Digital-microfluidic lab-on-a chip (LoC) technology offers a platform for developing diagnostic applications with the advantages of portability, sample and reagent volume reduction, faster analysis, increased automation, low power consumption, compatibility with mass manufacturing, and high throughput. In addition to diagnostics, digital microfluidics is finding use in airborne chemical detection, DNA sequencing by synthesis, and tissue engineering. In this article, we review efforts to develop various LoC applications using electrowetting-based digital microfluidics. We describe these applications, their implementation, and associated design issues.

Measurement of ambient particulate organic carbon (POC) with quartz filters is prone to positive and negative sampling artifacts. One approach for estimating these artifacts is to sample with a backup quartz filter placed behind either the main quartz filter or a Teflon filter in a parallel line. Another approach is to use a denuder to reduce the positive artifact in combination with a highly adsorbent backup filter to capture any negative artifact. Results obtained using both of these approaches in parallel for over one year in Pittsburgh, PA are presented in this article. A sampler using an activated carbon monolith denuder has been developed and tested extensively. Transmission losses were found to be negligible, and the denuder is on average 94% efficient at removing gas-phase organics. Denuder breakthrough is corrected for each run using a dynamic blank in parallel with the sample line. Comparisons with the dynamic blank indicate that the denuder almost eliminates the positive artifact on the quartz filter. Negative artifact from the denuded quartz filter is quantified using a carbon-impregnated glass fiber (CIG) backup filter and was found to be small, typically less than 10% of the ambient POC. Compared to the denuded sampler POC, 24 h bare quartz samples showed an almost constant positive artifact of 0.5 μg-C/m³ for samples taken throughout the year-long study period. Sampling for shorter durations (4–6 h) resulted in a larger positive artifact. A quartz filter behind a Teflon filter (QBT) provides a consistent estimate of the positive artifact on the bare quartz filter irrespective of sample duration, though it overcorrects for the positive artifact by 16–20% (attributed to particulate matter volatilizing off the upstream Teflon filter). The quartz behind quartz (QBQ) approach provides a reasonable estimate of the positive artifact on the bare quartz filter for the 24 h samples but not for the shorter samples. A slight seasonal variation is observed in the absolute value of the positive artifact, with higher values observed during the summer months.

During the Pittsburgh Air Quality Study (PAQS) aerosol size distributions between 3 nm and 680 nm were measured between July 2001 and June 2002. These distributions have been analyzed to assess the importance of nucleation as a source of ultrafine particles in Pittsburgh and the surrounding areas. The analysis shows nucleation on 50% of the study days and regional-scale formation of ultrafine particles on 30% of the days. Nucleation occurred during all seasons, but it was most frequent in fall and spring and least frequent in winter. Regional nucleation was most common on sunny days with below average PM_2.5 concentrations. Local nucleation events were usually associated with elevated SO₂ concentrations. The observed nucleation events ranged from weak events with only a slight increase in the particle number to relatively intense events with increases of total particle counts between 50,000 cm⁻³ up to 150,000 cm⁻³. Averaging all days of the study, days with nucleation events had number concentrations peaking at around noon at about 45,000 cm⁻³. This is compared to work days without nucleation, when the daily maximum was 8 am at 23,000 cm⁻³, and to weekends without nucleation, when the daily maximum was at noon at 16,000 cm⁻³. Twenty-four-hour average number concentrations were approximately 40% higher on days with nucleation compared to those without. Nucleation was typically observed starting around 9 am EST, although the start of nucleation events was later in winter and earlier in summer. The nucleation events are fairly well correlated with the product of [UV intensity * SO₂ concentration] and also depend on the effective area available for condensation. This indicates that H₂SO₄ is a component of the new particles. Published correlations for nucleation by binary H₂SO₄—H₂O cannot explain the observed nucleation frequency and intensity, suggesting that an additional component (perhaps ammonia) is participating in the particle formation.

Ambient aerosol particles vary in size from a few nanometers to several micrometers. No instrument is currently available to cover such a wide size range, and so a combination of several instruments is usually used. One such combination is that of electrical mobility classifiers and an aerodynamic sizer. Because of the differences in measurement principles between the instruments, difficulties arise in the combination of the measurements into a single size distribution. Here we report a simple algorithm that was developed to combine aerosol size distributions measured with commercially available scanning mobility particle sizers (SMPS; TSI Inc.) and an aerodynamic particle sizer (APS; TSI Inc.). This algorithm was tested during July 2001 in the Pittsburgh Air Quality Study. The aerosol during the study had both urban and regional origin and is characteristic of urban atmosphere in the Northeastern U.S. The integrated volume concentrations from the SMPS–APS showed a good correlation with PM_2.5 mass concentration measurements using a TEOM. The relation of the aerosol mass to its volume is an “effective” density, a ratio of the bulk aerosol density to the shape factor. As a result of the comparison with the TEOM the ambient aerosol in the Pittsburgh area was found to have an effective density of 1.5 ± 0.3 g cm⁻³. Given that the aerosol during the study was found to always contain water, the particles are expected to be spherical and thus the shape factor may be assumed to be 1. This assumption has been supported by a comparison with the MOUDI, using the aerosol density of 1.5 g/cm³. It should be noted that the estimated aerosol density and the shape factor are applicable to this study only and may be different in other locations.