There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The field of chemical sensing is becoming ever more dependent upon novel materials. Polymers, crystals, glasses, particles, and nanostructures have made a profound impact and have endowed modern sensory systems with superior performance. Electronic polymers have emerged as one of the most important classes of transduction materials; they readily transform a chemical signal into an easily measured electrical or optical event. Although our group reviewed this field in 2000,1 the high levels of activity and the impact of these methods now justify a subsequent review as part of this special issue. In this review we restrict our discussions to purely fluorescence-based methods using conjugated polymers (CPs). We further confine our detailed coverage to articles published since our previous review and will only detail earlier research in this Introduction to illustrate fundamental concepts and terminology that underpin the recent literature.

Chemical sensors based on amplifying fluorescent conjugated polymers.

When considering new sensory technologies one should look to nature for guidance. Indeed, living organisms have developed the ultimate chemical sensors. Many insects can detect chemical signals with perfect specificity and incredible sensitivity. Mammalian olfaction is based on an array of less discriminating sensors and a memorized response pattern to identify a unique odor. It is important to recognize that the extraordinary sensory performance of biological systems does not originate from a single element. In actuality, their performance is derived from a completely interactive system wherein the receptor is served by analyte delivery and removal mechanisms, selectivity is derived from receptors, and sensitivity is the result of analyte-triggered biochemical cascades. Clearly, optimal artificial sensory sys-

Conjugated polymer-based chemical sensors.

Processable, single-layered graphene oxide (GO) is an intriguing nanomaterial with tremendous potential for electronic applications. We spin-coated GO thin-films on quartz and characterized their sheet resistance and optical transparency using different reduction treatments. A thermal graphitization procedure was most effective, producing films with sheet resistances as low as 102 −103 Ω/square with 80% transmittance for 550 nm light. Our experiments demonstrate solution-processed GO films have potential as transparent electrodes.

Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

We report two viable organic excitonic solar cell structures where the conventional In2O3:Sn (ITO) hole-collecting electrode was replaced by a thin single-walled carbon nanotube layer. The first structure includes poly(3,4-ethylenedioxythiophene) (PEDOT) and gave a nonoptimized device efficiency of 1.5%. The second did not use PEDOT as a hole selective contact and had an efficiency of 0.47%. The strong rectifying behavior of the device shows that nanotubes are selective for holes and are not efficient recombination sites. The reported excitonic solar cell, produced without ITO and PEDOT, is an important step towards a fully printable solar cell.

/pdf/organic-solar-cells-with-carbon-nanotubes-replacing-in2o3-sn-3lf1gfzq5o.pdf

Organic solar cells with carbon nanotubes replacing In2O3:Sn as the transparent electrode

The photoconductivity of films of single-wall carbon nanotubes has been studied under continuous-wave near-infrared illumination. The photocurrent exhibits a linear response with the light intensity and with bias voltage up to 5 V. The temporal photoresponse of on/off step illumination shows a relatively slow relaxation time (4.3 s for films with a thickness of ∼500 nm), which can be interpreted in terms of a kinetic model that takes into account the binding of photoelectrons with adsorbed oxygen. Possible applications of this photosensitive material are discussed.

Photoconductivity of single-wall carbon nanotubes under continuous-wave near-infrared illumination

The photophysical and energy transport properties of a poly(p-ethynylene), 1, were investigated in thin films. Highly aligned films of a precise thickness, prepared by sequential monolayer deposition using the Langmuir−Blodgett technique, were surface modified with luminescent traps (acridine orange, AO) for energy transfer studies. The degree of energy transfer to the traps was investigated as a function of the AO concentration and the number of polymer layers. An increased efficiency of energy transfer to the traps was observed with increasing numbers of layers until an approximate thickness of 16 layers. This behavior is consistent with a transition to a three-dimensional energy migration topology. A phenomenological model for the transport was proposed, and solutions were obtained by numerical methods. The model yields a fast (>6 × 1011 s-1) rate of energy transfer between polymer layers and a diffusion length of more than 100 A in the Z direction (normal to the film surface).

/pdf/energy-migration-in-a-poly-phenylene-ethynylene-4438lznt00.pdf

Energy Migration in a Poly(phenylene ethynylene): Determination of Interpolymer Transport in Anisotropic Langmuir−Blodgett Films

The present invention relates to a class of luminescent and conductive polymer compositions having chromophores, and particularly solid films of these compositions exhibiting increased luminescent lifetimes, quantum yields and amplified emissions. These desirable properties can be provided through polymers having rigid groups designed to prevent polymer reorganization, aggregation or π-stacking upon solidification. These polymers can also display an unusually high stability with respect to solvent and heat exposures. The invention also relates to a sensor and a method for sensing an analyte through the luminescent and conductive properties of these polymers. Analytes can be sensed by activation of a chromophore at a polymer surface. Analytes include aromatics, phosphate ester groups and in particular explosives and chemical warfare agents in a gaseous state. The present invention also relates to devices and methods for amplifying emissions by incorporating a polymer having an energy migration pathway and/or providing the polymer as a block co-polymer or as a multi-layer.

Emissive polymers and devices incorporating these polymers

The solvatochromic dye Nile Red dispersed in selected hydrogen bond acidic polymer matrixes demonstrated strong fluorescence enhancement at the presence of dimethyl methylphosphonate (DMMP) vapors. Two hydrogen bond acidic polymers were examined as dye matrixes, one with fluorinated alcohol groups on a polystyrene backbone (PSFA) and the other with fluorinated bisphenol groups alternating with oligo(dimethylsiloxane) segments (BSP3). The combination of hydrogen bond acidic polymer (a strong sorbent for DMMP) with the solvatochromic dye led to initial depression of the dye fluorescence and a significant red shift in the absorbance and fluorescence spectra. DMMP sorption changed the dye environment and dramatically altered the fluorescence spectrum and intensity, resulting in a strong fluorescence enhancement. It is proposed that this fluorescence enhancement is due to the competition set up between the dye and the sorbed vapor for polymeric hydrogen-bonding sites. The highest responses were obtained with B...

Igor A. Levitsky

Papers

Organic solar cells with carbon nanotubes replacing In2O3:Sn as the transparent electrode

Photoconductivity of single-wall carbon nanotubes under continuous-wave near-infrared illumination

Energy Migration in a Poly(phenylene ethynylene): Determination of Interpolymer Transport in Anisotropic Langmuir−Blodgett Films

Emissive polymers and devices incorporating these polymers

Rational design of a Nile Red/polymer composite film for fluorescence sensing of organophosphonate vapors using hydrogen bond acidic polymers.