Seongho Jeon

Recent functional material based approaches to prevent and detect counterfeiting

Technological advances made in constructing high-resolution instrumentation have enabled fabrication of sophisticated counterfeit products. As a result, the need to develop novel anti-counterfeiting materials and systems is greatly increasing. Accordingly, various functional materials that display distinct chemical, physical as well as optical and electrical properties have been investigated for preventing and detecting counterfeiting. In this feature article, we describe recent strategies that have been devised for anti-counterfeiting purposes that focus on colorimetric and fluorometric approaches that are employed in the determination of the authenticity of banknotes, documents, medicine, etc. In addition, methods of incorporating organic electronics on banknotes as active authentication tools as well as molecular imaging techniques based on mass spectrometry are briefly described.

Detection of adulterated gasoline using colorimetric organic microfibers

Gasoline adulteration poses serious economic and environmental problems in many countries. A “protective layer approach” has been developed for the colorimetric detection of fake and adulterated gasolines. In this approach, a distinct blue-to-red colour change is observed when a conjugated polydiacetylene (PDA)-embedded, electrospun fiber mat is exposed to fake gasoline (a mixture of thinner, toluene, and methanol) and adulterated gasoline (toluene or methanol added to commercial gasoline). The matrix polymer used for electrospinning is found to play a critical role as a protective layer for the embedded colorimetric PDA supramolecules. Thus, chromic transition occurs only when the protective matrix polymer is dissolved by the component of the gasoline. In fact, toluene and methanol were found to facilitate dissolution of the matrix polymer (toluene for polystyrene and methanol for polyacrylic acid) and allows penetration of the adulterated gasoline to the embedded PDA, causing blue-to-red phase transition of PDA. The colour change occurs almost instantaneously (<3 s) and is observable by the naked eye. Thus, complicated instrumental techniques are not required in this approach. Importantly, this smart sensor system is portable, inexpensive, sensitive and it requires minimal amounts (app. 40 µL) of test samples.

In situ synthesis of stimulus-responsive luminescent organic materials using a reactive inkjet printing approach

We have prepared luminescent organic materials using a reactive inkjet printing system. A stimulus (vapour/heat)-responsive fluorescent dye as well as a luminescent conjugated polymer were readily generated by printing of an aldehyde, a phenylenediacetonitrile, and potassium t-butoxide.

Responsive 3D Microstructures from Virus Building Blocks

Fabrication of 3D biological structures reveals dynamic response to external stimuli. A liquid-crystalline bridge extrusion technique is used to generate 3D structures allowing the capture of Rayleigh-like instabilities, facilitating customization of smooth, helical, or undulating periodic surface textures. By integrating intrinsic biochemical functionality and synthetic components into controlled structures, this strategy offers a new form of adaptable materials.

Virus-based surface patterning of biological molecules, probes, and inorganic materials

An essential requirement for continued technological advancement in many areas of biology, physics, chemistry, and materials science is the growing need to generate custom patterned materials. Building from recent achievements in the site-specific modification of virus for covalent surface tethering, we show in this work that stable 2D virus patterns can be generated in custom geometries over large area glass surfaces to yield templates of biological, biochemical, and inorganic materials in high density. As a nanomaterial building block, filamentous viruses have been extensively used in recent years to produce materials with interesting properties, owing to their ease of genetic and chemical modification. By utilizing un-natural amino acids generated at specific locations on the filamentous fd bacteriophage protein coat, surface immobilization is carried out on APTES patterned glass resulting in precise geometries of covalently linked virus material. This technique facilitated the surface display of a high density of virus that were labeled with biomolecules, fluorescent probes, and gold nanoparticles, thereby opening the possibility of integrating virus as functional components for surface engineering.

A side-chain crosslinking approach for the fabrication of conjugated polymer patterns

Poly(p-phenylene vinylene) (PPV) is a family of conjugated polymers that have been used in a variety of fields, including light-emitting diodes, light-emitting transistors, photovoltaic cells, and sensors. PPV is generally synthesized by a Wittig or a Knoevenagel condensation reaction. Owing to its poor solubility, it has been difficult to utilize PPV in solutionbased manufacturing processes. To overcome this issue, alkoxysubstituted PPVs, such as poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) (Scheme I), have been developed. The long alkyl substituents enhance the solubility in common organic solvents. In addition, there have been numerous attempts to tune the emission wavelength of PPV for display applications. Depending on the substituents and steric hindrance, the fluorescence of PPV can be tuned from blue to red. Many efforts have been devoted to fabricate patterned PPV images on solid substrates. For instance, Dirk and coworkers reported the patterning of PPV using a photolabile xanthate group. PPV patterns were produced by elimination of xanthate groups upon exposure to UV light. In addition, patterns can be fabricated by employing photolithography, micromoldingin-capillaries (MIMIC), and array spotting techniques. Among the various patterning methods, the photolithographic approach has been considered as the most reliable due to its high scalability and resolution. Conventional photolithographic methods generally utilize precursor PPVs that are converted to PPVs by UV irradiation. Polydiacetylenes (PDAs), discovered by Wegner in 1969, are optically and structurally unique conjugated polymers. When 254 nm UV light is irradiated on self-assembled diacetylene (DA) monomers, 1,4-addition polymerization occurs to form a colored (typically blue) PDA. Polydiacetylene (PDA) shows a color transition (typically blue to red) and exhibits self-fluorescence when exposed to external stimuli. In addition, the solubility of PDA in common organic solvents is drastically decreased after polymerization. We hypothesized that, if a thin film of PPV containing DA moieties (PPV-DA) is UV-irradiated with a photomask, photoinduced PDA formation should occur in the UV-exposed area (Figure 1). Subsequent development of the film in an organic solvent should remove any organic solvent-soluble (UV-unirradiated) part, leaving patterned PPV images behind.