Paper-Based Microfluidic Device with Integrated Sputtered Electrodes for Stripping Voltammetric Determination of DNA via Quantum Dot Labeling

Electrochemical Paper-Based Microfluidics: Harnessing Capillary Flow for Advanced Diagnostics

Electrochemical paper-based microfluidics has attracted much attention due to the promise of transforming point-of-care diagnostics by facilitating quantitative analysis with low-cost and portable analyzers. Such devices harness capillary flow to transport samples and reagents, enabling bioassays to be executed passively. Despite exciting demonstrations of capillary-driven electrochemical tests, conventional methods for fabricating electrodes on paper impede capillary flow, limit fluidic pathways, and constrain accessible device architectures. This account reviews recent developments in paper-based electroanalytical devices and offers perspective by revisiting key milestones in lateral flow tests and paper-based microfluidics engineering. The study highlights the benefits associated with electrochemical sensing and discusses how the detection modality can be leveraged to unlock novel functionalities. Particular focus is given to electrofluidic platforms that embed electrodes into paper for enhanced biosensing applications. Together, these innovations pave the way for diagnostic technologies that offer portability, quantitative analysis, and seamless integration with digital healthcare, all without compromising the simplicity of commercially available rapid diagnostic tests.

Photoelectrochemical sensors based on paper and their emerging applications in point-of-care testing

Point-of-care testing (POCT) technology is urgently required owing to the prevalence of the Internet of Things and portable electronics. In light of the attractive properties of low background and high sensitivity caused by the complete separation of excitation source and detection signal, the paper-based photoelectrochemical (PEC) sensors, featured with fast in analysis, disposable and environmental-friendly have become one of the most promising strategies in POCT. Therefore, in this review, the latest advances and principal issues in the design and fabrication of portable paper-based PEC sensors for POCT are systematically discussed. Primarily, the flexible electronic devices that can be constructed by paper and the reasons why they can be used in PEC sensors are expounded. Afterwards, the photosensitive materials involved in paper-based PEC sensor and the signal amplification strategies are emphatically introduced. Subsequently, the application of paper-based PEC sensors in medical diagnosis, environmental monitoring and food safety are further discussed. Finally, the main opportunities and challenges of paper-based PEC sensing platforms for POCT are briefly summarized. It provides a distinct perspective for researchers to construct paper-based PEC sensors with portable and cost-effective, hoping to enlighten the fast development of POCT soon after, as well as benefit human society.

Rapid on-site nucleic acid testing: On-chip sample preparation, amplification, and detection, and their integration into all-in-one systems

As nucleic acid testing is playing a vital role in increasingly many research fields, the need for rapid on-site testing methods is also increasing. The test procedure often consists of three steps: Sample preparation, amplification, and detection. This review covers recent advances in on-chip methods for each of these three steps and explains the principles underlying related methods. The sample preparation process is further divided into cell lysis and nucleic acid purification, and methods for the integration of these two steps on a single chip are discussed. Under amplification, on-chip studies based on PCR and isothermal amplification are covered. Three isothermal amplification methods reported to have good resistance to PCR inhibitors are selected for discussion due to their potential for use in direct amplification. Chip designs and novel strategies employed to achieve rapid extraction/amplification with satisfactory efficiency are discussed. Four detection methods providing rapid responses (fluorescent, optical, and electrochemical detection methods, plus lateral flow assay) are evaluated for their potential in rapid on-site detection. In the final section, we discuss strategies to improve the speed of the entire procedure and to integrate all three steps onto a single chip; we also comment on recent advances, and on obstacles to reducing the cost of chip manufacture and achieving mass production. We conclude that future trends will focus on effective nucleic acid extraction via combined methods and direct amplification via isothermal methods.

Wax-Printed Fluidic Controls for Delaying and Accelerating Fluid Transport on Paper-Based Analytical Devices

In this work, we explore a new method for controlling fluid transport rate on paper-based analytical devices that enables both the delay and the acceleration of fluid flow. The delays were incorporated by wax printing linear patterns of variable width within the flow channel and melted to penetrate the paper. In this manner, the surface tension of the fluid decreases while its contact angle increases, causing a pressure drop along the fluid path that reduces capillary flow. The acceleration of flow was accomplished by overlaying hydrophobic stripes (prepared by wax printing and melting the wax) on the hydrophilic path (top or top–bottom). In this manner, the fluid was repelled from two dimensions (vertical and applicate), increasing the flow rate. The combination of these methods on the same devices could adjust wicking time in intermediate time internals. The method enabled a wide timing of fluid transport, accomplishing a change in wicking times that extended from −41% to +259% compared to open paper channels. As a proof of concept, an enzymatic assay of glucose was used to demonstrate the utility of these fluid control methods in kinetic methods of analysis.

Advances in quantum dots as diagnostic tools

Quantum dots (QDs) are crystalline inorganic semiconductor nanoparticles a few nanometers in size that possess unique optical electronic properties vs those of larger materials. For example, QDs usually exhibit a strong and long-lived photoluminescence emission, a feature dependent on size, shape and composition. These special optoelectronic properties make them a promising alternative to conventional luminescent dyes as optical labels in biomedical applications including biomarker quantification, biomolecule targeting and molecular imaging. A key parameter for use of QDs is to functionalize their surface with suitable (bio)molecules to provide stability in aqueous solutions and efficient and selective tagging biomolecules of interest. Researchers have successfully developed biocompatible QDs and have linked them to various biomolecule recognition elements, i.e., antibodies, proteins, DNA, etc. In this chapter, QD synthesis and characterization strategies are reviewed as well as the development of nanoplatforms for luminescent biosensing and imaging-guided targeting. Relevant biomedical applications are highlighted with a particular focus on recent progress in ultrasensitive detection of clinical biomarkers. Finally, key future research goals to functionalize QDs as diagnostic tools are explored.

A Pencil-Lead Immunosensor for the Rapid Electrochemical Measurement of Anti-Diphtheria Toxin Antibodies

Diphtheria is a vaccine-preventable disease, yet immunization can wane over time to non-protective levels. We have developed a low-cost, miniaturized electroanalytical biosensor to quantify anti-diphtheria toxin (DTx) immunoglobulin G (anti-DTx IgG) antibody to minimize the risk for localized outbreaks. Two epitopes specific to DTx and recognized by antibodies generated post-vaccination were selected to create a bi-epitope peptide, biEP, by synthesizing the epitopes in tandem. The biEP peptide was conjugated to the surface of a pencil-lead electrode (PLE) integrated into a portable electrode holder. Captured anti-DTx IgG was measured by square wave voltammetry from the generation of hydroquinone (HQ) from the resulting immunocomplex. The performance of the biEP reagent presented high selectivity and specificity for DTx. Under the optimized working conditions, a logarithmic calibration curve showed good linearity over the concentration range of 10-5-10-1 IU mL-1 and achieved a limit of detection of 5 × 10-6 IU mL-1. The final device proved suitable for interrogating the immunity level against DTx in actual serum samples. Results showed good agreement with those obtained from a commercial enzyme-linked immunosorbent assay. In addition, the flexibility for conjugating other capture molecules to PLEs suggests that this technology could be easily adapted to the diagnoses of other pathogens.

Electroanalytical Paper-Based Nucleic Acid Amplification Biosensors with Integrated Thread Electrodes

Nucleic acid amplification tests (NAATs) are very sensitive and specific methods, but they mainly rely on centralized laboratories and therefore are not suitable for point-of-care testing. Here, we present a 3D microfluidic paper-based electrochemical NAAT. These devices use off-the-shelf gold plasma-coated threads to integrate electroanalytical readouts using ex situ self-assembled monolayer formation on the threads prior to assembling into the paper device. They further include a sandwich hybridization assay with sample incubation, rinsing, and detection steps all integrated using movable stacks of filter papers to allow time-sequenced reactions. The devices use glass fiber substrates for storing recombinase polymerase amplification reagents and conducting the isothermal amplification. We used the paper-based device for the detection of the toxic microalgae Ostreopsis cf. ovata. The NAAT, completed in 95 min, attained a limit of detection of 0.06 pM target synthetic DNA and was able to detect 1 ng/μL O. cf. ovata genomic DNA with negligible cross-reactivity from a closely related microalgae species. We think that the integration of thread electrodes within paper-based devices paves the way for digital one-time use NAATs and numerous other advanced electroanalytical paper- or textile-based devices.

Real-time quantitative monitoring of in vitro nasal drug delivery by a nasal epithelial mucosa-on-a-chip model

OBJECTIVES

A human nasal epithelial mucosa (NEM) on-a-chip is developed integrated with a novel carbon nanofibers-modified carbon electrode for real-time quantitative monitoring of in vitro nasal drug delivery. The integration of platinum electrodes in the chip also enables real-time measurement of transepithelial electrical resistance (TEER).

METHODS

The air-liquid interface culture of nasal epithelial RPMI 2650 cells in the NEM-on-a-chip was optimized to mimic the key functional characteristics of the human nasal mucosa. The epithelial transport of ibuprofen in the NEM-on-a-chip was electrochemically monitored in real-time under static and physiologically realistic dynamic flow conditions.

RESULTS

The NEM-on-a-chip mimics the mucus production and nasal epithelial barrier function of the human nasal mucosa. The real-time drug quantification by the NEM-on-a-chip was validated versus the high-performance liquid chromatography method. The drug transport rate monitored in the NEM-on-a-chip was influenced by the flow in the bottom compartment of the chip, highlighting the importance of emulating the dynamic in vivo condition for nasal drug transport studies.

CONCLUSION

This novel NEM-on-a-chip can be a low-cost and time-efficient alternative to the costly laborious conventional techniques for in vitro nasal drug transport assays. Importantly, its dynamic microenvironment enables conducting nasal drug transport tests under physiologically relevant dynamic conditions.

Affinity biosensors developed with quantum dots in microfluidic systems

Quantum dots (QDs) are synthetic semiconductor nanocrystals with unique optical and electronic properties due to their size (2-10 nm) such as high molar absorption coefficient (10-100 times higher than organic dyes), resistance to chemical degradation, and unique optoelectronic properties due to quantum confinement (high quantum yield, emission color change with size). Compared to organic fluorophores, the narrower emission band and wider absorption bands of QDs offer great advantages in cell imaging and biosensor applications. The optoelectronic features of QDs have prompted their intensive use in bioanalytical, biophysical, and biomedical research. As the nanomaterials have been integrated into microfluidic systems, microfluidic technology has accelerated the adaptation of nanomaterials to clinical evaluation together with the advantages such as being more economical, more reproducible, and more susceptible to modification and integration with other technologies. Microfluidic systems serve an important role by being a platform in which QDs are integrated for biosensing applications. As we combine the advantages of QDs and microfluidic technology for biosensing technology, QD-based biosensor integrated with microfluidic systems can be used as an advanced and versatile diagnostic technology in case of pandemic. Specifically, there is an urgent necessity to have reliable and fast detection systems for COVID-19 virus. In this review, affinity-based biosensing mechanisms which are developed with QDs are examined in the domain of microfluidic approach. The combination of microfluidic technology and QD-based affinity biosensors are presented with examples in order to develop a better technological framework of diagnostic for COVID-19 virus.

A multilayer microfluidic paper coupled with an electrochemical platform developed for sample separation and detection of dopamine

A new construction of a multilayer electrochemical microfluidic paper-based analytical device using a single drop of the sample solution was performed for highly selective detection of dopamine in the presence of ascorbic acid interference.

Engineering strategies for enhancing the performance of electrochemical paper-based analytical devices

Applications of electrochemical detection methods in microfluidic paper-based analytical devices (μPADs) has revolutionized the area of point-of-care (POC) testing towards highly sensitive and selective quantification of various (bio)chemical analytes in a miniaturized, low-coat, rapid, and user-friendly manner. Shortly after the initiation, these relatively new modulations of μPADs, named as electrochemical paper-based analytical devices (ePADs), gained widespread popularity within the POC research community thanks to the inherent advantages of both electrochemical sensing and usage of paper as a suitable substrate for POC testing platforms. Even though general aspects of ePADs such as applications and fabrication techniques, have already been reviewed multiple times in the literature, herein, we intend to provide a critical engineering insight into the area of ePADs by focusing particularly on the practical strategies utilized to enhance their analytical performance (i.e. sensitivity), while maintaining the desired simplicity and efficiency intact. Basically, the discussed strategies are driven by considering the parameters potentially affecting the generated electrochemical signal in the ePADs. Some of these parameters include the type of filter paper, electrode fabrication methods, electrode materials, fluid flow patterns, etc. Besides, the limitations and challenges associated with the development of ePADs are discussed, and further insights and directions for future research in this field are proposed.

Portable electrochemical carbon cloth analysis device for differential pulse anodic stripping voltammetry determination of Pb2

A novel electrochemical carbon cloth (CC) analysis device (eCAD) is proposed for the determination of Pb²⁺ in environmental water samples, which was assembled using a single-step functional CC as both the sensing and the substrate material. The modified CC was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectra, and electrochemical impedance spectroscopy. The increase in electrochemical activity is due to the increased defective extent and excellent electrochemical activity of CC. Under optimum conditions (viz. a pH value of 4.5, deposition time of 160 s), the sensor is capable of determining Pb²⁺ by differential pulse anodic stripping voltammetry (DPASV) at a typical working potential of - 1.0 V (vs. Ag/AgCl). Response is linear from 5.0 × 10^-9 to 3.0 × 10^-6 M Pb²⁺, and the detection limit is 4.8 nM (at S/N = 3). The sensor was successfully applied to the determination of Pb²⁺ in real samples, with apparent recoveries from 96.0 to 102.0% and a relative standard deviation of less than 3.4%. In addition, the integration of the sensor with signal collection components has enabled us to realize on-site analysis of Pb²⁺, which is highlighted as a new generation of electrode platform for the development of a portable analysis device.Graphical abstract.

Magneto-controlled aptasensor for simultaneous detection of ochratoxin A and fumonisin B1 using inductively coupled plasma mass spectrometry with multiple metal nanoparticles as element labels

The simultaneous detection of multiple mycotoxins is important for food safety. Here, a magneto-controlled aptasensor for quantitative analysis of ochratoxin A (OTA) and fumonisin B1 (FB1) using inductively coupled plasma mass spectrometry (ICP-MS) with multiple metal nanoparticles as element labels was proposed. Firstly, the OTA aptamer (Apt1) and the FB1 aptamer (Apt2) immobilized on the magnetic beads (MBs) were hybridized with probe DNA1-CdSe quantum dots (pDNA1-QDs) and probe DNA2-Ag nanoparticles (pDNA2-Ag NPs) labels, producing the MBs-Apt1-pDNA1-QDs and MBs-Apt2-pDNA2-Ag NPs conjugates, respectively. Then, the MBs-Apt1-OTA and MBs-Apt2-FB1 conjugates were generated with the addition of targets, resulting the pDNA1-QDs and pDNA2-Ag NPs labels released into the solutions. Finally, the signal intensities of ¹¹¹Cd and ¹⁰⁷Ag were detected by ICP-MS, achieving limits of detection of 0.10 and 0.30 ng mL^-1 for OTA and FB1, respectively. The assay showed high specificity and succeeded in wheat flour. The method provides an ideal model for sensitive analysis of multiple mycotoxins in food samples.

Quantum Dot Bioconjugates for Diagnostic Applications

Quantum dots (QDs) are a special type of engineered nanomaterials with outstanding optoelectronic properties that make them as a very promising alternative to conventional luminescent dyes in biomedical applications, including biomolecule (BM) targeting, luminescence imaging and drug delivery. A key parameter to ensure successful biomedical applications of QDs is the appropriate surface modification, i.e. the surface of the nanomaterials should be modified with the appropriate functional groups to ensure stability in aqueous solutions and it should be conjugated with recognition elements capable of ensuring an efficient tagging of the BMs of interest. In this review we summarize the most relevant strategies used for surface modification of QDs and for their conjugation to BMs in preparation of their application in nanoplatforms for luminescent BM sensing and imaging-guided targeting. The applications of conjugations of photoluminescent QDs with different BMs in both in vitro and in vivo chemical sensing, immunoassays or luminescence imaging are reviewed. Recent progress in the application of functionalized QDs in ultrasensitive detection in bioanalysis, diagnostics and imaging strategies are reported. Finally, some key future research goals in the progress of bioconjugation of QDs for diagnosis are identified, including novel synthetic approaches, the need for exhaustive characterization of bioconjugates and the design of signal amplification schemes.

Electrochemical paper-based devices: sensing approaches and progress toward practical applications

Paper-based sensors offer an affordable yet powerful platform for field and point-of-care (POC) testing due to their self-pumping ability and utility for many different analytical measurements. When combined with electrochemical detection using small and portable electronics, sensitivity and selectivity of the paper devices can be improved over naked eye detection without sacrificing portability. Herein, we review how the field of electrochemical paper-based analytical devices (ePADs) has grown since it was introduced a decade ago. We start by reviewing fabrication methods relevant to ePADs with more focus given to the electrode fabrication, which is fundamental for electrochemical sensing. Multiple sensing approaches applicable to ePADs are then discussed and evaluated to present applicability, advantages and challenges associated with each approach. Recent applications of ePADs in the fields of clinical diagnostics, environmental testing, and food analysis are also presented. Finally, we discuss how the current ePAD technologies have progressed to meet the analytical and practical specifications required for field and/or POC applications, as well as challenges and outlook.

Electrochemical DNA Biosensors Based on Labeling with Nanoparticles

This work reviews the field of DNA biosensors based on electrochemical determination of nanoparticle labels. These labeling platforms contain the attachment of metal nanoparticles (NPs) or quantum dots (QDs) on the target DNA or on a biorecognition reporting probe. Following the development of DNA bioassay, the nanotags are oxidized to ions, which are determined by voltammetric methods, such as pulse voltammetry (PV) and stripping voltammetry (SV). The synergistic effects of NPs amplification (as each nanoprobe releases a large number of detectable ions) and the inherent sensitivity of voltammetric techniques (e.g., thanks to the preconcentration step of SV) leads to the construction of ultrasensitive, low cost, miniaturized, and integrated biodevices. This review focuses on accomplishments in DNA sensing using voltammetric determination of nanotags (such as gold and silver NPs, and Cd- and Pb-based QDs), includes published works on integrated three electrode biodevices and paper-based biosystems, and discusses strategies for multiplex DNA assays and signal enhancement procedures. Besides, this review mentions the electroactive NP synthesis procedures and their conjugation protocols with biomolecules that enable their function as labels in DNA electrochemical biosensors.

3D Capillary-Driven Paper-Based Sequential Microfluidic Device for Electrochemical Sensing Applications

This article describes the device design and fabrication of two different configurations (flow-through and stopped-flow) of a sequential fluid delivery platform on a microfluidic paper-based device. The developed device is capable of storing and transporting reagents sequentially to the detection channel without the need for external power. The device comprises two components: an origami folding paper (oPAD) and a movable reagent-stored pad (rPAD). This 3D capillary-driven device eliminates the undesirable procedure of multiple-step reagent manipulation in a complex assay. To demonstrate the scope of this approach, the device is used for electrochemical detection of biological species. Using a flow-through configuration, a self-calibration plot plus real sample analysis using a single buffer introduction are established for ascorbic acid detection. We further broaden the effectiveness of the device to a complex assay using a stopped-flow configuration. Unlike other electrochemical paper-based sensors in which the user is required to cut off the device inlet or rest for the whole channel saturation before measurement, herein a stopped-flow device is carefully designed to exclude the disturbance from the convective mass transport. As a proof of concept, multiple procedures for electrode modification and voltammetric determination of serotonin are illustrated. In addition, the research includes an impedimetric label-free immunosensor for α-fetoprotein using the modified stopped-flow device. The beneficial advantages of simplicity, low sample volume (1 μL), and ability to perform a complex assay qualify this innovative device for use with diverse applications.

Electrochemical Detection and Characterization of Nanoparticles with Printed Devices

Innovative methods to achieve the user-friendly, quick, and highly sensitive detection of nanomaterials are urgently needed. Nanomaterials have increased importance in commercial products, and there are concerns about the potential risk that they entail for the environment. In addition, detection of nanomaterials can be a highly valuable tool in many applications, such as biosensing. Electrochemical methods using disposable, low-cost, printed electrodes provide excellent analytical performance for the detection of a wide set of nanomaterials. In this review, the foundations and latest advances of several electrochemical strategies for the detection of nanoparticles using cost-effective printed devices are introduced. These strategies will equip the experimentalist with an extensive toolbox for the detection of nanoparticles of different chemical nature and possible applications ranging from quality control to environmental analysis and biosensing.

Microfluidic paper-based photoelectrochemical sensing platform with electron-transfer tunneling distance regulation strategy for thrombin detection

This work reports a microfluidic paper-based photoelectrochemical (μ-PEC) sensing platform for thrombin (TB) detection with electron-transfer tunneling distance regulation (ETTDR) and aptamer target-triggering nicking enzyme signaling amplification (NESA) dual strategies. Specifically, paper-based TiO₂ nanosheets (PTNs) were prepared with an efficient hydrothermal process, serving as the direct pathway for the charge carriers transfer. When CeO₂-labeled hairpin DNA 3 (HP3) was closely located at the PTNs, the CeO₂-PTNs heterostructure was formed, which could great facilitate the photogenerated carries separation of CeO₂. In addition, with the aid of aptamer target-triggering NESA strategy, the input TB could be transducted to numerous output target of DNA (tDNA), achieving the goal of desirable signal amplification. In the presence of TB, the output tDNA could be further hybridized with HP3 and unfold its hairpin loop, which forced the CeO₂ away from the surface of PTNs and vanished the CeO₂-PTNs heterostructure, resulting in the obviously reducing of photocurrent signal. The as-designed sensing platform exhibited a linear range from 0.02 pM to 100 pM with a detection limit of 6.7 fM. Importantly, this μ-PEC sensing platform could not only realize the highly efficient TB detection, but also pave a luciferous way for the detection of other protein in bioanalysis.

Paper based modification-free photoelectrochemical sensing platform with single-crystalline aloe like TiO2 as electron transporting material for cTnI detection

By controlling target-induced signal quencher release, a label-free and modification-free microfluidic paper based photoelectrochemical analytical device (μ-PAD) for cardiac troponin-I (cTnI) detection was designed for the first time. To achieve it, cellulose paper based single-crystalline three-dimensional aloe like TiO₂ arrays (PSATs) were firstly fabricated as the electron transporting material, providing direct pathways for the charge carriers transfer, and subsequently coupled with CdS to form PSATs/CdS heterojunction for extending the solar spectrum response. Meanwhile, positive charged mesoporous silica nanoparticles (PMSNs) were prepared as the nanocarrier to efficient entrap the Cu²⁺ which could be regarded as signal quencher due to their reaction with CdS to form Cu_xS. Single stranded DNAs (ssDNAs), which could bind specifically with the target of cTnI, were then introduced to couple with the PMSNs and used as the bio-gate to encapsulate the signal quencher of Cu²⁺, endowing the functional PMSNs with responsiveness to cTnI. When the cTnI existed, the ssDNAs were dissociated from PMSNs due to the formation of cTnI-ssDNAs complexes, triggering controllable release of the trapped Cu²⁺, and further decreasing the photocurrent signal caused by the formation of Cu_xS. Accordingly, the concentration of cTnI could be accurately quantified via the photocurrent, realizing the target-induced modification-free μ-PAD assay. We believe this work could provide an ingenious idea to construct the easy-to-use novel modification-free μ-PAD.

Autocatalytic Metallization of Fabrics Using Si Ink, for Biosensors, Batteries and Energy Harvesting

Commercially available metal inks are mainly designed for planar substrates (for example, polyethylene terephthalate foils or ceramics), and they contain hydrophobic polymer binders that fill the pores in fabrics when printed, thus resulting in hydrophobic electrodes. Here, a low-cost binder-free method for the metallization of woven and nonwoven fabrics is presented that preserves the 3D structure and hydrophilicity of the substrate. Metals such as Au, Ag, and Pt are grown autocatalytically, using metal salts, inside the fibrous network of fabrics at room temperature in a two-step process, with a water-based silicon particle ink acting as precursor. Using this method, (patterned) metallized fabrics are being enabled to be produced with low electrical resistance (less than 3.5 Ω sq-1). In addition to fabrics, the method is also compatible with other 3D hydrophilic substrates such as nitrocellulose membranes. The versatility of this method is demonstrated by producing coil antennas for wireless energy harvesting, Ag-Zn batteries for energy storage, electrochemical biosensors for the detection of DNA/proteins, and as a substrate for optical sensing by surface enhanced Raman spectroscopy. In the future, this method of metallization may pave the way for new classes of high-performance devices using low-cost fabrics.

Emerging paper microfluidic devices

Paper has unique advantages over other materials, including low cost, flexibility, porosity, and self-driven liquid pumping, thus making it widely used in various fields in biology, chemistry, physics and materials science.

Quantum dot-based electrochemical biosensor for stripping voltammetric detection of telomerase at the single-cell level

Human telomerase is responsible for the maintenance of chromosome end structures and is a valuable biomarker for malignant growth. However, the accurate measurement of telomerase activity at the single-cell level has remained a great challenge. Here we develop a simple quantum dot (QD)-based electrochemical biosensor for stripping voltammetric detection of telomerase activity at the single-cell level. We designed a thiol-modified capture DNA which may be immobilized on the gold electrode by the gold-sulfur bond. The presence of telomerase enables the addition of the telomere repeats of (TTAGGG)_n to the 3' end of the primer, accompanied by the incorporation of abundant biotins in the extension product with the assistance of the biotin-tagged dATP. The subsequent hybridization of extension product with the capture DNA and the addition of streptavidin-coated QDs induce the assembly of large amounts of QDs onto the electrode via specific biotin-streptavidin binding. After the acidic dissolution of QDs, the released Cd (II) can be simply quantified by anodic stripping voltammetry (ASV). Due to the introduction of large amounts of QDs by telomerase-induced primer extension reaction and the synergistic signal amplification induced by the release of Cd (II) from the QDs, this biosensor can detect telomerase activity at the single-cell level without the involvement of any thermal cycling and extra enzymes for signal amplification. Moreover, this assay exhibits a large dynamic range over four orders of magnitude and it is very simple without the involvement of specific hairpin probe design and complicated labelling, holding great potential in point-of-need testing.

Flexible plastic, paper and textile lab-on-a chip platforms for electrochemical biosensing

Flexible biosensors represent an increasingly important and rapidly developing field of research. Flexible materials offer several advantages as supports of biosensing platforms in terms of flexibility, weight, conformability, portability, cost, disposability and scope for integration. On the other hand, electrochemical detection is perfectly suited to flexible biosensing devices. The present paper reviews the field of integrated electrochemical bionsensors fabricated on flexible materials (plastic, paper and textiles) which are used as functional base substrates. The vast majority of electrochemical flexible lab-on-a-chip (LOC) biosensing devices are based on plastic supports in a single or layered configuration. Among these, wearable devices are perhaps the ones that most vividly demonstrate the utility of the concept of flexible biosensors while diagnostic cards represent the state-of-the art in terms of integration and functionality. Another important type of flexible biosensors utilize paper as a functional support material enabling the fabrication of low-cost and disposable paper-based devices operating on the lateral flow, drop-casting or folding (origami) principles. Finally, textile-based biosensors are beginning to emerge enabling real-time measurements in the working environment or in wound care applications. This review is timely due to the significant advances that have taken place over the last few years in the area of LOC biosensors and aims to direct the readers to emerging trends in this field.