The precise and non-invasive control over single particles is key for an array of physical and bio-medical
applications, such as microfluidics and biophysics. In particular, the three-dimensional manipulation of
single rare-earth-doped luminescent particles is of great interest due to their biocompatibility and the
sensitivity of their luminescent properties to environmental conditions, which stand out among other
dielectric luminescent particles. The analysis of the damped rotation dynamics of an optically trapped
microparticle is a novel and powerful tool that allows not only the controlled and remote manipulation of
the sensor, but also an improved characterization of the medium and fast recording of its content.
Here, an optically trapped and rotated rare-earth-doped β-NaYF 4 :RE 3+ microparticle is presented as a novel
sensor to characterize the properties of a liquid medium at the microscale (temperature, viscosity and
detection of bio-objects).
3D cell culture resembles tissues better than traditional monolayer cultures, which differ greatly from in vivo models. We use this technique to develop multicellular spheroids from two cell lines: MCF-7 (adenocarcinoma) and U87mg (glioblastoma astrocytoma), by forced-floating method. In this work, we research the spheroid behaviour through two optical techniques: photovoltaic tweezers and laser irradiation. We use photovoltaic tweezers to manipulate spheroids and to explore their electric charge. We also investigate their biological response to laser irradiation depending on wavelength and laser power. Finally, cell viability of the spheroids after undergoing each of these optical/optoelectric treatments has been quantified.
Lanthanide-based upconverting nanoparticles (UCNPs) boast low thermal sensitivity and brightness, which, along with the difficulty in controlling individual UCNP remotely, make them less than ideal nanothermometers at the single-particle level. In this work we show how these problems can be elegantly solved using a thermoresponsive polymeric coating. Upon decorating the surface of NaYF4:Er,Yb UCNPs with poly(N-isopropylacrylamide) (PNIPAM), a >10-fold enhancement in optical forces is observed, allowing stable trapping and manipulation of a single UCNP in the physiological temperature range (20-45 ºC). This optical force improvement is accompanied by a significant enhancement of the thermal sensitivity reaching a maximum value of 7 % °C-1 at 31.5 ºC caused by the temperature-induced collapse of PNIPAM.
In this work we demonstrate laser cooling in liquids on a micrometric scale by using colloidal microparticles doped with ytterbium ions. These microparticles behave as cooling elements thanks to their anti-stokes luminescence. The novel point of our work is that laser radiation produces laser refrigeration and linear and angular transfer of momentum that allows cooling during remote particle manipulation and rotation. This dual function of laser radiation opens the door to new applications based on the use of portable micro-refrigerants. In addition, we demonstrate for the first time how analyzing rotation dynamics is a new and simpler way to determine, in real time, the magnitude of cooling. In other words, we show how laser light can heat, cool, move and rotate our particles simultaneously. All this without requiring complicated experimental systems.
Since Arthur Ashkin and coworkers found that focused laser beam could displace and levitate microsized particles, optical tweezers has turn out to be a reliable noncontact tool for 3D manipulation of micro-objects, allows sensing by using only a single particle. The further development of nanophotonics toward higher sensitivities and resolutions continues to stimulate optical trapping of smaller and smaller objects. While applied to sub 100 nm-particles, the optical force starts to get insufficient to trap or manipulate, and the trap potential starts to be comparable to the thermal energy. Although there are a lot of methods that have been proposed to enhance the optical force, few research gets insight of the temperature effect on optical trapping. In this work, we summarize our recent experimental results on thermal sensing experiments in which micro/nanoparticles are used as probes with the aim of providing a contemporary state-of-the-art about the temperature effects in the stability of potential trapping processes.
An optical system based on a photonic nanojet created by an optically trapped microsphere is proposed for enhanced trapping and remote manipulation of upconverting nanoparticles. The nanojet-induced enhancement in the optical forces acting on a 8 nm single upconverting nanoparticle are compared to those achievable by the traditional approach based on the use of high numerical aperture optics. The results presented in this work prove that this is a simple and effective method to remotely trap, manipulate, and detect single upconverting nanoparticles. This is an innovative approach to overcome the current challenges in the remote manipulation of probes in biosensing or in single particle studies.
Luminescence of a single upconverting particle (NaYF4:Er3+,Yb3+) can be used to determine the optical trap temperature due to the partial absorption of the trapping beam either by the medium (water) or the optically trapped particle itself. This fact is an important drawback can be reduced by shifting the trapping wavelength out of the water absorption band, or by using time-modulated laser trapping beams. Both approaches have been studied and the results have shown that the thermal loading due to the trapping radiation can be minimized.
Rheological parameters (viscosity, creep compliance and elasticity) play an important role in cell function and viability. For this reason different strategies have been developed for their study. In this work, two new microrheometric techniques are presented. Both methods take advantage of the analysis of the polarized emission of an upconverting particle to determine its orientation inside the optical trap. Upconverting particles are optical materials that are able to convert infrared radiation into visible light. Their usefulness has been further boosted by the recent demonstration of their three-dimensional control and tracking by single beam infrared optical traps. In this work it is demonstrated that optical torques are responsible of the stable orientation of the upconverting particle inside the trap. Moreover, numerical calculations and experimental data allowed to use the rotation dynamics of the optically trapped upconverting particle for environmental sensing. In particular, the cytoplasm viscosity could be measured by using the rotation time and thermal fluctuations of an intracellular optically trapped upconverting particle, by means of the two previously mentioned microrheometric techniques.
We have studied different strategies of use of luminescence thermometry with upconverting nanoparticles in the biological range of temperatures, among them, the thermal sensing ability of fluoresncent lifetime of Er,Yb:NaY2F5 nanoparticles. Er,Yb:NaY2F5O nanocrystals show great potentiality as thermal sensors at the nanoscale for biomedical applications due to the incorporation of additional non-radiative relaxation mechanisms that shorten the emission lifetime generated by the oxygen present in the structure. Here we report ex-vivo temperature determination by laser induced heating in chicken breast using lifetime-based thermometry in these up-conversion nanoparticles.
Strong ion migration in shown to enable the production of high refractive index contrast waveguides by fs-laser writing
in a commercial (Er,Yb)-doped phosphate based glass. Waveguide writing was performed using a high repetition rate fslaser
fibre amplifier operated at 500 kHz and the slit shaping technique. Based on measurements of the NA of
waveguides, the positive refractive index change (Δn) of the guiding region has been estimated to be ∼1-2 x10-2. The
compositional maps of the waveguides cross-sections performed by X-ray microanalysis evidenced a large increase of
the La local concentration in the guiding region up to ~25% (relative to the non-irradiated material). This large
enrichment in La was accompanied by the cross migration of K to a neighbouring low refractive index zone. The
refractive index of the La-phosphate glass increases linearly with the La2O3 content (Δn per mole fraction increase of
La2O3 ≈ 5x10-3) mainly because of the relative mass of the La3+ ions. The density increase without substantial
modification of the glass network was confirmed by space-resolved micro-Raman spectroscopy measurements showing
minor variations in the (PO2)sym vibration Raman band. These results provide evidence for the feasibility of adapting the glass composition for enabling laser-writing of high refractive index contrast structures via spatially selective
modification of the glass composition.
B. del Rosal, P. Haro-González, W. Ramsay, L. Maestro, K. Santacruz-Gómez, M. C. Iglesias-de la Cruz, F. Sanz-Rodríguez, J. Y. Chooi, P. Rodríguez-Sevilla, D. Choudhury, A. Kar, J. García Solé, L. Paterson, D. Jaque
Laser-induced thermal effects in optically trapped microspheres and single cells have been investigated by Luminescence
Thermometry. Thermal spectroscopy has revealed a non-localized temperature distribution around the trap that extends
over tens of microns, in agreement with previous theoretical models. Solvent absorption has been identified as the key
parameter to determine laser-induced heating, which can be reduced by establishing a continuous fluid flow of the
sample. Our experimental results of thermal loading at a variety of wavelengths reveal that an optimum trapping
wavelength exists for biological applications close to 820 nm. This has been corroborated by a simultaneous analysis of
the spectral dependence of cellular heating and damage in human lymphocytes during optical trapping. Minimum
intracellular heating, well below the cytotoxic level (43 °C), has been demonstrated to occur for optical trapping with 820 nm laser radiation, thus avoiding cell damage.
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