Molecularly Imprinted Au Nanoparticle Composites for Selective Sensing Applications - Designing Receptors for the Next Generation of Biosensors - page 201

concentration of 1 nM, for the analysis of the L -glucose substrate (20) on the same
matrix. Also, ca. 2 10 3 -fold lower association constant was derived for the binding

of (20)tothe(17)-imprinted matrix. The results were also reversed upon the imprint-
ing of L-glucose sites in the Au NPs composite, and lower detection limit and
enhanced sensing performance were observed in this case for the analysis of (20)by
the (20)-imprinted Au NPs composite [ 54 ]. The results suggest that the molecular
contours generated in the matrix during the imprinting process provide a unique steric
template for the imprinted substrate, and can be used to distinguish between
diastereoisomers or enantiomers. It should be noted that in another study, an analogous
system also employing boronic acid-functionalized Au NPs was used for the imprint-
ing of different diol-functionalized antibiotics, including Kanamycin, Neomycin, and
Streptomycin [ 55 ]. The imprinted matrices revealed impressive detection limits for
the respectively imprinted substrates, which were several orders of magnitude lower
than the maximum allowed residue limits (MRLs). Furthermore, the antibiotic-
imprinted Au NPs matrices also allowed the sensitive and selective analysis of the
target substances in real food samples, such as in milk [ 55 ].
Ligand-analyte interactions which constitute the grounds for imprinting via
complexation in electropolymerized Au NPs composites may also be used for the
detection of metal ions [ 56 ]. Au NPs were co-modified with the complexating
dithiothreitol (21) ligand and reacted with an alkaline-earth-metal ion, such as
Sr 2+ ,Mg 2+ ,Ca 2+ ,orBa 2+ . Following the primary complexation of the ion with
the ligand-modified particles, the Au NPs were electropolymerized onto a
thioaniline-modified Au surface, to yield an imprinted composite, which is
schematically exemplified in Fig. 8A . The removal of the metal ions from the
respective matrices by exposing them to an acidic, pH 1.5, solution, resulted in

vacant imprinted templates. This process presumably protonated the dithiotreitol
ligands, thus releasing the metal ions to the solution.
Figure 8B depicts the sensograms obtained upon the interaction of the Mg 2+ ,
Ca 2+ ,Sr 2+ , and Ba 2+ ions with the Mg 2+ -imprinted bis-aniline-cross-linked Au NPs
composite. Evidently, the Mg 2+ -imprinted matrix is most sensitive to the presence
of Mg 2+ ions, exhibiting a low detection limit of 20 fM and a linear dynamic
sensing range in the femtomolar region (Fig. 8C ). The high sensitivity is attributed
to the dielectric changes induced upon the binding of the Mg 2+ ions to the matrix,
and/or due to possible changes in the local plasmon intensity, caused by variations
in the inter-Au NPs distances, as a result of the association of the ions with the

¼

Fig. 7 (continued) d-galactose (concentrations a - c : 20-200 m M) on the D -glucose-imprinted bis-
aniline-cross-linked Au NPs matrix. (C) Calibration curves derived from the sensograms in (B).
The inset shows the lower concentration regions of the curves. (D) Sensograms, at y ¼ 63.5 ,
showing the reflectance changes obtained upon to the analysis of: ( a ) D -glucose (concentrations
a - l : 10 pM-100 m M), and ( b ) L -glucose (concentrations a - i : 1 nM-100 m M) on the D -glucose-
imprinted bis-aniline-cross-linked Au NPs matrix. The inset shows the respective calibration
curves derived from the sensograms. All measurements were performed in a 50 mM HEPES
buffer solution (pH ¼ 9.2). Reproduced from ref. 54 by permission of the Royal Society of

Chemistry (RSC)