Interaction of Polyoxometalates and Nanoparticles with Collector Surfaces—Focus on the Use of Streaming Current Measurements at Flat Surfaces

Johannes Lützenkirchen,Atiđa Selmani,Lionel Vayssières,Venkata Sai Kiran Chakravadhanula,Engelbert Redel,Gopala Krishna Darbha

doi:10.3390/colloids4030039

Abstract

Streaming current measurements were used to study the interaction of polyoxometalates (POMs) and nanoparticles (NPs) with flat surfaces as an alternative, innovative approach to infer POM and NP properties of potential sparse material in terms of charge and magnitude. With respect to POMs, the approach was able to reveal subtle details of charging properties of +7 vs. +8 charge at very low POM concentrations. For NPs, the sign of charge and even the zeta-potential curve was retrieved. Concerning NPs, mutual interaction between TiO2 and SiO2 surfaces was studied in some detail via macroscopic measurements. Post-mortem analysis of samples from electrokinetic studies and separate investigations via AFM and HRTEM verified the interactions between TiO2 NPs and SiO2 collector surfaces. The interactions in the SiO2/TiO2 system depend to some extent on NP morphology, but in all our systems, irreversible interactions were observed, which would make the studied types of NPs immobile in natural environments. Overall, we conclude that the measurement of streaming currents at flat surfaces is valuable (i) to study NP and POM collector surface interactions and (ii) to simultaneously collect NPs or POM (or other small mobile clusters) for further (structural, morphological or release) investigations.

Highlights

Streaming current measurements were used to study the interaction of polyoxometalates (POMs) and nanoparticles (NPs) with flat surfaces as an alternative, innovative approach to infer POM and NP properties of potential sparse material in terms of charge and magnitude
Post-mortem analysis of samples from electrokinetic studies and separate investigations via AFM and HRTEM verified the interactions between TiO2 NPs and SiO2 collector surfaces
We propose that the determination of zeta-potentials from streaming current measurements at flat surfaces could allow the determination of the IEP of NPs, and we emphasize that only very small quantities are required for the measurements

Summary

Introduction

The use of engineered NPs in science and technology arises from a strong interest in the surface properties of NPs. The NPs emerging in nature are expected to cause adverse effects in various compartments of the environment [1,2,3,4,5,6,7,8]. Among the most important groups of such emerging materials are TiO2 NPs. Various aspects of NP physico-chemical behavior or transport processes have been addressed, including the role of air–water interfaces [9], the effect of UV radiation [10], the desorption of adsorbed metal ions that can be transported by TiO2 NPs [11], and the respective influences of natural organic matter [12] or surfactants [13], to name only a few. Since NPs are emerging in so many environmentally relevant contexts, simple methods for detecting and characterizing NPs are required

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Conclusion