# Oligonucleotide surface attachment by tosylation for digital detection of microRNA using photonic resonator absorption microscopy

## TL;DR

This paper introduces a new method to attach DNA to surfaces for detecting microRNA with high sensitivity using a titanium dioxide biosensor.

## Contribution

A novel tosylation-based DNA immobilization method is introduced for stable and sensitive miRNA detection on photonic crystal biosensors.

## Key findings

- The tosylation method enabled attomolar detection of miRNA-148a-3p without silanes or PEG linkers.
- The method achieved linear signal amplification through toehold-mediated strand displacement reactions.
- The functionalization approach is broadly applicable to any titanium dioxide-based biosensor.

## Abstract

DNA capture probes attached to surfaces are necessary for many biosensing assays for the specific detection of nucleic acid target sequences. In this work, we present a chemically mediated direct oligonucleotide–titanium dioxide bond for stable DNA surface immobilization on photonic crystal (PC) biosensors. We validated the DNA–TiO2 functionalization using photonic resonator absorption microscopy (PRAM) to digitally detect hybridization-bound nanoparticles, then compared the tosylate-mediated chemical binding method to commonly used silanes (3-aminopropyl)triethoxysilane and (3-glycidyloxypropyl)trimethoxysilane. This surface functionalization process was then applied to the digital detection of nanoparticles for the ultrasensitive detection of microRNA (miRNA) sequences on PRAM. By immobilizing a single-stranded capture DNA sequence onto a titanium dioxide PC surface through a reaction with p-toluenesulfonic anhydride, we demonstrate the target recycling amplification process for digital detection of miRNA using PRAM through toehold-mediated strand displacement reactions with linear signal amplification. Using this method of DNA surface functionalization, we achieved attomolar levels of detection of target miRNA-148a-3p, a potential biomarker for certain cancers, without requiring silanes or polyethylene glycol linkers, which can be unstable or expensive. While demonstrated here using PCs for the detection of miRNA, this functionalization approach could be broadly applied to any biosensor with a titanium dioxide surface.