Microfluidic paper-based analytical device for aerosol oxidative activity

Abstract

Human exposure to particulate matter (PM) air pollution has been linked with respiratory, cardiovascular, and neurodegenerative diseases, in addition to various cancers. Consistent among all of these associations is the hypothesis that PM induces inflammation and oxidative stress in the affected tissue. Consequently, a variety of assays have been developed to quantify the oxidative activity of PM as a means to characterize its ability to induced oxidative stress. The vast majority of these assays rely on high-volume, fixed-location sampling methods due to limitations in assay sensitivity and detection limit. As a result, our understanding of how personal exposure contributes to the intake of oxidative air pollution is limited. To further this understanding, we present a microfluidic paper-based analytical device (μPAD) for measuring PM oxidative activity on filters collected by personal sampling. The μPAD is inexpensive to fabricate and provides fast and sensitive analysis of aerosol oxidative activity. The oxidative activity measurement is based on the dithiothreitol assay (DTT assay), uses colorimetric detection, and can be completed in the field within 30 min following sample collection. The μPAD assay was validated against the traditional DTT assay using 13 extracted aerosol samples including urban aerosols, biomass burning PM, cigarette smoke, and incense smoke. The results showed no significant differences in DTT consumption rate measured by the two methods. To demonstrate the utility of the approach, personal samples were collected to estimate human exposures to PM from indoor air, outdoor air on a clean day, and outdoor air on a wildfire-impacted day in Fort Collins, CO. Filter samples collected on the wildfire day gave the highest oxidative activity on a mass normalized basis, whereas typical ambient background air showed the lowest oxidative activity.

Figure 1

(A) Steps to perform DTT assay on a microfluidic paper-based analytical device (μPAD). (B) Actual μPADs used for DTT detection. The left device measured high oxidative activity and, therefore, produced a low-intensity image (more DTT consumed); the right device measured low oxidative activity resulting in more color intensity at the detection zone (less DTT consumed). μPAD dimensions are 2.5×2.5 cm. The sample and detection zone diameters are 10 and 4 mm, respectively.

Figure 2

Typical response curve obtained from DTT assay plotted remaining DTT signal (gray intensity) as a function of 1,4-NQ model oxidant added to the assay (n=3).

Figure 3

Optimization of DTT assay for analysis by μPAD using 1,4-NQ as a PM model oxidant (a) Reaction temperature study (b) Reaction time study.

Figure 4

The impact of initial DTT amount on the assay sensitivity using 1,4-NQ as a model oxidant (n=3).

Figure 5

Comparison of PM oxidative activity (DTT consumption rate, pmol min⁻¹ Dg⁻¹) between the traditional DTT assay and paper based DTT assay. Data represent aqueous extracts of 13 different aerosol samples.

Figure 6

Oxidative activity of personal PM_2.5 (red bar) and PM₁₀ (green bar) samples collected in various environments. Outdoor (clean day): PM collected on a typical day in Fort Collins, CO. Outdoor (smoky day): PM collected on a smoky day when the High Park wildfire (10 miles from Fort Collins, CO) was active. Indoor (Kitchen): PM collected in a restaurant kitchen.