Optofluidic refractive index sensor based on asymmetric diffraction

Real-time and high-sensitivity refractive index sensing with an arched optofluidic waveguide

Refractive index (RI) sensing plays an important role in analytical chemistry, medical diagnosis, and environmental monitoring. The optofluidic technique is considered to be an ideal tool for RI sensor configuration for its high integration, high sensitivity, and low cost. However, it remains challenging to achieve RI measurement in real time with high sensitivity and low detection limit (DL) simultaneously. In this work, we design and fabricate a RI sensor with an arched optofluidic waveguide by monitoring the power loss of the light passing through the waveguide, which is sandwiched by the air-cladding and the liquid-cladding under test, we achieve RI detection of the sample in real time and with high sensitivity. Furthermore, both numerical simulation and experimental investigation show that our RI sensor can be designed with different geometric parameters to cover multiple RI ranges with high sensitivities for different applications. Experimental results illustrate that our sensor is capable to achieve a superior sensitivity better than -19.2 mW/RIU and a detection limit of 5.21×10^-8 RIU in a wide linear dynamic range from 1.333 to 1.392, providing a promising solution for real-time and high-sensitivity RI sensing.

The Rise of the OM-LoC: Opto-Microfluidic Enabled Lab-on-Chip

The integration of optical circuits with microfluidic lab-on-chip (LoC) devices has resulted in a new era of potential in terms of both sample manipulation and detection at the micro-scale. On-chip optical components increase both control and analytical capabilities while reducing reliance on expensive laboratory photonic equipment that has limited microfluidic development. Notably, in-situ LoC devices for bio-chemical applications such as diagnostics and environmental monitoring could provide great value as low-cost, portable and highly sensitive systems. Multiple challenges remain however due to the complexity involved with combining photonics with micro-fabricated systems. Here, we aim to highlight the progress that optical on-chip systems have made in recent years regarding the main LoC applications: (1) sample manipulation and (2) detection. At the same time, we aim to address the constraints that limit industrial scaling of this technology. Through evaluating various fabrication methods, material choices and novel approaches of optic and fluidic integration, we aim to illustrate how optic-enabled LoC approaches are providing new possibilities for both sample analysis and manipulation.

A 3D-cascade-microlens optofluidic chip for refractometry with adjustable sensitivity

Refractive index (RI) sensing as a label-free and non-invasive method has been playing an important role in industrial metrology, biochemical detection, and environmental analysis. Due to the combined advantages of microoptics and microfluidics, optofluidic RI sensors have attracted growing interest. Despite a variety of prototypes of optofluidic RI sensors, comprehensive improvement in sensitivity, detection range, fabrication procedures and cost can still bring substantial benefits to the field. In this work, we fabricated a 3D-cascade-microlens optofluidic chip (3DCMOC) for RI sensing. Two-photon stereolithography was employed to fabricate the chip mold, with which the 3DCMOC could be easily manufactured via mold replication. By virtue of integrating four detection channels configured with different numbers (1, 3, 5, and 7) of cascaded microlenses within the 3DCMOC, adjustable sensitivity for RI sensing has been demonstrated through measuring standard sucrose solutions. It was found that the seven-microlens configuration achieved an excellent sensitivity (mean: 21 ± 5 AU·RIU (refractive index unit)^-1) and resolution (mean: 3.8 × 10^-5 ± 0.9 × 10^-5 RIU) at a cost of a narrow linear dynamic range (LDR, 1.3326-1.3548). In contrast, the single-microlens configuration led to an extended LDR (1.3326-1.5120 tested) despite the lower sensitivity (mean: 2.6 ± 0.2 AU·RIU^-1) and resolution (mean: 1.5 × 10^-4 ± 0.1 × 10^-4 RIU). Furthermore, the use of the 3DCMOC was investigated via real-time salinity sensing and analysis of urine specific gravity.