A Droplet Microfluidic-Based Sensor for Simultaneous in Situ Monitoring of Nitrate and Nitrite in Natural Waters

Abstract

Microfluidic-based chemical sensors take laboratory analytical protocols and miniaturise them into field-deployable systems for in situ monitoring of water chemistry. Here we present a prototype nitrate/nitrite sensor based on droplet microfluidics that in contrast to standard (continuous phase) microfluidic sensors, treats water samples as discrete droplets contained within a flow of oil. The new sensor device can quantify the concentrations of nitrate and nitrite within each droplet and provides high measurement frequency and low fluid consumption. Reagent consumption is at a rate of 2.8 ml/day when measuring every ten seconds, orders of magnitude more efficient than the current state of the art. The sensor's capabilities were demonstrated during a three-week deployment in a tidal river. The accurate and high frequency data (6 % error relative to spot samples, measuring at 0.1 Hz) elucidated the influence of tidal variation, rain events, diurnal effects, and anthropogenic input on concentrations at the deployment site. This droplet microfluidic-based sensor is suitable for a wide range of applications such as monitoring of rivers, lakes, coastal waters, and industrial effluents.

Emerging Trends in Microfluidics Based Devices

“…The design of the device reduced the reagent consumption to 2.8 mL per day with a measurement frequency of per 10 s. The use of MFs sensor reduced the amount of reagent used so that 1 L of the reagents was sufficient for 11 months of continuous functioning. [ 104 ] Recently, research has been directed towards the use of self‐powering devices that are capable of generating power for self‐functioning when left in the environment. [ 105 ] Characteristics of some of the FDS reported in the literature for environmental monitoring are listed in Table S2, Supporting Information.…”

Autonomous and In Situ Ocean Environmental Monitoring on Optofluidic Platform

“…Various techniques, such as colorimetry [22][23][24][25][26][27][28][29][30][31][32][33][34], chemiluminescence [35,36], fluorimetry [37][38][39][40], electrochemistry [41][42][43], and chromatography [44][45][46], have been proposed for nutrient determination. Among them, the colorimetric method using chromogenic agents is one of the most favored detection approaches due to its stability, excellent detection limits, simplicity, high cost efficiency, and analytical feasibility.…”

“…Among various colorimetric assay chemistries, the Griess assay method has been the mainstay for nitrite analysis for over a century [22][23][24][25][26][27][28][29][30][31][32][33][34]. The mechanism of the Griess assay method is that under acidic conditions, nitrite reacts with sulfanilic acid to produce a diazonium salt, which is then coupled to N-(1-naphthyl) ethylenediamine (NED), resulting in pink azo compounds that can be detected at 543 nm.…”

Easily fabricated monolithic fluoropolymer chips for sensitive long-term absorbance measurement in droplet microfluidics

“…A modied Griess assay for nitrite and nitrate measurement was formulated as previously described. 10,25 Briey, 1.25 g of vanadium(III) chloride (99.0%, Alfa Aesar, UK) was added to a 250 ml volumetric ask along with 50 ml of ultrapure water to form a dark brown solution. 15 ml of concentrated (37%) hydrochloric acid was added, causing the solution to turn dark turquoise, then 1.25 g of sulfanilamide ($99.0%) and 0.125 g of N-(1-naphthyl)ethylenediamine dihydrochloride (>98%) were added, dissolved, and the solution nally made up to the volumetric mark using ultrapure water.…”

“…325 C for PTFE, 315 C for PFA), that mean expensive specialised equipment is required for moulding procedures. 8,9 Consequently researchers oen opt to assemble uidic manifolds from off-the-shelf uoropolymer capillary tubing 6,7,10 as a more user-friendly option for long-term experimentation. In such systems uidic architectures cannot be arbitrarily designed however, meaning complex structures such as the extended path length ow cells needed for sensitive absorbance measurement are unobtainable.…”