We demonstrate a compact, low cost and practical fluorescence detection system for lab-on-a-chip applications. The system comprises a commercially available InGaN light emitting diode (501 nm) as light source, an organic or silicon photodiode detector, absorptive dye coated colour filters and linear and reflective polarisers. An injection moulded polystyrene microfluidic chip is used as the platform for fluorescence immunoassays for cardiac markers myoglobin and CK-MB. The optical limit of detection (LOD) is measured using a TransFluoSphere® suspension at 5.6 × 10(4) beads µl(-1) which can be equated to ∼3 nM fluorescein equivalent concentration. The LOD for the human plasma immunoassays is measured as 1.5 ng ml(-1) for both myoglobin and CK-MB.

Materials in which sub‐wavelength physical structures, rather than variations in chemical composition, are used to modify the nature of their interaction with electromagnetic radiation form the promising new class of metamaterials. For molecular materials one has an intriguing alternative, namely structuring the conformation or physical geometry of the molecule. In order for this to be an effective methodology one needs the change in conformation i) to engender a significant change in electromagnetic properties and ii) to be spatially controllable to allow patterning of practical structures. In this paper the potential of such an approach is demonstrated through spatial patterning, via masked solvent vapor exposure, of the β‐phase conformation in poly(9,9‐dioctylfluorene) (PFO). Significantly the conformation change approach preserves a planar film format and is found not to negatively impact on optical gain properties, both very attractive features for optoelectronic and photonic lightwave circuit applications. As a specific demonstration the ability to spatially control the lasing wavelength for samples in which a β‐phase conformation is selectively patterned in a glassy PFO film spin coated atop a one‐dimensional distributed‐feedback grating etched into a spectrosil substrate is shown.

We report an investigation of the effect of morphology on the gain properties of poly(9,9-dioctylfluorene) (PFO). The PFO β-phase morphology has previously been reported to be detrimental to lasing threshold, a conclusion at odds, however, with pump-probe measurements on PFO/poly(methylmethacrylate) blend films that show enhanced stimulated emission characteristics for β-phase chains. In order to understand these conflicting indications, we have undertaken a detailed study of the gain properties for spin-coated PFO films, including samples in which the fraction of β-phase chains is deliberately enhanced by post-deposition exposure to toluene vapour. We find that the amplified spontaneous emission (ASE) threshold (390 nm pump, 10 ns pulses, 10 Hz repetition rate) is of order 80 nJ/pulse, independent of the presence of a significant β-phase component. Surface emitting distributed feedback lasers comprising polymer-coated second-order gratings etched into silica substrates are also insensitive to the β-phase morphology: lasing threshold energies are equivalent so long as the structures are tuned to the correct peak gain wavelength for each film morphology. This occurs at the 0-1 vibronic peak in the corresponding photoluminescence emission spectra, namely 465 nm for films with and 450 nm for films without a significant β-phase component. We can thus conclude that whilst the introduction of β-phase chains leads to new lasing wavelengths (some 15 nm red shifted from those for films without β-phase chains) it is not obviously detrimental to lasing performance. An additional effect does occur, however, when the pump beam energy is increased by one to two orders of magnitude above the ASE threshold energy: the ASE peak position for the β-phase films then migrates from 465 to 450 nm. This phenomenon is irreversible and appears to be the result of exciton quenching on β-phase chains due to the photo-oxidative formation of fluorenone moieties.

The focus of this work is to study the refractive index in thin films of blue lightemitting poly(9,9-dioctylfluorene) (PFO) that contain extended rigid chains (βphase). Using a post-deposition exposure to toluene vapour, β-phase chains were induced in glassy PFO films that had been spin coated from a range of solution concentrations to provide thicknesses from 50-200 nm. With the aid of absorption spectroscopy, a semi-empirical calculation of the change in refractive index due to the presence of β-phase chains is undertaken. Over the spectral region where optical gain occurs, the refractive index of β-phase PFO is found to be appreciably larger than for as-spin-coated glassy PFO. Further development and explicit evaluation of the Kramers-Kronig relations lead to analytic and closed form expressions describing the change in index n which are used to highlight the key contributions to index change. We find that the spectral response of n is largely governed by the emergence of a characteristic absorption band associated with the presence of β-phase chains. The changes in refractive index are experimentally studied by tracking the spectral shifts of optical resonances observed from films spin coated directly onto sub-wavelength 1D grating structures. These measurements are supported and compared with rigorous calculations of the spectra, which incorporate the full optical descriptions for glassy PFO and β-phase PFO. The excellent agreement confirms the accuracy of our semi-empirical extraction approach. Similar polymer-coated grating structures are examined under lasing conditions and can also be used to support the expected spectral response and the decline of n for longer wavelengths.

A bulk-heterojunction (BHJ) structure of organic semiconductor blend is widely used in photon-to-electron converting devices such as organic photodetectors (OPD) and photovoltaics (OPV). However, the impact of the molecular structure on the interfacial electronic states and optoelectronic properties of the constituent organic semiconductors is still unclear, limiting further development of these devices for commercialization. Herein, the critical role of donor molecular structure on OPD performance is identified in highly intermixed BHJ blends containing a small-molecule donor and C60 acceptor. Blending introduces a twisted structure in the donor molecule and a strong coupling between donor and acceptor molecules. This results in ultrafast exciton separation (<1 ps), producing bound (binding energy ∼135 meV), localized (∼0.9 nm), and highly emissive interfacial charge transfer (CT) states. These interfacial CT states undergo efficient dissociation under an applied electric field, leading to highly efficient OPDs in reverse bias but poor OPVs. Further structural twisting and molecular-scale aggregation of the donor molecules occur in blends upon thermal annealing just above the transition temperature of 150 °C at which donor molecules start to reorganize themselves without any apparent macroscopic phase-segregation. These subtle structural changes lead to significant improvements in charge transport and OPD performance, yielding ultralow dark currents (∼10–10 A cm–2), 2-fold faster charge extraction (in μs), and nearly an order of magnitude increase in effective carrier mobility. Our results provide molecular insights into high-performance OPDs by identifying the role of subtle molecular structural changes on device performance and highlight key differences in the design of BHJ blends for OPD and OPV devices.