DNA-capped silver nanoparticles for stochastic nanoparticle impact electrochemistry

One of the major challenges in analytical chemistry is reducing the detection limit of an analyte down to a level where the specific identification of a single entity is possible. In this context, nano-impact electrochemistry is one of the most active and promising research areas in the field of single-entity experiments. This method is a versatile analytical procedure for characterization and real-time monitoring of bioconjugation and biomolecular recognition events as well as for ultrasensitive detection of a variety of biological species. The combination of a highly sensitive amplifier system and high-density microelectrode arrays allows detection of single silver nanoparticle impacts down to subpicomolar concentrations. For the analyte detection, silver nanoparticles are modified with biomolecular receptors alternating their impact frequency on the electrode surface. Thus, the particles serve as redox tags converting an otherwise redox-inactive target into an electrochemically detectable species. In this work, silver nanoparticles were modified with thiolated single stranded oligonucleotides with varying molar ratios of DNA to nanoparticles. The modified conjugation protocol resulted in stable DNA-nanoparticle conjugates. In depth characterization of these conjugates gave insight into their structural and physicochemical properties. In a next step, the impact behaviour of DNA-capped nanoparticles was evaluated and compared to citrate-capped nanoparticles. Different parameters were identified to influence the impact probability. First, the surface modification results in a higher nanoparticle stability by preventing particle aggregation, which increases the impact frequency, especially in the presence of high salt concentrations. Second, the redox activity is reduced in comparison to citrate stabilized particles. In particular, the ligand surface density as well as the conformation and size of the receptor molecule were found to play a crucial role. Furthermore, the composition of the electrolyte and the applied potential affect the impact probability, but to a different extent as for citrate stabilized particles. By carefully adjusting the surface density of ligands, a high particle stability is achieved while maintaining their desired redox activity. The results demonstrate that DNA-AgNPs possess impact characteristics different from standard citrate stabilized particles. In a last step, stochastic nanoparticle impact electrochemistry was probed for the detection of DNA hybridization events on the nanoparticle surface. The results disclose decreased hybridization efficiencies on the nanoparticle surface and reveal that a surface-bound process is more complicated when compared to hybridization in solution