Influence of experimental parameters on the laser heating of an optical trap

Scale-Dependent Rheology of Synovial Fluid Lubricating Macromolecules

Synovial fluid (SF) is the complex biofluid that facilitates the exceptional lubrication of articular cartilage in joints. Its primary lubricating macromolecules, the linear polysaccharide hyaluronic acid (HA) and the mucin-like glycoprotein proteoglycan 4 (PRG4 or lubricin), interact synergistically to reduce boundary friction. However, the precise manner in which these molecules influence the rheological properties of SF remains unclear. This study aimed to elucidate this by employing confocal microscopy and multiscale rheometry to examine the microstructure and rheology of solutions containing recombinant human PRG4 (rhPRG4) and HA. Contrary to previous assumptions of an extensive HA-rhPRG4 network, it is discovered that rhPRG4 primarily forms stiff, gel-like aggregates. The properties of these aggregates, including their size and stiffness, are found to be influenced by the viscoelastic characteristics of the surrounding HA matrix. Consequently, the rheology of this system is not governed by a single length scale, but instead responds as a disordered, hierarchical network with solid-like rhPRG4 aggregates distributed throughout the continuous HA phase. These findings provide new insights into the biomechanical function of PRG4 in cartilage lubrication and may have implications in the development of HA-based therapies for joint diseases like osteoarthritis.

Shining Light in Mechanobiology: Optical Tweezers, Scissors, and Beyond

Mechanobiology helps us to decipher cell and tissue functions by looking at changes in their mechanical properties that contribute to development, cell differentiation, physiology, and disease. Mechanobiology sits at the interface of biology, physics and engineering. One of the key technologies that enables characterization of properties of cells and tissue is microscopy. Combining microscopy with other quantitative measurement techniques such as optical tweezers and scissors, gives a very powerful tool for unraveling the intricacies of mechanobiology enabling measurement of forces, torques and displacements at play. We review the field of some light based studies of mechanobiology and optical detection of signal transduction ranging from optical micromanipulation-optical tweezers and scissors, advanced fluorescence techniques and optogenentics. In the current perspective paper, we concentrate our efforts on elucidating interesting measurements of forces, torques, positions, viscoelastic properties, and optogenetics inside and outside a cell attained when using structured light in combination with optical tweezers and scissors. We give perspective on the field concentrating on the use of structured light in imaging in combination with tweezers and scissors pointing out how novel developments in quantum imaging in combination with tweezers and scissors can bring to this fast growing field.

Probing Mitotic Chromosome Mechanics Using Optical Tweezers

During mitosis, cells compact their DNA into rodlike shapes, four orders of magnitude shorter than the DNA backbone contour length. We describe an experimental protocol to isolate and study these intricate mitotic chromosomes using optical tweezers. We touch upon the technical details of the required optical tweezers and microfluidics setup, including advanced force calibration procedures to accurately measure the high forces the chromosomes withstand. The procedure used to isolate mitotic chromosomes, including biotinylation of the telomeric ends to facilitate trapping them in optical tweezers, is described in detail. Finally, we provide a protocol for carrying out optical tweezers experiments on the isolated mitotic chromosomes.

Light-to-Heat Conversion of Optically Trapped Hot Brownian Particles

Anisotropic hybrid nanostructures stand out as promising therapeutic agents in photothermal conversion-based treatments. Accordingly, understanding local heat generation mediated by light-to-heat conversion of absorbing multicomponent nanoparticles at the single-particle level has forthwith become a subject of broad and current interest. Nonetheless, evaluating reliable temperature profiles around a single trapped nanoparticle is challenging from all of the experimental, computational, and fundamental viewpoints. Committed to filling this gap, the heat generation of an anisotropic hybrid nanostructure is explored by means of two different experimental approaches from which the local temperature is measured in a direct or indirect way, all in the context of hot Brownian motion theory. The results were compared with analytical results supported by the numerical computation of the wavelength-dependent absorption efficiencies in the discrete dipole approximation for scattering calculations, which has been extended to inhomogeneous nanostructures. Overall, we provide a consistent and comprehensive view of the heat generation in optical traps of highly absorbing particles from the viewpoint of the hot Brownian motion theory.

Temperature Measurement of Trapped, Thermally Sensitive Single Particles in an Optical Trap Using Raman Spectroscopy

Single particles trapped in an optical trap may experience temperature elevation, yet direct measurement of temperature and its distribution inside the optical trap of several to hundreds of microns in size remains a big challenge. We introduce a method that can measure the temperature inside a universal optical trap (UOT) using Raman spectroscopy of single trapped particles of high thermal conductivity. We measured temperature and temperature distributions inside the UOT using Raman shifts of single-walled carbon nanotubes (SWCNTs) and micron-sized diamonds (MSDs), which are heated by trapping laser beams directly or indirectly, depending on the location of the particle in the trap. We show that the temperature at the center of the UOT is much lower than the temperature along the hollow beams that form a hollow, cage-shaped UOT. In the range of the trapping laser power of 200-2950 mW, the surface temperature of particles trapped at the center of a UOT changes from 322 K to 830 K, correspondingly. This result gives a heating rate as a high thermal-absorbing particle trapped in the center of the UOT with 18.3 ± 0.4 °C/100 mW. In addition, the temperature gradient outside the UOT was also characterized by trapping SWCNT particles outside the UOT. Results show that when a light-absorbing particle is trapped for the study of material property, phase transitions, surface equilibrium process, chemical reactions, etc., this method can be used to measure temperature distribution and its variations in the trap and its surroundings.

Looking at Biomolecular Interactions through the Lens of Correlated Fluorescence Microscopy and Optical Tweezers

Understanding complex biological events at the molecular level paves the path to determine mechanistic processes across the timescale necessary for breakthrough discoveries. While various conventional biophysical methods provide some information for understanding biological systems, they often lack a complete picture of the molecular-level details of such dynamic processes. Studies at the single-molecule level have emerged to provide crucial missing links to understanding complex and dynamic pathways in biological systems, which are often superseded by bulk biophysical and biochemical studies. Latest developments in techniques combining single-molecule manipulation tools such as optical tweezers and visualization tools such as fluorescence or label-free microscopy have enabled the investigation of complex and dynamic biomolecular interactions at the single-molecule level. In this review, we present recent advances using correlated single-molecule manipulation and visualization-based approaches to obtain a more advanced understanding of the pathways for fundamental biological processes, and how this combination technique is facilitating research in the dynamic single-molecule (DSM), cell biology, and nanomaterials fields.

Intense femtosecond optical pulse shaping approaches to spatiotemporal control

For studying any event, measurement can never be enough; "control" is required. This means mere passive tracking of the event is insufficient and being able to manipulate it is necessary. To maximize this capability to exert control and manipulate, both spatial and temporal domains need to be jointly accounted for, which has remained an intractable problem at microscopic scales. Simultaneous control of dynamics and position of an observable event requires a holistic combination of spatial and temporal control principles, which gives rise to the field of spatiotemporal control. For this, we present a novel femtosecond pulse-shaping approach. We explain how to achieve spatiotemporal control by spatially manipulating the system through trapping and subsequently or simultaneously exerting temporal control using shaped femtosecond pulses. By leveraging ultrafast femtosecond lasers, the prospect of having temporal control of molecular dynamics increases, and it becomes possible to circumvent the relaxation processes at microscopic timescales. Optical trapping is an exemplary demonstration of spatial control that results in the immobilization of microscopic objects with radiation pressure from a tightly focused laser beam. Conventional single-beam optical tweezers use continuous-wave (CW) lasers for achieving spatial control through photon fluxes, but these lack temporal control knobs. We use a femtosecond high repetition rate (HRR) pulsed laser to bypass this lack of dynamical control in the time domain for optical trapping studies. From a technological viewpoint, the high photon flux requirement of stable optical tweezers necessitates femtosecond pulse shaping at HRR, which has been a barrier until the recent Megahertz pulse shaping developments. Finally, recognizing the theoretical distinction between tweezers with femtosecond pulses and CW lasers is of paramount interest. Non-linear optical (NLO) interactions must be included prima facie to understand pulsed laser tweezers in areas where they excel, like the two-photon-fluorescence-based detection. We show that our theoretical model can holistically address the common drawback of all tweezers. We are able to mitigate the effects of laser-induced heating by balancing this with femtosecond laser-induced NLO effects. An interesting side-product of HRR femtosecond-laser-induced thermal lens is the development of femtosecond thermal lens spectroscopy (FTLS) and its ability to provide sensitive molecular detection.

Thermal fluctuations of the lipid membrane determine particle uptake into Giant Unilamellar Vesicles

Phagocytic particle uptake is crucial for the fate of both living cells and pathogens. Invading particles have to overcome fluctuating lipid membranes as the first physical barrier. However, the energy and the role of the fluctuation-based particle-membrane interactions during particle uptake are not understood. We tackle this problem by indenting the membrane of differently composed Giant Unilamellar Vesicles (GUVs) with optically trapped particles until particle uptake. By continuous 1 MHz tracking and autocorrelating the particle's positions within 30µs delays for different indentations, the fluctuations' amplitude, the damping, the mean forces, and the energy profiles were obtained. Remarkably, the uptake energy into a GUV becomes predictable since it increases for smaller fluctuation amplitudes and longer relaxation time. Our observations could be explained by a mathematical model based on continuous suppression of fluctuation modes. Hence, the reduced particle uptake energy for protein-ligand interactions LecA-Gb3 or Biotin-Streptavidin results also from pronounced, low-friction membrane fluctuations.

Optical tweezers across scales in cell biology

Optical tweezers (OT) provide a noninvasive approach for delivering minute physical forces to targeted objects. Controlling such forces in living cells or in vitro preparations allows for the measurement and manipulation of numerous processes relevant to the form and function of cells. As such, OT have made important contributions to our understanding of the structures of proteins and nucleic acids, the interactions that occur between microscopic structures within cells, the choreography of complex processes such as mitosis, and the ways in which cells interact with each other. In this review, we highlight recent contributions made to the field of cell biology using OT and provide basic descriptions of the physics, the methods, and the equipment that made these studies possible.

Exploring cell and tissue mechanics with optical tweezers

Cellular and tissue biosystems emerge from the assembly of their constituent molecules and obtain a set of specific material properties. To measure these properties and understand how they influence cellular function is a central goal of mechanobiology. From a bottoms-up, physics or engineering point-of-view, such systems are a composition of basic mechanical elements. However, the sheer number and dynamic complexity of them, including active molecular machines and their emergent properties, makes it currently intractable to calculate how biosystems respond to forces. Because many diseases result from an aberrant mechanotransduction, it is thus essential to measure this response. Recent advances in the technology of optical tweezers have broadened their scope from single-molecule applications to measurements inside complex cellular environments, even within tissues and animals. Here, we summarize the basic optical trapping principles, implementations and calibration procedures that enable force measurements using optical tweezers directly inside cells of living animals, in combination with complementary techniques. We review their versatility to manipulate subcellular organelles and measure cellular frequency-dependent mechanics in the piconewton force range from microseconds to hours. As an outlook, we address future challenges to fully unlock the potential of optical tweezers for mechanobiology.

Foregut organ progenitors and their niche display distinct viscoelastic properties in vivo during early morphogenesis stages

Material properties of living matter play an important role for biological function and development. Yet, quantification of material properties of internal organs in vivo, without causing physiological damage, remains challenging. Here, we present a non-invasive approach based on modified optical tweezers for quantifying sub-cellular material properties deep inside living zebrafish embryos. Material properties of cells within the foregut region are quantified as deep as 150 µm into the biological tissue through measurements of the positions of an inert tracer. This yields an exponent, α, which characterizes the scaling behavior of the positional power spectra and the complex shear moduli. The measurements demonstrate differential mechanical properties: at the time when the developing organs undergo substantial displacements during morphogenesis, gut progenitors are more elastic (α = 0.57 ± 0.07) than the neighboring yolk (α = 0.73 ± 0.08), liver (α = 0.66 ± 0.06) and two mesodermal (α = 0.68 ± 0.06, α = 0.64 ± 0.06) progenitor cell populations. The higher elasticity of gut progenitors correlates with an increased cellular concentration of microtubules. The results infer a role of material properties during morphogenesis and the approach paves the way for quantitative material investigations in vivo of embryos, explants, or organoids.

Here, the authors present a method based on optical tweezers to measure mechanical properties of cells inside living zebrafish embryos. The measurement reveals spatiotemporally distinct mechanical properties, linking cell mechanics and morphogenesis.

Laser induced degradation of bacterial spores during micro-Raman spectroscopy

Micro-Raman spectroscopy combined with optical tweezers is a powerful method to analyze how the biochemical composition and molecular structures of individual biological objects change with time. In this work we investigate laser induced effects in the trapped object. Bacillus thuringiensis spores, which are robust organisms known for their resilience to light, heat, and chemicals are used for this study. We trap spores and monitor the Raman peak from CaDPA (calcium dipicolinic acid), which is a chemical protecting the spore core. We see a correlation between the amount of laser power used in the trap and the release of CaDPA from the spore. At a laser power of 5 mW, the CaDPA from spores in water suspension remain intact over the 90 min experiment, however, at higher laser powers an induced effect could be observed. SEM images of laser exposed spores (after loss of CaDPA Raman peak was confirmed) show a notable alteration of the spores' structure. Our Raman data indicates that the median dose exposure to lose the CaDPA peak was ∼60 J at 808 nm. For decontaminated/deactivated spores, i.e., treated in sodium hypochlorite or peracetic acid solutions, the sensitivity on laser power is even more pronounced and different behavior could be observed on spores treated by the two chemicals. Importantly, the observed effect is most likely photochemical since the increase of the spore temperature is in the order of 0.1 K as suggested by our numerical multiphysics model. Our results show that care must be taken when using micro-Raman spectroscopy on biological objects since photoinduced effects may substantially affect the results.

Temperature Quantification and Temperature Control in Optical Tweezers

Optical tweezers are widely used to investigate biomolecules and biomolecular interactions. In these investigations, the biomolecules of interest are typically coupled to microscopic beads that can be optically trapped. Since high-intensity laser beams are required to trap such microscopic beads, laser-induced heating due to optical absorption is typically unavoidable. This chapter discusses how to identify, quantify, and control thermal effects in optical tweezers. We provide a brief overview of the reported causes and effects of unwanted heating in optical tweezers systems. Specific details are provided on methods to perform a temperature-independent trap calibration procedure. Finally, an effective temperature-control system is presented, and we discuss the operation of this system as well as the methods to measure the temperature at the optically trapped particle.

Exploring Single-Nanoparticle Dynamics at High Temperature by Optical Tweezers

The experimental determination of the velocity of a colloidal nanoparticle (v_NP) has recently became a hot topic. The thermal dependence of v_NP is still left to be explored although it is a valuable source of information allowing, for instance, the discernment between ballistic and diffusive regimes. Optical tweezers (OTs) constitute a tool especially useful for the experimental determination of v_NP although they have only been capable of determining it at room temperature. In this work, we demonstrate that it is possible to determine the temperature dependence of the diffusive velocity of a single colloidal nanoparticle by analyzing the temperature dependence of optical forces. The comparison between experimental results and theoretical predictions allowed us to discover the impact that the anomalous temperature dependence of water properties has on the dynamics of colloidal nanoparticles in this temperature range.

Detachment of Dunaliella tertiolecta Microalgae from a Glass Surface by a Near-Infrared Optical Trap

We report on the observation of the detachment in situ and in vivo of Dunaliella tertiolecta microalgae cells from a glass surface using a 1064 nm wavelength trapping laser beam. The principal bends of both flagella of Dunaliella were seen self-adhered to either the top or bottom coverslip surfaces of a 50 μm thick chamber. When a selected attached Dunaliella was placed in the trapping site, it photoresponded to the laser beam by moving its body and flagellar tips, which eventually resulted in its detachment. The dependence of the time required for detachment on the trapping power was measured. No significant difference was found in the detachment time for cells detached from the top or bottom coverslip, indicating that the induced detachment was not due solely to the optical forces applied to the cells. After detachment, the cells remained within the optical trap. Dunaliella detached from the bottom were seen rotating about their long axis in a counterclockwise direction, while those detached from the top did not rotate. The rotation frequency and the minimal force required to escape from the trap were also measured. The average rotation frequency was found to be independent of the trapping power, and the swimming force of a cell escaping the laser trap ranged from 4 to 10 picoNewtons. Our observations provide insight into the photostimulus produced when a near-infrared trapping beam encounters a Dunaliella. The microalgae frequently absorb more light than they can actually use in photosynthesis, which could cause genetic and molecular changes. Our findings may open new research directions into the study of photomovement in species of Dunaliella and other swimming microorganisms that could eventually help to solve technological problems currently confronting biomass production. In future work, studies of the response to excess light may uncover unrecognized mechanisms of photoprotection and photoacclimation.

Real-Time Microscale Temperature Imaging by Stimulated Raman Scattering

Microscale thermometry of aqueous solutions is essential to understand the dynamics of local heat generation and dissipation in chemical and biological systems. A wide variety of fluorescent probes have been developed to map temperature changes with submicrometer resolution, but they often suffer from the uncertainty associated with microenvironment-dependent fluorescent properties. In this work, we develop a label-free ratiometric stimulated Raman scattering (SRS) microscopy technique to quantify microscale temperature by monitoring the O-H Raman stretching modes of water. By tracking the ratio changes of the hydrogen-bonding O-H band and the isosbestic band, we can directly quantify the temperature of water-based environments in real time without exogenous contrast agents. We demonstrate real-time measurement of localized intracellular and extracellular temperature changes due to laser absorption. This high-speed nonlinear optical imaging technique has the potential for in situ microscale imaging of thermogenesis in both chemical and biological systems.

Heat Generation in Single Magnetic Nanoparticles under Near-Infrared Irradiation

Heat generation by pointlike structures is an appealing concept for its implications in nanotechnology and biomedicine. The way to pump energy that excites heat locally and the synthesis of nanostructures that absorb such energy are key issues in this endeavor. High-frequency alternating magnetic or near-infrared optical fields are used to induce heat in iron oxide nanoparticles, a combined solution that is being exploited in hyperthermia treatments. However, the temperature determination around a single iron oxide nanoparticle remains a challenge. We study the heat released from iron oxide nanostructures under near-infrared illumination on a one-by-one basis by optical tweezers. To measure the temperature, we follow the medium viscosity changes around the trapped particle as a function of the illuminating power, thus avoiding the use of thermal probes. Our results help interpret temperature, a statistical parameter, in the nanoscale and the concept of heat production by nanoparticles under thermal agitation.

Manipulating rod-shaped bacteria with optical tweezers

Optical tweezers have great potential in microbiology for holding and manipulating single cells under a microscope. However, the methodology to use optical tweezers for live cell studies is still at its infancy. In this work, we determined suitable parameters for stable trapping of single Escherichia coli bacteria, and identified the upper limits of IR-exposure that can be applied without affecting viability. We found that the maximum tolerable IR-exposure is 2.5-fold higher when employing oscillating instead of stationary optical trapping (20 J and 8 J, respectively). We found that good stability of cells in an oscillating trap is achieved when the effective trap length is 20% larger than the cell length, the oscillation frequency higher than 100 Hz and the trap oriented perpendicular to the medium flow direction. Further, we show, using an IR power just sufficient for stable holding, that bacteria remain viable during at least 30 min of holding in an oscillating trap. In this work, we established a method for long-term stable handling of single E. coli cells using optical tweezers. This work will pave the way for future use of optical tweezers in microbiology.

Optical Tweezers: Phototoxicity and Thermal Stress in Cells and Biomolecules

For several decades optical tweezers have proven to be an invaluable tool in the study and analysis of myriad biological responses and applications. However, as with every tool, they can have undesirable or damaging effects upon the very sample they are helping to study. In this review the main negative effects of optical tweezers upon biostructures and living systems will be presented. There are three main areas on which the review will focus: linear optical excitation within the tweezers, non-linear photonic effects, and thermal load upon the sampled volume. Additional information is provided on negative mechanical effects of optical traps on biological structures. Strategies to avoid or, at least, minimize these negative effects will be introduced. Finally, all these effects, undesirable for the most, can have positive applications under the right conditions. Some hints in this direction will also be discussed.

High-precision temperature sensor based on weak measurement

High-precision temperature sensor is demonstrated based on weak measurement using spectrum domain analysis. By introducing an extra phase to the postselection, the operating temperature range and temperature precision can be conveniently modulated. Spectral shifts resulted from temperature variation are robust to practical imperfections. The precision of 2.4 × 10^-6°C can be achieved by a currently available spectrometer. The maximum operating range is associated with the nematic temperature range of nematic liquid crystals (NLCs) sample. Moreover, the temperature sensitivity of 16.16 nm/°C is obtained experimentally in different linear operating intervals.

Effects of near infrared focused laser on the fluorescence of labelled cell membrane

Near infrared (NIR) laser light can have important reactions on live cells. For example, in a macroscopic scale, it is used therapeutically to reduce inflammation and in a single-cell scale, NIR lasers have been experimentally used to guide neuronal growth. However, little is known about how NIR lasers produce such behaviours on cells. In this paper we report effects of focussing a continuous wave 810-nm wavelength laser on in vivo 3T3 cells plasma membrane. Cell membranes were labelled with FM 4-64, a dye that fluoresces when associated to membrane lipids. Confocal microscopy was used to image cell membranes and perform fluorescence recovery after photobleaching (FRAP) experiments. We found that the NIR laser produces an increase of the fluorescence intensity at the location of laser spot. This intensity boost vanishes once the laser is turned off. The mean fluorescence increase, calculated over 75 independent measurements, equals 19%. The experiments reveal that the fluorescence rise is a growing function of the laser power. This dependence is well fitted with a square root function. The FRAP, when the NIR laser is acting on the cell, is twice as large as when the NIR laser is off, and the recovery time is 5 times longer. Based on the experimental evidence and a linear fluorescence model, it is shown that the NIR laser provokes a rise in the number of molecular associations dye-lipid. The results reported here may be a consequence of a combination of induced increments in membrane fluidity and exocytosis.

Controlled Mechanical Motions of Microparticles in Optical Tweezers

Optical tweezers, formed by a highly focused laser beam, have intriguing applications in biology and physics. Inspired by molecular rotors, numerous optical beams and artificial particles have been proposed to build optical tweezers trapping microparticles, and extensive experiences have been learned towards constructing precise, stable, flexible and controllable micromachines. The mechanism of interaction between particles and localized light fields is quite different for different types of particles, such as metal particles, dielectric particles and Janus particles. In this article, we present a comprehensive overview of the latest development on the fundamental and application of optical trapping. The emphasis is placed on controllable mechanical motions of particles, including rotation, translation and their mutual coupling under the optical forces and torques created by a wide variety of optical tweezers operating on different particles. Finally, we conclude by proposing promising directions for future research.