Nonspecific interactions of a carboxylate-substituted PPE with proteins. A cautionary tale for biosensor applications

Deposition of anionic conjugated poly(phenylenevinylene) onto silica nanoparticles via electrostatic interactions — Assembly and single-particle spectroscopy

Fluorescent nanoparticles were prepared via adsorption of the conjugated polyelectrolyte poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene] (MPS-PPV) onto 50 and 100 nm aminosilane functionalized silica beads. The particles were investigated via ensemble and single-molecule or -particle spectroscopy techniques to quantify the effect of the silica bead core on the exciton migration efficiency within the polymer. Ensemble emission spectra and ensemble fluorescence quenching studies with methyl viologen are consistent with good exciton migration along the polymer in the polymer-coated bead. The silica nanobead scaffolding preserves the sensitivity of the free polymer and provides a controllable architecture that minimizes nonspecific interactions. Single-particle spectroscopy studies were conducted on particles immobilized onto the positively charged surface of glass cover slips. Particle immobilization enabled us to monitor the effect of oxygen scavenger solutions on individual particles by changing the surrounding solution. The intensity–time trajectories of individual beads provide a mechanism of signal transduction with potential applications in multiplexing studies. Hundreds of individual beads can be imaged in a rapid parallel fashion.

Nanofibers doped with dendritic fluorophores for protein detection

We report a solid-state, nanofiber-based optical sensor for detecting proteins with an anionic fluorescent dendrimer (AFD). The AFD was encapsulated in cellulose acetate (CA) electrospun nanofibers, which were deacetylated to cellulose to generate secondary porous structures that are desirable for enhancing molecular interactions, and thus better signaling. The protein sensing properties of the fibers were characterized by monitoring the fluorescence response of cytochrome c (cyt c), hemoglobin (Hgb), and bovine serum albumin (BSA) as a function of concentration. Effective quenching was observed for the metalloproteins, cyt c and Hgb. The effect was primarily due to energy transfer of the imbedded fluorescent dendrimers to the protein, as both proteins contain heme portions. Electron transfer, caused through the electrostatic effects in the binding of the anionic dendrimer to the positive patches of globular proteins, could be responsible as well. BSA, on the other hand, triggered a "turn-on" response in fluorescence, suggesting the negatively charged BSA reduces the pi-pi stacking of the partially dispersed, negatively charged dendritic fluorophores through repulsion forces, which results in an increase in fluorescence. Stern-Volmer constants (K(sv)) of the electrospun fibers were found to be 3.4 x 10(5) and 1.7 x 10(6) M(-1) for cyt c and Hgb, respectively. The reusability of the nanofibers is excellent: the nanofibers demonstrated less than 15% change of fluorescence intensity signal in a 5-cycle test.

Anionic conjugated polymer with aptamer-functionalized silica nanoparticle for label-free naked-eye detection of lysozyme in protein mixtures

An assay triggered by recognition-induced charge switching is developed for protein detection and quantification. Aptamer-functionalized silica nanoparticles (NPs) have been synthesized to capture lysozyme, resulting in an alternation of the surface charge from negative to partially positive. The binding event is then translated and monitored by the fluorescence signal of a highly fluorescent anionic poly(fluorene-alt-vinylene) (PFVSO(3)), which "stains" on protein/aptamer-NP complexes via electrostatic interaction. Blue-greenish fluorescence of PFVSO(3) is observed in the presence of lysozyme by the naked eye, while no fluorescence is obtained for NPs upon treatment with a mixture of foreign proteins. A linear relationship between NP fluorescence and lysozyme is observed in the concentration range of 0-22.5 microg/mL, which gives a limit of detection as approximately 0.36 microg/mL. This work demonstrates a convenient label-free conjugated polyelectrolyte (CPE)-based protein detection with high specificity and sensitivity, which has potential applications in medical diagnosis.

Sensitive, selective and label-free protein detection using a smart polymeric transducer and aptamer/ligand system

We have demonstrated a smart polymeric transducer and aptamer/intercalating dye system that allows the label-free detection of protein with high sensitivity and selectivity.

Supramolecular assemblies of amphiphilic homopolymers

Amphiphilic molecules self-assemble in solvents because of the differential solvation of the hydrophilic and lipophilic functionalities. Small-molecule surfactants have long been known to form micelles in water that can solubilize lipophilic guest molecules in their water-excluded interior. Polymeric surfactants based on block copolymers are also known to form several types of aggregates in water owing either to the mutual incompatibility of the blocks or better solvation of one of the blocks by the solvent. Incorporating amphiphilicity at smaller length scales in polymers would provide an avenue to capture the interesting properties of macromolecules and fine tune their supramolecular assemblies. To address this issue, we designed and synthesized amphiphilic homopolymers containing hydrophilic and lipophilic functionalities in the monomer. Such a polymer can be imagined to be a string of small-molecule surfactants tethered together such that the hydrophilic and lipophilic functionalities are located on opposite faces, rendering the assemblies facially amphiphilic. This feature article describes the self-assembly of our amphiphilic homopolymers in polar and apolar solvents. These homopolymers not only form micelles in water but also form inverse micelles in organic solvents. Subtle changes to the molecular structure have been demonstrated to yield vesicles in water and inverted micelles in organic solvents. The characterization of these assemblies and their applications in separations, catalysis, and sensing are described here.

Recent conceptual and technological advances in polydiacetylene-based supramolecular chemosensors

Owing to the color (blue-to-red) and fluorescence (non-to-fluorescent) changes that take place in response to environmental perturbations, conjugated polydiacetylenes (PDAs) have been actively employed as sensory materials for the detection of biologically-, environmentally- and chemically-important target molecules. Until recently, the majority of PDA sensors have been prepared in the form of aqueous suspensions or Langmuir-type thin films on solid substrates. In order to overcome the limitations associated with conventional solution/film sensors, conceptually new formats, such as immobilized PDAs in and on solid substrates, microarrayed PDA sensors, microfluidic PDA sensors, as well as PDA-embedded electrospun fiber sensors and resonance energy transfer (RET)-based PDA sensors, have been developed recently. In this tutorial review, the recent conceptual and technological achievements made in the area of conjugated PDA chemosensors are described.

Synthesis and characterization of a water-soluble carboxylated polyfluorene and its fluorescence quenching by cationic quenchers and proteins

We have developed a new intermediate monomer, 2,7-[bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(3-(tert-butyl propanoate))]fluorene, that allows the easy synthesis of water-soluble carboxylated polyfluorenes. As an example, poly[9,9'-bis(3''-propanoate)fluoren-2,7-yl] sodium salt was synthesized by the Suzuki coupling reaction, and the properties of the polymer were studied in aqueous solutions of different pH. Fluorescence quenching of the polymer by different cationic quenchers (MV(2+), MV(4+), and NO(2)MV(2+); MV=methyl viologen) was studied, and the quenching constants were found to be dependent on the charge and electron affinity of the quencher molecule and the pH of the medium. The largest quenching constant was observed to be 1.39 x 10(8) M(-1) for NO(2)MV(2+) at pH 7. The change in polymer fluorescence upon interaction with different proteins was also studied. Strong fluorescence quenching of the polymer was observed in the presence of cytochrome c, whereas weak quenching was observed in the presence of myoglobin and bovine serum albumin. Lysozyme quenched the polymer emission at low protein concentrations, and the quenching became saturated at high protein concentrations. Under similar experimental conditions, the polymer showed improved quenching efficiencies toward cationic quenchers and a more selective response to proteins relative to other carboxylated conjugated polymers.

Fluorogenic polydiacetylene supramolecules: immobilization, micropatterning, and application to label-free chemosensors

This Account describes a new strategy for the preparation of label-free sensor systems based on the fluorogenic properties of the conjugated polymer, polydiacetylene (PDA). PDA has been extensively investigated as a sensor matrix, owing to a brilliant blue-to-red color transition that takes place in response to environmental perturbations. It has been known for some time that "blue-phase" PDAs are nonfluorescent while their "red-phase" counterparts fluoresce. For the most part, however, the significance of the different fluorogenic properties of PDAs has been ignored in the context of sensor applications. In the course of developing PDA-based sensors, we discovered that PDA vesicles can be readily immobilized on solid substrates. This is an attractive property of PDAs since it leads to the combined advantages of the vesicle sensors (which have three-dimensional interactions between sensor and target molecules) and film sensors (which are applicable to a two-dimensional array or chip format). Stable blue-phase immobilized PDAs can be prepared by employing one of three strategies involving the formation of covalent adducts, biotin-avidin complexes, or complexes formed through nonspecific physical adsorption. A procedure for generating well-patterned fluorescence images is necessary for the immobilized PDAs to function in chip-based sensor systems. Patterned fluorescence images are readily constructed by employing (1) the photolithographic technique, (2) the micromolding in capillaries (MIMIC) method, or (3) an array spotting system. Heat treatment of the patterned "blue-phase" PDA vesicles transforms the nonfluorescent images into their fluorescent red forms. The observation that finely resolved fluorescence patterns can be generated by heat treatment of microarrayed PDAs is highly significant in that it indicates that fluorescence signals might be produced by specific molecular recognition events. Indeed, red fluorescence emission is observed when immobilized PDAs are subjected to specific molecular recognition events, such as ligand--cyclodextrin or protein-protein interactions. The facile immobilization of PDA vesicles on solid substrates and the affinity-induced fluorescence emission combine to make this system applicable to the fabrication of label-free PDA sensors. Since in theory any molecular recognition event that promotes the blue-to-red color transition of PDAs should result in the generation of fluorescence, it should be possible to reformat a variety of previously described colorimetric PDA sensors into fluorescence-based sensor systems. The fluorescence properties of PDAs, when combined with modern methods for the fabrication of microarrays, should stimulate the development of a number of new label-free chemosensor systems.

Amplified fluorescence quenching in high ionic strength media

We report a new cationic poly(phenylene ethynylene) (PPE) derivative that exhibits strong amplified fluorescence quenching in the presence of electron-deficient species, yielding high Stern-Volmer coefficients of 4.7 × 10 M in aqueous solutions. Importantly, with the addition of appropriate non-ionic surfactants, the polymer is found to retain its excellent sensitivity even when transferred to high ionic strength buffered media, which have previously been shown to suppress the amplified quenching effect in other polyelectrolyte systems. The cationic PPE derivative yields Stern-Volmer coefficients as high as 10 M in 25 mM buffer solutions of both tris(hydroxymethyl) aminomethane (Tris) and sodium acetate containing 150 mM sodium chloride, the optimal conditions for many enzymes such as phosphatases. The ability to maintain high Stern-Volmer coefficients in high ionic strength buffered media extends the applicability of ionic conjugated polymers to high sensitivity detection in biological media, and thus greatly enhances their versatility as biological sensors.