Chronic toxicity of core–shell SiC/TiO₂ (nano)-particles to <i>Daphnia magna</i> under environmentally relevant food rations in the presence of humic acid

To date, research on the toxicity and potential environmental impacts of nanomaterials has predominantly focused on relatively simple and single-component materials, whilst more complex nanomaterials are currently entering commercial stages. The current study aimed to assess the long-term and size-dependent (60 and 500 nm) toxicity of a novel core–shell nanostructure consisting of a SiC core and TiO2 shell (SiC/TiO2, 5, 25, and 50 mg L−1) to the common model organism Daphnia magna. These novel core–shell nanostructures can be categorized as advanced materials. Experiments were conducted under environmentally realistic feeding rations and in the presence of a range of concentrations of humic acid (0.5, 2, 5, and 10 mg L−1 TOC). The findings show that although effect concentrations of SiC/TiO2 were several orders of magnitude lower than the current reported environmental concentrations of more abundantly used nanomaterials, humic acid can exacerbate the toxicity of SiC/TiO2 by reducing aggregation and sedimentation rates. The EC50 values (mean ± standard error) based on nominal SiC/TiO2 concentrations for the 60 nm particles were 28.0 ± 11.5 mg L−1 (TOC 0.5 mg L−1), 21.1 ± 3.7 mg L−1 (TOC 2 mg L−1), 18.3 ± 5.4 mg L−1 (TOC 5 mg L−1), and 17.8 ± 2.4 mg L−1 (TOC 10 mg L−1). For the 500 nm particles, the EC50 values were 34.9 ± 16.5 mg L−1 (TOC 0.5 mg L−1), 24.8 ± 5.6 mg L−1 (TOC 2 mg L−1), 28.0 ± 10.0 mg L−1 (TOC 5 mg L−1), and 23.2 ± 4.1 mg L−1 (TOC 10 mg L−1). We argue that fate-driven phenomena are often neglected in effect assessments, whilst environmental factors such as the presence of humic acid may significantly influence the toxicity of nanomaterials.迄今为止，有关纳米材料（nanomaterials）毒性与潜在环境影响的研究大多聚焦于结构相对简单的单组分材料，而结构更为复杂的纳米材料目前已逐步进入商业化应用阶段。本研究旨在评估一种新型核壳纳米结构（core–shell nanostructure）——以碳化硅（SiC）为核、二氧化钛（TiO2）为壳（SiC/TiO2，设置5、25、50 mg L−1三个浓度梯度）——对常见模式生物大型溞（Daphnia magna）的长期毒性及其粒径依赖性（60 nm与500 nm两种粒径）。此类新型核壳纳米结构可归类为先进材料。实验在环境相关的投喂剂量条件下开展，并设置了一系列梯度浓度的腐殖酸（humic acid，以总有机碳（Total Organic Carbon，TOC）计：0.5、2、5、10 mg L−1）作为共存介质。研究结果显示，尽管SiC/TiO2的效应浓度较当前报道的应用更广泛的纳米材料的环境浓度低数个数量级，但腐殖酸可通过降低颗粒聚集与沉降速率，加剧SiC/TiO2的毒性。基于标称SiC/TiO2浓度计算得到的半数效应浓度（EC50，平均值±标准误（standard error））结果如下：对于60 nm粒径颗粒，在总有机碳浓度分别为0.5、2、5、10 mg L−1时，其EC50值依次为28.0±11.5 mg L−1、21.1±3.7 mg L−1、18.3±5.4 mg L−1及17.8±2.4 mg L−1；对于500 nm粒径颗粒，对应条件下的EC50值分别为34.9±16.5 mg L−1、24.8±5.6 mg L−1、28.0±10.0 mg L−1及23.2±4.1 mg L−1。本研究认为，在纳米材料的效应评估中，由归趋驱动的过程常被忽视，而腐殖酸这类环境共存因子可能会显著影响纳米材料的毒性。