Recent advances in non-enzymatic electrochemical detection of hydrophobic metabolites in biofluids

This review focuses on recent advances in non-enzymatic electrochemical biosensors for detection of hydrophobic metabolites. Electrochemical approaches have been widely applied in many established and emerging technologies and a large range of electrochemical biosensors have been used for detection of various hydrophobic metabolites. Despite the progress made in this field, some problems still exist, specifically, electrochemical detection of hydrophobic biomarkers can be challenging in complex biological fluids. In this review, we have highlighted some of the most representative surface modification technologies that have been employed in electrochemical biosensors to counter the problems of poor sensitivity and selectivity towards hydrophobic metabolites. The hydrophobic metabolites discussed in this review include uric acid, epinephrine, cortisol, cholesterol, tyrosine, adenine, guanine, cytosine, and thymine. This is followed by discussion on future research directions for electrochemical sensing of hydrophobic biomarkers

Analytical strategies based on quantum dots for heavy metal ions detection

Heavy metal contamination is one of the major concerns to human health because these substances are toxic and retained by the ecological system. Therefore, in recent years, there has been a pressing need for fast and reliable methods for the analysis of heavy metal ions in environmental and biological samples. Quantum dots (QDs) have facilitated the development of sensitive sensors over the past decade, due to their unique photophysical properties, versatile surface chemistry and ligand binding ability, and the possibility of the encapsulation in different materials or attachment to different functional materials, while retaining their native luminescence property. This paper comments on different sensing strategies with QD for the most toxic heavy metal ions (i.e., cadmium, Cd2+; mercury, Hg2+; and lead, Pb2+). Finally, the challenges and outlook for the QD-based sensors for heavy metals ions are discussedM.V.G. thanks the MICINN for the PhD grant (FPI, BES-2010-032652). C.C.C. acknowledges a postdoctoral fellowship from the Alexander von Humboldt FoundationS

Metal ion sensors using Tunable Resistive Pulse Sensing

There is a drive to develop rapid, portable and simple methods for detecting heavy metal ions. Due to their toxic nature, heavy metal ions are monitored in aqueous solutions such as drinking water. Standard methods for metal detection rely on instrumentation such as atomic absorption/emission and mass spectrometry. These are often costly and do not allow for rapid on-site or real-time measurements. The aim of this PhD was to develop and optimise tunable resistive pulse sensing (TRPS) for sensing metal ions. This combines nanomaterials, dual molecular recognition with an emerging nanopore technology. TRPS is a label-free portable sensor that allows characterisation of particles based on their size, concentration and charge. Monitoring changes upon the particle surface via changes to the particle charge could be a powerful analytical tool for studying metal ion binding and new sensors. Tuning functional groups on the nanoparticle surface will allow for an array of metal ions to be detected. Nanoparticles will be modified with functional groups that bind to metal ions in solution, in turn this will change the charge on the nanoparticle which will be studied using TRPS. Particle velocity through the pore is dependent on particle charge so changes on the nanoparticle surface can be monitored. The literature review in Chapter 1 focuses on the use of different ligands for the detection of metals focusing on aptamers and modified nanoparticles. The application of the theory of resistive pulse sensors (RPS), which is the main sensing platform within the thesis is covered in detail however these sensors to date have little use in metal ion detection. The theory behind RPS follows the literature review. This covers the theory of transport through a conical nanopore, a brief introduction to zeta potential and particle surface charge and ion current rectification. Before developing a metal ion sensor, the translocation of a particle through the pore, focusing on its relative velocity needed to be understood. Chapter 3 demonstrates how changes in the double layer can affect the measured particle velocity. Understanding how the double layer changes with ionic strength and pH is essential in designing a metal ion sensor where the velocity of the particle through the pore is being measured. The work presented in Chapter 3 gave confidence that TRPS could be used to monitor metal ion binding to the surface of nanoparticles. The nanoparticles were modified with a ligand (APTES) and DNA. The subsequent particle velocities differ to those of the unmodified particles, making TRPS a suitable platform for monitoring changes upon a nanoparticle surface. Building on the knowledge gained from Chapter 3, particle translocation velocities were used for the detection of copper (II) on the surface of modified nanoparticles, Chapter 4. Changes in particle velocity through the nanopore allows for detection of copper (II) as low as 1 ppm and at 10 ppm with competing metal ions present. Chapter 4 also presents the first use of studying pulse waveshape for the detection of an analyte. At low ionic strengths, particles passing through the conical pore generated a biphasic pulse containing a conductive pulse and resistive pulse. The biphasic pulse behaviour was used to monitor changes on the nanoparticle surface, and infer the presence of ions within the particles double layer. The method can be easily adapted to different analytes by altering the ligand used. As an alternative to a particle-based assay, a pore-based assay was developed which exploited the current rectification properties of the conical pores used in TRPS. Chapter 5 presents the use of Layer-by-Layer (LbL) assembly of polyelectrolytes onto the surface of the polyurethane pore for the modification of the pore wall, a DNA aptamer was then easily immobilized onto the pore wall. Vascular Endothelial Growth Factor (VEGF) was chosen as the analyte prior to developing a metal ion assay as it was a system studied in more detail in the literature and within the group. An advantage of TRPS is the particle-by-particle analysis. This allows for simple multiplex detection by using particles of two different sizes to detect two different analytes. In Chapter 6 the methodology and techniques from Chapter 4 is applied to the multiplexed detection of lead (II) and mercury (II) using particle translocation velocities to detect the metal ion binding to DNA aptamers. The method is applicable over a large range of ionic strengths with little interference from a high salt content. Finally, to advance the multiplexed concept, the two independent aptamer sequences used in Chapter 6 are merged together. While both aptamer halves retain their initial functionality and bind to the respective metals, the location of the binding and change in DNA structure with respect to the particles surface is the dominating factor in determining the sensitivity of the RPS technology

Molecular imprinting on nanozymes for sensing applications

As part of the biomimetic enzyme field, nanomaterial-based artificial enzymes, or nanozymes, have been recognized as highly stable and low-cost alternatives to their natural counterparts. The discovery of enzyme-like activities in nanomaterials triggered a broad range of designs with various composition, size, and shape. An overview of the properties of nanozymes is given, including some examples of enzyme mimics for multiple biosensing approaches. The limitations of nanozymes regarding lack of selectivity and low catalytic efficiency may be surpassed by their easy surface modification, and it is possible to tune specific properties. From this perspective, molecularly imprinted polymers have been successfully combined with nanozymes as biomimetic receptors conferring selectivity and improving catalytic performance. Compelling works on constructing imprinted polymer layers on nanozymes to achieve enhanced catalytic efficiency and selective recognition, requisites for broad implementation in biosensing devices, are reviewed. Multimodal biomimetic enzyme-like biosensing platforms can offer additional advantages concerning responsiveness to different microenvironments and external stimuli. Ultimately, progress in biomimetic imprinted nanozymes may open new horizons in a wide range of biosensing applications.The authors gratefully acknowledge funding from the European Commission through the project MindGAP (FET-Open/H2020/GA829040). The author ARC also acknowledges funding to National Foundation for Science and Technology, I.P., through the Ph.D. Grant, SFRH/BD/130107/2017.info:eu-repo/semantics/publishedVersio

Carbon nanotube (CNT)-based biosensors

This review focuses on recent advances in the application of carbon nanotubes (CNTs) for the development of sensors and biosensors. The paper discusses various configurations of these devices, including their integration in analytical devices. Carbon nanotube-based sensors have been developed for a broad range of applications including electrochemical sensors for food safety, optical sensors for heavy metal detection, and field-effect devices for virus detection. However, as yet there are only a few examples of carbon nanotube-based sensors that have reached the marketplace. Challenges still hamper the real-world application of carbon nanotube-based sensors, primarily, the integration of carbon nanotube sensing elements into analytical devices and fabrication on an industrial scale

Coordination of H2O2 on praseodymia nanorods and its application in sensing cholesterol

The advancement of functional nanomaterials has promoted the development of biomarker sensors underpinning promising analytical tools for a range of bioanalytes such as cholesterol. In this work, we established a light-on fluorescent probe for cholesterol in human serum by coordination of H2O2 on the surface of praseodymia nanorods (Pr6O11 NRs). The distinctive interactions of various nucleotides and H2O2 with praseodymia were examined, whereby good fluorescent quenching and recovery capability were observed. A highly sensitive and selective cholesterol detection was achieved in serum samples with a detection limit of 0.1 mu M and recovery of 97.2-101.3%, respectively, due to the high oxygen mobility of praseodymia. The result suggests strong potential for work towards a key probe for a portable clinical test system for cholesterol as well as other H2O2-deriving biomarkers, potentially addressing the ever-increasing demand for the prevention of cardiovascular disease. (C) 2022 Vietnam National University, Hanoi. Published by Elsevier B.V.This work was supported by the Natural Science Foundation of Shandong Province (Grant ZR2017LB028) , Key R&D Program of Shandong Province (Grant 2018GSF118032) , and Fundamental Research Funds for the Central Universities (Grant 18CX02125A) in China. The project with reference number of ENE2017-82451-C3-2-R from Ministry of Science, Innovation and Universities of Spain is also acknowledged. This work has been co-financed by the 2014-2020 ERDF Operational Programme and by the Department of Economy, Knowledge, Business and University of the Regional Government of Andalusia with reference number of FEDER-UCA18-107316

New trends in nanoclay-modified sensors

Nanoclays are widespread materials characterized by a layered structure in the nano-scale range. They have multiple applications in diverse scientific and industrial areas, mainly due to their swelling capacity, cation exchange capacity, and plasticity. Due to the cation exchange capacity, nanoclays can serve as host matrices for the stabilization of several molecules and, thus, they can be used as sensors by incorporating electroactive ions, biomolecules as enzymes, or fluorescence probes. In this review, the most recent applications as bioanalyte sensors are addressed, focusing on two main detection systems: electrochemical and optical methods. Particularly, the application of electrochemical sensors with clay-modified electrodes (CLME) for pesticide detection is described. Moreover, recent advances of both electrochemical and optical sensors based on nanoclays for diverse bioanalytes? detection such as glucose, H2O2, organic acids, proteins, or bacteria are also discussed. As it can be seen from this review, nanoclays can become a key factor in sensors? development, creating an emerging technology for the detection of bioanalytes, with application in both environmental and biomedical fields

Metal–Ligand Interactions in Molecular Imprinting

Molecular imprinting enables the design of highly crosslinked polymeric materials that are able to mimic natural recognition processes. Molecularly imprinted polymers exhibit binding sites with tailored selectivity toward target structures ranging from inorganic ions to biomacromolecules and even viruses or living cells. The choice of the appropriate functional monomer, crosslinker, and the nature and specificity of template–monomer interactions are critical for a successful imprinting process. The use of a metal ion mediating the interaction between the monomer and template (acting as ligands) has proven to offer a higher fidelity of imprint, which modulates the molecularly imprinted polymers (MIPs) selectivity or to endow additional features to the polymer, such as stimuli-responsiveness, catalytic activity, etc. Furthermore, limitations in using nonpolar and aprotic solvents are overcome, allowing the use of more polar solvents and even aqueous solutions as imprinting media, opening new prospects toward the imprinting of biomacromolecules (proteins, DNA, RNA, antibodies, biological receptors, etc.). This chapter aims to outline the beneficial pairing of metal ions as coordination centers and various functional ligands in the molecular imprinting process, as well as to provide an up to date overview of the various applications in chemical sensing, separation processes (stationary phases and selective sorbents), drug delivery, and catalysis

Development of paper-based microfluidic devices for environmental and food quality analysis, The

Includes bibliographical references.2016 Fall.Providing safe and nutritious food and water, both domestically and internationally, has long been a goal for improving global health. Recent legislations enacted within the United States have enabled government agencies to further regulate agricultural and industry standards, necessitating the need for more preventative approaches with regards to food and beverage quality and safety. Increasing detection speed and enabling field and production detection of point-source contamination are crucial to maintaining food and beverage safety as well as preventing detrimental disease outbreaks, such as those caused by bacterial contamination. The development of simple, inexpensive, and portable methods for detecting contamination indicators are key to reaching this goal. Moreover, recent developments into microfluidic approaches for analysis have shown great promise as platforms for providing faster simplified methods for detection. The work conducted within this dissertation focuses on the development of simple, inexpensive and disposable platforms for colorimetric and electrochemical analysis of food and beverage quality. Aside from more commonly studied polymer-based devices, recent advances in paper-based diagnostics have demonstrated use as an analytical platform capable of self-pumping, reagent storage, mixing, and implementation of various detection motifs. Herein, the development of microfluidic paper-based analytical devices (μPADs) is presented as a platform for the colorimetric detection of bacteria in food and water samples. Initial work was conducted for the paper-based, colorimetric detection of Listeria monocytogenes, Salmonella Typhimurium, and E. coli O157:H7 bacteria species, all of which have been associated with fatal, multistate food- and waterborne outbreaks. Detection was performed on ready-to-eat meats using a swabbing technique to collect and quickly culture surface contamination of bacteria using enzymatic assays within paper-based microwells. A scanner was used for imaging followed by use of image analysis software for semi-quantitative measurement determination. This method was further applied to the detection of bacteria in irrigation water, a known source of foodborne contamination, using a 3D-printed filter for collection and culture of bacteria present in low concentrations within water. Although colorimetric detection offers a simple, visual detection method, electrochemistry is an alternative, sensitive and portable method for detection. Use of common office materials such as transparency film and copy paper, as well as laboratory filter papers were studied and developed for optimal electrochemical platform performance. The use of microwires as a simple fabrication method for incorporating metallic or modified metallic electrodes into electrochemical paper-based devices (ePADs) was also developed. Electrochemical behavior in both well-based and flow-based ePADs was studied and implemented for the nonenzymatic detection of sugars in beverages using copper oxide modified microwires, and for the in-line flow detection of enzymatic assays using gold and platinum microwire electrodes respectively. Furthermore, the fast, inexpensive, and simple fabrication of carbon stencil-printed electrodes (CSPEs) on transparency film were demonstrated for the electrochemical detection of E. coli and Enterococci bacteria species, both indicators of fecal contamination, in food and water samples using enzymatic assays. These same assays could also be determined colorimetrically and a more portable cell phone was used to image and wirelessly send paper-based well-plate results. This method was developed for use in place of a more bulky and expensive plate reader, and results were used for comparison to electrochemical detection of bacteria from a single assay

Novel fluorescent tools and techniques for 3D imaging of the cleared brain

Background: To better understand the complexity of the brain and how it becomes impaired under different pathological states, a considerably large number of brains would be needed for imaging to generate highly detailed maps in 3D. Chemical probes can offer a readily scalable labelling method that is robust, easy to use with the quick operation, and feasible for human tissue where genetic viral and toxin tracers are inappropriate. The drawbacks of immunostaining methods have spurred interest in developing alternative strategies to visualize the optically transparent brain, especially from fixed archived samples or human autopsies that are not optimally fixed. Purpose: We envision AIE-based probes and techniques as robust tools when paired with clearing methods for visualizing the human brain. This thesis aims to develop alternative strategies to tissue labelling using novel AIE-based fluorescent chemical probes and methods that offer easy operation, high brightness, photostability and contrast suitable for 3D visualization of neurons and nerve fibers in mouse brains. Paper I: The novel water-soluble silver-ion sensitive AIE probe TPE-4TA achieved by tetrazole-Ag+ coordination, allowed for the development of a new fluorescent silver (silver-AIE) method to visualize separated proteins following sodium dodecyl-sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Compared with conventional silver nitrate stains, silver-AIE not only offers sensitive fluorogenic detection of proteins, but it is quantifiable, easy to use, has a broad linear dynamic range and a great contrast which rivals the popular commercial stain, SYPRO Ruby. Study II describes how to troubleshoot the fluorescent silver gel stain, alternative steps for rapid staining and techniques to carry out the procedure correctly to avoid suboptimal results. Paper II: We report a novel fluorescent silver stain for fixed mouse brain tissue compatible with multiplexed immunofluorescence imaging in paraffin sections. The Ag+-specific aggregation-induced emission (AIE) strategy outperforms the chromogenic detection employed by many conventional silver staining protocols to visualize neurites and fiber tracts in paraffin sections or passive Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging / Immunostaining / in situ-hybridization-compatible Tissue hYdrogel (CLARITY) -cleared tissue. This enables imaging using standard fluorescent widefield or optical sectioning microscopies. Not only does our method uses less hazardous reagents, but the highly sensitive TPE-4TA also uses silver nitrate concentrations up to two million-fold lower than the standard Yamamoto-Hirano’s modification of the Bielschowsky stain. Paper III: Development of the novel near-infrared AIE fluorescent probe PM-ML with D-π-A (donor-pi-acceptor) structure for the selective staining of myelinated fibers in the teased sciatic nerves, mouse brain cryosections and ClearT-cleared mouse brain tissue for 3D fluorescent imaging. We envision PM-ML as a potential tool for studying demyelination and evaluated its selectivity, photostability and signal-to-background (SBR) ratio which outperformed common commercial fluorescent myelin staining dyes