Researches

Introduction

Molecules often do not play alone, but assemble together via either covalent linkage or non-covalent interactions to have specific structures and functions. Examples of these molecular clusters and/or aggregates include various organic/inorganic self-assemblies, metal and/or semiconductor nanoclusters, oligomeric biomolecules, and so on.

In our researches, we emphasize the importance of the structures and interactions in the intermediate clusters and aggregates. The reasons are following: The structures of the final cluster or assembly often depend highly on 1) the size and shape of intermediate clusters/aggregates, 2) the covalent and non-covalent interactions which are formed or broken during the formation of intermediates, and 3) the interactions between intermediate species with surrounding environments such as solvent and surface ligands. Therefore, obtaining insight into the structure of those intermediate clusters/aggregates is of great importance, because it enables to understand the detailed mechanisms and driving forces of assembly process and further allows for controlling and manipulating the structures and functions of molecular clusters/assemblies.

However, the chemistry of those intermediate clusters and aggregates have not been explored intensively. It is mainly due to their intrinsic complexities and transient nature. Firstly, they usually exist in a wide distribution of cluster size and aggregation states. Secondly, an enormous number of structures and conformations are possible for each cluster or aggregation state. Thirdly, the cluster growth and the aggregation process usually involve a series of complex structural and conformational transitions. Unfortunately, conventional condensed-phase analytical methods for the structural study often have limitation in studying those clusters and aggregates, because they only yield ensemble averaged results over a wide distribution of clusters/aggregates as well as conformational isomers, and it is extremely hard or even impossible to separate a specific size of cluster or aggregate from the distribution.

For those reasons, the development and application of new and complementary experimental approaches to determine the structural elements and interactions in the specific individual molecular clusters/aggregates is needed. This requires the proper isolation of each cluster/aggregate of specific size and conformation and the structural analysis using spectroscopic methods. Therefore, we are using a gas-phase experimental approach which combines mass spectrometry (MS), ion mobility spectrometry (IMS), molecular ion optical spectroscopy, and theoretical methods. This combination enables to investigate the structures and interactions of the isolated individual atomic and molecular cluster/aggregate.

Our Research Methods

Mass Spectrometry

Mass Sepctrometry

Mass spectrometry (MS) is an gas-phase analytical method that ionize chemical species and separate those ions by their mass-to-charge ratio (m/z). MS enables to determine the masses of chemical species, and to isolate the specific molecule or molecular complex by mass in the gas phase. Therefore, MS is an essential tool to investigate the atomic and molecular clusters.

Ion Mobility Sepctrometry

In ion mobility spectrometry (IMS), molecular ions drift through a buffer gas under the influence of a weak electric field and are separated by their geometrical sizes and charge states, which enables to separate isomers and conformers. In addition, collision cross section (CCS) of a molecular ion can be determined, which is further compared to the structures from molecular modelling.

Ion Mobility Spectrometry

Optical Spectroscopy of Gaseous Ions

Ion spectroscopy can provide direct information about the underlying structure of the ion. For example, vibrational spectroscopy is highly sensitive to the functional groups and their interactions, and therefore is very useful to probe the roles of functional groups, internal interactions, and structural evolution during the cluster formation and growth.

Theoretical Approaches

To obtain atomic- and molecular-level structural information, we are also using theoretical approaches which include molecular modeling, molecular dynamics simulation (MD), and ab initio quantum chemical calculation.

Research Topics

With these experimental and theoretical methods on hands, we are interested in the following research themes:

Investigating structural evolution of atomic/molecular clusters in the early aggregation stages

　- Tracking structural changes of biomolecular complexes (e.g., peptides, nucleotides, proteins, etc.) during their self-assembly processes.

　- Studying the formation process of organic/inorganic supramolecular assemblies (e.g., MOF)

　- Investigating the nucleation process of various metal and/or semiconductor nanoparticles

Investigating atmospheric nano- and/or micro-paticulate pollutant ions

　- Studying the structures and interactions of nucleation intermediates - various organic and inorganic pollutant aggregates in the air

Developing new analytical instruments and methods using IMS-MS for nanoparticle researches

　- Building a new IMS-MS instrument for nanoparticle analysis

　- Size-specific separation and isolation of nanoparticle ions in the gas phase using IMS-MS

　- Soft-landing of the size-selected nanoparticle ions on various substrates