Paper-based Analytical Devices Research Articles

A family of Cd(II) coordination polymers constructed from 6-aminopicolinate and bipyridyl co-linkers: study of their growth in paper and photoluminescence sensing of Fe3+ and Zn2+ ions.

In this work, we report on five novel coordination polymers (CPs) based on the linkage of the [Cd(6apic)2] building block [where 6apic = 6-aminopicolinate] by different bipyridine-type organic spacers, forming different coordination compounds with the following formulae: [Cd(μ-6apic)2]n (1), {[Cd(6apic)2(μ-bipy)]·H2O}n (2), {[Cd(6apic)2(μ-bpe)]·2H2O}n (3), [Cd(6apic)(μ-6apic)(μ-bpa)0.5]n (4) and {[Cd2(6apic)4(μ-tmbp)]·7H2O}n (5) [where bipy = 4,4'-bipyridine, bpe = 1,2-di(4-pyridyl)ethylene, bpa = 1,2-di(4-pyridyl)ethane (bpa) and tmbp = 1,3-di(4-pyridyl)propane]. Most of the synthesized compounds form infinite metal-organic rods through the linkage of the building block by the bipyridine-type linker, except in the case of compound 4 whose assembly forms a densely packed 3D architecture. All compounds were fully characterized and their photoluminescence properties were studied experimentally and computationally through density functional theory (DFT) calculations. All compounds display, upon UV excitation, a similar blue emission of variable intensity depending on the linker employed for the connection of the building units, among which compound 2 deserves to be highlighted for its room temperature phosphorescence (RTP) with an emission lifetime of 32 ms that extends to 79 ms at low temperature. These good photoluminescence properties, in addition to its stability in water over a wide pH range (between 2 and 10), motivated us to study compound 2 as a sensor for the detection of metal ions in water, and it showed high sensitivity to Fe3+ through a fluorescence turn-off mechanism and an unspecific turn-on response to Zn2+. Furthermore, the compound is processed as a paper-based analytical device (PAD) in which the phosphorescence emission is preserved, improving the sensing capacity toward Fe3+ ions.

Development and Evaluation of a Noninvasive Microfluidic-Based Paper Analytical Device for Leptospirosis Diagnosis.

Leptospirosis is a re-emerging infectious disease that presents a diagnostic enigma for clinicians with frequent misdiagnosis due to lack of rapid and accurate diagnostic tests, as the current methods are encumbered by inherent limitations. The development of a diagnostic sensor with a sample-in-result-out capability is pivotal for prompt diagnosis. Herein, we developed a microfluidic paper-based analytical device (spin-μPAD) featuring a sample-in-result-out fashion for the detection of Leptospira specific urinary biomarker, sph2 sphingomyelinase, crucial for noninvasive point-of-care testing. Fabrication of paper devices involved precise photolithography techniques, ensuring a high degree of reproducibility and replicability. By optimizing the device's configuration and protein components, a remarkable sensitivity and specificity was achieved for detecting leptospiral sph2 in urine, even at low concentrations down to 1.5 fg/mL, with an assay time of 15 min. Further, the spin-μPAD was validated with 20 clinical samples, suspected of leptospirosis including other febrile illnesses, and compared with gold standard microscopic agglutination test, culture, Lepto IgM ELISA, darkfield microscopy, and Leptocheck WB spot test. In contrast to commercial diagnostic tools, the spin-μPAD was noninvasive, rapid, easy to use, specific, sensitive, and cost-effective. The results highlight the potential of this innovative spin-μPAD for an efficient and dependable approach to noninvasive leptospirosis diagnosis, addressing critical needs in the realms of public health and clinical settings.

A Programmable Microfluidic Paper-Based Analytical Device for Simultaneous Colorimetric and Photothermal Visual Sensing of Multiple Enzyme Activities.

New strategies for the simultaneous and portable detection of multiple enzyme activities are highly desirable for clinical diagnosis and home care. However, the methods developed thus far generally suffer from high costs, cumbersome procedures, and heavy reliance on large-scale instruments. To satisfy the actual requirements of rapid, accurate, and on-site detection of multiple enzyme activities, we report herein a smartphone-assisted programmable microfluidic paper-based analytical device (μPAD) that utilizes colorimetric and photothermal signals for simultaneous, accurate, and visual quantitative detection of alkaline phosphatase (ALP) and butyrylcholinesterase (BChE). Specifically, the operation of this μPAD sensing platform is based on two sequential steps. Cobalt-doped mesoporous cerium oxide (Co-m-CeO2) with remarkable peroxidase-like activities under neutral conditions first catalytically decomposes H2O2 for effectively converting colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxidized TMB (oxTMB). The subsequent addition of ALP or BChE to their respective substrates produces a reducing substance that can somewhat inhibit the oxTMB transformation for compromised colorimetric and photothermal signals of oxTMB. Notably, these two-step bioenzyme-nanozyme cascade reactions strongly support the straightforward and excellent processability of this platform, which exhibit lower detection limits for ALP and BChE with a detection limit for BChE an order of magnitude lower than those of the other reported paper-based detection methods. The practicability and efficiency of this platform are further demonstrated through the analysis of clinical serum samples. This innovative platform exhibits great potential as a facile yet robust approach for simultaneous, accurate, and on-site visual detection of multiple enzyme activities in authentic samples.

A Programmable Microfluidic Paper-Based Analytical Device for Simultaneous Colorimetric and Photothermal Visual Sensing of Multiple Enzyme Activities.

New strategies for the simultaneous and portable detection of multiple enzyme activities are highly desirable for clinical diagnosis and home care. However, the methods developed thus far generally suffer from high costs, cumbersome procedures, and heavy reliance on large-scale instruments. To satisfy the actual requirements of rapid, accurate, and on-site detection of multiple enzyme activities, we report herein a smartphone-assisted programmable microfluidic paper-based analytical device (μPAD) that utilizes colorimetric and photothermal signals for simultaneous, accurate, and visual quantitative detection of alkaline phosphatase (ALP) and butyrylcholinesterase (BChE). Specifically, the operation of this μPAD sensing platform is based on two sequential steps. Cobalt-doped mesoporous cerium oxide (Co-m-CeO2) with remarkable peroxidase-like activities under neutral conditions first catalytically decomposes H2O2 for effectively converting colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxidized TMB (oxTMB). The subsequent addition of ALP or BChE to their respective substrates produces a reducing substance that can somewhat inhibit the oxTMB transformation for compromised colorimetric and photothermal signals of oxTMB. Notably, these two-step bioenzyme-nanozyme cascade reactions strongly support the straightforward and excellent processability of this platform, which exhibit lower detection limits for ALP and BChE with a detection limit for BChE an order of magnitude lower than those of the other reported paper-based detection methods. The practicability and efficiency of this platform are further demonstrated through the analysis of clinical serum samples. This innovative platform exhibits great potential as a facile yet robust approach for simultaneous, accurate, and on-site visual detection of multiple enzyme activities in authentic samples.

A portable smartphone detection of ctDNA using MnB2 nanozyme and paper-based analytical device

The development of point-of-care testing (POCT) for circulating tumor DNA (ctDNA) is meaningful for the non-invasive cancers screening and diagnosis, particularly in resource-limited settings. The microfluidic paper-based analytical device (μPAD) provides an ideal platform, its application in ctDNA assays remains underexplored. In this work, a multifunctional μPAD was manufactured, which can enhance the efficiency and reduce the cost of ctDNA sensing. Additionally, a smartphone-based application analysis was fabricated for convenient, portable detection and colorimetric signal readout. Moreover, the novel oxidase-like MnB2 nanozyme was introduced in the sandwiches sensing strategy, utilizing its catalytic properties to effectively generate a colorimetric signal. The use of MnB2 nanozyme in sensing application is relatively novel, and its catalytic performance and mechanism was thoroughly evaluated via experiment and density functional theory (DFT) calculations. After optimizing the detection conditions, the proposed biosensor exhibited satisfactory results. Furthermore, the method was successfully used to detect ctDNA in tumor cell lysates and peripheral blood samples from tumor-bearing mice. The results were consistent with standard qPCR method, affirming the reliability of our POCT analysis device in ctDNA detection. Thus, this work not only provides a paper-based POCT device and intelligent analysis tool for portable cancers diagnosis, but it also paves a new application path for MnB2 nanozyme in the sensing filed.

Spontaneous aggregation-enhanced electrochemiluminescence via galvanic strategy

Researchers unremittingly strive to develop innovative luminophores to enhance intrinsic electrochemiluminescence (ECL) performance. However, the potential to harness facile strategies, such as manipulating the physical properties of luminophores while retaining functional chemical properties to fabricate cost-effective ECL complexes, remains underexplored. Herein, we reported a novel and efficient one-step galvanic technique to actualize aggregation-enhanced ECL (AEECL) of ruthenium complexes. It marked the first instance of the galvanic process being employed to synthesize aggregate luminophores through electrostatic attraction. The ECL intensity and efficiency of the prepared ruthenium complexes with AEECL properties surpassed traditional ruthenium complexes by 8.9 and 13.6 times, respectively, outperforming most reported luminophores. Remarkably, the target luminophore exhibited high stability across varied scan rates and temperatures. Furthermore, a binder-free and carbon paper-based AEECL analytical device for lidocaine detection was fabricated, achieving a satisfactory detection limit (0.34 nM) and selectivity. The convenient modulation strategy of aggregate structure, along with the transformative leap from insufficient ECL to AEECL, bring forth a new revenue in aggregate science. This research also promises a universally applicable and versatile protocol for future biological analysis and bioimaging applications.

Aptasensor platform based on electrochemical Paper-Based analytical device for histamine detection

This paper outlines a simple, affordable, and highly flexible rapid prototyping technique for creating electrochemical paper-based analytical devices (ePADs). The craft cutter printer used for ePAD fabrication offers high flexibility and simplicity, eliminating the need for specialized equipment such as a wax printer by generating both electrode patterns and hydrophilic barriers. The ePAD was utilized to construct an aptasensor for the determination of histamine, which is essential for several physiological functions, including gastric secretion, narcolepsy, cell differentiation, cell growth, neurotransmission, and neuromodulation. Following the preparation of an ePAD with a unique design specific to this work, surface characterization was conducted using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). A single-use aptasensor was created by immobilizing histamine-specific aptamer onto the working electrode surface of the ePAD. By optimizing all experimental parameters, histamine detection was successfully performed using the differential pulse voltammetry (DPV) technique. The reproducibility and selectivity of the aptasensor were examined, and its potential applicability to real samples was tested in an artificial saliva. These findings suggest that the ePAD-based aptasensor provides a cost-effective and reliable method for histamine detection, with potential applications in various bioanalytical contexts.

Origami-inspired microfluidic paper-based analytical device (μPAD) for microorganism detection

AbstractPathogenic microorganisms impose great risk especially in resource-limited settings due to inaccessibility of diagnostic tools and monitoring devices. This is mainly caused by current methods often being economically demanding and complex in practice; while these methods are sensitive and accurate, they rarely follow Point-of-care (POC) approaches, which is essential for rapid detection and intervention. Incorporating origami into paper-based analytical devices (μPAD) presents an innovative alternative, offering affordability, portability, and ease of disposal. Herein, a colorimetric origami μPAD that is suitable for use in POC applications was developed. The μPAD was fabricated via laser ablation utilizing PVDF and cellulose membranes. In order to develop the biosensor platform, fabrication parameters were optimized and hydrophilicity of PVDF membranes was improved using various solvents. The PVDF membranes were characterized through light microscopy imaging, protein adsorption assay and contact angle measurements. Then, optimization of the assay parameters was carried out in order to improve sensitivity and resolution of the μPAD, utilizing Box-Behnken experimental design. The responses generated by the origami μPAD in form of visible color development were then analyzed using image processing. After optimization is concluded, E. coli detection was carried out as a model system. Resulting calculations showed a limit of detection (LoD) of 2 CFU/mL and a dynamic working range up to 106 CFU/mL for E. coli. Overall, developed origami μPAD promises an economic advantage compared to conventional methods, and provides rapid and sensitive results without the requirement of expertise or complex equipment.

A novel indicator for lead poisoning beyond blood lead level: Facile diagnosis of lead poisoning using random urine with point-of-care testing

Lead (Pb) poisoning is estimated to account for 1% of the global disease burden. The gold standard for diagnosing lead poisoning in human body relies on blood lead level (BLL), which is always performed in hospitals using expensive instruments. However, there are still many countries and regions with a lack of medical resources (without enough professional medical staff and analytical instruments). To achieve a facile diagnosis of lead poisoning by ordinary residents (without any expertise), this study conducted a research study on 810 participants to discover and validate a new lead poisoning indicator (creatinine-corrected urinary lead level, cULL) beyond BLL in non-invasive samples. A point-of-care testing (POCT) device to measure cULL was developed, equipped with liquid-phase microextraction and electromembrane extraction on a paper-based analytical device for on-site separation of lead and creatinine in the urine, using a smartphone for the quantification of analytes. The cULL as a novel indicator and the POCT device developed could be effective in reducing the risk of damage from lead contamination. Environmental implicationLead (Pb) is a priority hazardous pollutant and a group B2 human carcinogen owing to its high toxicity toward organ systems. Lead poisoning is estimated to account for approximately 1% of the global disease burden. This study finds a new non-invasive indicator for diagnosing lead poisoning. A dual-channel paper-based analytical device was developed to achieve a facial measurement of the new indicator. This new lead poisoning diagnostic method addresses the limitations of traditional diagnostic methods. In summary, the new strategy proposed in this work aims to reduce the harm caused by environmental pollutants to people.

Unveiling the effect of paper matrix on the electrochemical response of diffusive redox probes

This study evaluates the influence of paper substrate selection on the electrochemical response of an electrochemical paper-based analytical device (ePAD). Various paper substrates commonly employed in this type of devices such as cellulose, nitrocellulose and glass-fibre based medical grade materials are evaluated using chronoamperometric measurements. Both theoretical modelling and experimental analyses are conducted to understand the diffusion behaviour of widely employed two redox probes, [Fe(CN)6]3− and FcMeOH. Findings show that glass fibre substrates show similar performance to liquid drop conditions while nitrocellulose cause a decrease in current after a short measurement period, mainly due to a thin-layer effect. Cellulose-based substrates decrease the diffusivity of redox species, especially for charged species, indicating potential limitations in their use for chronoamperometric measurements. The study offers valuable insights into the electrochemical behaviour of paper substrates in ePADs, laying the groundwork for future research in this area.

Hybrid photo paper-based microfluidic device for colorimetric detection of iodine in salt

Microfluidic paper-based analytical devices (µPADs) have gained widespread use in various analytical applications because they are low-cost and suitable for onsite testing. The development of µPADs, including fabrication methods, new materials, and enhancement functionality is crucial to advance their practical application in analytical chemistry. In this study, we introduce a new hybrid paper-based analytical device, the hybrid photo paper-based microfluidic device (hPPMD), which combines a photo paper-based microfluidic device (PPMD) with a µPAD. We conducted a systematic study that detailed hPPMD’s characteristics, including surface properties and fluidic transportation. The hPPMD showed two fluidic transportation behaviors: continuous flow and discontinuous flow at the device junction, depending on the orientation of the device combination. Our hPPMD could increase fluidic flow approximately four times the speed when six layers of guided channels were added. The customized hPPMD was used for colorimetric detection of iodine in table salt, and then the result was quantitatively analyzed using a computer and smartphone with color analysis software. The detection zones of the hPPMD showed a flawless circular color signal. Under optimum conditions, the hPPMD was sensitive enough to detect iodine in salt solutions at various concentrations ranging from 1 to 100 ppm. The developed hPPMD should be a simple and low-cost analytical device for onsite qualitative analysis of the iodine and other chemical contaminants in food and the environment.

Enhanced electrochemical detection of chikungunya virus through aptasensor modification with origami paper-based ternary nanocomposite

Electrochemical sensors have drawn significant attention due to their crucial significance in the early detection of infectious illnesses, signifying their critical importance in diagnostic applications. This study’s proposed novel aptasensor exhibits a distinctive origami-based ePAD design, utilizing a unique silver-gold and zinc oxide (Ag-Au-ZnO) ternary nanocomposite for enhanced charge-transfer properties. Composite nanomaterials have a lower toxicity level, a higher signal enhancement capacity, increased sensitivity, improved conductivity, superior solubility, enhanced resolution capabilities, and a more straightforward functionalization process than individual nanomaterials. Employing paper, an economically advantageous substrate amenable to large-scale production with an origami-based ePAD is a critical enhancement of the sensor’s commendable attributes. Categorized as precision tools with an eco-conscious design, these sensors stand out for their streamlined manufacturing of environmentally sensitive substrates. This research capitalizes on the inherent foldability of paper to construct a three-dimensional multi-angular aptasensor, demonstrating cutting-edge specificity for chikungunya virus (CHIKV) detection. X- ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), UV–vis spectroscopy, and Fourier transmission infrared spectroscopy (FTIR) characterized the nanocomposites. This study also includes aptamers as outstanding and sensitive instruments for quick diagnostic applications. With the use of Cyclic Voltammetry (CV), the analytical response of the aptasensor was carefully evaluated. The study demonstrates a broad linear detection range spanning from 1 ng/ml to 10 µg/ml, with an impressively low limit of detection (LOD) at 1 ng/ml for the CHIKV antigen (CHIKV-Ag), underscoring the novelty and heightened sensitivity of this aptasensor design. Moreover, the aptasensor successfully detects CHIKV antigens in serum samples, and a long stability test demonstrates its practical utility and long-term stability for clinical applications. The proposed study concludes by introducing a novel origami-based paper-based analytical device (ePAD) as a promising tool for the quick, precise, and selective identification of the CHIKV antigen as well as its affordability, easy fabrication, and suitability of POCT testing makes an excellent diagnosing technique in resource-limited settings.

Distance-based quantitative determination of pyrophosphate ions with a Haus-made microfluidic paper-based device using the metal–organic framework NH2-MIL-88 (Fe)

This study initially reports a distance-based quantitative assay of pyrophosphate ion (PPi) based on analyte-triggered decomposition of metal–organic framework (MOF) in microfluidic paper-based analytical devices (µPADs). The µPADs were created using a mini-cutter machine and a home-use hot-pressing machine for loading one type of MOF of NH2-MIL-88 (Fe) prepared with Fe3+ and fluorescent ligand of 2-aminoterephthalic acid (NH2-BDC). Upon sample introduction, the PPi could compete with NH2-BDC to bind Fe3+, leading to disintegration of the MOF and releasing of the fluorescent ligand in paper channel. The length (distance) of the NH2-BDC’s flow trace, when irradiated by a money tester pen, is proportional to the PPi level. The results show that the developed method only requires a ruler to linearly quantify PPi in a concentration range of 0–1000 μM, with a limit estimated to be ∼0.25 μM. Furthermore, recovery of PPi in orange juice and human urine was between 95.72 and 108.11 %, further validating its high detection accuracy. The fabrication method for µPADs offers many attractive advantages kike simplicity, batch production capability and low cost. Moreover, to our knowledge, this may be the first report of a portable quantitative MOF-based PPi assay with distance-based readout in µPADs.

Advances in Microfluidic Paper-Based Analytical Devices (µPADs): Design, Fabrication, and Applications.

Microfluidic Paper-based Analytical Devices (µPADs) have emerged as a new class of microfluidic systems, offering numerous advantages over traditional microfluidic chips. These advantages include simplicity, cost-effectiveness, stability, storability, disposability, and portability. As a result, various designs for different types of assays are developed and investigated. In recent years, µPADs are combined with conventional detection methods to enable rapid on-site detection, providing results comparable to expensive and sophisticated large-scale testing methods that require more time and skilled personnel. The application of µPAD techniques is extensive in environmental quality control/analysis, clinical diagnosis, and food safety testing, paving the way for on-site real-time diagnosis as a promising future development. This review focuses on the recent research advancements in the design, fabrication, material selection, and detection methods of µPADs. It provides a comprehensive understanding of their principles of operation, applications, and future development prospects.

Single-step batch fabrication of microfluidic paper-based analytical devices with a 3D printer and their applications in nanoenzyme-enhanced visual detection of dopamine.

Wax printing is the most widely used method for fabricating microfluidic paper-based analytical devices (μPADs), but it still suffers from disadvantages like discontinuation of wax printers and need for additional equipment for heating treatment. To address these issues, this work initially describes a new class of wax printing approach for high-precision, batch fabrication of μPADs using a household 3D printer. It only involves a one patterning step of printing polyethylene wax into rice paper body. Under optimized parameters, a fabrication resolution, namely the minimum hydrophilic channel width, down to ~189 ± 30 μm could be achieved. In addition, the analytical applicability of such polyethylene wax-patterned μPADs was demonstrated well with enhanced colorimetric detection of dopamine as a model analyte by combining metal-organic framework (MOF) based nanoenzymes (ZIF-67) with a smartphone (for portable quantitative readout). The developed nanosensor could linearly detect dopamine over a concentration range from 10 to 1000 μM, with a detection limit of ca. 2.75 μM (3σ). The recovery results for analyzing several real samples (i.e., pig feed, chicken feed, pork and human serum) were between 91.82 and 102.79%, further validating its good detection accuracy for potential practical applications in food safety and medical diagnosis.

An exonuclease I-assisted quencher-free 2-aminopurine aptasensor based on a multipath paper-based device for ultrasensitive detection of kanamycin

A quencher-free multipath microfluidic paper-based analytical device (µPAD) was constructed for ultrasensitive detection of kanamycin based on exonuclease I (Exo I)-assisted signal enhancement of 2-aminopurine (2-AP). Here, Exo I, a single-stranded DNA-specific nuclease, was introduced to fully liberate 2-AP mononucleotides to greatly enhance biosensing sensitivity. 2-AP, a fluorescent adenine analogue embedded in single-stranded DNA (ssDNA), was employed as the detection signal source. The fluorescence of 2-AP is strong in the mononucleotide state, while it can have low fluorescence and even no fluorescence in ssDNA and dsDNA, respectively. The 2-AP fluorescence probe included 2-AP DNA and kanamycin aptamer. When kanamycin was present, binding occurred between kanamycin and the aptamer, leading to ssDNA, which was further digested by Exo I. In this case, free 2-AP mononucleotides were liberated, indicating strong fluorescence. In addition, the captured kanamycin was released for binding with the new aptamer, which resulted in the formation of a binding-hydrolysis-release cycle with the aid of Exo I. Under optimal conditions, this µPAD exhibited sensitive and multipath detection of kanamycin at concentrations as low as 1.26 × 10−14 M with a wide range of 10−13–10−7 M. Furthermore, satisfactory results were achieved for analysing spiked kanamycin in milk and honey samples. This strategy is a very promising tool for monitoring antibiotics and evaluating the safety of animal-derived foods.

Molecularly Imprinted Polymers Coupled with Cellulosic Paper-Based Analytical Devices for Biosensing Applications

Molecularly Imprinted Polymers Coupled with Cellulosic Paper-Based Analytical Devices for Biosensing Applications

A Microfluidic Paper-Based Lateral Flow Device for Quantitative ELISA

This study presents an innovative lateral flow microfluidic paper-based analytical device (μPAD) designed for conducting quantitative paper-based enzyme-linked immunosorbent assays (p-ELISA), seamlessly executing conventional ELISA steps in a paper-based format. The p-ELISA device utilizes a passive fluidic circuit with functional elements such as a multi-bi-material cantilever (B-MaC) assembly, delay channels, and a buffer zone, all enclosed within housing for autonomous, sequential loading of critical reagents onto the detection zone. This novel approach not only demonstrates a rapid assay completion time of under 30 min, but also boasts reduced reagent requirements, minimal equipment needs, and broad applicability across clinical diagnostics and environmental surveillance. Through detailed descriptions of the design, materials, and fabrication methods for the multi-directional flow assay (MDFA), this manuscript highlights the device’s potential for complex biochemical analyses in a user-friendly and versatile format. Analytical performance evaluation, including a limit of detection (LOD) of 8.4 pM for Rabbit IgG, benchmarks the device’s efficacy compared to existing p-ELISA methodologies. This pioneering work lays the groundwork for future advancements in autonomous diagnostics, aiming to enhance global health outcomes through accessible and reliable testing solutions.

Microscale titration of acetic acid using digital colorimetry and paper-based analytical devices

A quantitative method for acid-base titrations in paper-based devices (PADs) is described to analyze acetic acid in vinegar samples. In this work, two different types of PADs were developed: a device for individual spot testing and a microfluidic device. Digital colorimetry was used as the detection method, and the images were acquired using a smartphone and a homemade box with LED lights for controlled image acquisition. Titration curves were built with just eight points, using the R channel based on the gradual color transition from red to blue of litmus, a natural indicator. The endpoint was accurately determined by second derivative calculations. Both systems were applied to fifteen vinegar samples of different types, and good concentration results were obtained in comparison to the reference method. The proposed methodology is simple, fast, environmentally friendly, and surpasses the need for calibration curve construction. Moreover, the subjective endpoint identification is eliminated, and the method was automated to provide a high throughput workflow, suitable for quality control processes and real-time measurements.

Preconcentrating extractive polymeric network in paper-based sensing of copper

The concept of using an extractive polymeric sensing network on a paper-based substrate for the pretreatment of samples in the colorimetric analysis of heavy metals is demonstrated with the determination of Cu2+ in aqueous samples. The newly developed paper-based device combines the advantages of both liquid–liquid extraction-based separation and the analytical capabilities of microfluidic paper-based analytical devices (μPADs). The preconcentrating extractive polymeric network composed of 53.5 wt% poly(vinyl chloride) (PVC), 45 wt% extractant (di-(2-ethylhexyl)phosphoric acid (D2EHPA)) and 1.5 wt% dye (1-(2′-pyridylazo)-2-naphthol (PAN)), coated on filter paper, shows good water-wicking properties, allowing the transmission of high sample volumes. This results in significant analyte preconcentration. The extractant (D2EHPA) selectively binds Fe3+ and Zn2+ which otherwise will compete with Cu2+ in its colour reaction with PAN at the chosen pH of 3.0. The device’s capacity to process higher sample volumes resulting in higher sensitivity is increased by the integration of several filter paper absorption layers. The optimized device is characterized by a limit of detection (LOD) for tap and environmental waters of 3.4 and 9.6 µg/L, respectively; stability of 51 days if stored in a refrigerator; and total analysis time (from sample insertion to colour detection) of less than 5 min.