Nanoparticle-based biosensors and bioassays - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 443

a 100-fold increase in sensitivity compared to analogous fl uorescence-based assays. The
silver enhancement relies on the chemical reduction of silver ions by hydroquinone to
silver metal on the surface of the gold nanoparticles. Such silver precipitation facilitated
visualization of the nanoparticle label and enabled quantitation of the hybridized target
based on the imaged grayscale values. In addition, the use of nanoparticle labels altered
the melting profi les to allow effective discrimination against single-base mismatches.
Pavlov et al. reported on the use of gold nanoparticles for amplifi ed optical transduction
of aptamer-protein interactions [14]. The gold nanoparticles were functionalized with
the thiolated aptamer (80 aptamers per particle). The aptamer binding to the thrombin
protein analyte caused the gold nanoparticles to aggregate and their plasmon absorbance
spectra to decrease.
Surface-enhanced Raman scattering (SERS) is another spectroscopic transduc-
tion mode that can greatly benefi t from the use of gold nanoparticles. Cao et al. used
nanoparticles functionalized with oligonucleotides and Ramanactive dyes for detecting
DNA hybridization [15]. The gold nanoparticles facilitated the formation of a silver
coating that acted as a promoter for the Raman scattering of the dyes. High sensitivity
down to the 20 fM DNA level was reported. Multiplexed detection was accomplished
by using different Raman dyes. The high fl uorescence intensity of semiconduc-
tor quantum dots (QDs) can also lead to remarkably sensitive bioassays. Hahn et al.
reported on a highly sensitive detection of the single-bacterial pathogen E. coli O157
using CdSe/ZnS core-shell QDs conjugated to streptavidin [16]. This system exhibited
two orders of magnitude increased sensitivity (along with higher stability) compared to
the common fl uorescent dyes.
Mirkin and coworkers have developed a novel gold nanoparticle-based bio-barcode
method for detecting proteins down to the low attomolar level [17]. This powerful
protocol relies on magnetic spheres functionalized with an antibody that binds spe-
cifi cally the target protein and a secondary antibody conjugated to gold nanoparticles
that are encoded with DNA strands that are unique to the target protein (Fig. 14.2,

Step 1.
Ta rget Protein
Capture with
MMP Probes

a b

Step 2.
Sandwich Captured Target
Proteins with NP Probes

1.

SH
Bevine Serum Albumin 2.

3.

Nanoparticle (NP) Probe

Ta rget Protein
(PSA)

13 nm NPs for Bio-Bar-Code PCR
30 nm NPs for PCR-less Method

1.

Step 5.
Chip-Based Detection
of Bar-Code DNA for
Protein Identification

2. Bevine Serum

Albumin

Step 4.
Polymerase
Chain Reaction

Step 3.
MMP Probe Separation
and Bar-Code DNA
Dehybridization

Magnetic Microparticle
(MMP) Probe

Bar-Code DNA

Ag
Au

Gold Nanoparticle SH Capture DNA Bar-Code DNA

Amine-Functionalized
Magnetic Particle

Monoclonal Anti-PSA Polyclonal Anti-PSA

Step 4.
PCR-less Detection
of Bar-Code DNA from
30 nm NP Probes

Magnetic
Field

FIGURE 14.2 The bio-bar-code assay method. (a) Probe design and preparation. (b) PSA detection and
bar-code DNA amplifi cation and identifi cation (reproduced from [17] with permission) (see Plate 14 for
color version).