Biosensor

Neuroscience

Receptor

Nanotechnology

Organic chemistry

Combinatorial chemistry

Serotonin

Tyramine

Biology

Colloidal gold

Adsorption

Aptamer

Physical chemistry

Nanoparticle

Isothermal titration calorimetry

Melamine

Neurotransmitter

Chemistry

Materials science

Biogenic amine

Octopamine (neurotransmitter)

Molecule

Dopamine

Genetics

Biochemistry

Analytical Chemistry

Target-directed aptamer adsorption by gold nanoparticles (AuNPs) has been widely used to develop label-free colorimetric biosensors. However, the potential interactions between target molecules and AuNPs have not been considered, which may lead to misinterpretation of analytical results. In this work, the detection of dopamine, melamine, and K+ was studied as model systems to address this problem. First, dopamine and two control molecules all induced the aggregation of citrate-capped AuNPs with apparent Kd's of 5.8 μM dopamine, 51.6 μM norepinephrine, and 142 μM tyramine. Isothermal titration calorimetry measured the aptamer Kd to be 1.9 μM dopamine and 16.8 μM norepinephrine, whereas tyramine cannot bind. Surface enhanced Raman spectroscopy confirmed direct adsorption of dopamine, and the adsorbed dopamine inhibited the adsorption of DNA. Using a typical salt-induced colorimetric detection protocol, a similar color response was observed regardless of the sequence of DNA, indicating the observed color change reflected the adsorption of dopamine by the AuNPs instead of the binding of dopamine by the aptamer. For this label-free sensor to work, the interaction between the target molecule and AuNPs should be very weak, while dopamine represents an example of strong interactions. For the other two systems, the melamine detection did not reflect aptamer binding either but the K+ detection did, suggesting melamine also strongly interacted with AuNPs, whereas K+ had very weak interactions with AuNPs. Since each target molecule is different, such target/AuNP interactions need to be studied case-by-case to ensure the sensing mechanism.

Dopamine and Melamine Binding to Gold Nanoparticles Dominates Their Aptamer-Based Label-Free Colorimetric Sensing