Quantum dot-based concentric FRET configuration for the parallel detection of protease activity and concentration

Ratiometric fluorometric assay triggered by alkaline phosphatase: Proof-of-concept toward a split-type biosensing strategy for DNA detection

Herein, a sensitive ratiometric and split-type fluorescent sensing platform has been constructed for DNA detection based on one signal precursor and two fluorescent signal indicators. In this assay, o-phenylenediamine (OPD) was selected as the signal precursor. On one hand, Cu2+ can oxidize OPD to produce 2, 3-diaminophenazine (DAP), which with an emission peak at 555 nm. On the other hand, ascorbic acid (AA) could react with Cu2+ to generate dehydroascorbic acid (DHAA), which could further react with OPD to form 3-(1, 2-dihydroxy ethyl)furo[3, 4-b]quinoxalin-1 (3H)-on (DFQ) with a strong emission peak at 420 nm. As a result, the formation of DAP was inhibited, and leading to the decrease of fluorescence intensity at 555 nm. Alkaline phosphatase (ALP) could catalyze the substrate l-ascorbic acid-2-phosphate (AA2P) to produce AA in situ. Inspired by the successful use of ALP as a biocatalytic marker in bioassay, a split-type ratiometric fluorescent assay has been designed for DNA detection by using H1N1 DNA as the target model. It was realized for ratiometric fluorescent determination of H1N1 in a linear ranging from 50 pM to 1.5 nM with a limit of detection of 10 pM. The novel strategy could reduce the mutual interferences between the biomolecular recognition system and the fluorescence signal conversion system, which improving the accuracy of detection and effectively reducing the background signal. Furthermore, the strategy provided a promising platform for biomarkers detection in the fields of ratiometric fluorescent biosensors and bioanalysis.

Multiplexed DNA and Protease Detection with Orthogonal Energy Transfer on a Single Quantum Dot Scaffolded Biosensor

Almost all pathogens, whether viral or bacterial, utilize key proteolytic steps in their pathogenesis. The ability to detect a pathogen's genomic material along with its proteolytic activity represents one approach to identifying the pathogen and providing initial evidence of its viability. Here, we report on a prototype biosensor design assembled around a single semiconductor quantum dot (QD) scaffold that is capable of detecting both nucleic acid sequences and proteolytic activity by using orthogonal energy transfer (ET) processes. The sensor consists of a central QD assembled via peptidyl-PNA linkers with multiple DNA sequences that encode complements to genomic sequences originating from the Ebola, Influenza, and COVID-19 viruses, which we use as surrogate targets. These are hybridized to complement strands labeled with a terbium (Tb) chelate, AlexaFluor647 (AF647), and Cy5.5 dyes, giving rise to two potential FRET cascades: the first includes Tb → QD → AF647 → Cy5.5 (→ = ET step), which is detected in a time-gated modality, and QD → AF647 → Cy5.5, which is detected from direct excitation. The labeled DNA-displaying QD construct is then further assembled with a RuII-modified peptide, which quenches QD photoluminescence by charge transfer and is recognized by a protease to yield the full biosensor. Each of the labeled DNAs and peptides can be ratiometrically assembled to the QD in a controllable manner to tune each of the ET pathways. Addition of a given target DNA displaces its labeled complement on the QD, disrupting that FRET channel, while protease addition disrupts charge transfer quenching of the central QD scaffold and boosts its photoluminescence and FRET relay capabilities. Along with characterizing the ET pathways and verifying biosensing in both individual and multiplexed formats, we also demonstrate the ability of this construct to function in molecular logic and perform Boolean operations; this highlights the construct's ability to discriminate and transduce signals between different inputs or pathogens. The potential application space for such a sensor device is discussed.

Highly Sensitive Detection of Chymotrypsin Based on Metal Organic Frameworks with Peptides Sensors

In this study, peptides and composite nanomaterials based on copper nanoclusters (CuNCs) were used to detect chymotrypsin. The peptide was a chymotrypsin-specific cleavage peptide. The amino end of the peptide was covalently bound to CuNCs. The sulfhydryl group at the other end of the peptide can covalently combine with the composite nanomaterials. The fluorescence was quenched by fluorescence resonance energy transfer. The specific site of the peptide was cleaved by chymotrypsin. Therefore, the CuNCs were far away from the surface of the composite nanomaterials, and the intensity of fluorescence was restored. The limit of detection (LOD) using Porous Coordination Network (PCN)@graphene oxide (GO) @ gold nanoparticle (AuNP) sensor was lower than that of using PCN@AuNPs. The LOD based on PCN@GO@AuNPs was reduced from 9.57 pg mL^-1 to 3.91 pg mL^-1. This method was also used in a real sample. Therefore, it is a promising method in the biomedical field.

Triple-FRET multi-purpose fluorescent probe for three-protease detection

A new, robust and reliable methodology for three-protease screening in a single-enzyme mode has been developed and verified, employing a multi-purpose peptide probe with three selectively cleavable sites furnished with four fluorophores. A triple-FRET-based single-excitation quadruple-emission concept for unambiguous sensing of trypsin, chymotrypsin and caspase-8 in the lowest detectable concentrations of 0.5 ng mL^-1, 0.2 μg mL^-1, and 2 U mL^-1, respectively, has been applied and graphically depicted. Then the developed 4-dye probe has been also studied from the perspective of simultaneous two-protease screening, which was found only partially feasible, primarily due to unselective chymotrypsin cleavage.

Developing FRET Networks for Sensing

Förster resonance energy transfer (FRET) is a widely used fluorescence-based sensing mechanism. To date, most implementations of FRET sensors have relied on a discrete donor-acceptor pair for detection of each analytical target. FRET networks are an emerging concept in which target recognition perturbs a set of interconnected FRET pathways between multiple emitters. Here, we review the energy transfer topologies and scaffold materials for FRET networks, propose a general nomenclature, and qualitatively summarize the dynamics of the competitive, sequential, homoFRET, and heteroFRET pathways that constitute FRET networks. Implementations of FRET networks for sensing are also described, including concentric FRET probes, other single-vector multiplexing, and logic gates and switches. Unresolved questions and future research directions for current systems are discussed, as are potential but currently unexplored applications of FRET networks in sensing.

A Dendrimer-Based Time-Gated Concentric FRET Configuration for Multiplexed Sensing

Förster resonance energy transfer (FRET) is widely used for the development of biological probes and sensors. In this context, the norm for multiplexed detection is deployment of multiple probes, each a discrete donor-acceptor pair. Concentric FRET (cFRET) probes enable multiplexed sensing with a single vector but, to date, have only been developed around semiconductor quantum dots, which may limit the scope of biological applications for such probes. Here, we demonstrate that dendrimers labeled with a luminescent terbium complex (Tb) are a viable and advantageous alternative platform for cFRET probes. Polyamidoamine dendrimers were functionalized with Tb, biotin, NeutrAvidin, and three types of dye-labeled oligonucleotide probes to establish a network of competitive and sequential Tb-to-dye and dye-to-dye FRET pathways. These probes were characterized physically and photophysically, and a time-gated multiplexed assay for DNA targets was demonstrated. The time-gating offered by the Tb allowed the rejection of background autofluorescence from serum. More broadly, this dendrimer-based architecture shows that cFRET is a general concept and is an important step toward a new generation of probes for biological sensing.

Photoluminescent Nanoparticles for Chemical and Biological Analysis and Imaging

Research related to the development and application of luminescent nanoparticles (LNPs) for chemical and biological analysis and imaging is flourishing. Novel materials and new applications continue to be reported after two decades of research. This review provides a comprehensive and heuristic overview of this field. It is targeted to both newcomers and experts who are interested in a critical assessment of LNP materials, their properties, strengths and weaknesses, and prospective applications. Numerous LNP materials are cataloged by fundamental descriptions of their chemical identities and physical morphology, quantitative photoluminescence (PL) properties, PL mechanisms, and surface chemistry. These materials include various semiconductor quantum dots, carbon nanotubes, graphene derivatives, carbon dots, nanodiamonds, luminescent metal nanoclusters, lanthanide-doped upconversion nanoparticles and downshifting nanoparticles, triplet-triplet annihilation nanoparticles, persistent-luminescence nanoparticles, conjugated polymer nanoparticles and semiconducting polymer dots, multi-nanoparticle assemblies, and doped and labeled nanoparticles, including but not limited to those based on polymers and silica. As an exercise in the critical assessment of LNP properties, these materials are ranked by several application-related functional criteria. Additional sections highlight recent examples of advances in chemical and biological analysis, point-of-care diagnostics, and cellular, tissue, and in vivo imaging and theranostics. These examples are drawn from the recent literature and organized by both LNP material and the particular properties that are leveraged to an advantage. Finally, a perspective on what comes next for the field is offered.

Interference-Free Duplex Detection of Total and Active Enzyme Concentrations at a Single Working Electrode

The duplex detection of both total and active enzyme concentrations without interferences at a single working electrode is challenging, especially when two different assays are combined. It is also challenging to obtain two different redox-cycling reactions without interference. Here, we present a simple but sensitive combined assay that is based on two redox-cycling reactions using two incubation periods and applied potentials at a single electrode. The assay combines an immunoassay for the determination of the total enzyme (total prostate-specific antigen, tPSA) concentration with a protease assay for the determination of the active enzyme (free PSA, fPSA) concentration. The immunoassay label and fPSA that are affinity-bound to the electrode are used for high sensitivity and specificity in the protease assay as well as the immunoassay. In the immunoassay, electrochemical-enzymatic (EN) redox cycling involving ferrocenemethanol is obtained at 0.1 V versus Ag/AgCl without incubation before the proteolytically released 4-amino-1-naphthol is generated. In the protease assay, EN redox cycling involving 4-amino-1-naphthol is obtained at 0.0 V after 30 min of incubation without ferrocenemethanol electro-oxidation. The detection procedure is almost the same as common electrochemical sandwich-type immunoassays, although the two different assays are combined. The duplex detection in buffer and serum is highly interference-free, specific, and sensitive. The detection limits for tPSA and fPSA are approximately 10 and 1 pg/mL, respectively.

A Heparin based dual ratiometric sensor for Thrombin

Thrombin is an important enzyme that plays a pivotal role in the blood clotting pathways. An imbalance in the activity of this enzyme is clinically known to be associated with various diseases, such as thrombosis, inflammation, atherosclerosis, and haemophilia, suggesting the need to devise sensors for Thrombin detection. However, the majority of the fluorescence-based Thrombin assays rely on fluorescence labelling assays or Thrombin specific recognition biomolecules, such as, aptamers or antibody which requires sophisticated techniques and makes it very expensive. Herein, we report a simple, selective, sensitive and label-free fluorescence detection scheme for Thrombin which is based on the interaction between Thrombin and a fluorescent complex of Heparin with a molecular rotor dye, Thioflavin-T. The detection scheme exploits selective interaction between cationic Thrombin and anionic Heparin to modulate the monomer-aggregate equilibrium of the Thioflavin-T-Heparin system. Importantly, the present system offers a ratiometric response that has the ability for robust quantification of Thrombin concentration even in complex medium. The involvement of all commercially available components is a crucial advantage of this detection scheme. Further, the detection scheme also shows reasonable response in diluted serum matrix.

Metal-Enhanced Fluorescence by Bifunctional Au Nanoparticles for Highly Sensitive and Simple Detection of Proteolytic Enzyme

Although fluorescence-based analytical methods have been used in intracellular analyses, their sensitivity is low for the precise analysis of intracellular proteolytic enzymes to observe cell apoptosis related to cancer and neurodegenerative diseases. In this study, a metal-enhanced-fluorescence (MEF)-based highly sensitive biosensor for the detection of proteolytic enzymes is proposed for the first time by using a bifunctional Au nanoparticle (AuNP), which is connected to the fluorophore by both single-stranded DNA (ssDNA) and a peptide. Once caspase-3, a proteolytic enzyme, cuts the peptide specifically, the fluorescence signal is drastically increased because the ssDNA maintains an optimal distance for the MEF. The proposed sensing method shows the highly sensitive detection of caspase-3 based on just a simple enzymatic cleavage reaction within 1 h, and caspase-3-related preapoptotic cell detection was successfully carried out with high sensitivity. The proposed sensing method is a rapid, simple, and one-step technique for the real-time monitoring of intracellular proteolytic enzymes and can be applied to the early diagnosis of cancer and neurodegenerative diseases.

Perspective on chymotrypsin detection

Chymotrypsin is one of the most extensively known proteases participating in the pathogenesis of various diseases, which can be used in drug discovery and clinical diagnosis.

Recent developments in protease activity assays and sensors

Proteases play a pivotal role in regulating important physiological processes from food digestion to blood clotting. They are also important biomarkers for many diseases such as cancers. The importance of proteases has led to extensive efforts in the screening of proteases and their inhibitors as potential drug molecules. For example, human immunodeficiency virus (HIV) patients have been treated with HIV-1 protease inhibitors to prolong the life expectancy of patients. Such a close relationship between diseases and proteases provides a strong motivation for developing sensitive, selective, and robust protease assays and sensors, which can be exploited to discover new proteases and inhibitors. In this aspect, protease assays based on levels of proteolytic activities are more relevant than protease affinity assays such as immunoassays. In this review, recent developments of protease activity assays based on different detection principles are discussed and compared. For homogenous assays, fluorescence-based techniques are the most popular due to their high sensitivity and quantitative results. However, homogeneous assays have limited multiplex sensing capabilities. In contrast, heterogeneous assays can be employed to detect multiple proteases simultaneously, given the microarray technology that is already available. Among them, electrochemical methods, surface spectroscopy techniques, and enzyme-linked peptide protease assays are commonly used. Finally, recent developments in liquid crystal (LC)-based protease assays and their applications for detecting proteases and their inhibitors are discussed.

Colorimetric detection of protein via the terminal protection of small-molecule-linked DNA and unmodified gold nanoparticles

A novel colorimetric strategy for protein detection was developed based on unmodified gold nanoparticles (AuNPs) and terminal protection from a target protein.

Elucidating Surface Ligand-Dependent Kinetic Enhancement of Proteolytic Activity at Surface-Modified Quantum Dots

Combining biomolecules such as enzymes with nanoparticles has much to offer for creating next generation synergistically functional bionanomaterials. However, almost nothing is known about how these two disparate components interact at this critical biomolecular-materials interface to give rise to improved activity and emergent properties. Here we examine how the nanoparticle surface can influence and increase localized enzyme activity using a designer experimental system consisting of trypsin proteolysis acting on peptide-substrates displayed around semiconductor quantum dots (QDs). To minimize the complexity of analyzing this system, only the chemical nature of the QD surface functionalizing ligands were modified. This was accomplished by synthesizing a series of QD ligands that were either positively or negatively charged, zwitterionic, neutral, and with differing lengths. The QDs were then assembled with different ratios of dye-labeled peptide substrates and exposed to trypsin giving rise to progress curves that were monitored by Förster resonance energy transfer (FRET). The resulting trypsin activity profiles were analyzed in the context of detailed molecular dynamics simulations of key interactions occurring at this interface. Overall, we find that a combination of factors can give rise to a localized activity that was 35-fold higher than comparable freely diffusing enzyme-substrate interactions. Contributing factors include the peptide substrate being prominently displayed extending from the QD surface and not sterically hindered by the longer surface ligands in conjunction with the presence of electrostatic and other productive attractive forces between the enzyme and the QD surface. An intimate understanding of such critical interactions at this interface can produce a set of guidelines that will allow the rational design of next generation high-activity bionanocomposites and theranostics.

Aptamer induced multicoloured Au NCs-MoS2 "switch on" fluorescence resonance energy transfer biosensor for dual color simultaneous detection of multiple tumor markers by single wavelength excitation

An aptamer induced "switch on" fluorescence resonance energy transfer (FRET) biosensor for the simultaneous detection of multiple tumor markers (e.g., AFP and CEA) combining molybdenum disulfide (MoS₂) nanosheets with multicolored Au NCs by a single excitation was developed for the first time. Here, AFP aptamer functionalized green colored Au NCs (510 nm) and CEA aptamer functionalized red colored Au NCs (650 nm) are used as energy donors, while MoS₂ is used as energy receptor. On the basis of recording the change of the recovered fluorescence intensity at 510 nm and 650 nm upon the addition of targets CEA and AFP, these two tumor markers can be simultaneously quantitatively detected, with detection limits of 0.16 and 0.21 ng mL^-1 (3σ) for AFP and CEA, respectively. In addition, it is noteworthy that the developed biosensor can not only realize accurate quantitative determination of multiple tumor markers by fluorescent intensity, but also be applied in semi-quantitative determination through photo visualization. More importantly, confocal microscope experiments prove that serums from normal and hepatoma patients can also be visually and qualitatively discriminated by this FRET-based biosensor with a single excitation wavelength, indicating promising potential of this assay for clinical diagnosis.

QD-Biopolymer-TSPP Assembly as Efficient BiFRET Sensor for Ratiometric and Visual Detection of Zinc Ion

In this work, we report a new type of quantum dot (QD)-based fluorescence resonance energy transfer (FRET) assembly and its utility for sensing Zn²⁺ in different media. The assembly on the QD scaffold is via first coating of poly(dA) homopolymer/double-stranded DNA, followed by loading of meso-tetra(4-sulfonatophenyl)porphine dihydrochloride (TSPP), both of which are electrostatic, offering the advantages of cost-efficiency and simplicity. More importantly, the biopolymer coating minimizes the interfacial thickness to be ≤2 nm for QD-TSPP FRET, which results in improvements of up to 60-fold for single FRET efficiency and nearly 4-fold for total FRET efficiency of the QD-biopolymer-TSPP assemblies in comparison with silica-coating-based QD-TSPP assemblies. On the basis of Zn²⁺-chelation-induced spectral modulation, dual-emission QD-poly(dA)-TSPP assemblies are developed as a ratiometric Zn²⁺ sensor with increased sensitivity and specificity. The sensor either in solution or on a paper substrate displays continuous color changes from yellow to bright green toward Zn²⁺, exhibiting excellent visualization capability. By utilizing the competitive displacement of Zn²⁺, the sensor is also demonstrated to have good reversibility. Furthermore, the sensor is successfully used to visualize exogenous Zn²⁺ in living cells. Together the QD-biopolymer-TSPP assembly provides an inexpensive, sensitive, and reliable sensing platform not only for on-site analytical applications but also for high-resolution cellular imaging.

A highly sensitive quantum dots-DNA nanobiosensor based on fluorescence resonance energy transfer for rapid detection of nanomolar amounts of human papillomavirus 18

A very sensitive and convenient nanobiosensor based on fluorescence resonance energy transfer (FRET) was developed for the detection of a 22-mer oligonucleotides sequence in Human Papillomavirus 18 virus (HPV18) gene. For this purpose, water-soluble CdTe quantum dots (QDs) were synthesized and, subsequently, amino-modified 11-mer oligonucleotide as one of the two necessary probes was attached to QDs surface to form functional QDs-DNA conjugates. Right after addition of the QDs-DNA and a second Cyanine5 (Cy5)-labeled 11-mer oligonucleotide probe to the DNA target solution, the sandwiched hybrids were formed. The resulting hybridization brings the Cy5 fluorophore as the acceptor to close proximity of the QDs as donor, so that an effective transfer of energy from the excited QDs to the Cy5 probe would occur via FRET processing. The fluorescence intensity of Cy5 found to linearly enhance by increasing the DNA target concentration from 1.0 to 50.0nM, with a detection limit of 0.2nM. This homogeneous DNA detection method does not require excessive washing and separation steps of un-hybridized DNA, due to the fact that no FRET can be observed when the probes are not ligated. Finally, feasibility and selectivity of the proposed one-spot DNA detection nanobiosensor were investigated by analysis of derived nucleotides from HPV18 and mismatched sequences.

A new two-mode fluorescence signal amplification strategy for protease activity assay based on graphene oxide

A new graphene oxide-based two-mode fluorescence signal amplification strategy for the detection of protease activity has been established.

Multifunctional Concentric FRET-Quantum Dot Probes for Tracking and Imaging of Proteolytic Activity

Proteolysis has many important roles in physiological regulation. It is involved in numerous cell signaling processes and the pathogenesis of many diseases, including cancers. Methods of visualizing and assaying proteolytic activity are therefore in demand. Förster resonance energy transfer (FRET) probes offer several advantages in this respect. FRET supports end-point or real-time measurements, does not require washing or separation steps, and can be implemented in various assay or imaging formats. In this chapter, we describe methodology for preparing self-assembled concentric FRET (cFRET) probes for multiplexed tracking and imaging of proteolytic activity. The cFRET probe comprises a green-emitting semiconductor quantum dot (QD) conjugated with multiple copies of two different peptide substrates for two target proteases. The peptide substrates are labeled with different fluorescent dyes, Alexa Fluor 555 and Alexa Fluor 647, and FRET occurs between the QD and both dyes, as well as between the two dyes. This design enables a single QD probe to track the activity of two proteases simultaneously. Fundamental cFRET theory is presented, and procedures for using the cFRET probe for quantitative measurement of the activity of two model proteases are given, including calibration, fluorescence plate reader or microscope imaging assays, and data analysis. Sufficient detail is provided for other researchers to adapt this method to their specific requirements and proteolytic systems of interest.

Silver enhanced ratiometric nanosensor based on two adjustable Fluorescence Resonance Energy Transfer modes for quantitative protein sensing

We developed a silver decahedral nanoparticles (Ag₁₀NPs)-enhanced ratiometric Fluorescence Resonance Energy Transfer (FRET) nanosensor based on two adjustable FRET modes. Alexa Fluor 488 (Alexa) and Cyanine3 (Cy3)-aptamer-Black hole quencher-2 (BHQ-2) were bound with Ag₁₀NPs to form the ratiometric FRET nanosensor (Ag-Alexa/Cy3/BHQ-2). Alexa act as donor and Cy3 as acceptor in the FRET mode 1 while Cy3 was donor and BHQ-2 was acceptor in the FRET mode 2. In the absence of platelet-derived growth factor (PDGF-BB), the fluorescence intensity of Alexa was lowest while that of Cy3 was highest. Upon the addition of PDGF-BB, Cy3-aptamer-BHQ-2 binds with PDGF-BB resulting in the change of structure of aptamer. The fluorescence intensity of Alexa increased while that of Cy3 decreased. In addition, the fluorescence intensity ratio of Alexa to Cy3 increased remarkably with PDGF-BB concentration in the range of 0.4-400ng/mL. A good linear response was obtained when the PDGF-BB concentrations were in the range of 3.1-200ng/mL, with the limit of detection at 0.4ng/mL. When compared to sensors without Ag₁₀NPs (Alexa/Cy3/BHQ-2) and one without BHQ-2 (Ag-Alexa/Cy3), the new nanosensor Ag-Alexa/Cy3/BHQ-2 showed remarkable increase in sensitivity.

Label-free fluorometric detection of chymotrypsin activity using graphene oxide/nucleic-acid-stabilized silver nanoclusters hybrid materials

Pancreatic function tests are used to determine the presence of chronic pancreatitis, particularly in the early stage of the disease. Chymotrypsin is an indicator of pancreatic function and is thus related to pancreatic diseases. A new fluorescent biosensing method for assay of chymotrypsin activity was developed using DNA (dC₁₂)-templated silver nanoclusters and graphene oxide (GO). A peptide probe was also designed using chymotrypsin-cleavable amino acid sequence and a cysteine terminus. The peptide probe formed Ag-S bond to dC₁₂-AgNCs to enhance the fluorescence of dC₁₂-AgNCs. After the addition of GO, the peptide was adsorbed to the negative GO surface and the fluorescence of dC₁₂-AgNCs was quenched by FRET. The peptide was then degraded into amino acid fragments upon addition of chymotrypsin; these fragments were released from the GO surface, and the FRET was terminated. The developed label-free method features lower cost and higher sensitivity to chymotrypsin activity assay compared with conventional fluorescence analysis. The method can be used to analyze chymotrypsin (as low as 3ng/mL, signal/noise =3) across a dynamic range of 0.0-50.0ng/mL. The proposed biosensing strategy can also be extended to other proteases by using different peptide substrates.

Emerging Physicochemical Phenomena along with New Opportunities at the Biomolecular-Nanoparticle Interface

Efforts to create new nanoparticle-biomolecule hybrids for diverse applications including biosensing, theranostics, drug delivery, and even biocomputation continue to grow at an unprecedented rate. As the composite designs become more sophisticated, new and unanticipated physicochemical phenomena are emerging at the nanomaterial-biological interface. These phenomena arise from two interrelated factors, namely, the novel architecture of nanoparticle bioconjugates and the unique physicochemical properties of their interfacial environment. Here we examine how the augmented functionality imparted by such hybrid structures, including accessing concentric energy transfer, enhanced enzymatic activity, and sensitivity to electric fields, is leading to new applications. We discuss some lesser-understood phenomena that arise at the nanoparticle interface, such as the complex and confounding issue of protein corona formation, along with their unexpected benefits. Overall, understanding these complex phenomena will improve the design of composite materials while uncovering new opportunities for their application.

A self-assembly aptasensor based on thick-shell quantum dots for sensing of ochratoxin A

A novel self-assembling aptasensor was fabricated by precisely attaching three phosphorothioate-modified capture aptamers onto a single thick-shell quantum dot in a controllable manner for monitoring of ochratoxin A (OTA), a poisonous contaminant widespread in foodstuffs. Herein, CdSe/CdS QDs coated in ten layer CdS shells were synthesized using a continual precursor injection method. Analysis of the prepared CdSe/CdS QDs showed a zinc-blende structure, high photoluminescence quantum yields (>80%), and a photoemission peak with a narrow full-width at half-maximum (about 29 nm), all qualities that render them as a superior choice for optical applications. By adjusting the number of phosphorothioate bases in the anchor domain, the tunable-valency aptasensor was able to self-assemble. In the sensing strategy, the thick-shell quantum dot was provided as an acceptor while OTA itself was used as a donor. In the presence of OTA, the capture aptamers drive the aptasensor function into a measurable signal through a fluorescence resonance energy transfer (FRET) system. The newly developed aptasensor had a detection limit as low as 0.5 ng mL(-1), with a linear concentration in the range of 1 to 30 ng mL(-1), and therefore meets the requirements for rapid, effective, and anti-interference sensors for real-world applications. Moreover, the high quality thick-shell QDs provide an ideal alternative for highly sensitive imaging and intensive illumination in the fields of biotechnology and bioengineering.

A long-wavelength quantum dot-concentric FRET configuration: characterization and application in a multiplexed hybridization assay

Quantum dot-based concentric Förster resonance energy transfer (cFRET) is a promising modality for the development of multifunctional fluorescent probes for bioanalysis and bioimaging. A new long-wavelength configuration and multiplexed hybridization assay format expands the scope of cFRET.

Spectroscopic investigation of alloyed quantum dot-based FRET to cresyl violet dye

Quantum dots (QDs), bright luminescent semiconductor nanoparticles, have found numerous applications ranging from optoelectronics to bioimaging. Here, we present a systematic investigation of fluorescence resonance energy transfer (FRET) from hydrophilic ternary alloyed quantum dots (CdSeS/ZnS) to cresyl violet dye with a view to explore the effect of composition of QD donors on FRET efficiency. Fluorescence emission of QD is controlled by varying the composition of QD without altering the particle size. The results show that quantum yield of the QDs increases with increase in the emission wavelength. The FRET parameters such as spectral overlap J(λ), Förster distance R0, intermolecular distance (r), rate of energy transfer k(T)(r), and transfer efficiency (E) are determined by employing both steady-state and time-resolved fluorescence spectroscopy. Additionally, dynamic quenching is noticed to occur in the present FRET system. Stern-Volmer (K(D)) and bimolecular quenching constants (k(q)) are determined from the Stern-Volmer plot. It is observed that the transfer efficiency follows a linear dependence on the spectral overlap and the quantum yield of the donor as predicted by the Förster theory upon changing the composition of the QD.

Thioflavin T as an Efficient G-Quadruplex Inducer for the Highly Sensitive Detection of Thrombin Using a New Föster Resonance Energy Transfer System

We report a new Föster resonance energy transfer (FRET) system that uses a special dye, thioflavin T (ThT), as an energy acceptor and a water-soluble conjugated polymer (CP) with high fluorescence as an energy donor. A simple, label-free, and sensitive strategy for the detection of thrombin in buffer and in diluted serum was designed based on this new system using ThT as an efficient inducer of the G-quadruplex. The difference between the blank and the positive samples was amplified due to distinctive FRET signals because thrombin has little effect on the intercalation of ThT into the G-quadruplex. In the absence of the target, ThT induces the aptamer to form a G-quadruplex and intercalates into it with strong fluorescence. The electrostatic attractions between the negatively charged G-quadruplex and positively charged CP allow a short donor-acceptor distance, resulting in a high FRET signal. However, in the presence of the target, the aptamer forms a G-quadruplex-thrombin complex first, followed by the intercalation of ThT into the G-quadruplex. A long distance exists between the donor and acceptor due to the strong steric hindrance from the large-sized thrombin, which leads to a low FRET signal. Compared with previously reported strategies based on the FRET between the CP and dye, our strategy is label-free, and the sensitivity was improved by an order of magnitude. Our strategy also shows the advantages of being simple, rapid (about 50 min), sensitive, label-free, and low-cost in comparison to strategies based on the FRET between quantum dots and dyes.