained relatively high, indicating that a substantial portion had not yet exited the column.

In summary, rhodamine B displayed the weakest migration ability, whereas fluorescein sodium salt and the CQDs exhibited comparable and stronger migration abilities (Fig. 8). The migration characteristics of rhodamine B and fluorescein sodium salt in water were consistent with previous research findings, which further validates the reliability of this study38. As a fluorescent tracer, the CQDs have significantly lower migration capacity in groundwater compared to ³H and soluble salts, but they outperform EDTA, boron, ammonium, dichromate, picric acid, and salicylic acid39,40.

Fig. 8: Change in fluorescence intensity of different solutions.
figure 8

Variation of fluorescence intensity over time for various tracer solutions.

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Although 3H and soluble salts are effective tracers, their environmental risks render them unsuitable for monitoring water seepage damage in grottoes, making the CQDs an excellent alternative. Compared with CQDs synthesized using a combination of citric acid and PSS polymer, the CQDs in this study exhibit stronger migration capacity in column experiments with less tailing effect41. While CQDs synthesized using xylitol as the precursor show good migration capacity in sand columns under high-concentration salt solution conditions, it is necessary to further investigate the migration capacity and tracing performance of the CQDs developed by our team in different rock types under high-salinity conditions in subsequent studies.

Discussions

The tracer method is widely used to explore the paths and sources of water seepage; however, tracers currently available cannot be used to investigate water seepage in grottoes. This study developed a large quantity of environmentally friendly CQDs to avoid damage to grottoes by implementing a hydrothermal method using Ginkgo biloba as the precursor. The synthesized CQDs exhibited uniform, nanoscale particle size, an amorphous structure, abundant surface hydrophilic functional groups, and strong fluorescence intensity detectable even at low concentrations. These characteristics make CQDs ideal candidates for tracing the sources and flow paths of water seepage in grottoes.

To further evaluate the feasibility of using CQDs as tracers in grottoes, laboratory experiments were conducted to investigate their chemical interactions with the surrounding rock, using the LGB as a representative case. Moreover, this study examined their migration properties and compared the advantages and disadvantages of these properties with those of other commonly used fluorescent tracers, namely, fluorescein sodium salt and rhodamine B.

The difference between the ions generated by the chemical reaction of the tracer solution and DI water with the rock was within the error range. Therefore, the main reactant in the chemical reaction of the tracer solution with rock was water rather than the tracer (CQDs, rhodamine B, or fluorescein sodium salt). The chemical reaction of CQDs with the surrounding rocks was minimal and could be ignored. The CQDs have a very low probability of causing damage to rocks due to chemical reactions during the tracing process.

In the migration experiments, the fluorescence intensity of fluorescein sodium salt and CQDs in the exudate during tracer solution injection exhibited a consistent trend, which could be divided into three stages: low fluorescence intensity (0–0.75 PV), rapid increase (0.75–1.75 PV), and high fluorescence intensity and stability (1.75–5 PV). The fluorescence intensity of rhodamine in the exudate was low (0–3 PV) and gradually increased (3–5 PV). During the injection of DI water, the fluorescence intensity of fluorescein sodium salt and the CQDs in the exudate exhibited the same trend: stable change followed by a rapid decline, finally reaching the regional background value. The fluorescence intensity of rhodamine B was high and gradually decreased, with some rhodamine B remaining in the exudate at the end of the experiment. Rhodamine B exhibited poor migration capability in the rock. CQDs demonstrated limited chemical reactivity with the surrounding rock and showed favorable migration performance.

The amorphous structure of the CQDs ensures that they do not undergo crystallization in seepage channels, which would otherwise lead to the enlargement of pores and fractures; their negligible chemical reactions with rocks further ensures that they cause minimal damage to grotto temples during the tracing process; additionally, their uniform nano-sized particles, migration performance comparable to that of fluorescein sodium salt, and excellent fluorescence performance collectively guarantee their favorable tracing effect. Therefore, CQDs offer a promising and environmentally safe solution for tracing seepage sources and mitigating related damage in grottoes.