Optofluidic waveguide bending by thermal diffusion for visible light control.

Optofluidics has inspired many promising optical devices. Among them, waveguide bending is an important element for guiding light. Here, we demonstrated the thermal-diffusion liquids, acting as a natural transformation optical material in an annular structure. Compared with conventional step-index waveguide bending, this thermal one enables real-time tunable visible light bends by extreme angles, with nearly no power loss and intensity distribution. This unique light bending is because gradient refractive-index profiles caused by thermal diffusion meet the requirements by transformation optics. The work demonstrates the thermal diffusion in liquids as a natural technology to realize optofluidic gradient-index designs and has potential for tunable optical systems.

Rapid nitrate determination with a portable lab-on-chip device based on double microstructured assisted reactors.

Determining the nitrate levels is critical for water quality monitoring, and traditional methods are limited by high toxicity and low detection efficiency. Here, rapid nitrate determination was realized using a portable device based on innovative three-dimensional double microstructured assisted reactors (DMARs). On-chip nitrate reduction and chromogenic reaction were conducted in the DMARs, and the reaction products then flowed into a PMMA optical detection chip for absorbance measurement. A significant enhancement of reaction rate and efficiency was observed in the DMARs due to their sizeable surface-area-to-volume ratios and hydrodynamics in the microchannels. The highest reduction ratio of 94.8% was realized by optimizing experimental parameters, which is greatly improved compared to conventional zinc-cadmium based approaches. Besides, modular optical detection improves the reliability of the portable device, and a smartphone was used to achieve a portable and convenient nitrate analysis. Different water samples were successfully analysed using the portable device based on DMARs. The results demonstrated that the device features fast detection (115 s per sample), low reagent consumptions (26.8 µL per sample), particularly low consumptions of toxic reagents (0.38 µL per sample), good reproducibility and low relative standard deviations (RSDs, 0.5-1.38%). Predictably, the portable lab-on-chip device based on microstructured assisted reactors will find more applications in the field of water quality monitoring in the near future.

Silver Nanoprism Enhanced Colorimetry for Precise Detection of Dissolved Oxygen.

Dissolved oxygen (DO) content is an essential indicator for evaluating the quality of the water body and the main parameter for water quality monitoring. The development of high-precision DO detection methods is of great significance. This paper reports an integrated optofluidic device for the high precision measurement of dissolved oxygen based on the characteristics of silver nanoprisms. Metal nanoparticles, especially silver nanoprisms, are extremely sensitive to their surroundings. In glucose and glucose oxidase systems, dissolved oxygen will be transformed into H2O2, which affects the oxidation and erosion process of nanoprisms, then influences the optical properties of nanoparticles. By detecting the shift in the plasma resonance peak of the silver nanoparticles, the dissolved oxygen (DO) content can be determined accurately. Great reconfigurability is one of the most significant advantages of the optofluidic device. By simply adjusting the flow rate ratio between the silver nanoprisms flow and the water sample flow, real-time continuous adjustment of the detection ranges of DO from 0 to 16 mg/L can be realized dynamically. The detection limit of this device is as low as 0.11 µM (3.52 µg/L) for DO measurement. Thus, the present optofluidic system has a wide range of potential applications in fields of biomedical analyses and water sensing.

Versatile biomimetic array assembly by phase modulation of coherent acoustic waves.

A high-throughput cell-assembly method, with the advantages of adjustability, ease of operation, and good precision, is remarkable for artificial tissue engineering. Here, we present a scientific solution by introducing high rotational symmetrical coherent acoustic waves, in order to enable the shape and arrangement of the acoustic potential wells to be flexibly modulated, and therefore to assemble on a large area diverse biomimetic arrays on a microfluidic platform. Ring arrays, honeycomb, and many other biomimetic arrays are achieved by real-time modulation of the wave vectors and phase relation of acoustic beams from six directions. In the experiments, human umbilical vein endothelial cells (HUVECs), arranged in ring structures, tend to connect with the adjacent cells and reach confluency, thus directing the in vitro two-dimensional vascular network formation. Higher rotational symmetry of the six coherent acoustic waves provides much more flexibility and diversity for acoustic cell assembly. With the advantages of efficiency, diversity and adjustability, this acoustic chip is expected to fulfill many applications, such as in biochemistry, bioprinting and tissue engineering related research.

Amplitude Holographic Interference-Based Microfluidic Colorimetry at the Micrometer Scale.

Quantitative molecular analysis is usually based on spectrophotometric methods using colorimetric assay. Conventional methods, however, rely on the direct uniform absorption of the sample under test, and the detection sensitivity is strictly limited by the length of the absorption cell at the millimeter scale. Here, we report a new methodology for colorimetric assay based on the amplitude holographic interference (AHI) caused by nonuniform absorption of light, with detection sensitivity at the micrometer scale. In our method, the curved surface of the microfluidics results in a phase profile with a high diffraction efficiency, and the nonuniform absorption of samples exactly matches with the amplitude modulation in the holographic interference. The signal intensity is affected by not only direct sample absorption but also the sequential optical interference behind the liquid level. Both single- and multiple-wavelength colorimetric analyses of the Griess-Saltzman dye (GSD) were carried out using this method, and we found that the sensitivity can be improved by approximately 2-fold in comparison to the conventional method. This interference-based method would be a useful tool for the colorimetric assay of chemical samples in highly integrated systems with better performance.

Autonomous and In Situ Ocean Environmental Monitoring on Optofluidic Platform.

Determining the distributions and variations of chemical elements in oceans has significant meanings for understanding the biogeochemical cycles, evaluating seawater pollution, and forecasting the occurrence of marine disasters. The primary chemical parameters of ocean monitoring include nutrients, pH, dissolved oxygen (DO), and heavy metals. At present, ocean monitoring mainly relies on laboratory analysis, which is hindered in applications due to its large size, high power consumption, and low representative and time-sensitive detection results. By integrating photonics and microfluidics into one chip, optofluidics brings new opportunities to develop portable microsystems for ocean monitoring. Optofluidic platforms have advantages in respect of size, cost, timeliness, and parallel processing of samples compared with traditional instruments. This review describes the applications of optofluidic platforms on autonomous and in situ ocean environmental monitoring, with an emphasis on their principles, sensing properties, advantages, and disadvantages. Predictably, autonomous and in situ systems based on optofluidic platforms will have important applications in ocean environmental monitoring.

A Portable and Accurate Phosphate Sensor Using a Gradient Fabry-Pérot Array.

Here, a portable and accurate phosphate sensor using a gradient Fabry-Pérot array (FPA) is proposed. It can form a bidirectional gradient concentration (absorbance) distribution in the gradient FPA, simplifying the complex operations to get a standard curve and saving time. The gradient FPA can effectively filter out the interference (bubbles, light intensity, and salinity) while improving the absorbance, achieving a highly accurate and stable detection. Besides, the smartphone simplifies data processing and makes sensors more portable. In this work, the detection errors of standard solutions (100, 50, and 30 µM) are 0.39, 1.48, and 1.84%, respectively, and it has also been demonstrated with errors of 2.46 (sample 1, seawater), 2.08 (sample 2, lake water), and 1.83% (sample 3, sewage) for natural samples detection, which is more accurate than a traditional analyzer. The sensor has a good performance when affected by bubbles, light intensity, and salinity. In addition, the detection time is shortened to 80 s, which is more time saving compared with traditional devices, and the limit of detection (LOD) is 0.4 µM. It can be predicted that the novel optofluidic sensor is conducive to build a smart nutrient monitoring system and will be applied in the field of biochemistry and environmental chemistry.

On-chip hydrogel arrays individually encapsulating acoustic formed multicellular aggregates for high throughput drug testing.

Multicellular aggregates in three-dimensional (3D) environments provide novel solid tumor models that can provide insight into in vivo drug resistance. Such models are therefore essential for developing new drugs and preventing the failure of clinical treatments. However, high-throughput cell cluster assembly and fabricating individual 3D environments that mimic the extracellular matrix (ECM) remain significant challenges. To rapidly produce mini 3D multicellular aggregate units, acoustic force assembly combined with ECM mimic hydrogel array encapsulation is developed and then integrated into a diffusion-based microfluidic device for high-throughput drug testing. The active acoustic force gathers human mononuclear leukemia cells (THP-1) into hundreds of multicellular clusters with a controllable size. Instead of continuous bulk materials, photosensitive gelatin methacryloyl (GelMA) hydrogel pillar arrays containing cell clusters at drug concentration gradients are obtained through selective area exposure. Ten azelaic acid (AZA) concentration gradient series are applied to 100 units to simultaneously test the multicellular cluster drug resistance to multiple drug conditions. Real-time green fluorescent protein (GFP) fluorescence is analyzed to monitor cell viability. The results show that cell aggregate activity is inversely related to the drug concentration in the hydrogel pillars, and shows lower sensitivity to drug toxicity than the activity of monolayer cultured cells. The 3D multicellular arrays provide numerous in vitro tumor models and can be directly used for downstream drug testing. This technology inherits the advantages of acoustic assembly, while being more flexible, practical, and high-throughput, and shows significant potential for use in further tumor related research and clinical practice.

Light Manipulation in Inhomogeneous Liquid Flow and Its Application in Biochemical Sensing.

Light manipulation has always been the fundamental subject in the field of optics since centuries ago. Traditional optical devices are usually designed using glasses and other materials, such as semiconductors and metals. Optofluidics is the combination of microfluidics and optics, which brings a host of new advantages to conventional solid systems. The capabilities of light manipulation and biochemical sensing are inherent alongside the emergence of optofluidics. This new research area promotes advancements in optics, biology, and chemistry. The development of fast, accurate, low-cost, and small-sized biochemical micro-sensors is an urgent demand for real-time monitoring. However, the fluid flow in the on-chip sensor is usually non-uniformed, which is a new and emerging challenge for the accuracy of optical detection. It is significant to reveal the principle of light propagation in an inhomogeneous liquid flow and the interaction between biochemical samples and light in flowing liquids. In this review, we summarize the current state of optofluidic lab-on-a-chip techniques from the perspective of light modulation by the unique dynamic properties of fluid in heterogeneous media, such as diffusion, heat transfer, and centrifugation etc. Furthermore, this review introduces several novel photonic phenomena in an inhomogeneous liquid flow and demonstrates their application in biochemical sensing.