Nanosensors - Nanotechnologies for Water Environment Applications - page 430

Similarly, BioVeris employs the electrogenerated chemiluminescence (ECL)
reaction to attain the amplification of the signal to detect pathogens with enhanced
sensitivity (Figure 13.8). Two redox active compounds, (Ru(bpy) 3 +2 and tripropylamine)
and antibody immobilized paramagnetic beads targeted for specific pathogens play a
central role in the detection and signal amplification process. The interaction among
pathogen, ECL labels, and paramagnetic particles forms a sandwich structure, which
gets attracted towards the electrode by the application of current. The ECL labels
(Ru(bpy) 3 +2 and tripropylamine) get oxidized at the electrode. An electron transfer
reaction from the deprotonated tripropyl amine radical to the oxidized state of
Ru(bpy) 3 +2 pushes it to an excited state *Ru(bpy) 3 +2 which, when returns back to a
ground state, emits light. The cyclic nature of oxidation, excitation, and deactivation
reaction results into the amplification in the light signal. The amount/number of
pathogenic cells can be determined by measuring the emitted light from the ECL labels.
In one of the examples the detection limit of BioVeris assay is 10 5 plaque forming
unit/ml (small pox) with an assay time of 30 minutes (Higgins et al., 1999). An
important aspect of these technologies is the ease of sample processing and automated
detection of the target species together in many samples.

Light

*Ru(bpy) 3 +2 Luminesce nce

TPA*

Emitting Light

-H +

Chemi-
Chemical Energy

Ru(bpy) 3 +2

TPA

Ru(bpy) 3 +3 TPA *+

e - e - Electro-

Electrochemically
Initiated

Electrode

Figure 13.8 Principle of electrogenerated chemiluminescence (ECL) reaction. TPA
stands for tripropylamine (* denotes the excited state of the chemical compound).

13.9 Label-Free Detection

Ideally, an assay should not use labels because it imposes extra time and cost,
and may be the cause for decreased specificity, sensitivity, and reproducibility. Hence a
number of label-free approaches are also being explored to detect biomolecules. Two of
these approaches, having excellent potential for online monitoring of waterborne
pathogens are described below.