Searching for 'Cryptoelectrons': Redox Chemistry of Insulating Materials

A redox process involves reduction - gain of electrons, and oxidation - loss of electrons. To be redox active a material must have electrons it can lose or energy levels that can be occupied by electrons. An insulating material does not conduct electricity as all of its electrons are occupied in bonding and so has been considered to be not redox active. However, in this project we are aiming to study the redox activity of such materials by exploiting what happens when make the material very small - i.e. as it becomes a nanoparticle.
To predict how a material will behave chemically we consider the reactivity of its constituent atoms. When a material has large dimensions, most of its atoms are within the bulk of the material with only a small proportion found at its surface. Thus the properties are dictated by the majority bulk atoms and the behaviour of the surface atoms can be ignored. In contrast, when the dimension of the material is small - at the nanoscale - there may be as many atoms at the surface as in the bulk, and surface chemistry can no longer be overlooked.
A good example is diamond. We have shown that diamond nanoparticles ('nanodiamond') undergo redox processes. However, diamond is well-known as an insulating material, as most of its electrons are occupied in bonding; so how can nanodiamond be redox active? The reason is the large surface area that allows surface properties to dominate. Surface atoms have fewer neighbours so don't use all their electrons in bonding - these are available for redox processes, as we observe. We cannot therefore assume a material at the nanoscale will retain its bulk chemical and electronic properties and this may impact on how we use the material.
Some of the inspiration for this project came from recent studies of causes of electrostatic charging, which as well as being of fundamental interest has wide application. It is a well-known phenomenon that when two different materials are rubbed together they become charged with static electricity. Although common, there is little agreement on how this charging occurs. Recently it has been shown that the charging may have a redox origin. This is initially quite surprising as the material involved (e.g. plastics) are usually insulators and hence do not readily lose or accumulate electrons. It was suggested that in some way these materials were able to store electrons in their structure, perhaps associated with the surface or maybe with structural defects. These extra electrons were termed 'cryptoelectrons' and hence the aim of this project is learn more about crytoelectrons and how and where they exist.
The nanomaterials under investigation will be attached onto the surface of an electrode and studied using electrochemical methods. The potential of the electrode will be varied and the resulting current flow recorded. The potential of the electrode represents the energy of electrons within it; if the energy is higher than empty energy levels on the nanomaterial surface, electrons will flow from the electrode to the nanomaterial and a reduction current will be observed. Conversely, if the electrons on the surface of the nanomaterial are of a higher energy than those in the electrode, electrons will flow from the nanomaterial to the electrode and an oxidation current will be obtained. In this way we can map out the redox characteristics of the different nanomaterials.
Combined with electrochemistry we will use spectroscopic techniques to determine the chemical identity of the surface groups responsible for the observed activity. The nanomaterials will be immobilised on top of a prism through which infrared radiation is reflected. The infrared is absorbed by the chemical groups on the surface of the nanomaterials. The energy of the absorbed radiation enables us to identify which groups are present.

One potential impact of this project is increased understanding the mechanisms behind electrostatic charging. Electrostatic discharge (ESD) is common phenomenon that takes place when current spontaneously and suddenly flows between two materials of different electronic properties. ESD is of huge cost to the semiconductor electronics industry in terms of device damage and the problem is predicted to get worse with increasing miniaturisation of circuitry. ESD is also of concern in the chemical, fuel and mining industries through ignition of flammable materials, so understanding and controlling ESD is required to prevent economic loss and harm to workers and public health. Understanding at a molecular level how charge is accumulated on insulating materials will increase fundamental understanding of this phenomenon and enable the future design of bespoke materials that accumulate less charge, reducing ESD events and the resulting economic loss. The techniques developed in this proposal may also lead to low cost and simple methods for testing the capacity for charge accumulation of different materials, which may be complimentary to the methods and procedures already in place.
Even in the absence of discharge, static electricity is problematic in many industries, such as the paper industry, as products stick together with loss of production time. In addition electrostatic charging attracts contaminants in clean environments, e.g. in clean rooms and operating theatres. This project will allow us to identify the types of surface chemistry that lead to charge accumulation and hence suggest materials best suited for use in these environments e.g. in fabrics for protective clothing.
Surface charging of insulating materials is exploited in applications such as laser printing and photocopying, where toner particles (a polymer nanoparticle coated with a dye material) are electrostatically charged and attracted to paper of the opposite charge. The conclusions from this project will aid in understanding how to design toner nanoparticles with the optimum properties in terms of performance. This is potentially of huge economic benefit considering the size of the printer / copier market and the desire for higher resolution images. The impact on human health of inhalation of such powders must be established, as well as the minimisation of release of these materials into the environment. Providing insight into the surface chemistry of these types of material will lead to easier future assessment of their health risks and environmental sustainability.
This project will also offer insight into the role of (nano)materials in medicine and human health. Many processes within the body are redox in origin. If materials are introduced into the body as implants, drug delivery vehicles or imaging agents and they have associated redox activity they may interfere with processes with the cells. An outcome of this project may be the means to screen materials for surface redox activity as a first step to assessing their biocompatibility. This may impact on government policy on the use of nanomaterials in medicine.
Also in medicine, effectiveness of drug delivery using inhalers is often influenced by the static interaction of the drug particles with inhaler walls and with the lung tissue, leading to doubts about dosage. Understanding the surface redox chemistry of inhaler materials and the drug formulation itself could lead to more effective treatment for conditions such as asthma.
The surface redox chemistry of atmospheric particulates needs to be understood to assess health risks of pollutants and their environmental impacts. Understanding how specific surface chemistry induces redox activity in these insulating particulate materials may lead to new analytical procedures for their detection and identification, which could also be extended to terrorist threats like bio-aerosols.