Laser desorption ionization of gramicidin S on thin silver films with matrix isolation in surface plasmon resonance excitation

Longitudinal surface plasmon resonance based gold nanorod biosensors for mass spectrometry

A "strategy" for analyte capture/ionization based on chemical derivatization of gold nanorods and infrared laser desorption ionization (IR-LDI) is described. This is the first example of laser desorption/ionization of biomolecules using gold nanorods irradiated with an IR laser. LDI is performed at wavelengths (1064 nm) that overlap with the longitudinal surface plasmon resonance (LSPR) mode of gold nanorods. The absorbed energy from the laser facilitates desorption and ionization of the analyte. The wavelength of the LSPR band can be tuned by controlling the aspect ratio (length-to-diameter) of the nanorod. For example, the SPR band for Au nanorods having an aspect ratio of 5:1 is centered at approximately 840 nm, and this band overlaps with the 1064 nm output of a Nd:YAG laser. We show that a variety of biomolecules can be efficiently desorbed and ionized by 1064 nm irradiation of nanorods. We also show that analyte capture can be controlled by surface chemistry of the nanorods. The results of these studies are important for designing nanomaterial-based capture assays for mass spectrometry and interfacing nanomaterials with imaging/spatial profiling mass spectrometry experiments.

Matrix-free laser desorption/ionization of ions landed on plasma-treated metal surfaces

We report new experiments in which laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) was applied to detection and characterization of gramicidin S and IgG pentapeptide (DSDPR) that were reactively landed on plasma-treated stainless steel surfaces. The distributions of [M+H](+), [M+Na](+) and [M + K](+) ion species in LDI-TOF for gramicidin S and IgG pentapeptide (DSDPR) were found to be markedly different from those in conventional MALDI-TOF spectra of the same samples. LDI-TOF mass spectra showed a strong preference for [M+K](+) adducts even in the presence of a large excess of sodium cations, or following surface treatment with trifluoroacetic acid. Alkali metal cations (K(+) and Cs(+)) can be exchanged in reactively landed peptide samples to provide the corresponding cationized peptide ions by LDI. Multiple charged trypsin cations were reactively landed into a layer of 2-(4-hydroxyphenylazo)benzoic acid and ionized by LDI. The ionization mechanisms for LDI of surface-deposited peptides are briefly discussed.

Biosensing with plasmonic nanosensors

Recent developments have greatly improved the sensitivity of optical sensors based on metal nanoparticle arrays and single nanoparticles. We introduce the localized surface plasmon resonance (LSPR) sensor and describe how its exquisite sensitivity to size, shape and environment can be harnessed to detect molecular binding events and changes in molecular conformation. We then describe recent progress in three areas representing the most significant challenges: pushing sensitivity towards the single-molecule detection limit, combining LSPR with complementary molecular identification techniques such as surface-enhanced Raman spectroscopy, and practical development of sensors and instrumentation for routine use and high-throughput detection. This review highlights several exceptionally promising research directions and discusses how diverse applications of plasmonic nanoparticles can be integrated in the near future.

Surface plasmon resonance-based immunoassays

Surface plasmon resonance (SPR) has been successfully incorporated into an immunosensor format for the simple, rapid, and nonlabeled assay of various biochemical analytes. Proteins, complex conjugates, toxins, allergens, drugs, and pesticides can be determined directly using either natural antibodies or synthetic receptors with high sensitivity and selectivity as the sensing element. Immunosensors are capable of real-time monitoring of the antigen-antibody reaction. A wide range of molecules can be detected with lower limits ranging between 10(-9) and 10(-13) mol/L. Several successful commercial developments of SPR immunosensors are available and their web pages are rich in technical information. This review highlights many recent developments in SPR-based immunoassay, functionalizations of the gold surface, novel receptors in molecular recognition, and advanced techniques for sensitivity enhancement. Furthermore, it describes the challenge of current problems and provides some insights toward the future technologies.

Silver cationization of thia fatty acids and esters in laser desorption/ionization time-of-flight mass spectrometry

A laser desorption/ionization (LDI) time-of-flight mass spectrometric (TOF-MS) technique was used for the molecular mass analysis of thia fatty acids and esters, samples without appreciable light absorption at the laser wavelength. After a sample overlayer is deposited by solvent evaporation on a thin silver film substrate, it is subjected to 355 or 532 nm Nd : YAG laser irradiation. Photoablation of the Ag film substrate occurs with sufficient laser fluence, producing silver cluster cations, which can react with the desorbed thia fatty acid or ester molecules in the gas phase. Silver cation attachment of thia fatty esters may produce a silver-cationized analyte and fragments of structural diagnostic value, whereas thia fatty acids would not. With oxygen(s) present on the sulfur in sulfoxy fatty acids and esters, a silver-cationized analyte and additional fragments are produced. Formation of these fragments is consistent with charge-remote mechanisms through simple cleavage and rearrangement pathways. The structural reactivity of these compounds with ablated silver cations is hence comprehensively analyzed.