Silver‐Ion‐Mediated DNAzyme Switch for the Ultrasensitive and Selective Colorimetric Detection of Aqueous Ag+ and Cysteine

Two states, two applications! An Ag⁺‐mediated DNAzyme switch has been designed to detect Ag⁺ and cysteine with high sensitivity and selectivity. In the closed state, Ag⁺ turns on the switch through the formation of cytosine–Ag⁺–cytosine base pairs, whereas adding cysteine turns off the open switch because it competitively binds to Ag⁺. This feature endows the DNAzyme switch with two sensing applications.

     

A Stimuli‐Responsive Nanogel‐Based Sensitive and Selective Fluorescent Sensor for Cr3+ with Thermo‐Induced Tunable Detection Sensitivity

Stimuli‐responsive poly(N‐isopropylacrylamide) nanogel with covalently labeled rhodamine B urea derivatives (P(NIPAM‐co‐RhBUA)) is utilized as a sensitive fluorescent probe for Cr³⁺ in aqueous solution, and its thermo‐induced tunable detection capacity is investigated. At 20 °C, non‐fluorescent nanogel can selectively bind with Cr³⁺ over some other metal ions, leading to prominent fluorescence OFF–ON switching due to the recognition of RhBUA with Cr³⁺. Upon heating above the phase transition temperature, enhanced fluorescence intensity is observed (≈61‐fold increase at 45 °C) for the nanogel in the presence of Cr³⁺, accompanied with an improved detection sensitivity, which suggest that hydrophobic microenvironment generated in the collapsed nanogel plays an active role for their detection performance.

     

A New Tridentate Sulfur Receptor as a Highly Sensitive and Selective Fluorescent Sensor for Cu2+ Ions

The introduction of Lawesson′s reagent into a bis‐rhodamine spirolactam system afforded a new fluorescent sensor for Cu²⁺ ions, SRR , which contained a new tridentate sulfur ligand. SRR showed excellent specificity for Cu²⁺ ions over other cations (including Cu⁺, Hg²⁺, and Fe³⁺), very high sensitivity (10 nM ), and a rapid response time (3 min). The detection mechanism was investigated by ¹H NMR, ¹³C NMR, ³¹P NMR, and ESR spectroscopy, MS, and Gaussian calculations. Coordination of a Cu²⁺ ion to the tridentate sulfur ligand, which promotes ring‐opening of the rhodamine groups, followed by a spontaneous reduction reaction (Cu²⁺ into Cu⁺), has been proposed as the sensing mechanism.     

A Computational Study of a Single‐Walled Carbon‐Nanotube‐Based Ultrafast High‐Capacity Aluminum Battery

Exploring suitable electrode materials is a fundamental step toward developing Al batteries with enhanced performance. In this work, we explore using density functional theory calculations the feasibility of single‐walled carbon nanotubes (SWNTs) as a cathode material for Al batteries. Carbon nanotubes with hollow structures and large surface area are able to overcome the difficulty of activating the opening of interlayer spaces as observed in graphite electrode during the first intercalation cycle. Our results show that AlCl₄ binds strongly with the SWNT to result in an energetically and thermally stable AlCl₄‐adsorbed SWNT system. Diffusion calculations show that the SWNT system allows ultrafast diffusion of AlCl₄ with a more favorable inner surface diffusion than outer surface diffusion. Our charge‐density difference and Bader atomic charge analysis confirm the oxidation of SWNT upon adsorption of AlCl₄, which shows a similar behavior to the previously studied graphite cathode. The average open‐circuit voltage and AlCl₄ storage capacity increases with increasing SWNT diameter and can be as high as 1.96 V and 275 mA h g⁻¹ in (25,25) SWNT relative to graphite (70 mA h g⁻¹). All of these properties show that SWNTs are a potential cathode material for high‐performance Al batteries and should be explored further.     

A Highly Selective and Sensitive Fluorescent Chemosensor for Detection of Al3+ in Totally Aqueous Media

A fluorescent chemosensor ( 1 ) based on 2‐hydroxy‐1‐naphthaldehyde Schiff‐base was developed for the detection of Al³⁺ in 100% aqueous solution. Upon addition of Al³⁺, a significant fluorescence enhancement was observed, which was not affected by other metal ions including Na⁺, K⁺, Ca²⁺, Mg²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cr³⁺, Ag⁺, Pb²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Fe²⁺ and Fe³⁺ under weak acid conditions. Moreover, the specific response to Al³⁺ was visible under natural light. The binding mode between 1 and Al³⁺ was clarified by ESI‐MS and ¹H NMR.     

Sensitive and Selective Fluorescent and Colorimetric Sensor for Ag+ Based on the Supramolecular Self‐Assembly in Semi‐Water

Specific recognition of ultratrace levels of ions in semi‐water using super‐quicker methods is still a challenge for environmental monitoring. Herein we report a fluorescent and colormetric sensor ( ZH ) based on supramolecular self‐assembly, whose structure was destroyed by the addition of ultratrace of silver ions. The process promoted either naked eye visible color changes or fluorescence intensity quenched in conjunction with a wide pH range. Systematic studies revealed very high selectivity (0.07 µmol/L) for silver ions, and other common cations, e.g ., Hg²⁺, Cu²⁺, Cd²⁺, Pb²⁺ had nearly no influence on the sensing behavior. This sensor also served as a multiple use of component in sensing materials by addition of I⁻ into the mixture of ZH and Ag⁺ (about 5 times). What's more, ZH containing filter paper emerged distinct color and fluorescence changes upon exposure to silver (Ag⁺), which could be used as a portable method to undertake field testing for Ag ⁺ .     

A Highly Selective Fluorescence Turn‐On Sensor for Extracellular Calcium Ion Detection

A highly water‐soluble, fluorescence turn‐on sensor for Ca²⁺ is reported. The sensor affords high selectivity in sensing Ca²⁺ over other biologically important metal cations. The dissociation constant of the sensor in binding Ca²⁺ is 0.92 mm . Fluorescence microscopy experiments demonstrate that the sensor is cell‐impermeable and capable of detecting extracellular Ca²⁺.     

Turn‐On Fluorescence Sensor Based on Single‐Walled‐Carbon‐Nanohorn–Peptide Complex for the Detection of Thrombin

Proteases play a central role in several widespread diseases. Thus, there is a great need for the fast and sensitive detection of various proteolytic enzymes. Herein, we have developed a carbon nanotube (CNT)‐based protease biosensing platform that uses peptides as a fluorescence probe for the first time. Single‐walled carbon nanohorns (SWCNHs) and thrombin were used to demonstrate this detection strategy. SWCNHs can adsorb a fluorescein‐based dye (FAM)‐labeled peptide (FAM‐pep) and quench the fluorescence of FAM. In contrast, thrombin can cleave FAM‐pep on SWCNHs and recover the fluorescence of FAM, which allows the sensitive detection of thrombin. This biosensor has a high sensitivity and selectivity toward thrombin, with a detection limit of 100 pM .     

Selective and Sensitive Fluorescent Chemosensors for Cu2+ Ion Based upon Bis‐1,8‐naphthalimide Dyads

A series of new fluorescent chemosensors 5a – 5e , composed of two aminonaphthalimide fluorophores and 2,6‐bis((N‐aminoalkyl)aminocarboxy)pyridines, were prepared, characterized, and their fluorescent properties towards heavy and transition metal (HTM) ions were investigated. Chemosensors 5c – 5e exhibited high selectivity and sensitivity for Cu²⁺ ion over other HTM ions with fluorescent quenching (green to colourless). It clearly demonstrated that the length of the linkers (diamines) between the aminonaphthalimides and 2,6‐dicarboxypyridine of 5a – 5e was very important for their sensitivity and selectivity for Cu²⁺ ion over other HTM ions.     

An Amphiphilic Polymer‐ and Carbon Nanotube‐Modified Indium Tin Oxide Electrode for Sensitive Electrochemical DNA Detection with Low Nonspecific Binding

We herein report an amphiphilic polymer‐, carboxylated multiwalled carbon nanotube (CNT)‐, silane polymer‐, and streptavidin‐modified indium tin oxide (ITO) electrode that allows low nonspecific binding and efficient immobilization of DNA, along with good electrocatalytic activities and low background‐current levels. The low nonspecific binding results from the well‐covering of the CNT and ITO surface with the amphiphilic polymer and silane polymer, as well as the poly(ethylene glycol) groups of the polymers. The streptavidin for DNA immobilization is covalently attached to the carboxylic acid groups of the amphiphilic polymer and CNT. A low surface coverage of CNT on the ITO electrode provides the good electrocatalytic activities and low background‐current levels. The fabricated electrode enables us to achieve a detection limit of 100 pM in DNA detection.     

Single‐Walled Carbon Nanotubes Modified Gold Electrodes as an Impedimetric DNA Sensor

A gold surface modified with a self‐assembled monolayer of 11‐amino‐1‐undecanethiol (AUT) was used for the covalent immobilization of oxidized single‐walled carbon nanotubes (SWNTs). The as‐described SWNTs‐modified substrate was subsequently used to attach single‐stranded deoxyribonucleic acid (ssDNA) used as a substrate for DNA hybridization. Electrochemical impedance spectroscopy measurements were performed to follow the DNA hybridization process by using the redox couple [Fe(CN)₆]^3−/4− as a marker ion. Specifically, changes in charge transfer resistance obtained from the Nyquist plots were used as the sensing parameter of DNA hybridization. The substrate sensitivity towards changes in target DNA concentration, its selectivity toward different DNA sequences and its reusability are successfully demonstrated in this report.     

Synthesis of Highly Sensitive Fluorescent Probe Based on Tetrasubstituted Imidazole and Its Application for Selective Detection of Ag+ Ion in Aqueous Media

A small molecule fluorescent probe, 4-[2-(4-chlorophenyl)-4,5-diphenyl-1H-imidazol-1-yl]aniline(L) for detecting Ag⁺ ion was gently synthesized via one-pot multi-component reaction catalyzed by H₃[PW₁₂O₄₀] under solvent-free microwave irradiation. When the concentration of Ag⁺ ion changed from 0 to 8.0×10⁻⁶ mol/L in the solution of H₂O/CH₃OH(19:1, volume ratio), the fluorescence emission spectrum was blue-shifted and accompanied by a gradual increase in fluorescence intensity with a low detection limit of 3.0×10⁻¹¹ mol/L. Moreover, UV-Vis absorption titration experiment demonstrated a 1:1 stoichiometric ratio and an association constant of (9.95±0.44)×10⁵ L/mol between probe L and Ag⁺ ion, and thus their complexation mechanism was also proposed and verified. More importantly, this fluorescent probe was remarkably specific for Ag⁺ ion under the interference of other metal ions and exhibited a wide pH application range of 4.0-8.0. Additionally, preliminary application of this probe was also carried out and satisfactory results were shown.     

A Highly Sensitive Electrochemical Immunosensor for Fumonisin B1 Detection in Corn Using Single‐Walled Carbon Nanotubes/Chitosan

A sensitive electrochemical immunosensor was developed for detecting fumonisin B₁ (FB₁) in corn using the single‐walled carbon nanotubes/chitosan. The detection mechanism of immunosensor was based on an indirect competitive binding to a fixed amount of anti‐FB₁ between free FB₁ and FB₁‐bovine serum albumin, which was conjugated on covalently functionalized nanotubes/chitosan laid on the glass carbon electrode. The anti‐rabbit immunoglobulin G secondary antibody labeled with alkaline phosphatase was then bound to the electrode surface through reactisubstrate α‐naphthyl phosphate, which produced electrochemical signal. Under optimized conditions, this method could detect FB₁ from 0.01 to 1000 ng mL^?1 with a detection limit of 2 pg mL^?1. This is well below the detection limit required from European Union legislation, 2–4 mg L^?1. Moreover, good recoveries were obtained for the detection of spiked corn samples and actual corn samples. As the method has good sensitivity and recovery for detecting FB₁, it is a practical detection method.     

Ultrahighly Sensitive Homogeneous Detection of DNA and MicroRNA by Using Single‐Silver‐Nanoparticle Counting

DNA and RNA analysis is of high importance for clinical diagnoses, forensic analysis, and basic studies in the biological and biomedical fields. In this paper, we report the ultrahighly sensitive homogeneous detection of DNA and microRNA by using a novel single‐silver‐nanoparticle counting (SSNPC) technique. The principle of SSNPC is based on the photon‐burst counting of single silver nanoparticles (Ag NPs) in a highly focused laser beam (about 0.5 fL detection volume) due to Brownian motion and the strong resonance Rayleigh scattering of single Ag NPs. We first investigated the performance of the SSNPC system and then developed an ultrasensitive homogeneous detection method for DNA and microRNA based on this single‐nanoparticle technique. Sandwich nucleic acid hybridization models were utilized in the assays. In the hybridization process, when two Ag‐NP–oligonucleotide conjugates were mixed in a sample containing DNA (or microRNA) targets, the binding of the targets caused the Ag NPs to form dimers (or oligomers), which led to a reduction in the photon‐burst counts. The SSNPC method was used to measure the change in the photon‐burst counts. The relationship between the change of the photon‐burst counts and the target concentration showed a good linearity. This method was used for the assay of sequence‐specific DNA fragments and microRNAs. The detection limits were at about the 1 fM level, which is 2–5 orders of magnitude more sensitive than current homogeneous methods.     

Reaction‐Based Colorimetric and Ratiometric Fluorescence Sensor for Detection of Cyanide in Aqueous Media

A stilbene‐based compound ( 1 ) has been prepared and was highly selective for the detection of cyanide anion in aqueous media even in the presence of other anions, such as F^?, Cl^?, Br^?, I^?, ClO₄^?, H₂PO₄^?, HSO₄^?, NO₃^?, and CH₃CO₂^?. A noticeable change in the color of the solution, along with a prominent fluorescence enhancement, was observed upon the addition of cyanide. The color change was observed upon the nucleophilic addition of the cyanide anion to the electron‐deficient cyanoacrylate group of 1 . The spectral changes induced by the reaction were analyzed by comparison with two model compounds, such as compound 2 with dimethyl substituents and compound 3 without a cyanoacrylate group. An intramolecular charge‐transfer (ICT) mechanism played a key role in the sensing properties, and the mechanism was supported by DFT/TDDFT calculations.     

A Highly Selective Mitochondria‐Targeting Fluorescent K+ Sensor

Regulation of intracellular potassium (K⁺) concentration plays a key role in metabolic processes. So far, only a few intracellular K⁺ sensors have been developed. The highly selective fluorescent K⁺ sensor KS6 for monitoring K⁺ ion dynamics in mitochondria was produced by coupling triphenylphosphonium, borondipyrromethene (BODIPY), and triazacryptand (TAC). KS6 shows a good response to K⁺ in the range 30–500 mM , a large dynamic range (F_max/F₀≈130), high brightness (?_f=14.4 % at 150 mM of K⁺), and insensitivity to both pH in the range 5.5–9.0 and other metal ions under physiological conditions. Colocalization tests of KS6 with MitoTracker Green confirmed its predominant localization in the mitochondria of HeLa and U87MG cells. K⁺ efflux/influx in the mitochondria was observed upon stimulation with ionophores, nigericin, or ionomycin. KS6 is thus a highly selective semiquantitative K⁺ sensor suitable for the study of mitochondrial potassium flux in live cells.     

A Highly Selective Potassium Sensor for the Detection of Potassium in Living Tissues

The development of highly selective sensors for potassium is of great interest in biology. Two new hydrosoluble potassium sensors (Calix‐COU‐Alkyne and Calix‐COU‐Am) based on a calix[4]arene bis(crown‐6) and an extended coumarin were synthesized and characterized. The photophysical properties and complexation studies of these compounds have been investigated and show high molar extinction coefficients and high fluorescence quantum yields. Upon complexation with potassium in the millimolar concentration range, an increase of one‐ and two‐photon fluorescence emission is detected. A twofold fluorescence enhancement is observed upon excitation at λ=405 nm. The ligands present excellent selectivity for potassium in the presence of various competitive cations in water and in a physiological medium. The photophysical properties are not affected by the presence of a large amount of competing cations (Na⁺, Ca²⁺, Mg²⁺, etc.). Ex vivo measurements on mouse hippocampal slices show that Calix‐COU‐Alkyne accumulates extracellularly and does not alter the neuronal activity. Furthermore, the sensor can be utilized to monitor slow extracellular K⁺ increase induced by inhibition of K⁺ entry into the cells.