3.1. Generalities

To date, silver sulfide (Ag2S) has been recognized as having three phases in its natural solid bulk state as follows: monoclinic acanthite (α-phase) that is stable below 179 °C, body-centered cubic (bcc) so-called argentite (β-phase) that is stable above 180 °C, and a high-temperature face-centered cubic structure (γ-phase) that is stable above 586 °C. At higher temperatures, these forms are electrical conductors [9], [10]. For some authors, the designations of the monoclinic and bcc phases are often confused, although for practical application as a photoluminescent semiconductor, the low-temperature monoclinic α-Ag2S (acanthite) phase in nanosized form is of the greatest interest [11].

3.2. Preparation methods

3.3. Biocompatibility

With regards to toxicity concerns, Hirsch et al. [23] investigated Ag2S in bulk, and its toxicity was tested on Hyalella azteca, the most abundant lake amphipod in North America. In this study, there were no statistically significant differences in the survival rates of Hyalella azteca when they were exposed to any concentration of Ag2S. In addition, the synthesis of Ag2S quantum dots was reported by Hocaoglu et al. [13], and these dots exhibit excellent cytocompatibility, even at 600 mg/mL in NIH/3T3 cells. This cell line comes from mouse embryonic fibroblast cells, and it has since become a standard fibroblast cell line, which is a good parameter for cytotoxicity measurements. In addition, the same research group reported their synthesis of Ag2S quantum dots using an FDA-approved heavy metal chelating drug that was coated with a slow sulfur-releasing agent (meso-2,3-dimercaptosuccinic acid). These quantum dots showed good cytocompatibility and hemocompatibility in the in vitroexperiments without affecting the integrity of erythrocytes; they did not interact with platelets and they were associated with only a slight alteration in the complement system and the intrinsic coagulation pathway [24]. Additionally, other researchers have investigated the hemocompatibility of these compounds, and they attributed an initial drop in the platelets and white blood cell to macrophage inhibition/damage to platelets and an acute immune reaction after the administration of Ag2S quantum dots. However, there is no cytotoxicity because the drop in these cells is followed by a return to normal levels [25]. In addition, Ag2S nanoparticles (13 ± 7 nm in diameter) coated with methyl polyethylene glycol were investigated in a rainbow trout cell line (Oncorhynchus mykiss). These results reveal that Ag2S showed neither genotoxic nor cytotoxic effects after 24 h of exposure [26].

Silver is not considered a toxic metal, but it is well-known that silver toxicity increases when it takes an ionic form. For example, it seems to be an accepted assumption that in argyria (a condition caused by high silver exposure for a prolonged period of time), silver is accumulated in the skin as silver sulfide [27], which is formed by the oxidation of metallic silver that is deposited throughout the body [28]. The toxic effects of silver ions should be reduced through the formation of insoluble silver sulfide [29], reducing their biological availability and consequently preventing their interference with normal enzymatic activities in tissues [30]. For this reason, silver sulfide nanoparticles (NPs) could exhibit the greatest potential for use in biological applications and the highest biocompatibility due to their minimal toxicity in comparison with Ag NPs.

3.4. Imaging, diagnostic and therapeutic agents

Near-infrared (NIR) treatment is known to be preferred because of its ability to interact less with and to penetrate deeper into tissues in comparison to ultraviolet and visible light. Fluorescence imaging with NIR-II (1000–1450 nm) is more desirable over visible light at 450–750 nm and traditional NIR-I imaging located at 750–950 nm. When quantum dots are excitable in NIR-II, they exhibit reduced photon scattering, deeper tissue penetration, and lower autofluorescence [18], [31]. By contrast, NIR-II imaging remains as a limited option, with few materials that work in its range. Functionalized Ag2S quantum dots are promising for use in imaging and diagnostics fields. Bioimaging and biolabeling, deep tissue imaging, diagnostics and photodynamic therapy are some of the many potential bio-application areas. The in vivo utilization of quantum dots as imaging agents and/or delivery vehicles requires a delicate balance regarding their toxicity, biocompatibility, stability, size, photoluminescence quality and excitation/emission wavelengths (Table 1). Considering all these requirements, Ag2S (NIR) quantum dots present great potential [24]. In biotechnology applications, quantum dots offer large advantages over fluorophores, given their higher sensitivity and narrow luminescence emission band, which enables the excitation of multiple quantum dots at a single wavelength and multiplexing, with a long luminescence lifetime [13]. Quantum dots are particles with sizes between 2 and 10 nm, and they have fluorescent properties, which unlike organic fluorophores, are based on inorganic semiconductor nanoparticles.

Some body tissues such as blood or adipose cells possess autofluorescence. In these tissues, the nanoparticles have a different excitation wavelength that allows an improved definition and resolution of images. When these inorganic quantum dots are photoexcited, electron-hole pairs are generated, and the emission of fluorescent light occurs through the recombination of these pairs [32]. It seems that the stoichiometry of silver with sulfide ions as well as the reaction temperature has an effect on the optoelectronic properties [13].

Cell imaging was successfully performed, demonstrating that functionalized Ag2S quantum dots could emit readily observable near-infrared fluorescence and then release nitrogen oxide into living cells [16]. This method would provide new perspectives for the use of multifunctional nanostructured materials in imaging since high-quality confocal images with a clean background were already obtained in a fluorescence microscopy study of a mammalian cell imaged with Ag2S quantum dots [13]. In another study, polyethylene glycol (PEG)-Ag2S quantum dots have enabled the in vivo (in mice) monitoring of lymphatic drainage and vascular networks, which could be useful in surgical treatments such as sentinel lymph node dissection as well in assessing the blood supply of tissues and organs and the screening of anti-angiogenic drugs [33].

Another interesting application for quantum dots is the marking and tracking of cells in vivo. Traditionally, organic dyes and proteins were used for tracking imaging and in experiments to obtain nanometer precision. However, these samples suffer from rapid photobleaching, limiting observation to only a few seconds before irreversible photobleaching occurs [34]. It may be thanks to the passage of the quantum dots from the cytoplasm of the stem cell to its two daughter cells. Each one retains a portion of the fluorescent cytoplasm, which could be fluorescently imaged in real time under continuous illumination [35]. A recent study shows that Ag2S quantum dots in the second NIR window (1000–1400 nm) are employed for the dynamic tracking of human mesenchymal stem cells in vivo. The in situ translocation and dynamic distribution of transplanted cells were monitored for up to 14 days with a temporal resolution of 100 ms. The in vivo high-resolution imaging indicates that the long-term retention of transplanted human mesenchymal stem cells in mice contributed to the liver failure treatment. These results suggested that there was a reduction in the inflammatory cytokine accumulation in mice and an improvement in liver function by transplanted stem cells [36]. Therefore, the Ag2S quantum dots offers the possibility of tracking stem cells in living animals with both high spatial and temporal resolution and encourages future clinical applications in imaging-guided cell therapies.

However, several difficulties remain to be resolved; for example, the ability of quantum dots to reach specific targets within cellular compartments or multi-component molecular complexes is still in question. This constraint appeared to be primarily related to the size of the quantum dots, which is increased due to their functionalization with large ligand agents, until reaching up to 100 nm [37], [38]. The routine use of quantum dots for imaging has a higher probability every day. In spite of that optimism, future research in this field should cover these issues and offer easy and feasible solutions at a large scale.

There are few investigations about the Ag2S NPs used for photothermal therapy in cancer treatment. To the best of our knowledge, there are only two studies about Ag2S NPs that have been used as transducing agents for cancer cell destruction. One of them, namely Ag2S nanoparticles, was used successfully for photothermal therapy, and the cytotoxicity to the surrounding tissues was minimal [39]. In another work, the researchers synthesized a doxorubicin into PEG-coated Ag2S quantum dots, and they exhibited a remarkable tumor suppression of 86% and showed high biocompatibility with non-target tissues [19].

3.5. Antibacterial activity

Ag2S nanoparticles were found to show bacterial growth inhibition. This is another potential application for Ag2S nanoparticles. Some studies have described the antibacterial properties of Ag2S nanoparticles. Kumari et al. [21]obtained Ag2S nanoparticles that possess photocatalytic activity under visible light and exhibit a moderate antimicrobial effect. A 0.1 μg/mL nanoparticle treatment showed a growth inhibition of > 75% against S. aureus and two different E. coli strains. Dasari et al. [22] performed an antimicrobial test for Ag2S nanoparticles against E. coli, P. aeruginosa, S. aureus and B. thuringiensis. In their conclusions, they claimed that the Ag2S nanoparticles show effective antibacterial activity against S. aureus gram-positive bacteria relative to other strains. In spite of that finding, the zones of inhibition for capped Ag2S nanoparticles did not support that statement in comparison with their positive controls. It seems that the capped Ag2S nanoparticles exhibit a low and moderate growth inhibition of tested bacteria, except for P. aeruginosa tests in which the inhibition halos have a very similar size.

By contrast, other research showed that the Ag2S nanoparticles did not exhibit a significant antibacterial effect against E. coli by themselves [40]. In addition, Suresh et al. [14] found that the Ag2S nanoparticles exhibit non-inhibitory effects against E. coli, S. oneidensis, and B. subtilis. For nanoparticles to present their antibacterial effect, it is not only their particle size that is important, but so is their synthesis route; consequently, the medium and the capping agents are both critical. Recently, the active species in the photo-oxidation process of Ag2S nanoparticles such as anionic ozone radicals [41] and OH– radicals were found to form under sunlight in the presence of H2O2 [22]. These mechanisms are most likely to explain their antibacterial effect.

3.6. Biosensors

Another novel use for Ag2S nanoparticles could be as biosensors. Prepared Ag2S nanoparticles could be used as an excellent and promising photoelectric active material in the photoelectrochemical biosensor [42]. Their simple preparation methods, high photocatalytic performance, and low toxicity increase the possible future applications of Ag2S NPs.

4.1. Generalities

The primary phases of copper sulfide minerals are Cu2S (chalcocite) and CuS (covellite). Eight copper sulfide minerals have been identified to date as follows: covellite (CuS), yarrowite (Cu1.12S), spionkopite (Cu1.40S), geerite (Cu1.60S), anilite (Cu1.75S), digenite (Cu1.80S), djurleite (Cu1.97S), and chalcocite (Cu2S) [43]. Some of these compounds such as covellite, anilite, digenite, djurlite, and chalcocite are stable at room temperature [44].

Copper sulfides (CuxS) are inherently p-type semiconductors due to the presence of copper vacancies in their lattice and band structures, crystalline structures, and thermal transport behaviors, which seem to be controllable if the copper content is regulated [2]. CuS has attracted great interest for its possible use in catalysis and photovoltaic devices. Among the prospects for future applications of CuS nanoparticles are primarily thermal phototherapy against cancer cells, use as a contrast agent for diagnostic imaging and use as an antimicrobial agent, as diverse recent studies have suggested.

4.2. Preparation methods

4.3. Biocompatibility

In addition, Guo et al. [55] performed a study that was focused on the cytotoxicity and biodegradability of CuS hollow spheres in comparison with Au hollow spheres, after demonstrating in a previous work [47] that CuS NPs could be used as a photothermal ablation agent. Both CuS and Au nanoparticles were formulated with similar particle sizes and morphology. The NPs were surface-modified with PEG to evade uptake by monophagocytic systems. The cytotoxicity against HeLa cells results for CuS NPs was similar to those of Au NPs. The low toxicity of CuS NPs appears to be due to the slow dissociation rate of Cu ions from CuS. By contrast, the comparative in vivo results between the two types of nanoparticles show that the nanotoxicity was related to the degradability of the nanoparticles. Although the Au NPs were non-metabolized particles, they were also more toxic compared to CuS NPs. Another study confirms these biocompatibility findings using human glioblastoma U87MG cells, which were incubated with CuS-ferritin nanocages in solution. No cytotoxicity was observed in the 1–10 μg/mL concentration range [48]. CuS nanoplates for photothermal therapy also show good compatibility of these particles. That study shows the tissue distribution results, which indicated that the CuS nanoplates were primarily spread in the liver, spleen, and lung due to the reticuloendothelial systems of these organs, which are responsible for eliminating foreign invaders [56].

4.4. Imaging, diagnostic and therapeutic agents

There is a recognized relationship between the band gap energy of the CuS nanoparticles and the crystalline phase, which depends on the synthesis method [57]. This relationship is important due to the impact on the energy band gap on which CuS NPs could act. Copper sulfide nanoparticles as well as silver sulfide nanoparticles present absorbance in NIR-II light. Their absorbance under NIR light is attractive because the most favorable light causes the smallest amount of thermal damage to normal cells. That property gives them potential applications in biomedicine.

For example, the Au/CuS core/shell nanoparticles were proposed for use in photothermal therapy for cancer treatment. The CuS nanoparticles in the shell have advantages such as low cost, simple and easy preparation, and small sizes for targeting. Conversely, the efficacy of photothermal therapy was improved via local field enhancement from Au NP surface plasmon coupling [46].

Tian et al. [49] reported the synthesis of CuS NPs that was used as photothermal agents for the ablation of cancer cells, and they exhibited a high NIR photothermal conversion efficiency. However, the size of these particles (approximately 500 nm) may limit their potential clinical translation. In addition, Li et al. [52] found that CuS NPs exhibit a quantum confinement phenomenon when they are irradiated with a laser beam at 808 nm. NIR light can generate heat for the photothermal ablation therapy of human cervical cancer HeLa cells and human embryonic kidney-293 cells. The cancer cells that were treated with CuS NPs plus an NIR laser experienced substantial cellular death, whereas both CuS and Au NPs had no cytotoxic effect on the human cells at concentrations up to 100 μM after 48 h of incubation.

Zhou et al. [58] reported on the efficient photothermal therapy of tumors in mice that were injected with 800 μg/mL PEG-CuS NPs (200 μL/mouse), which was achieved with a laser power density of 12 W/cm2 with 5 min of irradiation. In a later study by the same research group [50], PEG/[64Cu]CuS NPs reportedly worked as efficient radiotracers for pharmacokinetics, biodistribution, and positron emission tomography imaging, and PEG-CuS NPs alone acted as an efficient photothermal coupling agent. In the mice with U87 human glioblastoma xenografts that were treated with PEG-CuS NPs plus a laser, frequent effects of thermonecrosis, such as the loss of the nucleus, cell shrinkage, and coagulation, were found in the tumor tissues, resulting in 65% efficacy. In a subsequent study, the absorption band was shifted towards larger wavelengths by simply adjusting the stoichiometric ratio between CuCl2 and Na2S. For the above purpose, the absorption band of CuS NPs was shifted from 808 nm to 1064 nm NIR. At the 1064 nm wavelength, the tumor cells can clearly distinguish them from normal cells, enhancing the image. There is a potential application for image lesions in the human breast, skin or other superficial tissue at a depth of up to 40 mm by using photoacoustic tomography [51]. By contrast, in another study by the same research group, photothermal therapy with targeted folate-CuS NPs achieved significant tumor ablation at an injection dose of 200 μL of 200 μg/mL folate-CuS NPs per mouse and a laser power density of 1.5 W/cm2 for 2 min. The dose of injected NPs was reduced to 25% of that of the non-targeted PEG-CuS NPs, and the laser energy required with targeted NPs was reduced to 5% of what was needed with the non-targeted PEG-CuS NPs. These improvements in the CuS NPs have provided better conditions for experimentation in preclinical research [59].

4.5. Antimicrobial activity

The antimicrobial activity of Au/CuS NPs against B. anthracis spores and cells was investigated. Au/CuS NPs were found to be ineffective at inactivating B. anthracis spores. However, the Au/CuS NPs exhibited antimicrobial activity against B. anthracis cells, which depends on the NP concentration and treatment time. Furthermore, DNA efflux from bacteria treated with NPs confirmed the destruction of cell membranes by these NPs [53].

4.6. Biosensors

A sensor has been fabricated that has great potential applications in the detection of H2O2 in serum and urine samples and H2O2 released from living cells, which occurs in cancer. However, the accurate and sensitive detection of H2O2 has great importance due to its possible presence during the normal cellular process and in several diseases such as Alzheimer's, myocardial infarction, atherosclerosis, and Parkinson's. At present, this biosensor is promising for the detection of different H2O2 concentrations [54].

5.1. Generalities

Iron sulfides include several types of compounds containing iron and sulfur that have a formula of FexSy. The crystal shapes and physical properties of iron sulfides change significantly depending on the iron content [60]. Iron sulfide presents different crystalline phases as follows: FeS2 (pyrite and marcasite), Fe1-xS (pyrrhotite), Fe3S4 (greigite) and FeS (troilite), and their properties differ from one phase to another [4]. Iron sulfide (FeS2) is one of the 23 most abundant materials in the earth's crust, and it is inexpensive and considered non-toxic [2]. Its high natural abundance translates to an estimated 0.000002 ¢/W material extraction cost, which ranks FeS2 at the highest level with regards to material availability for semiconductor material systems [4].

The synthesis of well-defined inorganic nanoparticles in colloidal solution, which has evolved gradually since the 1950s, has now reached the point at which applications in both the research world and the wider world can be realized [61]. In particular, some metallic sulfides such as pyrite and marcasite are compounds that possess a long tradition, exist in nature and have been identified, at least empirically. Other metal sulfides such as triolite are more unstable, or they are rare or recent discoveries, such as minerals or compounds and others that are unusual materials. One of the mechanisms of mineral formation involves the solid state combination of large clusters of atoms or nanoparticles, or the addition of clusters or nanoparticles to larger crystals. The pieces are assembled in crystallographically specific ways so that interface elimination leads to the formation of a larger single crystal [62]. For this reason, mineralogy is important for some types of nanoparticles that are frequently formed in the environment, for example, the pyrite nanoparticles found in clay [63].

Recently, iron sulfides have been recognized as a valuable inorganic material in many areas, especially environmental remediation and photovoltaic cells, due to their chalcophilic nature and reducing capability [64]. Many current fundamental concepts about the evolution of the earth's surface environment are primarily based on the analysis and interpretation of iron sulfides, particularly pyrite, in ancient sedimentary rocks, which may have even been involved in the origin of life on earth [65].

5.2. Preparation methods

The phase at which FeS2 crystallizes depends on the parameters of the synthesis techniques, so it is necessary to control these parameters [66]. This pyrite exhibits a cubic phase in its chemical structure, and it exhibits greater stability and suitable properties for a specific application [67], [68].

5.3. Biocompatibility

Bare nanocrystal cores are highly reactive and toxic, resulting in a very unstable structure that is prone to photochemical degradation [78]. Therefore, it seems essential to use the biocompatible moieties that serve as capping for quantum dots, such as PEG, silica, and dextran. The ideal capping must therefore allow the capped nanoparticle to avoid bleaching from the reticuloendothelial system, given its ability to resist protein adsorption and its excellent biocompatibility [79]. With regards to safety issues, Yang et al. [75] reported on the synthesis, characterization, cytotoxicity and biodistribution of FeS/PEG nanoplates in vivo. After injecting mice with a high dose (100 mg/kg) of FeS/PEG nanoplates, high iron levels were found in the liver and spleen owing to the macrophage uptake of nanoparticles. Over time, they observed a persistent decrease in the iron levels for all the measured major organs. However, after 50 days, the iron levels dropped back to normal levels, except in the spleen. No obvious side effects were observed. The results of the blood tests showed that all of the parameters in the FeS-PEG-treated groups at different times appeared to be normal compared with those in the control group. The same was reported in relation to the liver and kidney function markers.

With respect to greigite NPs, Feng et al. [72] performed a cell cytotoxicity analysis to evaluate the Fe3S4 NPs, and the results indicated good cytocompatibility at a concentration of 75.0 μg/mL. Interestingly, HeLa cells show lower cellular activity than mouse embryo fibroblast cells. A hemolysis analysis demonstrated the blood's compatibility with a hemolysis of 2.1% after the incubation of Fe3S4 NPs at a 0.5 mg/mL concentration with human blood for 1 h, which is an acceptable hemolysis percentage. No evidence of adverse effects or injury can be induced by Fe3S4 NPs in the major organs of mice after NP administration for 2 weeks.

5.4. Imaging, diagnostic and therapeutic agents

Chang et al. [64], Feng et al. [62], and Yang et al. [65] reported the synthesis of quantum dots that exhibit plasmonic effects under specific wavelengths such as NIR, which generates heat through plasmonic photothermal conversion. Greigite is the most suitable phase of iron sulfides for that purpose, due to its magnetic properties. However, the size of the resulting nanoparticles is a great concern in relation to their biocompatibility in vivo.

5.5. Antimicrobial activity

However, to the best of our knowledge at the moment, there is a very small amount of research about the antimicrobial effect of iron sulfide nanoparticles that have been synthetically obtained. Despite that gap, there is available data related to natural iron sulfide particles from hydrothermal sources and clay, which has been extensively studied. These natural particles have an irregular shape and size (the average particle size is 200 nm or less, approximately). Agglomerations of particles with minuscule particles as small as 4 nm have been observed [63], [80]. Recently, the antibacterial action of mineral pyrite was explained by the formation of hydrogen peroxide in water, which is produced during the photocatalytic process, regardless of the oxygen presence [81]. When the bacteria are in contact with pyrite in aqueous solution, the rapid absorption of Fe2+ disturbs the bacterial metabolism by over-flooding the cell with excess Fe2+ and saturating its iron protein storage. As the intracellular Fe2+ is oxidized to Fe3+, the reactive oxygen species (ROS), which cause biomolecular damage and the precipitation of iron oxides, are produced [82], [83]. This process causes disruption in the nucleic acids, that is, RNA and DNA, as a result of a Fenton-type reaction [84], [85]. Therefore, the iron sulfide nanoparticles could have potential applications as antimicrobial agents. However, further studies are needed to determine if these nanoparticles can be effective antimicrobial agents, and their cytotoxicity must be tested.

Moreover, He Q. et al. [73] performed a study to find the bacteriostatic rates of the Fe3S4/Ag composite particles against E. coli and S. aureus, which were 86.2% and 90.6%, respectively. However, the Fe3S4 NPs themselves without silver showed no bacteriostatic effects on both bacteria.

5.6. Biosensors

Ding et al. [77] prepared magnetic Fe3S4 NPs with pseudo-enzymatic activity. This activity was applied to design a quantitative photometric enzymatic glucose assay in human serum, leading to the formation of a blue product from an enzyme substrate that is catalytically oxidized by H2O2 in the presence of Fe3S4 NPs. In this study, glucose can be detected in the 2 to 100 μM concentration range.