John C. deMello

Microfluidic routes to the controlled production of nanoparticlesElectronic supplementary information ESI available: image of the central portion of the micromixer chip. See http://www.rsc.org/suppdata/cc/b2/b202998g/

A microfluidic procedure for the controlled production of cadmium sulfide nanoparticles is described.

Monolithically integrated dye-doped PDMS long-pass filters for disposable on-chip fluorescence detection

We report the fabrication of high quality monolithically integrated optical long-pass filters, for use in disposable diagnostic microchips. The filters were prepared by incorporating dye molecules directly into the microfluidic chip substrate, thereby providing a fully integrated solution that removes the usual need for discrete optical filters. In brief, lysochrome dyes were added to a poly(dimethylsiloxane) (PDMS) monomer prior to moulding of the microchip from a structured SU-8 master. Optimum results were obtained using 1 mm layers of PDMS doped with 1200 microg mL(-1) Sudan II, which resulted in less than 0.01% transmittance below 500 nm (OD 4), >80% above 570 nm, and negligible autofluorescence. These spectral characteristics compare favourably with commercially available Schott-glass long-pass filters, indicating that high quality optical filters can be straightforwardly integrated into the form of PDMS microfluidic chips. The filters were found to be robust in use, showing only slight degradation after extended illumination and negligible dye leaching after prolonged exposure to aqueous solutions. The provision of low cost high quality integrated filters represents a key step towards the development of high-sensitivity disposable microfluidic devices for point-of-care diagnostics.

Solution processable bulk-heterojunction solar cells using a small molecule acceptor

We report a small-molecule electron-acceptor based on 2-vinyl-4,5-dicyanoimidazole [Vinazene™] for use in solution processed organic solar cells. The material has a favourably located LUMO level of −3.6 eV and absorbs strongly in the visible spectrum up to 520 nm—attractive properties compared to the widely used acceptor (6,6)-phenyl-C60-butyric acid methyl ester (PCBM). The Vinazene derivative was blended with a poly(2,7-carbazole) donor—chosen for its complementary absorption range and comparatively high-lying HOMO level of −5.6 eV—and incorporated into bulk heterojunction devices. The influence of the donor/acceptor composition and annealing temperature on device performance were then investigated. The best performing devices exhibited reasonable power conversion efficiencies of 0.75% and open-circuit voltages of more than 1.3 V, substantially higher than previously reported devices using small molecule acceptors.

On-line analysis of CdSe nanoparticle formation in a continuous flow chip-based microreactor

The online analysis of cadmium selenide nanoparticle formation in continuous-flow microfluidic reactors is described. The as-produced particles exhibit sharp excitonic absorption and emission peaks (∼30–40 nm) with relatively high quantum efficiencies (∼10%). The mean size and dispersity of the particles, determined using on-line fluorescence detection, may be controlled by varying the reaction temperature and/or the flow rate. The microfluidic approach provides considerable control over nucleation/growth processes and is a promising strategy for the direct production of near-monodisperse nanoparticles without recourse to further size selection.

Integrated thin-film polymer/fullerene photodetectors for on-chip microfluidic chemiluminescence detection

We report the use of solution-processed thin-film organic photodiodes for microscale chemiluminescence. The active layer of the photodiodes comprised a 1 : 1 blend by weight of the conjugated polymer poly(3-hexylthiophene) [P3HT] and [6,6]-phenyl-C(61)-butyric acid-methylester [PCBM]--a soluble derivative of C(60). The devices had an active area of 1 mm x 1 mm, and a broad-band response from 350 to 700 nm, with an external quantum efficiency of more than 50% between 450 and 550 nm. The photodiodes have a simple layered structure that permits facile integration with planar chip-based systems. To evaluate the suitability of the organic devices as integrated detectors for microscale chemiluminescence, a peroxyoxalate based chemiluminescence reaction (PO-CL) was monitored within a poly(dimethyl-siloxane) (PDMS) microfluidic device. Quantitation of hydrogen peroxide indicated excellent linearity and yielded a detection limit of 10 microM, comparable with previously reported results using micromachined silicon microfluidic chips with integrated silicon photodiodes. The combination of organic photodiodes with PDMS microfluidic chips offers a means of creating compact, sensitive and potentially low-cost microscale CL devices with wide-ranging applications in chemical and biological analysis and clinical diagnostics.

On the role of intermixed phases in organic photovoltaic blends

Recently, an intermixed phase has been identified within organic photovoltaic (OPV) bulk heterojunction (BHJ) systems that can exist in addition to relatively phase-pure regions, highlighting the need for a refined picture of the solid-state microstructure of donor–acceptor blends and for gaining further understanding of the exact nature and role such intermixed phases play in such devices. Here we manipulate the microstructure of polymer–fullerene systems via processing means and the selection of the molecular weight of the donor polymer. This manipulation is used as a tool to vary the fraction of intermixed phase present and its effects on the structure and subsequently the opto-electronic processes. We find clear relationships between the state of mixing and amount of exciton quenching and number of polarons generated per absorbed photon. Furthermore, we observe that blend systems incorporating higher molecular weight polymer result in a greater yield of dissociated polarons, likely due to the increase of the intermixed fraction.

Towards microalbuminuria determination on a disposable diagnostic microchip with integrated fluorescence detection based on thin-film organic light emitting diodes.

As a first step towards a fully disposable stand-alone diagnostic microchip for determination of urinary human serum albumin (HSA), we report the use of a thin-film organic light emitting diode (OLED) as an excitation source for microscale fluorescence detection. The OLED has a peak emission wavelength of 540 nm, is simple to fabricate on flexible or rigid substrates, and operates at drive voltages below 10 V. In a fluorescence assay, HSA is reacted with Albumin Blue 580, generating a strong emission at 620 nm when excited with the OLED. Filter-less discrimination between excitation light and generated fluorescence is achieved through an orthogonal detection geometry. When the assay is performed in 800 microm deep and 800 microm wide microchannels on a poly(dimethylsiloxane)(PDMS) microchip at flow rates of 20 microL min(-1), HSA concentrations down to 10 mg L(-1) can be detected with a linear range from 10 to 100 mg L(-1). This sensitivity is sufficient for the determination of microalbuminuria (MAU), an increased urinary albumin excretion indicative of renal disease (clinical cut-off levels: 15-40 mg L(-1)).

Improved organic semiconductor lasers based on a mixed-order distributed feedback resonator design

The authors present organic semiconductor distributed feedback lasers based on thin films of the conjugated polymer poly[9,9-dioctylfluorene-co-9,9-di(4-methoxy-phenyl)fluorene] and employing an improved resonator design. In order to combine the advantages of first- and second-order distributed feedback resonators, the authors utilize a mixed-order grating design: A second-order Bragg scattering region that provides efficient vertical outcoupling of the laser radiation is surrounded by first-order scattering regions that give rise to strong feedback. By optimizing the film thickness to obtain laser oscillation at the polymer maximum gain wavelength, a very low laser threshold of 45pJ∕pulse (≈36nJ∕cm2) was realized with this resonator concept.

Accelerated synthesis of titanium oxide nanostructures using microfluidic chips

The synthesis of one-dimensional titanium oxide nanostructures has been accelerated by performing the reaction in a microfluidic environment as opposed to a classical batch process.

Thin-film polymer light emitting diodes as integrated excitation sources for microscale capillary electrophoresis

We report the use of a thin-film polymer light emitting diode as an integrated excitation source for microfabricated capillary electrophoresis. The polyfluorene-based diode has a peak emission wavelength of 488 nm, an active area of 40 microm x 1000 microm and a thickness of similar 2 mm. The simple layer-by-layer deposition procedures used to fabricate the polymer component allow facile integration with planar chip-based systems. To demonstrate the efficacy of the approach, the polyfluorene diode is used as an excitation source for the detection of fluorescent dyes separated on-chip by electrophoresis. Using a conventional confocal detection system the integrated pLED is successfully used to detect fluorescein and 5-carboxyfluorescein at concentrations as low as 10(-6) M with a mass detection limit of 50 femtomoles. The drive voltages required to generate sufficient emission from the polymer diode device are as low as 3.7 V.