Green Synthesis of Fluorescent Ag Nanoclusters for Detecting Cu<sup>2+</sup> Ions and Its “Switch-On” Sensing Application for GSH

Shang, Yunpeng; Gao, Hui; Li, Lei; Ma, Chaoqun; Gu, Jiao; Zhu, Chun; Yang, Zichen; Wang, Chengwei; Zhang, Ye; Chen, Guoqing

doi:https://doi.org/10.1155/2021/8829654

Journal of Spectroscopy

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2021 | Article ID 8829654 | https://doi.org/10.1155/2021/8829654

Green Synthesis of Fluorescent Ag Nanoclusters for Detecting Cu²⁺ Ions and Its “Switch-On” Sensing Application for GSH

Yunpeng Shang,¹Hui Gao,^1,2Lei Li,¹Chaoqun Ma,^1,2Jiao Gu,^1,2Chun Zhu,^1,2Zichen Yang,¹Chengwei Wang,¹Ye Zhang,¹and Guoqing Chen^1,2

Academic Editor: Li-June Ming

Received09 Sept 2020

Revised21 Feb 2021

Accepted02 Mar 2021

Published15 Mar 2021

Abstract

Herein, we prepared the L-histidine- (His-) protected silver nanoclusters (Ag NCs) by the microwave synthesis method. The synthesis process was rapid, facile, and environmentally friendly. Under 356 nm excitation, the as-prepared Ag NCs exhibited the blue fluorescence, and the fluorescence emission peak was located at 440 nm. The Ag NCs could successfully detect trace copper (Cu²⁺) ions in the aqueous solution and the limit of detection (LOD) was as low as 0.6 pM. Interestingly, the Ag NCs showed a different pH-dependent selectivity for both Cu²⁺ and iron (Fe³⁺) ions with no responses to other heavy metal ions. Furthermore, the as-fabricated fluorescent sensing system was utilized to detect glutathione (GSH, the LOD was 0.8 nM) by using the “switch-on” fluorescence recovery of Ag NCs through adding glutathione (GSH) to the Cu²⁺-Ag NCs solution.

1. Introduction

Since heavy metal ions would combine with other toxins in water to produce toxic substances, they could cause serious water pollution leading to a destructive effect on the environment and human health. Among them, iron (Fe³⁺) and copper (Cu²⁺) ions are two of the important heavy metal ions. For children and adults, ingest excessive Fe³⁺ will lead to acute or chronic iron poisoning [1–3]. Excessive Cu²⁺ has toxic effects on brain neurons and causes serious diseases such as Alzheimer’s disease, Wilson’s disease, and Parkinson’s disease [4–6].

Conventional instrumental analysis methods required expensive instrumentation and complicated sample preparations for the trace determination of heavy metal ions [7]. Therefore, finding a fast, sensitive, and selective alternative method is one of the most important research subjects. In recent years, fluorescent nanomaterials have been widely used to develop sensitive fluorescent sensing probes due to their high sensitivity, high selectivity, and wide measurement range [8–13]. Silver nanoclusters (Ag NCs), as novel fluorescent nanomaterials, have been widely used as fluorescent sensing probes in sensing, biorecognition, chemical detection, and catalysis because of their high stability, good water solubility, easy modification, good biocompatibility, and strong bleaching resistance [14–17]. Various preparation methods of Ag NCs including UV-light mediated synthesis and microwave synthesis have been proposed [18–20]. The Ag NCs synthesized with different stabilizing ligands such as DNA, protein, peptide, and amino acid possess different optical properties and application areas [21–26].

For the detection, many methods of detection of Cu²⁺ using the Ag NCs as the probe have been reported. Jing Liu et al. synthesized Ag NCs with PMAA for the detection of Cu²⁺, and the limit of detection (LOD) was 100 nM [27]. Na Xiao et al. synthesized Ag NCs with glutathione for the detection of Cu²⁺, and the LOD was 27 nM [28]. Elaheh Babaee et al. synthesized a dual-emissive ratiometric nanohybrid probe for the detection of Cu²⁺, and the LOD was 7.0 nM [9]. The above-mentioned methods could detect very low concentrations of Cu²⁺, but it is still of great significance to sensitive detection of Cu²⁺.

In this research, we prepared an Ag NCs fluorescent probe for the detection of Cu²⁺ and Fe³⁺ by changing the pH value. The Ag NCs were synthesized using histidine as stabilizer and reductant by microwave synthesis method. Under 356 nm excitation, the Ag NCs solution showed the blue fluorescence with the fluorescent emission peak at 440 nm. The fluorescence intensity response of Ag NCs to the Cu²⁺ and Fe³⁺ was used as the detection signal. A simple pH-tuning method could achieve the selective detection of Cu²⁺ and Fe³⁺. According to this method, the Ag NCs selectively respond to Cu²⁺ and Fe³⁺ at pH = 4.3 and 7, respectively. In addition, as an important tripeptide, glutathione (GSH) has always been a popular detection material [35–37]. Since GSH with thiol bonds could easily form strong complexation with Cu²⁺ [29], we built a fluorescent sensing system to detect GSH. The fluorescence of the Ag NCs was recovered by forming a strong complex of GSH and Cu²⁺. It has application prospects in water pollution and dairy product detection.

2. Materials and Methods

2.1. Chemicals

Silver nitrate (AgNO₃, AR), L-histidine (His, BR), nitric acid (HNO₃, AR), hydrochloric acid (HCl, AR), copper chloride (CuCl₂, AR), barium chloride (BaCl₂, AR), calcium chloride (CaCl₂, AR), chromium chloride (CrCl₃, AR), iron chloride (FeCl₃, AR), potassium chloride (KCl, AR), magnesium chloride (MgCl₂, AR), manganese chloride (MnCl₂, AR), sodium chloride (NaCl, AR), lead chloride (PbCl₂, AR), aluminum chloride (AlCl₃, AR), sodium dihydrogen phosphate (NaH₂PO₄, AR), and disodium hydrogen phosphate (Na₂HPO₄, AR) were purchased from Sinopharm Group Chemical Reagent Co., Ltd. Glutathione reduced (GSH, 99%) and ferrous chloride tetrahydrate (FeCl₂·4H₂O, AR) were purchased from MACKLIN. Ultrapure water was used throughout all experiments.

2.2. Synthesis of Ag NCs

In the experiment, all glassware was thoroughly washed with freshly prepared aqua regia (HCl/HNO₃, 3 : 1) and rinsed with ultrapure water prior. In a typical synthesis, an aqueous solution of AgNO₃ (0.25 M, 1 mL) was mixed with an aqueous solution of L-histidine (0.125 M, 100 mL) and stirred quickly at room temperature for 10 min. Then the mixture solution was heated (700 W, 2450 MHz) in a microwave oven for 8 min. The color of the solution changed from colorless to light yellow. The solution was then allowed to cool to the ambient temperature before further purification by the dialysis (the dialysis membrane with a molecular mass of 500 Da) for 24 h. The dialyzed solution was freeze-dried to obtain Ag NCs powder and stored in the fridge for further use.

2.3. Apparatus and Characterization

Fluorescence emission spectra and excitation spectra were recorded on FLS920 (Edinburgh, Livingston, England). The UV-Vis absorption spectra were measured on the UV-Vis spectrophotometer (Shimadzu, Suzhou, China). The X-ray photoelectron spectra (XPS) were recorded on an ESCALAB 250xi X-ray photoelectron spectrometer (Waltham, MA, USA). Transmission electron microscopy (TEM) image was gained from a JEOL 2100F microscope (Peabody, MA, USA) operating at a maximum acceleration voltage of 200 kV. The domestic microwave oven was from Midea, Guangdong, China.

2.4. Quantum Yield Measurements

The quantum yield (QY) of the Ag NCs in water was determined by the reference method. The reference standard material is quinine sulfate (QY = 55% in 0.1 M H₂SO₄). And the formula of relative QY is as follows:where is the QY of the Ag NCs and A and I are the absorbance and the integral intensity (excited at 330 nm) of the Ag NCs. n is the refractive index (1.33 in water). The subscript “s” represents quinine sulfate, and “x” represents the Ag NCs. We prepared a series of different concentrations of the Ag NCs and quinine sulfate solution (adjusting the concentration so that their absorbance was between 0 and 0.2) to make the results more accurate.

2.5. Detection of Cu²⁺ and Fe³⁺ Ions

All fluorescence measurement conditions were set as follows: the excitation and emission slit were 4 nm and 6 nm, respectively, the excitation wavelength was 356 nm, and emission was recorded from 380 nm to 600 nm. The typical detection method of Cu²⁺ and Fe³⁺ ions was as follows. The Ag NCs powder was dispersed in water at a concentration of 10 mg/mL for further use. The pH value of the Ag NCs fluorescence probe solution was set at 4.3 for Cu²⁺ and 7.0 for Fe³⁺. Then a certain amount of Cu²⁺ and Fe³⁺ solution was added, and the fluorescence spectrum was recorded 30 minutes later. The ratio of I₀ and I (I₀/I or I/I₀) was considered to plot the calibration curve and prediction of spiked concentrations, where I₀ and I were the fluorescence intensity in the absence and presence of ions. For the selectivity and interference studies, an identical concentration (100 μM) of the stock solution of other metal ions (i.e., Na⁺, Mg²⁺, Ca²⁺, Mn²⁺, Cu²⁺, Cd²⁺, Pb²⁺, Ba²⁺, Fe³⁺, Fe²⁺, Al³⁺, and Cr³⁺) was mixed with the probe solution under the same conditions and recorded the fluorescence spectrum 30 minutes later.

2.6. Detection of GSH

All fluorescence measurement conditions were set as above. The typical detection method of GSH was as follows. The Ag NCs solution was prepared with a pH = 4.3; then the stock solution of Cu²⁺ (1 mM) was mixed. After 30 minutes, we added a certain amount of GSH solution with different concentrations and recorded the fluorescence spectrum 30 minutes later. The difference between I and I₀ (I – I₀) was considered; that is, the calibration curve and prediction could be drawn, where I₀ and I were the fluorescence intensity of the Cu²⁺-Ag NCs probe system in the absence and presence of GSH.

3. Results and Discussion

3.1. Characterization of Ag NCs

The fluorescent Ag NCs were prepared by the microwave method with the His as the reductant and stabilizer. The product exhibited a light-yellow solution. In the synthesis process of the Ag NCs, Ag⁺ was reduced to Ag⁰ by the imidazolyl groups of the His, and the carboxyl group played an important role in protecting the silver core [8]. All reactants were nontoxic, and the synthesis method was a simple and green process, as shown in Scheme 1. TEM image showed that the Ag NCs were the monodisperse spheres with an average particle size of 4.15 ± 0.8 nm (Figure 1). In the HR-TEM image, obvious lattice fringes could be seen with a lattice distance of 2.27 Å (Figure S1).

(a)

(b)

By XPS, surface composition and elemental analysis for the Ag NCs were characterized. The four peaks of the Ag NCs at 285, 368, 400, and 531 eV (Figure 2(a)) could be attributed to C_1s, Ag_3d, N_1s, and O_1s, respectively. The results showed that the Ag NCs were mainly composed of C, O, N, and Ag. As shown in Figure 2(b), the two peaks at about 367.8 and 373.9 eV were assigned to the Ag 3d levels. These two peaks could be deconvoluted into four distinct component peaks. The peaks at 368.8 eV (3d_5/2) and 375.2 eV (3d_3/2) shown by green line and light blue line were assigned to Ag⁺. The other peaks at 367.7 eV (3d_5/2) and 373.8 eV (3d_3/2) shown by red and dark blue lines were assigned to Ag⁰. Ag⁰ and Ag⁺ could be the core and Ag ions on the surface of the Ag NCs, respectively. Ag ions played an important role in adsorbing the His to stabilize the Ag NCs. The N_1s XPS spectrum could be deconvoluted into three distinct component peaks (Figure 2(c)). The peaks at 398 eV, 400 eV, and 408.1 eV shown by red line, green line, and blue line were attributed to C-N, N-H, and -NO₂. The peak that appeared at 408.1 eV showed that some of the amino groups of L-histidine reduced Ag⁺ to Ag⁰ and the amino groups were oxidized to -NO₂ groups [26].

(a)

(b)

(c)

3.2. The Optical Properties of the Ag NCs

Optical properties including UV-Vis absorption, fluorescence excitation and emission, and time-resolved fluorescence spectra of the Ag NCs were investigated. As shown in Figure 3(a), the absorption peak at about 300 nm of AgNO₃ disappeared, while a new absorption band appeared at 250–300 nm. In addition, the absorption peak near 411 nm was the characteristic surface Plasmon resonance (SPR) peak of large-size silver nanoparticles [30, 31]. There was no absorption peak emerging around 411 nm, as Figure 3(a) shows. Therefore, it could prove that the product was the Ag NCs with a core diameter of less than 2 nm. The fluorescence spectra of the Ag NCs solution, the His solution, and the solution mixed with His and AgNO₃ were compared under the excitation at 365 nm. The fluorescence of the His solution and the solution mixed with His and AgNO₃ could not be observed under the same excitation (Figure S2). Therefore, the Ag NCs were the fluorescence substance in the solution. As shown in Figure 3(b), the fluorescence emission reached the maximum at 356 nm excitation with the emission maximum at 440 nm. In addition, the Ag NCs solution under natural light had no obvious luminescence, while under ultraviolet (365 nm), it exhibited the blue fluorescence. Using quinine sulfate in 0.1 M H₂SO₄ as a reference, the QY was 5.2%. Time-resolved fluorescence spectrum (Figure 3(c)) showed that the fluorescence lifetime of the Ag NCs was 1.13 ns (13.87%), 3.98 ns (51.57%), and 8.25 ns (34.56%), and the average lifetime was 5.06 ns (Table S1).

(a)

(b)

(c)

3.3. Effects of Solution pH on Detection of Cu²⁺ and Fe³⁺ by the Ag NCs

The influence of the solution pH was examined to optimize the sensing conditions. It was found that the sensitivity and selectivity of the Ag NCs detection of Cu²⁺ and Fe³⁺ were dependent on the pH value. The Ag NCs had a high response to Cu²⁺ at pH = 4.3 (Figure 4(a)) and to Fe³⁺ at pH = 7 (Figure 4(b)). And at those two pH value conditions, the effect of other metal ions on the fluorescence intensity of the Ag NCs was very small. Therefore, pH = 4.3 and pH = 7 were selected as the detection condition for Cu²⁺ and Fe³⁺ ions, respectively. The results showed that the selectivity to Cu²⁺ and Fe³⁺ could be achieved by controlling the pH value of the Ag NCs fluorescent probe solution without adding other chelating agents.

(a)

(b)

After adding Cu²⁺ and Fe³⁺ ions, the Ag NCs solution exhibited different responses under different pH value. It could be attributed to the different binding mechanism between the two ions and His. According to the previous reports, His presented a strong binding affinity with Cu²⁺ via coordinating Cu²⁺ through the amino group and the imidazole ring [32]. The terminal amino and the imidazole nitrogen donors could form a six-membered chelate. In the presence of excess ligand, bis(ligand) complex could be formed in slightly acidic samples. When the pH increased, the amino group and the imidazole ring were deprotonated [33]. It could decrease the Cu²⁺-binding affinity with His. In addition, the Fe³⁺ could coordinate with carboxyl groups of the surface ligand. In acidic media, the carboxyl groups were easily protonated at a low pH value. Hence, neutral media were the most suitable condition for Ag NCs detecting Fe³⁺ [8, 34]. In alkaline media, OH⁻ and Fe³⁺ (Cu²⁺) could form precipitation. It could result in complexing capability getting weak. Hence, when pH = 4.3 and 7, the Ag NCs showed a high response to Cu²⁺ and Fe³⁺, respectively.

3.4. Calibration Curves and Detection Limits for Sensing Cu²⁺ and Fe³⁺

The above-mentioned optimal pH conditions for the Cu²⁺ and Fe³⁺ were then employed to construct their calibration curves based on the as-prepared Ag NCs. When the concentration of Cu²⁺ was less than 1 × 10⁻⁶ M (Figure 5(a)), the fluorescence intensity gradually increased with the increasing concentration of the Cu²⁺. When the concentration of Cu²⁺ was more than 1 × 10⁻³ M, the fluorescence intensity decreased with the increasing of the concentration of Cu²⁺ (Figure S3). Figure 5 showed that the fluorescence emission intensity ratio was sensitively and proportionately increased with an increasing concentration of Cu²⁺ at 440 nm. A linear relationship from 1 × 10⁻¹²∼1 × 10⁻⁶ M could be described (I/I₀) = 0.0155 [log C] + 1.294 (R² = 0.9897). And the LOD for Cu²⁺ was as low as 0.6 pM.

(a)

(b)

When the pH = 7 (Figure 6(a)), the fluorescence intensity decreased with the increasing concentration of Fe³⁺ ions. The fluorescence intensity ratio of the Ag NCs at pH = 7 (Figure 6(b)) possessed a linear relationship with the concentration of Fe³⁺ from 100∼1000 μM with a regression equation of (I₀/I) = 0.0008 [C_Fe³⁺] + 0.9353 (R² = 0.9944). The LOD for Fe³⁺ is 9.8 μM.

(a)

(b)

The Ag NCs showed different response mechanism for the above two Cu²⁺ concentration ranges. The fluorescence of Ag NCs was enhanced at low concentration (under 1 μM) of Cu²⁺ and quenched at high concentration (over 1 mM) of Cu²⁺. The different response of the Ag NCs toward the two kind Cu²⁺ concentration ranges could be attributed to the different binding degree of Cu²⁺ with His. As shown in Figure S4, after adding two concentrations of Cu²⁺ (1 mM and 1 nM), the lifetimes of the Ag NCs samples changed. It proved that the fluorescence dynamic quenching of the Ag NCs due to the Cu2+ was binding to His. The proposed mechanisms for the interaction of the Ag NCs by two concentrations of Cu²⁺ were further confirmed by the analysis of UV-Vis absorption spectrum and TEM images. As shown in Figure 7, when the concentration of Cu²⁺ was 1 mM, the aggregation of the Ag NCs was indicated because the absorbance of the absorption peak at 250–300 nm was enhanced. The aggregation of the Ag NCs in the presence of Cu²⁺ (Figure S5(a)) could be attributed to the interaction between the His shell of the Ag NCs and Cu²⁺. The quenched fluorescence could be attributed to aggregation of the Ag NCs by the simple coordination of Cu²⁺ with the amino groups of the His ligand. When the concentration of Cu²⁺ was 1 nM, it was indicated that the Ag NCs were without aggregation because the absorption spectra at 250–300 nm were unchanged (Figure 7). It could be seen that the Ag NCs still have good dispersion in the presence of Cu²⁺ (Figure S5(b)). This might be due to the fact that the low concentration of Cu²⁺ was not enough to induce the Ag NCs aggregation. In addition, the absorption peak of the π bond groups at 200 nm decreased (Figure 7). It could be attributed to the addition of Cu²⁺ which only improved the spatial structure of the functional groups on the surface of the Ag NCs. It was supposed that the fluorescence intensity was enhanced due to the spatial structural change of the Ag NCs.

3.5. Application to Water Samples

As mentioned above, the fluorescence of Ag NCs was enhanced in the presence of the Cu²⁺ at a low concentration without interference from other ions. Therefore, the proposed probe could be used for the detection of Cu²⁺ in real samples. To verify this, mineral water samples spiked with different concentrations of Cu²⁺ were examined. Table 1 summarizes the obtained results for the prediction of the concentrations of Cu²⁺. As shown in Table 1, there was an excellent percent recovery for Cu²⁺ in the mineral water samples. Therefore, the probe displayed high capability for the determination of the copper ions in real samples.

3.6. Comparison of the Ag NCs with Previously Reported Fluorescent Nanoprobes

The merits such as linear range, detection limit, and selectivity of the proposed Ag NCs for Cu²⁺ and Fe³⁺ are summarized in Table 2. Compared with other fluorescent probes reported for the determination of two ions, according to Table 2, the reported probes’ selective determination of two ions in the aqueous solution was to mask one of these ions with a masking agent such as EDTA. In most cases, EDTA as an efficient Cu²⁺ and Fe³⁺ chelating agent could not be used to selectively determine Cu²⁺ and Fe³⁺.

This work proposed a method for selective determination of Cu²⁺ and Fe³⁺ only by controlling the solution pH value. For example, in the acidic media, the easier protonation of carboxyl groups at low pH value resulted in the inability of Fe³⁺ to coordinate with carboxyl groups. Therefore, Fe³⁺ could not disturb the determination of Cu²⁺ ions.

3.7. Calibration Curves and Detection Limits for Sensing of GSH

Since GSH is a very important antioxidant, Cu²⁺-Ag NCs sensing system was further utilized for the detection of GSH. A “switch-on” effect on recovery of the fluorescence of Ag NCs could be expressed in this sensing system. A wide detection range and low detection limit could be obtained by this method. As shown in Figure 8, the fluorescence intensity of the Ag NCs was obviously recovered at 440 nm. A linear relationship for GSH was obtained from 1 nM to 8 mM with a regression equation of (I – I₀) = 12107.6–1234.49(−log10 (C)) (R² = 0.9888), where I₀ and I were the fluorescence intensity of the Cu²⁺-Ag NCs probe system in the absence and presence of GSH. The LOD for GSH was 0.8 nM.

(a)

(b)

4. Conclusions

In this research, L-histidine-protected Ag NCs were prepared by the simple synthesis process. The Ag NCs were the monodisperse spheres with an average particle size of 4.15 nm. The Ag NCs could be utilized for selective detection of Cu²⁺ and Fe³⁺ by simply adjusting the pH value of the Ag NCs solution without using the masking agent. And a fluorescent sensing system was further built for the detection of GSH. Based on the above method, a low LOD of the Cu²⁺ and GSH could be obtained.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest regarding the publication of the paper.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (Grant no. 2018YFC1604204), the Key Research and Development Program of Jiangsu Province (no. BE2020756), the National First-Class Discipline Program of Food Science and Technology (Grant no. JUFSTR20180302), and the Jiangsu Province Post-Doctoral Fund (Grant no. 2019K241).

Supplementary Materials

Table S1: the fluorescent lifetime of the Ag NCs. Figure S1: the HR-TEM image of the Ag NCs. The yellow line labels the lattice distance of the Ag. Figure S2: fluorescence spectrum of His solution (red), solution mixed with His and AgNO3 (blue), and Ag NCs solution (black) excited at 365 nm. Figure S3: fluorescence response of the Ag NCs upon addition of different concentrations of Cu2+ from top to bottom: 0 mM, 1 mM, 2 mM, 4 mM, 6 mM, and 8 mM. Figure S4: time-resolved fluorescence spectrum of the Ag NCs upon addition of different concentrations of Cu2+: 0,1 mM and 1 nM. Figure S5: TEM images of the Ag NCs in the presence of Cu2+: (a) 1 mM. (b) 1 nM. (Supplementary Materials)

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Copyright © 2021 Yunpeng Shang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Journal of Spectroscopy

Green Synthesis of Fluorescent Ag Nanoclusters for Detecting Cu2+ Ions and Its “Switch-On” Sensing Application for GSH

Abstract

1. Introduction

2. Materials and Methods

2.1. Chemicals

2.2. Synthesis of Ag NCs

2.3. Apparatus and Characterization

2.4. Quantum Yield Measurements

2.5. Detection of Cu2+ and Fe3+ Ions

2.6. Detection of GSH

3. Results and Discussion

3.1. Characterization of Ag NCs

3.2. The Optical Properties of the Ag NCs

3.3. Effects of Solution pH on Detection of Cu2+ and Fe3+ by the Ag NCs

3.4. Calibration Curves and Detection Limits for Sensing Cu2+ and Fe3+

3.5. Application to Water Samples

3.6. Comparison of the Ag NCs with Previously Reported Fluorescent Nanoprobes

3.7. Calibration Curves and Detection Limits for Sensing of GSH

4. Conclusions

Data Availability

Conflicts of Interest

Acknowledgments

Supplementary Materials

References

Copyright

Green Synthesis of Fluorescent Ag Nanoclusters for Detecting Cu²⁺ Ions and Its “Switch-On” Sensing Application for GSH

2.5. Detection of Cu²⁺ and Fe³⁺ Ions

3.3. Effects of Solution pH on Detection of Cu²⁺ and Fe³⁺ by the Ag NCs

3.4. Calibration Curves and Detection Limits for Sensing Cu²⁺ and Fe³⁺