Visual Detection of SARS-CoV-2 RNA by Conventional PCR-Induced Generation of DNAzyme Sensor

The gold standard for the diagnosis of SARS-CoV-2, the causative agent of COVID-19, is real-time polymerase chain reaction (PCR), which is labor-intensive, expensive, and not widely available in resource-poor settings. Therefore, it is imperative to develop novel, accurate, affordable, and easily accessible assays/sensors to diagnose and isolate COVID-19 cases. To address this unmet need, we utilized the catalytic potential of peroxidase-like DNAzyme and developed a simple visual detection assay for SARS-CoV-2 RNA using a conventional thermal cycler by the PCR-induced generation of DNAzyme sensor. The performance of RT-PCR DNAzyme-based sensor was comparable to that of real-time PCR. The pilot scale validation of RT-PCR DNAzyme-based sensor has shown ~100% sensitivity and specificity in clinical specimens (nasopharyngeal swab, n = 34), with a good correlation (Spearman r = 0.799) with the Ct-value of fluorescence probe-based real-time PCR. These findings clearly indicate the potential of this inexpensive, sensitive, and specific molecular diagnostic test to extend our testing capabilities for the detection of SARS-CoV-2 to curtail COVID-19 transmission.

Using Simple-Structured Split Aptamer for Gold Nanoparticle-based Colorimetric Detection of Estradiol

Demand for the detection of estradiol, which is a naturally occurring hormone, has been increasing. Gold nanoparticle-based colorimetric aptasensors have been developed for estradiol detection; however, the long sequence of aptamers due to the formation of the secondary structure likely affects the sensitivity of the aptasensors. Herein, a sensitive colorimetric biosensor is developed for label-free detection of estradiol by using an estradiol-specific split aptamer. The results demonstrate that a superior response is observed when a split aptamer with a high free energy of the secondary structure (ΔG > -3 kcal/mol) is used, in comparison to that observed using a split aptamer with a low free energy of the secondary structure (ΔG < -3 kcal/mol) at 27°C. After selecting the appropriate split aptamer, the standard calibration curve obtained for estradiol has a detection limit of 6.7 nM, with a linear range of 6.7 nM - 66.7 μM in the logarithmic scale. Furthermore, this assay is sensitive, easy-to-operate, inexpensive, and non-time-consuming (provides results within 50 min), thereby showing potential for clinical applications (detection of other small molecular targets).

Ultrasensitive Visual Detection of Glucose in Urine Based on the Iodide-Promoted Etching of Gold Bipyramids

Blood glucose monitoring is an essential but painful component of diabetes management, so it is urgent to develop simple, convenient, and noninvasive glucose monitoring methods as alternatives. Because the glucose level in urine is directly related to the blood glucose, urine can be an alternative for blood glucose monitoring. Herein, we report the development of a new and highly sensitive noninvasive colorimetric assay to detect the glucose content in urine samples using gold bipyramids (GBPs). The principle of this method is to utilize hydrogen peroxide (H₂O₂), the oxidation product of glucose, to etch GBPs, where the urine glucose will be quantified based on the displacement of the absorption peak of GBPs. The unique morphology (sharp tips) and etching mechanism (from tips) of GBPs determine the high sensitivity of this assay. Under optimal conditions, this colorimetric assay shows a dynamic range of 0.5-250 μM and a detection limit of 0.34 μM for artificial urine samples. This detection capability is ideal when sample dilution is necessary. Another advantage is that the color change of the GBP solution in this assay is convenient for the visual readout of the urine glucose semiquantitatively by the naked eye. Furthermore, it has been demonstrated here that the iodide ion has the horseradish peroxidase (HRP) activity and can be used alone to promote the reduction reaction of H₂O₂, which eliminates the use of HRP enzymes, simplifies the reaction, and reduces costs. The role of iodide ions has been studied and mainly attributed as a catalyst with I₂ as the reaction intermediate, which reduced the activation energy for the reduction of H₂O₂.

Fabrication of FeS2/SiO2 Double Mesoporous Hollow Spheres as an Artificial Peroxidase and Rapid Determination of H2O2 and Glutathione

Nanozymes as one of artificial enzymes show many advantages than natural enzymes. The high Michaelis-Menten constant (K_m) to H₂O₂ is the drawback for nanozymes, which means a high H₂O₂ concentration to oxidize 3,3',5,5'-tetramethylbenzidine (TMB). For this problem, FeS₂/SiO₂ double mesoporous hollow spheres (DMHSs) were first synthesized as an artificial peroxidase through a solid reaction. The experimental results demonstrate that Fe₃O₄ vulcanization and DMHS formation were effective strategies to enhance affinity to H₂O₂ for the nanozyme. The K_m of FeS₂/SiO₂ DMHSs (H₂O₂ as the substrate) is 18-fold smaller than that of FeS₂ nanoparticles (NPs). The catalytic efficiency (K_cat/K_m) of FeS₂/SiO₂ DMHSs is about 16 times higher than that of FeS₂ NPs. FeS₂/SiO₂ DMHSs can be used as a nanozyme to sensitively and rapidly detect H₂O₂ and glutathione within 1 min at room temperature.

Detection of toxin B of Clostridium difficile based on immunomagnetic separation and aptamer-mediated colorimetric assay

Clostridium difficile can cause antibiotic-associated diarrhoea or pseudo-membranous colitis in humans and animals. Currently, the various methods such as microbiological culture, cytotoxic assay, ELISA and polymerase chain reaction have been used to detect Clostridium difficile infection (CDI). These conventional methods, however, require long detection time and professional staff. The paper is to describe a simple strategy which employs immunomagnetic separation and aptamer-mediated colorimetric assay for the detection of toxin B of C. difficile (TcdB) in the stool samples. HRP-labelled aptamer against TcdB selected by SELEX was firstly captured on the surface of magnetic beads (MB) by DNA hybridization with a complementary strand. In the presence of TcdB, aptamer specifically recognized and bound TcdB, disturbing the DNA hybridization and causing the release of HRP-aptamer from MB. This reduced the catalytic capacity of HRP and consequently the absorption intensity. As there was a relationship between the decrease in the absorption intensity and target concentration, a quantitative analysis of TcdB can be accomplished by the measurement of the absorption intensity. Under the optimal conditions, the assay system is able to detect TcdB at a concentration down to 5 ng ml^-1 . Moreover the method had specificity of 97% and sensitivity of 66% and the system remained excellent stability within 4 weeks. The proposed method is a valuable screening procedure for CDI and can be extended readily to detection of other clinically important pathogens.

Optimization of a Colorimetric Assay to Determine Lactate Dehydrogenase B Activity Using Design of Experiments

Lactate dehydrogenase B (LDH-B) is overexpressed in lung and breast cancer, and it has been considered as a potential target to treat these types of cancer. Herein, we propose a straightforward incomplete factorial (IF) design composed of 12 combinations of two reaction buffers, three pH values, three salt (NaCl) concentrations, and three incubation times, which we called IF-BPST (Buffer/pH/Salt/Time), for the optimization of a colorimetric LDH-B assay in a final volume of 100 µL using 96-well plates. The assay is based on the absorbance change at ~570 nm and the color change of the reaction mixture due to the release of NADH that reacts with nitroblue tetrazolium (NBT) and phenazine methosulfate (PMS), resulting in the formation of a blue-purple formazan. The results obtained using the IF-BPST were comparable with those obtained by response surface methodology. Our work revealed that the NBT/PMS assay with some modifications can be used to measure the activity of LDH-B and other dehydrogenases in a high-throughput screening format at the early stages of drug discovery. LDH-B containing lysates cannot be assayed directly, however, due to the sensitivity of the method toward detergents. Thus, we suggest precipitating the proteins in the lysates to remove the interfering detergents, and then to dissolve the protein pellet in a suitable buffer and carry out the assay.

Enhancement of the Peroxidase-Like Activity of Iodine-Capped Gold Nanoparticles for the Colorimetric Detection of Biothiols

A colorimetric assay was developed for the detection of biothiols, based on the peroxidase-like activity of iodine-capped gold nanoparticles (AuNPs). These AuNPs show a synergetic effect in the form of peroxidase-mimicking activity at the interface of AuNPs, while free AuNPs and iodine alone have weak catalytic properties. Thus, iodine-capped AuNPs possess good intrinsic enzymatic activity and trigger the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), leading to a change in color from colorless to yellow. When added to solution, biothiols, such as cysteine, strongly bind to the interface of AuNPs via gold-thiol bonds, inhibiting the catalytic activity of AuNPs, resulting in a decrease in oxidized TMB. Using this strategy, cysteine could be linearly determined, at a wide range of concentrations (0.5 to 20 μM), with a detection limit of 0.5 μM using UV-Vis spectroscopy. This method was applied for the detection of cysteine in diluted human urine.

A Straightforward Assay for Screening and Quantification of Biosurfactants in Microbial Culture Supernatants

A large variety of microorganisms produces biosurfactants with the potential for a number of diverse industrial applications. To identify suitable wild-type or engineered production strains, efficient screening methods are needed, allowing for rapid and reliable quantification of biosurfactants in multiple cultures, preferably at high throughput. To this end, we have established a novel and sensitive assay for the quantification of biosurfactants based on the dye Victoria Pure Blue BO (VPBO). The assay allows the colorimetric assessment of biosurfactants directly in culture supernatants and does not require extraction or concentration procedures. Working ranges were determined for precise quantification of different rhamnolipid biosurfactants; titers in culture supernatants of recombinant Pseudomonas putida KT2440 calculated by this assay were confirmed to be the same ranges detected by independent high-performance liquid chromatography (HPLC)-charged aerosol detector (CAD) analyses. The assay was successfully applied for detection of chemically different anionic or non-ionic biosurfactants including mono- and di-rhamnolipids (glycolipids), mannosylerythritol lipids (MELs, glycolipids), 3-(3-hydroxyalkanoyloxy) alkanoic acids (fatty acid conjugates), serrawettin W1 (lipopeptide), and N-acyltyrosine (lipoamino acid). In summary, the VPBO assay offers a broad range of applications including the comparative evaluation of different cultivation conditions and high-throughput screening of biosurfactant-producing microbial strains.

The Colorimetric Isothermal Multiple-Self-Matching-Initiated Amplification Using Cresol Red for Rapid and Sensitive Detection of Porcine Circovirus 3

In 2016, a novel porcine circovirus (PCV), PCV3, was identified in USA. Subsequently, it was proved to be also epidemic in China, Poland, and Korea. To analyze and control the epidemic situation of PCV3, it is necessary to establish accurate and high-throughput detection methods. In this study, the colorimetric isothermal multiple-self-matching-initiated amplification (IMSA) using cresol red was developed to detect PCV3 for the first time. The reaction can be easily performed by incubating the tube at 63°C for 60 min. By the addition of pH-sensitive indicator dye cresol red, the initial color of the reaction mixture is red. When PCV3 capsid gene DNA was positive in the sample, the color of the reaction mixture changed from red to yellow after the isothermal incubation at 63°C, while the negative control maintained the red color. The colorimetric IMSA displayed good specificity in detecting PCV3, PCV2, and PCV1 and 4 porcine DNA pathogens. Moreover, it has a low and repeatable detection limit of 10 copies, which is consistent with TaqMan-based qPCR, but 10 times more sensitive than PCR. In diagnosing 128 clinical specimens, it not only showed 100% agreement with qPCR but also detected 15 positive results more than PCR. The colorimetric IMSA we offered might be a good choice for PCV3 epidemiological investigation and point-of-care testing.

A Microfluidic Paper-Based Analytical Device for Type-II Pyrethroid Targets in an Environmental Water Sample

A detection method for type-II pyrethroids in an environmental water sample using a microfluidic paper-based analytical device (µPAD) is reported here. The detection approach is based on the formation of cyanide from the hydrolysis of type-II pyrethroids and the colorimetric detection of cyanide on a layer-based µPAD. Parafilm and inexpensive laminating pouches were used to create a hydrophobic barrier for the assay on the µPAD. This detection approach was selective to type-II pyrethroids in water for which an environmental water sample was tested. The calibration curves for cypermethrin, deltamethrin, cyhalothrin, and fenvalerate ranged from 2 to 40 µg/mL without sample preconcentration. The lower concentrations of type-II pyrethroids can be assessed by including a preconcentration step prior to the detection on a µPAD. This detection system provides an alternative platform for fast, semiquantitative testing for pesticide contamination in environmental surface water by allowing for portability, low reagent/sample consumption, and low-cost testing.

Colorimetric Urinalysis for On-Site Detection of Metabolic Biomarkers

Over the past few decades, colorimetric assays have been developed for cost-effective and rapid on-site urinalysis. Most of these assays were employed for detection of biomarkers such as glucose, uric acid, ions, and albumin that are abundant in urine at micromolar to millimolar levels. In contrast, direct assaying of urinary biomarkers such as glycated proteins, low-molecular-weight reactive oxygen species, and nucleic acids that are present at significantly lower levels (nanomolar to picomolar) remain challenging due to the interferences from the urine sample matrix. State-of-the-art assays for detection of trace amounts of urinary biomarkers typically utilize time-consuming and equipment-dependent sample pretreatment or clean-up protocols prior to assaying, which limits their applicability for on-site analysis. Herein, we report a colorimetric assay for on-site detection of trace amount of generic biomarkers in urine without involving tedious sample pretreatment protocols. The detection strategy is based on monitoring the changes in optical properties of poly(3-(4-methyl-3'-thienyloxy)propyltriethylammonium bromide) upon interacting with an aptamer or a peptide nucleic acid in the presence and absence of target biomarkers of relevance for the diagnosis of metabolic complications and diabetes. As a proof of concept, this study demonstrates facile assaying of advanced glycation end products, 8-hydroxy-2'-deoxyguanosine and hepatitis B virus DNA in urine samples at clinically relevant concentrations, with limits of detection of ∼850 pM, ∼650 pM, and ∼ 1 nM, respectively. These analytes represent three distinct classes of biomarkers: (i) glycated proteins, (ii) low-molecular-weight reactive oxygen species, and (iii) nucleic acids. Hence, the proposed methodology is applicable for rapid detection of generic biomarkers in urine, without involving sophisticated equipment and skilled personnel, thereby enabling on-site urinalysis. At the end of the contribution, we discuss the opportunity to translate the homogeneous assay into a paper-based format.

Coupling Two Sequential Biocatalysts with Close Proximity into Metal-Organic Frameworks for Enhanced Cascade Catalysis

The encapsulation of multiple enzyme/nanoenzyme systems within mental-organic frameworks (MOFs) shows great promise for a myriad of practical applications. Herein, two sequential biocatalysts, oxidase and hemin, were coupled together with close proximity using a bifunctional polymer, poly(1-vinylimidazole) (PVI), and encapsulated into MOFs. As a demonstration of the power of such a protocol, glucose oxidase&PVI-hemin encapsulated in ZIF-8 showed significant enhancement of bioactivity for a cascade reaction compared to its counterpart without PVI. For the colorimetric assay of glucose, it showed a low limit of detection of 0.4 μM (S/N = 3), high selectivity, and excellent stability. Because there are numerous biocatalysts that can readily be coupled and encapsulated into MOFs, a myriad of interesting properties can be simply realized by encapsulating different sequential biocatalysts.

Colorimetric strategy for ascorbic acid detection based on the oxidase-like activity of silver nanoparticle single-walled carbon nanotube composites

A colorimetric assay for the determination and quantification of ascorbic acid (AA) is presented using silver nanoparticle (AgNP) single-walled carbon nanotube (AgNP/SWCNT) nanocomposites prepared using a microwave-assisted method. The AgNP/SWCNT nanocomposites possessed oxidase-like properties toward 3,3',5,5'-tetramethylbenzidine (TMB) and could catalyze the oxidation of TMB to form a blue oxidation product (λ_max = 652 nm) in the absence of H₂ O₂ . AA can specifically inhibit the oxidation of TMB, resulting in a decline of the absorbance value and blue colour fading. As such, amounts of AA can be assessed easily by the unaided eye and quantitatively using an ultraviolet-visible light spectrophotometer. Under the optimal reaction conditions, this strategy showed a good linearity ranging from 0.4 μM to 5.0 μM for AA detection, and the limit of detection was 130 nM. This assay was also applied for AA measurement in vitamin C tablets and juice samples that yielded satisfactory results.

Urinary PCA3 detection in prostate cancer by magnetic nanoparticles coupled with colorimetric enzyme-linked oligonucleotide assay

PCA3 is one of the most prostate cancer-specific genes described to date. Of note, PCA3 expression is detectable at high level in the urine of prostate cancer (PCa) patients. Accordingly, PCA3 is an ideal biomarker for PCa diagnosis. Several techniques for the measurement of this biomarker in urine have been developed but there are still some drawbacks. In this study, magnetic nanoparticle-based PCR coupled with streptavidin-horseradish peroxidase and a substrate for colorimetric detection was established as a potential assay for urinary PCA3 detection. The method provided a high specificity for PCA3 gene in LNCaP prostate cancer cell line. Additionally, this technique could detect PCA3 at femtogram level which was approximately 1,000-fold more sensitive than the conventional RT-PCR followed by agarose gel electrophoresis. The effectiveness of the method was assessed by PCA3 detection in clinical specimens. The relative PCA3 expression of PCa patients determined by this assay was significantly greater than that of benign prostatic hyperplasia (BPH) patients and healthy controls. The results of our test were comparable with the results of qRT-PCR. The proposed method is promising to distinguish between cancerous and non-cancerous groups. Altogether, this simple assay is practicable and useful for prostate cancer diagnosis.

Colorimetric Assay of Bacterial Pathogens Based on Co3O4 Magnetic Nanozymes Conjugated with Specific Fusion Phage Proteins and Magnetophoretic Chromatography

It is important to detect pathogens rapidly, sensitively, and selectively for clinical medicine, homeland security, food safety, and environmental control. We report here a specific and sensitive colorimetric assay that incorporated a bovine serum albumin-templated Co₃O₄ magnetic nanozyme (Co₃O₄ MNE) with a novel specific fusion phage protein and magnetophoretic chromatography to detect Staphylococcus aureus. The Co₃O₄ MNE was conjugated to S. aureus-specific fusion-pVIII (Co₃O₄ MNE@fusion-pVIII), screened from the S. aureus-specific phage AQTFLGEQD (the phage monoclone is denoted by the peptide sequence). The as-prepared triple-functional Co₃O₄ MNE@fusion-pVIII particles were capable of capturing S. aureus in sterile milk, which were then isolated from milk magnetically. Assisted by polyethylene glycol, the Co₃O₄ MNE@fusion-pVIII@S. aureus complex was separated from the free Co₃O₄ MNE@fusion-pVIII by magnetophoretic chromatography in an external magnetic field. After transferring the isolated Co₃O₄ MNE@fusion-pVIII@S. aureus complexes into a 96-well plate, diammonium salt of 2,2'-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) and H₂O₂ were added to develop color because of the peroxidase mimetics activity of the Co₃O₄ MNE. A S. aureus concentration within 10-10,000 cfu/mL in milk can be detected (detection limit: 8 cfu/mL). The as-developed method is simple, cost-efficient, and sensitive, which is useful for rapidly diagnosing pathogenic bacteria and helpful to prevent disease outbreaks induced by pathogens in developing countries.

Study of inhibitory potential and percent inhibition of oil of Syzygium aromaticum and leaves of Ocimum sanctum on ESBL enzyme from Escherichia coli in broilers of Jabalpur

OBJECTIVE:

The inhibitory potential and percent inhibition of Syzygium aromaticum oil and fresh juice of Ocimum sanctum leaves on beta-lactamase enzyme of cecal samples of healthy broilers were studied on samples phenotypically positive for extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli.

MATERIALS AND METHODS:

Four hundred cecal samples screened for ESBL-producing E. coli were collected from 38 poultry sale outlets located in Jabalpur. The effect of S. aromaticum oil and O. sanctum leaves was seen by colorimetric assay with CENTA and Nitrocefin as chromogenic substrate.

RESULTS:

Mean absorbance value was inversely propotional to the inhibitory potential. Syzigium aromaticum exhibited 0.4±0.02 and 0.41±0.03 mean absorbance value, 28 per cent and 27 per cent of inhibition with CENTA and Nitrocefin respectively. Ocimum sanctum mean absorbance value and per cent inhibition with CENTA and Nitrocefin was 2.03±0.02 and 10.0 ; 1.97±0.06 and 10.0 respectively (p>0.05) showing non- significant difference in CENTA and Nitrocefin activity. Tazobactum (100 μM) as standard control exhibited a mean absorbance value of 0.12 ± 0.01 and 0.13 ± 0.01 and percent inhibition of 99.88 and 98 against CENTA and Nitrocefin, respectively. Combination of Ocimum sanctum and Syzigium aromaticum showed range of 1.69±0.05 to 1.90±0.08 1.61±0.06 to 1.92±0.08 of absorbance value with per cent inhibition of 14 to 15.9 with CENTA and Nitrocefin respectively.

CONCLUSION:

The results depicted that the inhibition of beta-lactamase enzyme activity with S. aromaticum oil was higher than that of O. sanctum leaf juice, and combination of both the herbs showed not much difference in activity.

Colorimetric Assay for Uracil DNA Glycosylase Activity Based on Toehold-Mediated Strand Displacement Circuit

Herein, a novel enzyme-free and label-free strategy for colorimetric assay of uracil DNA glycosylase (UDG) activity, which relies on a target-activated toehold-mediated strand displacement (TMSD) circuit is described. The strategy employs a detection duplex probe composed of a uracil-containing strand (US) and a catalyst strand (CS). UDG present in a sample will cleave uracil bases within US and destabilize the detection duplex probe, which then leads to the dissociation of the detection duplex, releasing CS. The free CS promotes the TMSD reaction, consequently liberating a G-quadruplex DNAzyme strand (GS) which is initially caged by a blocker strand (BS). Notably, a fuel strand (FS) is supplemented to recycle the CS to promote another cycle of TMSD reaction. As a consequence, a large number of GSs are activated by UDG activity and a distinct colorimetric signal is produced through the oxidation of ABTS promoted by the peroxidase mimicking activity of the liberated GSs. Based on this design principle, UDG activity down to 0.006 U mL^-1 with excellent selectivity is successfully determined. The practical applicability of this assay is also demonstrated by reliably determining UDG activities in human serum.

Colorimetric Sensor Array for Human Semen Identification Designed by Coupling Zirconium Metal-Organic Frameworks with DNA-Modified Gold Nanoparticles

Rapid and accurate identification of semen is critical for male infertility diagnosis and the arrangement of personalized treatment. However, the complexity and diversity of samples impose lots of restrictions in detection. To solve this problem, we propose a colorimetric sensor array in this work by coupling zirconium metal-organic frameworks (Zr-MOFs) with single-stranded-DNA-decorated gold nanoparticles (ssDNA-AuNPs) for human semen identification. Because of the coordination interactions between the Zr₆ clusters and the DNA phosphate backbone, as well as π-π stacking and H-bonding, Zr-MOFs can absorb and precipitate AuNPs with the aid of single-stranded DNA. What's more, addition of semen samples in the test solution, proteins, or other contents in the samples will affect the co-precipitation of Zr-MOFs and ssDNA-AuNPs. Subsequently, the color of the supernatant will change and a method to identify human semen can be developed. Further studies reveal that the method can completely detect different semen cases based on the differences in inclusions, demonstrating the characteristics of simplicity, feasibility, and sensitivity in the application of male infertility diagnosis.

Colorimetric Determination of the Activity of Starch-Debranching Enzyme via Modified Tollens' Reaction

Nelson–Somogyi and 3,5-dinitrosalicylic acid (DNS) assays are the classical analytical methods for the determination of activity of starch-debranching enzymes, however, they have a narrow detection range and do not adapt to the quantitative measurement of linear polysaccharides. Herein, we developed a simple and accurate colorimetric assay for determining the activity of starch-debranching pullulanase through the modified Tollens’ reaction in combination with UV irradiation. Silver nanoparticles (AgNPs) were formed by reducing aldehyde groups in short-chain glucans (SCGs) generated by debranching of waxy maize starch using pullulanase through the modified Tollens’ reaction. In addition to providing a reducing moiety to the Tollens’ reaction, the debranching product, SCGs, also enhanced the colloidal stability of synthesized AgNPs, of which the amplitude of its surface plasmon resonance (SPR) absorbance peak was proportional to the concentration of SCGs ranging from 0.01–10 mg/mL. The detection limit of this system was 0.01 mg/mL, which was found to be 100 times higher than that of the conventional DNS assay. The purification of SCGs by recrystallization and gelatinization improved the selectivity of this colorimetric assay for debranching products, which provides a simple and accurate means of monitoring the debranching process and characterizing the activity of starch-debranching enzymes.

Microfluidic Actuation via 3D-Printed Molds toward Multiplex Biosensing of Cell Apoptosis

Multiplexed analysis of biochemical analytes such as proteins, enzymes, and immune products using a microfluidic device has the potential to cut assay time, reduce sample volume, realize high-throughput, and decrease experimental error without compromising sensitivity. Despite these huge benefits, the need for expensive specialized equipment and the complex photolithography fabrication process for the multiplexed devices have, to date, prevented widespread adoption of microfluidic systems. Here, we present a simple method to fabricate a new microfluidic-based multiplexed biosensing device by taking advantage of 3D-printing. The device is an integration of normally closed (NC) microfluidic valving units which offer superior operational flexibility by using PDMS membrane (E ∼ 1-2 MPa) and require minimized energy input (1-5 kPa). To systematically engineer the device, we first report on the geometrical and operational analysis of a single 3D-printed valving unit. Based on the characterization, we introduce a full prototype multiplexed chip comprising several microfluidic valves. The prototype offers-for the first time in a 3D-printed microfluidic device-the capability of on-demand performce of both a sequential and a parallel biochemical assay. As a proof of concept, our device has been used to simultaneously measure the apoptotic activity of 5 different members of the caspase protease enzyme family. In summary, the 3D-printed valving system showcased in this study overcomes traditional bottlenecks of microfabrication, enabling a new class of sophisticated liquid manipulation required in performing multiplexed sensing for biochemical assays.

Development of a Redox Dye-Based Rapid Colorimetric Assay for the Quantitation of Viability/Mortality of Pine Wilt Nematode

Control of pine wilt disease, which is caused by pine wilt nematode, Bursaphelenchus xylophilus, is heavily dependent on the use of chemicals such as abamectin. Although such chemicals are highly effective, demands for alternatives that are derived preferentially from natural sources, are increasing out of environmental concerns. One of the challenges to discovery of alternative control agents is lack of fast and efficient screening method that can be used in high-throughput manner. Here we described the development of colorimetric assay for the rapid and accurate screening of candidate nematicidal compounds/biologics targeting B. xylophilus. Contrary to the conventional method, which relies on laborious visual inspection and counting of nematode population under microscope, our method utilizes a redox dye that changes its color in response to metabolic activity of nematode population in a given sample. In this work, we optimized parameters of our colorimetric assay including number of nematodes and amount of redox dye, and tested applicability of our assay for screening of chemicals and biologics. We demonstrated that our colorimetric assay can applied to rapid and accurate quantification of nematode viability/mortality in a nematode population treated with candidate chemicals/biologics. Application of our method would facilitate high-throughput endeavors aiming at finding environment-friendly control agents for deadly disease of pine trees.

Gold Nanoparticle-Based Colorimetric Strategies for Chemical and Biological Sensing Applications

Gold nanoparticles are popularly used in biological and chemical sensors and their applications owing to their fascinating chemical, optical, and catalytic properties. Particularly, the use of gold nanoparticles is widespread in colorimetric assays because of their simple, cost-effective fabrication, and ease of use. More importantly, the gold nanoparticle sensor response is a visual change in color, which allows easy interpretation of results. Therefore, many studies of gold nanoparticle-based colorimetric methods have been reported, and some review articles published over the past years. Most reviews focus exclusively on a single gold nanoparticle-based colorimetric technique for one analyte of interest. In this review, we focus on the current developments in different colorimetric assay designs for the sensing of various chemical and biological samples. We summarize and classify the sensing strategies and mechanism analyses of gold nanoparticle-based detection. Additionally, typical examples of recently developed gold nanoparticle-based colorimetric methods and their applications in the detection of various analytes are presented and discussed comprehensively.

Development of Label-Free Colorimetric Assay for MERS-CoV Using Gold Nanoparticles

Worldwide outbreaks of infectious diseases necessitate the development of rapid and accurate diagnostic methods. Colorimetric assays are a representative tool to simply identify the target molecules in specimens through color changes of an indicator (e.g., nanosized metallic particle, and dye molecules). The detection method is used to confirm the presence of biomarkers visually and measure absorbance of the colored compounds at a specific wavelength. In this study, we propose a colorimetric assay based on an extended form of double-stranded DNA (dsDNA) self-assembly shielded gold nanoparticles (AuNPs) under positive electrolyte (e.g., 0.1 M MgCl2) for detection of Middle East respiratory syndrome coronavirus (MERS-CoV). This platform is able to verify the existence of viral molecules through a localized surface plasmon resonance (LSPR) shift and color changes of AuNPs in the UV-vis wavelength range. We designed a pair of thiol-modified probes at either the 5' end or 3' end to organize complementary base pairs with upstream of the E protein gene (upE) and open reading frames (ORF) 1a on MERS-CoV. The dsDNA of the target and probes forms a disulfide-induced long self-assembled complex, which protects AuNPs from salt-induced aggregation and transition of optical properties. This colorimetric assay could discriminate down to 1 pmol/μL of 30 bp MERS-CoV and further be adapted for convenient on-site detection of other infectious diseases, especially in resource-limited settings.

Enhanced Mimetic Enzyme Activity of Phosphorylated Porphyrin Nanocomposite Induced by Localized Surface Plasmon Resonance for Colorimetric Assay

Plasmon-enhanced light harvesting has been of great interest to enhance the catalytic efficiency of some composites or hybrids. The enhanced peroxidase-like activity of phosphorylated iron(III) porphyrin (TPPFe(III))-based nanocomposite, induced by localized surface plasmon resonance for a colorimetric assay, was developed in this study. Firstly, a phosphate group modification strategy was adopted to synthesize water-soluble iron(III) porphyrin materials. Then, the as-synthesized TPPFe(III) was covalently attached to core-shell gold nanorods (GNRs), GNR@Au₂S/AuAgS, to form TPPFe(III)-GNR@Au₂S/AuAgS nanocomposite, which shows greatly enhanced peroxidase-like activity compared to TPPFe(III). A mechanism for the enhanced peroxidase-like activity of TPPFe(III)-GNR@Au₂S/AuAgS was proposed, which results from a synergic effect of hot electrons excited by localized surface plasmon resonance and photogenerated electrons of the TPPFe(III), verified by experiments. Furthermore, a fast colorimetric assay for the detection of H₂O₂ and glucose was established based on the unique property of TPPFe(III)-GNR@Au₂S/AuAgS. This colorimetric assay was applied to determine practical human serum samples; satisfactory results demonstrate this method has high accuracy. The present study would not only provide some insights into the mechanism of plasmon-activated enzyme-like reactions, but also offer new strategies for improving the catalytic activity of a mimetic enzyme.

Distinctive Distribution of Secretory Phospholipases A₂ in the Venoms of Afro-Asian Cobras (Subgenus: Naja, Afronaja, Boulengerina and Uraeus)

The protein abundances of phospholipases A₂ in cobra venom proteomes appear to vary among cobra species. To determine the unique distribution of snake venom phospholipases A₂ (svPLA₂) in the cobras, the svPLA₂ activities for 15 cobra species were examined with an acidimetric and a colorimetric assay, using egg yolk suspension and 4-nitro-3-octanoyloxy benzoic acid (NOBA) as the substrate. The colorimetric assay showed significant correlation between svPLA₂ enzymatic activities with the svPLA₂ protein abundances in venoms. High svPLA₂ activities were observed in the venoms of Asiatic spitting cobras (Naja sputatrix, Naja sumatrana) and moderate activities in Asiatic non-spitters (Naja naja, Naja atra, Naja kaouthia), African spitters (subgenus Afronaja), and forest cobra (subgenus Boulengerina). African non-spitting cobras of subgenus Uraeus (Naja haje, Naja annulifera, Naja nivea, Naja senegalensis) showed exceptionally low svPLA₂ enzymatic activities. The negligible PLA₂ activity in Uraeus cobra venoms implies that PLA₂ may not be ubiquitous in all snake venoms. The svPLA₂ in cobra envenoming varies depending on the cobra species. This may potentially influence the efficacy of cobra antivenom in specific use for venom neutralization.