Simple Chip Electrophoresis Titration of Neutralization Boundary with EDTA Photocatalysis for Distance-Based Sensing of Melamine in Dairy Products

Smartphone-deployable and all-in-one machine vision for visual quantification analysis based on distance readout of electrophoresis titration biosensor

As a class of point-of-care (POC) assays with visible distance readout (thermometer style), the electrophoresis titration (ET) biosensor affords high robustness, versatility, and simplicity for point-of-care quantification. However, naked-eye observation of the distance readout is unreliable in POC settings and manual processing of distance readout is time-consuming. Herein, we developed a smartphone-deployable and all-in-one machine vision for four ET biosensors (bovine serum albumin, melamine, uric acid, glutathione) to classify and quantify the samples simultaneously. To ensure accurate and rapid quantification on the smartphone, we customized the decolorization methods and edge detection operators to balance the region of interest (ROI) extraction performance and processing speed. We then established a dataset of 180 distance readout images to endow our machine vision with the ability to classify four sample types. Consequently, our machine vision demonstrated high accuracy in determining the sample type (>97.2%) and concentration (>97.3%). Moreover, expanding its applications to other targets was readily achieved by including distance readout images of other ET biosensors (e.g., hemoglobin A1c) in the dataset. Therefore, our strategy of constructing machine vision is compatible with the versatile ET biosensor technique, suggesting that the same strategy can be used for other thermometer-style POC assays.

Step Pulse-Mediated Low-Triggering Potential Electrochemiluminescence of Polyfluorene Nanoparticles for Bioassay

When the electrochemiluminescence (ECL) reaction occurs at a triggering potential beyond ±1.0 V, the interference from the adverse oxidation-reduction reaction cannot be ignored. However, currently reported anode ECL usually occurs above +1.0 V. This study innovatively developed a convenient and simple step pulse (SP) method to modulate the low ECL triggering potential of poly [(9,9-dioctyl-fluorenyl-2,7-diacyl)-alt-co-(9-hexyl-3,6-carbazole)] (PFA) nanoparticles (NPs). Compared to cyclic voltammetry with a triggering potential exceeding +1.25 V for PFA NPs, SP scanning enabled PFA NPs to exhibit a strong and stable ECL emission with a triggering potential as low as +0.75 V and tripropylamine (TPrA) as a coreactant. PFA NPs coupled an efficient aptameric recognition-driven cascade nucleic acid amplification strategy to construct a sensitive biosensing platform for measuring phosphorylated Tau (p-Tau) protein as an Alzheimer's disease biomarker. p-Tau could release the secondary target (ST) chain through the aptameric recognition reaction with the aptamer, and the released ST could further trigger cascade catalytic hairpin assembly (CHA) and rolling circle amplification (RCA) at the PFA NP-modified electrode, producing a large number of long chains. The large amount of G-quadruplex/hemin formed by long chains and hemin will consume the ECL quencher H2O2 added in detection solution, thereby restoring the ECL signal and enabling the low potential quantitative analysis of p-Tau with a limit of detection of 4.15 fg/mL. SP technique provides a new way to reduce ECL triggering potential, and PFA NPs create a promising low-triggering potential ECL-sensing platform for bioanalysis.

A temperature-independent model of dual calibration standards for onsite and point-of-care quantification analyses via electrophoresis titration chip

Electrophoresis titration chip (ETC) is a versatile tool for onsite and point-of-care quantification analyses because it affords naked-eye detection and a straightforward quantification format. However, it is vulnerable to changes in environmental temperature, which regulates the electrophoretic migration by affecting the ion mobility and the target recognition by influencing the enzyme activity. Therefore, the quantification accuracy of the ETC tests was severely compromised. Rather than using the dry bath or heating/cooling units, we proposed a facile model of dual calibration standards (DCS) to mathematically eliminate the effects of temperature on quantification accuracy. To verify our model, we deployed the ETC device at different temperatures ranging from 5 to 40 °C. We further utilized the DCS-ETC to determine the protein content and uric acid concentration in real samples outside the laboratory. All the experimental results showed that our model significantly stabilized the quantification recovery from 35.31-153.44 % to 99.38-103.44 % for protein titration; the recovery of uric acid titration is also stable at 96.25-106.42 %, suggesting the enhanced robustness of the ETC tests. Therefore, DCS-ETC is a field-deployable test that can offer reliable quantification performance without extra equipment for temperature control. We envision that it is promising to be used for onsite applications, including food safety control and disease diagnostics.

Biomimetic mineralization-based In situ growth of AuNCs@ZIF-8 on paper fibers for visual detection of copper ions

As one of the essential trace elements in life activities, copper ion (Cu2+) plays a very important role in human health. However, copper-containing pesticides are usually used for sterilization and disinfestation in agriculture. Thus, the residues of copper-containing pesticides in agricultural samples increase the risk of excessive intake of Cu2+ for human. The development of an effective method for detecting Cu2+ is still an important task. Herein, a detecting system based on AuNCs@ZIF-8 modified paper and smartphone platform was developed for visual detection of Cu2+ in agricultural samples. Herein, a detecting system based on AuNCs@ZIF-8 modified paper and smartphone platform was developed for visual detection of Cu2+ in agricultural samples. In the detecting system, gold nanoclusters (AuNCs) were packaged by ZIF-8 to limit their molecular motion and enhance the fluorescence effectively. In the meanwhile, by ultrasound-assisted biomimetic mineralizing, AuNCs@ZIF-8 composites were uniformly synthesized in situ on the surface of the paper fibers to indicate Cu2+ by fluorescence quenching. A portable visual monitoring system consisted of the prepared Cu2+ paper sensor and a smartphone platform was then successfully built and applied to on site detecting Cu2+ in agricultural samples. The limit of detection (LOD) was 4.57 μM and recovery rate varied from 96.50 % to 121.58 %. The developed detecting system for Cu2+ has the advantages of easy preparation and operation, and is very suitable for the use in agricultural products and farmland.

Terminal deoxynucleotidyl transferase associated with split G-quadruplex/hemin deoxyribozyme amplification detection for various contaminants in milk based on pregnancy test strip platform

Contaminant residue analysis in milk can provide essential assistance for safety quality and contamination level management of milk production, which is critical for safeguarding public health. In this study, the pregnancy test strip is employed to achieve multiple analytes detection based on the specific recognition of aptamer and terminal deoxynucleotidyl transferase associated with split G-quadruplex/hemin deoxyribozyme system. Through the subsequent enzyme catalyzed reaction, the detection signal can be further amplified to improve the sensitivity. The method does not need to assemble test strip, prepare and purify antibodies/haptens, nor design complex probe sequences. By coupling human chorionic gonadotrophin with DNA probes and combining magnetic separation technology, the targets can be determined via the test strip. Under the optimized conditions, the visual detection limits for mercury ion, bisphenol A, and penicillin are 1, 0.1 and 0.05 nM, respectively. The detection results show that the method displays good accuracy and practicability in spiked milk sample. The method presents a simple scheme, low cost as well as good design versatility, which demonstrates great application prospect for the sensitive, low-cost, and convenient detection of food matrices.

Selective Plate-Based Assay for Trace EDTA Analysis via Boron Trifluoride-methanol Derivatization UHPLC-QqQ-MS/MS Enabling Biologic and Vaccine Processes

The employment of ethylenediaminetetraacetic acid (EDTA) across several fields in chemistry and biology has required the creation of a high number of quantitative assays. Nonetheless, the determination of trace EDTA, especially in biologics and vaccines, remains challenging. Herein, we introduce an automated high-throughput approach based on EDTA esterification in 96-well plates using boron trifluoride-methanol combined with rapid analysis by ultra-high-performance liquid chromatography-triple quadrupole tandem mass spectrometry (UHPLC-QqQ-MS/MS). Derivatization of EDTA to its methyl ester (Me-EDTA) serves to significantly improve chromatographic performance (retention, peak shape, and selectivity), while also delivering a tremendous enhancement of sensitivity in the positive ion mode electrospray ionization (ESI+). This procedure, in contrast to previous EDTA methods based on complexation with metal ions, is not affected by high concentration of other metals, buffers, and related salts abundantly present in biopharmaceutical processes (e.g., iron, copper, citrate, etc.). Validation of this assay for the determination of ng·mL^-1 level EDTA in monoclonal antibody and vaccine products demonstrated excellent performance (repeatability, precision, and linear range) with high recovery from small sample volumes while also providing an advantageous automation-friendly workflow for high-throughput analysis.

Ratiometric fluorescence determination of hydrogen peroxide using carbon dot-embedded Ag@EuWO4(OH) nanocomposites

A sheet-like carbon dot-embedded Ag@EuWO₄(OH) luminescent nanoprobe was successfully developed for assaying hydrogen peroxide. Firstly, the carbon dot-embedded EuWO₄(OH) nanosheets were prepared in a Eu(NO₃)₃·6H₂O-(NH₄)₁₀H₂(W₂O₇)₆·xH₂O-CS(NH₂)₂ hydrothermal synthetic system. Subsequently, the carbon dot-embedded EuWO₄(OH) was functionalized by Ag nanoparticles using an in situ photochemical deposition strategy upon ultraviolet light irradiation. Taking advantage of the dual emissions of the luminescence from carbon dots and characteristic red transitions of Eu³⁺ ions in the integrated system, the carbon dot-embedded Ag@EuWO₄(OH) luminescent composites exhibit ratiometric fluorescence responsive activity towards hydrogen peroxide. The luminescent intensity ratio of Eu³⁺ (614 nm) to carbon dots (389 nm) shows a polynomial function with changing hydrogen peroxide concentration. The corresponding detection limit is 60 μM at a signal-to-noise ratio of 3 (S/N = 3) implying the potential use of the carbon dot-embedded Ag@EuWO₄(OH) as nanoprobe. The method was applied to the quantification of H₂O₂ in real samples with satisfactory results. Graphical abstract A carbon dot-embedded Ag@EuWO₄(OH) luminescence ratiometric probe was successfully prepared through hydrothermal method and in situ photochemical deposition strategy. The luminescence intensity ratio of Eu³⁺ to carbon dots shows synergistic luminescence response activity towards H₂O₂ with detection limit of 60 μM.

Recent Advances and Applications of Microfluidic Capillary Electrophoresis: A Comprehensive Review (2017-Mid 2019)

Microfluidic capillary electrophoresis (MCE) is the novel technique resulted from the CE mininaturization as planar separation and analysis device. This review presents and discusses various application fields of this advanced technology published in the period 2017 till mid-2019 in eight different sections including clinical, biological, single cell analysis, environmental, pharmaceuticals, food analysis, forensic and ion analysis. The need for miniaturization of CE and the consequence advantages achieved are also discussed including high-throughput, miniaturized detection, effective separation, portability and the need for micro- or even nano-volume of samples. Comprehensive tables for the MCE applications in the different studied fields are provided. Also, figure comparing the number of the published papers applying MCE in the eight discussed fields within the studied period is included. The future investigation should put into consideration the possibility of replacing conventional CE with the MCE after proper validation. Suitable validation parameters with their suitable accepted ranges should be tailored for analysis methods utilizing such unique technique (MCE).

Versatile Electrochemiluminescence and Photoelectrochemical Detection of Glutathione Using Mn2+ Substitute Target by DNA-Walker-Induced Allosteric Switch and Signal Amplification

Glutathione (GSH) serves vital functions in biological systems and associates with various human diseases. In this work, a versatile electrochemiluminence (ECL) and a photoelectrochemical (PEC) "signal on" biosensing platform were developed for a sensitive assay of GSH by a Mn²⁺-powered DNAzyme amplification strategy combined with DNA-walker-triggered allosteric conversion. First, MnO₂ nanosheets were reduced to Mn²⁺ by GSH; then, Mn²⁺ as a substitute target triggered DNAzyme-assisted cleavage-cycling amplification to generate numerous DNA output (s3). Meanwhile, the DNA molecular machine was introduced to bridge signal probes for versatile biosensing, which included hairpin DNA as a track and an arm as a walker. The presence of DNA output (s3) activated the swing arm to hybridize with hairpin DNA and then cut it by Nt.BbvCI, which initiated autonomous walking of the arm for forming a large number of streptavidin (SA) aptamers. Thus, a large number of CdS:Mn-SA tags as versatile signal probes was linked to the electrode by specific SA-aptamer binding, generating highly enhanced ECL and PEC signals for sensitive detection of the target. The present biosensing system take advantage of metal ion-based DNAzyme amplification, a DNA walker machine, multi-signals of QDs, and specificity of aptamers, which can provide a universal and efficient biosensing method for detecting various targets. The designed strategy demonstrated good performance for a GSH assay in human serum samples, showing more promising applications than other reported methods.

Facile Counting of Ligands Capped on Nanoparticles via a Titration Chip of Moving Reaction Boundary Electrophoresis

Absolute quantification of ligand capped on the surface of nanoparticles (NPs) has faced a great challenge without the use of complex inner standards (CIS). Herein, we proposed a facile electrophoresis titration (ET) model, designed an ET device, and developed a relevant method for counting the ligand on NPs without the use of CIS, based on moving reaction boundary (MRB). Furthermore, we conducted the relevant ET runs by using 3-mercaptopropionic acid (MPA) and quantum dots (QDs) as the model ligand and NPs, respectively. The experiments revealed that the ligand content of 1518 ± 295 obtained via an ET was close to the one of 1408 ± 117 determined via NMR, validating the ET model. Moreover, the experiments showed fair stability (RSD < 5.62%) and simplicity of ET without the use of CIS. Evidently, the ET model opens a window for facile assay of ligand capped on NPs.

Double inner standard plot model of an electrophoresis titration chip for a portable and green assay of protein content in milk

High portability and environmental safety ("green") are two of the most important objectives pursued by microfluidic methods. However, there remain many challenges for the design of portable and visual microfluidic methods (e.g., chip electrophoresis) due to use of a cumbersome pump, power supply and detector. Herein, a facile double inner standard plot (DISP) model of electrophoresis titration (ET) was proposed for portable and visual assay of proteins in test milk samples without use of a pump, power supply or detector based on a moving reaction boundary (MRB) chip. The DISP-ET model predicted that: (i) by setting the upper limit (UL) and lower limit (LL) of double inner standard milk protein contents, points U and L were, respectively, achieved in the relationship D = -aC + b (D: MRB motion distance; C: protein content); and (ii) the two points divided both the C-axis and D-axis into "poor", "eligible" and "superior" rulers scaled for quantitative assay of test samples. To demonstrate the model of DISP-ET, an original portable device (120 mm × 78 mm × 30 mm, 341 g) was designed, which had a chip (25 mm × 25 mm × 4 mm) of three channels (15 mm × 200 μm × 80 μm), platinum electrodes, a lithium cell and touch screen. A series of experiments were undertaken based on the developed portable device. The relevant experiments demonstrated systemically the validity of the DISP-ET model, theory and method. In particular, the experiments clearly showed the advantages of the DISP-ET chip: portability, visuality, green use, rapidity, and flexibility for real-life use. Finally, the device was applied for a portable and visual assay of fresh milk from a cow on a dairy farm. The DISP-ET model opens a window for designing portable and visual quantitative methods of food-safety control and clinical diagnoses.

Convenient and sensitive colorimetric detection of melamine in dairy products based on Cu(ii)-H2O2-3,3′,5,5′-tetramethylbenzidine system

The illegal adulteration of melamine in dairy products for false protein content increase is a strong hazard to human health. Herein, a simple and sensitive colorimetric method was developed for the quantification of melamine in dairy products based on a Cu²⁺-hydrogen peroxide (H₂O₂)-3,3′,5,5′-tetramethylbenzidine (TMB) system. In this strategy, Cu²⁺ exhibits peroxidase-like activity and can catalyze the oxidation of TMB to oxidized TMB (oxTMB) in the presence of H₂O₂ with a blue colour change of the solution. However, the presence of melamine quickly interacts with H₂O₂ leading to the consumption of H₂O₂ and thus strongly hinders the oxidation of TMB. Under the optimal conditions, the absorbance change of oxTMB has a linear response to the concentration of melamine from 1 to 100 μM with a detection limit of 0.5 μM for melamine. The proposed method has many merits including more simplicity, good selectivity, and more cost-effectiveness without using any nanomaterials. The method was further successfully applied to detect melamine in dairy products including milk and infant formula powder.

Convenient and sensitive colorimetric detection of melamine in dairy products based on a Cu(ii)-H₂O₂-3,3′,5,5′-tetramethylbenzidine system was reported.