Due to their potential in applied science, quantum dots and their toxicity have been intensively studied and reviewed in the past few decades ^{[1][2][3][4][5][6][7]}[1,2,3,4,5,6,7]. Quantum dots (QDs) are nanosized (2–10 nm) semiconductor crystals with distinguished chemical and physical properties, enabling them to emit a wide range of bright, photobleaching-resistant light [8]. QDs’ fluorescence is size-tunable, which allows for simple adjustment of QDs’ size and composition to achieve desired color ^{[5][9][10][11][12][13][14][15][16][17][18][19][20]}[5,9,10,11,12,13,14,15,16,17,18,19,20]. It was found that increased diameter of QDs caused a redshift in fluorescence [17]. Additionally, quantum dots with greater height in dimension display a longer photoluminescence lifetime and increased emission wavelength [15]. Thus, larger quantum dots emit more stable fluorescence in higher emission ranges. In addition to QDs’ size, the shape of QDs also significantly influence QDs’ stability and optical property ^{[19][20][21][22][23]}[19,20,21,22,23]. Some of the most used quantum dot shapes include spherical QDs, cylindrical QDs, pyramidal QDs, conical QDs, tetrahedral QDs, and lens-shaped QDs ^{[19][20][21][24]}[19,20,21,24]. It was found that tetrahedral QDs have sharper edge absorption and are more confined than spherical QDs. On the other hand, spherical QDs at 3.1 nm showed to be most efficient in photon absorption and required less excitation energy compared to other types of QD shapes ^[19][21][19,21].

Quantum dots are typically sorted based on their core type, shape, structure, size, and ligands. Common quantum dot cores include cadmium, indium, and carbon encapsulated by chalcogenides such as selenides, tellurides, and sulfide ^{[25][26][27][28][29]}[25,26,27,28,29]. However, core-only quantum dots have been shown to be unstable due to the deterioration of materials ^[30][31][30,31]. For core-type QDs such as cadmium selenide quantum dots (CdSe QDs), an oxidizing environment causes oxidation of the selenide (Se) layer on the QDs’ surface, thus weakening the overall structure of QDs and leading to leakage of cadmium ions. When adding an additional shell layer such as zinc sulfide (ZnS), the oxidation of Se is reduced, thus decreasing the amount of Cd ion leakage overall [30]. Compared to core-only QDs, quantum dots with a core–shell structure are considered superior when it comes to structural stability and photoluminescent quantum yield (PLQY) ^[31][32][33][31,32,33]. It is known that long-term exposure to environmental factors such as blue light and UV light quenches QDs’ fluorescence. The addition of a protective shell has shown to be effective in increasing QD resistance against photobleaching ^{[34][35][36][37][38]}[34,35,36,37,38]. In addition to increased QDs’ stability, the presence of an exterior shell has been found to reduce the toxicity of quantum dots, which broadens their application range [39]. Therefore, a new type of quantum dot called core–shell quantum dots has been developed and is currently widely used. The shell of some quantum dots is decorated with ligands that provide further stability ^[40][41][42][40,41,42]. The addition of ligands allows QDs to have specific interactions with various environmental factors, which is useful in certain applications such as biosensing and particle detection ^{[43][44][45][46]}[43,44,45,46]. Appropriate ligand choice could also aid the dispersion of QDs in an aqueous solution, thus minimizing the aggregation of QDs and resulting in accurate emission of size-dependent QDs [47].

Due to their unique characteristics, quantum dots have become a promising candidate for a range of important applications. Quantum dots are sought after for their potential in biomedical science, particularly for biosensing, drug delivery, cell tracking, disease detection, and potential antimicrobial/antibiotic remedies ^{[48][49][50][51][52][53]}[48,49,50,51,52,53]. Furthermore, quantum dots are currently heavily utilized in several commercialized products, such as electronic devices, solar cells, LEDs, cosmetics, plastics, and other products essential to daily life ^{[54][55][56][57]}[54,55,56,57]. As quantum dots gained attention in recent years, a few studies have shown the potential toxicity of quantum dots to mammalian cells, fungal cells, plants, and other organisms ^{[25][58][59][60][61][62][63]}[25,58,59,60,61,62,63].

2. Core-Type Quantum Dots

As of today, a wide variety of quantum dots have been developed, each with slightly different properties and potential applications ^{[13][32][58][64][65][66][67][68][69][70][71]}[13,32,58,67,68,69,70,71,72,73,74]. Some of the most common quantum dots that have captured current interest in the scientific community are cadmium QDs, indium QDs, graphene QDs, and carbon dots ^{[26][56][63][69][72][73][74][75][76][77][78][79]}[26,56,63,72,75,76,77,78,79,80,81,82]. Each of these types of quantum dots possess a unique set of advantages and disadvantages, which make them suitable for slightly different applications (Figure 1). Among these, cadmium-based quantum dots are the most intensively researched. Due to their high photoluminescence yield and environmental stability, cadmium-based quantum dots have been widely utilized in biomedical and photovoltaic technology ^[25][72][25,75], in which cadmium selenide quantum dots (CdSe QDs) are more frequently used compared to other cadmium-based QDs, such as cadmium sulfide and cadmium telluride, due to the wider visible fluorescence range [23]. For this reason, CdSe QDs are also more well investigated compared to the other cadmium-based QDs. Due to their superior quantum yield, previous work has suggested that cadmium QDs have vast potential in biomedical applications. Yet, there have been reservations regarding in vivo usage due to their high toxicity profile ^{[18][55][56][79][80][81]}[18,55,56,82,83,84]. It has been assumed that the toxicity of cadmium quantum dots likely stems from the toxic cadmium ion content [23], and, therefore, research for a cadmium-free alternative has been ongoing in recent years ^[49][82][83][49,85,86]. Previously, a few studies have suggested that indium-based quantum dots, particularly indium phosphide zinc sulfide (InP/ZnS), could be an alternative to cadmium-based quantum dots ^[84][85][87,88]. It was found that with a similar amount of core leakage, InP/ZnS quantum dots seem to induce less cell damage than CdSe/ZnS, suggesting that InP (III) might be less toxic than cadmium ions ^[86][89]. Similar to that previously observed in cadmium QDs, the zinc shell has also been shown to play a crucial role in limiting InP-based QD toxicity ^[87][90]. Although considered inferior to cadmium-based quantum dots in fields such as bioimaging and LED performance due in part to lower quantum yield ^[88][91], the reduction in cytotoxicity makes indium-based quantum dots one of the most promising candidates for biomedical and industrial applications. Yet, Horstmann et al. showed that with the same amount of treatment concentration (100 µg/mL), InP/ZnS QDs inhibited yeast growth while CdSe/ZnS QDs prolonged the cell’s lag phase without affecting its final optical density ^[73][76]. Thus, the proposal of using InP-based quantum dots as an alternative to cadmium-based quantum dots needs additional assessments. Recently, a group of heavy-metal-free quantum dots called carbon dots have gained attention due to their promising future in biological applications ^[20][77][89][20,80,92]. These carbon dots are comprised of two main categories: carbon quantum dots and graphene quantum dots. Carbon quantum dots are sphere-shaped and are composed of widely available and eco-friendly carbon atoms, which make them less toxic than traditional semiconductor quantum dots. Due to their environmental compatibility, carbon dots have been the most recently proposed alternative to traditional quantum dots ^{[90][91][92][93]}[93,94,95,96]. In the last few years, the ability to synthesize carbon dots from biomass and waste products has also sparked researchers’ interest in using carbon dots in recycling and waste management ^[94][95][96][97,98,99]. However, studies have shown that although they have a low toxicity impact, carbon dots still induced shrinkage and the formation of holes on the surface of yeast cells ^[97][98][100,101]. Thus, it would be unwise to categorize carbon dots as safe before further investigation. Graphene quantum dots (GQDs), on the other hand, consist of a single layer of carbon atoms arranged in a two-dimensional honeycomb shape ^[74][99][66,77]. Most current studies on GQDs agreed that GQDs have a low toxicity effect, even in vivo testing ^{[100][101][102]}[102,103,104]. Therefore, the appeal of using GQDs for biomedical science and disease treatment has been the focus of research for this type of QD ^{[103][104][105][106][107]}[105,106,107,108,109]. In addition, carbon-based QDs are also desired for energy-related applications such as protogalactic energy conversion and super capacitator production ^[108][109][110,111]. Despite the potential in low toxicity use for biological processes, the research on GQDs is still in its infancy and needs further understanding before being commercially and industrially employed.

3. Quantum Dots in Fungal Cells

3.1. Interaction and Intracellular Trafficking

As decomposers and nutrient recyclers ^{[110][111][112][113]}[159,160,161,162], fungi are an important component of theour ecosystem. Therefore, before applying QDs in mass industrial-produced products, it is important to assess how QDs from discarded products will interact and affect fungal systems. For fungal cells, the rigid cell wall is a crucial factor when it comes to QD interactions. In a recent study, Qdots 625 ITKTM (QDs) were engineered on the surface of the budding yeast Saccharomyces cerevisiae by covalently binding yeast surface protein with the -SH group of modified QDs to investigate the interaction between QDs and the yeast surface. Results showed that Qdots 625 ITKTM (QDs) on the cell wall of the mother cell remained on its progeny for up to two generations. Although QDs remained attached to the cell surface for some time, there was no difference in yeast surface morphology when examined with an electron microscope. This study also found that engineered QD attachment on yeast surface did not affect the growth and viability of yeast ^[114][163]. In contrast, a different study found that small, free CdTe QDs, around 4.1–5.8 nm quantum dots, readily internalized into fungal cells and induced cytotoxicity by breaking cell wall components as well as causing apoptotic blebbing in the cytoplasm [52]. Consistently, recent data found that in the ascomycete fungus Fusarium oxysporum, CdSe/ZnS QDs easily internalized and uniformly distributed throughout hyphae after 3 h of incubation. It was also observed that QDs formed large defined clusters inside fungal cells after 16 h of incubation. Afterward, the team removed QDs from the media by washing cells and placing them in a QD-free media. Four hours after QDs were removed, a small amount of quantum dots remained inside the cells, while large aggregates were found on the cell surface and media. This led the team to hypothesize that internalized QDs are eventually secreted out, likely by exocytosis, but the exact mechanism of QD release remained unclear. Furthermore, the results in this study showed that only a high concentration, 500 nM of CdSe/ZnS QDs, compromised the growth and germination of Fusarium oxysporum [52]. Overall, in these findings, rwesearchers could conjecture that internalized QDs had more effect on yeast viability compared to surface-attached QDs. Consistent with data seen in mammalian systems, QDs are also exported out of the cell by exocytosis, yet the exact exit pathway in yeast remains a mystery. To the best of theour knowledge, the current data only indicate the uptake of QDs by fungal cells, however, the endocytic route and QDs’ exact intracellular trafficking are unknown. There is also a lack of studies demonstrating the trafficking of other types of QDs, such as InP-based and carbon-based quantum dots in yeast. This presents a knowledge gap in the field that requires immediate attention and research.

3.2. Toxicity Effect of QDs on Fungal Cells

Similar to the mammalian system, the protective zinc shell was also found to be effective in limiting QD toxicity in fungi. As demonstrated in a study, yeast cells treated with core-type CdSe QDs were found to have a significant reduction in mitochondrial membrane potential and acquired severe cell wall damage, while treatment of core–shell structure CdSe/ZnS QDs showed less impact on yeast cells overall ^[115][164]. The lower toxicity effect in the presence of a ZnS shell suggested that core leakage seems to be a major mechanism in QD toxicity. Interestingly, further studies show that the release of cadmium ions seems to be only one of the factors contributing to QD toxicity. In Saccharomyces cerevisiae, a systemic knockout mutant screening identified 114 KO mutant strains to develop tolerance to the presence of CdS QDs. These mutant strains were then tested with cadmium ions in the form of CdSO₄. The results showed that there were only 11 CdS-QD-tolerant mutant strains showing resistance to cadmium ions, supporting that the cellular response to cadmium ions and cadmium-based quantum dots is different ^[116][165]. Thus, the toxicity of CdS QDs was not solely due to the release of cadmium ions but rather resulted from the interaction with QDs. Another group of researchers demonstrated the impacts of CdS QDs against Saccharomyces cerevisiae. CdS QDs were found to induce the deletion of genes associated with stress response, metabolic processes, mitochondrial organization, DNA repair, and cellular transportation. Exposure to CdS QDs also led to an increased level of reactive oxygen species, reduced oxygen consumption, lowered both reduced and oxidized glutathione levels, and altered the mitochondrial membrane potential and mitochondrial morphology ^[117][166]. Although core–shell quantum dots are considered to cause less cellular damage, treatment of CdSe/ZnS QDs still altered the expression of many genes in S. Cerevisiae. Using RNA sequencing, it was found that most upregulated genes are associated with cellular component regulation, rRNA metabolic processes, macromolecule methylation, maturation of large and small subunits of ribosomes, and DNA replication in the cell cycle. Downregulated genes include genes involved in oxidation–reduction processes, small-molecule metabolic processes, proteolysis, transmembrane import and transportation, chemical responses, and electron transport chains ^[118][167]. For the mammalian cellular system, the majority of available data suggested that InP-based QDs are less toxic than Cd-based QDs. However, this was not observed in the fungal system. In 2021, Horstmann et al. were the first to compare the transcriptome of S. cerevisiae exposed to cadmium-based QDs (CdSe/ZnS QDs) to indium-based QDs (InP/ZnS QDs). They found that InP/ZnS inhibited yeast growth when treated at a high concentration of 100 µg/mL, while CdSe/ZnS prolonged the lag phase of yeast cells, out-compromising the final optical density at the same dosage. Furthermore, the team measured the ROS level of QD-treated cells to examine whether the inhibitory effect of InP/ZnS QDs was due to increased ROS level. They found that the ROS level in InP/ZnS-QD-treated cells decreased, while a significant elevation in ROS level was observed for CdSe/ZnS-treated cells. Furthermore, the team’s RNA-seq analysis revealed that InP/ZnS QD cells showed to have an increased expression in genes associated with antioxidant defense and peroxisome structure, while the expression of genes associated with metabolic activities was significantly decreased. On the other hand, CdSe/ZnS QDs altered genes associated with protein metabolic processes, cell wall organization, and cellular homeostasis. Interestingly, both QDs changed the level of genes associated with transmembrane transport and translation ^[73][76]. Unlike the increasing trend in the research effort on indium-based QDs in the mammalian system, available data regarding their interaction with the fungal system are limited. As the initial comparison of the two quantum dots points out the difference in the cellular response to each QD type, it is crucial to investigate the possible effect indium-based QDs have on fungal populations as rwesearchers examine the possibility of using them as an alternative to cadmium-based QDs. A few recent studies have found carbon dots to be a promising candidate for antimicrobial and antifungal drugs ^{[119][120][121][122][123][124]}[168,169,170,171,172,173]. It was found that while carbon dots were non-toxic to the human cells HCT-116 and TM4 at a low concentration of 3.5 mg/mL, they had dose-dependent antimicrobial activity on fungal cells ^[124][173]. Similarly, nitrogen-doped graphene quantum dots (NDGQDs) significantly inhibited the growth of several fungal strains, including P. citrinum, C. albicans, and Ammophilus fumigatus, while exerted minimal impact on mouse fibroblasts cells ^[119][168]. Additionally, combining high photothermal light and carbon-based quantum dots was found to enhance QDs’ antifungal activities ^[120][169]. All this evidence suggests that fungal cells are more sensitive to carbon-based quantum dots compared to mammalian cells, and thus could potentially become effective antimicrobial agents. It is worth noting that the above studies only focused on the viability of fungal cells, yet the mechanism by which carbon dots reduce fungal viability remains to be understood. Future research regarding the specific impact of carbon-based QDs on fungal transcriptomic, proteomic, and metabolic changes is greatly needed. While the impacts of different types of QDs are under investigation, it is important to also consider examining the interaction between QDs and fungal cells, as well as the mechanisms that induced these cellular responses. To the best of theour knowledge, currently, there are very few studies focusing on the specific QD–fungal cell interactions, the QD intracellular trafficking pathway in fungal cells, QD interactions with specific organelles, and how these interactions contribute to QD toxicity in fungal cells (Table 1). This knowledge would allow reusearchers to further understand the mechanisms of QDs’ toxicity and develop strategies to limit their toxicity for safer QD applications, therefore this could be the target of future research.