Photoelectrochemical DNA Biosensors

2D MXene-Based Biosensing: A Review

MXene emerged as decent 2D material and has been exploited for numerous applications in the last decade. The remunerations of the ideal metallic conductivity, optical absorbance, mechanical stability, higher heterogeneous electron transfer rate, and good redox capability have made MXene a potential candidate for biosensing applications. The hydrophilic nature, biocompatibility, antifouling, and anti-toxicity properties have opened avenues for MXene to perform in vitro and in vivo analysis. In this review, the concept, operating principle, detailed mechanism, and characteristic properties are comprehensively assessed and compiled along with breakthroughs in MXene fabrication and conjugation strategies for the development of unique electrochemical and optical biosensors. Further, the current challenges are summarized and suggested future aspects. This review article is believed to shed some light on the development of MXene for biosensing and will open new opportunities for the future advanced translational application of MXene bioassays.

Nanostructured Graphdiyne: Synthesis and Biomedical Applications

Graphdiyne (GDY) is a 2D carbon allotrope that features a one-atom-thick network of sp- and sp²-hybridized carbon atoms with high degrees of π conjugation. Due to its distinct electronic, chemical, mechanical, and magnetic properties, GDY has attracted great attention and shown great potential in various fields, such as catalysis, energy storage, and the environment. Preparation of GDY with various nanostructures, including 0D quantum dots, 1D nanotubes/nanowires/nanoribbons, 2D nanosheets/nanowalls/ordered stripe arrays, and 3D nanospheres, greatly improves its function and has propelled its applications forward. High biocompatibility and stability make GDY a promising candidate for biomedical applications. This review introduces the latest developments in fabrication of GDY-based nanomaterials with various morphologies and summarizes their propective use in the biomedical domain, specifically focusing on their potential advantages and applications for biosensing, cancer diagnosis and therapy, radiation protection, and tissue engineering.

SARS-CoV-2 detection by targeting four loci of viral genome using graphene oxide and gold nanoparticle DNA biosensor

The current COVID-19 pandemic outbreak poses a serious threat to public health, demonstrating the critical need for the development of effective and reproducible detection tests. Since the RT-qPCR primers are highly specific and can only be designed based on the known sequence, mutation sensitivity is its limitation. Moreover, the mutations in the severe acute respiratory syndrome β-coronavirus (SARS-CoV-2) genome led to new highly transmissible variants such as Delta and Omicron variants. In the case of mutation, RT-qPCR primers cannot recognize and attach to the target sequence. This research presents an accurate dual-platform DNA biosensor based on the colorimetric assay of gold nanoparticles and the surface-enhanced Raman scattering (SERS) technique. It simultaneously targets four different regions of the viral genome for detection of SARS-CoV-2 and its new variants prior to any sequencing. Hence, in the case of mutation in one of the target sequences, the other three probes could detect the SARS-CoV-2 genome. The method is based on visible biosensor color shift and a locally enhanced electromagnetic field and significantly amplified SERS signal due to the proximity of Sulfo-Cyanine 3 (Cy3) and AuNPs intensity peak at 1468 cm^-1. The dual-platform DNA/GO/AuNP biosensor exhibits high sensitivity toward the viral genome with a LOD of 0.16 ng/µL. This is a safe point-of-care, naked-eye, equipment-free, and rapid (10 min) detection biosensor for diagnosing COVID-19 cases at home using a nasopharyngeal sample.

Zwitterionic Polymers Coating Antibiofouling Photoelectrochemical Aptasensor for In Vivo Antibiotic Metabolism Monitoring and Tracking

Long-term in vivo monitoring and tracking of target molecules in living organism is essential to reveal vital physiological activity. However, undesirable contamination of protein and biological cells may bring serious biofouling issues. Herein, zwitterionic sulfobetaine methacrylate (SBMA) polymers are grafted on the TiO₂ nanotube (NT) surface with polydopamine (PDA) as linker to fabricate a TiO₂ NTs/PDA/SBMA photoelectrode. The TiO₂ NTs/PDA/SBMA/aptamer-based PEC aptasensor can be sensitive and have selective detection of target molecules with excellent antibiofouling activity. Beneficial from the above advantages, the implantable micro-PEC aptasensor has implemented in vivo tracking and monitoring of the metabolism of antibiotics in a living mouse. The robust antibiofouling property generates new inquiries and an approach for long-standing questions in a new way for reliable and long-term sensing of vital biomolecules in complex biological fluids and uncovers a promising advance of intrinsic physiological mechanisms.

Ultrathin Covalent Organic Framework Nanosheets/Ti3C2Tx-Based Photoelectrochemical Biosensor for Efficient Detection of Prostate-Specific Antigen

Designable and ultrathin covalent organic framework nanosheets (CONs) with good photoelectric activity are promising candidates for the construction of photoelectrochemical (PEC) biosensors for the detection of low-abundance biological substrates. However, achieving highly sensitive PEC properties by using emerging covalent organic framework nanosheets (CONs) remains a great challenge due to the polymeric nature and poor photoelectric activity of CONs. Herein, we report for the first time the preparation of novel composites and their PEC sensing properties by electrostatic self-assembly of ultrathin CONs (called TTPA-CONs) with Ti₃C₂T_x. The prepared TTPA-CONs/Ti₃C₂T_x composites can be used as photocathodes for PEC detection of prostate-specific antigen (PSA) with high sensitivity, low detection limit, and good stability. This work not only expands the application of CONs but also opens new avenues for the development of efficient PEC sensing platforms.

Biometric Photoelectrochemical-Visual Multimodal Biosensor Based on 3D Hollow HCdS@Au Nanospheres Coupled with Target-Induced Ion Exchange Reaction for Antigen Detection

Three-dimensional (3D) hollow photoactive nanomaterials can enhance light capture due to the light scattering benefiting from the unique hollow nanostructures, which contributes to the decrease in energy loss and the electron-hole recombination during the process of photoelectric conversion. Herein, a 3D hollow HCdS@Au nanosphere synthesized by the templated-assisted method and photodeposition is employed to construct a multimodal sensing platform by combining the photoelectrochemical (PEC) biosensor with colorimetric analysis and photothermal imaging. In the presence of target carcinoembryonic antigen (CEA), a sandwich structure is formed on magnetic beads based on the dual-aptamer recognition, followed by the initiation of rolling circle amplification (RCA) to bind numerous CuO-DNA probes. Upon stimulation by chlorhydric acidic, a large number of Cu²⁺ is released from CuO, which could interact with yellow HCdS@Au on electrode to produce dark CuS by ion exchange. As a result, with increased CEA level, the photocurrent is weakened and the color of electrode interface is changed from yellow to dark, which thus facilitates the PEC and colorimetric detection of CEA. Simultaneously, the formed CuS with highly photothermal effect can achieve qualitative visual analysis of CEA using a portable infrared thermal imager. This work exhibits an excellent performance for sensitive and selective detection of CEA in the dynamic working range from 0.015 to 2.4 ng/mL with a detection limit as low as 3.5 pg/mL. Moreover, the proposed PEC biosensor is successfully applied to CEA determination in human serum, which holds great promise in accurate analysis of biomarkers and early diagnosis of diseases in the clinic.

Functional nucleic acid engineered double-barreled nanopores for measuring sodium to potassium ratio at single-cell level

The use of double-barreled nanopipette (θ-nanopipette) to electrically sample, manipulate, or detect biomaterials has recently seen strong growth in single-cell studies, driven by the potential of the nanodevices and applications that they may enable. Considering the pivotal roles of Na/K ratio (R_Na/K) at cellular level, herein we describe an engineered θ-nanopipette for measuring single-cell R_Na/K. The two independently addressable nanopores, located within one nanotip, allow respective customization of functional nucleic acids but simultaneous deciphering of Na and K levels inside a single cell of a non-Faradic manner. Two ionic current rectification signals, corresponding to the Na- and K-specific smart DNA responses, could be easily used to derive the R_Na/K. The applicability of this nanotool is validated by practical probing intracellular R_Na/K during the drug-induced primary stage of apoptotic volume decrease. Especially, the R_Na/K has been shown by our nanotool to be different in cell lines with different metastatic potential. This work is expected to contribute to futuristic study of single-cell R_Na/K in various physiological and pathological processes.

Tuning the Surface Molecular Charge of Organic Photoelectrochemical Transistors with Significantly Improved Signal Resolution: A General Strategy toward Sensitive Bioanalysis

Nature makes use of molecular charges to operate specific biological synthesis and reactions. Targeting advanced opto-bioelectronic sensors, organic photoelectrochemical transistors (OPECTs), taking advantage of the light fuel substituting an external gate potential, is now debuting and expected to serve as a universal platform for studying the rich light-biomatter interplay for new bioanalytics. Given the ubiquity of charged biomolecules in nature, molecular charge manipulation should underpin a generic route for innovative OPECT regulation and operation, which nevertheless has remained unachieved. Herein, this work manifests the biological tuning of surface charge toward the OPECT biosensor, which was exemplified by a light-sensitive CdS quantum dot (QD) gate electrode interfaced by a smart DNA superstructure with adenosine triphosphate (ATP) responsiveness. Highly negative-charged supramolecular DNA concatemers were self-assembled via sequential hybridization, and the ATP-triggered disassembly of the DNA concatemers would cause a tandem change of the effective gate voltage and transfer characteristics with significantly improved resolution. The present opto-bioelectronic device translates the events of charged molecules into amplified electrical signals and outlines a generic format for the future exploitation of rich biological tunability and light-biomatter interplay for innovative bioanalytics and beyond.

Highly Sensitive Photoelectrochemical Biosensor for MicroRNA-21 Based on a Dumbbell-Shaped Heterostructure AuNRs@end-TiO2 Combined with Carbon Dots as Photosensitizers and Duplex-Specific Nuclease-Assisted Signal Amplification

TiO₂ was grown on both ends of gold nanorods (AuNRs) to form a dumbbell-shaped heterostructure (called AuNRs@end-TiO₂) first, and then assembled on the fluorine tin oxide (FTO) electrode surface through hydrophobic interactions to construct a concise photoelectrochemical microRNA-21 (miRNA-21, model target) biosensor using carbon dots (CDs) as photosensitizers. Hairpin probes (HPs) were fixed on the AuNRs@end-TiO₂-modified FTO electrode surface through the Au-S bond, and CDs-modified complementary DNA (CDs-cDNA) served as photosensitive probes. In the presence of the target, miRNA hybridized with the HP and triggered double-strand-specific nuclease to cleave the complementary part of the HP with miRNA, and miRNA was released, which can trigger another cycle to realize signal amplification. Many HPs were cleaved and the complementary sequence with cDNA was exposed, which can capture the photosensitive probes to the electrode surface and resulted in photocurrent enhancement. The photocurrent intensity system has a linear relationship with the logarithm of the miRNA concentration in the range of 0.1 fM to 100 pM with a low detection limit of 96 aM (S/N = 3). The biosensor has high sensitivity, good selectivity, and good reproducibility and shows satisfactory results in actual sample detection.

Ultrafine Silver Nanoparticle Encapsulated Porous Molecular Traps for Discriminative Photoelectrochemical Detection of Mustard Gas Simulants by Synergistic Size-Exclusion and Site-Specific Recognition

The rapid, discriminative, and portable detection of highly toxic chemical warfare agents is extremely important for response to public security emergencies but remains a challenge. One plausible solution involves the integration of porous molecular traps onto a photoelectrochemical (PEC) sensor. Here, a fast and facile protocol is developed to fabricate sub-1 nm AgNPs encapsulated hydrogen-bonded organic framework (HOF) nanocomposite materials through an in situ photoreduction and subsequent encapsulation process. Compared to traditional semiconductors and selected metal-organic frameworks (MOF) materials, these AgNPs@HOFs show significantly enhanced photocurrent. Most importantly, the portable PEC device based on AgNPs@HOF-101 can selectively recognize 13 different mustard gas simulants, including 2-chloroethyl ethyl sulfide (CEES), based on synergistic size-exclusion and specific recognition. The extremely low detection limit for CEES (15.8 nmol L^-1 ), reusability (at least 30 cycles), and long-term working stability (at least 30 d) of the portable PEC device warrant its use as a chemical warfare agents (CWAs) sensor in practical field settings. More broadly, this work indicates that integrating porous molecular traps onto PEC sensors offers a promising strategy to further develop portable devices for CWAs detection with both ultrahigh sensitivity and selectivity.

Tailoring of the photocatalytic activity of CeO2 nanoparticles by the presence of plasmonic Ag nanoparticles

The present study investigates basic features of a photoelectrochemical system based on CeO₂ nanoparticles fixed on gold electrodes. Since photocurrent generation is limited to the absorption range of the CeO₂ in the UV range, the combination with metal nanoparticles has been studied. It can be shown that the combination of silver nanoparticles with the CeO₂ can shift the excitation range into the visible light wavelength range. Here a close contact between both components has been found to be essential and thus, hybrid CeO₂@Ag nanoparticles have been prepared and analyzed. We have collected arguments that electron transfer occurs between both compositional elements of the hybrid nanoparticles.The photocurrent generation can be rationalized on the basis of an energy diagram underlying the necessity of surface plasmon excitation in the metal nanoparticles, which is also supported by wavelength-dependent photocurrent measurements. However, electrochemical reactions seem to occur at the CeO₂ surface and consequently, the catalytic properties of this material can be exploited as exemplified with the photoelectrochemical reduction of hydrogen peroxide. It can be further demonstrated that the layer-by layer technique can be exploited to create a multilayer system on top of a gold electrode which allows the adjustment of the sensitivity of the photoelectrochemical system. Thus, with a 5-layer electrode with hybrid CeO₂@Ag nanoparticles submicromolar hydrogen peroxide concentrations can be detected.

Ternary Z-Scheme Ag-Embedded TiO2-Ag2S Nanojunction as a Novel Photoelectrochemical Converter for CD44 Detection

Nanoarrays (NAs) with stable signal output have become the most promising photoelectrochemical (PEC) biosensing substrate. However, a general issue is that interfacial charge-carrier recombination in a single-component semiconductor cannot be easily prevented, resulting in a low photocurrent density. Herein, a biosensor utilizing a Ag-embedded TiO₂-Ag₂S nanojunction (TiO₂-Ag-Ag₂S) as a signal converter was developed for the detection of CD44 protein─a transmembrane glycoprotein highly expressed in breast cancer cells. The ternary Z-scheme heterojunction was prepared by a distinctive scheme in which the Ag layer is introduced onto the surface of rutile TiO₂ NAs by magnetron sputtering, whereas the Ag₂S is rooted in the local sulfuration of Ag. With a sufficient density of oriented nanorods, TiO₂-Ag-Ag₂S exhibits a smooth photocurrent output and minimal variation among different batches; it is undoubtedly a satisfactory PEC sensing carrier, which enables highly specific identification of target CD44 on the surface of MDA-MB-231 cells due to DNA strand displacement reactions (SDRs) and host-guest recognition between hyaluronic acid (HA) and CD44. The biosensor shows a sensitive PEC response to CD44 over a wide range of 37 to 5.0 × 10⁵ cells/mL. We can conclude that this approach will provide an alternative solution to breast cancer diagnosis.

Engineering carbon nanotubes for sensitive viral detection

Viral infections have been proven a severe threat to human beings, and the pandemic of Coronavirus Disease 2019 (COVID-19) has become a societal health concern, including mental distress and morbidity. Therefore, the early diagnosis and differentiation of viral infections are the prerequisite for curbing the local and global spread of viruses. To this end, carbon nanotubes (CNTs) based virus detection strategies are developed that provide feasible alternatives to conventional diagnostic techniques. Here in this review, an overview of the design and engineering of CNTs-based sensors for virus detection is summarized, followed by the nano-bio interactions used in developing biosensors. Then, we classify the viral sensors into covalently engineered CNTs, non-covalently engineered CNTs, and size-tunable CNTs arrays for viral detection, based on the type of CNTs-based nano-bio interfaces. Finally, the current challenges and prospects of CNTs-based sensors for virus detection are discussed.

State-of-the-Art Development in Liquid Crystal Biochemical Sensors

As an emerging stimuli-responsive material, liquid crystal (LC) has attracted great attentions beyond display applications, especially in the area of biochemical sensors. Its high sensitivity and fast response to various biological or chemical analytes make it possible to fabricate a simple, real-time, label-free, and cost-effective LC-based detection platform. Advancements have been achieved in the development of LC-based sensors, both in fundamental research and practical applications. This paper briefly reviews the state-of-the-art research on LC sensors in the biochemical field, from basic properties of LC material to the detection mechanisms of LC sensors that are categorized into LC-solid, LC–aqueous, and LC droplet platforms. In addition, various analytes detected by LCs are presented as a proof of the application value, including metal ions, nucleic acids, proteins, glucose, and some toxic chemical substances. Furthermore, a machine-learning-assisted LC sensing platform is realized to provide a foundation for device intelligence and automatization. It is believed that a portable, convenient, and user-friendly LC-based biochemical sensing device will be achieved in the future.

Enzyme-free nucleic acid dual-amplification strategy combined with mimic enzyme catalytic precipitation reaction for the photoelectrochemical detection of microRNA-21

A novel photoelectrochemical (PEC) biosensor based on an enzyme-free nucleic acid dual-amplification strategy combined with a mimic enzyme to catalyze the deposition of a quencher is reported for the ultrasensitive detection of miRNA-21. A limited amount of target miRNA-21 can trigger the formation of long DNA duplexes on the electrode, owing to the synergistic effect of the enzyme-free nucleic acid dual-amplification strategy of entropy-driven strand displacement reaction (ESDR) amplification and hybridization chain reaction (HCR) amplification. The embedded manganese porphyrin (MnPP) in the long DNA duplexes acts as a horseradish peroxidase (HRP)-mimicking enzyme to catalyze the transformation of benzo-4-chlorohexadienone on the electrode surface, resulting in a significant reduction in photocurrent intensity. As a photosensitive material, BiOCl-BiOI is used as a tag to provide strong initial PEC signals. Based on the cascade integration of the enzyme-free nucleic acid dual-amplification strategy and the mimic enzyme-catalyzed precipitation reaction, the current PEC biosensor exhibits outstanding performance for miRNA-21 detection with an ultralow detection limit (33 aM) and a wide quantification range (from 100 aM to 1 nM). This work provides a new avenue toward the ultrasensitive detection of miRNAs, and is expected to be used for clinical and biochemical samples. A unique PEC biosensor with the BiOCl-BiOI composite, as the photosensitive material, has been developed for ultrasensitive miRNA-21 determination based on the combination of an enzyme-free nucleic acid dual-amplification strategy and mimic enzyme catalytic precipitation reaction.

Dual-Engine Powered Paper Photoelectrochemical Platform Based on 3D DNA Nanomachine-Mediated CRISPR/Cas12a for Detection of Multiple miRNAs

This work proposed a novel double-engine powered paper photoelectrochemical (PEC) biosensor based on an anode-cathode cooperative amplification strategy and various signal enhancement mechanisms, which realized the monitoring of multiple miRNAs (such as miRNA-141 and miRNA-21). Specifically, C₃N₄ quantum dots (QDs) sensitized ZnO nanostars and BiOI nanospheres simultaneously to construct a composite photoelectric layer that amplified the original photocurrent of the photoanode and photocathode, respectively. Through the independent design and partition of a flexible paper chip to functionalize injection holes and electrode areas, the bipolar combination completed the secondary upgrade of signals, which also provided biological reaction sites for multitarget detection. With the synergistic participation of a three-dimensional (3D) DNA nanomachine and programmable CRISPR/Cas12a shearing tool, C₃N₄ QDs lost their attachment away from the electrode surface to quench the signal. Moreover, electrode zoning significantly reduced the spatial cross talk of related substances for multitarget detection, while the universal trans-cleavage capability of CRISPR/Cas12a simplified the operation. The designed PEC biosensor revealed excellent linear ranges for detection of miRNA-141 and miRNA-21, for which the detection limits were 5.5 and 3.4 fM, respectively. With prominent selectivity and sensitivity, the platform established an effective approach for trace multitarget monitoring in clinical applications, and its numerous pioneering attempts owned favorable reference values.

A near-infrared light-driven photoelectrochemical aptasensing platform for adenosine triphosphate detection based on Yb-doped Bi2S3 nanorods

Due to its capability of low spectral interference, high light stability, and minimal photodamage to biological species, near-infrared (NIR) light is advantageous in biosensing and biochemical analysis. This work developed a photoelectrochemical (PEC) aptasensor for adenosine triphosphate (ATP) detection using NIR light as the irradiation source. In order to utilize NIR light, we prepared Yb-doped Bi₂S₃ (Yb-Bi₂S₃) nanorods to act as photoelectric transducing materials. Due to the unfilled 4f orbitals of Yb which introduced the impurity level between the valence band and conduction band of Bi₂S₃, Yb-Bi₂S₃ exhibited admirable photo-to-current conversion efficiency under NIR light irradiation. The Yb-Bi₂S₃ modified electrode was employed to construct a NIR light-driven PEC sensor using an ATP-binding aptamer as the recognition element. When ATP was present, the photocurrent signal of the proposed aptasensor declined, owing to the formation of an ATP-aptamer complex which enhanced the steric hindrance of electron transfer on the electrode. Under optimal conditions, the sensor showed a sensitive response to ATP in the concentration range from 0.5 to 300 nmol L^-1 with a detection limit of 0.1 nmol L^-1. The proposed aptasensor exhibited high selectivity, good repeatability and desirable stability. Moreover, it was successfully applied to ATP detection in human serum samples.

Ti3C2 MXene-anchored photoelectrochemical detection of exosomes by in situ fabrication of CdS nanoparticles with enzyme-assisted hybridization chain reaction

Exosomes that carry large amounts of tumor-specific molecular information have been identified as a potential non-invasive biomarker for early warning of cancer. In this work, we reported an enzyme-assisted photoelectrochemical (PEC) biosensor for quantification of exosomes based on the in situ synthesis of Ti₃C₂ MXene/CdS composites with magnetic separation technology and hybridization chain reaction (HCR). First, exosomes were specifically bound between aptamer-labeled magnetic beads (CD63-MBs) and a cholesterol-labeled DNA anchor. The properly designed anchor ends acted as a trigger to enrich the alkaline phosphatase (ALP) through HCR. It catalyzed more sodium thiophosphate to generate the sulfideion (S²⁻), which combined with Cd²⁺ for in situ fabrication of CdS on Ti₃C₂ MXene resulting in elevated photocurrent. The Ti₃C₂ MXene-anchored PEC method was realized for the quantitative detection of exosomes, which exhibited the dynamic working range from 7.3 × 10⁵ particles per mL to 3.285 × 10⁸ particles per mL with a limit of detection of 7.875 × 10⁴ particles per mL. The strategy showed acceptable stability, high sensitivity, rapid response and excellent selectivity. Furthermore, we believe that the PEC biosensor has huge potential as a routine bioassay method for the precise quantification of exosomes from breast cancer in the future.

An enzyme-assisted photoelectrochemical (PEC) biosensor was established for quantification of exosomes based on the in situ synthesis of Ti₃C₂ MXene/CdS composites with magnetic separation technology and hybridization chain reaction (HCR).

Hybridization chain reaction for regulating surface capacitance of organic photoelectrochemical transistor toward sensitive miRNA detection

Photon-enabled bioelectronics has long been pursued in modern electronics due to their non-contact, remote-control, and even self-powered function interfacing the biological world with semiconductor devices. The debuting organic photoelectrochemical transistor (OPECT) relies on the photovoltage generated by the semiconductors to modulate the channel conductance, which enables light-fueled operation at zero gate bias. Inspired by the insulating nature of macrobiomolecules and surface capacitance mechanism, herein we demonstrate the biological regulation of the surface capacitance towards new OPECT biodetection, which was exemplified by a CdS quantum dots/TiO₂ nanotubes photoanode accommodating hybridization chain reaction (HCR) amplification with the target of biomarker miRNA-17. Formation of the non-conducting DNA layer from the miRNA-17-oriented HCR could decrease the surface capacitance and increase the corresponding fractional potential drop, shifting the transfer curve horizontally to higher gate voltage and thus producing different drain currents. The OPECT biosensor exhibited a linear relationship with the miRNA-17 concentration on the logarithmic axis in the range from 1 pm. to 10 μm with a detection limit of 1 pm. This work not only represented a generic methodology of miRNA detection, but also provided a universal mechanism for the operation of advanced OPECT bioanalytics.

Light-Controlled Ionic/Molecular Transport through Solid-State Nanopores and Nanochannels

Biological nanochannels perfectly operate in organisms and exquisitely control mass transmembrane transport for complex life process. Inspired by biological nanochannels, plenty of intelligent artificial solid-state nanopores and nanochannels are constructed based on various materials and methods with the development of nanotechnology. Specially, the light-controlled nanopores/nanochannels have attracted much attention due to the unique advantages in terms of that ion and molecular transport can be regulated remotely, spatially and temporally. According to the structure and function of biological ion channels, light-controlled solid-state nanopores/nanochannels can be divided into light-regulated ion channels with ion gating and ion rectification functions, and light-driven ion pumps with active ion transport property. In this review, we present a systematic overview of light-controlled ion channels and ion pumps according to the photo-responsive components in the system. Then, the related applications of solid-state nanopores/nanochannels for molecular sensing, water purification and energy conversion are discussed. Finally, a brief conclusion and short outlook are offered for future development of the nanopore/nanochannel field.

Photoelectrochemical aptasensors for detection of viruses

Photoelectrochemistry (PEC) is a dynamic discipline studying the effect of light on photoelectrode or photosensitive material, and the conversion from solar energy into electrical power. The basic PEC process refers to the oxidation or reduction reactions between electrochemical active species in solution and photoactive materials that occurred at the electrode/electrolyte interface during illumination. In recent years, the PEC biosensing approaches have also been developed by the combination of the PEC technique with bioanalysis, where the interaction between biological recognition element and analyte influences a photocurrent signal. This involves the charge and energy transfer of PEC reaction between electron donor/acceptor and photoactive material upon light irradiation. Coupling the advantages of PEC bioanalysis and aptamers has provided new concepts for highly selective and sensitive biosensors development, applicable in human health monitoring and environmental protection. In a typical assay, a photoactive material converts the affinity binding properties of aptamers into a detectable electrical signal, presenting an innovative method for probing numerous aptamer-analyte interactions. Using different aptamer probes aiming for specific purposes, more sensing strategies with rational design and exquisite signaling mechanisms have been proposed. This review concentrated on the current topic of PEC aptasensors that are used for the detection of viruses. The prospects in this area are also discussed.

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Metal organic frameworks for electrochemical sensor applications: A review

Metal-organic frameworks (MOFs) are broadly known as porous coordination polymers, synthesized by metal-based nodes and organic linkers. MOFs are used in various fields like catalysis, energy storage, sensors, drug delivery etc., due to their versatile properties (tailorable pore size, high surface area, and exposed active sites). This review presents a detailed discussion of MOFs as an electrochemical sensor and their enhancement in the selectivity and sensitivity of the sensor. These sensors are used for the detection of heavy metal ions like Cd²⁺, Pb²⁺, Hg²⁺, and Cu²⁺ from groundwater. Various types of organic pollutants are also detected from the water bodies using MOFs. Furthermore, electrochemical sensing of antibiotics, phenolic compounds, and pesticides has been explored. In addition to this, there is also a detailed discussion of metal nano-particles and metal-oxide based composites which can sense various compounds like glucose, amino acids, uric acid etc. The review will be helpful for young researchers, and an inspiration to future research as challenges and future opportunities of MOF-based electrochemical sensors are also reported.

Quantum dots based sensitive nanosensors for detection of antibiotics in natural products: A review

Residual antibiotics in food products originated from administration of the antibiotics to animals may be accumulated through food metabolism in the human body and endanger safety and health. Thus, developing a prompt and accurate way for detection of antibiotics is a crucial issue. The zero-dimensional fluorescent probes including metals based, carbon and graphene quantum dots (QDs), are highly sensitive materials to use for the detection of a wide range of antibiotics in natural products. These QDs demonstrate unique optical properties like tunable photoluminescence (PL) and excitation-wavelength dependent emission. This study investigates the trends related to carbon and metal based QDs preparation and modification, and their diverse detection application. We discuss the performance of QDs based sensors application in various detection systems such as photoluminescence, photoelectrochemical, chemiluminescence, electrochemiluminescence, colorimetric, as well as describing their working principles in several samples. The detecting mechanism of a QDs-based sensor is dependent on its properties and specific interactions with particular antibiotics. This review also tries to describe environmental application and future perspective of QDs for antibiotics detection.

Target-Induced In Situ Formation of Organic Photosensitizer: A New Strategy for Photoelectrochemical Sensing

Small-molecule photosensitizers have great application prospects in photoelectrochemical (PEC) sensing due to their defined composition, diversified structure, and adjustable photophysical properties. Herein, we propose a new strategy for PEC analysis based on the target-induced in situ formation of the organic photosensitizer. Taking thiophenol (PhSH) as a model analyte, we designed and synthesized a 2,4-dinitrophenyl (DNP)-caged coumarin precursor (Dye-PhSH), which was then covalently coupled onto the TiO₂ nanoarray substrate to obtain the working photoanode. Due to the intramolecular photoinduced electron transfer process, Dye-PhSH has only a very weak photoelectric response. Upon reacting with the target, Dye-PhSH undergoes a tandem reaction of the detachment of the DNP moiety and the intramolecular cyclization process, which leads to a coumarin dye with a pronounced photoelectric effect, thus achieving a highly selective turn-on PEC response to PhSH. For the first time, this study was to construct a PEC sensor by exploiting specific organic reactions for the in situ generation of small molecule-based photoactive material. It can be anticipated that the proposed strategy will expand the paradigm of PEC sensing and holds great potential for detecting various other analytes.

Homogeneous Photoelectrochemical Aptasensors for Tetracycline Based on Sulfur-Doped g-C3N4/n-GaN Heterostructures Formed through Self-Assembly

The complex synthesis of photoelectric materials and the difficulty of fixing the identification elements on the photoelectrode are long-standing problems in the field of photoelectrochemical (PEC) biosensing. In this work, a simple PEC aptasensor construction strategy based on a sulfur-doped g-C₃N₄ (SCN)/n-GaN heterostructure photoelectrode was proposed. The SCN/n-GaN heterostructure can be formed through self-assembly in solution since SCN can be uniformly dispersed in solution. In addition, as a dual-function mediate, an aptamer can be fixed on an SCN substrate automatically because of the good adsorption performance of SCN. Therefore, tedious steps of PEC electrode preparation and the fixing of recognition elements were both avoided. Compared with the traditional ones, the construction difficulty and time cost of the prepared PEC aptasensors are greatly reduced. The simplified experimental process improves the stability and reproducibility of the aptasensor. Finally, tetracycline (TET) was used as a model target to verify the sensing performance of the proposed PEC strategy. TET can consume the photogenerated holes of the SCN/n-GaN heterostructure, promote carrier migration, and result in the change in the photocurrent. The linear relationship between the change in the photocurrent intensity and the TET concentration can be used to detect TET. The aptasensor has a linear range of 0.10-10.0 nmol L^-1 and the detection limit is 0.030 nmol L^-1 (3S/N). The aptasensor was applied to the detection of TET in milk samples with satisfactory results.

A microfluidic cloth-based photoelectrochemical analytical device for the detection of glucose in saliva

Photoelectrochemical (PEC) detection is a widely used detection method that uses light to stimulate and photocurrent signals to detect the target. Due to the disengagement of the excitation unit and the detection unit, the PEC background signal is reduced, and the detection sensitivity is improved. In this work, we report the first demonstration of PEC detection for microfluidic cloth-based analytical devices (μCADs). Using PEC μCADs integrated with cadmium sulfide quantum dots (CdS QDs) and multiwalled carbon nanotubes (MWCNTs), the nonenzymatic, sensitive and rapid measurement of glucose in saliva has been achieved. For the cloth-based device, the PEC reaction zone and cloth-based electrodes can be fabricated by inexpensive wax-based and carbon ink-based screen-printing, respectively. By the layer-by-layer method, the as-prepared poly (dimethyl diadly ammonium chloride-functionalized) MWCNTs (PDDA-MWCNTs) and CdS QDs are successively adsorbed onto the working electrode surface of the cloth-based device. In the presence of an excitation source and glucose, the CdS QDs generate a strong oxidizing electron hole that can then continuously oxidize glucose to produce an electrical signal for glucose detection. Under optimized conditions, a linear dependence is obtained between the PEC signal and glucose concentrations in the range of 0.05-1000 μM with a detection limit of 15.99 nM. In the detection range, the cloth-based device also shows acceptable selectivity, reproducibility, and long-term stability. Moreover, the method has been implemented for the detection of glucose in real saliva samples, suggesting good potential for biochemical applications.

Advances in fluorescence sensing enabled by lanthanide-doped upconversion nanophosphors

Lanthanide-doped upconversion nanoparticles (UCNPs), characterized by converting low-energy excitation to high-energy emission, have attracted considerable interest due to their inherent advantages of large anti-Stokes shifts, sharp and narrow multicolor emissions, negligible autofluorescence background interference, and excellent chemical- and photo-stability. These features make them promising luminophores for sensing applications. In this review, we give a comprehensive overview of lanthanide-doped upconversion nanophosphors including the fundamental principle for the construction of UCNPs with efficient upconversion luminescence (UCL), followed by state-of-the-art strategies for the synthesis and surface modification of UCNPs, and finally describing current advances in the sensing application of upconversion-based probes for the quantitative analysis of various analytes including pH, ions, molecules, bacteria, reactive species, temperature, and pressure. In addition, emerging sensing applications like photodetection, velocimetry, electromagnetic field, and voltage sensing are highlighted.

Crystal-reconstructed BiVO4 semiconductor photoelectrochemical sensor for ultra-sensitive tumor biomarker detection

In this study, we developed a crystal-reconstructed-BiVO₄ aptamer photoelectrochemical (PEC) biosensor by a high-energy laser treatment technique. This biosensor achieves a limit of detection (LOD) (0.82 ag mL^-1), linear detection range (1 ag mL^-1 to 2 ng mL^-1), and resolution ratio (∼18 molecules per mL) for prostate-specific antigen (PSA) tumor biomarker detection. Furthermore, reconstructed surface microstructure and oxygen vacancy doping energy formation after crystal reconstruction induce the stereo-hindrance effect and photogenerated hole energy is reduced during PSA target detection. In this case, a photocurrent inhibition phenomenon for PSA detection is noticed. Based on this photocurrent inversion phenomenon, some dysoxidizable nucleonic acid tumor (miRNA-21) and virus biomarkers (RdRp-COVID) can be detected with a LOD level of ∼10^-16 M by linking the corresponding base paring probe on the surface of the crystal-reconstructed photoanode. In addition to high sensitivity, this PEC biosensor presents high detection specificity, stability, and accuracy in clinical verification. Thus, this crystal-reconstructed PEC biosensor shows application potential in the fields of multi-tumor or viral biomarker detection.

Targeting Natural Polymeric DNAs with Harmane: An Insight into Binding and Thermodynamic Interaction Through Biophysical Approach

DNA is one of the major molecular targets for a broad range of anticancer drugs. Hence, interaction studies involving cellular DNA and small molecules can be highly beneficial as they often lead to rational and efficient drug design. In this study, the binding interaction of Harmane (a naturally occurring, bioactive indole alkaloid) with two natural polymeric DNAs, that is, Calf thymus (CT) DNA and Herring testis (HT) DNA has been elucidated using biophysical techniques. A ground state, 1:1 complexation, was revealed by steady-state fluorescence spectroscopy. The thermodynamic profile and energetics of the associated reaction were evaluated by temperature-dependent fluorescence spectroscopy. The spontaneity of the binding was confirmed by the negative ΔG° values in both cases. Negative enthalpy change, along with stronger positive entropic contribution, indicated the dominant electrostatic nature of the interaction and finally the entropy-driven exothermic binding process throughout. Salt-dependent studies further demonstrated the significant contribution of electrostatic interactions in ligand binding toward DNA. Infrared data substantiated the structural information of the said interactions, leading to the exploration of the structure-function relationship.

Adopting Back Reduction Current as an Additional Output Signal for Achieving Photoelectrochemical Differentiated Detection

Photoelectrochemical (PEC) sensors are usually based on a single output signal, that is, the photocurrent change caused by the (photoelectro)chemical reaction between target analytes and photoelectrodes. However, the photocurrent may be influenced by redox species other than the target analyte; therefore, modifying the surface of photoelectrodes with probes that selectively bind to the analyte is essential. Moreover, even though various surface modification methods have been developed, distinguishing molecularly similar chemicals using PEC sensing systems remains a significant challenge. To address these selectivity issues, we proposed a photoanode-based PEC sensor that utilizes a cathodic transient current as a second output signal in addition to the photocurrent, which arises from the back reduction of photo-oxidized species. Factors influencing the back reduction were investigated by observing the transient photocurrent of hematite photoanodes in the presence of model redox probes. The chemical environment around the electrode-electrolyte interface was manipulated by altering the electrolyte composition or modifying the electrode surface. The favorable interaction between the electrode surface and redox species led to an increase in the extent of back reduction and the cathodic transient current. In addition, the extent of back reduction also depends on the chemical identity of the redox species, such as the kinetics of subsequent chemical reactions. Therefore, the synergistic combination of the photocurrent and the cathodic transient current enabled the differentiated detection of various catecholamine neurotransmitters with a single pristine photoelectrode, which has never been achieved using traditional PEC methods. Revisiting the transient photocurrent can complement conventional PEC applications and offers possibilities for more effective semiconductor-based applications.

Photoresponsive DNA materials and their applications

Photoresponsive nucleic acids attract growing interest as functional constituents in materials science. Integration of photoisomerizable units into DNA strands provides an ideal handle for the reversible reconfiguration of nucleic acid architectures by light irradiation, triggering changes in the chemical and structural properties of the nanostructures that can be exploited in the development of photoresponsive functional devices such as machines, origami structures and ion channels, as well as environmentally adaptable 'smart' materials including nanoparticle aggregates and hydrogels. Moreover, photoresponsive DNA components allow control over the composition of dynamic supramolecular ensembles that mimic native networks. Beyond this, the modification of nucleic acids with photosensitizer functionality enables these biopolymers to act as scaffolds for spatial organization of electron transfer reactions mimicking natural photosynthesis. This review provides a comprehensive overview of these exciting developments in the design of photoresponsive DNA materials, and showcases a range of applications in catalysis, sensing and drug delivery/release. The key challenges facing the development of the field in the coming years are addressed, and exciting emergent research directions are identified.

Biomedical IoT: Enabling Technologies, Architectural Elements, Challenges, and Future Directions

This paper provides a comprehensive literature review of various technologies and protocols used for medical Internet of Things (IoT) with a thorough examination of current enabling technologies, use cases, applications, and challenges. Despite recent advances, medical IoT is still not considered a routine practice. Due to regulation, ethical, and technological challenges of biomedical hardware, the growth of medical IoT is inhibited. Medical IoT continues to advance in terms of biomedical hardware, and monitoring figures like vital signs, temperature, electrical signals, oxygen levels, cancer indicators, glucose levels, and other bodily levels. In the upcoming years, medical IoT is expected replace old healthcare systems. In comparison to other survey papers on this topic, our paper provides a thorough summary of the most relevant protocols and technologies specifically for medical IoT as well as the challenges. Our paper also contains several proposed frameworks and use cases of medical IoT in hospital settings as well as a comprehensive overview of previous architectures of IoT regarding the strengths and weaknesses. We hope to enable researchers of multiple disciplines, developers, and biomedical engineers to quickly become knowledgeable on how various technologies cooperate and how current frameworks can be modified for new use cases, thus inspiring more growth in medical IoT.

Coenzyme-catalyzed electroinitiated reversible addition fragmentation chain transfer polymerization for ultrasensitive electrochemical DNA detection

Ultrasensitive detection of biomarkers at an early stage is generally limited by external influence factors such as high reaction temperature, complex operations, and sophisticated instruments. Here, we circumvent these problems by using nicotinamide adenine dinucleotide (NAD⁺) to control electroinitiated reversible addition fragmentation chain transfer (electro-RAFT) polymerization for biosensing that enables the detection of a few molecules of target DNA. In this coenzyme-catalyzed electro-RAFT polymerization, numerous ferrocenylmethyl methacrylate (FCMMA) as monomer with electrochemistry signal were linked to the biomarker on Au electrode. Afterwards, a strong oxidation peak appears at the potential of about 0.3 V that represents a typical oxidation potential of FCMMA. The sensitivity of this methodology was presented by detecting DNA from 10^-1 to 10⁴ fM concentration and detection limit (LOD) being down to 4.39 aM in 10 μL samples. This is lower by factors than detection limits of most other ultra-sensitive electrochemical DNA assays.

Singlet oxygen-based photoelectrochemical detection of DNA

The current work, designed for the photoelectrochemical detection of DNA, evaluates light-responsive DNA probes carrying molecular photosensitizers generating singlet oxygen (¹O₂). We take advantage of their chromophore's ability to produce ¹O₂ upon photoexcitation and subsequent photocurrent response. Type I, fluorescent and type II photosensitizers were studied using diode lasers at 406 nm blue, 532 nm green and 659 nm red lasers in the presensce and absence of a redox reporter, hydroquinone (HQ). Only type II photosensitizers (producing ¹O₂) resulted in a noticeable photocurrent in 1-4 nA range upon illumination, in particular, dissolved DNA probes labeled with chlorin e6 and erythrosine were found to give a well-detectable photocurrent response in the presence of HQ. Whereas, Type I photosensitizers and fluorescent chromophores generate negligible photocurrents (<0.15 nA). The analytical performance of the sensing system was evaluated using a magnetic beads-based DNA assay on disposable electrode platforms, with a focus to enhance the sensitivity and robustness of the technique in detecting complementary DNA targets. Amplified photocurrent responses in the range of 70-100 nA were obtained and detection limits of 17 pM and 10 pM were achieved using magnetic beads-captured chlorin e6 and erythrosine labeled DNA probes respectively. The presented novel photoelectrochemical detection can further be optimized and employed in applications for which enzymatic amplification such as polymerase chain reaction (PCR) is not applicable owing to their limitations and as an effective alternative to colorimetric detection when rapid detection of specific nucleic acid targets is required.

Fabrication of an imprinted polymer based graphene oxide composite for label-free electrochemical sensing of Sus DNA

An innovative label-free electrochemical sensor was developed for selective detection of Sus (pig) Deoxyribonucleic acid (DNA) through adenine imprinted polypyrrole fabricated on the surface of allyl mercaptan modified GO (MIP/mGO).

Spatially-extended 3D magnetic DNA nanodevice-based split-type photoelectrochemical strategy for sensitive and reliable miRNA detection in cancer cells

To improve the accuracy of PEC sensing. We developed a split-type “turn-off” PEC biosensor based on spatially-extended 3D magnetic DNA nanodevices with high-order DNA amplifiers for sensitive and reliable detection of miRNAs in cancer cells.

Recent advances in electron manipulation of nanomaterials for photoelectrochemical biosensors

This feature article discusses the recent advances and strategies of building photoelectrochemical (PEC) biosensors from the perspective of regulating the electron transfer of nanomaterials.