Paper-based solid-phase multiplexed nucleic acid hybridization assay with tunable dynamic range using immobilized quantum dots as donors in fluorescence resonance energy transfer

A facile and high-sensitive bio-sensing of the V617F mutation in JAK2 gene by GSH-CdTe-QDs FRET-based sensor

This study aimed to directly detect the V617F point mutation of the Janus kinase 2 (JAK2) gene in the target DNA using a FRET-based biosensor. The water-soluble GSH-CdTe-QDs were synthesized by a one-step process, then GSH-QD conjugated to the termini amino-modified oligonucleotides target via carboxylic groups on the QD surface. The prepared QDs-DNA biosensor was applied in the quantitative and rapid detection of V617F mutation with a detection limit of 3 × 10^-9 mol L^-1 based on the FRET mechanism. In other words, detecting the V617F mutation by bio-sensing technology would be much simpler, cheaper, time-saving, highly sensitive, and more convenient than molecular diagnostic tools. Furthermore, the nano-biosensor was applied to detect the V617F mutation in clinical samples compared to the common ARMS-PCR (Amplification Refractory Mutation System-Polymerase Chain Reaction) standard method. The results revealed that the GSH-capped biosensors would be effective for V617F mutation detection in samples distinguished with satisfactory analytical outcomes. Therefore, the designed fluorescence nanoprobe is suitable for the specific detection of V617F mutation of the JAK2 gene in clinical samples.

Overcoming Major Barriers to Developing Successful Sensors for Practical Applications Using Functional Nucleic Acids

For many years, numerous efforts have been focused on the development of sensitive, selective, and practical sensors for environmental monitoring, food safety, and medical diagnostic applications. However, the transition from innovative research to commercial success is relatively sparse. In this review, we identify four scientific barriers and one technical barrier to developing successful sensors for practical applications, including the lack of general methods to (a) generate receptors for a wide range of targets, (b) improve sensor selectivity to overcome interferences, (c) transduce the selective binding to different optical, electrochemical, and other signals, and (d) tune dynamic range to match thresholds of detection required for different targets; and the costly development of a new device. We then summarize solutions to overcome these barriers using sensors based on functional nucleic acids that include DNAzymes, aptamers, and aptazymes and how these sensors are coupled to widely available measurement devices to expand their capabilities and lower the barrier for their practical applications in the field and point-of-care settings.

Adsorption-desorption nano-aptasensors: fluorescent screening assays for ochratoxin A

In this study, a FRET-based fluorescent aptasensor for the detection of ochratoxin A (OTA) was optimized based on the quenching efficiency of single-walled carbon nanotubes (SWCNTs) and the binding affinity of aptamers. OTA aptamers were conjugated with quantum dots and adsorbed to the surface of both acid-modified and unmodified SWCNTs. The maximum fluorescence quenching efficiency of the SWCNTs were compared. Acid-modified SWCNTs (amSWCNTs) have moderate quenching efficiency, providing an optimal sensitivity for qualitative fluorescence-enhancement biosensor assays. The binding parameters of the QD-modified OTA aptamers (1.12.2 and A08min) on the surface of amSWCNTs were compared. Based on our results, the A08min aptamer is a better candidate for OTA detection. Using the A08min aptamer, the SWCNT method had a limit of detection (LOD) of 40 nM. The amSWCNT method had a significantly lower LOD of 14 nM. Turn-on fluorescent nano-aptasensors are emerging as an effective diagnostic tool for simple detection of mycotoxins. Nanocomplexes designed for the detection of mycotoxins in solution and paper-based tests have proven to be useful.

Fluorescent paper-based analytical devices for ultra-sensitive dual-type RNA detections and accurate gastric cancer screening

Demand on the quick screening of gastric cancer (GC) has significantly stimulated the development of biomarker sensing techniques. Herein, we report the novel fluorescent paper-based analytical devices (PADs) for detections of GC-related microRNA-21 (miRNA-21) and circular RNA from Hippocampus Abundant Transcript 1 gene (circRNA-HIAT1) with prominent reliability and sensitivity. The PADs, constructed by in-situ synthesis of blue-emissive carbon dots (CDs) and conjugations of probe DNAs, exhibit the superior uniformity and stability. In the presence of targets, rolling circle amplifications (RCA) are triggered to generate long DNA strands for the assemblies of green-/red-emissive labels. Consequently, remarkable blue-to-green and blue-to-red emission color transitions of the PADs are achieved, implementing the color-analysis of miRNA-21 and circRNA-HIAT1, respectively. Benefited from the efficient RCA, coupled with the drastic ratiometric fluorescent changes, the limit of detections (LODs) of PADs are found to be several fM with the upper limit of the linear detection range at 1 nM. More importantly, the fluorescent PADs possess excellent specificity, as well as anti-interference capability in biological settings, enabling their applications in accurate GC screening with plasma samples. Overall, the proposed fluorescent PADs are featured with robust sensing platform, facile signal readout, and exceptional dual-type RNA sensing performance, holding high potential in point-of-care testing (POCT).

Tunable Dual-Effector Allostery System for Nucleic Acid Analysis with Enhanced Sensitivity and an Extended Dynamic Range

In the last few years, studies have demonstrated the existence of dual-effector allosteric cooperativity in nature and the mechanism underlying enhanced activation/inhibition performance. In this work, we design an artificial dual-effector allostery system for the construction of a dynamic biosensor that can achieve nucleic acid detection with superior sensitivity and across an extraordinary broad detection range. Our dual-effector allostery-regulated biosensor is based on the multibranched hybridization chain reaction (mHCR) involving three hairpins (H1, H2, and H3). In the presence of the target nucleic acid, the mHCR is initiated via cascading strand displacement events. The products of mHCR are then captured on the electrode surface based on the mechanism of the multivalent proximity ligation assay (mPLA) and the multivalent binding assay (mBA). The subsequent conjugation of streptavidin-modified horseradish peroxidase (SA-HRP) can lead to an increase in the electrochemical signal. Importantly, two distinct allosteric activation sites and two distinct allosteric inhibition sites in H1 are designed to fine-tune the nucleic acid detection sensitivity and the dynamic range. Using this new dual-effector allostery tool, we report the detection of nucleic acid at a dynamic range spanning 10-10¹² aM, 11 orders of magnitude showing the broadest dynamic range reported to date with an allosteric regulation biosensor construct.

Paper-Based Microfluidic Sensors for Onsite Environmental Detection: A Critical Review

A newly developed research topic, fabricated paper-based microfluidic sensors, was discussed in the field of low-cost environmental detection. Distinguished with the traditional dipstick or lateral-flow setups, these paper-based microfluidic sensors can serve as a tool for onsite quantitative and semi-quantitative measurements, without risks to cause environmental pollution. They have attracted increasing interest since the first easy-fabricated paper-based setup reported by Whitesides group in 2007. Most of the publications utilized paper-based sensors in clinical detection. In recent years, some groups started to use these sensors in environmental measurement, leading to precise, easy operation, low-cost, and eco-friendly methods for onsite detection. In this review, paper-based microfluidic sensors were briefly introduced, followed by literatures review and discussion for future perspectives.

Resonance Energy Transfer-Based Biosensors for Point-of-Need Diagnosis-Progress and Perspectives

The demand for point-of-need (PON) diagnostics for clinical and other applications is continuing to grow. Much of this demand is currently serviced by biosensors, which combine a bioanalytical sensing element with a transducing device that reports results to the user. Ideally, such devices are easy to use and do not require special skills of the end user. Application-dependent, PON devices may need to be capable of measuring low levels of analytes very rapidly, and it is often helpful if they are also portable. To date, only two transduction modalities, colorimetric lateral flow immunoassays (LFIs) and electrochemical assays, fully meet these requirements and have been widely adopted at the point-of-need. These modalities are either non-quantitative (LFIs) or highly analyte-specific (electrochemical glucose meters), therefore requiring considerable modification if they are to be co-opted for measuring other biomarkers. Förster Resonance Energy Transfer (RET)-based biosensors incorporate a quantitative and highly versatile transduction modality that has been extensively used in biomedical research laboratories. RET-biosensors have not yet been applied at the point-of-need despite its advantages over other established techniques. In this review, we explore and discuss recent developments in the translation of RET-biosensors for PON diagnoses, including their potential benefits and drawbacks.

A novel trimodal system on a paper-based microfluidic device for on-site detection of the date rape drug "ketamine"

Paper-based microfluidic device was designed with wax-printing to combine potentiometric, fluorimetric and colorimetric detection zones. This newly developed trimodal paper chip has been used for on-site determination of ketamine hydrochloride (KET) as a date rape drug in beverages. The device employed polyaniline nano-dispersion as conducting polymer in ion sensing paper electrodes designed to fit USB plug connector. Carbon dots-gold nanoparticles and cobalt thiocyanate were used in fluorescence and color detection zones, respectively. Cellular phone's camera facilitated the on-site fluorimetric and color detection. The implemented trimodal detection system exhibited specificity for KET detection in the presence of several other beverage interferences i.e., biogenic amines. This innovative sensor brings together analytical figures of merit for effective KET detection in single aliquot of spiked beverages. The proposed paper-based chip also fulfils WHO criteria for point-of-care devices posing the proposed trimodal paper device as an active part for rapid, on-site drug diagnostics and to be applied further for other similar drugs.

Intelligent Platform for Simultaneous Detection of Multiple Aminoglycosides Based on a Ratiometric Paper-Based Device with Digital Fluorescence Detector Readout

A digital fluorescence detector (DFD), a handheld fluorescence detection device, can convert the fluorescence signal of samples into the corresponding fluorescer concentration. Herein, by adopting a DFD as the readout, a novel intelligent platform was developed based on a ratiometric paper-based device (RPD) for multiple aminoglycoside detection. There are five layers and four parallel channels contained in the designed RPD, functioning as reagent storage, fluidic path control and signal processing, respectively. The rationale of this design lies in the fact that aptamer/graphitic carbon nitride nanosheet (Apt/g-C₃N₄ NS) modified layers can catalyze o-phenylenediamine to fluorescent 2,3-diaminophenazine (DAP) in the presence of H₂O₂. When Apt was removed from nanosheets via the Apt-target reaction, the peroxidase-like activity would be decreased, thus decreasing the production of DAP. All the changes of the fluorescence DAP signal can be read out using a portable DFD. Based on the DFD signal change related to the concentration of the target, a quantitative reaction platform was established. Furthermore, the sample flow and Apt-target reaction time can be reasonably regulated using the H₂O₂-cleavable hydrophobic compound modified layer placed between the target recognition region and detection region. Then, the practicality of this platform was verified through realizing sensitive analysis of streptomycin, tobramycin, and kanamycin simultaneously. Overall, with merits including portability and ease of operation, the platform shows great potential in on-site simultaneous detection of multiple targets, especially in resource-limited settings.

Rapid Discovery and Structure-Property Relationships of Metal-Ion Fluorescent Sensors via Macroarray Synthesis

A macroarray immobilisation of fluorophores on filter papers for sensing metal ions by in-situ reductive amination and carbodiimide coupling is reported herein. Chemometric approaches resulted in a rapid discovery of sensors that can synergistically discriminate up to 12 metal ions with great prediction accuracies. Covalently bound on paper, sensoring scaffolds that were synthesised from the macroarray format can readily be adopted as practical paper-based sensors with great reusability and sensitivity, achieving the limit of detection at low nanomolar level with some repeating spotting. Lastly, the discovered scaffolds were also confirmed to be functional as unbound molecules, thus paving the way for more diverse applications.

Advances in DNA/RNA detection using nanotechnology

Specific nucleic acid detection in vitro or in vivo has become increasingly important in the discovery of genetic diseases, diagnosing pathogen infection and monitoring disease treatment. One challenge, however, is that the amount of target nucleic acid in specimens is limited. Furthermore, direct sensing methods are also unable to provide sufficient sensitivity and specificity. Fortunately, due to advances in nanotechnology and nanomaterials, nanotechnology-based bioassays have emerged as powerful and promising approaches providing ultra-high sensitivity and specificity in nucleic acid detection. This chapter presents an overview of strategies used in the development and integration of nanotechnology for nucleic acid detection, including optical and electrical detection methods, and nucleic acid assistant recycling amplification strategies. Recent 5 years representative examples are reviewed to demonstrate the proof-of-concept with promising applications for DNA/RNA detection and the underlying mechanism for detection of DNA/RNA with the higher sensitivity and selectivity. Furthermore, a brief discussion of common unresolved issues and future trends in this field is provided both from fundamental and practical point of view.

Nanoparticle-based Point of Care Immunoassays for in vitro Biomedical Diagnostics

In resource-limited settings, the availability of medical practitioners and early diagnostic facilities are inadequate relative to the population size and disease burden. To address cost and delayed time issues in diagnostics, strip-based immunoassays, e.g. dipstick, lateral flow assay (LFA) and microfluidic paper-based analytical devices (microPADs), have emerged as promising alternatives to conventional diagnostic approaches. These assays rely on chromogenic agents to detect disease biomarkers. However, limited specificity and sensitivity have motivated scientists to improve the efficiency of these assays by conjugating chromogenic agents with nanoparticles for enhanced qualitative and quantitative output. Various nanomaterials, which include metallic, magnetic and luminescent nanoparticles, are being used in the fabrication of biosensors to detect and quantify biomolecules and disease biomarkers. This review discusses some of the principles and applications of such nanoparticle-based point of care biosensors in biomedical diagnosis.

Detection of a cancer biomarker protein on modified cellulose paper by fluorescence using aptamer-linked quantum dots

The development of point-of-care bioassays for sensitive screening of protein-based cancer biomarkers would improve the opportunity for early stage diagnosis. A strategy for a fluorescence resonance energy transfer (FRET)-based bioassay has been investigated that makes use of modified cellulose paper for the detection of an epithelial cell adhesion molecule (EpCAM), which is a transmembrane glycoprotein that is overexpressed in several tumors of epithelial origin. The paper matrix was a substrate for immobilized aptamer-linked quantum dots (QDs-Apt) and Cy3 labeled complementary DNA (cDNA), which served as a donor and an acceptor, respectively. Competitive binding of EpCAM displaced the cDNA, resulting in the reduction of FRET. The paper-based bioassay was able to detect EpCAM in buffer solution as well as in 10% bovine serum solution using a reaction time of no more than 60 minutes. The dynamic range was 1-100 nM in buffer with a precision better than 4%, and the limit of detection was 250 pM in buffer and 600 pM in 10% serum.

Paper-based biodetection using luminescent nanoparticles

Point-of-care and in-field technologies for rapid, sensitive and selective detection of molecular biomarkers have attracted much interest. Rugged bioassay technology capable of fast detection of markers for pathogens and genetic diseases would in particular impact the quality of health care in the developing world, but would also make possible more extensive screening in developed countries to tackle problems such as those associated with water and food quality, and tracking of infectious organisms in hospitals and clinics. Literature trends indicate an increasing interest in the use of nanomaterials, and in particular luminescent nanoparticles, for assay development. These materials may offer attributes for development of assays and sensors that could achieve improvements in analytical figures of merit, and provide practical advantages in sensitivity and stability. There is opportunity for cost-efficiency and technical simplicity by implementation of luminescent nanomaterials as the basis for transduction technology, when combined with the use of paper substrates, and the ubiquitous availability of cell phone cameras and associated infrastructure for optical detection and transmission of results. Luminescent nanoparticles have been described for a broad range of bioanalytical targets including small molecules, oligonucleotides, peptides, proteins, saccharides and whole cells (e.g., cancer diagnostics). The luminescent nanomaterials that are described herein for paper-based bioassays include metal nanoparticles, quantum dots and lanthanide-doped nanocrystals. These nanomaterials often have broad and strong absorption and narrow emission bands that improve opportunity for multiplexed analysis, and can be designed to provide emission at wavelengths that are efficiently processed by conventional digital cameras. Luminescent nanoparticles can be embedded in paper substrates that are designed to direct fluid flow, and the resulting combination of technologies can offer competitive analytical performance at relatively low cost.

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.

Inorganic Nanoparticles as Donors in Resonance Energy Transfer for Solid-Phase Bioassays and Biosensors

Bioassays for the rapid detection and quantification of specific nucleic acids, proteins, and peptides are fundamental tools in many clinical settings. Traditional optical emission methods have focused on the use of molecular dyes as labels to track selective binding interactions and as probes that are sensitive to environmental changes. Such dyes can offer good detection limits based on brightness but typically have broad emission bands and suffer from time-dependent photobleaching. Inorganic nanoparticles such as quantum dots and upconversion nanoparticles are photostable over prolonged exposure to excitation radiation and tend to offer narrow emission bands, providing a greater opportunity for multiwavelength multiplexing. Importantly, in contrast to molecular dyes, nanoparticles offer substantial surface area and can serve as platforms to carry a large number of conjugated molecules. The surface chemistry of inorganic nanoparticles offers both challenges and opportunities for the control of solubility and functionality for selective molecular interactions by the assembly of coatings through coordination chemistry. This report reviews advances in the compositional design and methods of conjugation of inorganic quantum dots and upconversion nanoparticles and the assembly of combinations of nanoparticles to achieve energy exchange. Our interest is the exploration of configurations where the modified nanoparticles can be immobilized to solid substrates for the development of bioassays and biosensors that operate by resonance energy transfer (RET).

Bioresponsive Release System for Visual Fluorescence Detection of Carcinoembryonic Antigen from Mesoporous Silica Nanocontainers Mediated Optical Color on Quantum Dot-Enzyme-Impregnated Paper

An all-in-one paper-based analytical device (PAD) was successfully developed for visual fluorescence detection of carcinoembryonic antigen (CEA) on CdTe/CdSe quantum dot (QD)-enzyme-impregnated paper by coupling with a bioresponsive controlled-release system from DNA-gated mesoporous silica nanocontainers (MSNs). The assay was carried out in a centrifuge tube by using glucose-loaded MSNs with a CEA aptamer and a QD-enzyme-paper attached on the lid. Initially, single-strand complementary DNA to a CEA aptamer was covalently conjugated to the aminated MSN, and then glucose (enzyme substrate) molecules were gated into the pore with the help of the aptamer. Glucose oxidase (GOD) and CdTe/CdSe QDs were coimmobilized on paper for the visual fluorescence signal output. Upon target CEA introduction in the detection cell, the analyte specifically reacted with the immobilized aptamer on the MSN to open the pore, thereby resulting in the glucose release. The released glucose was oxidized by the immobilized GOD on paper to produce gluconic acid and hydrogen peroxide, and the latter quenched the fluorescence of CdTe/CdSe QDs, which could be determined by the naked eye on a portable smartphone and a commercial fluorospectrometer. Under optimal conditions, the PAD-based sensing system enabled sensitive discrimination of target CEA against other biomarkers or proteins in a linear range of 0.05-20 ng mL^-1 with a limit of detection of 6.7 pg mL^-1 (ppt). In addition, our strategy displayed high specificity, good reproducibility, and acceptable accuracy for analyzing human serum specimens with a commercial human CEA ELISA kit. Importantly, this methodology offers promise for simple analysis of biological samples and is suitable for use in the mass production of miniaturized devices, thus opening new opportunities for protein diagnostics and biosecurity.

Resonance Energy Transfer-Based Nucleic Acid Hybridization Assays on Paper-Based Platforms Using Emissive Nanoparticles as Donors

Quantum dots (QDs) and upconverting nanoparticles (UCNPs) are luminescent nanoparticles (NPs) commonly used in bioassays and biosensors as resonance energy transfer (RET) donors. The narrow and tunable emissions of both QDs and UCNPs make them versatile RET donors that can be paired with a wide range of acceptors. Ratiometric signal processing that compares donor and acceptor emission in RET-based transduction offers improved precision, as it accounts for fluctuations in the absolute photoluminescence (PL) intensities of the donor and acceptor that can result from experimental and instrumental variations. Immobilizing NPs on a solid support avoids problems such as those that can arise with their aggregation in solution, and allows for facile layer-by-layer assembly of the interfacial chemistry. Paper is an attractive solid support for the development of point-of-care diagnostic assays given its ubiquity, low-cost, and intrinsic fluid transport by capillary action. Integration of nanomaterials with paper-based analytical devices (PADs) provides avenues to augment the analytical performance of PADs, given the unique optoelectronic properties of nanomaterials. Herein, we describe methodology for the development of PADs using QDs and UCNPs as RET donors for optical transduction of nucleic acid hybridization. Immobilization of green-emitting QDs (gQDs) on imidazole functionalized cellulose paper is described for use as RET donors with Cy3 molecular dye as acceptors for the detection of SMN1 gene fragment. We also describe the covalent immobilization of blue-emitting UCNPs on aldehyde modified cellulose paper for use as RET donors with orange-emitting QDs (oQDs) as acceptors for the detection of HPRT1 gene fragment. The data described herein is acquired using an epifluorescence microscope, and can also be collected using technology such as a typical electronic camera.

A paper-based platform for detection of viral RNA

Viral detection presents a host of challenges for even the most sensitive analytical techniques, and the complexity of common detection platforms typically preclude portability. With these considerations in mind, we designed a paper microzone plate-based virus detection system for the detection of viral genetic material that can be performed with simple instruments. The sensing system can detect viral cDNA reverse-transcribed from total RNA extraction by utilizing a biotinylated capture probe and an Alexa Fluor® 647-labeled reporter probe. The biotinylated capture probe was linked to the paper surface via NeutrAvidin® that was physically adsorbed on the paper. After addition of reverse-transcribed sample and reporter probe in sequence, the reverse-transcribed target captured the reporter probe and tethered it to the capture probe in a bridged format. Fluorescence intensity was imaged using a Western blot imaging system, and higher target concentration was visible by the increased emission intensity from Alexa Fluor® 647. By utilizing paper, this detection setup could also serve as a sample concentration method via evaporation, which could remarkably lower the detection limit if needed. This detection platform used Epstein-Barr virus (EBV) RNA as a proof-of-concept by sensing cDNA resulting from reverse transcription and can be further expanded as a general method for other pathogens. EBV is a well-known human tumor virus, which has also recently been linked to the development of cervical cancer. The assay was accomplished within two hours including the room-temperature RNA extraction and reverse transcription steps. Also, this paper microzone plate-based platform can potentially be applicable for the development of point-of-care (POC) detection kits or devices due to its robust design, convenient interface, and easy portability. The experiment could be stopped after each step, and continued at a later time. The shelf-life of the modified paper plate setup was at least 3 months without a discernible change in signal, and the result from day 1 could be read at 3 months - both of which are important criteria for POC analytical testing tools, especially in resource-poor settings. All of the required assay steps could potentially be performed without any significant equipment using inexpensive paper microzone plates, which will be ideal for further development of POC testing devices. Although, this platform is not at the stage where it can be directly used in a point-of-care setting, it does have fundamental characteristics such as a stable platform, a simple detection method, and relatively common reagents that align closely with a POC system.

Rapid Immobilization of Oligonucleotides at High Density on Semiconductor Quantum Dots and Gold Nanoparticles

Oligonucleotide-coated nanoparticles (NPs) have been used in numerous applications such as bioassays, as intracellular probes, and for drug delivery. One challenge that is confronted in the preparation of oligonucleotide-NP conjugates is derived from surface charge because nanoparticles are often stabilized and made water-soluble with a coating of negatively charged capping ligands. Therefore, an electrostatic repulsion is present when attempting to conjugate oligonucleotides. The result is that the conjugation can be a slow process, sometimes requiring 1 to 2 days to equilibrate at the highest surface density. The effect is compounded by electrostatic repulsion between neighboring oligonucleotide strands on the NP surfaces, which tends to lower the surface density. Herein, we report a novel method that enables conjugation in less than 1 min with a surface density of oligonucleotides up to the theoretical physical limit of occupancy. Negatively charged NPs are first adsorbed onto the surface of positively charged magnetic beads (MBs) to create MB-NP conjugates. Oligonucleotides are subsequently electrostatically adsorbed onto the MB surfaces when added to a suspension of MB-NP conjugates. This creates an oligonucleotide concentration 10⁵ to 10⁶ greater than in bulk solution in the vicinity of the nanoparticles, resulting in the promotion of the kinetics by over 1000-fold and achieving the maximum density possible for the conjugation reaction.

Paper-based fluorescence resonance energy transfer assay for directly detecting nucleic acids and proteins

Paper-based fluorescence resonance energy transfer assay (FRET) is gaining great interest in detecting macro-biological molecule. It is difficult to achieve conveniently and fast detection for macro-biological molecule. Herein, a graphene oxide (GO)-based paper chip (glass fiber) integrated with fluorescence labeled single-stranded DNA (ssDNA) for fast, inexpensive and direct detection of biological macromolecules (proteins and nucleic acids) has been developed. In this paper, we employed the Cy3/FAM-labeled ssDNA as the reporter and the GO as quencher and the original glass fiber paper as data acquisition substrates. The chip which was designed and fabricated by a cutting machine is a miniature biosensor that monitors fluorescence recovery from resonance energy transfer. The hybridization assays and fluorescence detection were all simplified, and the surface of the chip did not require immobilization or washing. A Nikon Eclipse was employed as excited resource and a commercial digital camera was employed for capturing digital images. This paper-based microfluidics chip has been applied in the detection of proteins and nucleic acids. The biosensing capability meets many potential requirements for disease diagnosis and biological analysis.

A Paper-Based Sandwich Format Hybridization Assay for Unlabeled Nucleic Acid Detection Using Upconversion Nanoparticles as Energy Donors in Luminescence Resonance Energy Transfer

Bioassays based on cellulose paper substrates are gaining increasing popularity for the development of field portable and low-cost diagnostic applications. Herein, we report a paper-based nucleic acid hybridization assay using immobilized upconversion nanoparticles (UCNPs) as donors in luminescence resonance energy transfer (LRET). UCNPs with intense green emission served as donors with Cy3 dye as the acceptor. The avidin functionalized UCNPs were immobilized on cellulose paper and subsequently bioconjugated to biotinylated oligonucleotide probes. Introduction of unlabeled oligonucleotide targets resulted in a formation of probe-target duplexes. A subsequent hybridization of Cy3 labeled reporter with the remaining single stranded portion of target brought the Cy3 dye in close proximity to the UCNPs to trigger a LRET-sensitized emission from the acceptor dye. The hybridization assays provided a limit of detection (LOD) of 146.0 fmol and exhibited selectivity for one base pair mismatch discrimination. The assay was functional even in undiluted serum samples. This work embodies important progress in developing DNA hybridization assays on paper. Detection of unlabeled targets is achieved using UCNPs as LRET donors, with minimization of background signal from paper substrates owing to the implementation of low energy near-infrared (NIR) excitation.

Ratiometric fluorescence transduction by hybridization after isothermal amplification for determination of zeptomole quantities of oligonucleotide biomarkers with a paper-based platform and camera-based detection

Paper is a promising platform for the development of decentralized diagnostic assays owing to the low cost and ease of use of paper-based analytical devices (PADs). It can be challenging to detect on PADs very low concentrations of nucleic acid biomarkers of lengths as used in clinical assays. Herein we report the use of thermophilic helicase-dependent amplification (tHDA) in combination with a paper-based platform for fluorescence detection of probe-target hybridization. Paper substrates were patterned using wax printing. The cellulosic fibers were chemically derivatized with imidazole groups for the assembly of the transduction interface that consisted of immobilized quantum dot (QD)-probe oligonucleotide conjugates. Green-emitting QDs (gQDs) served as donors with Cy3 as the acceptor dye in a fluorescence resonance energy transfer (FRET)-based transduction method. After probe-target hybridization, a further hybridization event with a reporter sequence brought the Cy3 acceptor dye in close proximity to the surface of immobilized gQDs, triggering a FRET sensitized emission that served as an analytical signal. Ratiometric detection was evaluated using both an epifluorescence microscope and a low-cost iPad camera as detectors. Addition of the tHDA method for target amplification to produce sequences of ∼100 base length allowed for the detection of zmol quantities of nucleic acid targets using the two detection platforms. The ratiometric QD-FRET transduction method not only offered improved assay precision, but also lowered the limit of detection of the assay when compared with the non-ratiometric QD-FRET transduction method. The selectivity of the hybridization assays was demonstrated by the detection of single nucleotide polymorphism.

A fluorescent light-up platform with "AIE + ESIPT" characteristics for multi-target detection both in solution and on paper strip

We report a fluorescent light-up platform for multi-target detection in aqueous solution and on paper strip. The platform is based on a salicylaldazine fluorogen with aggregation-induced emission (AIE) and excited state intramolecular proton transfer (ESIPT) characteristics, which shows distinct advantages including ease of chemical modifications, free of self-quenching effect, excellent light-up ratio and large Stokes shift. To demonstrate the versatility of the platform, palladium cation and perborate anion, as well as UV light, were selected as the targets. The three representative probes, AIE-Pd, AIE-perborate and AIE-UV, light up specifically in the presence of the target both in aqueous solution and on paper strip. The immediate naked-eye response makes the probes ideal for instrument-free and power-free detection.

Control performance of paper-based blood analysis devices through paper structure design

In this work, we investigated the influence of paper structure on the performance of paper-based analytical devices that are used for blood analysis. The question that we aimed to answer is how the fiber type (i.e., softwood and hardwood fibers) influences the fiber network structure of the paper, which affects the transport of red blood cells (RBCs) in paper. In the experimental design, we isolated the influence of fiber types on the paper structure from all other possible influencing factors by removing the fines from the pulps and not using any additives. Mercury porosimetry was employed to characterize the pore structures of the paper sheets. The results show that papers with a low basis weight that are made with short hardwood fibers have a higher porosity (i.e., void fraction) and simpler pore structures compared with papers made with long softwood fibers. RBC transport in paper carried by saline solution was investigated in two modes: lateral chromatographic elution and vertical flow-through. The results showed that the complexity of the paper's internal pore structure has a dominant influence on the transport of RBCs in paper. Hardwood fiber sheets with a low basis weight have a simple internal pore structure and allow for the easy transport of RBCs. Blood-typing sensors built with low basis weight hardwood fibers deliver high-clarity assays. Softwood fiber papers are found to have a more complex pore structure, which makes RBC transport more difficult, leading to blood-typing results of low clarity. This study provides the principle of paper sheet design for paper-based blood analysis sensors.

Spectrally matched duplexed nucleic acid bioassay using two-colors from a single form of upconversion nanoparticle

Optical sensing can provide opportunity for simultaneous determination of multiple targets as well as for implementation of ratiometric methods that can improve accuracy and precision. Herein we report a paper-based two-color oligonucleotide detection assay with tunable sensitivity that is based on use of a single type of upconversion nanoparticle (UCNP). Water-soluble UCNPs were designed to concurrently offer green and red emission. These avidin functionalized UCNPs were adsorbed onto a cellulose support, and Cy3 was used as a green channel acceptor for Survival Motor Neuron (SMN1) target, and Cy5.5 was the red channel acceptor for the glucuronidase gene (uidA) target. Selective DNA hybridization of the labeled targets with the corresponding probe provided emission from dyes, which was the basis for concurrent quantification of both targets. The limit of detection (LOD) could be tuned by changing the relative ratio of the SMN1 and uidA probes. A higher proportion of a probe provided for a lower LOD. When the SMN1/uidA probe ratio was 1:4, the LOD for SMN1 and uidA target were 54.3 and 30.5 fmol, and when the probe ratio was 4:1, the LOD for the above targets were 22.1 and 1260 fmol, respectively. Selectivity evaluation showed that one base pair mismatched DNA for SMN1 and uidA could be discriminated in most cases. The assay showed resistance to nonspecific adsorption of interfering DNA and protein and was even functional for targets in undiluted serum. This work represents a significant step in the development of paper-based multiplexed UCNP luminescence assays.

Camera-based ratiometric fluorescence transduction of nucleic acid hybridization with reagentless signal amplification on a paper-based platform using immobilized quantum dots as donors

Paper-based diagnostic assays are gaining increasing popularity for their potential application in resource-limited settings and for point-of-care screening. Achievement of high sensitivity with precision and accuracy can be challenging when using paper substrates. Herein, we implement the red-green-blue color palette of a digital camera for quantitative ratiometric transduction of nucleic acid hybridization on a paper-based platform using immobilized quantum dots (QDs) as donors in fluorescence resonance energy transfer (FRET). A nonenzymatic and reagentless means of signal enhancement for QD-FRET assays on paper substrates is based on the use of dry paper substrates for data acquisition. This approach offered at least a 10-fold higher assay sensitivity and at least a 10-fold lower limit of detection (LOD) as compared to hydrated paper substrates. The surface of paper was modified with imidazole groups to assemble a transduction interface that consisted of immobilized QD-probe oligonucleotide conjugates. Green-emitting QDs (gQDs) served as donors with Cy3 as an acceptor. A hybridization event that brought the Cy3 acceptor dye in close proximity to the surface of immobilized gQDs was responsible for a FRET-sensitized emission from the acceptor dye, which served as an analytical signal. A hand-held UV lamp was used as an excitation source and ratiometric analysis using an iPad camera was possible by a relative intensity analysis of the red (Cy3 photoluminescence (PL)) and green (gQD PL) color channels of the digital camera. For digital imaging using an iPad camera, the LOD of the assay in a sandwich format was 450 fmol with a dynamic range spanning 2 orders of magnitude, while an epifluorescence microscope detection platform offered a LOD of 30 fmol and a dynamic range spanning 3 orders of magnitude. The selectivity of the hybridization assay was demonstrated by detection of a single nucleotide polymorphism at a contrast ratio of 60:1. This work provides an important framework for the integration of QD-FRET methods with digital imaging for a ratiometric transduction of nucleic acid hybridization on a paper-based platform.

Solid-phase covalent immobilization of upconverting nanoparticles for biosensing by luminescence resonance energy transfer

Monodisperse water-soluble upconverting nanoparticles (UCNPs) were immobilized onto modified glass substrates for development of biosensing surfaces that operated using luminescence resonance energy transfer (LRET). Amine modified UCNPs were prepared from oleic acid capped UCNPs by ligand exchange using o-phosphorylethanolamine (PEA). PEA-UCNPs were covalently immobilized on aldehyde functionalized coverslips. Environmental scanning electron microscopy (ESEM) images indicated a homogeneous distribution of UCNPs on surfaces with a high immobilization density of approximately 1.3 × 10(11) UCNP cm(-2). This is the first account of covalent immobilization of UCNPs for bioassay and biosensor development where the density is on par with the high immobilization densities reported for other types of nanoparticles. The functionality and stability of the immobilized NPs were demonstrated by examining an LRET-based bioassay. The well-known sandwich assay for the detection of thrombin was selected as a model in which UCNPs were used as donors and quantum dots (QDs) as acceptors. The closely packed UCNPs on the glass surface showed a 2.5-fold enhancement in assay sensitivity compared to less-densely packed surfaces. In addition, a 1.5-fold enhancement in energy transfer efficiency was shown for solid-phase compared to solution-phase LRET.