Towards low-cost QEPAS sensors for nitrogen dioxide detection

Increasing awareness of the adverse health effects of air pollution leads to a demand of low-cost sensors for the measurement of pollutants such as NO₂. However, commercially available low-cost sensors lack accuracy and long-term stability, and suffer from cross-sensitivity to other gases. These drawbacks can be overcome by the method of quartz-enhanced photoacoustic spectroscopy (QEPAS). In QEPAS modulated light is absorbed by the NO₂ molecules, which results in the production of a sound wave. The sound wave is detected by resonance of a quartz tuning fork, which results in a measurable electric signal. Due to the small size of the tuning forks, the gas sensing element can be smaller than 1 cm³. We present the first bare fork QEPAS setup for the ppb-level detection of NO₂, which is ideally suited for environmental trace gas detection without the need of using micro-resonators. Micro-resonators are commonly used to amplify photoacoustic signals. However, micro-resonators have different dependencies on environmental conditions than tuning forks, which makes them difficult to operate in changing conditions. In contrast, our bare fork QEPAS setup is more robust and easily adopted by the use of a low-cost temperature and humidity sensor. By using acoustic filters the integration time could be increased to offer higher sensitivity at a continuous flow rate of 200 std cm³ min^-1. The 1σ noise equivalent concentration is determined to 21 ppb NO₂ in synthetic air for 120 s measurement time, allowing detection which satisfies international health and safety standards thresholds.

A novel cluster of lectin-like receptor genes expressed in monocytic, dendritic and endothelial cells maps close to the NK receptor genes in the human NK gene complex

The NK gene complex is a region on human chromosome 12 containing several families of lectin-like genes including the CD94 and NKG2 NK receptor genes. We report here that the region telomeric of CD94 contains in addition to the LOX-1 gene the novel human DECTIN-1 and the CLEC-1 and CLEC-2 genes within about 100 kb. Sequence similarities and chromosomal arrangement suggest that these genes form a separate subfamily of lectin-like genes within the NK gene complex. DECTIN-1 is selectively expressed in dendritic cells and to a lower extent in monocytes and macrophages. mRNA forms with and without a stalk exon are observed. During functional maturation of dendritic cells the level of DECTIN-1 mRNA is down-regulated several-fold. CLEC-1 is found to be not only expressed in dendritic cells, but also in endothelial cells and in the latter aspect resembles the LOX-1 gene. Whereas recombinant full-length DECTIN-1 and LOX-1 are transported to the cell surface, CLEC-1 proteins accumulate in perinuclear compartments. We propose that this family of lectin-like genes encodes receptors with important immune and/or scavenger functions in monocytic, dendritic and endothelial cells.