Nano-Bio Interface Engineering Using Cyclovoltammetry and Design of Nanostructured Lanthanide Oxysulfide Thin Films
Authors: Meyniel Léna , Boissière Cédric , Krins Natacha , Carenco Sophie 
Published: September 2023

Nanoparticles (NPs) in which magnetic and optical properties coexist have been intensively explored for diverse biomedical applications including diagnostic, in vivo imaging and therapy. Gadolinium oxysulfide (Gd2O2S) NPs are promising candidates for biotechnologies as they have been used as MRI contrast agent or X-ray absorbing materials. When doped with cerium, they gain an additional functionality as visible light absorber due to the decrease in the material bandgap from 4.7 eV non-doped to 2.5 eV with 5% of Ce. Further biomedical applications may be envisioned thanks to the combined antioxidant properties of cerium and the magnetic properties of gadolinium.

Understanding the molecular-level interactions between organisms and nanoparticles to guide and enable the design of novel materials that are less toxic remains a challenge. In order to understand these interactions at the nanoscale, we studied the change in the physico-chemical properties of (Gd,Ce) oxysulfide NPs when they are immersed in a biological medium. In biological medium, pristine nanomaterials transform and acquire a corona of biomolecules, such as proteins, which can modify their identity. This is known as the nano-bio interface. A better understanding of how nanoparticles interaction with a biological system affect their properties is critical to the future development of multifunctional NPs.

Our recent work provides us with a privileged access to composition-controlled nanoplates with a narrow size distribution for a large range of Ln. In particular, we mastered the preparation of mixed-lanthanide nanoplates with tunable band gap and magnetic properties while providing an in-depth understanding of their structural and surface features. The challenge addressed here is to prepare and characterize ligand-free nanostructured thin films made from the NPs suspension. Compared to studies realized in solution, thin films allow to reach the surface of the nanoparticles without any interaction from the ligands. Moreover, it allows spectroscopic and electrochemical methods to be used for the monitoring of the NPs intrinsic properties.

The design and development of nanostructured thin films started with the colloidal synthesis of (Gd,Ce) oxysulfide NPs which provides a phase-pure powder. The nanoparticles were then dispersed into an optimized mix of solvents in order to get a stable colloidal suspension. Dip coating is an easy method to deposit matter on a substrate while having a fine control of the thickness and homogeneity. This process was optimized to obtain homogenous films with a withdrawal speed of 2 mm/s under low relative humidity. Thin films of optical quality with a thickness from 40 nm to 150 nm were obtained using single-layer and multilayer deposition on various substrates including silicon, FTO and kapton (Figure A).

In situ UV and IR Spectroscopic Ellipsometry were used to monitor the variations of thickness and optical constants, together with the chemical species present at the surface, upon heating the films in different conditions. This study allowed to finely tune the thermal treatment in order to get rid of the stabilizing ligands while preserving the integrity of the nanoplates, as confirmed by Transmission Electron Microscopy (Figure B shows the scraped film). As a result, we have access to the bare surface of the NPs, which was hardly possible in solution. The change in the band gap of the material was monitored through UV-Variable Angle Spectroscopic Ellipsometry. This non-invasive technique is the gold-standard for the characterization of optical quality thin films and gives refractive index of the material (Figure C shows the Tauc plot with the linear fit giving the band gap of the material).

The film can be immersed into a biological medium where proteins are expected to bind to the surface of the NP and significantly change their reactivity and mobility. Cyclovoltammetry helped us to probe the access to the NPs and monitor the position of the conduction band. We used redox probes of different standard potentials as electrolytes (Figure D). Electron exchange with the semiconductor electrode is supposed to occur through the conduction band as the valence band is too far in energy. For a redox couple with a standard potential negative to the conduction band edge, when the applied potential equals E0
redox, the semiconductor is accumulating electrons at its surface in a so-called space charge layer. Under these conditions, whether a positive or negative bias is applied (compared to E0
redox), the SC is maintained in an accumulation mode. This was very helpful to check whereas the electronical properties of the material changed upon exposition to biological medium. This could lead to a better understanding and prediction of the chemical interactions at the interface with biomolecular systems.

Figure 1