Prof. Katsuhiko Ariga is a world pioneer in the development of methods and concepts that have a significant ripple effect by focusing on self-assembly and supramolecular chemistry based on the control of the interfacial environment. These aspects are mainly exemplified by his pioneering work on alternate layer-by-layer (LbL) assembly and molecular recognition at interfaces. The following is a detailed description of his achievements.

1. Pioneering developments of layer-by-layer assembly
The basic concept of the alternate layer-by-layer (LbL) assembly method, proposed by Decher, has been demonstrated mainly in the preparation of assembled films of polyelectrolytes and surfactants. The LbL assembly was not widely used because the process of film formation mostly relied on X-ray diffraction and other instrumental methods. Prof. Ariga applied a simple in-situ quantification technique for nanogram-level weight changes using the quartz crystal microbalance (QCM) method to evaluation of LbL assembling processes. This extreme simplification of evaluation on the LbL assembling processes made the LbL assembly much easier method. The LbL assembly became recognized as a "nano-film preparation method that can be done with just tweezers and a beaker" by making it possible to prepare and evaluate films by the method on a lab bench on the spot. As a result, the LbL assembly with in situ QCM evaluation has advanced as the most convenient nano-film fabrication method that can be used by researchers in all fields around the world. In addition, the simplification of the experimental method has strongly promoted the application of the LbL assembly method to a wide range of materials from quantum materials and nanomaterials to biological materials (even virus). The most notable pioneering work by Prof. Ariga is the application of the LbL assembly method to biomolecules such as proteins. Prof. Ariga's research team systematically investigated the LbL assembly of a wide range of biomolecules (mainly protein molecules), and established a technological protocol standard that allows biomolecules to be universally converted into nanofilms by successfully balancing the pH of the medium and the isoelectric point of the biomolecule. In addition, they have demonstrated the world's pioneering demonstrations on successful LbL assemblies of silica nanoparticles, dye aggregates, clay nanosheets, and so on.

2. Exploratory research on molecular recognition at interfaces
Prof. Ariga reported the world-first example of permeation controls of materials through a single Langmuir monolayer in 1986. Few years later, more precise molecular interaction, molecular recognition, were systematically investigated by project team lead by Prof. Ariga and Prof. Kunitake. Their systematic experimental and theoretical research revealed surprisingly large enhancement of molecular recognition capability at interfacial media as compared with those in bulk aqueous solutions.
One distinct surprize in series of the research would be huge enhancement of binding constants. While binding constant between guanidinium and phosphate for molecularly dispersed systems in water is only 1.4 M^-1, binding constants of phosphates of nucleotides to guanidinium-functionalized monolayers at the air-water interface became as large as 10^6-10⁷ M^-1 range. In order to compensate this huge gap between aqueous molecular interface and macroscopic air-water interfaces, binding constants between guanidinium and phosphate at mesoscopic interface of surfaces formed by aqueous micelles and lipid bilayers were investigated by the equilibrium dialysis method (ultrafiltration method). The obtained binding constants for aqueous mesoscopic interfaces were ranged between 10² and 10⁴ M^-1. These systematic comparison revealed both (i) generality of enhancement of molecular recognition at interfacial media with low-dielectric phase and (ii) their significant dependences on types of interfaces. Enhanced molecular capability at interfacial environment is originated from electronic contributions from low dielectric phase within molecular vicinity.
These discoveries and mechanism proposals provide answers to mystery in biology, how bio-systems can accomplish molecular recognition through hydrogen bonding in highly polar aqueous medium. Molecular interactions at interfacial media such as cell surface, interior of enzymes, and DNA macromolecular interfaces are key for biological phenomena.

3. Recent challenge on molecular machine control at interfaces
Not limited to the above mentioned pioneering research accomplishments in 20th century, Prof. Ariga and co-workers are promoting innovative challenges on control of molecular machines and nanocars at interfacial media. Prof. Ariga pioneeringly proposed method to control of conformations of molecular machines embedded at the air-water interface which enables us to operate molecular machines by macroscopic mechanical forces like hand motions.
In addition, conformations of molecular receptors can be subtly tuned with mechanical processes to intentionally alter binding efficiency and selectivity to target guest molecules. For example, lateral compression on the monolayer of cholesterol-substituted triazacyclononane as a molecular receptor for nucleic acid bases optimizes discrimination between uracil and thymine derivatives, which cannot be distinguished by naturally occurring DNA and RNA. Reversion of enantio-selectivity of aqueous amino acids depending on lateral pressures was also realized through tuning of conformations of molecular receptor, cholesterol-armed cyclen complex, by mechanical compression of the receptor monolayers. In the past, basics of molecular recognition relies on one most stable complex structure between host (receptor) and guest. If it can be regarded as the first generation of molecular recognition, the second generation of molecular recognition should be assigned to switching of receptor structures by external stimuli as seen in pioneering example of photo-switchable azobenzene-type receptors. This switching mechanisms utilizes two or more stable states to control recognition capability by external stimuli. Unlike these two traditional mechanisms, receptor tuning at dynamic interface pioneered by Prof. Ariga is based on selection and optimization from numerous conformational candidates of receptors. In this third generation methodology, desirable efficiency and selectivity of molecular recognition can be tuned among numerous possibilities during continuous deformation of receptor structures.
In addition, his team developed research to precisely control molecular car (nanocar) on a solid interface. With this innovative mechanism, the research team directed by Prof. Ariga attended the world-first nanocar race as a Japanese-representative.

Thus, Prof. Ariga's great contributions deserves well to the Chemical Society of Japan (CSJ) Award.