Self-pressurised rapid freezing at arbitrary cryoprotectant concentrations

Self-pressurised rapid freezing (SPRF) has been proposed as a simple alternative to traditional high-pressure freezing (HPF) protocols for vitrification of biological samples in electron microscopy and cryopreservation applications. Both methods exploit the circumstance that the melting point of ice reaches a minimum when subjected to pressure of around 210 MPa, however, in SPRF its precise quantity depends on sample properties and hence, is generally unknown. In particular, cryoprotective agents (CPAs) are expected to be a factor; though eschewed by many SPRF experiments, vitrification of larger samples notably cannot be envisaged without them. Thus, in this study, we address the question of how CPA concentration affects pressure inside sealed capillaries, and how to design SPRF experiments accordingly. By embedding a fibre-optic probe in samples and performing Raman spectroscopy after freezing, we first present a direct assessment of pressure build-up during SPRF, enabled by the large pressure sensitivity of the Raman shift of hexagonal ice. Choosing dimethyl sulphoxide (DMSO) as a model CPA, this approach allows us to demonstrate that average pressure drops to zero when DMSO concentrations of 15 wt% are exceeded. Since a trade-off between pressure and DMSO concentration represents an impasse with regard to vitrification of larger samples, we introduce a sample architecture with two chambers, separated by a partition that allows for equilibration of pressure but not DMSO concentrations. We show that pressure and concentration in the fibre-facing chamber can be tuned independently, and present differential scanning calorimetry (DSC) data supporting the improved vitrification performance of two-chamber designs. Lay version of abstract for 'Self-pressurised rapid freezing at arbitrary cryoprotectant concentrations' Anyone is familiar with pipes bursting in winter because the volume of ice is greater than that of liquid water. Less well known is the fact that inside a thick-walled container, sealed and devoid of air bubbles, this pressure build-up will allow a fraction of water to remain unfrozen if the sample is also cooled sufficiently rapidly far below the freezing point. This phenomenon has already been harnessed for specimen preparation in microscopy, where low temperatures are useful to immobilise the sample, but harmful if ice formation occurs. However, specimen preparation cannot always rely on this pressure-based effect alone, but sometimes requires addition of chemicals to inhibit ice formation. Not enough is known directly about how these chemicals affect pressure build-up: Indeed, rapid cooling below the freezing point is only possible for small sample volumes, typically placed inside sealed capillaries, so that space is generally insufficient to accommodate a pressure sensor. By means of a compact sensor, based on an optical fibre, laser and spectrometer, we present the first direct assessment of pressure inside sealed capillaries. We show that addition of chemicals reduces pressure build-up and present a two-chambered capillary to circumvent the resulting trade-off. Also, we present evidence showing that the two-chambered capillary design can avoid ice formation more readily than a single-chambered one.

Sub millimetre flexible fibre probe for background and fluorescence free Raman spectroscopy

Using the shifted-excitation Raman difference spectroscopy technique and an optical fibre featuring a negative curvature excitation core and a coaxial ring of high numerical aperture collection cores, we have developed a portable, background and fluorescence free, endoscopic Raman probe. The probe consists of a single fibre with a diameter of less than 0.25 mm packaged in a sub-millimetre tubing, making it compatible with standard bronchoscopes. The Raman excitation light in the fibre is guided in air and therefore interacts little with silica, enabling an almost background free transmission of the excitation light. In addition, we used the shifted-excitation Raman difference spectroscopy technique and a tunable 785 nm laser to separate the fluorescence and the Raman spectrum from highly fluorescent samples, demonstrating the suitability of the probe for biomedical applications. Using this probe we also acquired fluorescence free human lung tissue data.

Ultra-low background Raman sensing using a negative-curvature fibre and no distal optics

Measuring Raman spectra through an optical fibre is usually complicated by the high intrinsic Raman scatter of the fibre material. Common solutions such as the use of multiple fibres and distal optics are complex and bulky. We demonstrate the use of single novel hollow-core negative-curvature fibres (NCFs) for Raman and surface-enhanced Raman spectroscopy (SERS) sensing using no distal optics. The background Raman emission from the silica in the NCF was at least 1000× smaller than in a conventional solid fibre, while maintaining the same collection efficiency. We transmitted pump light from a 785-nm laser through the NCF, and we collected back the weak Raman spectra of different distal samples, demonstrating the fibre probe can be used for measurements of weak Raman and SERS signals that would otherwise overlap spectrally with the silica background. The lack of distal optics and consequent small probe diameter (<0.25 mm) enable applications that were not previously possible.