microfluidic channel, the total volume of loaded sample, as well as
the temperature. In the present case, sample with a volume of
300 μl was loaded into the microfluidic channel at a flow rate of
2 μl min1
, and the temperature of the microchip was controlled
at 37 °C. Under these conditions, the linear range for E. coli detection
is from 103 to 107 CFU mL1
, with a correlation efficiency
of 0.986. Further decrease in the limit of detection could be expected
if more sample volume flew through the sensing chip.
Fluorescent staining method was employed to confirm the capture
of bacteria on pSi surface. Bacteria were stained with SYTO 9 green
fluorescent dye, which was excited by blue light at wavelength of
485 nm. As shown in Fig. 4, the fluorescent intensity within the
detection zone gradually increased with the increasing of bacteria
density in the samples. The fluorescent imaging also provided
unambiguous evidence that more bacteria were captured on the
pSi surface with the increasing of bacteria density in the sample
solutions.
3.4. The selectivity of indirect FT-RIS for bacteria detection
In practical application, bacteria contaminated sample always
contain different strain of bacteria. To discriminate and quantify a
target strain of bacteria, high selectivity is required. In this approach,
the selectivity is originated from the bio-affinity between
bacteria and its corresponding antibody. To confirm whether the E.
coli bacteria were specifically capture by the E. coli antibody,
sample solution with bacterial density of 107 CFU mL1 was
pumped through the microfluidic pSi chips with or without attached
antibody, respectively. After the capturing process, the
SYTO 9 fluorescent dye was introduced into the microfluidic chips,
and incubated in a dark room for 15 min. The residue dye was
removed with ultrapure water, and the stained bacteria were observed
with a fluorescent microscopy. Only few stained bacteria
can be found on the pSi chip without antibody. In contrast, high
density of stained bacteria can be observed on the pSi chip attached
with antibody (Fig. S4, ESI). The results confirmed that the
target bacteria were specifically captured on the pSi surface and
the non-specific adsorption can be almost ignored.