Microfluidic biochips for simple impedimetric detection of thrombin based on label-free DNA aptamers

Pre-enrichment-free detection of hepatocellular carcinoma-specific ctDNA via PDMS and MEMS-based microfluidic sensor

The growing interest in microfluidic biosensors has led to improvements in the analytical performance of various sensing mechanisms. Although various sensors can be integrated with microfluidics, electrochemical ones have been most commonly employed due to their ease of miniaturization, integration ability, and low cost, making them an established point-of-care diagnostic method. This concept can be easily adapted to the detection of biomarkers specific to certain cancer types. Pathological profiling of hepatocellular carcinoma (HCC) is heterogeneous and rather complex, and biopsy samples contain limited information regarding the tumor and do not reflect its heterogeneity. Circulating tumor DNAs (ctDNAs), which can contain information regarding cancer characteristics, have been studied tremendously since liquid biopsy emerged as a new diagnostic method. Recent improvements in the accuracy and sensitivity of ctDNA determination also paved the way for genotyping of somatic genomic alterations. In this study, three-electrode (Au-Pt-Ag) glass chips were fabricated and combined with polydimethylsiloxane (PDMS) microchannels to establish an electrochemical microfluidic sensor for detecting c.747G > T hotspot mutations in the TP53 gene of ctDNAs from HCC. The preparation and analysis times of the constructed sensor were as short as 2 h in total, and a relatively high flow rate of 30 µl/min was used during immobilization and hybridization steps. To the best of our knowledge, this is the first time a PDMS-based microfluidic electrochemical sensor has been developed to target HCC ctDNAs. The system exhibited a limit of detection (LOD) of 24.1 fM within the tested range of 2-200 fM. The sensor demonstrated high specificity in tests conducted with fully noncomplementary and one-base mismatched target sequences. The developed platform is promising for detecting HCC-specific ctDNA at very low concentrations without requiring pre-enrichment steps.

Impedimetric Aptamer-Based Biosensors: Applications

Impedimetric aptamer-based biosensors show high potential for handheld devices and point-of-care tests. In this review, we report on recent advances in aptamer-based impedimetric biosensors for applications in biotechnology. We detail on analytes relevant in medical and environmental biotechnology as well as food control, for which aptamer-based impedimetric biosensors were developed. The reviewed biosensors are examined for their performance, including sensitivity, selectivity, response time, and real sample validation. Additionally, the benefits and challenges of impedimetric aptasensors are summarized.

Impedimetric Aptamer-Based Biosensors: Principles and Techniques

Aptamers are a specific class of ligands with high affinities comparable to antibodies, which are selected and synthesized in vitro. In combination with impedance spectroscopy as sensitive measurement method, we gain a class of biosensors with high potential for handheld devices and point-of-care tests. In this review, we report on recent advances in aptamer-based impedimetric biosensors. Besides giving a short summary of electrochemical measurement techniques, the most exciting innovative developments of detection strategies in the last decades are reviewed. Finally, important criteria for the comparison of aptamer-based biosensors are discussed.

Laser-induced graphene interdigitated electrodes for label-free or nanolabel-enhanced highly sensitive capacitive aptamer-based biosensors

Highly porous laser-induced graphene (LIG) is easily generated in complex electrode configurations such as interdigitated electrodes (IDEs). Here, we demonstrate that their superior capacitive response at low frequencies can be exploited in affinity biosensors using thrombin aptamers as model biorecognition elements. Of specific interest was the effect of electrode surface area on capacitance detection, and the comparison between a label-free format and enhancement strategies afforded by carboxy group bearing polymeric nanoparticles or liposomes. Electrochemical impedance spectroscopy (EIS) was used to investigate the LIG performance and optimize the biosensor design. Interestingly, the label-free strategy performed extremely well and additional labels decreased the limit of detection or increased the sensitivity only minimally. It is assumed that the highly porous nature of the LIG structures dominates the capacitive response so that labels removed from the surface have only limited influence Also, while slight performance changes can be observed for smaller vs. larger electrode structures, the performance of a LIG IDE is reasonably independent of its size. In the end, a dynamic range of 5 orders of magnitude was obtained (0.01 nM-1000 nM) with a limit of detection as low as 0.12 pM. When measured in serum, this increased to 1.3 pM. The good reproducibility (relative standard deviation (RSD), 4.90%) and repeatability (RSD, 2.59%) and good long-term stability (>7 weeks at 4 °C) prove that a LIG-based capacitance sensor is an excellent choice for affinity-based biosensor. The ease-of-production, the simplicity of modification and the superior performance even in a label-free format indicate that LIG-based biosensors should be considered in point-of-care diagnostics in the future.

A self-calibrating electrochemical aptasensing platform: Correcting external interference errors for the reliable and stable detection of avian influenza viruses

Conventional electrochemical biosensing systems rely on a single output signal, which limits their certain practical application, specifically from the viewpoint of external interference factors causing electrochemical signal errors. This study reports a self-calibrating dual-electrode based electrochemical aptasensor for the reliable and independent detection of avian influenza viruses (AIVs), which are the primary cause of highly contagious respiratory diseases, under external interference factors. Both electrodes were fabricated using tungsten rods surface-modified with a 3D nanostructured porous silica film (3DNRE). Subsequently, methylene blue (MB) was loaded as a redox-active material into the pores and capped with corresponding aptamer. One electrode was capped with an anti-AIV nucleoprotein (NP) aptamer (Apt_AIV-MB@3DNRE) allowing target-specific binding, resulting in changes in electrochemical signal upon diffusional release of the loaded redox molecules. The other electrode was capped with a control aptamer (Apt_con-MB@3DNRE), serving as a reference to correct false responses generated by nonspecific aptamer detachment and MB release under environmental changes in pH and ion strength and presence of nontarget molecules from cell lysis debris. In the dual-electrode platform, Apt_con-MB@3DNRE provides a corrected baseline for the fluctuating original output signals from Apt_AIV-MB@3DNRE. Consequently, this dual-electrode platform exhibits excellent output-signal stability (relative standard deviation, RSD: 5.86%) compared to a conventional single-electrode platform (RSD: 30.13%) at equivalent concentrations of AIV NP samples under different reaction buffer conditions. Moreover, no further purification and washing steps were required, indicating that the strategy may represent a universal and reliable platform for the electrochemical aptamer-based detection of various biomolecules.

Parylene-Coated Polytetrafluoroethylene-Membrane-Based Portable Urea Sensor for Real-Time Monitoring of Urea in Peritoneal Dialysate

A portable urea sensor for use in fast flow conditions was fabricated using porous polytetrafluoroethylene (PTFE) membranes coated with amine-functionalized parylene, parylene-A, by vapor deposition. The urea-hydrolyzing enzyme urease was immobilized on the parylene-A-coated PTFE membranes using glutaraldehyde. The urease-immobilized membranes were assembled in a polydimethylsiloxane (PDMS) fluidic chamber, and a screen-printed carbon three-electrode system was used for electrochemical measurements. The success of urease immobilization was confirmed using scanning electron microscopy, and fourier-transform infrared spectroscopy. The optimum concentration of urease for immobilization on the parylene-A-coated PTFE membranes was determined to be 48 mg/mL, and the optimum number of membranes in the PDMS chamber was found to be eight. Using these optimized conditions, we fabricated the urea biosensor and monitored urea samples under various flow rates ranging from 0.5 to 10 mL/min in the flow condition using chronoamperometry. To test the applicability of the sensor for physiological samples, we used it for monitoring urea concentration in the waste peritoneal dialysate of a patient with chronic renal failure, at a flow rate of 0.5 mL/min. This developed urea biosensor is considered applicable for (portable) applications, such as artificial kidney systems and portable dialysis systems.

Microfluidic Technology for Nucleic Acid Aptamer Evolution and Application

The intersection of microfluidics and aptamer technologies holds particular promise for rapid progress in a plethora of applications across biomedical science and other areas. Here, the influence of microfluidics on the field of aptamers, from traditional capillary electrophoresis approaches through innovative modern-day approaches using micromagnetic beads and emulsion droplets, is reviewed. Miniaturizing aptamer-based bioassays through microfluidics has the potential to transform diagnostics and embedded biosensing in the coming years.

On-chip plasmonic immunoassay based on targeted assembly of gold nanoplasmonic particles

An on-chip, non-enzymatic immunoassay was developed via the targeted assemblies of gold nanoparticles with target proteins in degassing-driven microfluidic devices and simply quantified at the single particle level.

Protein detection in blood via a chimeric aptafluorescence assay: toward point-of-care diagnostic devices

Paper-based analytics allows building portable and disposable devices for point-of-care (POC) diagnosis. Conventional methods for quantifying proteins exhibit substantial disadvantages related to costs and difficulty of the technique when used in settings where fast and cost-effective assays are needed. We report the successful application of a simple, rapid, easy to use, and label-free aptasensor strategy based on the selective fluorescence of the NMM IX dye. For the probe design, the three-dimensional (3-D) structures of the DNA components were carefully analyzed using software for the 3-D visualization of crystallographic structures. The chimeric aptafluorescence molecule consists of two modules, a detection aptamer and a transduction sequence that induces the specific fluorescence of NMM IX. In the presence of thrombin, a fluorescent spot visible to the naked eye can be observed. The fluorescent response is directly proportional to protein concentration and can be easily quantified colorimetrically using a low-cost microscopy system. The recognition probe design might be adaptable to other relevant biological analytes by changing the sequence of the aptamer. This proof of principle represents a contribution to the development of useful, cheap, reliable, and simple protein quantification assays for POC testing.

Fabrication of a Urea Biosensor for Real-Time Dynamic Fluid Measurement

In this study, a portable urea sensor that monitors the urea concentration in flow conditions was fabricated. We propose an electrochemical sensor that continually measures the urea concentration of samples flowing through it at a constant flow rate in real time. For the electrochemical sensing, a porous silk fibroin membrane with immobilized urease was mounted in a polydimethylsiloxane (PDMS) sensor housing. The fabricated urea sensor elicited linear current–concentration characteristics in the clinically significant concentration range (0.1–20 mM) based on peritoneal dialysis. The sensor maintained the linear current–concentration characteristics during operation in flow conditions.

Microfluidic methods for aptamer selection and characterization

Nucleic acid aptamers have tremendous potential as molecular recognition elements in biomedical targeting, analytical arrays, and self-signaling sensors. However, practical limitations and inefficiencies in the process of selecting novel aptamers (SELEX) have hampered widespread adoption of aptamer technologies. Many factors have recently contributed to more effective aptamer screening, but no influence has done more to increase the efficiency, scale, and automation of aptamer selection than that of new microfluidic SELEX techniques. This review introduces aptamers as a powerful chemical and biological tool, briefly highlights traditional SELEX techniques and their limitations, covers in detail the recent advancements in microfluidic methods of aptamer selection and characterization, and suggests possible future directions of the field.