For the nanopore-based FT-RIS approach, the selective capture
of bacteria will lead to selective pore blockage effect and generate
a specific FT-RIS signal. In the present work, non-target bacteria
strain, NOX and P17, were employed to test the selectivity of indirect
FT-RIS measurement. Sample spiked with same density of E.
coli, NOX and P17 was introduced into the microfluidic sensing
chip attached with E. coli antibody, respectively. After sample
loading, the microfluidic chips were equilibrated with PBS buffer.
Afterwards, buffer solution containing BSA probe was pumped
through the chips, and the EOT value of pSi layer was monitored
with real-time FT-RIS respectively. As shown in Fig. 5a, the samples
spiked with NOX and P17 strain almost produce the same
ΔEOT value as the blank sample, whereas the ΔEOT found in the
E. coli sample significantly decreased. According to Eq. (2), the
relative EOT shift of the three types of samples can be calculated.
The optical signal generated from E. coli sample is around 10 folds
higher than that from the non-target bacteria (Fig. 5b). To further
explore the selectivity of this method, a more complicated sample
containing mixture of E. coli and Bacillus Subtilis (ATCC no. 6051)
was tested on the sensing chip. The optical data also showed that
the presence of Bacillus Subtilis did not cause significant interference
to the E. coli detection (Fig. S5, ESI). All these results indicate
that the pore blockage effect measured by the indirect
FT-RIS is highly selective to the target bacteria.