The workflow for constructing a pSi-based microfluidic sensor
is illustrated in Scheme 1a. Briefly, SU-8 mold was fabricated as a
master template for casting of microfluidic structure onto a PDMS
film using an UV photolithographic procedure. First, a layer of SU-
8 (thickness ∼400 mm) was spin-coated on a cleaned Silicon wafer.
Then, the SU-8 film was covered with a photo mask whose pattern
was designed with CorelDraw software (Corel Inc.) The film was
exposed to ultraviolet light followed by a development procedure.
PDMS (Sylgard 184, Dow Corning) was mixed with the as supplied
cross-linking reagent in its standard volume ratio (10:1) and degassed
at a vacuum of 0.88 bars for 45 min. The degassed PDMS
was poured on the SU-8 mold and was degassed again for 30 min
to remove bubbles. The wafer was baked at 70 °C for 1 h to completely
crosslink the PDMS. At last, the PDMS film patterned with
microchannel and holes was peeled slowly from the SU-8 master.
The patterned PDMS film was sealed with the porous Si chip before
both of them was treated by oxygen plasma for 30 s. The
position of the channel on the PDMS film should be carefully
aligned to the etched area of pSi chip. Finally, the patterned PDMS
film was sealed with another PDMS film but without channels.