Acute toxicity assessment of polyaniline/Ag nanoparticles/graphene oxide quantum dots on Cypridopsis vidua and Artemia salina

Nanotoxicology is argued and considered one of the emerging topics. In this study, polyaniline (PANI)/2-acrylamido-2-methylpropanesulfonic acid (AMPSA) capped silver nanoparticles (NPs)/graphene oxide (GO) quantum dots (QDs) nanocomposite (PANI/Ag (AMPSA)/GO QDs NC) as a nanoadsorbent has a potential for removal of toxic hexavalent chromium (Cr(VI)) ions from water. The acute toxicity of this NC was evaluated on Artemia salina and freshwater Ostracods (Cypridopsis vidua) larvae for 48 h. The measurements were made at 24 and 48 h with 3 repetitions. The 50% effective concentration (EC50) values of the NC were determined after the exposure of these organisms. According to the results of the optical microscope, it was found that both experimental organisms intake the NC. In the toxicity results of Ostracods, the NC had a highly toxic effect only at 250 mg/L after 48 h and the EC50 value was 157.6 ± 6.4 mg/L. For Artemia salina individuals, it was noted that they were less sensitive than the Ostracods and EC50 value was 476 ± 25.1 mg/L after 48 h. These results indicated that PANI/Ag (AMPSA)/GO QDs NC has low toxicity towards both investigated organisms.

www.nature.com/scientificreports/ Polyaniline (PANI) and its nanocomposites (NCs) with high stability, high electroactivity, high conductivity and low cost are candidate to apply in the fields of sensors 6,[26][27][28][29] , energy storage 10 and water treatment 5,30-32 . Yslas et al. reported that there is no toxicological effect of PANI nanofibers for Rhinella arenarum larvae exposed to PANI even at the highest concentration, 400 mg/L 33 . Ibarra et al. confirmed that the 50% lethal concentration (LC 50 ) values of PANI NPs dispersed in polyvinylpyrrolidone and polyN-isopropylacrilamide on the larvae of Rhinella arenarum were 1500 mg/L and 1170 mg/L, respectively 34 .
Chromium compounds are one of the highest toxic materials and consider as environmental pollutants. High levels of these compounds affect cellular structures. Therefore, there are great efforts to develop selective sensors and adsorbents for chromium with various nanomaterials, nanocomposites or hybrids materials 5,32,[35][36][37][38] .
Bioassay tests are important for determining the applicability of the synthesized materials in water treatment. The exposure to the nanomaterials and their possible effects should be evaluated in freshwater and saltwater. Cypridopsis vidua, also called Cypridopsis Müller are freshwater Ostracode. The most dominant species of freshwater Ostracods in Egypt is Cypridopsis vidua 39 . Ostracods have been used as one of freshwater crustaceans in the ecotoxicological studies and also for toxicity monitoring of water, soil and river sediment [40][41][42] .
Artemia salina is another crustacean from salt lakes and hypersaline ecosystems. Artemia salina is also considered as one of the test species by the United States Environmental Protection Agency (US-EPA) for acute toxicity testing 43 . Many studies have used Artemia salina as a model organism to study the ecotoxicity of some nanoparticles 19,25,[44][45][46] .
The current work is engaged with testing of PANI/2-acrylamido-2-methylpropanesulfonic acid capped Ag NPs/GO QDs (PANI/Ag (AMPSA)/GO QDs) toxicity which have been prepared and used in the detection and removal of toxic hexavalent chromium (Cr(VI)) from polluted water 6,32,47 . This test is conducted on larvae of freshwater Ostracode (Cypridopsis vidua) and saltwater Artemia salina. The acute toxicity of different concentrations of PANI/Ag (AMPSA)/GO QDs is studied for 24 h and 48 h of exposure. Also, the uptake and accumulation of the NC in the Ostracode and Artemia larvae are observed and discussed after 24 h of exposure.

Materials and methods
Preparation and characterization techniques. GO QDs were synthesized by direct glucose (BDH Prolabo Chemicals) pyrolysis and dodecylbenzene sulfonic acid (El-Gomhoria Chemical Company, Egypt) doped PANI was synthesized by chemical oxidative polymerization method, respectively. An amount of glucose (2 g) was put in a beaker and heated to 250 °C for 20 min and the orange liquid was added drop by drop to 100 mL of 12.5% NH 3 solution with stirring. This solution was heated at 70 °C for 3 h. 0.03 mL of aniline monomer (99.0%, Research Lab, India) was added to 10 mL deionized water and mixed with 10 mL of water (0.3 g DBSA and 0.1 g APS) through 1 h. Ag (AMPSA) NPs were prepared by the chemical reduction of silver nitrate (99.8%, PRS Panreac, Spain) using sodium borohydride (99.0%, Merck, Germany) as a reducing agent. 1.2 mL 10 mM sodium borohydride was inserted to 36.8 mL of deionized water in ice bath with stirring. Then, 0.4 mL 10 mM AgNO 3 was mixed and added dropwise and 0.3 mL 10 mM AMPSA (97.0%, Acros Organics, Germany) was added dropwise to the mixture with stirring for 10 min. PANI/Ag (AMPSA) NC was prepared by in situ oxidative polymerization of aniline in with Ag (AMPSA) NPs. Aniline (0.03 mL) was dissolved in 10 mL previously prepared Ag (AMPSA) NPs 47 .
In addition, PANI/Ag (AMPSA)/GO QDs NC was prepared by in situ oxidative polymerization of aniline in presence of the nanoparticles. PANI/Ag (AMPSA)/GO QDs NC was prepared with the same procedure PANI/ Ag (AMPSA) NC was prepared as above. The ternary NC was prepared by mixing 10 mL of AMPSA capped Ag NPs and 1 mL of the previously prepared GO QDs solution under magnetic stirring for 10 min 47 .
All details regarding the synthesis and characterization of PANI/Ag (AMPSA)/ GO QDs NC and its related materials in the present study can be found in our recent publications 32,47 . From our previous work 47 , it was noted that PANI/Ag (AMPSA)/GO QDs NC has the highest and sharpest PL intensity compared with each component of the NC of PANI, GO QDs and Ag (AMPSA) NPs. Therefore, PANI/Ag (AMPSA)/GO QDs NC was selected for Cr(VI) detection. PANI/Ag (AMPSA)/GO QDs NC was used as a sensitive fluorescence quenching probe for detecting Cr(VI). The explanation of the quenching mechanism of this NC is based on the synergistic effect of inner filter effect, the ground state compounds formation and ions exchange 6 .
Test organisms. The freshwater Ostracod was obtained from a local ornamental fish shop and was identified as Cypridopsis vidua. The identification of species was determined by the National Institute for Oceanography and Fisheries, Alexandria, Egypt. The Ostracods were reared in-house in 500 mL glass Jar at 28 ± 2 °C and fed with yeast powder and algae-containing water under the natural sunlight. The moderately hard synthetic freshwater (96 mg/L NaHCO 3 (99%, Acros Organics, Germany), 60 mg/L Ca(NO 3 ) 2 . 4H 2 O (99%, PRS Panreac, Spain), 123 mg/L MgSO 4 ·7H 2 O (99.0%, Fisher Scientific, UK) and 4 mg/L KNO 3 (98%, Fisher Scientific, UK)) was used as a test medium 48 . Adults of Ostracod were isolated in 50 mL beakers and allowed to produce offspring and the young Ostracods (neonates) produced were used in the experiments.
Commercially available dehydrated cysts of Artemia salina (origin: salt lake, U.S.) were obtained from Sera, Germany. Artemia life cycle begins as cysts, then emerged embryos, nauplii, finally larvae and adults. The hatching procedure followed the ARC-Test method 49 . The artificial seawater of 35 g/L was used for the hatching as well as a testing solution 50 .
Acute toxicity tests. Toxicity evaluation of PANI/Ag (AMPSA)/GO QDs NC was undertaken by determining the EC 50 for both of Artemia salina and freshwater Ostracod (Cypridopsis vidua) and using an immobilization process as an acute endpoint. Forty-eight hour acute toxicity test on Artemia salina was performed according to ISO/TS 20787 standard operating procedure with a slightly modification 51 53 reported that in the first Artemia larval stage (instar I), the digestive tract of the nauplius is not in contact yet with the external medium, and the larva only consumes its yolk. In the second larval stage (instar II), it starts the feeding on particulate matter. And, instar I larvae are significantly more resistant to chromic acid than instar II. So, they recommended that bioassays with Artemia larvae should be carried out as short-term toxicity tests with instar ӀӀ-ӀӀӀ stages 53 . Groups of 10 Artemia (instar ӀӀ-ӀӀӀ) stages and Ostracods larvae were placed in wells (flat bottom with lid, presterilized, Costar, US) contained 10 mL of the nanocomposite suspension. During the exposure, the tested larvae were not fed and kept under room temperature (28 °C). After 24 and 48 h, the numbers of immobilized larvae (completely motionless) were counted under a binocular microscope (PZ0, Poland) and the immobilization of each treatment was calculated. Immobilization percentage was calculated according to the following equation 54 : Negative controls exposure without PANI/Ag (AMPSA)/GO QDs NC were performed in parallel for both the test organisms and in the positive controls, solution of 50 mg/L potassium dichromate (99.0%, Merck, Germany) was used according to the ISO/TS 20787 method 51 . Ostracods culture and Artemia hatching media containing the same number of organisms (ten organisms) were used. Results were recorded and ensured that the percentage of immobilization in negative control did not exceed 10%. Acute toxicity test was conducted in triplicates and EC 50

Results and discussion
Acute toxicity of PANI/Ag (AMPSA)/GO QDs NC on the Ostracods. Table 1 shows the Ostracods mortality with different concentrations of the nanocomposite and the corresponding EC 50 . In addition, the toxicity unit (TU) values obtained after 24 and 48 h of exposure are depicted in Table 2. TU is equal to 100/EC 50 , which is the reciprocal of the concentration that causes 50% of the organisms to immobilize by the end of the acute exposure period. Results obtained from Table 1 indicate that the exposure to 10 mg/L of PANI/Ag (AMPSA)/GO QDs has not any toxic effects on the Ostracods. It is noted that further increasing of the NC concentration raises the immobilization. After 24 and 48 h of exposure, 100% immobilization is observed at concentration higher than 450 mg/L NC. Ag NPs and GO QDs dissolution and their aggregation are assumed to control the toxicity. The release of dissolved Ag + ions and GO from the NC surface to the media can be responsible for the toxicity 55 .
The results in Table 2

Immobilization values are the mean values of three replicates ± standard deviation. Ag NPs
have a toxic effect on aquatic organisms and this toxicity is highly depending on the particle size and different surface coatings. Several publications reported that polymer-coated Ag NPs significantly decreased the toxicity of the bare or citrate-coated Ag NPs. Moreover, they found that surface coating was the major factor that determines the toxicity compared with particle size [58][59][60] . On the other hand, it was found that there is no acute toxicity for the graphene derivatives demonstrated on the crustaceans although the optical microscope images showed the presence of these graphene derivatives aggregated in the gut 19,61 .
The main acute toxicity mechanisms governed the toxicity of the nanoparticles in aquatic organisms are the ion regulatory disturbance and competitive inhibition of K or Na ion-dependent adenosine triphosphate (Na + , K + -ATPase) activity [62][63][64] . For aquatic organisms in general, including invertebrates, there are very few studies which have addressed this issue. Meanwhile, they reported that after penetrating these NPs into the living cells, it produced reactive oxygen species and induced toxic effects such as membrane lipid peroxidation, mitochondrial damage, DNA damage, and consequently cell apoptosis 65 .
The low toxicity of PANI/Ag (AMPSA)/GO QDs NC is attributed to the presence of the nontoxic and good environmental stable PANI coating layer of the nanocomposite which decreases the dissolution of the Ag NPs and the release of ions 33,34 . Moreover, The AMPSA capping agent protects and preserves the Ag NPs from the dissolution.
The results regarding exposure of Artemia salina to different concentrations of PANI/Ag (AMPSA)/GO QDs are expressed in Table 3. A concentration of 250 mg/L of PANI/Ag (AMPSA)/GO QDs has a negligible acute toxic effect (0% immobilization) for Artemia salina after 24 h exposure and 20% immobilization is observed after 48 h. The concentration resulting in 100% immobilization of Artemia salina after 48 h is 1000 mg/L. The mean values of EC 50 and TU for PANI/Ag (AMPSA)/GO QDs after 48 h are 476 mg/L and 0.21, respectively as shown in Table 4.
It is found that EC 50 values of the nanocomposite for the Ostracods are lower than that for Artemia salina. This is may be due to the high aggregation of PANI/Ag (AMPSA)/GO QDs NC in saltwater to form micro-scale particles. The size distribution of the aggregated nanoparticles is usually not unimodal and the aggregated size increases as the pH approaches the point of zero charge 66      www.nature.com/scientificreports/ It is reported that nanoparticles suspended in the seawater tend to aggregate in the range from 400 nm up to several microns in diameter 44,67 and the Artemia salina larva are able to ingest them. Artemia salina are nonselective filter feeders, and they can readily ingest particles of up to 50 μm in diameter 68 . Several studies have reported that the uptake of nanoparticles by Artemia larvae is influenced by the NPs concentration and the time of the exposure while the size of the NPs was not a major factor 44,67 . Toxicity of the Cr(VI) treated water. In our recent publication 32 , PANI/Ag (AMPSA)/GO QDs NC was applied for the water purification of two water samples containing 60 mg L −1 Cr(VI) ions. The results demonstrated that more than 98% of the Cr(VI) ions were removed from the water samples. It was also found that presence of 60 mg L −1 of Cr(VI) with multiions did not significantly affect the removal % by PANI/Ag (AMPSA)/ GO QDs NC. The toxicity expressed in immobilization % of Cr(VI) solutions with different concentrations of 10, 30, 60 mg/L before and after treatment using PANI/Ag (AMPSA)/GO QDs NC on the Ostracods and Artemia salina larvae after 24 and 48 h is evaluated as shown in Fig. 1a,b. It is evident that Cr(VI) at these concentrations are completely toxic to both Ostracods and Artemia. However, using PANI/Ag (AMPSA)/GO QDs NC as Cr(VI) adsorbent plainly caused a significant reduction in the immobilization for the tested concentrations. The NC has reduced the immobilization by 100% for Cr(VI) of 10 mg/L, and 90 and 83% for Cr(VI) of 30 and 60 mg/L, respectively for the Ostracods after 48 h exposure as shown in Fig. 1a. For Artemia, up to 90% reduction in the immobilization % is observed in 48 h exposure for 60 mg/L Cr(VI) solution after treatment with 1 g/L PANI/Ag (AMPSA)/GO QDs (Fig. 1b). These immobilization reductions are due to the removal of toxic Cr  www.nature.com/scientificreports/ and external deposition. Figure 2 shows changes suffered by Ostracods exposed to a solution of 150 mg/L PANI/ Ag (AMPSA)/GO QDs NC for 24 h. As can be seen in comparison to the control (Fig. 2a), the dark coloration observed in the Ostracod indicates that these organisms ingested the solution of PANI/Ag (AMPSA)/GO QDs NC. It is noted that there are PANI/Ag (AMPSA)/GO QDs agglomerates impregnated in the carapace, antennules and other parts of the body (Fig. 2b). The shells of Ostracods are composed of low-magnesium calcite, and in some groups are semi-transparent so that the internal parts can be seen through the carapace 69 . Similar results were reported by using Heterocypris incongruens Ostracods as test organism for hazard evaluation of polystyrene nanoplastic and GO, respectively 70,71 . Figure 2c displays the dead Ostracods that contains PANI/Ag (AMPSA)/ GO QDs aggregates. Dead crustaceans are usually colorless, have their carapaces fully opened, and sometimes they are ripped apart and have their guts spilled over 72 . Ingestion of 150 mg/L PANI/Ag (AMPSA)/GO QDs after 24 h by Artemia salina is visually verified also under optical microscope as shown in Fig. 3. The gut is empty in the control (Fig. 3a), after exposure to PANI/ Ag (AMPSA)/GO QDs, Artemia salina larvae ingest the nanocomposite and the gut is almost entirely filled as manifested by a dark line inside the gut (Fig. 3b). Finally, the accumulated PANI/Ag (AMPSA)/GO QDs NC is excreted by Artemia salina (Fig. 3c). The accumulation of NPs inside the gut of Artemia salina has already been reported for the ecotoxicity of other NPs 19,44 . www.nature.com/scientificreports/

Conclusions
The acute toxicity effect of PANI/Ag (AMPSA)/GO QDs NC on the aquatic environment was studied using two organisms, freshwater Ostracods (Cypridopsis vidua) and saltwater Artemia salina larvae for 48 h. The mean values of EC 50 of PANI/Ag (AMPSA)/GO QDs NC powder after 48 h of exposure to the Ostracods and Artemia salina were 157.6 ± 6.4 and 476 ± 25.1 mg/L, respectively. PANI/Ag (AMPSA)/GO QDs NC was found to be not acutely toxic for both organisms (EC 50 ˃100), although the nanocomposite was accumulated inside the organisms. The Ostracods were appeared to be more sensitive towards PANI/Ag (AMPSA)/GO QDs than Artemia salina. It is recommended to fabricate this NC as a filter and exposed the polluted water for this filter to control the contamination of the NC and the adsorbed pollutants.

Data availability
All data generated or analysed during this study are included in this published article.