Synthesis of Palladium Nanoparticles by Electrode-Respiring Geobacter sulfurreducens Biofilms

Research Background

In modern industry and environmental science, palladium (Pd) serves as a crucial catalyst widely used in pharmaceuticals, agriculture, and chemical industries. However, traditional methods for synthesizing palladium nanoparticles (Pd NPs) typically rely on energy-intensive chemical and solid-state synthesis techniques, which are not only costly but also generate harmful chemical waste. Therefore, developing a more sustainable and environmentally friendly method for synthesizing palladium nanoparticles has become an important research direction.

In recent years, electroactive microorganisms, such as Geobacter sulfurreducens, have garnered significant attention due to their ability to oxidize organic electron donors and transfer electrons to external solid minerals or electrode surfaces. These microorganisms can not only form biofilms on electrode surfaces but also reduce soluble metal ions, such as palladium ions, to synthesize metal nanoparticles. Utilizing electroactive microorganisms for palladium nanoparticle synthesis can be conducted under physiological temperature, pressure, and pH conditions, avoiding the harmful waste produced by traditional methods. Thus, studying how to use Geobacter sulfurreducens biofilms to synthesize palladium nanoparticles on electrodes holds significant scientific and practical value.

Source of the Paper

This paper was co-authored by Marko S. Chavez, Magdalene A. Maclean, Nir Sukenik, Sukrampal Yadav, Carolyn Marks, and Mohamed Y. El-Naggar from the Department of Physics and Astronomy, Biological Sciences, and Chemistry at the University of Southern California. The paper was published on December 11, 2024, in the journal ACS Biomaterials Science & Engineering, titled Synthesis of Palladium Nanoparticles by Electrode-Respiring Geobacter sulfurreducens Biofilms.

Research Process and Results

1. Biofilm Cultivation and Electrochemical Activity Testing

The first step of the research involved cultivating Geobacter sulfurreducens biofilms in anaerobic electrochemical reactors. The researchers used graphite electrodes and gold interdigitated array (IDA) electrodes as working electrodes (WE) and monitored current changes during biofilm formation using chronoamperometry (CA). Current production was used as an indicator of biofilm growth and cell activity. Once the current reached a steady state, the researchers performed cyclic voltammetry (CV) scans to assess the electrochemical activity of the biofilms before adding palladium.

The results showed that the biofilms formed current-producing layers tens of microns thick on the electrodes, with a current density of approximately 1 mA/cm². CV scans revealed that electron transfer on the electrodes occurred through the outer membrane cytochromes of Geobacter sulfurreducens, consistent with the known extracellular electron transfer (EET) mechanism.

2. Addition and Reduction of Palladium Ions

After biofilm formation and reaching peak current, the researchers added 0.5 mM Na₂PdCl₄ solution to the reactors to study the electrochemical activity of the biofilms during the reduction of soluble palladium ions. By monitoring changes in palladium ion concentration using inductively coupled plasma optical emission spectroscopy (ICP-OES), the researchers found that the biofilms could completely reduce the added palladium ions within 24 hours, while the palladium ion concentration remained stable in control experiments (without biofilms in the medium and electrodes).

Additionally, CV scans showed that although the addition of palladium ions caused a significant drop in current, the biofilms retained partial electron transfer capability. This indicates that Geobacter sulfurreducens biofilms can simultaneously perform electrode respiration and palladium ion reduction.

3. Synthesis and Characterization of Palladium Nanoparticles

To confirm the localized formation of palladium nanoparticles in the biofilms, the researchers analyzed the biofilms on the electrodes using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). SEM images showed that palladium nanoparticles appeared as white spheres unevenly distributed on the biofilm surface. EDS spectra further confirmed the presence of palladium, indicating that the biofilms could indeed locally synthesize palladium nanoparticles.

To study the morphology and distribution of palladium nanoparticles in more detail, the researchers also used transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) combined with EDS. TEM images revealed that palladium nanoparticles were not only distributed on the biofilm surface but also penetrated into the biofilm interior, indicating that palladium ions could diffuse deep into the biofilm and be reduced by the cells. Particle size analysis showed that the average diameter of palladium nanoparticles was 4-5.5 nm.

4. Recovery and Reusability of Biofilms

After palladium ion reduction, the researchers conducted medium exchange experiments to assess whether the electrochemical activity of the biofilms could recover. The results showed that after medium exchange, the current production capacity of the biofilms significantly recovered, indicating that the addition of palladium ions did not cause long-term toxic effects on the biofilms. This finding suggests the potential for reusing biofilms in multiple palladium ion reduction experiments.

Conclusions and Significance

Through this study, the researchers demonstrated for the first time that Geobacter sulfurreducens biofilms can reduce soluble palladium ions while performing electrode respiration and locally synthesize palladium nanoparticles within the biofilms. This discovery not only expands the application of electroactive microorganisms in metal ion reduction and biomineralization but also provides new insights for developing novel cell-nanoparticle biomaterials.

Scientific Value and Application Prospects

1. Sustainable Nanomaterial Synthesis: Utilizing electroactive microorganisms for palladium nanoparticle synthesis can be conducted under mild conditions, avoiding the high energy consumption and harmful waste associated with traditional methods, offering significant environmental advantages.

2. Localized Material Formation in Biofilms: The localized synthesis of palladium nanoparticles in biofilms provides the possibility of constructing hybrid materials with unique electron transfer and catalytic properties, with broad application prospects in catalysis, environmental remediation, and energy storage.

3. Reusability of Biofilms: The study shows that Geobacter sulfurreducens biofilms can recover electrochemical activity after palladium ion reduction, offering the potential for reusing biofilms in multiple palladium ion reduction experiments, further enhancing the economic viability of this technology.

Research Highlights

1. First Synthesis of Palladium Nanoparticles in Electrode-Respiring Biofilms: Unlike previous studies, this research achieved the synthesis of palladium nanoparticles in electrode-respiring Geobacter sulfurreducens biofilms for the first time, showcasing the unique advantages of biofilms in nanomaterial synthesis.

2. Interdisciplinary Research Approach: The study combined electrochemical, spectroscopic, and electron microscopy techniques to comprehensively reveal the mechanisms of palladium ion reduction and nanoparticle synthesis in biofilms.

3. Potential Environmental Applications: By demonstrating the localized synthesis of palladium nanoparticles in biofilms, the researchers highlighted the potential applications of this technology in environmental remediation (e.g., heavy metal removal) and catalyst recovery.

Summary

This study not only provides new perspectives on the application of electroactive microorganisms in nanomaterial synthesis but also opens new avenues for developing more sustainable and environmentally friendly nanomaterial synthesis methods. In the future, by combining biofilm patterning techniques and genetic engineering, researchers can further optimize this process to develop hybrid materials with complex geometries and functionalities, advancing the application of microbial technologies in energy, environment, and materials science.