Catecholamines–dopamine, noradrenaline and adrenaline are important biomarkers of neurotransmitter metabolism, indicating neuroendocrine tumors and neurodegenerative diseases. Surface-enhanced Raman spectroscopy (SERS) is a promising analytical technique with unprecedented multiplexing capabilities. However, not all important analytes exhibit strong SERS signals on stable and robust nanostructured substrates. In this work, we propose a novel indicator system based on the formation of mixed ligand complexes with bispidine-based bis-azole ligands which can serve as pliers to trap Cu(II) ions and stabilize its complexes with catecholamines. Four synthesized ligands with different functional groups: carboxyl, amino, benzyl, and methoxybenzyl, were applied for forming stable complexes to shift maximum absorbance of catecholamines from the ultraviolet region to 570–600 nm. A new absorbance band in the visible range resonates with the local surface plasmon resonance (LSPR) band of metal nanoparticles and most used laser wavelengths. This match allowed use of Molecular Immobilization and Resonant Raman Amplification by Complex-Loaded Enhancers (MIRRACLE) methodology to measure intense Raman signals on a nanostructured silver-based SERS-active substrate. The synthesized plier-like ligands fixed and stabilized catecholamine complexes with Cu(II) on the SERS sensor surface, which facilitated the determination of dopamine in a 3.2 × 10−12–1 × 10−8 M concentration range.

One of the hallmarks of Alzheimer's disease (AD) pathogenesis is the production, aggregation, and deposition of amyloid-β (Aβ) peptide. Surface-enhanced Raman spectroscopy (SERS) is a promising analytical technique capable of providing valuable information on chemical composition and molecule conformations in biological samples. However, one of the main challenges for introducing the SERS technique into the practice is preparation of scalable and at the same time stable nanostructured sensors with uniform spatial distribution of nanoparticles. Herein, we propose SERS platforms for reproducible, sensitive, label-free quantification of amyloid-β aggregates for short-wavelength - 532 and 633 nm - lasers. A SERS sensor - based on silver nanoparticles immobilized into a chitosan film (AgNP/CS) - provided a uniform distribution of AgNPs from a colloidal suspension across the SERS sensor, resulting in nanomolar limits of detection (LODs) for Aβ42 aggregates with a portable 532 nm laser. The laser-induced deposition was used to obtain denser periodic plasmonic sensors (AgNP/LID) with a uniform nanoparticle distribution. The AgNP/LID SERS sensor allowed for 15 pM LOD for Aβ42 aggregates with 633 nm laser. Notably, both nanostructured substrates allowed to distinguish amyloid aggregates from monomers. Therefore, our approach demonstrated applicability of SERS for detection of macromolecular volumetric objects as amyloid-β aggregates for fundamental biological studies as well as for "point-of-care" diagnostics and screening for early stages of neurodegenerative diseases.

A unique approach based on Molecular Immobilization and Resonant Raman Amplification by Complex-Loaded Enhancers (MIRRACLE) on copper (II)–chitosan–modified SERS-active metallic nanostructured substrates is proposed for sensitive and rapid determination of the catecholamines (CA) dopamine, norepinephrine, and epinephrine. The ternary (CA)2Cu(4AAP)2 complexes were characterized by the appearance of new absorbance bands at 555, 600, and 500 nm for dopamine, norepinephrine, and epinephrine, respectively. The new absorbance band matched with a broad surface plasmon resonance band of utilized silver nanoparticles: 450–600 nm, and 633 excitation wavelength. We observed enhancement factors up to 3.6·106 due to the additional resonant enhancement. The multiplexing capabilities of quantitative spectral unmixing for Raman spectra of a group of CAs, which differ by only either hydroxy or methyl group, at the fingerprint region were successfully demonstrated with the direct classic least squares model. The achieved nM limits of detection with only 1.5 mW laser power and analysis of spiked human blood plasma samples proved the possibility of the multiplex determination of the catecholamines at the level of reference concentrations in the blood of healthy people as well as promise for the future facilitation in the precision diagnosis of neuroendocrine tumors and neurodegenerative diseases.

Toxic, carcinogenic, and mutagenic properties of polycyclic aromatic hydrocarbons (PAHs) and environmental pollution caused by polycyclic aromatic sulfur heterocycles (PASHs) postulate the importance of their selective and sensitive determination in environmental and oil fuel samples. Surface-enhanced Raman spectroscopy (SERS) opens up an avenue toward multiplex analysis of complex mixtures, however not every molecule gives high enhancement factors and, thus, cannot be reliably detected via SERS. However, the sensitivity can be drastically increased by additional resonant enhancement as a result of the analyte absorption band overlapping with the surface plasmon band of nanoparticles (NPs) and the laser excitation wavelength. Using this idea, we developed a dual-purpose SERS sensor based on trapping the target PAHs and PASHs into colored charge-transfer complexes (CTCs) with selected organic π-acceptor molecules on the surface of AgNPs. Studying, computing, and then comparing stability constants of the formed CTC served as a powerful explanation and prediction tool for a wise choice of π-acceptor indicator systems for the further silver surface modification. Moreover, we show that CTC formation can be effectively utilized for increasing both selectivity and sensitivity by simple liquid-liquid extraction prior to SERS measurements. For the first time, the dual-purpose SERS sensor allowed determination of two different classes of polycyclic aromatic fuel components down to 10 nM concentration, lower than that restricted by the ASTM regulation, and demonstrated multi-purpose capabilities of the developed approach.

Stoichiometry and stability of electron donor–acceptor complexes formed by dibenzothiophene and its alkylated derivatives with DDQ and TCNE in nonpolar organic solvents has been investigated. 4,6-Dimethyldibenzothiophene produces the most stable 1: 1 complex with DDQ in agreement with energy levels of the π-donors HOMOs and π-acceptors LUMOs calculated by DFT. The results make possible an employment of the complexes for multiplex determination of polycyclic aromatic pollutants using surface enhanced Raman spectroscopy.

The review summarizes authors' works of the last 15 year covering the possibilities, advantages, and prospects for the use of optical sensors based on peroxidase for the determination of a wide range of biologically active compounds, i.e., phenolic compounds and hydroperoxides of different structures, phenothiazines, catecholamines, and their metabolites, for the quality control of drugs, foodstuffs, biomedical research, and clinical diagnostics.

A sensitive, rapid, and simple fluorimetric procedure for the determination of artemisinin in a concentration range of 0.1–7 μM was developed with the use of microperoxidase-11 as a peroxidase biomimetic (RSD = 0.8% at LOQ, n = 5; LOD = 7.1 nM (3s0)). The determination is based on the fluorescence quenching of the cationic xanthene dye pyronin B (Stern–Volmer quenching constant, 0.101 μM–1) in the presence of microperoxidase-11. The procedure was tested in the analysis of a biologically active additive based on an Artemisia annua wormwood extract. The correctness of the results of the fluorimetric determination of artemisinin in a biologically active dietary supplement was confirmed by HPLC–mass spectrometry. The use of oligopeptide microperoxidase-11 instead of heme-containing proteins (hemoglobin, cytochrome c, and horseradish peroxidase) made it possible to shorten the duration of artemisinin determination by a factor of 2 with the retention of sensitivity and selectivity.

Solid-phase extraction of polycyclic aromatic sulfur heterocycles (PASHs) and their rapid determination in oil fuel without tedious sample pretreatment are of high interest. We propose porous and optically transparent hydrogels prepared from the covalently crosslinked chitosan (CS) as the basis for a sensor system for the rapid and robust monitoring of PASHs. We efficiently combined the ability of the crosslinked CS to sorb PASHs, the capacity of microcavities in a molecularly imprinted polymer to selectively recognize and trap analyzes, and the optical transparency of CS materials for selective sorption and solid-phase fluorometric determination of dibenzothiophenes. For the screening of PASHs in organic nonpolar media, ortho-phtalic dialdehyde appeared to be the most appropriate crosslinker. Synthetic and analytical procedures performed in microplate mode allowed obtaining CS hydrogels with suitable reproducible properties and their further time- and labor-efficient applying in analysis (particularly, as little as 2 mu M dibenzothiophene oxide can be determined).

A review of publications, mainly for the last 15 years, characterizing the advantages, limitations, and prospects for the development of modern methods and approaches to the determination of organic hydroperoxides and hydrogen peroxide, which are the most important markers of the oxidative stress level in living organisms.

A novel original biosensing system for the simultaneous multiplex determination of general markers of catecholamine-producing diseases - catecholamines (dopamine, epinephrine, norepinephrine) and their metabolites (homovanillic and vanillylmandelic acids) in biological liquids without preliminary separation of analytes, in the absence of specific antibodies and receptors and with minimum pretreatment of a samples has been developed. This outstanding approach includes the unique combination of obtaining highly fluorescent derivatives of the analytes as a result of their interaction with two different amines - benzylamine and 1,2-diphenylethylenediamine in the presence of peroxidase as a catalyst, with the application of first-order derivative fluorescence spectroscopy for the resolution of their spectra. Fluorescence is measured in 96-well microplates, which wells contain a bio-recognizing film consisted of horseradish peroxidase immobilized in the polymer chitosan. Spectra of the solutions are recorded in the range 400-500 nm (lambda(ex) similar to 305-356 nm). The proposed procedures provide sensitive (in the range of 3-200 nM), selective, and reproducible (RSDs <= 1%, n = 6) multiplex determination of the catecholamines and their metabolites in biological liquids were successfully applied for the rapid simultaneous (20 samples per 15-30 min) screening of human urine and mice blood plasma. The validated results showed good linearity, precision, accuracy and selectivity of this method. (C) 2018 Elsevier B.V. All rights reserved.

The review concerns the state of the art, the advances in and prospects for application of surface-enhanced Raman spectroscopy (SERS) in chemical analysis. Key advantages (nondestructive origin, high sensitivity and selectivity, easiness of sample preparation) and drawbacks (a relatively small number of analytes, insufficient metrological characteristics of certain objects) of the method are pointed out. Particular attention is paid to the development of novel chemical approaches and to the design of versatile optical sensors to significantly enhance the analytical potential of the method and to extend the range of analytes. Examples are given of the application of SERS in key branches of science and technology that require both qualitative and quantitative chemical analysis of complex objects, viz., environmental monitoring, fuel and energy area, medical diagnosis, biology and biochemistry. The bibliography includes 502 references.

In this work, the first use of a cellulose hydrogel film reconstituted from ionic liquid (IL) 1-butyl-3-methylimidazolium chloride for the fluorescent determination of a plant antimalarial endoperoxide artemisinin is reported. The sensing material was fabricated by noncovalent co-immobilization of the fluorescent cationic dye pyronin B and complex of Mn(II) with anionic surfactant sodium dodecyl sulfate into the film prepared by dissolution and regeneration of microcrystalline cellulose in ionic liquid. Artemisinin determination in a concentration range of 0.25-8 mu M is based on the dynamic quenching of pyronin B fluorescence (emission wavelength/excitation wavelength of 581/355nm) that is accelerated by the above-indicated complex. The developed disposable cellulose film exhibits a high sensitivity toward artemisinin (limit of detection of 30nM), sufficient selectivity for pharmaceutical analysis, a rapid response time (30s), and a strong stable fluorescent signal for over a month. The applicability of the cellulose film was demonstrated by analyzing a dietary supplement with an extract from the traditional Chinese herb Artemisia annua. The accuracy of the fluorescent determination of artemisinin in the dietary supplement was supported by a liquid chromatography-mass spectrometry technique. The developed fluorescent cellulose film is a biocompatible and easy to handle sensing material that makes it suitable for wide variety applications in pharmaceutical analysis.

New ternary complexes of europium with oxytetracycline possessing the intense fluorescence have been obtained for prospective applications in the high-sensitive detection of biogenic amines and their metabolites in biological fluids. Their stability constants have been determined using the Foster–Hammick–Wardley method based on the fluorescent spectroscopy data. The dependence between structure differences, stability constants of ternary complexes of europium–oxytetracycline with biogenic amines, and their metabolites, and sensitivity of their determination has been established.

This paper reviews the primary literature reporting the use of ionic liquids (ILs) in optical sensing technologies. The optical chemical sensors that have been developed with the assistance of ILs are classified according to the type of resultant material. Key aspects of applying ILs in such sensors are revealed and discussed. They include using ILs as solvents for the synthesis of sensor matrix materials; additives in polymer matrices; matrix materials; modifiers of the surfaces; and multifunctional sensor components. The operational principles, design, texture, and analytical characteristics of the offered sensors for determining CO2, O2, metal ions, CN-, and various organic compounds are critically discussed. The key advantages and disadvantages of using ILs in optical sensing technologies are defined. Finally, the applicability of the described materials for chemical analysis is evaluated, and possibilities for their further modernization are outlined.