Nanoparticle/DNAzyme based biosensors for heavy-metal ion detection: Effect of DNAzyme surface modifications on device sensitivity

Heavy metal ions, such as Lead (Pb), Chromium (Cr), Cadmium (Cd) etc. present a serious environmental threat when found in soil and water; thus their accurate and fast detection poses as a major challenge. Although several detection methods have been proposed in the literature, they are both expensive and time consuming. In this work we present a biosensing device for heavy-metal ion detection based on Platinum nanoparticles (Pt NPs) and DNAzymes. The biosensors feature two distinctive DNAzymes species with different chemical modification groups (i.e thiol and amino modified), that enable their attachment on the Pt NP layer.

The bio-sensing devices were characterized by measuring changes in resistance using a Keithley 2400 multimeter, under a 1 V bias. As a first step, buffer solution was drop-casted on top of the bio-sensors; this step was repeated until device resistance became immune to any further buffer addition. After the buffer-stabilization steps, a buffer solution containing a known heavy-metal ion concentration was drop-casted on the sensor, resulting in an increase in device resistance in accordance with the mechanism discussed in previous publicatios of this group. In this study, we compare the sensitivity of the two different DNAzymes for varying Pb²⁺ concentrations. Both techniques present a linear response range, for maximum concentrations of 225 nM and 250 nM in the case of thiol and amino modified DNAzymes respectively. The devices also exhibit a low limit of detection (LoD) of 25 nM, in agreement with the permitted levels of Pb²⁺ for the EU. Finally, thiol modified DNAzymes showcase higher sensitivity when compared to the amino modified DNAzymes; this can be related to the variations between the two chemical modifications and the quality of their attachment on the NP layer