Biofunctional paper via the covalent modification of cellulose

Advancements in clinical approaches, analytical methods, and smart sampling for LC-MS-based protein determination from dried matrix spots

Determination of proteins from dried matrix spots using MS is an expanding research area. Mainly, the collected dried matrix sample is whole blood from a finger or heal prick, resulting in dried blood spots. However as other matrices such as plasma, serum, urine, and tear fluid also can be collected in this way, the term dried matrix spot is used as an overarching term. In this review, the focus is on advancements in the field made from 2017 up to 2023. In the first part reviews concerning the subject are discussed. After this, advancements made for clinical purposes are highlighted. Both targeted protein analyses, with and without the use of affinity extractions, as well as untargeted, global proteomic approaches are discussed. In the last part, both methodological advancements are being reviewed as well as the possibility to integrate sample preparation steps during the sample handling. The focus, of this so-called smart sampling, is on the incorporation of cell separation, proteolysis, and antibody-based affinity capture.

One-step functionalization of paper and simplified antibody immobilization for on-the-spot immunocapture from dried serum in liquid chromatography-tandem mass spectrometry based targeted protein determination

This work aimed to simplify and improve the process of binding monoclonal antibodies (mAbs) covalently to filter paper for use in dried blood spot sampling, enabling instant capture of protein biomarkers for targeted protein determination. Incorporating the necessary immunocapture sample preparation step in the initial sampling stage saves time and reduces the workload. The biomarker human chorionic gonadotropin (hCG) was used as the model analyte. The antibody-based paper samplers were prepared by functionalizing paper discs (6 mm) through a simple reaction using divinyl sulfone (DVS). After DVS activation, the paper discs were incubated with E27 hCG mAbs, followed by 0.05% tween/phosphate buffer saline to block the surface. After sample application and drying, the discs only needed to be washed before tryptic digestion and finally analysed on a nanoliquid chromatography-tandem mass spectrometry system. The finished DVS-mAbs samplers could selectively capture hCG (100 ng/mL) from human serum, with a recovery of 50%. Sample clean-up reduced the number of identified proteins from 132 to 82 before and after wash, respectively, with a 70% reduction in serum albumin signal while still retaining hCG on the sampler during the washing protocol. An evaluation of the samplers revealed excellent linearity (R2  = 0.9995) for hCG in serum with relative standard deviations below 15%. This work has presented the first ever reported paper samplers immobilized with antibodies utilizing DVS chemistry, showing promise in the future of paper-based sampling.

Controlling Fluorescent Readout in Paper-based Analytical Devices

Paper is an ideal candidate for the development of new disposable diagnostic devices because it is a low-cost material, allows transport of the liquid on the device by capillary action, and is environmentally friendly. Today, colorimetric analysis is most often used as a detection method for rapid tests (test strips or lateral flow devices) but usually gives only qualitative results and is limited by a relatively high detection threshold. Here, we describe studies using fluorescence as a readout tool for paper-based diagnostics. We study how the optical readout is affected by light transmission, scattering, and fluorescence as a function of paper characteristics such as thickness (grammage), water content, autofluorescence, and paper type/composition. We show that paper-based fluorescence analysis allows better optical readout compared to that of nitrocellulose, which is currently the material of choice in colorimetric assays. To reduce the loss of analyte molecules (e.g., proteins) due to adsorption to the paper surface, we coat the paper fibers with a protein-repellent hydrogel. For this purpose, we use hydrophilic copolymers consisting of N,N-dimethyl acrylamide and a benzophenone-based cross-linker, which are photochemically transformed into a fiber-attached polymer hydrogel on the paper fiber surfaces in situ. We show that the combination of fluorescence detection and the use of a protein-repellent coating enables sensitive paper-based analysis. Finally, the success of the strategy is demonstrated by using a simple LFD application as an example.

Sustainable DNA Data Storage on Cellulose Paper

DNA is a promising material for high density and long-term archival data storage. In addition to algorithms for encoding digital information into DNA sequences, the DNA writing (chemical synthesis) and reading (DNA sequencing), the preservation of DNA mixtures with high sequence diversity is another critical issue for sustainable, long-term, and large-scale DNA data storage. Here, this work demonstrates a method for low-cost, convenient and sustainable DNA data storage on cellulose paper. A DNA pool comprising thousands of sequences, in which archival data are encoded, is conveniently stored on a cellulose paper with a calculated density as high as 15 TB per mm3 through electrostatic adsorption. This work demonstrates that these digitally encoded DNA pools can be stable for years on the cellulose paper after drying even when directly exposed to air. Furthermore, the reversible electrostatic adsorption enables repeated loading/retrieval of DNA on/off cellulose paper. Therefore, this sustainable DNA preservation on cellulose paper through the convenient electrostatic adsorption exhibits a great advantage in terms of storage capacity and cost that is crucial for practical systems to achieve large-scale and long-time data storage.

Glycosylated gold nanoparticles in point of care diagnostics: from aggregation to lateral flow

Current point-of-care lateral flow immunoassays, such as the home pregnancy test, rely on proteins as detection units (e.g. antibodies) to sense for analytes. Glycans play a fundamental role in biological signalling and recognition events such as pathogen adhesion and hence they are promising future alternatives to antibody-based biosensing and diagnostics. Here we introduce the potential of glycans coupled to gold nanoparticles as recognition agents for lateral flow diagnostics. We first introduce the concept of lateral flow, including a case study of lateral flow use in the field compared to other diagnostic tools. We then introduce glycosylated materials, the affinity gains achieved by the cluster glycoside effect and the current use of these in aggregation based assays. Finally, the potential role of glycans in lateral flow are explained, and examples of their successful use given.

Antibody-based lateral flow (immune) assays are well established, but here the emerging concept and potential of using glycans as the detection agents is reviewed.

Reducing Unspecific Protein Adsorption in Microfluidic Papers Using Fiber-Attached Polymer Hydrogels

Microfluidic paper combines pump-free water transport at low cost with a high degree of sustainability, as well as good availability of the paper-forming cellulosic material, thus making it an attractive candidate for point-of-care (POC) analytics and diagnostics. Although a number of interesting demonstrators for such paper devices have been reported to date, a number of challenges still exist, which limit a successful transfer into marketable applications. A strong limitation in this respect is the (unspecific) adsorption of protein analytes to the paper fibers during the lateral flow assay. This interaction may significantly reduce the amount of analyte that reaches the detection zone of the microfluidic paper-based analytical device (µPAD), thereby reducing its overall sensitivity. Here, we introduce a novel approach on reducing the nonspecific adsorption of proteins to lab-made paper sheets for the use in µPADs. To this, cotton linter fibers in lab-formed additive-free paper sheets are modified with a surrounding thin hydrogel layer generated from photo-crosslinked, benzophenone functionalized copolymers based on poly-(oligo-ethylene glycol methacrylate) (POEGMA) and poly-dimethyl acrylamide (PDMAA). This, as we show in tests similar to lateral flow assays, significantly reduces unspecific binding of model proteins. Furthermore, by evaporating the transport fluid during the microfluidic run at the end of the paper strip through local heating, model proteins can almost quantitatively be accumulated in that zone. The possibility of complete, almost quantitative protein transport in a µPAD opens up new opportunities to significantly improve the signal-to-noise (S/N) ratio of paper-based lateral flow assays.

A facile aptamer immobilization strategy to fabricate a robust affinity monolith for highly specific in-tube solid-phase microextraction

Developing a functional affinity monolithic column towards in-tube solid-phase microextraction (IT-SPME) for selective sample pretreatment is critical. Herein, a high-performance capillary affinity monolithic column with an ultra-high aptamer coverage density was rapidly fabricated via a simple adsorption strategy, in which aptamers with natural sequences were directly immobilized on an ammonium-based strongly cationic matrix. Limitations of the traditional biological or covalent methods such as time-consuming modification reactions, special requirement of active groups (e.g. -NH₂ and -SH) on the aptamer, and low aptamer coverage density levels were avoided. An ultra-high coverage density of 8616 pmol μL^-1 was achieved with excellent stability, and the highest aptamer-modification level among all the current methods was reached. Selective recognition and high recovery yields of the model mycotoxin ochratoxin A (OTA) were achieved in 95.9 ± 0.98%-97.9 ± 0.28% (n = 3). In particular, there was little cross-reactivity towards the OTB analogue of only 0.5% even in the case of 250 fold of the analogue OTB, which was not reported in previous affinity monoliths. Upon sample analysis, satisfactory discriminations of trace OTA were obtained at 93.7 ± 1.4%-95.5 ± 2.5% (n = 3) in beer and wheat. A facile and direct method for efficiently fabricating an aptamer-based affinity monolith towards online selective IT-SPME was proposed.

Drug Carriers Based on Graphene Oxide and Hydrogel: Opportunities and Challenges in Infection Control Tested by Amoxicillin Release

Graphene oxide (GO) was proposed as an efficient carrier of antibiotics. The model drug, amoxicillin (AMOX), was attached to GO using a peptide linker (Leu-Leu-Gly). GO-AMOX was dispersed in a hydrogel to which the enzyme responsible for releasing AMOX from GO was also added. The drug molecules were released by enzymatic hydrolysis of the peptide bond in the linker. As the selected enzyme, bromelain, a plant enzyme, was used. The antibacterial nature of the carrier was determined by its ability to inhibit the growth of the Enterococcus faecalis strain, which is one of the bacterial species responsible for periodontal and root canal diseases. The prepared carrier contained only biocompatible substances, and the confirmation of its lack of cytotoxicity was verified based on the mouse fibrosarcoma cell line WEHI 164. The proposed type of preparation, as a universal carrier of many different antibiotic molecules, can be considered as a suitable solution in the treatment of inflammation in dentistry.

Knitting Thread Devices: Detecting Candida albicans Using Napkins and Tampons

Reproducible and in situ microbial detection, particularly of microbes significant in urinary tract infections (UTIs) such as Candida albicans, provides a unique opportunity to bring equity in the healthcare outcomes of disenfranchised groups like women in low-resource settings. Here, we demonstrate a system to potentially detect vulvovaginal candidiasis by leveraging the properties of multifilament cotton threads in the form of microfluidic-thread-based analytical devices (μTADs) to develop a frugal microbial identification assay. A facile mercerization method using heptane wash to boost reagent absorption and penetration is also performed and is shown to be robust compared to other existing conventional mercerization methods. Furthermore, the twisted mercerized fibers are drop-cast with media consisting of l-proline β-naphthylamide, which undergoes hydrolysis by the enzyme l-proline aminopeptidase secreted by C. albicans, hence signaling the presence of the pathogen via simple color change with a limit of detection of 0.58 × 106 cfu/mL. The flexible and easily disposable thread-based detection device when integrated with menstrual hygiene products showed a detection time of 10 min using spiked vaginal discharge. The developed method boasts a long shelf life and high stability, making it a discreet detection device for testing, which provides new vistas for self-testing multiple diseases that are considered taboo in certain societies.

Thread integrated smart-phone imaging facilitates early turning point colorimetric assay for microbes

This study employs a commercial multifilament cotton thread as a low-cost microbial identification assay integrated with smartphone-based imaging for high throughput and rapid detection of pathogens. The thread device with inter-twined fibers was drop-cast with test media and a pH indicator. The target pathogens scavenge the media components with different sugars and release acidic by-products, which in turn act as markers for pH-based color change. The developed thread-based proof-of-concept was demonstrated for the visual color detection (red to yellow) of Candida albicans (≈16 hours) and Escherichia coli (≈5 hours). Besides that, using a smart-phone to capture images of the thread-based colorimetric assay facilitates early detection of turning point of the pH-based color change and further reduces the detection time of pathogens viz. Candida albicans (≈10 hours) and Escherichia coli (≈1.5 hours). The reported thread and smartphone integrated image analysis works towards identifying the turning point of the colorimetric change rather than the end-point analysis. Using this approach, the interpretation time can be significantly reduced compared to the existing conventional microbial methods (≈24 hours). The thread-based colorimetric microbial assay represents a ready-to-use, low-cost and straightforward technology with applicability in resource-constrained environments, surpassing the need for frequent fresh media preparation, expensive instrumentation, complex fabrication techniques and expert intervention. The proposed method possesses high scalability and reproducibility, which can be further extended to bio(chemical) assays.

A vinyl sulfone clicked carbon dot-engineered microfluidic paper-based analytical device for fluorometric determination of biothiols

A microfluidic paper-based analytical device integrating carbon dot (CDs) is fabricated and used for a fluorometric off-on assay of biothiols. Vinyl sulfone (VS) click immobilization of carbon dots (CDs) on paper was accomplished by a one-pot simplified protocol that uses divinyl sulfone (DVS) as a homobifunctional reagent. This reagent mediated both the click oxa-Michael addition to the hydroxyl groups of cellulose and ulterior covalent grafting of the resulting VS paper to NH₂-functionalized CDs by means of click aza-Michael addition. The resulting cellulose nanocomposite was used to engineer an inexpensive and robust microfluidic paper-based analytical device (μPAD) that is used for a reaction-based off-on fluorometric assay of biothiols (GSH, Cys, and Hcy). The intrinsic blue fluorescence of CDs (with excitation/emission maxima at 365/450 nm) is turned off via the heavy atom effect of an introduced iodo group. Fluorescence is turned on again due to the displacement of iodine by reaction with a biothiol. The increase in fluorescence is related to the concentration over a wide range (1 to 200 μM for GSH and 5-200 μM for Cys and Hcy, respectively), and the assay exhibits a low detection limit (0.3 μM for GSH and Cys and 0.4 μM for Hcy). The method allows for rapid screening and can also be used in combination with a digital camera readout. Graphical abstract Schematic representation of a μPAD based on click immobilized carbon dots and used for a reaction-based fluorometric off-on assay of biothiols. The intrinsic blue fluorescence of carbon dots is turned off via the heavy atom effect of an introduced iodo group and turned on by the displacement of this atom by reaction with a biothiol.

One-Step Surface Modification to Graft DNA Codes on Paper: The Method, Mechanism, and Its Application

Glass slides have been widely used for DNA immobilization in DNA microarray and numerous bioassays for decades, whereas they are faced with limitations of low probe density, time-consuming modification steps, and expensive instruments. In this work, a simple one-step surface modification method using 3-aminopropyl trimethoxysilane (APTMS) has been developed and applied to graft DNA codes on paper. Higher DNA immobilization efficiency was obtained in comparison with that in a conventional method using glass slides. Fluorescence detection, X-ray photoelectron spectroscopy (XPS), infrared spectra (FT-IR), and pH influence studies were employed to characterize the surface modification and subsequent DNA immobilization, which further reveals a mechanism in which this method lies in ionic interactions between the positively charged APTMS-modified paper surface and negatively charged DNA probes. Furthermore, an APTMS-modified paper-based device has been developed to demonstrate application in low-cost detection of a foodborne pathogen, Giardia lamblia, with high sensitivity (the detection limit of 22 nM) and high specificity. Compared with conventional methods using redundant cross-linking reactions, our method is simpler, faster, versatile, and lower-cost, enabling broad applications of paper-based bioassays especially for point-of-care detection in resource-poor settings.

One-Step Biotinylation of Cellulose Paper by Polymer Coating to Prepare a Paper-Based Analytical Device

Cellulose paper has strong potential as an analytical platform owing to its unique characteristics. In the present study, we investigated a procedure for functionalizing the surface of cellulose paper by dip-coating a mixture of a functional polymer and a perfluoroalkylated surfactant (surfactant 1). The functional polymer comprised a mixture of methyl methacrylate and poly(ethylene glycol) methacrylate monomers. The monomer ratio in the functional polymer affected the hydrophilicity and water absorbance of the cellulose paper after dip-coating. Furthermore, the presence of surfactant 1 during dip-coating promoted the surface segregation of poly(ethylene glycol) (PEG) moieties in the polymer, which enhanced the hydrophilicity, prevented nonspecific protein adsorption, and maintained the water absorbance of the dip-coated cellulose paper. Dip-coating with another functional polymer containing biotin groups produced a cellulose paper with a biotin-decorated surface in a one-step procedure. The displayed biotin groups immobilized avidin on the surface, and the PEG moieties in the polymer prevented nonspecific protein adsorption. We then immobilized a thrombin-binding DNA aptamer on the avidin-immobilized cellulose paper to prepare a paper-based analytical device. It is possible to visualize thrombin in model solutions and serum using the paper-based analytical device.

Filter paper grafted with epoxide-based copolymer brushes for activation-free peptide nucleic acid conjugation and its application for colorimetric DNA detection

Epoxide-bearing filter paper was first prepared by surface-initiated reversible addition-fragmentation chain transfer (RAFT) copolymerization of glycidyl methacrylate (GMA) and poly(ethylene glycol)methacrylate (PEGMA). Without the need for activation step, the capture peptide nucleic acid (PNA) probes carrying a C-terminal lysine modification can be directly immobilized on the surface-grafted poly[glycidyl methacrylate-ran-poly(ethylene glycol)methacrylate] (P(GMA-ran-PEGMA)) through ring-opening of epoxide groups in the GMA repeating units by amino groups in the PNA's structure. The success of P(GMA-ran-PEGMA) grafting on the filter paper and subsequent PNA immobilization was confirmed by fluorescence microscopy, Fourier transform-infrared spectroscopy and X-ray photoelectron spectroscopy. Colorimetric detection with signal amplification upon DNA hybridization relies on sandwich-hybridization assay employing another biotinylated PNA strand as a reporter probe together with streptavidin-horseradish peroxidase conjugate (SA-HRP) and o-phenylenediamine (OPD) substrate. It was found that increasing ionic strength during the DNA hybridization step by addition of NaCl can increase the signal intensity, which can be visualized by naked eye. The sensing platform showed the best performance in preventing non-specific adsorption from the non-complementary DNA and discriminating between complementary and single-mismatched targets of at least 50 fmol without the requirement for stringent hybridization or washing condition. This superior ability to suppress non-specific adsorption of non-target DNA as well as other non-DNA components may be explained as a result of hydrophilic PEGMA repeating units in the surface-grafted copolymer.

Paper-based diagnostics in the antigen-depletion regime: High-density immobilization of rcSso7d-cellulose-binding domain fusion proteins for efficient target capture

In this work, we report the development of a general strategy for enhancing the efficiency of target capture in immunoassays, using a bifunctional fusion protein construct which incorporates a substrate-anchoring moiety for the high-abundance immobilization of an antigen-binding domain. This approach was informed by the development of a pseudo first-order rate constant model, and tested in a paper-based assay format using a fusion construct consisting of an rcSso7d binding module and a cellulose-binding domain. These rcSso7d-CBD fusion proteins were solubly expressed and purified from bacteria in high molar yields, and enable oriented, high-density adsorption of the rcSso7d binding species to unmodified cellulose within a 30-second incubation period. These findings were validated using two distinct, antigen-specific rcSso7d variants, which were isolated from a yeast surface display library via flow cytometry. Up to 1.6 micromoles of rcSso7d-CBD was found to adsorb per gram of cellulose, yielding a volume-averaged binder concentration of up to 760μM within the resulting active material. At this molar abundance, the target antigen is captured from solution with nearly 100% efficiency, maximizing the attainable sensitivity for any given diagnostic system.

Enzymatic amplification of oligonucleotides in paper substrates

Several solution-based methods have recently been adapted for use in paper substrates for enzymatic amplification to increase the number of copies of DNA sequences. There is limited information available about the impact of a paper matrix on DNA amplification by enzymatic processes, and about how to optimize conditions to maximize yields. The work reported herein provides insights about the impact of physicochemical properties of a paper matrix, using nuclease-assisted amplification by exonuclease III and nicking endonuclease Nt. Bbv, and a quantum dot (QD) - based Forster Resonance Energy Transfer (FRET) assay to monitor the extent of amplification. The influence of several properties of paper on amplification efficiency and kinetics were investigated, such as surface adsorption of reactants, and pore size. Additional factors that impact amplification processes such as target length and the packing density of oligonucleotide probes on the nanoparticle surfaces were also studied. The work provides guidance for development of more efficient enzymatic target-recycling DNA amplification methods in paper substrates.

Antibody Engineering for Pursuing a Healthier Future

Since the development of antibody-production techniques, a number of immunoglobulins have been developed on a large scale using conventional methods. Hybridoma technology opened a new horizon in the production of antibodies against target antigens of infectious pathogens, malignant diseases including autoimmune disorders, and numerous potent toxins. However, these clinical humanized or chimeric murine antibodies have several limitations and complexities. Therefore, to overcome these difficulties, recent advances in genetic engineering techniques and phage display technique have allowed the production of highly specific recombinant antibodies. These engineered antibodies have been constructed in the hunt for novel therapeutic drugs equipped with enhanced immunoprotective abilities, such as engaging immune effector functions, effective development of fusion proteins, efficient tumor and tissue penetration, and high-affinity antibodies directed against conserved targets. Advanced antibody engineering techniques have extensive applications in the fields of immunology, biotechnology, diagnostics, and therapeutic medicines. However, there is limited knowledge regarding dynamic antibody development approaches. Therefore, this review extends beyond our understanding of conventional polyclonal and monoclonal antibodies. Furthermore, recent advances in antibody engineering techniques together with antibody fragments, display technologies, immunomodulation, and broad applications of antibodies are discussed to enhance innovative antibody production in pursuit of a healthier future for humans.

Covalent Binding of Antibodies to Cellulose Paper Discs and Their Applications in Naked-eye Colorimetric Immunoassays

This report presents two methods for the covalent immobilization of capture antibodies on cellulose filter paper grade No. 1 (medium-flow filter paper) discs and grade No. 113 (fast-flow filter paper) discs. These cellulose paper discs were grafted with amine functional groups through a silane coupling technique before the antibodies were immobilized on them. Periodate oxidation and glutaraldehyde cross-linking methods were used to graft capture antibodies on the cellulose paper discs. In order to ensure the maximum binding capacity of the capture antibodies to their targets after immobilization, the effects of various concentrations of sodium periodate, glutaraldehyde, and capture antibodies on the surface of the paper discs were investigated. The antibodies that were coated on the amine-functionalized cellulose paper discs through a glutaraldehyde cross-linking agent showed enhanced binding activity to the target when compared to the periodate oxidation method. IgG (in mouse reference serum) was used as a reference target in this study to test the application of covalently immobilized antibodies through glutaraldehyde. A new paper-based, enzyme-linked immunosorbent assay (ELISA) was successfully developed and validated for the detection of IgG. This method does not require equipment, and it can detect 100 ng/ml of IgG. The fast-flow filter paper was more sensitive than the medium-flow filter paper. The incubation period of this assay was short and required small sample volumes. This naked-eye, colorimetric immunoassay can be extended to detect other targets that are identified with conventional ELISA.

Amino acid-based anti-fouling functionalization of silica nanoparticles using divinyl sulfone

Natural amino acids are zwitterionic molecules and the good biocompatibility promises them potential candidates as anti-fouling materials. Here, we developed a new method to functionalize silica nanoparticles with a natural amino acid-based anti-fouling layer. Amino acids were covalently immobilized on 3-aminopropyltriethoxysilane modified silica nanoparticles using divinyl sulfone through a two-step reaction in aqueous solution at room temperature. The progress was monitored with NMR, X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM) and zeta potential measurements. A library of amino acids was screened and the nonspecific protein adsorption of bovine serum albumin (BSA) and fetal bovine serum (FBS) were investigated using dynamic light scattering method. The results showed that cysteine, lysine and arginine functionalized silica nanoparticles can effectively resist protein adsorption due to the zwitterionic structure. Among them, lysine functionalized silica nanoparticles had the best anti-fouling performance, which showed hydrodynamic diameter increases of only 10% after incubated in BSA solution and 20% after incubated in FBS solution for 24h. The neat aqueous modification process can conveniently create a thin zwitterionic layer on silica particles, and it has a great potential in biomolecule immobilization and biofunctional surface preparation.

STATEMENT OF SIGNIFICANCE

Zwitterionic polymer is an outstanding class of anti-fouling material; but the difficulty in synthesis is challenging its spread utilization. In this study, we developed a new method to create an amino acid-based zwitterionic layer on APTES functionalized silica nanoparticles through a two-step reaction in aqueous solution at room temperature. The surface chemistry was monitored with NMR, XPS, TEM and zeta potential measurements. With this method, a library of amino acid conjugated-silica nanoparticles was synthesized and their anti-fouling performance was evaluated using dynamic light scattering method. The results showed that the cysteine, lysine and arginine conjugated nanoparticles all can effectively resist nonspecific protein adsorption. Among them, lysine conjugated nanoparticles show the best anti-fouling performance, which showed hydrodynamic diameter increases of only 10% after incubated in BSA solution and 20% after incubated in FBS solution for 24 hours. These results indicates that the anti-fouling silica nanoparticles are of great potential in many biomedical applications, especially biosensing and diagnose imaging. The modification reactions in aqueous solution at room temperature are easily conducted in laboratory, indicating high potential in the functionalization of silica particles/surfaces with other biomolecules.

Electrical characterization of DNA supported on nitrocellulose membranes

Integrated DNA-based nanoscale electronic devices will enable the continued realization of Moore’s Law at the level of functional devices and systems. In this work, the electrical characterization of single and complementary base paired DNA has been directly measured and investigated via the use of nitrocellulose membranes. A radio frequency DAKS-3.5 was used to measure the reflection coefficients of different DNA solutions dotted onto nitrocellulose membranes. Each DNA solution was exposed to a radio frequency signal with a power of 10 dBm and with a sweep from 200 MHz up to 13.6 GHz. The conducted measurements show some distinctions between the homomeric and complementary bases due to their different electrical polarization. As revealed from the measurements conducted, with the addition of DNA oligonucleotides, the measured capacitance increased when compared with buffer medium alone. The DNA molecules could be modeled as dielectric material that can hold electrical charges. Furthermore, the complementary paired DNA molecule-based inks solutions had a higher capacitance value compared with single DNA molecules (A, C, G and T) solutions.

Cellulose-Based Biosensors for Esterase Detection

Cellulose has emerged as an attractive substrate for the production of economical, disposable, point-of-care (POC) analytical devices. Development of novel methods of (bio)activation is central to broadening the application space of cellulosic materials. Ironically, such efforts are stymied by the inherent biocompatibility and recalcitrance of cellulose fibers. Here, we have elaborated a versatile, chemo-enzymatic approach to activate cellulosic materials for CuAAC "click chemistry", to develop new fluorogenic esterase sensors. Gentle, aqueous modification conditions facilitate broad applicability to cellulose papers, gauzes, and hydrogels. Tethering of the released fluorophore to the cellulose surface prevents signal degradation due to diffusion and enables straightforward, sensitive visualization with a simple light source in resource-limited situations.

Writing and erasing hidden optical information on covalently modified cellulose paper

A strategy for preparing photoresponsive cellulose paper enabling the storage of short-lived optical data by covalent means is disclosed.

Low-cost bioanalysis on paper-based and its hybrid microfluidic platforms

Low-cost assays have broad applications ranging from human health diagnostics and food safety inspection to environmental analysis. Hence, low-cost assays are especially attractive for rural areas and developing countries, where financial resources are limited. Recently, paper-based microfluidic devices have emerged as a low-cost platform which greatly accelerates the point of care (POC) analysis in low-resource settings. This paper reviews recent advances of low-cost bioanalysis on paper-based microfluidic platforms, including fully paper-based and paper hybrid microfluidic platforms. In this review paper, we first summarized the fabrication techniques of fully paper-based microfluidic platforms, followed with their applications in human health diagnostics and food safety analysis. Then we highlighted paper hybrid microfluidic platforms and their applications, because hybrid platforms could draw benefits from multiple device substrates. Finally, we discussed the current limitations and perspective trends of paper-based microfluidic platforms for low-cost assays.

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.

Paper-based inkjet-printed microfluidic analytical devices

Rapid, precise, and reproducible deposition of a broad variety of functional materials, including analytical assay reagents and biomolecules, has made inkjet printing an effective tool for the fabrication of microanalytical devices. A ubiquitous office device as simple as a standard desktop printer with its multiple ink cartridges can be used for this purpose. This Review discusses the combination of inkjet printing technology with paper as a printing substrate for the fabrication of microfluidic paper-based analytical devices (μPADs), which have developed into a fast-growing new field in analytical chemistry. After introducing the fundamentals of μPADs and inkjet printing, it touches on topics such as the microfluidic patterning of paper, tailored arrangement of materials, and functionalities achievable exclusively by the inkjet deposition of analytical assay components, before concluding with an outlook on future perspectives.

Patterned paper sensors printed with long-chain DNA aptamers

There is growing interest in developing printable paper sensors to enable rapid testing of analytes for environmental, food safety, and clinical applications. A major challenge is to find suitable bioinks that are amenable to high-speed printing and remain functional after printing. We report on a simple and effective approach wherein an aqueous ink composed of megadalton-sized tandem repeating structure-switching DNA aptamers (concatemeric aptamers) is used to rapidly create patterned paper sensors on filter paper by inkjet printing. These concatemeric aptamer reporters remain immobilized at the point of printing through strong adsorption but retain sufficient segmental mobility to undergo structure switching and fluorescence signaling to provide both qualitative and quantitative detection of small molecules and protein targets. The convenience of inkjet printing allows for the patterning of internally referenced sensors with multiplexed detection, and provides a generic platform for on-demand printing of sensors even in remote locations.

Reactivity and kinetics of vinyl sulfone-functionalized self-assembled monolayers for bioactive ligand immobilization

A new vinyl sulfone (VS) disulfide, 1,2-bis(11-(vinyl sulfonyl)undecyl)disulfane, was synthesized to enable the preparation of VS-presenting self-assembled monolayers (VS SAMs) on Au substrates. The VS SAMs were used as a model system to assess the reaction kinetics of bioactive ligands, i.e., glutathione (GSH), N-(5-amino-1-carboxypentyl)iminodiacetic acid (ab-NTA), and mannose, toward the VS groups on the SAM surface. The VS SAMs and the ligand immobilization were characterized by X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and protein-binding experiments using a quartz crystal microbalance (QCM). Kinetic studies showed that the surface VS groups undergo pseudo-first-order reactions with various ligands, with the observed rate constant being 0.057 min(-1) for GSH at pH 7.5, 0.011 min(-1) for ab-NTA at pH 8.5, and 0.009 min(-1) for mannose at pH 10.5. This work advanced our understanding of the reactivity of VS-bearing functional surfaces and further demonstrated the versatile potential of VS chemistry to prepare ligand-immobilized bioactive surfaces.

Antibody-ligand interactions for hydrophobic charge-induction chromatography: a surface plasmon resonance study

This article describes the use of surface plasmon resonance (SPR) spectroscopy to study antibody-ligand interactions for hydrophobic charge-induction chromatography (HCIC) and its versatility in investigating the surface and solution factors affecting the interactions. Two density model surfaces presenting the HCIC ligand (mercapto-ethyl-pyridine, MEP) were prepared on Au using a self-assembly technique. The surface chemistry and structure, ionization, and protein binding of such model surfaces were characterized by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), contact-angle titration, and SPR, respectively. The influences of the surface and solution factors, e.g., ligand density, salt concentration, and solution pH, on protein adsorption were determined by SPR. Our results showed that ligand density affects both equilibrium and dynamic aspects of the interactions. Specifically, a dense ligand leads to an increase in binding strength, rapid adsorption, slow desorption, and low specificity. In addition, both hydrophobic interactions and hydrogen bonding contribute significantly to the protein adsorption at neutral pH, while the electrostatic repulsion is overwhelmed under acidic conditions. The hydrophobic interaction at a high concentration of lyotropic salt would cause drastic conformational changes in the adsorbed protein. Combined with the self-assembly technique, SPR proves to be a powerful tool for studying the interactions between an antibody and a chromatographic ligand.

A low cost design and fabrication method for developing a leak proof paper based microfluidic device with customized test zone

This article describes a fabrication process for the generation of a leak proof paper based microfluidic device and a new design strategy for convenient incorporation of externally prepared test zones. Briefly, a negative photolithographic method was used to prepare the device with a partial photoresist layer on the rear of the device to block the leakage of sample. Microscopy and Field Emission Scanning Electron Microscopy data validated the formation of the photoresist layer. The partial layer of photoresist on the device channel limits sample volume to 7 ± 0.2 μl as compared to devices without the partial photoresist layer which requires a larger sample volume of 10 ± 0.1 μl. The design prototype with a customized external test zone exploits the channel protrusions on the UV exposed photoresist treated paper to bridge the externally applied test zone to the sample and absorbent zones. The partially laminated device with an external test zone has a comparatively low wicking speed of 1.8 ± 0.9 mm/min compared to the completely laminated device with an inbuilt test zone (3.3 ± 1.2 mm/min) which extends the reaction time between the analyte and reagents. The efficacy of the prepared device was studied with colorimetric assays for the non-specific detection of protein by tetrabromophenol blue, acid/base with phenolphthalein indicator, and specific detection of proteins using the HRP-DAB chemistry. The prepared device has the potential for leak proof detection of analyte, requires low sample volume, involves reduced cost of production (∼$0.03, excluding reagent and lamination cost), and enables the integration of customized test zones.

Pyrrolidinyl peptide nucleic acids immobilised on cellulose paper as a DNA sensor

“Immobilisation of pyrrolidinyl peptide nucleic acids on paper resulted in a new DNA sensor with great specificity”.

Scalable low-cost fabrication of disposable paper sensors for DNA detection

Controlled integration of features that enhance the analytical performance of a sensor chip is a challenging task in the development of paper sensors. A critical issue in the fabrication of low-cost biosensor chips is the activation of the device surface in a reliable and controllable manner compatible with large-scale production. Here, we report stable, well-adherent, and repeatable site-selective deposition of bioreactive amine functionalities and biorepellant polyethylene glycol-like (PEG) functionalities on paper sensors by aerosol-assisted, atmospheric-pressure, plasma-enhanced chemical vapor deposition. This approach requires only 20 s of deposition time, compared to previous reports on cellulose functionalization, which takes hours. A detailed analysis of the near-edge X-ray absorption fine structure (NEXAFS) and its sensitivity to the local electronic structure of the carbon and nitrogen functionalities. σ*, π*, and Rydberg transitions in C and N K-edges are presented. Application of the plasma-processed paper sensors in DNA detection is also demonstrated.

Wire, mesh, and fiber electrodes for paper-based electroanalytical devices

Here, we report the use of microwire and mesh working electrodes in paper analytical devices fabricated by origami paper folding (oPADs). The important new result is that Au wires and carbon fibers having diameters ranging from micrometers to tens of micrometers can be incorporated into oPADs and that their electrochemical characteristics are consistent with the results of finite element simulations. These electrodes are fully compatible with both hollow channels and paper channels filled with cellulose fibers, and they are easier to incorporate than typical screen-printed carbon electrodes. The results also demonstrate that the Au electrodes can be cleaned prior to device fabrication using aggressive treatments and that they can be easily surface modified using standard thiol-based chemistry.

Cellulose: from biocompatible to bioactive material

The immobilization of biomolecules onto cellulose paper turns this environmentally friendly material into a platform for diagnostic devices.

DNA detection using origami paper analytical devices

We demonstrate the hybridization-induced fluorescence detection of DNA on an origami-based paper analytical device (oPAD). The paper substrate was patterned by wax printing and controlled heating to construct hydrophilic channels and hydrophobic barriers in a three-dimensional fashion. A competitive assay was developed where the analyte, a single-stranded DNA (ssDNA), and a quencher-labeled ssDNA competed for hybridization with a fluorophore-labeled ssDNA probe. Upon hybridization of the analyte with the fluorophore-labeled ssDNA, a linear response of fluorescence vs analyte concentration was observed with an extrapolated limit of detection <5 nM and a sensitivity relative standard deviation as low as 3%. The oPAD setup was also tested against OR/AND logic gates, proving to be successful in both detection systems.

Identifying human milk glycans that inhibit norovirus binding using surface plasmon resonance

Human milk glycans inhibit binding between norovirus and its host glycan receptor; such competitive inhibition by human milk glycans is associated with a reduced risk of infection. The relationship between the presence of specific structural motifs in the human milk glycan and its ability to inhibit binding by specific norovirus strains requires facile, accurate and miniaturized-binding assays. Toward this end, a high-throughput biosensor platform was developed based on surface plasmon resonance imaging (SPRi) of glycan microarrays. The SPRi was validated, and its utility was tested, by measuring binding specificities between defined human milk glycan epitopes and the capsids of two common norovirus strains, VA387 and Norwalk. Human milk oligosaccharide (HMOS)-based neoglycoconjugates, including chemically derived neoglycoproteins and oligosaccharide-glycine derivatives, were used to represent polyvalent glycoconjugates and monovalent oligosaccharides, respectively, in human milk. SPRi binding results established that the glycan motifs that bind norovirus capsids depend upon strain; VA387 capsid interacts with two neoglycoproteins, whereas Norwalk capsid binds to a different set of HMOS motifs in the form of both polyvalent neoglycoproteins and monovalent oligosaccharides. SPRi competitive binding assays further demonstrated that specific norovirus-binding glycans are able to inhibit norovirus capsid binding to their host receptors. A polyvalent neoglycoconjugate with clustered carbohydrate moieties is required for the inhibition of VA387 capsid binding to host receptor glycans, whereas both monovalent oligosaccharides and polyvalent neoglycoconjugates are able to inhibit Norwalk capsid binding to its host receptor. Binding of HMOS and HMOS-based neoglycoconjugates to norovirus capsids depends upon the specific strain characteristics, implying that HMOS and their polyvalent derivatives are potential anti-adhesive agents for norovirus prophylaxis.

Controlled hydrophobic functionalization of natural fibers through self-assembly of amphiphilic diblock copolymer micelles

The functionalization of natural fibers is an important task that has recently received considerable attention. We investigated the formation of a hydrophobic layer from amphiphilic diblock copolymer micelles [polystyrene-block-poly(N-methyl-4-vinyl pyridinium iodide)] on natural fibers and on a model surface (mica). A series of micelles were prepared. The micelles were characterized by using cryoscopic TEM and light scattering, and their hydrophobization capability was studied through contact angle measurements, water adsorption, and Raman imaging. Mild heat treatment (130 °C) was used to increase the hydrophobization capability of the micelles. The results showed that the micelles could not hydrophobize a model surface, but could render the natural fibers water repellent both with and without heat treatment. This effect was systematically studied by varying the composition of the constituent blocks. The results showed that the micelle size (and the molecular weight of the constituent diblock copolymers) was the most important parameter, whereas the cationic (hydrophilic) part played only a minor role. We hypothesized that the hydrophobization effect could be attributed to a combination of the micelle size and the shrinkage of the natural fibers upon drying. The shrinking caused the roughness to increase on the fiber surface, which resulted in a rearrangement of the self- assembled layer in the wet state. Consequently, the fibers became hydrophobic through the roughness effects at multiple length scales. Mild heat treatment melted the micelle core and decreased the minimum size necessary for hydrophobization.

Simple, distance-based measurement for paper analytical devices

Paper-based analytical devices (PADs) represent a growing class of elegant, yet inexpensive chemical sensor technologies designed for point-of-use applications. Most PADs, however, still utilize some form of instrumentation such as a camera for quantitative detection. We describe here a simple technique to render PAD measurements more quantitative and straightforward using the distance of colour development as a detection motif. The so-called distance-based detection enables PAD chemistries that are more portable and less resource intensive compared to classical approaches that rely on the use of peripheral equipment for quantitative measurement. We demonstrate the utility and broad applicability of this technique with measurements of glucose, nickel, and glutathione using three different detection chemistries: enzymatic reactions, metal complexation, and nanoparticle aggregation, respectively. The results show excellent quantitative agreement with certified standards in complex sample matrices. This work provides the first demonstration of distance-based PAD detection with broad application as a class of new, inexpensive sensor technologies designed for point-of-use applications.