Copper Nanoclusters for Heavy Metal Ions Detection: History
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Contributor:
Congying Shao
,
Shun Lu

Heavy metal ions (HMIs) can accumulate in the human body and cause poisoning. They can enter the biological chain from nature in many ways and are difficult to degrade in the body. HMIs can be detected by colorimetric methods, fluorescence methods, and point-of-care testing (POCT). POCT technology based on smartphone has the advantages of on-site diagnosis and portability. However, the technology relies on a wide variety of accessories that can be affected by environmental changes. Colorimetric methods for color evaluation are an important problem, and another problem is the stability of color. The fluorescence method is widely used in HMIs determination because of its advantages of simple operation, low cost, and high sensitivity. In addition, the fluorescence sensor has the characteristics of strong specificity, fast detection speed, and high accuracy. CuNCs are used as sensors to detect HMIs. The low-cost detection method brings convenience to the actual detection.

  • copper nanoclusters
  • fluorescent sensors
  • heavy metal ions

1. Silver Ion

Silver is used in cosmetics, industry, catalysts, and other fields. The discharge of waste liquid is widely used in industry and will also pollute water and soil [1]. Ag+ has a great influence on human health. The accumulation of Ag+ in the human body can cause cytotoxicity, body failure, and mitochondrial dysfunction [2]. Therefore, it is necessary to develop a simple and rapid silver ion detection method. The requirement of silver ion detection can be satisfied by using CuNCs as probes. Shao et al. made MMI-CuNCs as a probe to detect Ag+ [3]. The protective agent was MMI, and the reducing agent was hydrazine hydrate. The detection limit was 6.7 nM, which could be applied to the detection of Ag+ in human serum samples. Shao et al. also proposed to make a nanoprobe with dithiothreitol and eggshell membrane (ESM) [4]. Concurrently, they made a sensor to complete the detection of Ag+. It caused fluorescence quenching in the presence of Ag+. Zhang et al. synthesized CuNCs by sonochemistry using N-acetyl-L-cysteine (NAC) [5], which can be used for the selective detection of Ag+. The limit of detection (LOD) was as low as 7.76 × 10−11 M. The detection mechanism was attributed to dynamic quenching. The addition of Ag+ made the particles aggregate and the fluorescence lifetime of the NAC-CuNCs system decreased. Importantly, the synthesis process only takes 15 min, and the detection time is short in actual water samples. Glucose (Glc) was used as a reducing agent to prepare Glc-CuNPs [6], and the detection was completed by the fluorescence turn-off mechanism caused by the interaction between the probe and Ag+. The fluorescence of Glc-CuNPs changed significantly with increasing Ag+ concentration. In addition, CuNCs/ZIF-8(72) was prepared using CuNCs, Zn(NO3)2·6H2O, 2-methylimidazole (2-MIM,72 mg) as raw materials [7]. Interestingly, CuNCs/ZIF-8(72) can also be used for Ag+ detection by fluorescence turn-off mechanism. The mechanism is the strong complexation between sulfhydryl in CuNCs/ZIF-8 (72) and Ag+. The above-mentioned works (Table 1) prove the practicability and broad application prospect of CuNCs in Ag+ detection. In the reports the researchers investigated, the lowest detection limit of Ag+ detected by CuNCs was 1.2 pM [8]. These works are performed by fluorescent turn-off mechanism to complete Ag+ detection, which is applied to human serum or actual water samples. Using eggshell membrane as raw material to synthesize NCs has the advantages of green and low cost [9][10][11]. The synthesis of NCs by ultrasonic chemistry is also a useful way.
Table 1. Report on detection of Ag+ by CuNCs.
Type of CuNCs λex/λem
(nm)		 Read Out Sensing Mechanism Reaction
Time		 Limit
of Detection		 Published Time Reference Value Ref.
MMI-CuNCs 322/476 Turn-off AIQ, Static quenching 30 min 6.7 nM 2022 1.2 pM
2021
[8]		 [3]
CuNC/ESM 360/623 Turn-off high-affinity metallophilic interactions — — 2019 [4]
NAC-CuNCs 340/630 Turn-off dynamic quenching 2 min 7.76 × 10−11 M 2022 [5]
Glc-CuNPs 472/542 Turn-off AIQ 30 min — 2022 [6]
CuNCs/ZIF-8 360/627 Turn-off the formation of Ag-S bonds 3 min 0.33 μM 2022 [7]

2. Mercury Ion

Major sources of mercury include fossil fuel burning and mineral industry production. Mercury does not degrade naturally and enters the human body through the biological chain causing serious effects [12]. Mercury can accumulate in various forms in living organisms. Long-term exposure to mercury can cause neurological dysfunction and interferes with cell function [13]. Therefore, the detection of mercury levels is essential for environmental and biological health. The reducing agents were N2H4·H2O, NH2OH·HCl, and Vitamin C, and a strip sensor was made to detect Hg2+ [14]. The fabrication of sensing bands makes it easier and more convenient to monitor ions in real-time. Vasimalai et al. synthesized TG-CuNCs using 1-thio-β-glucose as a ligand [15]. The detection limit of Hg2+ is 1.7 nM. At the same time, a smartphone-assisted paper kit was designed for on-site monitoring of Hg2+ in tap water, rivers, and ponds. Furthermore, CuNCs prepared with turmeric root extract as a template can be used as a fluorescence probe to detect Hg2+, and its quenching mechanism was AIQ [16]. At room temperature, the linear range was 0.0005–25 µM, and the detection limit is 0.12 nM. The analysis of tap water, river water, and canal water demonstrated that the method had good accuracy. Importantly, the biomass used to synthesize CuNCs comes from nature and is more environmentally friendly. Yang et al. made carbon dots-CuNCs (CDs-CuNCs) with dual-emission wavelengths [17]. In the presence of Hg2+, the pink fluorescence changes to blue fluorescence. The addition of Hg2+ decreased the red fluorescence of CuNCs. A simple and sensitive test paper was developed for rapid detection and visualization. Zhang et al. used AgNO3 and Cu (NO3)2 to construct silver/copper bimetallic nanoparticles (AgCu-BNPs) [18]. The addition of Hg2+ enabled the bimetallic probe to perform selective detection by colorimetric and fluorescence modes. The minimum concentration limits of colorimetric and fluorescence methods were 89 nM and 9 nM, respectively. The blue fluorescence of AgCu-BNPs was quenched by Hg2+, and the system was accompanied by the visible color change. The quenching mechanism was attributed to IFE, static quenching, and dynamic quenching. Dual-mode detection made the detection more simple and more reliable. The above reports are related to the detection of Hg2+ by CuNCs (Table 2). The probes were designed into strip sensor belts or combined with a smartphone [19], which was more suitable for detection in complex environments. The NCs prepared by turmeric root extract extends the application value of biomass materials. CDs-CuNCs are nanocomposites with dual emission wavelengths assembled by simple electrostatic discharge. The synthesis of double-emission materials by green materials and chemical means is also worth studying. In a survey of the latest work [20][21], gold nanoparticles were combined with a resolution matrix-assisted laser desorption ionization time-of-flight mass spectrometer (MALDI-TOF MS) for measurement [20]. The detection limit of Hg2+ was 0.19 pmol/μL.
Table 2. Report on detection of Hg2+ by CuNCs.
Type of CuNCs λex/λem
(nm)		 Read Out Sensing Mechanism Reaction
Time		 Limit
of Detection		 Published Time Reference Value Ref.
CuNCs@ESM — Turn-off high-affinity metallophilic interactions 1 h (Hg2+: 500 µM) — 2018 0.19 pmol/μL
2023
[20]		 [14]
TG-CuNCs 350/430 Turn-off static and dynamic quenching 3 min 1.7 nM 2020 [15]
CuNCs 365/440 Turn-off AIQ 22 min 0.12 nM 2018 [16]
CD-CuNCs dual-
emission		 — strong affinity — 0.31 nM 2021 [17]
AgCu-BNPs 350/442 Turn-off IFE, static and dynamic quenching — 9 nM 2021 [18]

3. Iron Ions

The presence of Fe3+ allows many physiological processes to proceed normally. However, too much Fe3+ can lead to serious diseases such as cancer, Parkinson’s, and Alzheimer’s [22]. Among the methods for detecting iron ions (Table 3), the fluorescence method is a good choice for sensor manufacturing because of its high sensitivity [23]. Cao et al. recently assembled BSA-CuNCs @ [Ru(bpy)3]2+ from bovine serum albumin (BSA)-CuNCs and [Ru(bpy)3]2+ to detect Fe3+ in tap water and spirits [24]. BSA-CuNCs @ [Ru(bpy)3]2+ show two distinct emission peaks. [Ru(bpy)3]2+ did not respond to Fe3+, and fluorescence quenching occurred in BSA-CuNCs. Additionally, a POCT platform based on a smartphone was designed for the actual detection of Fe3+. Moreover, Cysteamine functionalized nanoconjugate materials (CA-CuNCs) can be used to determine Fe3+ in human urine [25]. The detection mechanism was based on electron transfer and AIQ. Importantly, the stratified test strips were designed, and the brightness changed greatly before and after adding Fe3+. Ai et al. used duplex oligonucleotide (dsDNA) as a template to prepare copper nanomaterials (dsDNA-CuNCs), which showed its applicability as a Fe3+ sensor; fluorescence quenching was caused by the aggregation of nanomaterial particles [26]. The detection limit was 5 μM when Fe3+ concentration was 5–100 μM. Hemmateenejad et al. synthesized Penicillamine-capped bimetallic Gold-Copper NCs (PA-AuCu-bi-MNCs). The milky water solution demonstrated orange fluorescence under a UV lamp [22]. It can be used for simple quantitative detection of Fe3+, utilizing the detection mechanism of IFE. It was important that the detection process is free from Fe2+ interference. In the literature investigated, the minimum detection limit of Fe3+ detected by CuNCs was 10 nM. CuNCs made of bovine serum albumin (BSA) by Debanjan Guin et al. could be applied to the detection of ions in wastewater and human serum samples [27].
Table 3. The recent report on the detection of Fe3+ by CuNCs.
Type of CuNCs λex/λem (nm) Read Out Sensing Mechanism Reaction
Time		 Limit
of Detection		 Published Time Reference Value Ref.
BSA-CuNCs@ [Ru(bpy)3]2+ Dual -emission — AIQ 3 min 0.086 μM 2022 10 nM
2022
[27]		 [24]
CA-CuNCs 385/467 Turn-off electron transfer,
AIQ		 — 423 nM 2020 [25]
dsDNA-CuNCs 312/400 Turn-off AIQ 1 h 5 μM 2020 [26]
PA-AuCu-bi-MNCs 275/605 Turn-off IFE 5 min 0.1 µM 2019 [22]

4. Cobalt Ion

Cobalt is an essential trace element, but ingesting high concentrations or prolonged exposure can still cause illness, including contact dermatitis, pneumonia, allergic asthma, and lung cancer [28]. Shao et al. used dithiothreitol (DTT) to make DTT-CuNCs to detect Co2+, and the detection mechanism was AIQ [29]. The DTT-CuNCs probe was fixed to the filter paper and designed as a visual paper sensor with the help of a mobile phone, which had good applications in water samples and other environments. Ling et al. found that GSH-AuNCs made from glutathione (GSH) can selectively detect Co2+ [30]. The detection process was accomplished by adjusting the pH. When the pH was 6, the presence of Co2+ can effectively quench the fluorescence of NCs, and the quenching mechanism was static quenching. He et al. used lysozyme (Lys) and hydroxylamine hydrochloride (NH2OH·HCl) as raw materials to make Lys-CuNCs, which showed strong yellow fluorescence under an ultraviolet lamp [31]. It can be used for sensitive and selective detection of Co2+ in an aqueous solution with a detection limit of 2.4 nM. Above are related reports on the application of MNCs in Co2+ detection (Table 4), which needs to be further developed. In recent work, Tong et al. used silicon nanoparticles/gold nanoparticles complex as a fluorescent probe to detect Co2+, and the detection limit was as low as 60 nM [32]. The detection of ions by GSH-AuNCs was completed by adjusting pH, indicating that pH value would have an impact on the detection process and was also an important factor affecting the fluorescence of substances.
Table 4. The list reported HMIs detection via CuNCs.
Metal Ions Type of CuNCs λex/λem (nm) Read Out Sensing Mechanism Reaction
Time		 Limit
of Detection		 Published Time Reference Value Ref.
Co2+ DTT-CuNCs 382/627 Turn-off AIQ 30 min 25 nM 2021 60 nM
2022
[32]		 [29]
  GSH-AuNCs 412/500 Turn-off Static quenching 15 min 0.124 μM (Co2+: 2.0–50.0 μM) 2021 [30]
  Lys-CuNCs 334/596 Turn-off — — 2.4 nM 2018 [31]
Cr6+ bi-ligand CuNCs 330/411 (Cu NC-2 a) Turn-off IFE — 0.03 mM — — [33]
Mn2+ CuNCs@T b 354/561 Turn-on AIE 40 min 10 μM — — [34]
Cd2+ GSH@CDs-CuNCs dual-
emission		 — AIE (750 nm) 15 min 0.6 μmol·L−1 — — [35]
Cu NC-2 a: TA-to-CysA molar ratio of 1:1, T b: Rich-thymine.

5. Other Ions

CuNCs have also been synthesized for the detection of other HMIs. For instance, Cr (VI) contamination is highly associated with atopic dermatitis, carcinogenesis, and mutations in animals and humans. Hu et al. synthesized CuNCs using thiosalicylic acid (TA) and cysteamine (CysA) as ligands [33]. Because the IFE effect quenches the fluorescence of CuNCs, a fluorescence sensor was designed to detect Cr (VI). In addition, He et al. synthesized water-soluble CuNCs stabilized by DNA single base thymine and performed selective detection of Mn2+ based on AIE [34]. Cadmium is highly toxic, known as a carcinogen, and in large quantities can damage the liver, bones, and kidneys. Furthermore, high levels of cadmium have been linked to diabetes, cancer, and heart disease [36]. Liu et al. used the glutathione@carbon dots (GSH@CDs) as a template for the synthesis of GSH@CDs-CuNCs, which can be used to detect Cd2+. The linear range of Cd2+ was 0~20 μmol·L−1, and the limit of detection was 0.6 μmol·L−1 [35].
Those listed above are just a few examples of CuNCs detecting HMIs. (Table 4) These works further elucidate the possibility of CuNCs for ions detection and provide a new approach for the detection of HMIs.

6. Selectivity Difference of CuNCs in Detecting HMIs

Heavy metal ions have a great influence on organisms and the environment, so fluorescence sensing for HMIs detection is of great significance. There were significant differences in CuNCs detection among different HMIs. For example, different fluorescence responses, and different fluorescence intensities. Huang et al. prepared CuNCs using bovine serum albumin (BSA) and thiosalicylic acid (TSA) as protective ligands [37]. The fluorescence changes produced by TSA/BSA-CuNCs interacting with various metal ions (incubated in sodium phosphate buffer) were observed. Only Cr6+ showed significant fluorescence quenching on TSA/BSA-CuNCs due to the IFE. Cr6+ provides sufficient oxidative etching of copper ions/atoms. Olga Garcia et al. designed a sensor for the detection of Hg2+ by taking advantage of the differences in the detection of metal ions by CuNCs [38]. CuNCs were tested on various metal ions under the same conditions, and the fluorescence attenuation was nearly 50% only in the presence of Hg2+, and the effect was small in the presence of other ions. CuNCs were incubated with a series of Hg2+ concentrations, and the hydrodynamic size was gradually increased. No changes in the hydrodynamic size were detected in the presence of Cd2+. This kind of CuNCs can be used as selective probes to detect Hg2+ in the presence of other ions. It has also been mentioned that the CDs-CuNCs synthesized by Yang et al. have dual emission wavelengths [17]. The addition of Hg2+ reduced the red emission of CuNCs, while the blue emission of CDs remained stable. As a result, the fluorescent color changes from pink to blue.

This entry is adapted from the peer-reviewed paper 10.3390/chemosensors11030159

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