Daigeun Kim

Highly Selective Cysteine Detection and Bioimaging in Zebrafish through Emission Color Change of Water-Soluble Conjugated Polymer-Based Assay Complex

A new concept for rapid, label-free cysteine sensing method is proposed via possible naked eye-detection of red-to-blue emission color change. Intermolecular exciton migration in conjugated polyelectrolyte-based assay complex is adopted to enhance selectivity and sensitivity for cysteine sensing by formation and dissociation of polymer-Hg(2+)-thymine assay complex in the absence and presence of cysteine, respectively. The assay complex shows red emission due to cooperative aggregation of conjugated polyelectrolyte, thymine, and Hg(2+). Upon exposure to cysteine, the assay complex dissociates into individual molecules showing transparent, blue-emitting solution, because cysteine extracts Hg(2+) from the assay complex via more favorable binding between cysteine and Hg(2+).

Simultaneous Detection and Removal of Mercury Ions in Aqueous Solution with Fluorescent Conjugated Polymer-Based Sensor Ensemble

A water-soluble, sulfur-containing fluorescent conjugated polymer exhibits a visible fluorescence color change for detection of mercury in the presence of thymine. A new concept provides the design of a sensor ensemble using a simple combination method. This strategy avoids the need for complicated design and synthesis of a recognition group, eliminating the tedious synthetic efforts for the preparation of a sensor material.

Conjugated Poly(fluorene-quinoxaline) for Fluorescence Imaging and Chemical Detection of Nerve Agents with Its Paper-Based Strip

Conjugated polymer of poly(fluorene-co-quinoxaline) was synthesized via Suzuki coupling polymerization. The emission color of the polymer can be tuned depending on the concentration of the polymer in solution. A low-energy bandgap is observed both in the concentrated solution and in the solid state, caused by aggregation of the polymer chains, resulting in long wavelength emission from the quinoxaline moiety, while short wavelength emission can be seen in diluted, well-dissolved solution. The presence of quinoxaline units enables us to demonstrate fluorescence switching and imaging. Paper-based strips containing the polymer are prepared via simple immersion of filter paper in the polymer solution for practical use in the detection of nerve agents. The emission of the paper-based strip is quenched upon exposure to diethyl chlorophosphate (DCP), a nerve agent simulant, and the initial emission intensity can be almost restored by treatment with aqueous sodium hydroxide solution, making a possible reversible paper-based sensor.

Cobalt Ion‐Mediated Cysteine Detection With a Hyperbranched Conjugated Polyelectrolyte as a New Sensing Platform

A highly efficient colorimetric and fluorescence turn-off probe for the sensitive and selective detection of the biologically important amino acid, cysteine (Cys), is demonstrated using a newly synthesized water-soluble hyperbranched polymer (HP) containing sulfonic acid groups. The detection mechanism involves two steps: (i) the slight quenching of HP in the presence of Co(2+) in advance; and (ii) the gradual quenching of the HP-Co(2+) complex according to the concentration of Cys due to the absorption screening effect of the formation of the Cys-Co(2+) complex, which prevents HP from absorbing excitation energy.

Conjugated Polymer Dots-on-Electrospun Fibers as a Fluorescent Nanofibrous Sensor for Nerve Gas Stimulant

A novel chemical warfare agent sensor based on conjugated polymer dots (CPdots) immobilized on the surface of poly(vinyl alcohol) (PVA)-silica nanofibers was prepared with a dots-on-fibers (DoF) hybrid nanostructure via simple electrospinning and subsequent immobilization processes. We synthesized a polyquinoxaline (PQ)-based CP as a highly emissive sensing probe and employed PVA-silica as a host polymer for the elctrospun fibers. It was demonstrated that the CPdots and amine-functionalized electrospun PVA-silica nanofibers interacted via an electrostatic interaction, which was stable under prolonged mechanical force. Because the CPdots were located on the surface of the nanofibers, the highly emissive properties of the CPdots could be maintained and even enhanced, leading to a sensitive turn-off detection protocol for chemical warfare agents. The prepared fluorescent DoF hybrid was quenched in the presence of a chemical warfare agent simulant, due to the electron transfer between the quinoxaline group in the polymer and the organophosphorous simulant. The detection time was almost instantaneous, and a very low limit of detection was observed (∼1.25 × 10(-6) M) with selectivity over other organophosphorous compounds. The DoF hybrid nanomaterial can be developed as a rapid, practical, portable, and stable chemical warfare agent-detecting system and, moreover, can find further applications in other sensing systems simply by changing the probe dots immobilized on the surface of nanofibers.

Aggregation-deaggregation-triggered, tunable fluorescence of an assay ensemble composed of anionic conjugated polymer and polypeptides by enzymatic catalysis of trypsin.

We prepared a water-soluble conjugated polymer composed of electron-donating units and electron-accepting groups in the backbone. The polymer exhibits a short wavelength (blue) emission in aqueous solution and long wavelength (red) emission in the solid state, because of intermolecular energy transfer. Considering this, we develop a new approach for the sensitive detection of trypsin, which is known to control pancreatic exocrine function, using an ensemble system composed of the anionically charged conjugated polymer and cationically charged polypeptides (such as polylysine and polyarginine). The blue-emitting, water-soluble conjugated polymer becomes aggregated upon exposure to the polypeptides, leading to a red-emitting assay ensemble. The red-emitting assay ensemble becomes dissociated in the conjugated polymer and polypeptide fragments by selective degradation of trypsin, which then exhibits recovery of blue emission. This emission-tuning assay ensemble allows for detection of trypsin at nanomolar concentrations, which enables naked-eye detection. Importantly, this strategy can be employed for label-free, continuous assay for trypsin.

Photoswitchable Emission Color Change in Nanodots Containing Conjugated Polymer and Photochrome

A simple approach for the preparation of conjugated polymer (CP)-based fluorescent nanodots containing photochrome (dithienylethene, DTE) is reported. The CP in the nanodots was designed to exhibit dual emissions of blue and green. The photochromic, fluorescent, composite nanodots (PNDs) were able to tune the emission color from green to blue using selective energy transfer from the CP to DTE under ultraviolet (UV) irradiation. The UV-irradiation-induced ring closure of the DTE within the PNDs provided a spectral overlap between the green emission of the CP and the absorption of DTE, leading to quenching of the green emission and, concomitantly, maintaining of the blue emission. The photoswitchable fluorescent PNDs with high on-off green fluorescence contrast were successfully applied in a living zebrafish imaging. Our design strategy provided a versatile tool for constructing a special photomodulated color-changeable nanostructure in bioimaging.

Synthesis of triphenylamine-containing conjugated polyelectrolyte and fabrication of fluorescence color-changeable, paper-based sensor strips for biothiol detection

A new water-soluble conjugated polyelectrolyte containing triphenylamine groups with aldehyde pendants was synthesized, which featured distinctly different emission colors according to its states, in aqueous solution and in the solid. Paper-based strips containing the polymer were prepared by simple immersion of filter paper in the polyelectrolyte solution for practical and efficient detection of biothiols including cysteine and homocysteine. The presence of aldehyde groups enables us to demonstrate noticeable fluorescence emission color changes (green-to-blue) because of the alterations in electron push–pull structure in the polymer via a reaction between the aldehyde group of the polymer and the aminothiol moiety in biothiol compounds. The presence of an aldehyde group and a sulfonate side chain was found to be indispensable for the cysteine reaction site and for a hydrophilic environment allowing the easy approach of cysteine, respectively, resulting in a simple and easy detection protocol for biothiol compounds.

Simultaneous detection and removal of radioisotopes with modified alginate beads containing an azo-based probe using RGB coordinates.

We prepared alginate beads that were modified with an azo-based probe molecule to monitor simultaneously the removal (by alginate) and probing (by the azo-probe molecule) of radioisotopes such as cobalt, strontium, and cesium ions. As an azo-probe molecule, Basic Orange 2 (BO2) was immobilized to the alginate bead. The BO2 in aqueous solution exhibited a slight red shift in absorption with a change in color from orange to dark orange upon addition of cobalt and strontium ions. In contrast, the color of BO2 did not change upon exposure to cesium ions. Thus, the covalently embedded BO2 in alginate beads could adsorb cobalt and strontium ions resulting in recognizable color change of the beads, which was induced by the formation of a complex between BO2 and metal ions. The color changes of the beads in the presence of metal ions were determined quantitatively using RGB color coordinate values. In addition to effectively removing metal ions, the colorimetric coordinate method provides a convenient and simple sensing technique for naked-eye metal ion detection.

Colorimetric detection and removal of radioactive Co ions using sodium alginate-based composite beads

We demonstrate a simple method for the visual determination and removal of Co ions using a bead-shaped, capturing probe based on hybridized sodium alginate. For Co ions, the designed protocol consisted of three main constituents: an azopyridine-based Co ion-probe for visual detection; sodium alginate as an adsorbent for the Co ion and a bead construct for removal and structure; silica as a linker for the probe and the alginate, leading to a robust structure. When the composite beads were exposed to Co ions, the yellow color of the beads turned to intensive violet and the color intensity was associated with the Co ion concentration. The color variation was quantified using red-green-blue (RGB) color values that were obtained with a scanner and evaluated with Photoshop. The technique achieved both visual recognition with obvious color change of the beads and efficient removal of the radioactive 60Co ion. The sensing and removal of any radioactive isotope could be achieved with an appropriate sensing probe, to provide a simple and universal platform for remediation.