Disposable DNA biosensor with the carbon nanotubes–polyethyleneimine interface at a screen-printed carbon electrode for tests of DNA layer damage by quinazolines

A screen-printed carbon working electrode within a commercially available screen-printed three-electrode assembly was modified by using a composite of multiwalled carbon nanotubes (MWCNT) dispersed in polyethylenimine (PEI) followed by covering with the calf thymus dsDNA layer. Several electrochemical methods were used to characterize the biosensor and to evaluate damage to the surface-attached DNA: square wave voltammetry of the [Ru(bpy)₃]²⁺ redox indicator and mediator of the guanine moiety oxidation, cyclic voltammetry and electrochemical impedance spectroscopy in the presence of the [Fe(CN)₆]^3−/4− indicator in solution. Due to high electroconductivity and large surface area of MWCNT and positive charge of PEI, the MWCNT–PEI composite is an advantageous platform for the DNA immobilization by the polyelectrolyte complexation and its voltammetric and impedimetric detection. In this respect, the MWCNT–PEI interface exhibited better properties than the MWCNT–chitosan one reported from our laboratory previously. A deep DNA layer damage at incubation of the biosensor in quinazoline solution was found, which depends on the quinazoline concentration and incubation time. Figure Impedance spectra for the modified electrodes. Conditions: 1 mM [Fe(CN)₆]^3–/4– in 0.1 M PBS (pH = 7.0), potential amplitude 10 m V, frequency range 12–1×10₄ Hz. Dedicated to Professor Jan Garaj on the occasion of his 75th birthday     

Voltammetric detection of damage to DNA caused by nitro derivatives of fluorene using an electrochemical DNA biosensor

An electrochemical DNA biosensor based on the screen printed carbon paste electrode (SPCPE) with an immobilized layer of calf thymus double-stranded DNA has been used for in vitro investigation of the interaction between genotoxic nitro derivatives of fluorene (namely 2-nitrofluorene and 2,7-dinitrofluorene) and DNA. Two types of DNA damage have been detected at the DNA/SPCPE biosensor: first, that caused by direct association of the nitrofluorenes, for which an intercalation association has been found using the known DNA intercalators [Cu(phen)₂]²⁺ and [Co(phen)₃]³⁺ as competing agents, and, second, that caused by short-lived radicals generated by electrochemical reduction of the nitro group (observable under specific conditions only).     

Detection of Aeromonas hydrophila DNA oligonucleotide sequence using a biosensor design based on Ceria nanoparticles decorated reduced graphene oxide and Fast Fourier transform square wave voltammetry

A new strategy was introduced for ssDNA immobilization on a modified glassy carbon electrode. The electrode surface was modified using polyaniline and chemically reduced graphene oxide decorated cerium oxide nanoparticles (CeO₂NPs-RGO). A single-stranded DNA (ssDNA) probe was immobilized on the modified electrode surface. Fast Fourier transform square wave voltammetry (FFT-SWV) was applied as detection technique and [Ru(bpy)₃]^2+/3+ redox signal was used as electrochemical marker. The hybridization of ssDNA with its complementary target caused a dramatic decrease in [Ru(bpy)₃]^2+/3+ FFT-SW signal. The proposed electrochemical biosensor was able to detect Aeromonas hydrophila DNA oligonucleotide sequence encoding aerolysin protein. Under optimal conditions, the biosensor showed excellent selectivity toward complementary sequence in comparison with noncomplementary and two-base mismatch sequences. The dynamic linear range of this electrochemical DNA biosensor for detecting 20-mer oligonucleotide sequence of A. hydrophila was from 1 × 10⁻¹⁵ to 1 × 10⁻⁸ mol L⁻¹. The proposed biosensor was successfully applied for the detection of DNA extracted from A. hydrophila in fish pond water up to 0.01 μg mL⁻¹ with RSD of 5%. Besides, molecular docking was applied to consider the [Ru(bpy)₃]^2+/3+ interaction with ssDNA before and after hybridization.     

Mapping the DNA Damaging Effects of Polypyridyl Copper Complexes with DNA Electrochemical Biosensors

Several classes of copper complexes are known to induce oxidative DNA damage that mediates cell death. These compounds are potentially useful anticancer agents and detailed investigation can reveal the mode of DNA interaction, binding strength, and type of oxidative lesion formed. We recently reported the development of a DNA electrochemical biosensor employed to quantify the DNA cleavage activity of the well-studied [Cu(phen)₂]²⁺ chemical nuclease. However, to validate the broader compatibility of this sensor for use with more diverse—and biologically compatible—copper complexes, and to probe its use from a drug discovery perspective, analysis involving new compound libraries is required. Here, we report on the DNA binding and quantitative cleavage activity of the [Cu(TPMA)(N,N)]²⁺ class (where TPMA = tris-2-pyridylmethylamine) using a DNA electrochemical biosensor. TPMA is a tripodal copper caging ligand, while N,N represents a bidentate planar phenanthrene ligand capable of enhancing DNA interactions through intercalation. All complexes exhibited electroactivity and interact with DNA through partial (or semi-) intercalation but predominantly through electrostatic attraction. Although TPMA provides excellent solution stability, the bulky ligand enforces a non-planar geometry on the complex, which sterically impedes full interaction. [Cu(TPMA)(phen)]²⁺ and [Cu(TPMA)(DPQ)]²⁺ cleaved 39% and 48% of the DNA strands from the biosensor surface, respectively, while complexes [Cu(TPMA)(bipy)]²⁺ and [Cu(TPMA)(PD)]²⁺ exhibit comparatively moderate nuclease efficacy (ca. 26%). Comparing the nuclease activities of [Cu(TPMA)(phen)] ²⁺ and [Cu(phen)₂]²⁺ (ca. 23%) confirms the presence of TPMA significantly enhances chemical nuclease activity. Therefore, the use of this DNA electrochemical biosensor is compatible with copper(II) polypyridyl complexes and reveals TPMA complexes as a promising class of DNA damaging agent with tuneable activity due to coordinated ancillary phenanthrene ligands.     

Nucleic Acid Biosensor for Detection of Human Immunodeficiency Virus Using Aquabis(1,10‐phenanthroline)copper(II) Perchlorate as Electrochemical Indicator

The electrochemical behavior of aquabis(1,10‐phenanthroline)copper(II) perchlorate [Cu(H₂O)(phen)₂]·2ClO₄, where phen=1,10‐phenanthroline, on binding to DNA at a glassy carbon electrode (GCE) and in solution, was described. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) results showed that [Cu(H₂O)(phen)₂]²⁺ had excellent electrochemical activity on the GCE with a couple of quasi‐reversible redox peaks. The interaction mode between [Cu(H₂O)(phen)₂]²⁺ and double‐strand DNA (dsDNA) was identified to be intercalative binding. An electrochemical DNA biosensor was developed with covalent immobilization of human immunodeficiency virus (HIV) probe for single‐strand DNA (ssDNA) on the modified GCE. Numerous factors affecting the probe immobilization, target hybridization, and indicator binding reactions were optimized to maximize the sensitivity and speed of the assay. With this approach, a sequence of the HIV could be quantified over the range from 7.8×10^?9 to 3.1×10^?7 mol·L^?1 with a linear correlation of γ=0.9987 and a detection limit of 1.3×10^?9 mol·L^?1.     

A label-free electrochemical biosensor for trace uranium based on DNAzymes and gold nanoparticles

A novel and highly sensitive electrochemical DNAzymes biosensor was fabricated using Au nanoparticles (AuNPs) immobilized on the surface of Au electrode that had been previously modified with self-assembled monolayers of 1,6-hexanedithiol. Different modified electrodes were prepared and characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The AuNPs were found to have a large surface area to anchor a large number of negatively charged phosphate backbones of DNAzymes, which further absorbed the electroactive indicator of hexaammineruthenium(III) ([Ru(NH₃)₆]³⁺) to amplify the electrochemical signal. In the presence of target molecules, a large amount of DNA partly associated with [Ru(NH₃)₆]³⁺ were removed from the electrode surface, leading to a significant decrease in peak current. Differential pulse voltammetry signals of [Ru(NH₃)₆]³⁺ provided quantitative measures of the concentrations of uranyl ion (UO₂ ²⁺), with linear calibration ranging from 13 pM to 0.15 nM and a detection limit of 5 pM. The presence of other metal ions did not affect the detection of UO₂ ²⁺, which indicated the high specificity of UO₂ ²⁺. Therefore, a new electrochemical DNAzymes sensor was designed with specific DNAzymes and AuNPs as immobilization platform and signal amplifier.     

Sensitively Electrochemical Sensing for Sequence‐Specific Detection of Phosphinothricin Acetyltransferase Gene: Layer‐by‐Layer Films of Poly‐L‐Lysine and Au‐Carbon Nanotube Hybrid

An electrochemical DNA sensing film was constructed based on the multilayers comprising of poly‐L ‐lysine (pLys) and Au‐carbon nanotube (Au‐CNT) hybrid. A precursor film of mercaptopropionic acid (MPA) was firstly self‐assembled on the Au electrode surface. pLys and Au‐CNT hybrid layer‐by‐layer assembly films were fabricated by alternately immersing the MPA‐modified electrode into the pLys solution and Au‐CNT hybrid solution. Cyclic voltammetry was used to monitor the consecutive growth of the multilayer films by utilizing [Fe(CN)₆]^3?/4? and [Co(phen)₃]^3+/2+ as the redox indicators. The outer layer of the multilayer film was the positively charged pLys, on which the DNA probe was easily linked due to the strong electrostatic affinity. The hybridization detection of DNA was accomplished by using methylene blue (MB) as the indicator, which possesses different affinities to dsDNA and ssDNA. Differential pulse voltammetry was employed to record the signal response of MB and determine the amount of the target DNA sequence. The established biosensor has high sensitivity, a relatively wide linear range from 1.0×10^?10 mol/L to 1.0×10^?6 mol/L and the ability to discriminate the fully complementary target DNA from single or double base‐mismatched DNA. The sequence‐specific DNA related to phosphinothricin acetyltransferase gene from the transgenically modified plants was successfully detected.     

Biosensor for Hepatitis B Virus DNA PCR Product and Electrochemical Study of the Interaction of Di(2,2′‐bipyridine)osmium(III) with DNA

The strategy for electrochemical detection of HBV DNA PCR product (181 bps) was designed by covalently immobilizing single‐stranded HBV DNA on preoxidized glassy carbon electrode surface. The immobilization of single stranded DNA was verified by AC impedance spectra. The following hybridization reaction on surface was evidenced by electrochemical methods using [Os(bpy)₂Cl₂]⁺ as an electroactive indicator. The interactions of [Os(bpy)₂Cl₂]⁺ with calf thymus single and double stranded DNA immobilized on preoxidized glassy carbon electrodes were studied. [Os(bpy)₂Cl₂]⁺ could bind preferentially to the duplex DNA by intercalating to base pairs. The intrinsic binding constant of [Os(bpy)₂Cl₂]⁺ with calf thymus DNA was calculated to be 1.21×10⁴ M^?1. Using [Os(bpy)₂Cl₂]⁺ as an electrochemical hybridization indicator, the HBV DNA sensor has been used to detect qualitatively target HBV DNA in solution with high sensitivity and selectivity.     

A label-free electrochemical DNA biosensor based on a Zr(IV)-coordinated DNA duplex immobilised on a carbon nanofibre|chitosan layer

A label-free electrochemical biosensor for detecting DNA hybridisation was developed by monitoring the change in the voltammetric activity of ferrocenecarboxylic acid at the biosensor–solution interface. The biosensor was constructed by initially immobilising on a glassy carbon electrode an anchoring layer consisting of chitosan, carboxyl group functionalised carbon nanofibres and glutaraldehye. Chitosan acted as an adhering agent and carbon nanofibres were strategically used to provide a large surface area with binding points for DNA immobilisation, while glutaraldehye was a linker for DNA probes on the electrode surface. Based on a two-factorial design, cyclic voltammetry of [Fe(CN)₆]^3−/4− was performed to optimise the composition of the anchoring layer. Next, a 17-base pair DNA probe was attached to the anchoring layer, followed by its complementary target. Zr(IV) ion, known to exhibit affinity for oxygen-containing electroactive markers, for example, ferrocenecarboxylic acid, was then coordinated in the DNA duplex. In this way, ferrocenecarboxylic acid was attracted towards the biosensor for oxidation. A change in the voltammetric oxidation current of ferrocenecarboxylic acid pre- and post-hybridisation was used to provide an indication of hybridisation. A linear dynamic range between 0.5 and 40 nM and a detection limit of 88 pM of DNA target were then achieved. In addition, the biosensor exhibited good selectivity, repeatability and stability for the determination of DNA sequences.     

Electrochemical Properties of a Conducting Film Derived from Iron(II) Tris(diaminopolypyridyl) Complex in the S(IV) Oxoanions Reduction

A new conducting film derived from the complex [Fe (diaphen)₃]²⁺, (diaphen=5,6‐diamino‐1,10‐phenanthroline) was electropolymerized by cyclic voltammetry onto a glassy carbon electrode. Poly‐[Fe^II (diaphen)₃] was studied by cyclic voltammetry, SEM, UV‐vis and micro‐Raman spectroscopy. Poly‐[Fe^II (diaphen)₃] shows electrocatalytic activity in HSO₃^? reduction in an ethanol/water solution. Electrocatalysis is centered at the π ring of phenanthroline. Rotating disk electrode studies showed a 0.117 V/dec Tafel slope, suggesting an EC process where the electrochemical process is the determining step. The chemical step was studied by UV‐vis spectroelectrochemistry. Amperometric behavior showed a linear range between 47.5 µM to 417 µM and the LOD was 19.5 µM.     

Impedimetric DNA‐Based Biosensor for Silver Ions Detection with Hemin/G‐Quadruplex Nanowire as Enhancer

A DNA‐based biosensor was reported for detection of silver ions (Ag⁺) by electrochemical impedance spectroscopy (EIS) with [Fe(CN)₆]^4?/3? as redox probe and hybridization chain reaction (HCR) induced hemin/G‐quadruplex nanowire as enhanced label. In the present of target Ag⁺, Ag⁺ interacted with cytosine‐cytosine (C? C) mismatch to form the stable C? Ag⁺? C complex with the aim of immobilizing the primer DNA on electrode, which thus triggered the HCR to form inert hemin/G‐quadruplex nanowire with an amplified EIS signal. As a result, the DNA biosensor showed a high sensitivity with the concentration range spanning from 0.1 nM to 100 µM and a detection limit of 0.05 nM.     

An Electrochemical Aptamer Biosensor for Bisphenol A on a Carbon Nanofibre-silver Nanoparticle Immobilisation Platform

A nanocomposite platform of silver nanoparticles and carbon nanofibres (AgCNFs) was used to immobilise a bisphenol A specific 63-mer ssDNA aptamer to form a biosensor. The fabrication process of the biosensor was studied with electrochemical impedance spectroscopy and cyclic voltammetry in the presence of [Fe(CN)₆]^3−/4− as redox probe. The biosensor detected bisphenol A in a linear range of 0.1–10 nM, with a limit of detection of 0.39 nM using square wave voltammetry (SWV). The biosensor exhibited good selectivity in the presence of interfering species at 100-fold concentrations and was used to detect BPA in real water sample.     

Electrically ‘wired’ tyrosinase enzyme inhibition electrode for the detection of respiratory poisons

The use of the solution redox species, [Os(bpy)₂Cl₂]^+/0, [Os(bpy)₂(MeIm)Cl]^2+/+ and [Fe(CN)₆]^4−/3−, where bpy is 2,2-bipyridine and MeIm is N-methylimidazole, as electron mediators in the enzymatic reduction of oxygen by tyrosinase is investigated. Co-immobilization of both enzyme and an osmium redox mediator in a hydrogel on glassy carbon electrodes results in a biosensor for the ‘reagentless’ addressing of enzyme activity, consuming only oxygen present in solution. Immobilized enzyme inhibition biosensors can thus be constructed for the detection of tyrosinase inhibitors, such as sodium azide, using this approach. The enzyme inhibition biosensor can detect levels of azide as low as 5 × 10⁻⁶ mol dm⁻³ in solution and may be useful in environmental monitoring applications and as an early warning poison sensor.     

Improved electrochemical performances of polyaniline nanotubes-poly-L-lysine composite for label-free impedance detection of DNA hybridization

A sensitive label-free DNA hybridization biosensing platform was fabricated based on the synergistic effect of polyaniline nanotubes (PANI_nt) and poly-L-lysine (pLys). The composite of pLys and PANI_nt was coated onto the carbon paste electrode (CPE) to form a uniform and very stable nanocomposite membrane. The pLys in the composite film not only acts as a membrane to retain good electron transfer capability of PANI_nt even at physiological pH, but also possesses fine biocompatibility for bio-analytes. DNA probes with negatively charged phosphate groups were readily linked to the positively charged pLys surface due to the strong electrostatic affinity. The synergistic effect of PANI_nt and pLys could significantly enhance the sensitivity of DNA hybridization recognition. The phosphinothricin acetyltransferase (PAT) gene fragment from transgenic corn and the polymerase chain reaction amplification of the terminator of nopaline synthase gene from the real sample of a kind of transgenic soybean were detected by this DNA electrochemical biosensor via label-free impedance method. This stable composite gives convenient permselectivity properties as a transducer material for the design of modern electrochemical impedance biosensor using [Fe(CN)₆]^3?/4? as an indicator.     

Carbon nanotubes/(pLys/dsDNA) n layer-by-layer multilayer films for electrochemical studies of DNA damage

Carboxylic group-functionalized carbon nanotubes (c-CNT) were modified on the surface of carbon paste electrode to obtain a conducting precursor film. Positively charged poly-l-lysine (pLys) and negatively charged double-stranded DNA (dsDNA) were alternately adsorbed on the c-CNT-modified electrode, forming (pLys/dsDNA) _n layer-by-layer (LBL) films. Cyclic voltammetry and electrochemical impedance spectroscopy of the electroactive probe [Fe(CN)₆]^3−/4− could give the valuable dynamic information of multilayer films growth. The oxidative DNA damage induced by cadmium ion (Cd²⁺) in the LBL multilayer films was studied by differential pulse voltammetry (DPV) with methylene violet (MV) as the intercalation redox probe. The electrochemical signals of MV on the multilayer films were effectively amplified via LBL technology. The specific intercalation of MV into dsDNA base pairs and the amplified electrochemical response of MV, combined with the unique feature of loading reversibility of MV in the DNA layer-by-layer films, made the difference in DPV response between the intact, and damaged dsDNA films become pronounced. This biosensor exhibited that the (pLys/dsDNA) _n films could be utilized for investigations of DNA damage.     

Elongated Thrombin Binding Aptamer: A G‐Quadruplex Cation‐Sensitive Conformational Switch

Aptamer‐based biosensors offer promising perspectives for high performance, specific detection of proteins. The thrombin binding aptamer (TBA) is a G‐quadruplex‐forming DNA sequence, which is frequently elongated at one end to increase its analytical performances in a biosensor configuration. Herein, we investigate how the elongation of TBA at its 5′ end affects its structure and stability. Circular dichroism spectroscopy shows that TBA folds in an antiparallel G‐quadruplex conformation with all studied cations (Ba²⁺, Ca²⁺, K⁺, Mg²⁺, Na⁺, NH₄⁺, Sr²⁺ and the [Ru(NH₃)₆]^2+/3+ redox marker) whereas other structures are adopted by the elongated aptamers in the presence of some of these cations. The stability of each structure is evaluated on the basis of UV spectroscopy melting curves. Thermal difference spectra confirm the quadruplex character of all conformations. The elongated sequences can adopt a parallel or an antiparallel structure, depending on the nature of the cation; this can potentially confer an ion‐sensitive switch behavior. This switch property is demonstrated with the frequently employed redox complex [Ru(NH₃)₆]³⁺, which induces the parallel conformation at very low concentrations (10 equiv per strand). The addition of large amounts of K⁺ reverts the conformation to the antiparallel form, and opens interesting perspectives for electrochemical biosensing or redox‐active responsive devices.     

Sensitive electrochemical DNA-based biosensors for the determination of Ag<Superscript>+</Superscript> and Hg<Superscript>2+</Superscript> ions and their application in analysis of amalgam filling

In the present paper, we used single-stranded poly-T (100% thymine bases) and poly-C (100% cytosine bases) nucleic acids as DNA probes for selective and sensitive individual electrochemical determination of Hg²⁺ and Ag⁺_, respectively, on the multi-walled carbon nanotube paste electrodes (MWCNTPEs) using [Fe(CN)₆]^3?/4? as electroactive labels. In the presence of Hg²⁺ and Ag⁺, the probe–Hg²⁺/Ag⁺ interactions through T–Hg²⁺–T and C–Ag⁺–C complexes formation could cause the formation of a unimolecular hybridized probe. This structure of probe led to its partial depletion from electrode surface and facilitation of electron transfer between [Fe(CN)₆]^3?/4? redox couple and electrode surface, resulting in the enhanced differential pulse voltammetry (DPV) oxidation current of [Fe(CN)₆]^3?/4? at the probe-modified electrode surface. We applied the difference in the oxidation peak currents of [Fe(CN)₆]^3?/4? before and after Hg²⁺/Ag⁺–DNA probe bonding (?I) for electrochemical determination of these heavy metal ions. Detection limits were 8.0?×?10^?12 M and 1.0?×?10^?11 M for Hg²⁺ and Ag⁺ ions determination, respectively. The biosensors were utilized to determine the weight percent of toxic metals, i.e., silver and mercury in dental amalgam filling composition. The results of their practical applicability in analysis of the amalgam sample were satisfactory.     

DNA-based biosensors with external Nafion and chitosan membranes for the evaluation of the antioxidant activity of beer,coffee, and tea

Novel electrochemical DNA-based biosensors with outer-sphere Nafion and chitosan protective membranes were prepared for the evaluation of antioxidant properties of beverages (beer, coffee, and black tea) against prooxidant hydroxyl radicals. A carbon working electrode of a screen-printed three-electrode assembly was modified using a layer-by-layer deposition technique with low molecular weight double-stranded DNA and a Nafion or chitosan film. The membrane-covered DNA biosensors were initially tested with respect to their voltammetric and impedimetric response after the incubation of the beverage and the medium exchange for the solution of the redox indicator [Fe(CN)₆]^3?/4?. While the Nafion-protected biosensor proved to be suitable for beer and black tea extracts, the chitosan-protected biosensor was successfully used in a coffee extract. Afterwards, the applicability was successfully verified for these biosensors for the detection of a deep degradation of the surface-attached DNA at the incubation in the cleavage agent (hydroxyl radicals generated via Fenton reaction) and for the evaluation of antioxidant properties of coffee and black tea extracts against prooxidant hydroxyl radicals. The investigation of the novel biosensors with a protective membrane represents a significant contribution to the field of electrochemical DNA biosensors utilization.      

离子强度对DNA媒介电荷转移的影响——基于Debye-Hückel 理论的实验结果分析

用电化学的方法研究了溶液离子强度对DNA媒介电荷转移的影响, 观察到[Ru(NH₃)₆]³⁺的还原峰电势随支持电解质的浓度增加向负方向移动. 分析发现微分脉冲伏安法(DPV)的峰电势与溶液离子强度间在一定范围内存在线性关系, 以式电势(E⁰')作为“桥梁”, 用Debye-Hückel理论给予了解释. 在高离子强度下, 峰电势对线性关系的偏移是由于超过了Debye-Hückel理论的适用范围, 而无强电解质存在时, DNA自身堆积的强负电荷对DNA媒介电荷转移起了推动作用.     

Tris(2,2′-bipyridyl)cobalt(III)-doped silica nanoparticle DNA probe for the electrochemical detection of DNA hybridization

A new and sensitive electrochemical DNA hybridization detection assay, using tris(2,2′-bipyridyl)cobalt(III) [Co(bpy)₃³⁺]-doped silica nanoparticles as the oligonucleotide (ODN) labeling tag, and based on voltammetric detection of Co(bpy)₃³⁺ inside silica nanoparticles, is described. Electro-active Co(bpy)₃³⁺ is not possible for directly linking with DNA, it is doped into the silica nanoparticles in the process of nanoparticles synthesis for DNA labeling with trimethoxysilylpropydiethylenetriamine (DETA) and glutaraldehyde as linking agents. The Co(bpy)₃³⁺ labeled DNA probe is used to hybridize with target DNA immobilized on the surface of glassy carbon electrode. Only the complementary sequence DNA (cDNA) could form a double-stranded DNA (dsDNA) with the DNA probe labeled with Co(bpy)₃³⁺ and give an obvious electrochemical response. A three-base mismatch sequence and non-complementary sequence had negligible response. Due to the large number of Co(bpy)₃³⁺ molecules inside silica nanoparticles linked to oligonucleotide DNA probe, the assay showed a high sensitivity. It allows the detection at levels as low as 2.0×10⁻¹⁰ mol l⁻¹ of the target oligonucleotides.