Effects of NaCl and MgCl2 Concentrations on the Adsorption of FAM-Labeled Single-Stranded DNA onto the Surface of Graphene Oxide Nanomaterial

Abstract

The interaction of single-stranded DNA (ssDNA) with carbon-based nanomaterials has attracted considerable attention because of the distinctive surface properties of these materials and their broad applications in biosensing and nanobiotechnology. Among carbon-based nanomaterials, graphene oxide (GO), with its layered structure, high specific surface area, and abundance of oxygen-containing functional groups, provides a suitable platform for investigating nucleic acid–nanomaterial interactions. In this study, the interaction of FAM-labeled single-stranded DNA (ssDNA-FAM) with the GO surface was investigated using fluorescence spectroscopy. The fluorescence emission spectrum of ssDNA-FAM was recorded over the wavelength range of 500–600 nm, with a maximum emission at approximately 520 nm. Upon addition of GO, a substantial decrease in the fluorescence emission intensity of ssDNA-FAM was observed, indicating pronounced quenching of the FAM fluorescence. This reduction in fluorescence is consistent with an interaction between ssDNA and the GO surface that brings the fluorophore into close proximity to the nanomaterial surface. The effect of ionic conditions on the fluorescence response of the ssDNA-FAM/GO system was subsequently evaluated by examining different concentrations of NaCl and MgCl₂. Increasing the NaCl concentration from 0 to 100 mM resulted in a progressive decrease in fluorescence emission intensity, indicating enhanced quenching of the FAM signal at higher NaCl concentrations. A similar trend was observed with MgCl₂. Increasing the MgCl₂ concentration from 0 to 10 mM led to a reduction in fluorescence intensity, with a more pronounced decrease observed at higher concentrations, particularly over the range of 1–10 mM. The observed changes may be associated with alterations in electrostatic interactions within the ssDNA-FAM/GO system. Increased ionic strength can reduce electrostatic repulsion between negatively charged DNA phosphate groups and negatively charged regions of the GO surface, thereby facilitating closer association of ssDNA with the nanomaterial. In the presence of Mg²⁺, charge screening may be further enhanced because of the higher charge density of divalent cations. These changes may promote closer proximity of the FAM fluorophore to the GO surface, thereby increasing fluorescence quenching. Overall, the results demonstrate that ionic composition is an important factor influencing the fluorescence response of the ssDNA-FAM/GO system. Increasing concentrations of both NaCl and MgCl₂ enhanced fluorescence quenching and reduced the emission intensity of FAM. Modulation of ionic conditions may therefore provide a useful approach for controlling ssDNA–GO interactions and optimizing the fluorescence response of GO-based nucleic acid sensing systems.