Direct in situ and real-time visualization of salt-dependent thiolated-DNA immobilization and biosensing on gold in nanofluidic channels

Abstract

Thiol-gold (Au) conjugation is perhaps the most routinely applied surface chemistry approach to immobilize receptors within Au-based molecular sensing platforms. However, no previous studies have investigated in situ immobilization in a nanofluidic systems. In this work, flowthrough receptor immobilization is demonstrated, using a nanoslit induced depletion zone to enable direct real-time visualization for characterizing the effect of sodium chloride (NaCl) concentrations during immobilization of thiolated single-stranded DNA on Au. The results show that the amount of fixed-DNA on Au, as well as the non-specific adsorption of DNA on fused silica surface, strongly depend on NaCl levels. Furthermore, the non-specific interaction at high NaCl levels reduces the DNA available for immobilization on Au, but enriches the DNA molecules in the channel under wash by deionized water. By adding Tween-20 to decrease the non-specific adsorption and increasing the driving pressure to increase the DNA flux under high-salt level, the immobilization time can be further decreased to 10 min or less, which is at least an order of magnitude faster than recently proposed bioconjugation methods for thiolated-DNA–Au conjugation. Moreover, this work demonstrates the capability of the platform for DNA- and aptamer-based sensing assays, thereby opening up new strategies for developing biosensors using nanofluidic platforms.