Sensitive detection and imaging in bio-microenvironment is highly desired in biophotonic and biomedical applications. However, conventional photonic materials inevitably show incompatibility and invasiveness to bio-systems. To address this issue, Scientists in China reviewed recent progresses of biophotonic probes, including bio-lasers, biophotonic waveguides, and bio-microlenses, made from biological entities with inherent biocompatibility and minimal invasiveness, with applications for bio-detection and imaging. These biophotonic probes open up entirely new windows for biophotonic researches and biomedical applications.
The rapid development of biophotonics and biomedical sciences makes a high demand on photonic structures that are capable of manipulating light at small scales for sensitive detection of biological signals and precise imaging of cellular structures in bio-microenvironment. Unfortunately, conventional photonic structures based on artificial materials (either inorganic or toxic organic) inevitably show incompatibility and invasiveness when interfacing with biological systems. The design of biophotonic probes from the abundant natural materials, particularly biological entities such as virus, cells and tissues, with the capability of multifunctional light manipulation at target sites can greatly increase the biocompatibility and minimizes the invasiveness to biological microenvironment.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Baojun Li and Professor Hongbao Xin from Institute of Nanophotonics, Jinan University, China, reviewed the intriguing progresses of emerging biophotonic probes made from biological entities, such as virus, bacteria, cells and tissues, for bio-detection and imaging. They systematically reviewed three biophotonic probes with different optical functions, i.e., biological lasers for light generation, cell-based biophotonic waveguides for light transportation, and bio-microlenses for light modulation.
To realize their potential biomedical applications of photonic probes, effective control and modulation of light generation are particularly important in various biochemical environments. In this regard, the unique properties of light emitted by lasers, including high intensity, directionality and monochromatic emission, have rendered lasers one of the most useful tools in biomedical applications. Unlike traditional laser devices, bio-lasers utilize biological entities such as cells, tissues and virus, as part of the cavity and/or gain medium in a biological system. Bio-lasers can be categorized into three types, i.e., cell lasers, tissue lasers and virus lasers. These bio-lasers avoid the biohazards of conventional laser devices. Since their optical output is tightly related to the biological structures and activities of the biological systems, bio-lasers can serve as highly sensitive tools in a range of biomedical applications, including cellular tagging and tracking, diagnostics, intracellular sensing, and novel imaging. For example, whispering gallery modes (WGM) microdisks with slightly different diameters resulted in obviously different lasing output spectra. Intracellular cell lasers realized by incorporating these microdisks into cells enabled tagging and tracking of individual cells from large cell populations at the same time.
In addition to bio-lasers for bio-detection and imaging in biological systems, optical waveguides also play important roles in bio-microenvironments. As the main component for light transportation, optical waveguides can deliver light signals in bio-microenvironments for further real-time analysis, and optical waveguides play irreplaceable role to break the tissue penetration limit of light by transporting light into deep tissues.
To solve the problem of invasiveness and low-biocompatibility of conventional materials-based optical waveguides, living cells hold huge potential for in situ formation of biophotonic waveguide that are inherently elastic, biocompatible, and biodegradable. The refractive index of biological cell (around 1.38) is slightly higher than that of water (around 1.33), thus allowing light guiding through a chain of cells by total internal reflection at the interface of the cell membrane and the water.
A feasible and noninvasive approach to assemble cell-based biophotonic waveguides is optical trapping. By using laser light launched by a tapered optical fiber, biophotonic waveguides can be formed by assembling a chain of bacteria cells through optical force. Light propagation is allowed through cell chains over tens of microns. In another case, nonlinear optical effects have also been applied for biophotonic waveguides formation based on living cells, including algae and red blood cells (RBCs), achieving stable long-distance propagation of light with low loss in biological environments. These cell-based biophotonic waveguides can be performed as a biophotonic probe for cell imaging and biological microenvironment detection.
For example, biophotonic waveguides formed by RBCs provide a potential detection technique for blood pH sensing and diagnosis of blood related disorders.
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