Effects of representative quantum dots on microorganisms and phytoplankton: a comparative study

Abstract

With the increasing application of semiconductor particles, especially metal-based quantum dots and carbon quantum dots (CQDs), these quantum dots will inevitably be released into the environment, and therefore their effects on biota should be assessed. Few studies on the effects of CQDs on microorganisms and phytoplankton have been performed. In this paper, we put forward more effective and convenient approaches to prepare three kinds of high-quality CQDs. Then we assessed their effects on Staphylococcus aureus and Microcystis aeruginosa, which were representative of microorganisms and phytoplankton, respectively, and compared the effects with the effects of metal-based QDs (CdSe-QDs or CdTe-QDs). The results showed that CQDs at low concentrations (<50 mg L⁻¹) had insignificant effects on the growth of Staphylococcus aureus and Microcystis aeruginosa, while the influence of the CQDs was observed significantly when the concentration was increased up to 100 mg L⁻¹. However, the negative impact of metal-based QDs was observed at any given concentration. In conclusion, the results demonstrated that any kind of carbon quantum dot has lower ecological risks than metal-based QDs, which may provide a reference for utilizing carbon quantum dots better and safely.

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Introduction

Due to the advantages in their physical characteristics, quantum dots (QDs) have found increasing use for commercial purposes during the last few decades. As semiconductor particles, quantum dots may be divided into two large subcategories, carbon-based quantum dots and metal-based quantum dots, which have been employed for many applications including in photoelectrical equipment, biological imaging, chemical sensors, and drug delivery. However, with these extensive applications, these semiconductor particles are discharged, directly or indirectly, into aquatic and terrestrial ecosystems. For example, CdTe-QDs are increasingly measured in natural water bodies.^1,2 Three sources of CdTe-QDs have been considered; industrial wastewater, medical sewage and research usage. Thus, the potential environmental risks of these semiconductor particles should be concerning. So far, a variety of research has been published to evaluate the potential toxicity of quantum dots with heavy metals on species such as³ human cells,⁴ rats,⁵ invertebrates and^6,7 so on. Compared with metal-based quantum dots, carbon-based quantum dots have attracted higher attention owing to their superior properties, as well as their promising applications in various fields. Up to now,^8,9 there are also many investigations focused on the toxicity of carbon quantum dots on human cells. But their conclusions could not be applied to all species, especially single-celled organisms, such as microorganisms and phytoplankton.¹⁰ These microbes play an important role in the ecological environmental system, and the proportion of bacteria is as high as 90%. In addition, phytoplankton serves as an important component of the aquatic ecosystem.¹¹ Potential environmental impacts on these autotrophs could decrease primary productivity, influence the entire food chain, and change the structure and function of the whole ecosystem. Both microorganisms and phytoplankton are widely existing, and their reflection of environmental change is also the most sensitive. So far, there have been few investigations into the biosecurity of carbon quantum dots in microorganisms and phytoplankton.^12–16 For instance,¹² nanocarbon synthesized from acetylene and benzene can destroy Escherichia coli and Staphylococcus aureus, and limit oxygen transport from the environment. In addition, Pereira et al.¹³ and Zhang et al.¹⁴ have investigated the interactions of carbon nanotubes with green algae. The results showed that the NPs inhibited algae growth by disturbing oxidation induction, disturbing ATP production, decreasing photosynthetic activity and physical stress. Yet, knowledge on the ecotoxicological effects of carbon quantum dots is still limited. Therefore, more experiments should be carried out to assess the effect of carbon quantum dots on microorganisms and phytoplankton.

In this work, we put forward more effective and convenient approaches to prepare three kinds of carbon quantum dots with superior optical properties. All of them exhibited small size, high aqueous solubility and bright blue fluorescence. Then, a series of assays were conducted to assess the effects of CQDs and metal-based QDs (CdSe-QDs or CdTe-QDs) on Staphylococcus aureus (S. aureus) and Microcystis aeruginosa (M. aeruginosa) which were representative of microorganisms and phytoplankton, respectively.^17,18 S. aureus is a kind of widely existing Gram-positive bacteria with high sensitivity to poisonous substances, while M. aeruginosa, as a kind of freshwater cyanobacteria, can reflect the condition of an aquatic ecosystem. This is the first time reporting the effects of carbon quantum dots passivated using various agents on S. aureus and M. aeruginosa, and it is hoped to provide a reference for understanding the effect of carbon quantum dots on the ecosystem and utilizing them better and safely.

Experimental

Materials

Activated carbon was made in the laboratory. Acetic acid (A.R., Nanjing Chemical Reagent Co., Ltd), hydrogen peroxide (30%, A.R., Sinopharm Chemical Reagent Co., Ltd), diethyl ether (A.R., Nanjing Chemical Reagent Co., Ltd.), PEG₂₀₀₀ (A.R., Xilong Chemical industry Co. Ltd), citric acid (CA, Nanjing chemical reagent co., Ltd, AR), glycine (Gly, Yika biotech, Nanjing, BR), N-butyl alcohol (Chinasun specialty products co., Ltd, AR), and sodium hydroxide (West Long chemical co., Ltd, AR) were used in this study. All chemicals were of analytical grade and used as received. Double distilled water was used throughout the experiments.

S. aureus (GIM1.142) samples were provided by the Guangdong Biological Germplasm Resource Bank, China. S. aureus was cultivated in Luria-Bertani (LB) culture medium (pH 7.4) which contains tryptone, 10 g L⁻¹ yeast extract and 5 g L⁻¹ NaCl.

The freshwater cyanobacteria M. aeruginosa (FACHB-1005) was obtained from the freshwater algae culture collection of the Institute of Hydrobiology (FACHB-Collection) of the Chinese Academy of Sciences. Cells of the cyanobacteria was incubated and maintained at 26 ± 1 °C under an illumination intensity of 2000 lux, with 12 h/12 h light/dark intervals. BG-11 medium was applied as the culture medium, which was adjusted to pH 8.0 with NaOH and HCl.

Instrumentation

A teflon lined autoclave (Zhenghong Plastics Co., Ltd.) was used to prepare the citric acid-passivated carbon quantum dots and PEG₂₀₀₀-passivated carbon quantum dots. S. aureus was cultivated using a rotary shaker (Peiying Co., Ltd.). An electron microscope (JEM-2100 120 kV, JEOL), UV-vis absorption spectrophotometer (UV 2100, Shimadzu, Kyoto, Japan) and fluorescence spectrometer (RF-5301PC, Hitachi, Tokyo, Japan) were used to characterize the properties of the CQDs. All pH values were measured with a pHS-25 pH meter (Shanghai INESA Scientific Instrument Co., Shanghai, China).

Preparation of PEG₂₀₀₀-CQDs, CA-CQDs, and Gly-CQDs

PEG₂₀₀₀-passivated carbon quantum dots (PEG₂₀₀₀-CQDs) were prepared via a simple chemical oxidation proposed earlier by our group.¹⁹ Briefly, activated carbon was dissolved in a mixture of hydrogen peroxide and acetic acid, then refluxed at 100 °C for 12 h. The obtained bare CQDs were purified using diethyl ether and passivated using PEG₂₀₀₀.

Citric acid-passivated carbon quantum dots (CA-CQDs) were prepared by the hydrothermal treatment of citric acid (CA). In a typical synthesis, CA (0.5 g) was added into H₂O (20 mL). Then the mixture was transferred into a 60 mL Teflon lined autoclave, heated at 200 °C for a period of 4 h and then cooled down to ambient temperature naturally. The obtained CA-CQDs were dispersed in water for further characterization and use.

Glycine-passivated carbon quantum dots (Gly-CQDs) were prepared using a method reported earlier by our group.²⁰ Briefly, 3.6 g of CA and 1.5 g of Gly were dissolved in 30 mL of distilled water and transferred to a 60 mL Teflon lined autoclave. The reaction was maintained at 200 °C for 4 h. The obtained burgundy solution was purified by extracting with N-butyl alcohol. Finally, the purified Gly-CQD solution was dried and stored for further study.

The comparative assays of S. aureus and M. aeruginosa

In order to evaluate the effects of the three kinds of CQDs on S. aureus and M. aeruginosa, CdSe-QDs and CdTe-QDs were adopted as representative metal-based materials, and were synthesized by our lab.^21–23 Typically, CdTe-QDs modified with TGA were prepared from cadmium chloride and NaHTe solutions. Tellurium powder was mixed with sodium borohydride under N₂ for 30 min at 45 °C. Then the mixture was injected into an N₂-saturated Cd²⁺–TGA precursor solution (the molar ratio of Cd : Te : TGA was 1 : 0.5 : 2.4). Finally, the mixture was refluxed at a temperature of 100 °C for 30 min. The synthesis of CdSe-QDs stabilized with TGA was similar to that of CdTe-QDs. Briefly, a mixture of Se powder and sodium borohydride was injected into a Cd²⁺–TGA solution. CdSe-QDs were obtained from the mixture that was refluxed for 60 min under a nitrogen atmosphere.

Exposure experiments were carried out at the same temperature as the stock cultures. The bacterial suspension and phytoplankton inoculum were prepared for each experiment from their respective fresh culture stocks sampled during the exponential growth phase. The experiments on the effects were performed in two parts: the effects of the nano-materials on S. aureus and the effects on M. aeruginosa. In part I, a quantity of CA-CQDs, Gly-CQDs and CdSe-QDs were dissolved in LB culture medium (final concentration = 500 mg L⁻¹). 0, 0.5, 1, 5, 10, and 50 mL of the above three kinds of solution were added into 250 mL Erlenmeyer flasks (final concentration: 0, 5, 10, 50, 100, and 500 mg L⁻¹). For each of these flasks, a certain volume of S. aureus solution was added accurately and they were placed on a rotary shaker (37 °C, 250 rpm) for different times ranging between 0 h and 24 h. Then, the absorptivity at 610 nm for each specimen was measured using a UV-vis absorption spectrophotometer.

In part II, an equal volume of the fresh culture medium, 100 mL, in the presence of PEG₂₀₀₀-CQDs at various concentrations was added to the phytoplankton pellets in 250 mL previously sterilized conical flasks. The corresponding PEG₂₀₀₀-CQD concentrations were 5, 10, 50, 100, and 500 mg L⁻¹. As for CdTe-QDs, all treatments were processed to be similar to PEG₂₀₀₀-CQDs while the corresponding CdTe-QD concentrations were 0.2, 0.5, 1.0, 10.0, and 20.0 mg L⁻¹ respectively. Phytoplankton culture without any quantum dots was used as a control. Samples were removed from the culture vessels at a predetermined time every 24 h. The cells were observed microscopically.²⁴ Chlorophyll and soluble proteins were analyzed using the standard method. The population growth rate (r) was calculated from Formula (1):

where N_t and N₀ are the population sizes at day 0 and day t, and t is the time in days when the population size is maximum. Each experiment was replicated three times. All the data analyses were carried out with the SPSS analytic package 16.0. Data were first tested for homogeneity (Levene’s test). Variables from the results of experiments I and II were examined using one-way analysis of variance (ANOVA) to identify the significant differences. All figures were produced using Sigmaplot Version 12.0.

Results and discussion

Characterization

The characterization of PEG₂₀₀₀-CQDs was reported earlier by our group.²⁵ As shown in Fig. 1 and 2, CA-CQDs and Gly-CQDs were characterized using TEM images, UV absorption spectra, FL spectra and FT-IR spectra. It can be seen that the two kinds of spherical particles were less than 10 nm in size and dispersed evenly (see Fig. 1).

Fig. 1 TEM images of CA-CQDs (a) and Gly-CQDs (b).

Fig. 2 (a) UV absorption spectrum and FL spectrum of CA-CQDs with excitation of 360 nm (a is UV spectrum, b is FL spectrum). Inset: UV absorption spectrum and FL spectrum of Gly-CQDs and image of the solution of Gly-CQDs under 365 nm UV light. (b) FT-IR spectra of CA-CQDs (curve a) and Gly-CQDs (curve b).

As shown in Fig. 2(a), CA-CQDs emitted blue light (485 nm) when excited with a 360 nm UV beam, while CQDs functionalized using glycine emitted blue light (485 nm) when excited with a 380 nm UV beam (see inset of Fig. 2(a)). Using quinine sulfate as a reference, the quantum yields of the prepared PEG₂₀₀₀-CQDs, CA-CQDs and Gly-CQDs measured were found to be 19.6%, 9.8% and 47%.

FT-IR spectra were recorded to provide further structural insights about the CA-CQDs and Gly-CQDs (Fig. 2(b)). As shown in curve a, the CQD sample treated with CA showed the characteristic absorption peak of OH at 3460 cm⁻¹ and the stretching vibration band of the C=O bond in carboxylic acid at 1610 cm⁻¹,^26,27 which contributes to good water-solubility.^28,29 According to literature, the O–H bond, –COOH and C=O of CA may interact with each other under pressure and high temperature, causing incomplete dehydration and carbonization. This may lead to the formation of fluorescent CQDs with the hydroxyl group, carbonyl group and carboxyl group on the surface and carbon–carbon bonds at the core. Compared with curve a, the FT-IR spectrum of Gly-CQDs showed many characteristic absorption bands, amide groups at 1628 cm⁻¹ and 3458 cm⁻¹, a methylene group at 2936 cm⁻¹, and a cyano group at 1186 cm⁻¹, but nearly no characteristic absorption for CA. Moreover, the stretching vibration of the C=O bond at 1713 cm⁻¹ was also detected, indicating that the Gly functionalized CQDs with an amide linkage (–CONH–).

Analysis of the effect on S. aureus. A series of assays were conducted to assess the effects of three different QDs (CA-CQDs, Gly-CQDs and CdSe-QDs) on S. aureus by means of the optical density method.

The optical densities of a series of LB media spiked with different concentrations of CdSe-QDs were also determined to investigate the effect of CdSe-QDs, which is a representative of quantum dots containing heavy metals. As shown in Fig. 3(a), S. aureus was significantly inhibited by even a low concentration of CdSe-QDs. For bacteria grown with 1 mg L⁻¹ CdSe-QDs, the bacteriostatic rate was approximately 40%, suggesting that the growth environment was fairly unsuitable for S. aureus. Growth rates of bacteria declined as the concentration of CdSe-QDs increased, and then the biomass stopped growing at concentrations over 5 mg L⁻¹. This indicated that CdSe-QDs exhibited strong inhibition of S. aureus.³⁰ The different core materials of the QDs were the principal reason for their different toxicities. Cadmium is toxic, even at a low concentration, and it can be released from the CdSe-QDs, while carbon was less toxic to the strain than cadmium.

Fig. 3 Growth curves of (a) CdSe-QD-treated S. aureus, (b) CA-CQD-treated S. aureus, and (c) Gly-CQD-treated S. aureus.

A series of the same amount of S. aureus suspensions were inoculated into fresh LB medium spiked with CA-CQDs ranging in initial concentration from 0 to 500 mg L⁻¹ and the results are shown in Fig. 3(b). It turned out that the growth tendencies of S. aureus in different concentrations of CA-CQDs are nearly in accordance with S. aureus suspensions without any CQDs. All of these growth curves peaked after nearly 20 h and the inhibition rates of S. aureus at that hour were 18.82%, 25.40%, 40.75%, 48.17%, and 88.56% (different concentrations of CA-CQDs: 5, 10, 50, 100, and 500 mg L⁻¹). These results indicated that S. aureus was insusceptible when the concentration was less than 10 mg L⁻¹, and the inhibition effect strengthened with an increase in CA-CQD concentration. As the initial concentration of CA-CQDs increased to 500 mg L⁻¹, the level of growth declined, indicating that a high-concentration can significantly inhibit S. aureus.

Compared with the blank control group, the growth status of S. aureus was different under different concentrations of Gly-CQDs. S. aureus was not suppressed by Gly-CQDs (<10 mg L⁻¹), but had increased growth, better than that of the blank control group, which may be partly due to Gly-CQDs containing nitrogen elements that contributed to the growth of the bacteria (see Fig. 3(c)). The inhibitory effects under low concentrations were dulled by the positive effects of the nitrogen element, hence, S. aureus increased rather than decreased. Growth inhibition of bacteria can be observed obviously with an increase in Gly-CQDs concentration, ranging from 50 mg L⁻¹ to 500 mg L⁻¹. The results show that the bacteriostatic rates of Gly-CQDs at 20 h were 30.59%, 56.14%, and 91.45% corresponding to the respective concentrations, 50, 100, and 500 mg L⁻¹. This indicated that the positive effects of the nitrogen element are not enough to offset the toxicity effects on the bacteria with an increase in the Gly-CQDs concentration, which can make the microflora decrease. However, the investigation of S. aureus is still at the primary stage, and the effects of CQDs on other aspects of S. aureus (e.g. metabolism) are unclear. We will carry out more experiments to deepen the research in the follow-up study.

Analysis of the effect on M. aeruginosa

The population growth curves of Microcystis aeruginosa for five concentrations of PEG₂₀₀₀-CQDs and CdTe-QDs are presented in Fig. 4. In general, the population increased over 144 h (6 days) under most concentrations except 500 mg L⁻¹ (Fig. 4(a)). The population dynamic at 5, 10, and 50 mg L⁻¹ indicated that the PEG₂₀₀₀-CQDs at the three concentrations did not produce significant inhibition on the growth of M. aeruginosa (p > 0.05, F-test). The cyanobacteria population in the three concentrations grew slowly in the first 48 h and then increased faster until the end of the experiment. The maximum population size was 10.2 ± 0.14, 10.07 ± 0.18, and 9.25 ± 0.39 × 10⁶ cells per mL, which was only 0.79%, 1.34%, and 4.96% lower than that of the control. However, PEG₂₀₀₀-CQDs produced a negative impact on the growth of M. aeruginosa at 100 mg L⁻¹; the maximum population size was 6.06 × 10⁶ cells per mL on average and the population growth rate was 0.30 d⁻¹, which was 76.97% and 58.33% of that of the control, respectively. In addition, the negative trend in the growth of M. aeruginosa was observed significantly when the concentration was 500 mg L⁻¹. The maximum population size was only 0.14 ± 0.02 cell per mL, which was only 1.35% of that of the control. In comparison, CdTe-QDs produced a negative impact on the growth of M. aeruginosa, regardless of the concentration (Fig. 4(b)). When the concentrations were lower than 1.0 mg L⁻¹, the population increased until the end of the experiment. The population dynamics at 0.2 and 0.5 mg L⁻¹ indicated that CdTe-QDs had a significant effect on the growth of M. aeruginosa (p < 0.05, F-test). The maximum population size was 6.31 ± 0.67 and 3.48 ± 0.68 × 10⁶ cells per mL on average, which was 61.62% and 33.98% of that of the control, respectively. In addition, the impact strengthened with increasing concentration. CdTe-QDs produced significant inhibition on the growth of M. aeruginosa when the concentration was 1.0, 10 or 20 mg L⁻¹. The population growth rate was −0.37, −0.68 and −0.69 d⁻¹ respectively. Most broken cells were observed microscopically.

Fig. 4 The population growth curves of M. aeruginosa under five concentrations of PEG₂₀₀₀-CQDs (a) and CdTe-QDs (b).

The chlorophyll-a content of M. aeruginosa, which varied under the given concentrations of the PEG₂₀₀₀-CQDs, and the control, is presented in Fig. 5(a). The trend of the PEG₂₀₀₀-CQDs was observed clearly. Chlorophyll-a was accumulated over 144 h (6 days) under most concentrations except 100 and 500 mg L⁻¹. Statistical analysis showed that the PEG₂₀₀₀-CQDs at 5, 10, and 50 mg L⁻¹ had no significant effect on the chlorophyll-a increasing (p > 0.05, F-test). In addition, the chlorophyll-a content of M. aeruginosa accumulated with 100 mg L⁻¹ of the QDs during the whole experiment time. The influence of the PEG₂₀₀₀-CQDs was observed clearly. The content of chlorophyll-a at 144 h was 0.37 ± 0.02 mg L⁻¹, which was 73.00% of that of the control. However, the chlorophyll-a content of M. aeruginosa declined significantly when exposed to the PEG₂₀₀₀-CQDs at 500 mg L⁻¹. It was 0.012 ± 0.01 mg L⁻¹ at the end of the experiment, only 2.42% of that of the control. In comparison, CdTe-QDs produced an obvious impact on the photosynthesis of M. aeruginosa, regardless of the concentration. The content of chlorophyll-a was almost lower than the control at any given concentration. Although the chlorophyll-a in M. aeruginosa was accumulated over 48 h at 0.2 and 0.5 mg L⁻¹, it declined rapidly from the second day. The content of chlorophyll-a at the end of the experiment was 0.21 ± 0.02 and 0.14 ± 0.02 mg L⁻¹ respectively, only 42.69% and 28.26% of that of the control. When the concentration of CdTe-QDs arrived at 1 mg L⁻¹ or higher, the content of chlorophyll-a dropped to near zero after 144 h.

Fig. 5 (a and b) The chlorophyll-a content of M. aeruginosa varied under given concentrations of the PEG₂₀₀₀-CQDs and CdTe-QDs. (c) The soluble protein content in the cells of M. aeruginosa varied under given concentrations of the PEG₂₀₀₀-CQDs and CdTe-QDs at 144 h.

The soluble protein content in the cells of M. aeruginosa under given concentrations of the PEG₂₀₀₀-CQDs and CdTe-QDs after 144 h is presented in Fig. 5(b). The soluble protein content of the cells was increased at relatively low concentrations (5 and 10 mg L⁻¹), whereas it was inhibited at relatively high concentrations (>10 mg L⁻¹). The results showed that the soluble protein content was 9.52% and 8.70% more than that of the control at 5 and 10 mg L⁻¹, respectively, while the value was 95.24% and 85.71% of that of the control at 50 and 100 mg L⁻¹. However, the concentration of 500 mg L⁻¹ produced a significant impact on the M. aeruginosa, which made the soluble protein content decline to 0.03 μg g⁻¹, only 13.81% of the control. In comparison, the soluble protein content was almost lower than the control at any given concentration of CdTe-QDs. The CdTe-QDs exhibited a significantly negative influence on the soluble protein of the plankton (p < 0.01, F-test). The soluble protein content decreased with increasing CdTe-QD concentration. When the concentration arrived at 1 mg L⁻¹ or higher, the content dropped to near zero.

The median effective concentrations (EC₅₀) of the PEG₂₀₀₀-CQDs and CdTe-QDs on the growth of M. aeruginosa are presented in Fig. 6. For the PEG₂₀₀₀-CQDs, the inhibition effect declined during the exposure time. Over the first 24 h, the EC₅₀ was 8.73 mg L⁻¹, implying a moderate toxicity. After 48 h, the EC₅₀ value increased rapidly and arrived at the maximum value (57.64 mg L⁻¹) after 144 h, seven times the 24 h-EC₅₀, indicating a low toxicity. Oppositely, the EC₅₀ of CdTe-QDs on the growth of cyanobacteria was much lower than that of the PEG₂₀₀₀-CQDs at any exposure time.

Fig. 6 The EC₅₀ of the PEG₂₀₀₀-CQDs and CdTe-QDs on the growth of M. aeruginosa.

A three-stepwise toxic effect was observed. When exposed to CdTe-QDs in the first 48 h, the EC₅₀ increased by 76.57%, which implied a reduced toxicity. In the second step, the EC₅₀ had no significant change from 72 to 120 h while it declined rapidly to 0.23 mg L⁻¹ finally, indicating a latent high-toxicity.

The growth inhibition of nanoparticles is related to their chemical composition.³¹ For example, QDs usually release their core metals into water.^32,33 The dissolved metal ions are known to be toxic to aquatic organisms even if at a relatively low concentration. The toxicity of CdTe-QDs is attributed to the core material, Cd.³⁴ A stepwise stress model (SSM) indicated that a serial sequence response of organisms was activated regularly by increased toxicant concentration or exposure time. In the present study, CdTe-QDs impacted the cyanobacteria in three steps. A declined inhibition in the first step implied the tolerance of M. aeruginosa when exposed to chemicals. However, stress gradually decreased the adaptation as time passed until M. aeruginosa could not overcome the threshold, which caused the inhibition effect to occur again. In contrast, the PEG₂₀₀₀-CQDs were synthesized from biological activated carbon, which was derived from an organic matrix, such as twigs and peels. This suggests that cyanobacteria had a possible metabolic effect on PEG₂₀₀₀-CQDs. The median effective concentrations (EC₅₀) of PEG₂₀₀₀-CQDs and CdTe-QDs, resulted in a 50% reduction in the growth rate of cyanobacteria within the given exposure time compared to the control. The EC₅₀ value comparison suggested that CQDs had a much lower inhibition effect and no latent impact, compared to CdTe-QDs.

³²Hormesis is the affected function, which is characterized as a response to toxicants from a low-concentration stimulation to a high-concentration inhibition.³⁵ In previous research, the population growth rate of P. tricornutum was stimulated when [QDs] ≤ 0.2 nM.^36,37 This effect is also reported in vivo with CdSe/ZnS-QDs and other nanoparticles. However, the effects of CQDs on organisms were less characterized. It’s worth noting that the positive effects on the growth and photosynthesis of M. aeruginosa at 5 and 10 mg L⁻¹ were not significant (Fig. 4 and 5), whereas the soluble protein in cells was increased at relatively low concentrations. The organism, not the chemical, is considered as the key factor in hormesis. The impact on or disruptions in homeostasis finally induced the organism to respond at different levels. Thus, this response can be considered a signal of cellular stress. We presume there could have been the possible occurrence of some interaction between cyanobacteria and CQDs, which should be considered deeply in future studies.

Conclusions

In this work, PEG₂₀₀₀-CQDs, CA-CQDs and Gly-CQDs were prepared via facile chemical oxidation and one-step thermal pyrolysis routes, respectively. All of these CQDs exhibited excellent water solubility, and favourable photostability. Whereafter, a series of comparative tests were conducted in order to investigate the inhibitory effects of different CQDs and metal-based quantum dots on S. aureus and M. aeruginosa. In part I, the effects of CA-CQDs and Gly-CQDs compared with CdSe-QDs were investigated on S. aureus using³⁸ the optical density method. Results showed that bacterial abundance was of positive relevance to the inhibitory effect of CQDs as the concentration increased. A low concentration of Gly-CQDs was benefitial to S. aureus. However, S. aureus was more susceptible to CdSe-QDs, which even in low concentration can inhibit bacteria significantly. It was demonstrated the CA-CQDs and Gly-CQDs were much less toxic compared with CdSe-QDs. In part II, this research was the first time M. aeruginosa was adopted to evaluate the potential environmental risks of PEG₂₀₀₀-CQDs and CdTe-QDs. The growth, chlorophyll-a accumulation and soluble protein content of the phytoplankton were evaluated. In general, CdTe-QDs had a significant inhibitory effect on the population growth and chlorophyll-a accumulation of M. aeruginosa, whereas CQDs had much less toxicity and latent impact than CdTe-QDs. In summary, any kind of carbon quantum dot has lower ecological risks than metal-based QDs on S. aureus and M. aeruginosa, which may provide a reference for the better and safe utilization of carbon quantum dots. However, more experiments should be done to deepen the research in the future.

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