title: Organic nanoparticles of malachite green with enhanced far-red emission: size-dependence of particle rigidity.
abstract: In tracing the biological processes using fluorescent probes, it is desirable to shift the excitation/emission energy to a far-red/near-infrared (FR/NIR) region. In this study, we successfully synthesize FR fluorescent organic nanoparticles via ion-association between the malachite green (MG) cations and tetrakis(4-fluorophenyl)borate (TFPB) anions in the presence of a neutral stabilizing polymer. Binding of MG with TFPB results in the prominent appearance of an absorption band that can be assigned to an H-aggregate of MG. The fluorescence intensity as well as the fluorescence lifetime shows a significant increase with a decrease in the nanoparticle size. Since the MG dye is known as a local viscosity or environmental rigidity probe showing a rotational friction dependence of the excited state lifetime, we find that the rigidity of the organic nanoparticle is strongly size-dependent; that is, the smaller the size of the nanoparticle, the greater the rigidity of the nanoparticle. We also reveal that surface regions of the ion-based organic nanoparticles are more rigid than inner regions. The presence of H-aggregates that are almost non-fluorescent is the major origin of aggregation-caused quenching (ACQ) and still avoids the enhancement of the fluorescence quantum yield of the MG nanoparticles, so we develop a new approach to prevent H-aggregation inside the nanoparticle by incorporating photochemically inert, bulky phosphonium cations, which results in a 430-fold enhancement of its fluorescence yield. We believe that such a methodology will open up an avenue in the development of new types of fluorescent nanomaterials for many applications.
authors.affiliation: Graduate School of Material Science, University of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan. yao@sci.u-hyogo.ac.jp.
authors.forename: Tomohito
authors.initials: T
authors.lastname: Funada
authors.forename: Takuya
authors.initials: T
authors.lastname: Hirose
authors.forename: Naoto
authors.initials: N
authors.lastname: Tamai
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Yao
mesh_terms: D003116:Color; D008958:Models, Molecular; D008968:Molecular Conformation; D053758:Nanoparticles / Q000737:chemistry*; D010316:Particle Size; D011205:Povidone / Q000737:chemistry; D012394:Rosaniline Dyes / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D013775:Tetraphenylborate / Q000031:analogs & derivatives / Q000737:chemistry; D005464:Fluorobenzenes; D005466:Fluorocarbons; D056831:Coordination Complexes
chemical_list: D011205:Povidone; D012394:Rosaniline Dyes; D013775:Tetraphenylborate; C421570:tetrakis(pentafluorophenyl)borate; D005464:Fluorobenzenes; D005466:Fluorocarbons; D056831:Coordination Complexes; C005095:malachite green