Diatom Nanotechnology

Progress and Emerging Applications

Dusan Losic, Dusan Losic

• Publisher: Royal Society of Chemistry
• Year: 2017
• ISBN:
  9781788012485
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Nanotechnology & MEMS
• Index: Technology & Engineering

Diatoms are single cell algae composed of silica. They represent one of the most outstanding natural materials with exceptional structural, mechanical, optical, photonic and chemical properties optimized through millions years of evolution. The unique nano and micro silica structures of the material combined with its availability as a low cost mineral from diatomaceous earth are attractive for solving many of today's environmental, energy and health problems.

Diatom Nanotechnology provides a comprehensive overview of the material and its uses. The first part of the book looks at the distinctive porous silica structure of diatoms, the mechanism of their formation and their properties. Individual chapters then explore the broad range of their applications in nanotechnology including nanofabrication, optical biosensors, gas sensors, water purifications, photonics, drug delivery, batteries, solar cells, supercapacitors, new adsorbents and composite materials.

With contributions from leading international experts, the book represents an important resource for academics, researchers, industry professionals, postgraduate and advanced level undergraduate students providing them with the latest developments on this emerging and dynamic field.

CHAPTER 1

Whence Is the Diversity of Diatom Frustules Derived?

JAMES G. MITCHELL

School of Biological Sciences, Flinders University, South Australia, Australia

*Email: [email protected]

1.1 Introduction

The silica frustules of diatoms have fascinated scientists since van Leeuwenhoek’s first reports of their structure in the early 18th century.¹ As microscopy and microbiology have advanced, other protists, along with sponges, have been found to produce silica-based structures, but none with as minute and intricate detail as diatoms. The outer silica diatom frustule became, and largely remains today, the basis for the group’s taxonomy, although this is beginning to be supplemented and supplanted by molecular studies.^2–5 During the last 300 years, little attention has been paid to the function of the frustule detail. Work shows that the frustule has protective and ballasting functions,^6,7 but these do not address the function of the intricate detail. There is recent work showing that frustules have good mechanical strength,^8,9 photonic properties,¹⁰ and hypothetical buffering capacity,¹¹ but again these are gross properties that would exist without resorting to the need for specific detail. There are specialized or rare structures, such as spines and excretion sites, but these are a tiny fraction of the frustule structure. Some work shows that the most surficial frustule details modify particle movement over the frustule surface and diffusion through the frustule.^12–17 However, none of this work explains why diatoms have a variety of minute and intricate structures found among their 100 000+ species.

The purpose of this review is to examine work directly and indirectly related to the frustule structure to show what we know about the function of the frustule to date. As such, this is meant to be a jumping-off place for those readers interested in understanding how the frustule interacts with the surrounding chemical environment, and the way that this interaction is influenced by fluid flow and by the cellular processes within the diatom.

1.2 The Frustule in Context

Diatoms have rigid, silicon-based exteriors that are similar to many micro/nanofluidic devices. The surface of the former always possesses distinct surface patterns. Figure 1.1 shows the rigid exterior (frustule) for a diatom, specifically the species Thalassiosira eccentrica. This basic structure appears repeatedly in diatoms and may explain their success in a variety of environments. Diatoms form the base of the marine food web and are among the most abundant phototrophs on Earth.^18,19 Their physiology and nutrient uptake capacities are moderately well studied,^20–22 but it is uniformly overlooked that the membranes are recessed below the frustule, essentially layers of what are effectively rigid, but porous patterned grids. The behaviour of particles near or in this grid system is virtually unknown, as is the role of the elaborate geometry. Although these groups are the dominant ocean and freshwater phytoplankton, photosynthetic single cells that drift in the ocean, we still lack fundamental information on how they identify and take up nutrients. Many studies have shown that diatoms are essential to phytoplankton ecology, and their role in the microbial loop and colloid dynamics is unparalleled.^23–26 However, here it will be shown that multiple disciplines are on the verge of providing insight into fundamental principles of particle surface interactions and that indeed progress has already been made in a variety of areas.

Figure 1.1 Diagram (A) and SEM (B and C) of a silica diatom frustule from Thalassiosira eccentric with a path of 0.25 µm particles diffusing over the surface. The opening (foramen) of the areolae is about 1 µm across. Preliminary results indicate that spines (white) along the frustule edge are particle ejection points. Adapted with permission from ref. 12. Copyright 2001 American Chemical Society.

1.2.1 The Chemical Milieu

Oceans and lakes are among the most complex of chemical environments on Earth, particularly when variation is considered over days, years and millennia. This includes the complex chemical composition of colloids and particles. Diatoms have had to cope with particles of all sizes at their surfaces for hundreds of millions of years. The basic tenets of natural selection in a particle-laden ocean suggest that the detailed, rigid patterns of their surfaces may help to control submicrometre particle behaviour near their surface. Cellular biologists do not consider this region and, because it is too small to fall within the realm of oceanography, it falls between discipline boundaries; thus, there are relatively few papers to review. However, this region may be crucial for understanding diatom ecology and physiology, since this is the region from which nutrients are drawn for uptake and in which pathogens and fouling bacteria and chemicals attach. It is also a region that is crucial to understanding micro/nanofluidics for microchip analysis methods²⁷ and nanostructure assemblage.^28,29 In fact, diatom frustules can be regarded as a prototype of a natural silica-based 3D microfluidic system.

The appreciation of diatoms as key components of the biosphere continues to increase. They fix 25% of global organic carbon and oxygen, host nitrogen-fixing symbionts, and migrate vertically more than a kilometre to transfer inorganic nitrogen to the ocean surface.^30,31 For this perspective, they fix more carbon than all of the rainforests combined.³¹ They reproduce and are consumed rapidly. The result is that the carbon they fix is rapidly passed through the food web compared to trees, grasses and seaweed, making them the primary biomass source of many marine, river, lake and some soil ecosystems.^31,32 Their importance far exceeds that of other microalgae. An important reason for their critical role in the biosphere appears to be their ability to precisely use silica to form complex frustules, which over evolutionary time have adapted to changing environments and chemical milieu.

Understanding how diatoms use their nanostructures to compete in ecosystems has become increasingly important as the importance of diatoms in the global food web and in biogeochemical processes has been realized. As stated above, they account for 25% of all primary production on Earth.^33,34 They are the primary cyclers of silica in the ocean.^33,34 Physiologically, they are the only group in which cadmium metalloenzymes have been found, and these are now known to be widespread as an apparent substitute for zinc in beta-class carbonic anhydrase.^35,36 This opens an unexplored area of how any cell can handle cadmium and avoid toxicity. On the opposite side of toxicity, diatoms produce toxins that can cause permanent memory loss in humans.^37,38 They are also models for nanotechnology...

Table of contents

• Cover
• Title
• Copyright
• Preface
• Contents
• Chapter 1 Whence Is the Diversity of Diatom Frustules Derived?
• Chapter 2 Interactions of Diatoms with Their Fluid Environment
• Chapter 3 Nanoengineering of Diatom Surfaces for Emerging Applications
• Chapter 4 Functionalization of Frustules From Diatom Cell Culture for Optoelectronic Properties
• Chapter 5 Micro- and Nano-optical Devices from Diatom Nanostructures: Light Control by Mother Nature
• Chapter 6 Immobilization of Proteins on Diatom Biosilica
• Chapter 7 The Potential of Modified Diatom Frustules for Solar Energy Conversion
• Chapter 8 Diatom Silica as an Emerging Biomaterial for Energy Conversion and Storage
• Chapter 9 Diatoms: A Natural Source of Nanostructured Silica for Drug Delivery
• Chapter 10 Diatomaceous Earth, A Natural Insecticide for Stored Grain Protection: Recent Progress and Perspectives
• Subject Index