3.1.1  Sample filtration
Generally, sample filtration relies on the principle of size discrepancy between the target pathogen and background species. Membranes, fibers, and channels have been used in size-selective sample filtration processes for biosensing applications. Biorecognition elements are commonly used to assist the separation process when the target species exhibits similar properties to background species or the matrix. For example, biorecognition elements that exhibit affinity to a broad group of pathogens, such as lectins, have been used in pre-concentration steps for pathogen detection (Zourob et al. 2008). Bacteria typically exhibit a net negative charge at physiological pH (7.4) because of an abundance of lipopolysaccharides or teichoic acids on the cell membrane (Gram-negative bacteria and Gram-positive bacteria, respectively) (Silhavy et al. 2010). This physical property of cell-based pathogens is leveraged in biofiltration processes, for example, using electropositive filters (Altintas et al. 2015). While the majority of the aforementioned separation processes involve manual handling steps, sample filtration processes are now being integrated with microfluidic-based biosensing platforms (Song et al. 2013). For example, Chand and Neethirajan incorporated an integrated sample filtration technique using silica microbeads for the detection of norovirus in spiked blood samples (Chand and Neethirajan 2017).