Dr. Kate Lawrence, Associate Editor for ChemElectroChem, talks to Professor Dr. Richard Compton, Oxford University, UK, about his recent article on non-invasive probing of nanoparticle electrostatics that was published in ChemElectroChem.
You have studied the unperturbed electrostatic interactions between charged indigo nanoparticles and a mercury electrode; what was the inspiration behind this study?
Electron transfer to and from nanoparticles lies at the heart of many energy storage and transformation technologies, yet the fundamental understanding of controlling key physical principles has been lacking. Whilst molecular electron transfer is well understood by electrochemists, extension to the nanoscale provides a challenge, both in terms of designing well-defined experiments and in their quantitative analysis.
Why is it important to focus on the unperturbed interactions and what other ways did you consider to circumvent perturbation?
The choice to study the reduction of negatively charged indigo nanoparticles at a mercury electrode arises because this process occurs at potentials both positive and negative of the potential of zero charge of the electrode. By contrasting the electron-transfer kinetics occurring at positively and negatively charged electrodes, the role of electrostatics in the process can be explored. In particular, two possible effects can be unscrambled. First, the attraction or repulsion between the negatively charged nanoparticles and the electrode changes the population of species close to the electrode. Second, a more negative electrode potential will increase the thermodynamic driving force of electron transfer.
Why was your attention focused on this particular area?
Many studies using nanoparticles and/or large charged biomolecules such as DNA have assumed that electrostatic attraction and repulsion is the primary consideration in governing electron-transfer rates. However, it has long been appreciated that, in the presence of electrolyte, screening reduces these effects significantly. Whilst such screening effects have long been accepted in molecular electrochemistry, where the reduction of a negatively charged molecule dissolved in an electrolyte solution at a negatively charged electrode is not thought to be exceptional, the role of screening in nanoparticle electron transfer needed exploration.
What is the main significance of your results?
Our experiments show that, for negatively charged indigo nanoparticles being reduced at a mercury electrode, the electrostatic attractions and repulsions from the electrode are very effectively screened, except with very dilute electrolyte concentrations of the order of a few millimolar or less. Thus, the dominant effect of changing the electrode potential is to alter the thermodynamic driving force for the reaction rather than the local nanoparticle population.
What is the broader impact of this work in terms of nanoscale studies?
We would like to see greater attention paid to quantitative measurements in an area that is becoming overwhelmed with empirical “try-it-and-see” work with little generic insight.
Do you have any plans for future work extending from this study? If so, could you provide some details?
The nanoimpact experiment offers tremendous opportunities for major progress in the understanding of the electrochemistry of single nanoparticles. For example, we have looked at the oxidation of single metal nanoparticles , the reductive dissolution of individual indigo particles , charge transfer to and from metal-oxide nanoparticles , and the doping of polymeric poly-vinylcarbazole nanoparticles . Very recently, we reported impacts of single drug-encapsulating liposomes and showed that the contents of single nanoparticles can be measured .
Supported by an ERC Advanced Grant, we are actively developing new applications and a deeper understanding of the associated basic science. Already, the application of electrochemistry to nanoparticles has led to revised or better appreciation of interfacial electron transfer. For example, the blocking effects of small adsorbed molecules is much more pronounced with nanoparticles compared to molecules , and the response of microelectrodes can be overwhelmed by adsorption on any surrounding insulation . Fundamentally, electrochemical studies with nanoparticles have emphasized the changed nature of diffusion near interfaces  and unambiguously identified changed reactivity at the nanoscale .
The article they talked about:
- Non-Invasive Probing of Nanoparticle Electrostatics,
K. Tschulik, W. Cheng, C. Batchelor-McAuley, S. Murphy, D. Omanović, R. G. Compton,
 The Electrochemical Detection and Characterization of Silver Nanoparticles in Aqueous Solution, Y.-G. Zhou, N. V. Rees, R. G. Compton, Angew. Chem. Int. Ed. 2011, 50, 4219–4221. DOI: 10.1002/anie.201100885
 Electrochemical Sizing of Organic Nanoparticles, W. Cheng, X.-F. Zhou, R. G. Compton, Angew. Chem. Int. Ed. 2013, 52, 12980–12982. DOI: 10.1002/anie.201307653
 Coulometric sizing of nanoparticles: Cathodic and anodic impact experiments open two independent routes to electrochemical sizing of Fe3O4 nanoparticles, K. Tschulik, B. Haddou, D. Omanović, N. V. Rees, R. G. Compton, Nano Res. 2013, 6, 836–841. DOI: 10.1007/s12274-013-0361-3
 Doping of Single Polymeric Nanoparticles, X.-F. Zhou, W. Cheng, R. G. Compton, Angew. Chem. Int. Ed. 2014. DOI: 10.1002/anie.201405992
 Investigation of Single-Drug-Encapsulating Liposomes using the Nano-Impact Method, W. Cheng, R. G. Compton, Angew. Chem. Int. Ed. 2014. DOI: 10.1002/anie.201408934
 Nanoparticle-Impact Experiments are Highly Sensitive to the Presence of Adsorbed Species on Electrode Surfaces, E. Kätelhön, E. Cheng, C. Batchelor-McAuley, K. Tschulik, R. G. Compton, ChemElectroChem 2014, 1, 1057–1062. DOI: 10.1002/celc.201402014
 Shielding of a Microdisc Electrode Surrounded by an Adsorbing Surface, S. Eloul, R. G. Compton, ChemElectroChem 2014, 1, 917–924. DOI: 10.1002/celc.201400005
 Understanding nano-impacts: impact times and near-wall hindered diffusion, E. Kätelhön, R. G. Compton, Chem. Sci. 2014. DOI: 10.1039/C4SC02288B
 Electrochemical Observation of Single Collision Events: Fullerene Nanoparticles, E. J. E. Stuart, K. Tschulik, C. Batchelor-McAuley, R. G. Compton, ACS Nano 2014, 8, 7648–7654. DOI: 10.1021/nn502634n