This paper reviews the past and recent studies on thermocapillarity in relation to microfluidics. The role of thermocapillarity as the change of surface tension due to temperature gradient in developing Marangoni flow in liquid films and conclusively bubble and drop actuation is discussed. The thermocapillary-driven mass transfer (the so-called Benard-Marangoni effect) can be observed in liquid films, reservoirs, bubbles and droplets that are subject to the temperature gradient. Since the contribution of a surface tension-driven flow becomes more prominent when the scale becomes smaller as compared to a pressure-driven flow, microfluidic applications based on thermocapillary effect are gaining attentions recently. The effect of thermocapillarity on the flow pattern inside liquid films is the initial focus of this review. Analysis of the relation between evaporation and thermocapillary instability approves the effect of Marangoni flow on flow field inside the drop and its evaporation rate. The effect of thermocapillary on producing Marangoni flow inside drops and liquid films, leads to actuation of drops and bubbles due to the drag at the interface, mass conservation, and also gravity and buoyancy in vertical motion. This motion can happen inside microchannels with a closed multiphase medium, on the solid substrate as in solid/liquid interaction, or on top of a carrier liquid film in open microfluidic systems. Various thermocapillary-based microfluidic devices have been proposed and developed for different purposes such as actuation, sensing, trapping, sorting, mixing, chemical reaction, and biological assays throughout the years. A list of the thermocapillary based microfluidic devices along with their characteristics, configurations, limitations, and improvements are presented in this review.

A novel bismuth (Bi)-biopolymer (chitosan) nanocomposite screen-printed carbon electrode was developed using a Bi and chitosan co-electrodepositing technique for detecting multiple heavy metal ions. The developed sensor was fabricated with environmentally benign materials and processes. In real wastewater, heavy metal detection was evaluated by the developed sensor using square wave anodic stripping voltammetry (SWASV). The nanocomposite sensor showed the detection limit of 0.1 ppb Zn²⁺, 0.1 ppb Cd²⁺ and 0.2 ppb Pb²⁺ in stock solutions. The improved sensitivity of the Bi-chitosan nanocomposite sensor over previously reported Bi nanocomposite sensors was attributed to the role of chitosan. When used for real wastewater samples collected from a mining site and soil leachate, similar detection limit values with 0.4 ppb Cd²⁺ and 0.3 ppb Pb²⁺ were obtained with relative standard deviations (RSD) ranging from 1.3% to 5.6% (n = 8). Temperature changes (4 and 23 °C) showed no significant impact on sensor performance. Although Zn²⁺ in stock solutions was well measured by the sensor, the interference observed while detecting Zn²⁺ in the presence of Cu²⁺ was possibly due to the presence of Cu-Zn intermetallic species in mining wastewater. Overall, the developed sensor has the capability of monitoring multiple heavy metals in contaminated water samples without the need for complicated sample preparation or transportation of samples to a laboratory.

Flexible sensors with low fabrication cost, high sensitivity, and good stability are essential for the development of smart devices for wearable electronics, soft robotics, and electronic skins. Herein, we report a nanocomposite material based on carbon nanotube and metal oxide semiconductor for ultraviolet (UV) sensing applications, and its sensing behavior. The sensors were prepared by a screen-printing process under a low-temperature curing condition. The formation of a conducting string node and a sensing node could enhance a UV sensing response, which could be attributed to the uniform mixing of functionalized multi-walled carbon nanotubes and zinc oxide nanoparticles. A fabricated device has shown a fast response time of 1.2 s and a high recovery time of 0.8 s with good mechanical stability.

Nanoporous gold (NPG) has remarkable catalytic activity and biocompatibility and could potentially be used in biomedical devices. Herein, we have assessed the long term effects of biofouling on NPG interface. Nanoporoes (25 nm) in gold electrode are fabricated using a de-alloying treatment resulting in an 18 fold increase in surface area as compared to the planar gold. The effects of biofouling on the planar gold interface were evidenced by the rapid decrease in faradaic current to 55% in just eight minutes of incubation in 2 mg ml⁻¹ of bovine serum albumin (BSA). On the other hand NPG showed barely any decline in the peak current when incubated in a similar biofouling solution. NPG upon incubation in a solution of higher concentration of BSA showed immediate peak current degradation which was subsequently recovered when the electrode was left idle in the biofouling solution. For instance, the peak current regenerated from (60% to 80%) when left idle for 60 minutes in 16 mg ml⁻¹ of BSA solution. The regeneration mechanism indicated that even after long term incubation in the biofouling solution, the accumulated organic layer on its interface is not impervious and allows the diffusion of small analytes molecules. Thereby, NPG could be used in biomedical devices such as biosensor or drug reservoir.

In this paper, a chitosan biopolymer-coated planar carbon electrode was developed for in situ determination of heavy metals (Zn ²⁺ and Pb ²⁺ ) using square-wave anodic stripping voltammetry. The experimental conditions were optimized with respect to deposition time, amplitude, and frequency. With 300-s deposition time, the heavy metal stripping was conducted at 0.05-V pulse amplitude, 20-Hz pulse frequency, and 0.004-V square-wave step voltage in 0.1-M acetate buffer at pH 4.6. Two distinguished peaks were observed at -0.86 and -0.37 V, which are associated with the stripping of Zn ²⁺ and Pb ²⁺ , respectively. The limit of detection was 0.6 and 1 ppb for Zn ²⁺ and Pb ²⁺ , respectively, and the relative standard deviations for repetitive measurements of Zn ²⁺ and Pb ²⁺ were in the range of 4.8%-5.4% (n = 30 with two identical electrodes). The chitosan biopolymer-coated carbon electrode was successfully applied for detecting Pb ²⁺ in tap water, mining wastewater, and soil leachate, and showed a reliable performance for measuring heavy metals with acceptable reproducibility. Overall, the developed biopolymer-coated carbon electrode exhibited excellent representativeness and reproductivity for in situ multiheavy metal ions detection in spiked samples, holding a great promise for on-site testing of heavy metals in drinking water.