Li Juan Fan

Construction of Response Patterns for Metal Cations by Using a Fluorescent Conjugated Polymer Sensor Array from Parallel Combinatorial Synthesis

Pattern-based strategy is an emerging field of interest for effective sensing applications. Seven different conjugated polymers from combinatorial synthesis were combined into a sensor array, and seven metal cations were selected as representative analytes. The response patterns for each cation were constructed by collecting the individual fluorescence responses from seven polymers in the array. Each ion owns a characteristic pattern. Some of them have similar modes of response with subtle differences, while some patterns are distinctively different. The family/period the metal cations belong to and the charges/electronic configurations they possess may account for such similarity/difference in the pattern.

Polydiacetylene-Polymethylmethacrylate/Graphene Composites as One-Shot, Visually Observable, and Semiquantative Electrical Current Sensing Materials

Aiming to develop pH-paper-like current sensing materials, we prepared irreversible electrochromic PDA-PMMA/graphene composites. The composites exhibited an excellent linear relationship between critical responsive currents and the amount of graphene in the system. In these composites, PDA acted as the electrochromic material and graphene as the conductive matrix. The presence of PMMA not only ensured mechanical performance but also made the color change more obvious to be observed by the naked eye.

Fluorescence Development of Latent Fingerprint with Conjugated Polymer Nanoparticles in Aqueous Colloidal Solution

Poly(p-phenylenevinylene) (PPV) nanoparticles in aqueous colloidal solution have been prepared via a modified Wessling method, with the addition of surfactant. The fluorescent colloidal solution was used as the developing solution to develop the fingerprints on different substrates. The developing process was accomplished simply by immersing the substrates into developing solution and then taking out, followed by rinsing with deionized water. The initial study about the fingerprints on the adhesive tapes showed that the developing solution is very effective in fluorescence development on both fresh and aged visible fingerprints; and such an effect was negligibly affected by treating the fingerprints with water or other organic solvents, whether before developing or after. Further study on latent fingerprints (LFPs) demonstrated that PPV nanoparticles in colloidal solution have high sensitivity in developing fingerprints to give very clearly fluorescent patterns. At least 6 months of storage of the colloidal solution did not reduce the developing effect; and each developing solution (3.6 mg/mL, 5.0 mL) can be used to develop at least 30 fingerprints without sacrificing the legibility of the pattern. The preliminary mechanism investigation suggested that selectivity achieved toward the ridge of the fingerprint is very likely due to the affinity between PPV molecules and oily secretions of the fingerprints. Digital magnification of the developed fingerprints provided more details about the fingerprint.

Preparation of electrospun luminescent polyimide/europium nanofibers by simultaneous in situ sol–gel and imidization processes

Luminescent polyimide (PI)/europium nanofibers have been successfully prepared by electrospinning combined with an in situ sol-gel technique. The possible reaction mechanism of the simultaneous imidization of polyamide acid and gelation of europium phase was analyzed by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). The results showed that chemical coupling and noncovalent interaction existed between the polymer and the europium which formed during the preparative process. Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and scanning electronic spectroscopy (SEM) studies also indicated the successful incorporation of europium into PI matrix and various morphologies could be achieved by controlling the calcination temperature and the europium content. Nanofibers with necklace-like structures were obtained after calcination under high temperatures. These PI/europium nanofibers were further demonstrated to have strong fluorescence emission. The intensity ratio for the PI/europium nanofibers, labeled as ((5)D(0)→(7)F(2))/((5)D(0)→(7)F(1)), which is well known as the asymmetry parameter, was lower than that in pure Eu(2)O(3) powder, indicating that there were highly symmetric coordination spheres around europium in the nanofibers.

Facile preparation of polydiacetylene-based uniform porous fluorescent microspheres for potential immunoassay applications

Fluorescent microspheres are prepared by attaching self-assembled polydiacetylene (PDA) vesicles with carboxyl side groups onto the substrate amino-modified poly(glycidylmethacrylate) (APGMA) microspheres. The characterization by SEM, confocal microscopy and flow cytometry demonstrated that the final resulting microspheres are highly uniform both in size (with a diameter of 5 μm) and in fluorescence emission (coefficient of variance < 3%). The Brunauer-Emmett-Teller (BET) surface area of these spheres is 114 m2 g-1. In addition, there are evenly distributed pores with an average size of 20.6 nm on the spheres. These spheres are found to have good thermal stability and photostability, and do not suffer from fluorophore leaching. Fluorescein isothiocyanate (FITC) labelled bovine serum albumin (BSA) as a representative biomolecule can be easily attached onto the fluorescent microspheres. All these characteristics possessed by the APGMA-PDA spheres allow them to be directly used as carriers of biomolecules in lab-on-a-chip immunoassay systems.

Preparation of cross-linked, multilayer-coated fluorescent microspheres with functional groups on the surface for bioconjugation.

This study is to develop a method for preparing fluorescent microspheres with steady and strong fluorescence as well as the surface functionality for bioconjugation. Layer-by-layer technique was employed to introduce poly(phenylenevinylene) (PPV) precursor, diazoresin (DAR), and polyanion, including poly(acrylic acid) (PAA) and poly(sodium-p-styrenesulfonate) (PSS), onto the substrate polystyrene-divinylbenzene microspheres with sulfonic groups on the surface (SPSDVB). The conversion of PPV precursor into fluorescent PPV as well as the cross-linking reaction between DAR and polyanion, were accomplished simultaneously in the following thermal treatment. After optimizing the DAR concentration, the selection of polyelectrolytes and the coating sequence, the cross-linked multilayer coated PPV microspheres, SPSDVB-(PPV/PSS/DAR/PAA) spheres, were prepared. These spheres were found to have uniform size with a clear core-shell structure and display even and strong fluorescence, based on the characterization by flow cytometry, microscopy, and photophysics. They were found to be stable and highly resistant to common solvents and even dissociation agent, as well as possess good thermal stability and photostability. The feasibility of conjugating biomolecules on the surface of spheres was also demonstrated.

Realization of fluorescence color tuning for poly(p-phenylenevinylene) coated microspheres via a heterogeneous catalytic thermal elimination process

Poly(p-phenylenevinylene) (PPV) fluorescent microspheres were prepared in two steps. First, the positive sulfonium-salt PPV precursor (pre-PPV) was coated onto the surface of negatively charged polymer substrate spheres; second, the pre-PPV was converted into fluorescent PPV via a heterogeneous catalytic thermal elimination process. A series of fluorescent microspheres were obtained with different apparent colors and different fluorescence emissions simply by varying the elimination temperature. A spectroscopy study showed that, compared to the direct solid elimination, the spheres obtained via catalyzed elimination gave a very large variation in the emission, such as a larger shift in the wavelength, and more delicate spectra profiles with peaks and shoulders. Overlapping of the signals from blue channels and green channels in the confocal microscopy study gave a direct view of the gradual change from the blue emission to the green emission for the spheres obtained at elimination temperatures from 40 °C to 120 °C. Flow cytometry measurements showed that spheres obtained at different temperatures exhibit different combinations of the intensities from four different receiving channels. These PPV spheres were also demonstrated to have a smooth surface, monodispersity, a clear core–shell structure, thermal stability and photostability.

Preparation of Fluorescent Conjugated Polymer Fibrous Membranes for Rapid Recognition of Aromatic Solvents

Fluorescent poly(phenylenevinylene) (PPV)/poly(vinyl alcohol) (PVA) fibrous membrane was prepared via electrospinning of PPV precursor and PVA aqueous solution followed by thermal elimination. Further cross-linking produced the cross-linked membrane PPV/CPVA. Both PPV/PVA and PPV/CPVA membranes were found to have similar morphology and photophysics. These membranes showed a great fluorescence quenching response to aromatic solvents and a much smaller response to other organic solvents. Water also effectively quenched the fluorescence of PPV/PVA but not that of PPV/CPVA. This was attributed to un-cross-linked PVA being able to dissolve in water and the cross-linking improving the resistance of the membrane toward water. The sensing behavior was found to have good reversibility. The contact angle study showed that addition of only about 1% of PPV into the matrix reduced the hydrophilicity of the membrane significantly, suggesting that the PPV chains would be located at the surface of the fibers. X-ray photoelectron spectroscopy (XPS) investigation further confirmed such surface enrichment of PPV in the binary polymer blends. The PPV chain on the surface facilitated the π-π interaction between the polymer backbones and the aromatic molecules, thus leading to good selectivity and fast response of the two fibrous membranes toward aromatic solvents.

Layer-by-layer introduction of poly(phenylenevinylene) onto microspheres and probing the influence from the weak/strong polyanion spacer-layers.

The layer-by-layer (LBL) technique was employed for preparing fluorescent microspheres with a core-shell structure by the alternating adsorption of positively charged poly(p-phenylenevinylene) precursor (pre-PPV) and the polyanions onto polymer substrate spheres, followed by the thermal elimination to convert pre-PPV into fluorescent poly(p-phenylenevinylene) (PPV). Weak polyelectrolytes poly(acrylic acid) (PAA) (usually in a partly ionized form) and strong polyelectrolytes poly(sodium-p-styrenesulfonate) (PSS) were used as the anions to space the PPV layers and reduce the fluorescence self-quenching. Flow cytometry, combined with spectroscopy and microscopy, were used to study the structure and photophysical properties of the resulting microspheres. Optimization of the processing factors was carried out. PAA and PSS as weak and strong polyelectrolytes, respectively, displayed very different influence on the final emission of the spheres. Such difference was attributed to different inherent characteristics of PAA and PSS after detailed investigation in many aspects. In addition, the fluorescent spheres were found to have excellent photostability and thermal stability.

Color Tuning of Core-Shell Fluorescent Microspheres by Controlling the Conjugation of Poly(p-phenylenevinylene) Backbone.

A series of poly(p-phenylenevinylene) (PPV)-coated microspheres with varied fluorescent emission colors have been prepared by controlling the average length of the conjugated segments on the polymer backbone. A modified Wessling method was used for preparing PPV with different conjugation segments. The labile sulfonium groups of the initial polymer precursor of PPV (pre-PPV) were partly substituted by relatively stable methoxyl groups. A series of precursors with different degrees of substitution were prepared by controlling the time of reaction; these precursors were adsorbed onto the negatively charged substrate spheres. Subsequently, heterogeneous thermal treatment eliminated the sulfonium groups selectively to form the conjugated segments on the PPV backbone with varied average conjugation lengths. Under UV exposure, the as-prepared PPV-coated microspheres displayed emission colors ranging from blue to green; a 65 nm shift in the emission maximum was observed in the fluorescence spectra. The gradual color change in emission of spheres was also confirmed in a confocal microscopy study. Further characterizations indicated that these microspheres possessed clear core-shell structure, good monodispersity in size, smooth surfaces, uniform emission, and superior thermal and photo stability. Flow cytometry measurements indicated that these spheres have very different patterns of intensity combination from four-signal receiving channels. The simple method reported herein, which can effectively and efficiently tune the emission color of the fluorescent microspheres, is a promising approach for preparation of microspheres used as encoded signal carrier in flow cytometry and other high-throughput techniques.