Advances in optical trapping techniques have brought about remarkable progress in the fields of biomechanics and biology in recent years by using focused laser beams to manipulate microscopic particles, such as cells and other biological entities. However, a significant challenge arises when attempting to trap and manipulate these particles because they lack a refractive index gradient with its surroundings, necessitating the use of high laser power, which, unfortunately, can lead to substantial photon damage to the delicate specimens. To address this challenge, researchers have turned to various innovative approaches, one of which involves combining optical forces with thermophoretic forces, a concept known as opto-thermophoretic trapping.1 By doing so, it becomes possible to reduce the laser power required by two to three orders of magnitude, thus substantially mitigating the risk of photon-induced damage. However, this approach also introduces a new concern: the increased potential for thermal damage due to the combined optical and thermophoretic forces.
Furthermore, most biological objects in aqueous solutions exhibit thermophobic behavior, meaning they naturally move away from areas that are heated by the laser towards cooler regions. This inherent tendency is addressed by modifying solution compositions and beam profiles, enabling more effective control and manipulation within aqueous environments.2-4
Here, we are introducing a small but significant modification to the opto-thermophoretic tweezers: Hypothermal Opto-Thermophoretic Tweezers (HOTTs). Just as we humans seek cooler or warmer environments depending on the external temperature, the same principle can be applied to the micro- and nanoscale world, especially when dealing with entities dispersed in aqueous solutions. In essence, HOTTs involve cooling down the sample before activating the trapping laser beam (Figure 1a), and this seemingly simple change yields three significant advantages. Firstly, and most crucially, cooling the sample environment leads to a substantial reduction in thermal damage inflicted upon microscale entities. Secondly, the cooling effect of HOTTs not only enables trapping but also enhances it across a wide range of colloidal and biological solutions. Finally, this broader applicability opens new possibilities for studying cell-cell interactions in their native solutions without the need for solvent or electrolyte modifications, further nearing opto-thermophoretic tweezers for practical applications.
In this study, we begin by showcasing the successful trapping of polystyrene particles in deionized water at sub-ambient temperatures using HOTTs. Subsequently, we conduct numerical analyses involving particles of varying sizes, solutions, and laser power levels. Our numerical analysis of experiments confirms that cooling the sample provides significant advantages, especially in turbid media with high colloidal concentrations which mimic complex biological fluids (Figure 1b). This advantage arises from the enhanced thermophilic behavior of particles in response to temperature gradients.
We then demonstrate the trapping of human red blood cells under different tonicities using HOTTs. We modify cell tonicities by adjusting the surrounding electrolyte concentration, which significantly impacts the size and shape of the red blood cells (Figure 1c). Additionally, changes in water permeation across the cell membrane influence the cells' response to temperature gradients. Despite the fragile nature of cell membranes and the potential for cell lysis at elevated temperatures, we successfully trap red blood cells in varying tonicities without any cell lysis. This achievement is particularly noteworthy since small changes in the red blood cells’ physiological conditions can lead to cell lysis. Future research is needed to assess the effects of reduced temperature on the pathophysiological conditions of the cells, including their ability to undergo cell division.
Finally, we expand the application of HOTTs to the safe trapping and manipulation of plasmonic vesicles for controlled cargo delivery. Plasmonic vesicles feature a tunable plasmonic coating of gold particles that can modify their optical and spectroscopic properties, making them valuable for opto-biomedical applications.5 However, the heat generated during optical manipulation often leads to the premature release of cargo within the vesicles. By cooling the environment, we increase the thermophilic force on the plasmonic vesicles, enabling their stable trapping at the laser focus point and allowing manipulation in three spatial dimensions. Additionally, we implement a dual laser beam setup for precise control over cargo delivery by rupturing the vesicle at specified locations (Figure 1d).
In summary, HOTTs represent a transformative development in opto-thermophoretic trapping techniques. By taking inspiration from our own temperature preferences, we've unlocked a powerful method that reduces thermal damage, expands trapping capabilities, and facilitates real-world applications in the fields of biology and nanotechnology. With this work, we are nearing opto-thermophoretic tweezers for practical biological applications such as diagnostics, thermal therapy, drug delivery, and surgery.
1 Chen, Z., Li, J. & Zheng, Y. Heat-Mediated Optical Manipulation. Chemical Reviews 122, 3122-3179, doi:10.1021/acs.chemrev.1c00626 (2022).
2 Wurger. Thermal non-equilibrium transport in colloids. Reports on Progress in Physics 73, doi:10.1088/0034-4885/73 (2010).
3 Lin, L. et al. Opto-thermoelectric nanotweezers. Nature Photonics 12, 195-201, doi:10.1038/s41566-018-0134-3 (2018).
4 Braun, M. & Cichos, F. Optically Controlled Thermophoretic Trapping of Single Nano-Objects. ACS Nano 7, 11200-11208, doi:10.1021/nn404980k (2013).
5 Xiong, H. et al. Probing Neuropeptide Volume Transmission In Vivo by Simultaneous Near‐Infrared Light‐Triggered Release and Optical Sensing**. Angewandte Chemie International Edition 61, doi:10.1002/anie.202206122 (2022).