DNA Adsorption by Indium Tin Oxide Nanoparticles Biwu Liu and Juewen Liu* Department of Chemistry, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada * S Supporting Information ABSTRACT: The high conductivity and optical transparency of indium tin oxide (ITO) has made it a popular material in the electronic industry. Recently, its application in biosensors is also explored. To understand its biointerface chemistry, we herein investigate its interaction with fluorescently labeled single- stranded oligonucleotides using ITO nanoparticles (NPs). The fluorescence of DNA is efficiently quenched after adsorption, and the interaction between DNA and ITO NPs is strongly dependent on the surface charge of ITO. At low pH, the ITO surface is positively charged to afford a high DNA adsorption capacity. Adsorption is also influenced by the sequence and length of DNA. For its components, In 2 O 3 adsorbs DNA more strongly while SnO 2 repels DNA at neutral pH. The DNA adsorption property of ITO is an averaging result from both components. DNA adsorption is confirmed to be mainly by the phosphate backbone via displacement experiments using free phosphate or DNA bases. Last, DNA-induced DNA desorption by forming duplex DNA is demonstrated on ITO, while the same reaction is more difficult to achieve on other metal oxides including CeO 2 , TiO 2 , and Fe 3 O 4 because these particles adsorb DNA more tightly. ■ INTRODUCTION DNA-functionalized nanomaterials have attracted extensive research interest. These hybrid materials combine the molecular recognition and programmable property of DNA with the physical properties of inorganic nanoparticles, showing promising applications in many fields including biosensing, 1-6 drug delivery, 7 materials science, 8,9 and nanotechnology. 10-13 Over the past two decades, many nanomaterials, such as metallic nanoparticles, 14 semiconductor quantum dots, 15 and nanoscale carbon materials (e.g., carbon nanotubes and graphene oxide), 16 have been modified with DNA. Each type of material has its own interaction force for DNA adsorption. A key step in constructing such materials is the attachment of DNA to the particle surface. Depending on the surface chemistry, several conjugation methods have been developed, including covalent bonding, metal coordination, and phys- isorption. 17 While covalent attachment provides a strong linkage with DNA, physisorption is attractive due to its simplicity, cost-effectiveness, and reversible binding. For example, DNA is readily adsorbed onto the graphene oxide, and the complementary DNA (cDNA) induces DNA desorption by forming a duplex. 16,18 For gold nanoparticles (AuNPs), while thiolated DNA is the main reagent for attachment, nonthiolated DNA has recently emerged as an alternative. 19,20 Metal oxides encompass a large number of important materials. In the past few years, the adsorption of DNA onto several metal oxides has been investigated. 21-26 For example, by forming a DNA layer, the catalytic property of CeO 2 was altered. 22 Fluorescently labeled DNA molecules were used to detect arsenate ions in water based on the competitive adsorption onto iron oxide nanoparticles. 23 Given this progress, our understanding on the DNA/oxide interaction is far from complete. Because each metal is different, and different oxides may interact very differently with DNA, it is impossible to predict DNA adsorption to a new oxide based on the current knowledge. Indium-doped tin oxide (ITO) is a very important material because it is both transparent and conductive. 27 ITO is used in electrical 28 and photoelectrochemical 29 biosensors for DNA as well as other targets. For example, Gao et al. developed a photoelectrochemical DNA sensor by conjugating an aldehyde- modified capture DNA onto a silanized ITO electrode. 30 The cDNA hybridization was followed by tagging a photoactive intercalator and the increased photocurrent. In other applications, direct DNA-ITO interaction was utilized for the detection of cDNA, 31 DNA methylation, 32 and pathogen. 33 At the same time, nanoscale ITO particles are particular interesting in making electrodes. 34,35 Some DNA-ITO nano- particle (NP) conjugates have been made into conductive networks for DNA detection. 36 Despite these analytical applications, little is known about the fundamental interactions between DNA and ITO. In this work, we study the adsorption of DNA oligomers on ITO NPs as a function of pH, salt concentration, and DNA sequence. By changing the oxide composition and displacement experiments, we also proposed the adsorption mechanism. Finally, ITO was used to achieve DNA-induced desorption, which was found to be more difficult with other types of metal oxides. Received: October 3, 2014 Revised: November 17, 2014 Published: December 18, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 371 DOI: 10.1021/la503917j Langmuir 2015, 31, 371-377