Self-Powered Biosensing Platform Based on “Signal-On” Enzymatic Biofuel Cell for DNA Methyltransferase Activity Analysis and Inhibitor Screening

A self-powered biosensor with cascade amplification capability facilitates ultra-sensitive detection of microRNA biomarkers

Sensitive microRNA (miRNA) detection is crucial for cancer diagnosis. Self-powered biosensors that are used for miRNA detection show the advantages of no external power supply, mild reaction conditions, portability, etc., but still face the challenges of low efficiency of solid electrode adsorption enzyme and insufficient enzyme active site. Here, the MnS@MoS₂ composite electrode substrate boosts enzyme load, accelerates electron transfer, and enhances detection. Catalytic hairpin self-assembly (CHA) and Hybridization chain reaction (HCR) cascade bio-signal amplification enables current signal amplification via molecular recognition. When miRNA-199a is present, CHA and HCR trigger signal cascade amplification, achieving high sensitivity and specificity while powering the system. The sensor has a 0.5 fmol/L - 100 pmol/L linear response and a 0.14 fmol/L limit of detection (LOD). Additionally, the test signal is transmitted to a smartphone interface via Bluetooth, enabling portable, enabling portable, real-time detection. Our work shows this self-powered biosensor offers a new path for ultrasensitive miRNA detection, aids rapid disease biomarker monitoring, and broadens self-powered sensor use in medical diagnosis.

Ultrasensitive COFs Functionalized Electrochemical Biosensor for DNA Methyltransferase Activity Detection by DNA Walking and Rolled Circular Strand Displacement Amplification

Assessing the activity of DNA methyltransferases (MTases) and screening for methyltransferase inhibitors not only allow for a deep exploration of the role of methylation regulation in disease initiation and progression but also provide an important experimental and clinical basis for the diagnosis and treatment of diseases. Herein, a new COFs functionalized electrochemical biosensor has been developed to detect DNA adenine methylation (Dam) MTase activity with high sensitivity and rapidity by taking advantage of the DNA walker and rolled circular strand displacement amplification (RC-SDA) reaction. Specifically, hairpin probe H1 was methylated by Dam MTase, followed by methylation site-specific cleavage of DpnI enzyme to generate the S5 probe. The padlock probes with two nucleic acid endonuclease sites were introduced to trigger the RC-SDA reaction under the action of the primer, releasing a large number of single-stranded S6 probes. The released probe worked synergistically with the Exo III enzyme to trigger DNA walking, which exposed the binding sites to enable the Au-COF-MB signal probes to bind effectively to the electrode surface, and the electrochemical signals were thus generated. The result showed that the designed electrochemical biosensor demonstrated excellent sensitivity and specificity in detecting the activity of Dam MTase, with the detection limit (LOD) of 6.85 × 10-5 U/mL. This method provides support for the development of new treatment strategies for methylation related diseases.

Specific Response Assembly of 3D Space-Confined DNA Nanoaggregates for Rapid and Sensitive Detection of DNA Methyltransferase

Rapid and sensitive detection of DNA adenine methyltransferase (Dam) activity is crucial for both research and clinical applications. Herein, we utilize two types of spherical nucleic acids (SNAs) to specific response assemble into 3D space-confined DNA nanoaggregates that enable the rapid and sensitive detection of Dam activity. The SNAs feature 3D order DNA scaffolds that serve as cores for anchoring signal hairpin probes (S-HPs) and target hairpin probes (T-HPs). Specifically, two distinct S-HPs are labeled with FAM fluorophores and BHQ1 quenchers and share identical hairpin sequences, while two types of T-HPs are designed with different linking sequences and specific recognition regions, resulting in the formation of two types of SNAs (SNA1 and SNA2). In the presence of Dam, the recognition region of the T-HPs is methylated and subsequently cleaved by auxiliary endonuclease, releasing the loop of the T-HP as a walking strand and exposing the linking sequence on the SNAs. Notably, the prior design of complementary linking sequences in the two types of SNAs facilitates their assembly into 3D DNA nanoaggregates, creating a confined space for walking strands to recover fluorescent signals. The 3D DNA nanoaggregate system not only provides highly ordered tracks but also enhances the spatial continuity of the walking strands, greatly improving the reaction kinetics for detecting Dam activity. This strategy enables the rapid and sensitive detection of Dam activity within 105 min, achieving a limit of detection of 2.9 × 10-4 U mL-1, demonstrating significant potential for advancing research in DNA methylation.

Novel self-powered sandwich type Aptamer sensor for estrogen detection assisted with peroxidase mimic Cu-MOF

A novel self-powered and flexible enzymatic biofuel cell (EBFC)-based aptasensor was developed for the sensitive and selective detection of 17 β-estradiol (E2). A flexible polyvinyl alcohol (PVA)-tannic acid‑carbon nanotube/reduced graphene oxide (PTCR) substrate was modified with gold nanoparticles (AuNPs) and thiolated aptamer 1 (Apt1) to yield Apt1@AuNPs/PTCR. A copper-based metal-organic framework (Cu-MOF) with peroxidase mimicking activity was employed to anchor glucose oxidase (GOD) and Apt2, forming the Cu-MOF@GOD/Apt2 tag. When E2 was recognized by Apt1, the anchored E2 quantitatively recognized Cu-MOF@GOD/Apt2 to create a Cu-MOF@GOD/Apt2-E2-Apt1 sandwich structure for glucose oxidation to generate electrical power. Increased E2 concentrations enhanced Cu-MOF@GOD/Apt2 capture and amplified the electrical signal. The electrical power increased linearly as the E2 concentration increased from 1.0 pM to 1.0 nM. The sensor was successfully applied to various food samples and blood serum detection. This work promoted the application of novel self-powered biosensors for food safety analysis.

A portability self-powered sensor facilitates sensitive Cd2+ detection: Dual mechanism and three quantitative mode

As a common heavy metal contaminant, Cd2+ has adverse effects on food safety and consumer health. It is very important for human health to realize highly sensitive Cd2+ detection methods. The self-powered sensing system based on enzyme biofuel cells (EBFCs) does not need an external power supply, which can simplify the experimental equipment and has great application value in portable detection. Thus, the biosensor is innovatively integrated into the screen-printed electrode to construct a new type of portable sensor suitable for on-site and real-time Cd2+ detection. Hybridization chain reaction (HCR) combined with the Cd2+-dependent deoxyribose (DNAzyme) signal amplification strategy is used to enhance the detection sensitivity while specifically recognizing the Cd2+. Moreover, the self-powered sensor combines with smartphones to realize quantitative Cd2+ detection without other instruments and has the characteristic of Effectively improving the hazard detection technology is essential to ensure food safety. Portability, simplicity, and speed are suitable for real-time Cd2+ detection in the field. The dual mechanism and three quantitative modes combining colorimetric and two electrical signals output modes are adopted to realize the visualization and accurate detection. A series of research results confirm that this strategy is of great significance to strengthen the development of intelligent Cd2+ technology, expand the application of self-powered sensing technology, and improve the safety detection system.

On-demand controlled bidirectional DNAzyme path for ultra-sensitive heavy metal ion detection

A bidirectional self-powered biosensor is constructed for the quasi-simultaneous detection of Pb2+ and Hg2+ based on MoS2@CuS heterostructures as an accelerator and hybridization chain reaction (HCR) as a signal amplification strategy. MoS2@CuS heterostructures significantly facilitate electron transfer between glucose and bioelectrodes, thereby greatly improving the detection signal of self-powered biosensors. This novel biosensor employs the unique sequences of DNAzymes to isolate Pb2+ and Hg2+ by the cleavage effect and thymine (T)-Hg2+-thymine (T) structures, respectively. In the process, Pb2+ cuts the sequence of DNAzyme at the bioanode to trigger glucose oxidation to monitor Pb2+. The as-formed T-Hg2+-T structures activate HCR to reduce [Ru(NH3)6]3+ to detect Hg2+ at the biocathode. It is noteworthy that this biosensor not only realizes Pb2+ or Hg2+ detection in a single-electrode, respectively, but also can quasi-simultaneously detect both Pb2+ and Hg2+ in the bioanode and the biocathode. The novel self-powered biosensor identifies Pb2+ in the range of 106 fM to 10 fM with a limit of detection (LOD) of 3.1 fM and Hg2+ in the range of 106 fM to 1 fM with an LOD of 0.33 fM.

Self-Powered FEN1 Biosensor Based on Accelerated CRISPR/Cas Trans-Cleavage around Porous Fe3O4 Nanoparticles

Flap endonuclease 1 (FEN1) is a structure-specific endonuclease that plays a critical role in the maintenance of genome integrity. In this work, we demonstrate a novel self-powered electrochemical FEN1 biosensor for potential applications in molecular diagnosis. Porous Fe3O4 nanoparticles are first prepared, and single-strand DNA probes are absorbed on the surface of the nanoparticles. Thus, electrochemical species of [Fe(CN)6]3- can be encapsulated inside the porous nanoparticles with the molecular gate of negatively charged DNA. On the other hand, a dumbbell structured DNA probe with 5' flap is designed. FEN1 is able to cleave the flap and activate the CRISPR/Cas system for the digestion of single-stranded DNA around Fe3O4 nanoparticles. As a result, the leakage of [Fe(CN)6]3- contributes to an enhanced electrochemical response, which can be used to reveal the level of FEN1. The high sensitivity of this biosensor is due to the application of porous nanomaterials and Mn2+ accelerated CRISPR/Cas cleavage. It succeeds in detection of biological samples and screening of FEN1 inhibitors. Therefore, this proposed method has potential applications in the early diagnosis of diseases and drug discovery.

CRISPR/Cas12a-triggered ordered concatemeric DNA probes signal-on/off multifunctional analytical sensing system for ultrasensitive detection of thalassemia

Based on CRISPR/Cas12a triggered ordered concatemeric DNA probes, a "on/off" self-powered biosensor is developed to achieve highly sensitive detection of thalassemia gene CD142 through open-circuit potential-assisted visual signal output. The ingeniously constructed glucose oxidase (GOD)-functionalized ordered concatemeric DNA probe structure can significantly amplify signal output, while the coupled CRISPR/Cas12a system is served as a "signal switch" with excellent signal-transducing capabilities. When the ordered concatemeric DNA probe structure is anchored on electrode, the response signal of the sensing system is in the "signal on" mode. While, the presence of the target activates the non-specific cleavage activity of the CRISPR/Cas12a system, causing the sensing system to switch to the "signal off" mode. In the detection system, GOD catalyzes the oxidation of glucose to produce hydrogen peroxide, which further catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to form a color product, enabling visual signal of the target through naked-eye color contrast. By employing a multifunctional analytical mode combining electrochemical and visual signal outputs, accurate determination of the target is achieved, with linear ranges of 0.0001-100 pM, and detection limits of 48.1 aM (S/N = 3). This work provides a reference method for sensitive detection of thalassemia genes and holds great diagnostic potential in biomedical applications.

Electrochemiluminescence in Graphitic Carbon Nitride Decorated with Silver Nanoparticles for Dopamine Determination Using Machine Learning

Electrochemiluminescence (ECL) luminophores with wavelength-tunable multicolor emissions are essential for multicolor ECL imaging detection and multiplexed analysis. In this work, silver nanoparticle (Ag NP)-decorated graphitic carbon nitride (g-CN@Ag) nanocomposites were synthesized. The morphology, chemical composition, structure, and ECL property of g-CN@Ag were investigated. The prepared g-CN, g-CN@Ag1, g-CN@Ag5, and g-CN@Ag10 can produce blue, blue-green, chartreuse, and yellow colored ECL emissions, respectively, by using K2S2O8 as the coreagent. The ECL emission wavelength of g-CN@Ag can be regulated from 460 to 565 nm by modulating the content of the immobilized Ag NPs. Then, a multicolor ECL detection array was fabricated by using g-CN, g-CN@Ag1, g-CN@Ag5, and g-CN@Ag10 as four ECL luminophores. Dopamine was detected based on its inhibition effect on the multicolor ECL emissions. The linear range is from 0.1 nM to 1 mM with the lowest detection limit of 44 pM. Then, machine learning-assisted multiparameter concentration prediction of dopamine was further carried out by combining the deep neural network (DNN) algorithm. This work provides a new avenue to regulate the ECL emission wavelength of g-CN by using the metal nanoparticle modification strategy and presents an effective machine learning-assisted multicolor ECL detection strategy for accurate multiparameter quantitative detection.

Fluorogenic RNA Aptamer-Based Amplification and Transcription Strategy for Label-free Sensing of Methyltransferase Activity in Complex Matrixes

DNA methyltransferase is significant in cellular activities and gene expression, and its aberrant expression is closely linked to various cancers during initiation and progression. Currently, there is a great demand for reliable and label-free techniques for DNA methyltransferase evaluation in tumor diagnosis and cancer therapy. Herein, a low-background fluorescent RNA aptamer-based sensing approach for label-free quantification of cytosine-guanine (CpG) dinucleotides methyltransferase (M.SssI) is reported. The fluorogenic light-up RNA aptamers-based strategy exhibits high selectivity via restriction endonuclease, padlock-based recognition, and RNA transcription. By combining rolling circle amplification (RCA), and RNA transcription with fluorescence response of RNA aptamers of Spinach-dye compound, the proposed platform exhibited efficiently ultrahigh sensitivity toward M.SssI. Eventually, the detection can be achieved in a linear range of 0.02-100 U mL-1 with a detection limit of 1.6 × 10-3 U mL-1. Owing to these superior features, the method is further applied in serum samples spiked M.SssI, which delivers a recovery ranging from 92.0 to 107.0% and a relative standard deviation <7.0%, providing a promising and practical tool for determining M.SssI in complex biological matrices.

Test strip coupled Cas12a-assisted signal amplification strategy for sensitive detection of uracil-DNA glycosylase

Uracil-DNA glycosylase (UDG) is a base excision repair (BER) enzyme, which catalyzes the hydrolysis of uracil bases in DNA chains that contain uracil and N-glycosidic bonds of the sugar phosphate backbone. The expression of UDG enzyme is associated with a variety of genetic diseases including cancers. Hence, the identification of UDG activity in cellular processes holds immense importance for clinical investigation and diagnosis. In this study, we employed Cas12a protein and enzyme-assisted cycle amplification technology with a test strip to establish a precise platform for the detection of UDG enzyme. The designed platform enabled amplifying and releasing the target probe by reacting with the UDG enzyme. The amplified target probe can subsequently fuse with crRNA and Cas12a protein, stimulating the activation of the Cas12a protein to cleave the signal probe, ultimately generating a fluorescent signal. This technique showed the ability for evaluating UDG enzyme activity in different cell lysates. In addition, we have designed a detection probe to convert the fluorescence signal into test strip bands that can then be observed with the naked eye. Hence, our tool presented potential in both biomedical research and clinical diagnosis related to DNA repair enzymes.

Advancements in Bio-inspired Self-Powered Wireless Sensors: Materials, Mechanisms, and Biomedical Applications

The rapid maturation of smart city ecosystems is intimately linked to advances in the Internet of Things (IoT) and self-powered sensing technologies. Central to this evolution are battery-less sensors that are critical for applications such as continuous health monitoring through blood metabolites and vital signs, the recognition of human activity for behavioral analysis, and the operational enhancement of humanoid robots. The focus on biosensors that exploit the human body for energy-spanning wearable, attachable, and implantable variants has intensified, driven by their broad applicability in areas from underwater exploration to biomedical assays and earthquake monitoring. The heart of these sensors lies in their diverse energy harvesting mechanisms, including biofuel cells, and piezoelectric, triboelectric, and pyroelectric nanogenerators. Notwithstanding the wealth of research, the literature still lacks a holistic review that integrates the design challenges and implementation intricacies of such sensors. Our review seeks to fill this gap by thoroughly evaluating energy harvesting strategies from both material and structural perspectives and assessing their roles in powering an array of sensors for myriad uses. This exploration offers a comprehensive outlook on the state of self-powered sensing devices, tackling the nuances of their deployment and highlighting their potential to revolutionize data gathering in autonomous systems. The intent of this review is to chart the current landscape and future prospects, providing a pivotal reference point for ongoing research and innovation in self-powered wireless sensing technologies.

Research Progress in Enzyme Biofuel Cells Modified Using Nanomaterials and Their Implementation as Self-Powered Sensors

Enzyme biofuel cells (EBFCs) can convert chemical or biochemical energy in fuel into electrical energy, and therefore have received widespread attention. EBFCs have advantages that traditional fuel cells cannot match, such as a wide range of fuel sources, environmental friendliness, and mild reaction conditions. At present, research on EBFCs mainly focuses on two aspects: one is the use of nanomaterials with excellent properties to construct high-performance EBFCs, and the other is self-powered sensors based on EBFCs. This article reviews the applied nanomaterials based on the working principle of EBFCs, analyzes the design ideas of self-powered sensors based on enzyme biofuel cells, and looks forward to their future research directions and application prospects. This article also points out the key properties of nanomaterials in EBFCs, such as electronic conductivity, biocompatibility, and catalytic activity. And the research on EBFCs is classified according to different research goals, such as improving battery efficiency, expanding the fuel range, and achieving self-powered sensors.

Advanced Self-Powered Biofuel Cells with Capacitor and Nanogenerator for Biomarker Sensing

Self-powered biofuel cells (BFCs) have evolved for highly sensitive detection of biomarkers such as noncodon micro ribonucleic acids (miRNAs) in the presence of interfering substrates. Self-charging supercapacitive BFCs for in vivo and in vitro cellular microenvironments represent the most prevalent sensing mechanism for diagnosis. Therefore, self-powered biosensing (SPB) with a capacitor and contact separation with a triboelectric nanogenerator (TENG) offers electrochemical and colorimetric dual-mode detection via improved electrical signal intensity. In this review, we discuss three major components: stretchable self-powered BFC design, miRNA sensing, and impedance spectroscopy. A specific focus is given to 1) assembling of sensors for biomarkers, 2) electrical output signal intensification, and 3) role of supercapacitors and nanogenerators in SPBs. We outline the key features of stretchable SPBs and the sequence of miRNA sensing by SPBs. We have emphasized the need of a supercapacitor and nanogenerator for SPBs in the context of advanced assembly of the sensing unit. Finally, we outline the role of impedance spectroscopy in the detection and estimation of biomarkers. We highlight key challenges in SPBs for biomarker sensing, which needs improved sensing accuracy, integration strategies of electrochemical biosensing for in vitro and in vivo microenvironments, and the impact of miRNA sensing on cancer diagnostics. This article attempts a specific focus on the accuracy and limitations of sensing unit for miRNA biomarkers and associated tool for boosting electrical signal intensity for a potential big step further.

Constructed a self-powered sensing platform based on nitrogen-doped hollow carbon nanospheres for ultra-sensitive detection and real-time tracking of double markers

Acute myocardial infarction (AMI) is acute necrosis of a portion of the myocardium caused by myocardial ischemia, which seriously threatens people's health and life safety. Its early diagnosis is a difficult problem in clinical medicine. Research has found that the abnormal expression of microRNA-199a (miR-199a) and microRNA-499 (miR-499) was closely related to AMI disease. In this work, we took advantage of the structural advantages of nitrogen-doped hollow carbon nanospheres (N-HCNSs) to design an ultra-sensitive, portable real-time monitoring visual self-powered biosensor system, which based on dual-target miRNAs triggered catalytic hairpin assembly (CHA) for sensitive detection of miR-199a and miR-499. In addition, the capacitor and the smartphone are introduced into the system to realize the secondary improvement of system sensitivity and portable real-time visual monitoring. Under optimized conditions, in the linear range of 0.1-100000 aM, the detection limits of miR-199a and miR-499 are 0.031 and 0.027 aM, respectively. At the same time, the ultra-sensitive detection of miRNAs is realized in the serum sample, and the recovery rate of miR-199a and miR-499 are 98.0-106.0% (RSD: 0.6-8.1%) and 94.0-109.7% (RSD: 1.8-7.7%), respectively. The method is simple, sensitive and can be used for real-time tracking and portable monitoring of related diseases.

Ultrasensitive fluorescence detection of multiple DNA methyltransferases based on DNA walkers and hyperbranched rolling circle amplification

The accurate and ultrasensitive detection of multiple methyltransferases was in great request for clinical diagnosis and epigenetic therapy. Here, a novel fluorescence assay was proposed for ultrasensitive CpG methyltransferase (M.SssI) and DNA adenine methyltransferase (Dam) activity detection based on hyperbranched rolling circle amplification (HRCA) and DNA walkers. The biosensor showed an extremely high sensitivity due to the dual-amplification strategy of HRCA and DNA walker. The LOD of the biosensor for M.SssI and Dam methyltransferase was estimated at 0.0004 U/mL and 0.001 U/mL, respectively. Without the presence of M.SssI methyltransferase, the corresponding recognition site of hairpin H_M was cleaved by HpaII endonuclease, generating a DNA fragment (T-DNA) and inducing the DNA walker-HRCA reaction. Since the HRCA products contained numerous double-strand DNA (dsDNA), SYBR Green I could be embedded in the dsDNA, leading to a high fluorescent signal. In the presence of M.SssI methyltransferase, the corresponding recognition site of hairpin H_M was methylated and the HpaII endonuclease-catalyzed stem of hairpin H_M dissociation was hindered, leading to no DNA fragment (T-DNA) present. Hence, the DNA walker-HRCA reaction was not initiated and the fluorescent signal of SYBR Green I remained at a low level. Similarly, DNA adenine methyltransferase (Dam) and its inhibitors could also be detected by redesigning hairpin H_D with the Dam recognition sequences. Furthermore, the sensing system was applied to analyze the endogenic Dam methyltransferase in the real samples such as E. coli cell lysate.

Self-Powered Biosensor for a Highly Efficient and Ultrasensitive Dual-Biomarker Assay

A dual-biomarker, self-powered biosensor was fabricated for the ultrasensitive detection of microRNA-21 (miRNA-21) and miRNA-155 based on enzymatic biofuel cells (EBFCs), catalytic hairpin assembly (CHA), and DNA hybridization chain reaction (HCR), with a capacitor and digital multimeter (DMM). In the presence of miRNA-21, the CHA and HCR are triggered and lead to the generation of a double-helix chain, which stimulates [Ru(NH₃)₆]³⁺ to move to the biocathode surface due to electrostatic interaction. Subsequently, the biocathode obtains electrons from the bioanode and reduces [Ru(NH₃)₆]³⁺ to [Ru(NH₃)₆]²⁺, which significantly increases the open-circuit voltage (E₁^OCV). When miRNA-155 is present, CHA and HCR cannot be completed, resulting in a low E₂^OCV. The self-powered biosensor allows for the simultaneous ultrasensitive detection of miRNA-21 and miRNA-155 with detection limits of 0.15 and 0.66 fM, respectively. Moreover, this self-powered biosensor exhibits the highly sensitive detection for miRNA-21 and miRNA-155 assay in human serum samples.

Tailoring enzymatic loading capacity on 3D macroporous gold by catalytic hairpin assembly and hybridization chain reaction: Application for ultrasensitive self-powered microRNA detection

It is important to develop effective strategies to construct enzymatic biofuel cell based self-powered biosensors. We report here the facile regulation of enzymatic loading capacity on the bioanode by utilizing a concatenated catalytic hairpin assembly (CHA)/hybridization chain reaction (HCR) and its application for self-powered microRNA-141 (miRNA-141) detection. To construct the bioanode, a concatenated CHA/HCR process triggered by miRNA-141 was conducted on the three-dimensional macroporous gold (3DMG) electrode to generate long double-stranded DNA nanowires for glucose oxidase immobilization. Quartz crystal microbalance study reveals that the enzymatic loading capacity on the bioanode increases at an increasing miRNA-141 concentration, leading to enhanced catalytic performance for glucose oxidation. The short-circuit currents of the assembled glucose/O2 biofuel cells increase at increasing miRNA-141 concentrations, enabling ultrasensitive detection of miRNA-141. The self-powered biosensor features a wide dynamic range for detecting miRNA-141 from 10-17 to 10-11 M, with an ultralow detection limit of 1.3 aM. This work provides a highly sensitive self-powered biosensing platform for miRNA detection.

Self-Powered Signal Transduction of Ion-Selective Electrodes to an Electronic Paper Display

Mobile integrated electrochemical sensors normally require a power supply for operation. Unfortunately, the practice of discarding batteries associated with these devices runs counter to our desire for a sustainable world. Self-powered sensing concepts that draw the energy directly from the measurement itself would overcome this limitation. Potentiometric sensors for the measurement of pH, many electrolytes, and gases are ubiquitous in analytical practice. However, in potentiometry, the voltage is acquired in the absence of current flow, making it seemingly impossible to draw power. Fortunately, it has been recently established that transient currents may be tolerated across potentiometric measurement cells to charge a capacitive or electrochromic element such as Prussian blue integrated in the measurement cell and whose absorbance then directly follows the potential changes in a reversible manner. We have shown here that commercial electronic paper (e-paper), widely used to make electronic ink and ebook readers, can directly be driven by a potentiometric measurement cell in a reversible manner at mild potentials of >100 mV typical for such sensors. The capacitance of the e-paper pixel studied here was found to be 0.53 μF mm^-2, 30 times smaller than that of Prussian blue films. The colorimetric absorbance of the e-paper was also more stable (observed drift over 2 h corresponding to 0.76 mV h^-1) and reproducible (corresponding to 1 mV standard deviation). The e-paper pixel was directly driven by a polymeric pH electrode as a model system. Choosing a basic inner solution (pH 12.9) behind the membrane gave sufficiently positive cell potentials for driving visible absorbance change in a sample pH range of 4-10, while a more acidic pH of 3.4 and alternating the connections to the e-paper were more suited for more basic samples of pH > 10. This convenient and cost-effective approach makes it possible to directly drive an optical display from the potentiometric measurement itself and should be suitable for moderate sensing membrane resistances of less than about 100 kΩ, depending on the area of the chosen pixel.

Direct Energy Transfer from a pH Glass Electrode to a Liquid Crystal Display

Self-powered sensors are attractive because the lack of a dedicated battery makes them environmentally friendly and allows them to be more easily miniaturized. Unfortunately, the development of self-powered potentiometric sensors is challenging because only very limited energy can be harvested from this measurement principle. For the first time, the potential of a high impedance glass pH electrode (130 M Ω) is shown here to be directly read out optically. This is accomplished by a liquid crystal display (LCD) as the electrochromic transducer, which changes its transmission upon imposing an external voltage in the range of 2-3 V. Importantly, owing to its low capacitance of about 50 pF, this process requires a very small transient charge on the order of 100 pC, which may be spontaneously imposable even across pH glass electrodes. For the LCD to be turned on, the cell voltage is boosted by additional Zn²⁺/Zn elements placed in series. The LCD is found to give a time-dependent absorbance decrease, which is mitigated by adding a high resistance element to attenuate the associated decay. The approach gives repeatable LCD absorbance values that allows one to directly visualize pH with a precision of about 0.01 pH units. The absorbance value depends inversely on pH in a much wider range (pH 1-13) than what is normally observed with optical sensors while based on the same underlying measurement as a potentiometric pH probe.

A glucose/O2 biofuel cell integrated with an exonuclease-powered DNA walker for self-powered sensing of microRNA

With the aid of an exonuclease-powered DNA walker, the amount of glucose oxidase immobilized on the bioanode can be facilely tailored by varying the concentration of microRNA-141, so a glucose/O₂ biofuel cell is employed as a self-powered sensor for sensitive and selective detection of microRNA-141.

Integration of a capacitor to a 3-D DNA walker and a biofuel cell-based self-powered system for ultrasensitive bioassays of microRNAs

A self-powered microRNA biosensor with triple signal amplification systems was assembled through the integration of three-dimensional DNA walkers, enzymatic biofuel cells and a capacitor. The DNA walker is designed from an enzyme-free target triggered catalytic hairpin assembly of modified gold nanoparticles. When triggered by the target microRNA, the DNA walker will move along the catalytic hairpin track, resulting in a payload release of glucose oxidase. The enzymatic biofuel cell contains the glucose oxidase bioanode and a bilirubin oxidase biocathode that bring a dramatic open circuit voltage to realize the self-powered bioassays of microRNA. A capacitor is further coupled with the enzymatic biofuel cell to further amplify the electrochemical signal, and the sensitivity increases 28.82 times through optimizing the matching capacitor. Based on this design, the present biosensor shows high performance, especially for detection limit and sensitivity. Furthermore, the present biosensor was successfully applied for serum samples, directly demonstrating its good application in clinical biomedicine and disease diagnosis.

A membraneless self-powered photoelectrochemical biosensor based on Bi2S3/BiPO4 heterojunction photoanode coupling with redox cycling signal amplification strategy

The photoelectrochemical (PEC) self-powered system has attracted great attention in disease detection. The determination of a simple and efficient approach for disease-related biomarkers is highly interesting and appealing. Herein, an ingenious visible light-induced membraneless self-powered PEC biosensing platform was constructed, integrating a signal amplification strategy for ultrasensitive split-type PEC bioanalysis. The system was comprised of a Bi₂S₃/BiPO₄ heterojunction photoanode and a platinum (Pt) cathode in a one compartment chamber. An alkaline phosphatase (ALP)-loaded sandwich immunoassay was used to generate the signal reporter ascorbic acid (AA) in a 96-well plate, and myoglobin (Myo) was used as a model protein. In the presence of AA, ferrocene (Fc), and Tris (2-carboxyethyl) phosphine (TCEP), the chemical-chemical redox cycling scheme was operated upon the initial oxidation of Fc by the holes in the Bi₂S₃/BiPO₄ photoelectrode, and Fc was regenerated from Fc⁺ by AA. Subsequently, AA was regenerated by TCEP after its oxidation, and cycling was triggered. As a result, the proposed self-powered PEC sensing exhibited excellent performance with a wide linear range from 5.0 × 10^-13 to 1.0 × 10^-7 g/mL, and a low detection limit of 2.0 × 10^-13 g/mL for Myo. This work provided a new design of a redox cycling strategy in the self-powered PEC biosensor, and showed an effective approach for the clinical diagnosis.

Exploiting the optical sensing of fluorophore-tagged DNA nucleobases on hexagonal BN and Al-doped BN sheets: a computational study

Hexagonal boron nitride (h-BN) sheets possess high fluorescence quenching ability and high affinity towards DNA/RNA, and they can be used as a sensing platform for rapid detection. We report the absorption and emission properties of DNA nucleobases such as adenine (A), cytosine (C), guanine (G), and thymine (T) tagged with benzoxazole on h-BN and aluminium-doped h-BN (Al_hBN) sheets. The binding affinity of studied nucleobases on h-BN sheets at the M062X/6-31G* level of theory showed the following adsorption trend: G ≥ T ≥ A > C, which is in good agreement with the previous results. The calculated stability trend of nucleobases on the Al_hBN sheet follows as C > G > A > T at the same level of theory. The physically adsorbed behavior of nucleobases to h-BN sheets was confirmed by the non-covalent interactions (NCIs) and the total density of states (TDOS) plots. The NCI results indicated that van der Waals interactions contribute significantly to the adsorption of nucleobases on h-BN sheets. Atoms in molecules (AIM) calculations revealed the electrostatic interactions between nucleobases and the Al_hBN sheet. The quenching phenomenon of nucleobase-tagged fluorophores on h-BN and Al_hBN sheets was investigated by TD-DFT calculations using the same level of theory. The thymine-tagged fluorophore upon adsorption to the pristine h-BN sheet was found to be blue-shifted (∼43 nm); however, the guanine-tagged fluorophore with Al_hBN showed a remarkable difference from other nucleobase-tagged fluorophores in the absorption and emission spectrum. Guanine-tagged fluorophores showed a smaller blue shift (∼7 nm) in the absorption spectrum; however, it showed a larger red shift (∼55 nm) than the other nucleobase-tagged fluorophores on Al_hBN sheets and can be useful in recognizing a sequence-specific phenomenon as a fluorescent biosensor of DNA and RNA to ascertain the presence of such nucleobases.

Construction of an Integrated Device of a Self-Powered Biosensor and Matching Capacitor Based on Graphdiyne and Multiple Signal Amplification: Ultrasensitive Method for MicroRNA Detection

The detection of microRNA (miRNA) in human serum has great significance for cancer prevention. Herein, a novel self-powered biosensing platform is developed, which effectively integrates an enzymatic biofuel cell (EBFC)-based self-powered biosensor with a matching capacitor for miRNA detection. A catalytic hairpin assembly and hybrid chain reaction are used to improve the analytical performance of EBFC. Furthermore, the matching capacitor is selected as an auxiliary signal amplifying device, and graphdiyne is applied as substrate material for EBFC. The results confirm that the developed method obviously increases the output current of EBFC, and the sensitivity can reach 2.75 μA/pM, which is 786% of pure EBFC. MiRNA can be detected in an expanded linear range of 0.1-100000 fM with a detection limit of 0.034 fM (S/N = 3). It can offer a selective and sensitive platform for nucleotide sequence detection with great potential in clinical diagnostics.

Tailored Biosensors for Drug Screening, Efficacy Assessment, and Toxicity Evaluation

Biosensors have been flourishing in the field of drug discovery with pronounced developments in the past few years. They facilitate the screening and discovery of innovative drugs. However, there is still a lack of critical reviews that compare the merits and shortcomings of these biosensors from a pharmaceutical point of view. This contribution presents a critical and up-to-date overview on the recent progress of tailored biosensors, including surface plasmon resonance, fluorescent, photoelectrochemical, and electrochemical systems with emphasis on their mechanisms and applications in drug screening, efficacy assessment, and toxicity evaluation. Multiple functional nanomaterials have also been incorporated into the biosensors. Representative examples of each type of biosensors are discussed in terms of design strategy, response mechanism, and potential applications. In the end, we also compare the results and summarize the major insights gained from the works, demonstrating the challenges and prospects of biosensors-assisted drug discovery.

Cyclometalated Iridium(III) Complex-Sensitized NiO-Based-Cathodic Photoelectrochemical Platform for DNA Methyltransferase Assay

This work reports an efficient [(C6)₂Ir(dppz)]⁺PF₆^- (C6 = coumarin 6 and dppz = dipyridophenazine)-sensitized NiO photocathode and its application in photoelectrochemical (PEC) bioanalysis field for the first time. This dye-sensitized NiO photocathode was found to exhibit a markedly enhanced cathodic photocurrent. A sensitive cathodic PEC platform was proposed integrating the as-prepared photocathode with enzyme-free cascaded amplification strategies of the catalytic hairpin assembly (CHA) and the hybridization chain reaction (HCR) for DNA methyltransferase (MTase) assay. A hairpin DNA(H_Dam) with specific recognition site of Dam MTase in its stem was designed. The site of H_Dam was methylated in the presence of Dam MTase and then cut by endonuclease DpnI. The released loop fragment, as an initiator, triggered the CHA circuit and the follow-up HCR circuit, resulting in long dsDNA concatemers on the ITO electrode. Numerous [(C6)₂Ir(dppz)]⁺PF₆^- were intercalated into dsDNA, and highly efficient signal amplification was realized. Benefiting from the superior iridium(III) complex-sensitized NiO photocathode and effective amplification strategy, a detection limit of 0.0028 U/mL for the determination of Dam MTase was achieved. Moreover, this work further demonstrated that these proposed tactics could be applied to screen Dam MTase activity inhibitors.

Photo-driven self-powered biosensors for ultrasensitive microRNA detection based on metal-organic framework-controlled release behavior

We developed a "signal-on" self-powered biosensing strategy by taking full advantage of both photoelectrochemical biofuel cells (PBFCs) and metal-organic framework (MOF)-controlled release behavior for ultrasensitive microRNA assay. PBFC-based self-powered sensors have the unique characteristics of non-requirement of external power sources, simple fabrication process, miniature size, good anti-interference ability and low cost. Furthermore, based on the target microRNA-induced release of the electron donor ascorbic acid and the high catalytic ability of the biocathode to catalyse the oxygen reduction reaction, photo-driven self-powered biosensors for ultrasensitive microRNA detection were successfully realized. The as-proposed signal-on biosensor not only provides a simple and effective strategy, but also possesses the merits of a wide dynamic concentration response range and high sensitivity for microRNA detection, with a limit of detection down to 0.16 fM.

Label-free and enzyme-free one-step rapid colorimetric detection of DNA methylation based on unmodified gold nanoparticles

DNA methylation has been identified as one of the important causes of tumorigenesis, so it is important to develop some advanced methods for detecting and quantifying DNA methylation. In this study, a label-free and enzyme-free one-step rapid colorimetric detection of DNA methylation based on unmodified Au nanoparticles(Au NPs)has been proposed. This method can quickly, efficiently, economically and easily colorimetric detect methylated DNA only by the color change of unmodified Au NPs solution without the covalent modification of Au NPs in advance or complicated instruments for implementation with practical limitations or expensive biological enzymes or traditional organic dyes during the reaction. The strategy employed the difference in electrostatic attraction of single-stranded DNA and double-stranded DNA against salt-induced aggregation of Au NPs. The method has a DNA methylated detection limit of 8.47 nM and it is distinctly visible to detect methylated DNA with the naked eye as low as 20 nM. Furthermore, the strategy has an ability to detect methylated DNA in the presence of abundant unmethylated DNA with the detection limit of 0.13% and as low as 1% methylated DNA can be distinguished in heterogeneous samples with the naked eye. Also, the stratagem provides a convenient and rapid platform for methylated DNA detection of human serum samples in one step, which displays a huge potential for clinical diagnosis and treatment of oncological diseases.

A self-powered aptasensor using the capacitor-amplified signal of a photofuel cell and a portable digital multimeter readout

A self-powered aptasensor for streptomycin detection was constructed with a photofuel cell combined with a capacitor and a digital multimeter. The sensitivity of the proposed sensor was 8.7 times of that without using a capacitor amplifier circuit.