8. Biofiltration

Absorption filters based on hydrogenated amorphous silicon-carbon alloys are developed for application as fluorescence filters in microarray and lab-on-a-chip systems. Fluorescence is one of the most commonly used methods for the detection of proteins, cells and DNA in microarrays [42, 43]. The method uses an external light source, which excites the fluorophores that label the entities of interest. The integration of a fluorophore sensor at each pixel of a microarray would allow for a rapid and real-time detection of the biological recognition event, while potentially increasing sensitivity and portability, and reducing costs. Fluorescence detection, however, requires the development of efficient light management to prevent the excitation light from reaching the detector and at the same time allow the emission light to be transmitted to the detector. Lipovesk et al. [40] proposed a layer of hydrogenated amorphous silicon carbide (a-SiC:H) as a suitable optical filter which can be easily integrated with a-Si:H photosensors for on-chip detection of biomolecules labelled with Alexa Fluor 430 or PyMPO. Simple fabrication of absorbing a-SiC:H filters (single-layer, low cost, with low dependence on the incidence angle) presents an important advantage compared to other filtering solutions, such as interference filters [44] where a large number of layers need to be tuned accurately during deposition. The challenge is to optimise the filtering characteristics of the a-SiC:H filter in order to match the excitation/emission wavelength fingerprint of any selected fluorophore. Therefore, a-SiC:H filters of different carbon content resulting in appropriate optical properties have to be designed. The relation between the carbon content in the a-SiC:H film and its optical filtering characteristics must be determined to enable the fabrication of the optimal filter for detection of a selected fluorophore.

The filtering characteristics of all filters are compared to the excitation/emission properties of numerous biologically relevant natural fluorophores. For each fluorophore, the most suitable a- SiC:H filter is necessary in order to ensure the highest rejection ratio between the transmitted emission/excitation light, thus achieving the optimal sensitivity of the fluorescence measuring system. As a proof-of-concept, one of the filters is tested to demonstrate the detection of the reduced form of nicotinamide adenine dinucleotide (NADH), an enzyme cofactor and an important marker for cell metabolic activity [45]. MEMS devices fabricated from bulk 6H-SiC wafers leverage heavily on the outstanding mechanical durability of the substrate, the chemical inertness of the 6H-SiC surface, the commercial availability of highquality water substrates and the versatile micromachining techniques available to render devices from them. As such, bulk SiC substrates offer nearly all the advantages of bulk Si substrates but with more robust properties. Like Si, porous SiC structures can be formed from 6H-SiC wafers by electrochemical etching. Under the proper conditions, the pore size and porosity of the resulting structures ofter the potential to use porous SiC in biofiltration applications. For biofilteration applications, the porous material was formed by bulk electrochemical etching of p-type and n-type, 6H-SiCsubstrates [13]. Rosenbloom et al. reported the development of porous SiC membranes for use as protein filters [46]. The performance of the porous SiC membranes was evaluated using protein-containing solutions with proteins .\*#%\*#z%\*z)+(!1(.z3!%#\$0z".+)zDJ\_CCCz0+zKC\_CCCz(0+\*/zcd^z 0z3/z"+1\* z0\$0z0\$!z)!)¥ branes were able to pass proteins with molecular weights of up to 29 kDa and were able to exclude proteins in excess of 45 kDa. Moreover, the porous SiC membranes exhibited lower ,.+0!%\*z/+.,0%+\*z/z+),.! z 0+z+))!.%((5z2%((!z,+(5)!.w/! z)!).\*!/z/,!¥ %"%((5z !/%#\*! z"+.z,.+0!%\*z/+.,0%+\*\_z%\* %0%\*#z0\$!z,+0!\*0%(z"+.z%z)!).\*!/z%\*z%+¥ filteration applications [13].

%.+)\$%\*! zFw%z)!).\*!/z(/+z,.+2% !z!4!((!\*0z/,!%)!\*/z0+z/01 5z0\$!z)!\$\*%¥ cal durability of 3C-SiC films since the adhesion of the film to the micromachined substrate is extremely high, due in part to the carbonization based growth process. Suspended 3C-SiC )!).\*!/z\$2!z,.+2!\*z 0+z!z \*z 00.0%2!z)!\$\*%(z /0.101.!z "+.z)%.+)\$%\*! z,.!/¥ sure sensors owing to their chemical inertness, mechanical durability and high temperature stability. In comparison with Si membranes, SiC membranes are much easier to fabricate [2].
