Use of Nanoparticle Decorated Surface-Enhanced Raman Scattering Active Sol-Gel Substrates for SALDI-MS Analysis

Spectroscopy and mass spectrometry techniques are sometimes combined into the same analytical workflow to leverage each technique's analytical benefits. This combined workflow is especially useful in forensic and medical contexts where samples are often precious in nature. Here, we adopt metal nanoparticle (NP) doped sol-gel substrates, initially developed for surface-enhanced Raman scattering (SERS) analysis, as surface-assisted laser desorption/ionization-mass spectrometry (SALDI-MS) substrates. Using dried blood and sample protocols previously developed for SERS analysis, we observe heme-related spectral features on both silver and gold NP substrates by SALDI-MS, demonstrating dual functionality for these orthogonal techniques. Modifying the dried blood extraction procedures also allows for the observation of blood triacylglycerols by SALDI-MS. This is the first demonstration of a SERS/SALDI-MS substrate based on a sol-gel scaffold and the first demonstration of a gold NP sol-gel substrate for SALDI-MS which features lower substrate-related SALDI-MS background compared to the silver substrate.

A Polymer-Based Matrix for Effective SALDI Analysis of Lipids

Surface-assisted laser desorption/ionization (SALDI) has become an attractive branch of matrix-assisted laser desorption/ionization (MALDI) and has been successfully applied for the detection of small molecules due to the lack of the interference of matrix. Herein, the low-cost and highly accessible polyvinylidene fluoride (PVDF) was modified using a facile alkali treatment and investigated as a SALDI matrix. The modified PVDF has a strong optical absorption and can be applied as a dual-mode substrate for both SALDI MS and SALDI imaging analysis. Modified PVDF powder showed superior performance in SALDI MS analysis of lipids, with good reproducibility, high sensitivity, and low background interference, especially for triacylglycerols (TAGs) and fatty acids. Additionally, the lipids in raw and extracted serum were both successfully determined with modified PVDF powder. A modified PVDF membrane (m-PVDF-m) showed excellent ability in lipids imaging in tissues due to its flat surface, mass signal enhancement, and elimination of matrix coating. The distribution of several TAGs and cholesteryl esters on mouse kidney section was presented by SALDI imaging directly on m-PVDF-m. These results demonstrated that modified PVDF materials presented exciting opportunities as matrices for the first time in SALDI MS acquisition and SALDI imaging.

Ultrathin Homogenous AuNP Monolayers as Tunable Functional Substrates for Surface-Assisted Laser Desorption/Ionization of Small Biomolecules

A series of ultrathin, homogenous gold nanoparticle (AuNP) substrates for surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) were prepared using a simple air/water interface approach. These SALDI substrates enabled soft ionization and provided significant improvements in terms of signal intensities and reduced background levels in comparison to other AuNP morphologies for different analytes such as fatty acids, peptides, amino acids, saccharides, and drugs. Through different microscopic and spectroscopic methods, we determined that the packing homogeneity of the [AuNP]_n substrates played a vital role in the efficiency of the SALDI process. We demonstrated that the signal intensities of the investigated analytes were readily optimized by manipulating the thickness of the [AuNP]_n substrates. The desorption/ionization efficiency increased as a function of the number of layers and then reached a saturation point. The optimized [AuNP]_n substrates not only exhibited high SALDI-MS desorption/ionization efficiencies but also showed excellent reproducibilities of the analyte signals.

A spherical covalent-organic framework for enhancing laser desorption/ionization mass spectrometry for small molecule detection

A spherical vinyl-functionalized covalent-organic framework (COF-V) was prepared at room temperature by a facile method and applied as a novel substrate for surface-enhanced laser desorption/ionization mass spectrometry (SELDI-MS).

Comparison of Clusters Produced from Sb2Se3 Homemade Polycrystalline Material, Thin Films, and Commercial Polycrystalline Bulk Using Laser Desorption Ionization with Time of Flight Quadrupole Ion Trap Mass Spectrometry

This study compared Sb₂Se₃ material in the form of commercial polycrystalline bulk, sputtered thin film, and homemade polycrystalline material using laser desorption ionization (LDI) time of flight mass spectrometry with quadrupole ion trap mass spectrometry. It also analyzed the stoichiometry of the Sb_mSe_n clusters formed. The results showed that homemade Sb₂Se₃ bulk was more stable compared to thin film; its mass spectra showed the expected cluster formation. The use of materials for surface-assisted LDI (SALDI), i.e., graphene, graphene oxide, and C₆₀, significantly increased the mass spectra intensity. In total, 19 Sb_mSe_n clusters were observed. Six novel, high-mass clusters-Sb₄Se₄⁺, Sb₅Se_3-6⁺, and Sb₇Se₄⁺-were observed for the first time when using paraffin as a protective agent.

Graphene-Coated Glass Substrate for Continuous Wave Laser Desorption and Atmospheric Pressure Mass Spectrometric Imaging of a Live Hippocampal Tissue

The atmospheric pressure mass spectrometric (AP-MS) imaging technology combined with an inverted optical microscopic system is a powerful tool for determining the presence and spatial distributions of specific biomolecules of interest in live tissues. Efficient desorption and ionization are essential to acquire mass spectrometric (MS) information in an ambient environment. In this study, we demonstrate a new and efficient desorption process using a graphene-coated glass substrate and a continuous wave (CW) laser for high-resolution AP-MS imaging of a live hippocampal tissue. We found that desorption of biomolecules in a live tissue slice was possible with the aid of a graphene-coated glass substrate and indirect application of a 532 nm CW laser on the graphene substrate. Interestingly, the desorption efficiency of a live tissue on the graphene-coated substrate was strongly dependent on the number of graphene layers. Single-layer graphene was found to be the most sensitive substrate for efficient desorption and reproducible high-resolution hippocampal tissue imaging applications. The subsequent ionization process using nonthermal plasma generated sufficient amounts of molecular ions to obtain high-resolution two-dimensional MS images of the cornu ammonis and the dentate gyrus regions of the hippocampus. Therefore, graphene-coated substrates could be a promising platform to induce an efficient desorption process essential for highly reproducible ambient MS imaging.

Enriching analyte molecules on tips of superhydrophobic gold nanocones for trace detection with SALDI-MS

The sensitivity of surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) analysis depends on the efficiency of desorption and ionization of analyte molecules, which is usually limited by the low utilization efficiency of laser energy. Herein we demonstrate an efficient method to increase energy utilization efficiency for improving the efficiency of desorption and ionization of analyte molecules in SALDI-MS analysis. To increase the utilization efficiency of energy, a superhydrophobic gold film covered silicon nanocone array is fabricated and used as SALDI substrate. The nanocone array increases the absorption up to 99.65% at the wavelength of 355 nm, which is applied for SALDI-MS detection. The superhydrophobicity promotes the analyte molecules concentrated on the tips of nanocones where photon energy is confined, therefore, more energy can be provided for desorption and ionization of analytes. The energy efficiency is increased by using this substrate. The sensitivity of SALDI-MS analysis is greatly improved. For example, 100 amol/μL of rhodamine 6G, 100 fmol/μL of polyethyleneglycol, 100 ymol/μL of glutathione and 100 ymol/μL arginine still can be analyzed. The lake water containing malachite green was used as the real sample. The regression equation (Log I = 0.39 Log C + 6.58, R² = 0.9811) was obtained when the concentration of analyte was in the range from 10^-4 mol/L to 10^-8 mol/L. Therefore, the calculated LOD and LOQ are 1.35 × 10^-14 mol/L and 1.35 × 10^-7 mol/L, respectively. In addition, the lower relative standard deviation (0.7%, n = 10), proper recovery (113% and 91%), and low matrix effect (-1.1% and -1.1%) all demonstrate the great potential of the designed substrate in practical analysis.

Osteoporosis risk assessment using multilayered gold-nanoparticle thin film via SALDI-MS measurement

A powerful technique to detect bone biomarkers has been developed for assessment of osteoporosis at the early stage. Two-dimensional multilayered gold-nanoparticle thin film (MTF-AuNPs) was demonstrated as a promising test platform for detection of bone biomarker, hydroxyproline (HYP), measured by surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). With strong surface plasmon resonance and excellent homogeneity, facilely prepared, highly ordered, and large-scale MTF-AuNPs revealed high sensitivity of HYP in the SALDI-MS measurement without additional matrixes, such as α-cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB). Furthermore, the mass spectrum of HYP with MTF-AuNPs was significantly improved in signal intensity enhancement, background noise reduction, and signal-to-noise ratio amplification. The excellent reproducibility of HYP spectra with only 9.3% relative signal variation could be attributed to MTF-AuNPs' high absorbance at a wavelength of 337 nm, low heat capacity, superior thermal conductivity, and outstanding homogeneity. The calibration curve showed high linear correlation between mass spectrum intensity and HYP concentration in the range of 1 to 100 μM, covering the whole level in healthy people and osteoporosis patients. In particular, the serum sample was directly deposited onto the MTF-AuNP sample substrate without any pretreatment and its HYP concentration was then successfully determined. We believe that the combination of SALDI-MS and MTF-AuNP sample substrates would be a potential approach for bone biomarker detection in the osteoporosis risk assessment. Graphical abstract.

Negative ion laser desorption/ionization time-of-flight mass spectrometric analysis of small molecules by using nanostructured substrate as matrices

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is an excellent analytical technique for rapid and sensitive analysis of macromolecules such as polymers and proteins. However, the main drawback of MALDI-TOF MS is its difficulty to detect small molecules with mass below 700 Da because of the intensive interference from MALDI matrix in the low mass region. In recent years there has been considerable interest in developing matrix-free laser desorption/ionization by using nanostructured substrates to substitute the conventional organic matrices, which is often referred as surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS). Despite these attractive features, most of the current SALDI-TOF MS for the analysis of small molecules employ positive ion mode, which is subjected to produce multiple alkali metal adducts, and thus increases the complexity of the analysis. Different from the complicated adducts produced in positive ion mode, mass spectra obtained in negative ion mode are featured by deprotonated ion peaks without matrix interference, which simplifies the interpretation of mass spectra and detection of unknown. In this review, we critically survey recent advances in nanostructured substrates for negative ion LDI-TOF MS analysis of small molecules in the last 5 years. Special emphasis is placed on the preparation of the nanostructured substrates and the results achieved in negative ion SALDI-MS. In addition, a variety of promising applications including environmental, biological, and clinical analysis are introduced. The ionization mechanism of negative ionization is briefly discussed.

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.

Nanomaterials as Assisted Matrix of Laser Desorption/Ionization Time-of-Flight Mass Spectrometry for the Analysis of Small Molecules

Matrix-assisted laser desorption/ionization (MALDI), a soft ionization method, coupling with time-of-flight mass spectrometry (TOF MS) has become an indispensible tool for analyzing macromolecules, such as peptides, proteins, nucleic acids and polymers. However, the application of MALDI for the analysis of small molecules (<700 Da) has become the great challenge because of the interference from the conventional matrix in low mass region. To overcome this drawback, more attention has been paid to explore interference-free methods in the past decade. The technique of applying nanomaterials as matrix of laser desorption/ionization (LDI), also called nanomaterial-assisted laser desorption/ionization (nanomaterial-assisted LDI), has attracted considerable attention in the analysis of low-molecular weight compounds in TOF MS. This review mainly summarized the applications of different types of nanomaterials including carbon-based, metal-based and metal-organic frameworks as assisted matrices for LDI in the analysis of small biological molecules, environmental pollutants and other low-molecular weight compounds.