Bioactive paper dipstick sensors for acetylcholinesterase inhibitors based on sol-gel/enzyme/gold nanoparticle composites

Paper-Based Microfluidic Chips for Food Hazard Factor Detection: Fabrication, Modification, and Application

Food safety and quality are paramount concerns for ensuring the preservation of human life and well-being. As the field of food processing continues to advance, there is a growing interest in the development of fast, instant, cost-effective, and convenient methods for detecting food safety issues. In this context, the utilization of paper-based microfluidic chips has emerged as a promising platform for enabling rapid detection, owing to their compact size, high throughput capabilities, affordability, and low resource consumption, among other advantages. To shed light on this topic, this review article focuses on the functionalization of paper-based microfluidic surfaces and provides an overview of the latest research and applications to colorimetric analysis, fluorescence analysis, surface-enhanced Raman spectroscopy, as well as their integration with paper-based microfluidic platforms for achieving swift and reliable food safety detection. Lastly, the article deliberates on the challenges these analytical methods and presents insights into their future development prospects in facilitating rapid food safety assessment.

Sensitive and simple multi-ion detection using organic nanocrystal enrichment in paper analytical devices

Microfluidic paper analytical devices (μPADs) are among the most promising platforms for heavy metal ion analysis. On the other hand, achieving simple and highly sensitive analysis of μPADs is challenging. In this study, we developed a simple enrichment method for sensitive multi-ion detection utilizing water-insoluble organic nanocrystals accumulated on μPAD. By combining the enrichment method with multivariate data analysis, three metal ion concentrations in the ion mixtures were simultaneously quantified with high sensitivity owing to the sensitive responses of the organic nanocrystals. In this work, we successfully quantified Zn²⁺, Cu²⁺, and Ni²⁺ at 20 ng L^-1 in the mixed ion solution using only two dye indicators with a larger sensitivity improvement than those reported in previous studies. Interference studies revealed possibilities for a practical application in real sample analysis. This developed approach also can be used for other analytes.

Gold Nanoparticles Are an Immobilization Platform for Active and Stable Acetylcholinesterase: Demonstration of a General Surface Protein Functionalization Strategy

Immobilizing enzymes onto abiological surfaces is a key step for developing protein-based technologies that can be useful for applications such as biosensors and biofuel cells. A central impediment for the advancement of this effort is a lack of generalizable strategies for functionalizing surfaces with proteins in ways that prevent unfolding, aggregation, and uncontrolled binding, requiring surface chemistries to be developed for each surface-enzyme pair of interest. In this work, we demonstrate a significant advancement toward addressing this problem using a gold nanoparticle (AuNP) as an initial scaffold for the chemical bonding of the enzyme acetylcholinesterase (AChE), forming the conjugate AuNP-AChE. This can then be placed onto chemically and structurally distinct surfaces (e.g., metals, semiconductors, plastics, etc.), thereby bypassing the need to develop surface functionalization strategies for every substrate or condition of interest. Carbodiimide crosslinker chemistry was used to bind surface lysine residues in AChE to AuNPs functionalized with ligands containing carboxylic acid tails. Using amino acid analysis, we found that on average, 3.3 ± 0.1 AChE proteins were bound per 5.22 ± 1.25 nm AuNP. We used circular dichroism spectroscopy to measure the structure of the bound protein and determined that it remained essentially unchanged after binding. Finally, we performed Michaelis-Menten kinetics to determine that the enzyme retained 18.2 ± 2.0% of its activity and maintained that activity over a period of at least three weeks after conjugation to AuNPs. We hypothesize that structural changes to the peripheral active site of AChE are responsible for the differences in activity of bound AChE and unbound AChE. This work is a proof-of-concept demonstration of a generalizable method for placing proteins onto chemically and structurally diverse substrates and materials without the need for surface functionalization strategies.

Sensory materials for microfluidic paper based analytical devices - A review

The microfluidic paper-based analytical devices (μPADs) have grown-up swiftly over the decade due to its low cost, simple fabrication procedure, resource-limitedness, non-toxicity and their environmentally benign nature. The μPADs, also identified as point-of-care devices or health care devices have successfully applied in several fields such as diagnostics, biological, food safety, environmental, electrochemical and most importantly colorimetric/fluorimetric sensors, owing to the attractive passive motions of analyte without any external forces. In recent years, a large number of colorimetric and fluorimetric probes have been reported that can selectively recognize the analytes in μPADs. However, there is no organized review on its structure-activity relationship. In this review, we have focused to summarize the colorimetric and fluorimetric probes utilized in μPADs. This review discuss about the relationships between the structure and functions of various probes as signaling units of the efficient μPADs. The probes including nanomaterials, nanozymes, polymers and organic molecules, their structural activity with regard to sensing performances along with their limit of detection are also discussed. This review is expected to assist readers for better understanding of the sensing mechanisms of various chemo and bio-probes utilized in μPADs, as well as promote their advancement in the field. On the other hand, this review also helps the researchers for enhancement of μPADs and paves way for synergistic application of existing molecular probes as an effective diagnostic tool for the worldwide pandemic novel corona virus COVID-19.

Rapid Detection of Dimethoate in Soybean Samples by Microfluidic Paper Chips Based on Oil-Soluble CdSe Quantum Dots

Given the imperative of monitoring organophosphorus pesticides (OPs) residues in the ecosystem, here a novel, facile and sensitive fluorescence sensor is presented for the rapid detection of dimethoate. In this work, surface molecularly imprinted polymer (SMIP) and microfluidic technology had been introduced to enhance the selectivity and portability of the described methodology. Oil-soluble CdSe quantum dots (QDs) synthesized in a green way were used as fluorescent material for the selective detection of dimethoate on the basis of static quenching and photoinduced electron transfer mechanism. Among many kinds of paper materials, glass fiber paper was used as the novel substrate of paper chip due to low pristine fluorescence and better performance when combining CdSe QDs. In the process of molecular imprinting, the interaction between several functional monomers and dimethoate molecule was investigated and simulated theoretically by software to improve the selectivity of the sensor. Consequently, the fabricated novel detection platform could effectively respond to dimethoate in 10 min with the concentration range of 0.45–80 μmol/L and detection limit of 0.13 μmol/L. The recovery in the spiked experiment soybean sample was in an acceptable range (97.6–104.1%) and the accuracy was verified by gas chromatography-mass spectrometry, which signified the feasibility and potential in food sampling.

Enzyme-polymeric/inorganic metal oxide/hybrid nanoparticle bio-conjugates in the development of therapeutic and biosensing platforms

Background

Because enzymes can control several metabolic pathways and regulate the production of free radicals, their simultaneous use with nanoplatforms showing protective and combinational properties is of great interest in the development of therapeutic nano-based platforms. However, enzyme immobilization on nanomaterials is not straightforward due to the toxic and unpredictable properties of nanoparticles in medical practice.

Aim of review

In fact, because of the ability to load enzymes on nano-based supports and increase their renewability, scientific groups have been tempted to create potential therapeutic enzymes in this field. Therefore, this study not only pays attention to the therapeutic and diagnostic applications of diseases by enzyme-nanoparticle (NP) bio-conjugate (abbreviated as: ENB), but also considers the importance of nanoplatforms used based on their toxicity, ease of application and lack of significant adverse effects on loaded enzymes. In the following, based on the published reports, we explained that the immobilization of enzymes on polymers, inorganic metal oxide and hybrid compounds provide hopes for potential use of ENBs in medical activities. Then, the use of ENBs in bioassay activities such as paper-based or wearing biosensors and lab-on-chip/microfluidic biosensors were evaluated. Finally, this review addresses the current challenges and future perspective of ENBs in biomedical applications.

Key scientific concepts of review

This literature may provide useful information regarding the application of ENBs in biosensing and therapeutic platforms.

Advanced Signal-Amplification Strategies for Paper-Based Analytical Devices: A Comprehensive Review

Paper-based analytical devices (PADs) have emerged as a promising approach to point-of-care (POC) detection applications in biomedical and clinical diagnosis owing to their advantages, including cost-effectiveness, ease of use, and rapid responses as well as for being equipment-free, disposable, and user-friendly. However, the overall sensitivity of PADs still remains weak, posing a challenge for biosensing scientists exploiting them in clinical applications. This review comprehensively summarizes the current applicable potential of PADs, focusing on total signal-amplification strategies that have been applied widely in PADs involving colorimetry, luminescence, surface-enhanced Raman scattering, photoacoustic, photothermal, and photoelectrochemical methods as well as nucleic acid-mediated PAD modifications. The advances in signal-amplification strategies in terms of signal-enhancing principles, sensitivity, and time reactions are discussed in detail to provide an overview of these approaches to using PADs in biosensing applications. Furthermore, a comparison of these methods summarizes the potential for scientists to develop superior PADs. This review serves as a useful inside look at the current progress and prospective directions in using PADs for clinical diagnostics and provides a better source of reference for further investigations, as well as innovations, in the POC diagnostics field.

Research progress on the applications of paper chips

Due to the modern pursuit of the quality of life, science and technology have rapidly developed, resulting in higher requirements for various detection methods based on analytical technology. Herein, the development, fabrication, detection and application of paper-based microfluidic chips (μPAD) are summarized. We aim to provide a comprehensive understanding of paper chips, and then discuss challenges and future prospects in this field.

A paper-based colorimetric sensor array for discrimination and simultaneous determination of organophosphate and carbamate pesticides in tap water, apple juice, and rice

A colorimetric paper-based sensor is proposed for the rapid monitoring of six major organophosphate and carbamate pesticides. The assay was constructed by dropping gold and silver nanoparticles on the hydrophilic zones of a paper substrate. The nanoparticles were modified by L-arginine, quercetin, and polyglutamic acid. The mechanism of sensing is based on the interaction between the pesticide and the nanoparticles. The color of nanoparticles changed during the interactions. A digital camera recorded these changes. The assay provided a unique response for each studied pesticide. This method can determine six individual pesticides including carbaryl, paraoxon, parathion, malathion, diazinon, and chlorpyrifos. The limit of detection for these pesticides were 29.0, 22.0, 32.0, 17.0, 45.0, and 36.0 ng mL^-1, respectively. The assay was applied to simultaneously determine the six studied pesticides in a mixture using the partial least square method (PLS). The root mean square errors of prediction were 11, 8.7, 9.2, 10, 12, and 11 for carbaryl, paraoxon, parathion, malathion, diazinon, and chlorpyrifos, respectively. The paper-based device can differentiate two types of studied pesticide (organophosphate and carbamate) as well as two types of organophosphate structures (oxon and thion). Furthermore, this sensor showed high selectivity to the pesticides in the presence of other potential species (e.g., metal ions, anions, amino acids, sugar, and vitamins). This assay is capable of determining the pesticide compounds in tap water, apple juice, and rice samples.Graphical abstract.

Nanotechnology-based recent approaches for sensing and remediation of pesticides

Pesticides are meant to control and destroy the pests and weeds. They are classified into different categories on the basis their origin and type of pest they target. Chemical pesticides such as insecticides, herbicides and fungicides are commonly used in agricultural fields. However, the excessive use of these agrochemicals have adverse effects on environment such as reduced population of insect pollinators, threat to endangered species and habitat of birds. Upon consumption; chemical pesticides also cause various health issues such as skin, eye and nervous system related problems and cancer upon prolonged exposure. Various techniques in the past have been developed on the basis of surface adsorption, membrane filtration and biological degradation to reduce the content of pesticides. However, slow response, less specificity and sensitivity are some of the drawbacks of such techniques. In recent times, Nanotechnology has emerged as a helping tool for the sensing and remediation of pesticides. This review focuses on the use of this technology for the detection, degradation and removal of pesticides. Nanomaterials have been classified into nanoparticles, nanotubes and nanocomposites that are commonly used for detection, degradation and removal of pesticides. The review also focuses on the chemistry behind the sensing and remediation of pesticides using nanomaterials. Different types of nanoparticles, viz. metal nanoparticles, bimetallic nanoparticles and metal oxide nanoparticles; nanotubes such as carbon nanotubes and halloysite nanotubes have been used for the detection, degradation and removal of pesticides. Further, various enzyme-based biosensors for detection of pesticides have also been summarized.

Low-cost Paper Analytical Devices for Environmental and Biomedical Sensing Applications

Over the last decade, the fabrication of analytical devices utilizing microfluidic structures and lab-on-a-chip platforms has shown breakthrough advancements, both for environmental and biological applications. The ASSURED criteria (affordable, sensitive, specific, user-friendly, robust, equipment-free, delivered), developed by the WHO for diagnostics devices, point towards the need of paper-based analytical devices (PAD) for diagnostics. On the other hand, cost-effective PADs owing the great advantage of affordable applicability in both resource-rich and -limited settings are recently employed for on-site environmental monitoring. In this book chapter, we will discuss about the brief history of paper analytical devices, fabrications, need, and its environmental and biomedical applications.

Cellulose nanocrystal (CNC)–inorganic hybrid systems: synthesis, properties and applications

Cellulose nanocrystal (CNC), a class of sustainable nanomaterial derived from forest and agro-biomass can serve as nature's storage for carbon dioxide.

The lab-on-PCB approach: tackling the μTAS commercial upscaling bottleneck

Commercialization of lab-on-a-chip devices is currently the "holy grail" within the μTAS research community. While a wide variety of highly sophisticated chips which could potentially revolutionize healthcare, biology, chemistry and all related disciplines are increasingly being demonstrated, very few chips are or can be adopted by the market and reach the end-users. The major inhibition factor lies in the lack of an established commercial manufacturing technology. The lab-on-printed circuit board (lab-on-PCB) approach, while suggested many years ago, only recently has re-emerged as a very strong candidate, owing to its inherent upscaling potential: the PCB industry is well established all around the world, with standardized fabrication facilities and processes, but commercially exploited currently only for electronics. Owing to these characteristics, complex μTASs integrating microfluidics, sensors, and electronics on the same PCB platform can easily be upscaled, provided more processes and prototypes adapted to the PCB industry are proposed. In this article, we will be reviewing for the first time the PCB-based prototypes presented in the literature to date, highlighting the upscaling potential of this technology. The authors believe that further evolution of this technology has the potential to become a much sought-after standardized industrial fabrication technology for low-cost μTASs, which could in turn trigger the projected exponential market growth of μTASs, in a fashion analogous to the revolution of Si microchips via the CMOS industry establishment.

Silica nanoparticle based techniques for extraction, detection, and degradation of pesticides

Silica nanoparticles (SiNPs) find applications in the fields of drug delivery, catalysis, immobilization and sensing. Their synthesis can be mediated in a facile manner and they display broad range compatibility and stability. Their existence in the form of spheres, wires and sheets renders them suitable for varied purposes. This review summarizes the use of silica nanostructures in developing techniques for extraction, detection and degradation of pesticides. Silica nanostructures on account of their sorbent properties, porous nature and increased surface area allow effective extraction of pesticides. They can be modified (with ionic liquids, silanes or amines), coated with molecularly imprinted polymers or magnetized to improve the extraction of pesticides. Moreover, they can be altered to increase their sensitivity and stability. In addition to the analysis of pesticides by sophisticated techniques such as High Performance Liquid Chromatography or Gas chromatography, silica nanoparticles related simple detection methods are also proving to be effective. Electrochemical and optical detection based on enzymes (acetylcholinesterase and organophosphate hydrolase) or antibodies have been developed. Pesticide sensors dependent on fluorescence, chemiluminescence or Surface Enhanced Raman Spectroscopic responses are also SiNP based. Moreover, degradative enzymes (organophosphate hydrolases, carboxyesterases and laccases) and bacterial cells that produce recombinant enzymes have been immobilized on SiNPs for mediating pesticide degradation. After immobilization, these systems show increased stability and improved degradation. SiNP are significant in developing systems for effective extraction, detection and degradation of pesticides. SiNPs on account of their chemically inert nature and amenability to surface modifications makes them popular tools for fabricating devices for 'on-site' applications.

Sensitive inkjet printing paper-based colormetric strips for acetylcholinesterase inhibitors with indoxyl acetate substrate

A new paper-based biosensing approach has been developed for sensitive and rapid detection of acetylcholinesterase (AChE) inhibitors. The biosensing zone of the paper strip is constructed with an inkjet printing method, and the biomolecule AChE is immobilized into two layers of biocompatible sol-gel-derived silica ink with a "sandwich" form. Indoxyl acetate (IDA) is used as a chromogenic substrate, which is colorless and can be catalytically hydrolyzed into blue-colored indigo dipolymer. When the enzymatic activity of AChE is inhibited after incubation with organophosphate pesticides (OPs), there is a decreased hydrolysis of IDA accompanying with a drop in color intensity. Paraoxon and trichlorfon are used as the representative OPs in the assay. Due to the low solubility and high molar absorption coefficient of the IDA dipolymer product, the paper-based strip can form a neat blue sensing zone and shows obviously improved sensitivity with a limit of detection (LOD) of 0.01ngmL^-1 paraoxon and 0.04ngmL^-1 trichlorfon (S/N=3) and the LODs for visual detection are 0.03ngmL^-1 for paraoxon and 0.1ngmL^-1 for trichlorfon comparing with the previously reported colorimetric methods. The concentrations of paraoxon in apple juice samples are also detected, and the results are in accord well with these results from high-performance liquid chromatography, showing great potential for on-site detection of OPs in practical application. The developed assay can be used to qualitatively and semiquantitatively estimate with naked eyes and quantitatively assess OPs through image analysis.

New Functionalities for Paper-Based Sensors Lead to Simplified User Operation, Lower Limits of Detection, and New Applications

In the last decade, paper analytical devices (PADs) have evolved into sophisticated yet simple sensors with biological and environmental applications in the developed and developing world. The focus of this review is the technological improvements that have over the past five years increased the applicability of PADs to real-world problems. Specifically, this review reports on advances in sample processing, fluid flow control, signal amplification, and component integration. Throughout, we have sought to emphasize advances that retain the main virtues of PADs: low cost, portability, and simplicity.

Surface Modification Chemistries of Materials Used in Diagnostic Platforms with Biomolecules

Biomolecules including DNA, protein, and enzymes are of prime importance in biomedical field. There are several reports on the technologies for the detection of these biomolecules on various diagnostic platforms. It is important to note that the performance of the biosensor is highly dependent on the substrate material used and its meticulous modification for particular applications. Therefore, it is critical to understand the principles of a biosensor to identify the correct substrate material and its surface modification chemistry. The imperative surface modification for the attachment of biomolecules without losing their bioactivity is a key to sensitive detection. Therefore, finding of a modification method which gives minimum damage to the surface as well as biomolecule is highly inevitable. Different surface modification technologies are invented according to the type of a substrate used. Surface modification techniques of the materials used as platforms in the fabrication of biosensors are reviewed in this paper.

Functional nanostructures for enzyme based biosensors: properties, fabrication and applications

A review describing functional nanostructures for portable and printable enzyme biosensors. Specific physicochemical and surface properties of nanoparticles used as carriers and sensing components and their assembly are discussed with an overview of current and emerging techniques enabling large scale roll-to-roll fabrication and miniaturization. Their integration in flexible, wearable and inexpensive point-of-use devices, and implementation challenges are also provided with examples of applications.

Portable Nanoparticle-Based Sensors for Food Safety Assessment

The use of nanotechnology-derived products in the development of sensors and analytical measurement methodologies has increased significantly over the past decade. Nano-based sensing approaches include the use of nanoparticles (NPs) and nanostructures to enhance sensitivity and selectivity, design new detection schemes, improve sample preparation and increase portability. This review summarizes recent advancements in the design and development of NP-based sensors for assessing food safety. The most common types of NPs used to fabricate sensors for detection of food contaminants are discussed. Selected examples of NP-based detection schemes with colorimetric and electrochemical detection are provided with focus on sensors for the detection of chemical and biological contaminants including pesticides, heavy metals, bacterial pathogens and natural toxins. Current trends in the development of low-cost portable NP-based technology for rapid assessment of food safety as well as challenges for practical implementation and future research directions are discussed.

Paper-based chemical and biological sensors: Engineering aspects

Remarkable efforts have been dedicated to paper-based chemosensors and biosensors over the last few years, mainly driven by the promise of reaching the best trade-off between performance, affordability and simplicity. Because of the low-cost and rapid prototyping of these sensors, recent research has been focused on providing affordable diagnostic devices to the developing world. The recent progress in sensitivity, multi-functionality and integration of microfluidic paper-based analytical devices (µPADs), increasingly suggests that this technology is not only attractive in resource-limited environments but it also represents a serious challenger to silicon, glass and polymer-based biosensors. This review discusses the design, chemistry and engineering aspects of these developments, with a focus on the past few years.

A cationic surfactant-decorated liquid crystal sensing platform for simple and sensitive detection of acetylcholinesterase and its inhibitor

In this paper, construction of the liquid crystal (LC)-based sensing platform for simple and sensitive detection of acetylcholinesterase (AChE) and its inhibitor using a cationic surfactant-decorated LC interface was demonstrated. A change of the optical images of LCs from bright to dark appearance was observed when the cationic surfactant, myristoylcholine chloride (Myr), was transferred onto the aqueous/LC interface, due to the formation of a stable surfactant monolayer at the interface. A dark-to-bright change of the optical appearance was then observed when AChE was transferred onto the Myr-decorated LC interface. The sensitivity of this new type of LC-based sensor is 3 orders of magnitude higher in the serum albumin solution than that only in the buffer solution. Noteworthy is that the AChE LC sensor shows a very high sensitivity for the detection of the enzyme inhibitor, which is around 1 fM. The constructed low-cost LC-based sensor is quite simple and convenient, showing high promise for label-free detection of AChE and its inhibitors.

Cobalt-porphyrin/dansyl piperazine complex coated filter paper for “turn on” fluorescence sensing of ammonia gas

Cobalt-porphyrin/dansyl piperazine-coated filter papers, as a series of “turn on” fluorescence sensors, show high sensitivity and fast response to ammonia gas.

Toward point-of-care diagnostics with consumer electronic devices: the expanding role of nanoparticles

A review of the role that nanoparticles can play in point-of-care diagnostics that utilize consumer electronic devices such as cell phones and smartphones for readout, including an overview of important concepts and examples from the literature.

Printed microwells with highly stable thin-film enzyme coatings for point-of-care multiplex bioassay of blood samples

A paper-based colorimetric biosensor suitable for point-of-care bioassay of blood samples is developed using highly stable enzyme thin-film coatings confined within inkjet printed polymeric microwells.

Bioactive paper sensor based on the acetylcholinesterase for the rapid detection of organophosphate and carbamate pesticides

In many countries, people are becoming more concerned about pesticide residues which are present in or on food and feed products. For this reason, several methods have been developed to monitor the pesticide residue levels in food samples. In this study, a bioactive paper-based sensor was developed for detection of acetylcholinesterase (AChE) inhibitors including organophosphate and carbamate pesticides. Based on the Ellman colorimetric assay, the assay strip is composed of a paper support (1 × 10 cm), onto which a biopolymer chitosan gel immobilized in crosslinking by glutaraldehyde with AChE and 5,5'-dithiobis(2-nitrobenzoic) acid (DTNB) and uses acetylthiocholine iodide (ATChI) as an outside reagent. The assay protocol involves introducing the sample to sensing zone via dipping of a pesticide-containing solution. Following an incubation period, the paper is placed into ATChI solution to initiate enzyme catalyzed hydrolysis of the substrate, causing a yellow color change. The absence or decrease of the yellow color indicates the levels of the AChE inhibitors. The biosensor is able to detect organophosphate and carbamate pesticides with good detection limits (methomyl = 6.16 × 10(-4) mM and profenofos = 0.27 mM) and rapid response times (~5 min). The results show that the paper-based biosensor is rapid, sensitive, inexpensive, portable, disposable, and easy-to-use.

Enzymatic etching of gold nanorods by horseradish peroxidase and application to blood glucose detection

Gold nanorods (AuNRs) have become some of the most used nanostructures for biosensing and imaging applications due to their plasmon-related optical response, which is highly sensitive toward minute changes in the AuNR aspect ratio. In this context, H2O2 has been used to trigger the chemical etching of AuNRs, thereby inducing a decrease of their aspect ratio. However, special conditions and relatively high concentrations of H2O2 are usually required, preventing the applicability of the system for biodetection purposes. To overcome this limitation we have introduced a biocatalytic species, the enzyme horseradish peroxidase (HRP) that is able to induce a gradual oxidation of AuNRs in the presence of trace concentrations of H2O2. Interestingly, the presence of halide ions has also been found to be essential for this process. As a consequence, other enzymatic reactions, such as those catalyzed by glucose oxidase, can be easily coupled to HRP activity, allowing the detection of different amounts of glucose. On the basis of these findings, we developed a highly sensitive and simple colorimetric assay that can be read out by the naked eye and allows the detection of physiological glucose concentrations in human serum.

Microwave-accelerated surface modification of plasmonic gold thin films with self-assembled monolayers of alkanethiols

A rapid surface modification technique for the formation of self-assembled monolayers (SAMs) of alkanethiols on gold thin films using microwave heating in <10 min is reported. In this regard, SAMs of two model alkanethiols, 11-mercaptoundecanoic acid (11-MUDA, to generate a hydrophilic surface) and undecanethiol (UDET, a hydrophobic surface), were successfully formed on gold thin films using selective microwave heating in (1) a semicontinuous fashion and (2) a continuous fashion at room temperature (24 h, control experiment, no microwave heating). The formation of SAMs of 11-MUDA and UDET was confirmed by contact angle measurements, Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The contact angles for water on SAMs formed by the selective microwave heating and conventional room temperature incubation technique (24 h) were measured to be similar for 11-MUDA and UDET. FT-IR spectroscopy results confirmed that the internal structures of SAMs prepared using both microwave heating and room temperature were similar. XPS results revealed that the organic and sulfate contaminants found on bare gold thin films were replaced by SAMs after the surface modification process had been conducted using both microwave heating and room temperature.

Recent advances in optical biosensors for environmental monitoring and early warning

The growing number of pollutants requires the development of innovative analytical devices that are precise, sensitive, specific, rapid, and easy-to-use to meet the increasing demand for legislative actions on environmental pollution control and early warning. Optical biosensors, as a powerful alternative to conventional analytical techniques, enable the highly sensitive, real-time, and high-frequency monitoring of pollutants without extensive sample preparation. This article reviews important advances in functional biorecognition materials (e.g., enzymes, aptamers, DNAzymes, antibodies and whole cells) that facilitate the increasing application of optical biosensors. This work further examines the significant improvements in optical biosensor instrumentation and their environmental applications. Innovative developments of optical biosensors for environmental pollution control and early warning are also discussed.

Portable bioactive paper-based sensor for quantification of pesticides

A paper-based biosensor was developed for the detection of the degradation products of organophosphorus pesticides. The biosensor quantifies acetylcholine esterase inhibitors in a fast, disposable, cheap, and accurate format. We specifically focused on the use of sugar or protein stabilizer to achieve a biosensor with long shelf-life. The new biosensor detected malathion with a detection limit of 2.5 ppm in 5 min incubation time. The operational stability was confirmed by testing 60 days storage at 4°C when glucose was used as stabilizer.

Surface modification of plasmonic nanostructured materials with thiolated oligonucleotides in 10 seconds using selective microwave heating

This study demonstrates the proof-of-principle of rapid surface modification of plasmonic nanostructured materials with oligonucleotides using low power microwave heating. Due to their interesting optical and electronic properties, silver nanoparticle films (SNFs, 2 nm thick) deposited onto glass slides were used as the model plasmonic nanostructured materials. Rapid surface modification of SNFs with oligonucleotides was carried out using two strategies (1) Strategy 1: for ss-oligonucleotides, surface hybridization and (2) Strategy 2: for ds-oligonucleotides, solution hybridization), where the samples were exposed to 10, 15, 30 and 60 seconds microwave heating. To assess the efficacy of our new rapid surface modification technique, identical experiments carried out without the microwave heating (i.e., conventional method), which requires 24 hours for the completion of the identical steps. It was found that SNFs can be modified with ss- and ds-oligonucleotides in 10 seconds, which typically requires several hours of incubation time for the chemisorption of thiol groups on to the planar metal surface using conventional techniques.

Recent advances in paper-based sensors

Paper-based sensors are a new alternative technology for fabricating simple, low-cost, portable and disposable analytical devices for many application areas including clinical diagnosis, food quality control and environmental monitoring. The unique properties of paper which allow passive liquid transport and compatibility with chemicals/biochemicals are the main advantages of using paper as a sensing platform. Depending on the main goal to be achieved in paper-based sensors, the fabrication methods and the analysis techniques can be tuned to fulfill the needs of the end-user. Current paper-based sensors are focused on microfluidic delivery of solution to the detection site whereas more advanced designs involve complex 3-D geometries based on the same microfluidic principles. Although paper-based sensors are very promising, they still suffer from certain limitations such as accuracy and sensitivity. However, it is anticipated that in the future, with advances in fabrication and analytical techniques, that there will be more new and innovative developments in paper-based sensors. These sensors could better meet the current objectives of a viable low-cost and portable device in addition to offering high sensitivity and selectivity, and multiple analyte discrimination. This paper is a review of recent advances in paper-based sensors and covers the following topics: existing fabrication techniques, analytical methods and application areas. Finally, the present challenges and future outlooks are discussed.