Investigation of Issues for the Accurate and Precise Measurement of an Analyte Using Surface-Enhanced Raman Scattering (SERS).

This paper builds on an earlier examination of the influence of sampling size and analyte surface density on the accuracy and precision of measurements using surface-enhanced Raman scattering (SERS) to read out heterogeneous immunoassays. Quantitation using SERS typically relies on interrogating a small area on the sample surface by using a micrometer-sized laser spot for signal generation. The information obtained using such a small portion of sample is then projected as being representative of the much larger sample, which can compromise the accuracy and precision of the measurement due to undersampling. For a heterogeneous immunoassay interrogated by SERS, quantitation is, therefore, sensitive to the size of the analyzed area and the surface density of the measured analyte. To identify conditions in which sampling error poses a threat to accuracy and precision, a simulation of a SERS immunoassay was developed and compared to experimental results. The simulation randomly distributes adsorbates across the capture surface and then measures the density of adsorbates inside areas of analysis of different sizes. This approach mimics the analysis of a heterogeneous immunoassay when using a Raman microscope with different laser spot sizes. The results of the simulations, which were confirmed experimentally by comparison to an immunoassay of human immunoglobulin G (IgG) show that the accuracy and precision of the measurement improved with larger analysis areas and higher analyte concentrations due to the increased apparent homogeneity of the analyte within the area of analysis. By imposing a threshold on precision (5%), we also begin to establish a framework for the parameters necessary to achieve reliable quantitative measurements (e.g., laser spot size, analyte concentration, and sample volume).

Influence of alternating L-/D-amino acid chiralities and disulfide bond geometry on the capacity of cysteine-containing reversible cyclic peptides to disperse carbon nanotubes.

Although single-walled carbon nanotubes (SWNTs) have exciting properties and potential applications, their hydrophobic nature makes them difficult to purify and manipulate. To fully realize the potential of SWNTs, strategies for the effective dispersion, separation, and organization of these materials must be devised. In this article, work involving the recent design and characterization of reversible cyclic peptides (RCPs) and RCP/SWNT composites will be described. The peptides in this work contain alternating L- and D-amino acid sequences, as well as N- and C-terminal cysteine residues (RCP-Cys) that allow for their covalent closure around the circumference of individual SWNTs. When RCPs are oxidized in the presence of SWNTs, dispersions are produced that are stable against dilution by dialysis without the formation of aggregates. The reported studies using Raman spectroscopy and UV/Vis/NIR were focused on answering the questions (1) does the chirality of the disulfide bond impact the capacity of the RCP-Cys to disperse SWNTs, and (2) is the alternating chirality of the amino acids in the RCP-Cys peptides important for SWNT dispersion. It was found that though Cys-containing RCPs are indeed able to disperse SWNTs, the chirality of the Cys residues on the N- and C- termini does not have a significant influence on the dispersed SWNT population. However, there is a large decrease in the dispersability by RCP-Cys when the alternating L/D-chiral pattern of amino acids is replaced with all L-amino acids.