Optical sensors for determining phenolic compounds with different structures

The literature data published in 1993–2012 on the development and application of optical sensors for the determination of phenolic compounds of various structures are summarized. The design, analytical characteristics, and advantages and disadvantages of the existing optical sensors are reviewed, and some examples of using optical sensors in the analysis of real samples, including those from field experiments, are considered in detail.     

Chemically modified electrodes with amperometric response in enantioselective analysis

Estimation of the enantiomeric purity of chiral biologically active compounds, as well as the determination of particular optical isomers, is very important for the control of medicines, food, and biological fluids. The main approaches to the development of electrochemical enantioselective sensors with the amperometric detection of the signal are considered in this review. Examples of the use of biochemical and supramolecular receptors providing enantiomer recognition and techniques of their inclusion into the corresponding sensors are given. The main characteristics of enantioselective sensors for the determination of optically active medicines, organic acids, aminoacids, carbohydrates, alcohols, and other biologically important compounds are considered.     

Visual pH Sensors: From a Chemical Perspective to New Bioengineered Materials

Many human activities and cellular functions depend upon precise pH values, and pH monitoring is considered a fundamental task. Colorimetric and fluorescence sensors for pH measurements are chemical and biochemical tools able to sense protons and produce a visible signal. These pH sensors are gaining widespread attention as non-destructive tools, visible to the human eye, that are capable of a real-time and in-situ response. Optical “visual” sensors are expanding researchers’ interests in many chemical contexts and are routinely used for biological, environmental, and medical applications. In this review we provide an overview of trending colorimetric, fluorescent, or dual-mode responsive visual pH sensors. These sensors include molecular synthetic organic sensors, metal organic frameworks (MOF), engineered sensing nanomaterials, and bioengineered sensors. We review different typological chemical entities of visual pH sensors, three-dimensional structures, and signaling mechanisms for pH sensing and applications; developed in the past five years. The progression of this review from simple organic molecules to biological macromolecules seeks to benefit beginners and scientists embarking on a project of pH sensing development, who needs background information and a quick update on advances in the field. Lessons learned from these tools will aid pH determination projects and provide new ways of thinking for cell bioimaging or other cutting-edge in vivo applications.     

New approaches in the development of DNA sensors: hybridization and electrochemical detection of DNA and RNA at two different surfaces

Up to now, the development of the electrochemical DNA hybridization sensors relied on solid electrodes, on which both the hybridization and detection steps have been performed. Here we propose a new method in which the DNA hybridization is performed at commercially available magnetic beads and electrochemical detection on detection electrodes (DE). Due to minimum nonspecific DNA adsorption at the magnetic beads, very high specificity of the DNA hybridization is achieved. Optimum DE can be chosen only with respect to the given electrode process. It is shown that high sensitivity and specificity in the detection of relatively long target DNAs can be obtained (a) by using cathodic stripping voltammetry at mercury or solid mercury amalgam DEs for the determination of purine bases, released from DNA by acid treatment, and (b) by enzyme-linked immunoassay of target DNA modified by osmium tetroxide,2,2'-bipyridine (Os,bipy) at carbon DEs. Direct determination of Os,bipy at mercury and carbon electrodes is also possible.     

Enantioselective Voltammetric Sensors: New Solutions

A review of new approaches and solutions in the development and application of enantioselective voltammetric sensors for the recognition of optical isomers of biologically active compounds and medicines is presented. The main methods of electrode modification by enantioselective selectors are discussed, i.e., the application of inclusion complexes, molecularly imprinted polymers, elements of living systems and their analogs, inorganic and organic materials with the effect of chirality, and also supramolecular structures. The main analytical characteristics of some sensors and sensor systems of the electronic tongue type for the recognition and determination of enantiomers in various samples are presented. Methods of processing of voltammetric data for the elimination of the effect the memory of measurements and the cleanup of analytical signals at low concentrations of enantiomers are considered.     

Fluorescent DNA-based enzyme sensors

Fluorescent sensors that make use of DNA structures have become widely useful in monitoring enzymatic activities. Early studies focused primarily on enzymes that naturally use DNA or RNA as the substrate. However, recent advances in molecular design have enabled the development of nucleic acid sensors for a wider range of functions, including enzymes that do not normally bind DNA or RNA. Nucleic acid sensors present some potential advantages over classical small-molecule sensors, including water solubility and ease of synthesis. An overview of the multiple strategies under recent development is presented in this critical review, and expected future developments in microarrays, single molecule analysis, and in vivo sensing are discussed (160 references).     

DNA hybridization at microbeads with cathodic stripping voltammetric detection

In electrochemical DNA hybridization sensors generally a single-stranded probe DNA was immobilized at the electrode followed by hybridization with the target DNA and electrochemical detection of the hybridization event at the same electrode. In this type of experiments nonspecific adsorption of DNA at the electrode caused serious difficulties especially in the case of the analysis of long target DNAs. We propose a new technology in which DNA is hybridized at a surface H and the hybridization is detected at the detection electrode (DE). This technology significantly extends the choice of hybridization surfaces and DEs. Here we use paramagnetic Dynabeads Oligo(dT)(25) (DBT) as a transportable reactive surface H and a hanging mercury drop electrode as DE. We describe a label-free detection of DNA and RNA (selectively captured at DBT) based on the determination of adenines (at ppb levels, by cathodic stripping voltammetry) released from the nucleic acids by acid treatment. The DNA and RNA nonspecific adsorption at DBT is negligible, making thus possible to detect the hybridization event with a great specificity and sensitivity. Specific detection of the hybridization of polyribonucleotides, mRNA, oligodeoxynucleotides, and a DNA PCR product (226 base pairs) is demonstrated. New possibilities in the development of the DNA hybridization sensors opened by the proposed technology, including utilization of catalytic signals in nucleic acid determination at mercury (e.g. signals of osmium complexes covalently bound to DNA) and solid DEs (e.g. using enzyme-labeled antibodies against chemically modified DNAs) are discussed.     

Peculiarities of microweighing of trace quantities of alkylamines on polymer and solid-state thin films

The development of various devices with sensing elements operating on the basis of quartz crystal microbalance technique for the determination of ammonia and volatile amines in air have been considered. The results on the formation of selective layers of piezoelectric sensors with the specified properties have been presented. The influence of the nature of modifier, solvent, sorbate, and method of their formation on the piezo element on the sorption properties of thin films has been considered. The mechanism of interaction in the studied systems has been discussed relying on the results of quartz crystal microbalance measurement and microstructural studies at different stages of sorption.     

Aptamer-based electrochemical sensors that are not based on the target binding-induced conformational change of aptamers

This study describes a new kind of aptamer-based electrochemical sensor that is not based on the target binding-induced conformational change of the aptamers by using a 15-mer thrombin-binding aptamer (5'-GGTTGGTGTGGTTGG-3') as the model oligonucleotide. The sensors are developed by first self-assembling the aptamer (i.e. a thrombin-binding aptamer) onto an Au electrode and then hybridizing the assembled aptamer with a ferrocene (Fc)-labeled short aptamer-complementary DNA oligonucleotide to form an electroactive double-stranded DNA (ds-DNA) oligonucleotide onto the Au electrode. The binding of the target (i.e. thrombin) towards the aptamer essentially destroys the Watson-Crick helix structure of the ds-DNA oligonucleotide assembled onto the electrode and leads to the dissociation of the Fc-labeled short complementary DNA oligonucleotide from the electrode surface to the solution, resulting in a decrease in the current signal obtained at the electrode, which can be used for the determination of the target. With the thrombin-binding aptamer as the model oligonucleotide, the current decrease obtained with the aptamer-based electrochemical sensors is linear with the concentration of thrombin within the concentration range from 0 to 10 nM (DeltaI/nA = 6.7C(thrombin)/nM + 2.8, gamma = 0.975). Unlike most kinds of existing aptamer-based electrochemical sensor, the electrochemical aptasensors demonstrated here are not based on the conformational change of the aptamers induced by the specific target binding. Moreover, the aptasensors are essentially label-free and are very responsive toward the targets. This study may pave a facile and general way to the development of aptamer-based electrochemical sensors.     

Homogeneous fluorescence assay for cyclic AMP

The objective of these studies was to develop a new homogeneous fluorescence assay for determining the concentrations of cAMP in biological samples. The assay is based on a novel general concept of using ligand-dependent sequence-specific DNA binding proteins as sensors for their respective ligands. CAP protein, a bacterial DNA binding protein whose DNA binding activity depends on cAMP, was used to develop the assay. In the presence of cAMP, DNA binding activity of CAP is greatly increased. Signaling of cAMP presence was achieved by detecting cAMP-dependent formation of CAP-DNA complex using a recently developed fluorescence assay for DNA binding proteins (Heyduk, T., and Heyduk, E. Nature Biotechnology 20,171-176, 2002). Both 96-well and 384-well black microplate formats of the assay were developed and used to detect cAMP in low nanomolar concentrations. The assay involves mixing of the sample with the assay solution containing all necessary components for cAMP determination followed by fluorescence intensity readout; no washing or reagent addition steps are necessary. Excellent reproducibility of fluorescence signal change as a function of cAMP concentration was observed. Experiments with HEK 293 cells stimulated with forskolin were performed to demonstrate that the assay could be used for cAMP determination in cellular extracts. In summary, the obtained data fully validated the new homogenous assay for measuring cAMP based on cAMP-dependent DNA binding activity of CAP protein. It is expected that the development of assays for many other ligands of DNA binding proteins will be possible using the same overall assay design developed for cAMP.     

Sensing nanomaterials of wearable glucose sensors

The metabolic disorder of glucose in human body will cause diseases such as diabetes and hyperglycemia.Hence the determination of glucose content is very important in clinic diagnosing.In recent years,researchers have proposed various non-invasive wearable sensors for rapid and real-time glucose monitoring from human body fluids.Unlike those reviews which discussed performances,detection environments or substrates of the wearable glucose sensor,this review focuses on the sensing nanomaterials since they are the key elements of most wearable glucose sensors.The sensing nanomaterials such as carbon,metals,and conductive polymers are summarized in detail.And also the structural characteristics of different sensing nanomaterials and the corresponding wearable glucose sensors are highlighted.Finally,we prospect the future development requirements of sensing nanomaterials for wearable glucose sensors.This review would give some insights to the further development of wearable glucose sensors and the modern medical treatment.     

Sensitive electrochemical monitoring of nucleic acids coupling DNA nanostructures with hybridization chain reaction

Methods based on metal nanotags have been developed for metallobioassay of nucleic acids, but most involve complicated labeling or stripping procedures and are unsuitable for routine use. Herein, we report the proof-of-concept of a novel and label-free metallobioassay for ultrasensitive electronic determination of human immunodeficiency virus (HIV)-related gene fragments at an ultralow concentration based on target-triggered long-range self-assembled DNA nanostructures and DNA-based hybridization chain reaction (HCR). The signal is amplified by silver nanotags on the DNA duplex. The assay mainly consists of capture probe, detection probe, and two different DNA hairpins. In the presence of target DNA, the capture probe immobilized on the sensor sandwiches target DNA with the 3′ end of detection probe. Another exposed part of detection probe at the 5′ end opens two alternating DNA hairpins in turn, and propagates a chain reaction of hybridization events to form a nicked double-helix. Finally, numerous silver nanotags are immobilized onto the long-range DNA nanostructures, each of which produces a strong electronic signal within the applied potentials. Under optimal conditions, the target-triggered long-range DNA nanostructures present good electrochemical behaviors for the detection of HIV DNA at a concentration as low as 0.5 fM. Importantly, the outstanding sensitivity can make this approach a promising scheme for development of next-generation DNA sensors without the need of enzyme labeling or fluorophore labeling.     

Magnetic beads as versatile tools for electrochemical DNA and protein biosensing

Magnetic beads (MBs) are versatile tools in the separation of nucleic acids, proteins and other biomacromolecules, their complexes and cells. In this article recent application of MBs in electrochemical biosensing and particularly in the development of DNA hybridization sensors is reviewed. In these sensors MBs serve not only for separation but also as a platform for optimized DNA hybridization. A hybridization event is detected separately at another surface, which is an electrode. The detection is based either on the intrinsic DNA electroactivity or on various kinds of DNA labeling, including chemical modification, enzyme tags, nanoparticles, electroactive beads, etc., greatly amplifying the signals measured. In addition to DNA hybridization, other kinds of biosensing in combination with MBs, such as DNA-protein interactions, are reviewed.     

Optical chemical sensors (Micro- and Nanosystems) for analysis of liquids

A number of peculiar analytical systems applied in production of detecting elements for chemical sensors were considered, in which organic reagents are immobilized on opaque and optically transparent polymeric matrices. Fibrous non-fabricated materials filled with powdered ion exchangers (Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences) and polymeric films coated with a high-purity gelatin layer (Mendeleev Russian University of Chemical Technology) were suggested as solidstate matrices. The conditions for determination of elements at MPC (maximal permissible concentration) levels were identified. Rapid, highly sensitive and selective techniques were developed for reflectance-spectroscopic determination of coexisting mercury(II), cadmium(II) and lead(II); chromium(VI), copper, and nickel; lanthanum, uranium(VI), and thorium, as well as for spectrophotometric determination of calcium, thorium, nitrite, and sulfate ions. Sensitive optical sensors suitable both for single control step and long-term monitoring of the content of the elements in water objects were proposed.     

Portable sensors for hydrogen analysis

Presently available methods for the determination of hydrogen in the gas phase are analyzed. Various procedures for the detection of hydrogen both at pre-explosion and at very low (less than 50 ppm) concentrations by means of thermocatalytic, semiconductor, amperometric, and potentiometric electrochemical sensors are considered.     

An effective approach to enhanced energy-transfer efficiency from up-converting phosphors and increased assay sensitivity

The energy transfer efficiency from up-converting donors was significantly enhanced by constructing beacon sensors, which led to increased sensitivities in DNA recognition and protein determination.     

Methods for the determination of adenosine triphosphate and other adenine nucleotides

The following methods for the determination of adenosine triphosphate reported in the past 25 years are considered: bioluminescence methods with the use of the firefly luciferase enzyme (with sensitivity to 10^?14 M); chromatographic methods (ion-exchange, thin-layer, and high performance liquid chromatography) for the determination of adenine nucleotides in mixtures with other nucleotides, nucleosides, and nitrogen bases; and fluorescence, spectrophotometric, and electrochemical techniques (including those with the use of sensors), which are promising but not commonly used for the determination of adenine nucleotides. The advantages and disadvantages of these methods are demonstrated.     

酶放大DNA电化学传感器的研究进展

DNA电化学传感器灵敏度高、选择性好、分析时间短和检测成本低,极大地推动了生物传感器的发展. 结合蛋白质酶、功能核酸酶的催化效率高与特异性好,可提高检测灵敏度和选择性. 本文评述了酶放大DNA电化学传感器的研究进展,并分析现存问题,展望发展趋势.     

E-DNA sensors for convenient, label-free electrochemical detection of hybridization

We review the development of reagentless, electrochemical sensors for the sequence-specific detection of nucleic acids that are based on the target-induced folding or unfolding of electrode-bound oligonucleotides. These devices, which are sometimes termed E-DNA sensors, are comprised of an oligonucleotide probe modified on one terminus with a redox reporter and attached to an electrode at the other. Hybridization of this probe DNA to a target oligonucleotide influences the rate at which the redox reporter collides with the electrode, leading to a detectable change in redox current. Because all sensing elements of this method are strongly linked to the interrogating electrode, E-DNA sensors are label-free, operationally convenient and readily reusable. As E-DNA signaling is predicated on a binding-specific change in the dynamics of the probe DNA (rather than simply monitoring the adsorption of a target to the sensor surface) and because electroactive contaminants (interferents) are relatively rare, this class of sensors is notably resistant to false positives arising from the non-specific adsorption of interferents, and performs well even when challenged directly with blood serum, soil and other complex sample matrices. We review the history of and recent advances in this promising DNA and RNA detection approach.