Nanosensors - Nanotechnologies for Water Environment Applications - page 426

level (Zhao et al., 2004) and the other at the DNA level (Lian et al., 2004). For cell level
labeling (Zhao et al., 2004), silica NPs were doped with Rubpy and the surface modified
with a specific antibody against E. coli O157:H7 resulting in a detection limit of 2.5 E.
coli cells/mL of sample. To detect the signal from single cells, the protocol did require
dilution in 96-well plates (perhaps cumbersome for routine analysis) but once the
protocol is established, the time taken to prepare such dilution and detect a single cell
was less than 20 min (Figure 13.6).
Table 13.3 Comparison of the two articles of RuBpy-doped silica NP labeling based
microbial detection.
Features Zhao et al., 2004 (cell based detection) Lian et al., 2004 (DNA based

detection)

NPs 16.667 mM RuBpy for fabrication 0.824 mM RuBpy for fabrication

60 ± 4 nm in diameter 70 nm in diameter

Scheme Whole cell detection; immunocytochemistry

(surface antigen interacted with antibody
conjugate on NPs);

Genomic DNA detection; DNA
hybridization;

Target E. coli O157:H7 cells Pseudomonas aeruginosa genomic

DNA: approximate dry weight of
DNA: 1.2 x 10 -14 g DNA/cell

Detection In solution (384-well microplate) On CMT-GAPs slide

Ar + laser (Omnichrome) Scanner GenePix 4000B scanner

Detection
limit

~0.25 cells/100L, i.e., 2.5 cells/mL. ~ 0.00282 μg genomic DNA, i.e.,

2.35 × 10 5 copies of gDNA /mL).

Advan-
tages

~4700-fold enhancement in sensitivity,
compared to Lian et al., 2004, due to:
(i) more recognition sites (many surface
antigens on one target cell). Thousands of
nanoparticles can bind to each bacterium
cell; and (ii) more concentrated dye
molecules doped: 20-fold RuBpy material
for NP fabrication, the other fabrication
conditions kept the same to that employed
by Lian et al., 2004.

The DNA hybridization recognition
scheme determined higher assay
specificity than immunological
recognition scheme, as Zhao et al.,
2004 used.

Poor sensitivity: ~2.35 x 10 5
cells/mL

Disadvan-
tages

(i) Specificity is determined to be low due
to the immunological recognition scheme.
(ii) The library of target cell surface
antigen-antibody is not readily available
for highly parallel pathogen detection.