Thermal Optofluidics: Principles and Applications

Abstract

Thermal optofluidics is an emerging field that promises to create numerous research and application opportunities in biophysics, biochemistry, and clinical biology. Innovation in plasmonic optics has led to the development of various invaluable tools in the fields of biosensing and microfluidic manipulation. The optothermal effect originates from light–matter interactions during photon–phonon conversion, which can lead to micro‐ or nanoscale inhomogeneities in the thermal distribution. This further induces a series of hydrodynamic phenomena such as natural convection, Marangoni convection, thermophoresis, the electrolyte Seebeck effect, depletion forces, and interfacial effects in colloidal particles. Light–matter interactions are particularly important for three aspects of microfluidics, namely the motion of colloidal particles, fluidic actuation, and biochemical reactions. This review first systematically elucidates the role of both nanoscale plasmonic thermal generation and heat‐induced fluidic motion in optofluidic microsystems. Then, recent state‐of‐the‐art thermal optofluidic applications of the above‐listed three aspects are presented. The paper aims to provide an insightful reference for future research in optofluidic biochemical systems.

Optothermophoretic flipping method for biomolecule interaction enhancement in biosensing

“…The two phenomena have greatly facilitated optical force induced tweezers 54 because of their abilities of long-range manipulation and facile energy-efficient trapping of small particles in recent years. [55][56][57] Working principle The interaction between antibody and antigen is a complex and dynamic phenomenon based on noncovalent forces. The interaction between antibody Ab and antigen Ag at equilibrium can be expressed as: 58 Ab + Ag ⇌ Ab − Ag When antibody Ab and antigen Ag tend to combine, several criteria are required: (i) The distance between Ab and Ag is small enough; (ii) The two molecules must be arranged in the proper spatial orientation for an effective binding.…”

Tailored optical propulsion forces for controlled transport of resonant gold nanoparticles and associated thermal convective fluid flows

“…This regime has been applied as a lithographic tool for controlled deposition of plasmonic NPs onto the same plasmonic substrate (a film of gold nano-islands fixed onto a glass wafer) required for microbubble generation 34 . Other optofluidic mechanisms, such as thermophoresis or the Seebeck effect with promising technological applications 19 , are outside the scope of the presented experimental study.…”

“…In particular, illumination of a metal NP or nanostructure with a wavelength close to the plasmon resonance transforms it into an efficient local heat source due to the enhanced light absorption. A resonant metal NP can easily reach a temperature that can alter the physical environment, for example, the viscosity of the fluid surrounding it 18 , 19 . This behaviour is the basis of numerous applications, such as photothermal therapy, drug delivery, photothermal and photoacoustic imaging and thermal optofluidics 18 – 28 .…”

“…Thermal optofluidics relies on the optothermal control of fluid motion, which can be achieved by using resonant plasmonic structures. Different physical phenomena are involved in the generation of such fluid flows 18 , 19 , 21 , 29 . A deeper understanding of the rather complex thermal mechanisms providing fluid motion at the microscale would allow control of the direction, velocity and extension of the fluid flows and, in turn, of the optothermal transport of colloidal particles.…”

“…It has been shown that an increase in the local temperature of a metal NP (or an NP assembly) is responsible for the appearance of convective fluid flow. Its configuration depends, in particular, on the position of the NP (usually attached on a floor or ceiling substrate) in the chamber and the width of the latter 19 , 21 , 29 . By illuminating periodic arrays of closely spaced plasmonic nanostructures (lithographically fixed on a floor glass substrate), the collective heating produces a short-range fluid convection motion with a speed of ∼1 µm/s, which can be increased up to ∼10 µm/s by using an optically absorptive substrate 30 .…”