Applications of silver nanoparticles capped with different functional groups as the matrix and affinity probes in surface-assisted laser desorption/ionization time-of-flight and atmospheric pressure matrix-assisted laser desorption/ionization ion trap mass spectrometry for rapid analysis of sulfur drugs and biothiols in human urine

Advances in LDI-MS Analysis: The Role of Chemical Vapor Deposition-Synthesized Silver Nanoparticles in Enhancing Detection of Low-Molecular-Weight Biomolecules

In this investigation, we detail the synthesis of silver nanoparticles (AgNPs) via a precise chemical vacuum deposition (CVD) methodology, aimed at augmenting the analytical performance of laser desorption/ionization mass spectrometry (LDI-MS) for the detection of low-molecular-weight analytes. Employing a precursor supply rate of 0.0014 mg/s facilitated the formation of uniformly dispersed AgNPs, characterized by SEM and AFM to have an average diameter of 33.5 ± 1.5 nm and a surface roughness (Ra) of 11.8 nm, indicative of their homogeneous coverage and spherical morphology. XPS and SEM-EDX analyses confirmed the metallic silver composition of the nanoparticles with Ag peak splitting, reflecting the successful synthesis of metallic Ag. Comparative analytical evaluation with traditional MALDI matrices revealed that AgNPs significantly reduce signal suppression, thereby enhancing the sensitivity and specificity of LDI-MS for low-molecular-weight compounds such as triglycerides, saccharides, amino acids, and carboxylic acids. Notably, the application of AgNPs demonstrated a superior linear response for triglyceride signals with regression coefficients surpassing 0.99, markedly outperforming conventional matrices. The study further extends into quantitative analysis through nanoparticle-based laser desorption/ionization (NALDI), where AgNPs exhibited enhanced ionization efficiency, characterized by substantially lower limits of detection (LOD) and quantification (LOQ) for tested standards. Particular attention was paid to lipids with a detailed examination of their fragmentation pathways. These results highlight the significant potential of AgNPs synthesized via CVD to transform the analytical detection and quantification of low-molecular-weight compounds using NALDI. This approach offers a promising avenue for expanding the scope of analytical applications in mass spectrometry and introducing innovative methodologies for enhanced precision and sensitivity.

Nanoparticle-based applications by atmospheric pressure matrix assisted desorption/ionization mass spectrometry

Recently, the development of atmospheric pressure matrix assisted desorption/ionization mass spectrometry (AP MALDI MS) has made contributions not only to biomolecule analysis but also to spatial distribution. This has positioned AP MALDI as a powerful tool in multiple domains, thanks to its comprehensive advantages compared to conventional MALDI MS. These developments have addressed challenges associated with previous AP MALDI analysis systems, such as optimization of apparatus settings, synthesis of novel matrices, preconcentration and isolation strategies before analysis. Herein, applications in different fields using AP MALDI MS were described, including peptide and protein analysis, metabolite analysis, pharmaceutical analysis, and mass spectrometry imaging.

Trends in monoliths: Packings, stationary phases and nanoparticles

Monoliths media are gaining interest as excellent substitutes to conventional particle-packed columns. Monolithic columns show higher permeability and lower flow resistance than conventional liquid chromatography columns, providing high-throughput performance, resolution and separation in short run times. Monolithic columns with longer length, smaller inner diameter and specific selectivity to peptides or enantiomers have been played important role in hyphenated system. Monolithic stationary phases possess great efficiency, resolution, selectivity and sensitivity in the separation of complex biological samples, such as the complex mixtures of peptides for proteome analysis. The development of monolithic stationary phases has opened the new avenue in chromatographic separation science and is in turn playing much more important roles in the wide application area. Monolithic stationary phases have been widely used in fast and high efficiency one- and multi-dimensional separation systems, miniaturized devices, and hyphenated system coupled with mass spectrometers. The developing technology for preparation of monolithic stationary phases is revolutionizing the column technology for the separation of complex biological samples. These techniques using porous monoliths offer several advantages, including miniaturization and on-line coupling with analytical instruments. Additionally, monoliths are ideal support media for imprinting template-specific sites, resulting in the so-called molecularly-imprinted monoliths, with ultra-high selectivity. In this review, the origin of the concept, the differences between their characteristics and those of traditional packings, their advantages and drawbacks, theory of separations, the methods for the monoliths preparation of different forms, nanoparticle monoliths and metal-organic framework are discussed. Two application areas of monolithic metal-organic framework and nanoparticle monoliths are provided. The review article discusses the results reported in a total of 218 references. Other older references were included to illustrate the historical development of monoliths, both in preparation and types, as well as separation mechanism.

Gold-activated luminol chemiluminescence for the selective determination of cysteine over homocysteine and glutathione

This work reports a chemiluminescence assay for the highly selective determination of cysteine in biological fluids without separation techniques. The method is based on the ability of cysteine to selectively enhance the metal-catalyzed chemiluminescence generated by the oxidation of luminol from gold tetrachloride anions under alkaline conditions. The selectivity of the method stems from the fact that, under strongly alkaline conditions, the formation of the four-membered ring transition state of cysteine is less favorable as compared to the formation of the respective 5- and 9- membered ring transition states of homocysteine and glutathione, respectively. These transition states exert stronger hindrance and hydrophobic interactions repelling the negatively charged luminol dianion and possibly exhibit lower reducing ability for dissolved oxygen, towards the formation of superoxide radicals, thus reducing the oxidation of luminol. Under the optimum experimental conditions, the linear range of the method extended from 0.5 to 20 μΜ while cysteine could be determined at concentrations as low as 0.5 μM, with good reproducibility (<3.5%) and recoveries between 80 and 93% in artificial and real biological fluids.

Pd-Fe3O4 Janus nanozyme with rational design for ultrasensitive colorimetric detection of biothiols

Although nanozyme-based colorimetric assays have been broadly used for biosensing, some limitations such as low catalytic activity of nanozyme, poor sensitivity to analytes and lack of understanding the structure-activity relationship remain unsolved. In this work, we developed an ultrasensitive colorimetric method for biothiols detection based on density functional theory-assisted design of janus Pd-Fe₃O₄ nanozyme. The Pd-Fe₃O₄ dumbbell-like nanoparticles (DBNPs) prepared by seed-mediated approach shows a uniform heterodimeric nanostructure. Ultrasensitive biothiols detection is achieved from two aspects. On one hand, due to the synergistic effect between Pd and Fe₃O₄ in the dumbbell structure, Pd-Fe₃O₄ DBNPs show enhanced peroxidase-mimic activity compared to the individual components. On the other hand, when the target biothiols molecule is present, its inhibition effect on the janus Pd-Fe₃O₄ nanozyme is also significantly enhanced. The above results are confirmed both in experiment and theoretical calculation. Based on the rational design, a simple, highly selective and urtrasensitive colorimetric and quantitative assay for biothiols is developed. The limit of detection (LOD) can reach as low as 3.1 nM in aqueous solution. This assay is also successfully applied to the detection of biothiols in real urine samples. Moreover, the Pd-Fe₃O₄ nanozyme is used to discriminate biothiols levels in normal and cancer cells with high sensitivity at the cell density of 15,000/mL, which demonstrates its great potential in biological and clinical analysis. This work not only shows the great promise of janus bimetallic nanozymes' excellent functionalities but also provides rational guidelines to design high-performance nanozymes for biosensing and biomedical applications.

Inorganic Matrices Assisted Laser Desorption/Ionization Mass Spectrometry for Metabolic Analysis in Bio-fluids

Metabolic analysis in bio-fluids interprets the end products in the bio-process, emerging as an irreplaceable disease diagnosis and monitoring platform. Laser desorption/ionization mass spectrometry (LDI MS) based metabolic analysis exhibits great potential for clinical applications in terms of high throughput, rapid signal readout, and minimal sample preparation. There are two essential elements to construct the LDI MS-based metabolic analysis: 1) well-designed nanomaterials as matrices; 2) machine learning algorithms for data analysis. This review highlights the development of various inorganic matrices to comprehend the advantages of LDI MS in metabolite detection and the recent diagnostic applications based on target metabolite detection and untargeted metabolic fingerprints in biological fluids.

Application and Perspectives of MALDI-TOF Mass Spectrometry in Clinical Microbiology Laboratories

Early diagnosis of severe infections requires of a rapid and reliable diagnosis to initiate appropriate treatment, while avoiding unnecessary antimicrobial use and reducing associated morbidities and healthcare costs. It is a fact that conventional methods usually require more than 24–48 h to culture and profile bacterial species. Mass spectrometry (MS) is an analytical technique that has emerged as a powerful tool in clinical microbiology for identifying peptides and proteins, which makes it a promising tool for microbial identification. Matrix assisted laser desorption ionization–time of flight MS (MALDI–TOF MS) offers a cost- and time-effective alternative to conventional methods, such as bacterial culture and even 16S rRNA gene sequencing, for identifying viruses, bacteria and fungi and detecting virulence factors and mechanisms of resistance. This review provides an overview of the potential applications and perspectives of MS in clinical microbiology laboratories and proposes its use as a first-line method for microbial identification and diagnosis.

3D paper-based microfluidic device: a novel dual-detection platform of bisphenol A

A novel platform of 3D paper-based microfluidic device (μPADs) was fabricated by a digital plotter for high precision analysis of bisphenol A using electrochemistry along with LDI-MS detection.

Size-selected silver nanoparticles for MALDI-TOF mass spectrometry of amyloid-beta peptides

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is one of the most efficient mass spectrometric techniques for the analysis of high-molecular-weight compounds with superior selectivity and sensitivity. Common MALDI matrices are low molecular weight (LMW) organics and will therefore produce a large amount of matrix-related ion peaks, which limits the use of MALDI-MS for the detection of LMW molecules. A major breakthrough of this limitation was made by the introduction of surface assisted desorption/ionization techniques, with graphite particles firstly as the matrix, followed by expansion into other types of nanoparticles or nanostructures. However, previous studies failed to address well the optimum size and concentration of Ag NPs to be used as the MALDI matrix. In this study, to explore and compare the efficiency of different sized silver nanoparticles (Ag NPs) as the MALDI matrix for the detection of LMW molecules, three different sized Ag NPs (2.8 ± 1.0, 12.8 ± 3.2 and 44.2 ± 5.0 nm) have been successfully developed as the MALDI time-of-flight MS (MALDI-TOF MS) matrix and amyloid-beta (Aβ) peptides, crucially involved in Alzheimer's disease and a variety of cancers, were chosen as an example of LMW molecules in our MALDI-TOF MS analysis with Ag NPs as matrices. The results showed size-selected MS signals with the smallest (2.8 ± 1.0 nm) Ag NP matrix producing the highest spectral intensities, when compared with other larger sized Ag NP matrices and conventional matrices such as SA and DHB. Furthermore, the optimal concentrations for different sized Ag NPs as matrices were determined as follows: 0.125 nM (2.8 ± 1.0 nm Ag NPs), 0.0625 nM (12.8 ± 3.2 nm Ag NPs), and 0.03125 nM (44.2 ± 5.0 nm Ag NPs), respectively. These results not only corroborated that Ag NPs could act as a very suitable matrix to assist in the desorption/ionization of LMW molecules but also revealed size-selected mass spectrometry signals with smaller Ag NPs as the MALDI matrix bearing more advantages than their larger counterparts. These novel findings paved the way for wider applications of MALDI-MS using Ag NPs as matrices for the analysis of LMW molecules.

Synthesis, Purification, Characterization, and Imaging of Cy3-Functionalized Fluorescent Silver Nanoparticles in 2D and 3D Tumor Models

Silver nanoparticles (AgNPs) have a high affinity for sulfhydryl (thiol) groups, which can be exploited for functionalization with various tracking and targeting moieties. Here, we describe how to reliably and reproducibly functionalize AgNPs with the fluorescent moiety cyanine3-polyethelyne glycol (5000 molecular weight)-thiol (Cy3-PEG₅₀₀₀-SH). We also demonstrate how to purify and characterize Cy3-functionalized AgNPs (Cy3-AgNPs). Additionally, we describe how these Cy3-AgNPs can be imaged in 2D and 3D tumor models, providing insight into cellular localization and diffusion through a tumor spheroid, respectively.

Overlapping MALDI-Mass Spectrometry Imaging for In-Parallel MS and MS/MS Data Acquisition without Sacrificing Spatial Resolution

Metabolomics experiments require chemical identifications, often through MS/MS analysis. In mass spectrometry imaging (MSI), this necessitates running several serial tissue sections or using a multiplex data acquisition method. We have previously developed a multiplex MSI method to obtain MS and MS/MS data in a single experiment to acquire more chemical information in less data acquisition time. In this method, each raster step is composed of several spiral steps and each spiral step is used for a separate scan event (e.g., MS or MS/MS). One main limitation of this method is the loss of spatial resolution as the number of spiral steps increases, limiting its applicability for high-spatial resolution MSI. In this work, we demonstrate multiplex MS imaging is possible without sacrificing spatial resolution by the use of overlapping spiral steps, instead of spatially separated spiral steps as used in the previous work. Significant amounts of matrix and analytes are still left after multiple spectral acquisitions, especially with nanoparticle matrices, so that high quality MS and MS/MS data can be obtained on virtually the same tissue spot. This method was then applied to visualize metabolites and acquire their MS/MS spectra in maize leaf cross-sections at 10 μm spatial resolution. Graphical Abstract ᅟ.

Highly oriented Langmuir-Blodgett film of silver cuboctahedra as an effective matrix-free sample plate for surface-assisted laser desorption/ionization mass spectrometry

The design of a homogeneous sample plate to solve the sweet heating spot issues is the key step to expand the applicability of surface-assisted laser desorption/ionization mass spectrometry (SALDI MS). Herein, large-scale and highly oriented Langmuir-Blodgett (LB) films of uniform silver nanocrystals have been fabricated as a highly efficient and matrix-free sample plate for SALDI MS. Three individual silver nanocrystals (cubes, cuboctahedra and octahedra) assembled LB films have been applied as the sample plates for glucose detection by SALDI MS without an additional matrix. The results show that the signal intensity, background noise, signal-to-noise ratio and reproducibility have been significantly improved using LB films as the sample plate in comparison with commercial matrixes of CHCA and DHB. In particular, a relative signal of 5.7% was obtained for LB films of silver cuboctahedra. The significant improvement in the SALDI MS measurement could be attributed to the homogenous dissipation of laser irradiation energy to create a large area of the sweet heating spot on well-oriented silver cuboctahedra-based LB film. This ready-to-use sample plate has the potential for widespread commercial applications in SALDI MS.

Laser Desorption/Ionization Mass Spectrometric Imaging of Endogenous Lipids from Rat Brain Tissue Implanted with Silver Nanoparticles

Mass spectrometry imaging (MSI) of tissue implanted with silver nanoparticulate (AgNP) matrix generates reproducible imaging of lipids in rodent models of disease and injury. Gas-phase production and acceleration of size-selected 8 nm AgNP is followed by controlled ion beam rastering and soft landing implantation of 500 eV AgNP into tissue. Focused 337 nm laser desorption produces high quality images for most lipid classes in rat brain tissue (in positive mode: galactoceramides, diacylglycerols, ceramides, phosphatidylcholines, cholesteryl ester, and cholesterol, and in negative ion mode: phosphatidylethanolamides, sulfatides, phosphatidylinositol, and sphingomyelins). Image reproducibility in serial sections of brain tissue is achieved within <10% tolerance by selecting argentated instead of alkali cationized ions. The imaging of brain tissues spotted with pure standards was used to demonstrate that Ag cationized ceramide and diacylglycerol ions are from intact, endogenous species. In contrast, almost all Ag cationized fatty acid ions are a result of fragmentations of numerous lipid types having the fatty acid as a subunit. Almost no argentated intact fatty acid ions come from the pure fatty acid standard on tissue. Graphical Abstract ᅟ.

Mechanisms of Nanophase-Induced Desorption in LDI-MS. A Short Review

Nanomaterials are frequently used in laser desorption ionization mass spectrometry (LDI-MS) as DI enhancers, providing excellent figures of merit for the analysis of low molecular weight organic molecules. In recent years, literature on this topic has benefited from several studies assessing the fundamental aspects of the ion desorption efficiency and the internal energy transfer, in the case of model analytes. Several different parameters have been investigated, including the intrinsic chemical and physical properties of the nanophase (chemical composition, thermal conductivity, photo-absorption efficiency, specific heat capacity, phase transition point, explosion threshold, etc.), along with morphological parameters such as the nanophase size, shape, and interparticle distance. Other aspects, such as the composition, roughness and defects of the substrate supporting the LDI-active nanophases, the nanophase binding affinity towards the target analyte, the role of water molecules, have been taken into account as well. Readers interested in nanoparticle based LDI-MS sub-techniques (SALDI-, SELDI-, NALDI- MS) will find here a concise overview of the recent findings in the specialized field of fundamental and mechanistic studies, shading light on the desorption ionization phenomena responsible of the outperforming MS data offered by these techniques.

Revisiting the quantitative features of surface-assisted laser desorption/ionization mass spectrometric analysis

Surface-assisted laser desorption/ionization (SALDI) coupled with mass spectrometry (MS) is frequently used to analyse small organics owing to its clean background. Inorganic materials can be used as energy absorbers and the transfer medium to facilitate the desorption/ionization of analytes; thus, they are used as SALDI-assisting materials. Many studies have demonstrated the usefulness of SALDI-MS in quantitative analysis of small organics. However, some characteristics occurring in SALDI-MS require certain attention to ensure the reliability of the quantitative analysis results. The appearance of a coffee-ring effect in SALDI sample preparation is the primary factor that can affect quantitative SALDI-MS analysis results. However, to the best of our knowledge, there are no reports relating to quantitative SALDI-MS analysis that discuss or consider this effect. In this study, the coffee-ring effect is discussed using nanoparticles and nanostructured substrates as SALDI-assisting materials to show how this effect influences SALDI-MS analysis results. Potential solutions for overcoming the existing problems are also suggested.This article is part of the themed issue 'Quantitative mass spectrometry'.

Silver nanoparticle functionalized glass fibers for combined surface-enhanced Raman scattering spectroscopy (SERS)/surface-assisted laser desorption/ionization (SALDI) mass spectrometry via plasmonic/thermal hot spots

Closely-packed silver nanoparticles with a size of 20–50 nm and an inter-particle nanoscale gap of less than 10 nm were effective for a simultaneously enhanced SERS/SALDI substrate via plasmonic/thermal “hot spots”.

Silver nanostructures in laser desorption/ionization mass spectrometry and mass spectrometry imaging

Silver nanoparticles have been successfully applied as a matrix replacement for the laser desorption/ionization time-of-flight mass spectrometry (LDI-ToF-MS).

Determination of aerosol oxidative activity using silver nanoparticle aggregation on paper-based analytical devices

Airborne particulate matter (PM) pollution significantly impacts human health, but the cellular mechanisms of PM-induced toxicity remain poorly understood. A leading hypothesis on the effects of inhaled PM involves the generation of cellular oxidative stress. To investigate PM-induced oxidative stress, analytical methods have been developed to study the chemical oxidation of dithiothreitol (DTT) in the presence of PM. Although DTT readily reacts with several forms of reactive oxygen species, this molecule is not endogenously produced in biological systems. Glutathione (GSH), on the other hand, is an endogenous antioxidant that is produced throughout the body and is directly involved in combating oxidative stress in the lungs and other tissues. We report here a new method for measuring aerosol oxidative activity that uses silver nanoparticle (AgNP) aggregation coupled to glutathione (GSH) oxidation in a paper-based analytical device. In this assay, the residual reduced GSH from the oxidation of reduced GSH to its disulfide induces the aggregation of AgNPs on a paper-based analytical device, which produces a reddish-brown product. Two methods for aerosol oxidative reactivity are presented: one based on change in color intensity using a traditional paper-based techniques and one based on the length of the color product formed using a distance-based device. These methods were validated against traditional spectroscopic assays for DTT and GSH that employ Elman's reagent. No significant difference was found between the levels measured by all three GSH methods (our two paper-based devices and the traditional method) at the 95% confidence level. PM reactivity towards GSH was less than towards DTT most likely due to the difference in the oxidation potential between the two molecules.

Silver Oxide Based Nanoparticle Assisted Laser Desorption/Ionization Mass Spectrometry for the Detection of Low Molecular Weight Compounds

A specific property of silver oxide-based nanoparticles permits the ionization of an analyte, giving rise to various applications of a smart analytical methodology. The nanoparticles (d=6.7 nm) contained an Ag2O core. The detection of several model componds (a nucleobase and two hair growth promoters) via the use of silver oxide nanoparticles is described. Adducts were produced between the target molecules and the two silver stable isotopes (Ag(107) and Ag(109)), resulting in the formation of specific signals as well as a protonated signal. Thus, it was possible to easily determine whether the given signals were correlated with the target molecule or not.

A lithium-rich composite metal oxide used as a SALDI-MS matrix for the determination of small biomolecules

A lithium-rich composite metal oxide material used as a SALDI matrix for high throughput analysis of small molecules.

Combined use of irreversible binding and MRM technology for low- and ultralow copy-number protein detection and quantitation

In this paper, we present a method for the determination of low- and ultralow copy-number proteins in biomaterials based on a combination of concentrating the protein from the sample onto cyanogen bromide-activated Sepharose 4B (via nonspecific binding of free amino groups) and MRM. The detection limit and the dependence of the MRM peak areas on the concentration of protein in the sample were determined using the proteins CYP102 and BSA, as a model system, both in solution and after their addition to human plasma. Nonspecific protein enrichment of proteins from diluted sample volumes of 10-50 mL was found to increase the range of linear dependence of the chromatographic peak area on concentration by more than three orders of magnitude, allowing a lower LOD limit (LLOD) of as low as 10(-18) M. At this LLOD, at least two tryptic peptides of CYP102 and BSA could be detected with S/N of ≥7.0. The results were equally good for samples containing pure protein mixtures and proteins spiked into diluted depleted human blood plasma.

Human serum albumin-modified Fe3O4 magnetic nanoparticles for affinity-SALDI-MS of small-molecule drugs in biological liquids

Here, we report on the use of human serum albumin (HSA)-modified Fe(3)O(4) nanoparticles (NPs) (HSA-Fe(3)O(4) NPs) for affinity-SALDI-MS of small drugs in human biological liquids. We demonstrated that HSA-Fe(3)O(4) NPs effectively captured small drugs from human urine and serum via the interactions between HSA and these drugs. The drugs adsorbed on HSA could then be identified by directly introducing the HSA-Fe(3)O(4) NPs into a mass spectrometer for SALDI-MS analysis. The ability of HSA to interact with multiple small drugs facilitated the simultaneous detection of a 4-drug-mixture in serum, viz., phenytoin, ibuprofen, camptothecin, and warfarin sodium, by affinity-SALDI-MS using HSA-Fe(3)O(4) NPs. In contrast, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) with an organic matrix could detect only warfarin sodium. We also demonstrated the capacity of affinity-SALDI-MS to quantify warfarin sodium in urine samples across a range of 50 - 1000 μM (R(2) = 0.998) when using HSA-Fe(3)O(4) NPs. The detection sensitivity was further improved to a range of 5 - 100 μM (R(2) = 0.999) by using denatured HSA. The open structure of denatured HSA may enhance the effective extraction of small drugs from biological liquids, and increase the detection-sensitivity of affinity-SALDI-MS. Affinity-SALDI-MS using protein-modified Fe(3)O(4) NPs can open up new approaches to the analytical detection of small drugs in biological liquids by SALDI-MS.

The application of semiconductor quantum dots for enhancing peptide desorption, improving peak resolution and sensitivity of detection in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry

The interest in quantum dots (QD) and a number of reported life sciences applications increased dramatically over the last decade. The popularity of QDs stems from better photostability, higher extinction -co-efficients, and unique optical properties such as superior light absorption. Here we report methods for improving matrix-assisted laser desorption/ionization desorption of crude tryptic protein digests by using CdSe/ZnS QDs. The addition of QDs to the matrix improves the signal-to-noise ratio, peak quality and increases the number of detected peptides and the overall sequence coverage.

Nanoparticle-based microextraction techniques in bioanalysis

Nanoparticles (NPs) have attracted a great deal of attention in the last decade due to their exceptional mechanical, optical and electronic properties. This article deals with the use of NPs as probes for the extraction of biomolecules from biological samples. In this context, NPs present some advantages compared with conventional sorbents. Their high surface-to-volume ratio, easy synthetic (especially for inorganic NPs) and derivatization procedures, and their biocompatibility make them a powerful alternative. In order to provide a systematic approach to the topic, NPs have been divided into two general groups attending to their chemical nature. Carbon-based (e.g., fullerene and nanotubes) and inorganic NPs (e.g., gold and magnetic NPs) are considered in depth, explaining their main properties and applications. After these critical considerations, the most important conclusions and essential trends in this field are also outlined.

3-Mercaptopropionic acid modified ZnSe quantum dots as the matrix for direct surface-assisted laser desorption/ionization mass spectrometric analysis of peptides/proteins from sodium salt solution

This study describes a strategy of using zinc selenium quantum dots (ZnSe QDs) modified with 3-mercaptopropionic acid (3-MPA) as the matrix for direct analysis of peptides and proteins from sodium salt solution in surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). The enhancement of detection sensitivity for these biomolecules was due to the adsorption of positively charged peptides or proteins onto the surfaces of negatively charged ZnSe-3MPA QDs via electrostatic interactions resulting in an increase in ionization efficiency for sodium adduct ions ([M+Na](+)). The applicability of the current approach was demonstrated for a variety of peptides, including leucine-enkephalin, methione-enkephalin, HW6, substance P and angiotensin II, and proteins (cytochrome c, myoglobin and lysozyme). Signal intensities of these peptides or proteins can be enhanced by 25-95 times compared with those obtained by LDI-MS in the absence of ZnSe-3MPA QDs. Applying ZnSe-3MPA QDs to serve as the matrix in SALDI-MS is a simple and effective approach for direct analysis of peptide and protein molecules from sodium salt solution without any pretreatment as the peptides and proteins can be successfully detected as sodium adduct ions ([M+Na](+)).

Quantitative determination of nicotinic acid in micro liter volume of urine sample by drop-to-drop solvent microextraction coupled to matrix assisted laser desorption/ionization mass spectrometry

Drop-to-drop solvent microextraction (DDSME) coupled with matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) for quantitative determination of nicotinic acid in one drop of urine sample has been proposed. All parameters, such as type of organic solvent, extraction time, exposure volume solvent, pH of the sample solution that affecting the separation and preconcentration of nicotinic acid were investigated. Under the optimal conditions, the detection limit of the method was 20 ng mL(-1) and the relative standard deviations (RSD) for determination of the nicotinic acid were in the range of 8.0-12.5%. The calculated calibration curves gave linearity in the range of 80-1000 ng mL(-1). The main advantages of the proposed method are simple, fast, and small amount of sample solution is used for separation and preconcentration of nicotinic acid. This method could be also useful for the analysis of other interested analytes in small volume of biological samples, like plasma, saliva and urine, where the availability of samples are limited.

Synthesis, characterization and application of modified Pd nanoparticles as preconcentration probes for selective enrichment/analysis of proteins via hydrophobic interactions from real-world samples using nanoparticle-liquid-liquid microextraction coupled to matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

We introduce a novel preconcentrating technique by using surface modification of palladium nanoparticles (Pd-NPs) with octadecane thiol (ODT) prepared in toluene for selective and sensitive extraction of proteins (insulin, ubiquitin, lysozyme) from a variety of real-world samples including pancreas, mushroom, soybean and milk using nanoparticle-liquid-liquid microextraction (NP-LLME) coupled to matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-MS). The limit of detection (LOD) values obtained for gramicidin D and insulin in water and urine are between 17-37 nM (17-37 fmol) (with RSDs ranging from 5.3-7.2%) which are 10-20-fold enhancement in detection sensitivity compared with conventional MALDI-MS. The optimal sample pH for highest extraction efficiency of insulin, ubiquitin and lysozyme from biological samples was observed at sample pH ∼ pI which could be due to the enhancement of hydrophobic interactions between proteins with the hydrophobic ligands of Pd-ODT NPs. In addition, we also found that with the addition of 1 M NaCl, signals could be significantly enhanced by using the current approach. It is an efficient, straightforward, sensitive and selective nanoprobe which can be widely applied for separation, enrichment and preconcentration of peptides or proteins from complex biological samples in proteome research.