Optical manipulation of gold nanoparticles has facilitated a wide range of innovative science and applications such as nanothermometry, cell poration, optical binding and optomechanics, with many exploiting the surface plasmon resonance. However, conventional gold nanoparticles usually depart from spherical shape and typically exhibit icosahedron, decahedron, triangular or hexagonal prism, thus making such studies less controlled and leading to potential artefacts in trapping behaviour. We successfully synthesise ultrasmooth gold nanoparticles of 50nm and 100nm in diameter with an improved monodispersity both in shape and size compared to conventional gold nanoparticles. We demonstrate the first optical manipulation of such ultrasmooth gold nanoparticles in a near infrared optical trap, and show trap stiffness with up to a three-fold reduction in standard deviation both in liquid and air, compared to conventional gold nanoparticles. Our trapping study highlights the exceptional sensitivity of the trapping parameters of gold nanoparticles on their morphology. Furthermore, we for the first time quantify the particle temperature of airborne gold nanoparticles for a range of optical powers based on the trap stiffness measurements and reveal the effects of particle morphology. We show our ultrasmooth gold nanoparticles exhibit lower temperature than the conventional counterparts due to the lack of additional morphological features that induce plasmonic heating. The use of ultrasmooth gold nanoparticles can pave the way for more controlled studies of optical binding and plasmon mediated light-matter interactions and novel applications in optomechanics such as optically controllable nanoprobes of weak forces and torques.
We present an optical spectroscopic technique, making use of both Raman signals and fluorescence spectroscopy, for the identification of five brands of commercially available extra-virgin olive-oil (EVOO). We demonstrate our technique on both a ‘bulk-optics’ free-space system and a compact device. Using the compact device, which is capable of recording both Raman and fluorescence signals, we achieved an average sensitivity and specificity of 98.4% and 99.6% for discrimination, respectively. Our approach demonstrates that both Raman and fluorescence spectroscopy can be used for portable discrimination of EVOOs which obviates the need to use centralised laboratories and opens up the prospect of in-field testing. This technique may enable detection of EVOO that has undergone counterfeiting or adulteration. One of the main challenges facing Raman spectroscopy for use in quality control of EVOOs is that the oxidation of EVOO, which naturally occurs due to aging, causes shifts in Raman spectra with time, which implies regular retraining would be necessary. We present a potential method of analysis to minimize the effect that aging has on discrimination efficiency; we show that by discarding the first principal component, which contains information on the variations due to oxidation, we can improve discrimination efficiency thus improving the robustness of our technique.
Moving towards label-free techniques for cell identification is essential for many clinical and research applications. Raman spectroscopy and digital holographic microscopy (DHM) are both label-free, non-destructive optical techniques capable of providing complimentary information. We demonstrate a multi-modal system which may simultaneously take Raman spectra and DHM images to provide both a molecular and a morphological description of our sample. In this study we use Raman spectroscopy and DHM to discriminate between three immune cell populations CD4+ T cells, B cells, and monocytes, which together comprise key functional immune cell subsets in immune responses to invading pathogens. Various parameters that may be used to describe the phase images are also examined such as pixel value histograms or texture analysis. Using our system it is possible to consider each technique individually or in combination. Principal component analysis is used on the data set to discriminate between cell types and leave-one-out cross-validation is used to estimate the efficiency of our method. Raman spectroscopy provides specific chemical information but requires relatively long acquisition times, combining this with a faster modality such as DHM could help achieve faster throughput rates. The combination of these two complimentary optical techniques provides a wealth of information for cell characterisation which is a step towards achieving label free technology for the identification of human immune cells.
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