Electrochemical stripping detection of DNA hybridization based on cadmium sulfide nanoparticle tags

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Abstract

We report on the detection of DNA hybridization in connection to cadmium sulfide nanoparticle tracers and electrochemical stripping measurements of the cadmium. A nanoparticle-promoted cadmium precipitation is used to enlarge the nanoparticle tag and amplify the stripping DNA hybridization signal. In addition to measurements of the dissolved cadmium ion we demonstrate solid-state measurements following a ‘magnetic’ collection of the magnetic-bead/DNA-hybrid/CdS-tracer assembly onto a thick-film electrode transducer. The new protocol combines the amplification features of nanoparticle/polynucleotides assemblies and highly sensitive stripping potentiometric detection of cadmium, with an effective magnetic isolation of the duplex. The low detection limit (100 fmol) is coupled to good reproducibility (RSD=6%). Prospects for using binary inorganic colloids for multi-target detection are discussed.

Introduction

The development of DNA biosensors has been the subject of an intense activity [1], [2], [3]. Such detection of DNA hybridization has greatly benefited from the recent use of nanoparticle materials [4], [5], [6], [7], [8], [9]. The unique properties of nanometer-sized materials make them excellent candidates for DNA sensing [11]. Recent activity in this area has involved the use of gold or silver nanoparticle probes for optical [4], [5] or electrochemical [6], [7], [8] monitoring of DNA hybridization, and the use of quantum-dot tagging for optical [9] or photoelectrochemical [10] gene sensing.

Here we report on a nanoparticle-based electrochemical detection of DNA hybridization based on cadmium sulfide (CdS) nanoparticle tags and potentiometric stripping measurements of the dissolved cadmium tracer. The use of metal labels (e.g., Au, Ag) for sensing DNA hybridization in connection to highly sensitive stripping detection has been demonstrated [6], [7], [8]. Combining the catalytic enlargement of the metal-particle tags, with the effective ‘built-in’ amplification of electrochemical stripping analysis, has thus paved the way to remarkably low (fmol) detection limits of target DNA. Analogous protocols based on binary inorganic colloids have not been reported, despite the rich chemistry of such inorganic nanoparticles. Particularly attractive for such bioassays are CdS and PbS semiconductor particles in view of the attractive stripping behavior of cadmium or lead ions [12]. Fig. 1 outlines the steps of the new bioelectronic protocol, including the binding of the biotinylated target to streptavidin-coated magnetic beads (A), its hybridization event to a CdS-labeled probe (B), dissolution of the CdS tag (C) and stripping potentiometric detection of the dissolved cadmium ion (D). We will also demonstrate that the dissolution step (C) can be eliminated and that solid-state detection of the cadmium can be accomplished through ‘magnetic’ collection of the DNA-linked nanoparticle network [13]. The performance characteristics of the new CdS-nanoparticle based nucleic-acid detection protocol are reported in the following sections.

Section snippets

Apparatus

Chronopotentiometric measurements were performed with a potentiometric stripping PSU20 system (Radiometer), controlled by a PC using the TAP2 software (Radiometer). The detection was carried out in a 1.5 ml electrochemical cell containing a mercury-coated glassy carbon disk working electrode (2 mm diameter), an Ag/AgCl reference electrode (CH Instruments, Austin, TX), and a platinum wire counter electrode. The magnetic bead assays were performed on an MCB 1200 Biomagnetic Processing Platform

Results and discussion

The new procedure couples the high sensitivity of nanoparticle-based stripping assays with a magnetic discrimination against non-complementary nucleic acids. The present protocol relies on binary inorganic (CdS) nanoparticles, compared to single (Au or Ag) metal colloids used in early stripping DNA procedures [6], [7], [8], [14]. Fig. 2 displays typical chronopotentiograms for solutions containing increasing target concentrations in 200 ng ml−1 steps (A–C), as well as to a large excess of

Conclusions

We have demonstrated for the first time the use of binary inorganic nanoparticles, such as CdS colloids, for electrochemical monitoring of DNA hybridization. The high sensitivity and selectivity make this protocol a useful addition to the armory of nanoparticle-based electrochemical genetic testing schemes. While the concept has been presented within the context of CdS particles, it could be readily extended to other inorganic colloids (e.g., ZnS or PbS) that can be similarly synthesized in

Acknowledgements

Financial support from the National Science Foundation and the US Army Medical Research (Award No. DAMD 17-00-1-0366) is gratefully acknowledged.

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