Cyclic Voltarefractometry of Single TiO₂ Nanoparticles in Large Ensembles in Nonaqueous Electrolyte

Single nanoparticle (NP) cyclic voltarefractometry (CVR), realized as wide-field surface plasmon resonance microscopy (SPRM) in combination with potential cycling, has been proposed and applied to the in situ study of TiO₂ NPs. Electrochemical activity of TiO₂ is mainly observed outside the electrochemical stability window of water. Therefore, the response of individual anatase (a-TiO₂) and rutile (r-TiO₂) NPs adsorbed on a gold layer was studied in 0.25 M LiClO₄ acetonitrile solutions. The use of acetonitrile allows us to exploit a much wider potential window compared to water, while due to the almost identical refractive index (n_D = 1.344 and 1.333 for acetonitrile and water, respectively), the conditions of the SPR are not changed. This greatly expands the variety of electrochemical reactions that can be studied by SPR techniques. Cyclic polarization of a-TiO₂ and r-TiO₂ NPs results in pronounced electrochemical and optical responses around -1.55 V and around -1.8 V vs Fc⁺/Fc, respectively. This specific optoelectrochemical response allows them to be distinguished from other NPs. Based on this difference in characteristic potentials, a mixture of a-TiO₂ and r-TiO₂ NPs can be analyzed by CVR as well. The proposed correction algorithm compensates for the drift in the SPRM background caused by the accompanying formation of insoluble compounds and separates the optical response of the NPs out of the background. The results obtained in the study of this complex system demonstrate the capabilities of the developed analytical method. The CVR can be applied to the quantitative analysis of many other types of NPs in nonaqueous solutions, providing information on the electrochemical properties of each individual particle on the electrode surface.