A comprehensive review of the application of Zr-based metal-organic frameworks for electrochemical sensors and biosensors.

A family of inorganic-organic hybrid crystalline materials made up of organic ligands and metal cations or clusters is known as metal-organic frameworks (MOFs). Because of their unique stability, intriguing characteristics, and structural diversity, zirconium-based MOFs (Zr-MOFs) are regarded as one of the most interesting families of MOF materials for real-world applications. Zr-MOFs that have the ligands, metal nodes, and guest molecules enclosed show distinct electrochemical reactions. They can successfully and sensitively identify a wide range of substances, which is important for both environmental preservation and human health. The rational design and synthesis of Zr-MOF electrochemical sensors and biosensors, as well as their applications in the detection of drugs, biomarkers, pesticides, food additives, hydrogen peroxide, and other materials, are the main topics of this comprehensive review. We also touch on the current issues and potential future paths for Zr-MOF electrochemical sensor research.

Improving design features and air bubble manipulation techniques for a single-step sandwich electrochemical ELISA incorporating commercial electrodes into capillary-flow driven immunoassay devices.

Integrating electrochemistry into capillary-flow driven immunoassay devices provides unique opportunities for quantitative point-of-care testing. Although custom electrodes can be inexpensive and are tunable, they require skilled fabrication. Here, we report the incorporation of a commercial electrode into a capillary-flow driven immunoassay (iceCaDI) device for a single end-user step sandwich electrochemical enzyme-linked immunosorbent assay (ELISA). The iceCaDI device is a pump-free portable microfluidic device with an integrated commercial screen-printed electrode and flow driven by capillary action. The iceCaDI device is composed of alternating polyester transparency film and double-sided adhesive film layers that are patterned with a laser cutter. This platform was designed to address known limitations of laminated device fabrication methods and operation. First, we developed a foldable laminated device fabrication using hinges for easy assembly and precise alignment. Second, reagent dispersing was achieved by incorporating a 1 mm wide arrow-shaped notch in the middle of the channel that trapped an air bubble and formed a baffle that facilitated reagent spreading to cover the detection area. Third, small vent holes were added to the top layer of the channels to prevent air bubbles from blocking flow. Finally, we fabricated a CRP immunosensor with a detection range of 0.625 to 10.0 µg mL-1 as a proof-of-concept to demonstrate an automatically driven sandwich electrochemical ELISA using the iceCaDI device. Three concentrations of CRP were successfully measured under flow conditions within 8 min. Our proposed device is a promising approach and a step forward in the development of point-of-care (POC) devices for techniques that traditionally require multiple user steps.

Electrogenicity of microbial biofilms of medically relevant microorganisms: potentiometric, amperometric and wireless detection.

Since the progression of biofilm formation is related to the success of infection treatment, detecting microbial biofilms is of great interest. Biofilms of Gram-positive Staphylococcus aureus and Streptococcus gordonii bacteria, Gram-negative Pseudomonas aeruginosa and Escherichia coli bacteria, and Candida albicans yeast were examined using potentiometric, amperometric, and wireless readout modes in this study. As a biofilm formed, the open circuit potential (OCP) of biofilm hosting electrode (bioanode) became increasingly negative. Depending on the microorganism, the OCP ranged from -70 to -250 mV. The co-culture generated the most negative OCP (-300 mV vs Ag/AgCl), while the single-species biofilm formed by E. coli developed the least negative (-70 mV). The OCP of a fungal biofilm formed by C. albicans was -100 mV. The difference in electrode currents generated by biofilms was more pronounced. The current density of the S. aureus biofilm was 0.9‧10-7 A cm-2, while the value of the P. aeruginosa biofilm was 1.3‧10-6 A cm-2. Importantly, a biofilm formed by a co-culture of S. aureus and P. aeruginosa had a slightly higher negative OCP value and current density than the most electrogenic P. aeruginosa single-species biofilm. We present evidence that bacteria can share redox mediators found in multi-species biofilms. This synergy, enabling higher current and OCP values of multi-species biofilm hosting electrodes, could be beneficial for electrochemical detection of infectious biofilms in clinics. We demonstrate that the electrogenic biofilm can provide basis to construct novel wireless, chip-free, and battery-free biofilm detection method.

Recent Advances in Cell Sheet Engineering: From Fabrication to Clinical Translation.

Cell sheet engineering, a scaffold-free tissue fabrication technique, has proven to be an important breakthrough technology in regenerative medicine. Over the past two decades, the field has developed rapidly in terms of investigating fabrication techniques and multipurpose applications in regenerative medicine and biological research. This review highlights the most important achievements in cell sheet engineering to date. We first discuss cell sheet harvesting systems, which have been introduced in temperature-responsive surfaces and other systems to overcome the limitations of conventional cell harvesting methods. In addition, we describe several techniques of cell sheet transfer for preclinical (in vitro and in vivo) and clinical trials. This review also covers cell sheet cryopreservation, which allows short- and long-term storage of cells. Subsequently, we discuss the cell sheet properties of angiogenic cytokines and vasculogenesis. Finally, we discuss updates to various applications, from biological research to clinical translation. We believe that the present review, which shows and compares fundamental technologies and recent advances in cell engineering, can potentially be helpful for new and experienced researchers to promote the further development of tissue engineering in different applications.

Paper-Based Screen-Printed Ionic-Liquid/Graphene Electrode Integrated with Prussian Blue/MXene Nanocomposites Enabled Electrochemical Detection for Glucose Sensing.

As glucose biosensors play an important role in glycemic control, which can prevent the diabetic complications, the development of a glucose sensing platform is still in needed. Herein, the first proposal on the in-house fabricated paper-based screen-printed ionic liquid/graphene electrode (SPIL-GE) modified with MXene (Ti3C2Tx), prussian blue (PB), glucose oxidase (GOx), and Nafion is reported. The concentration of PB/Ti3C2Tx was optimized and the optimal detection potential of PB/Ti3C2Tx/GOx/Nafion/SPIL-GE is -0.05 V. The performance of PB/Ti3C2Tx/GOx/Nafion modified SPIL-GE was characterized by cyclic voltammetry and chronoamperometry technique. This paper-based platform integrated with nanomaterial composites were realized for glucose in the range of 0.0-15.0 mM with the correlation coefficient R2 = 0.9937. The limit of detection method and limit of quantification were 24.5 µM and 81.7 µM, respectively. In the method comparison, this PB/Ti3C2Tx/GOx/Nafion/SPIL-GE exhibits a good correlation with the reference hexokinase method. This novel glucose sensing platform can potentially be used for the good practice to enhance the sensitivity and open the opportunity to develop paper-based electroanalytical devices.

Synergistic Behavior of Polyethyleneimine and Epoxy Monomers Loaded in Mesoporous Silica as a Corrosion-Resistant Self-Healing Epoxy Coating.

Corrosion is a significant problem and is, to a large extent, responsible for the degradation of metallic parts. In this direction, mesoporous silica particles (MSPs) were synthesized by a sol-gel technique and had an average pore diameter of ∼6.82 nm. The MSPs were loaded with polyethyleneimine (PEI) and epoxy monomers and, after that, carefully mixed into the epoxy matrix to formulate new modified polymeric coatings. The microstructural, compositional, structural, and thermal properties were investigated using various characterizing tools [Transmission electron microscopy, Fourier transform infrared spectroscopy, hermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy]. TGA confirms the loading of mesoporous silica with a corrosion inhibitor, and its estimated loading amount is ∼8%. The electrochemical impedance spectroscopy properties of the reference and modified coated samples confirm the promising anti-corrosive performance of the synthesized polymeric smart coatings. Localized electrochemical tests (scanning vibrating electrode technique and scanning ion-selective electrode technique) evidence the corrosion inhibition ability of the coating, and its self-healing was also observed during 24 h of immersion. The decent anti-corrosion performance of the modified coatings can be credited to the efficient synergistic effect of the PEI and epoxy monomer.

A comprehensive review on electrochemical and optical aptasensors for organophosphorus pesticides.

There has been a rise in pesticide use as a result of the growing industrialization of agriculture. Organophosphorus pesticides have been widely applied as agricultural and domestic pest control agents for nearly five decades, and they remain as health and environmental hazards in water supplies, vegetables, fruits, and processed foods causing serious foodborne illness. Thus, the rapid and reliable detection of these harmful organophosphorus toxins with excellent sensitivity and selectivity is of utmost importance. Aptasensors are biosensors based on aptamers, which exhibit exceptional recognition capability for a variety of targets. Aptasensors offer numerous advantages over conventional approaches, including increased sensitivity, selectivity, design flexibility, and cost-effectiveness. As a result, interest in developing aptasensors continues to expand. This paper discusses the historical and modern advancements of aptasensors through the use of nanotechnology to enhance the signal, resulting in high sensitivity and detection accuracy. More importantly, this review summarizes the principles and strategies underlying different organophosphorus aptasensors, including electrochemical, electrochemiluminescent, fluorescent, and colorimetric ones.

Simultaneous phenotyping of five Rh red blood cell antigens on a paper-based analytical device combined with deep learning for rapid and accurate interpretation.

Both the ABO and Rhesus (Rh) blood groups play crucial roles in blood transfusion medicine. Herein, we report a simple and low-cost paper-based analytical device (PAD) for phenotyping red blood cell (RBC) antigens. Using this Rh typing format, 5 Rh antigens on RBCs can be simultaneously detected and macroscopically visualized within 12 min. The proposed Rh phenotyping relies on the presence or absence of hemagglutination in the sample zones after immobilizing the antibodies targeting each Rh antigen. The PAD was optimized in terms of filter paper type, antibodies, and distance of the visualization zone. In this study, the optimal conditions were Whatman filter paper Grade 4; anti-D, -C, -E, -c, and -e antibodies; RBC suspension of 30%; and a visualization zone of 1 cm above the sample zone. The accuracy of simultaneously phenotyping the five Rh RBC antigens in the blood samples (n = 4692) was 99.19%, comparable with the accuracy of the gold-standard tube method used by blood bank laboratories in several regions of Thailand. Furthermore, decision making based on this method can be assisted by deep learning. After implementing a two-stage objective detection algorithm (YOLO v4-tiny) and classification model (DenseNet-201), the ambiguous images (n = 48) were interpreted with 100% accuracy. The PAD integrated with customized-region convolutional neural networks can reduce the interpretation discrepancies in RBC antigen phenotyping in any laboratory.

An origami paper-based peptide nucleic acid device coupled with label-free DNAzyme probe hybridization chain reaction for prostate cancer molecular screening test.

Prostate cancer associated 3 (PCA3) assay has been used to improve prostate cancer diagnosis and reduce unnecessary biopsies. In this work, we successfully developed a new PCA3 assay on an origami paper-based peptide nucleic acid device (oPAD). The PCA3 oPAD comprises an acrylic cassette and shutter slides to facilitate the molecular reaction and liquid control occurring on the paper surface. To quantify PCA3, a pyrrolidinyl peptide nucleic acid (acpcPNA) was immobilized onto the aldehyde-modified oPAD surface as a selective capture probe. A G-quadruplex (GQD) DNAzyme reporter probe was designed so that the PCA3 gene target binding triggered the hybridization chain reaction of the reporter probe, resulting in the accumulation of the GQD on the oPAD. The peroxidase activity of the GQD-hemin generated a deep green color of the oxidized ABTS substrate. Image analyses were performed in Adobe Photoshop CS6. The proposed oPAD was successfully applied in PCA3 detection ranges of 1-5 µM (r2 = 0.982) with a limit of detection of 0.5 µM. Our proposed oPAD was demonstrated to measure PCA3 samples in both urine matrix and human cancer cell lines. The results reveal the great potential of our origami paper-based platform to be an alternative approach for facile, rapid, and low-cost detection of PCA3 in real samples.

Smartphone-based technique for the determination of a titration equivalence point from an RGB linear-segment curve with an example application to miniaturized titration of sodium chloride injections.

A smartphone-based technique for determining the titration equivalence point from a linear-segment curve was developed for the first time. In this method, a titrant in an increasing microliter-volume was added to a set of sample aliquots containing an indicator covering both sides of the equivalence point. The solutions were subsequently photographed in one shot, in a dark box using a smartphone camera and an illuminating screen of a tablet or light emitting diode lamps arranged below a white acrylic sheet as a light source. After the colors of the solutions were delineated to Red, Green, and Blue (RGB) values, 1/log G was used to construct a plot in which the equivalence point was located at the intersection of the two lines in the region before and after the equivalence point. The technique was successfully applied to the miniaturized titration of sodium chloride injections, showing the good linear relationship of equivalence points to the sodium chloride concentration in the range of 0.4163-0.9675% w/v (R2 of 0.9998). The assay was accurate (% recovery of 98.92-100.52), precise (% relative standard deviation ≤ 1.20), and unaffected by the use of different types of microplates, smartphones, and RGB analysis tools. Additionally, it required no expensive nor complicated equipment and offered the possibility of performing analysis on a single smartphone device when it was used with a mobile application developed to aid data processing and immediate production of reports of analytical results.

A Simple Distance Paper-Based Analytical Device for the Screening of Lead in Food Matrices.

A simple and rapid distance paper-based analytical device (dPAD) for the detection of lead (Pb) in foods is proposed herein. The assay principle is based on competitive binding between carminic acid (CA) and polyethyleneimine (PEI) to Pb in a food sample. The paper channels were pre-immobilized with PEI, before reacting with a mixture of the sample and CA. Pb can strongly bind to the CA; hence, the length of the red color deposition on the flow channel decreased as a lower amount of free CA bound to PEI. The dPAD exhibited good linear correlation, with ranges of 5-100 µg·mL-1 (R2 = 0.974) of Pb. Although, the limit of detection (LOD) of this platform was rather high, at 12.3 µg·mL-1, a series of standard additions (8.0, 9.0, and 10.0 µg·mL-1) can be used to interpret the cutoff of Pb concentrations at higher or lower than 2 µg·mL-1. The presence of common metal ions such as calcium, magnesium, nickel, and zinc did not interfere with the color distance readout. The validity of the developed dPAD was demonstrated by its applicability to screen the contamination of Pb in century egg samples. The results obtained from the dPAD are in accordance with the concentration measured by atomic absorption spectroscopy (AAS) (n = 9). In conclusion, this proposed dPAD, combined with the standard addition method, could be applied for screening Pb contamination in food matrices. This platform is, therefore, potentially applicable for field measurements of Pb in developing countries, because it is cheap and rapid, and it requires no significant laborious instruments.

Paper-Based Competitive Immunochromatography Coupled with an Enzyme-Modified Electrode to Enable the Wireless Monitoring and Electrochemical Sensing of Cotinine in Urine.

This paper proposes a combined strategy of using paper-based competitive immunochromatography and a near field communication (NFC) tag for wireless cotinine determination. The glucose oxidase labeled cotinine antibody specifically binds free cotinine in a sample, whereas the unoccupied antibody attached to BSA-cotinine at the test line on a lateral flow strip. The glucose oxidase on the strip and an assistant pad in the presence of glucose generated H2O2 and imposed the Ag oxidation on the modified electrode. This enabled monitoring of immunoreaction by either electrochemical measurement or wireless detection. Wireless sensing was realized for cotinine in the range of 100-1000 ng/mL (R2 = 0.96) in PBS medium. Undiluted urine samples from non-smokers exhibited an Ag-oxidation rate three times higher than the smoker's urine samples. For 1:8 diluted urine samples (smokers), the proposed paper-based competitive immunochromatography coupled with an enzyme-modified electrode differentiated positive and negative samples and exhibited cotinine discrimination at levels higher than 12 ng/mL. This novel sensing platform can potentially be combined with a smartphone as a reader unit.

Paper-Based Analytical Device for Real-Time Monitoring of Egg Hatching in the Model Nematode Caenorhabditis elegans.

Caenorhabditis elegans is an in vivo model known for its easy handling and maintenance and lack of associated ethical issues. The release of chitinase can be used to monitor the egg-laying stage in C. elegans. The aim of this study was to develop a simple and cost-effective device to monitor the activity of chitinase in embryos of C. elegans. Colloid chitin azure (CCA), a substrate for chitinase, was preimmobilized on the detection area of paper, forming a purple region, to generate a CCA paper-based analytical device (CCA-PAD). The degradation of CCA by chitinase could be observed as the purple color became faint and the filter paper eventually became colorless. Under the optimum conditions, the proposed device quantified the chitinase enzyme in the range of 15.625-125 mU/mL within 48 h (R2 = 0.993). In this work, 10 young adult-staged wild-type C. elegans (Bristol N2) worms were analyzed on the CCA-PAD, which was supplemented with the laboratory food source E. coli OP50 on a gauze layer. The same strain treated with 5-fluoro-2'-deoxyuridine was used to prevent egg production in C. elegans. A significant difference in the color intensity was observed between these two groups at the end of the experiment (P = <0.001, independent t-test, n = 3). We successfully developed a simple and effective method for monitoring chitinase activity. The device may have potential applications in drug-screening studies as it efficiently distinguishes drugs that can impact egg laying.

Sensing by wireless reading Ag/AgCl redox conversion on RFID tag: universal, battery-less biosensor design.

Massive integration of biosensors into design of Internet-of-Things (IoT) is vital for progress of healthcare. However, the integration of biosensors is challenging due to limited availability of battery-less biosensor designs. In this work, a combination of nanomaterials for wireless sensing of biological redox reactions is described. The design exploits silver nanoparticles (AgNPs) as part of the RFID tag antenna. We demonstrate that a redox enzyme, particularly, horseradish peroxidase (HRP), can convert AgNPs into AgCl in the presence of its substrate, hydrogen peroxide. This strongly changes the impedance of the tag. The presented example exploits gold nanoparticle (AuNP)-assisted electron transfer (ET) between AgNPs and HRP. We show that AuNP is a vital intermediate for establishing rapid ET between the enzyme and AgNPs. As an example, battery-less biosensor-RFID tag designs for H2O2 and glucose are demonstrated. Similar battery-less sensors can be constructed to sense redox reactions catalysed by other oxidoreductase enzymes, their combinations, bacteria or other biological and even non-biological catalysts. In this work, a fast and general route for converting a high number of redox reaction based sensors into battery-less sensor-RFID tags is described.

A paper-based analytical device coupled with electrochemical detection for the determination of dexamethasone and prednisolone in adulterated traditional medicines.

The adulteration of herbal medicines by dexamethasone or prednisolone is regarded as a serious problem in many communities. Herein, a novel platform for the separation and quantification of both target steroids in herbal medicines based on electrochemical paper-based analytical devices (ePADs) has been created. The ePAD was composed of Whatman SG81 chromatography paper, 3D-printed devices and a commercial screen-printed electrode. Whatman SG81 silica-coated paper was used for the separation of dexamethasone and prednisolone based on the difference in their partition coefficients during the flow of the mobile phase. The optimal mobile phase was composed of 60% ethyl acetate in cyclohexane and required 7 min for separation. The separated steroids on the paper were then quantified by electrochemical detection using differential pulse voltammetry, in which the 3D-printed devices facilitated the measurement. Analytical detection ranges of 10-500 µg mL-1 were obtained for both dexamethasone and prednisolone (r2 = 0.988 and 0.994, respectively). The limits of detection for dexamethasone and prednisolone were 3.59 and 11.98 µg mL-1, respectively, whereas the limits of quantification were 6.00 and 20.02 µg mL-1, respectively. The amounts of both target steroids derived from real herbal medicine samples determined by the proposed method were comparable to those obtained with assays using standard high-performance liquid chromatography. In addition, a simple evaporation step can be used to increase the concentration of the samples before analysis. These ePADs are simple, low-cost, rapid, and very promising for on-site quantitative detection.

Exploring Matrix Effects on Binding Properties and Characterization of Cotinine Molecularly Imprinted Polymer on Paper-Based Scaffold.

Commercially available sorbent materials for solid-phase extraction are widely used in analytical laboratories. However, non-selective binding is a major obstacle for sample analysis. To overcome this problem, molecularly imprinted polymers (MIPs) were used as selective adsorbent materials prior to determining target analysts. In this study, the use of non-covalent molecularly imprinted polymers (MIPs) for cotinine adsorption on a paper-based scaffold was studied. Fiberglass paper was used as a paper scaffold for cotinine-selective MIP adsorption with the use of 0.5% agarose gel. The effects of salt, pH, sample matrix, and solvent on the cotinine adsorption and extraction process were investigated. Under optimal conditions, the adsorption isotherm of synthesized MIPs increased to 125.41 µg/g, whereas the maximum adsorption isotherm of non-imprinted polymers (NIPs) was stable at 42.86 µg/g. The ability of the MIP paper scaffold to absorb cotinine in water medium was approximately 1.8⁻2.8-fold higher than that of the NIP scaffold. From Scatchard analysis, two dissociation constants of MIPs were calculated to be 2.56 and 27.03 µM. Nicotine, myosmine, and N-nitrosonornicotine were used for selectivity testing, and the calculated selectivity factor of cotinine to nicotine, myosmine, and N-nitrosonornicotine was 1.56, 2.69, and 2.05, respectively. Overall, the MIP paper scaffold is promising for simple onsite sampling of cotinine and can be used to assess tobacco smoke exposure.

Multifunctional Paper-Based Analytical Device for In Situ Cultivation and Screening of Escherichia coli Infections.

Point-of-care testing (POCT) for uropathogen detection and chemical screening has great benefits for the diagnosis of urinary tract infections (UTIs). The goal of this study was to develop a portable and inexpensive paper-based analytical device (PAD) for cultivating bacteria in situ and rapidly testing for nitrite on the same device. The PAD was fabricated using a wax printing technique to create a pattern on Whatman No. 1 filter paper, which was then combined with a cotton sheet to support bacterial growth. Nitrite detection was based on the principle of the Griess reaction, and a linear detection range of 0-1.6 mg/dL (R2 = 0.989) was obtained. Scanning electron microscopy (SEM) analysis demonstrated that the bacteria were able to grow and formed a cluster on the cellulose fibres within 2 hours. The enzyme ß-glucuronidase, which is specifically produced by Escherichia coli, was able to convert the pre-immobilized 5-bromo-4-chloro-3-indolyl-ß-D-glucuronide sodium salt (X-GlcA), a colourless substrate, generating a blue colour. Under optimum conditions, the proposed device allowed bacterial concentrations in the range of 104-107 colony forming units (CFU)/mL to be quantified within 6 hours. Moreover, the use of this device enables the identification of E. coli pathogens with selectivity in real urine samples. In conclusion, the PAD developed in this study for UTI screening provides a rapid, cost-effective diagnostic method for use in remote areas.

A folding affinity paper-based electrochemical impedance device for cardiovascular risk assessment.

A novel affinity paper-based electrochemical impedance device (PEID) was first fully developed for cardiovascular risk assessment. Herein, a simple folding PEID comprising a dual screen-printed electrode was fabricated for label-free electrochemical impedance detection. The results demonstrated in a step-wise manner that the phosphocholine-modified screen-printed carbon electrodes were highly responsive to the clinically required range of C-reactive protein (CRP) (0.005 - 500 mg L-1; r2 = 0.993) levels with a detection limit (3σ/slope) of 0.001 mg L-1. The optimal binding frequency of CRP-phosphocholine interaction was determined to be 100 Hz. These results implied that our proposed system could be used for simultaneously measuring the CRP levels using a single PEID platform in combination with the specific recognition elements. When assaying two levels of CRP, the overall assay reproducibility for each concentration, expressed as relative standard error of the mean (RSE%; n = 30), was 1.21%. The variation in the measurements between individual electrodes, expressed as the relative standard deviation (RSD), was 2.82%. Using 2 measurement sites per device, the proposed sensor provided excellent precision for the simultaneous detection of CRP. Moreover, the RSD for the CRP levels measured on ten independently fabricated paper-based sheets was 2.11%, thereby offering an acceptable fabrication reproducibility. The presented folding PEIDs were used forthe determination of CRP in patient-derived blood samples with minimised bias and excellent correlation with a standard method (n = 15; CUSUM linearity test, p > 0.10), thus permitting the potential application of PEID for assessing cardiovascular risk in individuals.

In situ paper-based 3D cell culture for rapid screening of the anti-melanogenic activity.

Recently, paper has gained traction in the biotechnology research field due to its ability to be a substrate for 3D cell culture. In this work, we demonstrate the application of paper-based 3D cell culture for rapid and easy screening of the effect of natural compounds on melanin production. Whatman No. 1 filter paper was used as the substrate for B16F10 melanoma cell culture. The use of paper is beneficial for supporting the 3D structure of cells, which makes the result more reliable due to the similarity to in vivo conditions. Furthermore, paper is beneficial for melanin observation due to melanin's black color, which is easily in situ visualized after it is cultured on white paper. Matrigel was used to encapsulate cells before being pipetted onto the paper to prevent the passing of cells through paper pores. The intensity of melanin can then be observed with the naked eye and analyzed by scanning the paper. The analysis process took only 20 minutes, which is faster than that of the conventional absorbance spectroscopy, owing to the elimination of centrifugation, melanin solubilization, and the absorbance measurement step. The color intensity on the paper showed a direct proportion with increased α-MSH concentrations, confirming that the color on the paper was melanin. The 3D structure of cells was confirmed by using a scanning electron microscope. To demonstrate the application of the paper-based scaffold, paper-based 3D cell culture was used for screening the effects of Kojic acid and Arbutin on melanin production, which showed increased anti-melanogenesis effects with increased concentrations of natural compounds. High cell viability was observed over 120 hours. In conclusion, the developed paper-based scaffold can be used for screening the effect of natural compounds on melanin production, as a rapid and simple method with low cost.

Development of Paper-Based Analytical Devices for Minimizing the Viscosity Effect in Human Saliva.

Rationale: Saliva as a sample matrix is rapidly gaining interest for disease diagnosis and point-of-care assays because it is easy to collect (non-invasive) and contains many health-related biomarkers. However, saliva poses particular problems relative to more common urine and blood matrices, which includes low analyte concentrations, lack of understanding of biomolecule transportation and inherent viscosity variability in human samples. While several studies have sought to improve assay sensitivity, few have addressed sample viscosity specifically. The goal of this study is to minimize the effect of sample viscosity on paper-based analytical devices (PADs) for the measurement of pH and nitrite in human saliva. Methods: PADs were used to measure salivary pH from 5.0 to 10.0 with a universal indicator consisting of chlorophenol red, phenol red and phenolphthalein. Nitrite determination was performed using the Griess reaction. Artificial saliva with viscosity values between 1.54 and 5.10 mPa∙s was tested on the proposed PAD. To ensure the proposed PADs can be tailored for use in-field analysis, the devices were shipped to Australia and tested with human specimens. Results: Initial experiments showed that viscosity had a significant impact on the calibration curve for nitrite; however, a more consistent curve could be generated when buffer was added after the sample, irrespective of sample viscosity. The linear range for nitrite detection was 0.1 to 2.4 mg/dL using the improved method. The nitrite measurement in artificial saliva also showed a good correlation with the standard spectrophotometry method (p=0.8484, paired sample t-test, n=20). Measured pH values from samples with varying viscosities correlated well with the results from our pH meter. Conclusions: The inherent variation of salivary viscosity that impacts nitrite and pH results can be addressed using a simple washing step on the PAD without the need for complex procedures.