Environmental Science: Nano Blog

A phospholipid component of plants can potentially provide versatile, environmentally benign ligands in the synthesis of asymmetric gold nanoparticles (GNPs). This paper by Benjamin Ayres (Portland State University) and Scott Reed (University of Colorado Denver), demonstrates how this can provide benefits in terms of both possible application and environmental performance of these materials.

Asymmetric GNPs have been utilised for a wide range of applications including biomedical sensors and components of electronic and photonic devices. In order to move towards a large scale production of asymmetric GNPs, a synthetic method is required that is economical and reproducible, as well as displaying suitable green credentials.

The shape and size of GNPs are crucial to governing their electronic and optical properties. There is a need therefore to gain a good understanding of the synthetic process at a molecular level, in order to optimise the process from the perspective of both desired application and environmental impact, creating a method that can be easily controlled and reproduced.

Conventional synthetic methods traditionally involve seed-mediated synthesis using alkyl ammonium salts (e.g. CTAB) as ligands. These methods are unsustainable and non-biocompatible due to the toxic nature of the compounds used, and often require excess ligand and post-synthetic processing e.g. using ligand exchange in order to be used for in vivo applications.

The method described by Ayres and Reed involves reduction of bromoauric acid by ascorbic acid in the presence of ligands derived using biogenic phospholipid extracts from crude soybean lecithin. This provides a cheap, readily available and renewable feedstock for the process, avoiding use of fossil-fuel derived materials.

A key parameter of generated GNPs is their localized surface plasmon resonance (LSPR). In order for GNPs to be suitable for medical applications (e.g. biosensors, in-vivo imaging and phototherapeutic treatments) the LSPR needs to be in the near infrared (NIR) region.

It was shown, using UV-Vis spectroscopy and high resolution transmission electron microscopy (TEM) that the described method produces a mixture of spherical and triangular prismatic GNPs that exhibited an LSPR in the NIR region as well as extended stability with no aggregation.

Furthermore, the method includes chemical identification of the specific molecular component of soy lecithin that is the source of asymmetric growth. Soy lecithin is composed predominantly (~75%) of phosphatidylchorine (PC). However, separation of different GNP shapes using preparatory gel electrophoresis, and analysis of lipid extracts by LC-MS indicated phosphatidic acid (PA) was crucial to GNP asymmetric growth.

Indeed, it was shown that GNPs produced from lower purity PC (30%) showed NIR LPSR and were stable, while higher purity PC (95%) produced only spherical GNPs which aggregated over time and displayed LSPR only in the UV-Vis spectrum. When spiked with PA, the growth solutions of PC₉₅ displayed a second LSPR in the NIR, which shifted further red as more PA was added.

This study presents a biocompatible method for GNP production using environmentally benign ligands. It also demonstrates how gaining molecular level understanding of the synthetic process allows control over the GNP shape and size, based on lipid composition and/or specific plant material used. This therefore provides benefits both from the perspective of the desired application and the environmental impacts.

Environmental Science: Nano is providing free access upon registration* to all content published during 2014 and 2015. To get your free download of this paper, please follow the link below:

A minor lipid component of soy lecithin causes growth of triangular prismatic gold nanoparticles, Benjamin. R. Ayres and Scott. M. Reed, DOI: 10.1039/C3EN00015J

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Scientists at Clarkson University, USA, have investigated the effect of nanoparticles (NPs) on intestinal development of embryonic zebrafish (Danio rerio). They saw that levels of serotonin, a developmental marker, change according to exposure time and concentration.

NP use in commercial goods and food is ever increasing. Previously, studies have shown that NPs can alter skeletal development¹ and tissue formation,² but there is a need to find out more about what NPs might be doing to our bodies. The gastrointestinal (GI) tract has been identified as a hanging out point for ingested NPs that get trapped within our numerous intestinal folds; Özel and colleagues built on this observation and tested nickel (Ni) and copper oxide (CuO) NP exposure in the intestines of zebrafish embryos.

A zebrafish embryo

In their newly published paper³, Özel et al measured serotonin (5HT) levels to investigate intestinal development. 5HT, which is secreted by cells in the GI tract, is known to act as a growth factor during embryogenesis; it is particularly involved in brain development, with disruption of serotonin-related cellular processes leading to CNS (central nervous system) disorders.⁴ Intestinal levels of 5HT were measured as an indication of developmental physiology. 5HT concentration was determined via carbon filter microelectrodes, inserted directly into the embryo, and differential pulse voltammetry.

Immunohistochemistry (using rabbit anti-serotonin antibody) of 50ppm NP-exposed zebrafish embryos showed a decrease in fluorescence intensity of 5HT in Ni-exposed embryos, compared to the control. In contrast, CuO exposure was associated with an increase in 5HT fluorescence intensity (Fig 1).

Fig 1. Zebrafish embryos showing 5HT levels after NP exposure using anti-5HT immunohistochemistry

These findings were supported by in vivo electrochemical measurements, which showed significantly decreased 5HT for concentrations of 20ppm and higher Ni, and a general increase in 5HT for CuO exposure (Fig 2).

Özel and colleagues suggest that the different effects seen between Ni and CuO NP exposure demonstrate the complexity of the toxicity induced by NPs. As well as measuring 5HT levels, the team also showed that rates of embryo hatching from the chorion (an acellular membrane surrounding fish eggs) were generally decreased by increasing NP concentrations. At 50 to 100ppm CuO, more than 98% of embryos failed to hatch, whilst Ni produced a 70% failure rate.

Fig 2. Changes of in vivo intestinal 5HT concentrations after NP exposure

The findings of this paper indicate that environmental exposure to NPs may affect the physiology of developing organs, as well as the triggering of developmentally-timed events, such as hatching. In future, there is a need to extend these studies in order to fully understand how NPs in the environment may affect development, from fish to humans and everything in between. We are all exposed.

This paper is one of the first to be published in Environmental Science: Nano. We must say a big thank you to the authors, and the Nano Editorial Team, who worked hard to publish it. Environmental Science: Nano is providing free access upon registration* to all content published during 2014 and 2015. To get your free download of this paper, follow the links below:

R. E. Özel, K. N. Wallace and S. Andreescu, Alterations of intestinal serotonin following nanoparticle exposure in embryonic zebrafish, DOI: 10.1039/C3EN00001J

References:

1. C. Ispas et al, Environmental Science & Technology, 2009, 43, 6349-6356
2. D. Xiong et al, Science of The Total Environment, 2011, 409, 1444-1452
3. R. E Özel et al, Environmental Science Nano, 2014, DOI: 10.1039/C3EN00001J
4. M.S. Sodhi & E. Sanders-Bush, Int. Rev. Neurobiol., 2004, 59, 111-174

*Free access to individuals is provided through an RSC Publishing personal account. It’s quick, simple and more importantly free to register!