Siva H. Krishnadasan

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.

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.

Large-scale synthesis of nanocrystals in a multichannel droplet reactor

We report a multichannel microfluidic droplet reactor for the large-scale, high temperature synthesis of nanocrystals. The reactor was applied here to the production of CdTe, CdSe and alloyed CdSeTe nanocrystals, and found in all cases to provide high quality quantum dots with spectral properties that did not vary between channels or over time. One hour test runs yielded 3.7, 1.5 and 2.1 g of purified CdTe, CdSe and the alloy, respectively, using 0.4 M cadmium precursor solutions and carrier and reagent phase flow rates of 4 and 2 ml min−1. A further nine hour test-run applied to CdTe, utilizing increased carrier and reagent flow rates of 5 and 3 ml min−1, yielded 54.4 g of dry purified material, corresponding to a production rate of 145 g per day. The reactor architecture is inherently scalable and, with only minimal modifications, should allow for straightforward expansion to the kilogram-per-day production levels sought by industry.

Direct synthesis of dextran-coated superparamagnetic iron oxide nanoparticles in a capillary-based droplet reactor

We describe the controlled synthesis of dextran-coated superparamagnetic iron oxide nanoparticles (SPIONs) using a stable passively-driven capillary-based droplet reactor. High quality highly crystalline particles were obtained with a narrow size distribution of mean diameter 3.6 nm and standard deviation 0.8 nm. The particles were evaluated for use in MRI, and found to exhibit a large saturation magnetisation of 58 emu/g and a high T2 relaxivity of 66 mM−1s−1 at 4.7 T, signifying good MRI contrast enhancement properties.

Controlled synthesis of conjugated random copolymers in a droplet-based microreactor

We report the highly controlled synthesis of conjugated random copolymers in a droplet-based microfluidic reactor. Using two optically distinct polymers, poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS), a series of highly regioregular random copolymers is generated with physical properties intermediate to those of the parent homopolymers. Analysis by 1H nuclear magnetic resonance spectroscopy reveals the co-polymerisation process to follow ideal Bernoullian behavior.

Dietary bioavailability of cadmium presented to the gastropod Peringia ulvae as quantum dots and in ionic form

For quantum dots (QDs) synthesized in solvents that are immiscible in water, dietary, rather than aqueous, exposure is expected to be the primary route of uptake. The estuarine snail Peringia ulvae was presented with mats of simulated detritus spiked with oleic acid capped cadmium sulfide (CdS; 3.1 ± 0.4 nm) or cadmium selenide (CdSe; 4.2 ± 0.8 nm) nanoparticles, synthesized using a microfluidics method, or Cd(2+) (added as Cd[NO3 ]2 ) as a control. A biodynamic modeling approach was used to quantify parameters that describe the dietary accumulation of the Cd forms. Ingestion rates decreased across treatments at higher exposure concentrations, indicating a metal-induced stress response related to Cd dose rather than form. Although Cd was bioavailable from both CdS and CdSe QDs, uptake rate constants from diet were significantly lower than that of Cd(2+) (p < 0.05). After 72 h depuration, however, no loss of Cd was observed from snails that had accumulated Cd from either type of QD. In comparison, snails ingesting Cd(2+) -spiked detritus eliminated 39% of their accumulated body burden per day. The almost identical uptake and efflux rates for Cd in both QDs suggest no effect of the chalcogenide conjugates (S or Se). The findings of the present study indicate that the availability of Cd in the form of nanoparticles and its apparent in vivo persistence will lead to bioaccumulation. The implications of this are discussed.

Microfluidic Reactors for Nanomaterial Synthesis

Abstract The difficulty of preparing nanomaterials in a controlled, reproducible manner is a key obstacle to the proper exploitation of many nanoscale phenomena. An automated chemical reactor capable of producing (on demand and at the point of need) high-quality nanomaterials, with optimized physicochemical properties, would find numerous applications in nanoscale science and technology, especially in the areas of photonics, optoelectronics, bio-analysis, and targeted drug-delivery. In addition such a device would find immediate and important applications in toxicology, where it is essential to characterize the physiological effects of nanoparticles not only in terms of chemical composition but also in terms of size, shape, and surface functionalization. In this chapter, we describe recent advances in the development of microfluidic reactors for controlled nanoparticle synthesis and, more specifically, work in our group aimed at developing just such an automated reactor.

Photostable phosphorescent polymer nanospheres for high sensitivity detection

Time-gated interrogation of phosphorescent labels allows high sensitivity detection of analytes by excluding background fluorescence and stray excitation light. Here we report a method for preparing phosphorescent polymer nanospheres with optical properties specifically designed to enable time-gated detection with both organic and inorganic light-emitting diodes and photodiodes. The nanospheres were formed by co-precipitating the phosphorescent dye ruthenium diphenyl phenanthroline with the amphiphilic statistical co-polymer poly(styrene-co-maleic acid). To minimize aggregation and ensure effective encapsulation against water and oxygen ingress, the polymer chains were cross-linked with the small-molecule cross-linker bisphenol A diglycidyl ether, resulting in near-monodisperse nanospheres with excellent resilience against aggregation and quenching. The nanospheres exhibited long phosphorescence lifetimes of 5.3 ± 0.1 μs, high photoluminescence quantum efficiencies of 33 ± 5%, and excellent stability under UV irradiation. Using a blue light-emitting diode as a light-source and a silicon photodiode as a detector, a low detection limit of 4 μg ml−1 was achieved. The suitability of the nanospheres for bioanalysis was demonstrated by surface functionalising them with biotin to yield a simple biolabel for streptavidin.

Rapid Precipitation: An Alternative to Solvent Casting for Organic Solar Cells

Rapid precipitation, immersion of a liquid formulation into a nonsolvent, is compared with drop casting for fabricating organic solar cells. Blends comprising poly-3-hexylthiophene (P3HT), phenyl-C61-butyric acid methyl ester (PCBM), and chlorobenzene were processed into bulk samples by using two distinct routes: rapid precipitation and drop casting. The resulting structure, phases, and crystallinity were analyzed by using small-angle neutron scattering, X-ray diffraction, differential scanning calorimetry, and muon spin resonance. Rapid precipitation was found to induce a finely structured phase separation between PCBM and P3HT, with 65 wt % crystallinity in the P3HT phase. In contrast, solvent casting resulted in a mixed PCBM/P3HT phase with only 43 wt % P3HT crystallinity. The structural advantages conferred by rapid precipitation were shown to persist following intense thermal treatments.

Droplet Flow Techniques for the Scalable Production of Electronic Materials

Krishnadasan et al., Lab Chip, 2007, 7, 1434–1441; Nightingale et al., Lab Chip, 2011, 11, 1221; Nightingale et al., J. Mater. Chem. A, 2013, 1, 4067 Bannock et al. Adv. Funct. Mater., 2013, 23, 2123-2129; Bannock et al. Mater. Horiz., 2013 (accepted) Quantum Dots Polymers Separator Bannock, Phillips et al., Anal. Methods, 2013, 5, 4991-4998 www.afm-jou rnal.de 3 1 0 2 • 6 y a M • 7 1 . o N • 3 2 . l o V