A Novel Turn-On Fluorescent Sensor Based on Sulfur Quantum Dots and MnO2 Nanosheet Architectures for Detection of Hydrazine

In this paper, the SQDs@MnO2 NS as the probe was applied to construct a novel “turn-on” fluorescent sensor for sensitive and selective detection of hydrazine (N2H4). Sulfur quantum dots (SQDs) and MnO2 nanosheets (MnO2 NS) were simply mixed, through the process of adsorption to prepare the architectures of SQDs@MnO2 NS. The fluorescent emissions of SQDs@MnO2 NS play a key role to indicate the state of the sensor. According to the inner filter effect (IFE) mechanism, the state of the sensor at the “off” position, or low emission, under the presence of MnO2 NS, is which the ultraviolet and visible spectrum overlaps with the fluorescence emission spectrum of SQDs. Under the optimal conditions, the emission was gradually recovered with the addition of the N2H4, since the N2H4 as a strong reductant could make the MnO2 NS converted into Mn2+, the state of the sensor at the “on”. Meanwhile, the fluorescent sensor possesses good selectivity and high sensitivity, and the detection concentration of N2H4 with a wide range from 0.1 µM to 10 mM with a detection limit of 0.072 µM. Furthermore, actual samples were successful in detecting certain implications, indicating that the fluorescent sensor possesses the potential application ability to monitor the N2H4 in the water.


Introduction
Hydrazine (N 2 H 4 ) has attracted particular attention due to its strong reducibility and weak alkalinity in applications such as pesticides, pharmaceuticals, fuels, organic dyes, and so on [1,2]. Meanwhile, the toxicity and harm of N 2 H 4 could not be neglected due to its water-solubility. It could damage the lungs, eyes, skin, and some system diseases when exposed to the N 2 H 4 surroundings for an extended period of time [3,4]. Hence, the development of a facile and sensitive measure for N 2 H 4 is considerable. In the past decades, many analytical methods have been reported, including chromatography, electrochemical, fluorescent, titrimetric, colorimetry, and mass spectrometry [5][6][7]. The fluorescent method is a powerful technique to detect N 2 H 4 , due to a comprehensive consideration of the factors including the low cost, simple operation, and rapid analysis.
The fluorescent method consists of constructing a fluorescent probe to observe the fluorescence intensity enhancement, or quenching, for the qualitative and quantitative analysis present of the targets. The fluorescent probe materials are commonly applied in the fluorescent sensor field similar to quantum dots (QDs) [8][9][10], organics [11,12], metalorganic framework [13][14][15], and metal nanoclusters [16,17]. Therein, the sulfur quantum dots (SQDs) is a novel and attention the QDs, which retain the advantage of the traditional optical performance of QDs while overcoming potential issues of the toxicity of the heavy metal QDs. Thus, it is widely applied in the fluorescent probes, biological sensors, and cell imaging fields [18][19][20]. Lei et al., take the one-pot to prepare the polyvinyl alcohol-capped SQDs as the fluorescent probe for detection of Fe 3+ and temperature in cells [21].
Herein, we first introduced the IFE mechanism to establish a "turn-on" fluorescent sensor for the detection of N 2 H 4 . The sensing strategy is illustrated in Figure 1; SQDs combined with MnO 2 nanosheet (MnO 2 NS) to prepare SQDs@MnO 2 NS architectures. The SQDs alone have a strong fluorescence intensity and the MnO 2 NS has nearly no fluorescence under the same experimental conditions. The SQDs@MnO 2 NS possesses a lower intensity compared to the SQDs, due to the MnO 2 NS as a full-of-all adsorbed material in the ultraviolet and visible (UV-Vis) spectrum, which could overlap with the fluorescence emission spectrum of SQDs, led to the fluorescence intensity quenching. Meanwhile, at this stage, the state of the fluorescent sensor is off. However, the emission of fluorescent is recovered under the N 2 H 4 present condition, with the addition concentrations the state is gradually turned on. Benefits of the sensor for quantitatively detecting N 2 H 4 was successfully constructed by monitoring the fluorescent intensity of SQDs@MnO 2 NS. Furthermore, this approach possesses the potential for a practical application, due to its ability to effectively identify the N 2 H 4 in the real samples of water.
Herein, we first introduced the IFE mechanism to establish a "turn-on" fluorescent sensor for the detection of N2H4. The sensing strategy is illustrated in Figure 1; SQDs combined with MnO2 nanosheet (MnO2 NS) to prepare SQDs@MnO2 NS architectures. The SQDs alone have a strong fluorescence intensity and the MnO2 NS has nearly no fluorescence under the same experimental conditions. The SQDs@MnO2 NS possesses a lower intensity compared to the SQDs, due to the MnO2 NS as a full-of-all adsorbed material in the ultraviolet and visible (UV-Vis) spectrum, which could overlap with the fluorescence emission spectrum of SQDs, led to the fluorescence intensity quenching. Meanwhile, at this stage, the state of the fluorescent sensor is off. However, the emission of fluorescent is recovered under the N2H4 present condition, with the addition concentrations the state is gradually turned on. Benefits of the sensor for quantitatively detecting N2H4 was successfully constructed by monitoring the fluorescent intensity of SQDs@MnO2 NS. Furthermore, this approach possesses the potential for a practical application, due to its ability to effectively identify the N2H4 in the real samples of water.

Synthesis of SQDs and MnO 2 NS
SQDs were synthesized according to a literature method [34]. Briefly, the sublimed sulfur powder (1.4 g) was added to a mixed solution of PEG-400 (3 mL) and NaOH (50 mL, 0.08 g mL −1 ) stirring at 70 • C for 24 h. During the period, the color of the solution changed gradually from dark-yellow to light-yellow, and then added H 2 O 2 (3 mL) to each, the obtained solution was termed as SQDs. The prepared SQDs were introduced in the dialysis membrane with the molecular weight of 1000 Da to remove unreacted molecular dialysis for 72 h each 12 h to change the water. Then, the light-yellow solid was acquired by freeze-drying at −20 • C for 24 h, and the SQDs were stored at 4 • C for further use.
MnO 2 NS were prepared with reference to previous literature [35]. Firstly, TMA·OH (12 mL, 1.0 M) solution was introduced in MnCl 2 ·4H 2 O (10 mL, 0.3 M) at the 50 mL round-bottomed flask. Afterward, the H 2 O 2 (2 mL, 30%) solution was slowly added to the mixed solution vigorously stirring at room temperature for 24 h. The acquired dark brown solution was centrifuged and rinsed with ultra-water and CH 3 OH several times. Last, the obtained product of MnO 2 NS was dried at room temperature.

The SQDs@MnO 2 NS Fluorescent Probe Detection N 2 H 4
The mixture solution of SQDs@MnO 2 was obtained by SQDs and MnO 2 NS mixed to stand for 1 h at room temperature. Next, the different concentrations of N 2 H 4 solution (0.1 µM-10 mM) were added to the SQDs@MnO 2 (1 mL) to react for 10 min at room temperature and perform fluorescence spectroscopy tests. Finally, a standard curve line was constructed between various concentrations of N 2 H 4 and the recovery value of fluorescence intensity. In addition, the fluorescence probe selectivity, stability, and repeatability were studied under the optimal conditions.

Detection of Actual Samples
The fluorescence probe of SQDs@MnO 2 NS was selected specifically for N 2 H 4 . To verify the performance in the actual sample of the probe, this was applied to detect the environmental water samples. Actual samples were acquired from the lake and river in Yantai. Briefly, the water samples were filtered with the 0.45 µm filter membrane to remove impurities. Then, to detect the N 2 H 4 in the lake and river were used to prepare various concentrations of N 2 H 4 (0.1 µM, 10 µM, and 10 mM) reaction for 10 min to test fluorescence spectroscopy, respectively. Three experiments were performed in parallel, and RSD was calculated.

Characteristics of SQDs, MnO 2 NS, SQDs@MnO 2
The morphology of SQDs, MnO 2 NS, and SQDs@MnO 2 architectures was characterized by HR-TEM and TEM. As shown in Figure 2a,b, the morphology of SQDs was spherical particles with good distribution, and the size of SQDs was calculated mainly to be 3.5 ± 0.5 nm. Next, the morphology of MnO 2 NS was investigated presenting a large two-dimensional ultrathin planar structure (inset of Figure 2c). Meanwhile, the struc-ture of MnO 2 NS under the size of 100 nm of TEM appears to wrinkle and aggregation ( Figure 2c). Additionally, as shown in Figure 2d, SQDs@MnO 2 retained the planar structure but have a stronger aggregate phenomenon compared with MnO 2 NS (Figure 2c), and the SQDs were distributed on the surface of MnO 2 NS, indicating that the SQDs@MnO 2 was successfully prepared.

Characteristics of SQDs, MnO2 NS, SQDs@MnO2
The morphology of SQDs, MnO2 NS, and SQDs@MnO2 architectures was characterized by HR-TEM and TEM. As shown in Figure 2a,b, the morphology of SQDs was spherical particles with good distribution, and the size of SQDs was calculated mainly to be 3.5 ± 0.5 nm. Next, the morphology of MnO2 NS was investigated presenting a large twodimensional ultrathin planar structure (inset of Figure 2c). Meanwhile, the structure of MnO2 NS under the size of 100 nm of TEM appears to wrinkle and aggregation ( Figure  2c). Additionally, as shown in Figure 2d, SQDs@MnO2 retained the planar structure but have a stronger aggregate phenomenon compared with MnO2 NS (Figure 2c), and the SQDs were distributed on the surface of MnO2 NS, indicating that the SQDs@MnO2 was successfully prepared. To further study the elements of SQDs and MnO2 NS, X-ray photoelectron spectroscopy (XPS) was analyzed. In Figure S1a, the MnO2 NS was composed of four elements of C, O, N, and Mn. In the spectrogram of the Mn 2p element in Figure S1b, the band energy peaks located at 641.8 eV belonged to MnO2, and the characteristic peaks of Mn 2p appeared at 644.3 eV, 649.1 eV, which was identified with the previously reported work [36]. As can be seen in Figure S1c, the XPS survey spectrum of SQDs was recorded, which peaks corresponding to the elements of C, O, and S, respectively. The spectrum of the S 2p region in Figure S1d exhibits two peaks at 162.3 eV and 163.2 eV, which were due to the elemental S. The band peaks at 166.5 eV, 168.2 eV, and 169.3 eV were respective corresponding to the SO3 2− (2p2/3), SO3 2− (2p2/3) or SO2 2− (2p1/2), and SO3 2− (2p1/2), which demonstrated that the prepared SQDs the surface has an amount of sulfite group by adsorbing since the huge To further study the elements of SQDs and MnO 2 NS, X-ray photoelectron spectroscopy (XPS) was analyzed. In Figure S1a, the MnO 2 NS was composed of four elements of C, O, N, and Mn. In the spectrogram of the Mn 2p element in Figure S1b, the band energy peaks located at 641.8 eV belonged to MnO 2 , and the characteristic peaks of Mn 2p appeared at 644.3 eV, 649.1 eV, which was identified with the previously reported work [36]. As can be seen in Figure S1c, the XPS survey spectrum of SQDs was recorded, which peaks corresponding to the elements of C, O, and S, respectively. The spectrum of the S 2p region in Figure S1d exhibits two peaks at 162.3 eV and 163.2 eV, which were due to the elemental S. The band peaks at 166.5 eV, 168.2 eV, and 169.3 eV were respective corresponding to the SO 3 2− (2p 2/3 ), SO 3 2− (2p 2/3 ) or SO 2 2− (2p 1/2 ), and SO 3 2− (2p 1/2 ), which demonstrated that the prepared SQDs the surface has an amount of sulfite group by adsorbing since the huge surface and small volume [34]. Additionally, the XPS survey spectrum of SQDs@MnO 2 was shown in Figure 3a surface and small volume [34]. Additionally, the XPS survey spectrum of SQDs@MnO2 was shown in Figure 3a, in which elements of S 2p (Figure 3b) and Mn 2p (Figure 3c) correspond to the SQDs and MnO2, indicating the SQDs@MnO2 was successfully prepared. To further verify the SQDs, MnO2 NS, and SQDs@MnO2 NS were successful in preparation, the UV-Vis spectra were shown in Figure 4. The broad absorption bands of MnO2 NS the range from 280 to 650 nm a weak peak around 360 nm, which is due to the d-d transition of Mn 4+ ions [37]. The UV-Vis absorption spectra of SQDs and SQDs@MnO2 both have peaks at 313 nm and 350 nm, which might be ascribed to the S2 2− and S8 2− adsorbed on the surface of SQDs [34]. However, the values of peaks of SQDs@MnO2 were lower than SQDs due to the adsorption of SQDs on MnO2 NS. The excitation (Ex) and emission (Em) spectra of fluorescence of SQDs@MnO2 were shown in Figure 4b, the Em wavelength at 484.2 nm under the excitation wavelength of 380 nm, which is like the previous work [38].

Optimization of Experimental Parameters
We have investigated the experimental parameters to acquire the optimal conditions, including the excitation wavelength for SQDs, the concentration of MnO2 NS, the volume ratio of N2H4 to MnO2 NS, and the pH of the SQDs and SQDs@MnO2 NS solution. As illustrated in Figure 5a, the synthesized of SQDs detected under the different excitation wavelengths at 330-420 nm, the intensity of fluorescent behaved a general trend of rising first and then falling, and the maximum emission at 400 nm. Thus, the excitation wavelength of SQDs at 400 nm was chosen as the optimal wavelength. As shown in Figure 5b, To further verify the SQDs, MnO 2 NS, and SQDs@MnO 2 NS were successful in preparation, the UV-Vis spectra were shown in Figure 4. The broad absorption bands of MnO 2 NS the range from 280 to 650 nm a weak peak around 360 nm, which is due to the d-d transition of Mn 4+ ions [37]. The UV-Vis absorption spectra of SQDs and SQDs@MnO 2 both have peaks at 313 nm and 350 nm, which might be ascribed to the S 2 2− and S 8 2− adsorbed on the surface of SQDs [34]. However, the values of peaks of SQDs@MnO 2 were lower than SQDs due to the adsorption of SQDs on MnO 2 NS. The excitation (Ex) and emission (Em) spectra of fluorescence of SQDs@MnO 2 were shown in Figure 4b, the Em wavelength at 484.2 nm under the excitation wavelength of 380 nm, which is like the previous work [38].
surface and small volume [34]. Additionally, the XPS survey spectrum of SQDs@MnO2 was shown in Figure 3a, in which elements of S 2p (Figure 3b) and Mn 2p (Figure 3c) correspond to the SQDs and MnO2, indicating the SQDs@MnO2 was successfully prepared. To further verify the SQDs, MnO2 NS, and SQDs@MnO2 NS were successful in preparation, the UV-Vis spectra were shown in Figure 4. The broad absorption bands of MnO2 NS the range from 280 to 650 nm a weak peak around 360 nm, which is due to the d-d transition of Mn 4+ ions [37]. The UV-Vis absorption spectra of SQDs and SQDs@MnO2 both have peaks at 313 nm and 350 nm, which might be ascribed to the S2 2− and S8 2− adsorbed on the surface of SQDs [34]. However, the values of peaks of SQDs@MnO2 were lower than SQDs due to the adsorption of SQDs on MnO2 NS. The excitation (Ex) and emission (Em) spectra of fluorescence of SQDs@MnO2 were shown in Figure 4b, the Em wavelength at 484.2 nm under the excitation wavelength of 380 nm, which is like the previous work [38].

Optimization of Experimental Parameters
We have investigated the experimental parameters to acquire the optimal conditions, including the excitation wavelength for SQDs, the concentration of MnO2 NS, the volume ratio of N2H4 to MnO2 NS, and the pH of the SQDs and SQDs@MnO2 NS solution. As illustrated in Figure 5a, the synthesized of SQDs detected under the different excitation wavelengths at 330-420 nm, the intensity of fluorescent behaved a general trend of rising first and then falling, and the maximum emission at 400 nm. Thus, the excitation wavelength of SQDs at 400 nm was chosen as the optimal wavelength. As shown in Figure 5b,

Optimization of Experimental Parameters
We have investigated the experimental parameters to acquire the optimal conditions, including the excitation wavelength for SQDs, the concentration of MnO 2 NS, the volume ratio of N 2 H 4 to MnO 2 NS, and the pH of the SQDs and SQDs@MnO 2 NS solution. As illustrated in Figure 5a, the synthesized of SQDs detected under the different excitation wavelengths at 330-420 nm, the intensity of fluorescent behaved a general trend of rising first and then falling, and the maximum emission at 400 nm. Thus, the excitation wavelength of SQDs at 400 nm was chosen as the optimal wavelength. As shown in Figure 5b, with the increase of the concentration of MnO 2 NS, the quenching emission values of SQDs were increased, and the fluorescent intensity of SQDs was nearly all the quenched at the concentration of MnO 2 NS at 10 mg mL −1 . Hence, 10 mg mL −1 was selected as the optimum concentration of MnO 2 NS for the next use. In addition, the quenching behavior of SQDs@MnO 2 about different concentrations of MnO 2 NS for better visualization in Figure S2, which obviously noted that the MnO 2 NS possesses a huge surface that could package the SQDs. The volume ratio of N 2 H 4 to MnO 2 NS was shown in Figure 5c, the Nanomaterials 2022, 12, 2207 6 of 10 N 2 H 4 volume-specific gravity increased the emission was gradually recovered, and the volume ratio reached 2:1 of N 2 H 4 to MnO 2 NS the emission intensity reached the maximum recovery values. Furthermore, the SQDs increased with pH from 5 to 12, which had no influence on its emission, while introducing the MnO 2 NS the emission of SQDs values significantly decreased (Figure 5d). However, with the increased pH, the quench of emission degree was decreased. On this basis, we selected the pH = 7 as the experiment condition, considering the pH of the environment water. As shown in Figure 5e, the fluorescence of SQDs intensity was decreasing when the MnO 2 was added. The molar ratio of SQDs@MnO 2 was increased to 10:4 the fluorescence intensity reached its lowest. After, the molar ratio of SQDs@MnO 2 over 10:4 the fluorescence intensity was a tiny increase. Thus, the molar ratio of 10:4 has been chosen for the further experiment. In addition, the response time of SQDs@MnO 2 with N 2 H 4 was recorded in Figure 5f, when 10 min of reaction was the ∆I = 30 (∆I = intensity (2 min)-intensity (1 min)), and the value of ∆I was nearly stable. Therefore, the SQDs@MnO 2 with N 2 H 4 10 min of reaction as the optimal react time.
with the increase of the concentration of MnO2 NS, the quenching emission values of SQDs were increased, and the fluorescent intensity of SQDs was nearly all the quenched at the concentration of MnO2 NS at 10 mg mL −1 . Hence, 10 mg mL −1 was selected as the optimum concentration of MnO2 NS for the next use. In addition, the quenching behavior of SQDs@MnO2 about different concentrations of MnO2 NS for better visualization in Figure  S2, which obviously noted that the MnO2 NS possesses a huge surface that could package the SQDs. The volume ratio of N2H4 to MnO2 NS was shown in Figure 5c, the N2H4 volume-specific gravity increased the emission was gradually recovered, and the volume ratio reached 2:1 of N2H4 to MnO2 NS the emission intensity reached the maximum recovery values. Furthermore, the SQDs increased with pH from 5 to 12, which had no influence on its emission, while introducing the MnO2 NS the emission of SQDs values significantly decreased (Figure 5d). However, with the increased pH, the quench of emission degree was decreased. On this basis, we selected the pH = 7 as the experiment condition, considering the pH of the environment water. As shown in Figure 5e, the fluorescence of SQDs intensity was decreasing when the MnO2 was added. The molar ratio of SQDs@MnO2 was increased to 10:4 the fluorescence intensity reached its lowest. After, the molar ratio of SQDs@MnO2 over 10:4 the fluorescence intensity was a tiny increase. Thus, the molar ratio of 10:4 has been chosen for the further experiment. In addition, the response time of SQDs@MnO2 with N2H4 was recorded in Figure 5f, when 10 min of reaction was the ∆I = 30 (∆I = intensity (2 min)-intensity (1 min)), and the value of ∆I was nearly stable. Therefore, the SQDs@MnO2 with N2H4 10 min of reaction as the optimal react time.

Fluorescence Spectra Analysis of N2H4 Sensing
The MnO2 NS nearly a total absorption in UV-Vi's spectrum at the 280 nm to 650 nm in this study, which could effectively quench the fluorescence of SQDs due to the IFE mechanism. However, with the N2H4 was introduced once the emission was recovered,

Fluorescence Spectra Analysis of N 2 H 4 Sensing
The MnO 2 NS nearly a total absorption in UV-Vi's spectrum at the 280 nm to 650 nm in this study, which could effectively quench the fluorescence of SQDs due to the IFE mechanism. However, with the N 2 H 4 was introduced once the emission was recovered, demonstrating that the MnO 2 NS was reduced to Mn 2+ in the presence of N 2 H 4 . Beneficial from this result, a simply "turn-on" sensor was constructed.
Under the optimum experiment condition, the analytical performance of the fluorescent sensor was investigated to detect N 2 H 4 with various concentrations. As exhibited in Figure 6a, the fluorescence intensity was increased with the N 2 H 4 concentration gradually added, indicating that the more reduction matter the more Mn 2+ in the detected solution.
The recovery values of fluorescence intensity of the logarithm of N 2 H 4 concentration in the range from 0.1 µM to 10 mM, with a limit of detection (LOD) were calculated to be 0.072 µM according to the 3σ/s. Figure 6b demonstrates that the linear equation was I = 1010.4 logc(N 2 H 4 ) + 8116.2 with a correlation coefficient of 0.9972, where I was the recovery intensity value of fluorescence. The comparison of the proposed methods to detect N 2 H 4 with previous reports was listed in Table 1. It was significantly observed that the SQDs@MnO 2 NS probe possessed the lower LOD and satisfactory linear range over other approaches.
Under the optimum experiment condition, the analytical performance of the flu cent sensor was investigated to detect N2H4 with various concentrations. As exhibit Figure 6a, the fluorescence intensity was increased with the N2H4 concentration grad added, indicating that the more reduction matter the more Mn 2+ in the detected solu The recovery values of fluorescence intensity of the logarithm of N2H4 concentrati the range from 0.1 µM to 10 mM, with a limit of detection (LOD) were calculated 0.072 µM according to the 3σ/s. Figure 6b demonstrates that the linear equation w 1010.4 logc(N2H4) + 8116.2 with a correlation coefficient of 0.9972, where I was the reco intensity value of fluorescence. The comparison of the proposed methods to detect with previous reports was listed in Table 1. It was significantly observed tha SQDs@MnO2 NS probe possessed the lower LOD and satisfactory linear range over approaches.

Selectivity, Stability, and Repeatability
To evaluate the specificity of the probe of SQDs@MnO2 NS, the selective as one o most important factors was investigated under similar reaction conditions. The va ions including Ni 2+ , Co 2+ , K + , Ca 2+ , Fe 2+ , Na + , Cd 2+ , Cu 2+ , Cr 2+ , SO4 2− , NO 3− , Cl − , OH − , were used as interference agents, these ions are the common positive ions and anions sent in the environment. As shown in Figure 7a, the fluorescence intensity was negli present the interference agents compared to have N2H4, indicating that the prepar probe has a strong anti-interference ability and accuracy detect N2H4 in environmen ter.
In addition, to further assess the stability of the SQDs@MnO2 NS fluorescent p the good stability of SQDs was an important means to verify. As depicted in Figur

Selectivity, Stability, and Repeatability
To evaluate the specificity of the probe of SQDs@MnO 2 NS, the selective as one of the most important factors was investigated under similar reaction conditions. The various ions including Ni 2+ , Co 2+ , K + , Ca 2+ , Fe 2+ , Na + , Cd 2+ , Cu 2+ , Cr 2+ , SO 4 2− , NO 3− , Cl − , OH − , CO 3 2− were used as interference agents, these ions are the common positive ions and anions present in the environment. As shown in Figure 7a, the fluorescence intensity was negligible present the interference agents compared to have N 2 H 4 , indicating that the preparation probe has a strong anti-interference ability and accuracy detect N 2 H 4 in environment water. the fluorescence intensity of SQDs was continuous detection for 14 days under similar experimental conditions, it was noticed that the intensity have a slow decrease and the degree was insignificant. Interestingly enough, after a month of observing the intensity of SQDs was only a tiny different compared with them before a month, illustrating that the SQDs@MnO2 NS possessed a high stable fluorescence performance. For reproducibility, as can be seen from Figure 7c, the test was performed under the five sets of parallel solutions of SQDs in the same environment, all of the measured fluorescence intensities possess the semblable value with an outstanding RSD of 1.1%. This result was successful in confirming that the SQDs have preeminent reproducibility. Meanwhile, they have the potential benefit to the synthesis and application of the SQDs@MnO2 NS. These results demonstrated that the proposed sensor has good selectivity, stability, and repeatability for the analysis of N2H4.

Detection of N2H4 in Real Water Samples
To investigate the practicability of the probe of SQDs@MnO2 NS, it was applied to detect N2H4 in real samples. Three parallel water samples were obtained from the local lake and river for conducting the standard recovery test. The results were shown in Table  2, the N2H4 was detected in the lake, river, serum, and saliva, where the recovery ranged In addition, to further assess the stability of the SQDs@MnO 2 NS fluorescent probe, the good stability of SQDs was an important means to verify. As depicted in Figure 7b, the fluorescence intensity of SQDs was continuous detection for 14 days under similar experimental conditions, it was noticed that the intensity have a slow decrease and the degree was insignificant. Interestingly enough, after a month of observing the intensity of SQDs was only a tiny different compared with them before a month, illustrating that the SQDs@MnO 2 NS possessed a high stable fluorescence performance. For reproducibility, as can be seen from Figure 7c, the test was performed under the five sets of parallel solutions of SQDs in the same environment, all of the measured fluorescence intensities possess the semblable value with an outstanding RSD of 1.1%. This result was successful in confirming that the SQDs have preeminent reproducibility. Meanwhile, they have the potential benefit to the synthesis and application of the SQDs@MnO 2 NS. These results demonstrated that the proposed sensor has good selectivity, stability, and repeatability for the analysis of N 2 H 4 .

Detection of N 2 H 4 in Real Water Samples
To investigate the practicability of the probe of SQDs@MnO 2 NS, it was applied to detect N 2 H 4 in real samples. Three parallel water samples were obtained from the local lake and river for conducting the standard recovery test. The results were shown in Table 2, the N 2 H 4 was detected in the lake, river, serum, and saliva, where the recovery ranged from 90.21% to 109.1%, and the RSD was 0.9% to 4.5%, demonstrating that the fluorescent probe possesses practicability with promise for future applications.

Conclusions
In summary, we have developed a "turn-on" fluorescent sensor based on the SQDs@MnO 2 NS architectures for the detection of N 2 H 4 . The MnO 2 NS has a broad absorption band of MnO 2 NS at 280 to 650 nm, which could effectively quench the emission of fluorescence of SQDs, owing to the IFE mechanism. However, the fluorescent emission was recovered presenting the N 2 H 4 analysis target with a concentration in the range of 0.1 µM to 10 mM, with a LOD of 0.072 µM. In addition, the fluorescent sensor was successfully applied in real samples indicating the SQDs@MnO 2 NS probe was possess the potential ability to detect the N 2 H 4 in the environmental water samples.