Molecularly Imprinted Au Nanoparticle Composites for Selective Sensing Applications - Designing Receptors for the Next Generation of Biosensors - page 199

The imprinting of cross-linked Au NPs composites through ionic interactions
provides a general paradigm which is not limited for the sensing of aromatic
explosives. For example, using cysteine-modified Au NPs, imprinted matrices
were generated for a series of amino acids [ 53 ]. The matrices showed high
sensitivities, in the nM concentration range, toward the detection of the substrate
analytes, and exhibited remarkable stereoselectivity and chiroselectivity during the
analysis of different amino acids.

2.3 Imprinted Electropolymerized Au NPs Composites Based on

Ligand-Analyte Complexation Processes for Sensing

An alternative approach for imprinting recognition sites in cross-linked Au NPs
matrices involves the primary modification of the NPs with target-specific ligand
units. To this end, the ligand units are incorporated as a co-additive capping agent in
the synthesis of the Au NPs. Figure 7A schematically presents the electropoly-
merization of Au NPs, modified with a mixed monolayer consisting of thioaniline
(1) electropolymerizable units, thioethane sulfonate (2) stabilizing units, and
thiophenyl boronic acid (16) ligation units, in the presence of the saccharide D -glucose,
(17), analyte. The interaction between the ligand (16) and the vicinal diols of the sugar
(17), under basic conditions, results in the formation of a boronic-ester bond, leading
to the formation of a complex between the saccharide and the modified Au NP.
The (17)-complexated Au NPs were, then, electropolymerized onto a thioaniline
monolayer-modified Au surface in the presence of an excess saccharide, and under
basic conditions, to yield the (17)-imprinted bis-aniline-cross-linked Au NPs elec-
trode. The subsequent removal of (17) from the matrix was carried out by acidifying
the electrolyte to pH

1.3, which induces the dissociation of the boronate-ester bond,
thus releasing the sugar units. Following this stage, a vacant (17)-imprinted Au NPs
composite is obtained. Figure 7B depicts the analysis of D -glucose (17) by the resulting
imprinted matrix. The time-dependent reflectance changes correlate with the
concentrations of (17). The detection limit of (17)is40pM,Fig. 7C , and an association
constant corresponding to K a ¼

¼

10 9 M 1 between (17) and the matrix was
estimated. Also, only minute reflectance changes are observed upon challenging the
(17)-imprinted matrix with D -mannose, (18), or D -galactose, (19), consistent with the
impressive stereoselectivity provided by the (17)-imprinted sites.
In addition to the stereoselectivity for sensing the imprinted saccharides, the
imprinting methodology leads, also, to chiroselective recognition of sugars. Figure 7D
depicts the sensograms obtained upon the analyses of D -glucose, (17), and L -glucose,
(20), on the D -glucose (17)-imprinted Au NPs matrix. Whereas significant reflectance
responses are observed at D -glucose (17) concentrations as low as 10 pM on the
(17)-imprinted matrix, no substantial reflectance changes are observed, up to a

8 : 3