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/mm<sup>2</sup>. 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.
We achieved four types of laser emissions with quantum dots (QDs) using the same high-Q-factor optofluidic ring resonator (OFRR) platform. In the first type, 2 μM QDs dissolved in toluene that filled the entire OFRR cavity volume were employed as the gain medium. The lasing threshold was 15-22 μJ/mm<sup>2</sup>. In the second type, 2 μM aqueous QDs were in bulk buffer solution that filled the entire OFRR cavity volume. The lasing threshold was 0.1 μJ/mm<sup>2</sup>, over 3 orders of magnitude lower than the state-of-the-art. In the third type, the aqueous QDs were immobilized as a single layer on the interface between the OFRR inner wall and buffer solution with a surface density as low as 3×10<sup>9</sup> − 10<sup>1</sup>0cm<sup>−2</sup>. The lasing threshold of 60 μJ/mm<sup>2</sup> was achieved. In the fourth type, we achieved optofluidic FRET lasing using aqueous QDs as FRET donors and Cy5 dye molecules as acceptors. We observed lasing from Cy5 emission band in QD-Cy5 pair when excited at QD absorption band, far away from Cy5 absorption maximum. We also report a comprehensive theoretical analysis of optofluidic FRET lasers that was performed based on a Fabry-Perot microcavity using a rate equation model. By comparing FRET lasingbased sensors with conventional sensors using FRET signals obtained by spontaneous fluorescence emission, we show that for optimal pump fluence and FRET pair concentration, FRET lasing can lead to more than 20-fold enhancement in detection sensitivities of conformation changes for linker lengths in the Förster radius range.
The integration of optofluidic laser and FRET mechanism provides novel research frontiers, including sensitive biochemical analysis and novel photonic devices, such as on-chip coherent light sources and bio-tunable lasers. Here we investigated an optofluidic FRET laser using quantum dots (QDs) as FRET donors. We achieved lasing from Cy5 as the acceptor in the QD-Cy5 pair with excitation at 450 nm where Cy5 has negligible absorption by itself. The threshold was approximately 14 μJ/mm<sup>2</sup>. The demonstrated capability of QDs as the donor in a FRET laser greatly improves the versatility of optofluidic laser operation due to the broad and large absorption cross section of QDs in the blue and UV spectral region. The excitation efficiency of the acceptor molecules through FRET channel was also analyzed, showing that the energy transfer rate and the non-radiative Auger recombination rate of QDs plays a significant role in FRET laser performance.
Optofluidic lasers combine the advantages of microfluidics and laser technology. Unlike traditional lasers, optofluidic lasers obtain the optical feedback from microfluidic channels with gain media (<i>e.g.</i>, dyes) inside. Due to the small size of microfluidic channels, optofluidic lasers own the unique capabilities in terms of handling liquid of ρL~ μL volumes. Therefore, there is currently a great deal of interest in adapting optofluidic lasers for compact laser light sources and micro-total-analysis systems. Here, we use two examples to demonstrate the feasibility of using optofluidic lasers to sensitively detect DNA and protein. In the first example, the optofluidic laser is used to detect small conformational change in DNA Holliday junctions. The DNA Holliday junction has four branched double-helical arm structures, each of which is conjugated with Cy3 or Cy5 as the donor/acceptor pair. The conformational changes of the Holliday junction lead to the changes of fluorescence resonance energy transfer (FRET) between the donor and the acceptor. Using the optical feedback provided by the optofluidic laser, we are able to achieve nearly 100% wavelength switching. The FRET signal generated by the optofluidic laser is about 16 times more sensitive to DNA conformational changes than the conventional method. The second example is concerned with a fluorescent proteins laser. Green, yellow, and red optofluidic lasers based on fluorescent proteins are demonstrated, and the lasing threshold of 3 μmCitrine is only 1 μJ/mm<sup>2</sup>. This work will potentially open a door to study protein-protein interactions via the sensitive intra-cavity laser detection method.