Interaction of Tris with DNA molecules and carboxylic groups on self-assembled monolayers of alkanethiols measured with surface plasmon resonance

Functional materials employing organic coatings on inorganic substrates are perceived as potential platforms for applications in a variety of fields. Therefore, the investigation of interactions of such systems with the microenvironment has become an important research direction in surface science. Herein, we study the interaction of one of the buffers most commonly used in biological studies, Tris buffer, with self-assembled monolayers (SAMs) of alkanethiols and short DNAs using a surface plasmon resonance (SPR) biosensor. We show that the interaction between Tris and carboxylic groups of SAMs is a complex multiphasic process. We demonstrate that Tris base binds to the protonated carboxylic groups. When those groups become deprotonated, Tris base dissociates and Tris acid is attracted, which results in the formation of a diffuse layer over the charged surface. In addition, we show that the interaction of Tris with the immobilized DNA molecules biases the determination of surface concentrations of the immobilized DNA molecules and thus also the determination of hybridization efficiencies

Analyte transport to micro-and nano-plasmonic structures

The study of optical affinity biosensors based on plasmonic nanostructures has received significant attention in recent years. The sensing surfaces of these biosensors have complex architectures, often composed of localized regions of high sensitivity (electromagnetic hot spots) dispersed along a dielectric substrate having little to no sensitivity. Under conditions such that the sensitive regions are selectively functionalized and the remaining regions passivated, the rate of analyte capture (and thus the sensing performance) will have a strong dependence on the nanoplasmonic architecture. Outside of a few recent studies, there has been little discussion on how changes to a nanoplasmonic architecture will affect the rate of analyte transport. We recently proposed an analytical model to predict transport to such complex architectures; however, those results were based on numerical simulation and to date, have only been partially verified. In this study we measure the characteristics of analyte transport across a wide range of plasmonic structures, varying both in the composition of their base plasmonic element (microwires, nanodisks, and nanorods) and the packing density of such elements. We functionalized each structure with nucleic acid-based bioreceptors, where for each structure we used analyte/receptor sequences as to maintain a Damk\uf6hler number close to unity. This method allows to extract both kinetic (in the form of association and dissociation constants) and analyte transport parameters (in the form of a mass transfer coefficient) from sensorgrams taken from each substrate. We show that, despite having large differences in optical characteristics, measured rates of analyte transport for all plasmonic structures match very well to predictions using our previously proposed model. These results highlight that, along with optical characteristics, analyte transport plays a large role in the overall sensing performance of a nanoplasmonic biosensor