Toxicity Evaluation of Boron- and Phosphorus-Doped Silicon Nanocrystals toward Shewanella oneidensis MR‑1 Bo Zhi, † Sadhana Mishra, ‡ Natalie V. Hudson-Smith, † Uwe R. Kortshagen, ‡ and Christy L. Haynes* ,† † Department of Chemistry and ‡ Department of Mechanical Engineering, University of Minnesota Twin Cities, Minneapolis, Minnesota 55455, United States * S Supporting Information ABSTRACT: Silicon nanocrystals, also known as silicon quantum dots, are regarded as green alternatives to traditional quantum dots composed of heavy metal elements. While it is well-known that the semiconductor properties of these materials can be tuned by doping with p/n-type dopants (i.e., boron and phosphorus), there is a lack of systematic understanding of their potential environmental impact if released into the ecosystem. Here, we demonstrate that introduction of dopants, especially phosphorus, cause doped silicon nanocrystals to produce reactive oxygen species, resulting in significant toxicity to a model microorganism, Shewanella oneidensis MR-1. In addition, the interaction between bacteria cells and silicon nanocrystals was investigated using dark field microscopy and bio-TEM. Interestingly, boron-doped silicon nanocrystals tended to attach to the cell surface while this phenomenon was not observed for undoped or phosphorus-doped silicon nanocrystals. KEYWORDS: silicon nanocrystals, Shewanella oneidensis MR-1, nanotoxicity, boron and phosphorus doping, reactive oxygen species ■ INTRODUCTION First discovered in the 1980s, semiconductor nanocrystal quantum dots (QDs) exhibit unique tunable luminescent properties due to quantum confinement effects. 1,2 Other features, such as a broad absorption range, high molar extinction coefficients, high quantum yield, and satisfying photostability, render colloidal QDs competitive alternatives to organic dyes in applications ranging from electronic displays to biomedical research. 3-8 However, the rapid growth of consumer products that make use of QDs, especially group II-VI QDs like CdSe- or PbS-based QDs, has aroused safety concerns regarding their environmental impact as well as their adverse effects on human health due to the release of potentially toxic degradation products such as heavy metal ions (e.g., Cd 2+ and Pb 2+ ). 9, 10 To achieve maximal technological impact, QD synthesis should use green and cost-effective syntheses; however, QD precursors, such as cadmium acetylacetonate and bis(trimethylsilyl) sulfide, are known to be toxic or/and expensive, limiting their mass production and, as such, broad use in our daily life. 11,12 In this context, it is desirable to find new materials that exhibit comparable luminescent performance but are suitable for large- scale synthesis in a sustainable manner. In recent decades, group IV element (carbon, silicon, and germanium) based fluorescent materials have drawn great attention as they are conventionally considered nontoxic elements with minimal environmental impact. 13-18 Among this family, luminescent Si nanocrystals (Si NCs), also known as Si QDs, are emerging as novel and eco-friendly semi- conductor nanomaterials due to their promising optoelectronic properties, including size-tunable emission and acceptable quantum yield, comparable to their metal based counterparts (e.g., CdSe, InAs, and PbS) that are regarded as potentially hazardous. 19-26 Especially in recent years, the quantum yield of Si NCs has been greatly improved, and in some cases, it can be as high as 90%. 23,27,28 There are a number of methods to prepare Si NCs, including thermal annealing, 29 electrochemical etching, 30 laser ablation, 31 solution-phase reduction, 32 and thermolysis of silane. 33 Compared to products generated by other methods, Si NCs prepared by nonthermal plasma methods are characterized by being solvent/ligand-free, displaying narrow particle size distribution, and having efficient inclusion of dopants into the NC structure. 34,35 By fine-tuning the p/n-type doping level, the free carrier concentration of Si NCs is controllable, facilitating adjustment of their localized surface plasmon resonance (LSPR) performance, realizing a wide range of applications, including solar cells, 36 electronic devices, 37,38 and light-emitting devices. 39,40 Meanwhile, anoth- er consequence of p/n-type doping is the drop/rise of the Fermi level of doped Si NCs by introduction of free holes Received: June 22, 2018 Accepted: August 20, 2018 Published: August 20, 2018 Article www.acsanm.org Cite This: ACS Appl. Nano Mater. 2018, 1, 4884-4893 © 2018 American Chemical Society 4884 DOI: 10.1021/acsanm.8b01053 ACS Appl. Nano Mater. 2018, 1, 4884-4893 Downloaded via UNIV OF MINNESOTA on September 28, 2018 at 05:17:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.