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.
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 [
Conventional instrumental analysis methods required expensive instrumentation and complicated sample preparations for the trace determination of heavy metal ions [
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 [
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 [
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.
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.
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.
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:
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
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
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 [
Schematics of the formation of the Ag NCs.
(a) TEM images of the Ag NCs. (b) The particle size distribution histogram of the Ag NCs.
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
(a) XPS survey spectrum of the Ag NCs. (b-c) High-resolution XPS spectra of Ag3d and N1s, respectively.
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
(a) UV-Vis absorption spectrum of His (black), AgNO3 (red), and the Ag NCs (blue) solution. (b) The PL excitation and emission spectra of the Ag NCs. The inset in (b) is the Ag NCs solution under natural light (left) and under ultraviolet (right). (c) Time-resolved fluorescence spectrum of 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
Selectivity patterns of the Ag NCs to Cu2+ at pH = 4.3 (a) and Fe3+ at pH = 7 (b). All metal ions concentrations were 100
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 [
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
(a) Fluorescence response of the Ag NCs upon addition of different concentrations of Cu2+ from bottom to top: 0.1 × 10−12 M, 1 × 10−11 M, 1 × 10−10 M, 1 × 10−9 M, 1 × 10−8 M, 1 × 10−7 M, and 1 × 10−6 M. (b) The linear calibration ranges of the fluorescence intensity ratio to Cu2+ concentrations.
When the pH = 7 (Figure
(a) Fluorescence response of the Ag NCs upon addition of different concentrations of Fe3+ from top to bottom: 0 mM, 0.1 mM, 0.2 mM, 0.4 mM, 0.6 mM, 0.8 mM, 1 mM, and 5 mM. (b) The linear calibration ranges of the fluorescence intensity ratio to Fe3+ concentrations.
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
UV-Vis absorption spectrum of the Ag NCs upon addition of different concentrations of Cu2+.
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
Application of the probe for Cu2+ in real samples (data are average of three replicate measurements).
Sample | Cu2+ (pM) | Recovery (%) | |
---|---|---|---|
Added | Found | ||
Mineral water | 1 | 1.07 (±0.1) | 107 |
100 | 103 (±11) | 103 | |
10000 | 10073 (±110) | 101 |
The merits such as linear range, detection limit, and selectivity of the proposed Ag NCs for Cu2+ and Fe3+ are summarized in Table
Comparison of Ag NCs probe for the detection of Cu2+ and Fe3+ with previously reported fluorescent-based probes in aqueous solution.
Probe | Detected ion | Linear range | LOD (nM) | Selectivity | Ref |
---|---|---|---|---|---|
Ag NCs | Cu2+ | — | 10 | With no selectivity over ions even with the help of EDTA | [ |
Hg2+ | 0∼100 nM | 5 | |||
Au-Ag NCs | Cu2+ | 0.5 nM∼2.5 | 0.3 | Selective determination of Hg2+ by masking Cu2+ with EDTA | [ |
Hg2+ | 0.2 nM∼2.5 | 0.1 | |||
Ag NCs | Cu2+ | 0.1 nM∼20 | 28 | Masking Hg2+ with thymine and masking Cu2+ with potassium pyrophosphate | [ |
Hg2+ | 0.1 nM∼10 | 35 | |||
Ag NCs | Cu2+ | 0∼1 | 2.8 | Masking Cu2+ with EDTA for selective determination of Hg2+ | [ |
Hg2+ | 0.01∼0.5 | 1.0 | |||
Ag NCs | Fe3+ | 0.5 | 0.12 | — | [ |
Au NCs and Au/Ag NCs | Fe3+ | 5 | 1.11 | Masking Fe3+ with EDTA for selective determination of Hg2+ | [ |
Hg2+ | 5 nM∼5 | 1.56 | |||
Ag NCs | Cu2+ | 1 pM∼1 mM | 0.6 | Switching the selectivity by controlling solution pH (pH = 4.3 for Cu2+ determination and pH = 7 for Fe3+ determination) | This work |
Fe3+ | 100∼1000 | 9.8 |
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.
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
(a) GSH from bottom to top: 0 nM, 1 nM, 5 nM, 10 nM, 50 nM, 100 nM, 500 nM, 1
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.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest regarding the publication of the paper.
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.
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).