Gold-decorated titania nanotube arrays as dual-functional platform for surface-enhanced Raman spectroscopy and surface-assisted laser desorption/ionization mass spectrometry

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.

Hierarchical Superhydrophobic Device to Concentrate and Precisely Localize Water-Soluble Analytes: A Route to Environmental Analysis

An efficient superhydrophobic concentrator is developed using a hierarchical superhydrophobic surface on which the evaporation of a sessile droplet (6 μL) drives the nonvolatile elements it contains on a predefined micrometric analytical surface (pedestal of 80 μm diameter). This hierarchical silicon surface exhibits a surface texture made of etched nanopillars and consists of micropillars and guiding lines, arranged in radial symmetry around the central pedestal. The guiding lines ensure the overall convergence of the sessile droplet toward the central pedestal during evaporation. The nanopillar texturing induced a delay in the Cassie-Baxter to Wenzel regime transition, until the edge of the droplet reaches the periphery of the pedestal. Experiments performed with polymer microparticles suspended in ultrapure water or with DNA molecules solubilized in ultrapure water at sub-fM concentrations demonstrated that the totality of the nonvolatile elements in the liquid microvolume is delivered on or close to the pedestal area, in a very reproducible manner. The very high concentration capacity of the device enabled the discrimination of the degree of purity of ultrapure water samples from different origins. The concentrator also turned out to be functional for raw water samples, opening possible applications to environmental analysis.

Surface-assisted laser desorption/ionization mass spectrometry imaging: A review

In the last decades, surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) has attracted increasing interest due to its unique capabilities, achievable through the nanostructured substrates used to promote the analyte desorption/ionization. While the most widely recognized asset of SALDI-MS is the untargeted analysis of small molecules, this technique also offers the possibility of targeted approaches. In particular, the implementation of SALDI-MS imaging (SALDI-MSI), which is the focus of this review, opens up new opportunities. After a brief discussion of the nomenclature and the fundamental mechanisms associated with this technique, which are still highly controversial, the analytical strategies to perform SALDI-MSI are extensively discussed. Emphasis is placed on the sample preparation but also on the selection of the nanosubstrate (in terms of chemical composition and morphology) as well as its functionalization possibilities for the selective analysis of specific compounds in targeted approaches. Subsequently, some selected applications of SALDI-MSI in various fields (i.e., biomedical, biological, environmental, and forensic) are presented. The strengths and the remaining limitations of SALDI-MSI are finally summarized in the conclusion and some perspectives of this technique, which has a bright future, are proposed in this section.

Development of efficient SALDI substrate based on Au-TiO2 nanohybrids for environmental and forensic detection of dyes and NSAIDs

Herein, a matrix-free approach is presented for comprehensive environmental and forensic analysis of dyes and nonsteroidal anti-inflammatory drugs (NSAIDs) using Au-TiO₂ nanohybrids coupled with surface-assisted pulsed laser desorption ionization-mass spectrometry (SALDI-MS). The Au-TiO₂ nanohybrids was prepared and characterized using inductively coupled plasma-optical emission spectrometry (ICP-OES), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), surface area measurements, ultraviolet-visible (UV-vis) spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS). Initially, the optimal Au content was assessed using the survival yield (SY) method, confirming that 7.5% Au content on the TiO₂ surface offered the highest ionization efficiency. Subsequently, environmental analyses of dyes and NSAIDs in water samples were performed, and sensitive detection of all analytes was achieved with limits of detection (LODs) ranging from 10.0 ng mL^-1 to 10.0 fg mL^-1 and good spot-to-spot reproducibility. Additionally, the effect of potential contaminants commonly found in environmental samples, such as salts, surfactants and pesticides was also considered. Despite signal intensity reduction at high concentrations of some salts, the target analytes were detected, while the presence of surfactants and pesticides did not cause significant signal intensity reduction. Additionally, dyed and undyed Tetoron fibers and the effect of adhesive tape were evaluated. Direct analysis of the dyed Tetoron fibers on the target plate, using the nanohybrids, enabled higher detection sensitivity of the dyes, in addition to adducts of polystyrene and cellulose, the main components of the fiber. Finally, NSAIDs in oral fluid were analyzed and sensitive detection of the analytes was observed using the nanohybrids with LODs and LOQs in the range of 0.1-10 ng mL^-1 and 1-20 ng mL^-1, respectively. The trueness of the exact mass was in the range of 0.64-6.2 ppm while the recovery of the spiked samples was in the range of 82.90-107.54%% indicating the efficiency of the Au-TiO₂ nanohybrids as SALDI substrate. Thus, the Au-TiO₂ nanohybrids hold considerable promise in terms of sensitivity, reproducibility, and LOD, and may significantly contribute to environmental and forensic identification.

Imaging lipids in biological samples with surface-assisted laser desorption/ionization mass spectrometry: A concise review of the last decade

Knowing the spatial location of the lipid species present in biological samples is of paramount importance for the elucidation of pathological and physiological processes. In this context, mass spectrometry imaging (MSI) has emerged as a powerful technology allowing the visualization of the spatial distributions of biomolecules, including lipids, in complex biological samples. Among the different ionization methods available, the emerging surface-assisted laser desorption/ionization (SALDI) MSI offers unique capabilities for the study of lipids. This review describes the specific advantages of SALDI-MSI for lipid analysis, including the ability to perform analyses in both ionization modes with the same nanosubstrate, the detection of lipids characterized by low ionization efficiency in MALDI-MS, and the possibilities of surface modification to improve the detection of lipids. The complementarity of SALDI and MALDI-MSI is also discussed. Finally, this review presents data processing strategies applied in SALDI-MSI of lipids, as well as examples of applications of SALDI-MSI in biomedical lipidomics.

Nanocomposite-Based Matrices in Laser Desorption/Ionization Mass Spectrometry for Small-Molecule Analysis

The efficient detection of small molecules is of significance for environmental monitoring, pharmacology, metabolomics, and lipidomics. The laser desorption/ionization mass spectrometry (LDI MS) platform enables high sensitivity, accuracy, resolution, and throughput in molecular analysis, but its analytical capability with respect to small molecules is limited due to inherent drawbacks arising from conventional organic matrices. The selection of an appropriate matrix is thus a precondition for small molecule detection by LDI MS. To date, various inorganic matrices have been developed, with a growing interest in composite materials displaying synergetic effects. This Minireview highlights the development of nanocomposites as LDI matrices driven by numerous innovations in material science, and their emerging use in small-molecule analysis.

FeOOH@Metal-Organic Framework Core-Satellite Nanocomposites for the Serum Metabolic Fingerprinting of Gynecological Cancers

High-throughput metabolic analysis is of significance in diagnostics, while tedious sample pretreatment has largely hindered its clinic application. Herein, we designed FeOOH@ZIF-8 composites with enhanced ionization efficiency and size-exclusion effect for laser desorption/ionization mass spectrometry (LDI-MS)-based metabolic diagnosis of gynecological cancers. The FeOOH@ZIF-8-assisted LDI-MS achieved rapid, sensitive, and selective metabolic fingerprints of the native serum without any enrichment or purification. Further analysis of extracted serum metabolic fingerprints successfully discriminated patients with gynecological cancers (GCs) from healthy controls and also differentiated three major subtypes of GCs. Given the low cost, high-throughput, and easy operation, our approach brings a new dimension to disease analysis and classification.

SERS monitoring of the Fenton degradation reaction based on microfluidic droplets and alginate microparticles

Surface-enhanced Raman scattering (SERS) spectroscopy as a powerful tool has been used to explore different catalysis degradation reactions, whereas some drawbacks caused by ferric ions still exist in the current SERS monitoring of the Fenton reaction process. In this work, microfluidic droplet- and alginate microparticle-based methods were, respectively, applied to realize SERS monitoring of the Fenton degradation process in a relatively stable environment, which benefited from reduction of the loss of ferrous ions and the aggregation of the SERS substrate. As expected, the spectroscopic evidence at the molecular level directly revealed the degradation mechanism of rhodamine dyes, showing that the chemical bonds between xanthene and carboxybenzene broke continuously during the reaction. Afterward, the degradation mechanism determined by SERS was verified via mass spectrometry detection, which confirmed the validity of the SERS-based method. More broadly, the microfluidic droplet- and microparticle-based methods are potentially applicable for SERS monitoring of more Fenton degradation reactions.

Immobilization and 3D Hot-Junction Formation of Gold Nanoparticles on Two-Dimensional Silicate Nanoplatelets as Substrates for High-Efficiency Surface-Enhanced Raman Scattering Detection

We synthesize a high-efficiency substrate for surface-enhanced Raman scattering (SERS) measurements, which is composed of gold nanoparticles (AuNPs) on two-dimensional silicate nanoplatelets acting as an inorganic stabilizer, via the in-situ reduction of hydrogen tetrachloroaurate (III) by sodium citrate in an aqueous solution. Silicate platelets of ~1-nm thickness and various sizes, viz. laponite (50 nm), sodium montmorillonite (Na⁺⁻MMT, 100 nm), and mica (500 nm), are used to stabilize the AuNPs (Au@silicate), which are formed with uniform diameters ranging between 25 and 30 nm as confirmed by transmission electron microscopy (TEM). In particular, the laponite SERS substrate can be used in biological, environmental, and food safety applications to measure small molecules such as DNA (adenine molecule), dye (Direct Blue), and herbicide (paraquat) as it shows high detection sensitivity with a detection limit of 10-9 M for adenine detection. These highly sensitive SERS substrates, with their three-dimensional hot-junctions formed with AuNPs and two-dimensional silicate nanoplatelets, allow the highly efficient detection of organic molecules. Therefore, these Au@silicate nanohybrid substrates have great potential in biosensor technology because of their environmentally-friendly and simple fabrication process, high efficiency, and the possibility of rapid detection.

A direct assay of carboxyl-containing small molecules by SALDI-MS on a AgNP/rGO-based nanoporous hybrid film

Silver nanoparticles (AgNPs) and reduced graphene oxide (rGO) hybrid nanoporous structures fabricated by the layer-by-layer (LBL) electrostatic self-assembly have been applied as a simple platform for the rapid analysis of carboxyl-containing small molecules by surface-assisted laser desorption/ionization (D/I) mass spectrometry (SALDI-MS). By the simple one-step deposition of analytes onto the (AgNP/rGO)9 multilayer film, the MS measurements of various carboxyl-containing small molecules (including amino acids, fatty acids and organic dicarboxylic acids) can be done. In contrast to other energy transfer materials relative to AgNPs, the signal interferences of a Ag cluster (Agn(+) or Agn(-)) and a C cluster (Cn(+) or Cn(-)) have been effectively reduced or eliminated. The effects of various factors, such as the pore structure and composition of the substrates, on the efficiency of D/I have been investigated by comparing with the (AgNP)9 LBL nanoporous structure, (AgNP/rGO)9/(SiO2NP)6 LBL multilayer film and AgNP/prGO nanocomposites.

Reduced Ensemble Plasmon Line Widths and Enhanced Two-Photon Luminescence in Anodically Formed High Surface Area Au-TiO2 3D Nanocomposites

Localized surface plasmon resonances (LSPR) in TiO₂ nanorod and nanotube arrays decorated by gold nanoparticles can be exploited to improve photocatalytic activity, enhance nonlinear optical coefficients, and increase light harvesting in solar cells. However, the LSPR typically has a low quality factor, and the resonance is often obscured by the Urbach tail of the TiO₂ band gap absorption. Attempts to increase the LSPR extinction intensity by increasing the density of gold nanoparticles on the surface of the TiO₂ nanostructures invariably produce peak broadening due to the effects of either agglomeration or polydispersity. We present a new class of hybrid nanostructures containing gold nanoparticles (NPs) partially embedded in nanoporous/nanotubular TiO₂ by performing the anodization of cosputtered Ti-Au thin films containing a relatively high ratio of Au:Ti. Our method of anodizing thin film stacks containing alternate layers of Ti and TiAu results in very distinctive LSPR peaks with quality factors as high as 6.9 and ensemble line widths as small as 0.33 eV even in the presence of an Urbach tail. Unusual features in the anodization of such films are observed and explained, including oscillatory current transients and the observation of coherent heterointerfaces between the Au NPs and anatase TiO₂. We further show that such a plasmonic NP-embedded nanotube structure dramatically outperforms a plasmonic NP-decorated anodic nanotube structure in terms of the extinction coefficient, and achieves a strongly enhanced two-photon fluorescence due to the high density of gold nanoparticles in the composite film and the plasmonic local field enhancement.

"RaMassays": Synergistic Enhancement of Plasmon-Free Raman Scattering and Mass Spectrometry for Multimodal Analysis of Small Molecules

SiO₂/TiO₂ core/shell (T-rex) beads were exploited as "all-in-one" building-block materials to create analytical assays that combine plasmon-free surface enhanced Raman scattering (SERS) and surface assisted laser desorption/ionization (SALDI) mass spectrometry (RaMassays). Such a multi-modal approach relies on the unique optical properties of T-rex beads, which are able to harvest and manage light in both UV and Vis range, making ionization and Raman scattering more efficient. RaMassays were successfully applied to the detection of small (molecular weight, M.W. <400 Da) molecules with a key relevance in biochemistry and pharmaceutical analysis. Caffeine and cocaine were utilized as molecular probes to test the combined SERS/SALDI response of RaMassays, showing excellent sensitivity and reproducibility. The differentiation between amphetamine/ephedrine and theophylline/theobromine couples demonstrated the synergistic reciprocal reinforcement of SERS and SALDI. Finally, the conversion of L-tyrosine in L-DOPA was utilized to probe RaMassays as analytical tools for characterizing reaction intermediates without introducing any spurious effects. RaMassays exhibit important advantages over plasmonic nanoparticles in terms of reproducibility, absence of interference and potential integration in multiplexed devices.

Bifunctional 4MBA mediated recyclable SERS-based immunoassay induced by photocatalytic activity of TiO2 nanotube arrays

We present a recyclable SERS-based immunoassay for CA19-9 with a low detection limit of 5 U mL⁻¹. The linking between TiO₂-NTs and 4MBA was destroyed by catalyzing 4MBA into 4-sulfobenzoate upon UV irradiation, which was clarified by UPLC/ESI-tqMS and density functional theory.

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”.

Microfluidic electrochemical growth of vertically aligned TiO2 nanotubes for SERS optofluidic devices

The growth of a TiO₂ nanotubes (NTs) array into a microfluidic electrochemical reactor is here demonstrated.