Introduction

Nowadays, the environmental pollution caused by the discharge of industrial, agricultural, and household wastes has attracted people’s attention. One of them is the water pollution caused by heavy metal ions. Among various heavy metal ions, chromium (III) ion (Cr3+) plays a leading role in glucose metabolism and lipid metabolism, and is one of the essential trace elements for human body [1]. Deficiency of Cr3+ may lead to different health disorders, including disorders of lipid metabolism and diabetes. Likewise, high concentration levels of Cr3+ can also be harmful to our health and negatively affect cellular structure [2]. Another polluting heavy metal ion is mercury (II) ion (Hg2+). Hg2+ readily penetrates the skin of respiratory and gastrointestinal tissues and causes irreversible damage to the central nervous system, leading to renal failure and various cognitive and motor impairments [3]. Meanwhile, the non-biodegradability and bioconcentration of Hg2+ in the environment bring about various serious diseases, including kidney diseases, skin diseases, and genotoxicity [4]. Even when present in trace amounts, it poses a serious biological threat. In consequence, it is necessary to explore an effective and rapid method to detect Cr3+ and Hg2+ in environmental samples. Until now, researchers have developed various methods for the detection of trace metal ions, such as atomic absorption\emission spectrometry [5], electrochemical techniques [6], and fluorescence spectrophotometry [1]. Among them, the fluorescence method has attracted much attention for its unique advantages such as simplicity, fast response, good selectivity, and high sensitivity.

MXene, whose chemical formula is Mn+1Xn, is a two-dimensional (2D) transition metal carbide or nitride, where M is the early transition metal, including Sc, Ti, and V, and X represents C and/or N. MXene is usually obtained by etching the Mn+1AXn phase (where A usually stands for elements III A or IV A), such as the typical material Ti3AlC2. Due to its abundant active sites, good biocompatibility, chemical stability, and surface hydrophilicity, MXene has attracted the attention of many researchers. It shows very good potential for applications in batteries, supercapacitors, photocatalytic applications, sensors, water treatment, and other fields [7,8,9]. The zero-dimensional (0D) MXene QDs derived from 2D MXene nanosheets (NSs) are able to inherit the advantages of MXene NSs in addition to showing unique optical and optoelectronic properties. It has promising applications in bioimaging, biosensing, antioxidant, antibacterial, ion detection, etc. [10,11,12,13]. Potassium permanganate (KMnO4) is a common oxidant with a wide range of applications. After increasing the interlayer spacing by alkalinizing MXene NSs with potassium hydroxide (KOH), KMnO4 was added and metal ions were further inserted into the interlayer to provide tunable properties. Furthermore, MXene can act as a reducing agent and undergo a redox reaction with KMnO4, and the generated manganese dioxide (MnO2) provides more active sites for the material [14, 15].

In the previous literatures, there are a lot of sensing materials for detecting metal ions. For example, Chen Lin et al. studied the organic framework of double layer interpenetrating double emission luminescent metal as Cr3+ ratio sensor [16]. Yi Zhang et al. synthesized CdS/CdS QDs as fluorescent sensors and liposome carriers for signal amplification to detect Hg2+ [17]. However, fluorescence sensors are often designed for one target detection, while those designed for simultaneous detection of two or more metal ions are rarely reported [18,19,20,21]. Besides, there are few reports on nanoprobes for the detection of Cr3+ and Hg2+ ions based on the MXene material field. Therefore, it is still a challenge to explore the relationship between MXene and multi-ion detection of heavy metals.

In this paper, Ti3C2 QDs (QDs) were firstly synthesized by ammonia-assisted hydrothermal method. Using ammonia (NH3·H2O) as nitrogen source and KMnO4 as functionalized reagent, KMnO4-functionalized Ti3C2 QDs (Mn-QDs) were synthesized by hydrothermal shearing. The Mn-QDs exhibit bright blue-green under ultraviolet light and have an emission peak at 435 nm. Interestingly, compared with QDs without KMnO4 treatment, Mn-QDs can specifically bind Cr3+ or Hg2+. In the range of 0–2600 μM Cr3+, Mn-QDs showed fluorescence enhancement by fluorescence measurement. After the addition of 0–550 μM Hg2+, Mn-QD fluorescence decreased in the range of 0–50 μM and fluorescence increased in the range of 50 to 350 μM. As a result, we mainly explored a “turn-on” type nanoprobe for Mn-QDs that particularly detects double ions of Cr3+ and Hg2+. To the best of our knowledge, this is the first attempt to functionalize QDs with KMnO4 and apply them to the detection of bimetallic ions. This study opens up a new approach for the functional synthesis of Mn-QD nanomaterials and expands the application of MXene in ion monitoring of environmental pollution.

Experimental section

Chemical and apparatus

Related reagents and instruments have been included in the Electronic Supporting Information.

Preparation of Ti3C2 QDs (QDs)

The synthesis of QDs is an improvement on the previously reported process [22, 23]. Two grams of Ti3AlC2 powder was added to 40 mL 40% HF, stirred at 60 °C for 20 h, washed with pure water, centrifuged at 10,000 rpm for 10 min to collect precipitation, and vacuum dried at 60 °C for 24 h to obtain Ti3C2 MXene powder. Then, 0.25-g powder was placed in 17.5 mL deionized water and ultrasound was performed for 30 min, and 7.5 mL NH3·H2O was added subsequently. The mixture was transferred to a Teflon lined stainless steel autoclave and heated at 120 °C for 6 h. Finally, after the hydrothermal reaction has cooled to room temperature and centrifuged at 14,000 rpm for 30 min, the colorless and clear supernatant was retained and heated at 90 °C for 2 h to remove excess ammonia. QDs were obtained by 100-nm membrane filtration. The prepared colorless and clear solutions of QDs were stored at 4 °C for further characterization and application.

Synthesis of KMnO4-functionalized Ti3C2 QDs (Mn-QDs)

The synthesis procedure of Mn-QDs is shown in Scheme 1. Briefly, the etched Ti3C2 MXene powder was added into 100 mL 3 mM KOH and stirred for 8 h. The turbid solution was centrifuged at 1000 rpm for 10 min, washed for several times, and dried, and the alkalized MXene powder was gained. The treated MXene powder was mixed with 10 mL 0.12 M KMnO4 and 12 mM KCl solution, stirred for 30 min, then transferred to the reaction kettle, and heated at 140 °C for 8 h. The reaction was cooled before performing a series of centrifugation and washing and drying procedures. And KMnO4-functionalized MXene powder was gained. Afterwards, Mn-QDs can be obtained by the above QD synthesis operation. Similarly, the prepared material solution with light yellow was stored at 4 °C for later use. Mn-QD powder was obtained by freeze-drying for the further experimental characterization.

Scheme 1
scheme 1

Schematic representation for the preparation of Mn-QD nanoprobes and their applications for sensitive and selective fluorescence detection of Cr3+ and Hg2+ ions

Full size image

Determination of Cr3+ and Hg2+ ions

In this experiment, we took 10 μL Mn-QDs and added a certain concentration of Cr3+ or Hg2+, mixing with pure water to a total volume of 400 mL. After reaction at room temperature for 10 min, FL measurements were recorded with 246-nm excitation. To test the selectivity of this material, other metal ions including Na+, K+, Al3+, Mg2+, Mn2+, Co2+, Ni2+, Fe3+, Fe2+, Cu2+, and Ag+ were also examined by the same procedure. To determine whether the non-functionalized QDs interact with metal ions, the same series of ions (Na+, K+, Al3+, Mg2+, Mn2+, Hg2+, Co2+, Cr3+, Ni2+, Fe3+, Fe2+, Cu2+, and Ag+) mixed with 100 μL QDs were also performed. The FL (λex = 232 nm) of different ions were measured and compared with the above results. The emission and excitation slit widths of both QDs and Mn-QDs are 5 nm and 10 nm, and the measurement voltage is 700 V.

Fluorescence detection of nanoprobes in real samples

The practicality and feasibility of the Mn-QD nanoprobes for the detection of Cr3+ and Hg2+ in environmental samples were tested using two water samples, Runxi Lake and tap water, collected from the campus of Nanchang University. Prior to analysis, the water sample was treated by centrifugation to remove suspended particles. Then, Mn-QDs and known concentrations of Cr3+ and Hg2+ were added to the sample to determine the FL response of the solution. Calculate the recovery rate using the relationship in Formula 1:

$$\mathrm{Recovery}\left(\%\right)=\hspace{0.17em}\frac{\mathrm{Concentration}\;\mathrm{of}\;\mathrm{Cr}^{3+}\;\mathrm{or}\;\mathrm{Hg}^{2+}\;\mathrm{found}}{\mathrm{Concentration}\;\mathrm{of}\;\mathrm{Cr}^{3+}\mathrm{or}\;\mathrm{Hg}^{2+}\;\mathrm{added}}\times100$$
(1)

Results and discussion

Material characterization

On the one hand, the morphology, structure, and composition of the material were characterized in detail by TEM, XRD, XPS, and FTIR. The TEM images of the QDs and Mn-QDs are shown in Fig. 1. QDs and Mn-QDs exhibited nearly spherical particles with well monodispersity. The average lateral size of QDs was 1.64 ± 0.30 nm (n = 100), and the particle size distribution was mainly in the range of 1.2 to 2.4 nm. The average size of Mn-QDs was 2.24 ± 0.43 nm (n = 100), and the particle size distribution was mainly in the range of 1.6 to 3.2 nm. The crystallographic features of Mn-QD nanomaterials were further revealed by HRTEM images. Lattice fringes with spacing of 0.26 nm in the inner plane were measured (interpolation in Fig. 1E), corresponding to the (010) facet of Ti3C2 MXene [24, 25], which highlighted the crystalline nature of Mn-QDs.

Fig. 1
figure 1

TEM images of A, B QDs and D Mn-QDs. E HRTEM images of Mn-QDs with a lattice spacing of 0.26 nm. The size distribution of C QDs and F Mn-QDs

Full size image

The strongest peak (104) of Ti3AlC2 standard card is 39°, which is the characteristic peak of aluminum (Al) atom [26]. As shown in Fig. 2B, after HF etching, the 39° peak of QDs is significantly reduced or even disappears, indicating that the Al layer is successfully etched off. After the MXene hydrothermal reaction increased the layer spacing, the peaks of 41.7°, 28.6°, and 19° were enlarged and changed to the small angle direction of 40°, 28.1°, and 17.1° [27]. Furthermore, the two intense diffraction peaks located at 17.1° and 34.7°, which correspond to the (004) and (101) peaks of Ti3C2 MXene [28], respectively, indicate that QDs are successfully prepared and still maintain the 2D layered structure of MXene. On the basis of QDs, Mn-QDs exhibited the above phenomenon of weakening or disappearance of the strong peaks. After expanding the layer spacing of MXene by KOH alkalization treatment, the Mn-QDs derived from MXene were further modified with KMnO4, and the insertion of foreign substances caused corresponding changes in the material structure [29,30,31]. The XRD images of the two do not match well, mainly due to the insertion of the exogenous material causes a corresponding change in the material structure. The peak appearing at 24° is TiO2 formed by oxidation of MXene [32], indicating that QDs were successfully functionalized with KMnO4.

Fig. 2
figure 2

A FTIR spectra of QDs and Mn-QDs. B XRD spectra of QDs and Mn-QDs. XPS spectra of C QDs and D Mn-QDs

Full size image

FTIR was used to characterize the composition and chemical bond of the material, as shown in Fig. 2A, stretching vibration of hydroxyl (-OH) at 3420 cm−1 and 1401 cm−1, vibration at 3113 cm−1 attributed to amino group (-NH), peak of 1627 cm−1 attributed to carbonyl group (C = O), and 1007 cm−1 band belongs to C-F bond [33,34,35]. In particular, the peaks at 705 cm−1 and 576 cm−1 are related to the vibrations of Ti–O and Ti-C bonds, respectively [36]. It is clear that the wavelengths of the functional groups of QDs and Mn-QDs fluctuate between 3500 and 2800 cm−1. Due to the fact that the addition of KMnO4 may have changed the surface structure of Mn-QDs, thus, changing the wavelengths and intensities of the functional groups also corroborates the change in the structure of their physical phases. The functional groups of QDs and Mn-QDs at 1600 to 600 cm−1 are matched one by one. Beyond that, the existence of bands 576 and 702 is also attributed to Mn–O vibration [37]. In summary, it can be indicated that Mn-QDs have been successfully prepared.

The chemical composition and overall structure of QDs and Mn-QDs were further investigated by XPS. The full-range XPS spectra of QDs showed five significant peaks at 285.2 (C 1 s), 401.8 (N 1 s), 532.2 (O 1 s), 684.9 (F 1 s), and 456.1 eV (Ti 2p), demonstrating the successful synthesis of QDs [12, 24]. The full-range XPS spectra of Mn-QDs showed five significant peaks at 285.0, 407.1, 532.1, 685.1, 455.3, 293.1, and 654.0 eV corresponding to C, N, O, F, Ti, K, and Mn elements [15, 38, 39], further indicating KMnO4 successfully functionalized QDs. The enlarged characteristic spectra of C 1 s, O 1 s, K 2p, and Mn 2p are displayed in Fig. S1, respectively. And the detailed narrow spectrum analysis has been given in the supporting information. On the other hand, the optical properties of QDs and Mn-QDs were characterized. In Fig. 3A, the FL of QDs has two excitation peaks at about 232 nm and 321 nm and one emission peak (black line) at about 413 nm. The FL of Mn-QDs has two excitation peaks at about 246 nm and 314 nm and one emission peak at about 435 nm (red line). An obvious absorption peak located at 209 nm could be observed in the absorption spectrum of Mn-QDs (violet curve) when compared with that of QDs (blue curve), because of the trapping of the excited state energy by the surface states [40]. These changes of optical properties may be related to functionalization, and the embedding of external functional groups improves their optical properties. QDs are colorless and transparent under natural light (first from the left in the inset) and blue under 365-nm UV irradiation (first from the right), and Mn-QDs are a light yellow solution under natural light (second from the left) and blue-green under 365-nm UV light (second from the right). The excellent luminescence of Mn-QDs proves to be an ideal fluorescent probe material for ion detection. On the side, we performed an optimization test regarding the excitation wavelength of Mn-QDs, and as shown in Fig. 3B, the best display wavelength is 245 nm. In detail, the excitation wavelength of our subsequent experiments was concentrated at 246 nm.

Fig. 3
figure 3

A Absorption spectra of QDs (blue curve) and Mn-QDs (violet curve), fluorescence spectra of QDs (black curve) and Mn-QDs (red curve); insets show pictures of QDs and Mn-QDs under natural light and 365-nm UV light. B FL emission spectra recorded at diverse excitation wavelengths for Mn-QDs

Full size image

Condition optimization and stability performance

To ensure the best FL performance, the synthesis conditions of Mn-QDs were firstly optimized. As shown in Fig. S2, the results showed that the best FL performance of Mn-QDs was achieved when the temperature was 120 °C, the time was 6 h, KMnO4 doping amount was 10 mL, and ammonia/water ratio was 3/7. And then, in order to explore the practical application of Mn-QDs as nanoprobe in ion detection, it is necessary to examine the changes of their FL intensity under different concentrations of NaCl and pH environments. As shown in Fig. S3(A), the material can still maintain the same FL intensity at high and low salt concentrations. Figure S3(B) shows that the FL intensity of the material basically remained unchanged at pH 4–10, but decreased slightly when the pH exceeded 10. Figure S3(C) shows that the FL intensity of the material hardly changed after 55 days of storage, which indicates that the material is stable and can be stored for a long time. Finally, Mn-QDs also appeared outstanding optical stability. In Fig. S3(D), the FL intensity of Mn-QDs did not change significantly under 70-min continuous scanning (λex = 246 nm). This indicates that Mn-QDs have good stability and are expected to be used in environmental applications.

In order to save the experimental cost, we optimized the usage of Mn-QDs. The solutions of 10, 20, 35, 50, and 65 μL Mn-QDs were taken separately and supplemented with pure water to 400 μL. FL was measured at 246-nm excitation. In Fig. S4, when the amount of Mn-QDs decreased, the emission peak of Mn-QDs appeared blue shift. This may be caused by the reduced content of Mn-QDs in the cuvette. Therefore, from the perspective of saving experimental costs, we selected 10 μL Mn-QDs for the subsequent ion experiments.

Determination of Cr3+ and Hg2+ ions by Mn-QDs

In this study, a novel fluorescence nanoprobe based on Mn-QDs was developed to respond to Cr3+ and Hg2+ ions simultaneously. We first evaluated the feasibility of the experiment. As shown in Fig. S5(A), when Mn-QDs were mixed with 500 μM Cr3+ or Hg2+, the experimental phenomena of FL enhancement and peak blue shift of the mixed solution could be clearly observed, whereas the FL of the mixed solution did not change significantly when 500 μM Cr3+ or Hg2+ was added to QDs. Blank solutions of QDs and Mn-QDs were also measured and were able to exhibit lower FL intensities. Thus, the experiment was judged to be feasible. So as to explore possible practical applications, the effects of 13 common metal ions (Na+, K+, Al3+, Mg2+, Mn2+, Co2+, Hg2+, Ni+, Cr3+, Fe3+, Fe2+, Cu2+, and Ag+ at 300 μM concentrations) on QDs and Mn-QD FL responses were studied. In Fig. S5(B), for Mn-QDs, FL enhancement efficiency of Cr3+ and Hg2+ ions calculated using the formula (F – F0)/F0 was found to reach 1.75 and 0.95, respectively. F0 was the FL intensity of QDs or Mn-QDs, and F was the blank FL intensity of QDs or Mn-QDs reacting with metal ions. Figure S6 shows the fluorescence spectra of QDs and Mn-QDs in a series of different ions. As illustrated in Fig. 5B and Fig. S6, the addition of Cr3+ or Hg2+ has an obviously enhancing effect on the FL response of Mn-QDs; however, the others show minor influence on the response of Mn-QDs. Therefore, Mn-QDs have good selectivity for Cr3+ and Hg2+.

Further analysis was performed for Cr3+ and Hg2+ ions. Figure 4A shows the FL response of Mn-QDs over the 0–2600 μM Cr3+ concentration range. The inset in Fig. 4A shows the photographs under daylight lamp and 365-nm UV lamp regarding the blank Mn-QD solution and the solution after the addition of 500 μM, 1500 μM, and 2600 μM Cr3+. After the addition of Cr3+, Mn-QDs reacted with them resulting in the mixed solution gradually changing its color from light yellow to colorless. And the color of the solution changed from dark blue-green to bright blue under UV lamp. The FL intensity of Mn-QDs increased significantly with the increase of (0–2600 μM) Cr3+ concentration, while the blue-shift phenomenon occurred in the highest site peak. The linear range for Cr3+ is 50–2600 μM with a linear equation of F0/F = 1.4670 – 0.3520 Log [Cr3+] (μM) (R2 = 0.9762, n = 5). The limits of detection (LOD) of the nanoprobe for Cr3+ was 0.80 μM, which was estimated from the formula of LOD = 3σ/K (σ shows the standard deviation of blank signal (n = 5) and K is the slope of the linear equation). The comparison of other methods for the detection of Cr3+ with this experiment is shown in Table 1. Compared with other materials, our work has a good LOD and a wider detection range. Besides, different from the FL enhancement method for Cr3+ detection in this paper, most other detection mechanisms are FL quenching.

Fig. 4
figure 4

A FL emission spectra of Mn-QDs in the presence of Cr3+ ion over 0–2600 μM. The inset shows photographs of Mn-QDs and the solution after the addition of Cr3+ under sunlight and UV lamps. B Plots of the FL versus Cr3+ concentration; the inset is the calibration curve of F0/F and Cr3+ concentrations in the 50 to 2600 μM range. F0 is the FL response of Mn-QDs; F is the FL response of Mn-QDs after the addition of Cr3+. The error bars represent the standard deviation based on five independent measurements

Full size image
Table 1 Comparison work of linear range and LOD for the detection of Cr3+ by FL assay using different materials
Full size table

The inset in Fig. 5A shows photographs about the blank Mn-QD solution and the solutions after the addition of 200 μM, 400 μM, and 600 μM Hg2+ under daylight lamp and 365-nm UV lamps. After adding Hg2+, Mn-QDs reacted with them, resulting in the color of the mixed solutions gradually changing from yellow to light yellow. And the color of the solution changed accordingly under the UV lamp. Figure 5A shows that the FL response of Mn-QDs decreases with increasing Hg2+ concentration from 0 to 50 μM and increases from 50 to 550 μM Hg2+. With the increase of Hg2+ concentration, the FL intensity of Mn-QDs decreases, while the peak undergoes a blue-shift phenomenon. When the Hg2+ concentration reached 65 μM, the FL intensity gradually showed an increasing trend. The top left inset in Fig. 5B shows the linear plot of the FL quenching of the Mn-QD solution after the addition of 0–50 μM Hg2+. The linear equation for this interval is F = 1212.58 – 5.5320 [Hg2+] (μM) (R2 = 0.9837, n = 5). According to the illustration in the lower right corner, the linear range for Hg2+ is 50–550 μM and the linear equation is F0/F = 2.7399 – 0.8152 Log [Hg2+] (μM) (R2 = 0.9948, n = 5). The LOD was 0.16 μM by calculating the 3 σ/K formula. The comparison of other methods for the detection of Hg2+ with this experiment is shown in Table 2. Compared to most of the fluorescence-off assays in the table, the FL “turn-on” type of this experiment has a wider detection range and good detection limits.

Fig. 5
figure 5

A FL emission spectra of Mn-QDs in the presence of Hg2+ ion over 0–550 μM. The inset shows photographs of Mn-QDs and the solution after the addition of Hg2+ under sunlight and UV lamps. B Plots of the FL versus Hg2+ concentration. The inner illustration in the upper left corner is a linear FL quenching diagram of 0–50 μM Hg2+. Another inset is linear relationship between F0/F and Hg2+ concentrations in the range 50 to 550 μM. F0 is the FL response of Mn-QDs and F is that of Mn-QDs with Hg2+. The error bars are the same as those in Fig. 4

Full size image
Table 2 Comparison work of linear range and LOD for the detection of Hg2+ by FL assay using different materials
Full size table

In this experiment, with the increase of the concentration of Cr3+ and Hg2+, the maximum emission peak of Mn-QDs gradually shifted blue. Through zeta potential analysis, we can judge the binding between metal ions and Mn-QDs through electrostatic interaction, and the DLS data showed that average hydrodynamic radius of ultra-small Mn-QDs was also increased upon the addition of Cr3+ and Hg2+ ions (Supporting Information of Fig. S7A and S8). The strong interaction between Cr3+/Hg2+ ion and Mn-QDs with abundant functional groups on the surface leads to the formation of ground state complex [53]. Furthermore, the average fluorescence decay time of Mn-QDs is 4.77 ns; however, it decreases to 3.57 and 1.71 ns by the addition of Cr3+ and Hg2+ ions (Supporting Information of Fig. S7D), which further confirms the formation of complexes between Mn-QDs and Cr3+/Hg2+.

When appropriate metal ions are added to the functionalized Mn-QDs, the D-orbital electrons of the existing metal ions can repel these trapped conduction band electrons, which is caused by the binding between the surface functional groups and the metal ions [54]. This association of metal ions promotes the complexation of electron–hole pairs, which restores the fluorescence intensity of the functionalized Mn-QDs. This is why the fluorescence intensity of each quantum dot increases with increasing metal ion concentration and shifts in wavelength.

Investigation into the response mechanism for Cr3+ and Hg2+

To understand the fluorescence-on detection of Cr3+ and Hg2+ ions based on Mn-QD sensing nanoprobes, we performed a preliminary study of the response mechanism using zeta potential, fluorescence lifetime measurements, and absorption spectroscopy. As shown in Fig. S7(A), the zeta potential of Mn-QDs was − 47.9 mV. After mixing Mn-QDs with Cr3+ or Hg2+ solutions, the zeta potential increased to − 36.8 mV and − 13.65 mV, respectively. This indicates that there is an electrostatic interaction between Mn-QDs and Cr3+/Hg2+ [55]. This also demonstrates that Mn-QDs and Cr3+/Hg2+ can bind together. Furthermore, the small size and high quantum confinement effects of Mn-QDs lead to the formation of unsaturated bonds at their edges and surfaces, which exerted a higher chance to attach with other atoms [56].

The enhancement mechanism may be caused by passivation, absorption, and modification of the surface of Mn-QDs. The surface of Mn-QDs is rich in functional groups, which interact with Cr3+/Hg2+ to modify the surface traps, resulting in passivation of defects and exhibiting photoluminescence enhancement [57, 58]. In addition, due to the similarity of Al atoms, the alkali metal ions (K element) may have the ability to inhibit the passivation defects generated during the preparation process, resulting in the FL enhancement of Mn-QDs [59].

To investigate the FL quenching mechanism of Mn-QDs and Hg2+, the Stern–Volmer equation (F0/F = 1 + Ksv [Q], where Ksv represents the quenching constant and [Q] represents the concentration of the quenching agent) was used to determine the quenching mechanism [60]. As shown in Fig. S7 (B), a linear relationship between the F0/F ratio and concentration of Hg2+ (10–50 μM) indicates that there is a dynamic quenching between Hg2+ and Mn-QDs. Meanwhile, the UV–vis of Mn-QDs showed some changes of absorption peaks after the addition of Hg2+, which indicated that the FL quenching might be caused by the capture of Hg2+ by the amino groups on the surface of Mn-QDs to form Mn-QDs@Hg2+ complex [13, 61, 62]. The average fluorescence lifetime of QDs and Mn-QDs is 3.68 ns and 4.77 ns, respectively. Due to the fact that the functionalization of KMnO4 may change the properties of the surface chemical states of QDs, the lifetime of Mn-QDs is prolonged [63]. After the addition of Hg2+, the fluorescence lifetime of Mn-QDs decreased significantly to 1.71 ns. Besides, the value of τ0/τ for static quenching (average fluorescence lifetime τ0: QDs; τ: Mn-QDs@Hg2+) should be 1, while F0/F = τ0/τ should be valid for dynamic quenching [64]. It is calculated that τ0/τ = 4.77/1.71 = 2.79. Therefore, the type of FL quenching of Mn-QDs induced by Hg2+ was dynamic quenching, which was consistent with the conclusion obtained from the study of the Stern–Volmer equation.

Fluorescence detection of Cr3+/Hg2+ ions in real samples

The practicality and reliability of the Mn-QDs probe in detecting environmental samples were tested using samples from Runxi Lake and tap water of Nanchang University. In the present experiments, the Cr3+/Hg2+ solution with different concentrations was dropped into the pretreated real samples by standard addition method and then analyzed via the proposed approach. The specific procedure is that when a certain concentration of Cr3+ or Hg2+ ions was added to the Runxi Lake and tap water samples, their signals are measured by fluorescence spectrometry. Their recovery values can then be calculated by the corresponding linear equations. The measured results are summarized in Table S1 and Table S2. It can be clearly seen that the recoveries of Cr3+ and Hg2+ in water samples were 93.79–105.10% and 93.91–102.05%, respectively, with the relative standard deviations (RSD) of 0.86–3.32% and 0.66–5.00%. The results indicate that the method has great potential for the analysis of Cr3+ and Hg2+ ions in real samples.

Conclusion

In summary, Mn-QDs were prepared from KMnO4-functionalized MXene NSs by a hydrothermal reaction. The Mn-QDs displayed well-monodispersed near-spherical particles with average size of 2.24 nm and exhibited brightly blue-green emission and good long-term storage stability. In addition, Mn-QDs were used as a simple and reliable turn-on fluorescent nanoprobe for the sensitive and simultaneous detection of Cr3+ and Hg2+ ions. The surface trap of Mn-QDs was modified by binding Cr3+/Hg2+ through electrostatic interaction, which led to the passivation of surface defects and enhanced photoluminescence. This work not only leads to the development of a sensitive fluorescence assay for the detection of heavy metal ions, but also opens up a new and exciting channel for functionalized applications of MXene.