Direct Fluorescent Detection of Blood Potassium by Ion-Selective Formation of Intermolecular G-Quadruplex and Ligand Binding

Pattern Recognition of Alkaloids by Inhibiting the Catalytic Activity of Dopzymes for Dopamine

Exploiting the specific recognition probe for all of the biomolecules is difficult in "lock-and-key" biosensors. The cross-reaction or the semispecific probe in pattern recognition mode is an alternative strategy through extracting a multidimensional signal array from recognition elements. Here, we design a pattern recognition sensor array based on the alkaloid-inhibited catalytic activity of dopzymes for the discrimination and determination of six alkaloids. In this sensor array, three different G-rich sequences, i.e., G-triplex (G3), G-quadruplex (GQ1), and the G-quadruplex dimer (2GQ1) possessing various peroxidase activities, conjugated with a dopamine aptamer and the dopzymes (G3-d-apt, GQ1-d-apt, and 2GQ1-d-apt) were obtained with an enhanced catalytic performance for the substrate. Through the interactions between six target alkaloids and G3, GQ1, and 2GQ1 regions, the pattern signal (6 alkaloids × 3 dopzymes × 5 replicates) was obtained from the diverse inhibited effect for the dopzyme activity. In virtue of the statistical method principal component analysis (PCA), the data array was projected into a new dimensional space to acquire the three-dimensional (3D) canonical scores and grouped into their respective clusters. The sensor array exhibited an outstanding discrimination and classification capability for six alkaloids with different concentrations with 100% accuracy. In addition, the nonspecific recognition elements of the sensor array showed high selectivity even though other alkaloids with similar structures to targets existed in the samples. Importantly, the levels of the six targets can be analyzed by the most influential discrimination factor, which represented the vector with the highest variance, evidencing that the sensor array has potential in drug screening and clinical treatment.

MoS2 nanosheets supported on anodic aluminum oxide membrane: An effective interface for label-free electrochemical detection of microRNA

The interesting adsorption affinity of two-dimensional nanosheets to single stranded over double stranded nucleic acids have stimulated the exploration of these materials in biosensing. Herein, MoS₂ nanosheets decorated anodic aluminum oxide (AAO) membrane was simply prepared by suction filtration. The MoS₂/AAO hybrid membrane was initially applied to the electrochemical detection of microRNA using let-7a as the model. When let-7a was incubated with its complementary DNA, double stranded DNA-RNA formed and which displayed weak adsorption capability to the hybrid membrane. And thus the steric effect combining the electrostatic repulsion of the backbone phosphate of nucleic acids for [Fe(CN)₆]^3- transport across the hybrid membrane varied with the concentration of let-7a. In this way, a label-free electrochemical detection method for microRNA was established by monitoring the change of the redox current of [Fe(CN)₆]^3-. To further improve the detection sensitivity of the method, we proposed two separate strategies focusing on the amplification of the target-induced steric hindrance with DNA nanostructure and the magnification of the electrode sensitivity for [Fe(CN)₆]^3- by electrode modification. By using the two strategies, the hybrid membrane based-detection method exhibited broad linear range, low detection limit and good selectivity as well as reproducibility. Therefore, this study provided a proof-of-concept for the application of two-dimensional material to nucleic acids detection.

A cyanine dye probe for K+ detection based on DNA construction of G-quadruplex

Potassium ion (K⁺) plays an important role in the maintenance of cellular biological process for human health. Thus, the detection of K⁺ is very important. Here, based on the interaction between thiamonomethinecyanine dye and G-quadruplex formation sequence (PW17), K⁺ detection spectrum was characterized by UV-Vis spectrometry. The single-stranded sequence of PW17 can fold into G-quadruplex in the presence of K⁺. PW17 can induce a dimer-to-monomer transition of the absorption spectrum of cyanine dyes. This method shows high specificity against some other alkali cations, even at high concentrations of Na⁺. Further, this detection strategy can realize the detection of K⁺ in tap water.

Recent Developments in G-Quadruplex Binding Ligands and Specific Beacons on Smart Fluorescent Sensor for Targeting Metal Ions and Biological Analytes

The G-quadruplex structure is crucial in several biological processes, including DNA replication, transcription, and genomic maintenance. G-quadruplex-based fluorescent probes have recently gained popularity because of their ease of use, low cost, excellent selectivity, and sensitivity. This review summarizes the latest applications of G-quadruplex structures as detectors of genome-wide, enantioselective catalysts, disease therapeutics, promising drug targets, and smart fluorescence probes. In every section, sensing of G-quadruplex and employing G4 for the detection of other analytes were introduced, respectively. Since the discovery of the G-quadruplex structure, several studies have been conducted to investigate its conformations, biological potential, stability, reactivity, selectivity for chemical modification, and optical properties. The formation mechanism and advancements for detecting different metal ions (Na⁺, K⁺, Ag⁺, Tl⁺, Cu⁺/Cu²⁺, Hg²⁺, and Pb²⁺) and biomolecules (AMP, ATP, DNA/RNA, microRNA, thrombin, T4 PNK, RNase H, ALP, CEA, lipocalin 1, and UDG) using fluorescent sensors based on G-quadruplex modification, such as dye labels, artificial nucleobase moieties, dye complexes, intercalating dyes, and bioconjugated nanomaterials (AgNCs, GO, QDs, CDs, and MOF) is described herein. To investigate these extremely efficient responsive agents for diagnostic and therapeutic applications in medicine, fluorescence sensors based on G-quadruplexes have also been employed as a quantitative visualization technique.

Newly found K+-Thioflavin T competitive binding to DNA G-quadruplexes and the development of a label-free fluorescent biosensor with extra low detection limit for K+ determination in urine samples

The determination of potassium ion K⁺ in body fluids is important in health monitoring and diagnoses. One of the interesting and simple methods for K⁺ detection is the use of label-free biosensors based on DNA G-quadruplexes (GQs) coupled with a specific fluorescent probe, such as Thioflavin T (ThT), which lights up when bound with K⁺-stabilized GQs. However, these biosensors are not generally sensitive. In this work, we found a solution: at a low concentration, K⁺ competes with ThT in binding to a bimolecular GQ or a tetramolecular GQ, resulting in a decrease in ThT fluorescence emission with increasing K⁺. Therefore, we developed a label-free turn-off fluorescent K⁺ sensor. The sensor provides a very low detection limit of 21.87 ± 0.59 nM. Other possible interfering components in urine did not exert any effect even at quantities that were 10-fold greater than their upper limit of normal concentrations found in urine samples. With its only requirement of diluting samples, the developed low-cost label-free probe and simple sensor was successfully applied to the direct detection of K⁺ in normal urine samples with high accuracy (recoveries ranged from 90% to 100%).

Insights from Binding on Quadruplex Selective Carbazole Ligands

Polymorphic G-quadruplex (G4) secondary DNA structures have received increasing attention in medicinal chemistry owing to their key involvement in the regulation of the maintenance of genomic stability, telomere length homeostasis and transcription of important proto-oncogenes. Different classes of G4 ligands have been developed for the potential treatment of several human diseases. Among them, the carbazole scaffold with appropriate side chain appendages has attracted much interest for designing G4 ligands. Because of its large and rigid π-conjugation system and ease of functionalization at three different positions, a variety of carbazole derivatives have been synthesized from various natural or synthetic sources for potential applications in G4-based therapeutics and biosensors. Herein, we provide an updated close-up of the literatures on carbazole-based G4 ligands with particular focus given on their detailed binding insights studied by NMR spectroscopy. The structure-activity relationships and the opportunities and challenges of their potential applications as biosensors and therapeutics are also discussed. This review will provide an overall picture of carbazole ligands with remarkable G4 topological preference, fluorescence properties and significant bioactivity; portraying carbazole as a very promising scaffold for assembling G4 ligands with a range of novel functional applications.

Highly Selective Detection of K+ Based on a Dimerized G-Quadruplex DNAzyme

Potassium ion (K⁺) plays a crucial role in biological systems, such as maintaining cellular processes and causing diseases. However, specifically, the detection of K⁺ is extremely challenging because of the coexistence of the chemically similar ion of Na⁺ under physiological conditions. In this work, a K⁺ specific biosensor is constructed on the basis of a dimerized G-quadruplex (GQ) DNA, which is promoted by K⁺, and the enzymatic activity of the resulting DNAzyme depends on the concentration of the K⁺. The K⁺ in a 1-200 mM concentration range can be selectively detected by visual color, UV-Vis absorbance or fluorescence even if the concentration of the accompanying Na⁺ is up to 140 mM at an ambient condition up to 45 °C. In addition, this system can also be used to selectively detect NH₄⁺ in a 5-200 mM concentration range. This dimerized DNAzyme offers a new type of biosensor with a potential application in the biological system.

Detection of Tetraplex DNA and Detection by Tetraplex DNA

G-quardruplex (G4) DNA forms through the gathering together of G-quartet planes formed with four guanine (G) bases. G4 DNA stabilizes with potassium ions (K+) by coordination with the G-quartet center. Fluorometric G4 DNA carrying the fluorescence resonance energy transfer (FRET) chromophore pair at both termini has been applied for the fluorometric sensing or imaging of K+ under a homogeneous aqueous medium. This system has realized non-conventional K+ selectivity over the sodium ion (Na+). The selectivity of the fluorescence G4 was converted to Na+ from K+ with a modification of its sequence. On the other hand, G4 DNA detection has been achieved in terms of cancer diagnosis because of a strong relationship of G4 DNA and cancer development. Ligands interacting with G4 are expected to have anti-cancer potential. In addition, fluorometric G4 ligands have been developed and tested as tools for the dynamic monitoring of G4 in living cells. Moreover, fluorometric G4 DNA has been utilized to evaluate the G4 ligand performance.

A guanosine-based 2-formylphenylborate ester hydrogel with high selectivity to K+ ions

Guanosine-based supramolecular hydrogels are particularly of interest for biomaterial and biomedical purposes, as they are generally biocompatible and stimuli-responsive. We found a strong and long-life transparent hydrogel made by mixing guanosine (G) with 1 equiv. of 2-formylbenzeneboronic acid (2FPB) and KOH. Alkali cations can assist the stacking of individual G-quartet to give extended nanowires, but only K⁺ ion induces the formation of a stable and self-supporting network hydrogel for a couple of months. Data from variable temperature NMR indicated that guanosine 2-formylbenzeneborate ester and G are the key components of the self-assembly. Further, G-2FPB-K⁺ hydrogel solution can induce berberine (BBR) fluorescence, showing high selectivity to K⁺ ion and anti-ion interference capability. A good linear relationship between fluorescent intensity and K⁺ concentration allowed us to directly detect K⁺ levels in human blood serum.

A new self-assembling system prepared from guanosine and 2-formylbenzeneboronic acid showed a high selectivity to potassium ions and has a fluorescent enhancement effect on berberine, which can be used for serum potassium detection.

Quick stabilization of capillary for rapid determination of potassium ions in the blood of epilepsy patients by capillary electrophoresis without sample pretreatment

Mutations in the potassium channel genes may be linked to the development of epilepsy and affect the blood potassium levels. Therefore, accurate determination of potassium in the blood will be critical to diagnose the cause of epilepsy. CE is a competent technique for the fast detection of multiple ions, but complicated matrices of a blood sample may cause significant variation of migration times and the peak shape. In this work, a procedure for rapid stabilization of the capillary inner surface through preflushing of a blood sample was employed. The process takes only 40 min for a capillary and then it can be used for more than 2 weeks. No pretreatment of the blood sample or other surface modification of the capillary is needed for the analysis. The RSDs of the migration time and peak area were reduced to 1.5 and 5.1% from 12.6 and 14.5%, respectively. The proposed method has been successfully applied to the determination of the potassium contents in the blood sample of patients with epilepsy at different stages. The recoveries of potassium ions in these blood samples are in a range from 86.5 to 104.5%.

NIR Remote-Controlled "Lock-Unlock" Nanosystem for Imaging Potassium Ions in Living Cells

Despite great achievements in sensitive and selective detection of important biomolecules in living cells, it is still challenging to develop smart and controllable sensing nanodevices for cellular studies that can be activated at desired time in target sites. To address this issue, we have constructed a remote-controlled "lock-unlock" nanosystem for visual analysis of endogenous potassium ions (K⁺), which employed a dual-stranded aptamer precursor (DSAP) as recognition molecules, SiO₂ based gold nanoshells (AuNS) as nanocarriers, and near-infrared ray (NIR) as the remotely applied stimulus. With the well-designed and activatable DSAP-AuNS, the deficiencies of traditional aptamer-based sensors have been successfully overcome, and the undesired response during transport has been avoided, especially in complex physiological microenvironments. While triggered by NIR, the increased local temperature of AuNS induced the dehybridiztion of DSAP, realized the "lock-unlock" switch of the DSAP-AuNS nanosystem, activated the binding capability of aptamer, and then monitored intracellular K⁺ via the change of fluorescence signal. This DSAP-AuNS nanosystem not only allows us to visualize endogenous ions in living cells at a desired time but also paves the way for fabricating temporal controllable nanodevices for cellular studies.

Label-free and enzyme-free detection of microRNA based on a hybridization chain reaction with hemin/G-quadruplex enzymatic catalysis-induced MoS2 quantum dots via the inner filter effect

A new simple, sensitive and specific strategy for microRNA analysis has been described based on a hybridization chain reaction with hemin/G-quadruplex enzymatic catalysis-induced MoS2 quantum dots via the inner filter effect. The target microRNA triggers the hybridization chain reaction between two DNA probes to generate long dsDNA with many hemin/G-quadruplex DNAzymes in the presence of hemin. With the assistance of H2O2, the produced hemin/G-quadruplex DNAzyme could oxidize o-phenylenediamine (OPD) to 2,3-diaminophenazine (DAP) directly, resulting in the fluorescence quenching of MoS2 quantum dots via the inner filter effect. As an example, the fluorescence response of MoS2 quantum dots is linearly related with the logarithm of the microRNA let-7a concentration with a detection limit of 42 fM. The proposed label-free assay has promising potential to be applied in practical diagnosis.

Advancements in SPR biosensing technology: An overview of recent trends in smart layers design, multiplexing concepts, continuous monitoring and in vivo sensing

Inspired by the rapid progress and existing limitations in surface plasmon resonance (SPR) biosensing technology, we have summarized the recent trends in the fields of both chip-SPR and fiber optic (FO)-SPR biosensors during the past five years, primarily regarding smart layers design, multiplexing, continuous monitoring and in vivo sensing. Versatile surface chemistries, biomaterials and nanomaterials have been utilized thus far to generate smart layers on SPR platforms and as such achieve oriented immobilization of bioreceptors, improved fouling resistance and sensitivity enhancement, collectively aiming to improve the biosensing performance. Furthermore, often driven by the desires for time- and cost-effective quantification of multiple targets in a single measurement, efforts have been made to implement multiplex bioassays on SPR platforms. While this aspect largely remains difficult to attain, numerous alternative strategies arose for obtaining parallel analysis of multiple analytes in one single device. Additionally, one of the upcoming challenges in this field will be to succeed in using SPR platforms for continuous measurements and in vivo sensing, and as such match up other biosensing platforms where these goals have been already conquered. Overall, this review will give insight into multiple possibilities that have become available over the years for boosting the performance of SPR biosensors. However, because combining them all into one optimal sensor is practically not feasible, the final application needs to be considered while designing an SPR biosensor, as this will determine the requirements of the bioassay and will thus help in selecting the essential elements from the recent progress made in SPR sensing.

Exploration of intramolecular split G-quadruplex and its analytical applications

Distinct from intermolecular split G-quadruplex (Inter-SG), intramolecular split G-quadruplex (Intra-SG) which could be generated in a DNA spacer-inserted G-quadruplex strand has not been systematically explored. Not only is it essential for the purpose of simplicity of DNA-based bioanalytical applications, but also it will give us hints how to design split G-quadruplex-based system. Herein, comprehensive information is provided about influences of spacer length and split mode on the formation of Intra-SG, how to adjust its thermodynamic stability, and selection of optimal Intra-SG for bioanalysis. For instances, non-classical Intra-SG (e.g. 2:10, 4:8 and 5:7) displays lower stability than classical split strands (3:9, 6:6 and 9:3), which is closely related to integrity of consecutive guanine tract; as compared to regular Intra-SG structures, single-thymine capped ones have reduced melting temperature, providing an effective approach to adjustment of stability. It is believed that the disclosed rules in this study will contribute to the effective application of split G-quadruplex in the field of DNA technology in the future.

DNA-Templated Fluorescent Nanoclusters for Metal Ions Detection

DNA-templated fluorescent nanoclusters (NCs) have attracted increasing research interest on account of their prominent features, such as DNA sequence-dependent fluorescence, easy functionalization, wide availability, water solubility, and excellent biocompatibility. Coupling DNA templates with complementary DNA, aptamers, G-quadruplex, and so on has generated a large number of sensors. Additionally, the preparation and applications of DNA-templated fluorescent NCs in these sensing have been widely studied. This review firstly focuses on the properties of DNA-templated fluorescent NCs, and the synthesis of DNA-templated fluorescent NCs with different metals is then discussed. In the third part, we mainly introduce the applications of DNA-templated fluorescent NCs for sensing metal ions. At last, we further discuss the future perspectives of DNA-templated fluorescent NCs in the synthesis and sensing metal ions in the environmental and biological fields.

Polarity inversion sensitized G-quadruplex metal sensors with K+ tolerance

Due to the high abundance of K⁺ in environments and K⁺-induced high stability of G-quadruplex (G4), developing a selective G4-based fluorescent sensor for other metal ions with K⁺ tolerance is a great challenge. Herein, we found that even in the presence of 15000-fold excess of K⁺, Ba²⁺ exhibits a highly specific binding with a human telomeric G4 (htG4) in comparison with other G4-binding metal ions such as Pb²⁺ and Sr²⁺. This specific binding event can be recognized by a natural fluorophore of hypericin with a lighting-up fluorescence response. Interestingly, inverting the polarity of the most 3' G in htG4 can sensitize the Ba²⁺ response with the retaining Ba²⁺ specificity and K⁺ tolerance. This polarity inversion of htG4 causes a G4 conformation change in K⁺ and the polarity-inverted htG4 tends to favorably dimerize in response to the Ba²⁺ specific binding. To our knowledge, this is the first report that polarity inversion of G4 can be applied to construct a selective metal sensor with K⁺ tolerance. Our findings will open a new way to conveniently regulate the G4 conformation and stability by polarity inversion towards developing high-performance sensors.

Direct detection of potassium and lead (II) ions based on assembly-disassembly of a chiral cyanine dye /TBA complex

A highly selective and sensitive direct detection of potassium (K⁺) and lead (Pb²⁺) ions was developed by using the assembly and disassembly of a chiral cyanine dye/TBA complex. The dye DMSB (3-ethyl-2-[3-(3-ethyl-3H-benzoselenazol-2-ylidene)-2-methylprop-1-enyl] benzoselenazolium bromide) loses the ability of self-assembly, but it can be activated by thrombin-binding aptamer (TBA) G-quadruplex structure. And only the TBA G-quadruplex formed in the presence of K⁺, can strongly induce J-aggregate signals of DMSB. Because the Pb²⁺ ions can bind and stabilize the TBA G-quadruplex with much higher efficiency than K⁺, the J-aggregate signals of DMSB falls sharply when the Pb²⁺ is present. As a result, the assembly and disassembly of DMSB allows the selective detection of 10 μM K⁺ and 20 nM Pb²⁺ respectively, even the competitive sodium ion (Na⁺) was as high as 145 mM. The linear correlation existed between the J-aggregate intensity and the concentration of K⁺ and Pb²⁺ over the range of 0.5-5.0 mM and 200-2000 nM, respectively. Moreover, the concentration of K⁺ (∼3 mM) and Pb²⁺ (below 20 nM) in human blood serum samples were determined by the present method, which agreed well with inductively coupled plasma mass spectrometry (ICP-MS). This work not only opens a door for the further development of G-quadruplex-based aptasensor in complex real system, but also provides a simple and versatile sensing platform for ion detection in clinic.

G-Quadruplex-Based Fluorescent Turn-On Ligands and Aptamers: From Development to Applications

Guanine (G)-quadruplexes (G4s) are unique nucleic acid structures that are formed by stacked G-tetrads in G-rich DNA or RNA sequences. G4s have been reported to play significant roles in various cellular events in both macro- and micro-organisms. The identification and characterization of G4s can help to understand their different biological roles and potential applications in diagnosis and therapy. In addition to biophysical and biochemical methods to interrogate G4 formation, G4 fluorescent turn-on ligands can be used to target and visualize G4 formation both in vitro and in cells. Here, we review several representative classes of G4 fluorescent turn-on ligands in terms of their interaction mechanism and application perspectives. Interestingly, G4 structures are commonly identified in DNA and RNA aptamers against targets that include proteins and small molecules, which can be utilized as G4 tools for diverse applications. We therefore also summarize the recent development of G4-containing aptamers and highlight their applications in biosensing, bioimaging, and therapy. Moreover, we discuss the current challenges and future perspectives of G4 fluorescent turn-on ligands and G4-containing aptamers.

Real-Time Visualizing Mitophagy-Specific Viscosity Dynamic by Mitochondria-Anchored Molecular Rotor

Mitophagy, as an evolutionarily conserved cellular process, plays a crucial role in preserving cellular metabolism and physiology. Various microenvironment alterations assigned to mitophagy including pH, polarity, and deregulated biomarkers are increasingly understood. However, mitophagy-specific viscosity dynamic in live cells remains a mystery and needs to be explored. Here, a water-soluble mitochondria-targetable molecular rotor, ethyl-4-[3,6-bis(1-methyl-4-vinylpyridium iodine)-9 H-carbazol-9-yl)] butanoate (BMVC), was exploited as a fluorescent viscosimeter for imaging viscosity variation during mitophagy. This probe contains two positively charged 1-methyl-4-vinylpyridium components as the rotors, whose rotation will be hindered with the increase of environmental viscosity, resulting in enhancement of fluorescence emission. The results demonstrated that this probe operates well in a mitochondrial microenvironment and displays an off-on fluorescence response to viscosity. By virtue of this probe, new discoveries such as the mitochondrial viscosity will increase during mitophagy are elaborated. The real-time visualization of the mitophagy process under nutrient starvation conditions was also proposed and actualized. We expect this probe would be a robust tool in the pathogenic mechanism research of mitochondrial diseases.

Microfluidic DNA-based potassium nanosensors for improved dialysis treatment

BACKGROUND

Patients with end-stage renal disease (ESRD) have failed kidney function, and often must be treated with hemodialysis to extend the patient's life by artificially removing excess fluid and toxins from the blood. However, life-threatening treatment complications can occur because hemodialysis protocols are adjusted infrequently, as opposed to the kidneys which filter blood continuously. Infrequent blood tests, about once per month on average, are used to adjust hemodialysis protocols and as a result, patients can experience electrolyte imbalances, which can contribute to premature patient deaths from treatment complications, such as sudden cardiac death. Since hemodialysis can lead to blood loss, drawing additional blood for tests to assess the patient's kidney function and blood markers is limited. However, sampling multiple drops of blood per session using a microfluidic device has the potential to reduce not only the amount of blood drawn and avoid unnecessary venipuncture, but also reduce costs by limiting medical complications of hemodialysis and provide a more comprehensive assessment of the patient's health status in real time.

RESULT

We present preliminary proof-of-concept results of a microfluidic device which uses DNA-based fluorescence nanosensors to measure potassium concentration in a flowing solution. In a matter of minutes, the flowing potassium solution reduced the fluorescence intensity of the nanosensors to a steady-state value.

CONCLUSIONS

These proof-of-concept results demonstrate the ability of our DNA-based nanosensors to measure potassium concentration in a microfluidic device. The long-term goal is to integrate this technology with a device to measure potassium and eventually other blood contents multiple times throughout a hemodialysis session, enabling protocol adjustment similar to a healthy kidney.

Progress in biosensor based on DNA-templated copper nanoparticles

During the last decades, by virtue of their unique physicochemical properties and potential application in microelectronics, biosensing and biomedicine, metal nanomaterials (MNs) have attracted great research interest and been highly developed. Deoxyribonucleic acid (DNA) is a particularly interesting ligand for templating bottom-up nanopreparation, by virtue of its excellent properties including nanosized geometry structure, programmable and artificial synthesis, DNA-metal ion interaction and powerful molecular recognition. DNA-templated copper nanoparticles (DNA-CuNPs) has been developed in recent years. Because of its advantages including simple and rapid preparation, high efficiency, MegaStokes shifting and low biological toxicity, DNA-CuNPs has been highly exploited for biochemical sensing from 2010, especially as a label-free detection manner, holding advantages in multiple analytical technologies including fluorescence, electrochemistry, surface plasmon resonance, inductively coupled plasma mass spectrometry and surface enhanced Raman spectroscopy. This review comprehensively tracks the preparation of DNA-CuNPs and its application in biosensing, and highlights the potential development and challenges regarding this field, aiming to promote the advance of this fertile research area.

Electrochemical aptasensor based on a potassium ion-triggered DNA conformation transition and self-assembly on an electrode

A sensitive and selective electrochemical aptasensor was developed for the detection of potassium ions based on a simple sensing principle and straightforward operation.

Highly Sensitive and Selective Potassium Ion Detection Based on Graphene Hall Effect Biosensors

Potassium (K⁺) ion is an important biological substance in the human body and plays a critical role in the maintenance of transmembrane potential and hormone secretion. Several detection techniques, including fluorescent, electrochemical, and electrical methods, have been extensively investigated to selectively recognize K⁺ ions. In this work, a highly sensitive and selective biosensor based on single-layer graphene has been developed for K⁺ ion detection under Van der Pauw measurement configuration. With pre-immobilization of guanine-rich DNA on the graphene surface, the graphene devices exhibit a very low limit of detection (≈1 nM) with a dynamic range of 1 nM–10 μM and excellent K⁺ ion specificity against other alkali cations, such as Na⁺ ions. The origin of K⁺ ion selectivity can be attributed to the fact that the formation of guanine-quadruplexes from guanine-rich DNA has a strong affinity for capturing K⁺ ions. The graphene-based biosensors with improved sensing performance for K⁺ ion recognition can be applied to health monitoring and early disease diagnosis.

G-quadruplex based Exo III-assisted signal amplification aptasensor for the colorimetric detection of adenosine

Adenosine is an endogenous nucleotide pivotally involved in nucleic acid and energy metabolism. Its excessive existence may indicate tumorigenesis, typically lung cancer. Encouraged by its significance as the clinical biomarker, sensitive assay methods towards adenosine have been popularized, with high cost and tedious procedures as the inevitable defects. Herein, we report a label-free aptamer-based exonuclease III (Exo III) amplification colorimetric aptasensor for the highly sensitive and cost-effective detection of adenosine. The strategy employed two unlabeled hairpin DNA oligonucleotides (HP1 and HP2), where HP1 contained the aptamer towards adenosine and HP2 embedded the guanine-rich sequence (GRS). In the presence of adenosine, hairpin HP1 could form specific binding with adenosine and trigger the unfolding of HP1's hairpin structure. The resulting adenosine-HP1 complex could hybridize with HP2, generating the Exo III recognition site. After Exo III-assisted degradation, the GRS was released from HP2, and the adenosine-HP1 was released back to the solution to combine another HP2, inducing the cycling amplification. After multiple circulations, the released ample GRSs were induced to form G-quadruplex, further catalyzing the oxidation of TMB, yielding a color change which was finally mirrored in the absorbance change. On the contrary, the absence of adenosine failed to unfold HP1, remaining color unchanged eventually. Thanks to the amplification strategy, the limit of detection was lowered to 17 nM with a broad linear range from 50 nM to 6 μM. The proposed method was successfully applied to the detection of adenosine in biological samples and satisfying recoveries were acquired.

Ionophore-Based Titrimetric Detection of Alkali Metal Ions in Serum

While the titrimetric assay is one of the most precise analytical techniques available, only a limited list of complexometric chelators is available, as many otherwise promising reagents are not water-soluble. Recent work demonstrated successful titrimetry with ion-exchanging polymeric nanospheres containing hydrophobic complexing agents, so-called ionophores, opening an exciting avenue in this field. However, this method was limited to ionophores of very high affinity to the analyte and exhibited a relatively limited titration capacity. To overcome these two limitations, we report here on solvent based titration reagents. This heterogeneous titration principle is based on the dissolution of all hydrophobic recognition components in a solvent such as dichloromethane (CH₂Cl₂) where the ionophores are shown to maintain a high affinity to the target ions. HSV (hue, saturation, value) analysis of the images captured with a digital camera provides a convenient and inexpensive way to determine the end point. This approach is combined with an automated titration setup. The titrations of the alkali metals K⁺, Na⁺, and Li⁺ in aqueous solution are successfully demonstrated. The potassium concentration in human serum without pretreatment was precisely and accurately determined as 4.38 mM ± 0.10 mM (automated titration), which compares favorably with atomic emission spectroscopy (4.47 mM ± 0.20 mM).

RNA nanoparticles harboring annexin A2 aptamer can target ovarian cancer for tumor-specific doxorubicin delivery

A novel modified nucleic acid nanoparticle harboring an annexin A2 aptamer for ovarian cancer cell targeting and a GC rich sequence for doxorubicin loading is designed and constructed. The system utilizes a highly stable three-way junction (3WJ) motif from phi29 packaging RNA as a core structure. A phosphorothioate-modified DNA aptamer targeting annexin A2, Endo28, was conjugated to one arm of the 3WJ. The pRNA-3WJ motif retains correct folding of attached aptamer, keeping its functions intact. It is of significant utility for aptamer-mediated targeted delivery. The DNA/RNA hybrid nanoparticles remained intact after systemic injection in mice and strongly bound to tumors with little accumulation in healthy organs 6 h post-injection. The Endo28-3WJ-Sph1/Dox intercalates selectively enhanced toxicity to annexin A2 positive ovarian cancer cells in vitro. The constructed RNA/DNA hybrid nanoparticles can potentially enhance the therapeutic efficiency of doxorubicin at low doses for ovarian cancer treatment through annexin A2 targeted drug delivery.

In Vivo Biosensing: Progress and Perspectives

In vivo biosensors are emerging as powerful tools in biomedical research and diagnostic medicine. Distinct from "labels" or "imaging", in vivo biosensors are designed for continuous and long-term monitoring of target analytes in real biological systems and should be selective, sensitive, reversible and biocompatible. Due to the challenges associated with meeting all of the analytical requirements, we found relatively few reports of research groups demonstrating devices that meet the strict definition in vivo. However, we identified several case studies and a range of emerging materials likely to lead to significant developments in the field.

Tetraphenylethene derivative modified DNA oligonucleotide for in situ potassium ion detection and imaging in living cells

The monitoring of K⁺ is very important and emergency because of their unique relationship in various disease diagnosis and treatment. G-quadruplex analogue is a classical recognition unit for K⁺ detection and has been widely applied in K⁺ relevant research. Common fluorescent dyes were employed for design of G-quadruplex structure-based K⁺ probes which suffered from the aggregation-caused quenching effect, and possibly limited the biological applications in living systems. Herein, we report an aggregation-induced emission (AIE) effect-based fluorescent probe for cellular K⁺ analysis and imaging. Benefitting from the K⁺ triggered AIE phenomenon, the designed TPE derivative modified guanine (G)-rich oligonucleotide fluorescent probe (TPE-oligonucleotide probe) exhibits high sensitivity (∼10-fold higher than most reported G-quadruplex-based probes) with extended photostability which facilitates the prolonged fluorescence observations of K⁺ in living cells. On the basis of these advantages, the TPE-oligonucleotide probe serves as a promising candidate for the functional study and analysis of K⁺.

Novel fluorescent cationic benzothiazole dye that responds to G-quadruplex aptamer as a novel K+ sensor

A fluorescent cationic benzothiazole dye that selectively targets a G-quadruplex aptamer was designed and synthesized as a K⁺ sensor.