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

Laser desorption ionization (LDI)/laser ablation (LA) time-of-flight mass spectrometry (TOFMS) studies are performed via back-illumination on thin silver films deposited with gramicidin S, a cyclic decapeptide (1141 Da). Experimental results indicate an electronic excitation mechanism at low laser fluences and a thermal process at high laser fluences. In a perpendicular orientation of the thin silver film with respect to the drift tube axis, better success of chemical ionization (CI) of neutral silver atoms/clusters and gramicidin S molecules by Ag+ monomer cations within the ablation plume is evident. When these LDI/LA studies are repeated under surface plasmon resonance (SPR) conditions, the neutral atoms/clusters/molecules undergo SPR LDI/LA via a different laser energy transfer process compared to direct LDI/LA. These results also indicate an electronic excitation mechanism, only with a lower laser fluence requirement. Upon addition of a matrix, 2,5-dihydroxybenzoic acid (DHB), CI of gramicidin S molecules by Ag+ monomer cations is hindered while the CI of gramicidin S molecules by Na+ cations can occur at an even lower laser fluence. Thus, SPR LDI/LA with matrix isolation is an excellent technique for the mass analysis of gramicidin S

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

Stimulated Raman scattering activity of organic compounds

The stimulated Raman scattering (SRS) activity of over one hundred organic compounds was investigated at room temperature. Spectra were obtained successfully for the colourless neat liquids but not for the solid chemicals in solution, and their SRS shifts were measured systematically. The results confirmed that SRS Stokes shifts are usually the same as those observed in spontaneous Raman scattering, with occasional differences in frequency from the strongest spontaneous Raman bands. Empirical trends have been observed in functional group recognition, compound classification, degenerate vibrational modes, substituent group effects, hydrogen bonding and concentration threshold. Other new observations such as multiple peaks, exceptional SRS activity and distributed spectra are also reported. SRS offers several advantages over normal Raman spectroscopy in industrial process monitoring, and a potentially interesting scope of analytical utility is discussed

A 2-aminopyridine molecularly imprinted polymer surrogate micro-column for selective solid phase extraction and determination of 4-aminopyridine

Poly(methacrylic acid-ethylene glycol dimethacrylate) was prepared using 2-aminopyridine as the imprinting molecule. This molecularly imprinted polymer (MIP) was ground and packed into a micro-column for selective solid phase extraction (SPE) of 2-aminopyridine from 20μl of sample solution. Non-specific adsorption was also confirmed for a structural analogue. Interestingly one of the isomers, 4-aminopyridine, bound most strongly to the MIP. The implication of resonance and basicity of this isomer molecule can be used to explain its strong binding with the self-assembled functional methacrylic acid (MAA) monomer. The monomer template complexion process was evaluated by Scatchard plots to determine a binding constant. The binding constant value is important for predicting the selectivity of a new MIP. After optimization of the molecular recognition process, a molecularly imprinted solid phase extraction-differential pulsed elution (MISPE-DPE) method was developed for the selective determination of 4-aminopyridine in serum with an analysis time of less than 3min using a 2-aminopyridine micro-column for surrogate binding. The concentration detection limit was 0.5μg/ml, which corresponded to an absolute detection limit of 10ng. A larger sample vol