Smartphone-Enabled Fluorescence and Colorimetric Platform for the On-Site Detection of Hg2+ and Cl− Based on the Au/Cu/Ti3C2 Nanosheets

Smartphone-assisted fluorescence and colorimetric methods for the on-site detection of Hg2+ and Cl− were established based on the oxidase-like activity of the Au–Hg alloy on the surface of Au/Cu/Ti3C2 NSs. The Au nanoparticles (NPs) were constructed via in-situ growth on the surface of Cu/Ti3C2 NSs and characterized by different characterization techniques. After the addition of Hg2+, the formation of Hg–Au alloys could promote the oxidization of o-phenylenediamine (OPD) to generate a new fluorescence emission peak of 2,3-diaminopenazine (ADP) at 570 nm. Therefore, a turn-on fluorescence method for the detection of Hg2+ was established. As the addition of Cl− can influence the fluorescence of ADP, the fluorescence intensity was constantly quenched to achieve the continuous quantitative detection of Cl−. Therefore, a turn-off fluorescence method for the detection of Cl− was established. This method had good linear ranges for the detection of Hg2+ and Cl− in 8.0–200.0 nM and 5.0–350.0 µM, with a detection limit of 0.8 nM and 27 nM, respectively. Depending on the color change with the detection of Hg2+ and Cl−, a convenient on-site colorimetric method for an analysis of Hg2+ and Cl− was achieved by using digital images combined with smartphones (color recognizers). The digital picture sensor could analyze RGB values in concentrations of Hg2+ or Cl− via a smartphone app. In summary, the proposed Au/Cu/Ti3C2 NSs-based method provided a novel and more comprehensive application for environmental monitoring.


Introduction
Many anions and cations often have a significant influence on physiological functions and the environmental system. For example, Cl − is a common anion with essential physiological functions which takes part in many important biological processes, such as the regulation of cell volume, membrane potential, and intracellular pH. However, excess Cl − can cause severe dehydration and even plant death, as well as health issues such as high blood pressure and various heart diseases [1][2][3]. Hg 2+ as global environmental pollutants were found in the air, soil, and water. Inorganic mercury (Hg 2+ ) can transform into neurotoxin methylmercury (MeHg) through bacterial conversion within water bodies, which can damage the human brain, lungs, and central nervous system via the food chain [4][5][6][7]. Therefore, the detection of trace Cl − and Hg 2+ is particularly urgent and necessary for disease diagnosis, environmental monitoring, and food safety evaluation [8][9][10][11][12][13]. Various conventional methods have been developed for the detection of Hg 2+ and Cl − , including inductively coupled plasma mass spectrometry (ICP-MS), energy dispersive X-ray spectroscopy (EDX), and ion chromatography [14][15][16][17][18][19]. Despite the significant accuracy and precision of the above methods, they suffer from limitations including expensive equip-

Characterization of Au/Cu/Ti3C2 NSs
Cu/Ti3C2 NSs was prepared by the one-step Lewis acidic etching route. Ti3AlC2 and CuCl2 were mixed evenly in a mortar and grinded for 10 min. The pulverized mixture was transferred to a porcelain crucible and roasted at 730 °C for 1 h. After cooling and quenching, FeCl3 solution and deionized water were added to wash the powder. The power was collected and dried in vacuum at 70 °C to obtain the products. The synthesized multilayer Cu/Ti3C2 NSs and Au/Cu/Ti3C2 NSs were characterized by SEM, TEM, XPS, and XRD. Figure 2a showed the XRD patterns of Ti3AlC2, Cu/Ti3C2 NSs, and Au/Cu/Ti3C2 NSs. Compared with the pristine Ti3AlC2, most diffraction peaks disappeared in the final Cu/Ti3C2 NSs. There were several wide low diffraction peaks in the 2θ ranging from 5° to 80°, which indicated that Ti3AlC2 had been successfully exfoliated into layered Cu/Ti3C2 NSs. The corresponding diffraction peak of Ti3C2 (002) shifted from 9.63° to 7.98°, demonstrating that the interlayer spacing had been expanded. The diffraction peaks at 2θ of 43.30°, 50.46°, and 74.09° corresponded to Cu, while those at 2θ of 38.1°, 44.5°, 65.1°, and 77.6° corresponded to Au, which demonstrated the successful loading of Au on the surface of Cu/Ti3C2 NSs [35,36]. Figure 2b shows the SEM image of multilayer Cu/Ti3C2 NSs, revealing that Cu/Ti3C2 NSs material had a good layered microstructure, which was consistent with the XRD data analysis and similar to what was previously reported for MXenes and obtained through etching methods with the HF-or F-containing electrolyte. Figure 2c,d shows EDS diagrams of multilayer Cu/Ti3C2 NSs. The element distribution map of Cu/Ti3C2 NSs showed that the nanosheets contained C, O, Cu, Ti, and a small amount of Al, which demonstrated that the majority of Al was etched by this method.

Characterization of Au/Cu/Ti 3 C 2 NSs
Cu/Ti 3 C 2 NSs was prepared by the one-step Lewis acidic etching route. Ti 3 AlC 2 and CuCl 2 were mixed evenly in a mortar and grinded for 10 min. The pulverized mixture was transferred to a porcelain crucible and roasted at 730 • C for 1 h. After cooling and quenching, FeCl 3 solution and deionized water were added to wash the powder. The power was collected and dried in vacuum at 70 • C to obtain the products. The synthesized multilayer Cu/Ti 3 C 2 NSs and Au/Cu/Ti 3 C 2 NSs were characterized by SEM, TEM, XPS, and XRD. Figure 2a showed the XRD patterns of Ti 3 AlC 2 , Cu/Ti 3 C 2 NSs, and Au/Cu/Ti 3 C 2 NSs. Compared with the pristine Ti 3 AlC 2 , most diffraction peaks disappeared in the final Cu/Ti 3 C 2 NSs. There were several wide low diffraction peaks in the 2θ ranging from 5 • to 80 • , which indicated that Ti 3 AlC 2 had been successfully exfoliated into layered Cu/Ti 3 C 2 NSs. The corresponding diffraction peak of Ti 3 C 2 (002) shifted from 9.63 • to 7.98 • , demonstrating that the interlayer spacing had been expanded. The diffraction peaks at 2θ of 43.30 • , 50.46 • , and 74.09 • corresponded to Cu, while those at 2θ of 38.1 • , 44.5 • , 65.1 • , and 77.6 • corresponded to Au, which demonstrated the successful loading of Au on the surface of Cu/Ti 3 C 2 NSs [35,36]. Figure 2b shows the SEM image of multilayer Cu/Ti 3 C 2 NSs, revealing that Cu/Ti 3 C 2 NSs material had a good layered microstructure, which was consistent with the XRD data analysis and similar to what was previously reported for MXenes and obtained through etching methods with the HF-or F-containing electrolyte. Figure 2c,d shows EDS diagrams of multilayer Cu/Ti 3 C 2 NSs. The element distribution map of Cu/Ti 3 C 2 NSs showed that the nanosheets contained C, O, Cu, Ti, and a small amount of Al, which demonstrated that the majority of Al was etched by this method.
The TEM images of Cu/Ti 3 C 2 NSs and Au/Cu/Ti 3 C 2 NSs clearly demonstrated that Cu nanoparticles and Au nanoparticles were uniformly distributed on the surface of Ti 3 C 2 NSs (Figure 3a,b). The HRTEM image in the upper right corner of Figure 3b showed a lattice spacing of 0.29 nm, which corresponds to the (111) crystal plane of Au. The elemental composition and chemical bond of Au/Cu/Ti 3 C 2 NSs were characterized by XPS spectra (Figure 3c). Au/Cu/Ti 3 C 2 NSs contained five characteristic peaks according to the full spectrum analysis of XPS: Cu 2p (977.6 eV), O 1s (530.9 eV), Ti 2p (458.9 eV), C 1s (284.5 eV), and Au 4f (85.5 eV). It proved that Au/Cu/Ti 3 C 2 NSs was successfully synthesized. The chemical bonds of Au/Cu/Ti 3 C 2 NSs were analyzed by high-resolution XPS spectra, including Cu 2p, Ti 2p, C 1s, and Au 4f. As shown in Figure 3d-g, a deconvolution peak corresponding to C-C existed in the high-resolution XPS spectrum of C 1s. The highresolution XPS spectrum of Ti 2p contained two deconvolution peaks corresponding to Ti-C and Ti-O. However, there were two deconvolution peaks in the high-resolution XPS spectra of Au 4f, indicating that Au mainly existed in the form of 0 valence and 1 valence. Molecules 2023, 28, x FOR PEER REVIEW 4 of 12 The TEM images of Cu/Ti3C2 NSs and Au/Cu/Ti3C2 NSs clearly demonstrated that Cu nanoparticles and Au nanoparticles were uniformly distributed on the surface of Ti3C2 NSs (Figure 3a,b). The HRTEM image in the upper right corner of Figure 3b showed a lattice spacing of 0.29 nm, which corresponds to the (111) crystal plane of Au. The elemental composition and chemical bond of Au/Cu/Ti3C2 NSs were characterized by XPS spectra (Figure 3c). Au/Cu/Ti3C2 NSs contained five characteristic peaks according to the full spectrum analysis of XPS: Cu 2p (977.6 eV), O 1s (530.9 eV), Ti 2p (458.9 eV), C 1s (284.5 eV), and Au 4f (85.5 eV). It proved that Au/Cu/Ti3C2 NSs was successfully synthesized. The chemical bonds of Au/Cu/Ti3C2 NSs were analyzed by high-resolution XPS spectra, including Cu 2p, Ti 2p, C 1s, and Au 4f. As shown in Figure 3d-g, a deconvolution peak corresponding to C-C existed in the high-resolution XPS spectrum of C 1s. The high-resolution XPS spectrum of Ti 2p contained two deconvolution peaks corresponding to Ti-C and Ti-O. However, there were two deconvolution peaks in the high-resolution XPS spectra of Au 4f, indicating that Au mainly existed in the form of 0 valence and 1 valence.

Oxidase-Like Activity of Au/Cu/Ti3C2 NSs
The feasibility of Au/Cu/Ti3C2 NSs + Hg 2+ to catalysis substrate OPD was investig by absorption spectra (Figure 4a) and fluorescence spectra (Figure 4b). In the absorp spectra, a new absorption peak at 420 nm appeared after the oxidation of OPD to 2+ Figure 3. (a) TEM image of Cu/Ti 3 C 2 NSs, (b) TEM image of Au/Cu/Ti 3 C 2 NSs, the inset of (b) was the HRTEM of Au/Cu/Ti 3 C 2 NSs, (c) survey XPS spectra of Au/Cu/Ti 3 C 2 NSs and spectra of Ti 3 C 2 MXene/Cu/Au NSs, (d) C 1s, (e) Ti 2p, (f) Cu 2p, and (g) Au 4f.

Oxidase-like Activity of Au/Cu/Ti 3 C 2 NSs
The feasibility of Au/Cu/Ti 3 C 2 NSs + Hg 2+ to catalysis substrate OPD was investigated by absorption spectra (Figure 4a) and fluorescence spectra (Figure 4b). In the absorption spectra, a new absorption peak at 420 nm appeared after the oxidation of OPD to 2,3-diaminopenazine (DAP) with the catalysis of Au/Cu/Ti 3 C 2 NSs + Hg 2+ . In the fluorescence spectra, only when Au/Cu/Ti 3 C 2 NSs and Hg 2+ co-exist, OPD could be rapidly oxidized and a new fluorescence emission peak of DAP was generated at 570 nm. The oxidization process of OPD by dissolved oxygen was slow. The addition of Hg 2+ to the solution resulted in the formation of Hg-Au alloys on the surface of Au/Cu/Ti 3 C 2 NSs, which could accelerate the oxidization of OPD to generate DAP with both new absorption and a fluorescence signal [37]. Therefore, we intended to develop an absorption and fluorescent method for the sensing of Hg 2+ via this phenomenon. To achieve the sensitive detection of Hg 2+ , several enzymatic factors that may influence the enzyme-substrate interactions were studied. The reaction time dependence of the Hg 2+ -induced fluorescence of DAP increasing was investigated. The fluorescence intensity increased when the reaction time was from 0 to 9 min and then kept stable with the reaction time from 9 to 12 min (Figure 4c,d). Therefore, 9 min was chosen as the optimal reaction time. The pH dependence of the Hg 2+ -induced fluorescence of DAP increasing was investigated in the pH range of 1.0 to 10.0. The fluorescence intensity increased abruptly from pH 1.0 to 6.0, and then decreased from pH 6.0 to 10.0 ( Figure 4e). Therefore, a pH of 6.0 was chosen as the optimal pH.

Detection of Hg 2+
The feasibility of this method for the detection of Hg 2+ was verified. Au/Cu/Ti 3 C 2 NSs reacted with different concentrations of Hg 2+ for 9 min to obtain the oxidase-like Au-Hg alloy, which catalyzed the oxidation of OPD to produce DAP. As shown in Figure 5a, when the concentration of Hg 2+ gradually increased in the range from 0 to 200 nM, the fluorescence emission at 570 nm from the catalytic product DAP also increased continuously. Therefore, we established the fluorescence method for the detection of Hg 2+ . The linear relationship between the fluorescence intensity of DAP and the Hg 2+ concentration was I = 1.434C + 159.575 (C is the concentration of Hg 2+ , nM). The linearly range for the detection of Hg 2+ was 8.0 to 200.0 nM, with the detection limit of 0.8 nM. Since the new absorption peak of DAP at 420 nm also enhanced with the increased concentration of Hg 2+ , the correlation between the absorbance and concentration of Hg 2+ was analyzed, as shown in Figure 5c. Therefore, we also established the colorimetric method for the detection of Hg 2+ . Within the concentration range of 24.0 to 200.0 nM, the linear equation of absorbance and Hg 2+ concentration was A = 1.82 × 10 −4 C + 0.0334 (C is the concentration of Hg 2+ , nM), and the detection limit was 2.4 nM.

Detection of Cl −
To study whether the Cl − can be detected via the influence on the oxidase-like activity of Hg-Au alloys on the surface of Au/Cu/Ti 3 C 2 NSs, a series of Cl − with concentrations in the range from 0 to 350 µM was added to the system Au/Cu/Ti 3 C 2 NSs + Hg 2+ ( Figure 6). Figure 6a showed that as the concentration of Cl − increased, the fluorescence intensity of DAP decreased gradually. As shown in Figure 6b, there were two linear relationships between the fluorescence intensity of DAP and Cl − concentration. Therefore, we established the fluorescence method for the detection of Cl − . When Cl − concentration increased from 10.0 µM to 150.0 µM, the linear equation of fluorescence intensity and Cl − concentration was I = −1.076C + 749.776 (C is the concentration of Cl − , µM). When the concentration of Cl − increased gradually from 150.0 µM to 350.0 µM, the linear equation of fluorescence intensity and Cl − concentration was I = −0.546C + 676.657 (C is the concentration of Cl − , µM). The detection limit was 27.0 nM. In addition, the relationship between the absorbance and the added Cl − concentration was also explored, as shown in Figure 6c. Therefore, we also established the colorimetric method for the detection of Cl − . In the range of 0.0 to 350.0 µM, the absorbance of the system had a good response to Cl − concentration. The

Detection of Hg 2+
The feasibility of this method for the detection of Hg 2+ was verified. Au/Cu/Ti3C2 NSs reacted with different concentrations of Hg 2+ for 9 min to obtain the oxidase-like Au-Hg alloy, which catalyzed the oxidation of OPD to produce DAP. As shown in Figure 5a, when the concentration of Hg 2+ gradually increased in the range from 0 to 200 nM, the fluorescence emission at 570 nm from the catalytic product DAP also increased continuously. Therefore, we established the fluorescence method for the detection of Hg 2+ . The linear relationship between the fluorescence intensity of DAP and the Hg 2+ concentration was I = 1.434C + 159.575 (C is the concentration of Hg 2+ , nM). The linearly range for the detection of Hg 2+ was 8.0 to 200.0 nM, with the detection limit of 0.8 nM. Since the new absorption peak of DAP at 420 nm also enhanced with the increased concentration of Hg 2+ , the correlation between the absorbance and concentration of Hg 2+ was analyzed, as shown in Figure 5c. Therefore, we also established the colorimetric method for the detection of Hg 2+ . Within the concentration range of 24.0 to 200.0 nM, the linear equation of absorbance and Hg 2+ concentration was A = 1.82 × 10 −4 C + 0.0334 (C is the concentration of Hg 2+ , nM), and the detection limit was 2.4 nM.

Detection of Hg 2+
The feasibility of this method for the detection of Hg 2+ was verified. Au/Cu/Ti3C2 NSs reacted with different concentrations of Hg 2+ for 9 min to obtain the oxidase-like Au-Hg alloy, which catalyzed the oxidation of OPD to produce DAP. As shown in Figure 5a, when the concentration of Hg 2+ gradually increased in the range from 0 to 200 nM, the fluorescence emission at 570 nm from the catalytic product DAP also increased continuously. Therefore, we established the fluorescence method for the detection of Hg 2+ . The linear relationship between the fluorescence intensity of DAP and the Hg 2+ concentration was I = 1.434C + 159.575 (C is the concentration of Hg 2+ , nM). The linearly range for the detection of Hg 2+ was 8.0 to 200.0 nM, with the detection limit of 0.8 nM. Since the new absorption peak of DAP at 420 nm also enhanced with the increased concentration of Hg 2+ , the correlation between the absorbance and concentration of Hg 2+ was analyzed, as shown in Figure 5c. Therefore, we also established the colorimetric method for the detection of Hg 2+ . Within the concentration range of 24.0 to 200.0 nM, the linear equation of absorbance and Hg 2+ concentration was A = 1.82 × 10 −4 C + 0.0334 (C is the concentration of Hg 2+ , nM), and the detection limit was 2.4 nM. intensity and Cl − concentration was I = −0.546C + 676.657 (C is the concentration of Cl − , µM). The detection limit was 27.0 nM. In addition, the relationship between the absorbance and the added Cl − concentration was also explored, as shown in Figure 6c. Therefore, we also established the colorimetric method for the detection of Cl − . In the range of 0.0 to 350.0 µM, the absorbance of the system had a good response to Cl − concentration. The linear equation was A = −1.24 × 10 −4 C + 0.109 (C is the concentration of Cl − , µM), and the detection limit was 1.0 µM.

POCT for Hg 2+ and Cl −
In order to simplify the detection procedure and realize instrument-free detection, portable test strips were prepared for the detection of Hg 2+ and Cl − . The smartphone-assist platform was established and consisted of test strips containing Au/Cu/Ti3C2 NSs and a smartphone with an app as the signal reader and analyzer for the test strips. As the concentration of Hg 2+ increased, the fluorescence color of the test strips changed from purple to pink (Figure 7a). Figure 7b

POCT for Hg 2+ and Cl −
In order to simplify the detection procedure and realize instrument-free detection, portable test strips were prepared for the detection of Hg 2+ and Cl − . The smartphone-assist platform was established and consisted of test strips containing Au/Cu/Ti 3 C 2 NSs and a smartphone with an app as the signal reader and analyzer for the test strips. As the concentration of Hg 2+ increased, the fluorescence color of the test strips changed from purple to pink (Figure 7a). Figure 7b

Selectivity for Hg 2+ and Cl −
Selectivity is an important factor to evaluate the practicality of the method. In order to explore the practicability of this system for the detection of Hg 2+ , different anions and cations, including Na + , Ca 2+ , Mg 2+ , K + , Zn 2+ , Cu 2+ , CO3 2− , F − , Ac − , SO4 2− , and OH − were added to the Au/Cu/Ti3C2 NSs + OPD system. As shown in Figure 8a, the Au/Cu/Ti3C2 NSs + OPD system had an obvious response to Hg 2+ , while the fluorescence intensity of the system did not change significantly when detecting other ions.

Selectivity for Hg 2+ and Cl −
Selectivity is an important factor to evaluate the practicality of the method. In order to explore the practicability of this system for the detection of Hg 2+ , different anions and cations, including Na + , Ca 2+ , Mg 2+ , K + , Zn 2+ , Cu 2+ , CO 3 2− , F − , Ac − , SO 4 2− , and OH − were added to the Au/Cu/Ti 3 C 2 NSs + OPD system. As shown in Figure 8a, the Au/Cu/Ti 3 C 2 NSs + OPD system had an obvious response to Hg 2+ , while the fluorescence intensity of the system did not change significantly when detecting other ions.
Selectivity is an important factor to evaluate the practicality of the method. In order to explore the practicability of this system for the detection of Hg 2+ , different anions and cations, including Na + , Ca 2+ , Mg 2+ , K + , Zn 2+ , Cu 2+ , CO3 2− , F − , Ac − , SO4 2− , and OH − were added to the Au/Cu/Ti3C2 NSs + OPD system. As shown in Figure 8a, the Au/Cu/Ti3C2 NSs + OPD system had an obvious response to Hg 2+ , while the fluorescence intensity of the system did not change significantly when detecting other ions.
In addition, the reliability of the Ti3C2 MXene/Cu/Au + Hg 2+ + OPD system for the detection of Cl − concentration was also discussed. Different anions and cations were added to the Ti3C2 MXene/Cu/Au + Hg 2+ + OPD system, and the fluorescence intensity of the detection system was recorded by a fluorescence spectrometer. As shown in Figure 8b, the Ti3C2 MXene/Cu/Au + Hg 2+ + OPD system had good specificity for the analysis of Cl − , while there was no obvious change in fluorescence intensity when detecting other ions. Therefore, it can be concluded that the detection system has good selectivity for Hg 2+ and Cl − . Meanwhile, the influence of Clconcentration on the detection of mercury ions was studied ( Figure S1). When the concentration of Clranged from 0 to 500 µM, there was no obvious effect on the detection of mercury ions.  In addition, the reliability of the Ti 3 C 2 MXene/Cu/Au + Hg 2+ + OPD system for the detection of Cl − concentration was also discussed. Different anions and cations were added to the Ti 3 C 2 MXene/Cu/Au + Hg 2+ + OPD system, and the fluorescence intensity of the detection system was recorded by a fluorescence spectrometer. As shown in Figure 8b, the Ti 3 C 2 MXene/Cu/Au + Hg 2+ + OPD system had good specificity for the analysis of Cl − , while there was no obvious change in fluorescence intensity when detecting other ions. Therefore, it can be concluded that the detection system has good selectivity for Hg 2+ and Cl − . Meanwhile, the influence of Cl − concentration on the detection of mercury ions was studied ( Figure S1). When the concentration of Cl − ranged from 0 to 500 µM, there was no obvious effect on the detection of mercury ions.

Real Sample Detection
In order to verify the applicability of this method in detecting Hg 2+ concentration in actual samples, water from Shahu Lake, East Lake, and Yangtze River was selected as actual samples, and the content of Hg 2+ was evaluated using test strips. The specific results are shown in Table 1. The recoveries of Hg 2+ concentration was 93.3 to 109.7%, and RSD ranged from 1.4% to 5.5%. The real sample detection for chloride ions was also investigated. As shown in Table S1, the recoveries of Cl − concentration vary from 93.3 to 109.7% with the RSDs of recoveries less than 5.5%. The results demonstrate that the developed method is practical and reliable for the detection of Hg 2 and Cl − . (Shanghai, China). All solutions were freshly prepared before use.

Instuments
The UV-vis absorption measurements were observed by a Lambda 35 UV analyzer (Perkin-Elmer, Waltham, MA, USA). The excitation and emission spectra were obtained on a LS55 spectrophotometer (Perkin-Elmer, Waltham, MA, USA). The transmission electron microscopy (TEM) was performed using a TecnaiG20 transmission electron microscope (FEI, Hillsboro, OR, USA) and LEPL-Model 2100 F instrument. High-resolution transmission electron microscopy (HRTEM) was measured by a JEM-2100 UHR (JEOL, Tokyo, Japan). X-ray photo electron spectroscopy (XPS) was performed with a VGEscalab 200 spectrometer using an aluminum anode (AlKα) operating at 510 W with a background pressure of 2 × 10 −9 mbar (Escalab, Waltham, MA, USA).

Preparation of Au/Cu/Ti 3 C 2 NSs
Cu/Ti 3 C 2 NSs was synthesized by a one-step Lewis acidic etching route. After being placed in a mortar, 0.5 g of Ti 3 AlC 2 and 2.6 g of CuCl 2 ·2H 2 O were mixed evenly and ground for 10 min. The pulverized mixture was transferred to a 50 mL porcelain crucible and roasted at 730 • C for 1 h. After cooling and quenching, 50.0 mL of 3.0 M FeCl 3 solution was added to the powder and stirred continuously for 3 h. The precipitate was washed with deionized water, collected, and dried for 12 h in vacuum at 70 • C.
Then, 100.0 mg of Cu/Ti 3 C 2 NSs was dispersed in 25.0 mL of deionized water and ultrasonicated for 5 h. The suspension with Cu/Ti 3 C 2 NSs was gently collected with centrifugation at 5000 r for 8 min. Then the suspension was further centrifuged at 9500 r for 30 min and the collected precipitates were dispersed in 5 mL of deionized water for future use.
Subsequently, 500 µL of 0.5 mol/L HAuCl 4 was added to the above solution, followed by 250 µL of 0.1 M NaBH 4 quickly, and stirred for 1 h. Then, the precipitates of Au/Cu/Ti 3 C 2 NSs were collected by centrifugation at 9500 r for 30 min and dispersed in 6 mL of deionized water for future use.

Fluorescence and Colorimetric Method for the Detection of Hg 2+ and Cl −
The method for the detection of Hg 2+ is described below. First, 60 µL of Au/Cu/Ti 3 C 2 NSs and 30 µL of Hg 2+ with different concentrations were successively added into 880 µL of deionized water. The Au-Hg alloys with oxidase activity were obtained after being incubated for 5 min. Finally, 30 µL of 0.1 M OPD was added into the mixture. The concentration of Hg 2+ was monitored by fluorescence and colorimetric signals after incubation at 37 • C for 9 min.
The method for the detection of Cl − is described below. First, 60 µL of Au/Cu/Ti 3 C 2 NSs, 50 µL of 8 µM Hg 2+ , and 20 µL of Cl − with different concentrations were successively added to 840 µL of deionized water. Then, 30 µL of 0.1 M OPD was added into the mixture after incubation for 5 min. The concentration of Cl − was monitored by fluorescence colorimetric signals after incubation at 37 • C for 9 min.

Preparation of Test Papers
Commercial fiber filter paper was used to make test paper with a diameter of 1 cm. The fiber filter paper was immersed in Au/Cu/Ti 3 C 2 NSs solution for 10 min and then dried in an oven at 60 • C to obtain the test paper. Afterwards, 30 µL of 0.1 M OPD and 30 µL of Hg 2+ with different concentrations were successively dropped on the test paper. Then, the colorimetric signals were recorded by a Lambda 35 UV analyzer. The colorimetric images were collected with a smartphone under 365 nm ultraviolet light. The test paper for Cl − was prepared with the same process with test paper for Hg 2+ .

Real Sample Analysis
The potential application of the proposed method was demonstrated by applying it to detecting Hg 2+ and Cl − in water samples. All the water as the real sample was taken from a nearby lake and river in Wuhan (China). The water from East Lake, Shahu Lake, and Yangtze River was selected as the research objects. The samples were filtered by 0.22 µm microporous membrane and diluted to 20 times as the real samples to be tested.

Conclusions
In this work, an efficient fluorescence and colorimetric method with the favor of a smartphone for the on-site detection of Hg 2+ and Cl − was fabricated based on Au/Cu/Ti 3 C 2 NSs. Characterization techniques confirmed the sucessful synthesis of Au/Cu/Ti 3 C 2 NSs. Because of the formation of the Au-Hg alloy with oxidase-like activity, the OPD was transferred into ADP with a fluorescence emission at 570 nm and an obvious color change. Therefore, a turn-on fluorescence and colorimetric method for the detection of Hg 2+ was established. As the addition of Cl − , the fluorescence intensity was constantly quenched to achieve the continuous quantitative detection of Cl − . A series of color variations of the paper strip can be observed by the naked eye and digital images with the introduction of different concentrations of Hg 2+ or Cl − . The smartphone with the app (color recognizers) could favor the digital images of Hg 2+ or Cl − . The Au/Cu/Ti 3 C 2 NSs-based platform displayed potent sensitivity, broad applicability, favorable selectivity, and significant simplicity. Our proposed Au/Cu/Ti 3 C 2 NSs-based method is a promising method for environmental monitoring.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28145355/s1. Figure S1: The influence of Cl − concentration on the detection of mercury ions (n = 3, the concentration of Hg 2+ is 40 nM); Table S1: Analytical results for the detection of Cl − in real samples (n = 3).

Conflicts of Interest:
The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are available from the authors.