Current Analytical Chemistry - Volume 18, Issue 2, 2022

Background: Fluorescence-based sensing is considered highly sensitive and fluorescent probes with improved properties are always preferred. Fluorescent carbon dots (CDs) are newly emerging quasi-spherical nanoparticles of less than 10 nm in size and belong to the carbon nanomaterial’s family. CDs have great potential as fluorescent probes and currently are under open discussion by the researchers due to their striking properties such as low environmental hazard, high selectivity, greater sensitivity, good biocompatibility, tunable fluorescent properties and excitation dependent multicolor emission behavior. Introduction: This review demonstrates various available methods for fabrication of fluorescent CDs, capping of CDs and characterization with various techniques, including UV-visible, FT-IR, and TEM. Analytical applications using CDs for the sensing of small organic molecules, specifically nitroaromatic compounds in the environmental samples, are complied. Methods: The review covers literature related to synthesis and characterization of carbon dots. It includes around 171 research articles in this field. Results: Carbon dots can be synthesized using numerous routes. In all cases, CDs possess spectral properties with little variation in wavelength maxima. The optical properties of CDs can be tuned by compositing these with metallic quantum dots or by modifying their surface with desired functionalities. HR-TEM is needed to see the morphology and size of particles, whereas UV-Visible and FTIR are indispensable tools for this kind of research. These particles are successfully applied to sense small molecules in some matrices. Conclusion: Carbon dots are bright stars in fluorescent sensing of small molecules. However, more research is needed to determine small organic molecules in diversified areas of analysis.

Background: Chemosensor compounds are useful for sensitive, selective detection of cations and anions with fluorophore groups in an attempt to develop sufficient selectivity of the sensors. Although familiar fluorescent sensors utilize inter-molecular interactions with the cations and anions, an extraordinary endeavor was executed in the preparation of fluorescent-based sensor compounds. 4,4-difluoro-4- bora-3a,4a-diaza-s-indacene (BODIPY) and its derivatives were first used as an agent in the imaging of biomolecules due to their interesting structures, complexation, and fluorogenic properties. Among the fluorescent chemosensors used for cations and anions, BODIPY-based probes stand out, owing to the excellent properties such as sharp emission profile, high stability, etc. In this review, we emphasize the BODIPY-based chemosensor compounds, which have been used to image cations and anions in living cells because of their biocompatibility and spectroscopic properties. Methods: Research and online contents related to chemosensor online activity are reviewed. The advances, sensing mechanisms and design strategies of the fluorophore, exploiting selective detection of some cations and anions with BODIPY-based chemosensors, are explained. It could be claimed that the use of BODIPY-based chemosensors is very important for cations and anions in bio-imaging applications. Results: Molecular sensors or chemosensors are molecules that show a change that can be detected when affected by the analyte. They are capable of producing a measurable signal when they are selective for a particular molecule. Molecular and ion recognition is important in biological systems such as enzymes, genes, environment, and chemical fields. Due to the toxic properties of many heavy metal ions, it is of great importance to identify these metals due to their harmful effects on living metabolism and the pollution they create in the environment. This process can be performed with analytical methods based on atomic absorption and emission. The fluorescence methods among chemosensor systems have many advantages such as sensitivity, selectivity, low price, simplicity of using the instrument and direct determination in solutions. The fluorescence studies can be applied at nanomolar concentrations. Conclusion: During a few decades, a lot of BODIPY-based chemosensors for the detection of cations and anions have been investigated in bio-imaging applications. For the BODIPY-based fluorescent chemosensors, the BODIPY derivatives were prepared by different ligand groups for the illumination of the photophysical and photochemical properties. The synthesized BODIPYbased chemosensors have remarkable photophysical properties, such as a high quantum yield, strong molar absorption coefficient, etc. Moreover, these chemosensors were successfully implemented on living organisms for the detection of analytes.

Background: The optical properties of nanomaterials have evolved enormously with the introduction of nanotechnology. The property of materials to absorb and/or emit specific wavelength has turned them into one of the most favourite candidates to be effectively utilized in different sensing applications e.g, organic light emission diodes (OLEDs) sensors, gas sensors, biosensors, and fluorescent sensors. These materials have been reported as a sensor in the field of tissue and cell imaging, cancer detection, and detection of environmental contaminants, etc. Fluorescent nanomaterials are helping in rapid and timely detection of various contaminants that greatly impact the quality of life and food that is exposed to these contaminants. Later, all the contaminants have been investigated to be the most perilous entities that momentously affect the life span of the animals and humans who use those foods which have been contaminated. Objective: In this review, we will discuss various methods and approaches to synthesize the fluorescent nanoparticles and quantum dots (QDs) and their applications in various fields. The application will include the detection of various environmental contaminants and bio-medical applications. We will discuss the possible mode of action of the nanoparticles when used as a sensor for the environmental contaminants as well as the surface modification of some fluorescent nanomaterials with anti-body and enzyme for specific detection in the animal kingdom. We will also describe some RAMAN based sensors as well as some optical sensing-based nanosensors. Conclusion: Nanotechnology has enabled us to play with the size, shape and morphology of materials in the nanoscale. The physical, chemical and optical properties of materials change dramatically when they are reduced to the nanoscale. The optical properties can become choosy in terms of emission or absorption of wavelength in the size range and can result in the production of the very sensitive optical sensor. The results show that the use of fluorescent nanomaterials for sensing purposes is helping a great deal in the sensing field.

Background: The selection of capping agent depends on the method of synthesis, nature of nanoparticles (NPs), and type of the compounds to be analyzed. Therefore, different types of capping agents such as surfactants, drugs, amino acids, fatty acids, and polymers are used to increase stability of NPs, avoid aggregation, keep NPs away from one another, thereby achieving desired morphology as well as the size of NPs. Introduction: Recently, the fabrication of NPs has been extensively carried out using synthetic chemical routes in a wide range of materials. In this review, a comprehensive assessment of the colorimetric and fluorescent sensing of metal nanoparticles using different capped agents, such as surfactants, drugs, amino acids, fatty acids, and polymers has been summarized for the present and future strategies. Method: For the synthesis of metal nanoparticles, different methods, metals, and a variety of capping agents are used to obtain new properties and explore opportunities for innovative applications. Result: Capping agents perform their significant role as stabilizers to avoid the over-growth and coagulation of nanoparticles. Conclusion: Capping agents play an essential role in the colorimetric and fluorescent sensing of metal nanoparticles for particular analytes.

Aim: Selective and sensitive visual detection of Cu²⁺in aqueous solution at PPB level using an easily synthesized compound. Background: The search for a chemosensor that can detect Cu²⁺ is very long owing to the fact that an optimum level of Cu²⁺ is required for human health and the recommended amount of Cu²⁺ in drinking water is set to be 1-2 mgL-1. Thus, it is very important to detect Cu²⁺ even at a very low concentration to assess the associated health risks. Objective: We are still seeking the easiest, cheapest, fastest and greenest sensor that can selectively, sensitively and accurately detect Cu²⁺ with the lowest detection limit. Our objective of this work was to find one such Cu²⁺ sensor. Methods: We have synthesized a quinoline derivative following very easy synthetic procedures and characterized the compound by standard methods. For the sensing study, we used steady state absorption and emission spectroscopy. Results: Our sensor can detect Cu²⁺ selectively and sensitively in an aqueous solution instantaneously, even in the presence of an excess amount of other salts. The pale-yellow color of the sensor turns red on the addition of Cu²⁺. There is no interference from other cations and anions. A 2:1 binding mechanism of the ligand with Cu2+ is proposed using Jobs plot with binding constant in the order of 10⁹ M^-2. We calculated the LOD to be 18 ppb, which is quite low than what is permissible in drinking water. Conclusion: We developed a new quinoline based chemosensor following a straightforward synthetic procedure from very cheap starting materials that can detect Cu²⁺ visually and instantaneously in an aqueous solution with ppb level sensitivity and zero interference from other ions.

Background: The traditional methods for the detection and quantification of Cu²⁺ and Fe³⁺ heavy metal ions are usually troublesome in terms of high-cost, non-portable, time-consuming, specialized personnel and complicated tools, so their applications in practical analyses is limited. Therefore, the development of cheap, fast and simple-use techniques/instruments with high sensitivity/selectivity for the detection of heavy metal ions is highly demanded and studied. Methods: In this study, a fluorene-based fluorescent ''turn-off'' sensor, methyl 2-(2-((((9H-fluoren-9- yl)methoxy)carbonyl)amino)-3- phenylpropanamido) acetate (probe FLPG) was synthesized via onepot reaction and characterized by ¹H-NMR, ¹³C-APT-NMR, HETCOR, ATR-FTIR and elemental analysis in detailed. All emission spectral studies of the probe FLPG have been performed in CH3CN/HEPES (9/1, v/v, pH=7.4) media at rt. The quantum (φ) yield of probe FLPG decreased considerably in the presence of Cu²⁺ and Fe³⁺. The theoretical computation of probe FLPG and its complexes were also performed using density functional theory (DFT). Furthermore, bio-imaging experiments of the probe FLPG was successfully carried out for Cu²⁺ and Fe³⁺ monitoring in living-cells. Results: The probe FLPG could sense Cu²⁺ and Fe³⁺ with high selectivity and sensitivity, and quantitative correlations (R2>0.9000) between the Cu²⁺/Fe³⁺ concentrations (0.0−10.0 equiv). The limits of detection for Cu²⁺ and Fe³⁺ were found as 25.07 nM and 37.80 nM, respectively. The fluorescence quenching in the sensor is managed by ligand-to-metal charge transfer (LMCT) mechanism. Job’s plot was used to determine the binding stoichiometry (1:2) of the probe FLPG towards Cu²⁺ and Fe³⁺. The binding constants with strongly interacting Cu²⁺ and Fe³⁺ were determined as 4.56×10⁸ M^-2 and 2.02×10³⁺ M^-2, respectively, via the fluorescence titration experiments. The outcomes of the computational study supported the fluorescence data. Moreover, the practical application of the probe FLPG was successfully performed for living cells. Conclusion: This simple chemosensor system offers a highly selective and sensitive sensing platform for the routine detection of Cu²⁺ and Fe³⁺, and it keeps away from the usage of costly and sophisticated analysis systems.

Background: Derivatization of analytes is a quite convenient practice from an analytical perspective. Its vast prevalence is accounted by the availability of distinct reagents, primarily pragmatic for obtaining desired modifications in an analyte structure. Another reason for its handiness is typically to overcome limitations such as lack of sensitive methodology or instrumentation. The past decades have witnessed various new derivatization techniques including in-situ, enzymatic, ultrasound-assisted, microwave-assisted, and photochemical derivatization, which have gained popularity recently. Methods: The online literature available on the utilization of derivatization as prominent analytical tool in recent years with typical advancements is reviewed. The illustrations of the analytical condition and the structures of different derivatizing reagents (DRs) are provided to acknowledge the vast capability of derivatization to resolve analytical problems. Results: The derivatization techniques have enabled analytical chemists throughout the globe to develop an enhanced sensitivity method with the simplest of the instrument like High-performance Liquid Chromatography (HPLC). The HPLC, compared to more sensitive Liquid chromatography coupled to a tandem mass spectrometer, is readily available and can be readily utilized for routine analysis in fields of pharmaceuticals, bioanalysis, food safety, and environmental contamination. A troublesome aspect of these fields is the presence of a complex matrix with trace concentrations for analyses. Liquid chromatographic methods devoid of MS detectors do not have the desired sensitivity for this. A possible solution for overcoming this is to couple HPLC with derivatization to enable the possibility of detecting trace analytes with a less expensive instrument. Running cost, enhanced sensitivity, low time consumption, and overcoming the inherent problems of analyte are critical parameters for which HPLC is quite useful in high throughput analysis. Conclusion: The review critically highlights various kinds of derivatization applications in different fields of analytical chemistry. The information primarily focuses on pharmaceutical and bioanalytical applications in recent years. The various modes, types, and derivatizing reagents with mechanisms have been ascribed briefly. Additionally, the importance of HPLC coupled to fluorescence and UV detection is presented as an overview through examples accompanied by their analytical conditions.

Background: Glucosinolates (GLS) are important secondary metabolites in Cruciferae vegetables and herbs. Currently, the assays of total GLS determination are cumbersome (requiring acidic or enzymatic hydrolysis and addition of staining reagents), time-consuming, and indirect. High concentrations of inorganic salts are inevitably incorporated into the GLS products during separation. There is a need for a quantitative method for simple and rapid determination of total GLS after desalting process. Methods: A 96-well plates-based UV spectrophotometric method for determination of total GLS of Isatis indigotica roots was developed in the present study. The detection wavelength is set at 230 nm using quartz plates. This assay was validated using gluconapin and sinigrin as reference standards, and applied to determine the total GLS of I. indigotica roots prepared from five different desalting methods. Results: This assay is specific for total GLS prepared from I. indigotica roots, and it has acceptable accuracy (91.76–98.18% for quality control, and 95.59–102.52% for addition/recovery), precision (0.24–0.70% pooled RSD), reproducibility (0.31–1.84% RSD), and stability (0.24–1.45% RSD) over a 72-h period. Conclusion: The 96-well plates-based UV spectrophotometric assay is simple and accurate for high-throughput determination of total GLS.