Design and optimization of a cost-effective paper-based voltammetric sensor for the determination of trinitrotoluene (TNT) utilizing cysteamine-linked Fe3O4 @Au nanocomposite

This paper presents the development and optimization of a cost-effective paper electrochemical sensor for the detection of TNT using Fe3O4-Au core-shell nanoparticles modified with cysteamine (Fe3O4@Au/CA). The sensor was constructed by modifying a graphite paste with the aforementioned nanoparticles, which facilitated the formation of a Meisenheimer complex between cysteamine and TNT as an electron donor and an electron acceptor, respectively. The central composite design was employed to optimize four key parameters pH, modifier percentage, contact time, and buffer type to enhance the performance of the sensor. The detection limit was found to be 0.5 nM of TNT, while the linear range of the electrode response spanned from 0.002 μM to 10 μM. The simplicity and low cost of the sensor make it highly attractive for practical applications, particularly in scenarios where rapid and on-site TNT detection is required.

Detection of H2O2 and catalase on a paper-based flow sensor constructed with borate cross-linked PVA hydrogel

The detections of H2O2 and catalase play an important role in daily life. This study introduces a paper-based flow sensor that is specifically designed to detect H2O2 and catalase. The sensor utilizes a hydrogel composed of cross-linked 4-carboxyphenylboronic acid and polyvinyl alcohol. When H2O2 is in contact with the hydrogel, the B-C bonds of the hydrogel undergo a reactive process, causing decomposition of the hydrogel. The pH indicator strip enables the visual monitoring of the viscosity change that occurs during the gel-sol transition. The quantification of H2O2 is accomplished by assessing the proportion of water coverage on the pH indicator strip. The sensor shows a detection limit of 0.077 wt% and is applicable for the quantitative measurement of H2O2 in routinely used disinfectants. Furthermore, the presence of catalase is effectively identified and the detection of catalase in milk is successfully fulfilled. In summary, this work proposes a simple, user-friendly, label-free, and cost-effective method for constructing a paper-based flow sensor using borate cross-linked polyvinyl alcohol hydrogel, showing great potential for detecting H2O2 and catalase in various practical scenarios.

Hydrazone-linked covalent organic framework functionalized with cysteine as a fluorescence sensor and Exploration of paper chip for p-nitrophenol detection

The large use and emission of p-nitrophenol (p-NP) seriously pollute the environment and endanger human health. In this work, a hydrazone-linked fluorescent covalent organic framework (BATHz-COF) was simply synthesized at room temperature and covalently linked N-acetyl-L-cysteine (NALC) via the "thiol-ene" click reaction, where carboxyl groups were introduced to improve dispersion and fluorescence intensity. As a rapid, good selectivity and reusability fluorescence sensor, the obtained COF-NALC has been used for quantitative analysis of p-NP predicated on the internal filtering effect (IFE). Under optimal conditions, COF-NALC enabled quantitative detection of p-NP with a linear range of 5-50 μM and the detection limit was 1.46 μM. The application of COF-NALC to the detection of p-NP in river water samples was successful, and the satisfactory recoveries were 98.0%-109.3%. Furthermore, the fluorescent COF paper chips constructed by in situ growth were combined with a smartphone to build a visual platform for the quick and real-time detection of p-NP, providing an excellent illustration for the development of intelligent fluorescence sensing in environmental analysis.

Facile and selective recognition of sulfonylurea pesticides based on the multienzyme-like activities enhancement of nanozymes combining sensor array

Traditional identification methods based on cholinesterase inhibition are limited to recognizing organic phosphorus and carbamate esters, and their response to sulfonylurea pesticides is weak. Residual sulfonylurea pesticides can pose a threat to human health. So, it is very important to develop an effective, rapid and portable method for sulfonylurea pesticides detection. Herein, we first found that sulfonylurea pesticides have activity-enhancing effects on copper-based nanozymes, and then combined them with the array technology to construct a six-channel sensing array method for selectively identifying sulfonylurea pesticides and detecting total concentration of sulfonylurea pesticides (the limit of detection was 0.03 µg/mL). This method has good selectivity towards sulfonylurea pesticides. In addition, a smartphone-based colorimetric paper sensor analysis method was developed to achieve the on-site detection of the total concentration of sulfonylurea pesticides. And this array can also be used for individual differentiation (1-100 µg/mL). Our work not only investigates the specific responses of copper-based nanozymes to sulfonylurea pesticides, but also develops a simple method that contributes to directly detect sulfonylurea pesticides at the source of pollution, providing insights for further research on sulfonylurea pesticides detection and filling the gap in pesticide residue studies.

Non-invasive paper-based sensors containing rare-earth-doped nanoparticles for the detection of D-glucose

Today, diabetes mellitus is one of the most common diseases that affects the population on a worldwide scale. Patients suffering from this disease are required to control their blood-glucose levels several times a day through invasive methods such as piercing their fingers. Our NaGdF4: 5% Er3+, 3% Nd3+ nanoparticles demonstrate a remarkable ability to detect D-glucose levels by analysing alterations in their red-to-green ratio, since this sensitivity arises from the interaction between the nanoparticles and the OH groups present in the D-glucose molecules, resulting in discernible changes in the emission of the green and red bands. These luminescent sensors were implemented and tested on paper substrates, offering a portable, low-cost and enzyme-free solution for D-glucose detection in aqueous solutions with a limit of detection of 22 mg/dL. With this, our study contributes to the development of non-invasive D-glucose sensors, holding promising implications for managing diabetes and improving overall patient well-being with possible future applications in D-glucose sensing through tear fluid.

G-quadruplex DNA-based colorimetric biosensor for the ultrasensitive visual detection of strontium ions using MnO2 nanorods as oxidase mimetics

Strontium-90 (90Sr) is a major radioactive component that has attracted great attention, but its detection remains challenging since there are no specific energy rays indicative of its presence. Herein, a biosensor that is capable of rapidly detecting Sr2+ ions is demonstrated. Simple colorimetric method for sensitive detection of Sr2+ with the help of single-stranded DNA was developed by preparing MnO2 nanorods as oxidase mimic catalysis 3,3',5,5'-tetramethylbenzidine (TMB). Under weakly acidic conditions, MnO2 exhibited a strong oxidase-mimicking activity to oxidize colorless TMB into blue oxidation products (oxTMB) with discernible absorbance signals. Nevertheless, the introduction of a guanine-rich DNA aptamer inhibited MnO2-mediated TMB oxidation and reduced oxTMB formation, resulting in blue fading and diminished absorbance. Upon the addition of strontium ions to the system, the aptamers formed a stable G-quadruplex structure with strontium ions, thereby restoring the oxidase-mimicking activity of MnO2. Under the best experimental conditions, the absorbance exhibits a linear relationship with the Sr2+ concentration within the range 0.01-200 μM, with a limit of detection of 0.0028 µM. When the concentration of Sr2+ from 10-8 to 10-6 mol L-1, a distinct color change gradient could be observed in paper-based sensor. We successfully applied this approach to determine Sr2+ in natural water samples, obtaining recoveries ranging from 97.6 to 103% with a relative standard deviation of less than 5%. By providing technical solutions for detection, our work contributed to the effective monitoring of transportation of radioactive Sr in the environment.

Specific separation and sensitive detection of foodborne pathogens by phage-derived bacterial-binding protein-nano magnetic beads coupled with smartphone-assisted paper sensor

Foodborne pathogen infection poses a significant threat to public health and is considered as one of the most serious hazards in global food safety. Herein, a sensitive and efficient method for on-site monitoring of foodborne pathogens was developed by using a smartphone-assisted paper-sensor combined with phage-derived bacterial-binding proteins-nano magnetic beads (PBPs-MBs). PBPs including tail fiber protein (TFP:gp13), cell-wall binding domain (CBD) of endolysin and tailspike protein (TSP) coated on the surface of MBs were applied for rapid separation and enrichment of targeted bacteria (Escherichia coli O157:H7, Staphylococcus aureus and Salmonella typhimurium, respectively) from food samples in 20 min before detection on paper-based sensors. The paper-based sensor was loaded with the lytic agent (polymyxin B) to induce bacterial lysis and release specific endogenous enzymes. Subsequently, three distinct chromogenic substrates were hydrolyzed by their corresponding enzymes, resulting in characteristic color changes on the paper, respectively. In addition, a smartphone APP for red-green-blue (RGB) color analysis of paper was able to directly detect three foodborne pathogens. As a result, the limit of detection (LOD) values for three foodborne pathogens were found to be 2.44 × 102, 2.68 × 104 and 4.62 × 103 CFU/mL, respectively, which were much lower than other studies (106-108 CFU/mL) based on enzymes. Moreover, the feasibility of this approach was further assessed through the successful detection of targeted bacteria in real samples with satisfactory recovery rates. In conclusion, this smartphone-assisted biosensor offers promising application potential for point-of-care testing (POCT) of foodborne pathogens in resource-scarce areas.

Self-calibrated HAp:Tb-EDTA paper-based probe with dual emission ratio fluorescence for binary visual and fluorescent detection of anthrax biomarker

Development of the portable device is significant for sensitive and rapid detection of an anthrax biomarker dipicolinic acid (DPA), existing in the B. anthracis. In this work, a novel HAp:Tb-EDTA paper-based ratiometric fluorescent sensor was obtained by a simple one-pot method for rapid and sensitive DPA detection. With the increased DPA concentration, the luminescence intensity of HAp (hydroxyapatite) remained constant, and thus applied as the stable reference signal, while the luminescence signal of Tb3+-EDTA was significantly enhanced due to the antenna effect. Therefore, the HAp:Tb-EDTA paper-based sensor was endowed with self-calibrated and ratiometric fluorescent detection performance for DPA. The proposed sensor showed excellent detection performance with a detection limit as low as 10.8 nM in the linear range of 0.5-30 μM. After combination with a smartphone, rapid visual and fluorescent detection of DPA was achieved. The proposed sensor was successfully applied to detect DPA from B. subtilis spore real samples, showing the application prospects of the paper-based sensors and opening a new horizon to develop novel paper-based point-of-care testing (POCT) devices.

Designing of efficient two-armed colorimetric and fluorescent indole appended organosilicon sensors for the detection of Al(III) ions: Implication as paper-based sensor

Metal ions have significant roles in diagnosis, industry, human health, and the environment. To design and develop new lucid molecular receptors for the selective detection of metal ions is important for environmental and medical applications. In the present work, two-armed indole appended Schiff bases conjoined with 1,2,3-Triazole bis-organosilane and bis-organosilatrane skelton sensors for naked eye colorimetric and fluorescent detection sensors for Al(III) are developed. The introduction of Al(III) in sensor 4 and 5 show red shift in UV-visible spectra, changes in fluorescence spectra and immediate color change from colorless to dark yellow. Furthermore, the pH and time response studies were explored for both sensors 4 & 5. The sensors 4 and 5 exhibited significantly low detection limit (LOD) in nano-molar range 1.41 × 10-9 M and 0.17 × 10-9 M respectively from emission titration. The LOD form absorption titration was found to be 0.6 × 10-7 M for sensor 4 and 0.22 × 10-7 M for sensor 5. In addition, the sensing model is developed as paper based sensor for its practical applicability. The theoretical calculations were performed on Gaussian 03 program by relaxing the structures using Density functional theory.

A ratiometric fluorescent paper sensor based on dye-embedded MOF for high-sensitive detection of arginine

Ratiometric fluorescent sensors can suppress the interference of factors unrelated to analysis due to their built-in self-calibration characteristics, which exhibit higher sensitivity and more obvious visual detection in the process of qualitative and quantitative analysis. Herein, we constructed a ratiometric fluorescence probe based on fluorescent/colorimetric dual-mode method for the determination of arginine by encapsulating rhodamine B in-situ into UiO-66-NH2 MOFs (UiO-66-NH2@RhB). The as-prepared probe showed dual-emission characteristics under a single excitation wavelength. The fluorescence intensity of UiO-66-NH2 was increased significantly by arginine, while the emission peak intensity of rhodamine B remained stable, resulting in a single-signal response with fixed reference. Furthermore, the practicality of the presented sensor was successfully validated by quantitative detection of arginine in human serum. More significantly, paper-based sensors for arginine detection were devised by using carboxymethyl cellulose modified filter papers. Under the irradiation of ultraviolet light, the paper-based sensors would produce obvious color variation from lightpink to bluish violet. This work provided a convenient and efficient method for on-site detection of arginine.

Paper-based chiral biosensors using enzyme encapsulation in hydrogel network for point-of-care detection of lactate enantiomers

Chiral analysis is of pivotal importance in a variety of fields due to the different biological activities and functions of enantiomers. Here, we develop a simple paper-based chiral biosensor that can perform sample-to-answer simultaneous analysis of lactate enantiomers in human serum samples. By modification of alginate hydrogel with "egg-box" three-dimensional network structure on a glass microfiber paper, reagents of enantiomer-selective enzymatic reactions are efficiently encapsulated forming the sensing regions for chiral analysis. Dual enzyme catalytic system (lactate dehydrogenase and glutamic pyruvic transaminase) is utilized to enhance the response of the biosensor. A smartphone with color analysis software is used to collect and analyze the fluorescence signal from the product nicotinamide adenine dinucleotide. The results show that the sensor has excellent selectivity toward lactate enantiomers with low limit-of-detection of (30.0 ± 0.7) μM for L-lactate and (3.0 ± 0.2) μM for D-lactate, and wide linear detection range of 0.1-3.0mM and 0.01-0.5 mM for L-lactate and D-lactate respectively. The proposed method is successfully applied to the simultaneous detection of L-/D-lactate concentrations in human serum with satisfactory accuracy. Our study provides a robust approach for developing chiral biosensors, which would have promising application prospect in point-of-care testing (POCT) analysis of various biological and food samples.

A Smartphone-Based Sensing for Portable and Sensitive Visual Detection of Hg (II) via Nitrogen Doped Carbon Quantum Dots Modified Paper Strip

The development of portable and cost-effective sensing system for Hg2+ quantitation is highly demanded for environmental monitoring. Herein, an on-site, rapid and portable smartphone readout device based Hg2+ sensing system integrating nitrogen-doped carbon quantum dots (NCDs) modified paper strip was proposed, and the physicochemical properties of NCDs were characterized by high resolution TEM, FTIR, UV-vis absorption spectrum and fluorescence spectral analysis. The modified paper strip was prepared via "ink-jet" printing technology and exhibits sensitive fluorescence response to Hg2+ with fluorescence color of bright blue (at the excitation/emission wavelength of 365/440 nm). This portable smartphone-based sensing platform is highly selective and sensitive to Hg2+ with the limit of detection (LOD) of 10.6 nM and the concentration range of 0-130 nM. In addition, the recoveries of tap water and local lake water were in the range of 89.4% to 109%. The cost-effective sensing system based on smartphone shows a great potential for trace amounts of Hg2+ monitoring in environmental water samples.

Sensitive and highly rapid electrochemical measurement of airborne coronaviruses through condensation-based direct impaction onto carbon nanotube-coated porous paper working electrodes

Rapid detection of indoor airborne viruses is critical to prevent the spread of respiratory diseases. Herein, we present sensitive, highly rapid electrochemical measurement of airborne coronaviruses through condensation-based direct impaction onto antibody-immobilized, carbon nanotube-coated porous paper working electrodes (PWEs). Carboxylated carbon nanotubes are drop-cast on paper fibers to make three-dimensional (3D) porous PWEs. These PWEs have higher active surface area-to-volume ratios and electron transfer characteristics than conventional screen-printed electrodes. The limit of detection and detection time of the PWEs for liquid-borne coronaviruses OC43 are 65.7 plaque-forming units (PFU)/mL and 2 min, respectively. The PWEs showed sensitive and rapid detection of whole coronaviruses, which can be ascribed to the 3D porous electrode structure of the PWEs. Moreover, water molecules condense on airborne virus particles during air sampling, and these water-encapsulated virus particles (<4 µm) are impacted on the PWE for direct measurement without virus lysis and elution. The whole detection takes ∼10 min, including air sampling, at virus concentrations of 1.8 and 11.5 PFU/L of air, which can be due to the highly enriching and minimally damaging virus capture on a soft and porous PWE, demonstrating the potential for the rapid and low-cost airborne virus monitoring system.

Fluorescence Naphthalene Cationic Schiff Base Reusable Paper as a Sensitive and Selective for Heavy Metals Cations Sensor: RSM, Optimization, and DFT Modelling

Heavy metals are particularly damaging contaminants in the environment, and even trace concentrations represent a risk to human health due to their toxicity. To detect the heavy metals of Mn2+ and Co2+ ions, a novel selective reusable paper-based Fluorescence naked-eye sensor based on naphthalene cationic Schiff base (NCSB) was synthesized and confirmed using FT-IR, 1 H-NMR, and MS tools. Based on a blue to colorless color change in the aqueous solution, the NCSB sensor is utilized to Mn2+ and Co2+ cations selectively among other metal ions (Fe2+, Cu2+, Mg2+, Ni2+, Zn2+, Cd2+, Hg2+, Pb2+, Sn2+ and Cr3+). In the aqueous medium, the NCSB sensor displayed high sensitivity, with limits of detection (LOD) values of 0.014 µM (14.08 nM) and 0.041 µM (41.47 nM) for Mn2+ and Co2+ cations, respectively. The paper-based sensor naked-eye detected Mn2+ and Co2+ cations in water at concentrations as low as 0.65 µM (65 nM) and 0.086 µM (86 nM), respectively. It was discovered that 5 min of incubation time and a pH range of 7 to 11 were optimal for the complexation reaction between the Mn2+ and Co2+ ions and the NCSB sensor. Through a static quenching process, the interaction of the different metal ions with the Schiff base group in the NCSB molecule results in the development of a ground-state non-fluorescent complex. NCSB sensor was also successfully applied in analysis of Mn2+ and Co2+ in environmental water with good recoveries of 94.8-105.9%. The theoretical calculations based on density functional theory (DFT) studies are in support of experimental interpretations. The links between the input factors and the anticipated response were evaluated using the quadratic model of the response surface methodology (RSM) modeling.

Cyanostilbene-based fluorescent paper array for monitoring fish and meat freshness via amino content detection

The detection of biogenic amines released from degraded meats is an effective method for evaluating meat freshness. However, existing traditional methods like titration are deemed tedious, while the use of sophisticated analytical instruments is not amenable to field testing. Herein, a cyanostilbene-based fluorescent array was rapidly fabricated using macroarray synthesis on a cellulose paper surface to detect amines liberated from spoiled beef, fish, and chicken. The fluorescence changes of immobilized molecules from the interaction with gaseous amines were used to monitor changes in freshness levels. Thanks to the high-throughput nature of macroarray synthesis, a set of highly responsive molecules such as pyridinium and dicyanovinyl moieties were quickly revealed. Importantly, this method offers flexibility in sensing applications including (1) sensing by individual sensor molecules, where the fluorescence response correlated well with established titration methods, and (2) collective sensing whereby chemometric analysis was used to provide a cutoff of freshness with 73-100% accuracy depending on meat types. Overall, this study paves the way for a robust and cost-effective tool for monitoring meat freshness.

A simple paper-based nickel nanocluster-europium mixed ratio fluorescent probe for rapid visual sensing of tetracyclines

In this work, a ratiometric fluorometric sensor based on nickel nanoclusters (NiNCs)-europium complex (NiNCs-Eu³⁺) was constructed for the highly selectivity detection of tetracyclines (TCs) in water samples. In the presence of TCs, the blue fluorescence of the sensor NiNCs-Eu³⁺ was quenched at 430 nm and the characteristic red fluorescence of Eu³⁺-TCs appeared at 620 nm because of the combined help of inner filter effect (IFE) and antenna effect. Under the optimized conditions (100 mM Eu³⁺ (100 µL); temperature (25℃); reaction time (10 min), HEPES buffer solution (pH = 7.0)), the sensor offered a wide detection range of tetracycline (TC) and oxytetracycline (OTC) from 0.1 to 50 μM with the detection limit (LOD) of 25 nM and 21 nM, respectively. Moreover, the sensor was able to detect of TC and OTC in tap and lake water with high recovery rate (89.10%-97.60%). In addition, the portable paper-based sensor was constructed using filter paper embedded with NiNCs-Eu³⁺. The distinct fluorescent color of the paper-based sensor varied from bright blue to red against different concentrations of TC and OTC. These above findings demonstrated the potential for wide application of as-prepared ratio metric fluorescence sensor for visual detection of TCs in water samples.

Integration of on-chip lysis and paper-based sensor for rapid detection of viral and exosomal RNAs

In recent years, paper-based nucleic acid sensors have been demonstrated for the ability to detect DNA and RNA molecules extracted from viruses and bacteria. In clinical samples, these nucleic acids are mostly encapsulated in lipid membranes and need to be released before being analyzed using paper-based sensors. For the nucleic acid amplification tests (NAATs), it is also desirable to remove the interfering molecules that can inhibit the nucleic acid amplification. To achieve a field deployable NAAT, we report a portable sensor system that combines the thermolysis and paper-based NAATs to detect target RNA molecules carried by viral and exosomal nanoparticles. The sensor cartridge includes a lysis chamber with a pressure-controlled diaphragm valve, paper flow channels, and three paper-based NAAT reaction chambers to extract, transport, and detect nucleic acids respectively. A compact instrument was prototyped to automate the assay, collect fluorescence images of the nucleic acid amplification, and generate amplification curves for NAATs. The pump-free and paper-based sensor achieved quantitative analysis of influenza A virus (IAV) RNA and exosome microRNA within 1 h, with the lowest detect concentration of 10⁴ TCID₅₀/mL and 10⁶ EV/mL for IAV and exosome, respectively. Owing to the advantages of easy storage, simple operation, and low cost, such as system has great potential to be used as a point-of-care test for in-field diagnosis of viral and bacterial infections.

A convenient paper-based fluorescent aptasensor for high-throughput detection of Pb2+ in multiple real samples (water-soil-food)

Lead ion (Pb²⁺) is one of the most toxic and widely polluted heavy metal ions. Given the potential health risks and economic losses associated with Pb²⁺, the rapid detection of Pb²⁺ using fluorescent aptasensors is of significant importance in evaluating food safety. A rapid, facile and economic fluorescent aptasensor using convenient paper as the sensing substrate was designed to high-throughput detect Pb²⁺ in complex samples within about 45 min. The Pb²⁺ changed the conformation of FAM-modified Apt from a random coil to a stable G-quadruplex structure. And then Dabcyl-labeled cDNA was added to form double-stranded DNA with the Apt that did not form a G-quadruplex structure, resulting in a weak fluorescence due to the fluorescence resonance energy transfer (FRET). The fluorescent aptasensor showed a positive correlation with Pb²⁺ concentration, and a linear relationship was obtained in the range of 0.01-10 μM with LOD of 6.1 nM. In addition, this method has been successfully used for the determination of Pb²⁺ in water, soil and various foods containing complex substrates. Meanwhile, the high-throughput detection of Pb²⁺ has also reached an acceptable level. Therefore, this convenient strategy has potential application value for on-site rapid detection of Pb²⁺.

Capsulation of red emission chromophore into the CoZn ZIF as nanozymes for on-site visual cascade detection of phosphate ions, o-phenylenediamine, and benzaldehyde

Accurate on-site profiling of the pollutants is of vital significance for estimating environmental pollution. Herein, we propose a paper-based fluorescence-sensing system to precisely report the level of multiple pollutants. A high-performance fluorescence-sensor for apparatus-free and visual on-site tandem precisely reporting phosphate ions (Pi), o-phenylenediamine (OPD), and benzaldehyde (BA) levels have been fabricated successfully by introducing synthesized red emission (>600 nm) fluorescent chromophore 10-(diethylamino)-3-hydroxy-5,6-dihydrobenzo [c]xanthen-12-ium (HTD) into the environment of CoZn zeolitic imidazolate framework (CoZn ZIF) by a simple stirring method. CoZn ZIF@HTD with the bimetallic nodes not merely provided main Zn²⁺ sites for specific recognition of Pi to generate an enhanced red fluorescent optical signal, Co³⁺/Co²⁺ exhibited excellent peroxidase-like activity for the catalytic oxidation of OPD substrate in the presence of H₂O₂ resulting in color changing from red to yellow. Subsequently, the obvious yellow fading of the OPDox species took place with the addition of BA. By virtue of the sensitively visual tandem detection of Pi, OPD, and BA, the sensor can be applied to real wastewater samples. Meanwhile, this fluorescent sensor was further adopted for practical application in confocal cell imaging and security inks. Overall, this work established a fluorescent sensing system with integrated multifunctional applications for environmental and biological samples, implying the great potential for simultaneous real-time cascade detection of various important pollutants with the merit of low-cost, time-saving, and easy-to-use.

High enrichment and near real-time quantification of airborne viruses using a wet-paper-based electrochemical immunosensor under an electrostatic field

Conventional airborne virus measurement usually requires appreciable sampling and detection times. Viral aerosols should also be collected or prepared in a liquid medium whose volume typically ranges from milliliters to tens of milliliters; hence, many sampling and detection steps need to be taken with the unit horizontal or immobile. Moreover, viral aerosols need to be sufficiently enriched, which makes real-time monitoring difficult. Herein, we present a near real-time enrichment and quantification system of airborne viruses that consists of a wet-paper-based electrochemical immunosensor with a gel electrolyte and a modified electrostatic particle concentrator. A small amount of phosphate-buffered saline flowed on the electrode, which resulted in sensor electrodes that are barely wet (covered in a thin buffer film measuring several micrometers) to ensure antigen-antibody interaction and the removal of non-target particles on the electrode surface. This system ensures that airborne viruses are highly enriched on the working electrode of the immunosensor, and it is possible to measure the MS2 virus particle concentrations every 10 min for 60 min stably and selectively against non-target airborne viruses and bacteria at horizontal and tilted measurement configurations. This system thus has the potential to be used in the real-time mobile monitoring of airborne microorganisms.

Paper-based colorimetric glucose sensor using Prussian blue nanoparticles as mimic peroxidase

A novel paper-based colorimetric glucose sensor was proposed employing Prussian blue nanoparticles (PB NPs) as mimic peroxidase. The sensor was manufactured by spraying solution containing PB NPs, glucose oxidase and chromogenic agents into a paper, then coating the filter layer and spreading layer on the top. The layer-by-layer structure enabled the sensor detect glucose in whole blood, as well as elimination of the coffee-ring effect which ensure the performance. As a powerful alternative to natural peroxidase, PB NPs showed the mimic enzymatic activity well preserved in dry environment. The manufacture process of the sensor is easy to be industrialized. Under optimal conditions, the sensor exhibited a linear range from 2.5 mM to 25 mM for glucose in blood with satisfactory reproducibility (the coefficient of variant <4%), great storage stability (1 month at 45 °C) and excellent linearity compared with those commercial kits (R > 0.99). Coupled with a handhold device, the PB NPs-based test strip realized the goal of personal operation, user-friendly control, automatic readouts, and data storage, and able to link the Cloud, showing unique potential in clinical application, especially in community-level medical scenarios.

Fully inkjet-printed paper-based Pb2+ optodes for water analysis without interference from the chloramine disinfectant

We developed a paper-based colorimetric sensor for facile and cost-effective detection of Pb²⁺ in drinking and environmental water samples. The Pb²⁺ ion-selective optodes are fabricated by inkjet printing of ionophore, chromoionophore, and ion exchanger on cellulose paper. Pb²⁺ in water samples induces deprotonation of the pH chromoionophore and changes the optode color, which is acquired and analyzed by a smartphone. The paper-based optode without any plasticizer or polymer has a dynamic range and selectivity comparable to those of traditional optodes using PVC polymer and/or plasticizer. Furthermore, the response time of the plasticizer/polymer-free paper-based optode is much shorter than those of plasticized PVC-based optodes on paper and glass (5 min vs. 15 and 50 min). Moreover, the plasticizer/polymer-free optode preserves the water-wicking capability of porous cellulose paper, allowing for the design of pump-free microfluidic devices. Chloramine, a widely used disinfectant in drinking water, was found to be a strong and generic interference species for heavy metal ion detection via ion-selective optodes. A fully inkjet-printed lateral-flow paper-based device consisting of a sodium thiosulfate-based chloramine elimination zone and a plasticizer/polymer-free sensing zone was designed for Pb²⁺ detection in tap water disinfected by chloramine. The dynamic range of the Pb²⁺ sensor may be shifted from the current 10^-6 to 10^-5 M to lower concentrations by using stronger ionophores, but this work lays a foundation for the design of paper-based heavy metal ion sensors without detrimental interference from disinfectants.

Rational construction of deep-red fluorescent probe for rapid detection of HClO and its application in bioimaging and paper-based sensing

Hypochlorous acid (HClO), the core bactericidal substance of the human immune system, plays a vital role in many physiological and pathological processes in the human body. In this work, we designed and synthesized a novel deep-red fluorescent probe TCF-ClO for the determination of hypochlorous acid through theoretical analysis. The results showed that probe TCF-ClO exhibited excellent characteristics of long-wavelength emission (635 nm), fast response (< 1 min), and low detection limit (24 nM). In addition, it had been successfully used for imaging of HClO in living HeLa cells. More importantly, the TCF-ClO composited paper-based sensing material was successfully constructed. The RGB/gray value was obtained from a mobile phone and computer, which could achieve the quantitative detection of HClO, with a linear detection range of 0-50 μM and a detection limit of 1.09 μM (RGB mode)/3.38 μM (gray mode). The function of the paper-based sensor extended from qualitative to quantitative detection of HClO, and it is expected to become a portable device widely used in the environmental area.

Green-emitting carbon quantum dots as a dual-mode fluorescent and colorimetric sensor for hypochlorite

In this work, green-emitting carbon quantum dots were successfully prepared through a facile one-step solid-state reaction method. The obtained green-emitting carbon dots (G-CDs) showed good fluorescence stability in NaCl aqueous solution and different pH values. Moreover, the G-CDs showed high sensitivity and selectivity for detecting hypochlorite by both fluorometry and colorimetry. Under the optimized condition, a highly sensitive detection of hypochlorite was established in the range of 0.2-100 μM and 10-150 μM for fluorescent and colorimetric methods, respectively. The corresponding limits of detection (LOD) were 0.0781 μM and 1.82 μM, respectively. Therefore, the G-CDs were successfully applied to determinate hypochlorite in actual water samples. In addition, a paper-based sensor loading with the G-CDs was also developed for rapid visual detection of hypochlorite. The results suggested that the G-CDs could be a promising candidate to detect hypochlorite.

A highly sensitive ratiometric fluorescent sensor for copper ions and cadmium ions in scallops based on nitrogen doped graphene quantum dots cooperating with gold nanoclusters

Based on the electrostatic interaction, we constructed a ratiometric fluorescence nanomixture of graphene quantum dots-gold nanoclusters (GQDs-AuNCs) for the quantitative detection of Cu²⁺ and Cd²⁺. When Cu²⁺ or Cd²⁺ was added into the reaction system, the fluorescence of GSH-AuNCs at 565 nm can be quenched by Cu²⁺ and enhanced by Cd²⁺ while the intensity of N-GQDs at 403 nm stayed constant. Under the optimized conditions, the fluorescence intensity ratio (I₅₆₅/I₄₀₃) of the GQDs-AuNCs system was proportional to the concentration of Cu²⁺ and Cd²⁺ in the range of 8×10^-8 mol/L-6×10^-6 mol/L and 1×10^-6 mol/L-4×10^-5 mol/L, respectively, with detection limits of 4.12×10^-9 mol/L and 9.43×10^-7 mol/L, respectively. In the presence of Cu²⁺ and Cd²⁺, the paper-based vision sensor would produce visible fluorescent color changes, which can be used for rapid detection on site. The method has been successfully applied to the determination of Cu²⁺ and Cd²⁺ in scallops with satisfactory results.

Structure engineering of lanthanide functionalized metal-organic frameworks: A versatile tool for the early diagnosis of pheochromocytomas and paragangliomas

As a main extracellular metabolite of dopamine, 3-methoxytyramine (3-MT) is considered a potential biomarker of pheochromocytomas and paragangliomas. Therefore, the determination of 3-MT is of great significance in the early diagnosis of disease. However, it remains challenging for detecting 3-MT in consideration of sensitivity and accuracy. Here, a luminescent  Eu³⁺ functionalized metal-organic frameworks （Eu³⁺@Al-MOF）with ultra-high chemical stability was constructed based on postsynthetic modification. Such a rational design greatly enhances the fluorescence signal of Eu³⁺@Al-MOF and it is endowed with excellent properties as a luminescent sensor to detect 3-MT in urine system. Intriguingly, the strong red emitting derived from antenna effect was significantly interdicted upon addition of 3-MT through the interaction between 3-MT and the ligand. The proposed sensing system exhibited many appealing analytical performances, such as excellent selectivity, high sensitivity and quick response. Remarkably, the developed paper-based sensor not only provides a portable and reliable strategy for direct detection of 3-MT but also expands the application of visual analysis tools. This work represents the first effort in designing a luminescent sensor to determine the metabolite biomarker 3-MT level and provides a new method for biomedical analysis.

A disposable gold foil paper-based aptasensor for detection of enteropathogenic Escherichia coli with SERS analysis and magnetic separation technology

Rapid and sensitive detection of enteropathogenic Escherichia coli (EPEC) in fluids with complex background is an important task for safety quality control in the field of medicine, environment, and food. In this study, a gold foil paper-based aptasensor was developed for the detection of enteropathogenic EPEC O26:K60 with surface-enhanced Raman spectroscopy (SERS) and magnetic separation technology mediated by Fe₃O₄@Au composite. The gold foil paper was firstly modified with thiolated capture probe and SERS tag. The thiolated aptamer probe for EPEC was immobilized onto a Fe₃O₄@Au composite. In the presence of EPEC, highly specific recognition between the aptamer probe and EPEC made the Fe₃O₄@Au composite partially dissociated from the gold foil paper. This led to a decreased Raman intensity response, which showed an obvious negative linear correlation with increasing concentration of EPEC over a wide concentration range from 10 to 10⁷ CFU/mL under an excitation wavelength of 633 nm. The detection limit was about 2.86 CFU/mL in a buffer solution and a licorice extractum and the detection time was only 2.5 h. The results demonstrate that the gold foil paper-based aptasensor can be an excellent biosensing platform that offers a reliable, rapid, and sensitive alternative for EPEC detection.

N, Cl-doped carbon dots for fluorescence and colorimetric dual-mode detection of water in tetrahydrofuran and development of a paper-based sensor

N, Cl-doped carbon dots (N, Cl-CDs) were prepared by hydrothermal method from rhodamine B (RhB) and ethylenediamine (EDA). The resulting N, Cl-CDs exhibited fascinating solvent dependence and strict excitation independence. As the polarity of the solvent increased (from tetrahydrofuran (THF) to water), the emission spectrum of N, Cl-CDs was redshifted and the fluorescence efficiency decreased, which were attributed to hydrogen bond-induced aggregation. Taking advantage of these attributes, the N, Cl-CDs were used as suitable probes for fluorescence and colorimetric dual-mode detection of water in THF. The linear relationship was 0.5-100% water with the detection limit down to 0.093%. Moreover, the sensing platform was converted into a paper-based sensor for handy, real-time, and visible humidity sensing. N, Cl-CDs/PVA films were fabricated and realized continuously tunable solid-state fluorescence, further expanding their practical application.

Paper-based sensor from pyrrolidinyl peptide nucleic acid for the efficient detection of Bacillus cereus

Bacillus cereus is one of the most common foodborne pathogens found in various kinds of staple foods such as rice and wheat. A rapid and accurate detection method for this pathogen is highly desirable for the sustainable production of relevant food products. While several classical and molecular-based detection methods are available for the identification of B. cereus, they suffered one or more limitations such as the requirement for a tedious and time-consuming process, less than ideal specificity, and the lack of portability. Herein, we developed the first paper-based sensing device that exhibits high species specificity with sufficiently low limit of detection for the visual detection of specific DNA sequences of B. cereus. The success is attributed to the strategic planning of fabrication in various dimensions including thorough bioinformatics search for highly specific genes, the use of the pyrrolidinyl peptide nucleic acid (PNA) probe whose selectivity advantage is well documented, and an effective PNA immobilization and DNA-binding visualization method with an internal cross-checking system for validating the results. Testing in rice matrices indicates that the sensor is capable of detecting and distinguishing B. cereus from other bacterial species. Hence, this paper-based sensor has potential to be adopted as a practical means to detect B. cereus in food industries.

Development of a surface-modified paper-based colorimetric sensor using synthesized Ag NPs-alginate composite

There has been an increase in the discovery and usage of sensors for the detection of chemical compounds in the field of analytical chemistry since the last several years. This has led to progressive research in nanotechnology for developing efficient nanomaterials for bio-chemical sensing applications. Thereby, a deft synthesis of silver nanoparticles (Ag NPs) under microwave irradiation was achieved using sodium alginate as a reducing and capping agent in a fast and cost-effective approach. As per the X-ray diffraction analysis, the average particle size of Ag NPs was found to be 10 nm. X-ray photoelectron spectroscpopy analysis showed characteristic peaks at binding energies of 368.10 and 374.11 eV indicating the formation of Ag NPs. The synthesized Ag NPs-alginate composite was further used to develop a paper-based sensor for the detection of H₂O₂. Detection of H₂O₂ is based on the discolouration of the Ag NPs-alginate composite modified paper sensor as a function of H₂O₂ concentration. The analysis of the decoloured paper strips was done by a smartphone camera and an RGB Colour Reader application (app) to measure colour intensity. The sensing characteristics were found in the range of 0.1-10 mM. The colour analysis revealed piecewise linear relationship of intensity of RGB to H₂O₂ concentration in the range of 0.1-1.5 and 2-10 mM with R² values of 0.97 and 0.9778, respectively. Owing to the high sensitivity, selectivity, and cost-effectiveness, the developed paper sensor can be a potential tool for real-time analysis of H₂O₂.

Selective and easy detection of microcystin-LR in freshwater using a bioactivated sensor based on multiwalled carbon nanotubes on filter paper

Microcystin-LR (MC-LR) is a cyanobacterial toxin produced as a result of eutrophication in polluted water in warm weather conditions. The MC-LR could cause health problems in mammal organs such as the liver, heart, and muscle. Therefore, the World Health Organization (WHO) has stipulated a limit of <1.0 ng/mL in drinking water. Thus, detection and quantification are vital, but current techniques require complex and expensive offsite analysis. We have developed an inexpensive, sensitive, and field-deployable sensor based on bioactivated multiwalled carbon nanotubes (MWCNTs, diameter 20 nm) and micropore filter paper (0.45-μm pore size) for the detection of MC-LR. A specially designed DNA oligonucleotide (5-NH₂-C₆-AN₆) was used as the MC-LR targeting aptamer (MCTA). For bioactivation, MCTA was immobilized on the carboxylated MWCNTs via the formation of amide bonds. The bioactivated MWCNTs were deposited on the micropore filter paper by suction filtering. The detection of MC-LR in freshwater was possible within 1.5 h, achieved by measuring the changes in electrical resistance caused by the selective MC-LR and MCTA interactions. Despite suffering from some matrix effects, the detection limit of the sensor was 0.19 ng/mL for low-concentration MC-LR (≤0.5 ng/mL). This method is much cheaper (biosensor price: < $2.5) than liquid chromatography coupled with tandem mass spectroscopy analysis (ca. $50/sample) which is a standard method for MC-LR detection in a modern laboratory. Thus, this cheap and straightforward MC-LR sensor has applications for detection in remote locations.

Equipment-free quantitative determination of urea based on paper-based sensor via urease-mediated chitosan viscosity change

In this study, a paper-based sensor combined with visual distance-readout technique for point of-care testing (POCT) of urea was developed by urease-mediated chitosan viscosity change. A series of factors that affect the performance of the sensor were investigated, including the type of filter paper, chitosan concentration, acetic acid concentration and enzymatic reaction conditions. Under optimal conditions, the proposed method for urea determination has good linearity between 3.8-15.1 mM. The limit of quantitation is 3.8 mM. Finally, the paper-based sensor was successfully applied to the determination of urea in two diesel exhaust fluid (DEF) samples. The recoveries of urea were 91.4 % and 109.9 % in DEF-1 and DEF-2, respectively. The present study provides a novel approach, which integrates paper-based sensor and visual distance-readout technique, for monitoring urea in POCT application, especially in remote or resource-limited regions.

A colorimetric paper-based ATONP-ALP nanobiosensor for selective detection of Cd2+ ions in clams and mussels

A colorimetric paper-based enzyme-coupled antimony tin oxide nanoparticle (ATONP) nanobiosensor for selective detection of Cd²⁺ ions in clams and mussels is presented. Alkaline phosphatase (ALP) was immobilized on ATONPs via 16-phosphonohexadecanoic acid (16-PHA) to develop ATONP-ALP nanobiosensor. The biosensor was characterized using XPS, Raman spectroscopy, SEM, and EDX. ATONP-ALP nanobiosensor exhibited high selectivity towards detection of Cd²⁺ ion with a LOD 0.006 μg L^-1 and linear range of detection 0.005-1 μg L^-1. The developed biosensor was further integrated into a low-cost paper-based format. A visual color change was obtained for Cd²⁺ ion in the range 0.1-10 μg L^-1. The developed biosensor was successfully demonstrated for the analysis of Cd²⁺ ions in clams with recoveries 101-104%. The ATONP-ALP nanobiosensor was validated using mussel tissue (BCR-668) and the conventional ICP-OES and ICP-MS techniques.

An IoT-enabled paper sensor platform for real-time analysis of isothermal nucleic acid amplification tests

Paper-based sensors can be exploited to develop low-cost, disposable, and rapid assays for the detection of a large variety of analytes. We report a paper-based sensor system for a point-of-care (POC) nucleic acid amplification test that can quantitatively detect multiple genes from different pathogens. The POC system combines a paper sensor chip and a portable instrument, which is built on an Internet of Things (IoT) platform. The paper-based sensor provides the functions of reagent storage, sample transportation, and nucleic acid amplification. The IoT instrument uses an Arduino microcontroller to control temperature, collect fluorescence images, and store the data in cloud storage via a WiFi network. A compact fluorescence reader was designed to measure fluorescence images of the amplicons during a loop-mediated isothermal amplification reaction in real-time. The real-time detection capability enables the quantitative analysis of target genes. The results show that the paper-based sensor cam distinguish multiple genes of the genomic DNA extracted from Escherichia coli and Campylobacter jejuni, with the concentration as low as 2 × 10³ copies/μL. The affordable instrument, in conjunction with the disposable paper sensor chip, would have a great potential for POC detections of pathogens.

Modulating the mixed potential for developing biosensors: Direct potentiometric determination of glucose in whole, undiluted blood

The growing demand for tools to generate chemical information in decentralized settings is creating a vast range of opportunities for potentiometric sensors, since their combination of robustness, simplicity of operation and cost can hardly be rivalled by any other technique. In previous works, we have shown that the mixed potential of a Pt electrode can be controlled with analytical purposes using a coating of Nafion, thus providing a way to develop a potentiometric biosensor for glucose. Unfortunately, the linear range of this device did not match the relevant clinical range for glucose in blood. This work presents a novel strategy to control the mixed potential that allows the development of a potentiometric biosensor for the direct detection of glucose in whole, undiluted blood without any sample pretreatment. By changing the ionomer, the analytical response can be tuned, shifting the linear range while keeping the sensitivity. Aquivion, a polyelectrolyte from the same family as Nafion, is used to stabilize the mixed potential of a platinized paper-based electrode, to entrap the enzyme and to reduce the interference from negatively charged species. Factors affecting the generation of the signal and the principle of detection are discussed. Optimization of the biosensor composition was achieved with particular focus on the characterization of the linear range and sensitivity. The accurate measurement of blood sugar levels in a single drop of whole blood with excellent recovery is presented.

Paper microzone plate based on DPPH as a simple colorimetric assay of the total antioxidant content of herbal extracts

The human body needs an antioxidant-rich diet that comes from foods, beverages, and herbal products to support the physiological antioxidant systems. Thus, the development of an analytical tool for a simple assay of the total antioxidant capacity (TAC) of the rich antioxidant samples is crucial. The current work demonstrates a simple colorimetric assay of TAC of the herbal extract on the paper microzone plate (PµZP) that was constructed in the 70-well of patterned paper using the screen printing technique. The PµZP was constructed by immobilizing DPPH (2,2-diphenyl-1-picrylhydrazyl) onto 70-well of PµZP as a sensing zone for colorimetric detection. The purple-sensing zone exhibited a good response to gallic acid (GA), by producing slightly gray to pale yellow color that can be captured using a scanner and then analyzed using the ImageJ program. The paper-based sensor showed a linear response toward GA at 0.05-0.6 mM (r = 0.9895), reproducible response (RSD < 4%), and accurate measurement with 91 to 106% recovery for measuring TAC of herbal extract, presented as mM gallic acid equivalent and showed a good agreement with the standard DPPH method. The results suggested that the proposed method can be applied for simple TAC measurement in the herbal extract.

Visual genotyping of thalassemia by using pyrrolidinyl peptide nucleic acid probes immobilized on carboxymethylcellulose-modified paper and enzyme-induced pigmentation

A simple probe pair was designed for the detection of hemoglobin E (HbE) genotype, a single-point mutation that leads to abnormal red blood cells commonly found in South East Asia. The key to differentiation is the use of a conformationally constrained peptide nucleic acid (PNA) that was immobilized on carboxymethylcellulose-modified paper. This was then used for target DNA binding and visualization by an enzyme-catalyzed pigmentation. The biotinylated target DNA bound to the immobilized probe was visually detected via alkaline phosphatase-linked streptavidin. This enzyme conjugate catalyzed the dephosphorylation of the substrate 5-bromo-4-chloro-3-indolyl phosphate, leading to a series of reactions that generate an intense, dark blue pigment. The test was validated with 100 DNA samples, which shows good discrimination among different genotypes (normal, HbE, and heterozygous) with 100% accuracy when optimal conditions of analysis were applied. The method does not require temperature control and can be performed at ambient temperature. This is an attractive feature for diagnosis in primary care, which accounts for a large part of affected population. Graphical abstract Schematic representation of a paper-based sensor for the detection of the gene Hemoglobin E. The interaction between an immobilized peptide nucleic acid and a DNA target leads to enzymatic pigmentation, allowing simple visual readout with up to 100% accuracy.

Pixelated colorimetric nucleic acid assay

Conjugated polyelectrolytes (CPEs) have been widely used as reporters in colorimetric assays targeting nucleic acids. CPEs provide naked eye detection possibility by their superior optical properties however, as concentration of target analytes decrease, trace amounts of nucleic acid typically yield colorimetric responses that are not readily perceivable by naked eye. Herein, we report a pixelated analysis approach for correlating colorimetric responses of CPE with nucleic acid concentrations down to 1 nM, in plasma samples, utilizing a smart phone with an algorithm that can perform analytical testing and data processing. The detection strategy employed relies on conformational transitions between single stranded nucleic acid-cationic CPE duplexes and double stranded nucleic acid-CPE triplexes that yield distinct colorimetric responses for enabling naked eye detection of nucleic acids. Cationic poly[N,N,N-triethyl-3-((4-methylthiophen-3-yl)oxy)propan-1-aminium bromide] is utilized as the CPE reporter deposited on a polyvinylidene fluoride (PVDF) membrane for nucleic acid assay. A smart phone application is developed to capture and digitize the colorimetric response of the individual pixels of the digital images of CPE on the PVDF membrane, followed by an analysis using the algorithm. The proposed pixelated approach enables precise quantification of nucleic acid assay concentrations, thereby eliminating the margin of error involved in conventional methodologies adopted for interpretation of colorimetric responses, for instance, RGB analysis. The obtained results illustrate that a ubiquitous smart phone could be utilized for point of care colorimetric nucleic acids assays in complex matrices without requiring sophisticated software or instrumentation.

MoS2 QDs-Based sensor for measurement of fluazinam with triple signal output

In this study, direct detection of fluazinam was realized using a fluorescent sensor using disulfide quantum dots (MoS₂ QDs) via inner filter effect (IFE). The maximum excitation of as-prepared MoS₂ QDs presented a complementary spectral-overlap with the maximum absorption of fluazinam. Thus the occurrence of inner filter effect led to the significant fluorescence quenching of MoS₂ QDs. Additionally, fluorescent quenching efficiency of MoS₂ QDs could be enhanced by the effects of π-π stacking, hydrogen bond and electrostatic interaction between fluazinam and MoS₂ QDs, and these non-chemical bond responses also promoted the selectivity for fluazinam detection. Under the optimum conditions, the IFE-based fluorescent sensor exhibited a relative wide linear range from 50 nM to 25 μM with the LOD of 2.53 nM (S/N = 3). In addition, a paper-based sensor was established by cross-linking the MoS₂ QDs into cellulose membrane for naked-eyed detection and digital analysis of fluazinam. The paper-based sensor presented a liner range from 10 μM to 800 μM for fluazinam detection with the LOD of 2.26 μM. Additionally, the acceptable recoveries were obtained for fluazinam detection in the spiked samples of tomato, potato and cucumber, indicating that the proposed method provided an effective sensing platform for real applications of fluazinam detection in food safety.

Paper sensor of curcumin by fluorescence resonance energy transfer on nitrogen-doped carbon quantum dot

Paper Sensor detection methods are attractive in wide analytical applications. Presented herein is a paper sensor and fluorescence methods that was firstly developed to detect curcumin (Cur) based on fluorescence resonance energy transfer (FRET) between nitrogen-doped carbon quantum dots (NCQDs) and Cur. The facile fluorescent method was demonstrated to detect Cur in the range of 0-2600 μM with a detection limit of 0.13 μM. And facile paper sensor of Cur was fabricated and displayed at concentration of 0 μM, 100 μM, 200 μM, 300 μM, 400 μM, 500 μM and 600 μM, respectively. In additions, it was realized for determination of Cur in real samples including orange juice and curry solution. Compared with the reported methods, the present method is simple, rapid and sensitive for detecting Cur.

Silver nanoparticles-embedded nanopaper as a colorimetric chiral sensing platform

Paper-based sensors open new avenue to produce simple, rapid, inexpensive and single-use analytical devices for a wide application fields involving medical diagnosis, food analysis and environmental monitoring. In this study, a new optical sensing platform for enantioselective recognition of chiral analytes was introduced by using in-situ synthesized silver nanoparticles-embedded transparent nanopaper. To this aim, nanopaper was obtained by environment-friendly approach using bacterial cellulose made of nanofibers and silver nanoparticles were embedded within nanopaper by an in-situ generation method. The chiral recognition capability of AgNPs was operated in both solution phase and nanopaper for the tested enantiomers. AgNPs showed a discriminative sensing response toward D-cysteine with a LOD value of 4.88 µM. The principle of optical sensing is the selective interaction of the inherently chiral AgNPs with enantiomers causing to aggregation of AgNPs to display a significant colour change from yellow to purple-brown in both aqueous phase and nanopaper. As for practical use, the obtained plasmonic nanopaper was punched into circular pieces and put on wax-printed PET film to produce disposable two-dimensional cuvette which could be inserted in an ordinary spectrophotometer. The enantiomeric percentage of D-cysteine was successfully determined by the fabricated nanopaper-based cuvettes.

Salicylyl Fluorene Derivatives as Fluorescent Sensors for Cu(II) Ions

Two derivatives of fluorene containing salicylic acid groups are successfully synthesized by palladium-catalyzed coupling reactions and subsequent hydrolysis of salicylate esters. The compounds are characterized by various spectroscopic methods. In phosphate buffer (pH 8.0) solutions, these compounds are well soluble. They show maximum absorption wavelengths in the range of 304-330 nm and exhibit maximum emission wavelength around 420 and 430 nm with the quantum yields of 2.7 and 4.4 %, respectively. The compound with alkynyl salicylate groups (2) exhibits a selective fluorescence quenching towards Cu(II) and Fe(II) with a relatively similar sensitivity. The selectivity favoring Cu(II) over Fe(II) and other metal ions can be achieved upon the addition of 30 μM Triton X-100. The Cu(II) detection limit in solution phase is 1.47 ppb. The fluorescence signal recovery upon the addition of EDTA indicate a reversible complexation between 2 and Cu(II) ion. Fabrication of 2 on filter paper using a 50 μM solution in THF affords a naked-eye detection for Cu(II) and Fe(II) in aqueous media at picomole level.

Highly selective and sensitive paper-based colorimetric sensor using thiosulfate catalytic etching of silver nanoplates for trace determination of copper ions

A novel, highly selective and sensitive paper-based colorimetric sensor for trace determination of copper (Cu(2+)) ions was developed. The measurement is based on the catalytic etching of silver nanoplates (AgNPls) by thiosulfate (S2O3(2-)). Upon the addition of Cu(2+) to the ammonium buffer at pH 11, the absorption peak intensity of AuNPls/S2O3(2-) at 522 nm decreased and the pinkish violet AuNPls became clear in color as visible to the naked eye. This assay provides highly sensitive and selective detection of Cu(2+) over other metal ions (K(+), Cr(3+), Cd(2+), Zn(2+), As(3+), Mn(2+), Co(2+), Pb(2+), Al(3+), Ni(2+), Fe(3+), Mg(2+), Hg(2+) and Bi(3+)). A paper-based colorimetric sensor was then developed for the simple and rapid determination of Cu(2+) using the catalytic etching of AgNPls. Under optimized conditions, the modified AgNPls coated at the test zone of the devices immediately changes in color in the presence of Cu(2+). The limit of detection (LOD) was found to be 1.0 ng mL(-1) by visual detection. For semi-quantitative measurement with image processing, the method detected Cu(2+) in the range of 0.5-200 ng mL(-1)(R(2)=0.9974) with an LOD of 0.3 ng mL(-1). The proposed method was successfully applied to detect Cu(2+) in the wide range of real samples including water, food, and blood. The results were in good agreement according to a paired t-test with results from inductively coupled plasma-optical emission spectrometry (ICP-OES).

Paper-based assay of antioxidant activity using analyte-mediated on-paper nucleation of gold nanoparticles as colorimetric probes

With the increasing interest in the health benefits arising from the consumption of dietary products rich in antioxidants, there exists a clear demand for easy-to-use and cost-effective tests that can be used for the identification of the antioxidant power of food products. Paper-based analytical devices constitute a remarkable platform for such expedient and low-cost assays with minimal external resources but efforts in this direction are still scarce. In this work we introduce a new paper-based device in the form of a sensor patch that enables the determination of antioxidant activity through analyte-driven on-paper formation of gold nanoparticles. The principle of detection capitalizes, for the first time, on the on-paper nucleation of gold ions to its respective nanoparticles, upon reduction by antioxidant compounds present in an aqueous sample. The ensuing chromatic transitions, induced on the paper surface, are used as an optical "signature" of the antioxidant strength of the solution. The response of the paper-based sensor was evaluated against a large variety of antioxidant species and the respective dose response curves were constructed. On the basis of these data, the contribution of each species according to its chemical structure was elucidated. For the analysis of real samples, a concentration-dependent colorimetric response was established against Gallic acid equivalents over a linear range of 10 μM-1.0 mM, with detection limits at the low and ultra-low μM levels (i.e. <1.0 μM) and satisfactory precision (RSD=3.6-12.6%). The sensor has been tested for the assessment of antioxidant activity in real samples (teas and wines) and the results correlated well with commonly used antioxidant detection methods. Importantly, the sensor performed favorably for long periods of time when stored at moisture-free and low temperature conditions without losing its activity thus posing as an attractive alternative to the assessment of antioxidant activity without specialized equipment. The use of the sensor by non-experts for a rapid assessment of natural products in field testing is envisioned. Importantly, we demonstrate for the first time that analyte-mediated growth of nanomaterials directly on the paper surface could open new opportunities in paper-based analytical devices.

A paper-based calorimetric microfluidics platform for bio-chemical sensing

In this report, a paper-based micro-calorimetric biochemical detection method is presented. Calorimetric detection of biochemical reactions is demonstrated as an extension of current colorimetric and electrochemical detection mechanisms of paper-based biochemical analytical systems. Reaction and/or binding temperature of glucose/glucose oxidase, DNA/hydrogen peroxide, and biotin/streptavidin, are measured by the paper-based micro-calorimeter. Commercially available glucose calibration samples of 0.05, 0.15 and 0.3% wt/vol concentration are used for comparing the device performance with a commercially available glucose meter (electrochemical detection). The calorimetric glucose detection demonstrates a measurement error less than 2%. The calorimetric detection results of DNA concentrations from 0.9 to 7.3 mg/mL and temperature changes in biotin and streptavidin reaction are presented to demonstrate the feasibility of integrating the calorimetric detection method with paper based microfluidic devices.