Chromatic immunoassay based on polydiacetylene vesicles

Colorimetric Detection of the SARS-CoV-2 Virus (COVID-19) in Artificial Saliva Using Polydiacetylene Paper Strips

The spread and resurgence of the SARS-CoV-2 virus (COVID-19 disease) threatens human health and social relations. Prevention of COVID-19 disease partly relies on fabricating low-cost, point-of-care (POC) sensing technology that can rapidly and selectively detect the SARS-CoV-2 virus. We report a colorimetric, paper-based polydiacetylene (PDA) biosensor, designed to detect SARS-CoV-2 spike protein in artificial saliva. Analytical characterizations of the PDA sensor using NMR and FT-IR spectroscopy showed the correct structural elucidation of PCDA-NHS conjugation. The PDA sensor platform containing the N-Hydroxysuccinimide ester of 10, 12-pentacosadiynoic acid (PCDA-NHS) was divided into three experimental PCDA-NHS concentration groups of 10%, 20%, and 30% to optimize the performance of the sensor. The optimal PCDA-NHS molar concentration was determined to be 10%. The PDA sensor works by a color change from blue to red as its colorimetric output when the immobilized antibody binds to the SARS-CoV-2 spike protein in saliva samples. Our results showed that the PDA sensing platform was able to rapidly and qualitatively detect the SARS-CoV-2 spike protein within the concentration range of 1 to 100 ng/mL after four hours of incubation. Further investigation of pH and temperature showed minimal influence on the PDA sensor for the detection of COVID-19 disease. After exposure to the SARS-CoV-2 spike protein, smartphone images of the PDA sensor were used to assess the sensor output by using the red chromatic shift (RCS) of the signal response. These results indicate the potential and practical use of this PDA sensor design for the rapid, colorimetric detection of COVID-19 disease in developing countries with limited access to medical testing.

Self-Assembled Peptide-Labeled Probes for Agglutination-Based Sensing

The use of polydiacetylene (PDA) vesicles in sensing systems are wide-spread due to the interesting optical properties of this stimuli-responsive material; however, agglutination based sensing with PDA have been relatively underutilized. To demonstrate the means for rapidly generating an agglutination probe based on peptide-displaying polydiacetylene vesicles, we implement here the use of a biotin mimetic peptide functionalized to a diacetylene amphiphile for proof-of-concept detection of a multivalent target, specifically streptavidin. Tuning of the vesicle composition revealed a distinct limit in the surface density of peptide amphiphile that could be displayed for this particular peptide sequence. A wide operational detection range was demonstrated, and the result also revealed an effective agglutination response of the PDA-based probe to streptavidin suggesting possible use of future formulations in profiling other multivalent targets.

Modifying Polydiacetylene Vesicle Compositions to Reduce Non-Specific Interactions

Polydiacetylene (PDA) vesicles provide useful stimuli-responsive behavior as well as by the modular structure afford a means for the design of sensing and delivery systems with tunable target specificity. To reduce inherent non-specific interaction with either anionic or cationic formulations of polydiacetylene vesicles, we explored the use of various lengths of poly(ethylene glycol) (PEG) amphiphiles for integration and polymerization within PDA vesicles. Our results established that as little as 1% of polyethylene glycol amphiphile integration into anionic vesicles was sufficient to significantly reduce non-specific association with mammalian cells. Similarly integrating a low percent of PEG amphiphile content within cationic vesicles could also significantly reduce non-specific cell association, and moreover reduced cytotoxicity. These results may be prove useful in augmenting PDA vesicles formulations for reduced non-specific interaction which is of particularly interest to enhancing selectivity in vesicles designed with integrated targeting moieties for sensing and drug delivery applications.

Structures and strategies for enhanced sensitivity of polydiacetylene(PDA) based biosensor platforms

Polydiacetylene (PDA) is a versatile polymer that has been studied in numerous fields because of its unique optical properties derived from alternating multiple bonds in the polymer backbone. The conjugated structure in the polymer backbone enables PDA to possess the ability of blue-red colorimetric transition when π-π interactions in the PDA backbone chain are disturbed by the external environment. The chromatic property of PDA disturbed by external stimuli can also emit fluorescence in the red region. Owing to the unique characteristics of PDA, it has been widely studied in facile and label-free sensing applications based on colorimetric or fluorescence signals for several decades. Among the various PDA structures, membrane structures assembled by amphiphilic molecules are widely used as a versatile platform because facile modification of the synthetic membrane provides extensive applications, such as receptor-ligand interactions, resulting in potent biosensors. To use PDA as a sensory material, several methods have been studied to endow the specificity to PDA molecules and to amplify the signal from PDA supramolecules. This is because selective and sensitive detection of target materials is required at an appropriate level corresponding to each material for applicable sensor applications. This review focuses on factors that affect the sensitivity of PDA composites and several strategies to enhance the sensitivity of the PDA sensor to various structures. Owing to these strategies, the PDA sensor system has achieved a higher level of sensitivity and selectivity, enabling it to detect multiple target materials for a full field of application.

Aptamer-Conjugated Polydiacetylene Colorimetric Paper Chip for the Detection of Bacillus thuringiensis Spores

A colorimetric polydiacetylene (PDA) paper strip sensor that can specifically recognize Bacillus thuringiensis (BT) HD-73 spores is described in this work. The target-specific aptamer was combined with PDA, and the aptamer-conjugated PDA vesicles were then coated on polyvinylidene fluoride (PVDF) paper strips by a simple solvent evaporation method. The PDA-aptamer paper strips can be used to detect the target without any pre-treatment. Using the paper strip, the presence of BT spores is directly observable by the naked eye based on the unique blue-to-red color transition of the PDA. Quantitative studies using the paper strip were also carried out by analyzing the color transitions of the PDA. The specificity of this PDA sensor was verified with a high concentration of Escherichia coli, and no discernable change was observed. The observable color change in the paper strip occurs in less than 1 h, and the limit of detection is 3 × 10⁷ CFU/mL, much below the level harmful to humans. The PDA-based paper sensor, developed in this work, does not require a separate power or detection device, making the sensor strip highly transportable and suitable for spore analysis anytime and anywhere. Moreover, this paper sensor platform is easily fabricated, can be adapted to other targets, is highly portable, and is highly specific for the detection of BT spores.

Biomolecule-Functionalized Smart Polydiacetylene for Biomedical and Environmental Sensing

Polydiacetylene (PDA) has attracted interest for use as a sensing platform in biomedical, environmental, and chemical engineering applications owing to its capacity for colorimetric and fluorescent transition in response to external stimuli. Many researchers have attempted to develop a tailor-made PDA sensor via conjugation of chemical or biological substances to PDA. Here, we review smart bio-conjugates of PDA with various biomolecules such as carbohydrates, lipids, nucleic acids, and proteins. In addition, materialization and signal amplification strategies to improve handling and sensitivity are described.

Polydiacetylene-Coated Sensor Strip for Immunochromatic Detection of Xylella fastidiosa subsp. fastidiosa

This study presents a sensor strip for user-friendly, naked-eye detection of Xylella fasitdiosa, the bacterial causal agent of Pierce's disease in grapevine. This sensor uses anti- X. fastidiosa antibodies conjugated to a polydiacetylene layer on a polyvinylidene fluoride strip to generate specific color transitions and discriminate levels of the pathogen. The detection limit of the sensor is 0.8 × 10⁸ cells/mL, which is similar to bacterial load in grapevine 18 days following bacterial inoculation. This sensor enables equipment-free detection that is highly desirable for in-field diagnostic tools in resource-limited settings.

Label-Free Colorimetric Detection of Influenza Antigen Based on an Antibody-Polydiacetylene Conjugate and Its Coated Polyvinylidene Difluoride Membrane

This study presents an antibody-conjugated polydiacetylene (PDA) and its coated polyvinylidene difluoride (PVDF) membrane. The M149 antibody was hybridized to nano-vesicles consisting of pentacosa-10,12-diynoic acid (PCDA) and dimyristoylphosphatidylcholine (DMPC). After photo-polymerization at 254 nm, the effects on the PDA by antigenic injection were investigated with UV-vis spectroscopy, fluorescence spectroscopy, dynamic light scattering and transmission electron microscopy. Because PDA, an alternating ene-yne molecule, induces a blue-to-red color transition and an interesting fluorescent response by the distortion of its backbone, the biomolecular recognition of an antibody–antigen can be converted into an optical and fluorescent signal. Thus, an influenza antigen was successfully detected with the proposed label-free method. Furthermore, the vesicular system was improved by coating it onto a membrane type sensing platform for its stability and portability. The proposed antibody-PDA composite PVDF membrane has potential for rapid, easy and selective visualization of the influenza virus.

Polydiacetylene-coated polyvinylidene fluoride strip aptasensor for colorimetric detection of zinc(II)

We report a new polydiacetylene (PDA) sensor strip for simple visual detection of zinc ions in aqueous solution. The specificity of this sensor comes from Zn²⁺ DNA aptamer probes conjugated onto PDA. Effects of aptamer length and structure on the sensitivity of PDA's color transition were first investigated. PDA conjugated with the optimal aptamer sequence was then coated onto a strip of polyvinylidene fluoride membrane and photopolymerized by UV exposure. The newly developed sensor successfully exhibited a blue-to-red chromatic change in a semi-quantitative manner in response to zinc ions. No discernable change was observed in solutions containing other common ions. Advantages of this sensor include its ease of fabrication, high specificity, and equipment-free detection, all of which are desirable for in-field applications and use in resource-limited settings.

Development and evaluation of a polydiacetylene based biosensor for the detection of H5 influenza virus

H5N1 avian influenza has caused serious economic losses as well as posed significant threats to public health, agriculture and wildlife. It is important to develop a rapid, sensitive and specific detection platform suitable for disease surveillance and control. In this study, a highly sensitive, specific and rapid biosensor based on polydiacetylene was developed for detecting H5 influenza virus. The polydiacetylene based biosensor was produced from an optimized ratio of 10,12-pentacosadiynoic acid and 1,2-dimyristoyl-sn-glycero-3-phosphocholine, with the anti-H5 influenza antibody embedded onto the vesicle surface. The optimized polydiacetylene vesicle could detect H5 influenza virus sensitively with a detection limit of 0.53 copies/μL, showing a dramatic blue-to-red color change that can be observed directly by the naked eye and recorded by a UV-vis spectrometer. The sensitivity, specificity and accuracy of the biosensor were also evaluated. The sensor could specifically differentiate H5 influenza virus from H3 influenza virus, Newcastle disease virus and porcine reproductive and respiratory syndrome virus. Detection using tracheal swabs was in accord with virus isolation results, and comparable to the RT-PCR method. These results offer the possibility and potential of simple polydiacetylene based bio-analytical method for influenza surveillance.

Vibrational and theoretical study of diacetylenic acids

Three selected diacetylenic acids (DAs) with side-chains of various length (CH3-(CH2)m-C≡C-C≡C(CH2)n-COOH, where m=7, 9, 11, and n=3, 8) were analyzed using vibrational spectroscopy and quantum-chemical calculations. The conformational analysis was followed by potential energy distribution (PED) calculations to gain deeper insight into their FT-Raman and FT-IR spectra. The analysis was focused on spectral features of the diacetylene system sensitive to the substitution. In particular, the electron donor-acceptor properties of the substituent and the influence of side-chain length were studied. FT-IR spectra were measured by using two techniques, i.e. transmission (with KBr substrate) and Attenuated Total Reflection (ATR), and the latter seems to be less adequate for DAs measurements because the bands in the fingerprint region as well as the ν(C≡C)as mode are relatively of low intensity. Additionally, polymerization process of DAs was recognized using FT-Raman spectroscopy and strong and well-separated bands of diacetylenic polymers. Temperature and exposure to the sunlight are the factors of an important influence on the polymerization process of DAs. Since the investigated DAs are carboxylic acids, the interpretation of experimental spectra was performed on the basis of monomer and dimer calculations.

Polydiacetylene-based high-throughput screen for surfactin producing strains of Bacillus subtilis

Although traditional mutation is still an attractive approach for strain improvement, it is tedious, time-consuming, and inefficient to screen for surfactin producing strains. To overcome this, we developed a high-throughput screening method for surfactin producing mutants by applying polydiacetylene (PDA) vesicles as sensors with visible chromatic change from blue to red, detected as colorimetric response (CR%) signal, which can even semi-quantify the yields of surfactin. Bacillus subtilis 723 was used as parent strain and multiply mutated with atmospheric and room temperature plasma (ARTP). Mutants were cultured in MicroFlask by Duetz (24 square deepwell plates, Applikon Biotechnology) and surfactin titers were tested in 96-well plates with PDA vesicles. Mutants with surfactin titers above150 mg/L (CR% value above 26%) were selected as high-yield strains and further quantified by HPLC. By integrating MicroFlask cultivation and the PDA vesicles detection, we screened 27,000 mutants and found 37 high-yield strains. From these, one mutant produced 473.6 mg/L surfactin (including 353.1 mg/L C15 surfactin), which was 5.4-fold than that of the parent strain. This method is efficient, cost-effective and provides wider application in screening for various surfactants.

A reinforced composite structure composed of polydiacetylene assemblies deposited on polystyrene microspheres and its application to H5N1 virus detection

In this study, we immobilized polydiacetylene vesicles (PDAVs) onto the surface of polystyrene (PS) microspheres (1 μm in diameter) by using both electrical charge and conjugated forces to form a reinforced composite structure. These reinforced complexes could be easily washed, separated by centrifugation, and resuspended by gentle agitation. After passing through a narrow 200 μm-diameter channel, the composite structures maintained their original shape, demonstrating their resilience and potential for use in microfluidic technologies. The number of PDAVs in the composite structure could be mediated by changing the extent of layer deposition, which affected the sensitivity of detection. It showed that PDAVs did not change their blue color after addition of detecting probes such as anti-H5N1, which was of great importance in the fabrication and modification of stable color-changeable biosensors based on PDAVs. By conjugating anti-H5N1 antibodies to the PS@PDAV via N -hydroxysuccinimide and 1-ethyl3-(3-dimethylaminopropyl) carbodiimide chemistry, a stable blue complex, anti-H5N1 microsphere (PS@PDAV-anti-H5N1) was formed. A target antigen of H5N1 (HAQ [H5N1 strain A/ environment/Qinghai/1/2008{H5N1} in clade 0]) was detected by PS@PDAV-anti-H5N1. At an optimal PDAV deposition level of three layers, the limit of detection was determined to be approximately 3 0 ng/mL of HAQ by using optical spectrum measurement and visual inspection, meeting the needs of fast and simple color-changeable detection. However, a much lower limitation of detection (1 ng/mL) was able to be obtained using laser-scanning confocal microscopy, which could be compared with the results obtained with other sophisticated equipment.

Design of polydiacetylene-phospholipid supramolecules for enhanced stability and sensitivity

We present polydiacetylene (PDA) liposome assemblies with various phospholipids that have different headgroup charges and phase transition temperatures (T(m)). 10,12-Pentacosadiynoic acid (PCDA)-epoxy was used as a base PDA monomer and the insertion of highly charged phospholipids resulted in notable changes in the size of liposome and reduction of the aggregation of PDA liposome. Among the various phospholipids, the phospholipid with a moderate T(m) demonstrated enhanced stability and sensitivity, as measured by the size and zeta potential over storage time, thermochoromic response, and transmission electron microscopy images. By combining these results, we were able to detect immunologically an antibody of bovine viral diarrhea virus over a wide dynamic range of 0.001 to 100 μg/mL.

Polydiacetylene incorporated with peptide receptors for the detection of trinitrotoluene explosives

Because of their unique optical and stimuli-response properties, polydiacetylene-based platforms have been explored as an alternative to complex mechanical and electrical sensing systems. We linked chromic responsive polydiacetylene (PDA) onto a peptide-based molecular recognition element for trinitrotoluene (TNT) molecules in order to provide a system capable of responding to the presence of a TNT target. We first identified the trimer peptide receptor that could induce chromic changes on a PDA backbone. We then investigated the multivalent interactions between TNT and our peptide-based receptor by nuclear magnetic resonance (NMR) spectroscopy. We further characterized various parameters that affected the conjugated PDA system and hence the chromic response, including the size of end-group motifs, the surface density of receptors, and the length of alkane side chains. Taking these necessary design parameters into account, we demonstrated a modular system capable of transducing small-molecule TNT binding into a detectable signal. Our conjugated PDA-based sensor coupled with molecular recognition elements has already proven useful recently in the development of another sensitive and selective electronic sensor, though we expect that our results will also be valuable in the design of colorimetric sensors for small-molecule detection.

Microbead-assisted PDA sensor for the detection of genetically modified organisms

A simple and sensitive approach for the detection of marker protein, phosphinothricin acetyltransferase, from genetically modified crops was developed based on the colorimetric transition of polydiacetylene (PDA) vesicles in combination with silica microbeads. PDAs have attracted a great deal of interests as a transducing material due to their special features that allow colorimetric response to sensory signals, as well as their inherent simplicity. However, most PDA-based biosensors require additional analytical equipment such as a fluorescence microscope or UV-Vis spectrometer. In this study, we report a new approach to increase the degree of color transition by coupling antibody-conjugated PDA vesicles with silica microbeads in an effort to monitor the results with the unaided eye or simple RGB analysis. By immobilizing PDA vesicles on silica microbeads, we were able to overcome the disadvantages of colloidal PDA-based sensors and increase the degree of colorimetric changes in response to target molecules to a concentration as low as 20 nM. The additional stresses were given to PDA vesicles by antigen-antibody bridging of PDA vesicles coupled with microbeads, resulting in enhanced blue-red color transition. All the results showed that PDA vesicles in conjunction with silica microbeads will be a promising transducing material for the detection of target proteins in diagnostic and biosensing applications.

Fluorescent nanoscale detection of biotin-streptavidin interaction using near-field scanning optical microscopy

We describe a nanoscale strategy for detecting biotin-streptavidin binding using near-field scanning optical microscopy (NSOM) that exploits the fluorescence properties of single polydiacetylene (PDA) liposomes. NSOM is more useful to observe nanomaterials having optical properties with the help of topological information. We synthesized amine-terminated 10,12-pentacosadiynoic acid (PCDA) monomer (PCDA-NH(2)) and used this derivatized monomer to prepare PCDA liposomes. PCDA-NH(2) liposomes were immobilized on an aldehyde-functionalized glass surface followed by photopolymerization by using a 254 nm light source. To measure the biotin-streptavidin binding, we conjugated photoactivatable biotin to immobilized PCDA-NH(2) liposomes by UV irradiation (365 nm) and subsequently allowed them to interact with streptavidin. We analyzed the fluorescence using a fluorescence scanner and observed single liposomes using NSOM. The average height and NSOM signal observed in a single liposome after binding were ∼31.3 to 8.5 ± 0.5 nm and 0.37 to 0.16 ± 0.6 kHz, respectively. This approach, which has the advantage of not requiring a fluorescent label, could prove highly beneficial for single molecule detection technology.

Glycopolydiacetylene nanoparticles as a chromatic biosensor to detect Shiga-like toxin producing Escherichia coli O157:H7

Shiga toxin-producing Escherichia coli organisms (STEC) were detected by Gal-alpha1,4-Gal glycopolydiacetylene (GPDA) nanoparticles through the selective binding between Shiga toxin and GPDA nanoparticles. The binding produced a colorimetric change in the absorption wavelength of the GPDA nanoparticles. This method provides a highly selective, rapid, sensitive, and quantitative approach for the detection of STEC.

Fluorometric immunoassay based on pH-sensitive dye-encapsulating liposomes and gramicidin channels

This article describes a new method for direct fluorometric immunoassay with a liposome array using pH-sensitive dye (BCECF [2',7'-bis(carboxyethyl)-4 or 5-carboxyfluorescein])-encapsulating liposomes immobilized on an avidin slip and gramicidin channels. The liposomes were composed of phosphatidylcholine (PC), cholesterol (Chol), biotinylated phosphatidylethanolamine (B-cap-PE), and recognition sites (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(2,4-dinitrophenyl) [DNP-PE], Fab' fragment of anti-substance P, and Fab' of anti-neurokinin A). The addition of gramicidin induced release of H(+) ions from the inner solution (pH 5.5) to the outer one (pH 7.8), enhancing fluorescence of BCECF (1.0mM) encapsulated in liposome. The binding of an analyte (anti-dinitrophenyl [anti-DNP], avidin, substance P, or neurokinin A) to the membrane-bound recognition sites caused further enhancement of fluorescence of BCECF due to a local distortion of the bilayer structure that affects the channel kinetics of gramicidin. The intensity of fluorescence from the immobilized liposomes 60 min after the addition of gramicidin (10 ng/ml) increased with an increase in the concentration of anti-DNP ranging from 1.2 x 10(-8) to 1.2 x 10(-6)g/ml, avidin ranging from 1.0 x 10(-8) to 1.0 x 10(-6)g/ml, substance P ranging from 1.0 x 10(-8) to 1.0 x 10(-6)g/ml, and neurokinin A ranging from 1.0 x 10(-8) to 1.0 x 10(-6)g/ml. The direct fluorometric immunoassay with a liposome array is simple and easy to carry out. The intensity of fluorescence emitted from the immobilized liposomes is directly measured after incubation with a sample solution and a gramicidin solution in sequence without washing steps. The assay allows simultaneous quantification of multiple components without labeling of antibody or antigen with a fluorescent tag. The liposome-based assay is discussed in terms of principle, sensitivity, and selectivity.

Polydiacetylene (PDA)-based colorimetric detection of biotin–streptavidin interactions

The natural binding of streptavidin (STA) for a small molecule, biotin, has made it a useful tool in specific targeting application, due to their most specific noncovalent biological interactions in nature. Here, we describe a polydiacetylene (PDA)-based colorimetric biosensor to detect the STA-biotin interactions. We synthesized two kinds of biotin-labeled 10,12-pentacosadiynoic acid (PCDA) monomers having 2(ethylene oxide) or 3(ethylene oxide) spacer and prepared biotin-modified PDA liposomes using them. Upon the addition of STA, the modified PDA liposomes showed a color change from blue to red in about 1h followed by their aggregation and precipitation. The aggregation was found to be due to the cross-linking between the PDA liposomes caused by the STA having four biotin binding sites, which was confirmed by their TEM analysis. Since the STA-biotin interactions are widely adopted for various biological detection systems; the biotin-modified PDA sensor developed in this study has enormous potentials for the development of various colorimetric biosensors.

Assembly of polydiacetylene vesicles on solid substrates

A method to create assembly of polydiacetylene vesicles on solid substrates in a novel fashion is described. The formation of the assembly is based on electrostatic layer-by-layer deposition using negatively charged 10,12-pentacosadiynoic acid (PCDA) vesicles and polyelectrolyte polyethylenimine, or positively charged PCDA-2(')-aminoethylamide (PCDANH2) vesicles. This is an efficient method for preparing the chromatic sensor films of polydiacetylene vesicles after ultraviolet light irradiation. The strategy would be useful in the development of polydiacetylene-based chemosensors and biosensors.