A colourful problem
The global textile industry is responsible for nearly 20% of industrial water pollution, with the extensive use of dyes releasing an estimated 40,000–50,000 tons of dye waste annually into aquatic systems. Synthetic dyes, such as azo dyes, dominlate the textile industry due to their bright colours, innate stability and low cost.
Azo dyes, as well as posing significant public health risks, have detrimental environmental impacts due to their persistence in ecosystems. This inherent stability stems from azo bonds within the chemical groups of azo dyes. Azo bonds are very strong double bonds between two nitrogen atoms (N=N) with each nitrogen bonded to an organic (carbon-based) group, making them resistant to degradation under normal environmental conditions. Consequently, their removal often requires energy-intensive and technologically advanced treatment methods; a compelling research area for chemical sustainability.
Overcoming a fundamental challenge
Photocatalysis is a way to use light to facilitate chemical reactions. A major challenge in photocatalysis is that photons (the packages of energy that make up light) are quantised. This means that a single event in photocatalysis converts one photon into only one electron which can then be used to drive chemistry. This constraint creates a critical issue of one-electron photochemistry because many useful transformations require several electrons. Azo-bond degradation is one example, as breaking an azo bond requires multiple electrons. To overcome this challenge, researchers sought a biohybrid photocatalyst (with natural and synthetic components) capable of collecting and storing several light-generated electrons before using them for the breakdown of azo dyes.
Current catalysis
Conventionally, a purely synthetic photocatalyst known as Ru(II)(bipyridine)3-TiO2 has been used in the degradation of azo dyes to perform a process called light-driven decolourisation. Catalysts are essential compounds in chemical industry that increase the speed of chemical reactions without being consumed. Photocatalysts, such as Ru(II)(bipyridine)3-TiO2, required light activation to perform catalysis. Here, light excites the ruthenium (Ru) metal complex, which subsequently injects electrons into TiO2 to drive chemical reactions.
The titanium dioxide (TiO2) element of the complex is a widely used synthetic photocatalyst due to TiO2's stability and non-toxicity. Despite its widespread use, TiO2 poses potential ecotoxicological risks and significant environmental challenges stemming from its energy intensive production and limited end-of-life options.
In the present study, scientists have developed a prototype biohybrid photocatalyst to overcome the sustainability obstacles presented by conventional TiO2 photocatalysts through a nature-inspired approach.
A best-of-both-worlds approach
Biohybrid photocatalysts represent a novel avenue for improving the efficiency of photocatalysis by combining artificial light-absorbing materials with biological proteins. In this study, scientists developed a specific type of biohybrid catalyst, substituting the synthetic TiO2 for a biological protein called a cytochrome (specifically MtrC extracted from Shewanella oneidensis bacteria) to create a Ru(II) (bipyridine)3-MtrC biohybrid assembly.
Cytochrome proteins play a vital role in the storage and transfer of electrons between cells, and promotion of biological reactions. Cytochromes contain several heme units that facilitate this electron transfer. Similar heme units are present in the hemoglobin protein in our red blood cells, which carry oxygen molecules around the body.
Nature's solar-powered reactor
This biohybrid photocatalyst can be described as a solar-powered reactor where the ruthenium metal complex (Ru(II)(bipyridine)3) acts as a solar antenna capturing sunlight by absorbing photons and converting them into energized electrons. These energised electrons are stored successively within the 10 heme units of the MtrC cytochrome protein. Once sufficient chemical energy has accumulated, it is utilised to drive azo dye degradation via breaking the azo bond. This demonstrates the ability of cytochrome proteins to power multi-electron reactions, artfully overcoming the major limitation of one-electron photochemistry.
An Ultra laser lab at the CLF's Lasers for Science Facility
Catalysing a cleaner future
This work was completed in international collaboration with the researchers from the University of East Anglia, UK; Leiden University, Netherlands; Nanjing University of Science and Technology, China; and the STFC Lasers for Science Facility (LSF). This study was enabled by the unique capability pioneered at the Lasers for Science Facility Ultra group and called time-resolved multiple-probe spectroscopy (TRMPS) which can follow electron transfers, kinetics and lifetimes on picosecond to microsecond timescales.
Inspired by the cytochrome mechanism of electron transfer in nature, this fundamental science experiment designing a prototype biohybrid catalyst has demonstrated the feasibility of a new route to more sustainable chemistry. This approach has the potential to help solve the problem of pollution in water systems and may present a more environmentally friendly catalysis route than preexisting ruthenium TiO2 complexes.
Dr. Igor Sazanovich, Senior Facility Scientist involved in this work, explains:
“Although the bio-hybrid material developed in the course of this work is a prototype aimed to demonstrate the approach to such type of materials, it opens new avenues to the research community to apply such techniques for other areas as well.
Read"Engineering cytochromes for photocatalysis: Biohybrid assemblies for light-driven dye decoloration" on the ScienceDirect website
