Recent Advances on Nanozyme-based Electrochemical Biosensors

Nanozymes are nanomaterials with enzyme-like catalytic activities. The unique features of nanozymes (such as high stability, low cost, large surface area for bioconjugation, ease of storage, and multi-functionalities) offer unprecedented opportunities for designing electrochemical biosensors. Recent years have witnessed the rapid development of nanozyme-based electrochemical biosensors. To highlight these achievements, this review first discusses the representative nanozymes including peroxidase mimics, oxidase mimics, hydrolase mimics, and superoxide dismutase mimics used in electrochemical biosensors. Then, it summarizes the bioanalytical applications for the detection of various analytes. Finally, current challenges and future research directions are summarized.     

薄膜基荧光气体传感器中的涂层化学

近年来,高性能薄膜基气体传感器的研制备受关注,所涉及的涂层化学已经成为物理化学学科发展的一个热点。传感因分析物与敏感层(涂层)物质相互作用引起薄膜特定静态及动态物理量变化而实现,因此,薄膜传感性能势必受到敏感层物质种类和敏感层微纳结构等因素影响。就薄膜基荧光传感而言,荧光敏感物质的结构和性质对薄膜传感性能起着至关重要的作用。同时,因毛细凝结、色谱效应、尺寸效应、分子间相互作用等因素的存在,敏感层微观结构也极大地影响着薄膜的传感性能。本文结合课题组近期研究工作,简要讨论薄膜基荧光气体传感器研究中的涂层化学基本问题,以及相关薄膜基荧光传感器在隐藏爆炸物、毒品、挥发性有机污染物检测/监测等方面的应用探索。最后,文章展望了薄膜基荧光气体传感器的发展前景和所面临的主要挑战。     

Paired pulse voltammetry for differentiating complex analytes

Although fast-scan cyclic voltammetry (FSCV) has contributed to important advances in neuroscience research, the technique is encumbered by significant analytical challenges. Confounding factors such as pH change and transient effects at the microelectrode surface make it difficult to discern the analytes represented by complex voltammograms. Here we introduce paired-pulse voltammetry (PPV), that mitigates the confounding factors and simplifies the analytical task. PPV consists of a selected binary waveform with a specific time gap between each of its two comprising pulses, such that each binary wave is repeated, while holding the electrode at a negative potential between the waves. This allows two simultaneous yet very different voltammograms (primary and secondary) to be obtained, each corresponding to the two pulses in the binary waveform. PPV was evaluated in the flow cell to characterize three different analytes, (dopamine, adenosine, and pH changes). The peak oxidation current decreased by approximately 50%, 80%, and 4% for dopamine, adenosine, and pH, in the secondary voltammogram compared with the primary voltammogram, respectively. Thus, the influence of pH changes could be virtually eliminated using the difference between the primary and secondary voltammograms in the PPV technique, which discriminates analytes on the basis of their adsorption characteristics to the carbon fiber electrode. These results demonstrate that PPV can be effectively used for differentiating complex analytes.     

Ionic liquids in sample preparation

Due to their unique properties, their good extractabilities for various target analytes, and the fact that many compounds are highly soluble in them, room-temperature ionic liquids (ILs) are used as promising alternatives to the traditional organic solvents employed in sample preparation. ILs have been used as extraction solvents for a wide range of analytes, from environmental contaminates to biomacromolecules and nanomaterials, and as dissolution solvents for various detection techniques. In this paper, the main applications of ILs in sample preparation are reviewed, and the problems and challenges in this area are described.     

Optical Multiplexed Bioassays for Improved Biomedical Diagnostics

Conventional analytical methods based on the detection of a single disease marker may not be sufficiently accurate because the progression of disease generally involves multiple chemicals and biomolecules. The drive for simultaneous analysis of multiple targets, which plays a key role in both basic biomedical research and clinical applications, demands the development of multiplexed bioassays with high‐throughput. In this minireview, we summarize the recent progress in optical multiplexed analytical techniques for improving biomedical diagnostics, in which fluorescence and surface enhanced Raman scattering (SERS) with distinctive optical features are chosen as the main readout signals. Focusing on multiplexed strategies in the biomedical field, a selection of recent contributions from biosensing of multiple analytes and multicolor cellular tracking to in vivo multiplexed bioimaging are highlighted. Finally, we frame the future challenges and opportunities for multiplexed bioanalysis.     

Progress and challenges in the detection of residual pesticides using nanotechnology based colorimetric techniques

Pesticides are generally used to control and prevent agricultural pests. Excessive and sporadic use of pesticides poses a serious threat to human and livestock life. Therefore, to safeguard the people’s health simple and sensible approaches for the determination of residual pesticides in food items are desperately required. The nanotechnology-based colorimetric approaches provide the options for detection of residual pesticides with high precision and speed. Localized surface plasmon resonance (LSPR) is one of the most prominent features of metal nanoparticles which provides unique optoelectronic characteristics in the visible region of the electromagnetic spectrum. Metal nanoparticles especially gold and silver have very high extinction coefficients, therefore a well-suited electrochemical interaction between target analytes and nanoparticles surfaces cause aggregation, which leads to a colorimetric response. Modification and functionalization of nanoparticles with other ligands enhances the sensitivity and selectivity of colorimetric assays. But still, there are major challenges which affect the efficacy of these techniques for onsite pesticides monitoring which need to be addressed. Therefore, a comprehensive review of progress and challenges in the application of nanotechnology-based colorimetric techniques for detection of residual pesticides is presented here. The mechanism behind the development of these analytical techniques is also discussed, briefly. In conclusion, potential future trends and prospects of colorimetric techniques are addressed.     

Challenges and trends in the determination of selected chemical contaminants and allergens in food

This article covers challenges and trends in the determination of some major food chemical contaminants and allergens, which-among others-are being monitored by Health Canada's Food Directorate and for which background levels in food and human exposure are being analyzed and calculated. Eleven different contaminants/contaminant groups and allergens have been selected for detailed discussion in this paper. They occur in foods as a result of: use as a food additive or ingredient; processing-induced reactions; food packaging migration; deliberate adulteration; and/or presence as a chemical contaminant or natural toxin in the environment. Examples include acrylamide as a food-processing-induced contaminant, bisphenol A as a food packaging-derived chemical, melamine and related compounds as food adulterants and persistent organic pollutants, and perchlorate as an environmental contaminant. Ochratoxin A, fumonisins, and paralytic shellfish poisoning toxins are examples of naturally occurring toxins whereas sulfites, peanuts, and milk exemplify common allergenic food additives/ingredients. To deal with the increasing number of sample matrices and analytes of interest, two analytical approaches have become increasingly prevalent. The first has been the development of rapid screening methods for a variety of analytes based on immunochemical techniques, utilizing ELISA or surface plasmon resonance technology. The second is the development of highly sophisticated multi-analyte methods based on liquid chromatography coupled with multiple-stage mass spectrometry for identification and simultaneous quantification of a wide range of contaminants, often with much less requirement for tedious cleanup procedures. Whereas rapid screening methods enable testing of large numbers of samples, the multi analyte mass spectrometric methods enable full quantification with confirmation of the analytes of interest. Both approaches are useful when gathering surveillance data to determine occurrence and background levels of both recognized and newly identified contaminants in foods in order to estimate human daily intake for health risk assessment.     

Dosimetric gelator probes and their application as sensors

Supramolecular gels formed by the self-assembly of organic molecules are useful in many areas from materials to medicine. Of the different applications, exploitation of gels for the visual detection of analytes is a fairly recent trend in gel chemistry. Most of the gel-based sensors rely on non-covalent interactions between the gelator molecules and the added chemical analytes and therefore, often suffer from less selectivity and long response time. In this context, dosimetric gelator probes are superior to other gel-based sensors with high selectivity and fast response time. Unlike non-covalent binding sites, dosimetric gelators typically contain a reaction centre and undergo a specific chemical reaction selective to an analyte resulting in either formation or rupturing of covalent bonds. In this review, we provide an up-to-date report of various reaction-based gel systems applied for the sensing of analytes. We elaborately discuss the concept, design principles, self-assembly properties, and reaction mechanisms of such gelators. We also highlight the limitations, challenges, and the necessity of further exploration of dosimetric gels in this domain.     

Fluoroalcohols as novel buffer components for basic buffer solutions for liquid chromatography electrospray ionization mass spectrometry: retention mechanisms

Two fluoroalcohols--1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol (HFTB)--were evaluated as volatile buffer acids in basic mobile phases for LC-ESI-MS determination of acidic and basic compounds. HFIP and HFTB as acidic buffer components offer interesting possibilities to adjust retention behavior of different analytes and expand the currently rather limited range of ESI-compatible buffer systems for basic mobile phases. Comparing with commonly used basic buffer components the fluoroalcohols did not suppress the ionization of the analytes, for several analytes ionization enhancement was observed. RP chromatographic retention mechanisms were evaluated and compared to traditional buffer system. All trends in retention of the acidic and basic analytes can be interpreted by the following model: the neutral fluoroalcohols are quite strongly retained by the stationary phase whereas their anions are less retained, thus their amount on the stationary phase is dependent on mobile phase pH; the anions of the fluoroalcohols form ion pairs in the mobile phase with the basic analytes; the fluoroalcohols on the stationary phase surface compete with acidic analytes thereby hindering their retention; the fluoroalcohols on the stationary phase bind basic analytes thereby favoring their retention.     

Graphitized carbons for solid-phase extraction

The objective of this review is to provide updated information about the most important features of graphitized carbonaceous sorbents used for solid-phase extraction (SPE) of organic compounds from liquid natural matrices or extracts. The surface characteristics of graphitized carbon blacks and porous graphitic carbons are described which are responsible for the various types interactions (hydrophobic, electronic and ion-exchange) with analytes. The method development is given which is based on the prediction from liquid chromatographic retention data obtained using porous graphitic carbon. Emphasis is placed on their capability for trapping very polar and water-soluble analytes from aqueous samples. Comparison is made between carbon-based SPE sorbents and other reversed-phase materials such as octadecyl silicas and highly cross-linked copolymers. Especially, the difficulty encountered for the desorption of some strongly retained analytes is explained by LC data and solutions are given for optimizing the composition and volume of the desorption solution. Many examples illustrate the various common features of graphitized carbons which are the extraction of very polar analytes and multiresidue extractions. Some applications are specific to graphitized carbon black due to the presence of surface functional groups. They include the extraction of anionic compounds such as benzene and naphthalene sulfonates or acidic pesticides. Other applications are specific to porous graphitic carbon due to its flat and homogeneous surface. One example is the trace extraction of coplanar polychlorinated biphenyls (PCBs), dibenzo-p-dioxins and dibenzofurans from other PCB congeners.     

Signal enhancement in laser diode thermal desorption‐triple quadrupole mass spectrometry analysis using microwell surface coatings

Laser‐diode thermal desorption (LDTD) is an ionization source usually coupled to triple quadrupole mass spectrometry (QqQMS) and specifically designed for laboratories requiring high‐throughput analysis. It has been observed that surface coatings on LDTD microwell plates can improve the sensitivity of the analysis of small polar molecules. The objective of the present study is to understand and quantify the effect of microwell surface coatings on signal intensity of small organic molecules of clinical, environmental, and forensic interest. Experiments showed that the peak areas of diclofenac, chloramphenicol, salicylic acid, and 11‐nor‐9‐carboxy‐Δ⁹‐tetrahydrocannabinol obtained by LDTD‐QqQMS increased by up to 3 orders of magnitude when using microwells coated with ethylenediaminetetraacetic acid (EDTA). Tests with different chelating agents and polytetrafluoroethylene as microwell surface coatings showed that nitrilotriacetic acid gave significantly higher peak areas for five out of the nine compounds that showed signal enhancement using chelating agents as coatings. Scanning electron microscopy studies of EDTA‐coated and uncoated microwells showed that analytes deposited in the former formed more uniform and thinner films than in the latter. The enhancement effect of surface coatings in LDTD‐QqQMS was explained mainly by the formation of homogenous and thinner layers of nanocrystals of analytes that are easier to desorb thermally than the layers formed when the analytes dry in direct contact with the bare stainless‐steel surface. Chemisorption of some analytes to the stainless‐steel surface of the microwell plate appeared to be a minor factor. Surface coatings widen the number of compounds analyzable by LDTD‐QqQMS and can also improve sensitivity and limits of detection.     

Plume Expansion Dynamics of Matrix‐Assisted Laser Desorption Ionization

High‐resolution angular and velocity distributions for neutral analytes (tryptophan and poly‐tryptophan) and matrix (2,4,6‐trihydroxyacetophenon, THAP) are measured by using 355 nm laser desorption. The information suggests that two separate mechanisms dominate the angular and velocity distributions at the beginning and before the end of desorption. A molecular jet‐like isentropic expansion dominates the plume expansion at the beginning of desorption. This only occurs at high surface temperature, thus resulting in a large velocity normal to the surface and a very narrow angular distribution. Most of the analytes are produced under these conditions. Before the end of desorption, the surface temperature decreases and the mechanism of thermal desorption at low vapor pressure takes over. The velocities become small and the angular distribution is close to cosθ. Only a very small amount of analytes are generated under these conditions. Compared to tryptophan, poly‐tryptophan has a much narrower angular distribution, thereby suggesting that it is only produced at the higher surface temperatures.     

Liquid extraction surface analysis in-line coupled with capillary electrophoresis for direct analysis of a solid surface sample

A surface-sampling technique of liquid extraction surface analysis (LESA) was in-line coupled with capillary electrophoresis (CE) to expand the specimen types for CE to solid surfaces. The new direct surface analysis method of LESA–CE was applied to the determination of organophosphorus pesticides, including glufosinate-ammonium, aminomethylphosphonic acid, and glyphosate on the external surface of a fruit such as apple. Without any sample pretreatment, the analytes sprayed on the surface of a half apple were directly extracted into a liquid microjunction formed by dispensing the extractant from the inlet tip of a separation capillary. After extraction, the analytes were derivatized in-capillary with a fluorophore 4-fluoro-7-nitro-2,1,3-benzoxadiazole and analyzed with CE-laser induced fluorescence (LIF). The limits of detection for glufosinate-ammonium, aminomethylphosphonic acid, and glyphosate were 2.5, 1, and 10 ppb, respectively, which are at least 20 times lower than the tolerance limits established by the U.S. Environmental Protection Agency. Thus, we demonstrated that LESA–CE is a quite sensitive and convenient method to determine analytes on a solid surface avoiding the dilution from sample pretreatment procedures including homogenization of a bulk sample.     

Capillary zone electrophoretic separation of neutral species of chloro-s-triazines in the presence of cationic surfactant monomers

Chloro-s-triazines are difficult to separate by capillary zone electrophoresis (CZE), due to their low pKa values. However, these analytes can be effectively separated by CZE in the presence of cationic surfactant monomers, such as tetradecylammonium bromide (TTAB) and dodecyltrimethylammonium bromide (DTAB). The separation mechanism based on a 1:1 binding of analytes to cationic surfactant monomers is proposed. The binding constants of chloro-s-triazines to cationic surfactant monomers are estimated. The results show that the strength of the interactions of these analytes with TTAB monomers is considerably strong, whereas that of the corresponding analyte with DTAB monomers is about 12- to 14-fold weaker. A linear correlation of binding constants with log P(ow) (the logarithm of the partition coefficient of analytes between 1-octanol and aqueous phases) indicates that the migration order of these chloro-s-triazines depends primarily on their hydrophobicity. Moreover, the skewed peaks of chloro-s-triazines observed may reveal the occurrence of adsolubilization of these analytes in the adsorbed cationic surfactant layer on the capillary surface.     

Analysis of amino acids and proteins using a poly(methyl methacrylate) microfluidic system

Plastic microchips are very promising analytical devices for the high-speed analysis of biological compounds. However, due to its hydrophobicity, their surface strongly interacts with nonpolar analytes or species containing hydrophobic domains, resulting in a significant uncontrolled adsorption on the channel walls. This paper describes the migration of fluorescence-labeled amino acids and proteins using the poly(methyl methacrylate) microchip. A cationic starch derivative significantly decreases the adsorption of analytes on the channel walls. The migration time of the analytes was related to their molecular weight and net charge or pI of the analytes. FITC-BSA migrated within 2 min, and the theoretical plate number of the peak reached 480,000 plates/m. Furthermore, proteins with a wide range of pI values and molecular weights migrated within 1 min using the microchip.     

Magnetic nanomaterials for the removal,separation and preconcentration of organic and inorganic pollutants at trace levels and their practical applications: A review

The presence of organic and inorganic pollutants in nature even in very small concentrations threatens human and other living bodies health and makes it significant to remove, separate and preconcentrate these pollutants. Recently magnetic nano sorbents has been used frequently in the separation and preconcentration of these pollutants. The use of magnetic nano sorbents modified with inorganic and organic species has widespread applications in the solid phase extraction (SPE) studies due to its many unique properties. These modified nano sorbents are preferred due to their advantages such as high adsorption capacities and large surface areas. This review examined different types of magnetic materials such as magnetic carbon-based nano sorbents, inorganic nano sorbents with magnetic properties, magnetized biosorbents, magnetic metal-organic frameworks, and magnetized ionic liquids used in SPE studies. In this study, a comprehensive and systematic review of the separation and preconcentration of analytes such as heavy metal ions, drug active substances, pesticides, dyes, hormone disruptors, etc with SPE methods using magnetic nanomaterials has been revealed. Future aspects and challenges that may be encountered are discussed.     

The effect of dipole moment and electron deficiency of analytes on the chemiresistive response of TiO2(B) nanowires

Nanostructured TiO(2)(B) thin films were found to have strong and fast chemiresistive response to nitro-aromatic and nitro-amino explosives recently. In this study, the effects of dipole moment and electron deficiency of the analyte molecules on the chemiresistive response are explored to understand the details of molecular interactions of analytes with the sensor surface which lead to charge depletion and the chemiresistive effect. It was found that the speed of the response is dominated by the polarity of the analytes and molecules with larger dipole moments produce faster responses. The degree of the response was found to be dominated by the electron deficiency of the analytes and molecules with greater electron deficiency produce stronger chemiresistive responses.     

Gelation-based visual detection of analytes

Molecular gels are one type of soft materials which in recent times have found widespread applications in areas such as drug delivery, biomaterials, tissue engineering, organic electronic devices, visual sensors, and others. Visual sensing of analytes using gels is a fairly new concept which holds a lot of promise. Typically, detection of analytes entails the use of expensive and sophisticated instrumentations which are often complex to perform and need specialized training for their operation. In contrast, gelation-based visual detection techniques are simple, convenient, inexpensive, and doesn’t require any instrument. This emerging research area has not been comprehensively reviewed so far. This review article will provide an in-depth and up-to-date summary of the various reports and highlight the advantages, limitations, challenges, and future prospects of gelation-based visual detection techniques.     

Characterisation of morphology of self-assembled PEG monolayers: a comparison of mixed and pure coatings optimised for biosensor applications

For detection of low concentrations of analytes in complex biological matrices using optical biosensors, a high surface loading with capture molecules and a low nonspecific binding of nonrelevant matrix molecules are essential. To tailor biosensor surfaces in such a manner, poly(ethylene glycols) (PEG) in varying lengths were immobilised covalently onto glass-type surfaces in different mixing ratios and concentrations, and were subsequently modified with three different kinds of receptors. The nonspecific binding of a model protein (ovalbumin, OVA) and the maximum loading of the respective analytes to these prepared surfaces were monitored using label-free and time-resolved reflectometric interference spectroscopy (RIfS). The three different analytes used varied in size: 150 kDa for the anti-atrazine antibody, 60 kDa for streptavidin and 5 kDa for the 15-bp oligonucleotide. We investigated if the mixing of PEG in different lengths could increase the surface loadings of analyte mimicking a three-dimensional matrix as was found using dextrans as sensor coatings. In addition, the effect on the surface loading was investigated with regard to the size of the analyte molecule using such mixed PEGs on the sensor surface. For further characterisation of the surface coatings, polarisation modulation infrared reflection absorption spectroscopy, atomic force microscopy, and ellipsometry were applied. All authors contributed equally to this work.     

Polyelectrolytes as new matrices for secondary ion mass spectrometry

Significant enhancements in ion yields in time-of-flight secondary ion mass spectrometry (TOF-SIMS) are observed when water-soluble analytes are mixed with a polyelectrolyte, e.g., poly(diallyldimethylammonium chloride) or poly(sodium 4-styrenesulfonate), and then deposited in the layer-by-layer method on a surface. This previously unobserved effect is demonstrated for 5-chloro-8-methoxyquinoline appended diaza-18-crown-6, 5-(2-aminoethoxy)methyl-5-chloro-8-methoxyquinoline appended diaza-18-crown-6, acridine, 9-anthracenecarboxylic acid, and ferrocenecarboxylic acid. By optical ellipsometry film thicknesses range from ca. 5-20 angstroms. X-ray photoelectron spectroscopy shows significantly less analyte in the polyelectrolyte-analyte films than in the neat analytes. However, TOF-SIMS generally shows significant enhancements in ion yields from the polyelectrolyte films compared with either the neat compounds or the compounds solubilized with acid or base and then dried on a surface. These significant enhancements in ion yields also appear to extend to analyte fragments and cationized molecular species. Some enhancement is also observed for dried droplets of analytes mixed with a polyelectrolyte on surfaces.