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

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

In-situ synthesis of floating ZnIn2S4/cellulose foam for facile photocatalysis

The delicate design of photocatalyst monoliths is of great significance for the practical applications of artificial photocatalysis. An in-situ synthesis to prepare ZnIn₂S₄/cellulose foam was developed. Cellulose is dispersed in a highly concentrated ZnCl₂ aqueous solution to prepare Zn²⁺/cellulose foam. Zn²⁺ ions are pre-anchored by hydrogen bonds on cellulose and become in-situ sites for synthesizing ultra-thin ZnIn₂S₄ nanosheets. This synthesis method makes ZnIn₂S₄ nanosheets and cellulose tightly bound and prevents ZnIn₂S₄ nanosheets from stacking in multiple layers. As a proof of concept, the prepared ZnIn₂S₄/cellulose foam exhibits a favorable performance for photocatalytic reduction of Cr(VI) under visible light. By adjusting the concentration of zinc ions, the optimal ZnIn₂S₄/cellulose foam is capable to completely reduce Cr(VI) in 2 h and the photocatalytic activities show no decrease after 4 cycles. This work could inspire people to build floating cellulose-based photocatalysts via in-situ synthesis.

Titania-Cellulose Hybrid Monolith for In-Flow Purification of Water under Solar Illumination

In this work, we report a versatile approach for the development of an in-flow purification water system under solar illumination. Cellulose nanofibrils (CNFs) were impregnated with TiO₂ nanoparticles using water as a solvent to obtain hybrid CNF/TiO₂ monoliths with 98% porosity. The opposite surface potential enables an electrostatically induced direct conjugation between TiO₂ and CNFs. Scanning electron microscopy analysis of the surface morphology of the CNF/TiO₂ monolith shows a homogeneous dense coating of titania nanoparticles onto the interconnected nanofibril network, providing a Brunauer-Emmett-Teller surface area of about 80 m²·g^-1 for the hybrid monolith. Furthermore, compression tests reveal a good shape recovery after unloading, thanks to the highly flexible and mechanically stable three-dimensional structure. Finally, the CNF-based hybrids were tested as catalysts for the decomposition of organic pollutants under solar illumination. The tests were performed using a continuous flow reactor with a customized holder, allowing the solution to pass through the monolith. The results reveal a good photocatalytic activity and a long-term stability of the hybrid CNF/TiO₂ monolith toward the decomposition of methyl orange and paracetamol. These features provide a proof of concept for the applicability of the hybrid CNF/TiO₂ monoliths for in-flow purification of water under solar illumination, not only for model dyes but also for organic pollutants of high practical relevance.

An aptamer cocktail-functionalized photocatalyst with enhanced antibacterial efficiency towards target bacteria

We developed TiO2 particles conjugated with an Escherichia coli surface-specific ssDNA aptamer cocktail (composed of three different aptamers isolated from E. coli) for targeted and enhanced disinfection of E. coli. We examined the target-specific and enhanced inactivation of this composite (TiO2-Apc), which were compared to those of TiO2 conjugated with a single aptamer (one of the three different aptamers, TiO2-Aps) and non-modified TiO2. We found that TiO2-Apc enhanced the inactivation of targeted E. coli under UV irradiation compared to both the non-modified TiO2 and TiO2-Aps. A higher number of TiO2-Apc than TiO2-Aps particles was observed on the surface of E. coli. The amount of TiO2-Apc required to inactivate ∼99.9% of E. coli (10(6) CFU/ml) was 10 times lower than that of non-modified TiO2. The close proximity of functionalized particles with E. coli resulting from the interaction between the target surface and the aptamer induced the efficient and fast transfer of reactive oxygen species to the cells. In a mixed culture of different bacteria (E. coli and Staphylococcus epidermidis), TiO2-Apc enhanced the inactivation of only E. coli. Taken together, these results support the use of aptamer cocktail-conjugated TiO2 for improvement of the target-specific inactivation of bacteria.

Tailor-made functional surfaces based on cellulose-derived materials

As one of the most abundant natural materials in nature, cellulose has revealed enormous potential for the construction of functional materials thanks to its sustainability, non-toxicity, biocompatibility, and biodegradability. Among many fascinating applications, functional surfaces based on cellulose-derived materials have attracted increasing interest recently, as platforms for diagnostics, sensoring, robust catalysis, water treatment, ultrafiltration, and anti-microbial surfaces. This mini-review attempts to cover the general methodology for the fabrication of functional cellulose surface and a few popular applications including bioactive and non-adhesive (i.e., anti-fouling and anti-microbial) surfaces.

Metal oxide nanostructures incorporated/immobilized paper matrices and their applications: a review

A comprehensive review on the metal oxide nanostructures incorporated/immobilized paper matrices byex situandin situmethods for various applications.

Microwave-assisted incorporation of silver nanoparticles in paper for point-of-use water purification

This work reports an environmentally benign method for the in situ preparation of silver nanoparticles (AgNPs) in paper using microwave irradiation. Through thermal evaporation, microwave heating with an excess of glucose relative to the silver ion precursor yields nanoparticles on the surface of cellulose fibers within three minutes. Paper sheets were characterized by electron microscopy, UV-Visible reflectance spectroscopy, and atomic absorption spectroscopy. Antibacterial activity and silver release from the AgNP sheets were assessed for model Escherichia coli and Enterococci faecalis bacteria in deionized water and in suspensions that also contained with various influent solution chemistries, i.e. with natural organic matter, salts, and proteins. The paper sheets containing silver nanoparticles were effective in inactivating the test bacteria as they passed through the paper.

Bacterial target-specific photocatalyst for the enhancement of antibacterial property to targets

A bacterial target-specific titanium oxide (TiO₂) photocatalyst was developed for the enhancement of selective inactivation of targeted bacteria. An antibacterial composition comprising TiO₂ particles immobilized with a bacterial-specific antibody having affinity to bacteria of interest was prepared via a carbodiimide hydrochloride/N-hydroxysulfosuccinimide (EDC/NHS) coupling reaction between polyacrylic acid (PAA) coated TiO₂ and an antibody. As a model case, an antibody to Escherichia coli was conjugated with the PAA-coated TiO₂ (TiO₂-Ab_E). We evaluated the enhancement of the antibacterial effect of TiO₂-Ab_E against target E. coli, compared with its effect on other bacteria that lack affinity for the antibody used. The TiO₂-Ab_E inactivated approximately 90% of the E. coli at 15 min, whereas the raw TiO₂ inactivated approximately 20% of the E. coli after the same period of time under UV irradiation. The TiO₂-Ab_E did not show an enhanced antibacterial effect against non-target bacteria. We infer that close contact between TiO₂ particles and E. coli, as a result of the specificity of the antibody, can enhance the direct transfer of reactive oxygen species (ROS) generated by TiO₂ particles to the cellular surface under UV irradiation and result in rapid and efficient inactivation of the targeted bacteria. The strategy presented here will facilitate the combination of other receptors and TiO₂ particles for the preparation of highly selective and photocatalytic composites to prevent or remediate contamination by unwanted bacteria in a wide variety of natural and man-made systems.

Disposable chemical sensors and biosensors made on cellulose paper

Most sensors are based on ceramic or semiconducting substrates, which have no flexibility or biocompatibility. Polymer-based sensors have been the subject of much attention due to their ability to collect molecules on their sensing surface with flexibility. Beyond polymer-based sensors, the recent discovery of cellulose as a smart material paved the way to the use of cellulose paper as a potential candidate for mechanical as well as electronic applications such as actuators and sensors. Several different paper-based sensors have been investigated and suggested. In this paper, we review the potential of cellulose materials for paper-based application devices, and suggest their feasibility for chemical and biosensor applications.

Adsorption of DNA oligonucleotides by titanium dioxide nanoparticles

Titanium dioxide (TiO2) or titania shows great promise in detoxification and drug delivery. To reach its full potential, it is important to interface TiO2 with biomolecules to harness their molecular recognition function. To this end, DNA attachment is an important topic. Previous work has mainly focused on long double-stranded DNA or single nucleotides. For biosensor development and targeted drug delivery, it is more important to use single-stranded oligonucleotides. Herein, the interaction between fluorescently labeled oligonucleotides and TiO2 nanoparticles is reported. The point of zero charge (PZC) of TiO2 is around 6 in water or acetate buffer; therefore, the particles are positively charged at lower pH. However, if in phosphate or citrate buffer, the particles are negatively charged, even at pH ∼2, suggesting strong adsorption of buffer anions. DNA adsorption takes place mainly via the phosphate backbone, although the bases might also have moderate contributions. Peptide nucleic acids (PNAs) with an amide backbone cannot be adsorbed. DNA adsorption is strongly affected by inorganic anions, where phosphate and citrate can strongly inhibit DNA adsorption. DNA adsorption is promoted by adding salt or lowering pH. DNA adsorption is accompanied with fluorescence quenching, and double-stranded DNA showed reduced quenching, allowing for the detection of DNA using TiO2 nanoparticles.