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

Abstract

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.

1. 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]. Hg²⁺ as global environmental pollutants were found in the air, soil, and water. Inorganic mercury (Hg²⁺) 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 Hg²⁺ 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 Hg²⁺ 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 equipment, tedious sample preparation, and on-site detection. Therefore, it is necessary to look for facile, portable, and cost-effective strategies for the detection of Cl⁻ and Hg²⁺.

The colorimetric method has attracted the most attention due to its simple readout and obvious color change which can be observed by the naked eye [20,21]. There is a great demand to achieve a high-sensitivity colorimetric method which can greatly expand its applications field [22]. Recently, the nanozyme-based colorimetric method was used for the detection of ion at the nmol/L level, which could be equivalent to the analytical results from expensive equipment. Zhou et al. developed the colorimetric detection of Hg²⁺ by Au nanoparticles formed with the H₂O₂ reduction of HAuCl₄ [23]. Logan et al. demonstrated amalgamated Au-nanoalloys with enhanced catalytical activity for the colorimetric detection of Hg²⁺ in seawater samples [24]. Wang et al. developed a single-nanozyme colorimetric array based on target-induced differential surface passivation for the quantification and discrimination of Cl⁻, Br⁻, and I⁻ [25]. However, Cl⁻ and Hg²⁺ usually coexist in the practical clinical or environmental matrix, with only a few works reported on colorimetric methods for the continuous monitoring of Cl⁻ and Hg²⁺. Yan et al. proposed an efficient strategy for the visual detection and removal of toxic Cl⁻ and Hg²⁺ based on the β-Cyclodextrin and graphene oxide co-strengthened AgRu bimetal mesoporous nanozyme [26]. Unfortunately, this method accelerated the oxidation of colorless 3,3′,5,5′-tetramethylbenzidine (TMB) with H₂O₂ in blue oxTMB. The instability of H₂O₂ could interfere with the accuracy of the corresponding sensing method. Therefore, it is still a challenge to explore the colorimetric methods for the continuous monitoring of Cl⁻ and Hg²⁺ based on oxidase-like nanozymes which catalyzed the substrate without H₂O₂.

As a new two-dimensional material, Ti₃C₂ nanosheets (NSs) as the most promising MXene material derived from the MAX phase have intriguing metallic conductivity, high stability, a large surface area and ease of functionalization, as well as biocompatibility, which has wide applications in the fields of biosensing, catalysis, energy, and nanomedicine [27,28,29,30,31,32]. MXene cannot be directly synthesized from M and X elements for their thermodynamic metastability. The employment of the HF etching strategy is generally used for the fabrication of MXene, although HF acid is severely corrosive and toxic. Therefore, it is very important to develop a fluorine-free method for MXene preparation for its applications. Recently, Ti₃C₂ NSs attracted wide attention for its enzyme-like activity. Chen et al. prepared nitrogen-sulfur-doped Ti₃C₂ NSs with peroxidase-like activity and an electrochemical ability for the quantitative detection of uric acid [33]. Wu et al. synthesized histidine-modified Ti₃C₂ NSs, which can simulate the catalytic performance of peroxidase for a colorimetric paper-based sensor of glucose [34]. But until now, reports about the Ti₃C₂ NSs-based nanozyme with oxidase-like activity as well as nanozyme-based sensing are limited.

Herein, we fabricated a smartphone-enabled fluorescence and colorimetric platform for the on-site detection of Hg²⁺ and Cl⁻ based on the oxidase-like activity of Au/Cu/Ti₃C₂ NSs (Figure 1). The Au nanoparticles (NPs) were in-situ loaded onto the surface of Cu/Ti₃C₂ NSs to form Au/Cu/Ti₃C₂ NSs. The Au/Cu/Ti₃C₂ NSs were characterized and their enzyme-like activities were studied. After the addition of Hg²⁺ with different concentrations, the formation of Hg–Au alloys could promote the oxidization of o-phenylenediamine (OPD) to generate a new fluorescence emission peak at 570 nm. Therefore, a turn-on fluorescence method for the detection of Hg²⁺ could be established. Given the interaction between Hg²⁺ and Cl⁻ and the inhibition of the formation of Hg–Au alloys, the fluorescence intensity was constantly quenched to achieve the quantitative detection of Cl⁻. Depending on the color change with the detection of Hg²⁺ and Cl⁻, a convenient colorimetric method for Hg²⁺ and Cl⁻ could be developed by using digital images combined with smartphones (color recognizers) to analyze RGB values in concentrations of Hg²⁺ or Cl⁻. The digital picture sensor based on a smartphone tested Cl⁻ and Hg²⁺ in water samples and held great promise for environmental monitoring.

2. Results and Discussion

2.1. 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.

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.

2.2. Oxidase-like Activity of Au/Cu/Ti₃C₂ NSs

The feasibility of Au/Cu/Ti₃C₂ NSs + Hg²⁺ 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/Ti₃C₂ NSs + Hg²⁺. In the fluorescence spectra, only when Au/Cu/Ti₃C₂ NSs and Hg²⁺ 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 Hg²⁺ to the solution resulted in the formation of Hg–Au alloys on the surface of Au/Cu/Ti₃C₂ 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 Hg²⁺ via this phenomenon. To achieve the sensitive detection of Hg²⁺, several enzymatic factors that may influence the enzyme-substrate interactions were studied. The reaction time dependence of the Hg²⁺-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 Hg²⁺-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.

2.3. Detection of Hg²⁺

The feasibility of this method for the detection of Hg²⁺ was verified. Au/Cu/Ti₃C₂ NSs reacted with different concentrations of Hg²⁺ 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 Hg²⁺ 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 Hg²⁺. The linear relationship between the fluorescence intensity of DAP and the Hg²⁺ concentration was I = 1.434C + 159.575 (C is the concentration of Hg²⁺, nM). The linearly range for the detection of Hg²⁺ 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 Hg²⁺, the correlation between the absorbance and concentration of Hg²⁺ was analyzed, as shown in Figure 5c. Therefore, we also established the colorimetric method for the detection of Hg²⁺. Within the concentration range of 24.0 to 200.0 nM, the linear equation of absorbance and Hg²⁺ concentration was A = 1.82 × 10⁻⁴C + 0.0334 (C is the concentration of Hg²⁺, nM), and the detection limit was 2.4 nM.

2.4. 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/Ti₃C₂ NSs, a series of Cl⁻ with concentrations in the range from 0 to 350 μM was added to the system Au/Cu/Ti₃C₂ NSs + Hg²⁺ (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 linear equation was A = −1.24 × 10⁻⁴C + 0.109 (C is the concentration of Cl⁻, μM), and the detection limit was 1.0 μM.

2.5. POCT for Hg²⁺ and Cl⁻

In order to simplify the detection procedure and realize instrument-free detection, portable test strips were prepared for the detection of Hg²⁺ 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 Hg²⁺ increased, the fluorescence color of the test strips changed from purple to pink (Figure 7a). Figure 7b showed the fluorescence color picture of the test strips with different concentrations of Cl⁻, and with the continuous addition of Cl⁻ with different concentrations, the fluorescence color of the test strips changed from yellow to light green. The app software (color recognizer) in the smartphone acquired the color parameters (R, G, B value) of the photos. The RGB ratio (R/B) had a linear relationship with the concentration of Hg²⁺ over the range of 8.0 to 200.0 nM (Figure 7c). The linear equation of R/B and Hg²⁺ concentration was R/B = 0.0048C + 0.79 (C is the concentration of Hg²⁺, nM). The detection limit was 0.8 nM. There are two linear relationships between R/B and Cl⁻ concentration, as shown in Figure 7d. Within the concentration range of 30.0 to 150.0 μM and 150.0 to 350.0 μM, the linear equations were R/B = −0.0016C + 1.309 and R/B = −7.396 × 10⁻⁴C + 1.191 (C is the concentration of Cl⁻, μM), and the detection limit was 3.0 μM. Compared with the fluorescence and colorimetric methods, the digital image method based on Au/Cu/Ti₃C₂ NSs was expected to be applied to the real-time and rapid detection of Hg²⁺ and Cl⁻ concentrations in the field.

2.6. Selectivity for Hg²⁺ 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 Hg²⁺, different anions and cations, including Na⁺, Ca²⁺, Mg²⁺, K⁺, Zn²⁺, Cu²⁺, CO₃²⁻, F⁻, Ac⁻, SO₄²⁻, 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 Hg²⁺, 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 + Hg²⁺ + OPD system for the detection of Cl⁻ concentration was also discussed. Different anions and cations were added to the Ti₃C₂ MXene/Cu/Au + Hg²⁺ + 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 + Hg²⁺ + 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 Hg²⁺ 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.

2.7. Real Sample Detection

In order to verify the applicability of this method in detecting Hg²⁺ concentration in actual samples, water from Shahu Lake, East Lake, and Yangtze River was selected as actual samples, and the content of Hg²⁺ was evaluated using test strips. The specific results are shown in

Table 1. Analytical results for detection of Hg²⁺ in real samples (n = 3).

Samples	Spiked (µM)	Found (µM)	Recovery (%)	RSD (%)
Shahu Lake	24	25.23	105.1	2.3
	80	87.78	109.7	2.1
	160	149.22	93.3	2.5
East Lake	24	23.04	96.0	2.8
	80	82.29	102.9	5.5
	160	159.1	99.4	1.4
Yangtze River	24	22.49	93.7	2.6
	80	83.39	104.3	3.1
	160	154.71	96.7	4.5

. The recoveries of Hg²⁺ 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 Hg² and Cl⁻.

3. Experimental

3.1. Materials

All reagents were of at least analytical grade and used as received without purification. Ti₃AlC₂ was acquired from Forsman Technology Co., Ltd. (Beijing, China). Hg(NO₃)₂, CuSO₄·5H₂O, and HAuCl₄ were supplied by Macklin Biochemical Co., Ltd. (Shanghai, China). O-Phenylenediamine (OPD) was purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). NaBH₄, NaCl, Na₂CO₃, FeCl₃, MgCl₂·6H₂O, ZnCl₂, KCl, CaCl₂, Na₂SO₄, and KNO₃ were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All solutions were freshly prepared before use.

3.2. 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).

3.3. Preparation of Au/Cu/Ti₃C₂ NSs

Cu/Ti₃C₂ NSs was synthesized by a one-step Lewis acidic etching route. After being placed in a mortar, 0.5 g of Ti₃AlC₂ and 2.6 g of CuCl₂·2H₂O 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 FeCl₃ 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/Ti₃C₂ NSs was dispersed in 25.0 mL of deionized water and ultrasonicated for 5 h. The suspension with Cu/Ti₃C₂ 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 HAuCl₄ was added to the above solution, followed by 250 µL of 0.1 M NaBH₄ quickly, and stirred for 1 h. Then, the precipitates of Au/Cu/Ti₃C₂ NSs were collected by centrifugation at 9500 r for 30 min and dispersed in 6 mL of deionized water for future use.

3.4. Fluorescence and Colorimetric Method for the Detection of Hg²⁺ and Cl⁻

The method for the detection of Hg²⁺ is described below. First, 60 µL of Au/Cu/Ti₃C₂ NSs and 30 µL of Hg²⁺ 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 Hg²⁺ 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/Ti₃C₂ NSs, 50 µL of 8 µM Hg²⁺, 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.

3.5. 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/Ti₃C₂ 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 Hg²⁺ 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 Hg²⁺.

3.6. Real Sample Analysis

The potential application of the proposed method was demonstrated by applying it to detecting Hg²⁺ 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.

4. Conclusions

In this work, an efficient fluorescence and colorimetric method with the favor of a smartphone for the on-site detection of Hg²⁺ and Cl⁻ was fabricated based on Au/Cu/Ti₃C₂ NSs. Characterization techniques confirmed the sucessful synthesis of Au/Cu/Ti₃C₂ 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 Hg²⁺ 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 Hg²⁺ or Cl⁻. The smartphone with the app (color recognizers) could favor the digital images of Hg²⁺ or Cl⁻. The Au/Cu/Ti₃C₂ NSs-based platform displayed potent sensitivity, broad applicability, favorable selectivity, and significant simplicity. Our proposed Au/Cu/Ti₃C₂ NSs-based method is a promising method for environmental monitoring.