Determination of total aflatoxin using cysteamine-capped CdS quantum dots as a fluorescence probe

Bifunctionalized nanobioprobe based rapid color-shift assay for typhoid targeting Vi capsular polysaccharide

Typhoid fever is an acute illness caused by Salmonella Typhi and the current diagnostic gap leads to inaccurate, over-diagnosis of typhoid leading to excessive use of antibiotics. Herein, to address the challenges we describe a new rapid color-shift assay based on a novel bifunctional nanobioprobe (Vi-AgNP probe) that is functionalized with specific biomarker Vi polysaccharide and also has the co-presence of Ag as urease inhibitor. The immunoreactions between the Vi with specific antibodies (Abs) present in typhoid patient sample forms a shielding barrier over Vi-AgNP probe rendering the urease to be active, generating colored output. Vi polysaccharide coating on the AgNP was visualized using HRTEM. TEM was performed to get insight into shielding barrier formation by the Abs. MST (microscale thermophoresis) data showed less binding Kd of 7.43 μM in presence of Abs whereas probe with urease showed efficient binding with Kd 437 nM. The assay was validated using 53 human sera samples and proven effective with 100% sensitivity. The assay showed relative standard deviation (RSD) of 4.3% estimated using rabbit anti-Vi Abs. The entire procedure could be completed within 15 min. Unlike lateral flow based assays, our assay does not require multiple combination of Abs for detection. The assay format was also found compatible in paper strip test that provides promising opportunities to develop low-cost on-spot assay for clinical diagnostics.

Size- and surface functionalization-driven molecular interaction of CdSe quantum dots with jack bean urease: multispectroscopic, thermodynamic, and AFM approach

Quantum dots (QDs) with distinctive optical properties have been extensively researched and developed for usage in solar cells, imaging, drug delivery, cellular targeting, etc. But the inevitable production of QDs can lead to their unavoidable release and increased environmental concentration. Depending on morphological and surface properties, QDs at the nano-bio interface considerably impact the activity and structure of bio-molecules. The present study investigates the interaction of metalloenzyme jack bean urease (JBU) and bi-sized CdSe QDs (2.43 nm and 3.63 nm), surface-functionalized to mercaptopropionic acid (MPA) (-COOH), L-cysteine (CYS), L-glutathione (GSH), N-acetyl L-cysteine (NAC) (-COOH, -NH₂), and cysteamine hydrochloride (CYST) (-NH₂) to assess any alterations in JBU's binding, microenvironment, structure, exciton lifetime, and activity. JBU catalyzes the hydrolysis of urea to produce ammonia and carbon dioxide; any changes in its properties could threaten the survival of several microbes and plants. Spectroscopy techniques such as UV-Vis, fluorescence, circular dichroism, synchronous, time-resolved fluorescence, atomic force microscopy, and JBU activity assay were studied. Results suggested highly spontaneous and energy-favored interactions, which involved static quenching and hydrophobic forces of varied magnitude, dependent on QDs properties. The size, surface modifications, and dosage of QDs significantly impacted the secondary structure and activity of JBUs. Even though the larger sizes of the relevant modifications demonstrated stronger binding, the smaller sizes had the greatest impact on α-helicity and activity. CYST-capped QDs with an average number of the binding site (n) = 1, reduced α-helicity by 16% and activity by 22-30% at 7 nM concentration. In contrast, MPA-capped QDs with n < 1 had the least effect on α-helical structure and activity. The smaller GSH-capped QDs increased the activity by 9%, via partially restoring JBU's α-helical content. The study thus thoroughly analyzed the impact of varied-size and surface-functionalized QDs on the structure and function of JBU, which can be exploited further for several biomedical applications.

Size Control and Improved Aqueous Colloidal Stability of Surface-Functionalized ZnGa2O4:Cr3+ Bright Persistent Luminescent Nanoparticles

Near-infrared (NIR)-emitting ZnGa₂O₄:Cr³⁺ (ZGO) persistent luminescent nanoparticles (PLNPs) have recently attracted considerable attention for diverse optical applications. The widespread use and promising potential of ZGO material in different applications arise from its prolonged post-excitation emission (several minutes to hours) that eliminates the need for continuous in situ excitation and the possibility of its excitation in different spectral regions (X-rays and UV-vis). However, the lack of precise control over particle size/distribution and its poor water dispersibility and/or limited colloidal stability required for certain biological applications are the major bottlenecks that limit its practical applications. To address these fundamental limitations, herein, we have prepared oleic acid (OA)-stabilized ZGO PLNPs with controlled size (7-12 nm, depending on the type of alcohol used in synthesis) and monodispersity. A further increase in size (8-21 nm), with a concomitant increase in persistent luminescence, could be achieved using a seed-mediated approach, employing the as-prepared ZGO PLNPs from the first synthesis as the seed and growing layers of the same material by adding fresh precursors. To remove their surface oleate groups and make the nanoparticles hydrophilic, two surface modification strategies were evaluated: modification with only poly(acrylic acid) (PAA) as the hydrophilic capping agent and modification with either PAA or cysteamine (Cys) as the hydrophilic capping agent in conjunction with BF₄^- as the intermediate surface modifier. The latter surface modifications involving BF₄^- conferred long-term (60 days and longer) colloidal stability to the nanoparticles in aqueous media, which is related to their favorable ζ potential values. The proposed generalized strategy could be used to prepare different kinds of surface-functionalized PLNPs with control of size, hydrophilicity, and colloidal stability and enhanced/prolonged persistent luminescence for diverse potential applications.

Photoluminescence-Based Bioassay With Cysteamine-Capped TiO2 Nanoparticles for the Selective Recognition of N-Acyl Homoserine Lactones

Currently available diagnostic procedures for infections are laborious and time-consuming, resulting in a substantial financial burden by increasing morbidity, increased costs of hospitalization, and mortality. Therefore, innovative approaches to design diagnostic biomarkers are imperative to assist in the rapid and sensitive diagnosis of microbial infections. Acyl homoserine lactones (AHLs) are ubiquitous bacterial signaling molecules that are found to be significantly upregulated in infected sites. In this pioneering work, we have developed a simple photoluminescence-based assay using cysteamine-capped titanium oxide (TiO2) nanoparticles for AHL detection. The PL intensity variation of the oxygen defect state of TiO2 was used for the biosensing measurements. The bioassays were validated using two well-studied AHL molecules (C4-HSL and 3-oxo-C12 HSL) of an important human pathogen, Pseudomonas aeruginosa. The developed system has a maximum relative response of 98%. Furthermore, the efficacy of the system in simulated host urine using an artificial urine medium showed a linear detection range of 10-160 nM. Also, we confirmed the relative response and specificity of the system in detecting AHLs produced by P. aeruginosa in a temporal manner.

Metal Nanoparticles-Enhanced Biosensors: Synthesis, Design and Applications in Fluorescence Enhancement and Surface-enhanced Raman Scattering

Metal nanoparticles (NP) that exhibit localized surface plasmon resonance play an important role in metal-enhanced fluorescence (MEF) and surface-enhanced Raman scattering (SERS). Among the optical biosensors, MEF and SERS stand out to be the most sensitive techniques to detect a wide range of analytes from ions, biomolecules to macromolecules and microorganisms. Particularly, anisotropic metal NPs with strongly enhanced electric field at their sharp corners/edges under a wide range of excitation wavelengths are highly suitable for developing the ultrasensitive plasmon-enhanced biosensors. In this review, we first highlight the reliable methods for the synthesis of anisotropic gold NPs and silver NPs in high yield, as well as their alloys and composites with good control of size and shape. It is followed by the discussion of different sensing mechanisms and recent advances in the MEF and SERS biosensor designs. This includes the review of surface functionalization, bioconjugation and (directed/self) assembly methods as well as the selection/screening of specific biorecognition elements such as aptamers or antibodies for the highly selective bio-detection. The right combinations of metal nanoparticles, biorecognition element and assay design will lead to the successful development of MEF and SERS biosensors targeting different analytes both in-vitro and in-vivo. Finally, the prospects and challenges of metal-enhanced biosensors for future nanomedicine in achieving ultrasensitive and fast medical diagnostics, high-throughput drug discovery as well as effective and reliable theranostic treatment are discussed.

A photoluminescence biosensor for the detection of N-acyl homoserine lactone using cysteamine functionalized ZnO nanoparticles for the early diagnosis of urinary tract infections

A urinary tract infection (UTI) is a recurrent infection that requires timely diagnosis and appropriate treatment. Conventional urinalysis methods are laborious and time-consuming, and lack sensitivity and specificity. In this context, photoluminescence (PL)-based biosensors have gained more attention due to their fast response time, and enhanced sensitivity and specificity. In relation to this, a PL-based biosensor was developed using ZnO nanoparticles obtained via a microwave-assisted process functionalized with cysteamine (ZnO-Cys) to detect the quorum sensing signalling molecules of Gram-negative bacteria, N-acyl-homoserine lactones (AHLs). These AHLs are involved in bacterial communication and are responsible for activating virulence and pathogenicity. Biosensing measurements based on PL intensity variations corresponding to the O₂ acceptor defect level of ZnO with reference to ZnO-Cys were considered. A maximum sensitivity of 97% was achieved in the detection of AHL. The linear detection range of the developed biosensor was 10-120 nM in artificial urine media (AUM). The effect of pH on the sensitivity of the biosensor in AUM was also investigated and reported. The developed sensor was validated using the AHLs produced by Pseudomonas aeruginosa (MCC3101) in real-time analysis, which further confirmed the overall specificity and sensitivity.

A facile one-pot green synthesis of β-cyclodextrin decorated porous graphene nanohybrid as a highly efficient adsorbent for extracting aflatoxins from maize and animal feeds

In this paper, β-cyclodextrin (β-CD) supported on porous graphene nanohybrid (β-CDPG) was obtained by self-assembly of functionalized graphene nanosheets into a three-dimensional network in the presence of ascorbic acid via an in situ graphene oxide reduction and β-CD functionalization process during a hydrothermal reaction. The prepared supramolecular nanohybrid was further packed into a reusable syringe filter holder and applied as an adsorbent for solid phase extraction of four aflatoxins (B1, B2, G1, G2). Under optimal conditions, the detection limits and linear dynamic ranges were achieved in the range of 0.0075-0.030 μg kg^-1 and 0.025-100 μg kg^-1, respectively and the relative standard deviations were less than 6.1%. Good recoveries were observed for analyzing target AFs in maize and cereal-based chicken feed samples ranged from 90.5 to 105%. The method offered simultaneous advantages of high supramolecular recognition and enrichment capability of β-CD and the high specific surface area of the porous graphene.

Citric Acid Capped CdS Quantum Dots for Fluorescence Detection of Copper Ions (II) in Aqueous Solution

Citric acid capped CdS quantum dots (CA-CdS QDs), a new assembled fluorescent probe for copper ions (Cu2+), was synthesized successfully by a simple hydrothermal method. In this work, the fluorescence sensor for the detection of heavy and transition metal (HTM) ions has been extensively studied in aqueous solution. The results of the present study indicate that the obtained CA-CdS QDs could detect Cu2+ with high sensitivity and selectivity. It found that the existence of Cu2+ has a significant fluorescence quenching with a large red shifted (from greenish-yellow to yellowish-orange), but not in the presence of 17 other HTM ions. As a result, Cu₂S, the energy level below the CdS conduction band, could be formed at the surface of the CA-CdS QDs and leads to the quenching of fluorescence of CA-CdS QDs. Under optimal conditions, the copper ions detection range using the synthesized fluorescence sensor was 1.0 × 10‒8 M to 5.0 × 10‒5 M and the limit of detection (LOD) is 9.2 × 10‒9 M. Besides, the as-synthesized CA-CdS QDs sensor exhibited good selectivity toward Cu2+ relative to other common metal ions. Thus, the CA-CdS QDs has potential applications for detecting Cu2+ in real water samples.

Early diagnosis of disease using microbead array technology: A review

Early diagnosis of diseases (before they become advanced and incurable) is essential to reduce morbidity and mortality rates. With the advent of novel technologies in clinical laboratory diagnosis, microbead-based arrays have come to be recognized as an efficient approach, that demonstrates useful advantages over traditional assay methods for multiple disease-related biomarkers. Multiplexed microbead assays provide a robust, rapid, specific, and cost-effective approach for high-throughput and simultaneous screening of many different targets. Biomolecular binding interactions occur after applying a biological sample (such as blood plasma, saliva, cerebrospinal fluid etc.) containing the target analyte(s) to a set of microbeads with different ligand-specificities that have been coded in planar or suspension arrays. The ligand-receptor binding activity is tracked by optical signals generated by means of flow cytometry analysis in the case of suspension arrays, or by image processing devices in the case of planar arrays. In this review paper, we discuss diagnosis of cancer, neurological and infectious diseases by using optically-encoded microbead-based arrays (both multiplexed and single-analyte assays) as a reliable tool for detection and quantification of various analytes.

Multiplexed microarrays based on optically encoded microbeads

In recent years, there has been growing interest in optically-encoded or tagged functionalized microbeads as a solid support platform to capture proteins or nucleotides which may serve as biomarkers of various diseases. Multiplexing technologies (suspension array or planar array) based on optically encoded microspheres have made possible the observation of relatively minor changes in biomarkers related to specific diseases. The ability to identify these changes at an early stage may allow the diagnosis of serious diseases (e.g. cancer) at a time-point when curative treatment may still be possible. As the overall accuracy of current diagnostic methods for some diseases is often disappointing, multiplexed assays based on optically encoded microbeads could play an important role to detect biomarkers of diseases in a non-invasive and accurate manner. However, detection systems based on functionalized encoded microbeads are still an emerging technology, and more research needs to be done in the future. This review paper is a preliminary attempt to summarize the state-of-the-art concerning diagnostic microbeads; including microsphere composition, synthesis, encoding technology, detection systems, and applications.

Designed Strategies for Fluorescence-Based Biosensors for the Detection of Mycotoxins

Small molecule toxins such as mycotoxins with low molecular weight are the most widely studied biological toxins. These biological toxins are responsible for food poisoning and have the potential to be used as biological warfare agents at the toxic dose. Due to the poisonous nature of mycotoxins, effective analysis techniques for quantifying their toxicity are indispensable. In this context, biosensors have been emerged as a powerful tool to monitors toxins at extremely low level. Recently, biosensors based on fluorescence detection have attained special interest with the incorporation of nanomaterials. This review paper will focus on the development of fluorescence-based biosensors for mycotoxin detection, with particular emphasis on their design as well as properties such as sensitivity and specificity. A number of these fluorescent biosensors have shown promising results in food samples for the detection of mycotoxins, suggesting their future potential for food applications.

New insight into the selective photocatalytic oxidation of RhB through a strategy of modulating radical generation

Rhodamine B (RhB) has often been used as a model pollutant, but its photocatalytic mechanism is still controversial. Herein, Ag NPs were sandwiched between CdS QDs and amorphous-TiO2 (a-TiO2) with the intent to build a CdS/Ag/a-TiO2 catalyst with highly selective oxidation activity. When rhodamine B (RhB) was used as the model organic compound, the CdS/Ag/a-TiO2 composite can not only modulate radical generation but also improve the conversion ratio of RhB to rhodamine 110 (Rh-110) to as high as 82% at 80 min during the visible-light irradiation. A series of the radical scavenging experiments revealed that CdS/Ag/a-TiO2 composites could modulate the effects of hydroxyl radicals (·OH) and superoxide anion radicals (·O2 -) at different reaction stages so that the overoxidation of RhB and Rh-110 were repressed. Therefore, the transient state protection mechanism of selective oxidation of RhB was proposed to explain the reaction selectivity for Rh-110. Although the effects of both ·O2 - and ·OH are important during the photocatalytic selective oxidation of RhB, it is shown that the selective oxidation of RhB would be performed when the effect of ·O2 - is bigger than the ·OH, if not, RhB would be oxidized unselectively. Meanwhile, this may provide a new strategy for modulating radical generation in the photocatalysis of water phases.

An On-Site, Ultra-Sensitive, Quantitative Sensing Method for the Determination of Total Aflatoxin in Peanut and Rice Based on Quantum Dot Nanobeads Strip

An on-site, ultra-sensitive, and quantitative sensing method was developed based on quantum dot nanobeads (QDNBs) and a test strip for the determination of total aflatoxins (AFTs) in rice and peanuts. The monoclonal antibody against AFT (mAbAFT) was homemade and labeled with QDNB. After the pre-coating of the AFT antigen on the test line (T line), the competitive immunoreactions were conducted between AFT and AFT antigen on the T line with QDNBs-mAbAFT. Under optimal conditions, this approach allowed a rapid response towards AFT with a considerable sensitivity of 1.4 pg/mL and 2.9 pg/mL in rice and peanut matrices, respectively. The put-in and put-out durations were within 10 min. The recoveries for AFT in rice and peanut sample matrices were recorded from 86.25% to 118.0%, with relative deviations (RSD) below 12%. The assay was further validated via the comparison between this QDNB strip and the conventional HPLC method using spiked samples. Thus, the design provided a potential alternative for on-site, ultra-sensitive, and quantitative sensing of AFT that could also be expanded to other chemical contaminants for food safety.

Thioglycolic Acid-Capped CdS Quantum Dots Conjugated to α-Amylase as a Fluorescence Probe for Determination of Starch at Low Concentration

In the present research, water soluble thioglycolic acid-capped CdS quantum dots (QDs) were synthesized by chemical precipitation method. The characteristics of prepared quantum dots were determined using X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The obtained results revealed that CdS QDs have 5.60 nm crystallite size, hexagonal wurtzite structure and spherical morphology with less than 10 nm diameter. The photoluminescence (PL) spectroscopy was performed in order to study the effect of the presence of starch solutions. Blue emission peaks were positioned at 488 nm and its intensity quenched by increasing the concentration of starch solutions. The result of PL quenches in range of studied concentrations (0-100 ppm) was best described by Michaelis-Menten model. The amount of Michaelis constant (Km) for immobilized α-amylase in this system was about 68.08 ppm which showed a great tendency of enzyme to hydrolyze the starch as substrate. Finally, the limit of detection (LOD) was found to be about 2.24 ppm.