Introduction: Chemical Sensors

Advances in flame synthesis of nano-scale architectures for chemical, biomolecular, plasmonic, and light sensing

Flame spray pyrolysis (FSP), a key technique under the broader category of flame aerosol synthesis, is being increasingly explored for the design of advanced miniaturized sensor architectures with applications including chemical, biomolecular, plasmonic, and light sensing. This review provides an overview of the advantages of FSP for the fabrication of nanostructured materials for sensing, delving into synthesis strategies and material structures that meet the increasing demands for miniaturized sensor devices. We focus on the fundamentals of FSP, discussing reactor configurations and how process parameters such as precursor compositions, flow rates, and temperature influence nanoparticle characteristics and their sensing performance. A detailed analysis of nanostructures, compositions, and morphologies made by FSP and their applications in chemical, chemiresistive, plasmonic, biosensing, and light sensing is presented. This review identifies the challenges and opportunities of FSP, exploring current limitations and potential improvements for industrial translation. We conclude by highlighting future research directions aiming to establish guidelines for the flame-based design of nano-scale sensing architectures.

Highly selective and sensitive histamine and tryptamine analysis using SiO2@AuNPs@PDA molecularly imprinted polymer coupled with SALDI-TOF MS

A combination of SiO2@AuNPs@PDA molecularly imprinted and surface-assisted laser desorption/ionization-time-of-flight mass spectrometry (SALDI-TOF MS) was devised as a method for highly specific and ultrasensitive detection of two biogenic amines-histamine (HIS) and tryptamine (TRP)-in real samples. In this strategy, AuNPs modified amino-abundant silica nanospheres (SiO2@AuNPs). The prepared SiO2@AuNPs were used as a substrate to synthesize a molecularly imprinted polymer (MIP) through in situ dopamine self-polymerization with HIS and TRP as the template molecules (SiO2@AuNP@PDA-MIP). The as-prepared MIP structure, properties, and target-analyte identification conditions were characterized and optimized and it was used as the matrix for MS. Compared to the case of nonimprinted materials, the imprinting function endowed the matrix with a higher selectivity for capturing the target molecules. The enriched analytes were directly and rapidly identified using SALDI-TOF MS without elution. Meanwhile, the proposed method has low background interference, good reproducibility and stability, high salt tolerance, and satisfactory linearity (R2 > 0.99), and it enables ultrasensitive detection of HIS and TRP (limits of detection for HIS and TRP were 0.2 and 0.1 ng mL-1, respectively). Moreover, the proposed method was applied to analyze samples of real beer, sausage, and chicken, and the results agreed with those obtained via liquid chromatography-MS, suggesting that the method has excellent practical applications in the field of food safety.

A review on solution- and vapor-responsive sensors for the detection of phthalates

Phthalic acid esters, largely referred to as phthalates, are today acknowledged as important pollutants used in the manufacture of polyvinyl chloride (PVC)-based plastics, whose use extends to almost every aspect of modern life. The risk of exposure to phthalates is particularly relevant as high concentrations are regularly found in drinking water, food-contact materials and medical devices, motivating an immense body of research devoted to methods for their detection in liquid samples. Conversely, phthalate vapors have only recently been acknowledged as potentially important atmospheric pollutants and as early fire indicators; additionally, deposition of these vapors can pose significant problems to the proper functioning of spacecraft and diverse on-board devices, leading to major space agencies recognizing the need of developing vapor-responsive phthalate sensors. In this manuscript we present a literature survey on solution- and vapor-responsive sensors and analytical assays for the detection of phthalates, providing a detailed analysis of a vast array of analytical data to offer a clear idea on the analytical performance (limits of detection and quantification, linear range) and advantages provided by each class of sensor covered in this review (electrochemical, optical and vapor-responsive) in the context of their potential real-life applications; the manuscript also gives detailed fundamental information on the various physicochemical responses exploited by these sensors and assays that could potentially be harnessed by new researchers entering the field.

2D quasi-layered material with domino structure

Interlayer coupling strength dichotomizes two-dimensional (2D) materials into layered and non-layered types. Traditionally, they can be regarded as atomic layers intrinsically linked via van der Waals (vdW) forces or covalent bonds, oriented orthogonally to their growth plane. In our work, we report a material system that differentiates from layered and non-layered materials, termed quasi-layered domino-structured (QLDS) materials, effectively bridging the gap between these two typical categories. Considering the skewed structure, the force orthogonal to the 2D QLDS-GaTe growth plane constitutes a synergistic blend of vdW forces and covalent bonds, with neither of them being perpendicular to the 2D growth plane. This unique amalgamation results in a force that surpasses that in layered materials, yet is weaker than that in non-layered materials. Therefore, the lattice constant contraction along this unique orientation can be as much as 7.7%, tantalizingly close to the theoretical prediction of 10.8%. Meanwhile, this feature endows remarkable anisotropy, second harmonic generation enhancement with a staggering susceptibility of 394.3 pm V-1. These findings endow further applications arranged in nonlinear optics, sensors, and catalysis.

Gold Nanoparticle-Based Colorimetric Sensors: Properties and Application in Detection of Heavy Metals and Biological Molecules

During the last decade, advances have been made in nanotechnology using nanomaterials, leading to improvements in their performance. Gold nanoparticles (AuNPs) have been widely used in the field of sensor analysis and are also combined with certain materials to obtain the desired characteristics. AuNPs are commonly used as colorimetric sensors in detection methods. In developing an ideal sensor, there are certain characteristics that must be met such as selectivity, sensitivity, accuracy, precision, and linearity, among others. Various methods for the synthesis of AuNPs and conjugation with other components have been carried out in order to obtain good characteristics for their application. AuNPs can be applied in the detection of both heavy metals and biological molecules. This review aimed at observing the role of AuNPs in its application. The synthesis of AuNPs for sensors will also be revealed, along with their characteristics suitable for this role. In the application method, the size and shape of the particles must be considered. AuNPs used in heavy metal detection have a particle size of around 15-50 nm; in the detection of biological molecules, the particle size of AuNPs used is 6-35 nm whereas in pharmaceutical compounds for cancer treatment and the detection of other drugs, the particle size used is 12-30 nm. The particle sizes did not correlate with the type of molecules regardless of whether it was a heavy metal, biological molecule, or pharmaceutical compound but depended on the properties of the molecule itself. In general, the best morphology for application in the detection process is a spherical shape to obtain good sensitivity and selectivity based on previous studies. Functionalization of AuNPs with conjugates/receptors can be carried out to increase the stability, sensitivity, selectivity, solubility, and plays a role in detecting biological compounds through conjugating AuNPs with biological molecules.

Laser-Carbonization - A Powerful Tool for Micro-Fabrication of Patterned Electronic Carbons

Fabricating electronic devices from natural, renewable resources is a common goal in engineering and materials science. In this regard, carbon is of special significance due to its biocompatibility combined with electrical conductivity and electrochemical stability. In microelectronics, however, carbon's device application is often inhibited by tedious and expensive preparation processes and a lack of control over processing and material parameters. Laser-assisted carbonization is emerging as a tool for the precise and selective synthesis of functional carbon-based materials for flexible device applications. In contrast to conventional carbonization via in-furnace pyrolysis, laser-carbonization is induced photo-thermally and occurs on the time-scale of milliseconds. By careful selection of the precursors and process parameters, the properties of this so-called laser-patterned carbon (LP-C) such as porosity, surface polarity, functional groups, degree of graphitization, charge-carrier structure, etc. can be tuned. In this critical review, a common perspective is generated on laser-carbonization in the context of general carbonization strategies, fundamentals of laser-induced materials processing, and flexible electronic applications, like electrodes for sensors, electrocatalysts, energy storage, or antennas. An attempt is made to have equal emphasis on material processing and application aspects such that this emerging technology can be optimally positioned in the broader context of carbon-based microfabrication.

Machine Learning Identification of Organic Compounds Using Visible Light

Identifying chemical compounds is essential in several areas of science and engineering. Laser-based techniques are promising for autonomous compound detection because the optical response of materials encodes enough electronic and vibrational information for remote chemical identification. This has been exploited using the fingerprint region of infrared absorption spectra, which involves a dense set of absorption peaks that are unique to individual molecules, thus facilitating chemical identification. However, optical identification using visible light has not been realized. Using decades of experimental refractive index data in the scientific literature of pure organic compounds and polymers over a broad range of frequencies from the ultraviolet to the far-infrared, we develop a machine learning classifier that can accurately identify organic species based on a single-wavelength dispersive measurement in the visible spectral region, away from absorption resonances. The optical classifier proposed here could be applied to autonomous material identification protocols and applications.

The Frontiers of Functionalized Nanocellulose-Based Composites and Their Application as Chemical Sensors

Chemical sensors are a rapidly developing technology that has received much attention in diverse industries such as military, medicine, environmental surveillance, automotive power and mobility, food manufacturing, infrastructure construction, product packaging and many more. The mass production of low-cost devices and components for use as chemical sensors is a major driving force for improvements in each of these industries. Recently, studies have found that using renewable and eco-friendly materials would be advantageous for both manufacturers and consumers. Thus, nanotechnology has led to the investigation of nanocellulose, an emerging and desirable bio-material for use as a chemical sensor. The inherent properties of nanocellulose, its high tensile strength, large specific surface area and good porous structure have many advantages in its use as a composite material for chemical sensors, intended to decrease response time by minimizing barriers to mass transport between an analyte and the immobilized indicator in the sensor. Besides which, the piezoelectric effect from aligned fibers in nanocellulose composites is beneficial for application in chemical sensors. Therefore, this review presents a discussion on recent progress and achievements made in the area of nanocellulose composites for chemical sensing applications. Important aspects regarding the preparation of nanocellulose composites using different functionalization with other compounds are also critically discussed in this review.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Plasmonic Sensing: Connecting the Dots

Plasmonic sensitivity of noble metals has often been attributed to the morphology of the nanostructures and dielectric effects of both the materials and the surrounding medium. The measurable plasmonic shift with respect to the change in local dielectric as a function of analyte concentrations within nanoscale volume forms the basis of plasmonic sensing. However, the situation of the surrounding medium in the presence of multicomponent systems and, moreover, inhomogeneous adsorption around the anisotropic nanostructures become seemingly complicated as the precise description of several individual components becomes nearly impossible. Therefore, we have designed a retrospective formalism through a critical condensation of the electromagnetic scattering theories, macroscopic mixing rules, and micromechanics at the metal-analyte interface that can be adopted as generalized model irrespective of morphology of the nanostructures and the nature of analytes to account for the response of all the individual (microscopic) components to the observed (macroscopic) plasmonic sensing.

Conductive Metallophthalocyanine Framework Films with High Carrier Mobility as Efficient Chemiresistors

The poor electrical conductivity of two-dimensional (2D) crystalline frameworks greatly limits their utilization in optoelectronics and sensor technology. Herein, we describe a conductive metallophthalocyanine-based NiPc-CoTAA framework with cobalt(II) tetraaza[14]annulene linkages. The high conjugation across the whole network combined with densely stacked metallophthalocyanine units endows this material with high electrical conductivity, which can be greatly enhanced by doping with iodine. The NiPc-CoTAA framework was also fabricated as thin films with different thicknesses from 100 to 1000 nm by the steam-assisted conversion method. These films enabled the detection of low-concentration gases and exhibited remarkable sensitivity and stability. This study indicates the enormous potential of metallophthalocyanine-based conductive frameworks in advanced stand-off chemical sensors and provides a general strategy through tailor-make molecular design to develop sensitive and stable chemical sensors for the detection of low-concentration gases.

Microplotter-Printed On-Chip Combinatorial Library of Ink-Derived Multiple Metal Oxides as an "Electronic Olfaction" Unit

Information about the surrounding atmosphere at a real timescale significantly relies on available gas sensors to be efficiently combined into multisensor arrays as electronic olfaction units. However, the array's performance is challenged by the ability to provide orthogonal responses from the employed sensors at a reasonable cost. This issue becomes more demanded when the arrays are designed under an on-chip paradigm to meet a number of emerging calls either in the internet-of-things industry or in situ noninvasive diagnostics of human breath, to name a few, for small-sized low-powered detectors. The recent advances in additive manufacturing provide a solid top-down background to develop such chip-based gas-analytical systems under low-cost technology protocols. Here, we employ hydrolytically active heteroligand complexes of metals as ink components for microplotter patterning a multioxide combinatorial library of chemiresistive type at a single chip equipped with multiple electrodes. To primarily test the performance of such a multisensor array, various semiconducting oxides of the p- and n-conductance origins based on pristine and mixed nanocrystalline MnO_x, TiO₂, ZrO₂, CeO₂, ZnO, Cr₂O₃, Co₃O₄, and SnO₂ thin films, of up to 70 nm thick, have been printed over hundred μm areas and their micronanostructure and fabrication conditions are thoroughly assessed. The developed multioxide library is shown to deliver at a range of operating temperatures, up to 400 °C, highly sensitive and highly selective vector signals to different, but chemically akin, alcohol vapors (methanol, ethanol, isopropanol, and n-butanol) as examples at low ppm concentrations when mixed with air. The suggested approach provides us a promising way to achieve cost-effective and well-performed electronic olfaction devices matured from the diverse chemiresistive responses of the printed nanocrystalline oxides.

Integrating Plasmonic Supercrystals in Microfluidics for Ultrasensitive, Label-Free, and Selective Surface-Enhanced Raman Spectroscopy Detection

Surface-enhanced Raman spectroscopy (SERS) microfluidic chips for label-free and ultrasensitive detection are fabricated by integrating a plasmonic supercrystal within microfluidic channels. This plasmonic platform allows the uniform infiltration of the analytes within the supercrystal, reaching the so-called hot spots. Moreover, state-of-the-art simulations performed using large-scale supercrystal models demonstrate that the excellent SERS response is due to the hierarchical nanoparticle organization, the interparticle separation (IPS), and the presence of supercrystal defects. Proof-of-concept experiments confirm the outstanding performance of the microfluidic chips for the ultradetection of (bio)molecules with no metal affinity. In fact, a limit of detection (LOD) as low as 10^-19 M was reached for crystal violet. The SERS microfluidic chips show excellent sensitivity in the direct analysis of pyocyanin secreted by Pseudomonas aeruginosa grown in a liquid culture medium. Finally, the further integration of a silica-based column in the plasmonic microchip provides charge-selective SERS capabilities as demonstrated for a mixture of positively and negatively charged molecules.

Perylene diimides: will they flourish as reaction-based probes?

Perylene diimides (PDI) are a well-studied class of functional organic dyes, and in recent years, they have been accepted as promising scaffolds for the design of small molecule/polymer-based chromogenic and fluorogenic reaction-based-probes because of their strong absorption combined with high fluorescence quantum yield in organic solvents, low reduction potential, good electron-acceptor properties, and broad color range properties. Undoubtedly, the intrinsically poor solubility of PDI-based derivatives in water greatly hampers their exploitation as reaction-based probes; however, a vast array of functionalizations now offer design strategies that have resulted in >50% solubility of PDI derivatives in water. A chemodosimeter, wherein chemical transformation is achieved by specific reactions, affords naked-eye visibility, fast response time, sensitivity, ratiometric response, and low cost. The present review focuses on the progress of PDI-based chemodosimeters achieved so far since the inception of this member in the rylene diimide family. This comprehensive review may facilitate the development of more powerful chemodosimeters based on PDI for broad and exciting applications in the future.

One-Component Array Based on a Dansyl-Modified Polylysine: Generation of Differential Fluorescent Signatures for the Discrimination of Human Cells

A one-component array-based sensor consisting of a dansyl-modified polylysine (PLL-Dnc) is capable of discriminating the types and compositional ratios of human cells using varying buffer conditions. The PLL-Dnc sensor array, which affords turn-on fluorescence responses against analyte cells that depend on the pH value and the ionic strength, generates differential fluorescence signatures of cells and successfully discriminates eight types of human cell lines (2.0 × 10⁴ cells/mL) with 100% accuracy using multivariate analysis. The array also allows differentiation of the composition of the cell mixtures that contain cells with the same tissue origin but of different subtypes. The good discrimination ability and simple platform of our "one-component"-based array allows an easy and rapid sensing of cells without requiring any information on specific biomarkers.