Nanoparticle-based biosensors and bioassays - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 445

14.4 NANOPARTICLE-BASEDELECTROCHEMICAL
BIOSENSORS AND BIOASSAY
Electrochemical devices have recently received considerable attention in the develop-
ment of immunosensors and sequence-specifi c DNA hybridization biosensors [27-29].
Electrochemical biosensors are devices that intimately couple a biological recogni-
tion element to an electrode transducer that relies on the conversion of the antibody-
antigen and Watson-Crick base-pair recognition event into a useful electrical signal.
Electrochemical devices offer elegant routes for interfacing - at the molecular level -
the DNA (or antibody-antigen) recognition and signal transduction elements and are
uniquely qualifi ed for meeting the size, cost, low volume, and power requirements of
decentralized DNA and protein diagnostics [27-29]. The high sensitivity of electro-
chemical biosensors, coupled with their inherent miniaturization, compatibility with
modern microfabrication technologies, low-cost and power requirements, and inde-
pendence of sample turbidity make such devices excellent candidates for centralized
and decentralized genetic and protein testing. Although the use of electrochemical bio-
sensors or chips is at an early stage, easy-to-use hand-held electrical DNA and protein
analyzers are already approaching the marketplace [30] and are expected to have a con-
siderable impact on future DNA and protein diagnostics. Electrochemical transduction of
DNA hybridization events has commonly been achieved in connection with electroactive
indicators/intercalators or enzyme tags. Electrochemical transduction of antibody-
antigen recognition events has been realized in connection with metal ion and enzyme
labels. The use of nanoparticle tracers is relatively new in electrical detection and offers
unique opportunities for electrochemical transduction of protein and DNA sensing events.
14.4.1 Nanoparticle-based electrochemical DNA biosensors and
bioassays
Nanoparticle-based electrical routes for gene detection have attracted much inter-
est. Such new protocols are based on the use of colloidal gold [31, 32], semiconduc-
tor quantum dot tracers [33, 34], iron/gold alloy nanoparticles [35], copper/gold alloy
nanoparticles [36], and silver nanoparticles [37]. These nanoparticle materials offer
elegant ways for interfacing DNA recognition events with electrochemical signal trans-
duction and for amplifying the resulting electrical response. Most of these schemes
have commonly relied on a highly sensitive electrochemical stripping transduction/
measurement of the metal tracers. Stripping voltammetry is a powerful electroanalyti-
cal technique for trace metal measurements [38]. Its remarkable sensitivity is attributed
to the “built-in” preconcentration step during which the target metals are accumulated
(deposited) onto the working electrode. The detection limits are thus lowered by three
to four orders of magnitude, compared to pulse-voltammetric techniques used earlier
for monitoring DNA hybridization. Such ultrasensitive electrical detection of metal
tags has been accomplished in connection with a variety of new and novel DNA-linked
particle nanostructure networks.