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Smaller silver nanoparticles more likely to be absorbed by aquatic life, UCLA study finds

Researchers studied zebrafish because they have some genetic similarities to humans and their embryos and larvae are transparent, which makes them easier to observe (credit: Tunde Akinloye/CNSI)

A study led by UCLA scientists has found that smaller silver nanoparticles entered fish’s bodies more deeply and persisted longer than larger silver nanoparticles or fluid silver nitrate.

More than 2,000 consumer products today contain nanoparticles — particles so small that they are measured in billionths of a meter. Manufacturers use nanoparticles to help sunscreen work better against the sun’s rays and to make athletic apparel better at wicking moisture away from the body, among many other purposes.

Of those products, 462 contain nanoparticles made from silver, which are used for their ability to kill bacteria. But that benefit might be coming at a cost to the environment. In many cases, simply using the products as-intended causes silver nanoparticles to wind up in rivers and other bodies of water, where they can be ingested by fish and interact with other marine life.

The new study by the University of California Center for Environmental Implications of Nanotechnology, published online in the journal ACS Nano, was intend to begin addressing the question: to what extent do organisms retain those particles and what effects might they have?

Absorption of silver nanoparticles by fish

According to Andre Nel, director of UCLA’s Center for Environmental Implications of Nanotechnology (CEIN) and associate director of the California NanoSystems Institute at UCLA, it is not yet known whether silver nanoparticles are harmful, but the research team wanted to first identify whether they were even being absorbed by fish.

Deposits of 20-nanometer silver nanoparticles in zebrafish gill filaments (outlined in red) (credit: Olivia J. Osborne et al./ACS Nano)

In the study, researchers placed zebrafish in water that contained fluid silver nitrate and two sizes of silver nanoparticles — some measuring 20 nanometers in diameter and others 110 nanometers. The researchers found that the two sizes of particles affected the fish very differently.

The researchers used zebrafish in the study because they have some genetic similarities to humans, and their embryos and larvae are transparent (which makes them easier to observe). In addition, they tend to absorb chemicals and other substances from water.

The team focused its research on the fish’s gills and intestines because they are the organs most susceptible to silver exposure.

The gills showed a significantly higher silver content for the 20-nanometer than the 110-nanometer particles, while the values were more similar in the intestines; both sizes of the silver particles were retained in the intestines even after the fish spent seven days in clean water.

The experiment was one of the most comprehensive in vivo studies to date on silver nanoparticles, as well as the first to compare silver nanoparticle toxicity by extent of organ penetration and duration with different-sized particles, and the first to demonstrate a mechanism for the differences.

Osborne said the results seem to indicate that smaller particles penetrated deeper into the fishes’ organs and stayed there longer because they dissolve faster than the larger particles and are more readily absorbed by the fish.

Nel said the team’s next step is to determine whether silver particles are potentially harmful. “Our research will continue in earnest to determine what the long-term effects of this exposure can be,” he said.

The research was supported by the National Science Foundation and the Environmental Protection Agency.

Abstract of Organ-Specific and Size-Dependent Ag Nanoparticle Toxicity in Gills and Intestines of Adult Zebrafish

We studied adult zebrafish to determine whether the size of 20 and 110 nm citrate-coated silver nanoparticles (AgC NPs) differentially impact the gills and intestines, known target organs for Ag toxicity in fish. Following exposure for 4 h, 4 days, or 4 days plus a 7 day depuration period, we obtained different toxicokinetic profiles for different particle sizes, as determined by Ag content of the tissues. Ionic AgNO3 served as a positive control. The gills showed a significantly higher Ag content for the 20 nm particles at 4 h and 4 days than the 110 nm particles, while the values were more similar in the intestines. Both particle types were retained in the intestines even after depuration. These toxicokinetics were accompanied by striking size-dependent differences in the ultrastructural features and histopathology in the target organs in response to the particulates. Ag staining of the gills and intestines confirmed prominent Ag deposition in the basolateral membranes for the 20 nm but not for the 110 nm particles. Furthermore, it was possible to link the site of tissue deposition to disruption of the Na+/K+ ion channel, which is also localized to the basolateral membrane. This was confirmed by a reduction in ATPase activity and immunohistochemical detection of the α subunit of this channel in both target organs, with the 20 nm particles causing significantly higher inhibition and disruption than the larger size particles or AgNO3. These results demonstrate the importance of particle size in determining the hazardous impact of AgNPs in the gills and intestines of adult zebrafish.

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Detecting infectious and autoimmune antibodies with a DNA nanomachine

A nanoscale DNA “machine,” shown in this illustration bound to an antibody (yellow), rapidly lights up when it recognizes specific target antibodies (credit: Marco Tripodi)

An international team of scientists has developed a nanomachine using synthetic DNA for rapid, sensitive, low-cost diagnosis of infectious and auto-immune diseases, including HIV, at the point of care. It aims to replace the current slow, cumbersome, and expensive current process of detecting the protein antibodies used for diagnosis.

An antibody causes a structural change (or switch) in the device, which generates a light signal. The sensor does not need to be chemically activated and is rapid — acting within five minutes — enabling the targeted antibodies to be easily detected, even in complex clinical samples such as blood serum.

The antibody-targeting sensor is composed of a light-emitting fluorophore (F) and quencher (green circle) connected to two single-stranded DNA tails joined to the appropriate recognition element (red hexagons) for a given test. When a target antibody is detected by the two recognition elements, they open the stem, activating the fluorophore. (credit: S. Ranallo et al./Angew. Chem. Int. Ed.)

The research is described in the October issue of the journal Angewandte Chemie.

“One of the advantages of our approach is that it is highly versatile,” said Prof. Francesco Ricci, of the University of Rome, Tor Vergata, senior co-author of the study. “This DNA nanomachine can be in fact custom-modified so that it can detect a huge range of antibodies; this makes our platform adaptable for many different diseases.”

“Our modular platform provides significant advantages over existing methods for the detection of antibodies,” added Prof. Vallée-Bélisle of the University of Montreal, the other senior co-author of the paper. “It is rapid, does not require reagent chemicals, and may prove to be useful in a range of different applications such as point-of-care diagnostics and bioimaging.”

The researchers plan to allow the light-emitting signal to be detected by a mobile phone.

A University of California, Santa Barbara scientist was also involved in the research.

Abstract of A Modular, DNA-Based Beacon for Single-Step Fluorescence Detection of Antibodies and Other Proteins

A versatile platform for the one-step fluorescence detection of both monovalent and multivalent proteins has been developed. This system is based on a conformation-switching stem–loop DNA scaffold that presents a small-molecule, polypeptide, or nucleic-acid recognition element on each of its two stem strands. The steric strain associated with the binding of one (multivalent) or two (monovalent) target molecules to these elements opens the stem, enhancing the emission of an attached fluorophore/quencher pair. The sensors respond rapidly (<10 min) and selectively, enabling the facile detection of specific proteins even in complex samples, such as blood serum. The versatility of the platform was demonstrated by detecting five bivalent proteins (four antibodies and the chemokine platelet-derived growth factor) and two monovalent proteins (a Fab fragment and the transcription factor TBP) with low nanomolar detection limits and no detectable cross-reactivity.