Magnetic Mesoporous Silica Microspheres Research Articles

Nanoengineering of Core-Shell Magnetic Mesoporous Microspheres with Tunable Surface Roughness.

Functional core-shell mesoporous microspheres with integrated functions, controlled structure, and surface properties and morphologies have received increasing attention due to their excellent physicochemical properties. Herein, core-shell magnetic mesoporous materials with cauliflower-like morphology and tunable surface roughness have been synthesized through a kinetics-controlled interface co-assembly and deposition of mesostructured nanocomposites on Fe3O4@RF microspheres (RF refers to resorcinol formaldehyde resin). The obtained microspheres, synthesized via this interface nanoengineering method, possess well-defined sandwich structure with a tunable rough morphology, uniform size (560-1000 nm), perpendicularly aligned mesopores (∼5.7 nm) in the outer shell, RF-protected magnetic responsive core, high surface area up to 382 m2/g, and large pore volume of 0.66 cm3/g. As a result of the unique surface features and magnetic properties, these microspheres exhibit excellent performance in stabilizing and oxygen-free manipulating aqueous solutions in petroleum ether by a magnetic field. They also exhibit superior cell uptake properties compared with traditional smooth core-shell magnetic mesoporous silica microspheres, opening up the possible applications in fast drug delivery in cancer therapy.

Rational synthesis of hierarchical magnetic mesoporous silica microspheres with tunable mesochannels for enhanced enzyme immobilization.

Hierarchical magnetic mesoporous silica microspheres (MMSMs) with a core-shell structure have been fabricated through an improved water/oil biphase synthesis strategy. We firstly reported that the mesopore size can be readily tuned from 6.1 to 11.4 nm by the synergistic effect of surfactant concentration and an amphiphilic agent, thus holding a bright future in many possible applications.

Synthesis of core–shell structured magnetic mesoporous silica microspheres with accessible carboxyl functionalized surfaces and radially oriented large mesopores as adsorbents for the removal of heavy metal ions

Core–shell structured mesoporous silica with accessible carboxyl functionalized surfaces and radial oriented large mesopores was fabricated as a recyclable adsorbent for the adsorption of Cd(ii), Cu(ii), and Pb(ii).

Preparation of phenyl-functionalized magnetic mesoporous silica microspheres for the fast separation and selective enrichment of phenyl-containing peptides.

Peptide enrichment before mass spectrometry analysis is essential for large-scale peptidomic studies, but challenges still remain. Herein, magnetic mesoporous silica microspheres with phenyl group modified interior pore walls were prepared by a facile sol-gel coating strategy, and were successfully applied for selective enrichment of phenyl-containing peptides in complex biological samples. The newly prepared nanomaterials possessed abundant silanol groups in the exterior surface and numerous phenyl groups in the interior pore walls, as well as a large surface area (592.6 m2 /g), large pore volume (0.33 cm3 /g), uniform mesopores (3.8 nm), strong magnetic response (29.3 emu/g), and good dispersibility in aqueous solution. As a result of the unique structural properties and size-exclusion effect, the core-shell phenyl-functionalized magnetic mesoporous silica microspheres exhibited excellent performance in fast separation and selective enrichment of phenyl-containing peptides, and the adsorption capacity for bradykinin reached 22.55 mg/g. In addition, selective enrichment of phenyl-containing peptides from complex samples that are consist of peptides, large proteins, and human serum were achieved by using the as-prepared microspheres, followed by high-performance liquid chromatography withultravioletdetection and electrospray ionization quadrupole time-of-flight mass spectrometry analysis. These results demonstrated the as-prepared microspheres would be a potential candidate for endogenous phenyl-containing peptides enrichment and biomarkers discovery in peptidome analysis.

An Interface Coassembly in Biliquid Phase: Toward Core-Shell Magnetic Mesoporous Silica Microspheres with Tunable Pore Size.

Core-shell magnetic mesoporous silica microspheres (Magn-MSMs) with tunable large mesopores in the shell are highly desired in biocatalysis, magnetic bioseparation, and enrichment. In this study, a shearing assisted interface coassembly in n-hexane/water biliquid systems is developed to synthesize uniform Magn-MSMs with magnetic core and mesoporous silica shell for an efficient size-selective biocatalysis. The synthesis features the rational control over the electrostatic interaction among cationic surfactant molecules, silicate oligomers, and Fe3O4@RF microspheres (RF: resorcinol formaldehyde) in the presence of shearing-regulated solubilization of n-hexane in surfactant micelles. Through this multicomponent interface coassembly, surfactant-silica mesostructured composite has been uniformly deposited on the Fe3O4@RF microspheres, and core-shell Magn-MSMs are obtained after removing the surfactant and n-hexane. The obtained Magn-MSMs possess excellent water dispersibility, uniform diameter (600 nm), large and tunable perpendicular mesopores (5.0-9.0 nm), high surface area (498-623 m(2)/g), large pore volume (0.91-0.98 cm(3)/g), and high magnetization (34.5-37.1 emu/g). By utilization of their large and open mesopores, Magn-MSMs with a pore size of about 9.0 nm have been demonstrated to be able to immobilize a large bioenzyme (trypsin with size of 4.0 nm) with a high loading capacity of ∼97 μg/mg via chemically binding. Magn-MSMs with immobilized trypsin exhibit an excellent convenient and size selective enzymolysis of low molecular proteins in the mixture of proteins of different sizes and a good recycling performance by using the magnetic separability of the microspheres.

Hydrophilic gallic acid–imprinted polymers over magnetic mesoporous silica microspheres with excellent molecular recognition ability in aqueous fruit juices

Hydrophilic molecularly imprinted polymers (MIPs) for gallic acid (GA) were prepared with excellent recognition ability in an aqueous solution. The proposed MIPs were designed by self-polymerization of dopamine (DA) on magnetic mesoporous silica (Fe3O4@SiO2@mSiO2, MMS) using GA as template. Resulting Fe3O4@SiO2@mSiO2@MIPs (MMS–MIPs) were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), thermo-gravimetric analysis (TGA), Brunauer–Emmett–Teller (BET), vibrating sample magnetometer (VSM), and evaluated by adsorption isotherms/kinetics and competitive adsorption. The adsorption behavior between GA and MMS–MIPs followed Langmuir and Sips adsorption isotherms with a maximum adsorption capacity at 88.7mg/g and pseudo-second-order reaction kinetics with fast binding (equilibrium time at 100min). In addition, MMS–MIPs showed rapid magnetic separation (10s) and stability (retained 95.2% after six cycles). Subsequently, MMS–MIPs were applied for the selective extraction and determination of GA from grape, apple, peach and orange juices (4.02, 3.91, 5.97, and 0.67μg/g, respectively). Generally, the described method may pave the way towards rationally designing more advanced hydrophilic MIPs.

Novel molecular imprinted polymers over magnetic mesoporous silica microspheres for selective and efficient determination of protocatechuic acid in Syzygium aromaticum

Improving sites accessibility can increase the binding efficiency of molecular imprinted polymers (MIPs). In this work, we firstly synthesized MIPs over magnetic mesoporous silica microspheres (Fe3O4@mSiO2@MIPs) for the selective recognition of protocatechuic acid (PCA). The resulting Fe3O4@mSiO2@MIPs were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectrometer (FT-IR), thermo-gravimetric analysis (TGA), Brunauer–Emmett–Teller (BET), and vibration sample magnetometer (VSM), and evaluated by adsorption isotherms/kinetics and competitive adsorption. The maximum adsorption capacity of PCA on Fe3O4@mSiO2@MIPs was 17.2mg/g (2.3 times that on Fe3O4@SiO2@MIPs). In addition, Fe3O4@mSiO2@MIPs showed a short equilibrium time (140min), rapid magnetic separation (5s) and high stability (retained 94.4% after six cycles). Subsequently, Fe3O4@mSiO2@MIPs were successfully applied for the selective and efficient determination of PCA (29.3μg/g) from Syzygium aromaticum. Conclusively, we combined three advantages into Fe3O4@mSiO2@MIPs, namely, Fe3O4 core for quick separation, mSiO2 layer for enough accessible sites, and surface imprinting MIPs for fast binding and excellent selectivity, to extract PCA from complex systems.

A mixed-function-grafted magnetic mesoporous hollow silica microsphere immobilized lipase strategy for ultrafast transesterification in a solvent-free system

A novel magnetic mesoporous hollow silica microspheres immobilized lipase is described for ultrafast transesterification of phytosterol with fatty acids and triglycerides in a solvent-free system.

Rational synthesis of superparamagnetic core–shell structured mesoporous microspheres with large pore sizes

Core–shell structured porous materials combining the functionalities of the core and shell have great application potential in various fields. Here, we report the synthesis of superparamagnetic core–shell structured microspheres which possess a core of nonporous silica-protected magnetite particle and an outer shell of ordered mesoporous silica with large pores. The synthesis adopts a co-surfactant templating approach under acidic conditions, with triblock-copolymer Pluronic P123 as a primary surfactant and a trace amount of cationic surfactant CTAB as an assistant template. The obtained magnetic mesoporous silica microspheres have uniform large pore size (4.5 nm), tuneable shell thickness (15–50 nm), high specific surface area (190–250 m2 g−1), large pore volume (0.17–0.38 cm3 g−1) and high magnetization (29.3 emu g−1). By using the obtained microspheres as an advanced magnetic absorbent, a fast, convenient, and efficient removal of large-size toxic microcystins in water solution was achieved in 60 seconds. The superparamagnetic properties and unique nanostructure enable the core–shell microspheres to be not only a novel absorbent for large size molecules, but also an ideal carrier for nanocatalysts like noble metallic nanoparticles and tools for peptide purification.

Sequential enrichment with titania-coated magnetic mesoporous hollow silica microspheres and zirconium arsenate-modified magnetic nanoparticles for the study of phosphoproteome of HL60 cells

As one of the most important types of post-translational modifications, reversible phosphorylation of proteins plays crucial roles in a large number of biological processes. However, owing to the relatively low abundance and dynamic nature of phosphorylation and the presence of the unphosphorylated peptides in large excess, phosphopeptide enrichment is indispensable in large-scale phosphoproteomic analysis. Metal oxides including titanium dioxide have become prominent affinity materials to enrich phosphopeptides prior to their analysis using liquid chromatography-mass spectrometry (LC-MS). In the current study, we established a novel strategy, which encompassed strong cation exchange chromatography, sequential enrichment of phosphopeptides using titania-coated magnetic mesoporous hollow silica microspheres (TiO2/MHMSS) and zirconium arsenate-modified magnetic nanoparticles (ZrAs–Fe3O4@SiO2), and LC-MS/MS analysis, for the proteome-wide identification of phosphosites of proteins in HL60 cells. In total, we were able to identify 11,579 unique phosphorylation sites in 3432 unique proteins. Additionally, our results suggested that TiO2/MHMSS and ZrAs–Fe3O4@SiO2 are complementary in phosphopeptide enrichment, where the two types of materials displayed preferential binding of peptides carrying multiple and single phosphorylation sites, respectively.

Amidoxime-functionalized magnetic mesoporous silica for selective sorption of U(vi)

Amidoxime-functionalized magnetic mesoporous silica (MMS-AO) microspheres were synthesized through co-condensation of tetraethyl orthosilicate (TEOS) and 2-cyanoethyltriethoxysilane (CTES) on the surface of silica-coated Fe3O4 followed by chemical modification of nitrile into amidoxime. The synthesized microspheres exhibit a typical sandwich structure with an inner core of Fe3O4, a middle layer of nonporous silica and an outer layer of amidoxime-functionalized mesoporous silica. Owing to the mesoporous structure and amidoxime functionalization, sorption of U(VI) by MMS-AO reaches equilibrium in 2 h of contact time with a maximum sorption capacity of 1.165 mmol g−1 (277.3 mg g−1) at pH = 5.0 ± 0.1 and T = 298 K, which is much higher than the results previously reported for other magnetic materials. The sorption process is strongly dependent on pH but independent of ionic strength, indicating that the predominant sorption mechanism is inner-sphere surface complexation. The selectivity of MMS-AO for U(VI) is remarkably improved in comparison with that of magnetic mesoporous silica without amidoxime functionalization. U(VI)-loaded MMS-AO can be conveniently separated from aqueous solutions with an external magnetic field and efficiently regenerated using 1 mol L−1 HCl with only a small decrease in U(VI) sorption capacity. These results suggest that MMS-AO shows promise as a future candidate for selective separation of U(VI) from aqueous solutions in possible real applications.

Synthesis and drug-loading properties of folic acid-modified superparamagnetic Fe3O4 hollow microsphere core/mesoporous SiO2 shell composite particles

A drug delivery system, which not only has superparamagnetic property, higher surface area but also has targeting function, has been developed. The core/shell structural magnetic magnetite mesoporous silica microspheres with amine groups (Fe3O4–SiO2–NH2) were first fabricated by a one-pot direct co-condensation method, then folic acid-modified magnetic mesoporous silica composite microspheres (Fe3O4–SiO2–NHFA) were obtained by the bonding of the Fe3O4–SiO2–NH2 with folic acid as targeted molecule. The resultant composite microspheres were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, low temperature nitrogen adsorption–desorption, and vibrating sample magnetometer. A well-known inflammational drug ibuprofen was used as a model drug to assess the loading and releasing behavior of the composite microspheres. Fe3O4–SiO2–NHFA system exhibits magnetic properties typical for superparamagnetic material with a higher saturation magnetization value of about 41.2 emu/g and has better capacity of drug storage (32.0 %) and sustained drug-release property. So this system has potential applications in biomedical field.

Synergistic Removal of Pb(II), Cd(II) and Humic Acid by Fe3O4@Mesoporous Silica-Graphene Oxide Composites

The synergistic adsorption of heavy metal ions and humic acid can be very challenging. This is largely because of their competitive adsorption onto most adsorbent materials. Hierarchically structured composites containing polyethylenimine-modified magnetic mesoporous silica and graphene oxide (MMSP-GO) were here prepared to address this. Magnetic mesoporous silica microspheres were synthesized and functionalized with PEI molecules, providing many amine groups for chemical conjugation with the carboxyl groups on GO sheets and enhanced the affinity between the pollutants and the mesoporous silica. The features of the composites were characterized using TEM, SEM, TGA, DLS, and VSM measurements. Series adsorption results proved that this system was suitable for simultaneous and efficient removal of heavy metal ions and humic acid using MMSP-GO composites as adsorbents. The maximum adsorption capacities of MMSP-GO for Pb(II) and Cd (II) were 333 and 167 mg g−1 caculated by Langmuir model, respectively. HA enhances adsorption of heavy metals by MMSP-GO composites due to their interactions in aqueous solutions. The underlying mechanism of synergistic adsorption of heavy metal ions and humic acid were discussed. MMSP-GO composites have shown promise for use as adsorbents in the simultaneous removal of heavy metals and humic acid in wastewater treatment processes.

Preparation of phenyl group-functionalized magnetic mesoporous silica microspheres for fast extraction and analysis of acetaldehyde in mainstream cigarette smoke by gas chromatography–mass spectrometry

Acetaldehyde is regarded as a toxic mainstream cigarette smoke constituent, and measurement of acetaldehyde in complex real samples is difficult owing to its high volatility and reactivity. In this work, phenyl group-functionalized magnetic mesoporous microspheres were developed as the solid-phase extraction sorbents for enrichment and analysis of acetaldehyde in mainstream cigarette smoke. The functional magnetic microspheres were first synthesized through a facile one-pot co-condensation approach. The prepared nanomaterials possessed abundant silanol groups in the exterior surface and numerous phenyl groups in the interior pore-walls, as well as a large surface area (273.5m2/g), strong superparamagnetism and uniform mesopores (3.3nm). Acetaldehyde in mainstream cigarette smoke was collected in water and derivatizated with O-2,3,4,5,6-(pentafluorobenzyl)hydroxylamine. The formed acetaldehyde oximes were extracted and enriched by the prepared adsorbents via π–π interactions and subsequently analyzed using GC–MS. Extraction conditions such as amounts of sorbents, eluting solvent, adsorption and desorption time were investigated and optimized to achieve the best efficiency. Method validations including linearity, recovery, repeatability, and limit of detection were also studied. It was found that the suggested methodology provided low detection limit of 0.04mg/mL, good recovery of 88–92%, intra-day and inter-day RSD values of 4.5% and 10.1%, and linear range of 0.25–4mg/mL (R2=0.999). The results indicated that the proposed method based on phenyl-functionalized magnetic mesoporous microspheres was rapid, efficient and convenient for the enrichment and analysis of acetaldehyde in tobacco.

Preparation and characterization of novel immobilized Fe3O4@SiO2@mSiO2–Pd(0) catalyst with large pore-size mesoporous for Suzuki coupling reaction

In this paper, core–shell magnetic mesoporous Fe3O4@SiO2@mSiO2 microspheres with double-shell structure were synthesized by two-step coating processes of silica on inorganic magnetic core (Fe3O4). The dense inner shell can effectively protect the magnetic core and enhance the stability of the magnetic core to some extent. And the porous outer shell can provide high specific surface area and increase the amounts of palladium loaded on the surface of the supports. An immobilized palladium nanoparticles catalyst (Fe3O4@SiO2@mSiO2–Pd(0)) was obtained after coordination and reduction of Pd2+ on the amine-modified core–shell magnetic mesoporous Fe3O4@SiO2@mSiO2 microspheres. Activities of this kind of catalyst were characterized by catalyzing a series of Suzuki coupling reactions between aromatic halides and phenylboronic acid. Research showed that the average particle diameter, BET specific surface area, pore volume and pore size of the obtained core–shell magnetic mesoporous Fe3O4@SiO2@mSiO2 microspheres is 420nm, 202.98m2/g, 0.2419cm3/g and ~20nm, respectively. Comparing to immobilized palladium nanoparticles catalyst on traditional magnetic mesoporous silica microspheres, palladium nanoparticles can not only be observed on the outer surface but also in the mesopore channels of the right kind of magnetic mesoporous microspheres. This is very conducive to the improvement of the catalytic activities and cycle stability, and the reactive yield of 4-chloroacetophenone reached almost up to 93.77%. The reusability of the supported catalyst was also investigated in catalyzing the Suzuki coupling reaction between iodobenzene and phenylboronic acid and between chlorobenzene and phenylboronic acid, respectively. The results show that the supported catalyst can be conveniently recovered by applying an external magnetic field and the activity of the supported catalyst for catalyzing the Suzuki coupling reaction of iodobenzene and chlorobenzene can still reach up to over 90% and 70% respectively after being reused at least six times.

Phosphotungstic acid anchored to amino–functionalized core–shell magnetic mesoporous silica microspheres: A magnetically recoverable nanocomposite with enhanced photocatalytic activity

H3PW12O40 was successfully anchored to the surface of amino-functionalized Fe3O4@SiO2@meso-SiO2 microspheres by means of chemical bonding to aminosilane groups, aiming to remove unwanted organic compounds from aqueous media. The resultant multifunctional microspheres were thoroughly characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, infrared spectroscopy, inductively coupled plasma, and N2 adsorption–desorption. The as-prepared microspheres possess unique properties including high magnetization (46.8emug−1), large BET surface area (135m2g−1), and highly open mesopores (∼5.0nm), and H3PW12O40 loading is calculated to be ∼16.8%; and as a result, the as-prepared microspheres exhibit enhanced performance in degrading dyes under UV irradiation compared with pure H3PW12O40. Additionally, the photocatalyst can be easily recycled using an external magnetic field without losing the photocatalytic activity.

Preparation of novel magnetic hollow mesoporous silica microspheres and their efficient adsorption

Magnetic hollow mesoporous silica microspheres (MHMSs) were successfully prepared by employing yolk-shell magnetic silica spheres as the template together with the coating of tetraethoxysilane (TEOS)/hexadecyl trimethoxysilane (C16TMS) hybrid. The microstructure and properties of MHMSs were studied by TEM, SEM, XRD, BET, and VSM characterizations. The average diameter of MHMSs was about 300nm and the mesoporous shell thickness was totally around 70nm. The 11nm magnetite nanoparticles were homogeneously embedded in the mesoporous silica shell. Meanwhile, the adsorption and separation process of Rhodamine B were carried out to demonstrate that the material could be used for the adsorbent.

Quantitative Mass Spectrometry Combined with Separation and Enrichment of Phosphopeptides by Titania Coated Magnetic Mesoporous Silica Microspheres for Screening of Protein Kinase Inhibitors

We describe herein the development of a matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry (MALDI-TOF-MS) approach for screening of protein kinase inhibitors (PKIs). MS quantification of phosphopeptides, the kinase-catalyzed products of nonphosphorylated substrates, is a great challenge due to the ion suppression effect of highly abundant nonphosphorylated peptides in enzymatic reaction mixtures. To address this issue, a novel type of titania coated magnetic hollow mesoporous silica spheres (TiO(2)/MHMSS) material was fabricated for capturing phosphopeptides from the enzymatic reaction mixtures prior to MS analysis. Under optimized conditions, even in the presence of 1000-fold of a substrate peptide of tyrosine kinase epidermal growth factor receptor (EGFR), the phosphorylated substrates at the femtomole level can be detected with high accuracy and reproducibility. With a synthetic nonisotopic labeled phosphopeptide, of which the sequence is similar to that of the phosphorylated substrate, as the internal standard, the MS signal ratio of the phosphorylated substrate to the standard is linearly correlated with the molar ratio of the two phosphopeptides in peptide mixtures over the range of 0.1 to 4 with r(2) being 0.99. The IC(50) values of three EGFR inhibitors synthesized in our laboratory were then determined, and the results are consistent with those determined by an enzyme-linked immunosorbent assay (ELISA). The developed method is sensitive, cost/time-effective, and operationally simple and does not require isotope/radioative-labeling, providing an ideal alterative for screening of PKIs as therapeutic agents.

Pseudomorphic synthesis of monodisperse magnetic mesoporous silica microspheres for selective enrichment of endogenous peptides

In this work, we describe a novel synthetic strategy of magnetic mesoporous silica spheres (Fe 3O 4@mSiO 2) for the selective enrichment of endogenous peptides. Fe 3O 4 particles were coated with silica shell by a sol–gel method, followed by pseudomorphic synthesis to transform nonporous silica shell into ordered mesoporous silica shell. The core/shell structure and mesostructure were individually fabricated in two steps, which can be expedient to independently optimize the properties of monodispersion, magnetization and mesostructure. Actually, it was confirmed that the produced Fe 3O 4@mSiO 2 particles possess good monodispersion, high magnetization, superparamagnetism, uniform accessible mesopores, and large surface area and pore volume. With these good properties, Fe 3O 4@mSiO 2 spheres were applied to the rapid enrichment of peptides. Based on the size-exclusion mechanism and hydrophobic interaction with siloxane bridge group mainly on the surface of inside pores, Fe 3O 4@mSiO 2 can selectively capture peptides and exclude high-MW proteins and salts. Furthermore, peptides in human plasma were successfully enriched by Fe 3O 4@mSiO 2.

Preparation of magnetic core‐mesoporous shell microspheres with C8‐modified interior pore‐walls and their application in selective enrichment and analysis of mouse brain peptidome

In this paper, magnetic mesoporous silica microspheres with C8-modified interior pore-walls were prepared through a facile one-pot sol-gel coating strategy, and were successfully applied for selective enrichment of endogenous peptides in mouse brain for peptidome analysis. Through the one-pot sol-gel approach with surfactant (CTAB) as a template, tetraethyl orthosilicate (TEOS) and n-ctyltriethoxysilane (C8TEOS) as the precursors, C8-modified magnetic mesoporous microspheres (C8-Fe(3)O(4)@mSiO(2)) consisting magnetic core and mesoporous silica shell with C8-groups exposed in the mesopore channels were synthesized. The obtained microspheres possess highly open mesopores of 3.4 nm, high surface area (162.5 m(2)/g), large pore volume (0.17 cm(3)/g), excellent magnetic responsivity (56.3 emu/g) and good dispersibility in aqueous solution. Based on the abundant surface silanol groups, functional C8 groups and the strong magnetic responsivity of the core-shell C8-Fe(3) O(4) @mSiO(2) microspheres, efficient and fast enrichment of peptides was achieved. Additionally, the C8-Fe(3)O(4)@mSiO(2) microspheres exhibit excellent performance in selective enrichment of endogenous peptides from complex samples that are consist of peptides, large proteins and other compounds, including human serum and mouse brain followed by automated nano-LC-ESI-MS/MS analysis. These results indicate C8-Fe(3)O(4)@mSiO(2) microspheres would be a potential candidate for endogenous peptides enrichment and biomarkers discovery in peptidome analysis.