A study on the fabrication of fluorescent sensors using molecularly imprinted nano-sized particles and fibers

Abstract

Molecular imprinting has been considered as a practical way to produce materials with molecular recognition properties due to its ease of use and low cost. In this study, the fluorescent sensors based on molecularly imprinted nanomaterials were developed using various fluorescent materials as a signal transducer. Firstly, a highly sensitive molecularly imprinted fluorescent sensor was prepared by using a CdSe quantum dot (QD) as a signal transducer and a mesoporous silica nanoparticle as an imprinting material. Bisphenol A (BPA) was chosen as a model template, which is known as an endocrine disruptor. Binding sites were selectively formed between the pores and CdSe QDs were encapsulated in the pores of the mesoporous silica. QD-encapsulated, molecularly imprinted mesoporous silica particles (QD-MIMS) exhibited excellent molecular recognition properties in terms of both sensitivity and selectivity. Owing to the proximity of the binding sites to the QDs, a significant, concentration-sensitive fluorescence quenching was observed in the presence of BPA. QD-MIMS showed a linear Stern-Volmer relationship for BPA and its analogs. QD-MIMS had a much larger quenching constant for BPA (by more than ten times) than for BPA analogs, demonstrating the high selectivity of QD-MIMS. Secondly, a molecularly imprinted polymer-based fluorescent sensor was fabricated through an organogelation process. The sensor was comprised of a molecularly imprinted nanofiber as a receptor and a CdSe/ZnS quantum dot as a signal transducer. Histamine was selected as a model template. An organogelator with two different polymerizable groups, an acrylate and a diacetylene was successfully synthesized. As a functional monomer for complexation with the template, an acrylate having a carboxyl group was used. The QD and template-containing organogel formed in n-decane was polymerized in the presence of a photoinitiator and a cross-linker by UV irradiation to produce highly cross-linked organogel nanofibers. The template molecules were removed by extraction with methanol/acetic acid (9:1 v/v) to give the QD-incorporated, histamine imprinted organogel nanofibers (QD-HIOGNF). QD-HIOGNF showed high molecular recognition properties toward histamine in respects to both sensitivity and selectivity. The fluorescence intensity of QD-HIOGNF decreased sensitively as the concentration of histamine increased. QD-HIOGNF could be reused for sensing after removing the bound analytes. Thirdly, a facile and versatile sensing assay of diethylstilbestrol (DES) was developed by fabricating a molecularly imprinted fullerene-silica nanocomposites (MIFSNCs). Fullerene encapsulated in a microemulsion with the aid of the non-ionic surfactant was incorporated into the silica network by the sol-gel reaction of triethyl orthosilicate and a triethoxysilane-DES complex as silica precursors. MIFSNCs exhibited a fast kinetic binding and high molecular recognition properties in terms of both sensitivity and selectivity. MIFSNCs showed a notable fluorescence quenching under all given concentrations of DES. On the other hand, non-imprinted fullerene-silica nanocomposites (NIFSNCs) showed only a few amount of quenching for the concentrations of DES. Lastly, a molecularly imprinted mesoporous silica in which the tetraphenylethylene based AIE active chromophore was selectively introduced into the inner pore of the silica network was prepared. DES was chosen as a target molecule and connected to the triethoxysilane moieties via the thermally reversible urethane bonds. DES-selective imprinted cavities which have two-point binding sites were successfully formed between the pores of the silica framework by the sol-gel reaction and subsequent removal of DES. The AIE chromophore-grafted, DES imprinted mesoporous silica nanoparticles (TFPE-DIMS) showed a specific binding ability for the target template and a fast kinetic binding profile. The degree of fluorescence quenching of TFPE-DIMS was concentration-sensitive. The sensitivity and selectivity of TFPE-DIMS were estimated by the Stern-Volmer equation. TFPE-DIMS displayed a much larger Stern-Volmer quenching constant for DES than for DES analogs with a high molecular imprinting factor. TFPE-DIMS also showed a great recovery of its initial fluorescence intensity even after several extraction and rebinding cycles.