Bimetallic Pd-Pt supported graphene promoted enzymatic redox cycling for ultrasensitive electrochemical quantification of microRNA from cell lysates

Overview on the Development of Electrochemical Immunosensors by the Signal Amplification of Enzyme- or Nanozyme-Based Catalysis Plus Redox Cycling

Enzyme-linked electrochemical immunosensors have attracted considerable attention for the sensitive and selective detection of various targets in clinical diagnosis, food quality control, and environmental analysis. In order to improve the performances of conventional immunoassays, significant efforts have been made to couple enzyme-linked or nanozyme-based catalysis and redox cycling for signal amplification. The current review summarizes the recent advances in the development of enzyme- or nanozyme-based electrochemical immunosensors with redox cycling for signal amplification. The special features of redox cycling reactions and their synergistic functions in signal amplification are discussed. Additionally, the current challenges and future directions of enzyme- or nanozyme-based electrochemical immunosensors with redox cycling are addressed.

Development of Electrochemical Biosensor for miR204-Based Cancer Diagnosis

With increase in cancer burden worldwide and poor survival rates due to delayed diagnosis, it is pertinent to develop a device for early diagnosis. We report an electrochemical biosensor for quantification of miRNA-204 (miR-204) biomarker that is dysregulated in most of the cancers. The proposed methodology uses the gold nanoparticles-modified carbon screen-printed electrode for immobilization of single-stranded DNA probe against miR-204. Colloidal gold nanoparticles were synthesized using L-glutamic acid as reducing agent. Nanoparticles were characterized by UV-visible spectroscopy and transmission electron microscopy. Spherical gold nanoparticles were of 7-28 nm in size. Biosensor fabricated using these nanoparticles was characterized by cyclic voltammetry after spiking 0.1 fg/mL-0.1 µg/mL of miR-204 in fetal bovine serum. Response characteristics of the miR-204 biosensor displayed high sensitivity of 8.86 µA/µg/µL/cm² with wide detection range of 15.5 aM to 15.5 nM. The low detection limit makes it suitable for early diagnosis and screening of cancer.

Machine learning improves our knowledge about miRNA functions towards plant abiotic stresses

During the last two decades, human has increased his knowledge about the role of miRNAs and their target genes in plant stress response. Biotic and abiotic stresses result in simultaneous tissue-specific up/down-regulation of several miRNAs. In this study, for the first time, feature selection algorithms have been used to investigate the contribution of individual plant miRNAs in Arabidopsis thaliana response towards different levels of several abiotic stresses including drought, salinity, cold, and heat. Results of information theory-based feature selection revealed that miRNA-169, miRNA-159, miRNA-396, and miRNA-393 had the highest contributions to plant response towards drought, salinity, cold, and heat, respectively. Furthermore, regression models, i.e., decision tree (DT), support vector machines (SVMs), and Naïve Bayes (NB) were used to predict the plant stress by having the plant miRNAs' concentration. SVM with Gaussian kernel was capable of predicting plant stress (R2 = 0.96) considering miRNA concentrations as input features. Findings of this study prove the performance of machine learning as a promising tool to investigate some aspects of miRNAs' contribution to plant stress responses that have been undiscovered until today.

Gold nanoparticles-based biosensor can detect drought stress in tomato by ultrasensitive and specific determination of miRNAs

Drought stress can significantly affect the yield and quality of tomato production. However, the development of a sensitive and specific method for the determination of drought stress is somehow challenging since plant common morpho-physiological and biochemical characteristic are not generally specific to biotic and abiotic stresses. As a solution, the concentration of miRNAs in plant tissues can be a selective and specific indicator of plant stress. In this study, an optical biosensor based on gold nanoparticles is introduced to determine miRNA-1886 in tomato plant roots. Results showed that irrigation levels from 100% to 60% of field capacity increased the concentration of miRNA-1886 in a range from ca. 100 to 6800 fM (fM) causing a linear change in the biosensor response (R² = 0.97). Results also revealed that in contrast with plant conventional morpho-physiological and biochemical characteristic, miRNA-1886 concentration was not significantly affected (P < 0.01) by other stresses, i.e., salinity and temperature during the growth period. The biosensor introduced in this study is a reliable method to study stress-related functions of miRNAs in plants and their application in specific plant stress determination.

A Novel Graphene Oxide-Based Aptasensor for Amplified Fluorescent Detection of Aflatoxin M1 in Milk Powder

In this paper, a rapid and sensitive fluorescent aptasensor for the detection of aflatoxin M1 (AFM1) in milk powder was developed. Graphene oxide (GO) was employed to quench the fluorescence of a carboxyfluorescein-labelled aptamer and protect the aptamer from nuclease cleavage. Upon the addition of AFM1, the formation of an AFM1/aptamer complex resulted in the aptamer detaching from the surface of GO, followed by the aptamer cleavage by DNase I and the release of the target AFM1 for a new cycle, which led to great signal amplification and high sensitivity. Under optimized conditions, the GO-based detection of the aptasensor exhibited a linear response to AFM1 levels in a dynamic range from 0.2 to 10 μg/kg, with a limit of detection (LOD) of 0.05 μg/kg. Moreover, the developed aptasensor showed a high specificity towards AFM1 without interference from other mycotoxins. In addition, the technique was successfully applied for the detection of AFM1 in infant milk powder samples. The aptasensor proposed here offers a promising technology for food safety monitoring and can be extended to various targets.

Molecular methods in electrochemical microRNA detection

High-throughput profiling/sensing of nucleic acids has recently emerged as a highly promising strategy for the early diagnosis and improved prognosis of a broad range of pathologies, most notably cancer. Among the potential biomarker candidates, microRNAs (miRNAs), a class of non-coding RNAs of 19-25 nucleotides in length, are of particular interest due to their role in the post-transcriptional regulation of gene expression. Developing miRNA sensing technologies that are quantitative, ultrasensitive and highly specific has proven very challenging because of their small size, low natural abundance and the high degree of sequence similarity among family members. When compared to optical based methods, electrochemical sensors offer many advantages in terms of sensitivity and scalability. This non-comprehensive review aims to break-down and highlight some of the most promising strategies for electrochemical sensing of microRNA biomarkers.

Size-dependent modulation of graphene oxide-aptamer interactions for an amplified fluorescence-based detection of aflatoxin B1 with a tunable dynamic range

Aflatoxin B1 (AFB1) is a common toxin found in many foods. While AFB1 sensors have been reported, few studies have shown amplified detection with tunable dynamic ranges. We herein report a simple and highly sensitive amplified aptamer-based fluorescent sensor for AFB1, which relies on the ability of nano-graphene oxide (GO) to protect aptamers from nuclease cleavage for amplified detection and on the nanometer size effect of GO to tune the dynamic range and sensitivity. The assay was performed by simply mixing the carboxyl-X-rhodamine (ROX)-labeled AFB1 aptamer, the GO, the nuclease, and the AFB1 samples. Modulating the size of the GO nanosheet resulted in three dynamic ranges, i.e., 12.5 to 312.5 ng mL(-1), 1.0 to 100 ng mL(-1), and 5.0 to 50 ng mL(-1), with corresponding limits of detection of 10.0 ng mL(-1), 0.35 ng mL(-1) and 15.0 ng mL(-1), respectively. The sensor was highly selective against other aflatoxins and common molecules in foods, and its performance was verified in corn samples spiked with known concentration of AFB1.

A palladium-platinum bimetal nanodendritic melamine network for signal amplification in voltammetric sensing of DNA

A sandwich-type electrochemical DNA sensor is described for the detection of oligonucleotides typical for MECP2 gene mutations. Palladium nanoparticles (PdNPs) and platinum nanoparticles (PtNPs) were used to synthesize flower-like PdPt nanodendrites (NDs) by a one-pot method. The PdPt NDs possess a high specific surface area and excellent catalytic capabilities. They served as the carrier for the signal DNA probe (SP) and simultaneously catalyze the reduction of hydrogen peroxide (H₂O₂). The PdPt NDs were modified with melamine, and this results in the formation of a PdPt-melamine network through stable interactions between the PdPt NDs and the three amino groups of each melamine molecule. The network exhibits excellent catalytic ability in enhancing the current signal response in the voltammetric detection of MECP2 gene mutation, best measured at -0.4 V vs. SCE and using H₂O₂ as the electrochemical probe. In addition, gold nanoflowers were electrodeposited on the electrode interface in order to accelerate electron transfer and to capture the capture probe. The sensor is stable and can detect MECP2 gene mutations in the 1 fmol·L^-1 to 1 nmol·L^-1 concentration range, with a 0.33 fmol·L^-1 lower detection limit at an S/N ratio of 3. Graphical abstract Schematic presentation of electrodes for the determination of the X-linked gene methyl-CpG-binding protein 2 (MECP2). The sensor is based on the electrooxidation of added H₂O₂ by using the melamine modified palladium platinum bimetal nanodendrites as network signal amplification strategy. This versatile platform expands studies on the detection of monogenic disease.

Utilization of hydroxypropyl carboxymethyl cellulose in synthesis of silver nanoparticles

Hydroxypropyl carboxymethyl cellulose samples having varying degrees of substitution and varying degrees of polymerization were used to reduce silver nitrate to silver nanoparticles. UV spectral analysis of silver nanoparticles colloidal solution reveal that increasing the pH of the reduction solution leads to improvement in the intensity of the absorption band for silver nanoparticles, to be maximum at pH 11. The absorption peak intensity also enhanced upon prolonging the reaction duration up to 60 min. The conversion of silver ions to metallic silver nanoparticles was found to be temperature-dependent and maximum transformation occurs at 60 °C. The reduction efficiency of hydroxypropyl carboxymethyl cellulose was found to be affected by its degree of polymerization. Colloidal solutions of silver nanoparticles having concentration up to 1000 ppm can be prepared upon fixing the ratio between silver nitrate and hydroxypropyl carboxymethyl cellulose at 0.017-0.3g per each 100ml of the reduction solution.