Inverted core/shell Nanocrystals: the future Nanomaterial for the Visualization of Neuron activity

Project Information

iNano

Grant agreement ID: 846764

DOI

10.3030/846764

Project closed

EC signature date 15 April 2019

Start date 1 July 2019

End date 30 June 2021

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 162 806,40

EU contribution

€ 162 806,40

Coordinated by

BUNDESANSTALT FUER MATERIALFORSCHUNG UND -PRUEFUNG
Germany

Periodic Reporting for period 1 - iNano (Inverted core/shell Nanocrystals: the future Nanomaterial for the Visualization of Neuron activity)

Reporting period: 2019-07-01 to 2021-06-30

Summary of the context and overall objectives of the project

In the last decades, the utilization of nanomaterials has enabled significant advances in several key technologies of the 21st century and in numerous (bio-) technological applications and thus has a significant influence on our daily life. Colloidal semiconductor nanocrystals (NCs), also termed quantum dots (QDs), are a particularly prominent example, which have attracted considerable research interest. QDs exhibit unique optoelectronic properties, which made them popular nanomaterials intensively used in in vitro diagnostics, bioimaging, photovoltaics, and light-emitting optoelectronic devices. However, the best-studied NCs, II/VI semiconductor NCs contain the toxic heavy metal element cadmium. Therefore, it is important to explore alternative materials that exhibit comparable optoelectronic properties and possess lower toxicity. A promising heavy-metal free alternative is indium phosphide (InP) NCs as demonstrated by the commercialisation of TV screens based on these NCs. However, synthetic strategies for InP NCs are still in their infancy compared to Cd-based NCs. So far, InP NCs suffer typically, e.g. from broader photoluminescence (PL) band caused by the broader size distribution, and the synthesis of InP NCs with a band gap emission at wavelengths > 750 nm failed, although the InP band gap is 1.34 eV, equalling 925 nm.
For in vivo imaging or image-guided surgery, there is an increasing interest in nanomaterials emitting above 750 nm. Furthermore, NCs possess an enhanced sensitivity to external electronic fields, which makes them attractive tools to explore the vast network and communication pathways of neuron cells in the brain. Currently used technologies to study the brain like electrode-based techniques or voltage-sensitive dyes (VSDs) have several shortcomings as they are not scalable, have slow kinetics, have a limited dynamic range, and can interfere with the membrane capacitance. NCs have shown in the past to outperform VSDs in terms of (PL) intensity changes in the presence of an electronic field. By changing the NC shape from spherical a rod-shaped the sensitivity even becomes stronger. However, up to now, there exists no single report of the synthesis of InP NCs with an anisotropic shape.
The iNano project aimed to develop a new synthesis approach for the preparation of spherical and rod-shaped InP-based NCs and thus responding to the challenges currently faced in the synthesis of InP NCs. A key feature is the use of magic-sized clusters (MSCs) as a seed and epitaxially grow an InP shell around this seed. Thereby the thickness of the InP shell allows to control the PL emission wavelength and potentially enables to reach an emission beyond 750 nm. Furthermore, depending on the crystal structure of the MSCs, the InP shell growth direction should be possible to control yielding either spherical or more elongated, rod-shaped NCs. These nanomaterials should be then explored for their potential as a new tool for neuroscientists to measure and activate the action potential of neuron cells.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The main work frame of this project can be divided into three major packages: i) the synthesis of the InP NCs; ii) rendering the prepared NCs water-dispersible; and iii) exploring the interaction of NCs with cells in in vitro measurements. Like the rest of the world, the Corona pandemic situation has also affected the research progress of this project. Nevertheless, new insights, results, and knowledge was built during the project duration. At the beginning of the project, the preparation of zinc selenide (ZnSe) MSCs was established. These MSCs are highly monodisperse, which makes them ideal seeds for the growth of an InP shell aiming at NCs with a similar InP thickness. Following published synthetic approaches for the preparation of ZnSe MSCs requires the utilization of highly toxic precursors like hydrogen selenide. Therefore, a new approach was developed in this project using much less toxic chemicals. This also enabled the preparation of a novel ZnSe MSCs size, not known so far. For the epitaxial growth of the InP shell, two different approaches were tested. The first approach was based on a layer-by-layer approach where only small amounts of indium and phosphorous precursor were used at a time followed by an annealing phase at elevated temperatures. The second approach uses a syringe pump to constantly add a small amount of the reactants to the reaction solution. Whereas the layer-by-layer approach was not successful and resulted in rather inhomogeneous InP shell growth, the constant supply of chemicals yielded ZnSe/InP core/shell NCs with improved quality. Surprisingly these NCs were not fluorescent. The epitaxial growth of a third layer of zinc-sulphide (ZnS) on the InP shell then led to luminescent NCs. ZnS shells are often used in NC chemistry for NC surface passivation and to reduce surface defects, which are known to quench the PL of NCs.
Although the preparation of NCs in organic solvents has been shown to result in high-quality optical properties, they need to be rendered water-dispersible for use in biological applications. For these applications, a small size of the NCs is preferred, which can be achieved by the exchange of organic ligands on the NC surface with ligands that enable water dispersibility. In this project, the influence of different organic molecules possessing one, two, or three sulphur groups on the stability and optical properties of NCs after ligand exchange was investigated. The hypothesis that a higher number of sulphur groups per molecule tightly binding to the ZnS outer shell results in more stable water dispersible NCs could be proven and the optical properties were similar to monodentate ligands. The last part of the project, focusing on the interaction of the NCs with neuron cells and evaluating their potential to stimulate and measure the action potential, is still ongoing. Due to the unforeseeable impact of the Corona pandemic situation, it could not be finished within the time frame of the project. The here obtained results were disseminated at conferences and publications are in preparation.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The results obtained in the iNano project enabled us to push further the frontiers in the synthesis of complex NC structures and gave new insights into the synthesis of MSCs. The here developed greener synthesis route to ZnSe MSCs is beyond the state of the art and might enable to use MSCs as building blocks to design new complex NCs. This could have a large impact on the utilization of NC in biomedical applications as their properties can be fine-tuned and less dangerous precursors are necessary to provide them with specific properties. Furthermore, the developed approach for the inverted ZnSe/InP core/shell structures can be easily scaled up and thus make preparation on an industrial scale feasible. The insights gained into the chemical and optical stability of water-dispersible NCs using ligands with different sulphur groups are beneficial for the utilization of NCs in biological applications as the decomposition of NCs in biological media, i.e. in cells, is a known promotor for their cytotoxicity. Providing NCs with a stable organic ligand shell will reduce their cytotoxic potential and thus help to provide safer NCs for in vivo imaging or drug delivery in commercial applications. In summary, this project has a wider societal impact as well as a socio-economic impact for the nanomaterial community.

Image of ZnSe/InP nanocrystals with increasing InP shell thickness illuminated with a UV lamp.