Salinity is an indispensable parameter for various applications such as biomedical diagnostics, environmental chemical analysis, marine monitoring, etc. Miniaturized salinity sensors have significant potential in portable applications in various scenarios and designs with highly desirable features of convenience, reliability, economy, and high sensitivity and also the capability of real-time measurements. Herein, we demonstrate a highly refractive index-sensitive sensor based on a microscale III-nitride chip that consists of a light emitter and a photodetector. This highly monolithically integrated chip shows an excellent sensitivity of salinity of 2606 nA/(mol/L) (or 446 nA/%) and a response time of 0.243 s. In addition, wireless communication technologies can be easily integrated with the sensing device, which enables automatic remote control for data collection and postprocessing. Remarkably, a polymer-based antifouling coating on the surface of the sensing chip has been established to significantly improve its long-term stability in mimicked marine water. The demonstrated ultrasensitive, ultracompact, cost-effective, fast response, wirelesscompatible, and easy-to-use features endow the current device with a huge potential for in situ salinity sensing under varying environmental conditions.

Airflow sensors are an essential component in a wide range of industrial, biomedical, and environmental applications. The development of compact devices with a fast response and wide measurement range capable of in situ airflow monitoring is highly desirable. Herein, we report a miniaturized optical airflow sensor based on a GaN chip with a flexible PDMS membrane. The compact GaN chip is responsible for light emission and photodetection. The PDMS membrane fabricated using a droplet-based molding process can effectively transform the airflow stimuli into optical reflectance changes that can be monitored by an on-chip photodetector. Without the use of external components for light coupling, the proposed sensor adopting the novel integration scheme is capable of detecting airflow rates of up to 53.5 ms−1 and exhibits a fast response time of 12 ms, holding great promise for diverse practical applications. The potential use in monitoring human breathing is also demonstrated.

The ability to quantitatively monitor various cellular activities is critical for understanding their biological functions and the therapeutic response of cells to drugs. Unfortunately, existing approaches such as fluorescent staining and impedance‐based methods are often hindered by their multiple time‐consuming preparation steps, sophisticated labeling procedures, and complicated apparatus. The cost‐effective, monolithic gallium nitride (GaN) photonic chip has been demonstrated as an ultrasensitive and ultracompact optical refractometer in a previous work, but it has never been applied to cell studies. Here, for the first time, the so‐called GaN chipscope is proposed to quantitatively monitor the progression of different intracellular processes in a label‐free manner. Specifically, the GaN‐based monolithic chip enables not only a photoelectric readout of cellular/subcellular refractive index changes but also the direct imaging of cellular/subcellular ultrastructural features using a customized differential interference contrast (DIC) microscope. The miniaturized chipscope adopts an ultracompact design, which can be readily mounted with conventional cell culture dishes and placed inside standard cell incubators for real‐time observation of cell activities. As a proof‐of‐concept demonstration, its applications are explored in 1) cell adhesion dynamics monitoring, 2) drug screening, and 3) cell differentiation studies, highlighting its potential in broad fundamental cell biology studies as well as in clinical applications.

The fabrication of a micro humidity sensor based on a GaN chip with silica opal is reported. The GaN chip containing InGaN/GaN multi-quantum well provides the two key functions of light emission and detection and the emitted light can be directly coupled into and out of the humidity-sensitive opal through the transparent sapphire substrate. The novel chip-scale integration scheme fully eliminates the complex assembly of external optical elements. The measured photocurrent signal quantitatively reflects the humidity change and has a linear relation of 0.24 µA/% over a wide humidity range of 10-90 %. The developed micro-sensor possesses the advantages of compact size, low cost, high repeatability, and ease of integration and operation, which paves the way for its widespread adoption in humidity sensing.Index Terms-Gallium nitride, optoelectronic integration, micro humidity sensor. I. INTRODUCTIONH UMIDITY is a critical parameter in numerous fields of industrial manufacturing, human activity, medical and chemical processing [1], [2]. With the rapid development of portable sensing equipment, the demand for a humidity sensor with comprehensive performances of fast response, high sensitivity, small size, and low cost, is emerging as an important trend. In addition to sensing schemes of humidity based on the variations of resistance [3], [4], capacitance [5], [6], and impedance [7], [8], optical detection via spectrum change has received increasing attention and is considered as a promising approach to meet the above criteria [9]. A variety of optically

Pressure sensing based on high-sensitivity and fast-response photonic devices is essential for various transient and dynamic processes in diverse fields. Therefore, a miniaturized device being capable of precise and reliable detection is highly desired for the development of optical pressure sensors. Here, we develop a compact pressure sensor, showing a sensitivity of 1 μA/kPa and a fast response time of <10 ms, based on a III-nitride photonic chip combined with a PDMS membrane on submillimeter-scale footprints. The emitter and detector are monolithically integrated on a GaN-on-sapphire chip consisting of InGaN/GaN multiquantum wells, enabling quantitative readout for pressure sensing. Self-assembled polystyrene nanospheres are embedded in the PDMS layer and function as an opal-based photonic crystal, transforming the received mechanical signals into optical signals which can be precisely determined through recorded photocurrent. This underlying mechanism of angle-dependent reflective characteristics via the photonic bandgap effect is well fitted by our theoretical simulation. Sensors with opal films embedded at different vertical positions are fabricated, and their corresponding performance is systematically studied and compared through a series of pressure loading/unloading tests. The demonstrated high repeatability, stability, and durability of the developed chip-scale optical pressure sensor, paving the way for its widespread usage.