Silver Inkjet-Printed Electrode on Paper for Electrochemical Sensing of Paraquat

Integrated microfluidic platforms for heavy metal sensing: a comprehensive review

Heavy metals are found naturally; however, anthropogenic activities such as mining, inappropriate disposal of industrial waste, and the use of pesticides and fertilizers containing heavy metals can cause their unwanted release into the environment. Conventionally, detection of heavy metals is performed using atomic absorption spectrometry, electrochemical methods and inductively coupled plasma-mass spectrometry; however, they involve expensive and sophisticated instruments and multistep sample preparation that require expertise for accurate results. In contrast, microfluidic devices involve rapid, cost-efficient, simple, and reliable approaches for in-laboratory and real-time monitoring of heavy metals. The use of inexpensive and environment friendly materials for fabrication of microfluidic devices has increased the manufacturing efficiency of the devices. Different types of techniques used in heavy metal detection include colorimetry, absorbance-based, and electrochemical detection. This review provides insight into the detection of toxic heavy metals such as mercury (Hg), cadmium (Cd), lead (Pb), and arsenic (As). Importance is given to colorimetry, optical, and electrochemical techniques applied for the detection of heavy metals using microfluidics and their modifications to improve the limit of detection (LOD).

The Role of Interdigitated Electrodes in Printed and Flexible Electronics

Flexible electronics, also referred to as printable electronics, represent an interesting technology for implementing electronic circuits via depositing electronic devices onto flexible substrates, boosting their possible applications. Among all flexible electronics, interdigitated electrodes (IDEs) are currently being used for different sensor applications since they offer significant benefits beyond their functionality as capacitors, like the generation of high output voltage, fewer fabrication steps, convenience of application of sensitive coatings, material imaging capability and a potential of spectroscopy measurements via electrical excitation frequency variation. This review examines the role of IDEs in printed and flexible electronics since they are progressively being incorporated into a myriad of applications, envisaging that the growth pattern will continue in the next generations of flexible circuits to come.

Printable and flexible integrated sensing systems for wireless healthcare

The rapid development of wearable sensing devices and artificial intelligence has enabled portable and wireless tracking of human health, fulfilling the promise of digitalized healthcare applications. To achieve versatile design and integration of multi-functional modules including sensors and data transmission units onto various flexible platforms, printable technologies emerged as some of the most promising strategies. This review first introduces the commonly utilized printing technologies, followed by discussion of the printable ink formulations and flexible substrates to ensure reliable device fabrication and system integration. The advances of printable sensors for body status monitoring are then discussed. Moreover, the integration of wireless data transmission via printable approaches is also presented. Finally, the challenges in achieving printable sensing devices and wireless integrated systems with competitive performances are considered, so as to realize their practical applications for personalized healthcare.

Numerical Simulation of Solvent Evaporation in a Reactive Silver Ink Droplet Deposited on a Heated Substrate

Understanding the movement of silver ions (Ag+) in the solvent of a thermally evaporated particle-free reactive silver ink droplet is essential for optimizing the electronic inkjet printing process. In this work, a numerical study based on the Navier-Stokes equations is used to examine the microflows inside the evaporating solvent of a reactive silver ink droplet and to predict the morphology of the resultant Ag particle aggregations that form during the heat-activated processes. The droplet evaporation of the water-ethylene glycol ink solvent (H2O-(CH2OH)2) is simulated using COMSOL Multiphysics software. The model assumes that the evaporating fluid is heterogeneous due to the mass transfer of ethylene glycol molecules throughout the droplet by capillary flow. A layer of concentrated ethylene glycol forms at the fluid-substrate interface during solvent evaporation if the substrate is heated. The concentrated ethylene glycol molecules are then transported inward by the capillary action, and the resultant Ag particles, arising from the thermally driven reactions, accumulate at the bottom center of the drying droplet. The numerical simulations demonstrate that the droplet evaporation process depends on the water concentration in the solvent, substrate temperature, surface tension, and natural convection. Furthermore, the capillary flow dominates the fluid flow inside the evaporating droplet, causing some Ag particles to accumulate at the contact line, the commonly observed "coffee-ring effect". The results provide new insights into the chemical reactions that produce experimentally observed silver particle aggregations during the reactive silver ink droplet evaporation process and help establish realistic process parameters for improving the quality of inkjet-printed conductive silver films and electronic circuit microtraces.

Graphene electrochemical transistors decorated by Ag nanoparticles exhibiting high sensitivity for the detection of paraquat over a wide concentration range

Paraquat (PQ) is a nonselective contact herbicide used in agriculture for the control of broad leaf weeds, which would cause irreversible damage to human organs even at very low concentrations. Therefore, the trace residue detection of PQ in the environment is of vital importance. Here, a novel graphene electrochemical transistor (GECT) for PQ detection is reported for the first time. The key to the device design is the application of a layer of Ag nanoparticle (Ag NP) modified monolayer graphene as the channel layer. Due to the good electrochemical activity of Ag NPs for PQ detection, the device exhibits excellent sensitivity for PQ with the detection limit of 0.068 nM and a wide linear range from 0.1 nM to 5 mM. The GECT sensor also reveals good selectivity toward several common interferents and exhibits satisfactory recoveries for PQ detection when using Chinese cabbage as a simulant to deduce the real detection situation. The GECT sensor not only provides an efficient method for the detection of PQ residues, but also provides an effective grafting platform for the construction of novel high-sensitivity electrochemical sensors.

Amphiphilic Silver Nanoparticles for Inkjet-Printable Conductive Inks

The large-scale manufacturing of flexible electronics is nowadays based on inkjet printing technology using specially formulated conductive inks, but achieving adequate wetting of different surfaces remains a challenge. In this work, the development of a silver nanoparticle-based functional ink for printing on flexible paper and plastic substrates is demonstrated. Amphiphilic silver nanoparticles with narrow particle size distribution and good dispersibility were prepared via a two-step wet chemical synthesis procedure. First, silver nanoparticles capped with poly(acrylic acid) were prepared, followed by an amidation reaction with 3-morpholynopropylamine (MPA) to increase their lipophilicity. Density functional theory (DFT) calculations were performed to study the interactions between the particles and the dispersion medium in detail. The amphiphilic nanoparticles were dispersed in solvents of different polarity and their physicochemical and rheological properties were determined. A stable ink containing 10 wt% amphiphilic silver nanoparticles was formulated and inkjet-printed on different surfaces, followed by intense pulsed light (IPL) sintering. Low sheet resistances of 3.85 Ω sq^-1, 0.57 Ω sq^-1 and 19.7 Ω sq^-1 were obtained for the paper, coated poly(ethylene terephthalate) (PET) and uncoated polyimide (PI) flexible substrates, respectively. Application of the nanoparticle ink for printed electronics was demonstrated via a simple flexible LED circuit.

Development of sensitive colorimetric aptasensor based on short DNA aptamer and its application to screening for paraquat residues in agricultural soil

Paraquat is a widely used herbicide for controlling weeds and grasses in agriculture, and its contaminated residues in agricultural areas are of increasing concern. This work reports the development of the sensitive and easy-to-use colorimetric aptasensor for screening paraquat residues in agricultural soil. The short DNA fragments derived from the original aptamer were analyzed for their capability to interact with paraquat by molecular dynamic simulation. The paraquat-aptasensor was developed using the selected DNA fragment and gold nanoparticles. Its limit of detection (LOD) for paraquat is 2.76 nM, which is more sensitive than the aptasensor with long-length aptamer (LOD = 12.98 nM). The developed aptasensor shows the selectivity to paraquat, but not to other tested herbicides; ametryn, atrazine, difenzoquat, 2,4-D-dimethyl ammonium, and glufosinate. The recovery rates of paraquat detection in the spiked soil samples were in a range of 99.5%-105.1%, with relative standard deviation values of <4%. The developed aptasensor was used to screen for paraquat residues in agricultural soils, and three out of 23 soil samples were tested positive for paraquat, which was confirmed by a high-performance liquid chromatography analysis. These results suggested the potential application of the developed aptasensor to detect paraquat residues in agricultural sites.

A Label-Free Electrochemical Aptasensor Based on Zn/Fe Bimetallic MOF Derived Nanoporous Carbon for Ultra-Sensitive and Selective Determination of Paraquat in Vegetables

Paraquat (PQ) has high acute toxicity, even at low concentrations. For most people, the main pathway of exposure to PQ is through the diet. Therefore, the development of simple and efficient methods for PQ testing is critical for ensuring food safety. In this study, a new electrochemical detection strategy for paraquat is proposed based on the specific binding of PQ to its nucleic acid aptamer. Firstly, the Zn/Fe bimetallic ZIF derived nanoporous carbon (Zn/Fe-ZIF-NPC) and nickel hexacyanoferrate nanoparticles (NiHCF-NPs) were sequentially modified onto the glassy carbon electrode (GCE). NiHCF-NPs served as the signal probes, while Zn/Fe-ZIF-NPC facilitated electron transfer and effectively enhanced the sensing signal of NiHCF-NPs. Au nanoparticles (AuNPs) were then electrodeposited on the NiHCF-NPs/Zn/Fe-ZIF-NPC/GCE and then the thiolated aptamer was assembled on the AuNPs/NiHCF-NPs/Zn/Fe-ZIF-NPC/GCE via Au-S bonding. When incubated with PQ, the formation of PQ–aptamer complexes delayed the interfacial electron transport reaction of NiHCF-NPs, which caused a decrease in the current signals. As a result, simple and highly sensitive detection of PQ can be readily achieved by detecting the signal changes. A linear range was obtained from 0.001 to 100 mg/L with a detection limit as low as 0.34 μg/L. Due to the recognition specificity of the aptamer to its target molecule, the proposed method has excellent anti-interference ability. The prepared electrochemical aptasensor was successfully used for PQ assay in lettuce, cabbage and agriculture irrigation water samples with recoveries ranging from 96.20% to 104.02%, demonstrating the validity and practicality of the proposed method for PQ detection in real samples.

Hydrothermally constructed AgWO4-rGO nanocomposites as an electrode enhancer for ultrasensitive electrochemical detection of hazardous herbicide crisquat

The advancements in electrode materials with high efficiency has been prioritized to effectively monitor the presence of harmful pesticides concerning the environment. In such a way, we hydrothermally constructed a hybrid AgWO₄-rGO nanocomposites for the rapid electrochemical detection of crisquat (CQT). The structural, compositional, morphological and topographical characterization for AgWO₄-rGO nanocomposites is thoroughly performed to understand its electrocatalytic properties. The AgWO₄-rGO nanocomposites are used as an electrode enhancer (rGO@AgWO₄/GCE) for the electrochemical investigations towards CQT detection. The results indicated that the rGO@AgWO₄/GCE possessed an excellent catalytic activity with a wide linear detection range 1-1108 μM coupled with an ultrasensitive limit of detection (LOD) 0.0661 μM for electrochemical CQT detection. The rGO@AgWO₄/GCE CQT sensor also expressed remarkable sensitivity of 0.6306 μAμM^-1cm^-2 in addition to good selectivity and reproducibility. Furthermore, the commercial CQT, river water, tap water and washed vegetable water are used as a representative for real world analysis using rGO@AgWO₄/GCE and results are highly appreciable for the real time CQT detection. Our work proposes a novel hybrid rGO@AgWO₄ nanocomposites reinforced electrodes for ultra-trace level CQT detection with good reliability and can be advocated for real time detection of pesticides.

Electrochemical paraquat sensor based on lead oxide nanoparticles

1,1-Dimethyl-4,4-bipyridinium dichloride known as paraquat is a popular well-known herbicide that is widely used in agriculture around the world. However, paraquat is a highly toxic chemical causing damage to vital organs including the respiratory system, liver, heart, and kidneys and death. Therefore, detection of paraquat is still necessary to protect life and the environment. In this work, an electrochemical sensor based on lead oxide nanoparticles (PbO-NPs) modified on a screen-printed silver working electrode (SPE) has been fabricated for paraquat detection at room temperature. The PbO-NPs have been synthesized by using a sparking method via two Pb metal wires. The electrochemical paraquat sensors have been prepared by a simple drop-casting of PbO-NPs solution on the surface of the SPE. The PbO-NPs/SPE sensor exhibits a linear response in the range from 1 mM to 5 mM with good reproducibility and high sensitivity (204.85 μA mM⁻¹ cm⁻²) for paraquat detection at room temperature. The PbO-NPs/SPE sensor shows high selectivity to paraquat over other popular herbicides such as glyphosate, glufosinate-ammonium and butachlor-propanil. The application of the PbO-NPs/SPE sensor is also demonstrated via the monitoring of paraquat contamination in juice and milk.

The PbO nanoparticles-based electrochemical sensor can be integrated into a smartphone for on-site field testing of paraquat with high sensitivity and selectivity.