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Resonators have been used in a wide range of fields, such as biochemical detection and microscale lasers. In recent years, optofluidic resonators have attracted a significant amount of attention owing to their unique liquid environments. Liquids containing biochemical samples can be designed to pass through the ring resonators or to directly form droplets, for sample sensing. Liquid diffusion is an important property in optofluidic applications, such as gradient refractive index lenses and waveguides. However, liquid diffusion has not been used in the study of optofluidic resonators, for both possible sensing characteristics, and unidirectional emission that is mostly acted as light sources. Here, we introduce a gradient refractive index profile formed by liquid diffusion in annular channels into a circular resonator, forming a gradient-index resonator with a tunable unidirectional emission. For both simulations and experiments, the squeezed and non-rotationally symmetrical light intensity profile was first obtained in a circular resonator. The squeezed light profile enables unidirectional emission in circular resonators, which is difficult to achieve in conventional ones. The squeezed light profile and unidirectional emission are determined by the refractive index difference of the liquids used, the dimension of the circular channels, and the working wavelengths. In experiments, different dimensions of bending radii were demonstrated and a tunable squeezed light intensity profile and unidirectional emission were exhibited. Interestingly, the squeezed coefficient of light, which was about 1.8 for a bending radius of 100 μm, enabled emission with a divergence angle as small as 14 degrees, which could be used for laser emission applications in the future. This work reveals the significant potential of the novel liquid gradient refractive index resonator, which provides a practicable approach for optofluidic resonator emission applications and also has potential for use in optofluidic sensing based on the squeezed light profile. [ABSTRACT FROM AUTHOR]

Leukocyte subpopulations contain crucial physiological information; hence, precise and specific leukocyte separation is very important for leukemia diagnosis and analysis. However, conventional centrifugation and immunofluorescence-based separation methods are inaccurate and inconvenient due to the overlapping cell size and density or complex marking processes. Herein, we report a new label-free technology for precise leukocyte subpopulation separation by synergy of acoustic and optical technologies. Standing surface acoustic wave (SSAW) solved the problem of gentle and precise focusing of cells in optical systems. In addition, SSAW was used for the separation of granulocytes, which have evident size distinction from other components. In case of lymphocytes and monocytes, which have overlap in size/density, optical force could distinguish them accurately based on the RI difference, with the convenience of acoustic pre-focusing. In this experiment, separation of three types of leukocyte subtypes with considerable throughput and purity was conducted, through which we obtained 99% pure lymphocytes, 98% pure monocytes, and 95% pure granulocytes. Experimental results prove that the device has robust ability in separating leukocyte phenotypes and have the advantages of being non-invasive, label-free and precise. In the future, this convenient hybrid method will be a potential powerful tool for auxiliary clinical diagnosis and analysis. [ABSTRACT FROM AUTHOR]

Nitrite detection plays a very important role in environmental monitoring and for industrial purposes. The commonly used colorimetric analysis requires the measurement of a system's calibration curve by asynchronously preparing and detecting a dozen standard samples, leading to time-consuming, slow and cumbersome procedures. Here, we present a differential colorimetry method that determines the nitrite level based on the paired chromaticity gradient, formed by coupling the colour reaction into the microfluidic network. The two gradients reshape each other and contain enough information for the quantitative analysis of the sample being tested, without the need for a calibration curve. The independence of the two gradients of the absorbance change caused by the detecting system and water quality results in a high stability and anti-interference performance, with the assistance of its self-correcting ability. This differential colorimetry method requires little time and energy consumption as only one sample is needed. Standard nitrite solutions of 0.50 mM and 0.33 mM have been determined with an error of 1.16% and 0.50%, respectively. These measurements are advantageous in terms of greater stability by up to 10 times and accuracy by 6 times, compared with the calibration curve approaches. It is foreseeable that this differential colorimetry method will find a wide range of applications in the field of chemical detection. [ABSTRACT FROM AUTHOR]

Real-time detection and monitoring of the drug resistance of single cells have important significance in clinical diagnosis and therapy. Traditional methods operate a number of times for each individual concentration, and innovation is required for the design of more simple and efficient manipulation platforms with necessary higher sensitivity. Here, we have developed a novel diffused total internal reflection (TIR) method to perform drug metabolism and cytotoxicity analysis of trapped myeloid leukemia cells. Molm-13 cells, a type of acute myeloid leukemia cell, were chosen and injected into the device and fittingly captured by cell traps. Differing from previous studies, a series of different concentrations of azelaic acid (AZA) drug could be used from 0 mM to 50 mM through convection and diffusion processes in a single chip, with each concentration region featuring 50 cells, with a total of 549 cell trapping units. Thanks to the high sensitivity of the TIR method, only cells with the same drug concentration could be illuminated in the detection process. By adjusting the incident angle, we could exactly detect and monitor the drug resistance of the cells using different drug concentrations and the experimental resolution of the drug concentration was as small as 5 mM. Images of the membrane integrity and morphology of the cells in the bright field were measured and we also monitored the cell viabilities in the dark field over 2 hours. The effects of AZA on the Molm-13 cells were explored in different concentrations at the single cell level. Compared with the results of the traditional MTT assay method, the experimental results are more simple and accurate. A cell death of 5% at an AZA concentration of 5 mM was observed after 30 minutes, while a concentration of 40 mM corresponded to a 98% cell death. The designed method in this study provides a novel toolkit to control and monitor drug resistance at the single cell level more easily with higher sensitivity and we believe it has significant potential application in single cell quality assessment and medicine analysis in clinical practice. [ABSTRACT FROM AUTHOR]

Real-time detection of phosphate has significant meaning in marine environmental monitoring and forecasting the occurrence of harmful algal blooms. Conventional monitoring instruments are dependent on artificial sampling and laboratory analysis. They have various shortcomings for real-time applications because of the large equipment size and high production cost, with low target selectivity and the requirement of time-consuming procedures to obtain the detection results. We propose an optofluidic miniaturized analysis chip combined with micro-resonators to achieve real-time phosphate detection. The quantitative water-soluble components are controlled by the flow rate of the phosphate solution, chromogenic agent A (ascorbic acid solution) and chromogenic agent B (12% ammonium molybdate solution, 80% concentrated sulfuric acid and 8% antimony potassium tartrate solution with a volume ratio of 80 : 18 : 2). Subsequently, an on-chip Fabry–Pérot microcavity is formed with a pair of aligned coated fiber facets. With the help of optical feedback, the absorption of phosphate can be enhanced, which can avoid the disadvantages of the macroscale absorption cells in traditional instruments. It can also overcome the difficulties of traditional instruments in terms of size, parallel processing of numerous samples and real-time monitoring, etc. The absorption cell length is shortened to 300 μm with a detection limit of 0.1 μmol L−1. The time required for detection is shortened from 20 min to 6 seconds. Predictably, microsensors based on optofluidic technology will have potential in the field of marine environmental monitoring. [ABSTRACT FROM AUTHOR]

Liquid–liquid (L2) microlenses have great potential for various applications in imaging and detection systems. Traditional L2 microlenses are almost two-dimensional (2D) due to the modulation of flow rates in planer chips. Fundamental difficulties in effective application to cell imaging and analysis arise due to the limitations of 2D profiles. Herein, we demonstrate the feasible design of three-dimensional (3D) L2 biconvex lenses to detect flowing cells. Using the auxiliary curved microchannels, a 3D L2 lens is formed using Dean flow. The shape of the 3D biconvex lens and its focal length can be modulated by tuning the flow rates of the liquids. 3D light focusing was successfully achieved and the focal length could be modulated by around 435 μm, from 3554 μm to 3989 μm, in the experiment. The numerical aperture of the 3D L2 lens was also measured and its range was 0.175–0.198. Compared to a traditional objective lens with the same magnification (4×/0.1), the resolution of the 3D L2 biconvex lenses was improved 1.79-fold due to being completely immersed in liquid. Mouse myeloma cells sp2/0 and acute promyelocytic leukemia cells NB4 were imaged in the contrast experiments. The time response of experimental manipulation was about 2.7 ms. This 3D biconvex lens has great application prospects for cell imaging and analysis systems in lab-on-a-chip settings. [ABSTRACT FROM AUTHOR]

A Fresnel zone plate (FZP) is a unique diffractive optical device and widely used in integrated optical systems such as interferometers and antennas. A traditional FZP utilizes solid materials and cannot be modulated in real time for desired focusing properties. This paper reports a tunable optofluidic FZP based on a solid–liquid hybrid structure. This FZP consists of two parts including a fast microfluidic mixer, which can adjust the refractive index of liquids from 1.332 to 1.432, and subsequently an optical FZP with a solid–liquid combination. Simulations and experiments successfully showed the real-time tunability of the focusing properties such as peak intensity, focal spot sizes and focal lengths. The focal spot size can be modulated from 16 μm to 80 μm at λ0 = 532 nm in experiments with focal length changes of approximately 700 μm. Moreover, it can be easily switched between focusing, defocusing and collimation. The dispersion with different wavelengths was also investigated, showing that these types of focusing properties are quite different from a traditional optofluidic lens by refraction or reflection. It is foreseeable that such a hybrid FZP may find wider applications in lab-on-a-chip systems and optical devices. [ABSTRACT FROM AUTHOR]

Conventional flow cytometry (FC) suffers from the diffraction limit for the detection of nanoparticles smaller than 100 nm, whereas traditional total internal reflection (TIR) microscopy can only detect few samples near the solid–liquid interface mostly in static states. Here we demonstrate a novel on-chip optofluidic technique using evanescent wave sensing for single nanoparticle real time detection by combining hydrodynamic focusing and TIR using immiscible flows. The immiscibility of the high-index sheath flow and the low-index core flow naturally generate a smooth, flat and step-index interface that is ideal for the TIR effect, whose evanescent field can penetrate the full width of the core flow. Hydrodynamic focusing can focus on all the nanoparticles in the extreme centre of the core flow with a width smaller than 1 μm. This technique enables us to illuminate every single sample in the running core flow by the evanescent field, leaving none unaffected. Moreover, it works well for samples much smaller than the diffraction limit. We have successfully demonstrated the scattering imaging and counting of 50 nm and 100 nm Au nanoparticles and also the fluorescence imaging and counting of 200 nm beads. The effective counting speeds are estimated as 1500, 2300 and 2000 particles per second for the three types of nanoparticles, respectively. The optical scattering spectra were also measured to determine the size of individual Au nanoparticles. This provides a new technique to detect nanoparticles and we foresee its application in the detection of molecules for biomedical analyses. [ABSTRACT FROM AUTHOR]

Multimode interference (MMI) is a typical self-imaging phenomenon and has been widely exploited for optical devices like couplers and splitters. Usually, it utilizes solid waveguides only and thus has very limited tunability in self-imaging properties. This paper reports our original work on tunable MMI using a hybrid optofluidic waveguide. It is generated by the diffusion between miscible flows in a microchannel and consists of two parts: gradient-index liquid–liquid waveguide for light modulation and step-index liquid–solid waveguide for image formation. Simulation and experiments have shown successful realization of the real-time tuning of self-imaging properties. For detail, the image point (focal spot) width could be modulated from 7 μm to 16 μm, and the self-imaging period changes in the range of 500 μm by varying the fluid properties. It is foreseeable that such a hybrid optofluidic waveguide may find wider applications in lab-on-a-chip systems and optical devices. [ABSTRACT FROM AUTHOR]