A signal amplification strategy and sensing application using single gold nanoelectrodes

New nanosensor fabricated on single nanopore electrode filled with prussian blue and graphene quantum dots coated by polypyrrole for hydrogen peroxide sensing

Hydrogen peroxide (H2O2) is a common oxidant that plays an important role in many biological processes and is also an important medium analysis in various fields. In this work, a new electrochemical nanosensor capable of detecting and quantifying hydrogen peroxide was introduced. This nanosensor was fabricated by electrodepositing prussian blue (PB)/graphene quantum dots (GQDs)/polypyrrole (PPy) on single nanopore electrode etched from single gold nanoelectrode. This prepapred nanosensor exhibits good electrochemical response to hydrogen peroxide with high sensitivity and stability, with a linear response in the 2.0 and 80 μM by using amperometric method and differential pulse voltammetry (DPV) method. The limit of detections are 0.33 μM (S/N = 3) for amperometric method and 0.67 μM (S/N = 3) for differential pulse voltammetry (DPV) method, respectively. This nanosensor can be used for the determination of hydrogen peroxide in human urine, and can serve as a new electrochemical platform to monitor H2O2 release from single living cells due to its small overal dimension and high sensitivity.

Functionalized nanopores based on hybridization chain reaction: Fabrication and microRNA sensing

Enzyme-free hybridization chain reaction (HCR) technology is often used as a signal amplification tool for the detection of different targets. In this study, an ultrasensitive and label-free method for detecting miRNA-21 was developed using the nanopore ionic current rectification (ICR) technology coupled with HCR technology. The probe oligonucleotide (DNA1) was combined with the gold-coated nanopore through the Au-S bond to form a DNA1-functionalized gold-coated nanopore (DNA1-Au-coated nanopore). Since miRNA-21 is partially complementary to DNA1, it can be selectively recognized by DNA1-functionalized gold-coated nanopores. The target (miRNA-21) can induce the opening of hairpin DNA and HCR reaction after the introduction of hairpin DNA H1 and H2. The concentration of miRNA-21 will affect the combination of H1 and H2 on the inner wall of the nanopore, and its surface charge will change with the internal modification, thereby changing the ion current rectification ratio. Under the condition that the concentration of H1, H2 and HCR reaction time are constant, the change of ICR ratio is linearly correlated with the logarithm of miRNA-21 concentration within a certain range, which shows that the sensing strategy we designed can achieve target miRNA-21 detection. This ultrasensitive miRNA holds great promise in the field of cancer diagnosis.

Electrochemical Redox Cycling Behavior of Gold Nanoring Electrodes Microfabricated on a Silicon Micropillar

We report the microfabrication and characterization of concentric gold nanoring electrodes (Au NREs), which were fabricated by patterning two gold nanoelectrodes on the same silicon (Si) micropillar tip. Au NREs of 165 ± 10 nm in width were micropatterned on a 6.5 ± 0.2 µm diameter 80 ± 0.5 µm height Si micropillar with an intervening ~ 100 nm thick hafnium oxide insulating layer between the two nanoelectrodes. Excellent cylindricality of the micropillar with vertical sidewalls as well as a completely intact layer of a concentric Au NRE including the entire micropillar perimeter has been achieved as observed via scanning electron microscopy and energy dispersive spectroscopy data. The electrochemical behavior of the Au NREs was characterized by steady-state cyclic voltammetry and electrochemical impedance spectroscopy. The applicability of Au NREs to electrochemical sensing was demonstrated by redox cycling with the ferro/ferricyanide redox couple. The redox cycling amplified the currents by 1.63-fold with a collection efficiency of > 90% on a single collection cycle. The proposed micro-nanofabrication approach with further optimization studies shows great promise for the creation and expansion of concentric 3D NRE arrays with controllable width and nanometer spacing for electroanalytical research and applications such as single-cell analysis and advanced biological and neurochemical sensing.

Influence of citrate buffer and flash heating in enhancing the sensitivity of ratiometric genosensing of Hepatitis C virus using plasmonic gold nanoparticles

Gold nanoparticles (Au NPs) based technology has been shown to possess enormous potential in the viral nucleic acid diagnosis. Despite significant advancement in this domain, the existing literature reveals the diversity in the conditions employed for hybridization and tagging of thiolated nucleic acid probes over the Au NPs. Here we employ the probe sequence derived from the Hepatitis C virus to identify the optimal hybridization and thiol-Au NP tagging conditions. In a typical polymerase chain reaction, the probes are initially subjected to flash heating at elevated temperatures to obtain efficient annealing. Motivated by this, in the current study, the hybridization between the target and the antisense oligonucleotide (ASO) has been studied at 65 °C with and without employing flash heating at temperatures from 75 to 95 °C. Besides, the efficiency of the thiolated ASO’s tagging over the Au NPs with and without citrate buffer has been explored. The study has revealed the beneficial role of flash heating at 95 °C for efficient hybridization and the presence of citrate buffer for rapid and effective thiol tagging over the Au NPs. The combinatorial effect of these conditions has been found to be advantageous in enhancing the sensitivity of ratiometric genosensing using Au NPs.

Dual-signaling amplification strategy for glutathione sensing by using single gold nanoelectrodes

A new nanosensor for glutathione (GSH) detection by use of single nanoelectrodes has been developed through a dual-signaling ratiometric amplification strategy. Ag nanoparticles (Ag NPs) metalized DNA1 was modified on an Au nanoelectrode surface. Due to the strong affinity between Ag NP and GSH, Ag NPs could be removed by the addition of GSH. The remaining metalized DNA1 could hardly form a double strand, while the de-metalized DNA1 could hybrid with DNA2 and DNA3 to form a complex structure to adsorb methylene blue (MB), and then the electrochemical signal of differential pulse voltammetry (DPV) from MB oxidation could be observed. With the addition of GSH, the peak current of MB oxidation at about -0.27 V (I_MB) increases, while the signal of Ag oxidation at about 0.1 V (I_Ag) decreases. It was found that there had a linear relationship between the ratio of dual-signal (I_MB/I_Ag) and the GSH concentrations, which could be used to detect GSH. The ratiometric nanosensor is label-free, easy to operate, and can eliminate inherent system errors. Considering the advantages of nanoelectrodes, such as low IR drop, fast response, and small overall dimension, this developed nanosensor can be used for GSH detection living systems (e.g., cell lysate).

Single gold nanoclusters: Formation and sensing application for isonicotinic acid hydrazide detection

Nano-sized electrodes have their special advantages for sensing applications, such as small overall dimension, fast response and low background current. In this work, single gold nanoclusters (AuNCs) were controllably prepared on single Pt nanoelectrode surface by electrodeposition method. The AuNCs covered Pt nanoelectrode (AuNCs/PtNE) had steady-state voltammetric response in redox species solution, which was similar to micro-/nano-sized electrodes. It was interesting to find isonicotinic acid hydrazide (INH, also known as isoniazid) showed good electrochemical response on AuNCs/PtNE surface, which had investigated carefully by square wave voltammetry (SWV) and chronoamperometry. Moreover, the prepared single AuNCs/PtNEs showed the capability for INH sensing with good sensitivity, reproducibility and selectivity, which was demonstrated for INH detection in human urine samples.

Nanobioconjugates for Signal Amplification in Electrochemical Biosensing

Nanobioconjugates are hybrid materials that result from the coalescence of biomolecules and nanomaterials. They have emerged as a strategy to amplify the signal response in the biosensor field with the potential to enhance the sensitivity and detection limits of analytical assays. This critical review collects a myriad of strategies for the development of nanobioconjugates based on the conjugation of proteins, antibodies, carbohydrates, and DNA/RNA with noble metals, quantum dots, carbon- and magnetic-based nanomaterials, polymers, and complexes. It first discusses nanobioconjugates assembly and characterization to focus on the strategies to amplify a biorecognition event in biosensing, including molecular-, enzymatic-, and electroactive complex-based approaches. It provides some examples, current challenges, and future perspectives of nanobioconjugates for the amplification of signals in electrochemical biosensing.

Analytes Triggered Conformational Switch of i-Motif DNA inside Gold-Decorated Solid-State Nanopores

The nanopore-based technique is a useful tool for single-molecule sensing and characterization. In this work, we have developed a new DNA-functionalized gold-modified nanopore, and analytes can induce the conformational switch of i-motif DNA formed on the inner surface of the nanopore. i-Motif DNA structure can be formed in the presence of silver ions (Ag⁺), which will result in the change in surface charge and structure of the nanopore tip and ion current rectification (ICR) ratio. The i-motif DNA structure on nanopore surface will be destroyed after the addition of glutathione (GSH) due to the strong interaction of Ag-S bond, which results in the recovery of surface charge, steric hindrance, and ICR ratio. This analyte-triggered conformational switch of i-motif DNA can help us deeply understand the DNA technology inside single nanopore and will benefit the possible applications in an ultrasensitive detection and biological/chemical analysis.

In vitro selection of tacrolimus binding aptamer by systematic evolution of ligands by exponential enrichment method for the development of a fluorescent aptasensor for sensitive detection of tacrolimus

Tacrolimus (TAC) is an immunosuppressant for preventing solid-organ transplant rejection. Because of its narrow therapeutic window, analytical methods which can detect TAC in serum samples with high accuracy and reliability are required. In this study, specific aptamers (Apt122 and Apt125) for TAC were isolated via systematic evolution of ligands by exponential enrichment method using magnetic beads to immobilize the target. After determination of binding constants of aptamers by flow cytometry analysis, Apt122 was selected and labeled with ATTO 647 N as a fluorophore to develop a fluorescent sensing platform for detection of TAC using graphene oxide (GO) as a fluorescence quencher. The designed aptasensor could detect TAC in phosphate buffer saline (10 mM PBS) and serum samples with detection limits as low as 1.4 and 2.5 nM, respectively.

Target-induced structure switching of aptamers facilitates strand displacement for DNAzyme recycling amplification detection of thrombin in human serum

To monitor the thrombin concentration under the condition of abnormal blood coagulation is of clinical significance for the diagnosis of various diseases. Here, on the basis of the aptamer structure switching induced by the target molecules and the signal amplification strategy via recycling of metal-ion dependent DNAzymes, we have established a sensitive and simple fluorescent aptasensor for detecting thrombin in human serum. The thrombin target specifically binds to the aptamer sequence and causes a corresponding conformational structure switching, which leads to the formation of a toehold sequence to facilitate the strand migration displacement reaction for the generation of functional metal-ion dependent DNAzymes. These DNAzymes further cleave the fluorescently quenched hairpin substrates cyclically to yield substantially amplified fluorescence recovery for sensitively detecting thrombin in the dynamic range from 0.01 nM to 50 nM. Such an aptasensor shows a detection limit of 6.9 pM and can achieve the monitoring of thrombin in diluted human serum with high selectivity, offering a universal sensing strategy for the construction of various sensitive and simple aptasensors to detect different biomarker molecules.

Amperometric sensing of hydrazine by using single gold nanopore electrodes filled with Prussian Blue and coated with polypyrrole and carbon dots

A nanoprobe for hydrazine sensing is described that is making use of a single gold nanopore electrode (SAuNPEs) that was modified by electro-deposition of Prussian Blue (PB) and then coated with a thin membrane of polypyrrole and carbon dots in order to enhance stability and catalytic activity. Best operated at a low potential of 0.3 V vs. Ag/AgCl, the nanosensor display good electrocatalytic activity towards the oxidation of hydrazine, with a linear response in the 0.5-80 μM hydrazine concentration range and a 0.18 μM detection limit (at S/N = 3). The method was applied to the determination of hydrazine in human urine. Graphical abstract Schematic presentation of the electrocatalytic oxidation of hydrazine using a single gold nanopore electrode that was modified by electro-deposition of Prussian Blue and then coated with a thin membrane of polypyrrole and carbon dots.