Optical barcodes have demonstrated a great potential in multiplexed bioassays and cell tracking for their distinctive spectral fingerprints. The vast majority of optical barcodes were designed to identify a specific target by fluorescence emission spectra, without being able to characterize dynamic changes in response to analytes through time. To overcome these limitations, the concept of the bioresponsive dynamic photonic barcode was proposed by exploiting interfacial energy transfer between a microdroplet cavity and binding molecules. Whispering-gallery modes resulting from cavity-enhanced energy transfer were therefore converted into photonic barcodes to identify binding activities, in which more than trillions of distinctive barcodes could be generated by a single droplet. Dynamic spectral barcoding was achieved by a significant improvement in terms of signal-to-noise ratio upon binding to target molecules. Theoretical studies and experiments were conducted to elucidate the effect of different cavity sizes and analyte concentrations. Time-resolved fluorescence lifetime was implemented to investigate the role of radiative and non-radiative energy transfer. Finally, microdroplet photonic barcodes were employed in biodetection to exhibit great potential in fulfilling biomedical applications.
Electrostatics plays a critical function in most biomolecules, therefore monitoring subtle biomolecular bindings and dynamics via the electrostatic changes of biomolecules at biointerfaces has been an attractive topic recently and has provided the basis in diagnosis and biomedical science. Here we present a bioelectrostatic responsive microlaser based on liquid crystal (LC) droplet and explored its application for ultrasensitive detection of negatively charged biomolecules. Whispering gallery mode (WGM) lasing from positively charged LC microdroplets was applied as the optical resonator, where the lasing wavelength shift was employed as a sensing parameter. With the dual impacts from whispering-gallery mode and liquid crystal, molecular binding signals will be amplified in such LC droplet sensors. It is found that molecular electrostatic changes at the biointerface of droplet triggered wavelength shift in lasing spectra. The total wavelength shift increased proportionally with the adhering target concentrations. Compared to a conventional polarized optical microscope, significant improvements in sensitivity and dynamic range by four orders of magnitude were achieved. Our work indicated that the surface-to-volume ratio plays a critical role in the detection sensitivity in WGM laser-based microsensors. Finally, bovine serum albumin and specific biosensing using streptavidin and biotin were exploited to demonstrate the potential applications of microlasers with a detection limit on the order of 1 pM. We anticipate this approach will open new possibilities for the ultrasensitive label-free detection of charged biomolecules and molecular interactions by providing a lower detection limit than conventional methods.
Optofluidic bio-lasers are currently of high interest for sensitive, intra-cavity, biochemical analysis. In comparison with conventional methods such as fluorescence and colorimetric detection, lasers provide us with a method for amplifying small concentration differences in the gain medium, thus achieving high sensitivity. Our previous research has demonstrated that sandwich IL-6 ELISA performed in capillary-based optofluidic laser cavity was able to achieve ultrahigh detection sensitivity (LOD between 1-10 fg/ml) with a small sample volume (~20 μL). However, such approach has several limitations such as low repeatability and long assay time (~8 hours in total, 7 hours for laser measurements). Here, we developed a novel on-chip ELISA laser platform by directly fabricating micro-wells on dielectric mirrors for immunosorbent reactions. Polystyrene microbeads of 30 μm in diameter were placed in the wells to optically enhance the resonance cavity during laser measurement, thus significantly improving reliability, shortening assay time (~1.5 hours, 30 minutes for laser measurements) while maintaining the attractive features such as small sample volume and very high sensitivity (LOD ~0.1 pg/mL for IL-6). This work pushes the ELISA laser one step closer to solving problems in realworld biochemical analysis.
We demonstrated the ultrasound modulated droplet lasers, in which the laser intensity from whispering gallery mode (WGM) of oil droplets can be reversibly enhanced up to 20-fold when the ultrasound pressure is beyond a certain threshold. The lasing enhancement was investigated with various ultrasound frequencies and pressures. Furthermore, the ultrasound modulation of the laser output was achieved by controlling the ultrasound pressure, the duty cycle, and the frequency of ultrasound bursts. Its potential application was explored via the study on a human whole blood vessel phantom. A theoretical analysis was also conducted, showing that the laser emission enhancement results from the directional emission from a deformed cavity under ultrasound pressure. Our studies reveal the unique capabilities of ultrasound modulated droplet lasers, which could lead to the development of laser emission-based microscopy for deep tissue imaging with high spatial resolution and detection sensitivity that may overcome the long-standing drawback of traditional fluorescence imaging.
Biolasers are an emerging technology for next generation biochemical detection and clinical applications. Progress has recently been made to achieve lasing from biomolecules and single living cells. Tissues, which consist of cells embedded in extracellular matrix, mimic more closely the actual complex biological environment in a living body and therefore are of more practical significance. Here, we developed a highly versatile tissue laser platform, in which tissues stained with fluorophores are sandwiched in a high-Q Fabry-Pérot microcavity. Distinct lasing emissions from muscle and adipose tissues stained respectively with fluorescein isothiocyanate (FITC) and boron-dipyrromethene (BODIPY), and hybrid muscle/adipose tissue with dual-staining were achieved with a threshold of only ~10 μJ/mm2. Additionally, we investigated how tissue structure/geometry, tissue thickness, and staining dye concentration affect the tissue laser. It is further found that, despite large fluorescence spectral overlap between FITC and BODIPY in tissues, their lasing emissions could be clearly distinguished and controlled due to their narrow lasing bands and different lasing thresholds, thus enabling highly multiplexed detection. Our tissue laser platform can be broadly applicable to various types of tissues/diseases. It provides a new tool for a wide range of biological and biomedical applications, such as diagnostics/screening of tissues and identification/monitoring of biological transformations in tissue engineering.
Lipid-correlated disease such as atherosclerosis has been an important medical research topic for decades. Many new microscopic imaging techniques such as coherent anti-Stokes Raman scattering and third-harmonic generation (THG) microscopy were verified to have the capability to target lipids in vivo. In the case of THG microscopy, biological cell membranes and lipid bodies in cells and tissues have been shown as good sources of contrast with a laser excitation wavelength around 1200 nm. We report the THG excitation spectroscopy study of two pure free fatty acids including oleic acid and linoleic acid from 1090 to 1330 nm. Different pure fatty acids presented slightly-different THG χ(3) spectra. The measured peak values of THG third-order susceptibility χ(3) in both fatty acids were surprisingly found not to match completely with the resonant absorption wavelengths around 1190 to 1210 nm, suggesting possible wavelengths selection for enhanced THG imaging of lipids while avoiding laser light absorption. Along with the recent advancement in THG imaging, this new window between 1240 to 1290 nm may offer tremendous new opportunities for sensitive label-free lipid imaging in biological tissues.