Improved assessment of accuracy and performance using a rotational paper-based device for multiplexed detection of heavy metals

The Development of a Selective Colorimetric Sensor for Cu2+ and Zn2+ in Mineral Supplement with Application of a Smartphone Paper-Based Assay of Cu2+ in Water Samples

Herein, we developed a colorimetric method for the determination of Cu2+ and Zn2+ using NBD-G as a novel selective metal sensor. NBD-G was easily synthesized by a nucleophilic substitution reaction between 4-chloro-7-nitro-2,1,3-benzoxadiazole (NBD-Cl) and Girard's Reagent P. The NBD-G solution is yellow, but when it reacts with Cu2+ and Zn2+, its color changes selectively to red (510 nm) and orange (480 nm), respectively. NBD-G was used as a sensor for Cu2+ and Zn2+, showing a high sensitivity down to 0.77 µM for Cu2+ and 1.66 µM for Zn2+. NBD-G could determine both metals simultaneously; thus, it was applied to determine them in multimineral supplements, which showed excellent recoveries. Next, a filter paper impregnated with NBD-G was prepared as a test paper, and a simple, selective, and rapid onsite method for quantifying Cu2+ was developed as, interestingly, the paper showed no change upon the addition of Zn2+. Next, Cu2+ could be quantified with high selectivity and accuracy by photographing the color change with a smartphone camera and processing the image with Image J. The detection limit for Cu2+ using this method was 3.9 µM. Finally, the NBD-G test paper method was able to satisfactorily quantify Cu2+ spiked into the rainwater.

A novel low-cost and simple fabrication technique for a paper-based analytical device using super glue

BACKGROUND

The microfluidic paper-based analytical devices (μPADs) have been highly regarded as effective tools that offer a cost-effective and portable solution for point-of-care testing (POCT) and on-site detection. Utilizing paper substrates such as cellulose and nitrocellulose membranes, μPADs have proven beneficial for a range of applications from medical diagnostics to environmental monitoring. Despite their advantages, the fabrication of μPADs often requires sophisticated techniques and equipment, posing challenges for widespread adoption, especially in resource-limited settings. This study addresses the need for a simplified, low-cost method for fabricating μPADs that is accessible without specialized training or equipment.

RESULTS

This research introduces a novel, efficient method for producing μPADs using 3D-printed slidable chambers and super glue vapor, bypassing traditional, more complex fabrication processes. The method utilizes super glue (ethyl-cyanoacrylate) vapor to create hydrophobic barriers on paper substrates. By optimizing the exposure sequence to super glue and water vapors and the heating conditions, we achieved rapid hydrophobization within 5 min, creating effective hydrophobic barriers and hydrophilic channels on paper substrates. The technique's simplicity allows for use by individuals without specialized training. The practical application of the fabrication method is demonstrated by the fabrication of μPADs that can detect multiple target analytes. We perform the simultaneous detection of glucose, proteins, and also the simultaneous detection of heavy metal ions nickel (Ni2+) and copper (Cu2+), highlighting its potential for broad applications in point-of-care diagnostics.

SIGNIFICANCE

This study is the first to report a method for selective exposure of ethyl-cyanoacrylate vapor for the fabrication of μPADs. This method significantly reduces the complexity, time, and fabrication cost, making it feasible for use in various settings. It also eliminates the need for specialized equipment and can be executed by individuals without specialized training. We believe that the proposed fabrication method contributes to the wider adoption and deployment of μPADs across various sectors.

Fill, Fold, Photo: Preconcentration and Multiplex Detection of Trace Level Heavy Metals in Water

Heavy metal contamination is an increasing global threat to human and environmental health, particularly in resource-limited areas. Traditional platforms for heavy metal detection are labor intensive and expensive and require lab facilities. While paper-based colorimetric sensors offer a simpler approach, their sensitivity limitations prevent them from meeting legislative requirements for many metals. Existing preconcentration systems, on the other hand, can achieve lower detection limits but typically focus on analyzing only one metal, making comprehensive monitoring difficult. We address these limitations by introducing a low-cost preconcentration system coupled with colorimetric analysis for the simultaneous detection of seven metal ions at low ppb levels without the need for external equipment outside a smartphone. The system achieved detection limits of 15 ppb (Ni(II)), 7 ppb (Cu(II)), 2 ppb (Fe(III)), 20 ppb (Cr(VI)), 13 ppb (Pb(II)), 26 ppb (Hg(II)), and 15 ppb (Mn(II)) with six out of seven limits of detection values falling well below EPA regulatory guidelines for drinking water. The user-friendly Fill, Fold, Photo approach eliminates complex pretreatment steps. Smartphone-based detection offers portable quantification within seconds. Employing masking strategies ensured higher selectivity for each assay on the card, while our packaging protocols enable system stability for over 4 weeks of study, facilitating mass production and deployment within a realistic time frame. To validate the sensor's performance in real-world scenarios, the sensor was tested with environmental water samples. The sensor demonstrated good recovery, ranging from 77% to 94% compared to the standard ICP-MS method. Furthermore, spike recovery analysis confirmed the sensor's accuracy, with a relative standard deviation (RSD) of less than 15%. This technology holds significant promise for future development as a convenient, portable solution for field-based monitoring of a broad spectrum of water contaminants, including pesticides, PFAS, fertilizers, and beyond.

Paper-based sensors: affordable, versatile, and emerging analyte detection platforms

Paper-based sensors, often referred to as paper-based analytical devices (PADs), stand as a transformative technology in the field of analytical chemistry. They offer an affordable, versatile, and accessible solution for diverse analyte detection. These sensors harness the unique properties of paper substrates to provide a cost-effective and adaptable platform for rapid analyte detection, spanning chemical species, biomolecules, and pathogens. This review highlights the key attributes that make paper-based sensors an attractive choice for analyte detection. PADs demonstrate their versatility by accommodating a wide range of analytes, from ions and gases to proteins, nucleic acids, and more, with customizable designs for specific applications. Their user-friendly operation and minimal infrastructure requirements suit point-of-care diagnostics, environmental monitoring, food safety, and more. This review also explores various fabrication methods such as inkjet printing, wax printing, screen printing, dip coating, and photolithography. Incorporating nanomaterials and biorecognition elements promises even more sophisticated and sensitive applications.

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).

Dual-spot ratiometric microfluidic paper-based analytical device for accuracy and precision improvement

Instrumental and environmental fluctuations are common sources of error in smartphone-based optical detection, significantly affecting the accuracy of analytical measurements. In this regard, spotting the sample and reference simultaneously and in close proximity compensates for the fluctuations. This "dual-spot" design is similar to the double-beam technique used in spectrophotometry, which reduces fluctuations in the results. The underlying hypothesis is that any instrumental and/or environmental factors influencing the color intensity in the detection zones will similarly impact the color intensity in the control zone under the same conditions. To test our design, a ratiometric microfluidic paper-based analytical device (μPAD), functionalized with a mixture of green-emissive carbon dots (CDs) and red-emissive ethidium bromide, was developed for the selective detection of ascorbic acid (AA). The green emission of the CDs is quenched by both AA and Fe3+; NaF was thus loaded onto the 3D connector as a masking agent to remove the interference effect of the Fe3+ ions. The color variations were monitored under a UV lamp, using a smartphone to capture the images, and the RGB intensities were processed using the Color Grab application. The proposed double-spot method greatly enhanced the analytical precision and accuracy of the device. A linear working range from 0 to 125 μM was obtained, and the limit of detection was 2.71 μM. The μPAD was successfully used for the detection of AA in human serum, with recoveries from 87.27 to 98.52 %.

A Facile Method to Fabricate an Enclosed Paper-Based Analytical Device via Double-Sided Patterning for Ionic Contaminant Detection

Microfluidic paper-based analytical devices (μPADs) have been developed for use in a variety of diagnosis and analysis fields. However, conventional μPADs with an open-channel system have limitations for application as analytical platforms mainly because of the evaporation and contamination of the sample solution. This study demonstrates the design and fabrication of an enclosed three-dimensional(3D)-μPAD and its application as a primary early analysis platform for ionic contaminants. To generate the hydrophobic PDMS barrier, double-sided patterning is carried out using a PDMS blade-coated stamp mold that is fabricated using 3D printing. The selective PDMS patterning can be achieved with controlled PDMS permeation of the cellulose substrate using 3D-designed stamp molds. We find the optimal conditions enabling the formation of enclosed channels, including round shape pattern and inter-pattern distance of 10 mm of stamp design, contact time of 0.5 min, and spacer height of 300 µm of double-sided patterning procedure. As a proof of concept, this enclosed 3D-μPAD is used for the simultaneous colorimetric detection of heavy metal ions in a concentration range of 0.1-2000 ppm, including nickel (Ni2+), copper (Cu2+), mercury (Hg2+), and radioactive isotope cesium-137 ions (Cs+). We confirm that qualitative analysis and image-based quantitative analysis with high reliability are possible through rapid color changes within 3 min. The limits of detection (LOD) for 0.55 ppm of Ni2+, 5.05 ppm of Cu2+, 0.188 ppm of Hg2+, and 0.016 ppm of Cs+ are observed, respectively. In addition, we confirm that the analysis is highly reliable in a wide range of ion concentrations with CV values below 3% for Ni2+ (0.56%), Cu2+ (0.45%), Hg2+ (1.35%), and Cs+ (2.18%). This method could be a promising technique to develop a 3D-μPAD with various applications as a primary early analysis device in the environmental and biological industries.

Tailoring Acid Free-Paper based Analytical Devices (Af-PADs) via radiation assisted modification of cellulose paper

A novel green fabrication approach is being proposed based on radiation assisted modification of Whatman filter paper 1 (WFP) for development of Acid Free-Paper based Analytical Devices (Af-PADs). Af-PADs exude immense potential as handy tools for on-site detection of toxic pollutants such as, Cr(VI), boron, etc., which have established detection protocols involving acid mediated colorimetric reactions that necessitate external acid addition. The proposed Af-PAD fabrication protocol asserts its novelty through elimination of external acid addition step, making the detection process safer and simpler. To achieve this, poly(acrylic acid) (PAA) was grafted onto WFP via a single step, room temperature process of gamma radiation induced simultaneous irradiation grafting, introducing acidic -COOH groups in the paper thereon. Grafting parameters namely, absorbed dose and concentrations of monomer, homopolymer inhibitor and acid were optimized. The -COOH groups incorporated in PAA-grafted-WFP (PAA-g-WFP) provide localized acidic conditions for colorimetric reactions between pollutants and their sensing agents, anchored on the PAA-g-WFP. Af-PADs loaded with 1,5-diphenylcarbazide (DPC) have been ably demonstrated for visual detection and quantitative estimation of Cr(VI) in water samples using RGB image analysis, with LOD value of 1.2 mg.L^-1 and a measurement range comparable to that of commercially available PADs based Cr(VI) visual detection kits.

MID-FTIR-PLS Chemometric Analysis of Cr(VI) from Aqueous Solutions Using a Polymer Inclusion Membrane-Based Sensor

A partial least squares (PLS) quantitative chemometric method based on the analysis of the mid-Fourier transform infrared spectroscopy (MID-FTIR) spectrum of polymer inclusion membranes (PIMs) used for the extraction of Cr(VI) from aqueous media is developed. The system previously optimized considering the variables membrane composition, extraction time, and pH, is characterized in terms of its adsorption isotherm, distribution coefficient, extraction percent, and enrichment factor. A Langmuir-type adsorption behavior with KL = 2199 cm3/mmol, qmax = 0.188 mmol/g, and 0 < RL < 1 indicates that metal adsorption is favorable. The characterization of the extraction reaction is performed as well, showing a 1:1 Cr(VI):Aliquat 336 ratio, in agreement with solvent extraction data. The principal component analysis (PCA) of the PIMs reveals a complex pattern, which is satisfactorily simplified and related to Cr(VI) concentrations through the use of a variable selection method (iPLS) in which the bands in the ranges 3451-3500 cm-1 and 3751-3800 cm-1 are chosen. The final PLS model, including the 100 wavelengths selected by iPLS and 10 latent variables, shows excellent parameter values with root mean square error of calibration (RMSEC) of 3.73115, root mean square error of cross-validation (RMSECV) of 6.82685, bias of -1.91847 × 10-13, cross-validation (CV) bias of 0.185947, R2 Cal of 0.98145, R2 CV of 0.940902, recovery% of 104.02 ± 4.12 (α = 0.05), sensitivity% of 0.001547 ppb, analytical sensitivity (γ) of 3.8 ppb, γ-1: 0.6 ppb-1, selectivity of 0.0155, linear range of 5.8-100 ppb, limit of detection (LD) of 1.9 ppb, and limit of quantitation (LQ) of 5.8 ppb. The developed PIM sensor is easy to implement as it requires few manipulations and a reduced number of chemical compounds in comparison to other similar reported systems.

Microfluidic Devices for Heavy Metal Ions Detection: A Review

The contamination of air, water and soil by heavy metal ions is one of the most serious problems plaguing the environment. These metal ions are characterized by a low biodegradability and high chemical stability and can affect humans and animals, causing severe diseases. In addition to the typical analysis methods, i.e., liquid chromatography (LC) or spectrometric methods (i.e., atomic absorption spectroscopy, AAS), there is a need for the development of inexpensive, easy-to-use, sensitive and portable devices for the detection of heavy metal ions at the point of interest. To this direction, microfluidic and lab-on-chip (LOC) devices fabricated with novel materials and scalable microfabrication methods have been proposed as a promising approach to realize such systems. This review focuses on the recent advances of such devices used for the detection of the most important toxic metal ions, namely, lead (Pb), mercury (Hg), arsenic (As), cadmium (Cd) and chromium (Cr) ions. Particular emphasis is given to the materials, the fabrication methods and the detection methods proposed for the realization of such devices in order to provide a complete overview of the existing technology advances as well as the limitations and the challenges that should be addressed in order to improve the commercial uptake of microfluidic and LOC devices in environmental monitoring applications.

A 3D sliding-strip microfluidic device for the simultaneous determination of mta

A simple paper-based microfluidic device was fabricated to simultaneously detect multiple targets. Microfluidic paper-based analytical devices (μPAD) comprise a single-layer moving sliding PAD (SPAD) to control the flow channel switch together with a folding origami PAD (OPAD) to test the target analytes. The facile assembly without any splicing materials avoids cross-contamination and non-specific adsorption of joining materials that may be caused by multi-target detection. The concentration of Fe(III), Ni(II), Cr(VI), and nitrite in standard solutions and actual aqueous solutions was successfully determined using the designed μPAD. The μPAD was able to achieve LOD of 3.3 mg/L, 1.3 mg/L, 0.35 mg/L, 0.28 mg/L for Fe (III), Ni (II), Cr (VI), and nitrite, respectively. The designed SOPAD exhibits improved stability, with a deviation of less than 7% compared to conventional analytical methods (ICP-OES and UV). Our work demonstrates that this 3D PAD holds great promise and a wide scope in environmental monitoring, biochemical analysis, food testing and other testing industries.

Capillary Flow-Driven Microfluidics Combined with a Paper Device for Fast User-Friendly Detection of Heavy Metals in Water

Human exposure to heavy metals is a concerning global problem because of its detrimental effect on our health and ecosystem. Assessing the levels of these metals is cost- and labor-intensive and nonuser friendly because current analysis approaches typically rely on heavy instrumentations like inductively coupled plasma-mass spectrometry, which is only possible in centralized labs. Hence, simple economical detection methods are in high demand in developing countries and areas with insufficient infrastructure, professional experts, and appropriate environmental treatment. Several microfluidic paper-based analytical devices have been reported as promising alternatives to conventional testing methods for on-site heavy metal detection. Paper-based microfluidics are advantageous because of their simple fabrication, biodegradability, low cost, and ability to operate without pumps. However, typical assay times for current platforms are slow, and they typically rely on pipetting a fixed volume into the assay cards. This adds complexity in actual field scenarios. Here, we report a novel, inexpensive, and straightforward capillary-driven microfluidic device combined with paper for rapid and user-friendly detection of Ni(II), Cu(II), and Fe(III) in water. A colorimetric approach was adopted to quantify these metals. The device was able to produce a homogeneous color signal within 8 s of sample insertion. The limit of detection and limit of quantification were calculated to be 2 and 6.67 ppm for nickel, 0.3 and 1 ppm for Cu, and 1.1 and 3.67 ppm for Fe, respectively. The majority (>90%) of the collected samples showed recovery in the 80-110% range with acceptable accuracy and precision (<15% RSD) for a colorimetric device. This technique can be beneficial for rapidly assessing heavy metal exposure in drinking and surface water at drastically reduced assay time and is the first example of capillary flow-driven microfluidic devices as a transport medium for heavy metal detection.

A device-independent method for the colorimetric quantification on microfluidic sensors using a color adaptation algorithm

A general and adaptable method is proposed to reliably extract quantitative information from smartphone images of microfluidic sensors. By analyzing and processing the color information of selected standard substances, the influence of light conditions, device differences, and human factors could be significantly reduced. Machine learning and multivariate fitting methods were proved to be effective for chroma correction, and a key element was the training of sample size and the fitting form, respectively. A custom APP was developed and validated using a high-sensitivity chromium ion quantification paper chip. The average chroma deviations under different conditions were reduced by more than 75% in RGB color space, and the concentration test error was reduced by more than half compared with the commonly used method. The proposed approach could be a beneficial supplement to existing and potential colorimetry-based detection methods.

Multi-responsive paper chemosensors based on mesoporous silica nanospheres for quantitative sensing of heavy metals in water

Exposure to low concentrations of heavy metal cations seriously harms living organisms, hence they are considered environmental toxins. Portable simple detection systems are required for field monitoring of multiple metal ions. In this report, paper-based chemosensors (PBCs) were prepared by adsorbing 1-(pyridin-2-yl diazenyl) naphthalen-2-ol (chromophore), which recognizes heavy metals, onto filter papers coated with mesoporous silica nano spheres (MSNs). The high density of the chromophore probe on the surface of PBCs resulted in ultra-sensitive optical detection of heavy metal ions and short response time. The concentration of metal ions was determined using digital image-based colorimetric analysis (DICA) and compared to spectrophotometry under optimal sensing conditions. The PBCs exhibited stability and short recovery times. The detection limits determined using DICA of Cd²⁺, Co²⁺, Ni²⁺ and Fe³⁺ were 0.22, 0.28, 0.44, and 0.54 μM; respectively. Additionally, the linear ranges for monitoring Cd²⁺, Co²⁺, Ni²⁺ and Fe³⁺ were 0.44-4.4, 0.16-4.2, 0.8-8.5, and 0.002-5.2 μM; respectively. The developed chemosensors showed high stability, selectivity, and sensitivity for sensing of Cd²⁺, Co²⁺, Ni²⁺ and Fe³⁺ in water under optimum conditions and hold potential for low cost, onsite sensing of toxic metals in water.

Optimization of a silver-nanoprism conjugated with 3,3',5,5'-tetramethylbenzidine towards easy-to-make colorimetric analysis of acetaldehyde: a new platform towards rapid analysis of carcinogenic agents and environmental technology

Acetaldehyde acts as an important mediator in the metabolism of plants and animals; however, its abnormal level can cause problems in biological processes. Although acetaldehyde is found naturally in many organisms, exposure to high concentrations can have effects on the eyes, respiratory system, etc. Due to the importance of detecting acetaldehyde in environmental samples and biofluids, determination of its concentration is highly demanded. There are some reports showing exposure to high concentrations of acetaldehyde for a long time can increase the risk of cancer by reacting with DNA. In this work, we presented a novel colorimetric method for rapid and sensitive detection of acetaldehyde with high reproducibility using different AgNPs with various morphologies. The redox reaction between AgNPs, 3,3',5,5'-tetramethylbenzidine (TMB) solution, and analytes endows a color change in 15 minutes that is detectable by the naked eye. UV spectrophotometry was further used for quantitative analysis. An iron mold with a hexagonal pattern and liquid paraffin were also used to prepare the paper-based microfluidic substrate, as a low cost, accessible, and rapid detection tool. Different types of AgNPs showed different lower limits of quantification (LLOQ). The AgNPs-Cit and AgNPrs could identify acetaldehyde with linear range of 10-7 to 10 M and an LLOQ of 10-7 M. The AgNWs showed the best color change activity with a linear range 10-5 to 10 M and the lowest diagnostic limit is 10-5 M. Finally, analysis of human biofluids as real samples were successfully performed using this system.

Electric yo-yo centrifugation combining with paper-based microfluidic immunoassay chip for inflammatory biomarkers detection in whole blood

At present, most countries or regions use commercial centrifuges for centrifugation, but this is out of reaching for limited-resource areas. To overcome this problem, a portable electric yo-yo as centrifuge was firstly proposed to obtain serum, and this device can be combined with paper-based analytical devices for enzyme-linked immunosorbent assay (ELISA) analysis from human whole blood. In this study, inflammatory biomarkers C-reactive protein (CRP) and serum amyloid A (SAA) were used as target biomarker to verify the performance of the proposed method. The results shows good performance and their detection limits were determined to be 580 pg/mL for CRP and 800 pg/mL for SAA, respectively. We believe this method provides a new platform of low cost and fast detection for inflammatory biomarkers in the limited-resource settings.

Syringe Paper-Based Analytical Device for Thiamazole Detection by Hedysarum Polysaccharides-Mediated Silver Nanoparticles

In this paper, silver nanoparticles (AgNPs) were successfully green-synthesized for the first time using Hedysarum polysaccharide (HPS) as a reducing agent, stabilizer, and modifier (HPS-AgNP). Thiamazole could induce the aggregation of HPS-AgNPs in the residue on a cellulose membrane. A syringe paper-based analytical device was creatively established to ensure the tightness, stability, and good repeatability of the test. The color information remaining on the cellulose membrane was converted into gray values using ImageJ software. Hence, the linear regression curve for thiamazole was established as y = 1 + 0.179x with a detection limit (LOD) of 24.6 nM in the relatively wide range of 0.1~10 μM. This syringe paper-based analytical device was successfully applied to the biological samples.

Colorimetric detection of heavy metal ions with various chromogenic materials: Strategies and applications

Detection of heavy metal ions has drawn significant attention in environmental and food area due to their threats to the human health and ecosystem. Colorimetry is one of the most frequently-used methods for the detection of heavy metal ions owing to its simplicity, easy operation and rapid on-site detection. The development of chromogenic materials and their sensing mechanisms are the key research direction in the area of colorimetric method. Since each chromogenic material has their unique optical and chemical properties, they have totally different colorimetric sensing mechanisms. This review focuses on the chromogenic materials and their sensing strategies for the colorimetric detection of heavy metal ions. We divide the chromogenic materials into three types, including organic materials, inorganic materials, and other materials. As for each type of chromogenic material, we discuss their detailed sensing strategies, sensing performance, and real sample applications. Moreover, current challenges and perspectives related to the colorimetry of heavy metal ions are also discussed in this review. The aim of this review is to help readers to better understand the principles of colorimetric methods for heavy metal ions and push the development of rapid detection of heavy metal ions.

Simultaneous Cross-type Detection of Water Quality Indexes via a Smartphone-App Integrated Microfluidic Paper-Based Platform

Water quality guarantee in remote areas necessitates the development of portable, sensitive, fast, cost-effective, and easy-to-use water quality detection methods. The current work reports on a microfluidic paper-based analytical device (μPAD) integrated with a smartphone app for the simultaneous detection of cross-type water quality parameters including pH, Cu(II), Ni(II), Fe(III), and nitrite. The shapes, baking time, amount, and ratios of reaction reagent mixtures of wax μPAD were optimized to improve the color uniformity and intensity effectively. An easy-to-use smartphone app was established for recording, analyzing, and directly reading the colorimetric signals and target concentrations on μPAD. The results showed that under the optimum conditions, the current analytical platform has reached the detection limits of 0.4, 1.9, 2.9, and 1.1 ppm for nitrite, Cu(II), Ni(II), and Fe(III), respectively, and the liner ranges are 2.3-90 ppm (nitrite), 3.8-400 ppm (Cu(II)), 2.9-1000 ppm (Ni(II)), 2.8-500 ppm (Fe(III)), and 5-9 (pH). The proposed portable smartphone-app integrated μPAD detection system was successfully applied to real industrial wastewater and river water quality monitoring. The proposed method has great potential for field water quality detection.

Rapid and accurate nanoelectrokinetic diagnosis of drug-resistant bacteria

Increased antimicrobial resistance presents a major threat to public health, and it is a global health problem due to the rapid globalization and transmission of infectious diseases. However, fast and precise diagnosis tool is lacking, and inappropriate antibiotic prescription leads to the unforeseen production of drug-resistant bacteria. Here, we report a Rapid and Accurate Nanoelectrokinetic Diagnostic System (RANDx) for detecting drug-resistant bacteria, which cause a common infectious disease called Urinary Tract Infection (UTI), within 7 min. We develop nanoelectrokinetic paper-based analytic device (NEK-PAD) as a sample prep module of RANDx and obtain >100-fold post-wetting preconcentration by balancing between ion concentration polarization (ICP) and radial imbibition for a constant flow rate. Simultaneously with preconcentration, our cathodic nanochannel design enables NEK-PAD to extract drug-resistant enzymes without denaturation and accelerate enzyme-linked reactions under electrical spontaneous heating at approximately 37 °C. Finally, using a cell phone camera, we detect label-free drug-resistant bacteria as low as 10⁴ cfu/mL, which is higher than clinically required threshold (>10⁵ cfu/mL) by enhancing 1000 times of the limit of detection (LOD) of colorimetric nitrocefin assay. We believe that the RANDx will be an innovative precision medicine tool for UTI and other infectious diseases in limited remote settings.

Hand-Operated, Paper-Based Rotational Vertical-Flow Immunosensor for the Impedimetric Detection of α-Fetoprotein

This study demonstrates a hand-operated, paper-based rotational vertical-flow immunosensor (rotational VFI) platform requiring fewer pipetting steps, designed for the electrochemical detection of α-fetoprotein with multiple and time-sequenced steps. The platform allows users to perform electrochemical measurements without interference from the convective component of fluid motion, which is unfavorable in most techniques. Users can freely transfer-switch-stop fluid flows by manually rotating the paper disk, evidencing the superior flexibility of this sensor compared to other biosensors. Furthermore, the overall assay duration can be considerably shortened to 9 min. The linear range (LR) is determined to be 0.01-500 ng/mL, with a limit of detection (LOD) of 1.65 pg/mL, and the sensitivity can be significantly enhanced simply by switching off the sample stream to ensure detention at the binding zone (for up to 30 min). This additional step can widen the LR to 0.5 pg/mL, with a LOD of 3.54 fg/mL, which is the lowest detectable level ever reported among paper-based sensors. The advantages of the designed rotational VFI qualify it as a suitable alternative to various biosensors.

Adsorptive colorimetric determination of chromium(VI) ions at ultratrace levels using amine functionalized mesoporous silica

There is an urgent need for a rapid, affordable and sensitive analytical method for periodic monitoring of heavy metals in water bodies. Herein, we report for the first time a versatile method for ultratrace level metal detection based on colorimetric sensing. The method integrates preconcentration using a nanomaterial with a colorimetric assay performed directly on the metal-enriched nanomaterial surface. This method circumvents the need for tedious sample pre-processing steps and the complex development of colorimetric probes, thereby reducing the complexity of the analytical procedure. The efficacy of the proposed method was demonstrated for chromium(VI) ions detection in water samples. Amine functionalized mesoporous silica (AMS) obtained from a one-pot synthesis was utilized as a pre-concentration material. The structural and chemical analysis of AMS was conducted to confirm its physico-chemical properties. The pre-concentration conditions were optimized to maximise the colorimetric signal. AMS exhibited a discernible colour change from white to purple (visible to the naked eye) for trace Cr(VI) ions concentration as low as 0.5 μg L^-1. This method shows high selectivity for Cr(VI) ions with no colorimetric signal from other metal ions. We believe our method of analysis has a high scope for de-centralized monitoring of organic/inorganic pollutants in resource-constrained settings.

Semi-quantitative analysis of nickel: counting-based μPADs built via hand drawing and yellow oily double-sided adhesive tape

Counting-based μPADs were fabricated by hand drawing and yellow oily double-sided adhesive tape, and then successfully applied for the semi-quantitative analysis of nickel.

Online and offline preconcentration techniques on paper-based analytical devices for ultrasensitive chemical and biochemical analysis: A review

Microfluidic paper-based analytical devices (μPADs) have attracted much attention over the past decade. They embody many advantages, such as abundance, portability, cost-effectiveness, and ease of fabrication, making them superior for clinical diagnostics, environmental monitoring, and food safety assurance. Despite these advantages, μPADs lack the high sensitivity to detect many analytes at trace levels than other commercial analytical instruments such as mass spectrometry. Therefore, a preconcentration step is required to enhance their sensitivity. This review focuses on the techniques used to separate and preconcentrate the analytes onto the μPADs, such as ion concentration polarization, isotachophoresis, and field amplification sample stacking. Other separations and preconcentration techniques, including liquid-solid and liquid-liquid extractions coupled with μPADs, are also reviewed and discussed. In addition, the fabrication methods, advantages, disadvantages, and the performance evaluation of the μPADs concerning their precision and accuracy were highlighted and critically assessed. Finally, the challenges and future perspectives have been discussed.

Automatic flow delay through passive wax valves for paper-based analytical devices

Microfluidic paper-based analytical devices (μPADs) have been widely explored for point-of-care testing due to their simplicity, low cost, and portability. μPADs with multiple-step reactions usually require precise flow control, especially flow-delay. This paper reports the numerical, mathematical, and experimental studies of flow delay through wax valves surrounded by PDMS walls on paper microfluidics. The predried surfactant in the sample zone diffuses into the liquid sample which can therefore flow through the wax valves. The delay time is automatically regulated by the diffusion of the surfactant after sample loading. The numerical study suggested that both the elevated contact angle and the reduced porosity and pore size in the wax printed region could effectively prevent water but allow liquids with lower contact angles (e.g., surfactant solutions) to flow through. The PDMS walls fabricated using a low-cost liquid dispenser effectively prevented the leakage of surfactant solutions. By controlling the quantity, diffusion distance, and type of the surfactant predried on the chip, the system successfully achieved a delay time ranging from 1.6 to 20 minutes. A mathematical model involving the above parameters was developed based on Fick's second law to predict the delay time. Finally, the flow-delay systems were applied in sequential mixing and distance-based detection of either glucose or alcohol. Linear ranges of 1-100 mg dL^-1 and 1-40 mg dL^-1 were achieved for glucose and alcohol, respectively. The lower limit detection (LOD) of glucose and alcohol was 1 mg dL^-1. The LOD of glucose was only 1/11 of that detected using μPADs without flow control, indicating the advantage of controlling fluid flow. The systematic findings in this study provide critical guidelines for the development and applications of wax valves in automatic flow delay for point-of-care testing.

Recent developments in sensors for wearable device applications

Wearable devices are a new means of human-computer interaction with different functions, underlying principles, and forms. They have been widely used in the medical and health fields, in applications including physiological signal monitoring; sports; and environmental detection, while subtly affecting people's lives and work. Wearable sensors as functional components of wearable devices have become a research focus. In this review, we systematically summarize recent progress in the development of wearable sensors and related devices. Wearable sensors in medical health applications, according to the principle of measurement, are divided into physical and chemical quantity detection. These sensors can monitor and measure specific parameters, thereby enabling continuously improvements in the quality and feasibility of medical treatment. Through the detection of human movement, such as breathing, heartbeat, or bending, wearable sensors can evaluate body movement and monitor an individual's physical performance and health status. Wearable devices detecting aspects of the environment while maintaining high adaptability to the human body can be used to evaluate environmental quality and obtain more accurate environmental information. The ultimate goal of this review is to provide new insights and directions for the future development and broader application of wearable devices in various fields.Graphical abstract.

Recent advances of environmental pollutants detection via paper-based sensing strategy

Paper has become one of the most promising substrates for building low-cost and powerful sensing platforms due to its self-pumping ability and compatibility with multiple patterning methods. Paper-based sensors have been greatly developed in the field of environmental monitoring. In this review, we introduced the research and application of paper-based sensors in environmental monitoring, focusing on the deposition and patterning methods of building paper-based sensors, and summarized the applications of detecting environmental pollutants, including metal ions, anions, explosives, neurotoxins, volatile organic compounds, and small molecules. In addition, the development prospects and challenges of promoting paper-based sensors are also discussed. The current review will provide references for the construction of portable paper-based sensors, and has implications for the field of on-site real-time detection of the environment.

Highly selective simultaneous determination of Cu(ii), Co(ii), Ni(ii), Hg(ii), and Mn(ii) in water samples using microfluidic paper-based analytical devices

A new paper-based analytical device design was fabricated by a wax printing method for simultaneous determination of Cu(ii), Co(ii), Ni(ii), Hg(ii), and Mn(ii). Colorimetry was used to quantify these heavy metal ions using bathocuproine (Bc), dimethylglyoxime (DMG), dithizone (DTZ), and 4-(2-pyridylazo) resorcinol (PAR) as complexing agents. The affinity of complexing agents to heavy metal ions is dependent on the formation constant (Kf). To enhance the selectivity for heavy metal ion determination, the new device was designed with two pretreatment zones, where masking agents remove the interfering ions. It was found that two pretreatment zones worked better than a single pretreatment zone at removing interferences. The reaction time, sample and complexing agent volumes, and complexing agent concentrations were optimized. The analytical results were achieved with the lowest detectable concentrations of 0.32, 0.59, 5.87, 0.20, and 0.11 mg L-1 for Cu(ii), Co(ii), Ni(ii), Hg(ii), and Mn(ii), respectively. The linear ranges were found to be 0.32-63.55 mg L-1 (Cu(ii)), 0.59-4.71 mg L-1 (Co(ii)), 5.87-352.16 mg L-1 (Ni(ii)), 0.20-12.04 mg L-1 (Hg(ii)), and 0.11-0.55 mg L-1 (Mn(ii)). The lowest detectable concentration and linearity for the five metal ions allow the application of this device for the determination of heavy metal ions in various water samples. The sensor showed high selectivity and efficiency for simultaneous determination of Cu(ii), Co(ii), Ni(ii), Hg(ii), and Mn(ii) in drinking, tap, and pond water samples on a single device and detection with the naked eye. The results illustrated that the proposed sensor showed good accuracy and precision agreement with the standard ICP-OES method.

A simple and low-cost paper-based colorimetric method for detecting and distinguishing the GII.4 and GII.17 genotypes of norovirus

In modern times, viruses still threaten people's lives. Among them, norovirus was the main pathogenic factor in the cause of gastroenteritis and foodborne illness, of which the GII.4 and GII.17 genotypes are prevalent in China and most parts of the world. A simple and low-cost platform for rapid and accurate norovirus detection remains a major challenge. After the cell-free system and paper-based chromogenic system were optimized, a rapid and specific norovirus detection method was established based on norovirus-specific sequences in combination with toehold switch elements. The development of a visible color change during detection eliminates the need for any complicated instruments. We validated this strategy and its specificity in differentiating GII.4, GII.17, Zika virus, and human coronavirus HKU1. The results showed that the optimized detection system not only provided a simple and rapid detection method for the sufficient differentiation of the two norovirus genotypes but also showed high specificity and no cross-reactivity with other viruses. Using nucleic acid isothermal amplification, this assay showed a limit of detection of 0.5 pM for the GII.4 genotype and 2.6 fM for the GII.17 genotype in reactions that could be observed directly with the naked eye. Our results suggested that this paper-based colorimetric method could serve as a simple and low-cost visual detection method for pathogens in clinical samples, especially in remote or rural areas.

Coordination array for accurate colorimetric sensing of multiple heavy metal ions

Heavy metals detection is of great significance to the environment and human health, and most of the detection methods require expensive instruments and laborious operation. Herein, we present a coordination array for accurate and rapid colorimetric heavy metal ions sensing. The array was composed of six chelates and has cross response to Hg²⁺, Cd²⁺, Pb²⁺ and Cu²⁺. The results could be observed by naked eyes or detected by plate reader combined with pattern analysis. Linear discrimination analysis was applied for the pattern analysis and the four heavy metal ions (Pb²⁺, Cd²⁺, Hg²⁺ and Cu²⁺) generated a clustering map at 1 μM. The coordination array demonstrates a great potential for sensing heavy metal ions simultaneously.

Paper-Based Microfluidic Sensors for Onsite Environmental Detection: A Critical Review

A newly developed research topic, fabricated paper-based microfluidic sensors, was discussed in the field of low-cost environmental detection. Distinguished with the traditional dipstick or lateral-flow setups, these paper-based microfluidic sensors can serve as a tool for onsite quantitative and semi-quantitative measurements, without risks to cause environmental pollution. They have attracted increasing interest since the first easy-fabricated paper-based setup reported by Whitesides group in 2007. Most of the publications utilized paper-based sensors in clinical detection. In recent years, some groups started to use these sensors in environmental measurement, leading to precise, easy operation, low-cost, and eco-friendly methods for onsite detection. In this review, paper-based microfluidic sensors were briefly introduced, followed by literatures review and discussion for future perspectives.

Paper Microfluidics and Tailored Gold Nanoparticles for Nonenzymatic, Colorimetric Multiplex Biomarker Detection

The plasmonic properties of gold nanoparticles (AuNPs) are a promising tool to develop sensing alternatives to traditional, enzyme-catalyzed reactions. The need for sensing alternatives, especially in underdeveloped areas of the world, has given rise to the application of nonenzymatic sensing approaches paired with cellulosic substrates to biochemical analysis. Herein, we present three individual, low-step, wet-chemistry, colorimetric assays for three target biomarkers, namely, glucose, uric acid, and free cholesterol, relevant in diabetes control and their translation into paper-based assays and microfluidic platforms for multiplexed analysis. For glucose determination, an in situ AuNPs synthesis approach was applied into the developed μPAD, giving semiquantitative measures in the physiologically relevant range. For uric acid and cholesterol determination, modified AuNPs were used to functionalize paper with a gold-on-paper approach with the optical properties changing based on different aggregation degrees and hydrophobic properties of particles dependent on analyte concentration. These paper-based assays show sensitivity ranges and limits of detection compatible for target analyte level determination and detection limits comparable to those of similar enzymatic, colorimetric systems, relying only on plasmonic transduction without the need for enzymatic activity or other chromogenic substrates. The resulting paper-based assays were integrated into a single 3D, multiplex paper-based device using paper microfluidics, showing the capability for performing different colorimetric assays with distinct requirements in terms of sample flow and sample uptake in test zones using a combination of both horizontal and vertical flows inside the same device. The presented device allows for multiparametric, colorimetric measures of different metabolite levels from a single complex sample matrix drop using digital color analysis, showing the potential for development of low-cost, low-complexity tools for diagnostics toward the point-of-care.

Nanomaterial-Integrated Cellulose Platforms for Optical Sensing of Trace Metals and Anionic Species in the Environment

The development of disposable sensors that can be easily adapted to every analytical problem is currently a hot topic that is revolutionizing many areas of science and technology. The need for decentralized analytical measurements at real time is increasing for solving problems in areas such as environment pollution, medical diagnostic, food quality assurance, etc., requiring fast action. Despite some current limitations of these devices, such as insufficient detection capability at (ultra)trace level and risk of interferent effects due to matrix, they allow low-cost analysis, portability, low sample consumption, and fast response. In the last years, development of paper-based analytical devices has undergone a dramatic increase for on-site detection of toxic metal ions and other pollutants. Along with the great availability of cellulose substrates, the immobilization of receptors providing enhanced recognition ability, such as a variety of nanomaterials, has driven the design of novel sensing approaches. This review is aimed at describing and discussing the different possibilities arisen with the use of different nanoreceptors (e.g., plasmonic nanoparticles, quantum dots, carbon-based fluorescent nanoparticles, etc.) immobilized onto cellulose-based substrates for trace element detection, their advantages and shortcomings.

Plug-and-play assembly of paper-based colorimetric and electrochemical devices for multiplexed detection of metals

We propose a “plug-and-play” (PnP) assembly for coupling paper-based colorimetric and electrochemical devices for multiplexed detection of metals.

Paper-based electroanalytical devices for stripping analysis of lead and cadmium in children's shoes

Children's shoes are potential sources of toxic heavy metals, especially for younger children. Electrochemical detection could be applied for effective stripping analysis of heavy metals (such as Cd and Pb). However, the substrates of working electrodes are still limited and it is not well known which property is critical. Herein ITO glass was used as the substrate and the working electrode was modified with carbon cement for stripping analysis of Cd and Pb. The electrochemical impedance spectra of the ITO modified electrodes suggested the connection between the resistance and the electrochemical responses of heavy metals in stripping analysis, depending on the dilution ratio of the carbon cement. After optimization, the ITO modified electrodes in paper-based analytical devices could be used to sensitively quantify Cd and Pb with the concentration ranging from 10 to 1000 ppb. The detection limit of Pb2+ could reach less than 1 ppb while that of Cd2+ could reach 5 ppb, depending on the pH value of the sample solution. The paper-based electroanalytical devices could be used to quantify the concentration of Cd and Pb in children's shoes. This study implied the impact of the electric conductivity of the electrode substrates on stripping analysis, which might help to find more materials for the fabrication of the working electrodes.

Simultaneous Multiplexed Detection of Protein and Metal Ions by a Colorimetric Microfluidic Paper-based Analytical Device

In order to improve the efficiency of disease diagnosis and environmental monitoring, it is desirable to detect the concentration of proteins and metal ions simultaneously, since the current popular diagnostic platform can only detect proteins or metal ions independently. In this work, we developed a colorimetric microfluidic paper-based analytical device (µPAD) for simultaneous determination of protein (bovine serum albumin, BSA) and metal ions [Fe(III) and Ni(II)]. The µPAD consisted of one central zone, ten reaction zones and ten detection zones in one device, in which reaction solutions were effectively optimized for different types of chromogenic reactions. Fe(III), Ni(II) and BSA can be easily identified by the colored products, and their concentrations are in good accordance with color depth based on the established standard curves. The detection limits are 0.1 mM for Fe(III), 0.5 mM for Ni(II) and 1µM for BSA, respectively. Best of all, we demonstrated the efficiency of the µPAD with accurate detection of Fe(III), Ni (II) and BSA from river water samples within 15 minutes. The µPAD detection is efficient, instrument-free, and easy-to-use, holding great potential for simultaneous detection of cross type analytes in numerous diagnostic fields.

Selective electromembrane extraction and sensitive colorimetric detection of copper(II)

In this study, an extremely effective electromembrane extraction (EME) method was developed for the selective extraction of Cu(II) followed by Red-Green-Blue (RGB) detection. The effective parameters optimized for the extraction efficiency of EME include applied voltage, extraction time, supported liquid membrane (SLM) composition, pH of acceptor/donor phases, and stirring rate. Under optimized conditions, Cu(II) was extracted from a 3 mL aqueous donor phase to 8 µL of 100 mM HCl acceptor solution through 1-octanol SLM using an applied voltage of 50 V for 15 min. The proposed method provides a working range of 0.1–0.75 µg·mL−1 with 0.03 µg·mL−1 limit for detection. Finally, the developed technique was applied to different environmental water samples for monitoring environmental pollution. Obtained relative recoveries were within the range of 93–106%. The relative standard deviation (RSD) and enhancement factor (EF) were found to be ≤4.8% and 100 respectively. We hope that this method can be introduced for quantitative determination of Cu(II) as a fast, simple, portable, inexpensive, effective, and precise procedure.

Novel approaches for colorimetric measurements in analytical chemistry - A review

Colorimetric techniques have been developed and used in routine analyses for over a century and apparently all their potentialities have been exhaustively explored. However, colorimetric techniques have gained high visibility in the last two decades mainly because of the development of the miniaturization concept, for example, paper-based analytical devices that mostly employ colorimetric reactions, and by the advances and popularity of image capture instruments. The impressive increase in the use of these devices was followed by the development and enhancement of different modes of color detection to meet the demands of making qualitative, semi-quantitative, and fully quantitative analyses of multiple analytes. Cameras, scanners, and smartphones are now being used for this purpose and have become suitable alternatives for different approaches to colorimetric analysis; this, in addition to advancements in miniaturized devices. On the other hand, recent developments in optoelectronics technologies have launched more powerful, more stable and cheaper light-emitting diodes (LEDs), which once again have become an interesting tool for the design of portable and miniaturized devices based on colored reactions. Here, we present a critical review of recent developments and challenges of colorimetric detection in modern analytical chemistry in the last five years, and present thoughts and insights towards future perspectives in the area to improve the use of colorimetric detection in different application approaches.

Fabrication of paper microfluidic devices using a toner laser printer

This paper describes a method to fabricate microfluidic paper-based analytical devices (μPADs) using a toner laser printer. Multiple methods have been reported for the fabrication of μPADs for point-of-care diagnostics and environmental monitoring. Despite successful demonstrations, however, existing fabrication methods depend on particular printers, in-house instruments, and synthetic materials. In particular, recent discontinuation of the solid wax printer has made it difficult to fabricate μPADs with readily available instruments. Herein we reported the fabrication of μPADs using the most widely available type of printer: a toner laser printer. Heating of printed toner at 200 °C allowed the printed toner to reflow, and the spreading of the hydrophobic polymer through the filter paper was characterized. Using the developed μPADs, we conducted model colorimetric assays for glucose and bovine serum albumin (BSA). We found that heating of filter paper at 200 °C for 60 min caused the pyrolysis of cellulose in the paper. The pyrolysis resulted in the formation of aldehydes that could interfere with molecular assays involving redox reactions. To overcome this problem, we confirmed that the removal of the aldehyde could be readily achieved by washing the μPADs with aqueous bleach. Overall, the developed fabrication method should be compatible with most toner laser printers and will make μPADs accessible in resource-limited circumstances.