Multivariate spectral analysis of pH SERS probes for improved sensing capabilities

Adsorption of functional groups to the surface of plasmonic nanoparticles provides a platform for localised optical sensing. Over the past decade, nanoscale sensors for intracellular pH measurement based on surface‐enhanced Raman spectroscopy (SERS) have been developed. However, the approaches by which pH‐SERS measurements are made and analysed can greatly impact the precision and accuracy of pH calibration. To improve pH nanosensors, the sources of experimental variation must be determined and the data must be optimally analysed. Here we report the plasmon‐induced decarboxylation of para‐mercaptobenzoic acid (pMBA) pH reporters attached to gold nanoparticles and conclude a strong association with laser power. The detrimental decarboxylation of pMBA has profound implications on the sensitivity and reliability of the pH sensor. Decarboxylation spectral signatures map directly onto those that are typically used to record pH changes, and, hence, the greatest implication of decarboxylation of pH sensors is inaccurate or false pH reporting. Here a more robust spectral analysis for pH sensing based upon an optimal spectral region for pH calibration is presented together with a unique application of the multivariate statistical technique, principal component analysis (PCA). PCA interprets complex spectral dynamics, and by direct comparisons with the typically employed ratiometric analysis, a significant improvement in generating accurate pH sensing is demonstrated. An application of these methods in determining the pH of internalised nanosensors in macrophage cells further promotes these step changes in pH measurement methodology via the avoidance of disruptive spectral signatures that arise in real applications. Copyright © 2016 John Wiley & Sons, Ltd.     

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Enhancement of lattice defect signatures in graphene and ultrathin graphite using tip‐enhanced Raman spectroscopy

Understanding the role of defects in graphene is the key to tailoring the properties of graphene and promoting the development of graphene‐based devices. Defects can affect the electronic properties of a device while also offering a means by which to functionalize the local properties. Using tip‐enhanced Raman spectroscopy (TERS), heightened defect sensitivity was demonstrated on graphene edges, folds, and overlapping regions. Measurements confirm that TERS can provide simultaneous structural and spectral information on a localized scale, hence offering defect characterization on a scale that is not obtainable using conventional Raman spectroscopy. This study observed preferential enhancement of the D band signal on multilayered graphene and ultrathin graphite; in addition, other key defect signatures were also enhanced and detected. We present our findings in relation to theoretical predictions of graphene defect signatures and an analysis of the sensitivity of TERS in measuring two‐dimensional structures. Copyright © 2013 John Wiley & Sons, Ltd.     

Strength of coherently strained layered superlattices

Electronic-grade single-crystal semiconductor structures provide a means to study the mechanical effects of coherency strain in isolation. In this work, thin coherently strained InGaAs superlattices grown on thick InP substrates were tested in three-point bending at 500°C. The force-deflection curves of the specimens were measured and the beams were found to be significantly strengthened by the presence of the superlattices. Analysis provides estimates of the stress supported by the thin superlattices in the plastic regime. The superlattices are found to display mechanical strength up to a hundred times greater than the strength of the bulk substrate material. This effect can be attributed only to the coherency strain in the superlattices.     

Luminescence and magnetic resonance in post-hydrogenated microcrystalline silicon

We report photoluminescence, EPR and ODMR studies of plasma hydrogenated LPCVD microcrystalline silicon thin films. Apart from the dangling bond EPR resonance which is similar to that observed in a-Si : H, and possibly the emission at 1.3 eV, we find emission bands and resonance signals quite unlike those observed in normal c-Si and a-Si : H. We conclude that μc-Si : H cannot be regarded purely as crystallites of pure silicon embedded in a matrix of a-Si : H.