Desorption/ionization On Porous Silicon Research Articles

Comparison of the performance of different silicon-based SALDI substrates for illicit drug detection

Surface-assisted laser desorption ionization mass spectrometry (SALDI-MS) is an emerging technique used for the detection of small molecules (<700Da) such as illicit drugs. In recent times, this technique has been employed for the detection of illicit drugs in various body fluids including saliva. Three common SALDI techniques, desorption ionization on porous silicon (DIOS), nanostructure-initiator mass spectrometry (NIMS) and nanostructured laser desorption ionization (NALDI™) are compared for the detection of four drug classes, amphetamines, benzodiazepines, opiates and tropane alkaloids. We focus in our comparison on structural and chemical characteristics, as well as analytical performance and longevity.

Surface-assisted laser desorption ionization mass spectrometry techniques for application in forensics.

Matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) is an excellent analytical technique for the rapid and sensitive analysis of macromolecules (>700 Da), such as peptides, proteins, nucleic acids, and synthetic polymers. However, the detection of smaller organic molecules with masses below 700 Da using MALDI-MS is challenging due to the appearance of matrix adducts and matrix fragment peaks in the same spectral range. Recently, nanostructured substrates have been developed that facilitate matrix-free laser desorption ionization (LDI), contributing to an emerging analytical paradigm referred to as surface-assisted laser desorption ionization (SALDI) MS. Since SALDI enables the detection of small organic molecules, it is rapidly growing in popularity, including in the field of forensics. At the same time, SALDI also holds significant potential as a high throughput analytical tool in roadside, work place and athlete drug testing. In this review, we discuss recent advances in SALDI techniques such as desorption ionization on porous silicon (DIOS), nano-initiator mass spectrometry (NIMS) and nano assisted laser desorption ionization (NALDI™) and compare their strengths and weaknesses with particular focus on forensic applications. These include the detection of illicit drug molecules and their metabolites in biological matrices and small molecule detection from forensic samples including banknotes and fingerprints. Finally, the review highlights recent advances in mass spectrometry imaging (MSI) using SALDI techniques.

Analysis of deprotonated acids with silicon nanoparticle‐assisted laser desorption/ ionization mass spectrometry

Chemically modified silicon nanoparticles were applied for the laser desorption/negative ionization of small acids. A series of substituted sulfonic acids and fatty acids was studied. Compared to desorption ionization on porous silicon (DIOS) and other matrix-less laser desorption/ionization techniques, silicon nanoparticle-assisted laser desorption/ionization (SPALDI) mass spectrometry allows for the analysis of acids in the negative ion mode without the observation of multimers or cation adducts. Using SPALDI, detection limits of many acids reached levels down to 50 pmol/µl. SPALDI of fatty acids with unmodified silicon nanoparticles was compared to SPALDI using the fluoroalkyl silylated silicon powder, with the unmodified particles showing better sensitivity for fatty acids, but with more low-mass background due to impurities and surfactants in the untreated silicon powder. The fatty acids exhibited a size-dependent response in both SPALDI and unmodified SPALDI, showing a signal intensity increase with the chain length of the fatty acids (C12-C18), leveling off at chain lengths of C18-C22. The size effect may be due to the crystallization of long chain fatty acids on the silicon. This hypothesis was further explored and supported by SPALDI of several, similar sized, unsaturated fatty acids with various crystallinities. Fatty acids in milk lipids and tick nymph samples were directly detected and their concentration ratios were determined by SPALDI mass spectrometry without complicated and time-consuming purification and esterification required in the traditional analysis of fatty acids by gas chromatography (GC). These results suggest that SPALDI mass spectrometry has the potential application in fast screening for small acids in crude samples with minimal sample preparation.

Functionalized pyrolytic highly oriented graphite polymer film for surface‐assisted laser desorption/ionization mass spectrometry in environmental analysis

The pyrolytic highly oriented graphite polymer film (PGS) was first employed to analyze low-mass analytes in environmental analysis by surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). PGS is a synthetic uniform and highly oriented graphite polymer film with high thermal anisotropic conductivity. We have found that negative ion mode SALDI-MS using oxidized PGS (PGS-SALDI-MS) can be used to detect [M-H]- ions from perfluorooctanoic acid (PFOA) and other perfluoroalkylcarboxylic acids when the PGS surface is modified with the cationic polymer polyethyleneimine (PEI). The signal intensity of PFOA when employing the PEI modification showed a ten-fold increase over that obtained from desorption/ionization on porous silicon (DIOS). PFOA was quantified using PGS-SALDI-MS and the calibration curve showed a wide linear dynamic range of response (20-1000 ppb). The combination of atmospheric pressure ionization and PGS (AP-PGS-SALDI) showed greater signal intensity than vacuum PGS-SALDI for deprotonated PFOA. Several other environmentally important chemicals, including perfluoroalkylsulfonic acid, pentachlorophenol, bisphenol A, 4-hydroxy-2-chlorobiphenyl, and benzo[a]pyrene, were also successfully used to evaluate PGS-SALDI-MS. In addition, we found that nonafluoro-1-butanesulfonic acid was able to produce protonated peptides in positive ion PGS-SALDI-MS, but that perfluoropentanoic acid and trifluoroacetic acid were not. It is suggested that perfluoroalkylsulfonic acids are better protonating agents than perfluoroalkylcarboxylic acids in SALDI-MS.

Comparison of inert supports in laser desorption/ionization mass spectrometry of peptides: pencil lead, porous silica gel, DIOS‐chip and NALDI™ target

In the search for alternative inert surfaces replacing silicon chips in Desorption/Ionization On porous Silicon (DIOS)-like mass spectrometry analyses, nanostructured silicon-based NALDI chips were evaluated in Laser Desorption/Ionization (LDI) of peptides. Comparisons were made using commercially available DIOS chips (MassPREP-DIOS-target), amorphous carbon powder from lead pencil and porous silica gel used for chromatographic purposes as reference supports. A set of synthetic model peptides presenting variable amino acid sequences of various lengths was analyzed under all conditions. The LDI responses of the four 'matrix-free' techniques were compared, especially in terms of peptide detection sensitivity and overall experiment robustness.

Study on the redox reactions for organic dyes and S‐nitrosylated peptide in electrospray droplet impact

Reduction of analytes in ionization processes often obscures the determination of molecular structure. The reduction of analytes is found to take place in various desorption/ionization methods such as fast atom bombardment (FAB), secondary ion mass spectrometry (SIMS), matrix-assisted laser desorption/ionization (MALDI) and desorption ionization on porous silicon (DIOS). To examine the extent of the reduction reactions taking place in electrospray droplet impact (EDI) processes, reduction-sensitive dyes and S-nitrosylated peptide were analyzed by EDI. No reduction was observed for methylene blue. While methyl red has a lower reduction potential than methylene blue, the reduction product ions were detected. For S-nitrosylated peptide, protonated molecule ion [M + H](+) and NO-eliminated molecular ion [M - NO + H](+*) were observed but reduction reactions are largely suppressed in EDI compared with that in MALDI. As such, the analytes examined suffer from little reduction reactions in EDI.

Electroless plating of silver nanoparticles on porous silicon for laser desorption/ionization mass spectrometry

An improved DIOS (desorption ionization on porous silicon) method for laser desorption/ionization mass spectrometry (LDI MS) by electroless plating of silver nanoparticles (AgNPs) on porous silicon (PSi) was developed. By addition of 4-aminothiophenol (4-ATP) into the AgNO 3 plating solution, the plating speed can be slowed down and simultaneously 4-ATP self-assembled monolayers (SAMs) on AgNPs (4-ATP/AgNPs) were formed. Both AgNPs and 4-ATP/AgNPs coated PSi substrates present much higher stability, sensitivity and reproducibility for LDI MS than the un-treated porous silicon ones. Their shelf life in air was tested for several weeks to a month and their mass spectra still displayed the same high quality and sensitivity as the freshly prepared ones. And more 4-ATP SAMs partly play a role of matrix to increase the ionization efficiency. A small organic molecule of tetrapyridinporphyrin (TPyP), oligomers of polyethylene glycol (PEG 400 and 2300), and a peptide of oxytocin were used as examples to demonstrate the feasibility of the silver-plated PSi as a matrix-free-like method for LDI MS. This approach can obtain limits of detection to femtomoles for TPyP, subpicomoles for oxytocin, and picomoles for PEG 400 and 2300, comparable to the traditional matrix method and much better than the DIOS method. It simplifies the sample preparation as a matrix-free-like method without addition of matrix molecules and homogenizes the sample spread over the spot for better and more even mass signals.

Laser Desorption/Ionization Mass Spectrometry on Porous Silica and Alumina for Peptide Mass Fingerprinting

We investigated a variant of desorption/ionization on porous silicon (DIOS) mass spectrometry utilizing an aqueous suspension of either porous silica gel or porous alumina (pore size of 60 and 90 A, respectively). Laser desorption/ionization (LDI) from samples directly deposited on a stainless steel surface without any inorganic substrates was also achieved. Synthetic peptides designed to cover large sequence diversity constituted our model compounds. Sample preparation, including material conditioning, peptide solubilization, and deposition protocol onto standard matrix-assisted laser desorption/ionization (MALDI) probe, as well as ionization source tuning were optimized to perform sensitive reproducible LDI analyses. The addition of either a cationizing agent or an alkali metal scavenger to the sample suspension allowed modification of the ionization output. Comparing hydrophilic silica gel to hydrophobic reversed-phase silica gel as well as increasing material pore size provided further insights into desorption/ionization processes. Furthermore, mixtures of peptides were analyzed to probe the spectral suppression phenomenon when no interfering organic matrix was present. The results gathered from synthetic peptide cocktails indicated that LDI mass spectrometry on silica gel or alumina constitutes a promising complementary method to MALDI in proteomics for peptide mass fingerprinting.

High Surface Area of Porous Silicon Drives Desorption of Intact Molecules

The surface structure of porous silicon used in desorption/ionization on porous silicon (DIOS) mass analysis is known to play a primary role in the desorption/ionization (D/I) process. In this study, mass spectrometry and scanning electron microscopy (SEM) are used to examine the correlation between intact ion generation with surface ablation and surface morphology. The DIOS process is found to be highly laser energy dependent and correlates directly with the appearance of surface ions (Si(n)(+) and OSiH(+)). A threshold laser energy for DIOS is observed (10 mJ/cm(2)), which supports that DIOS is driven by surface restructuring and is not a strictly thermal process. In addition, three DIOS regimes are observed that correspond to surface restructuring and melting. These results suggest that higher surface area silicon substrates may enhance DIOS performance. A recent example that fits into this mechanism is the surface of silicon nanowires, which has a high surface energy and concomitantly requires lower laser energy for analyte desorption.

Preparation of a biochip on porous silicon and application for label‐free detection of small molecule‐protein interactions

A new approach for the preparation of a biochip on porous silicon and the application of the biochip for detection of small molecule-protein interactions with desorption/ionization on porous silicon (DIOS) was demonstrated. The galvanostatically etched porous silicon substrates were chemically modified firstly to yield carboxylic acid terminated surfaces, and then the protein was covalently attached to the surface through amide bonding. By applying a solution of candidate chemicals to the surface and a subsequent wash step, the masses of captured compounds could be analyzed by DIOS. DIOS has advantages of being a direct detection tool compared to the classic fluorescence or chemiluminescence methods, because the process of labeling molecules employed in the fluorescence or chemiluminescence methods can sometimes alert the properties of the labeled molecule. The recognition between proteins and their binding partners is efficient and selective. A good tolerance to disturbance and high enrichment factor of the biochip to the analytes was observed. As an on-chip-based approach, the demonstrated method has a potential to perform in a high-throughput format.

Fingerprinting of 3, 4-Methylenedioxymethamphetamine Markers by Desorption/Ionization on Porous Silicon

Desorption/ionization on porous silicon (DIOS) is a method which extends the application range of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. This technique eliminates matrix background in the low mass range; DIOS is especially advantageous in research on small organic molecules and their metabolites in biological samples. DIOS mass spectrometry was applied for 3, 4-methylenedioxymethamphetamine, (MDMA, Ecstasy) impurities identification. Trace components profiling enables the identification of by-products characteristics for the synthesis route of MDMA. Ecstasy, a synthetic psychoactive drug, is highly popular among young people, and often used as a recreational drug, most commonly used during disco parties. MDMA enhances feeling of euphoria by increasing the level of neurotransmitters such as serotonin, dopamine and norepinephrine, and causes acute behavioral and psychological effects. MDMA is almost exclusively produced illegally, primarily in Western Europe. The new method for MDMA impurities profiling has been developed to trace the origin of MDMA pills. For comparison and classification of the impurity profiles, the principal components analysis was used.

Effective detection of peptides containing cysteine sulfonic acid using matrix‐assisted laser desorption/ionization and laser desorption/ionization on porous silicon mass spectrometry

Cysteine sulfonic acid-containing peptides, being typical acidic peptides, exhibit low response in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. In this study, matrix conditions and the effect of diammonium hydrogencitrate (DAHC) as additive were investigated for ionization of cysteine sulfonic acid-containing peptides in MALDI. A matrix-free ionization method, desorption/ionization on porous silicon (DIOS), was also utilized to evaluate the effect of DAHC. When equimolar three-component mixtures of peptides carrying free cysteine, cysteine sulfonic acid, and carbamidomethyl cysteine were measured by MALDI using a common matrix, alpha-cyano-4-hydroxycinnamic acid (CHCA), no signal corresponding to cysteine sulfonic acid-containing peptide could be observed in the mass spectrum. However, by addition of DAHC to CHCA, the peaks of cysteine sulfonic acid-containing peptides were successfully observed, as well as when using 2,4,6-trihydroxyacetophenone (THAP) and 2,6-dihydroxyacetophenone with DAHC. In the DIOS mass spectra of these analytes, the use of DAHC also enhanced the peak intensity of the cysteine sulfonic acid-containing peptides. On the basis of studies with these model peptides, tryptic digests of oxidized peroxiredoxin 6 were examined as a complex peptide mixture by MALDI and DIOS. In MALDI, the peaks of cysteine sulfonic acid-containing peptides were observed when using THAP/DAHC as the matrix, but this was not so with CHCA. In DIOS, the signal from cysteine sulfonic acid-containing peptides was suppressed; however, the use of DAHC significantly enhanced the signal intensity with an increase in the number of observed peptides and increased signal-to-noise ratio in the DIOS spectra. The results show that DAHC in the matrix or on the DIOS chip decreases discrimination and suppression effects in addition to suppressing alkali-adduct ions, which leads to a beneficial effect on protonation of peptides containing cysteine sulfonic acid.

Surface Modification and Laser Pulse Length Effects on Internal Energy Transfer in DIOS

Benzyl-substituted benzylpyridinium (BP) chloride salts were used as a source of thermometer ions to probe the internal energy (IE) transfer in desorption/ionization on porous silicon (DIOS). To modify their wetting properties and the interaction energies with the thermometer ions, the DIOS surfaces were silylated to produce trimethylsilyl- (TMS), amine- (NH2), perfluoroalkyl- (PFA), and perfluorophenyl-derivatized (PFP) surfaces. Two laser sources--a nitrogen laser with pulse length of 4 ns and a mode locked 3 x omega Nd:YAG laser with a pulse length of 22 ps--were utilized to induce desorption/ionization and fragmentation at various laser fluence levels. The corresponding survival yields were determined as indicators of the IE transfer and the IE distributions were extracted. In most cases, with increasing the laser fluence in a broad range (approximately 20 mJ/cm2), no change in IE transfer was observed. For ns excitation, this was in remarkable contrast with MALDI, where increasing the laser fluence resulted in sharply (within approximately 5 mJ/cm2) declining survival yields. Derivatization of the porous silicon surface did not affect the survival yields significantly but had a discernible effect on the threshold fluence for ion production. The IE distributions determined for DIOS and MALDI from alpha-cyano-4-hydroxycinnamic acid reveal that the mean IE value is always lower for the latter. Using the ps laser, the IE distribution is always narrower for DIOS, whereas for ns laser excitation the width depends on surface modification. Most of the differences between MALDI and DIOS described here are compatible with the different dimensionality of the plume expansion and the differences in the activation energy of desorption due to surface modifications.

Oxidation of Ferrocene Derivatives in Desorption/Ionization on Porous Silicon

In matrix-assisted laser desorption/ionization (MALDI), the true molecular structures of some analytes are not represented by the observed ions due to a redox reaction. In earlier reports, electron transfer from analyte to chemical matrix has been proposed for the oxidation of ferrocene derivatives in MALDI. To address such a redox phenomenon in laser desorption/ionization processes, two ferrocene derivatives, FcCH2CH2Fc and FcCH2NMe2 [Fc:(CsHs)Fe(CsH4)], were analyzed by a matrix-free method, desorption/ionization on porous silicon (DIOS). The oxidized species, Fc+CH2NMe2 and FcCH2CH2Fc+, were detected in the DIOS mass spectra. The results suggested that electron transfer from the analytes to the sample target occurs during the ionization process.

Quantitative analysis of polypropyleneglycol mixtures by desorption/ionization on porous silicon mass spectrometry

Mixtures of diol and triol types of polypropyleneglycol (PPG) bearing two and three hydroxyl end groups were analyzed quantitatively by matrix-assisted laser desorption/ionization (MALDI) and desorption ionization on porous silicon (DIOS) with the conventional dried droplet method. The reproducibility of MALDI mass spectra depended on the factors regarding sample preparation such as the analyte/matrix ratio, and the type of solvent and/or chemical matrix employed. For DIOS, the analyte concentration and the selection of solvents were important for good reproducibility. Optimization of these factors allowed reliable quantification of the polymer mixtures. Under optimized conditions, DIOS would be suitable than MALDI for this purpose.

Reduction of organic dyes in matrix‐assisted laser desorption/ionization and desorption/ionization on porous silicon

Reduction of analytes in matrix-assisted laser desorption/ionization (MALDI) often obscures the actual determination of molecular structure. To address the redox reactions in laser desorption/ionization processes, the organic dyes Methylene Blue, Janus Green B, Crystal Violet and Rhodamine B were analyzed by MALDI or by desorption/ionization on porous silicon (DIOS). Susceptibility to reduction in MALDI was dependent on both the reduction potentials of analytes and the molar ratio of analyte to matrix molecules. Addition of Cu(II) ions as an electron scavenger suppressed the reduction of Methylene Blue in MALDI. The results suggested that electron transfer to analytes from the sample target and/or from the matrix contributed to the reduction. In DIOS, the reductions of organic dyes were more prominent than in MALDI, and were not prevented by Cu(II) ion doping, probably due to direct contact of the analytes with silicon which had little electric resistance.

Reduction of Cu(II) and Riboflavin in DIOS Mass Spectrometry

Understanding ion-molecular reactions is suggested to be essential to delineating the mechanisms of matrix-assisted laser desorption/ionization (MALDI) and related ionization methods. To examine the implications of matrix-analyte reactions in the reduction observed in MALDI, copper(II) chloride and riboflavin were analyzed by desorption/ionization on porous silicon (DIOS) mass spectrometry. This system does not require a matrix for soft laser desorption/ionization. The two susbstances were reduced by DIOS as well, and the reduction of riboflavin was accelerated using water as a solvent, as reported in MALDI. The results indicated that target-analyte electron transfer occurs in LDI, although the possibility of matrix-analyte electron transfer in MALDI cannot be excluded.

ＤＩＯＳ法を利用した合成高分子の質量分析−ＤＩＯＳチップ作製の最適化−

Desorption/ionization on porous silicon (DIOS) is a novel matrix-free variant of laser desorption/ionization (LDI) techniques for mass spectrometry. The DIOS chips are produced by electrochemical etching of silicon wafers under light exposure. In the present report, the optimal conditions, regarding resistivity of silicon wafer, etching current density and etching time, for making DIOS chip with better ionization performance are described. In addition, the DIOS mass spectra of various synthetic polymers including polyethyleneglycol, nonylphenolpolyethoxylate, nonylphenolpolyethoxylatesulfate, polymethylmethacrylate are compared with the matrix-assisted LDI mass spectra.

Direct assay of Aplysia tissues and cells with laser desorption/ionization mass spectrometry on porous silicon.

Desorption/ionization on porous silicon (DIOS) is a form of laser desorption mass spectrometry that allows for the direct mass analysis of a variety of analytes without the addition of organic matrix. Protocols are described for the direct analysis of exocrine tissue and single neurons using DIOS-MS. The atrial gland of Aplysia californica was blotted on to porous silicon and analyzed with DIOS-MS in the range m/z 1000-4000. The ability to culture invertebrate neurons directly on porous silicon is also presented. Isolated bag cells regenerated neuronal processes in culture on porous silicon. DIOS-MS allowed the direct detection of the peptides contained in individual cultured neurons indicating that with appropriate protocols, DIOS can be used with biological samples with considerable thickness.

Experimental factors controlling analyte ion generation in laser desorption/ionization mass spectrometry on porous silicon.

Desorption/ionization on porous silicon (DIOS) is a relatively new laser desorption/ionization technique for the direct mass spectrometric analysis of a wide variety of samples without the requirement of a matrix. Porous silicon substrates were fabricated using the recently developed nonelectrochemical H2O2-metal-HF etching as a versatile platform for investigating the effects of morphology and physical properties of porous silicon on DIOS-MS performance. In addition, laser wavelength, mode of ion detection, pH, and solvent contributions to the desorption/ionization process were studied. Other porous substrates such as GaAs and GaN, with similar surface characteristics but differing in thermal and optical properties from porous silicon, allowed the roles of surface area, optical absorption, and thermal conductivities in the desorption/ionization process to be investigated. Among the porous semiconductors studied, only porous silicon has the combination of large surface area, optical absorption, and thermal conductivity required for efficient analyte ion generation under the conditions studied. In addition to these substrate-related factors, surface wetting, determined by the interaction of deposition solvent with the surface, and charge state of the peptide were found to be important in determining ion generation efficiency.